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Alumina-based aerogels as carriers for automotive palladium catalysts
Journal of Non-Crystalline Solids 145 (1992) 250-254 North-Holland
]OURNA
L OF
NON-CRYSTALLIN SOLIDS E
Alumina-based aerogels as carriers for automotive palladium catalysts C. Hoang-Van, B. Pommier 1, R. H a r i v o l o l o n a a n d P. P i c h a t URA au CNRS 'Photocatalyse, Catalyse et Enuironnement ; Ecole Centrale de Lyon, B.P. 163, 69131 Ecully Cddex, France
Alumina-based aerogels were made by supercritical drying of alcogels obtained by hydrolysisof organic derivatives of AI, Ce, La and Ba. Palladium catalysts supported on these aerogels were tested in the reaction CO + NO + 02. The effects of the Pd precursor, oxide additive, preparation procedure and thermal treatment on catalytic performance were examined.
1. Introduction The sol-gel technique combined with the supercritical drying method allows simple or mixed oxide aerogels to be obtained in the amorphous state with high surface area and large pore volume [1]. These properties favour interaction between the oxides of a multicomponent aerogel [1] and, possibly, that between the aerogel and a metallic phase deposited on its surface. A review on aerogel catalysts has been published recently [2]. In attempts to develop rhodium-free catalysts, we selected palladium as a component that could be substituted for rhodium on alumina-based aerogels supports. Palladium has been reported to exhibit good catalytic performance when compared with platinum or rhodium in catalysts containing individual noble metals [3]. This paper reports the activities of palladium catalysts supported on alumina-based aerogels in the reaction CO + NO + O2, in comparison with conventional Pd or P t - R h catalysts supported on commercial carriers. The effects of oxide additives, preparation conditions, palladium precursors and thermal treatments on the catalytic performance are discussed. 1 Present address: Institut de Recherches sur la Catalyse, CNRS, 2, avenue A. Einstein, 69626 Villeurbanne C4dex, France.
2. Experimental 2.1. Preparation o f aerogels
The alumina aerogel was prepared from aluminium sec-butoxide dissolved in sec-butanol by hydrolysis. The alumina alcogel was then dried in an autoclave under supercritical conditions with respect to sec-butanol [4]. The same method was applied to prepare a CeO 2 aerogel using a solution of Ce acetylacetonate dissolved in methanol. Binary aerogels were prepared by co-hydrolyzing aluminium sec-butoxide and the corresponding metal (Ce, La or Ba) acetylacetonate, both dissolved in methanol, and by subsequent supercritical drying. The proportion of CeO2, L a 2 0 3 or BaO incorporated into A120 3 was fixed at approximately 15 wt%. A binary C e O 2 / A 1 2 0 3 aerogel was prepared by using a two-step hydrolysis procedure. A120 3 prepared in a separate operation was introduced into a methanolic solution of Ce acetylacetonate which was then hydrolyzed and the binary alcogel thus obtained was supercritically dried. 2.2. Preparation o f palladium catalysts
The catalysts were prepared by impregnating the above aerogel supports with a solution of Pd acetylacetonate or PdCI 2 dissolved in methanol.
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
C. Hoang-Van et al. / Alum#la-based aerogels T h e Pd 1 wt%.
content
was
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at
approximately
2.3. Characterization S u r f a c e a r e a s w e r e d e t e r m i n e d f r o m N 2 ads o r p t i o n at - 196°C. P r i o r to the a d s o r p t i o n m e a s u r e m e n t s , t h e s a m p l e s w e r e t r e a t e d in a H e flow at 250°C for 2 h. T h e d i s p e r s i o n o f p a l l a d i u m was m e a s u r e d by C O c h e m i s o r p t i o n at r o o m t e m p e r a t u r e , a s s u m ing t h e s t o i c h i o m e t r y C O / P d s u r f a c e = 1, by use o f the p u l s e c h r o m a t o g r a p h i c t e c h n i q u e [5]. P r i o r to t h e C O c h e m i s o r p t i o n m e a s u r e m e n t s , the samples w e r e r e d u c e d by H 2 at 500°C for 3 h. T h e y a r e r e f e r r e d to as f r e s h ' catalysts. S o m e catalysts w e r e t r e a t e d at 1000°C in flowing 0 2 c o n t a i n i n g 10% o f w a t e r v a p o u r for 4 h. T h e s e s a m p l e s a r e d e s i g n a t e d as a g e d ' catalysts.
