Abstract: Non stoichiometric layered double hydroxides (LDHs) with the hydrotalcite-like structure and containing Co2+ and Cr3+ cations in the brucite-like layers have been prepared by coprecipitation at room temperature from aqueous solutions of the corresponding chlorides. Five samples have been prepared, with Co/Cr molar ratios ranging from 2/1 to 1/2. The solids have been characterised by element chemical analysis for cations, powder X-ray diffraction, differential thermal analysis, thermogravimetric analysis, temperature-programmed reduction, Fourier transform-infrared and visible-ultraviolet spectroscopies, and particle size and specific surface area determination. The colorimetric parameters have been also measured. Their calcination at high temperature to yield homogeneously mixed metal oxides has been carried out. Both the original and the calcined solids have been dispersed in two enamels and the colorimetric parameters have been determined too. The optimum composition to yield a black pigment (bulk or dispersed in colourless or white enamels) has been determined. (C) 2011 Elsevier Ltd. All rights reserved.
Abstract: Recommendations are given for reporting in the primary scientific literature of measurements involving phase equilibrium. The focus is on documentation issues, and many of the recommendations may also be applied to the more general fields of thermodynamic and transport properties. The historical context of the work and specific plans for implementation of the recommendations are discussed.
Abstract: Three series of hybrid organic-inorganic layered double hydroxide samples containing Zn2+ and/or Mg2+ and Al3+ in the brucite-like layers (nominal M2+/Al molar ratio 2:1) and Diclofenac, Ketoprofen or Chloramphenicol Succinate as counter-anions, were synthesized by coprecipitation using drug/Al molar ratios of 2:1 to prepare the starting solutions. All samples have been characterised by element chemical analysis, powder X-ray diffraction, MIR spectroscopy, thermal analysis and morphology analysis (Scanning Electron Microscopy). The nanohybrids here prepared show a rather high crystallinity degree and the drug molecules are strongly interacting with the brucite-like layers. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: Layered double hydroxides (LDHs) containing M-II, Al-III, and Cr-III in the brucite-like layers (M = Cd, Zn) with different starting Al/Cr molar ratios and nitrate/carbonate as the interlayer anion have been prepared following the coprecipitation method at a constant pH: Zn-II-Al-III-Cr-III-CO32- at pH = 10, and Cd-II-Al-III-Cr-III-NO3- at pH = 8. Two additional M-II,Al-III-LDH samples (M = Cd, Zn) with chromate ions (CrO42-) in the interlayer have been prepared by ionic exchange at pH = 9 and 8, respectively, starting from M-II-Al-III-NO3-. The samples have been characterised by absorption atomic spectrometry, powder X-ray diffraction (PXRD), Fr-IR spectroscopy and transmission electron microscopy (TEM). Their thermal stability has been assessed by DTA-TG and mass spectrometric analysis of the evolved gases. The PXRD patterns of the solids calcined at 800 C show diffraction lines corresponding to ZnO and ZnAl2-xCrxO4 for the Zn-containing samples, and diffraction lines attributed to CdO and CdCr2O4 and (AI,Cr)(2)O-3 for the Cd-containing ones. Additionally a minority oxide, Cd2CrO5, is observed to Cd-II-Al-III-LDH samples with chromate ions in the interlayer. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: Luminescent hybrid materials derived from kaolinite appear as promising materials for optical applications due to their specific properties. The spectroscopic behavior of terbium picolinate complexes covalently grafted on kaolinite and the influence of the secondary ligand and thermal treatment on luminescence are reported. The resulting materials were characterized by thermal analysis, element analysis, X-ray diffraction, infrared absorption spectroscopy, and photoluminescence. The thermogravimetric curves indicated an enhancement in the thermal stability up to 300 degrees C for the lanthanide complexes covalently grafted on kaolinite, with respect to the isolated complexes. The increase in the basal spacing observed by X-ray diffraction confirmed the insertion of the organic ligands into the basal space of kaolinite, involving the formation of a bond between Al-OH and the carboxylate groups, as evidenced by infrared spectroscopy. The luminescent hybrid material exhibited a stronger characteristic emission of Tb(3+) compared to the isolated complex. The excitation spectra displayed a broad band at 277 nm, assigned to a ligand-to-metal charge transfer, while the emission spectra presented bands related to the electronic transitions characteristic of the Tb(3+) ion from the excited state (5)D(4) to the states (7)F(J) (J = 5, 4, and 3), with the 4 -> 5 transition having high intensity with green emission.
Abstract: Layered double hydroxides (LDH) containing zinc and aluminium within the brucite-like layers and carbonate as the interlayer anion have been used as precursors for the preparation of mixed metal oxides by calcination. Zinc oxide and the ZnAl(2)O(4) spinel were formed with different degree of crystallinity depending of two parameters, i.e., the Zn(2+)/Al(3+) molar ratio and the calcination temperature (500-800 degrees C) of the LDH precursor. A pure spinel phase was obtained to remove ZnO upon a basic post-treatment in NaOH solution of the samples calcined at 800 degrees C. All the samples were tested for the photodegradation of 2-propanol in gas-solid regime. All the samples resulted active as heterogeneous photocatalysts. The photocatalytic activity increased by increasing the Zn(2+)/Al(3+) molar ratio and the calcination temperature of the solids. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: Lanthanide (Eu and Nd)-doped hydrocalurnite has been prepared by a coprecipitation method. All samples, with Eu(3+) and Nd(3+) contents up to 4% (molar ratio), show a single crystallographic phase, without segregation of secondary lanthanide-containing phases. Calcination affords lime and mayenite, with lanthanide cations embedded in the structure. The photoluminescence spectra of the noncalcined samples consist of Eu(3+) (in the visible region) and Nd(3+) (in the NIR) intra-4f transitions and a broad band ascribed to Al-related defects. The Eu(3+5)D(0) lifetime values and quantum efficiency of the Eu(3+)/Nd(3+) codoped hydrocalumite decrease ca. 10%, relative to the single doped material, suggesting that the Nd(3+) incorporation activates nonradiative channels for the (5)D(0) depopulation. High-temperature stable ceramic pigments, prepared from Eu(3+) and Nd(3+) doped hydrocalumite precursors, afford multiwavelength luminescent materials emitting in large vis/NIR spectral region, with potential applications as barcodes and in broadband amplifiers. The emission spectra of the calcined samples give unequivocal attributed to Eu(3+) sites in CaO and mayenite. Upon calcination, the absolute emission quantum yield of the Eu(3+)-doped sample increases from less than 0.01 to 0.06.
Abstract: Conventional and microwave heating routes have been used to prepare PET-LDH (polyethylene terephthalate-layered double hydroxide) composites with 1-10 wt% LDH by in situ polymerization. To enhance the compatibility between PET and the LDH, terephthalate or dodecyl sulphate had been previously intercalated in the LDH. PXRD and TEM were used to detect the degree of dispersion of the filler and the type of the polymeric composites obtained, and FTIR spectroscopy confirmed that the polymerization process had taken place. The thermal stability of these composites, as studied by thermogravimetric analysis, was enhanced when the microwave heating method was applied. Dodecyl sulphate was more effective than terephthalate to exfoliate the samples, which only occurred for the terephthalate ones under microwave irradiation. (C) 2011 Elsevier Inc. All rights reserved.
Abstract: Saponites containing divalent Mg, Ni, or Fe as octahedral cations and trivalent Al and Fe substituting Si in the tetrahedral sheet were synthesized using microwave radiation. Saponite with a high specific surface area was obtained in all the syntheses, although Fe-Al saponite crystallized was impurified by Fe(2)O(3) and analcime. The catalytic activity of the solids for the epoxidation of (Z)-cyclooctene by hydrogen peroxide was tested, the solids obtained being highly active (conversion up to 8.8%, and 100% selectivity to the epoxide). (C) 2010 Elsevier B.V. All rights reserved.
Abstract: The synthesis of saponites with Mg(2+) as octahedral cation by using microwave radiation and their modification by mechanical grinding as a new way of synthesis of these materials were studied. The effect of time of irradiation during the microwave-assisted synthesis, and of the time of grinding during the further mechanical treatments was considered. Microwave radiation allowed obtaining saponites with structural formulae very close to the theoretical one and good ordering and properties employing short times of irradiation. Mechanical grinding induced the breakage of particles and the ordering of the layers, that is, smaller particles were formed, but they became much better oriented along the c direction than those existing in the as-synthesized saponites. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Thermochimica Acta has been serving the thermal science community for 40 years and has now published its 500th volume. The editors provide a brief overview of the highly cited papers published by the journal. Analysis of the topics of the highly cited papers suggests that Thermochimica Acta possesses major strength in the areas of instrumentation, materials, and kinetics. (C) 2010 Published by Elsevier B.V.
Abstract: The effect of microwave-hydrothermal treatment (MWHT) on the properties of Mg,Al-layered double hydroxides with the hydrotalcite-like structure intercalated with carbonate and organic anions (terephthalate, adipate and dodecyl sulphate) and on their thermal decomposition products is studied. All solids have been characterised by element chemical analysis, powder X-ray diffraction, FT-IR spectroscopy, thermal analysis, transmission electron microscopy, specific surface area determination by N(2) adsorption at -196 degrees C and particle size distribution. The MWHT yielded a larger improvement of the crystallinity for carbonate-containing samples than for the organic-intercalated ones; however, although the crystallinity is not greatly modified for the latter, the thermal properties and particle size distribution are significantly affected by the ageing treatment. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Two series of mixed oxides with formula [Eu2-xMx][Sn2-xMx]O7-3x/2 (M = Mg or Zn) have been synthesized. The study by X-ray diffraction and Fourier transform infrared spectroscopy shows that the solids obtained are new non-stoichiometric solid solutions with the pyrochlore type structure. For both series a decrease of the cell parameter is observed when the degree of substitution, x, increases. The structural refinements (X-ray studies) were achieved in the space group Fd-3m, no. 227 (origin at center-3m) by using the Fullprof software. The Rietveld refinements show that the divalent cations M2+ (Mg2+, Zn2+) substitute isomorphically for Eu3+ and Sn4+ ions producing vacancies in the anionic sublattice. (C) 2009 Elsevier Ltd. All rights reserved.
Abstract: Naproxen, a non-steroidal anti-inflammatory drug, has been immobilised in different inorganic matrices, namely Mg, Al layered double hydroxide (LDH), MCM-41 mesoporous silica and mesoporous alumina. The synthesised systems were characterised using several physicochemical techniques and their delivery rates tested through in vitro experiments. It was found that the amounts of drugs incorporated in the mesoporous matrices are quite similar, ranging between ca. 15 and 20% in wt, mainly depending on their pore size, while the LDH can host up to ca. 41% wt. In all cases the release rate of naproxen for the mesoporous systems is lower than that detected measured for the LDH system and all of them can be well fitted to the Fick's law. (C) 2009 Elsevier Inc. All rights reserved.
Abstract: Catalysts with the hydrotalcite-like structure and general formula NiCuAlyz-HT having (Ni + Cu)/Al atomic ratio of 3.0 with varying Ni:Cu atomic compositions (100: 0-50: 50) were synthesized, characterized and tested for isomerization of eugenol to isoeugenol. The decrease in the activity upon addition of copper to NiAl lattice is investigated through temperature programmed reduction and phenol adsorption.
Abstract: Following different preparation routes, fenbufen has been intercalated in the interlayer space of layered double hydroxides with Mg(2+) and Al(3+) or Mg(2+), Al(3+) and Fe(3+) in the layers. Well crystallized samples were obtained in most of the cases (intercalation was not observed by reconstruction of the MgAlFe matrix), with layer heights ranging between 16.1 and 18.8 angstrom. The presence of the LDH increases the solubility of fenbufen, especially when used as a matrix. The dissolution rate of the drug decreases when the drug is intercalated, and is even lower in those systems containing iron; release takes place through ionic exchange with phosphate anions from the solution. Preparation of microspheres with Eudragit (R) S 100 leads to solids with an homogeneous, smooth surface with efficient covering of the LDH surface, as drug release was not observed at pH lower than 7. (c) 2010 Elsevier Inc. All rights reserved.
Abstract: AlMCM-41 materials with SiO2/Al2O3 molar ratios 20, 70, 110, 150, 200, and Si-MCM-41 were synthesized following standard procedures, and loaded with different amounts of H3PO4. The catalysts were well characterized by powder X-ray diffraction (XRD), nitrogen adsorption studies, and solid state Si-29, Al-27, P-31, Na-23 and H-1 MAS NMR spectroscopy. Acidity measurement by FTIR spectroscopy monitoring of pyridine adsorption reveals that the incorporation of Al in the framework generates Bronsted and Lewis acidity, which increases with the increase in metal content. By loading H3PO4 on the AlMCM-41, Bronsted sites increases substantially, but the Lewis sites decrease to a large extent. Liquid phase isopropylation of naphthalene with isopropanol in n-hexane under autogeneous pressure was performed in the temperature range 473-623 K. The optimum feed molar ratio was found to be 1: 2: 10 (naphthalene: isopropanol: n-hexane), where the naphthalene conversion reaches to 85.5%. The beta-and beta, beta-selectivities over 30 wt% H3PO4/AlMCM41(200) were 97.0 and 84.8, respectively, at 85.5% conversion. The main products of the reaction, mono and diisopropylnaphthalenes, were analyzed and identified by gas chromatography and confirmed by GC-MS. The conversion and selectivity of the products are discussed from the point of view of catalyst characteristics and reaction conditions.
Abstract: An acetate-intercalated Ni-Zn layered hydroxide salt (LHS) was exchanged with chromate anions, achieving a rather large oxometalate concentration, which almost completely balanced the positive charges of the layers, as concluded from element chemical analysis, powder X-ray diffraction, and FT-IR spectroscopy results. Thermal decomposition of the solid led to dehydration and chromate anion grafting, followed by dehydroxylation of the host and reduction of the chromate anions. The solid then lost the layer structure, forming solids with large micro and mesoporosity, which corresponded to poorly crystallized (Zn, Ni)-Cr spinels, as well as nickel and zinc oxides. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: This work reports the immobilization of Fenbufen, a nonsteroidal anti-inflamatory drug, into two different hexagonal mesoporous silicas (MCM-41) which exhibit some differences in terms of morphology and pore size, and their behavior as systems for sustained release at pH 7 5. The drug/mesoporous silica systems have been characterized by powder X-ray diffractometry (PXRD), Fourier transform infrared spectroscopy (FT-IR), N(2) adsorption desorption, and transmission electron microscopy (TEM). The results show that the drug is mainly incorporated inside the pores, and its loading is dependent on both the pore size and the impregnation temperature The Fenbufen/mesoporous-silica systems give a well-sustained release profile, releasing 100% of the initially loaded drug at the end of the in vitro assays. (C) 2010 Wiley-Liss, Inc and the American Pharmacists Association J Pharm Sci, 99 3372-3380, 2010
Abstract: A new nanocomposite was obtained by dispersing an adipate-modified layered double hydroxide (Ad-LDH) with adipic acid and hexamethylene diamine. These samples were polymerized in the solid phase under a nitrogen flow for 200 min at 190 degrees C. The structural and compositional details of the nanocomposite were determined by powder X-ray diffraction (PXRD), fourier transform infrared (FTIR) spectroscopy, focused ion beam (FIB), thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The PXRD patterns and FIB images show a partially intercalated and partially exfoliated dispersion of layered crystalline materials in the polyamide 6.6 matrix. The best dispersion level is achieved in polyamide 6.6/LDH nanocomposites with low LDH loading. Some residual tactoids and particle agglomerates are also evident at high concentration. The best thermal stability of the nanocomposites is shown by the sample with 0.1% LDH content, for which it is higher than that of pure polyamide. (C) 2010 Elsevier Inc. All rights reserved.
Abstract: A series of hydrotalcite-type compounds containing Cu(II) and Al(III) in the layers, and carbonate or different alkylsulfonates in the interlayer, have been prepared and studied. Calcination of these solids gives rise to formation of metallic copper and Cu2+ and Cu+ oxides or sulfates, depending on the calcination temperature and on the precise nature of the interlayer alkylsulfonate. (C) 2008 Elsevier Masson SAS. All rights reserved.
Abstract: The present work reports the preparation of samples based on Millennium PC-500 TiO2 loaded with different amounts of tungstophosphoric acid (up to 30% in weight) and their characterisation by means of different physicochemical techniques: powder X-ray diffraction, N-2 adsorption-desorption at -196 degrees C, transmission electron microscopy, UV-vis and FT-IR spectroscopies and thermogravimetric analyses. The acidity of samples was evaluated by FTIR using pyridine as probe molecule, and the catalysts activity was tested for the photodegradation of 4-nitrophenol in aqueous solution and for the photo-oxidation of propan-2-ol in vapour phase. The results show a noticeable improvement (8 and 2.5 times faster in the photodegradation rate of 4-nitrophenol and propan-2-ol, respectively), in both regimes for these specific substrates, when tungstophosphoric acid is dispersed over the oxide up to a certain amount (20% in weight). A partial leaching of the polyoxometalate was also detected in solution for samples loaded with the largest amounts on this compound. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: The preparation of new layered double hydroxides/unsaturated polyester (LDH/UP) nanocomposites was performed and the effect of LDH on the resin properties was studied. Two different organo-LDHs have been prepared, adipate-LDH (A-LDH) and 2-methyl-2-propene-1-sulfonate-LDH (S-LDH); in order to evaluate the influence of these nanofillers, samples with two different concentrations were dispersed in the matrix. The physical, thermal, mechanical and fire reaction properties of nanocomposites were studied. Intercalated layered structures were observed for the different organo-LDH loadings (1 and 5 wt%). Mechanical properties studied Under flexural tests show that incorporation of organo-LDH in the resin reduces the flexural strength of polyester resin while the flexural modulus is unchanged for the S-LDH/UP composites and increased with 1 wt% of A-LDH. Adding 1 wt% of A-LDH to the resin produces an important reduction on the flexural strength, but an increase of the flexural modulus. The study of fire using cone calorimeter, suggested a significant reduction in the UP flammability, by reaction properties, 46 and 32%, by incorporating 1 wt% of A-LDH and 5 wt% S-LDH, respectively. Mass loss curves show enhanced char formation with the different loads tested while the amount of evolved smoke remains quite unchanged. (C) 2009 Elsevier Ltd. All rights reserved.
Abstract: The study of the ternary systems A(2)O(3) - SnO2 - MO (A = Eu or Y); (M = Mg or Zn) shows four new solid solutions (1-x) Eu2Sn2O7 - xMgO, (1-x) Y2Sn2O7 - x MgO, (1-x) Eu2Sn2O7 - x ZnO and (1-x) Y2Sn2O7 - x ZnO. The X-ray crystallographic study shows that their structure presents a strong analogy with the cubic. pyrochlore structures A(2)Sn(2)O(7) (A = Eu or Y) and a cubic symmetry (Fd (3) over bar m space group). This study has showed that these structures are non-stoechiometric and the lattice parameter a decreases when the degree of substitution x increases.
Abstract: Synthetic saponite containing Ni(2+) as octahedral cations has been prepared by a simple hydrothermal procedure, and has been intercalated with Al(13)-polycations. The catalytic performance of the pillared solids in the epoxidation of (Z)-cyclooctene and the oxidation of cyclohexanone in the presence of benzonitrile, Baeyer-Villiger reaction, using hydrogen peroxide (70%) as a clean oxidant have been studied. For comparison, Mg-saponite was synthesized under the same conditions and tested for the same reaction. (C) 2008 Elsevier Inc. Ail rights reserved.
Abstract: Nickel-aluminum layered double hydroxides have been prepared by conventional coprecipitation and by coprecipitation in the presence of a surfactant. The solids have been characterised by several physicochemical techniques. Calcination leads to formation of homogeneously dispersed mixed oxides, which have been characterised as well. The colour properties (lightness and chromaticity coordinates) of both series of solids (layered precursors and calcined ones) have been measured. It has been found that both the preparation method and the calcination treatment have an important effect on the luminosity (whiteness/darkness) of the solids, although the effect on the precise chromaticity coordinates (green/red and blue/yellow) is less marked. (C) 2009 Elsevier Inc. All rights reserved.
Abstract: Layered double hydroxides (LDHs) with Mg(2+), Al(3+), and Fe(3+) in the layers (molar ratios Mg(2+)/M(3+) = 2, Fe(3+)(Fe(3+) + Al(3+)) = 0.1) and different non-steroid antiinflammatory drugs, NSAIDs (mefenamic and meclofenamic acids and naproxen) in the interlayer space have been prepared. Anion exchange and coprecipitation procedures have been used, and the solids obtained have been characterised using different physicochemical techniques. The drug content was between 25 and 45% (weight) and depended on the precise nature of the drug and on the preparation procedure. Studies in solution, carried out with samples prepared by anion exchange, have shown a slow dissolution of the drug when intercalated in the inorganic matrix, even slower than when Mg, Al matrices matrixes with the same structure (LDH) were used. Fitting of the experimental data to different kinetics models indicated that the most adequate one is that proposed by Weibull. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: Two types of organo-layered double hydroxides (LDH), with adipate or dodecyl sulfate in the interlayer space, have been prepared by the coprecipitation. In order to improve the homogeneity and to obtain well crystallised solids, the samples were submitted to microwave (MW) irradiation. Hydrotalcite (HT)-like compounds were obtained in all cases. The TEM and particle size distribution point out that even short irradiation times led to homogeneous and well defined solids with small particle size. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: Carboxylate-intercalated (terephthalate, TA and oxalate, ox) layered double hydroxides (LDHs) are aged under a microwave-hydrothermal treatment. The influence of the nature of the interlayer anion during the ageing process is studied. Characterization results show that the microwave-hydrothermal method can be extended to synthesize LDHs with anions different than carbonate, like TA. LDH-TA compounds are stable under microwave irradiation for increasing periods of time and the solids show an improved order both in the layers and in the interlayer region as evidenced by powder X-ray diffraction (PXRD), Al-27 MAS NMR and FT-IR spectroscopy. Furthermore, cleaning of the surface through removal of some organic species adsorbed on the surface of the particles also occurs during the microwave-hydrothermal treatment. Conversely, although the expected increase in crystallinity is observed in LDH-ox samples, the side-reaction between Al3+ and ox is also enhanced under microwave irradiation, and a partial destruction of the structure takes place with an increase in the M2+/M3+ ratio and consequent modification of the cell parameters. (C) 2008 Elsevier Inc. All rights reserved.
Abstract: Layered double hydroxides( LDHs) with the hydrotalcite-type structure containing Co and Al, or Zn, Co and Al in the brucite-like layers and carbonate in the interlayer have been prepared by coprecipitation. The Zn/Co molar ratio was kept to 1 in all samples, while the divalent/trivalent molar ratio was varied from 2/1 to 1/2. The samples have been characterised by element chemical analysis, powder X-ray diffraction, differential thermal and thermogravimetric analysis, temperature-programmed reduction and FT-IR spectroscopy. A single hydrotalcite-like phase is formed for samples with molar ratio 2/1, which crystallinity decreases as the Al content is increased, developing small amounts of diaspore and dawsonite and probably an additional amorphous phase. Calcination at 1200 degrees C in air led to formation of spinels; a small amount of NaAlO(2) was observed in the Al-rich samples, which was removed by washing. The nature of the spinels formed(containing CoII, CoIII, AlIII and ZnII) strongly depends on the cations molar ratio in the starting materials and the calcination treatment, leading to a partial oxidation of Co(II) species to Co(III) ones. Colour properties(L*a*b*) of the original and calcined solids have been measured. While the original samples show a pink colour( lighter for the series containing Zn), the calcined Co, Al samples show a dark blue colour and the Zn, Co, Al ones a green colour. Changes due to the different molar ratios with in a given calcined series are less evident than between samples with the same composition in different series. These calcined materials could be usable as ceramic pigments. (C) 2009 Elsevier Inc. All rights reserved.
Abstract: This paper describes the synthesis and characterization of a composite constituted by an antiarthritic agent (AA) intercalated into a layered double hydroxide (LDH) supported on magnesium ferrite. Core-shell nanocomposites were prepared by depositing Mg-Al-NO(3)-LDH on a MgFe(2)O(4) core prepared by calcination of a nonstoichiometric Mg-Fe-CO(3)-LDH. Intercalation of ibuprofen and glucuronate anions was performed by ion-exchange with nitrate ions. The structural characteristics of the obtained products were investigated by powder X-ray diffraction, element chemical analysis, Fourier transform infrared spectroscopy, and thermogravimetric analysis. Morphologies of the nanocomposite particles were examined by scanning electron microscopy and transmission electron microscopy. The products were shown to intercalate substantial amounts of AA with enhanced thermal stabilities. Room-temperature magnetic measurements by vibrating sample magnetometry revealed that the products show soft ferromagnetic properties suitable for potential utilization in magnetic arthritis therapy.
