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Reduction and oxidation kinetics of different phases of iron oxides
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e8
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Reduction and oxidation kinetics of different phases of iron oxides Min Hye Jeong, Dong Hyun Lee, Jong Wook Bae* School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon, Gyeonggi-do, 440-746, Republic of Korea
article info
abstract
Article history:
The redox properties and kinetic parameters of three different phases of as-received iron
Received 10 October 2014
ores such as g-FeOOH, Fe2O3, and Fe3O4 were investigated using isothermal experiments
Received in revised form
for reduction and oxidation reaction using hydrogen and H2O, respectively, to investigate
19 December 2014
the possibility for chemical looping process to produce pure hydrogen. Among the selected
Accepted 22 December 2014
kinetic equations of three different iron oxide ores for the reduction at 800 C and oxidation
Available online xxx
in the temperature ranges 700e850 C, three-dimensional diffusion kinetic model such as Jander equation was well fitted with the experimental redox reactions of the three different
Keywords:
iron ores. The calculated activation energies of iron ores with a main phase of g-FeOOH,
Iron ores
Fe2O3, and Fe3O4, for the oxidation reaction with H2O, were found to be around 21.5, 48.9,
Redox reactions
and 11.0 kJ/mol, respectively. The smaller activation energy and larger oxidation rate were
Kinetics
observed on the Fe3O4ephase iron ore owing to its large reduction rate and stable particle
Iron oxide phases
size maintenance even under high temperature redox reactions, and it is suitable for the
Hydrogen production
further application to chemical looping process.
Chemical looping process
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The production of hydrogen fuel from water by the chemical looping (CL) process has been recognized as an environmentally begin alternative process compared to the conventional reforming processes of fossil fuels [1]. The material developments of an efficient hydrogen production by the CL combustion process can be simultaneously contributed to the efficient utilization of energy and mitigation of greenhouse gases as well [2]. Furthermore, for resolving a global energy crisis and an environmental problem, the CL process can be of extreme significance in the near future for the larger amount of hydrogen production from alternative energy sources such as byproduct gases originated form steel industry without further energy consuming separation processes. One possible
way to meet the CL process requirements, the redox (reduction and subsequent oxidation of metal oxides) reaction particularly using the as-received iron ores, can be one of the promising process compared to other hydrocarbon-based processes owing to its advantage for the direct production of high-purity hydrogen fuel [1,2]. The CL process for the hydrogen production is based on the well-known gasesolid reaction using steam and iron oxide materials. H2 storage and recovery by iron oxides for the CL process can be described as the following separate reversible redox reactions [3,4]: Reaction (1): Chemical hydrogen storage (reduction of magnetite)
Fe3O4(s) þ 4H2(g) / 3Fe(s) þ 4H2O(g)
* Corresponding author. Tel.: þ82 31 290 7347; fax: þ82 31 290 7272. E-mail address: [email protected] (J.W. Bae). http://dx.doi.org/10.1016/j.ijhydene.2014.12.099 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Reaction (2): Hydrogen recovery (water-splitting reaction)
3Fe(s) þ 4H2O(g) / Fe3O4(s) þ 4H2(g) For the redox reactions of Fe3O4, iron oxides should be previously reduced into metallic iron according to Reaction (1), and the reduced metallic iron particles can sequentially generate hydrogen gas by the reduction of H2O vapor following Reaction (2) with the simultaneous oxidation of metallic iron to iron oxides such as Fe3O4 or Fe2O3 species. During the redox reaction, the theoretical amount of hydrogen evolution has been reported to be around 4.8 wt% for metallic iron, corresponding to 4.2 103 L (STP) hydrogen gas per 1 L of metallic iron by assuming no porosity and no pore structures of iron oxides [4]. These results interestingly show a relatively high hydrogen storage capacity of iron oxides compared to the possible hydrogen absorbing alloys [4,5]. In addition, some modifiers such as Cr, Al, Zr, Mo, or their bimetal mixtures can alter the capacity of hydrogen production with different thermal stability of iron oxides during the CL process, and Mo modifier has been reported as a proper modifier at a lower temperature application [4]. Moreover, commercially available iron ores can exist in the forms of magnetite (Fe3O4), hematite (Fe2O3), and wustite (FeO or FexO) depending on the different oxidation temperatures [5], which can be also sensitive for the hydrogen storage capacity depending on the redox reaction conditions. The reduction of iron oxides to metallic iron follows different routes according to the reduction temperature by forming complex phases of iron oxides, especially at around 570 C. Moreover, the redox behaviors of iron oxides can be strongly affected by the particle size, crystallinity, types of additives (or impurities in iron ores), and the pretreatment and reaction conditions [4,6,7], indicating the complicated kinetic behaviors of iron oxides. Since the solid-state phase transformation of iron oxides plays an important role in the production of hydrogen gas, a significant interest has been focused on the proper description of the kinetics such as the timetemperature behaviors of the phase transformation of iron oxides to properly design the CL process. Therefore, the main iron oxide phases of Fe2O3, Fe3O4, and FeO have been largely investigated for the derivation of kinetic models of the redox reaction of the synthesized iron oxides [7]. The experiments for the kinetic model derivation of iron oxides are generally performed by an isothermal or a nonisothermal method [8,9]. Nonisothermal kinetic analyses have been reported by increasing the temperature with a constant ramping rate such as TPR (temperature-programmed reduction) [8] or CRTA (constant rate thermal analysis) method [10,11]. For describing the multistage reduction of metal oxides, isothermal methods are also largely applied to derive the proper kinetic models by estimating activation energy (Ea) and rate constant of k at a specific conversion of redox reactions of iron oxides. Various kinetic models for the redox reaction of iron oxides have been largely reported with the assumptions of the phase nucleation by the growth model of active sites through the diffusionlimited or reaction-limited models, etc [12]. The present investigation is focused on the study of the redox properties of three different commercially available
iron ores in steel industries, which are main iron phases such as goethite (g-FeOOH), hematite (Fe2O3), and magnetite (Fe3O4) to select the proper iron ore for the possible application in the CL process. The kinetic parameters such as Ea and k through the isothermal redox reactions on these iron ores were derived using the three-dimensional diffusional Jander equation model, and we finally suggest the proper iron oxide phases for the production of hydrogen by water splitting reaction for the further application in the CL process.
Experimental Pretreatment of iron ores and redox activity tests Three different iron ores with different main iron oxide phases (denoted as FeOOH, Fe2O3, and Fe3O4) were purchased and dried at 80 C for 24 h followed by the calcination at 400 C for 3 h. The calcined fresh iron ores were sieved with a size distribution of 62e125 mm prior to the experiment. The redox reaction was performed in a continuous flow fixed-bed quartz reactor under atmospheric pressure. A sample of 1.0 g was packed between quartz wool in the middle of the quart tube reactor and was previously reduced using 10 vol%H2/N2 mixed gases at a flow rate of 20 cm3(STP)/min at 850 C for 1 h. After the reduction, the sample was subsequently purged until the residual hydrogen gas in the sample-bed was thoroughly removed using the inert N2 gas at the same temperature. Subsequent oxidation reaction was performed by feeding of water vapor using a liquid syringe pump under N2 flow. The flow rate of steam was fixed at 10 cm3(STP)/min, and N2 was used as the carrier gas with a flow rate of 20 cm3(STP)/min in the temperature range 700e850 C using the separate iron ore samples. The effluent gases of H2 and N2 from the reactor were analyzed using a gas chromatograph (YoungLin ACME6100 GC) equipped with a thermal conductivity detector connected with carbosphere packed column.
