ESE Radicals Adsorbed in A-Type Zeolites
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equations for gll, g,, All,and ALN were slightly modified. By using the Vierke equations, we obtain, however, almost the same results as above, the main difference being in the a2parameter which results 0.84 with respect to 0.85. This difference is in the limits of the uncertainty. Table I11 reports the calculated values of the bonding coefficients for the pentammine species together with those of other previously investigated copper-ammine comp l e ~ e s . ~ J ~The J ~ calculated J~ bonding coefficients are of the correct order with respect to other nitrogenous complexes. The small differences obtained with the Cu(NH3)$+ unit in free solution and after adsorption on silica reflect a change in the environment, i.e., the interaction with the surface of the supports results in an increase of the ionic character of the in-plane u bond with respect to the unadsorbed species. In many cases, the largest amount of the in-plane covalent bonding was found on the unadsorbed species (see Table 111). This trend accounts for the substitution of H 2 0 with surface deprotonated silanol groups which should indeed establish a more covalent character in the plane.
Acknowledgment. Thanks are due to the Italian Na-
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tional Council of Research (CNR) for financial support.
References and Notes (1) G. Martini and L. Burlamacchi, preceding article in this issue. (2) M. L. Hair, "Infrared Spectroscopy in Surface Chemistry", Marcel Dekker, New York, 1967, p 67. (3) V. V. Morariu, Z. Phys. Chem. (FrankfurtamMain),97, 235 (1975). (4) G. Vierke, Z. Nafurforsch. A , 26, 554 (1971). (5) E. F. Vansant and J. H. Lunsford, J . Phys. Chem., 76, 2860 (1972). (6) J. Bjerrum, C. J. Ballhausen, and C. K. dbgensen, Acta Chim. Sand., 8. 1275 (1954). (7) P: W. Schindie;, B. Furst, R. Dick, and P. U,Wolf, J. Colloid Intorface Sci., 55, 469 (1976). (8) J. C. Vedrine, E. G. Derouane, and Y. Ben Taarit, J. Phys. Chem., 78, 531 (1974). (9) D. R. Flentge, J. H. Lunsford, P. A. Jacobs, and J. B. Uytterhoeven, J . Phys. Chem., 79, 354 (1975). (10) C. Naccacheand Y. BenTaarit, Chem. Phys. Left., 11, 11 (1971). (11) F. Mazzi, Acta Ciystaliogr., 8, 137 (1955); B. Morosin, Acta Crystallogr., Sect. B , 24, 19 (1969). (12) D. Kivelson and R. Neirnan, J . Chem. Phys., 35, 149 (1961). (13) A. H. Maki and B. R. McGarvey, J . Chem. Phys., 29, 35 (1958). (14) H. R. Gersrnann and J. D. Swalen, J. Chem. Phys., 36, 3221 (1962). (15) A. A. G. Tomlinson and B. J. Hathaway, J. Chem. SOC.A , 1905 (1968). (16) A. B. F. Duncan and J. A. Pople, Trans. Faraday Soc., 49, 217 (1953); A. 9. F. Duncan, J. Chem. Phys., 27, 423 (1957). (17) P. Peigneur, J. H. Lunsford, W. DeWilde, and R. A. Schoonheydt, J . Phys. Chem., 81, 1179 (1977).
Electron Spin Echo of H Atoms and OH Radicals Adsorbed in A-Type Zeolites 8. A. Dikanov, R. I. Samoilova, and Yu. B. TSWetkQv" Institute of Chemical Kinetics and Combustion, Novosibirsk 630090, USSR (Received February 2, 1979)
Data on radical trapping sites in zeolite a cages have been obtained by analysis of modulation effects in the two-pulse electron spin echo of H atoms and OH radicals produced by irradiation of A-type zeolite with adsorbed water.
