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Adsorption, Desorption, and Reaction of Methyl Radicals on Surface Terminations of r-Fe2O3† Li Liu,§ Brian R. Quezada,| and Peter C. Stair*,‡ Departments of Chemistry and Materials Science and Engineering, Northwestern UniVersity, EVanston, Illinois 60208, Chemical Sciences and Engineering DiVision, Argonne National Laboratory, Argonne, Illinois 60439, and Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843 ReceiVed: April 29, 2010; ReVised Manuscript ReceiVed: August 16, 2010
The adsorption, desorption, and reaction of gas phase methyl radicals were studied on the (0001)-oriented R-Fe2O3 surface in ultrahigh vacuum. Two different surface terminations were compared: An Fe3O4 (111) layer and the so-called “biphase” surface thought to be a mixture of FeO and Fe2O3 terminations. Gas phase methyl radicals were prepared by pyrolysis of azomethane. On Fe3O4 (111) methyl radical adsorption forms surface methoxide species as determined by the C(1s) XPS binding energy. Temperature programmed reaction spectroscopy produced direct desorption of methyl radicals at all coverages and the formation of ethane at high coverages in two desorption peaks at 331 and 439 K. The activation energies for desorption were 84 and 133 kJ/mol in the two regimes. The two surface terminations exhibit saturation coverages that differ by ca., 30×: 1.5 × 1014 and 5.2 × 1012 per cm2 for the Fe3O4(111) and “biphase” terminations, respectively. These results are interpreted in terms of bonding models and differences in atomic structure for the two terminations. 1. Introduction The surface chemistry of methyl radical has received considerable attention in recent years because of its relevance to the catalytic utilization of methane.1 Methyl radical is proposed to be the surface reaction intermediate in the partial oxidation of methane to methanol and formaldehyde,2,3 and the gas phase intermediate in the oxidative coupling of methane to higher hydrocarbons.4 To unravel the details of a gas-solid interface reaction mechanism, the preferred approach is to employ welldefined single crystal surfaces under the ultrahigh vacuum (UHV) conditions. This approach has been used to understand the interactions of methyl radicals with metal and oxygenmodified metal surfaces.5-13 In spite of the fact that the partial oxidation and oxidative coupling of methane are carried out on metal oxide catalysts, there has been no experimental report on the interactions between the methyl radical and well-defined metal oxide surfaces under UHV conditions. The present paper describes the first such study. Supported iron oxide catalysts have shown activity for the partial oxidation of methane to synthesis gas.14 In addition, iron oxides are used as catalysts in the high temperature carbon monoxide shift conversion, as well as the dehydrogenation of ethylbenzene.15 More importantly, extensive studies have been performed with the goal of determining the geometric16-22 and electronic23-26 structures of different terminations of R-Fe2O3 surfaces. The possibility of relating the surface reactions to the corresponding surface geometric and electronic structures provides the opportunity to understand the methyl radical surface reactions at the molecular level. †
Part of the “D. Wayne Goodman Festschrift”. * Corresponding author. Tel: 1-847-491-5266. Fax: 1-847-467-1018. E-mail:
[email protected]. ‡ Department of Chemistry, Northwestern University and Argonne National Laboratory. § Texas A&M University. | Department of Materials Science and Engineering, Northwestern University.
