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Development of automotive palladium three-way catalysts
catalysis today Catalysis Today 22 ( 1994) 113-126
Development of automotive palladium three-way catalysts T. Sekiba*, S. Kimura, H. Yamamoto, A. Okada Materials Research Laboratory,
Nissan Research Center, Nissan Motor Co. Ltd., I, Natsushima-cho, 237, Japan
Yokosuka
Abstract A rhodium-free automotive three-way catalyst was investigated for the purpose of reducing precious metal costs and for the enhancement of availability. The relative catalytic activities of platinum, rhodium and palladium were compared, and revealed that palladium could be a possible rhodiumfree catalyst. From a practical viewpoint, the major problem was its low NO, conversion efficiency in a fuel-rich exhaust. In order to improve the NO, conversion activity of the palladium catalyst, the effects of additives were investigated. The addition of basic element (Cs, Ba and La) or transition metal oxide (Ni and Co) improved the activity, possibly due to reducing hydrocarbon poisoning by electrons being donated to the palladium. The performance of an improved palladium catalyst was as high as that of a conventional platinum-rhodium catalyst.
1. Introduction In recent years, environmental problems have become the object of public interest, and many countries have introduced automobile exhaust standards. In addition, we can expect that many more countries in Africa, Asia, and South America will enforce emissions legislation in the near future. It is also expected that regulations will become more stringent. For example, in California, tighter standards are scheduled for the current decade, and it is expected that this trend will be followed by other American states and nations. Since the introduction of emission standards, automobile factories have used catalysts to meet the regulations [ 11. The worldwide trend for strict standards will increase the number of catalysts, catalyst volume, and the quantity of precious metals used in catalysts. Therefore in the future, the demand for precious metals will increase. * Corresponding author. 0920-5861/94/$07.00 0 1994 Elsevier Science SSDIO920-5861(94)00070-I
B.V. All rights reserved
114
T. Sekiba etal. /Catalysis Today 22 (1994) 113-126
c§I 02) 7000 6000 5000 4000 3000 2000 1000 0
1989
1990
1991
1992
1993
Fig. 1. Prices of rhodium (London quot~~on).
In conventional automotive three-way catalysts, precious metals such as platinum, palladium and rhodium have been used previously. Especially with regard to rhodium, the relation between supply and demand tends to be tight, because the rhodium ratio in a conventional PtRh catalyst is much higher than the mine-mix ratio. In addition to this, as over 80% of the demand for rhodium is used in the making of autocatalysts, the price of rhodium is directly affected by the demand for automobiles. In 1990 the price of rhodium increased four- to fivefold. During the last two years the price of rhodium has gradually declined due to the effects of a weak economic climate and a slump in car sales (see Fig. 1 which shows the variation in the price of rhodium for the years 1989-1993). But the future of this problem is not easy. However, although rhodium has been thought to be an essential element in a three-way catalyst for the reduction of NO,, it has recently been attempted to develop a three-way catalyst without rhodium. Palladium, which is’cheap and has excellent activity at low temperatures, is the possible candidate to substitute rhodium. It has been attempted to use palladium as a three-way catalyst [Z-d]. Though catalyst poisoning by S or Pb has been one of the major problems, recent cleaner fuels are reducing this problem. In addition to poisoning, some practical problems have been reported and counter-measures have been proposed. It has been reported that durable ~~o~~ces are dependent on palladium loading and cerium oxide loading [ 71. The optimization of conditions has been investigated [S-lo]. It has been suggested that the air/fuel ratio control is important in improving the conversion activity. The addition of lanthana has been investigated to improve NO conversion [ 1l-l 31. To make up for a weakness in performance, it has been suggested that the palladium catalyst should be used combination with a platinum catalyst or a platinum-rhodium catalyst [ 7,14,15 1. In this paper. the catalytic performance of palladium as a three-way catalyst, the effects of additives and the pe~o~ance of an improved palladium catalyst are reported.
T. Sekiba et al. /Catalysis Today 22 (1994) 113-126
115
2. Ex~rj~ent~ 2.1. Catalysts preparation For the relative catalytic performance test, Pt (1.18 g/l), Rh (0.23 g/l) and Pd (1.88 g/l) catalysts were prepared. PtRh (1.41 g/l, PtlRh ratio 511) catalyst was also prepared as a reference. The Pt only catalyst and the Rh only catalyst corresponded to components of the PtRh catalyst. Pd loading was about 1.5 times as much as Pt loading. Catalysts were made by depositing slurry containing precious metal, alumina and ceria on monolithic substrates. After deposition, the catalysts were dried and calcined in air at 400°C for 1 h. The monolithic substrates was 1.3 1 ( 110 cm2, 11.8 cm) with 400 cells/in2. For the evaluation of the improved palladium catalyst, 0.7 1 (68.6 cm’, 10.2 cm) monolithic substrates with 300 cells/in2 was used. Conventional Pd catalyst and PtRh catalyst were also prepared as references. While the Pd loadings was about three times as much as the PtRh loading, the precious metal cost of Pd was considerably cheaper than that of PtRh. For laboratory evaluation, cylindrical cores (36 mm cross section) were cut from a full-size catalyst. 2.2. Catalyst aging Catalyst aging was carried out with a V8 4.4~liter engine. Two full-size catalysts were aged simultaneously. Two containers packed with eight cylindrical cores were also used for small catalyst samples. The aging cycle includes a 60 s cruise mode and a 5 s fuel cut mode. For the cruise mode, the engine was operated using a stoichiometric air/fuel ratio. Aging temperatures were 650, 750, and 850°C at the catalyst inlet. 2.3. Engine evaluation Engine evaluation was conducted using a L6 2.0-liter engine. Three-way performances were obtained by measu~ng conversion efficiencies of HC, CO and NO, against the air/fuel ratio. The air/fuel ratio was traversed from 13.8 to 15.4 with a modulation of nt 1.O, + 0.5 and + 0.2 A/F at 1 Hz. The catalyst inlet temperature was 400 and 480°C. Light-off performances were measured by raising the temperature from 350 to 450°C at the stoichiometric point without A/F modulation. 2.4. Laboratory evaluation The laboratory evaluation was conducted using a synthetic gas composition simulating engine exhaust gas. Performances of fresh and engine-aged cylindrical
116
T. Sekiba et al. /Catalysis Today 22 (1994) 113-126
core catalysts were measured. The effects of gas composition HC, CO and NO conversion were investigated.
and concentration
on
2.5. Vehicle evaluation Nissan Sunny equipped with a L4 1.6~liter engine was used. 0.7 liter catalyst was attached on the exhaust manifold. HC, CO and NO, emission was measured using a Japan 10 mode test cycle.
