4448
J. Phys. Chem. 1982, 86,4446-4450
e f f e ~ t ~ which , ~would ~ ~ allow ~ , the ~ computation ~ ~ ~ ~ ~of~a ~ more reliable potential energy surface for proton transfer. Such improvements are certainly desirable but are in no way essential as far as the scope of the present model study goes, for the results reported here (e.g., AE,and the tunneling time) already provide a sufficient support for the (57) 0. Tapia, F. Sussman, and E. Poulain, J. Theor. Biol., 71, 49 (1978). (58) 0. Tapia, Theor. Chim. Acta, 47, 157 (1978).
proposed mechanism. On the other hand, further extension of the numerical analysis is mandatory for a more detailed understanding of the dynamics of proton transfer.
Acknowledgment. We acknowledge Dr. M. Ammar for many helpful discussions, Dr. Langlet for the use of her PCILO program, Professor S. G. Christov and Professor G. Barone for their useful comments, and Miss C. Minichino for help in the computations. Financial support of CNR (Rome) is gratefully acknowledged by the Italian authors.
Application of Positron Annihilation to the Characterization of Zeolites Hlroshl Nakanishi and Yusuke UJlhlra’ Faculty of Engineerlng, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan (Received: November 17, 1981; I n Flnal Form: May 21, 1982)
Positron annihilation lifetime and Doppler-broadening measurements were carried out for synthetic zeolite 13X, SK-40, NH,-X, and NH4-Y by varying the evacuation temperature in order to study the character of the zeolite cages. Four components of the positron annihilation, derived from the lifetime spectra, were interpreted from the results of the authors’ measurements and other studies on zeolites. The 0-Ps lifetimes in the cages became longer as the desorption of water molecules proceeded. It was found that some active groups in zeolites interacted with 0-Ps and reduced the 0-Ps lifetime after all the water molecules had detached. Br~nstedacid in the zeolite acted not only as an oxidizer but also as an inhibitor of Ps formation. An attempt was made to estimate the amount of Br~nstedacids by the positron lifetime technique. The longest lifetime of 50 ns indicates 0-Ps annihilation in a pore with 60 A free diameter, which seems to exist irregularly in the faujasite zeolites. It was also found that 0-Ps was oxidized in this large cavity.
Introduction When positrons, antiparticles of electrons, are emitted from neutron-deficient nuclei such as 22Nainto a condensed medium, they slow down to thermal energy within the order of picoseconds. A portion of them form the bound state of a positron and an electron pair called positronium (Ps). A thermal positron or Ps interacts with the surrounding material and annihilates, emitting two or three y quanta in less than a microsecond. The lifetimes of a positron and Ps reflect the character and states of the surrounding materials. In solids, they are very sensitive to structural irregularities such as lattice defects or micro void. Extensive research has been successfully carried out on metals and crystalline solids. The potentia1 capabiIities of positron measurements have not yet been fully applied to the studies of porous or amorphous materials. This is partly due to the complexity of the positron annihilation process in those substances and the difficulties in the analyses of the data obtained and partly due to the lack of systematic investigations in the field. Hitherto several reports have been published on the application of positron techniques to the studies of zeolites or fine Unfortunately there are certain inconsistencies among the data or their interpretations. In this work, several kinds of synthetic zeolites, whose structures were well-known, were selected as samples, and (1) Perkal, M. B.; Walters, W. B. J. Chem. Phys. 1970, 53, 190-8. (2) Levin, M. V.; Shantarovich, V. P.; Agievskii, D. A.; Landau, M. V.; Chukin, G. D. Kinet. Katal. 1977, 18, 1542-7. (3) Gol’danskii, V. I.; Mokrushin, A. D.; Tatur, A. 0.;Shantarovich, V. P. Appl. Phys. 1975, 5, 379-82. (4) Gol’danskii, V. I.; Mokrushin, A. D.; Tatur, A. 0.; Shantarovich, V. P.Kinet. Katal. 1972,13,961-8. (5) Paulin, R.; Ambrosino, G. J. Phys. 1968, 29, 263-70. 0022-365418212086-4446$0 1.2510
the states of the cations in the cages and the oxidation processes of 0-Ps were investigated by measuring both the lifetime and the Doppler-broadening of annihilation y rays. Sufficient experimental data have been provided to resolve the discrepancies found in the interpretation of the preceding experimental results. The usefulness of positron measurement in porous materials was also demonstrated.
