THE CHEMISORPTION OF METHANOL ON CU FILMS ON ZNO(000(1)OVER-BAR)-O

The interactions of methanol with well-defined Cu films on the oxygen- terminated ZnO(0001BAR)-O surface have been studied, mainly using temp erature programmed desorption (TPD). The Cu films, which were from sub monolayer to multilayer in coverage, had been structurally characteriz ed in previous studies using XPS, LEIS, ARXPS, LEED and work function measurements, and by CO, H2O and formic acid adsorption. On clean Cu f ilms methanol is adsorbed reversibly, desorbing at 200-260 K from atom -thick Cu islands, and at approximately 155 K from multilayer islands preannealed to approximately 550 K. In this respect, the atom-thin isl ands resemble Cu(110) sites and multilayer islands resemble Cu(111), c onsistent with behavior of other adsorbates. On oxygen-predosed multil ayer films (preannealed to approximately 600 K), methanol reacts to fo rm methoxy species which decompose at 395 K to yield formaldehyde and hydrogen in TPD, also like Cu(111). Multilayer films preannealed to > 750 K show a decrease in the peak area for methoxy decomposition which correlates with the loss of Cu area due to severe clustering. Oxygen- predosed Cu islands which are but one Cu atom thick show no clear evid ence for a methoxy state in TPD. This suggests that oxygen atoms on su ch atom-thin Cu islands are poor Bronsted bases relative to O(a) on bu lk Cu surfaces, consistent with results for adsorbed water. Results on high-area Cu/ZnO catalysts are discussed in the light of these new re sults.