title: 3D structural characterization of mesoporous materials by electron tomography
Creators: name: Huang, Xiaohui
Publisher: name: UNSPECIFIED
Year: 2023

description: Mesoporous materials, with pore size of 2-50 nm, are gaining increasing attention in applications such as gas separation, purification of wastewater, energy storage, drug deliver and catalytic processes due to their high surface area, tunable pore structure and large pore volume. In all of these applications, the desired performance depends strongly on the pore structure, such as pore size, pore length, tortuosity, volume and connectivity. Understanding the structure-performance relationship and exploiting it to the development of advanced mesoporous materials require an unambiguous understanding of the three dimensional (3D) pore structures. This requires reliable relevant characterization techniques. Electron tomography (ET) is a powerful technique to obtain 3D morphological information at the corresponding length scale for mesoporous materials. A very promising feature of ET is that no prior assumptions on the pore shape are needed, which is normally inevitable when using traditional diffraction or diffusion based bulk characterization techniques, allowing it to be particularly suitable for analyzing complex disordered pore structures. However, an accurate quantitative interpretation of the solid/void network from ET is still a challenge. The reconstruction of a 3D volume from a tilt series is mathematically an underdetermined problem due to the discrete sampling and limited practical tilt range, which means that there is no analytical solution and an approximation to the original structure is therefore pursued using appropriate algorithms. In addition, constraints and imperfections (e.g. noise, limited tilting parameters and misalignment of projections) in an experimental setup inevitably cause artifacts and errors in the final reconstruction. The overall aim of the thesis is to develop and evaluate approaches to extract an accurate quantitative 3D morphological description of the pore network in mesoporous materials. To achieve this, two aspects are considered: evaluating the reconstruction performance and improving the experimental methodology. In a fundamental study, the reconstruction accuracy of the three main-stream algorithms simultaneous iterative reconstruction technique (SIRT), total variation minimization (TVM) and discrete algebraic reconstruction technique (DART) were systematically investigated for mesoporous materials using different realistic tilt-series based on a set of phantom simulations. While the reconstruction accuracy has been partially addressed in previous publications focusing on the residual number of misclassified pixels, this analysis has been extended to consider effects on the pore morphology and diffusion properties due to aggregated reconstruction artifacts (inhomogeneously distributed misclassified pixels) thereby developing a more relevant estimate of the reconstruction performance. It was found that DART outperforms the other two methods in reliably revealing small pores and narrow channels, especially when the number of projections and the tilt-range are limited. The accurately segmented reconstruction from DART makes it possible to achieve reliable quantification of pores structure, which in turn leads to a reliable evaluation of effective diffusion coefficients. Moreover, the influence of different acquisition and reconstruction parameters on the reconstructed 3D volume and a quantitative analysis of pore features is discussed. With this, a practical guide for optimizing acquisition and reconstruction parameters and how to evaluate the accuracy when describing the mesoporous structure is provided. As seen in the fundamental parameter study, one of the strongest limitations in electron tomography is the limited tilt-range. To ultimately solve this missing wedge problem, 360° ET can be used by tilting a needle-shaped specimen over the full tilt range and thus filling the missing information. Obviously, the necessity of specimen processing to a needle shape with a diameter of a few tens of nanometer limits this technique for a wide range of materials, e.g. porous materials and any material in form of loose powders. Driven by this consideration, a new universal, yet facile sample preparation method for 360° ET was developed. A single nanoparticle or a few separated nanoparticles are selected in a TEM or SEM and the selected objects with the supporting film are then transferred to an easily prepared sharp tungsten tip, which is mounted to a full-range tomography holder tip. This method shows great flexibility and works for almost all types of powder materials without invasive FIB processing directly on the sample. Test results for 360° tomography are shown using a Pt@TiO2 hollow cage catalyst. Finally, as an example for a real application, ET is applied to uncover the leaching behavior of Pd nanoparticles supported on mesoporous carbon (CMK3) during formic acid decomposition in batch and fixed bed reactors. Using the knowledge from the first part (fundamental study) as guideline, the DART algorithm with optimized reconstruction parameters was used to quantitatively characterize the spatial and size distribution of the Pd nanoparticles in the three Pd@CMK3 catalysts before and after the reaction. A quantitative analysis of the tomographic data enables precise tracking of the evolution of the supported particles with a statistical analysis of the distribution on the internal and external support surface. Based on this quantitative analysis, the evolution of Pd nanoparticles during the catalytic process is discussed and related to the catalytic performance differences observed for the fixed bed and batch reactor. In the future, this information can be used to design catalysts with improved properties to optimize the reaction conditions.