MÖSSBAUER STUDIES OF CHEMISORPTION OF CO AND H2 ON PARTICLES OF METALLIC IRON

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MÖSSBAUER STUDIES OF CHEMISORPTION OF CO AND H2 ON PARTICLES OF METALLIC IRON

S. Mørup, B. Clausen, H. Topsøe

To cite this version:

S. Mørup, B. Clausen, H. Topsøe. MÖSSBAUER STUDIES OF CHEMISORPTION OF CO AND

H2 ON PARTICLES OF METALLIC IRON. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-

331-C1-332. �10.1051/jphyscol:19801123�. �jpa-00219606�

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JOURNAL DE PHYSIQUE C~Ir'~qu4

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supp/lment au

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Tome

41, janvier 1980, page

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l@SsBAUER STUDIES OF CHEMISORPTION OF CO AND

Ha ON $bV&l,

PARTICLES OF METALLIC IRON

+ +

S. Mbrup, B.S. Clausen and H. Tops$e

Laboratory of applied Physics II, TechnicaZ University o f ~ewnark, DK-2800 Lyngby, enm mark

+

HaZdor Topsbe Researoh Laboratories, DK-2800 Lyvgby, penmark.

Small iron p a r t i c l e s have recently a t t r a c t e d much a t t e n t i o n as model c a t a l y s t s f o r alnmonia synr t h e s i s / I / and hydrocarbon synthesis reactions 12-51, In order t o elucidate t h e reactions leading t o the synthesis of hydrocarbons s t a r t i n g from GO and H2 t h e mechanisms of chemisorption pf these gases a r e of great i n t e r e s t . In the present work

we

have investigated t h e influence of chemisorption

qf CO

and H2 on the Missbauer spectra of small s i l i c a - supported iron p a r t i c l e s .

In e a r l i e r Missbauer studies of siliea-sup- ported 5 7 C ~ doped Ni p a r t i c l e s 16, 71 and magnesia- supported Fe p a r t i c l e s /8/ i t was found t h a t the superparamagnetic relaxation time decreased when hydrogen was chemisorbed. This was explained by a deqrease i n t h e surface contribution t o the magng- t i e ani sotropy energy constant.

The iron p a r t i c l e s were prepared by impregna- t i o n of high surface area s i l i c a (Cab-0-Sil

H5)

with an aqueous solution of Fe(N03)3. The drying

treatments p r i o r t o reduction were similar t o t h p w described in 141. The sample contained 10 w t

%

Fe.

The dried sample was pressed t o a p e l l e t and placed i n an

i n s i t u

c e l l / 9 / allowing MBssbauer spectra t o be obtained a t temperatures from 78K t o 725K 3 n vacuum and in various gaseous environments and with applied magnetic f i e l d s .

The sample was reduced in flowing hydrogen a t

725K

f o r 94 hours and Mksbauer spectra were ob- tained a t lower temperatures in order t o avoid f u r t h e r reduction o r s i n t e r i n g of the p a r t i c l e s during the measurements. The chemisorbed hydrogen was removed by evacuation a t 675K f o r 3 hours t o

10''

Pa. CO and

HZ

were chemisorbed a t 200K and 300K, respectively .

The Missbauer spectra of t h e reduced sample consist i n general of two components: a ferromagnet t i c ( s i x - l i n e ) component a r i s i n g from metallic iron p a r t i c l e s and a paramagnetic component with c o n t r i ~ butions from non-reduced iron

( ~ e ~ ' )

and in some cases a l s o superparamagnetic metallic iron p a r t i c l ~ g ,

The p a r t i c l e s i z e was determined from the

nlapgf

i c f i e l d dependence of the Missbauer spectra

# t 523K.

A t

t h i s temperature the spectrum abtaiegq

i p

zero applied f i e l d

i s

completely paramagn$tiq l n d i c a t i ng f a s t superparamagnetic relaxation i n $he iron p a r t i c l e s . The s i z e of the magnetic spl itJi,nq

f~'! thq

Rresence of applied magnetic f i e l d s then e s s e n f i a l l y follows a Langevin function, and frnm

fha

high-field spectra the p a r t i c l e s i z e can e~aily

VELOCITY

( M M I S )

Fig, 1

:

Missbauer spectra of 6.3 nm iron p a r t i g l e r a t 78K,

( a ) without chemisorbed

C O Y H=O;

( b ) with chemisorbed C O Y

H=O;

( c ) without chemisorbed

C O Y

H=l2.4

kGi

(d) w i t h chemisorbed

C O Y

H=12.4 kG.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801123

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cl-332 JOURNAL DE PHYSIQUE

be estimated /6,

lo/.

I n t h i s way an average par- t i c l e diameter o f 6.3

+

0.5 nm was found.

