Hydrogen Production by the Sodium Manganese Ferrite

Article pubs.acs.org/IECR

Hydrogen Production by the Sodium Manganese Ferrite Thermochemical CycleExperimental Rate and Modeling Maria Anna Murmura,*,† Francesca Varsano,‡ Franco Padella,‡ Aurelio La Barbera,‡ Carlo Alvani,‡ and Maria Cristina Annesini† †

University “Sapienza” of Rome, Department of Chemical Engineering, Materials & Environment, Via Eudossiana 18, 00184 Rome, Italy ‡ ENEA − Materials Chemistry and Technology Lab. − Casaccia Research Center, Via Anguillarese 301, 00123 Rome, Italy ABSTRACT: The sodium manganese ferrite thermochemical cycle for hydrogen production by water splitting can successfully operate in a relatively low temperature range (1023−1073 K) and has a high potential for coupling with the solar source using conventional structural materials. With the aim of implementing the cycle in a solar reactor, the hydrogen evolution rate from the reactive mixture measured in laboratory apparatus has been modeled by using a shrinking-core model. Such a model proved to adequately describe the rate of hydrogen production in the studied temperature and water concentration range. The model was extended to predict the behavior of the reactive mixture subjected to different experimental conditions.

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INTRODUCTION Thermochemical hydrogen production has been under study for several years, with a more recent emphasis on the possibility of coupling solar thermal energy to the process. Solar power represents a low-density energy source, which can be enhanced through solar concentration technologies; however, the high temperatures necessary to operate the water-splitting processes are often restricted by the need of adequate materials required to construct thermochemical reactors.1−6 From this perspective, the cycle that makes use of the MnFe2O4/Na2CO3 mixture represents an advantage by operating at temperatures of about 1023 K, thus allowing the use of conventional construction materials; it has been proved that such a temperature can be easily reached in a reactor receiver coupled with a parabolic dish solar concentrator.7 The water-splitting cycle is conventionally described according to the following reactions:8

2MnFe2O4 (s) + 2Na 2CO3(s) = 2(MnO·2NaFeO2 ) + 2CO2 2(MnO·2NaFeO2 ) + Na 2CO3 + H 2O = 6Na(Mn1/3Fe2/3)O2 + H 2 + CO2

(1)

6Na(Mn1/3Fe2/3)O2 (s) + 3CO2 (g) = 2MnFe2O4 (s) + 0.5O2 (g)

(2)

In reaction 1, manganese ferrite reacts with water vapor and sodium carbonate to produce carbon dioxide, hydrogen, and sodium-ferrimanganite. During this reaction, manganese is oxidized from the Mn(II) to Mn(III) state. The cycle is closed by reaction 2, in which the carbon dioxide and sodiumferrimanganite formed in reaction 1 regenerate the solid mixture of manganese ferrite and sodium carbonate, releasing oxygen. Reaction 1 is actually the result of two consecutive steps. Quantitative studies have shown that H2 evolution occurs only after a nonoxidative decarbonation reaction, during which 2/3 of the expected CO2 evolves.9,10 The decarbonation and hydrogen evolution steps are described by reactions 3 and 4. © XXXX American Chemical Society

(4)

According to reaction 4, manganese(II) oxide is oxidized by water in the presence of sodium carbonate, releasing hydrogen and carbon dioxide in equal amounts. Apparently, the sodium ferrite, NaFeO2, resulting from reaction 3 constitutes the structural frame in which oxidized manganese (formally NaMnO2) dissolves to form Na(Mn1/3Fe2/3)O2. The overall chemical frame is rather complex, and the detailed reaction mechanism is still under investigation. Several issues are still to be addressed before implementing a scaled-up device, such as long-term stability of the material mixture, evolved gas separations, and solar reactor design optimization. Differently from other metal oxide based thermochemical cycles, the sodium manganese ferrite cycle requires separation and recirculation of CO2. The energetic cost of this step strongly depends on the process adopted. Nonetheless, the possibility of operating at lower temperatures than other metal oxide thermochemical cycles, and under isothermal conditions, is of great advantage because of the reduced energetic losses associated with reradiation and temperature swings between oxidative and reductive steps. Reactor design is also simplified: conventional materials may be adopted for reactor construction, and the solid reactive mixture may be heated indirectly. These aspects are expected to overcome the additional costs linked to CO2 separation.

