Mesoporosity of Zeolite Y: Quantitative Three-Dimensional Study by Image Analysis of Electron Tomograms

Heterogeneities of the nanostructure of platinum/zeolite y catalysts revealed by electron tomography

To develop structure-performance relationships for important catalysts, a detailed characterization of their morphology is essential. Using electron tomography, we determined in three dimensions the structure of Pt/zeolite Y bifunctional catalysts. Optimum experimental conditions enabled for the first time high-resolution 3D imaging of Pt particles as small as 1 nm located inside zeolite micropores. Semiautomated image analysis of 3D reconstructions provided an efficient study of numbers, size distributions, and interparticle distances of thousands of Pt particles within individual zeolite crystals. Upon extending this approach to a number of zeolite crystals of one batch of Pt/zeolite Y catalyst, heterogeneities were revealed. The Pt loading, an important parameter for catalyst performance, varied between zeolite crystals up to a factor of 35. This discovery calls for re-evaluation of catalyst preparation methods and suggests potential for lowering the nominal loading with noble metals.

Tracing the migration history of metal catalysts in metal-assisted chemically etched silicon

Three-dimensional (3D) visualization of complex embedded nanopore networks in silicon requires expensive machinery and tedious sample preparation procedures such as electron tomography, also known as 3D transmission electron microscopy. In this article, we report a new, fast, powerful, and low-cost three-dimensional imaging technique with sub-5 nm resolution. This new imaging method is applied to metal-assisted chemically etched monocrystalline Si to demonstrate its capabilities. The AFEI (atomic layer deposition-fill-etch-imaging) technique consists of three simple process steps that are available in most material research settings. First the porous substrate is conformally coated with an atomic layer deposition (ALD) metal oxide layer. ALD is able to penetrate deep into complex, high aspect ratio pores, as it is a sequential gas-phase deposition process. Next, the cross-section of the ALD-filled porous Si substrate is etched with high-density fluorine-based plasma processing, which yields very high selectivity toward Si (e.g., >400:1 for Si:ZnO). This step removes the bulk Si and exposes the metal oxide structures grown inside the pores. In the last step, the sample cross-section is examined using a standard scanning electron microscope at various angles, which allows precise imaging of hidden features and reconstruction of a 3D model of the embedded pore network.

Recent advances in the application of electron tomography to materials chemistry

Nowadays, tomography plays a central role in pureand applied science, in medicine, and in many branches of engineering and technology. It entails reconstructing the three-dimensional (3D) structure of an object from a tilt series of two-dimensional (2D) images. Its origin goes back to 1917, when Radon showed mathematically how a series of 2D projection images could be converted to the 3D structural one. Tomographic X-ray and positron scanning for 3D medical imaging, with a resolution of ∼1 mm, is now ubiquitous in major hospitals. Electron tomography, a relatively new chemical tool, with a resolution of ∼1 nm, has been recently adopted by materials chemists as an invaluable aid for the 3D study of the morphologies, spatially-discriminating chemical compositions, and defect properties of nanostructured materials. In this Account, we review the advances that have been made in facilitating the recording of the required series of 2D electron microscopic images and the subsequent process of 3D reconstruction of specimens that are vulnerable, to a greater or lesser degree, to electron beam damage. We describe how high-fidelity 3D tomograms may be obtained from relatively few 2D images by incorporating prior structural knowledge into the reconstruction process. In particular, we highlight the vital role of compressed sensing, a recently developed procedure well-known to information theorists that exploits ideas of image compression and "sparsity" (that the important image information can be captured in a reduced data set). We also touch upon another promising approach, "discrete" tomography, which builds into the reconstruction process a prior assumption that the object can be described in discrete terms, such as the number of constituent materials and their expected densities. Other advances made recently that we outline, such as the availability of aberration-corrected electron microscopes, electron wavelength monochromators, and sophisticated specimen goniometers, have all contributed significantly to the further development of quantitative 3D studies of nanostructured materials, including nanoparticle-heterogeneous catalysts, fuel-cell components, and drug-delivery systems, as well as photovoltaic and plasmonic devices, and are likely to enhance our knowledge of many other facets of materials chemistry, such as organic-inorganic composites, solar-energy devices, bionanotechnology, biomineralization, and energy-storage systems composed of high-permittivity metal oxides.

Visualization of hierarchically structured zeolite bodies from macro to nano length scales

A major challenge in the implementation of laboratory-designed catalysts is the scale-up into technically relevant forms. Advanced characterization is essential to understand and optimize catalyst assembly and function in industrial reactors. This Article presents an integrated approach to visualizing millimetre-sized extrudates and granules of a hierarchical MFI-type zeolite, displaying trimodal networks of micropores (0.56 nm), intracrystalline mesopores (∼10 nm) and macropores (∼200-300 nm). As exemplified for the conversion of methanol to olefins, the hierarchical zeolite yields a superior performance compared to its conventional analogue. The combination of dedicated specimen preparation with state-of-the-art optical, X-ray and electron-based microscopic and tomographic techniques proves a powerful methodology to reveal otherwise inaccessible information regarding structural organization over the whole range of length scales. It is expected that these tools will play a crucial role in the rationalization of scale-up principles in catalyst development.