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

The selective formation of meta-stable FeO from ferrous sources by suppressing its oxidative conversion to the most stable hematite (α-FeO) is challenging under oxidative conditions for solid-state synthesis. In this work, we investigated the conversion of iron(II) chloride (FeCl) to magnetite (FeO) under inert atmosphere in the presence of steam, and the obtained oxides were analyzed by atomic-resolution TEM, Fe Mössbauer spectroscopy, and the Verwey transition temperature (). The reaction proceeded in two steps, with HO 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, substituting lattice chloride Cl with oxide O, 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 H through the oxidation of Fe 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 H 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 FeO 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) of 86 emu g and > 120 K without affording the thermodynamically stable α-FeO as the major final product. The study also discusses the influence of parameters such as reaction temperature, initial grain size of FeCl, the extent of hydration, and partial pressure of HO.