Highly active Ni2P catalyst supported on core-shell structured Al 2O3

Article Cite This: Ind. Eng. Chem. Res. 2017, 56, 12038-12045

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Highly Active Ni2P Catalyst Supported on Core−Shell Structured Al2O3@TiO2 and Its Performance for Benzofuran Hydrodeoxygenation Bolong Jiang,† Jing Gong,§ Jian Zhang,† Feng Li,†,‡ Jiaojing Zhang,† Yanxiu Liu,† Yanguang Chen,† and Hua Song*,†,‡ †

College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, China Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, Heilongjing, China § PetroChina Daqing Refining Petrochemical Company, Daqing 163318, Heilongjing, China ‡

S Supporting Information *

ABSTRACT: Highly active Ni2P catalyst supported on Al2O3@TiO2 (core@ shell) for hydrodeoxygenation (HDO) of benzofuran (BF) was described. The effect of the TiO2 shell thickness on the structure and HDO properties of Ni2P/Al2O3@TiO2 was studied. The results showed that an appropriate TiO2 shell thickness can effectively suppress the formation of AlPO4 and change the transmission mechanism of Ni2P particles, which is beneficial to the formation of highly dispersed Ni2P particles on the Al2O3 core. The Ni2P/A@T-25 showed the best HDO activity of 95% with oxygen-free products yield of 87% among the as-prepared Ni2P/Al2O3@TiO2 catalysts. As compared with Ni2P/Al2O3 (BF conversion of 78% with oxygen-free products yield of 47%), the BF conversion and yields of oxygen-free products over Ni2P/A@T-25 catalyst were increased by 22% and 85%, respectively.

1. INTRODUCTION The growing demand for fossil fuels, especially oil and its derivatives, has generated the need to seek new sources to offset oil depletion, including biomass.1 The pyrolysis is known as one method of biomass conversion. However, the oxygen content in liquid products, which is produced via pyrolysis, is high (35−50 wt %), which results in a serious problem because oxygen compounds degrade the product quality, leading to low pH, low heating value, high viscosity, and low chemical and thermal stability.2 The hydrodeoxygenation (HDO) is generally used to upgrade pyrolysis oil. The catalytic removal of oxygen used hydrogen, which is operated at a high hydrogen pressure (3−10 MPa) and moderate temperature (573−773 K).3 Conventional HDO catalysts which are based on sulfidized CoMo4 or NiMo,5 usually supported on Al2O3, exhibited a good performance for the hydrocarbons’ production from biooil. However, due to oxidation of the active phase, sulfide catalysts showed a progressive loss of their activities during the HDO reaction.6 Alternatively, catalysts based on noble metals have also been efficiently applied to the HDO of bio-oil.7,8 However, these are expensive, and therefore their application may affect the economy of the process. The low-cost transition metal phosphides, such as Ni2P, have been regarded as viable alternatives to precious metals.9,10 It is well-known that bulk Ni2P catalysts have a low surface area and that high reduction temperatures are needed to obtain the bulk material (because © 2017 American Chemical Society

of the strong P−O bond and its high reduction temperatures), which may lead to low dispersion and relatively low activity of the obtained catalyst.11 Therefore, the alteration of the catalytic activity on a high-surface-area support can be considered to be a result of changes in dispersion and morphology.12 In recent years, macroporous γ-Al2O3, which is often used as a support for commercial catalysts, has been tested as a support for Ni2P catalysts because of its relatively high surface area, high mechanical strength, good thermal stability, and affordability.13,14 However, phosphorus can strongly interact with γAl2O3 and lead to the formation of AlPO4 on the surface of the support. It was reported that TiO2 support can serve as both a conventional support and an electronic accelerator in the hydrogenation process.15 However, pure TiO2 supports have some drawbacks, such as low surface area (which is 25 μm. This result indicated that increasing x can effectively suppress the formation of AlPO4. 3.6. X-Ray Photoelectron Spectroscopy. The X-ray photoelectron spectroscopy (XPS) spectrum of fresh and spent samples was obtained, and the results were listed in Table 2 and Table 2. Spectral Parameters Obtained by XPS Analysis binding energy (eV)

