KR20010108495A - Compositions Used as NOx Trap, Based on Manganese and an Alkaline or Alkaline-Earth and Use for Treating Exhaust Gases - Google Patents

Abstract

The present invention relates to compositions for use as NO _x traps based on manganese and alkali or alkaline earth metals and their use in the treatment of exhaust gases. The composition comprises a carrier and an active phase, the active phase is based on at least one element selected from manganese, alkali metals and alkaline earth metals, and the manganese and element A are chemically bonded. The composition can be used in a process for reducing nitrogen oxide emissions by treating gas as a NO _x trap, which is from an internal combustion engine, in particular from a diesel engine or a lean-burn engine.

Description

Compositions Used as NOx Trap, Based on Manganese and an Alkaline or Alkaline-Earth and Use for Treating Exhaust Gases}

The present invention relates to compositions for use as NO _x traps based on manganese and alkali or alkaline 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 by using "three-way" catalysts which use quantitatively 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. That is, their optimum operating temperature range is a relatively low temperature range, generally 200-270 ° C., and they have little or no efficacy at temperatures above this temperature. Therefore, it is important to develop a system that can operate at temperatures higher than those at which existing systems operate. In addition, existing systems may exhibit low thermal stability in hot hydrothermal or oxidizing media. Therefore, improvement of thermal stability may be one advantage. In addition, existing systems are generally based on expensive metals. These metals are expensive and their purchase can be a problem. It is also of interest to reduce costs by using catalysts that do not contain expensive metals.

It is therefore an object of the present invention to provide a composition which can be used as a NO _x trap at high temperatures and which does not optionally use expensive metals. In addition, the present invention is to provide a NO _x trap excellent in thermal stability.

Compositions of the present invention that can be used as NO _x traps comprise a support and an active phase, the active phase being based on at least one element A selected from manganese, an alkali metal and an alkaline earth metal, the manganese and element A being chemically It is characterized by being coupled. However, permanganic acid with A being potassium, the support being cerium oxide, and having an atomic ratio of [K] / ([K] + [CeO ₂ ]) = 0.16 and [Mn] / ([Mn] + [CeO ₂ ]) = 0.16 Compositions in which manganese and potassium are provided by potassium; And a support based on cerium oxide, zirconium oxide and lanthanum oxide in which A is potassium and each weight ratio to oxide is 72/24/2, comprising a support having 2.8 ml of O ₂ / g oxygen storage capacity. Excluded from the invention.

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

As used herein, the term "rare earth metal" refers to an element selected from the group consisting of yttrium and atoms of the periodic table having 57 to 71 atoms.

The oxygen storage capacity referred to herein is measured by a test that continuously oxidizes the injected amount of carbon monoxide and consumes the injected amount of oxygen to evaluate the ability of the support or product to oxidize the product again. The method used is known as an alternative.

The carrier gas is pure helium with a flow rate of 10 l / hour. Inject using a loop containing 16 ml of gas. CO is injected using a gas mixture containing 5% CO diluted with helium, while O ₂ is injected using a gas mixture containing 2.5% O ₂ diluted with helium. The gas is analyzed by chromatography using a thermal conductivity detector.

The amount of oxygen consumed allows the oxygen storage capacity to be measured. The measured oxygen storage capacity is expressed in ml of oxygen per gram of product introduced (under normal temperature and pressure conditions) and measured at 400 ° C. The oxygen storage capacity shown here is the oxygen storage capacity of the product pretreated in air at 900 ° C. for about 6 hours in a muffle furnace.

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.

In one aspect of the invention, the active phase is based on manganese and at least one element A selected from alkali metals and alkaline earth metals. More specific examples of alkali metal elements are sodium and potassium. As the alkaline earth metal element, barium may be mentioned. Since the compositions of the present invention may include one or more elements A, it is to be understood that the remaining detailed description of element A includes the case where several elements A are present.

