CN101454065A - Catalyst for Reducing Nitrogen-Containing Pollution Gas in Diesel Engine Exhaust Gas - Google Patents

Abstract

In exhaust gas purification units that reduce nitrogen oxides in lean burn exhaust gas of internal combustion engines by selective catalytic reduction using ammonia, the introduction of excess ammonia leads to undesirable emissions of unused ammonia. These emissions may be reduced using an ammonia blocking catalyst. In the ideal case, ammonia is oxidized to nitrogen and water by these catalysts. They require additional space in the exhaust gas purification unit, which may have to occupy the space provided for the SCR main catalyst. Furthermore, the use of such ammonia blocking catalysts may result in the over-oxidation of ammonia to nitrogen oxides. To overcome these disadvantages, catalysts comprising two superposed layers have been proposed for removing nitrogen-containing polluting gases from diesel exhaust gases. The lower layer contains an oxidation catalyst and the upper layer can store at least 20 ml of ammonia per gram of catalyst material. This catalyst shows reduced ammonia breakthrough in the low temperature range with good SCR conversion. It can be used as an SCR catalyst with reduced ammonia breakthrough or as an ammonia blocking catalyst.

Description

Catalyst for Reducing Nitrogen-Containing Pollution Gas in Diesel Engine Exhaust Gas

The present invention relates to the removal of nitrogen-containing polluting gases from the exhaust gases of internal combustion engines operating with a lean air/fuel mixture (referred to as "lean burn engines"), especially from diesel engines.

The emissions present in the exhaust of motor vehicles operating with lean-burn engines can be divided into two categories. Thus, the term primary emissions refers to the polluting gases formed directly by the combustion process of the fuel in the engine and present in the raw emissions before passing through the exhaust gas cleaning device. Secondary emissions are polluting gases that can be formed as by-products in the exhaust gas cleaning unit.

The exhaust gas of a lean-burn engine contains the typical primary emissions carbon monoxide CO, hydrocarbons HC and nitrogen oxides NOx together with a higher oxygen content of up to 15% by volume. Carbon monoxide and hydrocarbons can be easily rendered harmless by oxidation. However, the reduction of nitrogen oxides to nitrogen is much more difficult due to the high oxygen content.

A known method for removing nitrogen oxides from exhaust gases in the presence of oxygen is the selective catalytic reduction method (SCR method) by means of ammonia over a suitable catalyst (SCR catalyst for short).

Here, depending on the engine principle and the design of the exhaust unit, a distinction is made between "active" and "inert" SCR methods in which the secondary ammonia emissions produced in a targeted manner in the exhaust unit are used as Reducing agent for removal of nitrogen oxides.

Thus, US 6,345,496 B1 describes a method for purifying engine exhaust gases in which lean and rich air/fuel ratios are repeatedly set alternately and the exhaust gas thus produced is passed through an exhaust unit containing NOx at the inflow end only under rich exhaust gas conditions A catalyst that converts to _NH3 , while containing another catalyst that absorbs or stores NOx under lean conditions and releases NOx under rich conditions, so that it can react with the _NH3 produced by the catalyst on the inflow side to form nitrogen gas on the outflow side. As an alternative, according to US 6,345,496B1, NH adsorption and oxidation catalysts that store _NH under rich conditions _and desorb _NH under lean conditions and oxidize it to form nitrogen and water by means of nitrogen oxides or oxygen can be placed at the outflow end.

WO 2005/064130 also discloses an exhaust gas unit comprising a first catalyst at the inflow end, which first catalyst generates NH ₃ from exhaust gas constituents during the rich phase. In the second downstream catalyst, NH ₃ is periodically stored. Nitrogen oxides present in the exhaust gas in the lean phase are reacted with this stored ammonia. The exhaust gas unit also contains a third noble metal-containing catalyst comprising at least platinum, palladium or rhodium on a support material capable of storing ammonia during the rich phase and desorbing it again during the lean phase.

WO 2005/099873 A1 claims a method for removing nitrogen oxides from the exhaust gas of a lean-burn engine in cyclic rich/lean operation, the method comprising the following sub-steps: storing NOx in a NOx storage component in the lean exhaust gas, and storing NOx in a NOx storage component in the rich exhaust gas In-situ conversion of stored NOx to _NH3 , storage of _NH3 in at least one _NH3 storage component and reaction of _NH3 with NOx under lean exhaust gas conditions, wherein the first and last sub-reactions are carried out at least partly Temporally and/or partially performed concurrently and/or in parallel. In order to carry out the method, an integrated catalyst system is required comprising at least one NOx storage component, _NH3 generating component, _NH3 storage component and SCR component.

