Efficient Methane Conversion to Hydrogen by the Force-Activated

Article pubs.acs.org/JPCC

Efficient Methane Conversion to Hydrogen by the Force-Activated Oxides on Iron Particle Surfaces Satoshi Motozuka,*,†,‡ Motohiro Tagaya,§ Toshsiyuki Ikoma,† Masahiko Morinaga,‡ Tomohiko Yoshioka,† and Junzo Tanaka† †

Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan ‡ Department of Mechanical Engineering, Gifu National College of Technology, 2236-2 Kamimakuwa, Motosu, Gifu 501-0495, Japan § Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1 Kamitomioka-chyo, Nagaoka, Niigata 940-2188, Japan S Supporting Information *

ABSTRACT: Mechanochemically activated iron oxide formation on α-iron (Fe) microparticles for the efficient reaction with methane (CH4) was investigated using a simple milling process to successfully clarify the efficient CH4 adsorption as well as the C−H bonding dissociation. First, the oxides with different oxidation degrees (i.e., disordered or ordered iron oxides) were formed on the Fe by mechanochemical milling under an oxygen atmosphere. The surface analyses by X-ray diffraction, Raman, and X-ray photoelectron spectroscopy (XPS) revealed that the surface iron oxides consisted of two phases (α-Fe2O3 and Fe3O4), and the Fe2O3-like structure was dominantly grown on the near-surface of the disordered iron oxides during the initial oxidation time (0.08 and 0.80 h). Furthermore, the signals from an electron spin resonance analysis suggested dangling bondings on the disordered oxides, indicating the successful formation of the active O sites. Second, the mechanochemical reaction of the resultant iron oxide surfaces with CH4 was investigated. As a result, the efficient CH4 adsorption as well H2 generation were prominently observed on the disordered oxides, indicating an effectively dissociative adsorption. XPS analysis of the resultant particles revealed that the C−H bonding dissociation dominantly occurred on the nearsurface oxygen atoms. Furthermore, using a molecular orbital calculation analysis, the activated O atoms with dangling bondings in the disordered iron oxides were found to affect the C−H dissociation of the adsorbate molecules (e.g., CH4 and CH3•) and exhibit an efficient H2 generation. On the other hand, the O atoms in the ordered lattice could not induce the C−H dissociation of CH3• to produce the equilibrium adsorption/desorption states and lower H2 generation amount, suggesting the importance of both the force-induced activated O atoms and closest interatomic distances. Therefore, we first achieved the efficient mechanochemical decomposition of CH4 and CH3• to generate H2 by the disordered iron oxide surfaces on α-Fe particles.

1. INTRODUCTION

to valuable products (e.g., methanol (CH3OH), H2) is very important for solving energetic and economic problems. It has been reported that the C−H activation of CH4 mediated through metals or metal oxides has been suggested for the direct conversion process to CH3OH.1−8 Although the activation by elemental transition metal ions15−17 is difficult due to the endothermic oxidative reaction by most ions, iron surfaces have been theoretically investigated for such catalytic applications, and the oxidation of CH4 on such surfaces with an exothermic activation energy barrier was precisely calculated.18 Thus, the lower barrier was a possible candidate for inducing the dehydrogenation reaction of CHx (x = 1−3) as the catalytic ability. The pyrolysis of natural gas to graphite fibers at high temperature was recently studied by Tibbetts.19,20 Although

The selective reduction/oxidation processes of alkanes in academics and industry have attracted increasing interest during the past few decades.1−8 In particular, the reaction of methane (CH4) has been the recent focus as the preferred source for the promising energy of hydrogen (H2) based on the high ratio of proton to carbon atoms.9 As a practical matter, a CH4 increase in the atmosphere will cause potential damage such as climate changes.10−13 However, the CH4 molecule is inert because eight electrons in the molecule stably form a symmetrical tetrahedral structure with occupation of the low-lying 1a1 and 3-fold degenerate 1t2 molecular orbitals. The 1t2 HOMO and 2a1 LUMO have remarkable bonding and antibonding characteristics in the C−H bondings, respectively, and the HOMO/LUMO gap based on density functional theory calculations is 10.6 eV.14 Thus, the basic physicochemical research on the efficient C−H activation of CH4 for conversion © 2013 American Chemical Society

Received: May 29, 2013 Revised: July 16, 2013 Published: July 16, 2013 16104

dx.doi.org/10.1021/jp405316t | J. Phys. Chem. C 2013, 117, 16104−16118

The Journal of Physical Chemistry C

Article

Scheme 1. Scheme of the Mechanochemically Induced CH4 Interfacial Reaction on Oxidized Iron Particles (OIPs) with the Different Oxidation States of This Study

many studies on the conversion of CH4 to CH3OH have been reported, a few studies on oxidized iron surfaces as the conceivable way of favorably influencing the reaction enthalpy have been reported.21−23 The starting point of studying the surface design was the evaluation of iron oxide, which has been the choice of catalysts for the selective partial oxidation process since the 1950s.21 Schröder et al. suggested an increase in the reactivity of alkanes using excited iron oxide (e.g., FeO+) generated by Fourier transform ion cyclotron resonance conditions.21 By the fabrication of oxygen defects that occur in the bulk oxidized iron crystals, C−H and C−C bondings can be activated, and then one hydrogen atom in the CH4 molecule is successfully abstracted from CH4,22 which would be attributed to the enhanced strength of the O−H bonding formation as compared with that of the C−H bonding based on a molecular orbital calculation.23 However, few papers have reported the reactivity of the different oxygen species with the CH4 molecules. Thus, it is of great importance to reveal how the reactivity is affected by the surface oxygen lattice structures. To efficiently increase the surface reactivity with CH4 molecules, the precise design of the iron oxide structures is very important. From the viewpoints of the activation of the C−H bondings, the mechanism of selective alkane oxidation using iron-oxo species has been investigated, mainly for developing a high-performance catalytic system; i.e., cytochrome P45024,25 and methane monooxygenase26,27 are wellknown biological enzymes that are able to catalyze the addition of molecularly active oxygen to nonactivated hydrocarbons under physiological conditions. These reports suggested the importance of nonordered oxygen atoms as well as the control of the Fe−O interactions. The ability of iron oxides to activate oxygen and store it as a selective species, combined with their low surface acidity (that is, a low density of strong acid sites at which alkanes can crack or form graphitic precursors), suggests that they may have some of the key characteristics required for the partial oxidation of alkanes.28 However, few studies have investigated the states of the surface oxygen species based on the design of only the pure iron oxides because a systematic investigation of the structure−reactivity correlations becomes possible if these parameters can be varied in a controlled manner, which turned out to be very difficult.29 To precisely control the near-surface lattice structure of the iron oxides to exhibit rich dangling bondings, we have successfully proposed for the first time that activated oxygen atoms produced by a mechanochemical surface treatment,33−35