Figure 3. TEM of samples: (a) Ni2P/Al2O3, (b) Ni2P/A@T-4 shell, (c) Ni2P/A@T-4 core, (d) Ni2P/A@T-25 shell, and (e) Ni2P/A@T25 core.

superficial atomic ratio

P 2p

Ni 2p3/2 sample

Ni2+

Ni2P

PO43−

Ni2P

Ni/P

Ni2P/Al2O3fresh Ni2P/A@T-4fresh Ni2P/A@T25-fresh Ni2P/Al2O3spent Ni2P/A@T-4spent Ni2P/A@T25-spent

856.4

852.3

134.3

129.3

1/2.39

856.2

853.4

134.5

129.3

1/2.79

1/19.63

856.2

852.3

134.5

129.3

1/2.64

1/10.21

856.3

852.3

134.3

129.3

1/2.24

856.4

852.2

134.3

129.3

1/2.19

1/21.16

856.4

852.3

134.3

129.3

1/2.07

1/11.75

Ti/Al

Table 3. Niδ+/Ni2+, Pδ−/ PO43−, and Ni2P Content Obtained by XPS Analysis

from 4 to 6 nm (Figure 3b), whereas those in the Ni2P/A@T25 shell ranged from 2 to 6 nm (Figure 3d). It can be seen that both the Ni2P/A@T-25 core (Figure 3c) and the Ni2P/A@T-4 core (Figure 3e) contained Ni2P particles. Compared with the Ni2P/Al2O3 catalyst, Ni2P/A@T-4 and Ni2P/A@T-25 cores show smaller and more uniform Ni2P particles. This confirms that the TiO2 shell can promote the dispersion of Ni2P, which is possibly because the TiO2 shell can change the transmission mechanism of Ni2P particles. 3.5. Infrared Absorption Spectroscopy Analysis. Figure 4 shows the IR spectrum of Ni2P/Al2O3 and Ni2P/A@T-x

sample

Niδ+/Ni2+

Pδ−/ PO43−

Ni2P (%)

Ni2P/Al2O3-fresh Ni2P/A@T-4-fresh Ni2P/A@T-25-fresh Ni2P/Al2O3-spent Ni2P/A@T-4-spent Ni2P/A@T-25-spent

0.10 0.14 0.23 0.34 0.48 0.71

0.06 0.07 0.08 0.06 0.11 0.33

1.01 1.37 2.08 2.82 3.61 4.62

Table 3. For all the samples, the Ni 2p3/2 core-level spectrum possessed three components. The first one, which centered at 851.6−853.5 eV, can be attributed to Niδ+ in the Ni2P phase. The second, which centered at 856.3−857.6 eV, can be assigned to the interaction of Ni2+ ions with phosphate ions, as a result of superficial passivation, along with the broad satellite at approximately 6.0 eV higher than that of the Ni2+ species, and this shakeup peak is attributed to divalent species.27 For the P 2p core-level spectrum, the peak at 129.5 eV can be attributed to Pδ− of the metal phosphides, and the peak at 134.5 eV can be attributed to surface metal phosphate species due to the superficial oxidation of the nickel phosphide.11 As shown in Figure 5a, the XPS pattern for the fresh Ni2P/ Al2O3 catalyst exhibited Ni 2p3/2 peaks at 852.3 and 856.4 eV, which can be assigned to the Niδ+ band of Ni2P and Ni2+species, respectively. Interestingly, for the fresh Ni2P/A@ T-x samples, the binding energy of Ni2+ was 856.2 eV, which shifted to a slightly lower value with increasing x, indicating the presence of an electronic effect which was caused by Ti species in the shell. This may result in electron enrichment of Ni species on the surface. Our previous study showed that the Ti incorporated into Ti-MCM-41-supported Ni2P samples could act as an electronic promoter.11 Compared with the respective fresh catalysts, the intensities of the Niδ+ species contributions of the corresponding spent samples were stronger, showing that more Ni2P particles were formed on the surface after HDO. In

Figure 4. IR spectra of Ni2P/Al2O3 and Ni2P/A@T(x) catalysts: (1) Ni2P/Al2O3, (2) Ni2P/A@T-1, (3) Ni2P/A@T-4, (4) Ni2P/A@T-25, (5) Ni2P/A@T-40.

