Patent Description:
In the industrial preparation of nitric acid by the Ostwald process, ammonia is reacted with oxygen over a noble metal catalyst to form oxides of nitrogen which are subsequently absorbed in water. In this process, ammonia and oxygen or air are reacted at from <NUM> to <NUM> over a catalyst gauze comprising noble metals in a reactor. The catalyst gauze generally comprises platinum and rhodium as active metals. In the catalytic reaction, ammonia is firstly oxidized to nitrogen monoxide which is subsequently further oxidized by oxygen to give nitrogen dioxide or dinitrogen tetroxide. The gas mixture obtained is cooled and then passed to an absorption tower in which nitrogen dioxide is absorbed in water and converted into nitric acid. The reactor for the catalytic combustion of ammonia also contains, downstream of the catalyst gauze, a recovery gauze for depositing and thus recovering catalyst metals which have been vaporized at the high reaction temperatures. A heat exchanger is located downstream of the recovery gauze to cool the gas mixture obtained. Absorption is carried out outside the actual reactor in a separate absorption column.

The combustion and the absorption can be carried out at the same pressure level. It is possible to employ an intermediate pressure of from about <NUM> to <NUM> kPa or a high pressure of from about <NUM> to <NUM> kPa. In the case of a process with two pressure stages, the absorption is carried out at a higher pressure than the combustion. The pressure in the combustion is then from about <NUM> to <NUM> kPa and the pressure in the absorption is from about <NUM> to <NUM> kPa.

An overview of the Ostwald process may be found, for example, in <NPL>).

The combustion of ammonia forms not only nitrogen monoxide and nitrogen dioxide or dinitrogen tetroxide but generally also N<NUM>O (nitrous oxide, dinitrogen monoxide) as by-product. In contrast to the other oxides of nitrogen formed, N<NUM>O is not absorbed by the water during the absorption step. If no further step for removing N<NUM>O is provided, N<NUM>O can be emitted into the environment in a concentration of from about <NUM> to <NUM> ppm in the waste gas.

Since N<NUM>O is a greenhouse gas, very substantial removal from the waste gas is desirable. Although N<NUM>O is not the major contributor to global warming (-<NUM>%), it is much more potent than either of the other two most common anthropogenic greenhouse gases, CO<NUM> and CH<NUM>. Due to its long lifetime of approximately <NUM> years in the atmosphere, N<NUM>O has <NUM> and <NUM> times the Global Warming Potential of CO<NUM> and CH<NUM>, respectively.

A number of methods of removing N<NUM>O from waste gas streams have been described. Development of N<NUM>O abatement systems aims at the achievement of high efficiency (><NUM>% N<NUM>O conversion) and selectivity (<<NUM>% NO loss). The approaches followed by industry, research institutes, and universities can be classified in four groups, according to the position in the process (<NPL>):.

<CIT> describes a process for removing nitrogen oxides from a gas stream, in which the nitrogen oxides apart from N<NUM>O are absorbed in an absorption medium and remaining N<NUM>O is subsequently decomposed catalytically at from <NUM> to <NUM> in a decomposition reactor. Since nitrogen oxides can be formed in this decomposition, a selective catalytic reduction (SCR) can follow.

<CIT> describes a process for preparing nitric acid by the Ostwald method, in which the N<NUM>O content is reduced by passing the gas stream after the oxidation over a catalyst bed of zirconium oxide in the form of cylindrical pellets at a temperature of at least <NUM>.

<CIT> describes a process for the reduction of N<NUM>O. For this purpose, a reactor for carrying out the Ostwald process is modified in such a way that the gases obtained after the catalytic combustion are subjected to a retention time of from <NUM> to <NUM> seconds before cooling by means of the heat exchanger. If desired, a catalyst for the selective decomposition of N<NUM>O can be additionally provided.

<CIT> discloses a process for the catalytic decomposition of nitrous oxide N<NUM>O in a gas mixture obtained in the preparation of nitric acid by catalytic oxidation of ammonia in a reactor which contains in this order in the flow direction a noble metal gauze catalyst and a heat exchanger over a catalyst for the decomposition of N<NUM>O which is installed between the noble metal catalyst and the heat exchanger so that the hot gas mixture obtained from the catalytic oxidation of ammonia is brought into contact with the catalyst for the decomposition of N<NUM>O before subsequent cooling. The catalyst for the decomposition of N<NUM>O contains copper aluminate CuAl<NUM>O<NUM> and preferably a further metal oxide, in particular ZnO. It is generally said that the catalyst is preferably used in the star extrudate form. However, a specific catalyst form is not disclosed. The object underlying the present invention is to provide an improved catalyst for the decomposition of N<NUM>O which has an improved geometry, combining high geometric outer surface area (GSA) with low pressure drop and preferably also with high mechanical stability, specifically high side and/or bulk crushing strength, under practical conditions in a packed catalyst bed.

