EP0785820A1

EP0785820A1 - Nitrogen oxide reducing catalyst compositions based on tantalum, vanadium, niobium, copper or antimony

Info

Publication number: EP0785820A1
Application number: EP95934177A
Authority: EP
Inventors: Philippe Barthe; Pierre Macaudiere; Thierry Seguelong
Original assignee: Rhone Poulenc Chimie SA
Current assignee: Rhodia Chimie SAS
Priority date: 1994-10-13
Filing date: 1995-10-06
Publication date: 1997-07-30
Also published as: BR9509354A; CA2202185A1; AU3656795A; MX9702654A; AU700120B2; KR970706888A; FI971497A; CN1171062A; WO1996011740A1; FI971497A0; JPH10502020A

Abstract

Catalyst compositions for reducing nitrogen oxide emissions during gas processing. In a first embodiment, the compositions are characterised in that they are based on at least one element selected from the group which consists of tantalum, vanadium, niobium and antimony. In a second embodiment, they are characterised in that they are based on at least one element selected from a first group which includes tantalum, vanadium, niobium and antimony, plus at least one other element selected from a second group which includes copper, silver and gold. In a third embodiment, the compositions are based on at least one element selected from a first group which includes tantalum, vanadium, niobium, antimony and copper, plus at least one other element selected from a second group which includes zinc and the elements in groups IIIb, IVb and Vb of the periodic table. In a fourth embodiment, the compositions include copper and at least one other element selected from group VIa of the periodic table.

Description

CATALYTIC COMPOSITIONS FOR THE REDUCTION OF TANTALUM-BASED NITROGEN OXIDES. OF VANADIUM. OF NIOBIUM. OF COPPER OR ANTIMONY

RHONE-POULENC CHEMISTRY

The present invention relates to catalytic compositions based on tantalum, vanadium, niobium, copper or antimony for the reduction of emissions of nitrogen oxides (NO _x ) in the treatment of a gas.

It is known that the reduction of NOx emissions from exhaust gases from automobile engines is carried out using “three-way” catalysts which use the reducing gases present in the mixture stoichiometrically. Any excess oxygen results in a sudden deterioration in the performance of the catalyst.

However, some engines such as diesel engines or gasoline engines operating in lean burn are fuel efficient but emit exhaust gases which permanently contain a large excess of oxygen of the order of 5 to 15% . A standard three-way catalyst therefore has no effect on the NOx emissions of these engines. In addition, limiting NOx emissions is made imperative by the tightening of standards in automotive post combustion.

There is therefore a real need for a catalyst for reducing emissions from

NOx.

Furthermore, it is advantageous to have catalysts which can start to operate at relatively low temperatures.

The object of the invention is therefore to provide a catalytic composition having an effect for the reduction of NOx.

For this purpose, the catalytic composition according to a first embodiment of the invention for the reduction of emissions of nitrogen oxides in the treatment of a gas with a high oxygen content is characterized in that it is based on '' at least one element chosen from the group comprising tantalum, vanadium, niobium and antimony.

According to another embodiment of the invention, the catalytic composition is characterized in that it is based on at least one element chosen from a first group comprising tantalum, vanadium, niobium and antimony and d '' at least one other element chosen from a second group comprising copper, silver and gold.

According to a third embodiment of the invention, the catalytic composition is characterized in that it is based on at least one element chosen from a first group comprising tantalum, vanadium, niobium, antimony and copper, and at least at least one other element chosen from a second group comprising zinc and the elements of groups IIIb, IVb and Vb of the periodic table.

According to a fourth embodiment of the invention, the catalytic composition is characterized in that it comprises copper and at least one other element chosen from the group Via of the periodic table.

The compositions of the invention have an action in reducing the emissions of nitrogen oxides either in the treatment of gases or in the presence or in the absence of a hydrocarbon and / or an organic compound containing oxygen. In some cases, these compositions are effective at low temperatures.

Other characteristics, details and advantages of the invention will appear even more completely on reading the description which follows, as well as various concrete but nonlimiting examples intended to illustrate it.

The periodic classification of the elements to which reference is made in the description is that published in the Supplement to the Bulletin of the French Chemical Society No. 1 (January 1966).

