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

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.