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EXAFS studies of Co-Mo/A12 03 catalysts (12), indicate that the Co-. Mo-S structure has a MoS2~like structure (see e.g., 1» 10). With respect to the ...
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5 X-Ray Absorption Fine Structure, Mössbauer, and Reactivity Studies of Unsupported CobaltMolybdenum Hydrotreating Catalysts BJERNE S. CLAUSEN, HENRIK TOPSØE, and ROBERTO CANDIA Downloaded by EAST CAROLINA UNIV on December 19, 2017 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch005

Haldor Topsøe Research Laboratories, DK-2800 Lyngby, Denmark BRUNO LENGELER IFF, Kernforshungsanlage Jülich, D-5170 Jülich, Germany Catalytic and structural information has been obtained for unsupported Co-Mo hydrotreating (HDS) catalysts. The structural information has been provided by means of in situ Mössbauer emission spectroscopy (MES) and in situ EXAFS (for both the Mo and the Co K-edges). By comparing these results with the thiophene HDS rate and the rate of secondary hydrogenation of bute­ nes the nature of the active sites for these reactions has been elucidated. The results suggest that the Co-Mo-S phase can be regarded as a MoS structure with Co atoms located at the edges. These Co atoms are found to strongly promote the HDS reactions but have a much less influence on the hydrogenation rate. 2

The i n c r e a s i n g need f o r e f f i c i e n t treatment o f v a r i o u s f o s s i l f u e l feedstocks has r e s u l t e d i n many s t u d i e s ( f o r a recent review, see e.g., Ref. ( l ) ) devoted t o the understanding o f the c a t a l y t i c pro­ p e r t i e s o f hydroprocessing c a t a l y s t s (e.g., Mo or W based c a t a l y s t s promoted by Co or N i ) . The e f f o r t s have been d i r e c t e d towards an understanding o f the s t r u c t u r a l form i n which the d i f f e r e n t atoms are present, and t o e s t a b l i s h connections between the s t r u c t u r a l information and the various c a t a l y t i c f u n c t i o n s ( h y d r o d e s u l f u r i z a t i o n (HDS), hydrogenation, hydrodenitrogenation (HDN), e t c ) . I t has, however, been very d i f f i c u l t t o make progress since f o r a long time d i r e c t information regarding the s t r u c t u r a l s t a t e o f the a c t i v e elements has been almost impossible t o o b t a i n . This i s pro­ bably the reason why g r e a t l y d i v e r g i n g views on the s t r u c t u r e e x i s t (2-5). Recently, i t has been shown that two techniques, Mössbauer emission spectroscopy (MES) (6-11 ) and extended X-ray absorption f i n e s t r u c t u r e (EXAFS) (l£, 13), can provide some o f the needed s t r u c t u r a l information. T h i s has not only r e s u l t e d i n a b e t t e r d e s c r i p t i o n o f the s t r u c t u r a l s t a t e i n such c a t a l y s t s but i t has a l s o allowed one t o understand some o f the c a t a l y t i c i m p l i ­ cations o f the d i f f e r e n t s t r u c t u r a l f e a t u r e s . In t h i s connection, 0097-6156/ 84/0248-0071 $06.00/0 © 1984 American Chemical Society

Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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i t i s important that "both o f the above techniques conveniently allow studies t o be c a r r i e d out i n s i t u . The MES i n v e s t i g a t i o n s showed that part o f the promoter atoms i n Co-Mo c a t a l y s t s i s g e n e r a l l y present i n a s t r u c t u r e a l s o con­ t a i n i n g molybdenum and s u l f u r atoms (6^). This s t r u c t u r e was termed the Co-Mo-S s t r u c t u r e (Q) and since the promotion o f the HDS a c t i ­ v i t y was found t o be a s s o c i a t e d with t h i s s t r u c t u r e (9., 11 ) much work has been i n i t i a t e d i n order t o c h a r a c t e r i z e f u r t h e r the pro­ p e r t i e s o f t h i s Co-Mo-S s t r u c t u r e . A l l o f the r e s u l t s obtained so f a r , i n c l u d i n g the recent Mo EXAFS studies o f Co-Mo/A1 0 c a t a l y s t s (12), i n d i c a t e that the CoMo-S s t r u c t u r e has a MoS2~like s t r u c t u r e (see e.g., 1» 10). With respect t o the l o c a t i o n o f the Co atoms i n the h i g h l y dispersed M0S2 s t r u c t u r e , the e a r l y MES s t u d i e s (6 8) showed that the Co atoms are l o c a t e d at surface p o s i t i o n s but i t was not p o s s i b l e t o d e f i n i t i v e l y conclude whether these p o s i t i o n s are at Mo s i t e s i n the M0S2 s t r u c t u r e or on the b a s a l or edge planes. Recent studies ( l , 10, 11, l U , ]L5) seem t o favor the l a t t e r p o s i t i o n s and since the Co-Mo-S s t r u c t u r e i s formed i n systems where s i n g l e M0S2 slabs (or l a y e r s ) dominate ( l 6 , I T ) , the Co p o s i t i o n s are most l i k e l y edge s u b s t i t u t i o n a l or i n t e r s t i t i a l p o s i t i o n s and not edge i n t e r ­ calation positions. The previous EXAFS studies were r e s t r i c t e d t o supported c a t a ­ l y s t s . Furthermore, the s t r u c t u r a l p r o p e r t i e s determined by MES and EXAFS were mainly r e l a t e d t o the HDS a c t i v i t y and not t o the other c a t a l y t i c f u n c t i o n s . P r e s e n t l y , we w i l l report EXAFS (both Mo and Co), MES, HDS and hydrogenation a c t i v i t y s t u d i e s o f unsup­ ported Co-Mo c a t a l y s t s . These c a t a l y s t s have been prepared by the homogeneous s u l f i d e p r e c i p i t a t i o n method ( l 8 ) which permits l a r g e amounts o f Co t o be present as Co-Mo-S. The choice o f unsupported c a t a l y s t s allows one t o avoid some o f the e f f e c t s which i n h e r e n t l y w i l l be present i n alumina supported c a t a l y s t s , where support i n ­ t e r a c t i o n s may r e s u l t i n both s t r u c t u r a l and c a t a l y t i c c o m p l e x i t i e s .

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2

3

9

Experimental Sample P r e p a r a t i o n . The p r e p a r a t i o n o f the unsupported Co-Mo c a t a ­ l y s t s has been c a r r i e d out using the homogeneous s u l f i d e p r e c i p i t a ­ t i o n (HSP) method as described e a r l i e r ( l 8 ) and only few d e t a i l s w i l l be given here. A hot (335-3^5 K) s o l u t i o n o f a mixture o f co­ b a l t n i t r a t e and ammonium heptamolybdate with a predetermined Co/Mo r a t i o i s poured i n t o a hot (335~3^5 K) s o l u t i o n o f 20$ ammonium s u l f i d e under vigorous s t i r r i n g . The hot s l u r r y formed i s c o n t i ­ nuously s t i r r e d u n t i l a l l the water has evaporated and a dry pro­ duct remains. This product i s f i n a l l y heated i n a flow o f 2% H2S i n H2 a t 675 Κ and kept at t h i s temperature f o r at l e a s t k nr. Ca­ t a l y s t s with the f o l l o w i n g Co/Mo atomic r a t i o s were prepared: 0.0, 0.0625, 0.125, 0.25, 0.50, 0.75, and 1.0. EXAFS Measurements.

The s u l f i d e d c a t a l y s t s were s t u d i e d i n s i t u

Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

CLAUSEN ET AL.