As indicated above and according to a first embodiment of the invention, the catalytic composition comprises at least one element chosen from tantalum, vanadium, niobium and antimony.

According to the second embodiment of the invention, the catalytic composition comprises two categories of elements. It in fact comprises at least one element chosen from a first group consisting of tantalum, vanadium, niobium and antimony. It also comprises a second element chosen from a second group consisting of copper, silver and gold.

According to the third embodiment of the invention, the catalytic composition also comprises two categories of elements. It first comprises at least one element chosen from a first group consisting of tantalum, vanadium, niobium, antimony and copper, and at least one other element chosen from a second group comprising zinc and the elements of groups Illb, IVb and Vb of the periodic table.

As regards the elements of group IIIb, use will be made more particularly of gallium and indium.

For group IVb, mention will be made of tin in particular and antimony and bismuth for group Vb.

The compositions according to the second embodiment and those of the third embodiment based on tantalum, vanadium, niobium and antimony have the advantage of being effective at low temperature. Thus, it has been possible to demonstrate an activity of these compositions at temperatures as low as 300 ° C. or more particularly as low as 200 ° C. According to the fourth embodiment of the invention, the composition comprises copper and at least one other element chosen from the Via group of the periodic table.

As an element of the Via group, there may be mentioned more particularly molydbene and tungsten.

According to a particular variant, the compositions of the invention can also comprise a support.

As support, any support usually used in the field of catalysis can be used, such as, for example, ZrO ₂ , AI ₂ O ₃ , TiO ₂ or SiO ₂ . lanthanide oxides such as CeO ₂ , these supports possibly being doped, or alternatively spinel type oxides, zeolites, silicates, crystalline silicoaluminum phosphates, crystalline aluminum phosphates, these silicates or phosphates which can comprise metal substituents such as titanium, iron, magnesium, zinc, manganese, cobalt, gallium, lanthanum, copper, molybdenum, chromium, germanium or boron.

One can use more particularly as support AI ₂ O ₃ , TiO ₂ , ZrO ₂ , SiO ₂ and spinels like for example MgAI ₂ O ₄ .

According to a particular embodiment of the invention, cerium oxide can be used as a support.

For alumina, mention may in particular be made of aluminas resulting from the rapid dehydration of at least one aluminum hydroxide such as bayerite, hydrargillite or gibbsite, nordstrandite and / or at least one aluminum oxyhydroxide such as boehmite, pseudoboehmite and diaspore.

According to a particular variant, it is possible to use a stabilized alumina. As stabilizing element, mention may be made of rare earths, barium, silicon and zirconium. As rare earth there may be mentioned very particularly lanthanum or the neodymium lanthanum mixture.

For titanium oxide, it is also possible to use an oxide stabilized for example by a rare earth such as lanthanum, barium, strontium, phosphorus, silicon, zirconium or aluminum.

The elements described above and constituting the composition may be present in the latter under different types of phases generally in the form of oxides or mixed oxides, these mixed oxides possibly containing certain elements of the support in the case of the supported compositions .

The amounts and in particular the respective amounts of the constituent elements of the compositions of the invention can vary within wide proportions. The invention therefore applies to compositions in which tantalum, vanadium, niobium, antimony and copper are in the majority in atomic percentage relative to the other elements such as those where these other elements are in the majority. In the case of supported compositions, this amount is generally between 1 and 50% and more particularly, in particular in the case of copper, between 10 and 50%, expressed in atomic content of element relative to the sum of atoms of support elements and moles.

The compositions of the invention can finally comprise precious metals of the type conventionally used in catalysis and in particular in catalysis of automobile afterburning.

Mention may be made, as an example of metals, of platinum, palladium and rhodium, palladium being preferred.

As particular embodiments of the invention, mention will finally be made of compositions which comprise copper and at least one other element chosen from the groups Va, IIIb, IVb and Vb of the periodic table and a cerium oxide support.

As another particular embodiment, mention may be made of the compositions essentially comprising the elements mentioned above, that is to say the compositions in which it is these only elements which have a catalytic action, optionally in combination with precious metals of the type described. above.

