Method for producing an SCR catalytic converter by way of pre-drying

Abstract

The present invention relates to a method for producing automobile exhaust gas catalytic converters, to the catalytic converters as such and to the use thereof. In particular, the method comprises a step which results, independently of the actual drying process, in the catalytically active material used being dried. The invention is especially used in the coating of wall-flow filters.

Claims

1. Method for producing a catalyst containing zeolites or zeotypes for the aftertreatment of exhaust gases of an internal combustion engine, characterized in that the zeolite or the zeotype is a metal ion-exchanged zeolite or zeotype and, after it is coated onto and/or into a support, is pre-dried in such a way that a gas stream is guided through the support for a sufficient period of time and with a sufficient intensity, so that the solids content in the applied washcoat layer is set to 45%-60% before the support is completely dried and/or calcined.

2. Method according to claim 1, characterized in that zeolites or zeotypes are those derived from structure types selected from the group consisting of CHA, LEV, AEI, AFX, AFI or KFI framework or a mixture thereof.

3. Method according to claim 1, characterized in that the zeolite or zeotype is exchanged with iron and/or copper ions.

4. Method according to claim 1, characterized in that the coating has a binder selected from the group consisting of aluminum oxide, titanium dioxide, zirconium dioxide, silicon dioxide or mixtures thereof.

5. Method according to claim 1, characterized in that the support is a wall-flow filter.

6. Method according to claim 1, characterized in that the gas stream lasts for a period of time of 10 to 120 seconds.

7. Method according to claim 1, characterized in that the moisture of the gas stream is reduced to values of less than 5 g of water per kilogram of gas.

8. Method according to claim 1, characterized in that the gas stream does not exceed a temperature of 60° C.

9. Method according to claim 1, characterized in that the gas stream is generated by a pressure difference of more than 20 mbar between the inlet and outlet sides of the support.

10. Method according to claim 1, characterized in that the gas stream is sucked through the support in the direction of coating.

11. Method according to claim 1, characterized in that the coating suspension is treated with ultrasound prior to coating the support.

12. Catalyst for the aftertreatment of exhaust gases of an internal combustion engine produced according to claim 1.

13. Catalyst according to claim 12, characterized in that the support is a wall-flow filter.

14. Catalyst according to claim 13, characterized in that the support has a loading with the coating of 30-200 g/l.

15. Catalyst according to claim 14, characterized in that the support has less than 10% of a gradient with respect to the washcoat concentration and/or the copper content in the axial direction.

16. A method for the aftertreatment of exhaust gases of an internal combustion engine comprising contacting the exhaust gases with the catalyst according to claim 12.

17. Method according to claim 16, characterized in that these are exhaust gases of a predominantly on average lean-operated internal combustion engine.

18. Method according to claim 16, characterized in that the catalyst is configured for the selective reduction of nitrogen oxides by means of ammonia.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically illustrates the implementation of the method according to the invention.

(2) FIG. 2 shows the NOx conversion of Examples 1-3 at T=175° C.−350° C.

DETAILED DESCRIPTION

(3) Surprisingly, it has been found that, with the pre-drying in accordance with the invention of a support, in particular a filter, coated with a metal ion-exchanged zeolite or zeotype, the solids content of the coating can be adapted by passing a gas through at a certain pressure difference and a certain temperature and a certain moisture content of the gas in such a way that better activity of the final catalyst can be achieved (FIG. 2). FIG. 1 schematically illustrates the implementation of the method according to the invention. Preferably air as gas (1) with a temperature of at most 60° C. (3) and (4) and a moisture content of less than 5 g of water/kg of air (2) with a pressure difference (7) of more than 50 mbar between the inlet and outlet of the support (5) results in fixing the washcoat on or in the cell walls of the channels of the substrate (5) and thus prevents the uncontrolled separation and migration of the moist washcoat and its components. After being passed through, the moist gas stream is guided over a separator (6). Compared to the drying processes conventionally used, this has the consequence that the washcoat has a significantly lower concentration gradient and the channels no longer become blocked and clogged. This has the result that, for example, wall-flow filters which have been coated and dried with zeolite or zeotype-containing washcoat according to this method have a significantly increased catalytic effectiveness (FIG. 2) and at the same time a lower exhaust gas back pressure. It is assumed that the lower decrease in viscosity as a result of the use of cool and dry air in combination with a certain gas velocity leads to the stabilization of the moist coating. In particular, it is advantageous if the air used for pre-drying is preconditioned to a moisture content of less than 5 g of water per kilogram of air. This can be done by dehumidification methods known to the person skilled in the art. Typically, in the art, physical methods such as condensation by temperature reduction or adsorption of water vapor on hygroscopic fluids (aqueous solutions of alkali metal chlorides) or solids (molecular sieves, silica gel, etc.) are used for dehumidifying air. The dehumidification of the drying air by adsorption on molecular sieves has proven to be particularly advantageous for the process according to the invention, since in this way a very low relative humidity can be realized with a simultaneously high gas velocity.

