ES2379144B2 - PEROVSKITAS GIVEN WITH COPPER FOR THE STORAGE AND REDUCTION OF NOx. - Google Patents

Abstract

Perovskites doped with copper for storage and reduction of NOx. # The present invention relates to a new catalyst free of noble metals, active and stable, for the storage and reduction of nitrogen oxides (NOx) and its method of preparation. The catalyst, consisting of a mixed oxide doped with copper and with a perovskite structure, can be part of the post-treatment system to purify oxygen-rich gas streams, such as the exhaust gases of vehicles with diesel-type engines.

Description

Perovskites doped with copper for storage and reduction of NOx.

Field of the Invention

The present invention relates to a new catalyst free of noble metals, active and stable, for the storage and reduction of nitrogen oxides (NOx) and their method of preparation. The catalyst, consisting of a mixed oxide doped with copper and with a perovskite structure, can be part of the post-treatment system to purify oxygen-rich gas streams, such as the exhaust gases of diesel-powered vehicles.

Background of the invention

Diesel engines have several advantages over gasoline, for example, their greater durability, less maintenance and better use of fuel, which results in a lower emission of greenhouse gases. The operation of a diesel engine is different from that of gasoline and, as a consequence, the requirements of the post-combustion system to be used to purify the exhaust gases are also different. The mode of operation of diesel engines, on the one hand, leads to the formation of greater amounts of carbon and, on the other, makes it difficult to reduce nitrogen oxides (NOx) in a purely oxidizing atmosphere, preventing the use of catalysts. Three way used as post-combustion system in gasoline engines.

In order to comply with the NOx emission standards under conditions of excess oxygen, various processes have been proposed, among which the NOx storage and reduction traps (systems called NSRC) stand out as described in the patent application US 2009/0241522. NOx storage and reduction traps work under periodic operating conditions. In a first stage, which occurs during the usual conduction period (conditions rich in oxygen in the exhaust), the nitrogen oxides are retained in the catalyst in the form of nitrates. When the adsorbent material is saturated, the stored NOx are desorbed and reduced by periodically changing the composition of the exhaust gases to a clearly reducing atmosphere for a short period of time [Twigg MV. Appl. Catal. B 70 (2007) 2], This is achieved by adding a reducing gas to the exhaust, such as carbon monoxide, ammonia, hydrocarbons, hydrogen or a precursor compound of these gases.

The components used in the formulation of NSRC type catalysts include a basic oxide, typically an alkaline earth metal oxide, (Ba, Sr, Ca or Mg) or alkaline (Cs, Rb, K, Na and Li) and a metal noble. Both components are dispersed in a carrier material of high speci fi c surface. These supports can be oxides such as Al2O3, ZrO2, CeO2, MgO or mixed oxides such as MgO-CeO2, MgO-Al2O3. One of the most used formulations as a catalyst in NSRC systems is the one composed of the Pt-BaO / Al2O3 system [Roy S et al. Chem. Rev. 109 (2009) 4054], Although this formulation is one of the most efficient, it has certain problems, such as low thermal stability at high temperature and the formation of barium carbonate at temperatures above 450ºC in the presence of CO2 , which, being more stable than barium nitrate, decreases the NOx adsorption capacity of the catalytic system. In addition, barium oxide tends to sinter and form compounds with the materials that make up the support, which causes a loss of NOx storage capacity. On the other hand, the use of a noble metal such as platinum greatly increases the application of NSRC technology, so it is desirable to develop new materials capable of being used as NSRC systems that do not contain noble metals.

The use of mixed oxides with perovskite-like structure as NSRC catalysts has been scarcely presented in the literature.

Patent application WO90 / 08589 refers to the use of a perovskite type catalyst for the conversion of NO to N2 useful only in gasoline engines.

