An investigation on the solid state pyrolytic decomposition of bimetallic oxalate precursors of Ca, Sr and Ba with cobalt: A mechanistic approach

Abstract

The mixed metal oxalate precursors, calcium(II)bis(oxalato)cobaltate(II)hydrate (COC), strontium(II)bis(oxalato)cobaltate(II)pentahydrate (SOC) and barium(II)bis(oxalato)cobaltate(II)octahydrate (BOC) have been synthesized and their thermal stability was investigated. The complexes were characterized by elemental analysis, IR spectral and X-ray powder diffraction studies. Thermal decomposition studies (TG, DTG and DTA) in air showed that the compound COC decomposed mainly to CaC 2O 4 and Co 3O 4 at 340 °C, and a mixture of CaCO 3 and Co 3O 4 identified at 510 °C. A mixture of CaCO 3 and Ca 3Co 2O 6 along with the oxides and carbides of both the cobalt and calcium were attributed at 1000 °C as end products. DSC study in nitrogen ascertained the formation of a mixture of CaO and CoO along with a trace of carbon at 550 °C. The mixture species, SrC 2O 4, CoC 2O 4 and Co 3O 4 were generated at 255 °C in case of SOC in air, which ultimately changed to CoSrO 3, SrCO 3 and oxides of strontium and cobalt at 1000 °C. The several mixture species also generated as intermediate at 332 and 532 °C. The DSC study in nitrogen indicated the formation of CoSrO x (0.5 < x < 1) as end product. In case of BOC in air, a mixture of BaCoO 2, BaO, CoO and carbides are identified as end product at 1000 °C through the generation of several intermediate species at 350 and 530 °C. A mixture of BaO and CoO is identified as end product in DSC study in nitrogen. The kinetic parameters have been evaluated for all the dehydration and decomposition steps of all the three compounds using four non-mechanistic equations. Using seven mechanistic equations, the kind of dominance of kinetic control mechanism of the dehydration and decomposition steps are also inferred. The kinetic parameters, Δ H and Δ S of all the steps are explored from the DSC studies. Some of the decomposition products are identified by IR and X-ray powder diffraction studies.