Al2O3 with Yttrium Modification

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Research Article Cite This: ACS Catal. 2019, 9, 8373−8382

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Improving Syngas Selectivity of Fe2O3/Al2O3 with Yttrium Modification in Chemical Looping Methane Conversion Yu Kang,†,‡ Ming Tian,*,† Chuande Huang,† Jian Lin,† Baolin Hou,† Xiaoli Pan,† Lin Li,† Alexandre I. Rykov,† Junhu Wang,† and Xiaodong Wang*,† †

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Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, People’s Republic of China ‡ University of Chinese Academy of Sciences, No. 19(A) Yuquan Road, Shijingshan District, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: It is of great significance to improve the syngas selectivity of Fe-based oxygen carriers (OCs), because of their sufficient lattice oxygen, low cost, and environmental compatibility in chemical looping partial oxidation of CH4. In this work, it was found that the addition of Y could remarkably increase CO selectivity of Fe2O3/Al2O3 to 98% with a CH4 conversion of ∼90%. X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterizations combined with Mössbauer spectroscopy illustrated that the incorporation of Y led to the Fe species gradually transferring from Fe2O3 into the garnet structure (Y3Fe2Al3O12), a newly formed phase, which was found to be highly active for syngas generation. Density functional theory (DFT) calculations demonstrated that such a high CO selectivity of confined Fe species in garnet originated from enhanced oxygen vacancy formation energy (Eov), compared with Fe2O3, which resulted from the lattice oxygen shared by not only reducible Fe ions but also nonreducible Al and Y ones in a garnet structure. Therefore, our work provides a meaningful guidance of new materials screening for methane partial oxidation in the chemical looping process. KEYWORDS: CH4 conversion, chemical looping reforming, Fe-based OCs, garnet, oxygen vacancy formation energy

1. INTRODUCTION Shale gas as a new unconventional energy source has attracted much attention during recent decades, because of its large recoverable reserves.1−3 The conversion of methane as the main component of shale gas to syngas (H2 + CO) is an important reaction for the production of methanol and Fischer−Tropsch synthesis in chemical industry.4−7 However, traditional steam/carbon dioxide reforming of methane is a thermodynamically limited and energy-intensive process with a large amount of carbon emission and unsuitable H2/CO ratio for the downstream application.8−10 Alternatively, partial oxidation of methane to syngas is exothermic and thermodynamically favorable, which is a promising technique for syngas production. However, a gas mixture of CH4 and O2 will lead to a risk of explosion and O2 supply from air separation is also highly costly,11 which restrict the commercialization of this process. As an alternative, chemical looping reforming (CLR) of methane is proposed for methane conversion.12−16 Such a novel process involves two steps. That is, CH4 first reacts with lattice oxygen of oxygen carriers (OCs) to form syngas and the reduced OCs are then oxidized by air for oxygen storage (see Scheme S1 in the Supporting Information). The redox process could not only convert methane to syngas with a H2/CO ratio of 2 in the reduction step, which is suitable for the production of methanol and Fischer−Tropsch synthesis (2H2 + CO = © XXXX American Chemical Society

CH3OH; 2nH2 + nCO = CnH2n + nH2O) without the need to tune the feed ratio, but also avoid complex gas separation, because of the use of air as an oxidant instead of pure O2. Moreover, the risk of explosion has also been prevented due to the separation of methane and air.17,18 Hence, CLR is considered as one of the most potential techniques for syngas generation. Undoubtedly, it is of great importance to select suitable OCs with high methane reactivity, syngas selectivity, and sufficient oxygen carrying capacity. Currently, Fe-based OCs was most reported for CLR, because of their sufficient lattice oxygen, low cost and environmental compatibility.19−22 However, iron oxide shows deactivation during multiple redox cycles, and the large amount of CO2 formation during the reduction of Fe2O3 to Fe3O4 leads to a decrease in CO selectivity for pure Fe2O3 and supported Fe2O3 such as Fe2O3/Al2O3 OCs.23−28 To this end, some researchers added a variety of dopants into the Fe2O3 phase to promote the dispersion of iron species and increase CO selectivity. For instance, Lee et al.29 embedded ceria into mesoporous Fe2O3/Al2O3 OCs, leading to high selectivity when cofeeding CH4 and CO2 during the reduction step in the CLR process. Li et al.30,31 found that Ce1−xFexO2−δ Received: June 28, 2019 Revised: July 19, 2019

