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Fe2O3-MgAl2O4 for CO production from CO2: Mössbauer spectroscopy and in situ X-ray diffraction Lukas Buelens, Antoon Van Alboom, Hilde Poelman, Christophe Detavernier, Guy B Marin, and Vladimir V. Galvita ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01036 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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ACS Sustainable Chemistry & Engineering

Fe2O3-MgAl2O4 for CO production from CO2: Mössbauer spectroscopy and in situ X-ray diffraction Lukas C. Buelensa, Antoon Van Alboomb, Hilde Poelmana, Christophe Detavernierc, Guy B. Marina, Vladimir V. Galvitaa* a

Laboratory for Chemical Technology, Ghent University, Technologiepark 125, B-9052, Ghent, Belgium

b

Department of Applied Physics, Ghent University, Valentin Vaerwyckweg 1, B-9000, Ghent, Belgium

c Department of Solid State Sciences, Ghent University, Krijgslaan 281, S1, B-9000 Ghent, Belgium *Corresponding author: e-mail: [email protected]; tel: +32-468-10-6004; fax: +32-93311759

Abstract Fe2O3/MgFeAlOx materials are promising oxygen storage candidates for chemical looping CO2 conversion. In this work, the cyclic stability of a 50Fe2O3/MgFeAlOx (containing 50 wt% Fe2O3 and 50 wt% MgAl2O4) oxygen storage material is investigated. The evolution of its bulk properties over the course of 1000 H2/CO2 redox cycles has been studied by means of 57Fe Mössbauer spectroscopy and in situ X-ray diffraction. As expected, all iron in the as prepared oxygen storage material was present as Fe3+, 64% of which in iron-rich phases α-Fe2O3 and α-FeOOH, and 36% in form of a MgFeAlOx spinel. In contrast, after 1000 redox cycles only 19% of iron was present in an iron-rich spinel such as Fe3O4, γ-Fe2O3 and MgFe2O4. The remaining 81% was present in form of Mg-Fe-Al-O, including MgxFe1-xO. ILEEMS measurements showed surface enrichment of Fe3+ in 50Fe2O3/MgFeAlOx after 1000 redox cycles, with 36% of all surface Fe present as Fe3+ in iron-rich spinel phases such as γ-Fe2O3 and/or MgFe2O4.

Key words: CO2 conversion, Oxygen Storage Material, Mg-Fe-Al-O spinel, Chemical looping

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Introduction The chemical industry depends on energy for all processes, from petrochemicals and pharmaceuticals to common everyday items. Climate change makes it mandatory that we discover ways of producing energy, chemicals and fuel in a sustainable manner, while replacing the fossil fuels that currently form the basis for our energy and chemical industry. Redox active metal oxides are applied in several fields of chemistry, materials science and chemical engineering, e.g. as promoter in three-way exhaust or methane reforming catalysts 1-5,

as catalyst support 3, 6, as oxygen storage material in (thermo)chemical looping 7-12 and as

solid electrolyte, cathode or anode for solid oxide fuel cells 13-19. When exposed to the high temperatures associated with these applications, often exceeding 900 K, material degradation phenomena such as crystallite growth, poisoning, carbon deposition and formation of less reactive phases may deteriorate the performance of such redox active metal oxides. Chemical looping CO2 conversion is increasingly studied as a strategy for CO2 recycling, and comes in different forms depending on the chosen process 20-26. A wide variety of chemical looping processes has been proposed, all of which make use of the oxygen exchanging potential, also termed oxygen storage capacity, of (transition) metal oxides in order to carry out redox reactions, e.g. between a reductant and oxidant gas (equation 1). At the heart of these processes are two crucial stages involving the oxygen storage material (OSM): (i) reduction of the OSM (lattice oxygen removal, equation 2) with concomitant oxidation of a reductant gas (e.g. H2 to H2O) and (ii) reoxidation of the OSM (lattice oxygen replenishment, equation 3) with reduction of an oxidant gas (e.g. CO2 to CO). oxidant + reductant reduced product + oxidized product

(1)

reductant + MO (s) oxidized product + M (s)

(2)

oxidant + M (s) reduced product + MO (s)

(3)

The OSM is chosen based on the envisaged overall redox reaction (equation 1), and its application in chemical looping processes allows to inherently separate the oxidized product gas and the reduced product gas by separating reactions (2) and (3), either in time or space. Recently, was showed the economic potential of waste biomass anaerobic fermentation, followed by methane enrichment of the produced gas, subsequent injection into the available 2 ACS Paragon Plus Environment

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natural gas grid to reform it with CO2 from point emissions 27. The here proposed approach based on the chemical looping CO2 and CH4 reforming process would allow to convert around 50% of current stationary point CO2 emissions into CO as a key platform chemical28. For the conversion of CO2 into CO, iron oxide based OSMs are highly suitable owing to the relative ease of iron oxide reduction and the high capacity for CO2 conversion of reduced iron species 22, 29-30.

In order to ensure a high specific surface area and increase the resistance to crystallite

growth, an inert or redox-active textural promoter material is generally added. The high operating temperature, however, generally causes incorporation of iron in ‘inert’ textural promoter materials (e.g. Al2O3 or SiO2) leading to reduced activity (by formation of e.g. FeAl2O4 or Fe2SiO4). The predominant redox-active phase in iron oxide based OSMs for chemical looping CO2 conversion is generally a pure iron oxide, such as Fe2O3 or Fe3O4, a mixed ferrite, such as CoFe2O4, NiFe2O4, spinel-like MgFeAlOx 23, 30-33, or iron-containing perovskites 34-35. The application of Fe2O3/MgFeAlOx OSMs in chemical looping CO2 conversion is of particular interest since this material contains two types of active phase 23, 36. An interesting property of MgFeAlOx is that reduction and oxidation can occur between Fe3+ and Fe2+ without major reorganization of the crystal structure 6. The latter allows to limit crystallite growth of MgFeAlOx, which hence acts as physical barrier in order to mitigate sintering of the pure iron oxide phase. The occurrence of material degradation through crystallite growth and the formation of a MgxFe1-xO phase over the course of 1000 redox cycles was evidenced through characterization techniques such as N2 adsorption, X-ray diffraction and electron microscopy37. In order to detail this evolution, in situ X-ray diffraction and spectroscopy were applied as complementary characterization

38.

57Fe

Mössbauer

Although traditional

applications of 57Fe Mössbauer spectroscopy are mostly in the field of soils characterization and mineralogy 38-40, it can provide highly valuable information with respect to the oxidation state and chemical environment of bulk Fe in functional materials such as catalysts

41

and

OSMs for chemical looping 23. This work presents detailed characterization of a 50Fe2O3/MgFeAlOx oxygen storage material, previously shown to maintain a high oxygen storage capacity per unit of mass over extended redox cycling

37.

In situ X-ray diffraction and

57Fe

Mössbauer spectroscopy,

transmission as well as surface sensitive, are applied to investigate the OSM in its as prepared state and after 100, 200, 500 and 1000 redox cycles of reduction by H2 and reoxidation by CO2. 3 ACS Paragon Plus Environment


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The formation of new phases is revealed and the evolution of the iron distribution among the iron-containing phases is estimated.

Experimental Material preparation A one-pot co-precipitation method using Fe(NO3)3, Mg(NO3)2 and Al(NO3)3 as metal precursors and NH4OH as precipitation agent was used to synthesize an OSM with a theoretical composition of 50w% Fe2O3 and 50w% MgAl2O4. A detailed description of the synthesis procedure can be found elsewhere23. In what follows, the 50Fe2O3/MgFeAlOx oxygen storage material will be referred to as 50FMA.

