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J. Phys. Chem. 1984, 88, 3560-3563
mental data indicate that there exist two different composition regions: nonmetallic (8 MPM).
At the concentrations between 4 and 8 MPM, the deviation from the Korringa relation is interpreted in terms of the enhancement factor, from which the correlation time, T,, is evaluated. This value is constant in this composition range, consistent with the prediction from the “strong-scattering” model. The temperature dependence of the Knight shift and the relaxation rate in the nonmetallic region arise mainly from the increase of the number of unpaired electrons due to the dissociation of the spin-paired species.
The Silicon/Aluminum Ratio and Ordering in Zeolite A Karl Seff* and Mark D. Mellum Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822 (Received: June 28, 1983; In Final Form: February 13, 1984)
X-ray diffraction measurements indicate that the space group of the aluminosilicateframework of zeolite A, ignoring distortions caused by cations and guest molecules, is Fm3c with a 24.6 A. Therefore, either Si/Al is exactly one in the framework or, if greater than one as has been proposed, the silicon enrichment must occur as bounded subregions of the zeolite framework, i.e. islands. 29SiNMR measurements indicate clearly that there are no such islands up to the level of detection, of the order of 2% in the Si/AI ratio. This is true for conventional and for large-crystal preparations of zeolite A and is supported by ESR, STEM, and FABMS results. The crystallographic work of Pluth and Smith, by assuming that islands exist and that Si/AI = 1.1/0.9 = 1.2, fails to account for up to 10% of the exchangeable cations.
Introduction Three serious errors have been made during the past 10 years in the relatively narrow (it might seem) area of zeolite A structure, each leading to several publications in leading chemistry journals. The first of these errors was made by this author, who reported1-I0 that some monopositive and dipositive cations were located remarkably far from any ligand atom and were therefore “zero-coordinate” or “near-zero-coordinate”. It is noteworthy that Cs+ and Rb+ ions2s3apparently do find, in the 8-rings15 of zeolite A, energy minima which are 0.3-0.5 8, farther from their nearest neighbors than the sums of their appropriate ionic radii would indicate, presumably due to the partial coalescence of four normal potential minima in close proximity to give a new minimum at their center. (Other ions may also be a little too far from their nearest neighbor^.^,^) However, the extreme result of zero-coordination, by an ion on a threefold axis deep in the large cavity and more than 1 A from any framework oxide ligand atom, was shown to be wrong by Pluth and Smith,l69l7and this author
(1) Leung, P. C. W.; Kunz, K. B.; Maxwell, I. E.; Seff, K. J. Phys. Chem. 1975, 79, 2157-2162.
Firor, R. L.; Seff, K. J . Am. Chem. SOC.1976, 98, 5031-5033. Firor, R. L.; Seff, K. J. Am. Chem. SOC.1977, 99, 1112-11 17. Firor, R. L.; Seff, K. J . Am. Chem. SOC.1977, 99, 4039-4044. Firor, R. L.; Seff, K. J . Am. Chem. SOC.1977, 99, 6249-6253. (6) Firor, R. L.; Seff, K. J. Am. Chem. SOC.1977, 99, 7059-7061. (7) Subramanian, V.;Seff, K. J. Phys. Chem. 1977, 81, 2249-2251. (8) Firor, R. L.; Seff, K. J. Am. Chem. SOC.1978, 100, 3091-3096. (9) Kim, Y.; Seff, K. J . Phys. Chem. 1978,82, 1071-1077. (10) McCusker, L. B.; Seff, K. J . Am. Chem. Soc. 1978,100,5052-5057. (11) Vance, T. B., Jr.; Seff, K. J. Phys. Chem. 1975, 79, 2163-2167. (12) Subramanian, V.; Seff, K. J. Phys. Chem. 1979, 83, 2166-2169. (13) Subramanian, V.; Seff, K. J . Phys. Chem. 1980, 84, 2928-2933. (14) Takaishi, T.; Hosoi, H. J. Phys. Chem. 1982, 86, 2089-2094. (15) For presentations of the nomenclature used, see: Yanagida, R. Y.; Amaro, A. A.; Seff, K. J. Phys. Chem. 1973, 77, 805-809. Seff, K. Acc. Chem. Res. 1976, 9, 121-128. (16) Pluth, J. J.; Smith, J. V. J . Phys. Chem. 1979, 83, 741-749. (2) (3) (4) (5)