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DOI: 10.1021/acscatal.9b02730 ACS Catal. 2019, 9, 8373−8382

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Typically, taking 1.5-Y2O3 as an example (Y:Fe molar ratio of 3:2), 2.298 g of Y(NO3)3·6H2O (Aladdin, China), 1.616 g of Fe(NO3)3·9H2O (Damao, China), and 2.2508 g of Al(NO3)3· 9H2O (Damao, China) were dissolved in deionized water. Citric acid and ethylene glycol were then added to the solution at a molar ratio of 2:1 with stirring for 1 h. The ratio of citric acid to total metal ions was 3:1. The mixture was stirred at 80 °C until a viscous gel formed. After being dried at 120 °C overnight, the sample was calcined at 1200 °C for 4 h with the heating rate of 5 °C/min. 0-Y2O3, 0.5-Y2O3, and 1.0-Y2O3 could also be prepared using the methods described above. 2.2. Characterization of OCs. The crystalline structure of materials were analyzed by PANalytical X’Pert-Pro powder Xray diffractometer with Cu Kα monochromatized radiation (λ = 0.1541 nm). The samples were scanned with a step size of 0.0334° in the range of 10°−80° 2θ. The working voltage and current of the X-ray tube were 40 kV and 40 mA, respectively. HighScore Plus software was used to analyze the phase structure of the materials. Scanning electron microscopy (SEM) images derived from an SEM system (Model JSM-7800F, Jeol, Japan). Transmission electron microscopy (TEM) experiments were performed on a TEM system (Model JEM-2100F, Jeol) that was equipped with energy-dispersive spectroscopy (EDS) instrument. The 57Fe Mössbauer spectra were recorded at room temperature with a spectrometer working in the mode of constant accelerations with 57Co γ-quantum source in Rh matrix. The absorbers were obtained by pressing the powdered samples. All spectra were computer-fitted to a Lorentzian shape with a least-squares fitting procedure. The isomer shifts (IS) were given with respect to the centroid of α-Fe at room temperature. H2-TPR was performed on Micromeritics Auto Chem II 2920 apparatus equipped with a thermal conductivity detection (TCD) device. A quantity of 0.1 g material was pretreated in Ar (30 mL/min) at 300 °C. After cooling to 50 °C, the sample was heated to 950 °C in 10% H2/He (30 mL/min) with the heating rate of 10 °C/min. 2.3. Redox Reactivity Tests. The reactivity tests were performed by using a quartz fixed-bed reactor (with an inner diameter (id) of 6 mm) under atmospheric pressure. A quantity of 0.2 g of OCs were used in the test. During the redox reaction, the reduction step was performed with a space velocity of 6000 h−1 g−1 in 5% methane balanced in helium for 5.5 min. The reactor was then purged with helium. 5% oxygen in helium was used as an oxidant medium with the flow rate of 30 mL/min. The entire cycle was conducted at 900 °C. To test the stability of OCs, 20 redox cycles were also performed. The gas concentration was measured using a quadrupole mass spectrometry (MS) system (IPI, Model GAM200). The characteristic m/z signals of CH4, CO2, CO, and H2 are selected as 15, 44, 28, and 2, respectively. The MS signals were calibrated with calibration gases before tests. The amount of gas is calculated by integrating the MS signals. The calculation of CO selectivity, CO yield, CH4 conversion, oxygen converted and H2 selectivity are described in the Supporting Information. 2.4. Calculation Details. All the density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP). The projector-augmented wave (PAW) method was used to represent the core−valence interaction.50,51 The plane wave energy cutoff was set to 450 eV. The generalized gradient approximation (GGA) with the