EP 0923990 refers to a perovskite type catalyst comprising Cobalt and Lantanium

Only some studies on perovskites have been carried out as NSRC catalysts, such as one in which the composition perovskite (La0.7Ba0.3) (Fe0.776Nb0.194Pd0.03) O3 [Ueda et al. Catal. Commun. 11 (2009) 34], Recently it has also been published that a Pd catalyst supported in perovskite La0.9Sr0.1MnO3 could be a substitute for Pt as an oxidation catalyst, although the ability to retain NOx [C.] is not indicated. Hwan Kim,

G. Qi, K. Dahlberg, W. Li, Science 327 (2010) 1624], In both cases, the proposed materials continue to incorporate a noble metal (Pd) in their formulation, and no information has been published that refers to employment of perovskites containing copper for application in NSRC type catalysts.

Therefore, there is a need to develop new materials that do not contain noble metals and that are active, effective and stable for the elimination of NOx in diesel engine exhaust streams using NSRC technology, in order to comply with legislation That is increasingly strict.

Explanation of the invention.

Thus, the present invention relates to the use of a catalytic system comprising a mixed oxide with perovskite type structure of general formula (I):

where:

A is an alkaline earth metal,

B is a tetravalent transition metal partially substituted by Cu,

in an NSRC system for the reduction of nitrogen oxides present in oxygen-rich gas streams.

In a more particular aspect of the present invention, A is SryBesTi partially substituted by Cu.

In another more particular aspect of the present invention, the Ti is partially replaced by Cu in a molar ratio between the Ti: Cu comprised between 0.9: 0.1 and 0.7: 0.3.

In another more particular aspect of the present invention, the catalyst has the formula:

In another aspect in particular, the NSRC system of the present invention comprises retention / storage and reduction of nitrogen oxides.

In another more particular aspect of the present invention, in the retention / storage phase the catalyst oxidizes NO in NO2 while retaining NO2 in the form of nitrates and in the reduction phase NOx and O2 are used as a reactive mixture and as a gas carrier the N2.

In a more particular aspect of the present invention, a gas having 500 ppm NOx + 5% Vol O2 + N2 is used in the storage phase.

In a more particular aspect of the present invention, a gas composed of 3% CO in N2 is used in the reduction phase.

In a more particular aspect of the present invention, a gas composed of 3% H2 in N2 is used in the reduction phase.

In another more particular aspect of the present invention, the gas is an effluent from the gases of a diesel engine.

Brief description of the fi gures

Figure 1: Shows the percentage of NOx elimination versus temperature in reaction conditions at programmed temperature (heating rate 5ºC / min) in tests carried out using a spatial speed of 21000 h − 1 and a gas mixture with 500 ppm NOx , 5% of O2 and N2 as carrier gas. A catalyst of composition SrTi0.89Cu0.11O3 has been used without any pretreatment and pre-reduced at 400 ° C with one with 30% vol of H2 in N2. For comparison, an equivalent catalyst without copper (SrTiO3) is included.

Figure 2: Shows the percentage of NOx removal under reaction conditions at constant temperature (300 ° C) in tests performed using the SrTi0.89Cu0.1113 catalyst, a spatial velocity of 21000 h-1 and a mixture of reaction gases containing 500 ppm NOx, 5% O2 and N2 as carrier gas. The catalyst has been pretreated with hydrogen at 400 ° C.

Figure 3: Shows the X-ray diffractograms of the SrTi0,89Cu0,11O3 and SrTiO3 catalysts.

Figure 4: Shows the relationship between NOx concentrations at the outlet and at the reactor inlet with respect to time in cyclic experiments. Under oxygen-rich conditions the gas is composed of: 500 ppm of NOx and 5% of O2 dissolved in N2. Under reducing conditions the gas is composed of 3% vol CO in N2 or 3% vol H2 in N2. Laboratory tests carried out at (a) 300ºC and (b) 350ºC. Catalyst: SrTi0,89Cu0,11O3 pretreated with hydrogen at 400 ° C.