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acknowledged his errore1* Pluth and Smith have gone on to study other members of the above series and to report for each that zero-coordination has not been prover^.'^-^^ The second error was made by Thomas et al., 23-25 who “confirmed” the tentative conclusions of Lippmaa et a1.,26who, in their pioneering applications of 29SiMAS NMR, had concluded that Loewenstein’s rule27was not obeyed by zeolite A. Although it was clear to several groups familiar with the past 15 years of X-ray diffraction results,16~19-22~28~29 beginning with the first report in 1967,j0 that this new result could not be correct, Smith and Pluth denounced it first in print.31 More recent neutron diffraction studies of zeolite A32-34and 29SiMAS N M R work on zeolites A and ZK-435336supported the long-standing X-ray diffraction result, and Thomas et al. acceded.37 (17) Smith, J. V. Proc. Int. ConJ Zeolites, 5th, 1980 1980, 201. (18) Seff, K. Proc. Int. Con$ Zeolites, Sth, 1980 1980, 214-215. (19) Pluth, J. J.; Smith, J. V. J . Am. Chem. SOC.1980,102,4704-4708. (20) Pluth, J. J.; Smith, J. V. J . Am. Chem. SOC.1982, 104, 6977-6982. (21) Pluth, J. J.; Smith, J. V. J. Am. Chem. SOC.1983,105, 1192-1195. (22) Pluth, J. J.; Smith, J. V. J . Am. Chem. SOC.1983, 105,2621-2624. (23) Thomas, J. M.; Bursill, L. A.; Lodge, E. A.; Cheetham, A. K.; Fyfe, C. A. J. Chem. SOC.,Chem. Commun. 1981, 276-277. (24) Bursill, L.A.; Lodge, E. A.; Thomas, J. M.; Cheetham, A. K. J. Phys. Chem. 1981, 85, 2409-2421. (25) Klinowski, J.; Thomas, J. M.; Fyfe, C. A.; Hartman, J. S.J. Phys. Chem. 1981, 85, 2590-2594. (26) Lippmaa, E.; Magi, M.; Samoson, A.; Tarmak, M.; Engelhardt, 0. J. Am. Chem. SOC.1981, 103, 4992-4996. (27) Loewenstein, W. Am. Mineral. 1954, 39, 92-96. (28) Gramlich, V.; Meier, W. M. 2.Kristallogr., Kristallgeorn., Kristallphys., Kristallchem. 1971, 133, 134-149. (29) McCusker, L. B.; Seff, K. J . Phys. Chem. 1981, 85, 166-174. (30) Seff, K.; Shoemaker, D. P. Acta. Crystallogr. 1967, 22, 162-170. (31) Smith, J. V.; Pluth, J. J. Nature (London)1981, 291, 265. (32) Adams, J. M.; Haselden, D. A,; Hewat, A. W. J . SolidState Chem. 1982, 44, 245-253. (33) Adams, J. M.; Haselden, D. A. J. Chem. Soc.,Chem. Commun. 1982, 822-823. (34) Cheetham, A. K.; Eddy, M. M.; Jefferson, D. A.; Thomas, J. M. Nature (London) 1982, 299, 24-26. (35) Melchior, M. T.; Vaughan, D. E. W.; Jarman, R. H.; Jacobson, A. J. Nature (London)1982, 298, 455-456. (36) Thomas, J. M.; Fyfe, C. A.; Rarndas, S.; Klinowski, J.; Gobbi, G. C. J . Phys. Chem. 1982,86, 3061-3064. (37) Cheetham, A. K.; Fyfe, C. A.; Smith, J. V.; Thomas, J. M. J. Chem. Soc., Chem. Commun. 1982, 823-825.
0 1984 American Chemical Society
Silicon/ Aluiminum Ratio and Ordering in Zeolite A
Figure 1. The Si.AI orderine in zeolite A. When the exchaneeable czions and possible guest molecules are ignored, as they are h this drawing, the space group is Fm3c (this figure courtesy of W. M. Meier).
