Tailoring the properties and the reactivity of the spinel cobalt oxide

Abstract

Pulsed spray evaporation chemical vapor deposition (PSE-CVD) was employed for the synthesis of cobalt-based spinel oxide thin films, Co_3−xFe_xO₄ with x = 0–1.56. XRD, Raman scattering and FTIR emission spectroscopy show that the normal spinel structure was retained for 0 ≤x≤ 0.65 by the selective insertion of Fe³⁺ in the octahedral sites. The spinel inversion was noticed above this range, whereas the insertion of Fe²⁺ was first indicated with x≥ 1. The room-temperature electrical resistivity of the thin films was controlled between 9 and 0.007 Ω cm by the adjustment of iron doping concentration. Furthermore an improvement of the thermal stability of the spinel was noticed upon doping by iron. The reducibility of the spinel in the presence of molecular hydrogen was efficiently adjusted by a shift of the reduction temperature by up to 110 °C upon the controlled insertion of iron in the octahedral sites of the spinel. The investigation of the catalytic oxidation of CO and ethanol over Co_3−xFe_xO₄ films with controlled structural modification enabled the confirmation of the Mars–van Krevelen mechanism for the oxidation of CO and the tight correlation between the selectivity of the conversion of ethanol to acetaldehyde and the abundance of surface basic sites. The controlled iron doping was demonstrated to be an efficient strategy to tune the reactivity and the selectivity of the cobalt-based spinel oxide. The doping-induced transition from normal to inverse spinel was observed to induce a clear discontinuity in the trend of all investigated physicochemical properties.