Catalyst and process for the decomposition of nitrous oxide as well as process and device in nitric acid preparation

A specific Perovskite-type compound represented by the general formula (1) is described. The invention relates to a catalyst containing a Perovskite-type compound. Furthermore a process for decomposing nitrous oxide (N ₂ O), a device for preparation of nitric acid as well as a process for preparation of nitric acid is disclosed. The invention also relates to the use of the Perovskite-type compound for decomposition of N ₂ O.

Nitrous oxide is a climate relevant trace gas which, along with carbon dioxide and methane, contributes directly to man made and enhanced greenhouse effect. Not until in the stratosphere N ₂ O is basically decomposed by photochemical processes wherein N ₂ O has a 310 fold higher resistance compared to carbon dioxide and a correspondingly higher contribution to global warming of the atmosphere. Due to the formation of nitric oxide (NO) in the stratosphere N ₂ O additionally significantly takes part in the depletion of the ozone layer.

Nitric acid industry is one of the major industrial sources contributing to the direct entry of N ₂ O into the atmosphere. In the so called Ostwald process ammonia is catalytically oxidized with atmospheric oxygen on multi layer stacked precious metal catalyst meshes based on platinum alloys wherein NO is formed as primary product. NO is oxidized with atmospheric oxygen to form nitrogen dioxide (NO ₂ ). This is absorbed in water at elevated pressure yielding mainly nitric acid of 50 - 68 %.

In the catalytic process N ₂ O is formed additionally to NO as an undesired secondary product. Unlike other nitrogen oxides formed N ₂ O is not absorbed in water in the following absorption process. Without further downstream steps for removal of N ₂ O it will enter the atmosphere, according to an analysis of the EFMA (European Fertilizer Manufactures Association) of 1995, with a exhaust gas concentration of 300 to 3500ppm, corresponding to a dimension of 1.2 to 13.8 kg per ton of product (HNO ₃ ).

Due to the resolutions of the Kyoto protocol assigning mandatorial reduction of the six greenhouse gases CO ₂ , CH ₄ , N ₂ O, partially halogenated hydrofluorocarbons (HFCs), perfluorinated hydrocarbons (PFCs) and SF ₆ in total, the contracting states of the United Nations Framework Convention on Climate Change are obliged to take measures for the reduction of N ₂ O emissions in nitric acid preparation.

This provision of the Kyoto protocol has initiated the examination and development of different techniques for N ₂ O reduction in nitric acid preparation which mainly differ in their set up in the diverse process steps. Among them in particular measures of integration in the process of catalytic oxidation of ammonia, catalytic decomposition directly downstream to the precious metal catalyst for ammonia oxidation and the decomposition in the exhaust gas after the absorption step.

After the absorption step the exhaust gas is heated to temperatures of 250 to 500 °C for heat recovery to gain energy via the exhaust gas expander. The relaxed exhaust gas is delivered to the atmosphere at a temperature of more than 100 ⁰ C. Processes of N ₂ O decomposition in the exhaust gas differ in their set up before or behind the expander and thus in the working temperature for the used decomposition catalyst.

Especially with respect to existing plants, such refitting with a decomposition unit in the exhaust gas is associated with considerable costs as only few of these plants achieve the necessary exhaust gas temperature so that by such exhaust gas preheating the operational costs would also significantly increase.

Low cost process approaches would reside in an alternative catalyst for ammonia oxidation, integration of a catalyst for selective N ₂ O decomposition in the process of catalytic ammonia oxidation or selective decomposition of N ₂ O in the product gas directly downstream to the precious metal catalyst.

Alternatively precious metal-free catalysts for ammonia oxidation can for example be found in US-A-4,812,300 and US-B-6,489,264.

In US-A-4,812,300 a catalyst represented by the general formula ABO ₃ with Perovskite structure is described. Wherein A is an alkali, alkaline earth, rare earth, lanthanide or actinide metal. B is

an element or a combination of elements selected of the group consisting of IB, IVB, VB, VIB, VHB or Vm of the periodic table of elements. Preferably used are La, Sr and mixtures thereof for A; Cr, Mn, Co, Ni, Cu and mixtures thereof for B. This catalyst is to allow for a selective oxidation of ammonia to form NO with a minimum of N ₂ O formation depending on temperature.

In US-B-6,489,264 a catalyst represented by the general formula (A _x B _y θ _3Z ) _k (Me _m O _n ) _f for the oxidation of ammonia is described. Wherein k and f refer to % by weight with a ratio of k to f of 0.01 to 1. A is Ca, Sr, Ba, Mg, Be, La or a mixture thereof and B is Mn, Fe, Ni, Co, Cr, Cu, V or a mixture thereof, x is 0 to 2, z is 1 to 2 and z is 0.8 to 1.7. Me _m O _n is a specific metal oxide.

