Methane Dissociation on Î±-Fe2O3(0001) and Fe3O4(111) Surfaces

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Methane Dissociation on #-FeO (0001) and FeO (111) Surfaces: First-Principles Insights into Chemical Looping Combustion Joseph W. Bennett, Xu Huang, Yuan Fang, David M. Cwiertny, Vicki H. Grassian, and Sara E. Mason J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08675 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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The Journal of Physical Chemistry

Methane Dissociation on α-Fe2O3(0001) and Fe3O4(111) Surfaces: First-Principles Insights Into Chemical Looping Combustion Joseph W. Bennett1† , Xu Huang1† , Yuan Fang2 , David M. Cwiertny3 Vicki H. Grassian2 and Sara E. Mason1∗∗ 1: Department of Chemistry, University of Iowa, Iowa City, Iowa 52242; 2: Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California 92093; 3: Department of Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa 52242 E-mail: [email protected]

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Abstract Chemical looping combustion (CLC) has drawn much attention in recent years for its near 100% efficiency in generating CO2 . Unlike conventional combustion processes, CLC uses transition metal oxides to transform hydrocarbons into carbon dioxide in the absence of air. Instead of an air atmosphere acting as a source of gaseous oxygen, the transition metal oxide surface acts as a solid reservoir of oxygen. This decreases the cost of CO2 production because the CLC process creates CO2 product that does not need to be separated from O2 , N2 , and other gases found in the atmosphere. While CLC can lead to clean, efficient gas production, there are still a few key needs to further optimize the process. The most pressing need is to understand the chemical changes that occur by fully characterizing reaction products and surface reconstructions. Here, we use DFT + U methodology to obtain an atomistic picture of the surface transformations and chemical reactions that take place during the initial dissociation of methane into CH3 and H on hematite α-Fe2 O3 (0001) and magnetite Fe3 O4 (111) surfaces at the beginning of the CLC process. We find that a homolytic adsorption pathway is energetically preferred over a heterolytic pathway, and that it is necessary to include Hubbard U corrections to both Fe and O to accurately describe surface processes, such as adsorbtion and transformations, at the atomistic level. After a comparison of the two surfaces, we go on to show that they may exhibit competitive adsorption, and that oxygen deficient hematite surfaces may result in enhanced methane dissociation, an intermediate that may be a key step to optimizing the CLC process.

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Introduction Chemical looping combustion (CLC) is an emerging technology poised to become an efficient alternative to the conventional production of CO2 from fossil fuel combustion. Conventional combustion methods produce syngas from sources such as petroleum and natural gas, which can then go on to industrial processes such as energy production. 1,2 The process can lead to incomplete combustion and syngas then needs to undergo purification before use. This is most times a series of expensive gas phase separations that make the overall syngas production cycle inefficient. The CLC process sidesteps this inefficiency by preventing direct contact between CO2 product and O2 in the air. It does so by using lattice oxygen from solid transition metal oxides (TMOs) as the fuel oxidant, instead of air, in an evacuated fuel reactor. 3–6 In situ generation of CO2 using TMO surfaces in the CLC process means that CO2 can be directly captured without additional gas phase separations, increasing the overall cost efficiency when compared to typical combustion processes. 5,6 This leads to the higher efficiency of the CLC process; the energy release from the oxidation of fuels by TMOs does not incur the penalty of subsequent separation that would entail from conventional syngas production.

Figure 1: Scheme of the chemical looping combustion (CLC) process. Figure 1 shows a schematic illustration of the transformations that take place in the CLC process. For hydrocarbon fuels of general formula Cn H2m , the full and partial oxidation

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reactions that produce syngas can be written as Equations 1 and 2, respectively:

(2n + m)My Ox + Cn H2m → (2n + m)My Ox−1 + mH2 O + nCO2

(1)

nMy Ox + Cn H2m → nMy Ox−1 + mH2 + nCO

(2)

After oxidizing the hydrocarbon Cn H2m in the fuel reactor, the reduced oxygen carrier My Ox−1 is put into the air reactor where it can be re-oxidized by O2 . This replenishes its lattice oxygen sites via the Mars and van Krevelen mechanism (MvK), 7 as shown in Equation 3.

1 My Ox−1 + O2 → My Ox + (air, N2 + unreacted O2 ) 2

(3)

This means that a high degree of My Ox oxygen carrier performance is essential for CLC. TMOs must therefore be materials with thermal and mechanical stability at high temperature, and demonstrate reversible reduction and oxidation reactions required for the 1000’s of CLC oxidation/reduction cycles. Well-studied oxygen carriers for CLC are 3d TMOs that include Mn, Fe, Co, Ni, and Cu-based materials. 4,6,8,9 Among them, hematite (α-Fe2 O3 ) has become a competitive candidate for its low cost, high mass (ratio) of active oxygen, and its favorable reactivities in both the fuel and air reactors shown in Figure 1. 6,9–11 Hematite is an abundant, naturally occurring stable mineral that is already used in commercial applications such as water treatment, 12 electrocatalysis, 13 and contaminant adsorption. 14 Experimental investigations of the performance of α-Fe2 O3 in the oxidation step of the CLC process have focused on using natural gas (CH4 ), 8,15–23 H2 , 24,25 and syngas mixtures that contain CO 26,27 as hydrocarbon feedstock. The composition of the hydrocarbon reactant feedstock can dictate the nature and purity

