DE2926894A1 - METHOD FOR PRODUCING DENITRATING CATALYSTS - Google Patents

Description

The invention "relates to a method of making a Denitration catalyst by immersing a catalyst carrier made of porous metal in a silicic acid-containing one Coating bath and drying of the support treated in this way to form a porous silica coating.

In particular, it relates to the manufacture of a denitration catalyst for selective catalytic Reduction of nitrogen oxides (NO) in exhaust gases with

-X.

Ammonia.

Numerous methods have already been proposed for rendering nitrogen oxides in exhaust gases harmless. Of these processes, so-called denitration is particularly interesting; in this case the nitrogen oxides are selectively transformed into harmless N _{? in the presence of a catalyst with NH- at certain temperatures?} reduced. Although there are already many catalysts

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have become known that can be used in this process, "they still need to be improved for example a catalyst as in US Pat. No. 40 4-0,981 described, made of porous metal by immersing this as a catalyst carrier in a silicic acid bath became.

After the material treated in this way had dried, a superficial coating of porous silica had formed thereon educated. Experiments showed that the catalyst has a high mechanical strength and its pronounced, stable Activity is not lost through poisoning, for example with KCl. The invention now aims to further improve such catalysts.

The object of the invention is to provide a catalyst with a To produce a silica coating, which has a higher porosity and consequently better permeability for reaction gases shows. Furthermore, denitration catalysts are sought, which are characterized by improved activity and by satisfactory Distinguish strength.

According to the invention, this object is achieved in that an active component is absorbed onto the carrier by means of it immersed in a solution containing the component, and then dries.

Further advantages and features of the invention result from the following description, in which the invention is explained in more detail with reference to the drawing in a preferred embodiment, as well as the subclaims.

Fig. 1 shows the relationship in a graphical representation between the pore size and the total pore volume.

Fig. 2 shows a graph of the relationship between the treatment time and the average Thickness removal.

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Pig. 3 and 4 show the relationship in a graphical representation between the reaction temperature and the denitration efficiency.

Pig. 5 shows the relationship in a graphical representation between the coating thickness and the denitration efficiency

Pig. 6 shows a graph of the relationship between the concentration of the vanadium compound and the Amount of vanadium absorbed on the carrier per unit amount of the adsorbed substances.

7 shows the relationship in a graphical representation between the denitration efficiency and the amount of vanadium per unit amount of the adsorbed substances is drawn on the carrier.

Catalyst supports which can be used according to the invention are obtained, for example, from metal bodies which are supported on a metallic Base material wear a surface layer of an aluminum alloy by placing the body with a liquid treated, which dissolves the aluminum. It is also possible to etch the surfaces of the metal bodies and thereby to form porous, rough surfaces. Examples of suitable metallic base materials are pure iron and alloys iron-based, steel, nickel, nickel-based alloys and copper-based alloys. Metal body with a Aluminum alloy as a surface layer are obtained, for example, by using a base material coated with aluminum is subjected to a heat treatment. Aluminum can be made from metallic materials that are superficial are coated with aluminum, dissolve by immersing the material in a solution that dissolves aluminum, or by spraying the solution onto the material. To the For example, aqueous solutions of alkali hydroxides such as NaOH, alkali carbonates,

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Alkaline earth hydroxides and mineral acids. The leaching of the aluminum forms a porous surface on the metallic one Base material.

The porous metallic material to be used as the catalyst carrier is preferably subjected to an oxidizing treatment and / or subjected to treatment with S0 ~. Even if the treatment temperature, the treatment time and the oxygen concentration for the oxidation treatment are not set to a certain value, the metallic Material is preferably treated for 0.1 to 20 hours at room temperature in an atmosphere containing 0.1 to 20.8 vol $ oxygen contains. The SO ^ treatment is preferably carried out for 0.1 to 20 hours at room temperature to 400 C in an atmosphere containing at least 100 ppm SO ".

"Colloidal silica" includes both alkaline colloids, which contain a lot of alkali, and acidic colloids to understand that contain a smaller amount of alkali; the latter are preferred. Are particularly cheap acidic colloids that are adjusted to a pH of 3 to 4. Concentration and temperature of the coating bath, The duration of immersing the carrier in the bath and the number of repeated immersions become such as appropriate determines that the finished coating will eventually have the desired thickness.

