Chemistry of Supported Palladium Nanoparticles during Methane Oxidation

Time-resolved in situ, energy-dispersive X-ray absorption spectroscopy and mass spectrometry are used to correlate changes in the chemical state of alumina- and ceria-supported palladium nanoparticles with changes in activity and selectivity for methane oxidation. Specifically, modulation excitation spectroscopy experiments are carried out by periodically cycling between net-reducing and net-oxidizing reaction conditions. The XANES and EXAFS data show that the palladium nanoparticles are readily bulk-oxidized when exposed to oxygen, forming a PdO-like phase, and reduced back to a reduced (metal) phase when oxygen is removed from the feed. The difference between the two support materials is most noticeable at the switches between net-oxidizing and net-reducing reaction conditions. Here, a brief reduction in conversion is observed for the alumina-supported catalyst, but for the ceria-supported catalyst, this reduction in conversion is minor or not observed at all. This difference is attributed to differences in the oxidation kinetics and the oxygen storage capability of ceria.