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: Immobilization of [60]fullerene onto gamma-Al2O3 surface provides new heterogeneous photocatalysts for the oxidation of organic Compounds under oxygen atmosphere. These catalysts have been prepared by simple or Successive incipient wetness impregnation (using an organic solvent) followed by air-heating at 180 degrees C. In the C-60/Al2O3 system. C-60 loading was varied in the range of 1-4% (w/w). Several experimental techniques including BET. XRD, DRS, TGA, microelectrophoresis, photoluminescence and kinetic extraction, have been used to characterize these catalytic materials. It was found that the quite high surface exposed by the supported C-60 increases with the amount of the supported C-60, while the dispersion of the supported C-60 decreases. The quite stable supported [60]fullerene phase is comprised from C-60 clusters, small and large aggregates. This non-uniform size distribution is reflected to a non-uniform distribution concerning the 'supported phase-support' interactions. These interactions decrease with the amount of the supported C-60. The photocatalysts prepared may be safely used Lip to 200 degrees C. Above this temperature the supported C-60 is sublimated/combusted in air. The photocatalytic activity of the so-obtained catalytic systems has been evaluated in terms of substrate conversion in the singlet oxygen 'ene' reaction of alkenes. The photooxygenation of 2-methyl-2-heptene has been examined as a probe reaction. It was round that the catalytic activity increases with the increasing amount of the supported C-60 up to the value of 3% (w/w) and then decreases. The intrinsic activity expressed as TON or TOF decreased monotonically with C-60. In all cases, however. the photocatalytic activity of the Al2O3-supported C-60 catalysts was substantially increased compared to the unsupported C-60 precursor, exhibiting quantitative conversion yields after short reaction times. The catalytic behavior was attributed to the aforementioned opposite trends which follow the surface exposed by the supported C-60 on one hand and the 'supported C-60-support' interactions and the C-60 dispersion on the other hand. The easy separation of these solid catalysts from the reaction mixture, the high activity and stability as well as the retained activity in subsequent catalytic cycles. make these supported catalysts suitable for a small-scale synthesis. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Four CoMo catalysts supported on Al2O3-SiO2 mixed materials of varying SiO2 content (1.5, 10, 20 and 30% SiO2 w/w) were prepared following the co-EDF methodology. The catalysts were characterized using various techniques (BET, potentiometric mass titrations, XRD, DRS, XPS, LRS, TPR, TPD of ammonia and NO chemisorption). Two additional catalysts were prepared and characterized on two mixed supports, which contain 1.5 and 20% SiO2 w/w, following the conventional impregnation procedure. The hydrodesulphurization (HDS) activities of the catalysts studied were determined using thiophene as a probe molecule. The co-EDF catalysts were proved to be more active than the corresponding ones prepared following the conventional impregnation procedure. This was attributed to the relatively high dispersion of the Mo supported phase achieved by applying co-EDF. The following activity trend was obtained over the co-EDF catalysts, 20Si > 30Si > 1.5Si > 10Si, which indicates that the activity is maximized over the catalyst prepared on a mixed support containing 20% SiO2 w/w. This zigzag trend was explained in terms of two parameters, the ratio "tetrahedral (monomer) /octahedral (polymer) Mo species" and the amount of cobalt aluminate formed, both regulated by changing the composition of the mixed support. (C) 2010 Elsevier BM. All rights reserved.
