CoOx-FeOx composite oxide prepared by hydrothermal method as a highly active catalyst for low-temperature CO oxidation

A series of CoOx-FeOx composite oxides with different Co content was prepared using the hydrothermal method. Low-temperature activity of CoOx-FeOx composite oxides for CO oxidation was significantly improved by the addition of 50 atm% CoOx into FeOx. Further increase in Co content up to 70 atm% caused a decrease in low-temperature activity. Structural characterizations by X-ray diffraction, transmission electron microscope and Fourier transform infrared (FT-IR) revealed that Co species, which were not substituted with Fe sites to form CoFe2O4, are highly dispersed on the surface of CoFe2O4 particles with a perimeter interface for the samples with 50 atm% CoOx. The surface valence state of CoOx-FeOx composite oxides was found to be di(f)ferent by FT-IR spectroscopy following NO adsorption. CoOx-FeOx with 50 atm% CoOx includes a relatively large amount of quasi-tetrahedrally coordinated Co2+ sites on the surface, whereas that with 10 atm% CoOx consists of Fe2+ sites on the surface. Temperature-programmed reduction by H-2 measurements suggested that CoOx-FeOx with 50 atm% CoOx possesses the largest amount of active oxygen species, which can be reduced by H-2 in the lower temperature region. The surface oxygen species of which the formation is related to the presence of quasi-tetrahedrally coordinated Co2+ sites was concluded to participate in CO oxidation reaction on CoOx-FeOx composite oxides. (C) 2017 The Ceramic Society of Japan. All rights reserved.