Oxygen Exchange Kinetics over Sr- and Co-Doped ... - ACS Publications

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J. Phys. Chem. C 2008, 112, 12468–12476

Oxygen Exchange Kinetics over Sr- and Co-Doped LaFeO3 John N. Kuhn,† Paul H. Matter,† Jean-Marc M. Millet,‡ Rick B. Watson,† and Umit S. Ozkan*,† Department of Chemical and Biomolecular Engineering, The Ohio State UniVersity, 140 West 19th AVenue, Columbus, Ohio 43210, and Institut de Recherches sur la Catalyse et l’EnVironnement de Lyon, IRCELYON, UMR 5256 CNRS-UniVersite’ Claude-Bernard, Lyon 1, 2 AVenue A. Einstein, F-69626 Villeurbanne Cedex, France ReceiVed: February 20, 2008; ReVised Manuscript ReceiVed: June 2, 2008

Oxygen exchange constants and oxygen diffusion coefficients were determined by relaxation (TGA-DSC and in situ XRD) and steady-state isotopic transient kinetic analysis (SSITKA) techniques as a function of oxygen partial pressure, temperature, and Co content for La0.6Sr0.4CoyFe1-yO3-δ (y ) 0.1, 0.2, and 0.3). When y ) 0.2, oxygen exchange constants and oxygen diffusion coefficients appeared to be enhanced once above the temperature at which the unit cell symmetry increased from rhombohedral to cubic. Oxygen partial pressure demonstrated minimal influence upon these properties over the higher range (near ambient) studied, but showed a major impact when results obtained near ambient oxygen partial pressures are compared to those obtained under N2. The direction of oxygen transport was also important. Oxygen exchange constants and oxygen diffusion coefficients calculated from oxidation steps were greater than values for the same properties calculated from reduction steps under all conditions examined. Explanations for this behavior are proposed with use of kinetic, transport, and ESR spectroscopy data. While the oxygen exchange constant increased with Co content, the oxygen diffusion coefficient progressed through a maximum for the formations studied. The nonlinear trend for oxygen diffusivity with Co content is explained by an electronic structure change over this compositional range, which aligned with differences in Mo¨ssbauer spectra. 1. Introduction Fe-based perovskite-type oxides possess unique properties due to a simultaneous ability to conduct electrons and oxide ions. These mixed conducting materials are important for solid oxide fuel cells (SOFCs),1,2 oxygen separation devices,1 and heterogeneous catalysis.1,3 In our previous work,4 we characterized two structural effects, for La0.6Sr0.4CoyFe1-yO3-δ (y ) 0.1, 0.2, and 0.3), which are expected to influence these materials’ kinetic and transport properties. First, a transition from rhombohedral to cubic unit cell symmetry occurred between 500 and 800 °C with the transition temperature decreasing as Co content increased. Second, altering the Co/Fe ratio on the B-site in Ferich formulations led to nonlinear reduction behavior. Oxygen transfer abilities are expected to depend upon the bulk structure because the covalent bonding involved with oxide ion hopping and the stability of oxygen vacancies are both structure sensitive. Overlap between the O 2p and the B-site 3d orbitals, the key covalent bond for electronic and anionic transport properties, decreases as lattice symmetry decreases. Moreover, oxygen vacancies can become ordered when lattice symmetry is not cubic. A well-studied example is the Brownmillerite phase (orthorhombic lattice symmetry), in which oxygen transport decreases due to ordered oxygen vacancies.5–11 Additionally, reducibility (as well as surface properties12) did not scale with Co content as La0.6Sr0.4Co0.2Fe0.8O3-δ was least likely to form oxygen vacancies under mild conditions (in air and initial reducibility in He and H2) and most likely to form oxygen vacancies in He at higher temperatures compared to * To whom correspondence should be addressed. E-mail: [email protected]. Phone: 614-292-6623. Fax: 614-292-3769. † The Ohio State University. ‡ UMR 5256 CNRS-Universite’ Claude-Bernard.

