Reduction of SO2 to Elemental S over Ceria Catalysts

Desulrurization of combustion exhaust gases is presently a costly process often involving .... Thus, more than 95% elemental sulfur yield was obtained...
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Chapter 31

Reduction of SO to Elemental S over Ceria Catalysts 2

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Wei Liu and Maria Flytzani-Stephanopoulos Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, M A 02139

Bulk cerium oxide, pure or doped with other rare earth oxides is a good catalyst for the reduction of SO with CO to elemental sulfur at temperatures above 500 ° C . The production of COS is negligible over these catalysts when a feed gas of stoichiometric composition([CO]/[SO ]=2) is used. The effect of dopant oxides on the catalytic activity of ceria is correlated with the change of oxygen ionic conductivity and vacancy energetics that the dopants impart into the ceria lattice. The highest activity is observed with a CeO catalyst doped with 1 at % La O3. Effects of water vapor on the catalyst activity and selectivity are discussed. 2

2

2

2

D e s u l r u r i z a t i o n o f c o m b u s t i o n exhaust gases is p r e s e n t l y a c o s t l y process often i n v o l v i n g c o m p l e x flow sheets a n d " t h r o w - a w a y " sorbents. D i r e c t l y r e d u c i n g s u l f u r d i o x i d e to e l e m e n t a l s u l f u r o v e r a c a t a l y s t i s attractive, because i t p r o d u c e s a salable p r o d u c t w i t h o u t a n y s o l i d w a s t e to d i s p o s e of. S e v e r a l processes h a v e b e e n p r o p o s e d a n d d e v e l o p e d o v e r the last t w e n t y y e a r s . T h e A l l i e d C h e m i c a l c a t a l y t i c S O 2 r e d u c t i o n w i t h n a t u r a l gas w a s a p p l i e d to S 0 2 - r i c h ( > 5 % ) s m e l t e r off-gases(2). D i r e c t f l u e gas r e d u c t i o n b y s y n t h e s i s gas o v e r a n u n d i s c l o s e d catalyst is u n d e r d e v e l o p m e n t b y the P a r s o n s C o m p a n y ( 2 ) . H o w e v e r , b o t h processes r e q u i r e a n e x p e n s i v e C l a u s p l a n t to c o m p l e t e the e l e m e n t a l s u l f u r r e c o v e r y . O t h e r processes u n d e r d e v e l o p m e n t are two-stage, d r y regenerative flue gas c l e a n u p processes, w h e r e b y d i l u t e - S 0 2 streams are s c r u b b e d i n a m e t a l o x i d e s o r b e r a n d t h e n , i n the regenerator the spent s o r b e n t is regenerated for re-use w i t h c o n c o m i t a n t e m i s s i o n of a concentrated S O 2 stream

0097-6156/94/0552-0375$08.00/0 © 1994 American Chemical Society Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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s u i t a b l e for s u l f u r r e c o v e r y . T h e regenerator-off gas has o n l y a fraction o f the f l u e gas v o l u m e , a n d contains n o o x y g e n . R e c o v e r y o f s u l f u r f r o m this t y p e gas i n a single-stage catalytic converter, a v o i d i n g the m u l t i s t a g e C l a u s p l a n t , w o u l d decrease the cost a n d accelerate the c o m m e r c i a l i z a t i o n o f the d r y r e g e n e r a t i v e f l u e gas c l e a n u p process. T h e d i r e c t r e d u c t i o n of S O 2 b y C O to e l e m e n t a l s u l f u r is t h e r m o d y n a m i c a l l y f a v o r a b l e b u t it proceeds v e r y s l o w l y i n the absence o f a catalyst. A secondary undesirable reaction can occur between C O a n d e l e m e n t a l s u l f u r f o r m i n g C O S , w h i c h m a y a l s o r e d u c e S O 2 to e l e m e n t a l s u l f u r t h r o u g h r e a c t i o n 3. C O S is m o r e toxic t h a n S O 2 a n d its p r o d u c t i o n s h o u l d b e m i n i m i z e d i n a s u l f u r r e c o v e r y process. S02 + 2CO = 2C02 + l / x S x

