Reduction with the Combinations on LNT and SCR in Diesel Applications
Stricter emission limitations for NOx and particulates in mobile diesel applications will require the combinations of active aftertreatment methods like Diesel Particulate Filters (DPF), Selective Catalytic Reduction (SCR) with urea and Lean NOx Trap (LNT) in the 2010's. A new concept is the combination of LNT+SCR, which enables on-board synthesis of ammonia (LNT), which is then removed on the SCR catalyst. The main application for this kind system will be light-duty vehicles, where LNTs are already used and the low temperature deNOx is a main target. That combinatory system was investigated by developing and selecting PtRh/LNT and SCR catalysts for that particulate application, where the maximum temperature may reach 800°C and SCR should proceed without NO2 assistance. Pt-rich, PtRh/LNT with reasonable high loadings above 80g/cft resulted in a high NOx efficiency in the experimental laboratory conditions which created also on LNTs a higher NH3 concentration for the SCR unit. The SCR catalyst with copper as an active metal (Cu- SCR) showed the good durability up to 800°C and a wide operation window without the NO2 assistance. Fe-SCR and VSCR catalysts were more dependent on NO2, which is not present after LNT and DPF. An optimized concept had an air injection after LNT to keep SCR condition always lean side, where the SCR reaction was promoted by oxygen with a high selectivity without NH3 emissions. The simulations in reaction conditions and system design resulted in the proposals for the optimal design and main reaction mechanism.