【作者】 李文静；

【导师】 马厚义；

【作者基本信息】 山东大学 ， 物理化学， 2011， 博士

【摘要】 纳米材料尤其是铂、钯和金等贵金属纳米材料,因其具有与普通材料所不同的表面效应、量子尺寸效应、小尺寸效应和宏观量子隧道效应等特点,使得它们在光学、电学、磁学和生物学及众多交叉领域有着重要的研究价值,在能源、化工等方面显示出巨大的潜在应用价值。目前制备纳米材料的方法有很多种,其中利用电化学方法制备纳米结构材料具有反应条件可控性好、反应条件温和、环境友好、适用范围广等优点,是一种非常有前景的制备方法。同时,纳米结构的铂、钯和金在催化剂领域占有非常重要的地位,特别是纳米结构的双金属或多金属材料在电催化方面显示出了优异的催化性能,目前已成为相关领域的研究热点。本论文旨在通过一些简单、新颖的电化学方法来制备出诸如细微的贵金属纳米颗粒和多孔的纳米结构材料,并研究这类贵金属纳米结构材料对醇类小分子和有机氯化物分子的电化学催化性能,探索它们在燃料电池和环境催化等方面的潜在应用价值。主要内容包括：(1)利用一种简单的湿化学在水相溶液中以PVP作稳定剂和形状控制剂、乙二醇作为弱还原剂通过化学还原来制备金纳米棱柱。在事先表面进行处理过的氧化铟锡(ITO)导电玻璃基底上生长出这种形状规则厚度均匀的金纳米棱柱,通过观测发现金纳米棱柱均匀的分部在1TO导电基底表面,其覆盖率超过80%,形成了金纳米棱柱薄膜电极；这比之前在基地表面用同类方法所合成的金纳米棱柱(金纳米盘)的覆盖率大有增长。另一方面这种金纳米棱柱薄膜电极可以作为基底电极,通过建立电化学欠电位沉积(UPD)铜-氧化还原置换的方法在其表面沉积超微量的Au、Pt和Pd亚单层,构建出双金属或三金属电极。基于这种以金纳米为基底的多金属电极体系,重点研究了电极作为阳极催化剂对甲醇的电化学氧化性能。在研究中发现,金纳米棱柱薄膜电极在酸性条件下对甲醇的催化活性表现不佳,而在碱性电解质中却对甲醇有一定的催化效果,尤其在催化过程中对CO中间体抗中毒方面表现出了很好的性能。然而,如果在电极表面修饰微量的Pt、Pd等贵金属制备成双金属或三金属电极后,对电极在酸性电解液中对甲醇的电催化氧化进行了一系列的研究,在酸性条件下电极对甲醇的催化能力大为改善,并且与商业铂催化剂相比其电催化活性不仅变高而且抗中毒性也有了显著的改善。其重要原因是在Au的存在下,金属之间的协同效应使得电极对甲醇的催化活性和抗中毒性能均有所帮助。通过这种简单的化学方法和电化学方法相结合所制备的金属电极的一大优势是大大降低了贵金属Pt;和Pd的使用量,在制备过程中贵金属的消耗量极低,另一方面此类电极在作为电化学催化剂时对甲醇表现出了优异的催化活性和强的抗中毒能力。因此这类贵金属纳米催化剂因其简捷的制备方法和优秀的催化性能使其在燃料电池催化剂方面有着潜在的应用价值。(2)通过电化学去合金法在分别在离子液体([BMIM]BF4)和硫酸溶液中用恒电位阳极溶解的方法选择性去除Pd-Cu合金中的铜组分而制得多孔纳米钯。与水溶液相比离子液体具有较宽的电化学窗口并且在电化学腐蚀过程中没有OH-和其他不利阴离子的参与从而影响多孔材料的形貌。实验证明在离子液体电解质中选择性去合金后制备的多孔纳米钯的形貌和结构均比在硫酸溶液中所制备的多孔钯要更好一些,其孔径主要分布在-148.4 nm,韧带宽度为-67.8 nm。此外,在进行去合金化的实验后发现纳米多孔钯内还有少量残余铜组分的存在,为了方便进一步研究,将样品浸泡在Pd(II)溶液中通过置换反应来去除残留的铜组分,这种简单的方法不仅达到了去除铜的目的,而且还可以在多孔结构骨架上重新获得一些新鲜的颗粒均匀细小的Pd纳米粒子,这种经浸泡处理后的多孔纳米钯的电催化活性更高。由于钯纳米材料具有非常优秀的储氢性能,在催化反应尤其是催化氢解反应中发挥着极其重要的作用。本论文利用纳米多孔钯材料作为电催化剂对广泛存在于工业废水和地表水中的持久性有毒污染物——有机氯化物进行电化学催化脱氯,本文选择四氯化碳(CT)和氯苯(CB)等典型的有机氯化物为目标污染物,根据钯纳米材料电极在氢反应区域的特征曲线,通过在不同的电位下(-0.06 V、-0.15 V和-0.25 V)进行恒电位脱氯反应。通过电催化反应发现,对于四种钯材料电极——在离子液体中制备的纳米多孔钯(np-Pdil),离子液体中制备的纳米多孔钯经Pd(Ⅱ)溶液处理后(t-np-Pdil),在硫酸溶液中制备的纳米多孔钯电极(np-Pd硫酸)以及体相钯电极(bulk-Pd),四种电极材料的在电化学脱氯反应中的大概活性顺序依次为：t-np-Pdil、np-Pdil、np-Pd硫酸和bulk-Pd。其中t-np-Pdil对四氯化碳和氯苯的催化效果最好,这也进一步验证了t-np-Pdil电极因其自身结构的稳定性和高的比表面积使得它的催化活性也随之大为改善。此外,有趣的是三种纳米结构的钯电极在选定的三个电位下的对CT和CB脱氯规律和趋势相同,即在-0.22 V处的催化效果最佳,其次为在-0.06 V,脱氯效果最差的是在-0.15 V处。bulk-Pd电极在三个电位下的脱氯趋势与纳米多孔Pd相比有很大的区别,这主要是由于纳米结构的钯电极在氢反应电位区域与体相钯的不同所致。电化学去合金法的优点是环境友好,反应条件温和可控,操作简单方便。通过这种方法所制备的多孔纳米钯材料与传统的钯纳米材料催化剂相比不需要基底负载,可以循环使用,在一定程度上降低了贵金属的损耗。并且此类多孔钯纳米材料在对有机氯化物的优异的电催化脱氯性能为其在环境催化和污水治理等方面的应用提供了显著的优势。(3)利用电化学去合金法选择性去除Pd-Cu合金和Pd-Au-Cu三合金中的铜组分制备出多孔纳米Pd和多孔纳米Pd-Au合金材料。其中制备多孔纳米钯材料的制备过程与上一节相同。对Pd-Au-Cu三元合金材料通过在硫酸溶液中采用两种不同的电位进行电化学选择性腐蚀去除铜组分而制得两种多孔纳米双元合金材料。