Preparation Of Tin-based Catalysts And Research On The Mechanism Of Electrochemical Reduction Of Carbon Dioxide

Excessive use of fossil fuels has caused a large amount of carbon dioxide（CO₂）to be emittedinto the atmosphere,causing global climate change problems.Using clean and renewable energy to converted CO₂ molecules into fuels and chemicals with high added value through the electrocatalytic carbon dioxide reduction reaction（CO₂R）,which can not only realize the conversion and storage of renewable energy,but also realize the closed cycle of carbon.However,it is necessary to overcome a higher reaction energy barrier during the reaction process due to the extremely stable chemical structure of CO₂ molecules,resulting in a higher overpotential.And the catalyst also has problems such as low product selectivity and poor Faraday efficiency.The key factors to realize the commercial application of electroreduction of CO₂ are the selection of materials with abundant resources and low cost,and the design of catalysts with high catalytic activity,high Faraday efficiency and high product selectivity.As an element with extremely abundant reserves,Sn element has good CO₂ reduction activity.Therefore,this thesis mainly focuses on the structure design and performance research of tin（Sn）-based catalysts.The specific content mainly includes the following two aspects:（1）A tin-doped ceria（CeO₂）catalyst was designed.Firstly,CeO₂ nanorods were prepared by a simple hydrothermal method,and then oxygen vacancies were generated by Ar/H₂ heat treatment.Secondly,a series of Sn-doped CeO₂ nanorod catalysts with different contents were prepared by controlling the amount of Sn Cl₂ solution added.Finally,Ar/H₂ treatment was also applied to increase the oxygen vacancy concentration.Through the analysis of electrochemical test,and compared with the catalytic performance of pure CeO₂ and SnO₂,the reasons for improving the catalytic performance of Sn doped CeO₂ were summarized.The experimental results show that in 0.1M Na HCO₃ electrolyte in H-type electrolytic cell,the Faraday efficiency of formate is 81.10%at-1.1V vs RHE when the Sn content is 10%,which inhibits the release of hydrogen and the generation of CO,and the geometric current density is 9.13 m A cm^-2.Combing density functional theory with the calculation of Gibbs free energy,the results show that compared with CeO₂ and SnO₂,Sn-doped CeO₂ can effectively decrease the energy barrier to form HCOOH from the HCOO*intermediate and improve the selectivity of formate.The calculation of H adsorption by the catalyst shows that excessive hydrogen adsorption may preven the active sites of CO₂ adsorption,which is not conducive to the reduction of CO₂ to produce formate.The surface of CeO₂ can accelerate the dissociation of water.According to calculations,Sn-doped CeO₂ has a relatively moderate adsorption energy for H*,which provides adsorption of H for the formation of formate from CO₂R.Thus,Sn doping CeO₂effectively improves the selectivity to formate.（2）Since SnO₂ is unstable and easily reduced to metallic Sn during the catalytic reduction reaction of carbon dioxide,the improve the stability of SnO₂ and dynamic reduction process still needs further exploration.In the paper,SnO₂ nanoparticles can be grown in-situ on carbon cloth（SnO₂/CF）through a simple hydrothermal method.X-ray powder diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and in-situ Raman spectroscopy（in-situ Raman spectroscopy）were applied to analyze the process of CO₂R electrolysis.It was found that the SnO₂ catalyst was partially reduced and reconstructed into SnO₂/Sn motschottky heterojunction,which showed excellent electrocatalytic activity for CO₂R.In the H-type electrolytic cell with the 0.5 M Na HCO₃ electrolyte,the heterostructure SnO_2/Sn electrode has a higher Faraday efficiency of 93±1%when formate is generated at-1.0 V compared with the reversible hydrogen electrode,and its current density is 28.7 m A cm^-2,and it can remain relatively stable for 9 h.After that,the catalyst was tested again in the gas diffusion-flow electrolytic cell.The partial current density of formate at-1.18 V on the gas diffusion electrode with the 1.0 M KOH electrolyte is 174.86 m A cm^-2.Density functional theory calculations show that the SnO₂/Sn heterostructures in situ formed under CO₂R conditions helped decrease the energy barrier of HCOOH formation through COOH*as compared to pristine SnO₂ and Sn,and the hydrogen evolution reaction in SnO₂/Sn has been inhibited to a certain extent,and has better catalytic activity and selectivity for the production of formate from CO₂R.In the article,through the preparation and studied the Sn-doped CeO₂ and its catalytic activity,and the SnO₂ catalyst supported on carbon cloth was prepared,and studied the kinetic process and catalytic activity of in-situ electrochemical reconstruction in CO₂R reaction.Based on density functional theory,the mechanism of catalyst performance improvement was analyzed,which lays the foundation for the preparation of high-performance tin based CO₂R catalyst.