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by p l a c i n g s e l f - s u p p o r t i n g wafers (1.125" i n diameter) of pressed c a t a l y s t powder i n s p e c i a l l y designed c e l l s , equipped with X-ray transparent windows (13 ). A f t e r s u l f i d i n g of the c a t a l y s t s i n 2% H2S i n H2 at 675 K, the c e l l s were sealed o f f p r i o r to the mea­ surements. EXAFS studies of the model compounds and the p a s s i v a t e d c a t a l y s t s were c a r r i e d out by p l a c i n g appropriate amounts of the sample i n t h i n aluminum frames equipped with Kapton windows. In order to ensure a homogeneous sample t h i c k n e s s , boron n i t r i d e was used as a low absorption f i l l e r . By p a r t l y immersing the a l u ­ minum, frames i n t o l i q u i d n i t r o g e n , EXAFS s p e c t r a of these samples could a l s o be recorded at 77 K. The absorber t h i c k n e s s , x, of a l l the samples was chosen such that μχ ~ 1 (u i s the l i n e a r absorp­ t i o n c o e f f i c i e n t ) on the high absorption s i d e of the edge, and great care was taken i n order to make the samples of homogeneous t h i c k n e s s . Indeed, the jump height at the absorption edge c o r r e ­ sponded n i c e l y ( i n most cases w i t h i n 10$) t o the t h e o r e t i c a l value (based on the amount of sample used). The EXAFS experiments were conducted at DESY i n Hamburg, us­ i n g the synchrotron r a d i a t i o n from the DORIS storage r i n g and the EXAFS-setup at HASYLAB. This spectrometer i s somewhat d i f f e r e n t from that used i n our e a r l i e r s t u d i e s (12). The X-rays, which were emitted by e l e c t r o n s i n the storage r i n g with an energy of 3.3 GeV and a t y p i c a l current of 60 mA, were monochromâtized by two S i ( i l l ) s i n g l e c r y s t a l s when EXAFS above the Co K-edge was measured and by two S i (220) s i n g l e c r y s t a l s i n the case of the Mo K-edge measurements. The beam i n t e n s i t y was measured, before (I ) and a f t e r ( i ) passing through the sample, by use of two i o n i z a t i o n chambers f i l l e d with one atmosphere of N2 (Co K-edge) or one atmos­ phere of Ar (Mo K-edge). In order to eliminate.changes i n i n t e n s i ­ t y due t o sample inhomogeneities, the sample t a b l e i s moved simul­ taneously with the beam during the scan t o ensure that the beam i s h i t t i n g the sample at the same place at a l l times. In the present study we have extracted the EXAFS from the ex­ p e r i m e n t a l l y recorded X-ray absorption s p e c t r a f o l l o w i n g the me­ thod described i n d e t a i l i n Ref. (l£, 20). In t h i s procedure, a value f o r the energy t h r e s h o l d of the absorption edge i s chosen to convert the energy s c a l e i n t o k-space. Then a smooth background de­ s c r i b e d by a set of cubic s p l i n e s i s subtracted from the EXAFS i n order to separate the n o n - o s c i l l a t o r y part i n l n ( l / l ) and, f i n a l ­ l y , the EXAFS i s m u l t i p l i e d by a f a c t o r k and d i v i d e d by a func­ t i o n c h a r a c t e r i s t i c of the atomic absorption cross s e c t i o n (20). The reason f o r m u l t i p l y i n g with a k weighting f a c t o r i s t o compen­ sate f o r the decrease of the EXAFS amplitudes at high k values due to the Debye-Waller f a c t o r , the b a c k s c a t t e r i n g amplitude, and the k* dependence of the EXAFS (see, e.g., Ref. (21)). In order to i n t e r p r e t an EXAFS spectrum q u a n t i t a t i v e l y , the phase s h i f t s f o r the absorber and b a c k s c a t t e r e r and the backscat­ t e r i n g amplitude f u n c t i o n must be known. E m p i r i c a l phase s h i f t s and amplitude f u n c t i o n s can be obtained from s t u d i e s of known s t r u c t u r e s which are chemically s i m i l a r to that under i n v e s t i g a t i 1

Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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on (22). C a l c u l a t e d phase s h i f t s and amplitude f u n c t i o n s have, however, r e c e n t l y been t a b u l a t e d f o r a l a r g e number o f elements

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(23). By F o u r i e r transforming the EXAFS o s c i l l a t i o n s , a r a d i a l s t r u c t u r e f u n c t i o n i s obtained (2k). The peaks i n the F o u r i e r transform correspond t o the d i f f e r e n t c o o r d i n a t i o n s h e l l s and the p o s i t i o n o f these peaks gives the absorber-scatterer d i s t a n c e s , but s h i f t e d t o lower values due t o the e f f e c t o f the phase s h i f t . The height o f the peaks i s r e l a t e d t o the c o o r d i n a t i o n number and t o thermal (Debye-Waller smearing), as w e l l as s t a t i c d i s o r d e r , and f o r systems, which contain only one k i n d o f atoms at a given d i s t a n c e , the F o u r i e r transform method may give r e l i a b l e informa­ t i o n on the l o c a l environment. However, f o r more accurate determi­ nations o f the c o o r d i n a t i o n number Ν and the bond distance R, a more s o p h i s t i c a t e d c u r v e - f i t t i n g a n a l y s i s i s r e q u i r e d . In the present study we have used the phase and amplitude f u n c t i o n s o f a b s o r b e r - s c a t t e r e r p a i r s i n known model compounds t o f i t the EXAFS o f the c a t a l y s t s . By use o f F o u r i e r f i l t e r i n g , the c o n t r i b u t i o n from a s i n g l e c o o r d i n a t i o n s h e l l i s i s o l a t e d and the r e s u l t i n g f i l t e r e d EXAFS i s then non-linear l e a s t squares f i t t e d as described i n Ref. (19, 20). Mössbauer Measurements. Co-Mo c a t a l y s t s cannot be s t u d i e d d i r e c t ­ l y i n absorption experiments s i n c e n e i t h e r cobalt nor molybdenum has s u i t a b l e Mössbauer i s o t o p e s . However, by doping with C o the c a t a l y s t s can be s t u d i e d by c a r r y i n g out Mössbauer emission spec­ troscopy (MES) experiments. In t h i s case information about the co­ b a l t atoms i s obtained by studying the F e atoms produced by the decay o f C o . The p o s s i b i l i t i e s and l i m i t a t i o n s on the use o f the MES technique f o r the study o f Co-Mo c a t a l y s t s have r e c e n t l y been discussed (£3, 25 ). The MES experiments were performed using a c o n s t a n t - a c c e l e r a ­ t i o n spectrometer with a moving s i n g l e - l i n e absorber o f K^FeiCNK* 3H2O enriched i n F e . Zero v e l o c i t y i s defined as the c e n t r o i d o f a spectrum obtained at room temperature with a source o f C o i n m e t a l l i c i r o n . P o s i t i v e v e l o c i t y corresponds t o the absorber mo­ v i n g away from the source. The i n s i t u MES s p e c t r a were recorded with the c a t a l y s t s p l a c e d i n a Pyrex c e l l (8) connected t o a gas handling system a l l o w i n g the c a t a l y s t s t o be s t u d i e d i n a H2S/H2 or i n a thiophene/H2 gas mixture. The use o f H2S i n s t e a d o f t h i o phene d i d not have any n o t i c e a b l e i n f l u e n c e on the MES s p e c t r a (6) 5 7

5 7

5 7

5 7

5 7

C a t a l y s t A c t i v i t y Measurements. A c t i v i t y measurements f o r t h i o phene HDS and the consecutive hydrogenation o f butene were c a r r i e d out i n a Pyrex-glass, fixed-bed r e a c t o r at 625 Κ and at atmospher­ i c pressure as described i n Ref. (9.). Before the measurements the c a t a l y s t s were p r e s u l f i d e d i n 2% H2S i n H2 at 675 K. For each c a ­ t a l y s t conversions were measured at d i f f e r e n t space v e l o c i t i e s o f the thiophene/H2 mixture (2.5$ thiophene) and the c a t a l y t i c a c t i ­ v i t i e s a r e here expressed as pseudo f i r s t - o r d e r r a t e constants a s -

