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MÖSSBAUER STUDIES OF
HYDRODESULPHURIZATION CATALYSTS : COMPARISON OF SOURCE AND ABSORBER
EXPERIMENTS
S. Mørup, B. Clausen, H. Topsøe
To cite this version:
S. Mørup, B. Clausen, H. Topsøe. MÖSSBAUER STUDIES OF HYDRODESULPHURIZATION
CATALYSTS : COMPARISON OF SOURCE AND ABSORBER EXPERIMENTS. Journal de
Physique Colloques, 1979, 40 (C2), pp.C2-88-C2-90. �10.1051/jphyscol:1979231�. �jpa-00218629�
JOURNAL DE PHYSZQUE Cofloque C2, supplhent au no 3, Tome 40, mars 1979, page C2-88
M6SSBAUER STUDIES OF HYDRODESULPHURIZATION CATALYSTS
:
COMPARISON OF SOURCE AND ABSORBER EXPERIMENTSS. Mbrup, B. S. Clausen and H. ~ o ~ s b e "
Laboratory of Applied Physics 11, Technical University of Denmark, DK-2800 Lyngby, Denmark ' ~ a l d o r Topsde Research Laboratories, DK-2800 Lyngby, Denmark
Rbsum6.- On a compare des spectres Miissbauer de catalyseurs Fe-Mo avec ceux de catalyseurs Co-Modop6s de 5 7 ~ o afin de ddterminer les sites et les Qtats chimiques des dlgrnents 3d dans ces catalyseurs, et afin d'estimer l'influence du pro!cessus de d6croissance radioactive sur les spectres des catalyseurs dop6s de 5 7 ~ o . Les r6sultats indiquent que dans les 6tats actives des catalyseurs, Co et Fe sont dans les mgmes sites et avec les miSmes &tats de valence.
Abstract.- Miissbauer spectra of Fe-Mo and 5 7 ~ o doped Co-Mo catalysts are compared in order to deter- mine the sites and chemical states of the 3d elements in these catalysts and to estimate the influence of the decay process on the spectra of the 5 7 ~ o doped catalysts. The results indicate that in the ac- tivated states of the catalysts Co and Fe are present in similar sites and in similar valence states.
Hydrodesulphurisation catalysts are used for removal of sulphur from oil fractions. Therefore, their application in the petrochemical industry can contribute to a desirable reduction in the sulphur poisoning of the atmosphere in connection with uti- lization of fossile fuels.
The catalysts are normally prepared by impreg- nation of a high surface area y-Al,03 with a solu- tion of Co and Mo. After calcination at about 800 K, the catalysts are activated, for example by exposure to a H2/H,S gas mixture at 500-600 K. The hydrode- sulphurization is carried out by passing the oil to- gether with hydrogen over the catalyst at high tem- perature and pressure. The sulphur of the original oil molecules then reacts with hydrogen and forms H,S, which can easily be removed.
In spite of extensive research on these cata- lysts there are still unsolved problems regarding their structure and their mode of operation. Recently, it has been shown that i n s i t u ~iissbauer spectroscopy can contribute to the understanding of these cata- lysts / I - 4 / . Since none of the isotopes normally present in the catalysts can be used for Miissbauer spectroscopy, the measurements were carried out by replacing part of the Co by 5 7 ~ o or 5 7 ~ e , or by re- placing all the Co by Fe.
In the present paper we shall compare the dif- ferent methods, primarily in order to get insight into the interpretation of the source experiments.
However, the comparison of the Co-Mo and Fe-?lo cata- lysts is also interesting, since both catalysts are in fact active for the hydrodesulphurization process.
When Co-Mo catalysts are doped with 5 7 ~ o atoms, these will undoubtedly be present at sites characte- ristic of Co, even after the decay to 5 7 ~ e since the atoms are not expected to be disrupted from their original sites during the decay process. However, the valence and spin states of the 5 7 ~ e atoms formed by the decay of 5 7 ~ o may of course be different from
those of the parent 5 7 ~ o atoms.
One reason for this is that iron and cobalt atoms have different chemical and electronic proper- ties and the stable state of Fe may therefore be different from that of Co. Secondly, the decay pro- cess itself may create unstable, but long-living changes in the chemical surroundings of the Miissbauer nucleus. For a discussion of these so-called after- effects, see for example references /5-7/. In conduc- ting materials the after-effects can usually be neg- lected, and the observed chemical state will be that of iron present in low concentrations. However, the after-effects may be quite important in insula- tors, and valence states different from the stable one may be observed.
