CA2010970A1

CA2010970A1 - Catalyst for the selective reduction of nitrogen oxides with ammonia

Info

Publication number: CA2010970A1
Application number: CA002010970A
Authority: CA
Inventors: Reinhold Brand; Bernd Engler; Wolfgang Honnen; Edgar Koberstein; Johannes Ohmer
Original assignee: Individual
Current assignee: Evonik Operations GmbH
Priority date: 1989-02-28
Filing date: 1990-02-27
Publication date: 1990-08-31
Also published as: KR900012674A; AU5002690A; DE3906136C1; RU2058814C1; ATE95443T1; EP0385164A3; DD296854A5; EP0385164A2; JPH02290250A; DK0385164T3; AU615375B2; DE59002961D1; BR9000900A; ES2046549T3; EP0385164B1

Abstract

ABSTRACT OF THE DISCLOSURE
In addition to titanium oxide as component A), a catalyst for the selective reduction of nitrogen oxides with ammonia contains at least one oxide of W, Si, B, Al, P, Zr, Ba, Y, La, Ce and at least one oxide of V, Nb, Mo, Fe, Cu as component B), the atomic ratio between the elements of components A) and B) being from 1 : 0.001 to 1. The catalysts can be obtained by kneading reactive titanium oxide having a high specific surface of, predominantly, anatase with the substances of component B) or precursors thereof with addition of processing aids to form a homogeneous kneaded paste, extruding this paste, drying the extrudate and calcining it in air at 300 to 800°C..

Description

2~3~0970 The pr~sent invention relates to a catalyst for the selective reduction of nitrogen oxides with ammonia.

Nitrogen oxides which are formed in combustion processes are among the main causes of acid rain or photosmog and the ensuing damage to the environment. Along with fluorochlorocarbons, nitrogen oxides in particular are suspected of being responsible for the observed depletion of the ozone layer above the polar region.

Sources of nitrogen oxide emission are vehicle traffic, stationary combustion engines, power stations, thermal power stations, steam generators for industrial purposes and industrial production plants.

Although, in boiler-fired power stations, a reduction in the nitrogen oxide concentration of the exhaust gases can be obtained by using very pure fuels or by optimizing the combustion systems (primary measures), there are both technical and economic limits to these primary measures. Accordingly, secondary measures also have to be taken to keep to the legally stipulated emission limits. Secondary measures for nitrogen oxide reduction are, typically, catalytic reduction processes in which ammonia is generally used as a selective reducing agent.

Catalysts are know for reducing nitrogen oxide emission 2~ hy catalytic reduction. Thus, DE-AS 12 59 298, 12 53 685 and 11 15 230 describe oxidic catalysts with no noble metal content while DE-OS 22 14 604 describes oxidic catalysts containing noble metals.

3~ DE-PS 24 S8 888 describes another catalyst which consists of an "intimate mixture" of the following components: A) titanium in the form of oxides, B) at least one metal from the group 2~

~-1 iron and vanadium in the form of oxides and/or from the group s-2 molybdenum, tungsten, nickel, cobalt, copper, chromium and uranium in the form of oxides C) tin in the form of oxides D) metals from the group consisting of beryllium, magnesium, zinc, boron, aluminium, yttrium, rare earth elements, silicon, niobium, antimony, bis-lo muth and manganese in the form of oxides.
The components are present in atomic ratios of A : B ~ C :
D of 1 : 0.01 to 10 : 0 to 0.2 0 to 0.15 nltrogen oxlde A catalyst having this composition is used for the-reduction ~ gas mixtures containing oxygen and ammonia at temperatures in the range from 150 to 550C and at space velocities of 300 to 100,000 h-1.
These catalysts may be produced by measures known per se. However, these measures must always guarantee that the components A) and B) and, optionally, C) can be obtained as an intimate mixture in the form of their oxides. The fol-lowing are mentioned as typical examples o~ such production processes:
la: homogeneous dissolving process lb: co-precipitation processes 2 : cGmbinations of the dissolving and precipitation processes 3 : mixed precipitate processes.
Solutions and/or precipitates, such as for example hydroxides or aqueous gels, are used as precursors of components A), B) and C), being intimately mixed and then subjected to calcination. The precursors are pyrolized, giving the desired intimate mixture of the oxides of the components critical to the catalysis process. The calcin-ation temperature has to be in the range from 300 to 800~C.
Below 300C, it is not possible to obtain an intimate 97~