catalysts. The IR bands at 1636 cm−1 can be attributed to the O−H stretching vibration, indicating the existence of absorbed water in the samples.25 The band at 579 cm−1 can be attributed to the characteristic Ni−P stretching vibration, and that at 1120 cm−1 can be ascribed to AlPO4.26 From Figure 4, it can also be seen that the Ni2P/Al2O3 catalyst showed a much stronger peak intensity of AlPO4 at 1120 cm−1 than the Ni2P/A@T-x catalysts. In addition, the AlPO4 peak intensity of Ni2P/A@T-x 12041

DOI: 10.1021/acs.iecr.7b02018 Ind. Eng. Chem. Res. 2017, 56, 12038−12045

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Industrial & Engineering Chemistry Research

Figure 5. XPS spectra in the Ni(2p) and P(2p) regions for Ni2P/Al2O3 and Ni2P/A@T-x catalysts. (a) Ni 2p core level spectra, (b) P 2p core level spectra, (1) Ni2P/Al2O3-fresh, (2) Ni2P/A@T-4-fresh, (3) Ni2P/A@T-25-fresh, (4) Ni2P/Al2O3-spent, (5) Ni2P/A@T-4-spent, (6) Ni2P/A@T-25spent.

Table 4. Results of BF HDO over Ni2P/Al2O3 and Ni2P/A@T-x Catalysts product distributions catalysts

conversion (%)

CO uptake (μmol·g−1)

yields of oxygen-free products (%)

MCH

ECH

EB

2,3-DHBF

2-EtPh

Ni2P/Al2O3 Ni2P/A@T-1 Ni2P/A@T-4 Ni2P/A@T-25 Ni2P/A@T-40

78 84 86 95 84

17

47 64 71 87 63

8 7 9 9 8

43 58 62 71 60

8 11 12 12 7

16 9 8 3 7

25 15 9 5 18

15

showed considerably higher Ni/P atomic ratios than that of the spent Ni2P/Al2O3. This shows that the P amounts on the surface of Ni2P/A@T-x were decreased significantly during the HDO process. In addition, the ratio of the surface Ti/Al increased with the increasing of thickness of the TiO2 shell showing that the TiO2 shell is uniform. Both the fresh Ni2P/ A@T-25 and Ni2P/A@T-4 samples showed slightly lower Ti/ Al ratios after reaction. The fresh Ni2P/A@T-x catalysts have comparatively higher Niδ+/Ni2+, Pδ−/ PO43−, and Ni2P content compared to the fresh Ni2P/Al2O3 catalyst, reaching a maximum over fresh Ni2P/A@ T-25, which indicated that more Ni2P phase was formed on core−shell Ni2P/A@T-x catalysts. In addition, for spent catalysts, the Ni2P contents were higher than those of corresponding fresh ones. Therefore, one concludes that the high Ni2P contents of all spent samples can be attributed to the formation of Ni2P particles by reduction of Ni2+ and PO43− during the HDO process. 3.7. CO Uptake. The CO uptakes at room temperature of the catalysts are listed in column 2 of Table 4. The CO molecules would mainly be adsorbed on Ni sites, and the amounts of CO molecules which are adsorbed on P sites are possibly very small.29 The CO uptake of Ni2P/A@T-25 was 15 μmol·g−1, which was slightly lower than that of Ni2P/Al2O3 (17 μmol·g−1). A similar result was obtained in our previous study, whereby the CO uptake of Ni2P/MCM-41 was found to be much higher than that of Ni2P/Ti-MCM-41. Oyama et al.24 found that a higher SBET of support was responsible for the greater dispersion of the Ni2P phase and increased CO uptakes, which is consistent with our results. The SBET of Ni2P/A@T-25 is 246 m2·g−1, which is lower than that of Ni2P/Al2O3 (257 m2· g−1). In addition, the Ni/P surface molar ratio for Ni2P/A@T25 is comparatively lower than that of Ni2P/Al2O3, indicating greater enrichment of P species on the surface of Ni2P/A@T25. This may be another reason for the lower CO uptake of Ni2P/A@T-25 when compared to the Ni2P/Al2O3. Oyama et