Experimental Section Sample Preparations. The powders of Linde-type synthetic zeolite 13X and SK-40, whose unit-cell compositions are Naes[(A102)86(Si02)10,1.276H20 and Na56[(Al02)56(Si02)13s]*264H20, respectively, and the powders of NH4-X- and NH,-Y-type zeolites, which were obtained by exchanging Na+ of 13X or SK-40 to NH4+following the manual of “Ion Exchanging Procedures” specified by Union Carbide Co Ltd., were used in the experiments. The details of ion-exchange procedures are described in procedures 1-3. Procedure 1 . The hydrated zeolite powder (100 g on an anhydrous basis) was slurried in 1 L of 2.23 M NH4Cl aqueous solution and heated to reflux temperature while stirring for 2 h. Procedure 2. The exchanged powder was filtered while hot, washed with an equal weight of deionized water, and contacted again with fresh NH4C1solution. Procedure 3. After the final exchange, the molecular sieve was washed free of all soluble salts and dried a t 50 “C to a free-flowing powder. Procedures 1and 2 were repeated several times to obtain a high exchange ratio. The exchange ratio of zeolite 13X after each exchange process was estimated by measuring the amount of residual Na by thermal neutron activation-yray spectrometry, a 23Na(n,y)24 Na nuclear reaction 0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 22, 1982 4447
Ps Annihilation for Zeolite Characterization 1.0
1I
L
0.0
'
0
I 1
2
3
4
No. of BATCH EXCHANGE
Figure 1. NH,'
exchange ratio for 13X at 100
OC.
and a 1.369-MeV 24Nay-ray detection being utilized. Apparatus. The details of the positron lifetime measurement system were described in the previous papere6 The time resolution of our system was 460 ps for 6oCoy rays with good symmetry. 3.2-6.5 X lo3 counts were collected at the time-zero channel in each lifetime measurement. The Doppler-broadening measurement system was composed of a pure-Ge semiconductor detector (7410-7600P),a spectroscopy amplifier (Camberra 1413), and an 8000-channel analyzer (Camberra 8100). The energy resolution was determined to be 1.10 keV for 514-keV 85Sr y rays. About 5 X lo6 counts were collected in the channels of a multichannel analyzer with an energy-perchannel value of 46 eV in each Doppler-broadening measurement. Positron Lifetime and the Doppler Broadening of 511-keV Annihilation y-Ray Measurements. A source of 10 pCi of sodium-22, a positron emitter, sandwiched between aluminum foil' (18.3 mg/cm2, thick) was placed in the zeolite powder contained in a greaseless quartz vessel (40 mm diameter, 55 mm height) designed for measurements in vacuo at high temperatures. The samples were slowly degassed at temperatures ranging from 20 to 600 "C for at least 8 h in an electric furnace. All positron lifetime and Doppler-broadening measurements were carried out at room temperature in vacuo (5 X torr). The obtained lifetime spectra were resolved into three components with the POSITRONFITEXTENDED program,E because the variance of the fit for the two-component solution was not good, and the fourth component was obtained by expanding the TAC (time to amplitude converter) time range ten times.
Results and Discussion The result of the ion exchange of the zeolite is shown in Figure 1. It is evident that the exchange ratio did not increase notably after the fourth exchange process. Therefore we repeated the previously mentioned exchange procedures four times for the preparation of NH4-X- and NH4-Y-type zeolite samples. Both 13X- and SK-40-type zeolites have a faujasite structure whose unit cell consists of 192 silica and alumina tetrahedra with a cubic unit-cell dimension of 25 A. Each 24 silica and alumina tetrahedra, which are connected together, form a cuboctahedron referred to as a sodalite unit or truncated octahedron. Each sodalite unit has a spherical void volume of 6.6 8, (free diameter) and is (6) Kobayashi, Y.; Nomizu, T.;Ujihira, Y. J. Am. Chem. SOC.1979, 101, 537-40.
