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: 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: The interfacial chemistry of the impregnation step involved in the synthesis of cobalt catalysts supported on titania was investigated with regard to the mode of interfacial deposition of the aqua complex [Co(H2O)(6)](2+) on the "titania/electrolyte solution" interface, the structure of the inner-sphere complexes formed, and their relative interfacial concentrations. Several methodologies based on the application of deposition experiments and electrochemical techniques were used in conjunction with diffuse-reflectance spectroscopy and EPR spectroscopy. These suggested the formation of mononuclear/oligonuclear inner-sphere complexes on deposition of the [Co-(H2O)(6)](2+) ions at the "titania/electrolyte solution" interface. The joint application of semiempirical quantum-mechanical calculations, stereochemical considerations, and modeling of the deposition data revealed the exact structure of these complexes and allowed their relative concentrations at various Coll surface concentrations to be determined. It was found that the interface speciation depends on the Coll surface concentration. Mononuclear complexes are formed at the compact layer of the "titania/electrolyte solution" interface for low and medium Coll surface concentrations. Formation of mono-hydrolyzed Ti2O-TiO and the dihydrolyzed TiO-TiO disubstituted configurations is very probable. In the first configuration one water ligand of the [Co(H2O)(6)](2+) ion is substituted by a bridging surface oxygen atom and another by a terminal surface oxygen atom. In the second configuration two water ligands of the [Co(H2O)(6)](2+) ion are substituted by two terminal surface oxygen atoms. Binuclear and trinuclear inner-sphere complexes are formed, in addition to the mononuclear ones, at relatively high Coll surface concentrations.
Abstract: In this article the "titanium oxide/electrolyte solution" interface is studied by taking in advantage the recent developments in the field of Surface and Interface Chemistry relevant to this oxide. Ab-initio calculations were performed in the frame of the DFT theory for estimating the charge of the titanium and oxygen atoms exposed on the anatase (1 0 1), (1 0 0), (0 0 1), (10 3)(f) and rutile (1 1 0) crystal faces. These orientations have smaller surface energy with respect to other ones and thus it is more probable to be the real terminations of the anatase and rutile nanocrystallites in the titania polycrystalline powders. Potentiometric titrations for obtaining "fine structured" titration curves as well as microelectrophoresis and streaming potential measurements have been performed. On the basis of ab-initio calculations, and taking into account the relative contribution of each crystal face to the whole surface of the nanocrystals involved in the titania aggregates of a suspension, the three most probable surface ionization models have been derived. These models and the Music model are then tested in conjunction with the "Stern-Gouy-Chapman" and "Basic Stern" electrostatic models. The finally selected surface ionization model (model A) in combination with each one of the two electrostatic models describes very well the protonation/deprotonation behavior of titania. The description is also very good if this model is combined with the Three Plane (TP) model. The application of the "A/(TP)" model allowed mapping the surface (hydr)oxo-groups [TiO(H) and Ti2O(H)] of titania exposed in aqueous solutions. At pH>pzc almost all terminal oxygens [TiO] are non-protonated whereas even at low pH values the non-protonated terminal oxygens predominate. The acid-base behavior of the bridging oxygens [Ti2O] is different. Thus, even at pH = 10 the greater portion of them is protonated. The application of the "A/TP" model in conjunction with potentiometric titrations, microelectrophoresis and streaming potential experiments allowed mapping the "titania/electrolyte solution" interface. It was found that the first (second) charged plane is located on the oxygen atoms of the first (second) water overlayer at a distance of 1.7 (3.4) angstrom from the surface. The region between the surface and the second plane is the compact layer. The region between the second plane and the shear plane is the stagnant diffuse part of the interface, with an ionic strength dependent width, ranging from 20 (0.01 M) up to 4 angstrom (0.3 M). The region between the shear plane and the bulk solution is the mobile diffuse part, with an ionic strength dependent width, ranging from 10 (0.01 M) up to 2 angstrom (0.3 M). At I>0.017 M the mean concentration of the counter ions is higher in the stagnant than in the mobile part of the diffuse layer. For a given I, removal of pH from pzc brings about an increase of the mean concentration in the interfacial region and a displacement of the counter ions from the mobile to the stagnant part of the diffuse layer. The mean concentration of the counter ions in the compact layer is generally lower than the corresponding ones in the stagnant and mobile diffuse layers. The mobility of the counter ions in the stagnant layer decreases as pH draws away from pzc or ionic strength increases. