MÃ¶ssbauer Spectroscopy and Its Chemical Applications

29 A Mössbauer Investigation of α-Fe O3 2

Microcrystals Supported on

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch029

DING Y I N G - R U , YEN Q I - J I E , H S I A and QIU JIN-HENG

γ-Al O 2

YUAN-FU,

3

JIN YONG-SHU,

Department of Chemistry, N a n k i n g University, N a n k i n g , C h i n a

Two samples of microcrystalline α-Fe O 2

γ-Al O 2

3

3

supported on

were prepared by an impregnation method. The

particle size distribution of α-Fe O 2

determined by

3

Möss

bauer spectroscopy and electron microscopy depends on the preparation conditions. Under CO conversion condi tions the microcrystallites of Fe O 3

4

α-Fe O 2

can be reduced to

3

and nonstoichiometric ferrous oxide, and the surface 3+

reaction Fe

-->

2+

Fe

is a reversible oxidation-reduction

process. The interaction between α-Fe O and γ-Al O 2

3

2

3

is

not as great as that between α-Fe O and MgO. 2

3

S u p p o r t e d catalysts a r e w i d e l y u s e d i n i n d u s t r y ; t h e y g i v e t h e user t h e ^

a d v a n t a g e o f c r e a t i n g a n d s t a b i l i z i n g a l a r g e surface a r e a ( i . e . h i g h

dispersion)

f o r t h e a c t i v e c o m p o n e n t s o f t h e catalysts, r e d u c i n g t h e

a m o u n t of n o b l e m e t a l u s e d , a n d i m p r o v i n g c a t a l y t i c a c t i v i t y , m e c h a n i c a l s t r e n g t h , a n d t h e r m a l a n d c h e m i c a l s t a b i l i t y . T h i s t y p e o f c a t a l y s t is also v a l u a b l e t o t h e s t u d y o f t h e surface s t r u c t u r e a n d r e a c t i o n m e c h a n i s m o f s u p p o r t e d catalysts. R e c e n t l y , m u c h a t t e n t i o n has b e e n g i v e n t o t h e s t u d y o f p r e p a r a t i o n m e t h o d s a n d p r o p e r t i e s o f s u p p o i t e d catalysts. T h e results o f a n u m b e r of these studies (1,2) s h o w t h a t a n " i n e r t s u p p o r t " i n f a c t is n o t i n e r t f o r t h e a c t i v e c o m p o n e n t s a n d c a t a l y t i c reactions. T h i s c h a p t e r studies t h e d e p e n d e n c e o f t h e p a r t i c l e size o n t h e preparation conditions, a n d the influence of the nature of the support a n d its p a r t i c l e size o n t h e r e d u c t i o n o f t h e samples t h r o u g h t h e i n s i t u Mossbauer spectra of microcrystalline a - F e 0 2

©

3

supported o n y - A l 0 .

0065-2393/8] /0194-0609$05.00/0 1981 Amercan Chemical Society

Stevens and Shenoy; Mössbauer Spectroscopy and Its Chemical Applications Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

2

3

610

MOSSBAUER SPECTROSCOPY A N D ITS C H E M I C A L A P P L I C A T I O N S

Experimental y - A l 0 , calcined at 823 K i n air for 6 h w i t h a surface area of 117 m / g , was used as the support material. T h e support was impregnated w i t h ferric nitrate solution enriched w i t h F e , and the p H value of the solution was adjusted to 1.5-2.0 w i t h N H O H . After impregnation overnight, the pre cursor was dried at 373 K and then calcined i n air at 823 K for 3 h . T h u s , Sample 1 contained 3 w t % F e 0 . Sample 2 was prepared under similar con ditions using the impregnation solution without N H . T h e spectrometer used is described elsewhere ( 3 ) . T h e source was 8 m C i C o i n palladium. A 25-/un thick a - F e foil was used as a standard to calibrate the velocity of the spectrometer. A l l isomer shifts were recorded w i t h respect to a - F e . T h e i n situ Mossbauer measurements at h i g h temperature a n d under C O conversion conditions ( H 0 / C O = 2 : 1 , 6 2 3 - 8 2 3 K , flow rate of gas = 250 m L / h ) were carried out i n the quartz cell. T h e absorber contained 1-2 m g Fe/cm . T h e Mossbauer measurements at temperatures below room temperature were carried out w i t h the "cold finger" method. T h e particle size of the microcrystallites also was determined using the electron microscopic technique. 2

2

3

5 7

4

2

3

4

+

5 7


2

5 7

2

Results

and

Discussion

Mossbauer

Spectra

at Different

Temperatures.

