Abstract: Simple or successive incipient wetness impregnation followed by heating at 180 degrees C is proved an efficient preparation method for dispersing effectively onto the silica surface various amounts of C-60 in the range 1-4% (w/w). BET, XRD, DRS, TGA, microelectrophoresis and photoluminescence have been used to characterize the photocatalysts prepared. A high dispersion was obtained for the quite stable supported C-60 phase, comprised mainly from relatively small or medium size C-60 clusters/aggregates. The photocatalytic activity was assessed in the singlet oxygen oxidation of alkenes by examining the photo-oxygenation of 2-methyl-2-heptene as a probe reaction. The catalytic tests were carried out at 0-5 degrees C in CH3CN, under oxygen atmosphere and using a 300 W xenon lamp as the light source. The heterogeneous catalysts obtained were proved to be active in the photocatalytic oxidation of olefins via a O-1(2) ene reaction. The catalysts exhibited significant conversion, turnover number and turnover frequency values, substantially higher than those achieved over the unsupported C-60. The conversion increases with the amount of the supported C-60 up to a value equal to 3% (w/w) and then it decreases whereas turnover number and turnover frequency decreases monotonically as the amount of the supported C-60 increases. The easy separation of these solid catalysts from the reaction mixture, the high activity and stability as well as the retained activity in subsequent catalytic cycles, make these supported catalysts suitable for a small-scale synthesis of fine chemicals. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Immobilization of [60]fullerene onto gamma-Al2O3 surface provides new heterogeneous photocatalysts for the oxidation of organic Compounds under oxygen atmosphere. These catalysts have been prepared by simple or Successive incipient wetness impregnation (using an organic solvent) followed by air-heating at 180 degrees C. In the C-60/Al2O3 system. C-60 loading was varied in the range of 1-4% (w/w). Several experimental techniques including BET. XRD, DRS, TGA, microelectrophoresis, photoluminescence and kinetic extraction, have been used to characterize these catalytic materials. It was found that the quite high surface exposed by the supported C-60 increases with the amount of the supported C-60, while the dispersion of the supported C-60 decreases. The quite stable supported [60]fullerene phase is comprised from C-60 clusters, small and large aggregates. This non-uniform size distribution is reflected to a non-uniform distribution concerning the 'supported phase-support' interactions. These interactions decrease with the amount of the supported C-60. The photocatalysts prepared may be safely used Lip to 200 degrees C. Above this temperature the supported C-60 is sublimated/combusted in air. The photocatalytic activity of the so-obtained catalytic systems has been evaluated in terms of substrate conversion in the singlet oxygen 'ene' reaction of alkenes. The photooxygenation of 2-methyl-2-heptene has been examined as a probe reaction. It was round that the catalytic activity increases with the increasing amount of the supported C-60 up to the value of 3% (w/w) and then decreases. The intrinsic activity expressed as TON or TOF decreased monotonically with C-60. In all cases, however. the photocatalytic activity of the Al2O3-supported C-60 catalysts was substantially increased compared to the unsupported C-60 precursor, exhibiting quantitative conversion yields after short reaction times. The catalytic behavior was attributed to the aforementioned opposite trends which follow the surface exposed by the supported C-60 on one hand and the 'supported C-60-support' interactions and the C-60 dispersion on the other hand. The easy separation of these solid catalysts from the reaction mixture, the high activity and stability as well as the retained activity in subsequent catalytic cycles. make these supported catalysts suitable for a small-scale synthesis. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Four CoMo catalysts supported on Al2O3-SiO2 mixed materials of varying SiO2 content (1.5, 10, 20 and 30% SiO2 w/w) were prepared following the co-EDF methodology. The catalysts were characterized using various techniques (BET, potentiometric mass titrations, XRD, DRS, XPS, LRS, TPR, TPD of ammonia and NO chemisorption). Two additional catalysts were prepared and characterized on two mixed supports, which contain 1.5 and 20% SiO2 w/w, following the conventional impregnation procedure. The hydrodesulphurization (HDS) activities of the catalysts studied were determined using thiophene as a probe molecule. The co-EDF catalysts were proved to be more active than the corresponding ones prepared following the conventional impregnation procedure. This was attributed to the relatively high dispersion of the Mo supported phase achieved by applying co-EDF. The following activity trend was obtained over the co-EDF catalysts, 20Si > 30Si > 1.5Si > 10Si, which indicates that the activity is maximized over the catalyst prepared on a mixed support containing 20% SiO2 w/w. This zigzag trend was explained in terms of two parameters, the ratio "tetrahedral (monomer) /octahedral (polymer) Mo species" and the amount of cobalt aluminate formed, both regulated by changing the composition of the mixed support. (C) 2010 Elsevier BM. All rights reserved.
