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Metal-Doped Zeolites for Selective Catalytic Reduction of Nitrogen Oxides in Combustion Gases
H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
METAL-DOPED ZEOLITES FOR SELECTIVE CATALYTIC REDUCTION O F NITROGEN O X I D E S I N COMBUSTION GASES
J . HAAS. J . STEINWANDEL and C. PLOG Dornier S y s t e m GmbH, P. 0. Box 1360, D-7990 F r i e d r i c h s h a f e n (FRG)
ABSTRACT
In t h e p r e s e n t work i t is d e m o n s t r a t e d t h a t t h e a c t i v i t y of z e o l i t e s c o n c e r n i n g t h e S C R - p r o c e s s can be i n c r e a s e d s i g n i f i c a n t l y by m e t a l d o p i n g . T h e SCR c a t a l y s t s p r e p a r e d by m e t a l ion e x c h a n g e o f m o r d e n i t e a n d Y-zeolite w e r e t e s t e d i n a l a b o r a t o r y p l a n t t o d e t e r m i n e t h e efficiency of t h e c o n v e r s i o n of n i t r i c o x i d e s w i t h ammonia. C o p p e r - d o p e d Y - z e o l i t e c a n b e u s e d a s a SCR c a t a l y s t i n a wide t e m p e r a t u r e r a n g e . T h e r e f o r e , t h e o p e r a t i o n is p o s s i b l e a s well in a l o w - d u s t region, s u c h a s behind a d e s u l p h u r i z a t i o n p l a n t . A c o m p l i c a t e d t e m p e r a t u r e c o n t r o l f o r o p t i m u m c o n v e r s i o n is not n e c e s s a r y . A l s o t h e e x c e l l e n t a m m o n i a s t o r a g e c a p a c i t i e s of t h e z e o l i t e c a t a l y s t s c a n be d e m o n s t r a t e d . T h e r e f o r e , c h a n g e s i n t h e ammonia f e e d o r t h e NO c o n t e n t in t h e exh a u s t g a s c a n be c o m p e n s a t e d .
1NTRODUCTlON Due to t h e p u b l i c d i s c u s s i o n c o n c e r n i n g new t y p e s of e n v i r o n m e n t d a m a g e ( W a l d s t e r b e n ) , a r a p i d d e v e l o p m e n t of t e c h n o l o g i e s h a s s t a r t e d i n t h e FRG c o n c e r n i n g t h e r e d u c t i o n of p o l l u t a n t s f r o m e x h a u s t g a s e s . M e a n w h i l e , p r o c e s s e s c o n c e r n i n g d e s u l p h u r i s a t i o n have b e e n i n t r o d u c e d w i d e l y . In c o n t r a s t , t h e various t e c h n o l o g i e s f o r n i t r i c o x i d e r e d u c t i o n a r e s t i l l u n d e r i n v e s t i gation.
In t h e USA a s well a s t h e FRG, n o t only t h e w e l l - e s t a b l i s h e d ( s i n c e 1970) J a p a n e s e p r o c e s s e s a r e t a k e n i n t o a c c o u n t . T h e J a p a n e s e t e c h n o l o g y o f NO r e d u c t i o n by u s i n g modified TiO, c a t a l y s t s s h o u l d b e r e p l a c e d by a l t e r n a t i v e s o l u t i o n s . C o n c e r n i n g t h e maximum e m i s s i o n v a l u e s ( N O x ) given by g o v e r n m e n t a u t h o r i t y a n d mainly r e l a t e d to l a r g e h e a t i n g p l a n t s (e.g. b l o c k h e a t i n g p o w e r p l a n t s ) , primary p r o c e s s e s d o not lead t o s a t i s f y i n g r e s u l t s . I t is ins t e a d n e c e s s a r y t o d e v e l o p new p r o c e s s e s f o r r e d u c i n g NOx e m i s s i o n s : f o r example wet p r o c e s s e s and reductive processes (using ammonia) m u s t be t a k e n i n t o a c c o u n t . T a b l e 1 s h o w s a s e l e c t i o n of s u c h p r o c e s s e s a l r e a d y u n d e r investigation.
338 TABLE 1 Selection o f secondary t r e a t m e n t s for NOx reduction.
_ _ _ _ _ _ _ _ _ _
* * .:
* * *
* *
Selective noncatalytic reduction (SNCR) Selective catalytic reduction (SCR) Active carbon process Oxidation/Absorption process Electron beam process Dry sorption Sulphonic a c i d l n i t r i c acid process Complex s a l t process
________----__-------------------------------------Oxidative methods produce nitrates or nitric acid, and problems arise concerning the back-transfer of t h e products into t h e product cycle. Therefore, t h e m o s t practicable methods consist of reductive processes, which finally produce nitrogen. For t h e selective reduction of nitric oxides by ammonia a t acceptable temperatures and conversion r a t e s , the use of catalytic processes (SCR) i s essential. A possible reaction scheme is shown i n table 2. TABLE 2 Selection of possible partial reactions, SCR process Selective Reduction:
4 N H 3 + 6 N 0 ---D 4 NH, + 4 NO + 0,
5 N,
--*
6 H,O 4 N, + 6 H,O
+
Production o f N,O:
2 NH,
+ 2 0, ----0 8 NH, + 12 N O + S 0,
Oxidation of ammonia: 4 NH, + 7 0, --+
4NH,+50, 4NH,+30,
0
--
N,O + 3 H,O 10 N,O + 12 H,O
--+
4 NO, + 6 H,O 4NO+6H20 2N2+6H,O
339 In view of t h e h e t e r o g e n e o u s l y c a t a l y z e d r e a c t i o n s , v a r i o u s t y p e s o f c a t a -
l y s t s have a l r e a d y been d e v e l o p e d . The c a t a l y s t t y p e is mainly r e l a t e d t o t h e r e q u i r e d l o c a t i o n in t h e p r o c e s s . In p r i n c i p l e , t h r e e p o s s i b l e c a t a l y s t l o c a t i o n s m u s t be t a k e n i n t o a c c o u n t , a s s h o w n in Fig. 1. F o r e x a m p l e , p r o c e s s e s a r e k n o w n , whereby nitric o x i d e s a r e r e d u c e d by a n o b l e - m e t a l - c o n t a i n i n g c a t a l y s t a t c o m p a r a b l y l o w t e m p e r a t u r e s , producing n i t r o g e n . H o w e v e r , r e l a t i v e l y high n o b l e - m e t a l c o s t s p o s e s o m e r e s t r i c t i o n s f o r t h e p r o c e s s . In a d d i t i o n , t h e c a t a l y s t s a r e s u b j e c t t o p o i s o n i n g by s u l p h u r d i o x i d e in t h e e x h a u s t g a s . T h e r e a r e c h e a p e r a n d m o r e s t a b l e c a t a l y s t s b a s e d on j a p a n e s e t e c h n o mainly c o n s i s t i n g of vanadium and t i t a n i u m o x i d e s . A g e n e r a l d i s a d v a n t a g e of t h e s e c a t a l y s t s , however, is t h e heavy-metal c o n t e n t of u s e d c a t a -
l y s t s . P r o c e s s e s a r e known which employ z e o l i t e c a t a l y s t s . A n a d v a n t a g e of m o l e c u l a r sieve c a t a l y s t s is t h e s t o r a g e c a p a b i l i t y of a m m o n i a . By t a k i n g adv a n t a g e of t h i s e f f e c t , s h o r t t i m e v a r i a t i o n s of t h e a m m o n i a c o n c e n t r a t i o n c a n be c o m p e n s a t e d . The p r o c e s s e s , however, r e q u i r e r e l a t i v e l y high o p e r a t i o n t e m p e r a t u r e s ( u p t o 48OoC), a n d t h e r e is no c o m p l e t e n i t r i c o x i d e c o n v e r s i o n due t o t h e comparatively reaction-inactive z e o l i t e c a t a l y s t s .
