Kinetic and thermodynamic study of the non-isothermal decompositions of cobalt malonate dihydrate and of cobalt hydrogen malonate dihydrate

Non-isothermal dehydrations and decompositions of cobalt malonate dihydrate and of cobalt hydrogen malonate dihydrate were studied between ambient temperature and 500 degrees C. Different thermal analyses techniques (including TG, DTG, DTA and DSC) were used. The thermal reactions of both salts were performed in different dynamic atmospheres (40 ml/min) of N-2, H-2 or air. Cobalt hydrogen malonate was found to decompose at an appreciably lower temperature compared with cobalt malonate. Kinetic and thermodynamic parameters for the reactions of both reactant salts were calculated. IR spectroscopy of the partially decomposed samples showed that cobalt acetate and cobalt carbonate were formed as reaction intermediates during the breakdowns of both salts. This formation of cobalt acetate, a probable participant in the reaction, took place at an appreciably lower temperature for the cobalt hydrogen malonate reactant. X-ray diffraction revealed that the solid decomposition products were identical from both cobalt malonate and cobalt hydrogen malonate, under the same dynamic atmosphere. Cobalt metal was the major solid product in H-2 and N-2, while Co3O4 was the product in an atmosphere of air. Gas chromatography was used to identify the volatile decomposition products which included CO2, CO, acetic acid, acetone and traces of esters. It is concluded that, following dehydration, cobalt hydrogen malonate decomposes to cobalt malonate, and subsequent reactions were closely similar to those for this prepared reactant. Decomposition of cobalt malonate is identified as being controlled by an electron transfer step and proceeding at the active surface of the solid product. Cobalt metal is formed by reaction in hydrogen and cobalt oxide, Co3O4, in air. The slightly higher temperatures required for reaction in nitrogen are ascribed to deactivation of the product surface by deposited carbonaceous residues. (C) 2000 Elsevier Science B.V. All rights reserved.