AU666235B2

AU666235B2 - Exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines

Info

Publication number: AU666235B2
Application number: AU51988/93A
Authority: AU
Inventors: Bernd Engler; Dieter Lindner; Egbert Lox; Klaus Ostgathe
Original assignee: Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 1992-11-27
Filing date: 1993-11-26
Publication date: 1996-02-01
Anticipated expiration: 2013-11-26
Also published as: HUT70018A; EP0601314A1; CN1096712A; NO934269D0; PL301228A1; CZ288516B6; TW303307B; DE4239875A1; FI104749B; CA2110139C; CZ255993A3; MX9307424A; BR9304864A; HU212892B; KR100283341B1; JP3816113B2; NO934269L; HU9303345D0; AU5198893A; CA2110139A1

Abstract

The invention relates to an exhaust gas purification system for decreasing the emissions of hydrocarbons during the cold start of internal combustion engines. The exhaust gas purification system contains a hydrocarbon adsorber and a downstream catalyst system which can be composed of a single three-way catalyst or of a combination of oxidation, reduction and/or three-way catalysts in one or more beds. The hydrocarbon emissions during the cold start phase can be significantly decreased by using oxidation catalysts or three-way catalysts which, in comparison with conventional catalysts, have at least twice the loading of platinum and/or palladium.

Description

L I Exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines Description The invention relates to an exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines. The exhaust gas purification system contains a hydrocarbon adsorber and a downstream catalyst system, which may comprise one individual three-way catalyst or a combination of oxidation, reduction, and/or three-way catalysts in one or more beds.

The future limits for pollutant emissions from motor vehicles are laid down in the regulations TLEV/1994 and LEV/1997 (LEV Low Emission Vehicle). They represent a substantial tightening of the limits, particularly for hydrocarbons. As present-day exhaust gas catalysts have reached a high level of pollutant conversion in the hot operating state, it is possible to comply with the future *...limits only by an improvement in pollutant conversion during the cold start phase. This is because a large proportion of the hydrocarbons released as a whole is emitted during the cold start phase of the test cycles laid down by law US FTP 75). In said phase, the catalysts have not yet reached the operating temperature of 300 to 400 °C required for conversion.

ooo The hydrocarbons emitted during the cold start phase are chiefly C 1 to C 10 compounds such as paraffins, isoparaffins, olefins and aromatics.

In order to reduce the pollutant emissions during the cold start phase, an exhaust gas purification system comprising a hydrocarbon adsorber and a downstream catalyst is proposed, for example, in US patent 5,078,979. The hydrocarbon adsorber has the task of adsorbing the hydrocarbons contained in the exhaust gas during the cold start phase at temperatures that are still relatively low.

Only when the adsorber becomes hotter are the hydrocarbons desorbed again and arrive with the now hotter exhaust gas at the catalyst which is now almost at operating temperature and are converted here to harmless water and carbon dioxide. An important requirement in respect of the adsorber is the ability to adsorb-hydrocarbons preferentially before the water vapour which is also present in abundance in the exhaust gas.

A disadvantage of said described solution is the desorption of hydrocarbons commencing even at relatively low temperatures, with the result that optimum conversion on the downstream catalyst cannot yet take place. Usually, there is a temperature gulf of more than 100 °C between the light-off temperature TA of the catalyst of 300 to 400 OC and the desorption temperature TD of the adsorber immediately upstream of about 150 to 200 TA TD 100 OC. Moreover, there is the risk of thermal destruction of the adsorber since it has to be incorporated near the engine in the exhaust gas purification system and is

A

therefore exposed to temperature loads of up to 1000 OC during continuous operation.

In order to overcome said disadvantages, there is a large number of proposals in the patent literature, for example, in German specification DE 40 08 789, in European patent application EP 0 460 542, and in US patent 5,051.244. Said documents likewise start out from the combination of a •gee hydrocarbon adsorber and a catalyst, but propose complex circuits for the exhaust gas in order to overcome the disadvantages described.

