S e l f - r e g e n e r a t i o n o f a - p e r o v s k i t e C a t a l y s t : A P h i l o s o p h e r ’ s S t o n e f o r T o d a y ’ s A u t o m o t i v e E n g i n e S e l f - r e g e n e r a t i o n o f a P d - p e r o v s k i t e C a t a l y s t : A P h i l o s o p h e r ’ s S t o n e f o r T o d a y ’ s A u t o m o t i v e E n g i n e T h e i m p o r t a n t r o l e o f a u t o m o t i v e c a t a l y s t s i s w i d e l y r e c o g n i z e d f o r t h e c o n v e r s i o n o f t h r e e p o l l u t a n t - e m i s s i o n s , s u c h a s c a r b o n m o n o x i d e ( C O ) , n i t r o g e n o x i d e s ( N O x ) a n d u n b u r n e d hydrocarbons ( HC ) in engine exhaust gas es. The c o n v e n t i o n a l c a t a l y s t s d i s p e r s e f i n e p a r t i c l e s o f p r e c i o u s m e t a l o n c e r a m i c - s u p p o r t m a t e r i a l s . However, the catalytic activity deteriorates owing to t h e a g g l o m e r a t i o n a n d g r o w t h o f m e t a l p a r t i c l e s during vehicle use (see the lower sequence in Fig. 1 ). Thus, an excess amount of precious metals is u s u a l l y i n c o r p o r a t e d t o g u a r a n t e e c o n t i n u e d c a t a l y t i c a c t i v i t y a f t e r r u n n i n g t h e v e h i c l e o v e r 80,000 km. A state-of-the-art automotive gasoline engine is operated close to the stoichiometric air-to- f u e l r a t i o t o c o n v e r t t h e p o l l u t a n t - e m i s s i o n s s i m u l t a n e o u s l y [ 1 ] , a c c o m p a n y i n g w i t h r e d o x ( r e d u c t i o n a n d o x i d a t i o n ) f l u c t u a t i o n s i n e x h a u s t - gas composition by adjusting the air-to-fuel ratio. A p e r o v s k i t e - b a s e d c a t a l y s t , L a F e 0. 57 C o 0. 38 P d 0. 05 O 3 , has maintained its high activity with high dispersion Fig. 1. Self-regeneration of the intelligent catalyst (upper sequence) and deterioration of the conventional catalyst (lower sequence) in the exhaust-gas of the state-of-the-art gasoline engine. of pr ec io us me ta ls . In th is st ud y, we ha ve fo un d s e l f - r e g e n e r a t i o n o f t h e p e r o v s k i t e - b a s e d c a t a l y s t w i t h e n v i r o n m e n t a l f l u c t u a t i o n s , u s i n g X - r a y d i f f r a c t i o n a n d X A F S p e r f o r m e d a t b e a m l i n e BL14B1 [2]. T h e p a l l a d i u m - c o n t a i n i n g p e r o v s k i t e ( P d - pe ro vs ki te ) ca ta ly st wa s pr ep ar ed by th e al ko xi de met hod [3, 4]. The the rma l age ing pro ced ure was c a r r i e d o u t i n t h r e e s t e p s t o s i m u l a t e t h e r e d o x fluctuations of an automotive exhaust gas: at first t h e p o w d e r e d c a t a l y s t w a s o x i d i z e d i n t h e a i r a t 80 0 C for 1 hour, then reduced in an atmosphere of 10% H 2 / 90% N 2 at 800 C for 1 hour, and finally r e - o x i d i z e d i n t h e a i r a t 8 0 0 C f o r 1 h o u r . F i g u r e 2 ( a ) depicts the powder diffraction pattern for the three s a m p l e s . T w o B r a g g r e f l e c t i o n s ( 1 0 0 ) a n d ( 1 1 0 ) were observed for the oxidized catalyst. The Bragg r e f l e c t i o n s s h i f t e d a n d a f e w a d d i t i o n a l p e a k s appeared for the reduced catalyst, indicating lattice exp ans ion and par