Characterization of Supported Iron Oxide Particles Using MÑssbauer

43

Downloaded by NATL UNIV OF SINGAPORE on May 5, 2018 | https://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch043

Characterization of Supported Iron Oxide Particles Using Mössbauer Spectroscopy and Magnetic Susceptibility 2

J. Phillips1, Y. Chen , and J. A. Dumesic3 1Departmentof Chemical Engineering, The Pennsylvania State University, University Park, PA 16802 2Department of Chemistry, Nanjing University, Nanjing, China 3Department of Chemical Engineering, University of Wisconsin, Madison,WI53706 The size and structure of iron oxide particles supported on Grafoil and used as water-gas s h i f t catalysts were studied using magnetic s u s c e p t i b i l i t y and Mössbauer spectroscopy. The use of a Mössbauer spectra modeling program which accounts for magnetic relaxation effects (both superparamagnetic and collective excitation) aided in the i d e n t i f i c a t i o n of the i r o n phase under reaction conditions (magnetite) and permitted a quantitative determination of p a r t i c l e s i z e . The particle size determined using Mössbauer spectroscopy was in good agreement with that obtained using the well established magnetic susceptibility technique. It was also shown that the Grafoil supported particles sintered slowly under water-gas shift reaction conditions. During recent decades, while significant advances have been made in understanding t h e b e h a v i o r o f s u p p o r t e d m e t a l c a t a l y s t s , r e l a t i v e l y l i t t l e a t t e n t i o n has been g i v e n t o s u p p o r t e d metal-oxide c a t a l y s t s . Yet, supported oxide c a t a l y s t s are p o t e n t i a l l y o f g r e a t i n d u s t r i a l s i g n i f i c a n c e , and work needs t o be done i n t h i s a r e a . The f i r s t r e q u i r e m e n t f o r t h e s t u d y o f s u p p o r t e d o x i d e c a t a l y s t s i s t h e development o f t e c h n i q u e s f o r measuring s u p p o r t e d m e t a l - o x i d e p a r t i c l e s i z e s and d i s t r i b u t i o n s . I n t h i s paper t h e a p p l i c a t i o n s o f Môssbauer s p e c t r o s c o p y and magnetic s u s c e p t i b i l i t y t o t h e measurement o f s u p p o r t e d i r o n - o x i d e p a r t i c l e s i z e s a r e d i s c u s s e d . I t i s demonstrated t h a t b o t h methods g i v e i m p o r t a n t p a r t i c l e s i z e i n f o r m a t i o n . Theory Môssbauer S p e c t r o s c o p y . S m a l l , s i n g l e domain, f e r r o - o r f e r r i magnetic p a r t i c l e s can show b o t h c o l l e c t i v e magnetic e x c i t a t i o n ( p r e c e s s i o n o f the magnetic moment) and superparamagnetic ( r e l a x a 0097-6156/85/0288-O518$06.00/0 © 1985 American Chemical Society Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

43.

PHILLIPS ET AL.

Supported Iron Oxide Particles

519

t i o n o f t h e magnetic moment) b e h a v i o r . These m o d i f i c a t i o n s i n t h e magnetic b e h a v i o r o f s m a l l p a r t i c l e s produce s i g n i f i c a n t changes i n t h e r e s u l t a n t Môssbauer s p e c t r a . Thus, i n f o r m a t i o n about p a r t i c l e s i z e (and shape) i s c o n t a i n e d i n t h e Môssbauer s p e c t r a o f s m a l l p a r t i c l e s . T h i s f a c t has been r e c o g n i z e d f o r some t i m e and many workers have a t t e m p t e d t o a n a l y z e Môssbauer s p e c t r a t o o b t a i n i n f o r m a t i o n about p a r t i c l e s i z e . The e a r l y e f f o r t s were based e x c l u s i v e l y on t h e a n a l y s i s o f superparamagnetic e f f e c t s . I n a s e r i e s o f papers Brown (1,2) and A h a r o n i (3» .) d e v e l o p e d t h e t h e o r y f o r t h e r e l a x a t i o n o f magnetic moments i n s i n g l e domain magnetic systems c o n t a i n i n g s e v e r a l e q u i v a l e n t low energy d i r e c t i o n s . They showed t h a t i n such systems there i s a f i n i t e p r o b a b i l i t y that the magnetization vector w i l l s p o n t a n e o u s l y change d i r e c t i o n s . The energy b a r r i e r f o r t h i s p r o c e s s i s dependent on t h e o r i g i n o f t h e magnetic a n i s o t r o p y ( e . g . , magneto c r y s t a l l i n e a n i s o t r o p y , shape a n i s o t r o p y , i n t e r f a c e a n i s o t r o p y , s u r f a c e a n i s o t r o p y ) . A l l o f t h e s e a n i s o t r o p y energy barriers are a function of p a r t i c l e s i z e . (The o r i g i n s and magnitudes o f v a r i o u s a n i s o t r o p i e s a r e d i s c u s s e d a t l e n g t h i n r e f e r e n c e 5.) The average l i f e t i m e TR o f a g i v e n s t a t e can be w r i t t e n :


