Ce_1-xRu_xO_2-δ (x=0.05, 0.10): a new high oxygen storage material and Pt, Pd-free three-way catalyst

Abstract

Nanocrystalline Ce_1-xRu_xO_2-δ (x=0.05 and 0.10) of 8-10 nm sizes have been synthesized by hydrothermal method using melamine as complexing agent. Compounds have been characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray analysis (EDX) and their structures have been refined by the Rietveld method. The compounds crystallize in fluorite structure and the composition is Ce_1-xRu_xO_2-x/2 where Ru is in +4 state and Ce is in mixed-valence (+3, +4) state. Substitution of Ru⁴⁺ ion in CeO₂ activated the lattice oxygen. Ce_1-xRu_xO_2-x/2 reversibly releases 0.22[O] and 0.42[O] for x=0.05 and 0.10, respectively, which is higher than the maximum possible OSC of 0.22 [O] observed for Ce_0.50Zr_0.50O₂. Utilization of Higher OSC of Ce_1-xRu_xO_2-δ (x=0.05 and 0.10) is also reflected in terms of low-temperature CO oxidation with these catalysts, both in the presence and absence of feed oxygen. The Ru⁴⁺ ion acts as an active center for reducing molecules (CO, hydrocarbon "HC") and oxide ion vacancy acts as an active center for O₂ and NO, leading to low-temperature NO conversion to N₂. Thus due to Ru⁴⁺ ion, Ce_1-xRu_xO_2-δ is not just a high oxygen storage material but also shows high activity toward CO, hydrocarbon "HC" oxidation, and NO reduction by CO at low temperature with high N₂ selectivity for three-way catalysis.