A comparative study of the C3H6 + NO + O2, C3H6 + O2 and NO + O2 reactions in excess oxygen over Na-modified Pt/γ-Al2O3 catalysts

The performance of 0.5 wt.% Pt (varying wt.% Na)/γ-Al2O3 catalysts in catalysing the C3H6 + NO + O2, C3H6 + O2 and NO + O2 reactions under simulated lean-burn engine exhaust conditions, was investigated over a wide range of temperature (∼100-500 °C) and sodium loadings (0-4.2 wt.%). For the first two reactions, depending on the Na loading, both promoting and poisoning effects were obtained: optimal promotion was achieved at a sodium loading of 2.6 wt.%. On the other hand, NO + O2 → NO2 reaction was inhibited over the whole range of Na loadings used. In the promoting regime, Na widened the temperature window of the C3H6 + NO + O2 reaction towards lower temperatures by ∼50 °C, accompanied by an enhancement in N2-selectivity by ∼40 additional percentage points. For the C3H6 + O2 reaction the propene light-off temperature and the temperature for 100% propene conversion decreased by 64 °C. Na loadings higher than the optimal loading caused a dramatic decrease in NO conversion over the whole temperature range in the case of C3H6 + NO + O2 reaction, and a substantial decrease in hydrocarbon conversion for both reactions. Comparison of the C3H6 conversion profiles for these two reactions indicates significant inhibition of hydrocarbon oxidation by NO for all catalysts: nitrogen oxide increases the propene light-off temperature and the temperature of 100% C3H6 conversion by ∼80 °C. The promoting and poisoning effects of Na for all three reactions are understandable in terms of the influence of alkali modifier on the relative adsorption strengths of reactant species. The inhibition of hydrocarbon oxidation caused by NO and the propene-induced inhibition of NO2 formation at low temperatures are understandable in terms of the competition of reaction intermediates for active surface sites.