Low-temperature ammonia oxidation on platinum sponge studied with positron emission profiling

The low-temperature deactivation of a platinum sponge catalyst used for ammonia oxidation was studied with the positron emission profiling technique (PEP). Evidence that irreversibly adsorbed nitrogen species deactivate the catalyst is presented. Two reactivity regimes are distinguished. Initial fast N2 production at low surface coverage and a relatively slow N2 and N2O production at steady state when the surface is fully covered. The fast deactivation of the platinum sponge is mainly caused by adsorbed nitrogen species. The formation of PtO is relatively slow compared to surface nitride. The fast initial deactivation of platinum sponge by nitrogen and oxygen species is greatly retarded at temperatures above 388 K. Temperature-programmed reaction together with temperature-programmed desorption experiments show reactivation of the catalyst above this temperature.