GB1563647A

GB1563647A - Oxidation-reduction catalysts

Info

Publication number: GB1563647A
Application number: GB25232/77A
Authority: GB
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1976-06-17
Filing date: 1977-06-16
Publication date: 1980-03-26
Also published as: FR2354813A1; IT1080900B; DE2727198C2; JPS5325291A; ES459861A1; JPS6117544B2; FR2354813B1; DE2727198A1; CA1101398A; SE7707026L; SE438448B

Description

(54) IMPROVEMENTS IN OR RELATING TO OXIDATION REDUCTION CATALYSTS (71) We, COMMISSARIAT A L'ENERGIE ATOMIQUE, a French company, of 29 rue de la Federation, 75752 Paris, Cedex 15 France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an oxidation-reduction catalyst for the treatment of combustion gases, in particular of exhaust gases of internal combustion engines, and more precisely to a catalyst which is capable of carrying out simultaneously the oxidation of hydrocarbons and of carbon monoxide and the reduction of nitrogen oxides which are present in the exhaust gases of motor vehicles or automobiles.

The presence of contaminants such as hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust gases of motor vehicles presents a number of problems in connection with atmospheric pollution. Up to the present time, problems of this nature have been solved by treating exhaust gases with suitable catalyst which are capable of removing these pollutants and converting them to non-injurious products by oxidation or by reduction.

One of the usual practices adopted for carrying out this treatment consists in successively passing the gases to be purified over a reduction catalyst and over an oxidation cataylst. In this case, the nitrogen oxides which are present in the exhaust gases are first reduced in an oxygen-deficient atmosphere in a first catalytic converter by means of the reduction catalyst. Oxidation of the carbon monoxide and of the hydrocarbons is then carried out in a second converter by means of the oxidation catalyst.

This method of treatment is subject to a certain number of drawbacks arising in particular from the need to process the gases in two catalytic converters. This accordingly entails the need for cumbersome devices, for the injection of air between the two converters and also makes it necessary to place the first converter as close as possible to the engine cylinders since the reduction catalyst has a high start-up temperature. Moreover. this method does not make it possible to ensure complete removal of the nitrogen oxides since it is difficult to prevent a more or less substantial fraction of the nitrogen oxides which are present in the exhaust gases from being converted to ammonia in the first catalyst converter and to prevent this ammonia from being partly re-oxidized to form nitric oxide in the second converter.

In order to alleviate the above-mentioned disadvantages. it has already been proposed to treat the exhaust gases with a single catalyst which is capable of carrying out both reduction of the nitrogen oxides and oxidation of the carbon monoxide and the hydrocarbon simultaneously. However, catalysts of this type which have been developed at the present time create further problems by reason of their instability in time or their unsatisfactory efficiency at low temperatures.

An aim of the present invention is to provide a catalyst which overcomes these various disadvantages since oxidation of hydrocarbons and carbon monoxide and reduction of nitrogen oxides can now be carried out simultaneously by means of this catalyst which is also endowed with very good stability in time as well having a good efficiency at low temperatures.

To this end, there is provided a catalyst in accordance with the invention comprising a support of inert material coated with lanthanum oxide and a catalytic material comprising a mixed oxide of the perovskite type of lanthanum platinum or palladium and by rhodium.

This catalyst is particularly well suited to the treatment of exhaust gases, the composition of which corresponds to a stiochiometric adjustment of the air-fuel ratio.

Furthermore, the structure of a catalyst according to the invention endows it with good stability of time, this stability being primarily due to the combination of ruthenium and lanthanum in the mixed oxide of the perovskite type.

In some cases, the catalytic material of the catalysts in accordance with the invention further may contain iridium and/or rhenium.

Preferably, a catalyst in accordance with the invention has a weight content with respect to the total weight of said catalyst, of 1 to 3% lanthanum oxide, 0.02 to 0.1% rhodium, 0.05 to 0.2% platinum or palladium, 0.05 to 0.2% ruthenium and if necessary 0.005 to 0.05 iridium and/or 0.005 to 0.05% rhenium.

