DE102007061441A1 - Selective catalytic reduction of nitrogen oxides in automobile exhaust gas, is carried out using ammonia obtained by decomposing guanidine salt solution over non-oxidizing catalytic coating - Google Patents

Abstract

Selective catalytic reduction of nitrogen oxides in automobile exhaust gas is carried out with ammonia (NH 3) obtained by decomposing solutions of guanidine salts (I) of NH 3 formation potential 40-850 g/kg (optionally containing urea, NH 3 and/or ammonium salts) in presence of non-oxidizing catalytic coatings (II) of titanium dioxide, aluminum oxide, silicon dioxide and/or hydrothermally stabile, at least partially metal-exchanged zeolite.

Description

The The present invention relates to a selective catalytic process Reduction of nitrogen oxides in exhaust gases of vehicles with the help of Guanidine salt solutions, from the corresponding guanidine salts by evaporation and Catalytic decomposition ammonia is generated and this as a reducing agent for the subsequent Selective catalytic reduction of nitrogen oxides is used.

According to the state of the art, ammonia (NH ₃ ) is used in the selective catalytic reduction of nitrogen oxides in oxygen-containing exhaust gases of vehicles as reducing agents, in front of a special SCR catalytic converter or upstream of a group of parallel-flowable SCR catalytic converter modules integrated in a silencer incineration plants and internal combustion engines, in particular those of internal combustion engines of vehicles, initiated and causes in the SCR catalysts, the reduction of the nitrogen oxides contained in the exhaust gas. SCR means selective catalytic reduction of nitrogen oxides (NO _x ) in the presence of oxygen.

For the generation of ammonia, especially in vehicles, are so far different liquid and solid Ammonia precursors become known, which are described in more detail below.

For commercial vehicles, the use of an aqueous, eutectic solution of urea in water (AdBlue ^™ ) containing 32.5% by weight of urea, a freezing point of -11 ° C. and an ammonia formation potential of 0.2 kg / kg has been described Enforced ammonia precursor substance. For operation of the SCR system at temperatures down to -30 ° C, ie up to the cold flow plugging point (CFPP, lower operating temperature) of the winter quality diesel fuel, a comparatively expensive supplementary heating of the tank, lines and valves for the AdBlue is prone to malfunctioning. Use and for AdBlue logistics in cold climates in winter required.

The ammonia required for the catalytic reduction of NO _x is formed during the thermal decomposition of the urea. The following reactions are relevant for this: Urea can not be vaporized, but decays on heating primarily in isocyanic acid (HNCO) and ammonia (NH ₃ ) according to equation [1]. (H ₂ N ₂ ) CO → HNCO + NH ₃ [1]

The isocyanic can easily become non-volatile Substances, such as cyanuric acid polymerize. Here you can operational disturbing Deposits on valves, on injection nozzles and in the exhaust pipe arise.

The isocyanic acid (HNCO) is hydrolyzed in the presence of water (H ₂ O) to ammonia (NH ₃ ) and carbon dioxide (CO ₂ ) according to equation [2]. HNCO + H ₂ O → NH ₂ + CO ₂ [2].

The reaction [2] takes place very slowly in the gas phase. On the other hand, it proceeds very rapidly to metal oxide and / or zeolite catalysts, somewhat more slowly to the metal oxide catalysts which are strongly acidic due to their WO ₃ content, such as the SCR catalysts based on a mixed oxide of vanadium oxide, tungsten oxide and titanium oxide.

at the known in connection with motor vehicles applications of Urea SCR catalyst systems will usually be the engine exhaust taking advantage of its heat content used for the thermal decomposition of urea after reaction [1]. In principle, the reaction [1] can already take place before the SCR catalyst expire while the reaction [2] must be catalytically accelerated. in principle can the reactions [1] and [2] also take place on the SCR catalyst whose SCR activity is reduced thereby.

For countries in a cold climate, it is advantageous to be able to use a freeze-proof ammonia precursor substance. By adding ammonium formate to the solution of urea in water, the freezing point can be lowered significantly. This additional heaters are superfluous and achieved considerable savings in manufacturing and logistics costs. A solution of 26.2% ammonium formate and 20.1% urea in water has a freezing point of -30 ° C and is commercially available under the designation Denoxium-30 and can advantageously replace the AdBlue ^™ in the cold season (SAE technical papers 2005 -01-1856).

