J . Chem. SOC., Faraday Trans. I , 1986, 82, 205-212 Electron Microscopy of Pt, Pd and Ni Particles in a NaX Zeolite Matrix Andreas Kleine and Peter L. Ryder Materials Physics and Structure Research Group, University of Bremen, 0-2800 Bremen 33, Federal Republic of Germany Nils Jaeger and Gunter Schulz-Ekloff Applied Catalysis Research Group, University of Bremen, 0-2800 Bremen 33, Federal Republic of Germany The morphology, size distribution and crystallography of metal precipitates up to 10 nm in diameter in an NaX zeolite (Si/Al = 1.2) loaded with Pt, Pd and Ni have been studied by electron microscopy. Using special techniques to avoid specimen damage by the electron beam, it was shown by bright and dark field microscopy, selected area diffraction, microdiffraction and lattice imaging that all three metals precipitate as equiaxial single crystals having the normal crystal structure of the bulk metal (f.c.c.) in an intact zeolite matrix.Pt and Pd showed a preferred orientation relationship, with greater scatter in the case of Pd. The thermal treatments used led to monodispersed metal phases with uniform particle size distributions, in the case of Ni to a double distribution. Increasing the degree of ion exchange led to an increase in the density rather than the size of the particles. The results are discussed with regard to possible nucleation and growth mech- anisms, and an explanation for the observed orientation relationship is proposed. Crystalline aluminosilicates of the zeolite type with an open lattice structure are used industrially as molecular sieves and as carriers for metal cata1y~ts.l~~ The present investigation was concerned with zeolite catalysts containing metallic platinum, palladium and nickel particles.The starting material was NaX zeolite, in which the contents of the unit cell may be represented by the sum formula Na,(AlO,),(SiO,),, .kH,O, with n+m = 192 and k z 264. X zeolites have Si/A1 ratios (m/n) in the range 1-1.5. The material used in the present investigation had m/n = 1.2. The structure is.cubic (space group Fd3m), and the lattice parameter for this composition is a, = 2.495 nm. The basic structural units are tetrahedra of oxygen atoms grouped around silicon or aluminium, each oxygen atom being shared by two adjacent tetrahedra, so that the basic formula units are (SO,) and (AlO,)-.In NaX zeolites the excess negative charge is compensated by Na+ ions. The linked tetrahedra form an open network structure with two types of roughly spherical cavities, the supercages (1.3 nm in diameter) and the cages (0.66 nm in diameter), connected by window^.^ The water molecules are free to move through the cavities. In the production of catalysts, the Na+ ions are partially replaced by ions of transition metals such as platinum, palladium or nickel. This ion exchange is followed by dehydration and reduction treatments, leading to the precipitation of the metal in form of finely dispersed particles. The dehydration causes no basic change in the framework structure. Research and development work in this field is directed towards understanding the factors affecting the reactivity and dispersion of the metal particles.The aim is to produce a fine, uniform size distribution which is maintained during the reaction being catalysed. 205206 Electron Microscopy of Ni, Pd, Pt in Zeolite Matrices Some of the factors which have been found to affect the size distribution and the stability of the metal particles the silicon-aluminium ratio (m/n) of the starting material, the degree of ion exchange, and the temperature and duration of the dehydration and reduction treatments. Since the metal particles are generally only a few nm in diameter, transmission electron microscopy (TEM) is the only method by which they can be observed directly.11-18 In addition to the high-resolution imaging of the particles and the surrounding matrix, TEM can provide a great deal of further information which is relevant to the investigation of nucleation and growth mechanism.The crystal structures of the matrix and the precipitates may be determined by electron diffraction which, in addition to direct lattice imaging, also indicates whether the precipitation processcauses damage to the surrounding zeolite