Catalyst for purifying the exhaust gases of internal combustion engines and method for making the catalyst

A catalyst for purifying the exhaust gases of internal combustion engines with improved light-off behavior and improved durability with reduced specific noble-metal content is disclosed which uses active aluminum oxide provided with customary promoters as carrier and platinum and/or palladium and rhodium in customary amounts as noble metals. In addition, it contains one or more base metals in amounts up to 5 times the mass of the total noble metal which are brought into the immediate vicinity of the rhodium by means of a special manufacturing technique.

INTRODUCTION TO THE INVENTION 
The present invention relates generally to a catalyst for purifying the 
exhaust gases of internal combustion engines by means of oxidation and/or 
reduction. 
As a result of an increased awareness of the need for environmental 
protection, legislative efforts for setting exhaust-gas limits for 
pollutants such as carbon monoxide, hydrocarbons and nitrogen oxides from 
internal combustion engines, especially from motor vehicles, are becoming 
more and more intense. This has the consequence that the so-called 
cold-start phase of engine operation has become quite significant in the 
test cycles specified in the US, Japan and Europe. Accordingly, there is a 
need for emission control catalysts which can convert the pollutants to a 
high degree into harmless components at exhaust-gas temperatures which are 
as low as possible. Since engine design development is exhibiting at the 
same time a trend toward concepts which increasingly utilize operating 
phases with an excess of oxygen (lean exhaust gas), more stringent 
requirements also apply for the long-term behavior of catalysts. Moreover, 
there is also a need to reduce the amount of noble metal used per catalyst 
for reasons of economy but also for reasons of the availability of 
platinum-group metals. However, losses of activity and durability can 
occur as a result of these activities which are unacceptable. 
SUMMARY OF THE INVENTION 
The present invention relates in particular to a catalyst, for purifying 
the exhaust gases of internal combustion engines, where aluminum oxide of 
the transition series is a carrier which includes as promoter substances 
3-70 % by weight CeO.sub.2, 0-20 % by weight La.sub.2 O.sub.3, optionally 
as a mixture with other rare-earth metal oxides, and 1-20 % by weight 
ZrO.sub.2, 0-10 % by weight Fe.sub.2 O.sub.3, and 0-20 % by weight NiO as 
promoter. A catalytically active phase is applied to the carrier. The 
catalytically active phase contains noble metal and comprises 0.02-3 % by 
weight platinum and/or palladium and rhodium, with a weight ratio between 
platinum and/or palladium and the rhodium of 1:2 to 15:1, preferably 1:1 
to 3:1; optionally, with a weight ratio between platinum and palladium of 
10:1 to 1:5. The optionally lattice-stabilized carrier is impregnated with 
a solution of a promoter compound and/or coated by means of mixing the 
same with a suspension of oxides, hydroxides or carbonates of the 
particular promoter element. The carrier is subsequently treated in air at 
300.degree.-900.degree. C., and then impregnated with a solution of 
compounds of the noble metals. After drying, the product obtained thereby 
is activated, optionally in a hydrogen-containing gas, or optionally under 
hydrothermal conditions, at a temperature in a range of 
250.degree.-1050.degree. C. The catalyst in the form of a coating on an 
inert ceramic or metal carrier is present in an amount of 5-30 % by weight 
relative to the weight of the inert carrier. 
The invention is based on catalytic compounds described in DE-PS 29 07 106 
(U.S. Pat. No. 4,294,726) and expands and modifies them in an advantageous 
manner. 
An object of the invention is to solve the problem of meeting the 
above-mentioned requirements by means of improving the known catalysts as 
regards the light-off behavior, the lean aging durability and a lowering 
of the specific noble-metal content. 
In addition to the catalysts of the composition in accordance with the 
present invention, another object is to provide a method for their 
production. 
DETAILED DESCRIPTION OF THE INVENTION 
An object of the present invention is to provide a catalyst for purifying 
the exhaust gases of internal combustion engines which has aluminum oxide 
of the transition series as a carrier and which further contains 3-70 % by 
weight CeO.sub.2, 0-20 % by weight La.sub.2 O.sub.3, optionally as a 
mixture with other rare-earth metal oxides, and 1-20 % by weight 
ZrO.sub.2, 0-10 % by weight Fe.sub.2 O.sub.3, and 0-20 % by weight NiO as 
promoter for the catalytic reaction, and further with a catalytically 
active, noble metal containing phase applied to the carrier. The 
catalytically active phase comprises 0.02-3 % by weight of a noble metal; 
i.e. platinum and/or palladium and rhodium, with a weight ratio between 
platinum and/or palladium and the rhodium of 1:2 to 15:1, preferably 1:1 
to 3:1; optionally, with a weight ratio between platinum and palladium of 
10:1 to 1:5. The carrier can be optionally lattice-stabilized as is known 
in the art. 
According to the method aspect of the invention, the carrier is impregnated 
with a solution of a promoter compound and/or the carrier is coated by 
means of mixing with a suspension of oxides, hydroxides or carbonates of 
the particular promoter elements. Subsequently the carrier is treated in 
air at 300.degree.-900.degree. C., and then the carrier is impregnated 
with a solution of one or more compounds of the noble metals, as the 
catalytically active component. After drying, the catalyst is activated by 
heating at a temperature in a range of 250.degree.-1050.degree. C., 
optionally in a hydrogen-containing gas, optionally under hydrothermal 
conditions. 
The catalyst can also be present in the form of a coating on an inert 
support comprising ceramics or metal, as for example a honeycomb, in an 
amount of 5-30% by weight relative to the weight of the inert support. 
The percentage amounts of CeO.sub.2 in the catalyst, when formed carriers 
of active Al.sub.2 O.sub.3 such as pellets are used, are 3 to 
approximately 20% by weight, preferably 4 to 10 % by weight. In the case 
of honeycomb support catalysts, the catalyst applied by means of 
dispersion coating onto the inert honeycomb support contains at least 15 % 
by weight of CeO.sub.2. 
"Aluminum oxide of the transition series" is a well known term as shown in 
Kirk Othmer, Encyclopedia of Chemical Technology, 3rd Edition, volume 2, 
pages 218-244. 
