DE102007061440A1 - Use of aqueous guanidinium formate solution, optionally with urea and-or ammonia or ammonium salt, for the selective catalytic reduction of nitrogen oxides with ammonia in motor vehicle exhaust gas - Google Patents

Abstract

The use of aqueous guanidinium formate (GF) solutions (optionally in combination with urea and/or ammonia or ammonium salts) for the selective catalytic reduction of nitrogen oxides with ammonia in motor vehicle exhaust gas. An independent claim is included for aqueous compositions containing 5-85 wt.% GF (optionally in combination with urea and/or ammonia as above) as material for the selective reduction of nitrogen oxide with ammonia in exhaust gas.

Description

The The present invention relates to the use of aqueous Guanidinium solutions for the selective catalytic reduction of nitrogen oxides in exhaust gases of vehicles, taking from the corresponding Guanidiniumformiat solutions by Evaporation and catalytic decomposition ammonia is produced and this as a reducing agent for the subsequent one Selective catalytic reduction of nitrogen oxides is used.

According to the prior art, ammonia (NH ₃ ) is used in the selective catalytic reduction of nitrogen oxides in exhaust gases of vehicles as a reducing agent, in front of a special SCR catalyst or in front of a built-in silencer group of parallel-flow SCR catalyst modules in the exhaust pipe of Combustion systems and internal combustion engines, in particular those of internal combustion engines of vehicles, is initiated and causes the reduction of the nitrogen oxides contained in the exhaust gas in the SCR catalysts. 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 AdBue ^™ 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 with a filling of the ammonia precursor substance is increased by half and created in passenger cars generally 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 be activated only at exhaust gas temperatures above 160 ° C, as 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 provide suitable ammonia precursors, which do not have the disadvantages mentioned in the prior art, but allow a technically simple production of ammonia for NO _x reduction according to the SCR process and no unwanted by-products in the Form decomposition.

These Task was inventively characterized solved, that one is watery Guanidinium solutions optionally in combination with urea and / or ammonia and / or ammonium salts for the selective catalytic reduction of nitrogen oxides with ammonia used in exhaust gases from vehicles. It has surprisingly showed that the invention used Guanidiniumformiat a higher compared to the prior art ammonia formation potential having. About that In addition, the corresponding aqueous guanidinium formate solutions can be technically evaporate easily and without formation of solid decomposition products, possibly could lead to encrustations and blockages in the exhaust system.

to selective catalytic reduction of nitrogen oxides with ammonia In optionally oxygen-containing exhaust gases from vehicles, aqueous guanidinium formate solutions are used according to the invention preferably a solids content of 5 to 85 wt .-%, in particular 30 to 80 wt .-%, and optionally with urea and / or ammonia or ammonium salts are combined. The mixing ratios of guanidinium formate with urea and ammonia or ammonium salts can can be varied within wide limits, but it has proven to be special proved advantageous that the mixture of guanidinium formate and Urea a Guanidiniumformiat content of 5 to 60 wt .-% and a Urea content of 5 to 35 wt .-% has. Furthermore are Mixtures of guanidinium formate and ammonia or ammonium salts with a content of guanidinium formate of 5 to 60% by weight and of Ammonia or ammonium salt of 5 to 40 wt .-% to be regarded as preferred.

In particular, compounds of the general formula (I) have proven to be suitable as ammonium salts R-NH ₃ ^⊕ X ^⊖ (I) in which
R = H, NH ₂ , C ₁ -C ₁₂ -alkyl,
X ^⊖ = acetate, carbonate, cyanate, formate, hydroxide, methylate and oxalate.

It is to be regarded as essential to the invention that the aqueous guanidinium formate solutions and, if appropriate, the further components are subjected to catalytic decomposition to ammonia in the preferred temperature range from 150 to 350 ° C., carbon dioxide and possibly carbon monoxide being formed as further components. This decomposition of guanidinium formate to ammonia is preferably carried out in the presence of catalytically active, non-oxidation-active coatings of oxides selected from the group consisting of titanium dioxide, aluminum oxide and silicon dioxide and mixtures thereof, hydrothermally stable zeolites which are completely or partially metal-exchanged, in particular iron zeolites from ZSM 5. or BEA type. 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 are preferably supported on metallic support materials such. B. Heizlei ter alloys (especially chromium-aluminum steels) applied.

alternative can the catalytic decomposition of Guanidiniumformiat solutions or the remaining components also take place to ammonia and carbon dioxide, wherein catalytically active Coatings of oxides selected from the group consisting of titanium dioxide, aluminum oxide and silica and mixtures thereof, hydrothermally stable Zeolites that are completely or partially metal exchanged, can be used with Gold and / or palladium are impregnated 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 guanidinium formate used according to the invention and optionally the other components to ammonia may preferably internal combustion gas in the main, partial or secondary flow of the vehicle exhaust gases or exhausts in an autobar and externally heated arrangement be made.

