Insights into the High Activity and Methanol Selectivity of the Zn/ZrO2 Solid Solution Catalyst for CO2 Hydrogenation

Abstract

Selective hydrogenation of CO2 to methanol is vital for mitigating the massive CO2 emission by utilizing the captured CO2 for chemical and fuel productions. Here, the key intermediates and mechanism of CO2 hydrogenation to methanol over the Zn/ZrO2 solid solution catalyst are thoroughly investigated by density functional theory calculations. Our calculations show that CO2 is highly activated when strongly adsorbed on the surface in a carbonate-like configuration, which may be the reason for the high CO2 conversion rate in methanol synthesis. In addition, CO formation from the dissociation of CO2 or COOH* is suppressed because of the stability of carbonate or the high energy barrier of COOH* formation, respectively. When compared with the traditional bi-HCOO route, where breaking the C-O bond is predicted to be the rate-determining step (RDS) with a modest energy barrier of 1.11 eV, a novel route is found to be kinetically much more favorable with a much lower energy barrier of 0.76 eV for the RDS of bi-H2CO* -> mono-H2CO*. This alternative route starts from a newly found HCO3* species in a tetrahedral configuration with the central C atom surrounded by an H atom and three O atoms, denoted as tri-HCOO*. It can be stepwise hydrogenated to methanol through the bi-HCO*, bi-H2CO*, mono-H2CO*, and H3CO* intermediates. These theoretical predictions suggest the high activity of the carbonate species in methanol synthesis over the Zn/ZrO2 solid solution catalyst, different from that on the pure metal oxide surfaces.