(7) The positron lifetime in the aluminum foil was 0.23 ns and the intensity of this source component was about 10%. As the same positron source was used in this series of experiments, any correction of source component was not done. (8) Kirkegaard, P.; Eldrup, M. Comput. Phys. Commun. 1974, 7, 401-9.
Flgure 2. The faujasite structure. The silicon or aluminum ions are located at the corners and the oxygen near the edges. The supercage is in the center. The sodalie cages are in truncated octahedra around the supercage.
w LL5
_I
L
40
1 0
1 1
I
2
3
4
5
O
6
TEMPERATURE/lOO°C
Flgure 3. Variation of lifetime (73)and intensities (I2and 1 3 )vs. the evacuation temperature for 13X.
0
1
2
3
,
1
I
4
5
6
'
0
TEMPERATURE/lOO°C
Figure 4. Variation of lifetime (7J and intensities ( I , and evacuation temperature for SK-40.
I,) vs. the
connected to four other sodalite units by six bridge oxygen atoms as shown in Figure 2. The sodalite units are piled like carbon atoms in a diamond. The supercages, which is seen in the center of Figure 2, is surrounded by 10 sodalite units and is a spherical void of 1 2 8, diameter. It is connected to four other supercages through windows which are rings of six sodalite units, so that the faujasite zeolite has a highly porous framework s t r u ~ t u r e . ~
4440
The Journal of Physical Chemistry, Vol. 86, No. 22, 1982
Nakanishi and Ujihira
.
6
50
C
r5
-
40
- 30 \s
1
1
2
5 PORE
_ I _
0
1
I_
2
L
3
4
5
-
I
0
6
TEMPERATURE/IOOY
Figure 5. Variation of lifetime (7.J and intensities ( I , and I,) vs. the evacuation temperature for NH,-X
w
E 4 Iw
k3 - LO
2
-
6
I
1
2
~
--.-
3
- __. -L 0
4
5
6
TEMPERATURE /lOO°C
Figure 6. Variations of lifetime (7,) and intensities (I, and I,) vs. the evacuation temperature for NH,-Y.
The variations of the positron lifetimes and intensities vs. the evacuation temperatures for 13X, SK-40, NH4-X, and NH4-Y are shown in Figures 3-6, respectivly. The 7 1 and 7 2 values were remarkably constant for each sample even after evacuating a t high temperatures, and thier values were 0.3 and 0.9 ns, respectively, while the 73 values changed from 3 to 9 ns as a function of evacuation temperature. As the lifetime for self-annihilation of p-ps, 0.125 ns, is independent of external conditions and that of free positrons is about 0.4 ns in Si02powder,1° the 71 component with intensity Zl can be attributed to the annihilation of free positrons in the bulk of the synthetic zeolites and the self-annihilation of p-Ps. The values of lifetimes in several porous materials reported by Perkal and Walters,l Ito et al.,” Gol’danskii et a1.,I2 Chuang and Tao,13 and Hsu et al.14 were plotted as a function of mean pore radius in Figure 7 , which shows a good correlation between the mean pore radius and the (9) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. “Chemistry of Catalytic Processes”; McGrow-Hill: New York, 1979; pp 49-78. (IO) Brandt, W.; Paulin, R. Phys. Rev. B 1972, 5, 2430-5. (11)Ito, Y.; Yamashina, T.; Nagasaka, M. Appl. Phys. 1975,6,323-6. (12) Gol’danskii, V. I.; Levin, B. M.; Mokrushin, A. D.; Kaliko, M. A,; Pervushina, M. N. Dokl. Akad. Nauk SSSR 1970, 191, 855-8. (13) Chuang, S. Y.; Tao, S. J. J . Chem. Phys. 1971,54, 4902-7. (14) HSU,F. H.; Yang, M. M.; Ymg, C. C. Appl. P h y ~1978,15,85-7 .
