Visualizing Catalytic Dynamics of Single-Cu-Atom-Modified SnS 2 in CO 2 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 2 (Cu 1 /SnS 2 ) in the electrochemical CO 2 reduction reaction (CO 2 RR). Utilizing the newly established RFQ 119 Sn Mössbauer methodology, we clearly identified the dynamic transformation of Cu 1 /SnS 2 to Cu 1 /SnS and Cu 1 /Sn during the CO 2 RR, resulting in an outstanding Faradaic efficiency for formate production (∼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 /SnS 2 can accelerate the reduction of SnS with in situ formation of Cu 1 /Sn under CO 2 RR conditions, which effectively promote the generation of *CO 2 - /*OCHO intermediates. Theoretical calculations further support that in situ formed Cu 1 /Sn works as active sites catalyzing the CO 2 RR, which reduces the energy barrier for the CO 2 activation and formation of the *OCHO intermediate, thereby facilitating the conversion of CO 2 to formate. The results of this work provide a thorough understanding of the dynamic evolution of Sn-based catalytic sites in the CO 2 RR 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.