J. Phys. Chem. 1993,97, 8165-8170
8165
Spectroscopic Studies of Electron Trapping by Sodium Cationic Clusters in Zeolites Kai-Kong I u , ~Xinsheng Liu, and J. Kerry Thomas' Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received: March 9, 1993; In Final Form: May 18, I993
High-energy irradiation of sodium zeolites, both short pulses of 0.4-MeV electrons and steady-state W o y-rays, results in the formation of electrons trapped in Na+ clusters, e.g. Na43+, Na32+, and Na2+. ESR spectroscopy is used to identify this species. Only Nad3+ is observed in zeolites X and Y at all temperatures; however, other trapped species are observed at low temperatures in zeolite A, e.g. NaS2+ and Na2+ as well as Na43+,and in sodalite, Na43+and Nas2+. As the temperature is elevated, the smaller clusters, e.g. Na2+ in zeolite A and Nas2+ in sodalite, become less stable. The spectral absorption peaks are -550 nm for Na43+and -680 nm for Na32+, while the Na2+ species absorbs in the near-IR regime (>750 nm). The activation energies for hole/electron (Na43+) neutralizationare 12.58 f l.lOkcal/mol(433-469K) forzeoliteY and7.10f0.71 kcal/mol(308-363 K) for zeolite X. In zeolite A, the activation energy for hole/electron (Na32+) is 15.32 f 3.10 kcal/mol (373-421 K), and that for Na2+ is 11.13 f 0.91 kcal/mol (253-307 K). The temperature range of the measurements is also given. The rate constant for oxygen quenching of Na43+ in NaX is (3.23 f 0.26) X 104 T o r r ' s-l, and that of Na32+ in zeolite A is 26.08 f 6.42 Torr' s-l. The species Nad3+ is located inside the supercage of NaX and NaY and in the sodalite cage of zeolite A. In zeolite A, the other two clusters, Na32+ and Na2+, are located in the a-cage and sodalite cages, respectively. The Gvalue (Le., radiolytic yield) of Na,3+ in NaY is 2.76 f 0.30 for 6oCoy-ray irradiation. The study shows the great utility displayed by zeolites is providing unique sites and environments for the creation of new chemical species.
Introduction The trapping of electrons in condensed media has been of concern for several decades,' where interest has centered on the trapping of electrons in liquids,2 where the prime candidate is the hydrated electron. In fluid or glassy media, polarization of the medium in the immediate vicinity of the electron leads to a lowenergy well where the electron is trapped. Good theoretical descriptionsof these systems are available.3 Of allied interest is the trapping of electrons in well-defined clusters or pools4 and in organized structures such as zeolites. Early work5 showed colored absorption spectra for zeolites which had coadsorbed sodium metal vapor or where sodium had been diffused into the zeolte from solutions of sodium metal. Recent work6 confirms these early findings and suggests ways of calculating the observed spectra. Recently,' studies in our laboratory have shown that several arenes in zeolites may be photoionized to produce the arene cation and a trapped electron species which is remarkably like that reported in thermal studies. The electron trapping site is a cluster of four sodium ions, Na44+, which leads to a trapped electronof the form Nad3+. Other reports* show that y-ray irradiation of zeolites also gives rise to trapped electrons of the type Nas3+. In the present studies,pulse radiolysis and 6oCoy-ray radiolysis are used to produce several different trapped electron species in zeolites. UV-vis and ESR spectroscopies are used to identify these species and to study their properties. Experimental Section heparation of Zeolite Samples. Sodium zeolite X (NaX, Si/ A1 = 1.4) and A (NaA, Si/Al= 1.O) were obtained from Aldrich; sodium zeolite Y (Nay, Si/Al = 2.50) was kindly supplied by UOP; sodium sodalite (NaSod, Si/Al = 1.0) was synthesized according to the procedures described in Breck.9 About 50 mg of zeolite sample was pressed under 5000 1b/im2to a semitransparent disk in a l/z-in. diameter die. The sample disk was t Currentaddress: H-PCompany, 16399 West Bernard0Drive, San Diego,
CA 92127-1899. To whom correspondence should be addressed.
