Copper-Ammonia Complexes Adsorbed on Silica Gels
The Journal of Physical Chemistry, Vol. 83, No. 19, 1979 2511
Acknowledgment. The authors thank the Italian Council of Research (CNR) for the financial support of this work.
(18) (19) (20) (21)
References and Notes
(22) (23) (24) (25) (26) (27j (28) (29)
(1) R. Burwell, Jr., R. G. Pearson, G. H. Haller, P. B. Tjok, and S.P. Chock, Inorg. Chem., 4, 1123 (1965). (2) S. Fujiwara, S. Katsumata, and T. Seki, J. Phys. Chem., 71, 115 (1967). (3) D. M. Clementz, T. J. Pinnavaia, and M. M. Morthnd, J. Phys. Chem., 77, 196 (1973). (4) E. F. Vansant and J. H. Lunsford, J. Phys. Chem.,76, 2860 (1972). (5) H. Tominaga, Y. Ono, and K. Keji, J . Cafal., 40, 197 (1975). (6) D. R. Flentge, J. H. Lunsford, P. A. Jacobs, and J. B. Uytterhoeven, J . Phvs. Chem.. 79. 354 11975). (7) P. Peigneur, J. H. Lunsford: W. De Wilde, and R. A. Schoonheydt, J. Phys. Chem., 81, 1179 (1977). (8) G. Martini and M. F. Ottaviani, Z . Nafurforsch. 5, 33, 62 (1978). (9) C.Naccache and Y. Ben Taarit, Chem. Phys. Left., 11, 11 (1971). (10) I.R. Leith and H. F. Leach, Proc. R. Soc. London, Ser. A , 330, 249 (1972). (1 1) J. C. Vedrine, E. G. Derouane, and Y. Ben Taarit, J . Phys. Chem., 78, 531 (1974). (12) V. Bassetti, L. Burlamacchi, and G. Martini, J . Am. Chem. Soc., in press. (13) G. Martini and L. Burlamacchi, Chem. Phys. Left., 41, 129 (1976). (14) G. Martini and V. Bassetti, following paper In this issue. (15) J. Bjerrum, C. J. Ballhausen, and C. K. JtYgensen, Acta Chim. S a n d . , 8, 1275 (1954). (16) N. N. Tikhomirova, K. I.Zamaraev, and V . M. Berdnikov, J . Strucf. Chem. (Engl. Trans/.),4, 407 (1963). (17) W. W. Schmidt and K. G. Breitschwert, Chem. Phys. Left., 27, 527 (1974).
(30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44)
R. Wilson, Thesis, University of California at Berkeley, 1975. J. B. Spencer, Thesis, University of California at Berkeley, 1975. G. Vierke, Z . Nafurforsch. A , 28, 554 (1971). A. A. G. Tomlinson and B. J. Hathaway, J . Chem. Soc. A , 1905 (1968). E. H. Carlson and R. D. Spence, J . Chem. Phys., 24, 471 (1956). F. Mazzi, Acta Clysfallogr., 8, 137 (1955). B. Morosin, Acta Clysfallogr., Sect. 5, 24, 19 (1969). H. P. Fritz and H. J. Keller, Z . Naturforsch. 6 , 20, 1145 (1965). H. Yokoi and T. Isobe. Bull. Chem. Soc. Jon.. 41. 2835 119681. B. J. Hathaway and D. E. Billing, Coord. Chem. dev.,'5, 143 i1970j. J. J. Fripiat, Cafal. Rev., 5, 269 (1971). J. J. Fripiat, C.Van der Meersche, R. Touillaux, and A. Jelly, J. Phys. Chem., 74, 382 (1970). J. J. Fripiat, A. Leonard, and J. B. Uytterhoeven, J . Phys. Chem., 69, 3274 (1965). M. R. Basila and T. R. Kantner, J. Phys. Chem., 71, 467 (1967). V. I.Klividze, R. A. Brants, V. F. Kisselev, and G. M. Bliznakov, J . Catal., 13, 255 (1969). G. Biyholder and E. A. Richardson, J . Phys. Chem., 66, 2597 (1962). M. Nagao and T. Morimoto, Bull. Chem. SOC.Jpn., 49,2977 (1976). H. A. Reslng, Adv. Mol. Relaxation Processes, 1, 109 (1967-1968). Landolt-Bornstein, Zahlenwerte und Funktionen, I1 Band, 2. Teil, Bandteil 2b, Springer-Verlag, Berlin-Gottingen-Heidelberg, 1962, p 3.12. M. L. Hair, "Infrared Spectroscopy in Surface Chemistry", Marcel Dekker, New York, 1967, p 67. V. V. Morariu, Z. Phys. Chem. (FrankfurtamMain),97, 235 (1975). 0. Y. Samollov, Zh. Fiz. Khim., 20, 12 (1946). E. Forslind, Proc. Int. Congr. Rheol., 2nd, 1953, 50 (1954). H. S. Franks and W. Y. Wen, Discuss. Faraday Soc., 24, 133 (1957). M. Prigogine and J. J. Fripiat, Chem. Phys. Lett., 12, 107 (1971). M. Prigogine and J. J. Fripiat, Bull. SOC.R. Sci. Liege, 43, 449 (1974). A. A. Antoniu, J. Phys. Chem., 88, 2754 (1965).
