Studies On Electrocatalytic Reduction Of CO₂ By Nano Carbon-supportedmetal Alloys

The conversion of CO₂ into formic acid driven by low-grade renewable electricity is of significance,since it can not only reduce CO₂ emission by the utilization of CO₂ as feedstock,but also store renewable energy as value-added chemicals or fuels.In this thesis,a series of activated carbon（AC）-supported Pd-Sn alloys as the catalysts for CO₂ electroreduction were synthesized by a wetting chemistry reduction method.The bulk-phase compositions of Pd_xSn NPs were well-controlled to the desired alloy phases through the accurate wetting chemistry processes.The alloy nanoparticles comprising the bimetallic alloy phases of Pd₃Sn,Pd₂Sn,PdSn,PdSn₂ and PdSn₄ were present in the Pd_xSn/C catalysts（the subscript x represents the molar ratio of Pd/Sn in the preparation）,which were evidenced by HRTEM and XRD characterizations.XPS spectra showed the presence of substantial oxide species（PdO and/or SnO₂）in both Pd/C and Pd_xSn/C catalysts.Variation of the relative intensity ratios of Pd（0）/Pd（Ⅱ）and Sn（0）/Sn（Ⅳ）with respect to the molar ratios of Pd/Sn in Pd,Sn and alloy Pd_xSn NPs were shown in different variation trends.The Pd（0）/Pd（Ⅱ）ratio exhibited the inverse volcano-shaped variation while the Sn（0）/Sn（Ⅳ）ratios monotonically decreased with decreasing Pd/Sn molar ratios.The surface oxide species were quite stable,and the ratios of Pd（0）/Pd（Ⅱ）and Sn（0）/Sn（Ⅳ）in all catalysts were almost identical before and after CO₂electrolysis reactions.The linear sweep voltammetry curves were used to determine the appropriate electrolysis potentials for CO₂ electroreduction over Pd/C and Pd_xSn/C catalysts,in which hydrogen evolution reaction（HER）were greatly suppressed.The potentiostatic CO₂ electrolysis results showed that the main products were CO,formic acid,and hydrogen over Pd/C and Pd_xSn/C.The former（CO and formic acid）were from CO₂ reduction,and the latter was from HER.It was found that the faradaic efficiencies（FEs）and overpotentials of formic acid were closely related to the Pd（0）/Pd（Ⅱ）ratios of the catalysts.When x=1,i.e.,over the catalyst PdSn/C,the FE of more than 99%towards formic acid at an overpotential of-0.26 V was achieved,showing a superior performance for CO₂ electroreduction.The possible mechanisms of the formation of formic acid and CO were also explored by density functional theory（DFT）calculations.On the other hand,to improve the affordability of CO₂ electroredution over alloys,three kinds of non-noble metal alloy catalysts Pd-Sn,Co-Sn,and Cu-Sn supported on AC were studied,respectively.It was found that CuSn/C showed the better capacity for the productions of formic acid and ethanol,which showing a promising potential for CO₂ electroreduction by replacing Pd with Cu.