DE19743850A1 - Exhaust gas purification catalyst with improved sales for hydrocarbons - Google Patents

Description

The invention relates to an exhaust gas purification catalyst improved sales for hydrocarbons, which as catalytically active components platinum group metals contains and two applied on a support body has mutually lying functional layers.

Internal combustion engines emit pollutants through suitable emission control catalysts to harmless Connections must be implemented. The main pollutants are carbon monoxide, nitrogen oxides and unburned carbon hydrogen. They can contain platinum group metal Catalysts for water, carbon dioxide and nitrogen be implemented. A distinction is made between three-way catalysts, Oxidation and reduction catalysts. The The boundary between these catalysts is fluid and depends in particular also on the exhaust gas composition.

Three-way catalysts are able to do all three mentioned Pollutants under stoichiometric exhaust gas conditions to implement at the same time. Here, the nitrogen oxides with simultaneous oxidation of carbon monoxide and Hydrocarbons reduced to nitrogen. Three Weg catalysts contain essential catalytic ones Rhodium component in combination with platinum and / or Palladium. Such a rhodium and palladium containing Catalyst is, for example, in DE 38 35 184 C2 described. The presence of rhodium in the three away catalyst is important for its reduction function. Catalysts containing only platinum and / or palladium, have only an unsatisfactory reductive effect and are therefore mainly used as oxidation catalysts used.  

Today's three-way and oxidation catalysts are in the Able to find the appropriate pollutants after reaching their Operating temperature with conversion rates of more than 70% too to implement harmless products. An essential one However, the cold start of Combustion engines. During the so-called cold start phase, which is about the first 100 seconds after the engine starts includes, the exhaust gas purification catalyst is still cold and therefore inactive. The exhaust gas temperature warms up the catalyst. The increasing activity of the catalyst for the implementation of pollutants by the lightning characterized the temperature of the respective pollutant. It is the temperature at which the corresponding pollutant is implemented to 50%. The Starting temperatures of modern catalysts are in the Temperature interval between 200 and 400 ° C.

The main pollutant component during the Cold start phases are unburned hydrocarbons. To assess the cleaning effect of exhaust gas cleaning Various test methods have been developed for catalysts. A frequently used test method is the so-called FTP 75 exam developed in the USA. In Europe the ECE test procedure is standardized.

The FTP 75 exam spans a period of 2500 seconds after the cold start of the internal combustion engine and is divided into three sections. The one in these three Sections of pollutants emitted are in three bags collected and then analyzed. The first paragraph includes the actual cold start phase and is after 500 Seconds complete. The following two sections simulate changing load conditions and a hot start.

For the final assessment of an emission control system systems, which consists of several catalysts and adsorbers can exist, are collected in all three bags  Quantities of pollutants used. It has been shown that significant improvements in the overall assessment especially through improvements in pollutant conversion can be reached during the cold start phase. The Conversion rates of the warm catalysts only allow still minor improvements to the Overall assessment according to the FTP 75 test method hardly can influence. This is how an internal combustion engine emits during the first 100 seconds after the cold start two thirds of the total during the FTP 75 exam amount of hydrocarbon emitted.

To reduce these hydrocarbon emissions different combinations of hydrocarbon adsorber proposed with catalysts.

US 5,078,979 describes a method for exhaust gas cleaning, in which the exhaust gas is first Hydrocarbon adsorber and then one Catalyst is passed. The contained in the cold exhaust gas Hydrocarbons are absorbed by the adsorber this has reached a temperature of about 150 ° C. The hydrocarbons begin above this temperature desorb again from the adsorber and then from that already warmer catalyst to harmless products implemented. Molecular sieves (zeolites) are used as adsorbers suggested that the hydrocarbons preferred before Exhaust gas also absorb water vapor and have a high temperature stability.

