ABSTRACT
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 (Tv). 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 O2-, 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 FeII 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 (σs) of 86 emu g-1 and Tv > 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.
ABSTRACT
The stability constants of metal(M)-ligand(L) complexes are industrially important because they affect the quality of the plating film and the efficiency of metal separation. Thus, it is desirable to develop an effective screening method for promising ligands. Although there have been several machine-learning approaches for predicting stability constants, most of them focus only on the first overall stability constant of M-L complexes, and the variety of cations is also limited to less than 20. In this study, two Gaussian process regression models are developed to predict the first overall stability constant and the n-th (n > 1) overall stability constants. Furthermore, the feature relevance is quantitatively evaluated via sensitivity analysis. As a result, the electronegativities of both metal and ligand are found to be the most important factor for predicting the first overall stability constant. Interestingly, the predicted value of the first overall stability constant shows the highest correlation with the n-th overall stability constant of the corresponding M-L pair. Finally, the number of features is optimized using validation data where the ligands are not included in the training data, which indicates high generalizability. This study provides valuable insights and may help accelerate molecular screening and design for various applications.
ABSTRACT
We have studied the influence of low concentrations (0.1 M) of the ionic liquid 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) on suppressing the hydrogen evolution reaction (HER) using polycrystalline Ag, Cu, and Fe electrodes in aqueous acidic and basic media. HER suppression is generally desired when aiming to catalyze other reactions of interests, e.g., CO2 electro-reduction. Cyclic voltammetry and chronoamperometry measurements were performed at potentials between -0.2 and -0.8 V versus the reversible hydrogen electrode (RHE) to investigate HER activity in a simulated CO2 electrolysis environment without the CO2. In an acidic electrolyte, a decrease in HER activity was observed for all three electrodes with the largest effect being that of Fe, where the HER activity was suppressed by 75% at -0.5 V versus RHE. In contrast to the effect of [EMIM]Cl in an acidic electrolyte, no HER suppression was observed in basic media. Using 1H nuclear magnetic resonance spectroscopy on the electrolyte before and after electrolysis, it was determined that [EMIM]Cl breaks down at both the working and counter electrodes under reaction conditions under both acidic and basic conditions. These results underscore the challenges in employing ionic liquids for electrochemical reactions such as CO2 reduction.