In addition, manganese and element A are present in the composition of the present invention in chemically bonded form. This means that these two elements are not only present together in a simple mixture, but there is a chemical bond between manganese and element A resulting from the reaction between them. Thus, manganese and element A may exist in the form of compounds or mixed oxide types. This compound or phase can be represented by A _x Mn _y O ₂ ± δ (Formula 1), in particular, when the value of δ depending on the properties of the element A and the oxidation state of manganese is 0.5 < y / x < 6. Examples of phases or compounds of formula (I) include, but are not limited to, vernadite, hollandite, romanechite or psilomelane, birnessite, todorokite, Buserite or lithophorite type compounds. The compound may optionally be hydrated. The compound may have a plate structure of CdI ₂ type. Formula 1 is described herein, and the scope of the present invention also includes compounds of other formulas, of which manganese and element A are chemically bound.

X-ray analysis or electron microscopy can verify the presence of these compounds.

The oxidation state of manganese may be 2 to 7, more specifically 3 to 7.

When element A is potassium, potassium and manganese may be present in the form of a compound of type K ₂ Mn ₄ O ₈ . When element A is barium, barium and manganese may be BaMnO ₃ type compounds.

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

Compounds of the invention also include a support. The support can be any porous support that can be used for catalysis. Preferably, the support is sufficiently chemically inert to manganese and element A so that there is no substantial reaction of the support with one or more elements so that the support cannot prevent the formation of a chemical bond between manganese and element A. However, if there is a reaction between the support and these elements, large amounts of manganese and element A can be used to form the desired chemical bonds there between.

The support may be based on alumina. Any type of alumina can be used that can provide a specific surface sufficient for 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.

Stabilized alumina can also be used. Stabilizing elements may be rare earth metals, barium, silicon, titanium and zirconium. Rare earth metals may in particular be lanthanum or a mixture of lanthanum-neodymium.

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

The support may be based on an oxide selected from cerium oxide and zirconium oxide or mixtures thereof.

Specific mixtures of cerium oxide and zirconium oxide that may be mentioned are disclosed in European patent applications EP-A-0 605 274 and EP-A-0 735 984, the contents of which are hereby incorporated by reference. More specifically, it is possible to use a support based on cerium oxide and zirconium oxide present in at least one cerium / zirconium atomic ratio. In the case of these supports, it is also possible to use solid supports in solution form. In this case, the X-ray diffraction spectrum of the support shows that a single uniform phase is present in the support. In the case where the support contains the most cerium, this phase corresponds to a crystalline cubic cerium oxide CeO ₂ phase with slightly offset lattice parameters compared to pure cerium oxide, so that zirconium is incorporated into the crystalline network of cerium oxide to form a true solution Produces a solid.

Mixtures of cerium oxide and zirconium oxide based on the above two oxides, and rare earth metals other than scandium oxide or cerium, in particular the earth metals described in international patent application WO 97/43214, the contents of which are incorporated herein. Mixtures may also be mentioned. Specifically, the application is based on cerium oxide, zirconium oxide and yttrium oxide having a cerium / zirconium atomic ratio of at least 1, or at least one other oxide selected from rare earth metal oxides other than cerium oxide and zirconium oxide except scandium oxide and cerium oxide. A composition is disclosed. After calcination at 900 ° C. for 6 hours the specific surface area of these compositions is at least 35 m ² / g and the oxygen storage capacity at 400 ° C. is at least 1.5 ml O ₂ / g.

In a specific embodiment of the invention, this support is based on cerium oxide and also comprises silica. Supports of this type are disclosed in patent applications EP-A-0 207 857 and EP-A-0 547 924, the contents of which are hereby incorporated by reference.

The total content of manganese, alkali metals and alkaline earth metals can be within a wide range. The minimum content is the content when NO _x adsorption activity is no longer observed. This content can be between 2% and 50%, more specifically between 5% and 30%, expressed as an atomic percentage (%) of the total moles of oxides of the support and elements contained in the active phase. The content of each manganese, alkali metal and alkaline earth metal may be within a wide range, in particular the manganese content may be about or about the same as the content of alkali or alkaline earth metal.

According to an interesting embodiment of the invention, the alkali metal is potassium, which may be included in a content of 10 to 50%, more specifically 30 to 50% (calculated as above).

The compositions of the present invention are prepared by contacting a support with manganese and one or more element A or with manganese and one or more precursors of element A and then calcining at a temperature sufficient to form a chemical bond between manganese and element A. can do.

