NOx decomposition mechanism on the electrodes of a zirconia-based amperometric NOx sensor

In multi-electrode zirconia-based amperometric NOx sensors, a Pt–Au alloy is typically employed as the working electrode in the first cell to lower the oxygen concentration without decomposing the NO. In order to elucidate the mechanism for the preferential oxygen pumping, the NO decomposition performance of different zirconia cells with various electrodes was investigated. We used a newly developed method, which operates at 973 K, to simultaneously measure the reaction products at the electrodes and the cell currents on a millisecond timescale. This method enabled us to confirm that the electrochemical reduction of NO on a Pt electrode proceeds as follows. The dissociative adsorption of NO on the electrode and the desorption of N2 to the gas phase occur quite rapidly, followed by the gradual ionization of the adsorbed oxygen atoms. The same reaction process could be observed on a Pt–Au electrode, except that the N2 was generated at a different rate. The rate of N2 generation on the Pt–Au electrode was slower than that on the Pt electrode, even under conditions where more current was made to flow in the cell with the Pt–Au electrodes. From an estimation of the activation energies for N2 generation, it was concluded that an increase in the activation energy for NO dissociation enables the preferential oxygen pumping on the Pt–Au electrode.