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Reduction of Nitric Oxide by Carbon Monoxide on Palladium Based Bimetallic Catalysts
Guni, L e~ al. (Editors),New Frontiers in Caralysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rightP reserved
REDUCTION OF NITRIC OXIDE BY CARBON MONOXIDE ON PALLADIUM BASED BIMETALLIC CATALYSTS J. Massardier,A. El Hamdaoui, G. Bergeret and A. Renouprez
Institut de Recherches sur la Catalyse, 69626 Villeurbanne Cedex, France
Abstract Modifications of the Pd reactivity for the CO-NO reaction in presence of 0 have been studied after addition of Cr and Ag. Addition of Cr enhances the P8 activity for this reaction. This effect is discussed in terms of modifications of Pd electronic properties by Cr. 1. INTRODUCTION The reduction of pollution from automotive vehicles is now usually achieved by using catalysts where rhodium is associated with palladium or platinum to perform the reduction of NO in the presence of air. This depollution process implies the reduction of NO, in competition with the oxidation of CO by 0 2 .
+ NO -----> C 0 2 + co + +co----> c 0 2 CO
b
+N2
It is also known [l-2 that palladium alone is active for reaction (2). but far less for reaction (I), proba ly because of its low ability to dissociate NO [3]: it is thus expected that reactions (I) and 2) would not occur in the same temperature range in the presence of oxygen. T e aim of this work is to investi ate the effect of the addition of Cr and silver to palladium on its reactivity or the CO-NO reaction in presence of 0 2 .
h
k
2. SAMPLE PREPARATION AND CHARACTERIZATION 2.1. Palladium-chromium The preparation and characterization of the Pd-Cr/SiO and Pd-Cr/charcoal were described elsewhere [4]. A prereduced monometa lic Pd catalyst was impregnated with chromium nitrate, calcined at 600 K and reduced at 900 K in hydrogen in the case of the silica support or directly reduced for the carbon support. EXAFS has shown that 50 % of the chromium is reduced and incorporated to palladium for silica supported samples whereas this metal is fully reduced in the case of the charcoal su port, The composition range is limited to Pd87-Cr13 on silica and P d g ~ C r 5on c arcoal.
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271 0
2.2. Palladium-Silver catalysts The silica supported Pd-Ag catalysts were prepared by coimpregnation with nitrates; the samples were dried, calcined under oxygen at 600 K and reduced in flowing hydrogen between 500 and 900 K. The formation of the solid solution was followed by in situ X-ray diffraction performed under hydro en at various temperatures. Figure l a shows that alloying already begins at 500 . Between 700 and 900 K. no appreciable increase of the particle size measured by electron microscopy was observed, but one can detect on Figure l b a sharpening of the X-ray profile with a better separation between the ( 1 1 I ) and (200) lines: an homogenization of the alloy has occurred.
k
Figure 1. X-ray spectra of Pd-Ag/SiOz upon heating at 500 K (a) and 620 K (b). EDX-STEM shows that the composition of the particles is distributed in a 20 % ran e about the mean value. Most of the particles have a diameter of 2 nm and are P enriched, but a small number of 4-5 nm particles are Ag enriched. f
d
Table 1 ComDosition and disDersion of the catalysts Pd/SiO2(0,41%) Pdg7Cr13BiOz Samples
Pdg~CrslCh
Pds&s@iO2
4
5 Pdz$C;30
~
a(nm)[EM, S A X S ] STEM (at YO) H/MS (H2 Chem.)
1.5
=1
3.2 Pd94Cr6 = 0.40
3. CHEMISORPTIVE PROPERTIES 3.1. Hydrogen chemisorption The H adsorption is strongly reduced on both types of catalysts compared to pure Pd &able 1). In the case of Pd-Cr. the proportion of Cr is too low to cover an appreciable amount of the surface of the particles with 50 Yo dispersion. Moreover, EXAFS has shown that Cr is in interaction with Pd. The reduced so tion capacity can thus be attributed to an electronic effect. The situation IS different for the Pd-Ag catalysts where the Ag concentration is 50 %. Moreover, it is known from the study of bulk alloys, that Ag has a marked tendency to segregate at the surface. The reduction of the hydrogen adsorption can thus be attributed to silver enrichment of the surface.
