Hwang Bing Joe, Chen Ching-Hsiang, Sarma Loka Subramanyam, Chen Jiun-Ming, Wang Guo-Rung, Tang Mau-Tsu, Liu Din-Goa, Lee Jyh-Fu
Nanoelectrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, Republic of China.
J Phys Chem B. 2006 Apr 6;110(13):6475-82. doi: 10.1021/jp0563686.
The understanding of the formation mechanism of nanoparticles is essential for the successful particle design and scaling-up process. This paper reports findings of an X-ray absorption spectroscopy (XAS) investigation, comprised of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions, to understand the mechanism of the carbon-supported Pt-Ru nanoparticles (NPs) formation process. We have utilized Watanabe's colloidal reduction method to synthesize Pt-Ru/C NPs. We slightly modified the Watanabe method by introducing a mixing and heat treatment step of Pt and Ru oxidic species at 100 degrees C for 8 h with a view to enhance the mixing efficiency of the precursor species, thereby one can achieve improved homogeneity and atomic distribution in the resultant Pt-Ru/C NPs. During the reduction process, in situ XAS measurements allowed us to follow the evolution of Pt and Ru environments and their chemical states. The Pt LIII-edge XAS indicates that when H2PtCl6 is treated with NaHSO3, the platinum compound is found to be reduced to a Pt(II) form corresponding to the anionic complex [Pt(SO3)4]6-. Further oxidation of this anionic complex with hydrogen peroxide forms dispersed [Pt(OH)6]2- species. Analysis of Ru K-edge XAS results confirms the reduction of RuIIICl3 to [RuII(OH)4]2- species upon addition of NaHSO3. Addition of hydrogen peroxide to [RuII(OH)4]2- causes dehydrogenation and forms RuOx species. Mixing of [Pt(OH)6]2- and RuOx species and heat treatment at 100 degrees C for 8 h produced a colloidal sol containing both Pt and Ru metallic as well as ionic contributions. The reduction of this colloidal mixture at 300 degrees C in hydrogen atmosphere for 2 h forms Pt-Ru nanoparticles as indicated by the presence of Pt and Ru atoms in the first coordination shell. Determination of the alloying extent or atomic distribution of Pt and Ru atoms in the resulting Pt-Ru/C NPs reveals that the alloying extent of Ru (JRu) is greater than that of the alloying extent of Pt (JPt). The XAS results support the Pt-rich core and Ru-rich shell structure with a considerable amount of segregation in the Pt region and with less segregation in the Ru region for the obtained Pt-Ru/C NPs.
了解纳米颗粒的形成机制对于成功进行颗粒设计和放大过程至关重要。本文报道了一项X射线吸收光谱（XAS）研究的结果，该研究包括X射线吸收近边结构（XANES）和扩展X射线吸收精细结构（EXAFS）区域，以了解碳载Pt-Ru纳米颗粒（NPs）形成过程的机制。我们利用渡边的胶体还原法合成了Pt-Ru/C NPs。我们对渡边方法进行了轻微修改，在100℃下引入Pt和Ru氧化物种的混合和热处理步骤8小时，以提高前驱体物种的混合效率，从而可以在所得的Pt-Ru/C NPs中实现更好的均匀性和原子分布。在还原过程中，原位XAS测量使我们能够追踪Pt和Ru环境及其化学状态的演变。Pt LIII边XAS表明，当用NaHSO3处理H2PtCl6时，发现铂化合物被还原为对应于阴离子络合物[Pt(SO3)4]6-的Pt(II)形式。用过氧化氢进一步氧化该阴离子络合物形成分散的[Pt(OH)6]2-物种。Ru K边XAS结果分析证实，加入NaHSO3后，RuIIICl3还原为[RuII(OH)4]2-物种。向[RuII(OH)4]2-中加入过氧化氢会导致脱氢并形成RuOx物种。[Pt(OH)6]2-和RuOx物种混合并在100℃下热处理8小时，产生了一种含有Pt和Ru金属以及离子贡献的胶体溶液。如第一配位层中存在Pt和Ru原子所示，在300℃氢气气氛中还原该胶体混合物2小时形成Pt-Ru纳米颗粒。对所得Pt-Ru/C NPs中Pt和Ru原子的合金化程度或原子分布的测定表明，Ru的合金化程度（JRu）大于Pt的合金化程度（JPt）。XAS结果支持所获得的Pt-Ru/C NPs具有富Pt核和富Ru壳结构，在Pt区域有相当数量的偏析，而在Ru区域偏析较少。
