Abstract: The interfacial chemistry of the impregnation step involved in the preparation of nickel catalysts supported on titania is presented. Several methodologies based on deposition data, pH measurements, potentiometric mass titrations, and microelectrophoresis have been used in conjunction with diffuse reflectance UV/Vis/NIR spectroscopy, simulations, and semiempirical quantum chemical calculations. Three mononuclear inner-sphere complexes were formed at the compact layer of the "titania/electrolyte solution" interface: A monosubstituted, dihydrolyzed complex above a terminal oxo group, a disubstituted, dihydrolyzed complex above two terminal adjacent oxo groups, and a disubstituted, nonhydrolyzed complex above one terminal and one bridging adjacent oxo groups. The monosubstituted, dihydrolyzed complex predominates. The contribution of the disubstituted configurations is also important at very low Ni-II surface concentration, but it decreases as the Ni-II surface concentration increases. In addition, bi- and trinuclear inner-sphere complexes were formed. The receptor site involves one bridging and two terminal oxo groups in the first case and two bridging and three terminal oxo groups in the second case. The relative surface concentrations of these configurations increase initially with Ni-II surface concentration and then remain practically constant. The understanding of these interfacial processes at a molecular level is very important to shift the catalytic synthesis from an art to a science as well as to obtain strict control of the impregnation step and, to some extent, of the whole preparative sequence. This study is very relevant to the synthesis of submonolayer/monolayer nickel catalysts supported on TiO2 following equilibrium deposition filtration (otherwise called equilibrium adsorption).
Abstract: We have achieved the preparation of five catalysts with various Co and Mo radial profiles in γ-Al2O3 extrudates and loadings similar to the industrial hydrotreatment catalysts. EDS microanalysis was used for determining the profiles achieved. The catalyst bodies prepared were powdered and characterized using N2 adsorptionâdesorption, temperature programmed reduction, X-ray powder diffraction, UVâvis diffuse reflectance spectroscopy and NO adsorption. The hydrodesulfurization of thiophene under atmospheric pressure was used as a probe reaction. The selectivity for the butane produced was taken as a measure of the hydrogenation ability of the catalysts.
The different procedures inevitably used for the preparation of the different profiles affect somewhat the microscopic characteristics of the catalysts (Co and Mo dispersion, concentration of the coordinatively unsaturated sites, cobalt species formed) and to some extent their intrinsic catalytic behavior (determined in the powdered samples). The catalytic activity of the catalyst bodies depends on the aforementioned microscopic characteristics and the internal mass transfer resistance associated with the location of active phases and pores blocking. The combination of these factors becomes the sample with uniform Co and Mo profiles the most active one. DRS, TPR and XRD results indicated high Co dispersion in this sample.
The hydrogenation ability of the CoMo/γ-Al2O3 catalysts is higher over the catalyst bodies than over the powdered samples and enhanced on the catalysts with egg-shell Co profiles.
Abstract: The interfacial impregnation chemistry involved in the synthesis of Cr-VI catalysts supported on titania is presented. The mode of interfacial deposition of the Cr-VI oxo-species at the titania/electrolyte solution interface, the interfacial species, and the local structure of the deposited species were investigated. Several methodologies based on potentiometric titrations, microelectrophoresis, and adsorption experiments were used. Modeling of the interfacial deposition based on experimental results provided an integrated picture concerning the deposition features. The interfacial species were confirmed by using laser Raman spectroscopy. The deposited CrO42-, HCrO4-, and Cr2O72- ions are retained above the positively charged bridging hydroxyl groups (Ti2OH) of the titania surface as electrostatic forces cause the formation of ion-pairs. Each CrO42- or HCrO4- ion is located above a bridging hydroxyl, and each Cr2O72- ion above two bridging hydroxyl groups. Only the CrO42- and HCrO4- ions are deposited, with a preference for the CrO42- ions, at low CrVI surface concentrations ( up to 0.3 mu mol Cr(VI)m(-2)). Cr2O72- ions were deposited in addition to CrO42- and HCrO4- ions at higher CrVI concentrations. The direct probing of the interfacial species by using laser Raman spectroscopy confirmed the interfacial species determined by modeling the deposition data. The deposition model developed describes all of the experimental data ( from adsorption, titration, and microelectrophoresis experiments) very well. Moreover, it accurately predicts the displacements of pzc and iep caused by the presence of these species in the solution.
Abstract: The mechanism of retention of vanadium oxo-species at the âtitanium oxide/aqueous solutionâ
interface was investigated over a wide pH range (4-9) and V(V) solution concentration (10-5-2Ã10-2 M)
by combining equilibrium deposition experiments, potentiometric titrations, microelectrophoresis and
âprotonâionâ titration curves. It was inferred that the adsorbed V(V) oxo-species are retained inside
the compact layer of the interface through hydrogen/coordinative bonds forming very probably innersphere
complexes with the titania surface groups.
Abstract: The pzc of titania (anatase) was regulated by changing the suspension temperature. A decrease of about 1.5 pH unit was achieved, when the temperature increases from 10 to 75 °C. This decrease in pzc causes an increase of the negatively charged surface hydroxyls of titania, which act as receptor sites for the deposition of the cationic [Ni(H2O)6]2+ species, upon the preparation of Ni/TiO2 catalysts following the equilibrium adsorption methodology. Thus, a considerable increase of well dispersed Ni species on titania surface was obtained by regulating the temperature of the impregnation suspension.
Abstract: The evolution of the molecular structure of molybdena/titania catalysts prepared by the equilibriumâdepositionâfiltration (EDF) method is monitored by in situ Raman spectroscopy in the temperature range 25â430 °C. The study is based on a recently proposed picture for the interfacial speciation of Mo(VI) oxo-species deposited at the âtitania/electrolytic solutionâ interface of âwetâ catalyst pastes and on the temperatureâdependent features of in situ Raman âsnap-shotsâ obtained with increasing temperature. The configurations of the deposited Mo(VI) oxo-species on âwetâ (i.e. prior to drying) samples conform to the theoretically previewed speciation for the respective pH's and concentrations of the precursor electrolyte and impregnating solutions; for initial Mo(VI) concentrations up to 2.5Ã10â2 M the predominant deposited species evolve from monomeric MoO42â ions retained above a bridging surface hydroxyl through a hydrogen bond [Ti2OHâ¦O-MoO3] (for solution pH=9) to a mixture with inner sphere mononuclear mono-substituted (with the terminal surface oxygen of titania) complex [Ti-OMoO3] (for solution pH=6) and to a simultaneous occurrence of the former monomeric species with Mo7O246â and HMo7O245â (for solution pH=4). A gradual transformation of the complex configurations is monitored with rising temperature by in situ Raman snapâshots and the spectra of the final calcined samples exhibit the features of dispersed mono-oxo (TiâO)3Mo=O species. A low presence of associated species (possessing MoâOâMo linkages) is evident for the higher loaded samples.
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: A thorough study is presented concerning the interfacial chemistry of the impregnation step involved in the preparation of molybdenum(VI) supported titania catalysts. This is based on a recently developed picture for the "titania/electrolyte solution" interface. In the frame of this work, we investigated the mode of interfacial deposition of the Mo(VI) oxo-spccies at the titania/electrolytic solution interface, the Mo(VI) interfacial speciation, and the structure of the deposited Mo(VI) oxo-species. Several methodologies based on potentiometric titrations, microelectrophoretic mobility, and macroscopic adsorption measurements were applied. The deposition model developed describes very well the experimental "proton-ion" linear curves and the "adsorption edges". Moreover, it was verified by laser Raman spectroscopy. At Mo(VI) solution concentration up to 3 x 10(-2) M and in the pH range 9-5, the mounted Mo(VI) is practically deposited as monomer MoO42- species in two configurations: an inner sphere mononuclear monosubstituted complex with the terminal surface oxygen atoms of titania [TiOMoO3](0.35-) and a surface species where the MoO42- ions are retained above a bridging surface hydroxyl through a hydrogen bond [Ti2OH center dot center dot center dot O-MoO3](1.57-). In both configurations, the Mo atom is situated between the surface plane and plane 1, whereas the solution oriented oxygen atoms are situated at plane 1 of the compact layer of the interface. The concentration of the [Ti2OH center dot center dot center dot O-MoO3](1.57-) increases with pH, while the concentration of the [TiOMoO3](0.35-) decreases. Thus, at pH > 8, the [Ti2OH center dot center dot center dot O-MoO3](1.57-) predominates, whereas at pH < 5.5 the [TiOMoO3](0.35-) is the most important species. In the pH range 5-4 and for the maximum initial Mo(VI) solution concentration, the contribution of the polymer species to the whole deposition process is not negligible. The deposited polymer species, Mo7O246- and HMo7O245-, are adsorbed through electrostatic forces and located in a range extended from plane 1 up to the first layers of the stagnant-diffuse layer being close to plane 2 of the interface. The adsorption sites involve five bridging and five terminal neighboring (hydr)oxo-groups. A preferential deposition of the monomers, MoO42-, with respect to the polymer ones was generally found. The above findings could prove useful for controlling the impregnation-equilibration step involved in the preparation of the molybdenum supported titania catalysts by equilibrium deposition filtration.