The catalyst of the present invention is characterized in that it contains 
at least one base metal selected from the series of lithium, potassium, 
rubidium, magnesium, calcium, barium, lanthanum, cerium, praseodymium, 
neodymium, samarium, iron, cobalt and nickel in an amount of up to 5 times 
the total mass of the noble metal. The catalyst can be obtained by means 
of impregnating the promoter-containing carrier with a solution (A) of 
compounds of platinum and/or of palladium in an amount corresponding to at 
least 30 % of the total mass of platinum and/or palladium, drying and 
activating as described. This step is followed by impregnating in a second 
step with a solution (B) which contains the compound of rhodium, and 
optionally the residual amount of platinum and/or palladium compound, that 
was not added in the first step with solution (A). Then there follows a 
renewed drying and activating. Solution (B) contains one or more of the 
above mentioned base-metal compounds. 
An essential feature of the new catalyst in accordance with the present 
invention is the use at the same time of certain base metals in the 
immediate vicinity of rhodium. In other respects, the basic composition of 
the catalyst generally corresponds to known formulations with the 
platinum-group metal rhodium, as well as platinum and/or palladium, 
applied onto a carrier of aluminum oxide of the transition series which is 
provided with oxidic phases of oxygen-storing and/or carrier-stabilizing 
additives, so-called promoters. 
Both the elements of the platinum group and of the promoters are used in 
certain amounts which are essentially within known limits, even though 
these amounts are set forth in a more precise manner for the purpose of 
the present invention. 
The placing of the base metals Li, K, rubidium, Mg, Ca, Ba, La, Ce, Pr, Nd, 
Sm, Fe, Co and/or Ni in the immediate vicinity of rhodium, which placing 
is essential for the invention, is associated with a special method of 
production described herein as an aspect of the invention. In this method 
the promoter-containing aluminum oxide carrier is first impregnated with a 
solution of Pt and/or Pd compounds and then dried and activated, 
optionally in a gas containing H.sub.2 and optionally under hydrothermal 
conditions, that is, e.g. steaming in an atmosphere containing water 
vapor. There, the carrier is impregnated with a second solution containing 
the rhodium and the specified base metals and is thereafter activated. At 
least 30 % of the total mass of Pt and/or Pd is provided in the first 
solution (A) so that a large part of these elements remains removed from 
the vicinity of the specified base metals. 
The specified base metals added in accordance with the method of the 
present invention can thus enter in an intensified manner into a 
reciprocal action with the rhodium, which brings about the reduction 
effect of the catalyst. 
A quantitative separation of the local spheres of rhodium and Pt and/or Pd 
does not have to be present in the catalyst of this invention. It is much 
more essential that the two species of noble metal are primarily not 
present in an alloyed state or in a state in which they are interlocked 
with one another. This condition is achieved by the method of production 
in accordance with the present invention. 
The catalyst of the invention can be in the form of a carrier catalyst, 
e.g. in spherical form or in some other form as bulk material. However, it 
can also be present as a coating on an inert, structure-reinforcing 
support of ceramics or metal. Such inert support can be the well known 
honeycombs of cordierite, mullite, .alpha.-aluminum oxide or of a ferritic 
steel alloy. Compact form bodies of the specified materials can also be 
used. 
An advantageous embodiment of the catalyst will now be described that 
applies in the case of honeycomb catalysts as well as bulk-material shell 
catalysts with an inert carrier core. In this embodiment, 30-100 % of the 
total mass of the aluminum oxide carrier containing the promoters is 
deposited before the application of solution (A) and 0-70 % of this total 
mass of carrier is deposited before the application of solution (B) as the 
coating. As a result of the partial deposition of the carrier, the 
promoters in the deposited portions of the carrier exhibit differing 
concentrations and the carrier portion deposited before the application of 
solution (B) preferably exhibits lower concentrations of the promoters 
within the framework of the given concentration ranges than does the 
residual mass of the carrier. 
A further variation of the catalyst of the invention has also proven to be 
advantageous in which the promoter CeO.sub.2 is added in the form of a 
water-soluble compound into the carrier mass present before the 
application of solution (A). In the embodiment the specified promoter is 
added into the carrier mass present before the application of solution (B) 
by means of mixing it with a suspension of oxides, hydroxides or 
carbonates of cerium as well as by means of a subsequent decomposition. 
The following advantages are achieved with the invention: 
1. Lower cold start temperatures for all three pollutant components, to 
wit, carbon monoxide, hydrocarbons and nitrogen oxides; 
2. Increased degrees of conversion for all three pollutant components, to 
wit, carbon monoxide, hydrocarbons and nitrogen oxides; 
3. A better durability. 
The given increase in performance when the catalysts of the invention are 
used also creates the possibility of reducing the noble-metal content, 
especially the platinum content, per liter of catalytic volume without any 
significant loss of activity. 
The invention is explained further in the following using examples of 
embodiments. 
EXAMPLES 
Comparative example 1 
A honeycomb body of cordierite with 62 cells/cm.sup.2 was loaded with 180 
g/l catalytic volume of an oxide mixture. An aqueous suspension with a 
solid content of 48 % by weight and the following composition was used to 
this end for the oxide mixture present after activation: 
70.3 parts by weight .gamma.-aluminum oxide 
15.0 parts by weight cerium oxide as ammonium cerium (IV) nitrate 
4.2 parts by weight zirconium oxide as zirconyl acetate 
8.0 parts by weight nickel oxide 
2.5 parts by weight iron oxide. 
After the honeycomb body had been coated with the oxide layer, it was dried 
at 120.degree. C., and activated 2 hours at 400.degree. C. Then the 
honeycomb body coated in this manner was immersed into an aqueous solution 
of hexachloroplatinic acid, palladium (II) chloride, and rhodium (III) 
chloride in a weight ratio of Pt:Pd:Rh of 6:4:1 and dried. After a 
two-hour tempering at 350.degree. C. in air, the reduction of the 
noble-metal salts deposited on the carrier material finally took place in 
a current of hydrogen at a temperature of 450.degree. C. for a period of 4 
hours. The total noble-metal content was 1.77 g/l carrier volume after 
this treatment.

