One of the major problems associated with the development and use of internal combustion engines is the noxious exhaust emissions from such engines. Two of the most deleterious materials, particularly in the case of diesel engines, are particulate matter (primarily carbon) and oxides of nitrogen (NOX). Increasingly severe emission control regulations are forcing internal combustion engine and vehicle manufacturers to find more efficient ways of removing these materials in particular from internal combustion engine exhaust emissions.

Unfortunately, in practice, it is found that techniques which improve the situation in relation to one of the above components of internal combustion engine exhaust emissions tend to worsen the situation in relation to the other. Even so, a variety of systems for trapping particulate emissions from internal combustion engine exhausts have been investigated, particularly in relation to making such particulate emission traps capable of being regenerated when they have become saturated with particulate material.

Examples of such diesel exhaust particulate filters are to be found in European patent application EP 0 010 384; US patents 4,505,107; 4,485,622; 4,427,418; and 4,276,066; EP 0 244 061; EP 0 112 634 and EP 0 132 166.

In all the above cases, the particulate matter is removed from diesel exhaust gases by a simple physical

trapping of particulate matter in the interstices of a porous, usually ceramic, filter body, which is then regenerated by heating the filter body to a temperature at which the trapped diesel exhaust particulates are burnt off. In most cases the filter body is monolithic, although EP 0 010 384 does mention the use of ceramic beads, wire meshes or metal screens as well. US patent 4,427,418 discloses the use of ceramic coated wire or ceramic fibres.

GB patent 2,274,3412 discloses a method and apparatus for removing particulate and other pollutants from internal combustion engine exhaust gases, in which the exhaust gases are passed through a bed of charged pellets of material, preferably ferroelectric, having high dielectric constant (that is of the order of at least 1,000). In addition to removing particulates by oxidation, especially electric discharge assisted oxidation, there is disclosed the reduction of NOX gases to nitrogen, by the use of pellets adapted to catalyse the NOX reduction as exemplified by the use of barium titanate as the ferroelectric material for the pellets.

Also, US patents 3 983 021,5 147 516 and 5 284 556 disclose the catalytic reduction of nitrogen oxides.

However, US 3 983 021 is solely concerned with the reduction of NO to N in a silent glow discharge, the temperature of which is kept below a value at which the oxidation of N or NO to higher oxides of nitrogen does not occur. There is no mention of any simultaneous removal of hydrocarbons.

Although, so-called contact bodies are used in the process of US 3 983 021, and some of those disclosed may have some catalytic properties, catalysis does not appear

to be a necessary feature of the process of US 3 983 021.

Other surface properties, such as adsorption on large surface area materials, are the basis of the process of US 3 983 021.

US patent 5 147 516 does refer to the use of catalysts to remove NOX, but the catalytic materials involved are defined very specifically as being sulphur tolerant and deriving their catalytic activity from their form rather than their surface properties.

Also, the operating conditions are very tightly defined. There is no specific mention of the type, if any, of electric discharge involved. All that is disclosed is that the NOX removal depends upon electron- molecule interactions, facilitated by the structure of the'corona-catalytic'materials not the inter-molecular interactions involved in the present invention. There is no mention of the simultaneous removal of hydrocarbons from the gas streams being treated by the invention of US 5 147 516.

US patent 5 284 556 does disclose the removal of hydrocarbons from internal combustion engine exhaust emissions. However, the process involved is purely one of dissociation in an electrical discharge of the so- called'silent type, that is to say, a discharge which occurs between two electrodes at least one of which is insulated. The device described is an open discharge chamber, not a packed bed device. Mention is made of the possible deposition of a NOX-reducing catalyst on one of the electrodes.

In a broader context, the precipitation of charged particulate matter by electrostatic forces also is known.

However, in this case, precipitation usually takes place upon larger planar electrodes or metal screens.

It is an object of the present invention to provide an improved method for the removal of particulates and nitrogenous oxides from the exhaust gases emitted by internal combustion engines.

According to the present invention there is provided a reactor for the plasma assisted processing of a gaseous medium, comprising a reactor chamber including at least two gas permeable electrodes between which there is disposed a gas permeable bed of active material and means for constraining the gaseous medium to pass through the active material, characterised in that the active medium consists at least primarily of titania in either the anatase or textured rutile phase or a mixture of both phases.

According to the present invention in one aspect there is provided a reactor for reducing the emission of pollutants from an internal combustion engine, comprising a reactor chamber adapted to form part of an internal combustion engine exhaust system, the reactor chamber including at least two gas permeable electrodes between which there is disposed a gas permeable bed of active material, and means for constraining exhaust gases to pass through the bed of active material characterised in that the bed of active material consists at least primarily of titania in the anatase or textured rutile phases or a mixture of both phases.