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of the oxidized product(s), as can the properties of the α-Fe2 O3 surface. For example, combining hematite with inert supporting materials, such as Al2 O3 , ZrO2 , TiO2 , NiAl2 O4 , 4,15,16,26 CeO2 17 and MgO 18 can prevent surface agglomeration during CLC. Another factor to consider is that variations in experimental conditions, such as reactor temperature and the pressure of the hydrocarbon fuel stream, will also influence the nature of the oxide surface structures formed. Careful adjustment of these parameters can help direct the reactive Fe2 O3 surface to reduce over the course of CH4 oxidation, and result in the following series of reduced forms of hematite surfaces: Fe2 O3 → Fe3 O4 → FeO → metallic Fe. 6,8,21 The first of the stepwise reductions results in magnetite (Fe3 O4 ), and it is believed that this transition is caused by an unreacted shrinking core model (USCM). 28 In the USCM, a reduced product layer (magnetite) is formed on the surface of a particle and grows inward until the shrinking core (hematite) is depleted. It is conventionally believed that magnetite will demonstrate a decreased reactivity to hydrocarbons (CH4 , syngas, coal 29,30 ) when compared to hematite, because of its decreased O/Fe ratio. However, this may not be the case for most reaction CLC reaction conditions. Reduction of hematite to magnetite will create oxygen vacancies at the surface, which should dictate a transfer of oxygen from the core to replenish (in part) some of the surface states. The mass transfer of O will cause surface reconstructions and changes in the electronic structure, which could result in potentially competitive surface reactivities towards hydrocarbons. This suggests that analogous computational investigations of hydrocarbon oxidation on both Fe2 O3 and Fe3 O4 surfaces, as well as surface with vacancies, would yield atomistic insights into the energetics of hydrocarbon oxidation reactions and how to potentially control the surface transformations that take place in CLC. The reactivity of reduced Fe2 O3 (0001) surfaces with oxygen vacancies was studied by Cheng et al. using density functional theory (DFT), and revealed a correlation between the surface defect coverage and the CH4 oxidation energy. 31,32 The reaction mechanisms in these studies initiated from CH4 dissociative adsorption of CH3 * and H* on the surface of metal

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oxides. This also implies that the initial surface geometry of iron oxides, as caused by both preparation and reaction conditions, can influence adsorption reactivity. 33,34 Surface and bulk O atoms (in the metal oxide) act as the source of oxygen for the CLC reaction, and the surface O (or O vacancies) can also act as potential adsorption sites for hydrocarbons and their dissociated products. Previous computational studies of CLC have used density functional theory DFT to model the adsorption, dissociation, and oxidation reactions of small molecules, such as CO, on Fe2 O3 35–37 and Fe3 O4 38 surfaces and CH4 on various Fe2 O3 (0001) surface terminations. 39,40 To the best of our knowledge, none have yet compared dissociation mechanisms (through distinct metal and oxygen surface sites) for Fe2 O3 and Fe3 O4 . This points towards the need to understand the interplay between metal atom oxidation state, reactivity towards oxygen, surface geometry, and temperature effects. We put forth that parallel comparisons of hydrocarbon dissociation on Fe2 O3 and Fe3 O4 could differentiate the initial reaction steps that take place (on each surface) in a CLC fuel reactor, guiding us towards a better understanding of surface specific oxidation mechanisms. Here we choose the most commonly observed surfaces for both iron oxides, α-Fe2 O3 (0001) 41,42 and Fe3 O4 (111), 43,44 both of which are oxygen rich surfaces that can provide multiple CH4 adsorption sites. We focus on the Fe and O-terminated surfaces of each iron oxide, as they have been characterized across multiple experiments 45–47 and are the most likely surfaces to be formed using the surface preparation conditions presented in Ref; 28 synthetic air of composition ≈ 80%N2 /20%O2 is introduced during surface pre-heating. It is also the case that this allows for a comparison of the reactivity of similar surface structures common to distinct bulk oxides. That is, the systems modeled here will provide insight into the role of surface structure and oxygen availability, and go beyond using bulk O/Fe ratios to assess oxygen carrier performance. Given that the typical reaction temperature for CLC is in the range of 600 to 1200 degrees Celsius, 48 we do not consider hydroxylation of the surfaces that is known to occur under ambient conditions. 41,43 In the present study, we use periodic DFT + U d+p calculations to study the geometry and energetics of CH4 dissociative adsorptions on

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α-Fe2 O3 (0001) and Fe3 O4 (111) surfaces to model the initial steps of the CLC process. Both the Fe- and O-terminated surfaces are included with two types products via heterolytic and homolytic reaction routes, as shown in Figure 2. We find that in the early steps of CH4 oxidation of the CLC process O-terminated surfaces of hematite and magnetite show a preference for dissociative adsorption when compared to Fe-terminated surfaces, and that this preference is coverage dependent. While surface reconstructions are present in all surfaces studied here, they are larger for hematite than magnetite, and are directly linked to trends in adsorption energy. We also include a reactivity comparison for hematite surfaces with O defects and link this to recent mechanistic work. 28 Furthermore, our DFT results are compared to Fourier transform infrared spectroscopy (FTIR) studies of CH3 I dissociation on hematite surfaces and determine the preferred CLC reaction route and products.

Methods Computational Details Spin-polarized DFT calculations were carried out using the QUANTUM-ESPRESSO (QE) open source software package. 49 The generalized gradient approximation (GGA) of Perdew, Burke and Ernzerhof (PBE) 50 was used as the exchange-correlation functional. All atoms were represented as ultrasoft pseudopotentials 51 with a plane wave cutoff of 35 Ry and a 280 Ry cutoff for the charge density. All atoms were allowed to fully relax during structural optimizations, in the bulk and surface calculations, with no fixed layers as per the surface relaxation procedures outlined in Ref. 52 The convergence criterion for structural optimizations are the self-consistent change in total energy below 10−3 eV and a maximum residual A−1 per atom. force of 10−2 eV ·˚ An 8×8×8 Monkhorst–Pack k-point mesh 53 was used in bulk α-Fe2 O3 and Fe3 O4 calculations of rhombohedral primitive cells (shown in Figure 3). For the α-Fe2 O3 (0001) and 7



