The role of ZnO in Cu/ZnO methanol synthesis catalysts - morphology effect or active site model?

We examined whether or not the presence of ZnO changed the morphology of Cu particles in Cu/ZnO methanol synthesis catalysts using physical mixtures of Cu/SiO2 + ZnO/SiO2. The yield of methanol produced by the CO2 hydrogenation over the physical mixture increased by the reduction with Hz at 573-723 K. The promotion of the methanol synthesis activity was due to migration of Zn from ZnO onto Cu/SiO2. leading to the Cu-Zn active sites, as previously reported. On the other hand, no activity change was observed upon the reduction treatment for the reverse water gas shift (RWGs) reaction over the physical mixture: this result cannot he explained by the morphology change. The activation energy of the methanol synthesis and the RWGS reaction over the physical mixture did not vary upon reduction, further supporting the absence of any morphology change. It was thus clearly proved that the role of ZnO was not to change the morphology of Cu, but to create the Cu-Zn active sires for methanol synthesis only. (C) 2001 Elsevier Science B.V. All rights reserved.