ZA200108713B

ZA200108713B - Composition usable as NOx trap, based on manganese and an alkanline-earth or a rare earth and use in the treatment of exhaust gases.

Info

Publication number: ZA200108713B
Application number: ZA200108713A
Authority: ZA
Inventors: Thierry Birchem; Catherine Hedouin; Thierry Seguelong
Original assignee: Rhodia Chimie Sa
Priority date: 1999-04-23
Filing date: 2001-10-23
Publication date: 2002-08-22
Also published as: WO2000064580A1; FR2792547A1; BR0009995A; KR20020007386A; EP1181094A1; CN1351518A; JP2002542029A; CA2371276A1; CN1132681C; NO20015161D0; NO20015161L; FR2792547B1; KR100420909B1

Description

COMPOSITIONS FOR USE AS A NO, TRAP, BASED ON MANGANESE AND AN

ALKALINE-EARTH OR A RARE EARTH, AND USE IN TREATING EXHAUST GASES

RHODIA CHIMIE

The present invention relates to a composition for use as a NOy trap, based on manganese and an alkaline-earth or a rare earth, and its use in treating exhaust gases.

Emissions of oxides of nitrogen (NO,) particularly from exhaust gases from automotive engines can be reduced using “three way” catalysts which stoichiometrically use the reducing gases present in the mixture. Any excess oxygen results in substantial deterioration of catalyst performance.

Some engines, however, such as diesel engines or lean burn gasoline engines, economise on fuel but emit exhaust gases which permanently contain a large excess of oxygen, for example at least 5%. A standard three way catalyst is thus useless for NO, emissions in this case. Further, limiting

NO, emissions has been rendered imperative by the tightening of automobile post combustion regulations which now extend to such engines.

To overcome the problem, systems known as NO, traps have been proposed which can oxidise NO to NO, then adsorb the NO, formed. Under certain conditions, the NO, is lcached out then reduced to N, by reducing species contained in the exhaust gases. These NO, traps have a number of disadvantages, however. They age poorly insofar as they operate less satisfactorily when subjected to high temperatures. Further, they may have poor sulphation resistance.

Thus the invention aims to provide NO, traps with an improved resistance to ageing.

The invention also aims to provide NO, traps with an improved resistance to sulphation. .

To this end, a composition for use as a NO, trap of the invention comprises a support and an active phase, and is characterized in that the active phase is based on manganese and at least one other clement A selected from alkaline-carths and rare earths, and in that it has or is capable of having a specific surface area of at least 10 m’/g after calcining for 8 hours at 800°C.

Further characteristics, details and advantages of the invention will become more apparent from the following description and non limiting examples given by way of illustration.

The compositions of the invention comprise a support and an active phase. The term “support” should be taken in its broadest sense of designating in its composition the major element or elements and/or either with no catalytic activity or trapping activity proper, or with a catalytic or trapping activity not equivalent to that of the active phase; and on which the other element or elements are deposited. For simplification, the remainder of the description will discuss the support and the active phase or supported phase but it should be understood that the scope of the present invention also encompasses the case where an element described as forming part of the active phase or supported phase is present in the support, for example having been introduced thereto during 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 alkali or an alkaline-earth. Barium can be cited as the alkaline-earth element.

More particularly, the rare earth can be selected from cerium, terbium, gadolinium, samarium, neodymium and praseodymium. The total manganese, alkaline-earth or rare earth contents can be in the range 1% to 50%, more particularly in the range 5% to 30%. These proportions are expressed in atomic % with respect to the sum of the moles of the oxide(s) of the support and the elements concerned in the supported phase. The respective manganese, alkaline-earth or rare earth contents can also fall within a wide range; in particular, the manganese content can be equal to or close to that of element A.

The invention covers the case where the active phase consists essentially in manganese and in at least one or several elements A selected from alkaline-earths and rare earths. By "consists essentially in" it is meant that the composition of the invention can have a NO, trap activity in the absence in the active phase of any element other than manganese and the element or the elements A, ." such as for instance an element of the type of precious metal or other metal used usually in catalysis.

