Formation of Iron Clusters in Zeolites with Different Supercage Sizes

25 Formation of Iron Clusters in Zeolites with Different

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Supercage Sizes F. SCHMIDT, W. GUNSSER, and J. ADOLPH Institute of Physical Chemistry, University of Hamburg, Laufgraben 24, D-2000 Hamburg 13, West Germany

ABSTRACT Mössbauer spectroscopy, electron microscopy and magnetic measurements of Na reduced Fe -X, Fe -Y, and Fe +-A zeolites show formation of iron particles outside the zeolite cavities with mean diameter of 60 Å, but preferably formation of iron clusters with extremely narrow particle size distribution and diameters less than 13 Å. 2+

2+

2

Introduction T r a n s i t i o n metal ion-exchanged z e o l i t e s have been used to obtain w e l l dispersed metal c a t a l y s t s . Some c a t i o n i c forms of dehydrated z e o l i t e s can be r e duced by heating the z e o l i t e i n a reducing atmosphere l i k e hydrogen. This leads e i t h e r to h i g h l y dispersed metal atoms i n the channels and c a v i t i e s o f the zeol i t e or to e x t e r n a l deposition o f small metal c r y stallites. Some methods o f reduction o f t r a n s i t i o n metal ion-exchanged z e o l i t e s are l i s t e d i n Table I . Some t r a n s i t i o n metal ions may be s t a b i l i z e d i n unusual oxidation states - i . e . N i ( l ) , P d ( l ) . Table I Me-Zeol + 1/2 H2 Me = Pb,Ni,Cu,Ag,Pt,Pd

H-Zeol

+ Me

0

L e w i s Q ) ,Rabo e t a l . (2) .Yates(^) ,Bredikhina e t a l . (4) Romanowski(5) .RiekertTo) ,Richardson(7),Reman e t al.T§ Kudo e t al.T^J,Dalla Betta e t a l . (107«Minchev e t a l . " " (11),Beyer e t al.(12). 291

Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

292

MOLECULAR SIEVES—II

Table I(continued) Ni(ll)-Zeol + Na -> Rabo et al.(1j5) 2 Cu(II)-Zeol + CO-* Naccache et al.(14)·

Na(l)-Zeol

+ Ni(l)

2 Cu(l)-Zeol +

C0

p

As reported by Kuspert (15) hydrogen reduction of ferrous ion-exchanged zeolites i s possible only at and above 1023 K. But these conditions lead to l a t t i c e destruction. Furthermore, X- and A-type zeo l i t e s i n the hydrogen forms are not known to be stable. So we reduced our samples by sodium vapour at 673 and 573 K. Thus the i n i t i a l sodium form of zeo l i t e was prepared, containing reduced electroneutral metal. In the present study we were especially inter ested i n the magnetic properties of small metallic iron clusters. These particles should be formed with in the zeolite cavities and their diameters should be limited by the supercage sizes. f

Experimental Methods Sodium A,X, and Y-zeolites were obtained from the Linde Division of Union Carbide. A l l steps of preparation were made i n a nitrogen atmosphere. Only oxygen free d i s t i l l e d water was used. The iron X and Y zeolites were prepared by treating the sodium form of Linde zeolite with an aqueous solution of ferrous sulphate i n a nitrogen atmosphere at p 5 for 3 hours as described by Delgass (16). For preparing the ferrous exchanged form of Atype zeolite the ferrous salt solution was obtained by dissolving 1,88 g ferrous sulphate hepta hydrate i n 45 cm* water thus reaching a p of 4. A few grains of ascorbic acid were added to refiuce any f e r r i c ions present. The exchange was allowed to proceed for 2 and 20 hours respectively at 293 Κ and at p 8 as described by Dickson et a l . (17)· Because of the poor crysÎallinity thus obtained we chose another method for ferrous sulphate preparation. By dissolving between 5 and 200 mg iron i n a calculated volume of 2N sulphuric acid diluted i n approximately 6 cm3 water we got a ferrous sulphate solution of p 5· The ferrous ion-exchanged form of A-type zeolite was prepared by treating 0,5 g oxygen free zeolite with the sulphate solution at 293 Κ for 3 h and at 343 Κ for 0,1 and 0,3 h respectively. H

H

H

H


25.

