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Effects of sintering and of additives on the oxygen storage capacity of PtRh catalysts
A. Frennet and J.-M. Bastin (Eds.) Catalysis and Automotive Pollution Control 111
Studies in Surface Science and Catalysis, Vol. 96 1995 Elsevier Science B.V.
801
EFFECTS OF SINTERING AND OF ADDITIVES ON THE OXYGEN STORAGE CAPACITY OF PtRh CATALYSTS.
D. Martin, R.Taha and D. Duprez Laboratoire de Catalyse en Chimie Organique, URA 350 CNRS 40 Av. du Recteur Pineau, 86022 Poitiers Cedex, France
ABSTRACT : PtRh/A1203 and PtRh/CeO2-AI203 bimetallic catalysts (Pt+Rh = 60 lamolg-1) were prepared via chlorine-free precursors. Oxygen storage capacities (OSC) were measured on the fresh (calc. 723K) and on the sintered catalysts (1%O2 + 10%H20, 2h, 973K and 1173K). On alumina catalysts, only Rh can promote OSC which is extremely sensitive to sintering. OSC values are higher on alumina-ceria catalysts, but do not depend on the composition of the bimetallics. Moreover ceria renders the catalysts resistant to sintering. PtRh/A1203 and PtRh/CeO2-Al203 were modified by C1, SO42- and K. On A1203, OSC variations due to the additives follow the same trend as the variations of oxygen mobility (deduced from 180/160 isotopic exchange). Chlorine and sulfur are inhibitors of OSC while K, at low content, is a promotor.
1. INTRODUCTION
Rare earth oxides, especially cerium oxide, are used to improve the oxygen storage capacity (OSC) of three-way catalysts. Noble metals, in particular rhodium, play ml active role in promoting the OSC of the support [1-5]. The mechanism of OSC can be described in terms of three principal steps : - dissociative adsorption and desorption of molecular oxygen on the metals - transfer of active oxygen species from the metal to the support and surface migration of these species on the support - storage of oxygen by cerium oxide.
802 Duprez m~d al. investigated the first and the second step by means of 160/180 isotopic exchange [6-8]. The rate of step 1 can be deduced from the rate of the 1602/1802 equilibration reaction that occurs on the metal : 1802(gas) + 1602(gas) ~
2 18O160(gas )
(1)
while the rate of step 2 is in direct relation with the rate of isotopic exchange of 1802 with the 160 of the support 9 1802(gas) + 160(sup.) -+ 180160(gas) + 180(sup.)
(2)
The surface of ceria being readily reduced at low temperature (< 473K) in the presence of noble metals [1,4], step 3 is expected to be very rapid under the usual conditions of exhaust gas catalysis (T=673-773K). Duprez and Kacimi [7] showed that rhodiuln was the metal which has the highest intrinsic rate in reaction (1). Accordingly the rate determining step (rds) of the mechanism of OSC oll rhodium should be the transfer and the migration of oxygen at the surface of the support. Our aim, in the first part of this paper, is to study the effect of the sintering of PtRh/A1203 and PtRh/CeO2-A1203 catalysts on the OSC and to correlate the OSC with the surface migration of oxygen measured by 1802(gas)/160(support) isotopic exchange [6,8]. In the second part of this paper, we study the effects of certain additives (SO42-, C1) on the OSC values and on the oxygen mobility in the catalysts. As we have shown recently that the surface mobility of oxygen was linked to the basicity of oxides used as supports [8], we have also investigated the effect of potassium.