Abstract: f[60] Fullerene is a promising deposit for decreasing the rate of the 'electron-hole' recombination's occurring in the context of the photooxidation of pollutants and the photocatalytic splitting of water. We have demonstrated that simple or successive incipient wetness impregnation followed by heating at 180 degrees C is a simple and efficient method for dispersing effectively various amounts of C-60 (in the range 1-4%, w/w) on the titania surface used in photocatalysis. The photocatalysts prepared were characterized using BET, XRD, DRS, and microelectrophoresis. The supported C-60 nanoparticles, comprised from 13 to 215 C-60 molecules, have mean radius ranged between 0.8 and 3.1 nm which increases with the C-60 loading of the sample. The supported phase was proved to be quite stable against sublimation/combustion. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Nickel supported on alumina catalysts have been prepared using various synthesis methods (dry impregnation, co-precipitation, sol-gel) and evaluated for the hydrogenation of benzene contained in gasoline. The evaluation was carried out in a laboratory scale high pressure fixed bed reactor fed with a stream of surrogated reformate gasoline consisted by a mixture of hexane, benzene and toluene. All catalysts have been characterized by the joint use of various physicochemical characterization methods (XRF, BET, TGA, SEM, XRD, UV-vis DRS and XPS) in order to correlate their catalytic performances to their physicochemical properties. The results obtained revealed that sol-gel procedure, especially when it is followed by supercritical drying (aerogel), produced the most promising catalysts for the aforementioned catalytic process. Sol-gel methodology ensured the best compromise between dispersion of the nickel phase and its interaction with the support surface. Co-precipitated catalysts exhibited important activities but lower than those of the sol-gel catalysts. The catalyst prepared by dry impregnation proved to be the less active. Calcination of the catalysts before their activation by reduction decreased their activities. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: Two series of MnOx/Al2O3 catalysts with varying Mn loading (0-1.2%, w/w Mn) prepared by equilibrium deposition filtration (EDF) and wet impregnation (WI) methodology were used for studying the influence of the composition and the preparation method on their activity for the reduction of benzaldehyde with ethanol. The prepared catalysts were characterized by BET measurements, X-ray photoelectron spectroscopy and diffuse reflectance UV-vis spectroscopy. It was found that following EDF methodology, the deposition of MnOx species on the alumina takes place via adsorption of [Mn(H2O)(6)](2+) and [Mn-Ac](+) species on the negatively charged surface sites. On the contrary, following wet impregnation the via adsorption of deposition takes place mainly via precipitation in the step of the solvent evaporation. The extent of the interactions exerted between the support and supported phase is higher in the EDF samples. These interactions created during the impregnation step were detected by DRS after drying and calcination. Higher dispersion of the MnOx phase is achieved when it is deposited on the gamma-Al2O3 surface by the EDF than the WI method. The catalysts of the first series exhibited higher activity related well with the dispersion of MnOx, supported nano-particles.) 2007 Elsevier B.V. All rights reserved.
