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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.
