【作者】 李东玮；

【导师】 丁轶；

【作者基本信息】 山东大学 ， 物理化学， 2014， 博士

【摘要】 近年来,利用脱合金法制备的纳米多孔金属材料因其独特的多孔结构、特殊的性能,而被广泛应用于催化、电子、光学、药物输送等领域。进一步拓宽脱合金法,合成新型纳米多孔金属材料,研究它们结构和发展它们的应用对纳米材料和工业应用领域都有非常重要的意义。本论文以脱合金法为手段,制备了多种纳米多孔金属材料,对材料的结构、形貌进行表征,研究了材料的形成机理,并系统研究了这些材料在CO催化氧化、富氢体系下CO选择性氧化、醇类的气相选择性催化氧化、甲酸的电氧化等领域的催化特性。主要研究内容如下：1、纳米多孔金高效催化活性起源的探索：金的固有属性和残留杂质的结合脱合金化法得到的纳米多孔金对于很多典型的金基催化反应具有异常的催化活性,但目前研究结果发现其他因素可以影响这种独特的催化活性,其中金固有属性和残留杂质的影响是该领域国际讨论的热点问题。本章中我们利用AuAg\AuAl、AuCu的脱合金化得到三种典型的纳米多孔金,将合成的材料用于CO氧化反应的研究。另外我们引入AuAl基的三元合金体系,利用AuAgAl的脱合金化制备一系列不同含银量的纳米多孔金。理论分析表明,这种具有小孔隙率和大曲率的独特纳米多孔结构包含高密度的位于角、边上的低配位的Au原子。纳米多孔金高效催化活性主要源自于金的固有属性,残留杂质也会对催化活性有所贡献,但是残留杂质不是必须存在的。2、纳米多孔金低温时催化富氢体系下一氧化碳选择性氧化低温时将脱合金化得到的纳米多孔金用于富氢体系下一氧化碳选择性氧化。通过改变反应温度、添加CO2/H2O、改变残留银含量等反应参数系统的表征材料的催化性能。结果证实纳米多孔金是一种高效的催化富氢体系下一氧化碳选择性氧化反应的催化剂,尤其是在低温下具有高转化率和选择性。在反应气空速为120,000mL h-1g-1cat.,温度200C情况下,多孔金催化一氧化碳选择性氧化的转化频率为4.1×10-2s-1,混合气出口处CO的浓度被降为2ppm以下,并且这种高效的催化活性可以保持24h以上。多孔金中残留银的存在看起来不会对多孔金催化一氧化碳选择性氧化反应的固有属性产生有利的影响,但是对于多孔金的结构稳定性和表观催化活性有促进作用。这些结果表明纳米多孔金可被应用于燃料电池中氢气的纯化过程。3、脱合金法制备纳米多孔铂基合金及其催化性能研究利用PtCoAl的脱合金化制备纳米多孔铂钴合金(NP-PtCo),将合成的材料应用于一氧化碳氧化和富氢体系下一氧化碳选择性氧化。NP-PtCo的结构有趣,由几百纳米左右的分层纳米阵列组成,将图像放大之后发现,每个纳米阵列都是由孔壁/孔尺寸在3nm以下的三维双连续网状结构构成。NP-PtCo这种有趣的纳米多孔结构的形成有利于反应物和产物的扩散,利于催化反应的进行。NP-PtCo表现出高效的催化一氧化碳氧化反应的活性和稳定性,稳定性的提高与材料表面形成的Co氧化物种层和具有低扩散速率的Pt掺杂有关。动力学研究表明该过程具有较低的表观活化能(31.6kJ mol-1),反应遵循Langmuir-Hinshelwood机理。另外,我们利用脱合金化法制备了一系列低Pt载量的三维纳米多孔AuPt合金(NP-AuPt),该方法可以得到高度分散的、稳定的Pt原子。X-射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)和X射线电子能谱(XPS)用于表征合成材料的结构和表面组成。与商业Pt/C相比,这种低Pt载量的NP-AuPt具有高效的甲酸电催化氧化性能,包括较高的电流密度和较好的稳定性。高效的催化性能归因于孔状结构、单原子效应、电子性能和集团效应。该类材料有望应用于直接甲酸燃料电池。4、纳米多孔金银合金选择性氧化甲醇和乙醇纳米多孔金和纳米多孔银作为新型的纳米材料,对很多反应有异常的催化活性。最近的研究指出当多孔金中残留的银含量在很低的浓度时,残留银对于一氧化碳氧化反应起到决定性作用。本章中我们利用AuAgAl脱合金化制备出一系列的纳米多孔金银合金(NP-AuAg),包括低Ag载量的样品(Au99.5Ag0.5,Au99Ag1, Au98Ag2,Au95Ag5)、低Au载量的样品(Ag99.5Au0.5,Ag99Au1,Ag98Au2,Ag95Au5)、 Au20Ag80和Au50Ag50,将制备的样品应用于甲醇和乙醇的气相选择性氧化反应。低Ag载量的NP-AuAg催化甲醇气相选择性氧化过程中,转化率、选择性和转化频率随着Ag含量的增加而降低,不过该样品催化乙醇气相选择性氧化过程中这些参数随着Ag含量的增加而升高。在所有样品催化反应过程中,催化活性均随着Au/Ag比例的不同而改变,当Au/Ag的比例为1：4时,样品的催化活性最高。更多还原

