【作者】 王超；

【导师】 邹广田；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2016， 博士

【摘要】 材料是人类社会发展的物质基础,为人类社会经济、物质文明和精神文明的发展提供了源源不断的动力。由于量子尺寸效应、小尺寸效应和表面效应,当材料的尺寸小到纳米级别时,材料将表现出与其相应体材料完全不同的物理化学特性,并且这些特性与其形貌和尺寸密切相关。贵金属纳米材料具有较好的物理化学特性,在成像、传感、电子、医药尤其是催化方面表现出了潜在的应用价值,这引起了人们广泛的研究兴趣。由于纳米材料的物理和化学特性极大地依赖于其尺寸和结构,因此,在过去的这些年中制备具有特定形貌的贵金属纳米晶已经成为纳米技术和纳米科学领域热门的研究课题之一。尽管大量的不同形貌和尺寸的贵金属纳米晶已经被成功制备,然而高昂的价格、有限的催化活性和稳定性限制了它们在工业领域的规模化使用。在本论文中,我们主要研究溶剂热法制备具有较高催化活性的Ir基贵金属纳米晶以及其形成机理,并进一步探索了所合成的Ir基贵金属纳米晶在催化领域的潜在应用。首先,基于导向吸附机制我们发展了一个简单且高效的制备具有高催化活性树枝状Ir纳米晶(Ir NDs)的方法。反应溶液中不同时间间隔取样得到的样品的透射电子显微镜(TEM)照片,表明Ir NDs是通过最初形成的小Ir纳米粒子的不完美导向吸附形成的。改变反应参数发现,反应溶液中前驱体浓度和反应温度对Ir NDs的形貌和尺寸几乎没有影响,但是反应溶液中配体的类型却能够极大地改变Ir NDs的形貌和尺寸。负载到金属氢氧化物和金属氧化物载体上的Ir NDs对CO的催化氧化表现出了增强的催化活性。尤其是负载到Fe(OH)x上的Ir NDs在Ir的载入质量比重为4%时,对CO的催化氧化表现出了较高的催化活性,其CO的完全转化温度为120 oC。此外,与球形Ir纳米粒子和商用Ir黑催化剂相比,Ir NDs对氨的电催化氧化也表现出了增强的催化性能。值得注意的是,Ir NDs对氨电催化氧化的比活度和质量活度分别是球形Ir纳米粒子的1.7倍和7倍。我们认为较大的比表面积和丰富的高活性位点是Ir NDs表现出高催化活性的主要原因。我们预期我们所制备的Ir NDs在质子交换膜燃料电池和直接氨燃料电池方面具有潜在应用价值。其次,基于一个发展了的置换反应机理,以Cu纳米粒子为模板,我们使用一个简单的热注射方法制备出了单晶Cu-Ir合金多面体纳米笼结构。反应过程中不同时间间隔取样所得样品的TEM照片、局部表面等离子体共振吸收光谱、随时间演化的晶体结构和结晶性证实了从Cu纳米粒子到Cu-Ir合金纳米笼的演变过程。与实心Ir纳米粒子相比,空心多孔纳米结构消除了大量埋藏在实心粒子内部不起催化作用的Ir原子,提高了贵金属Ir的使用效率。此外,将Ir与Cu形成合金不仅能够降低Ir在催化剂中的载入量,并且Cu的引入能够调节Ir Ox的电子结构,从而进一步提高了其催化性能。在0.05 M的H2SO4溶液中,我们制备的Cu-Ir合金纳米笼结构对电解水氧析出反应(OER)表现出增强的催化性能。我们制备的Cu1.11Ir纳米笼结构仅需要286 m V的过电位就可以使OER的电流密度达到10 m A/cm2,这一过电位要小于Ir NDs的303 m V和商用Ir黑催化剂的311 m V。对于Cu1.11Ir纳米笼催化剂,在过电位为280 m V时,其Ir基质量活度可以达到73 m A/mgIr,这一质量活度是商用Ir黑催化剂的2.65倍。因此,我们认为Cu1.11Ir纳米笼结构在未来聚合物电解质膜水电解剂中具有潜在的应用价值。最后,我们沿用制备Cu1.11Ir纳米笼的方法,制备出了多晶Ni-Ir空心多孔纳米笼。TEM照片以及元素分布图证实了Ni2.53Ir纳米笼结构的空心多孔特性。通过调节前驱体摩尔比,我们得到了以Ni纳米粒子为核,Ni-Ir合金为壳的Ni@Ni-Ir核壳结构和表面Ir分枝修饰的空心多孔纳米结构。与实心纳米粒子相比,空心多孔Ni-Ir纳米粒子具有高的表体积比,较大的内部空间和金属间的协同作用,提高了其催化活性。在H2SO4溶液中循环伏安氧化后,Ni掺杂的Ir Ox以较小的塔菲尔斜率(46.6 m V/decade)和较低的过电位(电流密度达到10 m A/cm2时的过电位为302 m V)对OER表现出了增强的催化活性。因此,我们制备的Ni2.53Ir在实际电催化水分解系统方面具有潜在的应用价值。更多还原