Abstract: 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: 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: 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: 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: 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: 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: 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: 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 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: 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, for the first time, the kinetics of adsorption of the Co(H2O)(6)(2+) species on the "electrolytic solution/gamma-Al2O3" interface at pH = 7 and 25 degrees C for a very broad range of Co(II) surface concentrations ranged from 0.03 to 6 theoretical Co(H2O)(6)(2+) surface layers. Moreover, we studied the surface dissolution of gamma-alumina in the presence of the Co(H2O)(6)(2+) ions in the impregnating solution, the contribution of the Co(II) desorption on the whole deposition process and the deposition isotherm. It was found that under the conditions where the deposition has taken place, the dissolution of the gamma-alumina surface is negligible even in the presence of the Co(H2O)(6)(2+) species in the impregnating solution. It was, moreover, inferred that the Co(II) desorption does not participate significantly to the whole deposition process. It was found that the deposition kinetics may be described by the following kinetic expression r(Co.bulk) = k'C-Co,bulk(2), which relates the rate of disappearance of the Co(H2O)(6)(2+) ions from the impregnating solution, r(Co,bulk), with their concentration C-Co,C-bulk. This kinetic expression may be derived assuming the following deposition scheme: nS + 2[Co-(H2O)(6)(2+)] -> S-n - [Co(H2O)(x,x < 6)(2+)](2), where S represents the surface reception sites. The above expressions indicated that two Co(H2O)(6)(2+) ions are involved, from the side of the interface, in the reaction with the reception sites. It seems probable that the deposition step involves the simultaneous adsorption and dimerization of the two interfacial Co(H2O)(6)(2+) ions through (hydr)oxobridges. On the other hand, the sigmoidal form of the deposition isotherm and the dependence of the apparent rate constant, k', on the interfacial Co(II) concentration suggested that the already deposited Co(II) species may be involved in the reception sites, S, promoting the adsorption and resulting to the formation of multinuclear complexes and Co(II) surface precipitates. Finally, reasonable interface potential values for oxides were determined for the first time using kinetic results.
Abstract: In the present work we studied the influence of the 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 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 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: 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: 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: 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: 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: 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: The equilibrium deposition filtration technique was properly developed in order to allow the deposition, via adsorption, of a relatively large amount of the Co2+ ions on the gamma-alumina surface. This, in turn, permitted the preparation of a Co2+/gamma-Al2O3 catalyst with a very high active surface as compared to that achieved by preparing the corresponding catalyst using the conventional pore volume impregnation technique. In the so-prepared EDF catalyst, the joint use of nitrogen adsorption, X-ray photoelectron spectroscopy, temperature programmed reduction, and diffuse reflectance spectroscopy showed that the gamma-alumina surface is mainly covered by very well dispersed Co2+ oxo-species with octahedral symmetry.
Abstract: A novel technique for determining the point of zero charge (pzc), called 'potentiometric mass titrations technique', has been developed and used for determining the pzc of several industrially used catalytic supports (SiO2, TiO2, gamma-Al2O3 and MgO).
Abstract: The aim of this study 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: The kinetics of deposition (adsorption, surface reaction) of molybdenum-oxo species an the surface of gamma-alumina was studied at;various pH's, impregnation temperatures, solute concentrations, and ionic strengths. The kinetic study confirmed the findings of the thermodynamic study performed previously concerning the influence of the aforementioned parameters on the extent of the Mo-(vi) deposition and the mechanism of that deposition as well. It was found that the decrease in pH and ionic strength and the increase in the impregnation temperature increase the time required in order for the surface of the support to be saturated by the molybdenum-oxo species. Moreover, the kinetic study suggested that, at low pH's where adsorption predominates, the polymeric oxo species are exclusively adsorbed in the first 20 min and the deposition proceeds through the adsorption of the monomeric oxo species. This is expected to be very useful in designing the preparation of the molybdenum-supported gamma-alumina catalysts. The simple model developed allowed the interpretation of the kinetic curves achieved at various pH values and the determination of the kinetic constants. These constants may be used to model the development of the Mo profiles on gamma-alumina extrudates, to interpret the influence of pH on the characteristics of the kinetic curves achieved at various pH values, and to understand better the deposition mechanism.