2.4. Activity measurements Catalytic e x p e r i m e n t s w e r e p e r f o r m e d in a flow system at a t m o s p h e r i c p r e s s u r e . R e a c t a n t a n d p r o d u c t gases w e r e a n a l y z e d by gas c h r o m a t o g r a phy with a t h e r m a l conductivity d e t e c t o r ( 0 2 , C O , CO2, N 2 0 ) or i n f r a r e d s p e c t r o s c o p y (NO). A c t i v a t i o n o f t h e catalyst (100 mg) m i x e d with inactive c~-A120 3 (400 mg) c o n s i s t e d in a t r e a t m e n t at 500°C in a flowing r e a c t i o n m i x t u r e
251
(20 l / h ) for 3 h. T h e c o n v e r s i o n s o f C O a n d N O w e r e m e a s u r e d at i n c r e a s i n g t e m p e r a t u r e s in t h e r a n g e 1 5 0 - 5 0 0 ° C at a p r o g r a m m e d r a t e of 2 ° C / m i n . T h e gas mixture u s e d (0.75% C O + 0.1% N O + 0.75% 0 2 + N 2 ( d i l u e n t ) ) was m a r k e d l y oxidizing since t h e s t o i c h i o m e t r y n u m b e r was s = [ 2 ( 0 2) + ( N O ) ] / ( C O ) = 2.13.
3. Results and discussion
3.1. Textural properties of palladium catalysts T a b l e 1 shows t h e s u r f a c e a r e a s a n d m e t a l l i c d i s p e r s i o n s o f p a l l a d i u m catalysts p r e p a r e d by i m p r e g n a t i o n o f a e r o g e l s with Pd a c e t y l a c e t o n a t e , with t h e e x c e p t i o n o f t h e s a m p l e s Ib, o b t a i n e d by i m p r e g n a t i o n of A l z O 3 a e r o g e l with PdCI2, a n d II, m a d e f r o m Pd a c e t y l a c e t o n a t e a n d g a m m a A I 2 0 3 x e r o g e l (Degussa), for c o m p a r i s o n p u r poses. T h e t e x t u r a l p r o p e r t i e s o f a e r o g e l s d e p e n d on p r e p a r a t i o n p a r a m e t e r s such as the a m o u n t of w a t e r i n t r o d u c e d into t h e alcoholic solution to hydrolyze t h e o r g a n i c d e r i v a t i v e of the m e t a l o r t h e c o n c e n t r a t i o n o f t h a t derivative in t h e solvent [4]. Single a e r o g e l s p r e p a r e d in this w o r k (samples Ia a n d VI) exhibit lower B E T s u r f a c e a r e a s t h a n t h e b i n a r y a e r o g e l s ( t a b l e 1). In p a r t i c u l a r , high s u r f a c e a r e a a e r o g e l s a r e o b t a i n e d by co-hy-
Table 1 Textural characteristics of aerogel supported palladium catalysts Catalyst a) la: Pd/AI203 Ib: Pd/AIzO3(CI) b) II: Pd/A1203-D c) Ilia: Pd/CeO 2/AI203 dl IIIb: P d / C e O 2 - A I 2 0 3 IV: Pd/La203-AI203 V: Pd/BaO-AI203 VI: Pd/CeO 2
Surface area (m 2 g ~)
Palladium dispersion (%)
Fresh catalyst
Aged catalyst
Fresh catalyst
130 130 106 173 505 333 446 65
69.0 70.0 79.0 64.0 71.0 69.0 89.0 8.5
28.1 25.3 25.0 20.7 14.0 15.2 8.5 25.2
~) Palladium loading: ~ 1 wt%. b) Pd precursor: PdCl 2. °) AlzO 3 xerogel (Degussa, gamma structure), d) Mixed aerogel obtained by a two-step hydrolysis procedure (see text). °) -: not determined.