271 1
3.2. CO and NO adsorption, CO-NOcoadsorption Silver has a strong mfluence on the TDS of CO. After a 300 K adsorption it weakens the bonding of CO, by suppressin the multibonded species. Not so important effect has been observed with r, where the multibonded species desorb near 520 K. Conversely neither Ag or Cr have influence on the chimiso tion of NO which is desorbed without dissociation. Upon coadsorption of a C O X 0 equimolecular mixture at 300 K, the gas are also desorbed without decomposition but the ratios of the surface coverages CO/NO are strongly influenced by presence of Cr as shown in Table 2 and not by Ag. If the coadsorption is performed at 425 K, only C 0 2 and N are evolved at the same temperature from the PdCr catalyst, whereas NO is t e main product evolved from Pd5dg50. An intermediary situation is observed on Pd/SiO2.
E
t
Table 2 Ratios of the N O K O surface coverages for 300 and 425 K coadsomtion Samples Pd pag PdCr/Si02 COads/NOads(300K) CO /N (STR425K) CO?NC$ reactive species)
= 0.05
= 0,05
= 0.12
0.58 0.29
s 0.07
0.7
0.35
s 0,035
From the ratio between C 0 2 and N2 evolved during the surface thermoreaction (STR) at 425 K, the ratio between the reactive adsorbed CO and NO s ecies may be deduced. Indeed assuming a reaction occurring between adsorled CO and molecularly or dissociatively adsorbed NO, the CO2/N2 ratio is expected to be 2, if the respective adsorbed reactive species amounts were similar. The measured CO2/N2 ratio .is always smaller. Therefore the reactive adsorbed CO amount is always smaller than the reactive NO adsorbed amount but it increases from Pd to PdCr and is very weak on PdAg sample (Table 2).
c
CI
!
.acn loo
.a '00
5 c
Figure 2. CO-NO conversion on the various samples in the presence of oxygen. Pd
1---'
CO-NO co-02
I *-..-* *
Co-No co-02
PdCr
I --.. . . ...
CO-NO co-02
271 2
4. CONVERSION OF NO-CO IN THE PRESENCE OF OXYGEN
This reaction was studied under flow, as a function of temperature, in a gas mixture where CO/NO = I and 2(02) + (NO)/CO a 1. I . As can be seen for Figure 2, the temperature of half conversion for reaction (1) is decreased from 580 K on ure Pd to 545 K on Pd-Cr catalysts. On the contrary it reaches 675 K on bd59Ag5 Sam les. It is notworth also that carbon supported Pd-Cr are more efficient t an d-Cr/Si02 probaby because more Cr IS inco orated into the network of Pd. With respect to the direct oxidation of CO by 0 (reaction 2) the temperatures of half conversion increase from PdCr/Ch to PdAglSi02 (Figure 2 Thus, Cr has a positive effect and the PdCr samples have a behaviour compara le to that of Rh.
II
$ 8
T
b.
5. DISCUSSION AND CONCLUSION
a
The addition of Cr to Pd increases the activity of the noble metal for the NOCO reaction. The ex lanation, in agreement with our results would, be an increase of the CO/N ratio between the adsorbed species (Table 2). Moreover, the CO-NO reaction is greatly favoured on this catalyst (Figure 2) although the CO-02 reaction, in absence of NO, is expected to be faster [ 5 ] . Therefore NO would inhibate strongly the direct oxidation de CO. The enhanced reactivity of PdCr sample against the CO-NO reactivity would result of two factors: a Pd6- as deduced of XPS results with a downwards shift of the Pd3d levels of 10.7 eV [4], which influences the de ree of char e transfer from Pd to n* antibonding orbital of NO, leading to P N o s - adsor ed species, as discussed by Oh and coworkers on Rh [6]. the presence of Cr where the CO adsorption could occur. With these assumptions, the dissociation of NO is expected to be fast, the rate limiting step occurring between NO and 0 adsorbed species. With such a limiting step the kinetic equation can be written as:
-
d
-
dNO = k P N o / [ 1 + K l P c o + K ~ P N o+ a dt
%
PNo/Pco
+ p1
which agrees with the orders with respect to CO and NO which are (for P c d P ~ =o 1) ne ative for CO and positive for NO. (The two last terms in the denominator are c fue to the coverages by 0 and N atoms formed by the NO dissociation).
6. REFERENCES
L. M. Carballo. T. Haher and H.G. Linz, A pl. Surf. Sci., 40 (1989) 53. W.F. Egelhoff, The Chemical Physics of lolid Surfaces and Heterogeneous Catalysis, D . A . King and D.P. Woodruff (eds. ,) Elsevier Pub., Amsterdam, 4 (1984) 107. H. Conrad, G. Ertl and J. Kup er, Surf. Sci. 110 (1981) 227. A. Bor na, B. Moraweck, J. assardier and A. Renouprez, J. Catal., 128 1991) f 9 H. 0 h a n d J . E . Carpenter, J. Catal., 101, (1986) 114. W.C. Hecker and A. T. Bell, J. Catal., 84 (1983) 200.
