WO2020193678A1

WO2020193678A1 - Redox-stable perovskite for storage of nitrogen oxides from offgases

Info

Publication number: WO2020193678A1
Application number: PCT/EP2020/058476
Authority: WO
Inventors: Jürgen DORNSEIFFER; Sabrina ECKER; Wilhelm Albert Meulenberg; Olivier Guillon; Stefan Baumann
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 2019-03-27
Filing date: 2020-03-26
Publication date: 2020-10-01
Also published as: DE112020001428A5

Abstract

The invention relates to the use of a perovskite of the general formula (I) or (II) A_1-xA'_xB_1-xB'_xO₃ (I) A_1-xA'_xB_1-x/2B'_xO₃ (II), as NOx storage means, especially for storing nitrogen oxides from offgases, and to oxide ceramic materials comprising the perovskites of the formula (I) or (II) and an inert porous carrier material, to NOx storage means and catalysts comprising them, and to processes for producing them and to selected novel perovskites.

Description

Redox-stable perovskite for storing nitrogen oxides from exhaust gases

BACKGROUND AND PRIOR ART

Combustion processes in industrial combustion or processing plants as well as in vehicle combustion machines or in gas turbines are known to produce undesirable gaseous emissions that are harmful to the environment and health, which must be removed from the combustion gases at great expense or converted into harmless substances before they can get into the environment. In addition to carbon monoxide (CO), unburned or partially burned hydrocarbons (HCs) and fine dust (soot particles), there is also a special focus on nitrogen oxide compounds (nitrogen oxides; NOx), which arise from NO and N0 ₂ together with nominal amounts of N ₂ 0, and their removal or rendering harmless in exhaust gases is becoming increasingly important.

In the field of vehicle internal combustion engines in particular, an important means of increasing the efficiency of modern engines is unthrottled (lean) operation with excess air, which, however, requires a considerable amount of extra effort to clean the exhaust gases compared to the stoichiometric operation of gasoline engines. The particular challenge here is less the removal of carbon monoxide, hydrocarbons and soot particles, but rather compliance with the nitrogen oxide limit value specified by the legislator, which cannot be achieved by engine measures alone. The catalytic exhaust gas aftertreatment (AGN) required for this, however, forms the limiting link in the process chain, as nitrogen oxides (NOx) can only be efficiently converted back into molecular nitrogen by using reducing agents in the nominally oxygen-rich exhaust gas.

So far, both a continuous and a discontinuous process have been technically implemented. In lean-burn engines with continuous catalytic nitrogen oxide conversion, ammonia is used as a reducing agent, which is produced in situ by thermal hydrolysis of urea, which is injected into the exhaust gas in the form of an aqueous solution (Adblue) in front of an SCR catalytic converter (SCR: Selective Catalytic Reduction). In a discontinuous process that is also in use, the nitrogen oxides are first stored in a NOx storage catalytic converter (NSK) during normal operation of the engine (lean phase), which must be periodically regenerated. During this regeneration, the so-called rich phase, the air / fuel ratio is briefly reduced by an additional fuel injection into the engine until a sufficient concentration of carbon monoxide and hydrogen is available to reduce the stored nitrogen oxides in nitrogen. Commercial NOx storage catalytic converters (NSK) for lean-burn combustion engines contain platinum-activated cerium oxides and alkaline earth oxides as active storage components as well as platinum-coated magnesium spinel (MgAI ₂ 0 ₄ ) powders (produced by calcining hydrotalcites (Mg ₂ xAI ₂ (0H) _4x + 4C0 ₃ ^» nH ₂ 0)), which in various combinations and proportions in the washcoat cover a wide temperature range from 200 to 500 ° C for the storage of nitrogen oxides. These materials are present in the catalysts either as highly porous, pure compounds or compounds doped with lanthanides for thermal stabilization or partly in the form of disperse coatings on inert oxide-ceramic support structures such as aluminum oxides. The task of the platinum within these storage catalytic converters is to convert the nitrogen monoxide mainly found in the exhaust gas into storable nitrogen dioxide. However, this noble metal also catalyzes the oxidation of carbon monoxide and hydrocarbons when the engine is running lean and to a certain extent also the reduction of the stored NOx during regeneration. The catalytically active layers consisting of these materials are usually layered on the surface of metallic or ceramic monolithic flow-through bodies and, in addition to platinum, also contain other noble metals such as rhodium, which primarily catalyzes the reduction of the stored NOx during regeneration, or palladium, one of them has high activity in the oxidation of hydrocarbons even at low exhaust gas temperatures.

In addition to the above-mentioned storage compounds, oxide ceramic compounds with a perovskite structure of the general formula AB0 ₃ , AA'B0 _3I ABB'0 ₃ or AA'BB'0 ₃ can also be used in these catalysts. In addition to iron, cobalt and manganese or combinations of these metals on the BG itterplatz of these redox-active compounds, both lanthanoids and alkaline earth metals and other elements can be used as substituents or dopants in a large number of possible combinations.

From the publication DE10201 11 19129 A1, for example, manganites and cobaltates such as LaCo0 ₃ , LaMn0 ₃ and Lao.gSro.-iCoOa, and from DE10201 1 121280 A1 manganites, cobaltates and ferrites such as LaCo0 ₃ , LaMn0 ₃ , LaFe0 ₃ , La _0.9 Sro _{, i} Co0 ₃ , La _0, 9Sr _{0, i} MnO ₃ , and La _0.9 Sr _{0 1} FeO ₃ . In addition, DE102011121222 A1 discloses perovskite-based catalysts with possible perovskite structures AB0 ₃ , AA'B0 ₃ , ABB'O _s and AA'BB'0 ₃ . Specific examples are those with the structure Lai. _x Sr _x Co0 ₃ and La ^ SrxMnOs or LaCo ₃ , Lao _{, g} Sr _{0 i} Co0 ₃ and La Mn0 ₃ disclosed. The general description also mentions the possible meaning of B / B '= Fe, but without disclosing specific perovskites with B = Fe. The publication also discloses the use of the perovskite catalysts described for the oxidation of NO to N0 ₂ . Use or suitability as a NOx Memory is not written to. As described in these publications, due to their high redox activity, they catalyze the oxidation of nitrogen monoxide in N0 ₂ effectively even at low temperatures and can therefore be used for this task in the catalyst layers in order to save some of the expensive noble metals in the catalysts by this measure.

