Spectroscopic identification of active centers and reaction pathways on MoS2 catalyst for H2 production via water-gas shift reaction

In previous reports, it was proposed that the oxygen-substituted Mo(SxOy)zc site formed in situ by water is active center for water-gas shift reaction catalyzed by MoS2. However, water is also hypothesized to be the driving force for sulfide catalyst deactivation. This irreconcilable dispute stems from the limited understanding about the reaction mechanism and the lack of relevant in situ or/and operando characterization. In this work, the different reactivity of the two preferentially exposed MoS2 edge sites, M-edge and S-edge sites, with CO and H2O is revealed by means of in situ CO adsorption followed by IR spectroscopy. Isotopic reactants (13CO/12CO; H218O) were used to account for the origin of the formed products and catalyst surface modification. In particular, upon H2O feed, S/O exchange occurs on M-edge leading to Mo(SxOy)zc sites that are not reactive towards subsequent CO feed in contradiction with a redox mechanism in which the catalyst surface is first exchanged by H2O and then reduced by CO. Moreover, the M-edge sites hardly give vacancy under CO treatment. Conversely, the S-edge sites are much less prone to S/O exchange upon H2O feed but are sensitive to CO to form vacancies and release COS. In addition, IR operando studies are in accordance with a formate pathway and a novel redox mechanism via COS formation. This insight into the catalytic active sites under reaction conditions allows to identify the M-edge sites as the ones leading to the deactivation of the catalyst and the S-edge sites as the redox active sites. Thus, the work gives the direction for the rational design of high-performance and stable sulfide catalysts for reactions involving H2O dissociation and CO conversion.