Documents <CIT> and <CIT> generally disclose star shaped catalyst bodies having six lobes.

The invention is based thereon that the inventors have now been able to provide a star-shaped extrudate, having an optimum GSA and pressure drop, preferably combined with a high side and/or bulk crushing strength.

The object is achieved by a process for the catalytic decomposition of nitrous oxide N<NUM>O in a gas mixture obtained in the preparation of nitric acid by catalytic oxidation of ammonia, in a reactor which contains in this order in the flow direction a noble metal gauze catalyst and a heat exchanger, over a catalyst for the decomposition of N<NUM>O which is installed between the noble metal gauze catalyst and the heat exchanger so that the hot gas mixture obtained from the catalytic oxidation of ammonia is brought into contact with the catalyst for the decomposition of N<NUM>O before subsequent cooling, characterized in that the catalyst is in the form of a star-shaped body having six lobes, wherein the ratio of the maximum radius r2 in the star to radius r1 of a circle connecting the intersections of the lobes is in the range from <NUM> to <NUM>, the ratio of the area F1 inside this circle to the summed area F2 of the lobes outside this circle is in the range of from <NUM> to <NUM>, the ratio of the distance x2 between the two intersections I of one lobe with neighboring lobes and the radius r1 of the circle is in the range of from <NUM> to <NUM>.

Preferably, in the star-shaped catalyst body having a cross-section having six lobes, used according to the present invention, the ratio of the maximum radius r2 in the star to radius r1 of a circle connecting the intersections of the lobes is in the range of from <NUM> to <NUM> or <NUM> to <NUM>, more preferably from <NUM> to <NUM>.

Preferably, the cross-section of the body has six axes of mirror symmetry, so that the six lobes have an identical shape.

The mirror symmetry allows for a slight deviation from mirror symmetry. Preferably, at most one or two lobes deviate from the six axes of mirror symmetry. Thus, one or two lobes might be of different size or slightly inclined with regard to the other lobes, while still fulfilling the above geometrical requirements for the star-shaped catalyst body.

The mirror symmetry allows for a maximum of <NUM>% deviation from ideal mirror symmetry, more preferably of <NUM>% or less deviation from ideal mirror symmetry. Most preferably, the cross-section of the body has six axes of mirror symmetry without deviation from mirror symmetry. The mirror symmetry can be seen in <FIG>, where six identical lobes are depicted.

According to the present invention, N<NUM>O can be reacted directly in the reactor for the catalytic oxidation of ammonia when a suitable catalyst is located between the noble metal gauze catalyst and the heat exchanger. In this way, N<NUM>O formed as by-product is decomposed immediately after it is formed. The decomposition occurs at the temperature prevailing in the catalytic oxidation of ammonia. Heating or cooling of the gaseous reaction mixture is thus unnecessary. The catalyst for the decomposition of N<NUM>O which is used according to the present invention is located directly in the reactor, preferably between the position of a noble metal recovery gauze located downstream of the noble metal catalyst and the position of the heat exchanger. Reactors for the Ostwald process are usually provided with inserts for accommodating the noble metal catalyst and the noble metal recovery gauze. These reactors can easily be modified by additionally providing a holder for the N<NUM>O decomposition catalyst.

The low catalyst bed height required according to the present invention allows installation in existing reactors without great rebuilding of the reactors. Thus, existing reactors can be modified to enable the process of the present invention to be carried out, without replacement of the reactor being necessary. The Ostwald process can be carried out at one pressure level or at two pressure levels, as described above. The height of the catalyst bed is preferably from <NUM> to <NUM>, particularly preferably from <NUM> to <NUM>. In production, the residence time over the catalyst is preferably less than <NUM>. The pressure drop caused by installation of the catalyst is therefore very low, a small amount of catalyst can be employed, and the gas has to be held at a high temperature level for only a short time after the oxidation, so that secondary reactions can largely be suppressed.

According to the present invention, the decomposition of N<NUM>O is carried out in the reactor for the oxidation of ammonia at the oxidation temperature, generally at a temperature in the range from <NUM> to <NUM>. , preferably from <NUM> to <NUM>. , in particular from <NUM> to <NUM>. The pressure is, depending on the pressure level at which the Ostwald process is carried out, generally from <NUM> to <NUM> bar.

