Dependence of copper particle size and interface on methanol and CO formation in CO2 hydrogenation over Cu@ZnO catalysts

Abstract

Although Cu-based catalysts have been intensively investigated for methanol synthesis via CO₂ hydrogenation, the active sites for methanol and CO formation are still a subject of debate. In this work, Cu–ZnO interfacial effects on product selectivity were revealed using a series of Cu@ZnO catalysts prepared by a deposition–precipitation method. The results indicate that Cu and ZnO alone show hardly any conversion of CO₂. In contrast, the combination of Cu and ZnO exhibits a remarkable higher activity, because the efficient spillover of dissociative H atoms on the Cu surface facilitates the conversion of abundant CO₂ adsorbed on ZnO. In particular, increased Cu–ZnO interfacial sites favor methanol production, via two ways: on the one hand, the formation of the Cu–ZnOH interface due to H spillover promotes methanol production relative to the Cu–ZnO interface, and on the other hand, the electron donation from ZnO to Cu strengthens the Lewis acidity of ZnO to stabilize formate intermediate *HCOO and enhance its hydrogenation to methanol. In addition, it was found that CO prefers to form with a notable higher rate on the low-coordinated Cu sites as a result of a large number of stronger H–Cu bonds at the edge/corner sites. These active sites compared to the coordinately saturated Cu sites show their merit for enhancing the formation of *COOH and subsequent shifting to *CO and *OH due to stronger C–Cu bonds. Methanol is relatively difficult to form on Cu sites due to its weak affinity to CO₂. This work provides mechanistic insights into the design of efficient catalysts for CO₂ conversion into methanol.