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: Nickel supported on alumina catalysts have been prepared using various synthesis methods (dry impregnation, co-precipitation, sol-gel) and evaluated for the hydrogenation of benzene contained in gasoline. The evaluation was carried out in a laboratory scale high pressure fixed bed reactor fed with a stream of surrogated reformate gasoline consisted by a mixture of hexane, benzene and toluene. All catalysts have been characterized by the joint use of various physicochemical characterization methods (XRF, BET, TGA, SEM, XRD, UV-vis DRS and XPS) in order to correlate their catalytic performances to their physicochemical properties. The results obtained revealed that sol-gel procedure, especially when it is followed by supercritical drying (aerogel), produced the most promising catalysts for the aforementioned catalytic process. Sol-gel methodology ensured the best compromise between dispersion of the nickel phase and its interaction with the support surface. Co-precipitated catalysts exhibited important activities but lower than those of the sol-gel catalysts. The catalyst prepared by dry impregnation proved to be the less active. Calcination of the catalysts before their activation by reduction decreased their activities. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: Four supported catalysts with the same tungsten loading were prepared by depositing decatungstate species W10O324-, through wet impregnation, on the surface of gamma-alumina and silica at different pH values. The prepared samples were characterized using BET measurements as well as XRD, UV-vis DR, and XP spectroscopies. Higher dispersion of W(VI) oxo-species was obtained in the silica-supported catalysts compared with the corresponding alumina-supported ones. Within the same support, the dispersion was higher when the impregnation pH is lower than the point of zero charge (pzc) of the support. The decatungstate anions were present mainly on the silica surface without any modification, whereas these underwent a partial depolymerization on their deposition on the gamma-alumina surface. The extent of depolymerization was less in the sample prepared at pH above pzc. These findings were explained in terms of the mode of deposition of the W(VI) species from the solution onto the support surface. The photocatalytic activity of the aforementioned catalysts, concerning the photooxidation of 1-phenylethanol, depends on the fraction of the W10O324- supported species rather than on the W(VI) dispersion. Thus, extremely high conversions have been obtained over the silica-based catalysts and also over the gamma-alumina-based catalyst prepared at relatively high pH. These catalysts also are very effective in the photooxidation of a series of secondary and primary benzyl alcohols, in which benzyl ketones and benzoic acids were formed as the only or major products, respectively. The easy separation of the solid catalyst from the reaction mixture, the high activity, selectivity, and stability as well as the retained activity in subsequent catalytic cycles, make these supported catalysts suitable for a small-scale synthesis. Based on product analysis and kinetic data on the heterogeneous oxidation of benzyl alcohols, we suggest that a hydrogen abstraction transfer (HAT) mechanism predominates with respect to an electron transfer (ET) one in these reactions. (C) 2007 Elsevier Inc. All rights reserved.
Abstract: Two series of MnOx/Al2O3 catalysts with varying Mn loading (0-1.2%, w/w Mn) prepared by equilibrium deposition filtration (EDF) and wet impregnation (WI) methodology were used for studying the influence of the composition and the preparation method on their activity for the reduction of benzaldehyde with ethanol. The prepared catalysts were characterized by BET measurements, X-ray photoelectron spectroscopy and diffuse reflectance UV-vis spectroscopy. It was found that following EDF methodology, the deposition of MnOx species on the alumina takes place via adsorption of [Mn(H2O)(6)](2+) and [Mn-Ac](+) species on the negatively charged surface sites. On the contrary, following wet impregnation the via adsorption of deposition takes place mainly via precipitation in the step of the solvent evaporation. The extent of the interactions exerted between the support and supported phase is higher in the EDF samples. These interactions created during the impregnation step were detected by DRS after drying and calcination. Higher dispersion of the MnOx phase is achieved when it is deposited on the gamma-Al2O3 surface by the EDF than the WI method. The catalysts of the first series exhibited higher activity related well with the dispersion of MnOx, supported nano-particles.) 2007 Elsevier B.V. All rights reserved.
