CA2103647C

CA2103647C - A catalytic automotive emission control process with improved cold-start behavior

Info

Publication number: CA2103647C
Application number: CA002103647A
Authority: CA
Inventors: Dieter Lindner; Egbert Lox; Bernd Engler; Klaus Ostgathe
Original assignee: Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 1992-08-10
Filing date: 1993-08-09
Publication date: 1999-03-23
Anticipated expiration: 2013-08-09
Also published as: ZA935775B; FI933511A0; UA27233C2; HU212759B; NO932831L; AU664911B2; MX9304814A; TW322428B; AR247829A1; CA2103647A1; NO932831D0; JPH06154538A; DK0582971T3; DE59307696D1; ATE160295T1; DE4226394A1; KR940003601A; CN1084611A; ES2109401T3; FI104750B

Abstract

A considerable reduction in hydrocarbon emissions during the cold start of internal combustion engines can be achieved if the actual exhaust gas catalyst is preceded in the exhaust pipe by a hydrocarbon adsorber of which the heat capacity is made greater than the heat capacity of the following catalyst through a suitable choice of material and configuration of the adsorber.

Description

2~3~7 This invention relates to a catalytic automotive emission control process with improved hydrocarbon suppression during the cold start phase using a threeway catalyst known per se and a hydrocarbon adsorber which is arranged before the catalyst in the exhaust gas stream and which, after cold starting, adsorbs hydrocarbons present in the exhaust gas until the three-way catalyst has reached its full operating temperature and efficiency and only desorbs them to the exhaust gas after heating so that the desorbed hydrocarbons can be converted into harmless components by the now relatively active three-way catalyst.

The fu,~ure limits for automotive pollutant emissions are laid down in the regulations TLEV/1994 and LEV/1~97 (LEV =
low emission vehicle). They represent a significant tightening of the limits, particularly for hydrocarbons.
Since modern automotive emission control catalysts have reached a high level of pollutant conversion in their operationally warm state, it will only be possible to keep to future limits by improving pollutant conversion during the cold start phase. This is be~ause a large part of the total pollutants released is emitted during the cold start phase of the legally stipulated test cycles (e.g. US FTP 75), because in this phase the catalysts have not yet reached the operating temperature of 300 to 400~C required for conversion of the hydrocarbons.

Emission control systems consisting of a hydrocarbon adsorber and a following catalyst have already been proposed with a view to reducing emissions during ~,~

the cold start phase. The function of the hydrocarbon adsorber in these systems is to adsorb the hydrocarbons - present in the exhaust gas at the relatively low temper-atures prevailing during the cold start phase. It is only after fairly significant heating of the adsorber that the hydrocarbons are desorbed and pass with the now relatively hot exhaust gas to the catalyst already close to its operating temperature where they are effectively converted into harmless water and carbon dioxide. One of the key requirements which the adsorber is expected to satisfy is that it should be able to adsorb hydrocar-bons preferentially to the steam already present in abundance in the exhaust gas.
The disadvantage of this known solution is that the desorption of the hydrocarbons actually begins at temperatures around 250~C so that optimal conversion still cannot take place on the following catalyst. In addition, the adsorber is in danger of being destroyed by heat because it has to be installed in the exhaust system near the engine and, accordingly, is exposed to temperatures of up to 1000~C in long-term operation.
Numerous proposals have been put forward in the patent literature with a view to overcoming this problem, for example in DE 40 08 789, in EP 0 460 542 and in US
5,051,244. These documents also start out from a combination of a hydrocarbon absorber and a catalyst, but propose elaborate circuits for the exhaust gas to overcome the described disadvantages.
Thus, according to US 5,051,244, the actual catalyst is preceded by a zeolite adsorber which adsorbs the pollutants, particularly hydrocarbons, in the exhaust gas in the cold state and releases them again with increasing heating of the exhaust system. The adsorber is protected against destruction by overheating in long-term operation of the engine by switching on a _ 21~3~

short circuit line from the engine directly to the catalyst.

During the first 200 to 300 seconds after starting, the entire exhaust gas is passed over the adsorber and the catalyst. In this operational phase, the hydrocarbons are taken up by the adsorber. The adsorber and the catalyst are increasingly heated by the hot exhaust gas. The adsorber is short-circuited if, through an increase in temperature, desorption begins to overtake adsorption. The exhaust gas then flows directly over the catalyst. On reaching the operating temperature, part of the hot exhaust gas is passed over the ad~rber until the pollutants have been completely desorb~d 50 that they can then be efficiently converted by the catalyst. After desorption, the adsorber is short-circuited again so that it is protected against thermal overloading.