In t h e p r e s e n t work it is d e m o n s t r a t e d t h a t t h e a c t i v i t y of z e o l i t e s c o n c e r n i n g t h e S C R p r o c e s s c a n b e i n c r e a s e d s i g n i f i c a n t l y by m e t a l d o p i n g . T h e r e f o r e , t h e u s e of t h e c a t a l y s t a t l o w e r t e m p e r a t u r e s will b e p o s s i b l e .
Vessel
1 2 3
1
Dust removal
2 -'
Desulph. plant
High-Dust catalyst Low-Dust catalyst LT catalyst
Fig. 1. P o s s i b l e l o c a t i o n s of SCR c a t a l y s t s .
3 8 -
Flue
340 EXPERIMENTAL
A s s t a r t i n g m a t e r i a l s f o r t h e p r e p a r a t i o n of t h e S C R - c a t a l y s t s , c o m m e r c i a l ly a v a i l a b l e z e o l i t e s of m o r d e n i t e t y p e a n d Y - z e o l i t e s w e r e u s e d ( B a y e r A C a n d N o r t o n C o m p . ) . T h e d o p i n g of t h e z e o l i t e s w i t h m e t a l s w a s p e r f o r m e d by t h e m e t h o d of i o n e x c h a n g e c i t e d i n t h e l i t e r a t u r e . S C R c a t a l y s t s can b e used a s b u l k - c o n t a c t honeycomb c a t a l y s t s o r a s c o a t ed h o n e y c o m b c a t a l y s t s . From t h e p o i n t of v i e w of p r o c e s s d e v e l o p m e n t , w e d e c i d e d t o use t h e c o a t e d h o n e y c o m b t y p e . A f t e r t h e p r e p a r a t i o n of t h e c a t a lytic a c t i v e p o w d e r s by ion e x c h a n g e a n d s u b s e q u e n t t h e r m a l t r e a t m e n t o r r e d u c t i o n , c o r d i e r i t e h o n e y c o m b s w e r e c o a t e d w i t h t h e z e o l i t e p o w d e r s by a s p e c i a l d e v e l o p e d p r o c e s s . The g e o m e t r y of t h e c o r d i e r i t e h o n e y c o m b s c o r r e s ponded t o t h a t u s u a l l y used i n t h e a u t o m o b i l e i n d u s t r y ( t e s t m o n o l i t e s , 3.15 inch l o n g , 1 inch d i a m e t e r , 400 S q . c e l l s p e r S q . i n c h ) .
The SCR c a t a l y s t s p r e p a r e d by t h e m e t h o d d e s c r i b e d a b o v e w e r e t e s t e d in a l a b o r a t o r y p l a n t t o d e t e r m i n e t h e e f f i c i e n c y of t h e c o n v e r s i o n of n i t r i c o x i d e s w i t h ammonia. A s a m o d e l s u b s t a n c e f o r t h e g r o u p of n i t r i c o x i d e s , NO w a s used. T h i s can b e j u s t i f i e d b e c a u s e t h e NO c o n t e n t of n o r m a l e x h a u s t g a s is
a b o u t 9 0 %of t h e N O x . The l a b o r a t o r y p l a n t c o n s i s t s of a g a s - m i x i n g s t a t i o n , a g a s - h e a t i n g s y s t e m , a r e a c t i o n c h a m b e r c o n t a i n i n g t h e c a t a l y s t , and f i n a l l y a gas-analysis system. By u s i n g a g a s c y l i n d e r b a t t e r y a n d e l e c t r o n i c m e a s u r e m e n t a n d c o n t r o l c o m p o n e n t s i t is p o s s i b l e to vary t h e e x h a u s t g a s c o m p o s i t i o n o v e r a w i d e r a n g e . T h e s y n t h e t i c e x h a u s t g a s c o n t a i n s a m a x i m u m of 9 d i f f e r e n t c o m p o n e n t s . The t e s t p l a n t a l l o w s f o r a maximum gas v o l u m e s t r e a m u p t o SO I / m i n . The s t a n d a r d c o m p o s i t i o n of t h e s y n t h e t i c e x h a u s t g a s is s h o w n i n t a b l e 3. TABLE 3 S t a n d a r d c o m p o s i t i o n of e x h a u s t g a s f o r t h e p r e s e n t e x p e r i m e n t s .
79.86 10.0
4.0 6.0
0.07 0.07
341 T h e g a s h e a t i n g s y s t e m p e r m i t s e x h a u s t t e m p e r a t u r e s b e t w e e n 150 a n d 450 OC. P r o g r a m m a b l e h e a t i n g using s t a n d a r d i z e d t e m p e r a t u r e p r o g r a m s a l l o w s f o r m e a s u r e m e n t s o f t h e c a t a l y s t behaviour w i t h r e l a t i o n t o t h e d i f f e r e n t e x h a u s t - g a s t e m p e r a t u r e s . T h e g a s c o m p o s i t i o n d o w n s t r e a m a n d ups t r e a m of t h e c a t a l y t i c c o n v e r t e r is c o n t r o l l e d by t h e g a s - a n a l y s i s s y s t e m . The s y s t e m c o n s i s t s o f a GC w i t h FID a n d HCD d e t e c t o r s , a n d a c h e m i l u m i n e s c e n c e NOx a n a l y z e r . T h e chemical c o m p o s i t i o n of t h e c a t a l y t i c a c t i v e p o w d e r s w a s a n a l y z e d by
EDX. The i n f o r m a t i o n d e p t h
o f EDX is a b o u t 1 t o 2 pm. W i t h a c r y s t a l l i t e s i z e
of t h e z e o l i t e s o f 1 t o 4 pm, t h e volume c o m p o s i t i o n is a l s o given by t h e EDX method.
RESULTS A N D DISCUSSION A s a l r e a d y s t a t e d i n t h e i n t r o d u c t i o n , t h e s t a t e of t h e a r t c o n c e r n i n g n o b l e
m e t a l c a t a l y s t s a l l o w s f o r a c c e p t a b l e n i t r i c o x i d e c o n v e r s i o n a t low t e m p e r a t u r e s . A s is a l r e a d y known f r o m work o n t h r e e - w a y c a t a l y s t s ’),
i t is p o s s i b l e ,
by f i n e d i s p e r s i o n of p l a t i n u m i n n e a r - s u r f a c e r e g i o n s of m o r d e n i t e c r y s t a l l i t e s , t o p r o d u c e a c a t a l y s t c h a r a c t e r i z e d by high a c t i v i t y w i t h low n o b l e m e t a l c o n t e n t . I n a d d i t i o n , d u e t o t h e a d s o r p t i o n of a m m o n i a m o l e c u l e s a t t h e acid z e o l i t e s i t e s , t h e r e e x i s t s t h e a d d i t i o n a l e f f e c t of s y n e r g y . T h e r e f o r e , we s t a r t e d t h e i n v e s t i g a t i o n by using P t - d o p e d m o r d e n i t e a s SCR c a t a l y s t , b e c a u s e a r a t h e r low o p e r a t i o n t e m p e r a t u r e in c o n j u n c t i o n w i t h
high c o n v e r s i o n r a t e s s h o u l d t h u s be e x p e c t e d . Fig. 2. s h o w s t h e n i t r i c o x i d e c o n v e r s i o n a s a f u n c t i o n o f t h e g a s t e m p e r a t u r e f o r t w o d i f f e r e n t c a t a l y s t s of t y p e P t H - m o r d e n i t e . C a t a l y s t s w e r e p r e p a r e d f o r i d e n t i c a l Pt c o n t e n t of a b o u t 0.2 g/l.