Abstract: In 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: In the present work we studied the influence of the preparation method and the Co loading on the physicochemical properties and the catalytic activity of the cobalt oxide/gamma-alumina catalysts for the reduction of NO by propene under net oxidizing conditions. Two series of catalysts containing 1 and 5% w/w Co, respectively, were prepared using three preparation methods, namely, the equilibrium deposition filtration (EDF), the conventional incipient wetness impregnation (IWI) and the IWI adding nitrilotri acetic acid (nta) in the impregnating solution (IWInta). The catalysts were tested at various temperatures in the range 300-550 degrees C using a fixed-bed microreactor for the NO reduction by propene under lean burn conditions. The evolution of the Co species on the alumina surface was followed after each preparation step by diffuse reflectance spectroscopy (DRS). It was found that the catalysts of the first series were more active for the title reaction than those of the second one. The EDF sample of the first series was proved to be the most active and selective one followed by the IWI and then the IWInta sample of the same series. The DRS results indicated that the enhanced activity and selectivity of the EDF sample could be attributed to the increased concentration of isolated Co(II) inner sphere complexes of octahedral coordination, which are formed on the support surface by adsorption of the corresponding aqueous complexes, [Co(H2O)(6)](2+), being in the impregnating solution. These inner sphere complexes are transformed upon thermal treatment into a CoAl2O4 like phase with high dispersion. On the other hand, the [Co(H2O)(6)](2+)(NO3)(2-) and [Co-nta](-) NH4+(or H+) and/or [Co-2nta](4+).4NH(4)(+)(or 4H(+)) complex salts, precipitated on the alumina surface upon drying in the cases of the IWI and IWInta samples, are transformed upon calcination into CoAl2O4 like phases with lower dispersion. All the samples of the second series promoted the propene combustion as well as the oxidation of NO and N-2, used as carrier gas, to NO2. DRS results revealed that in all these samples cobalt(III) oxo species are formed in addition to the CoAl2O4 phase. These species are considered to be responsible for the enhancement of the rates of the oxidation reactions mentioned above. (c) 2005 Elsevier Inc. All rights reserved.
Abstract: The mechanism of the protonation of solid metal (hydr)oxides in aqueous media, which is closely interrelated to many processes of great technological and environmental importance, has been elucidated using simulation and experimental work. The electrical potential, smeared out at the interfacial region, changes the concentration of the H+ ions on the surface of the (hydr)oxide, thus promoting or hindering protonation. This is manifested by the shifts of the protonation peaks of the various kinds of surface sites and the appearance of an extra peak in the differential potentiometric, titration curve. (c) 2005 Elsevier Inc. All rights reserved.
Abstract: The contribution of the Interface Science to the preparation of supported catalysts during the last two decades is presented. It is illustrated how the concepts and the methodologies of the Interface Science could be effectively used for an in-depth understanding of the phenomena involved in the initial preparation step. This, extremely critical step, concerns the deposition of transition metal species containing the active elements, from an impregnation solution, onto the surface of common catalytic supports. Moreover, the aforementioned concepts and methodologies allow the regulation of the mode of interfacial deposition and the local structure of the deposited species and, thus, the surface characteristics and the catalytic behaviour of the resulted catalysts.
Abstract: The mechanism of the protonation of solid metal (hydr)oxides in aqueous media was investigated using simulation and experimental work. It was found that the apparent acidity/basicity of each kind of surface sites of metal (hydr)oxides in aqueous suspensions is strongly influenced by the overall surface charge of the (hydr)oxide and thus by the electrical potential smeared out at the interfacial region. Depending on its sign this increases or decreases the hydrogen ion concentration on the surface, thus promoting or hindering protonation. This is manifested by the shifts of the protonation peaks of the various kinds of sites with respect to the - pK values of the corresponding intrinsic protonation constants and the appearance of an extra peak in the d[H-cons,surf(+)]/dpH vs. pH curves. Potentiometric titrations experiments performed for four technologically important oxides showed that the proposed protonation mechanism describes indeed the protonation of polycrystalline (hydr)oxides in aqueous media. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: In the present work we studied the influence of the initial concentration of the impregnating solution used for mounting Co(II) species on the gamma-alumina surface by equilibrium deposition filtration method (edf) on the physicochemical properties and the catalytic activity of the "cobalt oxide" /gamma-alumina catalysts. The complete oxidation of benzene has been used as a model reaction. Two series of catalysts (edf-X-A and edf-X-B) of varying Co content (X: up to 21 wt.