By adding ammonium formate to the solution of urea in water, in a solution of 35% ammonium formate and 30% urea in water, the ammonia formation potential can be increased from 0.2 kg / kg to 0.3 kg / kg. Thus, the range of the vehicle is increased by one-third with a filling of the ammonia precursor substance and in passenger cars generally increases the possibility of a permanent filling between the inspection intervals. A disadvantage of this measure is the rise in the freezing point of the solution to the range of -11 to -15 ° C (Denoxium January 2005, www.kemira.com ).

In the EP 487 886 A1 A method for the quantitative decomposition of an aqueous solution of urea in water by hydrolysis of ammonia (NH ₃ ) and carbon dioxide (CO ₂ ) in a temperature range of 160 to 550 ° C is proposed, resulting in the formation of undesirable isocyanate acid and its solid secondary products is avoided. In this known method, the urea solution is first sprayed by means of a nozzle on a located in or outside of the exhaust gas evaporator / catalyst. The resulting gaseous products are passed for aftertreatment over a hydrolysis catalyst to achieve a quantitative formation of ammonia.

From the EP 555 746 A1 a method is known, wherein the evaporator due to its design, the urea solution homogeneously distributed so that the contact of the droplets is ensured with the channel walls of the decomposition catalyst. Homogeneous distribution prevents deposits on the catalysts and reduces the slip of excess reducing agent. The urea dosing should only be activated at exhaust gas temperatures above 160 ° C, as falling below this temperature unwanted deposits are formed.

The transfer of Ammonium formate as Ammoniakvoräufersubstanz in ammonia is by injection the aqueous solution in the hot Exhaust gas possible by simple sublimation without special pretreatment. adversely is a simultaneous release of very corrosive formic acid and the possible degeneration of ammonium formate on the surface of the SCR catalyst Exhaust gas temperatures below 250 ° C. The pore system of the SCR catalyst is clogged with temperature reversibility.

The present invention therefore an object of the invention to develop a method for the selective catalytic reduction of nitrogen oxides with ammonia in exhaust gases of vehicles, which does not have the disadvantages mentioned in the prior art, but wherein a technically simple generation of ammonia for NO _x Reduction is made possible by the SCR process and no unwanted by-products are formed in the decomposition.

These Task was inventively characterized solved, that you have solutions of guanidine salts with an ammonia formation potential of 40 to 850 g / kg optionally in combination with urea and / or ammonia and / or Ammonium salts in the presence of catalytically active, not oxidation active Coatings of oxides selected from the group titanium dioxide, Alumina, silica and hydrothermally stable metal zeolites or their mixtures decomposed catalytically.

It Surprisingly, it has turned out that with the aid of the method according to the invention achieved a reduction of nitrogen oxides in vehicle exhaust gases by about 90% can be. Besides that is with the inventively proposed Guanidine salts an increase the ammonia formation potential of 0.2 kg according to the state the technique up to 0.4 kg of ammonia per liter of guanidine salt at the same time Winter resistance (freezing point below -25 ° C) possible.

anzusehen, wobei

R

= H, NH₂, C₁-C₁₂-Alkyl,

X^⊝

= Acetat, Carbonat, Cyanat, Formiat, Hydroxid, Methylat und Oxalat, bedeuten.

For the selective catalytic reduction of nitrogen oxides with ammonia in possibly oxygen-containing exhaust gases of vehicles according to the invention guanidine salts are used which have an ammonia formation potential 40 to 850 g / kg, in particular 80 to 850 g / kg and more preferably 250 to 600 g / kg , Particularly preferred are guanidine salts of the general formula (I)

to look at,

R

= H, NH ₂ , C ₁ -C ₁₂ -alkyl,

X ^⊝

= Acetate, carbonate, cyanate, formate, hydroxide, methylate and oxalate.

It is within the scope of the present invention readily possible, a Mixture of two or more different guanidine salts to use. According to one preferred embodiment are used in the invention Guanidine salts with urea and / or ammonia and / or ammonium salts combined. The mixing ratios of guanidine salt with urea and ammonia or ammonium salts can be varied within wide limits, but it has proven to be special proved advantageous that the mixture of guanidine salt and urea a guanidine salt content of 5 to 60 wt .-% and a urea content from 5 to 35% by weight. Furthermore, mixtures of guanidine salts and ammonia or ammonium salts containing guanidine salt from 5 to 60% by weight and of ammonia or ammonium salt from 5 to 40 wt .-% to be regarded as preferred.

