【作者】 赵杰；

【导师】 陈卫祥；

【作者基本信息】 浙江大学 ， 物理化学， 2007， 博士

【摘要】 直接甲醇燃料电池（DMFC）具有较高的能量转化效率,被认为是理想的可移动或者小型化能源之一。但是DMFC研究中有两个问题急需解决:一是低温下甲醇在阳极上的电催化氧化活性不高;二是甲醇从阳极透过电解质膜渗透到阴极严重,这就需要制备对甲醇电催化氧化具有高活性的催化剂。过渡金属Pt以其优异的催化性能而成为DMFC的首选阳极催化剂。众所周知,催化剂的活性与其颗粒大小、分布、比表面积及其形貌等因素有着密切的关系,所以制备粒径适宜、分布均匀的催化剂是提高催化剂的催化活性的一个关键。论文的工作主要从碳负载Pt催化剂的制备方法入手,用微波多元醇方法制备了高分散的碳负载Pt纳米粒子催化剂,研究了合成溶液的pH值和添加少量的醋酸纳对Pt纳米粒子的粒径及其分布的影响;用微波多元醇方法一步合成了Pt-CeO₂/C和Pt-ZrO₂/C催化剂;用金属交换法在常温下制备了碳负载中空Pt纳米催化剂。用X射线衍射（XRD）、透射电子显微镜（TEM）和能谱（EDX）等研究手段对其结构和组成进行表征,并用电化学方法研究了催化剂对甲醇氧化的电催化性能。取得以下主要的创新性研究成果:1)用微波加热多元醇方法成功合成了XC-72碳和碳纳米管（CNTs）负载的铂纳米粒子催化剂。结果表明微波加热具有快速和均匀的特点,所合成的的Pt纳米粒子具有均匀的粒径,并高度分散在XC-72碳和碳纳米管载体上。TEM观察结果显示微波合成的Pt/XC-72和Pt/CNTs中铂纳米粒子的平均粒径分别为3.2和2.7 nm。电化学实验结果表明:与传统的NaBH₄还原方法制备的Pt/XC-72催化剂相比,微波多元醇方法制备的Pt/XC-72和Pt/CNTs催化剂具有更高的电化学活性比表面积,对甲醇的氧化具有更好的电催化性能。2)通过调整合成溶液的pH值和添加少量的醋酸钠溶液作为稳定剂,用微波多元醇方法合成了在XC-72碳负载的粒径可控的铂纳米粒子催化剂。研究了合成溶液的pH值和醋酸钠溶液的添加量对微波合成铂纳米粒子的粒径、粒径分布及其在碳载体表面分散均匀性的影响。结果表明通过调整合成溶液的pH值或者添加少量的醋酸盐稳定剂,可以控制微波合成铂纳米粒子的平均粒径大小。当合成溶液的pH值为9.5或者在50 mL合成溶液中加入0.5 mL 1M醋酸钠溶液时,微波合成的铂纳米粒子具有均匀的粒径,其平均粒径分别为2.7 nm和2.6 nm,并且高度分散在XC-72碳载体上,Pt/XC-72具有较高的电化学活性比表面积和良好的对甲醇氧化的电催化性能。3)通过调节合成溶液的pH值,用微波加热多元醇方法合成了在碳纳米管（CNTs）负载的粒径可控的铂纳米粒子。结果表明随着pH值的增加,微波合成Pt/CNTs催化剂的铂纳米粒子平均粒径逐渐减小,其尺寸的均匀性和在碳载体上分散的均匀性得到改善。电化学测试结果表明,当合成溶液的pH值由3.6增大到7.4时,所合成的Pt/CNTs催化剂的电化学比表面逐渐增大,对甲醇氧化的催化活性也逐渐增大。而当pH增大为9.2时,虽然Pt纳米粒子粒径继续减小,电化学表面积仍然增大,但对甲醇的电催化活性却减小。初步分析的结果认为在合适的pH值下能够合成出合适粒径的Pt/CNTs催化剂,而合适粒径的Pt纳米粒子能够使Pt解离水和吸附甲醇达到一个合适的比例,从而使电催化活性最高。4)通过金属交换法在室温下制备了中空的Pt/C催化剂,TEM表征结果显示Pt纳米球为中空结构,由一些不连续的更细小的Pt纳米粒子组成,其平均外径为24.5 nm,壳层厚度平均为2.3 nm。XRD与EDX表征结果表明中空Pt纳米球为面心立方晶体结构,载量为20%。其电化学活性比表面为138.7 m²/g,远远高于实心Pt/C催化剂（73 m²/g）。电化学测试结果显示中空Pt/C催化剂对甲醇氧化的电催化活性远高于实心Pt/C催化剂和商用E-TEK Pt/C催化剂。5)利用微波多元醇方法、以醋酸钠为稳定剂,利用氯铂酸、硝酸铈铵或者硝酸锆一步法合成了Pt-CeO₂/C和Pt-ZrO₂/C催化剂。表征结果显示所合成的Pt-CeO₂/C和Pt-ZrO₂/C催化剂中Pt纳米粒子均匀细小,并高度分散在碳载体上,而且CeO₂和ZrO₂纳米粒子也同样呈现出细小均匀分布的形貌。循环伏安实验显示:与Pt/C（E-TEK）催化剂相比,Pt-CeO₂/C和Pt-ZrO₂/C催化剂对甲醇电化学氧化的起始电位负移,而且具有的对甲醇电氧化的更高峰电流,说明Pt-CeO₂/C和Pt-ZrO₂/C催化剂具有较好的电催化活性。进一步研究显示当Pt与CeO₂的摩尔比为2:1时,Pt-CeO₂/C催化剂具有更高的电化学活性比表面和对甲醇氧化的更好的电催化性能。这是由于Pt与CeO₂或者ZrO₂氧化物之间的相互作用,在氧化物表面产生了大量的O空位,从而能够氧化CO,释放Pt表面的活性吸附位,提高Pt/C催化剂的抗CO中毒的能力,有效地改善其电催化性能。更多还原

【Abstract】 Direct methanol fuel cell（DMFC）is believed to be one of the ideal small or mobile energy sources due to its high energy conversion efficiency.However,there are two urgent problems faced in DMFC:one of which is that the electrocatalytic activity of methanol at anode is low at low termperature;the other is that the penetration of methanol from anode to cathode is quite severe.Therefore it is a urgent need to prepare a kind of anode catalyst with high electrocatalytic activity for methanol oxidation.Transition metal Pt is the first candidate for anode catalysts of DMFC due to their excellent catalytic activity.As is well known,catalytic activity depends on the size,particle distribution,surface area and the