The use of such "inert" SCR methods is limited to vehicles in which reducing ("rich") exhaust gas conditions can be generated in the engine without major difficulty. This applies to direct injection gasoline engines. Diesel engines, on the other hand, cannot readily operate with sub-stoichiometric ("rich") air/fuel mixtures. The creation of reducing exhaust gas conditions must be accomplished by means external to the engine, such as subsequent injection of fuel into the exhaust stack. This leads to problems in terms of adherence to the HC exhaust gas limit, to exothermic reactions in the downstream oxidation catalyst, to premature thermal aging of the oxidation catalyst and not least to a significant increase in fuel consumption. "Active" SCR methods have thus focused on the development and application of NOx removal from diesel engine exhaust.

In the "active" SCR method, the reducing agent is introduced into the exhaust gas system from an accompanying additional tank by means of injection nozzles. Compounds that can be easily decomposed into ammonia, such as urea, can be used for this purpose instead of ammonia. Ammonia must be added to the exhaust gas in at least a stoichiometric proportion to the nitrogen oxides.

The conversion of nitrogen oxides can usually be improved by introducing a 10-20% excess of ammonia, but this drastically increases the risk of higher secondary emissions, especially due to increased ammonia breakthrough. Since ammonia is a gas with a pungent odor even at low concentrations, in practice the aim is to minimize ammonia breakthrough. The molar ratio of ammonia to nitrogen oxides in the exhaust gas is usually represented by α:

$\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; NH</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; NO</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; )</span>}$

In internal combustion engines in motor vehicles, precise metering of ammonia presents great difficulties because of the greatly fluctuating operating conditions of the motor vehicle and sometimes leads to considerable ammonia breakthrough downstream of the SCR catalyst. In order to suppress ammonia breakthrough, an oxidation catalyst is typically arranged downstream of the SCR catalyst to oxidize the breakthrough ammonia. Such catalysts will hereinafter be referred to as ammonia barrier catalysts. The ammonia flameout temperature T ₅₀ (NH ₃ ) is reported as a measure of the catalyst's oxidizing ability. It indicates the reaction temperature at which the conversion of ammonia is 50% in the oxidation reaction.

Ammonia barrier catalysts arranged downstream of an SCR catalyst to oxidize breakthrough ammonia are known in various embodiments. Thus, DE 3929297 C2 (US 5,120,695) describes such a catalyst arrangement. According to this document, the oxidation catalyst is applied as a coating to the outflow end portion of the monolithic reduction catalyst configured as a fully active honeycomb extrudate, wherein the area coated with the oxidation catalyst accounts for 20-50% of the total catalyst volume. As catalytically active components, the oxidation catalyst comprises at least one of the platinum group metals platinum, palladium and rhodium, which are deposited on ceria, zirconia and alumina as support material.

According to EP 1 399 246 B1 it is also possible to apply the platinum group metals directly to the reduced catalyst component as support material by impregnation with soluble precursors of the platinum group metals.

According to JP2005-238199, it is also possible to introduce a noble metal-containing layer of an ammoxidation catalyst under a coating of titanium oxide, zirconium oxide, silicon oxide or aluminum oxide and a transition metal or a rare earth metal.

Especially when using highly active oxidation catalysts, the use of ammonia barrier catalysts carries with it the risk of excessive oxidation to nitrogen oxides. This phenomenon reduces the conversion of nitrogen oxides, which can be achieved by means of an overall system of SCR and barrier catalysts. The selectivity of an ammonia barrier catalyst is thus an important measure of its quality. For the purposes of this document, the selectivity to nitrogen is a concentration value and is calculated from the difference between all measured nitrogen components and the amount of ammonia introduced.

c(N ₂ )=1/2·[c _introduction (NH ₂ )-c _outlet (NH ₃ )-2-c _outlet (N ₂ O)-c _outlet (NO)-c _outlet (NO ₂ )]

If an ammonia barrier catalyst is required, space for another catalyst must be made available in the exhaust gas purification unit. Here, the ammonia barrier catalyst can be arranged in an additional converter downstream of the converter containing the SCR catalyst. However, such an arrangement is not common since space to install additional converters is generally not available in the vehicle.