Abstract: An organic-inorganic hybrid compound constituted by a layered double hydroxide (LDH) with the hydrotalcite-type structure intercalated with dodecyl sulfate (LDH-DS), was dispersed in poly(ethylene terephthalate), PET, through chemical reaction or by mechanical treatment in an agate mortar or a ball mill. Exfoliation and dispersion of the inorganic-organic hybrid in the polymer phase was achieved by chemical processes. Mechanical grinding of the LDH-DS and PET mixtures only led to partial dispersion after long milling times. both for the PET-LDH composites and for the LDH-DS hybrid. In situ preparation of the polymer by chemical reactions yielded optimum exfoliation at low LDH-DS loadings. As a result, mechanical processes are not recommended for preparation of these composites. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Both commercial and home prepared TiO2 samples impregnated with tungstophosphoric acid (H3PW12O40) were prepared and used for the photo-assisted degradation of 2-propanol in gas-solid regime. The characterization results evidenced a good coverage of the polyoxometalate (POM) onto the surface of both types of TiO2 samples along with a marginal effect of the presence of ethanol or HCl during the POM impregnation step on the specific surface area, porosity, morphology, crystallinity and acidity of the samples. Propene was the main intermediate product found in 2-propanol photocatalytic degradation by using the samples containing POM as the photocatalyst, whereas propanone was mainly obtained when the photocatalyst was bare TiO2. Acetaldehyde was also an intermediate product and its amount was always significantly smaller with respect to propanone and propene (when formed). Carbon dioxide and water were the ultimate oxidation products. The selectivity to propene has been attributed to the acidity of the POM supported samples. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Nanoporous sol gel glasses were used as host materials for the encapsulation of apomyoglobin, a model protein employed to probe in a rational manner the important factors that influence the protein conformation and stability in silica-based materials. The transparent glasses were prepared from tetramethoxysilane (TMOS) and modified with a series of mono-, di- and tri-substituted alkoxysilanes, R(n)Si(OCH(3))(4-n) (R = methyl-, n = 1; 2; 3) of different molar content (5, 10, 15%) to obtain the decrease of the siloxane linkage (-Si-O-Si-). The conformation and thermal stability of apomyoglobin characterized by circular dichroism spectroscopy (CD) was related to the structure of the silica host matrix characterized by (29)Si IWAS NMR and N(2) adsorption. We observed that the protein transits from an unfolded state in unmodified glass (TMOS) to a native-like helical state in the organically modified glasses, but also that the secondary structure of the protein was enhanced by the decrease of the siloxane network with the methyl modification (n = 0 < n = 1 < n = 2 < n = 3;0 < 5 < 10 < 15 mol %). In 15% trimethyl-modified glass, the protein even reached a maximum molar helicity (-24,000 deg. cm(2) mol(-1)) comparable to the stable folded home-bound holoprotein in solution. The protein conformation and stability induced by the change of its microlocal environment (surface hydration, crowding effects, microstructure of the host matrix) were discussed owing to this trend dependency. These results can have an important impact for the design of new efficient biomaterials (sensors or implanted devices) in which properly folded protein is necessary. (C) 2009 Wiley Periodicals, Inc. Biopolymers 91: 895-906, 2009.
Abstract: Microwave-hydrothermal treatment (MWHT), a modification of conventional hydrothermal treatment, has been used during post-treatment of different layered double hydroxides (LDHs). In some cases, microwaves (MWs) have been used simultaneously with urea hydrolysis or for reconstruction of the LDH structure. The main advantages of replacing the conventional furnaces by MW ovens are a noticeable reduction in the time required to complete the process to obtain well-crystallized materials, and modification of their particle size distribution and textural and thermal properties. MW radiation leads to an increase in the rate of urea hydrolysis and consequently to fast precipitation of LDHs. Finally, the memory effect of Ni,Al-based LDHs is also improved.
Abstract: Paracetamol was synthesized via the environmentally benign liquid phase Beckmann rearrangement of 4-hydroxyacetophenone oxime over Al-MCM-41 modified with H3PO4. Al-MCM-41 materials were synthesized using different aluminium sources. Aprotic solvents with a high dielectric constant or high polar nature are preferred for this transformation, acetone being the optimal solvent. The conversion of 4-hydroxyacetophenone oxime to paracetamol increased with the H3PO4 content in the catalysts up to a maximum value for a H3PO4 loading of 30 wt% and decreased for higher loadings. Recovery of the catalysts was also investigated. (C) 2009 Elsevier B. V. All rights reserved.
Abstract: Co-Zn-Al mixed oxides obtained from layered double hydroxide (LDH) precursors were synthesized and tested in the formation of multi-walled carbon nanotubes (MWNTs) by catalytic chemical vapour deposition (CCVD) of methane. The precursors were synthesized by co-precipitation from Co, Zn and Al nitrates with carbonate and submitted to microwave-hydrothermal treatment, in order to study the effect of time treatment on the catalyst performance. The microwave ageing treatment does not only play a role on some features of the precursors, such as crystallinity and textural properties, but also on the properties of the catalysts obtained by controlled calcination. The microwave-hydrothermal treatment affects to the distribution of the cations within the layers because of an improved ordering and this effect leads to better dispersed active species in the catalyst. In this sense, it was found that the aged catalysts reach better activity and stability levels during the methane decomposition reaction. Furthermore, the duration of microwave-hydrothermal treatment also produces a change of the kind of carbon nanofilaments produced. When the catalyst is not aged, herringbone carbon nanofibers (diameter similar to 14 nm) were found. On the other hand, multi-walled carbon nanotubes (diameter similar to 20-30 nm) could be observed in the sample after reaction with the 300 min aged catalyst. Finally, the application of a kinetic model based on the growing mechanism of nanocarbonaceous materials (NCMs), allows determining the influence of the microwave-hydrothermal treatment time on the kinetic parameters. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: The effect of a Mg- and Al-containing layered double hydroxide (LDH), used as an additive or as a matrix, on the dissolution and solubility of two non-steroidal anti-inflarrituatory drugs (NSAID-), namely, rnefenamic and meclofenamic acids, is reported. The use of MgAl-LDH enhances the Solubility of mcfenamic acid at the three pH values tested, and the value is larger when the acid is intercalated as opposed to simply being mixed with the layered material. For pure mefenainic acid, the most unfavourable pl-I is 1.2, while for the physical mixture and the sample with the intercalated drug, the most unfaVOUrable pH is 4.5, for which the minimum increase in the arnount of dissolved acid is recorded. The (dissolution) release-rate for both drugs when they are intercalated between the layers is much slower than for the physical mixture or for the dissolution of the pure drug.
Abstract: Organically-modified siloxanes were used as host materials to examine the influence of surface chemistry on protein conformation in a crowded environment. The sol-gel materials were prepared from tetramethoxysilane and a series of monosubstituted alkoxysilanes, RSi(OR')(3), featuring alkyl groups of increasing chain length in the R-position. Using circular dichroism spectroscopy in the far-UV region, apomyoglobin was found to transit from an unfolded state to a native-like helical state as the content of the hydrophobic precursor increased from 0 to 15%. At a fixed molar content of 5% RSi(OR')(3), the helical structure of apomyroglobin increased with the chain length of the R-group, i.e. methyl < ethyl < n-propyl < n-butyl < n-hexyl. This trend also was observed for the tertiary structure of ribonuclease A, suggesting that protein folding and biological activity are sensitive to the hydrophilic/hydrophobic balance of neighboring surfaces. The observed changes in protein structure did not correlate with total surface area or the average pore size of the modified glasses, but scanning electron microscopy images revealed an interesting relationship between surface morphology and alkyl chain length. The unexpected benefit of incorporating a low content of hydrophobic groups into a hydrophilic surface may lead to materials with improved biocompatibility for use in biosensors and implanted devices. (c) 2008 Elsevier Ltd. All rights reserved.
Abstract: The microwave-assisted reconstruction of Ni,Al hydrotalcite-like compounds (HTIcs) with Ni/Al molar ratios 2/1 and 3/1 has been studied. Mixed oxides obtained after calcination of the HTIcs are immersed in three different solutions containing carbonate, distilled water and an aqueous NH3 solution, and then heated at different temperatures for increasing periods of time under microwave radiation. The evolution of the structure during the treatment is followed by powder X-ray diffraction, FT-IR and vis-UV spectroscopies and SEM and TEM microscopies. Full recovery of the original layered structure is achieved in short periods of time for the 2/1 samples when the calcined HTlcs are rehydrated in the Na2CO3 Solution, but more drastic conditions are necessary for the 3/1 samples and the reconstruction seems not to be complete. Finally, only a partial reconstruction is observed in distilled water or NH3 aqueous solution. (C) 2008 Elsevier Inc. All rights reserved.
Abstract: A polyethyleaterephthalate-hydtotalcite (PET-FIT) composite was prepared by in silts polymerisation, using polyethylenterephthalate (PET) and different amounts of a hydrotalcite with dodecylsulphate in the interlayer space (DS-HT). The resulting material has been characterised by powder X-ray diffraction (PXRD) and thermogravimetric (TG) analysis. The thermal stability of PET is not greatly enhanced upon incorporation of the inorganic component, despite PXRD shows that the HT is exfoliated in the composite. Thus with 2-10% shows a complete dispersion of the layered component in the polymeric matrix, while it the inorganic loading is between 20-50 (larger loadings were not tested) no complete dispersion is achieved and PXRD-detectable agglomerates are formed.
Abstract: Vanadium-magnesium mixed oxides have been prepared by thermal decomposition of decavanadate-intercalated Mg, Al layered double hydroxides (LDHs), MgAIVO-LDH, and by impregnation of MgO (mixed, MgAIVO-1, or not, MgVO-I, with Al2O3) With an aqueous decavanadate solution. The activity of catalyst MgAlVO-LDH in oxidative dehydrogenation of propane is higher than that measured for the mixed oxides obtained by conventional impregnation. However, the largest normalised conversion measured over sample MgAIVO-1 is assigned to the higher surface density of V centers, which are stabilised along the reaction runs. Normalised selectivity and conversion profiles show that propene is first formed and then oxidized to CO on catalyst MgAIVO-1, a process favoured by the high acidity of this solid. However, catalyst MgVO-I, with a medium strength surface acidity, is rapidly deactivated, forming CO2, by coke deposition on exposed V sites. The low selectivity to propene of catalyst MgAIVO-LDH is related to its low surface density of V sites. (c) 2008 Elsevier B.V. All rights reserved.
Abstract: Zn,Al-CO(3) compounds with the hydrotalcite-like (layered double hydroxide, LDH) structure were prepared by a co-precipitation method followed by hydrothermal treatment under microwave irradiation. The influence of the ageing treatment was studied in two series of samples with different Zn(2+)/Al(3+) ratios, namely, 3/1 and 2/1. Moreover, the effects of the heating temperature and of the irradiation time were studied in order to select the optimum preparation conditions. The solids were fully characterised by powder X-ray diffraction (PXRD), (27)Al MAS-NMR, FT-IR spectroscopy, thermal analyses (DTA and TG), N(2) adsorption/desorption at -196 degrees C and TEM. The results showed that whatever the chemical composition of the starting mixture, HTlcs with Zn(2+)/Al(3+) ratio equal to 2 were obtained, and it was impossible to overcome the ZnO segregation when the molar ratio was >2. These compounds were stable and their crystallinity could be quickly enhanced when the temperature treatment was 100 degrees C, whereas at 125 degrees C the ZnO segregation was not prevented. (C) 2007 Elsevier Inc. All rights reserved.
Abstract: Mesoporous silica, prepared in basic conditions, has been loaded (20% weight) with 12-molybdophosphoric (PMo) or 12-tungstophosphoric (PW) acid and calcined at different temperatures ranging between 250 and 550 degrees C. The samples have been characterised by N(2) adsorption-desorption at -196 degrees C, transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), UV-visible diffuse reflectance, Raman spectroscopy and temperature programmed reduction (TPR). The acidity and catalytic activity have been, respectively, examined by monitoring the adsorption of pyridine and 2-butanol by FT-IR spectroscopy. The results indicate that PW and PMo acids are highly dispersed on mesoporous silica MCM-41 spherical nanoparticles. While PMo retains its Keggin structure up to 550 degrees C, PW decomposes at this temperature into crystalline W03 and phosphorous oxides. In both cases, the morphology, hexagonal symmetry and long-range order observed for the support are preserved with calcination up to 450 degrees C. The Bronsted-type acid sites found in all samples, whose surface concentration decreases as the calcination temperature increases, are responsible for the selective formation of cis-butene detected upon adsorption of 2-butanol. The sample containing PW calcined at 450 degrees C also shows selectivity to methyl ethyl ketone. (C) 2008 Elsevier Inc. All rights reserved.
Abstract: Acetate-containing nickel zinc layered hydroxysalts LHS-[Ni-Zn-AcO] have been exchanged with M2+-edta complexes (M2+= Cu2+, Cd2+ and Mn2+) at different pHs. Upon a careful selection of the reaction conditions, a rather large concentration of metal chelate units in the interlayer space is achieved, as concluded from element chemical analyses, powder X-ray diffraction and FT-IR spectroscopy results. Thermal decomposition of the edta-intercalated compounds leads to dehydration and dehydroxylation of the hydroxide matrix and to the oxidative decomposition of the edta anion. Immediately after edta decomposition the solids loose their layered structure, forming solids with low external surface areas and moderate micro- and meso-porosity, which correspond to poorly crystallised phases of the corresponding metal oxides, as demonstrated by powder X-ray diffraction analysis. (C) 2007 Elsevier Inc. All rights reserved.
Abstract: A simple procedure for preparing active TiO(2) photocatalysts is presented. The starting materials were unusual TiO(2) gels formed from TiCl(4). The use of microwaves for a very short time enhanced the TiO(2). crystallinity preventing an increase of particle size and minimizing the decrease of specific surface area. This result makes this preparation very attractive. The formation of the gels was monitored through measurements of viscosity. All the samples were characterized by means of X-ray diffraction, diffuse reflectance spectroscopy and BET specific surface area measurements. The photoactivity of the samples was evaluated using the photodegradation of 4-nitrophenol in liquid-solid regimen and gaseous 2-propanol as probe reactions. Commercial TiO(2) Degussa P25 was used for the sake of comparison. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: Acetate containing nickel-zinc hydroxysalts (LHS-Ni-Zn) have been synthesized by coprecipitation and hydrothermal treatment. The acetate anions were exchanged with PW(12)O(40)(3-) anions, and optimum conditions to attain the maximum level of W in the Compound have been identified. The W intercalated compound was characterized by powder X-ray diffraction, FT-IR spectroscopy, thermogravimetric and differential thermal analyses, scanning electron microscopy and transmission electron microscopy. The exchange of LHS-Ni-Zn with PW(12)O(40)(3-) at pH = 3 for 72 It leads to a solid with a basal spacing of 9.62 angstrom and a W content (weight) of 37%. The hydrothermal treatment at 90 degrees C for 24h increases this value to 48% with a W/Zn molar ratio of 1.38, which corresponds to a layered Compound with lacunary tungstophosphate anions in the interlayer space. The intercalated solid is stable up to 250 degrees C, the layer structure collapses on dehydroxylation and amorphous compounds were identified at 500 degrees C. Two crystalline phases, NiO (rock salt) and a solid solution (Zn(1-x)Ni(x))WO(4), were identified by powder X-ray diffraction at high temperature (ca. 1000 degrees C). (C) 2008 Elsevier Inc. All rights reserved.
Abstract: The secondary structures of two proteins were examined by circular dichroism spectroscopy after adsorption onto a series of organically modified silica glasses. The glasses were prepared by the sol-gel technique and were varied in hydrophobicity by incorporation of 5% methyl, propyl, trifluoropropyl, or n-hexyl silane. Both cytochrome c and apomyoglobin were found to lose secondary structure after adsorption onto the modified glasses. In the case of apomyoglobin, the alpha-helical content of the adsorbed protein ranged from 21% to 28%, well below the 62% helix found in solution. In contrast, these same glasses led to a striking increase in apomyoglobin structure when the protein was encapsulated within the pores during sol-gel processing: the helical content of apomyoglobin increased with increasing hydrophobicity from 18% in an unmodified glass to 67% in a 5% hexyl-modified glass. We propose that proteins preferentially adsorb onto unmodified regions of the silica surface, whereas encapsulated proteins are more susceptible to changes in surface hydration due to the proximity of the alkyl chain groups.
Abstract: Two series of hybrid organic-inorganic hydrotalcites containing Mg2+ and Al3+ in the brucite-like layers (Mg/Al molar ratios of 2:1, 3:1 and 4:1) and lactate (Lc) as counter-anion were synthesized by coprecipitation using Lc/Al molar ratios of 0:1 and 3:1 to prepare the starting solutions. All samples have been characterised by element chemical analysis, powder X-ray diffraction, FT-IR spectroscopy, thermal analysis and morphology analysis (specific surface area measurement, particle size analysis and transmission electron microscopy). Delamination of the synthesized samples after dispersion in water and ultrasound treatment was followed by X-ray diffraction. Delamination has been compared to that shown by samples containing Zn and Al in the brucite-like layers, and it has been found that, contrary to the Zn-containing samples, restacking takes place in all cases upon solvent evaporation. (C) 2008 Elsevier Masson SAS. All rights reserved.
Abstract: The synthesis of poly(ethylene terephthalate) (PET)/layered double hydroxide (LDH) nanocomposites through microwave methods has been investigated. To enhance the compatibility between the PET polymer and the LDH, dodecyl sulfate was intercalated in the lamellar structure. The organo-LDH structure was confirmed by powder X-ray diffraction (PXRD) and Fourier transform infrared spectroscopy (FTIR). PET nanocomposites were prepared with 0-10 wt % of LDH content by in situ microwave-assisted polymerization. PXRD was used to detect the formation of the exfoliated PET/LDH nanocomposites. Transmission electron microscopy was used to observe the dispersed layers and to confirm the exfoliation process. FTIR spectroscopy confirmed that the polymerization process had occurred. TG and DTA are used to study changes in thermal stability of the nanocomposites, which resulted enhanced by well dispersed LDHs layers. (C) 2008 Wiley Periodicals, Inc.
Abstract: Layered double hydroxides with Mg and Al cations in the layers (Mg/Al molar ratio 2) and with intercalated mefenamic and meclofenamic acid anions have been prepared by coprecipitation, ion exchange and reconstruction. The solids have been characterized by X-ray diffraction, FTIR spectroscopy and assessment of specific surface area; their thermal stability has been studied by differential thermal and thermogravimetric analyses. Intercalation has been achieved by all three methods used, the gallery height ranging between 16.5 and 17.5 angstrom, depending on the particular drying conditions, these values suggesting the organic molecules form bilayers in the interlayer space. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: The mictoporosity of granite stones has been studied in both the untreated condition as well as after being treated with a consolidation treatment using two commercial products. A decrease in the specific surface area and pore size distribution (especially micropores) has been found in samples treated with an organic silicate consolidant. Treatment with a consolidant containing polysiloxane (a water repellent agent) leads to variable results, depending on the precise pore structure of the untreated stone. The use of consolidation treatments may restrict flow of fluids (e.g., water, saline solutions, etc.) through the pores of the stones, which may decrease degradation.
Abstract: Co-containing layered double hydroxides at different pH have been prepared, and aged following different routes. The solids prepared have been characterized by element chemical analysis, powder X-ray diffraction, thermogravimetric and differential thermal analyses (both in nitrogen and in oxygen), FT-IR and Vis-UV spectroscopies, temperature-programmed reduction and surface area assessment by nitrogen adsorption at -196 degrees C. The best conditions found to preserve the cobalt species in the divalent oxidation state are preparing the samples at controlled pH, and then submit them to ageing under microwave irradiation. (c) 2006 Elsevier Inc. All rights reserved.
Abstract: Layered double hydroxides with different cations in the layers, MgAl, ZnAl and CoAl, have been prepared. The solids were obtained by the coprecipitation method and then they were submitted to the microwave radiation in order to improve the crystallinity and minimize the particle size of the solids. The samples were characterized by element chemical analysis, powder X-ray diffraction, FT-IR spectroscopy, particle size distribution, and transmission electron microscopy.
Abstract: The rotational molecular dynamics of water confined to layered oxide materials with brucite structure was studied by dielectric spectroscopy in the frequency range from 10(-2) to 10(7) Hz and in a broad temperature interval. The layered double hydroxide samples show one relaxation process, which was assigned to fluctuations of water molecules forming a layer, strongly adsorbed to the oxide surface. The temperature dependence of the relaxation rates has an unusual saddlelike shape characterized by a maximum. The model of Ryabov et al. (J. Phys. Chem. B 2001, 105, 1845) recently applied to describe the dynamics of water molecules in porous glasses is employed also for the layered materials. This model assumes two competing effects: rotational fluctuations of water molecules that take place simultaneously with defect formation, allowing the creation of free volume necessary for reorientation. The activation energy of rotational fluctuations, the energy of defect formation, a pre-exponential factor, and the defect concentration are obtained as main parameters from a fit of this model to the data. The values of these parameters were compared with those found for water confined to nanoporous molecular sieves, porous glasses, or bulk ice. Several correlations were discussed in detail, such as the lower the value of the energy of defect formation, the higher the number of defects. The pre-exponential factor increases with increasing activation energy, as an expression of the compensation law, and indicates the cooperative nature of the motional process. The involvement of the surface OH groups and of the oxygen atoms of the interlayer anions in the formation of hydrogen bonds was further discussed. For the birnessite sample, the relaxation processes are probably overlaid by a dominating conductivity contribution, which is analyzed in its frequency and temperature dependence. It is found that the conductivity of birnessite obeys the characteristics of semiconducting disordered materials. Especially the Barton/Nakajima/Namikawa relationship is fulfilled. Analyzing the temperature dependence of the direct current (dc) conductivity sigma(0) in detail gives some hint that sigma(0)(T) has also an unusual saddlelike form.
Abstract: This work reports on the characterisation and determination of the photocatalytic activity of some TiO2 catalysts prepared using TiCl4 as precursor. The samples, consisting of suspensions, dispersions or powders of nanostructured anatase or rutile phases, were obtained in very mild conditions, i.e., by boiling the solution obtained after hydrolysis of TiCl4 at atmospheric pressure; it is worth noting that no calcination was necessary to obtain photoactive phases. The samples were characterised by X-ray diffraction (XRD), determination of the BET specific surface area (SSA), scanning electron microscopy (SEM) observations and diffuse reflectance spectroscopy (DRS). FT-IR spectroscopy was used to study the surface acidity of the solids. Photodegradation of 4-nitrophenol and gemfibrozil (5-(2,5-dimethylphenoxy)-2,2-dimethyl-pentanoic acid) was used as probe reactions to evaluate the photoactivity and the obtained results showed that the home prepared samples are effective to degrade both substrates. The photoreactivity results were compared to those obtained using commercial photocatalysts.
Abstract: Several mesoporous aluminosilicate molecular sieves with the MCM-41 structure (SiO2/Al2O3 = 20-200) have been synthesized using different aluminum sources and modifying several synthesis parameters during the preparation process, such as the temperature and the content of water and sulfuric acid in the gel mixture. All samples were characterized by element chemical analysis, X-ray diffraction, N-2 physisorption, thermal analyses, and electron microscopy. These Al-MCM-41 materials have BET surface areas up to 940 m(2) g(-1). The catalytic properties of their H3PO4-treated derivatives for Friedel-Crafts alkylation of toluene with benzyl alcohol have been evaluated. Toluene benzylation preferentially gave p-benzyl-toluene. The conversion of toluene to monoalkylated products increased with increasing the H3PO4 content in the catalysts and reached a maximum value for a H3PO4 loading of 25%, a further increase in H3PO4 leading to a decrease in the activity for alkylation. The influence of H3PO4 loading and of the operating parameters on the performance of the catalysts was also investigated. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: Layered double hydroxides with the hydrotalcite-like structures, containing Mg2+ and Al3+, doped with Cr3+ and Y3+, have been prepared by precipitation at constant pH. The weight percentages of Cr3+ and Y3+ were 1, 2, or 3%, and 0.5 or 1%, respectively. Single phases were obtained in all cases, whose crystallinity decreased as the content in Cr and Y was increased. The solids have been characterised by element chemical analysis, powder X-ray diffraction, thermal analyses (differential, thermogravimetric and programmed reduction), FT-IR and UV-vis spectroscopies; the specific surface areas have been determined from nitrogen adsorption isotherms at -196 degrees C. Upon calcination at 1200 degrees C for 5 h in air all solids display a mixed structure (spinel and rock salt for MgO); these solids have also been characterised by these techniques and their chromatic coordinates (CIE - L*a*b*) have been determined. Their pink colour makes these solids suitable for being used as ceramic pigments. (c) 2007 Elsevier Masson SAS. All rights reserved.
Abstract: Layered double hydroxides (LDHs) containing Cd(II), Al(III), and Fe(III) in the brucite-like layers with different starting Fe/Al atomic ratios and with nitrate as counteranion have been prepared following the coprecipitation method at a constant pH value of 8. An additional Cd(II)Al(III)-LDH sample interlayered with hexacyanoferrate(III) ions has been prepared by ionic exchange at pH 9. The samples have been characterized by elemental chemical analysis, powder X-ray diffraction (PXRD), and FT-IR spectroscopy. Their thermal stability has been assessed by thermogravimetric and differential thermal analyses (TG-DTA) and mass spectrometric analysis of the evolved gases. The PXRD patterns of the solids calcined at 800 degrees C show diffraction lines corresponding to Cd(Al)O and spinel-type materials, which precise nature (CdAl(2)O(4), Cd(1-x)Fe(2+x)O(4), or Cd(x)Fe(2.66)O(4)) depends on location and concentration of iron in the parent material or precursor. (C) 2007 Elsevier Inc. All rights reserved.