which is a powerful and efficient tool to modify a solid state and to obtain new defective states unachievable by other means, is an important field and is mainly stimulated by modern materials science and the needs of industry for new nanomaterials in green technologies.30 The investigation goal involving the mechanism of the mechanochemical reactions is finding the major factors determining the dynamics of the process as well as a revelation of the possibilities of regulating the structure and properties of a product, that is, the traditional goal of solid state chemistry to adjust reactions in time and space.31,32 For the compaction and sintering of the powders into dense inorganic materials, finally determining the properties and understanding the physicochemical processes taking place during powder consolidation is also necessary for realization of the mechanochemical method. Thus, a new approach to the development of materials was the mechanochemical method with heterogenetic atoms such as carbon materials.33−35 In particular, intensive mechanical treatments of hematite (Fe2O3) and magnetite (Fe3O4) can induce the transformation to the lowest ferrous oxides on metallic iron.36,37 Therefore, these controllable defects and the formation of dangling bondings in the lattice structure will provide novel activation properties by the activated oxygen atoms.35 In this study, experimental and theoretical evaluations of the interfacial specific reactions between the CH4 and activated iron particles functionalized by nonlattice structures of the iron oxides on the surfaces, which were induced by a mechanochemical force, were investigated and further clarified to provide an efficient interfacial C−H bonding dissociation with the rich dangling bondings of the oxygen atoms as shown in Scheme 1. In particular, we demonstrate that the patterns of the activation behavior indicate that transitory adsorbed oxygen specieswhich have yet to form lattice oxide ionswill be capable of a selective catalytic reaction as well as the deep oxidation of CH4. Therefore, this study provides a new insight into the surface reactivity of well-established and extensively studied iron oxides and will be applicable as an efficient reactor source for the production of promising energy using H2.

2. EXPERIMENTAL SECTION Preparation of Oxidative Iron Particles and the Reaction with CH4. Pure iron particles (Kojundo Chemical Laboratory Co., Ltd.) with a purity of >99% were used. The 16105



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thermal conductivity detector (TCD), and reporting integrator (C-R3A, Shimadzu Co., Ltd.). The column temperature was kept at 313 K for the first 15 min and then raised to 473 K at the rate of 8 K/min. The supply current in the TCD was 60 mA. A 2 mL sample gas from the vessel after the reaction was extracted using a microsyringe and then injected into the argon gas (purity: >99.99%) carrier flow at the velocity of 40 mL/s. The integrated peak areas were calculated to obtain the partial pressure of the generated hydrogen by the reaction. As the reference gas, 2 mL of pure hydrogen (purity: >99.99%) was used for calibration. The generated amount of hydrogen (μmol·(g of sample)−1) was calculated based on the integrated peak area ratio of the sample to that of the reference using the constant of the ideal gas volume (22400 mL), ideal gas temperature (273 K), and sample temperature (293 K). Characterization of the Particles. The particles were characterized by FE-SEM, X-ray diffraction (XRD), Krypton (Kr) adsorption and desorption isotherm analysis, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The morphology of the particles was observed using S4700 (Hitachi Co., Ltd.) at an acceleration voltage of 15 kV. The XRD patterns were recorded by an M03X-HF (Mac Science Co., Ltd., model No. 1031) using monochromatized Cu Kα radiation. The primary optics is equipped with a divergence slit (0.5°) and scattering slit (0.5 °), and the secondary optics is equipped with a receiving slit (0.15 mm). The specific surface area was determined by adsorption of Kr molecules at 77 K using a volumetric gas adsorption apparatus (BEL Japan, Belsorp-max). A glass sample tube was filled with a sample (100 mg) and thoroughly dried under vacuum at room temperature for two days and further heated to 393 K for 3−4 h. For each sample, the measurements were performed 3 times after conducting a 3 min leak check to ensure data reproducibility, sufficient sealing, and complete drying of the specimens. The saturation vapor pressure, p0, of the Kr molecules is 0.257 kPa at 77 K. The adsorption isotherm at the lower pressures (p/p0 < 0.3) was systematically analyzed by the Brunauer−Emmett−Teller (BET) surface area evaluation method using commercially available software (BEL Japan, BEL Master) with the parameters for Kr (molecular weight: 83.800, cross-sectional area: 0.202 nm2). The Raman spectra were recorded by an NRS-7200 (JASCO Co., Ltd.) equipped with a holographic grating (L400) and a Nd:YVO4 laser at the wavelength of 532 nm for the excitation. The laser power was 3.56 mW to suppress any degradation. The exposure time, resolution, scan times, and measuring wavenumber were 4 s, 6.8 cm−1, 100 times, and 100−1800 cm−1, respectively. The XPS spectra were measured by a PHI5000 Versa Probe (ULVAC-PHI, Inc.) using a monochromatized Al X-ray source (Al Kα line at 1.4866 keV) at 15 kV and 25 mW. The atomic orbitals of iron (Fe), oxygen (O), and carbon (C) were expressed in the order of atomic symbol, main quantum number, and azimuthal quantum symbol to be Fe 2p, O 1s, and C 1s. These spectra were measured in the ranges between 700− 730, 525−535, and 282−287 eV, respectively. An X-ray beam was irradiated into the particle surface at the incident angle of 22.5° with respect to the sample stage surface at the spot size of 100 μm and step size of 0.2 eV to detect the generated photoelectrons at the takeoff angle of 45°. The spectra were recorded through the pass energy of 23.5 eV, and the charge correction of the sample was conducted by setting the binding energy of the adventitious carbon (C 1s) at 284.8 eV for