(x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) exhibited high CO selectivity of more than 80%, which was attributed to the interaction between highly dispersed iron oxide and Ce−Fe solid solution enhancing the syngas production rate effectively.32 However, only a small amount of Fe3+ (x < 0.2) could form a solid solution with ceria and the rest separated from the lattice in the form of Fe2O3, which resulted in a large amount of CO2 formation during reduction to Fe3O4, thereby limiting the increase in CO selectivity of the OCs. Recently, some dopants (La, Sr, etc.) in Fe2O3 were also reported to form mixed oxides such as perovskite in the CLR of CH4 and show high selectivity to syngas.15,27,28,33−39 Zhang et al.40 reported SrFeO3−δ−CaO nanocomposites were effective redox materials, with up to 90% syngas selectivity. Dai et al.41 found that LaFeO3 exhibited high CO selectivity of >95% in redox cycles of partial oxidation of CH4. Later, other groups reported improvements in the LaFeO3 reactivity by partial substitution of La or Fe. The effect of Sr doping on the performance of La1−xSrxFeO3 was investigated and found Sr incorporation could inhibit methane decomposition, the optimum amount of Sr was x = 0.3−0.5.42 Huang et al. also revealed that La0.6Sr0.4Fe0.8Al0.2O3 exhibited superior reactivity with CO selectivity of ∼95% and higher oxygen converted than LaFeO3, which resulted from the decrease of adsorbed surface oxygen and deep reduction from Fe4+ to Fe0 without carbon deposition.43 However, not all of the Fe species dispersed into the mixed oxides are highly selective to syngas production.16,24,44 For instance, Zhu et al. found that the introduction of La into Fe2O3/Al2O3 led to the formation of Lahexaaluminate, and only Fe3+ in trigonal bipyramid (Fe5) and tetrahedral (Fe4) sites could partially oxidize methane to syngas, while the ones in the mirror plane are nonselective.45,46 Until now, however, the underlying factor that determines the selectivity of Fe species with different surrounding environment was still unknown, Moreover, much of the work has focused on La-doped mixed oxides, and it is also highly desired to search for new additives for enhancing syngas selectivity of Fe-based OCs for CLR of CH4. Yttrium, as an important member of the rare-earth family, has properties similar to those of lanthanum. It was reported the addition of Y into the Fe species led to the formation of the Y−O−Fe structure in catalytic methane oxidation,47−49 which indicated that the interaction between Y and Fe might tune the surrounding environment of the Fe species and, thus, their selectivity. Therefore, in this paper, we reported an yttriumpromoted strategy in order to improve the syngas selectivity of Fe2O3/Al2O3 (0-Y2O3) OC. For the first time, it was observed that the OC with a molar ratio Y2O3:Fe2O3 of 1.5 (1.5-Y2O3) exhibited the highest CO selectivity of 98% with a CH4 conversion of ∼90% in CLR process. This could be attributed to the fact that Fe species completely entered into a newly formed garnet structure (Y3Fe2Al3O12), which was determined to be highly selective for CO formation. Subsequent H2-TPR characterization combined with density functional theory (DFT) calculations were conducted and demonstrated that the oxygen vacancy formation energy (Eov) played a key role for Fe ions in garnet being highly selective for CO generation.

2. EXPERIMENTAL SECTION 2.1. Preparation of OCs. Fe2O3/Al2O3 and Y-added OCs (x-Y2O3, where x indicates the molar ratio of Y2O3 to Fe2O3 (x = 0, 0.5, 1, and 1.5)) were prepared via the sol−gel method. The amount of Fe2O3 for each sample was fixed to 24.5 wt %. 8374

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Figure 1. Reaction kinetic curves of (a) the fresh 0-Y2O3, (b) 0.5-Y2O3, (c) 1.0-Y2O3, and (d) 1.5-Y2O3.

Figure 2. (a) CO selectivity and yield of fresh OCs, (b) methane conversion, (c) CO selectivity, and (d) H2/CO ratio of OCs during 20 redox cycles.

computational models are described in the Supporting Information. The computational models of perfect α-Fe2O3 (001) and Y3Fe2Al3O12 (111̅) surfaces are described in Figure S1 in the Supporting Information.

Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional was used in our calculations.52 The Brillouin zone was sampled at the gamma point with the 2 × 2 × 1 and 1 × 1 × 1 k-point meshes for α-Fe2O3 (001) and Y3Fe2Al3O12 (111̅) surfaces, respectively. The energy and force criterion for convergence of the electron density are set at 10−6 eV and 0.05 eV/Å, respectively. To accurately treat the highly localized Fe 3d-orbitals, we conducted spin-polarized DFT+U calculations with a value of Ueff = 3.6 eV applied to the Fe 3d state. The

3. RESULTS 3.1. Reactivity Tests. Figure 1 shows the kinetic curves of x-Y2O3 (x = 0, 0.5, 1, and 1.5) in the first reduction process. For unadded OC, almost complete oxidation of CH4 occurred, 8375

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Figure 3. SEM images and line scans of (a, m) fresh 0-Y2O3, (b, n) 0.5-Y2O3, (c, o) 1.0-Y2O3, and (d, p) 1.5-Y2O3. STEM and EDS mappings of (e, f) fresh 0-Y2O3, (g, h) 0.5-Y2O3, (i, j) 1.0-Y2O3, and (k, l) 1.5-Y2O3.

distinctly higher activity than the unadded one. Especially, 1.5-Y2O3 OC presented the highest reactivity with both CH4 conversion and CO selectivity of ∼90% and a H2/CO ratio of ∼2, which is suitable for subsequent methanol production and Fischer−Tropsch synthesis. To the best of our knowledge, iron-based OCs usually suffer from low methane conversion in CLR of CH4,8 although Lee’s group has reported that CH4 conversion and CO selectivity of >90% could be obtained by coupling CH4 decomposition and dry reforming process with CLR over Fe2O3−NiO/La0.8Sr0.2FeO3 and Ni-doped Fe2O3/ Al2O3, respectively,55,56 and our 1.5-Y2O3 exhibited comparable results with theirs, which made it a promising OC in the CLR process. In addition, it was interesting to note that the reactivity of 0.5-Y2O3 and 1.0-Y2O3 increased with the number of redox cycles (see Figure S3 in the Supporting Information). For instance, CH4 conversion and CO selectivity of 0.5-Y2O3 were 18% and 41%, respectively, in the first reduction, and they increased to 41% and 69% after the fifth reduction. As for 1.0Y2O3, CH4 conversion and CO selectivity increased from 66% and 67% to 82% and 72%, respectively, after two redox cycles. These results indicated that the addition of Y improved the activation of 0.5-Y2O3 and 1.0-Y2O3 to reach their best reactivity and the activation period became shorter (five cycles vs two cycles) with the increase in amount of Y. The reason for the enhanced reactivity would be discussed in the following section. 0.5-Y2O3 and 1.0-Y2O3 after five and two redox cycles were denoted as 0.5-Y2O3-A (activated) and 1.0-Y2O3-A, respectively.

followed by the formation of a very small amount of CO (Figure 1a), which was consistent with the results obtained in the previous work.53,54 When increasing the amount of Y (x) to 0.5, CO formation slightly increased (Figure 1b). By further increasing the amount of Y to x = 1, significant CO formation could be observed (Figure 1c) after a large amount of CO2 formed (reaction time of >160 s). For 1.5-Y2O3, nearly only partial oxidation of CH4 occurred, accompanied by a small amount of CO2 formation in the initial of reaction (Figure 1d). The results indicated that the addition of Y to Fe2O3/Al2O3 OC was favorable for the partial oxidation of CH4 to syngas. CO selectivity and the yield of fresh x-Y2O3 OCs, relative to the amount of Y (x), are given in Figure 2a. It could be seen that the selectivity and yield of CO increased as the amount of Y added to the OCs increased. For example, for 0-Y2O3, CO selectivity and yield were as low as ∼20% and 0.078 mmol/g, respectively. The addition of Y of x = 0.5 resulted in the increase of CO selectivity and yield to 45% and 0.1 mmol/g. The highest CO formation was obtained for 1.5-Y2O3 with CO selectivity and yield of 98% and 1.1 mmol/g, which is more than one order magnitude larger than that observed for unadded OC. In addition, the H2 selectivity was also given in Figure S2 in the Supporting Information, and more Y incorporation led to higher H2 selectivity. These results suggested that the addition of Y could significantly improve the formation of syngas in CLR of CH4. The redox stability of x-Y2O3 OCs was also investigated, and the results are shown in Figures 2b−d. Although all the samples exhibited good redox stability during 20 cyclic tests, the reactivity differed greatly. Y-added samples showed 8376

