Electrochemical Reduction Of CO₂ By Carboxyl Group Based IRMOF Materials

Ever since the Industrial Revolution,the excessive discharge of carbon dioxide（CO₂）has caused a series of social and environmental problems,such as global climate change and greenhouse effect.Therefore,reduction of CO₂ level in the atmosphere is in urgent need.Electrochemical CO₂ reduction reaction（CO₂RR）is considered to be an effective method to convert CO₂ into high value chemicals.Metal-organic frameworks have aroused increasing interest in electrochemical CO₂RR because of their high specific surface area and controllable structure.Meanwhile,Zn based materials have been manifested to be promising electrocatalysts for CO₂RR.However,IRMOF materials,with Zn as metal center and carboxylic organics as ligands,have not been widely noticed in the field of CO₂RR.Therefore,a systematic study was carried out to investigate the electrochemical CO₂RR by IRMOFs and their derivatives.First of all,IRMOF-8,with Zn（NO₃）₂·6H₂O as metal precursor and2,6-naphthalenedicarboxylic acid as ligand,was prepared by solvothermal method.The specific surface area of IRMOF-8 was 557.72 m²/g.The results showed that when Nafion dosage was 40（μL/m L）,catalyst loading was 1.91（mg/m L）,pre-reduction potential was-0.9V vs.RHE and working potential was-1.2 V vs.RHE,IRMOF-8 had the best performance with a CO Faradaic efficiency of 86%,and no obvious attenuation was found within 8 hours.The Faradaic efficiency of H₂ was basically less than 10%,and a small amount of formic acid was detected.The Tafel slope of CO was 169 m V/dec,indicating that CO₂ activation was the rate-determining-step（RDS）.In addition,the electrochemical surface area of IRMOF-8gradually decreased after the reaction,which could be due to the collapse of the skeleton.Furthermore,according to the characterization results,isolated Zn²⁺species was converted to Zn（0）nanoparticles（NPs）during electrochemical CO₂RR.Compared with the electrodeposited Zn/C materials,the Zn/C derived from IRMOF-8（IR8-Zn）exhibited higher catalytic activity owing to the suitable size and uniform distribution of Zn NPs.Moreover,the exposure of Zn（101）face also contributed to the high selectivity for CO.The CO₂RR performance of other IRMOF-8 derivatives was also studied.By using urea as the N source,N was successfully doped in IRMOF-8.However,the catalytic effect was significantly decreased,which was speculated to be associated with N species.In order to study the influence of N doping on electrocatalytic CO₂RR.IRMOF-3 was prepared by Zn（NO₃）₂·6H₂O and N-contained 2-aminoterephthalic acid,with a specific surface area of 459.4 m²/g,but the FE_CO obtained by CO₂ RR was only about 70%.Then,IRMOF-3 was carbonized at the temperatures of 600,700,800 and 900℃（named as IR3-T,T=600,700,800,900）,respectively.Among the carbonized samples,IR3-900 possessed the highest specific surface area of 1131.3 m²/g.Electrochemical CO₂RR results showed that IR3-900 not only decreased the optimal working potential to-0.74 V vs.RHE,but also improved the selectivity for CO,whose Faradaic efficiency could reach 90%for at least 7hours.XPS results implied that the distinct catalytic activity of these materials were associated with the N species.Pyridine-N and graphite-N could promote the production of CO,while pyrrole-N was prone to produce hydrogen.In addition,catalysts with high specific surface area could increase the local p H near the electrode,which might be conducive to CO₂ reduction.Due to the varieties of ligands in IRMOF materials,extensive studies could be conducted to further promote electrochemical CO₂RR by IRMOFs and their derivatives.The research above can provide a good foundation for the application of IRMOF in the field of CO₂ reduction.