Electronic Structure Controls Reactivity of Size-Selected Pd Clusters Adsorbed on TiO2 Surfaces

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Science  06 Nov 2009:
Vol. 326, Issue 5954, pp. 826-829
DOI: 10.1126/science.1180297

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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 (Pdn, 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.

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