Catalyst for exhaust gas purification

A three-way catalyst is generally used in an automotive exhaust system. This catalyst has a sufficient total content of noble metal active species and is prevented from heat deterioration so that the activity of the noble metal active species can be well utilized at low temperatures to enable the catalyst to exhibit improved purification performance at the low temperatures. The catalyst comprises a first catalyst layer formed on the surface of a honeycomb-shaped carrier and containing Pd (palladium) and alumina (.gamma.-Al.sub.2 O.sub.3), and a second catalyst layer formed on the outer surface of the first catalyst layer and containing Pd and ceria (CeO.sub.2).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a catalyst for purifying exhaust 
gases in automotive exhaust systems and, more particularly, to a three-way 
catalyst capable of enhancing the purification performance with respect to 
HC (hydrocarbons) and the like at a low temperature region. 
2. Description of Related Art 
For use as a catalyst for purifying automotive exhaust gases, a three-way 
catalyst has long been well known in the art. The three-way catalyst is of 
a type capable of simultaneously oxidizing CO (carbon monoxide) and HC 
(hydrocarbons) and reduce NOx (nitrogen oxides). This three-way catalyst 
generally comprises .gamma.-alumina (.gamma.-Al.sub.2 O.sub.3) carried by 
a carrier and deposited with, for example, Pt (platinum) and Rh (rhodium) 
as noble metal active species, and is known to exhibit a relatively high 
purifying efficiency with exhaust gases which have resulted from burning 
of an air-fuel mixture of about 14.7 in stoichiometric air-to-fuel ratio. 
One example of such three-way catalyst is disclosed in, for example, 
Japanese Laid-Open Patent Publication (unexamined) No. 58-36634. This 
catalyst comprises ceria (CeO.sub.2) and at least one of Pt and Pd 
(palladium) which are carried by a catalyst carrier. The component ceria 
has an O.sub.2 storage effect such that when the exhaust gases are in a 
lean condition, the ceria adsorbs O.sub.2 in the exhaust gases, whereas 
when the exhaust gases are in a rich condition, the adsorbed O.sub.2 is 
released, so that the ceria can contribute to oxidation and purification 
of HC and CO. Through such O.sub.2 storage effect can the exhaust gas 
atmosphere be brought close to the stoichiometric air-to-fuel ratio and, 
therefore, Pt and/or Pd is enabled to fully exhibit the activity thereof 
to thereby effectively purify the exhaust gases. 
However, such prior art three-way catalyst is subject to heat deterioration 
at high temperatures because Pt and Pd components as noble metal active 
species may become alloyed, or individual Pt components or Pd components 
may become sintered. As a consequence, the activity of Pt and/or Pd 
components is lowered, with the result that the catalyst will not exhibit 
any such purification capability as expected at low temperatures. 
In this regard, theoretically it may be conceivable that if the total 
content of noble metal active species in the catalyst is reduced, any heat 
deterioration due to alloying, sintering or the like is unlikely to occur. 
As a matter of fact, however, where the total content of such metal is 
reduced, the absolute purification capability of the catalyst is lowered. 
This will in no way be acceptable from the standpoint of practical use. 
SUMMARY OF THE INVENTION 
The present invention has been developed to overcome the above-described 
problem, and an objective of the present invention is to provide an 
exhaust gas purifying three-way catalyst which has a sufficient total 
content of noble metal active species and yet is adapted to be prevented 
from heat deterioration so that the activity of the noble metal active 
species can be well utilized at low temperatures to enable the catalyst to 
exhibit an improved purification performance at the low temperatures. 
In accomplishing the above and other objectives, according to a first 
aspect of the present invention, there is provided an exhaust gas 
purifying catalyst dispersedly containing Pd as noble metal active species 
in the thickness direction of catalyst layers. 
More specifically, the catalyst comprises a catalyst carrier, a first 
catalyst layer formed on the catalyst carrier and containing Pd and 
alumina, and a second catalyst layer formed on the outer surface of the 
first catalyst layer and containing Pd and ceria. 
Preferably, the weight ratio of the Pd content of the second catalyst layer 
to the Pd content of the first catalyst layer is within the range of 3/7 
to 9/1. 
Advantageously, at least one of the first and second catalyst layers 
contains Ir in the form of a composite with an alkali earth metal or a 
rare earth metal. 
Also advantageously, the alumina has a specific surface area of 300 m.sup.2 
/g or more and contains dispersed therein at least one of substances 
selected from the group consisting of La, Ba and Zr. 
Preferably, at least one of the first and second catalyst layers contains 
at least one of substances selected from the group consisting of Si, Mg, 
Cr and Mo. 