(1)

CO + l/xS =COS

(2)

x

2COS + SO2= 2 C 0 2 + 3/x S

x

(3)

A l t h o u g h r e a c t i o n 1 has c o n s i d e r a b l e b a c k g r o u n d , n o s u b s t a n t i a l c o m m e r c i a l e x p e r i e n c e exists a n d o n l y s o m e l a b o r a t o r y s t u d i e s h a v e b e e n r e p o r t e d . R y a s o n a n H a r k i n s ( 3 ) tested C u , P d , A g , C o o r N i s u p p o r t e d o n a l u m i n a as catalysts for the s i m u l t a n e o u s r e d u c t i o n o f S O 2 a n d N O b y C O . The S O 2 r e d u c t i o n b y C O o n F e / A l 2 0 3 a n d C U / A I 2 O 3 catalysts was e x t e n s i v e l y s t u d i e d b y K h a l a f a l l a , et alf4,5) a n d S h o r t , et a l (6,7), r e s p e c t i v e l y . B a z e s et al(8) i n v e s t i g a t e d the p e r o v s k i t e s L a C o 0 3 a n d C u C o 2 0 4 , a n d the o x i d e m i x t u r e C e 0 2 - C o 3 0 4 as catalysts for the s a m e r e a c t i o n . T h e p r o d u c t i o n o f C O S u s u a l l y p r o c e e d s to a s u b s t a n t i a l extent o n these catalysts. H a p p e l et al(9,10) l o w e r e d the C O S f o r m a t i o n rate b y u s i n g the p e r o v s k i t e L a T i 0 3 . M o r e recently, H i l b b e r t a n d C a m p b e l l i n , 2 2 ) f o u n d t h a t L a i - S r C o 0 3 is a n active catalyst for S O 2 r e d u c t i o n b y C O . T h e h i g h e s t s e l e c t i v i t y t o w a r d e l e m e n t a l s u l f u r w a s o b t a i n e d o n a catalyst w i t h x=0.3. i

x

x

I n a d d i t i o n to the f o r m a t i o n of C O S d u r i n g the S O 2 r e d u c t i o n b y C O , the p r e s e n c e of w a t e r i n the feed gas has a major effect o n b o t h the catalytic a c t i v i t y a n d s e l e c t i v i t y . T h e w a t e r m a y p o i s o n the catalyst a n d p a r t i c i p a t e i n the r e a c t i o n s , f o r e x a m p l e , the w a t e r - g a s s h i f t r e a c t i o n . It w a s r e p o r t e d ( 5 , 7 ) t h a t w a t e r s i g n i f i c a n t l y p o i s o n e d the C U / A I 2 O 3 a n d F e / A l 2 0 3 c a t a l y s t s , a n d a l s o l o w e r e d the r e a c t i o n s e l e c t i v i t y t o w a r d e l e m e n t a l sulfur. T h e w a t e r v a p o r effects o n the o t h e r catalysts p r e v i o u s l y studied, however, have not been reported. The redox reaction mechanism h a s b e e n p r o p o s e d f o r the S O 2 r e d u c t i o n b y C O C 2 0 , 22). A c c o r d i n g to t h i s m e c h a n i s m , t h e r e d u c t a n t r e m o v e s o x y g e n f r o m the catalyst surface w h i l e the o x i d a n t g i v e s u p its o x y g e n t o the c a t a l y s t , so that the catalyst surface is constantly r e d u c e d / o x i d i z e d d u r i n g the c o u r s e o f the r e a c t i o n . B u l k c e r i u m o x i d e

Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

31.