The catalytic compositions of the invention can be prepared by any process which makes it possible to obtain an intimate mixture of the constituents of the compositions of the invention. Various methods can be mentioned by way of example.

According to a first method, these compositions are obtained by chamotte of precursors of the elements and, where appropriate, of the support.

These precursors are generally oxides, hydroxides, carbonates or oxalates. They are mixed and ground then optionally shaped under pressure, for example pellets. The mixture is then calcined.

According to a second method, a solution or a slip of salts of the elements and, where appropriate, of the support is first formed.

As salts, one can choose the salts of inorganic acids such as nitrates, sulfates or chlorides.

It is also possible to use the salts of organic acids and in particular the salts of saturated aiiphatic carboxylic acids or the salts of hydroxycarboxylic acids. By way of examples, mention may be made of formates, acetates, propionates, oxalates or citrates.

Then, either the solution or the slip is precipitated by adding a precipitating agent in the presence, if necessary, of the support, or it is atomized before calcination.

In the latter case, a soil can be used instead of a salt of the elements.

According to another method and in the case of supported compositions, the process is carried out by impregnating the support with a solution or a soil of the abovementioned elements. After impregnation, the support is optionally dried and then it is calcined. The solutions that can be used are the same as those described above. For tantalum and niobium, alcoholic solutions of these elements are more generally used, in particular solutions of chlorides.

For the preparation by impregnation of a composition according to the second or third embodiment of the invention, it is possible either to co-impregnate the elements of the different groups, or to proceed in two stages.

In this case, the support is first impregnated with a solution of one of the two groups of elements. The support is optionally dried. In a second step, the support is impregnated with a solution of an element from the other group. Optionally drying and calcining the support thus impregnated.

More particularly, dry impregnation is used. Dry impregnation consists in adding to the product to be impregnated a volume of an aqueous solution of the element which is equal to the pore volume of the solid to be impregnated.

The compositions of the invention can be in various forms such as granules, balls, cylinders or honeycomb of variable dimensions.

The invention also relates to a catalytic system comprising a composition of the type which has been described above, for example a system comprising a coating of known composition, in particular based on a refractory oxide (wash coat) and based on these compositions, on a substrate of the type, for example metallic or ceramic monolith.

The systems are mounted in a known manner in catalytic devices such as vehicle exhaust pipes in the case of application to the treatment of exhaust gases.

The invention therefore applies to the manufacture of catalysts or catalytic devices using the compositions or systems described above.

The gases capable of being treated by the compositions of the present invention are, for example, those originating from gas turbines, boilers of thermal power stations or else from internal combustion engines, in particular from diesel engines or engines operating in lean mixture .

The invention applies to the treatment of gases which have a high oxygen content and which contain nitrogen oxides, with a view to reducing the emissions of these oxides. By gas having a high oxygen content, one understands gases having permanently an excess of oxygen compared to the stoichiometric value λ = 1. The value λ is correlated to the air / fuel ratio in a manner known per se in particular in the field of internal combustion engines. In other words, the invention applies to the treatment of gases from systems of the type described in the previous paragraph and operating continuously under conditions such as λ is always strictly greater than 1. The invention also applies to the treatment of gases which have an oxygen content (expressed by volume) of at least 5%, more particularly at least 10%, this content being able for example be between 5 and 20%.

The gases can contain a reducing agent or be treated in the presence of a reducing agent such as a hydrocarbon and, in such a case, one of the reactions which it is sought to catalyze is the reaction HC (hydrocarbon) + NO _x .

The hydrocarbons which can be used as a reducing agent for the elimination of NOx are in particular the gases or liquids of the families of saturated carbides, ethylenic carbides, acetylenic carbides, aromatic carbides and hydrocarbons from petroleum fractions such as for example methane , ethane, propane, butane, pentane, hexane, ethylene, propylene, acetylene, butadiene, benzene, toluene, xylene, kerosene and gas oil.

The gases can also contain, as reducing agent, organic compounds containing oxygen or be treated in the presence of these. These compounds may especially be alcohols of the type, for example saturated alcohols such as methanol, ethanol or propanol; ethers such as methyl ether or ethyl ether; esters such as methyl acetate and ketones.