Fe3 O4 (111) surface simulations, with and without CH4 adsorptions, the k-point mesh was reduced to 4×4×1 for 1×1 surface dimensions and 2×2×1 in for 2×2 surface dimensions. We employ a 4×4×4 k-point mesh for the surface slab calculations because the difference in total energy between a 4×4×4 and 6×6×6 k-point mesh was less than 1 meV. All surface slab calculations contain at least 15˚ A of vacuum between surface slabs. CH4 adsorption coverage tests on α-Fe2 O3 (0001) surfaces are shown in Figure 4 and 6. All the surface and CH4 adsorption configurations contain either 18 (Fe-term) or 16 (O-term) atomic layers, because modeling surface slabs without sufficient slab thickness (or by fixing part of the slab to reduce computational cost) may lead to inaccurate relaxed geometries and electronic structure. 52 The surface slabs obey inversion symmetry, which is why our figures enumerate up to 9 atomic layers. Since iron oxides are strongly correlated systems where Fe 3d and O 2p-orbitals hybridize significantly, we employ DFT + U 54–56 methodology to obtain accurate surface and electronic structures. The dependence of surface properties of α-Fe2 O3 (0001) on the details of the DFT +U approach, compared with standard and hybrid functionals, has been investigated recently. 57 The results suggest that all functionals provide similar structural details. While relative surface stabilities and details of the electronic structure are variable against the choice of exchange correlation functional, the lack of specific experimental information makes it difficult to truly assess which approach provides the most accurate description of the surface. Prior studies on bulk iron oxides 58,59 have applied U exclusively to Fe and obtained improved calculated physical properties and restored the band gap. However, making correction to only Fe 3d-orbitals can also create d-electrons that are too localized, significantly weakening the hybridization between d- and p-orbitals, 60–62 which yields inaccurate electronic states. To obtain a better description of the electronic structure it is necessary to apply a Hubbard U correction to O 2p-orbital. This is referred to here as the U d+p method, and has been used in modeling other TMO systems. 63–71 The modification of the surface specific U d+p method 72 used here, where U p =5.90 eV is not added to the top surface layer of oxygen is

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discussed in detail in Ref. 73 All U d values are derived using first-principles linear-response theory implemented by Cococcioni and de Gironcoli. 74 The specific-U d values for various Fe sites on hematite and magnetite surfaces are shown in Table 1, where the values for hematite surfaces were discussed in previous work. 72 Table 1: Surface-specific derived U d values for Fe atoms (by layer) in different surface models. All values are reported in eV. The Fe atomic layer numbering follows the scheme depicted in Figure 4, 5 and 6. α-Fe2 O3 (0001) Fe layer Fe-term O-term 1 5.90 3 4.20 5.39 4 4.26 5.56 6 4.30 4.17 7 4.22 4.00 9 4.38 4.04

Fe3 O4 (111) Fe layer Fetet1 1 5.67 3 3.95 5 4.29 6 4.56 7 4.11 9 4.27

O1 3.83 4.53 4.55 4.27 4.24

First-principles thermodynamics is used to calculate the surface free energy, γ, in the framework of Reuter and co-workers, 75–77 and is summarized in our previous work on the (0001) surface terminations of α-Fe2 O3 . 72 What we highlight here is that the surface free energies are based on chemical potentials (µFe and µO ) and DFT-computed free energies (Gbulk and Gslab ) values, with a sign convention that the more negative value of γ corresponding to a more stable surface stoichiometry. The values are used in forming a relative comparison, as inherent errors in DFT total energies prohibit considering absolute values alone.

Experimental Section Methyl iodide (CH3 I, Fisher Chemical, 99.5 %) and hematite (α-Fe2 O3 , Alfa Aesar, α-phase, 99% metal basis) were used as received. Surface area measurements using BET analysis showed a specific surface area of 23 ± 2 m2 /g of the as-received hematite. The adsorption of methyl iodide on hematite surfaces is studied using a modified Teflon coated infrared cell coupled with transmission Fourier transform infrared (FTIR) spec9



troscopy, as described in detail in previous studies. 78,79 In this experiment, approximately 5 mg of hematite was pressed onto half of a tungsten grid held by two Teflon coated jaws in the sample holder. The infrared cell sat on a linear translator which allowed either the sample (i.e., the coated portion) or blank half of the tungsten grid to be moved into the infrared beam path. The infrared cell was then evacuated for 6 hours using a turbo-molecular pump to clean the cell and sample surface. After evacuation, hematite surfaces was exposed to 2.5 Torr gaseous CH3 I under dry conditions. The single beam spectra of surface- and gas phase (250 scans) were acquired at 296 K, using a resolution of 4 cm−1 , for the spectral range of 800 to 4000 cm−1 . Data was collected using OMNIC software prior to and after the exposure of methyl iodide. Absorbance spectrum of adsorbed methyl iodide on hematite is reported as the difference in the hematite spectra before and after exposure. Absorption bands due to gas phase methyl iodide were subtracted to obtain the FTIR spectra from particle surface.

Results and Discussion General Mechanisms Figure 2 shows two types of CH4 dissociative adsorption products, labeled heterolytic and homolytic reaction pathways. The initial surface products are assumed to be CH3 and H, and it is their location on the Fe-O surfaces that differ. In the heterolytic pathway, initial CH3 and H adsorption sites are over surface Fe and O, respectively, while for the homolytic pathway initial CH3 and H adsorption sites are over different O. The heterolytic adsorption is also referred to as middle product-I since in all cases studied here, CH3 diffuses away from surface Fe to a nearby O-site. This results in final product-II, whose relaxed geometry is very close to the initial adsorption structures used to model the homolytic pathway.

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Figure 2: Two CH4 dissociative adsorption routes producing either heterolytic or homolytic products.

Bulk Iron Oxide Structures Bulk hematite, α-Fe2 O3 , is an insulator with a corundum-type structure (space group R3m), where the oxygen layers are parallel and separated by two Fe layers (shown in Figure 3a). The stoichiometric repeating unit R can be written as -(Fe–O3 –Fe)–, and the packing of R along the [0001] surface normal is shown in Figure 3b. In hematite, all Fe are octahedrally bound to six neighboring O and the spin states vary in an alternating ↑↑↓↓ manner along [0001] between oxygen layers (shown in the arrow in Figure 3b). This packing results in the anti-ferromagnetic (AFM) nature of bulk hematite. For bulk α-Fe2 O3 the U d+p method (U (Fe) = 3.81 eV and U (O) = 5.90 eV) yields calculated lattice constants are of 5.076 ˚ A and 13.820 ˚ A (Figure 3(a)), with a 3.67 µB magnetic moment per Fe, and a band gap of ∼ 2.2 eV; all of these values are in good agreement with experimental data. 80,81 Bulk magnetite (Fe3 O4 ) is a conductor at room temperature and forms in an inverse spinel structure (space group F d3m). There are two distinct types of Fe in Fe3 O4 system. A-type Fe is tetrahedrally bound to four neighboring O, represented as blue spheres in Figures 3(c) and (d)). B-type Fe is octahedrally bound to six neighboring O, represented as grey spheres. One-third of the total Fe in magnetite are found in A-sites and assigned a 3+ charge. The remaining Fe are found in B-sites and contain equal amounts of 3+ and 2+ Fe. The A- and

