This thesis aims at synthesizing various molybdenum (Mo) sulfides and oxysulfides (MoxOySz) species, exploring the whole range of y and z possible numbers, and to analyze the effects of the Mo surface concentration, chemical composition (z/y) and sizes of such species for CO2 photoreduction applications. These species are supported on both an optically inert support, gamma-alumina (y-Al2O3), to analyze if they can be self-sufficient to realize CO2 photoconversion, and on the titania (TiO2 P25) semiconducting support to study the impact of a possible heterojunction on the CO2 photocatalytic activity. Different classes of Mo precursors are explored through three syntheses approaches: 1) the conventional approach using a peroxomolybdate precursor with various sulfo-reductive treatments, 2) the surface organometallic chemistry inspired approach using an alkoxide precursor with various sulfo-reductive treatments, and 3) the molecular approach using well-defined Mo oxysulfide molecular precursors without any sulfidation treatment. These materials are tested on the photocatalytic unit present at IFP Energies Nouvelles, which operates in the gas-solid state, with a continuous flow-type reactor. While all catalysts are very selective in methane production, we show that an optimal surface concentration of Mo species is required to reach the best catalytic activity. By using various characterizations (TEM, XPS, UV-visible spectroscopy, …), we study the link between chemical composition, nanostructure, opto-electronic properties, and the reactivity of these systems. Operando XAS studies are also developed and conducted at the ROCK beamline of the synchrotron SOLEIL, they highlight for the first time interesting features at the Mo edge during the photocatalytic process.
Gratuit
Disciplines