From Fig. 2. it is s e e n , t h a t f o r b o t h c a t a l y s t s a l m o s t 100%n i t r i c o x i d e c o n version o c c u r s . H o w e v e r , t h e t e m p e r a t u r e r a n g e f o r o p t i m u m c o n v e r s i o n , b u t a l s o i n c r e a s e a n d d e c r e a s e of t h e conversion s l o p e a r e q u i t e d i f f e r e n t . T h i s is d u e t o t h e d i f f e r e n t S i / A l - r a t i o of t h e t w o m o r d e n i t e c a t a l y s t s . T h e m o r d e n i t e o f c a t a l y s t (11) w a s d e a l u m i n a t e d by acid t r e a t m e n t b e f o r e P t - d o p i n g . T h i s
r e s u l t e d i n a m o d u l u s ( m o l a r r a t i o SiO,/AI,O,)
of a b o u t 2 0 . T h e m o r d e n i t e of
c a t a l y s t ( 1 ) w a s n o t modified ( m o d u l u s o f a b o u t 12). By t h e dealumiiiatioii of t h e anionic l a t t i c e of t h e moi-denite in conji:::cti:,-
with a s i n i u l t a n e o u s c h a n g e o f t h e s t e r i c c o n d i t i o n s in t h e p o r e s y s t e m . a c h a n g e i n a c i d i t ) r e s u l t s . C h a n g e s i n s p e c i f i c s u r f a c e w e r e d e t e r m i n e d b)
342 using BET. T h e l o w e r c o n t e n t of n e g a t i v e p a r t i a l c h a r g e s o f t h e l a t t i c e a f t e r d i s s o l v i n g t h e a l u m i n u m r e s u l t s i n a d e c r e a s e in n u m b e r o f acid s i t e s . T h e i r s t r e n g t h , h o w e v e r , i n c r e a s e s . Both e f f e c t s i n f l u e n c e t h e n u m b e r of a d s o r p t i o n c e n t r e s f o r a m m o n i a a s well a s t h e d i f f u s i o n i n t o t h e p o r e s y s t e m . T h i s r e s u l t s i n a much b e t t e r c o n v e r s i o n c h a r a c t e r i s t i c f o r t h e d e a l u m i n a t e d P t H - m o r d e n i t e .
0 1I
150
i
I
200 Temperature
250
300
["Cl
Fig. 2. Activity b e h a v i o u r of P t H - m o r d e n i t e o f d i f f e r e n t m o d u l i a s a f u n c t i o n of g a s t e m p e r a t u r e . C a t a l y s t (1) s h o w s 90%c o n v e r s i o n o v e r o n l y a r e l a t i v e l y n a r r o w t e m p e r a t u r e r a n g e , a n d t h e a c t i v i t y s t r o n g l y d e c r e a s e s a t l o w e r o r h i g h e r t e m p e r a t u r e s . In c o n t r a s t , t h e d e a l u m i n a t e d c a t a l y s t s (11) s h o w s a l m o s t 100%c o n v e r s i o n b e t ween 170 and 200 O C . I n a d d i t i o n , t h e maximum c o n v e r s i o n i s m a i n t a i n e d a l s o a t h i g h e r g a s t e m p e r a t u r e s . T h e d e c r e a s e in c o n v e r s i o n f o r b o t h c a t a l y s t s a t h i g h e r t e m p e r a t u r e s is r e l a t e d t o t h e onset of t h e N O - p r o d u c i n g a m m o n i a o x i d a t i o n . T h i s r e s u l t s in a s i g n i f i c a n t d e c r e a s e of t o t a l NO c o n v e r s i o n . C a t a l y s t (11) s h o w s a h i g h e r
SCR a c t i v i t y a t l o w e r t e m p e r a t u r e s a n d a l o w e r a c t i v i t y
(desired) concerning ammonia oxidation a t higher t e m p e r a t u r e s . A s an a l t e r n a t i v e t o noble metal containing z e o l i t e c a t a l y s t s f o r selective
r e d u c t i o n of n i t r i c o x i d e , z e o l i t e c a t a l y s t s d o p e d w i t h t r a n s i t i o n m e t a l s o f t h e
343 p e r i o d i c s y s t e m w e r e p r o d u c e d . A s b a s i s m a t e r i a l s , Y-zeolite a n d m o r d e n i t e w e r e u s e d . Z e o l i t e s w e r e d o p e d w i t h e i t h e r vanadium o r c o p p e r . F i r s t , vanadium is known a s a c t i v e c o m p o n e n t of t h e T i 0 2 - b a s e d S C R - c a t a l y s t of J a p a n e s e t e c h n o l o g y . C o p p e r , o n t h e o t h e r h a n d , is k n o w n to a c t a s c e n t r a l ion in a m monia c o m p l e x e s , r e s u l t i n g i n a d d i t i o n a l s t o r a g e o f a m m o n i a in t h e z e o l i t e .
n
P
Y
!
'C
i Y
i 3
Fig. 3. C o n v e r s i o n c h a r a c t e r i s t i c of d i f f e r e n t d o p e d z e o l i t e s a s a f u n c t i o n o f t h e gas temperature. Fig. 3. s h o w s a s i g n i f i c a n t d i f f e r e n c e in t h e a c t i v i t y b e h a v i o u r o f t h e t h r e e m e t a l - d o p e d z e o l i t e s a s a f u n c t i o n of e x h a u s t t e m p e r a t u r e . I t c a n b e s e e n t h a t t h e H-mordenite (vanadium-doped) d o e s n o t show significant activity in cont r a s t t o t h e Pt-doped f o r m . Over t h e e n t i r e t e m p e r a t u r e r a n g e 200 t o 400 OC, o n l y NO-conversion r a t e s of 10 X a r e r e a c h e d . Vanadium is p r e s e n t in t h e z e o lite in a c o m p l e t e l y d i f f e r e n t f o r m , a s is t h e c a s e f o r t h e TiO, c a t a l y s t . T h e r e f o r e , vanadium c a n n o t play t h e identical role. In a d d i t i o n , i t is o b v i o u s t h a t t h e H - m o r d e n i t e a l o n e e x h i b i t s e x t r e m e l y l o w a c t i v i t y a s a n SCR c a t a l y s t .
