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

Abstract

AbstractThe 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.