On the role of oxygen defects in the catalytic performance of zinc oxide.

Metal oxides are highly important as components in heterogeneous catalysts. It is also well established that as the building blocks of nanoscaled materials get smaller, surfaces become increasingly important in determining their properties. Very early on, researchers found that catalytic activity is only indirectly related to the surface area; in fact it depends on the density of active sites. Although solid-state defects have been proposed to be the active sites in heterogeneous catalysis, active centers have rarely been conclusively identified and the rational design of catalysts is still out of reach. Processes in heterogeneous catalysis associated with the production of energy-storage molecules, for example, methanol as a storage molecule for hydrogen, is presently of great interest. Low-pressure production of methanol is conducted industrially with catalysts containing Cu and ZnO as the active phases and alumina as the support. Composites (Cu + ZnO), however, exhibit higher activities than that expected based on the performance of the single components. This was attributed to a strong metal–support interaction (SMSI) effect. Nevertheless, the active sites in Cu/ZnO and even in the pure ZnO have not yet been identified experimentally. Some important information concerning pure ZnO catalysts is already available: Generally, the catalytic activity of ZnO does not increase linearly with the increasing BET surface area. It was concluded that the catalytic formation of methanol over ZnO is a structure-sensitive reaction requiring the presence of polar ZnO facets. According to the most recent models of the methanol-synthesis reaction on the polar ZnO(0001̄) surface, oxygen vacancies formed on this surface of ZnO crystals serve as the active sites (Scheme 1). However, there has been no definite experimental proof for this assumption.