The interaction of oxygen with Pd particles, vapor deposited onto a thin alumina film grown on a NiAl(110) substrate, was studied by STM, AES, LEED, XPS, TPD and molecular beam techniques. The results show that O2 exposure at 400-500 K strongly influences the oxide support. We suggest that the oxygen atoms formed by dissociation on the Pd surface can diffuse through the alumina film and react with the NiAl substrate underneath the Pd particles, thus increasing the thickness of the oxide film. The surface oxygen inhibits hydrogen adsorption, and readily reacts with CO at 300-500 K. For large and crystalline Pd particles, the system exhibits adsorption-desorption properties which are very similar to those of the Pd(111) single crystal surface. The molecular beam and TPD experiments reveal that, at low coverage, CO adsorbs slightly stronger on the smaller Pd particles, with an adsorption energy difference of ca. 5-7 kJmol-1 for 1 and 3-5 nm Pd particles studied.
Corresponding author: M. Bäumer. Reprints also available from M. Schmid (schmid).
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