Thermal decomposition of hexaamminecobalt (III) dibromide permanganate: Formation of cobalt-manganese oxide spinel and catalytically active intermediates

The first-time controlled thermal decomposition of synthesized hexaamminecobalt (III) dibromide permanganate [Co(NH3)6]Br2(MnO4) was studied as a precursor for Co–Mn composite. TG-MS results revealed redox interactions, which were further confirmed by XRD, IR and Raman analysis of the solid-phase and solvent decomposition residues. Solid State thermal decomposition of the compound up to 500 °C under air and inert atmospheres produced intermediates and products with the Co: Mn atomic ratio of 1:1. In particular, the decomposition product at 500 °C under air atmosphere, exhibited Co1.5Mn1.5O4 spinel structure, whereas inert conditions resulted in bromide retention and formation of different crystalline phases, as confirmed by SEM-EDX. Analysis of the partially cooled intermediate formed at 300 °C, indicated the presence of hydrated cobalt (II) bromide whereas at 500 °C, cobalt (II) oxide emerged as the dominant crystalline phase. Further investigation into the catalytic activity of the decomposition intermediates and products exhibited their effective catalytic performance in the degradation of Congo Red dye under UV light. We examined the effect of pH on the degradation process at values above and below the dye's pKa. Air-derived samples generally exhibited faster degradation rates at pH 5.2 than at pH 3.6. The intermediate formed at 155 °C showed the highest activity, degrading Congo Red 26.1 times faster. At the optimal dosage, Co1.5Mn1.5O4 showed a moderate degradation rate of 6.8. In contrast, nitrogen-derived samples performed significantly better at pH 3.6, with the intermediates formed at 300 °C and 500 °C degrading Congo Red 12.5 and 22.7 times faster, respectively. © 2025 The Author(s)