J . Phys. Chem. 1993,97, 7331-7336
7331
Pyridine Adsorbed on Na-Faujasite: A FT-Raman Spectroscopic Study Roeland Ferwerda’ and John H. van der Maas Department of Analytical Molecular Spectrometry, Utrecht University, P.O. Box 80083, 3508 TB Utrecht. The Netherlands
Patrick J. Hendra Department of Chemistry, University of Southampton, Highfeld, Southampton SO9 SNH, U.K. Received: March 1, 1993; In Final Form: April 22, I993
The adsorption of pyridine as a probe molecule in the characterization of Na-faujasites has been investigated with FT-Raman spectroscopy. It appears that the interaction of pyridine is strongly affected by the presence of adsorbed water within the zeolite structure. At pyridine coverages around 180 mg/g of zeolite a signal at 998 cm-* is observed, attributed to pyridine adsorbed onto coordinated water. At higher coverages this signal decreases, whereas a band at slightly higher wavenumber arises due to the interaction with physisorbed water. At low coverages an increase in wavenumber is observed, assigned to pyridine adsorption on relatively more acidic OH sites or N a + cations.
Introduction The use of infrared spectroscopy in the characterization of catalyst surfaces is well established. The possibility to measure at elevated temperatures and under reaction conditions makes infrared spectroscopy an important in situ technique. However, the infrared spectrum is dominated by the absorption of the framework vibrations of the adsorbent. Raman spectroscopy does not suffer from this nuisance, because many metal oxides show almost no Raman activity.14 Owing to the appearance of intense backgrounds, mainly caused by fluorescence, conventional Raman spectroscopy (excitation with a visible source) has never became a routine technique in the analyses of adsorbed species on catalysts. These backgrounds often emerge with particular intensity during activation of the catalyst in vacuum and with even more persistence when elevated temperatures are used. Peculiarly, the use of very high activation temperatures in an oxygen atmosphere frequently reduces the problem, but the catalyst can be changed by doing so. The reasons for the appearance have been the subject of discussion but are often not completely understood and cannot be avoided in many Since the development of near-infrared-excited Fourier transform Raman spectroscopy (FT-Raman spectroscopy) the technique has been resurrected. On excitation with a near-infrared source, viz., a Nd:YAG laser with a wavelength of 1064 nm, fluorescence has been shown to be much less of a problem, because the laser line is well separated from the electronic level^.^ Some authors already reported on the use of this new technique in catalysis research. However, again problems have been observed, uiz., heating and, once more, fluorescence.l”l2 Nevertheless improvements are obvious. Burch et a1.10 and Hendra et al.11 describe the use of FT-Raman spectroscopyin the characterization of zeolites using different probe molecules. Their results look promising, although they mention limitations caused by sensitivity and heating problems. The probe molecule pyridine is often used to examine the acidity of metal oxide catalysts. As a result of its electron donor capabilities, the strength of electron pair acceptor sites can be monitored.13-18 These sites can be studied with infrared spectroscopy in the region from 1700 to 1400 cm-1. In this part of the spectrum pyridine has strong absorptions, i.e., vga, VBb, V19a, and v19b (following Wilson’s moden~mbering~~), wheremost metal oxides show an infrared window. These pyridine modes are weak
* Author to whom correspondence should be addressed.
TABLE I: Vibrational Modes (cm-I) of Pyridine at Different Adsorption Sites adsorption site utaa ugha v192 U19ha v12* none 1583 1575 1480 1440 1030 OH group 1599 sh -1485 1447 1035 Branstedsite -1640 -1610 -1485 1540 Lewissite -1630 -1585 1490 1465 1046 a IR assignments from ref 13. b Raman assignments from ref
vib ~
991 1000 1010 1020 18.
