Response Characteristics of a Stable Mixed Potential Ammonia Sensor in Simulated Diesel Exhaust

Advancements in both combustion system design and emission control technologies have allowed more efficient diesel engines to meet stringent exhaust emissions standards. With very stringent emission regulations currently in place, combustion system design and combustion management are used in conjunction with available exhaust after treatment technologies for optimal reduction of emissions to meet regulatory targets. Current diesel vehicles in the US are equipped with a diesel oxidation catalyst (DOC) to control CO and HC emissions, a diesel particulate filter (DPF) to control particulate matter (PM) and selective catalyst reduction (SCR) or lean-NO_x trap (LNT) to control NO_X emissions. In SCR, NO_x is reduced to H₂O and N₂ over a catalyst through the injection of an ammonia source provided by the onboard storage of diesel exhaust fluid (DEF), typically a mixture of urea and de-ionized water.  These emission control technologies add significant complexity and cost to a vehicle and some result in additional fuel penalty needed for their operation.

Meeting the EPA Tier 3 emissions reduction requirements while simultaneously increasing fuel economy to meet new CAFE standards will require optimization of advanced combustion strategies and emissions control technologies as well their state of operation under transient vehicle operation. NO_x sensors are needed to reduce variations in emissions control, lower the cost of closed loop DEF dosing control, improve consumption of both fuel and DEF. NH₃ sensors are needed to monitor DEF injection and prevent NH₃ slip—a condition under which unreacted ammonia from the SCR is emitted in the tailpipe exhaust.  The ammonia slip is treated as a more harmful pollutant than the NO_x emissions.  Ideally, both NH₃ and NO_x sensors would be incorporated into a closed-loop system to maintain ammonia close to stoichiometry in the SCR system, and could also serve to monitor engine out and tailpipe out emissions in order to ensure regulatory compliance.

Mixed-potential sensors are electrochemical devices that develop a non-Nernstian potential due to differences in the redox kinetics of various gas species at each electrode/electrolyte gas interface^1,2. The difference in the steady-state redox reaction rates between the “fast” and “slow” electrodes gives rise to the measured mixed potential response. A patented LANL sensor design incorporates dense electrodes and a porous electrolyte and reproducible and stable sensors are prepared using a simple ceramic tape cast approach³. The use of dense electrodes minimizes deleterious heterogeneous catalysis, and the increased morphological stability of dense electrodes yields a robust electrochemical interface, increasing lifetime durability⁴. The use of a dense gold wire-working electrode (e.g. the slow electrode), imparts preferential selectivity to NH₃, and avoids the stability problems one finds when using thin film, porous Au electrodes⁵.

In this work, we present recent results from testing a Au/YSZ/Pt tape cast sensor in ORNL’s high flow reactor at the NRTC that was especially constructed to simulate the exhaust gas constituents produced by lean burn vehicles powered by CIDI diesel engines with, and without emissions controls. The sensor was characterized at 525 and 625°C for NH₃, CO, C₃H₆, C₃H₈, and NO_x in a pre-heated base gas composition of 1%O₂, 5%H₂O, 5%CO₂ flowing at 15 slpm. The sensor exhibited fast response time equal to the response time of the system’s switching valve (T₉₀<0.6s). Moreover, in simulations of ammonia slip resulting from overdosing an SCR, the sensor was able to easily measure 20ppm injections of NH₃ in the presence of the interferent gas species at combined concentrations ten times higher than the NH₃.   These data provide experimental confirmation of the concept of using a mixed-potential electrochemical NH₃ sensor for tail pipe out applications. New data from recent engine out testing of pre-commercial prototype sensors that utilize design principles of tape sensors but are based on High Temperature Ceramic Co-Fire (HTCC) construction⁶  will be presented and discussed.

References

 

1. J. W. Fergus, Journal of Solid State Electrochemistry 15 (5), 971-984 (2010).
2. F. H. Garzon, R. Mukundan and E. L. Brosha, Solid State Ionics 136-137, 633-638 (2000).
3. R. Mukundan, E. L. Brosha, and F. H. Garzon, Tape Cast Sensors and Method for Making, US Patent no. 7,575,709.
4. R. Mukundan, E. L. Brosha and F. H. Garzon, Journal of The Electrochemical Society 150 (12), H279 (2003).
5. J. Tsitron et. al., Sensors and Actuators B 192 (2014) 283-293.
6. P.K.Sekhar et. al.,  Sensors and Actuators B: Chemical 144, 112 (2010).

 

 

Acknowledgments

 

The authors would like to thank Roland Gravel and the DOE Vehicle Technology Office for providing funding for this work.