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

Abstract

The thermolysis of the ill-known c-MnC 2 O 4 Á 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 DH = 149 kJ mol -1 (reported for the first time). Primary decomposition product is Mn 3 O 4 ([Mn 2? Mn 2 3? ]O 4 ) with high specific surface area. During its formation, the oxidation of Mn 3? species to Mn 4? starts, followed by oxidation of Mn 2? -Mn 3? , thus leading to formation of metastable Mn 5 O 8 ([Mn 2 2? Mn 3 4? ]O 8 ) and Mn 2 O 3 . Before complete precursor decomposition under non-isothermal conditions, the reduction of Mn 4? -Mn 3? occurs, forming an additional amount of Mn 3 O 4 . After isothermal annealing of c-MnC 2 O 4 Á2H 2 O in the range of 573-823 K, Mn 3 O 4 is detected as a major product in all samples. Mn 5 O 8 is identified after calcination at 573, 673, 723 K, and its amount decreases gradually in this order. Cubic Mn 2 O 3 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 Mn 3 O 4 at 823 K. The application of the same experimental conditions as in the investigation of a-MnC 2 O 4 Á2H 2 O and MnC 2 O 4 Á3H 2 O allows an objective comparison of the nature and the peculiarities of the thermal decomposition of different crystal forms of manganese oxalate.