FI98274B - Catalyst for purification of exhaust gases from a lean mixer engine and process for producing the same - Google Patents

Abstract

The invention relates to a catalyst for cleaning of exhaust gases from a lean-burning engine, in particular a diesel engine, the catalyst comprising a catalyst body having a plurality of flow channels for exhaust gas and being coated with a carrier comprising stabilised aluminium oxide and zeolite and having at least one catalytically active substance constituted by a precious metal. The invention relates as well to a method for manufacturing said catalyst.

Description

98274

The present invention relates to a catalyst for purifying the exhaust gases of a lean-burn engine, in particular a diesel engine. The present invention relates to a catalyst for purifying the exhaust gases of a lean-burn engine, in particular a diesel engine. The invention also relates to a process for preparing this catalyst 10.

The catalyst generally consists of a body material, which may be ceramic or metal strip or plate, so that the exhaust gas to be cleaned entering the catalyst enters a flue formed by a large number of parallel flow channels, i.e. holes. The walls of the fragrance are coated with a washcoat with a high specific surface area, on the porous surface of which the precious metals and / or other catalytically active substances, such as Cu, Co, Cr, Mn, Ni and Fe, are highly dispersed as discrete particles. The efficiency of the catalyst depends on e.g. from the contact surface of the flowing gas and the precious metal particles: the larger the contact surface, the more efficient the catalytic purification. In addition, the efficiency is affected by at least the composition of the exhaust gas, the temperature, the flow ratios and the purity (poisoning) of the catalyst.

·· *: There are several special features in cleaning the exhaust gas of a diesel engine compared to, for example, cleaning the exhaust gas of a petrol engine »· 30. Such special features are, for example, low exhaust gas temperature. In the latest diesel engines, it is even below 200 ° C, for example in city driving, but at · / motorway speeds the western temperature can be up to 750eC. The corresponding figures for the petrol engine are 300 ° C and 35 ° C.

··· ···

Another special feature of diesel engine exhaust is its; * ·. sulfur dioxide from diesel fuel »» · ·· • · 2 98274 sulfur. In some countries (eg Finland and Sweden) the sulfur content of diesel fuel is very low (less than 0.05%), but in general it is as high as 0.5% worldwide. The sulfur contained in the fuel forms sulfur dioxide SO2 from the gas-5 countries in the engine. This can stick to the catalyst; surface and further oxidized to sulfur dioxide. Under the influence of the presence of moisture in the catalyst (carbon dioxide and water are formed during the purification of unburned hydrocarbons), sulfur dioxide is converted to sulfuric acid HzSO * and reacts with the easily present cationic impurities to the corresponding sulphates. Sulfates tend to clog the catalyst scales along with any soot that may form.

15 The third special feature is the high oxygen content of the exhaust gas.

The exhaust gas of a diesel engine or a lean-burn engine contains a quantity of oxygen that clearly exceeds the (stoichiometric) oxygen used for catalytic reactions. In a diesel engine, this generally varies according to driving conditions in the range of 5 to 15% O 2, while in a gasoline engine it is about 0.6 to 0.8% O 2. Diesel catalysts differ considerably from gasoline engine catalysts due to different conditions.

Highly specific surface area gamma alumina has been used as a support for the cleaning of the exhaust gas of a diesel engine, and platinum, for example platinum, has been used as a noble metal on its surface. Alumina has a high tendency to bind SC 2 gas to the surface, so the accumulation of sulfate described above can be a significant problem.

30

Other inorganic compounds such as silica, zirconia and titanium dioxide have also been proposed for use in a diesel catalyst. These have a low tendency to store sulfur, but in such catalysts the operating temperature of the catalyst is insufficient or the support has • poor stability on the surface of the backbone material.

e m # 3 98274

Zeolites, which are 3-dimensional network-like composite structures of alumina and silica, have been extensively studied over the last ten years with the goal of making them a good catalyst for diesel or lean-burn exhaust gases. The use of zeolites is limited by their upper limit of thermal duration, about 650 to 750 ° C, so they are not suitable for the purification of petrol engine exhaust gases, at least as such and without very good thermal stabilization. In die-cast gas, zeolites with either a precious metal or 10 other catalytically active substances, e.g., copper, can function quite well fresh. Instead, the so-called after hydrothermal aging, in which the catalyst has been aged at high temperature in the presence of water vapor, the activity of the catalyst, in particular the reduction of nitrogen oxides, is impaired.

