Thermodynamic properties of Co(OH)2 from hydrothermal equilibria in cobalt oxide systems

Abstract

The preparative aspects of cobaltous and cobalitic oxides have been the subject of extensive investigations by various workers [1-6] because of their numerous applications in the field of ceramic, magnetic materials, catalysis and even in the extraction of cobalt metal. However, all these studies were confined to either 1 atm pressure or under vacuum conditions. Because hydro- thermal method has not so far been explored for the stabilization of CoO, a systematic study of the systems CoO HzO and CO304-H20 were taken up. Taking advantage of the experimental data, it was possible to determine the standard thermodynamic properties of Co(OH)z. The starting material, Co(OH)z , was precipitated by hydrolysis of cobalt nitrate solution with aqueous ammonia at room temperature. The precipitate during filtration was washed well with hot water in order to remove the possible nitrate ion contamination. The precipitate was dried in an air oven at 90°C and was confirmed to be Co(OH)2 by X-ray diffraction. Co304 was obtained by decomposing Co(OH)z in air at 350 ° C. Hydrothermal runs were carried out using the tempress unit HR-1B-4 fitted with stellite test-tube reactors. The starting material, Co(OH)2 or C0304, was placed in a platinum ampoule (5cm × 5mm diameter, 0.1 mm wall thickness) along with a few drops of distilled water and the ampoule was sealed. The ampoule was checked for leaks by extended heating at 100 ° C. The duration of runs ranged between 48 and 60h. The upper pressure and temperature limits used in the present study were 2000 bar and 800 ° C, respectively. The phase diagrams for the system CoO- HzO and Co304-HzO are given in Figs 1 and 2 and the cor- responding data of experimental points are given in Table I. In the system CoO-HzO, the cobalt hydroxide decomposes to cobaltous oxide at considerably lower temperature (i.e. at 2000 bar it decomposes at 230 ° C), whereas in the Co3 O4-HzO system the decomposition to cobaltous oxide takes place at relatively high temperature (at 200bar it decomposes at 675 °C). However, it may be noted that the hydrothermal stabilization of CoO was possible at much lower tem- peratures than those reported from the decomposition studies in air. It is well known that the decomposition of Co(OH)2 to CoO in air proceeds through an inter- mediate phase of Co304. Furthermore, the decompo- sition of Co3 04 to CoO in air was found to be above 800°C and that under vacuum was at 350 ° C. The decomposition of Co(OH)2 directly to CoO was, how- ever, possible under vacuum conditions even at 140 ° C [5]. We first thought that the low temperature for the hydrothermal stabilization of CoO was due to hydro- gen diffusion through the platinum ampoule, as was previously reported for hydrothermal synthesis of Fe304 [7, 8]. The hydrogen liberated from the reaction of HzO with the metal of the reactor penetrates through the platinum tube. But this possibility was eliminated by conducting two experiments in Co(OH)z-HzO systems using quartz tube (375 ° C and 1000 bar) as well as a teflon-lined low-pressure bomb (250°C and 100bar) wherein the product was CoO and not