experimental setup for the mechanochemical reaction and photograph of the top view during the milling by centrifugal grinding are shown in the Supporting Information, Figure S1. The milling vessel made of a stainless steel rod (SUS304) was used. The inner volume with a hemispherical bowl-like shape (ϕ 54 mm) has a 70 mL volume and 38 mm height. SUJ2 steel spherical balls (AISI52100) as the milling medium with a diameter of 9.5 mm were used. The vessel was surrounded by a water jacket to suppress the heat generated by the milling and maintain the temperature at 20 °C. First, the iron particles (2.0 g), which have cubic-like shapes with the average size of 69.5 ± 17.9 μm based on field emission scanning electron microscopy (FE-SEM) observations shown in the Supporting Information, Figure S2 (a), were added to the vessel at the weight ratio of the medium balls to iron particles of 35. The vessel was mounted on a centrifugal ball mill (NEV-MA-8, Nisshin Giken Co., Ltd.) and purged for 1 day to remove the water molecules and other substances adsorbed on the particle surfaces, then argon gas (purity: >99.999%) was introduced into the vessel until the pressure increased to 203 kPa and the vessel was immediately swung at the speed of 59.1 rad/s for 32 h. The particle size decreased by the milling due to fracture by fatigue, and the homogeneous iron particles were prepared having the average size of 4.97 ± 3.01 μm based on the FE-SEM observations shown in the Supporting Information, Figure S2 (b). Next, the resultant iron particles were milled under oxygen gas (purity: >99.999%), which was introduced into the vessel to the initial inner pressure of 152 kPa, at the speed of 59.1 rad/s for the different times of 0.08, 0.80, and 8.0 h to form an oxidative layer on the particles, which produced average particle sizes of 6.32 ± 3.87, 5.59 ± 2.40, and 0.50−4.82 μm (very broad distribution), respectively. The oxidized iron particles for the times of 0.08, 0.80, and 8.0 h were labeled as OIP-0.08, OIP-0.80, and OIP8.0, respectively. Finally, the resultant OIPs were milled under CH4 (purity: >99.9%), which was introduced into the vessel until the initial inner pressure reached 152 kPa, at the speed of 59.1 rad/s for 1.2 h. When the gas was exchanged, the main and anterior vessel chambers were completely purged, and then another gas was introduced into the chambers at the same pressure for all the experiments. The ball medium moves in a circumferential orbital on the inside vessel wall to subsequently press against the wall by centrifugal force. When the ball compresses the trapped particles between itself and the wall, a kinetic energy transfer from the ball to the particle surfaces occurs. The compressed force (F) can be expressed by eq 1 F = mrω 2

(1)

where m is the ball mass; r is the orbit radius of the ball; and ω is the angular velocity during the milling. Therefore, the value of F in this study was calculated using m of 3.5 × 10−3 kg, r of 2.7 × 10−2 m, and ω of 40.1 rad/s to be 153 mN. Monitoring of the CH4 Reaction. The inner pressure was in situ measured by a pressure sensor (E8MS-N1, OMRON Co., Ltd.) attached to the inside of the vessel. Using a state equation, the adsorbed and generation amounts of the gas on the particle surfaces (μmol·(g of sample)−1) were calculated from the reduced pressure values assuming the constant vessel volume (70 mL) and temperature (293 K). The generated gas in the vessel was analyzed by a gas chromatograph (GC-2014, Shimadzu Inc.) equipped with a packed column (Shincarbon ST, Shimadzu GLC Co., Ltd.), 16106



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measuring the Fe 2p and O 1s spectra.38 The background from each spectrum was subtracted using Shirley-type background to remove most of the extrinsic loss structure.39 On the basis of the Gaussian function using software (OriginPro, Ver. 8.6j), the spectra were fitted to each of the characteristic band curves; the component energies, number of peaks, and peak positions and widths were initially fixed according to previous reports40−54 and then refined only for the peak heights. During the final optimization, the component energies and peak width were refined again to reduce the residual values less than 3.0 × 10−4−8.0 × 10−4 to obtain the deconvoluted and separated spectra with respect to each component. In the final spectra, the spectra after subtraction of the background from the raw data, the separated spectra, and the summation are denoted by the black, green, and red curves, respectively. Also, the satellite peaks, which generally reflect their environmental electronic structures, were abbreviated as “Sat.”. In the deconvoluted XPS spectra, the ratio of the metallic iron’s (Fe0) 2p3/2 peak intensity (I(Fe0 2p3/2)) to the sum of the total Fe 2p peak intensity (I(Fe total)) to the reaction time with oxygen molecules was calculated using the integrated peak areas to obtain the I(Fe0 2p3/2)/I(Fe total) values, leading to the investigation of the bare Fe atom coverage on the near-surface layer. The ratio of the O 1s peak intensity (I(O 1s)) to the Fe0 2p3/2 intensity (I(Fe0 2p3/2)) was calculated using the integrated peak areas to obtain the I(O 1s)/I(Fe0 2p3/2) values, leading to the investigation of the reacted O atom coverage on the near-surface layer. The ratio of the C 1s peak intensity (I(C 1s)) to the (I(Fe total)) for the CH4 reaction was calculated using the integrated peak areas to obtain the I(C 1s)/I(Fe total) values, leading to the investigation of the reacted C atom coverage on the near-surface layer. Investigation of the Interfacial Reaction by Molecular Orbital Calculation. On the basis of the experimental results of the iron oxide layer reaction with CH4, the possible reaction mechanism of the CH4 on the OIP surfaces was calculated using a discrete-variational (DV)-Xα molecular orbital calculation method, which has been used as a powerful tool for the calculation of the electronic states of molecules and local lattices of crystals and has been described in previous reports.55−58 In the orbitals of the clusters, the bond overlap population (BOP), which indicates the strength order of the covalent bondings (based on the charge between the atoms), and the contour map, which indicates the levels in the wave function, were investigated to model the interfacial reaction between the CH4 and OIPs. The DV-Xα method is a first-principle calculation and gives the molecular orbital wave function using the Schrödinger equation. The l-th molecular orbital wave function (ϕl(r)) at the electron arbitrary position (r), which constitutes the orbits, can be expressed by the linear combination of the atomic orbitals, which is represented by eq 2.