Detailed description of the invention

The invention consists of a new active and stable catalyst, useful for use in the storage and reduction of nitrogen oxides in oxygen-rich gas streams, such as diesel engine exhaust. The catalyst, which does not incorporate any noble metal in its composition, consists of a mixed oxide with perovskite (ABO3) type structure doped with copper. Position A of the perovskite-like structure can be filled with an alkaline earth metal (Ba, Sr, Ca, Mg) and position B with a tetravalent transition metal, such as Ti. In a particular formulation strontium (Sr) has been used in position A of the structure and titanium (Ti) as a transition metal located in position B, which is partially replaced by copper (Cu).

This catalyst is capable of operating under the typical conditions of an NSRC system, where the composition of the gas to be treated is periodically alternated between rich and poor conditions in reducing gas. In poor operating conditions, this catalyst is capable of oxidizing NO from the exhaust stream in NO2 and retaining these nitrogen oxides in the form of nitrates. Under poor operating conditions, nitrogen oxides stored in the catalyst are reduced. The oxidation capacity of NO to NO2 and reduction of NOx is attributed to the structural modification caused by the partial substitution of titanium atoms with copper and the storage capacity of nitrogen oxides to the presence in the structure of a ground alkaline oxide like strontium

The catalyst of the invention is useful for the storage of nitrogen oxides in oxygen-rich gas streams, such as the leaks of diesel-powered vehicles (for example with 500 ppm NOx, 5% O2 in N2 as carrier gas See examples 3 and 4 and Figure 1). The inventors observed that the NOx retention process begins at a temperature of 150 ° C and reaches a maximum NOx removal in a temperature range between 280 and 340 ° C (Figure 1). At temperatures above 380 ° C, the NOx profile in Figure 1 showed negative values, indicating that NOx stored at low temperatures were emitted at temperatures above this value. It was also observed in Figure 1 that when the B position of the perovskite was not partially replaced by copper (SrTiO3 catalyst) the adsorption capacity of the perovskite was very low. The NOx adsorption capacity of the catalyst system of the present invention was significantly increased by introducing copper and reducing the catalyst (see Figure 1 and Examples 3 and 4). As seen in Figure 1, the adsorption process was extended up to 450 ° C with the reduced material.

The NSRC process with the catalyst of the invention was carried out in an isothermal manner with a mixture of effluent gas of similar composition to that of Example 5. In this sense, the reaction temperature was kept constant at 300 ° C (see Example 5 and Figure 2). Figure 2 shows the removal of NOx from the gas stream against time at 300 ° C. The NOx removal profile decreased slowly over a period of 25 minutes and fell rapidly when the sample was saturated. The amount of NOx stored in the catalyst SrTi0,89Cu0,11O3 per mass unit was comparable to the published values for catalysts containing noble metals (see example 5 and Table 1 of example 5). However, the developed catalyst showed a greater retention capacity of NOx stored per unit of surface area (41 μmol / m2) than the other systems based on conventional compositions incorporating noble metals.

Example 6 shows the behavior of the SrTi0,89Cu0,11O3 catalyst in two consecutive cycles of storage and reduction of NOx under isothermal conditions (see Figure 4). As an example, two reducing gases (Hydrogen (H2) and carbon monoxide (CO)) were used for periods with poor oxygen conditions, and two temperatures (300 and 350ºC).

The ability to store NOx of the SrTi0.89Cu0.1113 catalyst (example 6, Figure 4) was greater at 300 ° C than at 350 ° C and all NOx removal profiles showed the same shape. On the contrary, catalyst regeneration was more effective at 350 ° C than at 300 ° C. It was also established that the catalytic system of the invention was stable under the conditions evaluated, retaining the same storage capacity of nitrogen oxides after regeneration.

Examples

Example 1

Preparation of the catalysts SriTiO3 and SrTi0,89Cu0,11O3

A catalyst system was prepared by means of the sol-gel method using a titanium (IV) citrateperoxide precursor prepared from titanium (IV) isopropoxide, isopropyl alcohol, citric acid and hydrogen peroxide. As precursors of strontium and copper metals, Sr (NO3) 2 and Cu (NO3) 2.3H2O, respectively, were used. Gel formation was performed at a temperature of 90 ° C for a period of 4 hours. The sample was dried at 110 ° C and calcined in air at 850 ° C for 6 hours (heating rate 5 ° C / min). The molar concentration of titanium citrateperoxide (IV) was 0.4 M and the molar ratio of citric acid: Ti4 + and hydrogen peroxide: Ti4 + was 2.7: 1 and

1.2: 1 +, respectively. The final material has a composition of SrTi0,89Cu0,11O3. As a reference material, a perovskite of SriTiO3 composition was prepared with the same procedure but without adding the copper nitrate precursor.

