FR2793162A1 - Compound for reducing nitrogen oxides in vehicle exhaust has two compositions each having support and active phase including manganese and either alkaline or alkaline earth element - Google Patents

Abstract

A compound includes two compositions each having a support and an active phase including manganese and another element. In the first composition the element is either an alkaline or alkaline earth element and is bound chemically to the manganese. In the second composition the element is an alkaline, alkaline earth or rare earth element. The second composition has a specific surface area of \- 10 m<2>/g after being calcined for 8 hours at 80 deg C. Independent claims are included for the following: (a) a system having a substrate supporting two layers containing the respective compositions (b) a system having the compound in a layer containing a mixture of the two compositions; (c) a system having two substrates each with a layer carrying one or other of the compositions; (d) reducing nitrogen oxides in a flue gas using a system as in (b), (c) or (d); and (e) the use of the above compound for treating the exhaust gas from an internal combustion engine.

Description

COMPOUND THAT CAN BE USED AS A NOx TRAP, COMBINING TWO

COMPOSITIONS BASED ON MANGANESE AND ANOTHER CHOSEN ELEMENT

AMONG ALKALINES, ALKALINO-EARTHS AND RARE EARTHS AND

The present invention relates to a compound which can be used as a Nox trap, combining two compositions whose active phases are based on manganese and on another element chosen from alkalis, alkaline earths and rare earths 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 of

catalyst performance.

However, some engines such as diesel engines or gasoline engines operating on a lean burn (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 the NOx emissions in this case. In addition, the limitation of NOx emissions is made imperative by the tightening of automotive post combustion standards which

now extend to these engines.

To solve this problem, systems called NOx traps have been proposed in particular, which are capable of oxidizing NO to NO2 and then of adsorbing the NO2 thus formed. Under certain conditions, the NO2 is released and then reduced to N2 by reducing species contained in the exhaust gases. However, these NOx traps still have certain drawbacks. Thus, as a result of their aging, it has been observed that they have a tendency to operate only in a high temperature range and they therefore have a reduced operating window. It would therefore be advantageous to be able to have a system that could operate over a wider temperature range. In addition, they are generally based on precious metals. However, these metals are expensive and their availability can be problematic. It would also be interesting to be able to have catalysts without precious metals for

reduce costs.

The object of the invention is therefore the development of a compound which can be used in a wider range of temperatures and, optionally, in the absence of precious metal. For this purpose, the compound of the invention, which can be used as an NOx trap, is characterized in that it comprises a combination: - of a first composition comprising a support and an active phase, the active phase being based on manganese and at least one other element A chosen from alkalis and alkaline earths, manganese and element A being chemically bonded; - of a second composition comprising a support and an active phase based on

manganese and at least one other element B chosen from alkalis, alkaline-

earth and rare earths, this second composition exhibiting or being capable of exhibiting a specific surface area of at least 80 m 2 / g after calcination for 8 hours at

800 C.

Other characteristics, details and advantages of the invention will appear further.

more fully on reading the following description, as well as an example

concrete but not limiting intended to illustrate it.

For the whole of the description, the term “rare earths” is understood to mean the elements of

group consisting of yttrium and the elements of the periodic table with number

atomic included between 57 and 71.

Furthermore, the term “specific surface” is understood to mean the specific surface area 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 Chemical Society, 60, 309 (1938) ".

The compound of the invention is characterized by the combination of two compositions

specific which will now be described more particularly.

These compositions comprise a support and an active phase. The term support should be taken in a broad sense to denote, in the composition, the major element (s) and / or either without catalytic activity or specific trapping activity, or exhibiting a catalytic or trapping activity not equivalent to that of the phase. active; and on which or on which the other elements are placed. To simplify,

we will speak in the following of the description of support and active or supported phase

but it will be understood that it would not be departing from 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 having been introduced there during the preparation.

even support.

The first composition is described below.

This composition comprises an active phase which is based on manganese and on at least one other element A chosen from alkalis and alkaline earth metals. As alkaline element, there may be mentioned more particularly sodium and potassium. As an alkaline earth element, there may be mentioned in particular barium. As the composition may include one or more A elements, any reference in the following

of the description in element A should therefore be understood as potentially applicable

also in the case where there are several elements A. Furthermore, the elements manganese and A are present in this first composition in a chemically bound form. This is understood to mean that there are chemical bonds between manganese and element A resulting from a reaction between them, these

two elements not being simply juxtaposed as in a simple mixture.