For example, USP 5,051,244 proposes to provide a molecular sieve adsorber upstream of the actual catalyst, which in the cold state adsorbs the pollutants in the exhaust gas, particularly hydrocarbons, and releases them again as the exhaust gas purification system becomes hotter. In order to protect the adsorber from destruction by overheating when the engine is in continuous operation, provision is made for a short circuit line which can be connected from the engine directly to the catalyst.

During the first 200 to 300 seconds after the start, the exhaust gas is passed completely over the adsorber and the catalyst. In said operating phase, the hydrocarbons are adsorbed by the adsorber. Adsorber and catalyst are heated to an increasing extent by the hot exhaust gas. The adsorber is short-circuited if desorption begins to exceed adsorption as a result of the temperature increase. The exhaust gas now flows directly over the catalyst. When the operating temperature is reached, part of the hot exhaust gas is passed over the adsorber until complete desorption of the pollutants, which may now be converted by the catalyst with a good level of efficiency. After desorption has taken place, the adsorber is short-circuited again in order to protect it from destruction by thermal overload.

Adsorbers proposed by USP 5,051,244 and USP 5,078,979 are natural or synthetic zeolites with an Si/Al atomic ratio of at least 2.4. Suitable zeolites mentioned are silicalite, faujasite, clinoptilolite, mordenite, chabizite, ultrastable Y-zeolite, Y-zeolite and ZSM-5 and mixtures thereof. The zeolite adsorber may, moreover, contain finely divided, catalytically active metals such as platinum, palladium, rhodium, ruthenium, and mixtures thereof.

Said solutions known from the state of the art are either technically very complex, expensive and susceptible to breakdown or, as in the case of USP 5,078,979, lack a solution for bridging the temperature gulf between the -4desorption temperature of the adsorber and the light-off temperature of the downstream catalyst.

Object of the Invention It is an object of the present invention to overcome or substantially ameliorate the above disadvantages.

Summary of the Invention There is disclosed herein an exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines, comprising: a hydrocarbon adsorber; and a downstream catalyst system comprising a three-way catalyst or a combination of oxidation, reduction and/or three-way catalysts in one or more beds, wherein: said downstream catalyst system contains an oxidation catalyst with platinum 15 and/or palladium and a three-way catalyst with platinum and/or palladium and/or rhodium, wherein the loading of said oxidation catalyst with platinum and/or palladium is at least 3.5 g of platinum and/or palladium per litre of catalyst volume; said oxidation catalyst is placed immediately after said adsorber; and wherein the difference between the light-off temperature of the oxidation catalyst for conversion of hydrocarbons and the desportion temperature of the adsorber is less than 0

C.

There is further disclosed herein an exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines, comprising: 25 a hydrocarbon adsorber; and a downstream catalyst system comprising: a three-way catalyst in one or more beds, wherein said three-way catalyst comprises the platinum group metals platinum and/or palladium and/or rhodium in an amount of at least 3.5 g of platinum and/or palladium per litre of catalyst volume; and wherein the difference between the light-off temperature of the oxidation catalyst for conversion of hydrocarbons and the desportion temperature of the adsorber is less than 50 0

C.

8BFD iN JB1Too00748:BFD 14- There is still further disclosed herein an exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines, comprising: a hydrocarbon adsorber in immediate contact with an oxidation catalyst and a downstream three-way catalyst in one or more beds, wherein: said oxidation catalyst and adsorber are present in the form of superimposed coatings on a monolithic honeycomb body, the adsorber coating lying on the catalyst coating and the oxidation catalyst contains platinum and/or palladium in an amount of at least 3.5 g platinum and/or palladium per litre of catalyst volume; and wherein the difference between the light-off temperature of the oxidation catalyst for conversion of hydrocarbons and the desportion temperature of the adsorber is less than The light-off temperature TA of the catalyst is the exhaust gas temperature upstream of the catalyst, at which temperature the catalyst converts exactly J0 of the hydrocarbons.