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Figure 3: Bulk α-Fe2 O3 structures are depicted in (a) and its side view in (b) shows the (0001) surface normal. Bulk structures of Fe3 O4 are depicted in (c) and its side view in (d) shows the (111) surface normal, as ate the notations used to specify different atomic layer terminations. Grey and blue spheres represent octahedrally- and tetrahedrally-coordinated Fe, respectively; red spheres represent O. The up and down arrows indicate the spin states of different Fe-sties repeating in periodic system. B-site Fe have opposite spin directions, so the formula of magnetite can also be written as (Fe3+ ↓)A (Fe3+ ↑,Fe2+ ↑)B O4 . This produces an overall ferrimagnetic (FM) spin structure and the repeating unit, R0 , along [111] can be written as –(Fetet1 –O1 –Feoct1 –O2 –Fetet2 –Feoct2 )–. R0 has six unique atomic layers, as shown in Figure 3(d), and the labels will be used to specify different layer terminations of the Fe3 O4 (111) surface. The U d+p method (UAd = A and 14.622 ˚ A for 4.12 eV, UBd = 3.98 eV, U p = 5.90 eV) yields lattice constants of 5.969 ˚ the hexagonal cell (Figure 3b), and a magnetic moment of 3.63 µB per primitive cell. This is in agreement with experimental values 43,82 and comparative details are discussed in the Supporting Information (SI).

Surface Models of Iron Oxides Figure 4 shows the initial configurations of α-Fe2 O3 (0001) surfaces: Fe-term, with a single Fe layer (Fe-1) as the terminal atomic layer, and O-term, where the surface is created by

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Figure 4: (a) Side views of the Fe-term and O-term α-Fe2 O3 (0001) surfaces where each atomic layer is numbered. (b) The top views of the first 3 layers in these two surfaces. The green parallelogram indicates the size of the (1 × 1) cell and the yellow dashed lines shows the size of the (2 × 2) supercell. The labels i, ii and iii indicate the adsorption sites for CH3 and H groups. The parallelograms in (c), from left to right, show the full-coverage surface with one CH4 adsorbed on one (1 × 1) cell, 50% coverage of two CH4 adsorbed on one (2 × 2) cell in a check-board manner, and 25% coverage of one CH4 adsorbed on one (2 × 2) cell, respectively. removing the Fe-1 layer to expose the O-2 layer. Each of these surfaces are considered Orich (more O-sites to interact with CH4 molecules), have been previously predicted to be stable 59,72,83–85 using ab initio thermodynamics, 75–77 and observed with low-energy electron diffraction (LEED). 47,83,86–89 Since CH4 adsorption configurations are inversion symmetric, both sides of the surface slab contribute to the total energies. Therefore the dissociative adsorption energy for a single CH4 can be calculated using the equation below:

EI/II−ads =

1 (EI/II−product − Esurface − nECH4 ) n

(4)

where n is the total number of the adsorbed CH4 . Full-coverage adsorption in the (1 × 1) cell has one adsorbed CH4 per each side of the slab, which means n = 2 in Equation 4. Lowercoverages (50% and 25%) employ a (2 × 2) super cell. 50% adsorption has two CH4 per

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each side (shown in Figure 4c), making n = 4 in Equation 4, and n = 2 for 25% adsorption with only one CH4 on each side. In Figure 4(a), different atomic layers are labeled from the exterior layer (Fe-1) to the interior layer (Fe-9) which is above the inversion center of the surface slab. Layers 1-3 are selected in Figure 4b to show adsorption sites in the (1 × 1) surface cell, which is enclosed by the green parallelogram outline. i(Fe)-CH3 & ii(O)-H indicate the adsorption sites of the heterolytic product-I, and ii(O)-CH3 & iii(O)-H indicate the adsorption sites of the homolytic product-II. Defect α-Fe2 O3 (0001) surfaces are also considered here. Figure 5a shows the O-deficient (O-def) α-Fe2 O3 (0001) surfaces, where one of the layer-2 O atoms is removed from each of the (1 × 1) cells. In Figure 5b, layers 1-3 from both surfaces are selected in (b) to show adsorption sites in the (1 × 1) surface cell, outlined by a green parallelogram outline. i(Fe)CH3 & ii(O)-H indicate the adsorption sites of the heterolytic product-I, and ii(O)-CH3 & iii(O)-H indicate the adsorption sites of the homolytic product-II. Lower-coverages (50% and 25%) of CH4 adsorption are not included in the present work.

Figure 5: (a) Side views of the O-defect (O-def.) Fe-term and O-term α-Fe2 O3 (0001) surfaces, where different atomic layers are numbered. (b) Top views of the first 3 layers in the two O-def. surfaces. The green parallelogram outline indicates the size of the (1 × 1) cell and the red dashed empty circles show the positions of the oxygen defects. The i, ii and iii labels indicate the adsorption sites for CH3 and H groups. For the Fe3 O4 (111) surfaces we investigate only the Fetet1 and the O1 terminations because both were observed in either LEED or scanned-tunneling microscopy (STM), 90–95 the 14


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Fetet1 surface was also predicted to be the most stable for a wide range of oxygen chemical potentials, and the O1 surface (with one less Fetet1 terminated layer) was stable only near the high limit of oxygen chemical potential. 96–99 We include both because they are O-rich and can be used as comparative analogs to the α-Fe2 O3 (0001) Fe- and O-term surfaces. In Figure 6(a), different atomic layers are labeled according to their depths in the surface model (like hematite in Figure 4). There are six total oxygen layers in both surface models because the surface slabs are inversion symmetric. As discussed before, the three exterior layers are selected for the top views in (b), where the green parallelogram outlines indicate the (1 × 1) surface cell. Only the full-coverage CH4 adsorption was considered in this case. For the CH4 heterolytic dissociative adsorption on both surfaces, we found the lowest energy adsorptions are for the i(Fe)-CH3 & ii(O)-H for product-I, and ii(O)-CH3 & iii(O)-H for product-II.