By reason of the percentage of platinum or palladium. of rhodium and of ruthenium contained in the catalyst, a good efficiency at low temperatures with respect to the principle pollutants which are carbon monoxide, the nitrogen oxides and the hydrocarbons is provided.

Preferably, ruthenium, platinum or palladium and rhodium are present in the catalyst in the respective ratios (by weight) of 3/5/1.5.

The support of inert material which can be employed in a catalyst according to the invention can be fabricated either in particulate form such as beads, right cylinders or pellets, or in a monlithic form for example, a honeycomb structure.

When the support is in a particulate form, the refractory material is peferably a refractory ceramic material, for example, alumina, magnesia, zirconia, an alumino-silicate, such as cordierite, on a spinel and more preferably a transition alumina having a high specific surface area.

When the catalytic support is in monolithic form, the inert material can be a ceramic material of the type mentioned in the foregoing but can also be formed of a metal alloy.

Monolithic supports are preferably provided with a coating of refractory oxide, for example, a transition alumina having a high specific surface area in order to increase the specific surface area of the support. In this case, the layer of alumina preferably represents 2 to 15% by weight of the total weight of the catalyst.

The invention is also concerned with a method of preparation of a catalyst which is suitable for the treatment of exhaust gases of internal combustion engines.

Further in accordance with the invention there is provided a method for producing a catalyst for treating exhaust gases comprising depositing a layer of lanthanum oxide La2O3 on a support of inert material of suitable shape; then depositing Ruthenium on the support of refractory material which has thus been coated, the deposition being preferably carried out by the impregnating of the support with an aqueous solution (which may or may not be acidified) which contains a soluble salt of ruthenium, subjecting the impregnated support to a treatment with a reducing gas at a temperature of the order of 400 to 700"C over a period of several hours; then subjecting the support to a heat treatment at a temperature within the range of 600"C to 11000C in the presence of air during a period of time which depends on the temperature and ranges from two hours to one half hour in order to synthesize a mixed oxide of the perovskite type of ruthenium and lanthanum by reaction under the conditions of treatment of ruthenium and landthanum oxide which was initially present on the support of refractory material; successively depositing on the support which has thus been treated either platinum or palladium and then rhodium; this deposition is preferably carried out by impregnating the support with an aqueous solution which may or may not be acidified and contains a soluble salt of platinum or palladium or a soluble salt or rhodium. Each impregantion is peferably followed by a treatment for reduction of the impregnated support by a reducing gas at a temperature of the order of 400 to 700"C over a period of a few hours.

In accordance with the alternative embodiment of the invention, platinum or palladium and rhodium can be deposited simultaneously on the support which has been subjected to the heat treatment. this deposition preferably being carried out by impregnating the support with an aqueous solution containing a soluble salt of platinum or palladium and a soluble salt of rhodium and by subsequent reduction of the impregnated salts to the metallic state.

In accordance with another embodiment of the invention, iridium and/or rhenium are introduced into the catalyst. In this case, deposition of rhodium, iridium and/or rhenium can be carried out either simultaneously or separately by impregnating the support with one or a number of aqueous solutions containing at least one soluble salt of rhodium, iridium and rhenium followed by reduction of the impregnated salts to the metallic state by a reducing gas, the reduction process being carried out at a temperature of the order of 400 to 700"C over a period of a few hours.

Deposition of lanthanum oxide on the support of inert material can be carried out from an aqueous solution of lanthanum nitrate. The support is impregnated with this solution, then dried and calcined in air in two steps. The first step is carried out over a period of a few hours at a temperature within the range of 350 to 600"C in order to convert the lanthanum nitrate to lanthanum oxide. The second step also lasts a few hours and is carried out at a temperature within the range of 600 to 11000C in order to stabilize said oxide.

The starting lanthanum nitrate solution contains a quantity of lanthanum which is necessary in order to obtain 1 to 3 % oxide by weight with respect to the total weight of catalyst.

