WO1998016303A1

WO1998016303A1 - Improvements in catalytic processes

Info

Publication number: WO1998016303A1
Application number: PCT/GB1997/002783
Authority: WO
Inventors: Jonathon Charles Frost; Ann-Marie Jones; Ian Carmichael Wishart
Original assignee: Johnson Matthey Public Limited Company
Priority date: 1996-10-11
Filing date: 1997-10-09
Publication date: 1998-04-23
Also published as: AU4630697A; GB9621214D0

Abstract

An air purification and air conditioning system comprises, in combination, a particle filter (2), a first low temperature light-off catalytic convertor for the oxidation of CO to CO2 (6), a second catalytic convertor (7) for at least one of the reactions hydrocarbons to carbon dioxide and water, VOCs to carbon dioxide and water and NOx to N2, and a heat pump (8). The system is particularly suited to use in vehicles.

Description

IMPROVEMENTS IN CATALYTIC PROCESSES

The present invention concerns improvements in catalytic processes. More particularly, it concerns the use of catalytic processes to improve climate or environment, especially within a vehicle or building.

The interior environment of a vehicle is much higher in polluting gases and particulates than is generally realised, and may surprisingly be appreciably more polluted than the environment surrounding the vehicle. Inside the vehicle, there are generally present dusts, including particulates emitted from diesel and gasoline-powered vehicles, gases including hydrocarbons, carbon monoxide and vapours including benzene and other vapours emitted from the interior fittings. To date, there have been few practical attempts to improve the interior environment, these being limited essentially to the provision of "pollen filters" for ventilation air, and to air conditioning which does improve the environment to a small extent as well as providing a more comfortable temperature. There are not dissimilar problems arising in buildings, especially modern large buildings which have given rise to the term "sick building syndrome". We believe, therefore, that there is a need for a system which is capable of providing a real improvement for a vehicle or building internal environment.

The present invention, therefore, provides an air purification and air conditioning system comprising, in combination, a particulate filter, a first low temperature light-off catalytic convertor for the oxidation of carbon monoxide to carbon dioxide, a second catalytic convertor for at least one of the reactions NOx to N₂, hydrocarbons to carbon dioxide and water and volatile organic compounds ("VOCs") to carbon dioxide and water, and a heat pump.

In one preferred embodiment, the components of the system are positioned in series in ducting, but the benefits of the invention may be gained in large part by one or more components of the system operating largely independently. For example, in a vehicle, a catalytic convertor for hydrocarbons, VOC and/or NOx may be mounted to treat air within the passenger cabin, while the other system components treat air from outside the vehicle before being passed into the passenger cabin. Similarly, a particle filter may be mounted to treat cabin air. In all cases, the air to be treated may be "fresh" air, drawn wholly from the external environment, wholly re-circulated air, or a blend of "fresh" air and re-circulated air.

Hereinafter, reference will be made to vehicles, and it is to be understood that this applies to private cars, vans, minivans, light and heavy trucks and buses of all sizes, tractors and agricultural and earth-moving equipment all having an enclosed driver and/or passenger compartment, but not to vehicles which are open to the air such as motorcycles and similar. For convenience, the invention will be described hereafter with particular reference to vehicles, but it is believed to be applicable to at least rooms in buildings of most types also, to aircraft, enclosed parts of ships and naval or merchant craft of all types, trains, and may be adapted for personal environment control in hazardous or appropriate other conditions, for example for individual suits or even enclosed helmets.

Another preferred embodiment of the invention includes an absorber module, which is desirably effective to absorb or adsorb odours, especially sulphur compounds such as H₂S, acetaldehyde and formaldehyde, hydrocarbons and VOCs, particularly xylenes, benzene and MTBE. Such an absorber module may be connected with a ducted system, for example after a particle filter and before a first catalytic convertor. Depending upon the detailed system design and all of the environmental factors expected, the absorber may be re-generated by the use of a stream of hot air and the accumulated vapours and odours vented to the external environment, or the vapours and odours may be passed over a catalytic convertor, if amenable to reaction on the catalytic convertor. Rather than regenerating the absorber in situ, the absorber may be replaced at regular service intervals. It may also be desirable for the absorber to be effective to absorb moisture, especially if the catalytic convertor(s) is/are sensitive to water. In general, absorbing water reduces humidity with a considerable improvement in perceived comfort levels. It may also reduce the load on the heat pump unit, and possibly also reduce the volume of air required to be treated according to the invention, with consequential benefits in equipment size, volume, weight and cost. A suitable absorber may be a zeolite and/or active carbon, mounted in a canister. The particulate filter is desirably a sub-micron filter, capable of removing pollens and the small particles generated by gasoline engines, as well as smokes from diesel engines and other dust particles. Such filters are commercially available and include wall flow filters and ceramic foams. The filter may be replaced at regular intervals and will be sized according to the service intervals and the expected duty. The filter may be incorporated in an absorber module or a catalytic convertor canister.

