Prasai Binay, Ren Yang, Shan Shiyao, Zhao Yinguang, Cronk Hannah, Luo Jin, Zhong Chuan-Jian, Petkov Valeri
Department of Physics, Central Michigan University, Mt. Pleasant, Michigan 48858, USA.
Nanoscale. 2015 May 7;7(17):8122-34. doi: 10.1039/c5nr00800j.
An approach to determining the 3D atomic structure of metallic nanoparticles (NPs) in fine detail and using the unique knowledge obtained for rationalizing their synthesis and properties targeted for optimization is described and exemplified on Pt-Ru alloy NPs of importance to the development of devices for clean energy conversion such as fuel cells. In particular, PtxRu100-x alloy NPs, where x = 31, 49 and 75, are synthesized by wet chemistry and activated catalytically by a post-synthesis treatment involving heating under controlled N2-H2 atmosphere. So-activated NPs are evaluated as catalysts for gas-phase CO oxidation and ethanol electro-oxidation reactions taking place in fuel cells. Both as-synthesized and activated NPs are characterized structurally by total scattering experiments involving high-energy synchrotron X-ray diffraction coupled to atomic pair distribution functions (PDFs) analysis. 3D structure models both for as-synthesized and activated NPs are built by molecular dynamics simulations based on the archetypal for current theoretical modelling Sutton-Chen method. Models are refined against the experimental PDF data by reverse Monte Carlo simulations and analysed in terms of prime structural characteristics such as metal-to-metal bond lengths, bond angles and first coordination numbers for Pt and Ru atoms. Analysis indicates that, though of a similar type, the atomic structure of as-synthesized and respective activated NPs differ in several details of importance to NP catalytic properties. Structural characteristics of activated NPs and data for their catalytic activity are compared side by side and strong evidence found that electronic effects, indicated by significant changes in Pt-Pt and Ru-Ru metal bond lengths at NP surface, and practically unrecognized so far atomic ensemble effects, indicated by distinct stacking of atomic layers near NP surface and prevalence of particular configurations of Pt and Ru atoms in these layers, contribute to the observed enhancement of the catalytic activity of PtxRu100-x alloy NPs at x ∼ 50. Implications of so-established relationships between the atomic structure and catalytic activity of Pt-Ru alloy NPs on efforts aimed at improving further the latter by tuning-up the former are discussed and the usefulness of detailed NP structure studies to advancing science and technology of metallic NPs - exemplified.
描述了一种详细确定金属纳米颗粒（NPs）三维原子结构的方法，并利用所获得的独特知识来合理化其合成过程，并针对优化目标对其性质进行优化。以对清洁能源转换装置（如燃料电池）开发具有重要意义的Pt-Ru合金纳米颗粒为例进行了说明。具体而言，通过湿化学合成了x = 31、49和75的PtxRu100-x合金纳米颗粒，并通过在受控的N2-H2气氛下加热的合成后处理进行催化活化。如此活化的纳米颗粒被评估为燃料电池中发生的气相CO氧化和乙醇电氧化反应的催化剂。通过涉及高能同步加速器X射线衍射与原子对分布函数（PDFs）分析的全散射实验，对合成态和活化态的纳米颗粒进行结构表征。基于当前理论建模的原型Sutton-Chen方法，通过分子动力学模拟建立了合成态和活化态纳米颗粒的三维结构模型。通过反向蒙特卡罗模拟针对实验PDF数据对模型进行优化，并根据主要结构特征（如Pt和Ru原子的金属-金属键长、键角和第一配位数）进行分析。分析表明，尽管合成态和相应活化态纳米颗粒的原子结构类型相似，但在对纳米颗粒催化性能重要的几个细节上存在差异。将活化态纳米颗粒的结构特征与其催化活性数据进行了并排比较，发现有力证据表明，NP表面Pt-Pt和Ru-Ru金属键长的显著变化所表明的电子效应，以及到目前为止几乎未被认识到的原子团簇效应（由NP表面附近原子层的独特堆积以及这些层中Pt和Ru原子的特定构型的普遍存在所表明），促成了在x ∼ 50时观察到的PtxRu100-x合金纳米颗粒催化活性的增强。讨论了如此建立的Pt-Ru合金纳米颗粒原子结构与催化活性之间的关系对通过调整前者来进一步改善后者的努力的影响，并举例说明了详细的纳米颗粒结构研究对推进金属纳米颗粒科学技术的有用性。