Characterizations of iron ores Powder X-ray diffraction (XRD) patterns of the three different iron ores were obtained using a Bruker X-ray diffractometer system (D8 Advance) equipped with 2.2 kW Cu anode with a long fine focus ceramic X-ray tube for generating CuKa radi˚ ), which was used as an X-ray source (40 kV and ation (1.5405 A 100 mA) to obtain the XRD patterns of the as-received iron ore samples. The intensity of the diffracted radiation and peak position of the samples as a function of 2q in the ranges of 20e80 at a scan rate of 4 /min were recorded. The obtained XRD patterns were compared to the standard peaks to identify the crystalline phases of iron ores. The grain sizes of the iron oxide species were also calculated using the values of fullwidth at half maximum (FWHM) by the Scherrer's equation. Moreover, the elemental analysis of three different iron ores was performed by X-ray fluorescence (XRF) spectrometry (Bruker AXS S4 Pioneer) to verify the iron oxide phases and concentrations of impurities. The surface areas of the as-received iron ores were characterized by the BrunauereEmmetteTeller (BET) method from the nitrogen adsorptionedesorption isotherms obtained at
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Table 1 e Elemental analysis of the three different iron ores by XRF analysis. Iron ores
FeOOH Fe2O3 Fe3O4
Atomic ratio of (O/Fe)a
Elements (atomic percentage) Fe
O
Si
Al
Others
24.4 33.4 35.8
66.4 61.9 58.5
4.8 2.6 4.7
4.0 1.3 0.2
0.4 0.8 0.8
2.08 1.64 1.37
Surface area (m2/g)
93.5 7.5 1.7
XRD (size/nm) Before
After
37 104 85
47 87 85
a
Atomic ratios of O/Fe on the three different iron ores were calculated by considering the concentration of O and Fe atoms in the iron ores with the assumptions of the presence of SiO2 and Al2O3 metal oxide phases.
196 C using a Micromeritics ASAP 2000 instrument. The pore size distributions of the iron ores were also determined by the BarretteJoynereHalenda (BJH) method from the desorption isotherm branch of the nitrogen sorption method. Temperature-programmed reduction (TPR) patterns of the three different iron ores were investigated to determine the reducibility of the iron oxides and the possible phase transformation during the reduction and oxidation steps. Prior to TPR experiment, iron ore was previously pretreated under He flow at 300 C for 2 h to remove the adsorbed water followed by cooling to 50 C. The reducing gas containing 10%H2/Ar mixed gas was passed over the sample at a flow rate of 30 mL(STP)/ min at a heating rate of 10 C/min from 50 to 800 C. The effluent gas was passed over a molecular sieve trap to remove water formed during the TPR experiment, and they were analyzed by GC equipped with TCD. Temperatureprogrammed oxidation (TPO) was also performed under the same conditions of TPR experiment using 10%O2/He mixed gas. The binding energy (BE) of the as-received iron ores was characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB MK-II). During the experiment, the AlKa monochromatized line (1486.6 eV) was adopted, and the vacuum level was kept at around 107 Pa. The iron ore sample was previously pressed into a thin pellet for the analysis, and the BE was corrected with the reference BE of C1s (284.4 eV).
93.5, 7.5, and 1.7 m2/g for the iron ores with the main phases of FeOOH, Fe3O4, and Fe2O3, respectively, as summarized in Table 1. In addition, the average pore diameters of iron ores were found to be around 3e4 nm for the iron ores with the phases of FeOOH and Fe3O4 as shown in Fig. 1. However, these different surface areas and pore size effects of iron ores were not directly related to the redox properties according to the types of iron ores. The impurities of Al, Si, and others can also affect the redox properties of iron ores; however, the effects of impurities were not considered in this investigation because of their lower concentration with similar composition for insignificantly changing the reduction and oxidation behaviors of the main iron phases on the as-received iron ores. Fig. 2 shows the XRD patterns of the three different iron ores before and after the redox reactions at 850 C. The main phases of iron ores were observed at the characteristic peaks at 2q values of 21.2 and 35.5 , 33.1 and 35.5 , and 35.4 and 35.5 for FeOOH, Fe2O3, and Fe3O4, respectively, as shown in Fig. 2(a), and they were well correlated to the results of the XRF analyses. After the redox reaction on the three different iron ores, all the characteristic XRD peaks were transformed to the
Results and discussion Characterization of iron ores by XRD and XRF analysis The chemical compositions of the three different iron ores were determined by XRF, and the results are summarized in Table 1. The observed atomic ratios of O/Fe in the iron ores were found to be around 2.08, 1.64, and 1.37 for the FeOOH, Fe2O3, and Fe3O4 containing iron ores, respectively, which were calculated by compensating the oxygen contents in the other metal oxides with the assumption of the main phases of Al and Si for the fully oxides stable metal oxides such as SiO2 and Al2O3. These values indicate that the iron ores are mainly in the phases of FeOOH, Fe2O3, and Fe3O4, respectively, and they are closely corresponding to the theoretical values of O/ Fe atomic ratios of the pure FeOOH, Fe2O3, and Fe3O4 phases, respectively. Moreover, the iron ores also contain other impurities in the range 0.4e0.8 atomic%, increasing in the order of FeOOH > Fe3O4 > Fe2O3 without significant variation. The surface areas of the as-received samples were found to be
Fig. 1 e Pore size distribution of the as-received iron ores with different phases of FeOOH, Fe2O3, and Fe3O4.
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Fig. 2 e XRD patterns of the three different iron ores (a) before and (b) after the redox reaction.
magnetite (Fe3O4) phases. Interestingly, even after the redox reactions using hydrogen and water, the main bulk phases of iron ores were not completely transformed to the fully oxidized hematite (Fe2O3) phase because of the restricted oxygen transfer to the bulk iron ores possibly [10,11]. The calculated grain sizes of the as-received iron ores were found to be around 37, 104, and 85 nm for FeOOH, Fe2O3, and Fe3O4, respectively. After the redox reaction at 850 C as shown in Fig. 2(b) and Table 1, the grain sizes of Fe3O4 were significantly altered for the iron ores having the phases of FeOOH and Fe2O3 with the values of 47 and 87 nm, respectively. During the redox reactions, the increased grain size of FeOOH would be attributed to the possible aggregation of iron grain particles
because of its originally high surface area of the as-received iron ore. However, the reduced grain size from 104 to 87 nm of Fe2O3 seems to be possibly induced from the redox step by the rearrangement of iron oxide phases. However, the insignificant variation in the grain size on magnetite (Fe3O4) with the value at around 85 nm was observed because of the stabilized Fe3O4 phase during the redox reaction even at a high temperature of 850 C. To further confirm the incomplete oxidation of iron ores even after high temperature redox reactions, XPS analysis was also performed using the iron ores after the oxidation reaction, and the results are shown in Fig. 3. The observed characteristic binding energy of Fe2p3/2 at around 710.5 eV for all the tested iron ore samples indicates that the surfaces of iron ores after the oxidation reaction mainly exist in the hematite (Fe2O3) phase, and the partial oxidation of iron ore's outermost surfaces can be attributed to the limited oxygen transfer from water to the bulk phase of iron ores, as observed by forming a bulk main phase of Fe3O4 confirmed by the XRD analysis.
Characterizations of iron ores by TPR and TPO analysis
Fig. 3 e Results of the XPS analysis on the three different iron ores after the redox reaction.
To investigate the reduction behaviors of the three different iron ores, TPR analysis was performed, and the reduction patterns are shown in Fig. 4. The reduction peaks showed a Gaussian distribution with different maximum reduction temperatures. In general, the two distinct reduction peaks of Fe2O3 have been reported with the separate reduction stages by the sequential reduction through Fe2O3 / Fe3O4 / (FeO) / Fe [13]. As shown in Fig. 4(a), the first reduction peak was observed at 382 and 556 C for FeOOH and Fe2O3, respectively; however, the first reduction peak of Fe3O4 was observed at much higher temperature of 785 C, revealing that the FeOOH phase can be easily reduced because
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Fig. 4 e Patterns of (a) TPR and (b) TPO of the as-received iron ores. of its higher surface area with facile transport of hydrogen to the inner pores of FeOOH. Based on the TPR experiment, the FeOOH phase can be assigned to be the three-step reduction by showing the first reduction peak for a partial surface reduction of FeOOH at a lower temperature of 382 C with a possible dehydration to Fe2O3 [14], and the second reduction peak at 597 C for the reduction to Fe3O4 combined with the formation of FeO phase, and finally complete reduction to metallic iron at 731 C through the multistage reduction of FeOOH / Fe2O3 / Fe3O4 (FeO) / Fe. In the case of Fe2O3, the reduction peaks were deconvoluted to two characteristic peaks at around 556 and 721 C for the reduction steps of Fe2O3 / Fe3O4 (FeO) and the subsequent reduction to iron metals, respectively. Even though both the FeOOH and Fe2O3 phases seem to be completely reduced to metallic iron below 900 C, there possibly remains some unreduced Fe3O4 grains in the core of iron ores after the pretreatments as confirmed by the XRD analysis. Interestingly, the higher reduction temperature of Fe3O4 iron ore was observed, and the complicated reduction behaviors of Fe3O4 are consistent with the previously reported literature [13] by showing the reduction steps of Fe3O4 / FeO / Fe. As shown in Fig. 4(a), geothite (FeOOH) and hematite (Fe2O3) ores were almost reduced in our reduction condition of 850 C; however, magnetite (Fe3O4) partially reduced in our reduction condition. Fig. 4(b) shows the results of the TPO experiments of the asreceived three different iron ores. FeOOH iron ore showed a narrow oxidation peak at ~300 C, with the two deconvoluted peaks at 291 and 351 C. The oxidation behaviors of FeOOH can be described by two-step oxidations as follows; FeO / Fe3O4 / Fe2O3 as reported previously [15]. However, the fully oxidized Fe2O3 iron ore showed no oxidation peak owing to the stable final oxidation state. Interestingly, TPO
profile of Fe3O4 iron ore showed the complicated oxidation steps by the three-step oxidation. The first peak at the maximum peak around 326 C can be assigned to the desorption of adsorbed OH; the second peak at around 567 C is for the oxidation of Fe3O4 to Fe2O3 at the outer surfaces; the third peak at around 843 C is for the bulk oxidation by forming Fe2O3 phases. Even though these different oxidation behaviors of the different iron ores can be possibly attributed to the different surface areas and different phases, the masstransfer rate of hydrogen and oxygen from water to the bulk iron ores can more dominantly change their different redox properties, indicating that the consideration of mass-transfer properties should not be overlooked for deriving the proper kinetic equations for the redox reaction of iron ores.
