GB1568370A - Catalysis - Google Patents

Description

IMPROVEMENTS IN AND RELATING TO CATALYSIS (71) We, JOHNSON, MATTHEW & CO.

LIMFED, a British Company of 43 Hatton Garden, London, EC1N 8EE 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 catalysis; more particularly, it relates to catalytic reactions which are useful in exhaust gas purification, especially where the exhaust gases contain one or more oxides of nitrogen and a stoichiometric excess of oxygen.

In the purification of automobile exhaust gases two separate catalytic stages are frequently envisaged: firstly a reduction stage in which the unreacted hydrocarbon and carbon monoxide components reduce the oxides of nitrogen to nitrogen and water in the presence of a reduction catalyst and secondly, after the admission of air through a secondary air intake, an oxidation stage in which any unreacted hydrocarbon and carbon-monoxide are oxidised to carbon dioxide and water in the presence of an oxidation catalyst.

Alternatives to this envisage controlled fuel injection into the engine in such a way that stoichiometric conditions are operative substantially all the time. Exhausts from such engines may also be treated by the dual bed concept as described above or by use of a single combined bed in which either (a) an oxidation and a reduction catalyst are deposited on separate supports within a single catalyst unit with no secondary air intake, or (b) oxidation and reduction catalysts are simultaneously deposited upon the same catalyst support in such a way that they form a mixture on the surface of the support which subsequently exhibits combined beneficial oxidation and reduction properties.

Platinum is a known oxidation catalyst and platinum-rhodium alloys are known to be useful for reduction of oxides of nitrogen, particularly when supported upon a high surface area refractory ceramic or metallic honey comb ultimate support.

One disadvantage of this type of supported rhodium-platinum alloy catalyst is that although the ration of the two metals present is carefully selected for most effective catalysis under operating conditions, the surface of the alloy tends to become Rh-enriched by migration of the rhodium component in the alloy.

It is an object of the present invention to provide catalysts in which such enrichment does not occur.

It is a further object of the invention to provide catalysts which will remove oxides of nitrogen under oxidising conditions. Such catalysts would avoid the aforementioned disadvantages of requiring either two separate catalytic stages or controlled fuel injection.

According to one aspect of the present invention, a catalyst for the purification of automobile exhaust gases comprises a compound selected from the group represented by the general formula AXMyOz in which A is one or more metals selected from Li, Na, K, Mg, Ca, Sr, Ba, Al, Sc, Y, the lanthanides, Ti, Zr, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn; M is one or more metals, selected from Ir, Rh, Pt, Pd and Ru; y has an integral or fractional value within the range from 0.1 to 3.0, z is an integer with a value from 2 to 7 and x has a value such that the compound is electrically neutral, the compound being supported on a support with an intermediate layer of a refractory metal oxide applied to the support.

Examples of compounds according to the general formula defined on the previous page are LaRhO3, BaRuO3, LaPtO2, MgRh2 04, Ba4 PtO6, Co(Al,Rh)2 04, CdPd3 04 and LaNao SIrO 503 Of these examples, those in which x and y and z in the general formula A M 0 have a value of 3, have the perovskite structure whereas the remainder are spinels.

According to another aspect of the present invention, a process for the purification of an exhaust gas which includes the simultaneous reduction of an oxide of nitrogen, in the presence or absence of a gaseous fuel, and the oxidation of carbon monoxide and one or more organic compounds in the presence of oxygen by passage of said gases through a supported catalyst comprising a support which is impregnated or coated with a compound having the general formula AXMyOz in which A,M, x, y and z are as defined above, an intermediate layer of a refractory metal oxide having been applied to the support. The support is preferably an intert material in the form of a unitary porous refractory ceramic or metallic honeycomb or similar structure, but may be particulate, for example, in the form of granules. Preferably also, the inert material has the intermediate layer of a relatively high surface area refractory metal oxide and one or more of the compounds AXMyOz is deposited upon the said intermediate layer. If desired, the support may be made from a metal or alloy, for example, "KANTHAL" DSD (Registered Trade Mark) foil supplied by the Kanthal Company of Sweden.

In the operation of this invention we have sometimes found it desirable to deposit an oxidation catalyst such as platinum either on the same support or on a separate support in the same catalyst bed, so that under stoichiometric conditions oxidation of CO and hydrocarbons and reduction of NOX proceeds at the same time in one bed. However, depending on the actual catalyst used, it is not always desirable to deposit a separate oxidation catalyst since some catalysts according to the invention not only oxidise hydrocarbons and carbon monoxide but also remove oxides of nitrogen under oxidising conditions.

