Reaction pathway of the reduction by CO under dry conditions of NO x species stored onto Pt [sbnd]Ba/Al 2O 3 Lean NO x Trap catalysts

Abstract

The reduction by CO under dry condition of NO x species stored at 350 °C onto a Pt Ba/Al 2O 3 Lean NO x Trap catalyst is investigated by means of transient response methods (CO-TPSR and CO-ISC experiments) and complementary FTIR spectroscopy. It is shown that the reduction by CO under nearly isothermal conditions of nitrates stored onto Pt Ba/Al 2O 3 occurs through a Pt-catalyzed pathway which leads mainly to nitrogen and does not proceeds via the thermal decomposition of stored nitrates with release of NO x in the gas phase. The CO-TPSR and CO-ISC experiments and the combined FTIR study show that (i) the reduction of nitrates by CO occurs via a consecutive reaction scheme with formation of nitrites and then of surface isocyanate/cyanate species followed by the reaction of these species with residual nitrites to give nitrogen; (ii) the reaction of NCO species with nitrites to give nitrogen is slower than the reduction of nitrates to give nitrites and then NCO species; (iii) upon contact with oxygen at increasing temperature, NCO species can be re-oxidized to surface nitrites at first and then to surface nitrates; (iv) the formation of nitrogen is observed during the reduction of stored NO x by CO, and during the oxidation of surface NCO species upon oxygen addition and upon admission of NO in the presence of excess O 2. In all cases, the formation of nitrogen can be explained by the reaction between surface NCO species and nitrites. The mechanism for the reduction of stored NO x by CO under dry and near isothermal conditions is discussed.