DE102008016177A1 - Harnstoffhydrolysekatalysator - Google Patents

Abstract

The present invention relates to a urea hydrolysis catalyst comprising a catalytically active composition containing a metal-exchanged zeolite and / or zirconia, and to the use thereof.

Description

The The present invention relates to a urea hydrolysis catalyst comprising a catalytically active composition containing a metal-exchanged Zeolites and / or zirconium dioxide and its use in one Emission control system for mobile and stationary Incinerators.

by virtue of the harmful effect of nitrogen oxide emissions the environment is important to further reduce these emissions. Clear lower NOx emission limits for stationary and automotive exhaust gases now commonplace in the United States States envisaged in the near future and will also be in the European Union discussed.

The denitrification of exhaust gases is also referred to as DeNOx. In automotive engineering, selective catalytic reduction (SCR) is one of the most important DeNOx techniques. As the reducing agent commonly used hydrocarbons (HC-SCR) or ammonia (NH ₃ -SCR) and NH ₃ precursors such as urea (AdBlue ^®).

The Elimination of HC (hydrocarbons) and CO emissions from the Diesel exhaust can be comparatively easy by an oxidation catalyst respectively. Diesel oxidation catalysts (DOC) exist in the Essentially from a support structure made of ceramic, a Oxide mixture (washcoat) and from the catalytically active noble metal components such as platinum, palladium and rhodium.

The DOC fulfills the function that CO and HC are oxidized on the catalyst to CO ₂ and H ₂ and the emitted particles, which consist partly of hydrocarbons, are desorbed from the particle core at increasing temperatures. The oxidation of these hydrocarbons in the DOC reduces the particle mass.

Of the DOC can also be used as a catalytic burner (cat burner) to boost the exhaust gas temperature are used for. B. in the particle filter regeneration.

Next requires an emission control system, as for example in the WO 2004/079170 or the WO 2004/022935 as well as the WO 03/054364 described is a NOx storage catalyst (NSC) of NO ₂ , but can not save NO.

Therefore, the NO components are first oxidized to NO ₂ in an upstream or integrated oxidation catalyst.

The NO ₂ is typically stored by reacting with compounds on the catalyst surface (eg, barium carbonate BaCO ₃ as a storage material) and oxygen from the diesel exhaust to form nitrates.

At the end of the injection phase, the catalyst typically needs to be regenerated. For this purpose, specific conditions must be set in the exhaust filter (Λ1). In the exhaust gas is then so much reducing agent present (CO, H _2, and various hydrocarbons) that the nitrate binding dissolved abruptly and the released NO ₂ is reduced directly to the noble metal catalyst to N _2.

The Regeneration typically takes about 2 to 10 seconds Selective catalytic reduction is based on the principle that selected reducing agents in the presence of oxygen selectively reduce nitrogen oxides. Selective here means that the oxidation of the reducing agent is preferred (selective), the with the oxygen nitrogen oxides and not with that in the exhaust gas substantially abundant molecular oxygen available. Ammonia or Ammonia precursors have themselves as reducing agent proven with the highest selectivity.

typically, In this case, urea is used in place of the pure ammonia in particular used because of its non-toxicity. Urea also points a very good solubility in water and therefore can either as a solid or as an easy-to-dose aqueous Solution are added to the exhaust gas.

at a mass constellation of 32.5 wt .-% urea in water has the freezing point at -11 ° a local minimum, wherein a eutectic forms, thereby segregating the solution is excluded in the case of freezing.

Robert Bosch GmbH has developed the so-called DeNoxtronic1 system for the precise metering of the reducing agent. Urea water solutions are offered under the brand name AdBlue ^® .

Before the actual SCR reaction, ammonia must first be formed from urea. This is done in two reaction steps, collectively referred to as the hydrolysis reaction. First, NH ₃ and isocyanic acid are formed in a thermolysis reaction: (NH ₂ ) CO → NH ₃ + HNCO (thermolysis)

Subsequently, in a catalytic hydrolysis reaction, the isocyanic acid is reacted with water to form ammonia and carbon dioxide. HNCO + H ₂ O → NH ₃ + CO ₂ (hydrolysis)

to Avoiding solid precipitates requires that the second reaction by choosing suitable catalysts and enough high temperatures (from 250 °) sufficiently fast.

The ammonia produced by the thermohydrolysis reacts on the SCR catalyst according to the following equations: 4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (1) NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O (2) 6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O (3)

At low temperatures in the exhaust system (<300 °), the conversion proceeds predominantly via the reaction (2). For a good low-temperature conversion, it is therefore necessary to set a NO ₂ : NO ratio of about 1: 1. Under these circumstances, the reaction (2) can already take place at temperatures from 170 ° to 200 °.