other formulations studied.4 In work by other researchers, trends with B-site composition for oxygen vacancy formation in air depended upon the compositional regime studied for La1-xSrxCoyFe1-yO3-δ compounds.13–15 These studies demonstrated increasing oxygen vacancy formation with Co content for Sr-rich formulations14 and for Co-rich formulations when x ) 0.4.15 However, nonlinear behavior occurred for Fe-rich formulations when x ) 0.2.13 Moreover, catalytic activity for hydrocarbon combustion and hydrogen peroxide decomposition progressed through a maximum as Co content varied.16,17 Since nonlinear behavior occurred over a wide range of properties (e.g., surface and bulk, kinetic and thermodynamic), the cause was speculated to be differences in electronic structure. Literature support for an electronic structure transition in this compositional regime, based upon electrons being more localized in Fe-rich formulations and more delocalized in Co-rich formulations,2 was previously discussed 4,12 (note the error in the conclusions of the former reference due to switching of the words “localized” and “delocalized”). Recent research has also proposed that this transition led to changes in the mechanism of oxygen exchange as Co-based perovskite oxides appeared limited by oxygen dissociation whereas Fe-based ones are controlled by chemisorption of molecular oxygen.18 The present work aims at further probing the influence of these effects upon kinetic and oxide ion transport properties of Fe-based mixed conductors. These properties, namely the chemical diffusion coefficient (neutral mass transfer under chemical potential gradients) and the oxygen exchange constant, are determined by subjecting mixed conductors to step changes in PO2. Fitting the transient responses with analytical solutions to Fick’s second law is a common method to determine kinetic and transport properties for these types of materials.19–31 These

10.1021/jp801521y CCC: $40.75  2008 American Chemical Society Published on Web 07/23/2008

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analyses are complemented by additional data from techniques such as isotopic analysis, in situ XRD, and Mo¨ssbauer spectroscopy. 2. Experimental Section 2.1. Synthesis and Structure. Synthesis and bulk structure of La0.6Sr0.4CoyFe1-yO3-δ samples were previously described.4,12 After ball milling, materials were synthesized from La2O3, SrCO3, Fe2O3, and Co3O4 precursors by a solid-state reaction at 1000°C. Samples were a single-phase perovskite-type structure with rhombohedral unit cell symmetry under ambient conditions. 2.2. Isothermal Oxygen Equilibration Experiments. Oxygen equilibration experiments were monitored by using a Setaram TG-DSC111, an instrument with simultaneous thermogravimetric and calorimetric capabilities (TGA-DSC). Samples (∼80 mg) were loaded into Pt crucibles. Before each set of experiments, the instrument was flow balanced at room temperature. After a steady mass was obtained at room temperature, a flow of 16% O2/N2 at 20 mL/min began and then samples were heated to the desired temperature (between 500 and 700 °C) at 5 deg/min. After a steady mass was achieved at the elevated temperature, a four-port valve was used to subject samples to step changes in oxygen partial pressure. At all times, the total flow rate was 20 mL/min. The purity level of the pure N2 gas (Praxair) used for this experiment and for in situ XRD was 99.999%. Each value was repeated multiple times and error bars were calculated by using pooled standard deviation for all data collected at the same temperature. 2.3. Temperature-Programmed Reoxidation. Experiments were performed with an Autochem II 2920 instrument equipped with a TCD. Evolution of oxygen under inert environments was discussed in an earlier publication.4 Changes in oxygen content were higher in the present work because the complete temperature program (isothermal hold at 900 °C) was not previously shown. After oxygen evolution, samples were cooled in He to near-ambient conditions ( 100). However, eq 2 leads to a more accurate solution when the two parameters are on the same order of magnitude after accounting for particle size (L ≈ 1). When the rate constant is small relative to the diffusion coefficient (L < 100), only kinetic limitations exist (i.e., negligible concentration profile within particle). Particle radii were estimated as the geometric mean of one-half the crystallite size calculated from the cubic (110) line4 and of the particle radius calculated from

Figure 4. SSITKA for La0.6Sr0.4Co0.2Fe0.8O3-δ at (a) different temperatures and (b) a more in-depth look at 600 °C. Species indicated by 0 (m/z 32), O (m/z 34), 4 (m/z 36), and [ (m/z 40).

the mass specific surface area (using Kr as a probe molecule4,12) assuming dense particles. TEM micrographs (not shown) confirmed the accuracy of the crystallite size estimation. The fit of the data (Figure 1b) improved until the exchange constant was on the order of 2 × 10-7 cm/s. When it increased, it no longer factored into the results as eqs 1 and 2 arrived at the same value. Due to the long time required for the mass to equilibrate in N2, the response was also monitored by in situ XRD, a nontraditional technique for transient analyses. Results for several cubic diffraction lines are shown in Figure 2a. As discussed in more detail previously,4 lines shifted to lower angles, due to thermal and chemical (caused by gradual creation of oxidation vacancies and simultaneous reduction of transition metals) expansion during heating. During cooling, the opposite behavior occurred as the lattice contracted. Additionally, a phase transition between cubic and rhombohedral unit cell symmetry,

Oxygen Exchange Kinetics

Figure 5. TGA-DSC monitoring of the responses to a series of step changes in PO2 (numerical values indicate % O2/N2) at 1 atm and 600 °C for La0.6Sr0.4Co0.2Fe0.8O3-δ.

indicated by the sharpening of lines during heating and broadening of lines during cooling, occurred in air. Changes in the shapes evolved from changes in the amount of closely situated diffraction lines which increased as unit cell symmetry decreased. For this sample, the transition occurred near 550 °C4 regardless of atmosphere so unit cell symmetry was cubic during the environmental changes at 700 °C.

