The kinetic and thermodynamic study of KNiPO4·H2O from DSC and TG data

Abstract

The DSC and TG data showed the dehydration process occurring over the range of 160–300 °C. The XRD patterns of the synthesized KNiPO4·H2O and the calcined product at 350 °C with exposing in the air over 8 h are indexed as the KNiPO4·H2O structure, whereas at 600 °C is indexed as KNiPO4 structure. Hence, these data confirmed that the water molecule was eliminated from the structure at 300 °C, after that the spontaneously reversible hydration–rehydration process was observed. The activation energy and pre-exponential factor were calculated by Kissinger, Ozawa, and KAS equations. According to the DSC curves, the enthalpy change (ΔH) of dehydration process can be calculated and was found to be 100.12 kJ mol−1. Besides, we suggested another new method to determine the isokinetic temperature value using spectroscopic data. The surface area of synthesized hydrate and its calcined product at 350 °C with exposing in the air at over 8 h were found to be 21.48 and 134.3 m2 g−1, respectively. The reversible hydration–rehydration process was observed, and the surface area of final product at 350 °C (aging time over 8 h) is higher than that of the synthesized compound. This behavior is important to develop alternative desiccant materials or other process based on the rehydration mechanism with increasing the surface area.