Figure 6: (a) Side views of the Fetet1 and O1 terminated (111) surfaces of Fe3 O4 , where different atomic layers are labeled as numbers. (b) Top views of the first 3 layers in the two surfaces. The green parallelogram outline indicates the size of the (1 × 1) cell; The i, ii and iii labels indicate the adsorption sites for CH3 and H groups. The ab initio thermodynamics phase diagram shown in Figure 7 is generated using the surface specific U d+p method discussed in Refs. 72,73 We find similarities for the four key surfaces that we focus on in this work: at the O-rich limit, both of the oxygen-terminated surfaces, O-term of α-Fe2 O3 (0001) (blue solid line in (a)) and O1 of Fe3 O4 (111) (blue dashed line in (b)) are stable surface domains (with the lowest relative values of γ), consistent 15



with experimental observations for both iron oxides. 47,72,86,91,93 The inset of Figure 7a is a correlation of the oxygen chemical potential, µrel,O , to experimentally achievable conditions. For the Fe-terminated surfaces of both iron oxides, γ < 0 meV/˚ A2 < 0 for the range of µO investigated here, meaning that they are stable surface terminations even if they are not the favored terminations. At the O-rich limit of µO , O-terminated surfaces are preferred, but as µO decreases from ambient to UHV conditions, Fe-terminated surfaces become the favored terminations of both iron oxides.

Figure 7: Ab initio phase diagram of (a) hematite Fe- and O-term, and (b) magnetite Fetet1 and O1 surfaces at 600 K. The shaded region in both phase diagrams highlights the experimentally attainable oxygen rich regime in which µrel,O is greater than ∼ -1.4 eV, as determined by the pressure conversion bar at the top of (a).

CH4 Adsorption on Ideal Iron Oxide Surfaces Figure 8 compares the side views of the four iron oxide surfaces (left hand side) for hematite (a, b) and magnetite (c, d). The relaxed structures for product-I (middle) and -II (right hand side) configurations at full surface coverage are shown next to each surface. Here we include only the top few atomic layers for clarity, where each of the atomic layers are numbered. The C–Fe/C–O distances were also listed under each side view of the relaxed product structure. We find that the dissociative adsorption of CH4 causes surface ”reconstruction,” where atomic layers can be ”pulled out” or ”pressed in” in response to a change in chemical environment. We find that atomic layer displacements are dependent on both 16


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specific adsorption sites and surface terminations. The product-I configuration (Figure 8a, middle) on an Fe-term α-Fe2 O3 (0001) surface maintains the original order of atomic layers. Here the CH3 is directly above the Fe site while the surface H–O bond is tilted. However, the product-II configuration (Figure 8a, right) has significant surface reconstructions where layer-1 Fe sinks beneath the layer-2 O. This forms a closely packed subsurface Fe-trilayer where all the the octahedral sites under layer-2 are occupied by layer-1,3,4 Fe. For O-term reaction products, the adsorbed CH3 group in product-I Figure 8b, middle) almost pulled the sublayer-3 Fe out above layer-2 O. Product-II with in-plane H–O bond does not have any significant surface reconstruction (Figure 8b, right).

Figure 8: Side views of the top three atomic layers from the optimized structures of bare surfaces and their CH4 adsorption products-I and -II for (a) Fe-term, (b) O-term, (c) Fetet1 and (d) O1 surfaces. These are for full-coverage. The atomic layers were labeled in the left in each side view. The C–Fe/C–O distances in each product are also listed. The colors of different atomic species are the same as in Figures 4 and 6. For the reaction products on the Fe3 O4 (111) Fetet1 surface, neither product-I or -II induces significant reconstruction (Figure 8c). Unlike hematite, the adsorbed CH3 group in productII did not cause layer-1 Fe sink into the surface. For O1 surface product-I, however, we 17



obtained reconstructions similar to the same product on O-term hematite. The CH3 adsorbed on one third of the layer-3 Fe sites were pulled out forming an extra layer-3’ above layer-2 O, while the other layer-3 Fe remained in their original positions (Figure 8d, middle). ProductII on the O1 surface maintains its original surface configuration (Figure 8d, right). We find that the product-I surfaces produce very similar C–Fe distances (2.00 ˚ A or above), among which only the product on O-term hematite has a longer C–Fe bond of ∼ 2.2 ˚ A. All of the product-II produce very close C–O distance of ∼ 1.42-1.44 ˚ A. The C–O bonds are 0.6 ˚ A shorter than the C–Fe bonds and most likely differentiable in experiment.

Figure 9: Changes in layer-spacings after structural optimization. Depicted here are the bare surface and adsorption configurations for full-coverage, relative to bulk values. The x-axis is the interlayer positions using the same labels shown in Figure 4 and 6; The y-axis is changes of layer-spacing calculated as percent difference. Panels (a) to (d) show Fe-term, O-term, Fetet1 and O1 surfaces, respectively. In each panel, the grey lines correspond to the bare surface. In (a) and (b), type-I results are the light blue lines and type-II results are the orange lines. In (c) and (d), type-I results are the green lines and type-II results are the dark red lines. The interlayer distances of the bare and reacted relaxed surface are also evaluated to compare with their bulk values. The results are shown in Figure 9, where for the bare α-Fe2 O3 (0001) Fe- and O-term surfaces, the changes in interlayer distance are shown as 18


