DE2440433A1 - Nickel-ceric oxide catalysts - for redn. of nitrogen oxides in exhaust and waste gases, active over wide temp. range - Google Patents

Abstract

Catalysts for the redn. of N oxides contain CeO2 and NiO in a molar ratio of 0.05-2.5:1, esp. 0.1-1.5:1. They can also contain 7.5-25, pref. 10-20 wt. % Rh or 10-50, pref. 20-30 wt. % Ru or 7.5-25, pref. 10-20 wt. % Rh and 15-50, pref. 20-30 wt. % Ru (w.r.t. 100 pts. NiO+CeO2) and also Cu, Ca, Ag, Zn, Cd, Co and/or Ce sulphides in an amt. of 0.1-2 mole/mole Ru or Mg, Ca, Sr, Ba, Al, Ti, Zr, Mn, Fe, Ni, La, Ce, Nd, Sm, Gd and/or Yb oxide(s) in an amt. of 0.5-2 mole/mole Ru. The catalysts are used for the redn. of N oxides at a temp. >500 degrees C, esp. in exhaust and waste gases from IC engines, power stations, chemical works etc. These catalysts have high activity over a wide temp. range, resistance to catalyst poisons and acceptable or low cost. They also minimise the formation of NH3 during redn.

Description

Catalysts for the Reduction of Nitrogen Oxides The invention relates to a catalyst for the reduction of nitrogen oxides and a method for the reduction of nitrogen oxides, in particular a reduction catalyst and a process for the reduction of nitrogen oxides in exhaust gases and exhaust gases from Internal combustion engines, power plants, chemical plants and the like.

Oxides of nitrogen, hereinafter referred to as NOx, are reduced to free nitrogen (N2) by the following reactions: The reduction of nitrogen oxides has hitherto been carried out using catalysts, for example palladium or nickel oxide. However, palladium is expensive and susceptible to catalyst poisons, for example lead compounds or sulfur compounds, which are contained in the exhaust gases and exhaust gases, so that this susceptibility considerably deteriorates its catalytic activity and limits its practical use as a reduction catalyst. Nickel oxide, on the other hand, is rarely used alone, but in combination with an accelerator such as barium oxide or neodymium oxide. This combined catalyst is somewhat more favorable than palladium in terms of resistance to catalyst poisons, but is unsatisfactory in terms of service life due to the severe deterioration in catalytic activity over time when used at high temperatures.

In many cases, find out by the reaction schemes above (1), (2) and (3) described reduction reactions in general under reaction conditions instead, which are difficult to keep constant. In particular, the cleaning takes place of the exhaust gases from internal combustion engines of vehicles under conditions under which it is impossible to consider various factors affecting the speed of the chemical Affect reaction, e.g.

the composition, concentration, amount and temperature of the reactive Gases, keep constant, so that catalysts are used for this purpose should, have to meet various requirements: their catalytic activity must remain high over a wide range of temperatures, and they must be against Catalyst poisons contained in the exhaust gases must be resistant.

Furthermore, the cost of these catalysts must be acceptable or lower be.

The invention has the task of providing one for the reduction of Oxides of nitrogen specific catalyst meeting the above requirements fulfilled and at the same time the formation of ammonia during the reduction of nitrogen oxides Able to prevent as much as possible, make available. The invention is further to a process for the reduction of nitrogen oxides using the inventive catalyst directed.

It has been found that the objects set by the invention by a catalyst consisting of a mixture of nickel oxide and cerium oxide, or by a catalyst of this type which also has other various catalyst components contains, or by a catalyst of this kind, which on certain physico-chemical Paths that have been dealt with can be drawn.

The catalysts according to the invention can be prepared by Impregnation of porous carrier materials, e.g. aluminum oxide, silicon dioxide, Magnesium oxide and zirconium oxide, with aqueous solutions of salts of nickel and cerium, e.g. with aqueous solutions of acetates, nitrates1, carbonates and sulfates, or through Baking of the nickel and cerium salts on a carrier made of a heat-resistant Sheet metal is made of aluminum oxide on its surface with a slightly rough film or the like. Is coated, and then calcining the catalyst components for about 20 minutes to 3 hours at a temperature in the range from about 600 to 9000C in an oxidizing atmosphere, e.g. air. The nickel and cerium acetates are preferred. By using these acetates, catalysts with a small particle size can be produced and produce a large surface area easily.

