Kinetic approach of multi-step thermal decomposition processes of iron(III) phosphate dihydrate FePO4∙2H2O

In this study, we have reinvestigated the thermal decomposition kinetics of iron(III) phosphate dihydrate FePO4∙2H2O in air atmosphere by TG/DTG and DTA techniques using non-isothermal experiments. The apparent activation energy Eα was determined by the differential and integral isoconversional methods and suggested that the decomposition reaction is complex and follows multi-step processes. A mathematical deconvolution technique using Fraser-Suzuki equation was applied to the DTG curves and allowed the separation of two distinct processes. The apparent activation energies determined by the isoconversional Friedman's method were 93.05 ± 3.80 kJ mol−1 and 73.41 ± 3.14 kJ mol−1 for the first and second process respectively. Using Malek's procedure for each process, the characteristics of y(α) and z(α) functions showed that the kinetic reaction follows the Johnson-Mehl-Avrami model (JMA(n)). One-dimensional nucleation and growth mechanism occurred firstly with f(α1) = 1.272(1 − α1)[−ln(1 − α1)](1 − 1/1.272) and pre-exponential factor A1 = 9.11 × 1010 min−1. After 27% of total conversion, a two-dimensional nucleation and growth mechanism becomes predominant with f(α2) = 2.306(1 − α2)[−ln(1 − α2)](1 − 1/2.306) and A2 = 1.28 × 108 min−1. It was concluded that the two decomposition processes of FePO4∙2H2O are closely interrelated and thus neglected the first process leads to incoherent results.