On the inverse problem associated with the observation of electrochemical processes by the RF cavity perturbation method

Electrochemical reactions occuring at high temperatures and inside bulk materials are often observed indirectly. For instance, one commonly estimates the state of the three-way catalyst (TWC) in the exhaust pipe of a gasoline engine from the measured air-to-fuel ratios in the exhaust gases entering and leaving the catalyst. Our goal is to directly observe the catalyst state in situ by a non-contact RF approach. It is based on two facts: first, the reacting medium and its stainless steel canning form a filled cavity resonator at microwave frequencies; second, the medium changes its dielectric and conductive properties depending on its electrochemical state. Hence, the resonator is perturbed when the catalyst changes its conductivity as a function of oxygen loading. The estimation of the catalyst state involves the solution to the inverse problem, i.e., the inference of the catalyst material parameters from the measured RF parameters. We will first show that the characteristics of the RF resonator S-parameters indeed mirror the oxidation/reduction state of the catalyst and will then discuss possible approaches to the solution of the inverse problem.