Response Characteristics of Stable Mixed-Potential NH3 Sensors in Diesel Engine Exhaust
Eric Brosha, Los Alamos National Laboratory
Mixed-potential sensors fabricated via well-established commercial manufacturing methods present a promising avenue to enable the widespread utilization of nitrogen oxide (NOx), hydrocarbon (HC), and ammonia (NH3) sensing technology. These devices are fundamentally simple in design and construction, inexpensive to manufacture, and robust owing to their close relationship to the well-established and ubiquitous automotive Lambda sensor. Previous engine testing results presented last year focused on testing pre-commercial mixed-potential NOx/HC sensors in a lean burn, gasoline exhaust and it was suggested that the ability to actively shift preferential selectivity of the sensor from NOx to HC species using current bias may prove extremely attractive as a way to simplify emissions controls systems while accommodating potential future mandates such as the requirement for on-board diagnostics of HC traps and other emissions hardware related to low temperature combustion (LTC) strategies. In this presentation, we show the mixed-potential platform is extended to NH3 sensing by introducing a new gold-based working electrode. A planar, pre-commercial NH3 sensor utilizing LANLâ€™s controlled interface approach, a Pd-Au alloy working electrode, and a protective porous overcoat was tested in engine exhaust gas of a 1.9L GM turbo-diesel sampled downstream (20 L/min) of the engineâ€™s diesel oxidation catalyst (DOC). 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 levels of injected NH3 and transients measured via Fourier transform infrared (FTIR) analyzer, using a calibration curve derived from an ammonia staircase response measured in the engine exhaust at low NOx and HC concentrations (<20 ppm) and steady-state engine operation. Exhaust gas recirculation (EGR) switching and EGR 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 qualitative agreement between the sensor response and the actual NH3 in the exhaust gas composition, and fast response time such that transients at the tested lower limit (2.5 ppm) could be easily discerned from baseline without the need for sensor signal processing. In parallel experiments conducted at ORNL, an Au/YSZ/Pt tape-cast version of the NH3 sensor was tested in a high flow reactor with simulated diesel exhaust gas to quantitatively measure an upper limit of sensor response time (T90<0.6s) and to understand the ammonia response in exhaust gas mixtures with well-controlled gas species concentration. These data provided independent experimental confirmation of the engine testing results and of the overall concept of using a mixed-potential electrochemical NH3 sensor for SCR control and tailpipe-out applications.