It has been shown that by changing sample preparation conditions an R-Fe2O3 (0001) single crystal can be terminated by a thin layer of FeO (111),16 Fe3O4 (111),17,19,27 or the socalled “biphase”18 structure owing to the crystallographic similarities of the oxygen sublattices in FeO (111), Fe3O4 (111), and R-Fe2O3 (0001) single crystals orientated along the indicated directions. In this paper, we present temperature programmed reaction spectroscopy (TPRS) studies on Fe3O4 (111) and “biphase” terminated surfaces after methyl radical exposure. Based on quantifying the saturation coverages obtained on these two surfaces, methyl radicals are proposed to adsorb at regular surface sites on the Fe3O4 (111) surface but at defect sites on the “biphase” surface. Both C(1s) XPS binding energies and the carbon-surface bond energies are consistent with formation of a surface methoxide species. This behavior is shown to be consistent with methyl radical surface reactions proposed by Lunsford,28,29 a simple molecular orbital picture of the C-O bond, and recent structural models for the R-Fe2O3 (0001) surface terminations.30 2. Experimental Section 2.1. Instrumentation. The experiments were performed in two separate UHV chambers with base pressures of 1-3 × 10-10 Torr. The chamber used for TPRS experiments has been described previously.31 It was equipped with low energy electron diffraction (LEED), also used for retarding field Auger electron spectroscopy (AES), and a sputtering gun. An EXTREL quadrupole mass spectrometer (QMS), oriented perpendicular to the sample normal, was used to perform the TPRS experiments and residual gas analysis. The QMS ionizer and top portion of the quadrupole assembly was surrounded by a cylindrical glass shield with a 5 mm aperture in the side pointing at the sample. The shield discriminates against detecting desorption from surfaces other than the sample. The closed, axial construction of the ionizer blocks direct line-of-sight to the sample and minimizes exposure of the sample to hot electrons. The chamber used for XPS was equipped with a VG Microtech
10.1021/jp1039018 2010 American Chemical Society Published on Web 09/03/2010
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hemispherical electronic energy analyzer and a dual-anode (Al/ Mg) X-ray source, LEED, sputtering, and a QMS. 2.2. Sample Mounting. A 10 × 10 × 1 mm R-Fe2O3 (0001) single crystal sample (Commercial Crystal Laboratories Inc.) was polished on one side to a mirror finish with an orientation within (0.5° of the crystallographic (0001) plane as confirmed by Laue X-ray diffraction. The other side of the sample was attached to a 0.125 mm thick nickel (Ni) foil (Goodfellow Corp., annealed, 99.98% purity) by ceramic glue (Dupont 7095) mixed with 60 wt % fine gold powder. The gold enhances the electrical and thermal conductivity of the ceramic glue and bridges the difference in thermal expansion coefficients between the semiconductive R-Fe2O3 sample and the metallic Ni foil. Two 0.5 mm diameter Tantalum (Ta) wires (Goodfellow Corp., annealed, 99.9% purity) were spot-welded to the back of the Ni foil and attached to the sample holder. The sample was heated resistively by passing electric current through the Ta wires. The sample temperature was measured by a K-type thermocouple spotwelded to the Ni foil. Details of sample heating and cooling can be found in previous publications.32,33 2.3. Methyl Radical Dosing. Methyl radicals were generated by pyrolysis of azomethane.34 In this study, the quartz tube described in previous publications7,34 was replaced by a single crystal sapphire tube (1.5 mm ID × 157 mm long, Saphikon Corp.) to eliminate contamination by sodium which is a common impurity in the fused silica pyrolysis reactors. The pyrolysis product distribution was found to be very similar to the results of reference 34 when operated at a temperature of 1143 K. The axis of the sapphire tube was aligned with the center of the aperture in the QMS glass shield. The sample could be rotated into this path either facing the sapphire tube or the QMS. Portions of the manipulator could also be positioned in this path. The QMS signal for a nonreactive gas such as ethane was found to be higher by a factor 9.6 when the path between the sapphire tube and the shield aperture was direct, line-of-sight than when the gas flow was scattered by either the sample or a portion of the manipulator. In the latter case we believe that the QMS registers only the background gas. The value 9.6 was taken to be the pressure enhancement factor in subsequent calculations of methyl radical exposures. Interference from adsorption by the side products of azomethane pyrolysis, such as CH4, C2H6, N2, and H2, can be excluded on the basis of dosing experiments on a room temperature sample using these molecules. 