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: 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: Following a methodology based on the ''2pK/one site'' and ''triple-layer'' models, we further investigated the mechanism of deposition of the CrO42-, HCrO4-, and Cr2O72- species on the titania/electrolyte solution interface. The surface of the titania used is consisted of anatase and rutile patches. This methodology involved the following steps: (i) The assumption of tentative deposition equilibria. (ii) The derivation of the corresponding equations. (iii) The calculation of the concentration of each of the deposited Cr-VI-oxo species as well as the total concentration of the deposited Cr-VI at various pH's and bulk solution concentrations. (iv) The calculation of the variation with pH of the difference in the hydrogen ions consumption by the titania surface in the presence and absence of the Cr-VI-oxo species in the impregnating solution. (v) The calculation of the variation with pH of the zeta-potentials developed above the anatase and rutile patches. (vi) The comparison of the calculated parameters and variations mentioned above with the corresponding ones achieved from deposition experiments, potentiometric titrations, and microelectrophoresis. It was found that the deposition takes place following the equilibria: TiOH2+ + CrO42- <----> TiOH2+... CrO42-; TiOH2+ + HCrO4- <----> TiOH2+... HCrO4-; TiOH2+ + Cr2O72- <----> TiOH2+... Cr2O72-; TiOH + CrO42- <----> Ti-O-(CrO3-) + OH-; TiOH + HCrO4- <----> Ti-O-(CrO2)-OH + OH-; TiOH + Cr2O72- <----> Ti-O-(Cr2O6-) + OH-. At pH's 7.5 and 8.0 the deposition occurs exclusively by surface reaction (fourth and fifth equilibria). In the pH range 6.0-4.0 all the above equilibria contribute to the deposition. The only exception is that the fourth equilibria does not practically occur at pH less than or equal to 4.5. However, in the pH range 6.0-4.0 the deposition by adsorption of the Cr-VI species, mainly of the HCrO4- species, is the predominant process. It should be noted that the extent of the deposition in the anatase region is much larger than that on the rutile region. Lateral, attractive, interactions are exerted between the deposited Cr-VI-oxo species through water molecules being in the inner Helmholtz plane (MP). The intensity of these interactions is proportional to the charge of the deposited Cr-VI-oxo species.
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: 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: The mechanism of the deposition of the W-(vi) species from aqueous solutions on the gamma-alumina surface is refined in this work by investigating critical mechanistic points. A methodology recently developed to investigate the deposition of the Co-(II), Ni-(II), Cr-(VI), and Mo-(VI) on gamma-alumina has been applied. This methodology is based on the ''2pK/one site'' and ''triple-layer'' models and involves the writing down of various deposition equilibria, the derivation of the corresponding equations, the calculation of the amount of the deposited W-(vi) (through the calculated concentrations of the W-(vi) species formed on gamma-alumina) at various values of the impregnating parameters, the calculation of the variation, with pH, of the zeta-potential and of the difference in the H+ ions consumption by the support surface in the presence and absence of the W-(vi) species in the impregnating solution. The comparison of the calculated values of the aforementioned parameters with the corresponding ones achieved from deposition experiments, potentiometric titrations, and microelectrophoresis allowed us to establish the mechanism of deposition of W-(vi) species on gamma-alumina. It was found that although eight different W-(vi) species are present in the impregnating solution under the conditions of deposition, the mechanism of deposition was proved to be quite simple: AlOH2+ + WO42- <-> AlOH2+...WO42- AlOH2+ + HW6O20(OH)(2)(5-) <-> AlOH2+...HW6O20(OH)(2)(5+) AlOH + WO42- <-> Al-O-(WO3)(-) + OH- 2AlOH + WO42- <-> Al-O-(WO2)-O-Al + 2OH(-). According to this mechanism only two W-(vi) species contribute to the whole deposition. These species move from the bulk solution to the inner Helmholtz plane (IHP) of the double layer developed between the surface of the support particles and the impregnating solution and then are adsorbed on sites created in the IHP by protonated surface hydroxyl groups of the support. Moreover, the monomeric WO42- species, being in the IHP, may react with a single or a pair of adjacent neutral surface hydroxyl groups of the support resulting from the formation of a charged or uncharged W-(vi) species, respectively. Lateral intractions are exerted between W-(vi) species formed through water molecules also located in the IHP. The study of the variation of the saturation surface concentration of the species illustrated in the rhs of the above equilibria showed that in the pH range 10-6 the deposition occurs practically via reaction (equilibria [c] and [d]). The concentration of Al-O-(WO2)-O-Al and Al-O-(WO3-) species is maximized at pH's 7 and 6, respectively. In the pH range 6-5 both surface reaction and adsorption (equilibria [c] and [a]) contribute to the whole deposition process. The concentration of the AlOH2+...WO42- is maximized at pH 5. Finally in the pH range 5-3.5 the deposition takes place exclusively by adsorption (equilibria [a] and [b]). The concentration of the W-(vi) species illustrated in the rhs of the second equilibrium is maximized at pH 3.5. The selective deposition of the WO42- species with respect to the various polymeric species present in the impregnating suspension was attributed to the relatively low negative charge of these species and the negative potential developed at the IHP. (C) 1996 Academic Press Inc.
