ES2356459B2 - MODIFIED CERTIFIED RHODIUM AND OXIDE CATALYTICS SYSTEM FOR THE DECOMPOSITION OF N2O IN N2 AND O2. - Google Patents

Abstract

Catalytic systems of rhodium and cerium oxide modified for the decomposition of N2O into N2 and O_ {2}.

The present invention describes a system catalytic comprising an active rhodium phase supported on a mixed oxide of cerium and one or more metals selected from groups of transition metals and internal transition for example Pr, La, Zr, Y, Th or Sm and a support, such as alumina. The invention also describes a procedure for obtaining the system catalytic, and its use for elimination and / or reduction of the N2O content of a gas comprising it, for example a effluent from an anthropogenic source such as industry or a diesel engine

Description

Catalytic systems of rhodium and cerium oxide modified for the decomposition of N2O into N2 and O_ {2}.

Field of the Invention

The present invention relates to new catalytic systems, efficient and stable, useful for nitrous oxide removal, by decomposition in oxygen and nitrogen, from effluent gases from industry, from combustion plants or vehicle emissions. This type of effluents are characterized by containing dilute nitrous oxide (typically 500-5000 ppm), relatively low temperature (<525ºC) and presence of inhibitory gases.

Background of the invention

Nitrous oxide (N2O) is a compound gaseous harmful to the environment, which contributes to the destruction of the ozone layer as well as global warming of the planet, being in this aspect much more harmful than CO2. Emissions of N 2 O to the atmosphere due to activities humans come mainly from the production plants of nitric and adipic acid (intermediate product in the manufacture of nylon polymers), biomass combustion and fuels of origin fossil, agricultural activities and vehicles. In the case of vehicles, the N_ {2} O is generated as a result of the measures of control introduced for the elimination of other contaminants, as the three-way catalysts used in vehicles with Gasoline engine to remove NO, CO and hydrocarbons. In this type of catalytic systems, the emission of N 2 O has been related with the aging of the catalyst.

The amount of N 2 O in the atmosphere, whose half-life is 150 years, increases in approximately 0.2% per year, so there is a need for develop methods for its elimination and / or controlled emission.

One of the proposed methods consists in the elimination of N 2 O by its decomposition in oxygen (O2) and molecular nitrogen (N2). The main disadvantage of this decomposition is that it requires temperatures higher than 625 ° C to occur spontaneously. However, it You can break down the molecule at a lower temperature using a suitable catalyst. Numerous have been described in the literature. studies on the mechanism of decomposition of N2O as well as various supported or unsupported catalysts useful for decrease the decomposition temperature.

Numerous catalysts have been studied to decompose N 2 O, including unsupported and supported metals, pure and mixed oxides and zeolites [ Kapteijn et al. Appl. Catal. B 9 (1996) 25 ], Among these catalysts are systems containing metals such as Pt, Pd, Ag, Au and Ge, active at temperatures above 375 ° C or mixed oxide catalysts containing a first Mn oxide, Fe , Co, Ni, Cu or Zn on a metal oxide support such as MgO, CaO, ZnO, TiO 2, MoO 3 -CoO-Al 2 O 3, ZnO-Al 2 O_ { 3} or TiO2 -MgO, among others. Other supported catalysts have also been developed, such as the nickel oxide and cobalt oxide catalyst on a zirconium substrate, the Fe catalyst supported on zirconia, or mixtures of Fe and a second metal, selected from Co, Ni, Rh, Pd, Ir, Pt, Mn, La and Ce, supported on zirconia. Rh catalysts supported on various substrates have also been described in the literature, such as cerium oxides or cerium zirconium oxides [ Imamura et al. J. Mol. Catal. A 139 (1999) 55 ], and more recently, studies have been published regarding aspects of Rh catalysts supported on cerium oxide modified with La or Pr [ Bueno et al. J. Catal. 244 (2006) 102 ] such as the effect of the calcination temperature of the modified cerium oxide support on the catalytic activity.

