Catalyst Characterization Science - American Chemical Society

29 Atomic Imaging of Particle Surfaces 1 , 3

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L. D. Marks and David J. Smith 1

Department of Physics, Arizona State University, Tempe, AZ 85287 High Resolution Electron Microscope, Department of Metallurgy, University of Cambridge, Cambridge CB2 3RQ, England

Downloaded by UNIV LAVAL on July 3, 2014 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch029

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High resolution electron microscopy has recently demonstrated the capability to directly resolve the atomic structure of surfaces on small particles and thin films. In this paper we briefly review experimental observations for gold (110) and (111) surfaces, and analyse how these results when combined with theoretical and experimental morphological studies, influence the interpretation of geometrical catalytic effects and the transfer of bulk surface experimental data to heterogeneous catalysts. During the l a s t twenty years, small metal p a r t i c l e systems, often model c a t a l y s t s or commercial heterogeneous c a t a l y s t s , have been extensively studied by electron microscopy. The primary objective has been t o characterise t h e i r chemical and s t r u c t u r a l nature, with the intention of eventually understanding the nucleation and growth of small c l u s t e r s as well as heterogeneous c a t a l y s i s . In the process, e s s e n t i a l l y the whole range o f electron microscope imaging techniques have been used. Conventional bright f i e l d and dark f i e l d techniques are i l l u s t r a t e d by the c l a s s i c work o f Ino ( 1_), and Ino and Ogawa (2). More sophisticated dark f i e l d techniques have also been developed which give improved p a r t i c l e contrast, such as selected zone dark f i e l d ( 3 ) , annular dark f i e l d 0 0 , and weak beam dark f i e l d C5). Other approaches, p r i m a r i l y using a scanning trans mission instrument, have also been developed which produce a n a l y t i c a l information (e.g. 6-8). Some recent reviews o f these techniques can be found elsewhere (9-12) and reference should be made t o the a r t i c l e by J.M. Cowley i n these proceedings. A l l o f these methods suffer from one serious shortcoming - the s p a t i a l resolution i s r e l a t i v e l y poor (~1θ8) and information about the c a t a l y t i c a l l y i n t e r e s t i n g region of the p a r t i c l e , namely the surface structure, i s then not a v a i l a b l e . One p a r t i c u l a r technique, high resolution electron microscopy, has f o r many years been slowly 3

Current address: Department of Materials Science and Ipatieff Laboratory, Northwestern University, Evanston, IL 60201 Current address: Center for Solid State Science, Arizona State University, Tempe, AZ 85287

