Effect of basic metal additives on NOx reduction property of Pd-based three-way catalyst

The three-way catalytic (TWC) performance, especially CO and NOx conversion, of a Pd-only catalyst was improved by the addition of basic elements, and the activity increased as the electronegativity of the additive cations decreased (Ba>Sr>La), indicating that the basicity or electron-donating ability of the additives played a significant role in the improvement of the TWC performance. Thermal decomposition temperature of PdO in promoted catalysts became higher with decreasing the ionic electronegativity of the promoters. XPS analysis proved that the electron density around Pd(II) in the Ba-promoted catalyst was higher than that of Pd-only catalyst. The improvement of the TWC performance by the addition of basic promoters is discussed in terms of the stabilization of PdO, which may suppress the sintering, as well as of electron enrichment, which endows palladium with the Rh-like catalytic behavior.