Disadvantageously is, besides the poor product selectivity, in particular the low amount of process heat generated which would necessitate a separate pre heating of the educt mixed gas with respective additional costs and which could not be realized in that way in the existing plants used for nitric acid preparation.

Examples for an integration approach are found in WO 99/64352 and WO 01/87771. The former relates to a partial substitution of the precious metal catalyst by a cobalt oxide-containing catalyst, the latter to a combination with a modified palladium/rhodium alloy. Both patent applications describe a reduction of N ₂ O emissions. They have, however, the disadvantages of high sensitivity towards contaminations or rapid aging behaviour.

A pure catalytic decomposition of N ₂ O has been known for catalysts, such as silicon dioxide, titanium dioxide, aluminium dioxide, thorium dioxide, platinum foil and charcoal. Examples for catalysts that are supposed to be suitable for the decomposition of N ₂ O and which are applied directly downstream to the precious metal catalyst for ammonia oxidation, are found in DE-A- 198 41 740, US-A-2004/0179986, US-A-2005/0202966, US-B-6,723,295 and WO 2004/052512.

In DE 197 00 490 Al a catalyst for the decomposition of N ₂ O into N ₂ and O ₂ is described, containing a lanthanum-containing Perovskite. The catalyst consists of a mixture of an anion defective Perovskite of the composition Lai- _x CuχCoO _3- δ' with x = 0 to 0.5 and a spinel of the composition Co ₃ O ₄ in the mass ratio up to 1:1.

In one embodiment US-A-5,562,888 relates to a process for catalytic decomposition of N ₂ O in the presence of oxygen by using a solid oxide solution of the formula La _O sSr ₀₂ MO ₃ ^. M is a

transition metal, preferably Cr, Mn, Fe, Co or Y and δ is the deviation from the balanced stoichiometry.

Both types of catalysts described above fail, however, at high temperatures in the range of 800 to 1200 ⁰ C, as they prevail in particular for the reduction of the content of N ₂ O in the process gases in nitric acid preparation.

A series of catalysts for the decomposition of NO _x from car exhaust gases is known in the art.

US-A-3, 884,837 discloses a catalyst represented by the general formula RE _1-x M _x M _n θ ₃ wherein RE is one or more of the elements La, Pr and Nd, M is a monovalent ion, e.g. Na, K or Rb, and x ranges from 0.05 to 0.5. This compound is to catalyse decomposition of NO _x toxics into non harmful products. N ₂ O and nitrogen are listed as non harmful products. A decomposition of N ₂ O is, however, not described in the US-A-3,884,837.

In US-A-4, 126,580 a specific catalyst represented by the general formula ABO ₃ having Perovskite crystal structure is described which is said to be, among others, suitable for the reduction of nitrogen oxides to form compounds with a lower oxidation state.

US-B-6,569,803 relates to a catalyst for the purification of exhaust gases, where at least one specific precious metal catalyst component is supported on a complex oxide of the Perovskite type comprising at least two different metal elements. Wherein the complex oxide has the formula La _1-x K _x Bθ ₃ In one preferred embodiment wherein B is at least one of the elements Mn, Co, Fe and Ni and wherein 0 < x < 1. This catalyst is to allow for a high NO _x purification performance at high temperatures.

EP-A-O 089 199 relates to a catalyst consisting basically of a Perovskite represented by the general formula La _d . _x /2Sr _(1+X) /2Co _1-x Me _x O ₃ . Me is an element selected of the group consisting of Fe, Mn, Cr, V and Ti, and x is a number from 0.15 to 0.90. The catalyst is said to be suitable for the simultaneous treatment of oxidative and reductive gases.

The catalytic oxidation of NO from car exhaust gases to form NO ₂ and/or NO ₃ has also been described in the state of the art.

US-A-5,990,038 relates to a specific catalyst for the purification of exhaust gases. The catalytic layer of the catalyst comprises two types of granule. One including a double oxide represented by the formula (Lai _-x A _x )i. _α BO _δ supporting a precious metal selected of the group consisting of Pt and Pd. A is at least one element selected of the group consisting of Ba, K and Cs. B is at least one transition metal selected of the group consisting of Fe, Co, Ni and Mn. x is a number between 0 and 1, α is a number between 0 and 0.2, and δ is a number selected such that the total charge of the first double oxide is 0. This catalyst is to allow for a smooth oxidation from NO to form NO ₂ and/or NO ₃ due to the interaction between the double oxide and the precious metal.

