Ceria in automotive exhaust catalysts: I. Oxygen storage

Abstract

Oxygen storage capacities of CeO 2, CeO 2 Al 2O 3 , and PM/CeO 2/Al 2O 3, measured by a pulse injection method, are affected by pretreatment temperature, pulsing temperature, partial pressure of CO, presence of precious metals (PM), and the concentration of CeO 2 on alumina. They are lowered by higher pretreatment temperature but increase with the pulsing temperature in the range of application. At a pulsing temperature ≤500 °C, the capacities are not affected by oxygen pressure but increase with partial pressure of CO. The presence of PM lowers the reduction temperature and increases the oxygen storage capacity of CeO 2. TPR was used to measure the oxygen removal at various temperatures. At 900 °C, the maximum amount of oxygen removed from unsupported or alumina-supported ceria is about 25%. The TPR traces of the unsupported ceria show two peaks at 500 and 750 °C which are associated with the reduction of surface capping oxygen and bulk oxygen anions, respectively. For alumina-supported ceria, the TPR traces show a third peak at 850 °C which is associated with the reduction of the shared oxygen anions at the interface. The presence of PM lowers only the reduction temperature of the capping oxygen anions but not of the other two oxygen species. Both oxygen chemisorption and TPR were used to measure the oxygen anion restoration at various temperatures following the reduction at 500 and 900 °C, respectively. Chemisorption data show that the oxygen uptake per CeO 2 is highest at the lowest CeO 2 concentration. The TPR traces show that a new oxygen species, probably a molecular oxygen anion, is formed at 25 °C which converts slowly at 500 °C to the capping oxygen anion. Complete restoration of all three types of oxygen anions is accomplished at 850 °C in air.