Hwang Bing-Joe, Sarma Loka Subramanyam, Wang Guo-Rung, Chen Ching-Hsiang, Liu Din-Goa, Sheu Hwo-Shuenn, Lee Jyh-Fu
Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Tapei 106, Taiwan.
Chemistry. 2007;13(21):6255-64. doi: 10.1002/chem.200700126.
The ability to alter the surface population of metal sites in bimetallic nanoparticles (NPs) is of great interest in the context of heterogeneous catalysis. Here, we report findings of surface alterations of Pt and Ru metallic sites in bimetallic carbon-supported (PtRu/C) NPs that were induced by employing a controlled thermal-treatment strategy. The thermal-treatment procedure was designed in such a way that the particle size of the initial NPs was not altered and only the surface population of Pt and Ru was changed, thus allowing us to deduce structural information independent of particle-size effects. X-ray absorption spectroscopy (XAS) was utilized to deduce the structural parameters that can provide information on atomic distribution and/or extent of alloying as well as the surface population of Pt and Ru in PtRu/C NPs. The PtRu/C catalyst sample was obtained from Johnson Matthey, and first the as-received catalyst was reduced in 2 % H2 and 98 % Ar gas mixture at 300 degrees C for 4 h (PtRu/C as-reduced). Later this sample was subjected to thermal treatment in either oxygen (PtRu/C-O2-300) or hydrogen (PtRu/C-H2-350). The XAS results reveal that when the as-reduced PtRu/C catalyst was exposed to the O2 thermal-treatment strategy, a considerable amount of Ru was moved to the catalyst surface. In contrast, the H2 thermal-treatment strategy led to a higher population of Pt on the PtRu/C surface. Characterization of the heat-treated PtRu/C samples by X-ray diffraction and transmission electron microscopy reveals that there is no significant change in the particle size of thermally treated samples when compared to the as-received PtRu/C sample. The electrochemical properties of the as-reduced and heat-treated PtRu/C catalyst samples were confirmed by cyclic voltammetry, CO-adsorption stripping voltammetry, and linear sweep voltammetry. Both XAS and electrochemical investigations concluded that the PtRu/C-H2-350 sample exhibits significant enhancement in reactivity toward methanol oxidation as a result of the increased surface population of the Pt when compared to the PtRu/C-O2-300 and PtRu/C as-reduced samples.
在多相催化领域，改变双金属纳米颗粒（NPs）表面金属位点的数量备受关注。在此，我们报告了通过采用可控热处理策略诱导的双金属碳负载（PtRu/C）NPs中Pt和Ru金属位点的表面变化情况。热处理程序的设计方式使得初始NPs的粒径不变，仅Pt和Ru的表面数量发生变化，从而使我们能够推断出与粒径效应无关的结构信息。利用X射线吸收光谱（XAS）来推导结构参数，这些参数可以提供有关原子分布和/或合金化程度以及PtRu/C NPs中Pt和Ru的表面数量的信息。PtRu/C催化剂样品购自庄信万丰公司，首先将收到的催化剂在300℃的2% H₂和98% Ar气体混合物中还原4小时（还原后的PtRu/C）。随后，该样品在氧气（PtRu/C-O₂-300）或氢气（PtRu/C-H₂-350）中进行热处理。XAS结果表明，当还原后的PtRu/C催化剂采用O₂热处理策略时，大量的Ru迁移到了催化剂表面。相比之下，H₂热处理策略导致PtRu/C表面的Pt数量增加。通过X射线衍射和透射电子显微镜对热处理后的PtRu/C样品进行表征，结果表明与收到的PtRu/C样品相比，热处理样品的粒径没有显著变化。通过循环伏安法、CO吸附溶出伏安法和线性扫描伏安法确认了还原后的和热处理后的PtRu/C催化剂样品的电化学性质。XAS和电化学研究均得出结论，与PtRu/C-O₂-300和还原后的PtRu/C样品相比，由于Pt表面数量增加，PtRu/C-H₂-350样品对甲醇氧化的反应活性显著增强。
