【作者】 李会会；

【导师】 俞书宏；

【作者基本信息】 中国科学技术大学 ， 无机化学， 2014， 博士

【摘要】 面临能源短缺和环境污染,全球致力于清洁能源技术的开发和发展。在众多新能源中,燃料电池因其具有零排放、无污染、效率高、燃料来源多元化等优势,同时由于燃料电池在交通运输、便携式移动电源、发电系统等领域的迅速发展,因此被认为是最具潜力和有望全面市场化的选择。然而,高成本和运行稳定性依然是目前燃料电池商业化道路上的最大障碍。当前,纳米尺度上催化剂的结构和组分的调控对降低成本、提高铂利用率和改善催化性能均具有重要的研究意义,逐渐成为催化领域研究的重点。同时,对合成反应机理的理解和研究有利于特定结构、尺寸和组分催化剂的有效制备。本论文将集中阐述部分模板牺牲法宏量制备一维纳米结构催化剂,并系统研究其催化性能。为了进一步降低成本推动燃料电池商业化,我们发展了一种改良的模板牺牲法,即部分模板牺牲法,它充分利用了模板材料构建一维结构催化剂的同时,未被取代的原子与贵金属形成了合金而产生的配位效应和应力效应有利于催化性能的提升和铂载量的降低；基于对反应的合成机理的理解,充分利用一维结构模板材料的高反应活性、均一分散性和可控性,通过组分调控和在一维纳米结构尺度上的精细结构调控,有针对性的合成一系列高活性面积和结构稳定的一维纳米结构催化剂,体现了部分模板牺牲法的独特优势和广泛适用性；基于对表面活性位点的深入理解,对催化剂进行进一步的去合金等后处理,增加表面粗糙度和改善铂电子结构,从而提高催化剂活性和稳定性,满足实际应用要求。本论文所取得主要研究成果总结如下：1.发展了部分模板牺牲法合成一系列不同组分比例的一维纳米结构催化剂的制备技术。以铜纳米线作为部分牺牲模板,当未被取代的铜原子与贵金属形成合金后,其对贵金属的电子结构的修饰和晶格常数的差异所带来的应力效应,在降低载量的同时提高了催化性能。研究结果表明,成功合成表面颗粒组成的纳米管催化剂的关键因素是铜纳米线的高反应活性和较高的反应温度。结合柯肯达尔效应,我们最终得到表面由颗粒组成和内部中空的一维纳米管催化剂,并系统的研究和讨论了组成与结构对氧气电还原(ORR)和甲醇氧化(MOR)反应的催化影响。2.利用部分模板牺牲法制备不同组分比例的超细一维纳米结构催化剂。选择超细和高长径比(大于104)的碲纳米线为牺牲模板,它具有高反应活性、好的柔韧性和可在不同的溶剂中(诸如水,乙醇,乙二醇,正己烷,二甲哑砜等)易分散等优势,为设计和制备不同结构和组成的催化剂创造了条件。我们选择了铂和钯为前驱物与碲进行置换取代反应得到不同组分比例的超细PtPdTe和PtTe纳米线催化剂,这些纳米线继承了碲纳米线的超细和高长径比的结构特点,同时结合柯肯达尔效应形成了多孔结构。更重要的是,PtPdTe纳米线表面分布了大量单晶段,这种高结晶性和光滑的表面有利于催化性能的提高。此外,碲纳米线的合成是采用我们课题组已经发展成熟和可规模化的制备技术,为下一步催化剂的规模化制备提供了可能性。3.设计并成功制备了集超细、超长、内部多孔、表面有颗粒等于一身的一维点画线结构催化剂。我们发现并证明了铂前驱物中间态在低温下可稳定存在并可用于置换取代反应,通过紫外可见吸收光谱和X射线吸收光谱证明了Pt-Pt键的形成。氯铂酸根离子和长链聚合物PtmCln的共存以及氧化速率的差异是成功制备Pt/PtTe点画线结构催化剂的关键所在。采用氯铂酸钾的乙二醇溶液为前驱物,对不同置换取代反应时间的产物Pt/PtTe和长链聚合物PtmCln为前驱物得到的PtTe催化剂的物相分析,探明了其反应机理。前驱物的调控可扩展到制备其他组成的点画线结构的催化剂,进一步丰富和扩展了模板牺牲法。4.制备了核壳结构的一维纳米电催化剂,并且探讨和研究了铂壳层厚度和基底材料组分对催化性能的影响。首先采用部分模板牺牲法制备超细Pd和PdAu纳米线催化剂,然后结合欠电位沉积铜和置换取代的方法得到表面包覆铂壳层的催化剂。因为铜也可以欠电位沉积到铂表面,因此将欠电位沉积铜和置换取代的过程循环不同的次数,得到不同铂壳层厚度的催化剂。核壳结构的催化剂可充分利用铂原子,其中铂单层催化剂能够实现降低铂载量的同时最大化提高的铂利用率。更重要的是,核通常是由非贵金属或非贵金属合金组成,利用它们对铂的配位效应和应力效应可在很大程度上改善催化性能。更多还原