mixture of the oxides and therefore no active catalyst;
above 800 C, sinterilly occurs and results in loss of the acti~e catalyst surface.
Ille starting mate~ials used for titanium as component A) include, for example, various titanium acids, titanium hydroxide and various titanium salts, such as halides, titanium sulfate, titanyl sulfate and the like. Organic compounds of titanium, for example titanium alkoxides, may also be used as starting material for the titanium. Titan-ium oxide in the calcined rutile or anatase form is not suitable.
The catalyst material described in DE-PS 35 31 809 is based on a further development of these catalyst production processes described in DE-AS 24 58 888. Titanium dioxide is used as starting material for the production of this catalyst material and is ground together with vanadium oxide and one or more oxides of the elements tungsten, molybdenum, phosphorus, chromium, copper, iron, uranium and then ~ bc~ed to at least one heat treatment. In ~his le~-$æea$~e~t, the tungsten and molybdPnum are completely or partly replaced by phosphorus in the form of its oxides or phosphates. The mills used are preferably eccentric disc mills and ring chamber mills. The calcination step takes place at a temperature in the range from 200 to 900C.
With all hitherto known catalysts of this type, com-ponents A) and B) (for example A = Ti; B = W, Mo. etc.) must be present in the form of an intimate mixture of their oxides. This mixture is then subjected to a shaping pro-cess from which the catalysts can be obtained in the form of loose material or honeycomb monoliths by compre5sion moulding or extrusion.
This means that shaping of the catalyst, for example by extrusion, has to be preceded by a process of which the object is to form the intimate mixture as starting materi-20~0970 al. Thls procedure, which is widely used at the present time, is attended by the followlng disadvantages:

- elaborate, multlstage, energy-intenslve process steps are necessary - the productlon of the lntlmate mixture ln accordance with DE-AS 24 58 888 is not environment-friendly on account of emission and wastewater problems;

- the operatlon of mills for grIndlng activatlon is energy-o lntenslve and requlres elaborate noise control measures and dust control systems;

- the preliminary process steps for formlng the lntlmate ~mixture are a serious cost factor in the production of the catalyst.

Accordingly the present invention greatly simplifies and, hence, dlstlnctly reduces the cost of catalyst productlon by 2U saving process steps in the productlon of so-called solld type catalysts~whlch conslst throughout of catalytically active materlal) for the selectlve reduction of nltrogen oxides in oxygen-containing exhaust gases in the presence of ammonia. In addltlon, the inventlon enables the~propertles~of the catalyst to be dlrectly and finely controlled through the management of the process.
~:
According to the present inventlon there ls provlded a catalyst for the selective reductlon of nitrogen oxides with ammonia contalning the followlng components: A) titanium oxide, 3U Bl) at least one oxide of tungsten,~sillcon, boron, aluminum, phosphorus, zirconium, barium, yttrium, lanthanum, cerium, and B2) at least one oxide of vanadium, nlobium, molybdenum, iron copper, wlth an atomlc ratlo between the elements of components A) and B) of 1 : 0.001 to 1 obtalned by lntenslvely kneadlng ~ 109~0 component A) in the form of a reactive high-surface titanlum oxide having a BET surface of 40 to 500 m2/g, whlch is completely or predomlnantly present ln the anatase modlfication, together wlth components sl) and s2) or thelr precursors, wlth the addition o of the additlves normally used for the compresslon moulding or extrusion of ceramlc pastes, moistening agents, supporting substances, blnders, mouldlng aids and. to form a homogeneous kneaded paste, extruding the kneaded paste to mouldings, drying the mouldlngs with a gradual increase in temperature to at most 60C and then calcinlng them with a stepwise lncrease in temperature in ambient air at:flnal lU temperatures ln the range from 300 to 800C. Suitably 2. A
catalyst is obtalned by using hydroxides, oxides, salts or heteropoly acids or salts thereof as starting material for components sl) and B2).
l!;
Thus according to the present invention certain titanium oxide materials are processed by kneading and there is.
no longer any need for the already known measures for the formation of on intimate oxlde mlxture.

