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半导体光催化技术有望直接利用太阳光与溶解氧生成活性氧物种（ROS）从而净化废水，然而半导体光生电子?空穴易复合、羟基自由基（?OH，氧化性最强的ROS）产量很低，严重限制了太阳光净水技术的发展。本论文将可见光催化与低浓度臭氧耦合，并使用石墨相氮化碳（g-C3N4）作催化剂构建g-C3N4可见光催化-臭氧耦合过程，?OH产量和综合氧化效率远超g-C3N4光催化和臭氧氧化过程，可快速彻底矿化水体中绝大多数的有机污染物。主要研究内容及结果如下：（1）率先将块状g-C3N4用于可见光催化?臭氧耦合过程，发现g-C3N4能引发可见光与臭氧之间很强的协同氧化效应，耦合过程对草酸的去除速率最高可达到g-C3N4光催化与臭氧分别去除草酸速率之和的95.8倍，并且在相同光照强度下“可见光/臭氧/块状g-C3N4”的氧化能力明显强于“紫外光/臭氧/块状g-C3N4”。进一步证实了由于具有高导带位置和窄带隙，块状g-C3N4的催化活性高于WO3、TiO2这些经典催化材料。（2）使用非模板法合成纳米多孔g-C3N4，并研究典型芳香族污染物在不同氧化过程的矿化规律。以对羟基苯甲酸为例，光催化氧化对其矿化率几乎为零，臭氧的矿化能力中等，耦合过程对其矿化既快速又彻底，其总有机碳去除率比光催化氧化、臭氧分别处理时总有机碳去除率之和高近45%。耦合过程继承了臭氧能将芳香族有机物氧化至小分子羧酸的能力，同时能产生大量?OH不仅能加速以上过程，而且能将臭氧难降解的小分子羧酸快速氧化至CO2和H2O，因此耦合过程显示出比单一过程明显更强的矿化能力。（3）通过原位电子顺磁共振波谱仪在真实工作条件下监测块状及纳米片状g-C3N4中光激发生成电子、电子被氧及臭氧分子捕获、活性氧自由基演变等连续过程。研究发现，在O2中混入仅2.1 mol% 的O3能多捕获1?2倍数量的光生电子，并改变?OH的主要生成路径，由原来低效的O2的三电子还原路径（O2→?O2?→HO2?→H2O2→?OH）转化为高效的O3的单电子还原路径（O3→?O3?→HO3?→?OH），因此耦合过程中?OH产量显著提高，从而更快、更彻底、且无选择性地矿化有机污染物。在此基础上，制备了一系列能带结构有序变化的g-C3N4材料，并揭示能带结构影响催化活性的构效关系，指明未来优化g-C3N4能带结构的方向，即同时提高导带底位置和缩小带隙。 （4）研究了g-C3N4在耦合过程中的结构与催化稳定性。首次发现g-C3N4对?OH的化学不稳定性，并揭示了?OH氧化分解g-C3N4的反应路径。由于在实际废水处理中有机污染物能与g-C3N4竞争?OH，且?OH通常优先矿化小分子有机污染物，因此可维持g-C3N4的催化活性以及可见光催化-臭氧耦合过程的稳定运行。本论文对开发新一代高效光催化-臭氧耦合过程催化剂、利用原位表征技术在真实工作条件下研究复杂光催化反应机理以及发展太阳光净水技术具有重要意义。;Photocatalytic oxidation that utilizes sunlight and dissolved oxygen in water to generate reactive oxygen species (ROS) is one of the best solutions for wastewater treatment. However, due to the rapid recombination of photoinduced charge carriers and low production of hydroxyl radical (?OH) being the most effective ROS, photocatalytic water decontamination is far from practical application. In order to boost the ?OH yield and consequently increase the overall oxidation capacity, herein we couple a low concentration of ozone into visible-light photocatalysis and use graphitic carbon nitride (g-C3N4) catalysts to build a photocatalytic ozonation (Vis/O3/g-C3N4) system which has proven to be capable of rapidly mineralizing a wide variety of organic pollutants in water. The main contents and conclusions are as follows.(1) Bulk g-C3N4 was first used in visible-light photocatalytic ozonation which has triggered a super synergy between photocatalysis and ozonation toward much faster degradation of oxalic acid. The apparent pollutant removal rate constant in Vis/O3/bulk g-C3N4 can reach up to 95.8 times as high as the sum of those in photocatalytic oxidation (Vis/O2/bulk g-C3N4) and ozonation, and photocatalytic ozonation under visible light (420?800 nm) is more robust than that under UV with the same light intensity. As a result of high conduction band minimum position and narrow bandgap, bulk g-C3N4 proves to be more active than WO3 and TiO2, the state-of-the-art catalysts in the field of photocatalytic ozonation. (2) Nanoporous g-C3N4 was synthesized via a non-template method, and the mineralization rules of a typical aromatic pollutant in various oxidation