Item

Abstract

Small gold nanoparticles supported on ZnO have been identified as highly active and selective catalysts for the green synthesis of methanol from CO₂ and H₂. Furthermore, they can serve also a model system for the mechanistic understanding of methanol synthesis on the industrial Cu/ZnO catalyst. The dynamic changes in the structure of Au/ZnO upon exposure to methanol synthesis gas mixtures were studied using a combination of TAP reactor and near edge X-ray absorption spectroscopy (XANES) measurements at the Zn L_III edge, both in CO₂/H₂ and CO/H₂ gas mixtures. TAP measurements indicated that CO can create significant amounts of O-vacancy defects in ZnO at 240°C, while CO₂ can re-oxidize a pre-reduced catalyst or maintain this state in the presence of s
trongly reducing gases (CO and H₂). Furthermore, CO₂ present as reactant or resulting from the reactive removal of surface lattice oxygen by interaction with CO can be deposited on the pre-reduced Au/ZnO surface as stable adsorbed carbon containing species, e.g., as surface carbonates, which decompose at T≥250°C. In situ XANES measurements at the Zn L_III edge revealed that ZnO is significantly reduced during reaction, both in CO₂/H₂ and CO/H₂ gas mixtures, but with the extent of the reduction being more pronounced in CO/H₂ than in CO₂/H₂. These results will be critically discussed in the light of previous findings on the role of ZnO reduction in the activity of methanol synthesis catalysts.