Regeneration of NOx trap catalysts

We have investigated the regeneration of a nitrated or sulphated model Pt/Ba-based NOx trap catalyst using different reductants. H2 was found to be more effective at regenerating the NOx storage activity especially at lower temperature, but more importantly over the entire temperature window after catalyst ageing. When the model NOx storage catalyst is sulphated in SO2 under lean conditions at 650 °C almost complete deactivation can be seen. Complete regeneration was not achieved, even under rich conditions at 800 °C in 10% H2/He. Barium sulphate formed after the high temperature ageing was partly converted to barium sulphide on reduction. However, if the H2 reduced sample was exposed to a rich condition in a gas mixture containing CO2 at 650 °C, the storage activity can be recovered. Under these rich conditions the S2− species becomes less stable than the CO32−, which is active for storing NOx. Samples which were lean aged in air containing 60 ppm SO2 at <600 °C, after regeneration at λ=0.95 at 650 °C, have a similar activity window to a fresh catalyst. It is, therefore, important that CO2 is present during the rich regenerations of the sulphated model samples (as of course it would be under real conditions), as suppression of carbonate formation can lead to sulphide formation which is inactive for NOx storage.