Preferably the inert material may have a first deposit of a refractory metal oxide which is itself then impregnated or coated with AXMyOz and Pt. Suitably the ratio of M to Pt deposited considered as weight of metal alone is within the range 1-50% by weight with preferred M concentrations in the range 5-10% by weight.

The inert structure used in the process of the present invention and upon which the refractory metal oxide is deposited may be particulate i.e. granular or pelletised but we prefer to use a unitary rigid honeycomb structure of the ceramic type (see Talsma USP 3,255,027) or of the metallic type (see Heathcote et al German DOS 2,450,664) having a corrugated cellular form (see Corning British Patent 882,484). Alternatively the inert support may simply have macro pores or channels extending throughout the length of its body in the direction of gas flow.

The refractory metal oxide is deposited on the support (either continuously or discontinuously) and preferably the deposit is in the form of a film of from 0.0004 to 0.001 inches thick.

Such an oxide is a calcined refractory metal oxide which itself is characterised by a porous structure and which possesses a large internal pore volume and total surface area and is therefore referred to as an "active" (i.e.

catalytically active) refractory metal oxide.

The preferred active refractory metal oxides contain members of the gamme or activated alumina family which can be prepared, for instance, by precipitating a hydrous alumina gel and, thereafter, drying and calcining to expel water of hydration and provide the active gamma-alumina. A particularly preferred active refractory metal oxide is obtained by drying and calcining at temperatures of 3000 C. and 8000 C. a precursor mixture of hydrous alumina phases predominating in crystalline trihydrate, that is, containing in excess of 50% by weight of the total alumina hydrate composition, preferably from 65% to 95% by weight of one or more of the trihydrate forms gibbsite, bayerite and nordstrandite by X-ray diffraction.

Other suitable active refractory metal oxides include, for example, active or calcined beryllia, zirconia, magnesia or silica, and combinations of metal oxide such as boria-alumina or silica-alumina. Preferably the active refractory oxide is composed predominantly of oxides of one or more metals of Groups II, III and IV of the Periodic Table. The active refractory metal oxide deposits may constitute from 1 to 50 weight per cent of the unitary support, preferably from 5 to 30 percent.

The active refractory metal oxide of the present invention may be deposited on the support in several ways. One method involves dipping the support into a solution of the salt of the refractory metal and calcining to decompose the salt to the oxide form. Another and preferred method comprises dipping the support into an aqueous suspension, dispersion or slurry of the refractory oxide itself, drying and calcining. This method is known in the art (see Houdry, BP 690825).

In alternative methods the preferred refractory oxide is a transitional alumina which may be dispersed in colloidal aluminium hydroxide stabilised with nitric acid prior to dipping the support as above or alumina may be deposited onto the inert support from alkaline sodium aluminate solution. In these methods suspension or dispersions can be used to deposit a suitable amount of a refractory metal oxide on the support in a single application.

EXAMPLES 1-3 In these examples three, different catalyst systems for automobile exhaust purification were compared. All three were deposited upon a metal support; "Kanthal" DSD Registered Trade Mark) foil .002 inches thick supplied by the Kanthal Company, Sweden. The foil was made up into two pieces of corrugated honeycomb 3 inches long and 4 inches in diameter having 400 cells per square inch. Loading of precious metal in all cases amounted to a total of 40 grams per cubic ft. of support.

EXAMPLE I a. Split Bed A split bed catalyst system was constructed having a first oxidation catalyst consisting of platinum metal deposited by known methods upon an alumina wash-coated monolithic metal support described above and a second reduction catalyst consisting of a separate washcoated metallic monolith having LaRhO3 deposited from lanthanum nitrate and rhodium nitrate solutions. The mixed solutions were used to impregnate the support which was then dried and fired at 6500C. The alumina washcoat contained 5% by weight of barium (present as the oxide) as stabilizer. Loadings were as follows: Loading of Pt on Ist monolith 74 gm per cu ft.

Loading of Rh on 2nd monolith 6 gm per cu ft.

The formation of LaRhO3 was confirmed by X-ray diffraction analysis.

