Ohio State Researchers Patent Zeolite Filter for NOx Chemical Sensors

Researchers at Ohio State University are developing a sensor that can help control emissions from cars, power plants, and other combustion processes.

The matchtip-sized device is a prototype for even smaller sensors that could one day enable new ways of controlling combustion.

Prabir Dutta, professor and chair of chemistry at Ohio State, said the newly-patented sensor detects the total amount of a pollutant commonly referred to as NOx, which is primarily a combination of nitrogen oxide and nitrogen dioxide. It also removes the interference from carbon monoxide that can cause sensors to produce inaccurate readings.

The Environmental Protection Agency wants to curb production of NOx, because these compounds contribute to ground-level ozone, smog, and acid rain.

There are many gas sensors on the market -- and all new cars carry some form of sensor to comply with emissions standards -- but the new sensor represents an improvement, because it can pick out specific gases from the complex mixtures that make up combustion exhaust, detect these chemicals in small amounts, and do so rapidly.

Rather than design a new sensor from scratch, the Ohio State scientists employed what Dutta called a “chemical trick” -- they added an innovative filter to a typical electrochemical sensor. They made the filter out of a zeolite, one of a family of porous minerals that are used as water softeners, and to create gasoline from crude oil.

Zeolites are often called cage-like solids or molecular sieves, Dutta explained, because the solid form has tiny container-like structures that can capture other chemicals or be the site for chemical reactions.

“The chemistry happens inside those cages,” he said.

Though these compounds of silica and alumina can be found in nature, scientists create zeolites in the lab that are tailored for specific tasks.

Dutta and former graduate student Nicholas Szabo designed the zeolite filter with catalysts inside to remove carbon monoxide and provide a combined mixture of NOx that the sensor can easily detect. The NOx passes through the filter to an electrode that registers a signal. Szabo is now a scientist at Conductive Technologies, Inc. of York, PA.

The sensor can survive temperatures greater than 1,000 degrees F -- a prerequisite for working near an engine or furnace. In laboratory tests, the sensor accurately detected the presence of NOx in concentrations above 100 parts per million -- sensitivity adequate for car and power plant exhaust. The sensitivity of the sensor can be increased by altering the temperature difference between the filter and the sensor, and the scientists are currently developing electrode materials and catalysts to increase sensitivity.

Their goal: to develop a sensor that will detect concentrations well below 100 parts per million, which Dutta said will be needed for more environmentally friendly engines in the future. A more sensitive detector would also be needed for turbine engines for aircraft and power applications, he said.

The sensor currently measures half a centimeter (about a quarter of an inch) across, but eventually the scientists want to shrink it thousands of times smaller.

“There’s no reason we can’t do that,” Dutta said.

Several commercial companies are trying out the sensor for various applications.

Meanwhile, Henk Verweij, professor of materials science and engineering at Ohio State, is studying the sensor packaging to make it easier to manufacture. And Giorgio Rizzoni, professor of mechanical engineering and director of the university’s Center for Automotive Research and Intelligent Transportation, leads a team that is testing its performance at monitoring car engines.

The scientists are also working on sensor designs to detect carbon dioxide, carbon monoxide, oxygen, and various hydrocarbons.

Future plans include miniaturizing the sensors and building them into an array to monitor the combustion process in the boiler of power plants. With the right software, data from sensors at different locations in the boiler can be used to balance individual burners to better control emissions and improve efficiency. The software algorithms are being developed at the university’s Center for Industrial Sensors and Measurements (CISM), where Dutta is co-director.

Carl Palmer, engineering leader at GE Reuter Stokes of Twinsburg, OH, and his colleagues will test the sensor array.

Other collaborators include Sheikh Akbar, professor of materials science and engineering and founding director of CISM, and Bruce Patton, professor of physics.

“This is a truly multidisciplinary project,” Dutta said. “This is something a chemist can’t do alone.”

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