lattice. With the aid of modern microdiffraction techniques, crystallographic information may even be obtained from single particles. Orientation relationships between the precipitates and the matrix, which may throw considerable light on possible nucleation mechanisms, may also be detected and evaluated by electron diffraction. Finally, information concerning the chemical composition may be obtained with high spatial resolution by X-ray microanalysis.Despite these advantages, relatively few high-resolution TEM investigations of zeolite catalysts have been published. This is probably due to the fact that the preparation of these materials is difficult and time-consuming, and to the sensitivity of zeolites to radiation damage in the electron beam. The present investigation has shown, however, that these difficulties are not insurmountable. Using suitable preparation and operating techniques it was found that all the usual TEM imaging and diffraction methods could be applied successfully to these materials. Even lattice imaging was possible in some cases without the use of special preparation techniques, such as the substitution of uranyl ions.l9? 2o The present paper describes the techniques used and the results obtained in an investigation of platinum, palladium and nickel precipitates in NaX zeolites, the aim of which was to elucidate the possible nucleation and growth mechanisms of the metal particles. Experimental Specimen Preparation The materials investigated and the ion exchange and reduction treatments used are summarized in table 1 .Platinum and palladium were introduced in the form of their tetrammine complex ions from aqueous solutions of the chlorides. In the case of platinum, four degrees of ion exchange, ranging from 13-52% were investigated. Two palladium-loaded materials (7 and 43% ion exchange) and one material with nickel (23.5%) and calcium (25%) were also studied.Calcium is thought to impede the coarsening of the metal particles by sintering during operation as a ~ata1yst.l~ Nickel and calcium were exchanged from solutions of the acetates. In the case of the platinum and palladium loaded specimens, the dehydration and reduction treatment was carried out at 600 "C for the former and 300 "C for the latter under vacuum or an inert gas. In these materials the precipitation takes place by temperature programmed reduction.21T 22 Nickel was reduced with hydrogen at 300 "C. The specimen number 5 had the same composition as 3, but was reduced for a longer time (4 h) and under helium instead of in a vacuum. After ion exchange and reduction all specimens were sealed in glass capillaries under vacuum to protect them from oxidation during storage.The TEM specimens were embedded in epoxy resin (Epon 812) and sectioned in an ultramicrotome. This embedding material was found to give excellent results, but has the disadvantage that it has to be left for three months at room temperature to harden completely. For a firstA . Kleine, P. L. Ryder, N . Jaeger and G. Shulz-Eklofl 207 Table 1. Materials and treatments degree of ~ ~~ ~ sample ion exchange reduction no. metal (atom % j ion-exchange agent treatment 6 7 8 Pt Pt Pt Pt Pt Pd Pd Ni Ca 13 1 heated to 600 "C at 5 "C min-l held 15 min, vacuum held 4 h, He held 16 h, Ar 5 "C min-l to 600 "C 5 "C min-l to 300 "C 5 "C min to 300 "C, held 25 h, H, 23.5 Ni(CH,COO), 25 Ca( CH,COO), investigation specimens of lesser quality were therefore prepared by one of the following methods: the resin was hardened rapidly (1 week) at 60 "C.Alternatively, a suspension of the fine crystalline powder was dropped onto support films on copper grids and allowed to dry. The edge regions of some crystals were found to be thin enough for electron microscopy. Electron Microscopy The micrographs and diffraction patterns shown in this paper were taken in a Philips EM 420 electron microscope, equipped with an LaB, cathode, at 120 kV. For dark field imaging, tilted-beam illumination was used. Several techniques were used for obtaining electron diffraction patterns and lattice images of the beam-sensitive zeolite crystals. In the first place, the exposure times required were minimized by slightly underexposing and using special developing methods.In searching for