Theoretical investigation of methane oxidation on Pd (111) and other metallic surfaces

Density functional theory and microkinetic modeling are employed to investigate CH₄ oxidation to CO, CO₂, CH₂O, and CH₃OH on Pd(111) under mildly oxidizing conditions. Although our energetic analysis indicates that the metallic site on Pd(111) is more active than O* and OH* on the Pd surface for C–H bond activation, our microkinetic analysis indicates that the metallic site produces mostly CO, whereas the O* site produces mostly CH₂O. In addition, we show that the product selectivity can change significantly depending on the pressures of the products (CO, CO₂, CH₂O, and CH₃OH). Increasing the product pressures leads to the promotion of CO₂ production, because CO oxidation becomes more active than CH₄ oxidation. We then extend the study to other FCC(111) surfaces by incorporating the linear scaling relations in the mean-field microkinetic model. We find that most transition-metal surfaces cannot effectively activate CH₄ under the reaction conditions employed. We find that the kinetics of CH₄ oxidation to CO, CO₂, CH₂O, and CH₃OH can be described generally as a function of two descriptors, enabling identification of the most promising catalyst surface for selective production of the desired product.