(681d) Controlling the Activation of Methane for the Value-Added Products on Ir-Doped RuO2 and TiO2 (110)

Converting methane to value-added chemicals is widely regarded as an energy-efficient and eco-friendly use of natural gas. However, the difficulty in breaking the robust C–H bonds of methane and controlling subsequent reactions that govern product selectivity have hindered the adoption of methane-based chemicals. Studies have shown that IrO₂(110) is highly reactive for methane activation at low temperatures (< 100 K) but mainly produces complete oxidation products (CO₂ and H₂O) [1]. Density functional theory (DFT) calculations show that CH₄ molecules form strong dative bonds with coordinatively unsaturated Ir atoms (Ir_cus), and the desorption of the dative-bound CH₄ requires more energy than its activation. While rutile TiO₂(110) is almost inert for this process, it is isostructural to IrO₂, suggesting the possibility of synthesizing mixtures of IrO₂ and TiO₂. Similarly, RuO₂(110) can form mixed metal oxides with IrO₂, providing additional control over the Ir content. In this work, we aim to elucidate the effects of Ir isolation using DFT calculations to control methane activation on Ir-doped RuO₂ and TiO₂ catalysts. We assess the binding energies of CH₄*, CH₃*, H*, CO*, O*, OH*, and H₂O* on these surfaces. Furthermore, we contrast potential methane activation mechanisms to form products such as C₂H₄, CH₂O, and CO₂. Our data suggest that binding energies of species on isolated Ir within TiO₂ surfaces are more exothermic than those on pure IrO₂ surfaces and much more exothermic than those on pure TiO₂ surfaces. While the formation of CO₂ on pure IrO₂ surfaces requires the diffusion of C-derived species from one Ir_cus site to the next, isolated Ir within TiO₂ surfaces prevents this diffusion. Therefore, well-dispersed Ir single atoms on TiO₂ surfaces may be viable candidates for cost-effective methane conversion.