EXAMPLE 1 
A ceramic honeycomb reinforcer body (62 cells/cm.sup.2) was coated with the 
same oxide layer and in the same manner as described in comparative 
example 1, dried and activated. Then, the reinforcer coated in this manner 
was first immersed into an aqueous solution (A) containing 
hexachloroplatinic acid and palladium (II) chloride, dried and activated 2 
hours at 350.degree. C. in air. The amount of platinum applied was 100 % 
by weight, of palladium 35 % by weight, relative to the total amount of 
the particular noble metal in the finished catalyst. The residual amount 
of the palladium as well as the rhodium were fixed to the carrier by means 
of a second impregnation step with a solution (B), which contained the 
base metal lithium in the form of lithium chloride in addition to the 
noble-metal salts palladium chloride and rhodium chloride, dried and 
activated. The structure was then reduced 4 hours at 450.degree. C. in a 
current of hydrogen. The total noble-metal content of the finished 
catalyst was 1.77 g/l carrier volume. The weight ratio between the noble 
metals Pt:Pd:Rh was 6:4:1. The doping amount of lithium was 2.5 times the 
total amount of noble metal. 
EXAMPLE 2 
The preparation of this catalyst took place in the same way as in example 
1. However, the base metal additionally added here by means of noble-metal 
impregnation solution (B) was samarium (samarium (III) acetate). The 
doping amount of samarium was 0.5 times the total mass of noble metal. 
EXAMPLE 3 
The preparation of this catalyst took place in the same way as in example 
1. However, the base metal additionally added here by means of noble-metal 
impregnation solution (B) was cobalt (cobalt nitrate). This doping amount 
of cobalt was 2 times the total mass of noble metal. 
EXAMPLE 4 
The preparation of this catalyst took place in the same way as in example 
1. However, the base metal additionally added here by means of noble-metal 
impregnation solution (B) was cerium (cerium (III) acetate). This doping 
amount of cerium was 4 times the total mass of noble metal. 
Comparative example 2 
A honeycombed ceramic monolith (62 cells/cm.sup.2) was coated with a total 
of 160 g/l carrier volume of an oxide mixture. To this end, an aqueous 
suspension with a solid content of 51 % by weight and the following 
composition was used for the oxide mixture present after activation: 
59 parts by weight .gamma.-aluminum oxide 
30 parts by weight cerium oxide (cerium (III) 
3 parts by weight zirconium oxide (zirconyl nitrate) 
8 parts by weight lanthanum oxide (lanthanum (III) nitrate) 
After the coating of the carrier body with the oxide layer, it was dried at 
130.degree. C. and activated 2 hours at 650.degree. C. Then, the honeycomb 
body coated in this manner was immersed into a solution of 
hexachloroplatinic acid and rhodium nitrate in a weight ratio of Pt:Rh of 
2.5:1, dried and activated 2 hours at 600.degree. C. in air. The total 
noble-metal content was 0.88 g/l carrier volume after this treatment. 
EXAMPLE 5 
A ceramic honeycomb reinforcer body (62 cells/cm.sup.2) was coated with the 
same oxide layer and essentially in the same manner as described in 
comparative example 2. In contrast to comparative example 2, however, the 
honeycomb reinforcer was coated at first with only 2/3 of the total amount 
of the oxide layer. The honeycomb body coated in this manner was then 
immersed into an aqueous solution (A) containing hexachloroplatinic acid, 
dried and activated 2 hours at 600.degree. C. in air. Thereafter, the last 
third of the total amount of the oxide layer was applied in the manner 
already described, dried and activated before the reinforcer loaded in 
this manner was immersed into an aqueous solution (B) containing rhodium 
chloride and nickel acetate, dried and activated 2 hours at 600.degree. C. 
The total noble-metal content of the finished catalyst was 0.88 g/l 
carrier volume. The weight ratio of Pt:Rh was 2.5:1. The doping amount of 
nickel was twice the total mass of noble metal. 
EXAMPLE 6 
The preparation of this catalyst took place in the same manner as in 
example 5 with the difference that the base metal additionally added here 
by means of noble-metal impregnation solution (B) was barium (barium 
nitrate). This doping amount of barium was 3 times the total amount of 
noble metal. 
EXAMPLE 7 
The coating of the ceramic honeycomb body took place with the same oxide 
layer and essentially in the same manner as described in comparative 
example 2. In contrast to comparative example 2, however, the honeycomb 
reinforcer was coated at first with only 1/3 of the total amount of the 
oxide layer. The honeycomb coated in this manner was then immersed into an 
aqueous solution (A) containing hexachloroplatinic acid, dried and 
activated 2 hours at 600.degree. C. in air. Thereafter, the remaining two 
thirds of the total amount of the oxide layer was applied in the manner 
already described, dried and activated 2 hours at 600.degree. C. before 
the reinforcer charged in this manner was immersed into an aqueous 
solution (B) containing rhodium nitrate and lanthanum nitrate, dried and 
activated 2 hours at 600.degree. C. The total noble-metal content of the 
finished catalyst was 0.88 g/l carrier volume. The weight ratio of Pt:Rh 
was 2.5:1. The doping amount of lanthanum was 1.5 times the total mass of 
noble metal. 
EXAMPLE 8 
A ceramic honeycomb body with 62 cells/cm.sup.2 was coated with the same 
oxide layer and essentially in the same manner as described in comparative 
example 2. In contrast to comparative example 2, however, the honeycomb 
reinforcer was coated at first with only one half of the total amount of 
the oxide layer. The honeycomb body coated in this manner was then 
immersed into an aqueous solution (A) containing hexachloroplatinic acid, 
dried and activated 2 hours at 600.degree. C. in air. Thereafter, the 
second half of the total amount of the oxide layer was applied in the 
manner described, dried and activated before the reinforcer loaded in this 
manner was immersed into an aqueous solution (B) containing rhodium 
nitrate and barium nitrate, dried and activated 2 hours at 600.degree. C. 
The total noble-metal content of the finished catalyst was 0.88 g/l 
carrier volume. The weight ratio of Pt:Rh was 2.5:1. The doping amount of 
barium corresponded to the total amount of noble metal. 
