Process for treatment of exhaust gases

This specification discloses a process for treatment of exhaust gases from an internal combustion engine. The process has the following general steps. A hydrocarbon fuel is burned in an internal combustion engine to generate exhaust gases containing various amounts of hydrocarbons, carbon monoxide, and oxides of nitrogen. The generated exhaust gases are passed over an improved catalyst system characterized as follows. The catalyst system has a support medium for supporting the same. The support medium has both an upstream support portion over which exhaust gases initially flow and a downstream support portion over which exhaust gases flow after passing over the upstream support portion thereof. Finely divided palladium is dispersed by itself on the upstream support portion of the support medium. The downstream support portion of the support medium has a platinum/rhodium three-way catalyst thereon.

TECHNICAL FIELD 
This specification is directed to the teaching of a process for treatment 
of internal combustion engine exhaust gases. In particular, the process is 
for treatment of exhaust gases generated by burning a hydrocarbon fuel or 
a fuel containing hydrocarbons and alcohol blends in an internal 
combustion engine. Such burning action generates exhaust gases from the 
internal combustion engine containing various amounts of hydrocarbons, 
carbon monoxide, and oxides of nitrogen depending upon operating 
conditions of the internal combustion engine. 
BACKGROUND ART AND PRIOR ART STATEMENT 
A search conducted in the U.S. Patent Office resulted in the citation of 
the following patents: U.S. Pat. Nos. 2,956,865; 3,180,712; 3,503,715; 
4,118,199; 4,225,561; German Nos. 2,452,717; 2,555,038; 2,628,439; and 
Japanese No. 1978-3962. 
U.S. Pat. No. 2,956,865 discloses a rather complex apparatus for purifying 
exhaust gases. The structure includes elements which are adapted to add 
additional fuel to the exhaust gases and structure in which the exhaust 
gases and additional fuel are burned. 
U.S. Pat. No. 3,180,712 discloses a two-stage converter muffler structure. 
This device has two different catalyst beds which are separated by 
structure through which auxiliary air may be added to the converter. 
U.S. Pat. No. 3,503,715 is directed to an apparatus for treating an exhaust 
gas stream with different catalyst beds. One catalyst layer comprises 
platinized alumina particles and the next adjacent layer comprises 
platinized alumina particles containing a barium, calcium or strontium 
component. 
U.S. Pat. No. 4,118,199 is directed to a monolithic carrier catalyst and 
arrangements of such a catalyst for the purification of exhaust gases from 
an internal combustion engine. The structure taught is one in which an 
increased concentration of catalytically active material is found on the 
monolithic catalyst carrier. The concentration of the catalytic component 
increases in the direction of flow of the exhaust gases. 
U.S. Pat. No. 4,225,561 is directed to a catalytic converter for 
transforming gases from one condition to another. The structure disclosed 
is a rather complex one involving inner and outer canning structure 
members and means for supporting the same. 
German No. 2,452,717 is directed to a catalytic engine exhaust gas 
treatment system. The system is characterized in that the exhaust gases 
are first passed through an oxidation catalyst to oxidize part of their 
hydrocarbons and carbon monoxide content and to reduce their oxygen 
content to a predetermined amount. Thereafter, the exhaust gases are 
passed over a reducing catalyst where the nitrogen oxides are reduced. A 
final catalytic oxidation stage then follows to oxidize any remaining 
hydrocarbons and carbon monoxide. 
German No. 2,555,038 is also directed to a catalytic converter for use with 
an internal combustion engine. This device includes a twin type structure 
in which the exhaust gas purifier has a noble metal section followed in 
the same housing by a non-noble metal section. 
German No. 2,628,439 is directed to a catalytic converter structure. The 
catalytic converter contains a silver or palladium plated grid, the inner 
parts of which permit expansion in the chamber and absorption of energy of 
the sound pressure waves in the exhaust gas by forming an artificial 
turbulence in the gas stream. 
Japanese No. 1978-3962 is directed to a catalytic gas purifier for use with 
internal combustion engines. A primary catalytic unit is used along with 
an auxiliary catalyst unit which is disposed upstream of the primary 
catalytic unit. 
We are also aware of teachings contained in commonly assigned U.S. patent 
applications Ser. No. 284,759, now abandoned, entitled "Palladium Catalyst 
Promoted by Tungsten"; Ser. No. 284,762, now U.S. Pat. No. 4,374,103, 
entitled "Low Cost Catalyst System"; and Ser. No. 284,763, now U.S. Pat. 
