Operando Spectroscopic Analysis of Axial Oxygen-Coordinated Single-Sn-Atom Sites for Electrochemical CO 2 Reduction.

Sn-based materials have been demonstrated as promising catalysts for the selective electrochemical CO 2 reduction reaction (CO 2 RR). However, the detailed structures of catalytic intermediates and the key surface species remain to be identified. In this work, a series of single-Sn-atom catalysts with well-defined structures is developed as model systems to explore their electrochemical reactivity toward CO 2 RR. The selectivity and activity of CO 2 reduction to formic acid on Sn-single-atom sites are shown to be correlated with Sn(IV)-N 4 moieties axially coordinated with oxygen (O-Sn-N 4 ), reaching an optimal HCOOH Faradaic efficiency of 89.4% with a partial current density ( j HCOOH ) of 74.8 mA·cm -2 at -1.0 V vs reversible hydrogen electrode (RHE). Employing a combination of operando X-ray absorption spectroscopy, attenuated total reflectance surface-enhanced infrared absorption spectroscopy, Raman spectroscopy, and 119 Sn Mössbauer spectroscopy, surface-bound bidentate tin carbonate species are captured during CO 2 RR. Moreover, the electronic and coordination structures of the single-Sn-atom species under reaction conditions are determined. Density functional theory (DFT) calculations further support the preferred formation of Sn-O-CO 2 species over the O-Sn-N 4 sites, which effectively modulates the adsorption configuration of the reactive intermediates and lowers the energy barrier for the hydrogenation of *OCHO species, as compared to the preferred formation of *COOH species over the Sn-N 4 sites, thereby greatly facilitating CO 2 -to-HCOOH conversion.