KR20020007386A - Compositions Usable as NOx Trap, Based on Manganese and an Alkaline-Earth or a Rare Earth and Use in the Treatment of Exhaust Gases - Google Patents

Abstract

The present invention relates to a composition that can be used as a NO _x trap in the treatment of exhaust gas. The composition comprises a support and an active phase, wherein the active phase is based on at least one element A and manganese selected from alkaline-earth metals and rare earth metals, and after calcination at 800 ° C. for 8 hours, the specific surface area is 10 m ² / g It may be abnormal or abnormal. The support may be based on alumina or alumina stabilized with silicon, zirconium, barium or rare earth metals, or may be based on silica or silica and titanium oxides.

Description

Compositions Usable as NOx Trap, Based on Manganese and an Alkaline-Earth or a Rare Earth and Use in the Treatment of Exhaust Gases}

The present invention relates to compositions for use as NO _x traps based on manganese and alkaline earth metals or rare earth earth metals, and to their use in the treatment of exhaust gases.

In particular, the emissions of nitrogen oxides contained in the exhaust gases emitted from automotive engines can be reduced using "three-way" catalysts which use stoichiometrically the reducing gases present in the mixture. Excess oxygen substantially reduces catalyst performance.

However, some engines, such as diesel engines or lean burn gasoline engines, are economical in terms of fuel consumption, but mainly produce exhaust gases containing excess oxygen, such as at least 5% oxygen. Thus, trifunctional catalysts are not useful for NO _x emissions in this case. In addition, NO _x emissions are now necessarily limited by the strengthening of post-combustion control that affects the engine.

In order to overcome this problem, a system known as a NO _x trap capable of absorbing NO ₂ formed after oxidation of NO to NO ₂ has been proposed. Under certain conditions, NO ₂ leaks and is then reduced to N ₂ by reducing species contained in the exhaust gas. However, these NO _x traps have many disadvantages. These NO _x traps do not age only as long as they operate less satisfactorily when exposed to high temperatures. In addition, these NO _x traps may have weak sulfation resistance.

Accordingly, it is an object of the present invention to provide an NO _x trap with improved aging resistance.

It is also an object of the present invention to provide a NO _x trap with improved sulfate resistance.

Compositions of the invention which can be used as NO _x traps comprise a support and an active phase, which is based on manganese and at least one element A selected from alkaline earth metals and rare earth metals, calcined at 800 ° C. for 8 hours. The specific surface area may then be greater than or equal to 10 m ² / g.

Other features, details, and advantages of the invention will become more apparent from the non-limiting examples described and described below.

The composition of the present invention comprises a support and an active phase. The term "support" refers to a main element or elements that do not have catalytic activity or trap activity in the composition, and / or a main element or element that has catalytic activity or trap activity that is not the same as the catalytic activity or trap activity on the active phase. It should be understood in a broad sense that other elements or elements are deposited on the support. For simplicity, the remainder of the description will discuss the support and the active or supported phases, but the scope of the present invention is that when an element is present in the support that is described as forming part of the active or supported phase, for example, It is to be understood that this also includes the case where elements are introduced into the support during the manufacture of the support itself.

The active phase of the present invention is based on manganese and at least one element A. This element A may be an alkaline earth metal or a rare earth metal. As the alkaline earth metal element, barium may be mentioned. More specifically, the rare earth metal may be selected from cerium, terbium, gadolinium, samarium, neodymium and praseodymium. The total content of manganese, alkaline earth metals or rare earth metals may be 1% to 50%, more specifically 5% to 30%. This ratio is expressed in atomic percent relative to the total moles of oxide (s) of the support and elements contained in the supported phase. Each content of manganese, alkaline earth metal or rare earth metal may be within a wide range, specifically, the content of manganese may be the same as or similar to that of element A.

The present invention includes the case where the active phase consists essentially of manganese and at least one other element A selected from alkaline earth metals and rare earth metals. “Consisting essentially of” means that the composition of the present invention is NO in the absence of the active phase of any element except manganese and element (s) A, such as expensive metals or other metal type elements that are typically used for catalysis. _x may indicate trap activity.

As noted above, one feature of the present compositions is that after calcining at 800 ° C. for 8 hours, the specific surface area may be at least 10 m ² / g or greater. This surface may in particular be at least 20 m ² / g after calcination at the same temperature for the same time. More specifically, this specific surface area is at least 80 m ² / g and even more specifically at least 100 m ² / g after calcining at 800 ° C. for 8 hours.

