CeO2-based Passive NOx Adsorbers for Low Temperature Applications

Samantha  Jones, University of Kentucky’s Center for Applied Energy Research

Reduction of NOx emissions from lean-burn engines during cold starts represents a significant challenge to the automotive industry. Recently, it has been shown that a combined system consisting of a passive NOx adsorber (PNA) device with a downstream SCR catalyst holds promise for the mitigation of cold start NOx emissions [1]. In the present study, Pt/CeO2- and Pd/CeO2-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/CeO2 was higher than for Pd/CeO2. During subsequent NOx-TPD two NOx desorption events were apparent, the first occurring below 300 oC and the second occurring in the range 300 – 500 oC. The promotion of CeO2 with Pd caused the first desorption peak to shift to higher temperature; moreover, unlike the Pt sample, relatively more NOx was desorbed at low temperature (<300 °C) for the Pd sample compared to the high temperature release. To rationalize these results, DRIFTS measurements were performed. During NO storage on Pt/CeO2, strong nitrate bands appeared, accompanied by weaker nitrite bands. During subsequent TPD, the nitrite bands disappeared by 300 °C, while the nitrate bands initially increased in intensity and then started to diminish above 400 oC. During NO storage on Pd/CeO2, a strong chelating nitrite band rapidly formed, while relatively weak nitrate bands appeared. Upon TPD, the nitrite species were removed by ~250 oC, unlike the nitrate species, which required higher temperatures for their removal. These results suggest that comparatively more NO is stored as nitrites on Pd/CeO2 compared to Pt/CeO2. Given that these nitrites are less thermally stable than nitrates, this finding explains why relatively more NOx is desorbed at low temperature (<300 °C) for Pd/CeO2 than for Pt/CeO2.

1. Henry, C., Langenderfer, D., Yezerets, A., Ruth, M., Chen, H.-Y., Hess, H., Naseri, M., Presented at the 2011 DEER Conference, Detroit, MI.

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