A disadvantage of such a system is the fact that the adsorber itself heat the exhaust gas during heating phase removed, which is missing the downstream catalyst, so that it is slower than without upstream Warmed adsorber. The combination of spatially separated Adsorber and catalyst reduce the emission of Hydrocarbons during the cold start phase, in the  Cut overall assessment using the FTP 75 test method however, they perform worse than a single three away catalyst because the hydrocarbon initially adsorbed substances after the desorption temperature has been exceeded desorbed and not yet sufficiently active Catalyst to a large extent without implementation harmless components happen.

To improve this situation, the DE 42 39 875 A1 proposed an oxidation catalyst with a hydrocarbon adsorber on a support body combine. Oxidation catalyst and hydrocarbon adsorbers are in the form of superimposed coatings applied to the support body, the adsorber coating lies on the catalyst coating and comes into direct contact with the exhaust gas.

The oxidation catalyst contains platinum and / or palladium as catalytically active components. The adsorber contains a mixture of a dealuminated Y zeolite with a Zeolite ZSM5, the Y zeolite having an Si / Al ratio of over 40 and the zeolite ZSM5 an Si / Al ratio of over 20 has.

EP 0 716 877 A1 also suggests the combination of one Hydrocarbon adsorber with a catalyst in the form two superimposed coatings. The The catalyst coating lies on the adsorber coating and contains one or as catalytically active components several platinum group metals from the platinum group, Palladium, rhodium, ruthenium and iridium on aluminum oxide, cerium oxide and zirconium oxide. A is used as the adsorber material Zeolite with a weight ratio of silicon dioxide to Alumina used by more than 300. As a supporting body serves a monolithic honeycomb body made of cordierite parallel flow channels for the exhaust gas. The adsorbers coating is the first layer directly on the  Inner wall surfaces of the flow channels applied. Because of the poor adhesion of zeolite coatings on their Underlay, contains the adsorbent layer in addition to zeolite same amount of colloidal silicon dioxide as a binder. Because of this high proportion of colloidal material the danger that the pores of the zeolite are partially blocked and thus its adsorption capacity is deteriorated.

The proposed solutions are regarding their coals hydrogen suppression across all three sections of FTP 75 exam still unsatisfactory. The hydrocarbon adsorption during the first section satisfactory, but there are shortcomings in the dynamic pollutant conversion during section 2 and 3 of the FTP 75 exam. In particular, the Aging stability of pollutant conversion unsatisfactory.

The object of the present invention is therefore a Specify exhaust gas purification catalyst, which is an improved Hydrocarbon suppression across all three sections the FTP 75 test method with improved at the same time Has aging stability.

This task is accomplished by an exhaust gas purification catalytic converter solved which as a catalytically active components Contains platinum group metals and two on a supporting body applied superimposed functional layers having. This is the exhaust gas purification catalyst characterized in that the first functional layer Mixing a first catalyst with at least one Zeolites and those on the first functional layer applied second functional layer a mixture of one second catalyst with at least one zeolite included, the zeolites having a molar ratio of Have silicon dioxide / aluminum oxide of more than 40 and that as platinum group metals at least palladium and  Rhodium are present, separated from each other in each only one of the two catalysts are included.

As a functional layer in the context of this invention Understanding coating that have a specific effect on the Exhaust gas exerts. This can be the adsorption of Pollutant components or to have a catalytic effect or a combination of both. Both Functional layers of the exhaust gas purification catalytic converter contain at least for the adsorption of hydrocarbons a zeolite with a molar ratio of silicon dioxide / Alumina of more than 40. The adsorber material of the Functional layers themselves have no catalytically active ones Precious metal components, however, it is each with a Mixed catalyst. Adsorber effect and catalyst The effect is spatially narrow in the functional layers adjacent and thus guarantee optimal sales the desorbed again with increasing exhaust gas temperature Hydrocarbons. In the invention Exhaust gas purification catalytic converters therefore have an adsorber function acting zeolites no catalytically active components on their specific surface. This will make the otherwise frequently observed coking of the zeolites avoided.