Abstract: In this work we have examined whether the re-impregnation of CoMo/-gamma-alumina catalysts or the replacement of the conventional non-dry impregnation step by "equilibrium deposition filtration" (EDF) may be used for improving their surface characteristics and thus their catalytic activity. Two samples were prepared. In the first sample (EDF) the molybdenum species were mounted by "equilibrium deposition filtration" whereas in the second sample these species were mounted by non-dry impregnation (NDI). In both cases the Co was deposited on the calcined Mo/gamma-A1(2)O(3) precursor solid by simple dry impregnation. An aliquot of each sample was impregnated again with an amount of pure water equal to its pore volume and then it underwent drying and calcination. The catalysts prepared were characterized using N-2 adsorption measurements (BET), UV-vis diffuse reflectance spectroscopy (DRS), laser Raman spectroscopy (LRS) and NO chemisorption. The hydrodesulfurization (HDS) activities over the catalysts studied were determined using a continuous-flow tubular fixed-bed microreactor operating in a differential mode at atmospheric pressure. It was confirmed that the replacement of the conventional impregnation by equilibrium deposition filtration results to catalysts with relatively high active surface and high portion of the well-dispersed octahedral cobalt and thus, to catalysts with 30% higher HDS activity. The re-impregnation resulted to partial dissolution and re-dispersion of the Mo and Co supported oxidic phases. Concerning the NDI catalyst re-impregnation resulted to an increase of the active surface and of the portion of the well-dispersed octahedral cobalt and thus to 25% higher catalytic activity. The opposite effects were observed for the EDF catalyst which exhibited almost 7% lower activity after re-impregnation. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: In the present work we attempt to optimize the size of the supported "molybdenum oxide"/titania and "cobalt oxide"/gamma-alumina nanoparticles formed after calcination by "selecting", respectively, the proper mode of deposition and the local structure of the deposited species achieved upon the impregnation step of catalyst preparation. Concerning the first system, it was found that the disubstituted Mo inner sphere surface complexes, which are bound on the support surface stronger than the monosubstituded ones, resist more effectively to the sintering taking place during calcination where the above complexes are transfortned progressively into MoO3 supported nanoparticles. This leads to a catalyst with very small MoO3 nanoparticles and thus with very high activity for the selective reduction of NO by NH3. Concerning the "cobalt oxide" /gamma-alumina catalysts, it was found that a relatively large (small) size of the supported nanocrystallites is imposed by the bulk deposition (formation of inner sphere surface complexes). A quite small size of the supported "cobalt oxide" nanocrystallites, not strongly interacted with the support surface, is imposed by the interface precipitation. This is the optimum supported phase for the complete oxidation of benzene. () 2007 Elsevier B.V. All rights reserved.
Abstract: One-component samples containing K, Ni, and various concentrations of Mo were prepared by incipient wetness impregnation technique. Bi-and tri- component samples have been prepared from the calcined one- and bi-component samples following the same procedure. The samples were characterized by SSA, FT-IR, DRS and Raman measurements. The decrease of the ratio Ni2+(Td)/Ni-2+(Oh) with the increase of Mo loading and the re-dispersion of Mo phase after the Ni addition on the one-component Mo containing samples proved the occurrence of interactions between these components in the bi-component NiMo-containing samples. Most probably Ni-Mo-O surface species with different number of Mo-oxo and Ni-oxo entities are formed, depending on the Ni/Mo atomic ratio. It may be accepted, that the Ni2+ ions are partially localized into the molybdate layer without forming well-defined phases. Two kinds of K+ ions have been determined in the corresponding samples, interacting ("bounded" K+ ions) and not interacting ("free" K+ ions) with the rest of the supported phases. The potassium deposition increased the dispersion of Mo phase and influenced the coordination state of the Mo6+ and Ni2+ ions in the bicomponent KMo-, KNi-containing samples and the tri-component KNiMo-containing samples. It provoked partial transformation of the polymeric Mo6+(Oh) species into monomeric Mo6+(Td) ones and Ni2+(Td) species predomination over Ni2+(oh) ones. Most probably, new complex K-Ni-Mo-O species were formed on the support surface in the case of the tri-component samples. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: In the present work we studied the influence of the preparation method and the Co loading on the physicochemical properties and the catalytic activity of the cobalt oxide/gamma-alumina catalysts for the reduction of NO by propene under net oxidizing conditions. Two series of catalysts containing 1 and 5% w/w Co, respectively, were prepared using three preparation methods, namely, the equilibrium deposition filtration (EDF), the conventional incipient wetness impregnation (IWI) and the IWI adding nitrilotri acetic acid (nta) in the impregnating solution (IWInta). The catalysts were tested at various temperatures in the range 300-550 degrees C using a fixed-bed microreactor for the NO reduction by propene under lean burn conditions. The evolution of the Co species on the alumina surface was followed after each preparation step by diffuse reflectance spectroscopy (DRS). It was found that the catalysts of the first series were more active for the title reaction than those of the second one. The EDF sample of the first series was proved to be the most active and selective one followed by the IWI and then the IWInta sample of the same series. The DRS results indicated that the enhanced activity and selectivity of the EDF sample could be attributed to the increased concentration of isolated Co(II) inner sphere complexes of octahedral coordination, which are formed on the support surface by adsorption of the corresponding aqueous complexes, [Co(H2O)(6)](2+), being in the impregnating solution. These inner sphere complexes are transformed upon thermal treatment into a CoAl2O4 like phase with high dispersion. On the other hand, the [Co(H2O)(6)](2+)(NO3)(2-) and [Co-nta](-) NH4+(or H+) and/or [Co-2nta](4+).4NH(4)(+)(or 4H(+)) complex salts, precipitated on the alumina surface upon drying in the cases of the IWI and IWInta samples, are transformed upon calcination into CoAl2O4 like phases with lower dispersion. All the samples of the second series promoted the propene combustion as well as the oxidation of NO and N-2, used as carrier gas, to NO2. DRS results revealed that in all these samples cobalt(III) oxo species are formed in addition to the CoAl2O4 phase. These species are considered to be responsible for the enhancement of the rates of the oxidation reactions mentioned above. (c) 2005 Elsevier Inc. All rights reserved.