Redox activity and kinetic analysis of three different iron ores The kinetic models for the redox reaction of iron oxides have been largely investigated by many researchers with the assumption of a gasesolid reaction using two or threedimensional diffusion models [16e20]. The well-known procedures to properly fit the experimental results with kinetic models are the “model fitting integral method”. Based on those models, the rate constant (k(T)) and activation energy (Ea) can be determined by fitting the integral form of g(a) with experimental data, where a represents the extent of reaction (conversion of H2 or H2O for the reduction and oxidation reaction). The integral form of g(a) can be derived from the integration of the kinetic model of f(a) as listed in Table 2. The kinetic models for the redox reaction of iron oxides can be described using a conventional power law equation such as first-order and nth-order reactions and three-dimensional
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Table 2 e Selected kinetic equations of f(a) and the integral forms of g(a) for a gasesolid redox reaction of iron oxides [11,18e20]. Reaction model Conventional kinetics (n ¼ 0, zero order): Polany-Wigner equation Conventional kinetics (n ¼ 0.5, half-order) Phase-boundary controlled reaction Conventional kinetics (n ¼ 1, first order) Conventional kinetics (n ¼ 1.5, 1.5th order) Conventional kinetics (n ¼ 2, 2nd order) Jander equation: Three-dimensional diffusion
diffusion Jander equation [11,18e20]. The temperaturedependent rate constant k can be determined from the Arrhenius equation by using the results of the isothermal experiments with a proper mathematical model. Therefore, we carefully investigated the fittability of the listed kinetic models for the redox reaction of iron ores listed in Table 2 including conventional ones and Jander equation. Table 2 also shows some representative mathematical kinetic equations of f(a) and their integral forms of g(a) in this study. The best equation was selected by fitting various kinetic models of g(a) with the experimental data and by finding out the higher R2 value, indicating a good linearity. The selected best fitting model can be further used to derive the kinetic parameters of k and Ea on the three different iron ores. As shown in Fig. 5, the cumulative hydrogen consumption with time on the stream during the reduction stage at an isothermal temperature of 850 C for about 80 min is shown in Fig. 5(a) for FeOOH, Fe2O3, and Fe3O4, respectively. The total amounts of consumed hydrogen were found to be 10.5, 9.3, 5.9 mmol/gcat for FeOOH, Fe2O3, and Fe3O4, respectively, corresponding to the integrated area of each plot. The hydrogen consumption increased in the following order: FeOOH > Fe2O3 > Fe3O4, and it is well correlated to the results of the TPR experiments. However, the hydrogen storage capacity of iron ores seems not to be directly correlated to the reduction capacity. As shown in Fig. 5(b), the amounts of hydrogen production by the water oxidation at 850 C showed the similar trends with the production capacity of 2.27 > 1.98 z 1.85 mmol gcat1 for the FeOOH, Fe3O4, and Fe2O3, respectively. This can be possibly attributed to the transformation of all the tested iron ores to the same bulk phase of Fe3O4 irrespective to the phases of the as-received iron ores. Even though the impurities such as Mo, Rh, and Co have been reported to show a positive effect by reducing activation energies and enhancing the stabilities of iron oxides [4,21e23], the other factors should not be overlooked to explain the different redox properties of the present asreceived iron ores. Therefore, the complete oxidation of all the tested iron ores at the outermost surfaces of Fe2O3 phase by maintaining the bulk Fe3O4 phase as confirmed by the XRD and XPS analyses, seems to be the main reason for the similar hydrogen production capacity because of the restricted diffusion of oxygen species by water-splitting reaction through the first formed dense Fe2O3 outermost surfaces for the further reduction of bulk Fe3O4 phase [14,21]. It suggests that the kinetic model possessing the three-dimensional diffusion mechanism such as Jander equation seem to be more proper to describe the redox kinetics of as-received iron ores.
f(a) functions
g(a) functions
1 2(1a)1/2 (1a) 2(1a)3/2 1(1a)3/2 3(1a)1/3/2[(1a)1/31]
a 2[1(1-a)]1/2 ln(1a) [1(1a)1/2]$(2) (1a)1/21 [1(1a)1/3]2
As shown in Fig. 6, the integrated kinetic models of g(a) functions are compared using the selected equations listed in Table 2. The Jander equation, represented as g(a) ¼ [1(1a)1/ 3 2 ] ¼ kt where a is for the cumulative conversion of water, showed a good linearity among the selected models. Because the Jander equation is based on the three-dimensional diffusion mechanism for a gasesolid noncatalytic reaction system, this model shows a good fittability compared to the other
Fig. 5 e Activity of the redox reaction of the as-received iron ores, (a) the consumed amount of H2 with time on stream at 850 C, and (b) the generated amount of H2 by water splitting reaction at the representative temperature of 850 C.
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Fig. 6 e The fitted kinetics models listed in Table 2 for the redox reactions of the three different iron ores at (a) reduction at 850 C and (b) oxidation by water at 850 C.
conventional kinetic models in the tested oxidation temperature of 850 C while using the data for 0.3 a 0.7. Using the Jander equation, the rate constants for the oxidation at 850 C for the reduced iron ores were found to be 0.0857, 0.0450, and 0.0894 min1 for FeOOH, Fe2O3, and Fe3O4, respectively, as summarized in Table 3. It seems to be irrespective to the surface areas of iron ores. Moreover, the rate constants of the reduction at 850 C of the as-received iron ores were observed with the values of 0.0740, 0.0435, and 0.1274 min1 for FeOOH, Fe2O3, and Fe3O4, respectively. These variations in the redox properties seem not to be directly correlated to the results of
the N2 sorption, TPR, and TPO analyses by showing higher rate constants for Fe3O4 iron ores, because a higher surface area and a higher reducibility was observed on iron ore having main FeOOH phase. Therefore, the kinetic model of the threedimensional diffusion Jander equation seems to be proper to describe the redox reaction of the present iron ores by properly considering the transport phenomena of hydrogen and oxygen through outermost dense phase of Fe2O3 formed. The calculated activation energies for the oxidation reaction at the temperatures of 700, 750, 800 and 850 C are also listed in Table 3. By using the Arrhenius plot, the activation energies of FeOOH, Fe2O3, and Fe3O4 for the oxidation were found to be 21.5, 48.9, and 11.0 kJ/mol, respectively. These values are similar to the previously reported activation energies of iron oxides such as 50e100 kJ/mol for Fe2O3 / Fe3O4 or the Fe species [19,20]. In addition, the effects of a higher reduction temperature of 900 C were also investigated on FeOOH, Fe2O3, and Fe3O4 and the rate constants for the reduction and oxidation with their fittability using various kinetic models are shown in supplementary Table S1 and Figure S1. The somewhat lower rate constants for the reduction (0.0479, 0.0329, and 0.0530 min1 for FeOOH, Fe2O3, and Fe3O4, respectively) and the oxidation (0.0581, 0.0571, and 0.0860 min1 for FeOOH, Fe2O3, and Fe3O4, respectively) seem to be possibly attributed to the facile aggregation of iron grains on all the iron ores reduced at 900 C. However, the trend of rate constant variations between the different phases of iron ores is found to be similar with the previous results (reduced iron ores at 850 C) by showing the higher redox properties on the magnetite (Fe3O4). In addition, the results conducted at much lower reduction temperature of 600 C are also shown in supplementary Table S2 and Figure S2 by showing the higher rate constants on magnetite. These results suggest that the magnetite is the proper phase for the possible application for CL process to produce hydrogen by water oxidation. Therefore, the iron ore having a main magnetite phase by showing a higher oxidation rate constant with a lower activation energy without significant aggregation of iron grains even after the redox reaction seems to be proper for the application in the CL process to produce pure hydrogen fuel. The redox reaction rates of iron ores were found to be strongly depending on the types of iron phases, particularly for the activation energy, by mainly changing the mass transport rate through the formed dense phase of the fully oxidized Fe2O3 surfaces on the iron ores irrespective to the surface areas. Even though the reduction and oxidation rates of iron ores proceed through the complicated steps, the proper kinetic model to describe redox reaction seems to be Jander equation and a three-dimensional diffusion mechanism for the further CL applications for the production of hydrogen fuel by the water-splitting reaction.
