Influence of reductant on the thermal stability of stored NOx in Pt/Ba/Al2O3 NOx storage and reduction traps

The stability and reactivity of NOx stored over Pt/Ba/Al2O3 storage and reduction catalysts have been compared in the presence of hydrogen, carbon monoxide, and propene as reductants under stoichiometric conditions while heating between 298 and 873 K. The order of efficiency in terms of conversion of stored NOx to N-2 was H-2 > CO > propene. Additionally the use of hydrogen led to the lowest NO2: NO ratio. Four potential mechanisms to explain the initial release of stored NOx are discussed. In the absence of reductant, less than 10% of released NOx was in the form of NO, whereas the presence of reductant led to significant NO release which was independent of the presence of Pt. Results suggest that while formation of N-2 from stored NOx requires the presence of reduced noble metal sites, formation of NO can take place over the support alone, probably by direct interaction between reductant and stored nitrate. The role of Pt in providing activated spill over reductant is important for the decomposition of certain forms of NOx stored by the alumina. Both hydrogen and propene oxidation were inhibited by the presence of NOx whereas CO was able to compete for Pt sites, even in the presence of high NOx concentrations. (C) 2004 Elsevier Inc. All rights reserved.