Material characterization 57Fe 57Fe

Mössbauer spectroscopy

Transmission Mössbauer spectra (TMS) at room temperature (RT) and 80 K were

measured for as prepared 50FMA as well as for 50FMA after 100, 500 and 1000 redox cycles. For the sample recovered after 1000 redox cycles, an Integral Low-Energy Electron Mössbauer spectrum (ILEEMS) was also measured at RT. ILEEM spectroscopy is an electron emission mode of 57Fe Mössbauer spectroscopy and, with a probing thickness of ca. 5 nm, is particularly appropriate for the study of surfaces 42. The ILEEM spectrum was recorded with the sample and the electron detector (channeltron) both mounted in a vacuum chamber. The channeltron was subjected to a bias voltage of +146 V. All spectra were conventionally recorded in 1024 channels with the spectrometers operating in the constant acceleration mode using triangular reference signals and having excellent linearity. While a 57Co(Rh) source was used, center shift values  reported hereafter are obtained using α-Fe at RT as a reference. The velocity calibration for TMS was based on the RT spectrum of a standard hematite (α-Fe2O3). For the ILEEM spectrum an enriched 57Fe foil was used. In TMS mode, data were collected until a background of at least 106 counts per channel was reached. In ILEEMS mode, the measurement ran until a background of at least 4.0104 counts per channel was obtained. For the TMS, a velocity (𝜈) range −11 to +11 mm s-1 was covered with a velocity increment of approximately 0.044 mm s-1 per channel. For the 4 ACS Paragon Plus Environment

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ILEEMS, the velocity varied between ca. −10 and +10 mm s-1 with an increment of ca. 0.040 mm s-1 per channel. All spectra were reproduced using components consisting of Lorentzian-shaped absorption lines. Doublet and Fe3+ sextet spectra are the most common types of 57Fe Mössbauer spectra (MS) and are typical for not magnetically ordered (ferrous or ferric) and magnetically ordered (ferric) materials, respectively. Spectral lines in MS are due to transitions between certain 57Fe nuclear energy levels and their positions are defined by so-called Mössbauer parameters. These parameters characterize the splitting and/or shift of the relevant nuclear energy levels due to the, whether or not magnetically ordered, crystallographic environment of the

57Fe

nuclei in the sample. Line positions in doublet spectra are defined by the center of the two lines relative to the center of the calibration spectrum, i.e. the center shift (δ), and their mutual distance, i.e. the quadrupole splitting (ΔEQ). Line positions in Fe3+ sextet spectra are defined by the center shift (δ), the quadrupole shift (2εQ) and the magnetic hyperfine field (Bhf). Quadrupole splitting and quadrupole shift are due to the quadrupole interaction by which certain nuclear energy levels split and shift, while the magnetic hyperfine field determines the splitting of the relevant nuclear energy levels due to the nuclear Zeeman effect caused by an internal magnetic field. For a more comprehensive description of Mössbauer spectra is referred to specialized literature 43.

Electron microscopy Bright-field (BF) and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) was used to study the nanoscale morphology and obtain chemical information. A Cs corrected JEOL JEM-2200FS equipped with a Schottky-type field-emission gun (FEG) and JEOL JED-2300D energy-dispersive X-ray (EDX) detector was operated at 200 kV. Specimen preparation consisted of immersing a lacey carbon film supported on a copper grid into the sample powder. Large particle agglomerates were blown off, while small ones adhering to the carbon film were studied.



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In situ X-ray diffraction Temperature programmed reduction (TPR) and oxidation (TPO) experiments were performed within a home-built in situ XRD setup, equipped with a Bruker-AXS D8 Discover apparatus (Cu Kα radiation with 𝜆 = 0.154 nm) and a linear Vantec detector 44-45. About 10 mg of powdered sample was loaded onto a silicon wafer and placed on a conductive heating strip, the temperature of which is monitored and controlled by a thermocouple. Samples were subjected to 5 in situ XRD redox cycles before performing H2-TPR and CO2-TPO experiments in order to exclude the potential influence of prolonged sample exposure to air. After each step, a full XRD scan was taken at room temperature under the same gaseous atmosphere as the experiment, i.e. H2 after H2-TPR and CO2 after CO2-TPO. The evolution of the crystalline structures during temperature-programmed reactions was also monitored by fitting Gaussians (equation 5), thus allowing to estimate the peak position (2𝜃𝑚𝑎𝑥), peak intensity ( 𝐼 ∙ 𝐶0 𝛤∙ 𝜋

) and peak width (full width at half maximum, 𝛤) throughout the experiment. 𝐼𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 = 𝐼 ∙

𝐶0 𝛤∙ 𝜋

∙ 𝑒𝑥𝑝

(

― 𝐶0 ∙ (2𝜃 ― 2𝜃𝑚𝑎𝑥)2 𝛽2

)

(4)

By comparing these descriptors for different diffractions during H2-TPR and CO2-TPO of 50FMA after different exposure to redox cycling (after 5, 105, 205, 505 and 1005 redox cycles), additional information regarding the mechanisms of degradation can be obtained.

Reactor setup and procedures Step response reactor The activity and stability of the 50FMA oxygen storage material was studied by performing up to 1000 redox cycles, using H2 as reductant and CO2 as oxidant, in a quartz tube microreactor setup. The inner reactor diameter was 10 mm, and the reactor was heated by an electric furnace. Temperature control was ensured by three heating modules (below the bed, at bed height and above the bed) and three K-type thermocouples touching the outside of the reactor. A fourth thermocouple was located inside of the reactor and touched the OSM bed. Prior to a redox cycling experiment, 0.2 g of 50FMA was loaded into the reactor on a quartz wool plug, and the reactor bed was heated to 1023 K under inert gas flow. At 1023 K, 6 ACS Paragon Plus Environment

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redox cycles comprising 1 min reduction (10% H2 in He), 2 min purging (He), 1 min oxidation (40% CO2 in He) and 2 min purging (He) were performed. The flow rate was kept constant at 2.4 10-4 mol s-1 using Brooks mass flow controllers. Online product analysis, in which H2, He, H2O, CO and CO2 were followed at 2 AMU, 4 AMU, 18 AMU, 28 AMU and 44 AMU, was performed using an OmniStar mass spectrometer (Pfeiffer Vacuum). Fragmentation patterns were taken into account when determining the product composition, while He was used as internal standard. Following equation was used for calculating the activity of 50FMA in terms of its CO space-time yield STYCO: ―1 ) STY𝐶𝑂 (mmol CO s ―1 kgFe =

FCO (mmol CO s ―1) WFe (kg Fe)

=

𝑥𝐶𝑂 (𝑚𝑜𝑙 𝐶𝑂 𝑚𝑜𝑙 ―1) ∙ 𝐹0 (𝑚𝑜𝑙 𝑠 ―1) WFe (kg Fe)

(5 )

In order to study the evolution of OSM properties of 50FMA over the course of 1000 redox cycles, different redox cycle runs using 50FMA involved 100, 200, 500 and 1000 redox cycles.

Results and discussion Characterization of the oxygen storage material Electron microscopy Fig. 1, A to E, shows a typical HAADF-STEM image of an as prepared 50FMA aggregate and its corresponding EDX elemental mappings. In the HAADF-STEM image, iron oxide rich clusters typically appear brighter, while MgFeAlOx spinel clusters appear less bright. This is caused by Z contrast and may be further enhanced by the typically larger crystallite size of iron oxide compared with MgFeAlOx spinel

23, 28.

Iron-rich clusters (red) of the order of 50 nm can be

distinguished (Fig. 1B and Fig. 1E), while the MgFeAlOx spinel clusters are typically smaller (around 10 to 20 nm). Moreover, Fig. 1, B to D, suggests a homogeneous dispersion of Mg and Al in the spinel phase. These findings in terms of iron oxide and spinel cluster sizes as well as MgFeAlOx homogeneity in STEM-EDX elemental mapping are in accordance with previous studies 23, 28, 37.



