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Geometrical-Site-Dependent Catalytic Activity of Ordered Mesoporous Co-based Spinel for Benzene Oxidation: in situ DRITFS Study Coupled with Raman and XAFS Spectroscopy Xiuyun Wang, Yi Liu, Tianhua Zhang, Yongjin Luo, Zhixin Lan, Kai Zhang, Jiachang Zuo, Lilong Jiang, and Ruihu Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03547 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017
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Geometrical-Site-Dependent Catalytic Activity of Ordered Mesoporous Co-based Spinel for Benzene Oxidation: In Situ DRIFTS Study Coupled with Raman and XAFS Spectroscopy Xiuyun Wang,† Yi Liu,†Tianhua Zhang, † Yongjin Luo, ‡* Zhixin Lan, † Kai Zhang, † Jiachang Zuo, ‡ Lilong Jiang†* and Ruihu Wang§* †
National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou
University, Fuzhou, Fujian, 350002, China. ‡
Fujian Key Laboratory of Pollution Control & Resource Reuse, Fujian Normal
University, Fuzhou 350007, China. §
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, China. ABSTRACT: Co3O4 spinel has been widely investigated as a promising catalyst for the oxidation of volatile organic compounds (VOCs). However, the roles of tetrahedron-coordinated Co2+ sites (Co2+Td) and octahedron-coordinated Co3+ sites (Co3+Oh) still remain elusive, because their oxidation states are strongly influenced by the local geometric and electronic structures of the cobalt ion. In this work, we separately studied the geometrical-site-dependent catalytic activity of Co2+ and Co3+ in VOCs oxidation based on a metal ion-substitution strategy, by substituting Co2+ and Co3+ with inactive or low-active Zn2+(d0), Al3+(d0) and Fe3+(d5), respectively. A thorough Raman spectroscopy, X-ray Absorption Fine Structure (XAFS) and in situ DRIFTS spectra were applied to elucidate the active sites of Co-based spinel catalyst. The results demonstrate that octahedron-coordinated Co2+ sites (Co2+Oh) are more easily oxidized to Co3+ species compared to Co2+Td, and Co3+ are responsible for the oxidative breakage of the benzene rings to generate the carboxylates intermediate species. CoO with Co2+Oh and ZnCo2O4 with Co3+Oh species have demonstrated good catalytic activity and high TOFCo values at low-temperature. Benzene conversion for CoO and ZnCo2O4 are more than 50% at 196 and 212 °C, respectively. However, CoAl2O4 with Co2+Td sites shows poor catalytic activity and low TOFCo value. In 1 ACS Paragon Plus Environment
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addition, ZnCo2O4 exhibits good durability at 500 oC and strong H2O resistance ability. KEYWORDS: Spinel; Ordered mesopore; Benzene oxidation; Sustainable chemistry; DFT calculation.
1. INTRODUCTION Volatile organic compounds (VOCs) are carbon-based chemicals that can cause environmental and health problems.1-5 To meet more and more stringent regulations of VOCs emissions, several promising techniques, including conventional control processes (biological degradation, adsorption, etc.) and emerging technologies (plasma catalysis, catalytic oxidation, etc.) have been proposed for VOCs removal.6-8 Among these techniques, the catalytic oxidation is considered to be one of the most effective pathway for the removal of VOCs owing to its low operating temperature and high efficiency.9-11 Catalysts based on precious metals can exhibit outstanding catalytic performance at relatively low temperatures, but high noble metal loading is generally required, which greatly limits their practical application because of high cost and easy sintering at high temperature. A promising alternative to the precious metal-based catalysts is the use of transition metal oxides (TMOs) including MnO2, Co3O4, NiO, etc, among which Co3O4 is one of the most efficient catalysts in the many catalytic reaction, such as CH4 oxidation and catalytic oxidation of VOCs.2, 12-15 It was reported that the oxidation of VOCs over TMOs catalysts occurs according to a Mars-van Krevelen type redox cycle.15-18 Co3O4
consists
of
one
tetrahedron-coordinated
Co2+
sites
and
two
octahedron-coordinated Co3+ sites,19, 20 the adsorption over Co3+ or Co2+ sites and the activation of C-H are two crucial steps in the VOCs oxidation over Co3O4.15 Additionally, the C-H bond breaking on metal oxides occurs via direct interaction of σ and σ* C-H orbitals with d-type orbitals of cobalt cations.16,21 However, the roles of Co3+ or Co2+ sites in the activation of C-H and oxidative breakage of the C-H bond have remain elusive because their oxidation states are strongly influenced by the local 2 ACS Paragon Plus Environment
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geometric and electronic structures of the cobalt ion. For instance, Co3O4 nanorods with predominately exposed Co3+Oh sites show superior catalytic activities toward ethylene oxidation.17 Similar studies have shown that Co3O4 nanocrystals display high low-temperature activity and selectivity in the catalytic oxidation of C3H8, which is contributed by Co3+species.18 Conversely, some other researchers have found that a good correlation between the specific catalytic activity and the relative proportion of Co2+ ions during VOCs oxidation.14,
21
Thus, it is very essential to elucidate the