The impregnated catalyst support is dried at a temperature of 50 to 150.degree. Preferably, the coating using a colloidal silica with a content of 10 to 30 percent is. ^ Siop carried out as a bath, by the catalyst support for about 10 minutes at the room temperature in the bath, taken out and for 1 hour at about 90 ⁰ G is dried, whereupon the dipping and drying processes are repeated 1 to 6 times until finally a coating

from 7 to 20 u thickness. f

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You can greatly increase the porosity of the coating if you a mixture of colloidal silica and an emulsion of a high molecular weight substance is used as the coating bath, the catalyst support immersed in this bath, the moist support to form a coating on it The surface dries and the coated carrier burns.

Preferred high molecular weight substances are, for example, acrylic compounds that do not contain harmful gases when they burn hand over. The concentration of the high molecular substance in the plating bath is preferably according to desired mechanical strength, the component and the particle size of the dust contained in the reaction gas certainly. Preferably 10 to 50 parts by weight becomes high molecular weight Substance used on 100 parts by weight of SiO in the colloid. The coated carrier is exposed to air Fired for 1 to 5 hours at a temperature of 450 to 700 ° G, preferably 500 to 650 G. Through this strong The high molecular weight substance is removed from the coated carrier by heating.

If the coating bath contains a titanium compound in addition to the colloidal silica and the emulsion of a high molecular weight substance, the properties of the resulting catalyst carrier are further improved by finally obtaining a catalyst of excellent activity. If a mixture of colloidal silica, emulsified high molecular weight substance, a titanium compound and a tin compound is used for the coating bath, a catalyst with high resistance to sulfuric acid is obtained. Suitable titanium compounds are, for example, water-soluble organic titanium compounds, such as the ammonium salt of Ti (OH) - (OCH (CH) C00H and TiOCO (NH.) _ _{O , A.}

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Examples of useful tin compounds are organic tin compounds such as dibutyl tin laurate. Such titanium and tin compounds are thermally decomposed when the catalyst support is fired, forming TiO and SnO, respectively. The resulting coating preferably contains 40 to 100 parts by weight of TiO ₀ and 30 to 70 parts by weight of SnO ₀ , based in each case on 100 parts by weight of SiO ₀ .

As already said, the solutions for immersion treatment contain of the catalyst carrier advantageously has an active component. Examples include vanadium compounds such as Vanadyl sulfate, vanadyl oxalate, ammonium metavanadate or the like, hydrolyzable titanates such as tetra-isobutyl titanate and the Sulphates or halides of copper, iron or antimony, as well as tungstate or chromate. When the catalyst support in a bath is immersed that contains such additives, is also the formed coating contain a compound of V, Ti, Fe, Cu, Sb, W and / or a Cr compound.

The concentration and temperature of the solution, immersion time and similar conditions depend on the amount of active components that are to be applied to the carrier. Preferred application rates are from 0.15 to 1.5 f> for V, 0.15 to 1.5% for Ti, 0.16 to 1.6 f> of Fe, 0.17 to 1.7 $ for Cu, 0 , 1 to 3 fo for Sb, 0.15 to 1.5 f> for W and 0.2 to 2.0 fo for Cr.

The catalysts obtained in this way all have a high denitration activity; especially the catalysts that come with a vanadium compound or a titanium compound are, have an even higher activity and, in addition, a high resistance to sulfuric acid.

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Reference example 1

Measurement of the porosity of the coating

A colloidal silica solution with a pH of 3 »5 and 22 wt / ^ SiO _? was mixed with three different emulsions a, b_ and £ of high molecular weight acrylic substance while varying the ratio, so that 11 coating baths A, B ... K were obtained - see Table 1. Bath A contained no emulsion. The 11 batches were each placed approximately 4 mm high in 11 stainless steel dishes. The bowls had an inside diameter of 5 cm. Each shell was ^{heated to 90 °} C. for 1 hour in order to drive off the water, and the solid residue obtained was calcined in ^{air at 500 ° C. for 1 hour.} The coating formed on the cup surface was peeled off, and the porosity of the coating was measured using a high pressure mercury porosimeter. Table 2 summarizes the results.