J. Phys. Chem. C, Vol. 112, No. 32, 2008 12471 In our previous work,4 it was difficult to isolate chemical and thermal components as temperature changed and only combined expansion values were determined. These combined terms are essentially temperature and atmosphere dependant thermal expansion coefficients (TECs). Strictly speaking, the term “thermal expansion coefficient” does not accurately represent the data (e.g., refs 33 and 34) since both temperature and atmosphere influence oxidation state and consequently chemical expansion, nevertheless it is used as a practical term. In the present study, chemical expansion was isolated by examining structural changes in XRD under isothermal conditions (700 °C). As compared to the changes during temperature ramps, only slight shifts of the diffraction lines occurred following environment changes at 700 °C, indicating that, under these conditions, thermal expansion was more influential during the temperature ramps than chemical expansion. These trends were also quantified with Figure 2b presenting the cubic lattice parameter calculated from diffraction patterns shown in Figure 2a. During heating to 700 °C (and cooling back to 100 °C from 700 °C), the lattice parameter changed by 0.82%. The lattice parameter further increased by 0.15% during the switch from air to N2 at 700 °C. On the basis of these lattice size and oxygen

Figure 6. Parameters calculated from experiments similar to those shown in Figure 5. Exchange constant (a), diffusion coefficient (b), and energetics (c) from desorption and exchange constant (d), diffusion coefficient (e), and energetics (f) from adsorption, respectively. The values 500, 600, 650, and 700 indicate temperature (°C)

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Figure 7. Superoxide ion (O2-) detected over La0.6Sr0.4Co0.2Fe0.8O3-δ by ESR following various treatments.

Figure 8. Parameters calculated from experiments similar to as shown in Figure 5 for La0.6Sr0.4CoyFe1-yO3-δ. Exchange constant (a) and diffusion coefficient (b) for desorption.

vacancy concentration changes over this step change in oxygen partial pressure, “oxygen vacancy chemical expansivity” was estimated to be 0.059 (value of 1/3βc(xv)34), which agreed with reported values for other perovskite-type oxides.34 Since lattice parameters were monitored temporally following the change in oxygen partial pressure, the transient response is compared in Figure 3 to TGA data presented in Figure 1. TGA data are truncated to compare results over a similar time frame. Lattice parameters are reported at the intermediate time (i.e., values shown every 32 min starting at 16 min). Despite a small difference in initial oxygen partial pressure (16% for TGA and 21% for XRD), diffusion coefficients calculated from eq 1 agreed very well.

Kuhn et al. On the basis of results presented so far, several points are worth discussing. As observed by in situ XRD (Figure 2) and by similar TGA experiments (not shown), reoxidation occurred much more rapidly than reduction (formation of oxygen vacancies). Since the absence of mass transfer limitations was not verified under these conditions, the possibility exists that mass transfer effects may influence the results. However, we will discuss in the following section that the mechanism and the stability of surface species could also be a cause for differences between oxygen adsorption and desorption. Additionally, properties were calculated by inducing a measurable structural change by a large step in oxygen partial pressure. Since properties are structure dependent, it is desirable to eliminate or to minimize structural changes so that the influence of environmental parameters (e.g., oxygen partial pressure) can be better studied as is discussed further in section 3.2. In addition to the observation of structural changes in Figures 1 and 2, Mo¨ssbauer spectra also showed major differences following similar treatments. After thermal treatments under inert conditions, Fe4+ ions were reduced to Fe3+ species and magnetic properties changed. These results will be discussed in more detail when additional Mo¨ssbauer spectra are presented in section 3.4. 3.2. Properties of La0.6Sr0.4Co0.2Fe0.8O3-δ Examined by Isotopic Labeling and Quasi-Steady State Methods. Isotopic exchange with SSITKA (e.g., refs 35, 36, and 37), a technique for measuring oxygen mobility without inducing structural changes, is shown in Figure 4 for La0.6Sr0.4Co0.2Fe0.8O3-δ. At temperatures below where oxygen vacancies form (∼500 °C), little oxygen was exchanged as the unlabeled oxygen signal relaxed almost as quickly as the Ar profile, which signaled gasphase hold-up. The slight difference between the Ar profile and the relaxing oxygen signal is related to molecular oxygen-surface interactions. However, at 600 °C, oxygen was easily transferred as breaking of the oxygen-oxygen bond was facile. In fact, integration of the transient signals (after correction for gas-phase hold-up) indicated that all or nearly all oxygen contained within the sample was transferred. From these results, a composition of ABO2.96 was calculated for the material. This value for the oxygen content was only slightly lower (expected due to lower oxygen partial pressure in the present study) than previously reported4 by other techniques and confirmed that the sample was stoichiometric under ambient conditions. Additionally from the data at 600 °C, insight into the oxygen exchange mechanism and estimates of the properties were made. Using the SSITKA analysis method (i.e., ref 35), the oxide ion diffusion coefficient was estimated to be 9.5×10-12 cm2/s, which aligned well with data from relaxation technique at similar oxygen partial pressures (presented later in this section). Moreover, catalytic activity for the oxygen exchange was determined by a process similar to other work in the literature.38 An initial oxygen exchange rate of 4.8×10-5 mol of oxygen m-2 s-1 was calculated based on the initial slopes (over first 90 s) of the transient signals. A turnover frequency between 4 and 28 s-1 was determined based either upon one active site per unit cell (assuming the surface is equivalent to bulk) or upon previous methanol chemisorption data.12 Since the signal for cross-labeled oxygen (m/z 34) increased more rapidly than that of doubly labeled oxygen (m/z 36 for first switch and m/z ) 32 for second switch), the simple heteroexchange appeared to dominate the multiple heteroexchange (either place-exchange or four-atom oxygen surface species) and the homoexchange (no contribution from the oxide lattice) mechanisms. The simple heteroexchange mechanism suggested that O-O bond scission