Thus, the elements manganese and A can be present in the form of a compound or a phase of mixed oxide type. This compound or this phase can in particular be represented by the formula AxMnyO2 ô (1) in which 0.5 <y / x <6. As phase or compound of formula (1), mention may be made, by way of example, of those of the vemadite type,

hollandite, romanechite or psilomelane, birnessite, todorokite, buserite or lithiophorite.

The compound can optionally be hydrated. The compound can moreover have a lamellar structure of the Cdl2 type. Formula (1) is given here by way of illustration, it would not be departing from the scope of the present invention if the compound had a different formula to the extent of course o the manganese and the element A would be well

chemically linked.

The analysis by X-ray or by electron microscopy makes it possible to highlight

the presence of such a compound.

The degree of oxidation of manganese can vary between 2 and 7 and, more

particularly between 3 and 7.

In the case of potassium, this element and the manganese can be present in the form of a compound of the type K2Mn4O8. In the case of barium, it may be

of a compound of BaMnO3 type.

The first composition further comprises a support. As support, it is possible to use any porous support which can be used in the field of catalysis. It is preferable that this support has a chemical inertia with respect to the elements manganese and A sufficient to avoid a substantial reaction of one or these elements with the support which would be liable to hinder the creation of a chemical bond between the manganese and element A. However, in the case of a reaction between the support and these elements, it is possible to use larger amounts of manganese and of element A to obtain the desired chemical bond

between these elements.

More particularly, the support is based on an oxide chosen from the oxide of

cerium, zirconium oxide or mixtures thereof.

Mention may in particular be made, for mixtures of cerium oxide and of oxide

zirconium those described in patent applications EP-A- 605274 and EP-A-

735984 whose teaching is incorporated herein. It is more particularly possible to use the supports based on cerium and zirconium oxide in which these oxides are present in a cerium / zirconium atomic proportion of at least 1. For these same supports, it is also possible to use those which are present under form of a solid solution. In this case, the X-ray diffraction spectra of the support reveal the existence of a single homogeneous phase within the latter. For the supports richest in cerium, this phase corresponds in fact to that of a crystallized cubic ceric oxide CeO2 and whose mesh parameters are more or less offset compared to a pure ceric oxide, thus reflecting the incorporation of zirconium in the crystal lattice

cerium oxide, and thus obtaining a true solid solution.

Mention may also be made, for the mixtures of cerium oxide and of zirconium oxide based on these two oxides and in addition on scandium oxide or of a rare earth other than cerium, and in particular those described in the application WO 97/43214, the teaching of which is incorporated herein. This application describes in particular compositions based on a cerium oxide, on a zirconium oxide and on an yttrium oxide, or else, in addition to cerium oxide and zirconium oxide based on at least one other oxide chosen from scandium oxide and rare earth oxides to

except cerium, in a cerium / zirconium atomic ratio of at least 1.

These compositions have a specific surface area after calcination for 6 hours at 900 C of at least 35 m 2 / g and an oxygen storage capacity at 400 C of at least

less 1.5ml of O2 / g.

According to a particular embodiment of the invention, the support is based on cerium oxide and it further comprises silica. Supports of this type are described in patent applications EP-A-207857 and EP-A-547924, of which

teaching is incorporated here.

The total manganese, alkali, and alkaline earth content can vary widely. The minimum content is that below which NOx adsorption activity is no longer observed. This content may be in particular between 2 and%, more particularly between 5 and 30%, this content being expressed in atomic% relative to the sum of the moles of oxide (s) of the support and of the elements concerned of the active phase. . The respective contents of manganese, alkali, and alkaline earth metal can also vary widely, the content of

manganese may in particular be equal to, or close to that of alkali or alkaline

earthy. The first composition of the compound 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 o the whole is calcined at a temperature sufficient to create a chemical bond between the manganese and the element A. A method which can be used for the aforementioned contacting is impregnation. This first forms a solution or a slip of salts or compounds of the

elements of the supported phase.

