【作者】 黄武；

【导师】 易清风；

【作者基本信息】 湖南科技大学 ， 应用化学， 2008， 硕士

【摘要】 由于有机小分子的电催化氧化研究具有重要的理论和实际意义,自上世纪九十年代以来就受到国内外许多研究工作者的高度重视。有机小分子的电催化氧化过程的深入研究,对于认识电化学现象,发展电极过程动力学,丰富电化学氧化模型,促进物理化学学科相关理论与实验研究方法的发展,均具有重要的科学意义。铂等贵金属电极上有机小分子的电催化氧化已被广为研究。但作为一种有效的非贵金属电催化剂,镍电极主要应用于有机物的电催化合成以及电分析等领域,很少报道该类电极材料上有机小分子氧化的电化学现象。寻找对有机小分子电化学氧化具有好的催化活性的催化剂及其载体是十分重要的课题。水热法是在密闭体系中,以水或其它溶剂作为反应介质,在一定温度和自生压力作用下,使反应物转化为产物的过程。在水热反应条件下,物质的物理和化学性质会发生非常大的改变,所以经常会产生意料之外的反应结果。反应溶剂（如水）在合成过程中,首先是产生压力的作用,其次是提供了一个均匀的反应空间,经常使反应混合物处于溶解或半溶解的状态,使之类似于均相反应,这就使得反应容易进行。水热法作为一种软化学制备方法应用到电极材料的制备中,可得到结构新颖、性能优越的电催化剂。目前,多数电催化剂均采用电化学沉积法负载在碳基体上,本研究中则是以水合肼为还原剂,采用水热法将近似纳米大小的金属颗粒负载在钛基体上,从而制备出新型的钛基电极材料,它们对某些有机小分子的氧化反应表现出高度的电催化活性。本论文利用新方法—水热法在钛基体上制备了镍系列电催化剂（Ni/Ti、Ni-Co/Ti、Ni-Sn/Ti）,并通过电化学测试方法对各钛基镍系列电极电催化氧化甲醇的性能进行了对比性研究,从而得出高催化性能、稳定性较好的电极催化剂,同时研究了Ni-Co/Ti电极在碱性介质中对葡萄糖电氧化的催化活性。论文的主要内容和研究结论如下:1.对镍电极的应用和有机小分子的电催化氧化进行了概述,重点阐述了甲醇、乙醇和葡萄糖氧化的电催化机理,系统介绍了镍电极研究进展,总结了镍电极在有机电合成及其它方面的应用,并指出催化剂纳米化是当前对催化剂进行研究开发的主要思路。2.确定了制备钛基镍系列电极的具体工艺过程。本研究中以水合肼为还原剂,利用水热法制备了Ni/Ti、Ni-Co/Ti、Ni-Sn/Ti电极。这一过程的主要特点是:金属催化剂能够以纳米大小的颗粒直接沉积于基体钛表面上,从而一步完成这类电极材料的制备,不仅过程简单,而且催化剂颗粒稳定,可以反复使用。3.利用扫描电镜（SEM）和能谱分析（EDS）测试技术,对水热法制备的Ni/Ti、Ni-Co/Ti和Ni-Sn/Ti电极的形貌、结构与成分等进行了表征和分析。主要的研究结论如下:（1）对Ni/Ti电极而言,SEM和EDS分析表明,在钛基表面沉积了尺寸均匀的镍金属球形小颗粒,大小约500nm,并且这些球形小颗粒相互连接,形成链状结构,具有巨大的表面积。（2）对Ni-Co/Ti电极而言,SEM和EDS分析表明,在钛基表面沉积了尺寸均匀的镍钴金属球形小颗粒,并且这些球形小颗粒相互紧密连接呈多孔网状结构,相比单独沉积的镍颗粒,大小形状差不多,约500nm球形体,但是球体连接更加致密形成多孔结构,具有巨大的活性位点。（3）对Ni-Sn/Ti电极而言,SEM和EDS分析表明,Ni-Sn/Ti电极表面呈现纳米结构的类似花瓣片形状,长度约120～130 nm,厚度约40nm。双金属的Ni-Sn片状粒子较小,相互连接形成层状叠起的结构,从而增大了比表面积,催化活性大大提高,有利于甲醇吸附氧化。4.采用循环伏安、线性扫描、电位阶跃和交流阻抗等方法,在NaOH碱性介质中研究了在Ni/Ti、Ni-Co/Ti和Ni-Sn/Ti电极上甲醇的电催化氧化过程,并对电极过程动力学进行了分析研究,同时还研究了葡萄糖在Ni-Co/Ti电极上的电催化氧化反应。主要结论如下:（1）伏安特性研究表明,在碱性溶液中,多晶镍电极和Ni/Ti电极对甲醇氧化的电催化活性具有明显的差异。与多晶镍电极相比,Ni/Ti电极对甲醇电化学氧化起始电位提前为0.36V,而前者为0.42V;在电极几何面积相同的条件下,Ni/Ti电极上甲醇氧化峰电流为0.75mA,是前者的近5倍。电位阶跃实验表明,多晶镍电极和Ni/Ti电极上阶跃稳态电流都随甲醇的浓度增加而增大,并且成良好的线性增长。这种依赖关系在Ni/Ti电极上表现更为迅速,响应斜率为8.18,前者仅为1.40。电化学交流阻抗实验表明Ni/Ti电极上甲醇氧化电荷传递电阻R_ct很低,几乎只有多晶镍电极上的1/10。（2）将水热法制备的Ni-Co/Ti电极与同样方法制备的Co/Ti电极和Ni/Ti电极进行比较,电化学测试实验结果表明:Co/Ti电极对甲醇无催化活性,掺杂20%的Co的Ni₈Co₂/Ti电极对甲醇氧化表现出更为优异的催化性能,与Ni/Ti电极相比,Ni₈Co₂/Ti电极对甲醇氧化的起始电位提前了0.11V,为0.25V,氧化峰的电流增加到3.3倍,为2.5mA。Co的加入明显改善了电催化剂对甲醇电化学氧化性能,Co在较低电位下能够生成大量的含氧物种,与Ni产生协同作用,促进了氧化反应的发生。（3）研究了结构新颖的Ni₈Co₂/Ti电极在碱性溶液中对葡萄糖的电催化氧化情况。电化学测试实验结果表明:Ni₈Co₂/Ti电极对葡萄糖氧化表现出极高的催化性能,并且葡萄糖在该电极上电化学氧化速度是受扩散控制,得到葡萄糖在该体系中的扩散系数为6.50×10^-6cm²·s^-1,葡萄糖在该电极上电化学氧化反应速率常数为3.57×10⁴ cm³·mol^-1·s^-1。葡萄糖在Ni₈Co₂/Ti电极上的安培响应表明葡萄糖在该电极上具有良好的依赖关系,这种依赖关系从拟合曲线来看是成线性变化的。拟合曲线线性相关系数为0.9995,直线斜率为6.85×10^-4A·（mmol/L）^-1,显示Ni₈Co₂/Ti电极对葡萄糖检测具有灵敏度较高、检测下限较低的特点。（4）Ni-Sn/Ti电极对甲醇的电化学氧化也具有极高的电催化性能。电化学测试结果证明,与Ni/Ti电极相比,Ni-Sn/Ti电极对甲醇具有更高的氧化反应速率和更低的起始氧化电位,并且电荷传递电阻较低。当Ni:Sn比例为8:1时,Ni-Sn/Ti电极表现最优异的催化活性。目前,虽然对小分子有机物电催化氧化的研究报道较多,但制备性能优良的新型催化剂仍是这一领域的主要研究内容。本文制备的Ni/Ti、Ni-Co/Ti和Ni-Sn/Ti电极,其催化剂颗粒具有新颖的结构,对甲醇或葡萄糖的电催化氧化具有优异的活性,在国内外文献中未见报道。这些新颖的电极材料对今后燃料电池阳极催化剂的制备、以及新型小分子有机物传感器的研究开发开辟了新的思路,为制备纳米电催化剂的研究工作提供了一定的技术依据与理论指导,对研究有机小分子的电催化氧化在理论上具有一定的借鉴与指导意义。更多还原

