Development of platinum catalysts and their use in the control of vehicle exhaust emissions

Abstract

Metallic platinum is an excellent catalyst for a range of reaction types, and has a long history associated with the early development of catalysis. When it was necessary to have additional exhaust emissions control for cars, platinum-based oxidation catalysts were used to remove hydrocarbon and carbon monoxide. Later, nitrogen oxides were also removed in a two-stage process involving a platinum/rhodium catalyst followed by a platinum-based oxidation catalyst. When exhaust gas composition could be kept stoichiometric, hydrocarbons, carbon monoxide and nitrogen oxides were simultaneously converted over a single platinum/rhodium three-way catalyst. Modern three-way catalysts contain combinations of these metals, and emissions are exceptionally low. Nitric oxide is not catalytically dissociated under lean conditions, and its control from lean-burn gasoline engines involves forming nitrate that is stored and periodically reduced to nitrogen by enrichment of the exhaust gas. This method is being modified for diesel engines, and a platinum component has the key role of oxidising nitric oxide to nitrogen dioxide, the nitrate precursor. Diesel engines produce soot, and filtration is being introduced to remove it. The exhaust temperature of heavy-duty diesels is high enough (250–400°C) for nitric oxide to be converted to nitrogen dioxide over an upstream platinum catalyst, and this smoothly oxidises retained soot in the filter. The temperature of passenger car diesels is too low for this to take place all the time so trapped soot is periodically burnt in oxygen above 550°C. Here, a platinum catalyst is used to oxidise higher than normal amounts of hydrocarbon and carbon monoxide upstream of the filter to give sufficient temperature for soot combustion to take place.