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光化学反应中光生电子及共生自由基的调控与利用研究
Alternative TitleThe Research on Regulation and Utilization of Photo-induced electron and Concomitant Free radical in Photochemical Reaction
李曈
Subtype博士
Thesis Advisor曲久辉
2018-06
Degree Grantor中国科学院生态环境研究中心
Place of Conferral北京
Degree Name工学博士
Degree Discipline环境工程
Keyword半导体光催化,界面离子交换,体相光生载流子,协同氧化还原,自由基协同氧化 Semiconductor Photocatalysis, Interfacial Ion Exchange, Bulk Charge Transfer, Synergistic Redox, Synergistic oxidatIon Of Free Radical
Abstract

      光催化技术利用光激发半导体材料产生的光生电子和光生空穴,既可以氧化降解有机污染物,又可以还原重金属、硝酸盐等,是一种绿色清洁的多功能技术。由于绝大部分光生电子和光生空穴在体相中复合消耗,只有极少部分可以到达表面参与催化反应,其分离效率低、数量少严重制约了光催化技术的应用。目前的研究更关注于调控半导体光催化材料的界面和电子结构,增加光生载流子的分离效率、数量、迁移速率以提高其光电转换性能,而对催化过程中半导体材料的界面与污染物之间的作用关系缺乏认识。在半导体材料界面催化污染物的降解过程中,探究污染物的界面吸附、光生电子界面传递过程,将有助于我们深入理解污染物在界面的迁移转化机制,从而实现污染物的高效去除。

      本文首先制备了BiOBr0.75I0.25光催化材料,通过可见光激发生成的光生电子可以将六价铬(铬酸根离子)在中性条件下还原成三价铬。由于BiOX特殊的层状结构导致其暴露卤素层的{010}和{110}晶面具有离子交换的特性,通过XRD、XPS和UV-vis diffusion reflectance的分析发现,铬酸根离子(CrO42—)可以与层间的氢氧根发生离子交换,插层吸附在BiOBr0.75I0.25的晶格层间,并作为掺杂能级提高了BiOBr0.75I0.25对光的吸收范围和光生载流子的分离效率。同时,层间的CrO42—与Bi2O22+形成=Bi-O-Cr-O3—络合结构,在光激发下CrO42—发生荷移跃迁,导致电子从O-II转移到CrVI形成O-I-CrV的缺电子结构,直接从BiOBr0.75I0.25体相中获得光生电子被还原成Cr(III)。这种体相光催化还原过程避免了光生电子从体相到界面的分离-迁移,以及界面电子传递的过程。还原后生成的三价铬离子(Cr3+)在电荷排斥力的作用下重新释放到溶液中。基于此,本文揭示了CrO42?在BiOX光催化材料的插层吸附-体相催化还原-释放的反应历程。

       研究证明,由于层间的CrO42—可以直接从体相中获得光生电子,从而在层间产生大量的光生空穴。为此,我们选择Cr(VI)和双酚A(BPA),以及Cr(VI)和As(III)的复合体系,通过两种污染物分别利用BiOBr0.75I0.25体相中的光生电子和光生空穴,实现二者同步高效去除。研究发现,在Cr(VI)和BPA的共存体系中,两种污染物表现出很强的协同氧化-还原效果,在不同pH值(2、4、7)条件下Cr(VI)的还原效率分别提高了12.1、37.3和7.7倍;BPA的降解效率分别提高了54,12和2.4倍。这一结果表明,Cr(VI)和BPA分别直接利用BiOBr0.75I0.25体相中的光生电子和空穴,其协同去除作用远远高于在其他光催化材料表面产生的协同作用(光生电子和光生空穴迁移到催化剂表面再分别被利用)。此外,在Cr(VI)和As(III)的共存体系中进一步确定了协同氧化-还原过程,在pH=4的条件下光照10min后As(III)的氧化去除率从18.6%提高到82.2%。同时,Cr(VI)嵌入和Cr(III)嵌出导致As(III)和As(V)的交换过程,进一步揭示了层间的离子交换作用。此外,还推测了BiOBr0.75I0.25光催化体系中氧活性物种的形成过程,以及单线态氧和光生空穴协同降解BPA的氧化机制。

      在光催化氧化体系中,有机污染物的降解矿化主要是多种自由基协同氧化的结果。因此,研究不同种类的活性物种,特别是非氧活性自由基(?Cl、?SO42—)的氧化作用和协同机制十分重要。本文基于紫外/氯(UV/Cl)体系,通过调节pH控制活性氯的形态(Cl2、HOCl、OCl—),在紫外光解下生成了不同浓度的氯自由基和羟基自由基。根据腐植酸分子的形态转化及中间氯代产物的生成,分析了氯自由基的氧化作用以及氯自由基和羟基自由基协同氧化机制。结果表明,氯自由基主要与腐植酸发生夺氢反应,而羟基自由基则可以实现腐植酸分子的氧化开环。在二者协同作用下,改变了腐植酸分子生成中间氯代产物(DBPs)的前驱体结构,导致生成的中间氯代产物(DBPs)与氯化作用的结果有显著差异,其中间氯代产物主要以二氯丙酮DCP、三氯丙酮TCP和二氯乙酸DCAA为主。

Other Abstract

      Photocatalysis, as a green and clean technology, is a multifunctional method for reduction and oidation by using photogenerated-electrons and photogenerated holes respectively generated by photoexcited semiconductor. It can both oxidize organic pollutants as well as reduce heavy metals, nitrates and so on. Since most of the photogenerated electrons and photogenerated holes are recombination and consumed in the bulk phase, only a minority of photo-induced charges can reach the surface to participate in the catalytic reaction. The low separation efficiency and the minority of photogenerated electrons seriously restrict the application of the photocatalytic technology. The present researches focus on regulating the interface and electronic structure of semiconductor photocatalytic materials for increasing the separation efficiency, concentration of photo-induced charges at surface, and migration rate of photo-carriers to improve their photoelectric conversion performance. However, the interaction between the interfaces of semiconductor materials and contaminants was left out of consideration. In catalytic processes, Research on the interfacial adsorption of contaminants and the transfer path of photoelectron-electronic will help us to understand the mechanism of the migration and transformation of contaminants at the interface and improve the removal efficiency of contaminant.

