Reduction of NOx over a NOx-Trap Catalyst and the Regeneration Behaviour of Adsorbed SO2

A conventional NOx-trap catalyst containing platinum, rhodium, barium and lanthanum was conditioned with oxygen at 500°C, preloaded with NO under standard oxidising conditions and then subjected to regeneration with the reductants H2, CO and C3H6, either alone or as a mixture. Hydrogen is the most efficient reductant in terms of NOx conversion efficiency and reductant usage efficiency. There is a temperature optimum for CO between 300 and 400°C and a catalyst loading optimum (mols reductant added)/(mols NOx adsorbed) between 1.5 and 3.0. The behaviour of the catalyst towards sulphur poisoning was examined in supplementary trials with the adsorption of SO2 in the presence or absence of water vapour. When water is not present in both adsorption and reduction steps, very stable sulphates are formed, unattacked by reductants even at 1000°C. Sulfates are more easily reduced when water is present in the reductant mixture.