Mechanistic and kinetic studies of NOx storage and reduction on Pt/BaO/Al2O3

NOx storage and NO oxidation experiments have been carried out on a model Pt/BaO/Al2O3 catalyst powder in order to elucidate kinetic and thermodynamic limitations. Uptake of NO on a pre-oxidized catalyst reveals the existence of multiple regimes. The short-term storage is a sensitive function of storage time, but is nearly independent of temperature whereas the long-time storage has much slower kinetics and is a non-monotonic function of temperature. A simple steady-state reactor model is developed that predicts NO oxidation conversion and the limiting NOx storage. The model predicts that full barium utilization is thermodynamically feasible even under kinetically limiting conditions of incomplete NO oxidation. However, apparent NOx diffusion limitations prevent complete utilization. Storage and reduction (with propylene) experiments indicate that less than 10% of the barium is needed to achieve high NOx conversion (>85%) at moderate space velocity. At higher temperature both NO oxidation and barium nitrate formation are thermodynamically limited, both of which adversely impact the NOx reduction. Thermogravimetric measurements corroborate a physical picture in which only a small fraction of barium is utilized during lean/rich cycling protocols giving high NOx conversion.