intrinsic roles of two types of cobalt sites Herein, we separately studied the catalytic activity of Co2+ and Co3+ for VOCs oxidation based on a metal ion-substitution strategy. Three-dimensionally (3D) ordered mesoporous Co3O4 was prepared by nanocasting method, then Co2+ or Co3+ sites were replaced with catalytically inactive or low-active Zn2+ (d0), Al3+ (d0) and Fe3+ (d5), respectively. Benzene, one of carcinogenic VOCs, was used as a target toxic gas for testing the catalytic activity of catalysts. Our results indicate that Co2+Oh sites are more easily oxidized to Co3+ species compared to tetrahedron-coordinated ones, and Co3+ is responsible for the oxidative breakage of the benzene rings to generate the intermediate species (i.e. carboxylates), giving rise to good catalytic activity and high TOFCo values at low-temperature. 2. Experimental Sections 2.1 Catalyst preparations 3D ordered mesoporous Co3O4 was synthesized according to the modified literature method.22 0.5 g KIT-6 (purchased from nanoscience and technology companies of Jicang in Nanjing) was added to a stirring solution of 1.0 g Co(NO3)2⋅6H2O in ethanol (40 mL), the suspension was stirred at room temperature for 12 h. After the removal of solvent, the remaining solid was dried at 120 oC overnight and followed by a calcination at 400 oC for 2 h. The KIT-6 hard template was removed using 2 M NaOH solution, the resultant solid was dried at 120 oC for 24 h and finally calcined at 400 oC for 2 h. The obtained sample was denoted as Co3O4. When the as-obtained Co3O4 was treated with 3.5 vol % H2 at 250 °C for 2 h, the resultant reduced sample was denoted 3 ACS Paragon Plus Environment
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as CoO. The synthetic procedures of ZnCo2O4, CoAl2O4 and CoFe2O4 were similar to that of Co3O4 except that Co(NO3)2⋅6H2O was partially replaced by salt nitrates with the 1:2 molar ratios of Zn: Co, Co: Al and Co: Fe, respectively. 2.2 Catalytic Activity Tests The catalytic activities of samples were evaluated in a continuous flow fixed-bed quartz reactor with 0.1 g of catalyst. A thermocouple was inserted inside the catalytic bed to measure the reaction temperature. The gas mixture composed of 498 ppm benzene, 20% O2 and balance N2 was fed into the reactor, and the total flow rate was kept at 150 mL/min by a mass flow controller, equivalent to a weight hourly space velocity (WHSV) of 90,000 mL/(g. h). After the steady operation for 30 min, the activity of catalyst was tested. Benzene conversions were analyzed by a gas chromatograph equipped with a flame ionization detector. In the case of water vapor addition, 9.5 vol% of H2O was introduced at 350 oC via a mass flow controller using a water saturator. The benzene conversion (Xbenzene) and turnover frequency of TOFCo were calculated according to the following equations:
X
= C c− C *100% in
benzene
out
[1]
out
TOF = C Co
benzene
*X n
benzene
*V [s-1] gas
[2]
co
where Cin and Cout are the inlet and outlet benzene concentration, respectively; Vgas is the total molar flow rate; Cbenzene is the benzene concentration in the inlet gas; nCo is the molar amount of Co in total catalyst calculated by Inductively Coupled Plasma OES spectrometer results. 2.3 Characterizations Powder X-ray diffraction (XRD) was performed on a Panalytical X’Pert Pro diffractometer using a Co-Kα radiation. N2 physisorption measurement was performed on an ASAP 2020 apparatus; the sample was degassed in vacuo at 180 oC at least 6 h before each measurement. H2 temperature-programmed reduction (H2-TPR) was performed on AutoChem II 2920 equipped with a TCD detector, in which the sample 4 ACS Paragon Plus Environment
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was pretreated under Ar flow (30 mL/min) at 400 oC for 0.5 h. After cooling to room temperature, the temperature was increased to 800 oC at 5 oC/min in a gas flow of 10 vol% H2/Ar (30 mL/min). Oxygen-temperature-programmed surface reaction (O2-TPSR) experiments were performed on the same apparatus as that of H2-TPR. Before the experiment, the sample was pretreated in He at 300 °C for 0.5 h. After the sample was cooled to 50 °C, the He flow was switched to a pulse injection of 10 vol% benzene/Ar until complete adsorption of benzene, followed by a purging in He for 10 min. Finally, the TPSR run was started under a flow of 40 mL/min of 3 vol% O2/He ramping at 5 °C/min to 500 °C. A mass spectrometer (Cirrus) was used for on-line monitoring of effluent gases. The signals at mass-to-charge (m/z) ratios of 18 (H2O), 28 (CO), 44 (CO2) and 78 (C6H6) were monitored, and the profile of CO had been deducted from the contribution of m/z =28 fragment of CO2. X-ray photoelectron spectroscopy (XPS) analysis was performed on Physical Electronics Quantum 2000, equipped with a monochromatic Al-Kα source (Kα = 1,486.6 eV) at 300 W under UHV. Catalyst charging during the measurement was compensated by an electron flood gun. Transmission Electron Microscope (TEM) and high-resolution transmission electron microscopy (HR-TEM) measurements were carried out on a JEM-2010 microscope operating at 200 kV in the mode of bright field. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis was carried out using an Ultima2 spectrometer. The component of Si was determined using a PANalytical Axios X-ray Fluorescence (XRF) spectrometer with a rhodium tube as the source of radiation. Raman spectra of samples were collected at ambient condition on a Renishaw spectrometer. A laser beam (λ= 532 nm) was used for an excitation. X-ray absorption fine structure (XAFS) measurements were performed on the 1W2B beam line of Beijing Synchrotron Radiation Facility. The spectra of the Co K-edge of the samples and reference compounds were recorded at room temperature in transmittance and fluorescence mode, respectively. A Si (111) double-crystal monochromator was used to reduce the harmonic content of the monochrome beam. 2.4 In Situ DRIFTS 5 ACS Paragon Plus Environment
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In situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) was recorded on a Nicolet Nexus FT-IR spectrometer in the range of 650-4000 cm−1 with 32 scans at a resolution of 4 cm−1. Prior to each experiment, the sample was pretreated at 300 oC for 0.5 h in a gas flow of N2 to remove any adsorbed impurities, and then cooled down to 150 oC. The background spectrum was collected under N2 and automatically subtracted from the sample spectra. Afterward, 1000 ppm benzene balanced with N2 was introduced to the cell in a flow rate of 30 mL/min at 150 oC, DRIFTS spectra were recorded. After physisorbed benzene was removed by flushing wafer with N2 for 3 h, subsequently 20% O2/N2 were introduced to investigate the reactivity of pre-adsorbed benzene with N2 + O2 at 150 or 250 oC. 2.5 DFT calculation All spin-polarized DFT calculations were carried out using the Vienna ab Initio Simulation Program (VASP) with the gradient-corrected PW91 exchange-correction function. For valence electrons, a tight convergence of the plane-wave expansion was obtained with a kinetic energy cutoff of 500 eV, and the ionic cores were described with the projector augmented-wave (PAW) method. The Brillouin zone of the Monkhorst-Pack grid was set at 2 × 2 × 1. For energy calculation, the electronic energy was converged to 10−5 eV, and the positions of the atoms were allowed to relax until all forces were smaller than 0.02 eV/ Å. 3. Results and Discussion 3.1 Structural, Textural Properties and Reducibility
