Strategies for total NOx measurement with minimal CO interference utilizing a microporous zeolitic catalytic filter

A strategy is presented for determining total NOx (NO+NO2) in gas streams at temperatures greater than 400 °C. By using a Pt-loaded zeolite Y as a catalyst filter bed placed before a sensor, NOx species in the gas stream are brought to an equilibrium concentration of NO and NO2 that is determined by the background oxygen concentration and the filter temperature. The equilibrated NOx is then measured with a yttria stabilized zirconia (YSZ) sensor using a chromium oxide sensing electrode. In the absence of the filter NO and NO2 produced responses in opposite directions, while in the presence of the filter, NO and NO2 gave signals of the same magnitude and direction. By varying the temperature of the filter relative to the sensor, the signal magnitude could be increased. Another advantage of the filter is that CO is oxidized to CO2 and thus little interference of CO upon NOx was observed. By introducing a layer of the Pt-zeolite catalyst filter material directly onto a Pt electrode, a pseudo-reference to NOx was formed. This design provides the opportunity for miniaturization as well as removes the need for an air reference.