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

A series of CoO x FeO x composite oxides with different Co content was prepared using the hydrothermal method. Low-temperature activity of CoO x FeO x composite oxides for CO oxidation was significantly improved by the addition of 50 atm % CoO x into FeO x . 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 CoFe 2 O 4 , are highly dispersed on the surface of CoFe 2 O 4 particles with a perimeter interface for the samples with 50 atm % CoO x . The surface valence state of CoO x FeO x composite oxides was found to be different by FT-IR spectroscopy following NO adsorption. CoO x FeO x with 50 atm % CoO x includes a relatively large amount of quasi-tetrahedrally coordinated Co 2+ sites on the surface, whereas that with 10 atm % CoO x consists of Fe 2+ sites on the surface. Temperature-programmed reduction by H 2 measurements suggested that CoO x FeO x with 50 atm % CoO x 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 Co 2+ sites was concluded to participate in CO oxidation reaction on CoO x FeO x composite oxides.