Metal Ion Site Geometry and Oxidation State in Zeolites - American

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Downloaded by CORNELL UNIV on October 11, 2016 | http://pubs.acs.org Publication Date: May 17, 1983 | doi: 10.1021/bk-1983-0218.ch019

Metal Ion Site Geometry and Oxidation State in Zeolites T. I. MORRISON, P. J. VICCARO , and G. K. SHENOY 1

Argonne National Laboratory, Argonne, IL 60439

The combined results of EXAFS and Mössbauer effect spectroscopies have been used to deduce the oxidation state and local structure of iron in iron containing Na, K- and H-chabazites. In the mineral Na,K(Fe) chabazite iron is found to be present as oxo- or hydroxo-bridged Fe(III) oligimers, while i n the H-chabazite it is found to be present as Fe(III) ions bridged by oxo- or hydroxo-groups to the zeolite framework.

Metal c o n t a i n i n g z e o l i t e s are of great i n t e r e s t as p r a c t i c a l c a t a l y s t s f o r petrochemical r e a c t i o n s . Such c a t a l y s t s are u s u a l l y considered to be b i f u n c t i o n a l , with the z e o l i t e framework a c t i n g as a support for the metal c e n t e r s . In order to optimize r e a c t i o n c o n d i t i o n s and parameters i n the processes f o r which m e t a l - z e o l i t e c a t a l y s t s are used, it is necessary to understand the chemistry of the support and the metal and t h e i r i n t e r a c t i o n . Much can be learned about the chemistry of the metal centers by a d e s c r i p t i o n of the environment of the c a t i o n . In broad terms, the task is to determine the l o c a t i o n of the c a t i o n s i t e or s i t e s w i t h i n the z e o l i t e and thereby deduce i t s l o c a l s i t e e n v i ronment, or conversely to determine the l o c a l s i t e environment of the c a t i o n and i n f e r i t s p o s i t i o n i n the z e o l i t e . The former task is o f t e n undertaken by x-ray or neutron s c a t t e r i n g technique, and the l a t t e r by spectroscopic techniques. O n leave from Instituto de Fisica, UFRGS, Porto Alegre RS, Brazil

This chapter not subject to U.S. copyright. Published 1983, American Chemical Society Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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A l a r g e number of x-ray s c a t t e r i n g studies have been made on metal c o n t a i n i n g z e o l i t e s and have met with varying degrees of success ( 1 ) . In cases where the number of c a t i o n s per u n i t c e l l is high enough to give an appreciable (and unambiguous) s i g n a l , where s u i t a b l y large s i n g l e c r y s t a l s can be grown, and when there is a degree of ordering among the c a t i o n s , these studies have been able to provide complete and r e l i a b l e d e s c r i p t i o n s of the c a t i o n environment. Nevertheless, there e x i s t s the p o s s i b i l i t y of m i s assignment of e l e c t r o n d e n s i t y peaks, a t t r i b u t i n g c r y s t a l c o o r d i nation to wrong atoms or to "average s i t e s " only p a r t i a l l y f i l l e d


