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This paper describes a newly developed NOx trap catalyst that achieves cleaner exhaust gas using much smaller quantities of precious metals. The precious metal loading of this NOx trap catalyst has been halved by developing a technique for inhibiting precious metal sintering even under exposure to high temperature exhaust gas and a trap material with improved catalyst functions at low temperature. This NOx trap catalyst is used on the Nissan X-TRAIL fitted with a diesel engine, which was the first vehicle to comply with Japan's Post New Long term Exhaust Emission Regulations.

This paper describes a new catalyst powder structure that inhibits the sintering of the precious metals used in emission control catalysts even under vehicle aging conditions and keeps their particle size in the single-digit nanometer range. This catalyst powder structure consists of a support material for stabilizing precious metals by promoting their chemical bonding and a wall material that physically inhibits aggregation of support material particles. This new structure makes it possible to halve the precious metal loading compared with that of conventional catalysts, while providing the same level of catalyst conversion performance. This paper describes the effects obtained with this new powder structure for an underfloor catalyst, a close-coupled catalyst and a lean NOx trap catalyst.

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.