Quantitative 3D-analysis of real pore structures using advanced electron tomography techniques

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The aim of the project (over the full 6 years of the SPP) is to apply and further develop advanced electron tomography (ET) techniques for quantitative 3D analysis of porous structures for catalytic applications with the goal to (i) contribute to an optimized synthesis and design of the catalyst and (ii) directly use the 3D data for modelling and simulation of relevant diffusion and reaction processes. The project has a strong methodological focus and aims at implementing ET as a valuable tool for evaluating and improving pore structures for catalytic applications. Apart from ET the project employs other transmission electron microscopy (TEM) methods as well as scanning electron microscopy (SEM) and focused ion beam (FIB) techniques. In the first funding period a dedicated 360° ET holder was installed and put into operation, which allows taking tilt series across the full tilt range thus avoiding missing wedge artefacts typically present in 3D reconstructions derived from ET. Parallel to this new implementation conventional ET has been applied to obtain quantitative data of the 3D pore structure of mesoporous TiO2 layers. Such data have already been used in modelling studies of gas diffusion through such real pore systems. In the second funding period FIB pillar preparation has been established and applied to mesoporous structures for 360° ET giving improved 3D data of the pore system without missing wedge artefacts. Moreover, a new preparation technique using manipulation tools in a FIB has been devised which enable the transfer of individual (porous) micro and nanoparticles from a TEM grid onto the tip of a 360° tomography stub thus enabling 360° tomography of such pre-selected particles without any further preparation. The method has been successfully demonstrated for micrometer sized zeolite particles with open macropores as well as for mesoporous hematite nanoparticles (50 - 100 nm) showing closed pores and resulted in excellent 3D reconstructions of the pore network based on 360° ET. The main goal of the third funding period is to further establish the combined application of individual particle transfer and 360° ET and to apply this new technique to porous particles (e.g. zeolites) loaded with catalytic nanoparticles (e.g. Pt, Co3O4) for specific catalytic reactions. Such particle systems will be provided by partners within the SPP. Using 360° ET in tandem with advanced high-resolution TEM technique as well as electron diffraction is expected to precisely reveal the 3D structure of such support particles enabling in particular the determination of the exact locations and accessibility of catalytic nanoparticles within the porous network. Such investigations will help, on the one hand, to evaluate and optimize the fabrication route of such loaded particles. On the other hand, the 3D data will be used as input for modelling of hierarchical catalyst structures using such particles as fundamental building blocks.