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these two phases, a new one attributed to a garnet structure (JCPDS No. 1-082-0575) was also identified. Compared with the typical garnet with Y3Al5O12 formula, the cell parameters were relatively larger (see Table S1 in the Supporting Information), indicating that Fe species entered into the garnet structure by substituting part of the Al sites. With further increases in the amount of Y, no Fe2O3 phase was observed, except garnet, and AlFeO3 ones for 1.0-Y2O3, illustrating that Fe species entered these two phases. Through the enlargement of XRD patterns of 0.5-Y2O3 and 1.0-Y2O3 between 32° and 34° (see Figure S5a in the Supporting Information), it could be seen that the [420] diffraction peaks of these two samples are located between that of Y3FeAl4O12 and Y3Fe2Al3O12, indicating the number of Fe atoms substituting into the Al sites in the garnet lattice ranged between 1 and 2 for the 0.5-Y2O3 and 1.0-Y2O3 samples. For 1.5-Y2O3, the pure garnet (Y3Fe2Al3O12) structure formed without the Fe2O3 phase and its cell parameters were larger than those of Y3Al5O12, indicating that all Fe species entered and highly dispersed into the garnet lattice. XRD results presented that the Fe species gradually transferred from Fe2O3 into the garnet structure with the addition of Y. That was why the dispersion of Fe species could be greatly improved. In addition, the XRD patterns of OCs after 20 cycles (Figure S5b in the Supporting Information) were almost the same as those of fresh ones, suggesting the stability of their structure during multiple redox cycles. The XRD patterns of 0.5-Y2O3-A and 1.0-Y2O3-A are also provided in Figure 4. After activation, Fe2O3 and AlFeO3 phases in 0.5-Y2O3 and 1.0-Y2O3 disappeared completely. This might be resulted from the addition of Y favorable for the formation of garnet by the occurrence of some solid phase reactions during redox cycles. Considering that 0.5-Y2O3-A and 1.0-Y2O3-A exhibited higher reactivity than 0.5-Y2O3 and 1.0Y2O3 (Figure S3), the formation of garnet might play a key role for the improvement of performance. At present, few literature has identified the amount of Fe2O3 and Fe ions in the mixed oxides, and their quantitative correlation with reactivity in the CLR of CH4, which might be due to the lack of efficient characterization technique. In this work,57 Fe Mössbauer spectroscopy characterization was employed to quantify the content of different iron species in x-Y2O3 OCs. The results are shown in Figure 5, and the corresponding Mössbauer parameters are given in Table 1. For 0-Y2O3, it was found that a magnetic sextet and a doublet with relative areas of 42% and 58%, respectively, were observed, which were ascribed to hematite (with larger crystal size) and superparamagnetic Fe2O3 (with smaller crystal size) or Fe species in Al2O3, respectively. For 0.5-Y2O3, the amount of hematite with a larger crystal size decreased to as low as 16%, and Fe2O3 with a smaller crystal size or Fe species in Al2O3 increased to 72%, indicating that the addition of Y significantly alleviated the sintering of Fe 2 O 3 . Besides these two interactions, two new doublets with IS of 0.06 mm/s, quadrupole splitting (QS) of 0.89 mm/s (corresponding to a relative area of 11%) and IS of 0.39 mm/s, QS of 0.34 mm/s (corresponding to an area of 1%) were observed, attributing to the Fe3+ in the tetrahedral and octahedral Al sites of garnet, respectively. This suggested that the incorporation of Y resulted in part of the Fe2O3 being transferred into the garnet structure. By increasing the amount of Y to x = 1, the sextet ascribed to hematite completely disappeared and two doublets attributed to Fe3+ in AlFeO3, considering XRD results, were

