金属纳米材料氧化行为的原位透射电镜研究

发布时间：2018-01-14 22:01

本文关键词：金属纳米材料氧化行为的原位透射电镜研究　出处：《浙江大学》2017年博士论文　论文类型：学位论文

【摘要】：作为材料科学中重要的研究对象,金属纳米材料在催化工业中有着广泛的应用。然而由于尺寸的减小、表面能的增加,使得金属纳米材料在实际应用时,不可避免地会被氧化,影响其催化效率。原位原子尺度研究金属纳米材料的氧化行为,探究其氧化机理,有助于设计和开发性能更好的纳米金属催化剂材料,提高催化应用效率。本文利用环境透射电子显微镜,借助高分辨成像技术,原位原子/纳米尺度研究了 Pd、Co金属纳米材料在反应环境、电子束辐照作用下的氧化反应行为,对氧化反应的起始位点,氧化产物的结构、氧化产物的生长、氧化反应机理、电子束的作用等进行了系统研究。在Pd纳米颗粒的原位氧化研究中发现,在室温、低氧压条件下,高能电子束的辐照分解作用将02分子分解为原子氧,直接以非热方式激活Pd纳米颗粒的表面氧化反应,生成纳米岛状PdO氧化物,PdO(101)//Pd(111)。类球形结构Pd纳米颗粒在低指数晶面顶点位置开始氧化,PdO优先生长在Pd(111)表面上。八面体结构Pd纳米颗粒在氧化时,若有突出的表面台阶,则氧化反应发生在表面台阶位置,否则也从晶面顶点位置开始反应,PdO生长在Pd(111)和Pd(200)表面上。在提供的实验条件下,电子束辐照只能激活Pd纳米颗粒表面有限原子层的氧化反应。由于电子束的辐照还能引起原子的脱附过程,当氧化反应进行到一定程度,继续辐照会引起氧的脱附,导致表面PdO的还原。在10-2 Pa O2分压下,实验观察到Pd八面体纳米颗粒的表面氧化还原过程可以往复周期进行。通过对表面反应产物面积占比的分析得出,纳米颗粒粒径越小,氧化反应速率越快;电子束束流强度越大,氧化反应速率越快;O2分压越大,氧化反应开始所需要的时间越短。对石墨包覆球形Co纳米颗粒的原位氧化研究发现,电子束辐照首先会在某些位点破坏表面石墨层的结构,Co原子与氧的接触会导致该位点初始氧化物的形核长大。由于Co是易氧化金属,电子束辐照会导致Co纳米颗粒发生完全氧化。氧化时,首先会生成CoO相,呈核壳结构,随后发生CoO相向Co304相的结构转变过程,最终形成中空结构的Co304氧化产物。表面的氧化产物都为多晶结构,且在氧化过程中没有观察到氧化物生长的晶体学取向等特征。对氧化反应的动力学分析发现,Co纳米颗粒氧化时氧化层厚度的增长符合抛物线速率规律,证实了氧化反应由离子的扩散控制。同时,通过计算得到了氧化反应速率常数。根据柯肯达尔效应理论,中空结构氧化物的形成是由金属离子在氧化层中的向外扩散导致的,所以Co纳米颗粒的氧化行为是由Co在CoO层中的扩散控制。电子束激活Co原子发生氧化反应的机理与Pd的相同,即直接将O2分子分解为原子氧。电子束束流强度越大、颗粒直径越小,氧化反应速率越快。加热过程也会加快氧化反应的速率。
[Abstract]:As an important research object in material science, metal nanomaterials are widely used in catalytic industry. However, due to the reduction of size and the increase of surface energy, metal nanomaterials are widely used in practical applications. It is inevitable to be oxidized and its catalytic efficiency is affected. In situ atomic scale study of the oxidation behavior of metal nanomaterials and study of their oxidation mechanism will be helpful to design and develop nanometallic catalysts with better performance. In this paper, using environmental transmission electron microscope and high resolution imaging technology, in situ atomic / nano scale study of PD Co metal nanomaterials in the reaction environment. The oxidation reaction behavior under electron beam irradiation, the initial site of the oxidation reaction, the structure of the oxidation product, the growth of the oxidation product, and the oxidation reaction mechanism. The effects of electron beam were systematically studied. In situ oxidation of PD nanoparticles, it was found that at room temperature and at low oxygen pressure, the irradiation decomposition of high energy electron beam could decompose 02 molecule into atomic oxygen. The surface oxidation reaction of PD nanoparticles was directly activated in a non-thermal manner to form nano-island PdO oxides. PD nanoparticles with spherical structure begin to oxidize at the top of the low exponent crystal plane. PdO preferentially grows on the surface of PD 111). When the octahedron PD nanoparticles are oxidized, if there are prominent surface steps, the oxidation reaction occurs at the surface step position. Otherwise, the growth of PdO on the surface of PdC111) and PdC200) also starts from the vertex position of the crystal plane, under the experimental conditions provided. Electron beam irradiation can only activate the oxidation reaction of the limited atomic layer on the surface of PD nanoparticles. Since electron beam irradiation can also cause the desorption process of atoms, when the oxidation reaction is carried out to a certain extent. Continuous irradiation will cause oxygen desorption, resulting in the reduction of surface PdO at 10-2 Pa O 2 partial pressure. It is observed that the surface redox process of PD octahedral nanoparticles can be carried out in a reciprocating cycle. The analysis of the area ratio of the surface reaction products shows that the smaller the particle size is, the faster the oxidation rate is. The higher the beam intensity, the faster the oxidation rate. The larger the partial pressure of O2, the shorter the time required to start the oxidation reaction. The in-situ oxidation of graphite coated spherical Co nanoparticles shows that electron beam irradiation first destroys the structure of graphite layer at some sites. The contact of Co atom with oxygen will lead to nucleation and growth of the initial oxide at this site. As Co is an oxidizing metal, electron beam irradiation will lead to the complete oxidation of Co nanoparticles. At first, the CoO phase is formed, which is core-shell structure, and then the structural transition process from CoO phase to Co304 phase occurs. Finally, the Co304 oxidation products with hollow structure were formed, and the surface oxidation products were polycrystalline. The crystal orientation of oxide growth was not observed in the oxidation process. The kinetic analysis of the oxidation reaction showed that the thickness of oxide layer increased in accordance with the parabola rate law during the oxidation of Co nanoparticles. It is confirmed that the oxidation reaction is controlled by the diffusion of ions. At the same time, the rate constant of the oxidation reaction is calculated. The formation of hollow oxide is caused by the outward diffusion of metal ions in the oxide layer. Therefore, the oxidation behavior of Co nanoparticles is controlled by the diffusion of Co in the CoO layer. The mechanism of electron beam activation of Co atoms is the same as that of PD. The larger the electron beam intensity, the smaller the particle diameter, the faster the oxidation reaction rate, and the faster the oxidation reaction rate is during heating.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TB383.1;TN16

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