Energy Fuels 2010, 24, 4034–4045 Published on Web 06/25/2010
: DOI:10.1021/ef100351j
Biomass Gasification with Catalytic Tar Reforming: A Model Study into Activity Enhancement of Calcium- and Magnesium-Oxide-Based Catalytic Materials by Incorporation of Iron L. Di Felice,*,†,‡ C. Courson,† D. Niznansky,§ P. U. Foscolo,‡ and A. Kiennemann† † Laboratoire des Mat eriaux, Surface et Proc ed es pour la Catalyse, ECPM, UMR 7515, 25 rue Becquerel, 67087, Strasbourg Cedex 2, France, ‡Chemical Engineering Department, University of L’Aquila, Via Campo di Pile, Zona industriale di Pile, 67100 L’Aquila, Italy, and §Department of Inorganic Chemistry, Faculty of Sciences, Charles University, Hlavova 2030, 128 40, Praha 2, Czech Republic
Received March 4, 2010. Revised Manuscript Received May 28, 2010
The development of efficient catalysts for the in situ elimination of tar produced in the gasification of biomass represents an important stage in the production of a clean hydrogen-rich gas suitable for use in fuel cells. With this in mind, the incorporation of iron, a readily available, low-cost, and non-toxic material, in calcined dolomite and its constituent compounds, CaO and MgO, was investigated. The Fe/CaO, Fe/ MgO, and Fe/dolomite systems were prepared using two impregnation techniques to generate the Fe2þ and Fe3þ species, which were then carefully characterized. Tar conversion tests in the presence of these prepared catalysts were carried out in a microreactor, using toluene as the tar model component. The catalysts were characterized before and after the reactor runs [using X-ray diffraction (XRD), temperature-programmed reduction (TPR), and M€ ossbauer spectroscopy] to better identify the catalytically active phases and the competitive interaction of iron with CaO, MgO, and calcined dolomite under reaction conditions.
perties.7 This characteristic in particular makes the gasification process possible at lower temperatures (650-700 °C), the so-called adsorption enhanced reforming (AER) process,8 than the more conventional 850-900 °C range, without a significant increase in downstream tar content.9 However, the catalytic activities of dolomite and olivine for tar conversion leave room for improvement, hence, the motivation for the search for catalytic additives. Nickel-based catalysts are very active for tar abatement.10 Nickel, however, suffers from deactivation by carbon deposition and, because of its toxicity, needs to be filtered out from the gasifier product gas.11 Other transition-metal-based catalysts, such as Pt, Ru, or Rh, show good activity but are expensive. It has been demonstrated12-23 that iron in various oxidation states is a potentially active catalyst for biomass gasification
Introduction The conversion of tar is a crucial step in the development of processes for the gasification of biomass or coal, in which the resulting H2-rich gas may be used to fuel gas turbines and engines or solid oxide and molten carbonate fuel cells or upgraded for synthesis processes. Promising methods for carrying this out involve the use of catalysts capable of converting the high-molecular-weight hydrocarbon tars to produce H2 and CO, thereby performing the double function of cleaning the gas and, at the same time, increasing its useful energy content. Various catalytic materials for the reforming of tar at temperatures in the range of 650-900 °C have been used.1-3 Among these, char,4 olivine, and dolomite appear particularly attractive because they are all inexpensive and relatively abundant materials.5 Olivine is less active in gasification5,6 but shows the optimal hardness required for the fluidized-bed reactor used in the process. Dolomite seems to be more active in tar reforming and is well-known for its CO2-capture pro-
(7) Harrison, D. P. Ind. Eng. Chem. Res. 2008, 47, 6486–6501. (8) Marquard-M€ ollenstedt, T.; Sichler, P.; Specht, M.; Michel, M.; Berger, R.; Hein, K. R. G.; H€ oftberger, E.; Rauch, R.; Hofbauer, H. Proceedings of 2nd World Conference on Biomass for Energy, Industry and Climate Protection; Rome, Italy, 2004; pp 758-762. (9) Marquard-M€ ollenstedt, T.; Zuberb€ uhler, U.; Specht, M. Proceedings of 16th European Biomass Conference and Exhibition From Research to Industry and Markets; Valencia, Spain, 2008; pp 684-689. (10) Swierczynski, D.; Courson, C.; Kiennemann, A. Chem. Eng. Process. 2007, 47, 508–513. (11) Rapagna, S.; Gallucci, K.; Di Marcello, M.; Foscolo, P. U.; Nacken, M.; Heidenreich, S. Energy Fuels 2009, 23, 3804–3809. (12) Swierczynski, D.; Courson, C.; Bedel, L.; Kiennemann, A.; Guille, J. Chem. Mater. 2006, 18, 4025–4032. (13) Mondal, K.; Piotrowski, K.; Dasgupta, D.; Hippo, E.; Wiltowski, T. Ind. Eng. Chem. Res. 2005, 44, 5508–5517. (14) Simell, P. A.; Leppalahti, J. K.; Bredenberg, J. B. Fuel 1992, 71, 211–218. (15) Uddin, Md. A.; Tsuda, H.; Wu, S.; Sasaoka, E. Fuel 2008, 87, 451–459. (16) Polychronopoulou, K.; Bakandritsos, A.; Tzitzios, V.; Fierro, J. L. G.; Efstathiou, A. M. J. Catal. 2006, 241, 132–148.