in Raman spectroscopy,where the ring breathing region (YIZ and v1 mode) is a better choice to study the interactions of pyridine with surfaces. The assignment of thevibrational modes of pyridine on different sites is presented in Table I. In this study FT-Raman spectroscopy has been used to study the adsorption of pyridineon zeolites, i.e., (hydrated) Na-faujasites with general molecular formula NaxA1$i192-xO~s,-yH20. Neither NaX nor NaY exhibit Brransted or Lewis acidity. In these cases adsorption is limited to O H groups and cations present at the surface and, as a consequence, interpretation of the vibrational spectra should be relatively simple compared to catalysts which show Brransted or Lewis acidity. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) has been applied as a complementary technique. We believe that more detailed information will be obtained on the differences between the adsorption sites present at the surface, because of the smaller half-bandwidth in Raman spectroscopy. Furthermore we expect that Raman spectroscopy might help in the assignment and understanding of the infrared spectrum of probe molecules adsorbed onto metal oxide surfaces.
Experimental Section FT-Raman spectra were measured on a Perkin Elmer 1760 X FT-IR spectrometer, equipped with a Raman module. The InGaAs detector was operated at room temperature. Scans, 128, were co-added at a resolution of 2 cm-1 (measurement time approximately 18 min). A laser power of 200 mW was used for the highest loading and was increased to 500 mW to record the subsequent spectra. The Raman cell and vacuum system are described in detail elsewhere.lOJ1 Two Na-zeolites were used, viz., NaX (Union Carbide) with x = 85 and y = 255 and NaY (Crosfield Catalysts) with x = 56 and y = 249. As can be derived from the molecular formulas the Si/Al ratio is lower in the X than that in the Y type zeolites
0022-365419312097-7331$04.00/0 0 1993 American Chemical Society
Ferwerda et al.
7332 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993
Figure 1. DRIFT spectrum of NaX and NaY zeolite after activation at 150 O C (KM = Kubelka-Munk).
(respectively 1.3 and 2.4). For that reason the distribution of cations and water in the (de)hydrated structures varies (vide infra). The catalysts were stored under atmospheric conditions. Zeolites were activated a t 120 OC in vacuum (pressure I Torr) overnight. After pretreatment the catalysts were cooled to 24 OC in vacuum and then exposed to 20 Torr of pyridine (Fluka Chemika dried over molecular sieves, 3A) for at least 3 h. Aliquots of pyridine were desorbed, and Raman spectra with ever lower pyridine loadings were taken at room temperature. The Nd:YAG laser heats the sample to varying and unknown degrees in this type of experiment. Where the sample is of low density and held in a vacuum, the problem is potentially severe. To minimize this nuisance, which in extreme cases can cause desorption of the adsorbate away from the viewed part of the specimen, the cell was filled with dry air prior to each measurement, resulting in increased heat removal from the sample. The catalyst system was invariably catalyst/pyridine adsorbed/ pyridine vapor/dry air at 1 atm. The pyridine loading was estimated by weighing the cell and corrected for the weight loss of the catalyst during activation (inaccuracy 3 mg/g). Due to thestrong interaction of pyridine with the surface a t low coverages, it was necessary to increase the desorption temperature (up to 200 "C) to achieve loadings lower than 200 mg/g. In order to obtain high spectral reproducibility, the cell was carefully replaced in the same position before each measurement.20 Measurements on one catalyst with differing loadings were carried out successively to prevent errors caused by the fluctuations of the instrument performance. The Raman spectra were not corrected for instrumental response, but the intensity of the Raman bands was scaled for differences in the laser power. Base line correction was accomplished by fitting the background in the region 1100-900 cm-I. Spectra were resolved by means of the Mattson FIRST curve fitting routine. Following Pemberton e? the bandshape of the individual bands was assumed to be a summation of Lorentzian and Gaussian functions (respectively 20% and 80%). Fitted spectra were verified with the aid of self-deconvolution and second derivative routines. The inaccuracy of the band areas is believed to be within 5% for an intense band and somewhat higher for a small one. DRIFTS measurements were performed on a Perkin Elmer 1760 X FT-IR instrument equipped with a controlled environment DRIFTS cell (SpectraTech, Model0030-100). Scans, 128, were co-added a t a resolution of 2 cm-I (measurement time approximately 18 min). Catalysts were activated at 150 OC overnight in dry air and then cooled in a 10% 02/Ar flow. Pyridine was introduced into the gas stream by means of a saturator operating
at 21 O C . Adsorption of pyridine onto the zeolites took place over 5 min until equilibrium was reached (no intensity changes were observed). In Raman experiments with the same flow system, this pretreatment appeared to be comparable to the settings used in the Raman experiments with the vacuum system. The pyridine vapor pressure above the surface of the catalysts a t different coverages was measured by means of a pressure transducer (Transamerica Instruments, Model BHL 4250/ 10) in isotherm desorption experiments a t 24 0C.22 We assumed that equilibrium was reached after 15 min.