0022-3654/93/2097-8 165$04.00/0
transferred to a holder contained in a square metal Dewar with optical ports (1S-in. diameter) on three sides, and an outlet for pumping the Dewar, the holder could be heated or cooled to liquid N2 temperature. Two ports on opposing side of the Dewar were sealed by quartz windows pressed against the wall via an O-ring. The third port was sealed by a thin (0.01 mm) titanium window pressed against the wall via O-rings; this was used for entry of the high-energy electron beam into the Dewar and to the sample disk. The sample disk was dehydrated at 210 OC under vacuum (10-3 Torr) for 2 h before taking data. Time-Resolved Higb-Energy Pulse Radiolysis Apparatus. A pulsed-electron beamdevice (Febetron, Model 706,Field Emission Corp.) was used to provide a 2-ns, high-energy (- 106 rad/pulse) electron pulse. This electron beam directly bombarded the zeolite sample disk in the Dewar after passing through the thin titanium window. Analyzing light from a pulsed xenon lamp was passed through the two side ports and through the sample disk; the light beam is at right angle to the electron beam. Because the analyzing light penetrated through the thin sample disk, all experimental data were recorded via the transmittance mode. The control electronics, xenon lamp pulsing unit, and computer interface have been reported el~ewhere.~Jo Steady-State Fray Irradiation. In low-temperature studies, 0.2 g of powdered zeolite samples was sealed in a 6-mm Suprasil quartz tube. Prior to sealing, the zeolites in the sample tubes were kept at 500 "Cunder Torr pressure for 8 h. A MCo Tray source with a dosage rate of 9.5 krad/min was used to irradiate the above samples for an hour. A UV-vis spectrometer (Varian, Cary 13) with a low-temperature reflectance apparatus was used to record all steady-state UV-vis reflectance spectra of irradiated samples. The samples were kept under liquid N2 or in acetone-dry ice baths during the course of the experiment. For room-temperature G value measurements, a semitransparent selfsupported NaY disk was calcined at 550 OC for 5 h and then quickly transferred to a 1 X 1 cmz quartz cell. The sample was further heated at 200 OC and under 10-4Torr for 2 h. The sample cell and sample were exposed to y-rays and for various dosages and theabsorbance takenat 500 nm, Le. theabsorption maximum of Na43+ in NaY ,798 A blank NaY disk, which was not exposed 0 1993 American Chemical Society
8166 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
Sodalite Cage
Iu et al.
@I
\
I \ ,
Zeolite A
-
Sodalite
0.w 400
Zeolite X and Y
Figure 1. Fundamental structure of sodalite and zeolites A, X, and Y. xxxxx
x
x x X
x x
I
I
1
MO
760
10
u xx
X
*" w * *
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.*I
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X X
xx -'-'
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O
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W a v e l e n g t h ( nm 1 F'igure 3. Temperature effect on the transient spectra of NaX taken 5 ps after the Febetren pulse. (-100 "C),0 (20 "C), and X (70 "C). Insertion is the time-resolved transient signalsat 510 nm (solid line) and 590 nm (dotted line).
4
0.00
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nio
x
xxxxx
X
I
M
I
I
I
I
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W a v e l e n g t h ( nm 1 Figure2. Transientspectraof sodiumzeolite taken 5 p after the Febetron pulse: X, sodalite; *, zeolite A; 0,zeolite X.
