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

Abstract

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 microkinetic 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 (H y MoO3–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 O2 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.

Learning from the past: Are catalyst design principles transferrable between hydrodesulfurization and deoxygenation?

“…To begin building an understanding of this complex material, we focus on the MoO 3 end of the Mo‐oxy‐carbide spectrum. MoO 3 has attracted wide‐spread interest as a potential HDO catalyst, because it can catalyze many redox reactions, such as partial oxidation, or DeNO X . For example, Choksi and Greeley used density functional theory (DFT) to study the partial oxidation of methanol on MoO 3 , and observed a strong influence of oxygen partial pressure on the state of the surface and the reaction kinetics.…”

“…MoO 3 has attracted wide‐spread interest as a potential HDO catalyst, because it can catalyze many redox reactions, such as partial oxidation, or DeNO X . For example, Choksi and Greeley used density functional theory (DFT) to study the partial oxidation of methanol on MoO 3 , and observed a strong influence of oxygen partial pressure on the state of the surface and the reaction kinetics. Grunwaldt and coworkers presented a systematic study on the influence of synthesis procedure and the morphology of MoO 3 on the selective oxidation of propylene .…”

Developments in the Atomistic Modelling of Catalytic Processes for the Production of Platform Chemicals from Biomass

“…For instance, ceria and molybdenum oxides have been described as active in the transformation of methanol (a byproduct of several biomass conversion processes) [106] into formaldehyde. [107][108][109][110][111][112][113][114] For CeO 2 , the structure sensitivity drives the selectivity: the closed-packed (111) and (110) surfaces produce formaldehyde, which on the (100) surface is further oxidized to CO. [115] For molybdenum oxides, the influence of Mo oxidation state and the degree of surface reduction were analyzed. It was shown that only the fully oxidized MoO 3 is selective for formaldehyde production.…”

“…In parallel, the study of the reactivity of biomass on oxide surfaces has many more fundamental problems arising from the complexity of the electronic structure of these materials but also for the much wider variability in the types of elementary steps that they allow. For instance, ceria and molybdenum oxides have been described as active in the transformation of methanol (a byproduct of several biomass conversion processes) into formaldehyde . For CeO 2 , the structure sensitivity drives the selectivity: the closed‐packed (111) and (110) surfaces produce formaldehyde, which on the (100) surface is further oxidized to CO .…”

Effect of particle shape on methanol partial oxidation in a fixed bed using CFD reactor modeling

“…A first principles study by Choksi and Greeley showed that the oxygen partial pressure has a strong impact on both the state of the catalyst surface and the rate of the reaction. They showed that the turnover frequency of methanol oxidation reaches a maximum at intermediate oxygen partial pressures and then again decreases.…”