Example 2

Characterization of the formulation catalysts SrTi0,89Cu0,11O3 and SriTiO3 prepared as described in example 1

After the preparation of the SrTi0.89Cu0.11O3 and SriTiO3 catalysts (example 1), these were characterized by X-ray diffraction and N2 adsorption at -196 ° C. Figure 3 presents the X-ray diffractograms, where the presence of a perovsquita-like phase is evidenced. The BET surface area calculated by adsorption of N2 at -196 ° C for the SrTi0.89Cu0.1113 catalyst was 6 m2 / gryde11m2 / gr for the SriTiO3 copper-free catalyst.

Example 3

NOx storage assays with SrTi0,89Cu0,11O3 and SriTiO3 formulation catalysts in programmed temperature experiments

NOx storage tests were carried out at atmospheric pressure, using a catalytic bed composed of 100 mg of SrTi0.89Cu0,11O3 or SriTiO3 catalysts (prepared in Example 1). The tests used a total gas flow rate of 500 ml / min (spatial speed 21000 h − 1) and a reactive mixture composed of the following gases: 500 ppm NOx and 5% O2, using N2 as carrier gas. Reactions were carried out at program temperature by heating at 5 ° C / min in a temperature range between 25 ° C and 550 ° C.

As can be seen in Figure 1, the catalyst SrTi0.89Cu0.11O3 is active for the adsorption of NOx in a gas stream composed of NOx and O2 in a range between 280 and 340 ° C. Adsorbed species are desorbed after reaching a temperature of 380 ° C. Considerable activity is only achieved when partial replacement of titanium with copper is performed.

Example 4

NOx storage assays with a SrTi0,89Cu0,11O3 formulation catalyst pretreated with hydrogen in programmed temperature experiments

The SrTi0.89Cu0.11O3 catalyst prepared as described in Example 1 was used for the storage of NOx, with hydrogen pretreatment to the catalyst at a temperature of 400 ° C under experimental conditions similar to those described in Example 3. The Gas stream used for treatment is composed of 30% vol of H2 in N2 as carrier gas. The NOx adsorption capacity of the catalyst of the present invention increases significantly by reducing it with hydrogen.

Example 5

NOx storage tests with a SrTi0.89Cu0.1113 formulation catalyst in isothermal conditions at 300 ° C

The ability to retain NOx of the SrTi0.89Cu0.11O3 catalyst has been determined in experiments performed under isothermal conditions at 300 ° C, using gas streams and catalyst amounts similar to those used in these examples3 and 4.

Table 1 shows the results of the amount NOx stored in the catalyst SrTi0.89Cu0.11O3 of the present invention, with respect to the mass of catalyst and per unit area BET. The results published in the literature corresponding to catalysts containing noble metals tested under similar conditions are included by comparison.

TABLE 1

Example 6

NOx storage and reduction tests with a SrTi0,89Cu0,11O3 formulation catalyst under isothermal conditions

The catalyst SrTi0.89Cu0.11O3, prepared as described in example 1, was used in cyclic NOx storage and reduction experiments. The catalytic system is evaluated in two consecutive cycles of storage and reduction. For storage under oxygen-rich conditions, a gas consisting of: 500 ppm NOx and 5% O2 in N2 was used. For the reduction, a gas composed of 3% CO in N2 or 3% H2 in N2 was used. A spatial velocity of 21000 h − 1 was used.

In a particular example, the storage and reduction of NOx was carried out at a constant temperature of 300 ° C (Figure 4 (a)). In another example it was carried out in an isothermal regime at 350 ° C (Figure 4 (b)).