The main areas of application for this type of perovskite, however, are the use as cathode materials in solid oxide fuel cells (SOFC) or as electron conductors together with gadolinium ceroxide (CGO, ion conductor) in ceramic gas membranes, mainly in the form of substituted and doped lanthanum strontium ferrites, cobaltates and manganites. There are therefore publications dealing with the production and use of these functional materials, such as US2005 / 0106447, US 4,997,725 and EP 3211703 A1 or EP 2647419 A1 for gas membranes. For example, US2005 / 0106447 describes perovskite structures of the general formula (A ^ A '^ - yFeOs with concrete example structures in Table 1, containing mixtures of iron with cobalt or nickel on the B-lattice position according to the formulas Lao.sSro ^ Coo.eFeo ^ Os, Lao.sSro ^ Feo.gCoo ^ Os and Lao ^ Sro ^ Feo _j Nio ^ Oa. US 4,997,725 describes Lao _{, 89} Sr _{0, i} Mn0 ₃ as concrete perovskite structures. In EP 321 1703 Al, perovskites of the formula LaSrCo0 and LaSrCoFe0 ₃ as well as those which fall under the general formula La _zx Sm _x Sri. _z Co0 _3. EP 2647419 A1 describes as concrete perovskite structures those of the formulas Lao.ssSro ^ Cco ^ Feo.aOs (LSCF) and Bao.gSro .sCoo Feo.aOs. Due to the area of application, the perovskites described in these documents are optimized to maximize their ion and electron conducting properties, based on the high redox activities of these compounds.

DE102011083545 A1 discloses perovskites comprising those with a structure according to the formula La _0, 5S r _0.5 Feo _{, 5} Tio _{, 5} 0 ₃ and their use as storage material in batteries. A possible suitability or use as NOx storage or their use in catalytic converters is not described.

0.F. Lohne et al. / Solid State lonics 225 (2012) 186-189 describe perovskite structures of the general formula La _{Q 2} S r ₀₈ Feo _{, 8} Bo _{, 2} 0 ₃ , in which B can be selected from Al, Ga, Cr, Ti, Ta and Nb, and their results for the investigation of this B-lattice site substitution for aging and temperature resistance in a hydrogen-containing atmosphere for their application as oxygen separation membranes.

However, the stated prior art has two decisive disadvantages. By implementing platinum-activated cerium oxides or perovskites of the type mentioned above in the catalytically active NOx storage layers, the efficiency of the regeneration is also significantly reduced at the same time. The reason for this negative property is the high redox activity of these materials or material combinations, which leads to a correspondingly high intrinsic oxygen availability. Due to the fact that these compounds can be easily reduced, some of the carbon monoxide or hydrogen required for converting stored NOx into nitrogen reacts to H ₂ O and C0 ₂ during the regeneration phase of the catalyst. The associated reduction in the concentration of reducing agents within the catalytically active storage layer leads, in particular at the beginning of the rich phase, to an incomplete reduction of the NOx and to a desorption of some of the stored nitrogen oxides back into the exhaust gas. In addition, the regeneration time for completely emptying the storage tank increases, which results in increased fuel consumption.

Another disadvantage is the insufficient resistance of some of the NOx storage materials, in particular alkaline earth oxides such as BaO or SrO, to catalyst poisons such as sulfur dioxide, which are also contained in the exhaust gas. The high affinity of these compounds for S0 ₂ paired with a high thermal stability of the reactive surface sulfates formed in this way leads to a continuous decrease in the NOx storage capacity. Therefore, these materials have to be desulphated periodically at temperatures above 600 ° C. As with the regeneration of the filled NOx reservoir, this so-called DeSOx phase occurs through a change in the fuel / air mixture in a specially initiated regeneration phase of the engine, which also increases fuel consumption.

The invention is based on the object of providing an improved NOx storage device for storing NOx from exhaust gases which does not have the disadvantages of the prior art. In particular, an object of the invention is to provide a sintering and redox-stable storage material for nitrogen oxides (NOx) from exhaust gases. Another object of the invention is to provide an improved NOx storage material for nitrogen oxides (NOx) from exhaust gases from internal combustion engines. A further object of the invention is to provide an improved NOx storage material for nitrogen oxides (NOx) from exhaust gases from internal combustion engines, which allow an improvement in the efficiency of the regeneration of catalysts and a reduction in the associated fuel consumption. The improved NOx stores should be distinguished in particular by a high or improved storage capacity and / or improved resistance to catalyst poisons, in particular by adequate deactivation resistance to sulfur compounds.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, the object is achieved by a redox-stable perovskite of the general formulas (!) And (II) AI- _X A ' _X BI- _X B' _x 03

(I)

wherein

A is selected from the group of lanthanoids, yttrium and

Scandium or mixtures thereof;

is selected from the group of the alkaline earth metals and alkali metals or mixtures thereof;

B is Fe;

B ‘is selected from the group comprising Ti, Nb, Ta, Hf and Zr or

Mixtures thereof;

X represents a value in the range from 0.01 to 0.99.

In particular, the object can be achieved by oxide-ceramic materials with a perovskite structure, which comprise the perovskites according to the invention and an inert, porous carrier material.

It has surprisingly been found that such perovskites and oxide-ceramic materials are outstandingly suitable as NOx storage materials and that the aforementioned disadvantages of the prior art can thus be overcome.

Such perovskites and oxide ceramic materials, which contain iron on the B lattice site, show an increased sulfur resistance.