As noble metal gauze catalyst, it is possible to use any noble metal gauze catalyst suitable for the catalytic oxidation of ammonia. The catalyst preferably comprises platinum and possibly rhodium and/or palladium as catalytically active metals. The noble metal recovery gauze is preferably made of palladium.

The catalyst used according to the present invention for the decomposition of N<NUM>O is selected from among catalysts which still have sufficient activity at above <NUM> to decompose N<NUM>O at this temperature in the presence of NO and/or NO<NUM>. Catalysts which are suitable for the purposes of the present invention are, for example, binary oxides such as MgO, NiO, ZnO, Cr<NUM>O<NUM>, TiO<NUM>, WOx, SrO, CuO/Cu<NUM>O, Al<NUM>O<NUM>, Se<NUM>O<NUM>, MnO<NUM> or V<NUM>O<NUM>, if desired doped with metal oxides, lanthanide complexes such as La<NUM>NiO<NUM>, La<NUM>CuO<NUM>, Nd<NUM>CuO<NUM> and multinary oxide compounds thereof, spinels, ternary perovskites, and also oxidic systems such as CuO-ZuO-Al<NUM>O<NUM>CoO-MgO, CoO-La<NUM>O<NUM>, CO-ZuO, NiO-MoO<NUM> or metals such as Ni, Pd, Pt, Cu, Ag.

The catalyst is preferably a copper-containing catalyst, containing a compound of the formula MxAl<NUM>O<NUM>, where M is Cu or a mixture of Cu with one or more further metals M, selected from the group consisting of Sn, Pb, Zn, Mg, Ca, Sr and Ba, preferably Zn, Mg, Ca, Sr and/or Ba, more preferably Zn and/or Mg, and x is from <NUM> to <NUM>.

The preferred catalysts consists essentially of an essentially spinel phase which may still contain small amounts of free oxides in crystalline form, such as MO (where M is Cu, Zn or Mg) and M<NUM>O<NUM> (where M is Al). The presence of a spinel phase can be detected by recording XRD spectra. The amount of the oxides in the catalyst is in general from <NUM> to <NUM>, preferably from <NUM> to <NUM>% by weight.

The amount of Cu and any Zn and/or Mg should be chosen such that a filled or virtually filled spinel is obtained. This means x in the formula MxAl<NUM>O<NUM> is from <NUM> to <NUM>, preferably from <NUM> to <NUM>, more preferably from <NUM> to <NUM>, particularly preferably from <NUM> to <NUM>. For x values below <NUM>, the thermal stability is substantially lost. x values above <NUM> likewise lead to a deterioration in the catalyst activity and catalyst stability. The catalyst having an x value of from <NUM> to <NUM>, preferably from <NUM> to <NUM>, more preferably from <NUM> to <NUM>, particularly preferably from <NUM> to <NUM>, in the formula MxAl<NUM>O<NUM> is thus a high temperature-stable catalyst for decomposition of N<NUM>O. The catalyst has advantageous aging behavior, i.e. the catalyst remains active for a long time without being thermally deactivated.

The catalyst contains copper in oxide form, calculated as copper oxide CuO, in an amount of in general from <NUM> to <NUM>, preferably from <NUM> to <NUM>, particularly preferably from <NUM> to <NUM>, % by weight, based on the total catalyst.

The catalyst may additionally contain further dopants, in particular Zr and/or La, in oxide form. Doping with Zr and/or La further increases thermal stability of the catalysts, but the initial activity is slightly reduced. It is particularly advantageous to introduce Zr and/or La dopants via corresponding element-doped aluminum oxides. The content of the dopant compounds in the novel catalyst is in general from <NUM> to <NUM>% by weight, preferably from <NUM> to <NUM>% by weight.

The catalyst may contain further metallic active components. Such metallic active components are preferably metals of the 8th subgroup of the Periodic Table of the Elements, particularly preferably Pd, Pt, Ru or Rh. As a result, it is possible to obtain catalysts which not only are very active at high temperatures but have a very high activity at temperatures as low as below <NUM>. The catalysts can therefore be used in a wide temperature range, which is a major advantage in the case of adiabatically operated N<NUM>O decomposition processes. The amount of the metals of the 8th subgroup in the novel catalyst is in general from <NUM> to <NUM>% by weight, preferably from <NUM> to <NUM>% by weight.

