Thermal evolution of cobalt hydroxides: a comparative study of their various structural phases

Abstract

Through an atmosphere-controlled method, a new phase of hydrotalcite-like Co hydroxide with mixed valent states has been synthesised, along with preparations of known α and β phases. Structural and thermal behaviours of all the Co hydroxides have been compared. Three major stages of decomposition are found: (i) 149-164°C for dehydration of interlayer water, (ii) 185-197°C and (iii) 219-222°C for dehydroxylation of hydrotalcite- and brucite-like phases, respectively. Intercalated nitrate anions in hydrotalcite-like phases decompose largely during stage (ii). The oxide Co ₃ O ₄ starts to form at temperatures as low as 165°C especially for hydrotalcite-like phases. An intermediate compound, HCoO ₂ , which is formed thermally, decomposes at 258-270°C. The Co ₃ O ₄ oxide converts into CoO at 842-858 and 935-948°C respectively in nitrogen and air, which is much lower than the previously reported range of 1000-1200°C. Surface areas of calcined samples are found to be proportional to the intercalated anion content. The catalytic activity of the resultant Co ₃ O ₄ oxides with nitrous oxide is 7.2-8.2 mmol N ₂ O g ^–1 h ^–1 at 375°C, which is comparable to some reported active catalyst systems.