Preparation Of N-Doped Graphene Materials And Their Catalytic Degradation Performances Towards Organic Pollutants

Two-dimensional nitrogen doped graphene（NG）has large surface area,large conductivity,structure variety and non-toxicity,which could effectively activate persulfate to produce reactive oxidiant species and apply to the catalytic degradation of organic pollutants in environment area.Graphitic N in NG had been regarded as one of the possible active site for persulfate activation.However,the graphitic N content in NG prepared by calcination or hyderthermal methods was usually lower than 3 at%,limiting the improvment of its catalytic performance.In this work,a supramolecule induced method was employed to prepare graphitic N-rich graphene（GNG）,which could generate a nonradical persulfate activation;Considering nonradical persulfate activation possessed lower oxidation ability and higher tolerance,while radical persulfate activation possessed higher oxidation ability and lower tolerance,we have anchored Fe⁰ on GNG,constructing biavtive centers for radical and nonradical persulfate activation.The biactive centers could synergisticlly promote persulfate activation.Meanwhile,a facile ZnNCN confinement strategy had been proposed to prepared N-rich graphene（NRG）through one-step prolysis of nitrogen-rich organics.The relationship between graphitic N content,and persulfate activation of NRG and catalytic degradation of organics was studied.The main research contents are as follows:（1）Melamine would self-assemble with cyanurate and formed melamine-cyanurate（MC）supramolecule,which combined with graphene oxide（GO）throughπ-πinteraction.After calcination of the above precursor at 800°C,GNG with a graphitic N content of 4.18at%was prepared.The obtained GNG exhibited excellent capability to peroxymonosulfate（PMS）activation for degradation of organic pollutants.20 mg/L Rhodamine B（Rh B）was almost completely degraded by 50 mg/L GNG and 0.2 m M PMS within 60 min.Results of quenching experiments,electron paramagnetic resonance（EPR）spectra and density functional theory（DFT）calculations revealed that the graphitic N in graphene was the intrinsic active site for the cleavage of O-O bond in PMS to produce singlet-oxygen（¹O₂）,responsible for organics degradation.Organic pollutants could be effectively be removed in GNG/PMS system in a wide temperature range（5-45°C）,and the common matrix species of Cl^-,NO₃^-,HCO₃^-and humic acid（HA）had negligible effect on the degradation performance.In addition,the catalyst possessed a high stability and versatility.Over 80%Rh B could still be degraded after five cycles and different kinds of organics,such as methylene blue（MB）,methyl orange（MO）and tetracycline（TC）could also be effectively removed（2）Based on the preparation procedures of GNG,biactive centers of Fe⁰anchored graphitic N-rich graphene（Fe-GNG）were constructed via coordination between iron and GO@MC precursor.The morphology and structure analyses confirmed that Fe⁰ was anchored on two-dimensional graphene sheets.Compared with GNG,Fe-GNG exhibited superior PMS activation ability to degrade organic pollutants.20 mg/L Rh B was almost completely degraded by 25 mg/L Fe-GNG and 0.1 m M PMS within 70 min,accompanying with a low activation energy of 5.28 k J/mol.Electron paramagnetic resonance and quenching experiments indicated that hydroxyl radical（·OH）and ¹O₂ were predominant two reactive species responsible for Rh B degradation.After comparatively analyzes of the XPS spectra of Fe⁰,GNG and Fe-GNG2 samples before and after activating PMS,it was found that Fe⁰ and graphitic N in Fe-GNG were the active sites to active PMS for·OH and¹O₂ generation.The radical and nonradical oxidation processes could work in wide p H（2.2-10.3）and temperature（5-45°C）application ranges,and had a high tolerance to common matrix species,which could be applied to natural water systems.Furthermore,the prepared Fe-GNG magnetic catalyst was easily recycled by using a magnet and still degraded over 90%Rh B after five runs.（3）NRG was prepared via one-step pyrolytic transformation of N-rich organics,by using ZnNCN confinement formed from the reaction between metal Znand N-rich organics.The formed ZnNCN inhibits the sublimation of N-rich organics and release of N-containing gases,resulting in effective carbonization of N-rich organics and a high N-doping.NRG with N content of 30.28 at%and NRG with graphitic N content of 4.89 at%were obtained by adjusting calcination temperatures,ME and Znmass ratios and ramp rates.20 mg/L TC was almost completely degraded by 20 mg/L NRG and 0.1 m M PMS within 30 min,and corresponding turnover frequency（TOF）was 5.50 g^-1·min^-1.¹O₂ was confirmed to be the predominant ROS for TC degradation.XPS analyses of the fresh and used NRG indicated that graphitic N was the active site for PMS activation to produce ROS.Linear sweep voltammetry and DFT calculation results displayed that graphitic N accepts electron transferred from PMS to give ¹O₂ for TC degradation.Based on the intensity of EPR signal,the produced ¹O₂ amount was semi quantitatively analyzed.The results showed that the relative amount of ¹O₂ was linear with the graphite nitrogen content（R²=0.938）,and the degradation rate constant of TC was almost linear with the graphite nitrogen content（R²=0.812）,indicating that NRG is a kind of graphite nitrogen dependent PMS activation material.