Two embodiments of the invention will now be described by way of example only with reference to the accompanying illustrative drawings in which: Figure 1 is a graph which illustrates the respective efficiences of conversion of NO, CO, C3H6 obtained by means of the catalyst of Example 2 as a function of the oxygen content of the gas mixture to be purified; Figure 2 is a graph which illustrates the respective efficiences of conversion of NO, CO, C3H obtained by means of the catalyst of Example 2 which is subjected to an accelerated ageing treatment; Figure 3 is a graph which illustrates the activity of the catalyst of Example 2 as a function of the temperature before and after the ageing treatment.

Example 1 This Example describes the preparation of a catalyst having a support of alumina coated with lanthanum oxide and a catalytic material comprising mixed oxide of ruthenium and lanthanum, platinum and rhodium. This catalyst is prepared in the manner which will now be described as follows: A deposit of lanthanum oxide is formed in the first place on the alumina support which is in particulate form. Spherical particles comprising a transition alumina of the gamma type, having a specific surface area of 100 square meters per gram, are impreganted with an aqueous solution of lanthanum nitrate. After impregnation, the spherical particles are dried, then calcined for a period of two hours at 500"C in an air stream in order to convert the lanthanum nitrate to lanthanum oxide and are then further calcined for a further period of two hours at 9000C, in air, in order to stabilize said oxide. The coating of lanthanum oxide represents approximately 1.5% of the total weight of the catalyst.

Deposition of ruthenium is then carried out by impregnating the coated spherical particles with an aqueous acid solution of ruthenium chloride and then reducing the salt thus impregnated by hydrogen, during a period of approximately four hours at a temperature of 500"C, in order to restore the ruthenium to the metallic state. The impregnating solution contains a sufficient quantity of ruthenium that the resulting catalyst contains 0.10% by weight of ruthenium with respect to the total weight of the catalyst.

After this reduction process, the support is subjected to a heat treatment in air at a temperature of 900"C for a period of approximately one hour. As a result of this treatment, the entire quantity of ruthenium combines with part of the lanthanum oxide% which has been previosuly deposited on the support. The ruthenium is now present in the form of mixed oxide of ruthenium and lanthanum of the perovskite type.

After completion of this treatment, deposition of platinum is carried out by impregnating the support with an aqueous acid solution of chloroplatinic acid followed by a reduction in hydrogen at 500"C for a period of approximately four hours. The impregnating solution contains a quantity of platinum such that the final catalyst contains 0.17% by weight of platinum with respect to the total weight of the catalyst.

Deposition of rhodium is then performed by impregnating the support with an aqueous solution of rhodium chloride followed by a reduction in hydrogen at 500"C for a period of approximately four hours. The impregnating solution contains a quantity of rhodium such that the final catalyst contains 0.06% by weight of rhodium with respect to the total weight of the catalyst.

By employing a weakly acidic impregnating solution for the deposition of rhodium leads to formation of the rhodium deposit at the surface of the support. However, the ruthenium and platinum deposits are formed in the mass of the support since the impregnation of the catalyst is carried out with strongly acidic impregnating solutions which are obtained by adding hydrochloric acid.

The catalyst thus obtained is efficient at low temperatures and endowed with good stability in time. The catalytic material of this catalyst comprises a mixed oxide of the perovskite of ruthenium and lanthanum, platinum and rhodium, the respective proportions of ruthenium, platinum and rhodium being 0.10CHo, 0.17CHc and 0.06cm% by weight with respect to the total weight of the catalyst.

The activity of said catalyst has been determined in a laboratory reactor in the presence of a mixture of synthesis gas which simulates the exhaust gases of motor vehicles or automobiles. The volumetric composition of this mixture was as follows: - carbon monoxide : 0.60% - nitric acid : 0.10% - hydrogen : 0.16% - propylene : 0.03% - oxygen : 0.47% - carbon dioxide gas : 10% - water vapour : 10% - complement of nitrogen.

