Fe3+ Redox Phenomena into Nanocrystalline Ce1

Sep 7, 2014 - The second component attributed to Fe2+ is the most intense one around 500 °C. Beyond this temperature, the coexistence of the three ...
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Dual Ce4+/Fe3+ Redox Phenomena into Nanocrystalline Ce1−xFexO2−x/2 Solid Solution Iona Moog,† Carmelo Prestipino,‡ Santiago Figueroa,§ Jérôme Majimel,† and Alain Demourgues*,† †

CNRS, University of Bordeaux, ICMCB, UPR 9048, 33600 Pessac, France Sciences chimiques de Rennes, UMR-CNRS 6226, Campus de Beaulieu, Bat 10B. 35042 Rennes, France § European Synchrotron Radiation Facility, 6, Rue Jules HOROWITZ, BP 220, 38042 Grenoble, France ‡

ABSTRACT: The Ce1−xFexO2−x/2 solid solution was synthesized using a microwave-assisted hydrothermal route. The solubility limit corresponds to a Fe (x) content equal to 0.15. Our previous works showed also that isolated Fe3+ distorted octahedral sites and Fe3+ clusters are randomly distributed into the ceria network. Thermogravimetric analysis (TGA) under Ar/5%H2 revealed a higher reduction rate for Fe-substituted ceria with changes of slope and the appearance of pseudoplateaus around 400 and 550 °C. The first Fe K-edge X-ray absorption near-edge spectroscopy (XANES) spectra recorded at several temperatures up to 700 °C shift gradually to lower energies as the temperature increases, with the formation of metallic iron starting at 550 °C, as confirmed by X-ray diffraction analysis. Moreover, on the basis of the principal component factor analysis, three K-edge structures associated with three oxidation states of iron (Fe3+, Fe2+, and Fe0) have been distinguished during this temperature-programmed reduction. The second component attributed to Fe2+ is the most intense one around 500 °C. Beyond this temperature, the coexistence of the three oxidation states Fe3+/Fe2+/Fe0 has to be mentioned up to 650 °C, the temperature at which metallic iron is mainly stabilized. Ce LIII-edge and Fe K-edge XANES spectra recorded in quick extended Xray absorption fine structure mode and at various temperatures show the progressive reduction of Ce4+ and Fe3+ starting beyond 250 °C, which is in good agreement with the change of slope observed on the TGA curves. The Ce LIII-edge XANES spectrum of pure CeO2 recorded at 700 °C under reducing atmosphere exhibits much less Ce3+ stabilized into the fluorite network than Fesubstituted ceria. Fe3+ ions randomly distributed into ceria strongly contribute to enhancing the Ce4+ reducibility properties. The Ce4+ reduction seems to appear at lower temperatures and is slower than the Fe3+ reduction, whose speed decreases around 400 °C. At this temperature, the Fe2+ and Fe3+ components dominate and the Ce3+ content remains high. However, a plateau with the stabilization of Ce4+/Ce3+ mixed valences around 500 °C appears where the Fe2+ contribution is also a maximum and the Fe reduction is slower. Because of the large content of either Ce3+ and Fe2+, a charge-transfer equilibrium Ce4+ + Fe2+ ⇔ Ce3+ + Fe3+ may occur at this temperature at which a change of slope on TGA curve also is observed with the appearance of a plateau at 550 °C associated with the Fe0 demixtion phenomenon. Then, after reaching the maximum Fe2+ rate, the Ce4+ reduction rate increases again up to 700 °C, the temperature at which all Fe atoms have been transformed into metallic iron.

