Natural and shale gas can contain varying concentrations of acidic gases such as carbon dioxide and hydrogen sulfide, in addition to small alkanes (C1-C4). H2S that can act as a severe poison in the presence of transition metal catalysts such as platinum and typically has to be removed prior to alkane activation. Transition metal sulfides, on the other hand, are resistant to, and sometimes are promoted by, co-feeding H2S. Further, the CO2 in the feed can potentially serve as a “soft oxidant” to remove the hydrogen and drive the equilibrium-limited dehydrogenation step. In view of these advantages, our goal is to unravel the mechanism and site requirements of upgrading alkanes in the presence of acidic gases on transition metal sulfide catalysts. To this end, in this talk, we will present the mechanism of conversion of a surrogate system (CO2 + H2 + H2S) on molybdenum sulfide catalysts. Using a combination of reaction kinetics experiments and density functional theory, we will specifically show that (i) coordinative unsaturated sites on the edges of the catalyst are very active relative to edge/brim sites for reducing CO2 but are poisoned by co-feeding even small amounts of H2S, (ii) C-O bond scission is rate-controlling, and (iii) at high co-feeds of H2S, the molecule tends to either decompose to produce additional hydrogen, or react with CO2 directly.

Elucidating the mechanism of reverse water gas shift reaction on molybdenum sulfides

2019 Spring National Meeting

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