US-B-6,395,675 discloses a device for purification of exhaust gases comprising a catalyst for purification of exhaust gas. The catalyst comprises, amongst others, a powder of a double oxide represented by the general formula (Lni _-α A _α )i- _β BO _δ . a and β are numbers between 0 and 1, δ is a number larger than 0. Ln is at least one first element selected of the group consisting of La, Ce, Nd and Sm. A is at least one second element selected of the group consisting of Mg, Ca, Sr, Ba, Na, K and Cs. B is at least one third element selected of the group consisting of Fe, Co, Ni and Mn. Wherein the third element is to oxidize NO _x to form NO ₂ .

Perovskites are also applied in other technical fields. US-A-5,447,705, for example, describes the use of specific catalysts of the composition Ln _x A _t . _y B _y O ₃ in the preparation of synthesis gas. x is between 0 and 10 and y is between 0 and 1. Ln is at least one element selected from the rare earths with the atomic numbers 57 through 71. A and B are metals that differ from each other selected of the groups IVb, Vb, VIb, VIIb and VHI.

Among the catalysts previously described, especially cobalt containing catalysts exhibit the disadvantage that they are very sensitive to contamination by sulphur. Further, these catalysts age particularly rapidly.

Another problem observed in association with a variety of catalysts so far proposed for the decomposition of N ₂ O is the inhibition by the oxygen present in the product gas and exhaust gas respectively. Operating performance of these catalysts is only guaranteed when a reducing agent is present in the gas stream for the removal of the physically or chemically adsorbed oxides.

It was an object of the present invention to provide a catalyst for the decomposition of N ₂ O. The catalyst was supposed to be particularly suitable for the use in the decomposition of N ₂ O in the product gas of ammonia oxidation in nitric acid preparation.

The invention relates to a catalyst comprising a carrier in form of honeycomb and a Perovskite- type compound represented by the general formula (1)

M ¹ M ² _Lx M ³ _X O ₃ (1)

wherein x ranges from 0.05 to 0.9, M ¹ is selected of the group consisting of La, Ce, Nd, Pr, Sm and combinations thereof, M ² is selected of the group consisting of Fe, Ni and combinations thereof, and M ³ is selected of the group consisting of Cu, Co, Mn and combinations thereof.

The present invention also relates to a process for decomposing N ₂ O wherein N ₂ O is brought into contact with a Perovskite-type compound represented by the general formula (1)

M ¹ M ² _Lx M ³ _X O ₃ (1)

wherein x ranges from 0.0 to 0.9, M ¹ is selected of the group consisting of La, Ce, Nd, Pr, Sm and combinations thereof, M ² is selected of the group consisting of Fe, Ni and combinations thereof, and M ³ is selected of the group consisting of Cu, Co, Mn and combinations thereof.

In another embodiment the invention relates to a device for preparation of nitric acid comprising a Perovskite-type compound represented by the general formula (1)

M ¹ M ² _Lx M^O ₃ (1)

wherein x ranges from 0.0 to 0.9, M ¹ is selected of the group consisting of La, Ce, Nd, Pr, Sm and combinations thereof, M ² is selected of the group consisting of Fe, Ni and combinations thereof, and M ³ is selected of the group consisting of Cu, Co, Mn and combinations thereof, and a catalyst for ammonia oxidation.

The use of a Perovskite-type compound represented by the general formula (1)

wherein x ranges from 0.0 to 0.9, M ¹ is selected of the group consisting of La, Ce, Nd, Pr, Sm and combinations thereof, M ² is selected of the group consisting of Fe, Ni and combinations thereof, and M ³ is selected of the group consisting of Cu, Co, Mn and combinations thereof, for the decomposition of N ₂ O is also described.

Figure 1 shows the N ₂ O decomposition rate and the activation energy of the process for LaMO ₃ - Perovskites.

Compounds of the Perovskite type

The Perovskite-type compound has the following general formula (1)

x is at least 0.0, preferably at least 0.05, more preferably at least 0.1, even more preferably at least 0.15. x is at most 0.9, preferably at most 0.6, more preferably at most 0.4, even more preferably at most 0.25.

M ¹ is selected of the group consisting of La, Ce, Nd, Pr, Sm and combinations thereof. In one embodiment, M ¹ is preferably La. Alternatively, M ¹ is preferably a combination of the indicated lanthanides, for example a combination of 50 to 55 % by weight Ce, 25 to 30 % by weight La, 10 to 15 % by weight Nd, 5 to 10 % by weight Pr as well as 0 to 1 % by weight Sm, calculated as oxide respectively. In the following, a combination of lanthanides is abbreviated as Ln.