(4) No publication of the prior art has so far described a drying method which sets the solids content in the washcoat layer applied here on the support to values of 45-60%, in which the wet layer is dewatered by passing dry, relatively cool air through at high flow rates due to the applied pressure difference. The method according to the invention may be referred to as low-temperature pressure difference pre-drying due to the process parameters.

(5) FIG. 1 1. Gas stream 2. Air dehumidification 3. Temperature setting 4. Temperature control 5. Substrate 6. Separator 7. Pressure difference measurement

EXAMPLES

(6) A basic embodiment of the method is reproduced in the following in FIG. 1. The process for producing a catalyst containing metal ion-exchanged zeolites or zeotypes for the aftertreatment of exhaust gases of an internal combustion engine consists advantageously of the following method steps: a) Determining the dry weight of the support, in particular a wall-flow filter; b) Coating the support with the required loading of washcoat using a conventional method; c) Determining the wet weight of the coated support; d) Optionally inserting the support into a holding device having gas-tight connections at the inlet and outlet sides of the filter (if different from step b)); e) Applying a pressure difference between the inlet and outlet sides of the support via a suction or pressure blower; f) Conditioning the air stream prior to entry into the support by dehumidifying the air stream to a water content of less than 5 g of water per kilogram of air; g) Tempering the gas stream to a temperature of less than 60° C.; h) Determining the water loss through the pre-drying after passing the conditioned air through; i) Drying the washcoat layer at temperatures greater than 60° C.; j) Calcination of the washcoat layer at temperatures greater than 400° C.

(7) The method according to the invention of low-temperature pressure difference pre-drying comprises process steps d) to h). The passage of the air can take place in the coating direction of the washcoat or by rotating the support opposite the coating direction. Pre-drying preferably takes place through suction of air in the coating direction.

(8) The weight loss during the pre-drying can be controlled automatically (inline) by a weighing unit integrated into the holding device for the support or in a separate weighing step after the pre-drying. The duration of the pre-drying step is controlled in this manner until a solids content in the wet washcoat layer from 45% to 60% has been established. The solids content of the washcoat layer is defined as the proportion of all solids in the total weight of the washcoat applied and is calculated after determination of the moisture loss by weighing according to the following formula:
FSG2=MWC*FSG1/(MWC−MH.sub.2O) FSG1: Solids content of the washcoat suspension FSG2: Solids content in the pre-dried layer MWC: Amount of washcoat suspension applied MH2O: Measured moisture loss

(9) As is customary in the production of catalytically active monoliths and filters, the coated supports are then finally dried in a convection oven at temperatures between 100 and 150° C. and calcined in a subsequent annealing step at temperatures between 400° C. and 700° C.

(10) Characterization of the Filters

(11) a) Determination of the Washcoat Gradient

(12) The washcoat gradient, i.e. the axial distribution of the mass of catalytic substance and in some cases also the radial concentration differences, generally have a negative influence on the catalysis, the pressure loss and the filtration efficiency due to the uncontrolled flow of the applied washcoat suspension in the conventional circulating air drying methods with hot air. The long drying times in the case of vertically standing supports result in considerable concentration differences in the axial direction due to gravity, and additionally the fact that the support is heated by hot air, preferably from the outside, also results in a gradient in the radial direction. In both cases, the low-temperature pressure differential pre-drying according to the invention brings about a significant improvement.

(13) The washcoat gradient is measured on the calcined support in an X-ray spectrometer (manufacturer Panalytical, type Axios, 4 kW Rh tube). For this purpose, for example, the calcined filter is divided into three segments in the axial direction and the concentration of copper oxide and aluminum oxide in relation to a reference is determined semi-quantitatively. The gradient given in Table 3 is calculated from the arithmetic mean of the maximum and minimum weight proportions of aluminum oxide and copper oxide and is a measure for the evenness of the distribution of the washcoat layer.

(14) b) Determination of the Exhaust Gas Back Pressure in a Wall-Flow Filter as Substrate

(15) The exhaust gas back pressure is the dynamic pressure which is established by the friction of the flowing gas stream when passing through the porous filter walls. The exhaust gas back pressure is determined in the cold flow (20° C.) on a dried or calcined filter support by measuring the pressure difference when flowing through with an air flow of, for example, 300 m.sup.3/h or 600 m.sup.3/h. The exhaust gas back pressure is given as a pressure difference in mbar between the inlet and outlet sides of the filter. The percentage dynamic pressure increase given in Table 3 is calculated in relation to the uncoated substrate.