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INTRODUCTION Cerium dioxide has been widely investigated because of the occurrence of Ce4+/Ce3+ mixed valences associated with the stabilization of oxygen vacancies depending on the oxygen partial pressure. This material, considered as an oxygen buffer, can be used as a three way catalyst (TWC) for diesel soot abatement, wet oxidation,1,2 and as solid oxide fuel cell (SOFC).3,4 To improve the oxidation rate of a diesel particulate matter at low temperatures, a combination of a particulate filter and a metal fuel additive is used in most of today’s vehicles. During the last 20 years, Ce and Fe are the main metal additives which have been tested for their activity at low temperature for soot oxidation.5 Reduction of Ce4+ under Ar/5% H2 of pure ceria can occur with a two-step process which begins between 300 °C and 500 °C and concerns the surface.6,7 The second reduction takes place at 800 °C and represents the onset of bulk reduction.6,7 © 2014 American Chemical Society

At this temperature, the surface area decreases drastically and a sintering phenomenon occurs.6,8 New synthesis routes and substitution of various cations into ceria have to be developed in order to get high surface area with many active sites and to avoid sintering at high temperatures which leads irreversibly to the deactivation of these catalysts. Zr4+ substitution into ceria is an interesting example in which high surface areas can be maintained at high temperatures.9,10 Moreover, ordered superstructures of the fluorite-type network can be stabilized with Zr4+, and the local environment of Ce4+ is also highly distorted, leading to an increase in the ionic character of Ce−O bonds and to a stabilization of a high rate of Ce3+ during the reduction process.11 The reducibility properties Received: May 28, 2014 Revised: August 25, 2014 Published: September 7, 2014 22746

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of ceria are enhanced by Zr4+ substitution, and their high surface areas are stabilized at high temperatures. In order to get high surface areas, a competition between the surface and Madelung energies occurs during the synthesis process. Then, metastable phases can be prepared and the local organization of substituted cations and cerium can also change. An enhancement of the reducibility properties can also be noted when tetravalent transition metals smaller than Zr4+, such as Ti4+, are substituted for Ce4+.12 Extended X-ray absorption fine structure (EXAFS) investigations have been carried out on the Ce0.6Ti0.4O2 solid solution, and the local environment of Ti is distorted as Zr4+/Ce4+ into tetragonal Zr0.8Ce0.2O2 phase with 4 + 4 coordination number and Ti−O bond distances at 1.9 and 2.5 Å, respectively.13 The onset bulk reduction temperature appears around 500 °C as the Zr4+ stabilized ceria.13 Numerous works have been devoted to the substitution of small trivalent transition metals such as Fe3+, and the solubility limit as well as the variation of the cell parameter will strongly depend on the synthesis route.14−28 Moreover, the Fe3+ incorporation into ceria contributes to enhancing the reducibility properties and the oxygen storage capacity. In a recent study, we demonstrated that the microwaveassisted hydrothermal route compared to the classical coprecipitation synthesis lead to various local organization of Fe3+ into ceria, different morphologies, and surface areas which influence the reducibility properties. The high concentration of isolated distorted octahedral sites of Fe3+, identified by electron paramagnetic resonance (EPR), Mossbauer, and Fe K-edge Xray absorption near-edge (XANES) spectroscopies, in Ce1−xFexO2−x/2 solid solution with high surface area around 100 m2/g and nanocubic shapes prepared by a microwaveassisted hydrothermal route allows the large rate of reduced Ce4+. At 550 °C, for a low Fe content (x = 0.05), the reduced cation rate is doubled (60%) compared to that of a pure ceria (30%) prepared under the same conditions. The released oxygen amount (ΔO) becomes equal to 0.30 in Ce0.95Fe0.05O1.97 at 550 °C just before the metallic Fe0 formation and the demixtion phenomenon associated with the sintering of ceria, as shown in a previous work. An increase of Fe content incorporated into ceria does not change the cation reduction rate and the released oxygen quantity in this solid solution. Because Fe is cheaper and more abundant than Ce, Ce0.85Fe0.15O1.93 is an interesting candidate for redox properties at low temperatures. Moreover, several reduction steps observed in our work and in the literature appear around 400 and 600 °C. Whereas the Fe3+ reduction in α-Fe2O3 hematite occurs around 400 °C,5 the reduction process into the as-prepared Ce0.85Fe0.15O1.93 solid solution is completely unknown. Several topics can be investigated, such as the occurrence of Ce4+/Fe3+ reductions and charge-transfer equilibrium Ce4+ +Fe2+ ⇔ Ce3+ + Fe3+ and the temperature range of such phenomena. In situ Ce LIII- and Fe K-edges XANES under reducing atmosphere from room temperature up to 700 °C (corresponding to the metallic Fe0 formation, the demixtion process, and the sintering of ceria) is a powerful tool for investigating simultaneously the variation of Ce and Fe oxidation states versus temperature. The aim of this paper is to follow the various steps of Ce and Fe reduction as a function of the temperature by in situ Ce LIII- and Fe K-edges XANES experiments, to establish some redox equilibria, and to conclude about the key role of Fe3+ stabilized into ceria which will allow the design of new materials with enhanced redox properties at low temperatures.