Desirably, the first catalytic convertor has a light-off temperature, defined as the temperature at which 50weight% of the gas concerned is converted, below ambient temperature, that is below 25 °C, for example at 5-20 °C. A preferred catalyst is a platinum- based catalyst, especially platinum promoted with manganese, most desirably a catalyst such as 1 weight%Pt on MnO₂/Al₂O₃. Other catalysts such as Pt on SnO₂ or "Hopcalite", believed to be a copper oxide/manganese oxide mixture, may be used as an alternative. It is believed necessary that the first catalytic convertor is effective under most start-up conditions, but perhaps not sub-zero temperatures, to convert 50weight% or more of CO in an amount of about 50ppm. Since the CO oxidation reaction is exothermic, the catalytic convertor quickly reaches light-off temperature even if starting sub-zero. Nonetheless, a pre-heater may be used to speed light-off.

Preferably, the first catalytic convertor is in high surface area form with low pressure drop characteristics. Although the catalyst may be in the form of pellets, it is preferably in the form of a coating on a metal or ceramic open-celled honeycomb, of the type well known as substrates for exhaust gas catalytic convertors. However, since the first catalytic convertor will not be exposed to the high operating temperatures of exhaust gas catalysts, which may be 800 °C or more, it is possible that savings may be made on materials. For example, a metal honeycomb may be made of aluminium rather than stainless steel. It is also believed to be advantageous to include the absorber module upstream of the first catalytic convertor, to reduce the amounts of potentially interfering substances present in the air stream being treated. The air leaving this first catalytic convertor will thus be substantially free of carbon monoxide, and will have been heated by the exothermic oxidation. Desirably, the treated air next meets a second catalytic system which is capable of catalysing the reduction of NOx to N₂ and/or capable of catalysing the oxidation of organic vapours and gases. A currently preferred system is effective to catalyse both reactions simultaneously, and such a preferred system is a photocatalyst combined with a suitable light source. We have found that a platinum on titanium dioxide catalyst system is particularly effective, and this may be deposited on one or more metal plates or a metal or ceramic honeycomb. We have found that a conventional mercury lamp operates to provide UV light in the correct and effective range for a TiO₂-based catalyst, but other light sources, such as a halogen lamp, may be used. Especially in the case of a honeycomb, light may be distributed over the catalyst by using a mirror and/or lenses. The use of a photocatalyst system significantly reduces the activation energy for the desired reactions, and thus the reactions take place at a lower temperature, speeding light-off and saving energy. The preferred catalyst system is a lweight% Pt on TiO₂ catalyst, which may be coated using known methods onto the plates or honeycomb. Other photocatalysts, however, may be used, but at present it is considered that TiO₂ or promoted TiO₂ offer the best performance.

Gases leaving the second catalytic convertor may be at a temperature of about 200°C.

Desirably, the hot gases are passed to an air conditioning heat pump before being passed into the vehicle. A particularly suitable unit is an adsorber-based system using water as adsorbate and utilising waste heat from the exhaust and/or exhaust gas catalyst, such as described in our co-pending application number 9613211.3, the disclosure of which is incorporated herein. A particular advantage of such an air conditioning unit is that high grade electrical and/or mechanical energy from the engine is not necessary to operate it. Current designs of air conditioner put a considerable load on the engine, with a consequent fuel consumption penalty. It may, however, be desirable to use a compression-type heat pump, eg using hydrocarbons or other non-CFC fluid, for size or weight considerations. The system of the present invention offers, we believe for the first time, a very considerably improved environment inside the vehicle, with major pollutants being removed and the passenger/driver compartment being maintained at a comfortable temperature. In its most preferred embodiment, the system is fully integrated with the air supply to the compartment, and routine design procedures provide a low-energy consumption system.

The present invention will now be described by way of example only with reference to the accompanying schematic drawing of one embodiment, Figure 1 , and accompanying charts Figures 2 to 5 which show performance of the catalytic convertors.

Referring to Figure 1, which is not to scale, external air is taken into the system through intake 1, and passes through a sub-micron filter, 2. The filter removes pollens, dusts, aerosols and particulates of all types; it will require replacing during the normal car servicing cycle. The filtered air is then passed through heater unit, 3, which may be electrically heated or use waste heat. Fans, 4, power the passage of air through the system. The filtered air passes through an absorber module, 5, which is effective to remove odours and hydrocarbons and other VOCs, before being passed to a first, low light-off temperature catalytic convertor, 6, which catalyses the oxidation of CO to CO₂. The catalyst used in this first convertor is desirably a lweight%Pt on MnO₂/Al₂O₃ catalyst, deposited on a high surface area alumina washcoat, coated onto a standard cordierite or metal honeycomb monolith.

