14. Donkova B., V. Petkova, Comparison between thermal behaviour of γ-MnC₂O₄.2H₂O in oxidative and inert media, Bulgarian Chemical Communications, (2015), Volume 47, Number 1, 185–189 IF=0.201

Abstract: Sparingly soluble manganese oxalate is an appropriate precursor for preparation of various nano-sized manganese oxides which explains the growing interest to its utilization and investigation of its thermolysis.

Recently, three crystal forms of manganese oxalate are known – monoclinic α-MnC₂O₄.2H₂O (SG C2/c), orthorhombic γ-MnC₂O₄.2H₂O (P2₁2₁2₁) and orthorhombic trihydrate MnC₂O₄.3H₂O (Pcca). While the mechanism and products of thermal decomposition of monoclinic dihydrate have been extensively studied by various methods, the literature data for the trihydrate and orthorhombic dihydrate are quite scarce.

The aim of the present study is to investigate and compare the decomposition mechanism of the slightly known γ-MnC₂O₄.2H₂O in oxidative and inert media under non-isothermal conditions with analysis of the evolved gases. Pure phase of pinkish orthorhombic dihydrate was obtained and characterized by X-ray diffraction and scanning electron microscopy. Le Bail whole pattern fitting analysis was performed in addition. The non-isothermal investigation was carried out in a static Air and Ar atmosphere (60 ml.min^-1) at a heating rate of 5^oC.min^-1 in the range of 25-800°C, using a coupling system TG-MS - instrument SETSYS2400 combined with an mass-spectrometer. According to TG-DTA-DTG curves, dehydration proceeds in the interval 140-187 °C in Ar and 140-182 °C in air. The corresponding intervals of decomposition are 335-434 °C in Ar and 230-361°C in air. In inert Ar atmosphere, CO and CO₂ are liberated simultaneously and MnO is obtained as a final product. In oxidative air atmosphere the evolved CO participate in additional oxy-reduction reactions and only CO₂ is detected. The decomposition is accompanied by oxidation of Mn(II) thus leading to formation of manganese oxides in higher and most probably mixed oxidation states. The investigations at other heating rates and calculation of the reaction enthalpy and activation energy of the processes under different atmospheres are in progress.