This gas mixture is circulated over the catalyst in the fresh state at a spatial velocity of 50,000 h-1 and the necessary treatment temperatures were determined in order to ensure respective conversions by the catalyst of 10CHo, 50ass and 90% of nitric oxide, of carbon monoxide and of propylene. The results obtained are grouped together in Table 1, which also records the results obtained with a catalyst which has been subjected to an 'accelerated ageing" treatment at 990"C for eighteen hours in the presence of a gas mixture having the following composition: - nitrogen : 74% - carbon dioxide gas : 12% - water vapour : 12% - oxygen : 2%.

After this "accelerated ageing" treatment, the activity of the catalyst was again determined as a function of the temperature under the same conditions as in the case of the catalyst in the fresh state.

TABLE 1 Pollutant to NO CO C3H6 be removed Conversion efficiency 10% 50% 90% 10% 50% 90% 10% 50% 90 % Conversion temp. "C 170 200 230 180 210 250 200 240 270 (fresh catalyst) Conversion temp. "C 265 335 400 240 310 385 330 370 410 (aged catalyst) The results obtained show that the activity of the catalyst is satisfactory despite the fact that it is slightly weakened by the ageing treatment which is particularly exacting, especially in regard to the temperature to be chosen and the oxygen content.

Example 2 This Example described the preparation of a catalyst comprising a monolithic support of cordierite coated with alumina and a catalytic material comprising a mixed oxide of the perovskite type of ruthenium and lanthanum. platinum and rhodium.

The catalyst is prepared as follows: In an initial step, the monolithic support of cordierite. having the formula 4(Mg, Fe).0.4 Awl203.10 SiO2.H2O is coated with a first layer of alumina having a high specific surface area and obtained by precipitation from sodium aluminate in a carbon dioxide gas stream followed by calcination at 1000"C.

In this case, the alumina layer represents 7.8sic by weight of the total weight of the catalyst.

The support, thus coated, is then impregnated with an aqueous solution of lantanum nitrate. After impregnation the support is dried, then calcined for a period of two hours at 500"C in an air stream in order to convert the lanthanum nitrate to lanthanum oxide and then for a further period of two hours at 900"C again in air in order to stabilize said oxide.

The lanthanum oxide coating represents approximately 1.5ic by weight of the total weight of the catalyst.

Deposition of ruthenium is then carried out by impregnating the coated support with an aqueous solution of ruthenium chloride and then reducing the impregnated salt by hydrogen for a period of approximately four hours at a temperature of 500"C in order to restore the ruthenium to the metallic state. The impegnating solution contains a quantity of ruthenium such that the catalyst contains 0.12% by weight of ruthenium with respect to the total weight of the catalyst.

After this reduction, the support is subjected to a heat treatment carried out in air at a temperature of 1000"C for a period of approximately one hour.

When this treatment has been completed, deposition of platinum is carried out by impregnating the support with an aqueous solution of chloroplatinic acid followed by a reduction for apprxomately four hours in hydrogen at 500"C. The impregnating solution contains a quantity of platinum such that the final catalyst contains 0.14% by weight of platinum with respect to the total weight of the catalyst.

Deposition of rhodium is then carried out by impregnating the support with an aqueous solution of rhodium chloride followed by a reduction for a period of approximately four hours in hydrogen at 500"C. The impegnating solution contains a quantity of rhodium such that the final catalyst contains 0.07% by weight of rhodium with respect to the total weight of the catalyst.

The activity of this catalyst has been determined in the laboratory in the presence of a gas mixture containing: carbon monoxide 1.5% nitric oxide 0.2% propylene 0.04% carbon dioxide gas 1ouzo water vapour 10% nitrogen complement to 100% and oxygen in different concentrations ranging from 0 to 1.5%.

In these tests, the gas mixture is circulated over the catalyst either in the fresh state or in the aged state at a spatial velocity of 50.000 h-1 and at a temperature of 450"C.

