Surface Dynamics and Size Sensitivity of Pt-Catalyzed Methane Oxidation Probed by Modulation-Excitation X-ray Absorption Spectroscopy

In situ X-ray absorption spectroscopy (XAS) is a powerful tool to probe the electronic and structural properties of materials under operating conditions. However, its bulk-sensitive nature often obscures the surface-specific dynamics critical to understanding catalytic processes. We herein demonstrate successful extraction of surface dynamics from bulk XAS spectra by applying modulation excitation spectroscopy to XAS (ME-XAS). Methane oxidation over Pt/Al₂O₃ was chosen as a model reaction to verify the strength of ME-XAS for extracting surface dynamics of Pt nanoparticles of different sizes. ME-XAS spectra revealed that during methane oxidation, only surface Pt atoms were repeatedly oxidized and reduced while the bulk Pt remained mostly metallic. Samples with low Pt loadings were more oxidizable and showed lower methane oxidation activity, in line with metallic Pt being more active for methane dissociation than Pt oxides. This study underscores the potential of ME-XAS to enhance our understanding of surface-bulk dynamics and rationalize catalytic phenomena, offering a pathway to the development of more efficient and selective catalysts.