The third error is now being made predominantly by Pluth, Smith, et al.,16,1p22*38 who have concluded that the aluminosilicate frameworks of the large single crystals which they study have a Si/Al ratio of approximately 1.1/0.9 = 1.2, and by others32-34,37 who report various values greater than 1.00 for this ratio for conventional small-crystal preparations. The large crystals of zeolite A studied by Pluth and Smith have been prepared by the same method, C h a r n e l l ' ~ ,as ~ ~those studied in this laboratory continuously since 1971 and which we have, with one erroneous exception early on,4otreated as stoichiometric with Si/Al = 1.00. In some cases, we have studied crystals from the very same preparation, courtesy of George T. Kokotailo. The purpose of this report is to show that these crystals, like any conventional preparation of zeolite A, have Si/Al = 1.O within the limits of experimental error and that there is no evidence that stoichiometry (Si/Al = 1) does not exist. Accordingly, Pluth and Smith have failed to locate up to 10% of the exchangeable cations in their recent reports. It is hoped that the future literature can be freed of its current scatter16,'7J9-22*3234*37,38 of Si/Al ratios from 1.01 to 1.2. This is a problem which has arisen because the various investigators, believing Si/Al to be a variable, have felt it necessary to redetermine it for each of their samples, usually by wet chemical or electron microprobe methods, neither of which is reliable unless extreme care is taken. Wet chemical analysis is not easy to do either for Si or Al, and microprobe methods are destructive to the zeolite and inaccurate due to the high mobility of ions in the zeolite, a particular problem for this surface analysis technique. Also contributing are unreliable population refinements of positions of low occupancy in crystallographic least squares. These often suffer from high correlations and are sensitive to approximations in the choice of space g r o ~ p ' ~(Fm3c , ' ~ and Pm3m are both only approximate space groups for zeolite A because of the perturbing effect of cations and guest molecules).
Discussion In carefully prepared diffraction patterns of highly crystalline zeolite A, "b" reflections are always observed in single cryst a l ~ , ~ ~ *in~the ~ crystalline - ~ ~ * ~p ~ w, d~ e~r , ,~and ~~ ,~~less ' directly by Rietveld analysis of zeolite A p o ~ d e r . ~ ~ - ~ ~ The easiest explanation for their appearance, and the only one ever proposed by crystallographers, is that silicon and aluminum atoms occupy alternate tetrahedral sites with long-range order throughout each crystal or crystallite (Figure l).28,41 This long-range ordering requires that the number of silicon atoms be equal to the number of aluminum atoms, that is, that Si/Al = 1 intimately throughout the material. This is generally consistent with the results of 2 decades of wet chemical analyses and is nicely (38) Gellens, L. R.; Smith, J. V.; Pluth, J. J. J. Am. Chem. Soc. 1983,105, 51-55. (39) Charnell, J. F. J . Cryst. Growth 1971, 8 , 291-294. (40) Riley, P. E.; Seff, K.; Shoemaker, D. P. J. Phys. Chem. 1972, 76, 2593-2597. (41) Broussard, L.; Shoemaker, D. P. J . Am. Chem. SOC.1960, 82, 1041-105 1.
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3561
OZSi
*=A1
Figure 2. The central atom, formerly an AI atom, has been replaced by Si.
TABLE I: Relative Intensities of 29Si MAS NMR Lines for Small DeDartures from Si/AI = 1.00" Si/AI 1.OOb 1.02' 1.04d A14
SiAl, Si2AI2 Si3A1 Si4
100 0 0 0 0
96
92
4 0
0
0 1
0 2
8
a This assumes that silicon enrichment occurs only as the isolated point defects (shown in Figure 2). If the defects are not entirely isolated, they are most likely to occur as Si5Al3O5double 4-rings (Figure 3), and the SiA1, peak would be equally sensitive to small departures from stoichiometry. (The only alternative double-4-ringcompositions, Si6AI2O8,Si7A108,and Si808,are considered unlikely for small departures from Si/AI = 1.00.) A peak at %Alpwith an intensity 4/96 = 4% of the main AI4 peak would be well above the uncertainty of measurements. Its absence indicates that silicon enrichments, if present at all, cannot be more than that corresponding to Si/AI ca. 1.02. b X s , = 0.500, where X,, is the mole fraction of silicon, considering only Si and AI. cXs, = 0.505. dXs, = 0.510.
