Distinctly different active sites of ZnO-ZrO2 catalysts in CO2 and CO hydrogenation to methanol reactions

Abstract

The active site of a solid catalyst varies sensitively with the catalyzed reaction. Herein, using experimentally measured elementary surface reaction kinetics of CO₂ or CO hydrogenation reactions over a ZnO-ZrO₂ catalyst under working conditions in combinations with comprehensive structural characterizations and theoretical simulations, we unveil the distinctly different active sites in catalyzing the CO₂ or CO hydrogenation to methanol reaction. Zn²⁺ cations with different local environments are present on the ZnO-ZrO₂ surface, including Zn₁ single atoms exclusively with a Zn-O-Zr local structure and Zn_n clusters with both Zn-O-Zr and Zn-O-Zn local structures. The -Zr-O-Zr- structure bonded to the Zn_n clusters is more easily to be reduced than that bonded to the Zn₁ single atoms. The Zn₁-single atom (-Zr-O-Zn-O-Zr-) is the active site for catalyzing the CO₂ hydrogenation to methanol reaction, whereas the Zn_n cluster bonded to an in situ formed -Zr-V_o-Zr- structure (-Zn-O-Zn(-O-Zr-V_o-Zr-)-O-Zr-) is the active site for catalyzing the CO hydrogenation to methanol reaction. These results provide a reliable and effective methodology of elementary surface reaction kinetics for identifications of active sites of working catalysts in complex reactions and unveil how sensitively the active site structure varies with the catalyzed reaction.