suitable diffraction patterns showing both the metal phase and the matrix, the specimen stage was moved while observing the screen in diffraction mode with the lowest possible illumination intensity, and an exposure was made as soon as a satisfactory pattern was observed. For high magnification imaging the focusing was carried out using a very small spot size, so that only a small area of the specimen (50-100 nm in diameter) was irradiated. The specimen was then shifted slightly in the horizontal plane and left for a few minutes with the beam fully defocused in order to eliminate specimen drift during exposure. The lowest possible magnification compatible with the desired resolution was used. Results Platinum Plate 1 shows a typical bright-field electron micrograph from each of the four platinum loaded materials after autoreduction in vacuum (specimens 1 4 in table 1).The platinum precipitates are visible as approximately spherical particles. The effect of increasing the degree of ion exchange is to increase the density and slightly reduce the mean diameter of the particles. It is difficult to measure the diameter of the smallest particles, but a rough estimate indicates that the mean particle diameter is reduced by a factor of ca. 2 from 3 nm to 1.5 nm when the degree of ion exchange is increased from 13 to 52% . In all cases there is only a single particle size distribution with a fairly narrow range of sizes within the zeolite matrix.23208 Electron Microscopy of Ni, Pd, Pt in Zeolite Matrices Increasing the duration of the reduction treatment from 15 min to 4 h (under helium) results in a slight coarsening of the particles.The mean diameter of the platinum particles in specimen 5 (plate 2) was ca. 4 nm. Selected area diffraction from all five materials showed weak diffraction rings which fitted the face-centred cubic structure of metallic platinum with a, = 0.392 nm. In addition, spot patterns from the zeolite lattice were observed, which disappeared after a few minutes exposure to the electron beam at normal intensities. Using the techniques described above, it was, however, possible to obtain a few diffraction patterns showing both the metal and the zeolite matrix. An example (from specimen 5) is shown in plate 3, together with a schematic diagram indicating the Miller indices of the diffraction spots.The diffracted intensity from the metal particles is not distributed evenly around the rings, as one would expect if the orientation were completely random, but is concentrated in certain directions, indicating a preferred orientation of the particles with respect to the zeolite lattice. As can be seen in plate 3, the maxima on the { 1 1 1 ), (200) and (2201 rings of the platinum diffraction pattern lie on reciprocal lattice vectors of the same type with respect to the zeolite lattice. The intensity maxima may therefore be indexed as a single crystal pattern, and the orientation relationship is simply (loo), / / (loo),, (Z = zeolite), i.e. the crystallographic axes of the platinum crystals tend to be parallel to those of the zeolite matrix.Further crystallographic information concerning the precipitates may be obtained from dark field micrographs. As an example, plate 4 shows a bright field micrograph from material 2, the corresponding selected area diffraction pattern and two dark field micrographs from the same area, one of them taken with the objective aperture on an intensity maximum of the { 11 l),, ring and the other with the aperture on the same ring, but halfway between the maxima, As is to be expected, a greater number of particles appears bright in the dark field micrograph from the intensity maximum. However, a certain fraction of the particles seems to have no special orientation relationship with the matrix. The study of a large number of dark field micrographs from this and other materials indicates that there is no visible difference, e.g.in size or shape, between particles close to the preferred orientation and those with an apparently random orientation. In the dark field images, most of the particles showing strong diffraction are evenly illuminated, indicating that they consist of single crystals. The internal structure seen in some of the particles may be due