6,
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REDUCTION OF NITRIC OXIDE BY CARBON MONOXIDE ON PALLADIUM BASED BIMETALLIC CATALYSTS J. Massardier,A. El Hamdaoui, G. Bergeret and A. Renouprez
Institut de Recherches sur la Catalyse, 69626 Villeurbanne Cedex, France
Abstract Modifications of the Pd reactivity for the CO-NO reaction in presence of 0 have been studied after addition of Cr and Ag. Addition of Cr enhances the P8 activity for this reaction. This effect is discussed in terms of modifications of Pd electronic properties by Cr. 1. INTRODUCTION The reduction of pollution from automotive vehicles is now usually achieved by using catalysts where rhodium is associated with palladium or platinum to perform the reduction of NO in the presence of air. This depollution process implies the reduction of NO, in competition with the oxidation of CO by 0 2 .
+ NO -----> C 0 2 + co + +co----> c 0 2 CO
b
+N2
It is also known [l-2 that palladium alone is active for reaction (2). but far less for reaction (I), proba ly because of its low ability to dissociate NO [3]: it is thus expected that reactions (I) and 2) would not occur in the same temperature range in the presence of oxygen. T e aim of this work is to investi ate the effect of the addition of Cr and silver to palladium on its reactivity or the CO-NO reaction in presence of 0 2 .
h
k
2. SAMPLE PREPARATION AND CHARACTERIZATION 2.1. Palladium-chromium The preparation and characterization of the Pd-Cr/SiO and Pd-Cr/charcoal were described elsewhere [4]. A prereduced monometa lic Pd catalyst was impregnated with chromium nitrate, calcined at 600 K and reduced at 900 K in hydrogen in the case of the silica support or directly reduced for the carbon support. EXAFS has shown that 50 % of the chromium is reduced and incorporated to palladium for silica supported samples whereas this metal is fully reduced in the case of the charcoal su port, The composition range is limited to Pd87-Cr13 on silica and P d g ~ C r 5on c arcoal.
21
R
271 0
2.2. Palladium-Silver catalysts The silica supported Pd-Ag catalysts were prepared by coimpregnation with nitrates; the samples were dried, calcined under oxygen at 600 K and reduced in flowing hydrogen between 500 and 900 K. The formation of the solid solution was followed by in situ X-ray diffraction performed under hydro en at various temperatures. Figure l a shows that alloying already begins at 500 . Between 700 and 900 K. no appreciable increase of the particle size measured by electron microscopy was observed, but one can detect on Figure l b a sharpening of the X-ray profile with a better separation between the ( 1 1 I ) and (200) lines: an homogenization of the alloy has occurred.
k
Figure 1. X-ray spectra of Pd-Ag/SiOz upon heating at 500 K (a) and 620 K (b). EDX-STEM shows that the composition of the particles is distributed in a 20 % ran e about the mean value. Most of the particles have a diameter of 2 nm and are P enriched, but a small number of 4-5 nm particles are Ag enriched. f
d
Table 1 ComDosition and disDersion of the catalysts Pd/SiO2(0,41%) Pdg7Cr13BiOz Samples
Pdg~CrslCh
Pds&s@iO2
4
5 Pdz$C;30
~
a(nm)[EM, S A X S ] STEM (at YO) H/MS (H2 Chem.)
1.5
=1
3.2 Pd94Cr6 = 0.40
3. CHEMISORPTIVE PROPERTIES 3.1. Hydrogen chemisorption The H adsorption is strongly reduced on both types of catalysts compared to pure Pd &able 1). In the case of Pd-Cr. the proportion of Cr is too low to cover an appreciable amount of the surface of the particles with 50 Yo dispersion. Moreover, EXAFS has shown that Cr is in interaction with Pd. The reduced so tion capacity can thus be attributed to an electronic effect. The situation IS different for the Pd-Ag catalysts where the Ag concentration is 50 %. Moreover, it is known from the study of bulk alloys, that Ag has a marked tendency to segregate at the surface. The reduction of the hydrogen adsorption can thus be attributed to silver enrichment of the surface.
271 1
3.2. CO and NO adsorption, CO-NOcoadsorption Silver has a strong mfluence on the TDS of CO. After a 300 K adsorption it weakens the bonding of CO, by suppressin the multibonded species. Not so important effect has been observed with r, where the multibonded species desorb near 520 K. Conversely neither Ag or Cr have influence on the chimiso tion of NO which is desorbed without dissociation. Upon coadsorption of a C O X 0 equimolecular mixture at 300 K, the gas are also desorbed without decomposition but the ratios of the surface coverages CO/NO are strongly influenced by presence of Cr as shown in Table 2 and not by Ag. If the coadsorption is performed at 425 K, only C 0 2 and N are evolved at the same temperature from the PdCr catalyst, whereas NO is t e main product evolved from Pd5dg50. An intermediary situation is observed on Pd/SiO2.
