Tailoring ZnO x Species Confined on ZrO2 Support for Enhanced CO Hydrogenation.

Abstract

ZnZrO _x is a promising oxide component for direct syngas conversion via oxide-zeolite bifunctional catalysis, while rational design of active centers within the composite oxide remains limited. In this study, through ab initio thermodynamics, molecular dynamics, and microkinetic modeling, we find that diverse subnanometer ZnO _x species, including single-site, single-chain, and single-layer configurations, can form on active ZrO₂ surfaces under the reaction conditions. These confined ZnO _x species weaken CO adsorption but enhance heterolytic H₂ dissociative adsorption, favoring continuous hydrogenation of CO to methanol over direct or H-assisted CO dissociation. For single-layer ZnO _x structures, a double-chain film grows on a monoclinic ZrO₂ (m-ZrO₂) surface while a graphene-like film emerges on tetragonal ZrO₂ (t-ZrO₂). These single-layer ZnO _x species exhibit higher methanol formation activity than their single-chain or single-site counterparts, which benefit from sufficient sites for adsorption of intermediates and a suitable space for bonding of H with C in CHO. Furthermore, the double-chain ZnO _x film confined on m-ZrO₂ exposes octahedral Zn_oct sites, which are more reactive than the triangular Zn_tri sites in the graphene-like ZnO _x on t-ZrO₂, despite both sites being nominally three-coordinate. These findings provide insights for the precise design of composite oxide/oxide catalysts through fine-tuning overlayer coverage and/or support surface properties.