Table 3 e Estimated kinetic parameters for the reduction and oxidation reaction on the three different iron ores with the main phases of FeOOH, Fe2O3, and Fe3O4. Iron oxides
Rate constants of k (min1)
FeOOH Fe2O3 Fe3O4
Activation energy (Ea; kJ/mol) for oxidation (700e850 C)
R2 for Ea
21.5 48.9 11.0
0.94 0.96 0.98
Reduction (850 C)
Oxidation (850 C)
0.0740 0.0435 0.1274
0.0857 0.0450 0.0894
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Conclusions The proper kinetic model for the redox reaction of the three different iron ores having a main phase of FeOOH, Fe2O3, and Fe3O4 phases, respectively, was found to be the Jander equation based on the three-dimensional diffusion mechanism by the isothermal experiments, and the rate constant was not strongly affected by the surface area of iron ores. The Jander equation provide the best fittability of the experimental data, and the activation energy of FeOOH, Fe2O3, and Fe3O4 were found to be around 21.5, 48.9, and 11.0 kJ/mol, respectively, with the higher rate constants for the reduction and oxidation reactions of Fe3O4. The iron ore having a Fe3O4 phase by showing a higher rate constant with small activation energy without a significant aggregation of iron grains even after the redox reaction seems to be proper for the further application in CL process. Even though the redox reaction of iron ores proceeded by the complicated steps, the as-received iron ore having a main phase of magnetite (Fe3O4) can be properly applied for the CL process to produce pure hydrogen from water-splitting reaction at higher temperatures.
Acknowledgments This study was financially supported by the grant from the Industrial Source Technology Development Programs (201310042712) of the Ministry of Knowledge Economy (MKE) of Korea. The authors would like to acknowledge the financial support from the National Research Foundation of Korea (NRF) grant funded by the Korea government (NRF2014R1A1A2A16055557). This study was also supported by the R&D Center for Valuable Recycling (Global-Top R&D Program) of the Ministry of Environment with a project number of GT14-C-01-038-0. I would like to appreciate for the technical advices from Dr. Jonghwun Jung and Dr. Sang-Ho Yi with a research fund from POSCO (POSCO-2014X027).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.12.099.
references
[1] Bhavsar S, Najera M, Solunke R, Veser G. Chemical looping: to combustion and beyond. Catal Today 2014;228:96e105. [2] Lyngfelt A, Leckner B, Mattisson T. A fluidized-bed combustion process with inherent CO2 separation; application of chemical-looping combustion. Chem Eng Sci 2001;56:3101e13. [3] Pena JA, Lorente E, Romero E, Herguido J. Kinetic study of the redox process for storing hydrogen: reduction stage. Catal Today 2006;116:439e44.
[4] Hui W, Takenaka S, Otsuka K. Hydrogen storage properties of modified fumed-Fe-dust generated from a revolving furnace at a steel industry. Int J Hydrogen Energy 2006;31:1732e46. [5] Munteanu G, Ilieva L, Andreeva D. Kinetic parameters obtained from TPR data for a-Fe2O3 and Au/a-Fe2O3 systems. Thermochim Acta 1997;291:171e7. [6] Jankovic B. Isothermal reduction kinetics of nickel oxide using hydrogen: conventional and Weibull kinetic analysis. J Phys Chem Solids 2007;68:2233e46. [7] Takenaka S, Kaburagi T, Yamada C, Nomura K, Otsuka K. Production of pure hydrogen from methane mediated by the redox of Ni- and Cr-added iron oxides. J Catal 2004;228:405e16. [8] Jozwiak WK, Kaczmarek E, Maniecki TP, Ignaczak W, Maniukiewicz W. Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres. Appl Catal A e Gen 2007;326:17e27. [9] Vyazovkin S, Wight CA. Isothermal and non-isothermal kinetics of thermally stimulated reactions of solids. Int Rev Phys Chem 1998;17:407e33. [10] Tiernan MJ, Barnes PA, Parkes GMB. Reduction of iron oxide catalysts: the investigation of kinetic parameters using rate perturbation and linear heating thermo analytical techniques. J Phys Chem B 2001;105:220e8. [11] Gotor FJ, Criado JM, Malek J, Koga N. Kinetic analysis of solidstate reactions: the universality of master plots for analyzing isothermal and nonisothermal experiments. J Phys Chem A 2000;104:10777e82. [12] Khawam A, Flanagan DR. Solid-state kinetic models: basics and mathematical fundamentals. J Phys Chem B 2006;110:17315e28. [13] Zielinski J, Zglinickab I, Znaka L, Kaszkur Z. Reduction of Fe2O3 with hydrogen. Appl Catal A e Gen 2010;381:191e6. [14] Rosmaninho MG, Moura FCC, Souza LR, Nogueira RK, Gomes GM, Nascimento JS, et al. Investigation of iron oxide reduction by ethanol as a potential route to produce hydrogen. Appl Catal B e Environ 2012;115e116:45e52. [15] Basinska A, Jozwiak WK, Goralski J, Domka F. The behaviour of Ru/Fe2O3 catalysts and Fe2O3 supports in the TPR and TPO conditions. Appl Catal A e Gen 2000;190:107e15. [16] Galvita V, Sundmacher K. Redox behavior and reduction mechanism of Fe2O3eCeZrO2 as oxygen storage material. J Mater Sci 2007;42:9300e7. [17] Wimmers OJ, Arnoldy P, Moulijna JA. Determination of the reduction mechanism by temperature-programmed reduction: application to small iron oxide (Fe2O3) particles. J Phys Chem 1986;90:1331e7. [18] Wen F, Wang H, Tang Z. Kinetic study of the redox process of iron oxide for hydrogen production at oxidation step. Thermochim Acta 2011;520:55e60. [19] Pineau A, Kanari N, Gaballah I. Kinetics of reduction of iron oxides by H2 (Part I): low temperature reduction of hematite. Thermochim Acta 2006;447:89e100. [20] Lin HY, Chen YW, Li C. The mechanism of reduction of iron oxide by hydrogen. Thermochim Acta 2003;400:61e7. [21] Takenaka S, Kaburagi T, Yamada C, Nomura K, Otsuka K. Storage and supply of hydrogen by means of the redox of the iron oxides modified with Mo and Rh species. J Catal 2004;228:66e74. [22] Wang H, Wang G, Wang X, Bai J. Hydrogen production by redox of cation-modified iron oxide. J Phys Chem C 2008;112:5679e88. [23] Scheffe JR, Allendorf MD, Coker EN, Jacobs BW, McDaniel AH, Weimer AW. Hydrogen production via chemical looping redox cycles using atomic layer deposition-synthesized iron oxide and cobalt ferrites. Chem Mater 2011;23:2030e8.
