High oxygen storage capacity and high rates of CO oxidation and NO reduction catalytic properties of Ce_1-xSn_xO₂ and Ce_0.78Sn_0.2Pd_0.02O_2-δ

Abstract

Ce_1-xSn_xO₂ (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 H₂/temperature programmed redution (TPR) studies. The cubic fluorite structure remained intact up to 50% of Sn substitution in CeO₂, and the compounds were stable up to 700 °C. Oxygen storage capacity of Ce_1-xSn_xO₂ was found to be much higher than that of Ce_1-xZr_xO₂ due to accessible Ce⁴⁺/Ce³⁺ and Sn⁴⁺/Sn²⁺ redox couples at temperatures between 200 and 400 °C. Pd²⁺ ions in Ce_0.78Sn_0.2Pd_0.02O_2-δ are highly ionic, and the lattice oxygen of this catalyst is highly labile, leading to low temperature CO to CO₂ conversion. The rate of CO oxidation was 2 μmol g^-1 s^-1 at 50 °C. NO reduction by CO with 70% N₂ selectivity was observed at 200 °C and 100% N₂ selectivity below 260 °C with 1000-5000 ppm NO. Thus, Pd²⁺ ion substituted Ce_1-xSn_xO₂ is a superior catalyst compared to Pd²⁺ ions in CeO₂, Ce_1-xZr_xO₂, and Ce_1-xTi_xO₂ for low temperature exhaust applications due to the involvement of the Sn²⁺/Sn⁴⁺ redox couple along with Pd²⁺/Pd⁰ and Ce⁴⁺/Ce³⁺ couples.