CA1101638A

CA1101638A - Process

Info

Publication number: CA1101638A
Application number: CA288,201A
Authority: CA
Inventors: James G. Hansel; John J. Mooney
Original assignee: Engelhard Minerals and Chemicals Corp
Current assignee: Engelhard Minerals and Chemicals Corp
Priority date: 1976-10-07
Filing date: 1977-10-05
Publication date: 1981-05-26
Also published as: BR7706619A; ES462971A1; SE7711247L; AR212664A1; ES472787A1; GB1584998A; AU2942077A; BE859454A; FR2367188A1; PL201333A1; JPS5346515A; DE2745013A1; AU518302B2; DD133058A5

Abstract

PROCESS

Abstract of the Disclosure Exhaust gases from two-stroke (two-cycle) internal combustion engines which contain relatively high concentrations of carbon monoxide and hydrocarbon are catalytically treated to purify the exhaust gases and thus decrease pollution of the atmosphere. The catalytic treatment is conducted such that the relatively high concentrations of carbon monoxide and hydro-carbon in the exhaust gases can be reduced without deleteriously affecting the catalyst. The exhaust gases are passed to a first catalyst zone which contains insufficient catalyst for complete combustion of the carbon monoxide and hydrocarbon, and the first catalyst zone does not reach temperatures which are deleterious to the catalyst. The effluent from the first catalyst zone contains combustible values and is cooled and passed to a second catalyst zone, said cooling being sufficient to avoid reaching temperatures in the second catalyst zone which are deleterious to the catalyst.

Description

This invention relates to catalytic processes suitable for purifying exhaust gases from internal combustion engines in order to decrease pollution of the atmosphere, and the invention is especially concerned with overcoming problems of purifying exhaust gases from two-stroke, also known as two-cycle, internal combustion engines emitting relatively high concentrations of hydrocarbon and carbon monoxide.
In catalytically treating exhaust gases from internal combustion engines, problems may exist in that excessively high catalyst temperatures may be reached due to the high tempera-tures of the exhaust gases being treated and the presence of exothermic xeactions, and the high temperatures may be dele-terious to the catalyst. These problems are accentuated when the exhaust gases contain relatively high concentrations of carbon mono~ide and hydrocarbon along with molecular oxygen.
Gases of various sorts are discharged or exhausted by internal combustion engines, and frequently these gases contain undesirable materials which, if released to the atmosphere, would be pollutants. The problem of minimizing the release of such pollutants to the atmosphere has been under consideration for many years, and it is becoming increasingly important that means for abating emissions of pollutants be devised. Certain modes of engine operation which result in relatively large amounts of hydrocarbon and carbon monoxide being discharged from an internal combustion engine are particularly troublesome with respect to effectively abating emissions of these atmospheric pollutants. Exemplary of internal combustion engines which typically discharge such large amounts of hydrocarbon and carbon monoxide are the two-stroke combustion engines which are frequently employed as power sources for motorcycles, outboard boats, snow mobiles, chain saws and small electrical generators.
It is estimated that emissions from motorcycles account

-2-i38 -- .
for about 0.9 percent of the total mobile source of hydrocarbons and 0O6 percent of the total mobile source of carbon monoxide emissions annually in the United States. However, because of the seasonal use of motorcycles and the higher concentration of motorcycles in large cities, the effect of the emissions from motorcycles on air quality in certain regions during certain periods during the year may be substantially more significant.
For example, the raw emissions from a two-stroke motorcycle engine may frequently range from about 10 to 18 grams of hydro-carbon per mile and about 12 to 28 grams of carbon monoxide permile, and the hydrocarbon and carbon monoxide emissions produced by such motorcycles may even be as great as ten to twenty passenger cars employing catalytic exhaust treating units. A
typical exhaust from a two-stroke motorcycle engine operating at an air-to-fuel ratio of 12.8 may contain on a volume basis approximately 5 percent carbon monoxide, 1.7 percent hydrogen (estimated to be 1/3 of the amount of carbon monoxide in the exhaust gases), 3 percent oxygen, and 2,000 ppm hydrocarbon and may have a temperature of over 1000F. While general technology - 20 exists for the treatment of undesirable components in exhaust streams, the application of such general technology to the treatment of emissions from two-stroke enyines is faced with ; significant obstacles. Moreover, the exhaust gases from two-stroke engines also generally contain smoke which may comprise black carbon particles, white aerosol, and oil ash since lubri-cating oil is consumed with the fuel. The white aerosol (white smoke) could coat the catalyst and, in combination with black carbon particles, clog the catalyst. Oil ash could clog the catalyst as well as contribute to catalyst poisoning. The tendency of two-stroke engines to leave deposits can often be ; observed on examination of exhaust pipes after periods of use.

