How Catalytic Converters Work

BASF researchers improve cleanup of diesel emissions.

There are currently more than half a billion cars on the roads worldwide, plus around 200 million trucks. Although this intense automobile traffic raises many environmental issues, health hazardous exhaust gases do not necessarily have to be one of them. In theory, the hydrocarbons in the gasoline are combusted with atmospheric oxygen to give the nontoxic end products carbon dioxide and water. In the real world of the automobile this ideal combustion process is not that easy to accomplish. Incomplete combustion and the presence of minimal impurities in the fuel can result in the formation of toxic carbon monoxide, unburned hydrocarbons, nitrogen oxides and for diesels carbon particulates (soot). When optimally adjusted, modern engines can reduce emissions of these pollutants significantly. The key to clean exhaust gases, however, lies in the conversion of the harmful emissions into harmless end products by catalytic converters. The widespread introduction of such catalytic converter systems in North America (starting in 1976) and Europe (1986) resulted in a marked decrease in metropolitan air pollution from harmful tailpipe emissions despite a growing population of vehicles.

The development of the first three-way catalytic converter by the US American company Engelhard in 1979/80 marks a milestone in exhaust gas technology. 

This device was able to catalyze the conversion of the three main pollutants (unburned hydrocarbons, carbon monoxide and nitrogen oxides) simultaneously. 

How It Works

How It Works

Experts check the installation of a catalyst underneath a test vehicle.

Stress Testing

Stress Testing

In the engine laboratory catalysts are subjected to long term stress testing.

As a general principle, a catalyst allows chemical conversions to desired products to occur more rapidly and at lower temperatures. 

For this reason, in addition to pollution control, industrial catalysts find use in a wide field, be it the processing of petroleum to produce transportation fuels or the production of chemicals including polymers and pharmaceuticals. Ideally, the catalyst itself is not chemically consumed during this process. Automotive emissions catalytic converters consist of special combinations of precious metals such as platinum, palladium and rhodium dispersed on high surface area carriers which in turn are coated onto the walls of ceramic or metallic monolithic structures. In the modern three-way catalytic converter, uncombusted fuel residues are oxidized with oxygen to produce carbon dioxide and water, nitrogen oxides are converted to ubiquitous nitrogen, and toxic carbon monoxide is oxidized with oxygen to carbon dioxide. Like this, a typical catalytic converter is capable of destroying around 98 percent of hydrocarbons, carbon monoxide and nitrogen oxides produced by the car's engine.

"The principle of a catalyst may be simple, but it's the details of the technological implementation that cause the headaches", explains Bob Farrauto. "The catalyst needs the right operating temperature and an accurately adjusted residual oxygen content in the exhaust gas." In today's catalytic converters, the oxygen content is measured and regulated by lambda sensors. Through a computer controlled feedback system the mixture of air and fuel entering the engine is adjusted to meet the requirements for the three-way catalyst to function.