Such a catalyst can be prepared, for example, by combining CuAl<NUM>O<NUM> with one or more further metals M, selected from the group consisting of Sn, Pb, Zn, Mg, Ca, Sr and Ba as oxide or salt or in elemental form, and subsequently calcining the mixture at from <NUM> to <NUM> and a pressure in the range from <NUM> to <NUM> bar.

In one embodiment, the catalyst is preferably prepared using zinc, magnesium, calcium, strontium and/or barium as oxide or salt or in elemental form in addition to CuAl<NUM>O<NUM>. The catalyst is preferably free of noble metals.

The present invention also relates to a catalyst body for the decomposition of N<NUM>O as described herein, wherein the catalyst contains a compound of the formula MxAl<NUM>O<NUM>, where M is Cu or a mixture of Cu with one or more further metals M, selected from the group consisting of Sn, Pb, Zn, Mg, Ca, Sr and Ba, preferably Zn, Mg, Ca, Sr and/or Ba, more preferably Zn and/or Mg, and x is from <NUM> to <NUM>.

To prepare the catalyst, use is made of CuAl<NUM>O<NUM> of which from <NUM> to <NUM>% by weight, preferably from <NUM> to <NUM>% by weight, particularly preferably from <NUM> to <NUM>% by weight, is present as spinel. It is particularly preferably completely in the form of spinel. Mixing with one or more further metals M, selected from the group consisting of Sn, Pb, Zn, Mg, Ca, Sr and Ba, is preferably carried out at from <NUM> to <NUM>. , particularly preferably from <NUM> to <NUM>. , and preferably at pressures of from <NUM> to <NUM> bar, particularly preferably at atmospheric pressure. Mixing can be carried out, for example, by spraying, mechanical mixing, stirring or kneading of the milled solid of the composition CuAl<NUM>O<NUM>. Particular preference is given to impregnation of the unmilled solid. During the calcination after the mixing with the additive, the copper is preferably replaced at least partly by the additional metal. The finished catalyst preferably comprises at least <NUM>%, particularly preferably at least <NUM>%, in particular at least <NUM>%, of a spinel phase.

It is possible to use not only oxides of the metals or the metals in metallic form, but also their salts. Examples are carbonates, hydroxides, carboxylates, halides and oxidic anions such as nitrites, nitrates, sulfides, sulfates, phosphites, phosphates, pyrophosphates, halites, halates and basic carbonates. Preference is given to carbonates, hydroxides, carboxylates, nitrites, nitrates, sulfates, phosphates and basic carbonates, particularly preferably carbonates, hydroxides, basic carbonates and nitrates. The additional metal is particularly preferably in the oxidation state +<NUM>. Preference is given to using Zn, Mg, Ca, Sr and/or Ba, in particular Zn and/or Mg.

The preparation of the starting oxide of the composition CuAl<NUM>O<NUM>, preferably in the form of a spinel, is known from, for example, <NPL>. Preference is given to impregnating an Al<NUM>O<NUM> support with a solution of an appropriate salt. The anion is then preferably decomposed thermally to form the oxide. It is also possible to mix the salt with the aluminum compound (for example in suspension with subsequent spray drying), compact it and then bring it into the desired shape, followed by calcination.

The catalyst preferably comprises from <NUM> to <NUM>% by weight of CuO, from <NUM> to <NUM>% by weight of the further metal oxide M(II)O of the one or more of the further metals, selected from the group consisting of Sn, Pb, Zn, Mg, Ca, Sr and Ba, in particular ZnO, and from <NUM> to <NUM>% by weight of Al<NUM>O<NUM>.

Apart from the spinel, preferably small amounts of CuO and further metal oxide are also present. Preferably, not more than <NUM>% by weight of CuO and not more than <NUM>% by weight of ZnO are present.

Suitable catalysts are also described in <CIT> and <CIT>. Further examples of preparations of catalysts which can be used according to the present invention may be taken from the documents cited.

The catalyst preferably has a BET surface area of from <NUM> to <NUM><NUM>/g. The porosity is preferably in the range from <NUM> to <NUM>/g.

According to the present invention, the catalyst is preferably used in the star extrudate form described as a fixed bed. The thickness of the fixed bed is preferably from <NUM> to <NUM>, particularly preferably from <NUM> to <NUM>. The residence time over the catalyst for the decomposition of N<NUM>O is preferably less than <NUM>.