344
I n c o n t r a s t t o vanadium, c o p p e r a s doping metal s h o w s a positive e f f e c t f o r t h e H - m o r d e n i t e . A t t e m p e r a t u r e s up t o 300 OC, t h e r e is n o d i f f e r e n c e b e t w e e n t h e t w o c a t a l y s t s . A t t e m p e r a t u r e s a b o v e 300 OC, h o w e v e r , a s i g n i f i c a n t inc r e a s e i n t h e s e l e c t i v e r e d u c t i o n of n i t r o g e n o x i d e is f o u n d f o r t h e CuHm o r d e n i t e . A t 380 OC a maximum c o n v e r s i o n o f a b o u t 50 % is r e a c h e d . A f u r t h e r i n c r e a s e i n t e m p e r a t u r e , however, r e s u l t s in a s t r o n g d e c r e a s e of NO c o n version. T h i s is d u e t o t h e r a p i d i n c r e a s e o f NH, o x i d a t i o n . Above 400 OC, more ammonia molecules a r e reacting with oxygen t h a n with nitrogen oxide, f i n a l l y r e s u l t i n g in a n i n c r e a s e of NO e m i s s i o n in t h e t o t a l mass b a l a n c e . T h e p a r a l l e l r e a c t i o n o f a m m o n i a o x i d a t i o n c a n b e r e d u c e d by u s i n g a c o p p e r - d o p e d Y - z e o l i t e . a s s h o w n in f i g . 3. T h i s p a r t i c u l a r c a t a l y s t c o m b i n e s t h e c a p a b i l i t y of r e d u c i n g ammonia o x i d a t i o n w i t h a high a c t i v e NO r e d u c t i o n . T h i s l e a d s t o a r e l a t i v e l y high c o n v e r s i o n p l a t e a u o v e r a w i d e t e m p e r a t u r e r a n g e . Between 250 a n d 400 OC NO c o n v e r s i o n is b e t t e r t h a n 80 %.
1
I
I
Fig. 4 . C o m p a r i s o n of c o n v e r s i o n c h a r a c t e r i s t i c f o r d i f f e r e n t l y p r e p a r e d c o p per-doped Y-zeolites.
345
?., 9,? n
I
Y
6n -
I
i
'k;
i
\
\
I I
1
\
k
I !
\
i
I
\\
1:
Time iminl
Fig. 5. C o n v e r s i o n r a t e of t h e r e a c t i o n NO r u p t i o n o f t h e ammonia f e e d .
+
NH, v e r s u s t i m e ( m i n ) a f t e r i n t e r -
T h i s h i g h e r p o t e n t i a l o f t h e c o p p e r - d o p e d Y-zeolite a s SCR c a t a l y s t c a n a l s o b e s e e n f r o m fig. 4 . H e r e , NO c o n v e r s i o n i s p l o t t e d a g a i n v e r s u s g a s t e m p e r a t u r e , and t w o d i f f e r e n t l y p r e p a r e d CuNaY-zeolites a r e c o m p a r e d . I t c a n b e s e e n t h a t by o p t i m i z i n g t h e p r e p a r a t i o n t e c h n i q u e a f u r t h e r i n c r e a s e in c o n v e r s i o n c h a r a c t e r i s t i c s is p o s s i b l e . T h e c a t a l y s t CuNaY(2) s h o w s m o r e t h a n 80 % c o n v e r s i o n over a t e m p e r a t u r e r a n g e 2 0 0 - 400 OC. Between 250 a n d 390 OC a n a l m o s t 100 % c o n v e r s i o n of n i t r i c o x i d e is f o u n d .
-
T h e r e f o r e , t h e c o p p e r - d o p e d Y-zeolite c a n b e o p e r a t e d a s SCR c a t a l y s t a t l e a s t o n a l a b o r a t o r y s c a l e - t o b e u s e d o v e r a wide t e m p e r a t u r e r a n g e . O p e r a t i o n is p o s s i b l e a s w e l l in a l o w - d u s t r e g i o n s u c h a s b e h i n d t h e d e s u l p h u r i z a -
t i o n p l a n t , w h e r e t h e g a s t e m p e r a t u r e is a l r e a d y r e d u c e d s i g n i f i c a n t l y . A c o m -
p l i c a t e d t e m p e r a t u r e c o n t r o l f o r o p t i m u m c o n v e r s i o n is n o t n e c e s s a r y d u e t o t h e wide t e m p e r a t u r e w i n d o w of t h e c a t a l y s t .
346
An a d d i t i o n a l i m p o r t a n t c h a r a c t e r i s t i c of t h e z e o l i t e SCR c a t a l y s t s is s h o w n in f i g . 5. H e r e , T h e o p t i m i z e d C u N a Y - z e o l i t e w a s u s e d . During a l o n g - t e r m o p e r a t i o n a t a n e x h a u s t g a s t e m p e r a t u r e of 300 OC, t h e a m m o n i a f e e d w a s i n t e r r u p t e d , w h i l e t h e NO f e e d w a s m a i n t a i n e d c o n s t a n t . Fig. 5 s h o w s NO c o n v e r s i o n v e r s u s t i m e ( m i n ) a f t e r i n t e r r u p t i o n of t h e a m m o n i a f e e d . T h e e x c e l l e n t a m m o n i a s t o r a g e c a p a b i l i t y of t h e z e o l i t e , in p a r t i c u l a r t h e c o p p e r - d o p e d Y - z e o l i t e , c a n b e d e m o n s t r a t e d . Even u p t o 3 min a f t e r a m m o n i a f e e d i n t e r u p t i o n , a 100 % c o n v e r s i o n o f NO is m a i n t a i n e d . A f t e r t h i s t i m e t h e r e a c t i o n e f f i c i e n c y d e c r e a s e s . H o w e v e r , even a f t e r 10 m i n , t h e r e is still 2 0 % NO c o n v e r s i o n . A f t e r 2 0 min, t h e r e a c t i o n e f f i c i e n c y is z e r o . By u s i n g t h e e x c e l l e n t s t o r a g e c a p a b i l i t y , c h a n g e s in t h e a m m o n i a f e e d o r t h e NO c o n t e n t in t h e e x h a u s t g a s c a n be c o m p e n s a t e d . This h o l d s a l s o f o r s h o r t - t i m e c e s s a t i o n of t h e a m m o n i a f e e d .
T h e d i f f e r e n t l y modified z e o l i t e SCR c a t a l y s t s d e s c r i b e d i n t h i s w o r k c a n be u s e d f o r d i f f e r e n t o p e r a t i n g c o n d i t i o n s . S u c h a d v a n t a g e s a s e c o n o m i c a l basis s u b s t a n c e , excellent activity behaviour ( i n p a r t over wide t e m p e r a t u r e r a n g e s ) a n d , i n p a r t i c u l a r , t h e unique s t o r a g e c a p a b i l i t y f o r t h e r e a c t i o n c o m p o n e n t a m m o n i a have been d e m o n s t r a t e d . I n a d d i t i o n , d u e t o t h e c o m p o s i t i o n of t h e z e o l i t e s , t h e d i s p o s a l of used c a t a l y s t s is l e s s t r o u b l e s o m e c o m p a r e d
w i t h heavy-metal-containing
TiO, c a t a l y s t s .
ACKNOWLEDGEMENTS The a u t h o r s g r a t e f u l l y a c k n o w l e d g e financial s u p p o r t by t h e B u n d e s m i n i s t e r f u r F o r s c h u n g und T e c h n o l o g i e .
REFERENCES
1
DE-OS 26 17 744.