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0097-6156/85/0288-0341 $06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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progressing towards t h i s goal. Although useful r e s u l t s were obtained using the technique to unravel complicated p a r t i c l e structures (13-17), surface information remained unavailable except for a few unusually favorable circumstances (e.g. 18). As a r e s u l t of t e c h n i c a l improvements (1£) to the Cambridge High Resolution Electron Microscope (20), we have recently succeeded i n d i r e c t l y resolving the atomic structure of surfaces on small p a r t i c l e s and t h i n f i l m s (21-27)* This has included the f i r s t d i r e c t observation of a surface reconstruction, the so-called missing row model (28) of the 2x1 (110) gold surface (see Figure 1 and 21,22), e f f e c t s due to e l a s t i c and p l a s t i c deformations at surfaces (see Figures 2 and 3 and 23,26) and d e t a i l s of surface steps and facetting (see Figures 4,6 and 21,27). In t h i s paper we b r i e f l y describe the p r i n c i p l e of the technique, review these observations, and consider t h e i r implications with respect to geometrical e f f e c t s , l i n k i n g the experimental data with t h e o r e t i c a l analyses. Basis of the Technique The technique employs a beam of s w i f t (~500kV) electrons grazing the surface of i n t e r e s t . Provided that the beam i s accurately aligned along a c r y s t a l zone a x i s , and that the e l e c t r o n - o p t i c a l imaging system i s adequate, then images are obtained which appear to show the atomic surface structure i n p r o f i l e (see Figure 1). Interpretation of these images i s both complicated and simple. With any electron microscope technique, the f i n a l image i s the r e s u l t of a complicated d i f f r a c t i o n and lens aberration process and i t i s necessary to avoid the trap of naive i n t e r p r e t a t i o n , that seeing i s b e l i e v i n g . I t i s generally accepted that detailed c a l c u l a t i o n s are required to confirm image i n t e r p r e t a t i o n s , p a r t i c u l a r l y for high resolution imaging, but also f o r other techniques. Fortunately, i n most cases, high r e s o l u t i o n images are f a i t h f u l representations of the surface structure. The reasons for t h i s are discussed i n d e t a i l i n 25,29,30, and can be summarised thus. With s w i f t electrons and a heavy element, the electron waves channel (e.g. 30-32) down the atomic columns (for a c r y s t a l zone-axis orientation) with minimal cross-talk between adjacent columns. With optimal imaging conditions, p r i m a r i l y depending on the objective lens defocus, the spherical aberration and the damping e f f e c t s of the microscope i n s t a b i l i t i e s balance each other out (25,29). The f i n a l image i s then an accurate l o c a l representation of the object, and i t i s correct to believe what i s seen. Monolayer s e n s i t i v i t y and, with some care, l i m i t e d s e n s i t i v i t y to chemical impurities can be achieved (25). When these conditions are not met an averaged (over the object) image i s obtained rather than a l o c a l one and measurement of, for instance, surface relaxations i s w e l l nigh impossible. Results Gold (110). The gold (110) surface has been observed to undergo a 2x1 reconstruction, with every other surface column missing, as shown i n Figure 1. This p a r t i c u l a r image i s from a p a r t i c l e of approxi mately 30o8 i n radius, and elements of the reconstruction were also observed on smaller p a r t i c l e s (~10θ8 i n r a d i u s ) . One i n t e r e s t i n g feature of the reconstruction i s a 20 + 5% expansion at the top of

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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the corrugated structure which becomes apparent i n c a r e f u l , d i g i t a l comparisons of experimental and calculated images (22). This expansion has recently been confirmed by X-ray grazing incidence d i f f r a c t i o n and ion scattering experiments (33,34). Gold (111). The main feature of the gold (111) surface i s that the surface mesh expands r e l a t i v e to the bulk - there i s a tangential surface pressure (23,26,35). This behaviour manifested i t s e l f during an electron beam induced etching of contaminant carbon layers (by water vapour) as a " h i l l and v a l l e y " reconstruction, as shown i n Figure 2. This i s a roughening mechanism which provides space for an expansion to occur which i s eventually accommodated by surface d i s l o c a t i o n s , e s s e n t i a l l y m i s f i t d i s l o c a t i o n s to accommodate the surface pressure (see Figure 3). I t i s important to recognise here that the nature of the specimen used for electron microscopy d i f f e r s from the bulk surfaces studied by, for instance, LEED. With a bulk surface we might expect some manifestation of the surface expansion other than a h i l l and v a l l e y roughening. It i s i n t e r e s t i n g to consider t h i s surface as i f i t were an e p i t a x i a l system, that i s a monolayer of gold epitaxed on a gold (111) surface. With any e p i t a x i a l system, provided that the m i s f i t between the adsorbate and the substrate i s small, the adatoms are e l a s t i c a l l y strained to the substrate surface mesh y i e l d i n g pseudomorphic growth ( f o r a review see, for instance, 36). For an i n f i n i t e surface the s t r a i n i s purely homogeneous, but with a f i n i t e adsorbed layer there i s some buckling due to the i m p l i c i t boundary condition of no t r a c t i o n s at the edges of the layer (37,38). This buckling also appears i n some of the e a r l i e r analyses of surfaces using simple Morse p o t e n t i a l s (e.g. 39) since these have an i n b u i l t expansive surface pressure. I f the m i s f i t between the adsorbate and the substrate i s s u f f i c i e n t l y l a r g e , i t becomes e n e r g e t i c a l l y favorable to nucleate m i s f i t d i s l o c a t i o n s to r e l i e v e the s t r a i n s . Numerical c a l c u l a t i o n s by Snyman and Snyman (40) for the case of a (111) layer on a (111) substrate show that Shockley p a r t i a l dislocations and stacking f a u l t s are a low energy mechanism for t h i s s t r a i n r e l i e f , c o r r e l a t i n g with the case of s i l v e r epitaxed on gold (111) (40). These c a l c u l a t i o n s are i n excellent agreement with our results. Benzene on Gold (111). One chemically i n t e r e s t i n g event seen on the gold (111) surface was the formation of a benzene monolayer during the etching of the i n i t i a l carbon contaminant layer by water vapour (25,26). This i s a f l a t π-bonded l a y e r , with a benzene to benzene spacing of 7.3 0+0.2)8, and i s shown i n Figure 4. I t i s i n t e r e s t i n g to connect t h i s observation with the mechanism of the etching process, which i s probably s i m i l a r to that of the water-gas r e a c t i o n . We would hypothesise that the carbon acts as a temporary sink for the hydrogen during the etching with the i n i t i a l reaction products being hydrocarbons and carbon monoxide. With various cracking reactions taking place under the electron beam, benzene can be one of many reaction products. Since benzene i s probably r e l a t i v e l y r a d i a t i o n r e s i s t a n t , p a r t i c u l a r l y when π-bonded to a metal which can act as an energy sink, i t could be a favored metastable product. There could also be some c a t a l y t i c effect from the gold substrate.



