Abstract: The mechanism of retention of vanadium oxo-species at the âtitanium oxide / aqueous solutionâ interface was investigated over a wide pH range (4-9) and V(V) solution concentration (10-5 - 2Ã10-2 M) by combining equilibrium deposition experiments, potentiometric titrations, microelectrophoresis and âprotonâionâ titration curves. It was inferred that the adsorbed V(V) oxo-species are retained inside the compact layer of the interface through hydrogen/coordinative bonds forming very probably inner-sphere complexes with the titania surface groups.
Abstract: The interfacial chemistry of the impregnation step involved in the preparation of nickel catalysts supported on titania is presented. Several methodologies based on deposition data, pH measurements, potentiometric mass titrations and microelectrophoresis have been used in conjunction with Diffuse Reflectance UV-Vis-NIR Spectroscopy, simulations and semi-empirical quantum chemical calculations. Three mono-nuclear inner sphere complexes are formed at the compact layer of the âtitania / electrolyte solutionâ interface; a mono-substituted, di-hydrolyzed complex above a terminal oxo-group, a di-substituted, di-hydrolyzed complex above two terminal adjacent oxo-groups and a di-substituted, non-hydrolyzed complex above one terminal and one bridging adjacent oxo-group. The mono-substituted, di-hydrolyzed complex predominates. The contribution of the di-substituted configurations is also important at very low Ni(II) surface concentration, but it decreases as the Ni(II) surface concentration increases. In addition, a binuclear and a three-nuclear inner sphere complex are formed. The receptor site involves one bridging and two terminal oxo-groups in the first case and two bridging and three terminal oxo-groups in the second case. The relative surface concentrations of these configurations increase initially with the Ni(II) surface concentration and then remain practically constant. The understanding of these interfacial processes at molecular level is very important in order to shift the catalytic synthesis from the Art to the Science as well as to obtain a severe control of the impregnation step and, to some extent, of the whole preparative sequence. The present study is more relevant to the synthesis of sub-monolayer/monolayer nickel catalysts supported on TiO2 following equilibrium deposition filtration (otherwise called equilibrium adsorption).
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.