The desorption temperature TD of the adsorber is a parameter which may be determined from the engine only in dynamic operation. To this end, the crude hydrocarbon emission of the engine without the use of an adsorber is recorded as a function of time initially during the first 200 to 300 seconds after the cold start. Said S 20 crude emission typically shows a high and broad maximum during the first 60 to 100 seconds. As the engine becomes hotter, the hydrocarbon emission falls to the normal level when the engine is hot. In a second test run, the hydrocarbon emission after the connected adsorber and the temperature before the adsorber is then measured as a function of time.

25 As a result of the adsorber, the hydrocarbon emission is initially greatly :j suppressed by adsorption, but then increases as the exhaust gas becomes hotter as a result of increasing desorption by the adsorber and likewise passes through an emission maximum with a time delay compared with the crude emission, before it eventually falls to the value of the crude emission when the engine is hot. As a result of the time shift in the emission maxima of the crude emission and the emission with adsorber, the two emission curves intersect at a particular time within about 60 to 100 seconds after the cold start.

[N:\LIBTrI00748:BFD The exhaust gas temperature present before the adsorber at this point in time is known as the desorption temperature of the adsorber. It depends on the design of the exhaust gas system in each case and on the adsorber material itself, and is typically between 150 and 200 0

C.

Zeolites are used as preferred adsorber materials. As already disclosed in USP 5,051,244, however, only those zeolites that adsorb hydrocarbons preferentially before water, which are hydrophobic and, moreover, have a high temperature- and acid stability, are suitable for the adsorption of hydrocarbons from the exhaust gas of combustion engines.

The hydrocarbon adsorber preferably contain at least one hydrophobic, temperature- and acid-stable zeolite with an Si/Al ratio of more than 20. In a particularly favourable embodiment of the invention, two zeolites which have unequally steep temperature curves of their hydrocarbon adsorption capacities are combined with one another in the adsorber. At least two zeolites I and II should be combined, of 15 which zeolite I has a greater adsorption capacity at temperatures below 100 0 C than zeolite II, and zeolite II has a greater adsorption capacity above 100 0 C than zeolite I.

The zeolite I used is, for example, a dealuminised Y-zeolite with an Si/Al ratio of more than 40 and the zeolite II used is a zeolite ZSM5 with an Si/Al ratio of more than Dealuminised Y-zeolite and zeolite ZSM5 should be present in the adsorber in a weight ratio to one another of 1:10 to 10:1.

Zeolite Y belongs to the wide-pore zeolites with a pore diameter of 0.74 nm, a pore volume of 0.3 ml/g and a specific surface area of more than 700 m 2 Zeolite is a medium-pore zeolite with a pore diameter of approx. 0.55 nm. As a result of its large pore aperture, the Y-zeolite has a high initial adsorption capacity for the aromatics contained in the exhaust gas. The adsorption capacity falls very rapidly, however, as the temperature rises. Zeolite ZSM5, on the other hand, has a lower initial adsorption capacity for aromatics, bui exhibits a smaller fall in said capacity as [N:\LIBTT10074B:BFD -7the temperature rises. Moreover, said zeolite has a good adsorption capacity for other hydrocarbons still contained in the exhaust gas. The combination of the two zeolites according to the invention leads to an optimum adsorption behaviour in the temperature range concerned. The invention is not, however, confined to a mixture only of said two zeolites. Other zeolite mixtures may also be used if their components fulfil the requirements regarding temperature dependence of the adsorption capacity and pore diameters.

The high Si/Al ratio of the zeolites preferably used provides, on the one hand, high selectivity of the adsorption of hydrocarbons compared with water and, on th e other hand, good temperature stability to 1000 0 C and above, and good acid stability.

The temperature stability is necessary for the exhaust gas purification system according to the invention since the adsorber is placed near the engine and is thus exposed to high temperatures in operation.