Introduction Synthetic zeolites, possessing a high structural homogeneity of the silica-alumina network as compared to other sorbents, are well suited for investigations of trapped radicals produced by irradiation of various species in an adsorbed state. ESR analysis of irradiated zeolites with an adsorbed substance allows one to identify the type of the paramagnetic particles trapped after irradiation and to study their transformation and decay kinetics. However, ESR spectra usually do not yield any information on the location of radicals with respect to the zeolite network since the hyperfine interaction (hfi) of unpaired electrons with the magnetic nuclei of the zeolite network is usually very weak and does not manifest itself in ESR spectra due to inhomogeneous line broadening. New possibilities for weak hfi studies are now possible due to the development of the electron spin echo (ESE) technique. The weak anisotropic hfi between an unpaired electron and matrix magnetic nuclei, in the case of polycrystalline and amorphous species, is known to result in modulation effects in ESE. Analysis of the modulation effects allows one to identify the magnetic nuclei in the vicinity of a radical and to estimate the number of such nuclei and the distance to them.l Recently we have studied2 ESE modulation effects of CHzOH radicals adsorbed in A-type zeolites. By the analysis of ESE modulation due to aluminum magnetic nuclei involved in the 0022-3654/79/2083-2515$01 .00/0
zeolite framework and also the magnetic nuclei of the molecules located around CHzOH radical we have constructed the geometry of trapping sites for CHzOH radicals adsorbed in the A-type zeolite framework. The present paper considers the modulation effects in the ESE of PI atoms and OH radicals trapped in y-irradiated A-type zeolites with adsorbed water molecules.
Experimental Technique Na-A and K-A zeolites completely saturated with water were used in the experiment. The degree of Na/K exchange for K-A zeolite was about 50%. Prior to use the zeolite samples were dehydrated by placing them in thin-wall quartz ampules and pumping at 250-280 "C to (1-5) X torr. Then water vapor was adsorbed in the zeolites. The sealed ampules were irradiated with a 6oCo y source to a dose of 4-5 Mrd at 77 K. The experiments were carried out with an X-band ESE spectrometer1 at 15 K. It is known that after irradiation of water adsorbed in different types of A and X zeolites, H atoms, OH radicals, and trapped electrons are stabilized at 77 K.3 We studied the ESE signal which arose when microwave pulses excited the ESR lines of H atoms in low and high fields and also various parts of the OH ESR spectrum (its total width being approximately 80 Oe). These lines did not overlap those of trapped electrons, zeolite defects, and quartz 0 1979 American
Chemical Society
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The Journal of Physical Chemistry, Vol. 83, No. 19, 1979
S. A.
Dikanov,
R. I. Samoilova, and Yu. D. Tsvetkov
Flgure 1. Primary ESE modulation of the H atom in low (a) and high (b) H, fields, and OH radical (c) in Na-A zeolite.
Flgure 2. Primary ESE modulation of the H atom in low (a) and high (b) Ho fields, and OH radical (c) in K-A zeolite.
paramagnetic centers. The two-pulse ESE technique was used in the experiments. The ESE signal was recorded by an X-Y recorder as a function of the time interval T between the first and second pulses. Because of the nonlinear response of the microwave detector, the amplitude of the recorded ESE signal was proportional to [ V ( T ) ]where ~ x was determined by the detector characteristics and equal to 1.8 in our experiments.
high and low fields has 3.94- and 3.38-MHz modulation, respectively, and for OH radicals it has 3.61-MHz modulation. For disordered species the modulation effects in the primary ESE for the ease of weak anisotropic hfi of an unpaired electron with nuclei of an arbitrary spin at small T are described by the relation1