Not only would clogging in the exhaust system be adverse to the 63t3 performace of the engine and catalyst, but, also, the resultant back pressure may foster an increase in production of the pollutants in the engine.
The presence of relatively large amounts of carbon monoxide and hydrocarbon in typical exhausts from two-stroke engines may produce a significant thermal load on oxidative exhaust purification systems. For instance, the adiabatic temperature rise during catalytic combustion of 1 volume percent carbon monoxide in combustion gases is of-ten about 85 to 90C. ~ o/~/~6 The carbon monoxide concentration in exhaust gases~two-stroke ~ i engines may be as high as about 10 or more volume percent carbon monoxide. Two-stroke engines are often operated under fuel-rich conditions, and hence, their exhausts generally tend to contain increased amounts of hydrocarbon, say up to about 1 or more volume percent. ~oreover, two-stroke engines can misfire, in which case substantial amounts of hydrocarbon and oxygen are emitted from the engine, and the hydrocarbon concentration in the exhaust gases may be as high as about 2 percent or more during a misfire. ~ydrocarbons and oxygen are also present in the exhaust due to the normal scavenging process within the cylinder, during which a portion of the air, fuel and lubricating oil mixture passes from the inlet port to the exhaust port without signifi-cant combustion. The adiabatic temperature rise during catalytic co~ustion of 1000 ppm hydrocarbon in the exhaust gases is approximately 100C. Also, during periods of high fuel con-sumption, e.g., operations under heavy load conditions and/or high speeds, the volume of exhaust gases to be treated may result in an increase in catalyst temperature, for instancet due to the limited inherent cooling capacity of the catalyst. Thus, the thermal load on catalysts used to treat exhausts from two-stroke engines can be severe, particularly during periods of high fuel consumption and complete or partial misfiring, and theoretically L63~3 exceed the upper temperature limit:for efficient catalyst operation and even cause melting of the catalyst support.
The exhaust from a cylinder of an internal combustion engine is in a pulsating mode with exhaust being emitted during the exhaust phase of operation. Two-stroke engines usually employ a separate exhaust system for each cylinder of the engine. Thus, the pulsating flow environment is generally more pronounced than in combined exhaust streams from, e.g., multi-cylindered automobile engines. This pulsating flow can adverse-ly affect the mounting system for an exhaust catalyst as well asprovide strains on the catalyst structure itself. Moreover, it has also been found that, in general, two-stroke engines may tend to temporarily reverse the direction of the exhaust flow.
Thus, catalytically combusted exhaust effluent may be drawn back into the catalyst, thereby accentuating the pulsating effect a~d increasing the thermal load on the catalyst.
Houdry in U. S. Patent ~o. 2,664,340 discloses a catalytic apparatus and method for treating exhaust gases employing a plurality of catalytic elements arranged in series.
Each grouping of downstream catalytic elements is more catalyt-ically-activ~ than the preceding grouping of catalytic elements, and cooling zones are provided between each grouping of catalytic elements. Saufferer in U. S. Patent No. 3,440,817 discloses a catalytic treatment system for exhaust gases wherein a smaller catalyst is placed closely adjacent to the engine and a larger catalyst is pos~tioned downstream from the smaller catalystO An adjuskable diverter is provided upstream from the smaller catalyst to by-pass a sufficient portion of the exhaust gases around the smaller catalyst to avoid its achieving dele-teriously high temperatures. Xeith, et al., in U. S. Patent No. 3,896,616 describe a process and apparatus for treatingexhaust gases containing carbon monoxide, hydrocarbon and `nitrogen oxides comprising a irst and second catalytic convertor.
The first catalytic convertor is operated ~uring start-up under oxidizing conditions to heat the second catalytic convertor.
Then the flrst catalytic convertor is operated under reducing conditions by the addition of fuel to the exhaust gases entering the first catalytic convertor to reduce nitrogen oxides and the second catalytic convertor oxidizes combustible materials contained in the exhaust gases. These patents serve to illus-trate representative systems for catalytically treating exhaust gases in two or more catalyst beds.
By the present invention there are provided catalytic processes for treating exhaust gases from two-stroke internal ; combustion engines, which gases contain relatively high amounts of carbon monoxide and hydrocarbon r along with free or molecular oxygen. High carbon monoxide and hydrocarbon-content exhaust gases are catalytically processed using a suitable oxidation catalyst for treating exhaust gases from internal combustion engines without reaching unduly high temperatures which may adversely affect the catalyst, and these undesirable components of exhaust gases can be abated by oxidation over wide ranges of throughput volumes and engine operation conditions. The catalytic processes of this invention provide for rapid achieve-ment o~ temperatures suitable for activation of the catalyst after start-up and rapid adjustment to changes in operating conditions of internal combustion engines. Moreover, this ; invention provides for the abatement of undesirable components in such exhaust gases without undue adverse effect on the operation of the internal combustion engine, e.g., by causing an unduly high back pressure. The processes of this invention are particularly suitable for use in ~reating exhaust gases from ; two-stroke engines where pulsing flows of exhaust gases exist the exhaust gases may contain black carbon particles, white aerosol, oil ash, and the likei and engine misiring as well as cylinder scavenging may occur producing exhaust gases containing large amounts of uncombusted fuel and oxygen values.
In accordance with the processes of this invention for oxidizing constituents from two-stroke internal combustion engine exhaust gases having relatively high carbon monoxide and hydrocarbon contents, the exhaust gases from the engine contain-ing carbon monoxide, hydrocarbon, and free oxygen are passed without significant addition of additional free oxygen, to a first catalyst zone containing an oxidation catalyst wherein sufficient oxidation occurs to increase the temperature of the exhaust gases, but insufficient to produce temperatures within the catalyst which are deleterious to the catalyst. The gases exiting the first catalyst contain combustible values, e.g., carbon mono~ide and hydrocarbon, and are cooled and then passed to at least one downstream catalyst zone containing an oxidation catalyst wherein further oxidation of remaining combustible values takes place. Frequently, only a first catalyst zone and a second catalyst zone need be employed; however, expecially in treating exhaust gases having higher caxbon monoxide and hydro-carbon content, additional catalyst zones may be employed to avoid excessively high temperatures in the second catalyst zone while still obtaining an adequate decrease in the amount of undesirable components in the exhaust gases. The gases passing between each downstream catalyst zone, if more than one is employed, are preferably cooled before being passed to each immediately following catalyst zone. Sufficient heat is lost during cooling of the exhaust gases from a preceeding catalyst zone that the combustion taking place in the subsequent catalyst zone will not provide temperatures which are unduly deleterious to the catalyst; however, the decrease in temperature should not be so great that gases containing significant combustible values 63t3 .:
are cooled below the temperature re~uired for activa~ion of the catalyst for promotion of the oxidation of combustible values in the gases in the following catalyst zone.
In another aspect of this invention exhaust gases from two-stroke internal combustion engines are catalytically pro-cessed in a catalyst zone having suitable oxidation catalyst therein wherein the catalyst is in the form of at least two sequentially adjacent unitary bodies. Each of the unitary bodies has a plurality of passages, or channels, through a single piece of the carrier wherein the passages are open to fluid flow and thus are not blocked or sealed against flow from an inlet to a separate outiet. The first catalyst body in the direction of exhaust flow has passages sufficiently large in cross-section to facilitate entry of black carbon particles, white smo]ie aerosol, and the like, into the passages without undue deposition of these materials on the walls of the catalyst which may tend to restrict the flow of gases and even cause blockage of the passages. Combustion occurring in the fi~st catalyst body tends to heat the gases passing therethrough and thus reduce the tendency of black carbon particles, white smoke aerosol, and the like, to form deposits on the catalyst walls.
The heated gases pass from the first catalyst body to a sequent-ially adjacent second catalyst body having a greater number of ; flowthrough passages per unit of cross-section than the first catalyst body in order to provide additional active surface area to enhance catalytic combuskion of combustible values in the exhaust gases. Additional sequentiàlly adjacent catalyst bodies can be employed having essentially the same or a greater number of flowthrough passages per unit of cross-section than the sccond catalyst body. The greater active surface area in the second, and, if employed, subsequent, catalyst bodies enhances the degree of conversion per unlt volume. ~hu5, a srnaller volume of catalyst oan be employed as compared to the volume required i the flow-through passages were the same size as those in the first catalyst body, and the back pressure provided by the catalyst is reduced since undue formatlon of deposits is abated. Also, when the catalyst is cold, the first catalyst body can be relat$vely quickly heated to temperatures at which catalytic oxidation is promoted by the gases contacting it since the first catalyst body can contain a lesser volume of solid structure which is to be heated than that of an equivalent total volume catalyst body having a greater number of flowthrough passages. The first catalyst body which may be quickly heated to catalytically-active temperatures thus serves to increase the temperature of the gases passing to the second catalyst body and hasten reaching temperatures at which catalytic oxidation is promoted in the second catalyst body. During operation, generally the coolest portion of the catalyst is the initial portion of the catalyst where the coolex gases to be treated first contact the catalyst. Since the second catalyst ~ody has more catalytically-active surface area per unit volume, the temperatures reached therein due to the exothermic com-, . .
bustion reaction are sufficiently high that heat is radiated to the first catalyst body to enhance catalytic combustion therein. The catalyst of this aspect of the invention may be employed in the process of thisinvention wherein at least two catalyst zones are employed or in any suitable catalytic process for treating exhaust gases from two-stroke engines.
The processes and apparatus of this invention are particularly suited for treating two-stroke engine exhaust gases containing, say, at least about 2 volume percent carbon monoxide and at least about 0.05 volume percent hydrocarbon (c ~cul-ted ~', ~ , .
~ , .
9 _ ' ~

