A First-Principles Microkinetics for Homogeneous-Heterogeneous Reactions: Application to Oxidative Coupling of Methane Catalyzed by Magnesium Oxide

The oxidative coupling of methane (OCM), a major catalytic reaction in natural gas utilization, was examined by first-principles density functional theory (DFT) calculations combined with microkinetic and reactor simulations. We investigated magnesium oxide (MgO), which is a standard catalyst for the OCM reaction. We used the stepped MgO as the catalyst model, as this surface is a strong candidate for the active site for the CH3 generation from CH4. Previous experimental and kinetic simulations have shown that CH3 generated by the surface reactions couples into C2H6 in the gas phase, and thus the OCM is the homogeneous-heterogeneous process. Based on this, we conducted a theoretical study using DFT-based microkinetics by including both gas-phase and surface reactions. The calculations provide theoretical CH4 conversion and C-2 selectivity without using any kinetic experimental parameters, which are in reasonable agreement with the experiments. The dependence of these quantities on the reaction condition such as the temperature or inlet gas component is also analyzed, and it has been shown that the CH4 conversion increases at higher temperature; on the other hand, the C-2 selectivity decreases. The opposite trend was observed with respect to the partial pressure ratio of CH4 and O-2, as the C-2 selectivity increases when the partial pressure of CH4 is high. These tendencies are in agreement with the experiments. Consequently, our calculation supports the experimental suggestion that the stepped MgO is the active site for the OCM. We also carried out the analysis on the reaction network, indicating that a large portion of deep oxidation occurs via dehydrogenation of C-2 compounds.