Visualizing Catalytic Dynamics of Single-Cu-Atom-Modified SnS2 in CO2 Electroreduction via Rapid Freeze-Quench Mossbauer 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) Mossbauer spectroscopy as a powerful technique for quantitatively monitoring the catalytic dynamics of single-Cu-atom-modified SnS2 (Cu-1/SnS2) in the electrochemical CO2 reduction reaction (CO2RR). Utilizing the newly established RFQ Sn-119 Mossbauer methodology, we clearly identified the dynamic transformation of Cu-1/SnS2 to Cu-1/SnS and Cu-1/Sn during the CO2RR, resulting in an outstanding Faradaic efficiency for formate production (similar to 90.9%) with a partial current density of 158 mA cm(-2). Results from operando Raman spectroscopy, operando attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), quasi in situ electron microscopy, and quasi in situ X-ray photoelectron spectroscopy (XPS) measurements indicate that the anchored single Cu atom in Cu-1/SnS2 can accelerate the reduction of SnS with in situ formation of Cu-1/Sn under CO2RR conditions, which effectively promote the generation of *CO2-/*OCHO intermediates. Theoretical calculations further support that in situ formed Cu-1/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 Mossbauer spectroscopy will emerge as an advanced spectroscopic technique for enabling a genuine visualization of catalytic dynamics across various reaction systems.