M ² is selected of the group consisting of Fe, Ni and combinations thereof.

M ³ is selected of the group consisting of Cu, Co, Mn and combinations thereof. Preferred are Cu and Co, more preferred is Co.

In the case of M ² = Ni, M ³ is preferably Cu and Co, more preferably Co. x ranges preferably from 0 to 0.4 or from 0.15 to 0.4.

In the case of M ² = Fe or the combination of Fe and Ni, M ³ is preferably Cu. x ranges preferably from 0 to 0.4 or from 0.15 to 0.4.

Preferred compounds are:

M ¹ FeO ₃

preferably M ¹ Fe _C4 - _O sNi ₀₆ - _O aO ₃ , more preferably M ¹ Fe ₀₄ Ni ₀ OO ₃ and M ^l Feo. ₈ Nio _.2 O ₃

M'Feo.s.i.oCuo.a-o.oOa

In the formulas above, M ¹ is preferably La or Ln.

Although it is not desired to be bound by a particular theory, it is assumed that the incorporation of the element M ³ into the Perovskite crystal structure results in alteration of the nature of the 3d cation in its charge distribution or the formation of oxygen/cation lattice vacancies, a crystallographic defect. Thereby a new surface species is formed which is involved in the catalytic reaction and requires different activation energy for the catalytic N ₂ O decomposition which the special applicability of the compounds of the Perovskite type represented by the general formula (1) is based on.

According to the invention, the Perovskite-type compound can be used in different forms as a catalyst. The Perovskite-type compound can, for example, be used as such in form of regularly or irregularly shaped particles (powder, granulate, granules). Alternatively, the Perovskite-type compound is used in combination with a carrier. The carrier can have any known form. Thus, amongst others, granules, pearls or honeycomb carriers are possible. In one preferred embodiment, carriers in the shape of honeycomb are used. These offer a mostly approximately cylindrical outer shape, pervaded by a variety of parallel channels. Such carriers of the honeycomb type are known in the field of catalysis. The carrier itself consists mostly of oxidic material (e.g. cordierite) or metal. Both forms can be used according to the invention.

The Perovskite-type compound can be incorporated in the material of the carrier, the carrier can be impregnated with the Perovskite-type compound or the carrier can have a surface coating ("washcoat"), containing the Perovskite-type compound.

In the embodiment wherein the carrier has a surface coating containing the Perovskite-type compound the Perovskite-type compound can be applied to a carrier material contained in the surface coating.

The commonly used materials known in the art are suitable as carrier material. Preferably high- surface materials are used as carrier material. In the scope of this invention, materials are referred to as high-surface, the specific BET-surface of which being larger than 5 m ² /g. Suitable carrier materials are for example titanium oxide, aluminium oxide, silicon oxide, cerium oxide, zirconium oxide, zeolite and mixtures or mixed oxides thereof. Preferably, the carrier material comprises aluminium oxide, as this can additionally slow down the aging of the catalyst.

A carrier provided with a surface coating can be produced in different ways. In a first approach the carrier can be provided and then a coating comprising carrier material and Perovskite-type compound can be applied. In a second approach the carrier can be provided, a coating comprising carrier material can be applied and then the Perovskite-type compound can be applied. Processes for performing these steps are known in the state of the art.

In the first approach, for example a dispersion of the Perovskite-type compound and the other coating components, including carrier material or precursors thereof, can be prepared. Here, the Perovskite-type compound can already be present on the carrier material or it can be present separately in the dispersion. The carrier can be immersed in said dispersion once or several times. Then, residual dispersion can, wherever necessary, be removed from the channels of the carrier and the catalyst can be completed, for example by drying and calcination.

In the second approach a dispersion of the coating components as above is described, however is provided without the Perovskite-type compound and the carrier coated as described. Then the coated carrier is impregnated with a precursor compound for the Perovskite-type compound. The precursor compound will then be converted to form the Perovskite-type compound.

It has been observed that depending on the type of carrier different Perovskite-type compounds can preferably be used. In the embodiment wherein the carrier has a surface coating containing the Perovskite-type compound, the Perovskite-type compound preferably has the formulas M ¹ FeCS-I oCUo ₂ -OoO ₃ and M ¹ FeO more preferably M ¹ Fe _{O 4} NiO ₆ Os); wherein M ¹ is more preferably La. Surprisingly, these compounds are characterized a by high resistance towards modifications with components of the carrier material, such as the formation of less active spinels. The activity, particularly of the Perovskite-type compound with the formula LaFeo.8-i.oCuo. ₂ -o.o0 ₃ , is hardly influenced by changes in the contents of oxygen and water in the gas stream. An influence on selectivity in the presence of NO/NO ₂ in the gas stream is not detectable.