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: 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: The structure of the precursor Co(II) and N(II) aqua complexes and the Cr(VI), Mo(VI) and W(VI) monomer and polymer oxo-species formed upon impregnation at the interface developed between the surface of the titania grains and the impregnating solution was thoroughly elucidated. Moreover, the interfacial speciation was determined for various surface concentrations of the precursor species regulated by adjusting the corresponding solution concentrations and the pH of the impregnating solution.
Abstract: The interfacial chemistry of the impregnation step involved in the synthesis of cobalt catalysts supported on titania was investigated with regard to the mode of interfacial deposition of the aqua complex [Co(H2O)(6)](2+) on the "titania/electrolyte solution" interface, the structure of the inner-sphere complexes formed, and their relative interfacial concentrations. Several methodologies based on the application of deposition experiments and electrochemical techniques were used in conjunction with diffuse-reflectance spectroscopy and EPR spectroscopy. These suggested the formation of mononuclear/oligonuclear inner-sphere complexes on deposition of the [Co-(H2O)(6)](2+) ions at the "titania/electrolyte solution" interface. The joint application of semiempirical quantum-mechanical calculations, stereochemical considerations, and modeling of the deposition data revealed the exact structure of these complexes and allowed their relative concentrations at various Coll surface concentrations to be determined. It was found that the interface speciation depends on the Coll surface concentration. Mononuclear complexes are formed at the compact layer of the "titania/electrolyte solution" interface for low and medium Coll surface concentrations. Formation of mono-hydrolyzed Ti2O-TiO and the dihydrolyzed TiO-TiO disubstituted configurations is very probable. In the first configuration one water ligand of the [Co(H2O)(6)](2+) ion is substituted by a bridging surface oxygen atom and another by a terminal surface oxygen atom. In the second configuration two water ligands of the [Co(H2O)(6)](2+) ion are substituted by two terminal surface oxygen atoms. Binuclear and trinuclear inner-sphere complexes are formed, in addition to the mononuclear ones, at relatively high Coll surface concentrations.
Abstract: In the present work we have investigated the influence of the preparation method on the physicochemical and activity features of CoOx/gamma-Al2O3 catalysts used for the combustion of anode tail gas produced in a proton-exchange membrane (PEM) fuel cell. The catalysts prepared contain 21% (w/w) Co and have been calcined at 850 degrees C. Three different impregnation methodologies have been followed: incipient wetness impregnation (IWI) using a cobalt nitrate aqueous solution, incipient wetness impregnation using a mixed cobalt nitrate-nitrilotriacetic acid (IWI-nta) aqueous solution and equilibrium deposition filtration (EDF) using a cobalt nitrate aqueous solution. The catalysts were characterized using nitrogen adsorption for determining the specific surface area, the pore volume and the mean pore diameter as well as using X-ray powder diffraction (XRD), diffuse reflectance spectroscopy (DRS; UV-vis), LRS, X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR). Catalytic activity measurements were performed in the temperature range 250-850 degrees C using a continuous flow fixed-bed micro-reactor working under atmospheric pressure and fed with a reaction mixture consisted of 15% H-2/3% CO/1.1% CH4/20% O-2 balanced in He. The EDF methodology imposed interfacial deposition and resulted to the formation of an almost bi-dimensional surface precipitate. Upon calcination, this surface precipitate provided a very well-dispersed CoOx amorphous species strongly interacted with the support surface and thus hardly reducible as well as relatively small Co3O4 Supported nanocrystals (14.3 nm). The first phase is the predominant one. Therefore, EDF resulted to a catalyst with the highest cobalt surface and specific surface area. The conventional IWI imposed bulk (solution) precipitation and thus induced relatively large supported crystallites which upon calcination provided relatively large supported Co3O4 nanocrystals (19.8 nm) and CoAl2O4 as well. The formation of the relatively large nanocrystals and the insertion of cobalt inside the gamma-alumina lattice to form CoAl2O4 may be responsible for the lowest cobalt surface obtained in this catalyst. The presence of the nitrilotriacetic acid in the impregnating solution induced the exchange of the water ligands of the Co(II) aqua complex present in the cobalt nitrate solution with organic ligands and thus the bulk precipitation of an organometallic complex. The more bulky organic ligands decreased the cobalt-support interactions. Thus, the insertion of cobalt inside the gamma-alumina lattice and the formation of CoAl2O4 are inhibited upon calcination of the IWI-nta sample. This may be responsible for the relatively higher cobalt surface obtained with respect to that achieved in the IWI sample though the size of the Co3O4 nanocrystals is larger in the IWI-nta sample (25.8 nm). At relatively high reaction temperatures all catalysts exhibited almost the same activity for oxidation reactions. In contrast, at low reaction temperatures the EDF catalyst proved to be more active for the CH4 production as well as for the oxidation of H-2 and CO. This behaviour may be attributed to the favourable physicochemical characteristics of this catalyst. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: An integrated work concerning the interfacial chemistry of the impregnation step involved in the preparation of tungsten(VI) supported titania catalysts is presented. Specifically, we investigated the mode of interfacial deposition of the W(VI) oxo-species at the "titania/impregnation solution" interface, the W(VI) interfacial speciation and the structure of the deposited species. Several methodologies based on potentiometric titrations, microelectrophoresis and macroscopic adsorption measurements have been used in conjunction with laser Raman spectroscopy and a modeling of the interfacial deposition process. The modeling was based on a recently established ionization model for the titania surface and a modern picture for the "titania/electrolytic solution" interface in the absence of the W(VI) oxo-species. It was found that the monomers, WO42-, are exclusively deposited at the interface at relatively low W(VI) concentrations of the impregnation solution (<= 10(-3) M) and over the whole pH range studied (9-4). Three different monomers are formed: an inner sphere mono-substituted complex with the terminal surface oxygen atoms of titania (predominant species), a surface species where the WO42- ions are retained above a bridging surface hydroxyl through a hydrogen bond and an inner sphere di-substituted complex with two terminal surface oxygen atoms of titania. Their relative surface concentration depends strongly on the pH of the impregnation solution. At relatively high W(VI) concentrations of the impregnation solution (>10(-3) M) the polymers W7O246-, HW7O245- and H2W12O1042- are deposited, in addition, in the pH range 7-4. These species are adsorbed through electrostatic forces on adsorption sites that involve 5-7 bridging and 5-7 terminal neighboring (hydr)oxo groups. It was generally found a preferential deposition of the monomers, WO42-, with respect to the polymer ones. The mode of interfacial deposition, the interfacial speciation and the structure of the deposited W(VI) oxo-species, would be very useful for a tailor made preparation of the tungsten supported titania catalysts. (C) 2009 Elsevier Inc. 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: Mesoporous anatase was prepared following sol-gel and using urea as template. The influence of calcination temperature on the phase stability, nanocrystal/aggregate size, pore size distribution and specific surface area as well as on the acid-base behavior in aqueous solutions was studied using X-ray diffraction, laser-Raman and diffuse reflectance spectroscopies, scanning electron microscopy and laser scattering as well as N-2 adsorption-desorption isotherms and potentionnetric mass titrations. The crystal structure was kept constant upon calcination over the whole temperature range, 200-500 degrees C. In this range anatase is constituted from primary nanocrystals. These are assembled into larger, rather spherical, clusters of about 30-40 nm and then into aggregates of various sizes (0.2-0.3 mu m and 2-100 mu m) with a distribution centered at about 12 mu m. Increase of the calcination temperature caused an increase in the size of the primary nanocrystals from 8.1 nm at 200 degrees C to 17.1 nm at 500 degrees C, whereas calcination does not influence the morphology at micro-scale. Moreover, increase of the calcination temperature from 200 degrees C to 500 degrees C brings about a shift in the mean pore diameter from 47 nm to 91 nm accompanied by a decrease in the specific surface area and pore volume. The above effects were related with the aforementioned increase in the size of the primary nanocrystals. The value of pzc and the values of surface charge determined at various pH do not practically depend on the calcination temperature. The absence of pore space confinement effects was explained in terms of the structure and size of the interface development between the anatase surface and the electrolytic solution. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: In this article the "titanium oxide/electrolyte solution" interface is studied by taking in advantage the recent developments in the field of Surface and Interface Chemistry relevant to this oxide. Ab-initio calculations were performed in the frame of the DFT theory for estimating the charge of the titanium and oxygen atoms exposed on the anatase (1 0 1), (1 0 0), (0 0 1), (10 3)(f) and rutile (1 1 0) crystal faces. These orientations have smaller surface energy with respect to other ones and thus it is more probable to be the real terminations of the anatase and rutile nanocrystallites in the titania polycrystalline powders. Potentiometric titrations for obtaining "fine structured" titration curves as well as microelectrophoresis and streaming potential measurements have been performed. On the basis of ab-initio calculations, and taking into account the relative contribution of each crystal face to the whole surface of the nanocrystals involved in the titania aggregates of a suspension, the three most probable surface ionization models have been derived. These models and the Music model are then tested in conjunction with the "Stern-Gouy-Chapman" and "Basic Stern" electrostatic models. The finally selected surface ionization model (model A) in combination with each one of the two electrostatic models describes very well the protonation/deprotonation behavior of titania. The description is also very good if this model is combined with the Three Plane (TP) model. The application of the "A/(TP)" model allowed mapping the surface (hydr)oxo-groups [TiO(H) and Ti2O(H)] of titania exposed in aqueous solutions. At pH>pzc almost all terminal oxygens [TiO] are non-protonated whereas even at low pH values the non-protonated terminal oxygens predominate. The acid-base behavior of the bridging oxygens [Ti2O] is different. Thus, even at pH = 10 the greater portion of them is protonated. The application of the "A/TP" model in conjunction with potentiometric titrations, microelectrophoresis and streaming potential experiments allowed mapping the "titania/electrolyte solution" interface. It was found that the first (second) charged plane is located on the oxygen atoms of the first (second) water overlayer at a distance of 1.7 (3.4) angstrom from the surface. The region between the surface and the second plane is the compact layer. The region between the second plane and the shear plane is the stagnant diffuse part of the interface, with an ionic strength dependent width, ranging from 20 (0.01 M) up to 4 angstrom (0.3 M). The region between the shear plane and the bulk solution is the mobile diffuse part, with an ionic strength dependent width, ranging from 10 (0.01 M) up to 2 angstrom (0.3 M). At I>0.017 M the mean concentration of the counter ions is higher in the stagnant than in the mobile part of the diffuse layer. For a given I, removal of pH from pzc brings about an increase of the mean concentration in the interfacial region and a displacement of the counter ions from the mobile to the stagnant part of the diffuse layer. The mean concentration of the counter ions in the compact layer is generally lower than the corresponding ones in the stagnant and mobile diffuse layers. The mobility of the counter ions in the stagnant layer decreases as pH draws away from pzc or ionic strength increases. (C) 2008 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: 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: Four supported catalysts with the same tungsten loading were prepared by depositing decatungstate species W10O324-, through wet impregnation, on the surface of gamma-alumina and silica at different pH values. The prepared samples were characterized using BET measurements as well as XRD, UV-vis DR, and XP spectroscopies. Higher dispersion of W(VI) oxo-species was obtained in the silica-supported catalysts compared with the corresponding alumina-supported ones. Within the same support, the dispersion was higher when the impregnation pH is lower than the point of zero charge (pzc) of the support. The decatungstate anions were present mainly on the silica surface without any modification, whereas these underwent a partial depolymerization on their deposition on the gamma-alumina surface. The extent of depolymerization was less in the sample prepared at pH above pzc. These findings were explained in terms of the mode of deposition of the W(VI) species from the solution onto the support surface. The photocatalytic activity of the aforementioned catalysts, concerning the photooxidation of 1-phenylethanol, depends on the fraction of the W10O324- supported species rather than on the W(VI) dispersion. Thus, extremely high conversions have been obtained over the silica-based catalysts and also over the gamma-alumina-based catalyst prepared at relatively high pH. These catalysts also are very effective in the photooxidation of a series of secondary and primary benzyl alcohols, in which benzyl ketones and benzoic acids were formed as the only or major products, respectively. The easy separation of the solid catalyst from the reaction mixture, the high activity, selectivity, and stability as well as the retained activity in subsequent catalytic cycles, make these supported catalysts suitable for a small-scale synthesis. Based on product analysis and kinetic data on the heterogeneous oxidation of benzyl alcohols, we suggest that a hydrogen abstraction transfer (HAT) mechanism predominates with respect to an electron transfer (ET) one in these reactions. (C) 2007 Elsevier Inc. 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: The mechanism of the protonation of solid metal (hydr)oxides in aqueous media was investigated using simulation and experimental work. It was found that the apparent acidity/basicity of each kind of surface sites of metal (hydr)oxides in aqueous suspensions is strongly influenced by the overall surface charge of the (hydr)oxide and thus by the electrical potential smeared out at the interfacial region. Depending on its sign this increases or decreases the hydrogen ion concentration on the surface, thus promoting or hindering protonation. This is manifested by the shifts of the protonation peaks of the various kinds of sites with respect to the - pK values of the corresponding intrinsic protonation constants and the appearance of an extra peak in the d[H-cons,surf(+)]/dpH vs. pH curves. Potentiometric titrations experiments performed for four technologically important oxides showed that the proposed protonation mechanism describes indeed the protonation of polycrystalline (hydr)oxides in aqueous media. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: In situ Raman spectroscopy has been used at temperatures LIP to 400 degrees C under O-2. NH3/N-2, H-2/N-2), NH3/NO/O-2/N-2, O-2/H2O/N-2), and SO2/O-2/N-2 for Studying tile influence of these gases oil the molecular Structure Of V2O5/TiO2 catalysts with V surface density, it, in the range 2.5-18.7 VOx/nm(2). The catalyst activities for the SCR of NO by NH3 have been determined to derive structure-activity relationships in combination with the Raman data. Isolated monovanadates and polyvanadates are formed at various proportions (depending oil the loading) oil the catalyst Surface Under dehydrated conditions. The hand positions and characteristics are discussed in terms of possible configurations for the dispersed VOx, species. The bands observed, the surface composition, and the bond conservation rule allow to propose a small size for the V-O-V chains of polyvanadates (i.e., 2, 3). The reducing action of NH3 is favored in the presence of adjacent V sites; at low loadings. the presence of NH3 has not effect on the structural properties of surface VOx. The reducibility in H-2 follows an oppostite trend and is favored at low n(s), as indicated by both effect oil the structural properties of surface VOx. The reducibility in H-2 follows all opposite trend and is favored at low it, ill Situ Raman and H-2-TPR. The SO,) presence affects only the molecular Structure of catalysts with low n(s). for which a significant part of surface TiO2,) sites are vacant; the effect (judged from the ill Situ Raman data) is merely one of driving the dispersed vanadia species in a state of"virtually" high Surface density by crowding them together, thereby providing more adjacent V sites for activation of NH3 in SCR reaction conditions. The NO TOF Values initially increase with increasing its, suggesting that the number of active sites per V atom increases with increasing its below s or at conditions of "virtually" high n(s) (in the presence of monolayer. The formation of adjacent V-O-Ti sites is favored either at increasing n(s), or at conditions of "virtually" high n(s) (in the presence of SO2). The increase in the number of such centers per V atom correlates with the increase in TOF values with increasing its. (c) 2006 Elsevier Inc. All rights reserved.
Abstract: The mechanism of the protonation of solid metal (hydr)oxides in aqueous media, which is closely interrelated to many processes of great technological and environmental importance, has been elucidated using simulation and experimental work. The electrical potential, smeared out at the interfacial region, changes the concentration of the H+ ions on the surface of the (hydr)oxide, thus promoting or hindering protonation. This is manifested by the shifts of the protonation peaks of the various kinds of surface sites and the appearance of an extra peak in the differential potentiometric, titration curve. (c) 2005 Elsevier Inc. All rights reserved.
Abstract: The contribution of the Interface Science to the preparation of supported catalysts during the last two decades is presented. It is illustrated how the concepts and the methodologies of the Interface Science could be effectively used for an in-depth understanding of the phenomena involved in the initial preparation step. This, extremely critical step, concerns the deposition of transition metal species containing the active elements, from an impregnation solution, onto the surface of common catalytic supports. Moreover, the aforementioned concepts and methodologies allow the regulation of the mode of interfacial deposition and the local structure of the deposited species and, thus, the surface characteristics and the catalytic behaviour of the resulted catalysts.
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 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: 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 &AP; 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 &DEG; 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: A new methodology is presented, called differential potentiometric titration (DPT),which allows the determination of the point of zero charge (pzc) of metal (hydr)oxides using only one potentiometric curve. By performing extensive simulations of potentiometric titrations for various model (hydr)oxides, we found that an inflection point in a H-cons,surf(+) versus pH potentiometric curve (H+(cons,surf): hydrogen ions consumed on the surface of the (hydr)oxide) and a peak in the corresponding differential curve, dH(cons,surf)(+)/dpH versus pH, appear at a pH equal to the pzc assumed for a model (hydr)oxide. This distinguishable peak appears at the same position irrespective of the surface ionization and the interfacial model adopted as well as the assumed ionic strength. It was found that the aforementioned peak also appears in the high-resolution differential potentiometric curves experimentally determined for four oxides (SiO2, TiO2, gamma-Al2O3, and MgO) that are widely used in various environmental and other technological applications. The application of DPT to the above-mentioned oxides provided practically the same pzc values as the corresponding ones achieved by using four different techniques as well as the corresponding isoelectric point (iep) values determined by microelectrophoresis. Differences between the pzc and iep values determined using various techniques in the case of MgO were attributed to the increasing dissolution of this oxide as pH decreases and the adsorption of cations (Mg2+, Na+) on the MgO/electrolytic solution interface.