The use of zeolites has been described e.g. in the following patents: 20 0559021 A3 discloses a bilayer support in which the stabilized alumina and / or cerium oxide in the lower layer is impregnated with a precious metal alloy (Pt, Ir) and the zeolite in the upper layer contains Cu and / or Fe. The preparation of such a catalyst • 25 in many different steps is cumbersome and costly.

This catalyst is only intended for the reduction of nitrogen oxides in the exhaust of lean-burn engines. It is also assumed that the thermal stability of such a catalyst. is not optimal due to the top layer.

, BP 0566878 A1 discloses a catalyst • • in which the surface of the body material contains Al, Ti, Si oxide, zeolite or a mixture thereof, which has been prepared using ** 35 alcoholic solution and which also contains V-oxide and refined • *. Mews. Such a catalyst is at its best

. obtained in the diesel test with ignition values of 195 ° C (CO) and 210 ° C

• · ’(HC) for fresh catalyst.

m • 9 m 98274 4

BP 508513 A1 discloses a catalyst for a diesel engine having solid acidic substances such as zeolite or certain other, mainly silicate-based alloy oxides on the surface of the body, and noble metals on the surface of the support thus formed. The catalyst is only intended for the cracking of hydrocarbons in diesel soils.

EP 589393 A2 discloses a catalyst for a lean-blend engine with alumina, silica, zeolite and / or zirconia on the surface of the body and with Pt and / or Pd, Ba, and Fe, Ni, Co and / or Mg. The thermal duration of the zeolite has not been demonstrated in the present invention, all exemplary samples have been prepared using an alumina support. Only NOx conversions have been measured in the testing.

The use of zeolites in catalysts, especially diesel catalysts, has been described above. However, the problem with these has been to obtain low ignition temperatures and good aging resistance in the same catalyst. Another problem is often that the zeolite catalysts are only intended for the reduction of NOx gases, in which case a separate oxidation catalyst is also required for the CO and HC gases.

1 25: The object of the invention is to provide a catalyst for a lean-blend engine which avoids the above-mentioned drawbacks and in which the catalyst in the sauna combines low flammability. temperatures and good aging resistance and which effectively. * 30 reduces NOx gases and oxidizes carbon monoxide and hydrocarbon gases at low western temperatures. These goals can be achieved. with a catalyst according to the invention, the main features of which appear from the appended claim 1.

0 m *: 35 When studying various zeolite-containing supports die— *. for catalysts for reflux gas purification, it was surprisingly found that a mixture of Si-stabilized alumina and zeolite added with a precious metal such as Pt, »· ·· 5 98274 also proved to be subject to thermal aging (750 ° C 24 h atmospheric atmosphere). and after hydrothermal aging (750 ° C for 24 h, in an atmosphere of 10% H 2 O and 90% air) to a very low flammability with respect to CO and HC, and in addition the conversion to N 2 remained good. By Si-stabilized alumina, i.e. Si-alumina, is meant alumina in which a suitable amount of silicon has been added to the alumina crystal lattice to stabilize the alumina, making it more thermally resistant, i.e. crystal changes only occur at significantly higher temperatures than unstabilized. Instead of silicon, alumina can be stabilized with other substances, for example barium, lanthanum or zirconium.

The present invention therefore relates to a catalyst for purifying the exhaust gases of a lean-15 engine, in particular a diesel engine, comprising a catalyst body having a plurality of flow channels for the exhaust gas and coated with a catalytically active agent support comprising stabilized alumina and a stabilized alumina and silicon, barium, lanthanum or zirconium compounds. The catalyst contains at least one noble metal as catalytically active substance.

·· · • · i · # • ·. *! *. The body of the catalyst according to the invention can be a metal or a ceramic cell.

• 9 • m 9 · · • · J The support on the surface of the catalyst body has a single layer structure.

• The stabilizer is preferably a silicon compound. The concentration of the stabilizer in the alumina is preferably about 0.2 to 30 · »· t · · M k, V p- * · ··· • · · • m · 'λ ·. The weight ratio of stabilized alumina to zeolite in the support is preferably about 9: 1 to 1: 9.

··· • 9 9 9 999 \ The support may further comprise a lanthanide group oxide. Preferred elements of the lanthanide group are La, Ce, Y, Er, Eu and 6 98274

Nd. The amount of oxide of said lanthanide group in the support is preferably about 2 to 20% by weight based on the support.

