High Oxygen Storage Capacity and High Rates of CO Oxidation and NO Reduction Catalytic Properties of Ce_1−<i>x</i>Sn_<i>x</i>O₂ and Ce_0.78Sn_0.2Pd_0.02O_2-δ

Abstract

Ce 1− x Sn x O 2 ( x = 0.1−0.5) solid solution and its Pd substituted analogue have been prepared by a single step solution combustion method using tin oxalate precursor. The compounds were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and H 2 /temperature programmed redution (TPR) studies. The cubic fluorite structure remained intact up to 50% of Sn substitution in CeO 2, and the compounds were stable up to 700 °C. Oxygen storage capacity of Ce 1− x Sn x O 2 was found to be much higher than that of Ce 1− x Zr x O 2 due to accessible Ce 4+ /Ce 3+ and Sn 4+ /Sn 2+ redox couples at temperatures between 200 and 400 °C. Pd 2+ ions in Ce 0.78 Sn 0.2 Pd 0.02 O 2-δ are highly ionic, and the lattice oxygen of this catalyst is highly labile, leading to low temperature CO to CO 2 conversion. The rate of CO oxidation was 2 μmol g −1 s −1 at 50 °C. NO reduction by CO with 70% N 2 selectivity was observed at ∼200 °C and 100% N 2 selectivity below 260 °C with 1000−5000 ppm NO. Thus, Pd 2+ ion substituted Ce 1− x Sn x O 2 is a superior catalyst compared to Pd 2+ ions in CeO 2, Ce 1− x Zr x O 2, and Ce 1− x Ti x O 2 for low temperature exhaust applications due to the involvement of the Sn 2+ /Sn 4+ redox couple along with Pd 2+ /Pd 0 and Ce 4+ /Ce 3+ couples.