Abstract: The mechanism of deposition of MoO42- and Mo7O246- from electrolytic solution on the surface of industrial titania consisting of anatase and rutile patches was further investigated following a recently developed methodology, The methodology is based on the ''2pk/one site'' and ''triple-layer'' models. It involves the writing down of various deposition equilibria, derivation of the corresponding equations, calculation of the amount of each of the deposited Mo-(vi) species and the total amount of deposited Mo-(vi) at various pH and concentrations, calculation of the variation with pH of the zeta potentials and of the difference in H+ ion consumption by the support surface in the presence and absence of MoO42- and Mo7O246- species in the impregnating solution. Comparison of the calculated parameters and variations mentioned above with the corresponding ones obtained by deposition experiments, potentiometric titrations, and microelectrophoresis allowed us to establish the mechanism of deposition of the Mo-(vi) species on the surface of titania. It was found that the deposition of MoO42- and Mo7O246- species on the surface of ''anatase regions'' of the industrial titania takes mainly place by the following equilibria in the pH range 9.0-4.6: (TiOH)-O-a + MoO42- <-> Ti-a-O-(MoO3)(-) + OH- (TiOH2+)-O-a + MoO42- <-> (TiOH2+)-O-a ... MoO42- (TiOH2+)-O-a + Mo7O246- <-> (TiOH2+)-O-a ... Mo7O246. According to this mechanism, in the pH range 9-7 the deposition takes place exclusively through the first equilibrium, namely, by reaction of the MoO42- ions, located in the inner Helmholtz place (IHP) of the double layer developed between the support surface and the impregnating solution, and the neutral surface hydroxyls in the ''anatase regions'' of the support, The deposition through adsorption of the MoO42- and Mo7O246- species, on the protonated surface hydroxyls of the support, starts at pH 7 and 6.4, respectively. The same equilibria are mainly responsible for the deposition on the rutile patches of the support, but the extent of deposition in this case is too small. A preference for deposition of MoO42- with respect to Mo7O246- ions was observed in almost the whole pH range studied. This was attributed to the negative charge developed in the IHP, which inhibits the location in it of highly charged Mo7O246- species. Lateral, attractive, interactions are exerted between the deposited Mo-(vi) species through water molecules being in the IHP. The intensity of these interactions is proportional to the charge of the deposited Mo-(vi) species. (C) 1996 Academic Press, Inc.