ESR Study of Copper-Ammonia Complexes in Solution Adsorbed on Silica Gels. 2. Narrow-Pore Silica Gels Giacomo Martini" and Valerlo Bassettl Istitufo di Chimica Fisica, UniversitA di Firenze, 50 12 1 Firenre, Ita/y (Received February 5, 1979)
The adsorption of copper-ammonia complex solutions on narrow-pore silica gels (pore diameter 110 nm) was investigated by electron spin resonance. Cu(I1) surface chemisorption was the major effect observed and Cu(I1) species free to move in the solution filling the pores were observed only with the highest Cu(I1) concentrations. The surface adsorption was quantitatively checked by ESR intensity measurements and chemical analysis of the residual Cu(I1) in the mother liquid. Characterization of the surface complex was made in terms of the bonding coefficients calculated from the ESR parameters and compared with those of other copper complexes with nitrogenous ligands.
Introduction TABLE I: Physical Properties of t h e Silica Gels Used in T h i s Work When metal ions are adsorbed on a solid surface from liquid solutions, the process is by no mean simple and surface pore type area, m z / g diam, nm [OH], mol/g involves many steps which are seldom thoroughly considered in the current practice of impregnation of sup54 650 4 8.6 X ported catalysts or in other impregnation processes. S6 400 6 5.3 x 10-3 s10 In a previous paper in this series,l the behavior of copper 300 10 4 x 10-3 complexes, dissolved in water-ammonia mixtures, filling the pores of silica gels with pore diameter 1 20 nm was surface adsorbed species are derived. studied by electron spin resonance (ESR) and differential Experimental Section scanning calorimetry (DSC). It resulted that only a fraction of the Cu(1I) species was adsorbed on the surface Materials. Preparation of the C U ( N H ~ ) ~ ( H ~comO)~+ while the major amount was free to move within the liquid plex in 8.4 M ammonia solution with Cu(I1) concentrations solution filling the pores. in the range 10-3-10-1 M and impregnation and handling This paper reports an ESR study of C U ( N H ~ ) ~ ( H ~ O ) ~of+ the supports were carried out as previously described.l solutions adsorbed on narrow-pore silica gels (pore diEach adsorption was performed by putting 1 g of the ameter 510 nm), where copper adsorption on surface sites support into 10 cm3 of the appropriate solution. The is the major effect. From the calculation of bonding Cu(I1) residual content in the filtered solution after adparameters, the structure and essential features of the sorption was checked by measuring the Cu(I1) ESR signal 0022-3654/79/2083-2511$01 .OO/O
0 1979 American Chemical Society
2512
The Journal of Physical Chemistry, Vol. 83, No. 19, 7979
G. Martini and V. Bassetti
TABLE 11: Magnetic Parameters of Copper-Ammonia Complexes Adsorbed o n Silica Gels All, cm-I ( + 0 . 0 0 0 3 )
system CU(NH,),~+ on S4 Cu(NH,),'+ on S6 CU(NH,),~+ on ,910 Cu(NH,),(H,O)" in 8.4 M ammonia Cu(NH,)," in H,O-NH,-CH,OHa Cu(NH,)," on Y zeoliteb a
Reference 4.