The catalyst system downstream of the hydrocarbon adsorber may comprise a 15 three-way catalyst or a combination of oxidation, reduction and/or three-way catalysts in one or more beds.

aiy: Such catalysts and the preparation thereof are known to the expert. They usually comprise a support in the form of an open-cell honeycomb body made of 2 ceramic or metal. In order to accept catalytically active noble metals, said honeycomb 20 bodies are provided with an activity-increasing, high surface area oxide dispersion coating made of, for example, y-aluminium oxide in a quantity of 100 to 400 g, usually 160 g per litre of honeycomb body volume. The catalytically active noble metals may be deposited on said oxide coating by impregnation. In the case of oxidation catalysts, platinum and/or palladium are used in preference. Three-way catalysts contain 25 platinum and/or palladium and/or rhodium as catalytically active noble me~ils.

0 67 IN:\LIBTTIO074B:BFD

Y

In an exhaust gas purification system comprising an adsorber, oxidation catalyst and three-way catalyst, the loading of the oxidation catalyst with platinum and/or palladium compared with the loading of conventional oxidation catalysts of 0.01 to 1.8 g per litre of catalyst volume is at least doubled to at least 3.5 g of platinum and/or palladium per litre of catalyst volume. In preference, the loading should be 7 g per litre or more. Loadings with more than 10, or more than 20 g of noble metal per litre of catalyst volume are particularly effective. The oxidation catalyst is placed immediately after the adsorber. Said high loading with the catalytically active elements leads to a reduction in the light-off temperature by about 50 to 100 0 C compared with normally loaded catalysts.

If the catalyst system downstream of the adsorber is composed only of a threeway catalyst with the platinum group metals platinum and/or palladium and/or rhodium, the loading with platinum and/or palladium compared with the loading of conventional catalysts of 0.01 to 1.8 g per litre of catalyst volume can also be at least doubled with 15 said catalyst to at least 3.5 g of platinum and/or palladium per litre of catalyst volume oe in order to reduce the light-off temperature for the conversion of hydrocarbons.

The adsorber may be used as bulk material in the form of pellets, extruded •pieces or agglomerates. The use of the adsorber in the form of a dispersion coating on a monolithic honeycomb body in a quantity of 100 to 400 g per litre of honeycomb body is, however, preferred. The actual quantity of coating to be used depends on the hydrocarbon emissions of the combustion engine to be detoxified. The optimum quantity may be determined by any expert with few tests.

i. The dispersion coating is deposited on the honeycomb body, for example, by immersing the honeycomb body in an aqueous dispersion of the adsorber mixture 25 followed by blowing out excess dispersion, drying and, if necessary, calcining to fix the coating. In order to apply the desired quantity of adsorber, said coating may be repeated many times, if necessary.

A further possibility consists in an exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines, which contains a hydrocarbon adsorber in immediate contact with an oxidation catalyst, and a downstream three-way catalyst in one or more beds. This exhaust gas purification [N:\LIBTT00OO748:BFD I -9system is characterised in that the difference between the light-off temperature TA of the oxidation catalyst for the conversion of the hydrocarbons and the desorption temperature TD of the adsorber in immediate contact with the oxidation catalyst is less than 50 0 C, ie, TA-T D 50 0 C The immediate contact of the adsorber with the oxidation catalyst may be realised in the form of superimposed coatings on a monolithic honeycomb body, the adsorber coating lying on the catalyst coating.

The statement already made above applies to the choice and design of adsorber mixture and catalyst. Apart from reduced hydrocarbon emission, said exhaust gas purification systems also exhibit a substantially reduced carbon monoxide emission during the cold start phase.

Description of the Drawings 15 The present invention will now be described, by way of example only, with :reference to the accompanying drawings, wherein: Figure 1: Hydrocarbon emission of a combustion engine with exhaust gas purification system according to comparative example 3a during the cold start phase of the US test.

Figure la: Diagrammatic representation of the structure of the exhaust gas purification system, S 25 Figure 2: Hydrocarbon emission of a combustion engine with exhaust gas purification system according to comparative example 3b during the cold start phase of the US test.

Figure 2a: Diagrammatic representation of the structure of the exhaust gas purification system.

Figure 3: Hydrocarbon emission of a combustion engine with exhaust gas purification system according IN:\LIBTT)00748:BFD -a 9 1 ~-113-~11 to example 3 during the cold start phase of the US FTP-75 test.

