Rhodium and Iridium will catalyze the reaction of CO, H2 and hydrocarbons with NO as opposed to O2, which is important when an engine is run under lean (fuel rich) conditions. Pt reduces NO to NH3, which is ineffective.
Oxidation in the presence of O2 is relatively simple, but in a rich condition Pt is found to catalyze CO oxidation through the water-gas shift reaction, and hydrocarbon oxidation by the steam reforming reaction.
North American TWC systems use approx 10:1 Pt to Rh, with about 0.05 troy oz/converter of Pt.
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