The ratio of ceria to nickel oxide in the mixture of nickel oxide and ceria is an important factor affecting the catalytic activity of the catalyst determined according to the invention. The CeO2 / NiO molar ratio is about oS; 1 to 2.5: 1, preferably about 0.07: 1 to 2.0: 1. Particularly preferred is a Molar ratio of about 0.1: 1 to 1.5: 1. It should be noted, however, that for the intended purposes, suitable CeO2 / NiO molar ratio does not depend on the above mentioned limits is limited. These areas were determined with catalysts the nickel oxide / cerium oxide mixtures with different CeO2 / NiO molar ratios, those shown in Fig. I are contained. These catalysts were filled into a catalyst converter through which a sample gas consisting of 0.5% NO, 2% CO and 97.5% N2 at a space flow velocity of 20,000 cm3 gas / cm3 Catalyst bed was passed per hour while the converter was heated so that that the reaction temperature was kept at 5000C.

The percentage conversions of the nitrogen oxides used were determined and graphically on semi-log paper in the manner illustrated in Figure 1 shown, with the determined sales using the arithmetic Scale and using the CeO2 / NiO molar ratios of the catalysts used of the semi-logarithmic scale were plotted.

The catalysts containing the mixture of nickel oxide and cerium oxide according to the invention have high catalytic activity and high durability against catalyst poisons, e.g. lead and sulfur compounds. Even with continuous The nickel oxide-ceria catalysts according to show use at high temperatures of the invention a stable performance as a reduction catalyst, while the catalytic Activity decreases somewhat over time.

To illustrate these properties of the catalysts according to the invention, they were compared with conventional catalysts. A catalyst that removes the nickel oxide / Cerium oxide mixture containing a CeO2 / NiO molar ratio of 0.5: 1 was considered representative used for the invention. Catalysts, which are made of palladium on a support and consisted of a nickel oxide / barium oxide mixture on a carrier, were considered usual Catalysts used. The experiments were carried out by following the above said sample gas with a space flow velocity of 20,000 cm 3 / cm 3 catalyst bed per hour passed through a catalyst converter, each with the Catalytic converters was filled, whereby the above conditions were applied, unless otherwise stated. In Experiment I, the reduction reaction was used at carried out at a temperature of 3soOC. In experiment II, the reaction temperature was Maintained at 9000C for loo hours and then lowered to 3000C. In experiment III were the above-mentioned catalysts further with one containing 0.2 mol% of lead nitrate aqueous solution impregnated and dried, resulting in catalysts 3, the each contained 3 g, calculated as lead, per 100 cm of support with catalyst components. With these impregnated catalysts tests were carried out in which the Converter was heated so that the reaction temperature was kept at 5000C. In Experiment IV, the reduction of nitrogen oxides was used for 500 hours of a sample gas having the above composition and additionally contained 50 ppm sulfur dioxide. The conversion of the nitrogen oxides was then determined on the gases that escaped after carrying out the tests. the The results are given in Table 1.

Table 1 Catalyst amount of nitrogen oxides, ppm component Experiment I II III IV NiO + CeO2 500 700 900 800 Pd 1 ovo 4000- 6000 5000 NiO + BaO 3500 5000 4500 4000 As the values in Table 1 show, the palladium catalyst is very susceptible to catalyst poisons. The catalytic activity of the nickel oxide-barium oxide mixed catalyst decreases during operation at high temperatures, while the nickel oxide-ceria catalyst according to the invention excellent properties including high durability shows against catalyst poisons and maintenance of the catalytic activity.

When used at relatively low temperatures will, however, the conversion of nitrogen oxides by the catalysts according to the invention is which contain the nickel oxide-cerium oxide mixture, inadequate. This disadvantage, the the catalysts according to the invention can occur when they are relatively low temperatures are used, but is mitigated when low Amount of rhodium as an additional catalyst component together with the nickel oxide-cerium oxide mixture is used. Fig. 2 shows graphically the conversion of nitrogen oxides as a function of the reaction temperature varying from about 1500 to 6000C.

