Thermal kinetic analysis of a complex process from a solid-state reaction by deconvolution procedure from a new calculation method and related thermodynamic functions of Mn0.90Co0.05Mg0.05HPO4？3H2O
(Advanced Phosphate Materials and Alternative Fuel Energy Research Unit, Department of Chemistry, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok, Thailand)
Abstract: Three individual peaks of thermal solid-state reaction processes of the synthesized Mn0.90Co0.05Mg0.05HPO4？3H2O were observed corresponding to dehydration I, dehydration II and polycondensation processes. An alternative method for the calculation of the extent of conversion was proposed from the peak area of the individual DTG peak after applying the best fitting deconvolution function (Frazer–Suzuki function). An iterative integral isoconversional equation was used to compute the values of the apparent activation energy Eα and they were found to be 65.87, 78.16 and 119.32 kJ/mol for three peaks, respectively. Each individual peak was guaranteed to be a single-step kinetic system with its unique kinetic parameters. The reaction mechanism functions were selected by the comparison between experimental and model plots. The results show that the first, second and final individual peaks were two-dimensional diffusion of spherical symmetry (D2), three-dimensional diffusion of spherical symmetry (D3) and contracting cylinder (cylindrical symmetry, R2) mechanisms. Pre-exponential factor values of 3.91×106, 1.35×107 and 2.15×107 s-1 were calculated from the Eα values and reaction mechanisms. The corresponded standard thermodynamic functions of the transition-state (activated) complexes were determined and found to agree well with the experimental data.
Key words: solid-state reaction; calculation method; complex reaction; Frazer–Suzuki function; experimental and model plots