tia l dem oli tio n of the per ovs kit e crystal in a reductive atmosphere. Surprisingly, the 75 P d - p e r o v s k i t e S o l i d S o l u t i o n S e g r e g a t i o n o f 1 ~ 3 n m P d P a r t i c l e s P d - p e r o v s k i t e S o l i d S o l u t i o n N a n o - c o m p o s i t i o n S e l f - r e g e n e r a t i o n ! P r e c i o u s M e t a l s o n C o n v e n t i o n a l C e r a m i c s G r a i n G r o w t h o f P r e c i o u s M e t a l s F u r t h e r G r a i n G r o w t h o f P r e c i o u s M e t a l s C o n v e n t i o n a l C a t a l y s t P r e c i o u s M e t a l s I n t e l l i g e n t C a t a l y s t O x i d a t i o n ( I n i t i a l ) R e d u c t i o n O x i d a t i o n T i m e Fi g. 2. X- ra y po wd er di ff ra ct io n. (a ) Po wd er X- ra y di ff ra ct io n pa tt er n fo r thr ee age d cat aly sts , LaF e 0.57 Co 0.38 Pd 0.05 O 3 , whi ch wer e pre sse d int o pel let s with a BN binder after the ageing. Two Bragg peaks from the catalysts were a s s i g n e d a s a p s e u d o c u b i c c e l l o f t h e p e r o v s k i t e s t r u c t u r e . T h e p e r o v s k i t e crystal in a reductive atmosphere was partially destroyed and transformed into La(OH) 3 through La 2 O 3 . Energy dependence of the intensity for (b) (100) and ( c ) ( 1 1 0 ) r e f l e c t i o n s n e a r t h e P d K - e d g e i n d i c a t e s t h a t t h e B - s i t e o f t h e perovskite structure is occupied with Pd in the oxidized catalyst. d i f f r a c t i o n p a t t e r n r e c o v e r e d a t t h e r e - o x i d i z i n g s t e p : t h e B r a g g r e f l e c t i o n s s h i f t e d b a c k a n d t h e a d d i t i o n a l p e a k s d i s a p p e a r e d c o m p l e t e l y . T h e c a t a l y s t r e t a i n s a p r e d o m i n a n t l y p e r o v s k i t e structure throughout the redox cycle. The increase o f ( 1 0 0 ) a n d t h e d e c r e a s e o f ( 1 1 0 ) r e f l e c t i o n in te ns it ie s at th e Pd K -e dg e cl ea rl y sh ow th at Pd o c c u p i e s t h e B - s i t e ( 6 - f o l d c o o r d i n a t i o n ) o f t h e perovskite lattice in the oxidative atmosphere ( Figs. 2(b) and 2(c) ). F i g u r e 3 ( a ) s h o w s X A N E S s p e c t r a f o r P d - pe ro vs ki te ca ta ly st s af te r th e ea ch ag ei ng st ep to e s t i m a t e t h e v a l e n c e s t a t e o f P d . T h e c h e m i c a l shift of the edge position implies that the valence of Pd in the oxidized or re-oxidized catalyst is higher than the normal bivalence as seen in PdO. On the o t h e r h a n d , P d i s i n t h e m e t a l l i c s t a t e i n t h e r e d u c e d c a t a l y s t . T h e l o c a l s t r u c t u r e a r o u n d P d a l s o r e v e r s i b l y c h a n g e s , d e p e n d i n g o n t h e r e d o x atmospheres ( Fig. 3(b) ). The first peak of the radial 76 ( 1 1 0 ) 1 . 0 0 4 1 . 0 0 2 1 . 0 0 0 0 . 9 9 8 0 . 9 9 6 0 . 9 9 4 0 . 9 9 2 2 4 . 6 2 4 . 4 2 4 . 2 E n e r g y ( k e V ) 1 . 0 1 0 1 . 0 0 5 1 . 0 0 0 0 . 9 9 5 2 4 . 6 2 4 . 4 2 4 . 2 E n e r g y ( k e V ) ( 1 0 0 ) X - r a y I n t e n s i t y ( a r b . u n i t s ) 6 0 5 0 4 0 3 0 2 0 1 0 0 2 . 4 2 . 2 2 . 0 1 . 8 1 . 6 o x i d i z e d r e d u c e d r e - o x i d i z e d ( 1 0 0 ) L a 2 O 3 ( 1 0 0 ) L a 2 O 3 ( 1 0 1 ) L a ( O H ) 3 ( 1 1 0 ) B N L a ( O H ) 3 ( 1 0 1 ) ( 1 1 0 ) L a 2 O 3 ( 0 0 2 ) Q ( Å - 1 ) X - r a y I n t e n s i t y ( a r b . u n i t s ) ( a ) ( b ) ( c ) F i g . 