The oxidation of NO to NO _x occurs on the above-described upstream oxidation catalyst, which is therefore essential for optimum efficiency of an exhaust gas purification system for diesel engines.

Further include classic diesel engine emission control systems like this Particulate filter (DPF), which emitted by a diesel engine Soot particles from the exhaust gas with efficiencies of up to 95% can remove.

Currently commonly used urea hydrolysis catalysts mainly comprise titanium dioxide in anatase modification. As used herein, as used in the art, the term "urea hydrolysis catalyst" is used to refer to a catalyst which, strictly speaking, catalyzes only the hydrolysis of isocyanic acid, ie, the product of thermolytic decomposition of e.g. As urea or other NH ₃ forming reducing agents. However, the thermolysis also takes place mainly on the urea hydrolysis catalyst.

Examples of this can be found in the JP 2006122792 , of the JP 2004313917 as well as the US 2006/0257303 ,

Becomes this catalyst can be exposed to high exhaust temperatures, so it can come to a phase transition from anatase to rutile. The rutile phase has a considerably lower activity the catalysed reaction of isocyanic acid to ammonia on. In addition, the specific surface area decreases of the catalyst significantly. This catalyst aging occurs in particular at a temperature of 600 ° C.

Around to reduce or delay this aging either different additives are added to the titanium dioxide, in particular silicon oxides, lanthanum oxides or tungsten oxides. However, these additives have a negative impact on performance of the catalyst and reduce in particular the conversion of the Isocyanic acid to ammonia.

To circumvent this problem, various arrangements of the urea hydrolysis catalyst have been proposed in the exhaust gas purification system, such as in the EP 1852582 A1 and EP 1852583 A1 ,

Indeed It may be due to a relatively close-coupled arrangement of the urea hydrolysis in the Exhaust system to temperatures at the catalytic converter of up to 750 ° C come in, especially at full load points in engine operation Cars occur, for. B. at high acceleration or steep gradients. Alternatively, the urea hydrolysis catalyst may also be behind the diesel particulate filter are arranged. However, there are in the above-described thermal regeneration of the filter also temperature peaks, which also in a temperature range of 750 ° C and more.

It Therefore, there was a need to provide a urea hydrolysis catalyst to provide, which is particularly temperature stable and a low Has aging behavior and increased by short-term occurring Temperature peaks little in its catalytic activity being affected.

These Task is solved by a urea hydrolysis catalyst comprising a catalytically active composition containing a metal exchanged zeolites and / or zirconia.

It is known that the incorporation (doping) of metals into zeolites (ie, more specifically, this typically involves replacement of the lattice H or NH ₄ ions by the corresponding metal ions) contributes to good high temperature stability and hydrothermal stability of these species. The same applies to undoped or doped zirconium dioxide. In addition, both materials have a catalytically good activity with respect to the hydrolysis of isocyanic acid. Zirconium dioxide is extremely high temperature resistant and is also used as a material for refractory ceramics and technical ceramics, especially in mechanical engineering. A phase transition to catalytically inactive phases occurs in zirconium dioxide only from about 1173 ° C. From room temperature to this temperature is the catalytically relevant for the hydrolysis reaction monoclinic phase (Baddeleyit), which catalytically from 1173 ° C in one for the hydrolysis reaction almost in active tetragonal phase converts.

Especially begins the catalytic activity of the materials of the urea hydrolysis catalyst according to the invention already at about 120 ° C and is also caused by short-term high temperature peaks are not affected.

The term "zeolite" is used in the context of the present invention as defined by the International Mineralogical Association ( DS Coombs et al., Can. Mineralogist, 35, 1997, 1571 ) a crystalline substance from the group of aluminum silicates with spatial network structure of the general formula M ^{n +} [(AlO ₂ ) x (SiO ₂ ) y] x H ₂ O understood that consist of SiO ₄ / AlO ₄ tetrahedra, which are linked by common oxygen atoms to a regular three-dimensional network. The ratio of Si / Al = y / x is always ≥ 1 according to the so-called "Löwenstein rule", which prohibits the adjacent occurrence of two adjacent negatively charged AlO ₄ tetrahedra. Although there are more exchange sites for metals available at a low Si / Al ratio, the zeolite is becoming increasingly thermally unstable.

The Zeolite structure contains cavities and channels, which are characteristic of each zeolite. The zeolites become different structures according to their topology (see above) divided. Contains the zeolite framework open cavities in the form of channels and cages, usually of water molecules and extra framework cations, which can be exchanged are occupied. On an aluminum atom comes an excess negative charge that is compensated by these cations. The interior of the pore system represents the catalytically active surface. The more aluminum and the less silicon a zeolite contains, the denser is the negative charge in its lattice and the more polar its inner surface. The pore size and Structure is in addition to the parameters in the production (use or type of template, pH, pressure, temperature, presence of seed crystals) determined by the Si / Al ratio, which is the largest Determine part of the catalytic character of a zeolite.