The two catalysts of the functional layers point different catalytic effects. During the a catalyst using palladium and optionally platinum essentially an oxidative The catalyst shows the other effect Functional layer through the use of rhodium and optionally platinum has a reductive effect. Through the Arrangement of the two catalysts in different Layers are palladium and rhodium spatially apart Cut. This creates an alloy formation between Palladium and rhodium at high exhaust gas temperatures and the  deterioration associated with alloy formation of catalytic activity avoided.

The rhodium-containing reduction catalyst can either in the first or in the second one above Functional layer can be accommodated during the Palladium-containing oxidation catalyst in each another layer is arranged. The is preferred Reduction catalyst, however, in the second functional layer incorporated, which comes into direct contact with the exhaust gas. This allows the reduction catalytic converter to be in the exhaust gas optimally implement contained nitrogen oxides, which in the exhaust gas contained carbon monoxide and the hydrocarbons as well those during the heating of the adsorber material again desorbing hydrocarbons as reducing agents to serve.

In addition to palladium, rhodium and platinum, the catalysts of the functional layers can also contain other noble metals from the platinum group. The platinum group metals include ruthenium, rhodium, palladium, osmium, iridium and platinum. The catalysts contain active aluminum oxide as a carrier for the platinum group metals. Active aluminum oxide means aluminum oxides with specific surfaces (measured according to DIN 66 132) of more than 10 m ² / g. They have different crystal structures (gamma, delta, theta, kappa and eta aluminum oxide) which, when heated above 1150 K, change into the thermally stable alfa aluminum oxide (see Ullmann's Encyclopedia of Industrial Chemistry; 5th Edition; 1985; Vol. A1, 561-562).

The catalytically active platinum group metals are on the active alumina in a concentration of 0.1 to 10 % By weight based on the total weight of each Catalyst applied. For this, the active aluminum oxide in a separate manufacturing step with soluble  Impregnated precursors of the corresponding metals, dried and calcined. After this manufacturing step the platinum group metals lie on the aluminum oxide highly disperse form. Both catalysts can additional platinum group metal-free active aluminum oxide, Ceria and / or zirconia in an amount of 0 to 90 % By weight, based on the total weight of the respective Catalyst included.

A weight ratio of the respective is preferred Catalyst of a functional layer to those in it contained zeolites of 3: 1 selected. The weight However, ratio can be used to optimize the catalytic Effect of the exhaust gas purification catalyst in the area between 1: 5 and 5: 1 can be varied.

Zeolites are used as adsorber materials Molar ratios of more than 40 are used. Because of this high molar ratio preferentially adsorb the zeolites Hydrocarbons before water and have a high Temperature stability. Preference is given to the first Functional layer a zeolite ZSM5 with a molar ratio of more than 40 and in the second functional layer dealuminated Y zeolite with a molar ratio of more used as 100.

Can be used as a supporting body for the exhaust gas purification catalyst ceramic or metallic foams with open cells Pore structure or ceramic or metallic honeycomb body with parallel flow channels used for the exhaust gas will. These are supporting bodies that generally as a support body for exhaust gas catalytic converters be used. These support bodies are made with the help known method with the two functional layers coated. To do this, an aqueous one is used first Coating dispersion from the intended components of the relevant functional layer. Of the  Support body can, for example, by pouring over this Coating dispersion or by dipping into it Dispersion can be coated. Maybe after blowing out closed flow paths or flow channels the support body dried and then at temperatures calcined up to 500 ° C. The details here Techniques to be used are known to the person skilled in the art.

The functional layers are each in one Concentration from 10 to 300 grams per liter Carrier volume applied to the support body.

The exhaust gas purification catalyst according to the invention can alone or in combination with other catalysts in exhaust gas cleaning systems of internal combustion engines will.

