Size-dependent oxidation of Pdn (n ⩽ 13) on alumina/NiAl(110): Correlation with Pd core level binding energies

Small Pd clusters Pdn (n = 1, 4, 7, 10, 13) deposited on alumina/NiAl(110) at room temperature were examined by X-ray photoelectron spectroscopy (XPS), as-deposited and after exposure to O2 at temperatures ranging from 100 to 500 K. After O2 exposure at 100 K, the Pd clusters showed XPS shifts indicative of oxidation. The exception was Pd4, which did not oxidize under any conditions. The inertness of Pd4/alumina/NiAl(110) appears to be correlated with a significantly higher-than-expected Pd 3d binding energy, which we attribute to a particularly stable valence shell. None of the clusters examined oxidized during O2 exposures at 300 K or above, but He+ scattering showed that oxygen was bound on the cluster surfaces. Upon heating, all the oxygen associated with these small clusters appeared to spill over and react with the alumina/NiAl(110) support.