Spin-state transition of iron in ( perovskite
At room temperature, the iron in a high-performance perovskite for ceramic oxygen separation membranes is in a mixed valence state of 75% in the high-spin state and 25% predominantly in the low-spin state. When heated to 900 °C, a slight reduction of iron is observed that increases the quantity of species. However, the dominant occurrence is a gradual transition in the spin-state of trivalent iron from a mixed low-spin/high-spin to a pure high-spin configuration.
Introduction
Ceramic membranes can provide remarkable oxygen permeation fluxes at infinite selectivity without the need of external electrodes when they are based on heavily doped anion deficient cubic perovskite-type oxides exhibiting mixed oxygen-ion and electron conductivities at elevated temperatures () [1]. While the partial conductivity of electrons is distinctly higher than that of oxygen ions [1], [2], the oxygen deficit correlates directly with the obtainable permeation flux via the concentration of disordered oxygen vacancies. The perovskite lattice can tolerate a remarkable number of vacant oxygen sites if a reducible transition metal cation is located at the crystallographic B site in the center of the octahedron. Upon heating under constant pressure, entropy may free up even more oxygen from a previously equilibrated perovskite. The quantity of released oxygen may be quite high as the requirement of charge neutrality is no longer preserved solely by small concentrations of cation vacancies but by the possibly flexible redox behavior of the B site cation(s). However, one requirement for the oxygen vacancies to be mobile is to preserve a cubic perovskite structure [3], where the redox behavior of the B site cation(s) plays the key role as well. Hence, a better understanding of the redox behavior can be used to tailor improved membrane materials with excellent phase stability under strongly reducing conditions.
The pioneering work of Teraoka et al. in the 1980s [4], [5], [6], [7] exceeded the already high standards for obtainable oxygen fluxes. As a consequence, the search for high-flux materials (i.e., exhibiting oxygen permeation fluxes of one to two orders of magnitude higher than cubically stabilized zirconia equipped with shortcircuit external electrodes [4], [8]) focused until today mostly on complex perovskites hosting cobalt on their crystallographic B site. The current state-of-the-art material with respect to oxygen permeation and phase stability above is (BSCF) [3], [9], [10], [11]. Recently, in situ high-temperature electron energy-loss spectroscopy (HT-EELS) on BSCF has shown that cobalt is reduced from an average formal oxidation state of to and iron from to , if BSCF is heated in the vacuum chamber of a transmission electron microscope (TEM) from room temperature to [12]. The average valence of the B site cations, at room temperature and at , give oxygen contents of 2.3 and 2.2, respectively. That is in good agreement with oxygen stoichiometries estimated by thermogravimetric analysis (TGA) and neutron powder diffraction [13]. The experiments in [12] however, give direct proof of the easier reducibility of cobalt over iron in a highly doped perovskite-type oxide. Also, comparative TGA of different perovskite-type materials indicate that during heating the polyvalent B-site cobalt ions are reduced far more easily than iron, manganese or nickel [14].
The flexible redox behavior of cobalt provides on the one hand high oxygen fluxes at high temperatures. On the other hand it leads to two major problems hindering the reliable use of the BSCF material (and cobaltites in general) under important technical requirements. First, it causes a large coefficient of thermal expansion (CTE) that can lie in the range of over a wide temperature range [13], [15]. The resulting dilatation causes huge thermal stresses and thus cracks form easily in the membranes, especially if operated at steep oxygen potential gradients. Steep gradients can be obtained by making membranes very thin (i.e., ) [15], [16], [17]. This is of interest to increase flux densities for the design of compact membrane units. Second, the valence instability of cobalt introduces inherent phase instability to the cobaltites at intermediate temperatures (ITs, ca. ) [9], [18], [19]. This is due to a coupled valence/spin-state transition of cobalt, and cobaltites containing octahedra with in low-spin configuration tend to prefer face sharing (contributions of hexagonal stacking) rather than corner sharing (cubic stacking) [19]. The breakdown of the cubic perovskite structure principally limits the long-time stability of the BSCF material under the conditions required for the operation of a membrane material in the IT range. The IT range, however, is of special interest for membrane-based dehydrogenation processes in the synthesis of basic chemicals like ethylene [20] and propylene [21] at high selectivity, and for the novel concept of solid oxide fuel cells (SOFCs) [22].