to y-rays, was used as reference background for the visible absorption measurement. The results of the ESR measurements on these samples have been described elsewhere.'l
Results and Discussion We have selectively chosen a series of zeolites, namely zeolites X, Y,A, and sodalite, in which the sodalite cage is one of the fundamental building blocks of the whole zeolite structure. This ensures that the structural arrangement of the backbone controls the size and the location of the ionic cluster formed in the structure. Figure 1 shows the fundamental building block (i.e., sodalite cage) and the basic structures of the individual zeolite studied here. Figure 2 gives the transient spectra of the short-lived species in the pulse radiolysis of several sodium zeolites (X, sodalite; 0, zeolite X; *, zeolite A) taken 5 NS after the electron pulse. Although the zeolites in this figure have a common fundamental cage (Le. sodalitecage), they show quitedifferent transient spectra. These and subsequent studies determine the location of the ionic clusteninsidethezeolitematrixand the role they play in producing trapped electrons. Trapped Electron Species in Sodium Zeolite X. In earlier studies, we reported results of the laser flash photolysis of arenes in zeolite X, where trapped electron species absorbing at -550 nm were found.' These trapped species were determined to be ionic clusters of the form Na43+.* In the high-energy electron
+P
&&m
P-
I
Electron Trapping by Sodium Cationic Clusters
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8167
-
(u
X
1
a.f.0
* I
0
0 x
x xx
Y
X
a.151
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W a v e l e n g t h ( nm 1 Figwe 5. Temperatureeffect on the transient spectra of NaA taken 5 p
after the Febetron pulse:
X (-100
"C),0 (20 "C),and * with line
(70 OC).
0.00 400
.
.
TI*
.
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I.
,I
lnxll,r.ron*.l
I
l
a
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Wavelength
(
nm)
Elgum 7. Transient spectrum of NaA at 70 OC and under 150 Torr of
oxygen: * is taken 5 p after the Febetron pulse, 0 , 4 times enlarged, is taken 300 p after the Febetron pulse. Insertion is the timeresolved transient signals at 530 nm (solid lie) and 630 nm (dotted line). n
1
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4
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taken 5 ps after the radiation pulse is shown in Figure 5 at different temperatures; X (-100 "C), 0 (20 "C), * (70 "C). In order to determine the contribution of the various trapped species to the broad structureless spectrum in NaA at low temperature, both UV-vis and ESR studies were carried out at 77 and 196 K, the results are shown in Figure 6. At 77 K, the trapped electron species on irradiation of NaA is due to Na2+, and this species exhibits a characteristic seven-line ESR spectrum; the analysis of this seven-line ESR spectrum is the same as in the case of Na43+ above. Elevating the temperature of the NaA sample to 196 K produces a species with a 10-line ESR spectrum (vide ante), which indicates that the trapped species is due to N a P . TheNa32+showsadistinctiveabsorptionpeakat -640nm (Figure 6, symbol +). The disappearance of the near-IR absorption band (>750 nm) and of the characteristic seven-line ESR spectrum of NaA, after elevating the temperature from 77 to 196 K, suggests that this near-IR absorption band is due to Na2+. The temperature studies indicate that the smalltrapped electron cluster Na2+ is eliminated by elevating the temperature of the sample. If the electron species is formed in the sodalite cage, then any quenching of the species by oxygen is prevented as the entry pore of 2.6 A precludes oxygen. If the species is formed in the u-cage, where the entry pore size is larger, 4 A, then oxygen quenching may occur. With these ideas in mind, studies were conducted at high temperatures and in oxygenated NaA systems. Figure 7 shows the spectrum of the trapped species at 70 "C under 150 Torr of oxygen; symbol denotes the spectrum taken 5 ps after the high-energy pulse, and symbol 0 is the spectrum, 4 times enlarged, taken 300 ps after the high-energy pulse. A distinct spectral shift is shown 300 ps after high-energy pulse,
0.00 400
E40
7w
Wavelength
Mo
LO40
two
( nm )
Elgum 8. Temperatureeffect on the transient spectra of NaSod taken
* (-100 "C),0 (20 OC),and X (70 OC). Insertion is the time-resolved transient signals at 520 nm (solid line) and 710 nm (dotted line). 5 p after the Febetron pulse:
under 150 Torr of oxygen. The time-resolved absorption signals at 530 nm (line) and 630 nm (dot), given as inserts in Figure 7, show that a long-lived species absorbing at -550 nm; i.e. Na43+ is protected from oxygen quenching. This suggests that the ionic cluster of Nad3+is inside the sodalite cage and is not accessible to oxygen, while the Na32+ species is located in the a-cage. Trapped Electroa Species in sodium Sodalite. Compared to the other zeolites in this study, sodalite has only one fundamental building block, the sodalite cage. Here, all the exchangedcations are located inside the sodalite cage. Pulse radiolysis of sodalite produces two trapped electron specieswhich exhibit two absorption peaks at 560 and 700 nm, and these two peaks become more pronounced at low temperatures (e.g. -100 "C, symbol *) as shown in Figure 8. The insert of Figure 8 shows that the decay of the time-resolved signals at 520 nm (line) and 710 nm (dot) are not identical and that the two absorption peaks originate from two different species. The absorption band at the blue side of the spectrum (-560 nm), which is identical to the absorption of Na4'+ in NaX, is assigned to Na43+. A recent ESR study'l shows formation of Na32+in sodalite. The data from the present NaA studies (see Figure 6) indicate that the broad absorption band at -700 nm is due to the Na3Z+ species. As the temperature decreases, the
-
-
8168 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
Iu et al.