addition, as compared to the spent Ni2P/Al2O3 catalyst, the signal intensities of the Niδ+ for the spent Ni2P/A@T-x samples were stronger. Furthermore, with increasing x, the signal intensity of the Niδ+ for the spent Ni2P/A@T-x samples became distinctly stronger, showing that Ti species in the shell could significantly promote the formation of more Ni2P particles on the surface of Ni2P/A@T-x during the HDO process. As shown in Figure 5b, for all of the fresh catalysts, peaks centered at 134.6 eV for PO43− were detected, together with a weak peak at 129.3 eV which can be attributed to the Pδ− in Ni2P. Compared with the fresh Ni2P/A@T-x, the intensity of the Pδ+ signal for the corresponding spent Ni2P/A@T-x was increased with increasing x. However, this effect does not occur for Ni2P/Al2O3, indicating that Ti species present in the shell play a role in the formation of Ni2P during the HDO reaction. Interestingly, this result is consistent with the conclusion concerning Ni species drawn above, which reached a maximum at x = 25, showing that the higher the value of x, the stronger this effect. This is possibly because Ti4+ species present in the shell could be partially reduced to Ti3+ under the HDO reaction conditions. The 3d electron in Ti3+ might then be transferred through the conduction band of A@T-x and injected into the Ni 3d conduction band. The increase in electron density will lead to minor stabilization of the Ni 3d levels and a small Ni → P charge transfer,11,28 which is beneficial for the reduction of PO43− to Ni2P. As can be seen from Table 3, all samples showed lower Ni/P values than the theoretical Ni/P ratio of 1:2, which is likely due to the enrichment of phosphorus on the surface of the samples. The Ni/P ratios of all spent samples were higher than those of corresponding fresh samples. This possibly can be explained by a partial loss of phosphorus caused by the formation of volatile P species (such as PH3) during HDO reaction, which would cause a decrease in the P amount on the surface. In addition, both the spent Ni2P/A@T-4 and spent Ni2P/A@T-25 samples 12042


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Figure 6. Variation of the transformation of BF (a) and the yields of oxygen-free products (b). (Reaction temperature = 573 K, pressure = 3.0 MPa, WHSV = 4.0 h−1, and hydrogen/oil ratio = 500 (V/V).)

al.24 have reported similar findings. Surface P species could block some of the exposed nickel atoms and lead to a decrease in CO uptake. Chen et al.30 attributed this phenomenon to the ensemble effect of P. The presence of P species on the surface of the Ni2P catalysts reduced the number of CO chemisorption sites. In addition, surface nickel sites could also be blocked by surplus P. 3.8. Catalytic Results. Figure 6 presents the conversion of BF and the yields of oxygen-free products as a function of reaction time over the Ni2P/Al2O3 and Ni2P/A@T-x catalysts under conditions of 573 K, 3.0 MPa, weight hourly space velocity (WHSV) of 4.0 h−1, and hydrogen/oil ratio = 500 (V/ V). As expected, the conversion was greatly increased for all the catalysts during the initial stage of HDO reaction. It is wellknown that the HDS activity over Ni2P catalysts increased initially with time on stream and the explanation of this behavior as a result of the formation of an active superficial phosphosulfide phase with a stoichiometry represented by NiPxSy.31 Therefore, it is reasonable to explain the increase in HDO activity during the initial stage of HDO reaction as an intermediate phase, which is more active than Ni2P, also being formed. This intermediate phase was possibly formed by the surface reconstruction of Ni2P under the reaction conditions, which was confirmed by the change of the superficial atomic ratio after reaction. The Ni2P/A@T-x catalysts exhibited higher BF conversion with higher yields of oxygen-free products than the Ni2P/Al2O3 catalyst for tested reaction time. This is possibly because the TiO2 shell weakened the strong interaction between Al2O3 and Ni2P to suppress the formation of AlPO4 (Figure 4), and TiO2 species may act as an electronic promoter, favoring the formation of more uniform Ni2P particles. It is suggested that the unique core−shell structure of Ni2P/A@T-x could account for the superior HDO performance. Table 3 and Figure 7 show the reactivities of Ni2P/Al2O3 and Ni2P/A@T-x catalysts for HDO of BF after 8 h when the steady reaction conditions had been reached. The BF conversion increased with increasing x and then decreased slightly with further increasing x. The HDO activity reached a maximum of 95% at x = 25, which is increased by 22% as compared to that over Ni2P/Al2O3 (78%). All of the catalysts supported on an A@T-x core−shell structure showed enhanced HDO performance as compared to those directly supported on Al2O3; the ethylcyclohexane (ECH) selectivity especially increased remarkably, showing great promise for deep HDO catalysis. The selectivity for ECH was 71% over the Ni2P/A@ T-25 catalyst, which was increased by 65% as compared to that

