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J. Phys. Chem. 1983,87,94-97
in each case.27 These calculations utilized a cavity distribution function. More recently a value of -0.4 cm3mol-l was obtained by using a Monte-Carlo statistical approach.29 Both calculations utilized a Scott-Scheraga potential function for butane. Molecular volume changes with torsional angles in n-hexane and 1,1,2-trichloroethane have been calculated by assuming an overlapping sphere model and it is found that these changes significantly alter the internal rotation potential function, conformational energies, and populations of the conformers in solution in comparison with the gas phase. AV(anti-gauche) for hexane is -3.55 cm3 mol-l and for 1,1,2-trichloroethane it is 1.44 cm3 m01-l.~~In general, conformational volume changes are only -2 cm3mol-' for average substances with molar volumes 100 cm3 mol-'. Thus a change in internal pressure of 3000 atm would produce a shift of 150 cal in the conformer relative energiesa2 Since AV' for molecular internal rotations is roughly an order of magnitude larger, medium effects on AG* going from gases to solutions should be on the order of 1-2 kcal mol-' due entirely to packing forces in the liquid. This factor has been neglected in studies of phase dependence on activtion energies to internal rotation in molecules. AG* for nonpolar molecules present in dilute solutions of nonpolar solvents can differ significantly from the gasphase values. This effect is extremely important when comparing small barriers in liquids obtained from NMR long-range coupling constants with those obtained from gas-phase spectroscopic measurements. Values obtained for internal rotation barriers in 10% solutions of substituted ethyl-,31 i ~ o p r o p y l -and , ~ ~cycl~propylbenzene~~ in CS, are 1.2 (l),
-
-
(30) T. Bleha, J. GajdoB, and I. TvaroBka, J . Mol. Struct., 68, 189-98 (1980). (31) T. Schaefer, L. Kruczynski, and W. Niemczura, Chem. Phys. Lett., 38, 498-9 (1976).
2.0 (2), and 2.0 (3) kcal mol-', respectively. CS2 at 298 K has an internal pressure of 3715 atm.24 Gas-phase lowresolution microwave spectra of substituted isopropyl-34 and cyc1opropylbenzeneP are compatible with much lower internal rotation barriers of 250 (140) and 120 (90) cal mol-', respectively. The phase dependence of these energy barriers is compatible with expected internal solvent pressure effects. For systems with significant volume differences between conformers or conformers and transition states internal solvent pressure effects are a major determining factor in the medium dependence of thermodynamic and kinetic parameters. Acknowledgment. We are glad to acknowledge support from the National Institutes of Health through Grant 1 R01 GM 29985-01, the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the University of California-Davis Committe on Research. We thank Dr. G. B. Matson of the U.C. Davis NMR facility for providing technical help and valuable advice. We are very grateful to Professor N. L. Allinger of the University of Georgia who made valuable suggestions about this research. We are also grateful to Professor W. K. Musker of U.C. Davis for the SF, used in this study to Jesse Douglass Hartline, Jr., and J. Paul Chauvel, Jr., for help with calculations, and to Professor R. K. Bohn of the University of Connecticut for perceptive comments and suggestions about this study. Registry No. N,N-Dimethyltrifluoroacetamide,1547-87-1. (32) T. Schaefer, W. J. E. Parr, and W. Danchura, J . Magn. Reson., 25, 167-70 (1977).
(33) W. J. E. Parr and T. Schaefer, J. Am. Chem. Soc., 99, 1033-5 (1977). (34) N. S. True, M. S. Farag, R. K. Bohn, M. A. MacGregor, and J. Radhakrishnan, submitted for publication. (35) N. S. True, R. K. Bohn, A. Chieffalo, and J. Radhakrishnan, submitted for publication.