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suming that the HDS r e a c t i o n i s f i r s t order i n thiophene and that the hydrogenation of butène can be considered as a f i r s t order consecutive r e a c t i o n . Results Mössbauer Spectroscopy. Figure 1 shows room temperature Mössbauer emission s p e c t r a o f two o f the unsupported Co-Mo c a t a l y s t s which we have s t u d i e d i n t h e present i n v e s t i g a t i o n . I t i s observed that the MES spectra o f the two c a t a l y s t s are quite d i f f e r e n t . For the c a t a l y s t with the low Co/Mo r a t i o (0.0625) a quadrupole doublet with an isomer s h i f t o f 6=0.33 mm/s and a quadrupole s p l i t t i n g o f ΔΕ =1.12 mm/s are observed (spectrum a ) . These parameters are very s i m i l a r t o those observed p r e v i o u s l y f o r the Co-Mo-S phase i n oth­ er c a t a l y s t s (6-9). Furthermore, the spectrum o f an unsupported c a ­ t a l y s t with Co/Mo = 0.15 i s found t o be e s s e n t i a l l y i d e n t i c a l t o spectrum ( a ) . The MES spectrum (b) o f t h e c a t a l y s t with Co/Mo = 0.50 shows the presence o f a broad s i n g l e l i n e with apparent "shoulders" near the background absorption l i n e . The s i n g l e broad l i n e can be i d e n t i f i e d as o r i g i n a t i n g from C09S8 i n the c a t a l y s t , whereas t h e s p e c t r a l component, which shows up as the "shoulders" i n t h e spectrum, i s t y p i c a l of the spectrum o f the Co-Mo-S s t r u c ­ t u r e . Thus, i t i s observed that f o r the present unsupported c a t a ­ l y s t s , the Co-Mo-S s t r u c t u r e i s the only Co phase present at low Co c o n c e n t r a t i o n s , whereas CogSe i s a l s o formed at higher Co/Mo r a t i o s , and at very high Co content t h i s phase may be the dominat­ ing Co phase. The d i s t r i b u t i o n o f Co atoms among the two phases as obtained by computer a n a l y z i n g the Mössbauer emission spectra i s given i n Table I f o r the d i f f e r e n t unsupported c a t a l y s t s . A more d e t a i l e d a n a l y s i s o f the MES data f o r the unsupported c a t a l y s t s has been given i n Ref. (8, 11).

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Table I . The d i s t r i b u t i o n o f Co atoms among the two phases present i n the unsupported c a t a l y s t s as determined by MES. Co /Mo

Co as Co-Mo-S {%)

100 100 80 23

1

0.0625 0.15 0.25 0.50 2

2

1

2

From Ref. (26).

)

Co as C o S 9

8

(%)

0 0 20 77

From Ref. (11).

Mo EXAFS. In Figure 2a we have shown an X-ray absorption spectrum near t h e Mo K-edge o f the unsupported c a t a l y s t with Co/Mo = 0.125. The spectrum has been obtained i n s i t u and at room temperature. A f ­ t e r background s u b t r a c t i o n , m u l t i p l i c a t i o n by k and n o r m a l i z a t i o n ,

Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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CATALYTIC MATERIALS

— I

-2

1

L

0

2

Velocity (mm/s) Figure 1. Examples o f i n s i t u Mössbauer emission s p e c t r a o f unsupported Co-Mo c a t a l y s t s , a) Co/Mo = 0.0625; b) Co/Mo = 0.50.

Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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CLAUSEN ET A L .