No. 4,389,382, entitled "Simplified Low Cost Catalyst System", all filed 
on July 20, 1981. 
The Ser. No. 284,759 application teaches a specific catalyst system in 
which a palladium catalyst is promoted by tungsten. The catalyst system 
disclosed in that application has highly desirable characteristics in that 
it is effective in the catalytic oxidation of unburned hydrocarbons and 
the catalytic reduction of oxides of nitrogen without significant 
production of ammonia when an internal combustion engine with which it is 
associated is operated under fuel rich (oxygen deficient) conditions. 
The Ser. No. 284,762 catalyst system disclosed is one which teaches a 
catalyst formulation in which a catalyst substrate is provided with both 
an upstream catalyst portion and a downstream catalyst portion. The 
upstream and downstream catalyst portions contain palladium while the 
downstream catalyst portion additionally contains tungsten. 
In the Ser. No. 284,763 application the catalyst formulation is also found 
on a suitable catalyst substrate having both an upstream catalyst portion 
and a downstream catalyst portion. The upstream catalyst portion contains 
finely divided palladium thereon while the downstream portion contains 
only tungsten thereon.

DISCLOSURE OF THE INVENTION 
This invention relates to a process for treatment of exhaust gases from an 
internal combustion engine. More particularly, this invention relates to a 
process for treatment of exhaust gases from an internal combustion engine 
in which the following steps are carried out. 
A first step in the process for treating exhaust gases is one of burning a 
hydrocarbon fuel or a fuel containing hydrocarbons and alcohol blends in 
the internal combustion engine, thereby to generate exhaust gases from the 
internal combustion engine. These exhaust gases contain various amounts of 
hydrocarbons, carbon monoxide, and oxides of nitrogen depending upon the 
operating conditions of the internal combustion engine. 
In the second step of the process for treating exhaust gases, the generated 
exhaust gases are passed over an improved catalyst system which is 
characterized in the following manner. The catalyst system includes a 
support medium. The support medium has both an upstream support portion 
over which exhaust gases initially flow and a downstream support portion 
over which exhaust gases flow after passing over the upstream support 
portion thereof. Finely divided palladium is dispersed by itself on the 
upstream support portion of the support medium. The palladium is present 
on the upstream support portion of the support medium in an amount of from 
about 10 to about 40 grams per cubic foot, preferably at about 20 grams 
per cubic foot of the upstream support portion of the support medium. The 
downstream support portion of the support medium has a platinum/rhodium 
three-way catalyst thereon. The platinum to rhodium ratio of the three-way 
catalyst is in a range from about 11:1 to about 5:1. The three-way 
catalyst is present in an amount of from about 10 to about 40 grams per 
cubic foot, preferably about 20 grams per cubic foot of the downstream 
support portion of the support medium. 
By using the method of this invention, the three-way catalyst material is 
protected on its downstream support portion by the palladium which is on 
the upstream support portion of the support medium. The three-way catalyst 
material is protected from thermal shock and degradation by overheating. 
This will be discussed in greater detail hereinbelow. 
BEST MODE AND INDUSTRIAL APPLICABILITY 
The novel features that are considered characteristic of the invention are 
set forth with particularity in the appended claims. The invention itself, 
however, both as to its organization and its method of operation, together 
with additional objects and advantages thereof, will best be understood 
from the following description of specific embodiments. 
The following description is what we consider to be a preferred embodiment 
of the process of our invention. The following description also sets forth 
what we now contemplate to be the best mode of fabricating the catalyst 
system utilized in the process. The description is not intended to be a 
limitation upon the broader principles of the method of this invention. 
As part of the effort to reduce automotive emissions, three-way catalysts 
(TWC's) were developed to simultaneously convert hydrocarbons, carbon 
monoxide, and oxides of nitrogen. TWC's are currently used in most U.S. 
light duty passenger cars and their use is expected to continue for some 
time. 
Platinum and rhodium are the principal active metal components for TWC's. 
Both of these metals are imported and expensive. Platinum is susceptible 
to sintering at high temperatures. As an additional matter, rhodium in 
TWC's is known to interact with alumina washcoat at temperatures above 
1650.degree. F. under lean air/fuel ratio conditions to form a 
catalytically inactive spinel. Therefore, in order to maintain the high 
TWC activity and thermal resistance, the conventional method of making a 
TWC is to increase the precious metal loading thereon. As a result, the 
cost of TWC's can be relatively high. 