Suitable titanium oxides are reactive high-surface titanium oxldes having a specific BET surface of from 40 to 500 preferably 50 to 300 and more preferably 75 to 150 m2/g whlch are completely or predominantly present:in anatase form, These 2r materials may be commercial products produced by precipation or ~ by flame hydrolysis. Apart 3U .
.

- - 4a -2~ 3~"3 ~ ~3 from tll~ir higll specific surface, these generally highly disperse products have a very narrow particle siz~ distri-bution witll an averaye primary particle size of the order of 30 ~m.
The individual elements of group B) may be used, for example, in the form of the following starting compounds:
tungsten oxide, ammonium para- or meta-tungstate, silicon dioxide, silicotungstic acid, silica, boron oxide, boric acid, aluminium oxide, phosphorus oxide, ammonium phos-phate, phosphotungstic acid, phosphomolybdic acid, zir-conium dioxide, zirconium phosphate, barium oxide, yttrium oxide, lanthanum oxide, cerium acetate, cerium nitrate, cerium oxide, borotungstic acid, vanadium oxide, vanadyl oxalate, ammonium metavanadate, vanadotungstic acids, vanadomolyhdic acids, vanadophosphomolybdic acids, phospho-vanadic acid, niobium oxide hydrate, niobium oxalate, niobium vanadic acids, ammonium molybdate, molybdenum oxide, iron oxide, iron phosphate, iron hydroxide, organic iron salts, such as iron oxalate, copper acetate, copper-(II) oxide, Cu-, Ce-, La-containing heteropoly acids, etc.
The substances mentioned may be used in the form of solutions or even in the form of solid substances.
Apart from the actual catalyst material, a number of additives known ~er se are required for the production of solld type the whol~ catalysts according to the invention.
Deionized water, aqueous ammonia solution, monoethan-olamine and alcohols may be used as the moistening agent.
Glass fibers of various sizes, for example, may be used as supporting materials.
Suitable binders, which provide the paste to be pro-duced with sufficient stability in the so-called green state after shaping, are cellulose derivatives such as, for example, carboxymethyl cellulose or even unsubstituted celluloses.
Polyethylene, polypropylene, polyvinyl alcoholl poly-~ 3 ethylene oxide, polyacrylamide or polystyrene may also be used as the binder.

To facilitate compression moulding or to improve extrudabillty, mouldlng aids and/or lubricants such as, for example, bentonites, aluminas, organlc acids, paraffin, waxes, ; silicone oils, are added.

Finally, the porosity (pore volume, pore size distribution) of the whole catalyst may be ad~usted as required by the addition of suitable air-~ntraining agents. ~dditives Lu such as these are, for example, finely divided carbons or wood pulp, which burn out at the calcination temperatures applied.

Kneading units are used for intensively kneading the starting materials to a homogeneous kneaded paste. Kneaders with 1~ sigma or masticator blades are preferred. The use of a certain titanium oxide in accordance with the invention in conjunction with an intensive kneading step for preparing the catalyst paste affords considerable advantages over the conventional production 2U process. Thus, there is no longer any need for elaborate environment polluting and, hence, expensive process steps. Co-precipitation and grinding processes for preparing an intimate oxids mixture are no longer necessary. This leads to a distinct reduction in production costs and also eliminates dependence on expensively produced starting materials.
2~
In addition, contraction, a key parameter in regard to the tendency of solid type catalysts to crack and in regard to their breaking strength, can be regulated as required by the use of suitable starting materials and by adaption of the kneading 3~ process in regard to intensity, temperature and time, including intermediate conditioning steps.