According to a preferred embodiment, the support comprises Si-5 stabilized gamma alumina, zeollite and lanthanum oxide.

The invention also relates to a process for the preparation of a catalyst for cleaning the exhaust gases of a lean-burn engine, in particular a diesel engine, in which the catalyst body material is coated with a support slurry containing silicon, barium, lanthanum and then the calcined support is impregnated with at least one catalytically active substance which is a noble metal.

Said support slurry may further contain a compound of the lanthanide group 20 as an oxide or a soluble salt. The support slurry may further contain small amounts of aluminum and / or silicon compound as a sol or in soluble form.

The catalyst according to the present invention achieves the present invention. Very good conversions in 25 diesel engines already. . at low temperatures. The reason for the good thermal duration of this invention is not yet known, but it can be assumed that 9 · 9 zi: it is related to the fact that Si-stabilized gamma alumina and zeolite both contain both Si- that 30 ΆΙ-oxide. In this case, already at the manufacturing stage, when the support slurry crystallizes on the surface of the body material, the Si and Al crystal surfaces in the adjacent particles crystallize with each other. attached to it (-Si ... Si-, -AI ... AI- or -S1 ... A1-).

9 9 9 9 9 9 9 999 9 9 * ··· * 35 In adhesion, it is preferred that the slurry contain nitric acid.

: **]: poa which partially dissolves surfaces before crystallization (pH

: * · *: Less than 5). Attachment can be further enhanced by the addition of an Al and / or Si sol, which is known to be 98274 7 between soluble and particulate state, to the slurry. The sol acts as an adhesive between the various particles.

After crystallization, a composite structure is obtained consisting of Si-stabilized alumina and zeolite. Si-stabilizing gamma alumina withstands temperatures above 1000 ° C and thus also keeps the structure of the zeolite together even at high temperatures when crystal bridges are abundant. In this context, a crystal bridge means the adhesion of different solid particles to each other when I crystallizes between them.

a mediating crystallization of the slurry I adhering to both surfaces

when the moisture contained in it dries.

The catalyst according to the invention is, of course, having the function I

15 the better, the more precious metal is loaded into it

up to a certain limit, which is for a mischievous man skilled in the art

le clear. The amount of precious metal is greatly influenced by Catalyst I.

price, so it is appropriate in each case to determine the

va price / quality ratio. Precious metals such as Pt, Pd, Rh, Ir, I

20 Ru, in addition, other catalytically ak- I can also be used

substances such as those mentioned at the beginning of the description. Here- I

price-quality ratio is also essential, which I - in any case - decided

see the most suitable catalyst metal. I

A more detailed description of the invention will become apparent from the following Examples I

and the accompanying figures, in which I

Figure 1 shows a catalyst according to the invention (dark I

symbols) and the CO and HC ignition temperatures of the prior art catalyst (light symbols) as a function of Pt charge in fresh catalysts,

Figure 2 shows a catalyst according to the invention (dark I

35 symbols) and prior art catalysts.

CO and HC ignition of the transmitter (light symbols)

temperatures as a function of Pt charge thermally aging

and Figure 3 shows the CO and HC ignition temperatures of the catalyst of the invention (dark symbols) and the catalyst of the prior art (light symbols) as a function of Pt charge on hydrothermally aged catalysts.

Example 1

A series of experiments according to Table 1 was prepared in which the raw materials were: a. La nitrate (8% La in support) b. Alumina c. gamma alumina d. gamma alumina with 5% SiO 2 in the crystal lattice 15 e. zeolite ZSM-5 f. silica sol g. nitric acid

Table l. Catalysts of the test series. Catalysts 4, 5 20 and 6 are catalyst raw materials according to the invention support Pt transport g / m * g / cft * 25 ri 1 e + f 38 50 2 a + b + c + g 51 40 3 a + b + d + g 50 40 30 4 a + b + d + e + g, d: e - 2: 1 44 50 5 a + b + d + e + g, d: e -1: 1 47 50 6 a + b + d + e + g, d: e - 1: 2 41 50 35 * eft - Cubic foot (-28.3 1)

Each catalyst was prepared by slurrying the feedstocks into water to a dry matter content of 40.0% and grinding in a ball mill for 4 hours.

* 40 Metal foils, both smooth and shirred, were coated: with slurry on both sides and calcined at 550eC. for 4 hours.