This catalyst was exposed to an exhaust gas from a VW 1.7 litre "flat four" air cooled engine fitted with the "Bosch" (Registered Trade Mark) 'D' "Jetronic" (Registered Trade Mark) fuel injection system, with manual control, enabling the air fuel ratio to be varied in small steps from rich to lean tune (usually 14.2-15.5).

At each mixture setting, the conversion efficiencies of CO, HC and NOx were measured.

The catalyst was then subjected to a 50 hour ageing cycle in the exhaust of a 1.8 litre British Leyland "Dolomite" (Registered Trade Mark) engine. The cycle consisted of 1 minute at idle, and 1 minute at 360 rpm (equivalent to 70 m.p.h.). Thus an average speed of 35 m.p.h.

was maintained. A relay operated solenoid valve was used to direct the air intake for half of each cycle, so that the catalyst experienced a reducing atmosphere for 30 seconds of each cycle, and oxidising conditions for the remaining 30 seconds.

The air-equivalence ratio (X) range experienced on this cycle is as follows: Idle with air X = 1.10; CO % = 5-6% Idle without air = 0.73; CO % = 8-9% 3600 rpm with air X = 1.15;CO %= = 2.3-2.5% 3600 rpm without air X = 0.86; CO % = 2.8-3.0% After this ageing cycle, the catalyst was retested on the VW 1.7 litre engine and the results obtained for varying equivalence ratios are given in Figure 1.

EXAMPLE 2 b. Combined Bed.

In this example both the Pt and LaRhO3 were deposited on the same monolith. The combined Pt and Rh loading was still 40 gm.

per cu ft. and the same type of alumina washcoated metallic monolith was used as support.

Again readings were taken after ageing for 50 hours. Results are shown in Figure 2.

EXAMPLE 3 c. Alloy This example is not one of the present invention but of the prior art catalysts and is included for comparative purposes only. Agai using a washout and support of the same type a 711% Rh/Pt alloy was deposited at a loading of 40 gm. of platinum group metal per cubic foot of support. Results after ageing of the catalyst for 50 hours are shown in Figure 3.

Comparison of Results Average Conversion Efficiency (overall percentages) CO HC NOx Split bed 84 88 86 Combined 68 77 37 Alloy (comparison) 89 74 10 It can be seen that for hydrocarbon and for NOx conversion, especially the latter, results are appreciably better than for the prior art catalysts. For carbon monoxide conversio the split bed catalyst is nearly as good as 711% Rh/Pt alloy.

Overall the presence of Rh ad LaRhO3 must be considered to be a considerable improvement when used as a "three-way" catalyst.

EXAMPLES 4 - 10 In these comparative examples, seven different catalysts for automobile exhaust purification were compared. All were deposit ed on or associated with an inert particulate support of a refractory oxide, without an inte mediate catalytically active refractory oxide.

The inert refractory oxide selected was "Davison 70" silica. In each example, 0.1 g of catalyst was associated with sufficient silica to make a total weight of lg. Each catalyst was then texted in a simulated exhaust stream at a fixed value of R (where R is the ratio of oxidising to reducing species in the inlet gas stream and is calculated according to the equation R=2X%02 + %NOx %CO and with varying catalyst inlet temperature; the catalyst was then tested at the optimum temperature for removal of nitrogen oxides, as determined from the first run, carrying the value of R.

The composition of the feed gas was:nitric oxide in N2, 1900 v.p.m.; carbon mon oxide, 3%; oxygen, 2.76% at R = 1.9 (varied for different R values). The space velocity through the supported catalyst was 22,000 hr1 , calculated on the weight of actual catalytic material.

EXAMPLE 4. (See Figures 4a and 4b) The catalyst evaluated in this example was LaRhO3. Figure 4a shows that, at R = 1.9, maximum NOx removal occurred at just below 300"C. The isothermal experiment at 2810C (figure 4b) shows that this catalyst is reasonably resistant to oxygen poisoning.

EXAMPLE 5. (figures Sa and Sb) Catalyst LaPtO2 at R = 1.9 gave a maximum NOx removal of about 75% at 3740C (figure Sa). The isothermal results at this temperature (figure Sb) show that this catalyst is superior to LaRhO3 in its resistance to oxygen poisoning.

EXAMPLES 6 and 7. (Figures 6a,6b, 7a,7b) Catalysis LaRuO3 and BaRuO3respectively were poor in comparison to both previous Examples for resistance to oxygen poisoning.