The use of the catalyst directly in the reactor for the catalytic oxidation of ammonia leads to complete degradation of N<NUM>O, with nitrogen oxides being formed. The nitrogen oxides formed in the oxidation of ammonia are not degraded over this catalyst. The catalyst has a high activity. As a result of the low height of the catalyst bed and the star shape of the catalyst having six lobes, only a small pressure drop occurs in the reactor. No additional heating or cooling is required for the removal of N<NUM>O. Since the reactors are built for accommodating catalyst gauzes, rebuilding of a nitric acid plant is generally not necessary.

The catalyst body can be formed by various techniques including extrusion, additive manufacturing (like 3D printing) or tableting. Preferably, the ceramic bodies are prepared by extrusion.

Star-shaped bodies or extrudates can be defined as objects having some kind of central part or core, with three or more extensions on the circumference thereof. An advantageous property of the star-shaped extrudates is the fact that the ratio of geometric surface area to volume is more advantageous than in the case of conventional cylindrical extrudates or tablets.

According to the present invention, it has been found that by employing this specific six lobe geometry of the catalyst body, the geometric surface area GSA can be maximized while minimizing the pressure drop in a packed bed of the catalyst bodies, preferably extrudates, with regard to known star-shaped extrudates. Specifically, the gain in GSA can be higher than the penalty in the pressure drop experienced in such packed bed. Furthermore, a high side and/or bulk crushing strength can be obtained.

The gain in geometric surface area GSA in relation to a pressure drop is specifically achieved in a packed bed, so that not only the behavior of an individual extrudate is improved, but also the behavior of a packed bed of the extrudate.

The star-shaped catalyst bodies according to the present invention combine an advantageous property profile including high geometric surface area GSA and low pressure drop when in a packed bed. They preferably also are mechanically stable, and preferably have a high side crushing strength, high bulk crushing strength, and low attrition.

A high geometric surface area typically leads to a high activity of the catalyst bodies in particular in chemical reactions that are mass-transfer (diffusion) limited.

According to the present invention it has been found that a six-lobe star-shaped catalyst body, is superior to a five-lobe star-shaped extrudate.

Among other parameters, the size (diameter, length) of the catalyst body or catalyst extrudate, the slope of the lobes from intersection to top, the number of lobes, the sharpness of lobes, the depth of lobes and the size of the extrudates were varied, leading to the above improved star-shaped catalyst body or extrudate. The lobes can also be described as flutes or fingers of the stars.

The advantageous properties of the catalyst bodies, preferably extrudates, shall be maintained for a long time upon practical use, in which attrition of the catalyst bodies cannot be totally avoided. By employing the specific shape according to the present invention, however, a long-term stability of the properties of the catalyst bodies, can be achieved.

The geometry of the preferred star-shaped catalyst bodies used according to the present invention can be further illustrated with regard to <FIG> showing a cross-sectional view of the body or extrudate. The die used for extrusion will have openings of this shape, taking into consideration a possible shrinking of the paste after extrusion upon drying and possible calcination.

<FIG> show a six-lobe star shape, wherein the major part of the lobe outer walls is straight.

A circle connecting the intersections of the lobes, as shown in <FIG>, has a radius r1, whereas the maximum radius extends from the center of the cross-section to the maximum radius, i.e. the farthest end of the lobes, denoted r2. The ratio of the maximum radius r2 in the star to radius r1 of the circle connecting the intersections of the lobes is in the range of from <NUM> to <NUM>, preferably from <NUM> to <NUM> or <NUM> to <NUM>, most preferably from <NUM> to <NUM>.

The ratio x2 to r1 is <NUM> to <NUM>, preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, for example <NUM>.

The ratio of the area F1 inside this circle to the summed area F2 of the lobes outside the circle is in the range of from <NUM> to <NUM>, preferably from <NUM> to <NUM>, most preferably from <NUM> to <NUM>, for example <NUM>.

<FIG> also shows a radius r3 which extends to the endpoints of the straight part of the lobes, starting at the center of the cross-section and ending at the middle point of a straight line connecting the two endpoints of the straight sides of one lobe.

The ratio r2 to r3 is preferably from <NUM> to <NUM>, more preferably from <NUM> to <NUM>, most preferably from <NUM> to <NUM>, for example <NUM>.

Preferably, each lobe has straight outer walls and a rounded top. Preferably, each lobe has straight outer walls with a rounded top, wherein the ratio of the length x1 from the intersection I of one lobe with neighboring lobes to the end of the straight walls to the distance x2 between two intersections I of one lobe with neighboring lobes is from <NUM> to <NUM>, more preferably <NUM> to <NUM>, most preferably <NUM> to <NUM>, for example <NUM>. The distance x2 is shown in <FIG>.

