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The platinum starts off in an organometallic form which is soluble in fuel, but in the engine it is converted into metallic and ionic particles. The fuel additive is always used on a vehicle fitted with a soot filter, so the particles end up there (and very few are emitted).

The platinum acts as a heterogeneous catalyst, either as particles in the exhaust stream or (more likely) as immobilised particles in the filter, or deposited on surfaces within the cylinders.

Answer posted 31 July 2006