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Figure 2. a) X-ray absorption spectrum near the Mo K-edge of the Co/Mo = 0.125 unsupported Co-Mo c a t a l y s t recorded i n s i t u at room temperature; b) normalized Mo EXAFS spec­ trum; c) absolute magnitude o f the F o u r i e r transform; d) f i t o f t h e f i r s t s h e l l ; e) f i t o f the second s h e l l . The s o l i d l i n e i n d) and e) i s the f i l t e r e d EXAFS, and the dashed l i n e i s the l e a s t squares f i t .

Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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we o b t a i n the EXAFS o s c i l l a t i o n s as shown i n F i g u r e 2b. The EXAFS i s now F o u r i e r transformed from k = 2 Â " t o k = 21 Â " and the r e s u l t i n g r a d i a l s t r u c t u r e f u n c t i o n (Figure 2c) shows the presence of two d i s t i n c t peaks, one l o c a t e d at about 1.90 Â and the other at about 2.86 Â. I t should be noted here that the F o u r i e r t r a n s ­ formed EXAFS o f the model compound, M0S2, recorded i n the present study i s e s s e n t i a l l y i d e n t i c a l t o that o f M0S2 recorded by us on another EXAFS-setup (12). Furthermore, the l o c a t i o n s o f the two main peaks i n the transform f o r M0S2 are very c l o s e t o those o f the two peaks i n F i g u r e 2c. A l s o the heights o f the f i r s t s h e l l peak are s i m i l a r and only the second s h e l l peak f o r the c a t a l y s t i s reduced i n h e i g h t . Therefore, the phase and amplitude f u n c t i o n s of the a b s o r b e r - s c a t t e r e r p a i r Mo-S ( f i r s t s h e l l ) and Mo-Mo (sec­ ond s h e l l ) i n the model compound M0S2 recorded at room temperature have been used t o f i t the F o u r i e r f i l t e r e d EXAFS i n order t o ob­ t a i n the interatomic distances and the c o o r d i n a t i o n number o f the f i r s t and second neighbor s h e l l s i n the c a t a l y s t s . The F o u r i e r f i l ­ t e r e d EXAFS and the corresponding f i t o f the f i r s t s h e l l c o n t r i b u ­ t i o n are shown i n Figure 2d and those o f the second s h e l l are shown i n F i g u r e 2e. The c a l c u l a t e d bond lengths and c o o r d i n a t i o n s numbers f o r the two s h e l l s are given i n Table I I .

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1

1

Table I I . Bond lengths and c o o r d i n a t i o n s numbers obtained by f i t ­ t i n g the F o u r i e r f i l t e r e d Mo EXAFS o f the Co-Mo unsup­ ported c a t a l y s t recorded i n s i t u at room temperature.

1. s h e l l

2. s h e l l

Co/Mo

R(A)

Ν

R(A)

Ν

0.125

2.1+2

6.1

3.16

3.7

In order t o o b t a i n data with reduced temperature smearing, ex­ periments were a l s o c a r r i e d out at 77 K. However, such experiments c o u l d not be c a r r i e d out i n s i t u and the c a t a l y s t s were thus ex­ posed t o a i r before the measurements. EXAFS data o f three c a t a l y s t s with Co/Mo atomic r a t i o s o f 0.0., 0.25, and 0.50 were obtained. The r e s u l t s show many s i m i l a r i t i e s with the data recorded i n s i t u and were f i t t e d i n a s i m i l a r f a s h i o n u s i n g phase and amplitude func­ t i o n s o f the w e l l - c r y s t a l l i z e d model compound M0S2 recorded at 77 K. The r e s u l t s , which a r e given i n Table I I I , show that the bond lengths f o r the f i r s t and second c o o r d i n a t i o n s h e l l are the same f o r a l l the c a t a l y s t s and i d e n t i c a l t o the values obtained f o r the catalyst recorded i n s i t u (Table I I ) . The c o o r d i n a t i o n numbers f o r both s h e l l s appear, however, t o be somewhat s m a l l e r . Although coor­ d i n a t i o n numbers determined by EXAFS cannot be expected t o be de­ termined with an accuracy b e t t e r than + 20$, the observed r e d u c t i o n

Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

5.

