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evaporation of metal probably takes place more readily when the metal-support interaction is stronger, as this stabilizes the evaporated atoms on the support and increases the contribution of atomic migration to the size change of metal particles. In the particle migration mechanism, the migration process rather than coalescence is the rate-determining The surface movement of particles is due to the random thermal motion of metal atoms>17 and a possible mechanism involves two steps: the deformation of a particle to occupy an extended area and the re-formation into a stable form by a recession in some part of the edge, both of which are caused by the thermal motion of atoms. The deformation and the subsequent re-formation result in a elementary movement of the particle and the successive movements cause its Brownian motion on the upp port.^ The first step, the deformation, may occur by the thermal diffusion of atoms over the surface of the particle at low temperatures, while at higher temperatures, the mass fluctuation of atoms inside the particle causes its deformation. Baker et al.% have described liquidlike behavior of metal particles well below their melting points. In both the cases, the magnitude of metal-support interaction may affect slightly the deformation so long as the particle is not too small. On the other hand, the re-formation requires the breaking of some bonds between metal atoms and the support, so that particle movement is suppressed more by the stronger bond. Therefore, the particle migration mechanism would contribute less to the size change, when the metal-support interaction is stronger. From the above consideration, the activation energy for particle migration is expected to relate closely to that for atom diffusion over the surface of or inside the particle. This activation energy is probably smaller than that of the two-dimensional evaporation of atoms since more metalmetal bonds must be broken in the latter than in the former, provided that the metal-metal bonding is stronger
581
than the metal-support one. Hence, the atomic migration mechanism would predominate a t higher temperatures. The present results show that the size change of Ni particles occurs mainly through particle migration on M 2 0 3 support, while atomic migration also takes place on C and Si02supports concurrently with particle migration, especially a t higher temperatures. Atomic migration seems to be more significant on C than on SiOz. Kuo et al.B recently reported a similar dependence for nickel mobility on temperature in the Ni/Si02 system in either an N2 or an H2 atmosphere, based mainly on X-ray diffraction analysis. The idea of the concurrence of the two migration mechanisms is not so uncommon, but the experimental evidence obtained so far seems to be limited to the migration of Ni. Furthermore, the above-made estimation of the Nisupport interaction gives the oder Ni/C > Ni/Si02 > Ni/A1203. This is in accord with the order of the contribution of atomic migration mechanism given above, and supports the present view of the role of the metal-support interaction. In actual cases, however, the effect of support should be more complex than that discussed here. For example, the chemical and physical heterogeneity of the support surface should be taken into account. The surface property of the support may vary with temperature. In fact, an electron diffraction study of C and S O 2supports used here indicates a slight variation of the structure which changes from a highly disordered form to a less-disordered polycrystalline one a t higher temperatures. But this structural change is not so much evident that the mode of the migration of Ni particles on the supports may not be affected. The effect of atmospheres on particle growth is well studied for the platinum It is highly desirable to achieve such an experiment as the present one in gaseous environments, though the results given by Kuo et al. suggest that the effect of gases on Ni mobility is not so marked when reactive components such as oxygen are absent.
Twinning in Zeolite Y. The Conversion of Faujasite into a New Zeolitic Structure Marc Audler, John M. Thomas,” Jacek Kllnowskl, David A. Jefferson, and Leslie A. Bursill Department of Physlcal Chemistry, Universlt.v of Cambridge, CambrHge CB2 IEP, United Klngdom (Recelved: July 27, 1981; I n Flnal Form: October 9, 1981)
High-resolutionelectron microscopy of sodium and lanthanum cationic forms of the synthetic zeolite Y reveals a marked tendency to recurrent twinning, resulting in local formation of a new zeolitic structure. The twin plane is (111)and passes through the double six-membered rings between sodalite cages. A new hexagonal structure is generated within the cubic parent zeolite, with elliptical aperatures along [110]and an elongated “hypercage” the length of which depends on the extent of twinning. Multiply twinned zeolite Y may be of significancein catalysis and out to possess novel sorptive properties. It is likely that twinning occurs in a variety of natural and synthetic zeolites. Introduction One of the most important contributions of high-resolution electron microscopy (HREM) to structural studies has been to provide direct, real-space evidence, in the form of images with near-atomic resolution, for the occurrence of hitherto unknown structures. In particular, new structural types, composed of recurrent planar faults of various kinds, have been brought to light both for simple binary and ternary oxides’ and in pyroxenoid and am0022-3054/82/2086-0581$01.25/0
phibole silicate minerals and some of their synthetic analogue^.^^^ (1)R.J. D.Tilley, Chem. SOC.Spec. Period. Rep.: ‘Chemical Physics of Solids and Their Surfaces”, 8,121-201 (1980). (b)J. M. Thomas, D. A. Jefferson, L. G. Mallinson, D. J. Smith, and E. S. Crawford, Chem. Scr., 14, 167 (1978-79). (c) L.A. Bursill, ibid., 14,83 (1978-79). (d) J. M. Thomas and D. A. Jefferson, Endeavour, New Ser., 2,127-36 (1978). (e) D.A. Jefferson, J. M. Thomas, D. J. Smith, R. A. Camps, C. J. D. Catto, and J. R. A. Cleaver, Nature (London),281,51-2 (1979). (0J. M. Thomas, New. Sci., Aug 21 (1980).