In general, the activity of most of these catalysts has been studied by laboratory tests, in which an artificial gas stream composed of N 2 O diluted in an inert gas is used and, occasionally, also introducing O 2 . However, it is usual that in real gas streams, the N2O is accompanied by other gases such as O2, NOx, and / or H2O, and many of the catalysts mentioned suffer a significant inhibition when used in the presence of these gases, which makes it impossible to implement them on a real scale [ Centi et al. ChemTech 29 (1999) 48, Kapteijn et al. Appl. Catal. B 9 (1996) 25 ].

Therefore, in view of the above, there is the need in the state of the art to provide a catalyst that is active, effective and stable under real conditions of industrial operation, capable of reducing the content of N_ {2} O of gaseous effluents from an emitting source real, like industry or a combustion process.

Brief description of the figures

Figure 1: Shows the conversion of N 2 O (in N 2 and O 2) at different temperatures in tests carried out in the laboratory using the catalytic system 0.2% Rh / 49.9% Ce_ {0. 9} Pr_ {0. 1} O_ {x} (x \ leq 2) /49.9%\gamma-Al_ {2} O_ {3}, a speed spacing of 30,000 h -1 and gas mixtures containing one or more than the following components: 1000 ppm N 2 O; 1, 2.2 or 5% O2; 1000 ppm NO; 2.4% H2O.

Figure 2: Shows the conversion of N 2 O (in N2 and O2 versus temperature (a) (speed of heating 2ºC / min up to 450ºC) and against time (b) (in isothermal conditions at 350ºC) in tests carried out in a plant Production of nitric acid with the catalytic system: 0.2% Rh / 49.9% Ce_ {0. 9} Pr_ {0. 1} O_ {x} (x \ leq 2) /49.9%\gamma-Al_ {2} O_ {3} to different space velocities The gas mixture contains: 1000 ppm N 2 O + 750 ppm NOx + 1% H 2 O + 2% O 2.

Figure 3: Shows the X-ray diffractograms of the catalytic system 0.2% Rh / 49.9% Ce_ {0. 9} Pr_ {0. 1} O_ {x} (x \ leq 2) /49.9%\gamma-Al_ {2} O_ {3} before and after N2O catalytic decomposition tests performed in a nitric acid production plant (example 3).

Figure 4: Shows the Raman spectra of the Catalytic system 0.2% Rh / 49.9% Ce_ {0. 9} Pro_ {0. 1} O_ {x} (x \ leq 2) /49.9%\gamma-Al_ {2} O_ {3} before and after N2O catalytic decomposition tests performed in a nitric acid production plant (example 3).

Figure 5: Shows the XPS spectra corresponding to the electronic transitions Rh 3d of the system Catalytic 0.2% Rh / 49.9% Ce_ {0. 9} Pro_ {0. 1} O_ {x} (x \ leq 2) /49.9%\gamma-Al_ {2} O_ {3} before and after catalytic decomposition tests of N2O made in a nitric acid production plant (example 3).

Description of the invention

The invention relates in one aspect to a new active and stable catalytic system, useful for use in any polluting source emitting N 2 {O}, even when this gas is in the presence of species such as O2, NOx and / or H2O. This new catalytic system, hereinafter system catalytic of the invention, it is capable of decomposing directly nitrous oxide (N 2 O) in O 2 and N 2, and comprises:

(i) An active phase comprising rhodium supported on a mixed oxide of cerium and one or more metals selected from the transition metal groups and internal transition, and

(ii) A support.

In a preferred embodiment cerium oxide it is modified with a metal selected from Pr, La, Zr, Y, Th, Sm, and mixtures thereof, more preferably praseodymium (Pr).