As indicated above, one feature of the composition is that it has or is capable of having a specific surface area of at least 10 m’/g after calcining at 800°C for 8 hours. This surface may be especially of at least 20 m’/g after calcination at the same temperature and for the same time. More particularly, this specific surface area is at least 80 m’/g and still more particularly at least 100 m’/g after calcining at 800°C for 8 hours.

I. [) 3 ' M

The term “specific surface area” means the BET specific surface area determined by nitrogen adsorption in accordance with the 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 selecting a suitable support with a sufficiently high specific surface area.

This support can be based on alumina. Any type of alumina can be used which can have a specific surface area which is sufficient for an application in catalysis. Aluminas formed by rapidly dehydrating at least one aluminium hydroxide can be mentioned, such as bayerite, hydrargillite, gibbsite, nordstrandite, and/or at least one aluminium oxyhydroxide such as boehmite, pseudoboehmite or diaspore.

In one particular implementation of the invention, a stabilised alumina is used. Stabilising elements which can be cited include rare earths, barium, silicon and zirconium. Particular rare earths which can be mentioned are cerium, lanthanum or a lanthanum-neodymium mixture.

The stabilised alumina is prepared in a manner which is known per se, in particular by impregnating the alumina with solutions of salts such as nitrates, of the stabilising elements cited above or by co-drying a precursor of alumina with the salts of these elements followed by calcining.

Another method for preparing stabilised alumina which can be cited is one in which the powdered alumina formed by rapidly dehydrating an aluminium hydroxide or oxyhydroxide is subjected to an ageing operation in the presence of a stabilising agent constituted by a lanthanum compound and, optionally, a neodymium compound, this compound more particularly being a salt.

Ageing can be carried out by suspending the alumina in water then heating to a temperature in the ‘ range70°C to 110°C, for example. After ageing, the alumina undergoes a heat treatment.

A further preparation consists’of a treatment of a similar type but using barium.

The amount of stabiliser, expressed as the weight of stabilising oxide with respect to the : stabilised alumina, is generally in the range 1.5% to 15%, more particularly in the range 2.5% to 11%.

The support can also be based on silica.

Wt Y

It can also be based on silica and titanium oxide in a Ti/Ti+Si atomic proportion in the range 0.1% to 15%. More particularly, this proportion can be in the range 0.1% to 10%. Such a support has been described in International patent WO 99/01216, the contents of which are hereby incorporated. :

A further suitable support which can be used is one based on cerium oxide and zirconium oxide; these oxides can be present in the form of a mixed oxide or a solid solution of zirconium oxide in cerium oxide or vice versa. These supports are obtained in a first type of process which comprises a step in which a mixture comprising zirconium oxide and cerium oxide is formed and the mixture is washed or impregnated with an alkoxylated compound containing more than 2 carbon atoms. The impregnated mixture is then calcined.

The alkoxylated compound can be selected from products with formula (2) R;-((CH;)-O),-

R, where R; and R, represent linear or non linear alkyl groups or Hor OH or Cl or Bror I; nis a number in the range 1 to 100; and x is a number in the range 1 to 4; or those with formula (3) (R3,R4)-v-((CH,)x-0),-OH where v represents a benzene ring, R; and Ry are identical or different substituents on the ring and represent hydrogen or linear or non linear alkyl groups containing 1 to carbon atoms, x and n being defined as above; or those with formula (4) R4O-((CH;),-O),-H wherc Ry, represents a linear or non linear alcohol containing 1 to 20 carbon atoms and x and n are as defined above; and those with formula (5) Rs-S-((CH,)4-O),-H where Rs represents a lincar or non linear alkyl group containing 1 to 20 carbon atoms, x and n being as defined above. These products 20 are described in International patent WO 98/16472, the contents of which are hereby incorporated by reference.

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 selected from anionic surfactants, non ionic surfactants, polyethylene glycols, carboxylic acids and their salts is reacted, the reaction possibly taking place in the presence of a base and/or an oxidising agent. §

More particular anionic surfactants which can be used are carboxylates, phosphates, sulphates and sulphonates. Preferred non ionic surfactants which can be used are ethoxylated alkyl phenols and ethoxylated amines.

S

The zirconium and cerium salts can be reacted by heating the solution containing the salts in a thermohydrolysis reaction. It can also be reacted by precipitation by introducing a base into the solution containing the salts.

These products are described in International patent WO 98/45212, the contents of which are hereby incorporated by reference.

