Preparation And Catalytic Properties Of Iron Silicides By Direct Silification Method

Transition metal silicides is a kind of compound which have special physical and chemical properties, with high melting point, low resistivity, good electrical conductivity. It has been reported as a kind of new catalytic materials, and attracted more and more attention. Therefore, the controlled synthesis of high surface area metal silicides has become the focus of chemical researchers.In this paper, it has been successfully synthesized the iron silicides by direct silification method. The as-prepared samples were characterized by N2physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature programmed reduction of H2(H2-TPR), X-ray photoelectron spectroscopy (XPS), Mossbauer spectroscopy, and superconducting quantum interference magnetometer (SQUID). The catalytic property of dibenzothiophene hydrodesulfurization was also investigated on the iron silicides catalysts. Further thermal annealing method was applied to study the crystal transformation of FeSi2-540sample. These results showed that:(1) Iron silicides has been successfully synthesized by direct silane silification of Fe2O3nanoparticles. The XRD, TEM, XPS, Mossbauer results indicate the formation of FeSi2. TEM and XPS results indicate there is a thin shell layer of SiO2when the FeSi2sample exposed to the air. The SiO2layer can protect the core against oxidation and can be removed by etching in dilute HF solution. The magnetic results showed that all samples have ferromagnetic nature and the saturation magnetization values drastically decreased with increasing silicification temperature. This novel methodology is currently being applied to synthesis of CoxSi@SiO2and NixSi@Si02with core@shell structure.(2) In order to study the catalytic properties of transition metal silicides, the hydrodesulfurization (HDS) of dibenzothiophene (DBT) was performed. the catalytic activity was compared on the iron silicides and iron. The iron silicides catalysts proved to be more active in the HDS reactions than the iron catalyst. The iron silicides had higher catalytic activity, stability and sulfur resistance in the HDS reactions, indicating the silicon modified the electronic form and crystal structure of iron.(3) The thermal annealing method was used to study the phase crystal transformation of FeSi2-540sample. When the FeSi2-540sample was exposed to the air, we further treated it in the H2atmosphere for2h at600℃and700℃, respectively. There is not a phase crystal transformation during the heat treatment process, which may because the thin SiO2layer inhibit the diffusion of H2into the core. When the FeSi2-540sample was not exposed to the air, there is a phase crystal transformation during the annealing treatment in the Ar and H2atmosphere at600and700℃, respectively. It was found that, the reaction kinetics is mostly governed by diffusion of Fe and Si. The crystal transformation process is determined by the annealing time and temperature. If we control the annealing time and temperature more precisely, it would be possible to systematically synthesize FexSi materials.