(622b) In-Situ Spectroscopic Evidence for the Mars-Van Krevelen Mechanism in the Rh Single-Atom Catalyzed CO Oxidation

Single-atom catalysts have been garnering attention in recent years for a range of applications such as the oxidation of CO. Although surfaces of noble metals are well studied for CO oxidation, experimental in-situ investigations on single-atom catalysts are vastly limited. Current knowledge is primarily based on DFT calculations on structurally not well-defined single-atom environments. Hereby, we present operando Rh K-edge X-ray absorption spectroscopic experiments for atomically dispersed Rh on phosphotungstic acid. During the reaction, an oxygen vacancy in the heteropoly acid support was observed, and a rare five-coordination state of Rh was identified, indicating a Mars-van Krevelen mechanism. Furthermore, a structural model for the steady-state of the catalyst is proposed suggesting that the activation of O₂ is rate-limiting. We utilized in-situ DRIFT spectroscopy to further consolidate the understanding of the oxidation state of Rh during the reaction. This work adds solid experimental evidence to the general understanding of how CO and O₂ react over a single-atom site that is currently under debate. Atomically dispersed metal sites on structurally well-defined sites are prone to become particularly attractive tools for in-situ spectroscopy.