BioEd Online

A crystal with a structure resembling Swiss cheese has been developed to dramatically clean up exhaust emissions by trapping pollutants for the few minutes after the engine has been started.

Hydrocarbon emission from vehicles is a serious problem: the chemicals react with other pollutants in the air to form smog and ozone, which can induce asthma and breathing problems.

New cars are usually fitted with catalytic converters, which burn up most of the unreacted hydrocarbon fuel in exhaust gases and turn it into carbon dioxide. But one of the big problems in making cars cleaner is how to prevent hydrocarbons polluting the environment while the catalytic converters warm up.

Up to 80% of the hydrocarbons that make it into the atmosphere are emitted from vehicles during the one or two minutes that catalytic converters take to get going after a cold start. So the hydrocarbons need to be trapped until the converter has reached its operating temperature, typically 170-200°C.

Tatsuya Okubo of the University of Tokyo and colleagues have been investigating the problem and say that a material called SSZ-33 looks promising as a hydrocarbon trap. This substance is a zeolite: a crystal made from silicon, aluminium and oxygen that looks, at the atomic scale, like Swiss cheese. The team reports its findings in the Journal of Physical Chemistry¹.

The atoms in zeolites are linked into rings that form a three-dimensional framework riddled with tiny pores and channels. These channels can hold vast quantities of gases, just as the pores of a sponge hold water.

Zeolites have been investigated as hydrocarbon traps for vehicles before, and one of the promising candidates was a material called zeolite beta. But the pore network of zeolite beta tends to fall apart in moist air at high temperatures, which means the material loses its grip on the hydrocarbons.

The researchers found that the zeolite known as SSZ-33 fares better. It can hold 30% more hydrocarbons than zeolite beta, and it does not break down so readily when it is subjected to the kinds of high temperatures (around 800°C) that can be reached inside vehicle exhaust systems.