The presence of 2- or 3-valent cations as a tetrahedral center in the zeolite framework, the zeolite receives a negative charge in the form of so-called anion sites, in the vicinity of which are the corresponding cation positions. The negative charge is compensated by the incorporation of cations in the pores of the zeolite material. The zeolites are mainly distinguished by the geometry of the cavities formed by the rigid network of SiO ₄ / AlO ₄ tetrahedra. The entrances to the cavities are formed by 8, 10 or 12 "rings" (narrow, medium and large pore zeolites). Certain zeolites show a uniform structure structure (eg ZSM-5 with MFI topology) with linear or zigzag running channels, in others close behind the pore openings larger cavities, eg. B. in the Y and A zeolites, with the topologies FAU and LTA.

According to the invention preferred are zeolites with the topologies BEA, MFI, SAPO (especially SAPO-34) and TS (especially TS-1). TS-1 is strictly speaking not a zeolite, but a titanium silicalite. The three-dimensional grid or cage structure corresponds in Essentially those of ZSM5. For titanium silicalite that exists Lattice of Si and Ti tetrahedra. Important is the distinction that Ti is not present here by ion exchange in the zeolite, but instead is an integral part of the grid. Al is not present in the grid. However, the thermal stability of TS is very good.

It usually gives three different centers in zeolites, as so-called α-, β- and γ-positions which are the location of the exchange places (also referred to as "interchangeable positions or positions") define. Most preferred are MFI zeolites such as ZSM-5 or ZSM 12, as well as the topologies of SAPO-34 and TS-1.

The inventively preferred SiO ₂ / Al ₂ O ₃ modulus (in the case of TS this would be the SiO ₂ / TiO ₂ ratio) (molar ratio) is in the range from 5: 1 to 300: 1, particularly preferably in the range from 20: 1 to 200 1, and most preferably in the range of 30: 1 to 150: 1. Basically, the higher the modulus, the more temperature stable the zeolite is.

Prefers is the zeolite and / or ZrO 2 in an amount of 50 to 99 wt .-%, particularly preferably in an amount of 75 to 95 wt .-% in the catalytic active composition.

Next the amount of zeolite is as already explained above also on the type of zeolite used, including combinations the aforementioned preferred zeolites can be used. The catalysts mentioned and methods for their preparation are known in the art.

According to the invention the zeolite is a metal-exchanged zeolite, for example a iron-exchanged zeolite, as iron-exchanged zeolites particularly are temperature stable.

at Metal exchanged or doped zeolites becomes the metal content or the degree of exchange of a zeolite decisively determines the metal species present in the zeolite. This allows the Zeolite both with a single metal or with different ones Be doped metals. The preferred metals for the Exchange and doping are catalytically active metals such as Fe, Co, Cu, Ce and Ag. The zeolites can partially, be completely or else exchanged. From over-talking speaks if, for example, more iron is present in the zeolite than theoretically given by the negative charges of the grid is. This iron is then usually present as iron oxide and does not disturb the actual, catalytic reaction. About Exchanged Zeolites are relatively easy to exchange via solid ions produce.

The Manufacturing process for metal-exchanged zeolites, for example via solid or liquid phase exchange, are known in the art.

As well Advantageously, both components, namely the metal exchanged zeolite and zirconia, in the catalytic active composition.

All particularly preferably the metal-exchanged zeolite and / or the zirconia in a proportion of 80% by weight to 95% by weight in the catalytically active composition.

The Catalytically active composition is preferably supported applied. The carrier typically consists of a ceramic, a metal or a metal alloy.

Of the Carrier is in the form of a honeycomb body, a monolith, a pipe or a foam.

Instead of the above-mentioned carrier may be the urea hydrolysis catalyst as such already for hydrolysis in one of the usual components an exhaust gas cleaning system may be used, for example as Part of an evaporator unit, as part of a dosing, as Part of a feed line with the urea hydrolysis with the exhaust pipe is connectable or at least parts of a metering line for adding the gaseous substance mixture to the actual Urea hydrolysis catalyst or parts of the compound unit. Also For example, mixer elements can act as carriers that z. B. as simple baffles, generating vortex Internals to structured monoliths with cross-mixing over several channels (such as the type MX of the company Emitec) are designed.

This increases the conversion efficiency and allows it, the urea hydrolysis catalyst correspondingly smaller volume with a smaller catalytically active surface form.