For the primary storage of nitrogen oxides from exhaust gases, the perovskite according to the invention of the NOx storage according to the invention contains elements with a high Lewis basicity such as elements from the group of the alkaline earth metals Mg, Ca, Sr or Ba, or mixtures thereof on the A 'lattice site Elements, with Sr and barium being used as preferred alkaline earth metals and particularly preferably Sr. In principle, alkali metals such as Na, K, Rb and Cs as well as mixtures thereof are also suitable for this task. In principle, mixtures of elements from the group of the alkaline earth metals and the alkali metals can also be used. It is also possible to use oxides thereof.

The pure alkali and alkaline earth oxides used in commercial NOx storage catalysts for nitrogen oxide adsorption in the high temperature range up to 500 ° C are sensitive to sulfur dioxide. In connection with iron in a common cation lattice of a ferrite phase, this transition metal destabilizes the alkali / alkaline earth metal-sulfur bond and at the same time forms intermediate iron sulfate. This allows the DeSOx temperature of the perovskite to be reduced to below 500 ° C and thus to the normal exhaust gas temperature range of 200 to 500 ° C. Periodic desulfation of the perovskite of the type mentioned above is no longer necessary.

In order to ensure the necessary phase stability at high exhaust gas temperatures and at different atmospheres, the A-grid contains a mixture of the aforementioned alkaline earth and alkali metal elements (A) with elements from the group comprising lanthanoids, such as in particular lanthanum (La), cerium (Ce) , Samarium (Sm) etc., Ytrium and Scandium (A '). From this group (A ‘), preference is given to lanthanum and cerium, and lanthanum is particularly preferred.

Since in structures containing such mixtures (AA ') the iron on the B lattice site can be both trivalent and tetravalent, this would lead to a high redox activity or intrinsic oxygen availability. This is achieved according to the invention by the partial substitution of iron (B) with higher-valent metal ions from the group comprising Ti, Nb, Ta, Hf and Zr (B '), in particular Ti ⁴⁺ , Nb ⁵⁺ , Ta ⁵⁺ or Zr ⁴⁺ , prevented. From the group of elements (B '), titanium (Ti) and niobium (Nb) are particularly preferred. By increasing the concentration of these metals on the B'-square of the perovskite according to the invention, the iron is converted into the trivalent oxidation state to such an extent that the resulting perovskite has the necessary redox stability and thus no longer has any intrinsic oxygen availability. The iron is preferably completely converted into the trivalent oxidation state. A further reduction of the trivalent iron to Fe ²⁺ would only be possible through the complete conversion or disintegration of this compound. However, the high temperatures and high concentrations of reducing agents required for this cannot be achieved during the regeneration phase of the engine.

According to the invention, x represents a value in the range from 0.01 to 0.99. Preferably, x represents a value in the range from 0.5 to 0.9.

According to the invention it is preferred that x is a value s 0.5, preferably> 0.5, preferably £ 0.8. In particularly preferred embodiments, x = 0.5 or x = 0.75. According to the invention, it is preferred that in the perovskites of the present invention, as in the formulas (I) above, (1-1) below, (II) above and / or (11-1) below, the lattice sites with A, A 'and B 'have one or combinations of the following meanings;

A is lanthanum (La) and / or

A ‘is strontium (Sr) and / or

B 'is titanium (Ti) or niobium (Nb). In a further preferred embodiment, the perovskite corresponds to one of the general formulas (1-1) or (11-1)

La ^ xSrxFevxB xOa

(1-1)

La ·] _ _X S r _x Fe ₁ -x _ß B'x _ß Os

(H-1)

wherein B ‘and x have the meaning as defined in one of the embodiments herein.

In a further preferred embodiment, the perovskite corresponds to one of the general formulas (1-2) or (II-2)

Lai- _x Sr _x Fei _x Ti _x 0 ₃

(l-2)

Lai _x Si _x Fei_x / 2Nbxß03

(H-2)

wherein x has the meaning as defined in one of the embodiments herein.

In a further preferred embodiment, the perovskite corresponds to one of the general formulas (1-1-1) or (11-1-1)

La _0.5 SG _{0> 5} RQO, 75 N bo, 2503

(11-1-1)

The perovskites of the invention are preferably in crystalline form, e.g. as a powder.

Such perovskites have surprisingly turned out to be suitable NOx stores and accordingly the present invention relates to the new use of such perovskites as NOx stores, as described in detail herein.

In a further embodiment, the present invention also comprises oxide-ceramic materials with a perovskite structure, comprising a perovskite according to one of the general formulas (!), (! -1), (1-2), (1-1 -1), (!!), (11-1), (II-2) and (11-1- 1), as described herein, and an inert, porous support material.

For the production of the perovskite NOx storage according to the invention (the oxide-ceramic materials with a perovskite structure), basically all methods that form nanoparticles are suitable in order to generate high adsorption areas for the accumulation of nitrogen oxide. Processes based on metal precursors which are soluble in water or organic solvents such as alcohols are preferably used here. In addition to hydrothermal and polyol syntheses, microemlusion-assisted methods can also be used, for example. However, the production is not restricted to these processes. The required final temperatures during heat treatment for the thermal decomposition of the metal-precursor mixtures and the consolidation of the resulting crystalline perovskites depend on the composition and can be up to 1000 ° C. This results in different grain sizes in the resulting perovskite powders due to sintering effects, so that the adsorption interfaces that can be achieved can vary very widely.

In order to achieve sinter-stable NOx storage with very high adsorption areas for

To provide nitrogen oxides, in a preferred embodiment the perovskites according to the invention are deposited on the inner surface of inert, porous carrier materials, in particular of highly porous inert carrier materials, e.g. layered or deposited in the form of nanoparticles. The embedding of the perovskites according to the invention in such porous / highly porous support materials can e.g. in a simple way by impregnating the pore system of the carrier matrix with a concentrated solution of the metal compounds to form the perovskite structure, a so-called concentrated "metal precursor solution" with a stoichiometric composition of the metals forming the perovskite structure, and subsequent heat treatment to convert it into the corresponding perovskite. To produce this metal precursor solution, which is used for the coating / impregnation of the porous carrier materials ("coating solutions"), all soluble salts and compounds of these metals can be used as metal precursors, which in the subsequent heat treatment under an oxidative atmosphere are gases such as for example form nitrogen oxides, water or carbon dioxide. Nitrates and acetates are preferred.

