Adsorbate-assisted NO decomposition in NO reduction by C3H6 over Pt/Al2O3 catalysts under lean-burn conditions

The rates and product selectivities of the C3H6-NO-O-2 and NO-H-2 reactions over a Pt/Al2O3 catalyst, and of the straight, NO decomposition reaction over the reduced catalyst have been compared at 240 degrees C. The rate of NO decomposition over the reduced catalyst is seven times greater than the rate of NO decomposition in the C3H6-NO-O-2 reaction. This is consistent with a mechanism in which NO decomposition occurs on Pt sites reduced by the hydrocarbon, provided only that at steady state in the lean NOx reaction about 14% of the Pt sites are in the reduced form. However, the (extrapolated) rate of the NO-H-2 reaction at 240 degrees C is about 10(4) times faster than the rate of the NO decomposition reaction thus raising the possibility that NO decomposition in the former reaction is assisted by H-ads. It is suggested that adsorbate-assisted NO decomposition in the C3H6-NO-O-2 reaction could be very important. This would mean that the proportion of reduced Pt sites required in the steady state would be extremely small. The NO decomposition and the NO-H-2 reactions produce no N2O, unlike the C3H6-NO-O-2 reaction, suggesting that adsorbed NO is completely dissociated in the first two cases, but only partially dissociated in the latter case. It is possible that some of the associatively adsorbed NO present during the C3H6-NO-O-2 reaction may be adsorbed on oxidised Pt sites.