EP1181094A1

EP1181094A1 - COMPOSITION USABLE AS NOx TRAP, BASED ON MANGANESE AND AN ALKALINE-EARTH OR A RARE EARTH AND USE IN THE TREATMENT OF EXHAUST GASES

Info

Publication number: EP1181094A1
Application number: EP00920832A
Authority: EP
Inventors: Thierry Birchem; Catherine Hedouin; Thierry Seguelong
Original assignee: Rhodia Chimie SAS
Current assignee: Rhodia Chimie SAS
Priority date: 1999-04-23
Filing date: 2000-04-18
Publication date: 2002-02-27
Also published as: WO2000064580A1; FR2792547A1; BR0009995A; KR20020007386A; CN1351518A; ZA200108713B; JP2002542029A; CA2371276A1; CN1132681C; NO20015161D0; NO20015161L; FR2792547B1; KR100420909B1

Abstract

The invention concerns a composition usable as NOx trap in the treatment of exhaust gases. Said composition comprises a support and an active phase, and is characterised in that the active phase is based on manganese and at least another element A selected among alkaline-earths and rare earths, and it has or is capable of having a specific surface area of at least 10m<2>/g after being calcined for 8 hours at 800 DEG C. The support can be based on alumina or alumina stabilised with silicon, zirconium, barium or a rare earth; or based on silica or silica and titanium oxide.

Description

COMPOSITION FOR USE AS A NOx TRAP, BASED ON MANGANESE AND AN ALKALINE EARTH OR RARE EARTH, AND USE IN THE TREATMENT OF EXHAUST GASES

RHODIA CHEMISTRY

The present invention relates to a composition which can be used as a NOx trap, based on manganese and an alkaline earth or a rare earth and its use in the treatment of exhaust gases.

It is known that the reduction of emissions of nitrogen oxides (NOx) from the exhaust gases of automobile engines in particular 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, certain 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 at least 5% for example. A standard three-way catalyst therefore has no effect on NOx emissions in this case. Furthermore, the limitation of NOx emissions is made imperative by the tightening of standards in automotive post combustion which now extend to these engines.

To solve this problem, systems called NOx traps, potassium based in particular, have been proposed which are capable of oxidizing NO to O2 and then adsorbing the NO2 thus formed. Under certain conditions, NO2 is released and then reduced to N2 by reducing species contained in the exhaust gases. These NOx traps still have certain drawbacks, however. Thus, they age badly in the sense that their operation is less satisfactory when they are subjected to high temperatures. Furthermore, they may have a low resistance to sulfation.

The object of the invention is therefore to provide NOx traps which have improved resistance to aging.

Another object of the invention is also to provide NOx traps which have improved resistance to sulfation. For this purpose, the composition which can be used as a NOx trap, according to the invention, comprises a support and an active phase, and it is characterized in that the active phase is based on manganese and at least one other element A chosen among alkaline earths and rare earths, and in that it presents or is likely to have a specific surface of at least 10m ² / g after calcination for 8 hours at 800 ° C.

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 composition of the invention comprises a support and an active phase. The term support must be taken in a broad sense to designate, in the composition, the majority element (s) and / or either without catalytic activity or own trapping activity, or having a catalytic activity or trapping not equivalent to that of the phase active; and on which or on which the other elements are deposited. To simplify, we will speak in the following description of support and active or supported phase but it will be understood that it would not go beyond the scope of the present invention in the case where an element described as belonging to the active or supported phase was present in the support, for example by having been introduced into it during the preparation of the support itself.

The active phase of the composition is based on manganese and at least one element A. This element A can be an alkaline earth or a rare earth. As alkaline earth, there may be mentioned more particularly barium. The rare earth can be more particularly chosen from cerium, terbium, gadolinium, samarium, neodymium and praseodymium. The total manganese, alkaline earth or rare earth contents can vary between 1 and 50%, more particularly between 5 and 30%. These proportions are expressed in atomic% relative to the sum of the moles of oxide (s) of the support and of the elements concerned of the supported phase. The respective manganese, alkaline-earth or rare earth contents can also vary within wide proportions, the manganese content can in particular be equal or close to that of element A.

The invention covers the case where the active phase consists essentially of manganese and one or more other elements A chosen from alkaline earths and rare earths. By "consists essentially" is meant that the composition of the invention can have a NOx trap activity in the absence in the active phase of any element other than manganese and the element (s) A, such as for example an element of type precious metal or other metal usually used in catalysis.