【Abstract】 Materials is the foundation of society development and provide continuous driving force for the development of social economy, material civilization and spiritual civilization. Because of the quantum size effect, small size effect and surface effect, materials at the nanoscale could display very interesting shape and size dependent physical and chemical properties which are different from their bulk counterparts. Noble metal nanomaterials have attracted great interest due to their preferable properties and potential applications in imaging, sensing, electronics, medicine and especially in the field of catalysis. Because the physical and chemical properties of nanocrystals could be widely tuned by tailoring the morphology, as a consequence, over the past years, synthesis of noble metal nanocrystals with a special morphology has been a hot research topics in the field of nanotechnology and nanoscience. Although amounts of noble metal nanocrystals with different shape and size has been prepared, the higher cost and limited catalytic activity and stability prevent them from practical application. In this thesis, our research mainly concentrates on the preparation of Ir based nanocrystals with improved catalytic performance by solvothermal method and the formation mechanism of these nanocrystals. In addition, the catalytic performance of the as-prepared nanocrytals are discussed in detail.First of all, we developed an efficient and simple method to synthesize branched Ir nanodendrites(NDs) with enhanced catalytic performance based on the oriented attachment mechanism. Transmission electron microscopy(TEM) images of the samples which were got out from the reaction solution at different time intervals indicate that the Ir NDs were formed via an imperfect oriented attachment formation mechanism. The shape and size of the Ir NDs could be hardly tuned by changing the precursor concentrations and reaction temperatures. However, the binding affinity of the ligands could drastically tune the shape and size of nanocrystals. Metal hydroxide and oxide-supported Ir NDs display improved activity for the catalytic oxidation of CO. Particularly, Fe(OH)x-supported Ir NDs with a 4 wt% Ir loading exhibit great CO oxidation catalytic performance and the full conversion temperature of CO is 120 oC. Moreover, in comparison with Ir nanoparticles(NPs) and Ir black, Ir NDs exhibit enhanced performance for the catalytic oxidation of ammonia. It is worth noting that the specific and mass activity of Ir NDs are 1.7 and 7 times higher than that of Ir NPs for the catalytic oxidation of ammonia. The improved catalytic performance of Ir NDs are derived not only from their specific surface area, but also from their rich active sites. The excellent catalytic performance of Ir NDs may open new ways for direct ammonia fuel cells and proton-exchange membrane fuel cells.Moreover, we demonstrate the synthesis of single-crystalline Cu-Ir polyhedral nanocages by a facile ―hot-injection‖ method with Cu NPs as a template based on a modified galvanic replacement mechanism. TEM images, localized surface plasmon resonance absorption spectra and the temporal evolution of the crystal structure and crystallinity reveal the temperal growth process from Cu NPs to Cu-Ir nanocages. Compared with solid Ir NPs, hollow porous nanostructure diminish amounts of buried nonfunctional Ir atoms, improving the using efficiency of Ir. Furthermore, alloying Ir with Cu not only enables the reduction of Ir loading but also the modification of the electron structure of Ir Ox, resulting in an improved catalytic performance. The as-synthesized Cu-Ir nanocages display improved catalytic performance toward the oxygen evolution reaction(OER) in 0.05 M H2SO4. Affording a current density of 10 m A/cm2, Cu1.11 Ir nanocages just need a overpotential of 286 m V for the OER, which is lower than that of Ir NDs(303 m V) and commercial Ir black(311 m V). For Cu1.11 Ir nanocages at an overpotential of 280 m V, the Ir-based mass activity can reach 73 m A/mg Ir, which is 2.65 times higher than that of Ir black(27.5 m A/mg Ir). Therefore, we expect that Cu1.11 Ir nanocages could be a promising candidate for acid polymer electrolyte membrane water electrolyzers.Finally, we present the synthesis of poly-crystalline Ni-Ir hollow porous nanocages with the same method as the synthesis of Cu1.11 Ir nanocages. TEM images and elemental mapping images indicate the hollow porous sturcture of Ni2.53 Ir nanocages. By tuning the molar ratio of precursors, core-shell NPs with a Ni core and a Ni-Ir alloy shell or hollow porous Ni-Ir nanocages with Ir branches on their surface can be obtained. In comparison with solid NPs, hollow porous bimetallic NPs possess high surface to volume ratio, large void space and synergetic effects, thus improving their catalytic activity. After voltammetric cycling in H2SO4 aqueous solution, Ni doped Ir Ox hollow porous nanostructures exhibit improved catalytic performance toward the OER in 0.05 M H2SO4, with a smaller Tafel slope of 46.6 m V/decade, a smaller overpotential of 302 m V affording a current density of 10 m A/cm2. Hence, the synthesized Ni2.53 Ir could be a promising OER catalyst for practical electrocatalytic water splitting systems.更多还原