As an alternative, the ammonia barrier catalyst can be arranged in the same converter as the SCR catalyst ("integrated ammonia barrier catalyst"). Here, the space required to install the ammonia barrier catalyst disappears from the volume available for installing the SCR catalyst.

For example, it is possible to arrange two different catalysts in series in the converter. Such an arrangement is described in JP 2005-238195. In the embodiments disclosed there, the ammonia barrier catalyst occupies about 40% of the available space, and as a result, only about 60% of the available space is available for the SCR catalyst. US 2004/0206069 discloses a method for the thermal control of diesel exhaust gas purification systems in goods vehicles, wherein a converter for reducing nitrogen oxides by selective catalytic reduction is an integral part of the diesel exhaust gas purification system. The converter contains not only the SCR main catalyst but also an upstream hydrolysis catalyst that releases ammonia from urea and a downstream ammonia rejection catalyst.

In another embodiment of the "integrated ammonia barrier catalyst", a coating comprising the ammonia barrier catalyst is applied to a downstream guide component of the SCR catalyst. Applicant's WO 02/100520 describes an embodiment in which a noble metal based oxidation catalyst is applied to an SCR catalyst in the form of a monolithic fully active catalyst, wherein only 1-20% of the length of the SCR catalyst is used as a support for the oxidation catalyst .

In "active" SCR systems for the removal of nitrogen oxides from diesel engine exhaust, there is therefore firstly the problem of providing catalysts and conditions for efficient removal of nitrogen oxides by selective catalytic reduction. Second, incompletely reacted ammonia may not be allowed to be released into the environment. An exhaust unit that solves this problem must also be designed so that, firstly, very little space is required to install the required catalyst, but secondly, the system is as selective as possible for nitrogen.

The object of the present invention is to provide catalysts, exhaust gas purification units and/or methods with which nitrogen-containing polluting gases, whether in oxidized form or not, can be removed from completely lean exhaust gases of diesel engines by means of "active" SCR methods It is irrelevant whether nitrogen oxides, for example, or in reduced form, for example ammonia, are present in the polluting gas.

To achieve such an aim, EP 0 773 057 A1 proposes catalysts containing zeolites exchanged with platinum and copper (Pt-Cu zeolites). In a particular embodiment, the Pt-Cu zeolite catalyst is applied to a commonly used substrate. Furthermore, there is a second catalyst comprising a zeolite which has been exchanged only with copper.

According to the invention, this object is achieved by a catalyst comprising a honeycomb body and a coating consisting of two superimposed catalytically active layers, wherein the lower layer applied directly to the honeycomb body contains an oxidation catalyst and the upper layer applied thereon contains ammonia The storage material and has an ammonia storage capacity of at least 20 milliliters of ammonia per gram of catalyst material.

For the purposes of this document, an ammonia storage material is a compound that contains acidic sites that can bind ammonia. Those skilled in the art divide them into Lewis acid sites for ammonia physisorption and Bronsted acid sites for ammonia chemisorption. The ammonia storage material in the ammonia barrier catalyst according to the invention must contain a significant proportion of Bronsted acid sites and optionally Lewis acid sites to ensure sufficient ammonia storage capacity.

The ammonia storage capacity of the catalyst can be quantified using temperature programmed desorption. In this standard method of characterizing heterogeneous catalysis, the material to be characterized is first baked to remove any absorbed components such as water and then loaded with a defined amount of ammonia gas. This is done at room temperature. The sample is then heated under inert gas at a constant heating rate so that the ammonia previously absorbed by the sample is desorbed and can be quantitatively determined using a suitable analytical method. The amount of ammonia (in ml/g catalyst material) was obtained as a parameter for the ammonia storage capacity, where the term "catalyst material" always refers to the material used for characterization. This parameter depends on the selected heating rate. The values reported in this document are always based on measurements at a heating rate of 4 Kelvin/min.

The catalyst of the present invention is capable of storing at least 20 ml of ammonia per gram of catalyst material in the upper layer. Preference is given to using ammonia storage materials with an ammonia storage capacity of 40-70 ml/g of ammonia storage material, typically eg iron-exchanged zeolites.

The iron-exchanged zeolites used not only have the best ammonia storage capacity but also have good SCR activity. Addition of another component such as an additional SCR catalyst, nitrogen oxide storage material or oxides stable at high temperatures to improve thermal stability enables obtaining an upper layer with a very particularly preferred storage capacity of 25-40 ml ammonia/g catalyst material , wherein the term "catalyst material" refers to a mixture of ammonia storage material and the other component.

