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. 2021 Sep 1;6(36):23274–23280. doi: 10.1021/acsomega.1c02994

Preparation of Mn-Fe Oxide by a Hydrolysis-Driven Redox Method and Its Application in Formaldehyde Oxidation

Jie Ling †, Yaxin Dong ‡,*, Pan Cao ‡, Yixiang Wang ‡, YingYing Li ‡
PMCID: PMC8444290  PMID: 34549127

Abstract

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Homogeneous distribution of Mn-Fe oxides (xMn1Fe) with different Mn/Fe ratios was synthesized by a hydrolysis-driven redox method, and their catalytic activities in HCHO oxidation were investigated. The results showed that HCHO conversion was significantly improved after doping iron due to the synergistic effect between manganese and iron. The 5Mn1Fe catalyst exhibits excellent catalytic activity, achieving >90% HCHO conversion at 80 °C and nearly 100% conversion at 100 °C. The physicochemical properties of catalysts were characterized by BET, XRD, H2-TPR, O2-TPD, and XPS techniques. Experimental results revealed that the introduction of Fe into MnOx resulted in a large surface area, a high ratio of Mn4+, abundant lattice oxygen species and oxygen vacancy, and uniform distribution of Mn and Fe, thus facilitating the oxidation of HCHO to CO2 and H2O.

1. Introduction

The popularity of interior decoration has led to serious formaldehyde pollution. The average over-standard rate of formaldehyde in newly installed residential buildings is more than 70%, which is mainly derived from plywood, interior coatings, and furniture in decoration materials.1 Formaldehyde can cause immunity decline, allergic dermatitis, and even poisoning, seriously endangering people’s health.2,3 Therefore, reducing and eliminating formaldehyde are of great significance for protecting the indoor air and human health.

The methods of eliminating formaldehyde in indoor air are mainly plant purification,4 adsorption,5 plasma purification,6 photocatalysis,7 and catalytic oxidation.8 Catalytic oxidation is the most ideal method in eliminating formaldehyde compared to other physicochemical methods, which uses a catalyst to promote the reaction between formaldehyde and oxygen at low temperature to generate CO2 and H2O.9 It has the advantages of high efficiency, easy control of operating conditions, minimal secondary pollution, and low energy consumption. The key factor of this method lies in the selection of effective catalysts to eliminate HCHO. Thus, improving the low-temperature oxidation activity of catalysts in formaldehyde purification is still evolving.

Among the catalysts for formaldehyde degradation, noble metal catalysts and transition metal oxide catalysts exhibit better performance and have been widely studied. However, a high-efficiency noble metal catalyst was limited in its large-scale application due to lack of resources and high cost. In contrast, a cost-effective and efficacious transition metal oxide catalyst could be used as a promising catalyst with abundant resources, low cost, and good redox properties.10 Among them, manganese-based catalysts are excellent active components in various processes and are often used as catalysts or support materials.11 The outer electron structure of manganese is 3d54s2, and the convertible valence state can form oxides with different structures. Furthermore, the variable valence state of the manganese element in the manganese oxide will produce the internal defects and vacancies of the manganese oxide crystal, which is conducive to the movement and storage of oxygen. According to these literature studies, the preparation methods and synergistic effects among the active components play as key determinants of catalytic performance;12 constructing a uniform distribution between varying metal oxide particles is the preparation target. The hydrolysis-driven redox method uses the coupling of two simultaneous reactions. In this method, KMnO4 was reduced by H2O2 under acidic conditions, H+ produced by Fe3+ hydrolysis can be used for Mn redox, and the consumed H+ is beneficial to accelerate Fe3+ precipitation. At the same time, the conversion of H2O2 to H2O and O2 in the redox process is fully utilized. However, the conventional preparation methods such as impregnation and coprecipitation have disadvantages of easy agglomeration and difficult control of high dispersion of particles in mixed metal oxides. On the other hand, to improve the low-temperature oxidation activity of single metal oxides, metals, or metal oxides with a large atomic radius, weak M–O bond energy, and low electronegativity, elements such as Ce, Cu, Co, Fe, etc. are often added to manganese oxide to prepare a composite metal oxide catalyst. The Ce-MnO2 catalyst prepared by Zhu et al. could completely convert HCHO at 100 °C and had better activity at low temperature than a single component.13 Huang et al. reported that the CoxMn3–xO4 nanosheets exhibited excellent catalytic activity that could convert HCHO to CO2 at 100 °C.14 In addition, Ma et al.15 reported that iron oxides were the main active component to improve the catalytic activity of MnO2-Fe3O4 in the catalytic combustion of toluene. Nevertheless, the application of MnO2-Fe3O4 in catalytic oxidation of formaldehyde has not yet been widely studied.

