Inhibitor, Co-Catalyst, or Co-Reactant? Probing the Different Roles of H2S during CO2 Hydrogenation on the MoS2 Catalyst

Natural gas is often sour, containing varying amounts of H2S and CO2. Gas sweetening processes that are routinely used rely on processes such as amine-based adsorption–desorption cycles, membrane separation, and cryogenic technologies which are energy-intensive and reject CO2. This work explores the mechanism and nature of active sites for hydrogenation (HYD) of CO2 in the presence of H2S on MoS2, a sulfur-tolerant catalyst, using steady-state kinetics studies and density functional theory calculations. In the absence of H2S, this reaction is positive order in CO2 and H2 and negative order in CO and H2O; the system, further, shows no H/D kinetic isotope effect, thereby indicating that C–O scission is rate-controlling. The specific mechanism at play, namely, redox or associative, could, however, not be adequately resolved with the available data. In the presence of H2S, the rate first drops (until about 60 ppm co-feed) and, then, increases (at >200 ppm), indicating a complex functional relationship of the reaction system with H2S. A combination of the ab initio phase diagram, kinetic experiments, and H2S/D2 scrambling studies indicate that (i) the edge structure of the molybdenum sulfide catalyst varies substantially with reaction conditions and co-feeds, (ii) multiple types of sites are at play, in particular, a sulfur-poor site that is active but prone to H2S poisoning and a sulfur-rich site that is less active but tolerant to sulfur with a preponderance of data pointing to these being coordinative unsaturated (CUS) and brim sites, respectively, on the metal edge, and (iii) H2S inhibits CO2 HYD by dissociatively adsorbing on these CUS sites at low co-feeds (0–60 ppm), reacts with CO2 to produce COS and CO at high co-feeds (>1000 ppm) and assists (or co-catalyzes) HYD by probably acting as a hydrogen shuttle at intermediate co-feeds.