2. EXPERIMENTAL The support was a y-A1203 (100 m2g -1) supplied by IFP. A 12wt.% CeO2A1203 support was prepared by impregnating the 7-A1203 with aqueous solutions of ceric alnmonium nitrate. The catalysts used in the sintering studies were prepared by successive impregnations with aqueous solutions of dinitrodiamlnine platinum(II) and of rhodium(III) nitrate. After impregnation, the catalysts were dried and calcined at 723K (flesh catalysts). They are referred to here as PtRhXA
803 and PtRhXCA, where A desigamtes alumina, CA, ceria-alumina and X is the atomic percentage of rhodium : % (Rh / Pt+Rh). Aliquot samples of each catalyst were sintered for 2 h at 973K and at 1173K under a continuous flow of 1%02 + 10%H20 (vol%) in nitrogen. In the second phase of the experiments (effect of additives), Rh catalysts prepared on two different supports 9 y-A1203 (100 m2g "1, IFP), CeO2/y-AI203 (93 m2g-1, on IFP by impregnating a hydrochloric acid dried and calcined
alumina) were used. All the modified catalysts were prepared fresh catalyst with aqueous solutions of dimnmonium sulfate, or potassium nitrate. After modification, the catalysts were at 723K. They are referred to here as RhSyP, where y is the
weight percentage of the additive P (P=SO42-, C1 or K) and S is for the type of support (S=A for alumina and CA for ceria-alumina). The weight percentages of rhodium are 0.51 for catalysts supported on alumina and 0.53 on ceria-alumina. Dispersion measurements were carried out on fresh catalysts in a pulse chromatographic system previously described [9,10]. The catalysts were reduced at 723K in H2 degassed in Ar (impurities < l ppm) and cooled down to room temperature. Hydrogen chemisorption (HE), oxygen (OT) and hydrogen (HT) titrations were successively recorded. Metal dispersion was deduced from oxygen titration of the chelnisorbed hydrogen using the following stoichiometries 9H/Rhs =1, O/Rhs = 1.5 and H/Pts = O/Pts = 1. OSC and OSCC (Oxygen Storage Capacity Complete) measurements were carried out in a pulse chromatographic system. The catalyst sample (10 to 50 mg) was inserted in a U-quartz reactor, heated to 723K (4 Kmin-1) in a helium flow (30cm3min-1 ; less than 1 ppm impurities) and then oxidized to saturation by 02 pulses (0.268 cln 3) injected every other minute ("oxygen storage"). The CO pulses were then injected, at TOSC (423K< TOSC <723K), every other minute. OSC values were deterlnined from the amount of CO2 produced at the first pulse of CO (titration of the "fast" oxygen) and OSCC, from the amount of CO2 produced at the five first pulses of CO (titration of the "complete" oxygen storage). Isotopic exchange experiments were carried out in a recycle reactor coupled to a mass-spectrometer [11,12]. "In situ" pretreaments were 9 160 2 at 723K for 0.25 h, H2 at 723K for 0.25 h, outgassing at 723K for 0.5 h and cooling down to the temperature of exchange before admission of 1802. Mass spectra (1802 , 18016 0 and 1602, plus mass 18, 28 and 44 to detect possible leaks) were
804 recorded every nine seconds, which allowed us to determine the coefficient D s of surface diffusion [8,12] from equation (3) 9 Ne= 4Na
180
~o-~Dst
C
where Ne is the number of diffusing species, a is the radius and C 18 0 concentration of 180 of the N circular metal particles.
(3) the
3. RESULTS AND DISCUSSION 3.1 Catalyst characteristics 9
Table 1 gives the characteristics of the alumina-supported catalysts. Fresh catalysts and particularly RbJA1203 are well-dispersed. During treatments at 973 - 1173K in an oxidizing medium, two phenomena can occur with rhodium catalysts : (i) surface sintering by coalescence of Rh203 particles, (ii) fonnation of a non-reducible oxide phase in strong interaction with alumina (diffusion of Rh 3+ ions in the matrix [13] or refractory rhodium oxide [14]). On platinum only the mode of deactivation (i) can be usually observed. However Pt is significantly more sensitive than Rh to surface sintering in an oxidizing medium, which explains why PtA deactivates more rapidly at 973K. At higher temperatures, a second mode of deactivation (non-reducible oxide phase) predominates on Rh/A1203. However one may note the synergy effect between Pt and Rh which renders the bimetallic catalysts more resistant to sintering than the monometallics.
Table 1 PtRh/Al203 fresh and sintered catalysts characteristics Relative dispersion D/Do of Catalysts %Pt %Rh Fresh catalysts sintered catalysts at: wt. % wt. % /hnm2g-1 Do % 973K 1173K PtA 1.17 0 1.63 57 0.12 0.067 PtRh09A 1.01 0.05 1.63 60 0.29 0.055 PtRh22A 0.75 0.11 1.55 67 0.22 0.103 PtRh35A 0.70 0.21 2.07 80 0.25 0.101 PtRh60A 0.45 0.36 1.74 65 0.30 0.103 RhA 0 0.51 1.95 87 0.29 0.089
805 The characteristics of the flesh CeO2-A1203 supported catalysts are reported in Table 2. The stoichiometries for oxygen (OT) and hydrogen (HT) titrations were abnormally high, which could be explained by the fact that the ceria surface initially reduced in H2 at 723K, can be partially re-oxidized during OT. On the bare support, there is no hydrogen uptake on chemisorption (Hc) or titration (HT). This is not the case of the metal catalysts on which the oxygen (OT) taken both by the metal and by the support is titrable by H2. Accordingly, on ceria-alumina catalysts, only HC values can be used for dispersion measurements. Moreover the stoichiometries H/Pts and H/Rhs having not been verified by other teclmiques, chemisorption was not applied to sintered catalysts.
