Ellipsometric study of a palladium catalyst during the oxidation of carbon monoxide and methane

Spectroscopic ellipsometry is used to monitor the surface of a thick Pd-film catalyst during the oxidation of either carbon monoxide or methane. Dense PdO layers form under sufficiently lean conditions (excess oxygen) for both reactions. A stable metal surface exists in the case of CO, but a very porous PdO layer develops in the case of methane, under rich conditions. There 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, respectively.