R

= H, NH₂, C₁-C₁₂-Alkyl,

X^⊝

= Acetat, Carbonat, Cyanat, Formiat, Hydroxid, Methylat und Oxalat bedeuten.

In particular, compounds of the general formula (II) have proven to be suitable as ammonium salts R-NH ₃ ^⊕ X ^⊝ (II) in which

R

= H, NH ₂ , C ₁ -C ₁₂ -alkyl,

X ^⊝

= Acetate, carbonate, cyanate, formate, hydroxide, methylate and oxalate.

The guanidine salts used according to the invention and optionally the other components consisting of urea or ammonium salts are used in the form of a solution, with water and / or a C ₁ -C ₄ -alcohol being the preferred solvents. The aqueous and / or alcoholic solutions in this case have a preferred solids content of 5 to 85 wt .-%, in particular 30 to 80 wt .-%, on.

The solution the guanidine salt or the mixtures of guanidine salts possibly also in combination with urea in water in this case have a preferred Ammonia formation potential of 0.2 to 0.5 kg of ammonia per liter Solution, in particular 0.25 to 0.35 kg of ammonia per liter of solution.

It is to be regarded as essential to the invention that the guanidine salts and optionally the further components of a catalytic decomposition are subjected to ammonia in the preferred temperature range of 150 to 350 ° C, carbon dioxide and possibly carbon monoxide being formed as further components. This decomposition of the guanidine salts to ammonia is in this case in the presence of catalytically active, non-oxidation active coatings of oxides selected from the group of titanium dioxide, alumina and silica and mixtures thereof, hydrothermally stable zeolites which are completely or partially metal exchanged, in particular iron zeolites of ZSM 5 or BEA type, made. Suitable metals in this case are in particular the subgroup elements and preferably iron or copper. The corresponding Fe zeolite material is prepared by known methods such. B. the solid-state exchange method, for example. Made with FeCl ₂ , then applied in the form of a slurry on the substrate (such as, for example, cordierite monolith), dried or at higher temperatures (about 500 ° C) calcined.

The Metal oxides such as titanium oxide, alumina and silica and their mixtures are preferably on metallic support materials such as B. heating conductor alloys (especially chromium-aluminum steels) applied.

Preferably may be the catalytic decomposition of the guanidine salts or the other components also occur to ammonia and carbon dioxide and possibly carbon monoxide, in addition to a catalyst with non-oxidation-active coatings still a catalyst with oxidation-active coatings of oxides, selected from the group titania, alumina and silica and their Mixtures, hydrothermally stable zeolites, which are completely or are partially metal exchanged, is used, wherein the coatings are impregnated with gold and / or palladium as oxidation-active components. The corresponding catalysts with palladium and / or gold as Active components preferably have a precious metal content of 0.001 up to 2% by weight. With the help of such oxidation catalysts it is possible the unwanted Formation of carbon monoxide as a byproduct in the decomposition of Guanidine salts already in the ammonia production to avoid.

It In the context of the present invention, it is possible to use a catalyst consisting of two sections, the first section non-oxidation-active coatings and the second section oxidation-active Contains coatings. Preferably, 5 to 90% by volume of this catalyst does not consist oxidation-active coatings and 10 to 95% by volume of oxidation-active Coatings. Alternatively, one can see the catalytic decomposition also in the presence of two catalysts arranged one behind the other carry out, wherein the first catalyst is not oxidation active coatings and the second catalyst contains oxidation active coatings.

The catalytic decomposition of the guanidine salts used according to the invention as well as possibly the other components to ammonia can exhaust gas internally in the main, partial or secondary stream or exhausts in an autobaren and externally heated arrangement of the vehicle exhaust gases are made.

With Help the invention proposed Guanidine salts it is possible a reduction of nitrogen oxides in vehicle exhaust gases by approx. 90% to achieve. After all is also the risk of corrosion of the inventively used guanidine salt solutions in Comparison to solutions significantly reduced with ammonium formate.

The The following examples are intended to explain the invention in more detail.

