Systematic Study of Adsorption and the Reaction of Methanol on Three Model Catalysts: Cu(111), Zn–Cu(111), and Oxidized Zn–Cu(111)

The adsorption and reaction of methanol on Zn-modified Cu(111) surfaces were investigated by synchrotron-radiation X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). Dehydrogenation of methanol is not observed on clean Cu(111) or clean Zn–Cu(111) surfaces. The amount of more stable methanol molecules at defect sites is increased on a Zn–Cu(111) surface because surface roughening occurs as a result of surface-alloy formation, which leads to an increase in step density. All of the methanol is desorbed intact by heating to ∼220 K. On the other hand, when a Zn–Cu(111) surface is preoxidized by oxygen, methanol is partially dehydrogenated to methoxy species at 130–160 K. The reactivity of methoxy depends significantly on the adsorption sites. In particular, some reactive methoxy species are found, which are dehydrogenated to formaldehyde at 220–290 K, on a preoxidized Zn–Cu(111) surface. Methoxy species on the surface of a zinc oxide island are dehydrogenated to formaldehyde at 340–490 K. Dehydrogenation of methoxy above room temperature has been reported on both zinc oxide surfaces and oxidized Cu surfaces. Therefore, we conclude that there exists a synergetic effect between the zinc oxide islands and the partially oxidized Cu substrate for the partial oxidation (dehydrogenation) of methanol.