      In this paper, the photocatalytic material of BiOBr0.75I0.25 was prepared first. The photogenerated electron by visible light excitation can reduce Cr(VI) to Cr(III) under neutral conditions. Due to the special layered structure of BiOX, the {010} and {110} crystal faces has the characteristics of ion exchange. The resluts of XRD, XPS, and UV-vis diffusion reflectance shows that the chromate ion (CrO42?) could intercalate in the the lattice layers of BiOBr0.75I0.25 by ion exchange with hydroxyl and form a complex structure of =Bi-O-Cr-O3-, which increases the absorption range of BiOBr0.75I0.25 and the separation efficiency of photogenerated carriers. Therefore, in the process of photocatalytic reduction, chromate ions generate charge-transfer transitions under light excitation, which results in electrons transferring from O-II to Cr(VI) to form an O-I-CrV electron-deficient structure. This process allows the chromate ions to obtain photo-induced electron in the BiOBr0.75I0.25 bulk phase directly and could be reduced to Cr(V) and further to Cr(III). The bulk photocatalytic reduction process avoids the separation-migration of photoelectrons from the bulk phase to the interface and the reconbination process of interface electron transfer. After reduction, the trivalent chromium ions (Cr3+) was released into the solution again by charge repulsion in the layer. Based on this, this paper reveals the reaction process of intercalation adsorption, bulk phase catalytic reduction and release of CrO42? in BiOX photocatalytic materials.

      It has been vertified that photogenerated electrons can be directly trasfered in the bulk phase due to the chromate ions intercalated in the layers, leading to generating a large number of photogenerated holes in the layers. Thereby, we chose the coexisting systems of chromate ion (Cr(VI)) and bisphenol A (BPA) and chromate ion (Cr(VI)) and trivalent arsenic (As(III)), utilization photogenerated electrons and photogenerated holes in the BiOBr0.75I0.25 bulk phase by two types of pollutions respectively to simultaneous removal. The study found that two pollutants shows a strong synergistic oxidation-reduction effect in the coexistence system of chromate ions and bisphenol A. The reduction efficiency of chromate ions at different pH values (2, 4 and 7) was increased by 12.1, 37.3 and 7.7 times respectively; the degradation efficiency of bisphenol A was increased by 54, 12, and 2.4 times, respectively. This result indicates that the chromate ions and bisphenol A directly utilize the photogenerated electrons and holes in the BiOBr0.75I0.25 bulk phase, respectively, and their synergistic removal effect is much higher than that occurred on the surface of other photocatalytic materials(photogenerated electrons and holes migrated to the catalyst surface). In addition, the synergistic oxidation-reduction process was further confirmed in the coexistence system of Cr(VI) and As(III). The As(III) oxidation removal rate increased from 18.6% to 82.2% after light irradiation for 10 minutes at pH=4.. At the same time, the intercalation of Cr(VI) and deintercalation of Cr(III) results in the intercalated adsorption process of As(III) and As(V), which clearly reveal the interfacial ion exchange in the layers. In addition, it was also determined that the high efficient degradation of bisphenol A is mainly the result of the indirect oxidation of singlet oxygen and the direct oxidation of photogenerated holes, speculating that the formation of oxygen active species.

       In the photocatalytic oxidation system, the mineralization of organic pollutants is mainly the result of a variety of free radical synergistic oxidation. Therefore, it is very important to study different kinds of reactive oxygen species, especially for the oxidation and synergistic mechanisms of non-oxygen free radicals such as (Cl, ?SO42-). Based on the ultraviolet/chlorine (UV/Cl) system, the different forms of active chlorine (Cl2, HOCl, and OCl-) were controlled by pH to generate different concentrations of chlorine radicals and hydroxyl radicals under UV light. Based on the morphological transformation of humic acid molecules and the formation of intermediate chlorinated products, the oxidation of chlorine radicals and the synergistic oxidation mechanism of chlorine radicals and hydroxyl radicals were studied. The results showed that chlorine radicals mainly react with humic acid to generate HCl by H-abstract, while hydroxyl radicals can oxidate aromatic nucleus of humic acid molecules. Under the synergistic effect in both, the precursor structure of the humic acid molecules for intermediate chlorinated products (DBPs) was converted, resulting in significant differences in concentration and species of intermediate chlorinated products (DBPs) in contrary to chlorination. The intermediate chlorinated products is mainly consisted of dichloroacetone DCP, trichloroacetone TCP, and dichloroacetic acid DCAA.

Pages112
Language中文
Document Type学位论文
Identifierhttp://ir.rcees.ac.cn/handle/311016/41478
Collection环境水质学国家重点实验室
Recommended Citation
GB/T 7714
李曈. 光化学反应中光生电子及共生自由基的调控与利用研究[D]. 北京. 中国科学院生态环境研究中心,2018.
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