Thus according to one embodiment of the present invention the catalyst is obtained by mixing component A) and 3~

;~311Uf~70 component Bl) ln the kneader and subjecting the resulting mixture to dry preliminary kneading at pH 7 to ll to a residual moisture content of 3 to 12% by weight before component B2 is added.
Desirably the catalyst is obtained by precalcining the dry-prekneaded mixture of components A) and Bl) at temperatures of 400 to 700C before component B2) is added. Suitably the catalyst is obtained by precalcining the paste consisting of components A), B1) and B2) at 300 to ~00C before it is kneaded to a homogeneous kneaded paste.

In a further embodiment of the present invention the u catalyst is obtalned by components Bl) and B2) which are used in the form of a heteropoly acid or one of its salts the metals of groups Bl) and B2) present in the heteropoly acid being present in an atomic ratio of 12 : l to l : 12.
l!;
In a still further embodiment of the present invention the salt is the ammonium salt.

In addition, the new catalyst production process, by 2U saving several proces steps, guarantees more direct and, at the same time, more flexlble control of the catalyst 2~

- 6a -~ 139'~'~

prop~rties. Thus, t~le resistance of the catalysts to sintering can be improved by the choice of a suitable stabilizer component from group Bl). This is re~lected in the fact that the high BET surface of the final catalyst remains substantially constant under in-use conditions and in the fact that the temperature-induced phase transition of the titanium dioxide from the anatase to the rutile modification is suppressed. ~i~t-~etl~ -lo~ger--use~ul--~-~e~
~f tho ~ l}cte~ebt-~nablc in accordance with thc cl~
are the dire~t rcsult of thcGc--pro~crtic_.
Figure 1 (for Example 13) illustrates the reduction in activity fall-off during long-term use in flue gases of dry cjofal-burning plants with operating temperatures above 300~C
~crc sio2 is used as stabilizer in the catalyst.
Through the physicochemical properties of the startin~
materials ~Tio2 anatase, stabilizers from group Bl), acti-vators from group B2) and through the choice of the other additives and through processing in a kneader in accordance with the invention, the pore volume and pore radiu3 dis~
tribution of a given kneaded paste can be controlled by empirical variation of its moisture content during the size kneading proc~ss. The pore radiur~ distribution can be varied within wide limits over the mesopore and macropore range, mono- bi- and trimodal pore ra~ZuG distributions and transitional forms in between being adjustable as required.
The correct choice of these parameters leads to a consid-era~le increase in the activity of the catalyst. However, poreVdistribution and pore volume also have a major influ-ence.on resi~an~e to poisoning and hence directly on the statlonary ~ltetlme uroful~ 4-of the catalysts.
In this connection, particular significance is also attributed to the pK value of the solid surface. In the catalyst according to the invention, the pK value may be varied within wide limits through the choice of the stabil-izers or activators. The use of heteropoly acids as *) As a d;rect result of these propert;es catalysts obta;nable ;n accordance with the cla;ms have dist;nctly longer stationary lifetimes~