shape of the metal particles,and therefore the synthesis of well-controlled shapes and sizes of Pt nanocatalysts could be critical for improving catalytic activity.In this article,research has proceeded mainly with preparation methods of carbon supported Pt catalysts.Highly dispersed carbon supported Pt nanoparticle catalysts have been prepared by microwave polyol process and the effect of the synthesis solution pH and the addition of a small amount of sodium acetate on the Pt particle size and size distribution have been investigated;Pt-CeO₂/C and Pt-ZrO₂/C catalysts have been one-step synthesized by microwave polyol process;carbon supported hollow Pt nanosphere catalyst has been prepared by a metal replacement reaction at room temperature.The structrue and composition of catalysts have been characterized by energy-dispersive X-ray spectroscopy（EDX）,transmission electron microscopy（TEM）and X-ray diffraction（XRD）.And catalytic activity of methanol oxidation has been investigated by electrochemical analysis.The original results obtained are as follows: 1)Pt/XC-72 and Pt/CNTs nanoparticle catalysts have been successfully prepared by a microwave polyol process.The results demonstrate that the as-synthesized Pt nanoparticles are homogeneous and highly dispersed on surface of XC-72 carbon and carbon nanotubes due to rapidness and uniformity of microwave heating.TEM results show that the average Pt particle size of microwave-synthesized Pt/XC-72 and Pt/CNTs catalysts is 3.2 and 2.7 nm respectively.Electrochemical measurements show that microwave polyol synthesized Pt/XC-72 and Pt/CNTs catalysts have higher electrochemical active area and better electrocatalytic activity for methanol oxidation compared with KBH4-reduced Pt/XC-72 catalyst.2)XC-72 carbon supported size-controlled Pt have been synthesized by microwave polyol process through adjusting the synthesis solution pH or adding a small amount of sodium acetate as stabilizer.The effects of pH and the content of sodium acetate on the mean particle size,size uniformty and size ditribution of microwave synthesized Pt nanoparticles have been investigated.The results demonstrate that the mean size of microwave synthesized Pt nanoparticles can be controlled by adjusting pH value or the amount of acetate added in the synthesis solution.When the synthesis solution pH is 9.5 or 0.5 mL 1 M sodium acetate is added into 50 mL synthesis solution,microwave synthesized Pt particles have uniform particle size with the mean particle size of 2.7 nm and 2.6 nm respectively and highly disperse on the carbon support.Mcrowave polyol synthesized Pt/XC-72 catalysts have higher electrochemical active area and better electrocatalytic activity for methanol oxidation.3)Pt nanoparticles supported on carbon nanotubes（CNTs）have been synthesized by microwave polyol process