Study of TWC in NOx Adsorber Catalyst System for Gasoline Direct Injection Engine

Extensive research and development has been performed to develop the NOx-adsorber catalytic system, which would make Mitsubishi vehicles powered by the gasoline direct-injection (GDI™) engines comply with European Stage 4 emissions regulations. This NOx-adsorber catalytic system is a three-brick configuration, consisting of a three-way catalyst in the front (the front catalyst) and the rear catalytic converter, composed of a new NOx-adsorber catalyst and a conventional three-way catalyst (TWC). In the present research work, a special effort has been made to define the required performance of the front catalyst, particularly with HC reduction efficiency at the cold start, the steady-state leaner A/F and the transient phase of the A/F from leaner to stoichiometric. For HC reduction, it has been found that platinum (Pt) had the highest HC efficiency. In a Pt/rhodium (Rh) catalyst, if Rh and Pt are both included in the washcoat, the Rh content adversely effects Pt's HC reduction capability. From the aspect of HC control, palladium (Pd) is generally recommended over Pt. Recent trends in precious metals pricing in the market show a sharp increase in pricing due to an expanded demand for these metals for automotive emissions control. Accordingly, the use of precious metals, particularly a high load of Pd in the catalyst, can no longer be possible, and Mitsubishi has thus developed a front catalyst that has higher performance with less Pt and Rh loading. The test results suggest that the new NOx-adsorber catalytic system will have a high potential of meeting the EU Stage 4 emissions regulations.