Nitrous oxide formation during the reaction of simulated exhaust streams over rhodium, platinum and palladium catalysts

The formation of nitrous oxide during the reaction of a mixture of CO, NO, C₃H₆, C₃H₈, H₂ and O₂ over supported rhodium, platinum and palladium catalysts has been investigated under near-stoichiometric conditions. Rhodium gives the highest amount of N₂O with a peak selectivity near 70% at 250°C followed by a steady decline to low levels by 400°C. With Pt N₂O is seen in a narrow region just above 300°C and is removed at higher temperatures during oxidation of propane. Ammonia, probably produced via isocyanic acid, is the chief nitrogen-containing product of the reaction over Pt/Al₂O₃ below 300°C, but this route is much reduced when CeO₂/Al₂O₃ is the support. Palladium-containing catalysts give rise to N₂O in two regimes. It arises below 200°C by the reaction of NO with H₂ and above 300°C by another process which unlike Rh and Pt, persists through to 500°C. The behaviour of a trimetal catalyst containing Pd, Pt and Rh can be interpreted as a composite of that of the individual metals. Pt/Rh-containing catalytic converters recovered from vehicles show pronounced differences in N₂O production which correlate with a reduced ability to remove propane. The most probable cause of such changes is sintering of the platinum component as a result of exposure to high temperature. This may be the origin of the increased N₂O production seen in driving cycle tests with high mileage vehicles.