In cases where there is a low concentration of c a t i o n of i n t e r e s t , i f the c a t i o n s are h i g h l y disordered i n the z e o l i t e framework, or i f good c r y s t a l l i n e samples are u n a v a i l a b l e , atom s p e c i f i c or e n v i r o n m e n t - s p e c i f i c spectroscopic probes may be p r e f e r a b l e to determine l o c a l s t r u c t u r e s about the c a t i o n i n the zeolite. NMR ( 4 ) , IR (5,J&) ESR (7-10), o p t i c a l ( 9 , 1 0 ) , Mossbauer e f f e c t (11-15), and x-ray a b s o r p t i o n studies ( 2 , 1 6 , 1 7 , 1 8 ) have been used to determine c a t i o n microenvironments. In p a r t i c u l a r , it has been shown that EXAFS (Extended X-ray Absorption Fine Structure) of the c a t i o n can o f t e n be used to give d i r e c t s t r u c ture information about c a t i o n environments i n z e o l i t e s , but EXAFS techniques, while g i v i n g r a d i a l distances and r e l a t i v e c o o r d i n a t i o n numbers, are i n s e n s i t i v e to s i t e symmetry and cannot, i n g e n e r a l , give both c o o r d i n a t i o n numbers and r e l a t i v e s i t e p o p u l a tions. C l e a r l y it is d e s i r a b l e to use complementary spectroscopic techniques to f u l l y e l u c i d a t e the microenvironments i n d i l u t e , p o l y c r y s t a l l i n e z e o l i t e systems. We have addressed the problem o f determining the l o c a l e n v i ronments of i r o n centers i n i r o n c o n t a i n i n g chabazites by using two s p e c i f i c a l l y short range probes, Mossbauer and EXAFS s p e c t r o scopies. The Fe Mossbauer e f f e c t can be used to o b t a i n i n f o r mation on the o x i d a t i o n s t a t e s , magnetic s t a t e s and the l o c a l symmetry of i r o n (19) while the EXAFS of the i r o n can be used to determine r a d i a l distances to near neighbors, near neighbor atomic type, and within 1imits the r e l a t i v e numbers of near neighbors. Of primary i n t e r e s t i n t h i s study is the change i r o n undergoes from i t s s t a t e i n the f r e s h l y mined z e o l i t e , through ion exchange, c a l c i n a t i o n , and framework p r o t o n a t i o n . 5

Experimental The n a t u r a l mineral o r "as mined" Na,K(Fe) chabazite samples were prepared by simply l o a d i n g and r e s e a l i n g the z e o l i t e powder i n i t s sample c e l l . The protonated framework H(Fe) chabazite was prepared by i o n exchange with ammonium n i t r a t e , a i r d r y i n g , and c a l c i n a t i o n a t 350 ° C to d r i v e o f f the ammonia. To determine separately the e f f e c t s on the i r o n centers of i o n exchange and h e a t i n g , an NH^(Fe) chabazite sample was prepared by i o n exchange of the Na,K(Fe) chabazite i n 1 M NH4NO3 f o r 16 hours at room

Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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temperature with subsequent a i r drying of the sample. In a d d i t i o n , a sample of Na,K(Fe) chabazite was c a l c i n e d f o r 16 hours at 350 ° C to determining heating e f f e c t s . The Mossbauer e f f e c t experiments were performed using a Co/Rh source and a conventional electromechanical d r i v e . Data were c o l l e c t e d between 297 and 4.2 Κ on a Nuclear Data ND 6600 computer and were analyzed by l e a s t squares f i t t i n g of the appro p r i a t e l i n e shapes to the spectra d e s c r i b i n g the hyperfine i n t e r actions. The amount of Fe i n the chabazite was about 1 wt. %. Absorption path lengths were optimized i n sample preparation by uniformly d i s t r i b u t i n g 200 mg/cm of the c h a b a z i t e . Iron K-edge EXAFS experiments were a l s o performed on the Na,K(Fe) chabazite and H(Fe) chabazite samples. The experimental spectra were obtained at room temperature both i n transmission and fluorescence mode at the Stanford Synchrotron R a d i a t i o n Labora tory. Because of the d i l u t i o n of Fe i n the h o s t s , the f l u o r e scence data were an order of magnitude b e t t e r i n s t a t i s t i c s than the a b s o r p t i o n d a t a . M u l t i p l e runs were taken on each sample, and the i n c i d e n t and f l u o r e s c e n t i n t e n s i t i e s , I and I^, were summed r e s p e c t i v e l y to improve the s i g n a l - t o - n o i s e r a t i o .