SCHMIDT

E T

A L .

Iron Clusters in Zeolites

293

This method was changed t o prevent formation o f i r o n g e l completely. The suspension o f 0,5 g oxygen f r e e Α-zeolite i n about 2 cnP water was i n i t i a l l y ad j u s t e d t o PH 5 w i t h about 6 ml o f standard a c e t i c a c i d b u f f e r s o l u t i o n . Then the z e o l i t e was ion-ex changed s e v e r a l times. A f t e r being washed t e n times, the exchanged f e r r o u s Α-zeolite was dehydrated a t 673 Κ f o r 24 h. A f t e r dehydration under h i g h vacuum a t 673 Κ f o r 24 h the samples were reduced w i t h a c a l c u l a t e d amount o f sodium. Sodium was p l a c e d a t the bottom o f a g l a s s tube. The dehydrated z e o l i t e sample was supported 3 cm above i t on a s m a l l g l a s s f r i t . The top o f the tube was formed as a Mossbauer c e l l . A f u l l - l e n g t h h e a t i n g mantle was brought t o 673 K . A f t e r a h e a t i n g p e r i o d o f 5,10,15,20,25,30,48 h , r e s p e c t i v e l y the h e a t i n g mantle was removed and the tube was allowed t o c o o l . Then the z e o l i t e was brought i n t o the Mossbauer c e l l . This c e l l was sealed and then mounted i n a Mossbauer spectrometer. A n a l y s i s f o r i r o n was made o f a l l samples by Xray f l u o r e s c e n t spectroscopy. The c r y s t a l l i n i t y o f the f e r r o u s A,X and Y z e o l i t e a f t e r ion-exchange, a f t e r dehydration and a f t e r r e d u c t i o n was examined by X-ray powder d i f f r a c t i o n without exposure t o a i r . Sharp l i n e s i n the powder p a t t e r n s i n d i c a t e d t h a t no s t r u c t u r a l breakdown had occurred w i t h X- and Y-type z e o l i t e s and w i t h A-type z e o l i t e s u s i n g the second and t h i r d method o f p r e p a r a t i o n . The Mossbauer s p e c t r a were taken on a F r i e s e k e & Hoepfner constant a c c e l e r a t i o n spectrometer i n Con j u n c t i o n w i t h a m u l t i - c h a n n e l a n a l y s e r u s i n g a 57co i n Cu source. A l l isomer s h i f t s are r e p o r t e d w i t h r e spect t o standard oc-Fe. Spectra taken a t 300 Κ were made w i t h the g l a s s c e l l s . Spectra from 4 Κ t o 300 Κ were made w i t h a b e r y l l i u m sample holder mounted i n a Leybold c r y o s t a t . The z e o l i t e was placed i n the be r y l l i u m holder without exposure t o a i r . The data from each spectrum were analysed by a l e a s t squares f i t t i n g program. Magnetic s u s c e p t i b i l i t y data were obtained by the Felddifferenzen-method a t temperatures between 77 Κ and 650 Κ and v a r i a b l e f i e l d s up t o 1,25 T. We a l s o used a Foner magnetometer at. temperatures be tween 4 Κ and 500 K. R e s u l t s and D i s c u s s i o n The X-ray d i f f r a c t i o n p a t t e r n shows t h a t the Fe ion-exchanged A-type z e o l i t e was p a r t l y amorphous