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 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.

As described above, the composition wherein the support is cerium oxide and element A is potassium and the precursors of potassium and manganese used in the production process described above are potassium permanganate (the atomic ratio of Mn to K is as described above) of the present invention Excluded from the category.

As described above, calcination is carried out at a temperature sufficient to form a chemical bond between manganese and element A. This temperature depends on the nature of element A, but when calcined in air it is generally at least 600 ° C, more specifically at least 700 ° C, in particular 800 to 850 ° C. Higher temperatures are generally not necessary, since a chemical bond between manganese and element A is already formed and a higher temperature may cause a reduction at a particular surface of the support, which may reduce the catalytic properties of the composition. Because. The calcination time depends mainly on the temperature, so set it at a temperature sufficient to form chemical bonds between the elements.

The composition of the invention as described above is in powder form, but it can 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 quantitative combustion of the fuel, more precisely to an excess of oxygen-containing gas, i.e., to a quantitative value λ = 1. 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 abovementioned type which may further contain water in an amount of 10%.

The present invention also relates to a gas treatment system for reducing the emissions of nitrogen oxides, the gas may be of the type mentioned above and more particularly it may be a gas containing excess oxygen relative to quantitative values. This system is characterized by comprising the above-mentioned composition. Thus, the system can include a catalytic wash coat based on the present compositions on a monolithic metal or ceramic type substrate.

Finally, the present invention also relates to the use of the compositions of the present invention in the manufacture of such systems.

Examples will be described below.

In these examples, the test is performed as follows to evaluate the NO _X trap.

0.15 g of NO _X trap powder is charged to a quartz reactor. The powder used was squeezed, ground and sieved to isolate in fractions having a particle size of 0.125 to 0.25 mm.

The composition (volume) 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 and NO _X signals are obtained with a NO _X ECOPHYSICS analyzer using the principle of chemiluminescence.

NO _X traps are evaluated by measuring the total amount of NO _X adsorbed (expressed in mg of NO per gram of trap or active phase) until the trap phase is saturated. The experiment was repeated at various temperatures between 250 ° C and 500 ° C. Thus, the system can measure the optimal temperature range for the NO _X trap action.

<Examples 1 to 12>

Starting material

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

The support used was HSA5® cerium oxide (Rhodia), HSA1® cerium oxide (Rhodia), zirconium oxide (each weight ratio of ZrO ₂ / CeO ₂ is 80/20) and silica, and silica HSA514 (registered trademark; 99.15% CeO ₂ , 0.85% SiO ₂ ) cerium oxide (Rhodia). All supports were calcined at 500 ° C. for 2 hours.

Preparation of the composition :

The active phase was based on manganese with another element A which was K, Ba or Na.

The method is as follows.

Step 1 : Deposition of the Supported First Element

This step consists of a ratio of 10 atomic% to the number of moles of the element and the number of moles of oxide of the support, ie [Mn] / ([Mn] + [oxide of the support]) = 0.1, ie [Mn] = 0.1 and [oxide of the support] ] = 0.9 and manganese deposition.

Step 2 : Deposition of Supported Second Elements

This is deposited as element A at a rate of 10 atomic% relative to the sum of the moles of oxide supported, i.e., [A] / ([Mn] + [A] + [oxide of the support]) = 0.1 It consisted of making.

Here, A may be K, Ba or Na.

Consists of impregnating the support with a solution whose volume is equal to the pore volume (volume measured in water: 0.5 cm ³ / g), taking into account the supported elements to be dissolved in the solution and whose concentration is such that the desired doping can be achieved. Dry impregnation was performed.

In the present invention, the elements are sequentially impregnated into the support.

The following trial protocol was used.

Dry impregnation of the first element,

Oven drying (110 ℃, 2 hours)

Calcination at 500 ° C. for 2 hours (5 ° C./min)

Dry impregnation of the second element

Oven drying (110 ℃, 2 hours)

After impregnation, the product was calcined for 6 hours in air at 500 ° C, 600 ° C, 700 ° C, 800 ° C and 850 ° C.

Thus, the following composition was prepared.