Figure 1 (A) XRD patterns of Co3O4, ZnCo2O4, CoAl2O4, CoFe2O4, and CoO; (B) H2-TPR profile of Co3O4. 6 ACS Paragon Plus Environment
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Low-angle XRD patterns (Figure S1) of Co3O4, CoO, CoAl2O4, ZnCo2O4 and CoFe2O4 show a peak at 1.0-1.2o, indicating the existence of well-ordered mesoporous structure. Wide-angle XRD pattern (Figure 1A) of Co3O4 shows well-resolved reflections that can be assigned to the Co3O4 phase with cubic spinel-type structure (PDF: 00-042-1467). Interestingly, ZnCo2O4, CoAl2O4 and CoFe2O4 show similar peaks, while the peak intensities and positions are unavoidably affected because of different ionic radius and electronic state of foreign atoms. These results suggest the maintenance of the cubic-spinel structure after partial cobalt is substituted by zinc, aluminum or iron. After Co3O4 was treated by H2 at 250 oC, XRD pattern of CoO shows three diffraction peaks with 2θ values of 42.6, 49.6 and 72.8o, which can be assigned to the cubic CoO phase (PDF: 01-075-0393), indicating complete phase transformation from Co3O4 to CoO. H2-TPR profile (Figure 1B) of Co3O4 shows three reduction signals. The weak peak observed at 164 oC can be assigned to the reduction of surface oxygen species.13, 20 The second one, at approximately 249 °C, is associated with the reduction of Co3+ to Co2+ accompanying with the corresponding structural change to CoO. 13, 23 The high-temperature peak centered at 340 oC is associated with the reduction of Co2+ to Co0.
24
On the basis of these results, the reduction of
mesoporous Co3O4 to CoO can be realized at 250 °C under a 3.5 vol % H2 atmosphere.
Figure 2 TEM and HR-TEM images of A) KIT-6 template; B) Co3O4; C) CoO; 7 ACS Paragon Plus Environment
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D-F) CoFe2O4; G-I) ZnCo2O4 and J-L) CoAl2O4.
TEM images (Figure 2) of Co3O4, CoO, CoAl2O4, ZnCo2O4 and CoFe2O4 exhibit arrays of high-ordered mesoporous structure with cubic Ia3d symmetry, which are consistent with the low-angle XRD results. It's worth noting that 3D morphology of CoO is similar to that of Co3O4, indicating that the phase transformation occurs in a pseudomorphic manner, without reconstruction of the whole mesoporous framework. Moreover, the lattice fringes can be observed in HR-TEM images of these catalysts, revealing highly crystalline nature of mesoporous frameworks (The more details see supporting information).22,
25
These results indicate good replication of the KIT-6
template (Figure 2A). XRF analysis (Table S1) confirms that 4.8-5.1 wt% of silica residual is present in the prepared catalysts. Additionally, the texture properties such as surface area are also important parameter for catalysis. It is shown that the surface area of catalyst (Table 1) is close to each other, which is in the range of 76-83 m2/g. These results suggest that that only minor textural properties change over CoO, ZnCo2O4, CoAl2O4 and CoFe2O4 with respect to Co3O4.
Table 1 BET surface area, reaction temperature, TOFCo values and activation energy (Ea) of catalysts for benzene oxidation.
a
Sample
BET surface area (m2/g)
T10% (oC)
T50% (oC)
T90% (oC)
TOFCoa (10-3s)
Ea (kJ/mol)
Co3O4
80
178
215
245
0.30
70
CoO
76
145
196
263
0.67
64
CoAl2O4
79
201
-
-
0.14
112
ZnCo2O4
77
164
212
236
0.87
-
CoFe2O4
83
175
223
261
0.45
-
The TOFCo value at low conversions under a kinetically controlled regime 167 oC for benzene
oxidation.
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3.2 Cobalt Oxidation States and Structure 3.2.1 Cobalt Oxidation States
Figure 3
(A) Normalized Co K-edge XANES spectra over CoO powder (reference
sample), Co foil and Co3O4, CoO, CoAl2O4, CoFe2O4 and ZnCo2O4; (B)The energy position of main absorption of Co K-edge of Co3O4, CoO, CoAl2O4, CoFe2O4 and ZnCo2O4; (C) Pre-edge of XANES spectra at the Co K-edge of Co3O4, CoO, CoAl2O4, CoFe2O4 and ZnCo2O4; (D) XPS spectra of Co2p over Co(NH3)6Cl6, CoO reference and Co-based spinel catalysts. {Co foil, Co(NH3)6Cl6 and CoO reference were purchased from Sigma-Aldrich company}. (E) XPS spectra of O 1s over Co-based 9 ACS Paragon Plus Environment
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catalysts; (F) The relation of interatomic distance between atom (Oh) and atom (Td) in Co-based spinel structure.
The valence states of cobalt in Co3O4, CoO, CoAl2O4, ZnCo2O4 and CoFe2O4 are studied by X-ray absorption near edge structure (XANES), where the position of the absorption edge could be used as an indicator. Figure 3A suggests that the valence state of cobalt ions in catalysts follows the order of ZnCo2O4 > Co3O4 > CoFe2O4. It should be mentioned that Co2+ is presented in CoO and CoAl2O4, and an average oxidation state of cobalt in Co3O4is +2.67. Compared to pure Co3O4, the changes of main absorption edge position are observed in CoO, CoFe2O4, CoAl2O4 and ZnCo2O4, as shown in Figure 3B. The average oxidation state of cobalt in CoFe2O4 is +2.36, suggesting that the co-existence of Co2+ and Co3+ and the dominate state of Co ions is +2. The pre-edge of Co K-edge of CoO, CoAl2O4, CoFe2O4, ZnCo2O4 and Co3O4 are illustrated in Figure 3C. For 3d transition metal, the pre-edge peaks are assigned to the forbidden 1s→3d transition
26-28
, and the change of the pre-edge peak intensity is
indicative of the changes in the site occupation in tetrahedral and octahedral symmetry, namely narrower and more intense for the former, and broader and less intense for the latter.