EXAMPLE 9 
The preparation of this catalyst took place in the same way as in 
comparative example 2 with the difference that after the application of 
the oxide layer, the honeycomb body loaded in this manner was first 
immersed into an aqueous solution (A) containing hexachloroplatinic acid, 
dried and activated 2 hours at 600.degree. C. in air. The rhodium was 
fixed on the carrier by means of a second impregnation step with a 
solution (B), which contained the base metal iron in the form of iron 
(III) chloride in addition to rhodium nitrate, dried and activated 2 hours 
at 600.degree. C. 
The total noble-metal content of the finished catalyst was 0.88 g/l carrier 
volume. The weight ratio of Pt:Rh was 2.5:1. The doping amount of iron was 
twice that of the total mass of noble metal. 
EXAMPLE 10 
The preparation of this catalyst took place in the same manner as in 
example 9. However, the doping amount of iron additionally added with 
noble-metal impregnation solution (B) was five times that of the total 
mass of noble metal. 
COMATIVE EXAMPLE 3 
reinforcer of cordierite with 62 cells/cm.sup.2 was coated with 130 g/l 
carrier volume of an oxide mixture. To this end, an aqueous suspension 
with a solid content of 58 % by weight and the following composition was 
used for the oxide mixture present after activation: 
31 parts by weight .gamma.-aluminum oxide 
65 parts by weight cerium oxide 
2 parts by weight zirconium oxide (zirconyl nitrate) 
2 parts by weight nickel oxide (nickel (II) acetate) 
After the coating of the carrier body with the oxide layer, it was dried at 
150.degree. C. and activated 3 hours at 500.degree. C. Then, the 
reinforcer coated in this manner was immersed into an aqueous solution of 
tetraamine platinum (II) hydroxide, palladium (II) chloride and rhodium 
(III) chloride in a weight ratio of Pt:Pd:Rh of 2:1:1 and dried. Finally, 
after a one hour tempering at 500.degree. C. in air, the reduction of the 
noble-metal salts separated on the carrier material took place in a 
current of hydrogen containing water vapor (H.sub.2 O:H.sub.2 =1:3) at a 
temperature of 550.degree. C. for a period of 3 hours. The total 
noble-metal content was 0.64 g/l carrier volume after this treatment. 
EXAMPLE 11 
A ceramic honeycomb body (62 cells/cm.sup.2) was coated with the same oxide 
layer and in the same manner as described in comparative example 1, dried 
and activated. The reinforcer coated in this manner was then first 
immersed into an aqueous solution (A) containing tetraamine platinum (II) 
hydroxide and palladium (II) chloride, dried and activated 1 hour at 
500.degree. C. The amount of platinum applied therewith was 100 % by 
weight, the amount of palladium 60 % by weight relative to the total 
amount of the particular noble metal on the finished catalyst. The 
residual amount of the palladium as well as the rhodium were fixed to the 
carrier by means of a second impregnation step with a solution (B), which 
contained the base metals calcium and praseodymium (mass ratio Ca:Pr=2:1) 
in the form of their nitrates in addition to the noble-metal salts 
palladium chloride and rhodium chloride, dried and activated. The 
structure was then reduced 3 hours at 550.degree. C. in a current of 
hydrogen containing water vapor (H.sub.2 O:H.sub.2 =1:3). The total 
noble-metal content of the finished catalyst was 0.64 g/l carrier volume. 
The total amount of the doping elements calcium and praseodymium was four 
times the total mass of noble metal. 
EXAMPLE 12 
The preparation of this catalyst took place in the same manner as in 
example 11 with the difference that 90 % by weight of the platinum and 80 
% by weight of the palladium were fixed on the carrier with noble-metal 
impregnation solution (A) and 100 % by weight of the rhodium, 10 % by 
weight of the platinum and 20 % by weight of the palladium, together with 
the base metals potassium (as chloride), barium (as nitrate) and iron (as 
nitrate) in a mass ratio of K:Ba:Fe =1:3:1, were fixed on the carrier with 
noble-metal impregnation solution (B). The total amount of noble metal of 
the finished catalyst was likewise 0.64 g/l carrier volume, the doping 
amount of the base metals potassium, barium and iron was, in toto, five 
times the total mass of noble metal. 
COMATIVE EXAMPLE 4 
A honeycomb ceramic reinforcer with 62 cells/cm.sup.2 was coated with 145 
g/l carrier volume of an oxide mixture. An aqueous suspension with a solid 
content of 56 % by weight and the following composition was used to this 
end for the oxide mixture present after activation: 
56 parts by weight .gamma.-aluminum oxide 
31 parts by weight cerium oxide 
8 parts by weight zirconium oxide 
5 parts by weight lanthanum oxide (lanthanum (III) acetate) 
After the honeycomb body had been coated with the oxide layer, it was dried 
at 135.degree. C. and activated 1 hour at 350.degree. C. Then, the 
reinforcer coated in this manner was immersed into an aqueous solution of 
palladium nitrate and rhodium chloride in a weight ratio of Pd:Rh of 4:1, 
dried and treated 1 hour at 400.degree. C. in air and a further 3 hours at 
400.degree. C. in a current of hydrogen. The total noble-metal content of 
the finished catalyst was 0.78 g/l carrier volume. 
EXAMPLE 13 
A honeycomb body of cordierite with 62 cells/cm.sup.2 was coated with 145 
g/l carrier volume of an oxide mixture whose total composition 
corresponded to that in comparative example 4. However, in contrast to 
comparative example 4, 90 g/l carrier volume of an oxide layer were 
applied at first. An aqueous suspension with a solid content of 54 % by 
weight and the following composition was used thereby for the oxide 
mixture present after activation: 
45.8 parts by weight .gamma.-aluminum oxide 
38.9 parts by weight cerium oxide 
9.6 parts by weight zirconium oxide 
5.0 parts by weight lanthanum (III) oxide (lanthanum acetate) 
After the coating of the carrier body with this first oxide layer, it was 
dried at 135.degree. C. and activated 1 hour at 350.degree. C. Then, the 
honeycomb body coated in this manner was immersed into an aqueous solution 
(A) of palladium nitrate, dried and treated 1 hour at 400.degree. C. in 
air. 
Then, a further 55 g/l carrier volume of a second oxide layer were applied. 
