Thermal decomposition kinetics of Mn0.9Co0.1HPO4·3H2O using experimental-model comparative and thermodynamic studies

Abstract

Simple direct precipitation was used to synthesize the single-phase Mn0.9Co0.1HPO4·3H2O compound. Two thermal decomposition steps were observed corresponding to the dehydration and polycondensation processes, respectively. The pure-phase Mn1.8Co0.2P2O7 compound was obtained as the final decomposition product. The thermogravimetry/differential thermogravimetry/differential thermal analysis, Fourier transform infrared, atomic absorption spectrophotometry, X-ray diffraction and scanning electron microscope techniques were used to characterize the synthesized compounds. The iterative Kissinger–Akahira–Sunose method was carried out to calculate the exact activation energy $$E_{\alpha }$$ values. The first (overlapping between Regions I and II) and the final steps were confirmed to be single-step kinetic process with unique kinetic triplets. The experimental and model plots were compared to determine the reaction mechanisms. Regions I and II of the first step were found to be 3-D diffusion of spherical symmetry ( $$D_{3}$$ ) and cylindrical symmetry ( $$D_{4}$$ ) processes, respectively, while the final step was found to be an assumed random nucleation ( $$A_{2}$$ ) process. Pre-exponential factors were calculated from $$E_{\upalpha}$$ and reaction mechanism. The related thermodynamic functions of the transition state complexes were evaluated and found to agree well with the experimental data.