Chapter 31
Reduction of SO to Elemental S over Ceria Catalysts 2
Downloaded via TUFTS UNIV on July 11, 2018 at 11:24:00 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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.
ENVIRONMENTAL CATALYSIS
376
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.
LIU & FLYTZANI-STEPHANOPOULOS
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.
LIU & FLYTZANI-STEPHANOPOULOS
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
