| Bulk mixed metal oxide catalysts are used in industry for a number of oxidation processes. Some examples are bulk iron-molybdate for methanol oxidation to formaldehyde and bulk bismuth-molybdate for propylene ammoxidation to acrylonitrile. The reason behind their activity and selectivity, however, is still not well-understood. To address these fundamental questions, spectroscopic and kinetic studies were undertaken to provide quantitative bulk and surface information. Raman, low energy ion scattering (LEIS), CH3OH-IR and CH3OH-temperature programmed surface reaction (TPSR) spectroscopy along with steady state methanol oxidation were employed to investigate the fundamentals responsible for the activity and selectivity of bulk mixed molybdate and vanadate catalysts.;To examine the literature hypothesis that bulk M=O bonds are responsible for the activity of bulk mixed metal oxides, a series of bulk mixed vanadates and molybdates were synthesized. Their methanol oxidation activity was compared with the corresponding bulk M=O crystallographic bond lengths and Raman vibrations. No particular relationship, however, was found between the bulk M=O bond length and its specific catalytic activity (TOF). The study also indicated that the surface of bulk mixed vanadium and molybdenum oxides were enriched with vanadium and molybdenum oxide species, respectively.;These new surface insights motivated more detailed analyses of bulk iron-molybdate, Fe2(MoO4)3, and iron-vanadate, FeVO 4, catalysts and their performance for selective oxidation of methanol to HCHO. The stoichiometric iron-molybdate (Mo/Fe=1.5) catalyst is surface enriched with MoOx species and addition of excess MoO3, Mo/Fe ratio > 1.5, covers the exposed FeOx sites with surface MoOx species. The surface Mo/Fe ratio affects the reaction selectivity since the redox surface MoOx sites produce HCHO and the acidic surface FeOx sites form dimethyl ether. This was further confirmed by synthesizing a surface MoOx monolayer on a Fe 2O3 support and similar catalytic performance was found. Analogous behavior was found for the bulk FeVO4 catalyst. Both bulk Fe2(MoO4)3 and FeVO4 catalysts follow the Mars-van Krevelen reaction mechanism directly utilizing lattice oxygen rather than gas phase molecular O2. These studies demonstrate that the bulk mixed metal molybdate and vanadate catalysts are surface enriched with surface molybdena and vanadia species, respectively, that are the catalytic active sites for redox selective oxidation reactions. |
