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

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.