number (N). Equation 2 can be resolved using the simple eq 3 of the Schrödinger equation. hϕl = εlϕl

where εl is the energy eigenvalue of the l-th molecular orbital and h is a one-electron Hamiltonian. In the system of atomic units, eq 3 can be also expressed as the following eq 4. 1 h = − ∇2 + V (r1) 2

∑ Ci ,lχi (r) i=1

(4)

where −(1/2)∇2 is the kinetic energy and V(r1) is the potential energy at the coordinate (r1) worked in one electron. The potential energy that acts on one electron consists of the nuclear gravitation potential and the electrostatic potential by the electrons except for itself. Since the other electrons are constantly moving, the electrostatic potential is actually changing. Therefore, a huge calculation capacity is strictly needed for solving eq 4. Accordingly, the self-consistent field method suggested by Hartree63 was used in this study. This method is a general method expressing the electrostatic potential and expresses the potential averaged between the electrons except for itself. On the other hand, the nuclear gravitation potential can be expressed by eq 5. VN (r1) = −

Z r1

(5)

where z is the number of nuclear protons at r1. Thus, the charge density (ρ) containing itself in an arbitrary position (r2) (ρ(r2)) can be represented by eq 6. N

ρ(r2) = fi

∑ {χi (r2)}2 i=1

(6)

where f i is the number of electrons in the i-th atomic orbital. Thus, the average electrostatic potential, which acts on an electron (Vmean), can be expressed as eq 8, which is solved by the integration in the whole space. Vmean(r1) =

∫

ρ(r2) dr2 r1,2

(7)

r1,2 is the distance between the arbitrary positions r2 and r1. For example, if electron A is considered, the electron cloud is described in the Supporting Information, Scheme S1, which indicates the configuration-interaction illustration between r1, r2, and r1,2 in the electron cloud system, to give the potential energy at r1 produced by the cloud. However, the potential includes the action by the electron itself, and the exchange potential as represented by eq 864 is subtracted from the total potential. ⎤1/3 ⎡ 3 Vxc(r1) = −3α⎢ ρ(r1)⎥ ⎦ ⎣ 4π

N

ϕl(r ) =

(3)

(8)

α has been calculated by Schwarz et al.65 to indicate the dependence on the type of atomic element, and α = 0.7 significantly corresponds to the calculation for all the elements.55−58 In this study, α = 0.7 was used in the potential energy equation (eq 9).

(2)

where C is the scaling parameter obtained using the variation method by numerical calculation and Ci,l is the coefficients which show the magnitude of the linear combination of the i-th atomic orbitals in the l-th molecular orbital. χi indicates the i-th atomic orbital wave function, and the summation of all the atomic orbitals was calculated, where N is the total electron

V (r1) = − 16107

Z + r1

∫

⎤1/3 ⎡ 3 ρ(r2) dr2 − 3α⎢ ρ(r1)⎥ ⎦ ⎣ 4π r1,2

(9)



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This method using the potential is called as Xα. Taking into account eq 9, eq 4 can be calculated by the Rayleigh−Ritz’s variational method to obtain the secular equation.55−58 The equation can then be resolved by numerical calculation to obtain εl and Ci,l in the molecular orbital. The Mulliken population analysis was employed to obtain the BOP between atoms A and B (QA−B).55−58 For the electrons in the l-th molecular orbital, the charge (Qi,jl) of the overlap area between the i-th and l-th molecular orbitals can be represented by eq 10. Q il, j =

∫

Ci , lχi (r )Cj , lχj (r )dr

(10)

The obtained values from eq 10 can be summed for all the molecular orbitals to obtain eq 11. Q i,j =

∑ Q il,j

Figure 1. Lennard-Jones potential curves (atomic forces vs interatomic distances between Fe−C, Fe−H, O−C, and O−H), indicating that the closest approach distances at 153 mN are 0.045, 0.037, 0.028, and 0.021 nm, respectively.

(11)

l

the red line in Figures 6 and 7 and Figure S5 (Supporting Information). For the calculations, the interfacial approaching models of CH4 onto disordered and ordered iron oxide clusters are shown in Scheme 2. We now report that the iron oxide surfaces were

The BOP between the atom A and B orbitals (QA−B), which was extracted from the Qi,j, can be represented by eq 12 to obtain the BOP values of this study.