TABLE I: Absorption Maxima (nm),Hyperfine Coupling Constants (0, and g Values of the Sodium Cationic Clusters in Different Sodium Zeolites cluster Na43+ Na3*+ Na2+ 0
NaSod (2.32)"
NaA (2.32)O
560 nm, no data 700 nm, 31.5 G, ~ 1 . 9 9 8 7
NaX (2.42)O
NaY (2.60)O
550 nm, 27.5 G, ~ 1 . 9 9 9 0
570 nm, no data 680 nm, 39.5 G, ~ 2 . 0 0 2 6 >750 nm, 72 G, g=2.0050
500 nm, 30 G, g= 1.9990
Sanderson electronegativity.
intensity ratio 700 nm/550 nm increases. This indicates that Na32+ is the preferred trapping site at low temperature. The exact nature of the electronic transition that gives rise to the observed trapped electron spectra is debatable. The structured ESR signals and coupling constants in Table I show a significant interaction of e- and Na+ in the cluster Na43+. A simple particle in a box approachSand a more sophisticatedquantum mechanical calculation have been outlined.6 In the latter treatment, it was indicated that the experimental data were explained with a model which included an average electric field within the host sodalite framework. Table I shows that the spectral maxima,, ,A for several of the trapped electron speciesdepend on the particular zeolite which hosts the electron; for example, the A, of Na43+ is 550 nm in NaX, 500 nm in Nay, 570 nm in NaA, and 560 nm in NaSod. The ESR spectra indicate the exclusive structure of Na43+for all species. Basically, thesanderson e1ectronegati~ity'~J~ of a zeolite (S,) is calculated from the equation
where Mis the cation atomic electronegativity;AI, Si, and 0 are the atomic electronegatives for aluminum, silicon, and oxygen, respectively;14 and x is the number of aluminum atoms per unit cell of zeolite. In the present work, it is proposed that electrons are trapped in the following sodium ion configurations: Naz+, Nas2+,and Na43+. The observed spectral transitions are attributed to a ls2s transition of the electric field represented by a spherical well, i.e., the electrical field defined by the above Na+ configurations. The energies of the 1s and 2s transition are given approximately by the simple equation
E = nhz/8rzm?