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: Monolayer catalysts were prepared by supporting transition metal ionic (t.m.i.)-oxo species (VOx, CrOx, MoOx and WOx) on the TiO2 (anatase) surface using the equilibrium-deposition-filtration (EDF) technique. The prepared samples were characterized by N-2 adsorption, UV/Vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and tested for the selective catalytic reduction of NO by NH3 (NH3-SCR) in the temperature range 250-450 degreesC. Using the EDF method the transition metal ion (V, Cr, Mo or W) loading of the supported Titania catalysts achieved at monolayer coverage follows the order V > Mo > W > Cr. It is mainly determined by the deposition of polymeric t.m.i.-oxo species on the surface of titania in the impregnation step. The activity of the corresponding monolayers follows the order V > Cr > Mo > W. It was also demonstrated that the activity of the above catalysts correlates well with the intensity of a DRS absorption band appearing at ca. 400 nm. The intensity of this absorption band, which is considered as a measure of the extent of interactions exerted between the active phase and the support surface, depends on the kind and the amount of the transition metal. (C) 2004 Elsevier B.V. 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: 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: The reduction of benzene from benzene-rich real gasoline fractions has been studied in a high-pressure fixed-bed reactor using a Pt/TiO2 catalyst. It was found that the yield of this process decreases with the toluene content in the feedstock, but it is independent of the kind of the saturated hydrocarbons.
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: Monolayer catalysts containing binary active phases (VOx-CrOx, VOx-MoOx,) were prepared by simultaneous deposition of the corresponding transition metal-oxo species on the TiO2 (anatase) surface using the equilibrium deposition filtration technique. The prepared samples contained various amounts of each transition metal but almost the same total metal loading. They were characterized by atomic absorption spectroscopy, N-2 adsorption, UV-vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and tested for the selective catalytic reduction of NO by NH3 in the temperature range 250-450 degreesC. It was found that the transition-metal ionic species used for the preparation of these catalysts compete for the same surface sites of the TiO2 carrier upon co-deposition. Small amounts of the second phase (Mo- or Cr-oxo phase) are sufficient in order to promote the catalytic activity at relatively high temperatures, in contrast to what happens in the corresponding industrial catalysts prepared by conventional methods. An electronic interaction between V- and Cr-oxo species favored at a V/Cr atomic ratio around 3 is probably responsible for the relatively high catalytic performance of the corresponding TiCrV catalyst. The activity of the studied catalysts is well correlated with the intensity of a DRS absorption band that appeared at ca. 400 nm, which is considered as a measure of the magnitude of interactions exerted between the monolayer transition metal-oxo species and the TiO2 carrier. This correlation is independent of the transition metals combination used and follows the same linear relationship found previously for single-active-phase catalysts.
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: 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: 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: 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: MoO3/TiO2 catalysts of varying molybdenum content were prepared at various pHs and concentrations of the impregnating solution using the equilibrium deposition filtration (EDF) method. Moreover, a catalyst corresponding to the EDF one with the maximum Mo loading was prepared using the conventional non-dry impregnation (NDI) method. The above catalysts were characterized using X-ray powder analysis, diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy, and tested for the selective catalytic reduction of NO by NH3 in the temperature range 250-450degreesC. It was found that the application of EDF results in an improved MoO3/TiO2 catalyst exhibiting higher activity than the corresponding sample prepared by the conventional NDI method. The catalytic activity correlates well with the concentration of the Mo species strongly interacting with the anatase surface. The concentration of the above species is maximized when the EDF method is employed to prepare the catalysts. This is especially, so when low pH and Mo concentration of impregnating solution are used. (C) 2002 Elsevier Science B.V. All rights reserved.
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: In situ Raman spectroscopy was used for studying the ternary 2% CrO3-6% V2O5/TiO2 catalyst, for which a synergistic effect between vanadia and chromia leads to enhanced catalytic performance for the selective catalytic reduction (SCR) of NO with NH3. The structural properties of this catalyst were studied under NH3/NO/O-2/N-2/SO2/H2O atmospheres at temperatures up to 400degreesC and major structural interactions between the surface chromia and vanadia species are observed. The effects of oxygen, ammonia, water vapor and sulfur dioxide presence on the in situ Raman spectra are presented and discussed. (C) 2002 Elsevier Science B.V. All rights reserved.
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: 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.
Abstract: The aim of this study was to investigate the desulfurization of fluid catalytic cracking (FCC) gasoline using NiW supported on gamma -Al2O3 catalysts prepared by conventional non-dry impregnation (NDI) method and a new one, the equilibrium deposition filtration, (EDF). The latter has been modified (MEDF) to produce the required catalyst quantity for its evaluation in a hydrodesulfurization (HDS) pilot plant. The modification of the typical EDF method showed that it is possible to apply the principles of this technique for practical catalyst preparations. In the first part of this work, the catalyst preparation methodology and the catalyst characterization results are presented. In the second part, the catalyst evaluation is described, in relation to a commercial catalyst, concerning their HDS, hydrogenation (HYG) and isomerization (ISO) activities. The main conclusion of this study was that the MEDF method is more promising than the conventional NDI method for preparing NiW hydrotreatment catalysts. The catalyst prepared by MEDF exhibited higher HDS activity than that of the NDI one, comparable to that of a commercial CoMo catalyst and the highest HYG and ISO activities. The enhanced catalytic activity of the NiW-MEDF catalyst was attributed to the high dispersity of the supported phases achieved by the corresponding preparation method. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: Chromia-vanadia on titania catalysts containing 8 mol% (Cr + V) were prepared and tested for the selective catalytic reduction (SCR) of NO by NH3, in excess O-2, in the temperature range 250-450 degrees C. The physicochemical properties of the prepared catalysts as well as those of the corresponding single active phase-containing samples were investigated by specific surface area and pore volume measurements, X-ray powder analysis, UV-visible diffuse reflectance spectroscopy, laser-Raman microscopy, X-ray photoelectron spectroscopy, analytical electron microscopy, and temperature-programmed reduction experiments, The best catalytic performance was achieved by the catalyst containing 2 mol% Cr and 6 mol% V (TiCr2V6), while all the binary active phase-containing catalysts proved to be more active than the corresponding single active phase-containing catalysts, at reaction temperatures higher than 350 degrees C, The enhanced catalytic performance of the binary active phase-containing catalysts is attributed to a synergy developed between the two active phases, This synergy is maximized in the TiCr2V6 sample, where a new Raman band, at 971 cm(-1) has also been observed, This peak is attributed to isolated Cr6+ species decorated by well-dispersed V5+ species. Deposition of the chromia phase on the titania surface stabilizes the texture of the final catalysts against changes provoked by deposition of the vanadia phase. (C) 1999 Academic Press.
Abstract: V2O5/TiO2 catalysts of varying vanadium content were prepared at various pH's and concentrations of the impregnation solution using the equilibrium deposition filtration (EDF) method. Moreover, corresponding catalysts were prepared using the conventional wet impregnation (WI) method. The above catalysts were exhaustively characterized using BET, AEM, FT-IR, DRS, LRM, XPS, TPR, and TPD of NH3 and tested for the SCR of NO by NH3 in the temperature range 250-450 degrees C. It was found that the application of EDF results in V2O5/TiO2 catalysts with relatively high dispersity of the vanadia supported phase, homogeneous distribution of this phase on the support particles, and quite strong interactions between the supported vanadia phase and the surface of titania, and inhibits the formation of supported V2O5 crystallites. The catalysts prepared by EDF exhibited better activity and selectivity than those prepared by conventional WI. Decrease of the impregnation pH in the EDF preparations from 8 to 4.5 caused an increase in the vanadium content from 2.6 to 3.6 wt % V2O5 which, in turn, provoked very important differences in the physicochemical properties of the EDF catalysts (increase of the surface coverage of the titania by the vanadia phase and decrease of the mean value of the vanadia phase-support interactions). The above explained the increase, with the vanadium content, of the activity and selectivity of the V2O5/TiO2 EDF catalysts.
Abstract: A review with 69 refs.; the EDF method for preparing supported catalysts with improved physicochem. and catalytic properties, and a methodol. for elucidating the deposition mechanism are presented. The knowledge of the deposition mechanism, namely the nature of the deposited species formed, their relative concns. and their dependence on the impregnation parameters, permits the tailoring of the synthesis of supported catalysts by EDF, by adjusting the impregnation parameters to their optimal values.
Abstract: Chromia on titania catalysts containing chromium loading ranged from 0 up to 8 mol% Cr have been prepared and calcined at high temperature (500 degrees C). These catalysts have been tested for the selective catalytic reduction of NO by NH3 in excess of O-2 at relatively high reaction temperatures (250-450 degrees C). The physicochemical properties of the prepared samples have been investigated by specific surface area and pore volume measurements, X-ray powder analysis, UV-visible diffuse reflectance spectroscopy, laser-Raman microscopy, X-ray photoelectron spectroscopy, analytical electron microscopy, and temperature programmed reduction experiments. The best catalytic performance has been found for the catalyst containing the lowest amount of active phase (2 mol% Cr). The relatively high catalytic performance of this catalyst has been attributed to the isolated Cr-(vi) surface species strongly interacting with the anatase surface. In fact the physicochemical characterization mentioned above showed that these species are predominant at this low Cr-loading. Increase in the Cr-loading has been found to favor the formation of badly dispersed chromia phases with lower oxidation states. Besides the title reaction, the direct oxidation of NH3 by O-2 and the reaction of NH3 with NO producing N2O take also place at intermediate reaction temperatures (250-350 degrees C). At elevated reaction temperatures (400-450 degrees C) the direct oxidation of NH3 was found to be significant. (C) 1997 Academic Press.