2.4. Preparation of the Fe3O4 (111) and “Biphase” Terminations. X-ray fluorescence spectroscopy indicated that the mineral single crystal R-Fe2O3 (0001) sample had the composition 98.6% Fe2O3, 0.44% Cr2O3, 0.16% TiO2, and 0.14% CoO. The surfaces were cleaned by cycles of Ar+ sputtering (1 KeV energy at 10 µA) followed by electron beam annealing at 1173 K in various oxygen partial pressures. Impurities, such as chromium, titanium, and cobalt, were easily removed from the surface. However, 30 cycles of sputtering and annealing were required to lower the surface calcium level below the AES detection limit. A sharp hexagonal LEED pattern, identical to previous reports,16 appeared after the sputtered surface was annealed in UHV at 1173 K for 30 min. The surface termination associated with this LEED pattern is a selvedge of Fe3O4 (111) on top of either R-Fe2O3 (0001) for single crystal mineral samples16,19,27,35 or Pt (111) for epitaxially grown thin film samples.17,22 A “floreted” LEED pattern was observed after the sputtered sample was annealed in 1-5 × 10-6 Torr O2 at 1173 K for 30 min. The interpretation of this LEED pattern remains controversial. The satellite spots have been attributed to multiple
Liu et al. scattering at either a FeO (111)/R-Fe2O3 (0001) interface16 or a Fe3O4 slab/R-Fe2O3 (0001) interface30 or to diffraction from an ordered superlattice composed of FeO (111) and R-Fe2O3 (0001) islands.18 This surface has been dubbed “biphase” and will be referred to as such in the remainder of the paper. While the “biphase” designation has been applied primarily to the superlattice structure model, it should be pointed out that both of the multiple scattering interface models also involve two phases in the surface region. 2.5. TPRS Experiments. After surface preparation, the sample was aligned with the hot methyl radical source. Radiative heating of the sample above room temperature was partially compensated by initially cooling to 290 K with a liquid N2 reservoir attached to the manipulator. In a typical experiment the sample temperature rose to ∼310 K during methyl radical exposure. Methyl radical exposures were calculated from the ion gauge pressure readings, corrected for the typical pyrolysis gas composition, after accounting for the methyl radical mole fraction in the dosing gas and the pressure enhancement factor. For TPRS measurements, the sample was positioned ∼5 mm from the orifice of the QMS shield and heated resistively at 4 K s-1. To obtain a measure of the coverage that can be compared to the density of surface sites, the yield of desorbed gas molecules was estimated from the time-integrated peak intensities measured in TPRS. The QMS signal for a flux of ethane into the glass shield aperture was calibrated using the ion gauge pressure reading appropriate for ethane, the measured pressure enhancement factor of 9.6, and the effective pumping speed determined from the pumping time constant by the method of Johnson and Madix.36 The details of this procedure can be found in reference 32. QMS signals for the other desorbed products were calibrated against ethane from their published ionization cross sections as describe below. 2.6. XPS Measurements. The C(1s) XPS peak position and intensity were used to identify the methyl radical binding site and estimate the coverage, respectively, on the Fe3O4 (111) terminated surface. C(1s), O(1s), and Fe(2p) core level peaks were measured at normal emission on the clean surface and after saturation methyl radical coverage at room temperature. Shifts in the measured C(1s) binding energy due to sample charging were taken into account by assigning a binding energy of 530.0 eV to the O(1s) peak. 3. Results and Discussion 3.1. TPRS Spectra on the Fe3O4 (111) and “Biphase” Terminated Surfaces. The TPRS spectra are shown after exposing the freshly formed Fe3O4 (111) termination to 160 L (1 L ) 1 × 10-6 Torr s) of methyl radicals (Figure 1a) and the freshly formed “biphase” termination to 123 L of methyl radicals (Figure 1b). Note that the vertical scale is expanded by 3X in Figure 1b compared to Figure 1a. On both surfaces, desorption features for ions with various m/e ratios were monitored over a series of experiments to search for oxidation or dehydrogenation reaction products. No significant desorption peak is observed from the “biphase” termination. On the Fe3O4 (111) termination, only ions with m/e ratios of 15, 16, 28, 29, and 30 exhibit desorption features. These ions can be divided into two groups. Group one includes ions with m/e ratios of 15 and 16, and group two includes ions with m/e ratios of 28, 29, and 30. For both groups of ions, a low temperature feature peaked at 331 K, and a high temperature peak at 439 K was observed. Direct methyl radical desorption has been observed previously from an oxygen modified molybdenum (110) surface after
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Figure 1. (a) TPRS from the Fe3O4 (111)-terminated surface after a methyl radical exposure of 160 L. (b) TPRS from the “biphase”terminated surface after a methyl radical exposure of 123 L. Only ions with m/e values of 15, 16, 28, 29, and 30 showed desorption features. Note that the total intensity scale in (a) is 3× larger than in (b).
