The transient response of ZrO2 oxygen sensors to step changes in gas composition

The transient voltage response of ZrO₂ oxygen sensors was examined following step changes in gas composition. The experiments were performed on a laboratory flow reactor at 600° C. Composition changes between (a) 100% and (b) 1% O₂ in N₂ produced response curves whose symmetry varied between composition steps (a) from low-to-high oxygen and (b) from high-to-low oxygen. This difference is due to the logarithmic dependence of sensor voltage on oxygen partial pressure. Corresponding oxygen partial pressure-time curves, derived from experimental voltage via the Nernst equation, are symmetric with respect to the direction of composition changes. Abrupt transitions are found in voltage-time curves at 600° C following step changes of reactive gases; e.g. from O₂/N₂ mixtures to CO/N₂, H₂/N₂ or D₂/N₂ mixtures. These voltage-steps represent transitions in stoichiometry of the surface boundary layer on the ZrO₂ sensor. Delay times before the transition also reflect reaction stoichiometry. Response times with O₂/CO, O₂/H₂ and O₂/D₂ follow trends predicted by the kinetic theory of gases. A limited number of experiments were performed to examine the relationships between sensor response and sensor catalytic activity. Poorer oxidation catalytic activity parallels slower response characteristics.