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the grey lines in Figure 9(a) and (b). Fe-term hematite shows that layer-1 (Fe) are undercoordinated and therefore have relaxed into layer-2 (O) to decrease bond lengths in an effort to compensate for the lack of bonds at the surface. The bare O-term surface has no Fe layer above them to compensate their negative charge, and as a result this is considered to be a very polar surface. A consequence of this is that surface contractions mainly occurred between sublayer-3 and -4 Fe. All together, these findings are consistent with previous theoretical work. 83 After CH4 dissociation occurs on the surface, we find that product-I on Fe-term hematite helps satisfy the previously underbound layer-1 Fe, resulting in less contraction of layer-1 Fe into layer-2 O. This is shown as the blue line in Figure 9(a). On the contrary, product-II has made layer-1 Fe more underbound by adsorbing on layer-2 O. This means that the Fe have to sink into the sublayer to compensate for a lack of bonds, which is shown as the orange line in Figure 9(a). Similarly, product-II on the O-term surface satisfied the coordination of layer-2 O, causing less inward surface relaxation across the top three atomic layers. This is shown as the orange line in Figure 9(b). The adsorbed CH3 group and the layer-2 O are in competition for the bonds available within the sublayer Fe sites, resulting in a layer-3 Fe being pulled up. For bare Fe3 O4 (111) Fetet1 and O1 surfaces, the changes in interlayer distance are shown as the grey lines in Figure 9(c) and (d). The displacements are consistent with other theoretical work 96,98 where the terminal layers, layer-1 Fe on Fetet1 and layer-2 O on O1 , are under-coordinated. We observe similar surface reconstructions for product-I of the O1 surface of magnetite. The calculated CH4 dissociative adsorption energies for each of the four iron oxide surfaces are presented in Table 2, which also includes lower-coverage results for both hematite surfaces. All of the 100%-coverage reactions are exothermic except product-I on O-term surface, which is consistent with its longer C–Fe bond length shown in Figure 8. For the Fe terminated surfaces (Fe-term and Fetet1 ), product-II is about 1 eV lower in energy than product I, while for O terminated surfaces (O-term and O1 ), product-II is favored by more

19



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Table 2: CH4 dissociative adsorption energies calculated using Equation 4 for different ideal and O-defect (O-def.) α-Fe2 O3 (0001) and Fe3 O4 (111) surfaces. All of the adsorption energy values are in units of eV. Product-I Surface CH4 coverage Ideal O-def. Hematite Fe-term 100% -0.104 -0.874 50% -0.014 25% +0.077 O-term 100% +0.280 -0.545 50% -0.114 25% -0.415 Magnetite Fetet1 100% -0.254 O1 100% -0.597

Product-II Ideal O-def. -0.927 -2.201 -0.351 -0.199 -3.799 -2.955 -3.973 -4.223 -1.326 -3.662

than 3 eV. This implies that even the computed surface reconstruction would not be a barrier for the dissociative CH4 reaction on O terminated surfaces, or the formation of the type-II product. Two factors dominate the reaction energy. First, O terminated surfaces are more polar and prone to a high degree of reactivity; and second, product-II involves more adsorbate-surface band overlap than product-I. This overlap is due to the closer 2p-orbital energy levels between O and C, where the Fe 3d- and C 2p-orbitals have sizable energy differences. For O-term hematite with surface coverages lower than 100%, the reaction energies were further decreased for both product-I and -II, by up to 0.5 ∼ 0.6 eV for 25% coverage. This means that when the adsorbates have more space to separate from each other, reactions will be more favorable. For Fe-term hematite surfaces, both product types tend to stabilize each other when adsorbate coverage increases. The reaction energy of product-I on Fe-term switched from endothermic to exothermic when the coverage increases from 25% to 100%, and for product-II the reaction energy is lowered by ∼ 0.7 eV. Temperature can be a significant contribution of the reaction energy in the CLC process, as demonstrated by the high reactivity of CH4 oxidation over α-Fe2 O3 surfaces at temperatures up to 1000 K. 8,9,15,16,18,19,21 In the case of a molecular dissociation reaction, as 20


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considered here, the dominant thermodynamic consideration is the entropy of the gas-phase molecule. In going from low temperature to 1000 K, the calculated entropy of CH4 changes by > 50 cal/(mol·K), amounting to a T ∆S contribution of ≈2 eV 100 at the high temperatures (1000 K) relevant to CLC operation. In conjunction with the values of dissociative chemisorption reported in Table 2, this suggests that only product-II at oxygen sites will persist at high temperature. The interpretation is that the reactivity of the oxygen sites is then critical to enabling a favorable dissociation processes at high temperature, thus we aim to explain the differences in DFT energetics for product-I and II through electronic structure. We analyze the charge density of the surfaces with adsorbates through state-by-state atom-projected density of states(PDOS). The PDOS for each of the bare hematite and magnetite surfaces are shown in the top portions of Figures 10 and 11, respectively. In the middle and bottom portions are for adsorption configurations at full-coverage. Figure 10 shows the PDOS results for Fe- and O-term surfaces, and Figure 11 shows the PDOS results for Fetet1 and O1 surfaces. For each PDOS plot, the upper panel shows the comparison between the Fe 3d- and O 2p-orbitals of the top few layers of the corresponding bare surface, and the C 2p- and H 1s-orbitals in CH4 molecules before adsorption. The middle and bottom panels show the orbitals of the atoms directly involved in the formation of new bonds in product-I and -II. For Fe-term adsorptions in Figure 10(a), the formation of layer-1 Fe–C bond in product-I splits the C 2p-orbital into two bands to interact with Fe spin-up 3d-orbital, one at -5.5 eV, as bonding orbital, and another non-bonding orbital near the Fermi level. In Figure 10(b) on O-term surface, it is layer-3 Fe spin-down 3d-orbital that binds C 2p-orbital at -5.5 eV, and forms anti-bonding orbital at ∼ 2 eV. The band gap was filled by layer-3 Fe spin-up 3d-orbital. For both of the product-II at the bottom in Figure 10(a) and (b), C and O (red line) 2p bands are smearing upon a wide range of energy levels through the valence band between -8 to -1 eV, without strong anti-bonding above the Fermi level. The C 2p-orbital has strong hybridization with the 2p-orbital from lattice O, in contrary with the cases in

21



Figure 10: PDOS comparison between (a) Fe-term and (b) O-term hematite surfaces at full-coverage. The upper panels show the PDOS of clean surfaces and CH4 molecule before reaction, and the middle and bottom panels are showing corresponding PDOS from the orbitals directly involved in new bond formations in product-I and -II, respectively. The layer positions follow the labels shown in Figure 4.