Table 2 PtRh/CeO2-Al203 fresh catalysts characteristics Catalysts
CA (sup.) PtCA PtRhl0CA PtRh25CA PtRh40CA PtRh70CA RhCA
%Pt
%Rh
wt.% wt.% / / 1.00 0 0.90 0.05 0.75 0.13 0.60 0.21 0.30 0.37 0 0.53
chemis, and titrat. (lamol at. H or O g-1) HC 0 43 46 49 50 43 43
OT 90 150 113 152 160 146 161
HT 0 245 234 257 311 282 305
Hc/M
AII1
/ 0.8 0.9 1.0 1.0 0.8 0.8
(based on Hc/M-1) m2g-1 / 1.9 2.3 2.4 2.3 1.8 1.9
3.2 Effect of the temperature T O S C on the OSC metals' activities :
In these experiments, carried out on the monometallics, the temperature of oxidation (Tox = 723K) was kept constant while the temperature of OSC measurements (Tosc) was increased by step of 50K from 423K to 723K. The results shown in Fig.1 confirmed ceria as a good promotor of the oxygen storage [1-5,15]. Two temperature ranges can be distinguished for the two series of catalysts. For alumina catalysts, at TOSC < 523K platinum is more active than rhodium and at TOSC > 523K the opposite order is observed. For ceria-alumina catalysts, the same phenomenon is observed but the inversion temperature occurs for TOSC = 623K. After reduction and re-oxidation at 723K, it was shown by temperature programmed reduction in H2 that surface PtOx (l
806 formed in the alumina supported catalysts [16]. Because of the reduction of CeO2 together with the metals, no O/M stoichiometry could be proposed for the metals on ceria-alumina [16]. At low OSC temperature, on A1203, the platinum surface is readily reduced by CO and practically no oxygen uptake by the support can be observed whatever the temperature (PtA on Fig. 1). Reduction of Rh203 yields 75 lamol CO2 g-] and is achieved at about 600K (RhA, Fig. 1). However in RhA, alumina can store a significant amount of active oxygen at high temperature. On ceria-alumina, the metals (Pt and Rh) are readily reduced by CO at low temperature (2470K) and the support soon begins to store active oxygen. 200
'7,
9 ro
RhCA - k- -PtCA 9 9o- 9 RhA 150- .... a..-. PtA ~
~
/
~
/
~ __ _.~ ~'- A- - - ~" 4k.z" ~ -
&
~
/
O
o 100
"
~
Jl'_
/ ,
ig"
5O
" " " .D
" " " " 13._
...D
~
0
. .D"
"
D"
~ . . ...... ~ .... :.....~.: ........ A.......... ~ ......... A ......... A O"
0
400
El"
I
I
450
500
I
I
550 600 Tos c (K)
I
I
650
700
750
Figure 1. Effect of TOSC on the OSC of Rh or Pt catalysts supported on ceriaalumina and alumina 3.3 Effect of the sintering
on the OSC values of PtRhA
and PtRhCA
:
In what follows, the temperature of oxidation (Tox) and of OSC measurements were kept constant (723K). The results obtained on PtRh/A1203 catalysts are shown in Fig.2. On fresh samples, there is a quasi-linear increase of OSC values from pure Pt to pure Rh, which seems to indicate that there is no significm~t change in the surface composition of the bimetallics. Sintering at 973K induces a decrease of the OSC values more marked on Pt-rich samples than on those rich in Rh. Apparently, there is a profound change in the surface composition, bimetallics with X < 25 at.%Rh behaving like pure Pt. Sintering at 1123K causes a dramatic
807 decrease in the OSC values both on Pt and on Rh. However the bimetallics and particularly those in the 20 at.%Rh's region are more resistant to sintering than the other samples.
--
fresh[
- ~- - sintered at 973K o sintered at 1173K
160-
.
.
.
80
.
cg~
e~
60
~
120/
80-~,,
o" o
,
40 "~ 0
A
9 20 ~
40
:t.
0
0 0
o
.
O"
Q..
"'0 . . . . . . I 20
E
I I 40 60 X at.% (Rh/Pt+Rh)
I
80
0
100
Figure 2. Effect of sintering on the OSC of PtRh catalysts supported on alumina The effect of sintering on PtRh/CeO2-AI203 catalysts is shown in Fig. 3. Contrarily to what was observed on A1203-supported samples, ceria catalysts resist well to sintering (Table 3).
Table 3 Relative variations of OSC values with sintering. Catalysts sintered at"
OSC s i n t . / O S C fresh
(%) PtA
Rlvk
PtCA
RhCA
973K
24%
32%
68%
64%
1173K
8%
5%
36%
39%
The decrease in the OSC values of ceria-catalysts is the result of the sintering of both metal and ceria. These two components participating in the
808 oxygen storage are relatively little affected by sintering 9 ceria stabilizes the metals but is in tuna stabilized by alumina. 250
fresh[
m
a- - sintered at 973K c]. 9sintered at 1173K
250
~200
200
~ 1150
150 ~
0 0
'~ 100
100 -,~
o r~
C 9
o
~
.
9
[3
.
.
.
.
.
0
.
.
.
.
.
O
9
"
r~
"
.......
/50 0
I 20
0
I
I
40 60 X at.% (Rh/Pt+Rh)
I ....
