31 - Development of exhaust catalyst

Abstract

This chapter focuses on the three-way catalysts developed by using the technology of nanolevel powders. The three-way catalyst is used to keep up the stoichiometric composition of an exhaust gas, and a three-way catalyst system comprised of not only a catalyst itself but also other devices such as electronically controlled fuel injector, air flow meter, oxygen sensors, and computers to control their working. An oxygen sensor is set just upstream of the catalyst and sends a signal of oxygen concentration in an exhaust gas to the electronically controlled fuel injector for controling the A/F (air to fuel ratio) at the stoichiometric composition. As the A/F deviance from the stoichiometric composition worsens the conversions of HC, CO, and NOx, the catalyst itself has to have a function of buffer oxygen to keep the stoichiometric composition in the transient condition, in which the mechanical and electronic control cannot follow. Oxygen-storage materials in a three-way catalyst play an important role to give a function of oxygen buffer to a three-way catalyst. Two types of new technological developments have been made that are related to the nanolevel particles achieved in the three-way catalyst system. The first one is the improvement of oxygen storage capacity (OSC), in which ZrO2 is dissolved into CeO2 that improves the OSC remarkably. The second technology is aimed at the improvement of heat resistance of the OSC materials in which a nanolevel mixture of CeOz–ZrO2 solid solution particles and alumina particles is made by dispersing CeOz–ZrO2 solid solution particles among alumina particles