% Co) were prepared using the above-mentioned method and tested at various temperatures in the range 200-300 degrees C using a fixed-bed reactor. In the first series (A) various Co loadings were obtained by varying the initial Co(II) concentration of the impregnating solution. In the second series (B) the corresponding Co loadings were obtained by using the impregnating solution used for the preparation of the catalyst of A series with the maximum Co(II) content and varying the impregnation time. All catalysts were characterized using various techniques, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), X-ray powder analysis (XRD), nitrogen adsorption (BET) and temperature-programmed reduction (TPR). It was found that the initial Co(II) concentration of the impregnating solution used for depositing the corresponding species on the gamma-alumina surface by edf influences the catalytic activity of the "cobalt oxide" /y-alumina catalysts with respect to the complete oxidation of benzene. The increase of the initial Co(II) concentration of the impregnating solution brings about a change in the composition of the deposited phase formed in the impregnation step by decreasing the ratio "Co(II) surface inner-sphere complexes/surface Co(II) precipitates". Upon calcination, the Co(II) surface inner-sphere complexes are transformed to well-dispersed "cobalt oxide" phase strongly interacting with the support surface while the surface Co(II) precipitates are transformed to Co3O4 crystallites loosely interacting with the support surface. The former phase is responsible for the relatively high dispersion of Co observed in the A series of catalysts but it is rather inactive, while the latter exhibits lower dispersion but higher activity. Thus, the edf catalysts prepared using high initial Co(II) concentration exhibited higher catalytic activity than the corresponding ones prepared using low initial Co(II) concentration. (c) 2004 Elsevier B.V. All rights reserved.
Abstract: In the present work we studied the influence of the methodology used for mounting Co(II) species on the γ-alumina surface on the physicochemical properties and the catalytic activity of the 'cobalt oxide'/γ-alumina catalysts for complete oxidation of benzene. Three series of catalysts of varying Co content (up to 21 wt.% Co) were prepared using three preparation methods: pore volume impregnation (pvi), equilibrium deposition filtration (edf) and pore volume impregnation adding nitrilotri acetic acid (ma) in the impregnation solution. It was found that the catalytic activity for low, medium and high Co content follows, respectively, the orders, nta-pvi &MGT; pvi &MGT; edf, nta-pvi &MGT; edf &AP; pvi and edf > nta-pvi > pvi. The catalysts prepared were characterized using various techniques (BET, UV-vis/DRS, XRD and XPS) at each step of the preparation procedure, namely after the Co(II) mounting on the support surface, after drying as well as after calcination. It was inferred that the most active sites are located on CO3O4-supported crystallites, loosely or moderately interacting with the γ-alumina surface. Two critical parameters, related with the method followed for mounting Co(II) species on the γ-alumina surface, control the characteristics of the supported phase and thus the amount and the size of the above-mentioned CO3O4 crystallites: the ratio 'amount of Co(II) deposited in the impregnation step to that remaining in the liquid phase inside the pores precipitating thus in the drying step' closely related with the ratio 'amount of Co(II) in the deposited phase (isolated Co(II) surface inner sphere complexes and Co(II) surface precipitates)/amount of Co(II) in the precipitated phase formed in the drying step' as well as the composition of the precipitated phase. The application of the pvi technique resulted to low values for the above ratios and thus to the formation of a rather unstable precipitated phase consisted mainly by Co(H2O)(6)(2+) (.) 2NO(3)(-). Upon calcination it is transformed into loosely bounded CO3O4 crystallites of relatively big size. This is related with the low Co dispersion and thus with the low catalytic activity exhibited by these catalysts. The application of edf resulted to high values for the above-mentioned ratios. Therefore, the deposited phase is predominant. Upon calcination it is transformed to well (very well) dispersed cobalt phases strongly (too strongly) bounded with the support surface and thus reducible at high temperatures (non reducible up to 800 &DEG; C). Although these phases are responsible for the high Co dispersion achieved they do not contribute to the catalytic activity unless the deposited phase mainly comprises a Co(II) surface precipitate with relatively large number of layers as it is the case for the sample with the maximum Co content. The application of the nta-pvi technique resulted to very low values for the ratios mentioned above. This is because the [Co(Il)-nta](-) and [Co(Il)-2nta](4-) complexes, in which the Co(H2O)6 2, complex is completely transformed, are not practically adsorbed on the support surface. Therefore, in the nta-pvi catalysts a precipitated phase containing the [Co(II)-nta](- .) NH4+(or H+) and [Co(II)-2nta](4- .) 4NH(4)(+) (or 4H(+)) complex salts predominates. Upon calcination these are transformed into CO3O4 crystallites of small size, which are moderately interacting with the support surface. This is related with the relatively high Co dispersion, mainly that for the catalytically active species, and thus with high catalytic activity. © 2004 Elsevier B.V. All rights reserved.
