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

Research was carried out on the incorporation of divalent cobalt cations into the crystalline structure of MgF₂ to form Mg_xCo_1−xF₂ 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_xCo_1−xF₂ 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_xCo_1−xF₂ 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.

Results of the first research on structural and surface properties of binary fluorides MgF₂–CoF₂ are presented.

Nanoscale metal fluorides: a new class of heterogeneous catalysts

Nanoscale metal fluorides and hydroxide fluorides prepared according the fluorolytic sol–gel synthesis represent a powerful class of bi-acidic heterogeneous catalysts.

Investigation of stainless steel pickling liquor as a precursor for high Capacity battery electrode materials

Stainless steel and battery manufacturing industries can be united for mutual economic and environmental benefits by utilizing stainless steel pickling liquor waste as a precursor for high energy iron fluoride based positive electrode materials in batteries.

Surface Characterization of Chromia for Chlorine/Fluorine Exchange Reactions

The dismutation of CCl(2)F(2) was used to probe the effect of halogenation of chromia by Cl/F exchange reactions to find out the difference between the halogenated inactive and active catalysts. The heterogeneous reactions were performed in a continuous flow Ni reactor and also under simulated reaction conditions in a reactor where after the reaction X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES) analyses are possible without air exposure of the catalyst, i.e., under so-called "in situ" conditions. The Cr(III) 2p XP spectra, which revealed multiplet splitting features and satellite emission, were used for chemical analysis by using a simple evaluation procedure which neglects this inherent complexity. Chemical analysis was also applied by using chemical state plots for Cr 3s in order to cross-check Cr 2p related results. Both ex and in situ XPS show that as soon as Cr(2)O(3) is exposed to CCl(2)F(2) at 390 degrees C fluorination as well as chlorination takes place at the catalyst surface. When the XPS surface composition reaches approximately 4 at. % fluorination and 6 at. % chlorination, maximum catalytic activity was obtained. Application of longer reaction times did not change significantly the obtained surface composition of the activated chromia. The fluorination and chlorination of chromia was further investigated by various HF and HCl treatments. The activated chromia samples and the Cr(2)O(3), Cr(OH)(3), CrF(2)OH, CrF(3) x H(2)O, alpha-CrF(3), beta-CrF(3), and CrCl(3) reference samples with well-known chemical structures were also characterized by X-ray absorption near edge structure (XANES), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), pyridine-FTIR, wet chemical (F and Cl) analysis, X-ray powder diffraction (XRD), and surface area (BET) analysis. The results suggest that the formation of chromium oxide chloride fluoride species, e.g., chromium oxide halides, at the surface is sufficient to provide catalytic activity. The presence of any CrF(3) and/or CrCl(3) phases on the activated chromia samples was not found.