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.