to twins or subgrain boundaries. Microdiffraction showed no evidence for the existence of polycrystalline particles. Plate 5 shows a diffraction pattern in which the spots come mainly from a single platinum particles and belong to a single zone. The splitting of the spots may be due to diffraction from neighbouring particles or to the presence of low-angle grain boundaries in one particle.Palladium The palladium-loaded specimens (materials 6 and 7, table 1) both showed metal particles ca. 5 nm in diameter, similar in appearance to the platinum particles (see plate 6). Again, the main effect of increasing the degree of ion exchange, in this case from 7 to 43%, is to increase the density rather than the size of the particles. Plate 7 shows a selected area electron diffraction pattern from the material 7 (43% ion exchange). The rings may be attributed to a face-centred cubic phase with a, = 0.389 nm, being the normal crystal structure of metallic palladium. The spot pattern arises from the zeolite matrix. The intensity distribution in the ring pattern shows that palladium, like platinum, also has a preferred orientation with respect to the zeolite matrix.The orientation relationship is the same, i.e. (loo),, / / (loo),. The scatter of the individual orientations is, however, greater than in the case of platinum.J . Chrm. Soc., Faradajt Trans. I , Vol. 82, part 1 Plate 1 Plate 1. Transmission electron micrographs (bright field) of Pt-loaded NaX zeolites with different degrees of ion exchange reduced I5 rnin at 600 "C in vacuum (see table I , materials I to 4). Degree of ion exchange: ( a ) 13, ( b ) 25, ( c ) 42 and (d) 52';:). Magnification: 300000 x . A. KLEINE et crl. (Fucing p . 208)J . Chem. Soc., FurudujJ Trims. I , Vol. 82, purt I Plates 2 and 3 Plate 2. Transmission electron micrograph (bright field) of material 5 (42':,, ion exchange.reduced 4 h at 600 "C under He). Compare with plate 1 (c). Magnification 410000 x . Plate 3. Selected-area electron diffraction pattern from the specimen shown in plate 2, with Bragg reflections from the platinum particles and the matrix. The Miller indices are given in the schematic diagram on the right. 'The spots near the centre are from the matrix, and those on the rings from the metal phase. A. KLEINE cf NI.J. Chem. SOC., Faraday Trans. 1, Vol. 82,part I Plate 4 Plate 4. Material 2 (Pt, 25 ion exchange, 15 min 600 "C), (a) bright field, (b) diffraction pattern, (c) tilted-beam dark field from maximum on { 1 1 1 }Pt ring, (d) tilted-beam dark field with aperture between maxima on { 1 1 1 }Pt ring. Magnification 300000 x . A. KLEINE et al.J.Chern. SOC., Faraday Trans. I , Vol. 82, part I Plates 5 and 6 Plate 5. Microdiffraction pattern from the metal phase in material 2. Spot size ca. 20 nm. Plate 6. Transmission electron micrographs (bright field) of Pd loaded specimens with different degrees of ion exchange: (a) material 6 (7"10), (b) material 7 (43"<,). Magnification 4100C3 x . A. KLEINE et al.J . Chem. SOC., Faraday Trans. 1, Vol. 82, part 1 Plates 7 and 8 Plate 7. Selected-area electron diffraction pattern from the material shown in plate 6(h), with Bragg reflections from the palladium particles and the matrix. Plate 8. (a) Transmission electron micrograph (bright field) of nickel precipitates in material 8 (table l), showing two particle size distributions with mean diameters of c’u.10 and 1.5 nm, respectively. Magnification 300000 x . (b) Selected area electron diffraction pattern from the same specimen, showing Bragg reflections from the metal particles and the zeolite matrixJ . Chem. Sac., Faraday Trans. 1, Vol. 82,part 1 Plate 9 Plate 9. High magnification electron micrograph (bright field) of the specimen shown in plate 8. The electron beam is parallel to the [110] zone axis, and both sets of { 1 1 1 ) planes in this zone are clearly visible. Magnification 580000 x . A. KLEINE et al.A . Kleine, P. L. Ryder, N . Jaeger and G. Shulz-Eklof 209 Nickel The appearance of the nickel particles in the material loaded with nickel and calcium (material 8, table 1) is shown in plate 8(a). In this case two particle size distributions are observed: relatively coarse particles with a mean diameter of the order of 10 nm and fine particles with