Figure 2. (a) TPRS of ions with m/e of 28 (C2H6) from the Fe3O4 (111)-terminated surface after various methyl radical exposures. (b) TPRS of ions with m/e of 15 (CH3) from the Fe3O4 (111)-terminated surface after various methyl radical exposures. Saturation coverage on this surface is achieved at 11.2 L exposure.
dosing with methanol.37 The TPRS signal for ions with the m/e value of 16 was attributed to methane molecules formed in a reaction of methyl radicals with hydrogen adsorbed on the shield over the QMS. In our case, because the shield constrains gas phase methyl radicals in a limited volume, the chance that methyl radicals react with adsorbed hydrogen atoms is greatly enhanced. For group one ions, the intensity ratio with m/e value of 15 to m/e value of 16 is 0.9 for both desorption features. While ions with m/e value of 16 are generated by methane, ions with the m/e value of 15 can be due to either methane molecules or methyl radicals.34 The measured 15-16 intensity ratio is 0.54 for methane in our system. Because the 15-16 intensity ratio measured from the TPRS spectra is much bigger than that measured from pure methane, at least part of the ions with m/e value of 15 must be due to the direct desorption of methyl radicals from the surface. Furthermore, based on the fact that the shapes and positions of both low temperature and high temperature desorption features at 15 and 16 amu are the same, it is concluded that only methyl radicals desorb from the surface. Formation of methane on the surface involves reaction between adsorbed methyl and either surface hydrogen or another methyl
(methyl disproportionation). Such a surface reaction should produce desorption features that differ in position or shape from the species at 15 amu. Moreover, if surface hydrogen or the products of methyl dehydrogenation were present on the surface, additional desorption products such as H2, C2H4, and CO should be observed. No additional products were detectable up to a sample temperature of 800 K. As a consequence, we conclude that methyl radicals desorb directly from the surface. For group two ions, the intensity ratios of ions with the m/e values of 28, 29, and 30 for both low temperature and high temperature features are attributed to desorption of ethane. Ethane desorption spectra at different methyl radical exposures are shown in Figure 2a. The thermal desorption of ethane is only observed at saturation methyl radical coverage, which corresponds to a methyl radical exposure of 11.2 L or above (Figure 2b). This suggests that ethane is the product of methyl radical recombination on the surface. Under UHV conditions the recombination of methyl radicals in the gas phase is extremely unlikely. The observation that methyl radical and ethane desorption spectra resemble each other and the fact that ethane is produced only at saturated methyl radical coverage suggest that their kinetics are controlled by carbon-surface bond
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TABLE 1: Saturation Coverage of Adsorbed Methyl Radicals on Fe3O4 (111) and “Biphase” Terminated Surfaces •
CH3 exposures (L)
adsorbed •CH3 per cm2
160 123
1.5 × 1014 5.2 × 1012
Fe3O4 (111) “biphase”
breaking and that methyl radicals have limited mobility so that they react with each other only when in close proximity in the adsorbed state. 3.2. Estimation of Adsorbed Methyl Coverage. The QMS signal was calibrated to provide an ethane desorption yield as outlined above and described in reference 32. For the other products, such as methane and methyl radical, the yields were calculated using values of their ionization cross sections relative to ethane. The time-integrated QMS peak intensity IM can be written as
IM ) σIeDC TMGM
(1)
where TM is the transmission efficiency for ions with mass M, GM is the multiplier gain for ions with mass M, σ is the ionization cross-section for the molecule generating ions with mass M, Ie is the electron emission current of the QMS ionizer, D is the ionizing path length, and C is the concentration of the molecules. TM and GM for all of the fragment ions from ethane, methane, and methyl radical were approximately constant due to their small mass differences. Therefore, the concentration ratio of molecule A to B, CA/CB equates to the ratio of their absolute yields, NA/NB, according to
CA CB
NA IMA /σA MB ) ) MA NB IMB /σB
MB MA
(2)