2

Q

Results and D i s c u s s i o n In Figure 1, we present the Mossbauer spectrum of Na,K(Fe) chabazite mineral measured at room temperature. The spectrum is composed of a symmetric doublet. The s o l i d l i n e i n Figure 1 is a l e a s t squares f i t to the data assuming a s i n g l e quadrupole i n t e r action. The quadrupole s p l i t t i n g of O.49 ± O.02 mm/s and an isomer s h i f t of O.35 ± O.02 mm/s with respect to Fe metal obtained from t h i s f i t is t y p i c a l f o r a high s p i n f e r r i c (3d ) ion (20). The value of the isomer s h i f t does not permit us to a s s i g n unambi guously e i t h e r a t e t r a h e d r a l or octahedral c o - o r d i n a t i o n f o r t h i s f e r r i c ion (21). It must be pointed out that although the above a n a l y s i s assumes a unique c r y s t a l l o g r a p h i c s i t e f o r the f e r r i c i o n i n the c h a b a z i t e , the width of the resonance l i n e s (which is n e a r l y twice the minimum experimentally observable width) i n d i cates some d i s t r i b u t i o n i n the neighboring environment of Fe ions on a microscopic s c a l e . The spectrum of Na,K(Fe) chabazite measured at 4.2 Κ does not d i f f e r from the room temperature spectrum. T h i s is s u r p r i s i n g s i n c e with the low concentration of Fe i n the l a t t i c e , we expect to see some paramagnetic hyperfine s t r u c t u r e or at l e a s t l i n e broadenings (22) a s s o c i a t e d with slow r e l a x a t i o n of the F e ^ electronic spin. T h i s implies a f a i r l y strong coupling of Fe spins to the l a t t i c e or to other Fe s p i n s . Assuming the l a t e r case, we expect Fe to have other Fe neighbors w i t h i n about 4 Â . The room temperature spectrum of NH^(Fe) chabazite was nearly i d e n t i c a l to that of Na,K(Fe) c h a b a z i t e , i n d i c a t i n g that the process of ion exchange does not a l t e r the environment of Fe atoms in this l a t t i c e . 5

+



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The calcination of NH^(Fe) chabazite to produce H(Fe) chabazite, however, drastically changes the Mossbauer spectrum. In Figure 2 we present the spectrum of H(Fe) chabazite measured at room temperature. The solid line in Figure 2 is a least squares analysis assuming three distinct Fe sites with differing quadrupole interactions. The isomer shifts for each of the three sites is almost identical to that in Na,K(Fe) chabazite showing that iron is present in the high spin ferric state. The three quadurpole splittings are 1.90, 1.04 and O.81 mm/s and the three inequivalent sites have an approximate population ratio of 1.3:3.0:2.0, respectively. The broad width of the resonances again indicates the presence of a distribution in the local surroundings of each of these sites. The dramatic increase in the quadrupole interaction on protonation is noteworthy. These interactions, which are among the largest measured for f e r r i c compounds, are related to the local and near local charge dispositions around the iron sites. Â large quadrupole interaction in the protonated chabazite is indicative of significant gradients in the charge distributions of the sites. On cooling the sample to 4.2 Κ there are no indication of paramagnetic hyperfine s p l i t t i n g although the lines show some minor broadening. Again the Fe spins appear to be well coupled to the spin bath. The spectrum of calcined Na,K(Fe) chabazite (Figure 3) is similar in every respect to that of H(Fe) chabazite. This clearly indicates that the process of calcination is responsible for the large differences observed between the Na, K- and H-chabazite. Ion exchange alone produce l i t t l e effect at the Fe site environ ment. Both Na,K(Fe) chabazite and H(Fe) chabazite after dehydration show the onset of paramagnetic hyperfine structure in their Mossbauer spectra at 77 and 4.2 K. It is tempting to suggest that dehydration reduces the coupling of the spins in a major way or that the Fe atoms form a superparamagnetic oxide (23). We shall next discuss the EXAFS results and draw conclusions by comparing them with the Mossbauer results. In Figure 4 the EXAFS data are plotted in the wave-vector (k) space. The o s c i l l a tions were Fourier transformed to real space to resolve the major frequency components of the oscillations. The resulting phaseshifted radial distribution functions are shown in Figure 5. By backtransforming, or Fourier f i l t e r i n g , each peak representing a coordination shell, it is possible to determine from the backtransform envelope the type of near neighbor, and by least squares f i t t i n g the backtransformed oscillations it is possible to deter mine radial distances and relative coordination numbers about the iron (24). Figure 6 shows the backscattering amplitude functions for the f i r s t coordination shells of Fe in Na,K(Fe) chabazite and H(Fe) chabazite. The shape of this function, with the singularity at k = 0, is indicative of backscattering by f i r s t row elements and


ET

AL.



MORRISON

Figure 2.

