KR100392168B1 - Zinc Aluminate with High Specific Surface Area, Preparation Method and Use for Treating Motor Vehicle Exhaust Gases - Google Patents

Abstract

The present invention relates to zinc aluminate, characterized in that the specific surface area after firing at 800 ° C. for 8 hours is 85 m ² / g or more. The invention also relates to a precursor composition of the aluminate. Methods of making aluminates and compositions include the steps of contacting a salt, zinc sol or alkoxide and aluminum alkoxide in a solvent medium; Hydrolyzing the resulting mixture by adding excess water relative to aluminum alkoxide; Recovering the precipitate formed and optionally drying to obtain a precursor composition; And if desired, obtaining the aluminate by calcining the precipitate. Finally, the present invention relates to the use of aluminates for treating automotive exhaust.

Description

Zinc Aluminate with High Specific Surface Area, Preparation Method and Use for Treating Motor Vehicle Exhaust Gases}

The present invention relates to the use of aluminates in zinc aluminate, a precursor composition of this aluminate, aluminate and a process for producing the composition and gas treatment, in particular for the exhaust gas treatment of automobiles.

It is known that reducing the emission of nitrogen oxides (NOx) from the exhaust of automobile engines or industrial plants constitutes a major environmental protection problem. In the case of motor vehicles, a "three way" catalyst is used in particular, which uses stoichiometrically the reducing gas present in the mixture. However, excess oxygen causes a rapid deterioration of the catalyst.

However, certain engines, such as diesel or petrol engines operating in lean burn mode, save on fuel but emit exhaust gases that permanently contain a large excess of oxygen (eg, 5% or more). Thus, standard three way catalysts have no effect on NOx emissions from these engines. Moreover, strengthening standards on automotive post-combustion, which now extend to these types of engines, impose a limit on NOx emissions.

In these engines, catalysts in the form of spinel based on aluminum and zinc have been proposed. However, these catalysts have to be further improved because they do not have very large specific surface areas at high temperatures. Now, surface stability, that is, the ability to maintain large surface areas at high temperatures, is one aspect that can improve the performance of the catalyst.

Therefore, a spinel catalyst having a large specific surface area is required.

For this purpose, the zinc aluminate of the present invention is characterized by having a specific surface area of 85 m ² / g or more after firing at 800 ° C. for 8 hours.

The present invention also relates to a precursor composition for zinc aluminate, which composition comprises a compound of zinc and aluminum and is capable of forming zinc aluminate after firing, the aluminate being 800 ° C. It has a specific surface area of more than 85 m ² / g after firing for 8 hours at.

Another subject of the invention is a process for the preparation of the aluminate or composition of the above type, characterized in that it comprises the following steps.

Bringing together the zinc salt and aluminum alkoxide in the solvent medium;

Hydrolyzing the mixture formed by adding excess water relative to aluminum alkoxide;

Recovering the precipitate formed and optionally drying to obtain a precursor composition;

If desired, obtaining an aluminate by calcining the precipitate.

Additional features, details, and advantages of the invention will become more apparent by the various, specific but non-limiting examples of the following part of the specification and the intention to illustrate it.

The aluminate of the present invention is zinc aluminate. It has a spinel type structure, ZnAl ₂ O ₄ . It is one or more phases that satisfy the formulas Zn _1-x Al ₂ O _4-δ and Zn _{1 + x} Al ₂ O _{4 + δ} , where 0 <x ≤ 0.95, which is depleted or enriched in zinc compared to ZnAl ₂ O ₄ may be in the form of a phase. The value of x can more particularly satisfy the relationship of 0 <x ≦ 0.85, 0 <x ≦ 0.8, and more particularly 0 <x ≦ 0.5. Finally, x can satisfy the relationship of 0.4 ≦ x ≦ 0.85. The aluminate may further contain one or more additives. These additives may be selected from elements of groups IA, IIA, VII to IB of the periodic table, and tin, gallium and rare earths.