A Y-zeolite with an Si to Al atomic ratio of at least 2.4 is pxoposed as adsorber in US 5,051,244. The zeolite adsorber may contain fine-particle catalytically active metals, such as platinum, palladium, rhodium, ruthenium and mixtures thereof.
These solutions known from the prior art are technically very complex, expensive and susceptible to damage. The present invention provides a process by which the disadvantages known from the prior art are eliminated or at least mitigated and ensures very good hydrocarbon suppression during the cold start phase.

2103~7 More particularly, there is provided a catalytic automotive emission control process with improved hydrocarbon suppression during the cold start phase using a three-way catalyst known per se and a hydrocarbon adsorber which is arranged before the catalyst in the exhaust gas stream and which, after cold starting, adsorbs hydrocarbons present in the exhaust gas until the three-way catalyst has reached its full operating temperature and efficiency and only releases them to the exhaust gas after heating so that the desorbed hydrocarbons can be converted into harmless components by the now relatively active three-way catalyst. The process according to the invention is characterized in that the hydrocarbon adsorber has a greater specific heat capacity than the following three-way catalyst.

In the context of the invention, the specific heat capacity is not the specific heat of substances, but rather the heat capacity per unit volume of the adsorber or catalyst element. Adsorbers or catalysts may be present in the form of loose layers of granules, extrudates or pellets or in the form of monolithic foams or honeycombs. To calculate the specific heat capacity in the context o~ the invention, the heat capacity of these layers or monoliths is based on the geometric volume of the layers and elements, including all voids and pores. Accordingly, the specific heat capacity is not a material parameter in the physical sense, rather it is dependent upon the macroscopic form and the microscopic structure of the adsorber or catalyst. Accordingly, possibilities for influencing the specific heat capacity are available to the expert in the choice of the material for the adsorber or catalyst and in the processing and geometric configuration and, accordingly, leave the expert with considerable scope for carrying out the present invention.

~1~3~

The specific heats of a few materials suitable as starting materials for the manufacture of supports for 5 adsorber and catalyst coatings are mentioned by way of example below:

Table 1 Material Density Specific heat [g.cm 3] tJ-g-l.K-l]

Alpha-aluminium oxide 3.97 1.088 Mullite 2.80 1.046 Zirconium oxide, stabil. 5.70 0.400 Stainless steel, highly alloyed (20Cr; 7Ni) 7.86 0.544 As Table 1 shows, the specific heats of suitable materials cover a range of 0.400 to 1.088 J.g l.K 1, Taking into account the density of the particular materials, the expert is left - solely through the choice of material - with a design scope in regard to speci~ic heat capacity of the order of 1:2. ~his range can be further broadened by corresponding geometric configuration (different wall thicknesses) and the incorporation of porosities.

With increasing specific heat capacity of the hydrocarbon adsorber compared with that of the following three-way catalyst, heating up is delayed in relation to that of the catalyst. Accordingly, the adsorber retains its adsorption capacity for a longer period and only releases the adsorbed hydrocarbons after a certain time. If the ratio of the specific heat capacity of the hydrocarbon adsorber to that of the three-way catalyst is selected from values of 1.1 21~3~7 to 3.0:1 and preferably from values of 1.5 to 3.0:1, the effect of the delayed heating of the adsorber is that desorption of the hydrocarbons only begins when the following catalyst has almost reached its full effectiveness in regard to the conversion of hydrocarbons. For example, if the hydrocarbon adsorber has double the specific heat capacity of the following catalyst, the adsorber heats up only half as quickly as the catalyst for substantially the same energy input.

One particularly favorable embodiment of the process according to the invention is characterized in that the hydrocarbon adsorber and the three-way catalyst are monolithic honeycomb supports of which the heat capacities are 1.10 to 3.0:1 and to which hydrocarbon-adsorbing coatings or catalytically active coatings are applied in known manner.

A temperature-stable dealuminized Y-zeolite with an Si to Al ratio of greater than 50 and preferably greater than 100 is preferably used as the hydrocarbon adsorbex, being applied in a quantity of 100 to 400 g per liter honeycomb volume. A Y-zeolite such as this shows high temperature stability and does not lose its adsorption properties, even after repeated heating to around 1000~C - the operating temperature to be expected in the vicinity of the engine. In addition, an adsorber of this type shows selective adsorption behavior for hydrocarbons, i.e. it adsorbs hydrocarbons preferentially to the steam also present in the exhaust gas.