*To whom correspondence should be addressed: Chemical Engineering Departement, University of L’Aquila, Via Campo di Pile, Zona industriale di Pile, 67100 L’Aquila, Italy. Telephone: þ39-3204087804. Fax: þ39-0862701974. E-mail:
[email protected]. (1) Torres, W.; Pansare, S. S.; Goodwin, J. G., Jr. Catal. Rev. 2007, 49, 407–456. (2) Sutton, D.; Kelleher, B.; Ross, J. R. H. Fuel Process. Technol. 2001, 73, 155–173. (3) Abu El-Rub, Z.; Bramer, E. A.; Brem, G. Ind. Eng. Chem. Res. 2004, 43, 6911–6919. (4) Donnot, A.; Magne, P.; Deglise, X. J. Anal. Appl. Pyrolysis 1991, 21, 265–280. (5) Corella, J.; Toledo, J. M.; Padilla, R. Energy Fuels 2004, 18, 713– 720. (6) Rapagn a, S.; Jand, N.; Kiennemann, A.; Foscolo, P. U. Biomass Bionergy 2000, 19, 187–197. r 2010 American Chemical Society
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purposes, i.e., for tar and methane cracking/reforming, coke reforming, and the water-gas shift reaction (WGSR), and even if less active than the metal-based catalysts referred to above, it offers several advantages: it is inexpensive, abundant, and non-toxic and, thus, may be employed economically in larger quantities. Its role as a catalyst for tar and methane cracking and as a reforming catalyst has been well-reported in the literature. Simell et al.14 studied the catalytic activity of ankerite, a natural mineral containing iron, (Ca, Mg, and Fe)(CO3)2, comparing it to other synthetic iron- and nickel-based catalysts and natural calcined rocks, such as limestone and dolomite. They found that the presence of iron could improve the activity of carbonate rock for the decomposition of condensable tarry constituents of fuel gas. Tamhankar et al.,19 studying the catalytic cracking of benzene on an iron oxide-silica substrate, found this material to be very active, particularly when the iron is present in its metallic form. An important conclusion of this work is that metallic iron catalyzes the opening of the benzene ring toward primary coke; in the presence of H2, coke may react with it producing methane and impeding catalyst deactivation. Nordgreen et al.17,18 investigated the catalytic activity of iron for tar cracking in a biomass gasification process. They found that, at 900 °C, small sintered balls of hematite, prereduced to metallic iron, perform very well for tar abatement downstream of a gasifier. This result can be significantly improved by increasing the oxidizing power of the gasification atmosphere up to an equivalence ratio (the ratio between the experimental and stoichiometric fuel/oxidizer values) of 0.2. However, iron oxides, FeO, Fe3O4, and Fe2O3, did not show any catalytic activity. Murata et al.20 investigated CH4 decomposition over a Fe/ MgO/Al2O3 catalyst. They found that Fe3C carbide species are formed by carbon deposition on the catalyst surface, but catalyst deactivation is avoided by a MgFe2O4 phase when an oxidant agent and O2 or CO2 are also present in the product gas. Kiennemann et al.23 found iron/olivine catalysts to be efficient for model tar reforming under gasification conditions and ascribed this to specific interactions between the iron outside the olivine structure and the iron oxide of the structure itself. Polychronopoulou et al.16 studied steam reforming of phenol using various supported iron catalysts. They showed that iron activity strongly depends upon the kind of substrate used; for example, 5% Fe/MgO/γ-alumina does not perform in phenol steam reforming because a Mgx-Fe1-x-Al2O3 spinel inactive phase is formed. The best choice was found to be a CeO-MgO support containing 5% iron; the catalytic activity of this iron-based catalyst in the 600-700 °C temperature window was comparable to a commercial nickel-based
catalyst. This study at low temperature shows the feasibility of the absorption enhanced reforming (AER) process using an iron catalyst mechanically mixed with CeO-MgO-Fe and calcite in a microreactor. Uddin et al.15 have studied catalytic gasification of wood biomass using Fe2O3/Al2O3 catalysts in a microreactor at 600-850 °C. They bring to light the contribution of iron, in the form of a Fe3O4 phase, in tar cracking and WGSRs. In the reaction mechanism hypothesized by these authors, the carbon deposited on the iron surface is susceptible to removal by steam oxidation, producing H2 and CO. Matsuoka et al.,21 working on steam reforming of woody biomass in a fluidized bed at 500- 700 °C using a Fe/γalumina catalyst, found that a redox reaction takes place on the iron oxide surface Fe3 O4 þ CO f 3FeO þ CO2
ð1Þ
3FeO þ H2 O f Fe3 O4 þ H2
ð2Þ
CO produced during steam reforming was consumed to reduce Fe3O4 (eq 1), and the FeO resulting from this reaction might react with steam to form H2 (eq 2). The WGSR promoted by iron as the redox catalyst rather than by reforming of the coke seems to be the predominant pathway for H2 production. Moreover, it was hypothesized that iron may improve the catalytic activity of dolomite when present as an impurity in this substrate,24 although an investigation into chemical interactions and active phases of these materials was not carried out. From the available literature, it seems that the state of oxidation of the iron element is not very clear; some authors indicate that metallic iron is the active phase, while others indicate that Fe3O4 has an effect on tar cracking and the WGSR. This would appear to be an area in need of investigation. The aim of this work is to investigate new iron-based catalysts with efficient supports for biomass gasification. The choice of support is crucial, because deactivation of iron particles caused by sintering of the support surface is a well-known phenomenon.25 As highlighted in the above literature summary, calcined dolomite (CaMg)O as well as related materials CaO and MgO may perform as suitable substrates but the MgOFe and CaO-Fe interactions have to be thoroughly investigated. These points are discussed below in some detail in a study of iron-substrate interactions and with preliminary tests in a fixedbed microreactor using toluene as the model tar compound. Experimental Section Catalyst Preparation. The catalytic systems used in this work consist of precalcined dolomite (CaMg)O, lime (CaO), and magnesia (MgO) impregnated with 20% by weight iron. Because of the variability of the iron oxidation number, a key step in the preparation and characterization of the iron-based catalysts involves an understanding of the redox reactions affecting the iron species under different preparation conditions (iron salt, solvent for impregnation, and atmosphere of calcination). In this work, two preparation pathways have been developed (Table 1): an oxidative one, focused on the evaluation of the Fe3þ-substrate interaction, and a neutral one, for evaluating the Fe2þ-substrate and Fe2.5þ-substrate interactions. The prepared samples are described in Table 2.
(17) Nordgreen, T.; Liliedahl, T.; Sjostrom, K. Fuel 2006, 85, 689– 694. (18) Nordgreen, T.; Liliedahl, T.; Sjostrom, K. Energy Fuels 2006, 20, 890–895. (19) Tamhankar, S. S.; Tsuchiya, K.; Riggs, J. B. Appl. Catal. 1985, 16, 103–121. (20) Murata, K.; Inaba, M.; Saito, M.; Takahara, I.; Mimura, N. J. Jpn. Pet. Inst. 2003, 46, 196–202. (21) Matsuoka, K.; Shimbori, T.; Kuramoto, K.; Hatano, H.; Suzuki, Y. Energy Fuels 2006, 20, 2727–2731. (22) Wang, T.; Chang, J.; Lv, P. Energy Fuels 2005, 19, 22–27. (23) Kiennemann, A.; Courson, C.; Virginie, M. Catalyseurs pour le reformage de goudrons utilisables dans la vapogazeification de la biomasse. Demande Brevet Franc-ais, May 2009.
