几种典型有毒有机污染物的光催化降解研究

本文研究了氯代苯酚和苯胺类化合物的光催化降解,探讨了这些有机污染物分子的氯取代和化合物毒性与光催化降解之间的关系.研究表明,氯代苯酚和苯胺类化合物的光催化降解均符合表观一级反应动力学模型.有机物分子中氯取代基的引入加快了化合物的光催化降解,且反应速率随氯取代基个数的增多而增大.光催化降解的表观速率常数与正辛醇-水分配系数之间存在有较好的线性关系:logk=alogKow+b,该模型可以用于有机污染物的光催化降解性预测.     

水中硝基酚的纳米TiO_2光催化降解

以主波长254nm的紫外灯作为光源，研究了锐钛型纳米TiO2对邻硝基苯酚、2，4－二硝基苯酚的光催化降解行为，并与普通TiO2作了对比；结果表明，纳米TiO2表现出很高的光催化活性，催化降解过程符合一级动力学规律。     

超声-光催化降解水中有机污染物

超声-光催化是一项近年发展起来的废水处理的新型高级氧化技术。该技术利用超声的空化效应、自由基效应以及机械效应强化光催化的催化效能,实现超声和光催化对水中有机污染物的协同降解。本文从水中有机污染物的超声-光催化降解机理、降解动力学、影响因素（光催化剂类型和投加量、超声频率和强度、溶液pH值、温度、反应物初始浓度、溶解性气体和离子强度）和反应器类型（悬浮型、固定床型）4个方面介绍了相关研究进展,提出了目前存在的主要问题,并展望了超声-光催化降解水中有机污染物的发展方向。     

TiO2光催化反应机理及动力学研究进展

光催化处理环境污染物是基于催化反应过程中的一些自由基对污染物的氧化或还原作用,反应途径通常是HO·攻击或穴直接攻击,对可见光敏感的化合物也可能通过激发态来分解。动力学的表述多数符合L－H模式,广泛研究了L－H模式下的吸附与催化活性的关系,对动力学的研究也是了解其反应机理的重要途径。     

环境污染物的光催化降解:活性物种与反应机理

以二氧化钛为代表的光催化能使许多有毒的有机污染物发生降解或矿化,产生易被生物降解的小分子和二氧化碳,过程涉及羟基自由基等活性物种.本文在作者小组的工作基础上,试图总结近年来二氧化钛、(羟基)氧化铁、杂多酸和金属酞菁光催化或光敏化在水处理方面所取得的研究进展.综述的重点是二氧化钛半导体光催化,讨论的内容包括:(1)超氧自由基、羟基自由基和单线态氧的检测和各种生成机理;(2)金属离子、杂多酸和氟离子修饰催化剂表面对活性物种产生的影响和可能机理;(3)离子交换树脂和有机膨润土负载金属酞菁对吸附和可见光敏化降解水中有机物的影响.     

杂多酸光催化降解有机污染物

本文综述了杂多酸对有机污染物的光催化降解,包括自身光致催化作用,半导体协同光催化作用,H₂O₂存在下的类Fenton反应和超声辅助催化作用。杂多酸与有机物反应的机理主要是电子-空穴对的产生和·OH等自由基的生成。概述了杂多酸的结构类型和杂多酸光催化作用的优点和局限性,提出利用掺杂调变杂多酸的晶体结构,降低其能带宽度,提高光催化作用反应的量子效率;探索杂多酸与其它电子受体的组合,特别是与具有可见光活性的物质的组合,以扩大光响应范围,实现可见光催化反应。     

TiO2光催化反应机理及动力学研究进展

光催化处理环境污染物是基于催化反应过程中的一些自由基对污染物的氧化或还原作用，反应途径通常是HO·攻击或穴直接攻击，对可见光敏感的化合物也可能通过激发态来分解。动力学的表述多数符合L－H模式，广泛研究了L－H模式下的吸附与催化活性的关系，对动力学的研究也是了解其反应机理的重要途径。     

Ti基TiO_2纳米管阵列的改性及其光催化降解有机污染物

对近年发展的Ti基TiO2纳米管阵列改性的方法及其在光催化降解有机污染物方面应用的研究进展进行了全面的综述,分析了当前存在的问题,并对TiO2纳米管阵列材料研究的发展趋势进行了展望.     

有毒难降解有机污染物的光催化降解

有毒难降解有机污染物的光催化降解在原理、过程和效率等方面仍然存在着巨大的挑战,特别是光催化剂的构效关系、可见光的利用、分子氧的活化和污染物分子矿化分解的机理仍然是今后的研究热点.本文就近年来在半导体光催化和负载型铁离子光催化降解有毒有机污染物的两个重要方面的研究进展进行了评述.     

