Electronic Structure Controls Reactivity of Size-Selected Pd Clusters Adsorbed on TiO 2 Surfaces

Abstract

The catalytic activity of metal clusters of different sizes adsorbed on oxide surfaces can be explored systematically by using model catalysts. We studied the temperature-programmed reaction of CO with O₂ catalyzed by Pd clusters (Pd_n, for n = 1, 2, 4, 7, 10, 16, 20, and 25) that were size-selected in the gas phase and deposited on rutile TiO₂(110). X-ray photoemission spectroscopy revealed that the Pd 3d binding energy varied nonmonotonically with cluster size and that the changes correlated with strong size variations in CO oxidation activity. Taking final-state effects into account, low activity was correlated with higher-than-expected Pd 3d binding energy, which is attributed to a particularly stable valence electronic structure; electron transfer from the TiO₂ support to the Pd clusters also occurs. Ion scattering shows that small clusters form single-layer islands on the surface and that formation of a second layer begins to occur for clusters larger than Pd₁₀.