MXPA99006256A - Catalyst for the reduction of nitrogen oxides in oxidizing atmosphere and reduct - Google Patents

Abstract

The invention relates to a catalyst for the reduction of nitrogen oxides in an oxidizing and reducing atmosphere, which contains iridium on a support material using silicon oxide or a dealuminized zeolite in the Hacid form with a modulus of more than 20, preferably more than of 100, or mixtures thereof. The iridium is precipitated on the outer surface of these support materials with average particle size between 10 and 30 nm. The catalyst is especially suitable for the purification of exhaust gas from diesel engines or gasoline engines operated poorly.

Description

CATALYST FOR THE REDUCTION OF NITROGEN OXIDES IN OXIDIZING AND REDUCING ATMOSPHERE The present invention relates to a catalyst for the reduction of nitrogen oxides in an oxidizing and reducing atmosphere. The catalyst contains iridium on a carrier material.

Following the example of diesel engines, modern Otto engines are now being treated to reduce fuel consumption by running with poorer air / fuel mixtures. Fuel savings of up to 25% are expected for engines known as poor engines, especially those with direct naphtha injection compared to stoichiometrically operated combustion engines. But also poor engines have phases of operation with stoichiometric ratios and even more air / fuel rich. These relationships occur after cold start, accelerations and full load operation. Diesel engines, which operate virtually exclusively with poor air / fuel mixtures, also belong to the category of poorly operated combustion engines. Ref .: 30668 An essential problem of poor engines is the catalytic elimination of the oxides of nitrogen contained in the exhaust gas. Due to the high content of oxygen in the exhaust gases of these engines of up to 15% by volume, the nitrogen oxides (NOx) contained in the exhaust gas can not be simply converted together with the hydrocarbons (HC) and the carbon monoxide (CO) also contained in the lean exhaust gas by means of a conventional exhaust gas catalyst, and to what "in this case the reducing components (HC and CO as well as small amounts of H2 hydrogen) are directly oxidized with oxygen.

Exhaust gas catalysts for the simultaneous conversion of hydrocarbons, carbon monoxide and nitrogen oxides, so-called three-way catalysts, require a stoichiometrically composed gas with an oxygen content of about 0.7% by volume for transformation. The composition of the exhaust gas is usually described by the air number?, Which is defined as the air / fuel ratio normalized under stoichiometric conditions. The air / fuel ratio indicates how many kilograms of air are needed for the complete combustion of one kilogram of fuel. For usual fuels the air / fuel stoichiometric ratio is at a value of 14.6, which corresponds to an air number of 1.

For the conversion of the oxides of nitrogen into poor exhaust gas, two alternative routes are transited. It is attempted, with the help of so-called nitrogen oxide storage catalysts, to store the nitrogen oxides during the poor operation of the combustion machine in the form of nitrates. Preferred storage materials for this purpose are, for example, alkaline earth metal oxides, especially barium oxide. For storage, nitrogen oxides must be oxidized to nitrogen dioxide, depending on the construction of the engine and the type of engine running, between 50 and 90% by volume of nitrogen monoxide. can form nitrates with storage materials. The oxidation is carried out preponderantly in the storage catalyst itself, which for this purpose is provided, for example, with platinum as the catalytically active component.

Depending on the running activity, the storage material must be regenerated at certain intervals. For this, the combustion machine must be operated for a short time with rich air / fuel mixtures. Under the prevailing exhaust gas reducing conditions then the nitrates are decomposed, nitrogen oxides being released into nitrogen by the simultaneous oxidation of the reducing components. For the regeneration of the storage material, the acceleration phases can be used in part. But it is also necessary to achieve a regeneration even if there are no accelerations, which must be carried out by means of the corresponding motor drive. The fuel required for this reduces the theoretical fuel economy in the case of use of poor engines.

The current storage catalysts still show a high sensitivity with respect to the sulfur oxides contained in the exhaust gas of the combustion machines, which after oxidation in the sulfur trioxide storage catalyst reacts with the storage material producing sulphates very stable to temperature, reducing the storage capacity for nitrogen oxides continuously.

