Study On The Regulation Mechanism Of N Doping On The Cathode For The Performance Of Electrochemical Advanced Oxidation Process

Recently,electrochemical advanced oxidation processes?EAOPs?based on non-selective oxidation of organic pollutants by^·OH,have attracted great attention in the field of wastewater treatment due to their advantages of high mineralization efficiency for refractory organic pollutants.Among the main cathode oxidation technology,electro-Fenton?EF?is still limited by intrinsic drawbacks of narrow pH range and non-recyclable catalyst.At present,the research on 2e^-process for H₂O₂production in cathode is still limited,and there are few reports that the introduction of N element could promote the in-situ conversion of H₂O₂ to^·OH.Therefore,on the basis of raw cathode catalytic oxidation,a new cathode material and catalyst for the development of a new in-situ metal-free EAOPs is particularly important to expand the application of pollutant removal.However,the mechanism of the active site of this electro-catalyst is still unclear.It was necessary to determine its active mechanism in cathodic oxidation process and regulate the preparation method of catalyst,thereby regulating its removal of pollutants.Herein,nitrogen-doped graphene was synthesized by simple high-temperature pyrolysis method to anneal inexpensive melamine as nitrogen source and graphene.And the nitrogen-doped graphene and carbon black were mixed to modify graphite felt,which was used as cathode for the research of H₂O₂ production and pollutants removal.It was found that the mass ratio of melamine to graphene?N/C?was 3,which is the optimum doping ratio.At the same time,the nitrogen-doped graphene showed higher H₂O₂ yield?8.6 mg/h/cm²?and lower energy consumption?9.8kWh/kg?in the neutral solution.It was found that the generated free radicals at cathode include^·OH?80.72%?and O₂^-·?19.28%?for phenol removal.Most importantly,it proved that the introduction of graphite N could promote the 2e^-ORR process for H₂O₂ generation,and the presence of pyridinic N could catalyze H₂O₂ to the production of^·OH.Next,the optimal pyrolysis temperature?400°C?was determined by optimizing the pyrolysis temperature to control the performance of nitrogen-doped graphene,and it was found that the pyrolysis temperature had great influence on the N content and type:high temperature was beneficial to the construction of graphite N and it was mainly the formation process of pyridinic N and pyridine-N-oxide at low temperatures.Compared with the nitrogen-doped graphene prepared at 950°C,the concentration of^·OH from modified cathode increased by 2.46 times and the phenol removal increased by 52.22%when the pyrolysis temperature was 400°C.And N element did not participate the process of generated O₂^-·,but introduction of N in nitrogen doped graphene could promoted production of O₂^-·.The selectivity of H₂O₂to be catalyzed to produce^·OH by nitrogen-doped graphene was studied.With the increase of cathode potential and the increase of H₂O₂ concentration,the selectivity of generated^·OH by H₂O₂ was enhanced.When the H₂O₂ concentration reached 2.0 mM,the conversion selectivity of H₂O₂ to^·OH did not change with potential.And the XPS and FTIR spectra also showed that when the nitrogen-doped graphene undergo ORR process,graphite N promoted the adsorption of O₂ by nearby carbon atoms to form H₂O₂ by obtaining electrons and then desorb from the carbon.At the same time,the graphite N would be cracked to produce pyridinic N and pyrrolic N.In the catalytic process,pyridinic N catalyzed the formation of^·OH by H₂O₂ and pyridinic N simultaneously obtained oxygen atoms to produce pyridinic N-oxide.The 2e^-process was irreversible during the regeneration of the catalyst.However,regeneration could convert pyridinic N-oxide to pyridinic N to achieve catalyst reuse.Compared with conventional EF,the in-situ metal-free EAOPs was less affected by the initial pH.Moreover,in-situ metal-free EAOPs showed excellent stability,reusability and performance for various organic pollutants degradation.The result of effective pollutants removal by nitrogen-doped graphene in cathode and the analysis of its catalytic mechanism are expected to selectively control nitrogen-functional group to further enhance its application in EAOPs technology.