(24) Orio, A.; Corella, J.; Narvaez, I. Ind. Eng. Chem. Res. 1997, 36, 1545–1543. (25) Bleeker, M. F.; Kersten, S. R. A.; Veringa, H. J. Catal. Today 2007, 127, 278–290.
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TPR. To quantify the amount of reducible metal (iron), the reducibility of the catalyst has been followed by TPR performed on 50 mg of catalyst placed in a U-shaped quartz tube (6.6 mm inner diameter). The reductive gas mixture (H2, 0.12 L h-1; Ar, 3 L h-1) fed to the reactor was heated from room temperature to 900 °C at a rate of 15 °C min-1 and then maintained at 900 °C until all H2 had been consumed, indicating by a return to baseline. A thermal conductivity detector (TCD) was used for the quantitative determination of hydrogen consumption. M€ ossbauer Spectroscopy. The 57Fe M€ ossbauer spectra were recorded at room temperature (293 K) using a spectrometer with a triangular waveform and a 57Co source (50 mCi) dispersed in a rhodium matrix. From the obtained spectra, the isomeric shift was determined and compared to that for the metallic iron standard at room temperature. To identify the different forms of iron present in the sample, the spectra have been fitted using the NORMOS computer program. Experimental Apparatus for Reactivity Tests. The experiments for catalytic steam reforming of the tar components were carried out using a fixed-bed microreactor that has been described elsewhere.26 The catalyst was tested under an overall feed flow rate of 3 nL h-1, with the composition shown in Table 3. The feed gas mixture composition was adjusted to obtain a tar concentration of 30 g/Nm3, which corresponds to a high tar content for the gas phase in a dual fluidized-bed biomass gasifier; toluene was used as the model tar compound, in accordance with literature data.27 The steam/toluene ratio was slightly higher (18:1) than the stoichiometric value for complete toluene conversion to H2 and CO2 (14:1). The inert nitrogen flow acts as an internal standard for gas chromatography analysis. The outlet gas was analyzed using two gas chromatographs equipped with TCDs, with the former indicating the amount of H2, N2, CH4, and CO separated on a 5 A˚ molecular sieve and the latter quantifying Ar, CH4, and CO2 separated in a Hayesep Q column.
For both preparation methods, the iron salt is dissolved in the impregnation solvent and then the substrate is added and stirred to obtain a suspension. The solvent is evaporated at the appropriate temperature, and the solid is recovered, dried (120 °C, 5 h), and crushed (80 < dp < 300 μm). The material is then thermally treated under the appropriate atmosphere at 850 °C (or 1100 °C) for 4 h at a heating rate of 3 °C/min, to investigate also the influence of thermal treatment on the crystalline phases obtained. The starting materials consist of a natural dolomite (with elementary composition: 30.39 wt % of CaO and 20.56 wt % of MgO in fresh dolomite), a CaO powder (Prolabo), and a MgO powder (Prolabo) used as a substrate and iron(III) nitrate and iron(II) acetate salts (Acros Organics). In all of these cases, the substrate used was precalcined at 900 °C for 4 h at a 3 °C/min heating rate. Catalyst Characterization. The calcined dolomite formula (CaMg)O suggests that iron may interact with both magnesium and calcium oxides. Therefore, starting from iron acetate or iron nitrate salts and applying the neutral and oxidative pathways, the analysis of both iron(2þ, 2.5þ, and 3þ)-calcium oxide and iron(2þ, 2.5þ, and 3þ)-magnesium oxide interactions are required to clarify the nature of the iron-dolomite interaction. The Fe/substrate catalysts have been characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), and M€ ossbauer spectroscopy. XRD. The crystalline phases present in the samples and the structural modifications occurring during reduction and catalytic tests were examined by powder XRD on a Bruker D8 Advance diffractometer using a Ni detector side-filtered with Cu KR radiation (λ = 1.5406 A˚).
Table 1. Preparation Pathways iron salt impregnation solvent evaporation temperature (°C) thermal treatment atmosphere
oxidative
neutral
nitrate (Fe3þ) water 110 air
acetate (Fe2þ) ethanol 80 nitrogen
Results and Discussion Thermodynamic Study. Because of the variability of the iron oxidation number (0, 2þ, 2.5þ, or 3þ), iron may interact with the product gas of a biomass gasification process (H2, CO, CO2, and H2O), as indicated in Scheme 1. A preliminary thermodynamic study of iron species with respect to the oxidizing power of the product gas may be useful for an appreciation of the potentially active iron phases in catalytic tar abatement. A thermodynamic evaluation of the iron oxidation state with respect to these gaseous components has been carried out, making use of the thermodynamic data for reactants and products shown in Table 4.28 Thermodynamic equilibrium curves can be represented by a graph of the partial pressure ratio Pred/Pox as a function of the temperature (Figure 1) (Pred, partial pressure of the reducing agents H2 or CO; Pox, partial pressure of oxidant agents CO2 and H2O). To evaluate the driving force, Gibbs’ energy for the reactions was obtained starting from ΔH298 0
Table 2. Samples and Nomenclature preparation pathways oxidative neutral
substrate
nomenclature
CaO MgO calcined dolomite CaO MgO calcined dolomite
OxiCa OxiMg OxiDolo NeuCa NeuMg NeuDolo
Table 3. Feeding Composition for Fixed-Bed Microreactor Tests component
gas volume flow (nL/h)
gas composition (%)
Ar H2O toluene N2
2.02 0.36 0.02 0.60
67.2 12 0.8 20
Scheme 1. Main Redox Behavior of Iron in a Gasification Atmosphere
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Table 4. Thermodynamic Data of Reactants and Products in the Redox System of Fe and Product Gas compound Fe (s) FeO (s) Fe3O4 (s) Fe2O3 (s) H2O H2 CO CO2
heat of formation, ΔH, at 298 K (kJ/mol)
free energy of formation, ΔG, at 298 K (kJ/mol)
0 -270551.016 -1117456.92 -831079.8 -241988.248 0 -110598.509 -393776.914
0 -248612 -1014462 -749856 -228746 0 -137361 -394648
Figure 1. Thermodynamic equilibrium at different temperatures for iron species with respect to main gaseous phases involved in a gasification process.
