Substrate Media for Plasma Gas Processing Reactors The present invention relates to gas processing reactors in which a gaseous medium to be processed is subjected to the action of a plasma in the presence of a ceramic substrate material which acts to enhance the generation and maintenance of the plasma, and specifically to such reactors for the removal of noxious combustion products from the exhaust gases of internal combustion engines.

One such reactor, which is intended to remove pollutants from air so as to render it breathable, is shown in US patent 3 983 021 and another, which is intended for use in the purification of internal combustion engine exhaust emissions is shown in our earlier patent GB 2 274 412. In the former reactor, the substrate consists of a bed of pellets made of alumina, silica or zirconia. In the latter reactor, the substrate consists of pellets of a heat-resisting ferro-electric material such as barium titanate.

It has been proposed (European Patent Application 0 786 283) that a mixture of titania and zeolites should be used in paving stones to give them a capability for removing NOX from urban roads. However, there is no evidence that such an effect occurs at the temperatures and in the environments concerned.

It is an object of the present invention to provide an improved substrate material for use in plasma activated reactors for the processing of gaseous materials.

According to the present invention there is provided a plasma enhanced reactor for the processing of gaseous media, including a gas permeable bed of active material, means for constraining a gaseous medium to be processed to pass through the bed of active material and means for applying across the bed of active material a potential difference sufficient to establish a gaseous discharge in the gaseous medium in the interstices in the bed of active material characterised in that the bed of active material comprises an agglomeration of particles each of which includes a dielectric chemically active mixed metal oxide and at least ten weight per cent of a binder consisting of a second dielectric chemically active metal oxide.

For the avoidance of doubt, a mixed metal oxide is defined as an oxide material including ions of at least two different metals. For example, the substrate can consist of a mixture of alumina and barium titanate, or alumina and calcium titanate, or alumina or titania and a zeolite, preferably with gamma alumina as the binder.

Embodiments of 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 reactor for the purification of the exhaust emissions from an internal combustion engine.

Referring to the drawing, a plasma reactor for removing particulate carbonaceous and other combustion products and simultaneously controlling or removing NOX from the exhaust of an of internal combustion engine, consists of a cylindrical stainless steel chamber 1, which is arranged to be connected to an earthing point (not shown) and which has an inlet nozzle 2 by means of

which it can be connected to the exhaust system of an internal combustion engine, and a similar outlet nozzle 3. Mounted coaxially within the chamber 1 are perforated electrodes 4 and 11 of stainless steel. Inner electrode 4, is connected via high tension lead through connectors 12, 13 to a source of high potential, again not shown in the drawing. The electrodes 4 and 11 are supported in the chamber 1 by means of insulating supports 5 and 6.

The upstream end of the inner electrode 4 is closed off and the adjacent support 5 has a number of axial holes 7 in it so as to render it gas permeable with as little back pressure as is practicable. The other support, 6, is impermeable. Thus exhaust gases are constrained to pass radially as well as axially, through the space 8 between the outer electrode 11 and the inner electrode 4.

As indicated by the arrows some exhaust gas passes directly axially into the space 8 whilst some initially passes into the space between the wall of the chamber 1 and the outer electrode 11, then flowing radially through the latter into the space 8. The electrodes 4 and 11 and the supports 5 and 6 also act to retain a bed 9 of pellets 10 in the space 8 between the outer electrode 11 and the inner electrode 4. In use, the high potential applied across the electrodes 4 and 11 is such as to excite a non-thermal plasma in the exhaust gases in the interstices between the pellets 10. A convenient potential for this purpose is a potential of about 10 kV to 30 kV which may be a regularly pulsed direct potential or a 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.

The pellets 10 are made of a mixture of oxide materials such as alumina and barium titanate, or alumina and calcium titanate, or alumina and titania, or titania and a zeolite. Alternatively they can be made of a

mixed oxide material. The close proximity of the different metal oxides in the pellets 10 enhances their activity in relation to the NOx and other components of diesel exhaust emissions in particular compared with a bed of pellets made of either single metal oxide materials or a mixture of pellets of single metal oxide materials.

Specific examples are illustrated in Table 1 and Table 2 comprising respectively barium titanate/gamma- alumina and calcium titanate/gamma-alumina extrudates at 4 mm diameter, in which the gamma alumina serves as both active constituent and binder. The barium titanate and calcium titanate powders from which these mixed oxide extrudates were produced had permittivities of respectively 1410 (barium titanate) and 155 (calcium titanate) measured at 250C and 1 kHz.

The use of active alumina in the gamma alumina form as a binder provides beneficial effects, as for example when used in a ferroelectric bed reactor for the removal of NOX from diesel exhausts. Thus, for example: The strength of the extrudates can be adjusted by varying the amount of alumina binder. It is considered that a minimum alumina content in the region of 15% is required for sufficient practical strength.

Wear resistance of the extrudates can be increased by varying the amount of binder. A bed consisting of two or more materials in the form of separate beads or fragments may exhibit preferential wear of one component leading to dust formation when in use.

Although the examples described here involve two components (e.g. barium titanate and alumina) the use of binder would allow extrudates to be produced from more than two components for example extrudates from mixed titanate/alumina/zeolite powders. Because components are uniformly distributed within each extrudate the use of a bed of extrudates consisting of mixed powders avoids or reduces the effect of preferential settling of a component as is found to occur when a bed consists of a mixture of beads or fragments of different materials, for example barium titanate fragments and alumina beads.

Extrudates are generally less expensive to produce than spheres and cost savings can arise when producing extrudates of mixed materials.

The alumina binder can contribute a beneficial chemical effect during operation of the bed in addition to its role as a binder and aid NOX removal from the diesel exhaust stream.

When a ferroelectric component such as barium titanate is used, varying the amounts of alumina binder allows a control of the permittivity of the extrudates.

If an extrudate of alumina and y-alumina binder is produced then the extrudate will contain regions of differing surface chemistry. Thus the a-alumina has a basic surface chemistry while the binder is likely to have a more acidic surface. The presence of differing surface acidity/basicity in each extrudate is considered to affect the efficiency for removal of NOx Incorporation of alumina binder into the extrudates allows a control of extrudate porosity compared to use of

separate beads or fragments of for example alumina and a titanate.

Table 1 Properties of barium titanate:y-alumina extrudates Composition Density Surface Pore volume -3 2 -1 3 -1 g cm area m g cm g BaTiO3: 22 wt% γ-Al2O3 1.32 72 0.32 BaTiQ: 50 wt% t-AlOq 1.02 137 0.39 Table 2 Properties of calcium titanate:y-alumina extrudates Composition Density Surface Pore volume g cm-3 area m²g-1 cm g-1 CaTiO: 22 wt% y-Al,Q 0.93 84 0.30 CaTiO3: 50 wt% y-Al,Q 0.90 140 0.35