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Posted: August 27, 2007

High-sensitivity, rapid-response nanotechnology NOx sensor

(Nanowerk News) Koichi Hamamoto,a post-doctoral Research Scientist in the Functional Assembly Technology Group at the Advanced Manufacturing Research Institute of the National Institute of Advanced Industrial Science and Technology (AIST) has developed a high-sensitive nitrogen oxide (NOx) sensor with a rapid response.
Strenuous efforts are being made to develop clean-burn technologies for gasoline-fueled vehicles to comply with societal demands for reducing CO2 emissions and better fuel consumption. However, although clean-burn engines produce less CO2 emissions, they produce more NOx emissions than conventional engines. Existing three-way catalysts cannot be used to eliminate NOx emissions under lean combustion because of the high concentration of oxygen in the exhaust gases. Instead of the three-way catalyst, a practical clean-burn engine uses a NOc storage–reduction catalyst system. A NOc trap material in this catalyst absorbs NOx during lean-burn condition. When the catalyst becomes saturated with NOx, a rich spike (excessive fuel supply) is generated in the engine, and this excessive amounts of fuel reduces and purifies the absorbed NOx.
In the current system, the timing of when to add the rich fuel spike is based on the estimated amount of absorbed NOx obtained by means of model studies on the production of emission gases under normal operating conditions. Because the system does not rely on direct detection of the NOx concentration in the emissions, it is necessary to verify that it functions correctly under various operating conditions.
At the same time, there is a trade-off between emissions of NOx and those of unburned carbon (particulate matter: PM). Because emissions of PM increase if those of NOx are controlled. It is necessary to control NOx emissions at the regulated limit by using a NOx sensor, thereby minimizing emissions of PM. Therefore, the development of a small, high-performance, onboard NOx sensor capable of rapid and precise detection is highly desired. Such a sensor should permit monitoring of NOx emissions that are constantly changing, and would improve fuel economy by minimizing the number of rich fuel spikes required, thereby realizing highly efficient control of NOx emissions and reduction of fuel consumption.
Conventional NOx sensors do not have sufficient robustness or heat resistance in the harsh environment of engine exhaust gases. To solve this problem, a multi-chamber sensor that uses a solid oxide electrolyte (oxygen-ion conductor) has been developed. This, however, inherently does not give a high-speed response, because it has a rather complicated structure and it measures NOx concentrations by combining multistep electrochemical reactions. This poses a problem in relation to the purification of exhaust gases and reduction of fuel consumption.
The newly developed sensor has extremely high selectivity toward NOx molecules as a result of precise control of the nanostructure of the sensing electrode that detects NOx. This structural improvement to the electrochemical cell gives the speed of response for the direct detection of NOx molecules that is five times as fast as that of a conventional sensor and the detection sensitivity that is doubled.
The control of engine combustion with the aid of the high-performance sensor is expected to reduce NOx emissions, particularly from diesel vehicles, thereby contributing to preserving the atmospheric environment and reducing carbon dioxide emissions.
The results of this research were presented at the 16th International Conference on Solid State Ionics, held in Shanghai, PR China, on July 2-6, 2007.
Source: AIST
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