Development of a Diesel Emission Catalyst System for Meeting US SULEV Standards

In recent years, catalyst systems such as a lean NOx trap (LNT) catalyst system and a urea selective catalytic reduction (SCR) system have been developed to obtain cleaner diesel emissions. At Nissan, we developed an emission control system for meeting Tier 2 Bin 5 requirements in 2003. On the basis of that technology, a new HC-NOx trap catalyst system has now been developed that complies with the SULEV standards without increasing the catalyst volume and precious metal loading. Compliance with the SULEV standards requires a further reduction of HC (NMHC) emissions by 84% and NOx by 60% compared with the emission performance Tier 2 Bin 5 compliant catalyst system. Consequently high conversion performance for both HCs and NOx is needed. An investigation of HC emission behavior under the FTP75 mode showed that a reduction of cold-phase HCs was critical for meeting the standard. Large quantities of HCs above C4 are emitted in the cold state. Thus, a HC trap type three-way catalyst (HCT-TWC) which contains a relatively big pore size zeolite was selected for reducing cold-phase HCs corresponding to the size of HCs. This catalyst has the capability to convert not only HCs but also NOx effectively. The possibility of integrating the HCT-TWC and a LNT catalyst was investigated with an eye toward downsizing the catalyst system. This catalyst combination was also investigated with the expectation of obtaining functional synergies. That led to the development of an HC-NOx trap catalyst. The NOx reduction capability was improved by increasing the dispersion of precious metal and the NOx trapping agent in the LNT catalyst layer. It was found that this HC-NOx trap catalyst system maintains good conversion performance even after aging equivalent to driving 120,000miles. That confirmed the possibility of meeting the SULEV standards without increasing the catalyst volume and precious metal loading over the Tier 2 Bin 5 compliant system.