in accordance with Loewenstein's rule.27 It has been argued that, because the diffraction pattern of zeolite A is more involved than just a and b reflections, this Si,Al ordering of the aluminosilicate framework, and therefore the framework space group Fm3c, is i n ~ o r r e c t . ~ The ~ - ~ intensities ~ of these contributions to the diffraction pattern are I, >> Zb > Z,. I, represents minor departures from the systematic absence conditions of Fm3c: as violations of the c-glide condition (but inconsistently from structure to structure42)and of the F-centering condition.43 These violations are of the kind and of the magnitude to be expected from the perturbing effect of various cations and guest molecules on the zeolite f r a m e w ~ r k and ~ s ~do ~ not provide a basis for criticism of the presently assigned space group and ordering of the zeolite A framework. Nonetheless, the actual symmetry of zeolite A in any of its forms must be far less than Fm3c because of these effects, and in most samples ever studied, be they single crystals or powders, it is probably very low, usually P1. Because ICis present, Fm3c is not the correct space group of any form of zeolite A, only of its idealized (unperturbed by cations, etc.) aluminosilicate framework taken alone, and crystallography done using Fm3c must be interpreted with caution, just as work done using Pm3m, which ignores Zbas well as IC,must be. If the proposal is made that the zeolite is slightly or substantially silicon rich, such enrichment must occur within well-defined bounded sections of the aluminosilicate framework, like occlusions. Only by occurring in such localized regions, or islands, could silicon (42) Gramlich-Meier, R.; Gramlich, V. Acta Crystallogr.,Sect. A 1982, A38, 821-825. (43) Adachi, L. K.; Seff, K., unpublished work, 1976. (44) Cruz, W. V.; Leung, P. C . W.; Seff, K. J. Am. Chem. SOC.1978,100,
6997-7003.
(45) Lee, H. S.; Cruz, W. V.; Seff, K. J . Phys. Chem. 1982, 86, 3562-3569.
3562 The Journal of Physical Chemistry, Vol. 88, No. 16, 1984
Figure 3. The Si atom (front, top, right) shown near three other Si atoms may be considered to have replaced an A1 atom.
enrichment fail to interrupt the long-range ordering that the b reflections require. The simplest such island is the point island, which may be thought of as the result of replacing an aluminum atom in an alternating Si,Al array (Figure 1) with a silicon atom as shown in Figure 2. This is the mode of silicon enrichment assumed by Pluth and Smith16~19-Z2~38 to be the only one operative in zeolite A. Note that each newly introduced silicon is surrounded, ignoring oxygens, by four silicons. Each silicon adjacent to each newly introduced silicon is surrounded by one silicon and three aluminums. The remaining silicons are each surrounded by four aluminums. This should be directly observable experimentally by 29SiMAS N M R as a three-line spectrum with a distinctive intensity ratio and spacing (see Table I), corresponding to the three silicon environments (Si4, SiAI3, and A14). (If the aluminums replaced are relatively close together in the framework, more lines, up to five altogether, may be observed.) However, this is far from what is observed experimentally, either in the crystalline powder35or in large-crystal C h a r n e l l - m e t h ~ dzeolite ~ ~ A prepared in this labor a t ~ r y .Repeated ~~ experiments on various samples of zeolite A show only a simple one-line spectrum which indicates that all silicons have an A14 environment. Accordingly, we may conclude that there is no evidence from 29SiN M R measurements that such point islands exist and that zeolite A is not enriched with silicon in this way. Small concentrations of point islands which would allow Si/Al to deviate from unity by about 2%would be below the limits of detection and are not ruled out by 29SiN M R (see Table I). It is here assumed that large and small crystals of zeolite A from a single preparation have the same composition. A wide range of crystal sizes, perhaps 5-80 wm, results from a Charn e l l - m e t h ~ dsynthesis, ~~ with less than 1% by weight being large enough for single-crystal diffraction work. The 29SiMAS N M R measurements were made on the bulk product of this synthesis; crystallography has been done, of course, only with the largest crystals. Where silicon enrichment is seen, in the avowedly silicon-rich ZK-4 materials which are isostructural with zeolite A, the 29Si N M R spectra35show that it occurs by the general dilution of the aluminum atoms in the material. Initially, at lower levels of silicon enrichment such as Si/Al = 1.18, most silicon atoms not in a A14 environment are surrounded by SiA13, but all five silicon environments are seen. With increasing silicon content the population ~ of silicons in a &AI2 environment increases, and so 0 x 1 . ~Because a line corresponding to a Si4 environment for silicon is always observed for ZK-4,35point defects, possibly clustered, must exist even when the silicon enrichment is relatively small, Le. Si/Al = 1.18. Point defects are one of the modes of silicon enrichment in ZK.4 but are not seen in zeolite A. The only other entirely independent mode of silicon enrichment is indicated by the SiSAl3O8double 4-ring,I5 an arrangement of eight tetrahedral atoms approximately in the form of a cube. There is only one way to arrange these eight ions without violating Loewenstein’s rule (See Figure 3), and it is the same as the result (46) Fyfe, C. A.; Kennedy, G., private communication, 1983.