2MnFe2O4 (s) + 3Na 2CO3(s) + H 2O(g) = 6Na(Mn1/3Fe2/3)O2 (S) + 3CO2 (g) + H 2(g)

(3)

Received: March 4, 2014 Revised: May 30, 2014 Accepted: June 4, 2014

A

dx.doi.org/10.1021/ie500940z | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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present are MnFe2O4 and Na2CO3, as determined by comparison with ICCD-JCPDS database cards. Material characterization has been performed by scanning electron microscopy (SEM), using a high-resolution microscope (HRSEM LEO 130). Images of the solid mixture before and after its use in the water splitting reaction have been acquired. Reactor Apparatus. The rate of hydrogen production has been studied through Temperature-Programmed Desorption/ Reaction (TPD/TPR) analysis (Micromeritics 2920). The instrumental scheme is reported in Figure 2. The apparatus

Tests carried out on the system have proved that hydrogen can be produced in significant amounts (450−1300 μmol/ gmixture) when compared to other metal oxide cycles, under more favorable temperature conditions,1 therefore the thermochemical cycle is interesting for a full-scale application. The purpose of the present work is to propose a simple model that is able to adequately describe the rate of hydrogen production. To this end, reaction 4 has been studied in a Temperature-Programmed Desorption (TPD) device in the temperature range 973−1073 K. Analysis of the experimental data collected has allowed model validation.

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EXPERIMENTAL METHODS Materials Preparation. The experimental runs have been performed on materials produced in pilot scale batches, in view of a technological development. The mixtures have been produced starting from analytical grades of Fe2O3 (SigmaAldrich), MnCO3 (Sigma-Aldrich), and Na2CO3 (Carlo Erba) powders: the powders have been blended in a ZOZ Simoloyer CM01 mixer/mill for 1 h with ethanol. During each treatment, about 500 g of powder has been mixed using 3 kg of 5-mmdiameter stainless steel balls. The resulting slurry was dried and later granulated using an in-house granulator, forming composite porous pellets (1−2 mm diameter) possessing particle grains on the micrometer scale. A thermal treatment at 1023 K has been conducted by loading the material in an Inconel 625 tube (42 mm internal diameter), positioned in an oven (Lenton PTF 125/50/610). The treatment has been carried out in two steps: in air for the first 4 h, during which complete oxidation of manganese(II) is carried out, according to reaction 5:

Figure 2. Outline of the TPD/TPR apparatus utilized to measure hydrogen production. Purified Ar gas (bottle P) is water-saturated in vessel W, before entering the sample holder (C) inside the furnace (F). Unreacted water and carbon dioxide are trapped in (T) before the gas flow enters the TCD detector zone.

consists of a vessel containing water and maintained at a fixed temperature in order to control the partial pressure of water in the gas stream fed to the sample holder, a U-shaped quartz sample holder, placed within a furnace, and a trap. Gas composition at the outlet is measured as a function of the difference in thermal conductivity with respect to a reference gas, which in the case examined is argon. The sampling time is equal to 1 s, automatically measured by a controller card. The gas composition is obtained from the thermal conductivity measurements via calibration tests of gases of known composition. All the experiments have been carried out in an argon flow; the reactive solid mixture is placed within a small basket made of gold, placed in one of the branches of the sample holder. The gold basket is required to avoid the reaction of sodium carbonate with quartz. The sample temperature is measured by a thermocouple located immediately above the sample. A trap, formed by a molecular sieve (zeolites 5A from Chromatography Research Supplies Inc.) kept within an ice bath, is placed between the sample holder and the detector, to capture unreacted water and carbon dioxide formed during the reaction. The water vessel or trap may be bypassed, if required by the operating conditions. The experimental procedure consists of heating the sample to the desired temperature at a rate of 10 °C/min, while sending a flow of argon. The sample is then kept at constant temperature, in an argon flow, for about 30 min, to allow reaction 3 to reach completion. Throughout this first phase, the molecular sieve is bypassed, and carbon dioxide concentration at the outlet of the sample holder is measured. Once the measured CO2 concentration becomes negligible, water is sent to the sample by flowing argon through the water-containing vessel, and the molecular sieve trap is inserted. In this way, water and carbon dioxide are removed from the outlet gas stream, and the hydrogen concentration is measured by TCD.

