Detailed Characterization Studies of Vehicle- and Rapid-Aged Commercial Lean NOx Trap Catalysts

Jesus  De Abreu, Volvo Cars

Commercial Lean NO_x Trap (LNT) catalysts were aged and characterized in detail to elucidate the effect of aging on their performance and examine the deactivation of a rapid-aged catalyst towards an improved correlation with respect to a vehicle-aged catalyst. A vehicle-aged commercial LNT catalyst was driven in the vehicle chassis dynamometer for 100 000 km, while the rapid-aged catalyst was treated at 800 °C for a period of time equivalent to a driving distance of 160 000 km. Flow reactor measurements were performed to measure the deterioration of the WGS, O₂ storage, NO_x storage, NO_x release and NO_x reduction functions of the samples. In particular, the NO_x conversion was evaluated under dynamic conditions with three different reductants (H₂, CO and C₃H₆). Physicochemical characterization techniques were also employed to explore the fundamental relations between the state of the catalysts and their catalytic properties.

NO₂-TPD measurements revealed that aging resulted in deterioration of the NO_x storage capabilities of the catalysts, attributed to the loss of surface area and deactivation of the adsorber material. Similar trend was also observed under steady-state NO_x storage and reduction cycles. Elemental analysis and TEM data revealed two main explanations, which account for the degradation in LNT performance. First, significant precious metals sintering along the catalyst axis, which demonstrate the progressive thermal stress of the catalyst towards the outlet. Second, large concentration of poison species was found at the vehicle aged catalyst inlet and continually decreased towards the rear. Among the aged samples examined, the middle (lengthwise) vehicle-aged sample showed the highest NO_x conversion, while the oven-aged catalyst was the most active of the aged samples for WGS reaction and oxygen storage.

Moreover, the effect of sulfur species has been investigated on the modern LNT components by means of calorimetry experiments using nine different types of model catalysts, which mainly consisted of three compounds: i) Al₂O₃, ii) Mg/Al₂O₃ and iii) Mg/Ce/Al₂O₃ mixed with Pt, Pd and Pt-Pd.  It was observed that poisoning by sulfur is more pronounced in Pt containing catalysts and that the sulfur species were more stable in the bimetallic catalysts than in corresponding Pd or Pt catalysts.