A tin-based tandem electrocatalyst for ethanol synthesis via CO₂ reduction

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

Researchers from the Chinese Academy of Sciences, City University of Hong Kong and other Chinese institutes recently set out to design more efficient Cu-free electrochemical catalysts for CO reduction₂ Their article, published in Natural energyintroduced a new tin (Sn) catalyst, which was found to reduce CO₂ with ethanol (CH₃CH₂OH) with a selectivity of 80%.

“The discovery of DC coupling on the Sn₁-The O3G catalyst was not accidental, but rather built on our previous work on understanding CO₂RR behavior of transition metal single-atom catalysts,” Professor Bin Liu, co-author of the paper, told Tech Xplore.

“Specifically, we conducted preliminary experiments involving structural and electrochemical characterizations of various Sn-based COs.₂RR catalysts, including metal nanoparticles of Sn, 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 Sn₁-4N and Sn₁-O3G catalysts could reduce CO₂ to CO with KHCO₃as a proton donor in a CO₂RR solution. However, these catalysts exhibit different behavior in the presence of acid formate, only Sn₁-O3G ultimately producing ethanol.

“These observations lead us to believe that the difference in CO₂RR between the Sn₁-4N and Sn₁The -3OG catalysts could result from the different coordination environments of Sn,” said Professor Liu. “Subsequently, we focused our efforts on understanding the CC coupling mechanism on the O-coordinated Sn catalytic sites and constructed a tandem catalyst to produce selective CO.₂RR with ethanol.

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

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

“The electrochemical performances of SnS₂/Sn₁-O3G catalyst for CO₂RR was assessed by chronoamperometry in a type H cell containing CO₂-KHCO 0.5-M saturated₃“, said Professor Liu. “Our catalyst can reproducibly produce ethanol with a farada 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 above 70% over the potential window of -0.6 to -1.1 V._RHE“.

During initial evaluations, the catalyst developed by the researchers achieved very promising results, successfully producing ethanol from a CO₂High selectivity RR solution. In addition, the catalyst proved to be stable, retaining 97% of its initial activity after 100 h of operation.

“The double active centers of the Sn and O atoms in Sn₁-O3G serves to adsorb different C-based intermediates, which effectively reduces the DC coupling energy between *CO(OH) and *CHO,” explained Professor Liu. “Our tandem catalyst enables a formyl-coupling pathway bicarbonate, which not only provides a platform for CC bond formation during ethanol synthesis and overcomes the restrictions of Cu-based catalysts, but also offers a CO manipulation strategy₂ reduction paths to desired products.

Recent work by this team of researchers introduces an alternative Cu-free catalyst to induce the formation of CC bonds and enable CO reduction.₂ with ethanol. In the future, the proposed approach could be used to produce ethanol more reliably and could potentially also be applied to the synthesis of other desired chemicals via CO.₂ reduction reaction.

“The search for more efficient catalysts with two active sites should be continued through high-throughput experiments and theoretical calculations,” added Professor Liu. “The rate and selectivity of a catalytic reaction are also closely related to the coverage of reaction intermediates on the catalyst surface.

“Therefore, further study of the factors that affect the residence time of intermediates, such as the porous structure of the Sn support₁-O3G dual-active sites would contribute to a deeper understanding of the DC coupling process. We envision that tandem catalysis based on the concept of doubly active sites could be extended to CX coupling (X = N or S) to prepare other chemicals, such as urea and alanine.

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