Arsenic Poisoning of Hydrodesulfurization Catalysts - ACS Publications

1 Arsenic Poisoning of Hydrodesulfurization Catalysts Ruth N. Merryfield, Lloyd E. Gardner, and George D. Parks Downloaded by 80.82.77.83 on February 22, 2017 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch001

Phillips Petroleum Company, Bartlesville, OK 74004

X-ray photoelectron spectroscopy (XPS), Mössbauer emission spectroscopy (MES), and activity tests show that arsenic poisons hydrodesulfurization (HDS) catalysts by affecting the chemical nature of the sulfided catalyst. Activity tests show that Co-Mo/ Al2O3 and Mo/Al2O3 catalysts are deactivated when arsenic is added to the catalyst, either as a contaminant from the reactor feed or as As2O5 by laboratory impregnation. XPS shows one form of arsenic, As+5, on the calcined catalyst and two forms, probablyAs+3and Aso, on the sulfided catalyst. XPS also shows sintering of the molybdenum on the sulfided catalyst. We have used MES to study the effect of arsenic on the Co-Mo-S phase (believed to be active for HDS). Arsenic does not destroy this structure, but alters its electronic state. The arsenic appears to be interacting strongly with the cobalt, possibly filling the anion vacancies with atoms or clusters. A r s e n i c p o i s o n i n g o f c a t a l y s t s , p a r t i c u l a r l y r e f o r m i n g and hydrot r e a t i n g c a t a l y s t s , i s a l o n g s t a n d i n g problem. Interest i n shale o i l r e f i n i n g emphasized t h i s problem, as s h a l e o i l s o f t e n c o n t a i n 20-40 ppm a r s e n i c . I n t h i s s t u d y we have used s e v e r a l methods t o c l a r i f y t h e n a t u r e o f a r s e n i c p o i s o n i n g on h y d r o d e s u l f u r i z a t i o n (HDS) c a t a l y s t s . HDS a c t i v i t y t e s t s were used t o determine t h e e x t e n t o f p o i s o n i n g . X-ray p h o t o e l e c t r o n s p e c t r o s c o p y (XPS), X-ray d i f f r a c t i o n (XRD), and Môssbauer e m i s s i o n s p e c t r o s c o p y (MES) have been used t o s t u d y m e t a l s on t h e c a t a l y s t and t o i d e n t i f y s p e c i f i c compounds where p o s s i b l e . C0-M0/AI2O3 c a t a l y s t s have been s t u d i e d e x t e n s i v e l y , b o t h f o r t h e i r s t r u c t u r e and r e a c t i o n mechanisms, and many s t u d i e s have been r e p o r t e d i n t h e l i t e r a t u r e ( 1 - 1 4 ) . However, t h e HDS a c t i v i t y i s not c o m p l e t e l y understood and many c o n f l i c t i n g views have been r e p o r t e d . No attempt i s made here t o e x p l a i n the HDS mechanism,

0097-6156/85/0288-0002$06.00/0 © 1985 American Chemical Society Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1.

MERRYFIELD ET AL.

Arsenic Poisoning of Hydrodesulfurization Catalysts

but o n l y t o d e s c r i b e t h e s t a t e o f a r s e n i c on these c a t a l y s t s and i t s e f f e c t on a c t i v i t y . Experimental C a t a l y s t P r e p a r a t i o n . Most samples were p r e p a r e d u s i n g a K e t j e n a l u m i n a (1/16 i n c h e x t r u d a t e w i t h 280 m /g s u r f a c e a r e a and 0.71 ml/g pore volume) and t h e i n c i p i e n t wetness t e c h n i q u e f o r impregna tion. C a t a l y s t s were s t i r r e d on a h o t p l a t e u n t i l v i s i b l y d r y , d r i e d o v e r n i g h t i n a 100°C oven, and c a l c i n e d i n a i r a t 500°C f o r t h r e e h o u r s . Ammonium paramolybdate, c o b a l t n i t r a t e , and a r s e n i c p e n t o x i d e s o l u t i o n s were u s e d , w i t h d r y i n g and c a l c i n i n g a f t e r each a d d i t i o n . Molybdenum was always added f i r s t , f o l l o w e d by c o b a l t where a p p l i c a b l e , t h e n t h e a r s e n i c . On c a t a l y s t s w i t h h i g h l o a d i n g s o f a r s e n i c , some a r s e n i c was l o s t d u r i n g c a l c i n a t i o n and s u l f i d i n g . C a t a l y s t c o m p o s i t i o n s as d e t e r m i n e d by X-ray f l u o r e s c e n c e a r e g i v e n i n T a b l e I . A n o t h e r sample, a C a t a p a l Ν a l u m i n a , was coimpregnated w i t h 9.9% Co and 8.5 % As f o r use as a r e f e r e n c e m a t e r i a l .