In this figure, the solid curve A represents the results represent, which were obtained with a catalyst according to the invention, the nickel oxide and cerium oxide in a CeO2 / NiO molar ratio of 0.5: 1, to those in Example 1 and referred to as catalyst A. the Dashed curve B represents the conversion of nitrogen oxides with the nickel oxide / cerium oxide catalyst was achieved, of about 15 parts by weight of rhodium / 100 parts by weight of nickel oxide + cerium oxide and prepared in the manner described in Example 2 and below is referred to as catalyst B. The dash-dotted curve C represents the conversion represents the nitrogen oxides, which can be achieved with a conventional rhodium catalyst, the rhodium in approximately the amount of the rhodium contained in catalyst B. and hereinafter referred to as Catalyst C. As Fig. 2 shows, with the catalyst that contains the nickel oxide / cerium oxide mixture and the addition of rhodium according to of the invention contains a higher conversion of nitrogen oxides than with a conventional one Platinum group metal catalyst achieved at relatively low temperatures is relatively active, the catalyst according to the invention also when used an amount of rhodium that is much less than in a conventional rhodium catalyst, which contains the rhodium as the only catalyst component, excellent catalytic activity having.

The amount of rhodium to be added to the nickel oxide-ceria mixture is an important factor affecting the catalytic activity when used in proportionately low temperatures. Fig. 3 shows the relationship of the conversion of nitrogen oxides to the reaction temperatures varying from 1ovo0 to 3000e. Fig. 3 shows that the rhodium in an amount of about 7.5 to 25 parts, preferably about 10 up to 20 parts by weight per 100 parts by weight nickel oxide / cerium oxide mixture is used.

If the gas mixture contains hydrogen or moisture, the Reduction of nitrogen oxides when used with a reducing agent such as carbon dioxide or hydrocarbons are mixed, also to form ammonia gas.

It is believed that this ammonia gas is formed by reducing nitrogen oxides with hydrogen gas, which is formed by the reaction of hydrogen or carbon monoxide present in the gas mixture with water. The reduction of nitrogen oxides to nitrogen proceeds as follows: The ammonia gas formed in this way can easily be oxidized in an oxidizing atmosphere and converted back into nitrogen oxides. This means that this re-formation of nitrogen oxides does not contribute to a substantial reduction in the nitrogen oxides in the exhaust gases and exhaust gases. It is therefore sufficient in practice to allow the reaction to proceed according to reaction scheme (2) and not according to scheme (4).

Figures 4 and 5 both show graphically the conversion of nitrogen oxides in free nitrogen (N2) and ammonia (NH3) depending on the reaction temperature. In Fig. 4 where the catalyst was used and in Fig. 5 where the Catalyst B was used, illustrate the with solid oblique lines hatched areas show the conversion of nitrogen oxides to free nitrogen while the areas hatched with the dashed lines represent the conversion of nitrogen oxides to represent ammonia. These tests were carried out as follows: gas mixtures, the 750 ppm NO, 375 ppm hydrocarbon, 1.25% CO, 9% C02, 0.3% 02, 3% H20, 1% H2 and 85% N2 were over each of the catalysts to be tested at a space flow rate of 20,000 cm3 gas / cm3 catalyst bed / hour.

directed. For the escaping gases that passed over the catalytic converters the conversion of the nitrogen oxides to free nitrogen and ammonia was measured.

As FIGS. 4 and 5 show, remain when the reduction is carried out a temperature below 5000C the nitrogen oxides are not converted, i.e. they are converted into ammonia in free nitrogen, or they become predominantly converted into ammonia, while at reaction temperatures above from about 5000C the formation of ammonia gas is hardly noticeable and the conversion the oxides of nitrogen in free nitrogen is effectively achieved. Accordingly, the Conversion of the nitrogen oxides into free nitrogen is kept high and that way the formation of ammonia gas is largely eliminated when the catalytic reduction with a reduction catalyst, the nickel oxide-cerium oxide mixture or this Contains mixture and rhodium, carried out and a gas mixture over the catalyst, the nitrogen oxides and a reducing agent, e.g. carbon dioxide, hydrogen and Contains hydrocarbons, is passed at a temperature above about 500 ° C. It is believed that this phenomenon is due to the choice of a response determined by the Reaction scheme (2) and not represented by the reaction scheme (4) is.