;~:q~3 ~9'~'~

activators and/or st~bilizers is particular]y mentioned in this re~ard. Interestingly, it has been found that, when used i31 the particularly problematical flue gases of coal-fired sl~g-tap furnaces, catalyst~ made of these materials show a distinctly lower tendency to ~u~-~ arsenic and other catalyst poisons by comparison, for example, with the catalysts according to DE-PS 25 58 888. This is attri-butable to increased poisoning resistance through reduced heavy metal adsorption to the catalyst surface. As a result, catalysts can be used over industrially useful periods in the high-dust operation of coal-fired slag-tap fur~ces. By contrast, known comparison catalysts are rapidly deactivated mainly by heavy metals present in the flue gas.
Figures 2 and 3 show the surprisingly high increase in activity of catalysts according to the invention by com-parison with a catalyst conventionally produced in accord-ance with Comparison Example l.
The invention is further illustrated by the following Examples.
The catalysts were tested both in dust-free model exhaust gases in a laboratory test plant and also in the exhaust gas of an oil-burning plant. In addition, long-term tests were carried out in the flue gas of a dry coal-burning plant.
The catalyst tests were carried out at temperatures in the range from 200 to 500C. The space velocities were between 10,000 and 40,000 h-1. The reducing agent, ammonia, and the nitrogen oxide were used in the particular molar ratio found to bc fa~ h~ of 0.~ to 1.6t~ preferably 0.8 to 1.2t which was found to be favourable.

Comparison Example 20 Litres deionized water, 6 kg 15% by weight aqueous NH3 solution, 1.8 kg monoethanolamine and a solution of 2~
ammollium m~t~lv~nadate corresponding t~ 3~0 g V2o5 were added to 35 k~ of an intimate mixture of the oxides Tio2 and W03 in a r~tio by weight of 9 : 1 prepared in accordance with DE-PS 24 5~ 8~8. The mixture is intensively kneaded with a varying moisture conten-t a~d at temperatures of 70 to 90C. 620 g Sio2, 1.4 kg alkali-free clay and 3.5 kg glass fibres (length 1 to 8 mm) were then successively added.
The mixture is kneaded for 6 to 8 hours to form a homogene-ous kneaded paste, 410 g polyethylene oxide, 410 g carboxy-lo methyl cellulose, 230 g lactic acid and 11~5 kg fully deionized water additionally being added to establish the plasticity required for subsequent shaping.
The catalyst paste is then extruded in an extruder to honeycomb monoliths with passages of square cross-section (cell division: 3.4 mm or 7.4 mm).
The extrudates are dried at an increasing temperature of 20 to 60~C in an air-conditioned drying chamber and, after a stepwise increase in temperature, are finally calcined for 24 hours at 620C.
Examples 1 to 9 The composition of the catalysts is shown in Table 1.
The following basic procedure was adopted to prepare the catalysts:
4.4 kg ammonium paratungstate (APW), 22 litres deion-ized water, 7.5 kg 15% by weight aqueous NH3 solution, 1.8 kg monoethanolamine and a solution of ammonium metavanadate corresponding to 390 g VzOs are added to 35 kg of the Tio2 anatase mentioned in claim 1 with a BET surface of 98 m2/g.
670 g sio2, 2.5 kg glass fibres ~length 1 to 8 m~) and 1.5 kg alkali-free clay are then successively added with inten-sive kneading at a temperature in the range from 60 to 90C. The mixture is kneaded for 5 to 7 hours to form a homogeneous kneaded paste (Werner & Pfleiderer LUK 2.5 kneader), 450 g polyethylene oxide, 450 g carboxymethyl 20~037~

cellulose, 250 g lactic acid and 12.3 litres deionized water being additionally incorporated to establish the necessary plasticity. To fine-tune the moisture content and the plasticity of the kneaded paste, more ammonia water had to be added before the end of the kneading process.
The catalyst paste is then extruded in an extruder to honeycomb monoliths with passages of square cross-section (cell division: 3.4 mm). After drying at an increasing temperature of 20 to 60~C in an air-conditioned drying lo chamber, followed by a stepwise increase in temperature, the mouldings are calcined for 24 hours at 620-C.
In Examples 6 to 9, Tio2-P-25 produced by flame-hydrolysis (Degussa), tungsten oxide or boron oxide and Nb205, the latter in the form of niobium oxalate dissolved in water, were respectively used instead of Tio2 anatase, ammonium paratungstate (APW) and ammonium metavanadate (AMV).

