Synchrotron Single-Crystal X-Ray Diffraction Studies of [Co(NH3)6](MnO4)3 and its Heat-Induced Quasi-Intramolecular Solid-phase Redox Reaction Transformation into Mn-Rich Co–Mn Spinel Photocatalysts

The manganese-rich permanganate salt [Co(NH3)6](MnO4)3 was prepared as a single-source precursor for Co–Mn oxide photocatalysts and investigated by synchrotron single-crystal X-ray diffraction, vibrational spectroscopy, thermal analysis, and catalytic tests. In contrast to earlier reports on a cubic unit cell, the low-temperature crystal structure is described by a tetragonal superstructure containing two crystallographically distinct permanganate environments, with extensive N–H···O hydrogen bonding, and modulated anion arrangements. Detailed spectroscopic studies (IR, UV, Raman) and correlation analyses were done, and the resonance Raman effect of n1 mode was identified. The thermal decomposition proceeds through a solid-phase quasi-intramolecular redox reaction between the coordinated ammonia and the permanganate anions, producing amorphous Co–Mn oxide intermediates, which crystallize to Mn-rich (CoMn=1:3) tetragonal spinel oxides with 4-24 nm crystallite sizes up to 500 °C. The decomposition pathway involves transient ammonium nitrate formation, and the resulting Co–Mn oxide products catalysed the UV-A-assisted degradation of Congo Red and Methyl orange with 85.7% and 39.2% efficiency in 240 min. The XPS studies showed that the photocatalytic activity is controlled primarily by the formation of an oxygen-rich oxide-oxyhydroxide surface rather than solely by the Co-Mn ratio.