The periodic table of the elements mentioned is listed in Supplement au Bulletin de la Societe Chimique de France (Annex 1 of January 1996). The term "rare earth" should also be understood to mean elements of the group formed of elements of the periodic table with yttrium and atomic numbers inclusively between 57 and 71.

Manganese may be more particularly mentioned as an element of group VIIA, iron may be particularly mentioned as an element of group VIII, and copper and silver may be more particularly mentioned as an element of group IB.

These additives may in particular be present in the aluminate as a partial replacement for zinc or aluminate.

One feature of the aluminates of the present invention is the specific surface area. The term "specific surface area" is established throughout this specification by the Brunauer-Emmett-Teller method described in The Journal of the American Chemical Society, 60 , 309 (1938). It is to be understood that it refers to the BET specific surface area determined by nitrogen adsorption according to ASTM D 3663-78 standard.

Even after firing at high temperature, the aluminates of the present invention still have large surface area values. Therefore, after firing at 800 ° C. for 8 hours, the specific surface area is 85 m ² / g or more. It may be at least 90 m ² / g and more particularly at least 100 m ² / g after firing at 800 ° C. for 8 hours. A value of 120 m ² / g or more can be obtained.

Since the aluminate of the invention may have a specific surface area of at least 70 m ² / g, more particularly at least 80 m ² / g after firing at 900 ° C. for 2 hours, this large surface area value is maintained even at higher temperatures. In addition, it is possible to observe a specific surface area of 50 m ² / g or more, more particularly 70 m ² / g or more at 1000 ° C after 6 hours firing. This means that the surface area of the aluminate is stable over a wide range of temperatures. The values given above extend to firing in air. It can also be noted that the aluminates of the present invention have high ageing resistance. This means that the specific surface area changes relatively little under certain firing conditions. Thus, H ₂ O would be obtained after a 10% by volume of H ₂ O / N with the surface area after calcination at 1000 ℃ in a _second medium for 6 hours and firing for the same time at the same temperature in air (i.e., 50 m ² / g or more). The same result is obtained when firing in an O ₂ / H ₂ O / N ₂ medium having H ₂ O of 10% by volume and O ₂ of 10% by volume.

In addition, the aluminate of the present invention may have a void volume of 0.6 ml / g or more, and the porosity is determined by mercury injection porosimetry. For measurement, a Micromeretics Auto Pore 9220 machine was used on powder degassed overnight in an oven heated to 200 ° C. The operating parameters are as follows: permeability constant: 21.63, capillary volume: 1.1, contact angle: 140 °. The porosity may more particularly be at least 2 ml / g, for example between 2.5 and 3.5 ml / g.

The invention also relates to a precursor composition of aluminate, as described above.

This composition comprises a compound of zinc and aluminum and, if desired, a compound of the aforementioned additives. The main feature of the precursor composition is the ability to produce zinc aluminate after firing. The lowest firing temperature at which the aluminate is formed is approximately 500 ° C. In addition, the aluminate thus obtained has a characteristic as described above, that is, when fired at a temperature of 800 ° C. for 8 hours, it is 85 m ² / g or more, more particularly 90 m ² / g or more and even more particularly 100 m ² / g It is characterized by having a specific surface area. Of course, all the surface area values given above for the aluminate at temperatures of 800 ° C. and 900 ° C. also apply here.

The method for producing the aluminate and its precursor composition is described below.

The first step of this process is to collect zinc salts, sols or alkoxides and aluminum alkoxides together in a solvent medium, optionally with one or more salts, sols or alkoxides of the abovementioned additives. Zinc salts or alkoxides as well as salts or alkoxides of the additives must be soluble in the solvent medium. The salts of zinc salts or additives are for example inorganic salts such as nitrates or chlorides or organic salts such as citrate, oxalate or acetate. Aluminum alkoxides can be for example ethoxide, butoxide or isopropoxide.