Preferably, each lobe has straight outer walls with a rounded top, wherein the angle α between the straight wall and the straight line x2 between two intersections I of one lobe with neighboring lobes is from <NUM> to <NUM> degrees, preferably from <NUM> to <NUM> degrees, more preferably <NUM> to <NUM> degrees, most preferably <NUM> to <NUM> degrees, for example <NUM> degrees. This angle is also shown in <FIG>.

Preferably, the ratio of the length x2 between two intersections I of one lobe with neighboring lobes to the length x3 between the ends of the straight walls is from <NUM> to <NUM>, preferably from <NUM> to <NUM>, more preferably <NUM> to <NUM>. The respective lengths x2 and x3 as well as the intersections I are shown in <FIG>.

Preferably, each lobe has straight outer walls with a rounded top, and the ratio of the lobe area of the trapeze confined by the straight walls of a lobe and the outer-lobe area outside this trapeze is from <NUM> to <NUM>, preferably from <NUM> to <NUM>, more preferably <NUM> to <NUM>, most preferably <NUM> to <NUM>, for example <NUM>. The trapeze area F3 and the outer lobe area F4 are shown in <FIG>.

The rounded top has a radius of preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, most preferably <NUM> to <NUM>, for example <NUM>.

Preferably, the cross-section area of the extrudate is from <NUM> to <NUM><NUM>, preferably from <NUM> to <NUM><NUM>, preferred from <NUM> to <NUM><NUM>.

Preferably, the maximum radius r2 is from <NUM> to <NUM>, preferably from <NUM> to <NUM>, preferred from <NUM> to <NUM>, more preferably <NUM> to <NUM>.

As indicated above, the use of star-shaped extrudates is important in terms of pressure drop in relation to accessibility of the external surface of the extrudates. This also plays an important role in eliminating diffusion problems.

The present invention also relates to the use of a star shaped catalyst body as defined herein for the decomposition of N<NUM>O in N<NUM>O containing gas mixtures.

The present invention further relates to a reactor for the catalytic decomposition of nitrous oxide N<NUM>O in a gas mixture obtained in the preparation of nitric acid by catalytic oxidation of ammonia which contains in this order in the flow direction a noble metal gauze catalyst, a heat exchanger and a catalyst bed for the decomposition of N<NUM>O which is installed between the noble metal catalyst and the heat exchanger so that the hot gas mixture obtained from the catalytic oxidation of ammonia is brought into contact with the catalyst for the decomposition of N<NUM>O before subsequent cooling, characterized in that the catalyst bed contains the catalyst for the decomposition of N<NUM>O in the form of a star-shaped body having six lobes as defined above.

The table shows an overview of the calculated properties of the prior art geometries (<FIG>) and the geometry according to the invention (<FIG>) in two different sizes. The small trilobes have a high GSA/volume ratio, the large five-pointed stars have a low pressure drop. The new geometry combines these two beneficial features.

The values for delta p are calculated based on CFD (Computational Fluid Dynamics). GSA is the surface area of the modeled catalyst geometry in the simulations and the volume respectively. Both are the numerically calculated values from the catalyst model and based on the real geometries.

The geometric surface area (GSA) and pressure drop for a packed bed of the extrudates of different shapes and sizes were obtained from a detailed numerical simulation.

Claim 1:
A process for the catalytic decomposition of nitrous oxide N<NUM>O in a gas mixture obtained in the preparation of nitric acid by catalytic oxidation of ammonia, in a reactor which contains in this order in the flow direction a noble metal gauze catalyst and a heat exchanger, over a catalyst for the decomposition of N<NUM>O which is installed between the noble metal gauze catalyst and the heat exchanger so that the hot gas mixture obtained from the catalytic oxidation of ammonia is brought into contact with the catalyst for the decomposition of N<NUM>O before subsequent cooling, characterized in that the catalyst is in the form of a star-shaped body having six lobes, wherein the ratio of the maximum radius r2 in the star to radius r1 of a circle connecting the intersections of the lobes is in the range from <NUM> to <NUM>, the ratio of the area F1 inside this circle to the summed area F2 of the lobes outside this circle is in the range of from <NUM> to <NUM>, the ratio of the distance x2 between the two intersections I of one lobe with neighboring lobes and the radius r1 of the circle is in the range of from <NUM> to <NUM>.