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Figure 9. Temperature-programmed oxidation profiles (a) as a function of Co-content in La0.6Sr0.4CoyFe1-yO3-δ following oxygen evolution at 900 °C in He for 20 min and (b) compared to calculated profiles when y ) 0.2 for second order kinetic parameters.

TABLE 1: Kinetic Parameters for La0.6Sr0.4CoyFe1-yO3-δ Extracted from Temperature-Programmed Oxidation Experiments Shown in Figure 9 Co content

∆δ

Tp (°C)

∆W (°C)

EA (kJ/mol)

y ) 0.1 y ) 0.2 y ) 0.3

0.19 0.25 0.17

124 176 171

80 63 153

51.6 86.7 31.4

is rapid and takes place through three-atom oxygen surface species. A separate experiment where the same switch was performed without any catalyst showed no cross-labeled oxygen, verifying that the gas-phase hold-up time was short enough and that all of the labeled oxygen observed in the experiment actually involved exchange with the catalyst lattice. Differences in these mechanisms and oxygen surface species are discussed in more detail elsewhere38–40 and are also further addressed in this section using ESR spectroscopy data. In efforts to compare property differences between adsorption and desorption and to minimize structural changes, responses to smaller step changes in PO2 were monitored. An example of these experiments is shown in Figure 5. As PO2 is reduced, mass loss occurred due to oxygen desorption, which is also observed by an endothermic DSC peak. Upon an increase in PO2, the reverse occurred as the mass equilibrated to its initial value. Since steps in PO2 were much smaller than shown in section 3.1, effects induced by structural changes were reduced. Transient responses were analyzed in similar fashion as before and results are presented in Figure 6. Diffusion coefficients and surface exchange constants were close to values reported by other techniques over these temperatures.41 Differences between results presented here and those in ref 41 can be due to use of different experimental conditions and techniques (chemical versus selfdiffusion) or variations in the microstructure. In our experiments, parameters were assumed to be free of external mass transfer limitations because similar values (within error bars)