It should be noted however that according to an advantageous characteristic of the invention, the treatment process can be carried out on a gas without the presence of a reducing agent. This applies particularly to the case of compositions which comprise copper and at least one other element chosen from the groups Va, Illb, IVb and Vb of the periodic table and a support made of cerium oxide.

Examples will now be given.

In the examples given below and unless otherwise indicated, the compositions are tested in the following manner to evaluate their catalytic performance.

1.5 g of the powdered catalyst are loaded into a quartz reactor.

The reaction mixture at the inlet of the reactor has the following composition (by volume):

- NO = 300 vpm

- C ₃ H ₆ = 300 vpm

- O ₂ = 10%

- CO ₂ = 10%

- H ₂ O = 10%

- N ₂ = qs 100%

The overall flow rate is 10 Nl / h.

The VVH is of the order of 10,000 h ^'1 . The NO and NO _x signals (NO _x = NO + NO ₂ ) are permanently recorded as well as the temperature in the reactor.

The NO and NO _x signals are given by a NO _x ECOPHYSICS analyzer, based on the principle of chemistry-luminescence: it gives the values of NO and NO _x .

The catalytic activity is measured from the NO and NO _x signals as a function of the temperature during a programmed temperature rise from 20 to 700 ° C at a rate of 3.75 ° C / min and from the following relationships:

- The NO conversion rate (TNO) in% which is given by:

T (NO) = 100 (NO ° -NO) / NO ° with NO ° NO signal at time t = 0 which corresponds to the NO signal obtained with the reaction mixture during the bypass of the catalytic reactor and NO is the NO signal at time t.

- The overall conversion rate of NO _x (TNO _x ) to% which is given by:

T (NO _x ) = 100 (NO _x ° -NO _x ) / NO _x ° with NO _x ° signal of NO _x at time t = 0 which corresponds to the signal of NO _x obtained with the reaction mixture during the by- pass of the catalytic reactor and NO _x is the signal of NO _x at time t.

- The rate of conversion of NO _x to N ₂ O (TN ₂ O) in% which is given by:

T (N ₂ O) = 100 (N ₂ ON ₂ Oº) / NO _x ° with N ₂ Oº signal of N ₂ O at time t = 0 which corresponds to the signal of N ₂ O obtained with the reaction mixture during bypass of the catalytic reactor and N ₂ O is the signal for N ₂ O at time t.

Finally, by specific surface is meant the specific surface B.E.T. determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 established from the BRUNAUER - EMMETT-TELLER method described in the periodical "The Journal of the American Society, 60, 309 (1938)". EXAMPLE 1

This example relates to compositions based on niobium.

1) Synthesis of the catalysts Copper nitrate (Cu (NO ₃ ) ₂ , 3H ₂ O), tin chloride (SnCl4), gallium nitrate (Ga (NO ₃ ) ₃ ) in solution are used as raw materials. , indium nitrate (ln (NO ₃ ) ₃ ) in solution, zinc nitrate (Zn (NO ₃ ) ₂ ) in solution and niobium chloride (NbCl ₅ ).

The support used is undoped alumina, calcined at 1090 ° C for 8 h to reduce its specific surface to 28 m ² / g before deposition of the active elements.

The atomic content of active element is 10% calculated as follows:

([Nb] + [X]) / ([Nb] + [X] + [AI ₂ O ₃ ]) = 0.10 with X = Cu, Zn, Ga, Sn or In, where [] represents the number of moles of the species considered. The same niobium and second doping element content (5 atomic% of each) was deposited on the support, namely:

([Nb]) / [Nb] + [X] + [AI ₂ O ₃ ]) = 0.05

([X]) / ([Nb] + [X] + [AI ₂ O ₃ ]) = 0.05

with X = Cu, Zn, Ga, Sn or In.

To prepare the catalytic compositions, the operating protocol is as follows:

- Dry impregnation of the first element from an alcoholic solution of niobium resulting from the dissolution of NbCl ₅ with anhydrous ethanol, the alumina having a pore volume of 0.70 cm ³ / g.