In this work, we used the hydrolysis-driven redox reaction method to prepare uniform distribution of Mn-Fe oxides (xMn1Fe) with different molar ratios of Mn/Fe. This method utilizes the coupling of two simultaneous reactions to synthesize Mn-Fe binary oxides. Also, the catalytic performances of those catalysts for formaldehyde oxidation were investigated. The relationship between the structure and catalytic performance of the catalysts was illustrated by the combination of BET, XRD, TPR, TPD, and XPS characterization techniques.

2. Results and Discussion

2.1. Characterization of the Catalysts

2.1.1. N2 Adsorption and Desorption

The N2 adsorption–desorption isotherms of Mn-Fe oxides are illustrated in Figure 1, and the relative parameters are listed in Table 1. From Figure 1, it can be found that all the samples exhibited type III with H3 hysteresis, indicating the presence of a mesoporous structure. However, the isotherms of Fe2O3 are more similar to type V.18 Different Mn/Fe molar ratios have significant effects on the microstructure of Mn-Fe oxides. The specific surface area of MnO2 is 103.1 m2 g–1, which is much higher than that of Fe2O3 (54.3 m2 g–1). This result suggested that Fe2O3 samples prepared by the coprecipitation method have a lower SBET. Also, the SBET value can be further increased by introducing Fe ions; xMn1Fe showed a similar value at 159.7–202.2 m2 g–1, which might be because the interaction between manganese and iron can effectively inhibit the structure growth of mixed oxides.19 The values of pore diameter and pore volume for 5Mn1Fe were 21.2 nm and 1.1 cm3 g–1, respectively, which are larger than those of 3Mn1Fe and 7Mn1Fe. Overall, the larger pore volume and higher SBET are beneficial to the catalytic oxidation. Therefore, 5Mn1Fe exhibited excellent catalytic performance.

Figure 1.

Figure 1

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N2 adsorption/desorption isotherm curves of (a) Fe2O3, (b) MnO2, (c) 3Mn1Fe, (d) 5Mn1Fe, and (e) 7Mn1Fe.

Table 1. BET Surface Area, Pore Volume, and Pore Size of xMn1Fe Catalysts Together with Fe2O3 and MnO2.
catalyst SBET (m2 g–1) pore diameter (nm) pore volume (cm3 g–1)
Fe2O3 54.3 13.9 0.3
MnO2 103.1 18.2 0.5
3Mn1Fe 159.7 13.5 0.6
5Mn1Fe 184.5 21.2 1.1
7Mn1Fe 202.2 18.8 1.0
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2.1.2. XRD Patterns

The XRD measurement was performed to investigate the crystal structures of the MnO2, Fe2O3, and xMn1Fe samples. In Figure 2a, the Fe2O3 catalyst exhibits a typical pattern of α-Fe2O3 (JCPDS PDF 01-072-0469).20 The primary peaks of MnO2 appeared at 28.6, 37.4, and 56.8°, which is close to the pattern of α-MnO2 (JCPDS PDF 44-0141).21 Compared with MnO2 samples, the incorporation of Fe led to a significant change in the diffraction pattern of xMn1Fe catalysts. The intensity of the characteristic peak of MnO2 decreases greatly or even disappears. This phenomenon indicates that the interaction between manganese and iron oxides makes the crystal form of MnO2 change, showing an amorphous structure. According to the literature, poor crystallinity will lead to the loss of peaks and weakening of peak intensity.22 In addition, only one characteristic diffraction peak of MnO2 (37.1°) was observed in the three xMn1Fe catalysts, and no characteristic peak of the Fe element was found, which indicated that Fe ions were highly dispersed on the surface of manganese oxide and did not form large grains, which provided evidence for the synthesis of homogeneous Mn-Fe binary oxides.

Figure 2.

Figure 2

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XRD patterns of (a) Fe2O3, (b) MnO2, (c) 3Mn1Fe, (d) 5Mn1Fe, and (e) 7Mn1Fe.