Table 1

Emulsion glass transition particle size temperature (C) (nm)

a -45 b +25 C. -55

200

200

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Part 2

Bath Composition of the bath (parts by weight) high molecular weight substance

Emulsion a Emulsion Ib Emulsion £ (cm ^ / g)

A. 100 - B. 94.7 5.3 C. 90 10.0 D. 81.8 18.2 E. 75 25.0 F. 82 13.2 G 82 8.2 H 94.7 I. 90.0 J 75.0 K 90.0

5, 0 5.3 10, 0 10.0 - 18.2 - 5.0 - VJl 0

Total pore volume

0.028 0.100 0.119 0.231 0.319 0.220 0.210 0.083 0.190 0.432 0.127

The relationship between pore size and total pore volume was determined for the coatings <^, /?>, Γ, O and g formed from baths A, C, E, G and J. The results are shown in Fig. 1; the curve shows that the coating ov. from the emulsion-free bath has relatively few pores, while the porosity of the coatings from the emulsion-containing baths is high. Curve γ shows that the use of the emulsion A of a high molecular weight substance with a low glass transition temperature gives the coating pores which are predominantly 30 to 70 Å in size, but hardly exceed 1000 Å, so that the passage of the reaction gas through the coating is hindered . In contrast, the course of the curve ei shows that the emulsion b_ of high molecular weight substance with a high glass transition temperature results in a coating which has a relatively large number of pores of a size not less than 100 Å and accordingly has good gas permeability.

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Reference example 2 a) Carrier manufacture

Sheets made of steel grade SUS 304 of the Japanese industrial standard JIS were made in pieces of 2mm 33mm 50mm 20 min in immersed molten aluminum at 680 C in order to provide it with an aluminum coating. Each specimen was then heat-treated for 1 hour at 800 C under nitrogen gas, to allow the aluminum to diffuse into the sheet metal and a layer of an aluminum alloy on the sheet metal surface to build. The specimens were then each in 200 ml of an aqueous 10% strength by weight at 80 ° C. for 3 hours Sodium hydroxide solution so that the aluminum was dissolved out of the alloy and the surface layer was made porous. The test pieces were then washed with water, air-dried and then at 300 ° C. for 3 hours exposed to nitrogen gas containing 3 vol. $ oxygen contained. This oxidized the porous surface layer. In this way, catalyst carriers were made porous steel.

Some of the catalyst carriers were left for 10 minutes at room temperature immersed in the coating bath A prepared according to Reference Example 1, taken out and dried at 90 ° C. for 1 hour. The dipping and drying processes were repeated 3 times, so that finally a 7-1Ou thick porous silica coating formed on the catalyst supports. The carriers treated in this way were then left in the air for 1 hour Fired at 600 C to remove the high molecular weight organic component. The specimens obtained are marked with (a). In the same manner as before, supports (c) (d) (g) and (j) with coating baths C, D, G and J according to FIG Reference Example 1 treated.

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- ί 3 - b. Measurement of the strength of the beams

The test pieces (a) were placed in a stirred tank, which was crushed to a grain size of 60-80 mesh with a The silica gel was filled, and the silica gel was kept in motion to prevent abrasion on the surfaces of the Exercise specimens (a). Changes in the weight of the test pieces (a) were measured at certain time intervals, to determine the average thickness of the ablated layer. In the same way with the test pieces (c), (d), (g) and (j) proceed and the measurements became up closed the relationship between the average abrasion thickness and the exposure time. The results are in Table 2 shown. In general, the abrasion resistance, i.e. the mechanical strength, decreases with increasing porosity. However, Fig. 2 shows that specimens (c), (d), (g) and (j) prepared using the emulsion-containing baths had a higher porosity than the product (a) obtained with the emulsion-free bath, but with regard to are absolutely equivalent to their strength.

example 1

a. Manufacture of the catalysts

Raschig rings were used as the base material for the catalyst carrier made of steel with a diameter of 21 mm and a height of 20 mm. Six coupons of a catalyst carrier were prepared by forming a porous layer on the rings in the same manner as in Reference Example 2.