For this, an aqueous suspension with a solid content of 55 % by weight and 
the following composition was used for the oxide mixture present after 
activation: 
72.7 parts by weight .gamma.-aluminum oxide 
18.2 parts by weight cerium oxide (cerium (III) nitrate) 
5.5 parts by weight zirconium oxide (zirconyl acetate) 
3.6 parts by weight lanthanum (III) oxide 
After another drying and activation, the reinforcer coated in this manner 
was immersed into an aqueous solution (B) containing rhodium (III) 
chloride, lithium chloride and neodymium (III) nitrate, dried, activated 1 
hour at 400.degree. C. in air, and reduced a further 3 hours at 
400.degree. C. in a current of hydrogen. The total noble-metal content of 
the finished catalyst was 0.78 g/l carrier volume. The weight ratio of 
Pd:Rh was 4:1. The amount of base metals lithium and neodymium was 3.5 
times the total mass of noble metal. The mass ratio of Li:Nd was 1:1. 
Comparative example 5 
A ceramic honeycomb body (62 cells/cm.sup.2) was coated with 150 g/l 
carrier volume of an oxide mixture. An aqueous suspension with a solid 
content of 68 % by weight and the following composition was used to this 
end for the oxide mixture present after activation: 
49 parts by weight .gamma.-aluminum oxide 
41 parts by weight cerium oxide (cerium (III) oxalate nonahydrate) 
10 parts by weight zirconium oxide (zirconyl nitrate) 
After the honeycomb body had been coated with the oxide layer, it was dried 
at 150.degree. C. and activated 3 hours at 350.degree. C. Then, the 
reinforcer coated in this manner was immersed into an aqueous solution of 
tetraamine platinum (II) nitrate and rhodium (III) chloride, dried and 
activated 2 hours at 350.degree. C. The total noble-metal content of the 
finished catalyst was 0.33 g/l carrier volume. The weight ratio of the 
noble metals Pt:Rh was 1:1. 
EXAMPLE 14 
A ceramic reinforcer was coated with the same oxide layer and in the same 
manner as described in comparative example 5, dried and activated. The 
honeycomb body coated in this manner was then first immersed into an 
aqueous solution (A) containing tetraamine platinum (II) nitrate, dried 
and activated 2 hours at 350.degree. C. The amount of platinum applied 
therewith was 95 % by weight relative to the total amount of platinum on 
the finished catalyst. The residual amount of the platinum as well as the 
rhodium were fixed to the carrier by means of a second impregnation step 
with a solution (B), which contained the base metal rubidium (rubidium 
nitrate) in addition to the noble-metal salts hexachloroplatinic acid and 
rhodium (III) nitrate, dried and activated. The total noble-metal content 
of the finished catalyst was 0.33 g/l carrier volume. The weight ratio of 
Pt:Rh was 1:1. The doping amount of rubidium was 3 times the total amount 
of noble metal. 
Comparative example 6 
A honeycomb body of cordierite with 62 cells/cm.sup.2 was coated with 169 
g/l carrier volume of an oxide mixture. An aqueous suspension with a solid 
content of 62 % by weight and the following composition was used to this 
end for the oxide mixture present after activation: 
67.5 parts by weight .gamma.-aluminum oxide 
28.5 parts by weight cerium oxide (cerium (III) acetate) 
2.6 parts by weight zirconium oxide (zirconyl acetate) 
1.4 parts by weight iron (III) oxide 
The reinforcer coated with the oxide layer was dried at 135.degree. C. and 
activated 4 hours at 300.degree. C. Then, the honeycomb body coated in 
this manner was immersed into an aqueous solution of platinum nitrate and 
rhodium chloride and dried. After a two-hour tempering at 400.degree. C. 
in air, the reduction of the noble-metal salts separated on the carrier 
material finally took place in a current of hydrogen and nitrogen 
containing water vapor (volumetric ratio of N.sub.2 :H.sub.2 : H.sub.2 
O=87:3:10) at a temperature of 880.degree. C. for 2.5 hours. The total 
noble-metal content after this treatment was 1.06 g/l carrier volume. The 
weight ratio of Pt:Rh was 3:1. 
EXAMPLE 15 
A ceramic honeycomb body (62 cells/cm.sup.2) was coated with the same oxide 
layer and in the same manner as described in comparative example 6, dried 
and activated. The reinforcer coated in this manner was then first 
immersed into an aqueous solution (A) containing platinum nitrate, dried 
and activated 2 hours at 400.degree. C. The amount of platinum applied 
therewith was 80 % by weight of the total amount of platinum on the 
finished catalyst. The residual amount of the palladium and of the rhodium 
was fixed to the carrier in a second impregnation step with a solution 
(B), which contained the base metal magnesium in the form of magnesium 
chloride in addition to the noble-metal salts hexachloroplatinic (IV) acid 
and rhodium (III) chloride, dried and activated. The structure was then 
reduced 2.5 hours at 880.degree. C. in a current of nitrogen and hydrogen 
containing water vapor (volumetric ratio of N.sub.2 :H.sub.2 :H.sub.2 O 
=87:3:10). The total noble-metal content of the finished catalyst was 1.06 
g/l carrier volume. The weight ratio of the noble metals Pt:Rh was 3:1. 
The doping amount of magnesium was 1.5 times the total amount of noble 
metal. 
EXAMPLE 16 
The preparation of this catalyst took place in the same manner as in 
example 15 with the difference that the base metals nickel and barium were 
additionally added in a weight ratio of Ni:Ba of 1:1 as nickel nitrate and 
barium nitrate instead of magnesium by means of noble-metal impregnation 
solution (B). 
This doping amount of base metal was 3.5 times the total mass of noble 
metal. 
Comparative example 7 
A spherical catalyst of Al.sub.2 O.sub.3 with particle diameters between 2 
and 4 mm, a bulk density of 568 kg/m.sup.3, an average crush strength of 
52 N and a specific surface area of 102 m.sup.2 /g is impregnated with an 
aqueous solution of cerium acetate and zirconyl acetate, dried and 
tempered 1 hour at 550.degree. C. The catalytic precursor is subsequently 
coated with a solution of H.sub.2 PtCl.sub.6, PdCl.sub.2 and RhCl.sub.3, 
dried and activated 30 min. at 500.degree. C. in air. The finished 
catalyst contains 30 kg CeO.sub.2, 3 kg ZrO.sub.2, 570 g Pt, 228 g Pd and 
71 g Rh per m.sup.3 volume. 
