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Characterization of Catalysts by Analytical Electron Microscopy 1
C. E. Lyman
Central Research & Development Department, Experimental Station, Ε. I. du Pont de Nemours & Company, Wilmington, DE 19898 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 10nm diameter regions to better than +5% relative accuracy for the elements Bi and Mo. Digital x-ray images show qualitative 2-dimensional distributions of elements with a lateral spatial resolution of 10nm in supported Pd catalysts and ZSM-5 zeolites. Fine structure in CuL edges from electron energy loss spectroscopy indicate whether 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. 2,3
A n a l y t i c a l electron microscopy (AEM) permits elemental and s t r u c t u r a l data to be obtained from volumes of catalyst m a t e r i a l v a s t l y smaller i n size than the p e l l e t or f l u i d i z e d p a r t i c l e t y p i c a l l y used i n i n d u s t r i a l processes. Figure 1 shows three l e v e l s of analysis for catalyst m a t e r i a l s . Composite catalyst vehicles i n the 0.1 to 10mm size range can be chemically analyzed i n bulk by techniques such as electron microprobe, XRD, AA, NMR, IR, etc. The chemical and physical changes within 3nm of the surface of the p e l l e t or f l u i d i z e d bead can be studied by surface analysis techniques such as AES, XPS, ISS, SIMS, RBS, etc. However, these techniques may not detect important phenomena taking place on the surface of or within the i n t e r i o r of i n d i v i d u a l lnmto Ιμτη-diameter inorganic p a r t i c l e s that are synthesized s p e c i f i c a l l y for t h e i r c a t a l y t i c a c t i v i t y . AEM i s an extremely useful technique for analysis of the i n d i v i d u a l heterogeneous catalyst p a r t i c l e and i t s r e l a t i o n s h i p to various supporting materials. Structural and chemical analyses can be obtained from specimen regions nearly 1000 times smaller than those studied by conventional bulk analysis techniques. This high l a t e r a l s p a t i a l 1
Current address: Department of Metallurgy and Materials Engineering, Lehigh University, Bethlehem, PA 18015 0097-6156/ 85/0288-0361 $06.00/ 0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, Marvin L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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r e s o l u t i o n of analysis down to about 2nm i s a consequence of both the very t h i n specimens used and the specialized equipment employed. In general, AEM should be used i n conjunction with the other techniques mentioned for complete catalyst c h a r a c t e r i z a t i o n . Several previous studies have demonstrated the power of AEM i n various catalyst systems (1-11). Often AEM can provide reasons for v a r i a t i o n s i n a c t i v i t y and s e l e c t i v i t y during catalyst aging by providing information about the location of the elements involved i n the active c a t a l y s t , promoter, or poison. In some cases, d i r e c t quantitative c o r r e l a t i o n s of AEM analysis and catalyst performance can be made, This paper f i r s t reviews some of the techniques for AEM analysis of c a t a l y s t s and then provides some descriptions of applications to bismuth molybdates, Pd on carbon, z e o l i t e s , and Cu/ZnO c a t a l y s t s . A n a l y t i c a l Electron Microscopy Techniques Analysis of i n d i v i d u a l c a t a l y s t p a r t i c l e s less than lPm i n size requires an a n a l y t i c a l tool that focuses electrons to a small probe on the specimen. A n a l y t i c a l electron microscopy i s 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 a n a l y s i s . In most cases, an electron beam current of about 1 nanoampere i s required to produce an a n a l y t i c a l signal i n a reasonable time. Elemental analysis of t h i n specimens i n the AEM can provide a 1000-fold improvement i n s p a t i a l resolution of analysis over conventional electron microprobe analysis. The electron microprobe analyzes c h a r a c t e r i s t i c x-rays produced within a teardrop-shaped volume 1-5vm beneath the surface of a polished section of bulk m a t e r i a l . Thus, the best s p a t i a l resolution of analysis that can be achieved i s on the order of lym. By removing the teardrop-shaped electron d i f f u s i o n region, the s p a t i a l r e s o l u t i o n can be improved to l-20nm i n special cases where the specimen thickness, electron accelerating voltage, beam diameter, and average specimen atomic number are optimum. Figure 2 shows some of the signals available i n the AEM, Imaging signals such as transmitted electrons ( b r i g h t - f i e l d images), Bragg-diffracted electrons ( d a r k - f i e l d images), backscattered primary electrons, secondary electrons, cathodoluminescence ( l i g h t ) , heat, and specimen current are a l l available i n an AEM i f proper detectors are i n s t a l l e d . A n a l y t i c a l signals (12,13) such as electron energy loss electrons, c h a r a c t e r i s t i c x-rays, backscattered electrons, Auger electrons, o p t i c a l l i g h t emission (cathodoluminescence), and electron d i f f r a c t i o n have been used i n various instruments to analyze inorganic materials. Many of these a n a l y t i c a l signals can be used to form q u a l i t a t i v e images or maps of the l o c a t i o n of c e r t a i n elements and phases. Elemental imaging with the AEM i s important for the analysis of c a t a l y s t s because the l o c a t i o n of active phases, promoters, and poisons may not be evident from the normal electron image alone, Once an area of i n t e r e s t i n the t h i n specimen i s located, quantitative analysis of the volume penetrated by the electron beam In Catalyst Characterization Science; Deviney, Marvin L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
LYMAN
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SURFACE ANALYSIS (b)
ANALYTICAL E L E C T R O N MICROSCOPY (c)
Figure 1. Three l e v e l s of analysis for catalyst materials, a) bulk analysis of an entire catalyst p e l l e t , b) surface analysis and depth p r o f i l i n g from the surface inward, c) a n a l y t i c a l electron microscopy of i n d i v i d u a l catalyst p a r t i c l e s too small for analysis by other techniques.
Figure 2. Signals generated i n a t h i n specimen by a focused electron beam i n an a n a l y t i c a l electron microscope.
In Catalyst Characterization Science; Deviney, Marvin L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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can be obtained by several methods. For t h i n specimens, x-ray emission spectroscopy (XES) and electron energy loss spectroscopy (EELS) are the major a n a l y t i c a l modes for chemical analysis. These techniques analyze a through-section c y l i n d r i c a l volume under the electron probe which i s broadened somewhat due to beam spreading i n the t h i n specimen (13). D i f f r a c t i o n methods i n the AEM are important complementary techniques for phase analysis but w i l l not be discussed i n t h i s paper. For XES, q u a n t i t i f i c a t i o n techniques have been developed by C l i f f and Lorimer (14) for correcting the background-subtracted x-ray i n t e n s i t y r a t i o of two elements, I^/I^ by a s e n s i t i v i t y factor known as the C l i f f - L o r i m e r k-value, to y i e l d the elemental r a t i o C /C . Absorption [ACF] and fluorescence [FCF] correction factors are required when the sample i s t h i c k or i f the mass absorption c o e f f i c i e n t s of emitted x-rays are greatly d i f f e r e n t (15). Thus, for a t h i n specimen (generally
