In many heterogeneous catalytic processes it is conventional practice to facilitate interaction between the reactants (usually gases or liquids) and the catalyst by coating the latter on to ceramic monoliths.

Monoliths typically take different forms, for example circular or oval cross-sections.

Coating catalyst onto monoliths is preferred to the use of catalyst-coated ceramic pellets for vehicle exhaust catalysts because pellets in a container can suffer material loss due to mutual abrasion or can pack together to cause high back-pressure.

However ceramic monoliths still suffer from some inherent disadvantages. For example, they may sustain mechanical damage, leading in some cases to serious fracture or even to powdering. In addition, because of their poor thermal conductivity they can generate"hot spots"-i. e. regions of high temperature which can cause sintering of the catalyst, with loss of catalytic

activity.

An alternative approach is to manufacture the monolith from high-temperature resistant steel sheet. This has the obvious advantage of improving the mechanical durability. However, although the hot spot problem is improved it is not completely eliminated; and steel sheet is liable to suffer from edge corrosion.

The present invention is designed to eliminate all of these problems and to yield some additional significant advantages.

According to the present invention there is provided a monolith for use in catalytic reactors wherein the monolith is fabricated from wire.

Preferably the composition of the wire is an aluminium containing ferritic steel.

The invention further provides a monolith for use in catalytic reactors comprising knitted or woven high temperature oxidation-resistant wire.

In a preferred embodiment the invention provides a monolith comprising knitted or woven aluminium containing ferritic steel. Suitably the monolith may comprise Fecralloy steel or Kanthal steel.

Preferably the steel wire may further comprise a rare earth metal or metal oxide for stabilisation.

In a preferred embodiment the monolith comprises yttrium stabilised Fecralloy steel.

Suitably the FecralloyS steel comprises from 10% to 20%

chromium, from 1% to 10% aluminium and from 0.1% to 3% yttrium and the balance is iron.

In a particularly preferred embodiment the monolith comprises Fecralloy steel having a typical composition by weight of 15.5 to 16.5% chromium, 4.6 to 5.6% aluminium, 0.3 to 1% yttrium and the balance is iron.

The dimensions of the monolith of the invention can be similar to the dimensions of any conventional ceramic or steel monolith. The size of a monolith will depend on its use i. e. car, truck, motorbike etc.

Suitably the diameter of wire can be from O. lmm to 0.6mm.

The wire monolith may be used alone or in conjunction with a ceramic monolith (i. e. in tandem or ceramic within the wire).

The use of woven or knitted wire allows dispersion of heat so that hot spots do not develop.

Accordingly the invention also provides a monolith produced from knitted or woven high temperature oxidation-resistant wire, which is then coated with the catalyst. This offers the following advantages: (i) It can be fitted into a container easily. In fact by arranging for the diameter of the monolith to be slightly larger than that of the container, it can simply be pushed in and allowed to expand to hold itself securely within the container.

(ii) It eliminates hot spots. In contrast to

conventional monoliths, either ceramic or metal, the gas molecules are not confined to a single channel, but can swirl within the monolith, thereby gaining access to an increased number of catalytic sites. This ensures that a much larger proportion of the catalytic sites take part in the reactions, thereby improving the efficiency of the unit. Since hot spots are eliminated one of the major causes of catalyst deactivation has been removed.

(iii) Since the monolith is composed of wire, any edge corrosion associated with monoliths made from sheet metal is eliminated.

(iv) The monolith is resistance to mechanical damage.

Hence it is useful for a wide variety of vehicles, including those which are likely to be subjected to severe vibration, such as motorcycles.

(v) Back pressure can be minimised.

(vi) The monolith can be coated very uniformly with the catalyst, and the coating can be inspected easily.

(vii) Since the reactants have efficient access to the catalytic sites throughout the monolith the catalyst coating is used with high efficiency.

This is in contrast to conventional through- channel monoliths, in which the reactants are largely confined to a particular channel. The result is that the amount of catalyst applied to the monolith can be reduced significantly, as compared to ceramic or metal monoliths having

parallel channels.

(viii) The monolith can be heated electrically to ensure that the catalyst achieves its operating temperature quickly.

(ix) If desired the monolith can be composed of two components, a ceramic portion and a woven metallic portion.

Typically the monolith can be formed from a sheet knitted from wire. The sheet may be wound into monolith shape.

To coat a wire monolith formed from a sheet the monolith may be unwound and placed in catalyst mixture.

Alternatively the catalyst may be brush coated with the catalyst.

Example 1 Comparative performance of a monolith according to the invention was tested on a VG Golf 1.8 litre fuel injection engine, mounted on dynamometer. The engine ran at 2300 rpm with no load. The catalyst used is a palladium/zinc/zirconia/ceria catalyst containing 1.5wt% palladium. CeramicmonolithWovenWire Monolith (Cordierite) (Manufacturedby(FecralloySsteel) NGK,Japan)Compositionof wire:20% Cr;5% A1; 0.1% Y.Suppliedby ResistalloyLtd. Diaofwire, 0.32mm Weightof537g725g Monolith Dimensions4india,6in4.5in dia,6in long400cells/long in20.006inwall thinkness Wtofcatalyst35.4g5.lg applied ..wtofPd 0.53g0.077g Performance Light-off430°C430° temperature Hydrocarbons BeforeLight-off2000ppm1970ppm AfterLight-off10ppm8ppm |% Conversion 99% 99% CarbonMonoxide BeforeLight-off5%5% AfterLight-off0.06% 0.21% % Conversion 99% 96%

Example 2 Comparative backpressure measurement of a monolith according to the present invention was tested in a 2.0 litre fuel injection Vauxhall Cavalier. Measurements were taken at a constant speed of 80 km/h.

Average backpressure using a catalyst-coated ceramic monolith having dimensions 4 in diameter, 6 in long with 400 cells/in2 was 5.3 mbar.

The catalyst of the invention had an average backpressure measurement of 5.3 mbar.

Conclusion The catalyst on the woven wire monolith exhibits comparable performance to that on the ceramic monolith, in spite of being present in much lower concentration, without worsening the backpressure.