The amount of Rh present in the active phase it can vary between wide margins depending, among others factors, of the specific application to which the system is destined catalytic and the amount of catalytic system that is going to be used. However in a particular embodiment the amount of Rh is typically between 0.1 and 5% by weight of Rh with with respect to the total catalyst weight. In a preferred embodiment the amount is 0.2%, since Rh is a very expensive element and small amounts, for example around 0.2%, have proven to be sufficient to provide very effective catalytic systems, active and stable in the decomposition of N 2 O in various applications, such as in an acid production plant nitric (see Example 3).

The support that can be used in the present invention can be any conventional physical support stable under the conditions in which the system is used catalytic of the invention, that is, in an oxidizing atmosphere and at temperature equal to or less than 550 ° C. Between the stands include inorganic materials used usually as catalyst supports. Among these fits mention, by way of example, the pellets or particles of a inorganic material such as silica or alumina, or monoliths with honeycomb structure ("honeycomb"), for example of cordierite or silicon carbide, as well as their mixtures. In a particular embodiment the support are particles of γ-alumina, for example of size included between 0.4 and 0.71 mm.

The amount of active phase with respect to the amount of support may vary over a wide range and will depend of various factors, such as the type of support chosen or the application in which it will be used. In principle there is no limit clearly definable lower amount of active phase with Regarding the support. However, the lower the amount of active phase greater will be in general the amount of catalytic system necessary for a specific application. In one embodiment in particular the catalytic system contains an amount of 50% in active phase weight with respect to the total system weight catalytic.

In a particular embodiment the system Catalytic is 0.2% Rh / 49.9% Ce_ {0. 9} Pro_ {0. 1} O_ {x} (x \ leq 2) /49.9%\gamma-Al_ {2} O_ {3}.

The catalytic system of the invention is active, effective and stable in a stream of gases of real composition as the effluent from a nitric acid production plant (See Example 3 and the corresponding Figures 3, 4 and 5). The inventors have observed that the decomposition of N2O begins between 275 and 325 ° C, depending on the space velocity used, reaching complete decomposition between 380 and 425 ° C (Figure 2a). As seen in Figure 2b, in all conditions of used, the catalytic system maintains its activity constant over time, having accumulated up to a total of 40 non-consecutive hours of tests with the same system sample catalytic.

The stability of the catalytic system with respect to its ability to decompose N 2 O is consistent with the characterization of the physicochemical properties shown before and after catalytic tests (see example 4 and Figures 3-5). After 40 non-consecutive hours of catalytic tests in real conditions of use of the system catalytic in a nitric acid production plant, they have not observed changes in the following properties physicochemical:

?

Crystal structure of the oxide mixed Ce-Pr, studied by DRX and spectroscopy Raman

?

Catalyst surface area (BET), determined by absorption of N2 at -196 ° C.

?

Particle size of Rhodium, estimated by TEM microscopy.

?

Rhodium oxidation state, determined by XPS.

These results show that it is not observe changes in the structure and physical properties chemical system catalyst, which shows that the system Catalyst of the invention is stable.

In another aspect the invention relates to a procedure for the preparation of the catalytic system of the invention comprising the following steps:

(i)

impregnation of a support with a solution that comprises a cerium precursor salt and at least one precursor salt of a metal selected from among the groups of metals of transition and internal transition;

(ii)

calcination;

(iii)

impregnation of the mixed oxide of Ce and said metal supported obtained in the previous stage with a solution that comprises a precursor salt of rhodium and

(iv)

calcination.

In the preparation of this catalytic system no It is necessary to carry out the reduction of the noble metal.

In a particular embodiment the impregnation (i) of the support is carried out by the incipient moisture method with a solution of cerium precursor salt and salt or salts precursors of other metal / s are selected, in concentrations respective, which can be easily determined by an expert in matter, to achieve the desired molar relationship between elements. In a preferred embodiment the solution comprises Ce (NO 3) 3 • 6H 2 O and Pr (NO 3) 3 \ 6d2H and its respective concentrations are set to achieve a relationship molar Ce: Pr of 9: 1. However, such amounts may vary by wide margins depending on the amount of metal or metals that it is desired to present the mixed oxide obtained. The support can be any inorganic material, as described previously. In a particular embodiment the substrate is γ-alumina particles, for example of size 0.4-0.71 mm. The weight ratio between mixed oxide and support may vary between wide margins as has indicated above. In a particular embodiment the relationship by weight mixed oxide: γ-alumina is 1: 1.