Alternatively, the Perovskite-type compound is incorporated into the material of the carrier. The carrier preferably comprises the Perovskite-type compound in an amount of 50 to 90 % by weight, preferably of 55 % by weight to 70 % by weight, referring to the total weight of the carrier. Besides the Perovskite-type compound the carrier can additionally contain common components. Among them are oxidic materials that are, for example, resistant up to a temperature of 1200 ⁰ C. Examples of such compounds are cordierite, alumina, graphite, mullite, aluminium oxide, zirconium oxide, zirconium mullite, barium titanate, titanium oxide, silicon carbide and silicon nitrite. In a particularly preferred embodiment, the carrier comprises cordierite besides the compounds of the Perovskite type, preferably in an amount of 0 % by weight to 50 % by weight, preferably of 0 % by weight to 15 % by weight, alumina in an amount of 0 % by weight to 50 % by weight, particularly preferred 0 % by weight to 30 % by weight, and Al ₂ O ₃ in an amount of 0 % by weight to 50 % by weight, particularly preferred 10 % by weight to 30 % by weight. Preferably, extrudates contain 25 to 30 % by weight Al ₂ O ₃ , 8 to 10 % by weight cordierite and about 5 % by weight graphite.

In one embodiment the carrier consists of the Perovskite-type compound. In one preferred embodiment, the Perovskite-type compound has the formula M ¹ Ni _{O 9-07} Co _O i _{-O 3} O ₃ , preferably M ¹ Ni ₀₈ Co ₀₂ O ₃ wherein M ¹ is preferably La. Surprisingly, these compounds show a low activation energy of about 30 kcal/mol for the observed N ₂ O decomposition, allowing a universal use in a wide temperature range. Particularly in the typical temperature range from 850 to 900 ⁰ C for the decomposition of N ₂ O in nitric acid preparation, a doubling of the N ₂ O decomposition is found. Simultaneously, the examinations show a linear relationship of the

decomposition efficiency with the amount of catalyst used in said temperature range. An influence on selectivity in the presence of NO/NO ₂ in the test gas is not detectable.

Table 1: Reaction rate W, N ₂ O decomposition X and activation energy E _A at the N ₂ O decomposition for LaMC^ catalysts (at 900 °C). The N ₂ O decomposition X is defined as the quotient of reacted amount of N ₂ O to supplied amount of N ₂ O.

In the case of extrudates, preferred compounds of the Perovskite type are M ¹ Fe _O g- _I oCUo ₂ - _O oO ₃ and M ¹ Fe ₀₃ - _O gN^-0 ₁ O ₃ (preferably M ¹ Fe ₀₄ - _O sNi _O o-OaO ₃ , more preferably M ¹ FeO ₈ Ni ₀₂ O ₃ ). In these embodiments, M ¹ is preferably Ln. Carriers comprising respective extrudates, have a surprisingly high porosity. Furthermore, the formation of NiFe ₂ O ₄ or CuFe ₂ O ₄ spinels seems to significantly increase the catalytic activity with regard to the N ₂ O decomposition in these compounds. Besides, a minor dependence of the N ₂ O decomposition activity of varying contents of water, oxygen as well as the presence of N0/N0 ₂ in the reaction gas is observed.

In extrudates LaFeO ₃ has also proven to be particularly effective. This Perovskite-type compound has a high activity in N ₂ O decomposition.

Process for decomposing N ₂ O

Furthermore the present invention relates to a process for decomposing N ₂ O wherein N ₂ O is brought into contact with the Perovskite-type compound. In the decomposition basically N ₂ and O ₂ are formed.

The conditions where the Perovskite-type compound is brought into contact with the N ₂ O are not particularly limited. For example, the reaction temperature can be 800 to 1200 ⁰ C. The amount

of Perov ski te- type compound used conforms to the application and can suitably be appreciated by one with ordinary skill in the art. The contact time between catalyst and N ₂ O also depends on the application and is preferably more than 0.02 sec.

As the catalyst according to the invention can also be used when the N ₂ O containing gas stream contains water, oxygen, NO or NO _x , it can be used in a wide variety of applications wherein N ₂ O is to be decomposed. The process according to the invention is particularly suitable for the decomposition of N ₂ O in the product gas of the ammonia oxidation in nitric acid preparation.