Meanwhile, the peak strength in the MnO2 spectrum was decreased significantly with the increase in Fe content, indicating that the noncrystalline phase of MnO2 was produced and Fe ions were highly dispersed on the manganese oxide surface. We can infer that the Fe ions in the MnO2 lattice and the coexistence of manganese and iron oxides enhance this strong interaction, which are beneficial to produce more lattice defects and to increase the specific surface area of the samples.23 It provides evidence for the synthesis of homogeneous Mn-Fe binary oxides.

2.1.3. H2-TPR

The influence of different Mn/Fe ratios on xMn1Fe binary oxide is also reflected in the difference of reduction ability. As shown in Figure 3, for the Fe2O3 sample, the reduction peak of Fe2O3 to Fe3O4 appeared at 361 °C and the other broad peaks were seen at 500–700 °C, corresponding to the reduction of Fe3O4 to FeO.24 The MnO2 pattern exhibited two obvious peaks at 301 and 402 °C, corresponding to the reduction of MnO2 to Mn2O3 and Mn2O3 to MnO, respectively.25 Compared to pure MnO2 and xMn1Fe samples, Fe doping significantly improves the reduction performance of the samples. Three reduction peaks appeared at about 260, 290, and 400 °C for 3Mn1Fe, 5Mn1Fe, and 7Mn1Fe, respectively. The first and third reduction peaks are attributed to the reduction process of MnO2 → Mn2O3 → MnO, and the second peak is due to the reduction of Fe2O3 to Fe3O4. It can be seen that the reduction peaks of the three xMn1Fe catalysts shift toward lower temperatures. This result reflects the strong interaction between Mn-Fe oxides, and Li et al.38 also reached a similar conclusion in the study of NiMnFe mixed oxides.

Figure 3.

Figure 3

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H2-TPR profiles of (a) MnO2, (b) Fe2O3, (c) 3Mn1Fe, (d) 5Mn1Fe, and (e) 7Mn1Fe.

In addition, the peaks of 5Mn1Fe and 7Mn1Fe overlapped with each other at 250–300 °C, which was caused by the instantaneous strong exothermic reaction between MnOx and FeOx. With the increase in manganese content to 7Mn1Fe, it can be found that the overlapping area of reduction peaks increases and the reduction peak area decreases. The low-temperature reducibility facilitates the enhancement of catalytic oxidation.26,27 The order of reducibility increases as follows: 3Mn1Fe < 7Mn1Fe < 5Mn1Fe, which is in agreement with the activity test. High reducibility can encourage the mobility of oxygen species and promote the degradation of formaldehyde.28

2.1.4. O2-TPD

O2-TPD profiles of the catalysts were obtained to identify the mobility of the surface oxygen species. The physical adsorption of O2 desorb at low temperature (<200 °C), and chemically adsorbed oxygen (O2– and O–) desorbs between 200 and 400 °C, while the desorption peaks of lattice oxygen appear at high temperatures (>400 °C).29 The desorption capacity of oxygen species obeys the following sequence: O2(ads) > O2–(ads) > O–(ads) > O2–(latt), where O2(ads) refers to physically adsorbed oxygen, O2–(ads) means peroxy oxygen, O–(ads) represents monoatomic oxygen, and O2–(latt) indicates lattice oxygen.30 As presented in Figure 4B, the Fe2O3 catalyst displayed that the desorption of chemical oxygen was dominant (200–500 °C) and had a low amount of O2–(latt) at a high temperature (637 °C). From Figure 4A, the MnO2 and xMn1Fe catalysts showed desorption peaks, which are attributed to the desorption of lattice oxygen. Especially for the 5Mn1Fe catalyst, the desorption peak appeared at 510 °C, which is lower than other xMn1Fe catalysts. Meanwhile, it can be seen from Figure 4C that the desorption temperature of 5Mn1Fe is the lowest among the chemisorption oxygen peaks at 150–300 °C. It has been demonstrated that the desorption of surface active oxygen in 5Mn1Fe oxides is easier than those in pure MnO2, 3Mn1Fe, and 7Mn1Fe, indicating the enhanced mobility of oxygen on its surfaces.31

Figure 4.