Recently, the search for alternative materials has led to the development of the cobalt-free perovskite-type oxide (BSFZ) [23], [24]. Wang et al. [24] demonstrated its potential for use in a membrane reactor for the partial oxidation of methane (POM) at . The reported oxygen permeation flux of on 1.25 mm-thick membranes at indicates a high conductivity as well as high surface exchange rates for oxygen. Wei et al. [25], [26] quantified the latter by measuring low polarization resistances of 0.22, 0.46, and at 700, 650, and , respectively. They also used BSFZ as a cathode material in an IT-SOFC at at power densities up to . The room temperature structure of BSFZ has been refined in a cubic unit cell () by the Rietveld method [27]. In situ X-ray diffraction (XRD) has shown that BSFZ remains in the cubic structure if heated to in air or in low oxygen partial pressures down to [24]. Good phase stability of cubic BSFZ in the IT range has been demonstrated recently for up to 100 h [28] and it is further implied by the observation that the perovskite can be synthesized by a sol–gel-based method in the pure phase at [27], [29], [30]. This is contrary to the cobaltite BSCF that requires higher temperatures in an analogous synthesis () [19], [31]. The present work focuses on a thorough atomic level understanding of electronic effects in the redox behavior of the BSFZ perovskite. Combined in situ electron energy-loss spectroscopy (EELS) and Mössbauer spectroscopy show that a coupled valence and low-spin to high-spin transition play a key role. This is reflected in anomalies found in integrative investigations of temperature-dependent weight-losses and lattice dilatations.
Section snippets
Material and methods
The BSFZ material was synthesized from nitrate precursors via an ethylene-diamine-tetra-acetic acid (EDTA)/citrate acid complexing method at for 10 h as described in detail elsewhere [27], [29], [30]. BSFZ ceramics were sintered at for 10 h and shaped by cutting, grinding, and polishing into disks approximately 3 mm in diameter and in thickness. To obtain TEM specimens, these disks were dimple grinded from one side to a residual thickness of (dimple grinder, model 656, Gatan
Results and discussion
Fig. 1 illustrates the sol–gel-based synthetic process for BSFZ starting from an aqueous solution of stoichiometric amounts of nitrates with EDTA, citric acid, and ammonia. The stage of the gel (after 18 h at ) is characterized by an ultrafine dispersion of cross-linked metal–organic complexes (bright features in Fig. 1a). The fine-scale intermixing is considered a major advantage over classical solid-state routes if a homogeneous product of complex stoichiometry is desired, as in case of
Conclusions
The performed EELS, esp. ELNES analyses of the edge have revealed that iron in the BSFZ perovskite is in a mixed / valence state at room temperature. Upon heating to the reduction of iron is so weak that no species are involved. Moreover, at the O-K edge hybridization effects of O:2p orbitals with empty Fe:3d, Zn:4sp, Ba:4f, Sr:4d, and Fe:4sp orbitals are noticed. The relative amount of hybridization does not change upon heating of the BSFZ perovskite. Mössbauer
Summary
It has been shown that the iron in the BSFZ perovskite has a mixed 75% valence (3.75+) at room temperature. Upon heating to in air it is reduced to a 25% /75% valence (). The fraction is always in a high-spin state, and the fraction makes a transition from a predominantly low-spin to a pure high-spin configuration at intermediate temperatures. A decrease in the amount of Fe:3d–O:2p hybridization during lattice expansion is seen as the reason for the
Acknowledgments
We would like to thank Prof. Harald Behrens for putting his high-pressure apparatus at our disposal and Dr. Falk Heinroth for assistance in TGA measurements. Our discussions with Profs. Jürgen Caro and Haihui Wang were fruitful and are appreciated. This work was financially supported by the Deutsche Forschungsgemeinschaft (DFG) under Grant FE 928/1-2. V.Ŝ. thanks the DFG for supporting his work in the framework of the Priority Program “Crystalline Nonequilibrium Phases” (SPP 1415). Partial
References (74)
- et al.
Mixed ionic–electronic conducting (MIEC) ceramic-based membranes
J. Membr. Sci.
(2008) Dense ceramic membranes for methane conversion
Catal. Today
(2003)- et al.
Mixed ionic–electronic conductivity of perovskite-type oxides
Mater. Res. Bull.
(1988) - et al.
Investigation of the permeation behaviour and stability of a oxygen membrane
J. Membr. Sci.
(2000) - et al.
Phase stability and oxygen non-stoichiometry of
Solid State Ionics
(2006) - et al.
Fe-based perovskite-type oxides as excellent oxygen-permeable and reduction-tolerant materials
Solid State Ionics
(2006) - et al.
Performance of functional perovskite membranes for oxygen production
J. Membr. Sci.
(2006) - et al.
