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 support ed on SiO2, has been investigated in the temperature range 323 less-th an-or-equal-to T less-than-or-equal-to 523 K. The results have been co mpared 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. Th e maximum reactive sticking coefficient for CO is 0.85 on the film and apparently larger than unity on the island structure. The results obt ained from the island structure can be rationalized if O2 and H-2 diss ociate on the Pd islands and have the possibility to spillover onto th e 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 irrever sible with an apparent activation energy of 0.1 eV for the forward rea ction.