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The ammonia selectivity during the cycling NOx storage reduction process over a model Pt/Ba/Al2O3 catalyst was studied. Firstly, it was demonstrated that, whereas the presence of water or carbon dioxide in the gas mixture have a negative effect on the storage step, the effect of these components have different impacts on the NOx efficiency. Due to their involvement in the reverse water gas shift (RWGS) reaction, the absence of water in the gas mixture leads to a drop of the NOx removal whereas without CO2, an increase of the NOx conversion is observed.It was also showed that the reducer (H2) conversion during the short excursion in rich condition is directly correlated to the NH3 emission. NH3 is emitted since hydrogen is not fully converted, whatever the NOx conversion rate. The ammonia pathway is clearly demonstrated and it was claim that, when H2 remains in the reaction mixture, the ammonia production rate is higher than the ammonia reaction with the remaining NOx in order to form N2.

The influence of the ceria-zirconia mixed oxide composition in Pt/CexZr1-xO2 catalysts was studied toward NOx storage capacity, including sulfur poisoning and sulfur regeneration, and NOx reduction efficiency in lean-rich cycling conditions. The results are compared with a Pt/Ba/Al model catalyst. The samples were characterized by N2 adsorption, XRD and H2-TPR. The behaviors of the ceria-zirconia supported catalysts are quite similar whatever their composition. They are sensitive to a reducing pre-treatment which lead to an increase of (i) the cerium reducibility/oxygen mobility, (ii) the NO oxidation rate and (iii) the NOx storage capacity at 300 and 400°C. The sulfating treatment leads to a dramatic decrease of the NOx storage capacity for all catalysts, the decrease being more pronounced for the Zr-rich samples. H2-TPR experiments show that the sulfates amount and their stability tend to increase with the Zr content, but these sulfates are significantly less stable compared with Pt/Ba/Al. The sulfur elimination rates in rich mixture at 550°C are higher than 90% with the ceria-zirconia supported catalysts versus 56% with Pt/Ba/Al. The ceria-zirconia supported catalysts are able to convert NOx in lean-rich cycling condition. Compared with Pt/Ba/Al, the NOx conversions are a little lowered but the ammonia selectivity is significantly decreased with the Ce-Zr mixed oxides, with a beneficial influence of the cerium content.