Kinetics and Microstructural Evolution during High-Temperature Hydrogen Reduction of Cobalt(II) Oxide

Abstract

The high-temperature hydrogen reduction of unsupported cobalt(II) oxide (CoO) was investigated using an electrothermography-based approach that enables rapid heating, precise isothermal control, and fast quenching. A dense CoO layer formed on cobalt substrates was reduced isothermally at 1123–1573 K under low hydrogen pressures (0.667–6.67 kPa), providing access to reduction regimes not attainable using conventional techniques. The reduction follows an Avrami–Erofeev nucleation-and-growth model over the entire range of experimental conditions. Increasing temperature induces a transition from three-dimensional growth associated with heterogeneous internal reduction to predominantly two-dimensional growth characterized by the rapid formation of a continuous cobalt shell and inward propagation of a sharp reaction front. The reduction kinetics are strongly dependent on hydrogen pressure, with apparent activation energies of 137 ± 20 and 27.6 ± 4 kJ/mol at hydrogen pressures of 0.667 and 6.67 kPa, respectively, indicating a change in the rate-limiting step with increasing hydrogen availability. The phase composition of the reduced cobalt is governed by both the reduction conditions and the postreduction thermal history, with rapid quenching stabilizing metastable γ-Co and slower cooling promoting the formation of thermodynamically stable ε-Co. These findings provide new insights into the mechanisms of high-temperature CoO reduction and practical guidance for controlling cobalt polymorphism during hydrogen-based processing.