光催化具有潜在利用太阳光能，高效彻底去除污染物的能力，已逐渐成为一项绿色环保的环境净化新技术。石墨相碳化氮（g-C₃N₄）在可见光照射下就能有效地被激发，并且具有良好的稳定性和无毒等优点。目前对g-C₃N₄的研究还不够全面，人们还不是很清楚g-C₃N₄在光催化反应中的作用机理。本论文应用g-C₃N₄可见光催化降解优先污染物三氯酚和还原六价铬（Cr(VI)），并详细讨论了催化反应的机理。具体的研究内容如下：

1.     g-C₃N₄催化剂的制备和应用于可见光降解三氯酚的研究：采用加热聚合双氰胺的方法制备了g-C₃N₄催化剂。经XRD、XPS、TEM等手段对制备的淡黄色粉末状催化剂进行了表征，表明我们制备出了典型的石墨相碳化氮催化剂。制备的g-C₃N₄催化剂对三氯酚有很好的可见光降解效果：10^-4 M的三氯酚在1 g/L g-C₃N₄催化下，经可见光照射3 h，可完全降解。溶液TOC去除和Cl^-积累的数据表明，g-C₃N₄可见光催化对三氯酚有很好的去毒效果。

2.     g-C₃N₄光催化降解三氯酚机理探讨：g-C₃N₄具有不同于传统光催化剂TiO₂的独特能带结构，因此其光催化反应机理也必然具有特异性。自由基捕获实验和反应过程中活性物种的检测结果表明，超氧自由基是大气环境下g-C₃N₄可见光催化三氯酚降解的主要活性物种。在氮气环境下，金属离子作为导带电子捕获剂的实验证明，g-C₃N₄价带空穴可以直接氧化降解三氯酚。根据三氯酚降解过程中检测到的中间产物，提出了可能的降解反应路径。

3.     g-C₃N₄光催化同时还原Cr(VI)和降解三氯酚：Cr(VI)的还原和三氯酚的降解之间存在协同效应，只有当三氯酚存在时，g-C₃N₄才能可见光催化还原Cr(VI)；当Cr(VI)存在时，三氯酚的降解反应速率提高超过30%。考察了Cr(VI)和三氯酚的初始浓度、pH、溶解氧对光催化反应的影响，对Cr(VI)还原和三氯酚降解的过程进行了研究。反应中检测到了Cr(V)和增强的羟基自由基的信号，表明Cr(VI)的还原主要来自于自由基反应。强氧化性的羟基自由基的大量生成和光生空穴参与反应，是造成三氯酚降解速率提高的主要原因。

       Photocatalysis has the potential of utilizing solar energy, and removal contaminant efficiently and completely. So it appears to be a promising technology for environmental purification. Graphitic carbon nitride (g-C₃N₄) has an appropriate band gap for efficient visible light driven, reliable chemical inertness and stability，and non-toxic. But the existing works about g-C₃N₄ photocatalysis are not sufficient, and the mechanism of g-C₃N₄ in photocatalysis reaction still remains elusive. In this paper, g-C₃N₄ was used to catalyze 2,4,6-trichlorophenol degradation and Cr(VI) reduction under visible light irradiation. We also discussed the reaction mechanisms deeply and systematically. The contents of this thesis are as follows:

1.     g-C₃N₄ synthesis and visible light catalytic TCP degradation: g-C₃N₄ was synthesized by direct thermal condensation of dicyandiamide and characterized by XRD, XPS, SEM, TEM and FT-IR. The results indicated that the as-prepared catalyst possesses a typical graphite-like structure of carbon nitride. g-C₃N₄ had a good performance for TCP degradation: 10^-4 M TCP could be completely destroyed under 3 h visible light irradiation over 1 g/L g-C₃N₄ catalyzed. The statistic of TOC removal and Cl^- release illustrated this catalyst has excellent detoxication for TCP.

2.     The mechanism discussion for g-C₃N₄ photocatalytic TCP degradation: g-C₃N₄ possesses a unique energy-band structure which distinguishes from traditional photocatalyst such as TiO₂, so it must have different catalytic mechanism. The radical scavenged experiments and the detected active species indicated ·O₂^－ was the dominant active species in TCP degradation in the presence of molecular oxygen. When use N₂ gas to expel dissolved oxygen and metal ions as electron scavenger, g-C₃N₄ valence band (VB) holes were demonstrated oxidative degradation TCP directly. TCP degradation intermediates were examined and possible degradation pathway was proposed.

3.     g-C₃N₄ photocatalysis simultaneous Cr(VI) reduction and TCP degradation: A synergistic effect was observed between Cr(VI) reduction and TCP degradation over g-C₃N₄ photocatalysis: Cr(VI) reduction only occurred when TCP co-existed in the suspension, meanwhile above 30% TCP degradation rate increase was achieved compared to TCP/g-C₃N₄ single system. The effects of substrates initial concentration, pH, and dissolved oxygen were studied. Cr(VI) reduction and TCP degradation were well evaluated both during and after photocatalysis reaction. The strong ESR signal of Cr(V) and ·OH founded in the reaction system indicated Cr(VI) reduction mainly originated from free radical reactions. The higher ·OH concentration and g-C₃N₄ VB holes involved in the reaction may caused TCP degradation rate increase.