I
80
5o
0
100
Figure 3. Effect of sintering on the OSC of PtRh catalysts supported on ceriaalumina 3.4 E f f e c t o f a d d i t i v e s on the O S C and the o x y g e n m o b i l i t y of R h / A l 2 0 3 , Rh/CeO2-AI203
:
Five catalysts were prepared by addition of chlorine (0.1 and 0.5 wt.% C1), sulfur (4.5 wt.% SO42-) or potassium (0.2 and 1.5 wt.% K +) on the RhA parent catalyst. OSC values and coefficient Ds of surface diffilsion (deduced from the initial rates of exchange of 1802 with the 160 of the support) were measured on these samples. Fig. 4 gives the relative variations (A~%) in the values of OSC and of Ds of the modified catalysts compared to the parent catalyst. These variations of OSC and Ds follow the same tendency, which justifies the idea that oxygen storage and 180/160 are both controlled by the same rate determining step (transfer metal to support and/or surface diffilsion in the case of ~A). Potassium (at low content) is a promotor of OSC and of oxygen mobility while chlorine, sulfi~r and potassium at high content have inhibiting effects. The location of the additive at the surface has a determining effect. It was shown that chlorine tends to concentrate, even at low content, near the metal particles. This concentration amplifies ilflfibiting effect of C1 by blocking the transfer step.
809 Potassium at low content, would be essentially located on the support ; it increases the basicity and thus the surface mobility of oxygen [8]. At higher content K probably interacts strongly with the metal and decreases the catalyst activity. 20 ff] ~OSC (%) ]
- - . - -aDs (%)
10
-
60
-
40 20 0
-
-20
-
-40
-
-60
-
-80
o
-10
s 6
o
-20
!
9
-
-30
0.5C1 4.5S04 0.1C1
1,5K
None
0.2K
-100
% of additives on RhA
Figure 4. Effect of additives on the OSC and the mobilities of oxygen on RhA catalyst.
60
l"71 ~:)SC (%) ]
- - 0 - -~Ds (%)
50 40 30 20 10 _
-10
0.1C1
-
Q
None
"$ 4.5SO4
-100
% of additives on RhCA
Figure 5. Effect of additives on the OSC and the mobilities of oxygen on RhCA catalysts.
810 The behavior of modified Rh/CeO2-A1203 catalyst is shown in Fig.5. Contrarily to what was found on A1203, for modified catalysts supported on ceria-alumina, chlorine has only a slight effect on the OSC values. This difference would be result of the presence of ceria that could promote a better distribution of chlorine at the carrier surface. Sulfates would increase the oxygen storage capacity of the modified catalyst. This effect would only be apparent and would be due partly to the sulfitr reduction by CO which masks the inhibitor effect of sulfur on OSC. This can be observed by comparison with the oxygen mobilities.
4. CONCLUSION Noble metals play a definite role in storing oxygen on exhaust gas catalysts. Pt and Rh are able to store oxygen on CeO2-A1203 but only Rh is active on A1203 catalysts. On A1203, OSC values depend on the metal used (Rh > Pt) and are extremely sensitive to sintering (1%02 + 10%H20, at 973 and 1173K). On CeO2-A1203, OSC values depend little on the metal used (Rh ~ Pt) mad the catalysts resist well to sintering. On Rh/A1203, both OSC and oxygen mobility (deduced from 180/160 isotopic experiments) are inhibited by chlorine and sulfate and conversely promoted by potassium. Similar results are obtained with Rh/CeO2-A1203 except the apparent promotion of the OSC by sulfates.
ACKNOWLEDGMENT: This work was carried out within the Groupement Scientifique Catalyseurs de Postcombustion funded by the Centre National de la Recherche Scientifique, the Institut Frangais du P6trole and the Agence de l'Envirolmement et de la Ma]trise de l'Energie.