Abstract: The acid-base and red-ox properties of amorphous magnesium and zinc molybdates obtained through calcination at 400 degrees C of Mg(2)Al- and Zn(2)Al-heptamolybdate layered double hydroxides, which had been prepared by anionic exchange from the corresponding nitrate -hydrotalcite precursors, have been studied. Lewis-type acid and basic sites have been detected in both systems, in addition to Bronsted-type acid sites in magnesium molybdate; however, as studied by FT-IR spectroscopy, none of them is strong enough to yield propene through isopropanol decomposition. The higher reducibility of Mo(6+) cations in zinc-containing samples than the magnesium containing ones makes them more selective for acetone production. Moreover, a small amount of products formed on basic sites through acetone aldol reaction have been identified at high reaction temperatures. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: Nanosized cobalt mixed oxide, with high stability of Co(II), were obtained by controlled thermal decomposition of layered double hydroxides (LDHs) with Co, Zn and Al in the brucite-type sheets. The effect of the microwave ageing treatment of the LDHs into the properties of the calcined products was studied. For this reason, the precursors were submitted to increasing periods of time to microwave-hydrothermal treatment. The use of microwave ageing yields to well-crystallised compounds without oxidation of Co2+ species to Co3+. The structure features of oxides remain during whole ageing process; the microwave-hydrothermal treatment yield, in all cases, spinel-like compounds. Studies of surface properties, N-2 adsorption/desorption and the TEM micrographs, show that the textural properties are strongly modified depending on the irradiation time were submitted to, altering both the pore size distribution and the particle size of the nano-oxides. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: The Rh-TPPTS complex was heterogenized onto layered double hydroxides (LDH) Zn3AlCl and Co2FeCO3 using ion-exchange procedure. Before ion exchange, the Rh precursor [Rh(cod)Cl](2) was dissolved together with the TPPTS ligand in deionised water and stirred for 12 h. After stirring, the LDH was added to the solution and stirred for another 24 h, then filtered, washed with deionised water, and dried under vacuum at room temperature for 16 h. The characterization of the catalysts was carried out using several techniques: ICP-AES, N-2 adsorption-desorption isotherms at - 196 degrees C, thermal analysis, powder X-ray diffraction, XPS and DRIFTs. Substrates were prepared via Diels-Alder reaction of various 2H-pyran-2-ones with N-substituted maleimides in an aqueous medium with microwave irradiation. The catalytic tests were performed in a stirred stainless-steel autoclave using H-2 pressures in the range 20-40 atm, 30 mg catalyst and 30 mg substrate, at different temperatures. Hydrogenation of bicyclo[2.2.2]octenes showed that these strained and functionalized cycloadducts were inert to classic catalysts but could be hydrogenated by using ionic-immobilized Rh-ligand complexes on the LDH. The Rh-TPPTS/Zn3AlCl was found to exhibit a pretty high activity and selectivity for such reactions leading to heteropolycyclic derivatives. The heterogeneous catalyst was easily recycled with no leaching. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: Polyoxometalates (POMs) H2W12O406- and W4Nb2O194- have been intercalated between the brucite-like layers of Mg, Al and Zn, Al hydrotalcites by anion exchange, starting from the corresponding nitrate precursors. The solids have been characterised by Powder X-ray Diffraction (PXRD), Fourier Transform infrared (FT-IR) spectroscopy, N-2 adsorption-desorption at -196 degrees C and thermogravimetric (TG) and differential thermal analyses (DTA), and have been tested in the epoxidation of cyclooctene using H2O2 or t-BuOOH as oxidants. The results show that both anions are effectively located in the interlayer space maintaining their pristine structures without depolymerisation. Upon intercalation of such large anions microporosity is developed and subsequently an increase in the specific surface 06areas is also observed. In general, the prepared materials possess catalase and epoxidation activity, with ZnAl-intercalated H2W12O406- giving the best results in terms of epoxide yield (17% at 24 h). Product selectivity is different for the intercalated and free POMs, the latter yielding 1,2-cyclooctanediol as the only product, whereas the former produces only the epoxide. The epoxidation reaction seems to be catalysed in homogeneous phase by the POM. (c) 2007 Elsevier Ltd. All rights reserved.
Abstract: Hydrotalcite-type solids with different metal ions within the layers (M-II=Mg or Zn and M-III=Al and/or Fe) and carbonate or chloride between them have been synthesized. Chloride-LDHs and the solids obtained upon carbonate-LDHs calcination have been evaluated in the adsorptive removal of Cr(VI) from aqueous solutions. All the adsorption tests were carried out at 30 degrees C following two different stirring conditions: a) using a thermostatic bath with mechanical stirring at a speed of 52 rpm or b) in a sonicator bath under ultrasound waves. It was observed that no significant difference in the amount of adsorbed chromate is produced in both cases but the time spent to reach the equilibrium is much lower when the adsorbent/adsorbate mixture is submitted to ultrasounds. In all experiments L-type adsorption isotherms were obtained. The results show that chloride hydrotalcites are better adsorbents than calcined CO3-hydrotalcites and in these last ones their adsorption capacity increases with increasing the surface area: MgAlFe > MgAl > MgFe > ZnAl. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: The synthesis and characterization of Co,Al Layered Double Hydroxides (LDHs) containing carbonate is reported. The solids have been submitted to several thermal treatments, hydrothermal and microwave-hydrothermal, modifying both the treatment time and temperature, with the main aim of studying their influence on the crystallinity of the solids and on the stabilization of divalent cobalt oxidation state. The results show that whichever the temperature and time used the only phase detected was the hydrotalcite, and that the cobalt cations keep the divalent oxidation state.
Abstract: Paramolybdate-LDHs with MgAl or ZnAl cations within the layers have been prepared by the ion-exchange method from hydrotalcites with different interlayer anions (OH-, NO3-, and terephthalate). The samples and the oxides obtained after their calcination were characterized by element chemical analysis, PXRD, FT-Raman spectroscopy, thermal analysis (TG/DTA), N-2 adsorption at -196 degrees C, and SEM. The results show that layered solids with hydrotalcite-type structure were obtained in which the interlayer space is occupied by heptamolybdate with a small amount of MoO4(2-) units formed through hydrolysis of the polyanion; both oxomolybdenum species undergo a progressive distortion of the octahedral units from 50 degrees C but are roughly stable up to 250 degrees C as a consequence of the interaction between the polyanion and the brucite-like layers. This distortion is responsible for the observed decrease in the height of the gallery for samples heated in the temperature range, 50-250 degrees C, with respect to the original samples. Rehydration of the calcined solids allows recovering of their original structures and the initial values for the gallery heights. Calcination between 300 and 400 degrees C gives rise to a collapse of the layered structure, and amorphous phases are formed, in which molybdenum is both octahedrally and tetrahedrally coordinated. Crystalline magnesium and zinc molybdates (MgMoO4 and ZnMoO4) are formed at 450 and 600 degrees C, respectively. All solids have some microporosity, which decreases with increasing the calcination temperature.
Abstract: Layered double hydroxides with the hydrotalcite-like structure, containing Cu(II) and Al(III) in the layers, and different alkyl sulphonates in the interlayer, have been prepared and characterised by powder X-ray diffraction, FT-IR spectroscopy, differential thermal analysis and thermogravimetric analysis. Their magnetic properties have been also studied. Except for the sample containing octadecanesulphonate in the interlayer, for which an excess of sulphonate exists, pure crystalline phases have been obtained in the other cases. Upon heating, combustion of the organic chain takes place at lower temperature than for the corresponding sodium salts. A two-dimensional antiferromagnetic behaviour is observed at 200 K in all samples containing intercalated sulphonate. The T-X value is lower for the samples containing interlayer sulphonates (with layer-layer distances in the 21-31 angstrom range), than for a carbonate-containing analogue (basal spacing 7.51 angstrom). (c) 2005 Elsevier B.V. All rights reserved.
Abstract: The thermal stability of Ni,Al-CO3 hydrotalcite-like compounds synthesized by the coprecipitation method and aged upon microwave-hydrothermal treatment for different periods of time was studied. The samples prepared were characterized by Elemental Analysis, PXRD, Thermal analyses (DTA and TG) and Temperature Programmed Reduction (TPR). The results show that the use of microwave radiation as a source of heating during the ageing treatment leads to an increase in the crystallinity of the solids, which determines their thermal stability.
Abstract: The performance of Pd supported on alumina or titania, prepared by impregnation or sol-gel, for catalytic combustion of methane is studied. The addition of the Pd precursor after gelification of alumina has a beneficial effect on the catalytic properties of the catalysts. The better activity of the alumina catalysts is related to the high dispersion of the palladium particles, the low crystallite size and the high specific surface area. The catalytic activity of titania-based catalysts was only slightly affected by the preparation procedure. The addition of CO2 during the oxidation of methane promotes oxidation for the titania-based catalysts. (c) 2005 Elsevier B.V All rights reserved.
Abstract: Catalytic activity in the partial oxidation (CPO) of methane of some catalysts containing Ni or Rh/Ni active metals obtained by the calcination and reduction of hydrotalcite-like compounds was investigated. In particular, two hydrotalcite-like compounds subjected to the microwave-hydrothermal method (MWHT) were studied in order to evaluate the role of the synthesis method of the precursors on the catalytic activity and catalyst stability as compared to catalysts prepared by the conventional method. The tests carried out at 750 degrees C do not evidence any difference among catalysts. However, when the temperature is reduced to 500 degrees C a better catalytic performance is observed for the microwave-assited catalysts containing nickel, whereas for the bimetallic catalyst the best activity is achieved with the conventional method.
Abstract: Relative humidity and temperature data from six sensors in the inner and outer parts of the Zamora Cathedral (Spain) are analyzed to characterize different microenvironments in, as well as to determine the effect of the installation of central heating in 1999 and the closing of the cloister in 2000. When comparing the data from an external sensor of the Cathedral (Dome), with respect to the data of sensors inside the building, the fluctuations in relative humidity and temperature were found to be very low. Differences in temperature and relative humidity between the inner and the outer sides of the monument increased after installation of central heating and closing of Cloister. These rehabilitation works the monument have given rise to a smoothing in the fluctuations of the temperature and relative humidity along the year.
Abstract: Preservation of our Historical Cultural Heritage built in stone requires studies on the material itself, on the environment where the monument is located, and even on the microclimate affecting a particular stone in the building. The knowledge of the decay processes, as well as mechanisms governing these processes, and the real effect of factors depending on the intrinsic properties of the material to be studied, and external agents of decay, will allow a rationale use of these materials and to anticipate their behaviour beforehand, in order to success in restoration.
Abstract: PXRD (powder x-ray diffraction), FT-IR (Fourier Transform infrared), N-2 adsorption at 196 degrees C and TG/DTA (thermogravimetric and differential thermal analyses) techniques have been used for characterisation of MgAl- and ZnAl-CrO4 LDHs, which had been prepared by the ion exchange method from the corresponding chloride LDH (layered double hydroxides) precursors. The results indicates that the oxometalate intercalation in both systems produces interlayer microporosity and a basal spacing of 8.7 angstrom; This gallery height decreases when the samples are calcined in the temperature range 100-300 degrees C, due to a grafting process. A larger thermal stability is detected for MgAl-CrO4 sample than for the zinc-containing one.
Abstract: The suitability of granite conservation treatments (consolidation and/or water-repellency) widely used to protect heritage structures was evaluated on the basis of the changes occurring in the intrinsic characteristics of the stone (imbibition capacity, open porosity, total water-accessible porosity, water vapour permeability and capillar absorption coefficient). A statistical analysis conducted of the values obtained for these properties, which showed differences In fluid flow through the rocks, found that the application of consolidants followed by water repellency treatment was more effective than either procedure alone, substantially enhancing the properties of the stone from the standpoint of its use as a construction material.
Abstract: HTiNbO5, HTi2NbO7, HNb3O8, H4Nb6O17, and H2Ti3O7 nanosheets obtained by exfoliation of protonated cation-exchangeable layered metal oxides were examined as solid acids for the cyclodehydration of xylose into furfural. The exfoliated-aggregated composites were characterized by powder X-ray diffraction, N-2 gas adsorption, and scanning electron microscopy, and the acid proper-ties were measured by IR spectroscopy of adsorbed pyridine. In general, the materials had much higher catalytic activity than the nonexfoliated acid-exchanged layered materials. The initial catalytic activities of the exfoliated materials tended to increase with the total amount of acid sites (Bronsted + Lewis, B + L). Furfural yields of up to 55% were achieved after 4 h of reaction in a water-toluene solvent mixture under batch operation at 160 degrees C. Most of the materials could be recycled several times without loss of performance. (c) 2006 Elsevier Inc. All rights reserved.
Abstract: Ni containing layered double hydroxides (LDHs) have been prepared by precipitation and hydrothermally treated under microwave irradiation for different periods of time. The solids have been calcined at three temperatures corresponding to stable phases formed during thermal decomposition of LDHs. The properties of the irradiated samples and of the calcined products were studied in order to ascertain whether the ageing treatment under microwave irradiation modifies not only the properties of the layered materials, but also the properties of the calcined products. A structural and textural study was carried out by PXRD, FT-IR and Vis-UV spectroscopy, thermal analyses (DTA and TG), N-2 adsorption/desorption at - 196 degrees C and TEM microscopy; the reducibility of the nickel species was studied as well by TPR. The results show that the microwave treatment leads to better crystallized LDHs with modified thermal stability and reducibility. In addition, the degree of crystallinity of the layered precursors and their textural properties determine the properties of their thermal decomposition products. (c) 2006 Elsevier Inc. All rights reserved.
Abstract: Bismuth molybdenum titanium oxides were prepared in a highly dispersed state by the sol-gel method and in a largely crystallized state by coprecipitation and impregnation. The catalysts contained 14 or 25 wt.% of bismuth molybdate. Their catalytic performance in propene oxidation to acrolein was studied. The increase in activity of Bi-Mo-Ti oxides in propene oxidation as compared to that of bulk bismuth molybdate can be tentatively related, in the case of the sol-gel samples, to the stabilization of small aggregates of Bi- and Mo-containing phases due to the beneficial presence of the titania matrix. The catalytic performance of the samples prepared by coprecipitation or impregnation is enhanced once a crystal size of bismuth molybdate close to 23 nm is reached, i.e., the crystal size should not be too small (samples prepared by the sol-gel method) nor too large (samples prepared by impregnation and coprecipitation), but an optimum size should exist to enhance the catalytic performance. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: A MgAl hydrotalcite-like material intercalated with hexaniobate has been prepared by the anion exchange method from the corresponding nitrate precursor; the sample and the oxides obtained upon its calcination were characterized by element chemical analysis, powder X-ray diffraction, thermal analyses (thermogravimetric and differential), nitrogen adsorption-desorption isotherms at -196 degrees C, transmission electron microscopy, and FT-IR and UV/Vis spectroscopy. The results show the formation of a microporous hydrotalcite-type solidwitha gallery heoght of 7.2 angstrom where the H3Nb6O195- anions are orientated with their C-3 axes perpendicular to the layers. This materia; preserves its layered structure upon calcination up to 400 degrees C; calcination above this temperature causes decompositin of the hexaniobate and the layered structure collapses, giving rise to amorphous mesoporous solids (Nb-Mg-Al-O) with large specific surface area (157 m(2) g(-1)). At 800 degrees C crystillization of Mg4Nb2O9 takes place. FT-IR studies on the acid-basic properties carried out by pyridine and 2-propanol adsorption, showed that all solids obtained through hydrotalicite calcination present Lewis-type acid and Bronsted basic sites. ((c) Wiley-VCH Verlag GmbH & Co. KGbA).
Abstract: Hydrotalcites containing heptamolybdate (Mo7O246-) in the interlayer and Al combined with different divalent cations (Mg, Zn, Ni) in the brucite-like layers were prepared by ion exchange from suitable precursors containing terephthalate. To prepare these materials, a given amount of terephthalic acid was adjusted at pH 8.0 with NaOH and then mixed with a second solution, containing amounts of M(NO3)(2)center dot 6H(2)O (M = Mg, Zn, Ni) and Al(NO3)(3)center dot 9H(2)O corresponding to a M2+/M3+ molar ratio of 2, keeping the molar Al:terephthalate ratio to a value of 2:1. After refluxing under inert atmosphere and drying in vacuum, hydrotalcites were exchanged with aqueous solutions of ammonium heptamolybdate for an Al/Mo molar ratio of 4: 1. Characterization of these solids was made using element chemical analysis, adsorption-desorption isotherms of nitrogen at -196 degrees C, powder X-ray diffraction, and FT-IR spectroscopy. The catalysts were tested in liquid phase oxidation of bicyclo[2.2.1]-2,5-heptadiene (norbornadiene); benzobicyclo[2.2.1]-2-heptene (benzonorbornadiene) and cyclohexene using hydrogen peroxide and dioxane or dioxane-butyl maleate mixture as solvents. The selectivity to epoxides was found to depend on the hydrotalcite composition and on the reaction solvent. Hydrotalcites containing Ni/Al-Mo and Zn/Al-Mo were more selective than the Mg/Al-Mo one. The replacement of dioxane with a dioxane-butyl maleate mixture led to a decrease in the selectivity to epoxide. Recycling of the catalysts over six successive runs demonstrated their stability. (c) 2006 Elsevier Inc. All rights reserved.
Abstract: Layered double hydroxides containing Ni2+ and Al3+ have been synthesized by homogeneous precipitation through urea hydrolysis. The nature of the interlayer species changes according to the treatment of the samples, formation of interlayer (NH2)COO- species being observed immediately after precipitation, which undergo transformation to carbonate after hydrothermal treatment. Simultaneously, liberation of ammonia during decomposition under hydrothermal conditions gives rise to formation of [Ni(NH3)(6)](2+) species in solution.
Abstract: Organic-inorganic hybrid hydrotalcite-like compounds containing Zn2+ and Al3+ in the brucite-like layers (nominal Zn:Al molar ratios of 2:1, 3:1, and 4:1) and lactate (Lc) as counteranion were synthesized by coprecipitation using 0: 1 and 3: 1 lactic acid: Al molar ratios to prepare the starting solutions. Their physicochemical properties were studied by element chemical analysis, powder X-ray diffraction, infrared spectroscopy, and thermal analyses. Powder X-ray diffraction showed a pattern characteristic of a hydrotalcite-like structure. FT-IR spectroscopy confirmed the retention of the lactate anions in the interlayer without further contamination. Thermal analyses showed four stages of weight loss and five of heat change when recorded under oxidizing conditions. Dispersion of the dried samples in decarbonated water by ultrasound-assisted treatment at room temperature led to delamination of all wet fractions. Upon solvent evaporation, dried fractions restacked with a higher crystallinity and an increase in basal spacing.
Abstract: Small size, monodispersed hexagonal particles of Mg,Al-CO3 hydrotalcite-like compounds have been synthesized in short periods of time by the microwave-hydrothermal method. An in-depth study of the influence of the irradiation time and the process temperature has been carried out. Well-crystalized materials with relatively high specific surface area values have been prepared in 30 min at 150 degrees C.
Abstract: Two series of layered double hydroxides (LDHs) with compositions Mg,Al-CO3 and Mg,Cr-CO3 were prepared by microwave and conventional hydrothermal treatment to study the role of microwave radiation on their physicochemical properties. Aging temperature was 125 degrees C in all cases. The samples were fully characterised by element chemical analysis, powder X-ray diffraction, differential thermal analysis and thermogravimetric analysis, Al-27 MAS NMR, FT-IR and UV-Vis diffuse reflectance spectroscopies, and specific surface area and porosity assessment by N-2 adsorption at -196 degrees C. In all cases, the only crystalline phase obtained was hydrotalcite which crystallinity is further enhanced by microwave hydrothermal treatment than by conventional hydrothermal treatment. The microwave-aged samples also have larger amounts of interlayer water and smaller particle size than those prepared by conventional hydrothermal ageing. The textural properties of the solids are strongly modified during the ageing treatment, and large specific surface areas values are observed at shorter times for those samples subjected to microwave radiation. (c) 2006 Elsevier Inc. All rights reserved.
Abstract: A constant pH precipitation method has been applied to obtain solids with Ni/Fe molar ratios of 2/1, 3/2, 1/1, 2/3, and 1/2. In all cases, a phase with the hydrotalcite-like structure is obtained, containing Ni-II and Fe-III in the brucite-like layers and carbonate in the interlayer, and, for samples with a Ni/Fe molar ratio lower than 2/1, amorphous hydrated iron oxides, undetected by X-ray diffraction, are also formed. The solids have been characterized by element chemical analysis, powder X-ray diffraction, differential thermal analysis, thermogravimetric and differential thermogravimetric analysis, FT-IR spectroscopy, temperature-programmed reduction and assessment of specific surface area by nitrogen adsorption at -196 degrees C. In all cases reduction leads to zero-valent state for the metals, reduced nickel particles probably favouring reduction of Fe-III species; the specific surface area increases with the iron content, probably due to the amorphous nature of the hydrated iron oxides formed. Calcination at 1200 degrees C in air leads to well crystallized solids, formed by NiFe2O4 spinel and, additionally, rocksalt-type NiO for Ni/Fe ratios larger than 1/2. In this way, solids with tailored compositions of these two phases can be prepared.
Abstract: Mixed Mg-V oxides have been prepared following three different methods (calcination of precursors constituted by a layered double hydroxide, impregnation of MgO with vanadate, or mechanical MgO-V2O5 mixtures), and have been characterized following powder X-ray diffraction, X-ray absorption spectroscopy, vis-UV and FT-IR spectroscopies, thermal analysis, temperature-programmed reduction, and specific surface area assessment. Mg3V2O8 is mainly formed, although small amounts Of alpha-Mg2V2O7 are detected in some cases, in addition to MgO. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: Layered double hydroxides (LDHs) with Mg and Al in the layers and carbonate, nitrate or chloride in the interlayer, or with Zn and Al in the layers and chloride in the interlayer, have been prepared by coprecipitation, and have been used as precursors to prepare chromate-containing LDHs. All these systems, as well as those obtained upon their calcination up to 800 degrees C, have been characterised by powder X-ray diffraction, FT-IR and vis-UV spectroscopies, temperature-programmed reduction (TPR), nitrogen adsorption at -196 degrees C for surface texture and porosity assessment, and FT-IR monitoring of pyridine adsorption for surface acidity determination. The results obtained show that the crystallinity of the chromate-containing LDH depends on the precursor used. The layered structure of the Mg, Al systems is stabilised up to 400 degrees C upon incorporation of chromate; however, the Zn,Al-chromate samples collapse between 200 and 300 degrees C, with simultaneous formation of ZnO. Calcination of the samples above 400 degrees C gives rise to a reduction of Cr(VI) to Cr(III), as concluded from vis-UV spectroscopic studies. The TPR profiles show that chromate in ZnAl hydrotalcite is more easily reduced than that incorporated in the magnesium ones. Moderately strong surface Lewis acid sites exist in all samples calcined below 500 degrees C. (c) 2005 Elsevier Inc. All rights reserved.
Abstract: The aim of this work is to establish the original quarries used to construct the Roman bridge in Salamanca (Spain). Nineteen granite-type samples from the bridge were obtained. For comparison, 12 samples were taken from eight different quarries, where the original stones were probably exploited. The study is based on element chemical analysis for major elements. The results have been statistically analyzed using an inertia method based on an HJ-biplot. This method permits the characterization of three different clusters of samples and relates them to the original quarries, from the chemical composition of the samples. We conclude that the following three types of granite were used in the Roman bridge of Salamanca: (1) granodioritic granites from the Los Santos quarry, in Salamanca; (2) leucogranites from quarries in Villaseco de los Gamitos, Ledesma, Trabanca, and Zorita (Los Arribes, Salamanca); and (3) vaugneritic granites, probably from the quarry in Ledesma (Salamanca). The chemical changes produced by weathering of the different types of granites through the different decay processes in different areas of the Roman bridge in Salamanca have also been observed.
Abstract: Layered double hydroxides (LDHs), with the hydrotalcite-like structure containing Cu(II), Al(III) and Fe(III) in the layers, and different alkyl sulfonates in the interlayer, have been prepared and characterized by powder X-ray diffraction, FT-IR spectroscopy, differential thermal analysis and thermogravimetric analysis. Pure crystalline phases have been obtained in all cases. Upon heating, combustion of the organic chain takes place at lower temperature than the corresponding sodium salts. (c) 2005 Elsevier SAS. All rights reserved.
Abstract: Layered double hydroxides (LDHs, or hydrotalcites) with Mg2+ and Al3+ cations in the mixed metal hydroxide layer and paratungstate anions in the interlayer have been prepared. Different methods have been followed: anion exchange with Mg,Al LDHs originally containing nitrate or adipate, reconstruction of the LDH structure from a mildly calcined Mg2Al-CO3 LDH, and coprecipitation, In all cases, the tungsten precursor salt was (NH4)(10)H2W12O42, The prepared solids have been characterized by elemental chemical analysis, powder X-ray diffraction (PXRD), FT-IR spectroscopy, thermogravimetric (TG) and differential thermal (DTA) analyses, scanning electron microscopy (SEM) with EDX (energy-dispersive X-ray analysis), and nitrogen adsorption at -196 degreesC for surface area and surface texture. Most of the synthesis methods used, especially anion exchange starting from a Mg2Al-NO3 precursor at low temperature and short reaction times, lead to formation of a hydrotalcite with a gallery height of 9.8 Angstrom; increasing the reaction temperature to 70-100 degreesC and maintaining short contact times leads to a solid with a gallery height of 7.8 Angstrom. Both phases have been identified as a result of the intercalation of W7O246- species in different orientations in the interlayer space. If the time of synthesis or the temperature is increased, a more stable phase, with a gallery height of 5.2 Angstrom corresponding to a solid with intercalated W7O246-, is formed, probably with grafting of the interlayer anion on the brucite-like layers. All systems are microporous. Calcination at 300 degreesC leads to amorphous species, and crystallized MgWO4 is observed at 700 degreesC.