0 1982 American Chemical Society
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The Journal of Physhl Chemlsby, Vol. 66, No. 4, 1962
Flgure 2. (a)Brlght-feu image of perfect sbuctllre of zeoliie Y viewed
along [071]. (b) and (c)assochted elecbm dftractkm framework drawing (d).
patterns. and
Flgure 1. (a)and (b) Bright-fieldimages of twinned sbucbre of zeollte
Y viewed along [ 1lo]. The twin plane is (111). (c)Typical electron diffractionpattern and +heschematised index (d) a!mg the [oi 11 zone
axis.
Having demonstrated that HREM is of considerable value in the study of we have now used the technique to investigate in detail some properties of the synthetic zeolite Y, particularly in view of the discovery of its aptitude to undergo twinning. In the course of this work, which established that twinning occurs relatively easily in faujmibtype materials, we found that., depending upon the extent and precise nature of recurrent twinning, (2) D. A. Jefferson. N. J. Pugh, M. ALvio Franco, L. G.Mallinson, 0. R. Millward, and J. M. Thomas, Acta Cryatollogr., Sect. A, 36,1058-65 ,IO*",
(3) D. A. Jeffersonand J. M. Thom, J. Chem. Sw., Chem. Commn., 761-4.~ 119Rn)~ ....
( 4 ) J. M.'homas, 0. R Millward, and L. A. Bursill, Phil. ?taw. R. Soc. London. Ser. A, 300,43 (1981). (5) L. A. Buraill, J. M. Thomas,and K. J. Rao, Nature (London),289, 1.57-R (19R1) . _ .~ . . . .
I-
(6) L. A. Bursill. E. A. Lodge, and J. M. Thomas. Nature (London), 286. 111-3 (1980).
(7) J. M. Thomas.L. A. Bursill, E.A. Lodge, A. K. Cheetham, and C. A. We, J. Chem. Soc., Chem. Commun., 216 (1981). (8)L. A. Bursill. E. A. Lodge, J. M. Thomas, and A. K. Cheetham,J .
Phys. Chem.. 85.2409 (1981). (9) J. M. Thomas and L. A. Bursill, Angew. Chem., 19, 745 (1980). (10)J. M. Thomas, G. R. Millward, M. Audier, S.h d a s , and L. A. Bursill, Faraday Society Discussion No. 12 on Selectivity in Heterogene~w Catalysis. Nottingham, Sept 1981, in p r m .
Figure 3. (a) Bright-field image of twinned structure viewed almost
along [llO]. (b) Scalar framework drawing. Twin planes marked A and B run through double six-membered rings joining sodalite cages.
a new zeolitic structure is generated locally. HREM, together with electron diffraction and optical diffraction analyses, enabled us to identify the nature of the new local structure to the extent that the dimensions of new 'channels" and the unit cell dimensions of the new (local) structure have been specified. Intergrowths of two or more distinct structural types related by twinning may well be a common feature for a variety of other zeolites. Experimental Section Details of the general experimental methods used in the study of solids by high-resolution imaging have been described previously,"8 and specific details relating to the extra precautions mnceming preevacuation procedures and sample preparation for the rather hem-sensitive zeolites, A, X, Y, and ZSM-5 have also been described.',ea Briefly, the preevacuated samples were rapidly tilted into the appropriate orientation with a JEOL-200CX side-entry goniometer. The objective lens spherical aberration
Twinning in Zeolite Y
The Journal of Physical chemistry.