As above described, according to the present invention, Pd is dispersed in 
two separate catalyst layers, namely, the first catalyst layer formed on 
the surface of the catalyst carrier, and the second catalyst layer formed 
on the outer surface of the first catalyst layer. This means good 
dispersion of Pd in the direction of thickness of the first and second 
catalyst layers. In the second catalyst layer, ceria is present between Pd 
components and this assures better Pd dispersion. By virtue of such 
arrangement, any possible decrease of Pd activity due to sintering can be 
inhibited without involving a decrease in the total Pd content. Further, 
because of the fact that only one kind of noble metal active species, 
i.e., Pd, is used, it is unlikely that alloying will occur as in the case 
of Pd being used in combination with Pt, for example. Another advantage is 
that as the noble metal active species, Pd has higher heat resistance than 
Pt. On the other hand, ceria is present in the second catalyst layer, a 
position for ready contact with exhaust gases, so that it can go into fast 
reaction with exhaust gases. This fact permits the O.sub.2 storage effect 
of the ceria to be efficiently exerted, thus rendering the exhaust gas 
atmosphere to be brought close to the stoichiometric air-to-fuel ratio. 
Accordingly, the range of the air-to-fuel ratio within which purification 
factors are 80% or more with respect to HC, CO and NOx can be enlarged. 
The alumina component has a high ratio of surface to volume, and this 
helps increase the reactivity of the catalyst itself. Further, since the 
alumina is present in the first catalyst layer on which is placed the 
second catalyst layer, the influence of exhaust gas heat upon the alumina 
is alleviated by the second catalyst layer, so that the alumina can be 
prevented from being subject to crystal changes due to heat. Thus, any 
appreciable decrease in the specific surface area of the alumina due to 
such crystal change is inhibited and accordingly the reactivity of the 
catalyst can be well maintained. 
Where Pd is dispersed in two catalyst layers, i.e., the first and second 
catalyst layers, within a weight ratio (second catalyst layer/first 
catalyst layer) range of 3/7 to 9/1, the Pd dispersion in the thickness 
direction of the first and second catalyst layers and the total Pd content 
can be well balanced. If the weight ratio is less than 3/7 or more than 
9/1, Pd dispersion in the thickness direction and/or in the catalyst 
layers in which Pd is present is hindered, with the result that the 
catalyst would be no much different in respect of purification performance 
from any conventional catalyst in which Pd is contained in a single 
catalyst layer. 
Furthermore, the Ir component contained in at least one of the first and 
second catalyst layers has an inherent property such that it can readily 
adsorb NOx in exhaust gases. This greatly contributes to reduction and 
purification, and especially to improved purification performance relative 
to NOx in exhaust gases on the lean side. For this purpose, Ir is in the 
form of a composite with an alkali earth metal or a rare earth metal, so 
that the Ir component has improved heat resistance, thus preventing its 
activity from decreasing due to heat. 
Where the alumina has a specific surface area of 300 m.sup.2 /g or more, 
the catalyst is enabled to perform its reactivity at a high level. 
Further, because of the fact that the alumina contains at least one of 
substances selected from the group consisting of La, Ba and Zr which 
impart high heat resistance to the alumina, the alumina is prevented from 
undergoing a crystal change due to heat, which in turn inhibits a decrease 
in the specific surface area of the alumina. Thus, the catalyst can 
maintain high reactivity. 
Also, Si, Mg, Cr, and/or Mo contained in at least one of the first and 
second catalyst layers have an inherent property that they can more 
readily adsorb sulfides present in exhaust gases, whereby Pd is prevented 
from being adversely affected by the sulfides in the exhaust gases. Thus, 
possible decrease in the activity of Pd due to such unfavorable effect can 
be well prevented.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter the present invention will be described in connection with 
preferred embodiments thereof. 
First Embodiment 
The catalyst for purifying exhaust gases according to the first embodiment 
of the present invention comprises a honeycomb-shaped carrier 1 as a 
catalyst carrier, a first catalyst layer 2 formed on the outer surface of 
the carrier 1 and containing Pd and alumina, and a second catalyst layer 3 
formed on the surface of the first catalyst layer 2 and containing Pd and 
ceria, as shown in FIG. 1. The Pd content of the first catalyst layer 2 is 
4 g/liter (grams per liter of catalyst volume), and the Pd content of the 
second catalyst layer 3 is 6 g/liter. 
This exhaust gas purifying catalyst is produced in the following way. 
First, in order to form the first catalyst layer 2, 480 g of 
.gamma.-Al.sub.2 O.sub.3 powder, as alumina material, is added with 120 g 
of boehmite, 1 liter of water and 10 cc of nitric acid, and agitation is 
effected to provide a slurry. The honeycomb-shaped carrier 1 is dipped in 
the slurry and then withdrawn therefrom. After excess slurry present on 
the surface of the carrier 1 is removed by airblowing, the carrier 1 is 
dried at a temperature of 250.degree. C. for 2 hours and is then calcined 
at a temperature of 600.degree. C. for 2 hours. Thus, alumina, one of 
components contained in the first catalyst layer 2, is carried on the 
surface of the carrier 1. 