Claims (11)

1. Use of a catalytic system comprising a mixed oxide with perovskite type structure of general formula (I): where: A is an alkaline earth metal, B is a tetravalent transition metal partially substituted by Cu, in an NSRC system for the reduction of nitrogen oxides present in oxygen-rich gas streams. 2. Use of a catalytic system according to claim 1, wherein A is SryBesTi partially substituted by Cu.

3.

Use of a catalytic system according to any of claims 1-2, wherein the Ti is partially replaced by Cu in a molar ratio between the Ti: Cu comprised between 0.9: 0.1 and 0.7: 0.3.

Four.

Use of a catalytic system according to any of the preceding claims wherein the catalyst has the formula:

5.

Use of a catalytic system according to any of the preceding claims wherein the NSRC system comprises retention / storage and reduction of nitrogen oxides.

6.

Use of a catalytic system according to any of the preceding claims, wherein in the retention / storage phase the catalyst oxidizes NO in NO2 while retaining NO2 in the form of nitrates and in the reduction phase NOx and O2 are used as the reaction mixture and as carrier gas the N2.

7.

Use of a catalytic system according to any of the preceding claims wherein in the storage phase a gas having 500 ppm NOx + 5% Vol O2 + N2 is used.

8.

Use of a catalytic system according to any of the preceding claims wherein in the reduction phase a gas composed of 3% CO in N2 is used.

9.

Use of a catalytic system according to any of the preceding claims wherein in the reduction phase a gas composed of 3% H2 in N2 is used.

10.

Use of a catalytic system according to any of the preceding claims, wherein the gas is an effluent from the gases of a diesel engine.

SPANISH OFFICE OF THE PATENTS AND BRAND Application no .: 201001234 SPAIN Date of submission of the application: 27.09.2010 Priority Date: REPORT ON THE STATE OF THE TECHNIQUE 51 Int. Cl.: B01J21 / 06 (2006.01) B01J23 / 78 (2006.01) RELEVANT DOCUMENTS

Category

Documents cited Claims Affected

TO

LOPEZ-SUAREZ, F.E., et al., Role of surface and lattice copper species in copper-containing (Mg / Sr) TiO3 perovskite catalysts for soot combustion, Applied Catalysis B: Environmental, 2009, vol. 93, p. 82-89. Summary; sections: “2. Experimental ”,“ 3.5. Blank experiments and in situ DRIFTS ”and“ 4. Conclusions. " 1-10

TO

WANG, Q., et al., NOx storage and reduction over Cu / K2Ti2O5 in a wide temperature range: Activity, characterization and mechanism, Applied Catalysis A: General, 2009, vol. 358, p. 59-64. Summary; sections: “1. Introduction ”,“ 2. Experimental ”,“ 3.2. Storage and reduction over Cu / K2Ti2O5 ”,” 3.3. Storage and reduction mechanism ”and“ 4. Conclusions. " 1-10

TO

LI, Z., et al., Fe-substituted nanometric La0.9K0.1CoFexO3-delta perovskite catalysts used for soot combustion, NOx storage and simultaneous catalytic removal of soot and NOx ”, Chemical Engineering Journal, 2010, Vol. 164, pp. . 98-105. Summary. 1-10

Category of the documents cited X: of particular relevance Y: of particular relevance combined with other / s of the same category A: reflects the state of the art O: refers to unwritten disclosure P: published between the priority date and the date of priority submission of the application E: previous document, but published after the date of submission of the application

This report has been prepared • for all claims • for claims no:

Date of realization of the report 10.08.2011

Examiner M. García Poza Page 1/4

REPORT OF THE STATE OF THE TECHNIQUE Application number: 201001234 Minimum documentation sought (classification system followed by classification symbols) B01J Electronic databases consulted during the search (name of the database and, if possible, terms of search used) INVENES, EPODOC, WPI, XPESP, NPL, HCAPLUS State of the Art Report Page 2/4  WRITTEN OPINION Application number: 201001234 Date of Written Opinion: 10.08.2011 Statement