In s i t u ~Gssbauer spectra of a 5 7 ~ o doped Co- Mo catalyst and of a Fe-Mo catalyst in various ga-
seous environments are shown in figures 1 and 2, respectively. The spectra of 5 7 ~ e doped Co-Mo cata- lysts are essentially identi'cal to those of the Fe- Mo catalyst /1,3/. In the calcined states the spec- tra (Figs. la and 2a) are rather different, but ty- pical of 5 7 ~ o and 5 7 ~ e in A1,03 /8/. In the Fe-Mo catalyst only Fe3+ is present, whereas in the Co-Mo catalysts both Fez+ and ~ e ions are observed. ~ +
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979231
-6 -4 -2 0 2 4 6 VELOCITY (MM/S)
Fig. 1 : I n situ ~ z s s b a u e r s p e c t r a a t 300 K of a 5 7 ~ o doped Co-Mo c a t a l y s t s u p p o r t e d on y-A1203 o b t a i n e d w i t h a movable a b s o r b e r of K,Fe(CN);3H20 e n r i c h e d i n 5 7 ~ e . ( a ) c a l c i n e d s t a t e , (b) a c t i v a t e d s t a t e , (c) reduced s t a t e , (d) o x i d i z e d i n a i r a t room tem- p e r a t u r e , ( p o s i t i v e v e l o c i t i e s correspond t o t h e a b s o r b e r approaching t h e s o u r c e ) .
The d i f f e r e n c e may b e e x p l a i n e d e i t h e r by d t e r - e f f e c t s due t o t h e decay p r o c e s s i n t h e Co-Mo c a t a -
l y s t s o r by a d i f f e r e n c e i n t h e s i t e s which Co and Fe occupy i n t h e alumina. A d i s c u s s i o n o f t h e s e pos- s i b i l i t i e s w i l l be given elsewhere.
A f t e r a c t i v a t i o n of t h e c a t a l y s t s i n a H,/H,S gas mixture ( 1 % H,S) a t 600 K t h e room temperature s p e c t r a of t h e two samples (Figs. Ib and 2b) have q u i t e s i m i l a r parameters. I n b o t h s p e c t r a a low-spin Fe3+ and a high-spin ~ e component a r e p r e s e n t . An ~ + a n a l y s i s of t h e Fe3+ component shows t h a t i t i s pro- b a b l y due t o i r o n i n a MoSz-like s u r f a c e s t r u c t u r e
/ 2 , 3 / . The s p e c t r a lb and 2b i n d i c a t e t h a t Fe and Co a r e p r e s e n t i n s i m i l a r s i t e s i n t h e two samples and t h a t a f t e r - e f f e c t s do n o t s i g n i f i c a n t l y influenae t h e spectrum of t h e "CO doped c a t a l y s t . This i s i n aCcordance w i t h t h e r e l a t i v e l y high c o n d u c t i v i t y of MoS, /9/.
Also a f t e r r e d u c t i o n of t h e samples i n H, a t 600 K t h e room temperature Mb'ssbauer parameters
( F i g s . I c and 2c) a r e q u i t e s i m i l a r . The d i f f e r e n c e between t h e b and c s p e c t r a i n d i c a t e s t h a t changes i n t h e chemical s t a t e s of t h e MGssbauer atoms a l s o occur d u r i n g t h e c a t a l y t i c a l r e a c t i o n / 2 , 3 / .
VELOCITY (MM/S)
Fig. 2 : I n situ Mzssbauer s p e c t r a a t 300 K of a Fe-Mo c a t a l y s t supported on y-AlpO, o b t a i n e d u s i n g a s o u r c e o f 5 7 ~ o i n Rh. (a) c a l c i n e d s t a t e , (b) a c t i v a t e d s t a t e , ( c ) reduced s t a t e , (d) o x i d i z e d i n a i r a t room temperature,
This i s probably due t o o x i d a t i o n of t h e s u r f a c e l a y e r s and shows t h e n e c e s s i t y of c a r r y i n g o u t t h e
measurements in situ.
I t i s i n t e r e s t i n g t o n o t e t h a t although t h e s t r u c t u r a l and chemical p r o p e r t i e s of Fe and Co atoms i n Fe-Mo and Co-Mo c a t a l y s t s may b e d i f f e r e n t i n t h e c a l c i n e d s t a t e , g r e a t s i m i l a r i t i e s a r e found i n t h e a c t i v a t e d (sulphided) and reduced s t a t e s of t h e two types of c a t a l y s t s . T h e r e f o r e , i n t h e s e c a s e s t h e 3d atoms seem t o b e l o c a t e d i n s i m i l a r MoS2 s u r f a c e s t r u c t u r e s . Moreover, s i n c e b o t h Fe-Mo and Co-Mo c a t a l y s t s a r e a c t i v e f o r h y d r o d e s u l p h u r i z a t i o n i t i s h i g h l y probable t h a t t h e r o l e of Co i n Co-Mo c a t a l y s t s i s s i m i l a r t o t h a t of Fe i n Fe-Mo c a t a l y s t s .
Upon exposure t o a i r a t room temperature t h e s p e c t r a ( F i g s . Id and 2d) a r e d r a s t i c a l l y a l t e r e d .
JOURNAL DE PHYSIQUE
References
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