Experimental Results a n d Discussion Before analyzing experimental results we shall briefly describe the structure of silica-alumina network for A-type zeolite~.~J A104 and Si04tetrahedrons are the principal building blocks of the zeolite network. In the A-type zeolites used in these experiments the number of aluminum atoms equals that of silicon atoms, i.e., each oxygen atom is bound to one atom of aluminum and silicon. A104 and Si04 tetrahedrons form a cubooctahedron which is also called a sodalite or /3 cage. This cage has eight hexagonal and six square faces (Figure 3b). A number of 0 cages connected by cubic passages forms a big zeolite cage, the so-called a cage which has eight hexagonal and twelve square faces and also six octagonal windows leading to the neighboring a cages (Figure 3a). Each of these cages contains a 11.4-Adiameter void. The free diameter of the octagonal zeolite opening is 4.2 A, and the inlet diameter to the p cage is 2.2 A. Since A104 tetra.hedrons possess a surplus negative charge, the zeolite structure involves also an equivalent amount of metal cations. Usually A-type zeolites are synthesized in Na form, from which other zeolite types can be obtained by ion exchange, In A-type zeolites eight of the twelve monovalent,cations are localized in the vicinity of the centers of the octagonal openings leading from a to 0 cages and the other four cations are distributed statistically in twelve positions on the cubic surface near the octagonal zeolite openings. In hydrated zeolites these cations together with water molecules are localized in the octagonal opening^.^ Figures 1and 2 present curves of the signal decay of the primary echo for the ESR lines of H atoms in low (a) and high (b) Ho fields and OH radicals (c) stabilized in Na-A and K-A zeolites. The ESE signal decay for H atoms in
-
v(T)
= v(0) x g2P2 1 l)-X-(3 H: i rpi6
-
4
COS WIT
+ COS 2 ~ 1 ~ )
Here I is the nuclear spin, ri is the distance between an electron and the ith nucleus, and oIis the Zeeman nuclear frequency. To describe quantitatively the hfi with magnetic nuclei, a new parameter could be used, the ESE modulation amplitude
where Vminand V,,, are the echo signal envelopes which can be found from (1) at cos WIT= f l . A corresponding value of this parameter can be obtained from the experimental modulation curves (in Figures 1and 2 the signal echo envelopes, Aminand A,,,, are shown in dashed lines). Taking into account the detector characteristics, we can write X
1 - (Amin/Amax)l/X
(3)
The modulation amplitudes, A, for the experimental curves (Figures 1 and 2) measured at times, T , corresponding to the first of the echo signal minima are (accurate to kO.01) for zeolite Na-A, 0.19 (Ho,low), 0.15 (Ho, high), and 0.5 (Ho,OH), and for zeolite K-A, 0.16 (Ho,low), 0.13 (Ho,high), and 0.4 (Ho,OH). Here the values of the external field intensity at which the signals for hydrogen atoms and OH radicals are observed, are Ho, low = 3047 Oe, Ho, high = 3551 Oe, and Ho, OH = 3254 Oe. The decrease in the modulation amplitude for the radicals trapped in K-A zeolite can be explained if one takes into account that in Na-A zeolite the contribution
The Journal of Physical Chemistry, Vol. 83, No. 19, 1979 2517
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ESE Radicals Adsorbed in A-Type Zeolites
to X is made by magnetic aluminum and sodium nuclei involved in its lattice, whereas in K-A zeolite it results from aluminum and potassium nuclei and also by nonexchanged sodium nuclei. The Zeeman frequency of 23Na nuclei is very close to that of 27Alnuclei. For example, at an external magnetic field of Ho = 3300 Oe these frequencies equal 3.71 and 3.66 MHz which corresponds to a negligible difference of the modulation period, AT = 0.0037 ks. Therefore, in Na-A zeolite the ESE modulation amplitude, recorded at small T , equals the total contribution of sodium and aluminum nuclei. 39Knuclei have a frequency of 0.66 MHz at the same Ho which corresponds to a modulation period of about 1.5 ps. This value is close to the characteristic time of the primary ESE signal decay due to phase relaxation. Hence, modulation by 39Knuclei cannot be observed and thus the contribution to X in K-A zeolite is made by the aluminum and sodium nuclei which remain in the lattice after ion exchange. 23Nahas a nuclear spin I = 312 and 27A1has I = 512. In our calculations use is made of expression 2 because the experimental moddation curve has a simple form with the amplitude varying periodically with one frequency. Then the amplitude of modulation by aluminum and sodium nuclei in Na-A zeolite is
I
2
2
m* D:
Flgure 3. Structure of half the polyhedron bordering the a cage (a), the polyhedron bordering the /3 cage (b), and geometrical models of the parallel planes on which the polyhedron vertices of the a and /3 cage lie.
:.-
g2P2
= 287(S"'
HO
+ SNa)(4)
In K-A zeolite the first term is the same, whereas the second one is less by a quantity corresponding to the degree of cations exchange. Hence, one can determine the contributions of aluminum and sodium nuclei separately by the decrease of the modulation in K-A zeolite as compared to Na-A zeolite. Using relation 4 and experimental data on the modulation amplitude A, we found the lattice sums, S = S M + SNa,for Na-A and I
