George D. Panagiotou

Towards the local structure of the Co(II), Ni(II), Cr(VI) and W(VI) ionic species formed upon impregnation on titania

The mode of interfacial deposition and the local structure of the Co(II), Ni(II), Cr(VI) and W(VI) ionic species formed upon impregnation on titania surface has been studied using various techniques provided by the interface science.

Fullerene C60 supported on silica and γ-alumina catalyzed photooxidations of alkenes

Deposition of fullerene C60 (2% w/w) on silica and γ-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 γ-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.

Interfacial Impregnation Chemistry in the Synthesis of Chromium Catalysts Supported on Titania

The interfacial impregnation chemistry involved in the synthesis of CrVI catalysts supported on titania is presented. The mode of interfacial deposition of the CrVI oxo‐species at the titania/electrolyte solution interface, the interfacial species, and the local structure of the deposited species were investigated. Several methodologies based on potentiometric titrations, microelectrophoresis, and adsorption experiments were used. Modeling of the interfacial deposition based on experimental results provided an integrated picture concerning the deposition features. The interfacial species were confirmed by using laser Raman spectroscopy. The deposited CrO42−, HCrO4−, and Cr2O72− ions are retained above the positively charged bridging hydroxyl groups (Ti2OH) of the titania surface as electrostatic forces cause the formation of ion‐pairs. Each CrO42− or HCrO4− ion is located above a bridging hydroxyl, and each Cr2O72− ion above two bridging hydroxyl groups. Only the CrO42− and HCrO4− ions are deposited, with a preference for the CrO42− ions, at low CrVI surface concentrations (up to 0.3 μmol CrVI m−2). Cr2O72− ions were deposited in addition to CrO42− and HCrO4− ions at higher CrVI concentrations. The direct probing of the interfacial species by using laser Raman spectroscopy confirmed the interfacial species determined by modeling the deposition data. The deposition model developed describes all of the experimental data (from adsorption, titration, and microelectrophoresis experiments) very well. Moreover, it accurately predicts the displacements of pzc and iep caused by the presence of these species in the solution.

Optimization of the synthesis technique of molybdenum sulfide catalysts supported on titania for the hydrodesulfurization of thiophene

The present work deals with the optimization of the preparation method for the synthesis of unpromoted molybdenum catalysts supported on titania. Four methodologies have been followed: equilibrium–deposition–filtration (EDF), wet impregnation (WI), dry impregnation (DI) and controlled wet impregnation (CWI). The Mo surface density of all catalysts was 3.2 Mo atoms/nm2. The catalysts were characterized using N2 adsorption, XRD, XPS and DRS. The catalysts were evaluated in the hydrodesulfurization (HDS) of thiophene, using an atmospheric differential fixed bed reactor. EDF technique provided the most active catalyst followed by that prepared by WI and then by those prepared by DI or CWI. Although the impregnation technique does not influence the structure of the catalysts at nanoscale, a suitable choice of the impregnation technique leads to the increasing contribution of supported molybdenum clusters with small size and thus with high number of the Ti–O–Mo (Ti–S–Mo) bridges per Mo atom, stabilizing highly S-deficient structures. In order to confirm the favorable influence of titania on the catalytic performance, we have synthesized by DI a Mo catalyst supported on γ-alumina with the same loading. The relatively low activity exhibited by this catalyst was attributed to the different structure of the supported molybdenum species with respect to those formed on titania and to the lower Mo dispersion.

Elucidation of the surface configuration of the Co(II) and Ni(II) aqua complexes and of the Cr(VI), Mo(VI) and W(VI) monomer and polymer oxo–species deposited on the titania surface during impregnation

Abstract The structure of the precursor Co(II) and N(II) aqua complexes and the Cr(VI), Mo(VI) and W(VI) monomer and polymer oxo–species formed upon impregnation at the interface developed between the surface of the titania grains and the impregnating solution was thoroughly elucidated. Moreover, the interfacial speciation was determined for various surface concentrations of the precursor species regulated by adjusting the corresponding solution concentrations and the pH of the impregnating solution.