Q A−B =

∑

Q i,j (12)

i ∈ A, j ∈ B

Scheme 2. Interfacial Approaching Models of CH4 onto the Disordered and Ordered Iron Oxide Cluster Surfaces, Which Is the Same Process as the CH3• Approaching Models in This Study

Thus, the BOP value indicates the stability of the chemical bondings between two atoms,105 and the positive and negative values indicate the formation of bonding and antibonding molecular orbitals between the atoms, respectively.106 In this study, the changes in the BOP among the atoms as a function of the distance between the CH4 and iron oxides were calculated by taking into account the finite distance between the atoms. The milling force minimizes the interatomic distance, and the closest approaching distance can be calculated using the Lennard−Jones (LJ) potential (U(r)) at the interatomic distance (r),60 which is typically expressed by eq 13. ⎡⎛ σ ⎞12 ⎛ σ ⎞6 ⎤ U (r ) = 4ε⎢⎜ ⎟ − ⎜ ⎟ ⎥ ⎝r⎠ ⎦ ⎣⎝ r ⎠

(13)

where ε is the depth of the potential well, and σ is the interatomic limit distance at which the interatomic potential is zero. In the case of the approach between dissimilar atoms, based on the Lorentz−Berthelot combining rules, eq 13 was differentiated with respect to the distance r to give the force worked between the dissimilar atoms (F(r)), which can be expressed by eq 14. ⎡ σ 12 d σ6 ⎤ F(r ) = − F(r ) = 4ε⎢12 13 − 6 7 ⎥ dr r ⎦ ⎣ r

successfully achieved using the mechanochemical reaction, and the possibility of disordered and ordered iron oxide surfaces was suggested.59 In particular, the disordered lattice contains the active oxygen atoms with unpaired electrons. Furthermore, the neighboring oxidized Fe3+ ion clusters after the reaction with oxygen molecules are disordered, and the O atoms around the Fe ions simultaneously maintain the ordered structure, so that the defective Fe ion in the iron oxide cluster model is partially defected to investigate the surface reactivity with CH4. To put it more precisely, the major component on the surface oxide layers is α-Fe2O3, and the 0001 plane of α-Fe2O3 is very stable and often generated in the growth process,104 which was also described in our previous report.59 Thus, the cluster model of the 0001 plane of crystalline α-Fe2O3 was used, and the αFe2O3 cluster with the lattice defect and α-Fe2O3 crystal cluster by the DV-Xα method were designed as shown in the

(14)

The sum of the covalent radius was substituted into σ. ε was substituted for that of the noble gas atom62 with the larger atomic number in the same periodic table. According to eq 14, the closest distances between two atoms were defined by the LJ potential curves based on the balance between the interatomic repulsive force and the milling forces, and the calculated LJ potential curves (atomic forces vs interatomic distances between Fe−C, Fe− H, O−C, and O−H) are shown in Figure 1. At the force used in this study (153 mN), the curves indicate that the closest approaching distances between Fe−C, Fe−H, O−C, and O−H were 0.045, 0.036, 0.028, and 0.020 nm, respectively. The interatomic limit distances are expressed by 61

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Figure 2. (a) I(Fe0 2p3/2)/I(Fe total) and I(O 1s)/I(Fe0 2p3/2) changes calculated from the XPS Fe 2p and O 1s spectra of the iron particles as a function of the oxidation time and (b) Krypton adsorption (closed marks) and desorption (open marks) isotherms of the iron particles that reacted with oxygen molecules at different oxidation times.

the iron particle near-surfaces were effectively oxidized to predominantly form the α-Fe2O3 phase by the milling, and the Fe3+ ions would be octahedrally coordinated by the hexagonal closed-packed O2− ions.78 It is suggested that the oxygen atoms, which diffused onto the iron particles, specifically coordinated with the Fe3+ ions to effectively form the nonstoichiometric iron oxides on the iron particles. Figure 2(b) shows the krypton adsorption and desorption isotherms of the oxidized iron particles (OIP-0.08, OIP-0.80, and OIP-8). There are many different isotherm shapes depending on the type of adsorbent/absorbate and intermolecular interactions between the gas and solid. The isotherms can be classified; the Brunauer, Deming, Deming, and Teller (BDDT) classification has also become the basis of the modern IUPAC classification of adsorption isotherms.70,71 The isotherms in this study shown in Figure 2(b) are similar to Type II, indicating the adsorption on a nonporous or macroporous adsorbent with the unrestricted monolayer− multilayer adsorption state. The SBET of the particles varied from 0.36 to 1.07 m2/g as shown in Table 1. During the milling

Supporting Information, Schemes S6 (a) and (b), respectively. The wave functions of the model clusters were then calculated. The arrow in Scheme S6 indicates the oxygen atom in the main component of the molecular orbitals, and in the wave function models, the yellow and blue parts indicate the positive and negative values, respectively. The tetrahedral vertex H atom in CH4 and the C atom in the methyl radical (CH3•) approached the oxygen and iron atoms in the disordered and ordered iron oxide clusters as shown in Figures 6 and 7 and also the iron atoms in the α-Fe lattice cluster as shown in Figure S5 (Supporting Information).

3. RESULTS AND DISCUSSION The homogeneous iron microparticles with the size of 4.97 ± 3.01 μm in the equilibrium milling state shown in the Surpporting Information Figure S2 (b) were reacted under an oxygen atmosphere to obtain three types of oxidized iron particles, “OIP-0.08, OIP-0.80, and OIP-8.0”. The particle sizes were constant for 0.08−0.8 h at the size of 5−6 μm with the aggregation form, whereas that for the longer time (e.g., 8.0 h) slightly decreased to the size of 0.50−4.82 μm after dispersion based on the FE-SEM observations. Taking into account the crystallite sizes from the Williamson−Hall plots based on the XRD patterns, the iron particles in the equilibrium states were polycrystalline. This homogeneous iron particle fabrication technique by milling has already been described in detail in another journal.35 Figure 2(a) shows the I(Fe0 2p3/2)/I(Fe total) and I(O 1s)/ I(Fe0 2p3/2) changes calculated from the XPS Fe 2p and O 1s spectra of the iron particles as a function of the oxidation time. The penetration depth from the surface by the XPS analysis is generally 1−3 nm,66−68 and in particular, the inelastic mean free path of a photoelectron for Fe 2p3/2 is approximately 1.2 nm by the calculation using the kinetic energy of the photoelectron excited by the Al Kα1,2 lines,69 indicating the near-surface structural analysis of the iron oxide layer. In the spectra, the positions were located at around 710.6 eV for Fe 2p3/2 and 724.0 eV for Fe 2p1/2, which is almost similar to the shape and position of the α-Fe2O3 phase40−50 and is clearly different from those of the FeO phase as a comparison, indicating the main existence of α-Fe2O3 in the near-surface layer. The I(Fe0 2p3/2)/I(Fe total) decreased with the oxidation time, whereas the I(O 1s)/I(Fe0 2p3/2) increased. Therefore,