3 . C o m p a r i s o n o f X A F S . ( a ) X - r a y abs orp tio n spe ctr a nea r the Pd K-e dge , whi ch were measured in transmission mode, for three ag ed ca ta ly st s, La Fe 0. 57 Co 0. 38 Pd 0. 05 O 3 . Th os e f o r P d O a n d a P d - f o i l a r e a l s o s h o w n a s reference materials. The valence of Pd changes reversibly in a redox cycle. (b) Radial s tructure f u n c t i o n a r o u n d P d : M a g n i t u d e o f F o u r i e r t r a n s f o r m ( F T ) o f t h e k 3 - w e i g h t e d E X A F S o s c i l l a t i o n s f o r t h e c a t a l y s t s . N e i g h b o u r s seemingly appear closer to the X-ray absorbing a t o m b e c a u s e t h e p h a s e s h i f t o f t h e photoelectron was not taken into account. The l o c a l s t r u c t u r e a r o u n d t h e P d a t o m c h a n g e s reversibly, too. Yasuo Nishihata a and Hirohisa Tanaka b (a) SPring-8 / JAERI (b) Materials Research & Development Division, Daihatsu Motor Co., Ltd. E-mail: yasuon @ sp8sun.spring8.or.jp s t r u c t u r e f u n c t i o n c a n b e f i t t e d a s 6 oxygen atoms of a PdO 6 octahedron f o r t h e o x i d i z e d a n d r e - o x i d i z e d catalysts, while it can be fitted as Pd a n d C o a t o m s i n a P d - C o a l l o y particle for the reduced catalyst. It has been proved that the particle gr ow th of th e pr ec io us me ta l ca n be s u p p r e s s e d a s a r e s u l t o f t h i s P d m o v e m e n t b e t w e e n i n s i d e a n d o u t s i d e t h e p e r o v s k i t e l a t t i c e d u e t o the structural responses to the redox f l u c t u a t i o n s i n e x h a u s t - g a s c o m p o s i t i o n w i t h t h e s t a t e - o f - t h e - a r t g a s o l i n e e n g i n e ( s e e t h e u p p e r se qu en ce in Fi g. 1 ). Th er ef or e, th is c a t a l y s t c a n r e f r e s h b y i t s e l f t o maintain the activity while the vehicle is being driven , and can reduce by 70 - 90% the amount of precious metals needed to meet the ultra-low emission vehicle standards . References [ 1 ] R . M . H e c k a n d R . J . F a r r a u t o , C a t a l y t i c A i r P o l l u t i o n C o n t r o l : C o m m e r c i a l T e c h n o l o g y ( V a n Nostrand Reinhold, New York, 1995) 94. [2] Y. Nishihata, J. Mizuki, T. Akao, H. Tanaka, M. Ue ni sh i, M. Ki mu ra , T. Ok am ot o an d N. Ha ma da , Nature 418 (2002) 164. [3 ] H . T a n a k a e t a l . , S c i e n c e a n d T e c h n o l o g y i n C a t a l y s t s 1 9 9 4 ( e d s . Y . I z u m i , H . A r a i & M . Iwamoto ) ( Kodansya-Elsevier, Tokyo, 1994) 457. [4 ] H. Ta na ka et al . , To pi cs in Ca ta ly st s ( ed s N. Kruse, A. Frennet & J.-M. Bastin-Kluwer Academic, New York) 16-17 (2001) 63. 77 P d O P d - f o i l 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 2 4 . 3 3 2 4 . 3 4 2 4 . 3 5 2 4 . 3 6 2 4 . 3 7 2 4 . 3 8 E n e r g y ( k e V ) N o r m a l i z e d A b s o r p t i o n o x i d i z e d r e d u c e d r e - o x i d i z e d 1 5 2 0 1 0 5 0 0 1 2 3 4 5 D i s t a n c e ( Å ) | F T | P d - O P d - C o & P d - P d o x i d i z e d r e d u c e d r e - o x i d i z e d ( a ) ( b )
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