A kinetic study of the phase conversion of layered cobalt hydroxides

A kinetic study of the phase conversions of three layered α-type cobalt hydroxides, two pink forms {Co(OH)1.4(NCO)0.6·0.6H2O, Co(OH)1.5(NO3)0.5·0.6H2O} and a green form {Co(OH)1.6Cl0.4·0.4H2O} to a brucite-like β-Co(OH)2 in the presence of NaOH have been investigated using time-resolved, in situ energy-dispersive X-ray diffraction (EDXRD) and time-resolved, in situ simultaneous small/wide angle X-ray scattering (SAXS/WAXS). The kinetic data have been modelled using the Avrami–Erofe'ev model. This analysis suggests that conversion of Co(OH)1.4(NCO)0.6·0.6H2O to β-Co(OH)2 follows a two-dimensional diffusion-controlled model while the conversion of Co(OH)1.6Cl0.4·0.4H2O to β-Co(OH)2 is best modelled by a phase-boundary-controlled process. The in situ SAXS/WAXS experiments on the conversion of Co(OH)1.6Cl0.4·0.4H2O to β-Co(OH)2 suggests that an Ostwald ripening model is obeyed during the final stages of the reaction. The transformation from Co(OH)1.5(NO3)0.5·0.6H2O to β-Co(OH)2 proceeds via an intermediate crystalline phase, which is thought to be an α-cobalt hydroxide with an expanded interlayer separation containing tetrahedrally coordinated Co2+.