on the basis of C6 hydrocarbon). The gases al50 contain free oxygen, hydrogen, and inert diluents, e.g., nitrogen, and final products of combustion such as carbon dioxide and water. Free oxygen is generally present in the exhaust gases from the engine due to one or more of misfiring or other incomplete combustion of fuel in the engine, the normal cylinder scavenging process and the use of fuel lean combustible mixtures, the use of a stoichiometric excess of air in the fuel-air combustion mixture.
Often, the exhaust gases from two-stroke internal combustion engines contain on an average basis about 2 to 8 volume percent carbon monoxide, about 1 to 2.7 volume percent hydrogen, about 0.05 to 0.8 volume percent hydrocarbon, and about 0.5 to S
volume percent free oxygen during normal operating conditions.
During a misfire or during periods of start-up or high loads, the amounts of combustible components in the gases may be significantly higher. For instance, the exhaust gases ~rom a misfire may contain as much as about 2 or more volume percent hydrocarbon.
In accordance with this invention, oxidation reactions which occur within a given catalyst zone, whether it be the first catalyst æone or a subsequent, downstream catalyst zone, are insufficient to result in temperatures which would be deleterious to the catalyst. The temperatures achieved in a catalyst zone will, at least in part, be determined by the temperature of the gases entering the catalyst zone and the temperature increase due to the co~tbustion reactions occurring therein. Factors which influence the combustion reactions and the temperature increase resultiny therefrom include the amount of combustible components in the gases, the amount of free oxygen available for the combustion, the space velocity of the gases through the catalyst, and the level of catalytic activity per unit area of the catalyst.

63~3 By way of example, the factors which influence the amount of combustion in a catalyst zone are illustrated with respect to the first catalyst zone; however, it should be appreciated that subse~uent catalyst zones may operate in essentially the same manner in accordance with this invention.
For instance, one engine operating condition which may occur is idling in which a relatively small volume of exhaust gases is produced. The relatively slow volume flow rate may permit substantial cooling of the exhaust gases prior to impinging on the catalyst in the first catalyst zone, but the cooling is not sufficient to reduce the catalyst temperatures to below those which activate the catalyst to promote oxidation. Due to the reduced space velocity through the catalyst, a high portion of the combustible components in the exhaust gases can be reacted;
however, since the amount of gases being treated is relatively small and heat loss from the catalyst is significant, the temperature increase is insufficient to provide tempe*atures which are deleterious to the catalyst. Under high speed engine operating conditions, the volume flow rate of exhaust gases is much higher than the volume flow rate at idling conditions and the temperature drop between the engine and the first catalyst zone is generally less. The space velocity through the catalyst is also much greater, and, therefore, the catalyst is not able to effect a proportionately larger amount of combustion, i.e., the amount of catalyst employed is insufficient to e~fect com-plete combustion of the combustible materials passing there-through. Moreover, the additional gases passing through the catalyst which do not enter into the combustion reactions in the catalyst zone serve as a heat sink for heat provided by the oxidation in the catalyst zone. Accordingly, the temperature in the first catalyst zone remains below those which are deleteri-ous to the catalyst. Under conditions of misfire and sca~enging ' , . .

larger concentrations of combustible materials, especially hydrocarbon, with oxygen passed to the catalyst zone; however, due to the lower temperature of the gases passing to the catalyst zone and an insufficient amount of catalyst to effect complete conversion of the combustible materials, the tempera-tures reached in the catalyst zone are not deleterious to the catalyst.
Often, two-stroke internal combustion engines are operated under stoichiometric air-fuel ratios for theoretically complete combustion of the fuel to carbon dioxide and water or to the fuel-rich side of the stoichiometric ratio. The lack of sufficient free oxygen for complete combustion of the combustible materials in the exhaust gases may additionally assist in main-taining suitable temperature increases in the catalyst zone. In ; order to enhance the combustion of carbon monoxide and hydro-carbons in the exhaust gases, oxygen-containing gases may be added to the exhaust gases treatment system after the first catalyst zone. Generally, sufficient free oxy0en will be avail-able during the treatment of exhaust gases in accordance with this invention for substantially complete combustion on a stoichiometric basis of carbon monoxide and hydrocarbon contained therein to carbon dioxide-and water, and desirably, at least about 0.6 volume percent, generally about 0.6 to 3 volume percent, free oxygen in excess of that required on a stoichiometric basis for complete combustio~ of carbon monoxide and hydrocarbon components is provided.
Since significant cooling of the exhaust gases from the engine, especially during start-up when the engine is cold, may occur, the first catalyst is preferably positioned relatively close to the internal combustion engine such that the combustion ,., " .