heat capacity, Cp (J deg-1 mol-1), at constant pressure (T = K) 17.29 þ 26.71 10-3T (273 < T < 1041) 25.62 þ 0.014T (1041 < T < 1179) 52.84 þ 6.25 10-3T - 319034.2/T2 172.37 þ 0.079T - 4100971/T2 103.50 þ 0.067 - 1772691/T2 34.42 þ 6.28 10-4T þ 5.61 10-6T2 27.72 þ 34.21 10-4T 27.63 þ 0.005T 43.29 þ 0.0115T - 818519.4/T2
Figure 2. XRD spectra of (a) MgO, (b) Fe2O3, and (c) OxiMg calcined at 1100 °C, with the peaks of MgO (b), Fe2O3 (]), and MgFe2O4 (/).
and ΔG298 0 , indicated in Table 4, and integrating Cp values up to the corresponding reaction temperature. From Figure 1, it is clear that FeO is the thermodynamically favored iron species under gasification conditions (Pred/ Pox ≈ 1, and T = 650-850 °C). Only when Pred/Pox < 0.5 or >1.5-2, depending upon the temperature, does the thermodynamic data allow for the formation of Fe3O4 and metallic iron, respectively. On the other hand, the transition Fe3O4 f Fe2O3 is inhibited when steam or CO2 are used as oxidation ossbauer agents. Ohtsuka and co-workers29 have shown by M€ analysis that the following distribution of iron species occurs after 30 min of gasification: 15% metallic iron, 20% Fe3O4, and 65% FeO. These results are important, representing the clear possibility for discriminating between the oxidation states of iron in the reaction atmosphere of a biomass gasification process. Sample Characterization. Fe/MgO: Oxidative and Neutral Pathways. Many authors have carried out investigations on the characterization of the interaction between magnesium oxide and iron(III).30-32 Magnesium oxide has been shown to stabilize the supported iron particles and to retard sintering and may also help in maintaining the mechanical strength and growth of carbon in ethyl benzene dehydrogenation in
Figure 3. XRD spectra of (a) MgO, (d) Fe3O4-FeO, and (e) NeuMg calcined at 1100 °C, with the peaks of MgO (b), FeO (1), Fe3O4 (2), and MgO-FeO (4).
the presence of steam.30 XRD (Figure 2) enables the synthesized OxiMg to be related to the raw materials incorporated in this structure, i.e., MgO and Fe2O3, for the sample calcined at 1100 °C. In this way, it is well-evident that, following the oxidative pathway, a MgFe2O4 (magnesioferrite) phase is obtained, detectable as well-defined peaks and also as a shoulder to the right side of some MgO peaks, as clear from the zoom zone of Figure 2. Free Fe2O3 oxides are not detected. The Fe-MgO interaction has also been focused on in the investigation on reduced species between iron and MgO, following the neutral pathway discussed above. The XRD analysis in Figure 3 shows a comparison between NeuMg calcined at 1100 °C and the raw materials incorporated in this structure, i.e., MgO and FeO. A strong
(26) Di Felice, L.; Courson, C.; Jand, N.; Gallucci, K.; Foscolo, P. U.; Kiennemann, A. Chem. Eng. J. 2009, 154, 375–383. (27) Dayton D. Milestone Completion Report; National Renewable Energy Laboratory (NREL): Golden, CO, 2002; NREL/TP-510-32815. (28) Perry R. H.; Green D. Perry’s Chemical Engineers’ Handbook, 6th ed.; McGraw-Hill: New York, 2003. (29) Ohtsuka, Y.; Tamai, Y.; Tomita, A. Energy Fuels 1987, 1, 32–36. (30) Bond, G.; Molloy, K. C.; Stone, F. S. Solid State Ionics 1997, 101-103, 697–705. (31) Stobbe, E. D.; van Buren, R. J. Chem. Soc., Faraday Trans. 1991, 87, 1623–1629. (32) Stobbe, E. D.; van Buren, R. J. Chem. Soc., Faraday Trans. 1991, 87, 1631–1637.
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Figure 4. 57Fe M€ ossbauer spectra measured at room temperature of (a) OxiMg and (b) NeuMg calcined at 1100 °C. Table 5. M€ ossbauer Parameters of Components Present in the Fe/MgO System, Obtained by the Oxidative Pathway OxiMg (1100 °C) isomer shift, δ (mm/s) quadrupole splitting, ΔEQ (mm/s) hyperfine field, BHF (T) full line width at half-height (mm/s) relative area (%)
NeuMg (1100 °C)
sub-spectrum 1
sub-spectrum 2
sub-spectrum 1
0.24 0.06 45.5 2.7 45
0.33 -0.12 43.8 6.1 55
1.05 0.65 0.43 100
shift of the MgO peaks toward FeO is observed in the NeuMg diffractogram (see zoom zone in Figure 3); this experimental observation makes clear that most of the iron is integrated in the MgO substrate as FeO-MgO solid solution. The average composition of the solid solution has been determined from the Vegard law33 to be (Fe0.1Mg0.9)O. To further support the XRD data, M€ ossbauer analysis has been carried out for the synthesized samples OxiMg (Figure 4a) and NeuMg (Figure 4b), calcined at 1100 °C (see Table 5). The OxiMg sample contains only iron in the valence state Fe3þ (sub-spectra 1 and 2), and this iron is in the magnetically ordered form, in good agreement with spinel MgFe2O4 detected by XRD analysis. For the NeuMg sample, the paramagnetic doublet is observed; this parameter shows that all of the iron is in the valence state of Fe2þ in the structure, once again in accordance with the FeO-MgO solid solution detected by XRD. The samples calcined at 850 °C show the same general behavior, with the same phases appearing in the XRD analysis. However, in this case, the XRD analysis shows a generally broader peak shape, indicating that crystalline phases are less defined. Moreover, for the NeuMg sample, a small peak in the Fe3O4 zone is also detected, as highlighted in Figure 8, where XRD analysis of NeuMg, NeuCa, and Neudolo calcined at 850 °C is shown. It has been found32 that Mg2þ ions may replace Fe2þ in the Fe3O4 lattice; consequently, this phase may be attributed to free iron oxides (Fe3O4), a Fe3-xMgxO4 interaction, or a combination of both (Fe3O4Fe3-xMgxO4). This last suggestion is supported by the TPR
analysis (Figure 11), where two separate reduction peaks are associated with Fe2.5þ species. Fe/CaO: Oxidative and Neutral Pathways. The interaction between Fe2O3 and CaO has been investigated by XRD. Figure 5 shows that, for NeuCa calcined at 850 °C and OxiCa calcined at 1100 °C, the iron is present only as a Ca2Fe2O5 phase (calcium ferrite, a brownmillerite structure). Iron in the 2þ and 2.5þ oxidation states and free Fe2O3 iron oxides are not detected using both preparation conditions described in Table 1. This may be due to the possibility of the brownmillerite structure stabilizing the oxygen defects34 and, thus, resulting in high oxygen mobility.
