Response Characteristics of Stable Mixed-Potential NH₃ Sensors in Diesel Engine Exhaust

A mixed-potential, electrochemical sensor platform is extended to NH₃ sensing by the introduction of a new gold alloy working electrode. A planar, pre-commercial NH₃ sensor utilized LANL’s controlled interface approach, and a Pd-Au alloy working electrode was tested in exhaust of a GM 1.9 L diesel engine downstream of a diesel oxidation catalyst through a slipstream arrangement. A fraction of the exhaust was pulled across the sensor with a pump at 20 L/min. In order to simulate NH₃ slip inside of a full SCR emissions control system, NH₃ was injected immediately upstream of the sensor using a calibrated mass flow controller. The sensor response quantitatively tracked the NH₃ as measured via Fourier transform infrared (FTIR) analyzer. A calibration curve was obtained in the exhaust from an ammonia staircase response with the engine running at steady-state engine conditions resulting in low background concentrations of NO_x and HC (<20 ppm) during calibration. Exhaust gas recirculation (EGR) switching and sweeps were used to evaluate the NH₃ sensor response under different amounts of total background NO_x. The calibration curve was used to directly compare the [NH₃] calculated from sensor response to the gas phase composition measured via FTIR. In general, there was excellent quantitative agreement between the sensor response and the actual NH₃ in the exhaust gas, and fast response time such that transients (<5 ppm) could be easily discerned from baseline. A LANL pre-commercial NO_x sensor was tested simultaneously with the NH₃ sensor and the extent of cross-sensitivity between the two sensors will be discussed.