2Fe2O3(s) + 3Na 2CO3(s) + 2MnCO3(s) + 0.5O2(g) = 6Na(Mn1/3Fe2/3)O2 (s) + 5CO2 (g)

(5)

and in carbon dioxide for 10 h, to obtain a complete reduction of the mixture according to reaction 2. This treatment is necessary to allow intimate mixing of MnFe2O4 with Na2CO3, stabilize the material, and increase its mechanical robustness. At the end of the procedure, the pellets have been analyzed by X-ray diffraction (XRD) techniques in the angular range 4° < 2θ < 40°, using a Seifert Pad VI apparatus equipped with Mo Kα radiation and a LiF monochromator on the diffracted beam. From Figure 1, in which the XRD pattern of the synthesized material is shown, it is possible to see that the only phases

Figure 1. XRD pattern of the reactive mixture. The sample contains MnFe2O4 and Na2CO3. B



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Table 1. Experimental Conditions and Results expt. number 1 2 3 4a 5 6 7 8 9 10 11 12 13 14 15 (multiple cycles) a

T [K] (±5)

mixture mass [mg] (±0.1)

water pressure [mbar] (±0.1)

theoretical H2 yield [μmol]

H2 yield obtained [% of maximum]

approximate conversion time [min]

973

40.49 40.36 19.64 18.77 40.85 40.50 19.54 41.75 41.30 40.06 19.34 38.87 78.4 42.55 61.0

73.6 98.6 72.2 73.1 73.5 100.4 68.8 73.9 100.2 100.0 71.3 33.8 33.7 38.9 30.0

51.96 51.80 25.21 24.09 52.43 51.98 25.08 53.58 53.00 51.41 24.82 49.82 100.62 54.61 78.24

81 89 82 60 83 75 75 86 87 86 85 85 88 94 30

100 100 100 60 80 50 50 50 60 60 50 120 150 140 60

1023

1073

1023 1040 1073 1023

Data aquisition interrupted after 60 min.

The experiments were carried until hydrogen was no longer detected at the outlet. The capability of the solid mixture to undergo multiple cycles has been studied at 750 °C. The procedure used consists of three steps, which have been repeated multiple times. After the first complete oxidation, the solid mixture has been exposed to different gaseous environments for fixed times: 2 h in CO2, 30 min in Ar, for the decarbonation reaction, and 60 min in water and argon, with a partial pressure of water of 0.03 bar, for the hydrogen producing step of the reaction. By following this procedure, the mixture is only partially oxidized and reduced during each cycle.

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EXPERIMENTAL RESULTS Experiments have been carried out under different operating conditions. In particular, the variables that have been taken into consideration are temperature (between 973 and 1073 K), partial pressure of water (between 0.03 and 0.10 bar), and amount of solid reactive material (between 20 and 80 mg). All the experiments reported were carried out with an argon flow rate of 50 N mL/min. A summary of experimental conditions and hydrogen yields is reported in Table 1; it is worth noting that experiments 12−15 were performed after the development of the model and were used as validation tests. In all of the experiments, the hydrogen mole fraction in the outlet stream has been measured and the hydrogen production rate has been obtained from the total molar gas flow rate and the hydrogen mole fraction. The total amount of hydrogen produced has then been evaluated by integrating the hydrogen production rate with time. This value has been compared with the amount of hydrogen that could have been obtained if all the manganese present in the solid mixture had been available for the reaction. A typical experimental result is shown in Figure 3. In all the experiments, about 20 s after water vapor is sent to the system, hydrogen is detected at the outlet. The rate of hydrogen production reaches its maximum value in 10 s and then drops rapidly. Experiments conducted on the material for multiple cycles showed that hydrogen yield decreases at increasing cycle number. Reacting mixture degradation rate is strictly correlated to the working conditions (i.e., temperature, water pressure,

