Nature of the catalytic centers of porphyrin-based electrocatalysts for the ORR:: A correlation of kinetic current density with the site density of Fe-N4 Centers
Nature of the catalytic centers of porphyrin-based electrocatalysts for the ORR:: A correlation of kinetic current density with the site density of Fe-N4 Centers
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DOI：
10.1021/jp802456e
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发表时间：
2008-10-02
影响因子：
3.7
通讯作者：
Bogdanoff, Peter
中科院分区：
文献类型：
--
作者：
Koslowski, Ulrike I.;Abs-Wurmbach, Irmgard;Bogdanoff, Peter
在这项工作中，本工作表明，制备的催化剂的结构变化能够分配负责直接和间接氧还原反应的催化中心，分别负责基于卟啉的电催化剂。用所谓的发泡剂技术(FAT)制备了铁基卟啉(FeTMPPCl)催化剂和以H_2TMPP为基础的催化剂。得到的铁催化剂被用作后处理的通用材料。用Fe-57穆斯堡尔谱分析了化合物的结构变化。用旋转圆盘电极(R(R)DE)测定了其对氧还原反应(ORR)的催化活性。由于后处理，催化剂的铁含量在2.9~4.5wt%之间变化。结果表明，所有催化剂的穆斯堡尔谱都可以用两个不同的Fe-N-4中心进行拟合，一个是CFeN_2中心(Fe~(2+)，S=2)，另一个是Fe_3C中心(Fe-0)。在穆斯堡尔谱的强度与铁含量进行归一化后，发现与直接氧还原有关的动能电流密度与面内Fe-N-4中心的数量之间存在线性关系。此外，有证据表明，与过氧化氢形成有关的动力学电流密度与CFeN中心之间存在关联。作为标准物质制备的热处理碳支载铁卟啉，表现出与我们的脂肪催化剂相同的行为。这一关联使我们能够获得直接和间接氧还原反应的周转频率，并确定位置密度，其中我们达到了Gasteiger等人发表的目标值的三分之一。
In this work, it has been shown that structural changes of an as-prepared catalyst enable the assignment of the catalytic centers responsible for the direct and indirect oxygen reduction reaction, respectively, of porphyrin-based electrocatalysts. An iron porphyrin (FeTMPPCl)-based catalyst as well as a catalyst based on H2TMPP were prepared using the so-called foaming agent technique (FAT). The obtained iron catalyst was used as a generic material for the post-treatments. Structural changes were analyzed by Fe-57 Mossbauer spectroscopy. The catalytic activity toward the oxygen reduction reaction (ORR) was determined using rotating (ring) disc electrode (R(R)DE) experiments. The catalysts exhibit a variation in the iron content between 2.9 and 4.5 wt % caused by the post-treatments. It has been found that the Mossbauer spectra of all catalysts can be fitted assuming two different ferrous Fe-N-4 centers, a CFeN2 center (Fe2+, S = 2) and an Fe3C center (Fe-0). After the intensities found in the Mossbauer spectra were normalized relative to the iron content, a linear correlation between the kinetic current density related to the direct oxygen reduction and the amount of in-plane Fe-N-4 centers is found. Beside this, there is evidence for a correlation between the kinetic current density related to the hydrogen peroxide formation and CFeN2 centers. Heat-treated carbon-supported iron porphyrin, prepared as reference material, exhibits the same behavior as our FAT catalysts. The correlation enables us to obtain the turnover frequencies for both the direct and the indirect oxygen reduction reaction and to determine the site densities, in which we reach a third of the target value published by Gasteiger et al.