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

Abstract

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 NOx 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 NOx. 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 NOx sensor was tested simultaneously with the NH3 sensor and the extent of cross-sensitivity between the two sensors will be discussed.