3. Results and discussion 3.1. Relative catalytic performance
of platinum, rhodium, and palladium
First of all, the relative catalytic performances of Pt, Rh and Pd were investigated. Fig. 2 shows the light-off performance of each catalyst in engine dynamometer evaluation. The vertical axis indicates the temperature that the HC, CO and NO, conversion reaches 50%. For fresh catalysts, performances of three catalysts were almost the same. The light-off performance of Pd was better than the Pt or Rh catalyst by about 20°C. After engine aging, performance differences appeared. In spite of the small metal loading, Rh was the best of the three catalysts in light-off performance. The deterioration of the activity of Rh was small. The Pt only catalyst
0 m
Fresh After 85O’c,lOOhr
Engine Dynamometer Evaluation Catalyst 1 1.3L Metal Loadings Pt/Rh : 1.41&L-5/1 (1.18 +0.23) Pt: l.l8g/L Rh : 0.23& Pd : 1.88gfL
Pt
Rh
Pd
Fig. 2. Light-off performances.
T. Sekiba et al. /Catalysis
Today 22 (1994) 113-126
117
Engine Dynamometer Evaluation A/F Aging: 850°C 1OOhr 14.6f 1..a ’ Evaluation: 4OO’c
Pt
100 $?
80
‘.z 8 5 2
60
6
Rh
Pd
A/F 14.69~0 .5
40 20
Catalyst: volume 1.3L PtRh: 1.41&-S/l Pt: 1.1&/L Rh: 0.235giL Pd: 1.88gIL
100 8 8 ._ 6 & 3
80 A/F 14.6*0 .2
6o 40 20 0 PtRh
Pt
Rh
.
Pd
Fig. 3. Effect of A/F amplitude on conversion.
deteriorated considerably. Therefore, the light off performance of the PtRh catalyst depends on the activity of Rh. Fig. 3 shows the effect of the A/F amplitude on the conversion efficiencies of each catalyst at 400°C. Engine-aged catalysts were measured. The central air/fuel ratio was taken at the stoichiometric point. In the case of wide A/F amplitude ( f 1.O A/F), conversion efficiencies of the PtRh, Rh only, and Pd catalysts were almost the same (near 80%). But as the A/F amplitude decreased ( &OS A/F, 50.2 A/F), while the Pd only and Rh only catalyst performances improved, the Pt only catalyst performance severely deteriorated. Because the change in the gas composition was small, it is possible that the deterioration was due to poisoning by gas adsorption. Hydrocarbon is a gas which has strong adsorption properties. Bearing this point in mind, the effect of hydrocarbon concentration on conversion efficiency of each catalyst was investigated. Fig. 4 shows synthe-gas evaluation results. Propylene was used as a model hydrocarbon. The concentration of propylene and oxygen were modulated simultaneously to keep stoichiometric composition. As the propylene concentration increased, only the Pt catalyst performance deteriorated. Other catalyst performances were not affected by the propylene concentration. Therefore, it is found that hydrocarbon poisoning on a Pt surface could occur even in a stoichiometric atmosphere. In the engine dynamometer evaluation with large A/F modulation, as adsorbed hydrocarbon is removed from the surface under a strong oxidizing atmosphere, the
T. Sekiba et al. /Catalysis Today 22 (I 994) 113-126
Catalyst PlRh: 1.4lgL5/l Pt: l.lSgjL Rh: 0.235gL Pd: 1.88giL gas composition co: 1.53% H2: 0.53% NO: 0.10% 02: 0.93-1.23% C3H6: O-1665ppmC co2: 14.0% N2: Balance 0
1000 C3H6 concentration (ppmC)
Model Gas Evaluation
After engine Aging: 85O”c5Ohr Evaluation temperature: 350°C
Fig. 4. Effect of propylene concentration
ww0.3 ”
0.2”
x
0.1
2000
on conversion.
Car : Nissan Sunny Catalyst : , Paul o.wgjL-10/l Pd 2.83glL catalyst volume 0.7L after 750°C 1OOhraging .
wwO~ 2 8
1
0 (g/km> 0.3 * 0.2 5? 0.1 0.0 PtRh Fig. 5. Vehicle evaluation
Pd results (10 mode test)
T. Sekiba et al. /Catalysis Today 22 (1994) 113-126
119
Pt catalyst keeps a high conversion activity. Regarding the PtRh catalyst, a high performance even under a small A/F modulation is explained by the activity of Rh. Considering the high hydrocarbon conversion activity of Rh, Rh relieves the hydrocarbon poisoning effect on the Pt surface. From these results, it is recognized that the Pt only catalyst is sensitive to hydrocarbon self-poisoning. It is suggested that Pd may be a possible Rh-free threeway catalyst, rather than removing Rh from a conventional PtRh catalyst. 3.2. Three-way pe$ormance of palladium catalyst Using Pd as a three-way catalyst has been investigated by automobile and catalyst m~ufac~rers. It has been pointed out that there is a problem with NO, emissions. In this investigation, a Pd catafyst was evaluated on a vehicle mode test after engine aging, and also a conventional PtRh catalyst was evaluated as a reference. Fig. 5
mplitude
14.2
: *
14.6
1.0
(Amplitude
0’
15.0
: f
0.2A/F)
14.2
14.6
15.0
14.2
14.6
15.0
100 8
80
660 3 8
40
20 0
0
u
14.2
14.6
15.0
14.2
14.6
15.0
A/F
A/F
Catalyst:PtRh 0.88&10/l,
Pd 2.83gA Engine Evaluation after 850°C 3Ohr aging, Evaluation tcmpcrature Fig. 6. Performances of PtRh and Pd catalysts.
