Low-Temperature Activity and PdO-PdO_x Transition in Methane Combustion by a PdO-PdO_x/γ-Al₂O₃ Catalyst

The search to discover a suitable catalyst for complete combustion of methane at low temperature continues to be an active area of research. We prepared a 5 wt % PdO-PdO_x/γ-Al₂O₃ catalyst by a modified Vortex-assisted Incipient Wetness Method. X-ray Photoelectron Spectroscopy showed that the original catalyst contained PdO (38%) and PdO_x (62%) on the surface and indicated that PdO_x originated from the interaction of PdO with the support. Scanning Transmission Electron Microscopy confirmed the catalyst had an average particle size of 10 nm and was well-dispersed in the support. The catalyst exhibited exceptional low-temperature activities with 90–94% methane conversion at 300–320 °C. The catalyst was active and stable after several catalytic runs with no signs of deactivation by steam in this narrow temperature range. However, the conversion decreased in the temperature range 325–400 °C. The surface composition changed to some extent after the reaction at 325 °C. A tentative mechanism proposes PdO_x (Pd native oxide) as the active phase and migration of oxide ions from the support to PdO and then to PdO_x during the catalytic oxidation. The high methane conversion at low temperature is attributed to the vortex method providing better dispersion, and to catalyst–support interaction producing the active phase of PdO_x.