After impregnation, the support impregnated obtained is dried at a temperature typically around 110 ° C and then calcined. The calcination (ii) can carried out in air and at a temperature suitable for decompose the salts of Ce and the selected metal used as precursors In a particular embodiment using Ce (NO 3) 3 • 6H 2 O and Pr (NO 3) 3 • 6 O, the temperature of calcination has been of 600ºC, which is reached with a speed heating at 10 ° C / min and extending for 90 minutes.

In a particular embodiment, step (iii) of Cement-mixed mixed oxide impregnation supported obtained in (ii) is carried out with an aqueous solution of rhodium nitrate and then the resulting solid is dried at a temperature of 110 ° C. The nitrate concentration of Rh of the aqueous solution may vary depending on the amount of Rh that it is desired to incorporate into the catalytic system.

The calcination (iv) is carried out at temperatures generally around 500ºC, which ensure the stability of the catalyst to said temperature.

As mentioned in the background, anthropogenic sources emitting polluting gases that contain N 2 O include among others the production plants chemistry, for example, nitric acid, adipic acid, caprolactam, acrylonitrile, glyoxal, or processes that use acid nitric as an oxidizing agent or involving oxidation with ammonia, or combustion processes of fossil fuels such as coal, or biomass or waste, or the cultivation of land or emissions of vehicles. Vehicle emissions include those of the engines of gasoline, usually due to the aging of three-way catalysts, and those of diesel engines in the case of using a NOx catalytic reduction system with hydrocarbons with a Pt catalyst.

The catalyst system of the present invention It is suitable for decomposing the N 2 O of the effluents of these sources by decomposing into O2 and N2. So in another aspect the invention relates to the use of the system catalyst of the invention for the reduction and / or elimination of contained in N_ {2} O of a gas that contains it. In one embodiment particular said gas also contains other species such as NO_ {x}, H2O, O2, N2 or mixtures thereof. In other particular embodiment said gas has the following composition: 1000 ppm N 2 O + 750 ppm NO x + 1% H 2 O + 2% O 2 + N_ {2}.

A gas presenting these compositions can be typically an effluent, meaning effluent any of the gases from the aforementioned anthropogenic sources. Thus in another particular embodiment the catalyst system is used. for the reduction and / or elimination of the content in N 2 of a effluent, preferably an effluent from a plant production of nitric acid or a diesel engine. In particular the effluent from a nitric acid production plant comes from the direct extraction of the gas stream from the plant after the expansion turbine and presents the following composition: 1000 ppm N 2 O + 750 ppm NO x + 1% H 2 O + 2% O2 + N2.

In a further aspect, the invention will be It therefore relates to a heterogeneous catalysis procedure in which the catalyst system of the invention is contacted with a gas containing N2O. In a particular embodiment Said gas is an effluent. Such effluents can be generated at from the aforementioned anthropogenic sources.

The catalysis procedure of the invention is performs in general placing a system amount catalyst in a fixed bed reactor, and then feeding a current of a gas with a certain flow rate. The variables of the catalysis procedure of the invention such as temperature or gas flow can be easily determined by an expert in the field based on various factors such as Rh concentration of the catalytic system, the amount of system Catalytic used, type of effluent, space velocity and the residence time of the gas in the catalytic system, etc. In In this sense the composition of the effluent gas may vary depending on of the issuing source and this factor is also taken into account to Implement the catalysis procedure.