Abstract: A FT-IR spectroscopy Study on the surface reactivity of MgAl-paratungstate LDHs calcined at 500 and 700degreesC, prepared by anion exchange from a Mg(2)Al-nitrate hydrotalcite and precursor salt (NH(4))(10)H(2)W(12)O(42), for ethanol dehydrogenation, is reported. Calcination at 300degreesC of Mg(2)Al-W(7)O(24)(6-) LDH leads to amorphous species, and crystallisation of MgWO(4) is observed at 700degreesC. The solids calcined at 500 and 700degreesC exhibit surface Lewis acid sites and are selective to acelaldehyde formation during ethanol dehydrogenation or to oxidative dehydrogenation, especially in the sample calcined at 500degreesC, where carboxylate species are not detected in the temperature range tested (room temperature to 300degreesC).
Abstract: Solids containing Co(II) and Fe(III) with molar ratios of 211, 312, 1/1, 213 and 112 have been synthetised by coprecipitation at constant pH. All they displayed a hydrotalcite-like structure with interlayer carbonate, which crystallinity decreases as the iron content was increased. No other crystalline phase was identified, even in the Fe-rich samples. They have been characterised by powder X-ray diffraction, FT-IR spectroscopy, thermal analysis (differential thermal analysis, thermogravimetric analysis and temperature-programmed reduction), in addition to specific surface area assessment by nitrogen adsorption at -196degreesC. A minor oxidation of Co(II) to Co(III) is observed in the Co-rich samples, although it reverses again to Co(II) upon calcination in oxygen at ca. 850degreesC. Thermal decomposition takes place in a single step up to ca. 350degreesC, and the specific surface area increases with the iron content, probably because of the presence of hydrated amorphous iron oxides. The solids calcined at 1200degreesC in air contain crystalline CoO, Co3O4 and CoFe2O4 (spinel), this one being the dominant phase, and only phase detected for large Fe contents. Metallic species are more easily reduced in the original solids than in the calcined ones, and in all cases iron seems to be reduced at a higher temperature than cobalt.
Abstract: Layered manganese oxides prepared by sol-gel in both the Na and K form were ionic exchanged with Cu and characterized by chemical analysis, DTA, adsorption-desorption isotherms of N-2 at -196 degrees C, XRD, FTIR, and thermal analysis. They were tested in liquid phase oxidation of cyclohexane and cyclohexene with hydrogen peroxide and air showing high conversion of hydrocarbon (over 70%) and rather good selectivitity of H2O2.
Abstract: Birnessites containing Na+ or K' in the interlayer have been prepared following a sol-gel method, with an initial glucose/cation ratio of 1 or 1.5. The solids have been submitted to ion exchange with Li+, Mg2+, and Cu2+, and all the materials have been characterised by several physicochemical techniques (elemental chemical analysis, X-ray diffraction, FT-IR spectroscopy, thermal analysis and specific surface area assessment). Total cation exchange was not achieved in my case, although sodium is better exchanged than potassium. Exchange is larger for Li', lower for Cu2+ and even lower for Mg2+. Ionic exchange is topotactic, and the layered structure is maintained in all cases, with an interlayer spacing close to 7 Angstrom. The samples are stable up to 600 degreesC; above this temperature cryptomelane (a 2x2 tunnel structure) is formed in most of the samples containing potassium, while LiMn2O4 spinel is formed is Li was present in the birnessite.
Abstract: CuM(II)Al ternary hydrotalcites (M(II) = Ni, Co and Mg) with a (Cu + M(II))/Al atomic ratio of 3.0 and Cu/M(II) atomic ratio of 5.0 were synthesized by coprecipitation under low supersaturation. Powder X-ray diffraction of the as-synthesized samples showed a pattern characteristic of hydrotalcite-like (HT-like) structure (JCPDS: 41-1428). Thermal analyses of these samples showed four stages of weight loss/heat change when recorded in nitrogen. Analysis of the evolved gases characterized the nature of these transformations. The thermoanalytical effects differed significantly especially for the high-temperature transformations, when the treatment was performed in oxygen. In situ powder X-ray diffraction of the samples was carried out to elucidate the phase evolution of these compounds. Surprisingly formation of CuO was noted at temperatures around 200degreesC well below the destruction of the layered network. The nature of the resulting phases varied with both the nature of the co-bivalent metal ion and the heating atmosphere. FT-IR spectroscopy confirmed the retention of carbonate ions at higher temperatures (above 700degreesC), although the concentration of carbonate anion (most likely unidentate) varied with the calcination temperature. The crystallinity of CuO increased significantly above 600degreesC, probably through dissociation of copper oxycarbonate. Significant differences in the thermal transformation temperatures (for the third and the fourth transformations) of these samples containing different co-bivalent metal ions were not observed. This suggests that an association of the co-bivalent metal ions and/or trivalent metal ion in this phase is unlikely. A plausible thermal evolution scheme of these hydrotalcites is proposed. (C) 2003 Elsevier Inc. All rights reserved.
Abstract: The results obtained in the preparation of hydrotalcite-like compounds using microwave radiation are reported. The following systems were synthesized: Mg,Al-CO3, Mg,Cr-CO3, Ni,Al-CO3 y Zn,Ni,Al-CO3. The results obtained by the microwave-hydrotermal method have been compared with the conventional-hydrotermal one. A working protocol, which allows us to obtain reproducible results, has been elaborated, and this is a new aspect in the synthesis of inorganic solids. The obtained solids were characterized by several techniques (X ray difracttion, thermal analysis, chemical analysis and N-2 adsorption-desorption at -196 degreesC). Hydrotalcite-like compounds with the desirable chemical composition were obtained. The cristallinity depends on the microwave, radiation exposure time in all cases. Materials prepared using microwave radiation exhibit a better grain size homogeneity and a good crystalline degree and it also influences over the water molecules arrangement in the solid.
Abstract: Interlayer acetate anions in layered double hydroxyacetates of Ni2+ and Zn2+ have been exchanged by oxovanadates following three synthetic routes (at 60degreesC, under hydrothermal conditions and after preswelling with caprylate anions) and different pH; direct exchange at room temperature was not successful. Complete exchange was achieved under adequated conditions, and the precise nature of the interlayer anion depends on the pH during exchange: at low pH (4.5), the presence of alpha-VO3 chains, with anchoring (grafting) of the species to the hydroxide layers, is proposed. At higher pH (9.5) V2O74- species are present in the interlayer. Thermal decomposition of these vanadate-intercalated products leads to formation of orthorhombic Ni2+ and Zn2+ vanadates, together with NiO. (C) 2004 Elsevier Inc. All rights reserved.
Abstract: Transition metal loaded TiO(2) powders were characterised and tested for the photodegradation of some organic acids. Both Lewis and Bronsted surface acid sites were present in the samples containing large amounts of Mo, V and W. The Bronsted sites are associated to the presence of the transition metals as they do not exist in the unloaded support, and in particular to the presence of these metals in high oxidation states. The trend of the pzc values of the various powders matches rather well both with the surface acidity of the samples experimentally determined by NH(3) adsorption and with a scale of acidity/basicity for bulk binary oxides. XPS investigation indicates that only the presence of Cu(II) is able to modify the TiO(2) valence band spectrum. The photodegradation results have confirmed that the presence of transition metal ions generally reduces the photocatalytic activity of TiO(2). Surface acidity addresses the reactivity for the degradation of methanoic and benzoic acid: the more basic the surface of the powders the higher the rate of substrate disappearance. No relation with the acidity has been found in the case of ethanoic acid. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: Magnesium aluminium layered double hydroxides (LDH) with a molar Mg/Al ratio of 2.0 have been prepared with intercalated indomethacin following two routes: reconstruction from a previously calcined Mg2Al-CO3 LDH, and coprecipitation from the corresponding chlorides. The solids have been characterized by powder X-ray diffraction, FTIR, and C-13 CP/MAS NMR spectroscopies and thermal stability (differential thermal analysis and thermogravimetric analysis). Intercalation of the drug is attained by both routes; however, while coprecipitation leads to a single layered structure, contamination with another layered MgAl-CO3 phase occurs by the reconstruction method. The amount of drug intercalated, as well as the heigth of the gallery, are larger by the coprecipitation than by the reconstruction one. The data obtained support a somewhat tilted, upwards orientation of the drug molecules forming an interdigited bilayer, in the case of the sample prepared by coprecipitation, with the carboxylate groups pointing towards the hydroxyl layers. However, in the case of the sample prepared by reconstruction, the molecules are forming a tilted, upwards monolayer. The solids prepared are stable up to 250degreesC. Pharmacological studies in vivo show that intercalation of the drug in the LDH reduces the ulcerating damage of the drug. (C) 2004 Wiley-Liss, Inc. and the American Pharmacists Association.
Abstract: Mn and Co catalysts (approximate to10wt.% as MnO2 or Co3O4) Supported on Al-pillared clays (two saponites and a montmorillonite) have been prepared by wetness impregnation, using complex salts of the metals (Co(II) acetylacetonate, tris(ethylenediamine) Co(III) chloride, Mn(II) acetylacetonate and Mn(III) acetate dihydrate) as precursors. The incorporation of these metals onto the clays by co-intercalation of Al-Mn and Al-Co solutions was also considered, but this method was not useful to prepare the catalysts since only very small amounts of Mn or Co were fixed. All the solids have been thoroughly characterised (elemental chemical analysis, XRD, TG-DTA, FT-IR and UV-Vis spectroscopies, nitrogen adsorption, TPR and SEM). Impregnation led to solids which, in most cases, maintained the layered structure of the supports. However, their specific surface area markedly decreased due to blocking of the layered galleries by the organic moieties of the ligands. The surface area of some impregnated solids increased upon calcination at 500 degreesC due to removal of the organic moieties. Dispersed Mn2O3, Mn3O4 and Co3O4 particles were present in the calcined catalysts. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: Combined mid-IR and Raman spectroscopies indicate that intercalation of hexacyanoferrate (II) and (III) in the interlayer space of a Zn,Al hydrotalcite dried at 60degreesC leads to layered solids where the intercalated species correspond to both hexacyanoferrate(II) and (III). This is an indication that depending on the oxidation state of the initial hexacyanoferrate, partial oxidation and reduction takes place upon intercalation. The symmetry of the intercalated hexacyanoferrate decreases from O-h existing in the free anions to D-3d. The observation of a broad band around 2080 cm(-1) is indicative of the removal of cyanide from the intercalation complex to the outside surface of the crystals. Its position in the intercalation complex is probably filled by a hydroxyl group. (C) 2003 Elsevier Inc. All rights reserved.
Abstract: Layered double hydroxides (LDHs) with the hydrotalcite type structure and a Mg:Al ratio of two have been prepared, with salicylate or naproxen in the interlayer. Two synthetic routes have been used: reconstruction from a mildly calcined hydrotalcite-CO3 precursor, and a coprecipitation method with chlorides of the metals. The solids have been characterized using several physicochemical techniques, i.e., powder X-ray diffraction, FTIR and C-13 CP/MAS NMR spectroscopies and thermal analysis (therrnogravimetric and differential thermal analyses). The gallery height determined is in all cases larger than the size of the drug, 11.5 Angstrom for salicylate and 15.8 and 16.6 Angstrom for naproxen, depending on the specific synthesis route followed. Experimental data suggest the anion molecules form a tilted bilayer, with the carboxylate groups pointing towards the brucite-like layers. The solids are stable up to 230degreesC and their evolution from 350degreesC upwards is very similar to that observed for a carbonatc-containing hydrotalcite, forming mostly amorphous solids with a large specific surface area. (C) 2004 Elsevier Inc. All rights reserved.
Abstract: Colour is one of the parameters currently used to monitor the quality of a conservation treatment of ornamental stones, and its further ageing, whether artificially or by decay in the monuments. We report here a study of colour changes on three samples of granite, widely used in most of the buildings belonging to our Cultural Heritage in the city of Avila (Spain), once artificially aged in a simulation chamber under controlled conditions (25 fireezing/thawing cycles) after treated with a consolidant and a hydrofugant (RC80), a hydrofugant only (H224) and a consolidant (RC70), with or without further application of a hydrofugant (H224); all treatments reagents are commercially available. The results of the statistical study applied (MANOVA-Biplot) show that there are no differences in the values of the chromatic coordinates (L*,a*,b*) among the successive ageing treatments given to the samples, but differences are found among several samples grouped by combining the type of stone, treatment, and preliminary ageing to the samples before treatment or not. (C) 2004 Wiley Periodicals, Inc.
Abstract: Birnessites containing K+ and Na+ in the interlayer have been prepared by a sol-gel method using glucose and polyvinyl alcohol as reducing agents. The samples have been characterised by elemental chemical analysis, powder X-ray diffraction, thermal analysis (differential and thermogravimetric), FT-IR spectroscopy and specific surface area and texture assessment by nitrogen adsorption at -196degreesC. The solids obtained are more crystalline than those prepared by oxidation with H2O2 in a basic medium. The samples are thermally stable and evolution upon calcination at high temperature depends on the precise nature of the interlayer cation. So, calcination at 600 degreesC of a K-birnessite leads to formation of cryptomelane (2 x 2 tunnel structure), while the Na analogue leads to formation of mixed Mn-Na oxides with different tunnel structures. Ion exchange of interlayer cations takes place topotactically at room temperature; Li+ enters easily in the interlayer, while the entrance of Mg-2divided by is more difficult, probably because of its high stability in the hydrated form; increasing of the temperature and contacting time during ion exchange does not improve very largely the process. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: A comparative crystallochemical study was performed on natural and synthetic hydrotalcite-like compounds with similar compositions. The nature of the brucite-like sheet stacking was addressed by means of powder X-ray diffraction. From the resulting electron diffraction patterns it was possible to establish the order-disorder of the cations in the brucite-like sheet. The results show that a natural sample from Snarum is an intergrowth of hydrotalcite (3R(1) polytype) and manasseite (2H(1) polytype) at a ratio of 77:23 (wt.%). An aluminian serpentine is associated with the hydrotalcite and manasseite minerals. The structure of a synthetic sample, Mg:Al = 2: 1, was determined as space group Rim. For a few crystals in this sample, the octahedral cation distribution is compatible with the observed supercell (a = a'root3). A second synthetic sample showed the presence of stacking faults and was described as a random layer sequence of two polytypes (3R and 2H).
Abstract: WO3/Nb2O5-supported samples prepared by impregnation are characterised by X-ray diffraction (XRD), Raman spectroscopy and X-ray absorption spectroscopy (XAS) at the W-L-3 absorption edge, as well as temperature programmed reduction (TPR) and FT-IR monitoring of pyridine adsorption. Results are compared with those obtained for WO3/Al2O3 samples prepared in the same conditions, showing that niobia is able to disperse tungsta better than alumina does. Formation of a crystalline WO3 needs larger tungsten contents on niobia than on alumina, since tungsten solution into niobia is easier than into alumina. Raman and XAS spectra recorded under ambient conditions suggest that similar WO, species are formed on both supports at tungsten contents 0.5-1 theoretical monolayers; however, TPR results for the low tungsten loaded samples indicate that, when reduction starts (always at temperatures higher than 700 K under H-2/Ar flow) there is a larger concentration of tetrahedral [WO4] species on alumina, than on niobia. Samples with low tungsten loading have been tested in isopropanol decomposition and ethylene oxidation, following both processes by FT-IR of adsorbed species up to 673 K. Results show that adsorption of ethylene on WO3/Nb2O5 yields acetaldehyde and acetate at 473 K, while this adsorption is non-reactive either on the supports or on WO3/Al2O3. Isopropanol adsorbs dissociatively on both supports, leading to acetone and propene formation on tungsta-niobia, but only propene on tungsta-alumina, probably due to the larger reducibility of the tungsten-containing phases. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: A Mg,Al layered double hydroxide (LDH) with [Cr(C2O4)(3)](3-) anions in the interlayer has been synthesized following two different routes: reconstruction from a mildly calcined Mg,Al-carbonate LDH, and anion exchange from a Mg,Al-nitrate LDH. The solids prepared have been characterized by elemental chemical analysis, powder X-ray diffraction, FT-IR and UV-vis/DR (diffuse reflectance) spectroscopies, thermal methods, nitrogen adsorption at -196 degreesC, and FT-IR monitoring of pyridine adsorption. The results obtained indicate that the most appropriate method is anion exchange, leading to a well crystallized LDH with an interlayer spacing of 10 Angstrom. Due to the high pH value (>8) of the solution in the reconstruction method, however, a polyphasic system is obtained, where, in addition to a phase with the LDH structure, amorphous magnesium oxalate and chromium oxohydroxides are also formed due to hydrolysis of the complex. The interlayer complex is stable up to 200 degreesC, but the layered structure is stable up to 330 degreesC, probably because of the presence of interlayer oxalate anions formed during decomposition of the complex. Calcination leads to oxidation of Cr3+ ions to the six-valent state, which reverts to Cr3+ when the calcination temperature is further increased.
Abstract: Supported vanadium-containing catalysts were prepared by impregnation of Al-pillared clays with NaVO3 precursor aqueous solutions. A montmorillonite and a saponite, both natural as well as previously pillared with Al-13-Keggin polycations, were used as supports, being impregnated with solutions of the precursor by means of the incipient wetness technique. The layered structure of the supports is maintained after impregnation, and even after calcination of the impregnated solids at 500degreesC, although with a significant worsening of the textural properties of the solids, in particular, a loss of specific surface area. Three crystalline vanadium-containing phases were identified in the impregnated solids, namely, NaV3O8, AlVO4 and V2O5 (shcherbinaite). The reduction of these phases to V3+ species was observed as wide processes in the 450-750 degreesC range, all of them centred at ca. 600 degreesC. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: Birnessites containing Na, K or Li in the interlayer have been prepared by oxidation of Mn(II) cations with H2O2 in a basic medium with different alkaline cation/Mn molar ratios. The solids prepared have been characterised by elemental chemical analysis, powder X-ray diffraction, thermal analyses (differential thermal analysis and thermogravimetric analysis), FT-IR spectroscopy and surface texture assessment by adsorption of N-2 at -196degreesC. Crystalline birnessites are obtained for A/Mn ratios (A=K, Li) larger than 3.4, but MnO(OH) has been also identificed when such a ratio is smaller than 3.4. Ion exchange is topotactic, but is not complete for exchanging Na, K, or Mg for pre-existing Li. The solids are stable up to 400degreesC, and formation of spinels and solids with tunnel structures is observed at this temperature. Li-containing birnessites are transformed to LiMn2O4 spinel at 400degreesC, and co-crystallization of bixbyte (Mn2O3) is observed at higher temperatures. Bixbyte and cryptomelane are formed at 500degreesC for the K-containing birnessites. (C) 2003 Kluwer Academic Publishers.
Abstract: Hydroxylation of phenol was carried out over a series of CoNiAl ternary hydrotalcites (HTs) having a (Co + Ni)/Al atomic ratio close to 2.6 and a Co:Ni atomic ratios ranging from 1:0 to 0: 1 using H2O2 as oxidant and water as solvent. Both end members of this series, namely CoAl-HT and NiAl-HT, showed negligible conversion of phenol, while cooperative catalytic behavior was noted when both elements were present together. However, activity of the catalysts varied with Co/Ni atomic composition, wherein the activity decreased with a decrease in the concentration of cobalt. A variation in the activity with the crystallinity of the materials was noted wherein a highly ordered material showed maximum activity and dropped with a decrease in the orderliness of the layered structure. Temperature-programmed reduction (TPR) of the fresh samples substantiated the activity trend by exhibiting a decrease in the reduction maximum with a decrease in the concentration of cobalt. Probably an optimal configuration of sites involving cobalt, nickel, and Brphinsted basic hydroxyl groups is necessary for promoting the reaction. Among the catalysts studied, catalyst with a Co/Ni atomic ratio of 1:5 showed maximum conversion of phenol (14.2%, substrate: oxidant = 2:1, 65 degreesC) with a catechol/hydroquinone ratio of 3.8. Heating of these samples even at slightly elevated temperatures (> 120 degreesC in air for 5 h) led to complete loss in the activity necessitating the requirement of a well-ordered network. Prior to catalytic studies, the samples were characterized by various physicochemical techniques for structure-activity relationships. (C) 2003 Elsevier Inc. All rights reserved.
Abstract: Layered double hydroxides with the hydrotalcite-type structure containing Mg2+ and V3+ in the brucite-like layers, possessing different V contents, have been prepared and characterised by elemental. chemical analysis, powder X-ray diffraction, Fourier transform infrared (FT-IR) spectroscopy and specific surface area and porosity assessment by nitrogen adsorption; thermal decomposition was studied by Differential Thermal Analysis and Thermogravimetric Analysis. The solids obtained after calcination at 800 degreesC were tested as catalysts for oxidative dehydrogenation of propane and n-butane. Results indicate that the relative amounts Of Mg3(VO4) and MgO, depending on the V content in the starting hydrotalcite, determines the performance of the catalysts in oxidative dehydrogenation of propane and n-butane. (C) 2003 Elsevier Ltd. All rights reserved.
Abstract: Two commercial automobile three-way catalysts (RENAULT) of different configuration: (1) Pt, Rh active phase supported on ceria-promoted alumina washcoat, and (H) Pd, Rh active phase supported on zirconia-ceria-promoted alumina washcoat on cordierite supports, have been compared fresh and aged in a conventional automobile for 100,000 km in order to design and to adjust a catalyst ageing method suitable to imitate most relevant conditions of real tests. The characterization techniques were N-2 adsorption-desorption isotherms, powder X-Ray diffraction, elemental chemical analysis and activity tests. Activity tests were: (1) CO, NO and CH4 conversion versus temperature (from 120 to 500 degreesC), and (2) CO, NO and CH4 conversion versus the ratio between the concentrations of the oxidant and reducing species (lambda) at 500 degreesC (normal running temperature). Gas composition was close to conventional engine exhaust composition with and without water vapour. A strong deactivation has been observed as a consequence of thermal processes and loss of catalyst washcoat mainly for catalyst (II). Poison concentration along the catalyst has also been shown, however, its influence on catalyst deactivation was not as important as thermal processes were. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: A study is presented on different synthetic routes to prepare layered double hydroxides (LDHs) containing organic sulfonates hosted in the interlayer space. The solids prepared have been characterized by powder X-ray diffraction, FT-IR spectroscopy, mass spectrometry, and thermal methods. Anionic exchange and precipitation in the presence of NaOH have been used, but urea hydrolysis represents the best route to obtain crystalline solids, with low specific surface area, although coprecipitation of amorphous Al(III) oxohydroxides is only avoided when a large excess of urea is used.
Abstract: Layered Ni, Zn hydroxyacetates have been prepared by hydrothermal methods and have been used for anion exchange of the original acetate anions by chloride, bromide, carbonate, nitrate, sulfate, and phosphate. Exchange was complete in all cases. The solids have been characterised by elemental chemical analysis, powder X-ray diffraction, thermal analysis (thermogravimetric and differential), FT-IR spectroscopy and electron microscopy; XAS spectra were also recorded in some cases. Thermal decomposition in air leads to removal of interlayer water at 150-200 degreesC. Mixed NiO-ZnO oxides are formed at higher temperatures. Chloride, sulfate, and phosphate salts are also formed at intermediate calcination temperature, and phosphate remains even after calcination at 1000 degreesC.
Abstract: A review on recent results on characterisation of layered double hydroxides (LDHs) prepared by conventional coprecipitation methods is reported. Emphasis is made on the application of few experimental methods for characterising structural properties of these compounds. The use of powder X-ray diffraction, X-ray absorption, MAS-NMR, and FT-IR spectroscopies to characterise particular properties of these solids is described. Thermal analyses (thermogravimetric, differential thermal analysis and temperature-programmed reduction) are used, together with other techniques, to characterise LDHs and mixed oxides produced by thermal decomposition of LDHs. Partial or total reconstruction of the layered structure from medium-temperature calcined LDHs is achieved depending on the calcination temperature and the time of exposure to water vapour. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: Liquid phase catalytic hydroxylation of phenol was carried out over ternary hydrotalcites containing copper, nickel and aluminum using hydrogen peroxide as oxidant. The influence of various reaction parameters, namely, substrate: catalyst ratio, substrate: oxidant ratio, nature of oxidant, solvent, pH, time-on-stream, reaction temperature and calcination temperature on the activity and selectivity for the "sought for" reaction, were studied. The catalysts were synthesized by the coprecipitation technique using metal nitrates and a NaOH/Na2CO3 mixture. Hydroxylation of phenol over these catalysts resulted mainly in the formation of catechol and hydroquinone. Among the catalysts studied, CuNiAl3-5 ((Cu + Ni)/Al = 3.0; Cu/Ni = 5.0) and CuNiAl2-1 ((Cu + Ni)/Al = 2.0; Cu/Ni = 1.0) showed maximum activity with a catechol:hydroquinone ratio close to 1.6. An increase in the substrate: catalyst ratio enhanced the conversion of phenol over these catalysts. With respect to the influence of reaction temperature, the conversion increased up to 65 degreesC and decreased when the temperature was further increased. Oxidants other than H2O2 and solvents other than water have not showed measurable conversion of phenol. Time-on-stream studies indicated that around 90% of conversion of phenol was achieved in 10 rain and longer reaction time did not significantly enhance the conversion. Among the calcined samples studied, that calcined at 800 degreesC showed a maximum activity for phenol hydroxylation, however, the activities of calcined catalysts were lower than those of fresh hydrotalcites. The observed variation in the activity may be attributed to the copper concentration, especially those present on the surface. A reaction pathway involving hydroxy radical is proposed for the formation of dihydroxybenzenes. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: A new layered silicate (denoted as FLS1) has been prepared from reaction of H-magadiite in tetramethylammonium hydroxide (TMAOH) and water via hydrothermal treatment for 5 days at temperatures higher than 150 degreesC. When the product is heated to 500 degreesC, the layered silicate is converted to a three-dimensional network through condensation of hydroxyl groups and cross-linking of the layers. The calcined phase is stable up to 900 degreesC and has a surface area of 437 m(2) g(-1), more than 10 times larger than that of the starting H-magadiite (40 m(2) g(-1)). The external surface area and micropore volumes were determined using a new method, similar to that reported previously for pillared materials. The micropore volume of FLS1 calcined at 500 degreesC is about 0.130 mL (liquid nitrogen) g(-1). The solid-state transformation of FLS1 into a TMA-containing sodalite structure in a sodium aluminate and TMAOH(.)5H(2)O mixture at 150 degreesC for 2 h is also reported.