Downloaded by 80.82.77.83 on February 22, 2017 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch001

2

T a b l e I . C a t a l y s t C o m p o s i t i o n by Χ-Ray F l u o r e s c e n c e

Catalyst Μ0/ΑΙ2Ο3 Μ0/ΑΙ2Ο3 + As C0-M0/AI2O3 C0-M0/AI2O3 + As C0-M0/AI2O3 + As

(Used C a t a l y s t )

(Wt. %)

Mo 8.9 7.9 9.6 8.6

Co

2.4 2.2

3.9

8.4

1.9

3.6

As 3.6

A r s e n i c was added t o an American Cyanamid HDS-2 c a t a l y s t f o r comparison w i t h a used c a t a l y s t c o n t a i n i n g 3.6% A s . T h i s used c a t a l y s t was a l s o an American Cyanamid HDS-2 c a t a l y s t w h i c h had been i n s e r v i c e i n a r e f i n e r y d i s t i l l a t e HDS u n i t f o r about s i x y e a r s . A g a i n , X - r a y f l u o r e s c e n c e determined c o m p o s i t i o n s a r e i n Table I . The same K e t j e n a l u m i n a d e s c r i b e d e a r l i e r was used f o r t h e Môssbauer e x p e r i m e n t s . The samples were prepared i d e n t i c a l l y , w i t h the f o l l o w i n g e x c e p t i o n s . The e x t r u d a t e was ground t o 20-40 mesh b e f o r e i m p r e g n a t i o n , and 0.5 gram samples were prepared u s i n g 2 mCi o f Co->7. The samples were p r e p a r e d t o g i v e 8.9% Mo and 1.2% Co (Co/Mo = 0.21). These samples were n o t a n a l y z e d , b u t t h e a r s e n i c c o m p o s i t i o n s based on t h e p r e p a r a t i o n a r e g i v e n i n T a b l e I V . C a t a l y s t S u l f i d i n g . The c a l c i n e d samples were s u l f i d e d p r i o r t o XPS e x a m i n a t i o n by p u r g i n g t h e sample a t room t e m p e r a t u r e w i t h n i t r o gen, h e a t i n g t o 149°C, then s w i t c h i n g t o 10% H2S i n hydrogen and r a i s i n g t h e temperature g r a d u a l l y o v e r a f o u r hour p e r i o d t o 316°C. A f t e r c o o l i n g i n H2S/H2, t h e sample was f l u s h e d i n n i t r o g e n and p l a c e d i n a g l o v e box. There i t was loaded onto t h e XPS sample h o l d e r and t r a n s p o r t e d t o t h e s p e c t r o m e t e r i n an a i r - t i g h t c a r r i e r . S u l f i d i n g f o r t h e Môssbauer e x p e r i m e n t s was s i m i l a r . A l l c o n d i t i o n s were i d e n t i c a l e x c e p t an 8% H2S/H2 b l e n d was used. Figure

