Microanalysis of Supported

IMAGING AND MICROANALYSIS OF SUPPORTED METAL CATALYSTS IN THE ANALYTICAL ELECTRON MICROSCOPE 

In this chapter, we will only present a very basic introduction to some of the most important techniques available in these versatile instruments for the study of supported metal catalyst particles. Our intention here is not to present an exhaustive review, but rather to simply illustrate, by way of some examples, the types of information 

Since the specimen is thin compared to the mean free path of the incident electrons, most will not interact at all with the constituent atoms of the sample but will simply pass undeflected through the material. However, provided the sample is crystalline, some fraction of the incident electrons will be scattered from crystal planes within the material by Bragg diffraction, and give rise to characteristic spots or rings in an electron diffraction pattern. These diffracted electrons lose little or none of their incident energy in such Bragg scattering events and are said to be elastically scattered. 

F re 3.1 (a) A spot diffraction pattern from a single crystal of gold (b) A ring 

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This work was supported by the National Science Foundation under Grant DMR 1 9-20538, Microscope facilities were provided by the University of Illinois Center for Microanalysis of Materials which is supported as a national facility under National Science Foundation Grant DMR 89-20538 and DOE contract DE AC02-76-ER0M98. 

A transmission electron microscope (PHILIPS EM 420) equipped with an X-ray spectral analyzer (EDAX) was used to obtain the selected area electron diffraction pattern and for microanalysis of the catalyst. Speciments for microanalysis were prepared by dispersing the powders in ethanol, placing a drop of this suspension on a thin carbon support net and allowing the solvent evaporate. 

This work was partially funded by the United States Air Force Office of Scientific Research under grant number F49620-01-1-500. It was carried out in the Center for Microanalysis of Materials, of the Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, which is partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439. 

The 3 mm disc was covered by a 3 pm thick polycrystalline nickel foil, then a polycrystalline molybdenum disc was placed on top of the nickel foil. This composite sandwich was placed between two alumina supports and heated for 30 min at 1350°C. The cross-sections used for microanalysis were prepared by cutting with a wire saw in a direction perpendicular to the grain boundary, then mechanically and chemically polished. 

Another traditional method used for polymer support characterization is elemental analysis. Its use as an accurate quantitative technique for monitoring solid-phase reactions has also been demonstrated [146]. Microanalysis can be extremely valuable if a solid-phase reaction results in the loss or introduction of a heteroatom (usually N, S, P or halogen). In addition, this method can be used for determination of the loading level of a functional group (e. g. usually calculated directly from the observed microanalytical data). For example, in many cases, the displacement of chloride from Merrifield resin has been used as a guide to determine the yield of the solid-phase reaction. 

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