The catalysts of the present invention contain a substantial amount of ammonia storage material only in the upper layer. The lower layer does not contain this material. This is a considerable improvement over the solution proposed in EP 0 773 057 A1 with Pt-Cu zeolite in the lower layer and Cu zeolite in the upper layer and thus ammonia storage material throughout the thickness of the catalyst layer . In such an embodiment, the total amount of ammonia storage material in the catalyst is so large that if temperature fluctuations in dynamic operation occur, there is a risk of uncontrolled desorption of ammonia and, as a result, the increased ammonia breakthrough is surprisingly Occurs in dynamic operation, as in the experiments shown by the inventors (cf. comparative example 3). Contrary to this, limiting the ammonia storage material to the upper layer and at the same time limiting the amount used to particularly preferred values avoids "overloading" of the catalyst with ammonia and thus uncontrolled desorption.

In its preferred embodiment, the catalyst according to the invention comprises an oxidation catalyst having a strong oxidizing action in the lower layer. The oxidation catalyst generally comprises a noble metal and an oxidic support material, preferably platinum or palladium or a mixture of platinum and palladium, on a support material selected from the group consisting of activated alumina, zirconia, titania, silica and their mixtures or mixed oxides.

The catalysts of the present invention, when properly sized, can be used as SCR catalysts with reduced ammonia breakthrough compared to conventional catalysts. Furthermore, the catalysts of the invention are suitable as very highly selective ammonia barrier catalysts.

Depending on the size, the catalysts according to the invention are therefore able firstly to reduce nitrogen oxides (ie pollutant gases containing nitrogen in oxidized form) and also to eliminate ammonia (ie pollutant gases containing nitrogen in reduced form) by oxidation.

This versatility is probably attributable in detail to the following reactions, which are schematically illustrated in Figure 1:

1) Nitrogen oxides and ammonia in the exhaust gas are adsorbed on the upper layer (1) which is the SCR active coating and react in a selective catalytic reaction to form water and nitrogen, which are desorbed after the reaction is completed. Here, ammonia is present in suprastoichiometric amounts, ie in excess.

2) Excess ammonia diffuses into the upper layer (1). Ammonia is partially stored there.

3) Ammonia that is not stored passes through the upper layer (1) to layer (2), and has a strong oxidation effect below the layer (2). Here, nitrogen and nitrogen oxides are produced. The nitrogen formed diffuses unchanged through the upper layer (1) and into the atmosphere.

4) Before the nitrogen oxides formed in the lower layer (2) leave the system, they again pass through the coating (1) lying above the oxide layer. Here they react with the prestored ammonia NH _{3 —stored} in the SCR reaction to form N ₂ .

If the noble metals rely on diffusion processes from the lower layer into the upper catalyst layer, this leads to a reduction in the selectivity of the selective catalytic reduction, since the reaction then no longer proceeds as comproportionation to form nitrogen gas but as oxidation to form subvalent nitrogen oxides such as _N2 O. This noble metal diffusion process usually only takes place at high temperatures.

The catalysts of the present invention, when suitably sized, are remarkably suitable for use as SCR catalysts with reduced ammonia breakthrough at temperatures from 150°C to 400°C, especially preferably from 200°C to 350°C. In exhaust gas purification units in vehicles with diesel engines, such temperatures usually occur in the converter in the undercarriage position at the end of the exhaust system group. If a catalyst according to the invention having a sufficient volume is installed in such an exhaust unit at the end of the exhaust system group in the undercarriage converter, the nitrogen oxides produced by the diesel engine can be efficiently removed while avoiding ammonia High secondary emissions.

In a corresponding method for reducing nitrogen-containing polluting gases, ammonia or compounds which can be decomposed into ammonia are introduced into an exhaust gas system upstream of a catalyst according to the invention arranged in an undercarriage location. The use of an additional ammonia barrier catalyst can generally be dispensed with in this method.

The catalysts of the present invention can also be used as extremely effective ammonia barrier catalysts in combination with conventional SCR catalysts. Preference is given here to using SCR catalysts which comprise copper- or iron-exchanged zeolites or copper-and-iron-exchanged zeolites or mixtures thereof. In addition, it is possible to use SCR catalysts comprising vanadium oxide or tungsten oxide or molybdenum oxide on a titanium oxide-containing support material. Various embodiments of the exhaust unit are possible.