Example 17 
The catalyst precursor according to comparative example 7 is first 
impregnated with a solution of H.sub.2 PtCl.sub.6 and PdCl.sub.2 
containing Pd and one half of the entire platinum, dried and calcined 30 
min. at 850.degree. C. Then, RhCl.sub.3 and the residual amount of H.sub.2 
PtCl.sub.6 is applied together with barium nitrate, dried and activated 30 
min. at 550.degree. C. The doping amount of barium was 3 times the total 
amount of noble metal, which was selected in accordance with comparative 
example 7. 
EXAMPLE 18 
The catalytic precursor according to comparative example 7 is first 
impregnated with H.sub.2 PtCl.sub.6, dried and calcined 1 hour at 
500.degree. C. Then, the intermediate stage obtained is coated with a 
solution of RhCl.sub.3, PdCl.sub.2 and calcium nitrate, dried and 
activated 2 hours at 550.degree. C. in forming gas (N.sub.2 :H.sub.2 
=95:5). The amount of calcium corresponded to twice the amount of the 
total mass of noble metal, which was selected in accordance with 
comparative example 7. 
Testing of the catalysts 
The testing of the previously described catalysts as regards their 
qualities in the simultaneous conversion of the exhaust-gas pollutants 
carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NO.sub.x) 
took place largely in a testing system which operates with a synthetic gas 
mixture corresponding to that of an internal combustion engine. The 
dimensions of the test catalysts were cylindrical with the dimensions 
diameter .times. height=1".times.3". The space velocity was 50,000 
h.sup.-1. Propane was used by way of example as hydrocarbon component. A 
part of the catalytic specimens was tested on an engine test stand. 
The testing took place using freshly prepared catalysts and using specimens 
which had been previously aged 24 hours in air at 950.degree. C. in an 
oven. Experience has shown that this treatment (oven ageing) simulates 
rather well the rigorous requirements placed on the catalyst in a vehicle 
during rather long running periods under largely lean operation of the 
engine. A comparison of the results of the fresh and of the aged catalysts 
describes their durability. 
Testing of the cold-start behavior of the catalysts with synthetic exhaust 
gas 
In order to describe the light-off behavior of the catalysts, the 
temperature of the exhaust gas was driven up linearly from 75.degree. C. 
to 450.degree. C. at a heating rate of 15.degree. C./min. The exhaust-gas 
temperature was recorded thereby in comparison to the conversion of the 
pollutants CO, HC., NO. The temperatures at which a degree of conversion 
of 50 % is achieved is designated in short with the index 50 and functions 
as a measure of the starting ability of the catalyst in regard to the 
particular pollutant conversion. The lower these starting temperatures 
are, the more effective the catalyst is. 
The light-off behavior of the specimens was tested both with a rich 
(lambda=0.98) and with a lean (lambda=1.01) mixture of exhaust gas. The 
composition of the synthetic exhaust gas in the light-off test is shown in 
table 1. The gas mixture for the simulation of a rich exhaust gas 
(lambda=0.98) differed from the composition of a lean exhaust gas solely 
in that the oxygen component was selected to be correspondingly smaller 
and the nitrogen component correspondingly larger. 
TABLE 1 
______________________________________ 
Composition of the synthetic exhaust gas for 
the testing of the light-off behavior of the 
catalysts 
Exhaust-gas 
air/fuel ratio lambda 
components 
lambda = 0.98 (rich) lambda = 1.01 (lean) 
______________________________________ 
N.sub.2 73.24 72.55 
O.sub.2 0.73 1.42 
CO.sub.2 14.00 
CO 1.40 
H.sub.2 0.47 
C.sub.3 H.sub.8 0.06 
NO 0.10 
H.sub.2 O 10.00 
______________________________________ 
Testing the conversion behavior of the catalysts with synthetic exhaust gas 
In order to test the conversion behavior of the catalysts, the degree of 
conversion of the pollutants CO, HC and NO was measured as a function of 
the air/fuel ratio lambda at an exhaust-gas temperature of 400.degree. C. 
on a synthetic-gas test stand. In order to simulate the dynamic conditions 
in a vehicle, there was a sweep with a frequency of 0.5 Hz and an 
amplitude of .DELTA.lambda=.+-.0.059 around the average lambda value. The 
composition of the synthetic exhaust gas is characterized by a basic gas 
current and a sweep gas current (cf. table 2). 
Testing of the light-off behavior on an engine dynamometer 
The exhaust gas was generated by a 1.8 l Otto (gasoline internal 
combustion) engine (90 HP) and brought to the desired exhaust-gas 
temperature by means of a heat exchanger. The associated equilibrium 
concentrations were registered. The temperature range from 200.degree. to 
450.degree. C. was tested in increments of 10.degree. C. The temperature 
at which 50% of the particular pollutant is converted was determined by 
interpolation. The characterization took place for rich and for lean 
exhaust gas at lambda values of 0.984 and 1.02. 
TABLE 2 
______________________________________ 
Composition of the synthetic exhaust gas for 
testing the dynamic conversion behavior of the 
catalysts 
______________________________________ 
(A) Basic gas mixture 
Exhaust-gas component 
Contents (% by vol.) 
______________________________________ 
CO.sub.2 14.000 
CO 0.140 
H.sub.2 0.047 
C.sub.3 H.sub.3 (propane) 
0.060 
NO 0.100 
H.sub.2 O 10.000 
______________________________________ 
(B) Additional components of the basic gas mixture for 
maintaining the air ratio lambda 
Air ratio Exhaust gas components (% by vol.) 
lambda O.sub.2 N.sub.2 
______________________________________ 
0.97 0 72.293 
0.98 0.230 72.063 
0.99 0.460 71.833 
1.00 0.690 71.603 
1.01 0.920 71.373 
1.02 1.150 71.143 
______________________________________ 
(C) Sweep pulses with a frequency of 0.5 Hz 
Pulse Component Contents (% by vol.) 