Seff and Mellum
Figure 4. The smallest island of silicon enrichment which can be generated with Si5A1308double 4-rings.
Figure 5. A diagrammatical representation of Figure 4.
Figure 6. A diagrammatical representation of the next-to-the-smallest
island of silicon enrichment which can be generated with Si5Al3O8double 4-rings. of the replacement of one aluminum of an alternating array with a silicon. The Si atom (front, top, right) shown near three other Si’s may be considered to have replaced an A1 atom. If this new silicon completes its tetrahedron with a fourth silicon (of an adjacent double 4-ring), then the point center already discussed and dismissed for zeolite A would have been generated. If this tetrahedron is completed by an aluminum, a center of silicon enrichment different from the point center will have been generated. Topological considerations, which follow from the requirements of completing rings without violating Loewenstein’s rule, require that the resulting islands be larger than a Si5A1308 (or Si,&,,08, n > 5 and n + m = 8) double 4-ring. Because the rings of zeolite A contain even numbers of tetrahedral (silicon and aluminum) atoms, if a silicon is placed next to another silicon in a ring, this must happen again an odd number of times (at least once) or Loewenstein’s rule will be violated. All islands generated by the use of silicon-enriched double 4-rings are characterized by assemblages of one or more sodalite units (p cages) or large cavities (CYcages), all of whose outer 4-rings (not the 4-rings integral to the body of those cages, but the 4-rings attached to those, which make them “double”) are of composition SizAl2O4with tetrahedral atoms alternating; this allows the island to connect smoothly with the stoichiometric (Si/Al = 1) Fm3c continuum of the zeolite crystal. If the assumption of minimal silicon enrichment is made, as seems reasonable to avoid greater compositional discontinuities than necessary, then only normal Si4AI4O8double 4-rings and others of composition Si5Al3O8would exist. The simplest resulting island is surprisingly large and is depicted in Figure 4 (with surface
J . Phys. Chem. 1984,88, 3563-3566 4-rings not shown, each of cornpositon Si2A1204,Si and A1 alternating; please add one mentally at each of the 10 single 4-rings shown, so that each becomes a double 4-ring.) If Figure 4 is represented by Figure 5, then the next largest island may be represented by Figure 6. A large model (27 sodalite units) was used to make these observations. This exercise has begun to generate islands of silicon-rich structure and could continue on to generate larger such islands. In all of these, some silicon atoms would be tetrahedrally surrounded by groupings of composition SiA13 and Si3Al, which should be readily observable by 29SiMAS NMR. They are for ZK-4, but they are not for zeolite A, neither for the crystalline powder35nor for larger-crystal (Charnell-method) zeolite A.& For each only the single-line spectrum showing all silicons to be in an Alpenvironment is observed. It follows that there are no islands of silicon enrichment of any kind, up to the level of detection. If any exist, it is to an extent corresponding to a silicon enrichment not more than the experimental error of the 29Si N M R measurements, of the order of 2%. Lacking evidence to the contrary, it must be assumed, as is conventional in chemistry, that stoichiometry exists, Si/A1 = 1 in the aluminosilicate framework of zeolite A, and the structure shown in Figure 1 is correct. An ESR study of a sorption complex of zeolite 4A states, “Unlike ESR spectra of NO observed earlier with other zeolites, the ESR spectrum of NO monomer in Na-A zeolite was found
3563
to be well-defined and resolved, reflecting the compositional uniformity (Si/A1 = 1) of the material.”47 A similar result was observed for Zn2+-exchanged zeolite and is attributed to the presence of stoichiometry in the aluminosilicate s t r u ~ t u r e . ~ ’ Scanning transmission electron microscopy (STEM) measurements show that the Si/A1 ratio is the same at the surface as it is in the bulk of a single crystal of zeolite A.48 Fast atom bombardment mass spectrometry (FABMS) gives the same res ~ l t . Other ~ ~ zeolites which are nonstoichiometric show pronounced compositional variation as a function of depth by both methods. This supports the result that zeolite A is stoichiometric. In addition, the STEM work shows Si/A1 = 1.0.48 The synthesis conditions for ZK-4, whose Si/A1 ratios can range upward from, say, 1.2, are substantially different by the addition of the tetramethylammonium ion than those for zeolite A. That such changes in the stoichiometry of the zeolite A framework cannot be effected by lesser modification of the synthesis conditions, such as changing the Si/A1 ratio in the gel, argues for a stubborn stoichiometry that is not readily broken. (47) Kasai, P. H.; Gaura, R. M. J. Phys. Chem. 1982, 86, 4257-4260. (48) Lyman, C. E.; Betteridge, P. W.; Moran, E. F. In “Intrazeolite Chemistry”; Stucky, G. D., Dwyer, F. G., Eds.; American Chemical Society: Washington, DC, 1983; ACS Symp. Ser. No. 218, 199-215. (49) Dwyer, J.; Fitch, F. R.; Qin, G.; Vickerman, J. C. J . Phys. Chem. 1982, 86, 4574-4576.
