Partial Oxidation of Methanol on MoO3 (010): A DFT and Microkinetic Study

Methanol oxidation is employed as a probe reaction to evaluate the catalytic properties of the (010) facets of molybdenum trioxide (MoO3), a reducible oxide that exhibits a rich interplay of catalytic chemistry and structural transformations. The reaction mechanism is investigated with a combination of electronic structure calculations, using the BEEF-vdW and HSE06 functionals, and mean-field micro kinetic modeling. Considered pathways include vacancy formation and oxidation, monomolecular dehydrogenation of methanol on reduced and nonreduced surfaces, bimolecular reactions between dehydrogenated intermediates, and precursor steps for hydrogen molybdenum phase (HyMoO(3-x)) formation. Methanol dissociation begins with C-H or O-H scission, with the O-H route found to be kinetically and thermodynamically preferred. Dehydrogenation of CH2O* to CHO* is slow in comparison to desorption, leading to complete selectivity toward CH2O. C-H scission of CH3O* and recombination of dissociated OH* to form H2O* are kinetically significant steps exhibiting positive degrees of rate control, while oxidation of the reduced surface through adsorbed O-2 has a negative degree of rate control. The energetics of the latter elementary step are somewhat sensitive to the choice of density functional, and although this does not affect the predicted reaction orders, the overall rate may change. To estimate the impact of the surface oxidation state on the kinetics, the external pressure of oxygen is varied in the microkinetic model, and the reaction rate is found to follow a volcano-like dependency, with the optimum rate located where surface oxidation neither promotes nor inhibits the overall rate. The methodology demonstrated in this study should be more broadly applicable to modeling catalytic kinetics on reducible oxide single-crystal surfaces.