Atomic-level structural characterization of metal/gamma-alumina interfaces combining theory and experiments

非技术描述：γ-氧化铝(氧化铝的一种相)上的金属纳米颗粒在化学催化中很重要，因为它们在减少化石燃料燃烧的有害副产物和工业化学过程中的能源消耗方面起到了重要作用。这种材料体系的功能部分取决于金属与氧化物载体之间的相互作用，直到界面上的原子键。界面的原子级表征是具有挑战性的，因为界面是埋藏在主体中的复杂结构。在含有伽马氧化铝的体系中，表征尤其困难，因为它具有极高的比表面积，是催化金属纳米颗粒的良好载体，但它在结构上也很脆弱，而且对温度敏感。新的加工方法与非常先进的电子显微镜技术相结合，以揭示这种亚稳定相氧化铝与广泛使用的金属催化剂(如铂和钯)之间界面的原子结构。实验表征与计算模型相结合，这些模型基于描述材料之间成键的基本关系，以进一步阐明结构和性能之间的关系。该项目为研究生和本科生提供培训和研究机会，并吸引来自代表性不足群体的高中生参与科学和工程方面的探索。技术说明：伽马氧化铝是工业化工生产和催化转化器中广泛使用的金属纳米颗粒载体。虽然已知催化剂活性受金属-载体相互作用的影响，但对金属/γ-氧化铝界面的原子级结构和热力学性质的了解还不完全。这项研究的目的是考察界面金属物种的结构和化学对伽马氧化铝局部原子结构的影响程度。这项工作评估了具有高伽马-氧化铝/金属界面面积的致密纳米结构材料是否能够足够稳定系统，从而能够进行原子水平的表征。金属/氧化铝体系微观结构的复杂性、氧化铝相的亚稳性以及氧化铝在透射电子显微镜下对损伤的敏感性，限制了这些复杂体系中界面的原子级结构表征。在这个项目中，创新的加工路径被用来创造稳定的纳米结构材料，这种材料可以用像差校正的透射电子显微镜进行表征。利用像差校正的透射电子显微镜对氧化铝与某些金属(如铂、钯)之间的界面进行了定性的原子级结构表征。采用密度泛函理论(DFT)计算了界面候选结构模型的最优构型和界面能。研究的目的是利用实验表征和结构计算之间的重叠来验证DFT方法计算的界面结构和热力学稳定性范围，并将结构信息与界面的热力学性质，包括界面自由能联系起来。参与该项目的研究生接受了先进的瞬变电磁、数据分析技术和密度泛函计算方法的培训，以计算界面的原子结构。本科生也参与了这些主要的研究活动。面向科学和工程专业代表性不足群体的高中生的外联活动向他们介绍了材料表征的实验方法，以及如何使用计算技术来研究原子键。

NON-TECHNICAL DESCRIPTION: Metal nanoparticles on gamma-alumina (a phase of aluminum oxide) are important in chemical catalysis because of their role in reducing harmful by-products of fossil fuel combustion and energy consumption during industrial chemical processes. The functionality of such material systems depends partly on the interactions between the metal with the oxide support, down to the atomic bonding at the interface. Atomic-level characterization of interfaces is challenging because interfaces are complex structures buried within the bulk. Characterization is especially difficult in a system involving gamma-alumina, which is an excellent carrier for catalytic metal nanoparticles because of its extremely high surface area, but it is also structurally fragile and thermally sensitive. Novel processing methods are combined with highly advanced electron microscopy techniques to reveal the atomic structure at the interface between this metastable phase of aluminum oxide and widely used metal catalysts such as platinum and palladium. Experimental characterization is paired with computational models that are based on the fundamental relationships describing bonding between materials to further elucidate the relationship between the structure and properties. This project provides training and research opportunities for graduate and undergraduate students, and engages high school students from underrepresented groups in the exploration of science and engineering. TECHNICAL DESCRIPTION: Gamma-alumina is a widely used support for metal nanoparticles in industrial chemical production and catalytic converters. Although it is known that the catalyst activity is affected by metal-support interactions, the understanding of the atomic-level structure and thermodynamic properties of the metal/gamma-alumina interfaces is incomplete. The goal of this research is to examine the extent to which the local atomic structure of gamma-alumina is affected by the structure and the chemistry of the metal species at the interface. This work assesses if dense nanostructured material with a high gamma-alumina/metal interfacial area can stabilize the system enough to enable atomic-level characterization. The microstructural complexity of metal/alumina systems, the metastability of the alumina phase, and the susceptibility of alumina to damage during transmission electron microscopy (TEM) have limited atomic-level structural characterization of the interfaces in these complex systems. In this project innovative processing paths are used to create stabilized nanostructured materials that may be characterized with aberration-corrected TEM. Qualitative atomic-level structural characterization of the interfaces between the alumina and select metals (e.g., Pt, Pd) is achieved using aberration-corrected TEM. Density functional theory (DFT) is used to calculate the optimal configuration and interfacial energy of candidate structural models of the interfaces. The research objective is to use the overlap between the experimental characterization and the structural calculations to validate the interfacial structures and thermodynamic stability ranges calculated with DFT methods in these complex, metastable systems, and to connect the structural information to the thermodynamic properties of the interface, including the interfacial free energy. Graduate students involved in the project are trained in advanced TEM, data analysis techniques and in computational methods of DFT for calculating the atomic structure of the interfaces. Undergraduate students are also involved in these main research activities. Outreach activities for high school students from underrepresented groups in science and engineering introduce them to experimental methods for materials characterization and to how computational techniques are used to study atomic bonding.