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 codeposition of Mo-(VI) species and Ni2+ ions on the gamma-alumina surface, using the ''equilibrium deposition followed by filtration'' (EDF), has been studied in the pH range 4.1-6.2, An increase in the adsorptivity of the gamma-alumina for both the Mo-(VI) species and Ni2+ ions was observed at all pH values studied. This was attributed to the stronger lateral interactions exerted between the codeposited Mo-(VI) and Ni2+ ions as they compared with those exerted between the deposited Mo-(VI) species in the absence of the Ni2+ ions and between the deposited Ni2+ ions in the absence of the Mo-(VI) species. A codeposition model was developed and tested on the basis of the results obtained from deposition experiments, potentiometric titrations, and microelectrophoresis. According to this model the deposition of the Mo-(VI) species on the gamma-alumina surface in the presence of the Ni2+ ions takes place mainly via adsorption of the MoO42- and Mo7O246- ions on sites created by the protonated surface hydroxyls. Each adsorption site is created by one protonated surface hydroxyl, These sites are located in the inner Helmholtz plane (IHP) of the double layer developed between the surface of the support and the impregnating solution, The extent of deposition through reaction of one MoO42- ion with two neutral surface hydroxyls was proved to be negligible. Moreover, it was found that the extent of adsorption of MoO42- (Mo7O246-) decreased (increased) as pH decreased. Finally, it was shown that the total deposition of Mo-(VI) increased as pH decreased, Concerning the Ni deposition, according to the established model, it takes place through adsorption of one Ni2+ ion on a site created by one deprotonated surface hydroxyl in the IHP. As pH increased, the concentration of the deprotonated surface hydroxyls and thus the amount of the deposited Ni increased. (C) 1995 Academic Press, Inc.
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 rutile, anatase, and industrial titania(used in catalysis as support and consisting of rutile and anatase)-water solution interfaces were studied using potentiometric titration and microelectrophoretic mobility measurements. Concerning the simple oxides it was found that specific adsorption of the ions of the background electrolyte (NH4NO3) does not take place. The electrical double layer developed in the anatase/aqueous solution and rutile/aqueous solution interfaces is described by the diffuse layer model. On the basis of this model the surface acidity constants, the point of zero charge, and the concentration of the various surface hydroxyls in the pH range 4-8 were calculated using a computer program called SURFEQL. It was confirmed that the surface of rutile is more acidic than the surface of anatase. Concerning industrial titania used in catalysis as support it was demonstrated that its surface has a patchwise structure consisting of rutile (10%) and anatase (90%) regions. The acid-base behavior of this material is identical to a mechanical mixture of rutile and anatase of the same composition. Taking into account its patchwise structure we calculated the variation, with pH, of the concentration of each surface hydroxyl of industrial titanic [(TiOH2+)-O-r, (TiOH)-O-r, (TiO-)-O-r, (TiOH2+)-O-a, (TiOH)-O-a, and (TiO-)-O-a; a, r stand for anatase and rutile, respectively]. These variations allow one to model the deposition of species containing catalytically active ions on the surface of industrial titania. (C) 1995 Academic Press, Inc.
Abstract: The subject of the present lecture is the preparation of supported catalysts using the method of ''Equilibrium Deposition - Filtration'' for which we propose the term EDF. We propose the following structure for the lecture: First, to present the problems encountered when we use the usual impregnation techniques for preparing supported catalysts and to describe EDF. Second, to present the methodologies developed in order to increase the concentration of the deposition sites located on the surface of the industrial supports. This is necessary in order to prepare supported catalysts with high active surface using EDF. Third, to compare some surface and catalytic properties for catalysts prepared by dry or non-dry impregnation with corresponding catalysts prepared by EDF. Finally, to present the very recent developments concerning the mechanisms of deposition of species containing catalytically active elements from aqueous suspensions to the surface of the industrial, oxidic, supports.
