Electron beam, catalyst analysis

Analysis of individual catalyst particles less than IMm in size requires an analytical tool that focuses electrons to a small probe on the specimen. Analytical electron microscopy is usually performed with either a dedicated scanning transmission electron microscope (STEM) or a conventional transmission electron microscope (TEM) with a STEM attachment. These instruments produce 1 to 50nm diameter electron probes that can be scanned across a thin specimen to form an image or stopped on an image feature to perform an analysis. In most cases, an electron beam current of about 1 nanoampere is required to produce an analytical signal in a reasonable time. 

Figure 5 shows the Z-contrast scanning transmission electron microscope (STEM) image of a Ru/Sn02 nanocomposite catalyst prepared by the assembly process [18]. A combined EDX analysis, using an electron beam of... 

Energy-dispersive analysis can be used to perform spot analysis within a catalyst. The electron beam can be... 

It may be taken for granted that analysis for all elements on a catalyst support work equally well. We have performed experiments on many different catalysts and have found that elements such as Cl, K, Na, and S are very sensitive to the electron beam. Cl and S appear to volatilize in the vacuum while K and Na move away from the incident beam. This is especially true when the beam is spotted directly onto the particle versus using the less damaging raster mode. Examples of how elemental analyses of a BaS0A, zeolite, and NaCl particles vary as a function of time are shown in Figure 5. 

Another phenomenon commonly observed in the analysis of catalysts is the deposition of carbon in the area of beam concentration (23). In many instances this can be associated with residual organic material left on the carbon coated grids. We have also found that the catalyst particles themselves may have organic debris on them which subsequently react in the electron beam creating carbonaceous material. Unfortunately, this may disrupt observation of small crystallites and/or the quantitative analysis of the particle. For example, STEM examination of a Fischer-Tropsch catalyst which had wax deposited on the alumina particles containing Ru crystallites was made. Due to the reactivity of the wax in the electron beam the particles turned progressively darker as a function of exposure time, until finally the Ru crystallites were not... 

Polymers have been characterized by electron microprobe analysis to determine profiles of metal and functional groups (30). A catalyst particle (e.g., a spherical bead) Is sectioned and traversed with an electron beam In a vacuum system. The emitted x-ray signals allow quantitative elemental analysis of a roughly 1 )im3 volume of the catalyst. The results indicate the uniformity of incorporation of functional groups and metal In the polymer. 

For surface structure studies, perhaps the most popular technique has been LEED (373). Elastically diffracted electrons from a monoenergetic beam directed to a single-crystal surface reveal structural properties of the surface that may differ from those of the bulk. Some applications of LEED to electrocatalyst characterization were cited in Section IV (106,148,386). Other, less specific, but valuable surface examination techniques, such as scanning electron microscopy (SEM) and X-ray microprobe analysis, have not been used in electrocatalytic studies. They could provide information on surface changes caused by reaction, some of which may lead to catalyst deactivation (256,257). Since these techniques use an electron beam, they can be coupled with previously discussed methods (e.g. AES or XPS) to obtain a qualitative mapping of the structure and composition of a catalytic surface. 

A high percentage of the sputtered secondary particles are neutral and must be post-ionized for mass spectroscopy analysis (SNMS) (62). Post-ionization can be achieved by electron impact in a plasma or by an electron beam. Alternatively, resonant and nonresonant laser ionization can be applied. Applications of SNMS for catalyst characterization have still not been reported. 

Electron probe X-ray micro analysis measurements The distribution of molybdenum element on the used catalyst was measured by JCXA-733, at probe current 3.60x10 A, accelerated volt 25KV and beam diameter 10 tun. 

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