研究论文题目基于钯修饰电极的多氯联苯电催化还原脱氯.doc

论文题目：基于钯修饰电极的多氯联苯电催化还原脱氯研究作者简介：杨波，男，1975年12月出生，2002年09月师从于清华大学余刚教授，于2007年01月获博士学位。中 文 摘 要持久性有机污染物（POPs）是当今国际上环境污染物研究的前沿和重要课题，鉴于该类物质特殊的物化性质以及对人体健康和生态环境所造成的严重危害，2001年国际社会签署了本世纪来最重要的环境公约关于持久性有机污染物的斯德哥尔摩公约。而在该公约所规定的12种受控POPs名单中，多氯联苯（PCBs）具有显著代表性（高持久性、高毒性、全球迁移性等），是唯一被明确规定在全球范围内消除污染源的POPs，其消除期限为2025年。因此，开展PCBs污染控制技术以及受PCBs污染环境介质的修复研究，对于消除其环境危害和履约具有重要意义。另一方面，近年来许多传统的污染物处置方法都有了新的发展，绿色化学是它们发展的主要方向，所遵循的标准包括：在常规条件下即可实现低耗高效的去除效果，处置过程所使用的化学试剂环境友好且成本低廉，无毒性副产物生成等。其中，氢解还原法有别于将污染物彻底分解矿化的高温焚烧或高级氧化等方法，在常规条件下，以氢作为绿色试剂，通过其强还原作用有针对性地脱除污染物上致毒致危害原子，使得污染物在绿色低耗的过程中达到去毒的目的，因而成为环境污染控制研究的热点和重点。近年来，电催化氢解法作为主要的氢解还原方法之一，因其可连续稳定地原位产生强还原性的原子氢，高效地用于后续氢解去毒反应，已被越来越多地应用于各种污染物的降解去除。在氯代有机污染物的电催化氢解脱氯降解研究中，国际上所选用的目标污染物主要是氯酚、氯苯类的常规氯代芳烃，而对于分子结构更加稳定，氯原子很难脱除的POPs，相关报导非常少；另外，已有研究采用的催化电极主要是各种钯修饰的活性碳材料，电催化脱氯效率偏低。本论文工作基于上述研究现状，在国际上首先开展了高孔密度金属材料负载贵金属催化剂进行POPs电催化氢解脱氯的研究；从新型载体的筛选和高效催化剂材料的制备、相关影响因素的考察及其作用机制的解析、电催化脱氯界面反应机理的推测与验证等三个层面出发，深入系统地研究了PCBs电催化脱氯的增效机制和反应机理；所得结论和成果对有机污染物氢解还原去除研究和双金属材料催化机理的知识体系有新的贡献，并可为POPs的污染控制技术，特别是相关的环境修复技术提供可靠的数据支持和理论指导。本论文研究在电催化氢解还原降解污染物研究领域主要采用活性碳材料作为催化电极载体和通过电沉积工艺制备修饰电极的基础上，考虑到活性碳材料对有机化合物存在强吸附作用，使得氢解产物不易从电极高催化活性位点脱落，影响后续新的氢解反应效率，以及已报导的电沉积工艺过于复杂等特征，首先采用金属载体作为催化电极基材，选用多种金属网材、金属泡沫材料进行试验，并通过活性碳材料作为对照，系统研究并优化了钯负载工艺条件。结果表明，以高孔密度泡沫镍金属材料（99.9%，SBET: 1.4132 0.0796 m2 g-1，ppi: 140）作为基材，将PdCl2与NaCl按摩尔比3 : 1配制沉积液，通过自主研发的无电沉积新工艺，成功地制备出一种能高效稳定脱氯的钯修饰泡沫镍电极，由此建立了以新型金属载体制备高效电催化脱氯电极的方法。通过XRD、TEM、BET、SEM-EDX等催化剂表征手段对上述电极制备工艺和材料的脱氯效率差异进行了初步解析，结果表明所制备的钯负载泡沫镍材料，其钯颗粒细小，为纳米粒径（2-22nm），并高度分散于泡沫镍基材上，而部分钯颗粒存在无定形晶态，这些因素导致了该材料具有较高的催化脱氯活性。利用制备的钯修饰泡沫镍电极建立了PCBs电催化还原脱氯系统，在两室流通式隔膜反应器的基础上，全面研究了系统中溶液pH值、支持电解质、PCBs初始浓度、电流密度、反应温度、液流速度及电场施加方式等因素对PCBs电催化脱氯效率的影响规律，深入解析了上述影响因素在电催化脱氯过程中的作用机制。结果表明氢供体富余有利于电解析氢反应，从而促进PCBs脱氯反应；PCBs的初始浓度越高，脱氯速率越大；并获得了利用钯修饰泡沫镍电极对PCBs脱氯的优化参数，分别是电流密度2.23 mA cm-2、反应温度30 C、液流速度50 mL min-1；采用恒电流或恒电压的电场施加方式对脱氯效果影响不大，这主要与PCBs电催化还原脱氯反应是间接电还原过程有关。考虑到本研究可能作为处置采用土壤淋洗技术修复PCBs污染场地所产生的淋洗液的技术应用；并在已有研究认为疏水性氯代有机物采用表面活性剂增溶进行电催化脱氯时，阳离子型表面活性剂存在显著的脱氯增效作用的基础上，开展了通过醇类助溶剂和阳离子、非离子或阴离子型表面活性剂等模拟含PCBs土壤淋洗液的电脱氯实验。结果表明甲醇因其粘度低而脱氯增效作用最为显著，由于受甲醇溶解PCBs能力和水电解产氢脱氯机制的共同作用，水相中适宜的甲醇浓度为50%。对不同碳链长度的季铵盐型阳离子表面活性剂配制的增溶体系进行脱氯效果评价，碳链较长有利于PCBs的脱氯，但碳链过长或者碳链对称性不好，对脱氯反应有负效应。另外，电催化脱氯效率受表面活性剂的电荷属性影响不显著，而受表面活性剂空间构型的影响较大，在本研究体系中，当采用适宜的表面活性剂浓度可保证PCBs分子完全溶解和自由迁移时，阳/非/阴离子型表面活性剂和羟丙基-环糊精对PCBs脱氯具有相近的增效作用，但使用阳离子型表面活性剂时电解能耗较高。针对本研究所制备的可高效脱氯的钯修饰泡沫镍电极，通过开展PCBs分子在电极表面的吸附试验，采用线性伏安法对不同溶剂体系的伏安测试和进行对应脱氯效果的比较，以及解析电流密度与脱氯效率的关系，提出了钯修饰电极在电催化脱氯过程中新的界面反应机理，认为存在原子氢在双金属电极表面的氢溢流扩散机制，从而显著增加了电极表面的催化活性位点，提高了脱氯反应效率。钯修饰泡沫镍电极上氢溢流机理的提出和验证是对现有的电催化脱氯界面反应机理和双金属催化氢解反应机理及知识体系的补充和完善。另外，通过对具有不同氯取代位的PCBs单体进行脱氯试验表明，PCBs分子自身的空间位阻效应对脱氯效果有重要影响，脱氯难易程度依次为：邻位间位对位；利用GC/MS进行的产物分析结果表明，本研究系统在电催化脱氯过程中还能将脱氯产物联苯继续深度氢解生成苯基环己烷。关键词：氢解还原；电催化脱氯；多氯联苯；泡沫金属；钯修饰电极8Electrocatalytic Reductive Dechlorination of Polychlorinated Biphenyls by Palladium-Modified ElectrodeYang BoABSTRACTNowadays, the issue about persistent organic pollutants (POPs) is the frontier and important fields of scientific research for environmental pollutants in the world. Due to their special physicochemical properties and the severe risk to human health and globe ecosystems, the international society subscribed the most important environmental convention of this century, i.e. Stockholm Convention on Persistent Organic Pollutants, in 2001. In the list of 12 POPs controlled by the Stockholm Convention, polychlorinated biphenyls (PCBs) are the representative POPs, which are listed in this Convention as the only priority chemical for eventual elimination all over the world by 2025. Therefore, the concerned studies on PCB pollution control technologies and the remediation methods for PCBs-polluted field have the important significance to completely eliminate PCB environmental risk and successfully implement POP convention. On the other hand, there is much interest in developing technologies that are efficient at pollution prevention and degradation using green chemistry practices in recent years. Based on green chemistry practices, pollutant degradation technologies need to meet some or all of the criteria of pollutant destruction as follows: processes at room temperature and atmospheric