O2-assisted methane oxidation on single-atom Pd@SSZ-13: a combined first-principles and microkinetic study.

Complete catalytic oxidation of methane is an effective strategy for greenhouse gas mitigation and clean energy conversion. Yet, ensuring both high catalytic activity and stability using palladium (Pd)-based catalysts remains a challenge. In the present work, we performed a theoretical investigation of methane oxidation over single-atom Pd supported on the SSZ-13 zeolite (Pd@SSZ-13) using density functional theory (DFT) calculations, combined with climbing-image nudged elastic band (CI-NEB) calculations, to determine activation barriers. A systematic assessment of various aluminum (Al) distributions and Pd placements was carried out to identify the most stable configurations for Pd incorporation within the zeolite framework. Furthermore, two mechanistic routes for methane activation were evaluated: (i) direct dehydrogenation under dry conditions and (ii) O2-assisted oxidative dehydrogenation. Our results demonstrate that the direct (dry) pathway is energetically demanding and overall endothermic, whereas the O2-assisted route facilitates the exothermic energy profile, particularly in the C-H bond cleavage. The formation of stable hydroxyl and CO/CO2 intermediates was also studied. The results emphasize the role of oxygen-rich environments in enabling the complete methane oxidation with improved thermodynamic feasiblilty. Moreover, we propose an alternative low-energy pathway based on O2-assisted and multi-site mechanisms that reduce the overall reaction enthalpy. A detailed microkinetic analysis was carried out to evaluate the temperature and pressure dependent turnover frequency. In addition, we evaluated the surface coverage of carbon and oxygen species, rate of adsorption, production rate, and apparent activation barrier as a function of temperature. The results indicate that the conversion of CH4 and O2 to CO2 and H2O initiates at temperatures above 800 K. These insights provide the design principles for optimizing Pd-zeolite catalysts by highlighting how oxygen availability, water removal, and carbon-induced site blocking jointly control activity, stability and turnover frequencies during methane oxidation.