In both cases, considering the isothermal experiments at 3400C and 3250C respectively, NOx removal was falling rapidly before the stoichiometric point (R=1) was reached.

EXAMPLE 8 (Figures 8a and 8b) Catalyst Ba4 PTO6 is shown to have a certain measure of resistance to oxygen poisoning, retaining about 50% of its maximum activity in NOx removal at an R value of 2.

EXAMPLE 9 (Figures 9a and 9b) The catalyst tested in this example was a complex having the formula LaO 8Sr0 2Co0 9 Ruo .103. This catalyst was highly active in NOx removal under reducing conditions but at an R value of 1.9 NOx removal was zero.

EXAMPLE 10 (Figures 1 0a and lOb) This catalyst was the platinum analogue of that tested in the previous example and had the formula La0.8Sr0.2Co0.9pt0 1 3. The iso- thermal experiment (figure 10b) at 3600 C.

showed extremely good NOx removal under highly reducing conditions (right down to R=O) and reasonable resistance to oxygen poisoning at R=1.

Although specific reference has been made above to the use of Kanthal DSD (Registered Trade Mark) as the catalyst support, other metallic and non-metallic supports may be used. Another metallic support of particular interest is that sold under the Registered Trade Mark FECRALLOY which is described and claimed in British Patent Application No.

22707/73, Serial No. 1,471,138 (U.S. Patent No. 3,920,583).

We have found that the use of lanthanum rhodite is superior to rhodium oxide and this is summarised in Table 1. The superiority is also illustrated graphically in Figures 11 and 12.

The graphs of Figures 11 and 12 and the results given in Table 1 show the relative effectiveness of Rh2O3 and LaRhO3 for the catalytic oxidation of CO to CO2 and for the catalytic reduction of No to N2. The tabulated results and graphs also show the readiness with which Rh2 03 and LaRhO3 will take up oxygen.

The tests were carried out in a vertical reactor vessel in which was located a sintered glass plate upon which a powdered sample of the material under test could be placed. The aim of the tests was to determine the relative readiness with which the two materials will absorb carbon monoxide, nitric oxide and oxygen in that order. Readiness to absorb CO was determined by measuring the quantity of CO2 produced when a mixture of CO and helium was passed over a heated sample of the material under test. In the case of the other two gases, the proportion of NO and O2 actually removed from gas streams containing helium and nitric oxide and helium and oxygen respectively was determined when these gas streams were passed over the heated sample.

The carbon dioxide measurement gave a measure of the efficiency of the material in oxidising carbon monoxide; the NO absorption its readiness to reduce nitrogen oxides to nitrogen and its oxygen absorption the ease with which it may be re-oxidised in an oxidising environment so as to prepare it to carry out oxidising processes.

TABLE 1 CO, NO and O2 absorption capacities of Rh2 03 and LaRhO3 Volume of gas absorbed in 1 mien/ g of Rh CO NO O2 Rh203 14.7 1.71 0.98 LaRhO3 23.84 14.25 11.03 Initially a blank experiment was carried out on the reactor with nothing on the sintered glass plate. During this experiment the reactor was heated to 500"C and flushed through with helium gas, following which a gas consisting of 2% carbon monoxide in helium was passed through the heated reactor tube and the outflow fed to a mass spectrometer where the carbon dioxide content of the outflow was determined over a period of time. Next, the apparatus was again flushed through with helium and then 1% of nitric oxide in helium was passed through it, any reduction in the nitric oxide content in the gaseous outflow being determined by means of a mass spectrometer. Finally, this last stage was repeated using 1% of oxygen in helium (in place of the NO in He) after the reactor apparatus had again been purged with helium, the mass spectrometer on this occasion, of course, being used to determine any reduction in the oxygen content of the outflow from the reactor.

Following this, the procedures of these blank experiments were repeated, firstly with 66 gm of powdered Rh2 03 and then with 66 gm of powdered LaRhO3on the sintered glass plate.

The measurements made during these experiments were used to calculate the absorption capacities of Rh2O3 and LaRh03 for CO, No and 2 respectively in arbitrary units of volume per minute per gram of rhodium.

These figures are given in Table 1 where the outstandingly better performance of LaRhO3 compared with that of Rh203 is clearly shown.