Abstract: The mechanism of codeposition Of MoO42-, Mo7O246-, and Co2+ ions on the gamma-alumina surface, from aqueous solutions, has been studied in the pH range 4.1-6.1. A mutual promotion in the adsorption of Mo(VI) and Co2+ species was observed in all pH values studied. This was attributed to the strong lateral interactions exerted between the deposited Mo(VI) and Co2+ species. Results obtained from deposition experiments, potentiometric titrations, and microelectrophoresis allowed us to develop and test a codeposition model. The testing involved the following steps: (i) derivation of a number of equations corresponding to the deposition equilibria; (ii) calculation of the amount of the deposited Mo and Co; (iii) calculation of the difference of the hydrogen ions adsorbed on the gamma-alumina surface in the presence and absence of MoO42-, Mo7O246-, and Co2+ ions; (iv) calculation of the zeta-potential of the gamma-alumina particles in a suspension containing the aforementioned Mo(VI) and Co2+ species; (v) comparison of the aforementioned parameters with the corresponding ones determined experimentally. The agreement between the calculated and experimentally determined parameters over the pH range studied allowed us to establish the proposed mechanistic model for the title codeposition. According to this model the deposition of Mo(VI) on the gamma-alumina surface in the presence of Co2+ ions takes place through adsorption of one MoO42- or Mo7O246- ion on a site created by one protonated surface hydroxyl in the inner Helmholtz plane (IHP) of the double layer developed between the support surface and the impregnating solution. The deposition through reaction of one MoO42-ion with two neutral surface hydroxyls, observed previously in the absence of the Co2+ ions in the impregnating suspension, was proved now to be negligible. Moreover, the established codeposition model showed that the extent of adsorption Of Mo7O246-(MoO42- increased (decreased) as pH decreased. Finally, it was found that the amount of the deposited Mo(VI) increased as pH decreased. As regards the Co deposition, it was concluded that it takes place through adsorption of one Co2+ ion on a site created by one deprotonated surface hydroxyl in the IHP. The concentration of the deprotonated surface hydroxyls and thus the amount of the deposited Co increased as pH increased.
Abstract: The refinement of the mechanism of Mo(VI) deposition (proposed previously) from aqueous solutions on the gamma-alumina surface by investigating critical mechanistic points is the subject of the present work. A mechanistic model involving the adsorption of Mo7O246- and MoO42- ions on sites created by the protonated surface hydroxyls of gamma-alumina in the inner Helmholtz plane (IHP) of the double layer developed between the surface of the gamma-alumina particles and the molybdate solutions, as well as the deposition of the MoO42- ions through surface reaction with the neutral surface hydroxyls has been developed; ft has been tested over a wide range of impregnation parameters (pH = 8.5-4.1 at 25-degrees-C, temperature 25-50-degrees-C at pH = 5 and doping of the carrier with various amounts of Na+ and Li+ ions). The testing of the model included the derivation of various equations corresponding to the model deposition equilibria, the calculation of the amount of the deposited Mo, of the difference in the isotherms of the hydrogen adsorption in presence and absence of molybdates, and of the zeta-potential of the gamma-alumina particles in the molybdate solution (using an interactive code for the calculation of chemical equilibria in aqueous systems called SURFEQL), and the comparison of these parameters with the corresponding ones determined experimentally. The agreement between the calculated and experimentally determined parameters over a wide range of impregnating conditions allowed us to shed more light on the mechanism of the Mo deposition. It was established that both the adsorption of Mo7O246- and MoO42- ions and the deposition of MoO42- ions by surface reaction with two neutral hydroxyls of the support take place, but the contribution of each of these processes depends on the impregnating conditions. Specifically, hi the pH range 8.5-6. 1 the deposition practically occurs through the reaction of MoO42- ions with neutral surface hydroxyls of the support resulting in the formation of the surface complex [GRAPHICS] whereas at pH = 5 the adsorption of Mo7O246- and MoO42- ions is predominant and at PH = 4.1 the adsorption of Mo7O246- ions prevails. Increasing the impregnating temperature, at pH = 5, from 25 to 50-degrees-C increases considerably the adsorption of Mo7O246- ions and decreases further the deposition of MoO42- ions by surface reaction, whereas it does not change considerably the extent of adsorption of MoO42- ions. Finally, the doping with Li+ and Na+ ions increases both the extent of adsorption of Mo7O246- ions and the extent of MoO42- deposition by surface reaction, whereas it does not change considerably the extent of adsorption of MoO42- ions. Moreover, it was demonstrated that each protonated surface hydroxyl creates one adsorption site and that strong lateral interactions are exerted between the deposited species, mainly between the adsorbed MoO42- and Mo7O246- ions, through water molecules located at the IHP. On the other hand, comparison of the adsorption constant with the reaction constant demonstrated that the ionic sorptive band is much stronger than the covalent Al-O-Mo bonds on the above-mentioned surface complex. Finally, the fact that the ratio [MoO42-]b/[Mo7O246-]b calculated in the bulk solution was always lower than the ratio [MoO42-]adsorb+react/[Mo7O246-]adsorb calculated in the deposited state corroborated the finding reported previously that, concerning deposition, the selectivity of the support surface for the MoO42- ions is higher than the selectivity for the Mo7O246- ions. (C) 1993 Academic Press, Inc.