The SCR catalyst and the ammonia barrier catalyst according to the invention can thus be present in each case in the form of a coating on an inert honeycomb body, the two honeycomb bodies comprising an inert material, preferably ceramic or metal. The two honeycomb bodies can be present in two converters connected in series or in a common converter, wherein the ammonia barrier catalyst is always arranged downstream of the SCR catalyst. When the catalyst is arranged in a converter, the volume of the ammonia barrier catalyst typically occupies 5-40% of the space available in the converter. The remaining volume is occupied by the SCR catalyst or by the SCR catalyst and possibly a hydrolysis catalyst present at the inflow end. Additionally, an oxidation catalyst for oxidizing mononitrogen oxides to dinitrogen oxides may be arranged upstream of the SCR catalyst.

In a preferred embodiment of the exhaust gas unit, the two honeycombs of the SCR catalyst and the catalyst according to the invention for use as ammonia barrier catalyst form a unit with a front and a rear. The oxidation catalyst representing the lower layer of the ammonia barrier catalyst of the present invention is located only on the rear of the honeycomb body. The upper layer of the ammonia barrier catalyst of the present invention is designed as an SCR catalyst. It may have been deposited over the entire length of the honeycomb body, in which case it is covered with an oxidation catalyst-containing coating.

In a further embodiment of the exhaust unit according to the invention, the SCR catalyst can be in the form of a honeycomb body consisting entirely of SCR-active material (referred to as fully active extruded SCR catalyst). The ammonia barrier catalyst of the present invention is then applied as a coating on the back of the all active extruded catalyst such that the back of the SCR catalyst acts as a support for the ammonia barrier catalyst.

The invention is illustrated below with the aid of comparative examples and examples and FIGS. 1-7.

FIG. 1 : Functional principle of a catalyst according to the invention for removing nitrogen-containing pollutant gases from diesel engine exhaust gases, the catalyst comprising a honeycomb body and at least two superimposed catalytically active layers.

Figure 2: Improved NOx conversion of conventional SCR catalysts by increasing α

Figure 3: Concentration of nitrogen compounds formed in the oxidation of ammonia as a function of temperature on an exhaust gas purification system comprising a conventional SCR catalyst and a non-selective ammonia oxidation catalyst

Figure 4: Effectiveness of oxidation of ammonia on catalysts according to the invention (#2 and #3) compared to reference oxidation catalyst (#1)

Figure 5: Temperature dependence of the selectivity for the oxidation of ammonia to _N for catalysts according to the invention (#2 and #3) compared to a reference oxidation catalyst (#1)

Figure 6: Nitrogen oxide conversion and NH3 breakthrough after hydrothermal aging at 650°C for a catalyst according to the invention (#5) and a conventional SCR catalyst containing an iron-exchanged zeolite (#4).

Figure 7: NH measured on a catalyst according to the invention (#2) _and on a correspondingly pretreated catalyst according to EP 0 773 057 A1 (#6) with an _initial NH concentration of 450 ppm loaded at 200°C Desorption

Comparative Example 1:

In this comparative example, the improvement of the nitrogen oxide conversion over a conventional SCR catalyst due to an increase in the molar ratio α was examined. In this case, the increase in the ammonia concentration necessary to increase the value of α is achieved by introducing an excess of urea. The SCR catalyst comprises a coating of iron-exchanged zeolite on a ceramic honeycomb body. The volume of the honeycomb is 12.51. It has 62 cells/cm ² at a cell wall thickness of 0.17 mm.

The NOx conversion measurements were carried out on an engine test stand equipped with a 6.4 1 6 cylinder Euro3 engine. Six different exhaust gas temperatures (450°C, 400°C, 350°C, 300°C, 250°C, 200°C) are continuously generated using fixed engine points. At each constant engine point, the urea addition was increased stepwise and thus the molar ratio α was varied. Once the gas concentration at the outlet of the catalyst stabilized, the nitrogen oxide conversion and the ammonia concentration downstream of the catalyst were recorded. For example, Figure 2 shows the results for the exhaust gas temperature upstream of the catalyst at 250°C.