Figure 3. Example of hydrogen production rate measured in TPD apparatus (experiment 9: 1023 K, 100.2 mbar of water, 41.30 mg of solid mixture).

duration of the oxidative step, etc.). By using the procedure described in the Experimental Section, after 25 cycles the hydrogen yield dropped to 40% of its initial value. X-ray diffraction patterns performed on a sample no longer able to produce hydrogen in significant amounts (50 cycles)11 show the absence of competing parasitic phases. Only MnFe2O4, Na2CO3, and Na(Mn1/3Fe2/3)O2 are present, indicating that the sample has not been completely regenerated. It is very likely that morphological variations of the material are responsible for the decrease in hydrogen yield as evidenced by SEM imaging of the just-prepared (Figure 4a) and end-of-life (Figure 4b) solid mixture. In the as-prepared sample, the manganese ferrite, visible as light-colored portion of material, appears highly dispersed. On the contrary in the cycled sample it appears that the sodium carbonate (dark-colored) regenerated through reaction 2 coalesces and incorporates the manganese ferrite, reducing the number of interfaces between manganese ferrite, sodium carbonate, and water. Figure 4c and d show the manganese ferrite present in the material before and at the end of the cyclical procedure (sodium carbonate has been previously dissolved in water). Only slight ferrite particle growth is noticed, indicating that the reduced hydrogen C



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Figure 4. SEM image of MnFe2O4/Na2CO3 mixture (a,b) and of MnFe2O4 (c,d) for the pristine and the cycled sample. The image is divided into an INLens detector image (c,d) which is sensitive to the morphology and backscattered electron detector (a,b), which is more sensitive to the atomic number (heavier elements appear lighter). In images c and d, sodium carbonate has been removed by dissolution in water.

the growth of a layer of the reaction products (ash) that hinder the water access to the unreacted solid. Analogous reaction mechanisms have also been noticed in studies of the iron oxide,13,15 zinc ferrite,14 and zinc oxide15 cycles. At the same time, a significant dependence of the rate of hydrogen production on temperature has been observed, as reported in Figure 5 where the rate of hydrogen production during the first minutes of the reaction at two different temperatures has been evidenced. The strong dependence of the initial rate of hydrogen production on temperature indicates that intrinsic reaction kinetics do play a significant role in the overall hydrogen production rate. As in most metal-oxide thermochemical cycles, the hydrogen production rate is around 1−50 μmolH2 min−1 gmaterial−1. The heat associated with the reaction has been estimated to be about 330−370 kJ/molH2.16 This means that, in the performed measurements, the rate of heat absorption is fairly low and temperature gradients within the reacting particle and between

production observed in consecutive cycles is not due to sintering of the ferrite.

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KINETIC MODEL

The decrease in reaction rate is generally due to reduced reactant concentrations, diffusion resistances, or contracting area of the unreacted solid. In the present work, excess water has been continuously fed to the system. The incoming water flow rate has always been 2 orders of magnitude higher than the average rate of hydrogen evolution, meaning that water concentration is constant throughout the entire experiment and gas phase diffusion may be considered fast. Unreacted solid surface contraction has been accounted for, by applying a shrinking-core model in which the controlling resistance is considered to be the superficial reaction, but it is not sufficient to describe such a sharp decrease in the hydrogen production rate.12 A mass transport resistance increasing with time should therefore be considered to explain the fast decrease in the hydrogen production rate; such a resistance may be related to D



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By integrating eq 7, with the obvious initial condition that when t = 0, X = 0, the following expression is obtained: t=

0 nMn(II)

kd′·4πR2·pH0.5O

[1 − (1 − X )1/3 ]

2

1 = [1 − (1 − X )1/3 ] kdpH0.5O

(9)

2

where the constant is given by kd = kd′/(ρR), with ρ being the density, and therefore it also depends on the initial dimensions of the grain. On the other hand, for processes in which only diffusion of the reactants through the ash layer limits the process, the expression of the rate of conversion is given by eq 10:

Figure 5. Influence of temperature on the rate of hydrogen production. The dashed line refers to experiment 2 (973 K, 98.6 mbar of water, 40.36 mg of solid mixture); the continuous line refers to experiment 10 (1073 K, 100.0 mbar of water, 40.06 mg of solid mixture).