Table 1. SBET of OIPs, Coverage Rate by Adsorbed Molecules, and the Estimated Activated O Mole Number on the OIP Surfaces at the Equilibrium State of 1.2 ha

OIP-0.08 OIP-0.80 OIP-8.0

SBET/m2·g−1

coverage rate/%

activated O mole number/mmol·g−1

0.36 0.51 1.07

208 129 14.8

10.4 6.59 ―

a

The activated O mole number was described in the text and listed in the Supporting Information Table S1 to indicate no activated O atoms on OIP-8.0.

process of the iron particles, hydroxyl groups and adsorbed H2O molecules were removed from the near-surfaces to newly generate the activated bare surfaces.73 During the initial oxidation stage, the environmental oxygen molecules effectively adsorbed on the activated surfaces react with each other and form reactive atomic oxygen,74,75 which would be the dominant phenomena, although internal diffusion into the iron particles can slightly occur by the formation of vacancies and interstitial atoms on the near-surfaces.76,77 16109



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Figure 3. (a) Inner total pressure changes (≈apparent adsorption amounts) vs reaction time by the milling of OIPs under CH4 atmosphere and (b) the actual adsorption and H2 generation amounts at the equilibrium milling time of 1.2 h.

We have already reported the ESR spectra of the iron oxide particles treated for different oxidation times.35 There is no peak in the particles at 8.0 h, whereas there is a sharp peak at 330.6 mT (g = 2.03) with the integrated half-width of 0.1 mT at 0.08 h. It has been reported that the Fe3+ ion in the oxide matrix (e.g., glass structure) exhibits a broad peak at around g = 2.03 with the width of 300 mT,79 which is clearly different from the peak; the oxygen coordination number for Fe3+ is six. Taking into account no conformational changes in the environmental oxygen atoms for the Fe3+ ion (i.e., no oxygen vacancy around the Fe3+ ion) based on the XPS analysis, the peak in this study was not attributed to the Fe3+ ion. Therefore, the sharp peak would be attributed to the oxygen species with unpaired electrons that were environmentally generated by the Fe3+ ion defect. Accordingly, the iron particles reacted with the oxygen molecules for 0.08−0.8 h and effectively formed the reactive dangling bondings on the surfaces. Accordingly, the cluster model of the 0001 plane of the crystalline α-Fe2O3 was prepared and calculated as the near-surface in this study. The wave function contour figures at the initial and final reaction times are shown in the Supporting Information, Scheme S2. The environmental oxygen atom structures of the clusters were very disordered, suggesting a defect in the Fe3+ ion around the cluster to form the unpaired electrons at the initial reaction time as shown in Scheme S2 (a). As compared to the crystalline α-Fe2O3 surface in Scheme S2 (b), the defect of the Fe ion around the clusters containing the oxygen atom in the main orbitals, it was found that one side of the O 2p orbitals that formed the covalent bonding with the Fe 3d orbitals newly reformed the dangling bonding with unpaired electrons, suggesting the reactive surfaces at the initial reaction time from around 0.08−0.8 h. Figure 3 shows the inner total pressure changes (≈apparent adsorption amounts) on the OIPs vs the milling reaction time in the CH4 atmosphere. The pressure slightly decreased to a final equilibrium state. Although the complex phenomena (e.g, adsorption, other gas-evolving reactions) occur by the interfacial reaction, the dominant reaction at the initial time is that the CH4 from the gas phase adsorbs onto the adsorption site (σ) of the OIP surfaces to form the adsorbed CH4 (CH4σ). As a result, the equilibrium relation can be represented as follows.80 CH4 + σ ⇆ CH4‐σ

When the concentrations in the above equation are represented as CCH4, Cσ, and CCH4‑σ, the reaction rate can be represented as follows.81 d(CCH4)/dt = k(CCH4) ·(Cσ )

(16)

The CCH4 is the same in all the experiments and can be regarded as a constant, and eq 16 can be represented as follows. d(CCH4)/dt = k·(Cσ )

(17)

The reverse reaction (e.g., CH4 desorption) can be disregarded during the initial stage; therefore, the initial reaction rate due to the pressure reduction significantly corresponds to the number of crude adsorption sites. The reaction rates of CH4 onto the OIP-0.08, OIP-0.8, and OIP-8.0 were 232.8, 86.2, and 14.4 μmol/h, respectively, indicating the higher number of adsorption sites on OIP-0.08. Thus, the active surfaces (OIP0.08 and OIP-0.8) containing the dangling bondings with unpaired electrons affect the efficient reaction. Figure 3(b) shows the adsorbed amount of CH4 and generated amount of H2 at the equilibrium milling time of 1.2 h. There was no generation of oxygen, carbon monoxide, carbon dioxide, acethylene, ethylene, and ethane molecules based on the gas chromatography analysis, and surprisingly, only the generation of H2 molecules was detected. Thus, the actual adsorbed amounts were calculated based on the pressures by subtracting the H2 generation amounts (21.3, 17.7, and 4.60 μmol·(g of sample)−1 for OIP-0.08, OIP-0.8, and OIP-8.0, respectively) from the apparent adsorption amounts and formed to be 37.4, 32.7, and 7.90 μmol·(g of iron particles)−1 for OIP-0.08, OIP-0.8, and OIP-8.0, respectively. These results indicated that the activated OIP surfaces with the dangling bondings interacted with CH4 molecules to generate the H2 molecules. Here, the base area in the CH4 molecule regarded as a circular shape was calculated to be 0.033 nm2,72 and the surface occupation rates for OIP-0.08, OIP-0.8, and OIP-8.0 were estimated to be 208%, 129%, and 14.8%, respectively. Taking into account no change in the specific surface areas before and after the CH4 reactions, the occupation rate of the activated OIPs (OIP-0.08 and OIP-0.80) was very high as compared to that of OIP-8.0, indicating a multilayer adsorption as well as other specific interfacial reactions. Therefore, it was found that the surface reactivity of the mechanochemically prepared OIPs effectively affects the CH4 adsorption as well as H2 generation amounts, and especially, the