Figure 3a: Diagrammatic representation of the structure of the exhaust gas purification system.

Example 1 Adsorption properties of zeolite Y and The adsorption capacity of a DAY zeolite (dealuminised Yzeolite) with an Si/Al ratio of >100, and of two zeolites with the Si/Al ratios of >500 and 58 was determined for toluene at 20 and 80 OC. The results are shown in Table 1.

Table 1 Adsorption capacity of a DAY zeolite and of T[oC] Zeolite Si/Al MToluene [g/100 g] DAY >100 15.1 ZSM5 >500 6.2 20 ZSM5 58 7.1 80 DAY >100 0.8 *4*o44 80 ZSM5 >500 1.4 ZSM5 58 2.2 The data in Table 1 are applicable to a toluene concentration of 1 g/m air. "Toluene is the amount of Sadsorbed toluene which is in equilibrium with the surrounding atmosphere at the temperature given in each case, in grammes of toluene per 100 g of zeolite. Table 1 shows very clearly the different adsorption behaviour of the DAY zeolite and of zeolite ZSM5. Whilst the DAY zeolite exhibits an excellent adsorption capacity for toluene at low temperature, which falls very quickly, hoi:ever, as the temperature rises, the corresponding curve for the ZSM5 zeolite is much flatter. Even at 80 oC, zeolite ZSM5 is superior to the DAY zeolite. A mixture of 11 both zeolites gives a more uniform adsorption behaviour over a fairly large temperature range.

Example 2 The light-off temperatures TA for the conversion of hydrocarbons by palladium-oxidation catalysts with differing palladium loading and of a standard platinum/rhodium three-way catalyst in the fresh and aged state at space velocities of 75,000 h 1 and 60,000 hI and with an air ratio lambda of 1.15 were measured.

The catalysts were composed of the relevant oxide dispersion coating of 160 g y aluminium oxide per litre on ceramic honeycomb bodies made of cordierite and the catalytically active noble metals precipitated thereon.

The honeycomb bodies had a cell density of 62 cells per cm 2 The light-off temperatures are listed in Table 2.

20 Table 2 Light-off temperatures of various catalysts; lambda 1.15 Loading TA Space velocity [g Pd/l] Fresh Aged 3.53 226 237 75,000 5.30 227 232 75,000 7.06 220 235 75,000 10,59 219 220 75,000 191 209 60,000 40 189 204 60,000 5 Pt/l Rh standard three-way catalyst 1.41 g/l 251 286 60,000 In order to measure the light-off temperatures of the aged catalysts, they were operated for a period of 100 hours on the engine at exhaust gas temperatures before the catalyst of 850 In view of the heat of reaction, this leads to temperatures in the catalyst bed of 1000 OC.

The palladium-oxidation catalysts of ~Table 2 have a much lower light-off temperature for the conversion of hydrocarbons than the standard three-way catalyst. The high ageing stability of their light-off temperature, which can be attributed to the high palladium loading, is also worth noting. Table 2 also shows that the light-off temperature of the palladium-oxidation catalyst falls considerably with increasin ,alladium loading.

The light-off temperatures of said highly loaded palladium S"oxidation catalysts, with values of below 237 OC, lie just 15 above the typical desorption temperatures of the adsorbers and exhibit light-off temperatures at about 200 OC with very high loadings. They are capable of converting directly the hydrocarbons desorbing from the adsorber at S" about 200 0 C without the complex exhaust gas circuits known S 20 from the state of the art. The standard three-way catalyst is not able to do so because of its high light-off .oeooi temperature, particularly in the aged state.

*The hydrocarbon emissions of a motor vehicle with an Otto 25 motor (Mercedes 300 E; capacity: 3 i, power: 162 kW) were measured below during the cold start phase for different exhaust gas purification systems in accordance with examples 3 and the comparative examples 3a and 3b. The results of the residual emission measurements according to the US FTP-75 test are summarised in table 3.

