CH4 activation and C–C coupling on the Ti2C(100) surface in the presence of intrinsic C-vacancies: is excess good?

Abstract

Activation of methane, the main component in natural gas, and its conversion to useful products is an important chemical process because methane is not only one of the most important feedstocks for fuels and chemicals but is also one of the leading contributors to global warming. However, the presence of strong C–H bonds and absence of any dipole or quadrapole moments in methane make C–H bond scission difficult, making the molecule inert. In this work, using first-principles density functional theory based calculations, we have explored the most stable (100) surface of non-stoichiometric Ti₂C as a plausible catalyst for methane activation. This surface is characterized by the presence of lines of intrinsic C-vacancies. We find that these C-vacancies act as reaction centers. Our results show that while cleavage of the first three C–H bonds has very low barriers, the last C–H bond activation has a significantly large barrier. Further, our studies on the feasibility of formation of C₂ products like ethane, ethylene and acetylene show that these reactions are associated with large activation barriers, which suggests that C–C coupling reactions are not feasible on this surface. The above observation can be attributed to the presence of these C-vacancy lines on the surface. Further, taking into account both the reactions, our results suggest that the presence of such a substantial amount of C-vacancies on the surface would result in the formation of a large amount of CH and C species bound to the surface, thereby blocking the reaction sites and deactivating the catalyst. From our studies we anticipate that other non-stoichiometric TiC surfaces with lower concentrations of C-vacancies can be explored as catalysts for methane activation and C–C coupling reactions.