Catalyst for treatment of diesel exhaust particulate

A preferred catalytically-active solid acid material, zeolite, forms the basis of a washcoat effective for treating diesel-fuelled engine exhaust to reduce emission of particulate material therein. In a preferred method, hydrocarbons present in the exhaust are cracked and oxidised in the presence of a zeolite/silica washcoat mixture catalyst-coated with precious metal. A preferred Y-type zeolite provides acidic (cationic) sites having releasable cations which are exchanged with precious metal to form catalyst-coated zeolite. A preferred method of making the catalyst-coated zeolite/silica washcoat is provided, whereby colloidal silica is disposed as a barrier over the catalyst-coated washcoat, limiting the exposure of the catalyst metal in the washcoat to sulphur constituents in the exhaust.