TiO2/膨润土光催化降解有机污染物

用溶胶-凝胶法制备了一系列TiO2/膨润土光催化剂(不同负载量和不同焙烧温度), 以罗丹明B(RhB)为模型化合物, 通过测定染料吸光度和体系化学需氧量(COD)变化, 来研究它们在紫外光照射下降解有机污染物的性能. 评价结果表明, 负载量为50%和焙烧温度为400 ℃的催化剂Ti400样品降解RhB活性较好, 虽然其矿化活性略小于P25(光照4 h P25的COD变化为99.7%, 400 ℃焙烧的TiO2/膨润土催化剂Ti400的COD变化为97.0%), 但是TiO2/膨润土催化剂更易于回收再利用. 用Ti400做催化剂降解RhB, 连续循环使用7次, 其催化活性基本不变. 用XRD、BET和紫外可见漫反射(UV-Vis DRS)等方法对这些催化剂进行了表征. 表征结果表明催化剂比表面积大有利于催化活性的提高.     

Fe(III)参与TiO_2光催化降解X3B的反应机理研究

三价铁离子对TiO2 光催化降解X3B活性艳红染料具有明显的促进作用 .但是 ,当Fe(III)全部转化为Fe(II)离子以后 ,X3B的降解不再加快 .研究表明 ,Fe(III)捕获表面光生电子是导致X3B降解速率增加的主要原因 ,而Fe(III)光解产生羟基自由基使降解X3B的贡献则相对较小 .由于体系缺乏Fe(III) /Fe(II)循环 ,(Photo) Fenton反应参与X3B降解过程的可能性极小 .X3B和Fe(III)竞争吸附催化剂表面 ,促进了光生电子 -空穴对的分离和转移 .Fe(II)吸附相当微弱 ,这可能是导致Fe(II)难以被表面空穴或其它活性物质重新氧化的原因之一     

Boosting Visible-Light Photocatalytic Redox Reaction by Charge Separation in SnO2/ZnSe(N2H4)0.5 Heterojunction Nanocatalysts

In this work, environmentally friendly photocatalysts with attractive catalytic properties are reported that have been prepared by introducing SnO₂ quantum dots (QDs) directly onto ZnSe(N₂H₄)_0.5 substrates to induce advantageous charge separation. The SnO₂/ZnSe(N₂H₄)_0.5 nanocomposites could be easily synthesized through a one-pot hydrothermal process. Owing to the absence of capping ligands, the attached SnO₂ QDs displayed superior photocatalytic properties, generating many exposed reactive surfaces. Moreover, the addition of a specified amount of SnO₂ boosted the visible-light photocatalytic activity; however, the presence of excess SnO₂ QDs in the substrate resulted in aggregation and deteriorated the performance. The spectroscopic data revealed that the SnO₂ QDs act as a photocatalytic mediator and enhance the charge separation within the type II band alignment system of the SnO₂/ZnSe(N₂H₄)_0.5 heterojunction photocatalysts. The separated charges in the heterojunction nanocomposites promote radical generation and react with pollutants, resulting in enhanced photocatalytic performance.     

Filter‐based emulsification microextraction as an efficient method for the determination of chlorophenols by gas chromatography

In this work, an efficient microextraction method was applied for the extraction of some chlorophenols in water samples. This method, termed filter‐based emulsification microextraction, is based on the dispersion of an extractant into an aqueous sample solution to accelerate the extraction process and the utilization of a Nylon syringe filter to break the emulsion. After phase separation, the method is coupled with gas chromatography as a final analyzer instrument. The overall derivatization/extraction time was about 90 s. The proposed method is centrifuge‐free, and it also provides a suitable sample clean‐up by filtration of the extracting phase. The effective parameters involved in the extraction method were optimized. Under the optimal experimental conditions, the method provided a good linearity in the range of 2.0–2000 ng/mL, extraction repeatabilities (relative standard deviations) below 9.4%, enrichment factors of 180–203, and limits of detection between 0.5 and 1.2 ng/mL.     

Some Attempts to Employ the Singlet Oxygen Generated from H2O2

Some attempts to employ the singlet oxygen generated from molybdate-catalyzed decomposition of hydrogen peroxide are presented. Reduction of ascaridole with diimide is also described, along with the preliminary results of the cleavage study using Fe-cysteinate as a simple model for Fe-S type redox species. There were strong indications that S-alkylation occurred as observed in similar cleavage of the potent antimalarial qinghaosu.     

Selective endoperoxide formation by heterogeneous TiO2 photocatalysis with dioxygen

We report the selective formation of endoperoxides by aerobic TiO₂ photocatalysis through the cyclic addition of dioxygen and a non-conjugated diene, the first heterogeneous catalytic system for endoperoxide synthesis. This green protocol does not require any additive and the photocatalyst is abundant and recyclable, providing a yield up to 64% and >20:1 diastereoselectivity. Mechanistic investigations were carried out by using product analysis, kinetic studies, O-18 labelling experiments, electron-spin resonance and a set of quenching experiments. Superoxide (but not singlet oxygen, triplet oxygen or peroxide) is directly involved in the reaction cascade to form the endoperoxide product. The new findings may be helpful for future for designing eco-friendly and energy sustainable strategies for selective oxygenation reactions using semiconductors, O₂ and sunlight.     