For Examples 1-8, HSA5 (R) support was used, for Examples 9 and 10, HSA514 (R) support was used, and for Example 11, ZrO ₂ / CeO ₂ support was used. In the case of Example 12, an HSA1 (registered trademark) support was used.

Comparative Example 1: [Mn] = 10 atomic%; [K] = 10 atomic%, calcined at 500 ° C. for 2 hours, SBET = 115 m ² / g.

Example 2: [Mn] = 10 atomic%; [K] = 10 atomic%, calcined at 600 ° C. for 2 hours, SBET = 106 m ² / g.

Example 3: [Mn] = 10 atomic%; [K] = 10 atomic%, calcined at 700 ° C. for 2 hours, SBET = 15 m ² / g.

Example 4: [Mn] = 10 atomic%; [K] = 10 atomic%, calcined at 850 ° C. for 6 hours, SBET = 12 m ² / g.

Example 5: [Mn] = 10 atomic%; [Ba] = 10 atomic%, calcined at 500 ° C. for 2 hours, SBET = 112 m ² / g.

Example 6: [Mn] = 10 atomic%; [Ba] = 10 atomic%, calcined at 850 ° C. for 6 hours, SBET = 23 m ² / g.

Comparative Example 7: [Mn] = 10 atomic%; [Na] = 10 atomic%, calcined at 500 ° C. for 2 hours, SBET = 112 m ² / g.

Example 8: [Mn] = 10 atomic%; [Na] = 10 atomic%, calcined at 850 ° C. for 6 hours, SBET = 6 m ² / g.

Example 9: [Mn] = 10 atomic%; [K] = 10 atomic%, calcined at 800 ° C. for 2 hours, SBET = 6 m ² / g.

Comparative Example 10 The composition was the same as in Example 9, but calcined at 500 ° C. for 2 hours, SBET = 111 m ² / g.

Example 11: [Mn] = 10 atomic%; [K] = 10 atomic%, calcined at 850 ° C. for 6 hours, SBET = 11 m ² / g.

Example 12 [Mn] = 10 atomic%; [K] = 10 atomic%, calcined at 850 ° C. for 6 hours, SBET = 5 m ² / g.

SBET can be used to determine BET specific surfaces measured by nitrogen adsorption according to standard ASTM D 3663-78 established from the BRUNAUER-EMMETT-TELLER method described in "The Journal of the American Chemical Society" 60, 309 (1938). Refers to.

For the comparative example, X-ray analysis only showed CeO ₂ phase. For Examples 2, 3, 4, 9 and 11, X-ray analysis showed CeO ₂ phase and K ₂ Mn ₄ O ₈ phase (see JCPDS 16-0205 index). Microscopic analysis showed the presence of large crystals of about 200-300 nm consisting of Mn and K. Manganese was in oxidation states III and IV. For Example 6, X-ray analysis showed CeO ₂ phase and BaMnO ₃ type phases. For Example 8, X-ray analysis showed CeO ₂ phase and Na _0.7 MnO _2-δ type phase.

The results of the NO _X traps for the products of the examples are shown in the table below, where the values in the table correspond to the amount of stored NO _X expressed in mg of NO per gram of active phase.

T (℃) Comparative Example 1 Example 2 Example 3 Example 4 Comparative Example 5 Example 6 250 12.4 8.5 5.8 7.4 300 10.8 13.5 12.9 1.2 5.5 3.6 350 7.1 12.4 12.3 10.2 0.7 4.1 400 2.4 9.5 11.1 9.1 0 1.7 450 0 6.4 8.5 7.4 1.3 500 3.6 6.6

T (℃) Comparative Example 7 Example 8 Example 9 Comparative Example 10 Example 11 Example 12 250 8.9 300 7.8 1.2 1.6 12.2 6 1.7 350 3.1 3.0 5.9 10.5 13.7 10.6 400 0 3.0 7.4 8.3 11.1 9.3 450 1.0 7.5 4.4 10.7 7 500 5.9 0.6 7.6 6.4

The composition of the present invention showed a large displacement at T _max as the temperature was higher compared to the composition in which manganese and other elements were not chemically bound. In addition, these compositions are effective for the storage of NO _{x in} the absence of platinum or other expensive metals.