Article

SYNTHESIS

Preparation of the Ce1−xFexO2−x/2 Mixed Oxides Using a Microwave-Assisted Hydrothermal Route. The solid solution was prepared for x = 0, 0.05, 0.1, 0.15, and 0.17 (for this Fe (x) content, the presence of α-Fe2O3 has been detected on XRD patterns). The same precursors were used and were precipitated in a highly basic medium composed of sodium hydroxide and ammonia with a nbase/ncations molar ratio equal to 175. The molar ratio [NaOH]/[NH4OH] was equal to 1.8. The solution containing precipitated oxides and oxyhydroxides was then incorporated into a closed Teflon vessel, heated to 200 °C for 40 min using the microwave system (CEM, MARS) operating at 2450 MHz, and then cooled at room temperature. A self-generating pressure of 20 bar was induced by the increase of temperature inside the Teflon container. The precipitate was finally centrifuged at 5000 rpm, washed three times with 100 mL of distilled water, and dried overnight. In a first step, the processes of hydrolysis and nucleation in a basic medium of Ce3+ then Ce4+ ions occur in the microwave synthesis, involving the formation of complexes with water and hydroxyl molecules, such as [Ce(OH)x(H2O)y](4−x)+, where x +y is the coordination number of Ce4+. In a second step, condensation processes take place with the formation of CeO2, nH2O. This stage differs greatly depending on the chosen synthesis. The microwave-assisted route leads to fast and homogeneous nucleation because of the important interactions between the molecules where ionic and dipolar polarizations contribute to heating. Then, heating at the core of the polar molecules leads to the formation of nanocrystalline Ce1−xFexO2−x/2 with defined shapes. Cation ratios were checked with the inductively coupled plasma atomic emission spectroscopy (ICP-AES) technique and found to be in the right chemical composition. The samples were first dissolved using hydrochloric acid and heated at 200 °C for 15 min using a microwave digestion system (CEM, MARS).

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EXPERIMENTAL SECTION Powder X-ray diffraction patterns were recorded on a PANalytical X’Pert MPD apparatus equipped with a monochromated Cu Kα radiation (λ = 1.54059 Å) for accurate studies and an X’Celerator detector. Data were collected over a range of 10°−110° with 0.017° steps. The lattice parameters and the crystallite sizes were determined by profile matching (Le Bail fit) using the Thomson−Cox−Hastings function with the FullProf program package.29,30 X-ray absorption experiments (XANES) at the Fe K-edge (7112 eV) and at the Ce LIII-edge (5723 eV) were carried out for CeO2 and Ce0.85Fe0.15O1.93 compounds obtained by the microwave-assisted route. Data were collected using the QEXAFS acquisition mode on station BM2331 at ESRFGrenoble-France with beam currents between 150 and 250 mA in transmission mode mode using a Si(111) monochromator crystal. The samples in the form of pellets were placed in a reaction fluxed with Ar/5% H2. A first in situ XANES experiment was performed at Fe K-edge every 100 °C from room temperature to 700 °C in order to identify the temperature range for the stabilization of each oxidation state (Fe3+, Fe2+, and Fe0). Then, in situ time-resolved dispersive Ce LIII- and Fe K-edges XANES spectra were recorded from room temperature to 700 °C with a heating rate of 7 °C/min to follow simultaneously the reduction steps of Ce4+ and Fe3+ 22747