As an alternative & the catalysts for the storage of nitrogen oxides catalysts were developed, which have a higher selectivity than conventional catalysts in the case of the conversion of nitrogen oxides with hydrocarbons in an oxygen-rich exhaust gas. These include, for example, catalysts based on zeolites exchanged with copper or iron or catalysts containing iridium. These catalysts allow a permanent conversion of nitrogen oxides even in poor gases.

The activity of the reduction catalysts generally depends on the oxygen content of the exhaust gas and the temperature of the exhaust gas. In this regard, Chajar and others report in Catalysis Letters 28 (1994), 33-40, that a Cu-ZSM5 catalyst develops its optimum reducing activity with approximately 0.5% by volume of oxygen in the exhaust gas, ie, therefore under slightly substoichiometric conditions. In case of oxygen deficiency in the exhaust gas the NO conversion in this catalyst is, depending on the temperature of the exhaust gas, between 2% (at 250 ° C) and 8% (at 500 ° C).

In addition to the dependence of the oxygen content of the exhaust gas, the reduction catalysts show a marked dependence on the temperature of the conversion of nitrogen oxides. The starting temperature for the conversion of nitrogen oxides is found for the oxygen-rich gas at approximately 350 ° C. Start temperature is understood as that temperature at which the conversion rate of a harmful material reaches a certain value, usually 50%. With the temperature of the exhaust gas increased further, the conversion rate for the nitrogen oxides also rises in principle, passing for a certain temperature by a maximum and then falling for exhaust gas temperatures above 500 ° C again at almost zero.

Poorly operated gasoline engines, and especially diesel engines, reach under partial load temperatures of exhaust gases of frequently less than 350 ° C. Catalysts are needed, therefore, to develop their maximum conversion rate as far as possible already at low exhaust gas temperatures of less than 350, preferably less than 300 ° C.

EP 0 633 052 Bl describes a catalyst for the conversion of nitrogen oxides into oxygen-rich exhaust gases, which consist of a crystalline iridium silicate - with an Si / Ir atomic ratio of 50 to 800 and a Si / ratio Not less than 15. The maximum conversion rates of this catalyst are found for an exhaust gas oxygen content of 3.5% by volume at exhaust gas temperatures of less than 430 ° C and are therefore less of 430 ° C and are therefore less suitable for the case described. At the base of the manufacturing process selected for this catalyst, a definite bond is available between the silicate and the iridium, which leads to a very homogeneous atomic distribution of the iridium at this junction.

EP 0 832 688 A1 describes a catalyst containing iridium, sulfur and platinum necessary as catalyst active substances. Iridium and sulfur can be found with this catalyst in a carrier of common material, for example alumina. Alternatively a sulphate metal can also be carried as a carrier for the iridium. After impregnation of the carrier material with iridium chloride, the material is dried at 500 ° C for calcination, so that the iridium is available in the carrier material in fine particles. The catalyst that reaches the distance of the nitrogen oxide uses the oxidizing nitrogen oxide exhaust gas.

DE 196 19 791 A1 describes a catalyst that includes iridium, an alkali metal and at least includes as carriers, nitrides of metals and / or metal carbides. Here they are simultaneously impregnated with the carrier material, with iridium and alkali metal bonds, for example, with connections of soluble iridium precursors and alkali metals in the carrier. With an air / fuel ratio of 23, at a temperature with this catalyst for maximum production of oxygen oxide, of about 350 ° C.

JP 07080315 A also describes a catalyst at the distance of nitrogen oxide with oxidizing exhaust gases in poor engines and diesel engines. The catalyst contains iridium as a carrier material as the active component. As a carrier material, Y, X, they serve, among other things, silicon oxide and Zeolite A, ZMS-5, Mordenite and Silimanite.

The subject of the present invention is to provide a catalyst for the reduction of nitrogen oxide that is marked through a maximum of production assuming an exhaust gas temperature with conversion to low temperatures, also possessing a remarkable resistance with respect to poisoning by the sulfur oxides contained in the exhaust gas. In addition, this catalyst must be able to cope with the varying conditions of a poor engine and have sufficiently high activities for the reduction of nitrogen oxides in both poor running and rich running.