One Another object of the present invention are aqueous compositions, consisting of guanidinium formate with a concentration of 5 to 85 wt .-%, preferably 30 to 80 wt .-%, optionally in combination with Urea and / or ammonia or ammonium salts and the remainder water as a means for the selective catalytic reduction of nitrogen oxides with ammonia in exhaust gases of vehicles. Preferably, the Mixtures of guanidinium formate and urea have a guanidinium formate content from 5 to 60 wt .-% and a urea content of 5 to 35 wt .-% on. The mixtures of guanidinium formate with ammonia or ammonium salts preferably have a guanidinium formate content of 5 to 60% by weight and of ammonia or ammonium salts of 5 to 40% by weight.

With Help the invention proposed aqueous Guanidinium solutions Is it possible, a reduction of nitrogen oxides in vehicle exhaust gases by approx. 90% to achieve. Furthermore is with the inventively proposed guanidinium formate solutions a 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. Finally is also the risk of corrosion of the guanidinium formate solutions used in the invention 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 at orderly. 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) and a partial oxidation of the carbon monoxide is effected. At the FTIR gas analyzer 17 no increase in CO concentration is measurable.

Claims (17)

Use of aqueous guanidinium formate solutions, if necessary in combination with urea and / or ammonia or ammonium salts for the selective catalytic reduction of nitrogen oxides with ammonia in exhaust gases of vehicles. Use according to claim 1, characterized that the watery Guanidinium solutions a solids content of 5 to 85 wt .-%, in particular 30 to 80 % By weight. Use according to claim 1 or 2, characterized that the watery solutions Mixtures of guanidinium formate and urea with a guanidinium formate content from 5 to 60 wt .-% and a urea content of 5 to 35 wt .-% contain. Use according to one of Claims 1 to 3, characterized that the watery solutions Mixtures of guanidinium formate and ammonia or ammonium salts containing guanidinium formate of 5 to 60% by weight and of ammonia or ammonium salts of from 5 to 40% by weight. Use according to one of claims 1 to 4, characterized in that the ammonium salts consist of compounds of general formula (I) R-NH ₃ ^⊕ X ^⊖ (I) wherein R = H, NH ₂ , C ₁ -C ₁₂ alkyl, X ^⊖ = acetate, carbonate, cyanate, formate, hydroxide, methylate and oxalate. Use according to one of claims 1 to 5, characterized that the guanidinium formate and optionally the other components internal to the exhaust gas in the main, partial or secondary flow of the vehicle exhaust gases or exhausts in an autobar and externally heated arrangement is converted into ammonia by catalytic decomposition. Use according to claim 6, characterized that the catalytic decomposition of Guanidiniumformiats as well optionally the other components to ammonia, carbon dioxide and possibly carbon monoxide in the presence of catalytically active, non-oxidation active coatings of oxides, selected from the group titania, alumina and silica or their mixtures, hydrothermally stable zeolites, which are completely or partially metal exchanged, performs. Use according to claim 6, characterized in that for the catalytic decomposition of the guanidinium formate or the other components to ammonia and carbon dioxide catalytically active coatings of oxides selected from the group titanium dioxide, alumina so such as silica, hydrothermally stable metal zeolites and mixtures thereof, which are impregnated with gold and / or palladium as oxidation-active components. Use according to claim 6, characterized that for the catalytic decomposition of guanidinium formate and possibly the other components a catalytic coating with palladium and / or gold as active components with a noble metal content of 0.001 to 2 wt .-% is used. Use according to one of Claims 6 to 9, 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. Use according to claim 10, characterized that 5 to 90 vol .-% of the catalyst from not oxidation active Coatings and 10 to 95 vol .-% consists of oxidation-active coatings. Use according to one of Claims 6 to 11, 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. Use according to one of claims 1 to 12, characterized that the catalytic decomposition of Guanidiniumformiat solutions at 150 to 350 ° C performs. aqueous Compositions consisting of guanidinium formate with one concentration from 5 to 85% by weight, if appropriate in combination with urea and / or ammonia or ammonium salts and the balance of water as a means of selective catalytic reduction of nitrogen oxides with ammonia in exhaust gases of vehicles. aqueous Compositions according to claim 14, characterized in that the concentration of the guanidinium formate is 30 to 80% by weight. aqueous Compositions according to Claim 14 or 15, characterized from mixtures of guanidinium formate and urea with a guanidinium formate content of 5 to 60 wt .-% and a urea content from 5 to 35 wt .-%. aqueous Compositions according to any one of Claims 14 to 16, characterized in that mixtures of guanidinium formate and ammonia or ammonium salts containing guanidinium formate of 5 to 60% by weight and of ammonia or ammonium salts of from 5 to 40% by weight.