Theoretical study of Cu-based alloy catalysts for oxidative coupling of methane

Abstract

Methane, the main component of natural gas, is abundant but difficult to activate due to its stable structure and strong C–H bond. The oxidative coupling of methane (OCM) offers a direct route for methane conversion to valuable C₂ hydrocarbons such as ethane (C₂H₆) and ethylene (C₂H₄), providing both economic and environmental benefits. In this study, the catalytic activity of screened Cu-based intermetallic compounds (Cu₃Ir) and surface alloy (Cu₃Ir@Cu) catalysts as well as comprehensive reaction mechanisms for OCM were systematically evaluated by combining density functional theory (DFT) calculations and microkinetic modeling. The results showed that both Cu₃Ir and Cu₃Ir@Cu possessed low energy barriers for methane dissociation. Further electronic structure analysis reveals that the Ir atoms in Cu₃Ir and Cu₃Ir@Cu exhibit a stronger interaction with methane, facilitating the cleavage of the C–H bond to form methyl groups. In addition, the subsequent formation of both C₂H₆ and C₂H₄ follows the surface Langmuir–Hinshelwood mechanism, where ethane production is accomplished through the coupling of adsorbed with low activation energy (0.20 eV on Cu₃Ir and 0.38 eV on Cu₃Ir@Cu), while ethylene is generated mainly through the coupling of rather than dehydrogenation. Comprehensive microkinetic simulations revealed the distribution of reactive species by using the surface coverage, degree of rate control (DRC) of elementary steps and turnover frequency (TOF) for C₂ hydrocarbon production over screened alloy catalysts, where Cu₃Ir@Cu exhibited higher TOF over a wide range of temperatures and pressures compared to Cu₃Ir, especially at low temperatures. This study reveals the excellent activity of Cu₃Ir and Cu₃Ir@Cu alloys in methane activation and C₂ hydrocarbon generation, which provides a theoretical basis for the development of high-efficiency methane conversion catalysts and a potential direction for industrial applications.