Abstract: The deposition of the V-(v)-oxo species on the anatase/electrolyte solution interface was studied over a wide pH and bulk concentration V-(v) range using deposition experiments, microelectrophoresis, potentiometric titrations, and an advanced calculation procedure, further developed in the present work. It was found that the mechanism of deposition is quite complicated due to the fact that a change in pH or V-(v) bulk concentration causes dramatic changes in the relative concentrations of the V-(v)-oxo species in the bulk solution and thus of the corresponding concentrations in the ''TiO2/electrolyte solution interface''. In fact, 12 equilibria are needed in order to describe the title deposition over the pH range studied. However, a preference was found for the deposition of the monomeric V-(v) species (H2VO4 (-) and HVO42-) with respect to the deposition of the polymeric V-(v) species (V3O93-, V4O124-, V10O274-, HV10O285-) Howover, a preference was found for deposition through adsorption (reaction) at relatively low (high) pH values on sites created by single TiOH2+ (TiOH) groups. Significant deposition has not been observed on two adjacent TiOH2+ groups or TiOH2+-O-TiOH groups. The above were quantitatively explained on the basis of an equation derived to describe the deposition. The present work was based on the two-pK/one-site and triple-layer models.
Abstract: Results concerning the catalytic activity of catalysts prepared by Equilibrium Deposition Filtration (EDF) and corresponding catalysts prepared by conventional impregnation techniques are reported. These results show that the EDF and co-EDF methodologies may be used for preparing very active catalysts even with quite high loading.
Abstract: A general methodology for investigating the mechanisms of deposition of ionic species containing catalytically active elements on the oxidic support/electrolytic solution interfaces has been developed. The methodology is based on the ''two-pK/one-site'' and ''triple-layer'' models (for the charging mechanism of the surface of the support, and for the electrical double layer developed between the surface of the support, and the impregnating solution, respectively) and exploits the experimental data of the adsorption isotherms at various pH of the impregnation suspension. Starting from a quite general, ''postulated'', deposition mechanism, comprising all equilibria that could possibly take place at the ''support/impregnation solution'' interface, the application of this methodology leads to the ''proposed'' deposition mechanism, namely, the set of equilibria that actually take place, the kind of deposited species formed, their relative concentrations, and their dependence from the impregnating parameters (pH, concentration of the precursor, ionic strength, and temperature). The derived equations, describing the deposition of a given ionic species on the support surface, show that the concentration of the deposited species depends on various factors among which the most important are the charge of the ionic species, its concentration in the bulk impregnating solution, the surface concentration of the receptor sites on the support surface, the pH, and the Galvani and Volta potentials developed, respectively, on the surface of the solid particles and at the inner Helmholtz plane. As a test for the validity of the proposed deposition model, the theoretical values of some ''support/electrolyte interface'' parameters (calculated on the basis of the deposition mechanism revealed by applying this methodology) are compared with the corresponding experimental values determined by potentiometric titrations and microelectrophoresis.
Abstract: A series of W-(vi)/gamma-Al2O3 catalysts of varying W-(vi) content in the range 3.3-21.2% WO3 was prepared at various pH's using equilibrium deposition filtration (EDF). These catalysts were then characterized using various physicochemical techniques (BET, XPS, XRD, DRS, NO chemisorption, TPD of NH3, microelectrophoresis, and DTGA) and the surface characteristics achieved were related with the nature of the W-(vi) species deposited on the gamma-alumina surface during preparation. A close relation exists between the nature of the deposited W-(vi) species and the structure and dispersity of the W-(vi)-supported phase: Deposition through adsorption or reaction of the monomeric WO42- species in the pH range 7.4-4.9 resulted in a well-dispersed, presumably monolayered, W((vi)) phase relatively rich in tetrahedral W-(vi). In contrast, deposition through adsorption of the polymeric HW6O20(OH)(2)(5-) species at pH's 4.0 and 3.5 resulted in a supported phase with relatively low dispersity and a relatively small amount of tetrahedral W-(vi). Supported WO3 crystallites with size greater than 40 nm are formed in this case. Moreover, it was inferred that the mode of deposition of the monomeric WO42- species is reflected in the properties of the supported W-(vi) phase of the calcined samples. Thus, TPR and NO chemisorption data strongly suggested that the change in the deposition mechanism (upon decreasing pH from 6.1 to 4.9) from deposition through surface reaction of the monomeric WO42- species with neutral hydroxyls of the support into deposition through adsorption of these species on protonated surface hydroxyls of the support brought about a decrease in the ''supported phase-carrier'' interactions as well as an increase in the surface of the supported W-(vi). Three catalysts with W-(vi) loading corresponding to that of the three EDF catalysts (3.3, 11, and 21.2% WO3/gamma-Al2O3) were also prepared using the usual methodology of nondry impregnation (NDI) and characterized using the aforementioned techniques. It was found that NDI resulted in a supported phase with lower dispersity and less rich in tetrahedral W-(vi) species than EDF Moreover, catalytic tests, using the hydrodesulfurization of thiophene and the hydrogenation of cyclohexene as probe reactions, showed that EDF provides more active catalysts than NDI and this is another example showing that EDF with controlled regulation of the impregnation parameters is a promising methodology for preparing supported catalysts even with relatively low loadings. Finally, it was found that EDF provides more acidic samples than NDI. (C) 1996 Academic Press, Inc.
Abstract: Vanadium oxide supported on titania catalysts prepared by the ''Equilibrium Deposition followed by Filtration'' method (EDF) and the conventional Wet-impregnation method (WI) were extensively characterized by the joint use of various physicochemical techniques (BET, XRD, FT-IR, DRS, XPS, AEM, TPR, and TPD of NH3) and were tested for the partial gas-phase oxidation of o-xylene. It was found that the EDF method leads to more selective monolayer catalysts. The increased selectivity has been attributed to the reduction of the surface acidity, to the enhancement of the V(V)/V(IV) ratio and to the redox properties induced in this catalyst by the above preparation method.
Abstract: Samples of anatase, prepared using different methodologies, and an industrial sample were characterized using X-ray photoelectron spectroscopy, X-ray diffraction analysis, electron microscopy, diffuse reflectance spectroscopy, temperature-programmed desorption of ammonia, nitrogen adsorption, microelectrophoresis and potentiometric titrations. It was found that anatase prepared by hydrolysis of the titanium isopropoxide exhibited the largest specific surface area, the highest Lewis acidity and the highest concentration of the protonated plus neutral surface hydroxyl groups. These hydroxyl groups are considered to be the deposition sites for the VO3- ions in the preparation of the V2O5/anatase catalysts by equilibrium deposition-filtration. The above findings as well as the absence of foreign ions on the surface of the anatase prepared by the hydrolysis of titanium isopropoxide renders this material the most suitable for preparing the above catalysts used in NO reduction.
Abstract: The deposition of the negatively charged Cr(VI) species on TiO2 and SiO2 surfaces suspended in aqueous electrolyte media has been studied in the pH range 3.0-8.0. The investigation included deposition isotherm and microelectrophoretic mobility measurements. From the experimental results and a theoretical analysis of the deposition isotherms, it was concluded that the CrxOyz- ions taken up were located in energetically equivalent sites at the inner Helmholtz plane (IHP) of the electric double layer that had developed between the support particles and the impregnating solution. The deposition sites were mainly created by protonated surface hydroxyl groups in the case of SiO2 and by protonated and undissociated surface hydroxyl groups in the case of TiO2. The lateral interactions operative among the deposited Cr(VI) species were, in general, strong, being stronger in SiO2 supports in comparison with those of TiO2. Moreover, it was found that upon a pH decrease from 6.0 to 3.0, the extent of deposition of the Cr(VI) species on silica increased from 0.01 to 0.13 mu mol Cr(VI)m(-2). The extent of deposition of the Cr(VI) species on titania increased also from 0.23 to 1.68 mu mol Cr(VI)m(-2) with a pH decrease from 8.0 to 4.8. Further decrease in the pH in the case of titania resulted in a considerable decrease in the extent of deposition.
Abstract: The influence of vanadium loading and molybdenum presence on the catalytic performance of vanadia-titania (anatase) catalysts for the selective oxidation of toluene was investigated, Two series of V2O5/TiO2 and V2O5-MoO3/TiO2 catalysts were prepared. In the first series the loading of vanadium varied from 0 to 8 mole %, whereas in the second the atomic ratio V/(V+Mo) varied from 0 to 1 while the total loading of active elements (V+Mo) was kept constant and equal to 8 mole %. The samples were characterized by XPS, TPR, XRD and BET measurements. It was found that the activity for the oxidation of toluene and the selectivity for side chain partial oxidation products (benzaldehyde and benzoic acid) exhibited by V2O5/TiO2 catalysts increased with the vanadium loading up to monolayer coverage. This increase was attributed to the parallel increase of the surface concentration of easily reducible isolated vanadium species interacting with the anatase surface. When the vanadium content increased above monolayer coverage both activity and selectivity decreased. V2O5-MoO3/TiO2 catalysts were found to be less active and selective than the corresponding V2O5/TiO2 ones. Molybdenum species supported on anatase were less active for this reaction than vanadium and, in addition, the presence of molybdenum inhibited the interaction between vanadium and anatase leading to a poor vanadium dispersion.
