Pietrowski Mariusz, Zieliński Michał, Alwin Emilia, Suchora Agata, Gawarecka Joanna
Adam Mickiewicz University in Poznań, Faculty of Chemistry Umultowska 89B 61-614 Poznan Poland
RSC Adv. 2019 Feb 15;9(10):5711-5721. doi: 10.1039/c8ra09365b. eCollection 2019 Feb 11.
Research was carried out on the incorporation of divalent cobalt cations into the crystalline structure of MgF to form Mg Co F 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% CoF were prepared. The effects of treatment temperature (300, 400 °C) and atmosphere (oxidizing or reducing) on the structure (XRD, TPR-H, UV-Vis), texture (low-temperature N adsorption), surface composition (XPS) and surface acidity (NH-TPD) of the binary fluorides were determined. It has been found that in Mg Co F an isomorphic substitution occurs of Mg cations by Co 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 CoF to CoO even at 300 °C; therefore it is not possible to obtain pure Mg Co F 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), CoF 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 MgF results in a decrease in the surface acidity of the binary fluorides.
开展了关于将二价钴阳离子掺入MgF晶体结构以形成Mg-Co-F二元氟化物的研究，此前尚未对其进行过研究。上述氟化物通过用氟化铵从硝酸镁和硝酸钴的水溶液中沉淀而获得。制备了含0.6、7.5和37.7 mol% CoF的二元氟化物。测定了处理温度（300、400℃）和气氛（氧化或还原）对二元氟化物的结构（XRD、TPR-H、UV-Vis）、织构（低温N吸附）、表面组成（XPS）和表面酸度（NH-TPD）的影响。已发现，在Mg-Co-F中，Mg阳离子被Co阳离子发生同晶取代，这导致形成金红石型固溶体。所获得的二元氟化物具有介孔结构和相对较大的表面积。已发现，即使在300℃下，在氧化条件下对二元氟化物进行热处理也会导致CoF氧化为CoO；因此，在空气中不可能获得纯的Mg-Co-F二元氟化物。制备后者需要还原条件，即在300℃的氢气气氛中对干燥沉淀物进行热处理。如果在更高温度（400℃）下进行处理，CoF会部分还原为金属钴。XPS研究表明，在所研究的样品中存在羟基。然而，这些仅是表面基团，因为通过XRD测量未检测到它们的存在。通过我们的方法获得的二元氟化物的特征在于非常窄的光学能隙（5.31-3.50 eV），比块状氟化物记录的能隙窄得多。氨程序升温脱附测量表明，将钴阳离子掺入MgF晶体结构会导致二元氟化物的表面酸度降低。
