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

Abstract

Small Pd clusters Pd n ( 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 O 2 at temperatures ranging from 100 to 500 K. After O 2 exposure at 100 K, the Pd clusters showed XPS shifts indicative of oxidation. The exception was Pd 4, which did not oxidize under any conditions. The inertness of Pd 4/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 O 2 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.