where B stands for ethane and A stands for methane or methyl radical in this case. Ionization cross sections for ethane, methane, and methyl radical can be obtained from the National Institute of Standards and Technology (NIST) database. The saturation coverage of adsorbed methyl radicals per cm2, calculated from the sum of the detected methyl radicals, methane, and ethane, are listed in Table 1 for the Fe3O4 (111) and “biphase” terminated surfaces. Approximately 80% of adsorbed methyl is in the high temperature state. The saturation coverage on the Fe3O4 (111) terminated surface corresponds to approximately one methyl group for every two surface unit cells. It is comparable to the saturation coverage of 6 × 1014 per cm2 observed on a partially oxidized Mo (100) surface.7 This coverage implies that methyl radicals adsorb at regular surface sites. On the other hand, methyl radical thermal desorption peaks from the “biphase” termination are very small even at large exposures, and the corresponding saturation coverage is approximately 30× smaller than on the Fe3O4 termination. This suggests that methyl radicals adsorb at minority or defect sites on the “biphase” termination. From the data shown in Figure 2, saturation coverage is evidently produced at an exposure between 3 and 11 L. This corresponds to a sticking probability in the neighborhood of 0.02. This value is comparable to values of 0.002 and 0.1 obtained for methyl radical adsorption on O/Mo(100)38 and clean Ni(100),39 respectively.
Figure 3. C(1s) XPS peak measured from the freshly formed Fe3O4 (111)-terminated surface (gray line) and from the Fe3O4 (111)terminated surface after 160 L exposure to CH3.
3.3. XPS C(1s) Measurements. Figure 3 shows the C(1s) peaks measured on Fe3O4 (111) terminated surface at room temperature before and after saturation exposure to methyl radicals. The binding energy scale has been referenced to an O(1s) BE of 530.0 eV to account for sample charging. The small, broad carbon feature on the freshly formed surface is due to residual carbon that is not removed after 15 cycles of sputtering and annealing. The C(1s) peak intensity increased significantly after saturation methyl radical exposure. The C(1s) peak has a measured binding energy of 286.1 eV which is very close to the reported binding energy, 285.7 eV, for surface methoxide on oxygen-modified Mo(110)37 and significantly higher than the value reported for methyl bonded to surface Mo-atoms on oxygen-modified Mo(100), 284.6 eV.40 Therefore, it is apparent that methyl radical adsorbs on the Fe3O4 (111) termination by bonding to surface oxygen to form a surface methoxide species. The saturation carbon coverage was estimated using the procedure in reference 41 and the published sensitivity factors42 for carbon and oxygen and the electron inelastic mean free path (IMFP) at the O(1s) kinetic energy to compute the number of oxygen atoms that contribute to the O(1s) intensity.41 The value obtained is 5.7 × 1014 cm-2 with an uncertainty of 170% due to the scatter in IMFP. Given the uncertainty this value is in reasonable agreement with the result obtained by TPRS and serves to validate the TPRS method. 3.4. •CH3-Surface Bond Energy. It is apparent that methyl radicals adsorbed on the Fe3O4 (111) terminated surface desorb after carbon-oxygen bond scission in surface methoxide. The activation energy for this reaction is equal to the •CH3-surface oxygen bond energy plus the activation energy for methyl radical adsorption. We expect adsorption to have little or no activation energy as is typical for radical addition reactions. Consistent with this view, the activation energy for methyl radical adsorption on ZnO was measured to be only 11 kJ/mol.29 Threshold TPRS analysis43 was applied to calculate the activation energy for the high temperature peak. A simple Redhead analysis44 using the peak temperature was applied to the low temperature peak for which there was insufficient data to perform threshold TPRS analysis. The activation energies for desorption in the low and high temperature peaks are 84 and 133 kJ/mol respectively. By comparison the C-O bond energy reported for surface methoxide species on the oxygen modified Mo (110) surface is 145 kJ/mol.45 The C-O bond energies measured on Fe3O4 (111) and oxygen modified Mo (110) surfaces are very similar, but much
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Figure 4. (a) Side view of the Fe3O4 (111)-(1 × 1) unit cell. (b) Top view of Fe3O4 (111) surface terminated as Fetet1-O1. (c) Top view of Fe3O4 (111) surface terminated as Feoct2-Fetet1-O1. In (b) and (c), oxygen atoms, Fetet1 atoms, and Feoct2 atoms are represented as big open circles, small black circles, and small gray circles respectively. The cus surface oxygen atoms with vertical dangling bonds are marked as “×”.
