Synthesis and decomposition mechanism of γ-MnC2O4•2H2O rods under non-isothermal and isothermal conditions

The thermolysis of the ill-known gamma-MnC2O4 center dot 2H(2)O is investigated applying DTA, TG, DSC, FT-IR, SEM, BET and Rietveld XRD phase analyses for thorough characterization of the starting compound and its calcined products. The dehydration proceeds in one step with Delta H = 149 kJ mol(-1) (reported for the first time). Primary decomposition product is Mn3O4 ([Mn2+Mn23+]O-4) with high specific surface area. During its formation, the oxidation of Mn3+ species to Mn4+ starts, followed by oxidation of Mn2+-Mn3+, thus leading to formation of metastable Mn5O8 ([Mn22+Mn34+]O-8) and Mn2O3. Before complete precursor decomposition under non-isothermal conditions, the reduction of Mn4+-Mn3+ occurs, forming an additional amount of Mn3O4. After isothermal annealing of gamma-MnC2O4 center dot 2H(2)O in the range of 573-823 K, Mn3O4 is detected as a major product in all samples. Mn5O8 is identified after calcination at 573, 673, 723 K, and its amount decreases gradually in this order. Cubic Mn2O3 appears after heat treatment at 673 K; its content varies between 13 and 23 % in the range of 673-773 K and becomes close to that of Mn3O4 at 823 K. The application of the same experimental conditions as in the investigation of alpha-MnC2O4 center dot 2H(2)O and MnC2O4 center dot 3H(2)O allows an objective comparison of the nature and the peculiarities of the thermal decomposition of different crystal forms of manganese oxalate.