According to the invention, metal-ceramic carrier materials are preferably used.

Porous or highly porous support materials are particularly suitable so that the perovskites according to the invention can be layered or deposited on the inner surface of such porous / highly porous support materials. In principle, all inert porous or highly porous metals and ceramics with high internal surfaces that are sinter-stable even at high temperatures of up to 1000 ° C and have the lowest possible intrinsic absorption capacity for metal ions of the perovskites and, conversely, also a low release capacity in the perovskites are suitable as carrier materials at consolidation temperatures for the production of the oxide ceramic material with perovskite structure ("composite" or "infiltration composite") of 900 ° C. The “inert” carrier materials described herein are therefore particularly inert to ion migration into the other lattice, in particular at the selected operating temperatures under which the NOx storage device according to the invention is used or in the temperature ranges described herein (at least up to 800 ° C).

Oxides of aluminum, zirconium and silicon as well as mixtures of these elements are preferably used as carrier materials for this task. However, it is particularly preferred to use oxides whose open pore structures are mainly interspersed with pores whose diameter is less than 30 nm. This type of carrier material makes it possible to produce perovskite particles in the size range from 1 to 30 nm on the inner surface of the carrier materials, which are largely sinter-stable even at higher temperatures of up to 900 ° C. The bending density of these nanoparticles on this inner surface is not subject to any restrictions and can vary from a closed layer to individual isolated perovskite particles. However, a layer that is as closed as possible is advantageous in order to ensure a high adsorption area for nitrogen oxides. However, the layer thickness should not be so high that the pores are closed and thus the accessibility of the storage material for gases is restricted.

Depending on the pore structure and the pore volume of the support structure, the mass fraction of the perovskite in the resulting infiltration composite, i.e. in the oxide ceramic material with perovskite structure, be over 50%.

Examples of possible metal-ceramic carrier materials include zirconium oxide, aluminum oxide (Al2O3), magnesium oxide, silicon oxide and mixed materials thereof. Particularly preferred are aluminum oxide, zirconium oxide, silicon oxide and mixtures thereof.

In order to obtain a high mass fraction of storage-active perovskite in the carrier matrix, the use of highly concentrated coating solutions is necessary, depending on the pore volume of the carrier. In those cases in which the solvent power of the selected solvent is not sufficient to achieve the necessary metal precursor contents, complexing agents can also be added to the coating solution. In principle, all compounds are suitable as complexing agents which have the solubilities of the metal ions used in the Increase the chosen solvent to the required extent. However, when using protic solvents such as water, organic compounds containing one or more carboxyl groups such as citric acid, tartaric acid or ethylenediaminetetraacetic acid (EDTA) are preferred. Beta-diketones such as acetylacetone can be used in aprotic solvents such as alcohols. When using hydrocarbons as solvents, it is possible to use fatty acids such as, for example, 2-ethylhexylcarboxylic acid. However, the invention is not restricted to the use of these organic compounds. In principle, all soluble complexing agents can be used if necessary, which decompose without residue during the heat treatment under an oxidative atmosphere and form gases such as nitrogen oxides, water or carbon dioxide.

The grain sizes of the infiltration composites according to the invention (ie the oxide-ceramic materials with perovskite structure) for the storage of NOx from exhaust gases are in principle not subject to any restrictions, but powders with a narrow particle size distribution whose D50 values are between 2 and 5 μm are preferred in order to achieve a high adsorption capacity to obtain the best possible accessibility of the resulting catalyst layer for nitrogen oxides during storage and the reducing agents such as carbon monoxide and hydrogen during regeneration.

The oxide-ceramic materials with perovskite structure used for the NOx storage according to the invention can have an additional coating with a metal selected from the group of noble metals including platinum (Pt), rhodium (Rh), palladium (Pd) and ruthenium (Ru) (" coated infiltration composite ”). In such a case, an additional coating with platinum is preferred. It is also possible that the perovskites according to the invention and a further metal from the group of the aforementioned noble metals, preferably platinum, are present in the NOx storage in the form of a physical mixture.

The perovskites according to the invention described here are, as explained above, particularly suitable as storage or storage material for nitrogen oxides (NOx storage) and thus for the production of NOx storage. NOx stores based on perovskites are known in principle and can comprise further customary constituents in addition to the special perovskites and support materials according to the invention.

The perovskites and oxide ceramic materials according to the invention with a perovskite structure are particularly suitable for use as NOx stores and for use in catalytic converters or for use in the production of catalytic converters due to the properties described above. Thus, the present invention also includes NOx stores and catalysts that the Perovskites according to the invention of the formulas (I), (1-1), (I-2), (1-1-1), (II), (11-1), (II-2) and (11-1-1 ) or the oxide ceramic materials according to the invention with a perovskite structure. In particular, LNT / SCR hybrid catalysts (LNT; Lean NOx Trap; SCR: Selective Catalytic Reduction) are preferred. Such catalysts based on the inventive perovskites of the formulas (I), (1-1), (I-2), (1-1-1), (II), (11-1), (H-2) and ( 11-1-1) and oxide-ceramic materials with a perovskite structure can in principle be supported or unsupported catalysts, it being possible in principle to use any of the customary substrates for producing supported catalysts. Such catalysts can be present in customary configurations such as the supported and unsupported forms described in DE10 2011 119 129 A1.

The perovskites according to the invention of the formulas (I), (1-1), (i-2), (1-1-1), (II), (11-1), (H-2) and (11-1- 1), oxide ceramic materials with a perovskite structure, as well as NOx storage and catalysts comprising the same, are in principle suitable for storing NOx from all NOx-containing exhaust gases. Examples include exhaust gases from internal combustion engines, industrial incinerators, chemical processing plants, gas turbines, or medical devices. The perovskites according to the invention, oxide-ceramic materials with a perovskite structure, as well as NOx stores and catalysts comprising the same, can thus in principle be used in the aforementioned areas for NOx storage. The perovskites according to the invention, oxide-ceramic materials with a perovskite structure, as well as NOx storage and catalysts comprising the same, can also be used for treating a NOx-containing gas flow, in which the NOx-containing gas flow over the oxide-ceramic material with a perovskite structure or the NOx -Memory or the catalyst (which contain the perovskites) is allowed to flow.