【Abstract】 As we all know that the electrocatalytic oxidation of small organic molecules has been receiving extensive investigation since the end of 20th century, which is of significant application both in fundamental research as model systems and in the fuel cells（FC）. For example, study on electrocatalytic oxidation of small organic molecules has played an important role in recognizing the electrochemical phenomena, developing the kinetics of electrode processes, enriching the research content of electrooxidation model, and promoting the development of theory experimental methods concerned in physical chemistry. Electrocatalytic oxidation of small organic molecules on noble metals like Pt has been widely studied for about a half century. Nickel has been confirmed to present effective electrocatalytic activity towards some organic molecules such as methanol, ethanol, glucose and aspirin etc. Development of electrocatalysts with significantly electrocatalytic activities for electro-oxidation of small organic molecules has been considered as the most challenging problem in the study of fule cells（FC）.Hydrothermal method, using metal salts, oxides or hydroxides as a solution or suspension, offers an alternative synthesis route for the nano materials in the elevated temperature and pressure. Because the reactions are carried out in the liquid phase, the chemical properties of many reactions will change. As a soft chemistry method, hydrothermal synthesis method has been applied to preparing the novel electrode materials in this paper. The properties of electrocatalysts made by hydrothermal synthesis are different from that by the traditional solid-state method. Nowadays, most of catalysts are loaded on carbon with electrodeposition techniques. In this thesis, we reported the preparation of nano-sized metal particales deposited on the surface of titanium by the hydrothermal process using hydrazine hydrate as the reduction agent, so as to effectively develop novel electrocatalysts with high electroactivity for the oxidation of small organic molecules. Novel titanium-supported nickel Ni/Ti, binary Ni-Co/Ti and Ni-Sn/Ti electrodes were synthesized by the hydrothermal method. Electro-catalytic activities of the prepared electrodes for methanol oxidation are investigated by conventional electrochemical techniques like voltammetric responses, chronoamperometric measurements and electrochemical impedance spectra, etc. In addition, the Ni-Co/Ti electrode was examined as an electrocatalyst for the electrooxidation of glucose in alkaline solutions. The main contents and conclusions in the thesis are as follows:1. Application of nickel hydroxide electrodes and electrocatalytic oxidation of small organic molecules are reviewed briefly. The electro-catalytic mechanism of the oxidation of methanol, ethanol and glucose is elaborated according to corresponding reports of literatures. The research progress of nickel hydroxide electrodes are introduced. Application of nickel hydroxide electrodes