Room temperature Mossbauer spectrum of U(Fe) chabazite.



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Figure 3.

CHEMISTRY

Room temperature Mossbauer spectrum of calcined Na,K(Fe) chabazite.


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O.30-

-O.201 -O.25H -O.30"J 2.0

1

4.0

1 6.0

1 1 1 8.0 10.0 12.0 k (RECIR ANGSTROMS)

1 14.0

ι 16.0

18.0

Figure 4. k-weighted i r o n K-edge EXAFS spectra of Na,K(Fe) chabaz i t e and H(Fe) c h a b a z i t e . Spectra obtained at room temperature.


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O.I2l


O.114

R (ANGSTROMS)

Figure 5 . F o u r i e r transforms o f EXAFS s p e c t r a . Solid l i n e -» Na,K(Fe) c h a b a z i t e , dashed l i n e —• H(Fe) chabazite.



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Chabazite

3.0 4.0

5.0 6.0 7.0

8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0

MRECIP. ANGSTROMS) Figure 6. Backscattering amplitude functions f o r f i r s t c o o r d i n a t i o n s h e l l s i n Na,K(Fe) chabazite and H(Fe) c h a b a z i t e .


327


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we may conclude that the nearest neighbors to i r o n i n both z e o l i t e s are oxygen atoms (25)» T h i s is f u r t h e r supported by the r e s u l t s of the l e a s t - s q u a r e s f i t t i n g of the f i r s t peak i n each spectrum, shown i n Table I. Figure 7 shows the b a c k s e a t t e r i n g amplitude f u n c t i o n s f o r the second c o o r d i n a t i o n s h e l l s of Fe i n Na,K(Fe) chabazite and H(Fe) chabazite. The b a c k s c a t t e r i n g amplitude f o r the second s h e l l of the Na,K(Fe) chabazite i n d i c a t e s that t h i s s h e l l is made up com p l e t e l y of i r o n , and the r e s u l t s of the l e a s t - s q u a r e s f i t t i n g corroborate t h i s . In H(Fe) chabazite on the other hand, the b a c k s c a t t e r i n g amplitude f o r the second s h e l l i n d i c a t e s that t h i s s h e l l is composed of s i l i c o n and/or aluminum and oxygen. It was evident i n the l e a s t squares f i t t i n g o f t h i s s h e l l that the atoms were h i g h l y disordered r e l a t i v e to the absorbing i r o n , and r e l i a b l e s t r u c t u r a l parameters could not be e x t r a c t e d . TABLE I EXAFS RESULTS Na ,K(Fe)-chabaz i t e

H(Fe)-chabazite

8 Ri(A) Al σι (A) R2(A)

A2 σ (Α) 2

8

2.00 (.02) O.52 (.01) O.045 (.002) 26 3.12 (.03) O.09 (.03) O.045 (.029)

2.00 (.01) O.32 (.04) O.043 (.011)

8 1.46 O.15 O.02

(.02) (.03) (.02)

• i t h backscatterer atomic number R • r a d i a l distance to i t h backscatterer A^ = amplitude of o s c i l l a t i o n s due to i t h b a c k s c a t t e r e r o = (rms r e l a t i v e d i s p l a c e m e n t ) ' o f i t h backscatterer i

1

i

2

Conclusions By combining r e s u l t s of Mossbauer and EXAFS spectroscopies we are able to draw c e r t a i n conclusions about the i r o n s i t e s i n the chabazite samples. Considering f i r s t the i r o n In the Na,K(Fe) c h a b a z i t e , we f i n d that the i r o n is not exchangable with the ammonium s o l u t i o n s used, i n d i c a t i n g that the i r o n is e i t h e r i n an i n a c c e s s i b l e s i t e , is incorporated into the z e o l i t e framework, o r is present as species i n s o l u b l e under exchange c o n d i t i o n s used i n the z e o l i t e . The Mossbauer spectra t e l l us that all of the i r o n i n the z e o l i t e is i n nearly i d e n t i c a l environments, so only one o f the above options is p o s s i b l e . The EXAFS spectrum o f i r o n i n Na,K(Fe) chabazite shows that ( w i t h i n EXAFS r e s o l u t i o n ) each i r o n is surrounded e q u i d i s t a n t l y by



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O.016 O.014 O.012 O.010

50.008-1 Q

Na.K-Chabazite

O.006 0004

H-Chabazite O.002 O.000. 3.0 4Ό

5.0

6Ό

7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 k (RECIP. ANGSTROMS)

Figure 7. Backscattering amplitude functions f o r second c o o r d i n a t i o n s h e l l s i n Na,K(Fe) chabazite and H(Fe) c h a b a z i t e .