This objective is met by a catalyst for the reduction of oxides of nitrogen in an oxidizing and reducing atmosphere which contains iridium on a support material of silicon dioxide or zeolite. The catalyst is characterized in that the iridium is on the outer surface of the support material with an average particle size between 5 and 30, preferably between 10 and 25 nm.

This catalyst surprisingly shows already at very low exhaust gas temperatures of less than 350 ° C and with an oxygen content in the exhaust gas of 8% by volume optimum conversion rates for nitrogen oxides of more than 70%. An oxygen content of 8% by volume corresponds to an air number? of the exhaust gas of 1.5. There are stoichiometric exhaust gas conditions for an oxygen content of about 0.7% by volume.

It is important for the catalyst according to the present invention that the iridium is applied to a material having a high content of silicon dioxide as a carrier with a relatively large particle size between 10 and 30 nm on the outer surface of the same. Silicon dioxide itself or a dealuminized zeolite in the acidic H-form is therefore used as support materials. A zeolite ZSM-5 with a molar ratio (also referred to as a modulus) of silicon dioxide to aluminum oxide of more than 20, preferably more than 100, is preferred.

Zeolites are oxidic silico-aluminous compounds with a special crystalline structure. They have the general composition M2 / nO • A1203 • xSi02 • yH20 in which M represents a cation with a valence number n and x designates the module. The module is always greater than or equal to 2. The M cations fulfill the function of balancing the charges in the -la-z-eolite network. They can be replaced by ion exchange for other ions. In this case the new ion takes the place of the ion to be exchanged within the microporous structure of the zeoiite. The amount of ions that can be incorporated into the zeolite in this manner is therefore limited by the ion exchange capacity.

Zeolites are frequently marketed in their Na + or H + form. The theoretical ion exchange capacity of a zeolite is directly related to the amount of anions in the network. To increase their hydrothermal stability, the zeolites can be deacidified by special procedures. According to the type of zeolite used, zeolites with modules of more than 100 are thus obtained. With the dealuminization, however, the cation content of the zeolite is also reduced, since at lower aluminum contents a lower equilibrium is desired. loads. As a result, in the case of the zeolite, the ion exchange capacity drastically decreases.

The lowest temperatures for the optimum conversion of the nitrogen oxides in the case of the catalyst according to the present invention were not to be expected against the background of EP 0 633 052 Bl, since the catalyst described there consists of a silicate of iridium, which has the basic structure of a zeolite. According to this writing iridium is, for example, incorporated directly into the zeolite when preparing the zeolite. The zeolite does not fulfill the function here, therefore, of serving as a support for the iridium crystals with their specific surface, but forms a chemical compound with the iridium. Iridium is consequently distributed in this material very finely in the atomic plane.

The present invention transits a to-i-ally different path. Zeolite or silicon dioxide are used as support materials, on whose outer surface iridium precipitates.

For the iridium impregnation of the zeolite used as a carrier, the incorporation of the iridium in the zeolite network or the ion exchange is not used. These techniques would prevent on the one hand the accessibility of the catalytically active iridium to the reactive components of the exhaust gas and on the other hand there would be the amount of iridium with which the zeolite could be impregnated due to the small ion exchange capacity of the zeolite dealuminized strongly restricted.

According to the present invention, therefore, the catalytically active iridium is precipitated by impregnation with, for example, an aqueous solution of soluble iridium precursor compounds on the outer surface of the zeolite. Particularly advantageous is the so-called pore volume impregnation, in which the precursor compounds are dissolved in an amount of water, which corresponds to 70-100% of the previously determined water absorption capacity of the zeolite. . This solution is carried out on zeolite which is rotated in a container. The wet powder is dried at an elevated temperature. If the solubility of the precursor compounds is not sufficient, as to ensure the desired loading of the support material with iridium in a single impregnation step, the material may be impregnated repeatedly.

The impregnated support material is calcined after drying at 300-500 ° C for a time of 1 to 4 hours for the decomposition of the iridium precursor compound in a reducing atmosphere, preferably under reducing gas (5% by volume of H2; % in volume of N2).

