Schottky diodes with thin catalytic gate metals for potential use as ammonia sensors for exhaust gases

Selective catalytic reduction (SCR) is a method in which ammonia reacts with nitric oxides in a catalytic converter to form water and nitrogen. We show that catalytic Metal Insulator Silicon Carbide (MISiC) devices can be used as ammonia sensors for a SCR system in a diesel engine. Different catalytic metals, Pt and Ir, with a thickness of 30 or 50 nm were investigated. The temperature dependence of the ammonia response of the sensors was characterized. Maximum responses were found at temperatures between 225-250 degrees C. Preliminary experiments were performed to investigate how annealing in different gas ambient influences the response-temperature curve of the sensors. In synthetic diesel exhausts with ammonia added, the sensors showed very good selectivity for ammonia and a small interaction effect with oxygen. The influence of other gas components was almost negligible. Temperature in the diesel exhaust system can reach 550 degrees C; however, operating at temperatures above 400 degrees C limited the lifetime of the sensor. Anger electron spectroscopy (AES) revealed that island formation of the metal due to structural changes was the main reason for failure of the sensor.