The alumina is then impregnated with an aqueous solution of 
dinitrodiamminepalladium (Pd(NO.sub.2).sub.2 (NH.sub.3).sub.2) employed as 
a Pd material which is so adjusted that a given amount of Pd may be 
carried on the alumina. The alumina so impregnated is dried at a 
temperature of 250.degree. C. for 2 hours, and is then calcined at a 
temperature of 600.degree. C. for 2 hours. Thus, the first catalyst layer 
2 comprising the alumina and 4 g/liter of Pd carried on the alumina is 
obtained. 
Then, in order to form the second catalyst layer 3, an aqueous solution of 
dinitrodiamminepalladium which has been adjusted in the same manner as 
above noted is added to ceria, and agitation is effected. The resulting 
mixture is dried and calcined. The calcined material is subjected to ball 
milling. To 540 g of the resulting powder are added 60 g of boehmite, 1 
liter of water and 10 cc of nitric acid, and agitation is effected to 
provide a slurry. The honeycomb-shaped carrier 1 on which was formed the 
first catalyst layer 2 is dipped into the slurry. After it is removed from 
the slurry, the carrier 1 is dried at a temperature of 200.degree. C. for 
2 hours and is then calcined at a temperature of 600.degree. C. for 2 
hours. Thus, the second catalyst layer 3 containing 6 g/liter of Pd is 
formed on the outer surface of the first catalyst layer 2. 
The exhaust gas purifying catalyst obtained in the above described manner 
was subjected to aging at 1000.degree. C. over a time period of 50 hours. 
A catalyst according to the first embodiment of the present invention was 
thus obtained. The catalyst was examined for its purification capabilities 
with respect to HC, CO and Nox, respectively. For purposes of comparison, 
a conventional catalyst was prepared which comprised a single catalyst 
layer carrying ceria and alumina in combination and containing 10 g/liter 
of Pd which was subjected to aging in the same way as above described. 
Similar examinations were made with this conventional catalyst. The 
purification capabilities of the catalyst of the present invention with 
respect to HC, CO and NOx are shown in FIGS. 2, 3 and 4, respectively, 
with those of the conventional catalyst also shown in comparison. It can 
be seen from these drawings that the catalyst of the present invention has 
higher purification capabilities than the conventional catalyst, even at 
low temperatures. 
The range of the air-to-fuel ratio within which purification factors are 
80% or more with respect to HC, CO and NOx were examined with both the 
catalyst of the present invention and the conventional catalyst. The 
characteristic relationships between the air-to-fuel ratio and respective 
purification factors, as observed with the catalyst of the present 
invention, are shown in FIG. 5A, and those observed with the conventional 
catalyst are shown in FIG. 5B. As can be understood from the figures, the 
range referred to above for the catalyst of the present invention is 
A/F=14.30 to 14.74, or two times as high as that for the conventional 
catalyst (A/F=14.50 to 14.72). Especially, the range for the former is 
substantially enlarged on the rich side. 
The reason for this may be that Pd is dispersedly contained in the first 
and second catalyst layers 2 and 3, and that Pd and ceria are present in 
mixture in the second catalyst layer. The fact that Pd is dispersedly 
contained in the two catalyst layers, namely, the first catalyst layer 2 
formed on the surface of the honeycomb-shaped carrier 1, and the second 
catalyst layer 3 formed on the outer surface of the first catalyst layer 
2, permits good dispersion of Pd in the direction of thickness of the 
first and second catalyst layers 2 and 3. In the second catalyst layer 3, 
ceria is present between Pd components, and this assures better Pd 
dispersion. By virtue of such arrangement any possible decrease of Pd 
activity due to sintering can be inhibited without involving any decrease 
in the total Pd content, with the result that the activity of Pd can be 
well utilized at low temperatures, which in turn results in improvement of 
the purification performance of the catalyst at low temperatures. Further, 
because of the fact that only one kind of noble metal active species, 
i.e., Pd, is used, it is unlikely that alloying will occur as in the case 
of Pd being used in combination with Pt, for example. In addition, as the 
noble metal active species, Pd has higher heat resistance than Pt. 
On the other hand, ceria is present in the second catalyst layer 3, a 
position for ready contact with exhaust gases, so that it can go into fast 
reaction with exhaust gases. This fact permits the O.sub.2 storage effect 
of the ceria to be efficiently exerted, thus rendering the exhaust gas 
atmosphere to be brought close to the stoichiometric air-to-fuel ratio. 
Accordingly, the aforementioned range of the air-to-fuel ratio can be 
enlarged. This is considered to be another factor which contributes to the 
improved purification performance of the catalyst of the present invention 
at low temperatures. The alumina component has a high ratio of surface to 
volume, and this helps increase the reactivity of the catalyst itself. 