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

3

CATALYST CHARACTERIZATION SCIENCE

4


1 shows t h e r e a c t o r used f o r c a l c i n i n g and s u l f i d i n g t h e r a d i o a c t i v e samples. The sample was t r a n s f e r r e d t o t h e Môssbauer c e l l w i t h o u t exposure t o a i r and t r a n s p o r t e d t o t h e s p e c t r o m e t e r . P r e s u l f i d i n g f o r t h e a c t i v i t y t e s t s was a c c o m p l i s h e d by f i r s t h e a t i n g t h e c a t a l y s t i n n i t r o g e n a t 204°C. 10% H2S/H2 was i n t r o duced a t t h i s temperature and a l l o w e d t o f l o w over t h e c a t a l y s t f o r f i v e h o u r s . The temperature was then r a i s e d g r a d u a l l y t o 371°C and h e l d f o r an a d d i t i o n a l f i v e h o u r s . The c a t a l y s t was c o o l e d in nitrogen. XPS Measurements. XPS measurements were performed u s i n g a P h y s i c a l E l e c t r o n i c s Model 548 e l e c t r o n s p e c t r o m e t e r w i t h A l k a r a d i a t i o n (1486.6 e V ) . The s p e c t r o m e t e r was i n t e r f a c e d t o a H e w l e t t - P a c k a r d 21ΜΧ computer f o r d a t a a c q u i s i t i o n and m a n i p u l a t i o n . The i n s t r u m e n t was o p e r a t e d a t about 2 χ 10"^ t o r r , w i t h t h e samples b e i n g i n t r o duced i n t o t h e u l t r a h i g h vacuum from a prechamber evacuated t o about 1 χ 10'6 t o r r . B i n d i n g e n e r g i e s were r e f e r e n c e d t o Au 4 f ( 7 / 2 ) a t 84.0 eV. A t h i n f i l m o f g o l d was e v a p o r a t e d onto t h e sample a f t e r a complete s e t o f s p e c t r a had been o b t a i n e d , and a n o t h e r s e t o f s p e c t r a was then t a k e n . On t h e s u p p o r t e d samples, b i n d i n g e n e r g i e s were r e f e r e n c e d t o t h e A l 2s peak a t 119.6 eV, as d e t e r m i n e d by g o l d r e f e r e n c i n g . The s u r f a c e c o n c e n t r a t i o n s g i v e n i n T a b l e I I I a r e d e t e r m i n e d r e l a t i v e t o t h e A l 2s peak as 100 u s i n g S c o f i e l d ' s c r o s s s e c t i o n s ( L 5 ) and t h e method o f c a l c u l a t i o n d e s c r i b e d by C a r t e r et a l . ( 1 6 ) . P r e t r e a t m e n t o f t h e samples was performed i n t h e prechamber of t h e s p e c t r o m e t e r , e x c e p t f o r t h e s u l f i d i n g d e s c r i b e d p r e v i o u s l y . C a t a l y s t s were c a l c i n e d i n a i r a t 500°C f o r one h o u r , o r reduced i n hydrogen a t 310°C o r 350°C f o r up t o f o u r h o u r s . The prechamber was then evacuated and t h e sample i n t r o d u c e d i n t o t h e s p e c t r o m e t e r w i t h o u t exposure t o t h e atmosphere. F o r t h e XPS work, r e f e r e n c e m a t e r i a l s were examined t o e s t a b l i s h binding energies f o r the v a r i o u s arsenic o x i d a t i o n s t a t e s . A r s e n i c m e t a l , AS2O3, As2S2 5 AS2S3, a l l from V e n t r o n , and AS2O5 from J . T. Baker Chemicals were used. The a r s e n i c m e t a l powder was imbedded i n i n d i u m f o i l f o r e x a m i n a t i o n . An a r s e n i c m i r r o r formed on a r e a c t i o n f l a s k was a l s o examined. HDS A c t i v i t y Measurements. HDS a c t i v i t y measurements were made i s o t h e r m a l l y i n a 3/4 i n c h i . d . h i g h p r e s s u r e t r i c k l e bed r e a c t o r . C a t a l y s t s were ground t o 20-40 mesh and d i l u t e d w i t h alundum (37.5cc alundum t o 12.5cc c a t a l y s t ) . A f t e r p r e s u l f i d i n g , l i g h t c y c l e o i l (a c r a c k i n g p r o d u c t b o i l i n g between 177°C and 343°C and c o n t a i n i n g 1.7 wt % s u l f u r ) was i n t r o d u c e d a l o n g w i t h hydrogen (7 moles h^/mole f e e d ) . Most o f t h e s u l f u r i n t h e o i l was p r e s e n t as benzothiophenes and d i b e n z o t h i o p h e n e s . The r e a c t i o n was r u n a t 600 p s i g and 4.0 LHSV. A temperature s u r v e y was made from 257°C t o 357°C a t 14°C i n t e r v a l s over a s i x t y hour p e r i o d . Môssbauer, The Môssbauer e m i s s i o n s p e c t r o s c o p y measurements were made u s i n g t h e Co^7 doped c a t a l y s t as a s t a t i o n a r y s o u r c e . The moving a b s o r b e r was F e * e n r i c h e d K4Fe(CN)6·3H2O. Both t h e C o and t h e a b s o r b e r were o b t a i n e d from New England N u c l e a r . The con7


5 7

1.