811
REFERENCES :
9
10 11
12
13 14 15 16
H.C.Yao and Y.F.Yu Yao, J. Catal., 86 (1984) 256. E.C. Su, C.N.Montreuil and W.G.Rothschild, Appl. Catal., 17 (1985) 75. E.C. Su, and W.G.Rothschild, J. Catal., 99 (1986) 506. B.Harrison, A.F.Diwell and C.Hallett, Platinmn metals Rev., 32 (1988) 73. B.Engler, E.Koberstein and P.Schubert, Appl. Catal., 48 (1989) 71. D. Duprez, H. Abderrahim, S. Kacimi and J. Rivi6re in K.H. Steinberg, Editor, Proc. 2nd Int. Conf. Spillover, Leipzig, June 12-16, 1989, KarlMarx-Universitat, Leipzig, 1989, p. 127. S. Kacimi and D. Duprez in A. Crucq, Editor, Catalysis and Automotive Pollution Control II, Proc. 2nd Int. Symp. CaPoC 2, Brussels, Sept. 10-13, 1990, Stud. Surf. Sci. Catal., Vol.71, Elsevier, Amsterdaln, 1991, p. 641. D.Martin and D.Duprez, in T. Inui, K. Fujilnoto, T. Uchijima and M. Masai, Editors, New Aspects of Spillover Effects in Catalysis, Proc. 3rd Int. Conf. Spillover, Kyoto, Aug. 17-20, 1993, Stud. Surf. Sci. Catal., Vol. 77, Elsevier, Amsterdam, 1993, p. 201. D. Duprez, J. Chiln. Phys., 80 (1983) 487. R.Taha and D.Duprez, Catal. Letters, 14 (1992) 51. H. Abderrahim and D. Duprez, in A. Crucq and A. Fremlet, Editors, Catalysis and Automotive Pollution Control, Proc. 1st Int. Symp. CaPoC 1, Brussels, Sept. 8-11, 1986, Stud. Surf. Sci. Catal., Vol.30, Elsevier, Amsterdam, 19871, p. 359. D. Martin and D. Duprez, in S. Goldstein, E. Soulie and P. Louvet, Editors, Proc. Syrup. "Les Isotopes Stables Applicatoins-Production", Saclay, Nov. 24-25, 1993, Commissariat/l l'Energie Atomique, in press. H.C. Yao, S. Japar and M. Shelef, J. Catal., 50 (1970) 407. D.D. Beck, T.W. Capehart, C. Wong and D.N. Belton, J. Catal., 144 (1993) 311. S. Kacimi, J. Barbier Jr., R. Taha and D. Duprez, Catal. Lett., 22 (1993) 343. R. Taha, Thesis, Poitiers, 1994.
Studies in Surface Science and Catalysis, Vol. 96 1995 Elsevier Science B.V.
801
EFFECTS OF SINTERING AND OF ADDITIVES ON THE OXYGEN STORAGE CAPACITY OF PtRh CATALYSTS.
D. Martin, R.Taha and D. Duprez Laboratoire de Catalyse en Chimie Organique, URA 350 CNRS 40 Av. du Recteur Pineau, 86022 Poitiers Cedex, France
ABSTRACT : PtRh/A1203 and PtRh/CeO2-AI203 bimetallic catalysts (Pt+Rh = 60 lamolg-1) were prepared via chlorine-free precursors. Oxygen storage capacities (OSC) were measured on the fresh (calc. 723K) and on the sintered catalysts (1%O2 + 10%H20, 2h, 973K and 1173K). On alumina catalysts, only Rh can promote OSC which is extremely sensitive to sintering. OSC values are higher on alumina-ceria catalysts, but do not depend on the composition of the bimetallics. Moreover ceria renders the catalysts resistant to sintering. PtRh/A1203 and PtRh/CeO2-Al203 were modified by C1, SO42- and K. On A1203, OSC variations due to the additives follow the same trend as the variations of oxygen mobility (deduced from 180/160 isotopic exchange). Chlorine and sulfur are inhibitors of OSC while K, at low content, is a promotor.
1. INTRODUCTION
Rare earth oxides, especially cerium oxide, are used to improve the oxygen storage capacity (OSC) of three-way catalysts. Noble metals, in particular rhodium, play ml active role in promoting the OSC of the support [1-5]. The mechanism of OSC can be described in terms of three principal steps : - dissociative adsorption and desorption of molecular oxygen on the metals - transfer of active oxygen species from the metal to the support and surface migration of these species on the support - storage of oxygen by cerium oxide.
802 Duprez m~d al. investigated the first and the second step by means of 160/180 isotopic exchange [6-8]. The rate of step 1 can be deduced from the rate of the 1602/1802 equilibration reaction that occurs on the metal : 1802(gas) + 1602(gas) ~
2 18O160(gas )
(1)
while the rate of step 2 is in direct relation with the rate of isotopic exchange of 1802 with the 160 of the support 9 1802(gas) + 160(sup.) -+ 180160(gas) + 180(sup.)
(2)
The surface of ceria being readily reduced at low temperature (< 473K) in the presence of noble metals [1,4], step 3 is expected to be very rapid under the usual conditions of exhaust gas catalysis (T=673-773K). Duprez and Kacimi [7] showed that rhodiuln was the metal which has the highest intrinsic rate in reaction (1). Accordingly the rate determining step (rds) of the mechanism of OSC oll rhodium should be the transfer and the migration of oxygen at the surface of the support. Our aim, in the first part of this paper, is to study the effect of the sintering of PtRh/A1203 and PtRh/CeO2-A1203 catalysts on the OSC and to correlate the OSC with the surface migration of oxygen measured by 1802(gas)/160(support) isotopic exchange [6,8]. In the second part of this paper, we study the effects of certain additives (SO42-, C1) on the OSC values and on the oxygen mobility in the catalysts. As we have shown recently that the surface mobility of oxygen was linked to the basicity of oxides used as supports [8], we have also investigated the effect of potassium.