Aged catalyst 6.0 3.8 7.0 4.6 - °) _ o7 _ e) 5.0
C. Hoang-Van et aL / Alumina-based aerogels
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Temperature ( ° C) Fig. 1. CO and NO conversions as a function of the reaction temperature. Reaction mixture: 0.'75% CO + 0.1% NO + 0.75% 02. 1, Pd/CeO 2 aerogel; 2, P d / A l 2 0 3 aerogel; 3, Pd/AlzO3-Degussa. Dashed lines: dechlorinated 1% Pt-0.fi% Rh/A1203 [6].
drolysis o f a l u m i n u m s e c - b u t o x i d e w i t h C e , L a o r Ba acetylacetonate (300-500 m 2 g-l). The Pd d i s p e r s i o n o f f r e s h ' catalysts v a r i e s f r o m 15 to 3 0 % ( t a b l e 1), a p a r t f r o m s a m p l e V ( P d / B a O A 1 2 0 3) w h o s e d i s p e r s i o n was s i g n i f i c a n t l y s m a l l e r ( 8 . 5 % , t a b l e 1). I n all cases, t h e r m a l t r e a t m e n t at 1000°C in f l o w i n g 0 2 + 1 0 % H 2 0 f o r 4 h ( a g e d ' catalysts) p r o d u c e s a l a r g e d e c r e a s e o f t h e surf a c e a r e a as w e l l as t h e P d d i s p e r s i o n o f t h e samples. The binary BaO-A120 3 aerogel support
exhibits the highest surface area after b l e 1, 3rd c o l u m n ) .
a g i n g ' (ta-
3.2. Catalytic performance 3.2.1. Effect of the nature of single aerogel support Figure 1 shows the CO and NO conversions, for the mixture 0.75% CO + 0.1% NO + 0.75% 0 2, as a f u n c t i o n o f t h e r e a c t i o n t e m p e r a t u r e f o r t h e s a m p l e s Ia ( P d / A 1 2 0 3 a e r o g e l ) a n d V I
Table 2 Conversion efficiencies of catalysts with the mixture 0,75% CO+0.1% NO+0.75% 02 (s = 2.13) Catalyst a)
Ia: Pd/A1203 Ib: Pd/AI203(CI) II: Pd/A1203 - D Ilia: Pd/CeO 2 / A l 2 0 3 IIIb: Pd/CeO2-A120 3 IV: Pd/La203-AI203 V: Pd/BaO-AI203 VI: Pd/CeO 2 TWC: Pt-Rh/AI203 d) a) b) c) d)
-rco b) (oC) • 50%
NO maximum conversion (%)
Fresh catalyst
Aged catalyst
Fresh catalyst
Aged catalyst
220 240 240 190 222 210 255 190 278
235 257 250 235 _ c) c~ _ c) _ o 300
28 46 30 20 15 14 6 23 50
11 10 11 17 _ _ _ _ 46
See table 1 for the distinctive features of the catalysts. Temperature recorded at 50% CO conversion. _: not measured. Conventional dechlorinated three-way catalyst: 1%Pt-0.2%Rh/A1203 [6].
c) c) c) c~
C. Hoang-Van et al. / Alumina-based aerogels
( P d / C e O 2 aerogel). For comparison purposes, CO and NO conversions are also given on this figure for the P d / 1 1 2 0 3 - D e g u s s a catalyst (sample II) and for a conventional dechlorinated 1% Pt-0.2% R h / A 1 2 0 3 catalyst [6]. The activity of P d / C e O 2 aerogel (sample VI in table 1; fig. 1, curve 1) is higher than that of P d / 1 1 2 0 3 aerogel (sample Ia in table 1; fig. 1, curve 2) for the conversion of CO under oxidizing conditions (s = 2.13). Also, these aerogel-based catalysts are much more active than the conventional ones, especially the P t - R h / 1 1 2 0 3 conventional three-way catalyst (TWC). The total conversion of CO is reached in the temperature range 230-350°C, over all these catalysts. Under these oxidizing conditions, comparisons between the activities of the various catalysts for NO reduction are not straightforward. At temperatures < 270°C, the alumina aerogel supported Pd catalyst is more active for NO reduction than the P d / 1 1 2 0 3 - D e g u s s a catalyst and the TWC sample. By contrast, at high temperature, the TWC sample exhibits the highest NO conversion with a maximum of 50% at about 340°C, whereas the three Pd catalysts (samples Ia, II and VI) present similar NO conversion efficiencies. It is noteworthy that Pd catalysts produce very small amounts of N20.