Examples

example 1

Use of an aqueous, 40% strength by weight guanidinium formate solution, (GF) (mp <-20 ° C.) as ammonia precursor substance in the autobaric ammonia generator as described in US Pat 1

A car engine 1 generates an exhaust gas flow of 200 Nm ³ / h, which passes through the intermediate pipe 2 via a platinum oxidation catalyst 3 and a particle filter 4 in the exhaust intermediate pipe 6 is directed. The one with the FTIR gas analyzer 5 Measured exhaust gas composition in the intermediate pipe 6 is: 150 ppm Nitric oxide, NO; 150 ppm nitrogen dioxide, NO ₂ ; 7% carbon dioxide, CO ₂ ; 8% water vapor, 10 ppm carbon monoxide, CO.

In a tank container 7 there is a GF solution 8th by means of a metering pump 9 via a supply line 10 and a nozzle 12 in a reactor 11 is sprayed. The reactor 11 consists of a 250 ° C heated, vertical tube with 51 mm inner diameter made of austenitic stainless steel and has a heating jacket 15 , In the reactor 11 are the catalysts 13 and 14 , The catalysts are coated with titanium dioxide from the Fa. Südchemie AG, D-Heufeld coated metal support, diameter 50 mm, length 200 mm; Manufacturer of metal carriers: Emitec GmbH, D-53797 Lohmar. The catalyst 13 Based on a large-cell carrier type MX / PE 40 cpsi, 100 mm long. Downstream is the catalyst 14 made of the fine-celled carrier type MX / PE 200 cpsi, 100 mm long. The face of the large-cell catalyst 13 gets out of a nozzle 12 by means of pressure metering pump 9 with a GF solution 8th sprayed. The nozzle 12 is in the reactor 11 axially and upstream of the large-particle catalyst 13 arranged. At the catalyst 13 becomes the water content of the GF solution 8th evaporated and on the catalysts 13 and 14 the GF is thermohydrolytically decomposed so that the formation of the intermediates urea and isocyanic acid, HNCO is avoided.

The resulting mixture of ammonia, carbon dioxide, carbon monoxide and water vapor is supplied via the supply pipe 16 in the exhaust intermediate pipe 6 upstream of an SCR catalyst 18 at 300 ° C in the with the catalyst 3 and the filter 4 pretreated exhaust gas (200 Nm ³ / h) of the passenger car engine 1 initiated. The dosage of GF solution 8th This is how it works with the pressure metering pump 9 that regulates with the FTIR gas analyzer 17 an ammonia concentration of 270 ppm can be measured. At the same time, the CO concentration increases by 90 to 100 ppm due to the decomposition of the formate portion of the GF solution 8th , The increase in CO ₂ and water vapor content due to evaporation and decomposition of the GF solution 8th is expected to be low and almost impossible to measure. The catalytic hydrolysis of GF is complete, as with the gas analyzer 17 no isocyanic acid, HNCO can be detected and no deposits of urea and its decomposition products can be detected.

Downstream of the SCR catalyst 18 comes with the FTIR gas analyzer 19 a reduction in the concentration of NO and NO ₂ measured by 90% to 30 ppm. This is a complete implementation of the ammonia, NH ₃ with NO and NO ₂ to nitrogen, N ₂ . The concentration of ammonia after SCR catalyst 19 is <2 ppm.

The FTIR gas analyzers 5 . 17 and 19 allow simultaneous exhaust gas analysis of the components NO, NO ₂ , CO, CO ₂ , H ₂ O, ammonia, NH ₃ and isocyanic acid, HNCO.

Example 2

The procedure is analogous to Example 1, the titanium dioxide catalyst 14 However, it is replaced by a palladium oxide titanium dioxide catalyst, wherein the titanium dioxide was impregnated with an aqueous Pd (NO ₃ ) ₂ solution so that after drying and calcination (5 hours at 500 ° C), a catalyst is formed, the 1 wt .-% PdO (= about 0.9 wt .-% Pd), which causes a partial oxidation of the carbon monoxide. At the FTIR gas analyzer 17 no increase in CO concentration is measurable.

Example 3

The procedure is analogous to Example 1, but instead of the 40 wt .-% strength Guandiniumformiat solution, a 15 wt .-% strength di-guanidinium carbonate solution is used. The reactor 11 is also heated at 250 ° C, the catalysts 13 and 14 are identical to those of Example 1.

At the gas analyzer 17 no by-product is detected (<1 ppm), the CO ₂ increase corresponds to about 40 ppm of the expectation; at the gas analyzer 19 a reduction of the concentration of NO and NO ₂ by about 92% to 25 ppm is detectable.

