Role of the physicochemical properties of hausmannite on the hydrogen production via the Mn3O4–NaOH thermochemical cycle
Graphical abstract
Introduction
Hydrogen is expected to play a crucial role as an energy carrier in a world increasingly relying on renewable energy [1], [2]. In this context, thermochemical cycles constitute an interesting alternative for hydrogen production, because they can be driven by solar energy and they lead to the splitting of the water molecule with very high theoretical efficiencies [3], [4], [5], [6], [7], [8], [9]. Although more than 300 thermochemical cycles have been proposed, this list can be substantially reduced considering technical, environmental and economic constrains [10]. After applying these criteria, the MnO–NaOH thermochemical cycle appears as one of the most appealing processes due to its high theoretical exergy efficiency of 74% [11], [12], despite the fact that it consists of the following three stages:Mn2O3 (s) → MnO (s) + O2 (g)MnO (s) + NaOH (l) → NaMnO2 (s) + ½ H2 (g)2NaMnO2 (s) + H2O (l) → Mn2O3 (s) + 2NaOH (aq.)
However, significant technical barriers hinder the successful development of this cycle as a viable route for hydrogen production at large scale [1], [2], [10], [11], [12], [13], [14], [15], [16], [17], [18]. In this respect, previous studies have shown that high H2 yields can be only obtained if the textural properties of the material facilitate the complete contact between sodium hydroxide and manganese oxide [1], [18](b), [18]. Furthermore, even if particle size and other physicochemical parameters can be controlled in the first transformation, these properties are very likely to be modified in the following cycles due to the harsh thermal treatment required to reduce Mn3+ to Mn2+. Besides the incomplete hydrolysis and the high temperature required for the reduction step (at about 1600 °C under inert atmosphere) [1], [13], [18](b), [18], significant sodium losses can occur due to partial evaporation through successive cycles, causing a continuous decline of hydrogen yield [1], [13], [18](b), [18].
In order to circumvent these drawbacks different modifications of the thermochemical MnO–NaOH cycle have been suggested. Recently, the substitution of NaOH by Na2CO3 has been proposed by M. E. Davis and co-workers to limit the practical hurdles caused by NaOH corrosion at high temperature [19]. This thermochemical process uses hausmannite (Mn3O4) to reduce the maximum operation temperature to about 850 °C, and it has proved to be fully reproducible for at least five cycles. However, the kinetic of hydrogen release is slow and the need to recycle CO2 can be a hurdle for implementation. Accordingly, another interesting possibility is the transformation of Mn3O4 with the more reactive NaOH in the alternative thermochemical cycle, which two initial steps are the following:3Mn2O3 (s) → 2Mn3O4(s) + ½ O22Mn3O4 (s) + 6NaOH (l) → 6NaMnO2 (s) + 2H2O + H2(g)
In this case, the high temperature stage of the cycle is the Mn2O3 reduction to Mn3O4 (Eq. (4)). Subsequently, hydrogen production takes place during the reaction of Mn3O4 with NaOH to yield NaMnO2 as a solid product, as well as water vapor (Eq. (5)). This second step occurs at around 450 °C. Then, hydrolysis of the mixed oxide is carried out using H2O at temperatures below 100 °C and subsequently NaOH and Mn2O3 are recovered as, respectively, a solution and precipitated solid, in exactly the same way than for the MnO–NaOH cycle (Eq. (3)). The cycle of Mn3O4 was studied for the first time by P. Nuesch in 1998, and it showed a lower efficiency as compared to that based on MnO [11], [12], [20]. Nevertheless, the theoretical thermodynamic efficiency of this alternative cycle, 57%, is still attractive for achieving a feasible production of solar hydrogen. In addition, the use of Mn3O4 leads to a dramatic decrease in the reduction temperature (from around to 1600 °C to about 950 °C in air). These milder operation conditions can contribute to reduce the stress over the materials and to avoid important sodium losses.
Based on these considerations, in the present work the feasibility of applying the Mn3O4–NaOH thermochemical cycle for hydrogen production has been investigated. Specifically, the influence of the morphology and textural properties of Mn3O4 on hydrogen yield were evaluated. For this purpose, a series of samples were prepared by thermal treatment in argon of a commercial sample of Mn3O4. In this way oxides with differences on textural properties, but without any variation in the composition were obtained to gain a clear insight on the effects of these physicochemical characteristics on the hydrogen production yield. Additionally, a hausmannite sample was synthesized by precipitation, in order to have a more representative set of materials with wider range of properties. Finally, in order to explore the viability of the overall thermochemical cycle, the Mn3O4 sample which achieved the highest hydrogen production was hydrolysed and thermally treated to recover the initial compound.
Section snippets
Thermodynamic calculations
In order to evaluate the equilibrium of the different reactions, the values of the standard thermodynamic parameters ΔHf°, ΔSf° and Cp of the NaMnO2 phase are required. An estimation of these variables has been reported previously [1], [18](b), [18] and these values were introduced in the database of the HSC Chemistry 6.1 software for calculating the evolution of the equilibrium compositions with temperature.
Materials preparation
Two commercial Mn3O4 samples were used as reference materials for the hydrogen
Equilibrium simulation of the Mn3O4–NaOH cycle
An exploratory thermodynamic study of the variation of the equilibrium composition with temperature of both the thermal reduction and the hydrolysis steps have been reported in previous publications [1], [13], [14], [18](b), [18]. Experimentally, it has been found that the formation of Mn3O4 by thermal reduction of Mn2O3 occurs at about 950 °C in air [21], while in inert atmosphere takes place in the 650–900 °C interval [13], [14], [15].
Fig. 1a shows the variation of equilibrium composition
Conclusions
The influence of the textural and structural properties of the hausmannite samples in the hydrogen production stage of Na–Mn thermochemical cycle has been evaluated. The results obtained show a positive correlation between hydrogen production rate and the surface area of Mn3O4 due probably to the increase of the interface between both reagents. In fact, it seems that a minimum of specific surface is necessary since the Mn3O4 samples with surface area lower than 2 m2/g, are basically unreactive
Acknowledgments
The authors gratefully acknowledge the financial support from the Ministry of Economy and Competitiveness through the project MULTISTOR (ENE-2012-36937).
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