Figure 4

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(A) O2-TPD profiles of (a) Fe2O3, (b) MnO2, (c) 3Mn1Fe, (d) 5Mn1Fe, and (e) 7Mn1Fe; (B) enlarged O2-TPD profiles of (a) Fe2O3; and (C) enlarged O2-TPD profiles at 150–300 °C.

2.1.5. XPS Characterization

The atomic concentration and chemical states of the surface elements (O, Mn, and Fe) over as-prepared samples were analyzed by XPS. Figure 5 displays that the two peaks are discovered at 641.4 and 653.3 eV, which could be assigned to Mn-2p3/2 and Mn-2p1/2, respectively. Through the fitting analysis of the Mn2p3/2 curve, the species of Mn3+ (639.6–641.3 eV) and Mn4+ (641.1–642.9 eV) can be detected.32 Relative concentration ratios of Mnx/Mntotal were calculated and are summarized in Table 2. The Mn4+/Mn ratio in the MnO2 sample was 43.8% lower than the Mn3+/Mn ratio, indicating that the Mn species mainly exist in the form of Mn2O3. The introduction of Fe resulted in the increase in Mn4+ content, and the 5Mn1Fe catalyst exhibited a high value of 53.4%. This result may be due to the redox reaction between Mn and Fe oxides, leading to the transformation of Mn3+ to Mn4+. As reported,33 Mn4+ has the highest redox ability among all valence states of MnOx, and a high ratio of Mn4+/Mn is more favorable for the oxidation reaction.

Figure 5.

Figure 5

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Mn 2p spectra of (a) MnO2, (b) 3Mn1Fe, (c) 5Mn1Fe, and (d) 7Mn1Fe.

Table 2. Surface Atomic Composite of the Catalysts Determined by XPS.
    Mnx/Mntotal (%)
   Ox/Ototal (%)
   Fex/Fetotal (%)
samples   Mn3+ Mn4+   Oβ Oα   Fe2+ Fe3+
Fe2O3         47.8 52.2   43.3 56.7
MnO2   56.2 43.8   44.3 55.7      
3Mn1Fe   51.4 48.6   41.1 58.9   46.8 53.2
5Mn1Fe   46.6 53.4   37.2 62.8   57.1 42.9
7Mn1Fe   49.5 50.5   40.2 59.8   50.4 49.6
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Regarding the O 1s spectrum (Figure 6), two peaks featured at 527.7–529.0 and 529.2–531.3 eV correspond to lattice oxygen (named as Oα) and surface-absorbed oxygen (named as Oβ), respectively.34 Surface-adsorbed oxygen is mainly produced by gaseous oxygen adsorbed on oxygen vacancies on the catalyst surface, and lattice oxygen comes from surface oxygen in metal oxides. When lattice oxygen takes part in the reaction, an oxygen vacancy will be produced, thereby being immediately replenished by gaseous oxygen adsorption. From Table 2, the Oα/(Oα + Oβ) ratios are ranked as follows: Fe2O3 (52.2%) < MnO2 (55.7%) < 3Mn1Fe (58.9%) < 7Mn1Fe (59.8%) < 5Mn1Fe (62.8%), suggesting that 5Mn1Fe possessed the most abundant lattice oxygen species, produced a large number of oxygen vacancies, and consumed the most gaseous oxygen. Therefore, it has the best catalytic effect.

Figure 6.

Figure 6

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O 1s spectra of (a) Fe2O3, (b) MnO2, (c) 3Mn1Fe, (d) 5Mn1Fe, and (e) 7Mn1Fe.

The Fe-2p3/2 spectra of the FMC catalyst are presented in Figure 7. One peak located at around 708.7 eV might be assigned to Fe2+ ions, and the other that appeared at 710.8 eV might be attributed to Fe3+ ions.35 The contents of Fe2+ and Fe3+ in the samples are listed in Table 2. The Fe2+/Fe ratio of 5Mn1Fe was 57.1% and obviously higher than those of 3Mn1Fe (46.8%) and 7Mn1Fe (50.4%); the increase in Fe2+ can be attributed to the redox equilibrium reaction between Mn and Fe. The redox pair between Mn4+/Mn3+ and Fe3+/Fe2+ may enhance the redox cycle, thereby promoting the generation of more oxygen vacancies and ultimately facilitating the catalytic oxidation reaction.36,37

Figure 7.

Figure 7

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Fe 2p spectra of (a) Fe2O3, (b) 3Mn1Fe, (c) 5Mn1Fe, and (d) 7Mn1Fe.

