X-Ray Absorption Fine Structure, MÃ¶ssbauer, and ... - ACS Publications

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 .

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

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


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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


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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


(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 -


5.

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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).


5

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 ,



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.



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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 .


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78

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 .


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


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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 .


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

3Λ

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



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 .


C/3

2 £j g >

5.

CLAUSEN ET A L .


81


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,

X-Ray Absorption Fine Structure, MÃ¶ssbauer, and ... - ACS Publications

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