Ag-Cu和TiO_2催化材料的制备、相变以及性能研究

发布时间：2017-12-26 16:30

本文关键词：Ag-Cu和TiO_2催化材料的制备、相变以及性能研究　出处：《西北工业大学》2015年博士论文　论文类型：学位论文

【摘要】：氢燃料电池是先进的能源利用方式,能够把氢燃料中的化学能通过电化学反应直接转化为电能。然而在实际应用中,光催化制氢效率较低,而且氢的储存和运输成本较高,阻碍了氢燃料电池的大规模生产。在这种背景下,锌-空气燃料电池因使用丰富易获取的金属锌代替氢做为燃料,同时具有能量密度高和环保的优势,成为清洁能源领域的研究热点。针对锌-空气燃料电池的研究工作主要集中在寻求可替代Pt/C、Pd等贵金属的高效且成本低廉的氧还原电催化剂方面。另一方面,人们也在不懈的寻求低成本的制氢技术。作为光催化剂材料的代表,纳米TiO2因具备比表面积大、光催化及吸收性能好、表面活性高以及无毒无害等优势,不仅在光催化制氢领域,而且在环境污染治理领域有着广泛的应用前景。研究表明,由于混晶纳米TiO2具有比单相TiO2材料更好的光催化特性,因此合成TiO2混相纳米晶是高效光催化制氢以及有效治理环境污染的重要途径之一。本文采用脉冲激光沉积(Pulsed Laser Deposition,PLD)法,制备了高性能的Ag-Cu合金型氧还原催化剂。通过电化学性能测试并结合TEM表征,证实了薄膜内的Ag-Cu固溶体合金纳米颗粒是高催化活性的主要原因。当电流密度为20 mA cm-2,用该催化剂组装的组装的一次锌-空电池的比容量以及比能量密度分别升高至678 mAh g-1和725.5 mWh g-1,而采用该催化剂组装的二次锌-空气电池在经过400个充放电循环之后,电池的充放电电压基本保持不变,其电池能量效率仅由55%降低至52.7%,表明电池具有很好的稳定性。另外,采用电化学沉积法(Electrochemcial Deposition Method,EDM)制备了Ag-Cu纳米合金枝晶,获得了枝晶的晶体学特征和生长机制,TEM结果表明,所得的Ag-Cu纳米枝晶是Ag3Cu(L12)枝晶,其主枝和侧枝的生长方向均为?110?,即面心立方结构的密排方向。基于实验结果,提出了由定向附着机制和奥斯瓦尔德熟化机制控制的枝晶生长的机理模型。针对混相TiO2光催化剂方面的研究只要集中在以下三个部分。首先,利用TEM原位加热方法研究了水热法制备的H2Ti3O7(HT)至TiO2(B)(TB)以及TB至锐钛矿(TA)纳米纤维的相变过程中线形貌的变化规律和两相界面的原子结构,并采用不变应变单元原理(Invariant Deformation Element,IDE)模型计算了两个相变过程的晶体学特征。结果表明,HT/TB和TB/TA两相相变的晶体学取向关系分别为HT T B[001]//[001]、HT TB(020)//(020)及HT TB(200)//(200)和TB TA[001]//[100]、TB TA(200)//(002)以及TB TA(020)//(020)。由于结构非常相似,因而HT/TB的相界面呈完全共格状态,而TB/TA相变体系中存在一种共格界面和两种非共格界面,其界面类型和形态与煅烧温度密切相关。另外,通过基本构建块(Fundamental Building Block,FBB)模型揭示了两种相变的原子机制,并在此基础上利用IDE模型计算了两种相变的晶体学特征,计算结果与实验观察结果吻合。通过在大气和真空两种条件下煅烧TB纳米纤维,获得了不同煅烧气氛对相变产物的影响规律,基于TEM观察结果,揭示了TB至TinO2n-1的相变原子机制,并同时提出了一种制备新型TiO2基复相纳米材料的方法。结果表明,在空气中在650°C下煅烧1小时的TB纳米纤维中存在与TB具有相同空间群的两种中间相Ti3O5(T-I)和Ti6O11(T-II),而在真空下,当煅烧温度为650°C左右时,TB纳米纤维中仅发现有T-I相,可能是由于高真空下纳米纤维表面张力较大使得T-I无法进一步转变为T-II相。根据不同气氛下煅烧样品的结果,提出了TB?T-I?T-II?TA的相变顺序及相应的化学反应方程式。此外,利用高分辨TEM方法揭示了TB至TA相变过程中出现的TB/T-I、TB/T-II、T-II/TA及TB/TA4种共格相界面的晶体学位相关系:TB T-I[100]//[001]、TB T-II[100]//[100]、T-II TA[100]//[100]和TB TA[100]//[100],同时提出了一种新型的Ti3O5和Ti6O11相的制备方法。在TB纳米纤维发生相变的过程中,利用TEM观察得到了煅烧气氛对线表面纳米凹坑形态和分布的影响规律,探讨了纳米凹坑的存在对TB至TA和TA至TR两类相变过程的影响机制。结果表明,纳米凹坑是在TB至TA的相变过程中生成的,在大气和真空下产生的纳米凹坑形态和分布状态有所不同。在大气中煅烧的纳米纤维中出现的纳米凹坑较多,其形态呈六棱柱形,底面平行于TA{100},柱面分别平行于TA{011}和TA{010},而真空中生成的纳米凹坑数量很少,形态不规则。另外,当加热温度超过700°C后,TA表面纳米凹坑的数量急剧减少,同时形状向圆形变化。通过对比大气和真空下的相变进程发现,无论对于TB至TA还是TA至TR相变,纳米凹坑的存在均会阻碍相变的发生,其原因是由于纳米纤维的表面重构使得纳米凹坑消失,同时消耗了能量,使得相变点升高。
[Abstract]:Hydrogen fuel cell is an advanced way of energy utilization, which can directly convert chemical energy in hydrogen fuel into electric energy by electrochemical reaction. However, in practical applications, the efficiency of photocatalytic hydrogen production is low, and the storage and transportation cost of hydrogen is high, which hinders the large-scale production of hydrogen fuel cells. Under this background, zinc air fuel cell is a hot topic in clean energy area because of its high energy density and environmental protection. The research work of zinc air fuel cell is mainly focused on searching for efficient and inexpensive oxygen reduction electrocatalysts that can replace Pt/C, Pd and other precious metals. On the other hand, people are unremitting seeking low cost hydrogen production technology. As the representative of photocatalyst materials, nano TiO2 has wide application prospects in the field of photocatalytic hydrogen production and environmental pollution control because of its large specific surface area, good photocatalytic and absorption properties, high surface activity and innocuity and harmlessness. Research shows that, due to mixed crystal TiO2 nanoparticles have better photocatalytic properties than single-phase TiO2 material, so the synthesis of TiO2 mixed phase nanocrystalline high photocatalytic hydrogen production, is one of the important ways to effectively control environmental pollution. A high performance Ag-Cu alloy oxygen reduction catalyst was prepared by pulsed laser deposition (Pulsed Laser Deposition, PLD). The main reason for the high catalytic activity of the Ag-Cu solid solution alloy nanoparticles in the film was confirmed by the electrochemical performance test and TEM characterization. When the current density is 20 mA cm-2, a zinc - assembly with the catalyst of air battery capacity and energy density were increased to 678 mAh g-1 and 725.5 mWh g-1, while the two zinc air battery assembly of the catalyst after 400 charge discharge cycles, discharge voltage the pool remained basically unchanged, the battery energy efficiency only decreased