Methane oxidation over PdO x : on the mechanism for the hysteresis in activity and oxygen content

A conceptual picture is developed to explain the peculiar kinetic features of methane oxidation over supported Pd catalysts (observed by several investigators), notably the hysteresis in activity accompanying temperature cycles. Experiments were performed with supported Pdcatalysts to illustrate these features. The activity hysteresis is closely coupled with a hysteresis in oxygen content. The latter is in turn attributed to the properties of the PCT-diagram of the involved three-phase system; gas phase O₂ and the two solid phases, Pd and PdO_x. The two main ingredients in the mechanism are: (i) the so-called absorption and decomposition plateau pressures for the O₂-Pd-PdO _x system are different, i.e., show a hysteresis, (ii) these pressures are not independent ofx, but increase with increasing oxygen content. Both features are deviations from the ideal three-phase system and are frequently observed for H₂-metalmetal hydride systems.