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Examples 10 to 13 ~.4 kg ammonium paratungstate (APW) and 10 kg 15% by weight aqueolls N~l3 solution are added to 35 kg of the Ti anatase mentioned in claim 1, BET surface 75 m2/g, in a kneader which has been switched on. The suspension obtain-ed is kneaded for 3 hours at 80C to dryness (residual moisture 5 to 10~ by weight). 22 Lltres deionized water, 75 kg 15% by weight aqueous NH3 solution, 1.8 kg monoethan-olamine and a solution of ammonium metaYanadate (AMV) cor-responding to 390 g V2Os are then added to the mixture thus obtained. The resulting paste is further processed as described in Examples 1 to 9 and extruded to form the same honeycomb monoliths. Th ~ monoliths are also dried and calcined by the method described in Examples 1 to 9.
In Example 11, the ammonium metavanadate was replaced by ammonium molybdate (AM) and, in Examples 12 and 13, the ammonium paratungstate was replaced by BaO or sioz, as shown in Table 2.

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Exam~lP _ 1~ to 17 4.0 kg aluminium oxide and 12.5 kg 50% by weight aqueous Nll3 solution are added to 3S kg of the Tioz anatase mentioned in claim 1 with BET surface of 40 m2/g. The paste is kneaded for 2 to 3 hours at 80C to a residual moisture content of 5 to 10~ by weight. The powder is then precal-cined for ~ hours at 400C.
22 Litres deionized water, 75 kg ~4% by weight aqueous NH3 solution, 2 . O kg monoethanolamine, 210 g pulp (coarse-fibre cellulose~ and then a solution of ammonium metavana-date corresponding to 390 g V2O5 are added to the precal-cined oxide mixture in the kneader. 2.3 kg alkali-free clay, 2.2 kg glass fibres (length 1 to 8 mm), 200 g poly-ethylene oxidde~jt200 ~lcarboxymethyl cellulose and 250 y lactic acid are ~he~ added with intensive kneading at 60 to 90C. The mixture is kneaded for 5 to 7 hours to form a homogeneous kneaded paste, more ammonia water being added to establish the necessary plasticity. Finally, the cata-lyst paste is extruded in an extruder to honeycombs having square passages (cell division: 7.4 mm). After drying at an increasing temperature (20 to 60C) in an air-condi-tioned drying chamber, followed by a stepwise increase in temperature, the mouldings are calcined for 24 hours at 700C.
In Examples 15 to 17, the aluminium oxide was replaced by ammonium paratungstate or lanthanum oxide and, in Example 16, the ammonium metavanadate was replaced by copper(II) acetate dissolved in water, as shown in Table 3.

a ~ 2Q3 5~97'~3 o o ul o r~ O

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~ @ ~ i I r~ r 3~ 3 Exam~les 18 to 21 4.0 kg zirconiun- oxide, 390 g V2O5 and 15 kg 15% by weight aqueous Nl13 solution are added to 35 kg of the Tio2 anatase mentioned in claim 1 with a BET surface of 280 m2/g.
The thinly liquid paste i5 kneaded for 2 to 4 hours at 80C
to a residual moisture content of 5 to 10% by weight. The dry powder is then precalcined for 2 hours at 700C.
25 kg ~*~ deionized water, 75 kg 15% by weight NH3 solution and 2.0 kg monoethanolamine are added to the pre-calcined mixture, followed by further processing as in Examples 1 to 9. The final catalyst paste is extruded to honeycombs as in Examples 14 to 17.
In Examples 19 to 21, zirconium dioxide was replaced by ammonium paratungstate or phosphorus pentoxide and, in Example 20, V205 was replaced by iron(III) oxide, as shown in Table 4.

