C-Vacancy Mediated Methane Activation and C–C Coupling on TiC(001) Surfaces: A First-Principles Investigation

Abstract

Methane, the main component of natural gas, is one of the major greenhouse gases contributing to global warming. Therefore, capturing methane and converting it to other useful products are highly desirable. Methane activation is challenging due to the high energy of the C–H bonds and the nonpolar, nonreactive nature of the molecule. In this work, using density functional theory-based calculations and ab initio thermodynamic analysis, we have studied the role of C-vacancies on a TiC(001) surface toward methane activation and its nonoxidative coupling to form C2 hydrocarbons. Our C-vacancy concentration-dependent study of CH4 activation shows that (i) the first C–H bond cleavage is facile and less sensitive to the concentration of C-vacancy and (ii) the dissociation of the subsequent ones strongly depends on the vacancy concentration and becomes arduous in the presence of fewer vacancies. Among the two vacancy concentrations considered in this study, namely, 12.5 and 25%, we find that on the former though the first C–H bond cleavage is facile, the barriers for the subsequent C–H bonds are high suggesting that this might be a good candidate for further C–C coupling studies. Our C–C coupling studies show that this catalyst will yield acetylene at around 800 K. However, the rate-limiting step is the formation of H2 from the H atoms occupying the C-vacancies, which might block the vacancies, thereby deactivating the catalyst.