Combination of Wirebound and Microwave Measurements for In Situ Characterization of Automotive Three-Way Catalysts

Recently, a novel approach for the characterization of the state of automotive catalysts has been proposed, viz., the direct electrical characterization of the catalyst material itself. This direct approach has the potential to do away with the complex models needed by indirect measurement methods to infer the catalyst state, for instance from the oxygen partial pressure outside of the catalyst determined by lambda probes. In this work, we compare two ways of directly observing the catalyst state. Firstly, the electrical conductivity of the catalyst material is determined by wirebound sensors mounted inside of the catalyst and fabricated from catalyst-formulation coated planar interdigital electrodes. Secondly, we employ a contactless method based on the perturbation of the partially filled microwave cavity resonator formed by the catalyst and its metallic housing. For a better understanding of the observed effects, the data obtained at microwave frequencies from engine tests are compared with the local electrical conductivity measurements. It is shown that both methods give a coherent picture of the oxygen loading state of an automotive three-way catalyst and that both methods enable one to follow the progress of reaction fronts inside the catalyst. Furthermore, it is demonstrated that the resonance frequency is strongly correlated with the oxygen load.