The solvent medium is selected from any medium in which zinc salts or alkoxides and aluminum alkoxides can be dissolved. In general, alcoholic solvents are used. As alcoholic solvents, mention may be made of saturated monohydric alcohols and more particularly those having short chains (eg, up to C ₈ ) such as methanol, ethanol, propanol and butanol. It is also possible to use unsaturated alcohols and polyhydric alcohols such as for example ethylene glycol, propylene glycol, hexylene glycol, propanediol and butanediol. Ketones such as acetylacetone can also be used.

The reactions can be brought together in a solvent in any manner. However, according to one particular embodiment of the present invention, zinc salts and aluminum alkoxides are pooled together by addition to aluminum alkoxides, which pre-dissolve in the mother liquor, the zinc salts in the solvent medium, ie in this medium.

Here, it will be noted that according to another aspect of the invention, it is possible to heat the mixture thus obtained. This makes salts easier to dissolve and gives better control of the next step (hydrolysis and precipitation).

The second step of the process of the invention is to hydrolyze the mixture obtained in the previous step.

Hydrolysis occurs by adding water to the mixture. According to one feature of the process of the invention, hydrolysis is carried out by using excess water relative to aluminum alkoxide. This excess is determined by the H ₂ O / aluminum alkoxide molar ratio. In general, this ratio may be at least 6, more particularly at least 10 and even more particularly at least 20. However, this ratio may be lower when producing zinc-depleted aluminates. More specifically, when preparing an aluminate having a Zn / Al ratio of less than 0.4, the H ₂ O / aluminum alkoxide molar ratio may be at least 3, more particularly at least 4.

The water may be provided in the form of a water-alcohol mixture, and the alcohol may be especially selected from those mentioned above with regard to the solvent medium. More particularly mention may be made of ethanol.

Hydrolysis results in the precipitation of components.

The precipitate obtained is separated from the reaction mixture by any known means, in particular by centrifugation.

If necessary, the precipitate can be washed off.

Optionally, precipitation can then be dried.

In this step, the precursor composition of the present invention is obtained. Aluminate is manufactured by baking a precipitation (precursor composition) at the temperature of 500 degreeC or more.

If the aluminate or precursor comprises additives of the abovementioned type, mention may be made of another method of making the aluminate or precursor of the invention. This method is performed by immersing the precursor composition (eg dry precipitation) or the aluminate itself (ie calcined precipitation) without adding an additive during the synthesis of the aluminate. Immersion is carried out using a salt solution of an additive of the type given by way of example above.

Dry dipping is more particularly used. Dry soaking consists of adding an aqueous solution of a component equal to the pore volume of the solid to be immersed in the product to be immersed.

The invention also relates to a method for the treatment of automobile exhaust gases using a catalyst system comprising the aluminate.

The invention also relates to a method of using a catalyst system comprising this same aluminate for the treatment of gases which may comprise nitric oxide for the purpose of reducing the release of these nitric oxides.

In this case, the gases which can be treated are, for example, those coming from gas turbines, boilers of thermal power plants or internal combustion engines. In the latter case they may in particular be diesel engines or engines operating in lean burn mode.

The aluminates of the present invention are thus applied in the treatment of gases with high oxygen content and nitrogen oxides in order to reduce the emissions of these oxides. The expression "high oxygen content gas" refers to a gas having an excess of oxygen relative to the amount required for stoichiometric combustion of the fuel, and more particularly a gas having an excess of oxygen permanently relative to the stoichiometric value of λ = 1. It must be understood. The lambda value is correlated with the air-fuel ratio in a manner known per se, in particular in the art of internal combustion engines. In other words, the aluminates of the present invention are applied to the treatment of gases coming from systems of the type described in the previous paragraph and operating permanently under conditions such that λ is always strictly greater than one. Thus, for gases with high oxygen content, the aluminates of the present invention, on the one hand, operate in lean burn mode and process gases from engines having a high oxygen content (expressed in volume), typically between 2.5 and 5%. On the other hand higher oxygen content, such as for example diesel engines, i.e. at least 5%, more particularly at least 10% (for example between 5 and 20%). Branches are applied to the treatment of gases.