1

TR = ( l / 2 f ) e x p ( < v / k T ) 1

(1)

1 1

where f " i s o f t h e o r d e r o f 10~9 - i o " sec, κ i s the anisotropy energy c o n s t a n t (ergs/cm3) and ν i s t h e volume (cm3) o f t h e magnetic p a r t i c l e . Brown showed t h a t t h e p r e e x p o n e n t i a l f a c t o r i s a l s o a f u n c t i o n o f t h e a n i s o t r o p y energy c o n s t a n t s and t e m p e r a t u r e . I n t h e l i m i t o f l a r g e a n i s o t r o p y b a r r i e r s ( « n χρ(1/2·χ ) (1ησ) ) x

x

β

(14)

we f i n d t h a t / = 1.96 f3, where o=1.25. D i v i d i n g t h e measured v a l u e o f v / v by 1.96 and t a k i n g t h e cube r o o t , we f i n d t h a t t h e measured v a l u e o f t h e mean r a d i u s i s 89^. The agreement between t h e r e s u l t s o f magnetic s u s c e p t i b i l i t y and Môssbauer s p e c t r o s c o p y i s v e r y good ( T a b l e I I I ) . A s t u d y o f t h e r a t e o f s u p p o r t e d p a r t i c l e growth was a l s o conducted u s i n g l o w - f i e l d magnetic s u s c e p t i b i l i t y . The average r a d i u s o f p a r t i c l e s i n sample 3 ( s e e T a b l e I ) was measured a f t e r v a r i o u s l e n g t h s o f time i n C0/C0 (15:85) a t 663 K. The f i r s t measurement was made a f t e r t h e sample had been h e a t e d f o r a t o t a l of 8 1/2 hours a t 663 K. The second measurement was made a f t e r the sample had been t r e a t e d f o r a t o t a l o f 52 hours a t 663 K. The average p a r t i c l e r a d i u s i n c r e a s e d by o n l y 15 p e r c e n t d u r i n g t h e a d d i t i o n a l 44 hours o f h i g h temperature t r e a t m e n t (see T a b l e I V ) . The t h i r d measurement was made a f t e r t h e sample had been t r e a t e d f o r a t o t a l o f 146 hours a t 663 K. The a d d i t i o n a l 94 hours o f h i g h temperature treatment r e s u l t e d i n an average i n c r e a s e i n t h e p a r t i c l e r a d i u s o f l e s s t h a n 15 p e r c e n t . From t h e s e experiments we f i n d t h a t m a g n e t i t e s u p p o r t e d on G r a f o i l s i n t e r s s l o w l y b u t s t e a d i l y a t 663 K. F o l l o w i n g t h e c o m p l e t i o n o f t h e s i n t e r i n g s t u d y , a s t u d y was conducted t o demonstrate t h a t 663 K, t h e temperature a t w h i c h a l l magnetic s u s c e p t i b i l i t y measurements were t a k e n , was i n d e e d greater than T . As d i s c u s s e d i n t h e t h e o r y s e c t i o n , measurements were made a t s e v e r a l temperatures u n t i l i t was determined t h a t p l o t s o f M/M v e r s u s H/T c o l l a p s e d onto a s i n g l e c u r v e . I t i s c l e a r from F i g u r e 4 t h a t T must be l e s s t h a n 450 K. That i s , f o r any temperature above 450 Κ t h e average p a r t i c l e s i z e was measured t o be n e a r l y 200 Â i n d i a m e t e r . For s m a l l e r particles, T w i l l o f c o u r s e be an even lower t e m p e r a t u r e . T h i s proves t h a t a l l measurements made a t 663 Κ were indeed a c c u r a t e . 6

2

2

c m

s

c m

c m

Summary I n t h i s paper t h e use o f Môssbauer s p e c t r o s c o p y and magnetic s u s c e p t i b i l i t y t o measure t h e s i z e o f s u p p o r t e d o x i d e c a t a l y s t

Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

43.

