Preparation Of Defective Carbon Nitride Photocatalysts And Their Performance For Photocatalytic H₂O₂ Production And Photo-Fenton Process

H₂O₂ as an environmentally friendly oxidant,is widely used in wastewater treatment,disinfection,sterilization,bleaching,chemical synthesis and other fields.At present,more than95%of H₂O₂ is produced by the anthraquinone method,which involves multi-step processes with high energy consumption and environmental pollution.Additionally,the transport of concentrated H₂O₂ has safety risks.Photocatalytic technology has been considered as a promising and sustainable approach for the on-site production of H₂O₂ through the photocatalytic O₂ reduction process.Moreover,photocatalytic technology can be used to activate H₂O₂ and generate hydroxyl radical（·OH）with strong oxidizing ability for pollutant degradation through the photo-Fenton process.Among many reported photocatalysts,graphitic carbon nitride（C₃N₄）has been considered as one of the most promising photocatalysts for on-site production and activation of H₂O₂ because of the merits of chemical stability,visible light response and easy fabrication.However,the efficiency of C₃N₄ for photocatalytic production and activation of H₂O₂ is still very low due to the narrow light absorption range,poor charge separation,and slow transport of electrons and protons.To solve the above problems,we designed and fabricated several defects（alkali metal dopants,N vacancies and cyano groups）modified C₃N₄.Additionally,the light absorption and charge separation ability of C₃N₄,and the transport rate of proton/electron during photo-Fenton process were modulated by the synergistic effect of different defects.The mechanism of enhanced photocatalytic performance of C₃N₄ for H₂O₂ production and activation was revealed.The main research and results are shown as follows:（1）To improve the H₂O₂ production rate of C₃N₄ in the presence of alcohols as hole sacrificial agents,we prepared C₃N₄ comodified by alkali metal dopants and N vacancies（ACNN）by ionothermal method to enhance the light absorption and charge separation performance of C₃N₄.The photocatalytic performance of ACNN for H₂O₂ production under visible light was evaluated in the presence of 10 vol%isopropyl alcohol as hole sacrificial agents.The results show that the photocatalytic H₂O₂ production rate of ACNN reaches 10.1mmol·h^-1·g^-1,which is 88.6 times that of C₃N₄.Additionally,the photocatalytic H₂O₂production rate of ACNN in pure water reaches 45μmol·h^-1·g^-1,which is 4.5 times that of C₃N₄.Surface photovoltage spectrum,photoluminescence spectrum and UV-visible diffuse reflection spectrum were used to investigate the effect of alkali metal dopants and N vacancies on the light absorption and charge separation ability of C₃N₄.The results show that the alkali metal dopants and N vacancy not only enhance the light absorption ability of C₃N₄,but also improve the charge separation ability,thus improving the photocatalytic H₂O₂ production performance of ACNN.（2）To improve the H₂O₂ production rate of C₃N₄ in pure water,we prepared C₃N₄comodified by cyano groups and N vacancies（CCN-N_v）by Na BH₄ heat treatment to enhance charge separation performance of C₃N₄.Photochromic experiment shows that cyano groups as the redox mediators enable carbon nitride to capture,store and subsequently release photogenerated electrons.The photocatalytic performance of CCN-N_v for H₂O₂ production was evaluated under visible light in pure water.The results show that the photocatalytic H₂O₂production rate of CCN-N_v in pure water reaches 356μmol·h^-1·g^-1,which is 84.8 and 63.6 times higher than those of bulk C₃N₄ and C₃N₄ nanosheets,respectively.Photoluminescence spectrum and UV-visible diffuse reflection spectrum were used to investigate the effect of cyano groups and N vacancies on the light absorption and charge separation ability of C₃N₄.The results showed that the cyano groups as redox mediators can rapidly capture photogenerated electrons thus achieving effective separation of photogenerated electrons and holes.In addition,N vacancies can also promote the separation of photogenerated electrons and holes,and improve the photocatalytic performance of CCN-N_v for H₂O₂ production.The reason for the enhanced performance of CCN-N_v is that both cyano groups and N vacancies promote the charge separation process cooperatively.Compared with ACNN in Chapter 2(45μmol·h^-1·g^-1),the photocatalytic performance of CCN-N_v(356μmol·h^-1·g^-1)for H₂O₂ production in pure water increases by 7.9 times.（3）To improve the H₂O₂ activation rate of C₃N₄during photo-Fenton process under near-neutral condition,we prepared Fe doped C₃N₄ comodified by cyano groups and N vacancies（Fe-CCN-N_v）by thermal diffusion method to accelerate the transport of proton/electron.X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were used to demonstrate the introduction of cyano groups and N vacancies into Fe doped C₃N₄.The photo-Fenton performance of Fe-CCN-N_v under visible light was evaluated using phenol as the model pollutant.The kinetic constant of Fe-CCN-N_v under visible light irradiation reaches 0.12 min^-1under near-neutral conditions,which is 12.8 times that of Fe doped C₃N₄.Under dark condition,the kinetic constant of Fe-CCN-N_v reaches 0.060 min^-1.Furthermore,the influence of p H on the photo-Fenton performance of Fe-CCN-N_v was investigated.The results show that the kinetic constants of Fe-CCN-NV only slightly vary（less than 25%）over a wide p H range（3-9）,breaking the p H dependence of the traditional H₂O₂ activation processIn conclusion,introducing defects can regulate the light absorption,charge separation performance and surface catalytic process of C₃N₄ photocatalyst.And there is a synergistic effect between different types of defects,which can cooperatively improve the H₂O₂ production and photo-Fenton performance of C₃N₄.The methods proposed in this study provide new ideas for the rational design of high-performance photocatalysts for H₂O₂ production and activation.