The Application of the catalytically active composition of the urea hydrolysis catalyst takes place doing so by methods known in the art, for example by applying a washcoat, by coating in a dip or by spray coating.

Of the urea hydrolysis catalyst according to the invention Accordingly, it is ideal for use in emission control systems or to reduce nitrogen oxide emissions of mobile and stationary incinerators.

mobile Combustion devices in the context of the present invention are For example, internal combustion engines of motor vehicles, in particular Diesel engines, generating sets based on internal combustion engines or other units based on internal combustion engines.

at the stationary combustion devices are usually around power plants, combustion plants, Waste incineration plants and also to heating systems of Private households.

The The invention is explained below with reference to some examples, without these examples being understood as limiting should be.

1. Preparation of washcoats for the coating of hydrolysis catalysts according to the invention

1.1. ZrO ₂ washcoat

80 g of ZrO ₂ powder were mixed with 100 g of ZrO ₂ sol (solids content 20%, nitrate-stabilized) with 20 g of silicone resin emulsion.

1.2. Fe-ZSM5 washcoat

There were 80 g of Fe-ZSM5 powder (Fe content 6 wt .-%, Fa. Süd-Chemie), 45 g of SiO ₂ sol (solids content 40%, stabilized with ammonium hydroxide) and 0.5 g Tylose mixed together.

1.3. TiO ₂ washcoat

80 g of TiO ₂ powder, 170 g of TiO ₂ sol (solids content 12%, nitrate-stabilized) were mixed with 20 g of silicone resin emulsion.

Subsequently, a coating of a metallic monolith (available from Emitec, type designation ST400, standard structure 400 cpsi cell size).

The Coating was carried out by common coating methods. In general, the substrates were immersed in the washcoat suspension and then the excess washcoat emptied. It can be used both with overpressure (blowing out the monolith) or negative pressure (suction of the monolith) are worked. If Structured substrates were also used, the emptying of the Substrate proved by means of a centrifuge advantageous.

2. Activity investigations the catalysts of the invention.

For the following activity studies were measurements on the inventive according to Example 1 coated monoliths performed.

The activity measurements were carried out under the following conditions:
GHSV: 52,000 h ^-1 , 1000 ppm HNCO (isocyanic acid), 5% H ₂ O, 10% O ₂ , N ₂ as gas to adjust the equilibrium.

1 clearly shows the decrease in activity in the conversion of NH ₃ with respect to the temperature during the conversion of anatase to rutile in the case of TiO ₂ . Rutile shows a much lower activity in the low temperature range compared to anatase. This significantly worse cold start properties are connected because the urea dosing in the vehicle can be activated only at higher temperatures. The best conversion showed ZrO ₂ over the entire temperature range, followed by Fe-ZSM5. ZrO ₂ and Fe-doped zeolites are structurally known to be extremely hydrothermally stable and therefore particularly preferred for use according to the invention as a high-temperature-stable hydrolysis catalyst.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• WO 2004/079170 [0007]
• WO 2004/022935 [0007]
• WO 03/054364 [0007]
• - JP 2006122792 [0023]
• - JP 2004313917 [0023]
• US 2006/0257303 [0023]
• EP 1852582 A1 [0026]
• EP 1852583 A1 [0026]

Cited non-patent literature

• DS Coombs et al., Can. Mineralogist, 35, 1997, 1571 [0032]

Claims (9)

Urea hydrolysis catalyst comprising a catalytic active composition containing a metal-exchanged zeolite and / or zirconia. A urea hydrolysis catalyst according to claim 1, wherein the metal exchanged zeolite and / or the zirconia in a proportion of 50 wt .-% to 99 wt .-% in the catalytically active Composition are included. Urea hydrolysis catalyst according to claim 2, wherein the metal-exchanged zeolite has a SiO ₂ / Al ₂ O ₃ ratio of 5: 1 to 150: 1. A urea hydrolysis catalyst according to claim 3, wherein the zeolite is selected from the group consisting of MFI, BEA, SAPO and TS. A urea hydrolysis catalyst according to claim 4, wherein the zeolite in whole or in part with a metal from the group consisting of Fe, Co, Cu, Ce and Ag. A urea hydrolysis catalyst according to claim 5, wherein the catalytically active composition on a support is applied. A urea hydrolysis catalyst according to claim 6, wherein the carrier of a ceramic, a metal or a metal alloy consists. A urea hydrolysis catalyst according to claim 7, wherein the carrier in the form of a honeycomb body, a monolith, a Pipe of a mixer element or a foam is present. Emission control system for denitrification of exhaust gases containing mobile and stationary incinerators a urea hydrolysis catalyst according to any one of claims 1 to 8.