The gas contains a reducing agent, which may be one or more hydrocarbons, in which case one of the preferred reactions for carrying out the catalysis is the HC (hydrocarbon) + NOx reaction.

Hydrocarbons that can be used as reducing agents for removing NOx are, in particular, saturated carbides, ethylenic carbides, acetylenic carbides, aromatic carbides, and petoleum cut hydrocarbons (eg methane, ethane, propane, butane, pentane, hexane , Ethylene, propylene, acetylene, butadiene, benzene, toluene, xylene, kerosene and diesel).

The gas may also contain organic compounds containing oxygen as the reducing agent. These compounds are, for example, alcohols of saturated alcohols such as methanol, ethanol or propanol; Ethers such as methyl ether or ethyl ether; Esters such as methyl acetate and ketones.

The gas may also contain ammonia as the reducing agent.

In application to the treatment of gases, aluminates can be used in catalyst compositions that can have a variety of shapes and sizes, such as granules, beads, cylinders or honeycombs, such as for example ZrO ₂ , The aluminates of the invention may be included on any substrate commonly used in the field of catalysts such as Al ₂ O ₃ , TiO ₂ , CeO ₂ , SiO ₂ or mixtures thereof.

The invention also relates more particularly to the catalyst system for the gas treatment process described above. The system is characterized by including an aluminate on the substrate. Such systems generally comprise a wash coat incorporating an aluminate and a support of the above type, which wash coat is deposited on a substrate, for example a metal or ceramic honeycomb substrate.

The system is mounted in a known manner in the exhaust pipe of a car when applied to the treatment of exhaust gas.

Finally, the present invention also relates to the use of the aluminate or precursor compositions for producing such catalyst systems. Examples are described below.

Example 1

This example relates to the preparation of ZnAl ₂ O ₄ .

The following raw materials are used.

Zn (NO ₃ ) ₂ .6H ₂ 0, with 99% purity, molecular weight: 297.47.

97% pure aluminum tri-sec-butoxide (C ₂ H ₅ CH ₂ CH ₂ O) ₃ Al

99% pure hexylene glycol (2-methyl 2,4-pentanediol)

Anhydrous ethanol, molecular weight: 46.07; d: 0.79 g / cm ³ .

1.25 moles of zinc salt are dissolved in 1 L of hexylene glycol. This solution is added to the 2.5 moles of aluminum tri-sec-butoxide introduced previously in the reactor at one time and with vigorous stirring (500 rpm). The mixture is heated to 70 ° C. and maintained at this temperature for 2 hours. Next, a water / ethanol (50/50 volume ratio) mixture is added at a rate of 5 mL / min. The water / aluminum tri-sec-butoxide ratio is 28. The mixture is left to cool overnight with stirring. The obtained precipitate is separated by centrifugation. The paste is dried in a 70 ° C. oven for 48 hours to form a thin film. Finally, the product is calcined. The temperature increase is 5 ° C./min. The temperature is then maintained for the given time at a constant level of the given value.

After firing at 600 ° C. for 6 hours, the specific surface area of the product is 136 m ² / g.

After firing at 700 ° C. for 6 hours, the specific surface area of the product is 125 m ² / g.

After firing at 800 ° C. for 8 hours, the specific surface area of the product is 115 m ² / g.

After firing at 900 ° C. for 2 hours, the specific surface area of the product is 101 m ² / g.

After firing at 1000 ° C. for 2 hours, the specific surface area of the product is 76 m ² / g and after 6 hours at the same temperature it is 53 m ² / g.

After firing at 1000 ° C. for 6 hours in a H ₂ O / N ₂ mixture having H ₂ O of 10% by volume, the specific surface area is maintained at 53 m ² / g. The same result is obtained when firing at 1000 ° C. for 6 hours in an O ₂ / H ₂ O / N ₂ medium having H ₂ O of 10 vol% and O ₂ of 10 vol%.

