Paper SS+NS-WeM9
Encapsulation of Noble-Metal Clusters on ZnO(10-10) and Its Impact on Catalytic Reactivity

Wednesday, October 30, 2013, 10:40 am, Room 201 A

Supported metal clusters on ZnO are important catalysts for industrial synthesis of methanol from syn-gas. The active site of this reaction has long been debated. We have employed model single crystal catalysts and surface characterization tools to elucidate key aspects of the cluster structure and reactivity. Specifically, we have probed the CO oxidation reaction at high pressure (tens of Torr). In the case of Cu/ZnO(10-10), both LEIS and EELS show an encapsulation of the cluster with an ultrathin layer of ZnO which is thermally driven (up to 725 K) and promoted with oxygen exposure. Other spectroscopies, including XPS and UPS, suggest a self-limiting ZnO film thickness of 1 – 2 ML. UPS measurements on the bare (un-annealed) Cu/ZnO clusters show that CO adsorbs molecularly at 85K while it dissociates on an encapsulated cluster at the same temperature, indicating that the ZnO coating is crucial to reactivity. Similarly CO₂ reacts strongly with the encapsulated Cu clusters indicating that the ultra-thin ZnO is participating in the adsorption process. High-pressure studies at 50 Torr at RT show that there is a greatly enhanced reactivity for CO oxidation for the encapsulated clusters compared to either the bare ZnO substrate or the un-encapsulated clusters. Similar results have been seen in the case of Au clusters on ZnO(10-10) and corresponding encapsulation, however differences in reactivity and structure exist. These results offer a new understanding of the “support” role of ZnO on this heterogeneous catalyst system, and may lead to new means to drive the mechanism of methanol synthesis and help to develop new designs of this important industrial catalyst.