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 ¹¹⁹Sn Mössbauer measurements and theoretical simulations were performed to track and identify the metastable state of single-atom Sn in copper oxide (Sn₁-CuO) for highly selective CO₂ electroreduction to CO. A maximum CO Faradaic efficiency of around 98% at -0.8 V (vs. RHE) over Sn₁-CuO was achieved at an optimized Sn loading of 5.25 wt. %. Operando Mössbauer spectroscopy clearly identified the dynamic evolution of atomically dispersed Sn⁴⁺ sites in the CuO matrix that enabled the in situ transformation of Sn⁴⁺-O₄-Cu²⁺ to a metastable state Sn⁴⁺-O₃-Cu⁺ under CO₂RR 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 Sn⁴⁺-O₃ stabilized adjacent Cu⁺ site was evidenced. In addition, density functional theory calculations further verified that the in situ construction of Sn⁴⁺-O₃-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 CO₂ and the desorption of the *CO, thereby greatly facilitating the CO₂-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 CO₂ electroreduction.

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