To prevent the formation of ammonia gas when using the nickel oxide / cerium oxide catalyst or the nickel oxide / cerium oxide / rhodium catalyst in the catalyst converter, for example of an internal combustion engine is used, the gas temperature at which the The catalyst is brought into contact with the exhaust gases, kept above 5000C will. However, since the temperature of the exhaust gases at the time of starting and Start-up is relatively low, must or the like in the engine. a device is provided which heats the exhaust gases and increases their temperature to over 5000C. this would complicate the construction of the engine. When the catalyst converter is denser If placed at the exhaust port of the engine, the conversion of nitrogen oxides would occur occur before the temperature of the exhaust gases drops, causing the formation of ammonia gas is prevented at the time of start-up and start-up. The reaction temperature can rise to around loooOC for a long time if the converter is in continuous operation, so that there is a problem in the heat resistance of the catalyst can. It would therefore not be necessary to switch the motor or the like. with a device for Preheat the exhaust to provide if a catalyst would be found the nitrogen oxides can effectively convert into free nitrogen and thereby the Prevents the formation of ammonia gas, 7 Such a catalyst would also be the resist adverse change caused by the heat of the exhaust gases will. will.

/ even if it is used according to the invention at a relatively low temperature it has been found that this problem is solved when ruthenium is added to the nickel oxide-cerium oxide mixture or this mixture, which also contains rhodium, is added. When adding Ruthenium to the nickel oxide / cerium oxide mixture, the amount of ruthenium is about 10 to 50 parts, preferably about 20 to 30 parts by weight per 100 parts by weight of nickel oxide / cerium oxide mixture. When adding ruthenium to the nickel oxide / cerium oxide / rhodium system amounts to the amount of ruthenium is about 15 to 50 parts, preferably about 20 to 30 parts by weight per loo parts by weight of nickel oxide / cerium oxide mixture. Figures 6 and 7 are graphs the conversion of nitrogen oxides to free nitrogen and ammonia gas as a function on the reaction temperature. The results were obtained by using gas mixtures of the same composition as described above in connection with FIGS 5 was mentioned at a space flow velocity of 20,000 V / V / hour. passed through became. In the case of Figure 6, the nickel oxide / ceria-ruthenium catalyst system was used and in the case of Fig. 7, the nickel oxide / ceria / rhodium / ruthenium catalyst system used.

Figs. 8 and 9 illustrate the relationship of conversion of oxides of nitrogen in free nitrogen and its conversion into ammonia gas to the amounts of ruthenium at a reaction temperature of 4000C, In the case of F. 8 the ruthenium was dem Nickel oxide / cerium oxide mixture and, in the case of FIG. 9, the nickel oxide / cerium oxide / rhodium catalyst system added.

The formation of ammonia is suppressed more effectively when the nickel oxide / cerium oxide / ruthenium catalyst system or one or more of the nickel oxide / cerium oxide / rhodium / ruthenium catalyst system Metal sulfides, e.g. copper sulfide, potassium sulfide, silver sulfide, zinc sulfide, cadmium sulfide, Cobalt sulfide and cerium sulfide, being added, being copper sulfide, potassium sulfide and Silver sulfide are preferred, or if the catalyst is in a stream of sulfur compounds, e.g. hydrogen sulfide or carbon disulfide, about 30 minutes at about 400 kept up to 5000C or applied to the catalyst and fine sulfur powder the catalyst in a stream of a mixture of hydrogen with an inert gas is held at about 400 to 5000C for about 30 minutes to 2 hours.

The added to the catalyst components according to the invention Amount of metal sulfide can depend on the gas composition, the reaction temperature and space flow velocity of the gas to be treated, but is usually about 0.1 to 2 moles per mole of ruthenium. 10 to 16 graphically show the course of the conversion the nitrogen oxides in free nitrogen and its conversion into ammonia gas different reaction temperatures, the in connection with Fig. 4 and Fig. 5 mentioned gas mixtures at a room flow rate of 20,000 V / V / hour.

were passed and in the case of FIG. 10, the catalyst F, im In the case of FIG. 11, the catalyst G, in the case of FIG. 12, the catalyst H, in the In the case of FIG. 13, the catalyst I, in the case of FIG. 14, the catalyst J, in the case of of Fig. 15 the catalyst K and in the case of Fig.

