No abstract

Catalysts based on molybdenum carbide or nitride nanoparticles (2-5 nm) supported on titania were prepared by wet impregnation followed by a thermal treatment under alkane (methane or ethane)/hydrogen or nitrogen/hydrogen mixture, respectively. The samples were characterized by elemental analysis, volumetric adsorption of nitrogen, X-ray diffraction, and aberration-corrected transmission electron microscopy. They were evaluated for the hydrogenation of CO 2 in the 2-3 MPa and 200-300 • C ranges using a gas-phase flow fixed bed reactor. CO, methane, methanol, and ethane (in fraction-decreasing order) were formed on carbides, whereas CO, methanol, and methane were formed on nitrides. The carbide and nitride phase stoichiometries were tuned by varying the preparation conditions, leading to C/Mo and N/Mo atomic ratios of 0.2-1.8 and 0.5-0.7, respectively. The carbide activity increased for lower carburizing alkane concentration and temperature, i.e., lower C/Mo ratio. Enhanced carbide performances were obtained with pure anatase titania support as compared to P25 (anatase/rutile) titania or zirconia, with a methanol selectivity up to 11% at 250 • C. The nitride catalysts appeared less active but reached a methanol selectivity of 16% at 250 • C.

Molybdenum carbides supported on TiO2 or ZrO2 were prepared by temperature programmed reduction carburization method using mixtures of hydrogen and hydrocarbon (methane or ethane). All the materials exhibited molybdenum carbide with cubic crystallographic structure. The carbon content and MoC lattice parameter increased with the increase of hydrocarbon percentage (5-40%) and temperature (600-800°C) during carburization. All catalysts were significantly active in the hydrogenation of succinic acid to butyric acid and γ-butyrolactone. For the first time, a correlation between the degree of carburization and the catalytic activity for succinic acid hydrogenation was established. The selectivity depends strongly on the support. MoC/TiO2 favored the formation of butyric acid while MoC/ZrO2 and bulk MoC generated primarily γ-butyrolactone. The stability of MoC/TiO2 up to 50 h on stream in continuous reactor was demonstrated, showing the interest of carbide catalysts for future biorefinery processes.