Structure–activity relation of spinel-type Co–Fe oxides for low-temperature CO oxidation

Abstract

A series of cobalt ferrite thin films was prepared via pulsed spray evaporation chemical vapour deposition (PSE-CVD). The samples were comprehensively characterised in terms of structure, surface, morphology, and optical and redox properties. Both X-ray diffraction (XRD) and Raman analysis show that all samples exhibited an inverse spinel structure. X-ray photoelectron spectroscopy (XPS) results indicate that the films were mainly composed of Co, Fe and O species, and an increase in the Co : Fe ratio with Fe substitution by Co was observed. Helium ion microscopy (HIM) images show the film morphology to be dependent on the Co : Fe ratio. The investigation of the optical property using ultraviolet-visible spectroscopy reveals that the increase in the Co content results in an increase in the band gap energy. In situ emission FTIR spectroscopy was used to evaluate the redox properties of the samples, and a shift of the redox temperature to higher values was observed upon increase in the Co content. The effect of Fe substitution by Co in the mixed oxide systems on their catalytic performance for CO oxidation was investigated. Co–Fe oxides exhibit substantially better catalytic performance than the single α-Fe₂O₃. The catalytic performance of the Co–Fe oxides towards CO oxidation was discussed with respect to the participation of surface and lattice oxygen in the oxidation process. According to XPS and temperature-programmed reduction/oxidation (TPR/TPO) results, a suprafacial mechanism where CO molecules react with surface-adsorbed oxygen functions to form CO₂ was proposed as the dominant mechanism.