Kinetic mechanism of effects of hydrogen addition on methane catalytic combustion over Pt(1 1 1) surface: A DFT study with cluster modeling

Abstract

Effects of hydrogen addition on methane catalytic combustion over Pt(111) surface were investigated with density functional theory (DFT) calculations and micro-kinetic modeling. Optimizations of initial and transition states were performed for elementary reactions that involve feasible steps of the overall reaction. The elementary reactions include the dissociation process of hydrogen–methane, oxidation of intermediates and desorption of products on Pt. Many feasible pathways were investigated and the calculated results showed that the main reaction pathways are CH4→CH3∗→CH2∗→CH∗→CHOH∗→CHO∗→CO∗→COOH∗→CO2∗→CO2 and H2→H∗→OH∗→H2O∗→H2O. Chemical reaction kinetic theory was employed and showed that the rate determining step of methane oxidation is the elementary reaction CH4+2∗→CH3∗+H∗. This conclusion is in agreement with previous published results. In addition, the methane oxidation rate decreases with the increasing addition of hydrogen by analyzing a simple micro kinetic model. Meanwhile, the coverage density of the main species over Pt(111) surface were also studied. The results showed that hydrogen and methane keep a competitive relationship on dissociative adsorption over Pt(111). Even though H∗ is able to consume O∗ to release free site for methane adsorption, free sites are still rare for methane adsorption due to the addition of hydrogen. Therefore, even thought the heat release of hydrogen oxidation may accelerate the methane catalytic combustion, hydrogen addition will inhibit methane catalytic combustion in kinetically.