MERRYFIELD ET AL.


s t a n t a c c e l e r a t i o n mode Môssbauer s p e c t r o m e t e r was c a l i b r a t e d u s i n g a source o f C o i n p a l l a d i u m and an a d s o r b e r o f e n r i c h e d Fe57 i r o n foil. The c a t a l y s t samples were loaded i n t o a g l a s s c e l l w i t h a one i n c h d i a m e t e r b e r y l l i u m window. S p e c t r a were accumulated f o r a t l e a s t 24 hours and f i t by computer f o r L o r e n t z i a n c u r v e s u s i n g a l e a s t squares f i t . R e s u l t s and D i s c u s s i o n s


5 7

A c t i v i t y T e s t s . F i g u r e 2 shows r e s u l t s o f a c t i v i t y t e s t s f o r a commercial American Cyanamid HDS-2 c a t a l y s t w h i c h had been i n use f o r about s i x y e a r s . The c a t a l y s t was sampled a t v a r i o u s depths and r e s u l t s f o r t h r e e samples c o n t a i n i n g 0.01% As, 0.6% As, and 3.6% As show a d e c r e a s e i n a c t i v i t y w i t h i n c r e a s i n g a r s e n i c c o n t e n t . A s i m i l a r c a t a l y s t t o w h i c h 3.9% a r s e n i c had been added i n t h e l a b o r a t o r y was t e s t e d and i t s a c t i v i t y ( F i g u r e 3) compared t o t h e a c t i v i t y o f a f r e s h c a t a l y s t and a l s o t o t h a t o f t h e used c a t a l y s t . The a c t i v i t y l o s s o f t h e used c a t a l y s t c o n t a i n i n g 3.6% As c o r r e s p o n d s c l o s e l y w i t h t h a t f o r t h e prepared sample, i n d i c a t i n g t h a t a r s e n i c added by i m p r e g n a t i o n a c t s l i k e t h a t d e p o s i t e d under a c t u a l o p e r a t i n g c o n d i t i o n s . When t h e used c a t a l y s t s were r e g e n e r a t e d i n a i r a t 482°C, t h e a r s e n i c was n o t removed. The molybdenum on a l u m i n a c a t a l y s t was a l s o t e s t e d f o r a c t i v i t y w i t h and w i t h o u t a r s e n i c . A l t h o u g h t h i s c a t l y s t has a much lower i n t r i n s i c a c t i v i t y f o r HDS, t h e r e s u l t s i n F i g u r e 4 show t h a t 3.6% a r s e n i c almost completely d e a c t i v a t e s the c a t a l y s t . The s m a l l amount o f a c t i v i t y r e m a i n i n g i s t h a t e x p e c t e d f o r AI2O3 a l o n e . Thus a r s e n i c a l s o d e a c t i v a t e s c a t a l y s t s w i t h o u t c o b a l t promoters. XPS. S e v e r a l b u l k m a t e r i a l s and one supported sample were examined by XPS t o e s t a b l i s h b i n d i n g e n e r g i e s f o r t h e a r s e n i c . These v a l u e s , g i v e n i n T a b l e I I , c o r r e s p o n d c l o s e l y t o those r e p o r t e d i n t h e l i t e r a t u r e (17-19). The b i n d i n g energy found f o r AS2O5 on a l u m i n a i s comparable t o t h a t found on t h e b u l k AS2O5, i n d i c a t i n g t h a t t h e v a l u e s f o r supported a r s e n i c s h o u l d be s i m i l a r t o t h o s e f o r t h e bulk m a t e r i a l s . Table I I .

XPS B i n d i n g E n e r g i e s

As m e t a l As m i r r o r e d on f l a s k AS2O3 AS2O5

As 3d Be(eV) 41.9 41.7 45.2 45.6

f o r A r s e n i c Reference M a t e r i a l s

As2S2(As4S4)

As 3d Be(eV) 42.4

AS2S3 AS2O5/AI2O3

42.8 45.5

XPS s p e c t r a were o b t a i n e d f o r t h e c a t a l y s t s i n t h e c a l c i n e d , s u l f i d e d , and sometimes i n t h e reduced s t a t e , as d e s c r i b e d b e f o r e . T a b l e I I I g i v e s t h e b i n d i n g e n e r g i e s and r e l a t i v e s u r f a c e concen t r a t i o n s f o r t h e M0/AI2O3 and C0-M0/AI2O3 c a t a l y s t s , w i t h and w i t h out a r s e n i c . Data f o r t h e used c a t a l y s t s , w h i c h a r e n o t l i s t e d i n t h e t a b l e , a r e s i m i l a r t o those f o r t h e c a t a l y s t s prepared i n the l a b o r a t o r y .


5


R e l a t i v e Surface

Concentration,

a

6.04 1.70 3.15

7.15 1.72 4.00

6.99 1.45 3.22

9.93 1.92 4.71

RSC

r a t i o e d t o A l 2S as 100.

398.8 398.7 394.9

398.9 399.3 395.2

399.3 398.1 395.3

399.1 399.0 395.2

Mo3P BE(eV)

a.

