GB2122912A

GB2122912A - Exhaust gas purification catalyst

Info

Publication number: GB2122912A
Application number: GB08218792A
Authority: GB
Inventors: Alan Francis Diwell; Graham Butler
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 1982-06-29
Filing date: 1982-06-29
Publication date: 1984-01-25

Abstract

A lead tolerant catalyst system for purifying exhaust from a spark- ignition (e.g. petrol) engine comprises a support (preferably ceramic) having a high surface area refractory oxide coating for carrying a noble metal catalyst (platinum, palladium or rhodium) where greater lead tolerance is achieved by depositing the catalyst on particles of ceria which are then deposited on the refractory oxide coating. The combination of ceria and noble metal repels poisonous lead moieties. Also a method for making the catalyst system and its use in exhaust systems fitted to spark- ignition engines.

Description

SPECIFICATION Lead tolerant catalyst system for purifying exhaust from a spark-ignition engine This invention relates to a lead tolerant catalyst system suitable for purifying exhaust from a sparkignition engine such as a petrol engine and to a method for making the catalyst system.

Hitherto commercially available catalyst systems for purifying exhaust from spark-ignition engines have comprised a platinum, rhodium or palladium noble metal catalyst for catalysing the oxidation of pollutant gases such as hydrocarbons or carbon monoxide which are present in the exhaust. The noble metal catalyst is supported on a refractory metal oxide of high surface area (for example a-alumina) which in turn is supported on walls which are provided by either a metal or preferably a ceramic support structure and which define passageways passing through the support. The support is located in the flow of exhaust gas coming from the engine so that the exhaust gas is caused to flow through the support via its passageways whereupon the pollutant gases come into contact with the noble metal catalyst.A problem with such systems has been that the noble metal catalyst is easily poisoned by lead moiety derived from antiknock additives such as lead tetralkyls which are frequently present in fuels for sparkignition engines. The problem is especially acute if the catalyst contains palladium. In practice, noble metal exhaust purification catalysts have only been used satisfactorily with a spark-ignition engine if the engine is operated using a special so-called lead-free fuel. However in spark-ignition engines, lead-free fuels are less efficient than the conventional fuels which contain lead additives.

United States patent specifications 4,189,404 and 4,206,087 (the contents of both of which are herein incorporated by reference) describe attempts to reduce the poisoning by lead of exhaust purification catalysts comprising noble metals supported on refractory metal oxides by dispersing ceria into the refractory metal oxide to form a matrix which traps catalyst poisons. Presumably as soon as the matrix becomes saturated with poisons, it will cease to offer protection to the catalyst.

It has now been discovered that significant improvements in the lead tolerance of platinum, rhodium or palladium exhaust purification catalysts can be achieved if the precious metal is supported on ceria (CeO2) which in turn is supported by the refractory metal oxide. An object of this invention is to employ this discovery to provide a catalyst system suitable for purifying the exhaust from a sparkignition engine and which is more tolerant of lead. Another object is to provide a method for making the catalyst system.

Accordingly this invention provides a lead tolerant catalyst system suitable for purifying exhaust from a spark-ignition engine, the catalyst system comprising (a) a support preferably comprising a pair of spaced end portions and walls extending between the spaced end portions to define a plurality of passageways passing through the support from one end portion to the other.

(b) a refractory oxide of aluminium, silicon, zirconium, titanium and/or chromium having a surface area of not less than 10 m2 per gram of the refractory oxide (and preferably 100 to 300 m2/g) carried on the support and (c) a noble metal catalyst comprising platinum, rhodium and/or palladium (preferably 0.001 to 0.03 gram atoms of noble metal per cubic metre of the catalyst system) supported on ceria which in turn is supported by the refract ry oxide.

It appears that the combination of platinum, rhodium and/or palladium with ceria repels poisonous lead moieties and provided that the ceria is supported on the surface of a refractory oxide of high surface area, then the catalytic activity of platinum, rhodium or palladium is not unduly affected.