The invention also includes a method for producing the oxide ceramic materials according to the invention with the Perovskite structure according to the invention described herein, which comprises the following steps: a) Production of a solution of the metal compounds to form the perovskite structure, optionally with the addition of a complexing agent

("Coating Solution");

b) mixing the coating solution from step a) with an oxide ceramic carrier material;

c) removing the solvent and drying the mixture from step b); d) heat treatment or calcination of the dried mixture from step c) and obtaining the oxide-ceramic material with a perovskite structure

("I nfi iteration com posit"); e) optional coating of the oxide ceramic material with perovskite structure from step d) with a noble metal selected from the group comprising platinum (Pt), rhodium (Rh), palladium (Pd) and ruthenium (Ru) and obtaining a “coated infiltration composite”.

The method for producing the oxide ceramic materials according to the invention with a perovskite structure is described in more detail below.

The oxide-ceramic materials with perovskite structure are preferably produced as a so-called infiltration composite by capillary impregnation, in which highly porous, inert oxide-ceramic carrier materials are soaked or impregnated with solutions of corresponding precursor compounds of the metals (metal precursor solution; “coating solution”) (step b). Care must be taken to ensure that these precursors are sufficiently soluble in the limited volume of solvent, which results from the limited specific absorption capacity of the carrier materials for the coating solution. The subsequent drying and removal of the solvent result in the impregnated carrier materials in the form of a coated / impregnated powder or coated / impregnated particles, which are then subjected in step d) to a heat treatment or calcination to form the NOx storage-active oxide ceramic material with a perovskite structure . In principle, all ovens with a regulated air supply are suitable in step d). Calcination temperatures in the range from 600 to 1000 ° C. are preferred. When using readily soluble nitrate salts as precursor compounds for producing the coating solutions, an aftertreatment of the furnace gas is necessary to reduce the nitrogen oxides formed during the calcination of this compound. This is known in principle from the prior art and is quite common. For the production of an infiltration composite with a high proportion of NOx storage-active material in the resulting composite, the use of complexing agents in the coating solution according to step a) is advantageous in order to increase the solubility of the metal ions in the solvent used accordingly.

By adding complexing agents, the concentration of the precursor metals in the coating solution can be increased so that highly concentrated coating solutions can be used. This is particularly advantageous in the case of carrier materials with small pore volumes, since it enables a high proportion of active storage material to be generated even in such small-pore carrier materials.

With regard to suitable metal precursors, carrier materials, solvents and complexing agents, reference is made to the statements above. The invention also relates to a device for storing nitrogen oxides from exhaust gases.

The device comprises a NOx storage device with a gas inlet side and a gas outlet side. The storage device comprises a redox-stable perovskite for storing NOx. The redox-stable perovskite can therefore store NOx that is contained in the exhaust gas. The NOx storage device comprises one or more channels through which exhaust gas can flow from the gas inlet side to the gas outlet side. The redox-stable perovskite is located in the one or more channels.

A perovskite is redox-stable in the context of the present invention if no oxygen or no significant amount of oxygen can be released in the relevant temperature range by reduction. The relevant temperature range is the temperature range to which the perovskite is exposed during operation. The relevant temperature range can end at 600 ° C; the relevant temperature range usually ends at 500 ° C, since the operating temperatures usually do not exceed 500 ° C. If the operating temperature can reach 500 ° C, for example, then no oxygen, no significant amount of oxygen, can be released from the redox-stable perovskite by reduction.

If the perovskite thus comprises iron (III), as is provided according to the invention, then the perovskite no longer has any intrinsic oxygen availability. As stated above, iron (III) cannot become too in the relevant temperature range

Iron (II) are reduced so that the perovskites according to the invention are redox-stable due to the lack of intrinsic oxygen availability and a harmful release of oxygen does not occur. This is of particular importance for solving the task at hand.

In this context it has been found to be advantageous that the perovskite comprises Sr _x , where x is more than 0.5, preferably more than 0.6. For example, X can be 0.75, so that the perovskite comprises Sr ₀ 7 ₅ .

The device can comprise an exhaust gas generating device, for example an internal combustion engine of a motor vehicle. Exhaust gas generating device means a device which can generate NOx-containing exhaust gases. The device can comprise a gas supply through which exhaust gases from the exhaust gas generation device to the gas inlet side of the exhaust gas generation device be directed. The device can comprise a heat generating device by means of which the NOx storage device can be heated, specifically preferably to temperatures of 200 ° C. to 500 ° C.

The NOx storage device advantageously comprises a component with a large number of channels. In particular, the channels run parallel to one another. The component with the multiplicity of channels is in particular manufactured in one piece. A plurality of components has therefore not been produced which have subsequently been joined together to form the component with the plurality of channels. The component consists in particular of a ceramic in order to be suitably chemically resistant, especially in the temperature range from 200 ° C to 500 ° C. The redox-stable perovskite is located in the channels.

During the operation of the storage device, the exhaust gas flows through the channels. By providing many channels, a sufficiently large surface area is provided to be able to store NOx with a high degree of efficiency.

The channels preferably have a square cross section in order to enable particularly high levels of efficiency. The channels with a square cross section are preferably no more than 3 mm, particularly preferably no more than 2 mm high and wide. The channels with a square cross section are preferably at least 0.3 mm, particularly preferably at least 0.5 mm high and wide.

A layer on the inner wall of a channel can consist entirely or partially of the redox-stable perovskite. The layer can comprise inert, porous material as a support for the redox-stable perovskite. The inert, porous material can be an oxide ceramic.

The oxide-ceramic material can be designed as described above. The redox-stable perovskite can be designed as described above.