16 the catalyst L was used. The catalysts F to L are in the examples below.

That present in the catalyst components according to the invention Ruthenium can turn into ruthenium dioxide (Ru02) and further to ruthenium tetraoxide (Ru04) are oxidized if it occurs that the composition of the exhaust gases in the catalyst converter during the operation of the internal combustion engine or the like. so changes that the reducing atmosphere becomes an oxidizing atmosphere transforms. In particular, ruthenium tetraoxide is extremely toxic to humans, though it is emitted into the atmosphere. Furthermore, the volatilization of the ruthenium as ruthenium tetraoxide results in a reduction in the amount of ruthenium. Simultaneously the effectiveness of the catalyst is impaired, so that it is advisable to prevent the formation of ruthenium oxides. To this end, a material is made added, which is able to form a mixed oxide with ruthenium. As such materials are preferably oxides of metals that have an ionic radius greater than about 1.05 Angstroms, e.g.

Calcium, ~ strontium, barium, lanthanum, neodymium, samarium and gadolinium, preferred. Oxides of other metals, e.g. magnesium, aluminum, titanium, zirconium, Manganese, iron, nickel, cerium and ytterbium can also be used. These Metal oxides can form a solid solution with ruthenium at high temperatures and bound in the form of a mixed oxide. The use of these metal oxides thus prevents the oxidation of ruthenium and its volatilization as ruthenium tetraoxide, so that the ruthenium is retained in the reaction system. After the oxidizing Atmosphere has passed back into the reducing atmosphere, the ruthenium can serve again as a catalyst component having excellent catalytic activity. The metal oxide can be added in an amount of about 0.5 to 2 moles per mole of ruthenium will.

Table 2 shows the amount of ruthenium retained in%. The values were determined with catalysts in which the metal oxides are ruthenium dioxide were added in a metal atom / ruthenium atom molar ratio of 1: 1 and that Mixture was calcined in air at 11000C for 2 hours. The one left behind The amount of ruthenium was determined by qualitative analysis and the composition of the calcined Products determined by X-ray analysis.

The results are given in Table 2 below: Tabel 2 metal oxide ions. calcined dius, Å ruthenium,% main products magnesia 0.75 25 MgO, Ru02 calcium oxide 1.05 70 CaRu04, Ca0, RuO2 strontium oxide 1.18 98 Sr2RuO4, SrRu03 Barium Oxide 1.38 92 Ba2Ru04 Aluminum Oxide 0.55 25 A1203, RuO2 Titanium Dioxide 0.60 25 TiO2, RuO2 zirconium oxide 0.80 25 ZrO2, RuO2 manganese oxide 0.70 30 Mn2O3, RuO2 iron oxide 0.80 30 Fe2O3, RuO2 nickel oxide 0.74 30 #i; 0, RuO2 lanthanum oxide 1.22 99 La2Ru2O7 cerium oxide 0.94 30 CeO2, RuO2 neodymium oxide 1.15 95 Nd2Ru2O7 samarium oxide 1.13 95 Sm2Ru2O7 gadolinium oxide l.ll 94 Cd2RuO7 ytterbium oxide 1.00 70 Yb203, RuO2 It should be noted that through the Addition of the metal oxides mentioned to the catalyst components according to the invention the catalytic activity is not impaired. Table 3 compares the conversion the nitrogen oxides in the initial phase with the conversion during the time after Heat. In the underlying experiments, different metal oxides were used in each case the nickel oxide / cerium oxide / rhodium / ruthenium catalyst system in an amount of 2 Moles per mole of ruthenium added in the catalyst and gas mixtures, the had the composition given in connection with FIGS. 4 and 5, 2 hours at 11000C and a space velocity of 20,000 V / V #. For comparison, the same catalyst system was used without the addition of metal oxides. The results are given in Table 3 Table 3 Conversion of nitrogen oxides,% Metal in the beginning after the oxide phase heating SrO 99 95.5 comparative catalyst. 99.9 80 CaO 99 95 comparative catalyst 99.9 85 BaO 99 96.5 comparative catalyst 99.9 83 La2O3 99.9 96 Disintegration catalytic. 99.9 85 Nd2O3 99.9 95 9 Comparative cat. 99.9 85 Sm2O3 99.9 98.2 Comparative cat. 99.9 85 Gd2O3 99.9 93.3 Comparative cat. 99.9 85 The invention is further illustrated by the following examples.