Further, since the alumina is present in the first catalyst layer 2 on 
which is placed the second catalyst layer 3, the influence of exhaust gas 
heat upon the alumina is alleviated by the second catalyst layer 3, so 
that the alumina can be prevented from being subject to crystal changes 
due to heat. Thus, any appreciable decrease in the specific surface area 
of the alumina due to such crystal change is inhibited and accordingly the 
reactivity of the catalyst can be well maintained. This may be another 
factor which contributes to the improved purification capability of the 
catalyst at low temperatures. 
In addition to the above mentioned Pd dispersion, as one factor explanatory 
of the mechanism in which the presence of Pd and ceria in combination 
helps inhibit Pd sintering, it may be pointed out that possible 
dissociation reaction of Pd (PdO.fwdarw.Pd+1/2O.sub.2) will tend to shift 
toward the higher temperature side. When a dissociation reaction occurs, 
Pd is metallized and made ready for becoming sintered. Generally, it is 
considered that such dissociation reaction will take place at a 
temperature of about 900.degree. C. However, because of the fact that Pd 
is present together with ceria, such dissociation reaction will not take 
place unless there is a temperature rise to about 1000.degree. C., for 
example. Presumably, therefore, such dissociation reaction itself may have 
been inhibited, and accordingly sintering may have also been inhibited. 
Now, in conjunction with the above described embodiment of the present 
invention, a series of exhaust gas purifying catalysts was prepared 
wherein the weight ratio of the Pd content of the second catalyst layer 3 
to the Pd content of the first catalyst layer 2 (the second catalyst layer 
3/the first catalyst layer 2) was varied by 10% each within a weight ratio 
range of 10/0 to 0/10. With these catalysts, respective inlet gas 
temperatures were examined when the purification factor of each respective 
catalyst reached 50%. The results are shown in FIG. 6. As may be 
appreciated from the drawing, whereas such temperature exceeds 350.degree. 
C. in the case of the conventional catalyst, the exhaust gas purifying 
catalysts of the present invention in which the weight ratio is within the 
range of 3/7 to 9/1 achieved a purification factor of 50% with respect to 
HC, even at low temperatures of 330.degree. to 340.degree. C. In 
particular, an exhaust gas purifying catalyst of the present invention in 
which the weight ratio was 6/4 achieved 50% HC purification at the 
lowermost temperature level of 300.degree. C. 
The reason for this may be that Pd is dispersedly contained in two catalyst 
layers, i. e., the first and second catalyst layers 2 and 3 within the 
weight ratio range of 3/7 to 9/1, whereby satisfactory balance can be 
obtained between the Pd dispersion in the thickness direction of the first 
and second catalyst layers 2 and 3 and the total Pd content of the two 
layers as a whole. It may be considered that this enables the catalyst to 
efficiently exhibit its activity to a satisfactory degree, even at low 
temperatures. Basically, better Pd dispersion is obtained where the Pd 
contents of the first and second catalyst layers 2 and 3 are even with 
each other, or in other words, Pd dispersion goes better as the weight 
ratio goes closer to 5/5. However, when contact of Pd with exhaust gases 
is considered, it is believed that most satisfactory results can be 
obtained where the Pd content of the second catalyst layer 3 is slightly 
greater than that of the first catalyst layer 2, that is, the weight ratio 
is 6/4. If the weight ratio is less than 3/7 or more than 9/1, Pd 
dispersion in the thickness direction and/or in the first and second 
catalyst layers 2 and 3 in which Pd is present is hindered, with the 
result that the catalyst would be no much different in respect of 
purification performance from any conventional catalyst in which Pd is 
contained in a single catalyst layer. 
Further, with respect to various catalysts in which the weight ratio is 
varied, respective ranges of the air-to-fuel ratio referred to above were 
examined. The examination results are shown in FIG. 7. As may be 
appreciated from the drawing, there is a general tendency that the greater 
the Pd content of the second catalyst layer 3, the larger is the range. 
Especially where the weight ratio is 6/4, most satisfactory results are 
obtained. 
Second Embodiment 
FIG. 8 schematically shows an exhaust gas purifying catalyst according to 
the second embodiment of the present invention, wherein Ir (iridium), in 
the form of a composite with La (lanthanum), a rare earth metal, is 
contained in the first catalyst layer 2. Other structural features of the 
catalyst, including the weight ratio of the Pd content of the first 
catalyst layer 2 to the Pd content of the second catalyst layer 3, are 
identical with those of the foregoing first embodiment. 
The method of producing this exhaust gas purifying catalyst will be 
explained below. It is noted that the manner of forming the second 
catalyst layer 3 is identical with that in the first embodiment 1 and, 
therefore, description in that regard is omitted. 