0 α ·nMn,tot

particle and the bulk fluid can be considered to be around 3− 5°. On the basis of these observations, a shrinking-core model17−19 has been selected to describe the rate of hydrogen evolution according to reaction 4. The presence of the following phenomena has been assumed: diffusion of water through the ash layer (Na(Mn1/3Fe2/3)O2), whose thickness grows during the reaction, and reaction of water with MnO· 2NaFeO2/Na2CO3 mixture, at the surface between the unreacted solid and the oxidized compound (Na(Mn1/3Fe2/3)O2). In principle, water can interact with both metal oxide and sodium carbonate solid phases,and a specific study aimed at understanding the interaction between the reactive mixture and water in different condition is required. Within the scope of the present work, dissociation of water at the surface of the metal oxide has been assumed as the rate-limiting step of the surface reaction. This has been noticed during previous studies on the oxidation of Zn20 and other systems, as extensively reported by Henderson.21 Surface reactions of this kind generally present a reaction rate that depends on pHn 2O, with 0 < n < 1.20 The surface reaction has therefore been assumed to be a 0.5 order reaction with respect to water. The rate of the surface reaction may be expressed as

t=

2

2

t=

αR ash [1 − 3(1 − X )2/3 + 2(1 − X )] pH O 2

1 1 + [1 − (1 − X )1/3 ] kd pH0.5O

F

(7)

=1−

⎛ rc ⎞3 ⎜ ⎟ ⎝R⎠

pH

dX 0 0 2 = FH2 = 0.5αnMn,tot = 0.5αnMn,tot P dt ⎛ ⎞−1 ⎡1 1 −2/3 ⎤⎟ ⎜ 2αR ash [(1 − X )−1/3 − 1] + − X (1 ) ⎢ ⎥⎦⎟ ⎜ p kdpH O0.5 ⎣ 3 ⎝ HO ⎠ 2

where α is Mn(II) fraction in the reactive solid mixture, experimentally obtained from the measured reaction yield, and the conversion of Mn(II) has been defined as 0 nMn(II)

(12)

The mass balance of hydrogen is

dX = kd′·αS ·pH0.5O = k′d ·α 4πR2·(1 − X )1/3 ·pH0.5O 2 2 dt

0 nMn(II) − nMn(II)(t )

(11)

In eq 11, Rash = R2ρ9 T/(6+e ) accounts for the resistance to diffusion of water through the ash layer, and its value depends on the initial dimensions of the grain and the diffusivity of the gas through the solid product. Accounting for both the surface reaction kinetics and the resistance to diffusion in the product layer, the following relation is obtained:18,19

and if the process is under kinetic control, the manganese conversion is given by

X=

αR2ρ 9T [1 − 3(1 − X )2/3 + 2(1 − X )] 6+epH O

αR ash = [1 − 3(1 − X )2/3 + 2(1 − X )] pH O

(6)

2

(10)

where +e is the effective diffusivity of water through the ash layer and 9 is the gas constant. From the above equation, the following relationship between conversion and time is obtained:

2

r = Kd′·pH0.5O

0 α ·nMn,tot

+ dpH2O dX = −4πr 2· e · dt 9T dr

2

(13)

where F is the total molar flow rate. Therefore, once Rash and kd are known, the model allows one to evaluate, for each time, the corresponding manganese conversion from eq 12 and the hydrogen production from eq 13. Evaluation of Model Parameters. Model parameters have been evaluated by considering a dependency of kd and Rash from temperature. For each temperature, the model constants have been chosen so as to minimize the quadratic error in the hydrogen production rate:

(8)

n0Mn(II)

where nMn(II)(t) and indicate the number of moles of Mn(II) at time t and the initial number of moles of Mn(II) available for the reaction, respectively, rc is the radius of the unreacted core, and R is the initial grain radius. E