Example 2

The precipitate obtained in Example 1 is dried at 70 ° C. for 48 hours. This is then immersed in a SnCl ₂ .2H ₂ O solution dissolved in ethanol. The technique used is dry dipping. The amount of Sn deposited is 1.6% by weight relative to zinc aluminate oxide. The product thus obtained is oven dried at 110 ° C. for 2 hours and then calcined at 800 ° C. for 8 hours (rise rate, 5 ° C./min).

The specific surface area of the product thus obtained is 115 m ² / g.

Example 3

This example relates to the preparation of ZnAl _1.8 Ga _0.2 O ₄ .

The same raw material and 1.807 mol / L Ga (NO ₃ ) ₃ solution (d = 1.365 g / cm ³ ) as in Example 1 was further used.

Dissolve 1.25 moles of zinc salt in 1 L of hexylene glycol and then introduce 0.25 moles of gallium nitrate.

This solution is added to 2.25 moles of aluminum tri-sec-butoxide previously introduced into the reactor with vigorous stirring (500 rpm). The mixture is heated to 70 ° C. and maintained at this temperature for 2 hours. The water / ethanol (50/50 vol. Ratio) mixture is then added at a rate of 5 mL / min. The water / aluminum tri-sec-butoxide ratio is 25. The mixture is left to cool overnight with stirring. The obtained precipitate is separated by centrifugation. The precipitate obtained is dried at 70 ° C. for 48 hours to form a thin film and then calcined at 800 ° C. for 8 hours (rising rate 5 ° C./min).

The specific surface area of the product thus obtained is 113 m ² / g.

Example 4

This example relates to the preparation of Zn _0.95 Ca _0.05 Al ₂ O ₄ .

The same raw material and the Ca (NO ₃ ) ₂ .4H ₂ O solution of 98% purity is used as in Example 1.

Dissolve 1.19 moles of zinc salt in 1 L of hexylene glycol and then introduce 0.06 moles of calcium nitrate.

This solution is added to 2.5 moles of aluminum tri-sec-butoxide previously introduced into the reactor with vigorous stirring (500 rpm). The mixture is heated to 70 ° C. and maintained at this temperature for 2 hours. The water / ethanol (50/50 vol. Ratio) mixture is then added at a rate of 5 mL / min. The water / aluminum tri-sec-butoxide ratio is 28. The mixture is left to cool overnight with stirring. The obtained precipitate is separated by centrifugation. The precipitate obtained is dried at 70 ° C. for 48 hours to form a thin film and then calcined at 800 ° C. for 8 hours (rising rate 5 ° C./min).

The specific surface area of the product thus obtained is 119 m ² / g.

Example 5

This example relates to the preparation of Zn _0.95 Li _0.05 Al ₂ O ₄ .

The same raw material and Li (NO ₃ ) solution of 98% purity as in Example 1 is used.

Dissolve 1.19 moles of zinc salt in 1 L of hexylene glycol and then introduce 0.06 moles of lithium nitrate.

This solution is added to 2.5 moles of aluminum tri-sec-butoxide previously introduced into the reactor with vigorous stirring (500 rpm). The mixture is heated to 70 ° C. and maintained at this temperature for 2 hours. The water / ethanol (50/50 vol. Ratio) mixture is then added at a rate of 5 mL / min. The water / aluminum tri-sec-butoxide ratio is 28. The mixture is left to cool overnight with stirring. The obtained precipitate is separated by centrifugation. The precipitate obtained is dried at 70 ° C. for 48 hours to form a thin film and then calcined at 800 ° C. for 8 hours (rising rate 5 ° C./min).

The specific surface area of the product thus obtained is 108 m ² / g.

Example 6

This example relates to an aluminate of the formula ZnAl ₂ O ₄ comprising silver as an additive.