28
This is mainly because tetrahedral symmetry is highly noncentrosymmetric
and this enables p→d transitions that contributes to the pre-edge peak.26 When both tetrahedral and octahedral sites are occupied, the pre-edge peak will be the sum of these contributions, and the increase in intensity will be directly proportional to the tetrahedral site occupation. 28 Thus, the pre-edge intensities (Figure 3C) of cobalt ions in CoAl2O4 is higher than that of CoO, CoFe2O4 and ZnCo2O4, suggesting that more Co ions occupy tetrahedral sites in the former catalyst. The surface cobalt oxidation states were investigated by X-ray photoelectron spectroscopy (XPS). CoO powder and Co(NH3)6Cl3 were chosen as reference materials for pure Co2+ and Co3+ (Figure 3D), respectively. Pure Co2+ exhibits two shake-up peaks at ca. 785 and 802 eV,14, 16,
29
while pure Co3+ display only a very
weak shake-up peak at 791 eV. Compared to CoO reference, the Co 2p XPS spectra of 10 ACS Paragon Plus Environment
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mesoporous CoO, CoAl2O4 and CoFe2O4 exhibit two shake-up peaks at 786 and 804 eV, which are typical for Co2+. Co 2p XPS spectra of ZnCo2O4 indicate that the primary surface cobalt species are Co3+, which is similar to Co(NH3)6Cl3 reference. Fe 2p XPS spectrum (Figure S2) of CoFe2O4 demonstrates that the dominant state of Fe ions is +3. However, a relative weak band at 711.2 eV may also indicate the presence of small amounts of Fe2+.30 O 1s XPS spectra of catalysts are shown in Figure 3E, the peak at 530.4-532.5 eV can be assigned to lattice oxygen (Olatt),
14
while the one at 532.1-534.9 eV can be assigned to surface adsorbed oxygen species (Oads), resulting from the adsorption of gaseous O2 into oxygen vacancies. 31 The peak position of O1s in CoAl2O4, CoFe2O4 and ZnCo2O4 are different for that of Co3O4 and CoO. It is because that the structural defects of catalysts have been changed after Co2+ or Co3+ sites are replaced by Zn2+, Al3+ or Fe3+. The Oads/(Olatt+Oads) ratio (Table S1) in catalysts follows the trend of CoAl2O4 (56%) > CoFe2O4 (54%) > ZnCo2O4 (44%) > CoO (42%) > Co3O4 (37%), indicating the formation of more surface oxygen defects of CoAl2O4, ZnCo2O4 and CoFe2O4 compared to Co3O4. Additionally, structural transformation from Co3O4 to CoO procedure is also beneficial to enhance the formation of surface vacancies by oxygen removal from the surface oxygen ion binding with Co2+ and Co3+ sites,14,
32
resulting in more surface O-vacancies for
mesoporous CoO.
3.2.2 Electronic Structure
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Figure 4
Co K-edge EXAFS spectra of A) Co3O4, B) CoO, C) CoAl2O4, D)
ZnCo2O4 and
E) CoFe2O4; The interatomic distances are shorter than the actual
values owing to the fact that Fourier transform (FT) spectra were not phase corrected; F) Raman spectra of Co3O4, CoO, CoAl2O4, CoFe2O4 and ZnCo2O4. The coordination environments of mesoporous Co3O4, CoO, CoAl2O4, CoFe2O4 and ZnCo2O4 are confirmed by extended X-ray absorption fine structure (EXAFS) spectra. It should be noted that the radial distances in Figure 4 (A-E) are not phase corrected, and therefore, a typical value of 0.3-0.4 Å must be added to convert the apparent distance to real bond distance. To simplify our discussion, the distances in the rest of this paper are referred to apparent distances. Three main peaks can be seen below 4 Å in mesoporous Co3O4 (Figure 4A), the first peak below 2 Å corresponds to the Co-O shells, including Co2+ tetrahedrally coordinated by 1.33 O and Co3+ 12 ACS Paragon Plus Environment
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octahedrally coordinated by 4 O.33-35 Notably, the second and third peaks correspond to the Co−Co coordination shell at ~2.5 Å (coordination number, CN: 8) and ∼3.0 Å (CN: 4). If the metal ion is in the octahedral sites, there should exist two types of distances for this metal ion connecting the other neighboring metal ions: Atom (Oh)-Atom (Oh), and Atom (Oh)-Atom (Td). However, if the metal ion of interest sits at the tetrahedral site, the distance from this metal ion to its neighboring metal ion is only Atom (Td)-Atom (Oh). In order to more clearly understand the relationship of the interatomic distance between the two geometrical metal ions, the scheme of interatomic distance between atom (Oh) and atom (Td) in Co-based spinel structure is presented in Figure 3F. It can be found that octahedrally coordinated cations exhibit two different atom−atom bond distances (interatomic distances of ∼2.5 and ∼3.0 Å) from surrounding metal ions in octahedral and tetrahedral sites, respectively. On the other hand, the tetrahedrally coordinated cations only have