As indicated above, a characteristic of the composition is that it has or is likely to have a specific surface of at least 10 m / g after calcination for 8 hours at 800 ° C. This specific surface may in particular be at least 20 m 2 / g after calcination at the same temperature and for the same duration. More

2 in particular, this specific surface is at least 80 m / g and even more

2 particularly at least 100 m / g after 8 hours calcination at 800 ° C. The term “specific surface” is understood to mean the BET specific surface area determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 established on the basis of the BRUNAUER - EMMETT-TELLER method described in the periodical "The Journal of the American Chemical Society, 60 , 309 (1938) ". This surface characteristic is obtained by the choice of a suitable support, in particular having a sufficiently high specific surface.

This support can be based on alumina. Any type of alumina capable of having a specific surface sufficient for application in catalysis can be used here. Mention may be made of aluminas resulting from the rapid dehydration of at least one aluminum hydroxide, such as bayerite, hydrargillite or gibbsite, nordstrandite, and / or from at least one aluminum oxyhydroxide such as boehmite , pseudoboehmite and diaspore.

According to a particular embodiment of the invention, a stabilized alumina is used. As stabilizing element, mention may be made of rare earths, barium, silicon and zirconium. As rare earth there may be mentioned very particularly cerium, lanthanum or the lanthanum-neodymium mixture.

The preparation of the stabilized alumina is carried out in a manner known per se, in particular by impregnating the alumina with solutions of salts, such as nitrates, of the abovementioned stabilizing elements or else by co-drying of an alumina precursor and salts of these elements then calcination.

Mention may also be made of another preparation of stabilized alumina in which the alumina powder resulting from the rapid dehydration of an aluminum hydroxide or oxyhydroxide is subjected to a ripening operation in the presence of an agent stabilizer consisting of a lanthanum compound and, optionally, a neodymium compound, this compound possibly being more particularly a salt. Curing can be done by alumina in water suspension and then heating at a temperature of for example between 70 ^C and 110 C. After curing, the alumina is subjected to a heat treatment.

Another preparation consists of a similar type of treatment but with barium.

The stabilizer content expressed by weight of stabilizer oxide relative to the stabilized alumina is generally between 1.5 and 15%, more particularly between 2.5 and 11%.

The support can also be based on silica. It can also be based on silica and titanium oxide in an atomic proportion Ti / Ti + Si of between 0.1 and 15%. This proportion can be more particularly between 0.1 and 10%. Such a support is described in particular in patent application WO 99/01216, the teaching of which is incorporated here. As other suitable support, those based on cerium oxide and zirconium oxide can be used, these oxides possibly being in the form of a mixed oxide or of a solid solution of zirconium oxide in l cerium oxide or vice versa. These supports are obtained by a first type of process which comprises a step in which a mixture comprising zirconium oxide and cerium oxide is formed and the mixture thus formed is washed or impregnated with an alkoxylated compound having a number of carbon atoms greater than 2. The impregnated mixture is then calcined.

The alkoxylated compound can be chosen in particular from the products of formula (2) Rι - ((CH2) χ-O) n-R2 in which Ri and R2 represent alkyl groups, linear or not, or H or OH or Cl or Br or I; n is a number between 1 and 100; and x is a number between 1 and 4, or else those of formula (3) (R3, R4) - <^ - ((CH2) χ-O) _n - OH in which φ denotes a benzene ring, R3 and R4 are identical or different substituents of this ring and represent hydrogen or linear or non-alkyl groups having from 1 to 20 carbon atoms, x and n being defined as above; or also those of formula (4) R4θ - ((CH2) χ-O) _n -H where R4 represents a linear or non-linear alcohol group having from 1 to 20 carbon atoms; x and n being defined as above; and those of formula (5) R5-S - ((CH2) χ-O) _n -H where R5 represents a linear or non-linear alkyl group having from 1 to 20 carbon atoms, x and n being defined as above. Reference may be made for these products to patent application WO 98/16472, the teaching of which is incorporated here.

These supports can also be obtained by a second type of process which comprises a step in which a solution of a cerium salt, a solution of a zirconium salt and an additive chosen from anionic surfactants, non-surfactants, are reacted. ionic, polyethylene glycols, carboxylic acids and their salts, the reaction possibly being able to take place in the presence of a base and / or an oxidizing agent.

As anionic surfactants, carboxylates, phosphates, sulfates and sulfonates can be used more particularly. Among the nonionic surfactants, it is possible to use ethoxylated alkyl phenols and ethoxylated amines.

