Controlling the O-Vacancy Formation and Performance of Au/ZnO Catalysts in CO₂Reduction to Methanol by the ZnO Particle Size

Abstract

In a systematic approach to control and improve the performance of Au/ZnO catalysts in methanol synthesis from CO2, we have studied the effect of varying the ZnO particle size. We show that with increasing ZnO particle size (22–103 nm), while keeping the Au loading/Au particle size constant, the activity for methanol formation passes through a maximum in a volcano-shaped relation, while the selectivity increases steadily. This is explained by an increasing electronic modification of Au interface perimeter sites, due to electronic metal–support interactions (EMSIs), which occur together with partial overgrowth of a partly reduced ZnOx layer (SMSI effects); electronic modifications are proposed to arise from the increasing formation of O-vacancy defects in the ZnO lattice during reaction, whose concentration increases with increasing density of Au nanoparticles (NPs) on the support surface and thus increasing the ZnO particle size, as indicated by EPR spectroscopy, and charge transfer to adjacent Au sites. We propose that there is an optimum charge transfer and thus an optimum Au NP density, which results in the observed maximum in the methanol formation rate. Partial overgrowth is indicated by STEM imaging and quantified by in situ FTIR spectroscopy during CO adsorption at −140 °C, which revealed a significant decrease of the accessible Au surface area, which is more pronounced for smaller Au NP densities. Optimization of the support particle size at constant metal loading and thus of the Au NP density is proposed as an attractive approach for controlling the performance of supported catalysts.