E
t
Table 2 Ratios of the N O K O surface coverages for 300 and 425 K coadsomtion Samples Pd pag PdCr/Si02 COads/NOads(300K) CO /N (STR425K) CO?NC$ reactive species)
= 0.05
= 0,05
= 0.12
0.58 0.29
s 0.07
0.7
0.35
s 0,035
From the ratio between C 0 2 and N2 evolved during the surface thermoreaction (STR) at 425 K, the ratio between the reactive adsorbed CO and NO s ecies may be deduced. Indeed assuming a reaction occurring between adsorled CO and molecularly or dissociatively adsorbed NO, the CO2/N2 ratio is expected to be 2, if the respective adsorbed reactive species amounts were similar. The measured CO2/N2 ratio .is always smaller. Therefore the reactive adsorbed CO amount is always smaller than the reactive NO adsorbed amount but it increases from Pd to PdCr and is very weak on PdAg sample (Table 2).
c
CI
!
.acn loo
.a '00
5 c
Figure 2. CO-NO conversion on the various samples in the presence of oxygen. Pd
1---'
CO-NO co-02
I *-..-* *
Co-No co-02
PdCr
I --.. . . ...
CO-NO co-02
271 2
4. CONVERSION OF NO-CO IN THE PRESENCE OF OXYGEN
This reaction was studied under flow, as a function of temperature, in a gas mixture where CO/NO = I and 2(02) + (NO)/CO a 1. I . As can be seen for Figure 2, the temperature of half conversion for reaction (1) is decreased from 580 K on ure Pd to 545 K on Pd-Cr catalysts. On the contrary it reaches 675 K on bd59Ag5 Sam les. It is notworth also that carbon supported Pd-Cr are more efficient t an d-Cr/Si02 probaby because more Cr IS inco orated into the network of Pd. With respect to the direct oxidation of CO by 0 (reaction 2) the temperatures of half conversion increase from PdCr/Ch to PdAglSi02 (Figure 2 Thus, Cr has a positive effect and the PdCr samples have a behaviour compara le to that of Rh.
II
$ 8
T
b.
5. DISCUSSION AND CONCLUSION
a
The addition of Cr to Pd increases the activity of the noble metal for the NOCO reaction. The ex lanation, in agreement with our results would, be an increase of the CO/N ratio between the adsorbed species (Table 2). Moreover, the CO-NO reaction is greatly favoured on this catalyst (Figure 2) although the CO-02 reaction, in absence of NO, is expected to be faster [ 5 ] . Therefore NO would inhibate strongly the direct oxidation de CO. The enhanced reactivity of PdCr sample against the CO-NO reactivity would result of two factors: a Pd6- as deduced of XPS results with a downwards shift of the Pd3d levels of 10.7 eV [4], which influences the de ree of char e transfer from Pd to n* antibonding orbital of NO, leading to P N o s - adsor ed species, as discussed by Oh and coworkers on Rh [6]. the presence of Cr where the CO adsorption could occur. With these assumptions, the dissociation of NO is expected to be fast, the rate limiting step occurring between NO and 0 adsorbed species. With such a limiting step the kinetic equation can be written as:
-
d
-
dNO = k P N o / [ 1 + K l P c o + K ~ P N o+ a dt
%
PNo/Pco
+ p1
which agrees with the orders with respect to CO and NO which are (for P c d P ~ =o 1) ne ative for CO and positive for NO. (The two last terms in the denominator are c fue to the coverages by 0 and N atoms formed by the NO dissociation).
6. REFERENCES
L. M. Carballo. T. Haher and H.G. Linz, A pl. Surf. Sci., 40 (1989) 53. W.F. Egelhoff, The Chemical Physics of lolid Surfaces and Heterogeneous Catalysis, D . A . King and D.P. Woodruff (eds. ,) Elsevier Pub., Amsterdam, 4 (1984) 107. H. Conrad, G. Ertl and J. Kup er, Surf. Sci. 110 (1981) 227. A. Bor na, B. Moraweck, J. assardier and A. Renouprez, J. Catal., 128 1991) f 9 H. 0 h a n d J . E . Carpenter, J. Catal., 101, (1986) 114. W.C. Hecker and A. T. Bell, J. Catal., 84 (1983) 200.
6,
R