Exploring the activated state of Cu/ZnO(0001)-Zn, a model catalyst for methanol synthesis

The interaction of Cu clusters with ZnO(0001)-Zn terminated crystal faces is studied after reduction at high temperatures by a combination of scanning tunneling microscopy, scanning tunneling spectroscopy, X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy. We find that tiny amounts of Cu promote the reduction of the ZnO surface by means of triangular reconstruction. This metallization of the surface leads to a Cu-Zn alloy. The XPS results reveal that the degree of alloying depends strongly on the reduction temperature. This alloying stabilizes Zn from evaporation and H2 desorption below 500 K. On the basis of these results, we conclude that the activated catalyst surface consists of Zn-Cu alloy sites that form at 700-800 K. Moreover, the surface alloying enhances the activity of Cu/ZnO catalysts under typical methanol synthesis conditions.