through adjusting the synthesis solution pH. The research results demonstrate that the mean size of microwave synthesized Pt particles gradually decreases and the size uniformty and dispersion uniformty on CNTs surface get improved with increasing the synthesis solution pH. Electrochemical measurements demonstrate that the elctrochemical surface area and electrocatalytic acitivity for methanol oxidation of Pt/CNTs increases when the synthesis solution pH increases from 3.6 to 7.4.The electrocatalytic acitivity for methanol oxidation decreases,though the elctrochemical surface area and electrocatalytic acitivity for methanol oxidation of Pt/CNTs increases when pH increases to 9.2.The preliminary analysis propose that Pt particles with the right size can be synthesized under the right synthesis solution pH value,which can make Pt surface reach a right co-adsorption ratio of methanol and water to optimize the electrocatalytic activity for methanol oxidation.4)Hollow Pt/C catalyst has been prepared by a metal replacement reaction at room temperature.TEM results show that Pt nanospheres are hollow nanostructure, which are composed of some smaller discrete Pt nanoparticles.The average diameter of hollow Pt nanospheres is statistically calculated to be 24.5 nm and the thickess of shell is about 2.3 nm.XRD and EDX results show that hollow Pt nanospheres are faced-centered cubic（fcc）with the Pt loading of 20%.The electrochemical active surface area of hollow Pt nanospheres is 138.7 m²/g,which is much higher than that of solid Pt/C catalyst（73 m²/g）.Based on the study of methanol oxidation,it is found that the hollow Pt/C catalysts display much higher electrocatalytic activity for methanol electrooxidation than the E-TEK Pt/C and SN-Pt/C electrocatalysts.5)Pt-CeO₂/C and Pt-ZrO₂/C catalysts have been one-step prepared by microwave heating of polyol solutions of H₂PtCl₆,（NH₄）₂Ce（NO₃）₆ or Zr（NO₃）₄ and sodium acetate as stabilizer.Characterization results show that Pt nanoparticles of microwave polyol one-step synthesized Pt-CeO₂/C and Pt-ZrO₂/C catalysts are ultrafine and highly dispersed on carbon support.Compared with Pt/C（E-TEK） catalyst,the onset potentials of methanol oxidation of Pt-CeO₂/C and Pt-ZrO₂/C catalysts shift to the negative potential and the peak currents of Pt-CeO₂/C and Pt-ZrO₂/C catalysts for methanol oxidation become much higher.The further studies show that Pt-CeO₂/C catalyst has higher electrochemical surface area and better electrocatalytic activity for methanol oxidation.This is due to that the interaction of Pt and CeO₂ or ZrO₂ lead to form a large quantity of oxygen vacancies on the surface of oxide,which can oxidized CO and release active sites on Pt surface to enhance the ability to resist CO posioning and improve the electrocatalytic performance of Pt/C catalyst.更多还原