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

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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.