The component with the plurality of channels comprises on average in one embodiment at least 40 channels per square centimeter, preferably at least 60 channels per square centimeter, particularly preferably at least 80 channels per square centimeter, in order to achieve good efficiencies. In one embodiment, the component with the multiplicity of channels comprises on average no more than 150 channels per square centimeter, preferably no more than 120 channels per square centimeter, particularly preferably no more than 100 channels per Square centimeters in order to achieve good efficiencies without having to undertake an excessive technical effort.

The component with the multiplicity of channels can be cylindrical. The diameter of the cylinder can be at least 50 cm, preferably at least 10 cm. The diameter of the cylinder can be less than 30 cm, preferably less than 20 cm. The length of the cylinder can be at least 5 cm, preferably at least 10 cm. The length of the cylinder can be less than 30 cm, preferably less than 20 cm.

Another aspect of the invention relates to selected new perovskites of the general formulas (I), (II), (1-1), (11-1) or (II-2)

Lai. _x Sr _x Fei- _x B _c 0 ₃ Lai-xSrxFei.xßB'xßOs

(1-1) (11-1)

Lai_ _x Sr _x Fei_x / 2Nbx 203

(II-2)

wherein

A is selected from the group of lanthanoids, yttrium and

Scandium or mixtures thereof, preferably with the meaning of lanthanum (La);

A ‘is selected from the group of the alkaline earth metals and alkali metals or mixtures thereof, preferably with the meaning of strontium (Sr);

B is Fe;

B ‘is selected from the group comprising Nb, Ta, Hf and Zr or

Mixtures thereof, preferably meaning niobium (Nb); and x represents a value in the range from 0.01 to 0.99, preferably in the range from 0.5 to 0.9; or

wherein the perovskite corresponds to the formula (11-1-1)

La _0.5 Sro _{, 5} Feo _{, 7} sNbo _{, 25} 0 ₃

(11-1-1)

The invention is illustrated in more detail by the following examples, without being restricted to the embodiments described therein. EXAMPLES

A) Production of oxide ceramic materials according to the invention with a perovskite structure

An oxide ceramic material according to the invention with a perovskite structure according to the present invention (“infiltration composite”) for the storage of nitrogen oxides from exhaust gases on the basis of B-place substituted lanthanum strontium ferrites (NOx storage) can easily by impregnating large areas of aluminum oxide with pores a pore volume of 1.5 ml / g Al ₂ O _3, for example of the trademark Puralox L4 TH 100/150 from Sasol (Hamburg), using aqueous metal nitrate solutions of corresponding stoichiometric composition. In order, after heat treatment in air at temperatures above 700 ° C., a weight fraction of 20% of the NOx storage-active materials according to the following example 1 Lao ₅ Sro _{, 5} Feo _, 5Tio _, 50 ₃ (SPFT 0.5) and example 2 Lao.sSro To produce .sFeo sNbo ^ sOs (LSFN 0.25), concentrated coating solutions are used to which citric acid is added as a complexing agent to increase the solubility of the metal ions.

1st example: oxide ceramic material with 20 wt .-% Lao.sSro.sFeo.sTio.sOa

(SPFT 0.5) in Al ₂ 0 ₃ as a carrier material

For the production of 100 g of an oxide ceramic material (infiltration composite) with a weight proportion of 20% SPFT 0.5 on Puralox L4 TH 100/150, 20.315 g of lanthanum nitrate hexahydrate, 18.948 g of iron (III) nitrate nonahydrate,

9.928 g of strontium nitrate and 27.033 g of citric acid monohydrate dissolved in 80 ml of water with gentle heating, a dark red solution being formed. 44.915 g of titanyl citrate solution with a titanium content of 5% by weight were then added dropwise to this reaction mixture, a clear, light red solution being formed which was made up to 120 ml with water.

To prepare 100 g of the required titanyl citrate solution with a titanium content of 5% by weight, a solution of 21.94 g of citric acid monohydrate in 40 ml of water was placed in a round bottom flask with a reflux condenser and 29.68 g of titanium isopropoxide were added dropwise at room temperature . {Molecular ratio of citric acid to titanium isopropoxide 1: 1) A whitish, milky, viscous suspension of the intermediate insoluble titanium dioxide formed by the hydrolysis of the titanium compound was initially formed. The reaction mixture was then heated to boiling temperature and refluxed for 16 hours, a clear yellowish titanyl citrate solution which was made up to 100 g with water. The thus obtained metal precursor coating solution was treated portionwise with 80 g of alumina (Puralox L4 TH 100/150) mixed _"wherein a wet bright red free-flowing powder. This was dried within 6 hours in a drying cabinet at 130 ° C. until the solvent was completely removed.

To decompose the metal complexes and to form the perovskite structure on the aluminum oxide surface, a heat treatment was then carried out by slowly heating the coated / impregnated powder in a muffle furnace to 700 ° C at a heating rate of approx. 1 K / min under air with an aging time for this Temperature of 3 hours.

2nd example: oxide ceramic material with 20% by weight La _0.5 Sr _{0 5} Feo _{, 75} Nb ₀₂₅ 0 ₃

(LSFN 0.25) in Al ₂ 0 ₃ as a carrier material

For the production of 100 g of an oxide ceramic material (infiltration composite) with a weight proportion of 20% LSFN 0.25 on Puralox L4 TH 100/150, 19.456 g of lanthanum nitrate hexahydrate, 27.674 g of iron (III) nitrate nonahydrate, 9.49 g strontium nitrate and 28.181 g citric acid monohydrate dissolved in 60 ml water with gentle warming. 41.132 g of niobium peroxo citrate solution with a niobium content of 5% by weight were then added dropwise to this batch, a clear, light red solution being formed, which was made up to 120 ml with water.

To produce 100 g of niobium peroxo citrate solution with a niobium content by weight of 5%, 17.141 g of citric acid monohydrate were first dissolved in 61.6 g of H ₂ O ₂ solution (w = 30%) at room temperature with stirring. 27.114 g of ammonium niobate oxalate hydrate (H ₄ NbO (C ₂ 0 ₄ ) ₂ ^* x H ₂ O) were added in portions to this solution (molar ratio of citric acid to ammonium niobate oxalate 1: 1). For reactive decomposition of the oxalate, this light yellow suspension was then heated to 60 ° C. until the evolution of gas began and stirred at this temperature for 16 h. The colorless solution thus formed was then made up to 100 g with water.

