Analytical electron microscopy catalysts

Analytical electron microscopy permits structural and chemical analyses of catalyst areas nearly 1000 times smaller than those studied by conventional bulk analysis techniques. Quantitative x-ray analyses of bismuth molybdates are shown from lOnm diameter regions to better than 5% relative accuracy for the elements 61 and Mo. Digital x-ray images show qualitative 2-dimensional distributions of elements with a lateral spatial resolution of lOnm in supported Pd catalysts and ZSM-5 zeolites. Fine structure in CuLj 2 edges from electron energy loss spectroscopy indicate d>ether the copper is in the form of Cu metal or Cu oxide. These techniques should prove to be of great utility for the analysis of active phases, promoters, and poisons. 

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 (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

From the Co EXAFS results alone one cannot conclude whether the Co atoms are located at edges or basal planes but a comparison of the Co EXAFS data with the above Mo EXAFS results indicates that the edge position is the most likely one. This Co location is illustrated in Figure T For the unsupported catalysts, many of these "surface" positions may be present at internal edges (i.e., at the "domain" boundaries). Recently, direct evidence confirming the edge position has been obtained by combining MES results (to ensure that Co is present as Co-Mo-S in the samples studied) with ir spectroscopy (lU) or with analytical electron microscopy (l ) ... 

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. 

Although x-ray microanalysis in the STEM is the most developed form of analytical electron microscopy, many other types of information can be obtained when an electron beam interacts with a thin specimen. Figure 2 shows the various signals generated as electrons traverse a thin specimen. The following information about heterogeneous catalysts can be obtained from these signals ... 

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... 

Analytical Electron Microscopy Investigations of Equilibrium Catalyst Fractions... 

Characterization thus involves analytical electron microscopy, ordinary microprobe analysis or other techniques for localizing elements or chemical compounds (Scanning Auger Spectroscopy, Raman Microprobe, Laser Microprobe Mass Spectrometry). It also requires, in most cases, some physical separation of the catalyst for separate analysis (e.g., near surface parts and center of pellets, by peeling or progressive abrasion pellets present at various heights in the catalyst bed, etc.). 

A multifaceted characterization effort to stndy these materials as a function of thermal treatment has been undertaken. The techniques include BET surface area measurements. X-ray diffraction, chemisorption, scanning and high-resolution transmission electron microscopy, analytical electron microscopy, neutron activation analysis, atomic absorption spectroscopy, FTIR and isotopic tracer studies. The details of catalyst preparation have been previously... 

Furthermore, we investigated the effect of water on the activity of the above-mentioned sulfated zirconia catalysts and the observed activities were compared. We have extensively characterized the different catalysts by XPS, physical adsorption, analytical electron microscopy, and thermogravimetry. 

By XAS investigations of model and automotive catalysts (Pt, Rh / Ce02 / AI2O3) associated with analytical electron microscopy, it is shown that for both model and aged catalysts treated in air at high temperatures, true alloyed phases were formed. 

B.R. Powell "Analytical Electron Microscopy of a Vehicle Aged Automotive Catalyst", Applied Catal., 53, 233-250,1989. 

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. 

Gallezot P, Leclercq C. Characterization of catalysts by conventional and analytical electronic microscopy. In Imelik B, Vedrine JC, editors. Catalyst characterization, physical techniques for solid materials, fundamental and applied catalysis. Heidelberg Springer 1994. p. 509-58. 

Powell, B.R. and Chen, Y.-L. (1989) Analytical electron microscopy of a vehicle-aged automotive catalyst. Appl. Catal., 53, 233. 

The structure of the catalysts was characterized by X-ray diffraction, IR-spectroscopy and transmission electron microscopy, their thermal stability was followed by thermal analytical method. The specific surface area and pore size distribution of the samples were determined by nitrogen adsorption isotherms. 

Electron probe and X-ray fluorescence methods of analysis are used for rather different but complementary purposes. The ability to provide an elemental spot analysis is the important characteristic of electron probe methods, which thus find use in analytical problems where the composition of the specimen changes over short distances. The examination of the distribution of heavy metals within the cellular structure of biological specimens, the distribution of metal crystallites on the surface of heterogeneous catalysts, or the differences in composition in the region of surface irregularities and faults in alloys, are all important examples of this application. Figure 8.45 illustrates the analysis of parts of a biological cell just 1 pm apart. Combination of electron probe analysis with electron microscopy enables visual examination to be used to identify the areas of interest prior to the analytical measurement. 

A researcher in the field of heterogeneous catalysis, alongside the important studies of catalysts chemical properties (i.e., properties at a molecular level), inevitably encounters problems determining the catalyst structure at a supramolecular (textural) level. A powerful combination of physical and chemical methods (numerous variants x-ray diffraction (XRD), IR, nuclear magnetic resonance (NMR), XPS, EXAFS, ESR, Raman of Moessbauer spectroscopy, etc. and achievements of modem analytical chemistry) may be used to study the catalysts chemical and phase molecular structure. At the same time, characterizations of texture as a fairytale Cinderella fulfill the routine and very frequently senseless work, usually limited (obviously in our modem transcription) with electron microscopy, formal estimation of a surface area by a BET method, and eventually with porosimetry without any thorough insight. 

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