NOx storage and reduction properties of Pt/CexZr1-xO2 mixed oxides: Sulfur resistance and regeneration, and ammonia formation

The influence of the ceria-zirconia mixed oxide composition in Pt/CexZr1-xO2 catalysts was studied toward NOx storage capacity (NSC), 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 N-2 adsorption, XRD and H-2-TPR. The behaviors of the ceriazirconia supported catalysts are quite similar whatever their composition. They are sensitive to a reducing pretreatment which leads 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 degrees 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. H-2-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 degrees 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. (C) 2009 Elsevier B.V. All rights reserved.