to organic electrosynthesis and other fields are summarized. The thesis points out that the main problem in the research of catalysts is the development of nano-catalysts with high electroactivity.2. Ni/Ti, Ni-Co/Ti and Ni-Sn/Ti electrodes are synthesized by the hydrothermal process using hydrazine hydrate as the reduction agent. The electrocatalyst particles are directly deposited on the Ti surface under conditions of reaction temperature 120℃and reaction time 10 h.3. Scanning electron microscopy （SEM） and energy disperse spectroscopy （EDS） are employed to investigate the morphology and element compositions of Ni/Ti, Ni-Co/Ti and Ni-Sn/Ti electrodes obtained by the hydrothermal process. The main results are as follows:（1） For the Ni/Ti electrode, SEM and EDS images show that the surface of Ti substrate is partly covered by nickel particles which were present as small balls with the almost uniform size of around 500nm. An interesting finding is that some nickel particles are connected with each other to form alveolate and cateniform structures. Ti substrate surface was not completely covered by nickel particles.（2） For the Ni-Co/Ti electrodes, SEM and EDS images show that the surface of Ti substrate is almost totally covered by nickel-cobalt catalyst particles. The samples exhibit similar SEM image to Ni/Ti electrode with the almost uniform size of around 500nm. But the bimetallic Ni-Co particles are tightly combined with each other and form three-dimensional porous network structures, resulting in a significant surface area.（3） For the Ni-Sn/Ti electrodes, SEM and EDS images show that the surface of Ti substrate is totally covered by nano-scale particles with the sizes of ca. 120～130nm. The particles are present as nano-scale flakes which thicknesses are around 40 nm. These flakes look like petals of flowers, resulting in a significant surface area and providing considerable numbers of active sites which are necessary for the adsorption and electro-oxidation of methanol on the catalyst.4. Electro-catalytic oxidation of methanol in sodium hydroxide solution on Ni/Ti, Ni-Co/Ti and Ni-Sn/Ti electrodes, and electro-oxidation of glucose on Ni-Co/Ti electrode have been studied using cyclic voltammetry, pseudo-steady state polarization, chronoamperometry, and electrochemical impedance spectroscopy（EIS）. The results are as follows:（1） It was shown from cyclic voltammograms in alkaline solutions that the oxidation current of methanol on the Ni/Ti electrode was much higher than that on a polycrystalline nickel electrode （Ni）, and that the onset potential of methanol oxidation on Ni/Ti shifts to 0.36 V, which is less than that on the Ni, and that the oxidation peak current on the Ni/Ti was 5 times higher than on Ni. It was further observed from chronoamperometric measurements that the steady-state current (I_ss) on the Ni/Ti was also significantly higher than on Ni, and the I_ss is well linearly proportional to the methanol