In order to form the first catalyst layer 2, to an .gamma.-Al.sub.2 O.sub.3 
powder, as alumina material, is added iridium chloride (IrCl.sub.4), as Ir 
material, in such a way that the Ir component is carried at the rate of 1 
g/liter. This mixture is then mixed with 5 wt % of La, as rare earth 
metal, relative to the total Pd content. To 480 g of the resulting mixture 
powder are added 120 g of boehmite, 1 liter of water and 10 cc of nitric 
acid, and agitation is effected to provide a slurry. The honeycomb-shaped 
carrier 1 is dipped in the slurry and then withdrawn therefrom. After 
excess slurry present on the surface of the honey-comb-shaped carrier 1 is 
removed by airblowing, the carrier 1 is dried at a temperature of 
250.degree. C. for 2 hours and is then calcined at a temperature of 
600.degree. C. for 2 hours. Thus, alumina, and Ir+La, as component 
elements to be contained in the first catalyst layer 2, is carried on the 
surface of the carrier 1. 
The alumina is then impregnated with an aqueous solution of 
dinitrodiamminepalladium prepared in such a way that a given amount of Pd 
may be carried on the alumina. The alumina so impregnated is dried at a 
temperature of 250.degree. C. for 2 hours, and is then calcined at a 
temperature of 600.degree. C. for 2 hours. Thus, the first catalyst layer 
2 containing alumina, Pd, and an Ir-La composite is formed. Subsequently, 
the second catalyst layer 3 is formed on the outer surface of the first 
catalyst layer 2. The exhaust gas purifying catalyst obtained in the above 
described manner was subjected to aging at 1000.degree. C. over a time 
period of 50 hours. The catalyst according to the second embodiment of the 
present invention was thus obtained. The catalyst was examined for its 
purification capabilities with respect to HC, CO and NOx, respectively. 
For purposes of comparison, a conventional catalyst was prepared which 
comprised a single catalyst layer carrying ceria and alumina in 
combination, with Pd and the Ir-La composite contained therein in same 
proportions as in the second embodiment of the present invention, which 
was subjected to aging in the same way as above described. Similar 
examinations were made with this conventional catalyst. The purification 
capabilities of the catalyst of the present invention with respect to HC, 
CO and NOx are shown in FIGS. 9, 10 and 11, respectively, with those of 
the conventional catalyst also shown in comparison. It can be seen from 
these drawings that the catalyst of the present invention has higher 
purification capabilities than the conventional catalyst, at low 
temperatures. 
The range of the air-to-fuel ratio within which purification factors are 
80% or more with respect to HC, CO and NOx were examined with both the 
catalyst of the present invention and the conventional catalyst. The 
characteristic relationships between the air-to-fuel ratio and respective 
purification factors, as observed with the catalyst of the present 
invention, are shown in FIG. 12A, and those observed with the conventional 
catalyst are shown in FIG. 12B. As can be understood from the figures, 
such range for the catalyst of the present invention is A/F=14.34 to 
14.76, or nearly two times as high as that for the conventional catalyst 
(A/F=14.50 to 14.72). This range for the former extends widely, not only 
on the rich side but also on the lean side. 
The reason for this may be that the Ir contained in the first catalyst 
layer 2 is characteristically ready to adsorb NOx in exhaust gases, which 
fact can contribute substantially to reduction and purification of NOx, 
not to mention the reason explained with respect to the foregoing first 
embodiment. It is considered that this can enhance the purification 
capability of the catalyst with respect to NOx in particular in exhaust 
gases on the lean side. In this case, Ir in the form of a composite with 
La can exhibit improved heat resistance and this serves to prevent 
possible decrease in the activity of Ir due to heat. This is believed to 
be a factor contributive to purification capability improvement at low 
temperatures. 
Now, in conjunction with the above described embodiment of the present 
invention, a series of exhaust gas purifying catalysts was prepared 
wherein the Ir content was varied within the range of 0 g/liter to 2.0 
g/liter, and with these catalysts, respective inlet gas temperatures were 
examined when the HC purification factor of each respective catalyst was 
50%. The results are shown in FIG. 13. As can be seen from the drawing, 
the temperature tends to decrease as the Ir content increases, except that 
at the Ir content of 1.0 g/liter or more, the temperature will remain 
almost same. 
Again, with respect to exhaust gas purifying catalysts containing Ba 
(barium), as an alkali earth metal, instead of La, and those containing no 
such additive, respective inlet gas temperatures were examined when the HC 
purification factor of each respective catalyst was 50%. The results are 
shown in FIG. 14. As may be seen from the drawing, catalysts containing La 
or Ba had a good advantage in low temperature characteristics over those 
having no La or Ba content. Specifically, those containing La exhibited 
best low-temperature characteristics, say, at a temperature level of about 
270.degree. C., and those containing Ba came next, at a temperature level 
of about 290.degree. C. Those having no such additive content were active 
at a higher temperature, say, about 350.degree. C. 
