Surface science studies of catalyzed methanol synthesis on model copper and copper-zinc-oxygen surfaces.

摘要

Cu-Zn-O surfaces that are catalysts for methanol synthesis from CO, CO{dollar}sb2{dollar}, and H{dollar}sb2{dollar} are modeled using zinc oxide overlayers on copper single crystals. These studies were performed in ultra-high vacuum (UHV) utilizing Temperature Programmed Desorption, Auger Electron Spectroscopy, and Low Energy Electron Diffraction techniques.; The chemisorption of O{dollar}sb2{dollar}, CO, CO{dollar}sb2{dollar}, and D{dollar}sb2{dollar} were compared on a stepped Cu(311), and a flat Cu(110). At low pressures ({dollar}sim{dollar}10{dollar}sp{lcub}-6{rcub}{dollar} Torr), Cu(311) was found to be much more reactive than Cu(110) for the dissociative adsorption of CO{dollar}sb2{dollar} and D{dollar}sb2{dollar}, and the formation of CO{dollar}sb2{dollar} from surface oxygen and CO. Since these reactions are important in methanol synthesis, these results suggest that methanol synthesis over copper may be a structure sensitive reaction.; The interaction of copper, zinc, and oxygen were examined by the deposition of submonolayers to multilayers of zinc and oxygen in UHV on Cu(110). Carbon monoxide adsorbs well on copper at 150 K and low pressures ({dollar}<{dollar}10{dollar}sp{lcub}-6{rcub}{dollar} Torr), but only poorly on the oxides of copper and zinc. Carbon dioxide adsorbs on ZnO at 150 K and low pressures ({dollar}<{dollar}10{dollar}sp{lcub}-6{rcub}{dollar} Torr), but not on copper or oxidized copper. We used a combination of CO and CO{dollar}sb2{dollar} adsorption to follow the initial growth of two-dimensional ZnO{dollar}sb{lcub}x{rcub}{dollar} islands and the effects of heat and oxygen treatments on these islands. Heating above 300 K leads irreversibly to three-dimensional island formation. In addition, the behavior of ZnO{dollar}sb{lcub}x{rcub}{dollar} overlayers on Cu(311) and a high defect concentration Cu(111) were compared to ZnO{dollar}sb{lcub}x{rcub}{dollar} overlayers on Cu(110).; The interaction of methanol with these model Cu-Zn-O surfaces was also studied. Oxygen was adsorbed onto the exposed copper part of the surface to form ZnO{dollar}sb{lcub}x{rcub}{dollar}/y ML O/Cu(110) surfaces. The roles of ZnO{dollar}sb{lcub}x{rcub}{dollar} islands and chemisorbed oxygen on copper were investigated by monitoring methanol decomposition, into surface formate and methoxy species, on these ZnO{dollar}sb{lcub}x{rcub}{dollar}/y ML O/Cu(110) surfaces. The role of chemisorbed oxygen on Cu(110) in the ZnO{dollar}sb{lcub}x{rcub}{dollar}/y ML O/Cu(110) system is to keep the zinc oxidized and to increase the amount of formate formed on the ZnO{dollar}sb{lcub}x{rcub}{dollar} component of the surface. The presence of ZnO{dollar}sb{lcub}x{rcub}{dollar} increases the surface formate to methoxy ratio from that of O/Cu(110).