2. EXPERIMENTAL The support was a y-A1203 (100 m2g -1) supplied by IFP. A 12wt.% CeO2A1203 support was prepared by impregnating the 7-A1203 with aqueous solutions of ceric alnmonium nitrate. The catalysts used in the sintering studies were prepared by successive impregnations with aqueous solutions of dinitrodiamlnine platinum(II) and of rhodium(III) nitrate. After impregnation, the catalysts were dried and calcined at 723K (flesh catalysts). They are referred to here as PtRhXA
803 and PtRhXCA, where A desigamtes alumina, CA, ceria-alumina and X is the atomic percentage of rhodium : % (Rh / Pt+Rh). Aliquot samples of each catalyst were sintered for 2 h at 973K and at 1173K under a continuous flow of 1%02 + 10%H20 (vol%) in nitrogen. In the second phase of the experiments (effect of additives), Rh catalysts prepared on two different supports 9 y-A1203 (100 m2g "1, IFP), CeO2/y-AI203 (93 m2g-1, on IFP by impregnating a hydrochloric acid dried and calcined
alumina) were used. All the modified catalysts were prepared fresh catalyst with aqueous solutions of dimnmonium sulfate, or potassium nitrate. After modification, the catalysts were at 723K. They are referred to here as RhSyP, where y is the
weight percentage of the additive P (P=SO42-, C1 or K) and S is for the type of support (S=A for alumina and CA for ceria-alumina). The weight percentages of rhodium are 0.51 for catalysts supported on alumina and 0.53 on ceria-alumina. Dispersion measurements were carried out on fresh catalysts in a pulse chromatographic system previously described [9,10]. The catalysts were reduced at 723K in H2 degassed in Ar (impurities < l ppm) and cooled down to room temperature. Hydrogen chemisorption (HE), oxygen (OT) and hydrogen (HT) titrations were successively recorded. Metal dispersion was deduced from oxygen titration of the chelnisorbed hydrogen using the following stoichiometries 9H/Rhs =1, O/Rhs = 1.5 and H/Pts = O/Pts = 1. OSC and OSCC (Oxygen Storage Capacity Complete) measurements were carried out in a pulse chromatographic system. The catalyst sample (10 to 50 mg) was inserted in a U-quartz reactor, heated to 723K (4 Kmin-1) in a helium flow (30cm3min-1 ; less than 1 ppm impurities) and then oxidized to saturation by 02 pulses (0.268 cln 3) injected every other minute ("oxygen storage"). The CO pulses were then injected, at TOSC (423K< TOSC <723K), every other minute. OSC values were deterlnined from the amount of CO2 produced at the first pulse of CO (titration of the "fast" oxygen) and OSCC, from the amount of CO2 produced at the five first pulses of CO (titration of the "complete" oxygen storage). Isotopic exchange experiments were carried out in a recycle reactor coupled to a mass-spectrometer [11,12]. "In situ" pretreaments were 9 160 2 at 723K for 0.25 h, H2 at 723K for 0.25 h, outgassing at 723K for 0.5 h and cooling down to the temperature of exchange before admission of 1802. Mass spectra (1802 , 18016 0 and 1602, plus mass 18, 28 and 44 to detect possible leaks) were
804 recorded every nine seconds, which allowed us to determine the coefficient D s of surface diffusion [8,12] from equation (3) 9 Ne= 4Na
180
~o-~Dst
C
where Ne is the number of diffusing species, a is the radius and C 18 0 concentration of 180 of the N circular metal particles.
(3) the
3. RESULTS AND DISCUSSION 3.1 Catalyst characteristics 9
Table 1 gives the characteristics of the alumina-supported catalysts. Fresh catalysts and particularly RbJA1203 are well-dispersed. During treatments at 973 - 1173K in an oxidizing medium, two phenomena can occur with rhodium catalysts : (i) surface sintering by coalescence of Rh203 particles, (ii) fonnation of a non-reducible oxide phase in strong interaction with alumina (diffusion of Rh 3+ ions in the matrix [13] or refractory rhodium oxide [14]). On platinum only the mode of deactivation (i) can be usually observed. However Pt is significantly more sensitive than Rh to surface sintering in an oxidizing medium, which explains why PtA deactivates more rapidly at 973K. At higher temperatures, a second mode of deactivation (non-reducible oxide phase) predominates on Rh/A1203. However one may note the synergy effect between Pt and Rh which renders the bimetallic catalysts more resistant to sintering than the monometallics.