diameters in the range 1-2 nm.Selected area electron diffraction again showed rings from the metal phase and spots from the matrix [plate 8(b)]. The ring pattern was compatible with the usual structure of metallic nickel (face-centred cubic, a,, = 0.352 nm). In this case, however, no preferred orientation relationship was observed. Of all the materials investigated, the nickel-calcium specimens were the most stable under irradiation by the electron beam, and lattice imaging of the zeolite matrix was possible. Plate 9 shows a high magnification image of a zeolite crystal viewed along a [ 1101 zone axis. Particularly good contrast is seen for the two sets of (1 11) planes in this zone.There is no visible destruction of the zeolite lattice in the neighbqurhood of the metal particles. Discussion and Conclusions Summary of Results The results presented above and in previous investigations 11- show that electron microscopy provides a powerful tool for investigation of the form, size distribution and crystallographic properties of metal particles in zeolites. The experimental observations may be summarized as follows. The thermal treatments used (see table 1) lead to the precipitation of fine particles, 5 nm in diameter or smaller, distributed evenly throughout the zeolite matrix. In the case of nickel, two maxima in the particle size distributions are observed, differing by almost one order of magnitude in diameter. The effect of increasing the degree of ion exchange is to increase the density rather than the diameter of the particles.In the case of platinum, a slight reduction in particle diameter is observed. Slightly coarser platinum particles were obtained by prolonging the thermal treatment to 4 h in a helium atmosphere. In contrast to particles which have been observed to have nucleated on the surfaces of the zeolite crystaW4 the internally nucleated precipitates have no particular crystal- lographic form, but are approximately spherical in shape. All precipitates have the equilibrium crystal structure of the corresponding metallic phase. There is no evidence for the occurrence of intermediate, metastable phases during the reduction process. The platinum particles, and to a lesser extent the palladium precipitates, show a preferred orientation near the orientation relationship (lOO), / / ( (m = metal), whilst the nickel crystals are more or less randomly orientated.Although the metal particles sometimes show internal structure, they nevertheless consist essentially of single crystals. Single particles consisting of polycrystalline aggregates were never observed. The metal particles nucleate and grow in an intact zeolite matrix. Particle Growth Mechanisms The fact that the particles are single crystals shows that each precipitate grows from a single nucleus. Since the resulting particles are larger than the supercages, their growth must be associated with material transport within the zeolite lattice. It was proposed in a previous paperls that the accommodation of the debris in defect zeolite lattice sites and210 Electron Microscopy of Ni, Pd, Pt in Zeolite Matrices their ultimate saturation may be the factor limiting particle growth.This could explain why the degree of ion exchange and apparently also the duration of the thermal treatment have little effect on the final particle size. The reason for the existence of a bimodal particle size distribution in the case of nickel is not known. The observed distribution of the particles in the microtome sections shows clearly that they are situated within the bulk and not on the surface. This has been confirmed by X.P.S. meas~rements.