Ammonia oxidation catalyst having low N2O by-product formation

Abstract

The present invention relates to a catalytic composition comprising a noble metal on an acidic tungsten-containing mixed oxide, a method for producing the catalytic composition and the use of the catalytic composition as oxidation catalyst. The invention further relates to a catalyst shaped body, which has the catalytic composition on a support, a washcoat containing the catalytic composition according to the invention and the use of the washcoat to produce a coated catalyst shaped body.

Claims

1. A process for treating an exhaust gas, comprising the step of contacting the exhaust gas with an oxidation catalyst comprising a catalytic composition comprising a noble metal on an acidic tungsten-containing mixed oxide, wherein the acidic tungsten-containing mixed oxide is a Ce/Zr/W mixed oxide and wherein the mixed oxide has a tungsten content of from 5 to 10 wt.-%, relative to the total mixed oxide, wherein the exhaust gas is contacted with an SCR catalyst upstream of the oxidation catalyst, and wherein the oxidation catalyst provides a high selectivity for ammonia oxidation.

2. The process according to claim 1, wherein the SCR catalyst and the oxidation catalyst are present on a common support.

3. The process according to claim 1, wherein the noble metal is platinum.

4. The process according to claim 1, wherein the mixed oxide has 0.05 to 1.0 wt.-% platinum, relative to the total mixed oxide.

5. The process according to claim 1, wherein the Ce/Zr ratio is 80:20 to 50:50.

Description

(1) The invention will now be explained in more detail with reference to some embodiment examples, wherein however the content thereof is not to be understood as limiting the scope of the present invention. In addition, reference is made to FIGS. 1 to 3.

(2) There are shown in:

(3) FIG. 1 the ammonia conversion of the compound according to the invention in comparison with compounds not according to the invention. The ammonia light-off temperature can be determined using this diagram.

(4) FIG. 2 the NO.sub.x conversion of the compound according to the invention in comparison with compounds not according to the invention.

(5) FIG. 3 the N.sub.2O outlet concentration in ppm of the compound according to the invention in comparison with compounds not according to the invention.

EMBODIMENT EXAMPLES

(6) The invention is illustrated using Example 1 and the comparison examples. All of the following catalysts have 5 g/ft.sup.3 platinum (0.1766 g platinum/1 L honeycomb=0.1766 g platinum/250 g Ce/Zr/W oxide=0.0706%) on the respective honeycombs. The honeycombs all have 1 diameter and 2 length.

(7) In the example, the catalyst according to the invention is produced by coating a honeycomb with a W/Ce/Zr oxide, followed by Pt-impregnation. In comparison example 1 a catalyst based on Pt/titanium dioxide is produced, a catalyst without SCR properties. In comparison example 2 a catalyst with Pt on an Fe-BEA zeolite is produced, and in comparison example 3 a catalyst based on Pt on a V/W/Ti-SCR catalyst is produced. The comparative test is described in Example 4.

Example 1

(8) 381 g of a Ce/Zr mixed oxide (Ce/Zr=63:36) is impregnated with a solution of 37.5 g ammonium metatungstate in 190 mL of deionized water to 6.1% W content and calcined at 550 C. A washcoat is produced from this powder and a ceramic honeycomb is coated in several steps to 250 g/l washcoat loading (dry) and calcined at 550 C. Each coating step is followed by drying and calcining. (Honeycombs of 1 diameter and 2 length were coated, V=25.3 ml coated with 6.425 g washcoat.)

(9) Then the water absorption of the honeycomb is determined and the latter is dried again.

(10) A platinum-ethanolamine solution (ethanolammonium hexahydroxoplatinate) with 13.1% platinum content is diluted. There should be 5 g platinum/ft.sup.3 honeycomb volume of noble metal on the honeycomb (5 g/ft.sup.3=0.1766 g/l). This means that a platinum solution must be produced in which 0.00447 g platinum is contained in the volume of water absorbed. The honeycomb is then immersed completely for 10 s in a platinum-ethanolamine solution of this concentration, and in this way impregnated via incipient wetness. The honeycomb is dried and calcined at 550 C.

Comparison Example 1

(11) A washcoat is produced from 20 kg titanium dioxide DT 51D from Millennium and 26 kg titanium dioxide sol (12%, stabilized with nitric acid) from Sachtleben as binder. A honeycomb is coated to 32 g/l washcoat and calcined at 450 C.

(12) Then the water absorption of the honeycomb is determined and the latter is dried again.

