The NOx storage-reduction on Pt-K/Al2O3 Lean NOx Trap Catalyst

The nature of stored NOx and mechanistic aspects of the reduction of NOx stored over a model Pt-K/Al2O3 catalyst sample are investigated in this paper, and a comparison with a model Pt-Ba/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 Pt-K/Al2O3 and Pt-Ba/Al2O3. However, whereas bidentate nitrates are present in remarkable amounts on Pt-K/Al2O3, along with ionic nitrates, only very small amounts of bidentate nitrates were observed on Pt-Ba/Al2O3. Under nearly isothermal conditions, the reduction of the stored NOx with H2 occurs via an in series twosteps Pt-catalysed molecular process involving the formation of ammonia as an intermediate, like for the Pt-Ba/Al2O3 catalyst sample. However, higher N2 selectivity is observed in the case of the Pt-K/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 CO2 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.