Hollow fibre perovskite membranes for oxygen separation
J. Membr. Sci.
(2005) - et al.
Structural instability of cubic perovskite
Solid State Ionics
(2008) - et al.
Synthesis, electrical and electrochemical properties of perovskite oxide for IT-SOFC cathode
J. Power Sources
(2008)
The sol–gel synthesis of perovskites by an EDTA/complexing method involves nanoscale solid state reactions
Solid State Sci.
Advanced perovskite-type ceramics as oxygen selective membranes: evaluation of the synthetic process
Prog. Solid. State. Chem.
Oxidation states of Mn and Fe in various compound oxide systems
Micron
Measuring the absolute position of EELS ionisation edges in a TEM
Ultramicroscopy
Mixed oxygen ion and electron conducting hollow fiber membranes for oxygen separation
Solid State Ionics
Grain boundaries as barrier for oxygen transport in perovskite-type membranes
J. Membr. Sci.
Influence of grain size on the oxygen permeation performance of perovskite-type membranes
J. Membr. Sci.
X-ray absorption and dichroism of transition metals and their compounds
J. Electr. Spectrosc. Rel. Phenom.
Iron near-edge fine structure studies
Ultramicroscopy
High-resolution EELS study of the vacancy-doped metal/insulator system
J. Solid State Chem.
Systematic trends of the 57Fe Mössbauer isomer shifts in and polyhedra. Evidence of a new correlation between the isomer shift and the inductive effect of the competing bond T–X () where X is O or F and T any element with a formal positive charge
J. Phys. Chem. Solids
Synthesis structure and properties of : an intermediate spin state
J. Solid State Chem.
Charge compensation and oxidation in and studied by XANES
J. Solid State Chem.
Properties and performance of materials for oxygen transport membranes
J. Solid State Electrochem.
Oxygen permeation through perovskite-type oxides
Chem. Lett.
Mixed ionic–electronic conductivity of perovskite-type oxides
Chem. Lett.
Effect of cation substitution on the oxygen semipermeability of perovskite-type oxides
Chem. Lett.
Oxygen ion mobility in cubic
J. Am. Ceram. Soc.
A high-performance cathode for the next generation of solid-oxide fuel cells
Nature
Local charge disproportion in a high-performance perovskite
Chem. Mater.
Oxygen stoichiometry and chemical expansion of ( measured by in situ neutron diffraction
Chem. Mater.
Development of oxygen semipermeable-membrane using mixed conductive perovskite-type oxides. 2. Preparation of dense film of perovskite-type oxide on porous substrate
J. Ceram. Soc. Jpn.
Correlation of the formation and the decomposition process of the BSCF perovskite at intermediate temperature
Chem. Mater.
High selectivity of oxidative dehydrogenation of ethane to ethylene in an oxygen permeable membrane reactor
Chem. Commun.
Oxidative dehydrogenation of propane in a dense tubular membrane reactor
React. Kinet. Catal. Lett.
Materials for fuel-cell technologies
Nature
Cited by (58)
Effect of plasma atmosphere on the oxygen transport of mixed ionic and electronic conducting hollow fiber membranes
2021, Journal of Industrial and Engineering ChemistryEffect of catalyst preparation and storage on chemical looping epoxidation of ethylene
2021, Chemical Engineering JournalMicrostructure and diffusion behavior in the multilayered oxides formed on a Co–W electroplated ferritic stainless steel followed by oxidation treatment
2020, Acta MaterialiaCitation Excerpt :Using equation (1), we calculated the L3/L2 ratios of Cr, Co, and Fe for each layer. This ratio can reveal the valence state of each element in oxides by comparison with data for standard materials; the results are summarized in Table 5, together with the reported values [35,39–43]. The L3/L2 ratios of Cr (1.68 in Cr oxide and 1.69 in Cr–Fe–Co oxide) are slightly larger than that of standard Cr2O3 (1.63), suggesting that the valence state of Cr in the oxide layer formed in this study is +3.
Modelling of oxygen transport through mixed ionic-electronic conducting (MIEC) ceramic-based membranes: An overview
2018, Journal of Membrane Science
- 1
Now at: Nonmetallic Inorganic Materials, ETH Zurich, Wolfgang-Pauli-Str. 10, CH-8093 Zurich, Switzerland.
- 2
Now at: Suzuki Laboratory, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan.
- 3
On leave from the Institute of Geotechnics, Slovak Academy of Sciences, SK-04353 Košice, Slovak Republic.
- 4
Now at: Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany.