A kinetic study of the phase conversion of layered cobalt hydroxides

A kinetic study of the phase conversions of three layered alpha-type cobalt hydroxides, two pink forms {Co(OH)(1.4)(NCO)(0.6)center dot 0.6H(2)O, Co(OH)(1.5)(NO(3))(0.5)center dot 0.6H(2)O} and a green form {Co(OH)(1.6)Cl(0.4)center dot 0.4H(2)O} to a brucite-like beta-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)center dot 0.6H(2)O to beta-Co(OH)(2) follows a two-dimensional diffusion-controlled model while the conversion of Co(OH)(1.6)Cl(0.4)center dot 0.4H(2)O to beta-Co(OH)(2) is best modelled by a phase-boundary-controlled process. The in situ SAXS/WAXS experiments on the conversion of Co(OH)(1.6)Cl(0.4)center dot 0.4H(2)O to beta-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)(NO(3))(0.5)center dot 0.6H(2)O to beta-Co(OH)(2) proceeds via an intermediate crystalline phase, which is thought to be an alpha-cobalt hydroxide with an expanded interlayer separation containing tetrahedrally coordinated Co(2+).