The

Mossbauer

s p e c t r a of t w o samples at different t e m p e r a t u r e s are s h o w n i n F i g u r e s 1 a n d 2. F r o m t h e r o o m t e m p e r a t u r e s p e c t r a t h e i s o m e r shifts of t h e samples w e r e f o u n d to b e 0.328 a n d 0.321 m m / s , r e s p e c t i v e l y . A s s h o w n i n F i g u r e 1, t h e M o s s b a u e r s p e c t r u m of S a m p l e 1 e x h i b i t e d o n l y a s u p e r p a r a m a g n e t i c d o u b l e t , a n d e v e n at 120 K n o m a g n e t i c s p l i t t i n g w a s f o u n d . q u a d r u p o l e s p l i t t i n g v a l u e w a s 0.82 m m / s .

The

S a m p l e 2 at r o o m t e m p e r a

ture exhibited a magnetic splitting spectrum w i t h a doublet superimposed o n its center, a n d t h e h y p e r f i n e field c a l c u l a t e d w a s 495.7 k O e . I t w a s a l i t t l e l o w e r t h a n t h e 515 k O e r e p o r t e d f o r b u l k « - F e 0 2

q u a d r u p o l e s p l i t t i n g v a l u e w a s 0.77 m m / s . t h a t t h e t w o samples w e r e i n t h e a - F e 0 2

3

3

(2,4).

The

A l l of these v a l u e s s h o w e d

state.

A c c o r d i n g t o t h e K i i n d i g m o d e l ( 5 , 6 ) , t h e a v e r a g e p a r t i c l e size of S a m p l e 1 w a s e s t i m a t e d t o b e 5.6 n m , w h i c h is i n g o o d a g r e e m e n t w i t h the electron microscopic observation. I n F i g u r e 3 w e p l o t t e d t h e a r e a p e r c e n t of t h e s u p e r p a r a m a g n e t i c c o m p o n e n t as a f u n c t i o n of t e m p e r a t u r e — a t 230 K i t w a s 5 0 % .

The

a v e r a g e p a r t i c l e size of S a m p l e 2 w a s c a l c u l a t e d t o b e 12.8 n m , a c c o r d i n g to the equation:

In

( 2 X 1 0 "

4

K ) - ^


29.

DING

YING-RU

E T

a-Fe O

AL.

2

611

Microcrystals

s

LOO 096 092 QS8 #

o

I * "**

J

1.00 096 092


086

1.00 096 092 088

1.00 096 0.92 . 0«

wo """^ ""^6 """^

-2

0 ' 2 """J ' 6 ' 8 ' Jo Velocity (» /s*c) m

Figure I .

where

Mossbauer spectra of Sample 1 at different temperatures: at 123 K; (b) at 284 K; (c) at 623 K; (d) at 823 K

K = (8.2 ± 2.4) X 1 0 J / m 3

3

(5,6).

W h i l e the electron

(a)

micro

s c o p i c o b s e r v a t i o n s h o w e d t h a t S a m p l e 2 h a d a b r o a d p a r t i c l e size d i s t r i b u t i o n , t h e m a j o r i t y of t h e p a r t i c l e s w e r e a b o u t 7.0 n m , w i t h s o m e l a r g e r t h a n 20 n m . Mossbauer Study of the Reduction (Activation) of « - F e 0 3 M i c r o 2

crystallites Supported on y - A l ( > 3 U n d e r C O Conversion Conditions. 2

T h e results of t h e i n s i t u M o s s b a u e r m e a s u r e m e n t s u n d e r C O c o n v e r s i o n c o n d i t i o n s are s h o w n i n F i g u r e s 4 a n d 5. A c o m p a r i s o n of s p e c t r a u n d e r 623 K C O c o n v e r s i o n c o n d i t i o n s w i t h those i n a i r i n d i c a t e t h a t after b e i n g exposed t o C O + H 0 f o r 30 h , t h e center lines of S a m p l e 2 w e r e 2


612


M O S S B A U E R SPECTROSCOPY A N D ITS C H E M I C A L A P P L I C A T I O N S

Figure 2. Mossbauer spectra of Sample 2 at different temperatures: (a) at 823 K; (b) at 623 K; (c) at 373 K; (d) at 284 K; (e) at 178 K; (f) at 121 K broadened

significantly a n d the quadruple

(Figure 5a).

increased

2

sorbed on the F e 0 2

asymmetric.

splitting value

T h i s suggests t h a t C O a n d H 0 m o l e c u l e s w e r e c h e m i 3

surface, m a k i n g t h e e n v i r o n m e n t of F e

O n t h e o t h e r h a n d , t h e s p e c t r u m of

3 +

more

Sample 1 d i d not

c h a n g e , p r o b a b l y because of t h e s m a l l e r p a r t i c l e size. F i g u r e 4 b s h o w s t h a t after r e d u c t i o n at 673 K f o r 10 h , a F e

2 +

s h o u l d e r c a n b e seen a t t h e

r i g h t side of t h e s p e c t r u m .