3l3 exhaust gases have sufficiently high temperatures when they impinge on the ~irst catalyst that, whén cold, the catalyst is rapidly heated to catalytically-active temperatures. Generally, the first catalyst zone is positioned such that the gases contacting the catalyst are at temperatures in excess of about 250C., preferably about 300 to 600C., under operating conditions expected to be encountered during operation of the engine. Additional means for reaching temperatures sufficient for the catalyst in the first zone to achieve suitable catalytic activity such as electrical heating, ignition and thermal combustion of supplemental fuel upstream from the first catalyst zone, and the like, may be employed but are generally not used due to the complexity which they would add to the exhaust gases ; treatment system. Upon reaching temperatures sufficient to provide catalytic activity, the catalyst in the first catalyst zone promotes oxidation of carbon monoxide and hydrocarbon components in the exhaust gases to carbon dioxide and water.
The amount of oxidation occurring in the first catalyst zone is insufficient, under a wide range of operating conditions which may occur, to produce temperatures therein which are deleterious to the catalyst. Generally, the first catalyst zone serves to promote combustion of about 5 to 75, preferably about 15 to 50, volume percent of the combustible materials, i.e., carbon monoxide, hydrogen, hydrocarbons, and the like, in the exhaust gases.
The increase in temperature of the gases in the first ~` catalyst zone due to the oxidation therein is often at least about 50C. but is insufficient to pro~ide temperatures in the first catalyst zone which are deleterious to the catalyst.
Frequently, the temperature increase is less than about 500C., and often ranges, say, from about 50 to 300C. Generally, the .~ .

maximum temperature of the exhaust gases exiting the first catalyst zone is less than about 800C., e.g., ranges from about 400 to 800C., preferably about 450 to 650C. The maximum peak temperatures in the first catalyst zone are below tempera-tures which may be deleterious to the catalyst due to the use of an insufficient amount of catalyst in the first catalyst zone to promote complete combustion of the combustible values in the exhaust gases and, optionally, providing insufficient oxygen for complete combustion of the combustible values in the exhaust gases. Advantageously, the catalyst in the first catalyst zone is relatively small in volume, thereby facilitating its warming to active temperatures during start-up.
During operation, the temperature of the catalyst in the first catalyst zone is preferably maintained sufficiently high that the formation of deposits on the catalyst, if occur-ring at all, is negligible. In treating exhaust gases from two-stroke engines which contain black carbon particles and white aerosol, the temperature of the catalyst during use should be above about 300, preferably above about 500C., to minimize the formation of deposits. In the operation of two-stroke engines, the exhaust gases may contact the catalyst in a pulsing manner, and hot catalytic combustion gases may even be drawn back into the first catalyst zone. Desirable temperatures can be main-tained in the catalyst zone between the intermittent pulses of exhaust gases by further catalytic combustion of combustible ;~ values remaining in the hot combustion gases which may be drawn back into the catalyst.
~ he gases exiting the first catalyst zone contain combustible values, i.e., uncombusted carbon monoxide and hydro-carbon, under at least most engine operating conditions~Generally, the relative amount of uncombusted material in the .

3~3 effluent from the first catalyst zone is dependent upon the volume flow rate of the gases. For instance, over a range of operating conditions, the exhaust gases from the first catalys~
zone will contain carbon monoxide and most usually hydrocarbon, e.g., at least about 0.1, often about 1 to 4, volume percent carbon monoxide and at least about 0.02, o~ten abou~ 0.02 to 0.8, volume percent hydrocarbon. The gases e~iting the first catalyst zone are, e.g., by being passed through a cooling zone which interconnects the first catalyst zone and a second catalyst zone. The cooling serves to reduce the temperature of the exhaust gases such that further combustion can take place in the second catalyst zone without unduly high temperatures ~eing obtained which ma~ be deleteriows to the ca-talyst therein.
The cooling of the exhaust gases may be effected primarily by indirect heat exchange to the surrounding environ-ment and by radiation. Thus, the rate and amount of cooling will be dependent upon the throughput of gases in the cooling zone, the temperature of the gases, the temperature of the surrounding environment, and the surface area of the cooling zone. Additional cooling of the exhaust gases from the first catalyst zone may be accomplished by admixing cooler gases, e.g., air, therewith. The addition of air may also serve tG
insure that sufficient free oxygen for complete combustion of carbon monoxide and hydrocarbon values in the exhaust gases is ` provided. Advantageously, the addition of air is conducted ` between the first and second catalyst zones.
Generally, the temperature drop between the first and second catalyst zones is not 50 great that the second catalyst is not maintained at or near a temperature sufficient to initiate - 30 and maintain catalytic combustion. The drop in temperature of the gases between -the first catalyst zone and the second catalyst ~ `
:`
~ -15-63~3 zone is often, especially at low engine speeds, at least about 50C., for instance, about 50 to 400C., often about 150 to 350C., and the exhaust gases passing to the second catalyst zone are gene ~11 ~ led to temperatures in the range of about 300 to ~Un~C.j preferably about 350 to 550C.
In an aspect of this invention where the gases are exhausted from the system after the second catalyst zone, -the second catalyst is advantageously of sufficient surface area and activity to provide the desired level of reduction of carbon monoxide and hydrocarbons in the exhaust gases over most of the expected range of operating conditions of the internal combustion engine. General~y, sufficient combustion of carbon monoxide and hydrocarbons will occur in the first catalyst zone and the gases emitted therefrom sufficiently cooled, that the combustion occurring in the second catalyst zone will not reach temperatures which are deleterious to the catalyst therein even under conditions when the engine misfires. Under conditions of low throughput, there may be a lesser volume combustible values in the effluent from the first catalyst zone than under con-ditions of high exhaust throughput. Thus, the increase intemperature of the gases passing through the second catalyst zone may vary widely under varying engine operating conditions.
Generally, the increase in temperature of the gases passing through the second catalyst zone is at least about 25C., preferably at least about 50C., and oEken ranges between about 25 to 250C., preferably about 50 to 200C. The maximum temperatures of the gases obtained in the second catalyst zone are generally less than about 900C., and often are in the range of about 400 to 900C. The catalyst in the second catalyst zone is preferably maintained at a temperature of at least about 450C., preferably at least about 550C., in order to minimize deposition of undesired components on the catalyst surface and to effect catalytic combustion.
In an aspect of this invention when more than one sequentially subsequent catalyst zone is employed, the second catalyst æone generally has insufficient catalyst to promote such large amounts of oxidation that high temperatures which may be deleterious to the catalyst can be reached. In other respects such as maximum temperature of the gases in the second catalyst zone, the operation is similar to the aspect of the invention wherein the gases are exhausted from the system after the second catalyst zone. This second catalyst zone is preferably followed by a cooling zone through which the gases are cooled prior to impinging on the catalyst in a subsequent catalyst zone. The cooling may be effected in essentially the same manner as with the cooling zone between the first and second catalyst zones;
however, the amount of cooling may often be somewhat less, for instance, about 100 to 300C., to facilitate maintaining the temperature of the subsequent catalyst sufficiently high under the wide range of engine operating conditions which may be encountered so as to minimize the formation of deposits on the catalyst surface and to maintain the catalyst at or near ;` catalytically-effective temperatures.
In a further aspect of this invention, the catalytic treatment of the exhaust gases from the internal combustion engine may also serve to reduce the nitrogen oxides content `~ therein. ~xhaust gases containing nitrogen oxides~and an insufficient amount of oxygen on a stoichiometric basis for complete combustion of the carbon monoxide and hydrocarbon may enhance any reduction of nitrogen oxides in the first catalyst zone. The catalyst employed, especially in the first catalyst zone, may have catalytic activity for promoting oxidation as i3~3 well as reduction of nitrogen oxides. The oxidation reactions can serve to provide heat for the reduction reactions. Gener-ally, in the operation of two-stroke internal combustion engines under fuel-rich or near stoichiometric fuel-air ratio conditions, the production of nitrogen oxides during combustion in the internal combustion engine is relatively minor. For instance, the exhaust gases from two-stroke engines may produce about 0.0 to 0.08 gram per mile of nitrogen oxides.
The gases, after being treated by the processes of this 10 invention often contain less than about 0.02, preferably less than about 0.005, volume percent carbon mono~ide and less than about 0.002, preferably less than about 0.001, volume percent hydrocarbon.
Catalysts which are suitable for treating exhaust gases from internal combustion engines include macrosized catalysts. The catalysts have one or more metal components, especially a platinum group metal component, as a catalytic ; promoter combined with a high surface area, refractory oxide support. Depending upon the catalytically-active, promoting r ~- 20 metal components in the catalysts and the conditions of their ~ use, the catalysts may serve to promote both oxidation and ; reduction reactions simultaneously. The catalysts may thus serve to enhance the oxidation of hydrocarbons or carbon monoxide, while promoting the reduction of nitrogen oxides, to less noxious materials such as carbon dioxide, nitrogen, and water.
The catalytically-active promoting metal component of the catalysts which may be employed may comprise one or more -- metals which may be in elemental or combined form as in the case of alloys, salts, oxides and the like. The metals are generally the heavy or transition metals of Groups III to VIII having an \