2
4
US.-PS 3,279,884. U.S.-PS 4,010,238. EPA-A1 0 161 743
5
H a a s . J . . C. Plog a n d J . S t e i n w a n d e l , S t u d . S u r f . Sci. C a t a l . . i n p r e s s
3
METAL-DOPED ZEOLITES FOR SELECTIVE CATALYTIC REDUCTION O F NITROGEN O X I D E S I N COMBUSTION GASES
J . HAAS. J . STEINWANDEL and C. PLOG Dornier S y s t e m GmbH, P. 0. Box 1360, D-7990 F r i e d r i c h s h a f e n (FRG)
ABSTRACT
In t h e p r e s e n t work i t is d e m o n s t r a t e d t h a t t h e a c t i v i t y of z e o l i t e s c o n c e r n i n g t h e S C R - p r o c e s s can be i n c r e a s e d s i g n i f i c a n t l y by m e t a l d o p i n g . T h e SCR c a t a l y s t s p r e p a r e d by m e t a l ion e x c h a n g e o f m o r d e n i t e a n d Y-zeolite w e r e t e s t e d i n a l a b o r a t o r y p l a n t t o d e t e r m i n e t h e efficiency of t h e c o n v e r s i o n of n i t r i c o x i d e s w i t h ammonia. C o p p e r - d o p e d Y - z e o l i t e c a n b e u s e d a s a SCR c a t a l y s t i n a wide t e m p e r a t u r e r a n g e . T h e r e f o r e , t h e o p e r a t i o n is p o s s i b l e a s well in a l o w - d u s t region, s u c h a s behind a d e s u l p h u r i z a t i o n p l a n t . A c o m p l i c a t e d t e m p e r a t u r e c o n t r o l f o r o p t i m u m c o n v e r s i o n is not n e c e s s a r y . A l s o t h e e x c e l l e n t a m m o n i a s t o r a g e c a p a c i t i e s of t h e z e o l i t e c a t a l y s t s c a n be d e m o n s t r a t e d . T h e r e f o r e , c h a n g e s i n t h e ammonia f e e d o r t h e NO c o n t e n t in t h e exh a u s t g a s c a n be c o m p e n s a t e d .
1NTRODUCTlON Due to t h e p u b l i c d i s c u s s i o n c o n c e r n i n g new t y p e s of e n v i r o n m e n t d a m a g e ( W a l d s t e r b e n ) , a r a p i d d e v e l o p m e n t of t e c h n o l o g i e s h a s s t a r t e d i n t h e FRG c o n c e r n i n g t h e r e d u c t i o n of p o l l u t a n t s f r o m e x h a u s t g a s e s . M e a n w h i l e , p r o c e s s e s c o n c e r n i n g d e s u l p h u r i s a t i o n have b e e n i n t r o d u c e d w i d e l y . In c o n t r a s t , t h e various t e c h n o l o g i e s f o r n i t r i c o x i d e r e d u c t i o n a r e s t i l l u n d e r i n v e s t i gation.
In t h e USA a s well a s t h e FRG, n o t only t h e w e l l - e s t a b l i s h e d ( s i n c e 1970) J a p a n e s e p r o c e s s e s a r e t a k e n i n t o a c c o u n t . T h e J a p a n e s e t e c h n o l o g y o f NO r e d u c t i o n by u s i n g modified TiO, c a t a l y s t s s h o u l d b e r e p l a c e d by a l t e r n a t i v e s o l u t i o n s . C o n c e r n i n g t h e maximum e m i s s i o n v a l u e s ( N O x ) given by g o v e r n m e n t a u t h o r i t y a n d mainly r e l a t e d to l a r g e h e a t i n g p l a n t s (e.g. b l o c k h e a t i n g p o w e r p l a n t s ) , primary p r o c e s s e s d o not lead t o s a t i s f y i n g r e s u l t s . I t is ins t e a d n e c e s s a r y t o d e v e l o p new p r o c e s s e s f o r r e d u c i n g NOx e m i s s i o n s : f o r example wet p r o c e s s e s and reductive processes (using ammonia) m u s t be t a k e n i n t o a c c o u n t . T a b l e 1 s h o w s a s e l e c t i o n of s u c h p r o c e s s e s a l r e a d y u n d e r investigation.
338 TABLE 1 Selection o f secondary t r e a t m e n t s for NOx reduction.
_ _ _ _ _ _ _ _ _ _
* * .:
* * *
* *
Selective noncatalytic reduction (SNCR) Selective catalytic reduction (SCR) Active carbon process Oxidation/Absorption process Electron beam process Dry sorption Sulphonic a c i d l n i t r i c acid process Complex s a l t process
________----__-------------------------------------Oxidative methods produce nitrates or nitric acid, and problems arise concerning the back-transfer of t h e products into t h e product cycle. Therefore, t h e m o s t practicable methods consist of reductive processes, which finally produce nitrogen. For t h e selective reduction of nitric oxides by ammonia a t acceptable temperatures and conversion r a t e s , the use of catalytic processes (SCR) i s essential. A possible reaction scheme is shown i n table 2. TABLE 2 Selection of possible partial reactions, SCR process Selective Reduction:
4 N H 3 + 6 N 0 ---D 4 NH, + 4 NO + 0,
5 N,
--*
6 H,O 4 N, + 6 H,O
+
Production o f N,O:
2 NH,
+ 2 0, ----0 8 NH, + 12 N O + S 0,
Oxidation of ammonia: 4 NH, + 7 0, --+
4NH,+50, 4NH,+30,
0
--
N,O + 3 H,O 10 N,O + 12 H,O
--+
4 NO, + 6 H,O 4NO+6H20 2N2+6H,O
339 In view of t h e h e t e r o g e n e o u s l y c a t a l y z e d r e a c t i o n s , v a r i o u s t y p e s o f c a t a -
l y s t s have a l r e a d y been d e v e l o p e d . The c a t a l y s t t y p e is mainly r e l a t e d t o t h e r e q u i r e d l o c a t i o n in t h e p r o c e s s . In p r i n c i p l e , t h r e e p o s s i b l e c a t a l y s t l o c a t i o n s m u s t be t a k e n i n t o a c c o u n t , a s s h o w n in Fig. 1. F o r e x a m p l e , p r o c e s s e s a r e k n o w n , whereby nitric o x i d e s a r e r e d u c e d by a n o b l e - m e t a l - c o n t a i n i n g c a t a l y s t a t c o m p a r a b l y l o w t e m p e r a t u r e s , producing n i t r o g e n . H o w e v e r , r e l a t i v e l y high n o b l e - m e t a l c o s t s p o s e s o m e r e s t r i c t i o n s f o r t h e p r o c e s s . In a d d i t i o n , t h e c a t a l y s t s a r e s u b j e c t t o p o i s o n i n g by s u l p h u r d i o x i d e in t h e e x h a u s t g a s . T h e r e a r e c h e a p e r a n d m o r e s t a b l e c a t a l y s t s b a s e d on j a p a n e s e t e c h n o mainly c o n s i s t i n g of vanadium and t i t a n i u m o x i d e s . A g e n e r a l d i s a d v a n t a g e of t h e s e c a t a l y s t s , however, is t h e heavy-metal c o n t e n t of u s e d c a t a -
l y s t s . P r o c e s s e s a r e known which employ z e o l i t e c a t a l y s t s . A n a d v a n t a g e of m o l e c u l a r sieve c a t a l y s t s is t h e s t o r a g e c a p a b i l i t y of a m m o n i a . By t a k i n g adv a n t a g e of t h i s e f f e c t , s h o r t t i m e v a r i a t i o n s of t h e a m m o n i a c o n c e n t r a t i o n c a n be c o m p e n s a t e d . The p r o c e s s e s , however, r e q u i r e r e l a t i v e l y high o p e r a t i o n t e m p e r a t u r e s ( u p t o 48OoC), a n d t h e r e is no c o m p l e t e n i t r i c o x i d e c o n v e r s i o n due t o t h e comparatively reaction-inactive z e o l i t e c a t a l y s t s .
