The reduction by CO under dry conditions of NOx species stored at 350 °C onto a Pt-K/Al2O3 lean NOx trap catalyst is investigated by means of transient response methods (CO-TPSR and CO-ISC experiments) and complementary FT-IR spectroscopy. The results show that the pathway for the reduction of stored NOx by CO under dry and near isothermal conditions (and in the absence of CO2) is the same as that proposed by some of us in a previous work for a Pt-Ba/Al2O3 catalyst. In particular, (i) the reduction of stored NOx by CO occurs according to a Pt-catalyzed surface pathway that does not involve, as a first step, the release of NOx in the gas phase, is effective already at low temperature, and leads to nitrogen; (ii) the reaction scheme implies the formation of surface isocyanate species, followed by the reaction of these species with residual NOx to give nitrogen; and (iii) the reaction of NCO species with nitrates to give nitrogen is slightly slower than the reduction of nitrates to give NCO species; but on the Pt-K/Al2O3 catalyst, this last step is faster than on the Pt-Ba/Al2O3 system. As a consequence, the amount of isocyanate species present on the surface at the end of the reduction is lower for Pt-K/Al2O3 than for the Pt-Ba/Al2O3 catalyst.
The NOx reduction by CO on Pt-K/Al2O3 Lean NOx Trap Catalyst
MORANDI, Sara;GHIOTTI, Giovanna
2011-01-01
Abstract
The reduction by CO under dry conditions of NOx species stored at 350 °C onto a Pt-K/Al2O3 lean NOx trap catalyst is investigated by means of transient response methods (CO-TPSR and CO-ISC experiments) and complementary FT-IR spectroscopy. The results show that the pathway for the reduction of stored NOx by CO under dry and near isothermal conditions (and in the absence of CO2) is the same as that proposed by some of us in a previous work for a Pt-Ba/Al2O3 catalyst. In particular, (i) the reduction of stored NOx by CO occurs according to a Pt-catalyzed surface pathway that does not involve, as a first step, the release of NOx in the gas phase, is effective already at low temperature, and leads to nitrogen; (ii) the reaction scheme implies the formation of surface isocyanate species, followed by the reaction of these species with residual NOx to give nitrogen; and (iii) the reaction of NCO species with nitrates to give nitrogen is slightly slower than the reduction of nitrates to give NCO species; but on the Pt-K/Al2O3 catalyst, this last step is faster than on the Pt-Ba/Al2O3 system. As a consequence, the amount of isocyanate species present on the surface at the end of the reduction is lower for Pt-K/Al2O3 than for the Pt-Ba/Al2O3 catalyst.
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