Solid state mixed-potential sensors as direct conversion sensors for automotive catalysts

In order to determine directly the quantity “conversion of a catalyst”, we developed a new YSZ solid-electrolyte based mixed-potential sensor that enables to compare electrochemically two gas compartments. Core of the sensor is a self-heated YSZ disc that provides in the center sufficiently high temperatures for sensing. At the sensor rim, the temperatures are low enough to allow for applying polymer sealings to separate both gas chambers gas tightly. In this study, the YSZ sensor disc compares two gas mixtures emulating the propene concentrations that occur up- and downstream of a diesel oxidation catalyst (DOC). At a temperature of 500 °C at the center of the disc, the changing propene concentrations on one side of the sensor, which emulate a changing propene conversion of the catalyst, are reflected by the sensor signal. Up to a conversion of 90 %, the sensor signals follow a theoretical equation that is derived from mixed-potential theory. For higher propene conversions, the signal differs from that theory. An initial explanation for this may be the catalytic activity of the platinum electrode at these high operation temperatures. This is supported by the fact that when reducing the catalytic activity, which is achieved by reducing the sensing temperature, the sensor signal is meeting more and more the theoretically derived equation. At 425 °C and below, there is no more dependency on the feed gas concentration, and all measured data fit exactly with the above-derived theory, i.e., the sensor signal depends only on the “conversion of the catalyst” and not on the propene concentration. As result, a sensor that measures directly the quantity “conversion” is obtained.