载流子自分离型光催化微界面的构筑及其高效降解溴酚类污染物

It is difficult to simultaneously realize both complete debromination and deep mineralization of brominated phenols such as tetrabromobisphenol A. It is known that brominated phenols are easily reduced to lower-brominated and bromine-free products, and these products are relatively susceptible to oxidation. A highly efficient photocatalytic method for the complete debromination and deep mineralization of brominated phenols is proposed by coupling photocatalytic reduction and oxidation. For this purpose, TiO2 nanocrystals with co-exposed {001} and {101} facets are synthesized. Due to the formation of a surface heterojunction between {101} and {001} facets in a single TiO2 nanocrystal, photo-generated electron-hole pairs are spontaneously self-separated and directionally migrate to {101} and {001} facets, respectively. Therefore, photocatalytic reduction of brominated phenols and oxidation of their degradation products can be obtained on {101} and {001} facets of a single TiO2 nanocrystal, respectively. Then, the reduction ability of TiO2 nanocrystals is enhanced by Pd selectively confined on {101} facet of the TiO2 nanocrystals. The adsorption of brominated phenols on the surface of TiO2 based photocatalysts is enhanced and C-Br bond in brominated phenols is weakened and activated by strengthening the interactions between Br in the target pollutants molecular and the catalyst {101} facet confined Ag or Cu. Thus, brominated phenols can be efficiently reduced and oxidized. Further, the correlation between adsorption mode of brominated phenols and its reductive debromination performance, and the synergistic effect between photocatalytic reduction and oxidation of brominated phenols in the new-developed system are systematically investigated. Finally, reaction mechanism for complete debromination and deep mineralization of brominated phenols in the new-developed coupling system is clarified. The new molecular level insights on this efficient and harmless process for degradation of brominated phenols are very important to the disposal of other halogenated organic pollutants in both environmental science and technology.

四溴双酚A等溴酚类污染物很难同时完全脱溴和深度矿化。根据其易还原脱溴而脱溴产物易氧化的特点，本项目提出一种“先还原脱溴”与“后氧化矿化”的无缝对接实现其高效脱溴和矿化的新方法。为此，制备{101}和{001}晶面共同暴露的载流子自分离型TiO2光催化剂，构成晶面异质结实现{101}晶面还原区和{001}晶面氧化区的空间分离。在{101}晶面上选择性生长Pd和Ag（或Cu）助催化剂，利用Pd增强该晶面的还原能力，利用Ag（或Cu）与溴酚分子中Br原子间的强相互作用，弱化其C-Br键，为溴酚还原脱溴和脱溴产物氧化矿化创建合适的反应微环境。研究溴酚在催化剂表面的吸附模式对其还原脱溴途径的调控机制，阐明还原脱溴和氧化降解之间的协同效应，揭示在光催化还原脱溴-氧化矿化接力新体系中溴酚完全脱溴和深度矿化的反应机制，提出一种高效处置溴酚类污染物的新方法，为其他卤代有机污染物无害化处置提供指导。

对溴酚等溴酚类污染物和多溴联苯醚很难完全氧化降解和深度脱溴。为此，本项目采用KCl/LiCl熔融盐对块状g-C3N4进行处理，设计制备了一种高光催化活性的K元素掺杂的氮化碳（KCN）纳米棒。该KCN纳米棒层间的K离子和末端-C≡N基团的引入显著改善了其光生载流子分离效果。采用KCN光催化剂，氧化降解对溴酚90min基本实现了对其的完全氧化降解和脱溴。KCN光催化氧化对溴酚伪一级动力学常数为0.067min-1，是g-C3N4的22.3倍。此外，该光催化体系还能实现2,4-二溴酚和2,4,6-三溴苯酚的氧化和脱溴。在此基础上，通过化学还原法在KCN上沉积了贵金属Pd，制备了非均匀负载Pd的KCN（Pd/KCN）。在该催化剂中，纳米Pd颗粒在单个KCN纳米棒表面分布极不均匀，成团簇聚集；在不同纳米棒上分布也极不均匀，形成异质结结构，促进了载流子的自分离。采用Pd/KCN作为光催化剂，在无氧和30%甲醇溶液中能高效还原降解并完全脱溴4-溴酚和BDE47。其中，BDE47的降解速率和脱溴效果均高于文献报道的同等反应条件下的降解效果。在Pd/KCN光催化体系中，BDE47的降解速率达到了0.56 min-1，高于文献报道的数据（3.5×10−3~0.41 min-1）。机理研究表明Pd/KCN光催化还原体系活性物种以活性氢原子为主。这种活性氢原引发的还原脱溴途径不仅能快速还原脱溴，还能抑制低溴代产物的积累。该项目所取得的重要结果将有利于指导设计更高效的新型光催化材料和构造更高效溴酚类污染物的处置新方法。