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Available online at www.sciencedirect.com
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Reduction and oxidation kinetics of different phases of iron oxides Min Hye Jeong, Dong Hyun Lee, Jong Wook Bae* School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon, Gyeonggi-do, 440-746, Republic of Korea
article info
abstract
Article history:
The redox properties and kinetic parameters of three different phases of as-received iron
Received 10 October 2014
ores such as g-FeOOH, Fe2O3, and Fe3O4 were investigated using isothermal experiments
Received in revised form
for reduction and oxidation reaction using hydrogen and H2O, respectively, to investigate
19 December 2014
the possibility for chemical looping process to produce pure hydrogen. Among the selected
Accepted 22 December 2014
kinetic equations of three different iron oxide ores for the reduction at 800 C and oxidation
Available online xxx
in the temperature ranges 700e850 C, three-dimensional diffusion kinetic model such as Jander equation was well fitted with the experimental redox reactions of the three different
Keywords:
iron ores. The calculated activation energies of iron ores with a main phase of g-FeOOH,
Iron ores
Fe2O3, and Fe3O4, for the oxidation reaction with H2O, were found to be around 21.5, 48.9,
Redox reactions
and 11.0 kJ/mol, respectively. The smaller activation energy and larger oxidation rate were
Kinetics
observed on the Fe3O4ephase iron ore owing to its large reduction rate and stable particle
Iron oxide phases
size maintenance even under high temperature redox reactions, and it is suitable for the
Hydrogen production
further application to chemical looping process.
Chemical looping process
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The production of hydrogen fuel from water by the chemical looping (CL) process has been recognized as an environmentally begin alternative process compared to the conventional reforming processes of fossil fuels [1]. The material developments of an efficient hydrogen production by the CL combustion process can be simultaneously contributed to the efficient utilization of energy and mitigation of greenhouse gases as well [2]. Furthermore, for resolving a global energy crisis and an environmental problem, the CL process can be of extreme significance in the near future for the larger amount of hydrogen production from alternative energy sources such as byproduct gases originated form steel industry without further energy consuming separation processes. One possible
way to meet the CL process requirements, the redox (reduction and subsequent oxidation of metal oxides) reaction particularly using the as-received iron ores, can be one of the promising process compared to other hydrocarbon-based processes owing to its advantage for the direct production of high-purity hydrogen fuel [1,2]. The CL process for the hydrogen production is based on the well-known gasesolid reaction using steam and iron oxide materials. H2 storage and recovery by iron oxides for the CL process can be described as the following separate reversible redox reactions [3,4]: Reaction (1): Chemical hydrogen storage (reduction of magnetite)
Fe3O4(s) þ 4H2(g) / 3Fe(s) þ 4H2O(g)
* Corresponding author. Tel.: þ82 31 290 7347; fax: þ82 31 290 7272. E-mail address: [email protected] (J.W. Bae). http://dx.doi.org/10.1016/j.ijhydene.2014.12.099 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Reaction (2): Hydrogen recovery (water-splitting reaction)
3Fe(s) þ 4H2O(g) / Fe3O4(s) þ 4H2(g) For the redox reactions of Fe3O4, iron oxides should be previously reduced into metallic iron according to Reaction (1), and the reduced metallic iron particles can sequentially generate hydrogen gas by the reduction of H2O vapor following Reaction (2) with the simultaneous oxidation of metallic iron to iron oxides such as Fe3O4 or Fe2O3 species. During the redox reaction, the theoretical amount of hydrogen evolution has been reported to be around 4.8 wt% for metallic iron, corresponding to 4.2 103 L (STP) hydrogen gas per 1 L of metallic iron by assuming no porosity and no pore structures of iron oxides [4]. These results interestingly show a relatively high hydrogen storage capacity of iron oxides compared to the possible hydrogen absorbing alloys [4,5]. In addition, some modifiers such as Cr, Al, Zr, Mo, or their bimetal mixtures can alter the capacity of hydrogen production with different thermal stability of iron oxides during the CL process, and Mo modifier has been reported as a proper modifier at a lower temperature application [4]. Moreover, commercially available iron ores can exist in the forms of magnetite (Fe3O4), hematite (Fe2O3), and wustite (FeO or FexO) depending on the different oxidation temperatures [5], which can be also sensitive for the hydrogen storage capacity depending on the redox reaction conditions. The reduction of iron oxides to metallic iron follows different routes according to the reduction temperature by forming complex phases of iron oxides, especially at around 570 C. Moreover, the redox behaviors of iron oxides can be strongly affected by the particle size, crystallinity, types of additives (or impurities in iron ores), and the pretreatment and reaction conditions [4,6,7], indicating the complicated kinetic behaviors of iron oxides. Since the solid-state phase transformation of iron oxides plays an important role in the production of hydrogen gas, a significant interest has been focused on the proper description of the kinetics such as the timetemperature behaviors of the phase transformation of iron oxides to properly design the CL process. Therefore, the main iron oxide phases of Fe2O3, Fe3O4, and FeO have been largely investigated for the derivation of kinetic models of the redox reaction of the synthesized iron oxides [7]. The experiments for the kinetic model derivation of iron oxides are generally performed by an isothermal or a nonisothermal method [8,9]. Nonisothermal kinetic analyses have been reported by increasing the temperature with a constant ramping rate such as TPR (temperature-programmed reduction) [8] or CRTA (constant rate thermal analysis) method [10,11]. For describing the multistage reduction of metal oxides, isothermal methods are also largely applied to derive the proper kinetic models by estimating activation energy (Ea) and rate constant of k at a specific conversion of redox reactions of iron oxides. Various kinetic models for the redox reaction of iron oxides have been largely reported with the assumptions of the phase nucleation by the growth model of active sites through the diffusionlimited or reaction-limited models, etc [12]. The present investigation is focused on the study of the redox properties of three different commercially available
iron ores in steel industries, which are main iron phases such as goethite (g-FeOOH), hematite (Fe2O3), and magnetite (Fe3O4) to select the proper iron ore for the possible application in the CL process. The kinetic parameters such as Ea and k through the isothermal redox reactions on these iron ores were derived using the three-dimensional diffusional Jander equation model, and we finally suggest the proper iron oxide phases for the production of hydrogen by water splitting reaction for the further application in the CL process.
Experimental Pretreatment of iron ores and redox activity tests Three different iron ores with different main iron oxide phases (denoted as FeOOH, Fe2O3, and Fe3O4) were purchased and dried at 80 C for 24 h followed by the calcination at 400 C for 3 h. The calcined fresh iron ores were sieved with a size distribution of 62e125 mm prior to the experiment. The redox reaction was performed in a continuous flow fixed-bed quartz reactor under atmospheric pressure. A sample of 1.0 g was packed between quartz wool in the middle of the quart tube reactor and was previously reduced using 10 vol%H2/N2 mixed gases at a flow rate of 20 cm3(STP)/min at 850 C for 1 h. After the reduction, the sample was subsequently purged until the residual hydrogen gas in the sample-bed was thoroughly removed using the inert N2 gas at the same temperature. Subsequent oxidation reaction was performed by feeding of water vapor using a liquid syringe pump under N2 flow. The flow rate of steam was fixed at 10 cm3(STP)/min, and N2 was used as the carrier gas with a flow rate of 20 cm3(STP)/min in the temperature range 700e850 C using the separate iron ore samples. The effluent gases of H2 and N2 from the reactor were analyzed using a gas chromatograph (YoungLin ACME6100 GC) equipped with a thermal conductivity detector connected with carbosphere packed column.