one atom-atom bond distance of ca. 3.0 Å (Figure 3F).25 Therefore, the peak at ∼2.5 Å in CoO (Figure 4B) and CoFe2O4 (Figure 4E) suggest that both cobalt species are occupied in the octahedral sites, while the peak (Figure 4C) at ∼2.9 Å in CoAl2O4 indicate the presence of Co2+Td. The peaks (Figure 4D) at∼2.5 Å and ∼3.0 Å in ZnCo2O4 can be assigned to Co-Co (Oh) and Co-Zn (Td), respectively. Moreover, Co K-edge EXAFS spectra of Figure 4(B-E) indicate that CoO, ZnCo2O4, CoAl2O4 and CoFe2O4 still maintain the spinel structure. Additionally, the coordination environment of cobalt ions of catalysts was further investigated by Raman spectra, which are shown in Figure 4F. Typically, five Raman bands at 198, 479, 523, 613 and 686 cm-1 are visible in the range of 50-1000 cm-1, corresponding to the different modes of crystalline Co3O4. 36 Specifically, the band at 198 cm−1 is attributed to the characteristics of tetrahedral sites (CoO4), corresponding to the F2g1 symmetry. The Raman peaks centered at 479 and 523 cm−1 are associated with Eg and F2g2 symmetry, respectively. A weak band at 613 cm−1is related to the F2g2 symmetry.36, 37 The strong band at 686 cm−1 is attributed to the characteristics of octahedral sites (CoO6), matching well with the A1g species in the Oh7 spectroscopic 13 ACS Paragon Plus Environment
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symmetry. Compared to Co3O4, the Raman peak at 689 cm−1 is blue-shifted to 680 cm-1 in ZnCo2O4 and 681 cm-1 in CoFe2O4, and their peaks intensities decrease in comparison with Co3O4. The changes of Raman symmetry in ZnCo2O4 and CoFe2O4 suggest that the original coordinative environment of octahedral sites is changed, due to the change of cation-anion bond lengths and polyhedral distortion occurring in the spinel lattice after Co2+ (rionic radii =0.058 nm) or Co3+ (rionic radii =0.061 nm) in Co3O4 are replaced by Zn2+(rionic radii =0.060 nm) or Fe3+(rionic radii = 0.0645 nm)38. Interestingly, ZnCo2O4 and CoFe2O4 both show the absence of the characteristics Raman peaks of 198 cm − 1, revealing that cobalt species are both occupied in the octahedral coordination sites. A strong peak at 214 cm-1 with a very weak peak at 689 cm-1 is observed in CoAl2O4, suggesting that most Co2+ species are filled in the tetrahedral coordination sites. It should be mentioned that the Raman peak at 214 cm-1 for CoAl2O4 has a red shift (16 cm−1) in comparison with 198 cm−1 for Co3O4. The red shift suggests the occurrence of the lattice distortion or residual stress of the spinel structure,39 resulting from that Co3+ sites are substituted by smaller ionic radius Al3+ (rionic radii = 0.390 nm). Notably, an obvious Raman peak at 677 cm−1 is observed in CoO, suggesting that the Co2+ species are filled in the octahedral coordination sites, which is consistent with the Co K-edge EXAFS results. That is, the pretreatment of Co3O4 (can be written as (Co2+)[Co23+]O4) in H2 can make Co2+ species occupy the positions that are originally located by Co3+ species in Co3O4.40 Assuming all surface Co2+ species in CoO are oxidized to Co3+, their surface density could be higher with respect to the spinel structure, inducing structural defects and lattice distortion. Table 2
The coordination environments of Co-based spinel catalysts. Sample Co3O4 CoO CoAl2O4 ZnCo2O4 CoFe2O4
Tetrahedral coordination Co2+ Co2+ Zn2+ Fe3+
Octahedral coordination Co3+ Co2+ Al3+ Co3+ 2+ Co , Fe3+
3.2.3 Simulate structures 14 ACS Paragon Plus Environment
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Figure 5 The simulated structures of unit cells for Co-based spinels: (A) Co3O4; (B) CoO; (C) CoAl2O4; (D) ZnCo2O4 and (E) CoFe2O4; (red: oxygen, blue: cobalt, orange: Fe; green: Al; yellow: Zn atoms in spinel).
Based on the above EXAFS and Raman results, the coordination environments of catalysts are listed in Table 2, the corresponding simulated structures of unit cell in Co3O4, CoO, CoAl2O4, ZnCo2O4 and CoFe2O4 are shown in Figure 5. As for Co3O4 (Figure 5A), Co3+ species are located in octahedral sites (16a Wyckoff sites) and Co2+ species are in tetrahedral sites (8a Wyckoff sites).37 In the case of CoO (Figure 5B), the Co2+ species only occupy the octahedrally coordinated sites. For CoAl2O4 spinel (Figure 5C), Co2+ mainly occupies the tetrahedral coordination sites for the valence state of Al being +3. For ZnCo2O4 spinel (Figure 5D), Zn and Co occupy the tetrahedral and octahedral coordination sites, respectively. Notably, CoFe2O4 (Figure 5E)
adopts
an
inverse
spinel
structure
that
can
be
described
as
(Co1−2λFe2λ)A[Co2λFe2(1−λ)]BO4 (A and B represents A and B sites in AB2O4 structure, respectively) with λ between 0 and about 0.34.
