MXPA97008633A - Catalyst for the pestifying of gases deescape with an improved conversion for hydrocarb - Google Patents

Abstract

The present invention relates to a catalyst for the purification of exhaust gases with an improved conversion for hydrocarbons containing metals of the platinum group as catalytically active components and having two overlapping function layers applied to a carrier body, characterized in that the first layer of function contains a mixture of a first catalyst with at least one zeolite and the second function layer applied on the first function layer contains a mixture of a second catalyst with at least one zeolite, where the zeolites have a molar ratio between silicon dioxide / aluminum oxide of more than 40 and because at least palladium and rhodium are present as metals of the platinum group, which are separated from each other in only one of the two layers of function. Use of the catalyst for the purification of exhaust gases in an exhaust gas purification system of combustion engines

Description

Catalyst for the purification of exhaust gases with an improved conversion for hydrocarbons DESCRIPTION OF THE INVENTION The invention relates to a catalyst for the purification of exhaust gases with an improved conversion for hydrocarbons containing platinum group metals as catalytically active components and has two superimposed layers of function applied to a carrier body. 10 The combustion engines emit toxic substances, which can be converted into non-toxic compounds by means of catalysts for the purification of exhaust gases. The main toxic substances are carbon monoxide, nitrogen oxides and unburned hydrocarbons. 15 These can be converted into water, carbon dioxide and nitrogen in catalysts containing metals of the platinum group. Three-way catalysts, oxidation catalysts and reduction catalysts are distinguished. The boundary between these catalysts is fluid and also depends in particular on the composition of the exhaust gases.

The three-way catalysts are capable of simultaneously converting the three aforementioned toxic substances under stoichiometric exhaust gas conditions. In this case, nitrogen oxides are reduced under simultaneous oxidation of carbon monoxide and hydrocarbons to give nitrogen. The three-way catalysts contain as essential catalytic components __ rhodium in combination with platinum and / or palladium. A catalyst of this type which contains rhodium and palladium is described, for example, in the German patent document.

REF: 26129 DE 38 35 184 C2. The presence of rhodium in the three-way catalyst is important for its reduction function. Catalysts, which only contain platinum and / or palladium, only have an unsatisfactory reducing action and therefore are predominantly used as oxidation catalysts.

Three-way catalysts and current oxidation catalysts are capable of converting the corresponding toxic substances after having reached the operating temperature with conversion rates of more than 70% in non-toxic products, an essential problem is given by the cold start of combustion engines. During the so-called cold start phase, which comprises approx. the first 100 seconds after starting the engine, the exhaust gas purification catalyst is still cold and therefore not active. As the temperature increases, the catalyst is heated. The increased activity of the catalyst for the purification of the exhaust gases is characterized by the reaction temperature of the corresponding toxic substance. In this case it is the temperature at which the toxic substance becomes 50%. The reaction tempera- tures of modern catalysts range from temperatures of 200 to 400 ° C.

The main toxic component during the cold start phase are unburned hydrocarbons. To evaluate the purifying effect of catalysts for the purification of exhaust gases, different test processes have been developed. A frequently used test procedure is the so-called FTP 75 test, which has been developed in the United States. In Europe, the ECE test procedure has been standardized.

The FTP 75 test is extended for 2500 seconds after the cold start of the combustion engine and subdivided into three parts. The toxic substances emitted in these three stages are collected in three bags and then analyzed. The first stage comprises the cold start phase itself and ends after 500 seconds. The next two stages simulate changing load conditions and a hot start.

For the definitive evaluation of a system for the purification of the exhaust gases that can be composed of several catalysts and adsorbents, the quantities of toxic substances collected in the three bags are used. It has been shown that it is possible to achieve marked improvements in the overall assessment, especially through improvements in the conversion of toxic substances during the cold start phase. The conversion rates of hot catalysts per operation only allow for insignificant improvements, which can hardly influence the overall evaluation according to the FTP 75 test process. Thus, a combustion engine emits during the first 100 seconds after the cold start about two thirds of the total amount of hydrocarbons emitted. during the FTP 75 test.

To reduce these hydrocarbon emissions, different combinations of hydrocarbon adsorbents with catalysts have been proposed.