This invention also provides a method for making a lead tolerant catalyst system comprising a support (preferably having a pair of spaced end portions and walls extending between the spaced end portions which define a plurality of passageways passing through the support from one end portion to the other), wherein the method comprises (a) providing on the support a refractory oxide of aluminium, silicon, zirconium, titanium and/or chromium having a surface area of not less than 10 m2 per gram of the refractory metal oxide (and preferably 100 to 300 m2/g) or a precursor convertible to the refractory oxide by firing at over 2500C, (b) depositing onto dispersed (preferably colloidally dispersed) ceria particles a noble metal catalyst comprising platinum, rhodium and/or palladium or a precursor convertible to the noble metal catalyst by firing at over 2500C, applying the ceria particles and (c) applying the ceria particles and deposited noble metal to the surface of the refractory oxide or its precursor on the support and (d) firing the support to a temperature of over 2500C (preferably 400 to 8500 C) after the application of the ceria particles so ensuring that any precursor is converted to refractory oxide or noble metal catalyst.

Deposition of the precious metal onto the particles of ceria before the ceria particles are applied to the refractory oxide ensures intimate contact between the noble metal and the ceria. Preferably the catalyst system comprises from 5 to 25 g of ceria per cubic metre of the volume of the catalyst system.

Preferably the particles of colloidal ceria have a number average particle size of from 1 to 100 nm.

In practice it is easier to deposit a noble metal precursor such as a noble metal halide onto the ceria rather than to attempt to deposit the metal itself. Rhodium or palladium chlorides may be deposited directly by treatment of the ceria with aqueous solutions of the chlorides. Platinum chloride is preferably deposited by treatment of the ceria with chloroplatinic acid whereupon a spontaneous deposit of the chloride precursor occurs.

The noble metals may be used alone as catalysts or they may be used in combination with each other or in combination with base metals, preferably metals of Groups 3 to 8 and IB of the periodic table, especially nickel, cobalt or copper. More particularly platinum may be used in admixture with from 35 to 70% (by weight of the mixture) of palladium. A triple combination of platinum with 6 to 95% of a mixture of palladium and rhodium is especially useful and preferably the ratio of palladium to rhodium in the triple combination is from 1 to 4 to 1 to 8 by weight.

The particles of ceria carrying the deposit of noble metal or noble metal-precursor may be conveniently applied to the refractory oxide or its precursor on the support by simply dipping the support carrying its deposit of oxide or oxide-precursor into the ceria dispersion for preferably 1 to 5 minutes, removing the support from the dispersion and blowing excess dispersion from the passageways in the support and then drying that dispersion which remains in the passageways by gently heating the support to preferably 100 to 1 700C for 0.5 to 5 hours prior to firing.Usually firing for 5 to 100 minutes will ensure not only that any precursor is completely converted to refractory oxide or noble metal, but also that the noble metal catalyst is substantially permanently supported on the ceria and that the ceria is substantially permanently supported on the refractory oxide.

A colloidal dispersion of ceria which is especially useful in the performance of this invention can be made by treating non-dispersible cerium IV hydrate with dilute mineral acid (preferably HNO3) to deaggregate the cerium IV hydrate. The crystallite size of the de-aggregated cerium IV hydrate formed in this way is from 4 to 10 nm and usually about 6 nm. The addition of excess acid to the de-aggregated particles yields a clear supernatant liquor and a slurry. When the supernatant liquor is removed and water is added to the slurry, spontaneous peptization occurs to produce a dispersion of colloidal ceria.

Colloidal dispersions of ceria made in this way are preferred because the concentrations of ceria obtainable can be over 200 g/l and often as high as 800 g/l. Preferably the dispersions contain from 50 to 450 g/l of ceria.

The preferred refractory oxides of high surface area are y- or X-alumina although other suitable oxides include chromias, titanias or zirconias if their surface areas exceed 10 m2/g. Preferably the alumina is applied to the support at a loading of from 0.06 to 0.6 grams of alumina per m2 of wall surface area. Suitable precursors for y-alumina include amorphous alumina, alumina monohydrate or alumina trihydrate.