Analogously to the 1st example, 80 g of the aluminum oxide were then coated / impregnated with this metal precursor coating solution and the resulting moist, free-flowing light red powder was dried at 130 ° C. for 6 hours.

To form the perovskite structure on the aluminum oxide surface, a heat treatment was then carried out by slowly heating in a muffle furnace to 800 ° C. at a heating rate of approx. 1 K / min under air and with an aging time at this temperature of 3 hours. 3rd example: platinum-coated oxide ceramic materials

LSFT 0.5 and LSFN 0.25

To further increase the NOx storage capacity, the oxide ceramic materials of Examples 1 and 2 can additionally be coated with platinum or used in the form of a physical mixture with platinum. Such an additional platinum coating can also take place by means of pore impregnation. With an additional platinum coating, the oxidation of the NO primarily present in the exhaust gas into storable NO ₂ can be covered directly on these coating materials.

For the production of 100 g of platinum-coated LSFT 0.5 with a platinum content of 2.5% by weight, 97.5 g of the oxide ceramic material according to Example 1 with a determined pore volume of 1.1 ml / g were added in portions with a coating solution from 4, 96 g of tetrammine platinum nitrate Rί (NH ₃ ) ₄ ] (N0 ₃ ) ₂ are mixed in 107 ml of water and then dried in a drying cabinet at 130 ° C. for 6 hours.

In order to decompose the platinum precursor with the formation of platinum nanoparticles on the surface of the oxide-ceramic material according to Example 1, a heat treatment was then carried out by heating the coated powder in a muffle furnace to 550 ° C. at a heating rate of approx. 3 K / min under air with an aging time at this temperature for 2 hours.

Analogously, 100 g of platinum-coated LSFN 0.25 with a platinum content of 2.5% by weight was produced using the oxide-ceramic material according to Example 2, which, however, was mixed with a solution of 4.96 g of tetrammine platinum nitrate in 98 ml of water. since an absorption capacity of only 1 ml / g was determined for this compound.

B) NOx storage capacity

To determine the temperature-dependent NOx storage capacity of the oxide-ceramic materials according to the invention with a perovskite structure, the platinum-coated composite powders (comprising a platinum coating) produced in Example 3 were first ground in a ball mill to a grain size between 2 and 5 μm, then granulated using an extrusion process and sieved each fraction separated with a grain size of 1.5 to 2.5 mm. In a laboratory gas test stand, a gas mixture consisting of 500 ppm nitrogen monoxide, 500 ppm carbon dioxide, 200 ppm propylene, 6.5% oxygen, 9% water, 9% carbon dioxide (remainder nitrogen) was then passed through these granulates at a space velocity of 80,000 h ¹ the respective absorption capacity of nitrogen oxide is determined in the temperature range from 200 to 500 ° C. The result is shown in Figures 1 and 2

FIG. 1 shows a device with a NOx storage device

FIG. 2 shows the temperature profile of the specific NOx storage capacity obtained for the platinized oxide ceramic material LSFT 0.5 / Al ₂ O ₃ with a maximum absorption capacity between 250 and 300 ° C.

FIG. 3 shows the storage behavior of the platinum-coated oxide-ceramic material LSFN 0.25 / Al ₂ O ₃ , with a maximum storage capacity in the medium temperature range between 300 and 350 ° C.

FIG. 4 shows the storage behavior of a physical one

Mixture of an oxide ceramic material LSFT 0.5 / LI O with

Pt / Al ₂ 0 ₃ , with a maximum storage capacity in the medium temperature range between 250 and 350 ° C.

Explanation of the reference symbols

(1) Exhaust gas generating device (e.g. verb ren u ng engine)

(2) NOx storage device

(3) gas inlet side

(4) gas outlet side

(5) Gas supply (e.g. in the form of a pipe)

(6) one or more channels

(7) layer (of coated channels (6))

FIG. 1 shows an exemplary embodiment with an internal combustion engine (1) as the exhaust gas generating device (1). The device comprises a NOx storage device (2) with a gas inlet side (3) and an opposite gas outlet side (4). The device comprises a pipe as a gas supply (5) through which exhaust gases from the exhaust gas generating device (1) are conducted to the gas inlet (3) of the exhaust gas generating device (2). As shown in FIG. 1, the pipe (5) can widen like a funnel at one end in the direction of gas flow in order to keep the installation space required for the pipe (5) small. The tube (5) consists of a heat-resistant material such as steel, in particular stainless steel. The internal combustion engine (1) is also a heat generating device, which heats the NOx storage device (2) can be and that to temperatures of 200 ° C to 500 ° C through the hot exhaust gases. The storage device (2) comprises a multiplicity of channels (6). The channels (6) run parallel to one another. Redox-stable perovskite is located in the channels (6) as a component of layers (7) that are located on the channel walls. The layers (7) are shown hatched in FIG. The storage device (2) consists of a ceramic and is preferably made in one piece. The storage device (2) can be accommodated in a housing made of a heat-resistant material, for example steel, in particular stainless steel. During operation, hot exhaust gas flows from the internal combustion engine (1) through the pipe (5) and then further through the plurality of ducts (6). NOx from the exhaust gases is thereby stored in the layers (7).

Each layer (7) on an inner wall of a channel (6) comprises the redox-stable perovskite and, above it, porous material as a carrier for the perovskite. Each layer (7) can thus consist of a mixture of redox-stable perovskite and porous oxide-ceramic material.

The storage device (2) can be circular in section. The storage device (2) can, however, also be rectangular or square in section, for example. The storage device (2) comprises on average at least 40 channels 6 per square centimeter and not more than 120 channels 6 per square centimeter, particularly preferably not more than 100 channels per square centimeter, in order to achieve good efficiencies without incurring excessive technical effort have to.