: 480°C
120
T. Sekiba et al. /Catalysis
Today 22 (1994) 113-126
Catalyst : Pt Rh O.SSg/L, Pd 2.83gL Model Gas Evaluation : after 85Oc3Ohr engine aging, evaluation temperature 400°C
-0
1000
Xloo
3000
Gas composition NOx : ZOOOppmC C3H6 : 0-3000ppmC 02: 0.2-0.3546 H2: 0.5% co: 1.5%
C3H6 Concentration (ppmC C) Fig. 7. Effect of propylene concentration on NO conversion
shows the results of the vehicle 10 mode test. Though the HC emission of the Pd catalyst was better than that of the PtRh catalyst, for NO, emission, the Pd catalyst was considerably inferior to the PtRh catalyst. In order to understand this problem, the catalytic performances of the PtRh and Pd catalysts were evaluated on an engine dynamometer. Fig. 6 shows the catalytic performances of the PtRh catalyst and the Pd catalyst. In the case of a wide A/F amplitude, the performance of the Pd catalyst was superior to that of the PtRh catalyst. However, in the case of a narrow A/F amplitude, especially in NO, conversion efficiency in fuel-rich conditions, Pd was inferior to PtRh. The difference of the performance in this region affects the NO, emission. In fuel-rich, oxygen-lean conditions, hydrocarbon self-poisoning on the Pd surface occurs as in case of the Pt only catalyst at the stoichiometric point. The hydrocarbon poisoning effect on a Pd catalyst was investigated using a synthe-gas stream which simulated the exhaust gas composition. The effect of propylene concentration on NO conversion efficiency was measured (Fig. 7). In this investigation, the gas composition was not stoichiometric, and fuel-rich con-
260 3
40
20
evaluation temperature 400°C
I
Gas composition NOx : 2000ppmC C3H8 : O-2700ppmC 02: 0.2-0.358
C3H8 Concentration (ppmC ) Fig. 8. Effect of propane concentration
on NO conversion
T. Sekiba et al. / Catalysis Today 22 (1994) 113-126
121
80 8 .C1 fi-Rich: A/F=14.2f0.2 /$ ,+ m Stoichiometric : / A/F=14.6f0.2
20
g
n ”
Pd-Ce
PdCe-La Pd-Ce-Ba Pd-Cc-Cs
0
100
After 850°C 30hr Aging
80 60 40
K
B
20 n Pd-Ce
PdCc-La
Engine Evaluation at 480X,
Pd-Cc-Ba
PdCe-Cs
Pd : 2.838/L, Additives : 6.6gjL
Fig. 9. Effects of additives on NO, conversion.
ditions corresponding to an A/F ratio of 14.0 were used. The concentrations of propylene and oxygen were modulated simultaneously to keep the ratio of oxygen to reduction gas constant. In the absence of propylene, the PtRh and Pd catalysts showed a high NO conversion efficiency. As the propylene concentration increased, the NO conversion on the Pd catalyst decreased. At a propylene condensation of 2500 ppmC which corresponded to exhaust gas composition, a large difference in NO conversion efficiency was observed. A similar experiment was carried out substituting propylene for propane (Fig. 8). In this experiment the effect of propane concentration was very small as compared to that of propylene. This difference in the reaction inhibiting effect between propane and propylene could be due to the adsorption strength on the Pd surface. These results suggests that hydrocarbon poisoning occurs on a Pd catalyst in fuel-rich conditions. Therefore in order to use a Pd catalyst as a three-way catalyst, it is necessary to reduce hydroc~bon poisoning in order to improve NO, conve~ion efficiency in a fuelrich region. 3.3. Improvement of NO, conversion activity of palladium catalysts In order to improve the NO, conversion activity of the Pd catalyst in fuel-rich conditions, the effects of additives to a Pd--Ce/Al,O, catalyst were investigated.
122
T. Sekiba et al. /Catalysis
\,I,
Today 22 (1994) 113-126
_V”
g
80
.e
9
g
60
2
40
g
A/F=14.2*0.2 m Stoichiometric : A/F=14.6*0.2
20
0
0 (%) 100
After 850°C 30hr Aging
& ._ 5 g 6
80 60 .......................
40 20 0
Pd
Pd-Co304
Engine Evaluation
Pd-NiO
at 48O”c,
Pd-ZnO
Pd : 2.83gL
Pd-SnO2 Addition of Oxides : 2OgfL
Fig. 10. Effects of additives on NO, conversion.