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cations stabilized into the fluorite network. To perform XANES normalization, principal component factor analysis (PCA) and iterative transformation factor analysis (ITFA) has been used with the PrestoPronto code suite. Thermogravimetric analysis (TGA) was undertaken in a controlled reducing atmosphere (Ar/5% H2) on a Seteram thermal analyzer with the same experimental conditions used for the XANES experiments. Thermograms were recorded at 5 °C/min heating rate from room temperature to 700 °C. The use of CO/CO2 mixture would be probably more appropriate showing that the reduction process occurs at lower temperatures. However, the Ar/5% H2 gas mixture allowed the evaluation in a first approach of the reducibility performance of these complex oxides.

the crystallite size (from 20 to 10 nm) refined on the basis of XRD patterns. Moreover, the cell parameter varies from 5.413(1) Å for CeO2 to 5.374(7) Å for Ce0.85Fe0.15O1.93 compound showing the incorporation of smaller Fe3+ (Fe3+ ionic radius in Oh site, 0.65 Å) into ceria (Ce4+ ionic radius in cubic site, 0.97 Å). Furthermore, our previous works, based on EPR, Mossbauer, and XANES-EXAFS investigations, have also demonstrated that Fe3+ ions are stabilized in distorted octahedral sites which can be isolated or form clusters with a high disorder into the fluorite network. The average Fe−O bond distance determined by EXAFS spectroscopy is close to 1.98 Å and is shorter than the Ce−O bond length at 2.34 Å into CeO2, confirming the reduction of the cell parameter. Moreover, annealing at T > 600 °C under Ar/5%H2 of Fesubstituted ceria lead to a demixtion and sintering phenomena. In our previous works, the transformation of the Ce1−xFexO2−x/2 solid solution into metallic Fe0 and pure ceria with a cell parameter equal to 5.41 Å has been shown by differential thermal analysis, XRD, and transmission electron microscopy investigations. Thermogravimetric analyses were carried out under reducing atmosphere (Ar/5%H2,) up to 700 °C (Figure 2). The highest

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RESULTS AND DISCUSSION The refined (Le Bail fit) and experimental X-ray diffractograms of CeO2 and Ce0.85Fe0.15O1.93 compounds are represented in Figure 1. These diffraction patterns can be indexed with a cubic

Figure 2. Thermogravimetric analyses under reducing atmosphere (Ar/5% H2) of CeO2 (black) and Ce0.85Fe0.15O1.93 oxides (blue) and their derivatives.

H2O and CO2 quantities (several percents) adsorbed on the surface of Ce-based compounds appear before 300 °C. Beyond this temperature, when the slope of the curves change drastically for the Fe-substituted ceria, one can consider in a first approximation that mainly the reductions of Fe3+ and Ce4+ occur with the oxygen loss from the fluorite-type network. Two strong changes of the slope of the TGA curve of the Ce0.85Fe0.15O1.93 compound appear close to 350 and 550 °C before the formation of metallic iron. On the plateau around 550 °C, the released oxygen rate (3.2% weight loss) is around ΔO = 0.30, corresponding to a reduction rate of cations equal to 60%. This value decreases to 30% (reduced Ce4+) with ΔO = 0.15 of released oxygens for pure ceria prepared with the same synthesis route. Indeed, the incorporation of trivalent transition metals into the fluorite structure will create oxygen vacancies as well as distorted Ce4+ local environments with a weakening of the Ce−O bonds, hence improving the reducibility properties.

Figure 1. Experimental (red dots) and calculated (black solid line, Le Bail fit) XRD profiles for (A) CeO2 and (B) Ce0.85Fe0.15O1.93 compounds synthesized using the microwave-assisted hydrothermal route.

unit-cell related to the fluorite-type structure (SG: Fm3̅m). Diffraction lines related to α-Fe2O3 hematite were not observed, confirming the presence of the solid solution up to x = 0.15. For x = 0.15, peaks are significantly larger than those in the case of pure CeO2 and exhibit a higher surface area which increases from 40(4) m 2 /g (CeO 2 ) to 95(10) m 2 /g (Ce0.85Fe0.15O1.93), in good agreement with the decrease of 22748