Abstract: A new methodology is presented, called differential potentiometric titration (DPT),which allows the determination of the point of zero charge (pzc) of metal (hydr)oxides using only one potentiometric curve. By performing extensive simulations of potentiometric titrations for various model (hydr)oxides, we found that an inflection point in a H-cons,surf(+) versus pH potentiometric curve (H+(cons,surf): hydrogen ions consumed on the surface of the (hydr)oxide) and a peak in the corresponding differential curve, dH(cons,surf)(+)/dpH versus pH, appear at a pH equal to the pzc assumed for a model (hydr)oxide. This distinguishable peak appears at the same position irrespective of the surface ionization and the interfacial model adopted as well as the assumed ionic strength. It was found that the aforementioned peak also appears in the high-resolution differential potentiometric curves experimentally determined for four oxides (SiO2, TiO2, gamma-Al2O3, and MgO) that are widely used in various environmental and other technological applications. The application of DPT to the above-mentioned oxides provided practically the same pzc values as the corresponding ones achieved by using four different techniques as well as the corresponding isoelectric point (iep) values determined by microelectrophoresis. Differences between the pzc and iep values determined using various techniques in the case of MgO were attributed to the increasing dissolution of this oxide as pH decreases and the adsorption of cations (Mg2+, Na+) on the MgO/electrolytic solution interface.
Abstract: In the present work we studied, for the first time, the kinetics of adsorption of the Co(H2O)(6)(2+) species on the "electrolytic solution/gamma-Al2O3" interface at pH = 7 and 25 degrees C for a very broad range of Co(II) surface concentrations ranged from 0.03 to 6 theoretical Co(H2O)(6)(2+) surface layers. Moreover, we studied the surface dissolution of gamma-alumina in the presence of the Co(H2O)(6)(2+) ions in the impregnating solution, the contribution of the Co(II) desorption on the whole deposition process and the deposition isotherm. It was found that under the conditions where the deposition has taken place, the dissolution of the gamma-alumina surface is negligible even in the presence of the Co(H2O)(6)(2+) species in the impregnating solution. It was, moreover, inferred that the Co(II) desorption does not participate significantly to the whole deposition process. It was found that the deposition kinetics may be described by the following kinetic expression r(Co.bulk) = k'C-Co,bulk(2), which relates the rate of disappearance of the Co(H2O)(6)(2+) ions from the impregnating solution, r(Co,bulk), with their concentration C-Co,C-bulk. This kinetic expression may be derived assuming the following deposition scheme: nS + 2[Co-(H2O)(6)(2+)] -> S-n - [Co(H2O)(x,x < 6)(2+)](2), where S represents the surface reception sites. The above expressions indicated that two Co(H2O)(6)(2+) ions are involved, from the side of the interface, in the reaction with the reception sites. It seems probable that the deposition step involves the simultaneous adsorption and dimerization of the two interfacial Co(H2O)(6)(2+) ions through (hydr)oxobridges. On the other hand, the sigmoidal form of the deposition isotherm and the dependence of the apparent rate constant, k', on the interfacial Co(II) concentration suggested that the already deposited Co(II) species may be involved in the reception sites, S, promoting the adsorption and resulting to the formation of multinuclear complexes and Co(II) surface precipitates. Finally, reasonable interface potential values for oxides were determined for the first time using kinetic results.
Abstract: The adsorption of a model textile azo-dye, Acid Orange 7 (AO7), on the surface of titanium dioxide was extensively investigated in aqueous TiO2 suspensions over wide ranges of AO7 concentrations (1 X 10(-4)-3 X 10(-3) M) and PH values (2-10). Results obtained with the use of a variety of techniques, including potentiometric titrations, adsorption isotherms, adsorption edges, and microelectrophoresis, were used for the description of the "AO7 solution/TiO2 surface" interface. This has been achieved by taking into account the effects of PH on the speciation of the dye in solution and on the nature and population of the surface groups of TiO2. Results could be modeled very well with the use of the recently introduced CD-MUSIC approach. According to the model employed, the TiO2 surface is not considered homogeneous but is characterized by the presence of different types of surface groups, namely singly (TiOH-1/3), doubly (Ti2O-2/3), and triply (Ti3O0) coordinated. Surface complexes are not treated as point charges, but their charge is spatially distributed in the interfacial region. It has been found that adsorption of AO7 on the TiO2 surface occurs to a significant extent only at PH values lower than 7, via the sulfonic group of the azo-dye, through the formation of a bidentate innersphere surface complex. The determination of the adsorption mode of TiO2, which is supported by ex situ FTIR results, as well as of the adsorption constant, K-ads, allowed the description of the PH dependency of the AO7 adsorption over large PH and AO7 concentration ranges.
