Abstract
Keywords - lean NOx catalyst, alkali attack, thermal stability, precious metal, SOx
Abstract
A new type of lean NOx catalyst has been developed with improved thermal resistance. The lean NOx catalyst that we propose contains precious metals and NO2 adsorbents. By using this catalyst in a lean gas atmosphere (air-fuel ratio >18), NO is oxidized by the precious metals (NO+1/2O2 ¨NO2), and the generated NO2 is chemically adsorbed on the NO2 adsorbent. Then, by changing the air-fuel ratio of the gas to a rich atmosphere (air-fuel ratio <14.7), adsorbed NO2 is reduced.
In the past, the sintering of NO2 adsorbents was prevented by using mixed metal oxides including alkali metals as NO2 adsorbent in the catalyst; however, it has become clear that the performance decreased as the heat treatment conditions were more severe. This thermal degradation was ascertained to be caused by the alkali attack, namely, the reaction between alkali metals and Si in the cordierite honeycomb substrate. The alkali attack caused the degradation for NOx purification performance by decreasing the number of NO2 adsorption sites on the catalyst surface. Therefore, the alkali attack control was studied by noting improvement effects of the catalyst support. A good additive to the alumina support which controlled the alkali attack after heat treatment was found to be metal element Á. Examinations such as CO2 adsorption and EDX-SEM (Energy Dispersive X-ray and Scanning Electron Microscopy) etc., showed that the movement of the alkali metal to the cordierite substrate was controlled by addition of Á.
In addition, by optimizing the content and the composition of precious metals in the catalyst, the content could be cut while maintaining good purification performance.
The NOx conversion of the catalyst with the above mentioned improvements was measured in a model gas test. It was confirmed that thermal resistance for this catalyst was high. Furthermore, in a model gas SOx durability test, this catalyst showed good SOx durability compared with the conventional catalyst. The NOx conversion for this improved catalyst was measured by using exhaust gas from a commercial lean burn vehicle. Even after thermal treatment, this catalyst showed excellent performance for exhaust gas purification.