KINETICS OF THE THERMAL DEHYDRATIONS AND DECOMPOSITIONS OF SOME MIXED-METAL OXALATES

Both isothermal and programmed temperature experiments have been used to obtain kinetic parameters for the dehydrations and the decompositions in nitrogen of the mixed metal oxalates: FeCu(ox)2.3H2O, CoCu(ox)2.3H2O and NiCu(ox)2.3.5H2O, [ox = C2O4]. Results are compared with those reported for the thermal decompositions of the individual metal oxalates, Cuox, Coox.2H2O, Niox.2H2O and Feox.2H2O. X-ray pbotoelectron spectroscopy (XPS) was also used to examine the individual and the mixed oxalates. Dehydrations of the mixed oxalates were mainly deceleratory processes with activation energies (80 to 90 kJ.mol-1), similar to those reported for the individual hydrated oxalates. Temperature ranges for dehydration were broadly similar for all the hydrates studied here (130 to 180-degrees-C). Decompositions of the mixed oxalates were all complex endothermic processes with no obvious resemblance to the exothermic reaction of Cuox, or the reactions of physical mixtures of the corresponding individual oxalates. The order of decreasing stability, as indicated by the temperature ranges giving comparable decomposition rates, was NiCu(ox)2 > CoCu(ox)2 > FeCu(ox)2, which also corresponds to the order of increasing covalency of the Cu-O bonds as shown by XPS.