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

Abstract

The formation of nitrous oxide during the reaction of a mixture of CO, NO, C 3H 6, C 3H 8, H 2 and O 2 over supported rhodium, platinum and palladium catalysts has been investigated under near-stoichiometric conditions. Rhodium gives the highest amount of N 2O with a peak selectivity near 70% at 250°C followed by a steady decline to low levels by 400°C. With Pt N 2O 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/Al 2O 3 below 300°C, but this route is much reduced when CeO 2/Al 2O 3 is the support. Palladium-containing catalysts give rise to N 2O in two regimes. It arises below 200°C by the reaction of NO with H 2 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 N 2O 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 N 2O production seen in driving cycle tests with high mileage vehicles.