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Fig. 1 (A) HAADF-STEM image of as prepared 50FMA. (B) EDX elemental mapping corresponding with the HAADFSTEM image in A. (C) EDX elemental mapping of Mg. (D) EDX elemental mapping of Al. (E) EDX elemental mapping of Fe.

Mössbauer spectroscopy Experimental and fitted transmission Mössbauer spectra (TMS) of as prepared 50FMA are shown in Fig. 2. The spectra at RT (Fig. 2A) and 80 K (Fig. 2B) show asymmetric doublet absorption at the center, combined with sextet absorption due to magnetic phases in the sample. The TMS could be well reproduced by the superposition of a quadrupole splitting distributed (non-magnetic) component (D12) with two magnetic split components at RT (S1 and S2) and three magnetic split components at 80 K (S1, S2 and S3), which subsequently will be explained hereinafter. The parameters of each of these components are given in Table 1.


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Table 1 Results of the TMS analysis of as prepared 50FMA. For distributions, the listed parameter values are those with the highest probability in the respective distribution. S1 and S3 are attributed to hematite, S2 to goethite and D12 to MgFeAlOx spinel (together with other phases containing paramagnetic Fe3+). 𝑩𝒉𝒇 : Magnetic hyperfine field; 𝟐𝜺𝑸: quadrupole shift; 𝜟𝑬𝑸: quadrupole splitting; : center shift; : full line width at half maximum; RA: relative area of the absorption. T (K)

293

80

Component

Bhf

2εQ

EQ





RA

(kOe)

(mm s-1)

(mm s-1)

(mm s-1)

(mm s-1)

(%)

S1: DS (WF H)

Fe3+

512

-0.216

0.364

0.314

33.8

S2: HFD

Fe3+

424

-0.231

0.375

0.250

21.1

D12: QSD

Fe3+

0.282

0.564

45.1

S1: DS (WF H)

Fe3+

532

-0.108

0.481

0.401

11.5

S2: HFD

Fe3+

500

-0.138

0.461

0.250

27.6

S3: DS (AF H)

Fe3+

538

0.374

0.481

0.401

24.8

D12: QSD

Fe3+

0.375

0.754

36.1

0.760

0.785

DS: discrete sextet; HFD: hyperfine field distribution; QSD: quadrupole splitting distribution; RA = relative area WF H: weakly ferromagnetic spin state in hematite; AF H: antiferromagnetic spin state in hematite

The component with the outer sextet lines at RT is a well-defined discrete sextet (DS, S1) of which the Mössbauer parameters are typical for Fe3+ in the weakly ferromagnetic spin state in hematite (WF H, α-Fe2O3) 39. This component is also resolvable at 80 K. In comparison to S1, the absorption lines of the other magnetic component (S2) in the TMS at RT are spread over relatively broad velocity ranges, which is characteristic for a less ordered magnetic structure showing a distribution of magnetic hyperfine fields. This magnetic component (S2) of the TMS was hence reproduced by a superposition of sextet sub-spectra according to a modelindependent hyperfine field distribution (HFD), and is also present in the spectrum at 80 K, where it overlaps more or less with S1. Considering its Mössbauer parameters (𝐵ℎ𝑓: magnetic hyperfine field; 2𝜀𝑄: quadrupole shift; : center shift), S2 is attributed to ferric ions in goethite (α-FeOOH) 46. An additional discrete sextet (S3) related to S1 is resolved at 80 K and is, based on its parameters, attributed to the ferric antiferromagnetic spin state in hematite (AF H, αFe2O3). Indeed, it is well known that both magnetic states are present in hematite at relatively low temperature, while only the weakly ferromagnetic state remains at higher temperature 39.



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Fig. 2 Transmission Mössbauer spectra of as prepared 50FMA (A) at room temperature and (B) at 80 K. + experimental data; fitted spectrum; S1 discrete sextet (WF hematite); S2 hyperfine field distribution (goethite); D12 quadrupole splitting distribution (spinel); S3 discrete sextet (AF hematite). In situ XRD (C) H2-TPR and (D) CO2-TPO of 50FMA after 5 redox cycles with H2 and CO2.

As already mentioned before, the asymmetric doublet absorption at the center of the TMS could be adequately reproduced by a quadrupole splitting distributed component (D12) which is a superposition of a series of symmetric quadrupole doublet sub-spectra according to a model-independent quadrupole splitting distribution (QSD). In order to reproduce the observed asymmetry of the central doublet absorption the center shift () and quadrupole splitting (𝛥𝐸𝑄) of the constituent doublet sub-spectra were linearly correlated within this component during the fitting. The center shifts within this distribution are typical for Fe3+. The 𝛥𝐸𝑄 distribution is unimodal and symmetric in good approximation, which points to an

occupation of one type of site by Fe3+. The values of  and 𝛥𝐸𝑄 are in good agreement with the ones for Fe3+ at octahedral sites in the iron-containing natural MgAl2O4 spinel studied by Carbonin and coworkers (which was labeled TS2)

47.

The relatively high ferric 𝛥𝐸𝑄 at the

octahedral site is assigned to significant non-uniform distortions as a result of unordered cation distribution in the second coordination sphere of Fe3+ 23, which also explains small 10 ACS Paragon Plus Environment

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differences in  of the constituent doublets within the distribution. As a consequence of the linearly correlated  and 𝛥𝐸𝑄 within the QSD, the constituent doublets are slightly shifted relative to one another by which the asymmetry of the doublet absorption is reproduced. Fe2+ absorption is not resolved. The differences of the relative areas (RA) at 80 K compared with the values at 293 K are probably due to relaxation and depend among others on the size of the magnetic ‘particles’ in the powdered sample. Magnetic Fe3+ phases become more paramagnetic (no or less magnetic ordering) at higher temperature and therefore contribute to the doublet absorption at 293 K. At lower temperatures, they increasingly contribute to the magnetic split absorption as observed at 80 K (more magnetic ordering). The magnetic phases consequently have a larger RA at 80 K in comparison with 293 K, at the expense of the ferric doublet absorption which decreases from 293 K to 80 K. Therefore, the relative areas at 80 K give a more realistic view of the actual distribution of Fe3+ over the different sites and phases in the sample. It seems that at most 36 at% Fe is incorporated in the MgAl2O4 structure, thereby yielding a MgFeAlOx spinel. About 36 at% Fe is situated in hematite, while 28 at% Fe is present in the goethite phase. The latter may be underestimated because this phase is possibly not yet fully magnetically ordered at 80 K.