The term "specific surface area" is determined by nitrogen adsorption according to standard ASTM D 3663-78 established from the BRUNAUER-EMMETT-TELLER method described in the periodical publication "The Journal of the American Chemical Society" 60, 309 (1938). BET specific surface area.

This surface property is obtained by selecting a suitable support having a sufficiently high specific surface area.

This support may be based on alumina. Any type of alumina can be used that can provide a specific surface sufficient for use in catalysis. Examples of alumina that may be mentioned include one or more aluminum hydroxides such as bayerite, hydragillite or gibbsite, nordstrandite; And / or alumina formed from rapid dehydration of one or more aluminum oxyhydroxides, such as boehmite, pseudoboehmite, and diaspore.

In one particular embodiment of the invention, stabilized alumina is used. Stabilizing elements may be rare earth metals, barium, silicon, titanium and zirconium. Rare earth metals may in particular be mixtures of cerium, lanthanum or lanthanum-neodymium.

Stabilized alumina is specifically prepared by impregnating alumina with a solution of the stabilizing element, such as a nitrate, or by drying and calcining the precursor of alumina and the salt of these elements together.

Another process for producing stabilized alumina is the preparation of alumina powders formed by rapid dehydration of aluminum hydroxide or aluminum oxyhydroxide to lanthanum compounds and, optionally, neodymium compounds, more specifically salts of these compounds. It is a method of aging in the presence. Aging can be carried out by suspending alumina in water and then heating it, for example, at 70 ° C to 110 ° C. After aging, the alumina is heat treated.

Further preparation is similar but consists of treatment with barium.

The amount of stabilizer, expressed as weight of stabilized oxide relative to stabilized alumina, is generally 1.5% to 15%, more specifically 2.5% to 11%.

The support may be based on silica.

The support may be based on silica and titanium oxide having an atomic ratio of Ti / Ti + Si of 0.1% to 15%. More specifically, this ratio may be 0.1% to 10%. Such a support is described in international patent WO99 / 01216, the contents of which are incorporated herein.

Other suitable supports that can be used are based on cerium oxide and zirconium oxide, which oxides may be present in the form of mixed oxides, zirconium oxide solid solutions in cerium oxide or cerium oxide solid solutions in zirconium oxide. These supports are obtained by a first type of method comprising forming a mixture comprising zirconium oxide and cerium oxide and washing or impregnating the mixture with an alkoxylated compound containing at least two carbon atoms. The impregnated mixture is then calcined.

An alkoxylated compound is a compound of R ₁ -((CH ₂ ) _x -O) _n -R ₂ (Formula 2), wherein R ₁ and R ₂ are a straight or non-chain alkyl group, H, OH, Cl, Br or I, n is an integer from 1 to 100, x is an integer from 1 to 4); A compound of (R ₃ , R ₄ ) -v-((CH ₂ ) xO) _n -OH (Formula 3), wherein v represents a benzene ring, and R ₃ and R ₄ are hydrogen, 1 to The same or different substituents on the ring, which represent a straight or non-chain alkyl group of 20, wherein x and n are as defined above; A compound of R ₄ O — ((CH ₂ ) _x —O) _n —H (Formula 4), wherein R ₄ represents a straight or non-chain alcohol containing 1 to 20 carbon atoms, and x and n are As defined above); And a compound of R ₅ -S-((CH ₂ ) _x -O) _n -H (Formula 5), wherein R ₅ represents a straight or non-linear alkyl group having 1 to 20 carbon atoms, and x and n are As defined above). These compounds are described in international patent WO98 / 16472, the contents of which are incorporated herein by reference.

These supports are reacted with a solution of cerium salt, a solution of zirconium salt, and an additive selected from anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids and salts thereof (in the presence of bases and / or oxidants). By a second type of method).

More specific anionic surfactants that can be used are carboxylates, phosphates, sulfates and sulfonates. Preferred nonionic surfactants that can be used are ethoxylated alkyl phenols and ethoxylated amines.

Zirconium salt and cerium salt can be made to react by heating the solution containing a salt in a thermohydrolysis reaction. In addition, the zirconium salt and cerium salt can be reacted by introducing a base into the salt-containing solution and precipitating it.

These products are described in international patent WO98 / 45212, the contents of which are incorporated herein.