Vol. 86, No. 4.
1982 583
Flew0 4. (a) A '"hypercage"formed by recutrent twinning of the taulasite s@uciwe. Dashed lines denote Um twin planes. N stands for the normal a p m e s (7.4 A) viewed along [ 1lo], and T f M the elliptical apertures (7.4 by 6.9 A) of the twinned region. (b) View along [ l l 11 for the normal (lop)faujasiieand for the Wnwd (bonom) 81111clllre; the gap represents the d s w a m the hypercage. (c)(Top Wt)is a schematic representailon. side viaw, of the twinned structure (see text).
Fgun 5. (a)and (b) Examples of higlwesolutlon images of multiple twinning in zeolite y: (a) is given with an electron diffraction pattem. (b) wim an opacal dmaclion pattm and an inset identifying twin planes. (c) Interpretation of the electron dtfractkn pattem. Exba dtfraction spots marked by small (unindexed) circles are due to double diffraction;thick vertical dotted line denotes the twin plane. (d) Framework drawing of twinned Zeolite Y along [110]. Matrix and twin lamellae are marked by A and v. respectively; a vertical line denotes a twin plane.
coefficient C, was 2.5 mm, and images were recorded for the optimum lens defocus condition. Optical transforms were routinely used for further analysis of the high-resolution micrographs.'fi-" The La-Y sample was produced by exposing the parent Na-Y to an aqueous solution of LaC1, at 60 "C for 1h. Further details of the ordering of the cations in this material, as revealed by HREM,will be described in a later communication. A random selection of Na-Y and La-Y zeolites showed evidence of twinning on (111)planes.
Results and Discussion Real-Space Images. Figure 1, a and b, shows typical bright-field images of the twinned faujasite structure; the (11) G. R. Millward and J. M.Thomas in "Proceedingsof the Carbon and Graphite Conference",London, 1974, Society of Chemistzy and Induetry, 1975, p 492.
twin plane is (111). These low-resolution micrographs show the ultrastructure along the [llO] zone axis. Bright-field images at higher magnifications, together with the corresponding electron diffraction parameters, are given in Figure 2 (for the "perfect" structure) and Figure 3 (for the twinned structure). It is clear that the twin compoaition plane is (111). In the highest resolution images of faujasite that we can routinely record we see twin lamellae, bounded by two (111)twin planes. The corresponding projected Structure drawing clearly indicates that -. . twinning involves reflection acrossa Irirrm plane intersectine the suDercaees (marked A and B on the inset Figure3b) and'passing through the double six-membered rings joining the cuboodahedra (sodalite cages) rather than a mirror plane intersecting these cubooctahedra. Scalar Structural Drawings of Twinned Zeolite Y. Figure 4a the nature Of the framework viewed along [110] in the normal and (111)twinned crystal,
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J. Phys. Chem. 1982, 86, 584-585
denoted by N and T, respectively. It is clear that the dimensions of these apertures are significantly different; while the radius of the opening of the supercage in zeolite Y is 7.4 A, the aperture of the twinned structure is elliptical with the longitudinal and transverse diameters being 7.4 and 6.9 A, respectively. It is equally clear that the two openings will have a different kinetic diameter for sorption, and thus different sorption properties and selectivities. Ion-exchange properties are also likely to be affected for this and other reasons (see below). Figure 4b shows that in the [ l l l ] direction, with the twin plane parallel to the page, the size and geometry of the apertures are the same as in the untwinned zeolite Y. On some crystals, viewed along [110], a radically new structure has been identified (Figure 5). This new structure arises because of recurrent twinning on (111) according to the sequence schematized a t the foot of Figure 5 and in Figure 4c. It is composed of "hypercages", as distinct from the supercages in the precursor faujasite. The regular, untwinned faujasite may be represented by ...AAAAA ...,where each A denotes a unit repeat along [ l l l ] , (i-e.,14.2 A). The seauence ...AAIVIAAIVIAAIVIAA ...etc. s i d l e s that a twin lamella, V, bobnded by a pair of twin pianes denoted by vertical lines on each side is introduced between each flanking pair offaujasite r e p a t units along [1111, thereby generating the new structure composed of "hypercages" which are 49.6 A in length and have a diameter varying between 7.4 and 13 A. This new structure is hexagonal, and its principal characteristics are (cf. the parent cubic zeolite Y with a. = 24.70 A) a = dlIo(faujasite) = 17.46 A
