Preparation Of G-C₃N₄ Coordinated With Iron-based Catalysts And Its Photocatalytic Performance Under Visible Irradiation

With the development of modern science and technology,the pervasive water pollution problems has attracted wide attention regarding ecological environment and human health.The industrial wastewater often contains some organic compounds with low concentration,high toxicity and recalcitrant,such as antibiotic drugs,endocrine disruptors,chlorinated aromatic compounds and so forth.These organic compounds has been recognized as stubborn and long-term harm to human health by international community.Therefore,a technology with high efficiency and energy conservation is in need for the degradation of organic pollutants.Among various methods for wastewater treatment,photocatalytic degradation technology is popular with the researchers because of its promising potential for the employment of visible light or solar and efficient degradation of organic pollutants.Graphitic carbon nitride?g-C₃N₄?,as a photocatalyst with the properties of non-toxic,easy prepation and thermally stable,has garnered great concern in photocatalytic field for degradation of organic pollutants.However,the application of pure g-C₃N₄ has been limited for its poor absorption of visible light and low catalytic activity for conjugated compound removal.The modification of g-C₃N₄ is necessary for improvingthephotocatalyticperformance.Metallophthalocyanines?MPcs?and metalloporphyrins?MPs?has a similar conjugate macrocyclic structure with cytochrome P450,which usually employed as biomimetic catalyst for the acitivation of H₂O₂,O₂ and used in catalytic reactions.In nature,metalloporphyrins,such as chlorophyll,cytochrome P450 and so forth,are key materials in maintaining the ecological cycles,especially the carbon cycle,and play an important role in both photosynthesis and the catalytic oxidation of organisms.Inspired by these factors,a synergistic system is established by combining photocatalysis and biomimetic catalysis.g-C₃N₄is obtained by pyrolysing of urea,followed with the precursors g-C₃N₄-Br and g-C₃N₄-IMD which are prepared by g-C₃N₄ reacting with 5-Bromovaleroyl chloride and a substitution reaction between g-C₃N₄-Br and imidazole ligand?IMD?,respectively.Then,g-C₃N₄-IMD-hemin is obtained by g-C₃N₄-IMD axial coordination with hemin.To investigate the synergistic photocatalytic oxidation performance of g-C₃N₄-IMD-hemin in the presence of solar irradiation,p-chlorophenol?4-CP?is selected as the substrate and H₂O₂ is employed as the oxidant.Compared with pure g-C₃N₄,hemin and the mechanical mixture of the two?g-C₃N₄+hemin?,g-C₃N₄-IMD-hemin exhibits a good photocatalytic activity for 4-CP degradation and the stability of hemin is enhanced in the g-C₃N₄-IMD-hemin/H₂O₂ system.Furthermore,the effect of pH value,H₂O₂ concentration and recyclability of the catalyst are also explored.The results indicate that g-C₃N₄-IMD-hemin shows a good recyclability and 4-CP can be removed completely by g-C₃N₄-IMD-hemin in acidic,neutral and alkaline conditions compared with pure g-C₃N₄,in which the greatest difference of photocatalytic activity is in alkaline condition?pH 9?.An EPR spin trapping technique,adding capture agent,high-definition electrospray mass spectra?ESI-MS?analyses and gas chromatography/mass spectrometry?GC-MS?are utilized to probe the active species during the photocatalytic process.The findings show that high-valent iron?Fe?IV?=O?species,superoxide radicals?·O₂^-?,peroxy radicals?·OOH?and few·OH are generated in the catalytic system.To further improve the utilization of visible light,we combine g-C₃N₄-IMD with perchlorinated iron?II?phthalocyanine(FePcCl₁₆)through axial coordination and a visible-light-assisted catalyst g-C₃N₄-IMD-FePcCl₁₆ is prepared.In g-C₃N₄-IMD-FePcCl₁₆/H₂O₂system,the phthalocyanine ring can be actived to an exicited state FePcCl₁₆*under visible light irradiation.Then the Fe?IV?=O species which can be anchored to the catalyst itself are generated as FePcCl₁₆*contact with H₂O₂.Such a system provides good stability during the photocatalytic oxidation process.For comparision,g-C₃N₄-IMD-FePc is prepared by the same synthetic method and a contrast test is carried out in the presence of g-C₃N₄-IMD-hemin,g-C₃N₄-IMD-FePc and g-C₃N₄-IMD-FePcCl₁₆ for the degradation of carbamazepine?CBZ?.In pH 7,the results show that g-C₃N₄-IMD-FePcCl₁₆ can activate H₂O₂ efficiently for complete degradation of CBZ both in the presence of visible light and solar irradiation.This research combines photocatalysis and biomimetic catalysis skillfully and converts the traditional degradation mechanism based on·OH into anchored Fe?IV?=O species which provides good stability for the reaction system.