Assuming that the ammonia breakthrough will not be greater than 10 ppm, a nitrogen oxide conversion of approximately 45% can be achieved in the example shown. However, the conversion curves indicate that nitrogen oxide conversions as high as 57% will be achievable at higher values of α. In the case of the system examined (conventional SCR catalyst only) this was associated with a considerable ammonia breakthrough (225 ppm). In order to minimize ammonia breakthrough, the catalysts according to the invention should be used as SCR catalysts instead of conventional SCR catalysts or the system should be supplemented by suitable ammonia barrier catalysts.

Comparative example 2:

In this example, two catalysts connected in series were tested in a model gas unit. The two catalysts had the following composition and were applied as a coating on a ceramic honeycomb body with a cell density of 62 cm ⁻² :

First catalyst: conventional SCR catalyst based on V ₂ O ₅ /TiO ₂ ; dimensions of the honeycomb: 25.4 mm diameter, 76.2 mm length

Second catalyst: conventional ammonia barrier catalyst comprising 0.353 g/l Pt (= 10 g/ft ³ Pt) and a mixed oxide mainly containing titanium dioxide; dimensions of the honeycomb: 25.4 mm diameter, 25.4 mm length

Nine different constant temperature points were successively set on the model gas unit. The concentrations of the nitrogen components NH ₃ , N ₂ O, NO and NO ₂ obtained at the outlet of the system were measured with temperature changes using an FTIR spectrometer. The model gas has the following composition:

gas composition concentration Nitrogen oxides NOx 0vppm ammonia 450vppm oxygen 5% by volume water 1.3% by volume Nitrogen the rest Space velocity over total catalyst system: 30 000h ^-1 Space velocity over ammonia barrier catalyst: 120 000h ^-1 Gas temperature (inlet) 550; 500; 400; 350; 300; 250; 200; 175; 150

The measured concentrations of nitrogen components as a function of temperature are shown in graph form in FIG. 3 . Ammonia is efficiently removed from the exhaust gas mixture at temperatures greater than 200°C.

However, at higher temperatures (T > 300° C.), the formation of undesired by-products was observed. As the temperature increases, the formation of nitrogen components with higher oxidation states increases from +I( _N2O ) through +II(NO) to +IV( _NO2 ).

Example 1

The excessive oxidation of nitrogen oxides observed in Comparative Example 2 can be greatly reduced by using the catalyst according to the present invention as an ammonia barrier catalyst while maintaining the same oxidation capacity. The table below shows formulations according to the invention which were tested, for example, as ammonia barrier catalysts.

catalyst describe Precious metal content #1 Reference: Nonselective _NH3 Oxidation Catalysts Containing Platinum on Mixed Oxides Containing Primarily Alumina 0.353g/l #2 Upper layer (1): SCR catalyst based on iron-exchanged zeolite with _NH storage capacity of 58 ml/g catalyst material Lower layer (2): non-selective NH _oxidation catalyst as #1 0.353g/l #3 Upper layer (1): SCR catalyst based on iron-exchanged zeolite to which a barium-based nitrogen storage component is added; NH3 storage capacity of this layer is 29ml/g catalyst material Lower layer (2): non-selective NH3 oxidation as #1 catalyst 0.353g/l

The NH3 conversion activity and selectivity to nitrogen was tested on a model gas unit using the following gas composition:

gas composition concentration nitrogen oxides NOx, 0vppm ammonia 800vppm Propylene C ₃ H ₆ 40vppm CO ₂ 8% by volume oxygen 5% by volume water 1.3% by volume Nitrogen the rest airspeed 320 000h ^-1 gas temperature 550; 500; 450; 400; 350; 300; 250; 200

Compared to Comparative Example 2, a higher space velocity was selected. This amounts to the requirement that the volume of the ammonia barrier catalyst should be kept as small as possible. The selected ammonia concentration is higher than usual in practice and combined with lower noble metal content should ensure better discernibility of the results.

Figure 4 shows the oxidation effectiveness of ammonia: the curves of ammonia concentration versus temperature downstream of the catalysts clearly show that the ammonia quench temperature T ₅₀ (NH ₃ ) and non-selective The ammonia flameout temperature (about 380°C) of the reference NH ₃ oxidation catalyst is in the same range (370°C-390°C). The oxidation activity of all samples tested was equal. Despite the high space velocity, the NH ₃ flameout behavior is not affected by the upper layer. The observed residual _NH3 concentration of approximately 100 ppm at 550 °C may be the result of diffusion limitation attributed to the very high space velocity over the selected catalyst in this experiment.