The catalysis procedure of the invention it can be carried out in an isothermal way or on the contrary the temperature may vary during it, which allows its use in gaseous streams whose temperature can fluctuate for example between 300 and 500 ° C approximately. In this sense in a particular embodiment the temperature is kept constant at 350 ° C during the procedure and in another particular embodiment the temperature is increased during the procedure for example by reason from 2ºC / min until reaching 450ºC. Gas flow may vary also between wide margins. However, in one embodiment particular the flow rate is modified to achieve speeds Spatial between 3800 and 30000 h -1.

Results are shown in Figure 2 obtained in the catalysis procedure of the invention in a nitric acid production plant, through the curves that show the percentage of decomposition of N 2 O against the temperature (Figure 2 (a)) and versus time (Figure 2 (b)). Figure 2 (a) shows the procedure carried out out by increasing the temperature on a scheduled basis at the rate of 2 ° C / min and Figure 2 (b) shows the procedure led to out under isothermal conditions at 350 ° C. Space velocity in Each particular embodiment is indicated in the figures themselves. In All embodiments use the catalytic system: 0.2% Rh / 49.9% Ce_ {0. 9} Pr_ {0. 1} O_ {x} (x \ leq 2) /49.9%\gamma-Al_ {2} 3 and a gas mixture composition effluent 1000 ppm N 2 O + 750 ppm NO x + 1% H 2 O + 2% O 2 + N 2.

As mentioned above, and it follows of the results set forth, the present invention provides active and stable time catalytic systems useful by both for employment at the industrial level capable of reducing and / or remove N2O from polluting effluent gases.

Below are examples illustrative of the invention set forth for a better understanding of it and in no case should be considered a Limitation of its scope.

Examples

Example one

Preparation of a catalyst 0.2% Rh / 49.9% Ce_ {0. 9} Pro_ {0. 1} O_ {x} (x \ leq 2) /49.9%\gamma-Al_ {2} O_ {3}

A catalytic system was prepared by impregnation with incipient moisture using a solution of Ce (NO 3) 3 •
6H 2 O and Pr (NO 3) 3 · 6H 2 O of γ-alumina particles of size 0.4-0.71 mm. The sample was dried at 110 ° C and calcined in air at 600 ° C for 90 minutes (heating rate 10 ° C / min). The Ce: Pr molar ratio used was 9: 1 and the same amount by weight of mixed oxide and alumina was supported. Rhodium was introduced in a second stage by impregnation by incipient moisture of the alumina-supported Ce-Pr mixed oxide with an aqueous solution of Rh (NO3) 3. The sample was dried at 110 ° C and calcination was performed at 500 ° C for 2 hours. The amount of Rh supported, after the drying and calcination steps, was 0.2% by weight in the final catalyst.

Example 2

Catalytic tests of decomposition of N 2 O in the laboratory with a 0.2% Rh / 49.9% formulation catalyst Ce_ {0. 9} Pr_ {0. 1} O_ {x} (x \ leq 2) /49.9%\gamma-Al_ {2} and gas mixtures of different composition

Decomposition tests of N 2 O, at atmospheric pressure, using 1 g of the system catalytic obtained in Example 1 and using a total flow rate of 500 ml / min gas (spatial speed 30000 h -1). They were used different reactive mixtures with one or more of the following gases: 1000 ppm N 2 O; 1000 ppm NO x; 1, 2.2 or 5% of O2 and 2.4% H2O, using He as the majority gas. They were made isothermal reactions at different temperatures between 100 and 500 ° C, that lasted until they reached the steady state.

As seen in Figure 1, the system Catalytic is active for the decomposition of N2O in complex gas streams in which, accompanying the N2O, there is NO_ {x}, H2O and / or O2. It achieves a considerable activity at temperatures around 450ºC with the gas mixture more complex among those studied.