The same effect is illustrated in Figures 11 and 12 where the percentages of CO2 produced and of NO and 2 removed by 66 gm of Rh2 O3 and LaRhO3 respectively are plotted against time. In this case, however, the effect is not quite so marked because the proportion of rhodium by weight in a given weight of LaRhO3 is less than half that in the same weight of Rh2O3.

Reference is made to our prior British Patents Nos. 1,482,516 and 1,507,976 and the subject matter therein claimed is disclaimed herein. Subject to the foregoing disclaimer.

WHAT WE CLAIM IS: 1. A catalyst for the purification of automobile exhaust gases comprising a compound selected from the group represented by the general formula AXMyOz in which A is one or more metals selected from Li, Na, K, Mg, Ca, Sr, Ba, Al, Sc, Y, the lanthanides, Ti, Zr, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn; Mis one or more metals selected from Ir, Rh, Pt, Pd and Ru; y has an integral or fractional value within the range from 0.1 to 3.0, z is an integer with a value from 2 to 7 and x has a value such that the compound is electrically neutral, the compound being supported on a support with an intermediate layer of a refractory metal oxide applied to the support.

2. A catalyst according to claim 1 wherein y has the value 1 or 2.

3. A catalyst according to claim 1 wherein M is rhodium.

4. A catalyst according to claim 1 wherein x, y and z each have the value of 3 and the compound has perovskite structure.

5. A catalyst according to claim 1 wherein x, y, and z each have a value other than 3 and the compound has a spinel structure.

6. A catalyst according to claim 1 wherein the compound is selected from the group consisting of LaRhO3, BaRuO3, LaPtO2, MgRh2O4, Bad PtO6 Co(Al,Rh)204, CdPd304 and LaNa0.s IrO 5 3 7. A catalyst according to any one of claims 1 to 6 wherein the support is an inert support.

8. A catalyst according to any one of claims 1 to 7 wherein the inert support is in the form of particles or in the form of a unitary ceramic or metallic porous structure or honeycomb.

9. A catalyst according to any one of claims 1 to 8 wherein the intermediate layer presents a relatively high surface area for the catalyst compound.

10. A catalyst according to any one of claims 1 to 9 wherein the support is a refractory ceramic material.

11. A catalyst according to any one of claims 1 to 10 including separate oxidation and reduction catalyst compounds.

12. A catalyst according to any one of claims 1 to 10 including combined oxidation and reduction catalysts.

13. A catalyst according to any one of claims 1 to 12 wherein the catalyst compound includes Pt in addition to another metal selected from Ir, Rh, Pd and Ru.

14. A catalyst according to claim 13 wherein the ration of the metal selected from Ir, Rh, Pd and Ru, to Pt is 1-50 weight 5to and preferably 5-10 weight %.

15. A catalyst according to any one of claims 1 to 14 wherein the intermediate layer is in the form of a film having a thickness within the range 0.004 and 0.001 inch.

16. A catalyst according to any one of claims 1 to 15 wherein the refractory metal oxide contains at least one of gamma or activated alumina.

17. A catalyst according to any one of claims 1 to 16 refractory metal oxide is selected from active or calcined beryllia, zirconia, magnesia, silica; boria-thoria, silicaalumina, and oxides of at least one of the metals of Groups II, III and IV of the Periodic Table.

18. A catalyst according to any one of claims 1 to 17 wherein the refractory metal oxide is present in an amount of from 1 to 50 weight % of the support.

19. A catalyst according to claim 18 wherein the refractory metal oxide is present in an amount of from 5 to 30 weight % of the support.

20. A process for the purification of an exhaust gas which includes the simultaneous reduction of an oxide of nitrogen, in the presence or absence of a gaseous fuel, and the oxidation of carbon monoxide and one or more organic compounds in the presence of oxygen by passage of said gases through a supported catalyst comprising a support which is impregnated or coated with a compound having the general formula AXMyOz in which A, M, x, y and z are as defined in claim 1,2,4 or 5, an intermediate layer of refractory metal oxide having been applied to the support.