The process of this invention is one which reduces the cost of a TWC, but 
does not reduce the catalytic effect thereof. The method of this invention 
is one which employs the concept that the inlet portion of a catalyst 
should protect the outlet portion thereof from thermal damage when and if 
high catalyst temperatures occur. The inlet portion accomplishes the 
protection by limiting the spacial extent of these temperatures to the 
inlet catalyst as much as possible, thereby better maintaining the overall 
catalyst system performance. In addition, the method of this invention is 
one which does not reduce the catalytic effect thereof if high catalyst 
temperatures do not occur. 
PREATION OF CATALYST 
In order to demonstrate a best mode for the method of this invention, a 
catalyst was prepared on a cordierite monolith. The monolith had 400 
square channel cells per square inch with a 6 mil wall thickness. This is 
a standard monolithic catalyst substrate and is available on the 
commercial market. The catalyst substrate was divided into an upstream 
portion and a downstream portion, each portion being approximately 3 
inches in length. The substrate was divided by sawing it in half. 
Finely divided palladium was dispersed by itself on the upstream portion of 
the support medium in the following manner. The upstream support portion 
of the support medium was first coated with a gamma alumina washcoat by 
dipping the monolith in aqueous suspension of gamma alumina, which gamma 
alumina is also commercially available. The half of the support media was 
then dried and calcined in air at 600.degree. C. for 4 hours. The finely 
divided palladium was impregnated on the washcoated catalyst substrate 
using an aqueous solution of palladium chloride with 4% nitric acid by 
volume. Thereafter, the upstream support portion of the support medium was 
dried and calcined in air at 550.degree. C. for 20 hours. The precious 
metal loading on the upstream support portion of the support medium was in 
a range of about 20 grams of palladium per cubic foot of the upstream 
support portion of the support medium. 
The downstream support portion of the support medium had gamma alumina 
placed thereon in a manner identical to that described for the upstream 
support portion. Thereafter, the platinum/rhodium three-way catalyst was 
impregnated onto the gamma alumina washcoated downstream support portion 
of the support medium from a solution containing both chloroplatnic acid 
and rhodium nitrate. After impregnation the sample was dried and calcined 
in air for 4 hours at 500.degree. C. This produced a downstream support 
portion of the support media which had a platinum/rhodium ratio of 11:1 
and a precious metal loading of about 20 grams per cubic foot of the 
downstream support portion of the support medium. The two halves of the 
support medium were then packaged in a single package with the finely 
divided palladium being on the upstream portion of the substrate and the 
platinum/rhodium being on the downstream portion of the support medium. By 
this it is meant that the exhaust gases flowing into the catalytic 
converter first flow over the palladium portion and then over the TWC 
portion. This catalyst was tested against a normal catalyst which would 
have a precious metal loading of platinum and rhodium of about 20 grams 
per cubic foot along its entire length, the platinum to rhodium ratio 
being 11:1. 
Engine dynamometer tests were carried out to compare the standard TWC and 
the catalyst system used with the method of this invention. Both systems 
had good performance characteristics after high temperature lean spike 
aging. By this we mean that both catalyst systems were simultaneously 
exposed to sixty 2000.degree. F. high temperature episodes with 1.5 mole 
percent excess oxygen for about one minute each on a periodic basis. 
Interspersed with these episodes, the catalysts were aged for 12,000 miles 
using the U.S. emission durability cycle. The carbon monoxide and gross 
NOx conversion characteristics of the two systems were also nearly equal. 
The net NOx conversion is one of the major concerns in comparing the 
standard system to the catalyst system used with the method of this 
invention because the new catalyst system has less rhodium content and the 
rhodium is the primary material for accomplishing the net NOx conversion. 
However, our testing showed that the catalyst system used with the method 
of our invention suffers only a slight loss of net NOx conversion compared 
with a standard TWC, indicating that the palladium on the front half of 
the support media has adequate selectivity for the conversion of NOx to 
nitrogen. 
The hydrocarbon conversion efficiency of the two systems are about the same 
after the systems have been aged for 12,000 miles on an AMA cycle plus 60 
2000.degree. lean-spikes. In addition, both systems had good performance 
characteristics after 12,000 miles of aging on the U.S. emission 
durability cycle without high temperature lean spikes. Under this aging 
the maximum catalyst temperature was 1300.degree. F. 
While a particular embodiment of the invention have been illustrated and 
described, it will be obvious to those skilled in the art that various 
changes and modifications may be made without departing from the 
invention, and it is intended to cover in the appended claims all such 
modifications and equivalents as fall within the true spirit and scope of 
this invention. For example, the gamma alumina washcoat and precious metal 
may be placed on the substrate in a single operation instead of separate 
operations as described above.