Abstract: The mechanism of deposition, from aqueous suspensions, of the Cr(VI) species on the surface of gamma-alumina used as support for preparing chromia supported catalysts has been investigated in the pH range 7.5-4.0. The joint use of adsorption experiments, potentiometric titrations, and microelectrophoresis allowed the proposition of a tentative deposition model. This model is then tested. The testing involved the derivation of various equations describing the deposition equilibria, the calculation (using an interactive code for calculating chemical equilibria) of the amount of the deposited Cr(VI) species, the calculation of the difference in the isotherms of the hydrogen adsorption in presence and absence of the Cr(VI) species, the calculation of the zeta-potential of the gamma-alumina particles in the Cr(VI) solutions, and the comparison of these parameters with the corresponding ones determined experimentally. According to the established model, the deposition of the CrO42-, HCrO4-, and Cr2O72- ions (being in the impregnating suspension) on the surface of gamma-Al2O3 occurs by adsorption of these ions on sites created by the protonated surface hydroxyls of the support on the inner Helmholtz plane (IHP) of the double layer developed between the support particles and the impregnating solution, as well as by reaction of the Cr(VI) species with the neutral surface hydroxyls of the support. In the pH range 7.5-6.1 the deposition by reaction prevails whereas in the pH range 5.7-4.0 both reaction and adsorption participate considerably to the whole deposition. Moreover, the decrease in the pH decreases the relative amount of the deposited CrO42- ions. The above were explained in terms of the variation, caused by the pH change, of the relative concentration of the CrO42-, Cr2O72-, and HCrO4- ions on the IHP and of the ratio [neutral surface hydroxyls]/[protonated surface hydroxyls]. Decrease in pH brought about an increase in the concentration of the neutral plus protonated surface hydroxyls and thus in the extent of deposition. It was found that, considerable lateral interactions are exerted between the adsorbed species. The progressive diminution of the mean basicity of the neutral surface hydroxyls as pH decreases explains the similar trend observed for the equilibrium constants related with the reaction of the aforementioned Cr(VI) species with the neutral surface hydroxyls. Finally, the random change, with pH, of the equilibrium constants related to the adsorption was attributed to the different trends followed by the Galvani potential on the surface and the Volta potential at the IHP.
Abstract: Three different MoO3, samples have been prepared by spontaneous precipitation from (NH4)6Mo7O24 and Na2MoO4 aqueous solutions at pH 2.30 and 1.65. The supersaturated solutions were stable over the entire concentration range examined for pH values over 4.0 and below 1.50. The spontaneous precipitation process showed a high apparent order (p = 3) typical for a surface-controlled mechanism. The solution pH and the nature of the counter-ions influenced the size, texture and symmetry of Mo in the precipitated oxide. Catalytic tests over MoO3 Samples prepared by spontaneous precipitation followed by sulfidation, were performed using the hydrosulfurization of thiophene as a probe reaction. The tests showed that the activity increased with the ratio Mo (tetrahedral) : Mo (octahedral). The maximum value of this ratio was obtained for the sample prepared by ammonium heptamolybdate at pH 2.30.
Abstract: The development of a novel, simple technique allowed us to investigate the influence of various impregnation parameters on the axial Mo profiles achieved on γ-alumina extrudates. The effects of pH, temperature, volume, and concentration of the molybdate solutions, as well as the impregnation time, the rate of drying, the modification of γ-alumina with Fâ ions, and the nature and concentration of various competitors, have been systematically studied. It was found that decreases in pH as well as increases in the concentration of the molybdate solutions, of the impregnation temperature, and of the rate of drying cause a progressive transformation of the Mo profile from uniform to eggshell type. Doping of γ-alumina with Fâ ions and the use of NH4F, H3PO4, and citric acid as competitors transformed the Mo profiles from eggshell to uniform. The change in volume of the impregnating solution had no effect on the Mo profiles. The effect of the impregnation time was found to be complicated. Increases in the impregnation time, until a critical value sufficient for the complete imbibition of the extrudates, allow for the transfer of great amounts of molybdate to the interior of the extrudates, leading to more uniform profiles. Further increases in the impregnation time caused a considerable increase in the sharpness of the Mo profile. Most of the above observations were explained on the basis of derived equations, adopting a very simple macrodistribution model. Finally, it was demonstrated that small chromatographic columns filled with powder supports may be used to study active ion profiles on catalytic supports.
Abstract: The main goal of this work is to prepare, characterize, and determine the catalytic properties of molybdena supported γ-alumina extrudates with four Mo profiles. Concerning preparation, a procedure was established allowing us to prepare (both axially and radially) egg-shell, egg-white, egg-yolk, and uniform profiles with the same, relatively high, Mo loading. The preparation of the egg-shell and uniform profiles was achieved by impregnating γ-alumina extrudates with acidic and alkaline ammonium heptamolybdate solution, respectively. Moreover, it was found that an egg-white (egg-yolk) profile may be achieved from an egg-shell (uniform) profile by successive pore volume or simple nondry impregnations of the Mo-supported extrudates with NH4OH aqueous solutions. A qualitative mechanism, based on the relative rates of desorption and diffusion of the MoO42â ions occuring in the NH4OH impregnation, was developed to interpret the influence of desorption time, concentration of NH4OH solution, drying, and mode of impregnation on the characteristics of the egg-white and egg-yolk profiles obtained. The physicochemical characterization of the Mo/γ-alumina extrudates for which radial profiles have been achieved, done by using a number of techniques, showed a progressive increase of the âMo supported-supportâ interactions and Mo dispersion following the order egg-yolk < uniform < egg-white < egg-shell. A similar trend (uniform < egg-yolk < egg-white < egg shell) was obtained for the number of active sites as estimated by the amount of the adsorbed NO. The trends mentioned above were related with the values of the preparative parameters used for obtaining each profile. Kinetic experiments were done, using the hds of thiophene as a probe reaction, over the characterized samples being in the form of extrudates and powders produced by crushing the extrudates. It was found that hds activity was mainly determined by the number and the quality of active centers and not by the type of Mo profile, though these suggested the presence of no significant diffusional effects. The relative yield of butane, produced by the hydrogenation of the unsaturated hydrocarbons formed during hds, increased with the distance of the maximum of the Mo profile from the periphery of the extrudate to its center. Finally, it is important to note that the most active radial profile proved to be the egg-white for hds and the egg-yolk for hydrogenation.
Abstract: The mechanism of adsorption of tungstates on the surface of gamma-alumina was investigated using adsorption experiments, potentiometric titrations, microelectrophoretic mobility measurements, and electronic spectroscopy. It was found that under our experimental conditions the deposition of W(VI) ions takes place via adsorption of the tungstate ions on energetically equivalent sites of the inner Helmholtz plane (IHP) of the electrical double layer developed between the impregnating solution and the surface of the support. The protonated and neutral surface hydroxyls of the support and not the deprotonated hydroxyls should be mainly responsible for the creation of the adsorption sites. Moreover, between the adsorbed tungstate ions considerable lateral interactions are exerted. Finally, it was inferred that the adsorption constant of monomeric (WO42-) or oligomeric (HxWyOzn- where 0 < x < 5, 1 < y < 6, 4 < z < 22) tungstates should be higher compared to the adsorption constants of polymeric tungstates. The application of the "Stem-Langmuir-Fowler" equation to our experimental results allowed for the determination of the adsorption constant, the energy of lateral interaction, and the surface concentration of tungstate ions, corresponding to the saturation of the surface, at various pH values. The value of the latter parameter was found to increase with decreasing pH. The experimental results of this work showed that in the pH range between 3.0 and 4.0 supported gamma-alumina catalysts with extremely high W6+ content may be prepared by adsorption.
Abstract: The mechanism of the adsorption of molybdates on the titania surface has been investigated using adsorption equilibrium experiments, potentiometric titrations, and microelectrophoretic mobility measurements. Comparison of adsorption data with the surface charge of titania, regulated by changing the pH of the impregnating solution, demonstrated that the groups responsible for the creation of the adsorption sites are mainly the protonated surface hydroxyls of titania in addition to the neutral sites. Moreover, the results obtained by the combination of potentiometric titrations and microelectrophoretic mobility measurements, and the variation of pH before and after adsorption with the Mo(vI) concentration suggested that the MoO22â ions are adsorbed on the Inner Helmholtz Plane (IHP) of the electrical double layer, which is developed between the surface of the titania particles and the impregnating solution. Finally, from the analysis of the isotherms obtained it was concluded that the adsorbed MoO2â ions are located on energetically equivalent sites of the IHP and that relatively weak lateral interactions are exerted. On the basis of the abovementioned menhanism, an explanation of the dependence of the sorptive capacity of titania on the pH of the impregnating solution is provided. The combined use of NO and CO2 adsorption, as well as XPS, RAMAN spectroscopy, and temperature programmed reduction measurements, showed that at pH 7.3, corresponding to about 2 wt% MoO3, the titania surfaceis completely covered and the active surface reaches its maximum value. At pH values lower than 7.3, a second molybdena layer starts to form until the Mo03 content reaches the 5.4 wt% MoO3, a point at which Mo03 crystallites are formed.