Silver Atoms and Small Silver Clusters Stabilized in Zeolite Y: Optical Spectroscopy Geoffrey A. Ozln" and Francols Hugues Department of Chemistv, Lash Miller Chemical Laboratories, university of Toronto, Toronto, Ontario M5S 1A 1, Canada (Received: May 7, 1982; In Final Form: August 17, 1982)
Diffuse reflectance and fluorescence emission/excitation spectra of Ag-exchanged zeolite Y samples recorded under various pretreatment conditions and silver loadings which are known to favor the formation of silver atoms and small charged silver clusters are reported. Under low loading conditions, the spectral data are interpreted in terms of isolated silver atoms in site I by comparison with the corresponding data for silver atoms entrapped in rare gas solids. Higher silver loading favors the formation of an Ag2+cluster which appears to be best described as a silver atom in site I interacting with a silver ion in site 1'. Oxygen treatment at elevated temperatures (400-480 " C ) of completely Ag-exchanged zeolite Y induces silver ion migration and subsequent formation of an Aga2+cluster from the existing Agz+ cluster by occupying the vacant site I' adjacent to the silver atom in site I.
Introduction It has been shown recently by different techniques that small charged silver clusters can be formed and stabilized in the cavities of zeolites A, x , and y. Uflterhoeven et a1.1 have established by X-ray crystallography that a hy(1)L. R. Gellens, W. J. Mortier, R. A. Schoonheydt, and J. B. Uytterhoeven, J . Phys. Chem., 85, 2783 (1981).
drated silver-exchanged zeolite A, when subjected to a vacuum treatment at moderate temperatures contains the linear cluster Ag3'+ located in the sodalite cage of the lattice. Under vacuum treatment at higher temperature, these trinuclear cluster cations agglomerate to Ag64+also located in the sodalite cages. Optical absorptions observed in the region 300-600 nm have been attributed to and Ag,4+ by comparison with the known optical spectra
0022-3654/83/2087-0094$0 1.50/0 0 1983 American Chemical Society
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Spectroscopy of Ag-Exchanged Zeolites
TABLE I degree of exchange, Ag,Na-Y Ag,Na-Y Ag ,,Na-Y Ag,,Na-Y Ag,,Na-Y
%
Ag+ per unit cell
5.5 11.0 23.5
3 6 13
63.5
35 55
100.0
% Ag weight in the dry zeolite
2.5 5.0 10.0 24.0 34.0
of Ag, ( n = 1-6) stabilized in rare gas solids.2 In the case of silver-exchanged zeolites X and Y, thermal treatment under vacuum followed by an oxygen exposure was found to produce the clusters Ag2q+and linear Ag3P+ (p and q not defined) in completely exchanged zeolite Y and X, re~pectively.~The formation of these charged clusters arises from the simultaneous occupancy of the hexagonal prismatic site I and one or two adjacent sites I' in the sodalite cage. No optical data have been published concerning these charged clusters. However, Kellerman and Texter4 obtained evidence that isolated silver atoms are formed during the dehydration of low exchanged silver zeolite Y, that is, Ag,Na-Y. The appearance of an absorption band at 306 nm, a fluorescence emission at 490 nm excited in the UV range, as well as the excitation spectrum showing a maximum at 306 nm, constitute strong evidence for the existence of zeolite-entrapped silver atoms. The resistance of this species to oxidation by molecular oxygen led the authors to conclude that the isolated silver atom most probably resides in site I. In this article, we report our diffuse reflectance and fluorescence emission/excitation spectra related to the silver atoms and silver clusters Ag2q+and Agg+ stabilized in zeolite Y, and elucidate the mechanism of formation of the latter.
Experimental Section A series of silver-exchanged zeolite Y samples were prepared from zeolite Na-Y (Linde) by conventional ionexchange procedures, using dilute silver nitrate solutions. The notation and composition of the samples are given in Table I. The hydrated zeolites were placed in a quartz, flat cylindrical cell equipped with a valve for vacuum treatment and gas admission. The fluorescence and reflectance spectra were recorded at different steps of dehydration and oxygen treatment. The fluorescence spectra were recorded on a Perkin-Elmer MFP-44 fluorescence spectrometer by selecting both the excitation wavelength (Aex) and the emission wavelength (Aem) with scanning monochromators,the wavelength range being 200-900 nm. The diffuse reflectance spectra were recorded on the same samples used for fluorescence measurements, using a Cary 17 UV-visible spectrometer and a diffuse reflectance attachment working in the diffuse, nondisperse illumination mode. MgC03 was used as a reflectance standard. Only the range 320-700 nm could be recorded with the available accessories. Results and Discussion The fully hydrated samples do not show any fluorescence or absorption in the wavelength ranges examined. However, when submitted to vacuum treatment at in(2)G.A. Ozin, Symp. Faraday Soc., 14,7 (1980),and references cited therein. (3) L. R. Gellena, W. J. Mortier, and J. B. Uytterhoeven, Zeolites, 1, 11, 85 (1981). (4)R. Kellerman and J. Texter, J. Chem. Phys., 70,1562 (1974).
