Copper(I) Oxo Species in Cu/ZnO Catalysts and Its Role in the Methanol Synthesis from CO2 Hydrogenation

Abstract

This study reveals the presence of low-atomicity copper(I) oxo clusters strongly anchored to ZnO in commercial-type Cu/ZnO catalysts that, coexisting with metallic copper nanoparticles, play a critical role in CO2 hydrogenation. Their detection is challenging because metallic copper nanoparticles dominate the catalyst surface and these clusters appear in low amounts. Nevertheless, definitive evidence of their existence comes from the combination of in situ Raman spectroscopy, temperature-programmed CO surface reaction (TPSR-CO), time-resolved kinetic studies, and advance microscopy studies (STEM-XEDS and STEM-EELS). Catalytic studies combined with density functional theory (DFT) calculations demonstrate its critical role in methanol production and their size-dependent contribution to the total activity compared to traditional Cu-ZnO interfacial sites. According to DFT studies, CO2 adsorption is significantly more favorable on small Cu14O7 and Cu32O16 clusters than on extended Cu2O (111) (110) facets, metallic copper, or conventional metal-support interfacial sites. Moreover, the stabilization of carboxyl rather than formate-like intermediates follows a similar trend than the methanol production, being preferential on small Cu14O7 and Cu32O16 clusters, underlying a carboxyl-type intermediate path in methanol production.