Abstract: The effects of calcination temperature on structural and textural characteristics of two Greek bauxites were examined by different physicochemical techniques. Although the above minerals exhibited lower activity than a commercial CoMo/Al2O3 catalyst, they were evaluated as possible hydrotreating catalysts.
Abstract: Influence of fluorination on the HDS activity of unsupported MoS2 phase has been studied and correlated with the influence of fluoride on the CoMo and NiMo catalysts supported on gamma-Al2O3.
Abstract: The degree of conversion of the hydrodeoxygenation (HDO) of tetrahydrofuran and the hydrocracking of 2,4,4-trimethyl-1-pentene (HCG), used as probe reactions, was determined over the surface of the Ni-Mo/gamma-Al2O3 catalysts modified with various amounts of fluoride ions. Concerning HDO it was found that doping with small amount of fluoride ions (0.3-0.8% fluoride) increases the degree of conversion whereas further increase in the dopant concentration causes a drop in the conversion. Comparison of the graphs of degrees of conversion vs. fluoride content with the graph of nitric oxide uptake vs. fluoride content resulted in the conclusion that fluorination changes the HDO activity mainly by altering the number of active sites. The dependence of the apparent rate constant determined in the present work on the dopant concentration showed that fluorination also changes the activity of each active site. Concerning HCG no change in the catalytic activity was observed upon fluorination. Finally, based on results reported previously for the hydrodesulfurization (HDS) of thiophene it was concluded that, as in the case of HDO, fluorination modifies the HDS activity by altering both the number of active sites and the activity of each site.
Abstract: The point of zero charge, p.z.c.T, and surface charge, Ï, of titania over a wide pH range have been determined at various temperatures by potentiometric titration. It was found that a progressive increase in p.z.c.T can be achieved by increasing the temperature of aqueous suspensions from 10 to 45 °C. Therefore, an extension in the pH range in which titania can adsorb negative (positive) species is expected by increasing (decreasing) the impregnation temperature. The fact that the p.z.c.Tvs. 1/T curve does not obey the van't Hoff law, in disagreement with the literature, was explained considering two mechanisms for the charging of the titania surface, one for each phase (rutile and anatase). Morever, it was found that an increase (decrease) in the positive (negative) surface charge, proportional to the sites for adsorption of negative (positive) species, can be achieved by increasing the impregnation temperature.
Abstract: The mechanism of adsorption of molybdates on the surface of γ-alumina was investigated by use of adsorption equilibrium experiments, potentiometric titrations, microelectrophoresis and UV/VIS spectroscopy. From the experimental results and the theoretical analysis of the isotherms it was concluded that MoVI deposition was done by adsorption of the MoxOyzâ ions on energetically equivalent and distinct sites of the inner Helmholtz plane (IHP) of the double layer on the surface of the γ-alumina particles suspended in the aqueous medium. The creation of these sites was mainly attributed to the presence of the protonated surface hydroxyls of γ-alumina and not to the neutral hydroxyls. Analysis of the adsorption isotherms suggested that considerable lateral interactions are exerted between the adsorbed MoxOyzâ ions, resulting in the formation of MoxOyzâ ⦠MoxOyzâ ⦠oligomers in the IHP. It is proposed that these polymeric ions are deposited on the surface in the drying step.
Abstract: The kinetics of N2O decomposition on the surface of Fe3+ species supported on pure and Li+-doped Al2O3 have been studied. The supported FeIII species cause a transformation of the rate law from zero order, followed on pure support, to first order. Moreover, it has been concluded that the relatively high activity exhibited by the supported Fe2O3/Al2O3 catalysts compared with the activity of the Fe2O3 and Al2O3 should be attributed to the increase of the specific rate constant caused by the supported FeIII active phase. Finally, it was found that the deactivation observed after Li+ doping should be attributed to both the increase in the oxygen adsorptivity and the decrease in the dispersion of the supported FeIII species caused by the Li+ cations.
Abstract: The deactivation of alumina, with respect to N2O decomposition, brought about by doping with various amounts of Li+ and Fâ ions has been studied. It was demonstrated that the deactivation caused by Li+ is due to the fact that these ions cause a decrease in the specific surface area. On the other hand, it was concluded that the deactivation observed after doping with Fâ ions should be attributed to both the decrease of the specific surface area and the diminution of the surface concentration of active sites.
Abstract: The adsorption of MoxOyzâ species from aqueous solutions on the surface of pure and Na+ or Li+-doped γ-aluminas was studied over a pH range between 3.0 and 8.5 and temperatures ranging from 10 to 55°C. The variations in pH with the concentrations of the impregnating solutions observed and analysis of the isotherms demonstrated that the following findings reported for adsorption on pure γ-alumina at T = 25°C and pH 5 are also valid for adsorption on pure and sodium-doped aluminas performed at various pH values and temperatures: (i) The contribution of precipitation to the deposition is negligible. (ii) The adsorption constant for the MoO42â ions is larger than those for the isopolyanions. (iii) The adsorbed MoxOyzâ ions are located on energetically equivalent but distinct sites of the inner Helmholtz plane, created mainly by the protonated surface hydroxyls. (iv) Lateral interactions are exerted between the adsorbed MoxOyzâ ions resulting in the formation of MoxOyzâ ⦠MoxOzâ oligomers. Moreover, it was demonstrated that the regulation of the concentration of the protonated surface hydroxyls makes it possible to deposit by adsorption very large amounts of MoVI on the γ-alumina surface. Although the sodium doping was found to be the most attractive from the view point of maximization of the extent of adsorption, it may be suggested that it promotes the formation of the catalytically inert sodium molybdate. Therefore the second best method of regulation, namely the change in the impregnating temperature, may prove to be the most convenient in practice.
Abstract: The point of zero charge (p.z.c.) and the isoelectric point (i.e.p.) of a series of Fe2O3/Al2O3 catalysts of varying iron contents, used as a model system, have been determined using potentiometric titrations and microelectrophoretic mobility measurements, respectively. Simple equations allowing the calculation of the percentage surface coverage (p.s.c.) by determing p.z.c. or i.e.p. values have been derived. Although the p.z.c. was found to differ from the i.e.p., both techniques provided similar, although not identical, p.s.c. values. These values correlated very well with X-ray photoelectron spectroscopy results. This fact suggests that both potentiometric titrations and microelectrophoretic mobility measurements can be used to estimate the variation in p.s.c. of a supported-metal oxide catalyst caused by the variation of a preparative parameter. The main advantage of these two methods is the low cost involved. The present work suggests that the combination of the above-mentioned electrochemical techniques could prove extremely useful in investigating the formation of small amounts of non-oxidic supported phases hardly detectable by X-ray spectroscopy. Finally, it has been concluded that three iron-supported phases are formed on the surface of alumina: (i) small amounts of a non-oxidic supported phase (e.g. adsorbed Fe3+ or iron aluminate), (ii) loosely bound α-Fe2O3 and (iii) strongly associated α-Fe2O3.
Abstract: The kinetics of N2O decomposition on a series of specimens prepared by doping γ-Al2O3 with various amounts of Na+ ions has been studied at various temperatures using a flow-bed reactor working under atmospheric pressure. This doping promotes the adsorption of oxygen anions produced from surface decomposition, presumably via the formation of surface species [Na+OâNa+], bringing about a transformation of the rate equation from R=k into R=kbN2OPN2O/b2P2(where bN2O and bO2 are adsorption coefficients and PN2O and PO2 are partial pressures). Moreover, a decrease in catalytic activity, expressed either as fractional conversion or rate of reaction, was observed on increasing the surface coverage C, of γ-Al2O3 with Na+ ions determined by X-ray photoelectron spectroscopy. Specifically, the dependence of the catalytic activity on the surface coverage of γ-Al2O3 is described by the relationship ln (1/R)= 15.4 +(281/K)C(where K is a proportionality constant) and it was concluded that the deactivation observed is due to the promotion of the O2 adsorption caused by the Na+ ions. Finally, the linear dependence of the surface coverage of γ-Al2O3 on the sodium content strongly suggests that the dispersion of the sodium supported species is constant irrespective of the surface concentration of sodium.
Abstract: Deposition of F- ions, after the Ni(II) and Mo(VI) ions on γ-AI203 promotes the activity of (Ni,Mo)/γ-AI203 catalysts for the hydrodesulfurization of thiophene in the temperature range of 275-325 oC. X-ray diffraction and X-ray photoelectron spectra show that the fluorine ions do not form AIF 3 on the γ-Al203 surface.
