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

Subject Area Technical Chemistry
Term from 2011 to 2018
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 206081447
 
Final Report Year 2019

Final Report Abstract

The aim of the project over the entire duration of 6 years was to use advanced electron tomography (ET) techniques and develop them methodologically in a targeted manner in order to obtain quantitative 3D data of real pore structures that can be used for (i) the evaluation and optimisation of manufacturing processes and (ii) model considerations and calculations on the properties of pore systems in catalytic applications. The project had a strong methodological focus and was able to demonstrate the profitable use of ET for the development of optimised pore structures using model systems in cooperation with the partners in the SPP and to show the long-term potential of this approach. In addition to ET, other transmission electron microscopy (TEM) methods, but also scanning electron microscopy (SEM) procedures and FIB/SEM techniques (FIB = Focused Ion Beam) were used for this purpose. In the course of the first funding period, a special 360° ET holder was procured and put into operation. In parallel, tomographic analyses were carried out on mesoporous TiO2 layers using conventional ET. The resulting 3D data could be used as a basis for simulation calculations on gas transport in the real pore system [2]. In contrast to conventional ET, the 360° ET holder allows the use of the complete tilt-angle range and thus, in principle, an artefact-free reconstruction of the 3D pore structure. The feasibility of 360° ET as well as preliminary investigations on a FIB-prepared pillar sample of mesoporous TiO2 and on mesoporous nanoparticles (NP) could already be demonstrated in the first funding period. In the second funding period, the FIB preparation of dedicated pillar samples for 360°-ET was systematically optimised and conventional and new reconstruction methods (compressed sensing, CS) were applied comparatively. However, at the centre of the methodological developments was the elaboration and establishment of a novel preparation method with which individual (porous) micro- or nanoparticles can be specifically placed on a tailored tip and subsequently examined by means of 360°-ET with regard to their internal pore structure. This methodological development was motivated by the fact that in several subprojects of the SPP, porous micro- and nanoparticles were used as elementary building blocks for the construction of hierarchical pore structures and that these particles are accessible to direct 3D analysis using 360°-ET due to their dimensions. The method was successfully applied to three model systems of different particle size, namely spherical, colloidal polystyrene clusters (4-5 µm), mixed macro-/microporous zeolite particles of larger dimension (1-3 µm), as well as mesoporous hematite NP of smaller dimension (< 100 nm). Questions regarding the interconnectivity of the respective pores or the arrangement of the primary particles could be clarified. During the third funding period, the methodological developments of the first and second funding periods were specifically used to investigate selected pore systems from cooperating projects with regard to their 3D structure. In addition to the analysis of porous carriers without loading [8,12], porous carriers loaded with catalytic NP were addressed. Here, the segmentation of the tomographic data with regard to a reliable differentiation of the present materials (porous carrier material, catalytic NP) posed an additional challenge. In order to make efficient use of the methodological developments of the second funding period, the focus was on micro- and micro-/mesoporous zeolite particles, which were produced using different processes and loaded with catalytically active nanoparticles (metal or metal oxide particles). Possible problems of radiation damage (e.g. shrinkage of the particle due to compaction under the electron beam) were countered with a reduced number of projections (reduced dose) and a greatly reduced sample temperature using Cryo-ET. Apart from this dedicated project work, the equipment and expertise in the fields of TEM, SEM and FIB/SEM available within this subproject was used to support other projects by means of detailed microscopic analyses [1,5]. Furthermore, the methodological developments and gathered expertise in using ET techniques within the SPP 1570 found application in the 3D investigation of naturally-occurring macroporous photonic crystal structures in butterfly wing scales [4,6]. In addition to their scientific impact through peer-reviewed publications and contributions on international conferences, the research results of this subproject also attracted public outreach on open days of the university (“Lange Nacht der Wissenschaften”), lab tours for pupils and students, press releases in print media and blog articles.

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