Stuttgart, February 1996
Housewives have long appreciated the effects of highly porous substances like zeolites. As additives to detergents they substitute for environmentally harmful phosphates to soften wash water. Daimler-Benz researchers, too, have discovered zeolites for themselves."We use zeolites containing precious metals as precision sensors for the pollutants carried in exhaust emissions," says Dr. Carsten Plog, explaining the research activities of his group. "The zeolite sensor has a decisive advantage over conventional gas sensors. It accurately detects hydrocarbons even in the presence of carbon monoxide (CO) and hydrogen."
A characteristic property of the highly porous zeolites is their vast interior surface. One gram of the aluminium silicate has a surface of about 500 m² – about the size of a large swimming pool. Very small charged and uncharged particles can wander through the pores of the zeolites as if through an endless system of canals. All the time, numerous adsorptive and catalytic processes take place. The Daimler-Benz researchers took advantage of this.
Zeolites contain charged particles (ions). Like Tarzan in the jungle, they change places by "swinging" from place to place, with externally applied AC voltage serving as the liana in this case. If, in addition, gases flow through the pores and canals of the zeolites, they impede the mobility of the ions. This effect can be measured as a change in the sensor's resistance and capacitance.
The gas sensor developed by the Daimler-Benz researchers is only the size of a thumbnail. Exceedingly thin gold filament, to which the AC voltage is applied, intermeshes on its surface. The sensor is coated with zeolites containing precious metals. According to Plog, precision measurement of hydrocarbons in the presence of carbon monoxide and hydrogen is possible in a laboratory environment at present. A working specimen is being developed right now for measuring real emissions.
Gas sensors to detect hydrocarbons, ammonia and nitrogen oxides (NOx) play an important role in developing new strategies for secondary treatment of emissions, says Plog. The Daimler-Benz scientists are therefore conducting an intensive search for a suitable zeolitic material to be able also to measure the concentration of nitrogen oxides in emissions in future.
Back to the Newsarchive
© 1996 Daimler-Benz