The Preparation Of High Activity Catalysts And Their Performance For CO₂ Electroreduction

Recently,CO₂ electroreduction to renewable fuels through clean and economical chemical processes is regarded as a prospective pathway to relieve environmental pressure and utilize the intermittent energy（such as wind and solar energy）effectively.However,the kinetics of this reduction reaction is very slow due to the inert CO₂ molecule.Moreover,catalytic CO₂ reduction in aqueous solution inevitably competes with hydrogen evolution reaction,which results in a low product selectivity.To reduce the overpotential and suppress competitive hydrogen evolution reaction,it is particularly important to develop catalysts with high conversation efficiency and product selectivity.This paper is aimed to design efficient catalysts for CO₂ reduction by increasing the local CO₂ concentration the catalyst surface,adjusting the surface oxidation state,and increasing the CO₂ adsorption capacity.These methods achieve the desired goal and provide new insights into the further development of catalysts,which make a small step towards the practical applications.The main findings are as follows:1 Promoting ethylene selectivity from CO₂ electroreduction on CuO supported onto CO₂ capture materialsCu is a unique catalyst for CO₂ electroreduction,since it can catalyze CO₂ reduction to a series of hydrocarbons like alcohols,and carboxylic acids.Nevertheless,such Cu catalysts suffer from poor selectivity.High pressure of CO₂ is considered as an effective way to facilitate the activity and selectivity of CO₂ reduction.Herein,a new strategy is presented for CO₂ reduction to improved C2H4 selectivity on a Cu catalyst by using CO₂ capture materials as the support at ambient pressure.N-doped carbon（NxC）was synthesized through high-temperature carbonization of melamine and 1-lysine.We observed that the CO₂ uptake capacity of NxC depends on both the microporous area and the content of pyridinic N species,which can be controlled by the carbonization temperature（600-800 ℃）.The as-prepared CuO/NxC catalysts exhibit a considerably higher C2H4 faradaic efficiency（36%）than CuO supported on XC-72 carbon black（19%）,or unsupported CuO（20%）.Moreover,there is a good linear relationship between the C2H4 faradaic efficiency and CO₂ uptake capacity of the supports for CuO.The local high CO₂ concentration near Cu catalysts,created by CO₂ capture materials,was proposed to increase the coverage of CO intermediate,which is favorable for the coupling of two CO units in the formation of C2H4.This study demonstrates that pairing Cu catalysts with CO₂ capture supports is a promising approach for designing highly effective CO₂ reduction electrocatalysts.2 Effect of oxidation state of OD-Cu on selectivity of ethylene in CO₂ reductionOxide-derived copper catalysts（OD-Cu）are widely studied to promote ethylene selectivity in CO₂ reduction among all the methods.But the promoting mechanism and principle were still remained unclear.Obviously,the activity of copper and its selectivity to form ethylene are enhanced when CuO or Cu2O is existed.Therefore,in this part,a composite material of cubic Cu2O and monoclinic CuO,CuxOy was prepared by pyrolysis.Ethylene selectivity is controlled by regulating Cu+/Cu2+ ratio on the OD-Cu surface.The results show that when Cu+/Cu2+=1.2,the faradaic efficiency of ethylene is as high as 68%.At the same time,our results demonstrate that the difference ratio of the Cu+/Cu2+ only affects the selectivity of ethylene but the selectivity of methane won’t be impacted.Thus,it is inferred that the mechanism of ethylene formation should be a C2 path.The reason for this high ethylene selectivity may be that more defects were exposed on the OD-Cu surface in CO₂ reduction.Defects favorably promote the absorption of CO₂ and the formation of ethylene.This method of modulating the oxidation state of the Cu-based catalyst also provides a reference for the further development of a high ethylene-selective Cu-based catalyst.3 High selectivity reduction of CO₂ to CO on carbon materials with excellent tolerance to metalEco-friendly sucrose and melamine were served as carbon and nitrogen sources,respectively.Fe（SCN）3 was introduced as iron sources during pyrolysis to regulate nanostructure.This catalyst was found to be very high CO selectivity（99%）at a moderate overpotential of 0.44 V.At the same time,we demonstrate that CO₂ reduction and hydrogen evolution reactions in carbon materials compete with each other for active sites.The increase of CO selectivity in this catalyst stems from the high CO₂ adsorption performance.Excellent tolerance towards metal impurities was found in carbon materials,which ensure the good performance from the electrolyte purity interference in CO₂ reduction and make it potentially useful.4 The relationship between CO₂RR and ORR in carbon materialsCarbon materials were widely used in CO₂ electroreduction reaction（CO₂RR）and O₂ reduction reaction（ORR）.However,the active sites of carbon materials still open to debate in two reactions.Active sites all come out the introduction of heteroatoms N.Therefore,the relationship of CO₂RR and ORR is essential to provide references for the active sites study in carbon materials.Firstly,the different carbon materials were prepared by doping Fe,Co,and Ni.Among these carbon materials,Fe/N/C exhibited the best performance both in CO₂RR and ORR.We also find that the trends are various between CO₂RR and ORR for carbon materials.In order to further prove this opinion,we prepared Fe/N/C catalysts by controlling different oxidation at different temperature and tested their CO₂RR and ORR performance.After the oxidation treatment,the CO₂RR activity was enhanced while ORR activity was suppressed.There not are connections between CO₂RR and ORR for Fe/N/C,so we speculate that the active sites in two reactions are different.To further compare the trend of N species content with the CO₂RR activity,pyrrolic N is considered as an important role in active sites.