35
Part of Fe3+ is tetrahedrally
coordinated while leaving Fe3+ together with Co2+ ions is located in the octahedrally coordinated position. It should be mentioned that small fraction of Co3+ in CoFe2O4 could also occupy the octahedral coordination sites. 15 ACS Paragon Plus Environment
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3. 3 Catalytic Activity and Kinetic Studies 3.3.1 Catalytic Activity
Figure 6 A) Benzene conversions of Co3O4, CoO, CoAl2O4, CoFe2O4 and ZnCo2O4; B) O2-TPSR-MS profiles of Co3O4 at 300 oC; C) Thermal stability of mesoporous ZnCo2O4 at 500 oC; D) Arrhenius plots for benzene oxidation over Co3O4, CoO and CoAl2O4 (Test conditions: 11.2 mg catalysts, 1000 ppm benzene, 20% O2 and balance N2; total gas flow: 150 mL/min). Figure 6 shows the catalytic efficiency of various catalysts, which are performed in a fixed-bed flow reactor. Benzene conversion increases as the rise in temperature. The temperatures of T10%, T50% and T90% (corresponding to a benzene conversion of 10, 50 and 90%, respectively) are used to compare the catalytic activities, which are illustrated in Table 1. Apparently, the T10% and T50% values of CoO are 145 and 196 °C, which are 33 and 19 °C lower than that of Co3O4, respectively. However, T90% temperature of CoO (263 °C) is slightly higher than that of Co3O4 (245 °C). The slightly higher of T90% in Co3O4 compared to that in previously reported meso-Co3O4 16 ACS Paragon Plus Environment
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(T90=239 oC)14 should be related to the different WHSV. For mesoporous ZnCo2O4 and CoFe2O4, benzene conversions increase gradually with increasing temperature, reaching 50% at 212 and 223 °C, respectively. Further increasing the reaction temperature results in a significant increase in benzene conversion, and 90% conversion is achieved at 236 and 261 °C, respectively. Nevertheless, mesoporous CoAl2O4 presents poor catalytic activity at test temperatures. Notably, bare ZnO or Al2O3 (Figure S3) displays almost no catalytic activity at 120-437 oC, and benzene conversion in Fe2O3 is less than 20% at 120-340 oC. Meanwhile, benzene conversion in ZnCo2O4 is close to that of Co3O4. However, CoFe2O4 exhibits slightly lower catalytic activity compared to CoO because of the relatively low Co ratio in the former catalyst as confirmed by ICP-AES (Table S1). Additionally, O2-TPSR-MS experiment (Figure 6B) of Co3O4 suggests that the products detected are only carbon dioxide and water. According to the activity data and moles of Co in the catalysts, we calculated the TOFs, and the results are shown in Table 1. At 167 oC, the TOFCo value follows the order of ZnCo2O4 (0.87*10-3 s-1) > CoO (0.67*10-3 s-1) > CoFe2O4 (0.45*10-3 s-1) > Co3O4 (0.30*10-3 s-1) > CoAl2O4 (0.10*10-3 s-1), suggesting that ZnCo2O4 exhibits the highest TOFCo value. Based on XRD, TEM, Raman and XAFS results, Co2+Oh sites in CoO
exhibit high catalytic activity. ZnCo2O4 with Co3+Oh species displays good
catalytic activity and high TOFCo value, while CoAl2O4 with Co2+Td sites presents poor catalytic activity and low TOFCo value. Moreover, the thermal stability of ZnCo2O4 was tested by a time-on-stream experiment where the catalyst was kept on-line at 500 o
C for 48 h. Figure 6C indicates that no significant loss in catalytic activity is
observed. Besides, the effect of 9.5vol% water vapor on benzene conversion over ZnCo2O4 was tested at 350 oC. The addition of water vapor has little effect on benzene conversion with only a slight deactivation by less than 1% (Figure S4). These results indicate that ZnCo2O4 displays high thermal stability and strong tolerance against water vapor in benzene oxidation. 3.3.2 Kinetic Studies 17 ACS Paragon Plus Environment
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A dimensionless Weisz-Prater (W − P) parameter of less than 0.3 with an effectiveness factor higher than 0.95 and reaction order of 1 provide sufficient conditions for overcoming the significant pore diffusion limitations.35 The W−P value was calculated by the following equation:
− r' Cwp =
ρ R2
A(obs ) c DeC As
[3]
Herein, -r’A(obs) is the reaction rate, R and ρc represent the catalyst particle radius and solid catalyst density, respectively. De and CAs are effective gas-phase diffusivity and gas concentration of A at the catalyst surface, respectively.4, 14, 41 Obviously, the Weisz-Prater value obtained in our present investigations is 1.90*10-3, which is less than 0.3. Therefore, no significant mass transfer limitations existed in our catalytic systems. Under the presence of excess oxygen, the catalytic VOCs oxidation (VOCs conversion less than 20%) follows first-order kinetic, which is expressed in equation 4:
r = −kc = − Aexp[− E ]c RT a
[4]
where r and k are the reaction rate (mol. L-1 s-1) and rate constant (s-1), respectively, while A and Ea are the pre-exponential factor and the apparent activation energy (kJ/mol), respectively. Herein, mesoporous CoO, CoAl2O4 and Co3O4 are chose to investigate the activation energy due to their different coordination environments of cobalt species and catalytic activities. The linear plots of Arrhenius results (Figure 6D) reveal that the benzene oxidation only remains in the kinetically controlled region at the conversion less than 20%. On the basis of the slopes of the Arrhenius plots, the apparent activation energies (Ea) have been calculated and are summarized in Table 1. The results indicate that the Ea values decrease in the following order: CoAl2O4 (112 kJ/mol) > Co3O4 (70 kJ/mol) > CoO (64 kJ/mol). Obviously, CoO and Co3O4 exhibit lower activation energies than CoAl2O4, indicating that benzene oxidation over the former two catalysts is much easier. All the results confirm that the Co2+Oh species perform excellently in catalyzing the complete oxidation of benzene at low 18 ACS Paragon Plus Environment
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temperatures.
3.4 DFT Calculations
Figure 7 Optimized geometries of top and side views for A,E) Co3O4 (220); B,F) CoO (220); C,G) CoAl2O4 (220) and D,H) ZnCo2O4 (220) in benzene adsorption models; and I,M) Co3O4; J,N) CoO(220); K,O) CoAl2O4 (220) and L,P) ZnCo2O4 (220) in oxygen adsorption models. It is commonly accepted that the benzene adsorption and surface oxygen reactivity play important roles in the catalytic reaction.