294

MOLECULAR

SIEVES—II

i f prepared according to the f i r s t method. This was expected because of the arguments reported by lone (18) i n the last conference. The second method using exchange times of 3 h (at 293 Κ ) , 0,1 and 0,3 h (at 343 K) and the third method using exchange times shorter than 0,3 h (at 293 K) lead to complete cry stalline samples. Figure 1a shows the characteristic room tempe rature spectrum of crystalline, dehydrated Fe -A zeolite obtained by using the second method of pre paration for 0,3 hours. We attributed the doublet with i . s . = 0,19 mm/s ± 6% and q.s. = 0,91 mm/s ± 2% to iron gel. This i s i n agreement with the arguments of lone Γΐ8). Therefore,we chose the third method of ion-exchange : a buffer solution was used to prevent iron gel formation and the exchange time was shorten ed. The spectra of these samples show only two doublets (Figure 1b). Therefore,our supposition of iron gel formation was confirmed. In the resulting spectrum the doublet with i . s . = 0,61 mm/s ± 0,3%?+ and q.s. = 0,47 mm/s ± 0,8% i s associated with Fe ions on the sodalite window sites with 3-fold co ordination to l a t t i c e oxygens as reported by Dickson (17)· We cannot yet render a distinct explanation of the doublet with i . s . = 0,87 mm/s db 1,2% and q.s. = 2,26 mm/s ± 1%, but we suppose that a second site for iron ions exists i n A-type zeolites detectable only at small degrees of ion-exchange and using 57Fe. So i t seems to us that the best method preparing ferrous ion-exchanged A-type zeolite i s using the buffer solution, low temperatures and short exchange times. The spectra of Fe -X and Fe -Y zeolite are similar to those reported by Garten et a l . (19) and by Morice et a l . ( 2 0 ) . Detailed discussions of these spectra are given 13y those authors. A l l the reduced samples show the ferrous oxi dation state spectra and the superparamagnetic spec tra of small iron clusters. The ratio of area of these groups depends on the rate of reduction. In an assembly of noninteracting particles, the relaxation time for a spontaneous change of the direction of the magnetization vector i s given by the Nêel equation 2+

2+

(1)

7 -

1

_

2+

2 Κ ν kT



Figure 1. Môssbauer spectra at 295 Κ,υ = ±4 mm/s. (a), Fe-A, prepared according to the second method; (b), computerfit;(c), third method; (d), computer fit.

to

οι

to CO

co

f

es

Ν

s*

Ι

ε-

Ο

3 s

>

3

ox

296

MOLECULAR

SIEVES—II

in which a i s a geometrical factor, ν the volume, Κ the anisotropy constant of the particle. The frequen cy factor f i s the Larmor frequency of the magnetiza tion vector i n an effective f i e l d ; f=(KA)/(9N.jh) · A is the atomic weight. If the relaxation time i s greater than the time required for the measurement, one can see a snapshot; i . e . i f Τ $> Ι/νγ* Zeeman s p l i t t i n g i s observed - V i s the Larmor frequency of the nuclear spin around the effective f i e l d . For iron 1/V,—10- sec. For £" ^ 1/V only the time mean value i s observed - i . e. the Zeeman s p l i t t i n g w i l l disappear. For the fraction of the particles i n a sample which are superparamagnetic, the observed spectrum w i l l give a pure quadrupole-split center line i f any electric f i e l d gradient i s present. In Figure 2 a Mossbauer spectrum of a p a r t i a l l y reduced ferrous ion-exchanged Y-zeolite i s shown. The central doublet i s due to superparamagnetic iron clusters. Besides the Fe -doublet the usual magnetically s p l i t s i x peak spectral component of the larger iron particles i s to be seen. Taking the ratio of the area corres ponding to the quadrupole doublet to the t o t a l spectral area, the fraction of the small iron clusters can be estimated (assuming the f-values for the two states are similar). Using the Néel equation and taking the values for iron reported by Arnold (21), the maximum size of the small iron clusters canTe calculated from Mossbauer data: V'/3 < 15,6 A . In order to make sure that the central doublet i s not due to atomically dispersed iron and to study the small iron particles i n greater detail, electron micrographs of ultramicrotom cut reduced samples were taken at several magnifications from 40 000 to 200 000 times. These micrographs show a small number of larger particles with diameters up to about 500 A, but preferably they show a formation of iron clusters with extremely narrow particle size d i s t r i bution. The maximum of the particle size distribution function was found to be 20 A. But the corresponding real particles could be 15 A or less because of a diffraction zone of approximately 2 A on each side of the particle diameter. Furthermore, the electron micrographs do not show particles of this sizeneither between the zeolite crystalsnor on the outer surface of the zeolites,except a greater number of them i n the middle of the cut zeolite crystallites. So we can conclude that a l l detectable particles are only inside the cavities. The distribution of parL