The NO, conversion efficiency was evaluated for fresh and aged catalyst using an engine dynamometer (Fig. 9). In the evaluation, at the stoichiometric point, the addition of La, Ba or Cs did not affect the NO, conversion activity. But in the fuelrich region, these additives improved the activity. It is remarkable that the effects of the improvement coincide with the intensity of the basicity of each element. This result suggests that the addition of these basic elements may relieve hydrocarbon poisoning by the donation of electrons to Pd. After engine aging, Pd-Ce-Ba/Al,O, gave the highest NO, conversion efficiency. The improvement effect of La was somewhat less than that with Ba. This result suggests that La is diffused to Al,O, or CeO,. Though the addition of Cs gave the best performance with a fresh catalyst, the improvement decreased after aging probably due to the evaporation of Cs during engine aging. Next the addition of transition metal oxide was investigated (Fig. 10). NiO, Co304, ZnO and SnO, were used. Powders of these oxides were mixed with catalyst slurry and coated on monolithic substrates. With fresh catalysts at stoichiometric evaluation, the addition of oxides did not affect the NO, conversion activity. But in the fuel-rich region, it was found that the addition of NiO or Co,O, slightly improved the activity, and the addition of ZnO or SnOz decreased the activity. In
T. Sekiba et al. / Catalysis Today 22 (1994) 113-126
14.2
14.6
14.2
14.6
15.0
15.0
2 60 40 83 100 80 I
a2;i . . . . . 14.2
14.6
15.0
A/F
E 2 3 5 2
123
14.2
14.6
15.0
14.2
14.6
15.0
14.2
14.6
15.0
loo 80 60 40 20 0
A/F
Catalyst: PtRh 0.88g/L-10/l, Pd 2.83g/L Catalyst volume : 0.7L Engine Evaluation after 750°C 1OOhraging, Evaluation temperature : 480°C Fig. 11. The performance
of the improved Pd catalyst.
this connection, catalysts without Pd were evaluated as a blank test. These oxides showed negligible activity. Regarding these oxides as a semiconductor, there could be the electronic effect for Pd. That is to say, p-type semiconductor (NiO, Co,O,) may relieve hydrocarbon poisoning by electron donation to Pd, and n-type semiconductor (ZnO, SnOz) may increase hydrocarbon poisoning by electron acceptance from Pd. As it is thought that the amount of hydrocarbon or CO adsorption on a n-type semiconductor is greater than that on a p-type semiconductor, it is possible that gas adsorption characteristics of these oxides may influence the activity. After aging, the improvement effect of the addition of NiO or Co304 was obvious. About the addition of ZnO, NO, conversion efficiency in the fuel-rich region after aging was higher than that before aging. It is possible that the interaction between Pd and ZnO was weakened during the aging cycle. The performance of the SnOz
124
T. Sekiba et al. I Catalysis Today 22 (1994) 113-l 26
Pd PtRh Engine Evaluation after 750°C 1OOhr Aging Catalyst Volume : 0.7L Pt Rh : O%g/L-1 O/l Pd : 2.83g/L Fig. 12. The light-off performance
containing catalyst heavily deteriorated. to alloy formation.
of the improved Pd catalyst.
It is thought that this deterioration
is due
3.4. Per-$ormance ofan improved palladium catalyst We considered the above-mentioned results of our investigation, and developed a new Pd catalyst. For practical purposes, the thermal durability was improved along with the NO, conversion efficiency in a rich atmosphere. Conversion efficiencies were evaluated at the engine dynamometer after engine aging (Fig. 11) . The modified Pd catalyst showed the highest performance. The NO, conversion efficiency in the fuel-rich condition was improved and was found to be competitive with that of conventional the PtRh catalyst. Light-off performances were measured (Fig. 12). The conversion efficiency of the improved Pd catalyst was higher at lower temperatures than the PtRh catalyst. The improved Pd catalyst showed a good light-off performance. Vehicle emissions’ performances after engine aging (75O”C, 100 h) were evaluated by a 10 mode test (Fig. 13). The performance of the new Pd catalyst was considerably higher than those of conventional Pd and PtRh catalysts. As mentioned above, this investigation revealed that palladium could be used to manufacture a three-way catalyst without rhodium. But for worldwide use, different working conditions must be considered and various fuels containing S and Pb need to be looked into. Therefore improvements in thermal durability and resistance against poisoning (S, Pb) are required. We
T. Sekiba et al, I Ca falysis Today 22 (1994) 113426
125
10 mode test Q/km) 0.3
Car : Nissan Sunny Catalyst PtRh : 0.88g/L-10/l Pd : 2.83&L,-1O/l catalyst volume : 0.X after 750°C 1OOhraging
g 0.2 0.1 (g/km) of 2 &
1
(g/km)0.3
E0.1 0m2 0.0 PtRh (conv$ional)
(i!$roved)
Fig. 13. The performance of the improved Pd catalyst.
hope to keep on improving our Pd three-way catalyst. The development of an optimum engine control system will also assist the durability and the activity of the Pd catalyst. 4. Conclusion From the investigation, the following conclusions are obtained: (I) Because a Pt only catalyst is susceptible to hydrocarbon poisoning, when considering Rh-free three-way catalysts, Pd can be seen as a possible rhodium-free three-way catalyst rather than removing Rh from a PtRh catalyst. (2) The major problem in using a Pd three-way catalyst is its weak NO, conversion activity in a fuel-rich region, Pd is also affected by hydrocarbon poisoning in a fuel-rich, oxygen-lean atmosphere. (3) Some additives (e.g., La, Ba, Cs, NiO, Co,O,) improve the NO, conversion activity in a fuel-rich region. These may relieve hydrocarbon poisoning by electron donation to Pd. (4) The newly improved Pd three-way catalyst is competitive in peerformance with a PtRh catalyst. Acknowledgements The authors wish to thank Nissan ARC for catalyst analysis, the Itoh group for catalyst preparation, and the Irikura group for catalyst aging and evaluation.