2.2. Catalytic Activity

The catalytic activities of xMn1Fe catalysts with varying ratios evaluated for HCHO conversion are shown in Figure 8. With the increase in reaction temperature, HCHO conversion increased for all catalysts. Also, the different contents of the Mn-Fe mixed oxide catalyst have a great influence on the catalytic activity of HCHO oxidation. The HCHO conversion of pure Fe2O3 and MnO2 was very low in the test range, and the conversion values are 69.4 and 90.2% at 140 °C, respectively. Compared with a single oxide catalyst, the catalytic activity of the xMn1Fe mixed oxide catalyst was obviously higher. It is speculated that the high catalytic activity may be due to the interaction between MnOx and FeOy. Among all the catalysts, 5Mn1Fe exhibited the highest activity in HCHO conversion, achieving >90% HCHO conversion at 80 °C and nearly 100% conversion at 100 °C. In comparison, the MnO2 catalyst exhibited only 49.1% conversion at 80 °C. The sequence of the catalytic activity of various catalysts is as follows: Fe2O3 < MnO2 < 3Mn1Fe < 7Mn1Fe < 5Mn1Fe, which indicates the positive synergistic effect between Mn and Fe oxide. Meanwhile, manganese and iron oxide with a proper molar ratio could exhibit better catalytic activity, while an excessive amount of manganese has a negative influence. It is generally recognized that the metal state, structure and morphology, and redox properties are the main factors affecting the catalytic activity of the catalyst.38 According to the XRD results, the introduction of Fe not only disperses iron itself but also boosts the dispersion of manganese oxide; therefore, Mn-Fe binary oxide has a higher surface area and thus is advantageous to adsorb more reactants to participate in the oxidation reaction. Combining the XPS results indicated that the introduction of Fe increased the concentration of surface Mn4+ and lattice oxygen species, increased the amount of oxygen vacancies and lattice defects, and also contributed to the HCHO catalytic oxidation reaction. Therefore, it was concluded that there is better dispersion of Mn and Fe oxides, a high content of surface Mn4+, and a large number of lattice oxygen species. Moreover, the hydrolysis-driven method plays an important role in the synthesis of high-efficiency Mn-Fe binary oxides, which can lead to catalysts with homogeneous distribution of active components, a more exposed surface area, and lower crystallinity.

Figure 8.

Figure 8

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HCHO conversion over Fe2O3 (squares), MnO2 (circles), 3Mn1Fe (triangles), 5Mn1Fe (inverted triangles), and 7Mn1Fe (stars). Reaction conditions: 250 ppm HCHO and 21 vol % O2 and N2 (balance). The total flow rate and weight hourly space velocity (WHSV) are 30 mL min–1 and 36,000 mL gcat–1 h–1, respectively.

3. Experimental Section

3.1. Catalyst Preparation

MnO2 was fabricated by a redox reaction in acid solution.16 Typically, 13 mmol of KMnO4, 9.6 mmol of 98 wt % H2SO4, and 100 mL of deionized water were dissolved with stirring, and then a certain amount of H2O2 aqueous solution was slowly added into the above mixture. The mixture was continuously stirred for 6 h and then collected and washed with deionized water. After that, the drying treatment of products was performed at 100 °C for 12 h and heat treatment for 2 h at 400 °C.

A series of xMn1Fe (where x denotes the Mn/Fe molar ratio in Mn-Fe oxides) samples were prepared by the hydrolysis-driven redox reaction method, similar to the literature.17 KMnO4 (38.1 mmol) and Fe(NO3)·9H2O (12.6 mmol) were added into 300 mL of distilled water and stirred until completely dissolved. The mixed solution was added dropwise to a water solution of 570.4 mmol of H2O2 with vigorous stirring. After stirring at a constant speed for 6 h, the precipitate was collected by suction filtration and washed with distilled water several times. Finally, the product was dried in a vacuum oven to remove excess solution and roasted in a muffle furnace for 2 h at 400 °C. The obtained catalyst was denoted as 3MnOy-1FeOx (3Mn1Fe). It should be noted that hydrolysis of 1 mol of Fe(NO)3 can produce 3 mol of H+ in 3Mn1Fe to meet the theoretical requirement of nitric acid. Therefore, it is necessary to have a desired amount of HNO3 when preparing Mn-Fe mixed oxides with different molar ratios. According to the corresponding Mn/Fe molar ratio, the catalysts were labeled as 5MnOy-1FeOx (5Mn1Fe) and 7MnOy-1FeOx (7Mn1Fe).