To explain the invention, various exhaust gas purification catalysts were produced on ceramic carrier bodies. The supporting bodies were cordierite honeycomb bodies with a cell density (number of flow channels per cross-sectional area) of 62 cm ^-2 , a diameter of 11.84 cm, a length of 15.24 cm and a wall thickness of the flow channels of 0.2 mm. The volume of the support body was thus 1.68 l.

The cleaning effect of the catalysts was determined using the FTP 75 test procedure on a vehicle with 4 cylinders and 2 l displacement from the model year 1994 determined. The Ratio of installed catalyst volume to The displacement of the vehicle was 0.84.

The gas samples for the FTP 75 test were the exhaust system taken behind the test object.

Comparative Example 1

A single layer with the following composition was applied to one of the honeycomb bodies:
20 g / l La / Al ₂ O ₃ γ-aluminum oxide, stabilized with 2 to 4% by weight of lanthanum, calculated as lanthanum oxide
50 g / l La / Al ₂ O ₃ stabilized y-aluminum oxide with palladium and rhodium prefixed thereon in a weight ratio of 14: 1 and a concentration of 7% by weight, based on the total weight of the aluminum oxide plus palladium and rhodium
25 g / l cerium / zirconium mixed oxide (75% by weight cerium oxide, 25% by weight zirconium oxide)
25 g / l cerium oxide ex acetate
18 g / l zirconium oxide ex acetate
30 g / l dealuminated Y zeolite (molar ratio ≧ 200)
30 g / l zeolite ZSM5 (molar ratio ≧ 120).

The total load of the honeycomb body was thus 198 g / l, of which 3.5 g / l was palladium and rhodium.

For pre-fixing palladium and rhodium on one part of the stabilized alumina became the intended one Amount of alumina with an aqueous solution of Impregnated rhodium and palladium nitrate, at 120 ° C dried and then for a period of 4 hours 500 ° C calcined.

The powder thus obtained was used to prepare a Coating dispersion with the other components of the Catalyst dispersed in water and by grinding homogenized. After grinding, the average was Particle size of the solids of the coating dispersion about 6 µm. The solids content of the coating dispersion was 50% by weight.

The honeycomb body was immersed in the Coating dispersion coated, 10 minutes at 120 ° C dried and finally calcined at 300 ° C for 4 hours.  

Comparative Example 2

A single layer was applied to a second honeycomb body essentially the same composition as in Comparative Example 1 applied. In contrast to Comparative Example 1 was only palladium on the aluminum pre-fixed oxide. The necessary amount of rhodium was only added after coating by dipping the honeycomb body in an impregnation solution with rhodium nitrate in the coating brought in. To ensure direct contact between the rhodium and the Preventing palladium from becoming fixed became the impregnator Solution zirconium oxide sol for complexing the rhodium admitted. The finished impregnation solution contained 9% by weight Rhodium nitrate and 30 wt .-% zirconium oxide sol, each related on the total weight of the impregnation solution. After The catalyst was impregnated at 120 ° C. for 6 hours dried and then calcined at 500 ° C for 2 hours.

example

Another honeycomb body was used to manufacture a Exhaust gas purification catalyst according to the invention in succession applied two layers with the following compositions:

1st layer: (directly on the honeycomb body)

60 g / l La / Al ₂

O ₃

stabilized aluminum oxide with palladium pre-fixed thereon in a concentration of 5.45% by weight
15 g / l cerium / zirconium mixed oxide (75/25)
15 g / l cera oxide ex acetate
15 g / l zirconium oxide ex acetate
30 g / l zeolite ZSM5 (molar ratio ≧ 120)

2 layer

10 g / l La / Al ₂

O ₃

stabilized aluminum oxide with rhodium pre-fixed on it in a concentration of 2.3% by weight
15 g / l cerium / zirconium mixed oxide (75/25)
15 g / l La / Al ₂

O ₃

30 g / l dealuminized Y zeolite (Molar ratio ≧ 200).