Device for preparation of nitric acid

The present invention also relates to a device for preparation of nitric acid comprising a Perovskite-type compound and a catalyst for ammonia oxidation. Devices for the preparation of nitric acid are known in the art and are, for example, described in Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 17, 4 ^th edition (1996), pages 84 to 96. The device according to the invention differs from the previous devices in that additionally the Perovskite-type compound is used for the decomposition of N ₂ O. Thereby emissions of harmful N ₂ O can be reduced.

In one embodiment the Perovskite-type compound is downstream in the direction of flow to the catalyst for the oxidation of ammonia. Preferably only after the decomposition of N ₂ O does the absorption of nitrogen oxides in water take place in the device according to the invention.

The Perovskite-type compound can be present in one of the above described catalyst forms. It is, however, preferably used in combination with a carrier in the shape of honeycomb.

Such a honeycomb carrier with the Perovskite-type compound represented by the general formula (1) is, for example, placed in the reactor for catalytic ammonia oxidation on a carrier in the form of a grid, lattice or in a basket. If honeycomb carriers are used, one carrier or several carriers can be used. In such a device, the honeycomb carrier can take over the carrier function for the imposed precious metal catalyst and the precious metal recovery system, possibly located directly downstream. In the device according to the invention precious metal catalyst and precious metal recovery system can be separated from the honeycomb carrier by one or more separator meshes, for example made of non-precious metal. Because of the device according to

the invention, a uniform flow through the precious metal catalyst and the carrier in the shape of honeycomb is achieved.

The device according to the invention is advantageous, because the honeycomb carrier allows for a uniform flow at minimal pressure loss. An adaptation to the specific operating conditions of a nitric acid reactor and the N ₂ O contents in the process gas is readily possible by respective modifications of the channel openings, their number and wall thickness.

Preparation of the compounds of the Perovskite type

The preparation of the compounds of the Perovskite type is not particularly limited and any process for the preparation of compounds of the Perovskite type known in the art can be used. Among these processes are coprecipitation, impregnation, sol-gel techniques and mechanical mixing of oxides or precursors of oxides, followed by calcination.

In the following several processes which have been used for the preparation of the compounds of the Perovskite type used in the examples, are described for illustration.

(i) Pechini process

Compounds of the Perovskite type are produced by the Pechini process [M. P. Pechini, US-A- 3,330,697]. Herein, citric acid and ethylene glycol are added to a solution of the accordingly proportioned nitrate salts saturated at room temperature. The solution is evaporated at 80 to 100 °C. After tempering at 200 to 250 °C an amorphous Perovskite precursor will be formed. For formation of the Perovskite, the sample is calcined for 4 hours at 700 to 900 °C.

(ii) Mechanochemical process

For the preparation of the compounds of the Perovskite type commercially available transition metal oxides, such as Ot-Fe ₂ O ₃ , CoO, CuO, MnO, NiO and the respective carbonates of the lanthanides are mixed. The mixed oxides of the rare earth and transition metals are obtained by mixing for example Ln ₂ (CO ₃ ) ₃ / Fe ₂ O ₃ in a weight ratio of 4.5 : 1.0. Fe ₂ O ₃ can also be replaced by one or more further transition metal oxides. The mixture will then be fed to a disintegrator for

several times. After tempering at 700 to 900 ⁰ C over a period of 4 hours the resulting mixture is repeatedly triturated in the disintegrator.

Preparation of impregnated carriers

(i) Pechini process

Citric acid and ethylene glycol are added to a solution of the accordingly proportioned nitrate salts, saturated at room temperature. A commercially available honeycomb carrier based on cordierite is immersed in the solution for 5 to 10 min, blown off with air and dried at air at room temperature. Calcination will then be conducted at temperatures between 700 and 900 °C over a period of 4 hours.

(ii) Pasting process

A Perovskite-type compound is added to a cordierite or Al ₂ O ₃ paste for the preparation of a honeycomb carrier. Alternatively, a solution of accordingly proportioned nitrate salts of the Perovskite-type compound can be added to the paste. Besides cordierite and Al ₂ O ₃ respectively, the paste can contain an aqueous solution of methyl cellulose. As further components, alumina, talc and 3d oxides can be added in the required ratios. The paste is mixed in a mixer for a period of 40 min. The resulting paste has a humidity of 20 to 30 %. It will then be extruded through a pressure template to form a mould. The resulting monoliths are hardened at room temperature and dried at 300 to 400 °C over a period of 4 hours. Afterwards calcination is conducted at a temperature of 1000 to 1250 ⁰ C over a period of 4 to 5 hours.