Abstract: Monolayer catalysts containing binary active phases (VOx-CrOx, VOx-MoOx,) were prepared by simultaneous deposition of the corresponding transition metal-oxo species on the TiO2 (anatase) surface using the equilibrium deposition filtration technique. The prepared samples contained various amounts of each transition metal but almost the same total metal loading. They were characterized by atomic absorption spectroscopy, N-2 adsorption, UV-vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and tested for the selective catalytic reduction of NO by NH3 in the temperature range 250-450 degreesC. It was found that the transition-metal ionic species used for the preparation of these catalysts compete for the same surface sites of the TiO2 carrier upon co-deposition. Small amounts of the second phase (Mo- or Cr-oxo phase) are sufficient in order to promote the catalytic activity at relatively high temperatures, in contrast to what happens in the corresponding industrial catalysts prepared by conventional methods. An electronic interaction between V- and Cr-oxo species favored at a V/Cr atomic ratio around 3 is probably responsible for the relatively high catalytic performance of the corresponding TiCrV catalyst. The activity of the studied catalysts is well correlated with the intensity of a DRS absorption band that appeared at ca. 400 nm, which is considered as a measure of the magnitude of interactions exerted between the monolayer transition metal-oxo species and the TiO2 carrier. This correlation is independent of the transition metals combination used and follows the same linear relationship found previously for single-active-phase catalysts.
Abstract: Diffuse reflectance spectroscopy was used for the first time to investigate the adsorption of the [Co(H2O)(6)](2+) ions on the interface developed between the surface of the gamma-alumina particles and the electrolytic aqueous solutions used for the preparation of cobalt-supported gamma-alumina catalysts by equilibrium deposition filtration. The formation of inner-sphere Co(II) surface complexes in which Co(II) is in octahedral symmetry was confirmed. A deconvolution peak centered at similar to585 nm was attributed to the exchange of one aqua ligand with one AlxOHy (x = 1, 2, or 3; y = 0 or 1) negatively charged surface group resulting in the formation of mononuclear monosubstituted inner-sphere Co(II) complexes at a Co(II) surface concentration equal to 0.02, mumol of Co(II)/m(2). It was inferred that as the surface Co(II) concentration increases the formation of disubstituted and/or trisubstituted surface complexes is favored with respect to the formation of monosubstituted Co(II) surface complexes. A deconvolution peak centered at similar to640 nm was attributed to the exchange of one or more aqua ligands with bridging hydroxo ligands (Co-O-H). The relative magnitude of this peak increases with the Co(II) surface concentration, reflecting the increasing formation of binuclear, oligonuclear, and multinuclear Co(II) surface complexes and then the formation of the Co(II) surface precipitate.
