Monolith and TAP reactor studies of NOX storage on Pt/BaO/Al2O3:: Elucidating the mechanistic pathways and roles of Pt

The multiple roles of Pt are examined during NO and NO2 storage in the presence Of 02 on a series of Pt/BaO/Al2O3 catalysts spanning 0 - 3.7 wt.% Pt and employing both a bench-scale monolith and TAP (Temporal Analysis of Products) reactor system. Pt enhances the storage of both NO and NO2, although the extent of enhancement diminishes at higher Pt loadings. The promotional effect of Pt observed even for short-time NO2 storage suggests the importance of a Pt-Ba couple and associated spillover processes. During short-time exposure, NO and NO2 in O-2 store at comparable rates, suggesting the formation of barium mono nitro species in close proximity to Pt. At longer exposure times deeper nitration occurs, apparently involving Ba sites further removed from Pt. NO and NO2 TAP pulsing experiments reveal coupled NO/NO2 decomposition and storage on a pre-reduced catalyst which, as oxygen accumulates on the surface, evolves to a storage-only process. A phenomenological storage model is proposed that integrates previous theories from the Literature in an attempt to reconcile the observed effects of Pt and the similarities and differences in NO and NO2 storage. The trends in the data are consistent with the concurrent pathways of NOX storage; specifically, the "nitrite" and "disproportionation" pathways, both of which have been proposed in the literature. (c) 2006 Elsevier B.V. All rights reserved.