- Drying in the oven (110 ° C, 2 h).

- Dry impregnation of the second element X (X = Cu, Zn, Ga, Sn, In), in all cases in the form of an aqueous solution.

- Drying in the oven (110 ° C, 2 h).

- Calcination in air at 750 or 950 ° C for 2 h, mounted at 5 ° C / min.

The products obtained have the following characteristics:

Example 1-1: [Nb] = 5% and [Cu] = 5 atomic%, calcination at 750 ° C, SBET = 19.5 m ² / g.

Example 1-2: [Nb] = 5% and [Zn] = 5 atomic%, calcination at 950 ° C, SBET = 24.5 m ² / g.

Example 1-3: [Nb] = 5% and [Ga] = 5 atomic%, calcination at 750 ° C, SBET = 23.0 m ² / g.

Example 1-4: [Nb] = 5% and [Sn] = 5 atomic%, calcination at 750 ° C, SBET = 24.5 m ² / g.

Example 1-5: [Nb] = 5% and [In] = 5 atomic%, calcination at 750 ° C,

SBET = 21.5 m ² / g.

2) The catalytic performances are given in Tables I to V below:

Examples 1 -3, 1-4 and 1-5 show a very good level of activity, for the specific surface considered (<25 m ² / g), with a maximum NO _x conversion greater than 30% at 500 ° C approx. In addition, the NO _x conversion temperature range is very wide (between 350 ° C and 600 ° C approximately for an NO _x conversion greater than 20%). Note also a reaction initiation temperature below 200 ° C for Examples 1-3 and 1-4, which is very advantageous for a diesel engine.

In the case of Example 1-1, the conversion takes place over a wide temperature range between about 250 ° C and 400 ° C. In addition, the initiation temperature of the NO _x conversion reaction is also less than 200 ° C.

Example 1-2 gives intermediate results between the previous examples.

EXAMPLE 2

This example relates to tantalum-based compositions.

1) Synthesis of the catalysts

The same materials as those of the previous example are used as raw materials as well as tantalum chloride (TaCl ₅ ) in place of niobium chloride.

The content of tantalum and of other elements are identical to that of Example 1, the preparation of the catalysts is carried out according to the same protocol.

The products obtained have the following characteristics:

Example 2-1): [Ta] = 5% and [Cu] = 5 atomic%, calcination at 750 ° C,

SBET = 20.0 m ² / g. Example 2-2: [Ta] = 5% and [Zn] = 5 atomic%, calcination at 950 ° C, SBET

= 22.5 m ² / g.

Example 2-3: [Ta] = 5% and [Ga] = 5 atomic%, calcination at 950 ° C, SBET = 22.5 m ² / g.

Example 2-4: [Ta] = 5% and [Sn] = 5 atomic%, calcination at 950 ° C, SBET

= 22.5 m ² / g.

Example 2-5: [Ta] = 5% and [In] = 5 atomic%, calcination at 950 ° C, SBET

= 20.5 m ² / g.

2) The catalytic performances are given in Tables VI to X below.

Examples 2-3, 2-4 and 2-5 show a very good level of activity, for the specific surface considered (<25 m ² / g), with a maximum NO _x conversion greater than 30% at 500 ° C approx. In addition, for examples 2-4 and 2-5, the NO _x conversion temperature range is very wide between 350 ° C. and 550 ° C. approximately for an NO _x conversion greater than 20%. EXAMPLE 3

This example relates to a composition based on vanadium.

1) Synthesis of the catalyst

Copper nitrate (Cu (NO ₃ ) ₂ , 3H ₂ O) and sodium orthovanadate (Na ₃ VO ₄ ) are used in aqueous solution.

The support used is the same as that of Example 1.

The vanadium and copper contents are identical to those of Example 1, as well as the operating protocol.

The product obtained has the following characteristics:

Example 3-1: [V] = 5% and [Cu] = 5 atomic%, calcination at 750 ° C, SBET = 14.0 m ² / g.

2) The catalytic performances appear on table XI. These results show:

- a good level of activity, for the specific surface considered (<20 m ² / g), with a maximum conversion of NO _x of the order of 65% at around 500 ° C.

