The NOx storage-reduction on PtK/Al2O3 Lean NOx Trap catalyst

Article abstract of DOI:10.1016/j.jcat.2010.09.026

The nature of stored NO_x and mechanistic aspects of the reduction of NO_x stored over a model PtK/Al₂O₃ catalyst sample are investigated in this paper, and a comparison with a model PtBa/Al₂O₃ catalyst is also made. It is found that at 350 °C on both the catalysts the storage proceeds with the initial formation of nitrites, followed by the oxidation of nitrites to nitrates. A parallel pathway involving the direct formation of nitrates species is also apparent; at saturation, only nitrates are present on the catalyst surface over both PtK/Al₂O₃ and PtBa/Al₂O₃. However, whereas bidentate nitrates are present in remarkable amounts on PtK/Al ₂O₃, along with ionic nitrates, only very small amounts of bidentate nitrates were observed on PtBa/Al₂O₃. Under nearly isothermal conditions, the reduction of the stored NO_x with H₂ occurs via an in series two-steps Pt-catalysed molecular process involving the formation of ammonia as an intermediate, like for the PtBa/Al ₂O₃ catalyst sample. However, higher N₂ selectivity is observed in the case of the PtK/Al₂O₃ catalyst due to the similar reactivity of the H₂ + nitrate and NH₃ + nitrate reactions. Accordingly ammonia, once formed, readily reacts with surface nitrates to give N₂, and this drives the selectivity of the reduction process to N₂. Notably, a strong inhibition of H₂ on the reactivity of NH₃ towards nitrates is also pointed out, due to a competition of H₂ and NH₃ for the activation at the Pt sites. Finally, the effect of water and CO ₂ on the reduction process is also addressed. Water shows a slight promotion effect on the reduction of the nitrates by H₂, and no significant effect on the reduction by ammonia, whereas CO₂ has a strong inhibition effect due to poisoning of Pt by CO formed upon CO₂ hydrogenation.