It is noted that the catalyst in the "accelerated aged" state has been subjected to an "accelerated ageing" treatment at 1000"C for a period of 18 hours in the presence of the following gas mixture: carbon dioxide gas 12% water vapour 12% oxygen 2% nitrogen complement at an hourly spatial velocity of 50,000h-l.

The results obtained as indicated in Table 2 and shown in Figures 1 and 2 relate respectively to the catalyst in the fresh state and to the catalyst in the "accelerated aged" state.

In these Figures, the efficiency R of the conversion (in %) of the different pollutants NO, CO and C3H6 have been plotted as ordinates in these Figures as a function of the oxygen content of the gas mixture to be treated.

It is found that this catalyst has very good catalytic activity in its stoichiometry (0.85 % 02) in the fresh state. This corresponds to more difficult conditions than in a reducing medium both in regard to reduction is the nitrogen oxides (98% conversion efficiency in the case of NO) and in regard to oxidation of carbon monoxide (98inc conversion efficiency in the case of CO) and of hydrocarbons (98 % coversion efficiency in the case of C3H6).

Moreover, it is found that the conversion efficiency in respect to NO decreases only beyond the stoichiometric point in an oxidizing medium.

A feature which is also worthy of note is the fact that, when no oxygen is present (highly reducing medium), this catalyst has a very good conversion activity (reaction with H2O) in the case of CO and C3H6 which permits practically complete removal (more than 75%) of CO and 116 between 0 and 0.5% oxygen.

This feature is particularly advantageous in certain stages of operation of the engine with a rich mixture (starting, acceleration).

It is found in addition that, after "accelerated ageing at 10000C in an oxidizing medium, the catalyst retains very good activity in its stiochiometry (98cue conversion efficiency for CO and C3H8 - 85% conversion efficiency for NO) as well as good conversion activity at 0.5% oxygen, which shows that this catalyst is well stabilized.

The temperatures which correspond respectively to 10%, 50% and 90% conversion of nitric oxide, carbon monoxide and propylene have also been determined. The composition of the gas mixture which stimulates automobile exhaust gases and corresponds to a stoichiometric mixture (richness = 1) is as follows: CO 1.5% CO2 10% NO 0.2% H2O 10% C3H6 0.04% 02 0.8% N2 complement with a spatial velocity of 50,000 h-1.

These measurements have also been determined after accelerated ageing of the catalyst at 1000"C in an oxidizing medium (under the same conditions as before).

The results are grouped together in Table 3 and illustrated in Figure 3 which gives the conversion efficiencies R (in %) in respect to the pollutants : NO, CO and C3H6 as a function of the temperature in he case of the catalyst in the fresh state (curves 1) and in the case of the catalyst in aged state (curves 2).

It is consequently found that the starting temperatures (10% conversion) are particularly low in the case of fresh catalyst and in the case of the three principal pollutants (in the vicinity of 205"C in the case of NO, CO and C3H6) and that their removal is practically complete above 230"C.

It is also found that, after ageing at 1000"C, the catalyst retains very good activity despite the fact that it exhibits a slight reduction in activity which results in a slight displacement of the curves towards higher temperature values.

It is recalled that the conditions of test in a stiochiometric mixture are particularly unfavourable for the conversion of nitrogen oxides since it is in the vicinity of stiochiometry that the acitivity of the catalyst usually decreases very rapidly in the case of the nitrogen oxides.

TABLE 2 Concentration in 2 0.5So 0.8% 1% Pollutant to be removed NO CO C3H6 NO CO C3H6 NO CO C3H6 Conversion efficiency in % 98 95 80 98 98 98 30 99 99 (fresh catalyst) Conversion efficiency in % 98 70 90 85 98 98 35 99 99 (aged catalyst) TABLE 3 Pollutant removed CO NO C3H6 Conversion efficiency 10% 50% 90% 10% 50% 90% 10% 50% 90% Conversion temperature in "C (fresh 205 205 210 205 205 215 205 210 220 catalyst) Conversion temperature in "C (aged 215 250 280 240 250 270 240 270 330 catalyst) WHAT WE CLAIM IS: 1. A catalyst comprising a support of inert material coated with lanthanum oxide and a catalytic material comprising a mixed oxide of the perovskite type of ruthenium and lanthanum, platinum in the metallic form and rhodium.