Abstract: Flash vacuum pyrolysis (fvp) reactions of NH-pyrazole (1) and 3,5-diphenylpyrazole (2) were investigated in the presence of anionic clays having hydrotalcite structure (HT). Solid catalysts with Mg:Al ratio equal to 2:1 containing carbonate (HT-1), nitrate (HT-2), and silicate (HT-3) as interlayer anions were employed. Between 400 and 600 degreesC, compound 1 remained almost unchanged and only unidentified volatile products were detected in small amounts. In contrast, 2 afforded benzonitrile (3) and phenylacetonitrile (4) by a ring fragmentation reaction at 450 degreesC. At a higher temperature (660 degreesC), the same products obtained in homogeneous fvp reactions, i.e., 2-phenylindene (5) and 3-phenylindene (6), were obtained showing no catalysis by HT under these conditions. Results showed that the yield is strongly dependent on the nature of the interlayer anion in the hydrotalcite structure. In comparison with reactions of 2 over zeolites, HTs exhibit selectivity for ring fragmentation reaction.
Abstract: Thermal decomposition of hydrotalcite-like compounds with formula MgxAl1-x(CO3)(x/2)(.)nH(2)O has been studied varying the molar ratio (x) between Mg and Al cations. Compounds synthesized by coprecipitation method at fixed pH, have been fired at different temperatures between 500 and 1200 degreesC with different remaining times. This study has been carried out by Differential Thermal and Thermogravimetric Analysis (DTA/TG), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Al-27 Nuclear Magnetic Resonance (Al-27-MAS-NMR) Main results show that the optimum molar ratio (x) to hydrotalcite phase formation is between 0.20 and 0.35, obtaining spinel (MgAl2O4) and Periclase (MgO) as the decomposition compounds.
Abstract: Electron spin resonance spectroscopy (ESR), Fourier transform infrared spectroscopy (FTIR), and monitoring of pyridine (py) and boric acid trimethyl ester (BATE) adsorption for determining surface acidity and basicity, respectively, were used to carry out further characterization of mixed ZnO/TiO2 polycrystalline solids prepared by different methods. Moreover, the powders were tested in a batch photoreactor for a probe reaction, i.e., 4-nitrophenol photodegradation in aqueous medium. ESR results indicated the presence of signals attributable to Zn+ species in ZnO/TiO2 (anatase) solids, while in ZnO/TiO2 (rutile) samples the presence of zinc induced only the formation of signals probably due to Ti3+ centers. FT-IR spectra showed no significant differences of the surface hydroxylation degree of the various photocatalysts, whereas the surface acidic properties generally decrease by increasing the amount of ZnO, Coupling of ZnO and TiO2 semiconducting powders was not so beneficial, as expected on the basis of the intrinsic electronic properties, to enhance the photoreactivity for the studied reaction, although some of the powders showed photoactivities slightly higher than those of bare TiO2 and ZnO. Nevertheless, some of the samples (the mixed particles ZnO/TiO2 (rutile)) appear promising from an application point of view because no filtration tvas needed after the occurrence of the photoreactivity tests to separate them from the solution because they decanted easily. The mineralization of 4-nitrophenol was checked by determining the total organic carbon (TOC) and a complete photooxidation occurred in few hours in the presence of some of the samples.
Abstract: Polycrystalline ZnO/TiO2 solids have been prepared with four different methods using home prepared Tin, (anatase) or TiO2 (rutile) as supports and Zn(NO3)(2). 6H(2)O or Zn(CH3COO)(2). 2H(2)O as precursors for ZnO. The bulk and surface properties of the samples were investigated by means of TG-DTA, XRD, TEM, SEME-DAX, XPS, BET surface area determination, and porosity measurements. XRD and TEM results indicate that no significant defect structures exist in any of the samples. The ZnO crystallinity and its enrichment on the surface of TiO2 particles were dependent on the preparation method. The surface areas generally decrease by increasing the amount of ZnO except when ZnO from Zn(CH3COO)(2). 2H(2)O was supported on TiO2 (rutile). The samples prepared from Zn(CH3COO)(2). 2H(2)O were more porous than those prepared from Zn(NO3)(2). 6H(2)O. This was confirmed by BET surface area determinations and SEM observations. XPS spectra indicate that the atomic ratio between OH- and O2- on the particles surface is similar for samples with the same ZnO content independent of the precursor used for the samples' preparation. Moreover a much higher segregation of ZnO was found fur samples obtained by using the acetate precursor.
Abstract: Reaction of alkylammonium titanates with tetraethylorthosilicate (TEOS) resulted in the intercalation and polymerization of TEOS between the titanate sheets. After heating to 500 degreesC, the basal spacing of pillared silica titanates increased with increasing length of the alkylamine chains and the amount of silicon intercalated. Although the layers of pillared titanates were expanded by the silica species, and the specific surface area increased, only negligible micropores were formed, suggesting that the interlayer region was "stuffed" with the pillaring species. The adsorption of pyridine indicated the formation of strong Bronsted acid sites up to 300 degreesC. In addition some basic Lewis sites were detected on silica pillared titanates.
Abstract: A FTIR spectroscopy study on the surface reactivity of calcined layered double hydroxides (LDH), with different cations in the interlayer space (namely, carbonate, nitrate, silicate, and berate), for isopropanol oxidation is reported. The LDHs were previously calcined at 600 degreesC and then their surface acidity was quantitatively determined via volumetric determination of ammonia adsorption. Surface Lewis and/or Bronsted acid and basic sites were qualitatively studied by FTIR monitoring of the adsorption of pyridine and boric acid trimethyl ester. Carbonate and nitrate samples show surface acidity and basicity and hence the maximum activity for acetone formation through dehydrogenation of isopropanol. In contrast, although the silicate sample displays maximum surface acidity, the lack of an appreciable amount of surface basic sites gives rise to a lower activity in the named reaction, of the same order as that shown by the berate sample.
Abstract: Copper-cobalt-aluminium hydrotalcites with (Cu+Co)/Al atomic ratio of 3.0 and Cu/Co atomic ratios 80 : 20, 75 : 25, 50 : 50, 33 : 67, 25 : 75 and 20 : 80 were synthesized by coprecipitation under low supersaturation and characterized by various physicochemical techniques. Powder X-ray diffraction (PXRD) showed a single phase with a resemblance to hydrotalcite, whose crystallinity increased with increasing copper concentration. A continuous blue shift of the band with a concomitant red shift in the vibration of the carbonate was observed with increasing cobalt content. Thermal analysis of these samples showed a significant variation depending on the nature of the atmosphere, more marked at higher temperatures, owing to the possibility of oxidation of cobalt. UV-vis spectroscopy showed absorption maxima close to 750 and 530 nm with the former becoming stronger and the latter weaker as the copper content is increased. TPR of fresh samples substantiated the lack of oxidation of cobalt, as evidenced by an M/H-2 (M = Cu+Co) ratio for these samples close to 1.0. N-2 adsorption measurements showed type II isotherms with a hysteresis loop closing at P/P-o = 0.45. The total pore volume, the specific surface area and the area of the hysteresis loop increased with cobalt content. Thermal calcination of these samples at 500 degreesC showed a diffuse pattern exhibiting both a tenorite and a spinel phase, whose peak intensities varied with Cu/Co atomic ratio. TPR for these samples showed higher M/H-2 values, suggesting partial oxidation of Co2+. Liquid phase hydroxylation of phenol was carried out over these catalysts using H2O2 as oxidant. Among the catalysts studied, that with a Cu : Co atomic ratio of 3 : 1 showed the maximum activity with a catechol : hydroquinone ratio close to 3.4. An increase in substrate : catalyst ratio enhanced the conversion of phenol. Oxidants other than H2O2 and solvents other than water did not show measurable conversion of phenol. Time-on-stream studies indicated that 80% conversion of phenol was achieved in 10 min. The normalized activity of these catalysts indicated the influence of cobalt, probably together with surface properties, on the intrinsic activity of copper in controlling the course of reaction. A reaction pathway involving hydroxyl radical is proposed.
Abstract: In Rh/TiO2 systems, the dispersion of Rh particles depends on the reduction temperature and on the preparation method. The capacity for Hz adsorption is larger for those samples reduced at low than at high temperature.
Abstract: Hydrotalcite-type materials with different Ni:Mg:Al ratios have been prepared and characterized by different methods (powder X-ray diffraction, differential thermal analysis and thermogravimetric analysis, FT-IR and VIS-UV/diffuse reflectance spectroscopies and specific surface area and pore size distribution assessment). Samples calcined at 600 degreesC, where the layered structure has been destroyed, are formed by non-stoichiometric NiO-MgO oxides, and their catalytic activity in n-butane and propene oxidative dehydrogenation has been studied; production of methane and ethylene, in addition to CO(2) and H(2)O, is observed. The sample without Ni is not active in these reactions, but, among those containing Ni, that with the lowest content exhibits the largest activity. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: A study is reported on the thermal evolution of Mg, V layered double hydroxides with interlayer carbonate anions and different Mg/V molar ratios (from 1 to 4). Decomposition in O-2 occurs at lower temperature than that in N-2; the stability of the layered structure decreases as the V content increases. Oxidation of layer V3+ cations occurs in the same temperature range as dehydroxylation and decarbonation. The nature of the crystalline phases formed upon calcination at high temperatures strongly depends on the Mg/V molar ratio in the initial solid. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: The surface acid properties of WOx supported on different oxides (alumina, silica, magnesia, titania, zirconia, and niobia), as well as the decomposition of butan-2-ol on these systems, have been studied by FT-IR spectroscopy. Incorporation of supported tungsten-containing species significantly increases the surface acid properties, and new surface acid sites develop, except for in the magnesia-supported system. The increase in surface acidity is responsible for the high selectivity to butene formation on these systems, except for in the magnesia-supported system, where a larger selectivity to methyl ethyl ketone is observed.
Abstract: Ethyl cinnamate/montmorillonite intercalation complexes were obtained by gaseous adsorption of the drug on the clay surface. They were characterized by powder X-ray diffraction, differential thermal and thermogravimetric analyses and by visible-UV and IR spectroscopies. It was found that after 1 day of adsorption, most of the drug enters the interlayer space of the clay by substitution of water molecules, and is removed only after heating at high temperature. In addition, a portion is physisorbed on the external surface of the crystallites, being removed easily below 100degreesC. The interlayer complex improves the protecting ability of the pure clay or the pure drug against ultraviolet radiation, specially in the so-called 'C' range (290-190 nm).
Abstract: The thermal decomposition of hydrotalcite-like materials containing Zn(II), Fe(III), and Al(III) is studied. One of these samples contains zinc and aluminum in the brucite-like layers, while iron is incorporated as hexacyanoferrate(III) anions in the interlayer space. A second set of samples with Fe/Al atomic ratios in the range 0-1 contains zinc, iron, and aluminum cations in the layer and carbonate as the interlayer anion. When thermal decomposition is started from the latter precursors, X-ray diffraction only detects the formation of ZnO after calcination at 400-600 degreesC, while X-ray absorption spectra recorded at the Fe and AI-K absorption edges show that a fraction of trivalent cations migrates to tetrahedral holes in this temperature range. After calcination at 800 degreesC, the highest temperature achieved, two ZnFe(y)Al(2-y)O(4) spinels segregate, with a distribution of cations essentially normal, Fe/Al atomic ratios being different from the value of the starting material. On the other hand, thermal decomposition of the precursor that incorporates iron as hexacyanoferrate(III) yields already at 600 degreesC a mixture of ZnO and a single ZnFe(y)Al(2-y)O(4) spinel phase, whose Fe/Al atomic ratio coincides with the value of the original sample. Results suggest that the pyrolysis of the cyanide ligands at 400 degreesC leads to a coordination environment for iron highly disordered beyond the first-coordination shell, thus increasing the reactivity of iron and preventing the segregation of cations during the thermal decomposition of this sample.
Abstract: The response of five granite stones (fresh or submitted to accelerated aging treatments) to consolidation and hydrofugation treatments has been studied. Changes in free porosity and capillary absorption coefficient (CAC) have been monitored. Statistical analysis of the data indicates an important effect of these treatments on the changes on these physical properties, leading to different fluid-transport capabilities through the stone. Simultaneous hydrofuging and consolidating treatments have a larger effect than any treatment applied separately.
Abstract: Direct reaction of a colloidal suspension of an exfoliated layered titanate with an aluminium polyoxocation solution produced an ordered phase with a large basal spacing of 2.6 nm. Taking into account the starting Al/titanate molar ratio this swelling can be ascribed to the intercalation of double layers of Al-13 polyoxocations between the titanate sheets. Heat-treatment at 600 degrees C resulted in an alumina-pillared titanate with specific surface areas ranging from 270 to 300 m(2) g(-1) and a pore volume of 0.25 mL g(-1) (liquid nitrogen). The material was mainly mesoporous, the predominant pores having a diameter close to 4.0 nm. The acidity of the pillared titanate was ca. 0.60 mmol of protons g(-1).
Abstract: This present work tries to reproduce, through test of controlated conditions in accelerate aging, reproducing the most frequent ambients in places affected by a clima of continental tendency, the more commun forms of deterioration in historical interesting buildings, constructed with granitic rocks in those places. We analyze the changes in the rocks nature and properties (intensity and rapidity), produced in different experiences. In this study, realised on "sensu late" granites, we come to the conclusion that the deterioration due to cold/hot and frost/defrosting processes is quite slow and not much appreciable within a short time, whereas if the phenomena of salts cristalization in the rocs occurs, the deterioration is rapid and intense.
Abstract: V-Zr and V-Nb oxide systems were prepared by two methods, impregnation of the support (zirconia or niobia) with a salt precursor of vanadium, or by mechanically mixing vanadia with both supports. The solids obtained were characterized following several techniques. The state of vanadium on the surface of the support depends on the preparation method. In the mechanical mixtures the predominant species is slightly distorted crystalline V2O5. In samples prepared by impregnation the species detected are isolated vanadyl, polyvanadates and crystalline V2O5, depending on the vanadium content; the coordination of V ions in these species is tetrahedral, distorted octahedral or pentacoordinated, respectively. Formation of crystalline ZrV2O7 and V2O5 is observed in the case of the V-Zr sample with 7.5 wt.% vanadium. All these species are more easily reducible than bulk V2O5 (i.e., the onset temperature for reduction is lower), and show surface acidity, both Lewis and Bronsted, the Bronsted acidity increasing as the dispersion increases.
Abstract: Layered double hydroxides (LDHs) with the hydrotalcite-like structure have been prepared containing different amounts of Ni2+, Cu2+ and Al3+ in the brucite-like layers by a coprecipitation method. The samples have been characterized by elemental chemical analysis, powder X-ray diffraction, differential thermal analysis and thermogravimetric analysis, FT-IR and UV-VIS diffuse reflectance spectroscopies, temperature-programmed reduction, and specific surface area and porosity assessment by N-2 adsorption at -196 degrees C. The nature of the phase obtained, crystallinity of the HT-like phase and thermal behaviour of these materials were influenced by M-II/Al atomic composition and concentration of bivalent metal ion. Calcination at 500 degrees C leads to mostly amorphous solids containing NiO, and also CuO (tenorite) for large Cu2+ contents (Cu/Ni molar ratio equal to 4). When the calcination temperature is increased to 850 degrees C, crystallization of well defined phases (NiO, CuO and NiAl2O4, depending on the relative amounts of Ni, Cu and Al) takes place. Reduction of Cu2+ species in these samples takes place at a lower temperature than that for Ni2+, while the temperature of reduction of nickel is increased with an increase in aluminium and copper content of the samples.
Abstract: Differential thermal analysis and thermogravimetric analysis have been used to study EUROCAT-2 OXIDE (TiO2-V2O5-WO3). It has been found that water is removed at low temperature, and SO2 removal and surface reduction (O-2 evolution) take place at high temperature, both for a fresh and a used catalyst. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: Synthesis and physicochemical characterization (elemental chemical analysis, powder X-ray diffraction, FT-IR spectroscopy, thermal analysis and specific surface area determination) of Na, K and Li birnessites are described. The ionic exchange abilities of these materials have been studied, and the exchanged samples have been characterized following the same experimental techniques. An almost complete ionic exchange has been observed for Li, while exchange for Cu and Mg is rather low, probably because of ionic radii differences.
Abstract: Layered double hydroxides (LDHs) have been prepared with interlayer nitrate and with different Mg:Al ratios. Solids with low Mg:Al ratios show larger basal spacings than those with larger Mg:Al ratios, consistent with tilting of the nitrate ions, respectively. Nitrate/tetraborate and nitrate/silicate anionic exchange has been achieved only for those samples with "tilted" nitrate. The precise nature of the interlayer species in the exchanged samples has been concluded from MAS-NMR IR results. The layered structure of the exchanged samples collapses at temperatures higher than those for the nitrate-containing precursors. (C) 2000 Academic Press.
Abstract: A study is presented on the pore size distribution in granites, both fresh and artifically aged by freezing/thawing cycles (-20/110 degrees C), in the diameter range 0-25 nm, by nitrogen adsorption at -196 degrees C. The pore size distribution found permits a plausible explanation for the behaviour shown by these materials during degradation processes associated to fluid flows through their pores (thermoclasty and gelifraction). If low porous granites are used, artificial ageing has only a minor, limited effect on their pore network; however, changes in pore size distribution (diameter < 10 nm) are observed in samples where a pore network already exists in the unaltered samples. Changes in porosity observed upon artificial ageing are similar to those observed in historical buildings after natural decay. (C) 2000 Elsevier Science Ltd. All rights reserved.
Abstract: This paper reviews the synthesis, properties and applications of layered double hydroxides (LDHs), also known as anionic clays or hydrotalcite-like materials, containing intercalated anions constituted by metal complexes or oxometalates. After an introduction describing the main features of these compounds, emphasis is put on the synthesis methods, characterization and applications, (C) 1999 Elsevier Science S.A. All rights reserved.
Abstract: A titanate with lepidocrocite-like structure has been intercalated by hexylammonium (HA) and tetrabuthylammonium (TBA) organic cations, which were further exchanged with aluminum polyoxocations. The pillared structure of these materials was found to be stable up to 500 degrees C and exhibited a basal spacing of 1.4 nn. Nitrogen adsorption isotherms were measured to determine the surface areas and pore structure of these pillared titanates, and the effect of intercalation temperature and heat treatment on these surface characteristics is discussed. The micropore volume values were related to the amount of alumina and to the heat-treatment temperature. Most of the pillared titanates prepared from HA-Ti showed a sharp distribution of micropores with the average diameter of 2 nm, whereas the comparable materials obtained from TBA-Ti showed a broad distribution of pore diameter centered 4 nm in addition to micropores. Infrared spectroscopy monitoring the pyridine adsorption showed the presence of Lewis acid sites, and evolution of surface Bronsted acid sites upon incorporation of the alumina pillars. There is a good correlation between the amount of alumina and the acidity in terms of proton concentration (Bronsted) as determined by adsorption/desorption of cyclohexylamine.
Abstract: A layered titanate with a lepidocrocite-type structure has been pillared with Al-13 Keggin ions to prepare a porous and high-surface-area material. Pillaring was achieved by ion exchange of hexylammonium (HA-Ti) or tetrabuthylammonium (TBA-Ti) intercalated titanates with Keggin Al-13 complex. The thermal stability of the Al-13 intercalates depended on the amount of aluminum incorporated. The surface area and porosity can be tailored by controlling the amount of aluminum uptake and by the nature of base used to prepare the aluminium pillaring solution. In addition, the material derived from HA-Ti exhibited a sharp pore size distribution with an average diameter of 2 nm, while the pillared product obtained from TBA-Ti showed mostly a broad mesoporous distribution with an average pore diameter of 4 nm.
Abstract: In this paper the preparation of two sets of polycrystalline photocatalysts prepared by supporting TiO2 (anatase) on TiO2 (rutile) or Al2O3 is reported. The powders were prepared by a wet impregnation method using titanium(IV) isopropylate. The impregnation was followed by thermal treatment and this procedure was repeated several times in order to obtain samples with progressively increasing amounts of TiO2 (anatase). The photocatalytic activity of all of the specimens was tested by carrying out the 4-nitrophenol photodegradation, used as a "probe" reaction, in aqueous medium and in different kinds of batch photoreactors. The solids were characterised by X-ray diffraction (XRD), specific surface area (BET) and porosity determination, scanning electron microscopy observation (SEM) coupled with energy dispersive X-ray analysis (EDX), visible-ultraviolet diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR) monitoring of surface acidity. The characterisation results indicate an increase of TiO2 anatase phase with the number of impregnations. The porosity and the surface area of all the photocatalysts prepared by supporting TiO2 (anatase) on TiO2 (rutile) were always higher than that of the bare support, while those of the samples prepared by supporting TiO2 (anatase) on Al2O3 were lower than that of bare Al2O3 The FTIR measurements indicated that Lewis acid sites (Ti4+ or both Ti4+ and Al3+) were present on the surface of the two sets of supported samples, while no Bronsted acid sites were detected. The preparation method used allowed to obtain samples quite resistant to disaggregation when they were mixed in water Moreover they showed to be easily decantable after the occurrence of the runs. Both sets of TiO2 (anatase) supported samples resulted photoactive and the photoactivity increased by increasing the content of the anatase phase, although it was less significant than that of a pure home prepared and Degussa P25 TiO2 samples, tested for the sake of comparison. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: A layered double hydroxide with the hydrotalcite-like structure containing Mi(II) and Fe(III) cations in the brucite-like layers with the formula [Ni0.7Fe0.3(OH)(2)](CO3)(0.15).0.94 H2O has been prepared by coprecipitation. On hydrothermal treatment, a coproduct, identified as a NiFe2O4 spinel, is formed. Both samples, as well as the solids obtained from them by calcination at 450 degrees C (where only mixed oxides are present) and 750 degrees C (where crystallization of well defined phases has taken place), have been characterized by powder X-ray diffraction (PXRD), X-ray absorption (XAS) and FT-IR spectroscopies, thermal analysis (differential and thermogravimetric), temperature-programmed reduction, and specific surface area assessment. XAS results show that in both calcined samples all Ni(II) cations are in octahedral holes, while Fe(III) cations are equally distributed between octahedral and tetrahedral holes. For the sample calcined at 750 degrees C, coordination parameters at the first and second shells of Ni(II) and Fe(III) cations coincide with those expected for a mixture of NiO and the NiFe2O4 spinel, in agreement with the detection of both crystalline compounds by PXRD. Although PXRD only detects crystalline NiO in the sample calcined at 450 degrees C, the presence of Fe(III) cations in tetrahedral holes discards the formation of a Ni(II)-Fe(III) oxide solid solution with a rock salt structure, thus indicating that Fe(III) ions are forming an amorphous phase at this temperature. In this sample, coordination parameters at the second shell of Fe(III) are also different from those expected for a spinel-like structure. In agreement with the experimental data, a structure for the cluster present in the amorphous phase is proposed. Reducibility of cations in the sample calcined at 450 degrees C depends on the synthesis method and on the formation of spinel-like nuclei.