US Pat. No. 5,078,979 discloses a process for the purification of the exhaust gases in which the exhaust gas is first passed over a hydrocarbon adsorbent and then onto a catalyst. The hydrocarbons contained in the cold exhaust gas are retained by the adsorbent until it reaches a temperature of around 150 ° C. Above this temperature, the hydrocarbons are released from the adsorbent and converted into different products by the action of the catalyst that is already hotter. As adsorbents, molecular sieves (zeolites) have been proposed, which preferentially adsorb hydrocarbons before the water vapor which is also present in the exhaust gas and which have a high stability against temperature.

The disadvantage of such a system is that the adsorbent removes heat from the exhaust gas during the heating phase, which is missing from the subsequent catalyst, so that it heats more slowly than when there is an adsorbent in the past. . The combination between adsorbent and catalyst separated in space decreases the emission of hydrocarbons during the cold start phase, although in the general evaluation according to the FTP 75 test process it presents worse results than a single three-way catalyst, since the hydrocarbons first adsorbed are released after having exceeded the desorption temperature and pass the catalyst that is not yet sufficiently active with a large part without converting into non-toxic components.

To improve this situation in German patent document OE 42 39 875 A1 it is proposed to combine an oxidation catalyst with a hydrocarbon adsorbent on a carrier body. The oxidation catalyst and the hydrocarbon adsorbent are applied to the carrier body in the form of superposed layers, where the layer of adsorbent is on the catalyst layer and comes into direct contact with the exhaust gas.

The oxidation catalyst contains platinum and / or palladium as catalytically active components. The adsorbent contains a mixture of a Y-dealuminated zeolite with a ZSM5 zeolite, where the Y zeolite exhibits a _ Si / Al ratio of more than 40 and zeolite ZSM5 has a Si Al ratio of more than 20.

European patent document EP 0 716 877 A1 also proposes a combination of a hydrocarbon adsorbent in the form of two superposed layers. The catalyst layer is located above the adsorption layer. It contains as catalytically active components one or more metals from the platinum group composed of platinum, palladium, rhodium, ruthenium and iridium on aluminum oxide, cerium oxide and zirconium oxide. As an adsorbent material, a zeolite with a weight ratio between silicon dioxide and aluminum oxide of more than 300 is applied. A monolithic body in the shape of a Cordierite honeycomb with parallel current channels for the gas is used as the carrier body. escape The absorbent layer is applied as a first layer directly on the surfaces of the interior walls of the current channels. Due to the bad adhesion of the zeolite layers on its base, the adsorbent layer contains together with the zeolite in equal amount colloidal silicon dioxide as a binder. Due to this high proportion of colloidal material there is a danger that the pores of the zeolite become partially clogged and thereby worsen the adsorption capacity.

The proposed solutions remain unsatisfactory in terms of their removal of hydrocarbons throughout the three parts of the FTP 75 test.

The adsorption of hydrocarbons during the first stage is satisfactory, without However, there are deficiencies in the dynamic conversion of toxic substances during stage 2 and 3 of the FTP 75 test. In particular, the resistance to aging of the conversion of toxic substances is also unsatisfactory.

Therefore, it is the task of the present invention to provide an exhaust gas purification catalyst that has an improved hydrocarbon suppression throughout the three parts of the FTP 75 test process, as well as resistance to aging. also be improved.

This task is solved by means of the catalyst for exhaust gas purification, which contains diethyl group metals as catalytically active components and which has two superimposed layers of function applied to a carrier body. The catalyst for exhaust gas purification is characterized in that the first functional layer contains a mixture of a first catalyst with at least one zeolite and the second functional layer applied to the first functional layer contains a mixture of a second catalyst. with at least one zeolite, where the zeolites have a molar ratio between silicon dioxide aluminum oxide of more than 40 and because as metals of the platinum group are present at least palladium and rhodium, which separated from each other are contained in only one of the two catalysts.

As a function layer within the framework of this invention, it is to be understood that a layer exerts a certain effect on the exhaust gas. It can be the adsorption of toxic components or a catalytic effect or a combination of both. Both layers of catalyst function for the purification of exhaust gases contain for the adsorption of hydrocarbons by IO minus one zeolite with a motive ratio between silicon dioxide / aluminum oxide of more than 40. The adsorbent material of the function layers in it does not present catalytically active noble metal components, although in each case it is mixed with a catalyst. Therefore, the effect of the adsorbent and the catalyst are very close in the space in the function layers, and thus guarantee an optimal conversion of the. hydrocarbons that are desorbed again as the temperature increases. In the catalyst for the purification of exhaust gases according to the invention, the zeolites which function as adsorbents do not have catalytically active components on their specific surface. In this way, the carbonization of the zeolite, which is often observed, is avoided.

