Thermal Decomposition of Ferric Oxalate Tetrahydrate in Oxidative and Inert Atmospheres: The Role of Ferrous Oxalate as an Intermediate

Abstract

The thermal decomposition of ferric oxalate tetrahydrate Fe2(C2O4)3·4H2O was studied in dynamic oxidative and inert atmospheres by using a simultaneous thermogravimetric (TG) and differential scanning calorimetric (DSC) analytical device equipped with an evolved gas analyzer (EGA). Solid‐state decomposition products formed during the decomposition were analyzed by 57Fe Mössbauer spectroscopy, in situ and ex situ X‐ray powder diffraction, and magnetic measurements. In the dynamic inert atmosphere, we observed the formation of a tiny amount of superparamagnetic iron oxide (most likely Fe3O4) together with a majority of ferrous oxalate (FeC2O4) and remains of undecomposed Fe2(C2O4)3 after the first decomposition step, which finished at 210 °C. The astonishing presence of the oxidic phase at such low temperatures is a highly probable side effect of the main reduction action of the electrons on the FeIII cations in the ferric oxalate structure, thus resulting in the creation of intermediate FeC2O4. The final product of decomposition of the FeC2O4 intermediate in a dynamic inert atmosphere is a mixture of wüstite (FexO), α‐iron (α‐Fe), and magnetite (Fe3O4). Their proportion accurately reflects actual disproportionation/synproportionation/redisproportionation processes likely encouraged by the preserved size and morphology of the initial ferric oxalate crystals and that are dependent on temperature. In the oxidative atmosphere, the decomposition proceeds in the three overlapped stages that include dehydration, the astonishing reductive formation of FeC2O4 as an intermediate, and final decarboxylation to hematite (α‐Fe2O3). The principal effect of the experimental conditions on the amount of intermediate formation of FeC2O4 in the oxidative atmosphere is also discussed and evaluated from the isothermal experiments carried out at 180 °C.

Amorphous iron (II) carbonate: Crystallization energetics and comparison to other carbonate minerals related to CO2 sequestration

“…Navrotsky et al (2010) have argued that oxidation of FeO to Fe 3 O 4 is thermodynamically favored for nanosized particles. The endothermic peak at $575°C in the DSC curve of siderite was also observed in the work of Jozwiak et al (2007) and Hermankova et al (2010), and can be ascribed to a phase transition. It should be noted that the temperature of 577°C is the Curie temperature of magnetite, very similar to the temperature (570°C) below which FeO becomes unstable and disproportionates to Fe 3 O 4 and a-Fe (Redl et al, 2004).…”

Preparation of Magnetite by Thermally Induced Decomposition of Ferrous Oxalate Dihydrate in the Combined Atmosphere

“…Both the spectrum shape and the parameter values are typical for ferrous oxalate dihydrate. 23,34 The results obtained by XRD and Mössbauer spectroscopy of all 24 prepared samples are presented below. The individual samples are presented within the series, where each of them was prepared at different temperature (8 series, 3 samples each).…”

“…The temperature range was chosen in accordance with the previous thermogravimetric analysis (TGA) or DTA studies performed on the ferrous oxalate dihydrate. 23,28,[31][32][33][34] In addition, the samples were calcinated during 3 different time intervals: 1, 6 and 12 hours, for each of the temperature steps, to study the influence of calcination time on properties of the prepared samples. Overall, twenty-four samples with the different temperature and time combinations were prepared.…”

“…The thermal decomposition of ferrous oxalate dihydrate in oxygen rich atmosphere (e.g., in air) has been reported in many studies. [16][17][18][19][20][21][22][23][24] If the FeC 2 O 4 • 2H 2 O is decomposed in air, the reaction is straight-forward and proceeds in two consecutive steps. It was shown that these two steps (endo + exothermal) partially overlap in differential thermal analysis (DTA) in the range up to 450 °C.…”

“…Another decomposition process was reported to take place under the inert atmosphere (e.g., nitrogen or argon). 23,[29][30][31][32][33] The differential thermal analysis (DTA) of this process generally show two major events in the range up to 450 °C, which are well separated and both of them are endothermic. 29 The first event corresponds to the evaporation of water molecules (180 °C), while the other one, a large asymmetric peak that starts at approximately 320-350 °C, is attributed to the decomposition of oxalate group and the formation of oxides.…”