10 20
50
RADIUS~A
Figure 7. The relationship between the 0-Ps lifetime in the pore and the pore radius. Open circles are data reported by other workers: closed circles are data obtained from the present experiment.
lifetime corresponding to the 0-Ps annihilation in the pores. The values of 73 obtained by our measurements for 13X and 4AI5 were also plotted against the mean pore radius in Figure 7 . As they fall on the appropriate points of the correlation curve, the 73 component with intensity Z3 is attributed to the annihilation of 0-Ps in the cages of the synthetic zeolites. Annihilation processes of positrons and Ps in zeolites can be sorted as follows when the chemical reaction of 0-Ps does not occur: (1)positron annihilation in the bulk; (2) positron annihilation in the cages; (3) self-annihilation of p-Ps; (4)pick-off annihilation of 0-Ps in the bulk; (5) pick-off annihilation of 0-Ps in the cages. The 7 2 component, therefore, is concluded to originate from trapped positrons in the cages and 0-Ps which annihilate by the pick-off process in the bulk of the synthetic zeolites. Paulin and Ambrosino5obtained a 0-Ps lifetime of 1.8 ns in the bulk Si02 fine powder. The values obtained by us are smaller than 1.8 ns, which may indicate that 0-Ps hardly annihilates in the bulk of the zeolites and that almost all 0-Ps annihilate in the cages. The longest lifetime components (74 N 50 ns) were observed for 13X and SK-40 zeolites. From the correlation curve in Figure 7 , the lifetime of 50 ns corresponds to the annihilation of 0-Ps in a pore of diameter about 60 A. But such a large pore does not exist in the faujasite structure, which was studied in detail by X-ray diffractometry.16 Although Levin et a1.2 suggested that this long lifetime could be attributed to 0-Ps in the zeolite cages, this component does not seem to be due to the annihilation of 0-Ps in the cages since the long lifetime deviates from the correlation curve in Figure 7 . Brandt and Paulin17measured the positron lifetime for Si02powder (grain size 40-300 A) and obtained a lifetime of about 140 ns for the free annihilation of 0-Ps between the grains. The sizes of zeolite powders used for the present study are about 5 Fm in diameter. If the fourth component is due to the annihilation of 0-Ps in the boundary region of the particles, it must be observed for other types of zeolites whose particles sizes are as large as those of 13X. But the fourth component was not observed for 4A- and F-9-type zeolites.l8 Therefore this is neither the component for 0-Ps in the zeolite cages nor the com(15) Zeolite 4A was purchased from Wako Pure Chemical Industries, Ltd. The structure of 4A is the same as Linde type 4A. (16) Broussard, B. L.; Shoemaker, D. P. J . Am. Chem. SOC.1960,82, 1041-51. (17) Brandt, W.; Paulin, R. Phys. Reu. Lett. 1968,21, 193-5. (18) Zeolite F-9 was purchased from Wako Pure Chemical Industries, Ltd. The diameter of the cages in F-9 is about 9 A and the grain size is about 10 fim.