The total load on the honeycomb body was therefore 205 g / l, of these, 3.5 g / l were for palladium and rhodium.

Comparative Example 3

It became another honeycomb with a double layer provided as in the example according to the invention. The zeolites however, by increasing the others proportionately Coating components replaced.

1st layer: (directly on the honeycomb body)

77 g / l La / Al ₂

O ₃

stabilized aluminum oxide with palladium pre-fixed thereon in a concentration of 4.25% by weight
19 g / l cerium / zirconium mixed oxide (75/25
19 g / l ceria ex aceta
19 g / l zirconium oxide ex acetate

2 layer

18 g / l La / Al ₂

O ₃

stabilized aluminum oxide with Rh pre-fixed on it (1.28% by weight)
26 g / l cerium / zirconium mixed oxide (75/25)
26 g / l La / Al ₂

O ₃

.

The total load of the honeycomb body was therefore 204 g / l, of these, 3.5 g / l were for palladium and rhodium.

Application example

The following catalysts were checked for their cleaning performance:

• 1. Catalyst of Comparative Example 1
• 2. Catalyst of Comparative Example 2  
• 3. Catalyst of the example
• 4. Catalyst of Comparative Example 3.

The results of the FTP 75 test are listed in the following table. Before the test, the catalytic converters were aged at an exhaust gas temperature of 850 ° C for 50 hours on the engine.

As these results show, the catalyst according to the invention has the best conversion of the pollutants. This good result is achieved by using zeolites in both layers and by the spatial separation of palladium and rhodium by placing them in two different layers. In addition, the coating of the catalyst according to the invention has good adhesive strength on the honeycomb body without the use of colloidal silicon dioxide as the binder material. The good adhesive strength of the catalyst according to the invention is based on the mixture of the zeolite material of the first layer with further active layer components (La / Al ₂ O ₃ , cerium / zirconium mixed oxide, cerium oxide, zirconium oxide).

Claims (7)

1. Exhaust gas purification catalyst with improved conversion for hydrocarbons, which contains platinum group metals as catalytically active components and has two superimposed functional layers applied to a support body, characterized in that the first functional layer is a mixture of a first catalyst with at least one zeolite and the second applied to the first functional layer Functional layer contain a mixture of a second catalyst with at least one zeolite, the zeolites have a molar ratio of silicon dioxide / aluminum oxide of more than 40 and that at least palladium and rhodium are present as platinum group metals, which are contained separately from each other in only one of the two functional layers. 2. exhaust gas purification catalyst according to claim 1, characterized, that the first catalyst is palladium and optionally Platinum and the second catalyst rhodium and optionally platinum on an active aluminum oxide included, the concentration of the platinum groups metals on the active aluminum oxide 0.1 to 10 wt .-% based on the total weight of each Catalyst. 3. exhaust gas purification catalyst according to claim 2, characterized, that first and / or second catalyst additionally platinum group metal-free active aluminum oxide,  Ceria and / or zirconia in an amount of 0 to 90 % By weight based on the total weight of each Catalyst included. 4. exhaust gas purification catalyst according to claim 3, characterized, that the zeolite of the first functional layer is a ZSM5 Zeolite with a molar ratio of more than 40 and the a second zeolite of the second functional layer dealuminated zeolite Y with a molar ratio of is more than 100. 5. exhaust gas purification catalyst according to claim 4, characterized, that as a support body ceramic or metallic foams with open cell pore structure or ceramic or metallic honeycomb body with parallel flow channels be used for the exhaust gas, the inner wall surfaces with the functional layers in a concentration of 10 to 300 grams per liter of volume for each of the two functional layers are coated. 6. Emission control system according to one of the preceding Expectations, characterized, that the weight ratio of the catalyst one Functional layer to the zeolites of the functional layer 1: 5 to 5: 1. 7. Use of the exhaust gas purification catalyst after a the preceding claims in an exhaust gas purification system of internal combustion engines.