The reactivity test results demonstrated that the incorporation of Y greatly enhanced the CO selectivity and yield of Fe2O3/Al2O3 OC, and 1.5-Y2O3 exhibited the superior reactivity and cyclic redox stability over the other three samples for CLR of CH4. 3.2. Morphological and Structural Characterization of OCs. Figure 3 shows the results of SEM, STEM, EDS, and line scans of fresh x-Y2O3 OCs. For 0-Y2O3, large irregularly shaped particles with several micrometers (red circles) were observed, accompanied by some smaller ones (Figure 3a), indicating severe aggregation of Fe2O3 or supports. As the amount of Y increased, the particle size decreased significantly and uniform nanoparticles with an average size of ∼300 nm were observed, especially for 1.5-Y2O3 (Figure 3d), suggesting that the addition of Y could release the aggregation of Fe2O3/ Al2O3 OC. The results of EDS element mappings (Figures 3e− l) showed that Fe and Al species suffered from inhomogeneous distribution and severe phase separation (see Figures 3e and 3f). The incorporation of Y led to the improvement of dispersion of Fe species and the best dispersion was observed for 1.5-Y2O3, wherein Fe, Al, and Y species exhibited homogeneous distributions. This was also demonstrated by the results of line scans (Figures 3m−p). In addition, the SEM images of spent OCs were also provided in Figure S4 in the Supporting Information. It could be seen that the spent 0-Y2O3 (Figure S4a in the Supporting Information) and 0.5-Y2O3 (Figure S4b in the Supporting Information) encountered severe aggregation, with some of their particle size reaching ∼10 μm, in contrast to the fresh ones (Figures 3a and 3b). For comparison, the particles of spent 1.0-Y2O3 (Figure S4c in the Supporting Information) and 1.5-Y2O3 (Figure S4d in the Supporting Information) only exhibited slight aggregation with the size of ∼0.3−1 μm, much smaller than those of spent 0Y2O3 and 0.5-Y2O3OCs, suggesting the incorporation of Y could effectively alleviate the aggregation of particles for both fresh and spent OCs. X-ray diffraction (XRD) patterns of fresh x-Y2O3, 0.5-Y2O3A, and 1.0-Y2O3-A OCs are shown in Figure 4. For 0-Y2O3, Al2O3 (JCPDS No. 1-088-0826) and Fe2O3 (JCPDS No. 1084-0309) were identified with the crystal size of Fe2O3 of 65 nm calculated by the Scherrer equation. The addition of Y (x = 0.5) led to the decrease of their peak intensity remarkably especially for the Fe2O3 phase, indicating that its crystal size decreased significantly with the incorporation of Y. Besides

Figure 4. X-ray diffraction (XRD) patterns of fresh and activated OCs. 8377

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53% vs. 34%), which should be due to the addition of Y causing more Fe3+ entering into the garnet structure during the activation period for these two samples, this was consistent with the XRD results. 57Fe Mössbauer spectroscopy characterization displayed that the amount of Fe3+ entering in the garnet structure increased with that of Y, and the incorporation of Y led to a higher amount of Fe3+ in garnet for 0.5-Y2O3-A and 1.0-Y2O3-A, compared with fresh counterparts. Combined with the results of reactivity test (Figure 2a and Figure S3), Fe species in garnet might be highly active for the partial oxidation of CH4. In order to correlate the Fe species in garnet and the formation of syngas, CO yield of 0-Y2O3, 0.5-Y2O3-A, 1.0Y2O3-A, and 1.5-Y2O3 OCs are given in Figure 6. The enhancement of CO yield was paralleled by the increase in the amount of Fe ions in garnet for activated OCs, demonstrating that Fe ions in garnet was highly favorable to CO production.

4. DISCUSSION The results above showed that the addition of Y led to Fe species gradually transferring from Fe2O3 to the garnet lattice, which exhibited high reactivity for the generation of syngas. In the following section, the underlying reason for Fe3+ in garnet highly selective for CO formation would be uncovered. In the CLR of methane, it was reported that the loosely bound oxygen to metal cations was defined as reactive species for CO2 formation while those coordinated with metal cations confined tightly in the lattice was easier to produce CO.34,41,57 This indicated that the difficulty in the removal of lattice oxygen coordinated with metal ions was closely associated with selectivity. In our case, severe phase separation occurred for

Figure 5. 57Fe Mössbauer spectroscopy of (a) fresh 0-Y2O3, (b) 0.5Y2O3, (d) 1.0-Y2O3, (f) 1.5-Y2O3, and (c) activated 0.5-Y2O3-A and (e) 1.0-Y2O3-A.