The Journal of Physical Chemisfty, Vol. 86, No. 22, 1982 4449
Ps Annihilation for Zeolite Characterization
ponent for o-Ps between the boundary region of particles. In addition, the r4 component was observed for SK-40 which had been evacuated at 500 "C, exposed to air for 2 days, and evacuated at room temperature. Since the adsorption of water molecules to the cages in zeolites is generally reversible, this result supports the conclusion that the fourth component is not due to o-Ps annihilation in the cages. Therefore we concluded that the fourth component is due to the macrocavities which resulted from the destruction of the supercages in the faujasite zeolites by unidentified causes. Figures 3 and 4 show that the T~ values for 13X and SK-40 increase up to 400 "C and then start decreasing as the evacuation temperature is increased. The value of r3 can be calculated from the free volume theory19 as follows: 73
= (1/rro2c~o)[l+ J'(V,R)I
t
>
t
0 A
Y
a 2.5 w a
r
0
I
E u 1 2 3 4 5 6
2'o0
TEMPERATURE/IOVC
(1)
where ro is the classical electron radius, c is the velocity of light, po is the average electron density in the zeolite, F(V P ) is a function of the depth of the potential well, V , and the radius of the well or the mean radius of cages, R. Equation 1indicates that the r3value depends on the cage radius as well as the surface properties of the cages. When the properties of the cage surface are kept unchanged, that is, V is kept constant, the lifetime of o-Ps in the cage should depend only on the size of the cage. As the temperature increases the size of the cage also increases due to dehydration that takes place inside the cage. Therefore the corresponding increase in the r3value with temperature up to 400 "C can be attributed to the increase in the size of the cage due to dehydration. Shen and his collaboratorsz0examined the adsorption of water molecules in the cages by means of IR spectrometry and reported that their surfaces were covered with multilayers of water molecules. Considering the fact that the properties of the cage surfaces remain unchanged up to 400 "C, one can conclude that the higher the evacuation temperature becomes, the more the water molecules desorb and that the surface of the cages are just covered with a monolayer of OH groups when the evacuation temperature is about 400 "C. This explanation was confirmed by thermogravimetric analysis which showed that almost all water molecules detached above the calcination temperature of 350 "C. However, the decrease of r3 above 400 "C cannot be explained from this discussion. In order to explain this phenomenon, a change in the depth of the potential well of the cage surface must be considered. The decrease of r3 caused by further dehydration may be interpreted by the interaction of o-Ps with the surface of the cage as has been suggested by Chuang and Tao in their study of o-Ps reactions on a silica gel surface.13 By analogy with their results, the o-Ps in the cages is conjectured to be quenched by the chemical or the spin conversion reaction at the points of -=(Si,Al)-O. and =Si-. As the probability of a Ps chemical reaction with oxygen is approximately 1when the diameter of the pore containing Ps is less than 15 A,3 and as the fwhm of the annihilation y-ray peaks for 13X and SK-40 in Figure 8 do not increase, the chemical reaction of o-Ps seems likely to occur in the cages of zeolites. The T ~ 12, , and I3 values for NH4-X and NH4-Y are plotted against the evacuation temperatures in Figures 5 and 6. The r3 values are maximum at about 300 "C as are the r3'sfor 13X and SK-40. However, all the r3 values for (19)Brandt, W.; Berko, S.; Walker, W. W. Phys. Reu. 1960, 120, 1289-95. (20) Shen, J. H.; Zettlemoyer, A. C.; Klier, K. J.Phys. Chem. 1980,84, 1453-9.
13X SK-4C
Flgure 8. Variation of the fwhm's obtained from Doppler-broadening measurements for 13X and SK-40 vs. the evacuation temperature.
L
*'OO
1 2 3 4 5 6 TEMPERATURE/ 100°C
Flgure 9. Variation of the fwhm's obtained from Doppler-broadening measurements for NH,-X and NH4-Y vs. the evacuation temperature.
NH4-X and NH4-Y are considerably smaller than those for 13X and SK-40. The smaller r3values for NH4-X and NH4-Y at room temperature suggest that the NH4+-substituted zeolites have a smaller cage volume for certain reasons. It is well-known that the H-X or H-Y type zeolites, which are made from NH,-X or NH4-Y by heat treatment, have strong Brernsted acidity. Therefore, the smaller T~ values indicate the occurrence of the oxidation reaction of o-Ps in the cages of the heat-treated NH4-X and NH4-Y. The fwhm values for NH4-X and NH4-Y, which are plotted against the evacuation temperature in Figure 9, increase gradually after an initial decrease. The increase of the fwhm is caused by the inhibition of Ps formation or the oxidation of o - P s . ~The ~ occurrence of the oxidation reaction of o-Ps is also suggested by the results of Doppler-broadening measurements. When the oxidation of o-Ps occurs, one can describe the oxidation reaction mechanism in the cages as follows: Ps
+
ox -L ?SOX
i"p
*Y
21.