In t h e p r e s e n t work it is d e m o n s t r a t e d t h a t t h e a c t i v i t y of z e o l i t e s c o n c e r n i n g t h e S C R p r o c e s s c a n b e i n c r e a s e d s i g n i f i c a n t l y by m e t a l d o p i n g . T h e r e f o r e , t h e u s e of t h e c a t a l y s t a t l o w e r t e m p e r a t u r e s will b e p o s s i b l e .
Vessel
1 2 3
1
Dust removal
2 -'
Desulph. plant
High-Dust catalyst Low-Dust catalyst LT catalyst
Fig. 1. P o s s i b l e l o c a t i o n s of SCR c a t a l y s t s .
3 8 -
Flue
340 EXPERIMENTAL
A s s t a r t i n g m a t e r i a l s f o r t h e p r e p a r a t i o n of t h e S C R - c a t a l y s t s , c o m m e r c i a l ly a v a i l a b l e z e o l i t e s of m o r d e n i t e t y p e a n d Y - z e o l i t e s w e r e u s e d ( B a y e r A C a n d N o r t o n C o m p . ) . T h e d o p i n g of t h e z e o l i t e s w i t h m e t a l s w a s p e r f o r m e d by t h e m e t h o d of i o n e x c h a n g e c i t e d i n t h e l i t e r a t u r e . S C R c a t a l y s t s can b e used a s b u l k - c o n t a c t honeycomb c a t a l y s t s o r a s c o a t ed h o n e y c o m b c a t a l y s t s . From t h e p o i n t of v i e w of p r o c e s s d e v e l o p m e n t , w e d e c i d e d t o use t h e c o a t e d h o n e y c o m b t y p e . A f t e r t h e p r e p a r a t i o n of t h e c a t a lytic a c t i v e p o w d e r s by ion e x c h a n g e a n d s u b s e q u e n t t h e r m a l t r e a t m e n t o r r e d u c t i o n , c o r d i e r i t e h o n e y c o m b s w e r e c o a t e d w i t h t h e z e o l i t e p o w d e r s by a s p e c i a l d e v e l o p e d p r o c e s s . The g e o m e t r y of t h e c o r d i e r i t e h o n e y c o m b s c o r r e s ponded t o t h a t u s u a l l y used i n t h e a u t o m o b i l e i n d u s t r y ( t e s t m o n o l i t e s , 3.15 inch l o n g , 1 inch d i a m e t e r , 400 S q . c e l l s p e r S q . i n c h ) .
The SCR c a t a l y s t s p r e p a r e d by t h e m e t h o d d e s c r i b e d a b o v e w e r e t e s t e d in a l a b o r a t o r y p l a n t t o d e t e r m i n e t h e e f f i c i e n c y of t h e c o n v e r s i o n of n i t r i c o x i d e s w i t h ammonia. A s a m o d e l s u b s t a n c e f o r t h e g r o u p of n i t r i c o x i d e s , NO w a s used. T h i s can b e j u s t i f i e d b e c a u s e t h e NO c o n t e n t of n o r m a l e x h a u s t g a s is
a b o u t 9 0 %of t h e N O x . The l a b o r a t o r y p l a n t c o n s i s t s of a g a s - m i x i n g s t a t i o n , a g a s - h e a t i n g s y s t e m , a r e a c t i o n c h a m b e r c o n t a i n i n g t h e c a t a l y s t , and f i n a l l y a gas-analysis system. By u s i n g a g a s c y l i n d e r b a t t e r y a n d e l e c t r o n i c m e a s u r e m e n t a n d c o n t r o l c o m p o n e n t s i t is p o s s i b l e to vary t h e e x h a u s t g a s c o m p o s i t i o n o v e r a w i d e r a n g e . T h e s y n t h e t i c e x h a u s t g a s c o n t a i n s a m a x i m u m of 9 d i f f e r e n t c o m p o n e n t s . The t e s t p l a n t a l l o w s f o r a maximum gas v o l u m e s t r e a m u p t o SO I / m i n . The s t a n d a r d c o m p o s i t i o n of t h e s y n t h e t i c e x h a u s t g a s is s h o w n i n t a b l e 3. TABLE 3 S t a n d a r d c o m p o s i t i o n of e x h a u s t g a s f o r t h e p r e s e n t e x p e r i m e n t s .
79.86 10.0
4.0 6.0
0.07 0.07
341 T h e g a s h e a t i n g s y s t e m p e r m i t s e x h a u s t t e m p e r a t u r e s b e t w e e n 150 a n d 450 OC. P r o g r a m m a b l e h e a t i n g using s t a n d a r d i z e d t e m p e r a t u r e p r o g r a m s a l l o w s f o r m e a s u r e m e n t s o f t h e c a t a l y s t behaviour w i t h r e l a t i o n t o t h e d i f f e r e n t e x h a u s t - g a s t e m p e r a t u r e s . T h e g a s c o m p o s i t i o n d o w n s t r e a m a n d ups t r e a m of t h e c a t a l y t i c c o n v e r t e r is c o n t r o l l e d by t h e g a s - a n a l y s i s s y s t e m . The s y s t e m c o n s i s t s o f a GC w i t h FID a n d HCD d e t e c t o r s , a n d a c h e m i l u m i n e s c e n c e NOx a n a l y z e r . T h e chemical c o m p o s i t i o n of t h e c a t a l y t i c a c t i v e p o w d e r s w a s a n a l y z e d by
EDX. The i n f o r m a t i o n d e p t h
o f EDX is a b o u t 1 t o 2 pm. W i t h a c r y s t a l l i t e s i z e
of t h e z e o l i t e s o f 1 t o 4 pm, t h e volume c o m p o s i t i o n is a l s o given by t h e EDX method.
RESULTS A N D DISCUSSION A s a l r e a d y s t a t e d i n t h e i n t r o d u c t i o n , t h e s t a t e of t h e a r t c o n c e r n i n g n o b l e
m e t a l c a t a l y s t s a l l o w s f o r a c c e p t a b l e n i t r i c o x i d e c o n v e r s i o n a t low t e m p e r a t u r e s . A s is a l r e a d y known f r o m work o n t h r e e - w a y c a t a l y s t s ’),
i t is p o s s i b l e ,
by f i n e d i s p e r s i o n of p l a t i n u m i n n e a r - s u r f a c e r e g i o n s of m o r d e n i t e c r y s t a l l i t e s , t o p r o d u c e a c a t a l y s t c h a r a c t e r i z e d by high a c t i v i t y w i t h low n o b l e m e t a l c o n t e n t . I n a d d i t i o n , d u e t o t h e a d s o r p t i o n of a m m o n i a m o l e c u l e s a t t h e acid z e o l i t e s i t e s , t h e r e e x i s t s t h e a d d i t i o n a l e f f e c t of s y n e r g y . T h e r e f o r e , we s t a r t e d t h e i n v e s t i g a t i o n by using P t - d o p e d m o r d e n i t e a s SCR c a t a l y s t , b e c a u s e a r a t h e r low o p e r a t i o n t e m p e r a t u r e in c o n j u n c t i o n w i t h
high c o n v e r s i o n r a t e s s h o u l d t h u s be e x p e c t e d . Fig. 2. s h o w s t h e n i t r i c o x i d e c o n v e r s i o n a s a f u n c t i o n o f t h e g a s t e m p e r a t u r e f o r t w o d i f f e r e n t c a t a l y s t s of t y p e P t H - m o r d e n i t e . C a t a l y s t s w e r e p r e p a r e d f o r i d e n t i c a l Pt c o n t e n t of a b o u t 0.2 g/l.