Abstract: In the present work we studied the influence of the methodology used for mounting Co(II) species on the γ-alumina surface on the physicochemical properties and the catalytic activity of the 'cobalt oxide'/γ-alumina catalysts for complete oxidation of benzene. Three series of catalysts of varying Co content (up to 21 wt.% Co) were prepared using three preparation methods: pore volume impregnation (pvi), equilibrium deposition filtration (edf) and pore volume impregnation adding nitrilotri acetic acid (ma) in the impregnation solution. It was found that the catalytic activity for low, medium and high Co content follows, respectively, the orders, nta-pvi &MGT; pvi &MGT; edf, nta-pvi &MGT; edf ≈ pvi and edf > nta-pvi > pvi. The catalysts prepared were characterized using various techniques (BET, UV-vis/DRS, XRD and XPS) at each step of the preparation procedure, namely after the Co(II) mounting on the support surface, after drying as well as after calcination. It was inferred that the most active sites are located on CO3O4-supported crystallites, loosely or moderately interacting with the γ-alumina surface. Two critical parameters, related with the method followed for mounting Co(II) species on the γ-alumina surface, control the characteristics of the supported phase and thus the amount and the size of the above-mentioned CO3O4 crystallites: the ratio 'amount of Co(II) deposited in the impregnation step to that remaining in the liquid phase inside the pores precipitating thus in the drying step' closely related with the ratio 'amount of Co(II) in the deposited phase (isolated Co(II) surface inner sphere complexes and Co(II) surface precipitates)/amount of Co(II) in the precipitated phase formed in the drying step' as well as the composition of the precipitated phase. The application of the pvi technique resulted to low values for the above ratios and thus to the formation of a rather unstable precipitated phase consisted mainly by Co(H2O)(6)(2+) (.) 2NO(3)(-). Upon calcination it is transformed into loosely bounded CO3O4 crystallites of relatively big size. This is related with the low Co dispersion and thus with the low catalytic activity exhibited by these catalysts. The application of edf resulted to high values for the above-mentioned ratios. Therefore, the deposited phase is predominant. Upon calcination it is transformed to well (very well) dispersed cobalt phases strongly (too strongly) bounded with the support surface and thus reducible at high temperatures (non reducible up to 800 ° C). Although these phases are responsible for the high Co dispersion achieved they do not contribute to the catalytic activity unless the deposited phase mainly comprises a Co(II) surface precipitate with relatively large number of layers as it is the case for the sample with the maximum Co content. The application of the nta-pvi technique resulted to very low values for the ratios mentioned above. This is because the [Co(Il)-nta](-) and [Co(Il)-2nta](4-) complexes, in which the Co(H2O)6 2, complex is completely transformed, are not practically adsorbed on the support surface. Therefore, in the nta-pvi catalysts a precipitated phase containing the [Co(II)-nta](- .) NH4+(or H+) and [Co(II)-2nta](4- .) 4NH(4)(+) (or 4H(+)) complex salts predominates. Upon calcination these are transformed into CO3O4 crystallites of small size, which are moderately interacting with the support surface. This is related with the relatively high Co dispersion, mainly that for the catalytically active species, and thus with high catalytic activity. © 2004 Elsevier B.V. All rights reserved.
