【作者】 谢芳；

【导师】 侯浩波；

【作者基本信息】 武汉大学 ， 环境科学与工程， 2023， 博士

【摘要】 普遍存在于水环境中的典型有机污染物,对人体健康和生态环境造成了潜在的威胁,因此去除水体中的典型有机污染物成为了亟待解决的问题。基于硫酸根自由基的高级氧化技术（S-AOPs）是目前降解水中有机污染物最有效的方式之一。在S-AOPs的应用中,金属/石墨相氮化碳（g-C₃N₄）作为一种非均相催化剂具有简单易得、性质稳定、高性能等特点,同时g-C₃N₄作为一种光催化剂,可以利用太阳光在温和的反应条件下处理水中的污染物,在众多光催化材料中脱颖而出。然而由于g-C₃N₄特殊的2D层状结构,导致其具有催化活性低、比表面积相对不大、光生电子和空穴复合率较高、量子效率较低等缺陷,其在光催化和S-AOPs中的应用机理也尚未完全明确。综合上述问题,本文采用基于g-C₃N₄和激活过一硫酸盐（PMS）相耦合的可见光催化技术为处理手段,围绕环境中典型有机污染物的处理展开研究,主要研究如下:1)以Fe（Ⅱ）、磷钨酸（HPW）、g-C₃N₄为前驱体,通过光还原法成功制备了棒状纳米零价铁（Fe⁰）,并负载在片层状g-C₃N₄表面。Fe⁰/HPW/g-C₃N₄在可见光下耦合PMS降解罗丹明B（Rh B）,pH=3时降解效率为79.2%,pH=7时为23.8%。耦合降解比单独在可见光下催化或者单独激活PMS的降解效果都要高,协同系数为65.7%,且光催化作用在其中的贡献较大。淬灭实验证实了·OH和SO₄^·-是最主要的氧化活性物种。该体系对pH的依赖性较强,这归因于纳米零价铁的性质不够稳定,在环境中容易被氧化失活。2)为解决Fe⁰容易被氧化失活及材料对环境pH的适应性问题,以尿素、硫脲、二氰二胺、三聚氰胺四种不同的前驱体掺杂Fe（Ⅲ）,制备出四种不同的Fe/g-C₃N₄复合材料。表征结果表明,硫脲和尿素为前驱体的材料,主要成分是不同形貌和类型的三氧化二铁,而以三聚氰胺、二氰二胺为前驱体制备的材料主要成分是g-C₃N₄,样品中掺杂的铁主要为Fe（Ⅱ）。三聚氰胺为前驱体的催化剂Fe_0.05/M-CN在可见光下活化PMS降解Rh B的效率最高,当pH=3时,Rh B降解效率为97.9%,pH=7时,降解效率为92.5%。对比材料Fe⁰/HPW/g-C₃N₄,无论是可见光的催化还是PMS的活化效率均有所提高。这是因为Fe（Ⅱ）嵌合在g-C₃N₄中比分散在g-C₃N₄表面的Fe⁰更稳定,同时Fe_0.05/M-CN的禁带宽度为2.4e V,具有更强的光催化活性,且在Fe/M-CN/PMS/Vis体系中,¹O₂是最关键的活性物种,其次是空穴。这种非自由基活性物种的转变是体系在中性环境中依然维持高降解率的原因。3)氮化碳的结构对光催化性能至关重要,通过异烟酸（INA）对其有机框架进行改性,形成INA/Fe/M-CN的复合材料。将材料在可见光照射下激活PMS,并用于降解磺胺甲恶唑（SMX）,Fe_0.05/M-CN对SMX的降解效率为13.8%,INA_2.1Fe/M-CN对SMX的降解效率为100%,且可见光催化降解的贡献高于PMS的活化。分析结果表明,异烟酸的掺杂并未改变铁的价态,改性后的材料其内部主要成分仍为g-C₃N₄,但其形貌跟Fe/M-CN相比更为松散卷曲。INA_2.1Fe/M-CN对可见光的吸收边带红移最为明显,其禁带宽度为2.04 e V,能够有效的促进材料中光生电子-空穴对的分离,从而增加可利用的载流子数量,使其光催化性能提高。INA的掺杂进一步增强了非自由基活性物种¹O₂在催化中的作用,此外,O₂^·-超过了空穴成为体系中第二位的氧化活性物种。氧化活性物种的改变使得材料在广泛的pH范围内都有着较高的催化活性。上述三种材料激活PMS的效率较低,可归因为铁的含量较低,活性位点较少。4)为提高铁基材料对PMS的活化效率,增加铁的含量,在体系中引入具有高效稳定的磷酸铁,可以增加活性中心位点,提高催化效率。利用超分子自组装的方式形成空心六棱柱g-C₃N₄（HTCN）,并成功将铁以磷酸铁颗粒（FeP）的形式沉积、嵌入六棱柱的表面。复合材料FeP/HTCN-3在可见光下活化PMS降解SMX具有更高的催化活性,其反应速率是HTCN的22倍,是INA_2.1Fe/M-CN的1.2倍,且激活PMS在降解中的贡献为77.1%,比INA_2.1Fe/M-CN的激活效率提高73.6%,磷酸铁的引入使激活PMS成为高降解率的主要因素。通过EPR捕捉试验和自由基猝灭实验证明,Fe/HTCN-3/PMS/Vis体系在酸性环境中产生SO₄^·-及·OH,O₂^·-和¹O₂则在体系内一直都存在,且碱性的环境下更有利于O₂^·-和¹O₂的产生。5)双金属材料在活化PMS的过程中往往具有更高的效率,因此在材料FeP/HTCN的基础上,增加了Ce的掺杂,制备出Fe、Ce双金属掺杂的g-C₃N₄复合材料。当铈铁摩尔比为3:6时,催化剂Ce_0.9Fe_1.8/EG-HTCN在可见光下活化PMS对SMX的降解效率最高,且催化速率是FeP/EG-HTCN的3.5倍,是Ce P/EG-HTCN的4.2倍。表征分析表明,磷酸铁和磷酸铈形成均匀的颗粒沉积在六棱空心管柱表面,其可见光吸收边红移,具有更高的光催化活性。通过XPS分析证实了双金属加强了协同作用,多价金属物种和可见光辅助的耦合作用促进了反应活性的提高。通过测量反应中的中间产物来研究SMX的降解路径,苯环上的胺基（-NH₂）,甲基（-CH₃）以及磺胺键（-S-N）先被活性物种(·OH,SO₄^·-,O₂^·-,¹O₂等)部分氧化,然后进一步被氧化成中间产物对氨基酚和3-氨基-5-甲基异恶唑,进而开环后被氧化成小分子的中间体丁二酸、2-氨基-2-氧乙酸,最终氧化成CO₂和H₂O。该催化剂在3次循环后降解率为97%,5次循环后的降解率仍有89.2%。更多还原

【Abstract】 The prevalence of typical organic pollutants contaminants in the water environment poses a potential threat to the ecosystem and human health,making their removal from water bodies a pressing issue.Advanced oxidation techniques based on sulfate radicals（S-AOPs）are currently one of the most effective ways to degrade organic pollutants in water.In the application of S-AOPs,metal/graphite phase carbon nitride（g-C₃N₄）as a non-homogeneous catalyst with its easy availability,high performance,and stable physicochemical properties stands out among many photocatalytic materials as a photocatalyst that can treat pollutants in water under mild reaction conditions using sunlight.However,due to its special 2D layered structure,g-C₃N₄ has the defects of easy recombination of holes and electrons generated under the light irradiation,small specific surface area,low quantum efficiency,and low catalytic activity,and their application mechanisms in photocatalysis and S-AOPs have not been fully clarified.This paper investigates visible photocatalysis based on the coupling of