3.2.2. Effects of oxide additives, preparation conditions and 'aging' treatment In table 2, conversion efficiencies of the catalysts, are characterized by the light-off temperature' of CO t,, T5c0°%~J , which is the temperature at which 50% of the entering CO has disappeared, and the maximum conversion of NO. The results reported in table 2 substantiate the conclusions above and give some information about the effects of the additives, preparation procedures and aging' treatment. For fresh catalysts, the incorporation of CeO 2 or L a 2 0 3 into AI20 3 (samples IIIb and IV) by using the co-hydrolysis procedure leads to almost the same CO light-off temperature, whereas the maximum of the NO conversion slightly decreases, in comparison with the conversion efficiencies of the catalyst based on pure A120 3 aerogel (sample Ia). On the other hand, the in-
253
corporation of BaO into AI203 by co-hydrolysis brings about significant decreases of both the CO and NO conversion efficiencies (sample V). By contrast, addition of CeO 2 into A1203 by using the two-step hydrolysis procedure (sample Ilia) entails a 30°C decrease of T50~ c° as well as a slight decrease of the maximum of NO conversion, in comparison with the sample Ia. Therefore, the nature and the method of introduction of the additive influence the catalytic properties of deposited palladium. Pure CeO 2 aerogel supported Pd catalyst exhibits very high performance in CO conversion. The same efficiency is observed for the sample IIIa whose support was prepared using the twostep hydrolysis procedure (CeO2/A1203) , which tends to show that the two-step procedure entails a binary aerogel with an overlayer of the additive covering 11203 particles. By contrast, the co-hydrolysis procedure very probably brings about a good dispersion of the additive into the alumina phase leading to a solid (sample IIIb) with catalytic performance comparable to that based on pure 11203 aerogel. Results on sample V ( P d / B a O - A 1 2 0 3 ) show that the BaO additive is rather detrimental to the catalytic performance of Pd. Finally, the PdC12 precursor gives a catalyst (sample Ib) which is less active for CO oxidation but more efficient in NO reduction, in comparison with its counterpart obtained by impregnation of pure 11203 aerogel with the Pd acetylacetonate precursor. It is possible that, under the conditions of the catalytic test, residual chloride exerts an inhibiting effect on the CO oxidation but a promotional one on the reduction of NO. After the aging' treatment (1000°C, 4 h, 0 2 + 10% H 2 0 ) , the values of T50% c° increase by 1020°C, except for the most active catalyst (sample Ilia) which shows an increase of 45°C (table 2, columns 2 and 3). Simultaneously, the maximum of NO conversion decreases to a value of ~ 10%; however, samples Ilia ( P d / C e O 2 / 1 1 2 0 3 ) and TWC exhibit almost unchanged efficiencies with respect to the maximum of NO conversion (table 2, columns 4 and 5). The magnitude of the decreases in the catalytic conversion efficiencies of the aged catalysts
254
C. Hoang-Van et al. /Alumina-based aerogels
is much smaller than those observed for the surface area and the Pd dispersion (table 1). No correlations could be established between the catalytic conversion efficiencies of fresh or aged catalysts and their surfaces area and, more importantly, their metallic dispersion. It is likely that active sites of Pd catalysts in the CO + NO + 0 2 reaction are constituted of Pd ° atoms and Pd n-- ions created under the reaction mixture [7]. Further information should be obtained by taking into account the porosity of the catalysts and the effect of hydrocarbons on the conversions of CO and NO, which was beyond the scope of this work.
4. Conclusion
Alumina-based aerogel supported Pd catalysts show higher conversion efficiencies for CO oxidation in the range 150-500°C and also for NO reduction at temperatures less than ~ 270°C, as compared with catalysts deposited on commercial supports. However, for practical applications to automotive emission control, further research on
the optimization of the aerogel-based catalysts and their properties under real' conditions is needed. The authors thank Dr H. Praliaud (IRC, CNRS, Villeurbanne) for valuable suggestions and helpful discussions.