293 K -0.0186 -0.0188 -0,0187
gll ( t 0 . 0 0 3 )
77 K -0.0192 -0.0192 -0.0196 - 0.0182 -0.0192 175t 2 G
gi ( k 0 . 0 0 3 )
AlN, G
293 K
77 K
293 K
77 K
2.230 2.229 2.229
2.247 2.244 2.243 2.245 2.245 2.235
2.038 2.035 2.034
2.048 2.047 2.048 2.046 2.061 2.035
(k0.2) 12.4 12.4 12.4 13 14
Reference 5.
intensity and by titrimetric analysis with EDTA. Three kinds of silica gels (Merck adsorbents for chromatography) were used, which had monodispersed porosity of 4,6, and 10 nm diameter (henceforth labeled S4, S6, and SlO). Their physical properties are reported in Table I. The OH concentrations per gram of support were calculated assuming 8 OH/ 100 A' in fully hydrated silica gel~.~B Techniques. The ESR spectra were recorded with a Bruker 200tt spectrometer operating in the X-band and equipped with a Bruker B-ST 100/700 variable temperature assembly. Slow cooling (-1 OC/min) of the samples was usually carried out from room temperature to 77 K in order to allow eventual water crystallization. The optical spectra were measured with a Perkin-Elmer UV-VIS Model 200 spectrophotometer equipped with an internal reflection accessory. MgO was used as a standard. Results and Discussion ESR Spectra. Figure 1 shows the ESR spectra at room temperature and at 77 K obtained from a 1.25 X M C U ( N H ~ ) ~ ( H ~solution O ) ~ + adsorbed on S4, S6, and S10. The signals invariably consisted of a powder spectrum with axial symmetry and resolved low-field components. The ESR parameters both at 293 and at 77 K are reported in Table 11. Within the limits of experimental error, the magnetic parameters are independent of the Cu(I1) concentration and of the pore diameter. While the g values are close to those of C U ( N H ~ ) & H ~ Oin) ~the + frozen unadsorbed solution, appreciable differences are observed in the All value, which indicate a small but significant variation in the Cu(I1) environment. At the above copper concentration, no evidence is found for the presence of the isotropic four-line ESR absorption, due to the copperammonia complex free to move within the adsorbed liquid, previously observed in wide-pore systems.l The optical spectra in diffuse reflectance, carried out at room temperature on the same samples used in the ESR experiments, show maxima at 15800 cm-l to be compared with the transition at 15625 cm-l reported for CU(NH,)&H~O)~+ in aqueous solution at room temperature.6 Thus, on the basis of the magnetic parameters and of the optical absorption, the axial signal may be attributed to a pentaammine-copper complex adsorbed on silica surface sites with small structural changes with respect to C U ( N H ~ ) ~ ( H ~inO the ) ~ +unadsorbed aqueous solution. The sixth ligand substituting the axial water molecule for the pseudo-octahedral coordination should be a =Si-OH (or, more probably, its ionized form FSi-O-) on the surface. This accounts for the increase of the All value (see below). The resulting symmetry is therefore riot higher than C4".That metal ions can be adsorbed at silica-water interface giving rise to surface complexes with deprotonated surface silanol groups has been recently reported for Fe(III), Cu(II), Cd(II), and Pd(I1) hydrated ions.' Bridge complexes in which the Cu(I1) ion is directly linked to two OH groups of the same silanol unit are improbable in our case, because they are expected t o be in very low
-
A';
= - o 0 1 9 2 cm-'
4,
: - 0 0 1 8 6 cm"
g: = 2 2 3 0
t---i
V Figure 1. ESR spectra at 293 K (dashed line) and at 77 K (full line) of Cu-NH, complexes after adsorption of a Cu(I1) 1.25 X M ammonia solution ([NH,] = 8.4 M) on S4, SB, and S10.