- 2e2/r
(2)
where n = 1 for 1s and n = 2 for 2s. The positions of the experimental spectral maxima are calculated with the following values of r: 1.3, 1.4,and 1.8 A for Na43+, Na32+, and Naz+, respectively. These values are considerablysmaller than expected for knownconfigurationsof Na+ in thezeolites.ls Thus, a picture much akin to that suggested by Haug et al.13bis adopted, where the trapped electron experiences both the field of the Na+ cluster and the field of the zeolite framework. Simple calculations with either eq 1 or 2 do not predict the observed data. Oxygen Quenchingof the TrappedElectron Speciesin Zeolites. Oxygen quenching of the trapped species (i.e., electron in the sodium ionic cluster) requires close contact of the two reactants. The observed rate constant for oxygen quenching of the trapped species follows the equation koba= k,
+ k,x(O,
pressure)
(3)
where kobdenotes theobserved rate constant for decay, ko denotes the rate constant for decay without oxygen, and k, is the rate constant for oxygen quenching. The observed rate constant, kob, does not follow a simple single-exponential decay function but can be described in a Gaussian fashion as proposed by Albery et
0.00
0
1W
940
0,
Pressure
3W
4M
WO
(torr)
Figure 9. Plot of oxygen quenching rate of Nu3+ in NaX (X with line) and Naaz+ in NaA (0 with line) as a function of oxygen pressure. A linear least-squares fit is applied on the data.
TABLE Ik Oxygen Quenching Rate Constant of Sodium Cationic Cluster in Zeolite cluster
NaA (Torr'
Na43+ Na32+ Na2+
no quenching
NaX
5-11
(3.23 i 0.26) X l(r
(26.08 A 6.42)
no quenching
a1.16 and others.'' Basically, the function is the following:
I(t) = Z o / d l z ~ g ( X )dX
(4)
and g(A) = A-' exp(-(ln
A)z)(exp(-kobtA~)
+ exp(ko,tXr))
(5) The integration of eq 4 can be solved by an extended Simpson's rule, and three variables, IO (intensity of transience absorption signal at time zero), kob (observed decay rate of the transience), and y (distribution parameter) can be sought out by applying a least-squares fit on the experimental data. A detailed treatment of the above Gaussian function can be found in ref 17a. The observed rates of decay of Na4'+ in NaX and of Na32+in NaA vs oxygen pressure are plotted in Figure 9. The slope of the linear fit gives the oxygen quenching rate constant for Na43+ in NaX and that of Na32+in NaA and are listed in Table 11. The oxygen quenching rate of Na43+ in NaX is more than IO3 than in NaA. The large difference in rate constants for oxygen quenching in different zeolites is due to the subtle difference of the entry pore size of the su rcage in NaX (7 A) compared to thatof thea-cageinNaA (4 ). Diffusionoftheoxygenmolecule
R
intothea-cageofNaAwithadimensionof2.8Aismorerestricted compared to diffusion into the supercage of NaX. The largest oxygen quenching rate constant we observed in NaX18 is that for single excited pyrene, Le. (1.32 f 0.05) X lo7 Torr' s-1. A comparison of this rate constant with that for oxygen quenching rate of Na43+in NaX, Le. (3.23f 0.26)X 104 Torr's-1, indicates an intrinsic slow quenching process between oxygen and Na43+. The product of the reaction of the trapped electron species and oxygen is probably superoxide anion, and experiments to verify this are now under way.
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8169
Electron Trapping by Sodium Cationic Clusters
TABLE IIk Arrhenius' Preexponential Factors and Activation Energies of the Decay of Sodium Cationic Cluster in Zeolite cluster NaA NaX NaY ( d , kcal/mol) Na4)+ no data (3.82h0.22) X 108,7.10f0.71 (5.78h0.31) X109, 12.58h1.10 Na32+ (2.64h0.37) X10'2, 15.32f3.10 Na2+ (3.57h0.50) Xl0l2, 11.13h0.91 10.40
b
ionization of pyrene in zeolite X or Y? In the latter case electron disappears at a rate comparable to that of the pyrene cation ion Py*+,and pyrene is regenerated. Kochi et al.z' have indicated that photoproduced ion pairs are much more stable in zeolites than in other media. The similarity of pyrene photoionization in zeolite to the work of Kochi21 suggeststhat photolysisof pyrene leads to and Na43+, which reside in the same supercage. However, the high excitation energy in radiolysis ejectsan electron some distance beyond the site of excitation (20-50 A in liquids). Thus, in radiolysis the positive hole and Na43+are separated into different cages, and their recombination would be much slower than that of and electrons in photolysis. ESR studies show that thelossofthepositive hole matches that ofNad3+ore1ectron.l' The different activation energies for the trapped electron species are associated with the neutralization of the positive hole and electron and could reflect on the distinct properties with the hole or the trapped electron.
v+
v+
4 40
2
I
1
I
I
2 4
2 8
3 2
3 6
1
I/Temo ( K-' I ( x i 0 3, Figure 10. Arrhenius plot of Na43+at 520 nm in NaY (+ with line) and in NaX (0 with line); Na32+ at 650 nm (* with line) and Na2+ at 830 nm (X with line) in NaA.