References [1] S.J. Teichner, in: Proc. 2nd Int. Symp. on Aerogels, Rev. Phys. Appl. 24 (1989) C4-1. [2] G.M. Pajonk, Appl. Catal. 72 (1991) 217. [3] R.G. Silver, J.C. Summers and W.B. Williamson, in: Proc. 2rid Int. Congr. on Catalysis and Automotive Pollution Control, Brussels, Sept. 1990, Studies Surf. Sci. Catal. 71 (1991) 167. [4] S.J. Teichner, in: Proc. 1st Int. Symp. on Aerogels, ed. J. Fricke, Springer Proceedings in Physics, Vol. 6 (Springer, Berlin, 1986) p. 22. [5l C. Hoang-Van, C. Michel, B. Pommier and S.J. Teichner, React. Kinet. Catal. Lett. 13 (1980) 63. [6] J.L. Duplan, thesis no. 91-91, University of Lyon (1991). [7] J.L. Duplan and H. Praliaud, in: Proc. 2nd Int. Congr. on Catalysis and Automotive Pollution Control, Brussels, Sept. 1990, Studies Surf. Sci. Catal. 71 (1991) 667.
]OURNA
L OF
NON-CRYSTALLIN SOLIDS E
Alumina-based aerogels as carriers for automotive palladium catalysts C. Hoang-Van, B. Pommier 1, R. H a r i v o l o l o n a a n d P. P i c h a t URA au CNRS 'Photocatalyse, Catalyse et Enuironnement ; Ecole Centrale de Lyon, B.P. 163, 69131 Ecully Cddex, France
Alumina-based aerogels were made by supercritical drying of alcogels obtained by hydrolysisof organic derivatives of AI, Ce, La and Ba. Palladium catalysts supported on these aerogels were tested in the reaction CO + NO + 02. The effects of the Pd precursor, oxide additive, preparation procedure and thermal treatment on catalytic performance were examined.
1. Introduction The sol-gel technique combined with the supercritical drying method allows simple or mixed oxide aerogels to be obtained in the amorphous state with high surface area and large pore volume [1]. These properties favour interaction between the oxides of a multicomponent aerogel [1] and, possibly, that between the aerogel and a metallic phase deposited on its surface. A review on aerogel catalysts has been published recently [2]. In attempts to develop rhodium-free catalysts, we selected palladium as a component that could be substituted for rhodium on alumina-based aerogels supports. Palladium has been reported to exhibit good catalytic performance when compared with platinum or rhodium in catalysts containing individual noble metals [3]. This paper reports the activities of palladium catalysts supported on alumina-based aerogels in the reaction CO + NO + O2, in comparison with conventional Pd or P t - R h catalysts supported on commercial carriers. The effects of oxide additives, preparation conditions, palladium precursors and thermal treatments on the catalytic performance are discussed. 1 Present address: Institut de Recherches sur la Catalyse, CNRS, 2, avenue A. Einstein, 69626 Villeurbanne C4dex, France.
2. Experimental 2.1. Preparation o f aerogels
The alumina aerogel was prepared from aluminium sec-butoxide dissolved in sec-butanol by hydrolysis. The alumina alcogel was then dried in an autoclave under supercritical conditions with respect to sec-butanol [4]. The same method was applied to prepare a CeO 2 aerogel using a solution of Ce acetylacetonate dissolved in methanol. Binary aerogels were prepared by co-hydrolyzing aluminium sec-butoxide and the corresponding metal (Ce, La or Ba) acetylacetonate, both dissolved in methanol, and by subsequent supercritical drying. The proportion of CeO2, L a 2 0 3 or BaO incorporated into A120 3 was fixed at approximately 15 wt%. A binary C e O 2 / A 1 2 0 3 aerogel was prepared by using a two-step hydrolysis procedure. A120 3 prepared in a separate operation was introduced into a methanolic solution of Ce acetylacetonate which was then hydrolyzed and the binary alcogel thus obtained was supercritically dried. 2.2. Preparation o f palladium catalysts
The catalysts were prepared by impregnating the above aerogel supports with a solution of Pd acetylacetonate or PdCI 2 dissolved in methanol.
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
C. Hoang-Van et al. / Alum#la-based aerogels T h e Pd 1 wt%.
content
was
fixed
at
approximately
2.3. Characterization S u r f a c e a r e a s w e r e d e t e r m i n e d f r o m N 2 ads o r p t i o n at - 196°C. P r i o r to the a d s o r p t i o n m e a s u r e m e n t s , t h e s a m p l e s w e r e t r e a t e d in a H e flow at 250°C for 2 h. T h e d i s p e r s i o n o f p a l l a d i u m was m e a s u r e d by C O c h e m i s o r p t i o n at r o o m t e m p e r a t u r e , a s s u m ing t h e s t o i c h i o m e t r y C O / P d s u r f a c e = 1, by use o f the p u l s e c h r o m a t o g r a p h i c t e c h n i q u e [5]. P r i o r to t h e C O c h e m i s o r p t i o n m e a s u r e m e n t s , the samples w e r e r e d u c e d by H 2 at 500°C for 3 h. T h e y a r e r e f e r r e d to as f r e s h ' catalysts. S o m e catalysts w e r e t r e a t e d at 1000°C in flowing 0 2 c o n t a i n i n g 10% o f w a t e r v a p o u r for 4 h. T h e s e s a m p l e s a r e d e s i g n a t e d as a g e d ' catalysts.