(13) A platinum-ethanolamine solution (ethanolammonium hexahydroxoplatinate) with 13.1% platinum content is diluted. There should be 5 g platinum/ft.sup.3 honeycomb volume of noble metal on the honeycomb (5 g/ft.sup.3=0.1766 g/l). This means that a platinum solution must be produced in which 0.00447 g platinum is contained in the volume of water absorbed. The honeycomb is then completely immersed for 10 s in a platinum-ethanolamine solution of this concentration, and in this way impregnated via incipient wetness. The honeycomb is dried and calcined at 550 C.

Comparison Example 2

(14) A washcoat is produced from Fe-BEA zeolite, to which a colloidal silica sol is added as binder. A ceramic honeycomb is coated to 250 g/l washcoat and calcined at 550 C.

(15) Then the water absorption of the honeycomb is determined and the latter is dried again.

(16) A platinum-ethanolamine solution (ethanolammonium hexahydroxoplatinate) with 13.1% platinum content is diluted. There should be 5 g platinum/ft.sup.3 honeycomb volume of noble metal on the honeycomb (5 g/ft.sup.3=0.1766 g/l). This means that a platinum solution must be produced in which 0.00447 g platinum is contained in the volume of water absorbed. The honeycomb is then completely immersed for 10 s in a platinum-ethanolamine solution of this concentration, and in this way impregnated via incipient wetness. The honeycomb is dried and calcined at 550 C.

Comparison Example 3

(17) A washcoat is produced from W-stabilized titanium dioxide DT 52 and 2.5% V.sub.2O.sub.5. A ceramic honeycomb is coated to 200 g/l washcoat and calcined at 450 C.

(18) Then the water absorption of the honeycomb is determined and the latter is dried again.

(19) A platinum-ethanolamine solution (ethanolammonium hexahydroxoplatinate) with 13.1% platinum content is diluted. There should be 5 g platinum/ft.sup.3 honeycomb volume of noble metal on the honeycomb (5 g/ft.sup.3=0.1766 g/l). This means that a platinum solution must be produced in which 0.00447 g platinum is contained in the volume of water absorbed. The honeycomb is then completely immersed for 10 s in a platinum-ethanolamine solution of this concentration, and in this way impregnated via incipient wetness. The honeycomb is dried and calcined at 450 C.

Example 4

(20) The catalysts were tested in a tubular glass reactor (inside diameter 27 mm) under the following conditions:

(21) 250 ppm NO

(22) 50 ppm NH.sub.3

(23) 5% O.sub.2

(24) 5% CO.sub.2

(25) 5% H.sub.2O

(26) Remainder N.sub.2

(27) Space velocity=84,000 h.sup.1

(28) Measurement with cooling from 420 C. to 100 C.

(29) FIG. 1 shows the ammonia conversion versus the temperature. The ammonia light-off temperature can be determined from this (temperature at 50% conversion).

(30) It can be seen that here, although the Pt catalyst on the Fe-BEA zeolite (comparison example 2) is the best, the other two (VWT and the catalyst according to the invention) are not substantially poorer. Bearing in mind the fact that at least 200 C. is required to produce ammonia from the urea solution, it can be seen here that all catalysts, except the catalyst that does not contain an SCR component (TiO.sub.2, comparison example 1), fulfil this requirement.

(31) FIG. 2 shows the NO.sub.x conversion in this test. A negative NO.sub.x conversion means that additional NO.sub.x is formed from the ammonia by ammonia oxidation. At low temperature the reaction is selective, because here the SCR function predominates. With increasing temperature, the oxidation becomes so rapid that more ammonia is oxidized immediately, before it can react selectively with the NO.sub.x in the SCR. The higher this temperature at which this negative NO.sub.x conversion, thus more ammonia oxidation to NO.sub.x, takes place, the better the catalyst. It can clearly be seen that the catalyst according to the invention (squares) and the VWT catalyst (comparison example 3, stars) are the best.

(32) However, the decisive advantage of the catalyst according to the invention over the entire state of the art can clearly be seen in FIG. 3. FIG. 3 shows the N.sub.2O outlet concentration in ppm. N.sub.2O is an undesired by-product. It can clearly be seen that the catalyst according to the invention (squares) only exceeds 5 ppm N.sub.2O at 250 C., thus N.sub.2O only forms at higher temperature, and, with a maximum of 20 ppm at 300 C., also has the lowest N.sub.2O formation.