In situ X-ray diffraction The redox behavior of as prepared 50FMA has previously been studied through in situ XRD H2-TPR and CO2-TPO 23, 48. It has been shown that, upon reduction by H2, hematite (α-Fe2O3) is converted to magnetite (Fe3O4), wuestite (FeO) and iron (Fe) with increasing temperature. During reoxidation with CO2, magnetite is formed rather than hematite. In order to study the initial redox behavior, as prepared OSM was pretreated in situ with 5 redox cycles at 1023 K using H2 and CO2. Fig. 2 shows the in situ XRD H2-TPR (Fig. 2C) and CO2-TPO (Fig. 2D) experiments that were subsequently performed, showing the evolution of diffraction peaks in a 2𝜃 window between 32° and 46°. Characteristic diffractions (Table S1) of Fe3O4, MgFe2O4, γFe2O3, MgAl2O4, Mg(Fe,Al)2O4, FeO, (Mg1-xFex)O, MgO and Fe can be distinguished in the 2𝜃 window presented in Fig. 2C and D. The main transition in Fig. 2C originates from the reduction of Fe3O4 to FeO and Fe with increasing temperature during H2-TPR. The FeO phase appears as the Fe3O4 phase disappears between 723 K and 823 K (Fig. S1 and Fig. S3). The characteristic 11 ACS Paragon Plus Environment


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diffraction of Fe initially appears around 723 K (Fig. S4) together with the appearance of FeO diffractions, while a plateau in intensity of the Fe diffraction occurs around 1023 K. During CO2-TPO, shown in Fig. 2D, Fe is oxidized to Fe3O4 around 800 K. FeO diffractions are not observed. For the in situ XRD results presented in this work, elaborate data post-processing was performed in order to study the diffractions in the regions [35° – 36°] (Fig. S1), [36° – 37°] (Fig. S2), [41.5° – 43.5°] (Fig. S3) and [44° – 46°] (Fig. S4), by fitting a Gaussian to each XRD diffractogram in the respective region. This yields the evolution of the estimated diffraction angle at peak maximum (2𝜃𝑚𝑎𝑥), the peak amplitude (𝐼𝑚𝑎𝑥) and full width at half maximum (FWHM) during H2-TPR and CO2-TPO. Fig. S1 presents the evolution of Fe3O4, MgFe2O4 and/or γ-Fe2O3 diffraction during H2-TPR and CO2-TPO. The increase in peak position (Fig. S1A) and decrease in diffraction intensity (Fig. S1B) starting around 723 K indicate the onset of reduction of these phases. The decrease in peak width in Fig. S1C suggests an increasing periodicity in the crystallographic structure of the material, either through crystallite growth or homogenization. Properties of the MgFeAlOx spinel diffraction, relatively stable during H2-TPR and CO2-TPO, is presented in Fig. S2. Features of the diffraction of FeO, (Mg1-xFex)O and MgO can be followed in Fig. S3. Whether a phase occurs only as an intermediate, a reactant or a product during H2-TPR can be distinguished from the evolutions in peak position (Fig. S3A), diffraction intensity (Fig. S3B) and peak width (Fig. S3C). Diffraction data in Fig. S4 represent a convolution of MgFeAlOx and Fe diffractions. The increased diffraction intensity (Fig. S4B) and decreased peak width (Fig. S4C) during H2-TPR (above 773 K) can be linked to the appearance of Fe.

Activity and stability of oxygen storage material Fig. 3 presents the evolution in CO space-time yield (STYCO) during the oxidation half-cycle over 1000 redox cycles. The initial activity amounts to around 620 mmolCO s-1 kgFe-1. During the first 200 to 300 redox cycles, a significant decrease in maximum STYCO is observed. After this initial regime of continuous deactivation, the rate of decay in maximum STYCO decreases. From the 300th redox cycle onwards, it appears as if a dual regime occurs, with alternating redox cycles at high and low STYCO. This is indicated with the solid red lines, and suggests a residual activity after 1000 redox cycles that varies between roughly 30% and 60% 12 ACS Paragon Plus Environment

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of the initial activity. These results point to the occurrence of a dynamic redox behavior over the course of prolonged redox cycling. A more detailed characterization of the OSM after 100, 200, 500 and 1000 redox cycles is presented in what follows.

Fig. 3 Space-time yield of CO during 1000 redox cycles (p = 1.013 bar, T = 1023 K, Ftot = 2.4 10-4 mol s-1) using 50FMA. Each cycle comprises 1 min reduction (10% H2 in He), 2 min purging (He), 1 min oxidation (40% CO2 in He), 2 min purging (He).

Characterization of used oxygen storage material Mössbauer spectroscopy Experimental and fitted TMS of 50FMA after 100 and 1000 redox cycles are presented in Fig. 4. Fig. 4A and Fig. 4B correspond with the TMS at 293 K and 80 K of the OSM after 100 redox cycles, while Fig. 4C and Fig. 4D show the TMS at 293 K and 80 K of the OSM after 1000 redox cycles. The experimental spectra at 293 K and 80 K for both samples show asymmetric doublet absorption at the center and an additional asymmetric absorption band between ca. 1.0 and ca. 2.0-2.5 mm s-1, combined with sextet absorption due to magnetic phases. In accordance with the Mössbauer analysis of as prepared 50FMA (Fig. 2), the doublet and sextet absorptions would be expected to originate from MgFeAlOx and Fe2O3-related structures. The clearly observable decrease in sextet absorption intensity when comparing 50FMA after 100 redox cycles and after 1000 redox cycles (both at 293 K and 80 K) hence suggests an evolution towards a decreasing amount of iron present in (magnetic) Fe2O313 ACS Paragon Plus Environment


Page 14 of 34

related structures over the course of redox cycling. For both used samples, the asymmetric doublet absorption bands are fitted by discrete doublets (D2, D3 and D4). Two hyperfine field distributions (S1 and S2) allow to describe the sextet absorption at 293 K. For the spectra at 80 K, an additional discrete doublet (D1) and altered discrete sextet (S2’) are required to yield a good fit.

Fig. 4 Transmission Mössbauer spectra of 50FMA after 100 redox cycles (A) at 293 K and (B) at 80 K. Mössbauer spectra of 50FMA after 1000 redox cycles (C) at RT; (D) at 80 K. + experimental data; spectrum fit; S1 hyperfine field distribution (magnetite, maghemite or oxidized magnetite); S2 hyperfine field distribution (magnetite); D2 discrete doublet (Fe3+ in Mg-Fe-Al-O); D3 discrete doublet (Fe2+ in Mg2+ Fe-Al-O); D4 discrete doublet (Fe in Mg-Fe-Al-O). S2’ magnetic split subcomponent (Mg-Fe-Al-O); D1 discrete doublet (Fe3+ in Mg-Fe-Al-O).

At both temperatures and for both samples, the outer sextet component (S1) was reproduced by a superposition of sextet sub-spectra according to a model-independent hyperfine field distribution (HFD). The inner sextet component (S2), present only in the spectra at 293 K, was also reproduced by a HFD. For both of these fittings, the quadrupole shift of S1 and the center shift and quadrupole shift of S2 were fixed at the values for tetrahedral Fe3+ and octahedral Fe2.5+ in magnetite, respectively. The resulting parameters from the TMS fitting and analysis are listed in Table 2 and Table 3.


Page 15 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2 Results of the TMS analysis of 50FMA after 100 redox cycles using discrete doublets to reproduce the doublet absorption. For distributions, the listed parameter values are those with the highest probability in the respective distribution. f fixed value during the fitting. 𝑩𝒉𝒇: Magnetic hyperfine field; 𝟐𝜺𝑸: quadrupole shift; 𝜟𝑬𝑸 : quadrupole splitting; : center shift; : full line width at half maximum; RA: relative area of the absorption. Rat: relative atomic percentage with respect to the total iron content. T (K)

Component

293

80

Bhf

2εQ

EQ





RA

Rat

(kOe)

(mm s-1)

(mm s-1)

(mm s-1)

(mm s-1)

(%)

(%)