The air leaving catalytic convertor 6, is substantially free of CO and is warmed slightly to a temperature which depends upon the CO level in the air to be treated. This air then passes into a photocatalytic second catalytic convertor, 7, consisting of a mercury lamp, 7a, running from the car's electrical supply, and a high surface area plate or monolith carrying a 1 weight%Pt on titania catalyst. Our initial tests indicate that such a photocatalytic convertor operates best at elevated temperature eg up to 200 °C or more, and it is therefore desirable to heat the air, or the catalyst, for example using waste using waste engine or other process heat, to such a temperature. This is effective, for example at a steady state temperature of about 200 °C, to convert about 40-50% of NOx to N₂, and 80weight% or more of hydrocarbons.

The air leaving convertor 7 is at a temperature of about 200 °C. The purified air is passed through an adsorber air conditioning unit, 8, as described in detail in GB 9613211.3. The unit utilises waste heat from the car engine exhaust or from the exhaust catalyst (not shown). The air from convertor 7 is cooled to a comfortable temperature of about 20-25 °C, and is vented into the car interior. Waste air from the car interior is at least partially vented to the outside through vent, 10, and partially re-circulated and blended with "fresh" air from intake 1.

The absorber unit 5 can become saturated with absorbed odours and vapours, and it is desirable to regenerate it by blowing heated air therethrough. During a regeneration cycle, heater 3 is operative and a valve 11 is actuated to vent air-carrying desorbed vapours etc to outside the car through vent 12.

The system is designed in detail according to generally known heating and ventilating principles, and according to the car interior volume and other parameters.

The effectiveness of the preferred low light-off temperature catalyst used in convertor

3 is illustrated on a conversion against temperature plot shown in Figure 2, where the Pt/MnO₂/Al₂O₃ catalyst shows greater than 80% conversion of CO at all temperatures above 20 °C. Other catalysts tested do not show light-off until about 57-60 °C.

The effectiveness of the photocatalytic system used in convertor 4 is illustrated in accompanying Figure 3, which is a plot of the conversion of a hydrocarbon (C₃H₆) at 180 °C. To test the dependence of the conversion on the UV light source, this was switched on and off several times, and a dramatic difference in conversion is illustrated.

The effectiveness of the photocatalytic system for conversion of NOx is illustrated in accompanying Figure 4, which is a plot of the conversion of NO (500ppm by volume in helium) at 200°C, flowing at a rate of 50ml/min over a 50mm diameter disc coated with the catalyst. The approximate space velocity is between 1,000 and 2,000hr^"1. The gas emerging from the reactor is passed to a mass spectrometer which plots intensity (that is equivalent to concentration) against time. The light source was switched on and off, and a clear effect can be seen in the reduction of NO concentration, followed by its climb again to the original levels. The catalyst used in this test was lweight%Pt on TiO₂.

The foregoing test was repeated using TiO₂ as the photocatalyst, all other parameters being identical, and the results are shown in accompanying Figure 5.

It will be appreciated that the details of the present invention, as described, may be changed without departing from the overall inventive concept. For example, the first catalytic convertor may be composed such as to convert all or part of the hydrocarbons and/or other VOCs, or a single canister may include two separate monoliths carrying different catalytic coatings.

Claims

1. An air purification and air conditioning system comprising, in combination, a particulate filter, a first low temperature light-off catalytic convertor for the oxidation of carbon monoxide to carbon dioxide, a second catalytic convertor for at least one of the reactions NOx to N₂, hydrocarbons to carbon dioxide and water and volatile organic compounds to carbon dioxide and water, and a heat pump.

2. A system according to claim 1 , wherein the system also includes an absorber module effective to adsorb or absorb odours.

3. A system according to claim 2, wherein the absorber is also effective to remove water vapour.

4. A system according to any one of the preceding claims, wherein the first catalytic convertor comprises a platinum catalyst promoted with manganese.

5. A system according to any one of the preceding claims, wherein the second catalytic convertor comprises a photocatalyst system.

6. A system according to claim 5, wherein the photocatalyst system comprises a titania- based catalyst.

7. A system according to claim 6, wherein the titania-based catalyst is titania promoted by platinum.

8. A system according to any one of the preceding claims, wherein the particulate filter is a sub-micron filter.

9. A system according to any one of the preceding claims, wherein the heat pump is an adsorber-based system using waste heat.

10. A system according to claim 9, wherein the heat pump uses water as an adsorbate and a zeolite as the absorber.

11. A system according to claim 1 , substantially as hereinbefore described.

12. A method of air purification and air conditioning for an enclosed space, comprising passing air through, in series or partly in series and partly in parallel, a particulate filter, a first catalytic convertor to convert CO to CO₂, a second catalytic convertor to convert at least one of NOx to N₂, hydrocarbons to CO₂ and H₂O and VOCs to CO₂ and H₂O, and a heat pump effective to cool air from at least the second catalytic convertor.