Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to catalyst con-
verters ~or automobile exhaust systems.
Rhodium and iridum are known cata:lysts and are
known tQ be selective in reducing NO by CO in an environment
where the sum of the oxidizing species present, namely NO
and 25 iS slightly greater than the sum of the reducing species
present~ namely CO, H2 and HC. The rhodium and iridum bein~
selective means that they reduce NO to N2 rather than ammonia
NH3. In general~ any unburned hydrocarbons passing over
rhodium and iridum are not oxidized by these materials. In
essence, what occurs on these materials is that NO and CO
are chemlsorbed on adjacent sites and the oxygen of the NO
is taken by the CO or H2 thereby to form, on the overall,
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one molecule of nitrogen gas per two molecules o~ carbon
2 dioxide ~ormed.
3 On most base metal or noble metal catalysts~ NO
4 reduction requires an overall reducing atmosphere, not an
oxidizing or slightly oxidizing atmosphere as can be
6 tolerated by rhodlum and iridum. The reason that such
7 other catalysts require a reducing atmosphere is due to
8 the preferential oxidation of CO by any oxygen in the exhaust
9 gas stream. Such pre~erential oxidation leaves the NO un-
reacted. Hence, if most of the commonly known base or noble
11 metal catalysts are used and an NO reduction is desired~ the
12 exhaust gas stream ~ed to the catalyst has to be net reducing
13 in that the oxidizing species NO and 2 are slightly less
14 than the sum of the reducing species CO, H2 and HC.
In an application where a catalyst i9 used in
16 treating automobile exhaust for simultaneously reducing
17 NO, CO and HC, one approach i8 to add the selective metal
18 catalysts rhodium and iridum along with platinum and/or
19 palladium. These metals are depo lted uniformly along the
length o-f the-catalyst substrate either by impregnation
21 or sequential impregnation from their salt solutions in
22 a known manner. It is necessary to incorporate platinum
23 and/or palladium because rhodium and/or iridum do not ha~e the
24 desired activity for HC oxidation. However, in a catalyst
in which rhodium, iridum7 platinum and palladium are deposited
26 in a generally uni~orm manner throughout the extent of the
27 substrate, the selectivity of rhodium and iridum ~or reduction
28 of NO in a slightly oxidizing environment is not fully used.
29 This is because NO, CO and 2 molecules chemisorb and react
on all sites whether selective or non-selective. Therefore,
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any NO molecules, strjking platinum or palladium sites would
not be reduced selectivel~ by CO and simply would waste the
catalytic activity of such sites for oxidizing of HC and CO.
In accordance with the present invention, there
is provided a three-way catalyst converter which includes a
converter housing and a substrate means in the housing for
supporting catalyst materials thereon. A catalyst selected `~
from the group consisting of rhodium and iridum is deposited
on only an initial portion of the substrate means which
first receives the exhaust gases. A catalysk selected
consisting of platinum and palladium is deposited on only
a latter portion of the substrate means.
In this way, there is provided a three-way catalyst
converter in which NO, CO and HC are simultaneously
treated. Thus, the initial portion of the converter treats
the exhaust gases to reduce NO to N2 and to oxidize some or
all of the CO to CO2. The latter portion of the converter oxi
dizes the HC and any remaining CO.
In general, the initial portion of the substrate
means represents about one-third of the volume of the sub-
strate means and the latter portion of the substrate means
represents the remaining volume thereof.
The invention is described further by way of
illustration, with reference to the accompanying drawings, in
which:
Figures 1, 2 and 3 are schematic drawings of
catalytic converters formed in accordance with the teachings
of this invention; and
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Figures 4 and 5 are graphical presentations of
data obtained on exhaust gas streams being treated by ir~dum
alone in the case of Figure 4 and by a converter in accordance
with the teachings of this invention in the case o~ Figure
5.
In Figure 1~ a three-way catalyst converter
generally iden~ified by the numeral 10 is c;hown to include
a converter housing 11 and a substrate deviLce 12 therein.
~he substrate device is divided into an initial portion
which extends ~or about one third o~ the length of the
substrate in the direction in which the gases are passing
through the con~erter housing as indicated by arrow A.
The remaining length o~ the substrate ~orms the latter
portlon of the substrate.
A catalyst selected from the group consisting
of rhodium and iridum is deposited on only the initial
portion of the substrate device 12. Rhodlum and iridum
- may be present by themsel~e~ or both materials may be
present. These materials may be present in an amount
of 0.01 to 0. 2 percent by weight o~ the substrate on
which they are deposited. The latter portion o~ the
catalyst substrate 12 has a catalyst selected from the
group consisting of platinum and palladium deposited
thereon. Again platinum and palladium may be there
individually or in a combined manner. These materials
may be present in an amount equal to 0.1 to 0.5 percent
by weight o~ the substrate port ion which they are on .
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Figure 2 shows another embodiment of a three-way
catalyst converter generally desigr.ated by the numeral 20.
