Kinetic modelling of the first step of Mn2O3/MnO thermochemical cycle for solar hydrogen production

This work reports the kinetic study of the first step of the Mn2O3/MnO thermochemical cycle for hydrogen production by water splitting. The reaction kinetics of Mn (III) oxide thermal reduction has been evaluated using dynamic thermogravimetric analysis at constant heating rate under nitrogen flow. This way the reaction rate can be described as a function of temperature and different kinetic models were fitted to the experimental data obtained from thermogravimetric experiments. A good fitting can be observed for each experiment, although a significant disparity in the values estimated for the Arrhenius parameters has been found (activation energies and pre-exponential factors). Unique values for the kinetic parameters have been calculated by application of a multivariate non-linear regression method for the simultaneous fitting of data from all the experiments carried out at different heating ramps. However, also in this case the values of the Arrhenius parameters are significantly different depending on the chosen kinetic equation. Optimal kinetic parameters have been finally calculated through the estimation of activation energy values by model-free isoconversional methods and using a rigorous multivariate nonlinear regression for the calculation of the model-dependant pre-exponential factors.

► Kinetic modelling of Mn2O3 thermal reduction from TG data at constant heating rates. ► Traditional model fitting leads to a high disparity in the kinetic parameters values. ► Simultaneous fitting leads to unique values but dependent on the model. ► Novel method based on the preliminary calculation of model-free activation energies. ► Arrhenius pre-exponential factor estimated by multivariate non-linear regression.