Kinetic and thermodynamic parameters of the decomposition of chromium chromate in different gas atmospheres

Non-isothermal decomposition of chromium chromate hexahydrate, Cr-2(CrO4)(3). 6H(2)O, was studied on heating up to 600 degreesC in different dynamic atmospheres of N-2, O-2 and H-2, using thermogravimetry (TG), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC). The results obtained at various heating rates (2-20 degreesC min(-1)) were used to derive kinetic (E-a and lnA) and thermodynamic (DeltaH, C-p and DeltaS parameters. It has been found that the activation energies of the dehydration and decomposition steps in N-2 are generally larger than in H-2 atmosphere, and the reverse is true for the enthalpy change of the decomposition. Thus, it has been concluded that the reductive decomposition (in H-2) is easier than the thermal decomposition (in N-2 or O-2) of the chromate. Irrespective of the gas atmosphere applied, the eventual decomposition product was a mixture of alpha -Cr2O3 and non-crystalline chromate species, gamma -Cr2O3+x. Above 400 degreesC in H-2 atmosphere, more deoxygenation of the non-crystalline chromate takes place at high rates of heating to give alpha -Cr2O3.