Abstract :
The thermal decomposition of chromium(III) formate pentahydrate, Cr3(OH)2(HCO2)7·5H2O, in helium atmosphere and 20% O2 in helium atmosphere has been successfully studied by means of TG-DTA-MS, i.e., thermogravimetry-differential thermal analysis (TG-DTA) coupled with evolved gas analysis (EGA) using mass spectrometry (MS). The TG-DTA-MS is useful to interpret the complicated successive reactions and to determine the mechanism of the thermal decomposition. The thermal process in He atmosphere proceeded by four steps, while in 20% O2–He atmosphere it proceeded by three steps. The decomposition scheme of Cr3(OH)2(HCO2)7·5H2O in He atmosphere was proposed by the following decomposition mechanism via formation of three intermediates:Cr3(OH)2(HCO2)7·5H3O→Cr3(OH)2(HCO2)7·2H2O+3H2O Cr3(OH)2(HCO2)7·2H2O→Cr3(OH)2(HCO2)7+2H2O Cr3(OH)2(HCO2)7→Cr2O3+Cr(HCO2)3+CO+3CO2+3H2 2Cr(HCO2)3→Cr2O3+3CO+3CO2+3H2.
After the decomposition, a slight mass loss was observed with a sharp exothermic crystallization of Cr2O3 due to evolution of H2. In 20% O2–He atmosphere, a drastic exothermic mass loss accompanying evolution of CO2, H2O and H2 was explicable by one step combustion of the dehydrated formate, and a small amount of mass loss observed with a crystallization of Cr2O3 was attributed to evolution of O2. The decomposition process without an intermediate was indicated by the temperature increase of the specimen produced by combustion heat.
