Pt/C电极掺杂SnO₂对甲醇电催化氧化性能的研究

【作者】 郑丁琴；

【导师】 郭永榔；

【作者基本信息】 福州大学 ， 物理化学， 2006， 硕士

【摘要】 近年来,直接甲醇燃料电池（DMFC）以其独特的优越性可能成为未来可移动动力电源的首选。但目前DMFC仍然存在一些关键的问题,其中之一是阳极催化剂Pt活性较低,且易于被甲醇氧化中间产物CO毒化。许多研究者发现,在催化剂Pt基上掺杂一些金属氧化物对于甲醇的氧化催化有很好的效果,这是因为金属氧化物的加入可以促进Pt电极上活性氧OH的吸附和CO的脱附。本文深入地研究了Pt/C和Pt-Ru/C阳极催化剂对甲醇的电催化氧化过程,探讨甲醇在电极表面的动力学反应机理。另外,我们采用浸渍-液相还原法合成Pt-SnO₂/C催化剂,利用四方金红石型的SnO₂良好导电性和Sn本身d轨道富余的电子易向基体Pt转移的特性,通过改变催化剂的沉积顺序和选择不同的催化剂载体活性炭,以寻找对甲醇氧化具有高活性和耐CO中毒的阳极催化剂。通过循环伏安法（CV）和恒电流法（CP）研究,发现在甲醇的氧化过程中,反应速率的快慢取决于电极表面CHOHads,COads,OHads以及Oads的浓度。当电极表面COads和OHads的浓度都很高时,随着桥式吸附COads态的脱附,电极表面就有更多的Pt活性位空出来,促进反应的进一步进行,即“自催化”效应。阳极极化电流随着极化电位的降低反而升高,OHads的产生过程和COads的脱附过程为互相竞争反应。采用浸渍法合成催化剂,发现先制备SnO₂/C,然后再合成Pt-SnO₂/C催化剂,在Pt和SnO₂质量比为1:1时,催化氧化甲醇的活性最高、耐CO中毒也最好,SnO₂含量越少,催化氧化甲醇的活性和耐CO中毒性越差。而先制备Pt/C再合成Pt-SnO₂/C催化剂,在Pt和SnO₂质量比为20:1时,催化氧化甲醇的活性最高,SnO₂含量越高,催化氧化甲醇的活性和耐CO中毒性越差。两种催化剂沉积顺序的不同对氧化甲醇活性的影响与催化剂表面颗粒的形貌以及大小有很大关系。通过SEM扫描发现,先制备SnO₂/C再合成Pt-SnO₂/C催化剂,其颗粒较小,粒径约40nm。而先制备Pt/C再合成Pt-SnO₂/C催化剂,其颗粒大小高达300nm左右,催化剂Pt表面大部分被SnO₂所覆盖,因而活性降低。通过在不同温度下测试电极对甲醇的电催化氧化性能,发现温度越高,Pt-SnO₂/C电极对甲醇的氧化活性较Pt/C电极提高的更明显。说明在Pt/C中掺杂SnO₂,当Pt和SnO₂粒子处于相邻位置,而不是互相包覆时, SnO₂体相中的低浓度氧空位才有利于生成OH吸附,加速甲醇氧化中间体CO的脱附,从而大大提高催化氧化甲醇的电流,催化剂的活性和耐CO中毒性最好。更多还原

【Abstract】 In recent years, the direct methanol fuel cell （DMFC） has been paid more and more attention due to its many advantages such as the mobile power sources. However, it has some key problems. One of them is that the Pt, which is the normally used anodic catalyst, has the low electrocatalytic activity for the CH3OH oxidation and is easy to be poisoned with the intermediate of the methanol oxidation. Many investigations show that the Pt catalyst doping some metal oxides improves its electrocatalytic performance. This is due to the formation of more OHads on the surface of the doped metal oxides. It accelerates the desorption of the COads which is an intermediate product in the CH3OH oxidation. In this work, the kinetics and mechanism of the CH3OH oxidation on Pt/C and Pt-Ru/C electrodes have been studied. In addition, we synthesized the Pt-SnO₂/C catalysts through the liquid-saturated method. SnO₂ are tetrahedral rutile structure and excellent conductivity, their excess electrones on the orbit d of Sn atom can move to the Pt catalyst. So high electrocatalytic activities and the CO tolerance were found by changing the sequence of the Pt and SnO₂ and choosing different active carbons for the supported catalysts.The kinetic and mechanism of the CH3OH oxidation was studied by cyclic voltammetry（CV） and chronopotentiometry（CP）. It was found that the rate of the CH3OH oxidation depends on various adsorbates such as CHOHads, COads, OHads and Oads on the electrode surface.When the coverage of both COads and OHads is relatively high, an autocatalysis occurs by oxidizing the bridge-bonded adsorbates, which produces more vacant sites. The anodic current increase with the decreasing of the potential, and both the formation of the OHads and desorption of the COads are a couple of competitive reactions.For the preparation order of SnO₂/C and Pt-SnO₂/C, the Pt-SnO₂/C（Pt:SnO₂=1:1, quality ratio） catalyst gave the best results in electrocatalytic activity and the CO tolerance, and the lower the content of the SnO₂ was, the lower the electrocatalytic activity and the CO tolerance became. On the other hand, the Pt-SnO₂/C（Pt:SnO₂=20:1, quality ratio） showed the best electrocatalytic activity and the CO tolerance for the preparation order of Pt/C and Pt-SnO₂/C, the higher the content of the SnO₂ was, the lower the electrocatalytic activity and the CO tolerance更多还原