Effect of Length on the Performance of Lean NO_x Traps

The effect of monolith length on the NO_x performance of two different Lean NO_x Traps has been investigated using a bench flow reactor (BFR). The washcoat composition of one of the catalysts consists of Pt/K/γ-Al₂O₃; the major components of the other catalyst include Pt, Pd, Rh, barium, ceria and zirconia supported on γ-alumina washcoat. Samples of 2.22-cm in diameter and length of 2.54, 5.08 and 7.62-cm within each LNT were evaluated with long and short-cycle experiments at a fixed gas hourly space velocity, and results were compared between samples of different lengths. No significant difference in performance was observed in long and short-cycle experiments with full regeneration. On a contrary, significant difference was observed in short-cycle experiments with partial regeneration: the longer the sample the better the performance.

The intra-catalyst concentration of H₂ measured at different axial locations in short-cycle experiments indicated higher H₂ consumption in shorter samples. A series of experiments was carried out to ascertain different mechanisms of H₂ consumption, and results indicated different degrees of lean and rich front back-mixing for samples of different sizes. Higher back-mixing in shorter samples resulted in a higher H₂ loss via its oxidation by O₂ and lesser H₂ availability for reducing stored NO_x, which in turn affected catalyst’s performance when regeneration was limited by amount of H₂ available.