Catalytic reforming of methane with H₂S via dynamically stabilized sulfur on transition metal oxides and sulfides

Reforming of methane with H₂S is a promising path to directly utilize sour natural gas reserves, although some aspects of the mechanism and structure–function relations remain elusive. Here we show that metal oxides of group 4–6 elements, which are inert for steam and dry methane reforming reactions, are active and stable (pre)catalysts for the H₂S reforming of methane. The key active sites are sulfur species (S*) that are dynamically bound to metal cations during catalysis. Similar H/D isotope exchange patterns and universal rate inhibition by H₂ on representative catalysts indicate that H₂S decomposition and recombination of surface hydrogen atoms are quasi-equilibrated, whereas CH₄ dissociation steps are reversible, yet not quasi-equilibrated. An in-depth analysis of the kinetic data and isotopic substitution effects identifies S*-mediated C–H bond cleavage as the most plausible rate-limiting step common for all catalysts, with subtle yet essential differences between 3d and 4d/5d catalysts in the thermodynamic stability of S*. [Figure not available: see fulltext.]