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3-Dimensional Reconstruction of Catalysts by Scanning Transmission Electron Tomography

Final Activity Report Summary - CATALYSTS TOMOGRAPHY (3-Dimensional reconstruction of catalysts by scanning transmission electron tomography)

Objectives
The aim of the proposal was to use High Angle Annular Dark Field (HAADF) signal recorded under Scanning Transmission Electron Microscopy (STEM) illumination to acquire tomography tilt series of a number of catalytic systems. The alignment, reconstruction and visualisation of these data can provide unique information of the three dimensional distribution and morphology of the metallic nanoparticles over the support.

The host group at Cambridge University pioneered the use of HAADF-STEM images for Electron Tomography in materials science instead of bright field to avoid the problems arising from diffraction contrast during the data processing. The intensity seen in HAADF-STEM images is monotonically dependent on the sample thickness (close to linear) and, to a good approximation, proportional to the square of the atomic number, Z, of the material (the technique is also known as 'Z-contrast' imaging).

The catalyst used in heterogeneous catalysis for multiple applications are constituted by nanoparticles with high atomic numbers (e.g. Pt, Ru) distributed within a framework of low atomic number (SiO2, C). Thus a Z contrast image from such a system identifies the nanoparticles easily. This makes the data ideal for reconstruction. This kind of images can be used as well to characterise the shape of the oxide nanoparticles employed as support in some applications (e.g. supports based in CeO2, CeZrO4 or TiO2 for Three Way Catalysts, Solid Oxide Fuel Cells, etc).

To reconstruct the 3- dimensional image of an object, a series of HAADF-STEM images tilted 1 or 2 degrees relative to each other have to be acquired between high tilt angles , say +-80 degrees Celsius. These images need to be aligned and reproyected to obtain the 3 dimensional object.
Once the Fellow reached the experimental level of expertise in electron tomography of the host institution, the objective was to apply this knowledge to the field of heterogeneous catalysis to push the HAADF electron tomography approach to its limit to specify details of time-dependent morphology and spatial distribution of the nanoparticles catalysts.

Results
This methodology was successfully applied to the study of several catalytic systems in collaboration with other universities and research institutions. Some of these studies were focussed on the characterisation of the composition, morphology and three dimensional distribution within the support of a series of multimetallic catalysts PtRuSn,SnRu4 and PtRu4. An important effort was also made to characterise the morphology and faces exposed on the surface of small particles (between 5 and 30 nm) of CeO2 and CeZrO4 used as support and quantum dots of CdSe.
During this second year of the Fellowship a considerable effort was made to write a review article about tomography in a high impact index journal together with other members of the group. This review article provides a number of examples of nanotomography across the chemical, biological and materials sciences together with the summary of the principles of the technique.