2.4. Activity measurements Catalytic e x p e r i m e n t s w e r e p e r f o r m e d in a flow system at a t m o s p h e r i c p r e s s u r e . R e a c t a n t a n d p r o d u c t gases w e r e a n a l y z e d by gas c h r o m a t o g r a phy with a t h e r m a l conductivity d e t e c t o r ( 0 2 , C O , CO2, N 2 0 ) or i n f r a r e d s p e c t r o s c o p y (NO). A c t i v a t i o n o f t h e catalyst (100 mg) m i x e d with inactive c~-A120 3 (400 mg) c o n s i s t e d in a t r e a t m e n t at 500°C in a flowing r e a c t i o n m i x t u r e
251
(20 l / h ) for 3 h. T h e c o n v e r s i o n s o f C O a n d N O w e r e m e a s u r e d at i n c r e a s i n g t e m p e r a t u r e s in t h e r a n g e 1 5 0 - 5 0 0 ° C at a p r o g r a m m e d r a t e of 2 ° C / m i n . T h e gas mixture u s e d (0.75% C O + 0.1% N O + 0.75% 0 2 + N 2 ( d i l u e n t ) ) was m a r k e d l y oxidizing since t h e s t o i c h i o m e t r y n u m b e r was s = [ 2 ( 0 2) + ( N O ) ] / ( C O ) = 2.13.
3. Results and discussion
3.1. Textural properties of palladium catalysts T a b l e 1 shows t h e s u r f a c e a r e a s a n d m e t a l l i c d i s p e r s i o n s o f p a l l a d i u m catalysts p r e p a r e d by i m p r e g n a t i o n o f a e r o g e l s with Pd a c e t y l a c e t o n a t e , with t h e e x c e p t i o n o f t h e s a m p l e s Ib, o b t a i n e d by i m p r e g n a t i o n of A l z O 3 a e r o g e l with PdCI2, a n d II, m a d e f r o m Pd a c e t y l a c e t o n a t e a n d g a m m a A I 2 0 3 x e r o g e l (Degussa), for c o m p a r i s o n p u r poses. T h e t e x t u r a l p r o p e r t i e s o f a e r o g e l s d e p e n d on p r e p a r a t i o n p a r a m e t e r s such as the a m o u n t of w a t e r i n t r o d u c e d into t h e alcoholic solution to hydrolyze t h e o r g a n i c d e r i v a t i v e of the m e t a l o r t h e c o n c e n t r a t i o n o f t h a t derivative in t h e solvent [4]. Single a e r o g e l s p r e p a r e d in this w o r k (samples Ia a n d VI) exhibit lower B E T s u r f a c e a r e a s t h a n t h e b i n a r y a e r o g e l s ( t a b l e 1). In p a r t i c u l a r , high s u r f a c e a r e a a e r o g e l s a r e o b t a i n e d by co-hy-
Table 1 Textural characteristics of aerogel supported palladium catalysts Catalyst a) la: Pd/AI203 Ib: Pd/AIzO3(CI) b) II: Pd/A1203-D c) Ilia: Pd/CeO 2/AI203 dl IIIb: P d / C e O 2 - A I 2 0 3 IV: Pd/La203-AI203 V: Pd/BaO-AI203 VI: Pd/CeO 2
Surface area (m 2 g ~)
Palladium dispersion (%)
Fresh catalyst
Aged catalyst
Fresh catalyst
130 130 106 173 505 333 446 65
69.0 70.0 79.0 64.0 71.0 69.0 89.0 8.5
28.1 25.3 25.0 20.7 14.0 15.2 8.5 25.2
~) Palladium loading: ~ 1 wt%. b) Pd precursor: PdCl 2. °) AlzO 3 xerogel (Degussa, gamma structure), d) Mixed aerogel obtained by a two-step hydrolysis procedure (see text). °) -: not determined.