smaller than the gas phase methanol C-O bond energy (357 kJ/mol). Schiller et al.45 used the atom superposition and electron delocalization molecular orbital theory to model the •CH3 binding site and bond energy on oxygen-covered Mo (110). CH3 was found to favor binding to the 3-fold O2- surface ions, and the C-O bond energy was about 135 kJ/mol. Compared with the C-O bond energy in methanol, the bond in surface methoxide is significantly weakened because of the energy required to promote an electron to the Fermi level when methyl radicals bind to the filled-shell O2- ions. Therefore, the surfacecarbon bond strength calculation supports the conclusion that CH3 binds to surface O2- ions on the Fe3O4 (111) termination. 3.4.1. Structural Nature of r-Fe2O3 (0001) Terminations. In this section we consider some possible structural properties of surface oxygen ions in the Fe3O4 (111) and “biphase” terminations that may be responsible for the very large difference in saturation methyl coverage. Fe3O4 single crystal has the cubic inverse spinel structure where oxygen anions form a closepacked fcc sublattice. The tetrahedral sites are occupied by Fe3+ ions and the octahedral sites by Fe2+ and Fe3+ equally. Along the [111] direction, the stacking sequence can be represented by Fetet1-O1-Feoct1-O2-Fetet2-Feoct2-Fetet1-O1 (Figure 4a).22,46 Stacking of layers with opposite charges would accumulate a large dipole moment along the stacking direction which drives transformations such as surface reconstruction, surface relaxation, defect formation, etc.17,47 The surface structures of Fe3O4 (111) remain controversial. One proposed structure was based on LEED I/V analysis and STM of an epitaxial Fe3O4 (111) film grown on Pt (111).21,22 It has an unreconstructed but strongly relaxed surface, terminated by 1/4 ML of 4-fold-
coordinated iron cations over a close-packed oxygen layer, i.e. Fetet1-O1 in Figure 4a. Another structure was proposed based on STM studies of Fe3O4 (111) mineral single crystal samples,19 and was later confirmed as a selvedge of Fe3O4 (111) formed on top of a bulk R-Fe2O3 (0001) single crystal.20 The authors identified the coexistence of two terminations. Termination A had 3/4 ML of 6-fold-coordinated iron cations (Feoct1 in Figure 4a) capped by 1/4 ML of oxygen atoms. Termination B had both 1/4 ML of 6-fold-coordinated and 1/4 ML of 4-foldcoordinated iron cations over a close-packed oxygen layer, i.e. Feoct2-Fetet1-O1 in Figure 4a. Adsorption and desorption studies have been applied to help determine the structure of the Fe3O4 (111) termination. A combination of TPRS,48 XPS,49 AES,35 and STM50 studies of CCl4 adsorption/desorption on the Fe3O4 (111) selvedge over a R-Fe2O3 (0001) substrate favored the surface termination Fetet1-O1. On the other hand, the surface termination Feoct2-Fetet1-O1 was favored by a CO adsorption study.51 It is worth noting that both an ab initio periodic Hartree-Fock calculation52 and an ab initio density functional theory calculation53 concluded that the Feoct2-Fetet1-O1 termination is energetically favored. A schematic structure of the Fetet1-O1 termination is shown in Figure 4b. This surface is obtained by cleaving between the Feoct2 and Fetet1 layers in Figure 4a. From a simple covalent bond argument, each Feoct2 ion removed will leave one dangling bond on an oxygen ion which now binds only to Fetet1 ions. In particular, the oxygen ion marked by “×” has a dangling bond perpendicular to the surface. Fe3+ ions in the subsurface Feoct2 layer connect directly to these “×” oxygen ions.
