In situ investigation of the mechanochemically promoted Pd–Ce interaction under stoichiometric methane oxidation conditions

Abstract

The optimization of the supported Pd phase for CH₄ activation on Pd/CeO₂ catalysts has been a matter of great interest in the recent literature, aiming at the design of efficient methane abatement catalysts for Natural Gas fueled Vehicles (NGVs). Under lean conditions, a mixed Pd⁰/PdO combination has been indicated as exhibiting the best performance, while controversial results have been reported under stoichiometric conditions depending on the support oxide, where either Al₂O₃ or zeolite-based supports are usually considered. Here, by means of synchrotron-based in situ NAP-XPS and XRD measurements, we follow the evolution of Pd species on Pd/CeO₂ samples prepared by dry mechanochemical synthesis (M) under stoichiometric CH₄ oxidation feed, unravelling a stable Pd⁰/Pd²⁺ arrangement in a close to 1 : 1 ratio as the most active palladium state for CH₄ activation when excess oxygen is not available, in contrast to what was reported for Pd/alumina materials, where metallic Pd⁰ nanoparticles showed the highest activity. The combination of NAP-XPS analysis and activity test results highlights the promotional effect of the Pd–Ce interaction, resulting in enhanced oxygen transfer and improved activity and stability of the Pd/CeO₂ catalyst prepared by a novel mechanochemical approach even under low O₂ content, large excess of water vapor (10 vol%) and high temperature exposure (>700 °C).