Abstract: Monolayer catalysts were prepared by supporting transition metal ionic (t.m.i.)-oxo species (VOx, CrOx, MoOx and WOx) on the TiO2 (anatase) surface using the equilibrium-deposition-filtration (EDF) technique. The prepared samples were characterized by N-2 adsorption, UV/Vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and tested for the selective catalytic reduction of NO by NH3 (NH3-SCR) in the temperature range 250-450 degreesC. Using the EDF method the transition metal ion (V, Cr, Mo or W) loading of the supported Titania catalysts achieved at monolayer coverage follows the order V > Mo > W > Cr. It is mainly determined by the deposition of polymeric t.m.i.-oxo species on the surface of titania in the impregnation step. The activity of the corresponding monolayers follows the order V > Cr > Mo > W. It was also demonstrated that the activity of the above catalysts correlates well with the intensity of a DRS absorption band appearing at ca. 400 nm. The intensity of this absorption band, which is considered as a measure of the extent of interactions exerted between the active phase and the support surface, depends on the kind and the amount of the transition metal. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: In the present review article, we present the efforts done so far for elucidating the mechanism of adsorption of the Co(II) species, mainly Co(H2O)(6)(2+), on the interfacial region developed between metal oxide particles, used as catalytic supports, and aqueous electrolytic solutions. Specifically, we present: (i) the principal modes of deposition of the transition metal ionic species (TMIS) on the surface of oxidic supports related with the various methodologies used for the preparation of the supported catalysts; (ii) the state of the art concerning the general aspects of the adsorption mechanisms of the TMIS on the aforementioned interfacial region; and (iii) the works reported so far dealing with the adsorption of the Co(II) species on the surface of gamma-Al2O3 (gamma-alumina), alpha-Al2O3 (alpha-alumina), TiO2 (rutile), and SiO2 (silica). It was concluded that the mechanism of adsorption depends on two main factors: on the Co(II) surface concentration and on the nature of the support surface. It seems that, generally, the mechanism changes progressively along the Co(II) surface concentration from the deposition of monodentate-mononuclear inner sphere complexes, weakly evidenced in too low values of the Co(II) surface concentration, to multidentate, multinuclear inner sphere surface complexes at relatively low Co(II) surface concentrations, and then into surface Co(OH)(2)-like, eventually mixed precipitates, at relatively high Co(II) surface concentrations but at pH values lower than those required for bulk precipitation. In all cases, Co(II) forms surface species with Co(II) in octahedral symmetry. However, the exact Co(II) surface concentration values, in which the abovementioned two transitions (concerning the deposited phase) take place, depends on the kind of the support. Thus, SiO2 favors the formation of the Co(OH)(2)-like precipitates even at relatively low Co(II) surface concentrations. In contrast, TiO2 favors the formation of mononuclear or oligonuclear surface complexes. Finally, alumina, which exhibits the maximum adsorption capacity, favors the formation of highly defected Co(OH)(2)-like precipitates, probably mixed Co-Al precipitates. The exact local structure of the inner sphere Co(II) surface complexes, formed by exchanging the H2O ligands with surface oxygens, has been already approached but only for the surface planes of the alpha-Al2O3 and rutile monocrystals. This structure remains up to now rather unclear for the polycrystalline oxides used as catalytic supports. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: Two unpromoted Mo/gamma-Al2O3 catalysts containing almost the same amount of Mo were prepared using equilibrium deposition filtration, EDF, for depositing Mo-oxo species on the alumina surface at two different pH values (3.9 and 6.3). On these samples the same amount of Co ions was deposited by dry impregnation and thus, the corresponding CoMo/gamma-Al2O3 catalysts were prepared. The unpromoted and the Co-promoted samples were characterized using BET, DRS, LRS, TPR, and NO chemisorption. Moreover, the catalytic activity of the samples was determined at various temperatures using the hydrodesulfurization of thiophene as a probe reaction. The characterization of the specimens showed that the Mo deposition at pH = 3.9 favors the deposition of polymeric Mo-oxo species, in agreement with predictions drawn on the basis of two models related with the acid-base behavior of the gamma-Al2O3 surface. These species after calcination provide a well-dispersed Mo phase with relatively high reducibility. This phase, in which the concentration of the octahedral Mo is greater compared to that on the Mo/gamma-Al2O3 specimen prepared at pH = 6.3, hinders, in turn, the formation of the catalytically inactive CoAl2O4, whereas after sulfidation this provides a sulfided state with relatively high concentration of sulfur vacancies. The above explain why the unpromoted and the Co-promoted catalysts prepared at pH = 3.9 proved to be more active than the corresponding catalysts prepared at pH = 6.3.
