The NO x storage-reduction on Pt [sbnd]K/Al 2O 3 Lean NO x Trap catalyst

Abstract

The nature of stored NO x and mechanistic aspects of the reduction of NO x stored over a model Pt K/Al 2O 3 catalyst sample are investigated in this paper, and a comparison with a model Pt Ba/Al 2O 3 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/Al 2O 3 and Pt Ba/Al 2O 3. However, whereas bidentate nitrates are present in remarkable amounts on Pt K/Al 2O 3, along with ionic nitrates, only very small amounts of bidentate nitrates were observed on Pt Ba/Al 2O 3. Under nearly isothermal conditions, the reduction of the stored NO x with H 2 occurs via an in series two-steps Pt-catalysed molecular process involving the formation of ammonia as an intermediate, like for the Pt Ba/Al 2O 3 catalyst sample. However, higher N 2 selectivity is observed in the case of the Pt K/Al 2O 3 catalyst due to the similar reactivity of the H 2 + nitrate and NH 3 + nitrate reactions. Accordingly ammonia, once formed, readily reacts with surface nitrates to give N 2, and this drives the selectivity of the reduction process to N 2. Notably, a strong inhibition of H 2 on the reactivity of NH 3 towards nitrates is also pointed out, due to a competition of H 2 and NH 3 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 H 2, and no significant effect on the reduction by ammonia, whereas CO 2 has a strong inhibition effect due to poisoning of Pt by CO formed upon CO 2 hydrogenation.