(33) Denton, A. R.; Ashcroft, N. W. Phys. Rev. A: At., Mol., Opt. Phys. 1991, 43, 3161–3164.
(34) Hirabayashi, D.; Yoshikawa, T.; Mochizuki, K.; Suzuki, K.; Sakaib, Y. Catal. Lett. 2006, 110, 155–160.
Figure 5. XRD spectra of (f) OxiCa calcined at 1100 °C and (g) NeuCa calcined at 850 °C, with the peaks of CaO (0) and Ca2Fe2O5 (9).
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Figure 6. 57Fe M€ ossbauer spectra measured at room temperature of (a) OxiCa calcined at 1100 °C and (b) NeuCa calcined at 850 °C. Table 6. M€ossbauer Parameters of Components Present in the Fe/CaO System, with the OxiCa Sample Calcined at 1100 °C and the NeuCa Sample Calcined at 850 °C NeuCa isomer shift, δ (mm/s) quadrupole splitting, ΔEQ (mm/s) hyperfine field, BHF (T) full line width at half-height (mm/s) relative area (%)
OxiCa
sub-spectrum 1
sub-spectrum 2
sub-spectrum 3
sub-spectrum 1
sub-spectrum 2
0.35 0.53 51.2 0.29 48
0.18 0.72 43.5 0.28 47
0.13 0.99 N/A 0.60 5
0.35 -0.55 50.6 0.31 48
0.18 0.71 43.1 0.31 52
M€ ossbauer analysis, carried out for the samples OxiCa calcined at 1100 °C and NeuCa calcined at 850 °C, confirms this evidence, as summarized in panels a and b of Figure 6 and Table 6. For both catalysts, two sextets correspond to brownmillerite-type Ca2Fe2O5. Sub-spectrum 1 corresponds to the octahedral site Fe3þ, while sub-spectrum 2 corresponds to the tetrahedral site Fe3þ. For the sample NeuCa calcined at 850 °C, the sub-spectrum 3 (5% of iron) corresponds to trivalent iron in the paramagnetic phase; the larger width of the line may represent some phases without good crystallinity. Fe/Dolomite: Oxidative and Neutral Pathways. From the study carried out on Fe/MgO and Fe/CaO, it is to be expected that Fe/dolomite interactions take place as Ca2Fe2O5 and MgFe2O4 phases in the oxidative pathway and as Ca2Fe2O5, FeO-MgO, and Fe3O4-Fe3-xMgxO4 in the socalled neutral pathway. A study was therefore undertaken to compare these three phases for the respective preparation methods. Because no significant effect of the calcination temperature for the Oxi- and Neu- catalysts was observed, only the samples calcined at 850 °C were investigated. The XRD of the OxiDolo (diffractogram h in Figure 7) is compared to that of Ca2Fe2O5 (diffractogram i in Figure 7) and MgFe2O4 (diffractogram j in Figure 7). It is evident that the MgFe2O4 phase is absent but that the iron interacts strongly with CaO, forming Ca2Fe2O5. Because no other MgO-iron oxide is detected, it can be assumed that all Fe3þ reacted with CaO.
Figure 7. XRD spectra of (h) OxiDolo, (i) OxiCa, and (j) OxiMg calcined at 850 °C, with the peaks of CaO (0), MgO (b), Ca2Fe2O5 (9), and MgFe2O4 (/).
For the neutral pathway (Figure 8), XRD shows once again a strong presence of Ca2Fe2O5 but a peak in the Fe3O4 zone is also clearly visible that is perfectly superimposed on the peak of the NeuMg sample calcined at 850 °C. This means that the same Fe3O4-Fe3-xMgxO4 mixed phase has been detected. Superposition of TPR analyses between NeuMg and NeuDolo calcined at 850 °C (see Figure 13) also supports this evidence. The FeO-MgO solid solution is not detected by the shift in the MgO peaks, as it clearly was in the NeuMg sample. 4039
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To better interpret the XRD results, a M€ ossbauer analysis was carried out for both OxiDolo and NeuDolo (panels a and b of Figure 9 and parts a and b of Table 7) calcined at 850 °C; the former contains 88% of the iron as brownmilleritetype Ca2Fe2O5 (sub-spectra 1 and 2 in Table 7a), with the remaining 12% resulting in a Fe3þ valence state representing the paramagnetic iron phase (sub-spectrum 3). This latter phase contains 43% of the iron in the Ca2Fe2O5 phase (subspectra 1 and 3 in Table 7b), 25% in the spinel phase (subspectra 2 and 4), and the remaining 32% in the mixed valence state Fe2þ-Fe3þ (sub-spectrum 5). The spinel phase could be attributed to Fe3O4-Fe3-xMgxO4, as suggested by XRD; the sub-spectrum 2 represents iron(III) in the tetrahedral coordination state, while sub-spectrum 4 represents iron(II-III) in the octahedral coordination state. The observed interactions of Fe/CaO, Fe/MgO, and Fe/ dolomite catalysts suggest the following: (1) Fe/CaO strongly favors the Fe3þ species in a well-detectable interaction
with the substrate, a Ca2Fe2O5 brownmillerite structure, regardless of the starting degree of iron oxidation and the nature of the solvent. (2) The Fe/MgO interaction was found to take place with both the Fe2þ and Fe3þ species, as FeOMgO solid solution and MgFe2O4, but also with a small amount of Fe3O4-Fe3-xMgxO4. (3) The Fe/dolomite interaction, in particular with the OxiDolo sample, was found to be very dependent upon the Fe/CaO interaction. The NeuDolo catalyst may contain iron as the 2.5þ species that has been identified as Fe3O4-Fe3-xMgxO4 in addition to Ca2Fe2O5. In the reductive reaction atmosphere, these phases can change as a result of the thermodynamic constraints of iron redox reactions; Fe2þ is identified as the favored species under gasification conditions. For this reason, the reduction pathways of these types of iron catalysts have been investigated with TPR analysis. The identification of reduction intermediates of iron on the CaO, MgO, and dolomite surface and the evaluation of their stability as a function of the temperature are of great interest because they may represent the active forms for catalytic steam reforming and WGSRs, thus providing explanations for the possible evolution of the iron oxidation state in a reductive gasifier atmosphere. Reduction Behavior of Iron. The first step in the understanding of an iron-supported catalyst reduction pathway comes from the evaluation of free iron oxide reduction. In this respect, hematite reduction is known from the literature35 to take place in different ways, including direct reduction to Fe0 and successive reductions and disproportions involving Fe3O4, FeO, and Fe0.32 TPR of small particles of hematite and magnetite has been carried out as blank tests. Hematite shows a first reduction peak at 440 °C that may correspond to the Fe2O3 f Fe3O4 transition. A second peak in the Fe2O3 reduction pathway, appearing at around 590 °C, has been identified as Fe3O4 reduction by superposition with a pure magnetite sample,
Figure 8. XRD spectra of (k) NeuDolo, (g) NeuCa, and (l) NeuMg calcined at 850 °C, with the peaks of CaO (0), MgO (b), Ca2Fe2O5 (9), Fe3O4-MgFe3O5 (1), and MgO-FeO (4).
