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 O
2 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 TiO
2(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 TiO
2 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 Pd
10.