The reaction between the zirconium and cerium salts can be done by heating the solution containing the salts, it is then a thermohydrolysis reaction. It can also be done by precipitation by introducing a base into the solution containing the salts.

For these products, reference may be made to patent application WO 98/45212, the teaching of which is incorporated here. The composition of the invention can be prepared by a process in which the support is brought into contact with manganese and at least one other element A or with precursors of manganese and at least one other element A and in which the together at a temperature sufficient to transform the precursors or the elements into oxides. Generally, this temperature is at least 500 ° C., more particularly at least 600 ° C.

One method which can be used for the aforementioned contacting is impregnation. This first forms a solution or slip of salts or compounds of the elements of the supported phase. 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 aliphatic carboxylic acids or the salts of hydroxycarboxylic acids. By way of examples, mention may be made of formates, acetates, propionates, oxalates or citrates. The support is then impregnated with the solution or the slip.

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.

It may be advantageous to deposit the elements of the active phase in two stages. Thus, it is advantageous to deposit the manganese in a first step then the element A in a second.

After impregnation, the support is optionally dried and then it is calcined. It should be noted that it is possible to use a support which has not yet been calcined prior to impregnation. The deposition of the active phase can also be done by atomization of a suspension based on salts or compounds of the elements of the active phase and of the support. The atomized product thus obtained is then calcined.

The composition of the invention as described above is in the form of a powder but it can optionally be shaped to be in the form of granules, beads, cylinders or honeycombs of variable dimensions.

The invention also relates to a gas treatment process with a view to reducing the emissions of nitrogen oxides using the composition of the invention.

The gases capable of being treated in the context of the present invention are, for example, those from gas turbines, boilers of thermal power plants or even internal combustion engines. In the latter case, it may in particular be diesel engines or engines operating in a lean mixture. The composition of the invention functions as NOx traps when it is brought into contact with gases which have a high oxygen content. By gas having a high oxygen content is meant gases having an excess of oxygen relative to the quantity necessary for the stoichiometric combustion of fuels and, more specifically, gases having an excess of oxygen relative to the stoichiometric value λ = 1, that is to say gases for which the value of λ is greater than 1. The value λ is correlated with the air / fuel ratio in a manner known per se, in particular in the field of internal combustion engines. Such gases may be those of an engine operating in a lean burn mixture and which have an oxygen content (expressed by volume) for example of at least 2%, as well as those which have an even higher oxygen content, for example. example of diesel engine gases, that is to say at least 5% or more than 5%, more particularly at least 10%, this content being, for example, between 5% and 20%. The invention also applies to gases of the above type which may further contain water in an amount of the order of 10% for example.

The composition of the invention may be useful in the treatment of exhaust gases from internal combustion engines operating with a fuel containing sulfur, that is to say for fuels whose sulfur content is at least 50 ppm, more particularly at least 200 ppm (content expressed in sulfur element). The term sulfur must be taken in the broad sense, that is to say as designating sulfur but also the sulfur compounds which are present in fuels.

The invention also relates to a catalytic system for the treatment of an exhaust gas from an internal combustion engine comprising a composition according to the invention. More specifically, this system comprises a coating (wash coat) with catalytic properties and incorporating said composition, this coating being deposited on a substrate of the type, for example metallic or ceramic monolith.

The invention also relates to the use of a composition as described above in the manufacture of such a catalytic system. Examples will now be given.

In the examples, the compositions were prepared as follows.

Preparation of the compositions:

Manganese nitrate Mn (Nθ3) 2.4H ₂ O, potassium nitrate KNO3 99.5%, barium nitrate Ba (NO3) 2 99.5% are used. The support used is an SB3 Condea alumina.

There are two steps to filing.

1st step i Deposit of the first active element: This step consists in depositing the active element Mn in an amount equal to 10 atomic% and calculated as follows:

[Mn] / ([Mn] + [AI ₂ O ₃ ]) = 0.10 2Jth step: Deposit of the second active element: This step consists in depositing the second active element X which can be K

(comparative composition) or Ba in an amount of 10 atomic% and calculated as follows:

[X] / ([Mn] + [X] + [AI ₂ O ₃ ]) = 0.10 The deposition is carried out by the dry impregnation method. This method consists in impregnating the support considered with the element of the active phase dissolved in a solution of volume equal to the pore volume of the support and of concentration making it possible to reach the desired concentration.

