Thermocatalytic Behavior of TiO ₂ as a Dehydrogenation Catalyst: A Case Study of Methane Activation and Nonoxidative Coupling

The abundance of cheap natural gas has changed the energy supply landscape and spurred efforts to find alternative sources of energy to traditional fossil fuels. Methane (CH₄) is the primary constituent of natural gas, and its C-H bond activation remains a long-standing puzzle in the chemical industry. Transition-metal oxides exhibit intrinsic Lewis acid-base properties beneficial for activating the C-H bonds of CH₄. In this work, we investigated the nonoxidative coupling of CH₄ (NOCM) to C₂ hydrocarbons on the rutile TiO₂ (110) surface at 1240 K by using density functional theory (DFT) calculations. We explored three different CC coupling pathways for the formation of ethane after the sequential activation of two CH₄ molecules. We found that CH₃/CH₃ coupling involves high activation barriers, while the formation of C₂H₅ from the coupling of CH₃/CH₂ is kinetically and thermodynamically more facile. Considering ethylene formation routes, we found that the dehydrogenation of methyl species requires high energy barriers. However, the subsequent CC coupling of CH₂/CH₂ occurs at a lower activation barrier of 1.01 eV. Moreover, our calculations revealed that the dehydrogenation of C₂H₅ to form ethylene is favored over its hydrogenation to form ethane. This work provides various mechanistic pathways that can help in designing dehydrogenation catalysts with enhanced catalytic activity. However, our results indicate that despite low barrier coupling routes, rutile TiO₂ alone is not an effective catalyst for NOCM due to the energy-intensive C-H activation and limited stability of reactive intermediates. Rutile TiO₂ may have enhanced activity and selectivity in doped configurations or as a catalyst support within multifunctional catalytic systems.