The compositions of the present invention may be prepared by contacting a support with manganese and one or more elements A or with manganese and one or more precursors of element A and then calcining at a temperature sufficient to transform the precursor or element into an oxide. . In general, this temperature is at least 500 ° C, more specifically at least 600 ° C.

One method that can be used for such contact is impregnation. Thus, first, a solution or slurry of salts or compounds of the elements of the supported phase is formed.

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

Salts of organic acids, in particular salts of saturated aliphatic carboxylic acids or salts of hydroxycarboxylic acids can also be used. Examples of organic acids that may be mentioned are formate, acetate, propionate, oxalate and citrate.

Next, the support is impregnated with the solution or slurry.

More specifically, dry impregnation is used. Dry impregnation involves adding to the product to be impregnated an aqueous solution of element in the same volume as the pore volume of the solid to be impregnated.

It may be desirable to deposit the elements of the active phase in two steps. Thus, preferably, manganese is deposited in the first step and element A is deposited in the second step.

After impregnation, the support is optionally dried and then calcined. It should be recognized that a support that has never been calcined before impregnation should be used.

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

The compositions of the present invention as described above are in powder form but may optionally be formed into granules, beads, cylinders or honeycombs of various dimensions.

The invention also relates to a gas treatment method for reducing the emissions of nitrogen oxides using the compositions of the invention.

Gases which can be treated in the present invention are, for example, gases from gas turbines, power plant furnaces or internal combustion engines. The internal combustion engine may be a diesel engine or a lean burn engine.

The composition of the present invention acts as a NO _X trap when in contact with a high content of oxygen containing gas. The term "high oxygen-containing gas" refers to a gas containing excess oxygen relative to the amount required for stoichiometric combustion of the fuel, more precisely an excess oxygen-containing gas, i. It means gas which is 1 or more. The value λ is especially correlated with the air / fuel ratio in a manner known in internal combustion engines. Such gases are, for example, gases of a lean burn engine comprising an oxygen content of at least 2%, and gases having a higher oxygen content, such as gas from a diesel type engine, ie at least 5%, more specifically at least 10%, in particular It may be a gas containing 5% to 20% oxygen.

The invention is also applicable to gases of the above-mentioned type, which may further contain water, for example in an amount of 10%.

The compositions of the present invention can be used to treat exhaust gases from internal combustion engines using sulfur containing fuels, ie, fuels having a sulfur content of at least 50 ppm, more specifically at least 200 ppm (represented as elemental sulfur). The term “sulfur” is to be understood in its broadest sense, ie to refer to sulfur and sulfur containing compounds present in the fuel.

The invention also relates to a catalyst system for treating exhaust gases from an internal combustion engine comprising a composition according to the invention. More specifically, this system comprises catalytic washcoats and is based on these compositions, for example, on monolithic metal or ceramic type substrates.

The present invention also relates to the use of the composition in the manufacture of such catalyst systems.

Examples will now be described.

In the examples, the composition was prepared as follows.

Preparation of the composition

Manganese nitrate Mn (NO ₃ ) ₂ 4H ₂ O, 99.5% potassium nitrate KNO ₃ and 99.5% barium nitrate Ba (NO ₃ ) ₂ were used.

The support used was SB3 alumina, available from Condea.

Deposition was carried out in two steps.

Step 1 : Deposition of the First Active Element

This step consisted of depositing the active element Mn in a ratio of 10 atomic% calculated by the formula [Mn] / ([Mn] + [Al ₂ O ₃ ]) = 0.10.

Step 2 : Deposition of the Second Active Element

This step comprises the second active element X (X may be Ba, at a ratio of 10 atomic% calculated by the formula [X] / ([Mn] + [X] + [Al ₂ O ₃ ]) = 0.10 and the comparative composition May be K).

Dry deposition followed by deposition. This method considers the elements of the active phase to be dissolved in the solution, while the volume is equal to the pore volume of the support (volume measured in water: 0.5 cm ³ / g) and the concentration of the support into a solution where the concentration can achieve the desired concentration. It consists of impregnation.

In the present invention, the elements were sequentially impregnated into the support using the following trial protocol.

Dry impregnation of the first element,

Oven drying (110 ℃, 2 hours)

Calcined at 500 ° C. for 2 hours

Dry impregnation of the second element

Oven drying (110 ℃, 2 hours)

Calcined at 850 ° C. for 2 hours

The resulting composition

Composition Active phase BET surface area (after calcination at 850 ° C. for 2 hours) 1 (comparative) 10% Mn, 10% K 148 m ² / g 2 10% Mn, 10% Ba 112 m ² / g

<Example 1>

This example demonstrates the aging resistance of the compositions of the present invention.