5.44 1.50 2.55

4.94 1.24 2.16

a

15.4 15.9 4.7 18.9

45.1 44.4 42.0

45.9 44.8 41.9

RSC

Co-As/AlpO^ ( C o p r e c i p i t a t e d ) Dried 45.6 Presulfided 41.2 Reduced 44.8 40.9

C o - M o / A l ^ + As Calcined Presulfided

Co-Mo/Al?(h Calcined Presulfided

M0/AI2O3 4- As Calcined Presulfided

M0/AI2O3 Calcined Presulfided

As 3d BE(eV)

T a b l e I I I . XPS R e s u l t s on P r e p a r e d Samples

782.3 777.9 781.7 777.6

782.3 782.1 779.1

782.5 782.2 779.1

a

{23.2

17.1 16.2

{1.98

1.96

{2.21

2.29

Co 2P BE(eV) RSC


161.9

162.1

162.2

162.2

162.2

BE(eV)

S 2P a

12.1

10.1

12.5

8.7

11.9

RSC

1218 1226 1218 1226

1218 1218 1225

1217 1218 1224

As Auger KE(eV)

οm

Q m

1

Ν

2

m

χ > >

sο

î

ON


1.

MERRYFIELD ET AL.


REACTOR F i g u r e 1.

Sample r e a c t o r and Môssbauer c e l l .

PERCENT HDS

1

Ο 240

= J

260

1

280

' 300

' 320

—

1

340

360

TEMPERATURE(DEGREES C) F i g u r e 2.

HDS a c t i v i t y o f p o i s o n e d p l a n t

catalysts.


1



8


1.

MERRYFIELD ET AL.


R e s o l u t i o n o f the Mo 3p peaks showed molybdenum t o be p r e s e n t as Mo 6 f o r t h e c a l c i n e d c a t a l y s t s , and a m i x t u r e o f Mo ^ and Mo ^ f o r s u l f i d e d c a t a l y s t s . The Mo 3p peaks were used s i n c e b o t h a r s e n i c and s u l f u r peaks i n t e r f e r e w i t h t h e Mo 3d d o u b l e t . Although some a u t h o r s have been a b l e t o r e s o l v e a peak a t t r i b u t e d t o Mo ^ on s u l f i d e d c a t a l y s t s ( 8 - 9 ) , we were a b l e t o r e s o l v e o u r peaks u s i n g o n l y two component peaks. Curve r e s o l v i n g c o n s i s t e n t l y showed t h a t about 70% o f t h e molybdenum s i g n a l i n t h e s u l f i d e d c a t a l y s t s was due t o Mo+ . T a b l e I I I shows t h a t b o t h c o b a l t and a r s e n i c a f f e c t t h e Mo/Al r a t i o as measured by XPS. These elements cause t h e Mo/Al r a t i o on b o t h c a l c i n e d and s u l f i d e d c a t a l y s t s t o d e c r e a s e . A g a i n t h i s i s due e i t h e r t o c o v e r i n g o f molybdenum by promoters o r more l i k e l y t o some change i n molybdenum d i s p e r s i o n induced by t h e p r o m o t e r s . I t i s l i k e l y t h a t these m e t a l s d i s p l a c e molybdenum from s u r f a c e s i t e s on t h e a l u m i n a , so t h a t they i n t e r f e r e w i t h t h e s t r o n g i n t e r a c t i o n o f t h e molybdenum and aluminum w h i c h keeps t h e molybdenum well dispersed. The Mo/Al r a t i o measured by XPS always d e c r e a s e d upon s u l f i d i n g . B u l k a n a l y s i s showed no change i n molybdenum c o n c e n t r a t i o n upon s u l f i d i n g , i n d i c a t i n g t h a t no molybdenum was l o s t d u r i n g s u l f i d i n g . The drop i n i n t e n s i t y can b e s t be e x p l a i n e d by assuming t h a t t h e molybdenum s u l f i d e form i s s i n t e r i n g i n t o l a r g e c l u s t e r s . The model o f K e r k h o f and M o u l i j n (20) was used t o i n t e r p r e t t h i s d a t a . We assumed t h a t a l l o f t h e Mo ^~~on c a l c i n e d and s u l f i d e d c a t a l y s t s was p r e s e n t as a w e l l d i s p e r s e d monolayer. C r y s t a l l i t e s i z e s were c a l c u l a t e d based on t h e change i n Mo/Al r a t i o upon s u l f i d i n g ( t h i s method e l i m i n a t e s e r r o r s due t o i n a c c u r a c i e s i n p h o t o i o n i z a t i o n c r o s s s e c t i o n s ) . C r y s t a l l i t e s i z e s r a n g i n g from 2nm t o 4nm were c a l c u l a t e d f o r t h e s u l f i d e d s t a t e . These s i z e s s h o u l d be viewed w i t h c a u t i o n , due t o u n c e r t a i n t y i n e l e c t r o n mean f r e e paths and d i s p e r s i o n o f the oxide species. Incomplete d i s p e r s i o n o f Mo ^ o r p a r t i a l c o v e r i n g o f t h e M0S2 c r y s t a l l i t e s by promoter atoms would lead t o erroneous r e s u l t s . These c r y s t a l l i t e s i z e s a r e l a r g e r than those proposed by Topsoe e t a l . (21) based on i n f r a r e d and EXAFS d a t a . A l t h o u g h t h e e r r o r s mentioned above may a f f e c t t h e a b s o l u t e s i z e c a l c u l a t e d , i t does n o t a l t e r t h e c o n c l u s i o n t h a t some degree o f s i n t e r i n g i s o c c u r r i n g on s u l f i d i n g . An i n t e r e s t i n g e f f e c t i s n o t i c e d i f one l o o k s a t t h e r e l a t i v e d e c r e a s e i n t h e Mo/Al r a t i o upon s u l f i d i n g f o r each c a t a l y s t . When c o b a l t i s p r e s e n t t h i s r a t i o drops by about 20% a f t e r s u l f i d i n g (independent o f t h e presence o f a r s e n i c ) . Without c o b a l t , t h e r a t i o i s d e c r e a s e d by about 33%. I t t h e r e f o r e appears t h a t c o b a l t h e l p s t o lower t h e amount o f s i n t e r i n g w h i c h o c c u r s upon s u l f i d i n g ( a l t h o u g h i t causes some s i n t e r i n g o f t h e o x i d i c form o f t h e c a t a lyst). S u l f i d i n g has no e f f e c t on XPS Co/Al r a t i o s . The As 3d peak on c a l c i n e d c a t a l y s t s n o r m a l l y appeared as a s i n g l e peak c o r r e s p o n d i n g t o A s ^ ( F i g u r e 5 ) . S u l f i d i n g u s u a l l y gave two peaks, t h e peak a t h i g h e r b i n d i n g energy p r o b a b l y c o r r e s ponding t o A s 3 a r s e n a t e ) which i s t i e d up w i t h t h e support and d i f f i c u l t t o reduce. The peak a t lower b i n d i n g energy c o r r e s ponds c l o s e l y t o t h e v a l u e f o r z e r o v a l e n t a r s e n i c . The A s / A l r a t i o d e c r e a s e s when t h e sample i s s u l f i d e d . I t was found on r e c a l c i n a t i o n o f t h e used c a t a l y s t t h a t t h e A s / A l r a t i o r e t u r n e d t o t h e v a l u e +