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

Claims (20)

**WARNING** start of CLMS field may overlap end of DESC **. spectrometer on this occasion, of course, being used to determine any reduction in the oxygen content of the outflow from the reactor. Following this, the procedures of these blank experiments were repeated, firstly with 66 gm of powdered Rh2 03 and then with 66 gm of powdered LaRhO3on the sintered glass plate. The measurements made during these experiments were used to calculate the absorption capacities of Rh2O3 and LaRh03 for CO, No and 2 respectively in arbitrary units of volume per minute per gram of rhodium. These figures are given in Table 1 where the outstandingly better performance of LaRhO3 compared with that of Rh203 is clearly shown. The same effect is illustrated in Figures 11 and 12 where the percentages of CO2 produced and of NO and 2 removed by 66 gm of Rh2 O3 and LaRhO3 respectively are plotted against time. In this case, however, the effect is not quite so marked because the proportion of rhodium by weight in a given weight of LaRhO3 is less than half that in the same weight of Rh2O3. Reference is made to our prior British Patents Nos. 1,482,516 and 1,507,976 and the subject matter therein claimed is disclaimed herein. Subject to the foregoing disclaimer. WHAT WE CLAIM IS:

1. A catalyst for the purification of automobile exhaust gases comprising a compound selected from the group represented by the general formula AXMyOz in which A is one or more metals selected from Li, Na, K, Mg, Ca, Sr, Ba, Al, Sc, Y, the lanthanides, Ti, Zr, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn; Mis one or more metals selected from Ir, Rh, Pt, Pd and Ru; y has an integral or fractional value within the range from 0.1 to 3.0, z is an integer with a value from 2 to 7 and x has a value such that the compound is electrically neutral, the compound being supported on a support with an intermediate layer of a refractory metal oxide applied to the support.

2. A catalyst according to claim 1 wherein y has the value 1 or 2.

3. A catalyst according to claim 1 wherein M is rhodium.

4. A catalyst according to claim 1 wherein x, y and z each have the value of 3 and the compound has perovskite structure.

5. A catalyst according to claim 1 wherein x, y, and z each have a value other than 3 and the compound has a spinel structure.

6. A catalyst according to claim 1 wherein the compound is selected from the group consisting of LaRhO3, BaRuO3, LaPtO2, MgRh2O4, Bad PtO6 Co(Al,Rh)204, CdPd304 and LaNa0.s IrO 5 3

7. A catalyst according to any one of claims 1 to 6 wherein the support is an inert support.

8. A catalyst according to any one of claims 1 to 7 wherein the inert support is in the form of particles or in the form of a unitary ceramic or metallic porous structure or honeycomb.

9. A catalyst according to any one of claims 1 to 8 wherein the intermediate layer presents a relatively high surface area for the catalyst compound.

10. A catalyst according to any one of claims 1 to 9 wherein the support is a refractory ceramic material.

11. A catalyst according to any one of claims 1 to 10 including separate oxidation and reduction catalyst compounds.

12. A catalyst according to any one of claims 1 to 10 including combined oxidation and reduction catalysts.

13. A catalyst according to any one of claims 1 to 12 wherein the catalyst compound includes Pt in addition to another metal selected from Ir, Rh, Pd and Ru.

14. A catalyst according to claim 13 wherein the ration of the metal selected from Ir, Rh, Pd and Ru, to Pt is 1-50 weight 5to and preferably 5-10 weight %.

15. A catalyst according to any one of claims 1 to 14 wherein the intermediate layer is in the form of a film having a thickness within the range 0.004 and 0.001 inch.

16. A catalyst according to any one of claims 1 to 15 wherein the refractory metal oxide contains at least one of gamma or activated alumina.

17. A catalyst according to any one of claims 1 to 16 refractory metal oxide is selected from active or calcined beryllia, zirconia, magnesia, silica; boria-thoria, silicaalumina, and oxides of at least one of the metals of Groups II, III and IV of the Periodic Table.

18. A catalyst according to any one of claims 1 to 17 wherein the refractory metal oxide is present in an amount of from 1 to 50 weight % of the support.

19. A catalyst according to claim 18 wherein the refractory metal oxide is present in an amount of from 5 to 30 weight % of the support.

20. A process for the purification of an exhaust gas which includes the simultaneous reduction of an oxide of nitrogen, in the presence or absence of a gaseous fuel, and the oxidation of carbon monoxide and one or more organic compounds in the presence of oxygen by passage of said gases through a supported catalyst comprising a support which is impregnated or coated with a compound having the general formula AXMyOz in which A, M, x, y and z are as defined in claim 1,2,4 or 5, an intermediate layer of refractory metal oxide having been applied to the support.