The electrochemical reduction of carbon dioxide (CO₂) into various multi-carbon products is highly desirable, as it could help to easily produce useful chemicals for a wide range of applications. Most existing catalysts to facilitate CO₂ reduction are based on copper (Cu), yet the processes underpinning their action remain poorly understood.

Researchers at the Chinese Academy of Sciences, City University of Hong Kong, and other institutes in China have recently set out to design more efficient Cu-free electrochemical catalysts for the reduction of CO₂ Their paper, published in Nature Energy, introduces a new catalyst based on Tin (Sn), which was found to reduce CO₂ to ethanol (CH₃CH₂OH) with a selectivity of 80%.

"The discovery of C-C coupling over the Sn₁-O3G catalyst was not accidental, but instead built on our earlier works on understanding the CO₂RR behavior of transition metal single-atom catalysts," Prof. Bin Liu, co-author of the paper, told Tech Xplore.

"Specifically, we conducted preliminary experiments involving structural and electrochemical characterizations of various Sn-based CO₂RR catalysts, including metallic Sn nanoparticles, SnS₂ nanosheets, SnS₂ on nitrogen-doped graphene, single Sn atoms on nitrogen-doped graphene (Sn-4N) and single Sn atoms on O-rich graphene (Sn₁-O3G)."

In their preliminary experiments, the researchers found that both Sn₁-4N and Sn₁-O3G catalysts could reduce CO₂ to CO with KHCO₃, as a proton donor in a CO₂RR solution. However, these catalysts displayed different behavior in the presence of the acid formate, with only Sn₁-O3G ultimately producing ethanol.

"These observations led us to believe that the difference in CO₂RR between the Sn₁-4N and Sn₁-3OG catalysts could result from the different coordination environments of Sn," Prof. Liu said. "Thereafter, we focused our efforts on understanding the C-C coupling mechanism on O-coordinated Sn catalytic sites and constructed a tandem catalyst to realize selective CO₂RR to ethanol."

Prof. Liu and his colleagues fabricated their new Sn-based electrocatalyst by eliciting a solvothermal reaction between SnBr₂ and thiourea on a three-dimensional (3D) carbon foam. They subsequently examined their catalyst to characterize its structure.

Their examinations suggest that their catalyst is made up of SnS₂ nanosheets and atomically dispersed Sn atoms. These components are coordinated on the 3D O-rich carbon by binding with three O atoms (Sn₁-O3G).

"The electrochemical performance of the SnS₂/Sn₁-O3G catalyst for CO₂RR was evaluated using chronoamperometry in an H-type cell containing CO₂-saturated 0.5-M KHCO₃," Prof. Liu said. "Our catalyst can reproducibly yield ethanol with a Faradaic efficiency (FE) of up to 82.5% at -0.9?V_RHE and a geometric current density of 17.8?mA cm^–2. Additionally, the FE for ethanol production could be maintained at above 70% over the potential window from -0.6 to -1.1 V_RHE."

In initial evaluations, the catalyst developed by the researchers achieved highly promising results, successfully producing ethanol from a CO₂RR solution with a high selectivity. In addition, the catalyst was found to be stable, maintaining 97% of its initial activity after 100?h of operation.

"The dual active centers of Sn and O atoms in Sn₁-O3G serve to adsorb different C-based intermediates, which effectively lowers the C-C coupling energy between *CO(OH) and *CHO," Prof. Liu explained. "Our tandem catalyst enables a formyl-bicarbonate coupling pathway, which not only provides a platform for C-C bond formation during ethanol synthesis and overcomes the restrictions of Cu-based catalysts but also offers a strategy for manipulating CO₂ reduction pathways towards desired products."

The recent work by this team of researchers introduces an alternative Cu-free catalyst for eliciting the C-C bond formation and enabling the reduction of CO₂ to ethanol. In the future, their proposed approach could be used to produce ethanol more reliably and could potentially also be applied to the synthesis of other desired chemical products via the CO₂ reduction reaction.

"The search for more efficient catalysts with dual active sites should be pursued through high-throughput experiments and theoretical calculations," Prof. Liu added. "The rate and selectivity of a catalytic reaction are also closely related to the coverage of reaction intermediates on the catalyst's surface.

"Therefore, an in-depth study of factors that affect the residence time of intermediates, such as the pore structure of the support for the Sn₁-O3G dual-active sites, would help to deepen understanding of the C-C coupling process. We envisage that tandem catalysis based on the concept of dual-active sites could be extendable to C-X (X?=?N or S) coupling to prepare other chemicals, such as urea and alanine."

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