S1: HFD

Fe3+

493

0.00f

0.295

0.350

31.7

29.5

S2: HFD

Fe2.5+

458

0.16f

0.656f

0.900

16.6

15.5

D2: DD

Fe3+

0.814

0.311

0.464

25.7

23.9

D3: DD

Fe2+

0.986

0.893

0.444

10.9

13.0

D4: DD

Fe2+

1.742

0.942

0.550

15.1

18.0

S1: HFD

Fe3+

S2’: DS

516

-0.014

0.416

0.410

38.0

471

-0.074

0.708

1.775

19.6

D1: DD

Fe3+

0.447

0.286

0.306

3.8

D2: DD

Fe3+

0.880

0.392

0.503

16.2

D3: DD

Fe2+

2.155

1.083

0.833

13.2

D4: DD

Fe2+

2.902

1.039

0.359

9.2

HFD: hyperfine field distribution; DD: discrete doublet; DS: discrete sextet

While S1 and S2 can be attributed to tetrahedral Fe3+ and octahedral Fe2.5+ in magnetite based on their Mössbauer parameter values, the relative area of S1 should be half the relative area of S2 in perfect magnetite (RAS1,magnetite = ½ RAS2,magnetite). This proportion is clearly not observed for the TMS at 293 K of 50FMA after redox cycles. Indeed, the relative area of S1 is larger than half the one of S2 (RAS1 > ½ RAS2) for 50FMA after 100 and 1000 redox cycles. It is known, however, that the Mössbauer absorption of Fe3+ in maghemite cannot be distinguished from the ferric tetrahedral absorption (S1) in magnetite. The explanation for the higher than expected S1 over S2 ratio could therefore be found in (i) the simultaneous presence of maghemite (γ-Fe2O3) and magnetite (Fe3O4) in the sample: S1 is then due to both Fe3+ in maghemite and tetrahedral Fe3+ in magnetite, while S2 is only due to octahedral Fe2+ and Fe3+ in magnetite; (ii) the presence of oxidized magnetite, for which a higher than expected ratio in relative areas between S1 and S2 (RAS1 > ½ RAS2) has previously been observed

49.

Assuming the presence of two well-defined magnetic phases, maghemite and

perfect magnetite, the limit of their relative content can be estimated from the observed relative area of S1 and S2, based on the results of the adjustment at 293 K: 15 ACS Paragon Plus Environment


Page 16 of 34

𝑅𝐴𝐹𝑒, 𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑡𝑒 = 𝑅𝐴𝑆2 + 𝑅𝐴𝑆1, 𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑡𝑒 = 𝑅𝐴𝑆2 + ½ 𝑅𝐴𝑆2

(5-1)

𝑅𝐴𝐹𝑒, 𝑚𝑎𝑔ℎ𝑒𝑚𝑖𝑡𝑒 = 𝑅𝐴𝑆1 – 𝑅𝐴𝑆1,𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑡𝑒 = 𝑅𝐴𝑆1 – ½ 𝑅𝐴𝑆2

(5-2)

For 50FMA after 100, 500 and 1000 redox cycles, using the TMS adjustment data at 293 K (Table 2, Table 3 and Table S2), this leads to the results shown in Table 4. Under the used assumptions, these calculations indicate that the ratio between maghemite and magnetite is at most 1:1 for all OSMs after redox cycles. Because magnetite can also be present in a more oxidized form, intermediate between magnetite and maghemite, the ratio might in reality be lower. Table 3 Results of the TMS analysis of 50FMA after 1000 redox cycles using discrete doublets to reproduce the doublet absorption. For distributions, the listed parameter values are those with the highest probability in the respective distribution. f fixed value during the fitting. 𝑩𝒉𝒇: Magnetic hyperfine field; 𝟐𝜺𝑸: quadrupole shift; 𝜟𝑬𝑸 : quadrupole splitting; : center shift; : full line width at half maximum; RA: relative area of the absorption. Rat: relative atomic percentage with respect to the total iron content. T (K)

Component

Bhf

2εQ

(kOe) 293

80

(mm

s-1)

EQ (mm s-1)

 (mm

 s-1)

(mm

s-1)

RA

Rat

(%)

(%)

S1: DS

Fe3+

491.0

0.00f

0.281

0.557

14.8

12.8

S2: HFD

Fe2.5+

452.3

0.16f

0.656f

0.250f

6.7

5.8

D2: DD

Fe3+

0.770

0.378

0.342

22.1

19.1

D3: DD

Fe2+

0.952

0.928

0.612

49.5

54.8

D4: DD

Fe2+

2.060

0.838

0.326

6.9

7.6

S1: HFD

Fe3+

S2’: HFD

513

-0.024

0.409

0.533

18.5

301

-0.076

1.201

0.839

32.7

D1: DD

Fe3+

0.560

0.252

0.382

5.3

D2: DD

Fe3+

0.860

0.416

0.441

13.6

D3: DD

Fe2+

1.860

1.111

0.738

11.8

D4: DD

Fe2+

2.910

1.027

0.477

18.1

DS: discrete sextet; HFD: hyperfine field distribution; DD: discrete doublet

For the TMS at 80 K, the situation is more complicated. Because 80 K is lower than the Verwey transition temperature of magnetite

50,

octahedral Fe in magnetite (Fe2+ and Fe3+)

gives rise to individual magnetic split components of which the outer absorption lines approximately coincide with the ferric absorption of maghemite

49

and consequently

contribute to the relative area of S1. The absorption sub-spectrum corresponding with S2, observable at 293 K, therefore no longer exists at 80 K. However, another magnetic split subcomponent (S2’) is resolved in the TMS at 80 K, which can most likely be attributed to Fe in a 16 ACS Paragon Plus Environment

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MgxFe1-xO phase that could be partially magnetically ordered at 80 K. Indeed, according to Long and Grandjean, FeO has a magnetic ordering temperature of ca. 195 K

51.

While this

temperature could be lower for MgxFe1-xO, the complexity of the magnetic split spectrum of MgxFe1-xO, the unknown composition of the MgxFe1-xO phase and the overlap of S2’ with the other sub-spectra inhibit the accurate determination of its parameters as well as the relative areas of the other sub-spectra. The doublet absorption present in the spectra at 80 K is reproduced by two Fe3+ (D1 and D2) and two Fe2+ (D3 and D4) discrete doublets. At 293 K, on the other hand, the presence of two different Fe3+ doublets could not be resolved. Therefore, the doublet absorption in the TMS at 293 K is alternatively reproduced by one Fe3+ (D2) and two Fe2+ (D3 and D4) discrete doublets (see Fig. 4). The corresponding numerical results of the fitting are given in Table 2 and Table 3. Parameters for D1 and D2 match well with Fe3+ at octahedral sites, while those for D3 and D4 match with Fe2+ at tetrahedral sites both in iron-containing MgAl2O4 spinel. The difference between D1 and D2, respectively D3 and D4, reflects different cation distributions in the second coordination sphere 47. The decrease of the D3 and D4 contribution to the total relative area when lowering the temperature from 293 K to 80 K seems to happen in favor of S2’, suggesting that the doublet absorption at 293 K partly originates from MgxFe1-xO, present in a paramagnetic state. Similar to the overlap of the magnetic S2’ contribution of MgxFe1-xO with other sub-spectra, the paramagnetic MgxFe1-xO phase could not be resolved because of total overlap with the other doublet sub-spectra. The relative areas of D1, D2, D3 and D4 therefore reflect the contributions of Fe2+ and Fe3+ in both the MgFeAlOx spinel and paramagnetic MgxFe1-xO phase, which together are denoted as Mg-Fe-Al-O for simplicity. In the context of the TMS measurements performed for this work, Mg-Fe-Al-O thus represents a set of phases such as Mg(FeyAl1-y)2O4 (0 < y ≤ 1), (MgxFe1-x)Al2O4 (0 ≤ x < 1), (MgxFe1-x)(FeyAl1y)2O4

(0 < x, y < 1) and MgxFe1-xO (0 ≤ x < 1). Because the absorption of S2’ at 80 K seems to

occur mainly at the expense of the relative area of D3 and D4, the magnetic phase related to S2’ likely concerns a magnetic ordering of Fe2+ spins, most probably in the MgxFe1-xO phase. The center shift of S2’ confirms this. Since the MgxFe1-xO phase cannot be resolved in the spectra at 293 K nor determined accurately from the spectra at 80 K, it is only possible to differentiate and quantify the magnetic phases and non-magnetic phases at 293 K. Relative atomic percentages (Rat) with 17 ACS Paragon Plus Environment


