Abstract
Oxidation of H 2 and CO on large, oxygen preexposed Pd islands supported on SiO 2, has been investigated in the temperature range 323 ⩽, T ⩽ 523 K. The results have been compared with the corresponding reactions on a polycrystalline Pd film. A maximum reactive sticking coefficient of 0.9 for H 2 and an initial sticking coefficient for O 2 of 0.8 on both structures is concluded. The maximum reactive sticking coefficient for CO is 0.85 on the film and apparently larger than unity on the island structure. The results obtained from the island structure can be rationalized if O 2 and H 2 dissociate on the Pd islands and have the possibility to spillover onto the oxide, while CO adsorbs and reacts both on the Pd islands and on the oxide. Spillover of oxygen occurs in a precursor state and is irreversible with an apparent activation energy of 0.1 eV for the forward reaction.
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