concentrations, on the basis of the reported association constants, According to the suggestion by Vedrine et a1.8 for Cu(NH3)42+adsorbed on Na-Y zeolite, the decrease of both gli and All observed for the adsorbed complex with increasing temperature from 77 to 293 K (see Table 11)can be attributed to Cu-N molecular vibration dynamics, which is strongly influenced by the interactions between adsorbed complex and lattice vibrations. Superhyperfine Splitting. In some cases a superhyperfine structure (shfs) due to nitrogen nuclei is observed on the high-field component, both a t room temperature and at 77 K. Figure 2 shows the ESR signal at 77 K after M Cu(I1) water-ammonia adsorption of a 3.125 X solution on S4. Shfs is observed at low copper concentrations, the limits of appearance being about 2 X lo-', 1.25 and 8 X M, respectively, for S4, S6, and S10. X From the Figure 2b, nine lines (ALN = 12.4 G) can be identified, which account for four equivalent nitrogen nuclei, i.e., those of the NH3 molecules coordinated in the equatorial plane of the adsorbed complex. Shfs due to nitrogen nuclei from adsorbed copperammine complexes were previously observed by several
Copper-Ammonia Complexes Adsorbed
on
The Journal of Physical Chemistry, Vol. 83, No. 19, 1979 2513
Silica Gels
A:
= 124
G
Figure 2. (a) ESR spectrum at 77 K of Cu-NH, ([Cu(II)] = 3.125 X a.
authors after adsorption of nitrogenous ligands on copper-exchanged zeolite^.^@^^ The values of A L Nin the case of Cu-NH, complexes were in the range 12-14 G.53s-10 Lunsford and co-workersgwere able to detect resolved shfs in the mI = -3/2 low-field component of the spectrum of Cu(ND3)?+adsorbed on low-exchanged Y zeolite. On the basis of the apparent number of resolved shfs lines or of the ratio between adsorbed ligand molecules and copper ions introduced into the support, it was generally assumed that the copper ion was only coordinated with four nitrogenous ligands. Most probably, in our case the N nucleus of the fifth ammonia molecule in the axial position has a smaller coupling constant which can escape detection. Dependence of the C u ( I I )Line Shape on Concentration. Figure 3 shows the room temperature ESR spectra after adsorption of C U ( N H ~ ) ~ ( H ~solutions O ) ~ + with different copper concentrations on S10. An isotropic spectrum centered at g 2.1 with partial resoltuion of four hyperfine line (ICu = 3/2) appears at the highest concentrations. This spectrum was attributed to residual copper-ammonia species dissolved in the liquid mixture inside the p0res.l The resulting ratios between Cu(I1) ions and surface OH groups are in the range 1.1X 10-'-12.5 X When this ratio falls below 4 X practically only the powder signal is observed. Upon cooling, the isotropic spectrum transforms into an exchange-narrowedspectrum with three g values, due to segregated crystals of a copper-ammonia salt whose structure was suggested1 to be similar to that of [CU(NH,)~(H~O)].SO,.'~ All features of this t h r e e g value spectrum are identical with those found in wide-pore silica gels (pore diameter I20 nm) and they were described in a previous paper.' By analogy with the formalism used in that paper we will henceforth call A the powder-type spectrum (dotted trace in Figure 3) and B the isotropic spectrum (dashed trace in Figure 3). Simultaneously with the appearance of the spectrum B, chemical analysis and ESR check reveal the presence of residual copper ions in the mother liquid. The same results
f
\
A,, = - 00192 crn-I
\
/
M) adsorbed on S4; (b) expanded high-field component of spectrum
-
Figure 3. ESR spectra at room temperature of Cu-NH, on S10 at different Cu/OH ratios: (a) Cu/OH = 1.1 X (b) 1.6 X (c) 3.2 X lo-'; (d) 4.5 X (e) 6.4 X (f) 1.2 X lo-'. Superimposed on signal f a r e also reported (arbitrary intensity) the ESR spectra of the unadsorbed solution (dashed line) and of CU(NH,),~+ adsorbed on the surface (dotted line).
