Influence of the V2O5 content of the catalyst layer of a non-Nernstian NH3 sensor

Abstract

The sensing characteristic of a non-Nernstian ammonia sensor, which can be described as an electrochemical cell Au | YSZ | Au, V2O5–WO3–TiO2, is analyzed in dependence on the V2O5-content of the porous SCR-catalyst layer on top of one gold electrode. The ammonia sensitivities are determined at 550 °C for sensors with different amounts of V2O5 (0–3 wt.%). The slope of the semi-logarithmic characteristic curve increases with the vanadia content. The analysis of the half-cell electrode potentials of the half cells YSZ | Au, VWT confirms this behavior. All electrode potentials increase with increasing NH3 concentration and with the vanadia content of the catalyst layer. The V2O5 content of the catalyst layer affects the electrochemical processes at the three-phase boundary Au | YSZ | gas and the resulting voltage of the mixed potential. The gas composition at the three phase boundary can be modified by the catalyst layer. Both the NH3 sensitivity and the electrode potential shifts increase with the ammonia oxidation efficiency. The formation of an activated ammonia species on the catalyst layer, which is electrochemically oxidized, seems to be responsible for the electrode potential shift. According to this suggestion, more activated species are generated with increasing vanadia content. It is then subsequently electrochemically converted with oxygen ions from the YSZ to N2 and H2O.