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

摘要

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 similar 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.