Example 3

Catalytic tests of decomposition of N2O in a plant nitric acid production with a formulation catalyst 0.2% Rh / 49.9% Ce_ {0. 9} Pr_ {0. 1}) O_ {x} (x \ leq 2) /49.9%\gamma-Al_ {2} O_ {3}

The catalyst system prepared as and as described in example 1 to decompose N 2 O in a nitric acid production plant. 2 g of said system catalytic were placed in a fixed bed reactor located in a oven for temperature control. Gas feed used was extracted directly from the gas stream of the plant after the expansion turbine, using speeds Spatial between 3800 and 30000 h -1. The actual composition of Gas output at the inlet of the catalytic system was: 1000 ppm N 2 O + 750 ppm NOx + 1% H 2 O + 2% O 2.

In a particular example, the decomposition of N 2 O increasing the temperature at the rate of 2ºC / min up to 450ºC (Figure 2a). In another example it was carried out in isothermal regime at 350 ° C (Figure 2b).

Example 4

Formulation catalyst characterization 0.2% Rh / 49.9% Ce_ {0. 9} Pr_ {0. 1} O_ {x} (x \ leq 2) /49.9%\gamma-Al_ {2} O_ {3} before and after the catalytic tests described in example 3

After carrying out the catalytic tests described in example 3, and other equivalents not included, the Catalytic system 0.2% Rh / 49.9% Ce_ {0. 9} Pr_ {0. 1} O_ {x} (x \ leq 2) /49.9%\gamma-Al_ {2} O_ {3} used accumulated 40 non-consecutive hours of operation in the gas stream of the nitric acid production plant. During this time the catalytic system was subjected to nine cycles of heating-cooling and periods of diverse duration in isothermal conditions at 350 ° C.

Before and after the trials, the X-ray diffractograms (figure 3), Raman spectra (Figure 4) and the XPS spectra of the Rh 3d transition (Figure 5) of the catalytic system Also, Table 1 contains the results of Catalyst surface area (BET), determined before and after by absorption of N2 at -196 ° C as well as the size of Rhodium particle determined by TEM microscopy.

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TABLE 1

one

Claims (6)

1. Catalytic system comprising:

(i)

An active phase comprising rhodium supported on a mixed oxide of cerium and one or more metals between Pr, La, Zr, Y, Th, Sm and mixtures thereof, preferably Pr, where the amount of Rh is between 0.1 and 5% by weight with respect to the total weight of catalyst, preferably 0.2%, and

(ii)

A support that is stable in oxidizing atmosphere and at temperature equal to or less than 550 ° C, which is selected from pellets or particles of silica, pellets or particles of γ-alumina, pellets or carbide particles of silicon, lamb panel bee monoliths, panel monoliths silicon carbide bee and mixtures thereof,

characterized by presenting the following composition: 0.2% Rh / 49.9% Ce_ {0. 9} Pr_ {0. 1} O_ {x} (x \ leq 2) /49.9%\gamma-Al_ {2} 3 where the γ-alumina particles have a size between 0.4-0.71 mm.

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2. Procedure for system preparation catalytic according to claim 1, comprising the following stages:

(i)

impregnation of a support with a solution that comprises a cerium precursor salt and at least one precursor salt of a metal selected from the transition metal groups and internal transition;

(ii)

calcination;

(iii)

impregnation of the mixed oxide of Ce and said metal supported obtained in the previous stage with an aqueous solution comprising a precursor salt of rhodium and

(iv)

calcination.

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3. Use of the catalytic system according to a any of claims 1 to 2, for the reduction and / or elimination of the content in N 2 of a gas that contains it. 4. Use according to claim 3, wherein the gas also contains a species selected from NO_ {x}, H2O, O2, N2 and mixtures thereof. 5. Use according to claim 4, wherein The gas has the following composition: 1000 ppm N 2 O + 750 ppm NOx + 1% H 2 O + 2% O 2 + N 2. 6. Employment according to any one of the claims 3 to 5, wherein the gas is an effluent, preferably an effluent from a production plant of nitric acid or a diesel engine.