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The Ce4+ reduction occurs at the surface of ceria between 300 and 500 °C.6,7 At temperature superior to 300 °C, one should notice the greater weight loss observed in the case of iron-doped compounds. The derivative curves corresponding to pure CeO2 and Fe-substituted ceria show a different change of slope. Whereas the weight loss of pure CeO2 seems to decrease regularly with the temperature without any clear change of variation, several inflection points can be distinguished on the Ce0.85Fe0.15O1.93 derivative. Three temperatures (minima of the derivative curves) close to 370, 500, and 590 °C can be identified on the TGA curves and their derivatives. This first temperature (370 °C) is near that of the Fe3+ reduction in αFe2O3-hematite,5 which transforms into Fe3O4-maghemite (Fe2+/Fe3+ mixed valencies), whereas the last temperature (590 °C) is close to that found for the transformation of iron oxides into metallic iron. This two-step reduction process of αFe2O3-hematite has been described in the literature.32 After the formation of metallic Fe at T > 590 °C, the redox reaction is irreversible and the reoxidation leads to the stabilization of αFe2O3 hematite and ceria. Isolated Fe3+ octahedral sites and clusters are stabilized into ceria, but the local environments are clearly different from that of α-Fe2O3-hematite on the basis of our previous investigations by EPR, Mossbauer, and XANES spectroscopies. However, the average Fe−O bond distances around 1.98 Å, determined by the EXAFS study, are close to that found in α-Fe2O3-hematite. In a first approximation, the isolated octahedra containing Fe3+ can be more easily reduced than Fe3+ clusters because of the higher oxygen mobility around this locally distorted environment. Thus, the stabilization of Fe2+ in larger polyhedral sites replacing isolated Fe3+ octahedra becomes easier. Then, one can assume that various reduction phenomena between 300 and 500 °C will involve different Fe3+ local environments stabilized into ceria. To follow the variation of the Fe oxidation state versus temperature, XAFS experiments have been carried out under reducing atmosphere every 100 °C (Figure 3). A program was

data can be reduced to an interpretable set of basic functions. The data matrix Dr,c, constituted of c spectra of r points can be decomposed as follows: Dr,c = Rr,n·Cn,c + Er,c; Dr,c = Rr,n·Cn,c + Rr,j−n·Cj−n,c where n is the number of components in the series, j the number of spectra, R a row matrix which contains abstract spectral components, C the column matrix which represents the concentration of the components along the set, and E a matrix containing the errors. In order to obtain the decomposition, the diagonalization of the covariance matrix (Z = Dt·D) is performed. Eigenvector (Q) and eigenvalue (Λ) matrices can be determined. The following relationships can be established: Qt·Z·Q= Λ; C = Qt ; R = D·Q. The discrimination between real component and components corresponding to the noise is delicate, and several parameters have been proposed. The principal components are now made of an F test of the variance associated with eigenvalue and the summed variance associated with noise eigenvalues.34 The probability that an F value would be higher than the current value is given by percentage of significance level (% SL). Thus, the kth factor is accepted as a principal component if percentage of significance level is lower than some test level, equal here to 5%. In addition, an empirical function, IND, defined by Malinowski,34 will be used to help in deciding the number of principal components. The PCA results are given in Table 1. The Table 1. PCA Results for Ce0.85Fe0.15O2−δ Oxide during in Situ Thermal Treatment under He:H2 (95:5, 100 mL/min) T (°C)

eigenvalue

RT 300 400 500 550 600 700 700 RT (back)