Figure 9. 57Fe M€ ossbauer spectra measured at room temperature of (a) OxiDolo and (b) NeuDolo calcined at 850 °C.
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Table 7. 57Fe M€ossbauer Parameters of Components Present in the Fe/Dolomite System, with the (a) Oxidolo Sample Calcined at 850 °C and (b) Neudolo Sample Calcined at 850 °C Oxidolo (850 °C)
(a) sub-spectrum 1
sub-spectrum 2
sub-spectrum 3
0.35 -0.54 50.7 0.40 39
0.18 0.71 43.5 0.49 49
0.30 0.82 N/A 0.56 12
isomer shift, δ (mm/s) quadrupole splitting, ΔEQ (mm/s) hyperfine field, BHF (T) full line width at half-height (mm/s) relative area (%)
Neudolo (850 °C)
(b) isomer shift, δ (mm/s) quadrupole splitting, ΔEQ (mm/s) hyperfine field, BHF (T) full line width at half-height (mm/s) relative area (%)
sub-spectrum 1
sub-spectrum 2
sub-spectrum 3
sub-spectrum 4
sub-spectrum 5
0.35 -0.54 51.1 0.26 21
0.27 0.05 48.9 0.32 13
0.18 0.71 43.6 0.26 22
0.64 -0.03 45.8 0.46 12
0.59 0.39 N/A 0.90 32
Figure 10. TPR analysis for OxiMg calcined at (a) 850 °C and (b) 1100 °C.
which begins to reduce at this temperature. A final peak, the reduction of FeO to Fe, is detected at approximately 800 °C. A very different and more complex situation may occur with regard to supported iron phases, because of the development of the oxide-support interaction. Fe/MgO Reduction. The TPR of OxiMg and NeuMg is shown in Figures 10 and 11, respectively. TPR peaks of OxiMg (as well as NeuMg) are slightly shifted toward higher temperatures with respect to free iron oxide, as evidenced in particular in the transition zone FeO f Fe0, where the solid solution MgO-FeO reduction takes place. This interaction is known to preserve iron(II) species32 (the transition FeO f Fe0 may not be complete) even in a strongly reductive atmosphere. The sample calcined at 850 °C shows a first peak at 480 °C that is absent with the sample calcined at 1100 °C, indicating a more reducible (less associated to the support) iron oxide. This agrees with XRD analysis that shows a broader peak shape (less defined crystalline structures) for the lower calcination temperature. The sample calcined at 1100 °C indicates an almost unresolved reduction pathway, where the absence of free Fe2O3 is confirmed by the starting temperature of the reduction: 500 °C instead of 400 °C. More precisely, three reduction peaks are noticeable, which fit well with the OxiMg sample calcined at 850 °C: the first peak, appearing at 600 °C, may be ascribed to the MgFe2O4 f
Figure 11. TPR analysis for NeuMg calcined at (c) 850 °C and (d) 1100 °C.
Fe3O4-Fe3-xMgxO4 transition; the second reduction zone, between 650 and 800 °C, may be ascribed to Fe3O4Fe3-xMgxO4 f MgO-FeO; and the third (900 °C) corresponds to the above-mentioned FeO reduction zone. In the NeuMg sample calcined at 1100 °C (Figure 11), the only phase present, the FeO-MgO solid solution, is reduced at a very high temperature (900 °C) and it is clearly visible from the low intensity of this peak that the transition FeO f Fe0 is not completed, with the FeO-MgO interaction effectively stabilizing the Fe2þ species. For the sample calcined at 850 °C, the weak interaction predicted by XRD and the
(35) Messi, C.; Carniti, P.; Gervasini, A. J. Therm. Anal. Calorim. 2008, 91, 93–100.
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Figure 13. TPR analysis for (g) NeuDolo compared to (c) NeuMg calcined at 850 °C. Figure 12. TPR analysis for (e) OxiCa calcined at 1100 °C and (f) NeuCa calcined at 1100 °C.