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D′4h magnetic group, and four phonon lines were observed in the spectra, namely, an A1g mode at 668 cm−1, two T2g modes at 538 and 193 cm−1, and an Eg mode at 306 cm−1.82 On the other hand, α-Fe2O3 belongs to the D63d crystal space group, and seven phonon lines were observed in the spectra,83,84 namely, two A1g modes at 225 and 498 cm−1 and five Eg modes at 247, 293, 299, 412, and 613 cm−1.82 α-Fe2O3 is an antiferromagnetic material, and the collective spin movement can be excited in what is called a “magnon”.85 Thus, the intense peak at around 1320 cm−1 can be assigned to a two-magnon scattering which arises from the interaction of two magnons created on antiparallel close spin sites.82 The peak intensity attributed to the Fe3O4 phase decreased with the oxidation time to constructively form the α-Fe2O3 phase. If a microscopic high temperature is generated by the friction during the milling according to our and other previous reports,34,88 the Fe3O4 phase is transformed into γ-Fe2O3 (at around 200 °C) and then into α-Fe2O3 (at around 673 K).89,90 Thus, in this study, taking into account no peaks due to the γ-Fe2O3 phase at around 700 cm−1 in addition to the β-Fe2O3 phase at around 360 cm−1,91 the surface oxygen atoms would mechanochemically diffuse into the particles during the milling to directly reconstruct the iron oxide structures, suggesting the direct formation of the oxidation product, such as Fe3O4 and α-Fe2O3, from the near surfaces, whereas there are no carbon products due to the CH4 reaction on the surfaces based on the analysis. The phase constitutions of the surface layer in the OIPs were mostly the Fe3O4 and α-Fe2O3 phases and preserved by the CH4 reaction. Figure 5(a) shows the XPS Fe 2p spectra of the OIPs that reacted with the CH4 molecules for 1.2 h. Four peaks were clearly observed, and the strong two peaks can be attributed to Fe 2p3/2 and Fe 2p1/2. It seems that the spectral shape of the Fe 2p3/2 with four degeneracy states is narrower and higher than that of the Fe 2p1/2 due to only two in the spin−orbit coupling.92,93 Furthermore, the positions were located at around 710.6 eV for Fe 2p3/2 and 724.0 eV for Fe 2p1/2, which is almost similar to the shape and position of the αFe2O3 phase and is clearly different from those of the FeO phase as a comparison,92−94 indicating the existence of α-Fe2O3 in the near-surface layer. The representative separation results for the spectra are shown in the Supporting Information, Figure S3. It has been reported that several theoretical models for the satellite peaks were proposed in terms of the valence electron configurations51−54 to interpret the complex photoemission line shapes and reproduce the deconvolution that is good agreement with their experiment. In the vast majority of instances, the peak separation of Fe 2p3/2 has been suggested to be 4−6 peaks for the iron oxides.51−54 In particular, the existence of single lowintensity peaks on the low- and high-binding energy sides of the peak in addition to the peaks due to multiplets for the Fe 2p3/2 imply that the Fe ions have lower valences than the normal oxidation state with the valence number of three due to the production of defects in the neighboring sites,29 indicating the formation of unsettled reactive Fe ions. In this study, the widths of the spectral shape of Fe 2p3/2 are the same irrespective of the reaction time, indicating the instability factor of the near-surface Fe ion states. Moreover, in our detailed separation results, the Fe 2p spectra were composed of six peaks at 707.1 eV due to Fe0 2p3/2 (metallic state), 709.4−710.3 eV due to Fe2+ 2p3/2 (Fe3O4), 710.3−711.4 eV due to Fe3+ 2p3/2 (α-Fe2O3), 720.5 eV due to Fe0 2p1/2 (metallic state), 722.8−723.1 eV due to Fe2+ 2p1/2 (Fe3O4), and 723.6−724.6 eV due to Fe3+ 2p1/2 (α-

active O atoms of OIP-0.08 and OIP-0.80 would induce the higher adsorption and generation amounts. Figure 4(a) shows the XRD patterns of the OIPs that reacted with CH4 for 1.2 h. Only the diffraction peaks due to the (110)

Figure 4. (a) XRD patterns, (b) FE-SEM images (magnification: ×4000), and (c) Raman spectra of the OIPs that reacted with CH4 for 1.2 h.

of α-Fe in the OIP-0.08 and OIP-0.80 and the diffraction peaks due to the (311) of Fe3O4 in addition of the (110) α-Fe2O3 in OIP-8.0 were observed. This result indicates that the OIP structures were preserved during the CH4 reaction according to our previous report,34 suggesting that there are no structural changes by the reaction. Also, there are also no morphological changes during the CH4 reaction in the FE-SEM images (Figure 4(b)) of the OIPs that reacted with CH4 for 1.2 h according to our previous report.34 The average particle sizes of OIP-0.08, OIP-0.80, and OIP-8.0 were 5.30 ± 2.70, 6.12 ± 2.35, and 0.5−10 μm, respectively. Figure 4(c) shows the Raman spectra of the OIPs that reacted with the CH4 molecules for 1.2 h. The peaks at around 100−1800 cm−1 attributed to the Fe3O4 and α-Fe2O3 phases were observed for all the OIPs. The Raman spectroscopy is an optical technique based on scattering that probes the nearsurface layer of the sample at an optical skin depth (δ), which is determined by the absorptivity as represented by eq 18.