Preparation of Perovskite-containing carriers

Citric acid and ethylene glycol are added to a solution of the accordingly proportioned nitrate salts, saturated at room temperature, and said solution is evaporated at 80 to 100 ⁰ C. In this step, polymerized metal ether complexes form. After tempering at 200 to 250 °C the organic residue will be incinerated and an amorphous precursor of the Perovskite has formed. For obtaining the Perovskite-type compound, the amorphous precursor is tempered at 700 to 900 °C over a period of 4 hours.

For compounds represented by the formula Ln(FeiNi) _1-x Cu _x O ₃ , commercially available transition metal oxides Ot-Fe ₂ O ₃ , NiO and CuO as well as carbonates of mixed lanthanides (Ln) are used as starting material. The composition of these mixed lanthanides is 50 to 55 % by weight Ce, 25 to 30 % by weight La, 10 to 15 % by weight Nd, 5 to 10 % by weight Pr as well as 0 to 1 % by weight Sm, stated as their oxides CeO ₂ , La ₂ O ₃ , Nd ₂ O ₃ , Pr ₆ O ₁₁ and Sm ₂ O ₃ .

A mixture of the rare earth and transition metals is achieved by mixing 3.5 kg Ln ₂ (CO ₃ ) ₃ with 1.3 kg Fe ₂ O ₃ and CuO or NiO respectively. After mixing the mixture will be tempered at 700 to 900 °C over a period of 4 hours to obtain the Perovskite-type compound.

For obtaining a mouldable paste the Perovskite-type compound is plasticized with a binder based on Al ₂ O ₃ (Al ₂ O ₃ ^• n H ₂ O) or alumina (Al ₂ O ₃ / 20 - 25 % by weight, SiO ₂ / 55 - 60 % by weight, H ₂ O / 10 - 15 % by weight, rest admixtures) (as well as a peptisizing agent, for example an aqueous solution of different acids, such as nitric, oxalic and acetic acid). For the enhancement of the rheology of the paste ethylene glycol is added. Further, cordierite particles (2 MgO ^• 2 Al ₂ O ₂ ^• 5 SiO ₂ ) with a particle size of less than 0.5 mm and graphite can be added. Such components will increase the thermal shock stability and the porosity of the catalyst material respectively. The mixture is treated mechanically over a period of 10 min in a planetary ball mill and will then be extruded to form a honeycomb carrier. After drying at room temperature, the honeycomb carrier is calcined at 900 to 1150 ⁰ C over a period of 4 hours.

The described compounds of the Perovskite type represent a significant progress in N ₂ O decomposition as they have a high thermal shock resistance. This is of particular importance for a use in the Ostwald process as excessive temperature differences occur especially during starting up and shutting down the reactor. Furthermore, the compounds of the Perovskite type have a high aging stability even at high temperatures, as they prevail, for example, in the product gas stream of nitric acid preparation. They can also be used in applications wherein the gas stream contains sulphur and sulphur containing compounds, oxygen, water, NO or NO _x besides N ₂ O.

EXAMPLES

The invention is now illustrated by means of the following examples. It should not, however, be considered as being limited to these preferred embodiments.

Experimental assembly of the test reactor

The catalytic decomposition of N ₂ O is conducted in a U-shaped fused quartz flow reactor with an internal diameter of 11.4 mm at ambient pressure. For uniform tempering, the reactor is placed in an electrically heated sand bath.

Measurement and regulation of the reactor temperature takes place on the external wall of the reactor. Additionally, the catalyst temperature is taken by a temperature measurement in one of the honeycomb channels. The flow rates of the gas mixtures are set in a range of 0.5 to 3.2 1/min by mass flow controllers. Steam is added by a saturator at given temperature. In order to exclude the formation of potential condensates all elements and sampling systems flown through by gas are heat insulated and heated.

Example 1

A bulk of inactive fused quartz granules (1 to 2 mm in diameter, total mass of 1 g) is filled into the test reactor. Hereto a catalyst bed of 5 to 20 mg of particles of the Perovskite-type compound with a particle size of 0.25 to 0.50 mm is placed. The BET-surface of the Perovskite-type compound varies from 1 to 10 m ² /g. Each measurement is conducted at constant temperature in stationary state. For correction of the N ₂ O homogenous decomposition comparable measurement without catalyst will be conducted. For comparison the results, besides the absolute reaction of N ₂ O, are applied to the reaction rate W in mole N ₂ O / m ² s.

Table 2: Experimental conditions

Table 3: Activity of the compounds of the Perovskite type

The highest activities for the decomposition of N ₂ O have been found for Co- and Ni-comprising Perovskites, particularly for Perovskites containing both elements.

The catalytic properties of the examined Perovskites show a special dependency on the nature of the 3d cation, its charge and coordination. By additional elements M ³ , these properties can be modified, an effect which is probably due to point or crystallographic defects wherein the introduction of an element M ³ with several valences results in a change of the cation charge or in the formation of oxygen cation valences.