Abstract: In the present work we studied, for the first time, the kinetics of adsorption of the Co(H2O)(6)(2+) species on the "electrolytic solution/gamma-Al2O3" interface at pH = 7 and 25 degrees C for a very broad range of Co(II) surface concentrations ranged from 0.03 to 6 theoretical Co(H2O)(6)(2+) surface layers. Moreover, we studied the surface dissolution of gamma-alumina in the presence of the Co(H2O)(6)(2+) ions in the impregnating solution, the contribution of the Co(II) desorption on the whole deposition process and the deposition isotherm. It was found that under the conditions where the deposition has taken place, the dissolution of the gamma-alumina surface is negligible even in the presence of the Co(H2O)(6)(2+) species in the impregnating solution. It was, moreover, inferred that the Co(II) desorption does not participate significantly to the whole deposition process. It was found that the deposition kinetics may be described by the following kinetic expression r(Co.bulk) = k'C-Co,bulk(2), which relates the rate of disappearance of the Co(H2O)(6)(2+) ions from the impregnating solution, r(Co,bulk), with their concentration C-Co,C-bulk. This kinetic expression may be derived assuming the following deposition scheme: nS + 2[Co-(H2O)(6)(2+)] -> S-n - [Co(H2O)(x,x < 6)(2+)](2), where S represents the surface reception sites. The above expressions indicated that two Co(H2O)(6)(2+) ions are involved, from the side of the interface, in the reaction with the reception sites. It seems probable that the deposition step involves the simultaneous adsorption and dimerization of the two interfacial Co(H2O)(6)(2+) ions through (hydr)oxobridges. On the other hand, the sigmoidal form of the deposition isotherm and the dependence of the apparent rate constant, k', on the interfacial Co(II) concentration suggested that the already deposited Co(II) species may be involved in the reception sites, S, promoting the adsorption and resulting to the formation of multinuclear complexes and Co(II) surface precipitates. Finally, reasonable interface potential values for oxides were determined for the first time using kinetic results.
Abstract: In the present work we studied the influence of the initial concentration of the impregnating solution used for mounting Co(II) species on the gamma-alumina surface by equilibrium deposition filtration method (edf) on the physicochemical properties and the catalytic activity of the "cobalt oxide" /gamma-alumina catalysts. The complete oxidation of benzene has been used as a model reaction. Two series of catalysts (edf-X-A and edf-X-B) of varying Co content (X: up to 21 wt.% Co) were prepared using the above-mentioned method and tested at various temperatures in the range 200-300 degrees C using a fixed-bed reactor. In the first series (A) various Co loadings were obtained by varying the initial Co(II) concentration of the impregnating solution. In the second series (B) the corresponding Co loadings were obtained by using the impregnating solution used for the preparation of the catalyst of A series with the maximum Co(II) content and varying the impregnation time. All catalysts were characterized using various techniques, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), X-ray powder analysis (XRD), nitrogen adsorption (BET) and temperature-programmed reduction (TPR). It was found that the initial Co(II) concentration of the impregnating solution used for depositing the corresponding species on the gamma-alumina surface by edf influences the catalytic activity of the "cobalt oxide" /y-alumina catalysts with respect to the complete oxidation of benzene. The increase of the initial Co(II) concentration of the impregnating solution brings about a change in the composition of the deposited phase formed in the impregnation step by decreasing the ratio "Co(II) surface inner-sphere complexes/surface Co(II) precipitates". Upon calcination, the Co(II) surface inner-sphere complexes are transformed to well-dispersed "cobalt oxide" phase strongly interacting with the support surface while the surface Co(II) precipitates are transformed to Co3O4 crystallites loosely interacting with the support surface. The former phase is responsible for the relatively high dispersion of Co observed in the A series of catalysts but it is rather inactive, while the latter exhibits lower dispersion but higher activity. Thus, the edf catalysts prepared using high initial Co(II) concentration exhibited higher catalytic activity than the corresponding ones prepared using low initial Co(II) concentration. (c) 2004 Elsevier B.V. All rights reserved.