As salts, it is possible to choose the salts of inorganic acids such as nitrates, sulphates 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. As

examples that may be mentioned are formates, acetates, propionates, oxalates or citrates.

The support is then impregnated with the solution or the slip.

Dry impregnation is more particularly used. Dry impregnation consists of 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, one can advantageously deposit the manganese in a first

time then element A in a second.

After impregnation, the support is optionally dried and then it is calcined. Note that it is possible to use a support that has not yet been calcined

prior to impregnation.

The deposition of the active phase can also be carried out by atomization of a suspension based on salts or compounds of the elements of the active phase and of the support. We

then calcines the atomized product thus obtained.

The calcination is carried out, as indicated above, at a temperature sufficient to create a chemical bond between the manganese and the element A. This temperature varies according to the nature of the element A but, in the case of a calcination in air , it is generally at least 600 C more particularly at least 7000C, it can in particular be between 800 C and 850 C. Higher temperatures are generally not necessary insofar as the chemical bond between the manganese and the Element A is already formed but on the other hand they can lead to a reduction in the specific surface area of the support capable of reducing the catalytic properties of the composition. The duration of the calcination depends in particular on the temperature and it is also fixed so as to be sufficient to create a bond.

chemical elements.

The second composition of the compound of the invention will now be described.

This composition also comprises a support and an active phase.

What has been said above concerning the active phase of the first composition also applies here, in particular as regards the nature of the elements of this phase and their quantity. Thus, element B can be more particularly sodium,

potassium or barium.

The active phase of the second composition can also be based on manganese and on at least one rare earth. This rare earth can be more particularly chosen

from cerium, terbium, gadolinium, samarium, neodymium and praseodymium.

The total manganese, alkali, alkaline earth or rare earth content can vary between 1 and 50%, more particularly between 5 and 30%. This content is 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, alkali, alkaline earth or rare earth contents can also vary widely, the manganese content can be in particular equal to, or close to that of element B. As indicated above, a characteristic of the second composition is that it has or is capable of exhibiting a specific surface area of at least m2 / g after calcination for 8 hours at 800 C. More particularly, this surface

specific is at least 1 00m2 / g after calcination 8 hours at 800 C.

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 Chemical Society, 60, 309 (1938) ".

This surface characteristic is obtained by choosing a suitable support,

in particular having a sufficiently high specific surface.

This support can be based on alumina. Any type of alumina capable of exhibiting a sufficient specific surface area for an application in catalysis can be used here. Mention may be made of the 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. , the

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, it is possible to mention in particular the

cerium, lanthanum or lanthanum-neodymium mixture.

The preparation of the stabilized alumina is carried out in a manner known per se, in particular by impregnation of the alumina with solutions of salts, such as nitrates, of the aforementioned stabilizing elements or by co-drying of an alumina precursor.

and salts of these elements followed by 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 maturing 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 suspending the alumina in water and then heating to a temperature of for example between 70 and 110 C. After the

curing, the alumina is subjected to a heat treatment.

Another preparation is 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 Ti / Ti + Si proportion 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 the

patent application WO 99/01216, the teaching of which is incorporated here.

As another suitable support, it is possible to use those based on cerium oxide and on zirconium oxide, these oxides possibly being in the form of a mixed oxide or of a solid solution of the zirconium oxide in the mixture. 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 mixture

impregnated is then calcined.

The alkoxylated compound can be chosen in particular from the products of formula (2) R1 - ((CH2) x-O) n-R2 in which R1 and R2 represent alkyl groups, linear or not, or H or OH or Cl or Br or 1; n is a number between 1 and 100; and x is

a number between 1 and 4, or even those of formula (3) (R3, R4) -ó ((CH2) x-O) n-

OH in which 0 denotes a benzene ring, R3 and R4 are identical or different substituents of this ring and represent hydrogen or linear or non-linear alkyl groups having from 1 to 20 carbon atoms, x and n being defined as above; or else those of formula (4) R40 - ((CH2) x-O) n-H o R4 represents a linear alcohol group or not, having from 1 to 20 carbon atoms; x and n being defined as above; and those of formula (5) R5-S - ((CH2) x-O) n-H o R5 represents a linear or non-linear alkyl group having from 1 to 20 carbon atoms, x and n being defined as above. For these products, we can refer to the

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 and non-surfactants are reacted. ionic compounds, polyethylene glycols, carboxylic acids and their salts, the reaction possibly taking place in the presence of a base and / or a

oxidizing agent.

