The role of ZnO in Cu/ZnO methanol synthesis catalysts

Abstract

The methanol synthesis by the hydrogenation of CO 2 over Cu-based catalysts and Zn-deposited Cu(111) model catalysts was studied using XRD, TEM-EDX, reactive frontal chromatography, and surface science techniques such as XPS and AES. For the powder catalysts, a volcano-shaped relation between the oxygen coverage on the Cu surface and the specific activity for methanol formation was obtained, suggesting that a Cu +/Cu 0 ratio on the surface control the catalytic activity. Experiments using a physical mixture of Cu/SiO 2 and ZnO/SiO 2 showed that ZnO χ species migrated from the ZnO particles onto the Cu surface upon reduction with H 2, leading to the formation of the Cu + active species in the vicinity of the ZnO χ species on Cu. This model was proven by the surface science studies using partially Zn-deposited Cu(111), where the ZnO χ species on the Cu(111) surface promoted the catalytic activity of methanol formation, and a volcano-shaped relation between the Zn coverage on the Cu surface and the catalytic activity was obtained. The results definitely contradict the model of single Cu 0 active sites for methanol formation because the activity increased with decreasing Cu 0 surface area. On the other hand, the activity for the reverse water-gas shift reaction decreased with increasing Zn coverage.