The Fe2O3 catalyst was prepared using the coprecipitation method. In brief, 14.9 mmol of Fe(NO3)·9H2O was dispersed in 300 mL of distilled water and then mixed with 300 mL of aqueous solution containing 44.6 mmol of NH3·H2O with stirring at 30 °C. After that, the collection and treatment of brown solid were the same as the procedure described above.

3.2. Characterization

The Brunauer–Emmett–Teller (BET) specific surface area test was carried out using a micromeritics apparatus (ASAP2020HD88) at 77 K liquid nitrogen temperature under a N2 atmosphere. Before analysis, the sample was degassed for 4 h at 250 °C.

Temperature-programmed reduction of H2 (H2-TPR) and temperature-programmed desorption of O2 (O2-TPD) were performed on a GC1690 chemical adsorption instrument. Fifty milligrams of catalyst was required for each test. In the TPR experiment, the catalyst was pretreated in an O2/N2 flow (30 mL min–1) at 300 °C for 1 h and then cooled to room temperature. A 10 vol % H2/N2 reducing gas with a flow rate of 60 mL/min was introduced, and the TPR range was from room temperature to 700 °C at a rate of 10 °C/min. In the TPD experiment, the sample was first purged with helium at 200 °C for 1 h to purify the catalyst surface. Subsequently, the catalyst was allowed to react with O2 for 0.5 h at 70 °C, and then He was introduced for 0.5 h to remove the adsorbed physical O2. The adsorption of O2 was measured in a He atmosphere by increasing the temperature to 700 °C, and the O2 consumption was recorded continuously.

X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Advance with Cu Kα radiation and a scanning diffraction angle (2θ) range of 10–80° with a scan step size of 5 °C min–1.

X-ray photoelectron spectroscopy (XPS) measurement on a Thermo Scientific K-Alpha instrument used the C 1s peak position at 284.6 eV as the reference to calibrate the energy position of each peak.

3.3. Catalytic Activity Test

The catalyst (500 mg) with a size of 40–60 mesh was packed in a fixed-bed reactor with an inner diameter of 6 mm for the catalytic degradation of formaldehyde. Gaseous HCHO was produced by a flow of 21% O2/N2 over paraformaldehyde at 30 °C. The reaction feed contained 200 ppm HCHO and 21% O2 and N2 (constitute a balance). The total gas flow rate was maintained at 30 mL min–1, and the corresponding mass space velocity (GHSV) was 36,000 mL gcat–1 h–1. The products (such as CO and CO2) were analyzed by gas chromatography equipped with a thermal conductivity detector (TCD), hydrogen flame ionization detector (FID), and Ni catalyst converter. In the activity test, no other carbon compounds were detected in the catalyst products except CO2. Thus, HCHO conversion is equal to the yield of CO2 and calculated as follows:

3.3.

where [CO2]out is the outlet CO2 concentration and [HCHO]in is the inlet HCHO concentration.

4. Conclusions

Homogeneous distribution of Mn-Fe oxides (xMn1Fe) with different Mn/Fe ratios was developed by a hydrolysis-driven redox method. Among all the analyzed catalysts, the 5Mn1Fe catalyst exhibited superior catalytic activity for HCHO oxidation, achieving >90% HCHO conversion at 80 °C and nearly 100% conversion at 100 °C, results that reflect the positive synergistic effect between Mn and Fe oxide. According to the characterization results, it turns out that the enhanced catalytic activity can be ascribed to a high BET value, a large amount of Mn4+, abundant lattice oxygen and oxygen vacancy, and uniform distribution of Mn and Fe. Overall, the introduction of Fe improves the catalytic activity of Mn-Fe binary oxides for HCHO oxidation.

Acknowledgments

This work was funded by the Youth Project of Shaanxi Science and Technology Department (2021JQ-886), the Natural Science Foundation of Shaanxi Education Department (19JK0194), and the National Natural Science Foundation of China (51074122). Meanwhile, the modern analysis and testing center of Xi’an Shiyou University provided strong support.

The authors declare no competing financial interest.

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