As anionic surfactants, carboxylates, phosphates, sulfates and sulfonates can be used more particularly. Among the nonionic surfactants, it is preferably possible to use ethoxylated alkyl phenols and ethoxylated amines. The reaction between the zirconium and cerium salts can be carried out 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.

whose teaching is incorporated here.

The preparation of the second composition can be done using the same methods as those given above for the first. It will be noted that after bringing the support into contact with the elements of the active phase, the whole is calcined at a temperature sufficient for these elements to be present in the form of oxides. Usually this temperature is at least 500 C, plus

especially at least 600 C.

The compound 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, balls, cylinders or honeycombs of varying sizes.

The invention also relates to a system, which can be used for the piegage of NOx, which comprises a coating (wash coat) with catalytic properties and incorporating the compound according to the invention, this coating being deposited on a substrate of the type by

example metallic or ceramic monolith.

This system can be presented in different embodiments.

According to a first embodiment, the system comprises a substrate and a coating consisting of two superimposed layers. In this case, the first layer contains the first composition of the compound and the second layer contains the second composition. The order of the layers can be any, i.e. the internal layer in contact with the substrate can contain either the first or the second composition, the external layer deposited on the first then containing the other.

composition of the compound.

According to a second embodiment, the compound is present in the coating in the form of a single layer which in this case comprises the two compositions.

above, in the form of a mixture, obtained for example by mechanical mixing.

A third embodiment is conceivable. In this case, the system comprises two juxtaposed substrates, each substrate comprising a coating. The coating of the first substrate contains the first composition and the coating of the

second substrate contains the second composition.

The invention also relates to the use of a compound as described above.

in the manufacture of such a system.

The invention also relates to a gas treatment process with a view to reducing emissions of nitrogen oxides using the compound of the invention. The gases capable of being treated in the context of the present invention are, for example, those obtained from gas turbines, from thermal power station boilers or even from internal combustion engines. In the latter case, it may be in particular

diesel engines or lean burn engines.

The compound of the invention functions as NOx traps when contacted 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 precisely, gases having an excess of oxygen relative to the stoichiometric value. X = 1, that is to say the gases for which the value of X is greater than 1. The value X 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 from an engine operating on a lean burn (lean burn) 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 gas from engines of the diesel type, that is to say at least 5% or more than 5%, more particularly at least 10%, this content possibly being between 5%

and 20%.

An example will now be given.

In this example, the evaluation test of the NOx traps is carried out as follows: 0.15 g of each of the NOx traps (first composition and second composition) in powder form is charged into a quartz reactor. The powder used has been compacted beforehand then crushed and sieved so as to isolate the particle size slice.

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

-02: 10%

- C02: 10%

- H2O: 10%

- N2: qsp 100%

The overall flow rate is 30 NI / h.

The WH is of the order of 150,000 h-1.

The NO and NOx signals (NOx = NO + NO2) are continuously 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 the NOx traps is carried out by determining the total quantity of adsorbed NOx (expressed in mgNO / g of trap or active phase) until saturation of the

trap phase. The experiment is repeated at different temperatures between 250 C and 500 C.

It is thus possible to determine the optimum temperature zone for the

operation of NOx traps.

A) First composition Raw materials: Manganese nitrate Mn (NO3) 2,4H20 and potassium nitrate are used

KNO3 99.5%.

The support used is that described in Example 4 of patent application WO 97/43214, ie a support based on cerium oxide, zirconium oxide and lanthanum oxide in the proportions respective by weight with respect to oxides

from 72/24/4.

Preparation of the composition:

We proceed in two stages for the deposit.

1st step Deposition 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] + [CeO2] + [ZrO2] + [ La203]) = 0.10 2nd step: Deposit of the second active element: This step consists in depositing the second active element K in an amount of atomic% and calculated as follows: [K] / ([Mn] + [K ] + [CeO2] + [ZrO2] + [La203]) = 0.10 The deposition is done 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 (determined with water: 0.5 cm3 / g) and

concentration to achieve the desired concentration.

