(681g) Carbon-Based Catalysts for Non-Oxidative Coupling of Methane

Carbon materials have been studied as potential catalysts for methane conversion reactions.¹ Recently, it was determined that a variety of catalyst materials, after coking to the point of complete coverage, were still able to catalyze the non-oxidative coupling of methane (NOCM) reaction.² Previous experimental and computational work suggest that defects within the carbon structure, such as carbon vacancies in the surface or edges that are formed from sheet separation, resulting in individual carbon atoms with an electron-deficient valence structure, could act as the active sites in these reactions.^1,3 To test these hypotheses, we employ surface free energy and phase coexistence diagrams to determine the stability of model carbon-monolayer defects at partial hydrogen pressures appropriate for NOCM. These diagrams are generated using Density Functional Theory (DFT) calculations to determine the minimum grand free energy of defects with different levels of hydrogen passivation as a function of temperature and pressure. The results suggest that all defect models are most stable when fully passivated, and hydrogen binding to carbon atoms adjacent to vacancies renders their valence structure electronically satisfied. These structures, in turn, form the basis for a detailed analysis of the mechanisms, including thermodynamic energy changes and activation barriers, for NOCM. The results suggest that hydrogen-passivated sites can be active for NOCM and point to likely candidates for catalytically active defect structures on graphene-derived carbon surfaces.

References:

(1) Muradov, N., Catal. Comms. 2001, 2, 89-94

(2) Talpade, A., PhD Thesis 2021, Purdue University

(3) Huang, L. et al., J. Chem. Phys. 2008, 128, 214702