NOx storage and reduction with H2 on Pt/BaO/Al2O3 monolith: Spatio-temporal resolution of product distribution

The regeneration of a model Pt/BaO/Al2O3 monolith catalyst was studied with hydrogen as the reductant to elucidate the reaction pathways to molecular nitrogen and ammonia. NOx storage and reduction experiments (NSR) were conducted with a 2cm length monolith for a wide range of feed conditions. The NSR experiments were replicated for a series of monoliths of progressively decreasing length, enabling the construction of spatio-temporal profiles of reactant and product concentrations. The results show that there are two primary competing routes to the desired N2 product; specifically a direct route from the reduction of stored NOx by H2 (H2+NOx→N2) or by a sequential route through NH3 (H2+NOx→NH3; NH3+NOx→N2). A comparison between H2 and NH3 as reductant feeds during NSR revealed H2 is a more effective reductant in terms of NOx conversion for temperatures below approximately 230°C. At higher temperatures (230–380°C), the regeneration of stored NOx is feed-limited and the difference between the reductants H2 and NH3 is found to be small with H2 being a slightly superior reductant. Experimental measurements of the traveling front velocity are compared with a simple feed-limited model that assumes complete consumption of H2 as stored NOx is depleted. At lower temperatures the regeneration is limited by chemical processes at the Pt/Ba interface. The findings are pieced together to establish a phenomenological description of the spatio-temporal features of the lean NOx trap with hydrogen as the reductant.