,
1
300
356
500
550
600
650 nm
F w e 1. Fluorescence excitation and emission spectra of Ag,Na-Y, (where x = 3-55).
creasing temperature, the zeolite samples show fluorescence emission and excitation spectra whose intensity monotonically increases up to 500 "C. In Figure 1, the excitation and emission profiles for the samples Ag,NaYso0are presented. These profiles remain essentially invariant after oxygen treatment at 400 "C. In the case of low loading samples ( x = 3, 6) the excitation and emission bands at 3081312 and 4831490 nm, respectively (Figure 1, A and B), are in line with Kellerman and Texter's data4 and are best interpreted as arising from isolated silver atoms residing in an inaccessible site of the zeolite lattice, most probably site I at the center of the hexagonal prism. The dynamical and energetic properties of intrazeolitic silver atoms in site I appear to be quite similar to those recently observed for silver atoms entrapped within the cubooctahedral sites of the solid rare gases.5 This interesting observation will form the subject of a detailed study.8 In the case of the completely exchanged sample ( x = 551, the reflectance spectrum taken after oxygen treatment at 300 "C (Figure 2A) shows an intense and narrow absorption at 340 nm and the excitation spectrum shows a maximum at 340 nm associated with a broad emission centered around 544 nm (Figure 1F). At this level of exchange, Uytterhoeven's crystallographic results3 indicate the probable presence of a charged cluster AgPQ+ arising from the simultaneous occupancy (at least to a certain extent, measured by the excess of population in site 1') of one site I and one adjacent site I' by a silver ion. Therefore, we are tempted to associate the species giving rise to these optical spectra to the cluster Ag2q+. It is interesting to compare the spectrscopic properties of site 1,I' isolated Ag2q+in zeolite Y with those for Ag, entrapped in rare-gas solids.6 In the latter one finds a (5)G.A. Ozin, S. A. Mitchell, J. Farrell, and G . Kenney-Wallace, J. Am. Chem. SOC.,102, 7702 (1980). (6) S. A. Mitchell, G. A. Kenney-Wallace, and G. A. Ozin J.Am. Chem. SOC.,103,6030 (1981).
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The Journal of Physicel Chemistty, Vol. 87,No. 1, 1983
--
Ozin and Hugues
_______
__
t t t I
t
\
I EW)
w z a m
0
Kl
m Q
t
i4 F2 3
-
-
d
c -
,2
I
j
L
I
800
I
,,
----' I
70C
GOO
500
300
400
h (ntii)
s---
w r o gcs s -
IC
1
L
10 I
20 I
3 0 40 r(Ac-tqjA I 1
Figure 2. Diffuse reflectance spectra of Ag,,Na-Y,,, as a function oxygen treatment at dlfferent temperatves (oxygen pressure, 400 t w ) . Lower trace shows the baseline of the hydrated sample.
Figure 3. Qualitative representation of the X and A state potential energy curves for gaseous, rare gas solid, and zeolite Y isolated Ag, and Ag,'.