Surprisingly it was found that to achieve optimal catalytic activity of the iridium particles on the surface of the support material they should not be highly dispersed, as is usual in platinum catalysts. Rather, a certain minimum particle size was required. Optimal activities were achieved with average particle sizes between 10 and 30 nm. With average particle sizes below 10 nm the reducing activity of the catalyst is reduced, thus acting increasingly with decreasing particle sizes only as an oxidation catalyst for carbon monoxide and hydrocarbons. Average particle sizes of more than 50 nm also lead to a reduction in catalytic activity.

It was found that the particle size can be influenced by the choice of calcination conditions. To adjust the particle diameter in the order between 10 and 30 nm it is necessary to calcine under reducing conditions. The calcination temperature must be in the order between 300 and 500 ° C. The optimum calcining conditions can be easily established by the specialist by controlling the particle size obtained with a transmission electron microscope.

Another function of the calcination under reducing conditions is the removal of the chlorine from the catalyst, which is introduced into the catalyst by means of the preferred precursor compound of the iridium, i.e., the iridium chloride. It has been found that only highly active catalysts can be obtained by calcination in a reducing gas phase. A chemical reduction leads to bad results.

In the described manner, the support materials can be loaded with 0.01 to 5% by weight iridium, based on the total weight of the catalyst. Below 0.01% e "weight is the concentration of iridium in the catalyst too small for an effective conversion of the nitrogen oxides." Above 5% weight increases the growth of the particles because of the high concentrations, so that the iridium's catalytic potential can not be optimally exploited.

The catalyst is preferably applied in the form of a coating on the wall surfaces of flow channels of honeycomb ceramic or metal bodies. Such alveolar bodies are conventionally used as carrier bodies for automobile exhaust gas catalysts, having a plurality of parallel flow channels for the exhaust gas. The number of flow channels per square centimeter of cross-sectional area of the alveolar body is designated as cell density. The cell density of conventional alveolar bodies is between 10 and 250 cm.sup.2 As carriers for the catalysts, however, other structures can be used, such as open cell foams, for example. to 300 grams per liter of carrier body volume.

In order to improve the adhesion of the catalyst to the carrier bodies, other oxidic components such as aluminum oxide, titanium oxide, zirconium oxide or mixtures thereof can also be added in amounts of up to 50% by weight, based on the total weight of the catalyst.

The invention will be explained more in detail by means of the following examples. Figure 1: Conversion of nitrogen oxides for the catalyst of example 1 as a function of the temperature of the exhaust gas for different oxygen contents in the exhaust gas. Figure 2: Conversion of nitrogen oxides to the catalyst of Example 1 as a function of the temperature of the exhaust gas before and after loading with sulfur dioxide. Figure 3: Conversion of nitrogen oxides to the catalyst of Example 3 as a function of the temperature of the exhaust gas for different oxygen contents in the exhaust gas.

EXAMPLE 1 For the preparation of an iridium catalyst on a zeolite, a ZSM5 zeolite in the H-form was chosen with a "300-module, an impregnating solution being prepared by heating 0.92 g of IrCl3-3H20 in 100 ml of water. at reflux for a period of 24 hours 50 g of zeolite were mixed with an amount of the impregnation solution, which was exactly absorbed by the carrier, the wet mass was dried at 125 ° C, the impregnation being repeated so many times until All the solution had been applied on the carrier, The powder thus obtained was reduced in a reduction oven at 450 ° C for 6 hours with reducing gas (5% by weight of hydrogen, the rest nitrogen) in a gas stream of 5 Nl / in. The finished catalyst contained 1% by weight iridium, based on its total weight.

The catalyst thus prepared was homogenized as an aqueous dispersion "in a ball mill, then the dispersion was mixed with 36 parts by mass of binder (sodium silicate) per 100 parts of catalyst and diluted with water to a concentration of solids of 300 g / 1.

For the coating of a cordierite honeycomb body with a cell density of 62 cm "2, the latter was immersed in the dispersion, then the excess dispersion of the channels was blown off with air under pressure, then dried in a drying oven. The coated alveolar body had a coating concentration of 300 grams per liter of alveolar body.The coated alveolar body was reduced to 450 ° C for 6 hours in reducing gas.