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Figure 1. Area of rough gold (110) 2x1 reconstruction, with a numerical c a l c u l a t i o n inset - see 22. Atomic columns are black.

Figure 2. H i l l and v a l l e y roughening of a gold (111) surface i n a) before roughening (carbon covered) and b) following carbon removal.

Figure 3. Area of clean gold (111) surface showing a surface Shockley p a r t i a l d i s l o c a t i o n (arrowed) - see 26. Atomic columns are black.


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Geometric E f f e c t s


There are a number of concepts concerning the structure of small p a r t i c l e s which have a bearing upon geometrical c a t a l y t i c e f f e c t s (e.g. 4 1 - 4 3 ) . These follow both from the surface imaging r e s u l t s , and a d e t a i l e d experimental (13-15) and t h e o r e t i c a l ( 4 4 - 4 7 ) study of p a r t i c l e morphologies. F i n i t e s i z e e f f e c t s . I t i s w e l l known t h a t , as the s i z e of an atomic c l u s t e r drops, the r e l a t i v e proportion of edge and surface atoms increases. However, c a l c u l a t i o n s of the r e l a t i v e f r a c t i o n of these d i f f e r e n t surface s i t e s have to date made one important assumption, namely that the external morphology of the p a r t i c l e remains constant. This assumption i s not i n f a c t v a l i d ; edge atoms for example have a higher i n t r i n s i c energy than surface atoms, so i t i s possible that a morphological change could occur which would minimise the number of edge atoms. Detailed c a l c u l a t i o n s ( 4 7 ) show that e f f e c t s from the edge atoms are present and that there i s also a stronger e f f e c t which can be linked to sphere packing. The number of atoms on a p a r t i c u l a r face, or i n the c l u s t e r , deviates s u b s t a n t i a l l y from the continuum value (parameter!sed i n terms of the surface area and c l u s t e r volume respectively) when the p a r t i c l e s are small. This introduces further large edge-like terms i n the t o t a l p a r t i c l e energy which w i l l drive s u b s t a n t i a l morphological changes. For instance, for a simple broken bond model of an fee p a r t i c l e r e s t r i c t e d to having only (111) and (100) faces, the f r a c t i o n of (100) surface drops markedly as the c l u s t e r s i z e decreases as shown i n Figure 5. This e f f e c t only occurs when the c l u s t e r energy i s minimised with respect to i t s morphology. I t i s also possible to have d i s c r e t e microfacetting i n small p a r t i c l e s . For a large ( e s s e n t i a l l y continuum) c r y s t a l , v i c i n a l surfaces are important, t h e i r r o l e being w e l l understood through the Wulff construction (e.g. 48-50). However, the u n i t c e l l of a v i c i n a l surface i s l a r g e , and there may be i n s u f f i c i e n t space on a small p a r t i c l e . Only small u n i t mesh surfaces can be accommodated, and t h i s can lead to microfacetting, a possible example being shown i n Figure 6. We note that there i s a l i k e l y c a t a l y t i c p a r t i c l e s i z e e f f e c t here i n the disappearance of v i c i n a l surfaces. Boundary Conditions. The i m p l i c i t boundary condition of small surface area can a f f e c t surface reconstructions and chemisorption. Surface steps, for example, are important f o r reconstructions (e.g. 51)t and can determine the p a r t i c u l a r domain that occurs (e.g. 52). An example of a s t r u c t u r a l e f f e c t i s on the gold (111) surface where there i s an in-plane tangential surface pressure ( 2 3 , 2 6 , 3 5 ) . On extended surfaces, a h i l l - a n d - v a l l e y roughening occurs to accommodate the expansion, as described e a r l i e r . In contrast, small p a r t i c l e s accommodate the pressure by a surface buckle (26). We would expect s i m i l a r behaviour when there i s chemisorption i n v o l v i n g i n t e r a c t i o n s between the adsorbed molecules. Gas-Particle e f f e c t s . The gas environment and chemical i m p u r i t i e s , such as promotors or poisons, can strongly influence the t o t a l p a r t i c l e morphology ( 4 9 1 , 5 0 , 5 3 , 5 4 ) . E f f e c t s can occur v i a morphological changes which may eliminate or promote c e r t a i n