T. Sekiba et al. / Catalysis Today 22 (I 994) 113-126
126
References [ 1] [2] [3] ]4] [5] [6] ]7] [S] [9] [ 101 [ 111 [ 121 [13] [ 141 [ 151
KC. Taylor, Chemtech., 20 (9) ( 1990) 55 1. E.C. Su and W.G. Rothschild, J. Catal., 99 (1986) 506. Y.-F. Yu Yao. J. Catal., 87 (1984) 152. W.G. Durrer, H. Poppa and J.T. Dickinson, J. Catal., 115 (1989) 310. J.Z. Shyu, J. Catal., 114 (1988) 23. C.R. Henry and H. Poppa. J. Vacuum Sci. Technol. A, 6 (3) (1988) 1113. J.C. Summers, W.B. Williamson and M.G. Henk, SAE Paper 880 281 (1988). J.C. Summers, J.J. White and W.B. Williamson, SAE Paper 890 794 ( 1989). J.C. Summers and J.P. Hiera, SAE Paper 891 245 (1989). H. Mu&i, H. Sobukawa, M. Kimura and A. Isogai, SAE Paper 900 610 (1990). H. Mu&i, H. Shinjoh and Y. Fujitani, Appl. Catal., 22 (1986) 325. H. Mu&i, Appl. Catal., 48 (1989) 93. H.Muraki.SAEPaper910842 (1991). M. Skoglundh, L 0. Lowendahl and J.-E. Ottersted, Appl. Catal., 77 (1991) 9. J.C. Dettling and Y.K. Lui, SAE Paper 920 094 (1992).
Development of automotive palladium three-way catalysts T. Sekiba*, S. Kimura, H. Yamamoto, A. Okada Materials Research Laboratory,
Nissan Research Center, Nissan Motor Co. Ltd., I, Natsushima-cho, 237, Japan
Yokosuka
Abstract A rhodium-free automotive three-way catalyst was investigated for the purpose of reducing precious metal costs and for the enhancement of availability. The relative catalytic activities of platinum, rhodium and palladium were compared, and revealed that palladium could be a possible rhodiumfree catalyst. From a practical viewpoint, the major problem was its low NO, conversion efficiency in a fuel-rich exhaust. In order to improve the NO, conversion activity of the palladium catalyst, the effects of additives were investigated. The addition of basic element (Cs, Ba and La) or transition metal oxide (Ni and Co) improved the activity, possibly due to reducing hydrocarbon poisoning by electrons being donated to the palladium. The performance of an improved palladium catalyst was as high as that of a conventional platinum-rhodium catalyst.
1. Introduction In recent years, environmental problems have become the object of public interest, and many countries have introduced automobile exhaust standards. In addition, we can expect that many more countries in Africa, Asia, and South America will enforce emissions legislation in the near future. It is also expected that regulations will become more stringent. For example, in California, tighter standards are scheduled for the current decade, and it is expected that this trend will be followed by other American states and nations. Since the introduction of emission standards, automobile factories have used catalysts to meet the regulations [ 11. The worldwide trend for strict standards will increase the number of catalysts, catalyst volume, and the quantity of precious metals used in catalysts. Therefore in the future, the demand for precious metals will increase. * Corresponding author. 0920-5861/94/$07.00 0 1994 Elsevier Science SSDIO920-5861(94)00070-I
B.V. All rights reserved
114
T. Sekiba etal. /Catalysis Today 22 (1994) 113-126
c§I 02) 7000 6000 5000 4000 3000 2000 1000 0
1989
1990
1991
1992
1993
Fig. 1. Prices of rhodium (London quot~~on).
In conventional automotive three-way catalysts, precious metals such as platinum, palladium and rhodium have been used previously. Especially with regard to rhodium, the relation between supply and demand tends to be tight, because the rhodium ratio in a conventional PtRh catalyst is much higher than the mine-mix ratio. In addition to this, as over 80% of the demand for rhodium is used in the making of autocatalysts, the price of rhodium is directly affected by the demand for automobiles. In 1990 the price of rhodium increased four- to fivefold. During the last two years the price of rhodium has gradually declined due to the effects of a weak economic climate and a slump in car sales (see Fig. 1 which shows the variation in the price of rhodium for the years 1989-1993). But the future of this problem is not easy. However, although rhodium has been thought to be an essential element in a three-way catalyst for the reduction of NO,, it has recently been attempted to develop a three-way catalyst without rhodium. Palladium, which is’cheap and has excellent activity at low temperatures, is the possible candidate to substitute rhodium. It has been attempted to use palladium as a three-way catalyst [Z-d]. Though catalyst poisoning by S or Pb has been one of the major problems, recent cleaner fuels are reducing this problem. In addition to poisoning, some practical problems have been reported and counter-measures have been proposed. It has been reported that durable ~~o~~ces are dependent on palladium loading and cerium oxide loading [ 71. The optimization of conditions has been investigated [S-lo]. It has been suggested that the air/fuel ratio control is important in improving the conversion activity. The addition of lanthana has been investigated to improve NO conversion [ 1l-l 31. To make up for a weakness in performance, it has been suggested that the palladium catalyst should be used combination with a platinum catalyst or a platinum-rhodium catalyst [ 7,14,15 1. In this paper. the catalytic performance of palladium as a three-way catalyst, the effects of additives and the pe~o~ance of an improved palladium catalyst are reported.
T. Sekiba et al. /Catalysis Today 22 (1994) 113-126
115
2. Ex~rj~ent~ 2.1. Catalysts preparation For the relative catalytic performance test, Pt (1.18 g/l), Rh (0.23 g/l) and Pd (1.88 g/l) catalysts were prepared. PtRh (1.41 g/l, PtlRh ratio 511) catalyst was also prepared as a reference. The Pt only catalyst and the Rh only catalyst corresponded to components of the PtRh catalyst. Pd loading was about 1.5 times as much as Pt loading. Catalysts were made by depositing slurry containing precious metal, alumina and ceria on monolithic substrates. After deposition, the catalysts were dried and calcined in air at 400°C for 1 h. The monolithic substrates was 1.3 1 ( 110 cm2, 11.8 cm) with 400 cells/in2. For the evaluation of the improved palladium catalyst, 0.7 1 (68.6 cm’, 10.2 cm) monolithic substrates with 300 cells/in2 was used. Conventional Pd catalyst and PtRh catalyst were also prepared as references. While the Pd loadings was about three times as much as the PtRh loading, the precious metal cost of Pd was considerably cheaper than that of PtRh. For laboratory evaluation, cylindrical cores (36 mm cross section) were cut from a full-size catalyst. 2.2. Catalyst aging Catalyst aging was carried out with a V8 4.4~liter engine. Two full-size catalysts were aged simultaneously. Two containers packed with eight cylindrical cores were also used for small catalyst samples. The aging cycle includes a 60 s cruise mode and a 5 s fuel cut mode. For the cruise mode, the engine was operated using a stoichiometric air/fuel ratio. Aging temperatures were 650, 750, and 850°C at the catalyst inlet. 2.3. Engine evaluation Engine evaluation was conducted using a L6 2.0-liter engine. Three-way performances were obtained by measu~ng conversion efficiencies of HC, CO and NO, against the air/fuel ratio. The air/fuel ratio was traversed from 13.8 to 15.4 with a modulation of nt 1.O, + 0.5 and + 0.2 A/F at 1 Hz. The catalyst inlet temperature was 400 and 480°C. Light-off performances were measured by raising the temperature from 350 to 450°C at the stoichiometric point without A/F modulation. 2.4. Laboratory evaluation The laboratory evaluation was conducted using a synthetic gas composition simulating engine exhaust gas. Performances of fresh and engine-aged cylindrical
116
T. Sekiba et al. /Catalysis Today 22 (1994) 113-126
core catalysts were measured. The effects of gas composition HC, CO and NO conversion were investigated.