Then the mixture of colloidal silica and emulsion a according to reference example 1, the ammonium salt of Ti (OH) ₂ (OCH (CH ₅ ) COOHj ₂ and a mixture of dibutyltin laurate and emulsion a. Were mixed with one another in varying proportions so that four different Coating baths L, M, N and O were formed, as indicated in Table 3

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-H-

is. Of the six test pieces of the catalyst carriers produced as described above, one each was immersed in baths L and M and two each in baths S and 0 and then dried. As in Reference Example 2, this treatment was repeated and baking was also carried out under the same conditions as there. A porous coating had formed on each of the six catalyst carriers. _{TiO 2} and / or VpO were coated on these six slides with the exception of one obtained using Bath 0. To apply the TiOp, the carrier was immersed in liquid tetraisopropyl titanate at room temperature for 10 minutes, taken out, left to stand in saturated steam for 12 hours at room temperature to hydrolyze the titanate, and then dried at 100.degree. To apply VpOp-, the carrier was immersed for 10 minutes at room temperature in a solution of 1 mol of ammonium vanadate NH.VO ^ in 1 liter of a 15% aqueous solution of monoethanolamine and then burned in ^{air at 50O 0 C for 1 hour.} In the case of the catalysts that _{contain both TiO n} and V_0_, TiO_ was first applied to the carrier. In this way there were five catalysts

2
1, m, η., Η and ο. as shown in Table 3. The carrier, which was _{loaded neither with TiO n} nor with V _o 0, _ is also given in the table as catalyst ο for comparison.

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Table 3

Bath composition of the bath applied active (wt. ^) Component (wt. ^)

5.5 5.5 5.5 5.5 5.5

Si ₂ O Ti ₂ L. 100 M. 82 - N 57 25th N 57 25th 0 57 11

57 11

Sn ₂ O * Emul - **
sion a Ti 2 ° - - 2 , 3 18th 2 , 3 18th - 18th 2 , 3 14 18

14th

18th

catalyst

ⁿ 1 n ,.

₂ comparison

* calculated as the amount of metal oxide from the proportion of the corresponding organic metal compound that is used for Making the bath was used.

** Proportion of the relevant high molecular weight substance.

b. Activity test

The activity of the catalysts was tested using a quartz tube as a flow reactor. Of the Catalyst * C was placed in the adjusted reactor tube, and a test exhaust gas of the composition shown in Table 4 was blown at a rate of 15 m / h per Unit of the geometric surface area of the catalyst passed through the reactor tube.

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Table 4 Grass components proportions

NO 0.02

SO 0.02

NH "0.02

O ₂ 5.0

H ₂ O 10.0

CO ₂ 10.0

N ₂ remainder

The denitration effect was calculated from the difference between the NO concentration at the inlet and the outlet of the reactor tube. The experiment was repeated at different reaction temperatures in order to study the behavior of the catalyst at these temperatures. The denitration effect of the catalysts m, n ,, n ", o, and o" was determined in a similar manner. Pig. 3 gives the results and shows that the catalysts with VpO ₁ - all have high activity. The catalysts with a carrier which was produced using the emulsion-containing coating bath and with V _? 0 is loaded.

The catalysts obtained according to the present example were exposed to an atmosphere at 400.degree. C. for 2 hours from air with a content of 4000 ppm sulfuric acid vapor existed, and then examined again in the same way as before for their denitration effect. The results are in Pig. 4 reproduced. A comparison between the

Pig. 3 and 4 shows that the catalysts <£, m, n ,, n _?

ο and ο «at temperatures below 350 C a slightly reduced

Have activity, while the catalyst o ,, which contains SnOp in the coating, retains its activity and has a high resistance to sulfuric acid.

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Example 2

a. Manufacture of the catalysts

A number of test pieces of the same catalyst support as used in Example 1 were dipped several times in the same coating bath 0 as in Example 1 and each time subsequently dried. The pieces were then fired in the same way as in Example 1 and _{loaded with VpO 1} - so that a series of catalysts with different coating thicknesses resulted.

b. Relationship between coating thickness and denitration effect

The catalysts were under the same conditions as in Example 1 tested for its denitration effect at 300 ° C. in order to determine the relationship between the coating thickness and the denitration effect. Curve t in Figure 5 represents the results represent.

In the same way as before, the connection between the coating thickness and the denitration effect on Raschig rings made of cordierite and on Raschig rings made of alumina are determined, each had the same shape and size as those of Example 1. The curves u and ν in Fig. 5 give the results again. FIG. 5 shows that the denitration effect increases with increasing coating thickness, but that with coating thicknesses from 20 u or more, little or no improvement can be achieved. A comparison between the curves t on the one hand and u, ν on the other hand it can be seen that the catalyst support made of porous steel per se is already a remarkable Has denitration activity.