【Abstract】 Recently, nanoporous metal prepared by dealloying is sought for promising applications in catalysis, electronics, optic and drug delivery, due to its unique porous structure and property. Extending the facile method to prepare novel nanoporous material, further investigating its structure, and exploring its applications are very important to nanomaterial and industry field. Here we fabricate several nanoporous materials based on the dealloying process. The structure and possible formation mechanism are also investigated. Moreover, we explore their applications for the CO oxidation, preferential CO oxidation, gas-phase selective oxidation of alcohols, and electrooxidation of formic acid. The main informations are as follows:1. Insights into the reactivity of dealloyed nanoporous gold:combining the intrinsic activity of gold and residual impurityDealloyed nanoporous gold (NPG) has been found to possess remarkable catalytic activity for some of the typical gold-catalyzed reactions. It is suggested that there may be several effects contributing to the special catalytic properties. To date, no consistent picture regarding effects related to the intrinsic catalytic activity of gold and residual elements has arisen, so the debate still continues. In this work, we choose three typical NPG catalysts fabricated by dealloying of AuAg, AuCu, and AuAl alloy to investigate the catalytic activity of gold by using CO oxidation as the model reaction. Moreover, we first introduce AuAl-based ternary alloy to prepare NPG with different amounts of Ag content by dealloying of AuAgAl alloys. Theoretical analysis indicates that such unique nanostructure of NPG films, a smaller nanopore size with a closer interligament distance and large local curvature, possess a high concentration of low-coordination gold atoms (such as corner and step sites). Our data suggest that the most important effect is related to the intrinsic catalytic activity of gold. Residual impurity may also contribute to the special catalytic properties, but is not mandatory for catalytic reactions. 2. Nanoporous gold as an active low temperature catalyst toward CO oxidation in hydrogen-rich stream.Preferential CO oxidation (PROX) was investigated by using dealloyed nanoporous gold (NPG) catalyst under ambient conditions. Systematic investigations were carried out to characterize its catalytic performance by varying reaction parameters such as temperature and co-existence of CO2and H2O, which revealed that NPG was a highly active and selective catalyst for PROX, especially at low temperature. At20℃, the exit CO concentration could be reduced to less than2ppm with a turnover frequency of4.1×10-2s-1at a space velocity of120,000mL h-1g-1cat. and its high activity could retain for more than24hours. The presence of residual Ag species in the structure did not seem to improve the intrinsic activity of NPG for PROX; however, they contributed to the stabilization of the NPG structure and apparent catalytic activity. These results indicated that NPG might be readily applicable for hydrogen purification in fuel cell applications.3. Preparation of nanoporous platinum-based bimetallic nanocomposites by dealloying ternary alloys and research on their catalytic and electrocatalytic activity.Nanoporous PtCo (NP-PtCo) alloy was synthesized by dealloying a ternary PtCoAl alloy and applied in CO oxidation (COOX) and preferential CO oxidation (PROX). This interesting structure consists of layered nano-array around hundreds of nanometers, in which the array was composed of a three-dimensional bicontinuous network structure with a ligament/pore size down to3nm. NP-PtCo exhibited superior activity for COOX with good structure durability. The formed nanoporous structure facilitated the diffusion of the reactants and products, making contributions for the improved activity. This improvement of stability was related to the formation of a surface oxidized Co species layer and small surface diffusivity of Pt. Kinetic studies indicated that this reaction proceeded through Langmuir-Hinshelwood mechanism with low apparent activation energy (31.6kJ mol-1). Dealloying method is employed to fabricate three-dimensional nanoporous AuPt (NP-AuPt) alloys with low content of Pt, which favor the high dispersion of Pt atoms. The as-prepared alloys were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The NP-AuPt alloys composed of low Pt content show superior electrocatalytic activity and long stability toward the electrooxidation of formic acid in the acidic solution in comparison to the commercial Pt/C catalyst, due to the porous structure, the single-atom effect, the electronic property, and the assembly effect. This material can find potential applications in direct formic acid fuel cells.4. Gas-phase selective oxidation of methanol and ethanol over nanoporous AuAg alloys.Both nanoporous gold and nanoporous silver are interesting catalytic materials, because of their remarkable catalytic activities towards some reactions. Recent reports point to the crucial role of residual silver in very small concentrations in CO oxidation. Here we prepared a series of nanoporous AuAg alloys (NP-AuAg) prepared by dealloying of AuAgAl alloys, including NP-AuAg alloys with low content of Ag (Au99.5Ago.5, Au99Agi, Au98Ag2, Au95Ag5), with low content of Au (Ag99.5Auo.5, Ag99Au1, Ag98Au2, Ag95Au5), Au20Ag80, and Au50Ag50, which were applied in gas-phase selective oxidation of methanol and ethanol. In details, as for the catalytic oxidation of methanol on samples with low content of Ag, the conversion, selectivity and TOF decrease with the increase of Ag contents, however the catalytic activity of ethanol on the same samples exhibit contrary tendency. The activity varies with the Au/Ag molar ratios and attains the best activity when Au/Ag is1:4.更多还原