Example 13

This example illustrates the thermal stability of the composition of the present invention.

Although the composition used in Example 4 was used, the composition was calcined at 750 ° C. for 6 hours under a nitrogen atmosphere containing 10% by volume of hydrogen. The catalysis results for the compositions are shown in the table below, and the results of Example 4 are also reported in the table for comparison.

T (℃) Example 13 Example 4 250 300 2.3 1.2 350 10.4 10.2 400 9.3 9.1 450 6.9 7.4 500 5.4 6.6

There was no substantial difference between the results of the product of Example 13 and the product of Example 4 after the passage of time.

<Example 14>

In this example, a support calcined at 800 ° C. for 2 hours was used based on cerium oxide, zirconium oxide and lanthanum oxide, each having a weight ratio of CeO ₂ / ZrO ₂ / La ₂ O ₃ of 67/23/10. Under the conditions described above, the following molar ratios of manganese and potassium were dry impregnated.

[Mn] / ([Mn] + [oxide of the support]) = 0.1

[K] / ([K] + [Mn] + [Support Oxide]) = 0.4

After impregnation, the product was calcined at 850 ° C. for 2 hours. The surface SBET after calcination was 2 m ² / g.

The amount of stored NO _X indicated as above is shown in Table 4.

Temperature Q NO _X 300 ℃ 3.9 350 ℃ 11.5 400 ℃ 20.7 450 ℃ 34.3

In the case of this example, particularly large amounts of stored NO _x were observed.

<Example 15>

In this example, an alumina based support, calcined at 500 ° C. for 2 hours, was used. Under the conditions described above, the following molar ratios of manganese and potassium were dry impregnated.

[Mn] / ([Mn] + [Al ₂ O ₃ ) = 0.1 [K] / ([K] + [Mn] + [Al ₂ O ₃ ]) = 0.2

After impregnation, the product was calcined at 750 ° C. for 6 hours. The surface SBET after calcination was 129 m ² / g.

The amount of stored NO _X indicated as above is shown in Table 5.

Temperature Q NO _X 300 ℃ 23.3 350 ℃ 22.2 400 ℃ 18.8 450 ℃ 12.8

Claims (11)

Use as a NO _x trap comprising a support and an active phase, characterized in that the active phase is based on manganese and at least one element A selected from alkali metals and alkaline earth metals, and manganese and element A are chemically bonded. For compositions wherein A is potassium, the support is cerium oxide, and the atomic ratio is [K] / ([K] + [CeO ₂ ]) = 0.16 and [Mn] / ([Mn] + [CeO ₂ ]) = A composition in which manganese and potassium are provided by 0.16 potassium permanganate; and based on cerium oxide, zirconium oxide and lanthanum oxide, in which A is potassium and each weight ratio to oxide is 72/24/2, and 2.8 ml of O Composition comprising a support having a ₂ / g oxygen storage capacity is excluded). The composition of claim 1 wherein element A is potassium, sodium or barium. A composition according to claim 1 or 2, wherein the support is based on an oxide selected from alumina, cerium oxide, zirconium oxide or a mixture of cerium oxide and zirconium oxide. 4. A composition according to claim 3, wherein the support is based on cerium oxide and also comprises silica. The method according to any one of claims 1 to 4, wherein the support is contacted with manganese and one or more other elements A, or precursors of manganese and one or more other elements A, and the resulting mixture has a chemical composition between manganese and element A. Calcination at a temperature sufficient to form a bond. Process for reducing the emissions of nitrogen oxides, characterized in that the composition according to any one of claims 1 to 4 is used. The method according to claim 6, wherein the exhaust gas from the internal combustion engine is treated. The method of claim 6, wherein the treated gas contains excess oxygen relative to quantitative values. The method according to claim 7 or 8, wherein the oxygen content in the gas is at least 2% by volume. A treatment system for exhaust gas discharged from an internal combustion engine, comprising a composition according to any one of claims 1 to 4. Use of the composition according to any one of claims 1 to 4 for producing a treatment system for exhaust gas discharged from an internal combustion engine.