Catalyst Design for Meeting Stringent LEV-2 NOx Regulation 2002-01-0348
Beyond the ULEV regulation, a stringent NOx emission regulation like LEV-2 is enacted not only to control smog, but also to reduce environmental pollution like an acidic rain. LEV-2 is a typical example.
Rhodium (Rh) has been known for its superior NOx reduction activity but was restricted to limited usage because of its scarcity and high cost. Therefore, it is essential to maximize the Rh function the three-way catalyst (TWC) so that the usage of Rh can be minimized.
FTP results indicate that most NOx is emitted during high exhaust-gas flow rate, such as hard acceleration and high-speed cruising. The emission worsened in many cases during deceleration, especially after a fuel cut operation. Under this circumstance, (1) a quick restoration of the active Rh metallic species from an oxide form and (2) having sufficient Rh accessibility to exhaust gases, is important. For the first case, Rh interaction with additives was investigated, and the new materials were developed to stabilize the Rh metallic state and to enhance Rh's steam reforming activity. In the second case, the washcoat distribution of Rh layer in the catalyst was improved.
An effective catalyst was developed in this study by combining advanced materials and a new coating technology. NOx conversion efficiency was significantly improved without sacrificing HC performance, even with approximately 50% reduction in precious metal usage.
The developed technology could be adopted for various types of catalysts while the approach could be applied to areas where minimum Rh usage is required to meet the recent and future stringent NOx emission standards.