By measuring the electrical properties of the catalyst coating itself, one can observe directly and in situ the state of TWC, LNT, and SCR catalysts. Two principles are possible: a contact method, for which the coating is applied to planar electrodes and the electrical impedance is measured, and a non-contact method, in which the coating material is penetrated by radio frequency waves. In either case, the catalyst state is directly correlated with the measured transmission or reflection characteristics of the electrical sensors.

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The state of the three-way catalyst (TWC) of a gasoline engine is commonly estimated by measuring the air-to-fuel ratio in the exhaust gas. This indirect method suffers from several disadvantages that one could avoid by directly observing the catalyst state and in particular its oxygen loading. We have investigated an RF approach to this direct measurement problem which is based on the fact that the TWC and its stainless steel canning form a filled cavity resonator. It will be shown that the resonator is perturbed when the TWC changes its conductivity as a function of oxygen loading and that the characteristics of the resonator S-parameters mirror the oxidation/reduction state of the catalyst.

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