observed with their amount being 25%, indicating the increase in the amount of Y led to Fe2O3 partly entering into AlFeO3. In addition, the amount of Fe3+ in garnet increased to 34%. Further increasing the amount of Y to x = 1.5 led to all Fe3+ entering into the garnet lattice. Also note that the amount of Fe3+ in garnet for 0.5-Y2O3-A and 1.0-Y2O3-A was ∼2 times greater than that for 0.5-Y2O3 and 1.0-Y2O3 (23% vs. 12% and Table 1. Parameters of 57Fe Mössbauer Spectroscopy for OCs isomer shift, IS (mm/s)

quadrupole splitting, QS (mm/s)

hyperfine magnetic field, H (T)

relative area, A (%)

assignmenta

0-Y2O3

0.37 0.30

−0.20 0.54

50.8 −

42 58

Fe2O3 superpara-Fe2O3 or Fe3+ in Al2O3

0.5-Y2O3

0.36 0.33 0.06 0.39

−0.21 0.58 0.89 0.34

50.7 − − −

16 72 11 1

Fe2O3 superpara-Fe2O3 or Fe3+ in Al2O3 garnet (tetrahedral site) garnet (octahedral site)

0.5-Y2O3-5th

0.36 0.30 0.41 0.16

−0.19 0.51 0.36 1.06

51.1 − − −

19 58 3 20

Fe2O3 superpara-Fe2O3 or Fe3+ in Al2O3 garnet (octahedral site) garnet (tetrahedral site)

1.0-Y2O3

0.36 0.14 0.35 0.35 0.30

0.38 1.06 0.81 1.24 0.54

−

19 15 20 5 41

garnet (octahedral site) garnet (tetrahedral site) Fe3+ in AlFeO3 Fe3+ in AlFeO3 superpara-Fe2O3 or Fe3+ in Al2O3

1.0-Y2O3-2nd

0.15 0.37 0.30

1.05 0.30 0.57

−

37 16 47

garnet (tetrahedral site) garnet (octahedral site) superpara-Fe2O3 or Fe3+ in Al2O3

1.5-Y2O3

0.36 0.14

0.32 1.04

52 48

garnet (octahedral site) garnet (tetrahedral site)

sample

a

Superpara-Fe2O3 indicates superparamagnetic Fe2O3. 8378

DOI: 10.1021/acscatal.9b02730 ACS Catal. 2019, 9, 8373−8382

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calculation was performed to give the Eov value of fresh Fe2O3 (0-Y2O3) and garnet (1.5-Y2O3), with the corresponding computational models shown in Figures 8a and 8b. The Eov of the perfect 1.5-Y2O3 (111) surface without oxygen vacancy was 1.97 eV (Figure 8b), which is indeed larger than that of the perfect α-Fe2O3 (001) surface in 0-Y2O3 (1.4 eV in Figure 8a), strongly confirming that Fe3+ transferring from Fe2O3 to the garnet structure, because of the addition of Y, led to the increase in surface Eov (i.e., the difficulty of oxygen removal), which resulted from the fact that lattice oxygen was shared by not only reducible Fe ions but also nonreducible Al and Y ones. In order to quantitatively correlate the Eov value with the performance of OCs, CO selectivity corresponding to the Eov value of fresh Fe2O3/Al2O3 (0-Y2O3) and garnet (1.5-Y2O3) without oxygen vacancy on their surface should be obtained. Here, CO selectivity of these two OCs at a reduction time of 15 s was used as the one (see Figure S6a in the Supporting Information), since their surface could be expected to have little oxygen vacancy at this time, because of the small amount of lattice oxygen consumed (Figure S6b in the Supporting Information). It could be clearly seen in Figure 8d that fresh garnet with larger Eov values exhibited higher CO selectivity than fresh Fe2O3/Al2O3 did, suggesting that the enhanced Eov in garnet was responsible for its higher CO selectivity. To further illustrate their relationship between the Eov and CO selectivity, we also calculated the energy of the second oxygen vacancy formation after the first oxygen vacancy generated on the surface of garnet with corresponding computational model in Figure 8c, and it increased to 3.10 eV. Meanwhile, the corresponding CO selectivity should be that of garnet, which had been reduced for some time to produce oxygen vacancies on the surface. Here, 1.5-Y2O3 at a reduction time of 330 s might be expected to meet this requirement with its CO selectivity being enhanced to 98% (Figure S6a), indicating again that CO selectivity was positively related with the Eov (Figure 8d). In addition, it was reported CH4 conversion was negatively associated with the Eov.58,59 However, 1.5-Y2O3 with a larger Eov value exhibited higher CH4 conversion than 0-Y2O3 with lower Eov did in this work, which might be due to the better dispersion of Fe species in 1.5-Y2O3 than in 0-Y2O3 that resulted from Y modification (Figures 3a and 3d). Therefore, the present result demonstrated that CO selectivity of Febased OCs could be highly enhanced by doping Y to form garnet phase with increased Eov in the structure.