Selectivity to _N2 can be calculated from the difference between the measured total nitrogen components and the amount of ammonia introduced. This is shown in Figure 5 as a function of temperature.

If the temperature exceeds 400° C., nitrogen oxides are formed as by-products on the reference catalyst. _N2 formation is reversed in this way upon increasing temperature. In contrast, all bilayer catalysts (#2, #3) according to the present invention showed significantly improved selectivity to _N2 .

Example 2

The ammonia breakthrough observed in Comparative Example 1 can be reduced by using the catalyst according to the invention as the SCR catalyst. This is confirmed by a comparison of NOx, conversion and ammonia breakthrough of a conventional SCR catalyst comprising an iron-exchanged zeolite and a catalyst according to the invention. Check out the following catalysts:

#4: Conventional SCR catalyst based on iron-exchanged zeolite as in Comparative Example 1;

Dimensions of the honeycomb body: 25.4mm diameter, 76.2mm length

#5: Catalyst according to the invention; lower layer contains 0.0353 g/l Pd supported on zirconia and alumina (= 1 g/ft ³ Pd); upper layer: based on iron with NH storage capacity of 58 ml/g catalyst material SCR catalysts of exchanged zeolites;

Dimensions of the honeycomb body: 25.4mm diameter, 76.2mm length

Both catalysts were first subjected to synthetic hydrothermal aging at 650 °C in a furnace in an atmosphere of 10 vol% oxygen and 10 vol% water vapor in nitrogen. The SCR conversion activity and ammonia concentration downstream of the catalyst were then tested in a model gas unit under the following conditions:

gas composition concentration Nitrogen oxide NO: 500vppm Ammonia NH ₃ : 450vppm Oxygen O ₂ : 5% by volume Water _H2O : 1.3% by volume Nitrogen N ₂ : the rest airspeed 30 000h ^-1 Gas temperature [°C] 450; 400; 350; 300; 250; 200; 175; 150

The results of the study are shown in FIG. 6 . It is clear that catalyst #5 according to the present invention shows both improved nitrogen oxide conversion and Shows reduced _NH breakthrough.

Comparative Example 3

The catalyst described in EP 0 773 057 A1 was prepared. To this end, a coating of 35 g/l containing 1 wt% platinum and copper-exchanged ZSM-5 zeolite (SiO ₂ :Al ₂ O ₃ ratio 45) was first applied to cells with 62 cells/cm ² and 0.17 mm The zeolite contained 2.4 wt% copper on a ceramic honeycomb body of thickness . After drying and calcining the lower layer, an upper layer comprising 160 g/l copper-exchanged ZSM-5 zeolite (SiO ₂ :Al ₂ O ₃ ratio 45) containing 2.4 wt% copper was applied. This is followed by a fresh drying and calcination. The honeycomb body provided for the test had a diameter of 25.4 mm and a length of 76.2 mm and contained a total of 0.353 g/l of platinum, based on the volume of the honeycomb body.

The resulting catalyst #6 was tested in an ammonia desorption experiment in a model gas unit in comparison to catalyst #2 (upper layer: 160 g/l) from Example 1 according to the invention. To this end, the as-prepared catalyst was first exposed to a gas mixture containing 450 ppm ammonia at 200 °C for about 1 h at a space velocity of 30 000 1/h. The gas mixture additionally contained 5% by volume of oxygen and 1.3% by volume of water vapor in nitrogen. At the end of the loading time, complete breakthrough of the introduced amount of ammonia through the catalyst was observed. Stop introducing ammonia.

After two minutes at constant temperature, the catalyst was heated at a heating rate of 1 degree/second. The amount of desorbed ammonia was measured using an FTIR spectrometer.

Figure 7 shows the results obtained for catalyst #2 according to the invention and comparative catalyst #6 according to EP 0 773 057 A1. In addition to the ammonia concentration measured downstream of the catalyst, the temperature measured upstream of the catalyst during the experiment was also plotted. Only the desorption phase is shown.

In the case of both catalysts, ammonia desorption starts at about 210°C. It can be clearly seen that significantly more ammonia is desorbed from the comparative catalyst #6 compared to the catalyst #2 according to the invention. As stated above, this "overloading" of catalyst #6 with ammonia leads to uncontrolled ammonia desorption if temperature fluctuations occur during dynamic operation and thus to unwanted ammonia breakthrough during driving of the vehicle.

