Metal Nanoparticles Supported On Modified Carbon Nanotubes For Electrochemical Reduction Of Carbon Dioxide

Electrochemical reduction of CO2(CO2ER)by renewable electricity to produce high value-added chemicals or fuels is a sustainable and promising way to reduce the concentration of CO2 and alleviate the current problem of energy shortage.As a novel carbon material,carbon nanotubes(CNTs)have been widely reported as catalyst support applied in CO2ER due to the extraordinary electrical and mechanical properties.In this paper,based on the preparation of catalysts for CO2ER,the effects of oxygen-containing functional groups and heteroatom doping on the surface of CNTs on loading nanostructured metal catalysts and electrochemical conversion of CO2 were systematically investigated.The main research results obtained are as follows:(1)Three catalysts(Ag/CNT-COOH,Ag/CNT-OH and Ag/CNT-CO)were reasonably synthesized by loading Ag nanoparticles(NPs)on modified CNTs with different dominant oxygen functional groups(carboxyl,carbonyl and hydroxyl)via chemical in-situ reduction method.Ag NPs loaded on Ag/CNT-COOH are observed with the most uniform size and dispersion due to the highest content of surface carboxyl groups,which have been proved to be the effective adsorption sites for the nucleation of Ag NPs.In addition,the relatively lower pKa of carboxyl group endows it with the stronger ability of proton-donation compared to hydroxyl and carbonyl groups.Electrochemical results show that Ag/CNT-COOH shows the highest current density and Faradaic efficiency for CO production at high overpotentials(E<-1.0 V vs.RHE)while the opposite situation can be observed when Ag catalyst is not completely activated at low overpotentials(E>-0.9 V vs.RHE),which can be attribute to the the advantages of the stronger proton-donation ability and faster charge transfer for promoting the conversion of surface protons to the intermediates of CO2ER or H2 evolution reaction(HER).(2)Oxygen-,nitrogen-,and sulfur-doped CNTs were synthesized via different heat treatments,respectively.Electrochemical results show that oxygen-and sulfur-doped CNTs have poor activity for CO2ER,while the nitrogen-doped CNTs has good catalytic performance for CO2ER,and the maximum CO Faraday efficiency can reach to 94%at-1.0 V(vs.RHE).(3)Au/CNTs-OT,Au/CNTs-N and Au/CNTs-S catalysts were obtained by loading Au NPs on the three heteroatom-doped CNTs.The distinct anchoring effect of heteroatoms-doped CNTs leads to different size distributions of Au NPs.Electrochemical results show that the CO2ER activity of Au/CNTs-N is not significantly improved compared with CNTs-N while the HER activity is significantly increased.Au/CNTs-S shows the highest CO current density and Faradaic efficiency as well as excellent electrochemical stability.Notably,the maximum CO Faradaic efficiency can reach to 97%at-0.8V(vs.RHE).It is found that the catalytic performance of Au is not only related with the particle size,but also effected by the electronic effect between the heteroatoms-doped CNTs and Au NPs.Theoretical calculations show that the electron-withdrawing effect of nitrogen-doped CNTs promotes HER,while the electron-donating effect of sulfur-doped CNTs can significantly reduce the free energy required for the formation of*COOH intermediate,thereby effectively promoting the conversion of CO2 to CO.