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Figure 4. Area of benzene covered gold (111) .surface, for two d i f f e r e n t objective lens defoci as required for unique image interpretation (see 24, ?5). Tn a) the gold atomic columns are black, i n b) white. Moire f r i n g e s , rather than any true s t r u c t u r a l image, r e s u l t from the benzene monolayer. Simulations ( r i g h t ) have benzene overlay on top surface only.

Surface

Figure 5. Relative f r a c t i o n of the d i f f e r e n t surface atoms as a function of Log M, where Ν i s the number of atoms, for a (111) and (100) facetted f.c.c. c r y s t a l when the surface morphology i s e q u i l i b r i a t e d . F r e f e r s to the edges between the (100) and (111) faces, Ε to the edges between two (111) faces. The (111) curve i s drawn using the axes to the r i g h t . 1



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( c a t a l y t i c a l l y important) facets. (For a review of e f f e c t s on large surfaces see _12,49 and 50). This i s equivalent t o the blocking of an enzyme v i a a conformational change, rather than a d i r e c t attack upon the active s i t e . One example of t h i s type of process a r i s e s i n gold p a r t i c l e s . When grown i n UHV, the stable structures are i n the form of non-crystallographic p a r t i c l e s c a l l e d multiply twinned p a r t i c l e s or MTPs (l,2,J^,_l^,44-46,55 and see Figure 7)· I n - s i t u experiments by Yagi et a l (56), who observed the formation of MTPs both during growth and following coalescence, demonstrated that these p a r t i c l e s are thermodynamically preferred when small (see also 44). However, specimen c a t a l y s t s produced by Avery and Sanders (56) did not show s i g n i f i c a n t concentrations of MTPs, which would appear to contradict the s t a b i l i t y r e s u l t s of Yagi et a l (56). The difference can probably be a t t r i b u t e d to the e f f e c t s of trace water vapour, which experimentally i n h i b i t s gold MTP formation (26 and Figure 8 ) , and which we suspect was present during the preparation of Avery and Sanders. The t h e o r e t i c a l explanation i s through the change i n surface free energy and tangential surface pressure with contaminants, since both a f f e c t the energy balance between MTPs and single c r y s t a l s ( 4 £ ) . The surface pressure term i s probably dominant, with the surface expansion upon the clean gold

Figure 6. Microfacetted region of a gold c r y s t a l , with the facet indexing marked. The atomic columns are black.

Figure 7. A decahedral HTΡ o f gold showing white atomic columns, supported on an amorphous carbon f i l m - see 25. An explanation o f t h ^ g ^ a ^ ^ t i j ^ T P s i s given i n

Library 1155 16th St.,Deviney, N.W. M., et al.; In Catalyst Characterization Science; ACS Symposium Series; American ChemicalD.C. Society: Washington, DC, 1985. Washington, 20035


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Figure 8. Image and d i f f r a c t i o n pattern from an (100) e p i t a x i a l specimen of gold prepared in an unbaked UHV evaporator by deposition onto KC1 and then transfer onto amorphous carbon. Here water vapour was the dominant residual gas (determined by mass spectrometry). The p a r t i c l e s are square pyramidal single c r y s t a l s .