The exhaust gas purification systems comprised in each case 3 successive honeycomb bodies made of cordierite with 62 2 cells per cm 2 The honeycomb body on the engine side had a length of 154 mm and a volume of 1.8 i. The two rear honeycomb bodies each had a length of 102 mm and a volume of 1.2 i.

Said honeycomb bodies were coated as follows for comparison of an exhaust gas purification system according to the invention with conventional systems: Comparative example 3a (Fig. 1, la) 1.-3 Honeycomb body: Coating with a standard three-way catalyst according to example 2; aged r r e s s r s ssrss s r Comparative example 3b (Fig. 2, 2a) 1. Honeycomb body: Coating with 100 g/l of DAY zeolite (Si/A1 100) Coating with a standard three-way catalyst according to example 2; aged 2.-3 Honeycomb body: o Gooo

I..

o s ss soss D s s Example 3 (Fig. 3, 3a) 1. Honeycomb body: The first honeycomb body was replaced by two partial bodies.

The partial body 52 mm long on the engine side received a coating of 100 g/l of DAY zeolite (Si/Al 100). The second partial body was coated with an oxidation catalyst with 7 g of Pd per litre of honeycomb body volume. These coatings, too, were aged before the exhaust gas tests were carried out.

14 2.-3 Honeycomb body: Coating with a standard three-way catalyst according to example 2; aged.

Figures 1 to 3 show the emission curves for hydrocarbons during the first 250 s after the start when various exhaust gas purification systems are used (Figures !a 3a). The given hydrocarbon concentrations relate to an exhaust gas diluted to one tenth by air in accordance with the US FTP- 75 test specification.

Figure 1 shows that the aged three-way catalysts (according to Figure la) begin after about 50 seconds to convert the pollutants in the exhaust gas. At this point in time, the 15 temperature in the exhaust gas before the catalysts is 300 OC. Figure 2 shows the same relationships as Figure 1, but with an adsorber with a DAY zeolite upstream of the three-way catalyst in the exhaust gas purification system (Fig. 2a). The desorption of hydrocarbons by the adsorber S 20 begins after only about 30 seconds at a temperature of about 200 OC in front of the adsorber. The three-way oeeoe catalysts are not yet, however, capable of converting the S"majority of the desorbing hydrocarbons. Figure 3, in contrast, shows a marked decrease in the residual emission as a result of combining the adsorber with an oxidation catalyst highly loaded with palladium in conjunction with the two standard three-way catalysts (Fig. 3a). The hatched area in Figure 3 represents the reduction in hydrocarbon emission of the exhaust gas purification system according to the invention according to example 3 in comparison with the conventional exhaust gas purification system according to comparative example 3b.

The residual emission measurements of the exhaust gas purification systems according to comparative example 3b and according to example 3 are shown in Table 3. As these measurements show, the use of a highly loaded palladium oxidation catalyst in combination with hydrocarbon adsorber and standard three-way catalysts ha- a positive effect on the residual emissions of the exhaust gas system. During the cold start phase, not only are the hydrocarbon emissions reduced by 30%, but also the emission of carbon monoxide. Said positive effect also remains throughout the entir3 test.

Table 3 Residual emission measurement according to US Exhaust gas system Content of the first bag in g/miles (Cold start phase) CO HC NOx according to comparative example 3b*) 3.00 0.51 0.83 according to example 3 2.05 0.37 0.89 Exhaust gas system Total emission in g/miles CO HC NO x Saccording to Scomparative example 3b 1.29 0.22 0.46 S. according to example 3 0.48 0.10 0.38 Comparative example

Claims

1. An exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines, comprising: a hydrocarbon adsorber; and a downstream catalyst system comprising a three-way catalyst or a combination of oxidation, reduction and/or three-way catalysts in one or more beds, wherein: said downstream catalyst system contains an oxidation catalyst with platinum and/or palladium and a three-way catalyst with platinum and/or palladium and/or rhodium, wherein the loading of said oxidation catalyst with platinum and/or palladium is at least 3.5 g of platinum and/or palladium per litre of catalyst volume; said oxidation catalyst is placed immediately after said adsorber; and wherein the difference between the light-off temperature of the oxidation catalyst for conversion of hydrocarbons and the desorption temperature of the adsorber is less than 0 C. 15

