Effects of oxygen coverage on rates and selectivity of propane-CO2 reactions on molybdenum carbide

Abstract

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∗[sbnd]O∗, O∗[sbnd]∗, and ∗[sbnd]∗ 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∗[sbnd]∗ 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.