Figure 7. Reactivities of Ni2P/Al2O3 and Ni2P/A@T-x catalysts for BF HDO.

over Ni2P/Al2 O3 (43%). However, the selectivities to methylcyclohexane (MCH) and to EB over Ni2P/A@T-x catalysts were both changed little. This indicates that the transformation of ethylbenzene (EB) to ECH was improved significantly by an A@T-x core−shell structure support, whereas that of ECH to MCH almost remained unchanged at the reaction conditions. With increasing x, the yield of oxygen-free products (EB, ECH, and MCH) increased and then decreased (Figure 7). The yields of oxygen-free products reached a maximum of 87% over Ni2P/A@T-Ni2P/A@T-x with a shell thickness of 25 μm, which was increased by 85% as compared to that over Ni2P/Al2O3 (47%). With increasing x, the selectivities for the O-containing intermediates 2,3-DHBF and 2-EtPh decreased initially and then increased, reaching a minimum over Ni2P/A@T-25. This observation is consistent with the results of Wang et al.32 An overthick shell (>25 μm) increases the transport resistance and decreases the surface area of the catalyst. Therefore, there is an optimum value. For Ni2P/ A@T-x, the optimum thickness is 25 μm in the present case. As a model reaction, the BF HDO over Ni2P/MCM-41 was investigated in our previous work, and a reaction network was proposed.21 The results exhibited that the oxygen-free products detected were mainly MCH, EB, and ECH. The oxygencontaining intermediates detected were mainly 2-ethylphenol (2-EtPh) and 2,3-dihydrobenzofuran (2,3-DHBF). The first step in the transformation of BF over Ni2P/MCM-41 is hydrogenation of the CC bond of BF, which led to 2,3DHBF. And 2-EtPh is obtained as a secondary product by cleavage of the C−O bond in 2,3-DHBF. Further, 2-EtPh is converted into EB via the direct deoxygenation (DDO) pathway. The formed EB is transformed to ECH via 12043


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Industrial & Engineering Chemistry Research Scheme 1. Proposed Reaction Pathway for BF HDO over Ni2P/A@T-x

similar to our previously reported results observed with Ni2P/ Ti-MCM-41 catalysts.11 Ti can modify the surface electronic atmosphere of the Ni2P/MCM-41 catalyst and enhance the adsorption ability of the DBT thereon, thereby enhancing the catalytic activity of DBT HDS.