Results The differences in activation for the infrared and Raman experiments are believed to be negligible. On activation we observed a weight loss of approximately 23% for both samples. This is attributed to the disappearance of physisorbed water. Water coordinated in the zeolite lattice (water of crystallization) remains in the zeolite structure (see molecular formula). The DRIFT spectra (Figure 1) of both zeolites after activation show a sharp band around 3695 cm-I in the OH stretching region. This band is due to the stretching vibration of a single OH group of isolated and coordinated water.23 The broad band (3500-3 100 cm-I) is ascribed to thestretchingvibrations of bridged OHgroups, showing an Evans hole around 3280 cm-I caused by Fermi resonance with the first overtone of the H20 deformation mode a t about 1645 cm-1.24325The band at 3608 cm-I in the NaY zeolite is attributed to surface hydroxyl groups in close contact to lattice cations since it is affected by different cations.23 After adsorption of pyridine the O H stretching bands in the infrared spectra shift to lower frequency. In the 1700-1400-~m-~ region bands appear at 1606 (1614), 1591 (1595), 1573 (1575), 1490 (1490), and 1442 (1443) cm-I for NaX (Nay), whereas a decrease in absorption is observed a t about 1645 cm-l (Figure 2). The latter band is attributed to the bending mode of coordinated water.23 As mentioned above, signals in the region 1700-1400 cm-1 are weak in the Raman spectra. Hence Figure 3 parts a and b displays the Raman spectra of both zeolites in the region 1050-970 cm-1 with different pyridine loading. Two intense bands are observed a t approximately 1035 and 1000 cm-I. The former is attributed to the vI2 mode and the latter to the vl mode of pyridine, both adsorbed on a weak electron pair acceptor. As can be deduced from Table I, the spectra in Figure 3 correspond to the spectrum of pyridine interacting with an OH group. In contrast to the Raman spectra of pyridine on a NaY zeolite published by Burch et a1.I0 our spectra show no physisorbed pyridine (signal a t 990 cm-l ) .
Pyridine Adsorbed on Na-Faujasite
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 7333 1442
1591 h
Y
1490
.a
E
z
k5
D
.e
9 P
1645
'
1443
1595
4
86
1490
,
I
I
L
a 7.
a 260mug
T
6.
d
.
175 m u g
v
Raman Shift (cm-1)
b 2.5
.
2.0
.
9
253mg/g 179n1g/g
3 4
Raman Shift (cm-1)
Figure 3. FT-Raman spectra of pyridine adsorbed on NaX (a) and NaY (b) zeolite at different coverages of pyridine (ring breathing region).
The band maxima of the vl and v12 modes shift on desorption. Egerton et a1.6 also observed a shift of the same order of magnitude of the v1 mode of pyridine adsorbed onto faujasites, though they did not give an explanation for this phenomenon. As the effect is much smaller for the v12 than for the uI mode, we will not
discuss the former band in detail. Based on analysis of the band profiles (an example is given in Figure 4 parts a and b) we conclude that the band ascribed to v1 consists of three overlapping features-one a t 1003 cm-1 (due to a species that desorbs most easily), another at 998 cm-1, and the third near 1001 cm-l for the
Ferwerda et al.