and concentration
on
2.5. Vehicle evaluation Nissan Sunny equipped with a L4 1.6~liter engine was used. 0.7 liter catalyst was attached on the exhaust manifold. HC, CO and NO, emission was measured using a Japan 10 mode test cycle.
3. Results and discussion 3.1. Relative catalytic performance
of platinum, rhodium, and palladium
First of all, the relative catalytic performances of Pt, Rh and Pd were investigated. Fig. 2 shows the light-off performance of each catalyst in engine dynamometer evaluation. The vertical axis indicates the temperature that the HC, CO and NO, conversion reaches 50%. For fresh catalysts, performances of three catalysts were almost the same. The light-off performance of Pd was better than the Pt or Rh catalyst by about 20°C. After engine aging, performance differences appeared. In spite of the small metal loading, Rh was the best of the three catalysts in light-off performance. The deterioration of the activity of Rh was small. The Pt only catalyst
0 m
Fresh After 85O’c,lOOhr
Engine Dynamometer Evaluation Catalyst 1 1.3L Metal Loadings Pt/Rh : 1.41&L-5/1 (1.18 +0.23) Pt: l.l8g/L Rh : 0.23& Pd : 1.88gfL
Pt
Rh
Pd
Fig. 2. Light-off performances.
T. Sekiba et al. /Catalysis
Today 22 (1994) 113-126
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Engine Dynamometer Evaluation A/F Aging: 850°C 1OOhr 14.6f 1..a ’ Evaluation: 4OO’c
Pt
100 $?
80
‘.z 8 5 2
60
6
Rh
Pd
A/F 14.69~0 .5
40 20
Catalyst: volume 1.3L PtRh: 1.41&-S/l Pt: 1.1&/L Rh: 0.235giL Pd: 1.88gIL
100 8 8 ._ 6 & 3
80 A/F 14.6*0 .2
6o 40 20 0 PtRh
Pt
Rh
.
Pd
Fig. 3. Effect of A/F amplitude on conversion.
deteriorated considerably. Therefore, the light off performance of the PtRh catalyst depends on the activity of Rh. Fig. 3 shows the effect of the A/F amplitude on the conversion efficiencies of each catalyst at 400°C. Engine-aged catalysts were measured. The central air/fuel ratio was taken at the stoichiometric point. In the case of wide A/F amplitude ( f 1.O A/F), conversion efficiencies of the PtRh, Rh only, and Pd catalysts were almost the same (near 80%). But as the A/F amplitude decreased ( &OS A/F, 50.2 A/F), while the Pd only and Rh only catalyst performances improved, the Pt only catalyst performance severely deteriorated. Because the change in the gas composition was small, it is possible that the deterioration was due to poisoning by gas adsorption. Hydrocarbon is a gas which has strong adsorption properties. Bearing this point in mind, the effect of hydrocarbon concentration on conversion efficiency of each catalyst was investigated. Fig. 4 shows synthe-gas evaluation results. Propylene was used as a model hydrocarbon. The concentration of propylene and oxygen were modulated simultaneously to keep stoichiometric composition. As the propylene concentration increased, only the Pt catalyst performance deteriorated. Other catalyst performances were not affected by the propylene concentration. Therefore, it is found that hydrocarbon poisoning on a Pt surface could occur even in a stoichiometric atmosphere. In the engine dynamometer evaluation with large A/F modulation, as adsorbed hydrocarbon is removed from the surface under a strong oxidizing atmosphere, the
T. Sekiba et al. /Catalysis Today 22 (I 994) 113-126
Catalyst PlRh: 1.4lgL5/l Pt: l.lSgjL Rh: 0.235gL Pd: 1.88giL gas composition co: 1.53% H2: 0.53% NO: 0.10% 02: 0.93-1.23% C3H6: O-1665ppmC co2: 14.0% N2: Balance 0
1000 C3H6 concentration (ppmC)
Model Gas Evaluation
After engine Aging: 85O”c5Ohr Evaluation temperature: 350°C
Fig. 4. Effect of propylene concentration
ww0.3 ”
0.2”
x
0.1
2000
on conversion.
