GB2264242A

GB2264242A - Heating catalytic converters

Info

Publication number: GB2264242A
Application number: GB9202894A
Authority: GB
Inventors: James Anthony Cairns
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 1992-02-12
Filing date: 1992-02-12
Publication date: 1993-08-25
Also published as: GB9202894D0

Abstract

Catalytic apparatus, particularly for the treatment of the exhaust gas of an internal combustion engine in order to combat air pollution, comprises a catalyst and a radiant source mounted such that in operation radiant energy from the radiant source falls on at least part of the catalyst, whereby the catalyst is heated to reduce the time it takes to reach its effective reaction temperature. The radiant source is preferably an electrically heated quartz halogen tube or bulb.

Description

CATALYTIC APPARATUS This invention relates to catalytic apparatus and to a method of conducting a catalytic reaction employing the apparatus.

When a catalytic reaction occuring above ambient temperature is started up, it sometimes takes the catalyst a significant time to heat up to the point at which effective reaction begins to occur. This is particularly important in the case of the catalytic treatment of the exhaust gas of an internal combustion engine in order to combat air pollution.

Catalytic converters are employed to decrease gaseous pollutants (particularly hydrocarbons, carbon monoxide and nitrogen oxides) from the exhaust gas of internal combustion vehicles, and these converters take several minutes to heat up to their effective operating temperature from a cold start. In that time, a high proportion of the pollutants emitted during the whole of an average journey is emitted. The cold start phase of the standard United States Federal Test Procedure produces the major portion of the total emissions of the whole test (see Society of Automotive Engineers, Inc, paper 890799). Electrically heating the catalytic converter has been proposed in order to reduce the time it takes to achieve lightoff (see for instance the Society of Automotive Engineers paper referred to above).In such systems, a substrate serves simultaneously as a resistive heating device and the support for the catalytic coating. Such systems, however, require a high power level, which would probably necessitate an additional high power battery and associated recharging equipment to be fitted in vehicles.

The present invention provides catalytic apparatus comprising a catalyst and a radiant source mounted such that in operation radiant energy from the radiant source falls on at least part of the catalyst, whereby the catalyst is heated to reduce the time it takes to reach its effective reaction temperture.

The invention provides also a method of conducting a catalytic reaction wherein the time taken to heat up the catalyst to its effective reaction temperature is reduced by means of the catalytic apparatus.

By radiant source is meant a source of energy the majority of which is radiant energy. This contrasts with the prior art discussed above. The use of radiant energy in the present invention is advantageous in that it transfers energy very efficiently to heat up the catalyst. In addition, the present apparatus is simple and can employ existing equipment with little or no modification.

The apparatus is of particular interest where the reaction is exothermic, especially where the catalyst is for the treatment of exhaust gas, particularly the exhaust gas of an internal combustion engine, in order to combat air pollution. It has been found that the apparatus has good durability in such treatment. The engine can be a spark ignition or diesel engine. The engine is preferably that of a vehicle and is especially a petrol (gasoline) or methanol, particularly a petrol, engine.

The catalyst in the apparatus can be a known catalyst, especially a catalyst comprising at least one of platinum and palladium.

In a preferred embodiment, the catalyst comprises platinum and rhodium.

Preferably, the present catalyst is for the treatment of the exhaust gas of an internal combustion engine in order to combat air pollution. The catalyst then usually oxidises hydrocarbons and carbon monoxide to carbon dioxide and reduces nitrogen oxides to nitrogen. For this treatment, supplemental air can be injected up-stream of the catalyst if needed; usually it is needed.

The catalyst can be in known form, especially supported on a monolith. The catalyst is preferably dispersed on a high surface area carrier, especially alumina. The carrier in turn is preferably on a support, especially a monolith. The present monolith can be a honeycomb monolith and is usually ceramic. The catalyst unit can be mounted in known way, particularly in a box in the exhaust system of an internal combustion engine vehicle.

The radiant source in the present apparatus is preferably at least one electrically powered radiant bulb or tube, which bulb or tube usually has a quartz envelope, and is especially at least one quartz halogen electrically powered bulb or tube. For instance, 2-4 of the present bulbs or tubes, preferably connected electrically in parallel, can be employed. Existing such quartz halogen bulbs or tubes can conveniently be employed, and these are available in a variety of geometric forms and with different power outputs. The present bulb or tube preferably contains a filament. The present source preferably takes 8-28 volts. It preferably takes 100 to 1200, especially 200 to 900, watts of electrical power.

It is especially advantageous to employ the present apparatus where the catalyst is for the treatment of the exhaust gas of an internal combustion engine in order to combat air pollution and the engine is mounted in a vehicle, employing as the radiant source at least one quartz halogen electrically powered bulb or tube working off the vehicle's 8-28 volt supply, usually a conventional 12 or 24 volt supply to avoid having to increase the electrical power supply.

