Molybdenum carbide catalysts have demonstrated promising performance in the deoxygenation of biomass-derived oxygenates for the production of fuels and chemicals via ex situ catalytic fast pyrolysis (CFP). Mo2C possess multifunctional catalytic properties due to the presence of different types of sites including metallic-like sites, redox sites, and acidic sites, the relative abundance of which varies with synthesis procedure and reaction conditions. Some studies of carbide catalysts indicate that surface oxidation may increase the acidic character of the catalyst, though the structure and function of the proposed oxygen-induced acid sites are unknown. To probe the various catalytic functionalities of Mo2C and elucidate the role of surface oxygen, we use first principles calculations based on density functional theory (DFT) to determine the energetics of competing ethanol reaction pathways on Mo2C in the presence of 0.5 monolayer surface oxygen. This incorporation of significant oxygen coverage is consistent with experimental and theoretical studies of the composition of Mo2C under ex situ CFP conditions. The energetics of hypothesized oxygen-induced Brønsted acid site and oxygen-assisted exposed Mo-site catalyzed elementary steps are calculated for pathways leading to the major experimentally observed products, acetaldehyde and ethylene, at temperatures relevant to ex situ CFP (ca. 300 - 350 °C). The DFT-derived energetics are compared to experimentally collected kinetic data for ethanol and the results of the analysis reveal (i) the importance of O-assisted Mo-site catalyzed pathways and (ii) the role of surface oxygen coverage in key rate- and selectivity-controlling steps. This mechanistic information can ultimately be used to inform modifications to the Mo2C catalyst for improved deoxygenation performance.

On the role of surface adsorbed oxygen in ethanol reaction pathways on Mo2C

255th National Meeting (2018)

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