3 This catalyst converter once again has a converter housing
4 21 which, in turn, contains a first ca~alyst substrate 22
and a second catalyst substrate 23 with a space 24 there-
6 between. The first catalyst substrate 22 defines the
7 initlal portion of the substrate and it has a catalyst
8 selected from the group consisting of rhodium and iridum
9 deposited thereon. The second catalyst substrate 23 forms
~he latter portion of the substrate separated from the first
11 catalyst by some distance to allow remixing~ and i~ necessary,
12 secondary oxygen. This second substrate has a catalyst
13 selected from the group consisting of platinum and palladium
14 deposited thereon.
In Figure 3, a pelletized three-way converter ls
16 identi~ied generally by the numeral 30. This pelletized
17 catalyst converter has a converter housing 31 which is
18 divided into an lnitial portion and a latter portion by
19 screens 32, 33 and 34.
Between the screens 32 and 33 in the con~erter
21 housing 31, a pelletized catalyst substrate material 35
22 is located to de~ine the substrate for this portion o~
23 the converter. Thls initial portlon of the sub~trate has
24 been treated with rhodium and/or iridum catalyst. ~etween
the screens 33 and 349 a pelletized catalyst material 36
26 is located~ This pelletized catalyst contains thereon
27 a platinum and/or palladium catalyst material.
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1 The catalyst materials namely the platinum,
2 palladium~ rhodium and iridum can be deposited on their
3 respective substrates~ or individual portions of a slngle
4 substrate, or pellets by any o~ the well-known processes
for depositing such catalyst materials. For example,
6 these materials may be deposited by sequential impregnatlon
7 or colmpregnation, such as generally illustrated in IJ.S.
8 Patent 3,819,536 and Canadian Patent application Serial No.
9 228,647 filed June 5, 1975, respec~ively.
Re~erence is now made to Figure 4. This figure
11 illustrates the effect of the air-fuel ratio of a fuel mix-
12 ture being burned in an lnternal combustion engine on the
13 efficiency o~ N0, C0, HC conversion when lridum catalyst is
14 used by itself. In this situation, o.6 weight percent of ~
iridum catalyst was placed on a substrate and treated with a
16 slmulated exhaust gas stream containing C0, 2~ H2, H~0, C0
17 C3H6, C3Hg and N2 at 60,000 hr~l S.V. at 440~C. From the
18 graphical presentation, it i5 evident that iridum does not
19 do a very good ~ob on eliminating HC. It is also evident
from the graphical presentation that the iridum does a fairly
21 good ~ob on N0 reduction at any redox potential above 0.9
22 which indicates that one may operate on the lean or ~uel
23 deficient side and still get an effective treatment of N0.
24 Right at stoichiometric conditions where the redox potential
is 1.0, the NO conversion approaches 90%.
26 Reference is now made to Figure 5 which illustrates
27 the effect of air-fuel ratio on a conversion of N0~ C0~ HC
28 for a combined catalyst in which the initial portion of the
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1 catalyst is formed of 0.4 weight percent iridum and the
2 latter portion of the catalyst contains 0.35 weight percent
3 platinum and palladium ln a ratio o~ platinum to palladium
4 of about two ~o one. The catalyst portions were placed
back-to-back and treated in the same manner as described
6 for the iridum catalyst alone.
7 From Fi~ure 5 it is apparent now that the HC
8 conversion is extremely good and that it has been
9 increased greatly over that which was obtained on iridum
alone. The combined catalyst system is still almost 80%
11 efficient in converting N0 with a redox potential below 0.9
12 down to approximately 0.87. ~he catalyst is also extremely
13 selective in that the conversion of N0 results in nitrogen
14 gas rather than ammonia NH3. The Figure al~o
illustrates that one may obtain greater than 80% steady-state
16 conversion of all materials over a window width of 0.23 A/F
17 ratio unit or redox potential of 0.56. It should be noted thât
18 incorporation of the oxygen-storagecomponent into the three-
19 way catalyst (refer to Canadian Patent Application Serial No.
254,170 filed June 7, 1976) further ex~ends the three-way selectivit~
21 window during brief lean and rich transients. When such a
22 combined cataly~t system ls used, one has a wide margin over
23 which an air-fuel controller can control the alr-fuel ratio
24 being fed to the engine and still achieve a suitable conversion
o~ the HC, C0 and N0 contalned in the exhaust gas streamO
26 Rhodium will operate in the same manner as iri~um.
27 The catalyst system of this invention allows
28 pre~erential reduction of N0 by C0 in the initial portion of r
29 the catalyst converter in a close stoichiometric environment.
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The CO and HC are partlally oxldized on the initial catalyst
and are fully oxidized on ~the rear portion which contains
the platinum and/or palladium. The structure of this invention
al~o permlts a more efficient use of the less abundant metals
iridum and rhodium allowing them to perform in the smaller
~ront zone of the converter rather than dispersing the
materials ~hroughout the catalyst substrate thereby
increasing their overall e~ectiveness.
In view of this specification~ many modifications ~,
of this invention would be apparent to ~hose skilled in the
art. It is our intentlon that all modlYlcations
which ~all within the spirit and scope o~ this invention be
included within the scope o~ the intended claims.
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