Visualizing Catalytic Dynamics of Single-Cu-Atom-Modified SnS in CO 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 SnS (Cu/SnS) in the electrochemical CO reduction reaction (CORR). Utilizing the newly established RFQ Sn Mössbauer methodology, we clearly identified the dynamic transformation of Cu/SnS to Cu/SnS and Cu/Sn during the CORR, resulting in an outstanding Faradaic efficiency for formate production (∼90.9%) with a partial current density of 158 mA cm. Results from Raman spectroscopy, attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), quasi electron microscopy, and quasi X-ray photoelectron spectroscopy (XPS) measurements indicate that the anchored single Cu atom in Cu/SnS can accelerate the reduction of SnS with formation of Cu/Sn under CORR conditions, which effectively promote the generation of *CO/*OCHO intermediates. Theoretical calculations further support that formed Cu/Sn works as active sites catalyzing the CORR, which reduces the energy barrier for the CO activation and formation of the *OCHO intermediate, thereby facilitating the conversion of CO to formate. The results of this work provide a thorough understanding of the dynamic evolution of Sn-based catalytic sites in the CORR 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.