Table I I I .

Sample


1

P a r t i c l e S i z e Determined U s i n g B o t h Môssbauer S p e c t r o s c o p y and M a g n e t i c S u s c e p t i b i l i t y Methods

Avg. P a r t i c l e Radius (Hemispheres)

Technique

3

85A,105AC 95A,115A

5

Reduced S p l i t t i n g S i m u l a t i o n o f Môssbauer S p e c t r a

1A

d

95A,115A

d

5

90A,105A

C

5

100A,12of

S i m u l a t i o n o f Môssbauer S p e c t r a

1

IB

1

1C

Reduced S p l i t t i n g

1

ID

Reduced S p l i t t i n g S i m u l a t i o n o f Môssbauer S p e c t r a

2

a

Figure

529


PHILLIPS ET AL.

2A

120A,l40A 65A,75A

S i m u l a t i o n o f Môssbauer S p e c t r a Magnetic S u s c e p t i b i l i t y

d

d

90Â

S e e f i g u r e c a p t i o n s f o r d e s c r i p t i o n o f sample t r e a t m e n t .

^Average s i z e determined from t h e r e d u c t i o n i n t h e h y p e r f i n e f i e l d according t o the formula: measured s p l i t t i n g . kT bulk s p l i t t i n g 2KV w

C

A sites, Β sites

d

, l/3

h

e

p

e

κ

m

8

x

1

q

5

/

c

m

3

TABLE IV. P a r t i c l e s i z e v e r s u s Time i n C0/C0 a t 663 Κ as Measured U s i n g M a g n e t i c S u s c e p t i b i l i t y 2

T o t a l Time a t 663 Κ ( h r s )

8

1/2

Average P a r t i c l e R a d i u s

90Â

52

100A

146

11 OA


c


530

CATALYST CHARACTERIZATION SCIENCE

0

50

100

150

200

250

300

350

0

50

100

150

200

250

300

350

RADIUS

(A)

F i g u r e 3. Computer S i m u l a t i o n s o f t h e Môssbauer S p e c t r a o f F i g u r e 1. A) S i m u l a t i o n o f F i g u r e ΙΑ; B) S i m u l a t i o n o f F i g u r e I B ; C) D i s t r i b u t i o n o f p a r t i c l e r a d i i ; D) R e l a t i v e volume f r a c t i o n s as a f u n c t i o n of r a d i u s . For these s i m u l a t i o n s , the f o l l o w i n g parameters were used: o«1.25, mean r a d i u s « 95A, κ«8 χ 1θ5 e r g s / c m 3 . The Klebsch-Gordon c o e f f i c i e n t s used were 3:3:1. Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


PHILLIPS ET AL.


0

0.5

1.0

1.5

2.0

2.5

H/T F i g u r e 4. P l o t s o f M/M temperatures.

s

- Measurements atmosphere. - Measurements - Measurements - Measurements - Measurements

v e r s u s H/T f o r Sample 3 a t v a r i o u s

made w h i l e sample was a t 660 Κ i n a CO/CO2 made made made made

while while while while

sample sample sample sample

was was was was

at at at at

570 447 600 295

Κ k Κ Κ

in in in in

vacuum. vacuum. vacuum. vacuum.


532

CATALYST CHARACTERIZATION SCIENCE


p a r t i c l e s i s d i s c u s s e d i n t h e o r y and demonstrated i n p r a c t i c e . Môssbauer s p e c t r o s c o p y i s shown t o be a p a r t i c u l a r l y p o w e r f u l t e c h n i q u e because w i t h t h e c o r r e c t m o d e l i n g e q u a t i o n s , i t can be used t o measure t h e s i z e and phase o f c a t a l y s t p a r t i c l e s under r e a c t i o n c o n d i t i o n s . I t was a l s o shown t h a t t h e p a r t i c l e s i z e determined u s i n g Môssbauer s p e c t r o s c o p y i s i n v e r y good agreement w i t h t h a t o b t a i n e d u s i n g t h e w e l l e s t a b l i s h e d magnetic suscept i b i l i t y technique. Môssbauer s p e c t r o s c o p y and magnetic s u s c e p t i b i l i t y were used t o demonstrate t h a t m a g n e t i t e p a r t i c l e s s u p p o r t e d on a G r a f o i l s u b s t r a t e s i n t e r v e r y s l o w l y under water-gas s h i f t r e a c tion conditions.

