<i>Operando</i> Mössbauer Spectroscopic Tracking the Metastable State of Atomically Dispersed Tin in Copper Oxide for Selective CO₂ Electroreduction

Abstract

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, <i>operando</i> 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 (<i>vs.</i> RHE) over Sn₁-CuO was achieved at an optimized Sn loading of 5.25 wt. %. <i>Operando</i> Mössbauer spectroscopy clearly identified the dynamic evolution of atomically dispersed Sn⁴⁺ sites in the CuO matrix that enabled the <i>in situ</i> transformation of Sn⁴⁺-O₄-Cu²⁺ to a metastable state Sn⁴⁺-O₃-Cu⁺ under CO₂RR conditions. In combination with quasi <i>in situ</i> X-ray photoelectron spectroscopy, <i>operando</i> 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 <i>in situ</i> construction of Sn⁴⁺-O₃-Cu⁺ as the true catalytic site altered the reaction path <i>via</i> 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 <i>in situ</i> tuning the electronic structure of Cu-based catalysts, which may pave the way for the development of efficient catalysts for high-selectivity CO₂ electroreduction.