A Study On Electrochemical Reduction Of Carbon Dioxide To Syngas And Hydrocarbons Over Gold-based And Copper-based Catalysts

Currently,the necessity of reducing carbon dioxide(CO2)in atmosphere is highlighted due to the exhaustion of fossil resources resulting serious environmental problems.In recent years,the electrochemical reduction of CO2 to clean and sustainable fuels and platform chemicals has been viewed as a potential strategy for solving environmental pollution and received increasing attention.Electrochemical reduction of CO2 is one of the sustainable chemical routes because it can be driven by renewable electricity and pursued at ambient temperatures and pressures.However,the effective CO2 electrochemical reduction reaction(CO2ERR)is depended mainly on the development of effective catalysts.At present,Au-based catalysts are highly attractive for their high selective to carbon monoxide(CO)or syngas,while Cu-based catalysts are highly attractive due to their advantage to produce hydrocarbon products.Nevertheless,monometallic Au-based catalysts are expensive,low stability,and difficult to control the molar ratio of hydrogen(H2)to CO in the syngas product.And,the CO2 reduction over Cu-based catalysts required large overpotentials and the selectivity of hydrocarbons is limited because of the competing hydrogen evolution reaction.In this thesis,green and efficient Au-based bimetallic catalysts and oxide-derived Cu-based catalysts were designed and synthesized for the efficiently preparation of component tunable syngas and hydrocarbons under low overpotential.The electrocatalytic performance of these two kinds of catalysts in the CO2ERR was systematically studied as well as their mechanism.The specific works are summarized as follows:A series of Au-based bimetallic catalysts were synthesized by introducing Cu,Ag,In,Fe,Co,Ni,Zn and Bi as second metals into the Au using multi-walled carbon nanotubes and graphite phase carbon nitride as supports,respectively,with liquid phase reduction method.The performance of the catalysts for the electrochemical reduction of CO2 to syngas was evaluated,and the reaction conditions such as applied potential and category of electrolyte were investigated.The molar ratio of H2 to CO can be effectively controlled in a range of 0.12 to 17.04 by changing the supports and the M elements in AuM.At-0.5 V vs.RHE,AuAg/CNT showed the highest CO Faraday efficiency of 95.9%,besides,the highest CO partial current density is-16.4 mA/cm2,and the molar ratio of H2 to CO in syngas can be effectively controlled.At a low overpotential of 290 mV,the Faradaic efficiency of CO formation was 95.2%over AuCu/CNT.Moreover,a series of AuCu/CNT samples with different Cu/Au molar ratio were characterized by XRD,Raman spectroscopy,XPS,TEM and ICP-OES.The results showed that AuCu in AuCu/CNT samples was in the form of AuCu alloy structure,which showed a synergetic effect compared with monometallic catalysts.Meanwhile,their excellent electrochemical activity were confirmed by electrochemical impedance spectroscopy,double-layer capacitance tests and Tafel slope tests,providing a new way to control the ratio of syngas with bimetallic catalysts.Using Cu(CH3COO)2 as Cu source and urea as C/N source,a series of CuxO@CN catalysts were synthesized by controlling the temperature of calcination and weight ratio of precursors with thermal burning method.The performance of CuxO@CN for the electrochemical reduction of CO2 to hydrocarbons(CH4 and C2H4)was studied.XRD,Raman spectroscopy,XPS,TEM,XANES and ICP-OES confirmed the nanostructure of CuxO@CN.The ratio of Cu+to Cu2+and the amount and the structure of CN in CuxO@CN are controlled by the calcination conditions,which leads to different electronic migration and structure characteristics.By changing the applied potential at-0.8?-1.5 V vs.RHE,CuxO@CN sample prepared at 550? with a weight ratio of urea to Cu(CH3COO)2 of 10 has a high hydrocarbon Faraday efficiency of 80.0%,which is better than CuxO@C(44.1%),CuxO(31.9%),and commercial CuO(57.1%)and commercial Cu2O(49.6%).At last,the synthesized CuxO@CN catalyst showed good stability in 10 h test without any decay of catalytic activity.In addition,electrochemical impedance spectroscopy,electric double-layer capacitance tests and Tafel slope tests also prove its excellent electrochemical characteristics,which provided a theorectical basis for the design and development of Cu-based catalyst.