* 9 98274 The strips were then immersed in Pt saline, the Pt content of which was adjusted so that after impregnation for 30 minutes, the desired amount of Pt remained on the strips. The strips were calcined at 300 ° C after impregnation. Thereafter, the smooth and wrinkled tape superimposed were rolled into a tight roll to form a honeycomb with a number of holes of 500 holes per square inch.

The catalysts in Table 1 were tested on 10 laboratory scales with synthetic exhaust gas (in% by volume) propylene 0.05%, carbon monoxide 0.05%, carbon dioxide 10.0%, nitrogen oxides (NO) 0.10%, oxygen 15%, water vapor 7, 0 sulfur dioxide 25 ppm and the rest nitrogen. The test was performed under standard conditions, the test piece was 15 cylindrical, 14 mm in diameter and 75 mm long. The gas exchange in the catalyst was 50,000 / h.

From each catalyst, 3 identical side-by-side catalysts were prepared, one tested fresh, the other 20 tested after thermal aging and the third after hydrothermal aging.

The catalysts were impregnated with SO 2 gas prior to testing, which simulates prolonged surface saturation with SO 2 gas under a real situation. Fresh catalyst was maintained at: 420 ° C for 30 min in a test gas prior to: testing

• contained 25 ppm SO2. The thermally aged was impregnated at 420 ° C

. at 30 ° C in a gas mixture containing pro- ·, pen 0.05%, carbon monoxide 0.05%, carbon dioxide 10.0%, nitrogen oxide 0.10%, oxygen 5%, sulfur dioxide 100 ppm and the rest nitrogen . Hydrothermally aged was impregnated as termini, Estonian aged.

: Testing gave results according to Table 2, where *; 35 ignition temperature means the temperature at which the

• ·. gas conversion en 50%. N2 means the conversion of NO to N2.

m 9 9 9 »· • e 10 98274

It should be noted that instead of the lanthanum used in Example 1, other lanthanides commonly used in catalysts, such as Ce, Y, Er, Eu, Nd, can also be used.

Table 2. Test results 5 -------- cat-ignition ignition heat conversion% ly temperature ° C at 400 ° C CO HC CO HC NO N2 tori 10 ---- 1 fresh 153 190 95 100 1 thermal 207 240 95 100 2 fresh 196 211 94 100 45 13 15 2 thermal 181 199 95 100 56 18 2 hydrotherm. 182 199 95 100 56 20 3 fresh 175 195 95 100 3 thermal 190 215 95 100 20 4 fresh 173 173 94 100 61 15 4 thermal 159 174 94 100 71 18 4 hydrotherm. 171 191 95 100 60 17 25 5 fresh 157 170 94 100 64 16 5 thermal 160 176 94 100 61 15 5 hydrothermal. 174 190 95 100 56 10 6 fresh 163 171 95 100 68 19 30 6 thermal 171 186 94 100 6 hydrothermal. 171 193 94 100 66 19

It can be seen from the table above that the catalysts 4, 5 and 6 according to the invention, when fresh, were clearly better in terms of ignition rates than alumina-based catalysts; catalysts 2 and 3. After aging, catalysts 4, 5 and 6 according to the invention were also clearly better than zeolite-based catalyst 1.

»E F1 40 Thus, it has been possible in the invention to combine the good, flammability of fresh Si-stabilized alumina of zeolites with a good thermal resistance of 9 · ··. Alumina stabilization can also be performed with barium, lanthanum or zirconium.

• 9 • «·· · 1. 45 • 99 • · • · Φ 9 • 9 9 9 99 9 · 98274 11

Example 2

Catalyst 5 as shown in Example 1 was prepared with Si-stabilized alumina and zeolite 5 ZSM-5 in a weight ratio of 1: 1 with Pt charges of 1, 2, 5: 15, 40 and 100 g / cft. Catalyst 2 with unstabilized gamma alumina without zeolite addition is also used in the same series of experiments for comparison with corresponding Pt-la charges.

The catalysts were tested as in Example 1. The results according to Table 3 were obtained.

