Electron microscopy, analytical particles

Analytical electron microscopy (AEM) permits elemental and structural data to be obtained from volumes of catalyst material vastly smaller in size than the pellet or fluidized particle typically used in industrial processes. Figure 1 shows three levels of analysis for catalyst materials. Composite catalyst vehicles in the 0.1 to lOim size range can be chemically analyzed in bulk by techniques such as electron microprobe, XRD, AA, NMR,... 

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 1. Three levels of analysis for catalyst materials, a) bulk analysis of an entire catalyst pellet, b) surface analysis and depth profiling from the surface inward, c) analytical electron microscopy of individual catalyst particles too small for analysis by other techniques.

Analytical Electron Microscopy of Heterogeneous Catalyst Particles... 

Analytical electron microscopy of individual catalyst particles provides much more information than just particle size and shape. The scanning transmission electron microscope (STEM) with analytical facilities allows chemical analysis and electron diffraction patterns to be obtained from areas on the order of lOnm in diameter. In this paper, examples of high spatial resolution chemical analysis by x-ray emission spectroscopy are drawn from supported Pd, bismuth and ferric molybdates, and ZSM-5 zeolite. 

The origins of analytical electron microscopy go back only about 15 years when the first x-ray spectra were obtained from submicron diameter areas of thin specimens in an electron microscope [1]. Characterization of catalyst materials using AEM is even more recent[2,3] but is currently a very active research area in several industrial and academic laboratories. The primary advantage of this technique for catalyst research is that it is the only technique that can yield chemical and structural information from individual submicron catalyst particles. 

Analytical electron microscopy has been shown to be an effective technique for the chemical analysis of catalyst particles. In some cases AEM may be the only technique to provide chemical profiles across small particles. Analysis of thin sections of... 

This chapter discusses the range of analytical methods which use the properties of X-rays to identify composition. The methods fall into two distinct groups those which study X-rays produced by the atoms to chemically identify the elements present, and X-ray diffraction (XRD), which uses X-rays of known wavelengths to determine the spacing in crystalline structures and therefore identify chemical compounds. The first group includes a variety of methods to identify the elements present, all of which examine the X-rays produced when vacancies in the inner electron shells are filled. These methods vary in how the primary vacancies in the inner electron shell are created. X-ray fluorescence (XRF) uses an X-ray beam to create inner shell vacancies analytical electron microscopy uses electrons, and particle (or proton) induced X-ray emission (PIXE) uses a proton beam. More detailed information on the techniques described here can be found in Ewing (1985, 1997) and Fifield and Kealey (2000). 

Webb, S.M., Leppard, G.G., and Gaillard, J.-F., Zinc speciation in a contaminated aquatic environment Characterization of environmental particles by analytical electron microscopy, Environ. Sci. Technol., 34, 1926, 2000. 

Studies of ground and redispersed cement pastes by analytical electron microscopy (L28-L30,T17) showed wide variations between individual analyses, even within single particles of micrometre dimensions, and gave mean Ca/Si ratios of 1.5-2.0 for the C-S-H. The mean ratios of minor elements... 

The hydrated material has been analysed by X-ray microanalysis and analytical electron microscopy. In a 3-day old paste, that formed in situ from alite or belite did not differ significantly in composition from the corresponding product in pure Portland cement pastes (H4). but at later ages Ca/ Si is lower and Al/Ca higher (R25,R26,T44,U 17,U 18,R42). Ca/Si is typically about 1.55, but the value decreases with age and ratio of pfa to clinker. Uchikawa (U20,U17) reported a value of 1.01 for a 4-year-old paste with 40% replacement of cement by pfa. Several of the studies (R25,T44,U20,U 17) showed that the C-S-H was higher in alkalis if pfa was present, but one cannot tell to what extent potassium or sodium apparently present in the C-S-H has been deposited from the pore solution on drying. For material close to the pfa particles in a 10-year-old mortar. Sato and Furuhashi (S92) found a Ca/Si ratio of 1.1-1.2. 

In a previous paper (Anderson and Benjamin accepted for publication in Environmental Science and Technology), surface and bulk characteristics of amorphous oxides of silica, aluminum, and iron, both singly and in binary mixtures were described. The solids were characterized with an array of complementary analytical and experimental techniques, including scanning electron microscopy, particle size distribution, x-ray photoelectron spectroscopy (XPS),... 

In contrast to XRD methods that may introduce sample preparation artifacts (see Jiang et al. 1997 Li et al. 1998), TEM integrated with selected-area electron diffraction (SAED) and energy dispersive spectrometry (analytical electron microscopy, AEM) measurements, provides direct, in situ observations on rock microtextures, crystallite size distributions, lattice imperfections of crystallites and interstratification (see the extensive reviews by Peacor 1992 and Merriman and Peacor 1999). TEM observations on selected portions of thinned (ion-milled) whole rock samples contradict the fundamental particle theory of Nadeau et al. (1984a,b,c summarized recently by Nadeau 1998). The observations show that phyllosilicate domains with interstratified structures form coherent boundaries, and therefore, MacEwan-type crystallites do exist in quasi-undisturbed rocks (Peacor 1998). In addition, AEM studies may provide reliable mineral-chemical data on the phases devoid of any external or internal impurities. 

In a lung biopsy of a female aged 31 years, who had poUshed spectacle lenses with cerium oxide for three years, Sinico et al. (1982) found multiple macrophagic granulomas containing dust particles identified by analytical electron microscopy as cerium oxide. 

Analytical electron microscopy (AEM) permits the determination of the elemental composition of a solid catalyst at the microscopic level by energy-dispersive detection of the electron-induced X-ray emission (EDX). Energy dispersive spectroscopy (EDS) is sensitive for elements with atomic numbers Z > 11. For lighter elements (Z < 11), electron energy loss spectroscopy (EELS) is applied. An example is shown in Figure 7 (bottom), which exhibits the elemental composition by EDX of two individual Pt/Rh particles on a carbon film. This analysis clearly demonstrates the heterogeneous composition of the alloy particles. 

Basic analytical techniques such as light microscopy examination of histological slides (Fig. 3.4.1a,b) are easily accessible, quick, inexpensive and provide a very rough estimate of the particulate burden, which maybe sufficient for the pathological or clinical workup of a case. More specialised particle-by-particle determination using analytical electron microscopy (Fig. 3.4.1c) can provide extremely precise quantitative data about particle concentrations, types and sizes. As a general rule, these procedures can only be performed routinely in a small number of dedicated laboratories and are expensive and time consuming. 

The use of mixed-metal clusters has received attention, and it has recently been demonstrated, using high resolution analytical electron microscopy, that under certain conditions it is possible to obtain very small bimetallic particles having the same bulk composition as that of the starting cluster [26]. However, in many cases attempts to produce alloy particles have been frustrated by the predominance of segregation [27-31]. This general approach may nevertheless provide a valuable route to the production of metal particles in intimate contact with a monolayer of a metal oxide, itself coated onto a high surface area oxide. 

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