Table 1 PtRh/Al203 fresh and sintered catalysts characteristics Relative dispersion D/Do of Catalysts %Pt %Rh Fresh catalysts sintered catalysts at: wt. % wt. % /hnm2g-1 Do % 973K 1173K PtA 1.17 0 1.63 57 0.12 0.067 PtRh09A 1.01 0.05 1.63 60 0.29 0.055 PtRh22A 0.75 0.11 1.55 67 0.22 0.103 PtRh35A 0.70 0.21 2.07 80 0.25 0.101 PtRh60A 0.45 0.36 1.74 65 0.30 0.103 RhA 0 0.51 1.95 87 0.29 0.089
805 The characteristics of the flesh CeO2-A1203 supported catalysts are reported in Table 2. The stoichiometries for oxygen (OT) and hydrogen (HT) titrations were abnormally high, which could be explained by the fact that the ceria surface initially reduced in H2 at 723K, can be partially re-oxidized during OT. On the bare support, there is no hydrogen uptake on chemisorption (Hc) or titration (HT). This is not the case of the metal catalysts on which the oxygen (OT) taken both by the metal and by the support is titrable by H2. Accordingly, on ceria-alumina catalysts, only HC values can be used for dispersion measurements. Moreover the stoichiometries H/Pts and H/Rhs having not been verified by other teclmiques, chemisorption was not applied to sintered catalysts.
Table 2 PtRh/CeO2-Al203 fresh catalysts characteristics Catalysts
CA (sup.) PtCA PtRhl0CA PtRh25CA PtRh40CA PtRh70CA RhCA
%Pt
%Rh
wt.% wt.% / / 1.00 0 0.90 0.05 0.75 0.13 0.60 0.21 0.30 0.37 0 0.53
chemis, and titrat. (lamol at. H or O g-1) HC 0 43 46 49 50 43 43
OT 90 150 113 152 160 146 161
HT 0 245 234 257 311 282 305
Hc/M
AII1
/ 0.8 0.9 1.0 1.0 0.8 0.8
(based on Hc/M-1) m2g-1 / 1.9 2.3 2.4 2.3 1.8 1.9
3.2 Effect of the temperature T O S C on the OSC metals' activities :
In these experiments, carried out on the monometallics, the temperature of oxidation (Tox = 723K) was kept constant while the temperature of OSC measurements (Tosc) was increased by step of 50K from 423K to 723K. The results shown in Fig.1 confirmed ceria as a good promotor of the oxygen storage [1-5,15]. Two temperature ranges can be distinguished for the two series of catalysts. For alumina catalysts, at TOSC < 523K platinum is more active than rhodium and at TOSC > 523K the opposite order is observed. For ceria-alumina catalysts, the same phenomenon is observed but the inversion temperature occurs for TOSC = 623K. After reduction and re-oxidation at 723K, it was shown by temperature programmed reduction in H2 that surface PtOx (l
806 formed in the alumina supported catalysts [16]. Because of the reduction of CeO2 together with the metals, no O/M stoichiometry could be proposed for the metals on ceria-alumina [16]. At low OSC temperature, on A1203, the platinum surface is readily reduced by CO and practically no oxygen uptake by the support can be observed whatever the temperature (PtA on Fig. 1). Reduction of Rh203 yields 75 lamol CO2 g-] and is achieved at about 600K (RhA, Fig. 1). However in RhA, alumina can store a significant amount of active oxygen at high temperature. On ceria-alumina, the metals (Pt and Rh) are readily reduced by CO at low temperature (2470K) and the support soon begins to store active oxygen. 200
'7,
9 ro
RhCA - k- -PtCA 9 9o- 9 RhA 150- .... a..-. PtA ~
~
/
~
/
~ __ _.~ ~'- A- - - ~" 4k.z" ~ -
&
~
/
O
o 100
"
~
Jl'_
/ ,
ig"
5O
" " " .D
" " " " 13._
...D
~
0
. .D"
"
D"
~ . . ...... ~ .... :.....~.: ........ A.......... ~ ......... A ......... A O"
0
400
El"
I
I
450
500
I
I
550 600 Tos c (K)
I
I
650
700
750
Figure 1. Effect of TOSC on the OSC of Rh or Pt catalysts supported on ceriaalumina and alumina 3.3 Effect of the sintering
on the OSC values of PtRhA
and PtRhCA
:
In what follows, the temperature of oxidation (Tox) and of OSC measurements were kept constant (723K). The results obtained on PtRh/A1203 catalysts are shown in Fig.2. On fresh samples, there is a quasi-linear increase of OSC values from pure Pt to pure Rh, which seems to indicate that there is no significm~t change in the surface composition of the bimetallics. Sintering at 973K induces a decrease of the OSC values more marked on Pt-rich samples than on those rich in Rh. Apparently, there is a profound change in the surface composition, bimetallics with X < 25 at.%Rh behaving like pure Pt. Sintering at 1123K causes a dramatic
807 decrease in the OSC values both on Pt and on Rh. However the bimetallics and particularly those in the 20 at.%Rh's region are more resistant to sintering than the other samples.
--
fresh[
- ~- - sintered at 973K o sintered at 1173K
160-
.
.
.
80
.
cg~
e~
60
~
120/
80-~,,
o" o
,
40 "~ 0
A
9 20 ~
40
:t.
0
0 0
o
.
O"
Q..
"'0 . . . . . . I 20
E
I I 40 60 X at.% (Rh/Pt+Rh)
I
80
0
100
Figure 2. Effect of sintering on the OSC of PtRh catalysts supported on alumina The effect of sintering on PtRh/CeO2-AI203 catalysts is shown in Fig. 3. Contrarily to what was observed on A1203-supported samples, ceria catalysts resist well to sintering (Table 3).