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Reduction ofS0

2

to Elemental S

377

has b e e n i d e n t i f i e d as a n a c t i v e c a t a l y s t for the S O 2 r e d u c t i o n i n o u r p r e v i o u s w o r k ( 2 3 ) . It is w e l l k n o w n that C e 0 2 has h i g h o x y g e n v a c a n c y a n d m o b i l i t y , a n d these p r o p e r t i e s c a n be further e n h a n c e d b y d o p i n g o t h e r m e t a l i o n s i n t o its f l u o r i t e - t y p e c r y s t a l lattice. T h e p r e s e n t s t u d i e s a t t e m p t to test i f the c a t a l y t i c a c t i v i t y o f C e U 2 c a n b e i m p r o v e d b y i n c o r p o r a t i n g d o p a n t i o n s i n t o its lattice, a n d to e x p l o r e o t h e r C e 0 2 c o n t a i n i n g catalysts. Experimental A p p a r a t u s a n d P r o c e d u r e . A l l catalysts w e r e tested i n a l a b o r a t o r y - s c a l e , p a c k e d b e d f l o w reactor, w h i c h consists o f a 1.0 c m I D . χ 50 c m l o n g q u a r t z t u b e w i t h a p o r o u s q u a r t z frit p l a c e d at the m i d d l e for s u p p o r t i n g the catalyst. T h e reactor tube i s h e a t e d b y a L i n d b e r g furnace. T h e r e a c t i o n t e m p e r a t u r e is m o n i t o r e d b y a q u a r t z t u b e sheathed K - t y p e t h e r m o c o u p l e p l a c e d at the t o p o f the p a c k e d b e d a n d c o n t r o l l e d b y a W i z a r d temperature c o n t r o l l e r . T h e reacting gases, a l l c e r t i f i e d c a l i b r a t i o n gas m i x t u r e s b a l a n c e d b y h e l i u m ( M a t h e s o n ) , are m e a s u r e d w i t h r o t a m e t e r s a n d m i x e d p r i o r to t h e r e a c t o r i n l e t . T h e r e s u l t i n g gas m i x t u r e f l o w s d o w n w a r d t h r o u g h the p a c k e d b e d . W a t e r v a p o r is i n t r o d u c e d w i t h h e l i u m b u b b l i n g t h r o u g h a h e a t e d w a t e r b a t h . T h e p r e s s u r e d r o p of gas f l o w i n g t h r o u g h the a s s e m b l y is s m a l l . T h u s , e x p e r i m e n t s w e r e c a r r i e d o u t u n d e r n e a r l y a t m o s p h e r i c p r e s s u r e . A c o l d trap attached at the o u t l e t o f the reactor is u s e d to c o l l e c t the e l e m e n t a l s u l f u r f r o m the p r o d u c t stream. T h e p r o d u c t gas s t r e a m , free o f sulfur a n d particulates, is a n a l y z e d b y a H P 5 8 8 0 A G a s Chromatograph(GC) with a Thermal Conductivity Detector(TCD). H e l i u m is u s e d as the G C carrier a n d reference gas, e a c h at 30 c c / m i n . T h e detector a n d o v e n t e m p e r a t u r e s are set at 200 ° C a n d 60 ° C , r e s p e c t i v e l y . A 1/4" O . D . χ 6* l o n g p a c k e d c o l u m n o f C h r o m o s i l 310(from S U P E L C O ) p r o v i d e s g o o d s e p a r a t i o n o f C O , C O 2 , C O S , H 2 S , C S 2 , a n d S O 2 u n d e r these c o n d i t i o n s . T h e T C D s h o w s l i n e a r r e s p o n s e to a l l the c o m p o u n d s m e n t i o n e d a b o v e a n d a d e t e c t i o n l i m i t o f less t h a n 100 p p m b y v o l u m e . T h e fresh a n d u s e d catalysts w e r e t y p i c a l l y a c t i v a t e d b y h e a t i n g for one h o u r i n 10% C O / H e at 600 ° C . A f t e r a c t i v a t i o n , the r e a c t i n g gases w e r e i n t r o d u c e d a n d the r e a c t i o n t e m p e r a t u r e w a s r a i s e d to a b o u t 650 ° C . W h e n a steady-state r e a c t i o n w a s r e a c h e d , the t e m p e r a t u r e w a s l o w e r e d i n steps o f a b o u t 50 ° C u n t i l the r e a c t i o n w a s q u e n c h e d . I n catalysts e v a l u a t i o n test, the i n l e t gases o f 1 v o l % S O 2 a n d 2 v o l % C O w e r e u s e d , w h i l e the t o t a l f l o w rate w a s k e p t at 200 seem. T h e p a c k e d h e i g h t o f c a t a l y s t b e d w a s a r o u n d 7 m m a n d the contact t i m e w a s 0.01 to 0.05 g . s / c c ( S T P ) . T h e e l e m e n t a l s u l f u r y i e l d , i.e., the f r a c t i o n o f i n l e t S O 2 c o n v e r t e d i n t o e l e m e n t a l s u l f u r , w a s d e r i v e d f r o m the m a t e r i a l balance o f c a r b o n a n d s u l f u r , a n d o c c a s i o n a l l y c h e c k e d b y t i t r a t i o n o f the s u l f u r c o l l e c t e d i n the c o l d trap.

Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

ENVIRONMENTAL CATALYSIS

378

Catalyst Preparation. A l l b u l k catalysts w e r e p r e p a r e d b y the w e l l - k n o w n a m o r p h o u s citrate methodf24) f r o m nitrate p r e c u r s o r s ( f r o m A l d r i c h ) . T h i s m e t h o d p r o v i d e s w e l l dispersed m i x e d oxides or m i x e d oxide compounds. T h e c a t a l y s t p r e p a r e d i n this w a y has large f r a c t i o n o f m a c r o p o r e s ( > l m m i n d i a m e t e r ) w h i c h facilitates p o r e d i f f u s i o n . T h e p r e p a r a t i o n p r o c e d u r e is o u t l i n e d as f o l l o w s : m e t a l nitrates a n d citric a c i d o f m o l a r r a t i o o n e are d i s s o l v e d i n d e - i o n i z e d w a t e r , separately; the c i t r i c a c i d s o l u t i o n is a d d e d i n t o the n i t r a t e s o l u t i o n d r o p w i s e u n d e r c o n s t a n t m a g n e t i c s t i r r i n g ; the r e s u l t i n g m i x e d s o l u t i o n is c o n c e n t r a t e d i n t o a v i s c o u s f l u i d i n a r o t a r y e v a p o r a t o r ( 7 0 - 80 ° C ) ; the v i s c o u s fluid is t h e n t r a n s f e r r e d o n a d i s h i n a v a c u u m o v e n w h e r e a s o l i d f o a m is f o r m e d d u r i n g o v e r n i g h t d r y i n g ( 7 0 80 ° C , 15 K P a ) ; the s o l i d f o a m is c a l c i n e d at 600 ° C i n a m u f f l e furnace u n d e r f l o w i n g air for 2-3 h o u r s ; the r e s u l t i n g s o l i d is c r u s h e d a n d s i e v e d ; P a r t i c l e s b e t w e e n 20 to 35 mesh(420-840 μ π ι ) are t y p i c a l l y u s e d i n the tests. T h e s u p p o r t e d catalysts w e r e p r e p a r e d b y the c o n v e n t i o n a l w e t i m p r e g n a t i o n m e t h o d . T h e s l u r r y o f the s u p p o r t a n d metal nitrate s o l u t i o n w a s d e g a s s e d i n v a c u u m so that the n i t r a t e s o l u t i o n f u l l y f i l l e d the p o r e s o f the s u p p o r t d u r i n g i m p r e g n a t i o n . T h e i m p r e g n a t e d c a t a l y s t w a s d r i e d f o r 10 h o u r s at 100 ° C a n d t h e n c a l c i n e d for 3 h o u r s at 600 ° C . T h e catalysts tested i n this s t u d y are s h o w n i n T a b l e I. T h e catalysts w e r e c h a r a c t e r i z e d b y X - r a y p o w d e r d i f f r a c t i o n ( X R D ) for c r y s t a l l i n e p h a s e i d e n t i f i c a t i o n a n d B E T - N 2 d e s o r p t i o n for surface area measurement. W e l l crystallized C e 0 2 phase was identified b y X R D . The X R D a n a l y s i s a n d surface area m e a s u r e m e n t w e r e p e r f o r m e d o n a R i g a k u 300 X - r a y Diffractometer a n d F l o w S o r b Π 2300 M i c r o m e r i t i c s , r e s p e c t i v e l y . Results and Discussion C e 0 2 Catalyst. F i g u r e 1 s h o w s the e x p e r i m e n t a l results o f S O 2 r e d u c t i o n b y C O o n the b u l k C e 0 2 catalyst p r e p a r e d i n this w o r k ( T a b l e I). W h e n a feed gas o f l o w e r C O content t h a n the s t o i c h i o m e t r i c w a s u s e d , a n y S O 2 r e a c t e d w a s c o n v e r t e d i n t o e l e m e n t a l s u l f u r . W h e n the C O c o n t e n t e x c e e d e d the s t o i c h i o m e t r i c a m o u n t , 100% S O 2 c o n v e r s i o n w a s a c h i e v e d at l o w e r r e a c t i o n t e m p e r a t u r e , b u t C O S b e c a m e a m a j o r p r o d u c t . F o r e x a m p l e , w h e n the C O to S O 2 ratio w a s 3, the S O 2 c o n v e r s i o n w a s 100 % w i t h 36 - 62 % y i e l d of e l e m e n t a l s u l f u r o v e r the t e m p e r a t u r e r a n g e o f 530 to 700 ° C a n d c o r r e s p o n d i n g 64 - 38 % o f C O S y i e l d . T h e e l e m e n t a l s u l f u r y i e l d i n c r e a s e d w i t h the r e a c t i o n temperature. I n the f o l l o w i n g d i s c u s s i o n a n d f i g u r e s , o n l y the e l e m e n t a l s u l f u r y i e l d w i l l b e p r e s e n t e d . Because a feed gas of n e a r l y s t o i c h i o m e t r i c c o m p o s i t i o n w a s u s e d a n d the C O S f o r m a t i o n w a s a l w a y s n e g l i g i b l e i n these s t u d i e s , the e l e m e n t a l s u l f u r y i e l d c o r r e s p o n d s w i t h the S O 2 c o n v e r s i o n . D o p e d C e 0 2 Catalysts. It is w e l l k n o w n that the o x y g e n v a c a n c y a n d m o b i l i t y o f C e 0 2 c a n be e n h a n c e d b y i n t r o d u c i n g d i - o r t r i - v a l e n t m e t a l

Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

31.

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Reduction of S0 to Elemental S

i o n s i n t o its lattice. Y 2 O 3 d o p a n t w a s s t u d i e d b y W a n g , et al(15). their results are l i s t e d i n T a b l e Π. B o t h generated

the

activation enthalpy.

highest

Some of

the o x y g e n i o n i c c o n d u c t i v i t y a n d

a c t i v a t i o n e n t h a l p y v a r i e d w i t h the Y 2 O 3 d o p a n t c o n c e n t r a t i o n . dopant

379

2

oxygen

conductivity and

1% Y 2 O 3

the

lowest

Y 2 U 3 - d o p e d c e r i u m o x i d e c a t a l y s t s ( T a b l e I) w e r e

p r e p a r e d a n d tested i n this w o r k . F i g u r e 2 s h o w s the effect o f the Y 2 O 3 d o p a n t o n the catalyst a c t i v i t y . T h e 1% Y 2 O 3 d o p e d ceria catalyst ( C e 0 2 ( Y ) ) s h o w e d h i g h e r a c t i v i t y t h a n either the p u r e C e 0 2 o r a 10% Y 2 0 3 - d o p e d ceria(CeO2(10Y)). Thus, more

than

95% elemental

sulfur

yield

was

o v e r the C e 0 2 ( Y ) at 600 ° C , that i s , at 50 ° C l o w e r t e m p e r a t u r e

obtained

t h a n for the o t h e r t w o catalysts. W h e n t w o t r i - v a l e n t m e t a l i o n s are i n t r o d u c e d i n the C e 0 2 c r y s t a l structure, o n e o x y g e n v a c a n c y is created, [ ] + 2 M H M - [ ]-M, where [ ]

a n d M d e n o t e the o x y g e n v a c a n c y a n d

respectively.