Diffusion of Rhodium and Iridium within a Chromia Defective Lattice. An Electron Spin Resonance Investigation Leo Burlamacchi,* Italo Ferino, Bruno Marongiu, and Sergio Torrazza Dipartimento di Scienze Chimiche, Universitci di Cagliari, 091 00 Cagliari, Italy (Received: August 16, 1983)
Systems containing Rh or Ir ions deposited on oI-Crz03have been investigated by ESR spectroscopy in order to corroborate the diffusional hypothesis of the noble ions into the ar-Cr203lattice. The results are interpreted in terms of a defective structure of a-Cr203based on the presence of Cr3+vacancies whose mobility falls within the time scale of electron spin relaxation rate. The effect of the thermal treatments under different atmospheres and their influence on the diffusion process is also investigated.
Introduction The catalytic activity of platinum group metals supported on various oxides with respect to dealkylation of aromatic hydrocarbons has been investigated by several The corresponding kinetic results were also reported,36 together with catalysts behavior and ageing phenomena during the reaction performance. Among the investigated systems rhodium supported on a-Cr2O3 deserves particular a t t e n t i ~ ndue ~ . ~to its very low loss of activity. A detailed investigation on this system performed in our laboratory8 showed the existence of a limiting value of rhodium concentration in the catalysts corresponding to the maximum (1) L. M. Treiger, G. L. Rabinovich, and G. N. Maslyanskii, Petrol. Chem. USSR. 12. l(1972). (2) ’G. L. Rabinovich, G. N. Maslyanskii, and V. S . Vorob’ev, Petrol. Chem. USSR,13, 155 (1973). (3) P. Beltrame, I. Ferino, L. Forni, and S . Torrazza, Chim. Ind. Milan, 60, 3 (1978). (4) S . Kasaoka, M. Omoto, T. Watanabe, and K. Tokamatsu, Nippon Kagaku Kaishi, 141, 8 (1975). ( 5 ) K. Kochloefl, Proc. Int. Congr. Catal. 6th, 1122 (1977). (6) P. Beltrame, I. Ferino, L. Forni. and S. Torrazza. J . Catal.. 60. 472 (1979). (7) P. Beltrame, I. Ferino, L. Forni, B. Marongiu, and S. Torrazza, Chim. Ind. Milan, 62, 5 (1980). (8) I. Ferino, L. Forni, B. Marongiu, and S. Torrazza, J . Caral., 85, 169 (1984).
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catalytic utilization of the precious metal. Growth of the metal crystallites with increasing metal content is generally invoked to explain the low utilization of supported metals at high concent r a t i o n ~nevertheless ;~ our results for Rh/a-Cr203system seem to indicate that the existence of such a limiting value is correlated with the diffusion of the noble metal ions within the chromia lattice, rather than with the increase in size of surface rhodium crystallites. A similar behavior has been observed in Rh/yA1203,10in which Rh2+ ions diffused within the y-A1203. An ESR investigation of the diffusion of Rh or Ir ions within the magnetic concentrated system ar-CrzO, has been undertaken in the present work in order to get a better understanding of the physico-chemical interactions occurring between the noble metals and the support.
Experimental Section The a-Cr2O3 support was prepared from chromium nitrate (Cr(N03)3-9H20,C. Erba RPE, Merck for analysis) by Burwell’s method and carefully calcined at 1150 K in order to avoid the so-called “glow phenomenon”.” The surface area of our cu-Cr203 (9) F. Figueras, S. Fuentes, and C. Leclercq, “Growth and Properties of Metal Clusters”, Elsevier, Amsterdam, 1980. (10) H. C. Yao and M. Shelef, Proc. Int. Congr. Catal. 7rh, Part A, 329 (1981). (11) F. S . Baker, J. D. Carruthers, R. E. Day, K. S. W. Sing, and L. J. Stryker, Discuss. Faraday SOC.,52, 173 (1971).
0 1984 American Chemical Society