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Exam~les 22 to 26 422 g ammonium-2-hydrogen-12-vanadophosphate and 2~
litres deionized water are added to 35 kg of the Tio2 anatase mentioned in claim 1 with a BET surface of 98 mZ/g.
The paste is intensively kneaded at temperatures of 40 to 70OC, 670 g Sio2, 2.S kg glass fibres (length 1 to 8 mm) and 6.0 kg alkali-free clay being additionally introducPd.
In addition, 450 g polyethylene oxide, 900 g carboxymethyl cellulose, 250 g lactic acid and 15 litres deionized water are added to establish the necessary plasticity.
The. mixture is kneaded for 5 to 7 hours to a homogene-ous kneaded paste which is then processed as in Examples 1 to 9 to honeycomb monoliths.
In Examples 23 to 26, the ammonium-2-hydroxy-12-vanadophosphate was replaced by heteropoly acids, as shown in Table 5:

Table 5 __ ~
Example Comp. A Comp. Bl + B2 Ratio by weight A oxide/B2 oxide __ _ _ 22 Anatase (NH4)sH2[P(vl2o36] 9.99 : 0.01 23 Anatase (NH4)s[V6w6037] 9.9 : 0.1 24 Anatase H4[P(Mllv04o)] 9.99 : 0.01 Anatase H6~P(MogV3040)J 9.99 : 0.01 26 Anatase (NH4~6H~P(Mollcuo4o)] 9.99 : 0.01 . ~

Examples 27 to 31 4.3 kg ammonium paratungstate, 22 litres deionized water, 7.5 ky 15% hy weight aqueous NH3 solution and 1.8 kg monoethanolamine are added to 35 kg of the Tio2 anatase )9 ~ ~

mentionecl in Example 1 Wit}l a BET surface of 9~ m2/g. As in Examples 1 to 9, tlle paste is provided with additives ~plasticizers, supporting materials, etc.), intensively kneaded (2 to 7 hours at 60 to 90OC) and extruded to holleycomb monoliths which, in this case, count as catalyst precursors. The monoliths are dried and calcined as in Examples 1 to 9 and, after cooling ~ , 1.0 g vanadium pentoxide per 100 g titanium dioxide/tungsten oxide mixture is applied by impregnation with a solution of ammonium-2-hydrogen~12-vanadophosphate in a quantity of water corrdes~o~din~ to the water absorption capacity of the ~as mentlone 1 c alm honeycomb monolith~. The monoliths are dried at 150-C w~1~o 9 air ~Lows_~c~g~ and are subsequently conditioned for 2 hours at 400'C.
In Examples 28 to 31, the ammonium paratungstate and ammonium-2-hydrogen-12-vanadophosphate are respectively replaced by the quantities shown in Table 6 of ammonium metatungstate, yttrium oxide, zirconium dioxide or silicon dioxide and V2O5 (in the form of aqueous solutions of vanadium oxalate), ammonium-6-tungstato-6-vanadate or 11-molybdo-l-vanadophosphoric acid.

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Testin~ of the c~talysts roduced The catalysts prepared in accordance with Examples 1 to 31 were tested in the exhaust gas of an oil-burning plant WhiC}l was adjusted to meet the following test condi-tions by the introduction of additional pollutants (NO~ and SO2) and ammonia required for nitrogen oxide reduction.
Test conditions:
Exhaust gas composition: N0~ 800 ppm NH3 800 ppm S02 500 ppm 2 5.0 % by vol.
HzO 11.0 % by vol.
C02 12.0 % by vol.
N2 balance The catalyst tests were carried out at a temperature in the range from 250 to 500C and at a space velocity of 20,000 hl~ Selected results of the measurements and long-term tests in dry coal-burning plants under the conditions already described are shown in the form of graphs in Figures 1, 2 and 3. The basic measured data are shown in Tables 7 and 8.

