Low Temperature CO2 Hydrogenation on Unsupported Mo2C Catalysts

CO2 hydrogenation to methanol, a key reaction for decarbonizing the fuel and chemical industries, requires catalyst formulations that hydrogenate CO2 selectively to methanol at temperatures where methanol conversion is not significantly equilibrium limited (<423 K). Herein we report continuous CO2 hydrogenation at low temperatures (348-408 K, H2/CO2 = 0.1-50, 5-35 bar) with high selectivity to methanol (up to ca. 80%) over unsupported ..beta..-Mo2C catalysts. Active site density quantification via titration with trifluoroacetic acid at reaction temperatures enables an assessment of site-specific rates. Methanation and reverse water gas shift (RWGS) occur concurrently with methanol synthesis during CO2 hydrogenation over Mo2C. Reaction pathway analysis, product cofeeds, and reversibility formalisms show that all products form through primary reaction pathways from CO2, but secondary reactions of CO contribute significantly to rates of methanation. Dependences of forward rates on reactant and product concentration determined by independently varying the CO2, H2, CO, H2O, CH3OH, and CH4 pressure in conjunction with reversibility formalisms reveal that all products form through H-assisted CO2 activation and involve partially hydrogenated CO2-derived intermediates. These inferences were verified by quantitative agreement between measured site-time yields and site-time yields predicted by closed form kinetic rate expressions in an integral reactor model over widely varying conditions (85-2000 kPa H2, 80-1500 kPa CO2, 0-45 kPa H2O, 0-21 kPa CO, 0-25 kPa CH3OH, 0-75 kPa CH4, 5-87 mol Mos s mol CO2-1). Coverages calculated based on the kinetic model reveal that the Mo2C surface is covered with bidentate CO- and CO2-derived intermediates of the stoichiometry H2CO2 and H2CO, indicating that H2 and COx do not compete for surface occupancy but instead adsorb cooperatively to form partially hydrogenated intermediates. Hydrogenation of the CO-derived H2CO** intermediate favors methanation, while hydrogenation of CO2-derived H2CO2** favors methanol synthesis. Together, these findings demonstrate the ability of unsupported Mo2C to catalyze the hydrogenation of CO2 to methanol at low temperatures and provide insight into the reaction network and mechanisms involved in its formation.