A surface-science-based model for the selectivity of platinum–gold alloy electrodes in zirconia-based NO x sensors

Abstract

Recent surface chemistry experiments help explain the operation of an exhaust gas NO x sensor that uses a platinum–gold alloy electrode to selectively remove oxygen but not NO from an interior chamber. The electrode surface must dissociate O 2 for removal by electrochemical pumping, but must be at least 100 times less active for dissociating NO. In ultrahigh vacuum studies on stepped platinum single crystals, NO dissociation is strongly suppressed by surface gold coverages as low as 0.4 monolayer (ML), as well as by co-adsorbed oxygen and NO, probably because the dissociation process requires a special ensemble of several unoccupied Pt atoms. Oxygen dissociation is much less affected by surface modifications. In general, the surface composition of an alloy is very different from its bulk composition and depends on gas composition. By considering the energetics of the Pt–Au system we argue that under all conditions of interest for sensor operation the ensembles of free Pt atoms required for NO dissociation are strongly disfavored. A surface chemistry-based explanation is also suggested for the ammonia interference reported for this NO x sensor.