Visualizing Catalytic\nDynamics of Single-Cu-Atom-Modified\nSnS<sub>2</sub> in CO<sub>2</sub> Electroreduction via Rapid Freeze-Quench\nMossbauer Spectroscopy

Effective design and engineering of catalysts for an\noptimal performance\ndepend extensively on a profound understanding of the intricate catalytic\ndynamics under reaction conditions. In this work, we showcase rapid\nfreeze-quench (RFQ) Mössbauer spectroscopy as a powerful technique\nfor quantitatively monitoring the catalytic dynamics of single-Cu-atom-modified\nSnS<sub>2</sub> (Cu<sub>1</sub>/SnS<sub>2</sub>) in the electrochemical\nCO<sub>2</sub> reduction reaction (CO<sub>2</sub>RR). Utilizing the\nnewly established RFQ <sup>119</sup>Sn Mössbauer methodology,\nwe clearly identified the dynamic transformation of Cu<sub>1</sub>/SnS<sub>2</sub> to Cu<sub>1</sub>/SnS and Cu<sub>1</sub>/Sn during\nthe CO<sub>2</sub>RR, resulting in an outstanding Faradaic efficiency\nfor formate production (∼90.9%) with a partial current density\nof 158 mA cm<sup>–2</sup>. Results from <i>operando</i> Raman spectroscopy, <i>operando</i> attenuated total reflection\nsurface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), quasi <i>in situ</i> electron microscopy, and quasi <i>in situ</i> X-ray photoelectron spectroscopy (XPS) measurements indicate that\nthe anchored single Cu atom in Cu<sub>1</sub>/SnS<sub>2</sub> can\naccelerate the reduction of SnS with <i>in situ</i> formation\nof Cu<sub>1</sub>/Sn under CO<sub>2</sub>RR conditions, which effectively\npromote the generation of *CO<sub>2</sub><sup>–</sup>/*OCHO\nintermediates. Theoretical calculations further support that <i>in situ</i> formed Cu<sub>1</sub>/Sn works as active sites catalyzing\nthe CO<sub>2</sub>RR, which reduces the energy barrier for the CO<sub>2</sub> activation and formation of the *OCHO intermediate, thereby\nfacilitating the conversion of CO<sub>2</sub> to formate. The results\nof this work provide a thorough understanding of the dynamic evolution\nof Sn-based catalytic sites in the CO<sub>2</sub>RR and shed light\nfor engineering single atoms with an optimized catalytic performance.\nWe anticipate that RFQ Mössbauer spectroscopy will emerge as\nan advanced spectroscopic technique for enabling a genuine visualization\nof catalytic dynamics across various reaction systems.