g-C₃N₄ and activated PMS,and focuses on the treatment of typical organic pollutants in the environment:Firstly,the rod-shaped zero-valent iron was successfully loaded on the surface of lamellar g-C₃N₄ by photocatalytic reduction using divalent iron,phosphotungstic acid（HPW）,and g-C₃N₄ as precursors.Fe⁰@HPW/g-C₃N₄ coupled with PMS in visible light degraded rhodamine B（Rh B）with degradation efficiencies of 79.2%at pH=3,pH=7 23.8%at pH=7,and the coupled degradation was higher than either catalyzed in visible light alone or activated PMS alone,with a synergy coefficient of 65.7%.The quenching experiments confirmed that SO₄^·-and·OH were then the most dominant oxidizing active species.However,the strong pH dependence of the system was attributed to the fact that the nano-zero valent iron was not stable enough and easily oxidized and deactivated in the environment.For the problems of easily deactivation of zero-valent iron by oxidation and the adaptability of the materials to environmental pH,four different Fe/g-C3N4 composites were prepared by doping four different precursors including urea,thiourea,dicyandiamide,and melamine with trivalent iron.The materials with thiourea and urea as precursors were mainly composed of different morphologies and types of ferric trioxide.In contrast,the materials prepared with dicyandiamide and melamine as precursors were mainly g-C₃N₄,and their morphology was more loose compared to pure g-C₃N₄.The composite Fe_0.05/M-CN showed the highest efficiency with 97.9%for the degradation of Rh B by activating PMS under visible light when pH=3,92.5%when pH=7 in neutral environment.The comparison materials Fe⁰/HPW/g-C₃N₄,both the catalytic capacity of visible light and the activation capacity of PMS were improved.This is because Fe（II）embedded in g-C₃N₄ is more stable than Fe⁰ dispersed on the surface of g-C₃N₄,while Fe0.05/M-CN has more red-shift with a forbidden band width of 2.4 e V,which has a stronger photocatalytic activity.In addition,In the Fe_0.05/M-CN/PMS/Vis system,¹O₂was the most critical active species,followed by holes.This shift of non-radical active species was the reason why the system maintains a high degradation rate even in a neutral environment.The structure of carbon nitride is crucial to the photocatalytic performance.To improve the photocatalytic performance of Fe/M-CN,the organic framework was modified by isonicotinic acid（INA）to form INA/Fe/M-CN composites.The materials INA_2.1Fe/M-CN and Fe_0.05/M-CN were used to activate PMS under visible light irradiation under the same conditions and used to degrade sulfamethoxazole（SMX）,and the degradation efficiency of Fe_0.05/M-CN for SMX was 13.8%and that of INA_2.1Fe/M-CN for SMX was 100%,where the contribution of visible photocatalytic degradation was higher than the activation of PMS.Characterization analysis showed that the modified material INA_2.1/Fe/M-CN whose main internal structure was g-C₃N₄.The morphology was more loosely curled compared to Fe/M-CN.The absorption sideband of INA_2.1Fe/M-CN was significantly red-shifted,and the estimated band-gap was 2.04 e V,contributing to stronger photocatalytic activity.The photoluminescence pattern shows that the luminescence intensity gradually decreases with increasing the amount of doped INA,which confirms that the addition of INA can promote the separation of photogenerated electron-hole pairs effectively,thus more photogenerated carriers can be fully utilized.The doping of INA further enhances the role of the non-radical active species ¹O₂ in catalysis,and