6~8 atomic weight of at least about 45 The metals include, for ; instance, the iron group metals such as nickel and cobalt; the metals of Groups VB and VIB, e.g., vanadium, chromiurn, molybdenum and tungsten; copper; manganese; rhenium; and combinations of such metals. The previous metals may also be in the catalysts as catalytically-active components, and it is quite preferred that one or more metals from the platinum group be present. The useful platinum group metals include, for instance, platinum, ruthenium, palladium, and rhodium, and mixtures or alloys of such metals, e.g., platinum-palladium, platinum-rhodium, may serve in the platinum group metal component of the catalysts.
The amount of promoting metal is generally a minor portion of the catalytic composite based on the weight of the total promoting metal and the high surface area refractory oxide support, and the amount is sufficient to provide a desired catalytically-promoting effect during the use of the catalyst.
Such amounts may depend on the choice of metal and the intended use of the catalyst, and the amounts are generally at least about 0.01 weight % based on the total promoting metal and high area base. ~hese amounts may be up to about 30 or 40% or more, and preferably, the amounts are about 1 to 20%. In the case of the base or non-precious metals the amounts are frequently at least about 2%. In the case of platinum group metals, the amounts do not materially exceed about 5 weight percent based on the pro-moting metal component and the high area support. For instance, the amount may be about 0.01 to 4% and is preferably about 0.05 to 2~. Frequently, when the catalyst is in the form of pellets, ~ the amount of platinum group metal is about 0.02 to 2 or more ; wei~llt percent and is most often up to about 0.2 weight percent based on the total catalyst. When the platinurn group metal component of the catalysts contains more than one of such metals, ~:~6J11L6;~8 this component may, for instance, be composed o~ a major amount of platinum and a minor amount of one or more of the other platinum group metals, e.g., palladium, rhodium or ruthenium.
For example, this component of the catalyst may have about 55 to 95 weight % platinum and about 5 to 45 weight % palladium or rhodium based on the total of these metals. The amounts of the catalytically-promoting metals, whether they be base or precious metals, are stated herein on the basis of the metals regardless of their form.
Catalysts which may be particularly useful in systems in which it is desired to conduct both oxidation and reduction simultaneously, for instance, to reduce nitrogen oxides while oxidizing carbon monoxide and hydrocarbons which may be present in the reaction system comprise a platinum group metal and one or more base metal components which may be selected from those described above, and may particularly contain an iron group metal such as nickel, for instance, in the form of metal oxides, e.g., nickel oxide. The amounts of the platinum group metal present may be as indicated above, e.g., about 0.01 to 4%, preferably about 0.05 to 1.5 or 2%, while the base metal is often present in ` an amount greater than the platinum group metal, say at least about 2% and up to about 20%. These amounts are again based on the total weight of the promoting metal and the high area support.
The macrosize catalysts have alumina applied to their surfaces to provide protection against the poisoning effects of various materials such as lead, zinc, other metals, sulfur, phosphorus and the like. Often the surface-applied alumina ~A12O3 basis) comprises a minor amount, say, about 10 to 100%, of the total weight of the catalytic promoting metal and the hiyh area support, and preferably this amount is about 20 to 75 percent, and does not unduly adversely afect, if at all, the catalyst. The surface-applied alumina contains catalytic-ally-active alumina or a hydrous alumina precursor ~hereof, as an essential component. This active alumina component is of the high surface area-type, e.g., having a surface area of at least about 25, preferably at least about 100, square meters per gram as determined by the BET method, and is generally referred to as being catalytically-active. The active aluminas include the members of the gamma or activated alumina family, such as gamma and eta aluminas, as distinguished from relatively inactive, low surface area alpha-alumina. The surface-applied materials may be calcined or activated alumina or a hydrous alumina which can be converted to active alumina by calcination, or use, at high temperatures, for instance, amorphous hydrous alumina, alumina monohydrate, alumina trihydrate or their mixtures. These alumina materials mav contain minor amounts of other components such as rare earth oxides, e.g., ceria, silica and the likeO
The alumina i5 preferably a major amount of the surface-applied ~' material on a solids basis. Most desirably, the amount of alumina is at least about 75% of the total weight of the solids.
2Q If other ingredlents are added to the catalyst after the surface-; applied alumina component, it is preferred that they be -essentially free of catalytically-active, promoting metal ; components, e.g., platinum group metals, or other promoters, of substantially greater catalytic activity than the surface-applied alumina component.
The high area support with which the catalytically-` active, promoting metal component is combined in the catalysts of this invention, is comprised of one or more refractory oxides.
These oxides include, for example, silica and metal oxides such as alumina, including mixed oxide forms such as silica-alumina, aluminosilicates which may be amorphous or crystalline, alumina-zirconia, alumina-chromia, alumina~ceria and the like. Prefer ably, the support is composed to a major extent o~ alumina which especially includes the members of the gamma cr activated alumina family, such as gamma and eta aluminas. The support materials which are in admixture with the catalytically-active, ; promoting metal component in the catalysts of this invention are often referred to as being in catalytically-active form, but such activity is of a low order compared with that of the catalytically-active, promoting metal components.- The high surface area supports comprise a major amount based on the combined weight of the support and catalytically-active, ; promoting metal, and the surface area of the support is usually at least about 25 square meters per gram as determined by the ~`
BET method, preferably at least about lO0 square meters per gram.
The catalysts of this invention are in macrosize form as are the catalytic compositions to which the alumina is applied to impart the desired resistance to the effect of materials which would otherwise poison the catalysts to a greater extent. Generally, macrosize catalysts have minimum dimensions of at least 1/16 inch, and often at least one or all dimensions are at least 1/8 inch. The catalysts may be in particle form such as spheres, cubes, elongated pellets or the like, but preferably are in the form of monolithic or unitary structures having a plurality of gas flow paths through a single piece of catalyst.
The catalysts of the invention may have a carrier component which is relatively catalytically-inert compared with the high surface area support, and the carrier is generally of considerably lower total surface area than the support which is applied thereto. Thus, the carrier may have i38 a total surface area of less than about 5 or 10 square meters per ~ram, more often less than about 1 square meter per gram, as determin~d by the BET method. The carrier may be in macrosize particle form and preferably is in monolithic form, e.g., a honeycomb configuration. The high area support materi 1 i-Q
generally distribu~ed as a coating over most, if not all, of the surface of the carrier, and usually the high surface area support material is present in the catalysts in a minor amount based on the weight of the relatively inert carrier, say about 5 to 25, preferably about 10 to 20, weight percent.