Novelty (Art. 6.1 LP 11/1986)

Claims Claims 1-10 IF NOT

Inventive activity (Art. 8.1 LP11 / 1986)

Claims Claims 1-10 IF NOT

The application is considered to comply with the industrial application requirement. This requirement was evaluated during the formal and technical examination phase of the application (Article 31.2 Law 11/1986).  Opinion Base.- This opinion has been made on the basis of the patent application as published. State of the Art Report Page 3/4  WRITTEN OPINION Application number: 201001234 1. Documents considered.- The documents belonging to the state of the art taken into consideration for the realization of this opinion are listed below.

Document

Publication or Identification Number publication date

D01

LOPEZ-SUAREZ, F.E., et al., Role of surface and lattice copper species in copper-containing (Mg / Sr) TiO3 perovskite catalysts for soot combustion, Applied Catalysis B: Environmental, 2009, vol. 93, p. 82-89.

D02

WANG, Q., et al., NOx storage and reduction over Cu / K2Ti2O5 in a wide temperature range: Activity, characterization and mechanism, Applied Catalysis A: General, 2009, vol. 358, p. 59-64.

D03

LI, Z., et al., Fe-substituted nanometric La0.9K0.1CoFexO3-delta perovskite catalysts used for soot combustion, NOx storage and simultaneous catalytic removal of soot and NOx ”, Chemical Engineering Journal, 2010, Vol. 164, pp. . 98-105.

2. Statement motivated according to articles 29.6 and 29.7 of the Regulations for the execution of Law 11/1986, of March 20, on Patents on novelty and inventive activity; quotes and explanations in support of this statement The object of the invention is the use of a perovskite type catalyst for the storage and reduction of NOx. Document D01 discloses a catalytic system comprising a mixed oxide with perovskite type structure of formula ABO3, A being an alkaline earth metal and B a tetravalent transition metal partially substituted by Cu. Specifically discloses the catalyst SrTi0,89Cu0,11O3. This document discloses the use of this catalyst in soot combustion processes emitted by diesel engines (summary, “2.1.Sample preparation”). This document also discloses ("3.5. Blank experiments and in situ DRIFTS") that a sample of SrTiCuO3 subjected to a reactive gas mixture composed of NO + O2 stores NOx as a result of a chemisorption process. The mixture of NO + O2 is chemically absorbed by the surface of the sample, NO is oxidized and finally NO2 is released. That is, document D01 discloses the use of the catalyst SrTi0,89Cu0,11O3 for the storage of NOx but not for its reduction. Document D02 discloses a catalytic system comprising a mixed oxide with perovskite-like structure of formula ABO3, A being an alkali metal and B a tetravalent transition metal partially substituted by Cu, specifically disclosing the catalyst Cu / K2Ti2O5. This document discloses the use of this catalyst in NSCR systems for the reduction of nitrogen oxides present in oxygen-rich gas streams (Summary; sections: "1. Introduction", "2. Experimental", "3.2. Storage and reduction over Cu / K2Ti2O5 ”,“ 3.3. Storage and reduction mechanism ”and“ 4. Conclusions ”). Document D03 discloses a catalytic system comprising a mixed oxide with perovskite type structure, specifically discloses La0,9K0,1Co1-xFexO3-delta. This document discloses the use of this catalyst in NSCR systems for the reduction of nitrogen oxides present in oxygen-rich gas streams (Summary). In view of the cited documents it is concluded that the object of the invention, set forth in claims 1 to 10, that is, the use of a perovskite catalyst system of formula ABO3, with A being an alkaline earth metal and B a transition metal tetravalent partially substituted by Cu in an NSCR system for the reduction of nitrogen oxides present in oxygen-rich gas streams is new (Art. 6.1 LP). Nor would this use of the catalyst disclosed in D01 be apparent to the person skilled in the art from the information disclosed in the prior art. Therefore, the object of the invention included in said claims has inventive activity (Art. 8.1 LP). State of the Art Report Page 4/4