42
Thus, DFT calculation was
carried out to investigate the benzene and oxygen adsorption binding energies in Co3O4, CoO, CoAl2O4 and ZnCo2O4. The gas adsorption energy (Eads) was calculated by the following equation:
Eads = Esurface+gas − Esurface − Egas
[4]
where Esurface+gas, Esurface and Egas are the total energy of the optimized gas adsorbed on the surface, the energy of the naked surface and the energy of a gas molecule, respectively.43, 44 Therefore, the more negative value of Eads, the stronger adsorption ability will be. The optimized benzene and oxygen adsorption models of Co3O4, CoO, CoAl2O4 and ZnCo2O4 are shown in Figure 7. Benzene adsorbs with the ring parallel to the (220) surface up to saturation of the surface.45 The corresponding Eads-C6H6 19 ACS Paragon Plus Environment
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values (Table 3) of catalysts follows the order of ZnCo2O4 ( |-0.69 eV| ) > CoO ( |-0.39 eV| ) > Co3O4 ( |-0.28 eV| ) > CoAl2O4 ( |-0.06 eV| ). Besides, the Eads-o2 values (Table 3) of Co3O4, CoO, CoAl2O4 and ZnCo2O4 are -1.25, -0.98, -0.70 and -2.48 eV, respectively. These results indicate that the benzene and oxygen adsorptions are the easiest on ZnCo2O4 among these Co-based spinel catalysts. To be mentioned, benzene and oxygen prefers to adsorb on CoO (220) rather than CoAl2O4 (220) plane, suggesting that benzene and oxygen are more easily adsorbed in Co2+Oh with respect to Co2+Td sites. Moreover, the Co-O bond length in Co3O4, CoO, CoAl2O4 and ZnCo2O4 is 1.89, 1.84, 2.37 and 1.81 Å, respectively (Table 3), revealing that the bond length obviously increases after Co3+ species are replaced by Al3+. It is caused by the lattice expansion and the relaxation of Co-O bond in the CoAl2O4.
Table 3 Bond lengths of Co-C, Co-O and the adsorption energy of benzene and oxygen over Co3O4, CoO, CoAl2O4 and ZnCo2O4.
Sample
Co-C(benzene)
Eads-C6H6
Co-O (oxygen)
Eads-O2
(Å)
(eV)
(Å)
(eV)
Co3O4
2.42
-0.28
1.89
-1.25
CoO
2.38
-0.39
1.84
-0.98
CoAl2O4
2.35
-0.06
2.37
-0.70
ZnCo2O4
2.02
-0.69
1.81
-2.48
3.5 In Situ DRIFTS
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Figure 8 A-E) in situ DRIFTS spectra of benzene adsorption at 150 oC over Co3O4, CoO, ZnCo2O4, CoAl2O4, CoFe2O4 (test condition: 1000 ppm benzene+N2); F-J) in situ DRIFTS of O2 reacting with pre-adsorption benzene at 150 oC in Co3O4, CoO, ZnCo2O4, CoAl2O4, CoFe2O4 (test conditions: 20%O2 + N2); K-L) in situ DRIFTS of O2 reacting with pre-adsorption benzene at 250 oC in Co3O4 and CoFe2O4 (test conditions: 20%O2 + N2).
In situ DRIFTS experiment was employed to determine the intermediate species during the benzene oxidation, andthe results are shown in Figure 8. The bands at 963-1000 cm-1 are observed in all catalysts, which can be assigned to the lattice vibration of the Co-based spinel.
46
For mesoporous Co3O4 (Figure 8A), a weak band
located at 2929 cm−1 is associated with the C-H stretching vibrations. Notably, the strong bands at 1590 and 1418 cm−1 are characteristic of carboxylate species (1598 cm 21 ACS Paragon Plus Environment
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−1
: the ring vibrations; 1418 cm−1: symmetric COO stretching group),46, 47 resulting
from the oxidation of breaking ring. 46 The band at 1359 cm-1 is assigned to the acetate species.48 These bands intensities monotonically increase with time on stream, indicating that organic by-products are accumulated on the catalyst surface. The bands at 1589 and 1359 cm-1 are also detected in ZnCo2O4 (Figure 8B), another weak band attributable to adsorbed benzene is observed at 3248 cm-1, which is different from gaseous benzene (3038-3057 cm-1). The shifts in wavenumber for adsorbed benzene indicate that benzene strongly interacts with ZnCo2O4, as confirmed by DFT calculation. In the case of mesoporous CoO (Figure 8C) and CoFe2O4 (Figure 8D), the weak bands in the range of 1305-1307 cm-1 are assigned to the ring vibrations.48, 49 However, the band at 1308 cm-1 is absent for CoAl2O4 (Figure 8E). Moreover, introducing O2 at 150 oC cannot obviously increase the formation of carboxylate species in Co3O4 (Figure 8F) and ZnCo2O4 (Figure 8G), suggesting that the carboxylate species are mainly generated on the surface Co3+ sites and don’t change in the oxygen atmosphere. Interestingly, adding O2 can obviously increase the peak intensity in CoO and CoFe2O4. In the presence of 20% O2, the appearance of new bands at 1702 and 1438-1439 cm−1 are associated with the symmetric COO stretching group in CoO, suggesting that Co2+Oh sites have been oxidized to Co3+ sites. Then a faster breakage of the benzene rings occurs on Co3+ sites, resulting in the formation of carboxylate species. Similarly, the carboxylate species (1580 and 1438 cm-1) are observed in CoFe2O4 (Figure 8J). Nevertheless, no such bands are detected for CoAl2O4 (Figure 8I), and the bands intensities have no obvious change after the addition of 20% O2, indicating that Co2+Td sites are not easily oxidized at 150 oC. The decreasing trend for the carboxylates species presented in Figure 8 (K-L) suggests that the carboxylates species are the primary intermediate species in the catalytic oxidation of benzene, that are oxidized by O2 to generate CO2 and H2O as final products. This speculation is consistent with previously reported results for benzene oxidation.46 All these results indicate that Co3+ are the main active sites in the oxidation of breaking C-H. XPS spectra (Figure S5) of Co2p in Co-based spinel catalysts after activity test 22 ACS Paragon Plus Environment
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further prove that the octahedrally coordinated Co2+ species in CoO structure are oxidized to Co3+ in the benzene oxidation process. However, Co2p XPS spectrum of CoAl2O4 still displays two shake-up peaks at 786 and 804 eV, suggesting the tetrahedron-coordinated Co2+ sites are not easily oxidized to Co3+. The reason will be illustrated by the following discuss with ligand field theory.