The relatively inert carriers of the catalysts of this -invention may be made of one or more of a variety of materials, but preferably are composed primarily of one or more refractory oxides or other ceramics or metals. The preferred carriers are ; comprised of cordierite, cordierite-alpha alumina, silicon , n-tride, zircon mullite, spodumene, alumina-silica-magnesia or - zirconium silicate. Examples of other refractory ceramic materials utilizable in place of the preferred materials as a carrier are sillimanite, magnesium silicates, zircon, petalite, alpha-alumina and aluminosilicatesO Although the carrier may be a glass ceramic, it is preferably unglazed and may be essential-ly entirely crystalline in form and marked by the absence of any significant amount of glassy or amorphous matrices, for instance, of the type found in porcelain materials. Further, `` the structure may have considerable accessible porosity as distinguished from the substantially non-porous porcelain utilized in electrical applications, for instance in spark plugs, characterized by having relat,vely little accessible porosity. Thus, the carrier structure may have a water pore volume of at least about 10 weight percent. Such carriers are described, for example, in U. S. Patent No. 3,565,8`30, .~; . .
~ 23-i38 The monolithic carriers of the catalysts of this - invention are of ~he ~ype that have a plurality of passages through a sIngle piece of the carrier. The passages are open to fluid flow and thus are not blocked or sealed against flow from an inlet to a separate outlet, and, accordingly, the passages are not merely surface pores. The passages are generally rather large compared with the size of the surface pores in order that the fluids going through the passages are not subject to excessive pressure drop. The monolithic catalys~ carriers have a unitary, skeletal structure of macrosize with a minimum c.oss- ~
sectional dimension generally perpendicular to the direction of ~ r fluid flow therethrough of, for instance, at least about 2 centimeters, e.g., in honeycomb form, and have flow path lengths of at least about~10 millimeters, preferably at least about 20 -~
millimeters.
The flow passages of the monolithic carrier may be ~ thin-walled channels providing a relatively large amount of .~ . .
superficial surface area. The channels can be one or more of a variety of cross-sectional shapes and sizes. The channels can be of the cross-sectional shape of, for example, a triangle, trapezoid, rectangle, polygon or more than four sides, square, sinusoid, oval or ci~cle, so that cross-sections of the carrier represent a repeating pattern that can be described as a honey-comb, corrugated or lattice structure. The walls o~ the cellular channels are generally of a thickness necessary to provide a sufficiently strong unitary body, and the thickness will often fall in the range of about 2 to 25 mils. With this wall thickness, the structures may contain from about 50 to 2500 or more gas inlet openings for the flow channels per square inch of cross section and a corresponding number of the gas flow ~ $ - 24-3~3 channPls, preferably about 60 to 500 gas inlets and flow channels per square inch. The open area of the cross-section ma~ be in exce~s of about 60~ of ~he total area. The size and dimensions of the unitary refractory skeletal support of this invention can be varied. The support is unitary or monolithic in the sense that a significant portion, preferably a major portion or even essentially all, of its cross-section is composed of one inter-connected skeletal structure or unit. Generally, such a unit may have a cross-sectional area of at least about 1.5 square inches, preferably at least about 2 square inches.
In an aspect of this invention, the cataIyst in the first zone has about 50 to 300, prefera~ly about 60 to 300, gas flow channels per square inch of surface area of the face of the catalyst. Desirably, the first catalyst has large flow through openings, i.e., less gas flow channels per square inch, in order that minimal back pressure be provided and any formation of deposits on the catalyst surface would not rapidly cause ; plugging of the catalyst. The catalyst in the second and sub-sequent, if employed, catalyst zones may also employ large openinys.
In the aspect of this invention wherein the catalyst in a catalyst zone is comprised of a plurality of sequentially adjacent catalyst bodies, the first catalyst body generally may have less than about 100, frequently about 50 to 100, gas flow channels per square inch of cross section. The second and additional downstream catalyst bodies, if employed, generally may have at least about 150, often at least about 200, gas flow channels per ~.quare inch of cross-section. The catalyst bodies are sequentially adjacent, i.e., they may be integral or in contact, or they may be separated, but not by such distances that siynificant cooling of the gases occurs and are arranged such that the gases pass through the preceding catalyst body 63~
prior to passing the following catalyst boay. The ratio of volume of the first catalyst body to the overall cataly~t body may vary widely and frequently the ~irst catalyst body comprises about 10 to 90 volume percent of the total catalyst. Wh~n employed in the process of this invention for treating exhaust gases for two-stroke engines, the sequential adjacent catalyst bodies are employed at least in the first catalyst zone.
The invention is further described with reference to ;`
the drawings in which ` 10 FIGURE 1 is a schematic diagram of an apparatus of this invention having two catalyst zones, and FIGURE 2 is a schematic diagram of an apparatus of this invention having two catalyst zones and supplemental air supplied between the first and second catalyst zones.
: With reference to the drawings, the numeral 10 generally designates an exhaust port of an exhaust manifold from a two-stroke internal combustion engine. The exhaust gases from ; the internal combustion engine pass from the exhaust port to first catalyst interconnect line 12 to first catalyst zone 14.
The first catalyst zone 14 contains catalyst 16 mounted therein.
The gases exit first catalyst zone 14 and pass through cooling zone or line 18 whereat the gases are cooled and transported to second catalyst zone 20 having catalyst 22 mounted therein. The gases exit the second catalyst zone 20 via line 24 and may be exhausted to the atmosphere, transported to a muffler, or the like.
With reference to Figure 2, air line 26 is provided to supply air to the gases in cooling zone 18 flowing to the second catalyst zone 20. Thé rate of flow of air to the cooling zone 18 is controlled by control means 28 which may be a valve, volume flow adjustable pump, or the like, which is in turn 3~3 controlled by sensing means 30 which i5 in communlcatlon there~Jith.
Senslng means 30 may be in communlcatlon wlth at least one of the car-buretors of the engine to respond to fuel-rlch or fuel-lean conditlons, to the lgnitlon system or drive mechanism of the engine to respond to engine speed and thus the rate of flow of the exhaust gases, to a tem-perature sensor before or after the first catalyst zone 14, to a carbon monoxide and/or hydrocarbon sensor before or aEter the first catalyst zone 14, or oxygen sensor before or after catalyst zone 14. The supple-mental air may thus be supplied to the exhaust gases in amounts suitable for operation of the engine exhaust treatment system to provide suficient oxygen for combustion of the combustible impurities in the exhaust gases and~or to provide sufficient coollng of the gases prior to passing to a catalyst zone.
The invention is further illustrated by the following example. All parts and percentages are by volume unless otherwise designated.
A ~amaha motorcycle having a two cylinder, 350 cubic centimeter, two-stroke engine is provided with an exhaust treatment device similar to that depicted in Figure 2. The air supply is manually controlied to provide 1.5 cubic feet per minute of air at motorcycle speeds of less than about 30 miles per hour and 4.0 cubic feet per minute of air at motorcycle speeds greater than 30 miles per hour. The air is injected into the exhaust immediately downstream of the rear face of the first catalyst. The Eirst catalyst is positioned 15 inches downstream Erom the exhaust manifold port and contains two sequentially-abutting pieces of unitary catalyst providing a total catalyst volume of 181 cubic centimeters and cross-sectional area of 17.8 square centimeters. The upstream catalyst piece has a length of 1 inch and has 64 openings or gas flow passages per square inch and the adjacent downstream catalyst piece has a length of 3 inches and has 260 openings per square inch. The catalyst in the second catalyst zone is essentially .