Example 1 Aluminum granules with a grain diameter of about 4 mm, a porosity of about 40% by volume, a bulk density of about 0.8 and one specific surface area of about 160 m2 / g, measured by the BET method Maintained in pure boiling water for 30 minutes and removed any adhering to it Impurities 6 times washed with water. This pretreated The carrier was coated with a solution of 249 parts by weight of nickel acetate tetrahydrate Ni (CH3COO) 2.4H2O and 157 # ew.-parts cerium acetate monohydrate Ce (CH3C00) 2. H20 in 3000 parts by weight of water impregnated using the vacuum impregnation method. This impregnated carrier was placed in an electric furnace and calcined at 9000C for 1 hour. The calcined The carrier was impregnated again with the aqueous solution and then calcined. This treatment was repeated 6 times to obtain a catalyst which Nickel oxide plus ceria in an amount of about 3 g / 10000 cm3 of carrier. That The ratio of ceria to nickel oxide in the catalyst was 0.5: 1 (CeO2; NiO). This catalyst is referred to as Catalyst A for the sake of simplicity.

Example 2 The nickel oxide and cerium oxide produced according to Example 1 containing catalyst was impregnated with an aqueous solution, the o, 3 wt .-% Contained rhodium chloria, and then 10 minutes at 6000C in a nitrogen atmosphere, which contained 1% by volume of hydrogen, calcined.

This treatment was repeated 5 times to obtain a catalyst the rhodium in an amount of about 15 parts by weight per 100 parts by weight of nickel oxide plus ceria. This catalyst is referred to as Catalyst B.

Example 3 The nickel oxide and cerium oxide produced according to Example 1 containing catalyst was impregnated with an aqueous solution which 0.5 wt .-% Contained ruthenium chloride and then reduced in hydrogen at 6000C for 30 minutes. This treatment was repeated 6 times to obtain a catalyst which Contained 30 parts by weight of ruthenium per 100 parts by weight of NiO / CeO2 component. This Catalyst is referred to as Catalyst D.

Example 4 The nickel oxide and cerium oxide prepared according to Example 1 containing catalyst was impregnated with an aqueous solution, the o, 5 wt .-% Contained rhodium chloride and 0.8% by weight ruthenium chloride, and then in a nitrogen atmosphere, which contained 1% by volume of carbon nonoxide, reduced for 30 minutes at 6000 ° C. This treatment was repeated 4 times to obtain a catalyst containing 20 parts by weight Rhodium and 30 parts by weight of ruthenium per 100 parts by weight of nickel oxide / cerium oxide component contained. This catalyst is referred to as Catalyst E.

Example 5 The ternary nickel oxide produced according to Example 3 / Cerium oxide / RuthenilZ catalyst was in a stream of a gas mixture of 3 vol% Maintained hydrogen sulfide and 97% by volume nitrogen at 4200 ° C. for 1 hour. This Catalyst is referred to as Catalyst F.

Example 6 The nickel oxide / cerium oxide / rhodium-ruthenium catalyst system prepared according to Example 4 was in a stream of a gas mixture of 5 parts by volume of carbon dioxide and 95 parts by volume Nitrogen held at 4000C for 1 hour.

This catalyst is referred to as Catalyst G.

Example 7 The catalyst prepared according to Example 4 was with a suspension of 1 g of sulfur that is mechanically ground to a fine powder had been impregnated in 1 l of water and in a stream of a gas mixture Maintained 4 vol% hydrogen and 96 vol% nitrogen at 4000C for 1 hour and 20 minutes. This reduction catalyst is referred to as catalyst H.