29.

DING

YING-RU

E T A L .

a-Fe O 2

s

613

Microcrystals

U n d e r the C O c o n v e r s i o n c o n d i t i o n s a t 823 K the s p e c t r a ( F i g u r e s 4 c a n d 5 b ) m a y consist o f t h e a b s o r p t i o n c u r v e s o f F e 0 3

0.28mm/s)

a n d n o n s t o i c h i o m e t r i c ferrous

oxide

t r e a t e d f o r 5 h , t h e a r e a percentages o f F e 0 3

4

4

(isomer shift —

(7,8).

After

being

a n d F e O of the samples

were f o u n d to b e 5 6 % a n d 4 4 % , respectively, a n d d i d not change w i t h the increasing time of treatment. W h e n the temperature decreased t o 673 K , t h e F e

content decreased ( F i g u r e 4 d ) .

2 +

reduction of F e

3 +

to F e

2 +

T h i s suggests t h a t t h e

is reversible. W h e n the temperature decreased

t o 623 K , t h e F e * c o n t e n t d e c r e a s e d s t i l l f u r t h e r ( F i g u r e 4 e ) . 2

I . P . S u z d a l e v e t a l . ( 9 ) suggested t h a t t h e r e a c t i o n o f C O w i t h t h e surface o f F e 0 2

3

(8-nm particles) resulted i n C 0

2

formation and reduc


t i o n o f t h e surface f e r r i c i o n t o ferrous i o n . T h e l a t t e r w a s present i n a c o m p o u n d of t h e F e O t y p e , g i v i n g t w o n e w peaks i n t h e M o s s b a u e r s p e c t r u m , w i t h a q u a d r u p o l e s p l i t t i n g v a l u e o f 1.67 m m / s .

T h e subse

q u e n t o x y g e n t r e a t m e n t of t h e C O - r e a c t e d s a m p l e r e s u l t e d i n a r e t u r n o f t h e s p e c t r u m t o its o r i g i n a l f o r m , t h e F e O b e i n g o x i d i z e d b a c k t o F e 0 . 2

3

O u r e x p e r i m e n t a l results are consistent w i t h t h i s c o n c l u s i o n . A s a r e s u l t of t h e r e a c t i o n o f c h e m i s o r b e d o x y g e n o n t h e F e 0 2

oxidized back to F e

3 +

3

surface, F e

2 +

was

at a l o w e r t e m p e r a t u r e .

F i g u r e s 4 f a n d 5 d s h o w t h a t after t h e r e d u c t i o n , t h e M o s s b a u e r s p e c t r a o f the t w o samples a t r o o m t e m p e r a t u r e c h a n g e d m a r k e d l y . T h e c e n t r a l p o r t i o n o f t h e s p e c t r a is a t t r i b u t e d t o t h e presence o f some s u p e r p a r a m a g n e t i c c r y s t a l l i t e s o f f e r r i c a n d ferrous oxides.

2oo

Moo

Goo

Temperature, Figure

3.

Temperature

T h e spectrum of

K

dependence of the superparamagnetic age of Sample 2

percent-


614



04 036 094

Figure 4. Mossbauer spectra of Sample 1 exposed to a CO + H O mixture at various stages: (a) at 623 K; (b) after (a), at 673 K; (c) after (b), at 823 K; (d) after (c), at 673 K; (e) after (d), at 623 K; (f) after (e), at room temperature and exposed to COjCO mixture. t

t

S a m p l e 1 ( F i g u r e 4 f ) e x h i b i t e d a m a g n e t i c s p l i t t i n g of F e 0 , a n d t h e 3

4

h y p e r f i n e field w a s 490 k O e , a t t r i b u t a b l e t o t h e l a r g e r p a r t i c l e size of Fe 0 3

4

t h a n t h a t of F e 0 . F o r S a m p l e 2, t h e i n i t i a l m a g n e t i c s p l i t t i n g 2

3

s p e c t r u m of F e O d i s a p p e a r e d a n d t h e t r a c e of t h e l a r g e F e 0 2

a

3

4

particles

a p p e a r e d . T h e s u p e r p a r a m a g n e t i c l i n e s of c r y s t a l l i n e f e r r i c a n d ferrous oxides w e r e s u p e r i m p o s e d o n b o t h of these m a g n e t i c s p l i t t i n g l i n e s .