c = 6dlll(fauja~;te)= 85.56 A The chemical formula and the framework density (number of tetrahedral atoms per unit volume) are the same as for the parent material, and the unit cell volume is 22 604 A3. It is also noteworthy to comment on the fact that when a twinning sequence ...A ~ V ~ A ~ ... V is~ generated A~V the resulting hypercage now extends for the entire length of the recurrent twin in the [ l l l ] direction. Thus, if there are n twin planes, the length of the tunnel in the new structure is 14.2(n + 1) A + 6.95 A. The second term in the sum accounts for twice the distance between the extreme (111) plane and the wall of the supercage with which the tunnel terminates a t each end. [The new structure has a certain kinship with, but is distinct from, the so-called structure 6 discussed as a theoretical possibility by Breck.12] Preliminary investigation^'^ reveal that multiple twinning in zeolite Y can be induced artificially by a number of methods. A discussion of the factors affecting the twin process, together with its suggested mechanism and a consideration of how the twinning influences Si,Al ordering, is in course of preparation. Acknowledgment. We gratefully acknowledge support from the University of Cambridge, The ~~~~l Society European Exchange Scheme (for M.A.), BP Research Centre, and the Science Research Council for an equipment grant. (12)D.W.Breck, 'Zeolite Molecular Sieves", Wiley, New York, 1974, pp 56-7. (13)M. Audier, J. Klinowski, and J. M. Thomas, unpublished work.
COMMENTS Comment on "Metal-Ammonia Solutions. 14. Electron Spln Resonance at -65 OC"
Sir: In the cited paper,' Harris and Lagowski report electron spin susceptibilities and metal concentrations for solutions of K, Rb, and Cs in ammonia a t -65 "C. As a result, they conclude that the well-known spectral red shifts and the well-documented electron spin pairing which occur on increasing metal concentration in dilute metalammonia solutions are unrelated phenomena. They further conclude that the ion pair, M+.e-, is most likely responsible for the red shifts. In contrast, Rubinstein2 has shown that the spectral red shifts in Na-ammonia solutions a t -75 O C correlate well with corresponding changes in magnetic susceptibility at the same temperature, measured by H ~ s t e rand , ~ that the spectral red shifts for Na solutions at -55 "C correlate well with changes in magnetic susceptibility of K-ammonia solutions a t the same temperature, measured by Freed and Sugarman.4 Futhermore, Rubinstein, Tuttle, and Golden5 (1)R. L. Harris and J. J. Lagowski, J. Phys. Chem., 85,856 (1981). (2)G. Rubinstein, J. Phys. Chem., 79,2963 (1975). (3) E.Huster, Ann. Phys., 33, 477 (1938). (4)S.Freed and N. Sugarman, J. Chem. Phys., 11, 354 (1943). (5)G.Rubinstein, T.R. Tuttle, Jr., and S. Golden, J. Phys. Chem., 77,2872 (1973). 0022-3654/82/2086-0584$01.25/0
have shown that the spectral red shifts observed for dilute Na-ammonia solutions containing excess dissolved NaI cannot be accounted for by an ion-paired species. (In the presence of excess Na+ provided by dissolved NaI, the ion pair dissociation equilibrium requires [e-]/ [Na+-e-]to be essentially constant. Therefore, the observed persistence of red shifts for the salt-containingsolutions with changing metal concentrations5 rules out Na+.e- as responsible for these red shifts). There is an evident conflict between these results and conclusions of prior investigation^,^-^ and those reported by Harris and Lagowski,' which will require additional experimental data for its ultimate resolution. In the meantime, we present a partial examination of the data reported by Harris and Lagowski. This examination raises such doubts on the reliability of the data that any conclusions which may be drawn from them must be considered as highly questionable, a t best. First, Harris and Lagowski report spin susceptibility data for certain samples at both -33 and -65 "C. For each of nine samples observed their results imply that the concentration of spins appears to decrease with increasing temperature. The resulting implied increase of spin pairing with increasing temperature is quite incompatible with the behavior which has been observed in every previous investigation on this p ~ i n t . ~ , * , ~ - ~
0 1982 American Chemical Society