The radiant source can be chosen for its specific spectral output. For instance, the radiant output can be predominantly infra-red, to achieve rapid heat transfer, or it can have a significant ultra-violet component to stimulate specific chemical reactions, for example by changing the adsorptivity or catalytic properties of the catalyst on which it falls. The magnitude of such stimulation can be enhanced by incorporating dopants into the catalyst system. The catalyst system, for example a monolith support, can be adapted to promote absorption of the radiant energy, eg its visible or ultra-violet light.In a particular embodiment, the radiant source comprises at least one bulb or tube containing a filament, the bulb or tube emitting the majority of its radiant energy in the visible/infra-red regions, together with at least one discharge tube which contains electrodes across which an arc is struck, the tube emitting a major part of its radiant energy in the visible/ultra violet regions.

The radiant source can be mounted at any convenient point relative to the catalyst as long as radiant energy from the source falls on at least part of the catalyst to heat it up and hence reduce the time it takes to reach its effective reaction temperature. When the radiant energy falls on only part of the catalyst, this part heats up and the heat can then spread from that part to the remainder; this is particularly useful when the reaction is exothermic, since in that case the reaction itself produces heat which can spread through adjacent parts of the catalyst.

The source is usually close to but not touching the catalyst. In an alternative configuration, a thin catalyst coating can be present on the outside surface of an electrically powered radiant bulb or tube.

The radiant source and its position are so chosen as to achieve the desired reduction in the time it takes the particular catalyst employed to reach its effective reaction temperature. A measure of this time for an exothermic reaction is the time taken for the catalyst to achieve light-off. Light-off may be defined as the point at which there is a sudden increase in the rate of reaction, as indicated by a point of inflexion on a graph of percent conversion of a reactant plotted against time. The present apparatus is preferably such that the radiant energy is effective to reduce the light-off time by at least half, especially by at least 70%. When the catalyst is supported on a honeycomb monolith, the source can be mounted in the monolith, preferably at right angles to the direction of flow of gas through it.Thus, existing monoliths can be employed with the modification that they have a small hole or holes accommodating the radiant source. Preferably, however, the source is upstream of the monolith.

When the catalyst is employed for treating gas flowing over it, the source can be positioned in the gas stream. Alternatively, the source can be positioned outside the gas stream and the radiant energy directed thence onto the catalyst.

The present apparatus can incorporate means, usually a reflector, to focus the radiant energy. The reflector is usually a material which resists high temperature, eg quartz, carrying a coating which reflects radiant energy. For example, a curved such-coated quartz reflector can be positioned in the gas stream of a vehicle exhaust up-stream of the radiant source which in turn is up-stream of the catalyst. A reflector can advantageously be combined with all or part of the radiant source into a single unit, for instance a radiant bulb or tube can carry on the upstream side of its envelope the coating which reflects radiant energy, usually in the form of a deposit of a thin film. The present coating can be for example of gold.In a particular embodiment, the coating is itself catalytic, eg by comprising platinum, thus aiding the catalytic reaction, especially when this is exothermic (which it is in the case of vehicle exhaust) since the gas will thereby be heated and contribute to the main catalyst reaching its effective reaction temperature more quickly. It can be seen that in the usual arrangement in which gas to be treated impinges, en route to the catalyst, on the back of the reflector and not on its reflective face, the reflective quality of the reflective face can suffer no (for instance in the case of the coated bulb or tube) or little impairment from the gas, for instance from any carbon therein which might obscure the face.

Preferably, the apparatus is adapted so that the source can be replaced without disturbing the catalyst. It is an advantage of the apparatus that the source and the catalyst can be separate, in contrast to the prior art discussed above, in which the catalytic converter substrate is resistively heated.

The apparatus preferably incorporates means to turn off the source once the catalyst has reached the desired temperature, usually once light-off has been achieved. The temperature, for example at light-off, can be sensed, for example by at least one thermocouple. Conventional electrical circuitry associated with such temperature sensing can turn off the source.

When the catalyst is employed for treating engine exhaust gas, the apparatus can be adapted to turn on the radiant source, and hence start heating the catalyst, before the engine has started. This reduces the time to effective reaction temperature even further. For instance, opening the driver's door of a vehicle can trigger such a turn-on. In a preferred embodiment, however, the apparatus is adapted not to draw power from the engine's battery when it is needed to start the engine. The apparatus is preferably adapted to turn on the radiant source immediately after the engine has started, whether or not the radiant source has been turned on also before the engine has started. In a preferred embodiment, the radiant source is powered by a vehicle's alternator.

The present apparatus can be employed up-stream of other catalytic equipment, such as a further catalyst unit for the treatment of the exhaust gas of an internal combustion engine in order to combat air pollution, but this is not preferred. The present apparatus can incorporate additional means of heating the catalyst to reduce the time it takes to reach its effective reaction temperature, such as electrical resistive heating, besides the present radiant source, but again this is not preferred.

In the present invention, heat is transferred to the catalyst by means of radiant energy. Radiant energy is transferred between bodies not in physical contact. Even in the embodiment discussed above of catalyst coated on the outside surface of an electrically powered bulb or tube, the catalyst is heated predominantly by radiation.