- a relatively wide activity temperature range, of approximately 100 ° C., between 450 ° C. and 550 ° C. in which the NO _x conversion remains greater than 20%.

- an initiation temperature of the NO _x conversion reaction of less than 200 ° C.

EXAMPLE 4

This example illustrates compositions based on a single active element.

These compositions are prepared with the same support as Example 1 and for niobium and tantalum the same precursors as Examples 1 and 2. For antimony, the precursor is antimony tartrate; for vanadium, it is ammonium vanadate. The products obtained have the following characteristics:

- Example 4-1 [Sb] = 5% calcination at 950 ° C, 2 h.

- Example 4-2 [Nb] = 5% calcination at 950 ° C, 2 h.

- Example 4-3 [Ta] = 5% calcination at 950 ° C, 2 h.

- Example 4-4 [V] = 5% calcination at 750 ° C, 2 h.

The catalytic performances are given in Tables XII to XV below

EXAMPLE 5 This example illustrates a composition based on antimony and copper on a titanium support. The product is prepared in the same manner as in Example 1. For antimony, the precursor is tartrate. The support is TiO ₂ , a mixture of anatase and rutile with a surface area of 60 m ² / g. The product obtained has the following characteristics:

- [Sb] = 7% and [Cu] = 3% calcination at 750 ° C 2h. SBET 26 m ² / g

The catalytic performances are given in table XVI. To determine this performance, 0.75 g of catalyst was loaded into the reactor.

EXAMPLE 6

A composition based on niobium and gallium is prepared as in Example 1.

The support used is an alumina.

The product obtained has the following characteristics:

[Nb] = 20% and [Ga] = 20% calcination at 750 ° C 2h. SBET 118 m ² / g.

The test is carried out with a VVH of 100,000 h ^-1 and 150 mg of product.

The results are given in Table XVII.

EXAMPLE 7

This example relates to compositions based on copper.

1) Synthesis of the catalysts Copper nitrate (Cu (NO ₃ ) ₂ , 3H ₂ O), gallium nitrate (Ga (NO ₃ ) ₃ ) in solution, indium nitrate (ln ( NO ₃ ) ₃ ) in solution, tin chloride (SnCl ₄ ), zinc nitrate (Zn (NO ₃ ) ₂ ) in solution, bismuth nitrate in solution, ammonium metatungstate ((NH ₄ ) ₆ H ₂ W ₁₂ O ₄₀ ) and ammonium heptamolybdate ((NH ₄ ) ₆ Mo ₇ O ₂₄ ).

The support used is undoped alumina, calcined at 1080 ° C for 8 h to reduce its specific surface to 37 m ² / g before deposition of the active elements.

The atomic content of active element is 10% calculated as follows:

([Cu] + [X]) / ([Cu] + [X] + [AI ₂ O ₃ ]) = 0.10 with X = Mo, Ga, Sn, Zn, Bi, In and W. where [] represents the number of moles of the species considered

To prepare the catalytic compositions, the operating protocol is as follows:

- Dry impregnation of copper, alumina having a pore volume of 0.70 cm ³ / g.

- Drying in the oven (110 ° C, 2 h).

- Dry impregnation of the second element X.

- Drying in the oven (110 ° C, 2 h). - Calcination in air at 750 for 2h, mounted at 5 ° C / min.

The products obtained have the following characteristics:

Example 7-1: [Cu] = 5% and [Ga] = 5 atomic%, calcination at 750 ° C,

SBET = 23 m ² / g.

Example 7-2: [Cu] = 7% and [Mo] = 3 atomic%, calcination at 750 ° C,

SBET = 18.5 m ² / g.

Example 7-3: [Cu] = 9% and [Mo] = 1 atomic%, calcination at 750 ° C,

SBET = 18.5 m ² / g.

Example 7-4: [Cu] = 5% and [Sn] = 5 atomic%, calcination at 750 ° C,

SBET = 18m ² / g.

Example 7-5: [Cu] = 5% and [Zn] = 5 atomic%, calcination at 750 ° C,

BET = 23.5m ² / g.