Literature Cited 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

W. F. Brown, Jr., Phys. Rev. 130, 1677 (1963). W. F. Brown, Jr., J. Appl. Phys. 34, 1319 (1963). A. Aharoni, J. Appl. Phys. 33, 1324 (1962). A. Aharoni, Phys. Rev. 135A, 447 (1964). S. Mørup, J. A. Dumesic and H. Topsøe, in "Appl. of Mössbauer Spectroscopy," (R. L. Cohen, ed.) Vol. 2, p. 1, Academic Press, N.Y. (1980). H. H. Wickman in "Mössbauer Effect Methodology" 2, 39 (1966). S. Mørup, H. Topsøe and J. S. Lipka, Jnl. de Phys. 37, C6-287 (1976). S. Mørup and H.Topsøe, Appl. Phys. 11, 63 (1976). A. M. Van der Kraan, Phys. Stat. Sol. 18A, 215 (1973). T. Shinjo, T. Matsuzawa, T. Takada, S. Nasa and Y. Murakami, J. Phys. Soc. Japan 35, 1032 (1973). L. Gerward, S. Mørup and H.Topsøe, J. Appl. Phys. 47, 822 (1976). T. K. McNab, R. A. Fox and J. F. Boyle, J. Appl. Phys. 39, 5703 (1968). W. Kündig, H. Bömmel, G. Constabaris and R. H. Lindquist, Phys. Rev. 142, 327 (1966). W. Kündig, K. J. Ando, R. H. Lindquist and G. Constabaris, Czech. J. Phys. B17, 467 (1967). W. Kündig and R. S. Hargrove, Sol. State Comm. 7, 223 (1969). R. S. Hargrove and W. Kündig, Sol. State Comm. 8, 803 (1970). P. Roggwiller and W. Kündig, Sol. State Comm. 12, 901 (1973). H. Topsøe, J. A. Dumesic and M. Boudart, Jnl. de Phys. 35, C6-411 (1974). S. Mørup, B. S. Clausen and H. Topsøe, J. Phys. Colloq. Paris 40, C2-78 (1979). P. W. Selwood, "Chemisorption and Magnetization", Acad. Press NY (1975). J. W. Cahn, Trans. AIME 209, 1309 (1959). R. E. Dietz and P. W. Selwood, J. Chem. Phys. 35, 270 (1961). R. Pauthenet, Ann. Phys. 7, 710 (1952).


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24. 25. 26. 27. 28.


29. 30. 31. 32. 33. 34. 35. 36.


533

C. P. Bean and J. D. Livingston, J. Appl. Phys. 30, 120S (1959). J. Phillips, B. Clausen, J. A. Dumesic, J. Phys. Chem. 84, 1814 (1980). R. H. Bartholomew and M. Boudart, J. Cat. 26, 173 (1972). J. Phillips, Ph.D. Thesis, University of Wisconsin-Madison (1981). A. Muan and E. F. Osborn, "Phase Equilibrium Among Oxides in Steelmaking", Addison-Wesley Pub. Co., Reading, MA (1965). C. G. Granqvist and R. A. Burhman, J. Appl. Phys. 47, 2220 (1976). C. G. Granqvist and R. A. Burhman, Appl. Phys. Lett. 27, 693 (1976). C. G. Granqvist and R. A. Burhman, J. Appl. Phys. 47, 2200 (1976). C. G. Granqvist and R. A. Burhman, Sol. State Comm. 18, 123 (1976). C. G. Granqvist and R. A. Burhman, J. Cat. 42, 477 (1976). G. A. Sawatsky, F. van der Woude and A. H. Morrish, Phys. Rev. 183, (1969). F. van der Woude, G. A. Sawatsky and A. H. Morrish, Phys. Rev. 167, 533 (1968). T. Riste and L. Tanzer, J. Phys. Chem. Solids 19, 117 (1961).

RECEIVED March 28, 1985


Characterization of Supported Iron Oxide Particles Using MÑssbauer

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Characterization of Supported Iron Oxide Particles Using MÑssbauer