were obtained by using mixtures of O2/He instead of O2/N2. Additionally, by operating under higher PO2, behavior was controlled by oxygen reduction kinetics and lattice diffusion. As expected, the surface exchange constant and the diffusion coefficient both increased with temperature. For both properties, during adsorption and desorption, an enhancement appeared to occur near 550 °C. This enhancement or “jump” in properties was also verified by using Arrhenius plots, which showed different slopes at high and low temperatures (making calculations of activation energies not very useful), respectively. This temperature is near the reversible rhombohedral-cubic transition so the “jump” in properties is potentially caused by this structural change. Second, the effect of PO2 upon the properties was less conclusive. The surface exchange constant appeared mainly independent of PO2 during both adsorption and desorption. Although lack of dependence was suggested (but considered not very likely),42 it was likely caused due to evaluation of only three oxygen partial pressures over a limited range. It also apparently disagreed with the evidence of approximately half order dependence for La0.6Sr0.4Co0.2Fe0.8O3-δ21,28 and similar materials,23,24,43–45 which is the common assumption for models due to its basis on mass action kinetics.42,46 However, as demonstrated by other researchers (e.g., ref 18), knowledge of PO2 dependence does little to differentiate between potential mechanisms. Moreover, no clear correlations existed between diffusion coefficients and PO2. Although trends were reported in the literature,21,28 we did not detect them, possibly because of the narrow range of PO2 employed in these experiments. A lack of dependency was also reported for other formulations of (La, Sr)(Co,Fe)O3-δ oxides20,23,24,43,45,47 over even wider PO2 ranges. When compared to results from the previous section for large steps in PO2, these results demonstrated dependence upon PO2. On the basis of data points for switches from air to N2 and to 8% O2/N2, the diffusion coefficient showed a relationship of D ∝ PO20.63, which may be invoked by large structural changes associated with the large step change in PO2. Finally, results showed that the energy change was near 180 kJ/mol O2 exchanged, which was independent of varied parameters within experimental error. Exchange constants and diffusion coefficients were both higher when determined from oxygen adsorption data compared to desorption data. In (La,Sr)(Co,Fe)O3-δ oxides, the literature is not consistent with regards to whether exchange constants and diffusion coefficients are impacted by the direction of oxygen flow.20–22,24,28,30,43 Additionally, Fe-based perovskitetype oxides were more sensitive to the direction of oxygen flow than its Co-based counterparts.30 Our data suggested that the direction of flow was important and we have performed additional spectroscopic experiments (ESR) in efforts to better understand this phenomenon. Moreover, as presented in section 3.3 by using temperature-programmed studies, oxygen incorporation occurred at much lower temperatures than oxygen evolution, which may also support our observations that the direction of oxygen flow impacted the properties. Electron-spin resonance (ESR) spectroscopy, a technique to detect specific oxygen surface species,39,40 was used to gain additional insight into surface oxygen species. In the full scan range (not shown), ESR spectra exhibited resonances at g ≈ 4.3 and 2.1, which are generally ascribed to Fe3+ ions. These resonances are assigned to either distorted Fe3+ ions with rhombic symmetry in tetrahedral or octahedral coordination or exchange interactions between clustered Fe3+ species (refs 48

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Figure 10. Mo¨ssbauer spectra at 25 °C for La0.6Sr0.4CoyFe1-yO3-δ and after reduction when y ) 0.2. Solid lines are derived from least-squares fits.

and 49 and references therein). Additionally, a symmetric signal, with a resonance of 2.012, was detected that grew with evolution or incorporation of oxygen. These species are highlighted in ESR spectra shown in Figure 7. Symmetry and appearance of this resonance, in addition to its agreement with a cited value of 2.011,50 made a clear case for the presence of the superoxide ion (O2-). Since the superoxide ion can only been detected on nonparamagnetic cations,40 its detection following thermal treatments may align with a decrease in the amount of paramagnetic species. Intensity was stronger following adsorption indicating that this species may be more stable upon adsorption than desorption. Higher stability may indicate that different fundamental steps are limiting or different active sites are involved in determining the oxygen exchange rate depending on whether oxygen is being incorporated into or evolving from the structure. In either case, these explanations may support the higher oxygen exchange constants and oxygen diffusion coefficients observed during adsorption compared to desorption steps (Figure 6). While these explanations do not exclude all alternatives (e.g., superoxide ion may be a spectator ion or intensity differences may occur due to material changes induced by the treatments), they suggested that ionic interactions, which would dominate when the B-site cations are in a higher oxidation state, are not responsible for the behavior. 3.3. Effect of Co upon Oxygen Transfer Properties for La0.6Sr0.4CoyFe1-yO3-δ. Exchange constants and diffusion coefficients (calculated from desorption data) are shown in Figure 8 as a function of Co content. As was the case when y ) 0.2, values for both properties were higher when calculated from adsorption than desorption and increased with temperature (not shown). At 600 and 700 °C, the diffusion coefficient progressed through a maximum with Co content as was also observed for bulk oxygen vacancies.4 The higher diffusion coefficient also aligned with the highest energy of oxygen adsorption/desorption. The average value for all steps (again no trends with temperature or PO2 within experimental error) was 43, 180, and 51 kJ/mol O2 exchanged for y ) 0.1, 0.2, and