Example 7-6: [Cu] = 5% and [Bi] = 5 atomic%, calcination at 750 ° C.

SBET = 18m ² / g.

Example 7-7: [Cu] = 5% and [In] = 5 atomic%, calcination at 750 ° C,

SBET = 22m ² / g.

Example 7-8: [Cu] = 5% and [W] = 5 atomic%, in this case we proceeded first by dry impregnation of the support with ammonium metatungstate then in a second step we have copper impregnation, calcination at 750 ° C, SBET = 25.3m ² / g.

2) The catalytic performances are given in tables XVIII to XXV below

EXAMPLE 8

This example relates to compositions based on copper and comprising a CeO ₂ support.

The compositions are tested by using 50 mg of these in the form of powder with a particle size of 125-250 μm diluted in 150 mg of SiC with the same particle size.

The total gas flow rate is 30NI / h. The WH is 500000h ^-1 .

The composition of the treated gas mixture is that given above in the preamble to Example 1, with a CO content of 350 vpm.

In some cases, the treated gas mixture does not include any hydrocarbon and therefore corresponds to the following composition: NO = 300 vpm, O ₂ = 10%, CO ₂ = 10%, H ₂ O = 10% and N ₂ = qs 100%. 1) Synthesis of the catalysts

The raw materials used are copper nitrate (Cu (NO ₃ ) ₂ , 3H ₂ O), gallium nitrate (Ga (NO ₃ ) ₃ ) in solution, bismuth nitrate (Bi (NO ₃ ) ₃ 5H ₂ O), platinum (H ₂ PtCl ₆ ), an alcoholate and a niobium sol and a tin sol.

The niobium alcoholate is obtained by dissolving niobium chloride in an ethanolic medium at 70 ° C for 2 hours with stirring. The niobium sol is obtained by precipitation of the niobium alcoholate in an ammoniacal medium. The tin sol is prepared by adding volume to volume of a solution of NH ₄ OH

(1.70 mol / l) in a solution of tin chloride (SnCl ₄ 0.50 mol / l). After several washes with an ammonia buffer with a precipitation pH (close to 8.7) to remove the chlorides, the precipitate is prepared by centrifugation and resuspended in water to form a sol.

The supports used are cerium oxide CeO ₂ RHONE-POULENC

The atomic content of active element is 10% relative to the number of moles of the cerium oxide, ie:

([Cu] + [X]) / ([Cu] + [X] + [CeO ₂ ]) = 0.10 with X = Sn, Nb, Ga or Bi,

where [] represents the number of moles of the element considered,

either again:

[Cu] + [X] = 0.1 and [CeO ₂ ] = 0.9.

The preparation method is a dry impregnation which is carried out under the same conditions as those of Example 1. The calcination is carried out in air, at 750 ° C for 2 hours, mounted at 5 ° C / min.

The products obtained have the following characteristics:

Example 8-1 and 8-2: [Sn] = 5 atomic% and [Cu] = 5 atomic%. SBET = 63m ² / g.

Example 8-3 and 8-4: [Ga] = 5 atomic% and [Cu] = 5 atomic%. SBET = 60m ² / g.

Example 8-7: [Bi] = 5 atomic% and [Cu] = 5 atomic%, SBET = 31m ² / g.

Example 8-8: [Nb] = 3 atomic% and [Cu] = 7 atomic%, SBET = 63m ² / g. For this example, the impregnation was done on a support doped with platinum, the amount of platinum in the composition being 2500 ppm. Niobium is introduced by the alcoholate.

Example 8-9: this example is carried out with the composition of example 8-8 but the catalyst was aged 6 hours at 750 ° C. under a gas flow of 100NI. h ^-1 to 10% of O ₂ . 10% CO ₂ and 10% ₂ O.

For the other examples, the products were prepared by the atomization method. A slip is formed having a concentration of reagents expressed as an oxide of 180 g / l. Niobium is introduced as a soil. This slip is then atomized with a Buchi atomizer with an inlet temperature of 220 ° C and an outlet temperature of 130 ° C. We calcine as in the previous examples.