Abstract: A CoMo/gamma-Al2O3 catalyst, prepared by depositing on the Al2O3 carrier first the Mo species via equilibrium deposition filtration (EDF) and then the Co species by dry impregnation, was compared to three CoMo/gamma-Al2O3 samples prepared using various conventional impregnation methods. All samples had the same composition, cot-responding to an atomic ratio Co/(Co + Mo) equal to 0.3. The above samples were characterized using various physicochemical techniques (AAS, BET, DRS, LRS, XPS, TPR, and NO chemisorption), and their catalytic activity was determined using the hydrodesulfurization (HDS) of thiophene as a probe reaction. The EDF-prepared catalyst was about 30-43% more active in HDS than those prepared with the conventional impregnation techniques at all reaction temperatures studied. In contrast, the EDF catalyst exhibited the lowest hydrogenation activity. The higher HDS activity of the EDF sample is attributed to the higher number of active HDS sites formed on its surface. It is concluded that the increased number of active sites is due to the fact that the deposition of the Mo species by EDF results to a higher coverage of the support surface by supported molybdenum phase, which in turn, inhibits the formation of the catalytically inactive CoAl2O4 and favors the dispersion of octahedral cobalt on its surface. (C) 2003 Elsevier Inc. All rights reserved.
Abstract: Fullerene C-60 supported on silica and gamma-alumina (2% w/w C-60/SiO2 and C-60/Al2O3) sensitizes the photooxidation of alkenes via singlet oxygen and/or electron transfer mechanism, depending on the solvent and the substrate.
Abstract: Two MoO3/gamma-Al2O3 catalysts containing 11% (w/w) MoO3 prepared by equilibrium-deposition-filtration at two different pairs of pH and Mo concentration values (pH = 3.9, C-Mo = 1.1 x 10(-3) mol L-1 and pH = 6.3, C-Mo = 2.7 x 10(-2) mol L-1) were studied for the ethane oxidative dehydrogenation. The two catalysts demonstrated similar ethane conversions in a temperature range 450-600degreesC. Selectivity to ethene was 15% higher for the catalyst prepared at relatively low pH than for the sample prepared at a higher pH. The observed difference in ethylene selectivity was related to the different structural characteristics of the molybdena phase in the two catalysts. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: Diffuse reflectance spectroscopy was used for the first time to investigate the adsorption of the [Co(H2O)(6)](2+) ions on the interface developed between the surface of the gamma-alumina particles and the electrolytic aqueous solutions used for the preparation of cobalt-supported gamma-alumina catalysts by equilibrium deposition filtration. The formation of inner-sphere Co(II) surface complexes in which Co(II) is in octahedral symmetry was confirmed. A deconvolution peak centered at similar to585 nm was attributed to the exchange of one aqua ligand with one AlxOHy (x = 1, 2, or 3; y = 0 or 1) negatively charged surface group resulting in the formation of mononuclear monosubstituted inner-sphere Co(II) complexes at a Co(II) surface concentration equal to 0.02, mumol of Co(II)/m(2). It was inferred that as the surface Co(II) concentration increases the formation of disubstituted and/or trisubstituted surface complexes is favored with respect to the formation of monosubstituted Co(II) surface complexes. A deconvolution peak centered at similar to640 nm was attributed to the exchange of one or more aqua ligands with bridging hydroxo ligands (Co-O-H). The relative magnitude of this peak increases with the Co(II) surface concentration, reflecting the increasing formation of binuclear, oligonuclear, and multinuclear Co(II) surface complexes and then the formation of the Co(II) surface precipitate.