~~ The maintenance of an intact zeolite crystal structure during reduction may thus play an essential role in limiting particle growth.The present electron diffraction and lattice imaging observations show that there is no local destruction of the zeolite lattice even in the immediate neighbourhood of the metal particles. Crystallography The existence of the equilibrium metal structure in the precipitates has been demonstrated by X-ray diffraction. In the case of very small particles (ca. 1 nm), however, the diffraction lines from the metallic phase are so weak and broad that they are often undetectable. Nevertheless, by careful analysis of the measured intensities, Galle~ot~~v 25 succeeded in demonstrating the metallic structure of platinum particles with a diameter of 1 nm in Y zeolites. In the light of the results presented in this paper it may now be asserted with some confidence that the direct formation of the equilibrium crystal structure is a general property of the formation of metal particles in zeolites.The occurrence of an orientation relationship between the metal particles and the zeolite matrix cannot be explained by lattice matching between the two phases, since the structures have no recognizable similarity, and the precipitates exhibit no crystalline form. Since the larger particles embedded in the zeolite lattice cannot change their orientation appreciably during further growth, the process determining the orientation relationship must take place during the nucleation or the very first stages of growth. Any model for this process must account for: ( a ) the occurrence of the particular orientation relationship observed, (b) the fact that the orientation relationship is not obeyed exactly and ( c ) the differences between platinum, palladium and nickel.These facts are explained, at least qualitatively, by the following proposed mechanism. Since the metal atoms are free to move through the channels of the zeolite structure, it may be assumed that the first clusters form within the (super- or p) cages. These clusters are free to change their orientation within the cages and may at first adopt a random orientation. Once the particle size has reached the cage dimensions, however, further growth requires removal of material from the surrounding zeolite matrix. Since this process requires additional activation energy and is therefore slower, the addition of atoms to the cluster will continue as long as possible without destruction of the zeolite lattice, i.e.the initial growth process will proceed in such a way that the maximum possible number of metal atoms are fitted into the intact cage. Assuming that the metal atoms are arranged from the very beginning of cluster formation on the sites of a face-centred cubic lattice, the number of atoms fitting within the geometric limits of the cage will depend on the orientation of the metal lattice. As long as the cluster is free to change its orientation within the cage, the further addition of atoms will therefore lead automatically to that orientation which admits the maximum number of atoms. Both types of cages have cubic symmetry and may be represented by truncated cubo-octahedra or truncated octahedra, re~pectively.~ It may be assumed that optimum space filling is attained when the symmetry of the metal lattice matches that of the cage, which would give the observed orientation relationship.Once the maximum number of atoms have been packed into a cage, the exactness of the fit will depend on the size of the metal atoms in relation to the cage dimensions. For certain atom sizes the fit will be exact, leading to a precise orientation relationship. IfA . Kleine, P. L. Ryder, N . Jaeger and G. Shulz-Eklof 21 1 the atoms are a little smaller than this critical size, the cluster will have a certain freedom of movement, even when there is no room for a further atom. This may be the explanation for the observed scatter in the orientations and for the difference between atoms of different sizes, e.g.platinum (atomic radius 0.1387 nm) and palladium (0.1375 nm). As is predicted by the model, the slightly smaller atom shows the greater scatter. The particularly large scatter observed in the case of nickel, which has an even smaller radius (0.1246 nm) may also be a size effect, but there is another possible explanation in this case. It must be assumed that both platinum and palladium nucleate in the supercages, since the tetrammine complex ions do not fit into the smaller cages. Nickel, however, does not suffer from this restriction and may therefore nucleate also in the cages. As will be shown in a quantitative treatment (to be published), the scatter in the orientation resulting from a given size misfit is greater, the smaller the number of atoms in the cavity. Whereas about 100 nickel atoms can be accommodated in a super- cage, few more than 