F i g u r e 1 shows MiSssbauer s p e c t r a o f t h e sample obtained a t 78K w i t h o u t and w i t h chemisorbed CO and a t H=O and H=12.4 kG. From a comparison o f t h e s p e c t r a (a) and ( b ) i t i s found t h a t f o r H=O t h e i n t e n s i t y o f the ferromagnetic component i n - creases when CO i s chemisorbed. Such an e f f e c t m i g h t be explained by a l a r g e r amount o f non-reduced i r o n o r by t h e presence o f " m a g n e t i c a l l y dead l a y e r s " i n t h e absence o f chemisorbed molecules.

However, an a n a l y s i s o f t h e spectra ( c ) and (d) ob- t a i n e d i n an a p p l i e d magnetic f i e l d o f 12.4 kG shows i d e n t i c a l areas o f t h e ferromagnetic components i n d i c a t i n g t h a t t h e amount o f ferromagnetic i r o n i s u n a f f e c t e d by chemisorption o f CO. The d i f - ference between t h e zero f i e l d spectra must t h e r e - f o r e a r i s e from a decrease i n t h e superparamagnetic r e l a x a t i o n r a t e a n d i n t h e amplitude o f c o l l e c t i v e magnetic e x c i t a t i o n s when CO i s chemisorbed /11, 12/.

The r e s u l t s i n d i c a t e t h a t t h e s u r f a c e c o n t r i b u t i o n t o t h e magnetic a n i s o t r o p y energy constant K i n - creases when CO i s chemisorbed. Since a t 78K almost a l l t h e p a r t i c l e s a r e below t h e superparamagnetic b l o c k i n g temperature a q u a n t i t a t i v e estimate o f t h e e f f e c t may be obtained from t h e i n f l u e n c e o f c o l l e c - t i v e magnetic e x c i t a t i o n s , which l e a d t o a r e d u c t i o h i n t h e magnetic s p l i t t i n g o f t h e zero f i e l d s p e c t r a o f small p a r t i c l e s by a f a c t o r o f 1-kT/2KV /11, 12/, where k i s Boltzmanns constant, T i s t h e tempera- t u r e , V i s t h e volume and K i s t h e magnetic aniso- t r o p y energy constant.

The observed h y p e r f i n e f i e l d s i n t h e s p e c t r a shown i n F i g u r e 1 ( a ) and ( b ) a r e 329.5 c 1 kG and 336.0

+

1 kG, r e s p e c t i v e l y . The h y p e r f i n e f i e l d s i n the absence o f c o l l e c t i v e magnetic e x c i t a t i o n s can b e determined from t h e spectra obtained w i t h an a p p l i e d magnetic f i e l d o f 12.4 kG. A f t e r c o r r e c t i o n s f o r t h e i n f l u e n c e o f t h e a p p l i e d magnetic f i e l d , h y p e r f i n e f i e l d s o f 343.7 + 1 kG and 347.2

+

¹kG, were estimated f o r t h e s p e c t r a ( c ) and ( d ) , respec- t i v e l y . The d i f f e r e n c e between these values must be due t o t h e CO chemisorption /13/. The h y p e r f i n e f i e l d s are s l i g h t l y l a r g e r than t h a t o f an i r o n f o i l a t 78K ( = 339.0

+

1 kG). This can p a r t l y be explained by t h e i n f l u e n c e o f t h e demagnetizing f i e l d , which may g i v e r i s e t o an increase i n t h e magnetic s p l i t t i n g i n spectra o f single-domain par-

t i c l e s , corresponding t o about 7 kG, r e l a t i v e t o t h e value f o r an i r o n f o i l /14/.

By u s i n g t h e h y p e r f i n e f i e l d s g i v e n above we f i n d f o r t h e magnetic a n i s o t r o p y energy constants : Kvac = (1.0

+

0.1) x

l o 5

~ / m ~ and KCO = (1.3 k 0.1) x

l o 5

J/m3. A s i m i l a r a n a l y s i s of s p e c t r a w i t h and w i t h o u t chemisorbed hydrogen gives t h e r e s u l t : K = (1.2

+

0.1) x 105 J/m3. An a n a l y s i s

b f

t h e superparamagnetic behaviour o f the p a r t i c l e s a t Hz h i g h e r temperatures gave s i m i l a r r e s u l t s . Thus i t i s found t h a t t h e a n i s o t r o p y energy constant i n - creases when CO o r H2 i s chemisorbed. I t i s remark- a b l e t h a t t h i s e f f e c t i s o p p o s i t e t o t h a t found f o r hydrogen chemisorption on Ni p a r t i c l e s /6, 7/ and magnesia supported Fe p a r t i c l e s /8/. I n t h e l a t t e r study i t was p o i n t e d o u t t h a t such r e s u l t s c o u l d a l s o be e x p l a i n e d by p a r t i c l e shape changes occur- r i n g d u r i n g t h e chemi s o r p t i o n process. However, such changes should n o t occur i n t h e present study, s i n c e t h e e f f e c t was observed w i t h o u t h e a t i n g t h e p a r t i c l e s t o above 300 K.

Support from t h e Danish N a t u r a l Science Research Council i s g r a t e f u l l y acknowledged.

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