Aged catalyst 6.0 3.8 7.0 4.6 - °) _ o7 _ e) 5.0
C. Hoang-Van et aL / Alumina-based aerogels
252 100
.~_ >~ co
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~
90 80 70 60 50 40 30
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350
400
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500
Temperature ( ° C) Fig. 1. CO and NO conversions as a function of the reaction temperature. Reaction mixture: 0.'75% CO + 0.1% NO + 0.75% 02. 1, Pd/CeO 2 aerogel; 2, P d / A l 2 0 3 aerogel; 3, Pd/AlzO3-Degussa. Dashed lines: dechlorinated 1% Pt-0.fi% Rh/A1203 [6].
drolysis o f a l u m i n u m s e c - b u t o x i d e w i t h C e , L a o r Ba acetylacetonate (300-500 m 2 g-l). The Pd d i s p e r s i o n o f f r e s h ' catalysts v a r i e s f r o m 15 to 3 0 % ( t a b l e 1), a p a r t f r o m s a m p l e V ( P d / B a O A 1 2 0 3) w h o s e d i s p e r s i o n was s i g n i f i c a n t l y s m a l l e r ( 8 . 5 % , t a b l e 1). I n all cases, t h e r m a l t r e a t m e n t at 1000°C in f l o w i n g 0 2 + 1 0 % H 2 0 f o r 4 h ( a g e d ' catalysts) p r o d u c e s a l a r g e d e c r e a s e o f t h e surf a c e a r e a as w e l l as t h e P d d i s p e r s i o n o f t h e samples. The binary BaO-A120 3 aerogel support
exhibits the highest surface area after b l e 1, 3rd c o l u m n ) .
a g i n g ' (ta-
3.2. Catalytic performance 3.2.1. Effect of the nature of single aerogel support Figure 1 shows the CO and NO conversions, for the mixture 0.75% CO + 0.1% NO + 0.75% 0 2, as a f u n c t i o n o f t h e r e a c t i o n t e m p e r a t u r e f o r t h e s a m p l e s Ia ( P d / A 1 2 0 3 a e r o g e l ) a n d V I
Table 2 Conversion efficiencies of catalysts with the mixture 0,75% CO+0.1% NO+0.75% 02 (s = 2.13) Catalyst a)
Ia: Pd/A1203 Ib: Pd/AI203(CI) II: Pd/A1203 - D Ilia: Pd/CeO 2 / A l 2 0 3 IIIb: Pd/CeO2-A120 3 IV: Pd/La203-AI203 V: Pd/BaO-AI203 VI: Pd/CeO 2 TWC: Pt-Rh/AI203 d) a) b) c) d)
-rco b) (oC) • 50%
NO maximum conversion (%)
Fresh catalyst
Aged catalyst
Fresh catalyst
Aged catalyst
220 240 240 190 222 210 255 190 278
235 257 250 235 _ c) c~ _ c) _ o 300
28 46 30 20 15 14 6 23 50
11 10 11 17 _ _ _ _ 46
See table 1 for the distinctive features of the catalysts. Temperature recorded at 50% CO conversion. _: not measured. Conventional dechlorinated three-way catalyst: 1%Pt-0.2%Rh/A1203 [6].
c) c) c) c~
C. Hoang-Van et al. / Alumina-based aerogels
( P d / C e O 2 aerogel). For comparison purposes, CO and NO conversions are also given on this figure for the P d / 1 1 2 0 3 - D e g u s s a catalyst (sample II) and for a conventional dechlorinated 1% Pt-0.2% R h / A 1 2 0 3 catalyst [6]. The activity of P d / C e O 2 aerogel (sample VI in table 1; fig. 1, curve 1) is higher than that of P d / 1 1 2 0 3 aerogel (sample Ia in table 1; fig. 1, curve 2) for the conversion of CO under oxidizing conditions (s = 2.13). Also, these aerogel-based catalysts are much more active than the conventional ones, especially the P t - R h / 1 1 2 0 3 conventional three-way catalyst (TWC). The total conversion of CO is reached in the temperature range 230-350°C, over all these catalysts. Under these oxidizing conditions, comparisons between the activities of the various catalysts for NO reduction are not straightforward. At temperatures < 270°C, the alumina aerogel supported Pd catalyst is more active for NO reduction than the P d / 1 1 2 0 3 - D e g u s s a catalyst and the TWC sample. By contrast, at high temperature, the TWC sample exhibits the highest NO conversion with a maximum of 50% at about 340°C, whereas the three Pd catalysts (samples Ia, II and VI) present similar NO conversion efficiencies. It is noteworthy that Pd catalysts produce very small amounts of N20.