The invention will now be described, by way of example, with reference to the accompanying drawing which is a longitudinal section of a schematic plasma assisted

reactor embodying the invention for the treatment of internal combustion engine exhaust gases.

Referring to the drawing, a reactor 1 for the treatment of internal combustion engine exhaust gases to reduce emissions of pollutants such as carbonaceous and nitrogenous combustion products therefrom consists of a stainless steel reactor chamber 2 which has an inlet stub 3 and an outlet stub 4, by means of which it can be connected into the exhaust system of an internal combustion engine (not shown). The chamber 2 is arranged in use to be connected to an earthing point 5. Within the chamber 2 are a cylindrical inner electrode 6 made of perforated stainless steel sheet and an earthed outer electrode 14 co-axial with the inner electrode 6 and also made of perforated stainless steel sheet. The electrodes 6 and 14 are held in position by two insulating supports 7 and 8. The space 11 between the electrodes 6 and 14 and the supports 7 and 8 is filled with a bed of pellets illustrated highly diagrammatically at 12 made of titania or a mixture as discussed further below. The upstream electrode support 7 has a number of regularly spaced axially-oriented holes 13 around its periphery so that exhaust gases entering the reactor 1 pass into the space 15 between the outer electrode 14 and the wall of the chamber 2 before passing radially through the bed of pellets 12 and leaving the reactor 1 via the inner electrode 6, as shown in the drawing. An insulated feed- through 10 connects output from a source 9 of electrical potential to the inner electrode 6. The potential thus applied across the bed of pellets 12 is sufficient to excite a plasma within the interstices between the pellets 12. A convenient potential for this purpose is about 10 kV to 30 kV, and may be a pulsed direct or continuously varying alternating potential, or may be an

interrupted continuous direct potential. Typically we employ a potential of 20 kV per 30 mm of bed depth.

Anatase phase titania is particularly suitable for the removal of particulate carbon and nitrogenous oxides from diesel engine exhaust gases because anatase titania has photocatalytic properties in the presence of ultraviolet radiation, and such radiation is generated by the plasma discharges within the bed of pellets 12 of the reactor 1. The ultraviolet radiation excites electrons from the valence to the conduction band of the titanium atom. This excitation results in the formation of holes at the positions in the crystal lattice of the titanium atoms and free electrons. The free electrons cause reduction reactions to occur and the holes cause oxidation reactions to occur, both of which facilitate the removal of carbonaceous and nitrogenous combustion products from internal combustion engine exhaust gases.

Particularly suitable for use for the above purpose are anatase phase titania pellets manufactured by the Norton Chemical Process Products Corporation and marketed under their product code number XT 25376. These pellets contain 99% by weight titania and 0.1% by weight of SO4.

These pellets have a nominal surface area of 139 m a median pore diameter of 16 nm, a total pore volume of 0.38 cm g, and a density of 0.836 g cm. This material has the property that it does not adsorb water from the vapour phase, which is advantageous in connection with the treatment of internal combustion engine exhaust gases.

In the structured rutile phase, the permittivity e of titania is anisoptropic, and higher than the permittivity of isotropic titania phases (i. e. phases which do not

contain structued rutile phase). This isotropic titania has a permittivity of the order of 15 whereas for structured rutile phases quoted values are E, = 86, E= <BR> <BR> <BR> <BR> 9 6<BR> <BR> <BR> 86 and 170 for measurement at 300 K and 10-10 Hz.

High permittivity is beneficial for generating a non- thermal plasma, so if structured rutile phase is present improved performance can be anticipated.

Further, we have found that when pellets of zirconia are mixed in the gas permeable bed with pellets of anatase titania there is a symbiotic improvement in performance in the treatment of internal combustion engine exhaust gases as compared with the performance of the zirconia or anatase titania on their own. It is convenient for this purpose to provide equal parts by volume of titania and zirconia in the mixture.

The reduction of the emission of particulate carbonaceous and nitrogenous combustion products from the exhausts of internal combustion engines is illustrated by the following example.

A stream of exhaust gases from a diesel generator set was passed through a reactor of the type described above with reference to the accompabying drawing, in which the pellets 12 were pellets of anatase phase titania. The concentration of nitrogen oxides (NOX) was measured at the inlet and outlet ends of the reactor chamber 2, for different values of the electrical potential applied across the reactor bed of pellets 12.

The differences between corresponding inlet and outlet concentrations of NO are tabulated below.

Similar measurements were made in respect of carbon monoxide contained in the exhaust gases from the diesel generator set. An increase in the concentration of carbon monoxide is taken as a representation of the removal of soot from the exhaust gases by oxidation to carbon monoxide. These also are tabulated below. Electric Potential NO reduction CO production across Pellet Bed (ppm) (ppm) (kV) 0.0 52 5.3 52 7.5 53 9.7 85 11.0 75 9 13.0 86 24 15.0 88 30 17.0 101 51 19.0 107 58 21.0 117 75