通过对两种合金的表面形貌和成分分析发现,在低电位下(1.1V)去合金制备的多孔纳米Pd-Au合金的孔径较小,并且通过成分分析可以发现Pd与Au的原子比例与去合金前二者的原子比例相当,而Cu组分大部分都被成功的腐蚀去除掉。而在电位1.4 V下进行恒电位去合金所制备的Pd-Au双合金多孔纳米结构的孔径有所比在低电位下有所增大,而在成分组成分析中也可以发现Pd与Au的原子比例明显变小,约为1：2.7,而在XRD图谱分析中也可以观测到这种多孔纳米合金的XRD衍射峰位置比在1.1 V下制备的多孔材料更为向右偏移,接近于纯金的衍射峰,这也说明在高电位下进行电化学去合金过程中,除了Cu被去除之外部分Pd原子也随之被溶解掉,因此所制得的多孔纳米纳米材料为富金合金。近年来随着燃料电池的研究和应用越来越热,燃料电池所使用的催化剂也成为了人们关注的焦点,目前绝大多数催化剂依然是Pt材料催化剂,而由于Pt的地球储量比较低,并且价格相对昂贵因此寻找Pt的替代催化剂已是目前急需解决的一个问题。因为Pd的储量相对丰富而且化学性质与Pt极为相似,本论文利用多孔纳米Pd和Pd-Au合金代替Pt为阳极催化剂,以甲酸代替甲醇为燃料进行电化学氧化催化实验。对于多孔Pd电极来说,在酸性介质下与体相钯电极相比,t-np-Pdil对甲酸的阳极氧化电流密度约是体相钯的两倍。而利用多孔Pd-Au合金作为催化剂时,其对甲酸的催化活性不仅高,而且抗中毒效果也有所改善。此外,由于Au的存在纳米多孔Pd-Au合金对甲酸的催化活性也有所改善,Au与Pd原子比例不同时,其催化性能也表现出一定的差异。通过简单的电化学去合金法来制备的多孔纳米Pd或Pd-Au二元合金电极催化剂时,贵金属的损耗低,制备过程不需要有机表面活性剂的参与从而影响催化性能。并且所制备的金属材料因其特有的纳米多孔三维结构,其比表面积高,催化性能佳,可以广泛的应用在催化研究中。更多还原

【Abstract】 Nanomaterials, especially noble metals like platinum and palladium, have showed promising applications in many fields such as optics, electrics, magnetics, biology, and chemistry. Nowadays novel methods for the fabrication of nanomaterials are various, among which electrochemical method has many advantages like good controllability, mild reaction conditions, environmental benignancy, and wide range of applications, therefore it is an excellent method for preparing nanostructured materials. Meanwhile, nanostructured platinum, palladium and gold play very important roles in catalysis, what’s more, the bimetallic and multimetallic catalysts show excellent catalytic performance that has attracted many researches in the related fields. In this paper, we aim at utilizing simple and novel electrochemical methods to fabricate nanostructured noble metals, such as nanoparticles and porous materials, and at studying their applications in electrooxidation of methanol and formic acid for fuel cells or electrochemical degradation of chlorinated organic compounds. The main contents of this paper include:(1) Using a simple, wet chemical method to fabricate gold nanoprisms in aqueous solutions with PVP as stabilizer and shape controller, ethylene glycol as a weak reducing agent. The as-prepared gold nanoprisms grow on the treated indium tin oxide (ITO) glass, and the coverage of the gold nanoprisms on the ITO substrate is about 80%, which may act as gold prism film electrodes. The coverage of gold nanoprisms of this electrode is higher than other electrodes prepared by similar methods. Moreover, this kind of film electrodes can be used as the substrate to construct bimetallic or multimetallic electrodes by electrochemical underpotential deposition (UPD) of copper