2. A catalyst comprising a support of inert material coated with lanthanum oxide and a catalytic material comprising a mixed oxide of the perovskite type of ruthenium and

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

of the gas mixture which stimulates automobile exhaust gases and corresponds to a stoichiometric mixture (richness = 1) is as follows: CO 1.5% CO2 10% NO 0.2% H2O 10% C3H6 0.04% 02 0.8% N2 complement with a spatial velocity of 50,000 h-1.

These measurements have also been determined after accelerated ageing of the catalyst at 1000"C in an oxidizing medium (under the same conditions as before).

The results are grouped together in Table 3 and illustrated in Figure 3 which gives the conversion efficiencies R (in %) in respect to the pollutants : NO, CO and C3H6 as a function of the temperature in he case of the catalyst in the fresh state (curves 1) and in the case of the catalyst in aged state (curves 2).

It is consequently found that the starting temperatures (10% conversion) are particularly low in the case of fresh catalyst and in the case of the three principal pollutants (in the vicinity of 205"C in the case of NO, CO and C3H6) and that their removal is practically complete above 230"C.

It is also found that, after ageing at 1000"C, the catalyst retains very good activity despite the fact that it exhibits a slight reduction in activity which results in a slight displacement of the curves towards higher temperature values.

It is recalled that the conditions of test in a stiochiometric mixture are particularly unfavourable for the conversion of nitrogen oxides since it is in the vicinity of stiochiometry that the acitivity of the catalyst usually decreases very rapidly in the case of the nitrogen oxides.

TABLE 2 Concentration in 2 0.5So 0.8% 1% Pollutant to be removed NO CO C3H6 NO CO C3H6 NO CO C3H6 Conversion efficiency in % 98 95 80 98 98 98 30 99 99 (fresh catalyst) Conversion efficiency in % 98 70 90 85 98 98 35 99 99 (aged catalyst) TABLE 3 Pollutant removed CO NO C3H6 Conversion efficiency 10% 50% 90% 10% 50% 90% 10% 50% 90% Conversion temperature in "C (fresh 205 205 210 205 205 215 205 210 220 catalyst) Conversion temperature in "C (aged 215 250 280 240 250 270 240 270 330 catalyst) WHAT WE CLAIM IS: 1. A catalyst comprising a support of inert material coated with lanthanum oxide and a catalytic material comprising a mixed oxide of the perovskite type of ruthenium and lanthanum, platinum in the metallic form and rhodium.
2. A catalyst comprising a support of inert material coated with lanthanum oxide and a catalytic material comprising a mixed oxide of the perovskite type of ruthenium and