strongly blue-shifted gas to matrix X,lXg+ A,'&+ transition which appears as a broad, structureless absorption band, and for which photoexcitation produces atomic fluorescence as a result of a cage-assisted excited state Ag, dissociation process. The blue shift is due mainly to strong destabilization of the excited A state of Ag, (bound in the gas phase) where the energetics is such that the optically prepared state is unstable with respect to dissociation to excited atomic fragments. On the other hand, the ground state of Ag, is found not to be strongly perturbed in the rare-gas matrix. The instability with respect to excited-state dissociation arises from two factors. Firstly, there are repulsive (destabilizing) Ag,-rare gas interactions in the excited state of Ag2 which originate from electronic repulsion effects due to the diffuse charge density associated with the A state (Le., resulting in blue gas to matrix shifts). Secondly, the + ,P atomic fragments, which correlate with the optically prepared A state of Ag,, are strongly stabilized by matrix-cage relaxation effects in a manner analogous to that described in detail for ?P atomic Ago in rare gas s01ids.~The extent to which the destabilization of the photoexcited Ag, is successful in promoting net dissociation is determined by the detailed relaxation dynamics of the excited Ag,-rare gas cage complex.6 This idea is illustrated schematically in Figure 3. Compared to gaseous Ag,, the gaseous cation Ag2+is expected to have a less strongly bound X state (,E,+) and a dissociative (or weakly bound) A state (2Zu+)which correlate with the separated atom and ion limits ,S(Ag0) + 'S(Ag+) and ,P(Ag0) + lS(Ag+),respectively (Figure 3). For zeolite Y isolated Ag2+,repulsive interactions with the O(3) atoms at site 1,I' will act to further destabilize the A
state of Ag2+with respect to dissociation to excited Ago ,P atomic fragments (Figure 3), where the latter will be strongly stabilized by site I cage-relaxation effects primarily involving the six O(3) oxygen atoms. One therefore expects the energetic and dynamical properties of Ag2+in zeolite Y to be such that the A X transition should be blue shifted compared to that of Ag, in rare-gas solids6 (Ag2+: 340 nm, zeolite Y; Ag,: 389 nm, Ar; 390 nm, Kr; 391 nm, Xe; 435 nm, gas phase) and for which photoexcitation should produce cage-relaxed 2P Ago atomic fluorescence as a result of either an intrinsic/and or cage-assisted excited-state dissociation process of Ag2+in site 1,I'. The Ago emission from Ag2+in zeolite Y is expected to be somewhat red shifted (544 nm) compared to the emission from cage-relaxed 2PAgo atoms isolated in site I (483/490 nm) as a result of short-range repulsive interactions with the neighboring site I' Ag+ ion, the latter destabilizing the ,S state to a greater extent than the ,P state of Agothereby reducing the ,P ,S energy separation. Overall therefore, a net charge of 1+ seems reasonable and hence an Ag2+ formulation for the binuclear charged cluster in zeolite Y. In the case of Ag13Na-Y, the excitation and emission spectra (Figure IC)indicate the presence of a mixture of isolated atoms (313 and 495 nm, respectively, in line with the spectral properties of Ag3Na-Y and Ag6Na-Y) as well as charged clusters Ag2+associated with the shoulder at 334 nm in the excitation spectrum giving rise to the 530-nm emission band (Figure 1D). The Ag,Na-Y sample is found to contain mainly the species Agz+(excitation maximum at 338 nm, emission at 535 nm, Figure 1E) with a small amount of isolated silver atoms as seen by slight band broadening on the high-energy side of the excitation band.
-
+ -
-
The Journal of Physical Chemistry, Vol. 87, No. 1, 1983 97
Spectroscopy of Ag-Exchanged Zeolites
These results demonstrate that beginning with isolated silver atoms in the samples Ag,Na-Y and Ag,Na-Y, an increase in silver ion concentration induces a growth in the population of Ag2+species, by a progessive occupancy of site I’ adjacent to site I. Taking into account the site occupancies obtained by Uytterhoeven3for site I (12.8 Ag) and site I’ (13.1 Ag) in Ag,,-Y, together with our fluorescence results, we can deduce that in completely exchanged silver zeolite Y there exists a strong tendency for the silver ions to simultaneously occupy the site I and I’ positions to form Ag2+,up to a maximum of approximately 13 Ag2+/unitcell. In Figure 2 are presented the diffuse reflectance spectra obtained after oxidation of Ag,,Na-Y under O2 at increasing temperature. When the temperature of O2 treatment is raised above 300 “C, up to 480 “C, the sample turns yellow and one observes the progressive increase of a