from 55% to 52.7%, indicates that the battery has good stability. In addition, the electrochemical deposition method (Electrochemcial Deposition Method, EDM) Ag-Cu nano alloy dendrite was prepared and obtained crystallographic characteristics and growth mechanism of dendrite, TEM results show that the Ag-Cu nano dendrite is Ag3Cu (L12) dendrite growth direction, its main branches and lateral branches are? 110,? The direction of close packed face centered cubic structure. Based on the experimental results, a mechanism model for dendrite growth controlled by directional attachment mechanism and Oswald ripening mechanism is proposed. The research on the mixed phase TiO2 photocatalyst is focused on the following three parts. First of all, the study of hydrothermal synthesis method using TEM in situ heating H2Ti3O7 (HT) to TiO2 (B) (TB) and TB (TA) atomic structure to anatase nano fiber morphology midline phase change process and change rule of the interface, and the constant strain element principle (Invariant Deformation Element, IDE) model calculate the two crystallographic characteristics of phase change process. The results show that the relationship between the crystallographic orientation and TB/TA phase transformation in HT/TB were HT T B[001]//[001], HT TB (020) / (020) HT and TB (200) / (200) and TB TA[001]//[100], TB TA (200) / (002) TB and TA (020) / (020). Because the structure is very similar, the interface of HT/TB is perfectly coherent. However, there exists a coherent interface and two kinds of non coherent interfaces in TB/TA phase change system. Its interface type and morphology are closely related to calcination temperature. In addition, the atomic mechanism of two kinds of phase transitions is revealed by Fundamental Building Block (FBB) model. Based on that, the crystallographic characteristics of two phase transitions are calculated by IDE model, and the calculated results are in agreement with the experimental observations. Through the air and vacuum under the two conditions of calcination of TB nanofibers, obtaining the effect of different calcination atmosphere on transformation products, based on the TEM results, reveals the transformation mechanism of TB atoms to TinO2n-1, and also puts forward a new preparation method of TiO2 based nanocomposite materials. The results show that the TB has two kinds of intermediate same space group Ti3O5 exists in the air at 650 DEG C calcined under 1 hours of TB nanofibers (T-I) and Ti6O11 (T-II), and under vacuum, when the calcination temperature is about 650 DEG C, TB nano fibers found only T-I that may be due to a high vacuum surface tension nano fiber makes T-I cannot be further transformed into T-II phase. According to the results of the calcined samples under different atmospheres, the order of phase transition and the corresponding chemical reaction equation of TB? T-I? T-II? TA were put forward. In addition, the use of high resolution TEM method reveals TB to TA phase transition in TB/T-I, TB/T-II, T-II/TA and TB/TA4 phase interface crystallographic phase relationship: TB T-I[100]//[001], TB T-II[100]//[100], T-II TA[100]//[100] and TB TA[100]//[100] Ti3O5 and Ti6O11, and proposed a new phase preparation method. In the process of phase transformation of TB nanofibers, the influence of calcination atmosphere on the morphology and distribution of nano pits on the line surface was observed by TEM. The mechanism of the existence of nano pits on the two phase transformation processes from TB to TA and TA to TR was discussed. The results show that the nano pits are formed during the phase transition from TB to TA, and the morphology and distribution of the nanoscale in the atmosphere and the vacuum are different. There are more nano pits in the nanofibers calcined in the atmosphere, the morphology is six prism, the bottom surface is parallel to TA{100}, the cylinders are parallel to TA{011} and TA{010}, and the number of nano pits in vacuum is very small. In addition, when the heating temperature is over 700 C, the number of nanoscale pits on the surface of TA decreases sharply, and the shape changes to the circle. By comparing the transformation process in atmosphere and vacuum, it is found that the existence of nano pits will impede the occurrence of phase transition, no matter for TB to TA or TA to TR phase transition.
【学位授予单位】：西北工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TQ426

【参考文献】