Mechanistic Insight into the Direct Nonoxidative Conversion of Methane to Ethylene over Structure-Sensitive RhO₂/TiO₂ Catalysts

Abstract

Catalytic methane nonoxidative conversion to value-added products is of great practical significance and an important prototype catalytic reaction. Considerable experimental studies have suggested that TiO2(110)-supported Rh nanoparticle catalysts significantly improve the catalytic activity of methane dehydrogenation and the possibility of C–C coupling at mild temperatures, while the high cost prevents its application, and the relationship between loading amounts and loading modes with reaction activity and the stability of catalysts is still not clear. Herein, using TiO2(110)-supported RhO2 as a model catalyst, reaction mechanisms of methane conversion to ethylene were systematically investigated using first-principles density functional theory (DFT) calculations combined with atomistic thermodynamic analysis. Five TiO2(110)-supported RhO2 model catalysts, i.e., Rh4+/TiO2(110), RhO2/TiO2(110), [RhO2]in‑II/TiO2(110), [RhO2]out‑II/TiO2(110), and [RhO2]III/TiO2(110), were studied. DFT calculations indicate that the CH4 conversion over rutile RhO2/TiO2 catalysts is highly structure-sensitive. The greater surface reconstruction may lead to surface instability; however, the [RhO2]III/TiO2(110) surface with a remodeled three layer of surface atoms exhibits optimal catalytic performance for the conversion of methane. The distinctive supported catalytic model surface effectively prevents carbon deposition due to complete dehydrogenation and avoids the generation of byproducts. From the perspective of 100% atomic utilization, the cogenerated H2 may have further requirements on the reaction temperature due to the endothermic nature of the reaction process. These results provide available and well-performing TiO2-supported noble metal oxide catalysts for the nonoxidative conversion of methane.