(449g) Unsupported Mo2C Catalysts for Low Temperature CO2 Hydrogenation to Methanol

CO₂ conversion to higher-value products is a key component of technologies aimed at decarbonizing the fuel and chemical industries. The conversion of CO₂ to methanol over conventional Cu-based catalysts (473–573 K) is challenged by lower than desired yields, a consequence of the concurrent formation of CO through reverse water gas shift (RWGS) and equilibrium limitations on methanol conversion at these temperatures (X < 8–30%). Achieving higher methanol yields from CO₂ hydrogenation requires directing kinetics, and thereby selectivity, towards methanol and operating at low temperatures (< 423 K) where methanol conversion is not significantly equilibrium limited. Herein we report continuous CO₂ hydrogenation at low temperatures (373–408 K, H₂/CO₂ = 0.1–50, 5–35 bar) with high selectivity to methanol (~ 80%) over unsupported Mo₂C catalysts.

Methanation and RWGS occur concurrently with methanol synthesis during CO₂ hydrogenation over Mo₂C. Reaction pathway analysis in conjunction with co-feeding products indicates all products form through primary reaction pathways. Identical dependences of rates on CO₂ pressure for methanol synthesis and RWGS rates suggest a common CO₂-derived intermediate and shared active site, while methanation occurs through a distinct intermediate. Methanol selectivity increases with increasing hydrogen pressure and decreasing water pressure at fixed conversion, reflecting forward kinetic rates of methanol synthesis that are positive order in hydrogen and negative order in water. Such dependencies are rationalized by kinetically relevant hydrogenation of a CO₂-derived intermediate and high surface coverages of H* intermediates. Active site quantification via titration with trifluoroacetic acid at reaction temperatures enables rate normalization without exposing Mo₂C samples to ambient conditions. Together, these findings demonstrate the ability of unsupported Mo₂C to catalyze CO₂ hydrogenation to methanol at low temperatures and provide insight into the reaction network and mechanisms involved in its formation.