Comparative in Operando Studies in Heterogeneous Catalysis: Atomic and Electronic Structural Features in the Hydrogenation of Ethylene over Supported Pd and Pt Catalysts
Comparative in Operando Studies in Heterogeneous Catalysis: Atomic and Electronic Structural Features in the Hydrogenation of Ethylene over Supported Pd and Pt Catalysts
复制标题
DOI：
10.1021/cs501846g
复制
发表时间：
2015-03-01
期刊：
影响因子：
12.9
通讯作者：
Nuzzo, Ralph G.
中科院分区：
文献类型：
--
作者：
Jung, Ulrich;Elsen, Annika;Nuzzo, Ralph G.
实验研究方法的进步带来了一个新的机会，可以解决长期存在的问题，即在重要的多相催化过程中起作用的分子机制的性质，以及介导这些过程的复杂原子和电子结构特征的性质。在这方面特别感兴趣的是催化过程的动态属性的理解，可以允许设计原则被应用到优化的原子和电子结构的多相催化剂，以维持其性能，基本上在任何操作过程条件下的理解。目前的工作探讨这些想法突出的能力，在操作方法的光谱表征，适用于一个示例性的非均相催化过程，烯烃加氢。没有比烯烃加氢更深入研究的多相催化过程。可获得的大量文献确立了金属催化剂活化和有效转化氢和烯烃反应物的键合以产生产物烷烃的重要特征。即便如此，许多重要的机制问题仍然知之甚少，由于固有的多尺度复杂性与多相催化转化，以及缺乏适合其表征的方法在operando。最近的文献记载了原位和操作方法所提供的表征新能力的发展。其中，X射线吸收光谱（XAS）已成为研究催化反应机理的特别重要的技术，因为它能够阐明操作催化剂的原子和电子结构特征的性质。现在可以解决许多重要的问题，特别是那些在高压（非特高压）环境中发生的独特动力学影响和反应模式所带来的问题。在这个角度来看，我们研究了重要的结构-性能的相关性为一个示例性的模型反应-乙烯加氢-阐明了在操作中的两个有效的催化剂材料-纳米级的Pd和Pt簇载于SiO2。所检查的特征包括以下：金属簇的结构动力学及其对反应物进料的组成的敏感性;更一般地，在形成中间体和产物中，氢和金属-和/或载体-键合形式的吸附物的作用;吸附物键合状态（例如，氢）对反应性的影响;碳质沉积物所起的作用（及其形成机制）;存在于该催化反应的结构敏感性相关性内的原子特征的定量性质;以及介导在高压周围环境中操作的催化剂的烧结的机制。在这里，我们提出了一个比较概述的乙烯加氢超过约1纳米大小的Pd和Pt催化剂负载在SiO2上。该反应的特点是在各种混合的氢气和乙烯气氛中，在环境条件下，在operando XAS和补充与扫描透射电子显微镜（STEM）。两种催化剂的原子和电子结构发生明显变化（例如，发现在反应过程中发生氢和烃覆盖表面之间的定义的转变、碳化物相形成、氢（脱）插和颗粒粗化）。然而，催化剂特征的演变对反应性的大部分可逆模式只有最小的影响。这些研究结果表明显着的动态结构的复杂性在这两种类型的负载型催化剂的烷烃形成的机制。
There exists an emerging opportunity, engendered by advances made in experimental methods of research, to address long-standing questions about the nature of the molecular mechanisms that are operative in important heterogeneous catalytic processes, as well as the nature of the complex atomic and electronic structural features that mediate them. Of particular interest in this regard is the understanding of the dynamical attributes of catalytic processes-an understanding that might allow design principles to be applied to optimize the atomic and electronic structure of heterogeneous catalysts to sustain their performance in essentially any operating process condition. The current work explores these ideas-highlighting capabilities of in operando methods of spectroscopic characterization as applied to an exemplary heterogeneous catalytic process, olefin hydrogenation. No heterogeneous catalytic process has been studied more intensively than olefin hydrogenation. The extensive literature available establishes important features by which metal catalysts activate and efficiently transform the bonding of the hydrogen and alkene reactants to generate a product alkane. Even so, many important mechanistic questions remain poorly understood due to the inherent multiscale complexity associated with heterogeneous catalytic transformations, as well as the paucity of methods suitable for their characterization in operando. The recent literature documents the development of new capabilities for characterization afforded by in situ and in operando methods. Of these, X-ray absorption spectroscopy (XAS) has become a particularly important technique for studying the mechanisms of catalytic reactions due to its capabilities for elucidating the nature of the atomic and electronic structural features of operating catalysts. Many important questions can now be addressed, in particular those that follow from the unique dynamical impacts and patterns of reactivity that occur in higher pressure (non-UHV) environments. In this Perspective, we examine important structure-property correlations for an exemplary model reaction-ethylene hydrogenation-as elucidated in operando for two efficient catalyst materials-nanoscale Pd and Pt clusters supported on SiO2. The examined features include the following: the structural dynamics of the metal clusters and their sensitivity to the composition of the reactant feed; the role of hydrogen, and metal- and/or support-bonded forms of adsorbates more generally, in forming intermediates and products; the influences of adsorbate bonding states (e.g., hydrogen) on reactivity; the role played by carbonaceous deposits (and the mechanisms of their formation); the quantitative nature of the atomistic features that exist within the structure-sensitivity correlations of this catalytic reaction; and mechanisms that mediate the sintering of catalysts operating in high-pressure ambient environment. Here we present a comparative overview of the hydrogenation of ethylene over approximate to 1 nm-sized Pd and Pt catalysts supported on SiO2. The reaction was characterized in various mixed hydrogen and ethylene atmospheres at ambient conditions by in operando XAS and complemented with scanning transmission electron microscopy (STEM). Pronounced changes in the atomic and electronic structures of both catalysts (e.g., defined transitions between hydrogen- and hydrocarbon-covered surfaces, carbide-phase formation, hydrogen (de)intercalation, and particle coarsening) are found to occur during the reaction.The evolution of the catalysts features, however, has only minimal impact on the largely reversible patterns of reactivity. These findings demonstrate remarkable dynamic structural complexity within the mechanisms of alkane formation over both types of supported catalysts.