Visualizing Catalytic Dynamics of Single-Cu-Atom-Modified SnS₂ in CO₂ Electroreduction via Rapid Freeze-Quench Mössbauer Spectroscopy

Effective design and engineering of catalysts for an optimal performance depend extensively on a profound understanding of the intricate catalytic dynamics under reaction conditions. In this work, we showcase rapid freeze-quench (RFQ) Mössbauer spectroscopy as a powerful technique for quantitatively monitoring the catalytic dynamics of single-Cu-atom-modified SnS₂ (Cu₁/SnS₂) in the electrochemical CO₂ reduction reaction (CO₂RR). Utilizing the newly established RFQ ¹¹⁹Sn Mössbauer methodology, we clearly identified the dynamic transformation of Cu₁/SnS₂ to Cu₁/SnS and Cu₁/Sn during the CO₂RR, resulting in an outstanding Faradaic efficiency for formate production (∼90.9%) with a partial current density of 158 mA cm^-2. Results from <i>operando</i> Raman spectroscopy, <i>operando</i> attenuated total reflection surface-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 the anchored single Cu atom in Cu₁/SnS₂ can accelerate the reduction of SnS with <i>in situ</i> formation of Cu₁/Sn under CO₂RR conditions, which effectively promote the generation of *CO₂^-/*OCHO intermediates. Theoretical calculations further support that <i>in situ</i> formed Cu₁/Sn works as active sites catalyzing the CO₂RR, which reduces the energy barrier for the CO₂ activation and formation of the *OCHO intermediate, thereby facilitating the conversion of CO₂ to formate. The results of this work provide a thorough understanding of the dynamic evolution of Sn-based catalytic sites in the CO₂RR and shed light for engineering single atoms with an optimized catalytic performance. We anticipate that RFQ Mössbauer spectroscopy will emerge as an advanced spectroscopic technique for enabling a genuine visualization of catalytic dynamics across various reaction systems.