The invention is illustrated by the following Examples, and the accompanying drawings to which they refer. In the drawings: Figure 1 is a graph of relative intensity (in arbitrary units) against wavelength (lem) for the bulb employed in Example 1; Figure 2 is a graph of temperature ( C:) against time (minutes) for the catalytic apparatus employed in Example 1; Figure 3 is a diagrammatic plan view of the catalytic apparatus employed in Example 2; Figure 4 is a diagrammatic perspective view of that apparatus; Figure 5 is a graph of temperature (C C) against time (minutes) for that apparatus without the bulbs being switched on, together with an inset representation of the apparatus; and Figure 6 is a corresponding graph and representation with the bulbs being switched on.

EXAMPLE 1 A conventional Pt/Rh/Al2 O3 vehicle exhaust catalyst on a cordierite cylindrical monolith of dimensions 4in (10.2cm) diameter and 6in (15.2cm) long was employed. A small hole was drilled into the side of the monolith so as to accommodate a quartz halogen bulb (14 volts, 8.2 amps) at right angles to the flow of the exhaust gas and a few millimetres behind the front face of the monolith, which was fitted into the exhaust system of a British Leyland 1.5 litre 4-cylinder petrol engine. The spectral output of the bulb is shown in Figure 1 of the accompanying drawings, from which it can be seen that the majority of its radiant energy occurs in the visible/infra-red regions. Thermocouples were mounted in front of and behind the monolith. The engine was started from cold (17"C reading on the front thermocouple) and held at a speed of 1,000 rpm without choke, while monitoring the back thermocouple.

The sudden increase in the back temperature was taken as an indication of the catalyst having achieved light-off. The time required for this to occur was of the order of 7 min, as shown by curve B in Figure 2 of the accompanying drawings. The engine was then switched off and allowed to cool down to the same cold condition as before. It was then re-started and, at the same time, the bulb was switched on. It was found that light-off was achieved in approximately 2 min, as shown by curve A in the same figure of the accompanying drawings. This response could be made even shorter by (a) switching on the bulb for a short time before starting the engine; (b) using a more powerful bulb; or (c) using more than one bulb.

EXAMPLE 2 A conventional Pt/Rh/Al O3 vehicle exhaust catalyst on a metallic cylindrical monolith of dimensions 3.5in (9cm) diameter and 2in (5cm) length was employed. Two 24 volt, 250 watt, quartz halogen bulbs (Wotan Xenophot HLX 64655 A1/223 bulbs from Wotan in Germany) were mounted parallel to the front face of the monolith, at an angle of 55" to each other. The bulbs were connected in series (48 volts, 11.5 amps). The distance between the bulbs and the front face of the monolith was 0.20in (5mm). The whole assembly was held in a stainless steel container, which was fitted on to the exhaust system of a British Leyland 1.5 litre 4-cylinder petrol engine. Thermocouples were mounted in front of and behind the monolith.The catalytic apparatus is shown diagrammatically in plan view in Figure 3 of the accompanying drawings, in which 1 represents the catalyst on the monolith, 2 the quartz halogen bulbs, 3 ceramic bulb holders, 4 insulation around the monolith and 5 a stainless steel container. The stainless steel container is shown in diagrammatic perspective view in Figure 4 of the accompanying drawings, in which 6 represents electrical lead-throughs and 7 a silica observation window.

The engine was started from cold (17"C reading on the front thermocouple) and held at a speed of 1000 rpm without choke, while monitoring the back thermocouple. The sudden increase in the back temperature was taken as an indication of the catalyst having achieved light-off. The time required for this to occur was of the order of 13 min, as shown by the curve in Figure 5 of the accompanying drawings. The engine was then switched off and allowed to cool to the same cold condition as before. It was restarted on the following day, approximately 19 hours later, and, at the same time, the bulbs were switched on. It was found that light-off was achieved in less than 30 sec, as shown in Figure 6 of the accompanying drawings.

Claims

1. Catalytic apparatus comprising a catalyst and a radiant source mounted such that in operation radiant energy from the radiant source falls on at least part of the catalyst, whereby the catalyst is heated to reduce the time it takes to reach its effective reaction temperature.

2. Apparatus according to claim 1 wherein the catalyst is for an exothermic reaction.

3. Apparatus according to claim 2 wherein the catalyst is for the treatment of the exhaust gas of an internal combustion engine in order to combat air pollution.

4. Apparatus according to claim 3 wherein the engine is a petrol engine.

5. Apparatus according to any one of the preceding claims wherein the catalyst comprises at least one of platinum and palladium.

6. Apparatus according to claim 5 wherein the catalyst comprises platinum and rhodium.

7. Apparatus according to any one of the preceding claims wherein the catalyst is supported on a honeycomb monolith.

8. Apparatus according to any one of the preceding claims wherein the radiant source is at least one quartz halogen electrically powered bulb or tube.

9. Apparatus according to claim 8 wherein the source takes 100 to 1200 watts of electrical power.

10. Apparatus according to any one of the preceding claims wherein the radiant energy is effective to reduce the time taken to achieve light-off by at least one half.

11. Apparatus according to any one of the preceding claims adapted so that the radiant source can be replaced without disturbing the catalyst.

12. A method of conducting a catalytic reaction wherein the time taken to heat up the catalyst to its effective reaction temperature is reduced by means of catalytic apparatus claimed in any one of the preceding claims.