Q

L

2+


sdiipdz

ui stdisnjj uox\

τν xa

XOTWHOS


298

MOLECULAR

SIEVES—II

t i c l e sizes could be due to l a t t i c e imperfections like local derivations of the Si/Al ratio from the mean value resulting i n a distribution of supercage sizes. This matter i s s t i l l under investigation by STEM EDAX technique. The magnetic susceptibilities of the dehydrated ferrous ion-exchanged zeolites were measured as a function of temperature. Measured susceptibilities were corrected for the diamagnetism of the alumosilicate framework and of the sample holder and for the effects of a small amount of ferromagnetic impurity. The temperature dependence of the Fe2+-ion suscepti b i l i t y of a Y-zeolite could be represented by the Curie-Weiss-law with p f f = 5,54 /iu and θ = 105· Because of the two types of Fe +-ion sites ρ and θ are only mean values. But the large θ value which re flects the magnetic exchange interaction within the system, i s consistent with the model of the ion sites reported by Garten et a l . (19)· Typical results of magnetization versus magnetic f i e l d of a reduced sample are given i n Figure 3. Measured magnetizations were corrected for the dia magnetism. Taking the fraction of the unreduced iron from Mossbauer data, the f i e l d dependence of the magnetization of the reduced iron was calculated. The susceptibility of the small iron clusters which are paramagnetic from 150 Κ to 650 Κ and up to a f i e l d of 1 Τ waspcalculated by plotting the Δ C /λ Η values versus 1/H and extrapolating to 1/H —»0 according to the formula e

2

2

χ >4

(2)

Corrections of the paramagnetism of the small iron clusters thus obtained were made, and by plotting the resulting magnetization curves versus H/T superposi tion was obtained from approximately 150 Κ to 650 K. Furthermore, no remanence could be detected i n this temperature range. Analysing these curves, particle diameters of the large iron particles were obtained. To estimate the volume ν of the small clusters by means of the paramagnetic susceptibility the tem perature dependence of the spontaneous magnetization ^D^FeZeo * determined by equation (3)· These values were plotted versus Τ/Τ , with Τ the Curie temperature of bulk iron. The Weiss curves l i e dis t i n c t l y below those of bulk iron, calculated accord ing to the mean f i e l d theory. 111118

b

e


25.

SCHMIDT

E T A L .


299

•

KQE*K*x-l 0.24

Q.3Z

«1CT 0.4

0.48

0.

1

S6

0.64

0.7Z

Figure 3. Magnetization (Gauss cm /g) vs. H/T curves of Fe°-Y/Fe *-Y. Τ = 178 Κ; Q,T = 214K; Δ , Τ = 273 Κ; +, Τ = 295 Κ; χ , Τ = 316 Κ; 0 , Γ = 343Κ. 3

2


300

MOLECULAR SIEVES—II

ι„ (τ) Λ

(3>

=

(2 . τ)

1/2

(ΧΤ^) Φ

172

Ο

Taking the new Curie temperatures, new Weiss curves were calculated, from which the spontaneous magneti zation I p -cluster can be taken* Thus the particle size can be derived from susceptibility data» vV3 FeNaY = 9 A, v^/3 FeNaA = 8 X. S

(4

>

^sp-clust)

2

V/(3fkT)