Table 3 Relative variations of OSC values with sintering. Catalysts sintered at"
OSC s i n t . / O S C fresh
(%) PtA
Rlvk
PtCA
RhCA
973K
24%
32%
68%
64%
1173K
8%
5%
36%
39%
The decrease in the OSC values of ceria-catalysts is the result of the sintering of both metal and ceria. These two components participating in the
808 oxygen storage are relatively little affected by sintering 9 ceria stabilizes the metals but is in tuna stabilized by alumina. 250
fresh[
m
a- - sintered at 973K c]. 9sintered at 1173K
250
~200
200
~ 1150
150 ~
0 0
'~ 100
100 -,~
o r~
C 9
o
~
.
9
[3
.
.
.
.
.
0
.
.
.
.
.
O
9
"
r~
"
.......
/50 0
I 20
0
I
I
40 60 X at.% (Rh/Pt+Rh)
I ....
I
80
5o
0
100
Figure 3. Effect of sintering on the OSC of PtRh catalysts supported on ceriaalumina 3.4 E f f e c t o f a d d i t i v e s on the O S C and the o x y g e n m o b i l i t y of R h / A l 2 0 3 , Rh/CeO2-AI203
:
Five catalysts were prepared by addition of chlorine (0.1 and 0.5 wt.% C1), sulfur (4.5 wt.% SO42-) or potassium (0.2 and 1.5 wt.% K +) on the RhA parent catalyst. OSC values and coefficient Ds of surface diffilsion (deduced from the initial rates of exchange of 1802 with the 160 of the support) were measured on these samples. Fig. 4 gives the relative variations (A~%) in the values of OSC and of Ds of the modified catalysts compared to the parent catalyst. These variations of OSC and Ds follow the same tendency, which justifies the idea that oxygen storage and 180/160 are both controlled by the same rate determining step (transfer metal to support and/or surface diffilsion in the case of ~A). Potassium (at low content) is a promotor of OSC and of oxygen mobility while chlorine, sulfi~r and potassium at high content have inhibiting effects. The location of the additive at the surface has a determining effect. It was shown that chlorine tends to concentrate, even at low content, near the metal particles. This concentration amplifies ilflfibiting effect of C1 by blocking the transfer step.
809 Potassium at low content, would be essentially located on the support ; it increases the basicity and thus the surface mobility of oxygen [8]. At higher content K probably interacts strongly with the metal and decreases the catalyst activity. 20 ff] ~OSC (%) ]
- - . - -aDs (%)
10
-
60
-
40 20 0
-
-20
-
-40
-
-60
-
-80
o
-10
s 6
o
-20
!
9
-
-30
0.5C1 4.5S04 0.1C1
1,5K
None
0.2K
-100
% of additives on RhA
Figure 4. Effect of additives on the OSC and the mobilities of oxygen on RhA catalyst.
60
l"71 ~:)SC (%) ]
- - 0 - -~Ds (%)
50 40 30 20 10 _
-10
0.1C1
-
Q
None
"$ 4.5SO4
-100
% of additives on RhCA
Figure 5. Effect of additives on the OSC and the mobilities of oxygen on RhCA catalysts.
810 The behavior of modified Rh/CeO2-A1203 catalyst is shown in Fig.5. Contrarily to what was found on A1203, for modified catalysts supported on ceria-alumina, chlorine has only a slight effect on the OSC values. This difference would be result of the presence of ceria that could promote a better distribution of chlorine at the carrier surface. Sulfates would increase the oxygen storage capacity of the modified catalyst. This effect would only be apparent and would be due partly to the sulfitr reduction by CO which masks the inhibitor effect of sulfur on OSC. This can be observed by comparison with the oxygen mobilities.
4. CONCLUSION Noble metals play a definite role in storing oxygen on exhaust gas catalysts. Pt and Rh are able to store oxygen on CeO2-A1203 but only Rh is active on A1203 catalysts. On A1203, OSC values depend on the metal used (Rh > Pt) and are extremely sensitive to sintering (1%02 + 10%H20, at 973 and 1173K). On CeO2-A1203, OSC values depend little on the metal used (Rh ~ Pt) mad the catalysts resist well to sintering. On Rh/A1203, both OSC and oxygen mobility (deduced from 180/160 isotopic experiments) are inhibited by chlorine and sulfate and conversely promoted by potassium. Similar results are obtained with Rh/CeO2-A1203 except the apparent promotion of the OSC by sulfates.
ACKNOWLEDGMENT: This work was carried out within the Groupement Scientifique Catalyseurs de Postcombustion funded by the Centre National de la Recherche Scientifique, the Institut Frangais du P6trole and the Agence de l'Envirolmement et de la Ma]trise de l'Energie.
811
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12
13 14 15 16
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