The emission of hydrocarbons during the cold start phase can be further reduced if the adsorber itself shows catalytic properties. These can be achieved by providing the coating of hydrocarbon adsorber with additional amounts of a typical 2~3~7 6a catalytically active coating. Typical catalytically active coatings usually contain large-surface carrier oxides, such as for example lattice-stabilized or pure aluminium oxide of the transition series, doped or pure cerium oxide and doped or pure zirconium oxide. Catalytically active metal components from the group of platinum metals are applied to these carrier oxides. The ratio by weight between zeolite adsorber and carrier oxides in the coating should be from 4:1 to 1:2. The catalytically .,~,~

active components from the group of platinum metals should be finely distributed over all oxidic parts of the coating except for the zeolite adsorber. Preferred platinum metals are platinum, palladium and rhodium.
A coating of the type in question may be obtained by initially preparing a coating dispersion of zeolite, aluminium oxide, cerium oxide and zirconium oxide to which the catalytically active metal components are added in the form of their precursors, such as for example nitrates or chlorides. It is known that these precursors are deposited preferentially onto aluminium oxide, cerium oxide and zirconium oxide, but not onto zeolite. The monolithic support is coated with this dispersion by methods known per se, dried, calcined and optionally activated in a hydrogen-containing gas stream at temperatures around 600CC.
Even better separation of the noble metal com-ponents from the zeolites is obtained if a mixture of aluminium oxide, cerium oxide and zirconium oxide is first precoated with the noble metals in a separate impregnat~ng step and the final coating dispersion of zeolite and the other oxide components is only prepared after the noble metals have been fixed on those com-ponents by calcination.
2S In one particularly preferred embodiment of the invention, the support is coated with two different layers. The first layer is a catalyst coating of large-surface carrier oxides and catalytically active metal components from the group of platinum metals. The actual hydrocarbon adsorber coating is then applied to that coating. The quantities of coating to be applied range from 50 to 200 g per liter support volume for both layers.
Ceramic monoliths of cordierite or mullite are preferably used as supports for the hydrocarbon adsorber 2~03~7 and the three-way catalyst. Other suitable materials for the supports are zirconium mullite, alpha-aluminium oxide, sillimanite, magnesium silicates, petalite, spodumene, aluminium silicates, etc. or even stainless steel. The ratios between the specific heat capacities of these supports must correspond to 1.10 to 3.0:1. The data of a few commercially obtainable ceramic monoliths are set out in Table 2. The monoliths in question are honeycombs with a diameter of 93 mm and a length of 152.4 mm, with various wall thicknesses and with a cell interval of approximately 1.28 mm.

Table 2 Monolith Wall Specific Weight Heat thickness heat capacity tmm] [J g~l K~l] tg] [J K 1]

A 0.16 0.850 351 299 B 0.16 0.845 440 372 c 0.16 0.843 471 397 D 0.14 0.862 427 368 E 0.16 0.836 448 375 F 0.19 0.824 581 479 G 0.25 0.836 706 590 In another embodiment of the process according to the invention, the support for the hydrocarbon adsorber is a ceramic monolith while the support for the catalyst is a heatable metal monolith. The ratio between the heat capacities of the hydrocarbon adsorber and the three-way catalyst must again satisfy the conditions of 1.10 to 3.0:1.

The hydrocarbon emissions during the cold start phase can be further reduced in this way because the delayed desorption of the hydrocarbons is combined with accelerated heating of the catalyst.

The invention is further illustrated by the following Examples.
Example 1 The adsorption and desorption properties of an adsorber coating on various ceramic honeycombs of cordierite were compared with one another. The adsorber coating consisted of dealuminized Y-zeolite with an Si to Al ratio above 100.
This coating was applied in a quantity of 100 g/l honeycomb.
The adsorbers had a cell density of 68 cellsjcm2 , a diameter of 25.4 mm and a length of 152.4 mm. A total of three different ceramic honeycombs (A, C, G according to the above Table) with mass ratios of 1:1.34:1.98 and corresponding ratios between their specific heat capacities were used.
Testing was carried out in the model gas under the following conditions:

- Gas mixture before the adsorber 200 ppm toluene, rest N2 - Temperature heating of the gas mixture from So to 200~C at 10 K/min.
- Gas flow rate 1550 Nl/h - Measured quantities temperature before and after the adsober; toluene concen-tration after the adsorber The key results are set out in Table 3 which shows the quantities of toluene in percent released by the particular adsorber at the temperatures indicated, based on the quantities of toluene desorbed from the lightest adsorber at those temperatures.