The

dopant

ion

and

created

trivalent dopant cation, oxygen

vacancy

form

e n e r g e t i c a l l y associated p a i r s as d e n o t e d b y M - [ ] - M . D i f f e r e n t d o p a n t s b r i n g a b o u t different c r y s t a l s t r u c t u r e c h a n g e ion pair,

will

a n d i n t e r a c t i o n of v a c a n c y -

t h u s r e s u l t i n g i n different o x y g e n m o b i l i t y . T a b l e III lists s o m e

experimental

results

from

the

studies

by

Gerhardt-Anderson

and

N o w i c k f l 6 , ) , together w i t h the c a l c u l a t e d a s s o c i a t i o n e n t h a l p y b y Butler, et al(17). T h e 1% SC2O3 d o p a n t i n C e 0 2 g e n e r a t e d the strongest v a c a n c y - i o n a s s o c i a t i o n a n d h a d the l o w e s t o x y g e n c o n d u c t i v i t y a m o n g t r i v a l e n t i o n d o p e d c e r i a . T h e C e 0 2 ( L a ) h a d c o m p a r a b l e o x y g e n c o n d u c t i v i t y to C e 0 2 ( Y ) at l o w t e m p e r a t u r e , b u t h a d a l i t t l e l o w e r a s s o c i a t i o n e n t h a l p y t h a n

the

C e 0 2 ( Y ) . T h e e x p e r i m e n t a l r e s u l t s o f S O 2 r e d u c t i o n b y C O o n the b u l k c e r i u m o x i d e catalyst d o p e d w i t h these elements are c o m p a r e d i n F i g u r e 3 o n t h e s a m e t o t a l surface a r e a b a s i s . M o r e t h a n 9 5 % s u l f u r y i e l d

was

o b t a i n e d at 600 ° C for a l l three catalysts. W h e n the r e a c t i o n t e m p e r a t u r e w a s l o w e r e d , the s u l f u r y i e l d s o n the C e 0 2 ( S c ) a n d C e 0 2 ( L a ) catalysts f e l l off s t e e p l y at a b o u t 600 ° C a n d 560 ° C , r e s p e c t i v e l y , w h i l e the sulfur y i e l d o n the C e 0 2 ( Y ) catalyst d e c r e a s e d g r a d u a l l y f r o m 600 ° C T h e C e 0 2 ( L a ) catalyst s h o w e d the highest a c t i v i t y . S h o w n i n F i g u r e 4 is the l i g h t - o f f b e h a v i o r of the r e a c t i o n o f S O 2 w i t h C O o v e r different catalysts. F o r this s t u d y , w e started w i t h the fresh catalyst w i t h o u t a n y p r e t r e a t m e n t b y a r e d u c i n g a g e n t a n d r a i s e d the r e a c t i o n temperature

from

500

to

700 ° C i n a 50 ° C - s t e p ,

temperature for half an hour. temperature(around

holding

650 ° C ) o v e r a l l the L a 2 Û 3 - d o p e d c e r i a

i n d e p e n d e n t o f d o p a n t content a n d the a m o u n t o f catalyst reaction d i d not light-off h i g h as 690 ° C

at

each

T h e r e a c t i o n w a s l i g h t e d off at the s a m e used.

catalysts, B u t , the

o n the C e 0 2 ( S c ) catalyst e v e n at t e m p e r a t u r e s as

A f t e r o n e - h o u r h e a t i n g u n d e r r e a c t i n g a t m o s p h e r e at 690

Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

ENVIRONMENTAL CATALYSIS

380

Table I. Catalysts Tested in This Study Surface Area Catalyst 1. 2. 3. 4. 5. 6. 7. 8. 9.

Composition Ce02

Ce02(Y) CeO2(10Y) Ce