(99b) Evaluation of Low-Temperature Methane Oxidation Reaction over Pd Single-Atom Catalysts

Emissions of unburned methane from the natural gas engines need to be lowered, especially in certain conditions such as during a cold start. Catalytic oxidations of methane to less potent gases have been the current technology for controlling methane emissions from natural-gas-powered vehicles, and Pd is the commonly used active metal. However, it is precious and expensive and therefore needs to be minimized while still maintaining or enhancing the activity of methane oxidation catalysts.

This work adopts flame spray pyrolysis, a high-temperature synthesis method, to stabilize Pd on CeO₂ in different flame synthesis environments. The developed catalysts are tested for methane oxidation to elucidate Pd speciation and the reaction mechanism using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The 1 wt.% Pd/CeO₂ catalyst synthesized in the oxidizing environment has a lower T₅₀ and a higher methane oxidation rate than the reducing environment-synthesized catalyst. This is due to the atomically dispersed Pd²⁺ species formed on the oxidizing environment-synthesized catalyst compared to the highly dispersed Pdⁿ⁺, Pd^0, and Pd⁰ clusters formed on the reducing environment-synthesized catalysts. The in situ DRIFTS indicates that carbonate species at 1273 cm^-1 and 1540 cm^-1 increase during methane oxidation at 400 ^oC over the 1 wt.% Pd/CeO₂ catalyst. Formate and C-H methyl species at 1353 cm^-1 and 2966 cm^-1 also increase during the reaction, whereas CH₂ species at 1475 cm^-1 decrease. These results indicate that the conversion of adsorbed CH₂ species to carbonate and formate may be an essential elementary step in the methane oxidation reaction over the atomically dispersed Pd²⁺. In situ evaluation of the methane oxidation reaction will be extended to the low loading Pd/CeO₂ catalyst. The insight from these results can be used to elucidate the detailed reaction mechanism of methane oxidation reaction over single-atom Pd/CeO₂ catalysts.