Study of the hydrogen production step of the Mn2O3/MnO thermochemical cycle

•Thermodynamic simulation of hydrogen production validated by experimental data.•Experimental temperatures higher than predicted due to reaction kinetics and diffusion.•An excess of NaOH is required for total conversion.•Dependence of activation energy on conversion confirms diffusional control.•First order kinetics describe successfully the reaction rate for conversions below 80%.

In this work, a complete study of the second step of the Mn2O3/MnO thermochemical cycle for solar hydrogen production has been performed. It includes a complete thermodynamic calculation of the equilibrium phases between MnO, NaOH and H2, which shows that the reaction takes place theoretically at temperatures above 75 °C. However, the experimental results demonstrate that it is necessary at least 450 °C to achieve a satisfactory reaction rate. It indicates a dramatic influence of chemical kinetics and diffusion process, displacing the reaction to higher temperatures than those predicted by thermodynamics. The resultant solid of the reaction exhibits a phases distribution highly dependent on the temperature and the NaOH:MnO ratio and this is of great influence in the overall rate of the process. The kinetic study shows that the overall process involves not only the chemical reaction between MnO and NaOH, but also a number of physical processes (heat and mass transfer) and solid phase transformations. The apparent activation energy calculated is a composite value determined by the activation energies of those elementary processes.