Example 5

a. Manufacture of the catalysts

In the same manner as in Reference Example 2, 10 Samples of a catalyst carrier prepared with the exception that steel material of the quality SS 41 (JIS) is used

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became. The pieces were coated under the same conditions as in Reference Example 2, namely were under Using the coating bath L according to Example 1, ten test pieces were produced.

One of these specimens was immersed for 30 min at room temperature in 200 ml of a 2N oxalic acid solution of MH VO \, 0 mol / l), removed from the solution and then dried at 100 ° C. for 1 hour, so that a vanadium-containing catalyst was won.

Another specimen was placed in a solution of tetra-n-butyl titanate in n-butyl alcohol (1.5 mol / l) under the immersed and dried under the same conditions as above to prepare a Ti-containing catalyst.

The remaining eight test pieces of the catalyst carrier were as described above with the titanate solution, however, treated varying the concentrations as shown in Table 5. So it turned out differently catalysts loaded with Ti. The obtained carriers were then prepared in a similar manner using the same metavanadate solution treated as above, but with the various concentrations given in Table 5, so that they also absorbed vanadium. Finally, there were eight catalysts containing both Ti and V.

b. Activity test

The catalysts were tested for their denitration effect at 350.degree. C. in the same way as in Example 1. In the same manner as in Example 1, the catalysts were treated with sulfuric acid vapor and again on their Tested denitration effect. Table 5 also summarizes these results.

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Table 5

Active grain concentration concentration tjonente. im '· *. of the titanate of NH ₄ VO ₃ catalyst (mol / liter) (mol / liter) amount of active component *
calculated ₀ calculated ₅
as Ti (g / m ⁴⁴ ) as V (g / m.)

Denitrification effect

IF

ο ο

ZD CO

Q **

"2L * ^

V
Ti

Ti + V Ti + V Ti + V Ti + V Ti + V Ti + V Ti + V Ti + V

0.5 1/0 1.5 1.5 1.5 1.5 2.0 3.0

1.0

1.0

1.0

0.1

0.25

0.5

1.0

1.0

1.0 8.1
3.2
4.6
8.0
7.6
8.3
8.1
9.4
13.0

7.5

4.4

4.0
1.3
1.8
2.6
4.2
3.9
4.5

82.1 80.0 77.5 82.4 85.2 83.2 78.0 80.0 86.2 86.0

(58.1) **

(64.0)

(75.2)

(74.0)

(83.0)

(81.5)

(78.0)

(75.0)

(74.0)

(78.0)

A *

The amount of active component applied to the support, expressed in amounts by weight per unit of geometric Surface area of the catalyst.

The value in brackets is the denitration effect after treatment of the catalyst with sulfuric acid vapor.

Table 5 shows that the catalysts containing both a titanium and a vanadium compound, a have higher activity than those with only one of these compounds, and also more resistant to Are sulfuric acid.

Example 4

a. Manufacture of the catalysts

Seven test pieces of the same catalyst carrier having a porous surface layer as in Reference Example 2 were used using the coating bath 0 of Example 1 to prepare seven different catalysts in the same manner as in Reference Example 2. The test pieces were respectively measured at room temperature for 10 min-containing metal salt in the solutions (A) - immersed (G) shown in Table 6, then baked 1 hour at 100 C and finally dried for 1 hour at 30O ⁰ C. As a result, catalysts containing Pe, Cu, Sb, Sb + Fe, ¥ + Fe, and Cr were obtained.

b. Activity test

The catalysts were tested for denitration at 300 and 350.degree. C. in the same manner as in Example 1 tested with the change that the gas at a speed of 24 m / h per unit of geometric surface area of the catalyst was passed through the device. Table 6 summarizes the results.

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Tabel 6th : Concentration Denitri obtain solution Metal salt (Mol / liter) 500 ⁰ C 350 ⁰ C 0.5 42 65 (A) Peso 0.5 45 65 (B) CuSO 0.5 59 71 (C) CuCl ₂ 0.25 65 83 (D) SbBr. 0.25 PeBr ₂ 0.5 40 60 (E) SbBr ₄ 0.25 54 79 (P) Wed ₄ WE ₅ 0.25 PeSO _{4 7} 0.25 68 87 (G) (NH ₄ ) ₂ Cr ₂ 0 0.25 Pe ₂ SO

As Table 6 shows, these catalysts have improved activity at higher temperature.