+

+

+


4

+

+

+

+

o

r

a

n


9

CATALYST C H A R A C T E R I Z A T I O N SCIENCE


10

50.00

45.00

40.00

35.00

Binding Energy (eV)

Figure

5. X P S s p e c t r a

f o r A s o n C0-M0/AI2O3

catalyst.



1.

MERRYFIELD ET AL.


found on t h e o r i g i n a l c a l c i n e d sample, i m p l y i n g t h a t s i n t e r i n g was r e s p o n s i b l e f o r t h e lower a r s e n i c s i g n a l on t h e s u l f i d e d sample. The coimpregnated sample w i t h h i g h e r l e v e l s o f c o b a l t and a r s e n i c on a l u m i n a was s t u d i e d because t h e c o n c e n t r a t i o n o f t h e m e t a l s was h i g h enough t o g i v e XRD p a t t e r n s . The o n l y phase i d e n t i f i e d i n o t h e r c a t a l y s t s by XRD was gamma a l u m i n a . XRD showed the f o r m a t i o n o f a l l o y s (CoAs and C 0 2 A S ) when t h e c o p r e c i p i t a t e d sample was reduced i n hydrogen. However, on s u l f i d i n g , XRD gave o n l y a CoAs p a t t e r n w i t h broadened and s l i g h t l y d i s p l a c e d peaks, i n d i c a t i n g some d i s t o r t i o n o f t h e c r y s t a l l a t t i c e and a s m a l l e r c r y s t a l l i t e s i z e . The i n t r o d u c t i o n o f s u l f u r a p p a r e n t l y d i s r u p t e d the a l l o y f o r m a t i o n . XPS r e s u l t s f o r t h e c o p r e c i p i t a t e d c a t a l y s t , g i v e n i n T a b l e I I I , show a b i n d i n g energy o f 40.9 eV f o r t h e As 3d peak on t h e hydrogen reduced c a t a l y s t . A s l i g h t l y h i g h e r v a l u e , 41.2 eV, i s found f o r t h e s u l f i d e d sample. Môssbauer. The r a d i o a c t i v e samples, prepared as d e s c r i b e d above, were examined by MES a f t e r c a l c i n i n g and a l s o a f t e r p r e s u l f i d i n g , b o t h w i t h and w i t h o u t a r s e n i c . T a b l e IV g i v e s t h e quadrupole s p l i t t i n g , ΔΕ, and t h e isomer s h i f t , δ, measured f o r t h e two s e t s o f d o u b l e t s found i n each spectrum. On t h e s u l f i d e d sample, t h e d o u b l e t w i t h t h e s m a l l e r quadrupole s p l i t t i n g c o r r e s p o n d s t o t h e Co-Mo-S phase d e s c r i b e d by Topsoe e t a l . (11,22,23). The o t h e r d o u b l e t i s assigned t o the c o b a l t i n the alumina. As a r s e n i c i s added, a d e c r e a s e i s seen i n t h e quadrupole s p l i t t i n g f o r t h e Co-Mo-S phase ( F i g u r e 6 ) . A d e c r e a s e i n t h e i n t e n s i t y o f t h e Co:Al2Û3 phase i s a l s o o b s e r v e d as t h e a r s e n i c c o n c e n t r a t i o n i s i n c r e a s e d .