in addition,O₂^·-overtakes the hole as the second oxidation active species in the system.The change in oxidation active species resulted in a high catalytic activity of INA_2.1Fe/M-CN over a wide pH range.The lower efficiency of the material in activating PMS can be attributed to the lower iron content and fewer active sites.Given the low content of iron in the appealing material,this leads to low catalytic efficiency.Iron phosphate can be introduced into the system due to its efficient and stable characteristics.Meanwhile,a unique hollow hexagonal g-C₃N₄（HTCN）by supramolecular self-assembly and successfully depositing and embedding iron in the form of iron phosphate particles on the surface of it.The composite Fe/HTCN-3 has higher catalytic activity for the degradation of SMX by activated PMS under visible light,and its reaction rate is 22 times higher than that of HTCN and 1.2 times higher than that of INA_2.1Fe/M-CN,and the contribution of activated PMS in the degradation is 77.1%,which is 73.6%higher than the activation efficiency of INA_2.1Fe/M-CN,and the introduction of iron phosphate makes the activated PMS became the main factor for the high degradation rate.The Fe/HTCN-3/PMS/Vis system produced strong SO₄^·-and·OH in the acidic environment,while both radicals almost disappeared in the neutral and alkaline environment,as demonstrated by the EPR trapping test and the free radical burst experiment.However,O₂^·-and ¹O₂ are always present at higher levels in the system,and the alkaline environment is more favorable to produce O₂^·-and ¹O₂.Bimetallic materials tend to have higher efficiency in the activation of PMS,so Fe/Ce bimetallic-doped g-C₃N₄ composites were prepared by adding Ce doping to the material FeP/HTCN.Characterization analysis showed that iron phosphate and cerium phosphate formed uniform particles deposited on the surface of the hexagonal hollow tubular column.The highest efficiency for SMX degradation of Ce_0.9Fe_1.8/EG-HTCN reaction was achieved with the catalytic rate was 3.5 times higher than that of the binary composite FeP/EG-HTCN and 4.2 times higher than that of Ce P/EG-HTCN when the cerium-iron molar ratio was 3:6.Compared with the FeP/HTCN composite with the band-gap of 2.3 e V,the absorption edge of Ce_0.9Fe_1.8/EG-HTCN is further red-shifted and has higher photocatalytic activity.The combined effect of multivalent metal species and visible light assistance promoted the reaction activity.the ROS study was investigated by EPR trapping test and radical burst experiment.The results showed that¹O₂ was the most critical active species in the Ce_0.9Fe_1.8/EG-HTCN/PMS/vis system,and in addition,the active species SO₄^·-and·OH on the catalyst surface was also involved in the catalysis.The degradation path of SMX in the Ce Fe/EG-HTC/Vis/PMS system was investigated by LC-MS（High Performance Liquid Chromatography）measurements of intermediates in the reaction.The degradation path,in addition to polymerization,was firstly achieved through the amine group（-NH₂）,methyl group（-CH₃）and sulfonamide bond（-S-N）on the benzene ring by reactive species(·OH,SO₄^·-,O₂^·-,¹O₂,etc.)and then further oxidized to the intermediates p-aminophenol and 3-amino-5-methylisoxazole,which are then oxidized to the small molecule intermediates butanedioic acid,2-amino-2-oxyacetic acid,and finally to CO₂ and H₂O after ring opening.The catalyst degraded SMX under the same experimental conditions and the degradation rate of this catalyst was still at 97%after 3 cycles and 89.2%after 5 cycles at 60 min.更多还原