10 fit into a p cage.These qualitative arguments show that a study of the orientation relationship may provide important information concerning the mechanism of nucleation and growth of these particles. Outlook Computer calculations are being carried out to verify ( a ) that the observed orientation relationship is indeed that which allows the maximum number of metal atoms to fit into the cavity and (b) the observed dependence of orientation scatter on the atomic size ratios. The results of these calculations, when compared with the experimental results, njill provide a critical test of the model proposed above. The influence of the reduction conditions on the particle size also requires further investigation.It is not known, for example, whether the effect observed in the case of material 5 was due to the prolonged heat treatment or to the helium atmosphere. An influence of the inert gas atmosphere on the reduction reaction has been demonstrated by temperature-programmed desorption and differential thermal analysis22 and is thought to be due to interference of the gas with the diffusion of the reaction products in the channels of the zeolite structure. We thank Miss G. Wildeboer for providing the zeolite samples, Mrs U. Boseck for the preparation of the electron microscopy specimens, Mr D. Exner for making available unpublished measurements of size distributions, and a referee for his careful and thorough reading of our manuscript.N. J. and G. S-E. gratefully acknowledge financial support of the Deutche Forschungsgemeinschaft. References 1 A. P. Bolton, in Zeolite Chemistry and Catalysis, ed. J. A. Rabo, ACS Monograph 171 (American 2 E. Gallei, Chem. Ing. Techn., 1980, 52, 99. 3 H. Heinemann, in Catalysis: Science and Technology, ed. J. R. Anderson and M. Boudart (Springer, 4 D. W. Breck, Zeolite Molecular Sieves (J. Wiley, New York, 1974), p. 29. 5 Kh. M. Minachev and Ya. I. Irakov, in Zeolite Chemistry and Cataljsis, ed. J. A. Rabo, ACS Monograph 171 (American Chemical Society, Washington, 1976), p. 552. 6 J. B. Uytterhoeven, Acta Phys. Chem. (Szeged), 1978, 24, 53. 7 P. A. Jacobs, Carboniogenic Activity of Zeolites (Elsevier, Amsterdam, 1977).p. 183. 8 M. Briend-Faure, J. Jeanjean, M. Kermarec and D. Delafosse, J . Chem. Snc., Faraday Trans. 1, 1978, 9 M. Briend-Faure, J. Jeanjean, D. Delafosse, P. Gallezot, J. Phys. Chem.. 1980, 84, 875. Chemical Society, Washington D.C., 1976) p. 714. Berlin, 1981), vol. 1, p. 1. 74, 1538. 10 N. I. Jaeger, P. L. Ryder and G. Schulz-Ekloff, in Structure and Reactirity cfMon’lJedZeolites, ed. P. A. Jacobs et al., (Elsevier, Amsterdam, 1984) p. 299. 11 P. Gallezot, in Catalysis by Zeolites, ed. B. Imelik et al., (Elsevier, Amsterdam, 1980) p. 223.212 Electron Microscopy of Ni, Pd, Pt in Zeolite Matrices 12 D. Exner, N. Jaeger and G. Schulz-Ekloff, Chem. ing. Techn., 1980, 52, 734. 13 D. J. Elliott and J. H. Lunsford, J . Catal., 1979, 57, 11. 14 D. Exner, N. Jaeger, R. Nowak, H. Schriibbers and G. Schulz-Ekloff, J . Catal., 1982, 74, 188. 15 D. Exner, N. Jaeger, K. Moller, R. Nowak, H. Schriibbers, G. Schulz-Ekloff and P. L. Ryder, in Metal 16 F. Schmidt, in Metal Microstructures in Zeolites, ed. P. A. Jacobs et al. (Elsevier, Amsterdam, 1982), 17 D. Exner, N. I. Jaeger, R. Nowak, G. Schulz-Ekloff and P. L. Ryder, in Proceedzngs cf the 6th 18 P. Gallezot, in Proceedings of the 6th International Zeolite Conference, ed. D. Olson and A. Bisio 19 L. A. Bursill, J. M. Thomas and K. J. Rao, Nature (London), 1981, 289, 157. 20 J. M. Thomas, G. R. Millward, S. Ramdas And M. Audier, ASC Syrnp. Ser., 218, 1983, p. 181. 21 W. J. Reagan, A. W. Chester and G. T. Kerr, J . Catal., 1981, 69, 89. 22 D. Exner, N. Jaeger, K. Moller and G. Schulz-Ekloff, J . Chem. SOC., Faraday Trans. 1, 1982,78, 3537. 23 G. Schulz-Ekloff, D. Wright and M. Grunze, Zeolites, 1982, 2, 70. 24 P. Gallezot, A. 1. Bienenstock and M. Boudart, Now. J. Chim.. 1978, 2 , 263 25 P. Gallezot, Zeolites, 1982, 2, 103. Microstructures in Zeolites, ed. P. A. Jacobs et al. (Elsevier, Amsterdam, 1982), p. 205. p. 197. Zntrrnafional Zeolite Conference, ed. D. Olson and A. Bisio (Butterworths, Guildford, 1984), p. 387. (Butterworths, Guildford, 1984), p. 387. Puprr 51290; Remired 20th February, 1985