3.2.2. Effects of oxide additives, preparation conditions and 'aging' treatment In table 2, conversion efficiencies of the catalysts, are characterized by the light-off temperature' of CO t,, T5c0°%~J , which is the temperature at which 50% of the entering CO has disappeared, and the maximum conversion of NO. The results reported in table 2 substantiate the conclusions above and give some information about the effects of the additives, preparation procedures and aging' treatment. For fresh catalysts, the incorporation of CeO 2 or L a 2 0 3 into AI20 3 (samples IIIb and IV) by using the co-hydrolysis procedure leads to almost the same CO light-off temperature, whereas the maximum of the NO conversion slightly decreases, in comparison with the conversion efficiencies of the catalyst based on pure A120 3 aerogel (sample Ia). On the other hand, the in-
253
corporation of BaO into AI203 by co-hydrolysis brings about significant decreases of both the CO and NO conversion efficiencies (sample V). By contrast, addition of CeO 2 into A1203 by using the two-step hydrolysis procedure (sample Ilia) entails a 30°C decrease of T50~ c° as well as a slight decrease of the maximum of NO conversion, in comparison with the sample Ia. Therefore, the nature and the method of introduction of the additive influence the catalytic properties of deposited palladium. Pure CeO 2 aerogel supported Pd catalyst exhibits very high performance in CO conversion. The same efficiency is observed for the sample IIIa whose support was prepared using the twostep hydrolysis procedure (CeO2/A1203) , which tends to show that the two-step procedure entails a binary aerogel with an overlayer of the additive covering 11203 particles. By contrast, the co-hydrolysis procedure very probably brings about a good dispersion of the additive into the alumina phase leading to a solid (sample IIIb) with catalytic performance comparable to that based on pure 11203 aerogel. Results on sample V ( P d / B a O - A 1 2 0 3 ) show that the BaO additive is rather detrimental to the catalytic performance of Pd. Finally, the PdC12 precursor gives a catalyst (sample Ib) which is less active for CO oxidation but more efficient in NO reduction, in comparison with its counterpart obtained by impregnation of pure 11203 aerogel with the Pd acetylacetonate precursor. It is possible that, under the conditions of the catalytic test, residual chloride exerts an inhibiting effect on the CO oxidation but a promotional one on the reduction of NO. After the aging' treatment (1000°C, 4 h, 0 2 + 10% H 2 0 ) , the values of T50% c° increase by 1020°C, except for the most active catalyst (sample Ilia) which shows an increase of 45°C (table 2, columns 2 and 3). Simultaneously, the maximum of NO conversion decreases to a value of ~ 10%; however, samples Ilia ( P d / C e O 2 / 1 1 2 0 3 ) and TWC exhibit almost unchanged efficiencies with respect to the maximum of NO conversion (table 2, columns 4 and 5). The magnitude of the decreases in the catalytic conversion efficiencies of the aged catalysts
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C. Hoang-Van et al. /Alumina-based aerogels
is much smaller than those observed for the surface area and the Pd dispersion (table 1). No correlations could be established between the catalytic conversion efficiencies of fresh or aged catalysts and their surfaces area and, more importantly, their metallic dispersion. It is likely that active sites of Pd catalysts in the CO + NO + 0 2 reaction are constituted of Pd ° atoms and Pd n-- ions created under the reaction mixture [7]. Further information should be obtained by taking into account the porosity of the catalysts and the effect of hydrocarbons on the conversions of CO and NO, which was beyond the scope of this work.
4. Conclusion
Alumina-based aerogel supported Pd catalysts show higher conversion efficiencies for CO oxidation in the range 150-500°C and also for NO reduction at temperatures less than ~ 270°C, as compared with catalysts deposited on commercial supports. However, for practical applications to automotive emission control, further research on
the optimization of the aerogel-based catalysts and their properties under real' conditions is needed. The authors thank Dr H. Praliaud (IRC, CNRS, Villeurbanne) for valuable suggestions and helpful discussions.
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