29.

DING

YING-RU

E T

AL.

a-Fe O 2

s

615

Micwcrystak

I t is c l e a r t h a t at first, t h e r e d u c t i o n process of m i c r o c r y s t a l l i n e F e 0

3

o c c u r r e d at the surface of t h e l a r g e r p a r t i c l e s , a n d t h e o x y g e n of F e 0

3

2

2

reacted w i t h chemisorbed Fe

Fe 0 3

C O ; thus, C O - » C 0 , F e 2

3 +

- » F e , a n d the 2 +

component appeared. A s the reduction temperature increased, some

2 +

4

a n d F e O f o r m e d . B e c a u s e of t h e l a r g e r p a r t i c l e size of F e 0 , t h e 3

4

m a g n e t i c s p l i t t i n g s p e c t r u m a p p e a r e d at r o o m t e m p e r a t u r e . S e v e r a l authors (10,11)

h a v e suggested t h a t t h e r e d u c t i o n of t h e

s u p p o r t e d catalysts is affected b y t h e p a r t i c l e size. O u r s t u d y has s h o w n t h a t w i t h i n c e r t a i n p a r t i c l e size ranges, t h e r e d u c t i o n of t h e s a m p l e is


affected o n l y s l i g h t l y .

Vkloeiiy

c %^) m

Figure 5. Mossbauer spectra of Sample 2 exposed to a CO + H 0 mixture at various stages: (a) at 623 K; (b) after (a), at 823 K; (c) after (b), at 623 K; (d) after (c), at room temperature and exposed to a CO\CO mixture. 2

%


616


I t is w e l l k n o w n t h a t a n a c t i v e c o m p o n e n t , w h e n d i s p e r s e d t o a c e r t a i n degree, w i l l f o r m a n a c t i v e center.

A l t h o u g h p a r t i c l e s of s m a l l e r

sizes are m o r e a c t i v e , t h e r e is a n o p t i m u m p a r t i c l e size r a n g e t h a t m a n i fests t h e h i g h e s t a c t i v i t y a n d t h a t d e p e n d s n o t o n l y o n t h e n a t u r e of s u p p o r t b u t also o n t h e size of r e a c t i n g m o l e c u l e s .

F u r t h e r studies a r e

n e e d e d to e s t a b l i s h a n o p t i m u m p a r t i c l e size r a n g e f o r t h e r e d u c t i o n ( a c t i v a t i o n ) of t h e « - F e 0 2

3

supported on y - A l 0

3

w i t h a Support and Thermal Stability of

2

3

under C O conversion

conditions. Interaction of a - F e 2 0

I t is g e n e r a l l y c o n s i d e r e d t h a t b e c a u s e of t h e s i m i l a r i t y

the Sample. between F e Al O

a n d A l , t h e r e is a s t r o n g i n t e r a c t i o n b e t w e e n F e 0 3 +

(12,13).

2

and 3

s u p p o r t a n d suggested t h a t t h e i n t e r a c t i o n of m e t a l o x i d e w i t h y - A l 0

3

a

T . Y o s h i o k o (14)

3

s t u d i e d t h e different types of A 1 0

2


3 +

2

2

appears t o b e t h e weakest. O u r e x p e r i m e n t s h o w e d t h a t t h e r e d u c t i o n of S a m p l e 2 w i t h a n average p a r t i c l e size greater t h a n 10 n m w a s s i m i l a r t o t h a t of b u l k a - F e 0 , w h i c h is consistent w i t h t h e r e s u l t of K i i n d i g et a l . 2

(6).