Page 18 of 34

respect to the total iron content, presented in Table 2 and Table 3, are calculated from the relative areas at 293 K using a proportion factor of 0.78 for the Mössbauer recoil free fraction of Fe2+ with respect to Fe3+ 40. Based on these values the Fe distribution over the different phases in as prepared and redox cycled OSMs was estimated (Table 4). The results for 50FMA after 500 redox cycles, given in Fig. S5 and Table S2, are similar to those after 100 redox cycles. Table 4 Distribution of Fe over different phases (at% Fe) in 50FMA as prepared and after redox cycles, based on TMS and ILEEMS data at 293 K. Relative area (RA) of different Mössbauer absorptions at 80 K: S1 - Fe3+ in γ-Fe2O3 and Fe3O4; S2’ - Fe in MgxFe1-xO; D1 - Fe3+ in Mg-Fe-Al-O; D2 - Fe3+ in Mg-Fe-Al-O; D3 - Fe2+ in Mg-Fe-Al-O; D4 Fe2+ in Mg-Fe-Al-O (including MgxFe1-xO). a surface composition (ILEEMS). ‡ based on TMS data at 80 K. Whether the reported values should be interpreted as lower limits (min) or upper limits (max) typically depends on the effect of incomplete magnetization of iron species which adds to the doublet absorption intensity. % α-Fe2O3

% α-FeOOH

% γ-Fe2O3

% Fe3O4

% Fe3+

% Fe2+

hematite

goethite

maghemite

magnetite

Mg-Fe-Al-O

Mg-Fe-Al-O

(max)

(min)

(max)

(min)

(max)

(max)

As prepared‡

36

28

0

0

36

0

100 cycles

0

0

22

23

24

31

500 cycles

0

0

21

18

22

39

1000 cycles

0

0

10

9

19

62

0

0

36

0

18

46

RA(S1)

RA(S2’)

RA(D1)

RA(D2)

RA(D3)

RA(D4)

%

%

%

%

%

%

100 cycles‡

38

20

4

16

13

9

500 cycles‡

33

23

6

14

17

9

19

33

5

14

12

18

1000

1000

cyclesa

cycles‡

In order to evaluate whether the surface and bulk properties of iron in the redox cycled material would be similar, ILEEMS was performed. Fig. 5 shows the ILEEMS at 293 K of 50FMA after 1000 redox cycles. Except for S2, the same components with comparable parameters as for the TMS at 293 K could be resolved (Table 5). Hence, at 293 K, magnetic Fe2+ phases are not present at the surface of the particles, which suggests that the magnetic phase present at the surface of the particles is fully oxidized to Fe3+ in the form of maghemite (γ-Fe2O3), magnesioferrite (MgFe2O4) or a combination of both.


Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 5 ILEEMS of 50FMA after 1000 redox cycles at RT. + experimental data; spectrum fit; S1 hyperfine field distribution (magnetite, maghemite or oxidized magnetite); D2 discrete doublet (Fe3+ in Mg-Fe-Al-O); D3 discrete doublet (Fe2+ in Mg-Fe-Al-O); D4 discrete doublet (Fe2+ in Mg-Fe-Al-O).

As previously mentioned, Table 4 shows the percentage distribution of Fe among the different phases distinguished in TMS and ILEEMS at 293 K. The TMS results show that, over the course of 1000 redox cycles, the amount of Fe in iron-rich magnetically ordered oxides (hematite, goethite, magnetite, maghemite and magnesioferrite) decreases from 64% (as prepared) over 45% (after 100 redox cycles) and 39% (after 500 redox cycles) to 19% (after 1000 redox cycles). Considering the ILEEMS of 50FMA after 1000 redox cycles, however, shows a surface enrichment of these phases to about 36% rather than 19% in the bulk (TMS). In other words, these results suggest that the magnetic iron oxide rich phase is preferentially located at the surface in contrast to Mg-Fe-Al-O. Indeed, the observation of iron oxide enrichment at the surface of the 50FMA oxygen storage material after extensive redox cycling was also made during previously reported STEM-EDX measurements 37. Besides iron-rich magnetically ordered oxides, iron is present in form of various Mg-Fe-Al-O phases. After an initial decline of the amount of Fe3+ in the Mg-Fe-Al-O phases from 36% to 24% during the first 100 redox cycles, a slower decrease to 19% after 1000 redox cycles is noted. Hence, the strong decline of Fe in iron-rich oxides between the 500th and 1000th redox cycle originates from an increase in the amount of Fe2+ present in Mg-Fe-Al-O phases. More specifically, the increased amount of Fe2+ in MgxFe1-xO phases is suggested to account for the decrease of iron present in the form of iron-rich oxides. This is also apparent from the strong increase in importance of the S2’ absorption at 80 K (Table 4) over the course of redox cycling. 19 ACS Paragon Plus Environment


Page 20 of 34

Table 5 Results of the ILEEMS analysis of 50FMA after 1000 redox cycles using discrete doublets to reproduce the doublet absorption. f fixed value during the fitting. 𝑩𝒉𝒇: Magnetic hyperfine field; 𝟐𝜺𝑸: quadrupole shift; 𝜟𝑬𝑸 : quadrupole splitting; : center shift; : full line width at half maximum; RA: relative area of the absorption; Rat: relative atomic percentage with respect to the total iron content. T (K)

293

Component

Bhf

2εQ

EQ





RA

Rat

(kOe)

(mm s-1)

(mm s-1)

(mm s-1)

(mm s-1)

(%)

(%)

491

0.00f

0.354

0.856

40.0

35.9

S1: DS

Fe3+

D2: DD

Fe3+

0.810

0.386

0.432

19.7

17.7

D3: DD

Fe2+

1.029

0.967

0.707

35.5

40.9

D4: DD

Fe2+

1.996

0.838f

0.505

4.8

5.5

DS: discrete sextet; DD: discrete doublet

In all the above, the presence of maghemite or oxidized magnetite has been considered evident. Based on bulk thermodynamics, however, iron is not expected to be oxidized beyond Fe3O4 in presence of CO2 and absence of O2. Different explanations for this anomaly are possible, none of which can be excluded: (i) Fe3O4 could be partially oxidized to maghemite and/or oxidized magnetite due to exposure to ambient air. In this case, the fact that the maghemite to magnetite ratio is very similar for all of the used samples would be somewhat striking since their exposure time to ambient air, their structure and their morphology are all different. (ii) The thermodynamics result may be affected by surface tension in nanoparticles, which has previously been shown to cause significant differences in surface thermodynamics as compared with the expected bulk thermodynamics by Navrotsky and coworkers 52-54. This surface tension may occur both at the solid-gas and solid-solid (e.g. interactions between the iron-rich phase and Mg-Fe-Al-O) interface, and cause nanoparticles to be oxidized by CO2 beyond magnetite. Note that, while the used material may have a low BET (solid-gas) surface area, the solid-solid surface area between crystallites may still be high. (iii) Perhaps most likely, the presence of Mg doped Fe3O4 or MgFe2O4 could explain the high relative ratio between Fe3+ and Fe2+ in iron-rich magnetic phases.