Abstract: In this paper, we present a novel methodology, called the potentiometric mass titration (PMT) technique, for determining the point of zero charge (pzc) of mineral hydr(oxides) immersed in electrolytic solutions. Following PMT, the pzc is identified as the common intersection point (CIP) of the potentiometric curve of the blank solution with the corresponding curves of the impregnating suspensions containing different amounts of the immersed mineral (hydr)oxides. Full experimental results related to the determination of pzc using the PMT technique and four traditional techniques (potentiometric titrations, mass titrations, immersion, and micro-electrophoresis (for determining the isoelectric point, equal to pzc in cases where no specific adsorption takes place)) are presented for four oxides, namely, MgO, gamma-Al2O3, TiO2, and SiO2. The comparison of the pzc values determined by PMT, with the corresponding ones determined using the traditional methodologies, strongly suggested that the PMT technique can be used to determine the pzc of oxides. A simulation procedure of the PMT technique has been developed and applied to model oxides with properly selected acid-base characteristics and to various combinations of models related to the charging mechanism of the oxide surface (1 site/1 pK, 1 site/2 pK, multisite models) and to the description of the interfacial region (diffuse double layer, constant capacitance, basic Stern models). The intensive application of this simulation procedure offered a quantitative interpretation of the methodology. Specifically, it was demonstrated that (a) the application of the "quick scan" version of the PMT technique, realized by recording the titration curve of the blank solution (pH vs V-added (acid)) and the corresponding curve of a suspension of a given amount of the immersed oxide, indeed results in the determination of the pzc, provided that this is greater than a value of about 4; (b) the application of the "typical" version of PMT, realized by recording the titration curves of three different suspensions (pH vs V-consumed (acid)) containing different masses of the immersed oxide, provides the pzc value of this oxide over the whole pH range; and (c) the CIP that is determined, using PMT, corresponds to the pzc irrespective of the charging mechanism of the oxide surface and the structure of the double layer developed between the oxide surface and the solution. However, in the case where the basic Stern model is used to describe the interfacial region, the pzc value determined by PMT deviates slightly from the true value when the value of the affinity constants of the ion pairs formed between the positive counterions and the surface is different than the corresponding value of the negative counterions.
Abstract: The primary charging behavior of titanium oxide (anatase, rutile, and P25) and the ion pair formation of the electrolyte ions with the surface groups have been extensively studied. A large number of titration and electrokinetic data sets available in the literature have been successfully described, using the MUSIC (MultiSite Complexation) model with a Basic Stern double-layer option and applying the ion pair formation concept. The systematic analysis of the data, over a large number of different monovalent electrolytes and various ionic strength values, allowed the determination of a number of "best estimated" values for the ion pair formation constants. The values suggest that the interaction of the cations with the titania surface is stronger than that of the anions. This is in accordance with the observed shift of the IEP of titanium oxide to higher pH values, at high electrolyte concentrations. The binding of the cations follows the sequence Cs+ < K+ < Na+ < Li+ and that of the anions follows the sequence Cl- > NO3- > ClO4- > I-. Titanium oxide can be divided in two classes of materials, having a low (C = 0.9 F m(-2)) and a high (C = 1.6 F m(-2)) capacitance value, respectively. The low capacitance value corresponds with the low values, found for well-crystallized gibbsite and goethite. On the basis of the low capacitance value and the absence of correlation with the dielectric properties of the solids, it is hypothesized that the first layer of physically adsorbed water has a unique relative dielectric constant is an element of of about 40 on well-crystallized brides. The high capacitance may correspond to a situation with a distorted water layer, which has bulk water properties (is an element of = 78). No other significant differences between the interfacial charging parameters of anatase and rutile were found.
Abstract: Adsorption of polymers on mineral surfaces is of great interest. A representative important system is the molybdenum supported titania, used in catalysis. The well-known chemistry of molybdate polymers in aqueous solutions allows a detailed study of the contribution of these polymers to the Mo adsorption on titanium oxide surface. A large range of Mo concentrations was covered: from low levels where only Mo monomers are present to high levels where extensive polymerization takes place. Data from different techniques, like potentiometric titrations, proton-ion titrations, and adsorption edges, were used for the description of the interface "molybdate solution/titania surface". All the data could be modeled very well using the recently introduced CD-MUSIC approach. The charge of the surface complexes is spatially distributed in the interface. MoO42- monomers adsorb over the entire pH range forming inner sphere complexes, which are characterized by two types of structure, i.e., bidentate and monodentate. Below pH 5.5 and high total molybdate concentrations the heptamolybdate polyanion Mo7O23(OH)(5-) is also adsorbed, forming an outer sphere complex. Due to its relatively large size, it covers a significant number of surface groups.
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: 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: The EDF method for preparing supported catalysts with improved physicochemical and catalytic properties, and a methodology for elucidating the deposition mechanism are presented. The knowledge of the deposition mechanism, namely the nature of the deposited species formed, their relative concentrations and their dependence on the impregnation parameters, permits the tailoring of the synthesis of supported catalysts by EDF, by adjusting the impregnation parameters to their optimal values.
Abstract: 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 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: 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.