concentration. Electrochemical impedance spectra on the Ni/Ti reveal that the presence of methanol in 1.0mol/L NaOH enhances the charge transfer process of the oxidation of Ni（OH）₂ to NiOOH. In the activation range of methanol oxidation, the charge transfer resistance decreases with the increase of anodic potentials and methanol concentrations. This novel nickel electrode can be used repeatedly and exhibits stable electro-catalytic activity for the methanol oxidation.（2） Electrochemical measurements show that Co/Ti electrode doesn’t present catalytic activity for methanol oxidation. However, the oxidation current of methanol on the Ni₈Co₂/Ti electrode was much higher than that on Ni/Ti electrode, and the onset potential on Ni₈Co₂/Ti shifts about 0.25V toward the negative potential in comparison with that on the Ni/Ti, The oxidation peak current on the Ni₈Co₂/Ti was 2.3 times higher than on Ni/Ti electrode. Addition of cobalt to the nickel enhances the methanol oxidation due to the formation of conducting oxide in low potential, especially to the co-precipitation of cobalt and nickel hydroxide which enhances the methanol electro-oxidation.（3） The Ni₈Co₂/Ti electrode was examined as an electrocatalyst for the electrooxidation of glucose in alkaline solutions. The Ni₈Co₂/Ti electrode exhibits significantly high current of glucose oxidation, and that oxidation reaction of glucose is controlled by the diffusing step. A high catalytic rate constant of 3.57×10⁴ cm³·mol^-1·s^-1 and the diffusion coefficient 6.50×10^-6 cm²·s^-1 of glucose was calculated from amperometric responses on the Ni₈Co₂/Ti electrode. Furthermore, amperometric datas show a linear dependence of the current density for glucose oxidation upon glucose concentration in the range of 0.05～0.5mmol/L with a sensitivity of 6.85×10^-4A·（mmol/L）^-1. A detection limit of 0.0012 mmol/L（1.2 μmol/L） glucose was found. Results show that the prepared Ni₈Co₂/Ti electrode is a promising biosensor for the construction of enzyme-free glucose detection.（4） Electrochemical measurements show that Ni-Sn/Ti electrode presents much higher anodic currents and lower onset potential for methanol oxidation than Ni/Ti electrode. The EIS datas indicate that under conditions of various anodic potentials and methanol concentrations, Ni-Sn/Ti displays significantly lower charge transfer resistances. Results show that the electrode of Ni:Sn=8:1 exhibits high electrocatalytic activity towards methanol oxidation.According to our review of literatures involved in the electrooxidation of some small organic molecules, it can be concluded that the electro-catalytic oxidation of methanol on Ni/Ti, Ni-Co/Ti and Ni-Sn/Ti electrodes, and electrooxidation of glucose on the Ni-Co/Ti electrode in sodsium hydroxide solution have not been reported. Our study will be of significance in the development of liquid fuel cells and in the electrochemical study on the electrooxidation of small organic molecules.更多还原