Operando Spectroscopic Analysis of Axial Oxygen-Coordinated Single-Sn-Atom Sites for Electrochemical CO₂ Reduction

Abstract

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 Snsingle-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 Mossbauer 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.

Sn-based electrocatalysts for electrochemical CO2 reduction

“…The SAC has the distinctive advantage compared to bulk metal counterparts owing to its unique electronic structure, which not only can be further regulated easily, but also enables it to maximize the efficiency of atom utilization and therefore increase catalytic activity. 11,90–93…”

“…The SAC has the distinctive advantage compared to bulk metal counterparts owing to its unique electronic structure, which not only can be further regulated easily, but also enables it to maximize the efficiency of atom utilization and therefore increase catalytic activity. 11,[90][91][92][93] The Sn-based single-atom catalysts have also been reported for electrochemical CO 2 RR. Inspired by the fact that Pyridinic-N doped carbons could play a significant role in combining CO 2 and protons, Zhao et al 94 dispersed Sn on N-doped carbon nanofiber (AD-Sn/N-C) atomically and gave insight into the atomic structure of as-prepared catalysts (AD-Sn/N-C1000) by carrying out the aberration-corrected high-angle annual dark field scanning TEM (HAADF-STEM), which suggested that the isolated Sn atoms can be clearly visualized between carbon layers or on the surface of them.…”

“…This effective approach has demonstrated promising results in the rational synthesis of CO 2 RR electrocatalysts with abundant active sites. 136 Strikingly, dendritic structured electrocatalysts have been reported in various electrocatalytic systems, 37,38,137,138 which jointly offer a promising way to rationally fabricate Sn-based materials with higher catalytical activity. Notably, Won et al 139 prepared a series of Sn dendrite electrodes with large surface area and high porosity for optimal CO 2 RR activity via a simple electrodeposition method.…”

“…Apart from Cu nanoparticles, which can produce hydrocarbons and alcohols simultaneously through CO 2 transformation, [34][35][36] in recent years, Sn-based materials have gained significant attention as viable alternatives, offering economic viability compared to Bi-and In-based materials, low toxicity relative to Cd-, Pb-, Hg-, and Ti-based materials, ease of modification, and, crucially, environmental friendliness. These appealing characteristics have propelled Sn-based materials as highly promising candidates, [37][38][39][40][41] particularly in electrochemical CO 2 reduction applications. [42][43][44][45][46] By harnessing the unique properties of Sn-based materials, researchers can design rational and efficient electrocatalysts for the conversion of CO 2 , contributing to sustainable and cost-effective approaches.…”

Regulation of Electrocatalytic Behavior by Axial Oxygen Enhances the Catalytic Activity of CoN4 Sites for CO2 Reduction

“…[12][13] For example, Huang and co-workers have designed a series of single-Sn-atom catalysts with well-controlled coordination and electronic structure to explore the detailed structures of catalytic intermediates and the key surface species associated with CO 2 RR. [14] Very recently, Liao et al have studied the influence of small differences in the coordination environments of metal ions in the catalytic active centers on the selectivity of products in the electrocatalytic CO 2 reduction reaction (CO 2 RR). [15] This all reveals the importance of clarifying the structurefunction relationship.Recently, organometallic complexes with nitrogen-coordinated single atoms have been widely reported for electrocatalytic CO 2 RR.…”