Synthesis and characterization of MgF2-CoF2 binary fluorides. Influence of the treatment atmosphere and temperature on the structure and surface properties

Abstract

Research was carried out on the incorporation of divalent cobalt cations into the crystalline structure of MgF₂ to form Mg _x Co_1-x F₂ binary fluorides, which had not been investigated before. The above fluorides were obtained by the precipitation from aqueous solution of magnesium and cobalt nitrates with ammonium fluoride. Binary fluorides containing 0.6, 7.5 and 37.7 mol% CoF₂ were prepared. The effects of treatment temperature (300, 400 °C) and atmosphere (oxidizing or reducing) on the structure (XRD, TPR-H₂, UV-Vis), texture (low-temperature N₂ adsorption), surface composition (XPS) and surface acidity (NH₃-TPD) of the binary fluorides were determined. It has been found that in Mg _x Co_1-x F₂ an isomorphic substitution occurs of Mg²⁺ cations by Co²⁺ cations which results in the formation of a rutile-type solid solution. The obtained binary fluorides are characterized by a mesoporous structure and relatively large surface area. It has been found that thermal treatment of the binary fluorides in oxidizing conditions results in the oxidation of CoF₂ to Co₃O₄ even at 300 °C; therefore it is not possible to obtain pure Mg _x Co_1-x F₂ binary fluorides in the presence of air. The preparation of the latter requires reducing conditions, namely thermal treatment of dry precipitate at 300 °C in an atmosphere of hydrogen. If the treatment is conducted at a higher temperature (400 °C), CoF₂ undergoes a partial reduction to metallic cobalt. An XPS study has shown the presence of hydroxyl groups in the investigated samples. However, these are solely surface groups because their presence was not detected by XRD measurements. The binary fluorides obtained by our method are characterized by a very narrow optical energy gap (5.31-3.50 eV), considerably narrower than that recorded for bulk fluorides. Measurements of temperature-programmed desorption of ammonia have shown that the incorporation of cobalt cations into the crystal structure of MgF₂ results in a decrease in the surface acidity of the binary fluorides.