× × × × × × × × ×

8.88 6.78 8.76 1.94 1.35 8.76 6.86 6.09 4.43

2

10 100 10−1 10−2 10−2 10−3 10−3 10−3 10−3

IND 1.33 6.44 2.38 3.08 4.37 7.31 1.57 5.75 0.00

× × × × × × × × ×

signif. level −3

10 10−4 10−4 10−4 10−4 10−4 10−3 10−3 100

2.59 1.60 5.82 3.23 3.70 4.51 5.06 5.61 0.00

× × × × × × × × ×

−6

10 10−1 10−2 101 101 101 101 101 100

MAD 0 0 1 1 1 1 1 1

percentage variance associated with each eigenvalue also is reported in Table 1. Finally, a median absolute deviation (MAD) as statistical methods for identifying outliers in data, was chosen in this study. MAD is defined as MAD = median|xi −median(xi)|. An outlier (x0) is identified if |x0 − median(xi)|/ MAD > 5. There is an important jump in the significance level (%SL) between the third (400 °C) and the fourth (500 °C) eigenvalues. The IND function suggests also that a third component could be present. Then, three distinct components shown in Figure 4 can be identified (component 4 is shown to illustrate its negligibility); however, the three components are still without spectroscopic meaning and just describe the internal variance of the series. The results of the PCA are confirmed by a visual inspection of the variation of the set of Fe K-edge spectra in Figure 3. In fact, two isosbestic points can be detected, one at 7128 eV and another one at 7125 eV, confirming the existence of at least three spectral components. To generate a spectroscopic meaning component, it is necessary to perform a “rotation” of the real abstract component. One technique for determining the rotation matrix is the so-called iterative transformation factor analysis,33,34 consisting of the iterative refinement under some constraints of

Figure 3. Fe K-edge XANES spectra of Ce0.85Fe0.1501.93 oxide from room temperature to 700 °C under He/H2.

developed to analyze the XANES data using principal component factor analysis.33,34 The pre-edges were first subtracted with straight lines, and the K-edges were normalized by adjusting to one unit the averages of absorbance in the range from 30 to 150 eV from the threshold (inflection point). The so-called PCA assumes that a variable, the absorbance in a set of spectra, can be mathematically modeled as a linear sum of individual and uncorrelated components. Then a large body of 22749

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inflection point related to this intermediate component are near that observed for Fe2+ compounds. Then, one can assume in a first approximation that these spectra observed between 400 and 500 °C correspond mainly to Fe2+ valence state. Considering these three components, the variations of each contribution versus temperature have been represented in Figure 5b. The reduction of Fe3+ into Fe2+ is fast below 400 °C and becomes slower beyond this temperature up to 700 °C, where only metallic Fe is detected. The Fe2+ component is maximum between 400 and 550 °C, and for T > 600 °C metallic Fe is the major component. One should notice the coexistence of the three valence states (Fe3+, Fe2+, and Fe0) at 550 °C. Figure 4. (a) Representation of the three abstract components spectra deduced from PCA analysis.

the prealigned component obtained by VARIMAX rotation.35 In such a case, the constraints imposed on the loading are the presence of a single maximum and that concentration remains in the range between 0 and 1. The three spectra identified on the basis of the PCA and “rotation” are represented in Figure 5a. Whereas the extreme points are associated with Fe3+ at room temperature and metallic Fe at 700 °C, the maximum contribution of the intermediate one appears between 400 and 500 °C and should correspond to Fe2+. The comparison between various reference compounds (Figure 5) indeed shows that the edge and the

Figure 6. Representation of the Fe K-edge XANES spectra of the intermediate component and reference compounds.

To investigate in detail the redox behavior of this complex material, time-resolved QEXAFS Ce LIII-edge and Fe K-edge XANES experiments have been carried out. Ce-LIII XANES spectra in Figure 7 of Ce0.85Fe0.15O1.93 and CeO2 compounds at

Figure 7. Ce LIII-edge XANES spectra of Ce0.85Fe0.15O1.93 and CeO2 compounds at room temperature and after annealing at 700 °C under reducing atmosphere showing the variation of Ce valence states after thermal treatment.

room temperature and after annealing at 700 °C under reducing atmosphere illustrate the variation of Ce valence states with the two lines (5730 and 5740 eV with 4f1 and 4f0 electronic configurations of Ce3+ and Ce4+, respectively), the decrease of shake-down satellite (5740 eV), as well as the shift to lower energies when the Ce3+ content increases. It is then

Figure 5. (a) Representation of the three-component spectra corresponding to Fe3+/Fe2+/Fe0 deduced from PCA analysis. (b) Variation of the three component rates versus temperature. 22750

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clear that the Ce3+ rate at 700 °C of Ce0.85Fe0.15O1.93 oxide is largely superior to that of pure CeO2 without Fe3+ stabilized into the fluorite network. The various Ce LIII-edge XANES spectra versus temperature of Ce0.85Fe0.15O1.93 and CeO2 compounds are represented in panels a and b of Figure 8, respectively. In order to estimate the

Figure 9. Variation of the integral of a part of the Ce LIII XANES spectra characterizing the Ce3+ rate versus temperature. The integral curve corresponding to the Ce0.85Fe0.15O1.93 compound is in red, and that associated with pure ceria is in blue.

