Catalyst Ammonia Storage Measurements Using Radio Frequency Sensing¹

Motivated by increasingly strict nitrogen oxides (NO_x) limits, engine manufacturers have adopted selective catalytic reduction (SCR) technology to reduce engine-out NO_x. In the SCR process, NO_x react with ammonia (NH₃) to form nitrogen and water vapor. The reaction is influenced by several variables, including stored ammonia on the catalyst, exhaust gas composition, and catalyst temperature. Currently, measurements from NO_x and/or NH₃ sensors upstream and downstream of the SCR are used with predictive models to estimate ammonia storage levels on the catalyst and control urea dosing. This study investigated a radio frequency (RF)-based method to directly monitor the ammonia storage state of the SCR. This approach utilizes the catalyst as a cavity resonator, in which an RF antenna excites electromagnetic waves within the cavity to monitor changes in the catalyst state. Ammonia storage causes changes in the dielectric properties of the catalyst, which directly impacts the RF signal. Changes in the RF signal relative to stored ammonia (NH₃) were evaluated over a wide range of frequencies, temperatures, and exhaust conditions. The RF response to NH₃ storage, desorption, and oxidation on the SCR was well correlated with changes in the catalyst state. Calibrated RF measurements demonstrate the ability to monitor the adsorption state of the SCR to within 10% of the sensor full scale. The results indicate direct measurement of SCR ammonia storage levels, and resulting catalyst feedback control, via RF sensing to have significant potential for optimizing the SCR system to improve NO_x conversion and decrease urea consumption.