Operando Mössbauer Spectroscopic Tracking the Metastable State of Atomically Dispersed Tin in Copper Oxide for Selective CO 2 Electroreduction.

Metastable state is the most active catalyst state that dictates the overall catalytic performance and rules of catalytic behaviors; however, identification and stabilization of the metastable state of catalyst are still highly challenging due to the continuous evolution of catalytic sites during the reaction process. In this work, operando 119 Sn Mössbauer measurements and theoretical simulations were performed to track and identify the metastable state of single-atom Sn in copper oxide (Sn 1 -CuO) for highly selective CO 2 electroreduction to CO. A maximum CO Faradaic efficiency of around 98% at -0.8 V ( vs. RHE) over Sn 1 -CuO was achieved at an optimized Sn loading of 5.25 wt. %. Operando Mössbauer spectroscopy clearly identified the dynamic evolution of atomically dispersed Sn 4+ sites in the CuO matrix that enabled the in situ transformation of Sn 4+ -O 4 -Cu 2+ to a metastable state Sn 4+ -O 3 -Cu + under CO 2 RR conditions. In combination with quasi in situ X-ray photoelectron spectroscopy, operando Raman and attenuated total reflectance surface enhanced infrared absorption spectroscopies, the promoted desorption of *CO over the Sn 4+ -O 3 stabilized adjacent Cu + site was evidenced. In addition, density functional theory calculations further verified that the in situ construction of Sn 4+ -O 3 -Cu + as the true catalytic site altered the reaction path via modifying the adsorption configuration of the *COOH intermediate, which effectively reduced the reaction free energy required for the hydrogenation of CO 2 and the desorption of the *CO, thereby greatly facilitating the CO 2 -to-CO conversion. This work provides a fundamental insight into the role of single Sn atoms on in situ tuning the electronic structure of Cu-based catalysts, which may pave the way for the development of efficient catalysts for high-selectivity CO 2 electroreduction.