From Fig. 2. it is s e e n , t h a t f o r b o t h c a t a l y s t s a l m o s t 100%n i t r i c o x i d e c o n version o c c u r s . H o w e v e r , t h e t e m p e r a t u r e r a n g e f o r o p t i m u m c o n v e r s i o n , b u t a l s o i n c r e a s e a n d d e c r e a s e of t h e conversion s l o p e a r e q u i t e d i f f e r e n t . T h i s is d u e t o t h e d i f f e r e n t S i / A l - r a t i o of t h e t w o m o r d e n i t e c a t a l y s t s . T h e m o r d e n i t e o f c a t a l y s t (11) w a s d e a l u m i n a t e d by acid t r e a t m e n t b e f o r e P t - d o p i n g . T h i s
r e s u l t e d i n a m o d u l u s ( m o l a r r a t i o SiO,/AI,O,)
of a b o u t 2 0 . T h e m o r d e n i t e of
c a t a l y s t ( 1 ) w a s n o t modified ( m o d u l u s o f a b o u t 12). By t h e dealumiiiatioii of t h e anionic l a t t i c e of t h e moi-denite in conji:::cti:,-
with a s i n i u l t a n e o u s c h a n g e o f t h e s t e r i c c o n d i t i o n s in t h e p o r e s y s t e m . a c h a n g e i n a c i d i t ) r e s u l t s . C h a n g e s i n s p e c i f i c s u r f a c e w e r e d e t e r m i n e d b)
342 using BET. T h e l o w e r c o n t e n t of n e g a t i v e p a r t i a l c h a r g e s o f t h e l a t t i c e a f t e r d i s s o l v i n g t h e a l u m i n u m r e s u l t s i n a d e c r e a s e in n u m b e r o f acid s i t e s . T h e i r s t r e n g t h , h o w e v e r , i n c r e a s e s . Both e f f e c t s i n f l u e n c e t h e n u m b e r of a d s o r p t i o n c e n t r e s f o r a m m o n i a a s well a s t h e d i f f u s i o n i n t o t h e p o r e s y s t e m . T h i s r e s u l t s i n a much b e t t e r c o n v e r s i o n c h a r a c t e r i s t i c f o r t h e d e a l u m i n a t e d P t H - m o r d e n i t e .
0 1I
150
i
I
200 Temperature
250
300
["Cl
Fig. 2. Activity b e h a v i o u r of P t H - m o r d e n i t e o f d i f f e r e n t m o d u l i a s a f u n c t i o n of g a s t e m p e r a t u r e . C a t a l y s t (1) s h o w s 90%c o n v e r s i o n o v e r o n l y a r e l a t i v e l y n a r r o w t e m p e r a t u r e r a n g e , a n d t h e a c t i v i t y s t r o n g l y d e c r e a s e s a t l o w e r o r h i g h e r t e m p e r a t u r e s . In c o n t r a s t , t h e d e a l u m i n a t e d c a t a l y s t s (11) s h o w s a l m o s t 100%c o n v e r s i o n b e t ween 170 and 200 O C . I n a d d i t i o n , t h e maximum c o n v e r s i o n i s m a i n t a i n e d a l s o a t h i g h e r g a s t e m p e r a t u r e s . T h e d e c r e a s e in c o n v e r s i o n f o r b o t h c a t a l y s t s a t h i g h e r t e m p e r a t u r e s is r e l a t e d t o t h e onset of t h e N O - p r o d u c i n g a m m o n i a o x i d a t i o n . T h i s r e s u l t s in a s i g n i f i c a n t d e c r e a s e of t o t a l NO c o n v e r s i o n . C a t a l y s t (11) s h o w s a h i g h e r
SCR a c t i v i t y a t l o w e r t e m p e r a t u r e s a n d a l o w e r a c t i v i t y
(desired) concerning ammonia oxidation a t higher t e m p e r a t u r e s . A s an a l t e r n a t i v e t o noble metal containing z e o l i t e c a t a l y s t s f o r selective
r e d u c t i o n of n i t r i c o x i d e , z e o l i t e c a t a l y s t s d o p e d w i t h t r a n s i t i o n m e t a l s o f t h e
343 p e r i o d i c s y s t e m w e r e p r o d u c e d . A s b a s i s m a t e r i a l s , Y-zeolite a n d m o r d e n i t e w e r e u s e d . Z e o l i t e s w e r e d o p e d w i t h e i t h e r vanadium o r c o p p e r . F i r s t , vanadium is known a s a c t i v e c o m p o n e n t of t h e T i 0 2 - b a s e d S C R - c a t a l y s t of J a p a n e s e t e c h n o l o g y . C o p p e r , o n t h e o t h e r h a n d , is k n o w n to a c t a s c e n t r a l ion in a m monia c o m p l e x e s , r e s u l t i n g i n a d d i t i o n a l s t o r a g e o f a m m o n i a in t h e z e o l i t e .
n
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Fig. 3. C o n v e r s i o n c h a r a c t e r i s t i c of d i f f e r e n t d o p e d z e o l i t e s a s a f u n c t i o n o f t h e gas temperature. Fig. 3. s h o w s a s i g n i f i c a n t d i f f e r e n c e in t h e a c t i v i t y b e h a v i o u r o f t h e t h r e e m e t a l - d o p e d z e o l i t e s a s a f u n c t i o n of e x h a u s t t e m p e r a t u r e . I t c a n b e s e e n t h a t t h e H-mordenite (vanadium-doped) d o e s n o t show significant activity in cont r a s t t o t h e Pt-doped f o r m . Over t h e e n t i r e t e m p e r a t u r e r a n g e 200 t o 400 OC, o n l y NO-conversion r a t e s of 10 X a r e r e a c h e d . Vanadium is p r e s e n t in t h e z e o lite in a c o m p l e t e l y d i f f e r e n t f o r m , a s is t h e c a s e f o r t h e TiO, c a t a l y s t . T h e r e f o r e , vanadium c a n n o t play t h e identical role. In a d d i t i o n , i t is o b v i o u s t h a t t h e H - m o r d e n i t e a l o n e e x h i b i t s e x t r e m e l y l o w a c t i v i t y a s a n SCR c a t a l y s t .