and replacement of noble metals on the electrodes and used for electrooxidation of methanol in acid medium. In the study we found that the Au nanoprism films (Au-PF) electrode behaved poor electrochemical catalytic activity to methanol in acid solutions, while played good performance in the alkaline electrolyte, especially strong resistance to CO poisoning. When Au-PF electrodes were modified with tiny Pt, Pd or Au, the as-prepared bimetallic or multimetallic electrodes played excellent electrocatalytic oxidation activity of methanol in acid electrolyte. Compared with commercial platinum catalyst, these electrodes had wonderful electrocatalytic activity and good resistance of CO poisoning. The reason of this phenomenon is that the synergistic effect of noble metals could improve the catalytic activity of the electrodes. One of the most obvious advantages of the Au-PF electrodes is very low consumption of noble metals, and the other is that the electrodes display high electrochemical catalytic activity and strong resistance against CO poisoning.(2) Using electrochemical dealloying method to fabricate the nanoporous palladium (np-Pd) in ionic liquid or sulfuric acid solutions by selective removal Cu in Pd-Cu alloy. Ionic liquid has a wider electrochemical window than water, in which we able to select an appropriate anodic potential to induce fast dissolution of Cu while assuring appropriate surface diffusion kinetics of un-attacked Pd atoms to self-organize into a uniform porous network structure. In H2SO4 solutions, OH- ions and other anions usually participate in the selective anodic dissolution of Cu, forming deposits or salt films on the surface of a dissolving alloy, which will bring an adverse effect to the surface diffusion of Pd atoms. The nanoporous Pd fabricated in ionic liquid or in H2SO4 still contains residual Cu. To remove the residual Cu, the np-Pd sample was further immersed in Pd(Ⅱ) solution via galvanic replacement. This simple method not only can remove Cu, but also can refresh the porous structures due to formation of some fresh and tiny nanoparticles, which make the np-Pd electrode possess higher electrocatalytic activity. Because Pd nanostructures display very high catalytic activity towards the electrochemical reductive dechlorination of chlorinated organic compounds, we chose carbon tetrachloride (CT) and chlorobenzene (CB) as the target pollutants, and use the nanoporous Pd as the work electrodes to carry out electrochemical reductive dechlorination of CT or CB at different