Example 2 Another catalyst was prepared as in Example 1. Instead of ZSM5 with a modulus of 300, a ZSM5 zeolite with a modulus of 27 was used.

The catalyst was applied as described in Example 1 on an alveolar body.

Example 3 Another catalyst was prepared as in Example 1 Instead of ZSM5 with a modulus of 300, pure silicon dioxide was used.

The catalyst was applied as described in Example 1 on an alveolar body.

Application example: The conversion of nitrogen oxides of the catalysts of the previous examples was determined in a synthesis system as a function of the temperature of the exhaust gases for different concentrations of oxygen in the exhaust gas. For this purpose, a synthetic exhaust gas with a water content of 10% by volume, a concentration of nitrogen oxides of 270 ppm and a propene concentration of 1650 ppm in nitrogen was used. The alveolar bodies were charged with a space velocity of 51000 h_1.

Figures 1 and 3 show the conversion curves measured for the catalysts of Examples 1 and 3.

In the Ir-ZSM5 catalyst (module 300) it is observed for an oxygen content of 8% in --- volume, corresponding to an air number of approximately 1.5 (oxidizing conditions), a starting temperature for a conversion of oxides of 30% nitrogen of 270 ° C (Figure 1). The maximum conversion is at 320 ° C and reaches 65%. The temperature range, in which the conversion of nitrogen oxides exceeds 30%, ranges from 270 to 420 ° C. For an oxygen content of 0.7% by volume, corresponding to an air number of 1 (stoichiometric conditions), the starting temperature for a "conversion of nitrogen oxides of 50% is approximately 225 ° C. At temperatures above of 275 ° C the conversion of nitrogen oxides is greater than 90%.

In the Ir-SiO2 catalyst (FIG. 3), a starting temperature for a nitrogen oxides conversion of 30% of 290 ° C is observed for an oxygen content of 8% by volume. The maximum conversion takes place at 340 ° C and reaches 70%. The temperature range, in which the conversion of nitrogen oxides exceeds 30%, extends from 290 to 480 ° C. For an oxygen content of 0.7% by volume, the starting temperature for a conversion of nitrogen oxides of 50% is approximately 270 ° C. At temperatures above 380 ° C the conversion of nitrogen oxides is greater than 90%.

Figure 2 shows the influence of sulfur dioxide in the exhaust gas on the catalytic activity of the catalyst of Example 1 (Ir-ZSM5 with module 300). In the case of this catalyst, no deactivation was carried out after a charge of 350 ppm by volume of sulfur dioxide in air for a space of 2 hours and at a temperature of 450 ° C. On the other hand, there is a slight shift in the starting temperatures and temperatures for the maximum conversion of nitrogen oxides to lower values.

It is noted that in relation to this date, the best method known to the applicant, to implement said invention is that which is clear from the manufacture of the objects to which it refers.

Having described the invention as above, the content of the following is claimed as property.

Claims (7)

1. Catalyst for the reduction of oxides of nitrogen in an oxidizing and reducing atmosphere, containing iridium on a support material of silicon dioxide or zeolite, characterized in that the iridium is present on the outer surface of the support material with an average particle size between 5 and 30 nm.

2. Catalyst according to claim 1, characterized in that zeolite dealuminized in the acid form (form H) is used as the support material.

3. Catalyst according to the claim 2, characterized in that in the case of the dealuminated zeolite it is ZSM5 with a modulus of more than 20, preferably more than 100.

4. Catalyst according to any of the preceding claims, characterized in that it is applied in. the shape of a coating on the wall surfaces of the circulation channels of a ceramic or metal honeycomb body in a concentration of 50 to 300 g per liter of honeycomb body.

5. Catalyst according to claim 4, characterized in that the coating contains as additional components aluminum oxide, titanium oxide, zirconium oxide, aluminum oxide or mixtures thereof.

6. Process for the preparation of a catalyst according to any of claims 1 to 5 by impregnation of the support material with a soluble precursor of the iridium, drying of the impregnated material and calcination in a gas stream containing hydrogen at a temperature between 300 and 500 ° C for a time of 1 to 10 hours.

7. Use of the catalyst according to any of the preceding claims for the purification of the exhaust gases of diesel or gasoline engines.