200 °C < T < 450 °C, a faster reduction process occurs in the case of Fe-substituted ceria, followed by a plateau between 450 and 550 °C. Then the Ce3+ amount increases again at T > 550 °C. One should notice that a second minima of the first derivative TGA curve is observed at 500 °C. The variation of the reduced Ce4+ rate into Ce0.85Fe0.15O1.93 compound shows clearly that the reducibility properties are largely improved with Fe substitution and various mechanisms occur at selected temperatures. To better understand the kinetics and thermodynamics of these reactions between room temperature (RT) and 700 °C, a time-resolved dispersive Fe K-edge XANES experiment has been performed and spectra are represented in Figure 10. The integral curve characterizing the Fe reduction

Figure 8. Time-resolved dispersive Ce LIII-edge XANES spectra of Ce0.85Fe0.15O1.93 (a) and CeO2 (b) compounds versus temperature from room temperature to 700 °C. The area corresponding to the integral of the curve relating to the increase of Ce3+ content in these compounds is indicated. Arrows represent the change of shake down satellite intensity at 5740 eV and threshold position at 5730 eV with the Ce3+ content.

increase of Ce3+ rate versus temperature, integrals of the curves between 5712 and 5730 eV (this last energy corresponds to the intersection of each spectrum which can be considered as an isosbestic point) have been calculated. The variations of these integral values versus temperature for Ce0.85Fe0.15O1.93 and CeO2 compounds are shown in Figure 9. Whereas the reduction of Ce4+ is low in pure ceria and varies monotonously with the temperature, the Ce3+ content is higher in Fesubstituted ceria, and two domains of reduction can be distinguished before and after 450 °C. In this latter compound, the reduction process starts at 200 °C, whereas the same integral value associated with a Ce3+ content is identified at 300 °C for pure CeO2, in good agreement with the literature.6 For

Figure 10. Time-resolved dispersive F K-edge XANES spectra of Ce0.85Fe0.15O1.93 compound versus temperature from room temperature to 700 °C. The area corresponding to the integral of the curve relating to the increase of Fe2+/Fe0 content in these compounds is indicated. Arrows represent the change of second peak intensity at 7130 eV and threshold position at 7120 eV with the Fe2+/Fe0 content.

with the stabilization of mainly Fe2+ between 400 and 500 °C, then metallic Fe at T > 550 °C, as previously shown, is represented in Figure 10. Both the reductions of Fe3+ and Ce4+ phenomena are superposed in Figure 11. In the first part of the curves before 450 °C, the Fe3+ reduction seems to start after the Ce4+ reduction but is faster. 22751

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and oxygen mobility are optimized and redox reactions with molecules will occur.

