Catalytic converters reduce NO x emissions in conventional engines, but the exhaust from lean-burn engines requires more vigorous scrubbing. But substances that adsorb NO x also tend to take up sulphur oxides. These are produced from the sulphur impurities that all hydrocarbon fuels contain. Standard catalytic converters use metals such as platinum and rhodium to remove the NO x . Johan Martens and colleagues from the Catholic University of Leuven in Belgium have used a quite different material to develop an agent that works under lean-burn conditions.
Zeolites are mineral-like substances composed mostly of aluminium, silicon and oxygen.
They have been used for many years in the petrochemicals industry for separating and altering the components of crude oil. These new compounds get trapped inside the tiny, molecular-scale cages that pepper the zeolite's crystal structure. The new nitrogen oxides can be flushed out by exposing the zeolite to moist air or gas. This ability to trap and then release the compounds at will can be exploited using an appropriately designed exhaust system. Such a system could re-introduce the nitrogen oxides into the vehicle's carburettor at a suitable moment, where they would encounter combustion gases that convert them to nitrogen.