In situ X-ray diffraction In situ XRD H2-TPR and CO2-TPO results of used OSM after 100, 200, 500 and 1000 redox cycles are given in Fig. 6. As explained for 50FMA after 5 redox cycles (paragraph 0), characteristic diffractions of Fe3O4 (2𝜃 = 35.4° and 43.1°), MgFe2O4 (2𝜃 = 35.5° and 43.2°), γ20 ACS Paragon Plus Environment

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Fe2O3 (2𝜃 = 35.6° and 43.3°), MgAl2O4 (2𝜃 = 36.8° and 44.8°), Mg(Fe,Al)2O4 (2𝜃 = 36.3° and 44.1°), FeO (2𝜃 = 36.1° and 41.9°), Mg1-xFexO (2𝜃 = 36.5° and 42.4°), MgO (2𝜃 = 37.0° and 43.0°) and Fe (2𝜃 = 44.7°) can be distinguished in the presented 2𝜃 window between 32° to 46°.

Fig. 6 In situ XRD H2-TPR of 50FMA (A) after 100 redox cycles; (B) after 200 redox cycles; (C) after 500 redox cycles; (D) after 1000 redox cycles. In situ XRD CO2-TPO of 50FMA (E) after 100 redox cycles; (F) after 200 redox cycles; (G) after 500 redox cycles; (H) after 1000 redox cycles.

Characterization of MgFeAlOx spinel Fig. 6 presents the in situ XRD H2-TPR (A to D) and CO2-TPO (E to H) results for 50FMA after 100, 200, 500 and 1000 redox cycles. Diffractions of the MgFeAlOx spinel can be observed at angles between 36° and 37° (Fig. S2) and between 44° and 45° (Fig. S4). Upon reduction of iron oxide, however, the MgFeAlOx spinel diffraction around 44° typically overlaps with the intense Fe diffraction around 45°. With increasing temperature, the peak position generally decreases linearly due to thermal expansion of the lattice. For the 50FMA materials after 100 and especially after 200 redox cycles, however, an increase in peak position occurs around 773 K during H2-TPR (Fig. S2A), indicating lattice contraction due to a change either in structure or composition of the spinel. Even though this points to a (partial) removal of Fe from the spinel, the fact that the peak position at the start of the CO2-TPO experiment (Fig. S2B) is the same as the one at the start of the H2-TPR experiment (Fig. S2A) suggests that no such change 21 ACS Paragon Plus Environment


Page 22 of 34

occurred. One possible explanation for this phenomenon is the formation of a metastable MgFeAlOx phase under reducing environment at high temperature, which again disappears during cool-down between the H2-TPR and CO2-TPO treatments. The only used material that does not show such behavior to a significant extent is the 50FMA material after 500 redox cycles. Characterization of Fe3O4, MgFe2O4, γ-Fe2O3 Diffractions of Fe3O4, MgFe2O4 and γ-Fe2O3, generally referred to as iron-rich spinel, are expected at diffraction angles between 35° and 36° (Fig. S1). All of the samples typically show a peak positioned at 35.6° (Fig. S1A), corresponding closely to that expected for γ-Fe2O3. Because of the small difference with Fe3O4 and MgFe2O4 and the limited 2𝜃 resolution of in situ XRD, it is difficult to draw a definite conclusion regarding the phase based on the peak position. Nevertheless, Mössbauer spectroscopy suggests the presence of an iron-rich phase with Fe showing a higher degree of oxidation than that expected for Fe3O4, which suggests the presence of MgFe2O4 and/or γ-Fe2O3. The actual phase that occurs may be a combination of the above, depending on the local environment. During H2-TPR (Fig. S1, A to C), the diffraction caused by the iron-rich spinel starts to decrease in intensity around 723 K for as prepared 50FMA and around 773 K for 50FMA after 1000 redox cycles (Fig. S1B). The diffraction disappears around 823 K for the former and 923 K for the latter (Fig. S1, A and B), indicating a more difficult reduction of 50FMA after 1000 redox cycles compared with as prepared 50FMA. This may either be caused by kinetic limitations, e.g. due to sintering, or by formation of a new phase with a different behavior, e.g. phase transformation from Fe3O4 to MgFe2O4. It has previously been shown that the formation of MgFe2O4 from Fe3O4 impedes the reducibility 36, hence increasing the reduction temperature. During CO2-TPO (Fig. S1, D to F), the onset of formation of the iron-rich spinel phase occurs between 723 K and 773 K for all samples (Fig. S1, D to F). The oxidation by CO2 can be considered completed when the diffraction intensity reaches a plateau, as is the case for as prepared 50FMA around 873 K (Fig. S1E). Note that the temperature at which this occurs seems to increase with the number of cycles to which 50FMA is exposed. Indeed, such a plateau in intensity can be clearly distinguished only during CO2-TPO of as prepared 50FMA. This suggests that, after the reduction step, the extent of reoxidation by CO2 becomes limited 22 ACS Paragon Plus Environment

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and hence higher temperatures or a stronger oxidant (such as O2) may be required to reach complete oxidation. Characterization of MgxFe1-xO with 0 ≤ x ≤ 1 Fig. 6A and Fig. 6E present the in situ XRD H2-TPR and CO2-TPO results for 50FMA after 100 redox cycles. Similar to as prepared material (see section 0), FeO occurs as an intermediate between Fe3O4 and Fe during reduction by H2, while the oxidation of Fe to Fe3O4 by CO2 appears to occur without intermediate. As for 50FMA after 200 redox cycles, the in situ XRD H2-TPR result (Fig. 6B) initially shows a weak diffraction around 42.6°, corresponding with MgxFe1-xO rather than pure FeO or an iron-rich spinel. With increasing temperature, this diffraction gradually shifts towards 42° and gains intensity around 800 K, suggesting enrichment with Fe to form a MgxFe1-xO phase with a closer resemblance to pure FeO (Fig. S3A). At slightly higher temperatures, this phase again starts to disappear through formation of Fe. During CO2-TPO (Fig. 6F), both the iron-rich spinel and MgxFe1-xO phases reappear. The MgxFe1-xO diffraction appears around 42.2° (Fig. S3D), which is lower than the initial 42.6°, probably caused by a thermal lattice expansion effect. Interestingly, the occurrence of this MgxFe1-xO diffraction is no longer observable during H2-TPR (Fig. 6C) and CO2-TPO (Fig. 6G) of 50FMA after 500 redox cycles. Only a weak diffraction around 41.8-42.2° is observed around 873 K (Fig. S3A). After 1000 redox cycles, the MgxFe1-xO diffraction in H2-TPR (Fig. 6D) and CO2TPO (Fig. 6H) reappears. A relatively intense diffraction around 42.5° gradually shifts to 42.0° during H2-TPR (Fig. S3A). An increase in intensity, which would correspond to the formation of FeO as intermediate, is not apparent. Again, it should be noted that part of the decrease in diffraction angle can be explained by thermal effects. Nevertheless, the initial diffraction angle at 373 K is around 42.3° during CO2-TPO as compared with 42.5° at 373 K during H2-TPR (Fig. S3D). This indicates an actual shift in peak position of 0.2° during H2-TPR, which can be interpreted as the formation of a FeO phase or the enrichment of Fe in an (already present) MgxFe1-xO phase. During CO2-TPO of 50FMA after 1000 redox cycles, MgxFe1-xO appears as an intermediate towards the formation of iron-rich spinel.



Page 24 of 34

Characterization of Fe The presence of metallic Fe can be observed through a high-intensity diffraction close to 45° in the in situ XRD diffractogram (Fig. 6, A to H). While MgFeAlOx spinel and Fe diffractions overlap, the appearance of Fe can be recognized through an increase in diffraction peak intensity (Fig. S4B). During H2-TPR, the onset of this increase in intensity occurs between 723 K and 773 K, and corresponds with the onset of reduction of the iron-rich spinel phase. Similarly, a decrease in intensity during CO2-TPO around 723 K indicates the onset of Fe oxidation by CO2 which seems to be completed before 973 K for all samples (Fig. S4E). Electron microscopy Electron microscopy was performed on 50FMA after 1000 redox cycles to study the change in structure and morphology compared with as prepared material. Fig. 7A and Fig. 7B show a STEM-EDX elemental mapping and corresponding TEM image of the used OSM. The elongated iron-rich zone in Fig. 7A confirms the enrichment of iron-rich phases at the surface of the material, as was also concluded from the TMS and ILEEMS. The occurrence of surface enrichment of iron was also previously suggested based on STEM-EDX elemental mapping 37.