(111) surface (23,26) presumably being suppressed by water adsorption, thus favoring single c r y s t a l formation. There are good t h e o r e t i c a l reasons (35) for believing that s i m i l a r e f f e c t s can occur i n other systems. Conclusions We have discussed here, very b r i e f l y , some recent observations of small p a r t i c l e surfaces and how these r e l a t e to geometrical c a t a l y t i c e f f e c t s . These demonstrate the general conclusion that high r e s o l u t i o n imaging can provide a d i r e c t , s t r u c t u r a l l i n k between bulk LEED analysis and small p a r t i c l e surfaces. Apart from applications to conventional surface science, where the s e n s i t i v i t y of the technique to surface inhomogenieties has already yielded r e s u l t s , there should be many useful a p p l i c a t i o n s i n c a t a l y s i s . A useful approach would be to combine the experimental data with surface thermodynamic and morphological analyses as we have attempted herein. There seems no fundamental reason why r e s u l t s comparable to those described cannot be obtained from commercial c a t a l y s t systems. Acknowledgment s The authors would l i k e to thank Drs. A. Howie, V. Heine, E. Yoffe, R.F. W i l l i s , J.M. Cowley and J.A. Venables for advice and comments during the course of t h i s work. We acknowledge f i n a n c i a l support from the SERC, U.K. and L.D. Marks also acknowledges support on Department of Energy (USA) Grant No. DE-AC02-76ER02995.


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CATALYST CHARACTERIZATION SCIENCE van der Merwe, J . H . ; Ball, C.A.B. In "Epitaxial Growth"; Matthews, J.W., Ed.; Academic Press: New York, 1975; Part B, p. 493 (see also other articles in both Parts A and B). Neidermeyer, R.; Thin Films 1968, 1, 25. Snyman, J . A . ; van der Merwe, J . H . ; Surf. Sci. 1974, 42, 190. Wynblatt, P.; in Interatomic Potentials and Simulation of Lattice Defects, (Ed Gehlen, Beeler and Jaffee, Plenum Press, New York, 1972) pp. 633. Snyman, J . A . ; Snyman, H.C.; Surf. Sci. 1981, 105, 357. van Hardeveld, R.; Montfoort, V . ; Surf. Sci. 1966, 4, 396. Burton, J . J . ; Catal. Rev. Sci. Eng. 1974, 9, 209. Andersson, J.R. "Structure of Metallic Catalysts": Academic Press: London, 1975. Marks, L . D . ; J . Crystal Growth 1983, 61, 556. Marks, L . D . ; Phil Mag A 1984, 49, 81. Howie, Α.; Marks, L . D . ; Phil Mag. A 1984, 49, 95. Marks, L . D . ; Submitted to Surf. Sci. Linford, R.G.; In "Solid State Surface Science"; Green, M., Ed; Marcel Dekker: New York, 1973; Vol. 2, p.1. Flytzani-Stephanopoulos, M.; Schmidt L . D . ; Prog. Surf. Sci. 1979, 9, 83. Drechler, M.; In "Surface Mobilities on Solid Materials", Binh, V . T . , Ed; Plenum Press: New York and London, 1983; p. 405. Chabai, Y . J . ; Rowe, J . E . ; Christman, S.B.; Phys. Rev. B. 1981, 24, 3303. Krueger, S.; Monch, W.; Surf. Sci 1980, 99, 157. Lacmann, R.; Springer Tracts in Modern Physics 1968, 44, 1. Metois, J . J . ; Spiller, G.D.T.; Venables, J . A . ; Phil Mag A 1982, 46, 1015. Buttet, J . ; Borel, J . P . ; Helv. Phys. Acta, 1983, 56, 541. Yagi, K.; Takayanagi, K.; Kobayashi, K.; Honjo, G . ; J . Crystal Growth, 1975, 28, 117. Avery, N.R.; Sanders, J . V . ; J Catal. 1970, 18, 129.

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