Thermal decomposition of cobalt carbonyl complexes in viscous media

The thermal decomposition of Co₂(CO)₈ to Co₄(CO)₁₂ and further to metallic cobait and CO under an inert atmosphere at temperatures <90 °C in hydrocarbon solutions and in the solid state have been studied and are documented in the literature. The mechanistic aspects of the solid state decomposition were similar to those found with the solution decomposition, but the kinetic aspects were quite different, since the rate constant, k_obs, found for the solid state reaction, was two orders of magnitude lower than the one found for the solution reaction. The decomposition reaction of cobalt carbonyls is primarily governed by diffusion. The diffusion of the cobalt carbonyls through a certain medium is strongly dependent on the viscosity of that medium. In a solution containing a polymeric system, the viscosity is a dominant property, since it is directly proportional to the concentration of the polymer in the solution. Therefore, the solution and solid state decompositions may be viewed as two extreme cases, in which the only variable is the viscosity of the polymeric solution. In the solution case, in the absence of polystyrene, the viscosity of the solution is essentially the viscosity of pure hydrocarbon solvent (toluene), while in the solid state case, the viscosity of the composite polystyrene film (the film containing the cobalt carbonyl complex) may be approximated as the viscosity of solid polystyrene just before its melting point. There are no studies to date which examine the effect of the viscosity of the solution of the cobalt carbonyl complexes on their thermal decomposition reaction in an inert atmosphere. Therefore, a study of the variation of the rate constants of the decomposition reactions as a function of the concentration of the polystyrene component in the cobalt carbonyl toluene solutions was undertaken. The reaction rates decreased with increasing polystyrene concentration, but only after a critical polymer concentration, c^*, which is the coil overlap concentration, was reached. The data obtained are reported here with conclusions concerning the mechanism of the thermal decomposition reaction of cobalt carbonyl complexes to produce zero-valent cobalt particles.