Characterizations of iron ores Powder X-ray diffraction (XRD) patterns of the three different iron ores were obtained using a Bruker X-ray diffractometer system (D8 Advance) equipped with 2.2 kW Cu anode with a long fine focus ceramic X-ray tube for generating CuKa radi˚ ), which was used as an X-ray source (40 kV and ation (1.5405 A 100 mA) to obtain the XRD patterns of the as-received iron ore samples. The intensity of the diffracted radiation and peak position of the samples as a function of 2q in the ranges of 20e80 at a scan rate of 4 /min were recorded. The obtained XRD patterns were compared to the standard peaks to identify the crystalline phases of iron ores. The grain sizes of the iron oxide species were also calculated using the values of fullwidth at half maximum (FWHM) by the Scherrer's equation. Moreover, the elemental analysis of three different iron ores was performed by X-ray fluorescence (XRF) spectrometry (Bruker AXS S4 Pioneer) to verify the iron oxide phases and concentrations of impurities. The surface areas of the as-received iron ores were characterized by the BrunauereEmmetteTeller (BET) method from the nitrogen adsorptionedesorption isotherms obtained at
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Table 1 e Elemental analysis of the three different iron ores by XRF analysis. Iron ores
FeOOH Fe2O3 Fe3O4
Atomic ratio of (O/Fe)a
Elements (atomic percentage) Fe
O
Si
Al
Others
24.4 33.4 35.8
66.4 61.9 58.5
4.8 2.6 4.7
4.0 1.3 0.2
0.4 0.8 0.8
2.08 1.64 1.37
Surface area (m2/g)
93.5 7.5 1.7
XRD (size/nm) Before
After
37 104 85
47 87 85
a
Atomic ratios of O/Fe on the three different iron ores were calculated by considering the concentration of O and Fe atoms in the iron ores with the assumptions of the presence of SiO2 and Al2O3 metal oxide phases.
196 C using a Micromeritics ASAP 2000 instrument. The pore size distributions of the iron ores were also determined by the BarretteJoynereHalenda (BJH) method from the desorption isotherm branch of the nitrogen sorption method. Temperature-programmed reduction (TPR) patterns of the three different iron ores were investigated to determine the reducibility of the iron oxides and the possible phase transformation during the reduction and oxidation steps. Prior to TPR experiment, iron ore was previously pretreated under He flow at 300 C for 2 h to remove the adsorbed water followed by cooling to 50 C. The reducing gas containing 10%H2/Ar mixed gas was passed over the sample at a flow rate of 30 mL(STP)/ min at a heating rate of 10 C/min from 50 to 800 C. The effluent gas was passed over a molecular sieve trap to remove water formed during the TPR experiment, and they were analyzed by GC equipped with TCD. Temperatureprogrammed oxidation (TPO) was also performed under the same conditions of TPR experiment using 10%O2/He mixed gas. The binding energy (BE) of the as-received iron ores was characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB MK-II). During the experiment, the AlKa monochromatized line (1486.6 eV) was adopted, and the vacuum level was kept at around 107 Pa. The iron ore sample was previously pressed into a thin pellet for the analysis, and the BE was corrected with the reference BE of C1s (284.4 eV).
93.5, 7.5, and 1.7 m2/g for the iron ores with the main phases of FeOOH, Fe3O4, and Fe2O3, respectively, as summarized in Table 1. In addition, the average pore diameters of iron ores were found to be around 3e4 nm for the iron ores with the phases of FeOOH and Fe3O4 as shown in Fig. 1. However, these different surface areas and pore size effects of iron ores were not directly related to the redox properties according to the types of iron ores. The impurities of Al, Si, and others can also affect the redox properties of iron ores; however, the effects of impurities were not considered in this investigation because of their lower concentration with similar composition for insignificantly changing the reduction and oxidation behaviors of the main iron phases on the as-received iron ores. Fig. 2 shows the XRD patterns of the three different iron ores before and after the redox reactions at 850 C. The main phases of iron ores were observed at the characteristic peaks at 2q values of 21.2 and 35.5 , 33.1 and 35.5 , and 35.4 and 35.5 for FeOOH, Fe2O3, and Fe3O4, respectively, as shown in Fig. 2(a), and they were well correlated to the results of the XRF analyses. After the redox reaction on the three different iron ores, all the characteristic XRD peaks were transformed to the
Results and discussion Characterization of iron ores by XRD and XRF analysis The chemical compositions of the three different iron ores were determined by XRF, and the results are summarized in Table 1. The observed atomic ratios of O/Fe in the iron ores were found to be around 2.08, 1.64, and 1.37 for the FeOOH, Fe2O3, and Fe3O4 containing iron ores, respectively, which were calculated by compensating the oxygen contents in the other metal oxides with the assumption of the main phases of Al and Si for the fully oxides stable metal oxides such as SiO2 and Al2O3. These values indicate that the iron ores are mainly in the phases of FeOOH, Fe2O3, and Fe3O4, respectively, and they are closely corresponding to the theoretical values of O/ Fe atomic ratios of the pure FeOOH, Fe2O3, and Fe3O4 phases, respectively. Moreover, the iron ores also contain other impurities in the range 0.4e0.8 atomic%, increasing in the order of FeOOH > Fe3O4 > Fe2O3 without significant variation. The surface areas of the as-received samples were found to be
Fig. 1 e Pore size distribution of the as-received iron ores with different phases of FeOOH, Fe2O3, and Fe3O4.
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Fig. 2 e XRD patterns of the three different iron ores (a) before and (b) after the redox reaction.
magnetite (Fe3O4) phases. Interestingly, even after the redox reactions using hydrogen and water, the main bulk phases of iron ores were not completely transformed to the fully oxidized hematite (Fe2O3) phase because of the restricted oxygen transfer to the bulk iron ores possibly [10,11]. The calculated grain sizes of the as-received iron ores were found to be around 37, 104, and 85 nm for FeOOH, Fe2O3, and Fe3O4, respectively. After the redox reaction at 850 C as shown in Fig. 2(b) and Table 1, the grain sizes of Fe3O4 were significantly altered for the iron ores having the phases of FeOOH and Fe2O3 with the values of 47 and 87 nm, respectively. During the redox reactions, the increased grain size of FeOOH would be attributed to the possible aggregation of iron grain particles
because of its originally high surface area of the as-received iron ore. However, the reduced grain size from 104 to 87 nm of Fe2O3 seems to be possibly induced from the redox step by the rearrangement of iron oxide phases. However, the insignificant variation in the grain size on magnetite (Fe3O4) with the value at around 85 nm was observed because of the stabilized Fe3O4 phase during the redox reaction even at a high temperature of 850 C. To further confirm the incomplete oxidation of iron ores even after high temperature redox reactions, XPS analysis was also performed using the iron ores after the oxidation reaction, and the results are shown in Fig. 3. The observed characteristic binding energy of Fe2p3/2 at around 710.5 eV for all the tested iron ore samples indicates that the surfaces of iron ores after the oxidation reaction mainly exist in the hematite (Fe2O3) phase, and the partial oxidation of iron ore's outermost surfaces can be attributed to the limited oxygen transfer from water to the bulk phase of iron ores, as observed by forming a bulk main phase of Fe3O4 confirmed by the XRD analysis.
Characterizations of iron ores by TPR and TPO analysis
Fig. 3 e Results of the XPS analysis on the three different iron ores after the redox reaction.
To investigate the reduction behaviors of the three different iron ores, TPR analysis was performed, and the reduction patterns are shown in Fig. 4. The reduction peaks showed a Gaussian distribution with different maximum reduction temperatures. In general, the two distinct reduction peaks of Fe2O3 have been reported with the separate reduction stages by the sequential reduction through Fe2O3 / Fe3O4 / (FeO) / Fe [13]. As shown in Fig. 4(a), the first reduction peak was observed at 382 and 556 C for FeOOH and Fe2O3, respectively; however, the first reduction peak of Fe3O4 was observed at much higher temperature of 785 C, revealing that the FeOOH phase can be easily reduced because
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Fig. 4 e Patterns of (a) TPR and (b) TPO of the as-received iron ores. of its higher surface area with facile transport of hydrogen to the inner pores of FeOOH. Based on the TPR experiment, the FeOOH phase can be assigned to be the three-step reduction by showing the first reduction peak for a partial surface reduction of FeOOH at a lower temperature of 382 C with a possible dehydration to Fe2O3 [14], and the second reduction peak at 597 C for the reduction to Fe3O4 combined with the formation of FeO phase, and finally complete reduction to metallic iron at 731 C through the multistage reduction of FeOOH / Fe2O3 / Fe3O4 (FeO) / Fe. In the case of Fe2O3, the reduction peaks were deconvoluted to two characteristic peaks at around 556 and 721 C for the reduction steps of Fe2O3 / Fe3O4 (FeO) and the subsequent reduction to iron metals, respectively. Even though both the FeOOH and Fe2O3 phases seem to be completely reduced to metallic iron below 900 C, there possibly remains some unreduced Fe3O4 grains in the core of iron ores after the pretreatments as confirmed by the XRD analysis. Interestingly, the higher reduction temperature of Fe3O4 iron ore was observed, and the complicated reduction behaviors of Fe3O4 are consistent with the previously reported literature [13] by showing the reduction steps of Fe3O4 / FeO / Fe. As shown in Fig. 4(a), geothite (FeOOH) and hematite (Fe2O3) ores were almost reduced in our reduction condition of 850 C; however, magnetite (Fe3O4) partially reduced in our reduction condition. Fig. 4(b) shows the results of the TPO experiments of the asreceived three different iron ores. FeOOH iron ore showed a narrow oxidation peak at ~300 C, with the two deconvoluted peaks at 291 and 351 C. The oxidation behaviors of FeOOH can be described by two-step oxidations as follows; FeO / Fe3O4 / Fe2O3 as reported previously [15]. However, the fully oxidized Fe2O3 iron ore showed no oxidation peak owing to the stable final oxidation state. Interestingly, TPO
profile of Fe3O4 iron ore showed the complicated oxidation steps by the three-step oxidation. The first peak at the maximum peak around 326 C can be assigned to the desorption of adsorbed OH; the second peak at around 567 C is for the oxidation of Fe3O4 to Fe2O3 at the outer surfaces; the third peak at around 843 C is for the bulk oxidation by forming Fe2O3 phases. Even though these different oxidation behaviors of the different iron ores can be possibly attributed to the different surface areas and different phases, the masstransfer rate of hydrogen and oxygen from water to the bulk iron ores can more dominantly change their different redox properties, indicating that the consideration of mass-transfer properties should not be overlooked for deriving the proper kinetic equations for the redox reaction of iron ores.