Industrial & Engineering Chemistry Research j=1

E=

∑

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kJ/mol and 114 ± 6 kJ/mol, respectively; the pre-exponential factors are equal to 5.19 × 102 ± 7 bar−0.5 min−1 and 9.12 × 104 ± 3 × 103 bar−1 min−1. The value of activation energy of the water diffusion appears to be quite high for water diffusion in porous solids, and this suggests that what has been described macroscopically as a resistance to the transport of water toward the reactive surface may actually be the result of a more complex mechanism. If the solid product forms a dense layer around the reacting particle, the mass transport may occur through ionic diffusion. It has been reported that ionic diffusion can be the controlling process in the overall rate of reduction of metal oxides and the oxidation of metals.22 In the present case, both oxygen ions23−25 and sodium ions may play a role in the mass transport, considering the high mobility of sodium in the produced Na(Mn1/3Fe2/3)O2.26−28 In order to validate the kinetic model, three experiments (runs 12−14 in Table 1) have been carried out under different experimental conditions, and the experimental results (not used in the fitting of the model parameters) have been compared with model prediction of the hydrogen production rate. In particular, these experiments have been carried out with a lower water concentration in the feed. In experiment 13, a higher solid mass and a different temperature were also used. The values of the parameters used to model the hydrogen evolution rate at 1040 K were evaluated by applying the Arrhenius law with the value of the pre-exponential factors and activation energies reported above. For all these experiments, the agreement between the experimental data and the model prediction is satisfactory, with a quadratic error around 0.05. As an example, Figure 10 shows the comparison between experimental and predicted results relative to experiment 13. Under the experimental conditions adopted, the excess of water is not as strong as the previous cases, as a higher mass of the solid mixture and a lower water concentration were used. The good agreement obtained is an important result in the validation of the model, as it indicates that it may be safely extended to different experimental conditions. This is particularly important when considering the possibility of transferring the information acquired during these laboratoryscale experiments to a pilot-scale reactor, even though the extension of the model to a larger apparatus will necessarily require additional considerations on the fluid dynamics of the system.

N

∑i =j 1 (FH2, ij − FHexp2, ij)2

NTemp

Nj

(14)

where Nj is the number of points in which hydrogen concentration has been measured in experiment j, NTemp is the number of experiments conducted at the same temperature, and FH2,ij is the number of micromoles of hydrogen produced at time ti during experiment j. The error in the value of the parameters has been evaluated through the covariance matrix, which allows the definitions of the 95% confidence interval. In Table 2, the model parameters obtained are reported. Table 2. Model Parameters from Experimental Data Fitting temperature [K]

kd [bar−0.5 min−1]

Rash [min·bar]

973 1023 1073

6 × 10−2 ± 2 × 10−3 9 × 10−2 ± 8 × 10−3 1.4 × 10−2 ± 1 × 10−2

15 ± 2.5 8±1 4 ± 0.4

Figure 6 (a and b) reports, as an example, experimental (continuous line) and modeled (dashed line) hydrogen production rates for two tests conducted at 800 °C with similar water concentration but different solid masses. The integral production plot is reported for the two experiments in Figure 7. The quadratic error, defined in eq 14, for the two tests is equal to 0.04 and 3.72 for the hydrogen production rates and integral production, respectively. This agreement between experimental and calculated data is very good, more so if considering the extreme simplicity of the two-parameter model adopted. This is particularly true for the first minutes of the reaction, during which the hydrogen production rate is characterized by a sharp peak, immediately followed by a fast decrease. Modeling Results. According to the results obtained, the apparent constant for the chemical reaction increases with temperature, while the value of Rash decreases with increasing temperature. Having applied a shrinking-core model, the value of Rash is inversely proportional to the diffusivity of the reactant gas through the solid product layer. In this work, both kd and 1/Rash have been correlated with temperature using an Arrhenius-type law, as reported in Figures 8 and 9. The activation energies of the chemical reaction and of water diffusion through the product layer are equal to 73 ± 4

Figure 6. Comparison between experimental (continuous line) and calculated (dashed line) rate of hydrogen production for samples at 800 °C: (a) experiment 8: 73.9 mbar of water, 41.75 mg of solid mixture; (b) experiment 11: 71.3 mbar of water, 19.34 mg of solid mixture. F



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Figure 7. Comparison between experimental (continuous line) and calculated (dashed line) amount of hydrogen produced for samples at 800 °C: (a) experiment 8; (b) experiment 11.