diffractograms, respectively. In Figure 14a, it is seen that the shoulder peak to the right of MgO (magnesioferrite) is shifted toward the left (the FeO-MgO solid solution) after reduction, demonstrating that part of iron in the magnesioferrite reduction pathway from Fe3þ to Fe0 remains as Fe2þ because of the stability of the FeO-MgO interaction. This behavior is even more evident for the NeuMg sample; Figure 14b shows that the shifted peak of the MgO-FeO solid solution is still well-evident, confirming the low reduction of FeO-MgO into Fe0 detected by TPR. M€ ossbauer analyses for OxiMg and NeuMg have been carried out to quantify the relative amounts of iron species after TPR. The former sample contains 21% Fe2þ, with the largest amount (75%) being in the form of R-Fe (metallic iron), but there is also 4% Fe3þ. In the latter sample (NeuMg), the amount of metallic iron is much smaller; the sample contains 18% R-Fe (metallic iron) and 82% Fe2þ. Fe/CaO after TPR. For both OxiCa and NeuCa, the Ca2Fe2O5 phase is not detectable after TPR by XRD analysis and neither is iron in the 2þ form. This means that, even if the reduction of Ca2Fe2O5 takes place at a high temperature, the transition Fe3þ f Fe0 is quite complete. Fe/Dolomite after TPR. XRD analysis for both OxiDolo and NeuDolo shows similar results; because iron is extracted from the dolomite particle in the metallic form, Ca2Fe2O5 (for OxiDolo and NeuDolo) and Fe3O4-Fe3-xMgxO4 (for NeuDolo only) peaks do not appear in the diffractogram. M€ ossbauer analysis of these samples shows that, along with metallic iron as the predominant phase, 6% of iron is preserved as Fe2þ for OxiDolo and 20% of iron is preserved as Fe2þ for NeuDolo. This result is in agreement with previous considerations, which described a preferential Fe3þ f Fe0 reduction for the iron-calcium oxide interaction, as is the case of the OxiDolo catalyst, and a more complex pathway for the iron-magnesium oxide system, detected only in the NeuDolo sample. In this latter case, with a transition through the Fe2.5þ species that is partially extracted from the structure as the Fe3O4-Fe3-xMgxO4 phase, iron(III) is reduced to FeO and easily captured by the magnesium oxide matrix in the stabilized FeO-MgO solid solution, avoiding a complete reduction to the metallic form. A schematic model of the phases obtained in the OxiDolo and NeuDolo catalysts and their reduction behavior is shown in Figure 15. Catalytic Activity. The catalytic activity of supports and supported catalysts (calcined at 850 °C) was studied at 850 °C
evidence of some Fe2.5þ are once again confirmed, with two reduction peaks at 600 and 700 °C indicating the reduction pathway of a small amount of the Fe3O4-Fe3-xMgxO4 phase. In particular, the former peak fits well with the reduction temperature of free Fe3O4, and the second may be associated with Fe2.5þ more intimately mixed with the substrate in the Fe3-xMgxO4 phase, as indicated in Figure 11. Fe/CaO Reduction. TPR analysis of calcium ferrite, Ca2Fe2O5, detected by XRD for the Fe/CaO oxidative and neutral preparation pathways (Figure 12), shows a single broad reduction peak near 880 °C; this high reduction temperature reveals the stability of this phase. In this case, iron seems to be forced toward the unique transition Fe3þ f Fe0, demonstrating once again that Fe2þ is not a stable phase in the Fe-CaO interaction, as highlighted in the NeuCa sample analysis. Fe/Dolomite Reduction. A first qualitative characterization of the reduction behavior of the Fe/dolomite catalysts (calcined at 850 and 1100 °C) may be obtained from a superposition with their components, in the same way shown previously for XRD analysis. In particular, the OxiDolo sample shows a good superposition with the OxiCa sample, confirming the XRD and M€ ossbauer analyses. TPR of NeuDolo calcined at 850 °C is also consistent with previous analyses. A simple way of resolving TPR of this sample (Figure 13), which reveals the presence of four main transitions along reduction path, labeled from 1 to 4 in the figure, is by superposition with the sample NeuMg calcined at 850 °C. Peaks 1, 2, and 4 may be related to the Fe-Mg reduction pathway, being associated with Fe3O4-Fe3-xMgxO4 f FeO (600-750 °C) f Fe0 (900 °C) transitions. In this way, peak 3, appearing at 880 °C, may be easily related to the Ca2Fe2O5 f Fe0 þ CaO reduction, as highlighted in the Fe/ CaO and OxiDolo samples. Catalyst Characterization after Reduction. To complete the characterization of the Fe-CaO, Fe-MgO, and Fedolomite catalysts, these materials have been analyzed after TPR by XRD and M€ ossbauer analysis. Fe/MgO after TPR. After TPR, the magnesioferrite peaks of OxiMg detected by XRD are no longer evident; instead, metallic iron is well-detectable (not indicated here), and some shoulder appears now to the left of the MgO peaks. This behavior is clear from panels a and b of Figure 14, which show representative zoom zones of OxiMg and NeuMg 4042
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Figure 14. Diffractogram zoom zones of (a) OxiMg and (b) NeuMg samples (m and p) before and (n and o) after TPR, with the peaks of MgO (b), MgFe2O4 (/), and MgO-FeO (4) solid solution.
Figure 15. Schematic model of the phases obtained in the (a) NeuDolo and (b) OxiDolo catalysts and their reduction behavior.
authors,24 with the activity of dolomite being superior to CaO and MgO activities because of some degree of distortion in the array of Ca or Mg atoms, thereby generating more active sites. CaO and MgO are considerably less active than dolomite and, in addition, require an activation time in the reactive environment prior to reaching a plateau of reactivity. When iron is added to alkaline-earth oxides, i.e., calcined dolomite, magnesia, and lime, the improvement on catalytic activity of these materials is mainly related to the metal-support interaction and cooperation, because of the weak evidence (only for NeuMg and NeuDolo) of free iron oxides in such catalytic systems. OxiCa and OxiDolo show the same reactivity, whereas OxiMg shows a slightly superior toluene conversion, which decreases during the 6 h test from 70 to about 60% toluene conversion. For the OxiDolo sample, no substantial improvement on toluene conversion is observed with respect to raw dolomite. OxiCa, however, is able to raise the catalytic toluene conversion of raw CaO from below 30 up to 45-50%. This result is very interesting because lime has been indicated as the most recommended additive for steam gasification in a fluidized bed at low-medium (600800 °C) temperatures,36 being less soft and, therefore, less susceptible to erosion in a fluidized bed. Its disadvantage of being less active than dolomite for tar elimination can therefore be overcome by the addition of iron. The OxiMg system also improves catalytic activity over that of pure MgO, with toluene conversion rising from 28 to about 60% when iron is added to the substrate. This improvement upon reactivity is absent in the Fe/dolomite reactivity,
for toluene steam reforming. The main reactions that take place in the system are steam reforming of toluene (eq 3) and water-gas shift (eq 4): C7 H8 þ 7H2 O f 7CO þ 11H2
ð3Þ
CO þ H2 O f CO2 þ H2
ð4Þ
The overall conversion of toluene may be expressed as a function of the carbon-containing components in the gas phase, Xt (eq 5). CO þ CO2 þ CH4 Xt ¼ ð5Þ 7C7 H8 Catalysts were not pre-reduced before the tests. Results of the catalytic tests (see Table 3 for experimental conditions) using different catalytic systems are shown in panels a (for CaO, MgO, and calcined dolomite supports) and b (for OxiCa, OxiMg, and OxiDolo) of Figure 16 in terms of percent Xt (percentage of toluene conversion) as a function of time. Mean values for outlet concentrations of H2, CO, CO2, and CH4 are shown in Table 8. It is clear that the mean hydrogen production is proportional to conversion values and the CO2/CO ratio, being influenced by the extent of both steam reforming and WGSRs. Methane content that is probably a result of the cracking of toluene is of the same order of magnitude for iron-based catalysts, whereas it decreases considerably using raw CaO and (CaMg)O; raw MgO does not shows the same reactivity for methane reforming as the above-mentioned iron- and calcium-based catalysts. The reactivity tests with calcined dolomite, lime, and magnesia confirm the reactivity trends obtained by other
(36) Corella, J.; Toledo, J. M.; Molina, G. Ind. Eng. Chem. Res. 2006, 45, 6137–6146.