Redox activity and kinetic analysis of three different iron ores The kinetic models for the redox reaction of iron oxides have been largely investigated by many researchers with the assumption of a gasesolid reaction using two or threedimensional diffusion models [16e20]. The well-known procedures to properly fit the experimental results with kinetic models are the “model fitting integral method”. Based on those models, the rate constant (k(T)) and activation energy (Ea) can be determined by fitting the integral form of g(a) with experimental data, where a represents the extent of reaction (conversion of H2 or H2O for the reduction and oxidation reaction). The integral form of g(a) can be derived from the integration of the kinetic model of f(a) as listed in Table 2. The kinetic models for the redox reaction of iron oxides can be described using a conventional power law equation such as first-order and nth-order reactions and three-dimensional
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Table 2 e Selected kinetic equations of f(a) and the integral forms of g(a) for a gasesolid redox reaction of iron oxides [11,18e20]. Reaction model Conventional kinetics (n ¼ 0, zero order): Polany-Wigner equation Conventional kinetics (n ¼ 0.5, half-order) Phase-boundary controlled reaction Conventional kinetics (n ¼ 1, first order) Conventional kinetics (n ¼ 1.5, 1.5th order) Conventional kinetics (n ¼ 2, 2nd order) Jander equation: Three-dimensional diffusion
diffusion Jander equation [11,18e20]. The temperaturedependent rate constant k can be determined from the Arrhenius equation by using the results of the isothermal experiments with a proper mathematical model. Therefore, we carefully investigated the fittability of the listed kinetic models for the redox reaction of iron ores listed in Table 2 including conventional ones and Jander equation. Table 2 also shows some representative mathematical kinetic equations of f(a) and their integral forms of g(a) in this study. The best equation was selected by fitting various kinetic models of g(a) with the experimental data and by finding out the higher R2 value, indicating a good linearity. The selected best fitting model can be further used to derive the kinetic parameters of k and Ea on the three different iron ores. As shown in Fig. 5, the cumulative hydrogen consumption with time on the stream during the reduction stage at an isothermal temperature of 850 C for about 80 min is shown in Fig. 5(a) for FeOOH, Fe2O3, and Fe3O4, respectively. The total amounts of consumed hydrogen were found to be 10.5, 9.3, 5.9 mmol/gcat for FeOOH, Fe2O3, and Fe3O4, respectively, corresponding to the integrated area of each plot. The hydrogen consumption increased in the following order: FeOOH > Fe2O3 > Fe3O4, and it is well correlated to the results of the TPR experiments. However, the hydrogen storage capacity of iron ores seems not to be directly correlated to the reduction capacity. As shown in Fig. 5(b), the amounts of hydrogen production by the water oxidation at 850 C showed the similar trends with the production capacity of 2.27 > 1.98 z 1.85 mmol gcat1 for the FeOOH, Fe3O4, and Fe2O3, respectively. This can be possibly attributed to the transformation of all the tested iron ores to the same bulk phase of Fe3O4 irrespective to the phases of the as-received iron ores. Even though the impurities such as Mo, Rh, and Co have been reported to show a positive effect by reducing activation energies and enhancing the stabilities of iron oxides [4,21e23], the other factors should not be overlooked to explain the different redox properties of the present asreceived iron ores. Therefore, the complete oxidation of all the tested iron ores at the outermost surfaces of Fe2O3 phase by maintaining the bulk Fe3O4 phase as confirmed by the XRD and XPS analyses, seems to be the main reason for the similar hydrogen production capacity because of the restricted diffusion of oxygen species by water-splitting reaction through the first formed dense Fe2O3 outermost surfaces for the further reduction of bulk Fe3O4 phase [14,21]. It suggests that the kinetic model possessing the three-dimensional diffusion mechanism such as Jander equation seem to be more proper to describe the redox kinetics of as-received iron ores.
f(a) functions
g(a) functions
1 2(1a)1/2 (1a) 2(1a)3/2 1(1a)3/2 3(1a)1/3/2[(1a)1/31]
a 2[1(1-a)]1/2 ln(1a) [1(1a)1/2]$(2) (1a)1/21 [1(1a)1/3]2
As shown in Fig. 6, the integrated kinetic models of g(a) functions are compared using the selected equations listed in Table 2. The Jander equation, represented as g(a) ¼ [1(1a)1/ 3 2 ] ¼ kt where a is for the cumulative conversion of water, showed a good linearity among the selected models. Because the Jander equation is based on the three-dimensional diffusion mechanism for a gasesolid noncatalytic reaction system, this model shows a good fittability compared to the other
Fig. 5 e Activity of the redox reaction of the as-received iron ores, (a) the consumed amount of H2 with time on stream at 850 C, and (b) the generated amount of H2 by water splitting reaction at the representative temperature of 850 C.
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Fig. 6 e The fitted kinetics models listed in Table 2 for the redox reactions of the three different iron ores at (a) reduction at 850 C and (b) oxidation by water at 850 C.
conventional kinetic models in the tested oxidation temperature of 850 C while using the data for 0.3 a 0.7. Using the Jander equation, the rate constants for the oxidation at 850 C for the reduced iron ores were found to be 0.0857, 0.0450, and 0.0894 min1 for FeOOH, Fe2O3, and Fe3O4, respectively, as summarized in Table 3. It seems to be irrespective to the surface areas of iron ores. Moreover, the rate constants of the reduction at 850 C of the as-received iron ores were observed with the values of 0.0740, 0.0435, and 0.1274 min1 for FeOOH, Fe2O3, and Fe3O4, respectively. These variations in the redox properties seem not to be directly correlated to the results of
the N2 sorption, TPR, and TPO analyses by showing higher rate constants for Fe3O4 iron ores, because a higher surface area and a higher reducibility was observed on iron ore having main FeOOH phase. Therefore, the kinetic model of the threedimensional diffusion Jander equation seems to be proper to describe the redox reaction of the present iron ores by properly considering the transport phenomena of hydrogen and oxygen through outermost dense phase of Fe2O3 formed. The calculated activation energies for the oxidation reaction at the temperatures of 700, 750, 800 and 850 C are also listed in Table 3. By using the Arrhenius plot, the activation energies of FeOOH, Fe2O3, and Fe3O4 for the oxidation were found to be 21.5, 48.9, and 11.0 kJ/mol, respectively. These values are similar to the previously reported activation energies of iron oxides such as 50e100 kJ/mol for Fe2O3 / Fe3O4 or the Fe species [19,20]. In addition, the effects of a higher reduction temperature of 900 C were also investigated on FeOOH, Fe2O3, and Fe3O4 and the rate constants for the reduction and oxidation with their fittability using various kinetic models are shown in supplementary Table S1 and Figure S1. The somewhat lower rate constants for the reduction (0.0479, 0.0329, and 0.0530 min1 for FeOOH, Fe2O3, and Fe3O4, respectively) and the oxidation (0.0581, 0.0571, and 0.0860 min1 for FeOOH, Fe2O3, and Fe3O4, respectively) seem to be possibly attributed to the facile aggregation of iron grains on all the iron ores reduced at 900 C. However, the trend of rate constant variations between the different phases of iron ores is found to be similar with the previous results (reduced iron ores at 850 C) by showing the higher redox properties on the magnetite (Fe3O4). In addition, the results conducted at much lower reduction temperature of 600 C are also shown in supplementary Table S2 and Figure S2 by showing the higher rate constants on magnetite. These results suggest that the magnetite is the proper phase for the possible application for CL process to produce hydrogen by water oxidation. Therefore, the iron ore having a main magnetite phase by showing a higher oxidation rate constant with a lower activation energy without significant aggregation of iron grains even after the redox reaction seems to be proper for the application in the CL process to produce pure hydrogen fuel. The redox reaction rates of iron ores were found to be strongly depending on the types of iron phases, particularly for the activation energy, by mainly changing the mass transport rate through the formed dense phase of the fully oxidized Fe2O3 surfaces on the iron ores irrespective to the surface areas. Even though the reduction and oxidation rates of iron ores proceed through the complicated steps, the proper kinetic model to describe redox reaction seems to be Jander equation and a three-dimensional diffusion mechanism for the further CL applications for the production of hydrogen fuel by the water-splitting reaction.