Car : Nissan Sunny Catalyst : , Paul o.wgjL-10/l Pd 2.83glL catalyst volume 0.7L after 750°C 1OOhraging .
wwO~ 2 8
1
0 (g/km> 0.3 * 0.2 5? 0.1 0.0 PtRh Fig. 5. Vehicle evaluation
Pd results (10 mode test)
T. Sekiba et al. /Catalysis Today 22 (1994) 113-126
119
Pt catalyst keeps a high conversion activity. Regarding the PtRh catalyst, a high performance even under a small A/F modulation is explained by the activity of Rh. Considering the high hydrocarbon conversion activity of Rh, Rh relieves the hydrocarbon poisoning effect on the Pt surface. From these results, it is recognized that the Pt only catalyst is sensitive to hydrocarbon self-poisoning. It is suggested that Pd may be a possible Rh-free threeway catalyst, rather than removing Rh from a conventional PtRh catalyst. 3.2. Three-way pe$ormance of palladium catalyst Using Pd as a three-way catalyst has been investigated by automobile and catalyst m~ufac~rers. It has been pointed out that there is a problem with NO, emissions. In this investigation, a Pd catafyst was evaluated on a vehicle mode test after engine aging, and also a conventional PtRh catalyst was evaluated as a reference. Fig. 5
mplitude
14.2
: *
14.6
1.0
(Amplitude
0’
15.0
: f
0.2A/F)
14.2
14.6
15.0
14.2
14.6
15.0
100 8
80
660 3 8
40
20 0
0
u
14.2
14.6
15.0
14.2
14.6
15.0
A/F
A/F
Catalyst:PtRh 0.88&10/l,
Pd 2.83gA Engine Evaluation after 850°C 3Ohr aging, Evaluation tcmpcrature Fig. 6. Performances of PtRh and Pd catalysts.
: 480°C
120
T. Sekiba et al. /Catalysis
Today 22 (1994) 113-126
Catalyst : Pt Rh O.SSg/L, Pd 2.83gL Model Gas Evaluation : after 85Oc3Ohr engine aging, evaluation temperature 400°C
-0
1000
Xloo
3000
Gas composition NOx : ZOOOppmC C3H6 : 0-3000ppmC 02: 0.2-0.3546 H2: 0.5% co: 1.5%
C3H6 Concentration (ppmC C) Fig. 7. Effect of propylene concentration on NO conversion
shows the results of the vehicle 10 mode test. Though the HC emission of the Pd catalyst was better than that of the PtRh catalyst, for NO, emission, the Pd catalyst was considerably inferior to the PtRh catalyst. In order to understand this problem, the catalytic performances of the PtRh and Pd catalysts were evaluated on an engine dynamometer. Fig. 6 shows the catalytic performances of the PtRh catalyst and the Pd catalyst. In the case of a wide A/F amplitude, the performance of the Pd catalyst was superior to that of the PtRh catalyst. However, in the case of a narrow A/F amplitude, especially in NO, conversion efficiency in fuel-rich conditions, Pd was inferior to PtRh. The difference of the performance in this region affects the NO, emission. In fuel-rich, oxygen-lean conditions, hydrocarbon self-poisoning on the Pd surface occurs as in case of the Pt only catalyst at the stoichiometric point. The hydrocarbon poisoning effect on a Pd catalyst was investigated using a synthe-gas stream which simulated the exhaust gas composition. The effect of propylene concentration on NO conversion efficiency was measured (Fig. 7). In this investigation, the gas composition was not stoichiometric, and fuel-rich con-
260 3
40
20
evaluation temperature 400°C
I
Gas composition NOx : 2000ppmC C3H8 : O-2700ppmC 02: 0.2-0.358
C3H8 Concentration (ppmC ) Fig. 8. Effect of propane concentration
on NO conversion
T. Sekiba et al. / Catalysis Today 22 (1994) 113-126
121
80 8 .C1 fi-Rich: A/F=14.2f0.2 /$ ,+ m Stoichiometric : / A/F=14.6f0.2
20
g
n ”
Pd-Ce
PdCe-La Pd-Ce-Ba Pd-Cc-Cs
0
100
After 850°C 30hr Aging
80 60 40
K
B
20 n Pd-Ce
PdCc-La
Engine Evaluation at 480X,
Pd-Cc-Ba
PdCe-Cs
Pd : 2.838/L, Additives : 6.6gjL
Fig. 9. Effects of additives on NO, conversion.
ditions corresponding to an A/F ratio of 14.0 were used. The concentrations of propylene and oxygen were modulated simultaneously to keep the ratio of oxygen to reduction gas constant. In the absence of propylene, the PtRh and Pd catalysts showed a high NO conversion efficiency. As the propylene concentration increased, the NO conversion on the Pd catalyst decreased. At a propylene condensation of 2500 ppmC which corresponded to exhaust gas composition, a large difference in NO conversion efficiency was observed. A similar experiment was carried out substituting propylene for propane (Fig. 8). In this experiment the effect of propane concentration was very small as compared to that of propylene. This difference in the reaction inhibiting effect between propane and propylene could be due to the adsorption strength on the Pd surface. These results suggests that hydrocarbon poisoning occurs on a Pd catalyst in fuel-rich conditions. Therefore in order to use a Pd catalyst as a three-way catalyst, it is necessary to reduce hydroc~bon poisoning in order to improve NO, conve~ion efficiency in a fuelrich region. 3.3. Improvement of NO, conversion activity of palladium catalysts In order to improve the NO, conversion activity of the Pd catalyst in fuel-rich conditions, the effects of additives to a Pd--Ce/Al,O, catalyst were investigated.
122
T. Sekiba et al. /Catalysis
\,I,
Today 22 (1994) 113-126
_V”
g
80
.e
9
g
60
2
40
g
A/F=14.2*0.2 m Stoichiometric : A/F=14.6*0.2
20
0
0 (%) 100
After 850°C 30hr Aging
& ._ 5 g 6
80 60 .......................
40 20 0
Pd
Pd-Co304
Engine Evaluation
Pd-NiO
at 48O”c,
Pd-ZnO
Pd : 2.83gL
Pd-SnO2 Addition of Oxides : 2OgfL
Fig. 10. Effects of additives on NO, conversion.
