Carbon vacancy Mediated CH4 Activation and Carbon-Carbon Coupling on TiC(001) surface : A First Principles Investigation

Abstract

Methane is the main component of natural gas and one of the major greenhouse gases contributing to global warming. Therefore, capturing methane and utilizing it for chemical processes is highly desirable. Methane however, is a highly stable molecule, making methane activation a challenging process. As such finding a suitable catalyst for methane activation is one of the “holy grails” of catalysis. Catalysts that have been studied for methane activation exhibit a trade-off between activity and selectivity. On most catalysts the activation of the first C-H bond is difficult while the dissociation of the remaining C-H bonds is facile, leading to complete dissociation of methane into carbon and hydrogen. The C-deposition on the catalyst surface leads to catalyst poisoning and makes the surface unsuitable for repeated catalytic cycles. Additionally, the facile nature of the reaction poses a challenge to the direct conversion of methane into valuable products. Transition metal carbides such as Titanium carbide (TiC) have been widely used for small molecule activation. TiC is an easily available and a very cheap material. However it rarely exists in nature in the exact stoichiometric form, and mostly contains carbon vacancies. Methane activation is however not feasible on pristine TiC surfaces. Using Density Functional Theory based calculations, we have studied methane activation on TiC(001) surfaces containing carbon vacancies to understand how the concentration of C-vacancies affects the activation of methane on TiC(001). Dissociation of CH4 on the single vacancy surface is found to be less facile and hence more selective compared to CH4 dissociation on the divacancy surface. On the divacancy surface the facile nature of the reaction leads to deposition of a carbon atom in one of the two vacancies. The selectivity of CH4 activation on the single vacancy surface is desirable and hence this surface is explored further for the formation of C2 compounds. Acetylene or C2H2 formation is found to be thermodynamically and kinetically feasible on the single vacancy surface. For a catalyst to be economically viable it should be able to sustain repeated catalytic cycles. Hence, we explore the regeneration of the catalyst which involves the regeneration of the C-vacancy by the removal of H atoms in the form of H2 molecules. Finally, we have also studied the effect of temperature on the reactivity of the surface. We observe that while C2H2 formation is feasible at relatively low temperatures compared to the existing catalysts, one needs to go to higher temperatures for catalyst regeneration and C2H2 desorption.