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

The selective formation of meta-stable Fe 3 O 4 from ferrous sources by suppressing its oxidative conversion to the most stable hematite (α-Fe 2 O 3 ) is challenging under oxidative conditions for solid-state synthesis. In this work, we investigated the conversion of iron(II) chloride (FeCl 2 ) to magnetite (Fe 3 O 4 ) under inert atmosphere in the presence of steam, and the obtained oxides were analyzed by atomic-resolution TEM, 57 Fe Mössbauer spectroscopy, and the Verwey transition temperature ( T v ). The reaction proceeded in two steps, with H 2 O 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 H 2 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 H 2 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 Fe 2 O 3 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-Fe 2 O 3 as the major final product. The study also discusses the influence of parameters such as reaction temperature, initial grain size of FeCl 2 , the extent of hydration, and partial pressure of H 2 O.