In this example, the catalyst test was performed as follows.

0.15 g of each of said NO _x trap compositions in powder form were charged into a quartz reactor. The powder used was compacted, ground and sieved to isolate in fractions having a particle size of 0.125 to 0.25 mm.

The composition (volume ratio) of the reaction mixture at the reactor inlet is as follows.

NO: 300vpm

O ₂ : 10%

CO ₂ : 10%

H ₂ O: 10%

N ₂ : 100%

The total flow rate is 30 Nl / h.

HSV is about 150000 h ⁻¹ .

NO and NO _X signals (NO _X = NO + NO ₂ ) are recorded continuously according to the temperature in the reactor.

NO _X traps are evaluated by measuring the total amount of NO _X adsorbed (NSC) (expressed in mg of NO per gram of trap or active phase) until the trap phase is saturated. The experiment was performed at 250 ° C.

In addition, the composition was exposed to aging of hydrothermal redox type using the following protocol.

The temperature of the composition was raised over 60 minutes to 950 ° C. under N ₂ . The composition was then left at this temperature for 6 hours, followed by alternating 24 times of 15 minutes each under an atmosphere of oxygen and moisture in nitrogen and hydrogen and moisture in nitrogen. At the end of the treatment, the temperature was lowered to 80 ° C. in H ₂ / N ₂ and then left in N ₂ .

The results are shown in the table below.

Composition NSC, unaged product NSC, Aged Product Difference 1 (comparative) 9.2 2.9 -68% 2 7.0 3.6 -45%

It can be seen that the aging resistance of the composition of the present invention is improved.

<Example 2>

This example demonstrates the sulfate resistance of the compositions of the present invention.

The same composition (unaged) as used in Example 1 was used. The composition of the reaction mixture at the reactor inlet is the same as in Example 1 or 30 ppm of SO ₂ is added.

The results are shown in the table below.

Composition NSC, no SO ₂ NSC, SO ₂ Yes Difference 1 (comparative) 9.2 6.2 -33% 2 7.0 5.4 -22%

It can be seen that the resistance to SO ₂ of the composition of the present invention is improved.

Claims (14)

The active phase is based on manganese, at least one element A selected from alkaline earth metals and rare earth metals, and after calcination at 800 ° C. for 8 hours the specific surface area is at least 10 m ² / g, in particular at least 20 m ² / g or more A composition for use as a NO _x trap comprising a support and an active phase. The composition according to claim 1, wherein the specific surface area after calcination at 800 ° C. for 8 hours may be at least 80 m ² / g, in particular at least 100 m ² / g or more. The composition according to claim 1 or 2, wherein the element A is barium. The composition according to claim 1, wherein the support is based on alumina or alumina stabilized by silicon, zirconium, barium or rare earth metals. The composition according to any one of claims 1 to 3, wherein the support is based on silica. A composition according to claim 1, wherein the support is based on silica and titanium oxide in an Ti / Ti + Si atomic ratio of 0.1% to 15%. 4. The support according to claim 1, wherein the support is based on cerium oxide and zirconium oxide, the support forming a mixture comprising zirconium oxide and cerium oxide and the mixture comprising at least 3 carbons. A composition, which is obtained by a method of washing or impregnating with an alkoxylated compound containing an atom. The support according to any one of claims 1 to 3, wherein the support is based on cerium oxide and zirconium oxide, and the support is a cerium salt solution, a zirconium salt solution, and an anionic surfactant, nonionic surfactant, polyethylene Composition obtained by a method of reacting an additive selected from glycols, carboxylic acids and salts thereof, optionally in the presence of a base and / or an oxidizing agent. Process for reducing the emissions of nitrogen oxides, characterized in that the composition according to any one of claims 1 to 8 is used. 10. The method of claim 9, wherein the exhaust gas from the internal combustion engine is treated. The method of claim 10 wherein the gas containing excess oxygen relative to the stoichiometric value is treated. The method according to claim 10 or 11, wherein the oxygen content in the gas is at least 2% by volume. A catalyst system for the treatment of exhaust gases discharged from an internal combustion engine, comprising a composition according to any one of claims 1 to 8. Use of the composition according to any one of claims 1 to 8 for producing a catalyst system for treating exhaust gas discharged from an internal combustion engine.