T a b l e I V . Môssbauer Parameters f o r S u l f i d e d C0-M0/AI2O3 C a t a l y s t s . g. As Added on 0.5 g C a t a l y s t 0.000 0.016 0.032 0.048 a. b.

Co-Mo-S D o u b l e t AE _δ_^ 1.09 0.19 1.02 0.21 0.83 0.18 0.79 0.18 a

Co:Al203 D o u b l e t ΔΕ* §} 2.12 0.89 2.22 0.77 2.16 0.84 2.16 0.82

Quadrupole s p l i t t i n g , i n mms"* Isomer s h i f t , i n mms"*

S i n c e t h e s t r u c t u r e o f t h e Co-Mo-S phase i s n o t known, i t i s d i f f i c u l t t o propose v e r i f i a b l e e x p l a n a t i o n s f o r t h e changes i n the Môssbauer spectrum due t o a r s e n i c . However, s e v e r a l g e n e r a l o b s e r v a t i o n s might be made. The Co-Mo-S phase i s n o t d e s t r o y e d by a r s e n i c , i . e . , t h e a r s e n i c i s n o t p r e v e n t i n g t h e f o r m a t i o n o f the phase d u r i n g s u l f i d i n g . The c o b a l t i s n o t c h a n g i n g o x i d a t i o n s t a t e s and t h e geometry o f t h e c o b a l t i s e s s e n t i a l l y t h e same, b u t a change i n e l e c t r i c f i e l d a t t h e c o b a l t n u c l e u s i s i n d i c a t e d . T h i s change c a n be a t t r i b u t e d t o t h e presence o f t h e a r s e n i c , apparently i n t e r a c t i n g s t r o n g l y with the c o b a l t , p o s s i b l y f i l l i n g t h e a n i o n v a c a n c i e s i n t h e Co-Mo-S s t r u c t u r e .


11

12

CATALYST C H A R A C T E R I Z A T I O N SCIENCE


τ — η — ι — ι — ι — ι — r — r

-J ι

ι

ι—ι—ι—ι—ι—Ll

-4.0 -3 0 -2 0 -1.0

.0

1.0

20

3.0

4.0

VELOCITY — MM/SEC F i g u r e 6.

Môssbauer s p e c t r a o f s u l f i d e d

catalysts.


1.

M E R R Y F I E L D ET A L .

Arsenic Poisoning of Hydrodesulfurization Catalysts 13

Conclusions HDS a c t i v i t y t e s t s show c o n c l u s i v e l y t h a t a r s e n i c p o i s o n s the Co-Mo/ AI2O3 and the M0/AI2O3 c a t a l y s t s . XPS b i n d i n g e n e r g i e s i n d i c a t e a z e r o v a l e n t form o f a r s e n i c p r e s e n t on the s u l f i d e d c a t a l y s t s (as w e l l as an A s 3 f o r m ) . When s u l f i d e d samples a r e r e c a l c i n e d , a l l o f the a r s e n i c r e t u r n s t o the o x i d i z e d s t a t e . Môssbauer spec troscopy i n d i c a t e s that the a r s e n i c i s i n t e r a c t i n g s t r o n g l y w i t h the c o b a l t and t h a t w h i l e the Co-Mo-S phase s t i l l e x i s t s , i t s e l e c t r o n i c s t r u c t u r e has been a l t e r e d by the a r s e n i c . These d a t a i n d i c a t e that the a r s e n i c i s a l t e r i n g the e l e c t r o n i c s t r u c t u r e of the a c t i v e s i t e s , perhaps by o c c u p y i n g a n i o n v a c a n c i e s w i t h a r s e n i c atoms o r c l u s t e r s . S i m i l a r a n i o n v a c a n c i e s have been proposed by V a l y o n and H a l l (13) f o r t h e unpromoted molybdenum on a l u m i n a c a t a lyst. These v a c a n c i e s c o u l d be b l o c k e d by a r s e n i c j u s t l i k e those i n the promoted c a t a l y s t .