pressure are preferred with high destruction efficiency; any reagents should also be non-toxic and environmentally benign; the method should release non-toxic and environmentally benign products. According to the above criteria, the chemical hydrogenation method is a typical green process to treat the pollutants with strong reduction, which can effectively remove the toxic atoms from the pollutant molecule at mild condition and only uses hydrogen as the green reagent to finish the following reductive reaction. In addition, the hydrogenation process can also prevent from the production of toxic by-products. Therefore, the importance of hydrogenation methods is becoming apparent in the field of pollution control research.Recently, as one of hydrogenation methods, the electrochemical hydrogenation process is used to treat various pollutants because it involves the electrochemical reduction of H+ or H2O to produce continuously adsorbed nascent atomic hydrogen H and the following hydrogenation process of pollutant degradation can be stably provided with the strong reductive reagent, i.e. atomic hydrogen. In the study on the electrocatalytic hydrodechlorination of chlorinated organic compounds, the Pd-modified electrode has successfully been applied to treat a great variety of chloroorganics in aqueous solution mainly containing chlorophenols or chlorobenzenes. However, few studies explored the degradation of more refractory PCBs and other POPs in aqueous solution. In addition, Pd-modified electrode materials play a key role in electrocatalytic hydrodechlorination of chloroorganics. Various carbon materials are frequently used as cathode substrates supporting Pd catalyst because of their chemical inertness and strong adsorption capability, such as activated carbon fiber/cloth/felt, reticulated vitreous carbon, and carbon nanotubes. However, relatively low dechlorination performance was obtained when using the carbon materials as the support. Based on the above research status, a Pd-modified metal electrode was first applied to dechlorinate PCBs in our work, and high conversion efficiency was achieved. In this work, the high-performance Pd-modified electrode was prepared for PCB dechlorination; and then the influential factors on the electrocatalytic hydrodechlorination efficiency of PCBs were examined systematically; finally, the reaction mechanisms and pathways of the electrocatalytic hydrodechlorination of PCBs on Pd-modified electrodes were proposed. The research results add to the body of knowledge concerning both the degradation of organic pollutants with hydrogenation and the catalytic mechanism of bimetallic material. In addition, this paper can provide the enough data support and the theory guidance for the further development of PCB pollution control technologies, especially on the remediation technologies of PCBs-polluted field.Before this work, the activated carbon materials are the main catalyst supports and the electrodeposition crafts are the main preparation methods of Pd-modified electrode. However, the organic compounds can be firmly adsorbed on the surface of the palladized activated carbon during electrocatalytic hydrodechlorination so that the degradation products could hold the active sites on the electrode and hinder new dechlorination reaction. In addition, the electrodeposition processes are relatively complicated to prepare the Pd-modified electrode in the reported literatures. Therefore, various metallic materials were selected as the catalyst support and different preparation methods were tested and optimized in our work. After the evaluation of the preparation methods of Pd-modified electrode and the effects of cathode substrates including various metallic meshes, metallic foams, and activated carbon materials (as control experiment) on the conversion of PCBs, Pd-modified Ni foam (99.9%, SBET: 1.4132 0.0796 m2 g-1, ppi: 140) electrode prepared by electroless deposition in NaCl-PdCl2 (the molar rate is 3:1) solution shown the best dechlorination ability and catalytic stability. The characteristics of Pd catalyst on