Kuhn et al. 0.3, respectively. Since these results demonstrated higher oxygen mobility in less reducible samples, it implied that factors in addition to oxygen vacancy concentration, at least in this compositional range, influence the properties. Especially noticeable at higher temperatures, the exchange constant increased as Co content increased. These trends suggested an electronic structure transition, which is expected to occur as Co content is changed in these formulations,4,12 demonstrating an influence on the bulk properties (i.e., diffusion coefficient). However, its impact upon surface properties (i.e., exchange constant) was not important or masked by changes in Co content. Reasons for the transition are discussed in section 3.4. Samples were also evaluated for reoxidation kinetics following evolution of oxygen in He at 900 °C (He treatment shown previously4). These experiments and analyses were modeled after existing work in the literature.51 In Figure 9a, reoxidation profiles are compared. When evaluating these profiles against the oxygen evolution profiles,4 the temperature at which oxygen enters or exits the lattice is vastly different. Oxygen evolution did not occur until above 350 °C with the peak being above 750 °C. On the other hand, reoxidation occurred at much lower temperatures, another sign of more rapid oxygen incorporation than evolution, as indicated by the peak temperature being below 200 °C for all three samples. Oxygen consumption profiles showed differences based on peak temperature and shape, which are used to determine the activation energy. The only remaining question is the order of the kinetics. A better fit was obtained for second order kinetics (first order fit not shown) as shown in Figure 9b. Therefore, activation energies, as presented in Table 1, were calculated for second order kinetics. A maximum occurred for the sample with intermediate Co content indicating that the highest oxygen mobility, which would be low in this temperature range, is likely involved in limiting the process. 3.4. Effect of Co upon Magnetic Properties for La0.6Sr0.4CoyFe1-yO3-δ. 57Fe Mo¨ssbauer spectra of the La0.6Sr0.4CoyFe1-yO3-δ samples are shown in Figure 10 with the corresponding fitted parameters presented in Table 2. Results demonstrated differences in relative concentrations of Fe oxidation states and in magnetic behavior. Samples with higher Co content were fit with two single peaks while the sample with the lowest Co content was fit with a single peak and a distribution of sextets. These single peaks have parameter values comparable to those already published52–54 for similar compositions and are assigned to high spin Fe3+ and Fe4+ species, respectively. The sextet for the sample with the lowest Co content consisted of a distribution of sextets centered at 51.7 T, which is characteristic of Fe3+. While samples containing more Co showed single resonances indicative of paramagnetic behavior, the Fe-rich sample showed magnetic ordering behavior for the Fe3+ species. The onset of magnetic ordering was a possibility since the Curie temperature, a function of transition metal content, was near room temperature for perovskite-type oxides containing Sr and mixtures of La and Sr.55 Since the Curie temperature decreased with increasing Co content for pure La-based perovskite-type oxides54 and progressed through a maximum with increasing Co content for Sr and Sr-rich ones,55 A-site composition is important in determining the relationship between Co content and the Curie temperature in Fe-rich formulations. For the formulations in the present work, the Curie temperature appeared to increase as Co content increased yielding magnetic behavior only in the Fe-rich sample.

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TABLE 2: Mo¨ssbauer Parameters Calculated from Spectra in Figure 10 for La0.6Sr0.4CoyFe1-yO3-δ and after Reduction When y ) 0.2a sample y ) 0.1 y ) 0.2 y ) 0.3

∼ABO2.8

near an electronic structure transition. Although other reasons are not excluded, our data suggested that preferential oxidation of Fe over Co is involved in this behavior.

species rel intensity (%) δ (mm/s) ∆ (mm/s) H (T)c 3+

Fe Fe4+ Fe3+ Fe4+ Fe3+ Fe4+

effect of Co content 41 0.34 59 0.04 42 0.34 58 0.00 49 0.35 51 -0.01

0.00b

51.7

0.00 0.00 0.00 0.00 0.00

effect of oxygen content when y ) 0.2 Fe3+ 100 0.33 0.00b

51.1

δ, isomer shift (given with respect to R-Fe); ∆, quadrupolar splitting; H, internal magnetic field. b 2
, magnetic splitting. c mean value of hyperfine fields distributions. a