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Figure 17. Toluene conversion, as defined in eq 5, as a function of time for 20% iron catalysts prepared according to the neutral pathway.
the Ca2Fe2O5 phase. This is not surprising because of the stability of this phase confirmed by the high reduction temperature of the Fe3þ f Fe0 system. However, in the OxiDolo sample, 81% of iron oxide is in the Ca2Fe2O5 phase, whereas 19% is detected as Fe2þ, meaning that part of the iron may be subjected to the Fe3þ f Fe2þ reduction predicted by thermodynamic evaluation for 0.5 < Pred/Pox < 1.5. Therefore, MgO participates as a stabilizing agent for Fe2þ species. From the point of view of reactivity, the behavior of catalysts may be explained in light of these characterizations. When iron oxide is added to CaO and MgO substrates, it activates them as Fe3þ (Ca2Fe2O5) and Fe2þ (MgO-FeO solid solution), respectively. When it is added to dolomite, it neutralizes the beneficial distortion coming from the Ca or Mg atom arrangement and replaces it with the activity of Ca2Fe2O5. The progressive transition Ca2Fe2O5 f CaO þ MgO-FeO should slightly activate the Fe/dolomite system, because of the somewhat higher reactivity of FeO-MgO with respect to the Ca2Fe2O5 phase; however, this is not observed in Figure 16b. This means that the MgO-FeO sites are not available as active sites in catalyzing the reaction system when Ca2Fe2O5 is present. These points may be explained by considering the following: (1) At the beginning of the reaction, all of the iron oxides are found on the particle surface as a (CaO)2*Fe2O3 precipitate. During the reaction, iron is partly captured and solubilized in the MgO lattice, becoming virtually less accessible than the competitive sites of Ca2Fe2O5 at the surface, so that its contribution to enhancing the reaction kinetics becomes negligible. (2) Only 20% of the iron is susceptible to the Fe3þ f Fe2þ transition, so that the effect on reactivity is relatively unimportant.
Figure 16. Toluene conversion, as defined in eq 5, as a function of time for (a) supports and (b) iron catalysts prepared in the oxidative pathway.
Table 8. Mean Values of Percent Concentration in the Outlet Gas for Toluene Steam Reforming Using Substrates and 20% Fe/(Dolomite, CaO, and MgO) Oxidative Pathways OxiCa OxiMg OxiDolo CaO MgO (CaMg)O percentage of H2 (%) percentage of CO (%) percentage of CO2 (%) percentage of CH4 (%)
5.42 1.03 1.41 0.19
7.16 1.50 1.82 0.15
5.65 0.81 1.68 0.19
3.04 0.34 1.04 0.03
1.94 0.28 0.60 0.34
6.07 0.98 1.79 0.08
meaning that iron, even in a reductive atmosphere, remains mainly bound with calcium oxide as Ca2Fe2O5, as shown in the further section. Similar trends are obtained for the samples prepared with the neutral pathway (Figure 17); a slight increase (about 10%) in conversion with respect to Figure 16b has been recorded. However, because of the low reactivity difference and a high cost of manufacturing compared to the oxidative pathway, the neutral pathway does not appear suitable for scale up or further investigations. It has however been useful for providing a further understanding of the Fe/CaO-MgOdolomite interactions. Characterization after Tests. Characterization after the catalytic tests is useful for checking the phases of iron active in the various tested substrates during toluene steam reforming. XRD analysis reveals a FeO-MgO solid solution for OxiMg. This means that, under reaction conditions, the magnesioferrite phase of OxiMg is susceptible to reduction by CO and H2 to wurstite FeO, which is stabilized by MgO. Metallic iron as well as Fe3O4-Fe3-xMgxO4 phases are not detected, meaning that the solid solution represents a very stable form of the iron-substrate interaction. The iron(II) catalyst is able to catalyze steam reforming reactions even in the form of a FeO-MgO solid solution. A M€ ossbauer analysis was carried out for OxiCa and OxiDolo. The former shows that all of the iron remains in
Conclusions The removal of tar from the product gas of industrial biomass gasifiers, addressed to energy generation and/or fuel synthesis, is of considerable importance to the economic viability of the process. Tar production depends upon various factors, such as fuel composition, gasifying medium, operating temperature and pressure, gasifier construction, and the type of catalyst employed. The search for new catalytic materials is therefore a worthwhile research activity and one that should focus on realistic features of potential catalysts for commercial exploitation; these should consist of cheap and non-toxic materials to minimize manufacturing and final disposal costs. The development of catalysts with such characteristics has been the aim of the research presented in this paper. 4044
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In conclusion, it has been found that both Fe and Fe2þ in cooperation with CaO and MgO substrates are active in tar-reforming reactions. In view of the fact that raw CaO has been proposed as an ideal additive for biomass gasification applications,36 the conclusions of this work regarding the improved performance expected from the incorporation of iron could be of some significance for further developments.
Iron, an appropriately low-cost and non-toxic material, has been found to improve the catalytic activity of CaO and MgO substrates, rendering them comparable in activity to dolomite and, therefore, making them attractive for potential scale-up applications. It has been found that, when CaO is present in the substrate (as lime or calcined dolomite), iron may be preserved as the Fe3þ species but is reduced to Fe2þ in Fe/MgO systems. Thus, redox behavior in the product gas atmosphere is strongly dependent upon the kind of substrate adopted and the interactions that can take place. Regardless of the fact that Fe3þ is a thermodynamically unfavored oxidation state in the presence of steam and carbon dioxide oxidation agents, the Ca2Fe2O5 structure enables it to be stabilized. On the other hand, the Fe2þ species predicted by thermodynamic evaluation was detected in the Fe-MgO system, stabilized by the FeO-MgO solid solution.
Acknowledgment. The authors acknowledge the financial support received from Ente per le Nuove tecnologie, l’Energia e l’Ambiente (ENEA), the Italian national agency for non-conventional energy sources. L. Di Felice acknowledges the ItalianFrench University for his cotutelle Ph.D. grant and the European Doctoral College of Strasbourg for the support during his Ph.D. studies. Support from the Long-Term Research Plan of the Ministry of Education of the Czech Republic (MSM0021620857) is acknowledged.
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