Example 5

a. Manufacture of the catalysts

Four test pieces of the same catalyst carrier with a porous surface layer as was produced in Example 2 and the four types of plating baths 1 to 4 shown in Table 7, which are in the same manner as in Reference Example 2 were prepared in the same manner as in Reference Example 2 to produce four different Catalyst supports used.

Table 7 also shows the composition of the coating the specimens.

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Table 7

bath * composition
(Gew.Ji)
SiO ₂ TiO ₂ * SnO ₂ * 11 14 of the bathroom Composition of the coating
(Wt. Ji)
SiO ₂ TiO ₂ SnO ₂ 13.4 17.1 1 57 15 10 18th 69.5 18.3 12.2 IV) 57 7 18 18th 69.5 8.5 22.0 3 57 20th 18th 69.5 26.9 - 4th 57 Table 3 18th 74.0 as in Table 3 as in

Aqueous solutions of KH.VO ^ with concentrations that varied between 50 and 1000 mg V per liter were prepared, and using these solutions in the same way as in Example 4 catalysts prepared to establish the relationship between the concentration of Vanadium compound in this solution and the substance adsorbed per unit amount in the coating on the carrier to determine the amount of vanadium applied. This amount of vanadium is expressed by:

Weight of the vanadium on the carrier Weight of the coating χ (TiO ₂ ~ content + 1/2 SnO ₂ ~ G content)

The results are shown in FIG. 6. It can be seen that the vanadium compound is firmly adsorbed on the coating, the amount applied to the carrier increasing as the concentration of vanadium in the solution increases.

b. Activity test

The catalysts were tested in the same way as in Example 1 for their denitration effect at 300 ⁰ G,

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about the relationship between the denitration effect and the Vanadium amount to "determine that per unit of measure of the adsorbed substances in the coating was drawn onto the carrier. The results are shown in FIG. Man recognizes that the denitration effect increases with the increase in the pro Unit of amount of the adsorbed substances on the carrier amount of vanadium increases and the highest value achieved when the latter is in the range from 0.0t to 0.02, which - based on the amount of vanadium 0.15 to 1.5 ^ is equivalent to.

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-at*

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Claims (12)

Claims 1. Process for the preparation of a denitration catalyst by immersing a porous metal catalyst carrier in a silica-containing coating bath and drying the so treated carrier with the formation of a porous silica coating, characterized in that one is on the Carrier absorbs an active component by immersing it in a solution which contains this component, and then dries. 2. The method according to claim 1, characterized in that the porous catalyst carrier is obtained by a metallic Base material, which is a surface layer made of an alloy of aluminum and the metal of the base material is treated with an aluminum solvent until the aluminum is removed. S09883 / 0842 3 · The method according to claim 1, characterized in that the Coating bath is or contains acidic colloidal silica. 4. The method according to claim 1, characterized in that the Coating bath is a mixture of colloidal silica and an emulsion of a high molecular substance, and that the catalyst is fired after immersing the carrier in the bath and then drying it. 5 · The method according to claim 4, characterized in that the high molecular weight substance in the bath in an amount of 10 to 50 parts by weight per 100 parts by weight of SiO _? is present. 6. The method according to claim 4, characterized in that the coating bath is colloidal silica and a titanium compound contains. 7 · The method according to claim 6, characterized in that the amount of titanium compound, calculated as TiO _? , 40 to 100 parts by weight per 100 parts by weight of SiO ". 8. The method according to claim 5, characterized in that the coating bath is colloidal silica and a tin compound contains. 9. The method according to claim 8, characterized in that the amount of tin compound, calculated as SnO ₂ , is 50 to 70 parts by weight per 100 parts by weight of SiO ". 10. The method according to any one of claims 1-4, characterized in that that the active component is a vanadium compound. 11. The method according to claim 10, characterized in that the vanadium compound in an amount of 0.15 to 1.5 wt. $, calculated as V, is mounted on the carrier. 909883/0842 12. The method according to any one of claims 1-9, characterized in that that the active component is a mixture of a vanadium and a titanium compound. 13. Method according to one of Claims 1-9, characterized in that that as the active component a sulfate or a halide of iron, copper and antimony, a tungstate or a chromate, or a mixture of two or more such compounds can be used. 14. Method according to one of Claims 1-9, characterized in that that the coating has a thickness of 7-2Ou. 909883/084 2