Comparison of Pt-BaO/Al2O3 and Pt-CeO2/Al2O3 for NOx storage and reduction: Impact of cycling frequency

Abstract

NOx reduction under net lean and near-stoichiometric conditions was carried out on Pt/Al2O3, Pt/CeO2/Al2O3 and Pt/BaO/Al2O3 washcoated monoliths to compare performance features and identify reaction pathways. The impact of the storage components (BaO, CeO2) on the NOx conversion and byproduct (NH3 and N2O) yields was quantified for a range of feed temperatures, reductant types (H2, and C3H6), O2 feed concentrations, and cycle times. The NOx storage functionality is essential for NOx reduction under net lean conditions while the oxygen storage functionality promotes NOx reduction for near-stoichiometric conditions. NOx conversion by H2 under lean conditions is dependent on the NOx storage capacity of the catalyst, with Pt/CeO2/Al2O3 and Pt/BaO/Al2O3 exhibiting the highest NOx conversion below and above 300 °C, respectively. High NOx conversion is achieved over Pt/CeO2/Al2O3 for anaerobic rich feeds at temperatures above 400 °C. Increasing the O2 feed concentration enhances NOx conversion over Pt/CeO2/Al2O3 below 400 °C but inhibits NOx conversion above 400 °C. The former is attributed to promotion of NO oxidation leading to NOx storage while the latter is attributed to O2 inhibition of NO decomposition/reduction. Shorter cycle times increase the NOx conversion with C3H6 as reductant over Pt/BaO/Al2O3 under lean conditions and over PCA for the near-stoichiometric feed. The findings confirm that improved NOx storage utilization is mainly responsible for NOx conversion enhancement. A ceria redox pathway has only a secondary effect on NOx conversion under excess O2.