Kinetics of Mn₂O₃–Mn₃O₄ and Mn₃O₄–MnO Redox Reactions Performed under Concentrated Thermal Radiative Flux

Abstract

Manganese oxide based on the thermochemical cycle for splitting water is considered using concentrated solar energy. The high-temperature endothermic step is the thermal reduction of Mn 2 O 3 , which proceeds at above 1835 K via two sequential chemical reactions from Mn 2 O 3 to Mn 3 O 4 and then to MnO. The kinetic mechanisms of both reactions are investigated by a solar-driven thermogravimeter, with reactants directly exposed to high-flux thermal irradiation. With this arrangement, the overall kinetic rate laws are derived under similar heat-and mass-transfer characteristics existing in highly concentrating solar systems, such as solar towers or parabolic dishes. The experimental results suggest a nth-order rate for the conversion of Mn 2 O 3 to Mn 3 O 4 and a diffusion-controlled regime for the conversion of Mn 3 O 4 to MnO. Activation energies for both reduction steps are determined and compared to previous reported data.

High-flux optical systems for solar thermochemistry

“…Volumetric absorption increases the penetration of the flux into the reactant, reducing the temperature gradient through the reactant and allowing the heat to penetrate deeper for a more homogeneous heating. If the reactant is a powder, surface reaction is obtained by forming pellets which are fed to the hot cavity, or by lining the cavity inner surface with the powder (even dispersion can be obtained by rotation) (Abanades et al, 2007;Koepf et al, 2016;Levêque and Abanades, 2013;Möller and Palumbo, 2001;Alonso et al, 2013). This approach is generally satisfactory for reactions with consumption of the reactant.…”

A directly irradiated solar reactor for kinetic analysis of non-volatile metal oxides reductions

“…(4) Monitoring of reactions by means of the oxygen concentration at the carrier gas outlet Typical parameter to monitor reaction yield is mass loss, including solar-driven thermogravimeters [15,[27][28][29]. However, this procedure can be compromised with direct temperature measurement of the reactant because a thermocouple in direct contact with the reactant may distort the scale record.…”

“…Figure 7 shows a typical oxygen concentration curve obtained for an experiment with a C-type sample holder. Heterogeneous shape of the oxygen curve did not allow to accurately adjust it to one of the typical solid-state kinetic mechanism [15,39,40] according to the Arrhenius equation (Equation (14)). Moreover, high-temperature gradients were presented along the sample.…”

“…However, the conditions to supply heat to the reactants in this conventional analysis are substantially different from the concentrated solardriven process that might give rise to observe different kinetics [14,15].…”

“…The traditional approach to obtain the chemical kinetics of metal oxide reductions is to use conventional thermal analysis techniques, such as thermogravimetric analysis or thermal desorption [10][11][12][13]. However, the conditions to supply heat to the reactants in this conventional analysis are substantially different from the concentrated solar-driven process that might give rise to observe different kinetics [14,15]. Also, some devices and set-ups have been used for determining kinetics at high radiation fluxes of volatile [16] and non-volatile metal oxides [17,18].…”