Abstract: The effect of the Ni/Zn molar ratio on the activity, selectivity, and coke formation of NiO . ZnO . Al2O3 catalysts (modified with Fe3+ or Cr3+) during acetylene hydrogenation has been studied. Coke formation is decreased significantly in the presence of ZnO, and a similar effect is also found when the catalysts are doped with Cr3+ instead of with Fe3+. An optimum Ni/Zn ratio for activity, selectivity and coke formation performance has been found. The existence of this maximum implies the necessity of adding ZnO to the support in order to modulate the catalytic properties of Ni. Furthermore, if the Ni concentration is increased, the conversion, selectivity, and yield to ethylene not only fails to increase, but actually decreases, while coke formation simultaneously increases. The existence of the above-mentioned optimum is the consequence of a minimum concentration of the hydrogenolytic (naked) metallic sites, the majority being hydrogenating metallic sites covered by a monolayer of ethylidines. A kinetic model of coke growth is proposed assuming the existence of two types of coke associated with the hydrogenolitic and hydrogenating sites respectively. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: The reconstruction of layered double hydroxides with the hydrotalcite-like structure from material previously calcined at increasing temperatures has been studied. Both the calcination temperature and the time the samples spent equilibrating with a controlled water vapour pressure have been considered. The crystallinity of the samples and the crystalline phases formed have been assessed by powder X-ray diffraction, while the local environment of the Al ions has been probed by Al-27 MAS NMR. It has been found that reconstruction is complete when the sample has been calcined at or below 550 degrees C, after 24 h rehydration, while equilibration for 3 days is adequate to reconstruct samples previously calcined at 750 degrees C. Only partial reconstruction is observed after calcination at 1000 degrees C. However, the NMR results indicate that, in all cases, some of the Al ions remain in tetrahedral sites.
Abstract: The reaction between proline and carbon disulfide in a basic medium yields the corresponding dithiocarbamate thereafter dtc pro. Reaction of this compound with Na2MoO4. 2H(2)O leads to formation of a coordination compound with the formula MoO2(dtc Hpro)(2), while reaction with MoCl5, in an inert atmosphere leads to Mo2O4(dtc Hpro)(4). Mo K XANES data are consistent with the presence of Mo(VI) in the former compound, while a lower oxidation state of molybdenum in the latter, Mo(V), shifts the absorption edge to lower energies. EXAFS data indicate that, in these complexes, molybdenum ions are in a different coordination environment. In the first it is in a distorted octahedral coordination, and oxygen atoms are in cis positions, with an Mo-O distance of 1.68 A and an Mo-S distance of 2.48 Angstrom. However, in the dimer the Mo atoms (Mo-Mo distance 2.55 Angstrom) are bridged by two oxygen atoms (Mo-O distance 1.95 Angstrom), and the Mo-S distance is 2.43 Angstrom. The molybdenum atom in this case is pentacoordinated with a terminal oxygen atom distance Mo-O of 1.66 Angstrom. The dithiocarbamate unit behaves in all cases as a bidentate ligand, both sulfur atoms being equivalent. Both complexes are diamagnetic, and FT-IR spectroscopy provides evidence that the carboxylic group is in the acidic form, not being coordinated to the Mo cations. Only charge transfer bands are detected in the UV-Vis spectra. EPR spectra produced evidence of the presence of a small amount of a paramagnetic Mo(V) species, probably Mo(dtc Hpro)(3). (C) 1999 Elsevier Science S.A. All rights reserved.
Abstract: Layered double hydroxides (LDH) with the hydrotalcite-like structure, but containing Ni(II) and V(III) cations in the layers, have been prepared by the constant pH method. Characterization by elemental chemical analysis, powder X-ray diffraction, differential thermal analysis, thermogravimetric analysis, FT-IR spectroscopy, temperature-programmed reduction and specific-surface area assessment indicates the formation of well crystallized materials after hydrothermal treatment, without significant oxidation of V(III) species,and with interlayer, flat-oriented, carbonate anions. Reduction in H-2 takes place in a single step at cn. 400 degrees C[Ni(II)-->Ni(0)]; V(III) is not reduced. On calcination in air, Ni2V3O8 is formed, together with NiO, but crystallization is only evident after calcination at 750 degrees C. No major difference was found between samples with Ni:V molar ratios close to 2 or 4.
Abstract: The structure, surface acidity, and reducibility of WO(3)/SiO(2) systems, Obtained by impregnation of the silica support with aqueous paratungstate solutions, are described. The effect of tungsten loading and calcination temperature on the nature and physicochemical properties of tungsten-containing phases has been studied by FT-IR, Raman and W-L(3) XAS spectroscopies, X-ray diffraction, temperature-programmed reduction, and FT-IR monitoring of pyridine adsorption for surface acidity. Results obtained show that, due to a reaction between the tungstate and the support, calcination up to 723 K leads to formation of a Si-O-W crystalline species where tungsten is forming Keggin-type units similar to those found in dodecatungstosilicates. As the calcination temperature is raised, these species decompose leading to crystalline WO(3), due to the low dispersing ability of silica. Thermal satiability of this Si-O-W species is higher than that of commercial dodecatugstosilicic acid, although it slightly decreases as the tungsten content decreases. Si-O-W species possess large surface acidity, with both Lewis and Bronsted acid sites, and they are more easily reduced than crystalline WO(3).
Abstract: Zn-Al layered double hydroxides (LDHs) have been prepared with Cl- and CO32- interlayer anions and with Zn/Al ratios of 2 and 3. The corresponding polyoxovanadate-containing LDHs were then prepared by anion exchange reaction at different pH values. The products were characterized by atomic absorption, PXRD, FT-IR spectroscopy, TG analysis, and transmission electron microscopy (TEM). Thr resulting vanadate-containing LDHs, depending on the pH and the Zn/Al ratio, consisted of one or more phases. PXRD patterns corresponding to layered compounds with different gallery heights (7.0, 4.6, and 2.6 Angstrom for V10O286-, V4O124- and (VO3)(n)(n-) chains, respectively) were observed. X-ray absorption spectroscopy has been also used to assess the local geometry of vanadium ions in the different samples, and results indicate the presence of V5+ ions in octahedral and/or tetrahedral coordination depending upon the conditions of the synthesis.
Abstract: The spectroscopic properties of hydrotalcite-like compounds, containing Co(II) and Fe(III) in the layers, have been studied. In these materials, depending on the experimental conditions during synthesis, Co(II) becomes partially oxidized to Co(III). Although the environment of the cations is close to octahedral in all cases, Mossbauer data indicate a more symmetric environment for Fe(III) ions in samples not submitted to hydrothermal treatment. The FT-IR spectra show some distortion of the carbonate anions in the interlayer space.
Abstract: Cobalt and bismuth molybdates catalysts prepared by three different methods (coprecipitation, mechanical mixing and dispersion in n-pentane), have been studied by Laser Raman spectroscopy before and after testing the catalysts in the selective oxidation of isobutene to methacrolein. Raman bands ascribed to the component phases are observed in the spectra before test, while a phase transformation is detected after test.
Abstract: Cobalt(II)-iron(III) mixed hydroxides with hydrotalcite-like structure have been synthesized. Depending on the experimental conditions during synthesis, Co-II becomes partially oxidized to Co-III. The solids have been characterized by X-ray diffraction, FTIR spectroscopy, temperature-programmed reduction and nitrogen adsorption at low temperature for specific surface area assessment. Reduction of Fe-III takes place at lower temperatures than that of bulk Fe3O4. Thermal treatment in air leads to oxidation of Co-II to Co-III, in the same temperature range where collapsing of the layered structure by decarbonation is observed. The nature of the spinel phases formed upon calcination at high temperature depends both on the calcination temperature and the calcination time.
Abstract: The surface acidity of a range of lithium aluminium layered double hydroxides intercalated with carbonate, chloride, nitrate and decavanadate anions has been measured using Hammett indicators. In addition, the adsorption of pyridine and isopropanol has been monitored using infrared spectroscopy. The properties of the materials have been compared to their magnesium aluminium analogues. Surface measurements are compared with the observed catalytic activity for acetaldehyde self-condensation and 2-methylbut-3-yn-2-ol conversion. The MgAl matrix was found to be more stable than the LiAl matrix following calcination at the elevated temperatures which yielded the most active catalysts (ca. 450 degrees C). The order of acidity of the materials varied with the nature of the anion such that: carbonate < nitrate < chloride < decavanadate. Thus it was concluded that the nature of the interlayer anion had a large effect on the acidity of the materials, although the acidity was largely unaffected by the identity of the host matrix itself.
Abstract: Rh/TiO2 catalysts have been prepared under different experimental conditions (namely, crystallographic phase of the support, Rh loading, incorporation method, etc.) and reduced at 773 K. Hydrogenolysis of n-butane has been used as a reaction test to characterize these catalysts. The reaction parameters (activation energy and frequency factor) have been determined and the change in selectivity to methane, ethane and propane has been studied following the Kempling-Anderson formalism. (C) 1998 Elsevier Science B.V. All rights reserved.
Abstract: Oxidative dehydrogenation of n-butane and propene has been studied on six Mg-Al-Fe oxide catalysts prepared by thermal decomposition of layered double hydroxides with a hydrotalcite-like structure, possessing Mg2+ and Al3+ ions in the brucite-like layers, and hexacyanoferrate in the interlayers. In all cases, CO2 and H2O are formed, together with minor amounts of ethylene and methane. On the solids prepared from hexacyanoferrate(II), formation of benzene is also observed.
Abstract: Layered double hydroxides (,DH) containing Mg-II, Fe-III, and Al-III in the brucite-like layers and interlayer carbonate (with a constant M-II/M-III ratio but varying Al-III/Fe-III ratios) have been prepared and characterised by Xray diffraction, thermal analysis, FT-IR and UV-VIS/diffuse reflectance spectroscopies, temperature-programmed reduction and specific surface area assessment through low temperature adsorption of Nz. An Mg,AI-LDH, but with intercalated hexacyanoferrate (III), has been also prepared and characterised, in which simultaneous formation of the carbonate analogue did not occur. Thermal decomposition in air at 450 and 750 degrees C leads to MgO and poorly crystallised MgFe2O4 spinel (crystallinity increasing with the iron content), while for the hexacyano-containing sample, crystallization only is observed after calcination at 900 degrees C. This different behaviour has been related to the initial location of the iron ions.
Abstract: Ni-containing catalysts for selective acetylene hydrogenation to ethylene have been prepared by controlled calcination of hydrotalcite-like precursors. In addition to Ni and Al, Cr and Zn were added to improve the catalytic performance. The hydrotalcite-like precursors and the calcined catalysts have been characterized by powder X-ray diffraction, FT-IR and Vis-UV/Diffuse Reflectance spectroscopies, temperature-programmed reduction, and specific surface area assessment by nitrogen adsorption at 77 K. Despite addition of Zn hinders coke formation, the activity to gaseous products decreases as the Ni content is increased. An increase in coke concentration increases activity and selectivity to ethylene, specially in those samples with not too high Ni contents. The highest selectivity to ethylene is achieved for Zn/Ni approximate to 4 (molar ratio). (C) 1998 Elsevier Science B.V. All rights reserved.
Abstract: P(2)O(5)/ZrO(2), MoO(3)/ZrO(2) and MoO(3)-P(2)O(5)/ZrO(2) samples have been prepared by impregnation of a commercial zirconia support with ammonium phosphate and/or heptamolybdate aqueous solutions. The structural and morphological properties have been studied by XRD diffraction, nitrogen adsorption-desorption experiments and skeletal FT-LR and FT-Raman spectra. Original zirconia is in monoclinic phase with traces of tetragonal phase. Only Mo and P amount higher than the theoretical monolayer molybdite is observed. The incorporation of P and/or Mo causes a slight decrease in the value of the specific surface area with respect to pure support. All samples are mesoporous, whose texture can be described by tubular pores with widened parts. Surface Me(x)O(y) (where Me = P or Mo) groups have been detected. Fourier transform infrared spectra of adsorbed ammonia and carbon dioxide show that the addition of Mo and/or P on zirconia causes a relevant increase of the surface acidity and destroys the surface nucleophilic oxygen present on the pure support. (C) 1998 Elsevier Science S.A. All rights reserved.
Abstract: The interaction between phenyl salicylate and sepiolite has been studied using drug-clay systems obtained by melting and grinding. The samples have been characterized by powder Xray diffraction, differential thermal and thermogravimetric analyses, and Vis-UV and FT-IR spectroscopies. 'Free' water molecules are steadily substituted by the drug molecules, without any chemical change as shown by FT-TR. The systems prepared improved the protecting ability of the pure sepiolite or the pure drug against ultraviolet radiation, especially in the so-called 'C' range (290-190 nm).
Abstract: A series of polycrystalline WOx/TiO2 samples were prepared by means of a conventional impregnation method. The samples were characterized by means of X-ray diffraction, Vis-UV diffuse reflectance and Raman spectroscopies, nitrogen adsorption at 77 K for determining specific surface areas and surface texture, scanning electron microscopy, and FT-IR monitoring of pyridine adsorption for measuring the surface acidity. Catalytic activity of the samples has been assessed by carrying out as a "probe" reaction the photodegradation of 4-nitrophenol in aqueous medium. The results obtained indicate that incorporation of tungsten on titania leads to formation of different surface species, depending on the tungsten loading. Tungsta microcrystals were detected by X-ray diffraction when the nominal molar W/Ti ratio reached a value of 8.0%. FT-IR investigation indicated that the presence of tungsten induces formation of Bronsted and Lewis surface acid sites. The photoactivity results confirm the beneficial effect of tungsten in TiO2 for 4-nitrophenol photodegradation in aqueous medium. The reaction rates are higher than those reported in literature for another set of samples and maximum photoactivity was achieved for a sample containing 1.96 moles of W per 100 moles of Ti.
Abstract: Color is one of the parameters to be monitor-ed when determining the suitability of a protective or conservative treatment of stone. Application of a given treatment product should elicit the least possible effect in this parameter. Changes in color of five varieties of granite of highly varying characteristics due to their origin are studied in the present work. All the granites studied are widely used in buildings of historical interest in Avila (Spain). Some varieties are from unaltered facies (grey granifes of different grain size), and have a very low degree of porosity, while other are from facies that have undergone strong natural weathering processes, and that display important mineralogical and physical changes. In this sense, the ochre variety of granite contains clays (smectite and some kaolinite); the white variety contains kaolinite and opal, and the red variety contains kaolinite, opal, and iron oxihydroxides. Fresh cut and artificially aged (with 25 cycles of freezing/thawing and cold/heat treatment) samples were treated with consolidant (RC80 or RC70) and/or water repellent (H224) agents. After the corresponding statistical treatment of data obtained, significant changes in Delta E were observed with the treatmentsgiven; such changes are mol-e pronounced in unaged samples. Changes in at (lightness), however, despite being noticeable, seem to have only a minor effect on Delta E, which seem to be more dependent on changes in Delta a and Delta b. This was more pronounced when the stone was treated with RC80 and then with H224. (C) 1997 John Wiley & Sons, Inc.
Abstract: Layered double hydroxides (LDHs) containing Mg2+, Al3+, and Y3+ in the brucite-like layers and with carbonate as counteranion have been prepared following the coprecipitation method at constant pH. The Al/Y ratio depends on the concentrations in the starting solutions. The crystallinity of the layered materials decreases as the Al/Y ratio increases, probably due to the distortions introduced by the large difference in the ionic radii of the cations. On calcination at 750 degrees C MgO (probably containing Al3+ cations) and Y2O3 are formed from the sample richest in Y, while the other samples lead to formation of MgO and presumably Al2Y4O9. At 1000 degrees C, MgO, MgAl2O4, and Al2Y4O9 are formed in all cases. Exposure of samples calcined at 750 degrees C in air at room temperature leads to reconstruction of a Mg,Al hydrotalcite and segregation of Y-containing phases.
Abstract: FT-IR spectroscopy studies on the adsorption of pyridine on silica-supported heteropolyanions have shown the presence of surface acid sites, both Lewis and Bronsted ones, when starting from commercial phosphomolybdic acid, but only Lewis acid sites when starting from n-hexylammonium phosphododecamolybdate. A preferential formation of propene instead of acetone (i.e., a larger selectivity towards dehydration) has been observed during isopropanol decomposition, due to the strong surface acidity of the samples.
Abstract: Mixed Ni-Cr-V, Mg-Cr-V, and Ni-Al-V oxides have been prepared by thermal decomposition of layered precursors with the hydrotalcite structure (brucite-like layers containing divalent and trivalent cations, and interlayer anions balancing the extra positive charge of the layers) at 300 or 500 degrees C. Surface acidity (Bronsted and Lewis) has been assessed by FT-IR monitoring of pyridine adsorption. Ni-Al-V samples are more acidic (Bronsted and Lewis), while those with Cr contain only surface Lewis acid sites. Adsorption of 2-propanol takes place dissociatively, leading to alcoholate that is oxidized to acetone at 300 degrees C via a Mars-van Krevelen mechanism; such an oxidation is more extensive in the less acidic (Cr-containing) samples. Both the nature of the cations and the previous calcination temperature of the hydrotalcite control the oxidative properties of the mixed oxides.
Abstract: Zn,Al-hydrotalcites interlayered with hexacyanoferrate(II) and hexacyanoferrate(III) ions have been synthesized by different methods. These include rehydration of the product obtained by calcination of the carbonate form at 500 degrees C, anionic exchange of nitrate- and terephtalate-containing hydrotalcites, and direct synthesis. Both hexacyanoferrate(II) and (III) species are present in the interlayer space, but in different ratios, suggesting that a redox process takes place in this interlayer.
Abstract: Two different photo-catalysts consisting of tungsten oxide supported on zirconia and niobia have been studied. The photoactivity of the samples has been investigated in both the absence and presence of H2O2 for the photo-oxidation of 4-nitrophenol in aqueous suspensions and compared to that of WO3 and of the pure ZrO2 and Nb2O5 supports. Both catalysts were found to be photoactive, although no beneficial influence of the presence of tungsten oxide on the reaction rate was observed in the absence of H2O2. The presence of hydrogen peroxide was observed to be beneficial for all of the samples. Scanning electron microscopy, X-ray diffraction, diffuse reflectance and laser Raman spectroscopies, surface area and porosity determination, as well as Fourier transform infra-red spectroscopy monitoring of surface acidity were used to characterize the catalysts.
Abstract: New W-Mg-O systems have been obtained by impregnation of MgO with aqueous ammonium paratungstate solutions. The systems have been characterized by X-ray diffraction, Raman and W L-3 X-ray absorption (XAS) spectroscopies, FT-IR monitoring of pyridine adsorption for surface acidity, nitrogen adsorption at 77 K, and electron microscopy. Results obtained indicate that amorphous Mg tungstates are formed for low (723 K) calcination temperatures, but crystalline compounds are formed after calcination above 973 K. W6+ coordination in these solids depends on W content and calcination temperature. So, tetrahedral [WO4] species exist in amorphous species with low W content, and the crystalline ones obtained after calcination at 973 or 1073 K, while octahedral [WO6] species are present in amorphous solids with large (24 wt%) W content and in the crystalline phases formed after calcination at 1273 K, Formation of tungstates at rather low calcination temperature arises from the strong acid/base interaction between both reactants. Incorporation of tungsten leads to development of surface Lewis acid sites. The tungsten-containing species are less reducible in all cases than bulk WO3. (C) 1997 Academic Press.
Abstract: The reduction of nickel (Ni2+) and copper (Cu2+) to their metallic forms is reported for Ni-Al and Cu-Al layered double hydroxides (LDHs). Ethylene glycol is used as the reducing agent (the so-called polyol process). The degree of reduction of both cations was found to be dependent on the time and temperature of treatment and the nature of the anions located between the hydroxide layers. The process was found to be more efficient in reducing Cu2+ than Ni2+; substantial reduction of Cu2+ was obtained after 3 h at 200 degrees C, whereas the Ni2+ is not completely reduced after 192 h of treatment at this temperature.
Abstract: 'Casa Lis' is the most characteristic building in Salamanca, Spain, belonging to the modernist trend. It was built with Villamayor sandstone from nearby Salamanca, which has high porosity providing an easy medium for water absortion and capillarity. During a restoration process on the southern wall, near an underground water flow, two well defined, naturally developed layers were observed on the sandstone surface: an outer, hard crust with greyish shades and whitish salt patches resulting from rising damp, and an inner, green layer with organic material linking the sandstone to the inorganic crust. The microbiological study of this biofilm showed an ecologically obligate, stable mutualism between a dematiaceous mitosporic fungus (Septonema tormes sp. nov.), a coccoid cyanobacterium (Cyanothece-group) and a green alga (Gloeocystis rupestris), with the accumulation of different metabolites excreted by these microorganisms. The case reported here is one of the few studies where a microbial mat, in association with the external crust, avoids a further weathering of the stone because of an unforeseen biopreservation-effect due to the maintenance of humidity at constant levels under the crust avoids changes in clay swelling and subsequent surface arenization of the sandstone. The lichenized complex of a mitosporic mycobiont and two photobionts, in this case, has not been reported before as a stable association on this kind of substrate.
Abstract: Conductivity measurements have been carried out in order to follow the anion exchange process in layered double hydroxides. Measurements during chloride/decavanadate exchange have shown that it takes place through a first-order mechanism, without defoliation of the layered material. (C) 1997 Elsevier Science Ltd.
Abstract: Specimens of La2-xBa1+xCu2O6+delta have been synthesised with 0.25 greater than or equal to x greater than or equal to 0. The structure of La1.8Ba1.2Cu2O6.1 was determined by X-ray ponder diffractometry and high resolution electron microscopy to be isostructural to La2SrCu2O6, having an orthorhombic basic unit cell with a = 3.87, b = 3.90 and c = 20.81 Angstrom. The oxygen contents were determined by temperature programmed reduction. An 1 x 3 x 1 superstructure was observed It is believed that the non-superconducting properties of this material is resulted from the location of oxygen vacancies in the CuO2 planes.
Abstract: The structure of molybdenum oxocompounds formed in TiO2-supported MoO3 systems doped with alkaline metals have been studied by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) and X-ray absorption (XAS) spectroscopies. XRD, FT-IR, and XAS data give a detailed picture of the structure of molybdenum oxocompounds formed in these systems. Mo atoms are mostly octahedrally coordinated in an undoped TiO2 support, as a dispersed MoO3 phase. However, a high fraction of molybdenum cations are in tetrahedral coordination after the addition of Li, Na, or K as dopants. Thus, only MoO42- species exist in the 1 wt% Li-doped sample in the form of crystalline Li(2)MoO4, while tetrahedrally and octahedrally coordinated molybdenum form dispersed Mo2O72- chains in the 1 wt% Na- and K-doped systems. Octahedral polymolybdates, Rb2Mo3O10. H2O and Rb2Mo4O13, whose structures are based in [MoO6] units forming chains, are the species detected in the 1 wt% Rb-doped sample. Increasing the molybdenum content in K-doped samples favors formation of octahedral species, although in this case the structure of the supported phase is affected by the impregnation steps during the preparation of the sample. (C) 1996 Academic Press, Inc.
Abstract: Oxidative coupling of methane (OCM) has been studied on a LiCl-promoted Fe2O3 catalyst (atomic ratio Fe:Li = 4:1). LiFe5O8 exists in the fresh catalyst calcined at 1073 K. alpha-Fe2O3 is reduced to non-stoichiometric magnetite, and LiFe5O8 existing in the fresh catalyst calcined at 1073 K is partially reduced to Fe3O4 and converted to LiFeO2. Low selectivity in OCM to ethane and ethylene has been attributed to consumption of the LiCl promoter in a bulk interaction, resulting in the formation of new ferrite phases, which favour the destructive oxidation of methane.
Abstract: Titania- and alumina-supported molybdenum-containing systems have been prepared from mechanical mixtures of the support and molybdenum trioxide. The systems have been characterized by X-ray diffraction, temperature-programmed reduction and scanning electron microscopy, and Fourier transform-infrared monitoring of pyridine adsorption has been used to determine surface acidity. Dispersion of the supported phase, and its transformation to well-dispersed species has been attained by mechanical grinding and calcination in air, or by hydrothermal treatment of such mixtures and calcination. In the latter case, a higher dispersion is reached, despite the shorter time of calcination. Also hydrothermally treated samples show a larger concentration of surface Bronsted acid sites.
Abstract: Magnesia-supported tungsta catalysts have been prepared by impregnation, with different tungsta contents and calcined at different temperatures in air. Al samples are active for propan-2-ol dehydrogenation above 473 K, and selective towards acetone formation. At low W content, acetone is further oxidized. The behavior has been related to the W-containing phases existing in the samples.
Abstract: The interaction of methyl cinnamate/montmorillonite samples prepared by melting the former onto the second or by joint grinding, has been studied by x-ray diffraction, differential thermal analysis, thermogravimetric analysis and Fourier-transform infrared spectroscopy. Formation of an interlayer compound has been observed, leading to an increase of 4.15 or 3.42 Angstrom (samples obtained by melting or grinding, respectively) in the basal spacing of the clay. Formation of such a complex leads to a displacement of molecular water from the interlayer space, as concluded from the thermal studies. No chemical change is observed in the methyl cinnamate molecule, as confirmed by infrared spectroscopy. The systems prepared improve the shelter properties of the clay and the drug separately, mainly in the C zone of the ultraviolet spectrum (290-190 nm).
Abstract: Two nickel-manganese hydroxycarbonates with Ni/Mn ratios close to 2/1 (Ni2Mn) and 3/1 (Ni3Mn) have been synthesized by coprecipitation under an air atmosphere from aqueous solutions of Ni(II) and Mn(II). Characterization by powder Xray diffraction, transmission electron microscopy, and differential thermal and thermogravimetric analyses confirm a hydrotalcite-like structure for these brown colored compounds, indicating that Mn(II) is at least partially oxidized to Mn(III) during the synthesis process. Thermal decomposition in air at 450 degrees C leads to formation of NiO as the major product in both samples, At 700 degrees C, the Ni/Mn ratio determines the nature of the nickel-manganese mixed oxides present, in addition to NiO. Temperature-programmed reduction of calcined samples indicate that the major mixed oxide in sample Ni2Mn calcined at 700 degrees C is ilmenite, NiMnO3, while in Ni3Mn it is the spinel NiMn2O4. At 1000 degrees C, formation of NiO and NiMn2O4 spinel occurs whichever the Ni/Mn ratio in the starting material. (C) 1996 Academic Press, Inc.
