Ellipsometric study of a palladium catalyst during the oxidation of carbonmonoxide and methane

Spectroscopic ellipsometry is used to monitor the surface of a thick Pd-fil m catalyst during the oxidation of either carbon monoxide or methane. Dense PdO layers form under sufficiently lean conditions (excess oxygen) for bot h reactions. A stable metal surface exists in the case of CO, but a very po rous PdO layer develops in the case of methane, under rich conditions. Ther e is a large hysteresis in the conditions for PdO formation in the case of CO oxidation. Spontaneous oscillations in catalytic activity and Pd-surface composition occur for both reactions, the higher activities corresponding to O-atom-rich or PdO-rich surfaces for CO or methane oxidation, respective ly.