processes were investigated. Taking p-hydroxybenzoic acid as the model pollutant, photocatalytic oxidation (Vis/O2/nanoporous g-C3N4) exhibits negiligible mineralization ability; ozonation shows a moderate mineralization of p-hydroxybenzoic acid; best of all, photocatalytic ozonation (Vis/O3/nanoporous g-C3N4) can mineralize the pollutant faster and much more completely. The final total organic carbon removal by Vis/O3/nanoporous g-C3N4 was about 45% higher than the sum of those in Vis/O2/nanoporous g-C3N4 and ozonation. The significantlv enhanced mineralization of p-hydroxybenzoic acid in photocatalytic ozonation is caused by (i) the direct oxidation of aromatic compounds into carboxylic acids by ozone, and more importantly by (ii) further oxidation of these ozone-refractory carboxylic acids into CO2 and H2O by abundant ?OH generated in this process. (3) We developed an in-situ electron paramagnetic resonance (EPR) technique, by which the generation of photoexcited electrons, their further trapping by dissolved O2 and O3, and the evolution of ROS upon bulk and nanosheet g-C3N4 were semiquantitatively visualized. We found that the presence of only 2.1 mol % O3 in the inlet O2 gas stream can trap two to three times more photoexcited electrons than pure O2, and shift the ?OH generation pathway from the inefficient three-electron reduction of oxygen (O2→?O2?→HO2?→H2O2→?OH) to the robust one-electron reduction of ozone (O3→?O3?→HO3?→?OH). Due to the dramatically enhanced production of ?OH, Vis/O3/g-C3N4 can nonselectively mineralize various kinds of organic pollutants much faster and more completely in comparison to Vis/O2/g-C3N4. Furthermore, a series of g-C3N4 catalysts with well-tuned band structures were synthesized, and the relationships between the band structures and activities were revealed. On that basis, we point out the optimization direction of band structure, i.e., to simultaneously shift the conduction band edge up and narrow the band gap.(4) The structural and catalytic stabilities of g-C3N4 in photocatalytic ozonation were comprehensively studied. We have, for the first time, shown the chemical instability of g-C3N4 toward ?OH, and revealed its decomposition pathway. On a positive note, in the presence of organic pollutants which compete against g-C3N4 for ?OH, ?OH usually preferentially attack micromolecular organics rather than g-C3N4; therefore, the catalytic activity of g-C3N4 and stable operation of photocatalytic ozonation can be preserved.This work contributes to the upgrading of highly active catalysts used for visible-light photocatalytic ozonation, provides a powerful in-situ characterization technique to monitor complex photocatalytic reactions under realistic working conditions, and may advance the development of sunlight-driven wastewater treatment technologies.
|肖家栋. 石墨相氮化碳催化可见光-臭氧耦合矿化水体有机污染物的研究[D]. 中国科学院研究生院,2018.|
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