22


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both of their corresponding product-I where C 2p-orbital largely maintain discrete features as gas molecules, indicating poorer interaction between C and Fe.

Figure 11: PDOS comparison between (a) Fetet1 and (b) O1 magnetite surfaces at fullcoverage. The upper panels show the PDOS of clean surfaces and CH4 molecule before reaction, and the middle and bottom panels are showing corresponding PDOS from the orbitals directly involved in new bond formations in product-I and -II, respectively. The layer positions follow the labels shown in Figure 6. For the PDOS of Fetet1 and O1 adsorptions in Figure 11(a) and (b), the Fe 3d-, O and C 2p-orbitals on the top few atomic layers show similar distribution trend with hematite cases. However, in product-I formation, the C 2p-orbital interacts with both of the Fe spin-up and -down 3d-orbitals, which results in stronger C–Fe interaction on magnetite (111) surfaces. Both of the bare Fe terminated surfaces have a larger band gap, while both of the polar bare O terminated surfaces have no band gap, indicating the latter are more reactive.

23



Product-I always has a smaller band gap, or states on Fermi level, while product-II tended to open the band gap. In conclusion, the O terminated surfaces are more reactive, with stronger interaction between the O and C 2p-orbitals at similar energy levels. Those factors contribute to the energetically favored product-II formations on O terminated surface of both of the two types of iron oxide.

CH4 Adsorption on Defect Hematite Surfaces Recent work on oxygen deficient iron oxide surfaces has demonstrated that vacancies are a key piece in CH4 reactivity, 31 since they can act in multiple roles; as adsorption sites, to modify the geometric and electronic structure of the surface and to mediate the phase transitions that take place over the course of the CLC process. 28 Ref. 28 discusses how the unreacted shrinking core model 101 describes the transition of α-Fe2 O3 surfaces to reduced forms that are closer in stoichiometry Fe3 O4 . Here, we focus on the structure of and dissociative adsorption to oxygen deficient hematite. Figure 12 depicts the side views of both of the α-Fe2 O3 (0001) O-deficient (O-def.) Fe-term and O-term surfaces as well as their corresponding product-I and -II configurations. The dissociative reaction energy results are shown in Figure 13 and Table 2. Here we include only the top five atomic layers, with layers numbers labeled in Figure 12, for clarity. The C–Fe/C–O bond length information are also provided under each product side view. For the bare O-def. hematite surfaces and their corresponding product-I, there is no atomic-layer being pulling out or pressed down to compensate for underbound surface species. This can also be examined in Figure 13(a) and (b) where the grey lines indicate the O-def hematite surfaces, varying from the ideal surface (black lines, shown as reference). The Odef. surface contraction/expansion on the top few layers varies only slightly when compared to the ideal case. However, the O-def. Fe-term product shows significant re-optimization where the layer-2 O splits into two distinct layers, which was not observed on the ideal Fe-term hematite surface (see Figure 8). The lack of one layer-2 O can not balance the 24


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Figure 12: Side views of the top five atomic layers from the optimized structures of bare surfaces and their CH4 adsorption products-I and -II as (a) O-defect (O-def.) Fe-term and (b) O-def. O-term in (1 × 1) full-coverage using specific-U d+p (no U p on top O-layer). The atomic layers are labeled in the left in each side view. The C–Fe/C–O distances in each product were also listed. The colors of different atomic species are consistent with those shown in Figure 5. net positive charge of the adsorbed H and the layer-1 Fe, so there is a strong electrostatic repulsion on the top layers. The binding between C and Fe further induces Fe to be pulled away from the surface. This relaxation resulted in significant layer expansion toward the vacuum shown in Figure 13(a), blue line. On the contrary, the product-II in (a) shows more inward reconstruction, comparing with its ideal case: layer-4 Fe were pushed down to the same level of sublayer-5 O.

Figure 13: Changes in layer-spacings caused by structural optimizations on O-defect Fe-term and O-term surfaces and adsorption products for full-coverage, relative to bulk values. The x-axis indicates interlayer positions shown in Figure 5; The y-axis indicate changes of layerspacing in percentage. Black and grey lines show the results of the corresponding bare ideal and O-defect surface, respectively. Results from product-I are in light blue lines and -II are in orange lines. On the O-def. O-term hematite surface, product-I shows little deviation from the relaxed 25



bare surface (show in Figure 12(b), as a blue line). The lack of one layer-2 O caused the CH3 adsorption to be more favorable, with less spacial hindrance to bind with sublayer-3 Fe when compared to the ideal surface. In addition, the adsorption of a CH3 group compensates the under-bound layer-3 Fe, resulting in an exothermic reaction energy of -0.545 eV. Inspection of Table 2 shows that the dissociative adsorption of CH4 on oxygen deficient hematite is energetically more favorable in most of the cases investigated here. The exception is for product II on the O-term surfaces; adsorption is ≈ 0.80 eV higher in energy on the defect surface, but is still ≈ -3.0 eV. The trends in Table 2 suggest that even if product I is initially formed on a defect hematite surface, which was unfavorable on the pristine hematite surface, the reduced surface will be reactive towards adsorption and may be further indication of the competition adsorption between hematite and magnetite.

FTIR spectra of CH3 I Adsorption on Hematite To compare the DFT-calculated product preferences with experimental observations, FTIR measurements were carried out using methyl iodide (CH3 I) as precursor for adsorbed methyl groups on hematite. 102 The FTIR spectrum of CH3 I adsorption on hematite is shown in Figure 14. The peak assignments for the vibrational modes of adsorbed methyl groups following CH3 I adsorption are given in Table 3. The DFT-calculated vibrational frequencies for the four products on Fe2 O3 (0001) Fe- and O-term surfaces are also included. Since the phonon calculation has not been incorporated with the DFT + U approach, the calculation results were obtained using the DFT-GGA functional. Exposure of hematite to CH3 I leads to a negative peak at 3705 cm−1 which is due to terminated Fe-OH groups hydrogen bonding with CH3 I. The broad absorption band between 3000 to 3600 cm−1 corresponds to the O-H vibration mode between adsorbed CH3 I and surface hydroxyl groups involved in hydrogen bonding, suggesting that some CH3 I molecularly adsorbed on hematite via hydrogen bonding. The absorbance band at 3052 cm−1 is due to the C-H stretching from adsorbed CH3 I molecules. In addition, a new band formed at 1057 cm−1 (as shown in Figure 14) which can 26