potentials. Based on the electrocatalytic dechlorination results, we found that the four kinds of electrode materials followed this order in electrochemical dechlorination activity:t-np-Pdn, np-Pdn, np-Pdsuifuric acid and bulk-Pd. This further confirms that the stability and the high specific surface area of the t-np-Pdn electrode makes its catalytic activity greatly enhance. Besides, it is very interesting that the dechlorination efficiency of three nanostructure palladium electrodes has the same variation trends, namely the dechlorination efficiency at-0.22 V is the highest, followed by-0.06 V and the worst at-0.15 V. However this is not the same on the bulk Pd electrode. Electrochemical dealloying method has many advantages; fabricating nanoporous Pd electrodes using this method don’t need the substrate electrode and don’t cause obvious loss of precious metals. These kinds of nanoporous Pd materials display excellent dechlorination performance and therefore have potential applications in environment protection such as waste water treatment project.(3) Using electrochemical dealloying method to fabricate the nanoporous palladium (np-Pd) and nanoporous Pd-Au alloy. The fabrication method of nanoporous Pd-Au is similar as that of the nanoporous Pd. Pd-Au-Cu alloy was selectively corroded at two appropriate potentials to prepare nanoporous Pd-Au alloy electrodes. The nanoporous Pd-Au alloy fabricated at the potential of 1.1 V has smaller pore size, and the atom ratio of Pd and Au is close to 1:1. While the nanoporous treated at 1.4 V has bigger pore size and with the Pd/Au atomic ratio of 1:2.7. Besides the diffraction peak of the XRD of PdiAu2.7 shifts to right side more than that of PdjAui sample. This means that dealloying at 1.4 V would remove a part of Pd atoms except Cu atoms. Recently the common catalyst used in fuel cells is still Pt-based catalyst; however, Pt is very precious and expensive. To find an alternative to Pt catalyst is needed. The chemical properties of Pd is very similar to that of Pt, therefore, nanoporous Pd and Pd-Au alloy can be acted as the catalysts in fuel cells instead of Pt. In this paper, nanoporous Pd or Pd-Au alloy was used as anode catalyst and formic acid as fuel for electrochemical oxidation. The electrochemical activities of different electrodes were discussed. The nanoporous Pd-Au alloy fabricated by electrochemical dealloying has high surface area and excellent electrocatalytic activity, which displays promising applications in direct formic acid fuel cell (DFAFC).更多还原