Mechanistic Studies of the Steam Reforming of Methanol on Pd/ZnO Catalysts

2007 Spring Symposium

Eseoghene Jeroro and John M. Vohs
Department of Chemical and Biomolecular Engineering
University of Pennsylvania
Philadelphia, PA 19104-6393 USA

Abstract – Methanol and other alcohols are potential bio-renewable sources of hydrogen. The use of alcohols, however, as a source of H2 or for H2 storage requires stable reforming catalysts that have high activity 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. Iwasa and others have demonstrated that partial alloying of the Pd with Zn is required to obtain a highly selective Pd/ZnO SRM catalyst. While the importance of alloy formation has been established, the mechanism by which Zn alters the selectivity to produce CO2 rather than CO is not understood. Iwasa et al. have proposed, however, that the dramatic change in selectivity may result from the destabilization of η2-CH2O intermediates in which the carbonyl group is parallel to the surface and bonding is by both the C and O atoms in favor of η1-CH2O in which the carbonyl is perpendicular to the surface and only the oxygen interacts with the metal. Presumably this later species is more resistant to dehydrogenation and reacts with hydroxyl groups to produce CO2 and H2.

In order to elucidate how alloying with Zn affects the CH3OH dehydrogenation activity of Pd, the structure and reactivity of model catalysts consisting of submonolayer amounts of Zn supported on a Pd(111) single crystal have been investigated. I will present temperature programmed desorption (TPD) data for the reaction of methanol and formaldehyde 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 CO, CH2O, and CH3OH on Zn/Pd(111) surfaces. TPD data for the reaction of methanol on Pd supported on ZnO(0001) single crystal surfaces will also be presented. The TPD data shows that the formation of the Pd Zn alloy severely shuts down the dehydrogenation of CH3OH and CH2O which corresponds to the increase in η1-CH2O seen in the HREELS. Overall, these results provide fundamental insight into how Zn alters the reactivity of Pd for the dehydrogenation of CH3OH and this insight is useful in elucidating the mechanism of the steam reforming of methanol on Pd/ZnO catalysts.