The preferred support comprises one or more unitary elongate structures having walls which define passageways through which the exhaust flows during use. Because of the passageways defined by the walls, the structure presents in transverse section the appearance of a honeycomb except that the cells need not be hexagonal and are preferably mainly rectangular. Preferably the number of cells is from 0.3 to 1.0/mm2. The material from which the support is made may be any metal or ceramic with a commercially acceptable resistance to corrosion when used in the conditions encountered in the flow of exhaust from a spark-ignition engine. Suitable metals include stainless steels which contain aluminium and suitable ceramics include cordierite, cordierite-alpha alumina, silicon nitride, zircon mullite, spondumene, alumina-silica-magnesia or zirconium silicate.Other useful ceramics are disclosed in United States patent specification 3,565,830 (the contents of which are hereinincorporated by reference) and also in US 4,189,404 and US 4,206,087.

The invention is illustrated by means of the drawings of which Figure 1 is a diagrammatic perspective representation with part in transverse section of a support useful in the performance of this invention and Figure 2 is a transverse section on a larger scale taken through a central passageway of a support of the type shown in Figure 1 when the support forms part of a catalyst system according to this invention.

Figure 1 shows a unitary elongate cylindrical support 1 having spaced ends 2 and 3 and containing mutually orthogonal walls 4 and 5 which define passageways 6 passing through support 1 from end 2 to end 3. These passageways 6 cause support 1 to have a square "honeycomb" appearance in transverse section or end elevation.

Figure 2 shows in detail (though not to scale) a transverse section through a central passageway 6 of a support 1 which forms part of a catalyst system. A layer 7 of refractory oxide of high surface area is applied to walls 4 and 5 and supports particles 8 of ceria bearing a surface deposit of noble metal (not shown). Only a representative number of ceria particles 8 are shown and for clarity these are exaggerated in size relative to layer 7.

The invention is also illustrated by the following examples of which A and B are comparative.

EXAMPLE 1 Preparation of a Catalyst System: Ceria was made by de-aggregating cerium IV hydroxide with dilute nitric acid at 900C to produce an aqueous slurry having a pH of below 2.0 whereupon ceria particles formed. Supernatant liquor was decanted from the slurry and fresh water added to the undecanted residue. The residue and water were stirred mechanically to disperse the ceria particles so forming a colloidal dispersion containing 134 g/l of ceria.

A solution of 10 g of chloroplatinic acid (H2PtCle) in 50 cm3 of water was mixed with the ceria suspension at room temperature whereupon a spontaneous deposition of platinum chloride precursor onto the surface of the ceria particles occurres so creating an aqueous dispersion of platinised ceria particles.

The platinised ceria particles were then ready for application to a cylindrical support of the type shown in Figure 1. The support was composed of cordierite, was 76.2 cm long and 48.26 mm in diameter and contained 0.62 passageways per mm2 of its cross-sectional area. The walls of the passageways carried a deposit of y-alumina having a surface area of 1 50 m2/g and present in a loading of approximately 0.01 g/mm2 (although lower loadings may be preferred in future, for example 0.006 g/mm2). The platinised ceria particles were applied to the support by immersing the support in the aqueous dispersion of the particles for two minutes. The support was then removed from the dispersion and excess dispersion was blown from the passageways.The support was dried gently at 11 00C for 3 hours and finally fired at 6500C for 30 minutes to convert the precursor to platinum metal and to produce a support comprising platinum supported on ceria which in turn was carried by y-alumina provided on the walls of the passageways of the support. The catalyst system contained 0.007 gram atoms/m3, i.e. 1.3 g/m3 12%) of platinum and 10 g/m3 of ceria.

EXAMPLE 2 Evaluation of Catalyst Systems: A catalyst system made according to Example 1 was inserted into the exhaust pipe leading from a pulse flame combustion chamber designed to simulate the fuel combustion characteristics of a commercial petrol engine. The catalyst system made a close fit in the pipe.