The two catalysts of the function layers have different catalytic effects. While one of the catalysts develops essentially an oxidative effect due to the use of palladium and optionally platinum, the catalyst of the other functional layer has a reducing effect due to the use of rhodium and possibly platinum. Due to the arrangement of the two catalysts in different layers, palladium and rhodium are separated in space. In this way, the formation of an alloy between palladium and rhodium at high temperatures of the exhaust gas and a worsening of the catalytic activity that is accompanied by the formation of the alloy is avoided.

The reduction catalyst containing rhodium can be located either in the first function layer or in the second function layer that is above the first, while the palladium-containing oxidation catalyst is located in the other layer . However, preferably the second catalyst is introduced into the second function layer that comes into direct contact with the exhaust gas. That is why the reduction catalyst can optimally convert the oxides of nitrogen found in the exhaust gas, where the carbon monoxide and hydrocarbons found in the exhaust gas as well as the hydrocarbons that are desorbed during the heating Einto of the adsorbent material serve as reducing agents.

The catalysts of the function layers may contain, together with palladium, rhodium and platinum, other noble metals of the platinum group. The metals of the platinum group comprise ruthenium, rhodium, palladium, osmium, iridium and platinum. As a carrier for the metals of the platinum group the catalysts contain active aluminum oxide. Under active aluminum oxide are to be understood aluminum oxides with specific surfaces (measured in accordance with DIN 66 132) of more than 10 m 2 / g. They present different crystal structures (gamma, delta, teta, layer and eta aluminum oxide) that are converted to thermally stable alpha aluminum oxide when heated to 1150 K (see Ullmanp's Encyclopedia of Industrial Chemistry; 5. edition, 1985, Vol. A1, 561-562).

The catalytically active platinum group metals are applied over the active aluminum oxide in a concentration of 0.1 to 10% by weight based on the total weight of the corresponding catalyst. For this the active aluminum oxide is impregnated in a separate manufacturing step with previous steps of corresponding metal coughs, dried and calcined. After this manufacturing step, the metals of the platinum group are present on the aluminum oxide in highly dispersed form. Both catalysts can additionally contain active aluminum oxide free of metals of the platinum group, zirconium oxide and / or zirconium oxide in an amount ranging from 0 to 90% by weight, based on the total weight of the corresponding catalyst.

Preferably, a weight ratio of the corresponding catalyst of a function layer is chosen with respect to the zeolites contained therein of 3: 1. However, to optimize the catalytic effect of the catalyst for Exhaust gas purification can vary the weight ratio in a range that ranges from 1: 5 to 5: 1.

Zeolites with molar ratios of more than 40 are used as adsorbent materials. By means of this high molar ratio the zeolites adsorb preferably hydrocarbons before water and have a high content ID stability versus temperature. Preferably, a ZSM5 zeolite with a molar ratio of more than T40 is applied in the first functional layer and a dealuminated Y zeolite with a ratio of more than 100 in the second functional layer.

As carrier bodies for the catalyst for the purification of exhaust gases, ceramic or metal foams with a structure of open pore or body in the shape of honeycomb ceramic or metal with parallel current channels for the exhaust gas. In this case, they are carrier bodies which are generally used as carrier bodies for catalyst for the purification of exhaust gases. These carrier bodies are coated by means of known processes with the two support layers. For This first prepares an aqueous coating dispersion of the expected components of the corresponding function layer. The carrier body can be coated eg. by pouring on it this dispersed coating or immersing it in this dispersion. After blowing off any closed streams or channels, the carrier bodies are dried _ _ and then they are calcined at temperatures of 500 ° C. The various techniques that are used for this are known to the person skilled in the art.

The function layers are applied to the carrier body in each case in a concentration of 10 to 300 grams per liter of volume of the carrier body. & The catalyst for the purification of exhaust gases according to the invention can be applied alone or in combination with other catalysts in systems for the purification of exhaust gases from combustion engines.

