Identification of Active Phase for Complete Oxidation of Methane on Palladium Surface

Palladium (Pd)-based catalysts have received extensive attention for their outstanding performance in the complete oxidation of methane (CH₄). A fundamental understanding of the reaction mechanism, particularly the most active phase, is essential for the development of efficient catalysts and the optimization of reaction conditions. By employing time-resolved ambient pressure X-ray photoelectron spectroscopy (tr-APXPS) and on-line mass spectrometer (MS), we investigated the complete oxidation of CH₄ on the Pd(111) surface within a 1 mbar O₂ + 0.25 mbar CH₄ atmosphere, ranging from 600 to 900 K. Utilizing tr-APXPS enabled us to observe the phase transition of PdO → PdO_x → O/Pd on the Pd(111) surface during the linear heating process. On-line MS data revealed that the formation of PdO_x correlates with a significant increase in catalytic activity. By integrating the insights from APXPS and on-line MS, we propose that the catalytic activity for methane oxidation follows the order of PdO_x > PdO > O/Pd. Furthermore, we suggest that the PdO_x formed at the initial decomposition of bulk PdO is most active for dissociating methane.