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Effects of oxygen coverage on rates and selectivity of propane-CO2 reactions on molybdenum carbide
- Sullivan, Mark M., Bhan, Aditya
- Journal of catalysis 2018 v.357 pp. 195-205
- carbon, carbon dioxide, carbon monoxide, catalysts, catalytic activity, dehydrogenation, hydrogen, kinetics, methane, models, molybdenum, oxidation, oxygen, propane, propylene
- Mo2C catalyzes propane dehydrogenation and hydrogenolysis at 823 K; carbon selectivity can be tuned to >95% propylene via dehydrogenation in absence of H2, >95% CH4 via hydrogenolysis with H2 co-feed, or >80% CO via reforming pathways with H2 and CO2 co-feed. The changes in selectivity are mediated by an evolution in the coverage of oxidized (O∗) and carbidic (∗) surface sites which results in an evolution of O∗O∗, O∗∗, and ∗∗ site pairs that catalyze propane dehydrogenation. The fraction of O∗ in relation to ∗ was assessed from measured CO2/CO ratios because reverse water gas shift equilibrium exists under H2/CO2 co-feed steady state reaction conditions. Kinetic models based on the two-site dehydrogenation mechanism could be used to quantitatively describe measured rates of propane dehydrogenation at steady state with or without H2 and/or CO2 co-feed and the transient evolution in dehydrogenation rates upon removing H2 or CO2 in the influent stream to note that O∗∗ site pairs exhibit the highest rate per gram. This model also provides a rationale for O∗ inhibition of H-activated hydrogenolysis pathways and for promotion of oxidative dehydrogenation rates with the introduction of hydrogen into CO2-propane influent streams. This study extends concepts developed for examining the catalytic effects of O∗ coverage on oxidative light alkane conversion from transition metal catalysts to also include carbidic formulations.