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

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 alpha and beta phases. Structural and thermal behaviours of all the Co hydroxides have been compared. Three major stages of decomposition are found: (i) 149-164 degrees C for dehydration of interlayer water, (ii) 185-197 degrees C and (iii) 219-222 degrees C for dehydroxylation of hydrotalcite- and brucite-like phases, respectively. Intercalated nitrate anions in hydrotalcite-like phases decompose largely during stage (ii). The oxide Co3O4 starts to form at temperatures as low as 165 degrees C especially for hydrotalcite-like phases. An intermediate compound, HCoO2, which is formed thermally, decomposes at 258-270 degrees C. The Co3O4 oxide converts into CoO at 842-858 and 935-948 degrees C respectively in nitrogen and air, which is much lower than the previously reported range of 1000-1200 degrees C. Surface areas of calcined samples are found to be proportional to the intercalated anion content. The catalytic activity of the resultant Co3O4 oxides with nitrous oxide is 7.2-8.2 mmol N2O g(-1) h(-1) at 375 degrees C, which is comparable to some reported active catalyst systems.