A stoichiometric mixture of air and a commercial motor car petrol containing 0.4 g/l of lead moiety and 395 parts per million of sulphur, 1.0 theory* of ethylene dichloride and 0.5 theory of ethylene dibromide was injected into the chamber at a rate of 8 injections per second. The petrol was ignited by a pilot light and the exhaust was vented down the exhaust pipe and through the catalyst system.

Sufficient of the petrol/air mixture was injected to achieve a space velocity of 75,000/hr through the catalyst system. An additional quantity of air equal to 0.5% by weight of the air injected into the combustion chamber was injected into the exhaust pipe upstream of the catalyst system so as to ensure optimum exhaust purification.

Combustion was performed for three consecutive periods of 8 hours during which the lagging of the exhaust pipe upstream of the catalyst system was adjusted so that the temperature of the exhaust gas as it entered the catalyst system (i.e. the inlet temperature) was 6300C during the first 8 hour period, 5400C during the next period and 7700C during the last period. The concentration of methane, ethylene and carbon monoxide in the exhaust gas upstream of the catalyst system was measured as was their concentrations-in the exhaust leaving the catalyst system. The percentage reductions in the concentration of these gases subsequent to their passage through the catalyst system is shown in Table A. Measurements were taken after 1 hour and after 7.5 hours of operation at the temperatures shown.

COMPARATIVE EXAMPLES A AND B The procedure of Example 2 was repeated except a catalyst system was used in which the platinum catalyst had been deposited directly onto the y-alumina coating on the support by spontaneous deposition from either chloroplatinic acid (Example A) or from platinum tetrammine chloride (Example B). No ceria was present.

Table A shows that the presence of ceria inhibits the fall in the ability of the catalyst system to reduce the methane concentration at 6300C and 5400C and in the longer term also at 7700C. This is indicative of increased resistance to lead poisoning. The presence of ceria also inhibits loss of the ability to reduce the concentrations of ethylene and carbon monoxide at 7700C and this too is indicative of increased resistance to lead poisoning.

This invention also provides in combination, a spark-ignition engine, an exhaust system suitable for conveying exhaust gas from the engine and a catalyst system according to this invention located within the exhaust system. Preferably the combination also comprises means for introducing air into the exhaust system upstream (preferably not more than 20 cms upstream of) the catalyst system.

* A theory of a lead scavenger such as ethylene dihalide, is the weight of scavenger needed in theory for a stoichiometric conversion of all the lead present to the lead dihalide.

TABLE A

Inlet Temperature 0C 630 540 770 Example After % Reduction in % Reduction in % Reduction in hrs hrs CH4 CzHZ CO CH, C2H CO CH4 CzH CO 1 1 96 100 99 71 97 99 81 93 98 7.5 90 99 99 71 94 99 73 86 96 A 1 97 100 99 63 1 98 98 85 91 90 7.5 85 99 99 49 93 98 64 81 90 B 1 91 100 99 69 97 98 76 85 70 7.5 82 99 99 46 93 98 36 63 70 CLAIMS I 1.A lead tolerant catalyst system suitable for puifying exhaust from a spark-ignition engine, the catalyst system comprising (a) a support, (b) a refractory oxide of aluminium, silicon, zirconium, titanium and/or chromium having a surface area of not less than 10 m2 (per gram of the refractory metal oxide) carried on the support and (c) a noble metal catalyst comprising platinum, rhodium and/or palladium supported on ceria which in turn is supported by the refractory oxide.

2. A catalyst system as claimed in claim 1 wherein the catalyst system comprises from 5 to 25 g of ceria per cubic metre of the catalyst system.

3. A catalyst system according to claim 1 or claim 2 wherein the catalyst system comprises from 0.001 to 0.03 gram atoms noble metal catalyst per cubic metre of the catalyst system.

4. A catalyst system according to any one of the preceding claims wherein the catalyst comprises two or more of the metals platinum, rhodium or palladium.

