- Chemical Engineering; Chemistry
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Mo 2 C catalyzes propane dehydrogenation and hydrogenolysis at 823 K; carbon selectivity can be tuned to >95% propylene via dehydrogenation in absence of H 2, >95% CH 4 via hydrogenolysis with H 2 co-feed, or >80% CO via reforming pathways with H 2 and CO 2 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 CO 2 /CO ratios because reverse water gas shift equilibrium exists under H 2 /CO 2 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 H 2 and/or CO 2 co-feed and the transient evolution in dehydrogenation rates upon removing H 2 or CO 2 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 CO 2 -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.