Reactivity of Pd-O surface phases

COLL 454

J. Wang,, Y-R. Yen, yung-ruey.yen@yale.edu1, and E. I. Altman, eric.altman@yale.edu2. (1) Department of Physics, Yale University, PO Box 208260, New Haven, CT 06520, (2) Department of Chemical Engineering, Yale University, PO Box 208260, New Haven, CT 06520
Palladium-based materials have become the catalysts of choice for catalytic combustion. Still, even the most basic issues such as whether Pd oxide surfaces are more reactive than chemisorbed oxygen on metallic Pd are still a matter of debate. Catalytic studies have suggested that the oxide is more reactive while surface science studies have supported the opposite conclusion. We have found that Pd oxidation is a complex process proceeding through a number of surface phases including adsorbed oxygen, sub-surface oxygen, and surface oxides before a bulk oxide forms. We find that the surface oxides have much lower sticking coefficients for CO and propene leading to lower reactivities. On the other hand, once the hydrocarbons adsorb, the surface oxides favor a low activation energy, direct oxidation pathway rather than decomposition and oxidation of the fragments as favored on the metal surface. Exposure of the well-ordered surface oxides to atomic oxygen leads to poorly ordered bulk PdO. This PdO readily adsorbs propene while also favoring the direct oxidation pathway, although with a higher activation energy. As a result, the poorly ordered bulk PdO is far more reactive than the surface oxides. Ion scattering data indicate that the higher reactivity of the PdO is not due to greater accessibility of active Pd sites on the surface. To determine if the reactivity difference is due to the interaction of the surface oxide with the underlying Pd versus the lack of structural order in the PdO, we are working on growing highly ordered epitaxial PdO thin films.