CeO2-M2O3 Passive NOx Adsorbers for Cold Start Applications

Abstract

Pt/CeO2-M2O3 and Pd/CeO2-M2O3 (M = La, Pr, Y, Sm, or Nd) were prepared by co-precipitation and impregnation and were investigated for potential passive NOx adsorber (PNA) use. During NOx storage at 120 °C, it was found that the amount of NOx stored as a function of time for Pt-promoted materials was higher than the Pd-promoted counterparts. For Pt/CeO2-M2O3 samples doped at the 5% level, NOx storage efficiency (NSE) followed the order Pr > Nd > Sm > Ce (undoped) > Y, La. Increasing dopant content from 5 to 20% decreased NSE in most cases, although in the case of Pr, NSE was increased. During subsequent NOx-temperature-programmed desorption (TPD), two NOx desorption events were apparent in all cases, the first occurring below 350 °C and the second occurring in the range 350–500 °C. Doping with Pr promoted the release of increased amounts of NOx below 350 °C compared to samples doped with other lanthanides; moreover, increasing the content of all doping metals except Pr shifted desorption peaks to higher temperatures, while the opposite trend was observed for Pr. Promotion with Pd was also examined, resulting in an increase of NOx desorption at low temperatures (<350 °C) relative to Pt. These results can be rationalized in terms of the ability of Pr to create vacancies in the CeO2 lattice, which facilitate NOx adsorption, and by the superior NO oxidation activity of Pt relative to Pd, which promotes NOx storage as nitrates possessing high thermal stability.