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Figure 4c shows the surface structure for the Feoct2-Fetet1-O1 termination which is obtained by cleavage between the Fetet2 and Feoct2 layers in Figure 4a. When this surface is formed, the missing 1/4 ML of Fetet2 ions will leave the underlying oxygen (marked by “×”) coordinatively unsaturated (cus). Each of these oxygen ionss also has one dangling bond perpendicular to the surface. Both the Fetet1-O1 termination and the Feoct2-Fetet1-O1 termination have cus O2- ions with vertical dangling bonds and direct bonding to Fe3+ ions in the subsurface Feoct2 layer. The structure of the “biphase” termination remains controversial. Thornton et al. first proposed a structure, based on STM images from an R-Fe2O3 (0001) single crystal substrate.18,20 The images show a superlattice, proposed to be the coexistence of mesoscopic islands of R-Fe2O3 (0001) and FeO (111). Both FeO (111) and R-Fe2O3 (0001) phases are thought to be terminated by iron layers because of the low oxygen partial pressures and high temperatures used in the surface preparation.35,54 R-Fe2O3 single crystals have the corundum structure with Fe3+ ions located in 2/3 of the octahedral sites. The R-Fe2O3 (0001) islands in the proposed structure are terminated by 3-fold-coordinated Fe3+ ions. FeO single crystals have the wurztite structure with Fe2+ ions occupying octahedral sites. The FeO (111) islands are proposed to have 3-fold-coordinated Fe2+ ions on the surface. In this structure there are no cus surface oxygen ions with vertical dangling bonds available to bond with methyl radicals. A recent TEM study proposed that the “biphase” termination is composed of a Fe3O4 selvedge, less than one unit cell thick, on top of the R-Fe2O3 substrate.30 By tilting the sample away from the zone axis, the authors identified the “floreted” diffraction patterns as due to double-scattering from the epitaxial Fe3O4 layer and the R-Fe2O3 (0001) substrate. It is noteworthy that the iron cations in the epitaxial Fe3O4 layer adopt a nominal Fe2+ oxidation state to maintain surface charge neutrality.30 In spite of significant differences between these two structural models of the “biphase” surface, neither model possesses a surface with cus oxygen ions directly bonded to subsurface Fe3+. We propose that this structural feature is required for facile adsorption of methyl radical and the formation of surface methoxide as discussed next. 3.4.2. Methyl Radical Binding Sites. The calculated methylsurface bond energies and XPS C(1s) line position indicate the formation of a C-O bond when methyl radical adsorbs on the Fe3O4 (111) surface termination. On comparing the surface structures of the Fe3O4 (111) and “biphase” terminations, the difference in methyl radical adsorption properties between these surfaces appears to be caused by a requirement for reducible Fe3+ ions bonded to cus oxygen ions with vertical dangling bonds available to form an O-CH3 bond, i.e., for the binding of a methyl radical to a surface oxygen ion, an Fe3+ ion must be reduced as a result of electron transfer from methyl radical to the surface. Methyl radical assumes a planar structure where the unpaired electron occupies the carbon 2pz orbital. This nonbonding 2pz orbital plays a dominate role in •CH3-metal interactions. Hoffmann et al.55 found that methyl radical is bound to transition metal surfaces mainly through the interaction between the 2pz orbital of •CH3 and the band derived from transition metal dz2-s hybridized orbitals. A strong charge transfer from the transition metal to methyl radical was identified. On the other hand, the d bands of metal oxides are much more localized spatially and energetically due to the lack of s-d hybridization and the weak d-d overlap between neighboring metal ions.56,57 Therefore, methyl radical cannot effectively bond to metal ions through