1334 The Journal of Physical Chemistry, Vol. 97, No. 28, I993
a 2.50
.
'?.
3P
2.00.
A
1.50
'I
1
'
1"
0.50 .
lox)
1020
1010
loo0
990
980
970
Raman Shift (cm-1)
b 2.50
-?,
?
P
2.00
'I
3
1
1.50
1.00
0.50
0.00
Raman Shift (cm-1)
Figure 4. Examples of the deconvoluted spectrum (a) and the fitted spectrum (b) of pyridine adsorbed on the NaY zeolite (pyridine loading 253 mg/g).
NaX zeolite. For the NaY zeolite the equivalent features appear at 1001, 998, and 1004 cm-l, respectively. During the first desorptions at 24 "C only the first band disappears. Pyridine adsorbed onto alternative sites and sorbed more strongly can only be removed effectively at higher temperatures. Figure 5 shows the relative amount of pyridine contributing to the three different signals as a function of the total pyridine coverage. We have assumed that the Raman cross section remains unaltered for the different wavenumbers, resulting in a reasonable linear relationship between the total area of the V I mode and the pyridine coverage. Isotherm experiments a t 24 OC on both catalysts show that at coverages below, respectively, 210 mg/g for NaX and 230 mg/g for NaY the pyridinevapor pressure above the surface is negligible (PIP0< 0.02).
Discussion Both NaX and NaY have the faujasite structure and exhibit neither Bronsted nor Lewis acidity.23 The three-dimensional structure is obtained by linking sodalite units with hexagonal prisms. Each sodalite unit is surrounded by four other similar units in a tetrahedral configuration. Figure 6a presents one unit of the faujasite type. The drawing shows only three neighbors of one sodalite unit, but this is a consequence of the termination of the drawing. The circular 12-membered ring has a diameter of 7.4 A, while the supercages are 11.8 A across. The cation and water distribution in the sodalite cage and in the hexagonal prisms
is given in Figure 6b and Table I1and for dehydrated and hydrated faujasites.23 However, the distribution is complex and still subject to discussion, and the location of the cations and water in the supercage is not convincingly established. It is clear that the amount of adsorbed water strongly affects the morphology of the material^.^^ At 24 OC only the cations and the water situated in the supercage are accessible to pyridine (remember the pyridine molecule is -5 A). We will now compare our DRIFT spectra (Figure 2) with data reported in the 1 i t e r a t ~ r e . l ~As ' ~ only weak electron pair acceptors, uiz., OH groups or cations, are present at the surface of hydrated Na-faujasite, one expects the spectra to coincide with those of pyridine interacting with OH groups. However, the assignment of the vibrations of pyridine adsorbed on surfaces in the literature is confusing. Despite some resemblance to literaturevalues, wealso observe many discrepancies. Considering hydrogen bonded pyridine Parry,13 for example, mentions one band around 1599 cm-1 with a shoulder at 1575cm-l, while Parker et al.17 find two features near 1614 and 1593 cm-1 and nothing at 1575 cm-1. Furthermore the appearance or otherwise of a signal a t 1490 cm-I varies among different papers. The reason for the differences could be the diffuse nature and the overlap of the infrared bands, which make assignment difficult. We presume from our Raman spectra that all bands found in the DRIFT spectra must be attributed to pyridine on OH groups. We believe that pyridine interacts with (different) OH groups, depending on the pyridine coverage of the surface, resulting in the overlapping bands between 1630 and 1570 cm-1.
