In situ and operando transmission electron microscopy of catalytic materials

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Introduction Heterogeneous catalysis is a key process for converting reactants into products, and it plays a critical role in chemical and energy transformations.1,2 An industrial heterogeneous catalyst typically consists of nanostructured particles of variable shape and composition. The surfaces of catalytic particles interact with gas- or liquid-phase reactants and promote product generation. Reactions typically take place at temperatures up to 1000°C. As the catalytic materials convert gas- or liquid-phase reactants into products, the ambient gas or liquid environment also changes the surface and even the bulk form of the nanoparticles. During catalysis, the structure of the catalyst can vary in complex ways with changes in the pressures of the reactants and products as well as in the temperature. To build a structure–reactivity–performance correlation and achieve a fundamental understanding of catalytic mechanisms, the electronic and geometric structure of the catalyst must be determined at the atomic level. Environmental transmission electron microscopy (ETEM) is the only high-spatial-resolution technique available to perform in situ characterization of high-surface-area catalysts. In this approach, reactants are allowed to flow over the TEM sample (often during in situ heating), and the changes in the nanocatalyst particles are recorded using TEM imaging, diffraction, or spectroscopy techniques.

Catalysts are dynamic entities that change not only during each reaction cycle, but also on a longer time scale through phase transformations, surface restructuring, and particle growth. An example of this is illustrated in Figure 1, which shows a supported Ru catalyst for ammonia synthesis. In a vacuum, each Ru particle is encapsulated in a shell of BN, but under a reactive environment, this shell is no longer present.3 To build a direct correlation between catalyst structure and catalytic performance, structural information at the atomic or nanoscale level obtained under reaction conditions is critical. The value of the ETEM approach for characterizing highsurface-area materials was recognized by early workers in the field. Baker and co-workers were the first to extensively apply the technique to heterogeneous catalysts in a series of early works, elucidating the nanoscale mechanisms for carbon formation and catalyst sintering.4,5 In the current article, we limit ourselves to select examples drawn mostly from the past 10 years focusing on gas-phase reactions/products. For reviews of earlier work, see References 6–9.

Atomic-resolution environmental in situ microscopy A critical technical requirement for all gas systems compatible with TEM is that they maintain both a high gas pressure around a sample and a high vacuum throughout most of the

Peter A. Crozier, School of Engineering of Matter, Transport and Energy, Arizona State University, USA; [email protected] Thomas W. Hansen, Center for Electron Nanoscopy, Technical University of Denmark, Denmark; [email protected] DOI: 10.1557/mrs.2014.304