Example 8-5: [Nb] = 3 atomic% and [Cu] = 7 atomic%. SBET = 84m ² / g

Example 8- 6 [Nb] = 7 atomic% and [Cu] = 3 atomic%. SBET = 67m ² / g

2) The catalytic performances are given in tables XXVI to XXXIV below.

Examples 8-1 and 8-2 demonstrate that the Cu-Sn / CeO ₂ compounds are active with respect to the reduction of NO _x emissions in the presence or not of a reducing agent of the HC type and / or CO. It will be noted that the catalytic activity is both higher and obtained at lower temperature in the absence of a reducing agent of the HC and / or CO type in the reaction mixture.

Examples 8-3 and 8-4 demonstrate that the Cu-Ga / CeO ₂ compounds are active with respect to the reduction of NO _x emissions in the presence or absence of a reducing agent of the HC type and / or CO. It will be noted that the catalytic activity is both higher and obtained at lower temperature in the absence of a reducing agent of the HC and / or CO type in the reaction mixture.

Examples 8-5 and 8-6 demonstrate that systems of the Cu-Nb / CeO _{2 type} are active with respect to the reduction of NO _x emissions in the absence of a reducing agent of the HC type and / or CO. Formulations with an excess of Cu over Nb (expressed in atoms) are even more active under these conditions.

The comparison of Examples 8-8 and 8-9 makes it possible to demonstrate the stability of the performance of the catalyst after a thermal aging treatment for 6 hours at 750 ° C. in the presence of water, CO ₂ , and oxygen in the gas mixture. The catalytic activity did not decrease as a result of this treatment.

Claims

1- Catalytic composition for reducing emissions of nitrogen oxides in the treatment of a gas with a high oxygen content, characterized in that it is based on at least one element chosen from the group comprising tantalum, vanadium, niobium and antimony.

2- Catalytic composition for reducing emissions of nitrogen oxides in the treatment of a gas with a high oxygen content, characterized in that it is based on at least one element chosen from a first group comprising tantalum , vanadium, niobium and antimony and at least one other element chosen from a second group comprising copper, silver and gold.

3- Catalytic composition for the reduction of emissions of nitrogen oxides in the treatment of a gas with a high oxygen content, characterized in that it is based on at least one element chosen from a first group comprising tantalum , vanadium, niobium, antimony and copper, and at least one other element chosen from a second group comprising zinc and the elements of groups Illb, IVb and Vb of the periodic table.

4- Catalytic composition for the reduction of emissions of nitrogen oxides in the treatment of a gas with a high oxygen content, characterized in that it comprises copper and at least one other element chosen from the Via group of the classification periodic.

5- Composition according to claim 3, characterized in that the elements of the group Illb are gallium or indium, that of the group IVb tin and those of the group Vb, antimony or bismuth.

6- Composition according to claim 4, characterized in that the elements of the Via group are molybdenum and tungsten.

7- Composition according to one of the preceding claims, characterized in that it further comprises a support and, more particularly, a support chosen from alumina, silica, titanium oxide, zirconium oxide, lanthanide oxides, spinel-type oxides, zeolites, silicates, crystalline silicon phosphates, crystalline aluminum phosphates.

8- Composition according to claim 7, characterized in that it comprises a support in cerium oxide.

9- Composition according to one of the preceding claims, characterized in that it comprises copper and at least one other element chosen from the groups Va, Illb, IVb and Vb of the periodic table and a support made of cerium oxide.

10 - Catalytic system characterized in that it comprises a composition according to one of the preceding claims.

11- Process for the treatment of gases with a high oxygen content for the reduction of emissions of nitrogen oxides, in particular for the treatment of exhaust gases from internal combustion engines, characterized in that a catalytic composition according to l one of claims 1 to 9 or a system according to claim 10.

12- A method according to claim 11, characterized in that a gas is treated in the presence of a reducing agent of the type in particular hydrocarbon or organic compound containing oxygen.

13- A method according to claim 1 1, characterized in that a gas is treated in the absence of a reducing agent.

14- Use of a composition according to one of claims 1 to 9 or of a catalytic system according to claim 10 in the manufacture of catalysts or catalytic devices for automotive post combustion.