To illustrate the invention in more detail different catalyst have been prepared for the purification of exhaust gases on ceramic carrier bodies. In the case of the carrier bodies, these are Cordierite honeycomb-shaped bodies with a density of cells (number of current channels per surface of the cross section) of 62 cm ~ 2, a diameter of 11.84 cm , a length of 15.24 cm and a wall thickness of the current channels of 0.2 mm. The volume of the carrier bodies is 1.68 I. 5 The purifying effect of the catalysts was determined by means of the FTP 75 test process in a vehicle with 4 cylinders and 2 I displacement of a 1994 model year. The ratio between the installed catalyst volume and the vehicle displacement was 0 , 84. 0 Gas tests for the FTP 75 test were extracted from the exhaust gas system behind the test unit.

Comparative Example 1 On one of the honeycomb-shaped bodies a single layer with the following composition was applied: & 20 g 1 of La Al 2O 3 aluminum oxide? stabilized with 2 to 4% by weight of lanthanum, calculated as lanthanum oxide 50 g / 1 of La / Al 03 aluminum oxide? stabilized with palladium and rhodium fixed on it in a weight ratio of H 14: 1 and a concentration of 7% by weight based on the total weight of aluminum oxide plus palladium and rhodium g / 1 mixed oxide of cerium / zirconium (75% by weight of cerium oxide, 25% by weight of zirconium oxide) g / l cerium oxide ex acetate 18 g / l zirconium oxide ex acetate 0 30 g / 1 zeolite Y dealuminized (molar ratio greater than or equal to 200) g / l ZSM5 zeolite (molar ratio greater than or equal to 200) The total amount of the honeycomb body was then 198 g l of which 3.5 g / 1 corresponded to palladium and rhodium.

For the pre-fixation of palladium and rhodium on a part of the stabilized aluminum oxide, the expected amount of aluminum oxide was impregnated with an aqueous solution of rhodium nitrate and palladium nitrate, dried at 120 ° C and then calcined for 4 hours at 500 ° C.

The powder thus obtained was dispersed to prepare the coating dispersion with the other components of the catalyst in water and homogenized by grinding. After grinding, the average particle size of the solid substances in the coating dispersion was about 6 μm. The solids content of the coating dispersion was 5% by weight.

The honeycomb-shaped body was coated by immersion in the coating dispersion, dried for 10 minutes at 120 ° C and then calcined for 4 hours at 300 ° C.

Comparative example 2 A single layer essentially of the same composition as in Comparative Example 1 was applied to a second honeycomb body.

Unlike comparative example 1, only palladium was prefixed on aluminum oxide. The necessary amount of rhodium was newly introduced into the coating after coating the body in the form of a honeycomb by immersing it in an impregnation solution with rhodium nitrate. To avoid a direct contact of the rhodium with the pre-fixed palladium, zirconium sol oxide was added to the rhodium solder impregnation solution. The final impregnation solution contained 9% by weight of rhodium nitrate and 30% by weight of zirconium oxide, based on the total weight of the impregnation solution. After impregnation the catalyst was dried for 6 hours at 120 ° C and then calcined at 500 ° C for 2 hours.

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For the preparation of a catalyst for the purification of exhaust gases, two layers with the following composition were successively applied on another body in the form of a honeycomb: 1 layer: (directly on the body in the form of honeycomb): 60 g / l of La / Al 2O 3 stabilized aluminum oxide with palladium preset thereon at a concentration of 5.45% by weight g / 1 mixed oxide of cerium / zirconium (75/25) g / 1 cerium oxide ex acetate g / l zirconium oxide ex acetate g / 1 zeolite ZSM5 (molar ratio greater than or equal to 200) 10 g l of La / Al 2O 3 aluminum oxide stabilized with rhodium fixed thereon at a concentration of 2.3% by weight g / 1 mixed oxide of cerium / zirconium (75/25) g / l La / Al203 g / l De-alkylated zeolite Y (molar ratio greater than or equal to 200) The total load of the body in the form of honeycomb was therefore 205 g / 1 of which 3.5 g / 1 corresponded to palladium and rhodium.