+

Acknowledgments The a u t h o r s w i s h t o acknowledge the f o l l o w i n g i n d i v i d u a l s f o r t h e i r a s s i s t a n c e i n t h i s s t u d y : B i l l L e r o y f o r c a r r y i n g out the Môssbauer s t u d i e s , Ed Farmer f o r o b t a i n i n g the XPS s p e c t r a , Mike B r i g g s and Bobby Dodd f o r c o n d u c t i n g the HDS a c t i v i t y t e s t s , and M a r v i n J o h n s o n , Gary Nowack and P e t e r Gray f o r h e l p f u l d i s c u s s i o n s . Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Lipsch, J . M. J . G; Schuit, G. C. Α.; J . Catal., 1969, 15, 163. Friedman, R. M.; Declerck-Grimee, R. I.; F r i p i a t , J . J . ; J . Electron Spectrosc. Relat. Phenom., 1974, 5, 437. Brinen, J . S.; Armstrong, W. D., J . Catal., 1978, 54, 57. P h i l l i p s , R. W.; Fote, Α. Α.; J . Catal., 1976, 41, 168. Walton, R. Α., J . Catal., 1976, 44, 335. Delvaux, G.; Grange, P.; Delmon, B.; J . Catal., 1979, 56, 99. Gajardo, P.; Mathieux, Α.; Grange, P.; Delmon, B.; Appl. Catal., 1982, 3, 347. Patterson, Τ. Α.; Carver, J . C.; Leyden, D. E.; Hercules, D. M.; J . Phys. Chem., 1976, 80(15), 1700. Okamoto, Y.; Shimokawa, T.; Imanaka, T.; Teranishi, S.; J . Catal., 1979, 57, 153. Gates, B. C.; Katzer, J . R,; Schuit, G. C. Α.; "Chem. of C a t a l y t i c Processes," McGraw-Hill, Inc., New York, 1979. Topsoe, H.; Clausen, B. S.; Candia, R.; Wivel, C.; Morup, S.; J . Catal., 1981, 68, 433. Chung, K. S.; Massoth, F. E.; J . Catal., 1980, 64, 332. Valyon, J . ; H a l l , W. K.; J . Catal., 1983, 84, 216. Massoth, F. E.; Muralidhar, G.; Shabtai, J . ; J . Catal., 1984, 85, 53. S c o f i e l d , J . H., J . Electron Spectrosc. Relat. Phenom., 1976, 8, 129. Carter, W. J . ; Schweitzer, G. K.; Carlson, Τ. Α.; J . Electron Spectrosc. Relat. Phenom., 1974, 5, 827. Bahl, M. K.; Woodall, R. O.; Watson, R. L.; I r g o l i c , K. J . ; J . Chem. Phys.; 1976, 64(3), 1210.



14

18. 19. 20. 21.

22. 23.

Taylor, J . Α.; J . Vac. S c i . Technol.; 1982, 20(3), 751. Brundle, C. R.; and Seybold, D.; J . Vac. S c i . Technology, 1979, 16(5), 1186. Kerkhof, F. P. J . M.; Moulijn, J . Α.; J . Phys. Chem., 1979, 83(12), 1612. Candia, R.; Clausen, B. S.; Bartholdy, J . ; Topsoe, Ν. Y.; Lengeler, B.; Topsoe, H.; Proc. 8th Int. Congr. Catal., 1984, Vol. I I , p. 375. Wivel, C.; Candia, R.; Clausen, B.; Morup, S.; Topsoe, H,; J . Catal. 1981, 68, 453. Breysse, M.; Bennett, Β. Α.; Chadwick, D.; Vrinat, M.; B u l l Soc. Chim. Belg., 1981, 90, 1271.


R E C E I V E D December 6, 1984


Arsenic Poisoning of Hydrodesulfurization Catalysts - ACS Publications

Recommend Documents