lanthanum, palladium and the metallic form and rhodium.
3. A catalyst as claimed in claim 1 in which the percentage weight content of the constituents of the catalyst with respect to the total weight of the catalyst is as follows:1 to 3% lanthanum oxide, 0.05 to 0.2goo ruthenium, 0.05 to 0.2% platinum and 0.02 to t).1% rhodium.
4. A catalyst as claimed in claim 2 in which the percentage weight content of the constituents of the catalyst with respect to the total weight of the catalyst is as follows: 1 to 3% Lanthanum oxide, 0.05 to 0.2% ruthenium, 0.02 to 0.1% rhodium and 0.05 to 0.2% palladium.
5. A catalyst as claimed in claim 1 or claim 2 which the catalytic material additionally contains iridium in the metallic form and/or rhenium in the metallic form.
6. A catalyst as claimed in claim 5 in which the percentage weight content of the constituents of the catalyst with respect to the total weight of the catalyst is 0.005 to 0.05% iridium and/or 0.005 to 0.05% rhenium.
7. A catalyst as claimed in any one of the preceding claims in which the support is in particulate form.
8. A catalyst as claimed in any one of claims 1 to 6 in which the support is in monolithic form.
9. A catalyst as claimed in claim 7 or claim 8, in which the inert material is of ceramic.
10. A catalyst as claimed in claim 9 in which the ceramic is alumina.
11. A catalyst as claimed in claim 8, claim 9 or claim 10 wherein the inert material is a ceramic coated with alumina.
12. A catalyst as claimed in claim 8 in which the monlithic support is cordierite coated with alumina.
13. A catalyst as claimed in claim 11 or claim 12, in which the catalyst contains 2 to 15% by weight of alumina with respect to the total weight of the catalyst.
14. A catalyst as claimed in any one of claims 9 to 13, in which the inert material is a metal alloy coated with a refractory oxide.
15. A method of preparation of a catalyst which comprises depositing a layer of lanthanum oxide on support of inert material; depositing ruthenium on the coated support by impregnating the support with an aqueous solution containing a soluble salt of ruthenium and then reducing the impregnated salt with a reducing gas for a few hours at a temperature in the region of 400" to 7000C, subjecting the support to a heat treatment carried out in air at a temperature within the range of 600" to 1100 C over a period ranging from a half to two hours in order to form a mixed oxide of ruthenium and lanthanum and finally depositing platinum and rhodium on the support.
16. A method of preparation of a catalyst, which comprises depositing a layer of lanthanum oxide on a support of inert material; depositing ruthenium on the coated support by impregnating the support with an aqueous solution containing the a soluble salt of ruthenium and then reducing said impregnated salt with a reducing gas over a few hours at a temperature in the order of 400 to 700"C, subjecting the support to a heat treatment carried out in air at a temperature within the range of 600" to 1100"C over a period ranging from a half to two hours in order to form a mixed oxide of ruthenium and lanthanum and finally in depositing palladium and rhodium on the support which has thus been treated.
17. A method as claimed in claim 15 or claim 16, which comprises depositing the layer of lanthanum oxide by impregnating the support with an aqueous solution of lanthanum nitrate and by drying then calcining the impeganted support in air in two steps, the first step being carried out over a period of a few hours at a temperature within the range of 350 to 600"C and the second step being carried out at a temperature within the range of 600 to 11000C in order to stabilize the oxide.
18. A method as claimed in any one of claims 15 to 17, which comprises first in depositing the platinum or the palladium and then rhodium by impregnating the supprt with an aqueous solution containing a soluble salt of platinum or palladium and then reducing the impregnated platinum or palladium salt for a period of a few hours by a reducing gas at a temperature of the order of 400 to 700"C; and then impregnating the support with an aqueous solution containing a soluble salt of rhodium and reducing the impregnated rhodium salt for a few hours by a reducing gas at a temperature of the order of 400 to 700DC.
19. A method as claimed in any one of claims 15 to 17, which comprises depositing simultaneously the rhodium and either the platinum or the palladium by impregnating the support with an aqueous solution containing a soluble salt of platinum or palladium and a soluble salt of rhodium and subjecting the salts, which have been impregnated on the support, to a reduction by a reducing gas for a few hours at a temperature in the range 400 to 700"C.
20. A method as claimed in any one of claims 15 to 19 comprising depositing iridium and/or rhenium additionally rhodium by impregnating from at least one aqueous solution containing at least one soluble salt including rhodium and iridium and/or rhenium; and then reducing the salt or salts which have been impregnated on the support by a reducing gas for a period of a few hours at a temperature of the order of 400 to 700"C.
21. A method as claimed in any one of claims 15, 16 and 18 to 20 in which the reducing gas is hydrogen;
22. A method as claimed in any one of claims 15, 16 and 18 to 21, in which the temperature of reduction of the metallic salt or salts which are impregnated on the support is in the vicinity of 500"C.
23. A catalyst for the treatment of combustion gases substantially as herein described with reference to the examples.
24. A method of preparation of a catalyst substantially as herein described with reference to the examples.