Table 3 Monolith Temperature before the adsorber 120~C150~C 180~C 200~C

A 100%100% 100% 100%
C 53%95% 100% 100%
G 53% 83% 91~ 92%

During the test, a total of only 53~ of the quantity of toluene released by the monolith A up to a temperature of 120-C is found, for example, behind the monoliths C and G up to the temperature of 120~C. In conjunction with a following catalyst having a smaller specific heat capacity, this delayed release leads to a considerably improved hydrocarbon ConVersion in the cold start phase.

Example 2 The heating-up behavior of monoliths C and G was studied in a second test. To this end, air heated to 400~C was passed through the monoliths under flow conditions typical of 3~ automotive emission control catalysts. The increase in temperature as a ~unction ~f time at the exit end of the monoliths was measured by thermocouples. The results obtained are shown in Table 4 where the time taken to reach a certain temperature is shown in seconds. The slower heating-up behavior of the heavier monolith G can clearly be seen andleads to an adequate delay in its desorption behavior where 21~647 it is combined with a following catalyst on a ceramic monolith of the A type.

Table 4 Temperature after the adsorber MonolithlOO~C 150~C 200~C 250~C
C 18s 25s 32s 40s G 25s 34s 42s 55s

Claims

1. A method for hydrocarbon emission reduction during a cold start phase of an internal combustion engine using a three-way catalytic converter and a hydrocarbon adsorber arranged upstream of the catalytic converter in an exhaust gas stream, wherein the catalytic converter and the hydrocarbon absorber each have a specific heat capacity with reference to volume, and in which the hydrocarbon adsorber has a greater specific heat capacity per unit volume than the downstream catalytic converter.

2. A method as defined in claim 1, in which the specific heat capacity of the hydrocarbon adsorber is greater than the specific heat capacity of the catalytic converter by a factor of 1.1 to 3Ø

3. A method as defined in claim 2, wherein the factor is 1.5 to 3Ø

4. A method as defined in claim 1, 2 or 3, in which the hydrocarbon adsorber and the catalytic converter are monolithic supports with a honeycomb structure onto which are applied hydrocarbon-adsorbing or catalytically-active coatings, respectively.

5. A method as defined in claim 4, wherein the hydrocarbon adsorber is a thermally-stable dealuminized Y-zeolite with an Si to Al ratio greater than 50, which is applied in an amount of 100 to 400 g per liter of the honeycomb structure volume.

6. A method as defined in claim 5, in which the Si to Al ratio is greater than 100.

7. A method as defined in claim 5 or 6, in which the coating of hydrocarbon adsorber also contains additional proportions of a conventional catalytically-active coating of high surface area support oxides and catalytically-active active metal components from the group of platinum metals applied thereto, containing lattice-stabilized or pure aluminium oxide from the transition series, doped or pure cerium oxide and doped or pure zirconium oxide, and in which the zeolite hydrocarbon adsorber and the high surface area support oxides in the coating are present in a ratio by weight to one another of 4:1 to 1:2, and in which the catalytically-active metal components are finely distributed on the carrier oxides, but not on the zeolite hydrocarbon adsorber.

8. A method as defined in claim 4, in which said monolithic support of the hydrocarbon adsorber has a catalytically-active coating of high surface area support oxides and catalytically-active metal components from the group of platinum metals applied thereto, and in which a further coating of a hydrocarbon adsorber is applied to this coating, and wherein for both said coatings an amount of coating of 50 to 200 g per liter of support structure volume is used.

9. A method as defined in claim 8, wherein the hydrocarbon adsorber is a thermally-stable dealuminized Y-zeolite with an Si to Al ratio greater than 50.

10. A method as defined in claim 9, in which the Si to Al ratio is greater than 100.

11. A method as defined in any one of claims 4 to 10, wherein the monolithic support structure of the hydrocarbon adsorber and the catalytic converter are ceramic monoliths of cordierite or mullite.

12. A method as defined in any one of claims 4 to 10, wherein the monolithic support structure of the hydrocarbon adsorber is a ceramic monolith, and the support of the catalytic converter is a heatable metal monolith.

13. A method as defined in any one of claims 1 to 12, in which the specific heat capacity of the catalytic converter has a value between 0.29 J/kcm3 and 0.57 J/kcm3.