The increasing supply of natural gas from the rapid growth of hydraulic fracking and shale gas reserves provides substantial motivation for developing catalytic processes that can efficiently and directly transform methane (CH4) to value-added products. However, efficient activation of the strong C-H bond of CH4 at mild temperatures remains a challenge in converting CH4 to fuels and chemicals. Transition metal oxides have drawn enormous attention in CH4activation and conversion due to their inherent Lewis acidic (unsaturated metallic centers) and basic (lattice oxygen centers) properties. In this context, we explore the catalytic behavior (thermo- and photo-catalysis) of TiO2 for the nonoxidative coupling of methane (NOCM) using Density Functional Theory calculations with Hubbard U correction (DFT+U) and kinetic MonteCarlo (kMC) simulations. Our DFT+U calculations reveal that the activation barrier of the first C-H bond activation of CH4 decreases by 70% by photocatalysis compared to thermocatalysis (1.01 eV). Spin density distribution and Bader charge analysis confirm that the formation of hole polaron on the surface O2c●– by the absorption of photo hole (h+) promotes the C-H bond activation via a homolytic pathway (formation of methyl radical). Additionally, the activation barrier for light-driven NOCM to form ethane was significantly reduced compared to thermocatalysis. In the case of photocatalysis, NOCM is facilitated by coupling two methyl radicals in the presence of two self-trapped hole polarons on the surface O2c●–. Finally, kMC simulations reveal a high photocatalytic activity of TiO2 towards methane activation and coupling to ethane, whereas the methane thermocatalytic conversion is inactive, even at high temperatures. In summary, this work provides fundamental insights into the light-driven catalytic activity of TiO2 towards NOCM at mild conditions.

Photocatalytic nonoxidative coupling of methane: Insights from first principles calculations and kinetic Monte Carlo simulations

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