Table 3 e Estimated kinetic parameters for the reduction and oxidation reaction on the three different iron ores with the main phases of FeOOH, Fe2O3, and Fe3O4. Iron oxides
Rate constants of k (min1)
FeOOH Fe2O3 Fe3O4
Activation energy (Ea; kJ/mol) for oxidation (700e850 C)
R2 for Ea
21.5 48.9 11.0
0.94 0.96 0.98
Reduction (850 C)
Oxidation (850 C)
0.0740 0.0435 0.1274
0.0857 0.0450 0.0894
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Conclusions The proper kinetic model for the redox reaction of the three different iron ores having a main phase of FeOOH, Fe2O3, and Fe3O4 phases, respectively, was found to be the Jander equation based on the three-dimensional diffusion mechanism by the isothermal experiments, and the rate constant was not strongly affected by the surface area of iron ores. The Jander equation provide the best fittability of the experimental data, and the activation energy of FeOOH, Fe2O3, and Fe3O4 were found to be around 21.5, 48.9, and 11.0 kJ/mol, respectively, with the higher rate constants for the reduction and oxidation reactions of Fe3O4. The iron ore having a Fe3O4 phase by showing a higher rate constant with small activation energy without a significant aggregation of iron grains even after the redox reaction seems to be proper for the further application in CL process. Even though the redox reaction of iron ores proceeded by the complicated steps, the as-received iron ore having a main phase of magnetite (Fe3O4) can be properly applied for the CL process to produce pure hydrogen from water-splitting reaction at higher temperatures.
Acknowledgments This study was financially supported by the grant from the Industrial Source Technology Development Programs (201310042712) of the Ministry of Knowledge Economy (MKE) of Korea. The authors would like to acknowledge the financial support from the National Research Foundation of Korea (NRF) grant funded by the Korea government (NRF2014R1A1A2A16055557). This study was also supported by the R&D Center for Valuable Recycling (Global-Top R&D Program) of the Ministry of Environment with a project number of GT14-C-01-038-0. I would like to appreciate for the technical advices from Dr. Jonghwun Jung and Dr. Sang-Ho Yi with a research fund from POSCO (POSCO-2014X027).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.12.099.
references
[1] Bhavsar S, Najera M, Solunke R, Veser G. Chemical looping: to combustion and beyond. Catal Today 2014;228:96e105. [2] Lyngfelt A, Leckner B, Mattisson T. A fluidized-bed combustion process with inherent CO2 separation; application of chemical-looping combustion. Chem Eng Sci 2001;56:3101e13. [3] Pena JA, Lorente E, Romero E, Herguido J. Kinetic study of the redox process for storing hydrogen: reduction stage. Catal Today 2006;116:439e44.
[4] Hui W, Takenaka S, Otsuka K. Hydrogen storage properties of modified fumed-Fe-dust generated from a revolving furnace at a steel industry. Int J Hydrogen Energy 2006;31:1732e46. [5] Munteanu G, Ilieva L, Andreeva D. Kinetic parameters obtained from TPR data for a-Fe2O3 and Au/a-Fe2O3 systems. Thermochim Acta 1997;291:171e7. [6] Jankovic B. Isothermal reduction kinetics of nickel oxide using hydrogen: conventional and Weibull kinetic analysis. J Phys Chem Solids 2007;68:2233e46. [7] Takenaka S, Kaburagi T, Yamada C, Nomura K, Otsuka K. Production of pure hydrogen from methane mediated by the redox of Ni- and Cr-added iron oxides. J Catal 2004;228:405e16. [8] Jozwiak WK, Kaczmarek E, Maniecki TP, Ignaczak W, Maniukiewicz W. Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres. Appl Catal A e Gen 2007;326:17e27. [9] Vyazovkin S, Wight CA. Isothermal and non-isothermal kinetics of thermally stimulated reactions of solids. Int Rev Phys Chem 1998;17:407e33. [10] Tiernan MJ, Barnes PA, Parkes GMB. Reduction of iron oxide catalysts: the investigation of kinetic parameters using rate perturbation and linear heating thermo analytical techniques. J Phys Chem B 2001;105:220e8. [11] Gotor FJ, Criado JM, Malek J, Koga N. Kinetic analysis of solidstate reactions: the universality of master plots for analyzing isothermal and nonisothermal experiments. J Phys Chem A 2000;104:10777e82. [12] Khawam A, Flanagan DR. Solid-state kinetic models: basics and mathematical fundamentals. J Phys Chem B 2006;110:17315e28. [13] Zielinski J, Zglinickab I, Znaka L, Kaszkur Z. Reduction of Fe2O3 with hydrogen. Appl Catal A e Gen 2010;381:191e6. [14] Rosmaninho MG, Moura FCC, Souza LR, Nogueira RK, Gomes GM, Nascimento JS, et al. Investigation of iron oxide reduction by ethanol as a potential route to produce hydrogen. Appl Catal B e Environ 2012;115e116:45e52. [15] Basinska A, Jozwiak WK, Goralski J, Domka F. The behaviour of Ru/Fe2O3 catalysts and Fe2O3 supports in the TPR and TPO conditions. Appl Catal A e Gen 2000;190:107e15. [16] Galvita V, Sundmacher K. Redox behavior and reduction mechanism of Fe2O3eCeZrO2 as oxygen storage material. J Mater Sci 2007;42:9300e7. [17] Wimmers OJ, Arnoldy P, Moulijna JA. Determination of the reduction mechanism by temperature-programmed reduction: application to small iron oxide (Fe2O3) particles. J Phys Chem 1986;90:1331e7. [18] Wen F, Wang H, Tang Z. Kinetic study of the redox process of iron oxide for hydrogen production at oxidation step. Thermochim Acta 2011;520:55e60. [19] Pineau A, Kanari N, Gaballah I. Kinetics of reduction of iron oxides by H2 (Part I): low temperature reduction of hematite. Thermochim Acta 2006;447:89e100. [20] Lin HY, Chen YW, Li C. The mechanism of reduction of iron oxide by hydrogen. Thermochim Acta 2003;400:61e7. [21] Takenaka S, Kaburagi T, Yamada C, Nomura K, Otsuka K. Storage and supply of hydrogen by means of the redox of the iron oxides modified with Mo and Rh species. J Catal 2004;228:66e74. [22] Wang H, Wang G, Wang X, Bai J. Hydrogen production by redox of cation-modified iron oxide. J Phys Chem C 2008;112:5679e88. [23] Scheffe JR, Allendorf MD, Coker EN, Jacobs BW, McDaniel AH, Weimer AW. Hydrogen production via chemical looping redox cycles using atomic layer deposition-synthesized iron oxide and cobalt ferrites. Chem Mater 2011;23:2030e8.
Please cite this article in press as: Jeong MH, et al., Reduction and oxidation kinetics of different phases of iron oxides, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2014.12.099