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 7335
Pyridine Adsorbed on Na-Faujasite
a
200-
+
h
. 3 24
0
.e (I]
TABLE II: Cation (Ne+) and Water (Ox) Distribution in Hydrated and Dehydrated Faujasite Zeolites (Mole per Mole Zeolite)a site zeolite I I' 11' 11" other sites' Hydrated Faujasite Zeolite Na-faujasite 1.9Na 16.7Na 9 . 8 0 ~ 28.8Na 8.5Na, 620x 260x 24Na severalsitcs NaX(88) 9Na 8Na 120x 80x NaX(81) 9Na llNa 320x 22Na 20Na,280x
siteA (1001) site B (998) site C (1003)
v
8
3
.e
100-
a
120x 0-
200
100
0
pyridine (mg/g)
+ site A (1004) o site B (998) 0 siteC(1001)
.
P L
3
/
200
a
300 400 pyridine (mg/g) Figure 5. Amount of pyridine per site as a function of pyridine coverage (The lines do not render the real situation but serve as a guide for the eye) for NaX (a), and for NaY (b) zeolite. 0
100
200
(a) (b) Figure 6. One unit of the faujasite type zeolite (a) and the distribution of sites (b). The ring breathing region in our Raman spectra seems less complex than that in the infrared spectra. A signal around 990 cm-1 is not evident, because a different adsorption procedure has been used compared to that in former studies.lOJ1 In those studies a temperature gradient was applied between the cell and the pyridine vessel, resulting in the condensation of pyridine between the zeolite particles. Here, a lower partial pyridine pressure was used by keeping the complete system a t 24 OC. In this way no liquid pyridine condensed in the cell during the adsorption procedure. The bands a t 1000 cm-1 are attributed to pyridine adsorbed on weak electron pair acceptors, Le., the OH groups of H20 molecules, SiOH groups, or Na+ cations. Following the literature,l%l7we conclude that a stronger acceptor site shifts the pyridine vibrations to a higher frequency. Therefore we ascribe the bands at 1001 (NaX) and 1004 cm-1 ( N a y ) to pyridine
Dehydrated Faujasite Zeolite Na-faujasiteb lONa 9Na 32Na Na-faujasitec 9.3Na 16.7Na 31Na NaY(57) 7.8Na 20.2Na 3 1.2Na 32Na 8Na NaX(81) 3.1Na 32Na a Sites present in supercage and thus accessible for pyridine. * Dchydration at 350 OC. Dehydration at 420 OC.
TABLE IIL Assi ent of Different Raman Signals (cm-I) of the V I Mode of e d a e Adsorbed on Na-Faujasite site adsorption complex NaX NaY A Z-X.-pyridine (Na+ cation or OH group) 1001 1004 B Z-H2O-.pyridinc (chemisorbed water) 998 998 C Z-H2O-.H2O.-pyridine (physisorbed water) 1003 1001 adsorbed onto a slightly stronger acceptor site, i.e.,a Na+ cation or maybe an OH group which is more acidic than the bulk OH groups. Consequently, desorption is more difficult a t lower coverages and a greater shift is observed. On desorption a t coverages higher than 175 mg/g the intensity of the signal at 998 cm-l increases, whereas the signal just above 1000 cm-1 is decreasing. We believe that this phenomenon is caused by traces of water in the pyridine. The water readsorbs (physisorption) onto the surface of the catalysts during adsorption of pyridine a t 24 "C,whereas it disappears again in vacuum and at higher temperatures. After desorption of physisorbed water a pyridine molecule may readsorb onto coordinated water, which explains the increase of the signal a t 998cm-1. It seems impossible to overcome this problem, because of the strong hygroscopic properties of the zeolites and pyridine. Although only traces of water are present in the pyridine the hygroscopic character of the zeolite results in readsorption of water. Though not recognized, shifts are described in literature6 which we believe should be attributed to readsorption of water. The phenomenon is not observed after evacuation, because the interaction of physisorbed water is weak and it is easily desorbed. In fact we believe that many adsorption experiments a t lower temperatures with different probe molecules (pyridine, acetonitrile, ammonia, and carbon monoxide) suffer from this drawback, i.e.,shifting the frequencies of the vibrations of probe molecules adsorbed into the zeolite lattice. The assignment of the different vibrations is given in Table 111. The interaction of physisorbed water with pyridine is not yet completely understood. According to Zinth ef aLz6and C a b a p et al.27.28 the interaction of a pyridine molecule with two alcohol molecules results in a shift toward higher frequency compared to the results of interaction with one alcohol molecule. C a b a p et ~ 1 . 2 ~ ~ascribe 28 this shift to an increase in acidity of the OH groupof thedimer conformation. In HzO-pyridine model systems we observe the same change of frequency on increasing the water fraction (999 toward 1002 cm-1). Therefore we propose an interaction as presented in Figure 7. The OH group in the physisorbed species is probably slightly more acidic (Figure 7b), because the second H atom does not interact with another group. Regardless of the increase in frequency (according to Parry,I3 the result of a stronger bond) this complex is not stable, because the hydrogen bond between the physisorbed water and the coordinated water is relatively weak.
7336 The Journal of Physical Chemistry, Vol. 97, No. 28, I993
0 7
nsolSi I A1 (a)
(b)
Figure 7. Proposed model for the adsorption of pyridine in the supercage of faujasite zeolites at ambient.
TABLE IV: Amount of Pyridine per Mole of Zeolite (Calculated from Molecular Formulas of the Hydrated
Species) per mole Na+
per mole water
zeolite
site A
site B
site A
site B
NaX'J NaYb
0.11 0.08
0.46 0.58
0.036 0.017
0.16 0.13
per supercage site A site B 1.2 0.5
4.8 4.1
a Site A results in a signal at 1001 cm-I, and site B results in one at 998 cm-I. Site A at 1004 cm-I and site B at 998 cm-I.
Assuming that the relative Raman intensity of the different signals is almost constant, we calculate that NaX adsorbs -40 mg/g of pyridine on the strongest site (1001 cm-*) and at maximum -170 mg/g on the bulk H20 sites (998 cm-1). For NaY these estimates become -20 mg/g (1004 cm-l) and 150 mg/g (998 cm-1). Table IV shows the amount of pyridine per mole of Na+ and per mole of water. It appears that the amount of pyridine situated in the zeolite lattice is far less than the number of possible adsorption sites. However, as stated before, pyridine cannot penetrate into the sodalite cages nor into the hexagonal prisms, which restricts adsorption to the supercages. Further, the space within the supercages is itself limited. The maximum occupation of sites A and B for pyridine is given in Table IV (site C is left out of the calculation, because the amount of readsorbed physisorbed water is unknown, thus making the determination of the pyridine loading uncertain). It appears that the occupation of site B is between four and five pyridine molecules per supercage for the catalysts. Thedeviation is probably due to the uncertainty in the Raman data (possibly some of the strongly bonded pyridine (998 cm-') is desorbed during desorption of the more weakly bonded species in the NaY data) and to the deviation from an ideal zeolite lattice. The isotherm experiments show that the occupation could besomewhat higher. According totheliterature6 approximately five molecules can adsorb in each supercage, but when more than four pyridine molecules are present in the supercage, the interactions become weaker (mainly because of the presence of physisorbed water) making desorption possible at 24 OC. The amount of pyridine interacting with the strongest adsorption site (site A) points to changes in the zeolite structure. One should keep in mind that higher temperatures (upto 200 "C) were required for the desorption of pyridine at low coverages. Surface modification may well have taken place during the desorption process. As no differences were observed on repeating
-
Ferwerda et al. the experiment with the same catalyst, we therefore conclude that this modification has to be reversible. We further conclude that the structure of the coordinated water within the lattice may change on using higher desorption temperatures, which results in, for example, pyridine adsorbed onto cations. The frequency differences observed for pyridine adsorbed onto the two zeolites are probably due to the excess of Na+ cations of the X zeolite. These cations are situated in the supercages and give rise to a different charge distribution within the zeolite structure.15 Overlapping bands and large bandwidths hamper the interpretation of the infrared spectra of pyridine adsorbed on Nafaujasite surfaces. The sharp features and the simplicity of the Raman spectra in the ring breathing region makes the interpretation easier. Comparison to more detailed infrared data is planned, and Raman experiments on pyridine adsorbed onto acidic materials will be the subject of discussion to obtain a better insight into the interactions and the assignment of the different vibrational bands.