In the above described second embodiment, Ir is contained in the first 
catalyst layer 2, but alternatively Ir may be contained in the second 
catalyst layer 3, or in both the first and second catalyst layers 2 and 3. 
In the above second embodiment, Ir is used in the form of a composite with 
La or Ba, but alternatively Ir may be used in the form of a composite 
oxide or solid solution containing a rare earth metal other than La or an 
alkali earth metal other than Ba. 
Third Embodiment 
FIG. 15 schematically shows an exhaust gas purifying catalyst according to 
a third embodiment of the present invention, wherein the alumina in the 
first catalyst layer 2 has a specific surface area of 300 m.sup.2 /g and 
wherein the first catalyst layer 2 contains La as an additive for 
stabilizing the heat resistance of the alumina. Other structural features 
of the catalyst are identical with those of the foregoing first 
embodiment. In the present embodiment, the alumina is produced in 
accordance with the alkoxide process, and the above mentioned additive is 
added in the form of a compound based on La(NO.sub.3).sub.3, a nitroxide, 
in the stage of hydrolysis in the process for alumina production. 
The method of producing this exhaust gas purifying catalyst will be 
explained below. It is noted that the manner of forming the second 
catalyst layer 3 is identical with that in the first embodiment and, 
therefore, description in that regard is omitted. 
First, in order to produce .gamma.-Al.sub.2 O.sub.3, as alumina material, 
240 g of aluminum isopropoxide and 216 g of hexylene glycol were mixed 
together, and the mixture is heated and agitated in an oil bath at a 
temperature of 120.degree. C. for 4 hours. Thereafter, to the mixture is 
added 90 g of water and hydrolysis is carried out, and the hydrolyzate is 
then gelled. In this embodiment, La as an additive is added in such a 
condition that the la, together with Pd, is mixed into the water, during 
the stage of hydrolysis. In this case, the proportions of the additives 
are chosen to be 5 wt % each relative to the total amount of alumina 
production, and La is added in the form of La(NO.sub.3).sub.3, a 
nitroxide. The resulting mass is allowed to be aged overnight (for 16 
hours) while being kept at a temperature of 80.degree. C. After being 
subjected to drying under reduced pressure, the aged mass is calcined at 
600.degree. C. for 3 hours. The .gamma.-Al.sub.2 O.sub.3 thus obtained had 
a specific surface area of 350 m.sup.2 /g. 
In order to form the first catalyst layer 2, 15 g of boehmite, 125 cc of 
water, and 1.25 cc of nitric acid are added to 60 g of .gamma.-Al.sub.2 
O.sub.3, and agitation was effected to provide a slurry. The 
honeycomb-shaped carrier 1 is dipped in the slurry and is then removed 
therefrom. After excess slurry present on the surface of the carrier 1 is 
removed by airblowing, the carrier 1 is dried at a temperature of 
250.degree. C. for 2 hours and is then calcined at a temperature of 
600.degree. C. for 2 hours. Thus, the first catalyst layer 2 containing 
alumina, Pd and La is formed. Thereafter, the second catalyst layer 3 is 
formed on the outer surface of the first catalyst layer 2. 
The exhaust gas purifying catalyst obtained in the above described manner 
was subjected to aging at 1000.degree. C. over a time period of 50 hours. 
The catalyst according to the third embodiment of the present invention 
was thus obtained. The catalyst was examined for its purification 
capabilities with respect to HC, CO and NOx, respectively. For purposes of 
comparison, a conventional catalyst was prepared which comprised a single 
catalyst layer carrying ceria and alumina in combination, with Pd and La 
contained therein in same proportions as in this embodiment of the present 
invention, which was subjected to aging in the same way as above 
described. Similar examinations were made with this conventional catalyst. 
The purification capabilities of the catalyst of the present invention 
with respect to HC, CO and NOx are shown in FIGS. 16, 17 and 18 
respectively, with those of the conventional catalyst also shown in 
comparison. It can be seen from these drawings that the catalyst of the 
present invention has higher purification capabilities than the 
conventional catalyst, at low temperatures. 
The range of the air-to-fuel ratio within which purification factors are 
80% or more with respect to HC, CO and NOx were examined with both the 
catalyst of the present invention and the conventional catalyst. The 
characteristic relationships between the air-to-fuel ratio and respective 
purification factors, as observed with the catalyst of the present 
invention, are shown in FIG. 19A, and those observed with the conventional 
catalyst are shown in FIG. 19B. As can be understood from the figures, 
such range for the catalyst of the present invention is A/F=14.20 to 
14.74, or nearly two times as high as that for the conventional catalyst 
(A/F=14.50 to 14.72). This range for the former extends further on the 
rich side. 