Example 4

The procedure is analogous to Example 1, but the catalysts exist 13 and 14 from Al ₂ O ₃ and the reactor 11 is operated at a temperature of 350 ° C.

In the gas analyzer 17 As by-products, only CO (80 ppm) and HCN (<10 ppm) are measured; after the SCR catalyst are at the gas analyzer 19 a reduction of NO and NO ₂ by 85% to 45 ppm both no HCN measured.

Claims (18)

Method of selective catalytic reduction of nitrogen oxides with ammonia in exhaust gases of vehicles, characterized in that solutions of guanidine salts having an ammonia formation potential of 40 to 850 g / kg optionally in combination with urea and / or ammonia and / or ammonium salts in the presence of catalytically active, non-oxidation active coatings of oxides selected from the group titanium dioxide, alumina, silica or mixtures thereof and hydrothermally stable zeolites, which are completely or partially metal-exchanged, catalytically zer puts. Process according to Claim 1, characterized in that guanidine salts of the general formula (I) are used

wherein R = H, NH ₂ , C ₁ -C ₁₂ alkyl, X ^⊝ = acetate, carbonate, cyanate, formate, hydroxide, methylate and oxalate. Method according to claim 1 or 2, characterized that is a mixture of two or more different guanidine salts is used. Method according to one of claims 1 to 3, characterized that the guanidine salt has an ammonia formation potential of 80 to 1,000 g / kg, in particular 250 to 600 g / kg. Method according to one of claims 1 to 4, characterized that the mixture of guanidine salt and urea has a guanidine salt content of 5 to 60 wt .-% and a urea content of 5 to 35 wt .-%. Method according to one of claims 1 to 5, characterized that the mixture of guanidine salt and ammonia or ammonium salts a content of guanidine salt of 5 to 60 wt .-% and of ammonia or ammonium salt has 5 to 40 wt .-%. Method according to one of claims 1 to 6, characterized in that the ammonium salts consist of compounds of general formula (II) R-NH ₃ ^⊕ X ^⊝ (II) wherein R = H, NH ₂ , C ₁ -C ₁₂ alkyl, X ^⊝ = acetate, carbonate, cyanate, formate, hydroxide, methylate and oxalate. Method according to one of claims 1 to 7, characterized in that the guanidine salt and optionally the other components in a solvent consisting of water and / or a C ₁ -C ₄ alcohol are used. Method according to one of claims 1 to 8, characterized that the watery and / or alcoholic solution a solids content of 5 to 85 wt .-%, in particular 30 to 80 wt .-%, having. Method according to one of claims 1 to 9, characterized that the solution the guanidine salt or the mixtures of guanidine salts possibly also in admixture with urea in water an ammonia formation potential from 0.2 to 0.5 kg of ammonia per liter of solution, in particular of 0.25 to 0.35 kg / l. Method according to one of claims 1 to 10, characterized that the guanidine salt and possibly the other components are internally gas-exhaust in the main, partial or secondary flow of the vehicle exhaust gases or exhaust gases in an autobar and externally heated arrangement by catalytic Decomposition is converted into ammonia. Method according to one of claims 1 to 11, characterized that one for the catalytic decomposition of the guanidine salt and possibly the other Components to ammonia, carbon dioxide and possibly carbon monoxide in addition a catalyst with non-oxidation-active coatings one more Catalyst used with oxidation-active coatings. Method according to claim 12, characterized in that that is a catalyst with oxidation-active coatings of oxides, selected from the group titania, alumina and silica, used hydrothermally stable metal zeolites and mixtures thereof, which are impregnated with gold and / or palladium. Method according to claim 12, characterized in that that a catalyst with oxidation-active coatings consisting of palladium and / or gold as active components with a precious metal content of 0.001 to 2 wt .-% is used. Method according to one of claims 1 to 14, characterized that one uses a catalyst consisting of two sections, wherein the first section is not oxidation active coatings and the second section contains oxidation active coatings. Method according to claim 15, characterized in that that 5 to 90 vol .-% of the catalyst from not oxidation active Coatings and 10 to 95 vol .-% consists of oxidation-active coatings. Method according to one of claims 1 to 16, characterized that is the catalytic decomposition in the presence of two consecutively arranged catalysts, wherein the first catalyst from non-oxidation active coatings and the second catalyst consists of oxidation-active coatings. Method according to one of claims 1 to 17, characterized that the catalytic decomposition of Guanidinsalz solutions at 150 to 350 ° C performs.