2. The exhaust gas purification system according to claim 1, wherein said hydrocarbon adsorber comprises a mixture of a dealuminised Y zeolite with a Si/Al ratio of more than 40 and a zeolite ZSM5 with a Si/Al ratio of more than 20, and wherein dealuminised Y zeolite and zeolite ZSM5 are present in the adsorber in a weight ratio to one another of 1:10 to 10:1 and wherein said hydrocarbon adsorber is 20 present in the form of a coating on a monolithic honeycomb body in a quantity of 100 to 400 g per litre of honeycomb body volume.

3. An exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines, comprising: a hydrocarbon adsorber; and 25 a downstream catalyst system comprising: a three-way catalyst in one or more beds, wherein said three-way catalyst comprises the platinum group metals platinum and/or palladium and/or rhodium in an amount of at least 3.5 g of platinum and/or palladium per litre of catalyst volume; the difference between the light-off temperature of the oxidation catalyst for conversion of hydrocarbons and the desorption temperature of the adsorber is less than 0 C.

4. The exhaust gas purification system according to claim 3, wherein said hydrocarbon adsorber comprises a mixture of a dealuminised Y zeolite with a Si/Al ratio of more than 40 and a zeolite ZSM5 with a Si/Al ratio of more than 20, and wherein dealuminised Y zeolite and zeolite ZSM5 are present in said adsorber in a weight ratio to one another of 1:10 to 10:1, and wherein said hydrocarbon adsorber is present in the form of a coating on a monolithic honeycomb body in a quantity of 100 ,j to 400 g per litre of honeycomb body volume.

IN:\LIBTTI00748:BFD -17- An exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines, comprising: a hydrocarbon adsorber in immediate contact with an oxidation catalyst and a downstream three-way catalyst in one or more beds, wherein: said oxidation catalyst and adsorber are present in the form of superimposed coatings on a monolithic honeycomb body, the adsorber coating lying on the catalyst coating and the oxidation catalyst contains platinum and/or palladium in an amount of at least 3.5 g platinum and/or palladium per litre of catalyst volume; and wherein the difference between the light-off temperature of the oxidation catalyst for conversion of hydrocarbons and the desorption temperature of the adsorber is less than 500C.

6. The exhaust gas purification system according to claim 5, wherein said hydrocarbon adsorber contains a mixture of a dealuminised Y zeolite with a Si/Al ratio of more than 40 and a zeolite ZSM5 with a Si/Al ration of more than 20, and 15 dealuminised Y zeolite and zeolite ZSM5 are present in the adsorber in a weight ratio *to one another of 1:10 to 10:1. S"

7. An exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines substantially as hereinbefore described with reference to Figures 3 and 3a.

8. An exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines substantially as hereinbefore described with reference to the Examples, excluding the comparative examples. :DATED this Twentieth Day of November 1995 Degussa Aktiengesellschaft Patent Attorneys for the Applicant SPRUSON FERGUSON \B 0 F IN:\LIBTTIOO748:BFD ABSTRACT Exhaust Gas Purification System for Reducing Hydrocarbon Emissions During the Cold Start of Combustion Engines An exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines is disclosed. The system comprises a hydrocarbon adsorber and a downstream catalyst system composed of a three-way catalyst or a combination of oxidation, reduction and/or three-way catalysts in one or more beds. The catalyst system downstream of the hydrocarbon adsorber contains an oxidation catalyst with platinum and/or palladium and a three-way catalyst with platinum and/or palladium and/or rhodium, wherein the loading of the oxidation catalyst with platinum and/or palladium is at least 3.5 g of platinum and/or palladium per litre of catalyst volume, and the oxidation catalyst is placed immediately after the adsorber. *ft *e 0 6o f t o I *to a 0 [N:\LIBTTj00748:BFD