new band at 402 nm with a concomitant decrease of the 340-nm band associated with Ag2+. The behavior of these two bands indicates that the new species giving rise to the 402-nm absorption is generated at the expense of the Ag2+ clusters. Moreover, the position of the new band suggests that it corresponds to a small cluster.2 Unlike Ago and Ag2+,fluorescence emission could not be excited for this new silver cluster species anywhere in the range 200-700 nm (cf. Ag,/Ar photodissociation/cage recombination/ atomic emission induced by visible and UV excitation2y7). Taking into account that the linear cluster Agt+ stabilized in the sodalite cage of zeolite A has been shown to absorb at 446 nm’ and that Ag, isolated in rare gas solids absorbs in the range 400-440 nm,2,7we tentatively attribute our new 402-nm absorption to an Ag? cluster. Such a cluster could be envisaged to form from Ag2+by simply filling the vacant site I’ adjacent to a filled site I. It would in fact correspond to the Ag$+ cluster identified by Uytterhoeven by X-ray diffraction anal~sis.~ The spectral shift from 402 to 446 nm probably reflects the different environment of A g t + in the 0cage of zeolite A compared to site I + 2 sites I’ of Ag+ : in zeolite Y, and/or the slightly different silver-silver bond lengths (2.85-3.00 A in zeolite A,’ 3.08-3.17 A in zeolites X and Y3). A detailed investigation of the electronic properties of linear Ag3Q+(q = 0, 1 , 2 ) and Ag24+ ( q = 0, 1) based on SCF-Xa-SW molecular-orbital calculations will appear in forthcoming papers.8J2 The formation of Ag32+from Ag2+requires the filling of a vacant site I’ adjacent to a filled site I-site I’ couple, which implies the migration of a silver ion from another lattice site (I, 11, or 11’) to site 1’, under the influence of (7) G. A. Ozin, H. Huber, and S. A. Mitchell, Inorg. Chem., 18, 2932 (1979). (8) G. A. Ozin, F. Hugues, D. McIntosch, and S. Mattar in
‘Intrazeolitic Chemistry”, G. Stucky, Ed., ACS Symposium Series, Washington, DC, 1982; J . Phys. Chem., submitted for publication.
oxygen treatment at elevated temperature, in line with Uytterhoeven’s recent X-ray diffraction ob~ervations.~ Because site I1 shows the highest occupancy, it is likely that the migration depletes this site, defining the process as a migration from the supercage (11)to the sodalite cage (1’). Such a migration of ions has been shown by X-ray crystallography to also occur in the case of platinum- and palladium-exchanged zeolite Y9 under O2treatment; for example, in O2 at 300 OC most of the Pt2+ions are in the supercages whereas after O2 treatment at 600 “C most of them reside in the sodalite cages. We, therefore, presume that such a migration is likely to occur in the case of silver-exchanged zeolite Y and would be responsible for the formation of A g t + from Ag2+. All of our samples of Ag,Na-Y have been examined by ESR spectroscopy with conventional equipment at 298 and 77 K after vacuum and oxygen treatment. No signal corresponding to any of the species Ago, Ag2+,and Ag32+ discussed previously have been observed. The nonobservation of an ESR signal under conventional recording procedures, which has also been the experience of other workers with the same type of silver zeolite samples,lJo could arise from an exchange broadening phenomenon involving a paramagnetic center or impurity. Alternatively, the ESR silence could originate from a saturation phenomenon of the type reported by Kasai for Na:+ in zeolite Na-Y.l’ To overcome this saturation problem it would appear to be necessary to record the ESR spectra in the dispersion mode as described by Kasai.”
Conclusion In summary, the main result of this preliminary study is that, in completely exchanged silver zeolite Y, the formation of the cluster Agt+ occurs in two well-defined steps, the first one being the production of the cluster Ag2+under vacuum treatment from the simultaneous occupancy of a silver atom in site I and a silver ion in site 1’, the second one being the migration of a silver ion under oxygen treatment toward the cluster Ag2+filing the vacant site I’ thereby leading to Apt+. Acknowledgment. The generous financial assistance of the Natural Sciences and Engineering Research Council of Canada’s Operating and Strategic Energy Programmes is greatly appreciated. Registry No. Ag, 7440-22-4; Agz+, 12187-07-4; Ag?, 5222734-6. (9) P. Gallezot, Catal. Reu., 20,121 (1979); G. Bergeret, P. Gallezot, and B. Imelik, J. Phys. Chem., 85, 411 (1981). (10) P. H. Kasai, personal communication. (11) P. H. Kasai, J. Chem. Phys., 43, 3322 (1965). (12) G. A. Ozin, F. Hugues, D. McIntosh, and S. Mattar, manuscript in preparation.