δ = λ /(4πk)

(18)

where λ is the laser wavelength and k is the imaginary part of the complex refractive index of the sample. Thus, the δ values of the α-Fe, Fe3O4, and α-Fe2O3 were estimated to be 39, 75, and 77 nm, respectively. From the peaks, Fe3O4 belongs to the 16111



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Figure 5. (a) Fe 2p, (b) O 1s, and (c) C 1s XPS spectra of the OIPs that reacted with CH4 molecules for 1.2 h and (d) changes in the adsorbed amount of methane species onto the OIPs with the increasing I(OFe−O)/I(Fe total) and (e) the I(C)/I(Fe total) changes with the increasing adsorbed amount.

Fe2O3), indicating the existence of Fe0, Fe2+, and Fe3+ in the near-surfaces. Furthermore, there are three satellite peaks, and in particular, the satellite peak of α-Fe2O3 associated with Fe3+ 2p3/2 was clearly located at around 718 eV.95,96 The satellite peaks are used as fingerprints to identify the iron oxide phases. The cluster model can be theoretically applied to reproduce the XPS spectra of the 3d transition,96−99 and the core hole formed in the 3d electrons by the photoemission process is sufficient to redistribute the final-state electron configurations. As a result, the configuration interaction produced by the charge transfer from the ligand oxygen 2p orbitals to the iron 3d states effectively influenced the new specific satellite peaks.78 On the other hand, the Fe3O4 phase has no satellite peaks as seen in Figure 5(a) because the octahedral Fe2+, octahedral Fe3+, and tetrahedral Fe3+ coordination with the oxygen atoms in the inverse-spinel structure compensate the Fe3O4, resulting in no satellite peak.78 Therefore, the α-Fe2O3 phase was effectively formed in the surface by the milling through the mixed oxidation states of the Fe2+ and Fe3+ ions, and the Fe3+ ions would be octahedrally coordinated by the hexagonal closedpacked O2− ions.78 Therefore, the phase constitutions of the near-surface on the OIPs were the Fe3O4 and α-Fe2O3 phases: the α-Fe2O3 phase was dominant, and the near-surface states

were preserved during the CH4 reaction. As shown in Figure 5(d), the adsorbed amount of CH4 species decreased with the increasing I(OFe−O)/I(Fe total) value, suggesting a decrease in the reaction site (e.g., adsorption, desorption, hydrogen abstraction, etc.) on the surfaces with the increasing oxidation degree at the near-surface layer thickness (ca. 1.2 nm).69 Therefore, the further oxygen reaction with the iron surfaces induced the ordered arrangement and reconstruction of the iron oxide structure to reduce the surface reactivity with the CH4 molecules. Figure 5(b) shows the XPS O 1s spectra of the OIPs that reacted with CH4 molecules for 1.2 h. The observed peaks were deconvoluted to be at 530.0 eV due to Fe−O bonding in the iron oxides,40−50 531.5 eV due to hydroxyl groups (e.g., Fe− OH),100 and 533.0 eV due to C−O bondings,101 and these integrated peak intensities are abbreviated as IFe−O, IFe−OH, and IC−O, respectively. The occupation rates of IFe−O, IFe−OH, and IC−O in the peaks for OIP-0.08, OIP-0.80, and OIP-8.0 were 71.4, 72.0, and 72.7%, 28.6, 28.0, and 27.3%, and 5.3, 5.2, and 4.4%, respectively, suggesting the reflection in the oxygen reaction to form iron oxides and the C−O bonding formation at the near-surfaces. Furthermore, the C−O peaks would be attributed to the reaction products of the CH4 molecules on the 16112



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Figure 6. BOP changes with the interatomic distances by approaching of CH4 and CH3• onto the Fe atoms in the disordered Fe2O3 and ordered αFe2O3 lattice cluster surfaces and their possible reaction models at the interfaces.

Figure 7. BOP changes with the interatomic distances by approach of CH4 and CH3• onto the O atoms in the disordered Fe2O3 and ordered αFe2O3 lattice cluster surfaces and their possible reaction models at the interfaces.

iron oxides. Liu et al. analyzed the adsorption states of CH3• on the 0001 plane of an α-Fe2O3 single crystal under ultrahigh vacuum conditions to provide information about the selective adsorption onto the oxygen atoms of iron oxides that forms Feδ+···O−CH3• bondings.102 Accordingly, the CH4 molecules in this study effectively reacted with the oxygen atoms (σ) of the iron oxides to form the C−O bondings, which can be represented by the possible reaction formula as follows, although very complex reactions occur at the interface. 2CH4 ⇆ 2CH3•−OIPs + H 2↑

Therefore, the CH4 was effectively dissociated on the oxygen atoms of the iron oxides to form radical species (e.g., CH3•) with covalent bondings at the interfaces. Figure 5(c) shows the XPS O 1s and C 1s spectra of the OIPs that reacted with the CH4 molecules for 1.2 h. The peak can be derived from two peaks at 285.0 and 286.1 eV attributed to the C−C103 bondings of impurity hydrocarbons in the air and C−O bond (e.g., Fe+−O−CH)102 at the interface, respectively. The integrated peak intensities due to the C−O bond are calculated to show that the occupation rates in the peaks for OIP-0.08, OIP-0.80, and OIP-8.0 were 5.1, 4.8, and 3.3%, respectively, indicating that CH4 effectively reacted with the oxygen atoms of the disordered iron oxides to form the C−

(19)

This can also be supported by the H2 generation rate of the adsorbed amounts (40−50%) as described in Figure 3(b). 16113



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Supporting Information, Figure S5 (right). Therefore, it was found that the interfacial interactions between the vertex H atom of CH3• and the Fe atoms in the iron oxides also induce only the adsorption/desorption dynamic equilibrium states. Figure 7 shows the BOP changes with the interatomic distances by the approach of CH4 and CH3• to the O atoms in the disordered Fe2O3 and ordered Fe2O3 lattice surfaces and their possible reaction models at the interfaces. When the vertex H atom of CH4 approaches the O atoms in the disordered and ordered Fe2O3 (Figure 7, left), the BOP values at the wider H···O distance (

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