CO 2 Hydrogenation over Copper/ZnO Single-Atom Catalysts: Water-Promoted Transient Synthesis of Methanol.

The hydrogenation of CO 2 by renewable power-generated hydrogen offers a promising approach to a sustainable carbon cycle. However, the role of water during CO 2 hydrogenation is still under debate. Herein, we demonstrated that either too low or too high contents of water hampered the methanol synthesis over Cu/ZnO based catalysts. For Cu single atoms on ZnO supports, the optimal content of water was 0.11 vol. % under 30 bar (CO 2  : H 2 =1 : 3) at 170 °C. Upon the introduction of optimal-content water, the methanol selectivity immediately became 99.1 %, meanwhile the conversion of CO 2 underwent a volcano-type trend with the maximum of 4.9 %. According to mechanistic studies, water acted as a bridge between H atoms and CO 2 /intermediates, facilitating the transformation of COOH* and CH 2 O*. The enhanced activity induced the generation of more water to react with CO via water-gas shift reaction, resulting in the increase in methanol selectivity.