Abstract
The catalytic behavior of zeolite catalysts for the ammonia-based selective catalytic reduction (SCR) of nitrogen oxides (NOX) depends strongly on the type of zeolite material. An essential precondition for SCR is a previous ammonia gas adsorption that occurs on acidic sites of the zeolite. In order to understand and develop SCR active materials, it is crucial to know the amount of sorbed ammonia under reaction conditions. To support classical temperature-programmed desorption (TPD) experiments, a correlation of the dielectric properties with the catalytic properties and the ammonia sorption under reaction conditions appears promising. In this work, a laboratory test setup, which enables direct measurements of the dielectric properties of catalytic powder samples under a defined gas atmosphere and temperature by microwave cavity perturbation, has been developed. Based on previous investigations and computational simulations, a resonator cavity and a heating system were designed, installed and characterized. The resonator cavity is designed to operate in its TM010 mode at 1.2 GHz. The first measurement of the ammonia loading of an H-ZSM-5 zeolite confirmed the operating performance of the test setup at constant temperatures of up to 300 °C. It showed how both real and imaginary parts of the relative complex permittivity are strongly correlated with the mass of stored ammonia.
Highlights
The reduction of emissions like nitrogen oxides (NOX) from the exhaust gas of combustion engines are a continuous factor in forcing automotive manufacturers to improve the efficiency of their after-treatment systems for exhaust gases
In microwave cavity perturbation it is desirable for the sample to dominate the microwave losses, rather than other component parts
For the analysis of the microwave data, the volume occupied by the sample and not the volume of the whole fixed bed was used
Summary
The reduction of emissions like nitrogen oxides (NOX) from the exhaust gas of combustion engines are a continuous factor in forcing automotive manufacturers to improve the efficiency of their after-treatment systems for exhaust gases. For light and heavy duty diesel engines, which are operated leanly, the ammonia-based selective catalytic reduction (SCR) has become the major NOX control strategy to meet emission standards like the upcoming Euro 6 [1]. In this process, an aqueous urea solution (AdBlue) is injected into the hot exhaust, where it decomposes thermally to the reducing agent ammonia (NH3). Ammonia reduces NOX at the SCR-catalyst selectively to nitrogen (N2) and water (H2O). There are two main SCR reactions besides several subordinate reactions: the standard. SCR reaction (1) implying a 1:1-stoichiometry for NH3 and NO, and the fast SCR reaction (2) with equimolar amounts of NO and NO2 [2].
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