______________________________________ 
lean O.sub.2 1.000 
N.sub.2 2.360 
rich CO 2.52 
H.sub.2 0.84 
______________________________________ 
Testing of the dynamic conversion behavior on an engine dynamometer 
The tests of the dynamic conversion behavior were also carried out with a 
1.8 1 Otto engine (90 HP). The exhaust-gas temperature was 400.degree. C. 
and the composition of the exhaust gas corresponded to a lambda value of 
0.965 and 0.995 and 1.008. In order to simulate the dynamic conditions in 
a vehicle in street traffic, a sweep was made with a frequency of 1.0 
hertz and an amplitude of .DELTA.lambda=.+-.0.034 (.DELTA.A/F=.+-.0.5) 
around the average lambda value. The associated degree of conversion of 
the pollutant components CO, HC and NO.sub.x was registered. In tables 3 
to 14, the letter V represents the comparative example and the letter B 
represents the example of the present invention. 
TABLE 3 
______________________________________ 
Light-off behavior of the fresh catalysts in rich 
exhaust gas (lambda = 0.98) 
(synthetic gas test) 
Light-off temperature (.degree.C.) for 50% 
conversion 
Example CO.sub.50 HC.sub.50 
NO.sub.50 
______________________________________ 
V 1 215 287 217 
B 1 196 253 195 
B 2 189 247 193 
B 3 194 256 191 
B 4 192 251 196 
V 2 215 227 220 
B 5 192 211 198 
B 6 189 208 191 
B 7 195 215 195 
B 8 209 221 212 
B 9 196 207 196 
B 10 214 229 219 
V 3 211 219 214 
B 11 189 197 192 
B 12 183 191 185 
V 4 218 235 225 
B 13 187 198 190 
______________________________________ 
TABLE 4 
______________________________________ 
Light-off behavior of the aged catalysts in rich 
exhaust gas (lambda = 0.98) 
(synthetic gas test) 
Light-off temperature (.degree.C.) for 50% 
conversion 
Example CO.sub.50 HC.sub.50 
NO.sub.50 
______________________________________ 
V 1 272 441 274 
B 1 255 407 258 
B 2 251 417 255 
B 3 248 412 252 
B 4 257 421 256 
V 2 239 375 257 
B 5 204 327 217 
B 6 211 332 221 
B 7 201 325 213 
B 8 240 371 249 
B 9 209 341 233 
B 10 235 369 252 
V 3 245 381 260 
B 11 212 333 218 
B 12 205 326 208 
V 4 250 397 257 
B 13 215 359 219 
______________________________________ 
TABLE 5 
______________________________________ 
Light-off behavior of the fresh catalysts in lean 
exhaust gas (lambda = 1.01) 
(synthetic gas test) 
Light-off temperature (.degree.C.) for 50% 
conversion 
Example CO.sub.50 HC.sub.50 
NO.sub.50 
______________________________________ 
V 1 199 230 (not achieved) 
B 1 173 207 -- 
B 2 170 201 -- 
B 3 178 203 -- 
B 4 175 199 -- 
V 2 205 211 -- 
B 5 185 193 -- 
B 6 182 189 -- 
B 7 184 188 -- 
B 8 200 209 -- 
B 9 183 190 -- 
B 10 208 213 -- 
V 3 197 206 -- 
B 11 171 185 -- 
B 12 168 179 -- 
V 4 212 219 -- 
B 13 178 190 -- 
______________________________________ 
TABLE 6 
______________________________________ 
Light-off behavior of the aged catalysts in lean 
exhaust gas (lambda = 1.01) 
(synthetic gas test) 
Light-off temperature (.degree.C.) for 50% 
conversion 
Example CO.sub.50 HC.sub.50 
NO.sub.50 
______________________________________ 
V 1 263 359 (not achieved) 
B 1 245 327 -- 
B 2 240 313 -- 
B 3 247 309 -- 
B 4 248 312 -- 
V 2 231 236 -- 
B 5 192 198 -- 
B 6 198 207 -- 
B 7 190 196 -- 
B 8 228 233 -- 
B 9 199 210 -- 
B 10 233 240 -- 
V 3 227 243 -- 
B 11 195 200 -- 
B 12 189 194 -- 
V 4 240 257 -- 
B 13 208 223 -- 
______________________________________ 
TABLE 7 
__________________________________________________________________________ 
Dynamic conversion behavior of the fresh catalysts (synthetic gas test) 
% Conversion 
Lambda = 0.97 Lambda = 0.98 
Lambda = 0.99 
Lambda = 1.00 
Lambda = 1.01 
Lambda = 1.02 
Example 
CO HC NO CO HC NO CO HC NO CO HC NO CO HC NO CO HC NO 
__________________________________________________________________________ 
V 1 49 88 97 57 90 98 68 92 93 88 94 90 91 94 52 93 94 41 
B 1 57 93 99 68 95 99 79 98 99 91 99 95 99 99 60 99 99 50 
B 2 59 94 99 70 94 99 81 97 99 93 100 
97 100 
99 62 99 99 52 
B 3 55 96 99 67 96 99 79 99 99 91 100 
97 99 100 59 99 100 49 
B 4 57 93 99 69 93 99 79 97 98 92 99 96 100 
99 61 100 
99 52 
V 2 57 94 99 66 95 99 74 98 98 93 99 96 97 99 63 97 98 56 
B 5 65 98 99 78 98 100 
82 99 100 
98 99 99 100 
99 73 99 99 62 
B 6 69 99 100 
79 99 100 
86 99 100 
99 99 99 100 
100 74 100 
100 63 
B 7 63 98 100 
79 99 100 
85 99 100 
99 99 99 100 
100 71 100 
100 61 
B 8 59 96 99 68 98 99 77 99 99 95 99 98 99 99 68 99 99 59 
B 9 68 98 100 
81 99 100 
87 99 99 99 99 99 100 
100 71 99 99 62 
B 10 58 94 99 66 95 99 74 97 99 94 99 97 98 99 65 98 99 58 
V 3 53 91 99 64 94 99 74 98 99 95 99 97 99 99 66 99 99 53 
B 11 67 98 99 75 98 99 85 99 99 99 99 99 100 
100 71 99 99 65 
B 12 69 99 99 78 99 99 87 100 99 99 100 
99 100 
100 72 99 100 61 
