Axial length effects on Lean NO_x Trap performance

The effect of axial length on the NO_x reduction performance of two different commercial Lean NO_x Trap (LNT) monolithic catalysts was experimentally investigated in a bench flow reactor. The washcoat composition of one of the catalysts consisted of Pt and K on γ-Al₂O₃; whereas the other catalyst contained a complex mixture of Pt, Pd, Rh, Ba, Ce, Zr, Mg, Al and others. The NO_x removal characteristics of cylindrical monolith segments of constant diameter (2.22 cm) and axial lengths of 2.54, 5.08 and 7.62 cm were evaluated using a simulated lean engine exhaust containing water and carbon dioxide at a constant space velocity of 30,000 h^-1. No significant effects of length were observed when the catalysts were fully reduced with hydrogen between NO_x capture phases. However when the catalysts were only partially regenerated NO_x reduction efficiency increased with monolith length. Intra-catalyst H₂ measurements at different axial locations indicated that at least some of the efficiency loss during partial regeneration occurred when back-mixed H₂ was directly oxidized and became unavailable for nitrate reduction.