The i . s . values of the Fe° doublet of the Mossbayer spectra ( i . s . of Fe Y = 0,01 mm/s, i . s . of Fe A = 0,41 mm/s) are due to electron transfer from the surface shells of the iron clusters to the zeo l i t e framework, but the values indicate that this electronic interaction i s not very strong. The large q.s. values of Fe Y of 0,o9 mm/s show that not only the next nearest iron ions contribute to the electronic f i e l d gradient at the iron nuclei, but the zeolite framework also has an important i n fluence on the q.s. values of the iron clusters. The q.s. as well as the i . s . values indicate, that the small iron clusters must be inside the zeolite holes. Conclusion Iron(lI)-zeolites have been prepared by ion-ex change of faujasite-type zeolites with different Si/Al ratios under conditions preserving the zeolite structure and preventing iron gel formation. After dehydration the zeolites have been reduced with a l k a l i metal vapour at 573 and at 673 K. Mossbauer spectro scopy of the reduced samples shows formation of some iron particles with diameters greater than 20 A, but a prefered formation of iron clusters with extremely narrow particle size distribution and diameters less than 13 A. The greater iron particles are outside_the cavities. Their diameters are between 20 and 500 A as was shown by electron microscopy and magnetic measure ments. The iron clusters within the zeolite holes are superparamagnetic and their Mossbauer spectra show no HFS, even at 4 K. The iron particles outside the zeo l i t e framework show the usual magnetically s p l i t sixeak spectral component at a l l temperatures between and 300 K. By taking the ratio of the area corres ponding to the quadrupole doublet to the t o t a l spec-


25.

SCHMIDT

E T A L .


301

t r a l area, the fraction of the iron clusters inside the zeolite cavities has been estimated. The sponta neous magnetization of the small iron clusters has been measured from 4 to 650 K. The Weiss-curves l i e d i s t i n c t l y below those of bulk material. Particle sizes of both fractions have been calculated from the separated magnetization curves. Literature Cited 1. 2.

Lewis, P . H . , J.Phys.Chem. (1963) 67 2151 Rabo, J.A., Schomaker, V . , Discuss.Faraday Soc. (1966) 41 328 3. Yates, D.J., J.Phys.Chem. (1965) 69 1676 4. Bredikhina, T.N. and Evdokimov, V.B.,RUSS.J.Phys. Chem. (1967) 41 1601 5. Romanowski, W., Roczniki Chemie Ann.Soc.Chim.Po lonium (1971) 45 427 6. Riekert, L., Ber.d.Bunsenges. (1969) 73 (4) 331 7 · Richardson, J.T., J . C a t a l . (1971) 21 122 8. Reman, W.G., Ali, A.H. and Schuit, G.C.A., J. Catal. (1971) 20 374 9 · Kudo, T., B u l l . o f the Chem.Soc.of Japan (1972) 45 607 10. Dalla Betta, R . A . , Proc.Intern.Congr.Catalysis V Amsterdam (1972) 100 11. Minchev, H . , Steinbach, F., Penchev, V . , Z.Phys. Chem.NF (1976) 99 223 12. Beyer, H . , Jakobs, P . A . , Uytterhoeven, J.B., Trans. Faraday Soc. (1976) 674 13. Rabo, J . Α . , Agnell, C.L., Kasai, P . H . , Schomaker, V . , Discuss.Faraday Soc. (1966) 329 14. Naccache, C.M., Ben Taarit, Y . , J. Catal. (1971) 22 171 15. Küspert, B . , Dissertation Berlin (1970) 16. Delgass, W.N., Garten, R.L., and Boudart, Μ., J . Chem. Phys. (1969) 50 4603 17. Dickson, B . L . , Rees, L . V . C . , J.Chem.Soc., Fara day I (1974) 70 2038 18. Ione, K . G . , Third Int.Conf.on Molecular Sieves (1973) 2 330 19. Garten, R . L . , Delgass, W.N., and Boudart, Μ., J. Catal. (1970) 18 90 20. Morice, J . A . and Rees, L . V . C . , Trans.Faraday Soc. (1968) 64 1388 21. Arnold, D., Z. Chem. (1971) 11 409


Formation of Iron Clusters in Zeolites with Different Supercage Sizes

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