Microwave cavity perturbation studies on H-form and ion- exchanged SCR catalyst materials: correlation of ammonia storage and dielectric properties

Ammonia-based selective catalytic reduction (SCR) has become the major control strategy for NOx emissions from light and heavy duty diesel engines. Before reducing NOx on the SCR active material, ammonia storage on the active sites of the catalyst is crucial. The in operando measurement of the dielectric properties of the catalyst material using microwave cavity perturbation is a promising indicator of ammonia loading. In this work, results on two zeolite-based SCR materials, i.e.ZSM-5 in H-form and copper ion-exchanged, are presented. The catalyst powder samples were monitored by microwave cavity perturbation as a function of ammonia content at a frequency of approximately 1.2 GHz in a temperature range between 200 and 350 °C. Due to ion exchange, the NH3 storage behavior changes, what could be monitored in the sensitivity of the dielectric permittivity to NH3. In addition, the dependence of the complex dielectric permittivity on ammonia loading is decreased by ion exchange, hinting that mostly ammonia storage on Brønsted sites affects the dielectric permittivity. This finding adds new knowledge to the electrical conduction and polarization mechanisms occurring in these zeolite materials.