Scanning transmission electron limitations

The dedicated scanning transmission electron microscope (STEM) is an integral tool for characterizing catalysts because of its unique ability to image and analyze nano-sized volumes. This information is valuable in optimizing catalyst formulations and determining causes for reduced catalyst performance. For many commercial catalysts direct correlations between structural features of metal crystallites and catalytic performance are not attainable. When these instances occur, determination of elemental distribution may be the only information available. In this paper we will discuss some of the techniques employed and limitations associated with characterizing commercial catalysts. 

Figure 17.3.2 Detection limits, sampling depth, and spot size for several surface spectroscopic techniques. XRP (x-ray fluorescence) EMP (electron microprobe) EEL (electron energy loss), SAM (scanning Auger microprobe) STEM (scanning transmission electron microscopy). Other abbreviations in Figure 17.3.1. This figure is meant to provide a graphic summary of the relative capabilities of different methods modem instmments have somewhat better quantitative performance characteristics than the 1986 values given here. [From A. J. Bard, Integrated Chemical Systems, Wiley, New York, 1994, pp. 103, with permission adapted from Texas Instmments Materials Characterizations Capabilities, Texas Instmments, Richardson, TX, 1986, with permission.]...

The scanning transmission electron microscope concept was pioneered by von Ardenne at the same time as the early development of the TEM. However, his efforts were limited by the low brightness of available sources, lack of suitable... 

Wall, J. S. (1978). Limits on visibility of single heavy atoms in the scanning transmission electron microscopy—an experimental study. Chem. Scrip. 14, 271-278. 

Particle Induced X-Ray Emission (PKE) is similar to EDS analysis except that it uses high-energy particles instead of electrons to create the vacancies that lead to X-ray emission. It offers orders of magnitude better detection limits for trace elements than does EDS. Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) require extensive sample preparation. When coupled with another technique (EDS for example), they can be useful in determining the nature of surface films or in illuminating the microstructural causes of corrosion. Environmental Scanning Electron Microscopes (ESEM) that are equipped with EDS spectrometers allow SEM-EDS type data to be obtained from samples in the hydrated state. 

A titania-coated silica, as prepared and calcined, was used as support to prepare two An catalysts. The catalysts were characterized by several techniques. X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and tested in the CO Preferential Oxidation (CO PROX) reaction. The dispersion of Ti onto the silica was veiy homogeneous. On the other hand, the incorporation of Au was limited to less than 1 % and the particles size varied in the range 3-10 ran in both samples. Reaction studies were carri out on a fix bed reactor and on a Temporal Analysis of Products (TAP) reactor. The results reveal the participation of the surface -OH groups on the mechanism of the selective CO oxidation. 

---