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CONCLUSIONS Ce0.85Fe0.15O1.93 and CeO2 compounds have been prepared by a microwave-assisted hydrothermal route. XRD analysis confirms the occurrence of a solid solution. Our previous works showed that Fe3+ ions are randomly distributed into the fluorite network in isolated (with orthorhombic and axial distortion) octahedral sites and clusters. As far as the reducibility properties are concerned, various changes of slope of the TGA curve of Ce0.85Fe0.15O1.93 oxide are observed, whereas the weight loss of pure CeO2 varies quasilinearly versus temperature with a lower reduced Ce rate. The impact of Fe on reducibility properties is clear, and time-resolved Ce LIII and Fe K-edges XANES analyses versus temperature help to identify separately the reduction processes of Ce and Fe. On the TGA curve, three temperatures have been distinguished corresponding to the change of slopes at 370, 500, and 590 °C. The Ce4+ reduction occurs before the Fe3+ reduction but is slower. At 370 °C, the Fe2+ rate tends to a maximum, which probably corresponds to the reduced Fe3+ located in isolated octahedral sites, whereas the Ce3+ content still increases. At 500 °C, a charge-transfer equilibrium Ce3+ + Fe3+ ↔ Ce4+ + Fe2+ occurs and a Ce4+/Ce3+ mixed oxidation state without evolution of Ce valence states is stabilized between 450 and 550 °C. At this temperature, the Fe3+ reduction slows down and Fe3+ clusters are mainly reduced because the Fe−O bond covalency increases (compared to the isolated Fe3+ octahedral site) and the oxygen mobility decreases. Finally, at 590 °C, both Ce and Fe reduced rates tend to a maximum with the appearance of a large content of metallic Fe, the demixtion process, and the sintering of ceria. A question is then raised: are the stabilization of metallic iron and the sintering of ceria mainly due to the reduction of Fe3+ clusters or to the establishment of a charge-transfer equilibrium with a strong electronic exchange as well as a high oxygen mobility which contribute to transform this metastable phase. Because of the stabilization between 450 and 550 °C of two mixed valence states of Ce4+/Ce3+ and Fe3+/Fe2+ with high reduced cation rates, this activated temperature range is a key feature of this material which can be used for various redox reactions involving various pollutants and organic molecules. It is then clear that low-temperature reduction processes of Ce4+ and Fe3+ in this nanomaterial are strongly coupled and the Fe3+ reduction contributes toward the Ce4+ reduction and the stabilization of charge-transfer equilibrium.

Figure 11. Variation of the integral of a part of the Ce LIII and Fe−K edges XANES spectra characterizing the Ce3+ and the Fe2+/Fe0 rates versus temperature for Ce0.85Fe0.15O1.93 oxide. The integral curve corresponding to Ce3+ content is in red, and that associated with Fe3+ reduced content is in blue. The three temperatures of 370, 500, and 590 °C corresponding to the minima of the first derivative of the weight loss versus temperature (change of slope) are indicated.

After 450 °C, the Ce3+ and Fe2+ contents become high and the reduction processes slow down with the formation of a plateau in the case of Ce reduction. A high rate of oxygen vacancies around ΔO = 0.15 on the basis of TGA measurements is stabilized into fluorite network at 450 °C. Then, one can consider the establishment of a charge-transfer equilibrium: Ce3+ + Fe3+ ↔ Ce4+ + Fe2+ which contributes to slow the reduction process. At T > 550 °C after the plateau and the charge-transfer equilibrium, a large metallic Fe rate appears with the demixtion phenomenon. However, if the slope of the Fe reduction changes at 400 °C before the Ce plateau, the Fe2+ concentration slightly increases and passes through a maximum around 500 °C (Figure 5b) in the middle of the plateau when a very small amount of metallic Fe seems to appear (Figure 5b). The Fe3+ reduction into Fe2+ occurs simultaneously with the Ce4+ reduction but is faster. For 450 °C < T < 550 °C where the charge-transfer equilibrium between Ce and Fe cations takes place, no reduction of Ce can be noted despite the Fe3+/ Fe2+ cations continuing to be reduced into probably a small amount of metallic Fe. Then, taking into account the Le Chatelier law, the Fe reduction and the increase of Fe2+ content at T > 500 °C contribute to displace the charge-transfer equilibrium to the left, leading to the formation of Ce3+. In our recent paper, isolated Fe3+ octahedral sites and Fe3+ clusters have been identified into the ceria network. In a first stage, one can consider in a first approximation that the reduction of Fe3+ isolated octahedral sites occurs rapidly followed in a second step by the Fe3+ cluster reduction, which could be slower because of the enhancement of Fe−O covalency and the lower oxygen mobility, leading finally in a third step to the metallic Fe formation. After the reduction of all Fe3+ isolated octahedral sites into Fe2+, the charge equilibrium between Ce and Fe could take place. This equilibrium between rare earth and transition metal involving 4f levels and 3d bands is going to enhance the electronic conductivity, which may assist the oxygen mobility in this network. Therefore, 500 °C is a key temperature of this complex system, corresponding to the second inflection point on the TGA curve (minima of the first derivative) and to the plateau with the absence of Ce4+ reduction, because of a chargetransfer equilibrium. At this temperature, electronic exchange

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

REFERENCES

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