The NO, conversion efficiency was evaluated for fresh and aged catalyst using an engine dynamometer (Fig. 9). In the evaluation, at the stoichiometric point, the addition of La, Ba or Cs did not affect the NO, conversion activity. But in the fuelrich region, these additives improved the activity. It is remarkable that the effects of the improvement coincide with the intensity of the basicity of each element. This result suggests that the addition of these basic elements may relieve hydrocarbon poisoning by the donation of electrons to Pd. After engine aging, Pd-Ce-Ba/Al,O, gave the highest NO, conversion efficiency. The improvement effect of La was somewhat less than that with Ba. This result suggests that La is diffused to Al,O, or CeO,. Though the addition of Cs gave the best performance with a fresh catalyst, the improvement decreased after aging probably due to the evaporation of Cs during engine aging. Next the addition of transition metal oxide was investigated (Fig. 10). NiO, Co304, ZnO and SnO, were used. Powders of these oxides were mixed with catalyst slurry and coated on monolithic substrates. With fresh catalysts at stoichiometric evaluation, the addition of oxides did not affect the NO, conversion activity. But in the fuel-rich region, it was found that the addition of NiO or Co,O, slightly improved the activity, and the addition of ZnO or SnOz decreased the activity. In
T. Sekiba et al. / Catalysis Today 22 (1994) 113-126
14.2
14.6
14.2
14.6
15.0
15.0
2 60 40 83 100 80 I
a2;i . . . . . 14.2
14.6
15.0
A/F
E 2 3 5 2
123
14.2
14.6
15.0
14.2
14.6
15.0
14.2
14.6
15.0
loo 80 60 40 20 0
A/F
Catalyst: PtRh 0.88g/L-10/l, Pd 2.83g/L Catalyst volume : 0.7L Engine Evaluation after 750°C 1OOhraging, Evaluation temperature : 480°C Fig. 11. The performance
of the improved Pd catalyst.
this connection, catalysts without Pd were evaluated as a blank test. These oxides showed negligible activity. Regarding these oxides as a semiconductor, there could be the electronic effect for Pd. That is to say, p-type semiconductor (NiO, Co,O,) may relieve hydrocarbon poisoning by electron donation to Pd, and n-type semiconductor (ZnO, SnOz) may increase hydrocarbon poisoning by electron acceptance from Pd. As it is thought that the amount of hydrocarbon or CO adsorption on a n-type semiconductor is greater than that on a p-type semiconductor, it is possible that gas adsorption characteristics of these oxides may influence the activity. After aging, the improvement effect of the addition of NiO or Co304 was obvious. About the addition of ZnO, NO, conversion efficiency in the fuel-rich region after aging was higher than that before aging. It is possible that the interaction between Pd and ZnO was weakened during the aging cycle. The performance of the SnOz
124
T. Sekiba et al. I Catalysis Today 22 (1994) 113-l 26
Pd PtRh Engine Evaluation after 750°C 1OOhr Aging Catalyst Volume : 0.7L Pt Rh : O%g/L-1 O/l Pd : 2.83g/L Fig. 12. The light-off performance
containing catalyst heavily deteriorated. to alloy formation.
of the improved Pd catalyst.
It is thought that this deterioration
is due
3.4. Per-$ormance ofan improved palladium catalyst We considered the above-mentioned results of our investigation, and developed a new Pd catalyst. For practical purposes, the thermal durability was improved along with the NO, conversion efficiency in a rich atmosphere. Conversion efficiencies were evaluated at the engine dynamometer after engine aging (Fig. 11) . The modified Pd catalyst showed the highest performance. The NO, conversion efficiency in the fuel-rich condition was improved and was found to be competitive with that of conventional the PtRh catalyst. Light-off performances were measured (Fig. 12). The conversion efficiency of the improved Pd catalyst was higher at lower temperatures than the PtRh catalyst. The improved Pd catalyst showed a good light-off performance. Vehicle emissions’ performances after engine aging (75O”C, 100 h) were evaluated by a 10 mode test (Fig. 13). The performance of the new Pd catalyst was considerably higher than those of conventional Pd and PtRh catalysts. As mentioned above, this investigation revealed that palladium could be used to manufacture a three-way catalyst without rhodium. But for worldwide use, different working conditions must be considered and various fuels containing S and Pb need to be looked into. Therefore improvements in thermal durability and resistance against poisoning (S, Pb) are required. We
T. Sekiba et al, I Ca falysis Today 22 (1994) 113426
125
10 mode test Q/km) 0.3
Car : Nissan Sunny Catalyst PtRh : 0.88g/L-10/l Pd : 2.83&L,-1O/l catalyst volume : 0.X after 750°C 1OOhraging
g 0.2 0.1 (g/km) of 2 &
1
(g/km)0.3
E0.1 0m2 0.0 PtRh (conv$ional)
(i!$roved)
Fig. 13. The performance of the improved Pd catalyst.
hope to keep on improving our Pd three-way catalyst. The development of an optimum engine control system will also assist the durability and the activity of the Pd catalyst. 4. Conclusion From the investigation, the following conclusions are obtained: (I) Because a Pt only catalyst is susceptible to hydrocarbon poisoning, when considering Rh-free three-way catalysts, Pd can be seen as a possible rhodium-free three-way catalyst rather than removing Rh from a PtRh catalyst. (2) The major problem in using a Pd three-way catalyst is its weak NO, conversion activity in a fuel-rich region, Pd is also affected by hydrocarbon poisoning in a fuel-rich, oxygen-lean atmosphere. (3) Some additives (e.g., La, Ba, Cs, NiO, Co,O,) improve the NO, conversion activity in a fuel-rich region. These may relieve hydrocarbon poisoning by electron donation to Pd. (4) The newly improved Pd three-way catalyst is competitive in peerformance with a PtRh catalyst. Acknowledgements The authors wish to thank Nissan ARC for catalyst analysis, the Itoh group for catalyst preparation, and the Irikura group for catalyst aging and evaluation.
T. Sekiba et al. / Catalysis Today 22 (I 994) 113-126
126
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