lo-*;
are obtained on S4 and S6 samples and in any case sppctrum B and the presence of Cu(I1) in the mother liquid appears when Cu/OH I 4 X low2.Quantitative results for S10 are shown in Figure 4. The curve clearly shows that adsorptiv of CU(NH,),~+on the surface silanol groups
2514
The Journal of Physical Chemistry, Vol. 83, No, 79, 1979
G. Martini and V. Bassetti
TABLE HI: Bonding Coefficients of Copper Complex with Nitrogenous Ligands system
Figure 4. Concentrations of the surface Cu-NH, complex (H) and of the Cu(I1) residual in the mother liquid (A)as a function of the Cu/OH ratio.
tends to stabilize for Cu/OH ratio above 2 X IO-l. For S10 the limiting value of adsorbed copper-ammonia complexes is 3.2-3.4 X mol/g of support. Thus, one copperammonia surface complex seems to be formed every 10-12 silanol groups, that corresponds to a mean distance between Cu(I1) nuclei of 11-13 A. It is noteworthy that the resolution of the shfs from the nitrogen nuclei is observed i.e. -1/4 of only when the Cu/OH ratio is 12.5 X the maximum occupancy is reached on S10 (see Figure 4) and copper ions are at least 20-25 A apart, making spin-spin dipolar broadening negligible. In the above discussion, we did not consider the influence of ammonia adsorption at the surface OH groups. Ammonia itself can react with silanol groups as shown in ref 1. Almost complete coverage of the surface with NH3 molecules occurs only when the ",/OH ratio is >15, far above the limit reached in this work (-1.4 on S10 with [NH,] = 8.4 M), for which at least 90% of the surface sites are available for Cu(1I) chemisorption. Bonding Coefficients of the Surface Complex. In this section the magnetic parameters and the energy transitions reported in Table I1 are used in a calculation of the LCAO-MO bonding parameters of the copper-ammonia complexes. The appropriate spin Hamiltonian for a d9 ion with IB1) as the ground state in C10 symmetry is given by = PokIlHzS, + gi(H,S, + H y S y ) ] + AIII,S, + A,(SJ, + SJ,) (1) By limiting the treatment to the four ammonia equatorial ligands, the following molecular orbital scheme for the antibonding orbital can be set up: IB,) = ald,z-,z) - Y2a'[-ci,(') ciY ('1 a,(3)- ciy(4)]
+
IB,) = Pld,,)
-
+
1/(1 - P2)1/z[p,(1) + p,(')
- P , ( ~ )- P,'~']
IE) = Pi\&,) - (1- P12)1'z[~z(1) -~,'~'1/2 = Pild,,)
-
(1 - P12)1'2[pz(2) - ~ ~ ' ~ ' 1 / 2 (2)
The higher the values of a2,P2, and PI2, the higher will be the ionic character of the in-plane ci and T bond and the out-of-plane P bond, respectively. The resolution of the spin Hamiltonian (eq 1)on the basis of the wave functions in eq 2 was performed by several authors and the resulting equations are reported, for instance, in ref 12-14. The simplified equation12 (Y' = -All/P (911 - 2.0023) 3/7(gll - 2.023) + 0.04 (3)
+
+
01'
Cu(NH,),'+ adsorbed on silica gels Cu(NH,), z + in ",-water solution Cu( NH, )," in NH,-H'O--CH,OH Cu(NH,),'+ adsorbed on Y zeolite CU(PY h2+ in py-CHC1, CU(PY),'+ adsorbed on Y zeolite Cu(en),'+ in en solution Cu(en)''+ adsorbed on Y zeolite Cu(en),'+ in en solution Cu(en 3 2 + adsorbed on Y zeolite
0,'
02
note
0.85
0.71
0.67
a
0.81
0.79
0.72