hydrogenation. On the basis of our previous work, according to the product distribution and reaction routes which were previously reported in the literature,24 a possible reaction pathway for the BF HDO over Ni2P/A@T-x catalysts is shown in Scheme 1. There are a few mechanisms pertinent to C−O bond-cleavage reactions, including the Hofmann-type elimination reaction (E2) and nucleophilic substitution reaction (SN2).33 Our results have indicated that C−O bond cleavage on Ni2P/A@T-x is enhanced with core−shell structured catalysts. Edelman et al.34 reported that after ring-opening of 2,3-DHBF, the subsequent HDO of the 2-EtPh is the control step for overall BF HDO. The 2-EtPh in the hydrogenation pathway can, then, directly undergo an E2 or SN2 reaction to give the hydrocarbons. As evidenced by XPS measurements, the TiO2 shells possibly act as an electron donor and may transform the Ni2P into an electron-rich state (in accordance with the results of XPS analysis). The electron-rich Ni2P species could enhance both E2 and SN2 reactions. On the basis of the above experimental results and theoretical analysis, a possible pathway for the HDO of BF over the Ni2P/Al2O3@TiO2 core−shell structured catalysts is proposed, which accounts for the increase in performance in four ways. (i) During the HDO process, hydrogen molecules dissociate into hydrogen atoms, which are attached at the active sites of the catalyst and then transferred to the adjacent core surface via spillover and diffusion. Meanwhile, BF molecules are predominantly adsorbed on the surface of the catalyst shell, where both their diffusion and reaction can readily take place. Finally, the dissociated H atoms flow over the active sites and participate in the hydrogenation reaction to form water. According to the TEM results, Ni2P particles can be located on the TiO2 shell or on the Al2O3 core. The HDO activity of Ni2P/Al2O3 is expected to be low, because it is directly exposed to BF and H2O, resulting in a serious poisoning from water. Due to the unique core−shell structure of Ni2P/A@T-x catalysts, Ni2P particles located on the Al2O3 core can be less affected by water. (ii) The TiO2 shell can weaken the strong interaction between Al2O3 and P to suppress the formation of AlPO4 and change the transmission mechanism of Ni2P particles, which is beneficial to the formation of more dispersed Ni2P particles on the Al2O3 core. According to the TEM results, compared with the Ni2P/Al2O3 catalyst, Ni2P/A@T-4 and Ni2P/A@T-25 cores show smaller and more uniform Ni2P particles. This confirms that the TiO2 shell can promote the dispersion of Ni2P, which is possibly because the TiO2 shell can change the transmission mechanism of Ni2P particles. (iii) In addition, some of the Ti4+ may be partially reduced to Ti3+. As Ti3+ is an electron donor and Ni2P is an excellent electrical conductor, the electrons in Ti3+ can be easily transferred through the Ti3+ conduction band and injected into the Ni conduction band (in accordance with the results of XPS analysis). The Al2O3@TiO2 core surface is enriched with the dangling bonds of Ni and P atoms. This could enhance the adsorption ability of BF on the catalyst, ultimately resulting in an increase in catalytic performance. This phenomenon is

4. CONCLUSIONS In the present work, a series of Ni2P/Al2O3@TiO2 core−shell structured catalysts for HDO of BF was described. SEM images showed that TiO2 shells of different thicknesses were successfully deposited on the surface of Al2O3 cores. IR analysis showed that an appropriate shell thickness could effectively suppress the formation of AlPO4. The TEM analysis showed that deposition of a TiO2 shell on the surface of Al2O3 cores obviously promotes the formation of smaller and uniform Ni2P particles, facilitating their better dispersion. The Ni2P/A@T-25 catalyst ranged in Ni2P particle sizes from approximately 2 to 6 nm. With increasing x, the signal intensity of the Niδ+ in the spent Ni2P/A@T-x samples became distinctly stronger, showing that Ti species in the shell could significantly promote the formation of more Ni2P particles on the surface of Ni2P/ A@T-x during the HDO process. Compared with the Ni2P/ Al2O3 catalyst, the Ni2P/A@T-25 catalyst showed a high BF conversion of 95%, with an oxygen-free product yield of 87% under relatively mild conditions of 573 K, 3.0 MPa, WHSV 4 h−1, and a hydrogen/feed ratio of 500 (V/V). As compared with Ni2P/Al2O3 (BF conversion of 78% with oxygen-free products yield of 47%), the BF conversion and yields of oxygen-free products over Ni2P/A@T-25 catalyst were increased by 22% and 85%, respectively. A possible pathway to explain the improved BF HDO performance of Ni2P/ Al2O3@TiO2 core−shell structured catalysts involved three aspects: (i) The role of TiO2 shell is 2-fold; it can weaken the strong interaction between Al2O3 and P to suppress the formation of AlPO4 and change the transmission mechanism of Ni2P particles, which contributes to the formation of highly dispersed Ni2P particles on the Al2O3 core. (ii) During the HDO process, Ti4+ present in the shell is reduced to Ti3+ which acts as an electron donor during the reaction.

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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/acs.iecr.7b02018. Details on methods to describe the synthetic procedure of the A@T-x core−shell structured supports and demonstrate a uniform TiO2 shell which deposited on the surface of the Al2O3 core (PDF)

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Hua Song: 0000-0002-3062-9905 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial supports from the National Natural Science Foundation of China (21276048). REFERENCES

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