The precipitate obtained in Example 1 is dried at 70 ° C. for 48 hours. Then it is immersed in AgNO ₃ solution (99.8%). The technique used is dry dipping. The amount of silver deposited is 1.6% by weight relative to zinc aluminate oxide. The product thus obtained is oven dried at 110 ° C. for 2 hours and then calcined at 800 ° C. for 8 hours (rising rate 5 ° C./min).

The specific surface area of the product thus obtained is 90 m ² / g.

Example 7

The same method as in Example 1 was carried out to produce zinc aluminate having different Zn / Al ratios. The same raw material is used in the required proportions. For products 7-1 to 7-3, the H ₂ O / aluminum alkoxide molar ratio is 28. For products 7-4 to 7-6, the ratio is 4. The properties of the produced product are given below.

product Chemical formula Specific surface area in m2 / g (8 hours at 800 ° C)) 7-1 Zn _1.05 Al ₂ O ₄ 88 7-2 Zn _0.95 Al ₂ O ₄ 92 7-3 Zn _0.8 Al ₂ O ₄ 140 7-4 Zn _0.6 Al ₂ O ₄ 102 7-5 Zn _0.4 Al ₂ O ₄ 139 7-6 Zn _0.3 Al ₂ O ₄ 110

Example 8

In this example, the products obtained in the previous examples are tested to determine catalyst performance.

The quartz reactor is filled with 0.2 g of catalyst powder. The powder used was precompressed and ground and screened to isolate particle size ranges between 0.125 and 0.250 mm.

The reaction mixture entering the reactor has the following composition (volume):

NO = 300 ppmv

C ₃ H ₆ = 150 ppmv or 450 ppmv

C ₃ H ₈ = 150 ppmv or 450 ppmv

CO = 350 ppmv

O ₂ = 10%

CO ₂ = 10%

H ₂ O = 10%

N ₂ = qsp 100%

Total flow rate is 30 Nl / h.

HSV is about 200,000 h ⁻¹ .

HC (C ₃ H ₆ + C ₃ H ₈ ), NO and NOx (NOx = NO + NO ₂ ) signals are recorded continuously as the temperature in the reactor.

The HC signal is given by the total HC Beckman detector based on the principle of detection using flame ionization.

NO and NOx signals are given by an Ecophysics NOx analyzer based on the principle of chemiluminescence. This gives NO, NOx and NO ₂ values, the latter being calculated from the difference between the NOx and NO signals.

The catalytic activity is calculated from the HC, NO and NOx signals as a function of temperature during the programmed temperature increase of 150 to 700 ° C at a rate of 15 ° C / min and from the following equation.

% Conversion of NO (TNO):

TNO = 100 (NO ⁰ -NO) NO ⁰

In the formula, NO ⁰ is the NO signal at time t = 0, which corresponds to the NO signal obtained with the reaction mixture bypassing the catalytic reactor and NO is the NO signal at time t.

% Conversion of HC (THC):

THC = 100 (HC ⁰ -HC) / HC ⁰

In the formula, HC ⁰ is the HC signal at time t = 0 and corresponds to the HC signal obtained with the reaction mixture bypassing the catalytic reactor, and HC is the HC signal at time t.

% Conversion of NOx (TNOx):

TNOx = 100 (NOx ⁰ -NOx) / NOx ⁰

In the formula, NOx ⁰ is the NOx signal at time t = 0 and corresponds to the NOx signal obtained with the reaction mixture bypassing the catalytic reactor, and NOx is the NOx signal at time t.

Table 1 shows NO = 300 ppmv, C ₃ H ₆ = C ₃ H ₈ = 150 ppmv, the HC ₁ / NO ratio 3 (HC ₁ is carbon number) with the product of Example 1 calcined at 800 ℃ for 8 hours The results obtained for the reaction mixture expressed as, i.e. here 6 x 150/300).

