【作者】 王乐；

【导师】 殷宪强；

【作者基本信息】 西北农林科技大学 ， 环境科学， 2022， 硕士

【摘要】 随着工业的快速发展,各类重金属在生产过程中的广泛使用导致重金属废水在自然环境中大量排放。Cr（Ⅵ）广泛存在于电镀、制革等行业,大量含Cr（Ⅵ）废水的排放严重危害环境和人体健康。因此,如何治理Cr（Ⅵ）废水成为备受关注的问题。利用高效、清洁的光催化技术能够成功地将Cr（Ⅵ）转化为Cr（Ⅲ）,起到降低Cr毒性的作用。在众多催化剂中,石墨相氮化碳（g-C₃N₄）是一种新型的非金属半导体催化剂,具有合适的带隙和稳定的结构,是近几年光催化领域的热门材料,但其内部的光生载流子极易复合,导致在实际应用中具有一定的局限性。因此,本文通过表面修饰和与粘土矿物复合两种不同的策略对g-C₃N₄进行改性,调控材料的理化性质、电子能带结构,并对Cr（Ⅵ）的光催化性能进行探究,揭示不同改性材料对Cr（Ⅵ）的光催化反应机理。本文可为开发环境友好、高效的半导体光催化剂g-C₃N₄提供一定的参考价值。主要结论如下:1、采用水热合成法,成功合成酸改性氮化碳（H-CN）和碱改性氮化碳（OH-CN）,通过SEM、XRD、XPS、FT-IR、UV-vis、PL等表征方法得到证实。相比于g-C₃N₄,改性后的H-CN和OH-CN具有更高的光催化性能。这是由于酸的氧化刻蚀和质子化作用增大了H-CN的BET比表面积,并使材料的孔结构更加丰富,为光催化反应提供了更多的活性位点。此外,碱水热法在OH-CN表面引入了羟基,增强了材料在溶液中的分散性和可溶性,提高了材料表面和内部电子-空穴对的分离效率。在光催化还原Cr（Ⅵ）的试验中,光照75 min后,3H-CN和0.4OH-CN对Cr（Ⅵ）的去除率分别为99.4%和98.7%,表现出对Cr（Ⅵ）优异的光催化性能,被筛选为反应的最佳材料。此外还探究了p H、催化剂投加量等因素对光催化反应的影响,筛选出反应体系的最佳条件为p H=2、催化剂投加量为30 mg。经过5次循环利用试验后,3H-CN和0.4OH-CN对Cr（Ⅵ）仍然有95%和96%的去除率,表明制备的材料具有良好的结构稳定性。最后通过捕获试验探究了3H-CN和0.4OH-CN光催化还原Cr（Ⅵ）的反应机理,催化剂在光照条件下使反应体系产生大量自由电子,将以强氧化性Cr₂O₇^2-形式存在的Cr（Ⅵ）还原为Cr（Ⅲ）,自由电子在反应中起主导作用。2、通过溶液浸渍-热聚合法成功制备了伊利石/氮化碳复合材料（IL-CN）,该材料对Cr（Ⅵ）具有良好的光催化活性。对材料表面的形貌结构和电子能带结构进行表征（如SEM、TEM、XRD、XPS、UV-vis等）,结果表明伊利石成功地负载到了g-C₃N₄表面,构建了紧密的复合界面结构,加速了电子在界面结构中的的传输,提高了g-C₃N₄的光催化性能。以Cr（Ⅵ）为目标污染物,通过不同伊利石比例的IL-CN对Cr（Ⅵ）光催化还原的试验,表明随着伊利石含量的增加,制备的复合材料对Cr（Ⅵ）的去除率先升高后下降,其中2/IL-CN为最佳材料,光照60 min后对Cr（Ⅵ）的去除率为91.8%。通过连续5次循环利用试验后,2/IL-CN仍能去除85%的Cr（Ⅵ）,表明氮化碳和伊利石通过煅烧的方法能保证复合材料的稳定性。最后通过捕获试验证明了在2/IL-CN和Cr（Ⅵ）的反应体系中贡献最大的是自由e^-。伊利石的电子迁移到CN表面,同时CN的空穴迁移到伊利石上,这种界面结构间的光生载流子传输能够促进材料电子和空穴的分离,从而在反应体系中产生大量e^-,将Cr（Ⅵ）还原为Cr（Ⅲ）。更多还原