~ - 27 - ..

, ~" , .
Identical to that ln the flrst catalyst zone The first and second catalysts are separated by 10.5 lnches, and conventional metal exha~st tubing lnterconnects the catalyst zones. The effluent from the second catalyst zone passes to a conventional muffler for the mo~orcycle. The fuel employed in thls example for the motorcycle is a conventional com-binatlon of commercial two-stroke lubricating oil injected into gasoline having a 91 research octane number, about 0.02 to 0.025 gram of lead per gallon, an average of about 300 parts per million of sulfur, and less than about one part per million of phosphorous. ;

The use of the process of this invention on the two-stroke motorcycle described above is demonstrated using a motorcycle dual-roll chassis dynamometer (Clayton Model) with an inertia weight of 480 pounds. The motorcycle is cooled during testing with two 29 inch fans, one on each side of the front tlre and angled toward the engine.
The cooling air flow is approximately equivalent to the air flow at motorcycle speeds of 25 miles per hour.
The effluent gases from the exhaust system are analyzed essentially in accordance with the EPA Light Duty Vehicle Emission Test Procedure (1975-FTP). The duration of the test is about 31 minutes with about 14 minutes of operation under stop and go conditions ranging from idle to about 35 miles per hour, and about 10 percent of the test is conducted at a motorcycle speed of about 50 miles per hour.
During the emissions test, the emissions are determined to be 5.2 grams of hydrocarbon per mile, 10.4 grams of carbon monoxide per mile and 0.03 gram of nitrogen oxides per mile. Without employing the process of this invention, the emissions are determined to be about 1~3.4 .

~ - 2~

~1638 grams of hydrocarbon per mile, 22.6 grams of carbon monoxide per mile, and 0.03 gram of nitrogen oxides per mile. The maximum temperature profiles of the exhaust treatment system are determined for engine opera~ions at 56 miles per hour and under stop and go driving conditions at less than about 35 miles per hour. At about 56 miles per hour, the exhaust gases impinging on the first catalyst are about 710F., and the effluent from the first catalyst is about 1410F. The gases impinging on the second catalyst are about 1370F., and the effluent from the second catalyst is about 1640F. Under stop and go conditions, the exhaust gases impinging on the first catalyst are about 588F., and the effluent from the first catalyst is about 1050F. The gases impinging on the second , .
catalyst are about 685F., and the effluent from the second ~:~ catalyst is about 1090F.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for catalytically treating exhaust gases from a two-stroke internal combustion engine, said gases com-prising at least about 2 volume percent carbon monoxide and at least about 0.05 volume percent hydrocarbon, comprising passing the exhaust gases to a first catalyst zone to provide a first catalyst combustion effluent containing combustible values, said first catalyst zone having an insufficient amount of oxidation catalyst to promote complete combustion of the carbon monoxide and hydrocarbon in the exhaust gases and to produce temperatures in the first catalyst zone which are deleterious to the catalyst, wherein the temperature of the first catalyst combustion effluent is about 50 to 500°C. higher than the tem-perature of the exhaust gases to the first catalyst zone, cooling the first catalyst combustion effluent by at least about 50°C., and passing the cooled first catalyst combustion effluent to a second catalyst zone containing oxidation catalyst to provide a second catalyst combustion effluent wherein the temperature of the second catalyst combustion effluent is at least about 25°C.
higher than the temperature of the cooled, first catalyst combustion effluent, but is insufficient to produce temperatures in the second catalyst zone which are deleterious to the catalyst.

2. The method of claim 1 wherein the first catalyst com-bustion effluent ranges in temperature from about 400 to 800°C.

3. The method of claim 2 wherein the first catalyst com-bustion effluent is cooled about 50 to 400°C. prior to being passed to the second catalyst zone.

