Ethylene formation by methane dehydrogenation and C-C coupling reaction on a stoichiometric IrO2 (110) surface - a density functional theory investigation

The capability to activate methane at mild temperature and facilitate all elementary reactions on the catalyst surface is a defining characteristic of an efficient catalyst especially for the direct conversion of methane to ethylene. In this work, theoretical calculations are performed to explore such catalytic characteristic of an IrO2 (110) surface. The energetics and mechanism for methane dehydrogenation reactions, as well as C-C coupling reactions on the IrO2 (110) surface, are investigated by using van der Waals-corrected density functional theory calculations. The results indicate that a non-local interaction significantly increases the binding energy of a CH4 molecule with an IrO2 (110) surface by 0.35 eV. Such an interaction facilitates a molecular-mediated mechanism for the first C-H bond cleavage with a low kinetic barrier of 0.3 eV which is likely to occur under mild temperature conditions. Among the dehydrogenation reactions of methane, CH2 dissociation into CH has the highest activation energy of 1.19 eV, making CH2 the most significant monomeric building block on the IrO2 (110) surface. Based on the DFT calculations, the formation of ethylene could be feasible on the IrO2 (110) surface via selective CH4 dehydrogenation reactions to CH2 and a barrierless self-coupling reaction of CH2 species. The results provide an initial basis for understanding and designing an efficient catalyst for the direct conversion of methane to ethylene under mild temperature conditions.