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 ensemble is calcined at a temperature which is sufficient to transform the precursors or the elements into oxides. In general, this temperature is at least 500°C, more particularly at least 600°C.

One method which can be used to bring about the above contact is impregnation. Thus firstly, a solution or slurry of salts or compounds of the elements of the supported phase is formed.

The salts can be selected from inorganic acid salts such as nitrates, sulphates or chlorides.

Salts of organic acids, in particular salts of saturated aliphatic carboxylic acids or salts of 1s hydroxycarboxylic acids, can also be used. Examples which can be cited are formates, acetates, propionates, oxalates and citrates.

The support is then impregnated with the solution or slurry.

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

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

After impregnation, the support is optionally dried then calcined. It should be noted that it is possible to use a support which has not yet been calcined prior to impregnation. ;

The active phase can also be deposited by spray drying a suspension based on salts or compounds of the elements of the active phase and the support. The spray dried product obtained is then calcined.

CL Is

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

The invention also concerns a process for treating gases to reduce emissions of oxides of nitrogen using the composition of the invention.

Gases which can be treated in the present invention are, for example, those from gas turbines, power station furnaces or from internal combustion engines. In the latter case, they may be diesel engines or lean burn engines.

The composition of the invention acts as a NO, trap when brought into contact with gases with a high oxygen content. The term “gases with a high oxygen content” means gases with an excess of oxygen with respect to the quantity required for stoichiometric combustion of the fuel and, more precisely, gases with an excess of oxygen with respect to the stoichiometric value A = 1, i.e, gases for which the value of A is more than 1. The value A is correlated with the air/fuel ratio in a known manner in particular for internal combustion engines. Such gases can be those from a lean burn engine with an oxygen content (expressed by volume) of at least 2%, for example, and those with a higher oxygen content, for example gases from diesel type engines, i.e., at least 5% or more than 5%, more particularly at least 10%, this amount possibly being in the range 5% to 20%.

The invention applies also to the gas of the above mentioned type which may contain water in addition in a quantity in the region of 10% for instance.

The composition of the invention can be used to treat exhaust gases from internal combustion engines using a sulphur-containing fuel, i.e., for fuels with a sulphur content of at least 50 ppm, more particularly at least 200 ppm (expressed as elemental sulphur). The term “sulphur” should be taken in its broadest sense, i.e., to designate sulphur and also sulphur-containing ‘ compounds present in the fuels.

The invention also concerns a catalytic system for treating exhaust gases from an internal combustion engine comprising a composition in accordance with the invention. More precisely, this system comprises a wash coat with catalytic properties and based on these compositions on a monolithic metal or ceramic type substrate, for example.

The invention also concerns the use of a composition as described above in producing such a catalytic system.

Examples will now be given.

In the examples, the compositions were prepared as follows.

Preparation of compositions:

The following were used: manganese nitrate Mn(NOs),,4H,0, 99.5% potassium nitrate

KNO;, and 99.5% barium nitrate Ba(NOs),.

The support used was an SB3 alumina from Condéa.

Deposition was carried out in two steps. 1* step: Deposit of 1% active element

This step consisted of depositing the active element Mn in a proportion of {0 atomic % calculated as follows: [Mn}/([Mn] + [Al,05]) = 0.10 2" step: Deposit of 2" active element

This consisted of depositing the second active element X, which could be K (comparative composition) or Ba, in a proportion of 10 atom % of A and calculated as follows: [X)/([Mn] + [X] + [Al,04]) = 0.10 deposition was by dry impregnation. This method consisted of impregnating the support under consideration with the element of the active phase dissolved in a solution with a volume equal to the pore volume of the support and in a concentration which could achieve the desired concentration.

In the present case, the elements were impregnated into the support one after the other using ‘ the following operating protocol: o Dry impregnation of first element; o Oven drying (110°C, 2h); o Calcining for 2 h at 500°C; ; o Dry impregnation of second element; o Oven drying (110°C, 2 h); o Calcining for 2 h at 850°C.

Compositions obtained:

— €

Composition Active phase BET surface area (after calcining for 2 h at 850°C 10% Mn, 10% K 148 m’/g 10% Mn, 10% Ba 112 m’/g

EXAMPLE 1

This example demonstrates the resistance to ageing of a composition of the invention.