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Table_~*

. . . _ . _ _ Dry coal-burning plant (T=450C) t[}l~ Examples Comparison 1 6 13 Example .. .. ._ ._ . ..
Zero measurement 95 92 89 83 500 gl.5 86 84.5 75.5 1000 89.5 ~5 84 73.5 2000 88 85 83.5 73 3000 88 84.5 83.5 72.5 4000 87.5 84.5 83.5 71.5 . _ _ values * The figure~ shown represent NOX conversions (~NO ) in percent, based on the starting NOX concentration ~3

Claims

1. A catalyst for the selective reduction of nitrogen oxides with ammonia containing the following components: A) titanium oxide, B1) at least one oxide of tungsten, silicon, boron, aluminum, phosphorus, zirconium, barium, yttrium, lanthanum, cerium, and B2) at least one oxide of vanadium, niobium, molybdenum, iron, copper, with an atomic ratio between the elements of components A) and B) of 1 : 0.001 to 1 obtained by intensively kneading component A) in the form of a reactive high-surface titanium oxide having a BET surface of 40 to 500 m2/g, which is completely or predominantly present in the anatase modification, together with components B1) and B2) or their precursors, with the addition of the additives normally used for the compression moulding or extrusion of ceramic pastes, moistening agents, supporting substances, binders, moulding aids and to form a homogeneous kneaded paste, extruding the kneaded paste to mouldings, drying the mouldings with a gradual increase in temperature to at most 60°C and then calcining them with a stepwise increase in temperature in ambient air at final temperatures in the range from 300 to 800°C.

2. A catalyst as claimed in claim 1, obtained by using hydroxides, oxides, salts or heteropoly acids or salts thereof as starting material for components B1) and B2).

3. A catalyst as claimed in claim 2 in which the starting material is the ammonia salt.

4. A catalyst as claimed in claim 1 obtained by mixing component A) and component B1) in the kneader and subjecting the resulting mixture to dry preliminary kneading at pH 7 to 11 to a residual moisture content of 3 to 12% by weight before component B2 is added.

5. A catalyst as claimed in claim 1 obtained by mixing component A) and component B1) in the kneader and subjecting the resulting mixture to dry preliminary kneading at pH 8 to 10 to a residual moisture content of s to 10% by weight before component B2 is added.

6. A catalyst as claimed in claim 4 or 5 obtained by precalcining the dry-prekneaded mixture of components A) and B1) at temperatures of 400 to 700°C before component B1) is added.

7. A catalyst as claimed in claim 1, 2, 3, 4 or 5 obtained by precalcining the paste consisting of components A), B1) and B2) at 300 to 800°C before it is kneaded to a homogeneous kneaded paste.

8. A catalyst as claimed in claim 1, 2, 3, 4 or 5 obtained by precalcining the paste consisting of components A), B1) and B2) at 400 to 700°C before it is kneaded to a homogeneous kneaded paste.

9. A catalyst as claimed in claim 1, in which components B1) and B2) are used in the form of a heteropoly acid or one of its salts the metals of groups B1) and B2) present in the heteropoly acid being present in an atomic ratio of 12 : 1 to 1 : 12.

10. A catalyst as claimed in claim 9 in which the salt is the ammonium salt.

11. A catalyst as claimed in claim 1, obtained by leaving the starting material for component B2) out of the kneading process and applying it by impregnation in the form of a salt or a heteropoly acid or one of its salts, in aqueous solution to the catalyst precursor consisting of components A) and B1).

12. A catalyst as claimed in claim 11, in which the catalyst precursor is calcined.

13. A catalyst as claimed in claim 1, 2, 3, 4 or 5 in which the titanium oxides have a BET surface of 50 to 300 m2/g.

14. A catalyst as claimed in claim 1, 2, 3, 4 or 5 in which the titanium oxides have a BET surface of 75 to 150 m2/g.

15. A catalyst as claimed in claim 1, 2, 3, 4 or 5 in which the kneaded paste also contains an air destroying agent.