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79

Table I I I . S t r u c t u r a l parameters obtained by f i t t i n g the Mo EXAFS of various unsupported c a t a l y s t s recorded a f t e r exposure t o a i r .

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1.

2.

shell

shell

Co/Mo

R(A)

Ν

R(A)

Ν

0.00 0.25 0.50

2.U1

5.6

2.h2 2.h2

k.5

3.15 3.16 3.15

2.6 2.9

1+.8



i n the f i r s t s h e l l c o o r d i n a t i o n numbers i s probably due to the f a c t that these c a t a l y s t s were measured a f t e r exposure to a i r . S e v e r a l authors (27-29) have reported that s u l f i d e d Mo based HDS c a t a l y s t s have a c o n s i d e r a b l e O2 uptake and thus we t e n t a t i v e l y e x p l a i n the reduced f i r s t s h e l l c o o r d i n a t i o n number by an i n f l u e n ­ ce of oxygen atoms i n the l o c a l surroundings of the Mo atoms. The p o s s i b l e reasons f o r the smaller c o o r d i n a t i o n number f o r the sec­ ond s h e l l w i l l be discussed below. Co EXAFS. X-ray absorption s p e c t r a near the Co K-edge have a l s o been recorded f o r the Co/Mo = 0.125 unsupported c a t a l y s t i n order to get information about the l o c a l surroundings of the Co atoms. Figures 3a-c show the X-ray absorption spectrum, the normalized EXAFS, and the F o u r i e r transform, r e s p e c t i v e l y . Only one strong b a c k s c a t t e r e r peak i s observed i n the F o u r i e r transform i n d i c a t i n g h i g h l y disordered surroundings outside the f i r s t s h e l l . However, i t should be noted here that the Co EXAFS r e s u l t s are a s s o c i a t e d with g r e a t e r u n c e r t a i n t y due t o the much smaller s i g n a l - t o - n o i s e r a t i o compared to the Mo EXAFS, and c o n t r i b u t i o n s from backscat­ t e r e r atoms outside the f i r s t s h e l l - i f present - may escape de­ t e c t i o n i n the transform. A r e g i o n surrounding the observed back­ s c a t t e r e r peak was transformed back i n t o k-space (Figure 3d) and f i t t e d by use of the phase and amplitude f u n c t i o n s of the Co-S ab­ s o r b e r - s c a t t e r e r p a i r i n the C0S2 model compound. The values ob­ t a i n e d f o r the interatomic d i s t a n c e and number of atoms f o r the c o o r d i n a t i o n s h e l l around the Co atoms i n the c a t a l y s t are l i s t e d i n Table IV. The relevance of using a Co-S a b s o r b e r - s c a t t e r e r p a i r as i n C0S2 i s j u s t i f i e d by the i n s i t u MES r e s u l t s which show that a l l the Co atoms i n the unsupported c a t a l y s t s are surrounded by s u l f u r . In order to get f u r t h e r information on the l o c a t i o n of Co i n the c a t a l y s t s we have recorded an X-ray absorption spectrum near the Co K-edge of the Co/Mo = 0.125 unsupported c a t a l y s t a f t e r ex­ posure to a i r at room temperature (Figure h). This spectrum i s d i f ­ f e r e n t from the corresponding one recorded i n s i t u . This i s most e a s i l y seen at the absorption edge which shows a peak at apex f o r

Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

F i g u r e 3. ( a ) X - r a y a b s o r p t i o n s p e c t r u m n e a r t h e Co K-edge o f t h e Co/Mo = 0.125 c a t a l y s t r e c o r d e d i n s i t u a t room t e m p e r a t u r e ; ( b ) n o r m a l i z e d Co EXAFS; ( c ) a b s o l u t e magnitude o f the F o u r i e r t r a n s f o r m ; (d) f i t o f the F o u r i e r f i l t e r e d EXAFS. The s o l i d l i n e i s t h e f i l t e r e d EXAFS and t h e d a s h e d l i n e i s t h e f i t .

