Dynamics and selectivity of NOx reduction in NOx storage catalytic monolith

Several nitro-en compounds can be produced during the regeneration phase in periodically operated NO, storage and reduction catalyst (NSRC) for conversion of automobile exhaust gases. Besides the main product N-2, also NO, N2O, and NH3 can be formed, depending on the regeneration phase length, temperature, and gas composition. This contribution focuses on experimental evaluation of the NOx reduction dynamics and selectivity towards the main products (NO, N-2 and NH3) within the short rich phase, and consequent development of the corresponding global reaction-kinetic model. An industrial NSRC monolith sample of PtRh/Ba/CeO2/gamma-Al2O3 type is employed in nearly isothermal laboratory micro-reactor. The oxygen and NOx storage/reduction experiments are performed in the temperature range 100-500 degrees C in the presence of CO2 and H2O, using H-2, CO and C3H6 as the reducing agents. The spatially distributed NSRC model developed earlier is extended by the following reactions: NH3 is formed by the reaction of H-2 with NOx and it can further react with oxygen and NOx deposited on the catalyst surface, producing N-2. Considering this scheme with ammonia as an active intermediate of the NOx reduction, a good agreement with experiments is obtained in terms of the NOx, reduction dynamics and selectivity. A reduction front travelling in the flow direction along the reactor is predicted, with the NH3 maximum on the moving boundary. When the front reaches the reactor outlet, the NH3 peak is observed in the exhaust gas. It is assumed that the ammonia formation during the NOx reduction by CO and HCs at higher temperatures proceed via the water gas shift and steam reforming reactions producing hydrogen. It is further demonstrated that oxygen storage effects influence the dynamics of the stored NOx reduction. The temperature dependences of the outlet ammonia peak delay and the selectivity towards NH3 are correlated with the effective oxygen and NOx storage capacity. (C) 2007 Elsevier B.V. All rights reserved.