Sensor and method for determining the ammonia loading of a zeolite SCR catalyst

A sensor and a method for determining the NH3 loading of a base-metal zeolite catalyst are described. This catalyst material is used to reduce NOx emissions in diesel exhaust via reaction with injected NH3, a process known as selective catalytic reduction (SCR). Some of the injected NH3 is stored on the catalyst surface before it reduces NOx. We demonstrate that the catalyst material itself can be used as part of a sensor measuring the level of stored NH3. The AC conductivity of a thick film of the catalyst material was measured at 4 Hz and was found to increase when NH3 was introduced into the gas phase, eventually saturating presumably as the material reached its maximum NH3 storage capacity. The conductivity was monitored as the temperature was raised sufficiently to desorb (or oxidize) most of the stored NH3. The resulting change in the conductivity upon heating was used as a measure of the NH3 loading level on the zeolite catalyst material. We demonstrate this parameter is dependent on NH3 concentration, NH3 loading time and gas temperature.