Kinetics of Decomposition of Cobalt Coordination Complexes on Montmorillonite Surfaces

Abstract

Montmorillonite may adsorb cobalt(III)hexammine and cobalt(III)chloropentammine cations, and the C.E.C. determined for these complex cations is of the order of magnitude of that obtained with ammonium. The infrared spectra of these coordination complexes adsorbed by the clay are similar to those observed for the corresponding chloride salts. However, when montmorillonite treated with these coordination complexes is dehydrated in vacuum or under moderate temperature conditions (<100°C) in a dry atmosphere, the infrared spectra are deeply modified. The complex cations decompose with NH₃ evolution and ammonium formation. Cobalt(II)hydroxide is produced and ammonium cations balance the lattice electrical charge. The true decomposition rate was followed by IR absorbance of NH₄ which showed that the complex decomposition followed that for the nomothetic decomposition of solid particles. The apparent activation energy of the decomposition was 20.5 and 43 kcal/mole for the CoCl(NH₃)₅²⁺- and Co(NH₃)₆⁸⁺-montmorillonite complexes respectively. An analysis of the combined chemical and IR data on the hexammine complex permitted calculation of the NH₃ diffusion rate between collapsed montmorillonite sheets. When the interplanar gap approaches 1Å the diffusion coefficient is approximately 7 × 10^-19 cm²/sec. In contradiction to their stability in solutions, the cobalt(III)hexammine cation is less stable than the cobalt(III)chloropentammine cation when absorbed on montmorillonite.