In the present case, the elements are impregnated on the support one after the other. The operating protocol is as follows: - Dry impregnation of the first element - Drying in an oven (110 C, 2H) - Calcination 2h 600 C - Dry impregnation of the second element - Drying in an oven (110 C, 2H) - Calcination 2h 800 CB) Second composition Raw materials: Manganese nitrate Mn (NO3) 2,4H20 and potassium nitrateKNO3 99.5% are used.

The support used is a Condea SB3 alumina which has been stabilized by

impregnation at 10 atomic% of Ce followed by calcination for 2 hours at 500 C.

Preparation of the composition: One proceeds in two stages for the deposition in the same way as for the first composition (dry impregnation, pore volume of the support determined with water of 0.8cm3 / g) with the elements Mn and K in the same quantities, the first calcination taking place at 500 C and the second at 800 C. The composition thus obtained

has a BET surface of 117m2 / g.

The results in catalysis are given below.

T C Quantity of NOx stored (mg NOx / g active phase)

250 10

300 12

350 22

400 19

450 16

500 9

A NOx trap activity is therefore observed over the entire range of

temperatures between 2500C and 500 C. This activity takes place in the absence of precious metal.

Claims (14)

1- Compound which can be used as an NOx trap, characterized in that it comprises a combination: - of a first composition comprising a support and an active phase, the active phase being based on manganese and at least one other element A chosen from alkalis and alkaline earths, manganese and element A being chemically bonded; - of a second composition comprising a support and an active phase based on manganese and at least one other element B chosen from alkalis, alkaline earths and rare earths, this second composition exhibiting or being liable to exhibit a specific surface of at least 80m2 / g after calcination 8 hours at 800 C. 2- Compound according to claim 1, characterized in that the second composition has or is capable of exhibiting a specific surface area of at least 100 m 2 / g after calcination 8 hours at 800 C. 3- A compound according to claim 1 or 2, characterized in that the elements A and B are chosen from potassium, sodium or barium. 4- A compound according to one of the preceding claims, characterized in that the support for the second composition is based on alumina or alumina stabilized by silicon, zirconium, barium or a rare earth. - Compound according to one of claims I to 3, characterized in that the support of the second composition is based on silica. 6- A compound according to one of claims 1 to 3, characterized in that the support of the second composition is based on silica and titanium oxide in a proportion atomic Ti / Ti + Si between 0.1 and 15%. 7- A compound according to one of claims 1 to 3, characterized in that the support of the second composition is based on cerium oxide and zirconium oxide, this support having been obtained by a process in which a mixture comprising zirconium oxide and cerium oxide is formed and washed or the mixture thus formed is impregnated with an alkoxylated compound having a number of atoms of carbon greater than 2. 8- A compound according to one of claims 1 to 3, characterized in that the support of the second composition 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 a additive chosen from anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids and their salts, the reaction possibly taking place in presence of a base and / or an oxidizing agent. 9- A compound according to one of the preceding claims, characterized in that the support for the first composition is based on an oxide chosen from the oxide of cerium, zirconium oxide or mixtures thereof. - Compound according to one of the preceding claims, characterized in that the support for the first composition is based on cerium oxide and it comprises in besides silica. 11- System comprising a substrate and a coating incorporating the compound according to one of claims 1 to 10, on this substrate, the compound being present in the coating in the form of a first layer containing the first composition aforementioned and a second layer containing the second aforementioned composition. 12- System comprising a substrate and a coating incorporating the compound according to one of claims 1 to 10, on this substrate, the compound being present in the coating in the form of a mixture of the two aforementioned compositions. 13- System comprising two juxtaposed substrates, one coating on each said substrates and a compound according to one of claims 1 to 10, said compound being present in the coating on the first substrate in the form of the aforementioned first composition and in the coating on the second substrate in the form of the second aforementioned composition. 14- Gas treatment process with a view to reducing oxide emissions nitrogen, characterized in that a compound according to one of claims 1 to or a system according to one of claims 11 to 13. - Method according to claim 14, characterized in that a gas is treated internal combustion engine exhaust. 16- The method of claim 15, characterized in that a gas is treated having an excess of oxygen relative to the stoichiometric value. 17- Use of a compound according to one of claims 1 to 10 for the manufacture of a system for the treatment of an internal combustion engine exhaust gas.