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oxygen with next near neighbors of i r o n . Since the Mossbauer data i n d i c a t e that all irons are nearly i d e n t i c a l , it is most p l a u s i b l e to suggest an aquo, hyroxo, or oxo bridged i r o n o l i g i m e r . It is doubtful that the irons are aquo b r i d g e d , since such a species should be s o l u b l e and subject to ion exchange. The dimensions of the chabazite supercage put an upper l i m i t on the s i z e of the i r o n multimer, since t h i s cage would be the only reasonably s i z e d host f o r the multimer. It can a l s o be concluded that the c l o s e s t approach of an i r o n atom to the cage wall cannot be l e s s than approximately 3 Â, since no s i l i c o n , aluminum, or oxygen c o n t r i butes coherently to the second s h e l l b a c k s e a t t e r i n g . The Mossbauer spectra of the H(Fe) chabazite show c l e a r l y that the i r o n environment changes on protonation of the framework by ammonium exchange and deammoniation. The s i m i l a r i t y of the spectra of Na,K(Fe) chabazite and NH^(Fe) chabazite i n d i c a t e s that the change i n i r o n environment is not due to the ion exchange. However, c a l c i n e d Na,K(Fe) chabazite gives a spectrum very s i m i l a r to the deammoniated H(Fe) c h a b a z i t e , showing that the heating of the sample is responsible f o r the changes i n the i r o n environment. The Mossbauer spectrum of the H(Fe) chabazite shows three r e s o l v a b l e f e r r i c i r o n s i t e s , with l a r g e quadrupole i n t e r a c t i o n s i n d i c a t i v e of asymmetry i n the p o s i t i o n s of surrounding i o n s . The EXAFS data, i n d i c a t i n g two disordered iron-oxygen d i s t a n c e s , are c o n s i s t e n t with the Mossbauer r e s u l t s . The EXAFS a l s o shows that the i r o n multimer is completely d i s r u p t e d a f t e r h e a t i n g , since i r o n is no longer present i n the second s h e l l . Since the second s h e l l is composed e n t i r e l y of S i , A l , and 0, we may conclude that the i r o n has moved c l o s e r to the z e o l i t e framework. The p o s s i b l e l o c a t i o n s of the i r o n s i t e s i n the H(Fe) chabaz i t e can be i n f e r r e d by a process of e l i m i n a t i o n . It is improbable that the i r o n is l o c a t e d i n the center of the prisms forming supercages, since the s i t e is h i g h l y symmetric and there are no e l e c t r i c f i e l d gradients present that would give r i s e to the observed quadrupole s p l i t t i n g . It is also doubtful that the i r o n is l o c a t e d i n the center of a s i x - r i n g , since t h i s i r o n would be t e t r a - or p e n t a - , rather than hexa-coordinate. It is most l i k e l y that the irons are coordinated to the walls of the supercage v i a oxo- or hydroxo-bridges with waters of hydration coordinat i v e l y s a t u r â t ing the irons j u t t i n g i n t o the supercage. The complementarity and i n t e r p l a y of the r e s u l t s of EXAFS and Mossbauer spectroscopies provide means by which c r y s t a l chemistry of c e r t a i n cat ions i n z e o l i t e s may be s u c c e s s f u l l y s t u d i e d . While n e i t h e r technique is completely adequate as a " s t a n d - a l o n e " t o o l f o r such s t u d i e s , the combination of the two can be used to map the chemical nature, environment and l o c a t i o n of c a t i o n s where t h i s information would otherwise be i n a c c e s s i b l e .

Acknowledgments Work supported by the U . S . Department of Energy.