We can obtain the following equations: A3 = T3-I = A, + k[OX] (21) Nakanishi, H; Kobayashi, Y.; Ujihira, Y. Nippon Kagaku Kaishi 1979, 1198-203.
4450
The Journal of Physical Chemistry, Vol. 86, No. 22, 1982
I3 =
3 -P - A, - OX] 4 A0
A0
Nakanishi and Ujihira
- A,
where A. is the decay rate for annihilation in complex [PsOx], X, is the decay rate for pick-off annihilation of 0-Ps in the cages, k is the formation rate constant for the Psoxidizer complex, [Ox] is the concentration of oxidizer, and P is the probability of Ps formation. As A,,, A,, 12, and P can be considered constant, it is expected that the I3 values increase with an increase in the concentration of the oxidizing reagent. As can be seen from Figures 5 and 6, however, the I3 for NH4-X and NH4-Y do not increase, which may indicate that both the inhibition of 0-Ps formation and the oxidation of o-Ps occur a t the Brernsted acid sites. Wardz2investigated on the amount of Brernsted acid in NH4-Y by IR spectrometry and reported that the Brernsted acid sites increases with the evacuation temperature from 200 to 450 "C, become constant up to about 600 " C , and then decrease. In order to estimate the amount of acid sites by positron measurements, the values of the 73 for SK-40 minus the r3 for NH,-Y, which are considered to correlate with the amount of acid sites, are plotted as a function of the evacuation temperature and shown in Figure 10. The estimation of acidity obtained by positron measurements seems to agree with the results obtained by IR spectrometry. As is seen in Figure 11, the r4 values for SK-40 are about 50 ns in the temperature region from 100 to 400 "C and decrease slightly at 500 "C. The result seems to indicate that almost all water molecules have left the macrocavities of SK-40 and the adsorptivity of water molecules to the wall of the macrocavities is considerably small. As the 74 values for NH4-Y decrease with increasing evacuation temperature, the oxidation of 0-Ps seems to occur in the macrocavities as well as in the cages.
Conclusion Positron annihilation is an excellent tool for studying the sizes of the cages and the characteristics of the active sites in porous materials. In our experiments, we selected X- and Y-type zeolites as samples, and measured the positron lifetimes and the fwhm's of annihilation y-ray peaks by varying the evacuation temperature. The positron lifetime spectrum was resolved into three components to which five processes of positron annihilation can be attributed, considering the characteristics of zeolites. The fourth component ( T * ~ 50 ns) was observed for 13X and SK-40. It was considered to indicate the existence of irregular macrocavities with radii of about 60 A,although (22) W a d , J. W. J. C U ~ U1968, ~ . 11, 251-8.
p-i-
4 -
3 2 -1
9-P-i
0
TEMPERATURE1 1OO'C Figure 10. Variation of the values of [ ~ ~ ( s K - 4 0 )T~(NH~-Y)] vs. the evacuation temperature.
I
50
"
t 07K 0
40 -
h
I30 -
L
520-
o\o
Lo," NHYI
10 -
Figure 11. Variation of the fourth lifetimes for SK-40 and NH,-Y the evacuation temperature.
vs.
such large cavities could not be detected by X-ray diffractometry. From the lifetime-evacuation temperature curves for 13X and SK-40, it was found that all water molecules detached above 400 "C. The existence of active groups formed by a release of OH groups on the cages was suggested from the reduction of 73 above 400 "C. It was confirmed by both the lifetime measurements and the Doppler-broadening measurements that Brernsted acids in H-X and H-Y zeolites oxidized 0-Ps and inhibited the formation of 0-Ps. The amounts of Brernsted sites could be approximately estimated by the positron annihilation technique, which may suggest the possibility of quantitative analysis of active sites in porous materials. However, since the lifetime of 0-Ps in the cages decreases somewhat less rapidly than expected with the amount of Brernsted acids, it is suggested that a small portion of H+ should be projected into the supercages.