水催化HBrO与OH自由基反应的微观机理

采用密度泛函理论,分别在B3LYP/6-311++g(d,p)和B3LYP/aug-cc-PVTZ理论水平下,系统研究了无水和水催化的OH自由基与HBrO反应,即HBrO+OH和HBrO+OH+H_2O 2个反应的微观反应机理,给出了所有可能发生的反应路径,并指出能量最低的反应通道.对于没有水参与的反应,由于OH自由基进攻HBrO方式不同,存在顺式方向和反式方向2种进攻方式的反应路径;当有一分子水参与反应时,考虑HBrO H_2O复合物与OH自由基的反应和HBrO与H_2O OH复合物2种反应情况,共发现4条不同的反应路径.这2种反应的所有路径均是在OH自由基提取氢之前以氢键复合物形式存在,反应过程均为无势垒加合过程,总反应为放热反应.水对目标反应起催化作用,有效地降低了反应的势垒,可以加快OH自由基和HBrO的消耗速度.     

Gaseous reaction mechanism of C2F radical with water

The kinetic properties of the carbon-fluorine radicals are little understood except those of CFn (n =1-3). In this article, a detailed mechanistic study was reported on the gas-phase reaction between the simplest pi-bonded C2F radical and water as the first attempt to understand the chemical reactivity of the C2F radical. Various reaction channels are considered. The most kinetically competitive channel is the quasi-direct hydrogen-abstraction route forming P5 HCCF + OH. At the CCSD(T)/6-311+G(2d,2p)//B3LYP/6-311G(d,p)+ZPVE, CCSD(T)/6-311+G(3df,2p)//QCISD/6-311G(d,p)+ZPVE and Gaussian-3//B3LYP/6-31G(d) levels, the overall H-abstraction barriers (4.5, 4.7, and 4.2 kcal/mol) for the C2F + H2O reaction are comparable to the corresponding values (5.5, 3.7, and 5.7 kcal/mol) for the analogous C2H + H2O reaction. This suggests that C2F is a reactive radical like the extensively studied C2H, in contrast to the situation of the CF and CF2 radicals that have much lower reactivity than the corresponding hydrocarbon species. Thus, the C2F radical is expected to play an important role in the combustion processes of the carbon-fluorine chemistry. Furthermore, addition of a second H2O can catalyze the reaction with the H-abstraction barrier significantly reduced to a marginally zero value (0.5 kcal/mol). This is also indicative of the potential relevance of the title reactions in the low-temperature atmospheric chemistry.     

Theoretical Study on the Reaction Mechanism of F2 ＋ 2HBr = 2HF ＋ Br2

1 INTRODUCTION The replacement reactions between halide and hy- drogen halide, or halide and halide are basic reac- tions in chemistry. Goldfinger et al. have speculated by experiment that the gas reaction between chlorine and hydrogen bromide might be a two-step intermo- lecular reaction[1, 2]. But gas reactions between other halides and hydrogen halides haven’t been reported experimentally so far. About theoretical investiga- tion, colinearity quantum mechanics, vibrational tran- sitio…     

Rapid alkenylation of quinoxalin-2(1H)-ones enabled by the sequential Mannich-type reaction and solar photocatalysis

Herein, a rapid alkenylation of quinoxalin-2(1H)-ones enabled by a combination of Mannich-type reaction and solar photocatalysis is demonstrated. A wide range of functional groups are compatible, affording the corresponding products in moderate-to-good yields. Control experiments illustrate that the in situ generated ¹O₂ plays a central role in this reaction. This green and efficient strategy provides a practical solution for the synthesis of potentially bioactive compounds that containing a 3,4-dihydroquinoxalin-2(1H)-one structure.     

Chain transfer by addition–substitution–fragmentation mechanism. I. End–functional polymers by a single-step free radical transfer reaction: Use of a new allylic linear peroxyketal

A new chain transfer agent, ethyl 2-[1-(1-n-butoxyethylperoxy) ethyl] propenoate (EBEPEP) was used in the free radical polymerization of methyl methacrylate (MMA), styrene (St), and butyl acrylate (BA) to produce end-functional polymers by a radical addition–substitution–fragmentation mechanism. The chain transfer constants (C_tr) for EBEPEP in the three monomers polymerization at 60°C were determined from measurements of the degrees of polymerization. The C_tr were determined to be 0.086, 0.91, and 0.63 in MMA, St, and BA, respectively. EBEPEP behaves nearly as an “azeotropic” transfer agent for styrene at 60°C. The activation energy, E_atr, for the chain transfer reaction of EBEPEP with PMMA radicals was determined to be 29.5 kJ/mol. Thermal stability of peroxyketal EBEPEP in the polymerization medium was estimated from the DSC measurements of the activation energy, E_ath = 133.5 kJ/mol, and the rate constants, k_th, of the thermolysis to various temperature. © 1994 John Wiley & Sons, Inc.