Temperature (℃) THC (%) TNO (%) TNO _x (%) 350 0.6 1.1 1.3 400 One 2.1 2 450 6 8.8 7.9 500 24.8 23.3 23.7 550 46.4 44.5 44.3 600 75.6 59.5 59.4 650 99 28 25.8

Table 2 below shows a reaction mixture with the product of Example 1 calcined at 800 ° C. for 8 hours with NO = 300 ppmv, C ₃ H ₆ = C ₃ H ₈ = 450 ppmv, ie HC ₁ / NO ratio 9 The obtained result is shown.

Temperature (℃) THC (%) TNO (%) TNO _x (%) 350 0.8 0.2 1.4 400 1.8 0 1.4 450 7.5 4.8 6.2 500 29 32.1 32.8 550 50.9 71.5 70.9 600 71.9 87.9 86.4 650 95 69 63.9

Table 3 below shows the results obtained for the reaction mixture with the product of Example 1 calcined at 900 ° C. for 2 hours, the same as in Example 1, ie with a HC ₁ / NO ratio of 3.

Temperature (℃) THC (%) TNO (%) TNO _x (%) 350 0.3 0 0 400 0.7 0 0 450 2.1 1.2 2 500 10.5 11 11.3 550 3.1 28.2 28.5 600 68.6 52.6 52.9 650 97.9 28.7 27.5

Table 4 below shows the results obtained for the reaction mixture having the product of Example 1 calcined at 1000 ° C. for 2 hours, the same as in Example 1, ie with a HC ₁ / NO ratio of 3.

Temperature (℃) THC (%) TNO (%) TNO _x (%) 350 0 0 0 400 0 0 0 450 1.3 1.5 1.2 500 9.4 12.5 12.2 550 34.1 37.6 37.5 600 80.9 51.7 51.2 650 100 21.5 18.3

Table 5 below shows the results obtained for the reaction mixture having the product of Example 2 calcined at 800 ° C. for 2 hours and the same as the product of the first case of Example 1, ie the HC ₁ / NO ratio 3.

Temperature (℃) THC (%) TNO (%) TNO _x (%) 350 0 2.2 2.6 400 0.9 3.3 3.7 450 5.6 10.9 11.2 500 25 32 32.2 550 49 38.4 38.1 600 63.3 31.1 28.6 650 87.4 26.8 21.2

Table 6 below shows the reaction mixture with the product of Example 2 calcined at 800 ° C. for 2 hours with NO = 300 ppmv, C ₃ H ₆ = C ₃ H ₈ = 450 ppmv, ie HC ₁ / NO ratio 9 The obtained result is shown.

Temperature (℃) THC (%) TNO (%) TNO _x (%) 350 2.6 1.3 1.3 400 6.1 3.8 3.2 450 13.9 12.5 12.4 500 35.1 47.3 47.3 550 55.4 54.4 54.1 600 71.8 42 41.7 650 96.7 31.9 29.8

Table 7 below shows the results obtained for the reaction mixture with the product of Example 7 calcined at 800 ° C. for 2 hours and with an HC ₁ / NO ratio of 3. The product is also used in amounts of 100 mg of powder and 100 mg of SiC.

Temperature (℃) THC (%) TNO (%) TNO _x (%) 350 2.6 2.1 2.6 400 2.9 1.4 2.1 450 4.8 4.2 4.2 500 21.1 26.6 26.2 550 63 46.4 45 600 90 34.5 30.1 650 98.9 24.5 7

Table 8 below shows the maximum% NOx values and corresponding temperatures obtained for the reaction mixture with the product of Example 8 calcined at 800 ° C. for 2 hours with a HC ₁ / NO ratio of 3. The product is also used in amounts of 100 mg of powder and 100 mg of SiC.

product % NOx and temperature 7-3 35% 620 ℃ 7-4 38% 625 ℃ 7-5 42% 600 ℃ 7-6 46% 600 ℃

Claims (23)

