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Elektronenmikroskopische Untersuchungen an mehrschichtigen Nanopartikeln

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Swertz,  Ann-Christin
Service Department Lehmann (EMR), Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Swertz, A.-C. (2017). Elektronenmikroskopische Untersuchungen an mehrschichtigen Nanopartikeln. PhD Thesis, Bergische Universität Wuppertal, Wuppertal.


Cite as: http://hdl.handle.net/21.11116/0000-0001-1651-B
Abstract
Electron microscopy is a powerful and often used analytical method for the characterisation of nanomaterials and nanostructured catalysts. Different techniques can provide information about the structure, morphology and chemical composition of the material. For the design of novel materials and catalysts it is essential to understand their structure, as structure and reactivity are closely related. New developments in electron microscopy, like aberration correction, allow the characterisation of complex catalyst nanoparticles even with a small particle size (≤5 nm). In this work the application of electron microscopy methods for the investigation of three different catalyst systems is shown. The catalysts have been analysed by means of high-resolution imaging, electron diffraction and energy-dispersive X-ray analyses (EDX) with high spatial resolution. A brief summary of the obtained results for each of the three analysed systems is given below. Bimetallic PtCo nanoparticles encapsulated in hollow carbon spheres show a remarkably high activity as catalyst for the conversion of 5-hydroxymethylfurfural to 2,5-dimethylfuran, an interesting liquid fuel derived from biomass. The synthesis of this catalyst follows a simple strategy with three major steps: Formation of platinum nanoparticles encapsulated in hollow polymer shells (Pt@HPS), introduction of cobalt ions to the polymer shells (Pt@HPS-Co2+), followed by carbonisation of the shells and alloying of the bimetallic particles (PtCo@HCS). The intermediates and products from this synthesis were characterised using aberration corrected (scanning) transmission electron microscopy and EDX elemental mapping. The results of these characterisations lead to a better understanding of the reaction mechanism of the formation of PtCo@HCS and help to generalise this synthesis method to other bi- or multimetallic nanoparticles. Furthermore with the comparison of the structures of freshly prepared catalysts and samples used for a few catalytic cycles it is possible to obtain information about the stability of PtCo@HCS under reaction conditions. Metallic nanoparticles, especially gold, are known to offer a wide range of different shapes of single-crystalline or multiply twinned particles. In this work metallic nanoparticles of Au, Pt and Au@Pt have been analysed regarding their shape and crystal defects using high-resolution transmission electron microscopy (HR-TEM). HR-TEM images of the Pt-nanoparticles show spherical particles with an average diameter of 3 nm. For the Au-particles with an average diameter of 11 nm a great variety of different shapes could be observed. For the discussion of the observed shapes, models of common shapes of gold nanoparticles have been created. Selected area electron diffraction patterns (SAED) and Fast Fourier Transforms (FFT) were used to confirm the fcc-structure of single-crystalline particles. The Au@Pt core shell particles show similar shapes as observed for the Au particles. FFTs of single-crystalline Au@Pt particles synthesised with sodium citrate as reducing agent show an fcc-structure. The determined lattice constant for these crystals is in good agreement with the constants of platinum and gold crystals. However for single-crystalline Au@Pt particles reduced with sodium boron hydride no reasonable lattice constant for an fcc-structure could be determined. To explain this discrepancy a theoretical hcp-cell for Au and Pt was designed to compare calculated and measured lattice spacings. The measured lattice spacings from SAED patterns and FFTs are in good agreement with the theoretical spacings so that the structure of these Au@Pt particles is expected to be hcp rather than fcc. Pt-Ni particles embedded in hollow graphitic spheres are known as highly active catalysts for the oxygen-reduction reaction in fuel cell applications. The analysis of these particles with an average size of 3.5 nm was performed using an aberration corrected ultrahigh-resolution scanning transmission electron microscope. EDX line scans with a high spatial resolution (≤0.2 nm) allow the characterisation of individual nanoparticles. Before electrochemical degradation the structure of these particles could be identified as alloy with a homogenous distribution of Pt and Ni throughout the particles. After electrochemical degradation the line profiles evidence the formation of a core-shell like structure with a Pt-Ni alloy core surrounded by a 0.5-1 nm thin Pt-rich shell. To confirm the experimental data, theoretical EDX line profiles of Pt-Ni particles with different structures were calculated based on Monte-Carlo simulations. The simulated line scans of PtNi@Pt particles (0.5 nm shell and 3 nm core) show profiles similar to the experimental profiles of particles after electrochemical degradation. This agreement supports the formation of core-shell particles during the degradation of the catalyst.