In this example, the catalytic test was carried out as follows: 0.15 g of each of the above NO, trap compositions was charged in powder form into a quartz reactor. The powder used had been compacted then ground and sieved to isolate the fraction with a grain size in the range 0.125 to 0.250 mm.

The reaction mixture at the reactor inlet had the following composition (by volume): o NO: 300 vpm o 05: 10% o CO,: 10% o H,0: 10% o Nj: gsp 100%

The overall flow rate was 30 NI/h.

The HSV was of the order of 150000 h™.

The NO and NO, signals (NO, = NO + NO,) were continuously recorded along with the temperature in the reactor.

The NO, traps were evaluated by determining the total quantity of NO, adsorbed (NSC) (expressed in mg of NO/g of trap or active phase) until saturation of the trap phase. The experiment was carried out at 250°C.

Further, the compositions underwent hydrothermal redox type ageing using the following ) protocol.

The temperature of the compositions was raised under N; to 950°C over 60 minutes. The compositions were then kept at this temperature for 6 hours, by alternating 24 periods of 15 min in N atmospheres of oxygen and water in nitrogen and hydrogen and water in nitrogen respectively. At the end of the treatment, the temperature was dropped in Ho/N; to 80°C, then in N,.

The results are shown in the table below.

p

NSC, non aged product | NSC, aged product

It can be seen that the composition of the invention had an improved resistance to ageing.

EXAMPLE 2

This example demonstrates the resistance of a composition of the invention to sulphation. )

The same (non aged) compositions were used as those for Example 1. The reaction mixture at the reactor inlet had either the same composition as in Example 1 or this composition with an additional 30 ppm of SO,.

The results are shown in the table below.

NSC, no SO; NSC, with SO,

It can be seen that the composition of the invention had an improved resistance to SO,.

Claims

HO CLAIMS l. A composition for use as an NO, 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 selected from alkaline-earths and rare earths, and in that it has or is capable of having a specific surface area of at least 10 m®/g, more particularly of at least 20 m’/g, after calcining for 8 hours at 800°C.
2. A composition according to claim 1, characterized in that it has or is capable of having a specific surface area of at least 80 m’/g, more particularly of at least 100 m*/g after calcining at 800°C for 8 hours.
3. A composition according to claim 1 or claim 2, characterized in that element A is barium.
4. A composition according to any one of the preceding claims, characterized in that the support is based on alumina or alumina stabilised by silicon, zirconium, barium or a rare earth.
5. A composition according to any one of claims 1 to 3, characterized in that the support is based on silica.
6. A composition according to any one of claims 1 to 3, characterized in that the support is based on silica and titanium oxide in a Ti/Ti+Si atomic proportion in the range 0.1% to 15%.
7. A composition according to any one of claims 1 to 3, characterized in that the support is based on cerium oxide and zirconium oxide, the support having been obtained by a process in which a mixture comprising zirconium oxide and cerium oxide is formed and the mixture is washed or impregnated with an alkoxylated compound containing more than 2 carbon atoms.
8. A composition according to any one of claims 1 to 3, characterized in that the support is ‘ based on cerium oxide and zirconium oxide, the support having been obtained by a process in which a solution of a cerium salt, a solution of a zirconium salt and an additive selected from anionic surfactants, non ionic surfactants, polyethylene glycols, carboxylic acids and - their salts is reacted, the reaction optionally taking place in the presence of a base and/or an oxidising agent.

.. “1A LY : }
9. A process for treating gases to reduce emissions of oxides of nitrogen, characterized in that a composition according to any one of claims 1 to 8 is used.
10. A process according to claim 9, characterized in that an exhaust gas from an internal combustion engine is treated. 5s
11. A process according to claim 10, characterized in that a gas with an excess of oxygen with respect to the stoichiometric value is treated.
12. A process according to claim 10 or claim 11, characterized in that the oxygen content of the gas is at least 2% by volume.
13. A catalytic system for treating an exhaust gas from an internal combustion engine, characterized in that it comprises a composition according to any one of claims 1 to 8.
14. Use of a composition according to any one of claims 1 to 8 for producing a catalytic system for treating an exhaust gas from an internal combustion engine.