344
I n c o n t r a s t t o vanadium, c o p p e r a s doping metal s h o w s a positive e f f e c t f o r t h e H - m o r d e n i t e . A t t e m p e r a t u r e s up t o 300 OC, t h e r e is n o d i f f e r e n c e b e t w e e n t h e t w o c a t a l y s t s . A t t e m p e r a t u r e s a b o v e 300 OC, h o w e v e r , a s i g n i f i c a n t inc r e a s e i n t h e s e l e c t i v e r e d u c t i o n of n i t r o g e n o x i d e is f o u n d f o r t h e CuHm o r d e n i t e . A t 380 OC a maximum c o n v e r s i o n o f a b o u t 50 % is r e a c h e d . A f u r t h e r i n c r e a s e i n t e m p e r a t u r e , however, r e s u l t s in a s t r o n g d e c r e a s e of NO c o n version. T h i s is d u e t o t h e r a p i d i n c r e a s e o f NH, o x i d a t i o n . Above 400 OC, more ammonia molecules a r e reacting with oxygen t h a n with nitrogen oxide, f i n a l l y r e s u l t i n g in a n i n c r e a s e of NO e m i s s i o n in t h e t o t a l mass b a l a n c e . T h e p a r a l l e l r e a c t i o n o f a m m o n i a o x i d a t i o n c a n b e r e d u c e d by u s i n g a c o p p e r - d o p e d Y - z e o l i t e . a s s h o w n in f i g . 3. T h i s p a r t i c u l a r c a t a l y s t c o m b i n e s t h e c a p a b i l i t y of r e d u c i n g ammonia o x i d a t i o n w i t h a high a c t i v e NO r e d u c t i o n . T h i s l e a d s t o a r e l a t i v e l y high c o n v e r s i o n p l a t e a u o v e r a w i d e t e m p e r a t u r e r a n g e . Between 250 a n d 400 OC NO c o n v e r s i o n is b e t t e r t h a n 80 %.
1
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I
Fig. 4 . C o m p a r i s o n of c o n v e r s i o n c h a r a c t e r i s t i c f o r d i f f e r e n t l y p r e p a r e d c o p per-doped Y-zeolites.
345
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Fig. 5. C o n v e r s i o n r a t e of t h e r e a c t i o n NO r u p t i o n o f t h e ammonia f e e d .
+
NH, v e r s u s t i m e ( m i n ) a f t e r i n t e r -
T h i s h i g h e r p o t e n t i a l o f t h e c o p p e r - d o p e d Y-zeolite a s SCR c a t a l y s t c a n a l s o b e s e e n f r o m fig. 4 . H e r e , NO c o n v e r s i o n i s p l o t t e d a g a i n v e r s u s g a s t e m p e r a t u r e , and t w o d i f f e r e n t l y p r e p a r e d CuNaY-zeolites a r e c o m p a r e d . I t c a n b e s e e n t h a t by o p t i m i z i n g t h e p r e p a r a t i o n t e c h n i q u e a f u r t h e r i n c r e a s e in c o n v e r s i o n c h a r a c t e r i s t i c s is p o s s i b l e . T h e c a t a l y s t CuNaY(2) s h o w s m o r e t h a n 80 % c o n v e r s i o n over a t e m p e r a t u r e r a n g e 2 0 0 - 400 OC. Between 250 a n d 390 OC a n a l m o s t 100 % c o n v e r s i o n of n i t r i c o x i d e is f o u n d .
-
T h e r e f o r e , t h e c o p p e r - d o p e d Y-zeolite c a n b e o p e r a t e d a s SCR c a t a l y s t a t l e a s t o n a l a b o r a t o r y s c a l e - t o b e u s e d o v e r a wide t e m p e r a t u r e r a n g e . O p e r a t i o n is p o s s i b l e a s w e l l in a l o w - d u s t r e g i o n s u c h a s b e h i n d t h e d e s u l p h u r i z a -
t i o n p l a n t , w h e r e t h e g a s t e m p e r a t u r e is a l r e a d y r e d u c e d s i g n i f i c a n t l y . A c o m -
p l i c a t e d t e m p e r a t u r e c o n t r o l f o r o p t i m u m c o n v e r s i o n is n o t n e c e s s a r y d u e t o t h e wide t e m p e r a t u r e w i n d o w of t h e c a t a l y s t .
346
An a d d i t i o n a l i m p o r t a n t c h a r a c t e r i s t i c of t h e z e o l i t e SCR c a t a l y s t s is s h o w n in f i g . 5. H e r e , T h e o p t i m i z e d C u N a Y - z e o l i t e w a s u s e d . During a l o n g - t e r m o p e r a t i o n a t a n e x h a u s t g a s t e m p e r a t u r e of 300 OC, t h e a m m o n i a f e e d w a s i n t e r r u p t e d , w h i l e t h e NO f e e d w a s m a i n t a i n e d c o n s t a n t . Fig. 5 s h o w s NO c o n v e r s i o n v e r s u s t i m e ( m i n ) a f t e r i n t e r r u p t i o n of t h e a m m o n i a f e e d . T h e e x c e l l e n t a m m o n i a s t o r a g e c a p a b i l i t y of t h e z e o l i t e , in p a r t i c u l a r t h e c o p p e r - d o p e d Y - z e o l i t e , c a n b e d e m o n s t r a t e d . Even u p t o 3 min a f t e r a m m o n i a f e e d i n t e r u p t i o n , a 100 % c o n v e r s i o n o f NO is m a i n t a i n e d . A f t e r t h i s t i m e t h e r e a c t i o n e f f i c i e n c y d e c r e a s e s . H o w e v e r , even a f t e r 10 m i n , t h e r e is still 2 0 % NO c o n v e r s i o n . A f t e r 2 0 min, t h e r e a c t i o n e f f i c i e n c y is z e r o . By u s i n g t h e e x c e l l e n t s t o r a g e c a p a b i l i t y , c h a n g e s in t h e a m m o n i a f e e d o r t h e NO c o n t e n t in t h e e x h a u s t g a s c a n be c o m p e n s a t e d . This h o l d s a l s o f o r s h o r t - t i m e c e s s a t i o n of t h e a m m o n i a f e e d .
T h e d i f f e r e n t l y modified z e o l i t e SCR c a t a l y s t s d e s c r i b e d i n t h i s w o r k c a n be u s e d f o r d i f f e r e n t o p e r a t i n g c o n d i t i o n s . S u c h a d v a n t a g e s a s e c o n o m i c a l basis s u b s t a n c e , excellent activity behaviour ( i n p a r t over wide t e m p e r a t u r e r a n g e s ) a n d , i n p a r t i c u l a r , t h e unique s t o r a g e c a p a b i l i t y f o r t h e r e a c t i o n c o m p o n e n t a m m o n i a have been d e m o n s t r a t e d . I n a d d i t i o n , d u e t o t h e c o m p o s i t i o n of t h e z e o l i t e s , t h e d i s p o s a l of used c a t a l y s t s is l e s s t r o u b l e s o m e c o m p a r e d
w i t h heavy-metal-containing
TiO, c a t a l y s t s .
ACKNOWLEDGEMENTS The a u t h o r s g r a t e f u l l y a c k n o w l e d g e financial s u p p o r t by t h e B u n d e s m i n i s t e r f u r F o r s c h u n g und T e c h n o l o g i e .
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DE-OS 26 17 744.
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US.-PS 3,279,884. U.S.-PS 4,010,238. EPA-A1 0 161 743
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H a a s . J . . C. Plog a n d J . S t e i n w a n d e l , S t u d . S u r f . Sci. C a t a l . . i n p r e s s
3