Active phase of Cu-ZnO catalysts in methanol synthesis

The CO 2 hydrogenation activity of Cu-ZnO catalysts for methanol production is attributed to Cu δ+ sites formed on the interface between Cu nanoparticles and ZnO support. Currently, around 100 megatons of methanol are produced annually via CO or CO 2 hydrogenation on Cu-ZnO catalysts because of its applications as (potentially carbon-neutral) fuel and as a valuable feedstock for chemical synthesis. However, the nature of the active sites in the industrial Cu-ZnO catalysts has remained debated for decades, because Cu-ZnO catalysts undergo dynamic structural changes during the reaction. Various studies have proposed Cu-Zn bimetallic sites or interfaces between metallic Cu and ZnO support or ZnO overlayers as active phases for methanol synthesis. Herein, we conducted density functional (DFT) and microkinetic simulations of CO/CO 2 hydrogenation on three phases of Cu-ZnO catalysts: Cu-Zn alloy nanoparticles, ZnO films on Cu surfaces (ZnO/Cu), and Cu NPs on a ZnO support (Cu/ZnO). Cu-Zn alloys are calculated to be the most thermodynamically stable phase only at negligible CO 2 concentration in the reactant feed. In turn, Cu/ZnO becomes the most stable phase at CO 2 -rich conditions, whereas ZnO/Cu phase is stable at intermediate conditions. The microkinetic modelling suggests that CO 2 is the primary feedstock for methanol synthesis on a Cu-Zn alloy and Cu/ZnO, whereas ZnO/Cu interface exhibits dual reactivity towards CO 2 and CO. Importantly, the Cu/ZnO interface shows orders-of-magnitude higher methanol synthesis rates than Cu-Zn alloys or Cu-supported ZnO films, due to δ+ charge state of interfacial Cu atoms and stronger stabilization of intermediates at Cu-ZnO dual-sites. Our results explain the dynamic transformations of the Cu-ZnO catalysts and provide a foundation for optimizing their performance by stabilizing the most catalytically active phase through tailored activation treatments.