b
0.83
0.77
0.83
C
0.83 0.76
d
0.86
1
0.80
e d
0.81 0.63
0.61-0.73
f
0.79
0.56
0.54-0.76
f
0.77
0.67
0.67-0.78
f
0.76
0.64
0.62-0.76
f
a Parameter used for the calculation were as follows: gll = 2 . 2 4 6 ; g ~= 2.048;All = - 0 . 0 1 9 3 c m - ' ; S ( n ) = 0.093; T(n)=0.333; AE,. = 15800 cm-'; AExz = 16800 cm-'. Parameter used for the calculation were as follows: gii = 2 . 2 4 5 ; g l = 2.048;Ali = -0.0182 em-'; AE,, = 15625 cm-'; other parameters as in a. Reference 4. Reference 10. e Reference 14. f Reference 17.
where P = 0.036 cm-l, is used to obtain a first approximation of a2. By using AE,, = 15800 cm-I (from the optical spectra) and AE,, = 16800 cm-I, as reported15 for C U ( N H ~ ) ~ ( C ~one O ~can ) ~ ,obtain the coefficients p2 and Pi2 from the equations of ref 12-14 by an iterative procedure. The values of p2 and P12 calculated as above are used to obtain better values of a*. The same procedure is used for the calculation of a2,P2, and P12 for the pentaammine complex in frozen, unadsorbed aqueous solution. The superhyperfine splitting due to nitrogen nuclei can be used as a further check of the above calculation. With IB,) as the ground state, Maki and McGarveyl, obtained the following coupling energies:
where l$zs(0)12is the electron density of the N nucleus (l$2s(0)12= 33.4 X cm3); (r-,), is the electron-nucleus distance and it was evaluated to be ( r ) p - 3= 21.4 X loz4 ~ m - y~ is; the gyromagnetic ratio €or the N atom; a' may be obtained from the normalization of the IBl) orbital: cy2 d 2 2aa'S(n) = 1
+
+
where S(n) is the overlap integral, S(n) = 0.093.l' By using the ar2value calculated as described above, we obtained AiN = 11.9 G, in a good agreement with the experimental value A L N = 12.4 G. Vierke4 suggested a modified procedure for the calculation of the bonding coefficients for the copper complexes with ammonia as a ligand on the basis that sp2 hybridization used in the treatment of Kivelson and Nieman12 and of Maki and McGarveyl, is not a good approximation for NH, molecule. Recalculation of S(n), the overlap integral, and of T(n), the integral over the ligand functions, was carried out by using an arbitrary hybridization parameter n taken from the hybrid functions of Duncan and Pople16for the lone pair orbital. Accordingly, the resulting
ESE Radicals Adsorbed in A-Type Zeolites
equations for gll, g,, All,and ALN were slightly modified. By using the Vierke equations, we obtain, however, almost the same results as above, the main difference being in the a2parameter which results 0.84 with respect to 0.85. This difference is in the limits of the uncertainty. Table I11 reports the calculated values of the bonding coefficients for the pentammine species together with those of other previously investigated copper-ammine comp l e ~ e s . ~ J ~The J ~ calculated J~ bonding coefficients are of the correct order with respect to other nitrogenous complexes. The small differences obtained with the Cu(NH3)$+ 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-
The Journal of Physical Chemistry, Vol. 83, No. 19, 1979 2515
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
