Regulation Of Electrocatalytic Reduction Performance Of Nitrogen-doped Porous Carbon-based Materials And The Electrocatalysis In Pollutants Conversion And CO₂ Reduction

Porous carbon nanomaterials have the advantages of good electrical conductivity,large specific surface area,high stability,low cost and easy availability,which make them attractive in electrocatalytic conversion of refractory pollutants in water and CO₂ reduction.However,due to the lack of active sites and the vague understanding on the regulation mechanism of electrocatalytic performance,carbon-based electrocatalysts suffer from some drawbacks such as low reactivity and poor selectivity.Therefore,deep understanding on the regulation of electrocatalytic performance of carbon-based materials is necessary to guide the preparation of efficient carbon-based electrocatalysts.In this work,a series of nitrogen-doped porous carbon-based electrocatalysts were prepared by carbonization of different precursors.The electrocatalytic reduction property of the catalysts was adjusted by changing the doping amount of nitrogen,modifying with metal nanoparticles or atomically dispersed metal sites.The influences of the active components in the catalysts were explored with the reactions of refractory pollutants reduction,nitrate reduction and CO₂ reduction.And the improvement mechanism on the electrocatalytic reduction performance of the catalyst was further revealed.The obtained main conclusions are shown as follow:（1）To improve the electrocatalytic performance of carbon-based materials by adjusting the nitrogen content,a series of nitrogen doped porous carbon（NPC）electrocatalysts with different N contents were prepared by the carbonization of ZIF-8 precursor under different temperatures.The effect of the nitrogen content of NPC on its electrocatalytic reduction performance was investigated using nitrobenzene as a probe compound.The results showed that,under the premise of sufficient carbonization,the high nitrogen content in NPC was beneficial to improve the electrocatalytic reduction rate of nitrobenzene and promote the production of aniline.The NPC obtained with 800℃carbonization exhibited the optimal nitrobenzene reduction performance with the N content of 22.67 at.%.93.8%nitrobenzene could be reduced with a reaction rate constant of 1.00 h^-1 at-0.8 V（vs.Ag/Ag Cl）,while the Faradic efficiency of aniline production was 30.9%.The reaction rate of nitrobenzene removal and current efficiency of aniline production on NPC was 4.2 and 6.4 times higher than that on the commercial graphite electrode,respectively.The result of the quenching experiment showed that the electrocatalytic reduction of nitrobenzene on NPC was mainly an indirect hydrogenation reduction process,and the active hydrogen atom H*played a key role.（2）To improve the electrocatalytic performance of carbon-based materials by metal compounds,a series of Cu nanoparticle modified NPC composite electrocatalysts（Cu-NPC）were prepared by the carbonization of precursors composed with glucose,Cu Cl₂ and hydroxylamine hydrochloride.The mechanism of copper nanoparticles and NPC support on the electrocatalytic reaction process was investigated with nitrate reduction as a typical reaction.The optimized Cu-NPC showed 100%nitrate removal and 63.1%total nitrogen removal within9 h reaction in a single-cell reactor at-2.1 V（vs.Hg/Hg₂SO₄）.The kinetic constant of nitrate removal on the Cu-NPC was 3 and 19 times higher than that on commercial Cu sheet and graphite electrodes,respectively.The excellent nitrate removal performance of Cu-NPC was ascribed to the synergy of Cu nanoparticles and NPC carrier,which coupled the facilitation of nitrate conversion to nitrite on Cu nanoparticles and the acceleration of subsequent hydrogenation on NPC.（3）To improve the electrocatalytic performance of carbon-based materials by single-atom metal sites,the single-atom Cu modified nitrogen doped porous carbon(Cu _SANPC)was designed and prepared with the carbonization of Cu contained ZIF-8 precursor.The influence of introduced single-atom copper on the electrocatalytic nitrate reduction property of nitrogen-doped carbon-based materials was investigated with the comparation of the nitrogen-doped porous carbon（NPC）catalyst and the nitrogen-doped porous carbon-supported copper nanoparticles(Cu _NPNPC)catalyst.The experimental results showed that the introduced single-atom Cu enhanced the electrocatalytic performance of the catalyst on nitrate reduction,which positively shifted the onset potential and increased the selectivity of ammonia production.The onset potential of nitrate reduction on Cu _SANPC was-0.25 V（vs.RHE）,which was 0.27 V and0.15 V lower than that on NPC and Cu _NPNPC.The highest Faradic efficiency of Cu _SANPC for ammonia production was 87.2%,which achieved at-1.1 V（vs.RHE）with the ammonia production rate of 2602μg cm^-2 h^-1.The ammonia production rate was 2.3 times of NPC and2.3 times of Cu _NPNPC,respectively.The TOF numbers of Cu atoms in Cu _SANPC was 2.5 times of that in Cu _NPNPC.The selectivity of ammonia production on Cu _SANPC increased to 94.1%.（4）To improve the electrocatalytic performance of carbon-based materials by regulating the atomic dispersed metal sites,a diatomic Fe-Fe site modified nitrogen doped porous carbon（Fe₂NPC）was accurately prepared by direct carbonization of diiron nonacarbonyl contained ZIF-8 precursor.The electrocatalytic reduction of CO₂ was selected as the target reaction.The influence of the adjusted diatomic Fe-Fe configuration to the catalyst on the electrocatalytic reduction property was investigated with the discussion of electrocatalytic CO₂ reduction performances on nitrogen doped porous carbon（NPC）catalyst,nitrogen-doped porous carbon supported single-atom iron（Fe₁NPC）catalyst and Fe₂NPC.The results showed that Fe₂NPC had higher CO-producing Faraday efficiency(FE_CO)and CO-producing current density(j _CO)than Fe₁NPC and NPC.The FE_CO on Fe₂NPC reached a maximum of 96.0%at-0.6 V（vs.RHE）,and the corresponding j _CO was 2.96 m A cm^-2,the TOF number of Fe atoms in Fe₂NPC was 1.8 times than that in Fe₁NPC.The experimental and theoretical calculation results showed that the Fe₂N₆ site of Fe₂NPC had better CO₂ activation performance than the Fe N₄ site of Fe₁NPC.The two iron atoms at the Fe₂N₆ site cooperated with each other during the reaction,which reduced the energy barrier of the catalyst for CO production.In summary,the researches in this thesis revealed that the electrocatalytic reduction property of nitrogen doped carbon-based catalyst could be efficiently improved by tuning the nitrogen content and regulating the metal loading,which was discussed by preparing a series new nitrogen doped porous carbon-based materials.The researches in this thesis are helpful to understand the mechanism on the regulation of the electrocatalytic reduction performance of nitrogen-doped carbon-based nanomaterials.It has great significance in the design of high performance carbon-based electrocatalysts and the application in environmental fields such as water pollution control and CO₂ reduction.