In the present case, the elements are impregnated on the support one after the other according to the following operating protocol: - Dry impregnation of the first element

- Drying in the oven (110 ° C, 2H)

- Calcination 2h 500 ° C

- Dry impregnation of the second element

- Drying in the oven (110 ° C, 2H) - Calcination 2h 850 ° C.

Compositions obtained:

EXAMPLE 1

This example shows the resistance to aging of a composition according to the invention.

In this example, the catalytic test is carried out as follows:

0.15 g of each of the above NOx trap compositions in powder form is loaded into a quartz reactor. The powder used was previously compacted, then ground and sieved so as to isolate the particle size range between 0.125 and 0.250 mm. The reaction mixture at the inlet of the reactor has the following composition (by volume):

- NO: 300 vpm - O2: 10% - CO2: 10% - H2O: 10% - N2: qs 100% The overall flow is 30 Nl / h.

-1 The WH is of the order of 150,000 h ^" The signals of NO and NOx (NOx = NO + NO2) are permanently recorded, as well as the temperature in the reactor.

The NO and NOx signals are given by an ECOPHYSICS NOx analyzer, based on the principle of chemiluminescence.

The evaluation of NOx traps is carried out by determining the total amount (NSC) of adsorbed NOx (expressed in mg NO / g of trap or active phase) until the trap phase is saturated. The measurement is made at 250 ° C.

Furthermore, the compositions have undergone hydrothermal aging of the redox type according to the protocol which follows.

The compositions are raised in temperature under N ₂ in 60 minutes to 950 ° C. The compositions are then maintained at this temperature for 6 hours, alternating 24 periods of 15 min in atmospheres of oxygen and water in nitrogen and of hydrogen and water in nitrogen respectively. At the end of the treatment, the temperature drops under H ₂ / N ₂ to 80 ° C. and then under N ₂ .

The results obtained are given in the table below.

It can be seen that the composition according to the invention has improved resistance to aging.

EXAMPLE 2

This example shows the sulfation resistance of a composition according to the invention. The same compositions (not aged) are used as those of Example 1. The reaction mixture at the inlet of the reactor has either the same composition as in Example 1 or this composition with in addition 30 ppm of SO ₂ .

The results obtained are given in the table below.

It can be seen that the composition according to the invention has improved resistance to SO ₂ .

Claims

1- Composition usable as a NOx trap, comprising a support and an active phase, characterized in that the active phase is based on manganese and at least one other element A chosen from alkaline-earths and rare earths, and in that it has or is likely to have a specific surface of at least 10m ² / g, more particularly at least 20m ² / g, after calcination for 8 hours at 800 ° C.

2- Composition according to claim 1, characterized in that it has or is likely to have a specific surface of at least 80m ² / g, more particularly at least 100m ² / g after calcination 8 hours at 800 ° C .

3- Composition according to claim 1 or 2, characterized in that the element A is barium.

4- Composition according to one of the preceding claims, characterized in that the support is based on alumina or alumina stabilized by silicon, zirconium, barium or a rare earth.

5- Composition according to one of claims 1 to 3, characterized in that the support is based on silica.

6- Composition according to one of claims 1 to 3, characterized in that the support is based on silica and titanium oxide in an atomic proportion

Ti / Ti + Si between 0.1 and 15%.

7- Composition according to one of claims 1 to 3, characterized in that the support is based on cerium oxide and zirconium oxide, this support having been obtained by a process in which a mixture comprising of zirconium oxide and cerium oxide and the mixture thus formed is washed or impregnated with an alkoxylated compound having a number of carbon atoms greater than 2.

8- Composition according to one of claims 1 to 3, characterized in that the support is based on cerium oxide and zirconium oxide, this support having been obtained by a process in which a solution of '' a cerium salt, a solution of a zirconium salt and an additive chosen from anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids and their salts, the reaction possibly being able to take place in the presence of a base and / or an oxidizing agent.

9- A process for treating gas with a view to reducing the emissions of nitrogen oxides, characterized in that a composition according to one of claims 1 to 8 is used.

10- A method according to claim 9, characterized in that an internal combustion engine exhaust gas is treated.

11- A method according to claim 10, characterized in that a gas having an excess of oxygen relative to the stoichiometric value is treated.

12- Method according to one of claims 10 or 11, characterized in that the oxygen content of the gases is at least 2% by volume.

13- Catalytic system for the treatment of an exhaust gas from an internal combustion engine, characterized in that it comprises a composition according to one of claims 1 to 8.

14- Use of a composition according to one of claims 1 to 8 for the manufacture of a catalytic system for the treatment of an exhaust gas from an internal combustion engine.