Research On Modification And Performance Of Carbon Nitride Polymer-based Photocatalytic Materials

The conversion of renewable and clean solar energy into chemical energy through photocatalytic technology is an effective strategy to address energy and environmental challenges.Carbon nitride polymer materials are widely used in photocatalytic technology due to their suitable and tunable electronic energy band structures,excellent chemical stabilitise,environmental friendliness.Improving the photocatalytic efficiency of carbon nitride plolymers through various modification strategies has become a hot research topic.Meanwhile,the novel photocatalysis-self-Fenton advanced oxidation technology based on carbon nitride polymers is also a frontier research direction,and provides new ideas for environmental catalysis research.The paper focuses on the modification of carbon nitride polymers to improve their photoelectric properties and photocatalytic efficiencies.Based on these,we have investigated the inherent connections between the catalytic efficiency of carbon nitride polymers and their modification strategies such as loading cocatalysts,molecular structure doping,crystal structure regulution,and metal element doping,which is anticipated to provide theoretical guidance for the design of high-efficiency photocatalysts and the development of novel advanced oxidation technologies.The main research results achieved are as follows:Firstly,to investigate the impact of surface modification of carbon nitride polymer on catalytic efficiency,the active sites of monodisperse Ni‐cluster were anchored onto the surface of carbon nitride（CN）to obtain high-performance catalytic materials（Ni-cluster/CN）.Ni-cluster/CN exhibited efficient photocatalytic H₂ production and degradation performance,with a rate of up to 16.50mmol·h^-1·g^-1.Furthermore,the incorporation of Ni-cluster co-catalysts enhanced the utilization efficiency of metal atom,resulting in a total turnover frequency（TOF（H₂））of 461.14 h^-1.Based on X-ray absorption fine structure spectroscopy（XAFS）analysis and density-functional（DFT）theory calculations,it was shown that a stable and strong interfacial interaction was formed between the Ni-cluster and CN,which promoted the rapid separation and transfer of photogenerated charges.An in-depth understanding of the interfacial interaction between CN and Ni-cluster was revealed which would has important reference significance for cocatalyst mechanism research.Secondly,in order to investigate the effect of changes in the energy band structure on the catalytic efficiency,graphitic carbon rings（Cg）were doped into the conjugated network structure of a carbon nitride polymer（Cg-C₃N₄）by thermal polymerization,which altered theπ-electron delocalization within the conjugated system,thus changing the inherent optical/electrical properties of Cg-C₃N₄.Cg-C₃N₄ exhibited a more positive valence band position and a wider photoresponsive range.Meanwhile,the formed strong in-phase built-in electric field facilitated the separation and migration of photogenerated charges.The novel photocatalysis-self-Fenton system constructed by Cg-C₃N₄ exhibited significant degradation performance,with a 32.09-fold increase in the degradation rate compared to that of photocatalytic process of pristine carbon nitride.Meanwhile,the total organic carbon（TOC）removal efficiency reached 59.64%,which is 11.36 times than that of the traditional Fenton method（Fenton）.The conversion efficiency of H₂O₂ to hydroxyl radical（·OH）in the system reached 64.70%,which is 8.41 times than that of the Fenton system.The excellent performance stemed from the high utilization efficiency of H₂O₂,the efficient cycling of Fe³⁺/Fe²⁺,and the effective separation of photogenerated charges,which gave rise to sustainable generation of strong oxidizing·OH and holes for enhancing the degradation performance.Furthermore,in order to investigate the effect of conjugated structure on the catalytic efficiency,a crystalline carbon nitride polymer（poly-（heptazine imide）,Mel-PHI）was prepared by a high-temperature salt-assisted method to achieve efficient photocatalytic reduction of O₂ to generate H₂O₂.The strong built-in electric field induced by its high crystallinity and extendedπ-conjugate structure is beneficial for promoting exciton dissociation and accelerating the photogenerated carrier separation as well as the transfer from bulk phase to surface,resulting in a surface photovoltage of as high as 19.24 m V.Meanwhile,the formed nanocrystals shortened the charge migration distance,which facilitated the rapid migration of photogenerated charges.The abundant surface-C≡N/-OH groups provided more active sites for O₂ adsorption and reduction.The photocatalytic H₂O₂ production rate（16.01 m M/h）of Mel-PHI under visible light（λ>400 nm）increased by 77.31 times relative to that of pristine graphitic carbon nitride,and the quantum efficiency at 405 nm reached 25.1%.The photocatalytic H₂O₂production reached 80.36 m M after 8 hours under simulated sunlight irradition.Crystalline Mel-PHI with a strong built-in electric field throwed prospect on photocatalytic H₂O₂production,showing a new perspective for the design of carbon nitride materials.Finally,in order to further improve the catalytic efficiency of crystalline Mel-PHI and expand its application area,atomically dispersed Fe-N₃ active sites were doped the Mel-PHI（Fe-20/Mel-PHI）to construct a heterogeneous photocatalysis-self-Fenton system.The system realized the effective synergy and spatial separation of photocatalytic in-situ H₂O₂ generation and self-Fenton in-situ H₂O₂ activation.The formation of Fe-N₃ coordination structure by Fe active sites was obtained from the analysis of XAFS results.Structural characterization and DFT calculations confirmed that the Fe active sites with high electron density were favorable to promote the rapid activation of H₂O₂ to·OH.In-situ infrared spectroscopy and other characterizations validated the in-situ generation and activation of H₂O₂ in the reaction system,and also revealed the formation of the key intermediate species*OOH during the activation and decomposition of H₂O₂,which facilitated the activation of O-O bonds.Meanwhile,the effective charge separation enhanced the electron transfer between Fe active site and H₂O₂,which significantly improved the conversion efficiency from H₂O₂ to·OH.The Fe-20/Mel-PHI based photocatalysis-self-Fenton system achieved an average H₂O₂ conversion efficiency of85.99%,which was 3.77 times that of its Fenton（22.78%）.It also exhibited excellent degradation performance including phenols and antibiotics.This study provided a new insight into the construction of heterogeneous photocatalysis-self-Fenton systems for water treatment.