Figure 10. Comparison between experimental (continuous line) and predicted (dashed line) hydrogen production for experiment 13 (1040 K, 33.7 mbar of water, 78.4 mg of solid mixture).

Figure 8. Arrhenius plot for kd.

Figure 9. Arrhenius plot for 1/Rash. Figure 11. Comparison between experimental (continuous line) and predicted (dashed line) hydrogen production for experiment 15 (1023 K, 30.0 mbar of water, 61.0 mg of solid mixture). The result shown refers to the end-of-life mixture. For comparison purpose the y scale is fixed in 1−6 μmol/min interval. A magnification of the H2 production rate is reported for the initial production in a proper scale.

The model parameters obtained have also been used to describe the rate of hydrogen production using a sample for multiple cycles. Figure 11 shows the comparison between the experimental and predicted rate of hydrogen production for an end-of-life mixture. The quadratic error for the prediction is around 0.09. This low value indicates that the model is adequate for a general description of the water-splitting process; however, the figure shown suggests that the elementary mechanism may be more complicated than the one reported, especially in the first reaction stages (see insert in the figure), when described segregation of sodium carbonate strongly depresses the initial rate.

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CONCLUSIONS The sodium manganese ferrite thermochemical cycle, which operates at temperatures of about 1023 K, is an appealing process for hydrogen production coupled with solar thermal G



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(16) HSC chemistry 6.12; Outokumpu Research Oy: Pori, Finland. (17) Yagi, S.; Kunii, D. Fluidized-solids reactors with continuous solids feed − I: Residence time of particles in fluidized beds. Chem. Eng. Sci. 1961, 16, 364. (18) Yagi, S.; Kunii, D. Fluidized-solids reactors with continuous solids feed − II: Conversion for overflow and carryover particles. Chem. Eng. Sci. 1961, 16, 372. (19) Yagi, S.; Kunii, D. Fluidized-solids reactors with continuous solids feed − III: Conversion in experimental fluidized-solids reactors. Chem. Eng. Sci. 1961, 16, 380. (20) Ernst, F. O.; Steinfeld, A.; Pratsinis, S. E. Hydrolysis rate of submicron Zn particles for solar H2 Synthesis. Int. J. Hydrogen Energy 2009, 34, 1166. (21) Henderson, M. A. The interaction of water with solid surfaces: fundamental aspects revisited. Surf. Sci. Rep. 2002, 46, 1. (22) Szekely, J.; Evans, J. W.; Sohn, H. Y. Gas-Solid Reactions; Academic Press: New York, 1976. (23) Scheffe, J. R.; McDaniel, A. H.; Allendorf, M. D.; Weimer, A. W. Kinetics and mechanism of solar-thermochemical H2 production by oxidation of a cobalt ferrite−zirconia composite. Energy Environ. Sci. 2013, 6, 963. (24) Walker, L. S.; Miller, J. E.; Hilmas, G. E.; Evans, L. R.; Corral, E. L. Coextrusion of Zirconia−Iron Oxide Honeycomb Substrates for Solar-Based Thermochemical Generation of Carbon Monoxide for Renewable Fuels. Energy Fuels 2012, 26, 712−721. (25) Meng, Q. L.; Lee, C. I.; Shigeta, S.; Kaneko, H.; Tamaura, Y. Solar hydrogen production using Ce1‑xLixO2‑δ solid solutions via a thermochemical, two-step water-splitting cycle. J. Solid State Chem. 2012, 194, 343−351. (26) Varsano, F.; Padella, F.; La Barbera, A.; Alvani, C. The carbonatation reaction of layered Na(Mn1/3Fe2/3)O2: A high temperature study. Solid State Ionics 2011, 187, 19−26. (27) Shin, Y. J.; Park, M. H.; Kwak, J. H.; Namgoong, H.; Han, O. H. Ionic conduction properties of layer-type oxides NaxMx/2(II)Ti1‑x/2(IV)O2 (M=Ni, Co; 0.60 < x

Hydrogen Production by the Sodium Manganese Ferrite

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