Abstract: A thermogravimetric study of the decomposition of a Safaniya asphaltene deposited on various catalysts is described. The objective was to discern the influence of various functions of the catalyst surface on the evolution of light hydrocarbons and the formation of residual coke. The asphaltene was deposited, by pore-volume impregnation, from a solution in xylene, on sulphided CoMo catalysts supported on pure or sodium- or fluorine-modified γ-Al2O3. The presence of a catalyst, except fluorine-modified alumina, always diminishes the weight loss compared with that observed with the pure asphaltenes. The acidity of the support favours cracking to light hydrocarbons. Cobalt and molybdenum sulphides increase coke formation in a N2 atmosphere, whereas they favour decomposition to light products under H2/H2S.
Abstract: The dehydrogenation of cyclohexane to benzene on molybdena catalysts supported on γ-Al2O3 modified by various amounts of alkali cations has been studied. Kinetic experiments were carried out on the reduced form of these catalysts using a flow bed reactor. These allowed the determination of the fractional conversions in the temperature range 410-490°. It was found that the conversion increased with time of reduction while one âplateauâ of constant catalytic activity was attained after 5 h. Moreover, it was found that the modification of the y-A1203 causes a decrease in the catalytic activity.
Conductivity measurements obtained at various temperatures allowed the determination of the activation energy of conduction for the catalysts studied. No relation exists between this parameter and kind or content of the modifier. D.R.S. spectra of some catalysts obtained after catalytic tests for 12 h allowed the estimation of the concentration of the supported Mo ions with valence lower than six. Modification by Li+ cations does not practically affect this concentration, whereas doping by the remaining alkali cations inhibits the transformation of Mo(VI) into Mo with valence lower than six. Using Arrhenius law as a criterion the mechanistic scheme of the reaction has been established. On the base of this scheme apparent activation energies were determined for the catalytically active specimens. In our system, it was demonstrated that the apparent activation energy is not an appropriate measure of the catalytic activity.
The mechanism of the reaction was found to be independent of the geometrical and structural characteristics of the molybdenum phase as well as of the semi-conducting properties of the specimens.
The catalytic activity of the catalysts examined depends almost exclusively on the valence of the supported molybdenum. The dispersion of the molybdenum phase does not relate to the catalytic activity. Moreover, no relation exists between the semiconducting properties of the specimens as estimated by the activation energy of conduction and the catalytic activity.
Abstract: Two chromium-doped Raney nickel catalysts have been prepared from Al-Ni-Cr alloys where Cr was introduced in such a way as to substitute for nickel in the Ni2Al3 intermetallic phase. In comparison with the undoped catalysts, the BET and metallic surface areas remain little affected but more aluminium is retained after etching. X.P.S. shows that aluminium and nickel are in the metallic state whereas chromium is completely oxidized. Very large amounts of chromium are found on the surface, which suggests migration of chromium to the surface during leaching. Analytical electron microscopy indicates that the local chromium concentration is not perfectly uniform. Electron probe microanalysis of the precursor alloy reveals the segregation of part of chromium in a separate phase. These results are evaluated in the framework of the available literature data on the microstructure of Raney nickel catalysts.
Abstract: The influence of the iron(III) content and the calcination temperature on the dispersion of supported Fe3+ species on alumina, on the semiconducting properties and on the catalytic activity of Fe2O3/Al2O3 catalysts has been studied, with the decomposition of N2O into N2 into N2 and O2 being used as a probe reaction. Above 250 °C iron(III) is located on the surface of the carrier as α-Fe2O3 and âstrongly associated iron(III)â(denoted by Fe3+âS). The ratio of the amounts of these species remains constant from X= 0.172 to X= 0.615 mmol Fe3+ per g of alumina and then changes favouring Fe3+âS. The transformation of α-Fe2O3 into Fe3+âS is not accompanied by any detectable change in the Fe3+ dispersion but causes an increase in the activation energy of conduction. A rise in the calcination temperature and iron(III) content brings about an increase in the size of supported α-Fe2O3 crystals shown by a decrease in the dispersion of Fe+. It has been demonstrated that α-Fe2O3 and Fe3+âS exhibit similar activity. This increases linearly with the dispersion of the Fe3+ ions. Moreover, no relationship was found to exist between the semiconducting properties of the catalysts estimated by the activation energy of conduction and the catalytic activity. X-ray photoelectron spectroscopy was used to estimate the dispersion of the supported iron(III), conductivity experiments were performed to determine the activation energy of conduction and catalytic tests were carried out to determine catalytic activity.
Abstract: Doped supports were fabricated by dry impregnation of γ-A1203 with various amounts of MNO3 (M+: Li+, Na+, K+, Rb+ and Cs+). A quantity of molybdenum(vi) corresponding to 12 % wt MoO3 was deposited on each modified support.
The influence of the content of the modifier on the specific surface area of γ-A1203 was examined. A diminution in the specific surface area due to the toping has been observed. The magnitude of this effect generally increases with the modifier content. A model taking into account the above observation has been postulated.
The dispersion of the alkali cations on the γ-A1203 surface as well as the dispersion of Mo(vi) on the doped supports were studied using X-ray photoelectron spectroscopy:the dispersion of the alkali cations remains constant within the region 0.392-1.55 mmol of modifier per g of γ-A1203 followed by a considerable decrease at higher concentrations. As to Mo(vi),the variation in the dispersion with the dopant concentration depends on the kind of alkali cation. Thus, Li+ doping causes no considerable effect on the Mo(vi) dispersion. In contrast to that, doping by Na+, K+, Rb+ and Cs+ provokes a linear decrease in the Mo(vi) dispersion up to 1.55 mmol of the modifier per g of γ-A1203. Further increase in the dopant concentration has a quite weak effect on the dispersion. Diffuse reflectance spectroscopy measurements showed that the ratio of the K-M-S functions determined at 305 and65nm,F(Râ)305F(Râ)265,depends on the kind and content of the modifier: Li+ has no effect. The ratio mentioned decreases linearly with the Na+ (in the whole range of concentrations) and K+,Cs+and Rb+)up to 1.55 mmol per g of γ-A1203.
A model postulated to explain the above mentioned X-ray photoelectron spectroscopy and Diffuse reflectance spectroscopy results, suggests the formation of surface alkali molybdates at the expense of the supported polymeric Mo (vi).
Abstract: Modified carriers were prepared by dry impregnation of γ-Al2O3 with various amounts of M(NO3)2 (M2+ â¡ Be2+, Mg2+, Ca2+, Sr2+ and Ba2+). A quantity of molybdenum(VI) corresponding to 12 wt.% MoO3 was deposited on each modified support by dry impregnation of the doped support with aqueous solutions of ammonium paramolybdate. The structures of the doped supports and the active phase were investigated by the combined use of X-ray powder spectroscopy, diffuse reflectance spectroscopy and classical NO3â and NO2â analysis.
In the temperature region 110â400 °C the evolution of the M2+ species on the modified carrier surface follows the scheme
Full-size image (3K)
where Md2+ denotes M2+ diluted in the γ-Al2O3 lattice. The relative speeds of the individual steps are dependent on the kind of alkali earth cation present and the calcination temperature. A marked regulation of the MoVIh-to-MoVIt ratio in the catalysts (MoVIh denotes the polymeric, probably octahedral, form and MoVIt denotes the monomeric tetrahedral form) was obtained by modifying the carrier with various amounts of alkali earth cations. More precisely, this ratio decreased progressively with increasing dopant content in most cases. Moreover, when the dopant concentration was kept constant the ratio decreased from Be2+ to Sr2+ and then increased slightly. A mechanism to explain these phenomena is developed.
Abstract: Five series of modified carriers were prepared by the dry impregnation of γ-Al2O3 with various amounts of MNO3 (M+ ⡠Li +, Na+, K+, Rb+ and Cs+). Each modified carrier was used as a support in the preparation of three MoO3 (6%, 12% and 18%) supported catalysts.
The combined use of X-ray powder spectroscopy, diffuse reflectance spectroscopy and the classical analysis of NO3â and NO2â allows the structure of the modified supports to be investigated as well as the effects of the type of modifier and the modifier concentration on the molybdenum species formed on the carrier surface and on their valence and symmetry.
It was found that a large quantity of MNO3 remained on the carrier surface at temperatures below 400 °C. An increase in the calcination temperature from 400 to 500 °C produced a partial dissociation of MNO3 to MNO2. A further increase in the temperature from 500 to 600°C resulted in the formation of Li2Al2O4 and/or LiAl5O8 and the complete disappearance of LiNO3 and LiNO2. At this temperature some of the other alkali cations were still present on the γ-Al2O3 surface in the form MNO3 and MNO2. A critical alkali concentration must be present before surface crystallization of MNO3 and MNO2 commences.
The symmetry and valence of the molybdenum species can be regulated by changing the doping parameters. At 300 °C all the modifiers inhibited the reduction of molybdenum(VI) to molybdenum(V) and this effect increased with the size of the dopant cation and with the dopant concentration. At 300 and 600 °C, all the alkali cations except Li+ produced a transition from octahedral to tetrahedral coordination symmetry in the molybdenum(VI) species. The magnitude of this effect increased with increasing alkali content. These effects were independent of the MoO3 concentration. A mechanism explaining the doping effects is developed.