Moreover, when Co content increased, the relative content of Fe4+ slightly decreased. High fractions of Fe4+ suggested that oxygen vacancies did not exist under ambient conditions as previously measured4 and supported by isotopic analysis. Since the average oxidation state of the B-site components was the same for all three samples under ambient conditions,4 this trend suggested that a preferential oxidation of Fe over Co, as originally proposed by Tai et al. in similar formulations,13 occurred under ambient conditions. This effect became exaggerated as Co content decreased. As also presented in Figure 10 and Table 2, La0.6Sr0.4Co0.2Fe0.8O3-δ was characterized by Mo¨ssbauer spectroscopy after a reduction in He at 900 °C. The Mo¨ssbauer spectrum consisted of a distribution of sextets, which is characteristic of Fe3+. While the hyperfine field for the sextets was centered at 51.1 T, contributions existed between 42 and 55 T. All of the Fe4+ cations were reduced to Fe3+ during the reduction, but no further reduction occurred. The field distribution could be associated to the proximity of the magnetic ordering temperature as discussed for the fresh Fe-rich sample. On the basis of results for La0.5Sr0.5FeO3 following a mild reduction,52 attempts were made to fit the spectrum with a second sextet, but it was not possible. A second sextet would account for Fe3+ cations in a lower coordination than octahedral (i.e., CN ) 4 or 5). However, resolution was too low to provide evidence of anisotropic deformation caused by a change in the environment of the Fe3+ cations. 4. Conclusions Kinetic and mass transfer properties were evaluated as a function of oxygen partial pressure, temperature, and Co content during oxygen adsorption and desorption for La0.6Sr0.4CoyFe1-yO3-δ when y ) 0.1 to 0.3. Near the temperature for the rhombohedral to cubic transition, an enhancement of properties appeared to occur for the sample with the intermediate Co content (y ) 0.2). Oxygen partial pressure displayed a minimal effect except steps to very low values (PO2 ≈ 1×10-5 atm), which led to much lower properties. Hysteresis effects were also observed as oxygen incorporation into the lattice was much more rapid than oxygen evolution from the lattice. These differences may be related to stability of molecular and atomic oxygen surface species. In the compositional range examined, the oxygen exchange constant increased with Co content whereas the oxygen diffusion coefficient was highest for the sample with intermediate Co content (y ) 0.2). Nonlinear dependence upon Co content, which was also observed in DSC and temperature-programmed reoxidation, is related to being

Acknowledgment. Financial support provided for this work by the Ohio Coal Development Office and the Ohio Department of Development through a Wright Center of Innovation is gratefully acknowledged. P.H.M. acknowledges the National Science Foundation through grant NSF-CTS-0437451 for financial support as a graduate fellow. References and Notes (1) Gelling, P. J.; Bouwmeester, H. J. M. CRC Handbook of Solid State Electrochemistry; CRC Press: New York, 1997. (2) Adler, S. B. Chem. ReV. 2004, 104, 4791. (3) Pena, M. A.; Fierro, J. L. G. Chem. ReV. 2001, 101, 1981. (4) Kuhn, J. N.; Ozkan, U. S. Catal. Lett. 2008, 121, 179. (5) Deng, Z. Q.; Yang, W. S.; Liu, W.; Chen, C. S. J. Solid State Chem. 2007, 179, 362. (6) Wiik, K.; Aasland, S.; Hansen, H. L.; Tangen, I. L.; Ødegard, R. Solid State Ionics 2002, 675, 152–153. (7) Qui, L.; Lee, T. H.; Liu, L. M.; Yang, Y. L.; Jacobson, A. J. Solid State Ionics 1995, 76, 321. (8) Kruidhof, H.; Bouwmeester, H. J. M.; van Doorn, R. H. E.; Burggraaf, A. J. Solid State Ionics 1993, 816, 63–65. (9) McIntosh, S.; Vente, J. F.; Haije, W. G.; Blank, D. H. A.; Bouwmeester, H. J. M. Solid State Ionics 2006, 177, 1737. (10) Vente, J. F.; McIntosh, S.; Haije, W. G.; Bouwmeester, H. J. M. J. Electrochem. Soc. 2006, 10, 581. (11) Yang, Z. H.; Lin, Y. S. Solid State Ionics 2005, 176, 89. (12) Kuhn, J. N.; Ozkan, U. S. J. Catal. 2008, 253, 200. (13) Tai, L. W.; Nasrallah, M. M.; Anderson, H. U.; Sparlin, D. M.; Sehlin, S. R. Solid State Ionics 1995, 76, 259. (14) Stevenson, J. W.; Armstrong, T. R.; Carneim, R. D.; Pederson, L. R.; Weber, W. J. J. Electrochem. Soc. 1996, 143, 2722. (15) Lankhorst, M. H. R.; ten Elshof, J. E. J. Solid State Chem. 1997, 130, 302. (16) Seiyama, T.; Yamazoe, N.; Eguchi, K. Ind. Eng. Chem. Prod. Res. DeV. 1985, 24, 19. (17) Zhang, H.-M.; Shimizu, Y.; Teraoka, Y.; Miura, N.; Yamazoe, N. J. Catal. 1990, 121, 324. (18) Adler, S. B.; Chen, X. Y.; Wilson, J. R. J. Catal. 2007, 245, 91. (19) Reiss, I. Electrochemistry of Mixed Ionic-Electronic Conductors; CRC Press: New York, 1997; p 816. (20) ten Elshof, J. E.; Lankhorst, M. H. R.; Bouwmeester, H. J. M. Solid State Ionics 1997, 99, 15. (21) Lane, J. A.; Benson, S. J.; Waller, D.; Kilner, J. A. Solid State Ionics 1999, 121, 201. (22) Zeng, Y.; Lin, Y. S. Solid State Ionics 1998, 110, 209. (23) Katsuki, M.; Wang, S.; Dokiya, M.; Hashimoto, T. Solid State Ionics 2003, 156, 453. (24) Ten Elshof, J. E.; Lankhorst, M. H. R.; Bouwmeester, H. J. M. J. Electrochem. Soc. 1997, 144, 1060. (25) Kjaer, J.; Anderson, I. G. K.; Skou, E. Solid State Ionics 1998, 387, 113–115. (26) Mikkelsen, L.; Skou, E. J. Therm. Anal. Calorim. 2001, 64, 873. (27) Diethelm, S.; Closset, A.; Nisancioglu, K.; Van herle, J.; McEvoy, A. J.; Gur, T. M. J. Electrochem. Soc. 1999, 146, 2606. (28) Lane, J. A.; Kilner, J. A. Solid State Ionics 2000, 997, 136–137. (29) Badwal, S. P. S.; Jiang, S. P.; Love, J. G.; Nowotny, J.; Rekas, M.; Vance, E. R. Ceram. Int. 2001, 27, 431. (30) Badwal, S. P. S.; Jiang, S. P.; Love, J. G.; Nowotny, J.; Rekas, M.; Vance, E. R. Ceram. Int. 2001, 27, 419. (31) Ma, B.; Balachandran, U.; Park, J. H.; Segre, C. U. Solid State Ionics 1996, 83, 65. (32) Crank, J. The Mathematics of Diffusion; Clarendon Press: Oxford, UK, 1975. (33) Adler, S. B. J. Am. Ceramic Soc. 2001, 84, 2117. (34) Chen, X.; Yu, J.; Adler, S. B. Chem. Mater. 2005, 17, 4537. (35) Peil, K. P.; Goodwin, J. G.; Marcelin, G. J. Catal. 1991, 131, 143. (36) Driscoll, S. A.; Ozkan, U. S. J. Phys. Chem. 1993, 97, 11524. (37) Smith, M. R.; Ozkan, U. S. J. Catal. 1993, 142, 226. (38) Martin, D.; Duprez, D. J. Phys. Chem. 1996, 100, 9429. (39) Che, M.; Tench, A. J. AdV. Catal. 1982, 31, 77. (40) Che, M.; Tench, A. J. AdV. Catal. 1983, 32, 1. (41) Steele, B. C. H.; Bae, J. M. Solid State Ionics 1998, 106, 255. (42) Wilson, J. R.; Schwartz, D. T.; Adler, S. B. Electochim. Acta 2006, 51, 2006. (43) Preis, W.; Bucher, E.; Sitte, W. J. Power Sources 2002, 106, 116.