Example 2

In this example, a carrier in the shape of honeycomb which is impregnated with the Perovskite- type compound is examined.

For the examinations, a cordierite carrier with a surface area of 35 m ² /g and a pore volume of 0.4 cm ³ /g is selected. The carrier has a honeycomb structure with a hexagonal base area as well as isosceles, triangular channel openings having a side length of 2.5 mm and a wall thickness of 0.4 mm. For the examinations such carriers with a diameter of 9 to 10 mm are used. The results are set forth in table 5 to 7.

Table 4: Experimental conditions

Table 5: Experimental data for carriers which are impregnated with a Perovskite-type compound

The weight ratio is related to the total weight of the honeycomb carrier.

Table 6: N ₂ O decomposition depending on oxygen content (0.13 % N ₂ O)

Table 7: N ₂ O decomposition depending on NO content (0.13 % N ₂ O)

Example 3

Carriers in the shape of honeycomb containing a Perovskite-comprising extrudate are examined. Two different geometries are used for the examination and testing. A first series comprises hexagonal prisms with a side length of 6.0 cm and a triangular channel geometry with a side length of 2.5 mm and a wall thickness of 0.4 mm. A second series comprises hexagonal prisms with a side length of 2.8 cm, with a triangular channel geometry with a side length of 1.6 mm and a wall thickness of 0.6 mm. The samples were cut out of the produced honeycomb carriers respectively. The form was chosen in such way that the samples fit into the test reactor.

The activity of the catalyst is determined for a temperature range of 800 to 900 ⁰ C. A standard gas composition consists of 0.15 % N ₂ O, 3 % O ₂ and 3 % H ₂ O, balance helium. For the examination of the oxygen influence with reference to a possible inhibition of the catalyst, a gas composition of 0.13 % N ₂ O, 7 % O ₂ and 3 % H ₂ O, balance helium, is used. For examinations of the influence of H ₂ O on the N ₂ O decomposition, the portion of steam is increased to 10 to 12 %. All listed values are corrected for the value of the homogenous high temperature self- decomposition of N ₂ O.

Table 8: Experimental conditions

Table 9: Experimental data

The pore volume is measured with a high-pressure mercury porosimeter. The surface area is detected by the common BET procedure by using thermic Ar desorption.

Example 4

In order to examine the Effect of water concentration in the gas on the N ₂ O decomposition, the H ₂ O saturator is operated at different temperatures.

Table 10: Effect of water concentration on N ₂ O decomposition (X _N2O ), sample 57 % LnFeO ₃ + 28.6 % Al ₂ O ₃ + 9.5 % cordierite + 4.9 % graphite; height of the monolith = 12.0 mm; temperature 900 °C

Table 11: Effect of water concentration on N ₂ O decomposition (XN ₂ O). sample 57 % LnFe ₀₈ Cu ₀₂ O ₃ + 30 % Al ₂ O ₃ + 10 % cordierite + 5 % alumino-silicate fibres; height of the monolith = 6.60 mm; temperature 900 ⁰ C

Table 12; Effect of water concentration on N ₂ O decomposition (XN2 _O ), sample 57 % LnFe ₀₈ Ni ₀₂ O ₃ + 28.6 % Al ₂ O ₃ + 9.5 % cordierite + 4.9 % graphite; height of the monolith = 6.60 mm; temperature 900 °C

Table 13: Effect of oxygen concentration on N ₂ O decomposition, sample 57 % LnFeO ₃ + 28.6 % Al ₂ O ₃ + 9.5 % cordierite + 4.9 % graphite; height of the monolith = 12.0 mm; C° _N2 o = 0.13 %; temperature 900 ⁰ C

Table 14: Effect of oxygen concentration on N ₂ O decomposition, sample 57 % LnFeO ₃ + 28.6 % A All ₂₂ OO ₃₃ ++ 99..55 %% ccoorrddierite + 4.9 % graphite; height of the monolith = 6.6 mm; C° _N2 o = 0.118 %; temperature 900 ⁰ C

Example 15

In an industrial reactor for nitric acid production an extruded honeycomb carrier with a Perovskite-type compound LaFeO ₃ is set onto the grid of a basket. For spacing of the precious metal catalyst and the platinum recovery system two base Megapyr separator meshes are used.

Table 15: Geometry of the honeycomb carrier

Table 16: Industrial operating conditions

As a result the amount of N ₂ O in the exhaust gas was reduced by 65.8 % from 491 ppm to 168 ppm.