5. A method for making a lead tolerant catalyst system comprising a support wherein the method comprises (a) providing on the support a refractory oxide of allsminium, silica, zirconium and/or chromium having a surface area of not less than 10 m2 per gram of the refractory oxide or a precursor convertible to the refractory oxide by firing at over 2500C, (b) depositing onto dispersed ceria particles a noble metal catalyst comprising platinum, rhodium and/or palladium or a precursor convertible to the noble metal catalyst by firing at over 2500 C, (c) applying the ceria particles and deposited noble metal catalyst or its precursor to the surface of the refractory oxide or its precursor on the support and (d) firing the support to a temperature of over 2500C after the application of the ceria particles so ensuring that any precursor is converted to refractory oxide or noble metal catalyst.

6. A method according to claim 5 wherein the dispersion of ceria comprises a colloidal dispersion of ceria in water.

7. A method according to claim 5 or claim 6 wherein the dispersion of ceria comprises from 50 to 450 g/l of ceria.

8. A method according to any one of claims 5 to 7 wherein the support is fired to a temperature of from 400 to 850 C.

9. In combination, a spark-ignition engine, an exhaust system suitable for conveying exhaust from the engine and a catalyst system as claimed in any one of claims 1 to 5 or made by the method of any one of claims 6 to 8 located within the exhaust system.

10. A combination as claimed in claim 9 comprising means for introducing air into the exhaust system immediately downstream of the catalyst system.

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

Claims

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

TABLE A

Inlet Temperature 0C 630 540 770 Example After % Reduction in % Reduction in % Reduction in hrs hrs CH4 CzHZ CO CH, C2H CO CH4 CzH CO 1 1 96 100 99 71 97 99 81 93 98 7.5 90 99 99 71 94 99 73 86 96 A 1 97 100 99 63 1 98 98 85 91 90 7.5 85 99 99 49 93 98 64 81 90 B 1 91 100 99 69 97 98 76 85 70 7.5 82 99 99 46 93 98 36 63 70 CLAIMS I 1.A lead tolerant catalyst system suitable for puifying exhaust from a spark-ignition engine, the catalyst system comprising (a) a support, (b) a refractory oxide of aluminium, silicon, zirconium, titanium and/or chromium having a surface area of not less than 10 m2 (per gram of the refractory metal oxide) carried on the support and (c) a noble metal catalyst comprising platinum, rhodium and/or palladium supported on ceria which in turn is supported by the refractory oxide.
2. A catalyst system as claimed in claim 1 wherein the catalyst system comprises from 5 to 25 g of ceria per cubic metre of the catalyst system.
3. A catalyst system according to claim 1 or claim 2 wherein the catalyst system comprises from 0.001 to 0.03 gram atoms noble metal catalyst per cubic metre of the catalyst system.
4. A catalyst system according to any one of the preceding claims wherein the catalyst comprises two or more of the metals platinum, rhodium or palladium.
5. A method for making a lead tolerant catalyst system comprising a support wherein the method comprises (a) providing on the support a refractory oxide of allsminium, silica, zirconium and/or chromium having a surface area of not less than 10 m2 per gram of the refractory oxide or a precursor convertible to the refractory oxide by firing at over 2500C, (b) depositing onto dispersed ceria particles a noble metal catalyst comprising platinum, rhodium and/or palladium or a precursor convertible to the noble metal catalyst by firing at over 2500 C, (c) applying the ceria particles and deposited noble metal catalyst or its precursor to the surface of the refractory oxide or its precursor on the support and (d) firing the support to a temperature of over 2500C after the application of the ceria particles so ensuring that any precursor is converted to refractory oxide or noble metal catalyst.
6. A method according to claim 5 wherein the dispersion of ceria comprises a colloidal dispersion of ceria in water.
7. A method according to claim 5 or claim 6 wherein the dispersion of ceria comprises from 50 to 450 g/l of ceria.
8. A method according to any one of claims 5 to 7 wherein the support is fired to a temperature of from 400 to 850 C.
9. In combination, a spark-ignition engine, an exhaust system suitable for conveying exhaust from the engine and a catalyst system as claimed in any one of claims 1 to 5 or made by the method of any one of claims 6 to 8 located within the exhaust system.
10. A combination as claimed in claim 9 comprising means for introducing air into the exhaust system immediately downstream of the catalyst system.