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The Role Of ZnO In Cu/ZnO Methanol Synthesis Catalysts
Published 1996 · Chemistry
The methanol synthesis by the hydrogenation of CO2 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+/Cu0 ratio on the surface control the catalytic activity. Experiments using a physical mixture of Cu/SiO2 and ZnO/SiO2 showed that ZnOχ species migrated from the ZnO particles onto the Cu surface upon reduction with H2, leading to the formation of the Cu+ active species in the vicinity of the ZnOχ species on Cu. This model was proven by the surface science studies using partially Zn-deposited Cu(111), where the ZnOχ 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 Cu0 active sites for methanol formation because the activity increased with decreasing Cu0 surface area. On the other hand, the activity for the reverse water-gas shift reaction decreased with increasing Zn coverage.