Liu et al. the σ-donor-π-acceptor type interaction that is possible on metal surfaces. On the basis of studies of powder samples, Lunsford et al.28,29 found that metal ions with multiple oxidation states were reduced (Mn+1 to Mn) when methyl radicals bonded to O2- ions. They suggested that the Mn+1 ions were not accessible to methyl radicals during reactions because the surfaces were covered with oxygen. The accessibility argument can be safely ruled out in our case because both oxygen ions and Fe3+ ions with dangling bonds are present on Fe3O4 (111) surfaces, but the absence of • CH3-iron bonding on Fe3O4 (111) surfaces is likely due to the spatial and energetic localization of the d band in iron oxides (unable to bond via the σ-donor-π-acceptor interaction), and steric hindrance from the oxygen ions (preventing an effective σ bond formation between an iron ion and a methyl radical). In the molecular orbital picture, the half-filled 2pz carbon orbital in methyl radical can form a σ-bond with the band derived from filled 2pz oxygen orbitals. In this case, electrons should flow from C to O, as shown in a recent computational study.33 Because the 2pz orbital of O2- is filled, one electron should be promoted to the Fermi level of the metal oxides in association with C-O σ-bond formation. Local spin DFT calculations locate Fe (3d) derived states largely near the Fermi level, hence the promotion of an electron should form reduced iron.58,59 This bonding model is consistent with the reduction of Mn+1 observed by Lunsford et al.28,29 The proposed structures for Fe3O4 (111) have a common feature: the coexistence of Fe3+ ions and spatially accessible O2- ions. The “biphase” structures lack either spatially accessible O2- ions (the FeO (111)/R-Fe2O3 (0001) island model), or reducible Fe3+ ions bonded to the surface O2- ions (the epitaxial Fe3O4 layer model). The spatially accessible O2- ions are likely to be the open O2- ions marked by “×” in Figure 4, panels b and c, because methyl radicals can approach the surface along the vertical dangling bond direction. The two desorption features observed on Fe3O4 (111) surfaces may correspond to adsorption on two types of open oxygen ions associated with the Feter1-O1 and Feoct2-Fetet1-O1 terminations. On a perfect Fe3O4 (111) surface, the two kinds of open oxygen ions correspond to a coverage of 1/4 ML. The estimated saturation coverage of methyl radicals would occupy 61% percent of the open oxygen sites. STM studies revealed the presence of a FeO (111) phase on the Fe3O4 (111) surfaces even when a sharp Fe3O4 (111) LEED pattern was observed.50,60 In addition, adsorbates, vacancies, and defects were observed on the Fe3O4 (111) terminated area.19 Therefore, we attribute the 61% occupancy of adsorption sites to the imperfect nature of Fe3O4 (111) surfaces. 4. Conclusion TPRS and XPS have been used to study the adsorption, desorption and reaction of gas phase methyl radicals with Fe3O4 (111)- and “biphase”-terminated R-Fe2O3 (0001) single crystal surfaces. The “biphase”-terminated surface is inert to methyl radical adsorption except at minority or defect sites. The Fe3O4 (111) termination adsorbs methyl radicals to a saturation coverage of 1.5 × 1014 methoxide species per cm2. Surface methoxide reacts on heating by dissociation of the C-O bond to directly form gas phase methyl radicals in two temperature regimes. At saturation coverage small amounts of ethane are formed. C-O bond energies of 84 and 133 kJ/mol are obtained from the activation energies for desorption in the two temperature regimes. The difference in methyl radical reactivity between the Fe3O4 (111) and “biphase” terminations can be
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