Example 8 The catalyst prepared according to Example 3 was with an aqueous solution of 3 wt .-% copper sulfide and impregnated in a hydrogen atmosphere Reduced for 1 hour at 600 C. This treatment was repeated 3 times, one being Catalyst was obtained, the copper sulfide in an equivalent to the ruthenium Amount contained. This catalyst is referred to as Catalyst I.

Example 9 The catalyst prepared according to Example 4 was on treated in the manner described in Example 8 to add copper sulfide to it. This catalyst is referred to as Catalyst J.

Example lo The catalyst prepared according to Example 4 was with an aqueous solution of 5 wt .-% potassium sulfate and impregnated in Example 8 described manner reduced to apply potassium sulfide. This catalyst is referred to as catalyst K.

Example 11 The catalyst prepared according to Example 4 was with an aqueous solution of 1 wt .-% silver sulfate and impregnated in Example 8 described manner, wherein a catalyst with applied silver sulfide was obtained. This catalyst is referred to as Catalyst L.

Example 12 The catalyst prepared according to Example 1 was with impregnated with a solution of 3% by weight Strontium nitrate and in the air for 1 hour heated to 80006. This treatment was repeated 3 times. The catalyst was then with an aqueous solution of 0.5% by weight of rhodium chloride and 1% by weight of ruthenium chloride impregnated and in a nitrogen atmosphere, the 1 vol% carbon oxide contained, 3o M4 grooves reduced at 6000C. This treatment was repeated 6 times, whereby a catalyst was obtained comprising about 20 parts by weight of rhodium and about 40 parts by weight Parts by weight of ruthenium per 100 parts by weight of nickel oxide / cerium oxide component. When working in which the mixed solution of rhodium and ruthenium chloride is used, the strontium oxide is hardly reduced and remains in the form of the oxide. The amount of strontium oxide was about 2 moles per mole of ruthenium in the catalyst.

The letters A through L appearing in the figures correspond the letters which, for the sake of brevity, designate the according to Examples 1 - 11 produced catalysts are used.

Claims (10)

Claims 1. Catalysts for the reduction of nitrogen oxides, thereby characterized in that they contain nickel oxide and cerium oxide in the molar ratio of CeO2 to NiO in the range from about o, oS: 1 to 2.5 5 1. 2. Catalysts according to claim 1, characterized in that the CeO2 / NiO molar ratio about 0.07: 1 to 2.0: 1, preferably about 0.1: 1 to 1.5: 1. 3. Catalysts according to claim 1 and 2, characterized in that they added rhodium in an amount of about 7.5 to 25 parts by weight, preferably from about 10 to 20 parts by weight per 100 parts by weight of nickel oxide-ceria component contain. 4. Catalysts according to claim 1 and 2, characterized in that they added ruthenium in an amount of 10 to 50 parts by weight, preferably from about 20 to 30 parts by weight per 100 parts by weight of nickel oxide-ceria components contain. 5. Catalysts according to claim 3, characterized in that they are added Ruthenium in an amount of about 15 to 50 parts by weight, preferably from 20 to Contains 30 parts by weight per 100 parts by weight of nickel oxide-ceria component. 6. Catalysts according to claim 4 and 5, characterized in that they copper sulfide, calcium sulfide, silver sulfide, zinc sulfide, cadmium sulfide, cobalt sulfide and cerium sulfide in each case alone or in groups in an amount of about 0.1 to 2 Moles per mole of ruthenium included. 7. Catalysts according to claim 4 and 5, characterized in that them in a stream containing a sulfur compound, or after application of fine sulfur powder in a gas stream consisting of hydrogen and an inert gas have been heated. 8. Catalysts according to claim 4 and 5, characterized in that they oxides of magnesium, calcium, strontium, barium, aluminum, titanium, zirconium, manganese, Iron, nickel, lanthanum, cerium, neodymium, samarium, gadolinium and ytterbium, respectively included alone or in groups. 9. Catalysts according to claim 8, characterized in that they contain the metal oxides in an amount of about 0.5 to 2 moles per mole of ruthenium. 10. Use of the catalysts according to Arspruch 1 to 9 for the reduction of nitrogen oxides, whereby the nitrogen oxides are at a temperature above of about 5000C passes over the catalysts. blank page