Dynamics of storage and reaction in a monolith reactor:: lean NOx reduction

The NO, storage and reduction performance of a series of supported Pt/BaO washcoated monolith catalysts is investigated (y%Pt/x%BaO/Al2O3; y = 2-2.6%; x = 0-25%). Storage experiments over a range of exposure (storage) times and feed flow rates (space velocities) show the importance of the NO oxidation to NO2 reaction. At high space velocities, kinetic limitations reduce the NO conversion and NOx trapping efficiency (fraction of No-x trapped). Higher trapping efficiencies are achieved at lower space velocities when the NO oxidation approaches the equilibrium conversion limit. The Nos trapping efficiency increases with barium loading, but the extent of increase depends on the exposure time. Cycle-averaged NOx conversions and N-2 selectivities exceeding 80% are achieved over a range of operating conditions, including cycle time, space velocity, and reductant (propylene) concentration. The NOx conversion exhibits a maximum at an intermediate cycle time, and the location and magnitude of the maximum depends on the barium loading. The conversion also achieves a maximum at an intermediate barium loading, indicating that increases in dynamic storage capacity of the catalyst do not guarantee improved reduction performance. The dependence of the trapping efficiency on the exposure (storage) time provides a good estimate for the lean storage time needed to achieve a prescribed cycle-averaged NOx, conversion during cycling. The dependence of the cycle-averaged NOx conversion on barium loading parallels that of the short-time NOx storage, indicating the importance of the NOx trapping step in the overall monolith catalyst performance. (C) 2004 Published by Elsevier Ltd.