Using a MISiC-FET sensor for detecting NH3 in SCR systems

One way to decrease the emitted levels of NO, from diesel engines is to add NH3 in the form of urea to the exhausts after combustion. NH3 will react with NO, in the catalytic converter to form N-2 and water, which is called selective catalytic reduction (SCR). The amount of NH3 added may be regulated through closed-loop control by using an NH3 sensor. The metal-insulator silicon-carbide field-effect transistor (MISiC-FET) sensor has previously been tested for this application and has been shown to be sensitive to NH3. Here, the sensors have been further studied in engine SCR systems. Tests on the cross sensitivity to N2O and NO2, and studies concerning the influence of water vapor have been performed in the laboratory. The difference between Ir and Pt films, with regard to catalytic activity, has also been investigated. The sensors were found to be sensitive to NH3 in diesel engine exhausts. The addition of urea was computer controlled, which made it possible to add NH3 in a stair-like fashion to the system and detect it with the MISiC-FET sensors. The presence of water vapor was shown to have the largest effect on the sensors at low levels and the NH3 response was slightly decreased by a background level of NO,.