12476 J. Phys. Chem. C, Vol. 112, No. 32, 2008 (44) Endo, A.; Fununaga, H.; Wen, C.; Yamada, K. Solid State Ionics 2000, 135. (45) Van Doorn, R. H. E.; Fullarton, I. C.; De Souza, R. A.; Kilner, J. A.; Bouwmeester, H. J. M.; Burggraaf, A. J. Solid State Ionics 1997, 96, 1. (46) Adler, S. B.; Lane, J. A.; Steele, B. C. H. J. Electrochem. Soc. 1996, 143, 3554. (47) Lankhorst, M. H. R.; Bouwmeester, H. J. M. J. Electrochem. Soc. 1997, 144, 1261. (48) Gue´lou, E.; Barrault, J.; Fournier, J.; Tatiboue¨t, J.-M. Appl. Catal. B 2003, 44, 1. (49) Bingham, P. A.; Parker, J. M.; Searle, T.; Williams, J. M.; Smith, I. C. R. Chimie 2002, 5, 787.

Kuhn et al. (50) Zhao, Q.; Bao, X.; Wang, Y.; Lin, L.; Li, G.; Guo, X.; Wang, X. J. Mol. Catal. A: Chemical 2000, 157, 265. (51) Mai, A.; Tietz, F.; Stover, D. Solid State Ionics 2004, 173, 35. (52) Berry, F. J.; Marco, J. F.; Ren, X. J. Solid State Chem. 2005, 178, 961. (53) Fourne`s, L. A.; Wattiaux; Demourgues, A.; Grenier, J. C.; Pouchard, M.; Etourneau, J. J. Phys. IV, Colloque C1 1997, 7, C1. (54) Berry, F. J.; Gancedo, J. R.; Marco, J. F.; Ren, X. J. Solid State Chem. 2004, 177, 2101. (55) Kawasaki, S.; Takano, M.; Takeda, Y. Solid State Ionics 1998, 108, 221.

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