Acknowledgment. We thank Dr.C. J. G. van der Grift for numerous helpful discussions during the course of this work. References and Notes (1) Stencel, J. M. Raman spectroscopy for catalysis; van Nostrand Reinhold: New York, 1990. (2) Hendra, P. J.; Horder, J. R.; Loader, E. J. J. Chem. SOC.,Chem. Commun. 1970, 563. (3) Kagel, R. 0. J . Phys. Chem. 1970, 74,4518. (4) Hendra, P.J.; Turner, I. D. M.; Loader, E. J.; Stacey, M. J . Phys. Chem. 1974, 78 (3), 300. (5) Egerton, T. A.; Hardin, A. H.; Kozirovski, Y.; Sheppard, N. J . Catal. 1974, 32, 343. (6) Egerton, T. A.; Hardin, A. H.; Sheppard, N. Can. J . Chem. 1976, 54, 586. (7) Schrader, G.L.; Cheng, C. P. J. Phys. Chem. 1983,87, 3675. (8) Dutta, P. K.; Rao, K.M.; Park, J. Y. J. Phys. Chem. 1991,95,6654. (9) Chase, D. B. J. Am. Chem. Soc. 1986, 108, 7485. (10) Burch, R.; Passingham, C.; Warnes, G. M.; Rawlence, D. J. Spectrochim. Acta 1990,46A, 243. (1 1) Hendra, P. J.; Passingham, C.; Warnes, G. M.; Burch, R.;Rawlence, D. J. Chem. Phys. Lett. 1989,164, 178. (12) Mortensen, A.; Christensen, D. H.; Nielsen, 0. F.; Pedersen, E. J. Raman Spectrosc. 1991, 22, 47. (13) Parry, E. P. J . Catal. 1963, 2, 371. (14) Basila, M. R.; Kanter, T. R.; Rhee, K. H. J. Phys. Chem. 1964,68, 3197. (15) Freeman, J. J.; Unland, M. L. J . Catal. 1978, 54, 183. (16) Lercher, J. A.; Ritter, G.; Vinek, H. J . Colloid Interface Sci. 1985, 106 (l), 215. (17) Parker, L. M.; Bibby, D. M.; Burns, G. R. J . Chem. Soc., Faraday Trans. 1991.87 (19), 3319. (18) Crookell, A. Ph.D. Thesis, University of Southampton, 1989. (19) Wilson, E. B. Phys. Reo. 1934, 45, 706. (20) Ferwerda, R.; van der Maas, J. H.; Hendra, P. J. Manuscript in preparation. (21) Pemberton, J. E.;Bryant, M.A.;Sobocinski, R. L.;Joa,S. L.J.Phys. Chem. 1992, 96, 3776. (22) Passingham, C. Ph.D. Thesis, University of Southampton, 1990. (23) Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monograph Series 171; American Chemical Society: Washington, DC, 1976. (24) Odenbrand, C. U. I.; Brandin, J. G. M.; Busca, G. J . Catal. 1992, 135, 505. (25) Claydon, M. F.; Sheppard, N. J. Chem. SOC.,Chem. Commun. 1969, 1431. (26) Zinth, W.; Nuss, M. C.; Kaiser, W. Phys. Reu. A 1984,30 (2), 1139. (27) Cabap, M. I.; Besnard, M.; Y a r w d , J. Mol. Phys. 1992,75, 139. (28) Cabap, M. I.; Besnard, M.; Y a r w d , J. Mol. Phys. 1992,75, 157.
