Solid-State Schikorr Reaction from Ferrous Chloride to Magnetite with Hydrogen Evolution as the Kinetic Bottleneck

The selective formation of meta-stable Fe₃O₄ from ferrous sources by suppressing its oxidative conversion to the most stable hematite (α-Fe₂O₃) is challenging under oxidative conditions for solid-state synthesis. In this work, we investigated the conversion of iron(II) chloride (FeCl₂) to magnetite (Fe₃O₄) under inert atmosphere in the presence of steam, and the obtained oxides were analyzed by atomic-resolution TEM, ⁵⁷Fe Mössbauer spectroscopy, and the Verwey transition temperature (T_v). The reaction proceeded in two steps, with H₂O as the oxide source in the initial step and as an oxidant in the second step. The initial hydrolysis occurred at temperatures higher than 120 °C to release gaseous HCl, via substituting lattice chloride Cl^- with oxide O^2-, to give iron oxide intermediates. In the first step, the construction of the intermediate oxides was not topotactic. The second step as a kinetic bottleneck occurred at temperatures higher than 350 °C to generate gaseous H₂ through the oxidation of Fe^II by H⁺. A substantially large kinetic isotope effect (KIE) was observed for the second step at 500 °C, and this indicates the rate-determining step is the hydrogen evolution. Quantitative analysis of evolved H₂ revealed that full conversion of ferrous chloride to magnetite at 500 °C was followed by additional oxidation of the outer sphere of magnetite to give a Fe₂O₃ phase, as supported by X-ray photoelectron spectroscopy (XPS), and the outer phase confined the conductive magnetite phase within the insulating layers, enabling kinetic control of magnetite synthesis. As such, the reaction stopped at meta-stable magnetite with an excellent saturation magnetization (σ_s) of 86 emu g^-1 and T_v > 120 K without affording the thermodynamically stable α-Fe₂O₃ as the major final product. The study also discusses the influence of parameters such as reaction temperature, initial grain size of FeCl₂, the extent of hydration, and partial pressure of H₂O.