(630b) Mechanistic Studies of the Steam Reforming of Methanol on Pdzn Alloy Catalysts

Methanol and other alcohols are potential bio-renewable sources of hydrogen. The use of alcohols, however, as a H2 source or storage medium requires stable reforming catalysts that have high activity and selectivity at low temperatures. One such catalyst that has received much attention for steam reforming of CH3OH (SRM) [CH3OH + H2O → CO2 + 3H2] is Pd supported on ZnO. Pd/ZnO catalysts have unusually high selectivity (>95%) for the production of CO2 and H2 from methanol, in spite of the fact that bulk Pd exhibits nearly 100 % selectivity for the dehydrogenation of CH3OH to CO and H2 under typical SRM conditions. While it has been demonstrated that formation of a PdZn allow is required to obtain high selectivity, the mechanism by which Zn alters the reactivity of the Pd is not understood. In this talk we will present results of a study of model PdZn catalysts consisting of submonolayer amounts of Zn supported on Pd(111) that provide insight into the synergistic interactions between Zn and Pd and how they alter the reactivity of the Pd surface. Temperature programmed desorption (TPD) data for the reaction of methanol, formaldehyde and carbon monoxide on Pd(111) as a function of Zn coverage as well as results of a high resolution electron energy loss spectroscopy (HREELS) study of the bonding configurations of these molecules on Zn/Pd(111) surfaces will be presented. The TPD data show that the formation of the PdZn alloy significantly decreases the activity of the Pd surface for the dehydrogenation of CH3OH and CH2O. This change in reactivity is found to be due in part to a change in the bonding configuration of adsorbed formaldehyde intermediates. The experimental results also provide evidence for a strong electronic interaction between the Pd and Zn which affects the adsorption energies of CO and methanol.