3

T h e y suggested t h a t t h e i n t e r a c t i o n of t h e s u p p o r t w i t h F e 0 2

t i c l e size >

(par

3

13.5 n m ) is w e a k , a n d t h e r e d u c i b i l i t y is s i m i l a r t o t h a t of

b u l k F e 0 . O n t h e other h a n d , for t h e s m a l l p a r t i c l e s ( 6 n m ) of F e 0 , a 2

3

2

3

s t r o n g i n t e r a c t i o n w a s n o t s h o w n either. I t c o u l d b e r e d u c e d e a s i l y u n d e r the same C O c o n v e r s i o n c o n d i t i o n s . « - F e 0 2

on M g O (15)

3

microcrystallites supported

of t h e same p a r t i c l e size w e r e p r e p a r e d u n d e r s i m i l a r

conditions, b u t were f o u n d to be nonreducible under C O conditions.

conversion

F r o m t h e results of these t w o samples i t seems t h a t t h e

p a r t i c l e size a n d t h e n a t u r e of the s u p p o r t h a v e a n effect o n t h e r e d u c t i o n of « - F e 0 2

3

microcrystallites.

A f t e r f u r t h e r c a l c i n a t i o n at 823 K i n a i r f o r 10 h , M o s s b a u e r s p e c t r a of the t w o samples just m e n t i o n e d d i d n o t c h a n g e as c o m p a r e d w i t h the original. T h e interaction between « - F e 0 2

3

and y - A l 0 2

3

was weak, result

i n g i n t h e l i m i t e d t h e r m a l s t a b i l i t y of t h e t w o samples.

Conclusion The « - F e 0 2

3

m i c r o c r y s t a l l i t e s w i t h a n average p a r t i c l e size of a p

proximately 6 n m , supported on y - A l 0 2

y-Al 0 2

3

3

were prepared by impregnating

w i t h f e r r i c n i t r a t e s o l u t i o n . T h e p H v a l u e of t h e i m p r e g n a t i o n

s o l u t i o n w a s a d j u s t e d t o 1.5-2.0 w i t h N H O H . W h e n n o N H 4

4

+

ions w e r e

i n the i m p r e g n a t i o n s o l u t i o n , a s a m p l e w i t h a different p a r t i c l e size d i s tribution was obtained. T h e p a r t i c l e size of the a - F e 0 2

3

microcrystallites was determined b y

Mossbauer spectroscopy a n d electron microscopy.

W h e n the

p a r t i c l e sizes w e r e 6 n m a n d greater t h a n 10 n m u n d e r C O conditions, « - F e 0 2

3

c o u l d be reduced to F e 0 3

4

average

conversion

a n d F e O , but the reduci-


29.

DING

YING-RU

E T

a-Fe O

AL.

2

s

Microcrystals

617

b i l i t y of t h e t w o samples w i t h different p a r t i c l e sizes w a s n o t t h e same. It was shown that F e

3 +

-» Fe

2 +

is a r e v e r s i b l e o x i d a t i o n - r e d u c t i o n process.


Literature Cited 1. Carbuichio, M . J. Chem. Phys. 1979, 70(2), 784. 2. Dumesic, J.; Topsøe, H . Adv. Catal. 1977, 26, 186. 3. Ying-ru, Ding; Qi-jie, Yen; Yuan-fu, Hsia; Shun-hao, Yeh; Yi, Chen; Yongshu, Jin; Jin-heng, Qiu. J. Phys. (Paris) 1980, C - l - 3 4 1 . 4. Hobson, M . C.; Gager, H . M . J. Catal. 1970, 16, 254. 5. Kündig, W . ; Bömmel, H . ; Constabaris, G . ; Lindquist, R. H . Phys. Rev. 1966, 142, 327. 6. Kündig, W.; Ando, K. J . ; Lindquist, R. H.; Constabaris, G . Czech. J. Phys. B 1967, 17, 467. 7. Topsøe, H . ; Dumesic, J. A . ; Boudart, M . J. Phys. (Paris) 1974, C6-411. 8. Johnson, D. P. Solid State Commun. 1969, 7, 1785. 9. Suzdalev, I. P.; Shkarin, A . V . ; Zhabrova, G . M . Kinet. Katal. 1969, 20, 218. 10. Fujimoto, K.; Boudart, M . J. Phys. (Paris) 1978, C2-81. 11. Gager, H . M.; Hobson, M . C. Catal. Rev. 1970, 11 (1), 117. 12. Wertheim, G. K.; Remeika, J. P. Phys. Lett. 1964, 10, 14. 13. Bhide, V . G . ; Date, S. K. Phys. Rev. 1968, 172, 345. 14. Yoshioko, T . ; Koezuka, J.; Ikoma, H . J. Catal. 1970, 16, 264. 15. Ying-ru, Ding; Qi-jie, Yen; Yuan-fu, Hsia; Yong-shu, Jin; Jin-heng, Qui., unpublished data. RECEIVED

July 22, 1980.


MÃ¶ssbauer Spectroscopy and Its Chemical Applications

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