3.6 Reaction Mechanism
Figure 9
Schematic electronic configurations of the Co2+Td, Co2+Oh, Co3+Td and
Co3+Oh.△o and △t represent the octahedral and tetrahedral crystal field splitting in cobalt ions, respectively. P indicates electron pairing energy.
The oxidation ability tendency of Co2+Td to Co3+Td and Co2+ Oh to Co3+Oh can be explained by ligand field theory. The scheme of crystal field splitting and electron distribution over Co2+ and Co3+ are presented in Figure 9. Co2+ on the octahedral site is considered to be low-spinning, because the △o value for Co2+ is greater than the electron pairing energy.40 However, Co2+ on the tetrahedral site can be only high-spinning due to its △t value is much smaller than △o and thus less than the pairing energy.40 The crystal-field stabilization energies (CFSE) of Co2+ on octahedral and tetrahedral sites are 9/5△o-P and 24/45△o, respectively. The CFSE of Co3+ on octahedral site is 12/5△o-2P. Thus, the Co3+Oh (t2g6eg0) is energetically substantially supported Co2+Oh (t2g6eg1) in a moderate ligand field, thus removing an electron from an e orbital would oxidize Co2+ to Co3+. However, the ligand field stabilization of 23 ACS Paragon Plus Environment
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Co3+Td (eg3t2g3) to Co2+Td (eg4t2g3) is relatively small, resulting in that the oxidation of Co2+Td to Co3+Td is relatively difficult. Besides, there is a higher driving force for oxidation of Co2+Oh due to its location in a more open framework site than Co2+Td, which is also beneficial to make Co2+Oh more accessible to oxygen and more easily oxidized to Co3+. Combining the above analysis above, the reaction mechanisms are described as follows: 1) The Adsorption and the Activation of C-H Based on DFT calculation results (Figure 7), the adsorption of benzene over Co2+Oh is more easily than that over Co2+Td. The activation of the adsorbed benzene or oxygen species is rate-limiting.15 Compared to Co2+Td, lower apparent activation energy of Co2+Oh (Figure 6D) is beneficial to fast activate the adsorbed species. Additionally, according to Boreskov’s classification,50-52 oxidation reactions occur via an associative (concerted) or redox (step-wise) mechanism: the former is characterized by lower activation energies, whereas the latter involves lattice oxygen. The occurrence of one or other of the two mechanisms depends on lattice oxygen availability of the catalyst. In the present case, O1s XPS results (Figure 3E) display an apparently lower amounts of surface oxygen species for CoO, Co3O4 and ZnCo2O4 catalysts when compared with CoAl2O4, demonstrating the beneficial role of surface lattice oxygen towards benzene total oxidation.52, 53 The surface lattice oxygen species could be replenished by gaseous phase oxygen. Therefore, the mobility of oxygen is also an important parameter for catalyst performance. It was reported that one of the factors determining the oxygen mobility is the formation of oxygen vacancies in the spinel structure.54 Based on XPS results (Figure 3E), ZnCo2O4 and CoO possess more surface oxygen vacancies than Co3O4, which in turn makes a significant contribution to fast-oxygen mobility in the bulk. That is, the high oxygen mobility in the bulk of the ZnCo2O4 and CoO through oxygen vacancies in spinel structure, resulting in higher catalytic activity of CoO and ZnCo2O4 compared to that of Co3O4 at low temperature although they all contain the octahedron-coordinated cobalt species. 24 ACS Paragon Plus Environment
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2) The C-H Cleavage Step on Co-based Spinels Catalysts Based on the in situ DRIFTS results (Figure 8), the bands associated with C-H stretching vibrations at 2929 (Figure 8 A) and 1308 cm−1 (Figure 8 C-D) disappear in the presence of N2+O2 for 20 min at 250 oC (Figure 8 K-L), indicating that partial hydrogen atoms of the benzene ring have been abstracted. The results can be be interpreted as one hydrogen leaves as an H+ on the surface and probably links to an oxide ion, giving rise to an –OH. Other C-H groups links to another oxide ion, giving rise to the breakage of benzene rings. Then the oxidative breakage of the benzene rings on cobalt sites can occur via the interaction of σ and σ* C-H orbitals with d-type orbitals of Co3+, resulting in the formation of carboxylates species. Finally, the carboxylates species can be further oxidized by O2 to the final products (i.e., CO2 and H2O). Notably, Co2+Oh sites are much more easily oxidized to Co3+Oh species in the presence of oxygen at low temperature (i.e. 150 oC), promoting the oxidative of the cracked benzene rings to COO species as confirmed by in situ DRIFTS results. 4. Conclusions To summarize, we have successfully separately studied the roles of Co2+Oh, Co2+Td and Co3+Oh for benzene oxidation based on a metal ion-substitution strategy. The results demonstrate that not only the Co3+Oh species act as the active sites, but also the Co2+Oh sites exhibit high catalytic activity due to that they are easily oxidized to active Co3+ species. In contrast, Co2+Td species shows poor catalytic activity and low TOFsCo value. In situ DRIFTS results demonstrate that the Co3+ sites accounts for the formation of carboxylates species, which are the main intermediate species in the oxidation of benzene. This study provides a new strategy, tuning cobalt oxidation states and coordination environments, to promote oxidation activity. Supporting Information The Supporting Information (additional HR-TEM images, benzene conversion of reference ZnO, Al2O3 and Fe2O3, Data of ICP-AES, XRF and XPS) is available free of charge on the ACS Publications website.
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AUTHOR INFORMATION *L. Jiang: e-mail:
[email protected]; *Y. Luo: e-mail:
[email protected]; *R. Wang: e-mail:
[email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Basic Research Program of China (2013CB933202), the National Natural Science Foundation of China (21407025, 21576051) and the National High-tech R&D Program (2015AA03A402), National key research and development program (2016YFC0203903. We also thank the National Synchrotron Radiation Laboratory, BSRF for the synchrotron beamtime.
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