Sometimes you just can't win. Attempts to reduce fuel consumption, and thus the emission of carbon dioxide, by motor vehicles have run into the problem that 'lean-burn' engines produce more smog-generating nitrogen oxides. And efforts to destroy these noxious exhaust components catalytically are hampered by the 'poisoning' of the catalyst by another exhaust component: sulphur.

A solution may now be in sight. Chemists working in Belgium report in Angewandte Chemie1 that they have developed a catalyst that detoxifies lean-burn exhaust fumes without succumbing to sulphur sickness.

Lean-burn engines operate by burning fuel with more air than usual. This has the side effect that nitrogen compounds in the fuel are converted with even greater efficiency to oxides of nitrogen, collectively denoted 'NO_x'. These gases participate in chemical reactions that lead to the brownish pall of urban smog hanging over cities. Catalytic converters reduce NO_x emissions in conventional engines, but the exhaust from lean-burn engines requires more vigorous scrubbing.

One approach to removing NO_x- involves first capturing it on some adsorbent material and then either converting it to harmless nitrogen gas or re-injecting it into the engine. 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. Accumulation of sulphur oxides on the adsorbent material reduces its capacity to take up NO_x.

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. Some occur naturally; many others are made synthetically. They have been used for many years in the petrochemicals industry for separating and altering the components of crude oil.

Martens' team used a zeolite, 'zeolite Y', which is widely used for 'cracking' of petroleum -- converting large molecules into smaller ones. When infused with sodium ions, zeolite Y adsorbs the nitrogen-containing gases that make up NO_x -- nitrogen monoxide and dioxide -- and combines them into new kinds of nitrogen oxide. 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.

Crucially, the zeolite system is not affected by the presence of sulphur oxides. In laboratory tests with 'synthetic' exhaust gas spiked with sulphur dioxide, the zeolite retained its ability to capture the NO_x components. Chemically, then, the new adsorbent looks very promising. The challenge to put it to use in a real engine is now mainly an engineering one.