Fig. 7 (A) STEM-EDX elemental mapping and (B) corresponding TEM image. Squares represent regions of interest. Inset (C and D): magnification of region of interest with EDX linescan.

While Fig. 7B shows the presence of an additional feature (indicated with rectangle C), its chemical composition is not obvious from the corresponding elemental mapping in Fig. 7A. A linescan is therefore provided in Fig. 7C, showing that this feature corresponds with alumina, probably amorphous (γ-Al2O3). Likely, this alumina results from extraction of Al3+ from the MgFeAlOx spinel upon incorporation of Fe3+

55.

Fig. 7C also shows the EDX counts of the 24

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neighboring MgFeAlOx spinel particle, indicating similar amounts of Fe and Mg in the MgFeAlOx spinel and a higher amount of Al. A linescan of the interface between an iron-rich zone and the MgFeAlOx spinel is shown in Fig. 7D, revealing the presence of Mg in this iron-rich zone, albeit in a much lower ratio with respect to Fe than would be expected for MgFe2O4. Hence, while the iron-rich zone is not necessarily homogeneous in composition, it likely consists of a MgxFe3-xO4 and/or MgxFe1-xO phase (with low x). Summary An overview of the results is presented in Fig. 8. As found by Mössbauer spectroscopy, Fe3+ is present in as prepared material as α-Fe2O3, α-FeOOH and MgFe3+AlOx. Upon reduction, α-Fe2O3 and α-FeOOH are reduced to Fe3O4, FeO and Fe. Reoxidation by CO2 leads to formation of Fe3O4 (and possibly γ-Fe2O3). Subsequent reduction to Fe/FeO completes the first redox loop. In a second (parallel) redox loop, MgFe3+AlOx is reduced to MgFe2+AlOx by H2 and subsequently reoxidized by CO2. Over time, iron from the first redox loop is incorporated in the MgFeAlOx spinel phase, thus ending up in de second redox loop. At some point, MgFe2O4 is formed either by iron-enrichment of the MgFeAlOx phase to an extent at which it can be considered MgFe2O4, or by exchange of Fe2+ from Fe3O4 and Mg2+ from MgFeAlOx. The formation of MgFe2O4 goes hand in hand with the formation of alumina-enriched phases such as γ-Al2O3 and/or FeAl2O4. In the third redox loop, which increasingly comes into play after redox cycling, MgFe2O4 is periodically reduced to MgxFe1-xO (and perhaps Fe) by H2 and reoxidized by CO2. Depending on the local conditions, iron can also be segregated from the MgFeAlOx spinel, as was previously observed for 10Fe2O3/MgFeAlOx37. Overall, the results from Mössbauer spectroscopy indicate a net relocation of Fe from α-Fe2O3 and α-FeOOH (and the related Fe3O4 and/or γ-Fe2O3) to MgFeAlOx, MgFe2O4 and MgxFe1-xO over the course of 1000 redox cycles with H2 as reductant and CO2 as oxidant.



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Fig. 8 Schematic overview of the effect of 1000 redox cycles on the oxygen storage behavior of 50Fe2O3/MgFeAlOx. As prepared OSM consists of α-Fe2O3, α-FeOOH and MgFe3+AlOx. Different redox-active species, categorized in three different redox loops, come into play upon redox cycling.

Conclusions In this work, a 50Fe2O3/MgFeAlOx, subjected to 1000 redox cycles, has been characterized in detail by means of

57Fe

Mössbauer spectroscopy, in situ X-ray diffraction and electron

microscopy in order to investigate active phase transformation and material deactivation. The initial activity of the sample (620 mmolCO s-1 kgFe-1) significantly decreases during the first 200 to 300 redox cycles. After this initial regime of continuous deactivation, the rate of decay STYCO decreases. The residual activity after 1000 redox cycles varies between roughly 30% and 60% of the initial activity. These results point to the occurrence of a dynamic redox behavior over the course of prolonged redox cycling. Mössbauer spectroscopy revealed that iron in the as prepared 50Fe2O3/MgFeAlOx OSM, present as Fe3+, was distributed among iron-rich phases (α-Fe2O3 and α-FeOOH, around 64% of total Fe) and MgFeAlOx spinel (around 36% of total Fe). In the OSM after 1000 redox cycles, 26 ACS Paragon Plus Environment

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iron was distributed among iron-rich spinel phases such as Fe3O4, γ-Fe2O3 and MgFe2O4 as well as MgxFe1-xO and the more iron-lean MgFeAlOx. At least 19% of total Fe was present in phases such as Fe3O4, γ-Fe2O3, MgFe2O4 and MgxFe1-xO, while at most 81% of total Fe was present in MgFeAlOx (19% Fe3+ and 62% Fe2+). It should be stressed that a significant part of this Fe2+ probably corresponds with the MgxFe1-xO phase, which remained paramagnetic at 293 K but (part of which) appeared as a magnetic component at 80 K. Surface ILEEMS measurements on the OSM after 1000 redox cycles, revealed enrichment of iron-rich spinel phases (36% of total Fe compared with 19% during bulk measurements). At the same time, at most 64% of total Fe was present in Mg-Fe-Al-O (18% Fe3+ and 46% Fe2+). Interestingly, all of the surface iron in iron-rich spinel was in the Fe3+ oxidation state, pointing at γ-Fe2O3 and/or MgFe2O4. In situ X-ray diffraction during H2-TPR of used OSMs showed that, after an increasing number of redox cycles, the reduction temperature of iron-rich spinel (Fe3O4, γ-Fe2O3 and MgFe2O4, all of which share highly similar diffractions) increased, which may point to the presence of MgFe2O4. Even after 1000 redox cycles, metallic iron appeared upon reduction with no significant change in reduction temperature. After 1000 redox cycles, a MgxFe1-xO phase was present throughout both H2-TPR and CO2-TPO experiments. During CO2-TPO, FeO occurred as intermediate in the oxidation towards iron-rich spinel phases, in contrast to as prepared OSM where the oxidation of Fe to iron-rich spinel appeared to occur without crystalline intermediate. Electron microscopy was performed after redox cycles to study the change in structure and morphology compared with as prepared material. STEM-EDX elemental mapping revealed the presence of Mg in iron-rich surface phases, albeit not in relative amounts to iron that correspond with MgFe2O4. Furthermore, the presence of alumina (most likely amorphous γAl2O3) was observed, indicating the substitution of Al3+ by Fe3+ in the MgFeAlOx phase. A combination of crystallite growth with phase transformation are causes of loss in activity and oxygen storage capacity. Acknowledgements This work was supported by the Long Term Structural Methusalem Funding of the Flemish Government and the Fund for Scientific Research Flanders (FWO; project G004613N). L. C. Buelens acknowledges financial support from the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT Vlaanderen). The authors thank 27 ACS Paragon Plus Environment


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Geert Rampelberg, Davy Deduytsche and Ranjith K. Ramachandran for help with in situ XRD measurements (Department of Solid State Sciences, Ghent University). This work is dedicated to prof. E. De Grave†.

Supporting Information   

Evolution of the Gaussian fit parameters during in situ XRD H2-TPR and CO2-TPO. Transmission Mössbauer spectra of the sample after 500 redox cycles. Selection of crystalline phases with their characteristic diffractions.

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