Ni foam with electroless deposition by XRD, TEM, BET, and SEM-EDX indicated Pd particle presented nanometer size (2-22nm) and high dispersal degree on the surface of Ni substrate, part of which took on the amorphous crystal. These characteristics of Pd-modified Ni foam material could lead to its high performance for electrocatalytic hydrodechlorination.Based on the electrodechlorination system for PCBs using the above Pd-modified Ni foam and a membrane-separated flow-through cell operated in batch-recycle modes, the effects of pH, supporting electrolyte, PCBs initial concentration, current density, temperature, flow rate, and the mode of applied electric field on the dechlorination of PCBs were examined. Excessive hydrogen donor in catholyte and/or anolyte could enhance the dechlorination rate of PCBs due to the enhancement of hydrogen evolution reaction. Higher initial concentration, moderate current density, temperature, and flow rate were beneficial to improve the dechlorination rate of PCBs. The optimized operating parameters were the electrolysis conditions with current density of 2.23 mA cm-2 by catholyte flow rate of 50 mL min-1 at 30 C. The mode of applied electric field using constant voltage or constant current has only a minor effect on the dechlorination efficiency of PCBs since the electrocatalytic hydrodechlorination of PCBs is an indirect electrochemical reductive process.It is possible that this work would be developed as the remediation technology of PCBs-polluted field to treat the soil washing containing PCBs. In addition, the previous research considered that cationic surfactants should be the preferential surfactant to promote the dechlorination efficiency due to its cage effect when various surfactants-aided solutions were used to increase the apparent aqueous solubility of hydrophobic chloroorganics in electrocatalytic hydrodechlorination. Based on the above considerations, the solvent systems including alcohol cosolvents, cationic, nonionic, anionic surfactant or hydroxypropyl-cyclodextrin (HPCD), which were used to increase the solubility of PCBs and promote the dechlorination efficiency of PCBs, were evaluated in order to simulate the disposal process of soil washing containing PCBs. The results indicate that methanol was among the best cosolvents due to its low viscidity and was used in preferential concentration of 50 vol % in water for electrocatalytic hydrodechlorination of PCBs. The carbon chain length of the surfactant has a significant effect on the electrocatalytic hydrodechlorination efficiency because the length determines the transfer distance of PCBs molecule from the hydrophobic inner core to the outer surface of the surfactant micelle. And the branched chain of the surfactant molecule can also delays the transfer of the PCBs molecule in the micelle. In addition, PCB aqueous solution enhanced by cationic, nonionic (Brij 30), anionic surfactant (SDS) or HPCD, respectively, presented the similar efficiency of electrocatalytic hydrodechlorination in this electrolytic system. It means that the surfactant charge was not a significant factor for hydrodechlorination efficiency, and Brij 30, SDS, and HPCD were also favorable surfactants for electrocatalytic hydrodechlorination. However, cationic surfactant could lead to higher energy consumption than nonionic or anionic surfactant during electrolysis due to the effect of the positive charge in cationic surfactant.In order to explain the high performance of Pd-modified Ni foam electrode to dechlorinate PCBs, the concerned experiments were carried out including the adsorption experiment of PCBs on Pd-modified Ni foam electrode and activated carbon material, the characteristics of linear-sweep voltammetry for hydrogen evolution reaction on this electrode, and the effect of current densities on the dechlorination efficiency. On the basis of the above results, the hydrogen spillover mechanism was proposed for the explanation of high conversion efficiency of PCBs on Pd-modified Ni foam electrode, in which the high active hydrogen atom could transfer from Pd p