Abstract: Synergetic effects in the selective oxidation of isobutene to methacrolein have been studied over catalysts containing several molybdate phases. Two types of catalysts have been analysed: (1) pure bismuth molybdates with Bi/Mo ratios of 2/2 and 2/3 [BiMo(2/2) and BiMo(2/3)] and cobalt molybdate [CoMo(1/1)]; and (2) multiphasic molybdates consisting of three or four molybdates prepared by: (a) coprecipitation of cobalt and bismuth salts and calcination at high temperatures, (b) mechanically mixing in dry conditions the separate molybdates and calcining at high temperatures and (c) dispersing the separate molybdates in n-pentane and calcining at high temperatures. Synergetic effects are observed in multiphasic BiMo(2/2)CoMo and BiMo(2/3)CoMo catalysts. For BiMo(2/2)CoMo catalysts the highest synergetic effects in the conversion, yield and selectivity are observed. For the catalysts before test no new phase is formed after calcination while after test X-ray measurements indicate a practically complete transformation of alpha-Bi2Mo3O12 into gamma-Bi2MoO6 and MoO2 and traces of MoO3 and Bi2xMo1-xO3. Neither contamination between phases is observed. The synergetic effects are explained by the cooperation between separate gamma-Bi2MoO6, MoO3 and (alpha+beta)-CoMoO4 phases in contact. The cooperation between phases is more effective when MoO3 is present in a large proportion.
Abstract: The synthesis and full characterization of a new hydrotalcite-like compound with the formula [Mg-0.71 V-0.29(OH)(2)](CO3)(0.145). 0.72H(2)O and with V3+ in the layers are described. The influence of hydrothermal treatment and drying rate on the crystallinity of the materials obtained is discussed. The evolution to mixed oxides upon calcination at different temperatures (448, 548, 773, 1023, and 1273 K) under different atmosphere environments (air or nitrogen) for 2 h has been studied. Characterization of the original layered materials and of the calcination products has been carried out by X-ray diffraction, thermal analysis, Fourier-transform infrared spectroscopy, BET specific surface area determination, temperature-programmed reduction, and transmission electron microscopy. X-ray absorption spectroscopies (XANES and EXAFS) have also been used to assess the local geometry of vanadium ions in the different compounds prepared. All experimental data agree with a well-crystallized hydrotalcite-like compound after thermal treatment, and also a minor effect of the drying rate on the crystallinity has been found. Thermal decomposition yields poorly crystalline layered compound at 448 K that undergoes transformation to mostly amorphous materials when calcined at 548--773 K, finally leading to a mixture of MgO and Mg3V2O8, which has increasing crystallinity as the calcination temperature increases. XAS results indicate the presence of V3+ ions in an octahedral coordination in the parent sample calcined at 448 K and tetrahedrally coordinated V5+ species for samples calcined at higher temperatures, calcination giving rise to a better ordering of the second coordination sphere. Similar results were found when calcination was performed in nitrogen, although higher temperatures were needed to achieve the same result.
Abstract: A set of tungsten oxide/TiO2 polycrystalline samples (W/Ti) prepared by the sol-gel method has been characterised using several techniques, namely thermogravimetric analysis, gas-chromatography, mass spectrometry, thermogravimetric analysis combined with mass spectrometry or with gas chromatography/mass spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy, determination of specific surface area, scanning electron microscopy, Fourier-transform infrared spectroscopy and monitorisation of pyridine adsorption for surface acidity. Moreover the samples have been employed as catalysts for 4-nitrophenol photodegradation in aqueous suspensions used as a 'probe' reaction. Characterisation results indicate that the surface of the W/Ti particles is enriched with homogeneously dispersed tungsten as well as microcrystalline or amorphous species. Maximum photoactivity for 4-nitrophenol photodegradation was achieved for a sample containing 1.7 mol W per 100 mol Ti.
Abstract: Hydrotalcite-like compounds containing (a) Mg-II; Al-III and V-III in the layers and carbonate in the interlayers, or (b) Mg-II and Al-III in the layers and decavanadate in the interlayers, have been synthesized and characterized using powder X-ray diffraction, thermal analysis, FTIR spectroscopy, temperature-programmed reduction, and specific surface-area determination. The crystalline phases formed after calcination in air at 700 degrees C have been identified using these same experimental techniques. It has been found that the nature of the precursor determines the nature of the Mg-V-O phases formed; thus, when starting from (a), Mg-3(VO4)(2) containing isolated [VO4] units is formed, while when starting from (b), MgV2O6, containing pairs of edge-sharing [VO6] octahedra, is formed preferentially. Such a reactivity appears to depend on the nature of the support, and not on the stoichiometry (i.e., Mg/V atomic ratio) of the starting materials.
Abstract: Pharmaceutical-clay systems, used as solar radiation shields, obtained by interaction of Na montmorillonite and several pharmaceuticals using two methods of preparation, have been studied. Samples have been prepared by melting the drug onto the clay or by intimate mixing and grinding of both. The shielding ability against solar radiation is improved by use of these preparation methods. Differential thermal analysis and thermogravimetric analysis confirm the steady substitution of water molecules from the interlayer space of the clay by drug molecules, as the amount of the latter is increased. The melting method seems to facilitate access of the drug into the interlayer space of the clay.
Abstract: Hexacyanoferrate(II) and hexacyanoferrate(III) hydrotalcites have been synthesized by exchange of originally existing carbonate ions at pH = 4.5. The samples have been characterized by elemental chemical analysis, powder X-ray diffraction, FT-IR and X-ray absorption spectroscopies, nitrogen adsorption for specific surface area and porosity assessment, and their thermal stability has been characterized by Differential Thermal Analysis and Thermogravimetric Analysis. it has been found that during exchange hexacyanoferrate(II) is partially oxidized to hexacyanoferrate(III), a constant oxidation percentage being already attained after 3 h stirring during synthesis. Hexacyanoferrate(III) ions, however, are partially reduced during the same preparation process, but the reduction degree increases with the stirring time. Decomposition takes place in three consecutive partially overlapped steps: dehydration, decomposition of the interlayer cyanide ligands and dehydroxylation of the brucite-like layers. Upon calcination at 600 degrees C, a mixture of an Al-MgO solid solution is formed, together with MgFe2O4 spinel.
Abstract: Isopropanol on silica-supported heteropolyanions with the Keggin structure reacts mainly through dehydration, due to the presence of strongly acidic surface sites. The effect of preparation conditions of the catalysts on the reaction has been studied.
Abstract: Pharmaceutical-clay systems, used as solar radiation shields, obtained by interaction of natural sepiolite and several pharmaceuticals using two methods of preparation (melting the drug onto the clay or by intimate mixing and grinding of both), have been studied. The shielding ability against solar radiation is improved by using these preparation methods. Differential thermal analysis and thermogravimetric analysis confirm displacement of water molecules adsorbed onto the clay surface by the pharmaceutical molecules. The grinding method seems to facilitate adsorption and dispersion of the drug onto the clay surface.
Abstract: Hydrotalcite-like compounds containing Co(II) and Cr(III) in the brucite-like layers have been prepared. The interlayer anion was carbonate or decavanadate. The chemical formulas of the samples are [Co0.65Cr0.35(OH)(2)] (CO3)(0.175). 1.22H(2)O (sample CoCrC) and [Co0.61Cr0.39(OH)(2)](V10O28)(0.065). 1.35H(2)O (sample CoCrV). The compounds have been characterized by X-ray diffraction, XAS, vis-UV, FT-IR, and Raman spectroscopies, while the surface textures were assessed by nitrogen adsorption. Reducibility has been studied by temperature-programmed reduction. A similar characterization study has been carried out on samples obtained after calculation of the parent samples in air at increasing temperatures. Results indicate an ordered structure, with Co(II) and Cr(III) ions in octahedral holes of the brucite-like layers and, in CoCrV, decavanadate species with its main C-2 axis parallel to the layers. Thermal decomposition at increasing temperatures takes place, for CoCrC, through intermediate formation of Cr(VI) species, which are again reduced to Cr(III) at higher temperatures; simultaneously, Co(II) is oxidized to Co(III) (even at 673 K), thus leading to formation of (CoCoCrO4)-Co-II-Cr-III-O-III. However, decomposition of CoCrV takes place through partial depolymerization of decavanadate species and formation of (CoCr2O4)-Cr-II and (Co2V2O7)-V-II, Without intermediate formation of Cr(VI) species.
Abstract: Hydrotalcite-like compounds containing Mg(II) and Cr(III) or Ni(II) and Cr(III) cations in the brucite-like layers have been synthesized by the coprecipitation method. The layered structure has been confirmed by Powder X-ray diffraction, and characterization has been completed by elemental chemical analysis, FT-IR and vis-UV/diffuse reflectance spectroscopies, and thermal analysis (differential thermal analysis and thermogravimetric analysis). The behavior shown during the thermal studies depends on the atmosphere used to carry out the study; a weak endothermic effect, at temperatures coinciding with a small weight loss, is recorded when the analysis is performed in air but not when performed in nitrogen, and so it is ascribed to oxidation processes. Formation of chromate-like species in the samples calcined at intermediate temperatures has been confirmed by powder X-ray diffraction for sample MgCr-HT and is suggested by FT-IR spectroscopy for sample NiCr-HT; the lack of success in identifying chromate-like species for sample NiCr-HT calcined at different temperatures by X-ray diffraction suggests that these species an very well dispersed. Their presence has been undoubtedly established by temperature-programmed reduction analysis of the samples, showing a maximum oxidation state of Cr ions (+6) when the sample is calcined at 623 K. Calcination at high temperatures leads to formation of MCr(2)O(4) (M = Mg, Cr) spinel in addition to rock salt oxide of the divalent cation. Formation of amorphous materials at intermediate calcination temperature accounts for specific surface area development for these samples.
Abstract: Multiphasic Bi-Co-Mo oxide catalysts have been prepared by co-precipitation or mechanically mixing the corresponding molybdates. Characterization using X-ray diffraction and FT-IR and V-UV/diffuse reflectance spectroscopies has shown that no new phase is formed after mixing the parent molybdates, while a rather low specific surface area indicates a strong interaction between the phases. FT-IR monitoring of propene adsorption indicates a large selectivity towards acrolein formation.
Abstract: The transformation of isopropanol on MoO3/TiO2 catalysts doped with alkali-metal cations has been studied by the pulse technique. The FTIR studies have provided evidence of the dissociative adsorption of isopropanol and the formation of isopropoxide species which decompose at higher temperatures to acetone. Catalytic measurements have shown that the addition of alkali-metal cations leads to a drastic decrease in the yield of propene, owing to the elimination of Bronsted centres originally existing on the catalyst surface, whereas the rate of dehydrogenation to acetone is affected only slightly. The extent of the changes observed depends on the nature of the doping alkali-metal cation.
Abstract: Hydrotalcite-like compounds containing Zn-II and Cr-III in the brucite-like layers have been prepared, containing carbonate or decavanadate in the interlayer, with the formulae [Zn0.67Cr0.33(OH)(2)](CO3)(0.165). 1.34H(2)O and [Zn0.64Cr0.36(OH)(2)](V10O28)(0.06). 1.26H(2)O, respectively. The samples have been characterized by XRD, XAS, UV-VIS, FTIR and Raman spectrocopies, while the specific textures of the samples were assessed by nitrogen adsorption, and reducibilities were studied by temperature-programmed reduction. A similar characterization study has been carried out on samples obtained after calcination of the parent samples in air at increasing temperatures. Thermal decomposition of the ZnCr hydrotalcite containing carbonate leads to oxidation of Cr-III to CrO42- species at intermediate calcination temperatures, Zn-II cations then retaining their original octahedral coordination; at higher calcination temperatures ZnO and ZnCr2O4 are formed. However, the presence of interlayer decavanadate species inhibits the Cr-III-->CrO42- oxidation during thermal decomposition of the vanadate-containing hydrotalcite, owing to preferential formation of vanadates. In this case the final product is a mixture of Zn pyrovanadate, Zn2V2O7, and spinel ZnCr2O4, together with a phase consisting of a mixture of partially depolymerized decavanadate to beta-VO3- chains, together with monomeric VO43- species.
Abstract: A correlation exists between the number of surface acid sites and the catalytic activity of NiMo/TiO2-Al2O3 systems in thiophene hydrodesulfurization. The catalytic activity for thiophene hydrodesulfurization and surface concentration of Bronsted acid sites increase with the titania content in the TiO2-Al2O3 support.
Abstract: Grinding or melting of phenyl salycilate on montmorillonite and sepiolite leads to adsorption complexes suitable to prevent sun-burning by UV radiation. The protecting ability of the pure drug or of the clays are improved upon dispersion of the former onto the surface of the clays, specially in the 290-190 nm range.
Abstract: EXAFS and XANES studies have been performed on hydrotalcite-like materials containing vanadium species in the layers, as V3+ cations, or in the interlayer space as vanadate anions. It has been found that in the former sample V3+ isomorphically substitutes Mg2+ in the octahedral sites, while in the second case anions in the interlayer are as decavanadate clusters. Amorphous decomposition products obtained after calcination of the V3+ hydrotalcite contain V5+ cations as isolated VOa3- units.
Abstract: A study has been carried out on the selectivity of titania-supported rhodium catalysts in n-butane hydrogenolysis. It is concluded that incorporation of the rhodium precursor onto the titania surface via ion exchange leads to a less pronounced development of Strong Metal-Support Interactions and thus to an homogeneous distribution of active sites on the catalyst surface.
Abstract: Mg-Cr and Ni-Cr layered double hydroxides (LDHs) with carbonate anions in the interlayer were used as precursors to synthetize pillared derivatives with decavanadate (V10O28)(6-) anions without the use of any preswelling agent. Characterization was carried out by powder X-ray diffraction, chemical analysis, FT-infrared spectroscopy, thermal analysis, and transmission electron microscopy. The influence of the wetness degree of the starting Mg-Cr and Ni-Cr LDHs and of the pH conditions on the anionic-exchange rate of carbonate was studied. The pillared materials were constituted by a well crystalline phase, together with a coproduct with a lower degree of crystallinity, characterized by a broad X-ray diffraction peak close to 10 Angstrom. Anion-exchange was neither achieved without prewetting of the carbonate-containing LDH nor at pH values larger than 6.5. The thermal stability of the pillared materials and their surface properties are also presented.
Abstract: A study has been carried out on the extraction of salts from different types of granite using patent sepiolite formulation. The ability to remove salts greatly depends on the mineralogical compositions of the granites. Two types of equilibria are important: the absorption and/or adsorption of salts on the stone, and the partition of ions constituting the salt in the adsorbed state between the clay minerals in the rocks and that used as an extracting agent.
Abstract: Temperature-programmed reduction (TPR) has been used for characterization of anionic clays containing reducible cations. Results are reported on the reducibility properties of several anionic clays with the hydrotalcite or takovite structure containing transition metal cations, and of the products (usually a mixture of rock-salt-type oxides and spinels) thereof obtained by calcination. The systems studied include those possessing two reducible cations (e.g., Co2+ and Fe3+) and also others with only a reducible species (e.g., Ni2+ and Al3+). Carbonate, the most common interlayer anion in these compounds, is removed by decomposition before being reduced to C or CHx, as concluded from X-ray diffraction analysis of the residue after reduction. Reduction of the isolated cations in the brucite-like structure of the anionic clay (or in the decomposition products) proceeds at appreciable lower temperatures than that required for reduction of bulk spinels and other oxides formed upon calcination at high temperatures.
Abstract: Catalysts consisting of Mo and Ni oxides supported on Al2O3 and TiO2 and on mixtures of both have been prepared by co-impregnation of the supports with aqueous solutions of Mo(VI) and Ni(II) salts, and have been characterized by X-ray diffraction, Fourier transform-infrared (FTIR) and vis-UV/diffuse reflectance spectroscopies and surface texture by nitrogen adsorption; surface acidity has been measured by FT-IR monitoring of pyridine adsorption. Incorporation of TiO2 to the support Al2O3 leads to a larger dispersion of molybdena on the catalyst support. The solids (specially those with low TiO2 content) are not microporous, but have pores open at both ends. The increase in the TiO2 content leads to a larger concentration of surface Bronsted (Mo-OH) acid sites, associated with an increase in the number of dispersed molybdate species.
Abstract: Commercial anatase containing an appreciable amount of sulphur has been treated for sulphur removal by calcination at increasing temperatures or soaking with aqueous NaOH solutions. It has been found that both methods are active for sulphur removal, but the first one also leads to a sharp sintering of the titania particles. On the other hand, treatment with NaOHaq leads to incorporation of sodium cations. Both treatments lead also to changes in surface acidity, as measured by Fourier transform infrared (FT-IR) monitoring of pyridine adsorption.
Abstract: For the first time, the nickel-aluminum layered double hydroxide (LDH) [Ni1-xAlx(OH2] (CO3)(x/2). mH(2)O containing carbonate anions in the interlayer, has been pillared with polyvanadate in aqueous solution at pH = 4.5 by anionic exchange without the assistance of any swelling agent. Making use of the so-called memory effect of these materials, synthesis of similarly pillared compounds through recovery of the structure from calcined LDH in contact with an aqueous solution of sodium metavanadate was also achieved. Characterization of the materials obtained so far has been carried out by chemical analysis, X-ray diffraction, Fourier-transform infrared spectroscopy, and thermal analysis. The influence of experimental conditions during the calcination-reconstruction process of the pillared-layered structure, such as calcination temperature and pH during the reaction, as well as the temperature during hydrothermal treatment (to improve crystallinity), is reported; purity of the product depends on the calcination temperature of the starting precursor, and at pH values higher than 4.5, a (VO3)(n) polyvanadate chainlike pillared material is obtained. Exchange of carbonate anions depends on the pH of the solution, total exchange being only attained at pH = 4.5-5.5. Thermal stability of the pillared materials and the microstructure properties are also reported.
Abstract: The paper summarizes experimental results on the activity and selectivity of high-temperature (T-c) superconductors and some related cuprates in different heterogeneous catalytic reactions (ortho-para hydrogen conversion, iso-propanol and methanol dehydrogenation, nitric oxide decomposition and reduction with carbon monoxide, ammonia oxidation, selective oxidation of methanol and ethanol, partial oxidation of methane and of toluene, ammoxidation of toluene, oxidation of carbon monoxide). It contains also information for the structural peculiarities of YBa2Cu3O7-x superconductors and data for oxygen reactivity from temperature-programmed reduction, oxygen isotopic exchange and oxygen desorption measurements. A distinct correlation between the superconductivity of YBa2Cu3O7-x and its catalytic activity in ortho-para hydrogen conversion is observed. Data in the literature show an influence of the oxygen vacancies concentration in these solids and their activity and selectivity. However, in many cases no relationship seems to exist between the superconducting properties and the catalytic activity and selectivity. This is explained by: (i) the experiments were performed at temperatures much higher than T-c, and (ii) significant surface and bulk alterations in high-T-c superconductors during the catalytic reactions, as concluded from X-ray photoelectron spectroscopy and X-ray diffraction data (appearance of Cu+ and Cu-0 states in the surface layers, orthorhombic/ tetragonal phase transition, and, in some cases, decomposition and phase segregation). The outstanding importance of oxygen binding energy for complete oxidation reactions is already shown. From the practical point of view, the high activities and selectivities should be noted of high-T-c superconductors in selective oxidation of aliphatic alcohols to the corresponding aldehydes, in nitric oxide reduction by carbon monoxide and in ammoxidation of toluene to benzonitrile. Special attention should be paid to some related non-superconducting cuprates, which show high performances for complete oxidation reactions, with specific catalytic activities commensurable to those of the best catalysts known until now (e.g., Pt, Co3O4). Some possible future studies on the catalytic properties of high-T-c superconductors and related cuprates are outlined.
Abstract: Surface species formed on MoO3/SiO2 catalysts prepared by manually grinding mixtures of both oxides in the presence of water vapour and with molybdenum loadings between 2% and 12% were studied by X-ray diffraction, temperature-programmed reduction and scanning electron microscopy. Polymolybdate species have been identified for molybdenum loadings below 4%, these species condensing to form MoO3-like phases and orthorhombic MoO3, as the weight loading increases.
Abstract: Iron(III) doped specimens (0.01-5% atomic Fe :Ti) have been prepared by impregnating polycrystalline ''home prepared'' TiO2 (mainly anatase) and have been studied for photocatalytic degradation of 4-nitrophenol in aqueous dispersions. Some structural and surface properties have been studied by X-ray diffraction, specific surface area and porosity measurements and FTIR monitoring of pyridine, ammonia and methanoic acid adsorption for surface acidity and basicity. Their surface properties were compared with a corresponding series of photocatalysts prepared by a coprecipitation method. Adsorption of pyridine and ammonia indicates the presence of surface acid Lewis sites, a low concentration of surface Bronsted sites being present only in the samples prepared by impregnation. The photocatalytic experiments indicated that the presence of iron ions does not influence or is detrimental for the occurrence of the 4-nitrophenol photodegradation.
Abstract: Adsorption of pyridine on MoO3/TiO2 doped with alkali metal cations (Li, Na, K, and Rb) has been studied by FT-IR spectroscopy in order to assess the effect of the nature and concentration of alkali cations on the surface acidity. It has been found that incorporation of the alkali cations leads to a decrease in the surface acidity and of the ratio between the concentration of Bronsted and Lewis surface acid sites. These decreases are due to (i) formation of weak Lewis acid sites (alkali cations) with lower electron affinity than Mo6+ and Ti4+ and (ii) cancellation of Bronsted sites (Mo-OH) because of formation of alkali molybdates. These effects are more pronounced as the alkali content increases and/or the polarizing character of the alkali cation is increased. (C) 1994 Academic Press, Inc.
Abstract: Parallel studies of FeMoO and FeCrMoO catalysts for methanol oxidation to formaldehyde have shown that they possess commensurable activities and selectivities, the FeCrMoO catalyst being more stable. Mossbauer, ESR and XRD studies revealed that upon reduction with methanol, both catalysts have different behaviour. The presence of Cr-2(MoO4)(3) in the FeCrMoO catalyst retards its reduction. Under the experimental conditions used, the main intermediate reduction product of the FeCrMoO catalyst is alpha-FeMoO4, while under identical conditions reduction of FeMoO leads to alpha-FeMoO4 and beta-FeMoO4 phases.
Abstract: Layered double hydroxides with the hydrotalcite (Mg-(1-x)Al-x(OH)(2)(CO3)x/2.nH(2)O) structure intercalated with polyvanadate species have been prepared following different routes. X-ray diffraction, thermal analysis, transmission electron microscopy and infrared spectroscopy results indicate the layered structure of the material. A narrow mesopore size distribution was obtained when using terephtalate as swelling agent.
Abstract: The dispersion and nature of surface species formed upon calcination of MoO3-gamma-Al2O3 mixtures at 770 K for 10 h and with different molybdena loadings under different water vapour pressures [P(H2O)] have been studied by X-ray diffraction (XRD), zero-point charge (ZPC) measurements and temperature-programmed reduction (TPR); P(H2O) was in the range 25-45 Torr with MoO3 loadings up to 2 monolayers (1 monolayer=0.1681 g MoO3/g Al2O3). The intensity of the most intense XRD peak of MoO3 at 326 pm decreased as P(H2O) increased, indicating an increase in the concentration of surface species different from bulk MoO3. Up to 0.7 monolayer MoO3 loading, covering of the Al2O3 surface with Mo-containing species increases with increasing P(H2O); ZPC at P(H2O)=45 Torr was coincident with that of MoO3. P(H2O) has no effect on the dispersion of high loaded samples (above 1 monolayer), where all the support surface is covered even for P(H2O)=25 Torr. TPR results indicate the presence of species with different reducibilities, depending on P(H2O) and MoO3 loading; their nature has been assessed by comparison with previous results for samples obtained by impregnation; bulk MoO3 (responsible for high-temperature reduction peaks) formation has been observed at high MoO3 loadings.
Abstract: Layered double hydroxides (LDHs) with the hydrotalcite-like (HT) structure, and the formula [Mg1-xAlx(OH)(2)](CO3)(x/2).nH(2)O, have been synthetised following the constant pH method. Samples have been submitted to hydrothermal treatment (samples I) at 400 K or left under room conditions (samples 0) for 7 days. X-ray diffraction, thermal analysis, FT-IR spectroscopy and transmission electron microscopy results confirm the HT-like structure. Surface texture assessment by nitrogen adsorption at 77 K indicates type II nitrogen-adsorption isotherms with a narrow hystheresis cycle, due to pores open on both ends, but is not recorded in the isotherms of the solids obtained upon calcination of the samples at increasing temperatures. This treatment increases the specific surface area up to cn. 85 m(2)/g when the samples are calcined at 1100 K, where X-ray diffraction data indicate formation of MgAl2O4 and MgO. Such a change in the specific surface area (specially upon calcining at moderate temperatures) is even marked in the hydrothermally-treated sample.
