Influence of Catalyst Composition on NO_X Trap Performances

In the present work the influence of the type of noble metals present in NO_X storage catalysts was investigated regarding the NO oxidation, NO_X storage and NO reduction performances. Monolith samples with combinations of platinum, palladium and rhodium were tested in a flow-reactor. From this study, it was concluded that platinum is necessary for NO oxidation while palladium shows a better ability to oxidize NO to NO₂ at high temperature. The presence of Rh on Pt/Rh and Pt/Pd/Rh was found to have a negative effect on NO oxidation reaction. Results suggest also that the storage capacity is partially linked to the oxidation function: Surface area developed by the storage material and the Pt–Ba interaction have an important influence on NO_X storage function. The rich step was not efficient to purge completely the trap and leads to accumulation of NO_X and so to performances degradation which was assigned to accumulation of NO_X after each cycle and storage on of Ba-bulk sites. In presence of Rh, a high oxygen storage capacity leads to high exothermic process between reducing agent and stored oxygen which improves the ability of catalyst to release NO_X.