Self-sustained oscillations in oxidation of methane over palladium: the nature of “low-active” and “highly active” states

Abstract

The oxidation of methane was studied in a flow reactor at atmospheric pressure using palladium foil as a catalyst. It was shown that regular self-sustained reaction rate oscillations occur under oxygen-deficient conditions. CO, CO₂, H₂, and H₂O were detected as products when the catalyst was in a highly active state. The signals of CO and H₂ were small, indicating that the total oxidation is the main reaction. According to the operando X-ray diffraction and mass spectrometry study, the driving force for the oscillations is periodic oxidation and reduction of palladium. The highly active state is palladium in the metallic state, and the transition to the low-active state is accompanied by the growth of a PdO film on the palladium surface. The decomposition of PdO is accompanied by the formation of a Pd carbide layer. This is a result of accumulation of carbon on the highly active surface of metallic palladium and its subsequent dissolution in the bulk of palladium. The dissolution of carbon in the subsurface layers of palladium has no a significant effect on its catalytic activity. The oscillatory mechanism for the oxidation of methane over palladium is discussed.