Abstract: In the present review article, we present the efforts done so far for elucidating the mechanism of adsorption of the Co(II) species, mainly Co(H2O)(6)(2+), on the interfacial region developed between metal oxide particles, used as catalytic supports, and aqueous electrolytic solutions. Specifically, we present: (i) the principal modes of deposition of the transition metal ionic species (TMIS) on the surface of oxidic supports related with the various methodologies used for the preparation of the supported catalysts; (ii) the state of the art concerning the general aspects of the adsorption mechanisms of the TMIS on the aforementioned interfacial region; and (iii) the works reported so far dealing with the adsorption of the Co(II) species on the surface of gamma-Al2O3 (gamma-alumina), alpha-Al2O3 (alpha-alumina), TiO2 (rutile), and SiO2 (silica). It was concluded that the mechanism of adsorption depends on two main factors: on the Co(II) surface concentration and on the nature of the support surface. It seems that, generally, the mechanism changes progressively along the Co(II) surface concentration from the deposition of monodentate-mononuclear inner sphere complexes, weakly evidenced in too low values of the Co(II) surface concentration, to multidentate, multinuclear inner sphere surface complexes at relatively low Co(II) surface concentrations, and then into surface Co(OH)(2)-like, eventually mixed precipitates, at relatively high Co(II) surface concentrations but at pH values lower than those required for bulk precipitation. In all cases, Co(II) forms surface species with Co(II) in octahedral symmetry. However, the exact Co(II) surface concentration values, in which the abovementioned two transitions (concerning the deposited phase) take place, depends on the kind of the support. Thus, SiO2 favors the formation of the Co(OH)(2)-like precipitates even at relatively low Co(II) surface concentrations. In contrast, TiO2 favors the formation of mononuclear or oligonuclear surface complexes. Finally, alumina, which exhibits the maximum adsorption capacity, favors the formation of highly defected Co(OH)(2)-like precipitates, probably mixed Co-Al precipitates. The exact local structure of the inner sphere Co(II) surface complexes, formed by exchanging the H2O ligands with surface oxygens, has been already approached but only for the surface planes of the alpha-Al2O3 and rutile monocrystals. This structure remains up to now rather unclear for the polycrystalline oxides used as catalytic supports. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: Two unpromoted Mo/gamma-Al2O3 catalysts containing almost the same amount of Mo were prepared using equilibrium deposition filtration, EDF, for depositing Mo-oxo species on the alumina surface at two different pH values (3.9 and 6.3). On these samples the same amount of Co ions was deposited by dry impregnation and thus, the corresponding CoMo/gamma-Al2O3 catalysts were prepared. The unpromoted and the Co-promoted samples were characterized using BET, DRS, LRS, TPR, and NO chemisorption. Moreover, the catalytic activity of the samples was determined at various temperatures using the hydrodesulfurization of thiophene as a probe reaction. The characterization of the specimens showed that the Mo deposition at pH = 3.9 favors the deposition of polymeric Mo-oxo species, in agreement with predictions drawn on the basis of two models related with the acid-base behavior of the gamma-Al2O3 surface. These species after calcination provide a well-dispersed Mo phase with relatively high reducibility. This phase, in which the concentration of the octahedral Mo is greater compared to that on the Mo/gamma-Al2O3 specimen prepared at pH = 6.3, hinders, in turn, the formation of the catalytically inactive CoAl2O4, whereas after sulfidation this provides a sulfided state with relatively high concentration of sulfur vacancies. The above explain why the unpromoted and the Co-promoted catalysts prepared at pH = 3.9 proved to be more active than the corresponding catalysts prepared at pH = 6.3.
Abstract: A series of CoMo/gamma-Al2O3 catalysts have been prepared using various methodologies. One of them (EDF) was prepared by depositing the Mo species on the support via the equilibrium deposition filtration (EDF) technique and then the Co species by dry impregnation. Another catalyst (co-EDF) was prepared by depositing the Co and Mo species simultaneously via EDF. A third catalyst (co-WET) was prepared by depositing Mo and Co species simultaneously using the wet impregnation method. The fourth catalyst (WET) was prepared by depositing the Mo species through wet impregnation and then the Co species by dry impregnation. Finally, the fifth catalyst (s-DRY) was prepared by mounting the Mo species through successive dry impregnations and then the Co species by dry impregnation. In all cases the Mo and Co content was identical, giving a Co/(Co + Mo) ratio equal to 0.13. These catalysts were characterized using various physicochemical techniques (BET, NO chemisorption, DRS, LRS, TPR, and XPS), and their catalytic activity for the hydrodesulfurization of thiophene was determined. The trend observed for the HDS activity (namely, EDF > co-EDF > co-WET > s-DRY > WET) is attributed to similar trends observed for both the fraction of well-dispersed octahedral cobalt in the oxidic precursors and the concentration of the edge sulfur vacancies formed on the active phase of the sulfided samples. The EDF and co-EDF catalysts exhibited relatively low hydrogenating activity. The maximum HDS activity, achieved over the EDF catalyst, suggested the most suitable preparative strategy for the preparation of very active and less hydrogen-demanding CoMo/gamma-Al2O3 HDS catalysts. (C) 2003 Elsevier Science (USA). All rights reserved.
