A semiconducting metal oxide sensor array for the detection of NOx and NH3

In many fossil fuel burning systems, NOx emissions are minimized by a selective catalytic reduction (SCR) technique where NH3 is injected into the flue gas stream to react with NOx to form environmentally safe gases, such as nitrogen and water vapor. Unfortunately, this process is usually incomplete, resulting in either NOx emissions or excess NH3 (NH3 slip). Therefore, a critical need exits for an in situ sensor array near the stack to provide real-time control of the NH3 injection, and hence, minimize the NOx emissions released into the environment. In the present work, semiconducting metal oxide (SMO) film technology is used to engineer a small, robust, sensitive, and selective sensor array to detect NOx and NH3 emissions. Many thin film tungsten trioxide (WO3) based sensing elements were tested in order to identify two film recipes capable of sensitively and selectively detecting NOx and NH3. The critical parameters inherent in each film recipe are type of substrate material, film thickness, doping, deposition temperature, and operating temperature. The two element sensor array’s response characteristics analyzed include the response and recovery times, rates of reaction, dynamic range, sensitivity, repeatability and selectivity.