Abstract: Chromium ion-doped polycrystalline titania catalysts, mainly used in photoreactions, were studied by Fourier transform infrared (FTIR) spectroscopy and by X-ray photoelectron spectroscopy (XPS). Two series of catalysts prepared by two different methods, i.e., coprecipitation and impregnation, were analysed. The FTIR spectra recorded upon adsorption of ammonia and after outgassing at increasing temperatures indicated that Cr(x)O(y)/TiO2 samples, whichever preparation method was used, have two types of surface acid sites, Lewis and Bronsted sites. The Bronsted sites are associated with the presence of chromium, since they were not detected in pure titania. According to the X-ray photoelectron study, Cr(III) and Cr(VI) species are present in both series of catalysts, with the higher oxidation state being quite unstable under X-rays. As shown by quantitative XPS analysis, only the catalysts prepared by coprecipiatation and containing up to 2% Cr can be described by the Kerkhof-Moulijn monolayer model. (C) 1994 Academic Press, Inc.
Abstract: Molybdenum oxide supported on alumina and titania prepared by three different methods (impregnation, calcination and hydrothermal treatment of mechanical mixtures), have been studied by laser Raman spectroscopy. Different oxomolybdenum species have been detected depending on the method of preparation and on the calcination conditions. Heptamolybdate and polymolybdate species were found in the impregnated and in the uncalcined, hydrothermally treated samples supported on alumina and titania, respectively, whereas polymolybdate species were formed on the surface of alumina when the mechanical mixtures and the hydrothermally treated samples were calcined in the presence of water vapour. However, two kinds of species, polymolybdate and mono-oxomolybdenum, were found in the case of the mechanical mixtures and the hydrothermally treated samples calcined in the presence of water vapour when supported on titania.
Abstract: An X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD) and reduction (TPR) study of LaNiO3 and La2NiO4+delta perovskites has been carried out. The existence of at least two different forms of oxygen in these compounds is shown by both oxygen reactivity (TPD and TPR) and XPS characterisation. XP spectra have also revealed a surface enrichment in lanthanum and oxygen. TPR profiles have shown a reduction of LaNiO3 through the formation of La2Ni2O5 as an intermediate compound. Above 450-degrees-C, LaNiO3 and intergrowth nickelates (La2NiO4 and La2NiO4+delta) undergo a final reduction to metallic Ni and La2O3. Between 200 and 400-degrees-C all three compounds exhibit a high catalytic activity in the total catalytic oxidation of methanol.
Abstract: IR and Raman spectroscopic studies show that the catalyst EL10V1 consists of a 'monolayer' of surface vanadyl complexes spread on the TiO2 support surface. Such a 'monolayer' contains both Lewis and Bronsted acidic sites, while the nucleophilic sites of the support surface are suppressed. On the catalyst EL10V8, V2O5 particles are also present. UV-Vis spectra show that V5+ is predominant on the calcined catalysts; part of it can be converted easily into V4+. Evidence is also found for electronic interaction between the support and the surface vanadium oxide species.
Abstract: Methanol oxidation on V2O5 and V2O5-MoO3 catalysts supported on montmorillonite has been studied in the temperature range of 250-500 degrees C. The V2O5-MoO containing sample shows higher selectivity towards formaldehyde formation than the V2O5 catalyst.
Abstract: A magnesium-manganese hydroxycarbonate with the hydrotalcite structure has been synthesized by coprecipitation from aqueous solutions of Mg2+ and Mn2+. According to temperature-programmed reduction data, 84% of the manganese is oxidized to Mn3+ during synthesis. Fourier-transform infrared spectroscopy, powder X-ray diffraction and transmission electron microscopy data support the hydrotalcite structure. Thermal decomposition in air at 390-degrees-C leads to an amorphous material with a simultaneous increase of the specific surface area. Further calcination at higher temperatures (670 and 1000-degrees-C) leads to crystallization of MgO and Mg2MnO4 spinel and to sintering. Reduction of the spinel takes place in two separate steps, probably through formation of intermediate Mn3O4 and finally MnO.
Abstract: A bismuth (III) phosphate catalyst has been prepared by precipitation and characterized by X-ray diffraction, thermogravimetric analysis, and X-ray photoelectron and infrared spectroscopy. Its activity and selectivity toward selective methanol oxidation to formaldehyde has been studied by a flow method. A high selectivity has been found, similar to that reported for typical catalysts used for this reaction. The possible importance of the strength and covalency of the P-O bonds is discussed.
Abstract: The synthesis and characterization of polyoxovanadate-intercalated MgAl hydrotalcite-like layered double hydroxides (LDH) is described. Seven different methods have been used to intercalate the vanadate anion. These include exchange of the initial carbonate or terephthalate anions, as well as reconstruction of the layered structure from the carbonate form previously calcined at 550-degrees-C-the reconstruction being either directly to the vanadate or indirectly via a terephthalate intermediate. Preswelling with glycerol was also used in some cases. Characterization has been carried out by elemental chemical analysis, powder X-ray diffraction, differential thermal and thermogravimetric analyses, Fourier-transform infrared spectroscopy, transmission electron microscopy, and specific surface area and porosity assessment by nitrogen adsorption at -196-degrees-C. In all cases a layered material containing the [V10O28]6- anion in the interlayer is formed. Direct reconstruction from calcined MgAl LDH with vanadate at pH = 4.5 leads, in addition, to a fibrous material and partial dissolution of magnesium. Experimental results indicate total CO32-/C6H4(COO)22- exchange as well as the absence of glycerol in those cases where it had been used as a preswelling agent. The principal difference between the samples is found within the pore size distribution-a narrow distribution is present in samples prepared via a terephthalate intermediate, while for others the pore size distribution is broader.
Abstract: X-ray diffraction analyses were carried out by nine laboratories. The equipment and experimental conditions of analysis were generally different. However, the results obtained were very similar and coherent. Samples with a high vanadium content showed exclusively the presence of TiO2 (anatase) and crystallites of V2O5 as long as not pretreated. No TiO2 (rutile) or other vanadium oxide was observed. V2O5 was also observed in low vanadium content samples in two analyses. No other crystalline phases could be observed, although the presence of very weak lines were detected. These lines were not identified as vanadium oxide, titanium oxide or vanadium-titanium oxide. In the pretreated samples containing vanadium, the presence of V2O5 lowered the temperature of the phase transition of anatase to rutile.
Abstract: Eurocat V2O5-TiO2 catalysts containing 1% and 8% vanadia by weight, and their precursors, have been studied by thermogravimetry, differential thermogravimetry and differential thermal analysis. Decomposition of the precursors in air took place in two well-defined steps: the first occurred up to ca. 500 K, and corresponds to removal of water from the solid surface; and CO2 evolution was observed above this temperature, accompanied by a strong exothermic effect. CO2 was also detected in a non-oxidising atmosphere. Melting of vanadia was observed only in the V-rich samples, confirming the presence of crystalline V2O5; with the 1% sample diffusion of vanadium species probably hindered formation of large crystallites.
Abstract: The structure of well-dispersed molybdena moieties on the surface of a MoO3-TiO2 sample has been determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Octahedral coordination of molybdenum atoms is favored over the TiO2 support, and [MoO6] units join edges to form molybdenum rows that are similar to those found along the c-axis direction in bulk MoO3. The changes induced in this structure when the support is doped with different amounts (1, 3% w/w) of sodium have been studied by X-ray diffraction, temperature programmed reduction, and infrared and EXAFS spectroscopies. The addition of Na changes the coordination of Mo6+ ions from octahedral to tetrahedral, and simultaneously inhibits their reducibility. Mo-Mo bonds found in the Na-free sample are broken, and at low Na content isolated MoO42- units and ill-crystallized Mo2O72- chains are formed. Increasing the amount of the Na dopant increases the fraction of Mo atoms in tetrahedral coordination and crystalline Na2Mo2O7 coexists with dispersed phases in the 3% Na w/w sample. Also, an increase in the calcination temperature of the samples leads to the formation of crystalline Na-Mo-O compounds over the TiO2 surface. (C) 1994 Academic Press, Inc.
Abstract: The BET surface area was found to be 10.5 (+/-0.5), 10.1 (+/-0.5) and 11.0 (+/-0.5) m2 g-1 for sample EL10, EL10V1 and EL10V8, respectively. No microporosity was observed for any of the samples. In all cases, a bimodal distribution of porosity was observed, indicating intraparticle macroporosity with radii about 0.1-0.2 and interparticle porosity radii around 10 mum. The TiO2 support is composed of particles with radii of about 1 mum, these forming aggregates of loosely coherent particles (radii congruent-to 100 mum). Impregnation by the V2O5 compound does not modify appreciably either the surface area or the degree of aggregation.
Abstract: The activity and selectivity of a precipitated iron-chromium-molybdenum oxide catalyst (Mo/(Fe + Cr) = 2.5/(0.5 + 0.5)) towards methanol mild oxidation have been studied by a flow-circulation method. Commensurable activity and selectivity with those of the industrial Fe2 (MoO4)3-MoO3 catalysts as well as an enhanced stability have been found. The Mossbauer spectra of fresh and tested catalysts show that during the catalytic reaction a partial reduction occurs and a steady state composition differing from the initial one is formed.
Abstract: Adsorption of formic acid on Ca(OH)2 calcined at increasing temperatures has been monitored by FT-IR spectroscopy. Adsorption takes places dissociatively, forming bidentate formate species, coordinated to surface Ca2+ ions. The basicity decreases as the out-gassing temperature increases, due to removal of surface hydroxy groups.
Abstract: MoO2-TiO2 samples have been prepared from aqueous solutions of ammonium heptamolybdate and titania (by conventional impregnation and equilibrium adsorption) and by mechanically mixing both oxides (with and without further hydrothermal treatment). The samples have been characterized by X-ray diffraction, specific surface area assessment, Fourier-transform infrared (FTIR) and ultraviolet-visible diffuse refectance (UV-VIS DR) spectroscopies and temperature-programmed reduction (TPR). Surface properties have been studied following the adsorption of probe molecules by FTIR spectroscopy. Dispersed phases are formed preferentially in samples prepared by equilibrium adsorption and in physical mixtures submitted to hydrothermal treatment; in all other cases crystalline MoO3 is mainly formed. Dispersed phases are more easily reduced. They have been identified as surface molybdenyl species bonded to Ti cations through bridging oxygens, and their overall coordination depends on the degree of surface hydration. These species predominate quantitatively on the catalyst surfaces and have been found to be strong Lewis and Bronsted acids. They are also more easily reduced than bulk MoO3 under TRP conditions, and very active as oxidizing agents for methanol.
Abstract: Low-loaded vanadia-titania catalysts have been prepared by impregnation of titania P-25 (Degussa). The catalysts were characterized by X-ray diffraction, specific surface area and porosity assessment by nitrogen adsorption at 77 K. For very low vanadia contents the absence of surface acid Bronsted sites leads to dimerization on surface acid Lewis sites, whereas the presence of surface acid Bronsted sites when the vanadia content is increased leads to oxidation to carbonyl and carboxylate species.
Abstract: The reactivity of MoO3/TiO2 systems (pure or Na- or K-doped) with propene has been studied by FT-IR spectroscopy. Reactivity decreases in the presence of the alkaline cations, and thus reactive adsorption (leading to acrolein formation) is only observed in the doped catalysts above room temperature. This behaviour has been related to the decrease in surface acidity.
Abstract: An FTIR spectroscopy study of the adsorption and oxidation of ethene on MoO3/TiO2 catalysts (undoped or doped with alkali-metal cations, Li, Na, K and Rb) has been carried out. The undoped catalyst is less active, although it shows a larger selectivity towards acetaldehyde formation. Reactive adsorption is observed, even at room temperature, on the catalyst containing lithium, leading to formation of acetate.
Abstract: Hydrogenation of ethene has been studied at 185-230 K on a Rh/TiO2 catalyst reduced at 473-773 K. The activity decreases on increasing the reduction temperature, even at 500 K, but it is more evident when reduction is carried out at 773 K. Oxidation and reduction recovers the activity in hydrogenation.
Abstract: Surface species formed upon impregnation of silica, alumina and titania with aqueous solutions of ammonium molybdate and calcination at 770 K have been studied by X-ray diffraction, specific surface area assessment, scanning electron microscopy, laser Raman spectroscopy and temperature-programmed reduction. Dispersion decreases in the order TiO2 > Al2O3 much greater than SiO2. Polymolybdate species are formed for low loadings, that lead to bulk MoO3 for loadings of 0.4 monolayer on SiO2, but persist for higher loadings on titania and alumina. On this last support, isolated molybdate species have been also found.
Abstract: The synthesis of a new layered double hydroxide possessing the hydrotalcite structure and having Mg2+ and V3+ ions in the layers is described. The structure has been confirmed by X-ray diffraction and XANES and EXAFS spectroscopies. V3+ ions occupy octahedral sites in the brucite-like layers.
Abstract: Ln2O3CaO systems, where Ln=La, Sm, have been prepared by impregnation of calcia with aqueous solutions of Ln nitrates or by mixing both oxides. Decomposition at 1073 K leads to a slight decrease in the specific surface area when starting from Ln2O3, but to a sharp decrease when starting from the nitrates.
Abstract: Surface vanadium-containing species supported on titania (97% rutile) prepared by manually grinding both oxides and calcined between 773 and 973 K in the presence of water vapour have been studied by Fourier transform infrared spectroscopy. The application of band-shape analysis to the overlapping and weak bands using a computer programme has provided information about the surface vanadium species existing in these samples. Up to four different species have been identified in samples calcined above 923 K.
Abstract: A hydrotalcite with a Mg:Al ratio of 2.5:1 has been synthetized and characterized. The solids obtained upon calcination of this material at 770 and 1023 K have been also characterized, and have been found to be a mixture of MgAl2O4 and MgO. Surface acidity and basicity of the solid calcined at 1023 K have been assessed by FTIR monitoring of pyridine and formic acid adsorption. It has been found that the surface is not acid, although a residual acidity, larger than that in pure MgO, exists. Adsorption of formic acid takes place dissociatively on surface, isolated hydroxyl groups, contrary to previous results with MgO, where dissociative adsorption took place through interaction with exposed oxide ions.
Abstract: A high selectivity of iron(III) phosphate-based catalysts in the oxidation of methanol to formaldehyde has been established and explained on the basis of the high covalency of the P-O bonds.
Abstract: The synthesis and characterization of hydrotalcite-like materials is described. As the time to which the samples have been submitted to a hydrothermal treatment is prolonged, a change in the Mg/Al ratio is observed, together with a more ordered structure of the species existing in the interlayer space. Calcination at increasing temperatures leads to decomposition of this compound, with the final formation of MgO and MgAl2O4. Changes taking place during these processes have been related to the decrease observed in the specific surface area of the samples upon prolonging the hydrothermal treatment, as well as its development and further decrease upon thermal treatment.
Abstract: The adsorption of pyridine, for-mic acid, and acetic acid on MgO, MoO3 and MoO3/MgO (Mo:Mg=1:15) samples calcined at 773 K (where no extensive reaction between the two oxides seems to take place) or 1100 K (where the MgMoO4 species is formed) has been monitored by FT-IR spectroscopy. The adsorption of pyridine indicates the formation of Lewis sites on MgO upon incorporation of molybdena. However, this change in the surface acidity does not modify the adsorption of the two carboxylic acids, which takes place dissociatively, probably because the formation of surface MgMoO4 still leaves exposed Mg2+, where adsorption of the carboxylate ions takes place.
Abstract: Laser Raman Spectra of molybdena supported at different concentration on the surface of gamma-Al2O3 prepared by both impregnation and physical mixtures- have been recorded. Formation of polymolybdate species, depending on Mo loading and time and temperature of calcination, has been detected in both series of samples.
Abstract: Several series of molybdenum oxide catalysts supported on gamma-Al2O3 prepared by a mechanical mixture of both oxides have been studied by X-ray diffraction and Laser Raman spectroscopy. The formation of bulk molybdena is observed after monolayer coverage has been reached (0.71 theoretical monolayers). Calcination at 770K in wet oxygen favours the dispersion of molybdenum species leading to polymolybdate formation, while under dry oxygen a lower degree of dispersion is achieved, and the polymolybdate species are not formed. The formation of molybdenum species on the gamma-Al2O3 surface is also studied as a function of calcination time.
Abstract: Adsorption of the drug N-methyl 8-hydroxy quinoline methyl sulphate, used to prevent sunburning and to avoid the dangerous effect of erythematogenic radiations, on untreated and HCI-treated sepiolite has been studied. The adsorption complex formed extends the absorption properties of the drug in the visible and ultraviolet range. The behaviour shown by these samples is fairly similar in all cases with respect to light absorption. However, adsorption complexes prepared via a dry method, by mechanically mixing the drug and sepiolite, lead to samples with different light absorbent properties, due to the modification of the surface acid/base properties of the sepiolite and hence of the way the drug is adsorbed.
Abstract: The reaction of molybdena with the surface of magnesia in the presence of alkaline metal cations (Li, Na, K, Rb, and Cs) has been studied. Without the alkaline cation, formation of MgMoO4 has been found upon calcining at 1100 K and melting of molybdena on the surface of magnesia. However, in the presence of these alkaline cations formation of MgMoO4 is easier and takes place upon calcining even at 770 K. Crystallization of MgMoO4 crystallites is improved as the atomic weight of the alkaline metal increases. This behaviour has been related to the change in basicity of the different systems studied upon incorporation of the alkaline cation.
Abstract: Rh/TiO2 (rutile) samples have been prepared by impregnation (series I) and ion exchange (series C). Upon reduction at low temperature (473 K) the metal dispersity is larger for samples C than for samples I. Removal of surface OH groups in series C cancels the Strong Metal-Support Interaction, thus increasing the metal dispersity measured upon reduction at 773 K.
Abstract: Oxidative coupling of methane has been studied on Li/MgO catalysts prepared from three different MgO precursors and from lithium carbonate or nitrate. In samples prepared from Li2CO3, the persistence of this on the support surface favors formation of carbon oxides.
Abstract: Lanthanum and samarium oxides supported on CaO, prepared from lanthanide nitrates or oxides, have been tested for methane oxidation. Samples prepared from the oxides are more active for total oxidation. Those from nitrates lead mainly to C2 and C3.
Abstract: Molybdena/titania systems doped with potassium have been prepared by impregnation of the support with aqueous solutions of ammonium heptamolybdate and studied to assess the effect of the alkaline cation on their surface and structural properties. Simultaneous or consecutive (first potassium, then molybdenum) methods have been tested. Characterization has been carried out by X-ray diffraction, Fourier-transformed infrared and visible-ultraviolet (diffuse reflectance) spectroscopies and nitrogen adsorption at low temperature. It was concluded that both the relative amounts of Mo and K, and the method of preparation, determine the nature and dispersion of the species formed upon calcination at 773 K. Simultaneous incorporation of both cations favours formation of potassium polymolybdates, and if the amount of molybdenum is above the stoichiometric value for these Mo-K-O compounds, molybdena is also formed.
Abstract: MoO3/Al2O3 systems have been prepared by calcining at 770 K alumina impregnated with ammonium heptamolybdate (aq), or by calcining molybdena/alumina mixtures at 770 K. Surface polymolybdates have been observed by laser Raman spectroscopy in samples prepared by impregnation: for calcined MoO3/Al2O3 mixtures, X-ray diffraction indicates MoO3 readily spreading over the alumina surface in the presence of water vapour, and this being almost complete after calcination for 2 h.
Abstract: The activity in n-butane hydrogenolysis of Rh/TiO2 (0.6 wt.% Rh) catalysts prepared by impregnation and reduced at different heating rates is reported. The catalyst reduced at 5 K/min is ten times more active than that reduced at 20 K/min, despite the fact that the dispersion values are similar in both cases.
Abstract: The activity in n-butane hydrogenolysis of Rh/TiO2 (0.6 wt.% Rh) catalysts prepared by ion exchange and reduced at different heating rates is reported. The heating rate has no major effect on the properties of the catalysts (similar dispersions and activities are achieved). Carbonaceous residues formed at high temperature are removed by hydrogen at room temperature, thus leading to recovery of the activity.
Abstract: Molybdena-alumina and molybdena-siljca systems have been prepared by impregnating the support with aqueous solutions of ammonium heptamolybdate (samples I) or by mixing MoO3 with Al2O3 or SiO2 (samples M), and calcining at 770 K (to decompose the molybdate) and 1100 K (to melt molybdena). In all eight cases the composition corresponds to two monolayers of molybdena. The solids have been characterized by nitrogen adsorption, X-ray diffraction, electronic and infrared spectroscopies, temperature programmed reduction and electron microscopy. Whichever the preparation method, only molybdena is detected on silica, and molybdena and Al2(MoO4)3 on alumina. The dispersion of molybdena, however, depends on the preparation method and the nature of the support. Formation of ternary Mo-Al-O compounds is due to support-supported phase interaction, that stabilizes molybdena even upon calcining at 1100 K, while MoO3 is removed by sublimation from the silica surface, leaving only 6.5% of the original loading. This behaviour can be related to the differences in the acidities of the supports, and then reaction of the more basic alumina with molybdena is easier than reaction with silica, thus accounting for the weaker interaction of this support with molybdena.
Abstract: The adsorption of ethene and propene on titania and titania-supported rhodium has been studied, observing a continuous adsorption even after 24 h. The samples have been reduced at 623 or 773 K, but no difference was observed, whichever the reduction temperature or the presence of rhodium, on the amount of gas adsorbed per unit of surface area of the solid. A study of the polymerization has been done following the "equal reactivity hypothesis", and it has been concluded that the role played by the rhodium particles is to facilitate adsorption of the gaseous molecule, that is then transfered to the surface of the support, where it polymerizes.
Abstract: A study has been carried out on the adsorption of hydrogen and CO on rhodium supported on titania. Both rutile and anatase have been used, and the effect of the reduction temperature and of the presence of impurities (chloride in rutile and sulphate in anatase) has been studied. It has been found that the presence of sulphate leads to salification of the surface hydroxyl groups, thus decreasing the anchoring sites for rhodium and leading to large rhodium particles. In the case of rutile, the presence of chloride modifies the interaction of the gases with the metallic particles, retarding the anatase/rutile phase change.
Abstract: Hydrotalcite-like compounds containing Ni(II) and Al(III) cations and with different Ni/Al ratios in the brucite layers have been that prepared and studied using FT-IR and V-UV/DR spectroscopies. It has been found that the local environment of the Ni(II) cations is the same in all cases, occupying octahedral holes, but the orientation of the interlayer carbonate anions changes with the Ni/Al ratio.
Abstract: Vanadia samples supported on rutile and anatase have been prepared by manually grinding both oxides, and further calcination in air at temperatures ranging from 773 to 973K. Characterization has been performed by X-ray diffraction and FT-IR spectroscopy. It has been found that even under mild calcination conditions (773K), vanadia becomes well dispersed on the surface of anatase, while on rutile such a dispersion is only achieved upon calcination above the melting point of this oxide. These results are interpreted on the basis of the fit between the exposed crystallographic planes of vanadia and of anatase, but not of vanadia and rutile.
Abstract: Vanadia/titania samples have been prepared from two different titania supports (P-25 from Degussa and anatase from Tioxide) by impregnation with aqueous solutions of NH4VO3 and calcination at 723 K. A decrease in the specific surface area is observed on increasing the amount of vanadia; titania from Degussa being non-porous, only development of wide pores is observed upon supporting vanadia. However, while the unloaded Tioxide support shows pores with average diameters of 8 and 5 nm, these latter disappear upon incorporation of vanadia. The reactivity of the V2O5/TiO2 sample prepared using the P-25 support in olefin oxidation has been studied by FTIR spectroscopy in the temperature range 150-673 K. Ethylene and propene are adsorbed as such at low temperatures, become hydrated to alkoxy species at medium temperatures and later undergo oxidation to carbonyl compounds (acetaldehyde and acetone). Allylic oxidation of propene and oxidative breaking of the C = C bonds is also observed.
Abstract: The thermal decomposition of precursors of Li/MgO catalysts prepared from different Mg and Li compounds is reported. Dehydration and dehydroxylation is observed in both cases, in addition to decomposition of LiNO3, but not of Li2CO3. The properties of the solids are related to the melting and spreading of LiNO3 or Li2CO3 before or after Mg (OH)2 dehydroxylation.
Abstract: Li/MgO solids have been prepared from MgO, by thermal decomposition of Mg5(OH)2(CO3)4.4H2O or by precipitation from aqueous solutions of magnesium nitrate, and then impregnating the solid thus obtained with aqueous solutions of lithium carbonate or nitrate. Non-porous solids are obtained in all cases. Specific surface area development depends on the nature of the precursors, the MgO --> Mg(OH)2 --> MgO process being topotactic.
Abstract: The synthesis of hydrotalcite-like compounds containing cobalt(II) ions in the layer lattice is described. Hydrothermal treatment of freshly precipitated materials, which is normally carried out to improve crystallinity, leads to a partial oxidation of Co(II) to Co(III) species, with simultaneous formation of a well dispersed Co3O4 spinel phase. Formation of this compound seems to be responsible for the low thermal stability observed for the Co(II)-Al(III)-CO3(2)-hydroxytalcite.
Abstract: Hydrogenolysis of ethane, propane, n-butane and isobutane has been studied on a Rh/TiO2 catalyst after reduction at 773 K. The results have been analyzed following the Kempling-Anderson formalism and a kinetic analysis has been also performed. Positive reaction orders with respect to hydrogen have been found, probably due to removal of carbonaceous residues from the catalyst surface by hydrogen during the reaction, thus overcoming the negative effect that hydrogen usually has in hydrogenolysis.