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Smith, J. V. Proceedings of the F i f t h International Conference on Zeolites 1980, 194-204 and references cited therein. Morrison, Τ. I.; Reis, Α. Η., Jr.; Gebert, E.; Iton, L. E.; Stucky, G. D.; Suib, S. L. J. Chem. Phys. 1980, 72, 62766282. Pluth, Joseph J.; Smith, Joseph V. J. Chem. Phys. 1979, 83, 741-749. Lechert, H.; Henneke, H. W. "Molecular Sieves - II"; Katzer, James R., Ed., American Chemical Society: Washington; 1977; p 53-63. Brodskii, I. Α.; Zhadanov, S. P. Proceedings of the F i f t h International Conference on Zeolites 1980, 234-241. Flanigen, Edith M. "Zeolite Chemistry and Catalysis"; Rabo, Jule Α., Ed.; American Chemical Society: Washington; 1977; p 80-117. Tikhomirova, Ν. N.; Nikolaeva, I. V. Proceedings of the F i f t h International Conference on Zeolites 1980, 230-233. Kasai, Paul H.; Bishop, R. J., Jr. "Zeolite Chemistry and Catalysis"; Rabo, Jule Α., Ed.; American Chemical Society: Washington; 1977; p 350-391. Peigneur, P.; Lunsford, J. H.; de Wilde, W.; Schoonheydt, R. A. J. Phys. Chem. 1977, 81, 1179-1187. Garbowski, Edoward; Primat, Michel; Mathieu, Michel-Vital "Molecular Sieves - II"; Katzer, James R., Ed.; American Chemical Society: Washington; 1977; p 281-290. Schmidt, F.; Gussner, W.; Adolph, J. ibid., 291-301. Fitch, Frank R.; Rees, Lovat V. C. Zeolites 1982, 2, 33-41. Dickson, Bruce L.; Rees, Lovat V. C. J. Chem. Soc. Faraday Trans. 1 1974, 70, 2038-2050. Suib, S. L.; Zerger, R. P.; Stucky, G. D.; Emberson, R. M.; DeBrunner, G.; Iton, L. E. Inorg. Chem. 1980, 19, 1858-1862. Samuel, Ε. Α.; Delgass, W. N. J. Chem. Phys. 1975, 62 15901592. Morrison, T. I.; Reis, Α. Η., Jr.; Gebert, Ε.; Iton, L. Ε.; Stucky, G. D.; Suib, S. L. J. Chem. Phys. 1980, 73, 47054706. Morrison, Timothy I.; Iton, L. E.; Shenoy, G. K.; Stucky, G. D.; Suib, S. L. J. Chem. Phys. 1981, 75, 4086-4089. Morrison, T. I.; Shenoy, G. K.; Iton, L. E.; Stucky, G. D.; Suib, S. L. J. Chem. Phys. 1982, 76, 5665-5668. See, for example "Chemical Applications of Mössbauer Spectroscopy"; Gol'danskii, V. I.; Herber, R. H., Ed.; Academic Press: New York; 1968 and Gütlich, P.; Link, R.; Trautwein, A. "Mössbauer Spectroscopy and Transition Metal Chemistry"; Springer-Verlag: Berlin; 1978.


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Ingalls, R.; van der Woude, F.; Sawatzly, G. A. "Mössbauer Isomer Shifts"; Shenoy, G. K.; Wagner, F. Ε., Eds.; NorthHolland: Amsterdam; 1978; p 361-429. Nicholson, W. J.; Burns, G. Phys. Rev. A 1964, 133, 15681570. Wickman, Η. H. "Mössbauer Effect Methodology"; 2, Gruverman, I. J., Ed.; Plenum Press: New York; 1961. Suzdalev, I. P.; Afanasev, A. M.; Plachinda, A. S.; Gol'danskii, V. I.; Makarov, E. F. Zhur. Eksp. Teor. F i z . SSSR, 1968, 55, 1752. Stern, Ε. Α.; Sayers, D. E.; Lytle, F. W. Phys. Rev. Β 1975, II, 4836-4846. Lee, P. Α.; Beni, G. Phys. Rev. Β 1977, 15, 2862-2883.

RECEIVED December 16,

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Metal Ion Site Geometry and Oxidation State in Zeolites - American

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