Comparative example 3 Another body in the form of a honeycomb with two layers was prepared as indicated in the example according to the invention. However, the zeolite was replaced by proportionally increasing the other components of the coating. caoa: (directly on the honeycomb body): 7 g l of La / Al 2O 3 aluminum oxide stabilized with palladium fixed on it in a concentration of 4.25% by weight 1 g / 1 mixed cerium / zirconium oxide (75725) 19 g / 1 cerium oxide ex acetate 19 g / l zirconium oxide ex acetate 2. cap 18 g / 1 of Al ZO 3 stabilized aluminum oxide with rhodium fixed on it at a concentration of 1.28% by weight 26 g / I of mixed oxide of ceno / zirconium (75/25) 26 g / l La / Al20 3 The total load of the body in the form of honeycomb was therefore 204 g 1 of which 3.5 g / 1 corresponded to palladium and rhodium.

Application example The following catalysts were controlled for the purification of exhaust gases in order to determine their purifying efficiency: 1. catalyst of comparative example 1 2. catalyst of comparative example 2 3. example catalyst 4. catalyst of comparative example 3 The results of the FTP 75 test are listed in the following table. Before the test the catalysts in the engine were allowed to age at an exhaust gas temperature of 850 ° C for 50 hours.

Catalyst for Hydrocarbons in Total Hydrocarbons gas purification bag 1 (the 3 bags) of escape [g / mile] [g / mile] comparative example 1 0.69 0.19 comparative example 2 0.6 0.17 comparative example 3 0.76 0.18 example 0.5 0.12 As the results show, the catalyst according to the invention has the best conversion of substances toxic This good result is achieved by the application of zeolites in both layers and by separation in the space of palladium and rhodium by placing them in two different layers. Furthermore, the coating of the catalyst according to the invention has a good adhesion to the body in the form of a honeycomb without the use of colloidal silicon dioxide as a binder material. The good adhesion of the catalyst according to the invention is based on the mixture of the zeolite material of the first layer with the other active components (Al 2 O 3, mixed oxide of cerium / zirconium, cerium oxide, zirconium oxide). It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects or products to which it refers. Having described the invention as above, property is claimed as contained in the following:

Claims (7)

1. Claims 1. Catalyst for the purification of exhaust gases with an improved conversion for hydrocarbons containing platinum group metals as catalytically active components and having two overlapping function layers applied on a carrier body, characterized in that the first function layer contains a mixture of a first catalyst with at least one zeolite and the second function layer applied on the first function layer contains a mixture of a second catalyst with at least one zeolite, where the zeotites have a molar ratio between silicon dioxide aluminum oxide of more than 40 and because as metals of the platinum group are present at least palladium and rhodium, which separated between st are contained in only one of the two layers of function.
2. 2. Catalyst for the purification of exhaust gases according to claim 1, characterized in that the first catalyst contains palladium and optionally platinum and the second catalyst contains rhodium and optionally platinum on an active aluminum oxide, where the concentration of the metals of the group of the platinum on the active aluminum oxide ranges from 0.1 to 10% by weight based on the total weight of the corresponding catalyst.
3. 3. Catalyst for exhaust gas purification according to claim 2, characterized in that the first catalyst and / or the second catalyst additionally contains active aluminum oxide free of metals of the platinum group, cerium oxide and / or zirconium oxide in an amount ranging between 0 and 90% by weight based on the total weight of the corresponding catalyst.
4. 4. Catalyst for exhaust gas purification according to claim 3, characterized in that the zeolite of the first functional layer is a zeolite ZSM5 with a molar ratio of more than 40 and the second zeolite of the second functional layer is a zeolite And desiuminized with a molar ratio of more than 100.
5. 5. Catalyst for the purification of exhaust gases according to claim 4, characterized in that ceramic or metal foams with an open cell structure are used as the carrier body or ** Ceramic or metallic honeycomb bodies with parallel flow channels for the exhaust gas, the internal wall surfaces of which are covered with the function layers in a concentration ranging between 10 and 300 grams per liter of volume Carrier body for each of the two function layers.
6. 6. Catalyst for exhaust gas purification according to one of the preceding claims, characterized in that the weight ratio of the catalyst of a function layer with respect to the zeolites of the function layer ranges from 1: 5 to 5. : 1.
7. 7. Use of the catalyst for exhaust gas purification according to one of the preceding claims in an exhaust gas purification system of combustion engines.