CA1073363A

CA1073363A - Segmented three-way catalyst

Info

Publication number: CA1073363A
Application number: CA263,826A
Authority: CA
Inventors: Haren S. Gandhi; Mordecai Shelef
Original assignee: Ford Motor Company of Canada Ltd
Current assignee: Ford Motor Company of Canada Ltd
Priority date: 1975-11-24
Filing date: 1976-10-20
Publication date: 1980-03-11
Also published as: DE2649825B2; JPS5265177A; DE2649825C3; DE2649825A1; GB1548243A

Abstract

SEGMENTED THREE-WAY CATALYST

ABSTRACT OF THE DISCLOSURE

A three-way catalyst converter is disclosed for simultaneously treating carbon monoxide [CO], unburned hydro-carbons [HC] and oxides of nitrogen [NO] found in an exhaust gas stream from an internal combustion engine. The converter includes a converter housing having a catalyst substrate therein. A catalyst selected from the group consisting of rhodium and iridum is deposited only on an initial portion of the catalyst substrate. A catalyst selected from the group consisting of platinum and palladium is deposited on only a latter portion of the substrate. The rhodium and iridum are effected under stoichiometric or slight oxidizing conditions to effect a reduction of NO and an oxidation of some or all the CO. The platinum and palladium are effective under these conditions to oxidize HC and any CO which was not oxidized on the rhodium and iridum.

Description

~733~3 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, ~ .

~73363 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, ~L~73363 ~.

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 ~,, . ",,.

' ..' ' '. .' ~ ,, --" 1C9733~3 ,, ~
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 .

. . :~ , ;
.. , . ~ , ~733~;3 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.

1~733Çi3 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 ^
1~73~3 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.

~ - 7 -. I

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.

....

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

A three-way catalyst converter comprising:
a converter housing;
substrate means in said housing for supporting catalyst materials thereon;
a catalyst selected from the group consisting of rhodium and iridum deposited on only an initial portion of said substrate means, and a catalyst selected from the group consisting of platinum and palladium deposited on only a latter portion of said substrate means.

The converter of Claim 1 wherein: said substrate means is a monolithic catalyst substrate, said initial portion of said substrate means being about one-third of its length in the direction gases flow therethrough, and said latter portion being the remaining two-thirds of the length of said substrate means.

The converter of Claim 1 wherein: said substrate means is two separate monolithic catalyst substrates a first of said substrates defining said initial portion of said substrate means and the second of said substrates defining said latter portion of said substrate means.

The converter of Claim 3 wherein: said second substrate is about twice the volume of said first substrate.

The converter of Claim 1 wherein: said substrate means is a pelletized substrate material, and wherein a screen means is used in said converter housing to divide said pelletized substrate material into said initial portion and said latter portion.

The converter of Claim 5 wherein: the volume of said pelletized substrate material in said latter portion is about twice the volume of said pelletized substrate material in said initial portion.