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C/3

2 £j g >

5.

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81

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Table IV. S t r u c t u r a l parameters obtained by f i t t i n g t h e F o u r i e r f i l t e r e d Co EXAFS o f t h e Co-Mo unsupported c a t a l y s t recorded i n s i t u a t 300 K.

Co/Mo

R(A)

Ν

0.125

2.27

h.6

the a i r exposed c a t a l y s t (Figure k), whereas t h i s i s not present f o r the c a t a l y s t measured i n s i t u (Figure 3a). R e a c t i v i t y S t u d i e s . I n F i g u r e 5A t h e r a t i o between the hydrogénat i o n and the h y d r o d e s u l f u r i z a t i o n r a t e constants i s shown as a f u n c t i o n o f t h e Co/(Co+Mo) atomic r a t i o o f the unsupported c a t a l y s t s . This s e l e c t i v i t y r a t i o i s observed t o be very dependent on the Co/(Co+Mo) r a t i o w i t h a r e l a t i v e h i g h s e l e c t i v i t y f o r hydrogénation o f butane over HDS f o r t h e unpromoted M0S2 c a t a l y s t s , whereas i t i s much lower f o r t h e whole s e r i e s o f Co promoted c a t a l y s t s . I n the p l o t , we have a l s o i n c l u d e d the r e l a t i v e s e l e c t i v i t y f o r an unsupported CogSe c a t a l y s t ( i . e . t h e value at Co/(Co+Mo) = l . O ) . This c a t a l y s t shows a somewhat higher s e l e c t i v i t y r a t i o than the promoted c a t a l y s t s . A l s o t h e observed dependence o f the butane/ butene r a t i o on t h e conversion f o r CogSs was d i f f e r e n t from t h a t o f a l l t h e promoted c a t a l y s t s (30 ) i n d i c a t i v e o f d i f f e r e n t kinds o f k i n e t i c s . I n F i g u r e 5B, we have p l o t t e d s e p a r a t e l y the HDS and the hydrogenation r a t e constants as a f u n c t i o n o f t h e Co/(Co+Mo) atomic r a t i o . I t i s seen t h a t w h i l e t h e promotion w i t h Co has a l a r g e e f f e c t on t h e HDS r a t e parameter, t h e hydrogenation a c t i v i t y i s only s l i g h t l y i n f l u e n c e d . Discussion I t has p r e v i o u s l y been found (3., 11, 18, 31-3k) t h a t unsupported c a t a l y s t s e x h i b i t a HDS a c t i v i t y behavior q u i t e s i m i l a r t o t h a t o f supported c a t a l y s t s . This suggests t h a t although t h e support i s o f importance, i t does not have an e s s e n t i a l r o l e f o r c r e a t i o n o f the a c t i v e phase. Thus, i t i s very r e l e v a n t t o study unsupported c a t a l y s t s , both i n t h e i r own r i g h t and a l s o as models f o r t h e more el u s i v e supported c a t a l y s t s . Many d i f f e r e n t explanations have been proposed t o e x p l a i n t h e s i m i l a r i t y i n behavior o f unsupported and supported c a t a l y s t s (3., 31-3^ ). R e c e n t l y , we have observed t h a t f o r both types o f c a t a l y s t s t h e HDS a c t i v i t y behavior can be r e l a t ed t o t h e f r a c t i o n o f c o b a l t atoms present as Co-Mo-S (9~H , 35.). In the present study, MES was used t o e s t a b l i s h the c o b a l t phase d i s t r i b u t i o n . I n analogy w i t h previous r e s u l t s (jS,