Palladium catalysts are central to reducing unburned methane in natural-gas vehicle exhaust. Their performance depends sensitively on oxygen partial pressure and on co-reductants, which shift the Pd redox state and surface coverage during operation. In recent years, a Pd-Au nanoparticle catalyst showed promise for methanol oxidation reactions due to its enhanced activity for O₂ dissociation over Pd species. We hypothesize that the formation of partially reduced PdOx regions on the surface of Au nanoparticles will provide a low-barrier pathway for methane oxidation. Our approach is two-pronged. First, we modify the oxidation state of Pd in Au dynamically by alternating O₂ and H₂ pulses at 440 °C and investigate the resultant speciation of Pd using the modulation excitation method of X-ray absorption spectroscopy. We obtained evidence that surface oxides were formed dynamically, with a faster rate of oxide formation with respect to their decomposition. Due to the fast XAS scans (3 s) and short pulse times (15 s), their synchronization in the experiment was very challenging. To reconstruct the kinetics of the Pd reduction/oxidation, we applied k-means clustering, combined with principal component analysis, and obtained both the number of modulating Pd species (2) and their spectra. As a result, ca. 42% of all Pd atoms, that is, at and near the Au surface, were affected by the redox pulses. We also proposed that the oxidation of Pd was faster than the subsequent reduction. Finally, we applied the results obtained using redox pulses to quantify the number of active sites of Pd for modulation excitation studies of methane oxidation.

Akhil Tayal

Brookhaven National Laboratory

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