A high-angle annular detector provides a useful imaging signal for the detection of small metal catalyst clusters in light-element supports. The high-angle annular-dark-field (HAADF) signal is mainly generated by quasi-elastic scattering. The collected signal intensity depends on the atomic number of the scatterer, its amplitude of vibration and the total number of atoms encountered by the probe. This imaging mode can reveal high-Z sensitivity and eliminate most of the diffraction contrast present in low-angle ADF images. In a STEM instrument, secondary electron images with a resolution approaching the probe size ( ∼ 0.5 nm in diameter) can be obtained simultaneously with HAADF images, providing topographic information about the metal catalyst-support system. The combination of a variety of imaging and diffraction techniques available in a STEM instrument can yield important information about the distribution of small metal clusters in the supports, crystallographic relationships between the metal clusters and the support, and morphology as well as chemical compositions of the metal catalyst-support system. Samples of Pt/carbon, Pt/γ-alumina and Pt/NaY zeolite catalysts have been examined. Preliminary results reveal useful information about these catalyst systems.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Atomic and Molecular Physics, and Optics