TABLE Iv: C Values (Radiolytic Yield) of the Sodium Cationic Cluster ( N 4 9 in NaY for T o y-ray time dose [NaP+I (min) (X10-21, eV/L)" (X104, M)b G value 10 15 25 35 45
4.81 7.22 12.03 16.85 21.66
2.45 3.07 5.37 7.84 9.68
2.95 2.46 2.59 2.70 2.59 av 2.76 h 0.30
a The dosage of the 6oco y-source was calculated from the Fricke dosimeter method.l9 b This calculates from the absorbance at 500 nm where the extinction coefficient is 33 OOO cm-' M-' for N a P . 'The calculation is based on the description in ref 19.
Because of the small entry pore of the sodalite cage (2.6 A), little effect is expected on oxygen quenching of the ionic cluster located inside sodalite cage. Indeed, the experimental results of NaSod show that oxygen at 600 Torr has no effect on the rate of Na43+ decay. It is noteworthy that oxygen at 500 Torr has no effect on the rate of Na2+ decay in NaA, which indicates that Naz+ is located inside the sodalite cage of the NaA and is not accessible to oxygen. Effect of Temperature on the Decay of the Trapped Electron Species. Figure 10 shows Arrhenius plots of ln(k,,b) vs (1/ r ) for Na43+ in NaY (433-469 K) and in NaX (308-363 K), N a P (373-421K), and Na2+ (253-307K) in NaA. The slopes of the linear plots give the activation energy for decay of the trapped electron and are listed in Table 111. The reaction may be the recombination of the trapped electron and the positive species of the framework. A larger value of active energyof Nap2+compared to that of Naz+in NaA suggests that either Na32+is more stable or thea-cage is a morestable environmentfor the trapped electron. Furthermore, the G value (radiolytic yield) of Na43+in NaY was determined at 20 OC for ~ C y-ray O with different dosage, and the results are listed in Table IV. The magnitude of this G value, i.e. 2.76 f 0.30, is comparable to that of hydrated electrons in water and is indicative of an efficient radiolytic The higher activation energy of Na43+ in NaY compared to NaX is an indication that a more stable framework environment of NaY is provided for Na43+. The trapped electrons created by y-irradiation are more stable than those produced via photo-
Conclusion This study reports formation of trapped electrons in ionic clusters of sodium zeolites, namely zeoliteX, Y, and A andsodalite, the electrons being produced by high-energy radiation. Only Na43+clusters are found in zeolite X and Y and are located inside the supercage. Two clusters are found in the sodalite, namely Na43+ and Na32+. Three clusters are found in zeolite A, namely Na43+,N ~ J ~and + ,Naz+. An oxygen quenching study of zeolite A shows that Na43+and Na2+ reside in the sodalite cage and Na32+resides in the a-cage. Oxygen quenching of N a P in NaX is about lo3 times faster that of Na32+in NaA and reflects on the subtle difference of the entry pores of the cages (i.e. 7.4 A of supercage vs 4 A of a-cage). The small activation energies for decay of these ionic clusters are in agreement with the difficulties