Abstract: In order to elucidate the influence on the hydrodesulphurization (HDS) activity of the order of deposition of F- ions on CoMo/gamma-Al2O3-F catalysts, we prepared, characterized (using BET, NO chemisorption, DRS and XPS) and determined the HDS activity (at atmospheric pressure and various temperatures using the HDS of thiophene as model reaction) of four samples. Specifically, we prepared one undoped CoMo/gamma-Al2O3 catalyst (denoted as MC) and three fluorinated catalysts in which the F- ions were deposited before (FMC), after (MCF) and simultaneously with the Mo(VI) and Co(II) phases ( (MCF) ). All the specimen which were prepared contained a fixed amount of Mo (11.5 wt.-% MoO3), Co (3.5 wt.-% Co3O4) and F (0.8 wt.-% F). It was found that a change in the deposition order provoked a change in the specific surface area, in the dispersity of the supported Mo and Co species and in the intrinsic activity of each HDS active site. The last change was found to be related to the ratio Mo(octahedral)/Mo(tetrahedral). Moreover, it was inferred that fluorination increased the number of active sites per unit active surface. The above explained the changes in the density of the HDS active sites, in the number of these sites and finally on the HDS activity which follows the order MCF> FMC > MC > (MCF) in all temperatures studied. Finally, an attempt was made to convincingly develop pictures related to the preparation which explain the influence of the F doping on the specific surface area and dispersity of the Mo and Co supported species.
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: Using a model coking procedure and measuring the carbon content on fluorinated Ni-Mo/gamma-Al2O3 catalysts wet found that fluorination decreased markedly the extent of coke formation on the surface of the nickel-promoted MoO3/gamma-Al2O3 catalysts. The influence of fluorination on the acidity of the oxidic precursors of the title catalysts was also studied. Both temperature-programmed desorption of ammonia and an FTIR study of the chemisorbed pyridine were used to obtain information on the Lewis and total acidity. It was found that both decreased upon fluorination but different trends with F- content were followed. An excellent relationship found between the total amount of desorbed ammonia and the amount of carbon formed on the catalyst surface demonstrated that the extent of the coke formation is controlled by the total acidity of the oxidic precursors. Finally, sulfur analysis confirmed previous findings, namely that the fluorination inhibits the reduction and/or the sulfidation of Mo(VI) species as well as the sulfidation of the supported-nickel species.