【Abstract】 With the rapid development of industry,the extensive use of various heavy metals in the production process leads to the discharge of heavy metal wastewater in the natural environment.Cr（Ⅵ）widely exists in electroplating,leather and other industries,and the discharge of a large amount of wastewater containing Cr（Ⅵ）seriously harms the environment and human health.Therefore,how to treat Cr（Ⅵ）wastewater has become a concern.Using efficient and clean photocatalytic technology,Cr（Ⅵ）can be successfully converted into Cr（Ⅲ）,which can reduce the toxicity of Cr.Among many catalysts,graphite-phase carbon nitride（g-C₃N₄）is a new type of nonmetallic semiconductor catalyst,which has a suitable band gap and stable structure.It is a popular material in the field of photocatalysis in recent years.However,the photogenerated carriers in g-C₃N₄ are easily recombined,which leads to certain limitations in practical application.Therefore,in this paper,g-C₃N₄ was modified by surface modification and composite with clay minerals to control the physical and chemical properties and electronic band structure of the material,and the photocatalytic performance of Cr（Ⅵ）was explored to reveal the photocatalytic reaction mechanism of different modified materials on Cr（Ⅵ）.This study can provide some reference value for the development of environmentally friendly and efficient semiconductor photocatalyst g-C₃N₄.The main conclusions are as follows:1.Acid modified carbon nitride（H-CN）and alkali modified carbon nitride（OH-CN）were successfully synthesized by hydrothermal synthesis method,which was confirmed by SEM,XRD,XPS,FT-IR,UV-ⅥS,PL and other characterization methods.Compared with g-C₃N₄,the modified H-CN and OH-CN have higher photocatalytic performance.This is because the oxidation etching and protonation of acid increase the BET specific surface area of H-CN,enrich the pore structure of the material,and provide more active sites for photocatalytic reaction.In addition,alkaline hydrothermal method introduced hydroxyl groups on the surface of OH-CN,which enhanced the dispersion and solubility of the material in solution,and improved the separation efficiency of electron-hole pairs on the surface and inside of the material.In the experiment of photocatalytic reduction of Cr（Ⅵ）,the removal rates of Cr（Ⅵ）by 3H-CN and 0.4OH-CN are 99.4%and 98.7%respectively after 75 min of irradiation,showing excellent photocatalytic performance of Cr（Ⅵ）,and being selected as the best material for the reaction.In addition,the influence of p H,catalyst dosage and other factors on the photocatalytic reaction was also investigated.The optimal conditions of the reaction system were selected as p H=2 and catalyst dosage 30 mg.After 5 cycles,Cr（Ⅵ）removal rates of 3H-CN and 0.4OH-CN are 95%and 96%,indicating that the prepared materials have good structural stability.Finally,the mechanism of photocatalytic reduction of Cr（Ⅵ）by 3H-CN and 0.4OH-CN was investigated by active species trapping experiment.The catalyst produced a large number of free electrons in the reaction system under light condition,and reduced Cr（Ⅵ）in the form of strongly oxidized Cr₂O₇^2-to Cr（Ⅲ）.The free electrons played a leading role in the reaction.2.Illite/carbon nitride composite（IL-CN）was successfully prepared by solution impregnation-thermal polymerization,which has good photocatalytic activity for Cr（Ⅵ）.The surface morphology and electronic band structure of the material（such as SEM,TEM,XRD,XPS,UV-ⅥS,etc.）were characterized.The results showed that illite was successfully loaded onto the surface of g-C₃N₄,and a compact composite interface structure was constructed,which accelerated the transfer of electrons in the interface structure and improved the photocatalytic performance of g-C₃N₄.Taking Cr（Ⅵ）as the target pollutant,experiments on the photocatalytic reduction of Cr（Ⅵ）by IL-CN with different illite ratios showed that with the increase of illite content,the removal rate of Cr（Ⅵ）of the prepared composite first increased and then decreased,and 2/IL-CN was the best material.The removal rate of Cr（Ⅵ）was 91.8%after illumination for 60 min.After 5 cycles,85%Cr（Ⅵ）can still be removed by2/IL-CN,indicating that carbon nitride and illite can guarantee the stability of the composite by calcination.Finally,free e^-was proved to contribute most to the reaction system of 2/IL-CN and Cr（Ⅵ）by active species capture experiment.The illite electrons migrate to the surface of CN,while the CN holes migrate to the illite.This photogenerated carrier transport between the interface structures can promote the separation of electrons and holes in the material,thus generating a large amount of e^-in the reaction system and reducing Cr（Ⅵ）to Cr（Ⅲ）.更多还原