Abstract: Deposition of fullerene C-60 (2% w/w) on silica and gamma-alumina provokes a two orders-of-magnitude increase of its activity for the liquid-phase photooxidation of 2-methyl-2-heptene. Kinetic studies concerning the above photooxidation showed a first-order dependence of the reaction rate on the alkene concentration. The corresponding reaction-rate constant was found to be higher in the case where gamma-alumina was used as carrier. The nature of the carrier does not influence the mechanism and the selectivity of the reaction. High dispersion of the supported fullerene is achieved on the surface of the carriers, which increase the fullerene light absorbance especially in the visible range.
Abstract: In this paper, we present a novel methodology, called the potentiometric mass titration (PMT) technique, for determining the point of zero charge (pzc) of mineral hydr(oxides) immersed in electrolytic solutions. Following PMT, the pzc is identified as the common intersection point (CIP) of the potentiometric curve of the blank solution with the corresponding curves of the impregnating suspensions containing different amounts of the immersed mineral (hydr)oxides. Full experimental results related to the determination of pzc using the PMT technique and four traditional techniques (potentiometric titrations, mass titrations, immersion, and micro-electrophoresis (for determining the isoelectric point, equal to pzc in cases where no specific adsorption takes place)) are presented for four oxides, namely, MgO, gamma-Al2O3, TiO2, and SiO2. The comparison of the pzc values determined by PMT, with the corresponding ones determined using the traditional methodologies, strongly suggested that the PMT technique can be used to determine the pzc of oxides. A simulation procedure of the PMT technique has been developed and applied to model oxides with properly selected acid-base characteristics and to various combinations of models related to the charging mechanism of the oxide surface (1 site/1 pK, 1 site/2 pK, multisite models) and to the description of the interfacial region (diffuse double layer, constant capacitance, basic Stern models). The intensive application of this simulation procedure offered a quantitative interpretation of the methodology. Specifically, it was demonstrated that (a) the application of the "quick scan" version of the PMT technique, realized by recording the titration curve of the blank solution (pH vs V-added (acid)) and the corresponding curve of a suspension of a given amount of the immersed oxide, indeed results in the determination of the pzc, provided that this is greater than a value of about 4; (b) the application of the "typical" version of PMT, realized by recording the titration curves of three different suspensions (pH vs V-consumed (acid)) containing different masses of the immersed oxide, provides the pzc value of this oxide over the whole pH range; and (c) the CIP that is determined, using PMT, corresponds to the pzc irrespective of the charging mechanism of the oxide surface and the structure of the double layer developed between the oxide surface and the solution. However, in the case where the basic Stern model is used to describe the interfacial region, the pzc value determined by PMT deviates slightly from the true value when the value of the affinity constants of the ion pairs formed between the positive counterions and the surface is different than the corresponding value of the negative counterions.
Abstract: The equilibrium deposition filtration technique was properly developed in order to allow the deposition, via adsorption, of a relatively large amount of the Co2+ ions on the gamma-alumina surface. This, in turn, permitted the preparation of a Co2+/gamma-Al2O3 catalyst with a very high active surface as compared to that achieved by preparing the corresponding catalyst using the conventional pore volume impregnation technique. In the so-prepared EDF catalyst, the joint use of nitrogen adsorption, X-ray photoelectron spectroscopy, temperature programmed reduction, and diffuse reflectance spectroscopy showed that the gamma-alumina surface is mainly covered by very well dispersed Co2+ oxo-species with octahedral symmetry.
Abstract: A novel technique for determining the point of zero charge (pzc), called 'potentiometric mass titrations technique', has been developed and used for determining the pzc of several industrially used catalytic supports (SiO2, TiO2, gamma-Al2O3 and MgO).
Abstract: The aim of this study is to investigate the influence of incorporation of tungsten on an already prepared CoMo/gamma -Al2O3 hydrodesylfurization catalyst (base catalyst). The evaluation of W-incorporated CoMo/gamma -Al2O3 is described, in relation to them base catalyst, concerning their activities for hydrodesulfurization (HDS) of thiophene and hydrogenation (HYG) of its unsaturated products towards butane. The main conclusion of this study is that incorporation of a low amount of tungsten onto CoMo/gamma -Al2O3 catalyst increases its HDS and HYG activities. A synergy has not been found between Mo- and W-phase. The W-loading for which the maximum activity is observed highly depends on the details of the preparation method used. (C) 2001 Elsevier Science B.V. All rights reserved.