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be assigned to O-C stretching, suggesting that Fe-OCH3 species also formed after adsorption. Adsorption of CH3 I on hematite can be represented by R1 and R2:

CH3 I + Fe − OH → Fe − OH · · · ICH3 (R1)

(5)

CH3 I + Fe − OH → HI + Fe − OCH3 (R2)

(6)

Generally, product-II on both hematite surfaces have better agreement with experimental results. As shown in Table 3, the FTIR spectrum are in good agreement with literature values. The calculated stretching frequencies of the surface Fe-OH groups in each of the four products have up to 115 cm−1 variation from thee FTIR measured value centered at 3705 cm−1 . However, the hematite particles surface used in FTIR experiments were hydroxylated before reaction. 103 Therefore, 3705 cm−1 is an averaged value before and after the reactions. Our Product-II on Fe-term has the closest OH stretching at 3767 cm−1 . The calculated asymmetric and symmetric C-H stretching frequencies are generally ≈ 100 cm−1 higher than the experimental values, while the Product II of both surfaces show closer results. The CH3 group in-plane scissoring and rocking show more consistent results with literature values except that Product-I on Fe-term exhibits about 50 cm−1 lower frequencies. The formation of surface O-C bonds suggested by the absorbance band at 1057cm−1 in Figure 14 and Table 3, is in good agreement with our calculated vibrational modes for Product-II on both surfaces; 1031 cm−1 on the Fe-term surface and 1018 cm−1 on the O-term surface. However, no O-C bond formed from the calculation of Product-I on either surface, which is consistent with our energetic results that CH3 groups prefer to bind to surface O-sites to form surface methoxy.

27



Figure 14: FTIR spectrum of 2500 mTorr CH3 I(g) adsorbed on α-Fe2 O3 .

Conclusions As CLC methods have become a growing area of clean combustion technology, we studied the energetic and geometric results for the early step of CH4 dissociative adsorption on different α-Fe2 O3 (0001) and Fe3 O4 (111) surfaces. We consider the Fe and O-terminated surfaces that are most likely to be formed under the experimental surface preparation conditions given in the CLC studies of Ref. 28 Our results show that the homolytic adsorption route is the preferred reaction mechanism for CH4 dissociation in a CLC chamber, so both CH3 and H will adsorb onto surface O and not surface Fe. Computed reaction energies on O-terminated surfaces of hematite and magnetite are both energetically favored, which is consistent with our FTIR spectra of CH3 I dissociation on hematite at 296 K. As CLC is an inherently high-temperature process, temperature effects must also be considered in evaluating the DFT results. Dissociative adsorption, which is associated with the CLC mechanism, involves a positive value of ∆S that, at the high temperatures of CLC operation, can contribute at least ≈2 eV to the adsorption energy. This temperature effect

28


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Table 3: Comparison of the IR assignment between DFT-GGA calculated frequencies, FTIR (CH3 I precursor) measured and literature values on hematite surface. Mode assignment ν(OH) νas (CH) νs (CH) δ(CH3 )

ρ(CH3 ) ν(OC)

Frequency (cm−1 ) Fe-term O-term FTIR Prod.-I Prod.-II Prod.-I Prod.-II 3629 3767 3615 3590 3705 3052 2988 3089 3027 2946 3038 2976 3084 3014 2920 2940 2900 2956 2925 2821 1388 1460 1407 1437 1428 1376 1455 1397 1433 1405 1412 1096 1138 1135 1137 1175 1127 1125 1031 1018 1057

Literature

2927, 104 2926, 105 2922 106 2888 107 2832, 104 2819, 108 2818 106 1443 105

1150, 109 1155, 108 1142 106 1057 108

stemming from the entropy of the gas-phase adsorbate outweighs other competing factors from the surface. The DFT results presented here suggest that only dissociation over the oxygen sites of hematite and magnetite provide sufficient enthalpy of reaction to overcome the positive change in entropy that accompanies the dissociative chemisorption of CH4 . After hematite is reduced in the fuel reactor of the CLC process, magnetite surfaces form, and our DFT calculations find that they are also capable of oxidizing fuel gas molecules. Based on adsorption energies, we believe that the two materials may yield competitive combustion performances in the CLC process. This demonstrates the need to consider surface structure and surface mechanisms in assessing carrier performance, and that the bulk O/Fe ratio alone is not a reliable metric. The similar adsorption between hematite and magnetite, as shown in our comparative studies, is supported by recent reactivity studies 28 that link the two phases using an unreacted shrinking core model. We also model oxygen deficient surfaces and find that the adsorption energies are still exothermic. This indicates that even if the iron oxide materials are being actively reduced in the CLC process, the process should not incur an efficiency penalty while the oxides are still close to hematite and magnetite. An implication here is that stopping the CLC process before reduction to FeO occurs may 29



be beneficial in quickly regenerating the iron oxide catalysts, improving the life cycle of the process.

Associated Content Supporting Information: The PDOS comparison of (1 × 1) O-defect Fe-term and O-term surfaces are shown in Figure S1. A systematic comparison of the calculated physical properties of bulk Fe3 O4 (Figure S2) is presented in Table S2 and PDOS in Figure S3.

Author Information Corresponding Author †

These authors contributed equally to this work. ∗ (S.E.M.) E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgments This work was supported by National Science Foundation (NSF) Grant CHE-1509432 and the University of Iowa, College of Liberal Arts and Sciences. This research was supported in part through computational resources provided by The University of Iowa, Iowa City, Iowa and the National Science Foundation grant CHE-0840494. This work used the Extreme Science and Engineering Discovery Environment (XSEDE 110 ), which is supported by National Science Foundation grant number ACI-1548562 through allocation ID TG-GEO160006.

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Figure 15: TOC Graphic

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Methane Dissociation on Î±-Fe2O3(0001) and Fe3O4(111) Surfaces

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