Figure 6. Correlative relationships between CO yield and the amount of Fe3+ in garnet structure in the OCs.

Fe2O3/Al2O3 without Y modification (see Figures 3f and 3m) so that most of surface oxygen was just coordinated with reducible Fe ions (Fe−O−Fe) while the addition of Y led to Fe3+ entering into the garnet structure wherein lattice oxygen was shared by not only reducible Fe ions but also nonreducible Al and Y ones (Fe−O−Al and Fe−O−Y), which might result in more difficult removal of surface oxygen coordinated with Fe ions in garnet phase than that in Fe2O3/Al2O3 thus larger CO selectivity. To compare the mobility of lattice oxygen coordinated with Fe ions in garnet and Fe2O3/Al2O3, H2-TPR characterization of x-Y2O3 OCs was performed, and the results are shown in Figure 7. For 0-Y2O3, TPR profile presented two main peaks at

5. CONCLUSIONS In summary, the addition of Y greatly improved syngas selectivity of Fe2O3/Al2O3 OC (0-Y2O3) with the highest CO selectivity of 98% and CH4 conversion of ∼90% for CLR of CH4. This was due to Fe species gradually transferring from Fe2O3 to the garnet structure (Y3Fe2Al3O12), a newly formed phase, which was firstly found to be highly selective for CO formation. H2-TPR characterization combined with DFT calculation results demonstrated that the high CO selectivity of Fe species in garnet could be attributed to the increase in the oxygen vacancy formation energy (Eov), which resulted from the lattice oxygen shared by not only reducible Fe ions but also nonreducible Al and Y ones. In contrast, lower Eov values in 0-Y2O3 without Y modification led to its high CO2 selectivity. Overall, our present work proposed an effective strategy to enhance CO selectivity of Fe-based OCs by confining Fe species into characteristic mixed oxides, and made

Figure 7. H2 temperature program reduction profiles of the OCs.

530 and 750 °C, which corresponded to the reduction of Fe 2 O 3 to Fe 3 O 4 and Fe 3 O 4 to FeO, respectively (H 2 consumption was observed in Table S2 in the Supporting Information). The largest amount of H2 was consumed (35 mL/g), indicating that oxygen was easily removed in 0-Y2O3. The intensity of reduction peaks decreased with the addition of Y, suggesting increased difficulty in oxygen removal. For 1.5Y2O3 with the pure garnet phase, only a small reduction peak was observed before 890 °C with H2 consumption of 11 mL/g, indicating that oxygen coordinated with Fe3+ in the garnet structure was the most difficult to remove. The difficulty in the removal of oxygen coordinated with Fe3+ could be reflected by the oxygen vacancy formation energy (Eov), such that the DFT 8379

DOI: 10.1021/acscatal.9b02730 ACS Catal. 2019, 9, 8373−8382

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ACS Catalysis

Figure 8. Computational models of Eov of the first oxygen vacancy for (a) the fresh α-Fe2O3 (001), (b) the fresh 1.5-Y2O3 (111) surface, and (c) the second oxygen vacancy for the 1.5-Y2O3 (111) surface; (d) the relationship between CO selectivity of 0-Y2O3-15, 1.5-Y2O3-15, 1.5-Y2O3-330, and their corresponding Eov. Note: 0-Y2O3-15, 1.5-Y2O3-15, and 1.5-Y2O3-330 indicated the OCs were reduced for 15, 15, and 330 s in the first cycle, respectively.

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it clear that the increased oxygen vacancy formation energy in their structure contributed to the high selectivity of OCs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b02730. Quantitative calculation details and DFT computational models, schematic of the CLR process, reaction and characterization results (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M. Tian). *E-mail: [email protected] (X. Wang). ORCID

Lin Li: 0000-0002-3036-0934 Junhu Wang: 0000-0003-1987-2522 Xiaodong Wang: 0000-0002-8705-1278 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the National Science Foundation of China (NSFC) (Grant Nos. 21406225, 21573232, 21676269, and 21706254) and Postdoctoral Science Foundation of China (No. 2014M561261). We gratefully acknowledge Shenzhen Huasuan Technology Co., Ltd., for the DFT calculations. 8380

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