SPILLOVER OF HYDROGEN, OXYGEN AND CARBON-MONOXIDE IN OXIDATION REACTIONS ON SIO2 SUPPORTED PD

Oxidation of H-2 and CO on large, oxygen preexposed Pd islands supported on SiO2, has been investigated in the temperature range 323 less-than-or-equal-to T less-than-or-equal-to 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 O2 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 O2 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.