Claims (18)

1. from the waste gas of Diesel engine, remove the catalyst of nitrogenous dusty gas, the coating that this catalyst comprises honeycomb ceramics and is made up of two stacked catalytic active layers, it is characterized in that, be applied to directly that lower floor on this honeycomb ceramics comprises oxidation catalyst and the upper strata that is applied on it comprises the ammonia storage material and has the ammonia storage volume of at least 20 milliliters of ammonia/gram catalyst material.

2. according to the catalyst of claim 1, it is characterized in that this upper strata contains the zeolite of one or more iron exchanges.

3. according to the catalyst of claim 1, it is characterized in that this lower floor does not contain the ammonia storage material.

4. according to the catalyst of claim 3, it is characterized in that being present in oxidation catalyst in this lower floor and be included in the platinum on the carrier material or the mixture of palladium or platinum and palladium, this carrier material is selected from activated alumina, zirconia, titanium oxide, silica and their mixture or mixed oxide.

5. remove the exhaust purification unit of nitrogenous dusty gas from the waste gas of Diesel engine, this clean unit comprises the SCR catalyst and ammonia intercepts catalyst, it is characterized in that

This ammonia intercepts the coating that catalyst comprises honeycomb ceramics and comprises two stacked catalytic active layers, wherein be applied directly to lower floor on this honeycomb ceramics comprise oxidation catalyst and

The upper strata that is applied on it comprises ammonia storage material and has the ammonia storage volume of at least 20 milliliters of ammonia/gram catalyst material.

6. according to the exhaust purification unit of claim 5, it is characterized in that this SCR catalyst also exists with the coating form on honeycomb ceramics and two honeycomb ceramics all comprise the inert material of electing from pottery and metal.

7. according to the exhaust purification unit of claim 6, it is characterized in that two honeycomb ceramics form a unit and this oxidation catalyst with front and rear and are positioned on the rear portion of this honeycomb ceramics.

8. according to the exhaust purification unit of claim 7, it is characterized in that, two honeycomb ceramics form a unit and this oxidation catalyst with front and rear and are positioned on the rear portion of this honeycomb ceramics, and this SCR catalyst deposit is on the whole length of this honeycomb ceramics and cover oxidation catalyst on this honeycomb ceramics rear portion simultaneously.

9. according to the exhaust purification unit of claim 5, it is characterized in that this SCR catalyst is the honeycomb ceramics form, this honeycomb ceramics is made of this SCR catalyst fully.

10. according to the exhaust purification unit of claim 9, it is characterized in that the rear portion of this SCR catalyst intercepts the carrier of catalyst as ammonia.

11. the exhaust purification unit according to claim 5 is characterized in that, other oxidation catalyst is arranged in the upstream of this SCR catalyst, this other oxidation catalyst is used for oxidation of nitric oxide is become nitrogen dioxide.

12. the exhaust purification unit according to claim 5 is characterized in that, this SCR catalyst comprise with the zeolite of copper or iron exchange or with copper and the zeolite of iron exchange or their mixture.

13. the exhaust purification unit according to claim 5 is characterized in that, this SCR catalyst is included in vanadium oxide or tungsten oxide or the molybdenum oxide on the carrier material that contains titanium oxide.

14. from the waste gas of Diesel engine, remove the exhaust purification unit of nitrogenous dusty gas, this clean unit comprises the SCR catalyst, it is characterized in that, this SCR catalyst comprises honeycomb ceramics and comprises the coating of two stacked catalytic active layers, wherein be applied directly to lower floor on this honeycomb ceramics comprise oxidation catalyst and

The upper strata that is applied on it comprises the ammonia storage material and has the ammonia storage volume of at least 20 milliliters of ammonia/gram catalyst material.

15. the method for nitrogenous dusty gas is characterized in that in the minimizing diesel engine exhaust, uses exhaust purification unit, this exhaust purification unit to have to be arranged in each the converter of catalyst according to claim 1-4 that comprises of chassis upper/lower positions.

16. the method according to claim 15 is characterized in that, ammonia maybe can be decomposed the compound of ammonification and introduce in the waste gas stream of this catalyst upstream.

17. the method according to claim 15 is characterized in that, the temperature in this catalyst is 150 ℃-400 ℃.

18. the method according to claim 15 is characterized in that, does not use additional ammonia to intercept catalyst in the downstream of this catalyst.

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