V 4 49 92 99 60 93 99 71 96 99 92 99 98 99 99 63 99 99 49 
B 13 58 98 99 72 98 99 79 99 99 98 100 
99 100 
100 70 100 
100 59 
__________________________________________________________________________ 
TABLE 8 
__________________________________________________________________________ 
Dynamic conversion behavior of the fresh catalysts (synthetic gas test) 
% Conversion 
Lambda = 0.97 Lambda = 0.98 
Lambda = 0.99 
Lambda = 1.00 
Lambda = 1.01 
Lambda = 1.02 
Example 
CO HC NO CO HC NO CO HC NO CO HC NO CO HC NO CO HC NO 
__________________________________________________________________________ 
V 1 27 48 68 43 52 66 60 64 66 71 72 57 77 82 52 82 89 45 
B 1 38 62 73 57 69 73 71 79 74 81 88 64 87 91 59 93 97 51 
B 2 41 62 78 60 64 71 74 76 73 85 85 68 90 88 61 95 95 51 
B 3 36 65 75 54 66 73 73 78 73 83 87 66 89 91 60 95 97 50 
B 4 39 60 75 58 66 72 73 78 73 80 83 67 89 89 59 94 95 51 
V 2 43 86 94 54 91 90 63 94 83 73 95 70 74 96 57 73 96 47 
B 5 54 91 99 66 95 99 75 98 95 89 99 85 91 99 65 90 99 59 
B 6 56 95 99 69 97 99 78 99 93 90 99 89 91 99 68 91 99 59 
B 7 53 93 99 69 97 99 76 99 94 87 99 83 89 99 63 89 99 56 
B 8 48 88 95 56 92 94 64 95 86 76 95 72 76 97 55 75 96 49 
B 9 51 92 99 69 96 99 75 99 93 87 99 83 89 99 64 89 99 58 
B 10 45 83 95 54 90 93 62 92 85 74 94 74 74 95 56 74 96 47 
V 3 39 74 81 51 80 82 62 84 78 75 88 68 77 89 54 79 90 49 
B 11 44 86 94 63 88 94 70 91 91 86 97 75 89 98 59 92 98 55 
B 12 47 89 96 65 91 97 73 95 93 88 98 78 92 98 61 92 98 58 
V 4 41 79 85 53 84 85 63 85 80 77 86 70 79 90 52 80 91 48 
B 13 49 88 93 59 90 95 72 92 92 88 98 80 91 98 60 91 98 56 
__________________________________________________________________________ 
TABLE 9 
______________________________________ 
Light-off behavior of the fresh catalysts in rich 
exhaust gas (lambda = 0.984) (engine test) 
Light-off temperature (.degree.C.) for 50% 
conversion 
Example CO.sub.50 HC.sub.50 
NO.sub.50 
______________________________________ 
V 5 319 351 313 
B 14 295 333 284 
V 6 310 344 305 
B 15 288 337 278 
B 16 283 321 275 
______________________________________ 
TABLE 10 
______________________________________ 
Light-off behavior of the aged catalysts in rich 
exhaust gas (lambda = 0.984) (engine test) 
Light-off temperature (.degree.C.) for 50% 
conversion 
Example CO.sub.50 HC.sub.50 
NO.sub.50 
______________________________________ 
V 5 364 376 353 
B 14 335 342 320 
V 6 353 361 338 
B 15 324 337 305 
B 16 319 332 303 
______________________________________ 
TABLE 11 
______________________________________ 
Light-off behavior of the fresh catalysts in lean 
exhaust gas (lambda = 1.02) (engine test) 
Light-off temperature (.degree.C.) for 50% 
conversion 
Example CO.sub.50 HC.sub.50 
______________________________________ 
V 5 321 344 
B 14 289 300 
V 6 309 316 
B 15 278 283 
B 16 271 279 
V 7 342 345 
B 17 319 323 
B 18 325 327 
______________________________________ 
TABLE 12 
______________________________________ 
Light-off behavior of the aged catalysts in lean 
exhaust gas (lambda = 1.02) (engine test) 
Light-off temperature (.degree.C.) for 50% 
conversion 
Example CO.sub.50 HC.sub.50 
______________________________________ 
V 5 335 359 
B 14 311 317 
V 6 342 349 
B 15 321 329 
B 16 315 324 
V 7 &gt;450.degree. C. 
&gt;450.degree. C. 
B 17 &gt;450.degree. C. 
&gt;450.degree. C. 
B 18 &gt;450.degree. C. 
&gt;450.degree. C. 
______________________________________ 
TABLE 13 
______________________________________ 
Dynamic conversion behavior of the fresh catalysts 
(engine test) 
% Conversion 
Exam- Lambda = 0.965 
Lambda = 0.995 
Lambda = 1.008 
ple CO HC NO.sub.x 
CO HC NO.sub.x 
CO HC NO.sub.x 
______________________________________ 
V 5 62 36 98 93 81 99 99 92 65 
B 14 69 42 100 98 87 99 100 94 70 
V 6 76 35 99 96 82 99 100 92 64 
B 15 84 40 100 98 86 100 99 95 69 
B 16 82 45 100 98 88 100 100 97 68 
V 7 51 40 93 94 91 94 98 93 63 
B 17 55 44 95 95 94 95 98 94 65 
B 18 54 43 95 95 94 94 98 94 65 
______________________________________ 
TABLE 14 
______________________________________ 
Dynamic conversion behavior of the aged catalysts 
(engine test) 
% Conversion 
Exam- Lambda = 0.965 
Lambda = 0.995 
Lambda = 1.008 
ple CO HC NO.sub.x 
CO HC NO.sub.x 
CO HC NO.sub.x 
______________________________________ 
V 5 52 58 94 91 90 91 98 91 60 
B 14 59 67 95 99 93 94 100 94 67 
V 6 53 55 93 92 93 90 99 92 59 
B 15 58 63 95 99 96 95 99 95 63 
B 16 57 64 95 99 98 97 99 96 63 
V 7 43 53 72 65 77 59 76 84 51 
B 17 44 55 76 68 80 61 77 84 53 
B 18 46 55 75 68 79 62 77 84 54 
______________________________________ 
Further variations and modifications of the invention will become apparent 
to those skilled in the art from the foregoing and are intended to be 
encompassed by the claims appended hereto.