The reason for this may be that the alumina has a specific surface area of 
350 m.sup.2 /g which enables the catalyst itself to achieve a high level 
of reactivity, and that La is dispersedly contained in the alumina so that 
the La serves as a stabilizer for the alumina against heat to prevent the 
alumina from undergoing crystal changes under the influence of heat, not 
to mention the reason stated with respect to the foregoing first 
embodiment. Thus, possible decrease in the specific surface area of the 
alumina due to such crystal change can be prevented so that the reactivity 
of the catalyst can be maintained at a high level. This is considered to 
be an important factor which contributes to purification capability 
improvement at low temperatures. It is noted that the La also serves as a 
stabilizer for Pd against heat. 
During the stage of hydrolysis, with respect to various exhaust gas 
purifying catalysts wherein Ba, Zr (zirconium), Cr (chromium) and Fe 
(iron) were added respectively instead of La, and those having no such 
additive contained therein (w/o) were examined as to respective inlet gas 
temperatures when the HC purification factor was 50%. The results are 
shown in FIG. 20. As can be seen from the drawing, catalysts incorporating 
such additive as La, Ba, or Zr had an advantage in low temperature 
characteristics over those having no such additive component (w/o) and/or 
those containing such additive component as Cr or Fe. Specifically, one 
containing La was active at the lowermost temperature, say, about 
270.degree. C., and one containing Ba came next at about 280.degree. C. 
One containing Zr was active at about 290.degree. C. On the other hand, 
one containing Cr was active at about 300.degree. C. or about same 
temperature level as one with no additive. One containing Fe was active at 
about 320.degree. C. 
Also, catalysts with respect to which La addition was made at different 
stages, say, during a heating and agitating stage; during a hydrolysis 
stage; during a washcoating stage; and after the washcoating stage, were 
examined as to respective inlet gas temperatures when the HC purification 
factor was 50%. It is noted in this connection that La was added in the 
form of La(NO.sub.3).sub.3 in cases other than addition during the heating 
and agitating stage in which La was added in the form of La.sub.2 O.sub.3. 
The results are shown in FIG. 21. As may be understood from the drawing, 
all the catalysts exhibited good low-temperature characteristics and, in 
particular, those for which addition was made during the hydrolysis stage 
and during the washcoating stage respectively had an advantage, though 
slight, over the others. 
In the foregoing third embodiment, the additive is added to the first 
catalyst layer 2, but alternatively it may be added to the second catalyst 
layer 3 or both the first and second catalyst layers 2 and 3. For the 
purpose of adding to the first catalyst layer 2, the additive may be added 
in such a way that a solid solution or composite oxide powder made up of 
the additive and Pd is mixed with ceria powder to obtain a slurry and that 
the catalyst carrier is dipped into the slurry. 
Fourth Embodiment 
FIG. 22 schematically shows an exhaust gas purifying catalyst according to 
a fourth embodiment of the present invention, wherein Si (silicon) as an 
additive for preventing Pd from being affected by sulfides in exhaust 
gases is contained in the second catalyst layer 3. Other structural 
features of the catalyst are identical with those of the foregoing first 
embodiment. 
The method of producing this exhaust gas purifying catalyst will be 
explained below. It is noted that the manner of forming the first catalyst 
layer 2 is identical with that in the first embodiment and, therefore, 
description in that regard is omitted. 
In order to form the second catalyst layer 3, a powder material in the form 
of a solid solution or a composite oxide which is formed by Si, as an 
additive, and Pd in combination is mixed with ceria powder, and to 540 g 
of this mixed powder are added 60 g of boehmite, 1 liter of water, and 10 
cc of nitric acid, followed by agitation to obtain a slurry. The 
honeycomb-shaped carrier 1 on which is formed the first catalyst layer 2 
is dipped in the slurry, and is then dried at 200.degree. C. for 2 hours. 
Then, the honeycomb-shaped carrier 1 is calcined at a temperature of 
600.degree. C. for 2 hours. Thus, the second catalyst layer 3 containing 
ceria, Pd and Si is formed. The proportion of the Si is chosen to be 5 wt 
% relative to the total Pd content. 
The exhaust gas purifying catalyst obtained in the above described manner 
was subjected to aging at 1000.degree. C. over a time period of 50 hours. 
The catalyst according to the fourth embodiment of the present invention 
was thus obtained. The catalyst was examined for its purification 
capabilities with respect to HC, CO and NOx, respectively. For purposes of 
comparison, a conventional catalyst was prepared which comprised a single 
catalyst layer carrying ceria and alumina in combination, with Pd and Si 
contained therein in same proportions as in this embodiment of the present 
invention, which was subjected to aging in the same way as above 
described. Similar examinations were made with this conventional catalyst. 
