CeO2-M2O3 Passive NOx Adsorbers for Cold Start Applications
Samantha Jones, University of Kentucky Center for Applied Energy Research
Recently, it has been shown that a system consisting of a passive NOx adsorber (PNA) device combined with a downstream SCR catalyst holds promise for the mitigation of cold start NOx emissions [1-6]. In the present study, Pt/CeO2-M2O3 and Pd/CeO2-M2O3 (M=La, Pr, Y, Sm, or Nd) based materials prepared by co-precipitation and impregnation were investigated for potential use in PNA systems. During NOx storage at 120 oC, it was found that the amount of NOx stored as a function of time for Pt-promoted materials was higher than for the Pd-promoted counterparts. For Pt/CeO2-M2O3 samples doped at the 5% level, the order of NOx Storage Efficiency (NSE) followed the order Pr/Nd/Sm/Ce/Y/La. Notably, while increasing the dopant content from 5% to 20% decreased NSE in most cases, in the case of Pr NSE was increased. During subsequent NOx-TPD two NOx desorption events were apparent, the first occurring below 350 oC and the second occurring in the range 350 – 500 oC. Doping with Pr promoted the release of increased amounts of NOx below 350 oC compared to samples doped with the other lanthanides. Increasing the content of M shifted desorption peaks to higher temperature, while the opposite trend was observed for Pr. Finally, promotion with Pd caused the desorption peaks to shift slightly to higher temperatures; however, relative to the Pt samples, comparatively more NOx was desorbed at low temperatures (350 °C). These results can be rationalized by the ability of Pr to create vacancies in the CeO2 lattice and by the superior NO oxidation actvitiy of Pt relative to Pd, which promotes NOx storage as nitrates. DRIFTS data, which will be presented in this talk, support these ideas, and provide insights into the evolution of NOx species present during NOx adsorption and desorption.
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