(222a) Investigating the Support Effects In the Pd / ZnO Catalyzed Steam Reforming of Methanol

While methanol decomposition on all Pd surfaces produces hydrogen, the presence of ZnO as a support is necessary to activate the steam reforming pathway to selectively produce CO2 rather than CO. Early observations of PdZn alloy formation suggested that the support provides Zn to the Pd surface, which could inhibit the dehydrogenation pathway that produces CO. In the presence of H2O, oxidation of a CHxO intermediate could then become the favored pathway. More recent work from our group suggests that the steam reforming pathway is only active for small concentrations of Zn in the surface and that surfaces with large concentrations are inactive. This is in contrast with high surface area catalysts exhibiting large Zn fractions in highly active catalysts. The contradicting observations make apparent the need to more fully understand the role of ZnO, particularly its crystal plane dependence on the Pd surface properties. Ultrahigh vacuum studies in which Pd is deposited on ZnO single crystals to probe the surface properties as they relate to methanol decomposition have been performed by our group. We present the results of our investigations on the polar ZnO(0001) and nonpolar ZnO(101(bar)0) surfaces and discuss the implications for methanol steam reforming on high surface area Pd/ZnO catalysts.