· M 9 m 9 * 9 9 9 9 • t • 9 9 9 * 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 98274 12

Table 3. Effect of Pt charge on the catalysts of the test series Cat. 5 fresh 1,275,265 76 85 8 6 10 5 thermal 1,350 309 16 34 4 4 5 hydrotherm. 1,322,308 40 42 5 4 5 fresh 2,229,233 94 100 10 7 5 thermal 2,274,266 67 79 4 3 15 5 hydrotherm. 2 265 270 84 90 5 3 5 fresh 5 209 218 95 100 45 12 5 thermal 5 257 255 87 94 15 5 5 hydrothermal. 5 231 238 94 98 9 3 20 5 fresh 15 186 193 94 100 67 22 5 thermal 15 210 219 94 100 29 10 5 hydrothermal. 15 212 224 95 100 30 13 25 5 fresh 40 168 178 95 100 73 26 5 thermal 40 173 189 95 100 51 15 5 hydrotherm. 40 190 196 93 100 56 22 5 fresh 100 163 166 95 100 72 25 30 5 thermal 100 154 172 95 100 69 25 5 hydrothermal. 100 152 180 95 100 79 28 2 fresh 1 282 280 66 69 0 0 2 thermal 1 309 305 40 40 1 1 35 2 hydrothermal. 1 331 313 28 31 4 0 2 fresh 2 270 273 77 78 0 0 2 thermal 2 289 285 63 66 0 0: 2 hydrotherm. 2 298 299 52 52 1 0 40: 2 fresh 5 250 250 90 96 1 0 2 thermal 5 277 279 73 71 0 0 i 2 hydrotherm. 5 278 289 66 76 2 0 45 2 fresh 15 230 239 94 100 8 0 2 thermal 15 218 225 95 100 20 5. 2 hydrotherm. 15 234 250 93 97 15 5 •; 2 fresh 40 196 211 94 100 45 13 50 2 thermal 40 181 199 95 100 56 18. 2 hydrotherm. 40 182 199 95 100 56 20: 2 fresh 100 163 176 94 100 74 26 2 thermal 100 152 178 95 100 78 27: 55 2 hydrotherm. 100 152 181 94 100 76 27 r · 98274 13 The ignition results of Example 2 are also shown in Figures 1, 2 and 3.

From the series of experiments of Example 2, the same thing is observed as from Example 5 1, i.e. the superiority of the mixture of Si alumina and zeolite over alumina in the same Pt charges. Only a charge of 100 g / cft Pt gives similar results for both types of catalyst. The superiority of the catalyst according to the invention is particularly emphasized by the lower Pt charges. This is particularly advantageous for the catalyst according to the invention due to the considerable cost savings.

This means that in order to reach a certain ignition temperature, more platinum is required in a conventional alumina-based catalyst than in the catalyst according to the invention. As an example, it can be seen from Figures 1 (fresh catalyst) that Catalyst 5 with a charge of 2 g Pt / cft and Catalyst 2 with a charge of about 20 g Pt / cft are equivalent in terms of CO and HC ignition. In this case, the difference in Pt charge is significant in favor of the catalyst according to the invention.

Claims (9)

98274 A catalyst for purifying the exhaust gases of a lean-burn engine, in particular a diesel engine, comprising a catalyst body having a plurality of flow channels for the exhaust gas and coated with a support comprising at least one catalytically active substance, a noble metal, characterized in that and a zeolite, which alumina is stabilized with a silicon, barium, lanthanum or zir-10 conium compound. Catalyst according to Claim 1, characterized in that the support is monolayer. Catalyst according to Claim 1 or 2, characterized in that the content of stabilizer in the alumina is about 0.2 to 30% by weight. Catalyst according to one of the preceding claims, characterized in that the weight ratio of stabilized alumina to zeolite is about 9: 1 to 1: 9. : .J Catalyst according to one of the preceding claims, characterized in that the support further comprises a lanthanide group in an amount of oxide of about 2 to 20% by weight, based on the support. 9 9 9 9 Catalyst according to one of the preceding claims, characterized in that the support material comprises Si-stabilized alumina, zeolite and lanthanum oxide. • * 9 9 99 ί ί 7. Method of lean-blend engine, especially diesel engine ·. for the manufacture of an exhaust gas cleaning catalyst, characterized in that the catalyst body material is coated. 35 is supported by a support slurry containing alumina dia and zeolite stabilized with a silicon, barium, lanthanum or zirconium compound, the backing material coated with the support slurry is dried and calcined to form an oxide-containing support 98274, and then calcined to a single calcined support. which is a precious metal. Process according to Claim 7, characterized in that the support slurry further contains the lanthanide group compound as oxide or soluble salt. Process according to Claim 7 or 8, characterized in that the support slurry further contains the aluminum and / or silicon compound as a sol or in soluble form.