Nature of Hydride Species at the Surface of Ga-Based Oxides Used in the Methanol-Mediated Syngas to Light Olefins Conversion

Ga-based oxide catalysts have recently been proposed as an alternative to Zn-based oxides for the methanol-mediated syngas-to-light-olefins conversion through the OXZEO process, due to their high capacity to produce CH3OH intermediate and concomitant weaker activity for olefins hydrogenation. In this work, the nature and reactivity of surface Ga–H species were investigated by comparing in situ/operando DRIFTS measurements and DFT calculations to assess their role in achieving such selective hydrogenation. Like α-Ga2O3, GaZr mixed oxides were shown to favor formation of isolated gallium hydrides tri-coordinated to oxygen anions (Gatri–H) via a heterolytic dissociation mechanism of H2, with moderate but sufficient capacity to hydrogenate HCOO* to CH3OH. In contrast, on β-Ga2O3, both homolytic and heterolytic dissociative pathways of H2 led to Gatri–H species in close proximity, which were found much less stable and hence more prone to unselective hydrogenation reactions. These metastable gallium hydrides on coordinatively unsaturated Ga cations are proposed to strongly reduce the olefins selectivity in the OXZEO process but should be efficient for other reactions requiring consecutive (de)hydrogenation steps.