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

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-species 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 × 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···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···O−MoO3]1.57− increases with pH, while the concentration of the [TiOMoO3]0.35− decreases. Thus, at pH > 8, the [Ti2OH···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.