(16) c) Determination of the Conversion Rate

(17) To determine the catalytic effectiveness of the coated supports—here, a coated filter (Table 2)—these are aged after calcination for 16 hours under hydrothermal conditions (10% water) at 800° C. and then tested for their activity in the selective catalytic reduction of NOx in a model gas test stand, in the present case in a dynamic ammonia breakthrough criterium-based test with the gas conditions shown in Table 1.

(18) TABLE-US-00001 TABLE 1 Gas composition of testing of the catalytic activity at a model gas test stand at T = 175° C., 225° C., 250° C. and 350° C. Gas Proportion NOx 500 ppm NO2 0 ppm NH3 750 ppm O2 10% by volume CO 350 ppm C3H6 100 ppm H2O 5% by volume N2 Remainder

(19) A ceramic filter consisting of a support body made of silicon carbide (manufacturer NGK) with a porosity of 63% at an average pore size of 20 μm in the dimensions was used for the present tests:

(20) TABLE-US-00002 TABLE 2 Filter support data Diameter 165 mm Length 140 mm Cell density 300 cpsi Wall thickness 0.305 mm

(21) This is coated in a coating installation using the process described in WO2006021338. The following steps are carried out for this:

(22) a) aligning the flow channels of the wall-flow filter in vertical fashion such that one end surface lies at the bottom, and the second end surface lies at the top,

(23) b) introducing the coating composition into the filter body through the flow channels of the wall-flow filter which are open in the lower end face up to a desired height above the lower end face

(24) c) removing excess coating composition downward.

(25) The washcoat is at room temperature at the time of coating, which is usually 20° C. to 40° C., and consists of a suspension of a copper-exchanged zeolite (Chabasit) with a solids content of 37%. The suspension is ground in Examples 1-3 by a stirred ball mill (for example from the manufacturer Netsch or Hosokawa Alpine) using zirconium oxide grinding balls having a diameter of 1 mm. After coating, the loading of the support with washcoat is determined by weighing the support. 1. Example: Zeolite-containing washcoat ground with a ball mill, after coating dried standing in a convection oven at 120° C. for 30 minutes, annealed at 350° C. for 30 minutes, then calcined at 550° C. for 2 h (standard process, not according to the invention). 2. Example: Zeolite-containing washcoat ground with a ball mill, support after coating stored standing at room temperature for 24 hours, then dried at 120° C. for 30 minutes, annealed at 350° C. for 30 minutes and calcined at 550° C. for 2 h (not according to the invention). 3. Example: Zeolite-containing washcoat ground with a ball mill, pre-dried using the method according to the invention under the following conditions Pressure difference: 350 mbar Moisture content: 4 g water/kilogram air Air temperature at inlet: 20° C. Solids content after pre-drying: 53%; then dried at 120° C. for 30 minutes, annealed at 350° C. for 30 minutes and calcined at 550° C. for 2 h.

(26) Table 3 gives the results of the measurements of the concentration gradients, exhaust gas back pressures and NOx conversion rates of the differently produced wall-flow filters.

(27) TABLE-US-00003 TABLE 3 Data for characterization of the filters from Examples 1 to 3 Pressure X (NOx) Solids after increase Washcoat at 250° C. Loading pre-drying at 600 m.sup.3/h gradient after aging Example (g/L) (%) (%) (%) (%) 1 105 — 28% 11.1 41% 2 107 — 43% 14.3 31% 3 101 53.7% 18% 2.7 49%

(28) It can be seen from the examples that the low-temperature pressure difference pre-drying results in a marked improvement in the catalytic effectiveness in the selective catalytic reduction of NOx. This is shown in detail in FIG. 2 in the representation of the conversion over the temperature range of 175° C. to 350° C., in which Example 3 shows the highest conversion in NOx conversion despite the lowest loading of catalytically active material.

(29) FIG. 2: NOx conversion of Examples 1-3 at T=175° C.-350° C. at the model gas under the gas composition given in Table 1.

(30) The worst (highest) exhaust gas back pressure with a pressure increase of 43% is found for the filter slowly dried at room temperature. Compared to the filter dried conventionally with circulating air (Example 1, standard process), the filters which were treated using the low-temperature pressure difference pre-drying method (Example 3) show a marked improvement, i.e. a reduction of the exhaust gas back pressure.

(31) A significant improvement when using the pre-drying according to the invention can also be observed in particular in the axial distribution of the washcoat. The difference in the axial distribution of the mass of catalytic substance after the pre-drying is only 2.7%.