4. The method of claim 3 wherein supplemental air is admixed with the first catalyst combustion effluent prior to being passed to the second catalyst zone in an amount sufficient to provide an excess of free oxygen on a stoichiometric basis for combustion of the combustible values in the first catalyst combustion effluent.

5. The method of claim 4 wherein the exhaust gases con-tain an insufficient amount of free oxygen on a stoichiometric basis for complete combustion of the carbon monoxide and hydro-carbon contained therein.

6. The method of claim 3 wherein the second catalyst com-bustion effluent ranges in maximum temperature from about 400 to 900°C.

7. The method of claim 1 wherein the exhaust gases contain an insufficient amount of free oxygen on a stoichiometric basis for complete combustion of the carbon monoxide and hydrocarbon contained therein.

8. The method of claim 1 wherein the second catalyst com-bustion effluent contains combustible values and is cooled and then passed to at least one subsequent in series catalyst zone containing oxidation catalyst to provide a subsequent catalyst combustion effluent wherein the temperature of the second catalyst combustion effluent is reduced sufficiently during cooling such that the maximum temperature in the subsequent catalyst zone is below that which may deleteriously affect the catalyst.

9. The method of claim 1 wherein about 5 to 75 volume percent of combustible materials in the exhaust gases are com-busted in the first catalyst zone.

10. The method of claim 1 wherein the oxidation catalyst employed in the first and second catalyst zones is a macrosized catalyst having a minor amount of at least one metal component having catalytic activity to provide catalytic activity on a high surface area, refractory oxide support.

11. The method of claim 10 wherein the oxidation catalyst comprises platinum group metal as a metal component having catalytic activity.

12. The method of claim 11 wherein the oxidation catalyst in the first catalyst zone also comprises an iron group metal.

13. The method of claim 10 wherein the oxidation catalyst employed in the first catalyst zone is comprised of at least two sequentially adjacent unitary catalyst bodies each having a plurality of passages therethrough wherein the first catalyst body has less than about 100 passages per square inch of cross-section and the second catalyst body has at least about 150 passages per square inch of cross-section.

14. The method of claim 1 wherein the oxidation catalyst employed in the first catalyst zone is comprised of at least two sequentially adjacent unitary catalyst bodies each having a plurality of passages therethrough wherein the first catalyst body has passages sufficiently large in cross-section to facilitate entry of the exhaust gases therein and the second catalyst body has a greater number of passages.

15. The method of claim 14 wherein the first catalyst body has less than about 100 passages per square inch of cross-section and the second catalyst body has at least about 150 passages per square inch of cross-section.

16. The method of claim 15 wherein the second catalyst body has at least about 200 passages per square inch of cross-section.

17. The method of claim 15 wherein the oxidation catalyst employed in the second catalyst zone is comprised of at least two sequentially adjacent unitary catalyst bodies each having a plurality of passages therethrough wherein the first catalyst body has less than about 100 passages per square inch and the second catalyst body has at least about 150 passages per square inch of cross-section.

18. An apparatus for catalytically treating gases con-taining combustible values comprising an initial catalyst body for partially combusting said gases to provide a catalytic com-bustion effluent containing residual combustible values, a downstream catalyst body positioned sequentially with said upstream catalyst body for receiving gases combusted in said upstream catalyst body and further combusting the same to provide a combustion gas effluent, said catalysts being such that temperatures produced in said catalyst bodies are insufficient to be deleterious to the catalysts, and said initial catalyst body having a plurality of gas flow passages with inlet openings and outlet openings for passing the gases therethrough, said downstream catalyst body having a plurality of gas flow passages with inlet and outlet openings for passing the gases therethrough, the open, cross-sectional area of individual gas flow passages of said initial catalyst body being substantially greater than the open, cross-sectional area of individual gas flow passages of said downstream catalyst body.

19. The apparatus according to claim 18 wherein the open, cross-sectional area of each of said catalyst bodies is at least about 60 percent of its total cross-sectional area.

20. The apparatus according to claim 18 or 19 wherein said initial catalyst body has about 50 to 300 gas flow passages per square inch of cross-sectional area.

21. The apparatus according to claim 18 wherein said initial catalyst body has not more than about 100 gas flow passages per square inch of cross-section and said downstream catalyst body has at least about 150 gas flow passages per square inch of cross-section.

22. The apparatus according to claim 21 wherein said initial catalyst body is sequentially adjacent to and in the same exhaust gas treating zone as said downstream catalyst body.

23. The apparatus according to claim 22 wherein said gas flow passages in said catalyst bodies are formed by walls having a thickness of about 2 to 25 mils.

24. An apparatus for catalytically-treating exhaust gases from an internal combustion engine, wherein said gases comprise carbon monoxide and hydrocarbons, said apparatus comprising an initial catalyst body for partially combusting said gases to provide a catalytic combustion effluent containing combustible values, a downstream catalyst body positioned sequentially adjacent to said initial catalyst body and in the same exhaust gas treating zone as said initial catalyst body, for receiving exhaust gases combusted in said initial catalyst body and pro-viding combustion effluent, said catalysts being such that temperatures produced in said catalyst bodies are insufficient to be deleterious to the catalysts, said initial catalyst body having about 50 to 300 flow passages per square inch of cross-sectional area with inlet openings and outlet openings for receiving and emitting the exhaust gases, said downstream catalyst body having a greater number, and in any event at least about 150, gas flow passages per square inch of cross sectional area than said downstream catalyst body with inlet and outlet openings for receiving and emitting effluent gases, the gas flow passages of each of said initial and downstream catalyst bodies providing an open, cross-sectional area that is at least about 60% of the total cross-sectional area of the body, said passages being formed by walls that are about-2 to 25 mils thick.

25. The apparatus of claim 24 wherein there is positioned downstream of said catalyst bodies, another catalyst body having a plurality of gas flow passages therethrough and gas cooling means are positioned therebetween.

26. The apparatus of claim 25 wherein said cooling means comprises means for introducing an oxygen-containing gas into a passage means for said gases.

27. The method of claim 1 wherein said exhaust gases include free oxygen and further comprising passing said exhaust gases to said first catalyst zone without significant addition of additional free oxygen and wherein said first catalyst zone has an amount of oxidation catalyst sufficient to combust only about 15 to 50 volume percent of the combustibles in the exhaust gases under the conditions prevailing within said first catalyst zone, and the temperature of said second catalyst combustion effluent is at least about 50°C higher than the temperature of the cooled, first catalyst combustion effluent.