Zinc aluminate, characterized in that having a specific surface area of more than 85 m ² / g after firing at 800 ℃ for 8 hours. The zinc aluminate according to claim 1, having a specific surface area of at least 100 m ² / g after firing at 800 ° C. for 8 hours. Zinc aluminate, characterized in that having a specific surface area of at least 70 m ² / g after firing at 900 ℃ for 2 hours. Zinc aluminate, characterized in that having a specific surface area of at least 50 m ² / g after firing at 1000 ℃ for 6 hours. Zinc aluminate characterized in that it has a specific surface area of 50 m ² / g or more after calcining at 1000 ° C. for 6 hours in H ₂ O / N ₂ medium having H ₂ O of 10% by volume. 3. Zinc aluminate according to claim 1 or 2, comprising at least one additive selected from elements of Groups IA, IIA, VIIA to IB of the periodic table and tin, gallium and rare earths. A zinc aluminate precursor composition comprising a compound of zinc and aluminum, and after being calcined at 800 ° C. for 8 hours, zinc aluminate having a specific surface area of at least 85 m ² / g can be formed after firing. 8. The composition of claim 7, wherein the aluminate having a specific surface area of at least 90 m ² / g after firing at 800 ° C. for 8 hours can be formed. 9. A composition according to claim 7 or 8, further comprising at least one compound of elements of groups IA, IIA, VIIA to IB of the periodic table and an element selected from tin, gallium and rare earths. A method for producing an aluminate according to any one of claims 1 to 6 or a precursor composition according to any one of claims 7 to 9 Bringing together zinc salts, sols or alkoxides and aluminum alkoxides, together with one or more salts, sols or alkoxides of said additives, in a solvent medium; Hydrolyzing the mixture formed in the previous step by adding excess water relative to aluminum alkoxide; Recovering the precipitate formed and optionally drying to obtain a precursor composition; And If necessary, calcining the precipitate to obtain an aluminate. A method for preparing a precursor composition comprising an aluminate comprising an additive according to claim 6 or a compound of an element according to claim 9 Bringing together zinc salts, sol or alkoxides and aluminum alkoxides in a solvent medium; Hydrolyzing the mixture formed in the previous step by adding excess water relative to aluminum alkoxide; Recovering the precipitate formed and optionally drying to obtain a precursor composition; If necessary, obtaining an aluminate by calcining the precipitate; And -Immersing the precursor composition or aluminate with an additive or a salt solution of said element. The process according to claim 10 or 11, wherein an alcohol solvent is used as the solvent medium. 12. The method according to claim 10 or 11, wherein water in the form of a water-alcohol mixture is added. The process according to claim 10 or 11, wherein the zinc salt and the aluminum alkoxide are brought together by adding the zinc salt in the solvent medium to the aluminum alkoxide. The process according to claim 10 or 11, wherein the precipitate is calcined at a temperature of at least 500 ° C. A method for treating a gas for the purpose of reducing nitrogen oxide emissions, characterized by using a catalyst system comprising the aluminate according to any one of claims 1 to 6. A method for treating automobile exhaust gas, comprising using a catalyst system comprising the aluminate according to any one of claims 1 to 6. A method for treating gas from an automobile, characterized by using a catalyst system comprising the aluminate according to any one of claims 1 to 6 and having a high oxygen content of the gas. A catalyst system for immersion of the process according to claim 16, characterized in that it comprises the aluminate according to claim 1 on a substrate. Use of the aluminate according to any one of claims 1 to 6 or the precursor composition according to any one of claims 7 to 9 in a process. . The zinc aluminate according to claim 3, having a specific surface area of at least 80 m ² / g after firing at 900 ° C. for 2 hours. The zinc aluminate according to claim 4, having a specific surface area of at least 70 m ² / g after firing at 1000 ° C. for 6 hours. 9. A composition according to claim 8, which is capable of forming an aluminate having a specific surface area of at least 100 m ² / g after firing at 800 ° C. for 8 hours.