of detecting these species at room temperature in steady-state experiments;the larger value of activation energy of Na32+compared to that of Na2+in NaA suggests that NaBz+is either more stable itself or located in a more stable environment, i.e. a-cage. A G value (radiolytic yield) of 2.76 i 0.30 is determined for Na43+in NaY at 20 OC. Acknowledgment. We thank the National ScienceFoundation and EnvironmentalProtection agency for the support of this work. References and Notes (1) (a) Spink.9,J. W. T.; Woods, R. J. J. An Introduction to Radiation Chemistry Wiley: New York, 1964. (b) ODonnell, J. H.; Sangster, D. F.; Arnold, E. Principles of Radiation Chemistry; London, 1970. (c) Bednar, J. TheoreticalFoundationsofRadiation Chemistry;A c a d e m i c P : Bouton, 1990. (2) (a) Hart, E. J.; Anbar, M. The Hydrated Electron; Wiley: New York, 1970. (b) Del Buono, G.S.;Rwky, P. J.; Murphrey, T. H. J . Phys. Chem. 1992,96, 7761-7769. (3) Reference 2a, pp 39-73. (4) (a) Wong, M.; GrBtzel, M.; Thomas, J. K. Chem. Phys. Lett. 1975, 30, 329-333. (b) Thomas, J. K.; Grieacr, F.; Wong, M. Ber. Bunsen-Ges. Phys. Chem. 1978,82,937-949. (c) Bakale, G.; Beck, G.; Thomas, J. K.J. Phys. Chem. 1992, 96,2328-2334. ( 5 ) Baner, R. M.; Cole, J. F. J . Phys. Chem. Solids 1968, 29, 17551758. (6) Srdanov, I. V.; Haug, K.; Metiu, H.; Stucky, G. D. J. Phys. Chem. 1992, 96,9039-9043. (7) (a) Iu, K.-K.; Thomas, J. K. J. Phys. Chem. 1991,95,506-507. (b) Iu, K.-K.; Thomas, J. K. Colloids, Surf. 1992, 63, 39-48. (8) (a) Kasai, P. H. J. Chem. Phys. 1%5,43,3322-3327. (b) Rabo, J. A,; Angell, C. L.; Kasai, P. H.; Schomaker, V. Discuss. Faraday Soc. 1966, 41, 328-349.
8170 The Journal of Physical Chemistry, Vol. 97, No. 31,1993 (9) Brock, D. W.Zeolite Molecular Sieves; Wiley: New York, 1974. (10) Thoass, J. K. Chemlst~ofExcitationatInterfaces,ACSMotwgmph 181; ACS Washington, DC,1984. (11) Liu, X.;Thomas,J. K. Lungmuir 1992,8, 1750-1756. (12) Sanderson, R. T. Chemical Bonds and Bond Energy; Academic Pres: New York, 1971. (13) (a) Jacobs, P. A.; Mortier, W.J.; Uytterhoven, J. B. J. Inorg. Nucl. Chem. 1978,40,1919. (b)Haug, K.; Srdanov,I. V.;Stucky, G. D.; Metiu, H. J. Chem. Phy$.l992,95,3495-3502. (14) Reference 12, p 41. (15) The size of the clusters was calculated by the aizc of the sodalite cage, supercage, and a-cage from the literature data in ref 9.
Iu et al. (16) Albery, W.J.; Bartlett, P.N.;Wilde, C. P.;Dament, J. R. J. Am. Chem. Soc. 1985, 107, 18541858. (17) (a) Krasnansky, R.; Koikc, K.; Thomas, J. K. J. Phys. Chem. 1990, 91,45214528. (b) Draper, R. B.;Fox,M.A. Lungmuir 1990,6,1396-1402. (18) Liu, X.;Iu, K.-K.; Thomas,J. K. J. Phys. Chem. 1989, 93, 41204128. (19) Holm, N.W.;Berry, R. J., Ed. Manual on Radiation Dosfmetry; Marcel Dekker: New York, 1970; pp 313-317. (20) Reference 2a,p 18. (21) (a) Yoon, K. B.; Kochi, J. K. J. Am. Chem. Soc. 1989,111,11281130. (b) Yoon, K. B.; Kochi, J. K. 1.Phys. Chem. 1991,95,3780-3790. (c) Yoon, K. B. Chem. Rev. 1993,93, 321-339.