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 recently studied mechanism of adsorption of Co2+ and Ni2+ ions on the interface between pure or F--doped gamma-alumina and electrolyte solution has been investigated further. A quite general model has been developed and tested by comparing the experimental deposition isotherms obtained over various pH values and dopant concentrations with the corresponding ones calculated by applying to these models an interaction computer program for calculating chemical equilibria in aqueous systems (SURFEQL). The selected model showed that the deposition of the Co2+ or Ni2+ ions takes place by adsorption, following the extremely simple mechanism: AlO- + Co2+ (Ni2+) reversible arrow AlO-...Co2+ (Ni2+), where AlO- represents deprotonated surface hydroxy groups of the support and AlO-...Co2+ (Ni2+) a sorptive complex with the right-hand side being in the inner Helmholtz plane of the double layer developed between the support surface and the electrolyte solution. Moreover, the simple model selected has been confirmed using two additional independent methods. First, by comparing calculated and experimental values of the zeta potential in the presence of Co2+ and Ni2+ ions, and secondly, by comparing calculated and experimental values of the difference in gamma-alumina adsorptivity for hydrogen cations in the absence and presence of Co2+ or Ni2+ ions.
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 adsorption, from aqueous suspensions, of Co2+ and Ni2+ ions on the surface of gamma-alumina has been studied using potentiometric titrations, microelectrophoresis, and adsorption experiments. Correlation of the Co2+ or Ni2+ uptake corresponding to surface saturation, as obtained from the adsorption isotherms, with the concentration of the deprotonated and neutral surface hydroxyls of gamma-alumina, regulated by doping the support with various amounts of F- ions or by changing the pH of the impregnating solution, demonstrated that the deprotonated surface hydroxyls are mainly responsible for the creation of the adsorption sites. The combined use of microelectrophoresis and potentiometric titrations as well as the mathematical analysis of the adsorption isotherms showed that the adsorbed Co2+ and Ni2+ ions are located at the inner Helmholtz plane (IHP) of the double layer developed between the surface of the gamma-alumina particles and the impregnating solution. This analysis of the adsorption isotherms led also to the conclusion that the adsorbed Co2+ and Ni2+ ions are located at energetically equivalent sites of the IHP and that between the adsorbed ions relatively weak lateral interactions exist, provided that the density of the adsorption sites is relatively high. Moreover, it was found that fluorination increased the extent of adsorption, though it decreased the values of the adsorption constant and consequently the "adsorbed ions-support" interactions. Finally, it was found that the extent of adsorption of Co2+ and Ni2+ ions also increased by increasing the pH of the impregnating solution, but this method was less effective as compared to the fluorination for obtaining Co or Ni supported gamma-alumina catalysts with high Co or Ni surface.
Abstract: Molybdenum oxide (MoO3) supported catalysts are widely used for the catalysis of a number of processes. In the past oxidic carriers and polymers have been used as carriers. In the present work, a temperature resistant copolymer, poly(1,4-phenylene) terephthalamide (PPTA) was used as a substrate for the deposition of MoO3, selectively, from aqueous molybdate solutions, pH 2.25 at 25-degrees-C. MoO3 deposition followed the induction period and occurred on specific active sites of the copolymer. The rates of deposition, following the nucleation step, depended not only on the supersaturation of the aqueous solution with respect to MoO3, but on the chemical composition of the copolymer as well.
Abstract: The oxides gamma-Al2O3, SiO2 and TiO2, are the most commonly used carriers in the preparation of supported catalysts and are widely used in industry. The deposition of the active phase is the most critical step and is often done by wet impregnation. This phase, which may be anionic or cationic, is deposited on the cationic and anionic sites of the carrier, respectively. In the present work, we present an overview of the methods available for controlling the concentration of the positively or negatively charged sites on the surface of the oxide carriers. Doping with cations (Na+ or Li+) resulted in shifting the p.z.c. at more alkaline values, thus increasing the positively charged groups on the oxides, while doping with F- had the opposite effect. Adsorption of MoO4(2-) was, therefore, higher in the former samples while the adsorption of metal ions such as Co2+ and Ni2+ was higher in the latter samples. Similarly, increasing the suspension temperature shifted the p.z.c. to higher values. The variation of the surface concentrations of the charged groups of the carrier oxides provides a unique methodology for the preparation of the desired 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.