On the outlet side (4) there can be another pipe through which the gas emerging from the storage device (2) can be passed on.

As shown in FIG. 1, this tube can also advantageously have a funnel-shaped end which widens towards the storage device (2) and adjoins this storage device.

Claims

Expectations

1. Use of a perovskite of the general formula (I) or (II)

A _IX A ' _X B _{I .X} B' _x 0 ₃

(i)

A-i-xA'xBi-xßB'xßOa

(II)

wherein

A is selected from the group of lanthanoids, yttrium and

Scandium or mixtures thereof;

A ‘is selected from the group of alkaline earth metals and alkali metals or mixtures thereof;

B is Fe;

B ‘is selected from the group comprising Ti, Nb, Ta, Hf and Zr or

Mixtures thereof;

x represents a value in the range from 0.01 to 0.99

as NOx storage.

2, use according to claim 1, wherein in the formula (I) and / or (II) des

Perovskites

A has the meaning of lanthanum (La) and / or

A ‘has the meaning of strontium (Sr) and / or

B ‘has the meaning of titanium (Ti) or niobium (Nb)

3. Use according to one of claims 1 or 2, wherein the perovskite corresponds to one of the general formulas (1-1), (I-2), (11-1) or (II-2)

Lai- _x Sr _x Fe ^ B _x Oa

(1-1)

Lai-xSixFe ^ xTixOs

(1-2)

Lai _x Sr _x Fei_ _x 2B ^! _x / 203

(II-2) wherein B 'and x have the meaning according to one of claims 1 or 2.

4. Use according to one of claims 1 to 3, wherein in the formulas (1-1), (I-2), (11-1) and / or (II-2) of the perovskite x has a value in the range of 0, Represents 5 to 0.9.

5. Use according to any one of claims 1 to 4, wherein the perovskite corresponds to one of the formula (1-1-1) or (11-1-1)

6. Oxide ceramic material with perovskite structure comprising a perovskite as defined in one of claims 1 to 5 and an inert, porous carrier material which is selected from the group comprising zireonium oxide, aluminum oxide (Al3O3), magnesium oxide, silicon oxide and mixed materials thereof.

7. oxide ceramic material with perovskite structure according to claim 6, which also has a coating with a metal selected from the group of noble metals comprising platinum (Pt), rhodium (Rh), palladium (Pd) and ruthenium (Ru), preferably with platinum , or which is in the form of a physical mixture with a metal selected from the group of noble metals comprising platinum (Pt), rhodium (Rh), palladium (Pd) and ruthenium (Ru), preferably with platinum.

8. NOx storage or catalyst comprising a perovskite as in one of the

Claims 1 to 5 defined or an oxide ceramic material with a perovskite structure according to claim 6 or 7, preferably an LNT / SCR hybrid catalyst.

9. Use according to any one of claims 1 to 5 or use of the

oxide ceramic material with perovskite structure according to claim 6 or 7 or the NOx storage or catalyst according to claim 8 in

Internal combustion engines / engines, industrial incineration plants, chemical processing plants, gas turbines or medical devices used to store NOx from exhaust gases

10. A method for producing an oxide-ceramic material with a perovskite structure according to one of claims 6 or 7, comprising the steps of a) producing a solution of the metal compounds to form the perovskite structure, optionally with the addition of a complexing agent;

b) Mixing the solution according to step a) with an oxide ceramic

Carrier material;

c) removing the solvent and drying the mixture from step b); d) heat treatment or calcination of the dried mixture from step c) to obtain the oxide-ceramic material with a perovskite structure as defined in one of claims 6 or 7;

e) optional coating of the oxide-ceramic material with perovskite structure from step d) with a noble metal selected from the group comprising platinum (Pt), rhodium (Rh), palladium (Pd) and ruthenium (Ru) or

Production of a physical mixture with a noble metal selected from the group comprising platinum (Pt), rhodium (Rh), palladium (Pd) and ruthenium (Ru).

11. Device comprising a NOx storage device (2) with a

Gas inlet side (3) and a gas outlet side (4), wherein the NOx storage device (2) comprises one or more channels (6) through which a gas can flow from the gas inlet side (3) to the gas outlet side (4), characterized in that, that in the one or more channels (6) there is a redox-stable perovskite as defined in one of claims 1 to 5.

12. Device according to the preceding claim, characterized in that the device has an exhaust gas generating device (1), in particular one

Internal combustion engine (1) for a motor vehicle, and a gas supply (5) through which exhaust gases from the exhaust gas generating device (1) can be conducted to the gas inlet (3) of the exhaust gas generating device (1).

13. Device according to one of the two preceding claims, characterized in that the NOx storage device (2) comprises a one-piece component made of ceramic which comprises a plurality of channels (6) running parallel to one another with a square diameter, the channels ( 6) are coated, one layer (7) comprising the redox-stable perovskite as defined in one of claims 1 to 5 or consisting of the redox-stable perovskite as defined in one of claims 1 to 5.

14. Device according to one of the three preceding claims, characterized

characterized in that the redox-stable perovskite is located on a porous support structure as defined in one of claims 1 to 5.

15. Perovskite of the general formula (I), (II), (1-1), (11-1) or (II-2)

AI- _X A ' _X BI- _X B' _x 03

(i)

Al - _x A ' _x Bi -x / 2 B'X / 203

(II)

Lai. _x Sr _x Fei. _x B ' _x 0 ₃

(1-1)

Lai-xSrxFei-x _ß B'x _fi Os

(H-2)

wherein

A is selected from the group of lanthanoids, yttrium and

Scandium or mixtures thereof, preferably with the meaning of lanthanum (La);

B is Fe;

B ‘is selected from the group comprising Nb, Ta, Hf and Zr or

Mixtures thereof, preferably meaning niobium (Nb); and x represents a value in the range from 0.01 to 0.99, preferably in the range from 0.5 to 0.9; or wherein the perovskite corresponds to the formula (11-1-1)

Lao, 5Sr ₀₅ Feo, 75Nbo, 2503

(11-1-1).