【Abstract】 Access to clean and reliable energy has become the main strategy and cornerstone of dealing with the global energy and pollution problems. Fuel cells are regarded as the most potential choice and ideal solution for transportation, portable mobile power supply and power generation devices applications, due to their non-polluting, high efficiency and various fuels. However, the poor stability and high cost remain severe challenges to the ultimate commercialization of fuel cells. Till now, advanced stratigies have been developed and investigated to lower the cost, heighten platinum (Pt) utilization and improve the catalytic performence through composition and structure control. Understanding of the synthesis reaction mechanism is advantageous to synthesize the catalysts with specific structure, size and composition.The present dissertation will foucus on large-scale synthesis and catalytic application of one-dimensional (1D) electrocatalysts. We developed a modified template-scrificial method with low cost. The templates not only used to maintain1D morphology of the final products and the reducing agents, but also the not-reacted template atoms would be further form alloy with noble metal. The formation of the alloy and the addition of non-noble metal will improve the electronic structure of Pt and reduce the adsorption energy of the reaction intermediates to enhance the catalytic activity and stability. It is a broad applicable synthetic method with unique superiority owing to the high reactivity, well dispersity and controllability of the templates. It is efficient to prepare a series of ID material with high active surface area and structure stability. In addition, dealloying process is useful to expose more active sites, increase surface roughness and improve catalytic performance. The main results can be summarized as follows:1. A partial sacrificial template route was developed for preparing1D electrocatalysts with different composition by using cheap Cu nanowires (NWs) as templates. Although pure Cu NWs do not have high catalytic activity, alloying Cu with Pt, Pd or Ru metals would improve the activity of the alloy electrocatalysts owing the ligand effect and strain effect. The keys for successful synthesis of nanoparticle-on-nanotube structure catalysts are to use high active Cu NWs as templates and high reaction temperature coupling with Kirkendall effect. We also investigated the composition and structure effect on the activity of oxygen reduction reaction (ORR) and methonal oxidation reaction (MOR).2. One-dimensional ultrathin electrocatalysts with various compositions were prepared by partial sacrificial template route. Ultrathin tellurium (Te) NWs with high aspect ratio (>104), high reactivity, good flexibility and easy dispersion in different solvent (eg. water, ethanol, ethylene glycol, n-hexane etal.) were selected as templates for preparing ultrathin and porous PtPdTe and PtTe NWs catalysts. More importantly, the existence of single-crystalline segments (SCSs) of the PtPdTe NWs allowed for the preferential exposure of long segments with high crystallinity and low energy crystal facets, which is highly advantageous for the electrocatalytic reactions. In addition, these Te NWs can be synthesized in large scale by our group and provide the possibility for large-scale preparation of catalysts.3. We design and successfully synthesize ultrathin, ultralong and porous1D dotted line structure catalysts with NPs on the surface. The ultrathin Te NWs are used as template to confine the size in the range of5-7nm, to generate complex but tunable nanostructures from homogeneous PtTe to heteronanostructured and Pt decorated Pt/PtTe electrocatalysts. We revealed the formation of weaker oxidant PtmCln complexes from m×PtCl42-ions through the Pt-Pt bond during aging of K2PtCl4in ethylene glycol (EG) solution by using time-resolved in-situ UV-vis and X-ray absorption fine structure (XAFS) spectroscopies. The coexistence of PtCl42-ions and PtmCln complexes is the key of successful synthesis of ultrathin NWs with holes in the interior and Pt NPs on the surface of the NWs (increased structural complexity) coupling with the Kirkendall effect and improving upon Pt utilization and stability for electrocatalysis. The reaction kinetics is refined by tuning the coordination state of Pt precursors further enriched and expanded the template sacrificial method.4. The sacrificial template method was further developed for fabricating core-shell structure catalysts, including Pt monolayer catalysts. We examined the effects of the thickness of the Pt shell, lattice mismatch, and composition of the core on catalytic activities for ORR. Firstly, we prepared ultrathin PdTe and PdAuTe NWs using Te NWs as templates. Then, Pt-shell could be deposited on the PdTe and PdAuTe NWs combing the under potential deposition method with galvanic replacement reaction. The thickness of Pt shell could be tuned by repeat such processes. The synthesis of core-shell structure catalysts is useful to optimize the Pt utilization and loading.更多还原