(392d) Structure and Reactivity of Model Supported Pd-Zn Bimetallic Methanol Steam Reforming Catalysts

Pd supported on ZnO has been shown to have high activity for the steam reforming of methanol to produce hydrogen. A distinguishing characteristic of this catalyst is its high selectivity for the production of CO2 relative to CO. While the unique activity of this catalyst has been demonstrated, numerous questions concerning the structure of the catalyst and the role of the ZnO support have yet to be answered. For example, while alloying of the Pd with Zn is required for high activity, it is not clear if the ZnO support plays a role other than supplying the Zn. The mechanism by which the Zn affects the selectivity to produce CO₂ is also not well understood, although it has been proposed that it affects the bonding geometry of adsorbed aldehyde intermediates. In order to answer these questions we are using TPD, XPS, LEED, and HREELS to study both the structure and reactivity of model catalysts consisting of Pd and PdZn alloy particles and films supported on ZnO(0001) and Zn-modified Pd(111) surfaces. In this talk we will present results of our studies of the growth and thermal stability of vapor deposited Pd films on ZnO(0001). We observed that up to a critical thickness of ~0.5 ML Pd form 2D islands on ZnO(0001) while 3D growth occurs for higher coverages. Pd interacts weakly with the ZnO surface and agglomerates upon heating to relatively low temperatures. Formation of PdZn alloy particles, as determined by XPS, was observed upon heating above 650 K. Methanol TPD results will be presented which show that alloy formation was a significant effect on reactivity causing a decrease in the CO adsorption energy. The reaction of adsorbed CO with oxygen atoms supplied by the ZnO support to form CO₂ was also observed by TPD for supported PdZn particles.