The purification capabilities of the catalyst of the present invention 
with respect to HC, CO and NOX are shown in FIGS. 23, 24 and 25, 
respectively, with those of the conventional catalyst also shown in 
comparison. It can be seen from these drawings that the catalyst of the 
present invention has higher purification capabilities than the 
conventional catalyst, at low temperatures. 
The reason for this may be that the Si contained in the second catalyst 
layer 3 tends to more easily adsorb sulfides in exhaust gases than Pd. By 
virtue of this property of Si it is possible to prevent Pd from being 
adversely affected by sulfides in exhaust gases, to thereby prevent 
possible decrease of Pd activity due to the unfavorable effect of 
sulfides. This is considered to be an important factor which contributes 
to the improved purification performance at low temperatures. 
Now, with respect to exhaust gas purifying catalysts containing Mg 
(magnesium), Cr, or Mo (molybdehum), instead of Si, and those containing 
no such additive (w/o), respective inlet gas temperatures were examined 
when the HC purification factor of each respective catalyst was 50%. The 
results are shown in FIG. 26. As may be seen from the drawing, catalysts 
containing Si, Mg, Cr, or Mo had an advantage in low temperature 
characteristics over those having no such additive content (w/o). 
Specifically, those containing Si exhibited best low-temperature 
characteristics, say, at a temperature level of about 230.degree. C., and 
those containing Mg came next, at a temperature level of about 250.degree. 
C., followed by Cr at about 270.degree. C., Mo at about 290.degree. C. 
Those having no such additive content were active at a higher temperature, 
say, about 300.degree. C. 
While, in the foregoing fourth embodiment, additive such as Si is applied 
to the second catalyst layer 3, it may be applied to the first catalyst 
layer 2 or both the first and the second catalyst layers 2 and 3. In case 
that the additive is applied to the first catalyst layer 2, the additive 
may be applied in such a manner that a solid solution or composite oxide 
powder made up of the additive and Pd is mixed with alumina powder to 
prepare a slurry in which the catalyst carrier is dipped. 
As described above, where Pd is dispersed in the first and second catalyst 
layers 2 and 3, and Pd is present together with ceria in the second 
catalyst layer 3, improved Pd dispersion can be achieved without involving 
a decrease of the Pd content. Thus, possible activity decrease of Pd due 
to sintering is inhibited so that the activity of Pd can be exhibited at 
low temperatures to enhance the purification performance of the catalyst 
at the low temperatures. Further, the presence of ceria in the second 
catalyst layer 3 enables efficient achievement of the O.sub.2 storage 
effect of ceria to thereby enlarge the range of the air-to-fuel ratio 
within which purification factors are 80% or more with respect to HC, CO 
and N0x, thus contributing to improved purification performance at low 
temperatures. The alumina in the first catalyst layer 2 has a high ratio 
of surface to volume through which it serves to increase the reactivity of 
the catalyst, and is able to inhibit possible decrease of the specific 
surface area due to a thermal crystal change to maintain the catalytic 
reactivity. In this way, the alumina also contributes to improved 
purification performance at low temperatures. 
Where Pd is dispersed in the first and second catalyst layers 2 and 3, 
within a weight ratio (second catalyst layer/first catalyst layer) range 
of 3/7 to 9/1, the catalyst can efficiently exhibit its activity at low 
temperatures. This insures improved purification performance at the low 
temperatures. 
Furthermore, Ir, in the form of a composite with alkali earth metal or rare 
earth metal, may be contained in at least one of the first and second 
catalyst layers 2 and 3 to thereby enable improvement in the purification 
performance of the catalyst with respect to NOx in exhaust gases, and 
improvement in the heat resistance of the Ir. Thus, possible decrease of 
catalytic activity due to heat can be inhibited, and the purification 
performance of the catalyst at low temperatures can be enhanced. 
Also, where the alumina has a specific surface area of 300 m.sup.2 /g or 
more, the catalyst is enabled to perform its reactivity at a high level. 
Further, because La, Ba or Zr is dispersedly contained in the alumina, it 
acts as a heat resistance stabilizer for the alumina to enable the 
catalyst to maintain high reactivity. This provides for improvement in the 
reactivity of the catalyst which, in turn, results in improved 
purification performance at low temperatures. 
Where Si, Mg, Cr, and/or Mo is contained in at least one of the first and 
second catalyst layers 2 and 3, Pd is prevented from being adversely 
affected by sulfides in exhaust gases. Thus, possible decrease in the 
activity of Pd due to the effect of sulfides can be well prevented, and 
this will contribute to improved purification performance at low 
temperatures. 
Although the present invention has been fully described by way of examples 
with reference to the accompanying drawings, it is to be noted here that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless such changes and modifications otherwise depart 
from the spirit and scope of the present invention, they should be 
construed as being included therein.