本文關(guān)鍵詞：金屬納米材料氧化行為的原位透射電鏡研究　出處：《浙江大學(xué)》2017年博士論文　論文類型：學(xué)位論文

  更多相關(guān)文章： 納米金屬材料 鈀納米顆粒 鈷納米顆粒 環(huán)境透射電子顯微鏡 原位氧化 氧化機(jī)理 電子束輻照

【摘要】：作為材料科學(xué)中重要的研究對象,金屬納米材料在催化工業(yè)中有著廣泛的應(yīng)用。然而由于尺寸的減小、表面能的增加,使得金屬納米材料在實(shí)際應(yīng)用時,不可避免地會被氧化,影響其催化效率。原位原子尺度研究金屬納米材料的氧化行為,探究其氧化機(jī)理,有助于設(shè)計(jì)和開發(fā)性能更好的納米金屬催化劑材料,提高催化應(yīng)用效率。本文利用環(huán)境透射電子顯微鏡,借助高分辨成像技術(shù),原位原子/納米尺度研究了 Pd、Co金屬納米材料在反應(yīng)環(huán)境、電子束輻照作用下的氧化反應(yīng)行為,對氧化反應(yīng)的起始位點(diǎn),氧化產(chǎn)物的結(jié)構(gòu)、氧化產(chǎn)物的生長、氧化反應(yīng)機(jī)理、電子束的作用等進(jìn)行了系統(tǒng)研究。在Pd納米顆粒的原位氧化研究中發(fā)現(xiàn),在室溫、低氧壓條件下,高能電子束的輻照分解作用將02分子分解為原子氧,直接以非熱方式激活Pd納米顆粒的表面氧化反應(yīng),生成納米島狀PdO氧化物,PdO(101)//Pd(111)。類球形結(jié)構(gòu)Pd納米顆粒在低指數(shù)晶面頂點(diǎn)位置開始氧化,PdO優(yōu)先生長在Pd(111)表面上。八面體結(jié)構(gòu)Pd納米顆粒在氧化時,若有突出的表面臺階,則氧化反應(yīng)發(fā)生在表面臺階位置,否則也從晶面頂點(diǎn)位置開始反應(yīng),PdO生長在Pd(111)和Pd(200)表面上。在提供的實(shí)驗(yàn)條件下,電子束輻照只能激活Pd納米顆粒表面有限原子層的氧化反應(yīng)。由于電子束的輻照還能引起原子的脫附過程,當(dāng)氧化反應(yīng)進(jìn)行到一定程度,繼續(xù)輻照會引起氧的脫附,導(dǎo)致表面PdO的還原。在10-2 Pa O2分壓下,實(shí)驗(yàn)觀察到Pd八面體納米顆粒的表面氧化還原過程可以往復(fù)周期進(jìn)行。通過對表面反應(yīng)產(chǎn)物面積占比的分析得出,納米顆粒粒徑越小,氧化反應(yīng)速率越快;電子束束流強(qiáng)度越大,氧化反應(yīng)速率越快;O2分壓越大,氧化反應(yīng)開始所需要的時間越短。對石墨包覆球形Co納米顆粒的原位氧化研究發(fā)現(xiàn),電子束輻照首先會在某些位點(diǎn)破壞表面石墨層的結(jié)構(gòu),Co原子與氧的接觸會導(dǎo)致該位點(diǎn)初始氧化物的形核長大。由于Co是易氧化金屬,電子束輻照會導(dǎo)致Co納米顆粒發(fā)生完全氧化。氧化時,首先會生成CoO相,呈核殼結(jié)構(gòu),隨后發(fā)生CoO相向Co304相的結(jié)構(gòu)轉(zhuǎn)變過程,最終形成中空結(jié)構(gòu)的Co304氧化產(chǎn)物。表面的氧化產(chǎn)物都為多晶結(jié)構(gòu),且在氧化過程中沒有觀察到氧化物生長的晶體學(xué)取向等特征。對氧化反應(yīng)的動力學(xué)分析發(fā)現(xiàn),Co納米顆粒氧化時氧化層厚度的增長符合拋物線速率規(guī)律,證實(shí)了氧化反應(yīng)由離子的擴(kuò)散控制。同時,通過計(jì)算得到了氧化反應(yīng)速率常數(shù)。根據(jù)柯肯達(dá)爾效應(yīng)理論,中空結(jié)構(gòu)氧化物的形成是由金屬離子在氧化層中的向外擴(kuò)散導(dǎo)致的,所以Co納米顆粒的氧化行為是由Co在CoO層中的擴(kuò)散控制。電子束激活Co原子發(fā)生氧化反應(yīng)的機(jī)理與Pd的相同,即直接將O2分子分解為原子氧。電子束束流強(qiáng)度越大、顆粒直徑越小,氧化反應(yīng)速率越快。加熱過程也會加快氧化反應(yīng)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.
【學(xué)位授予單位】：浙江大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2017
【分類號】：TB383.1;TN16

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本文編號：1425462