**Yuyao Ma, Yuxia Ma, Min Liu, Yang Chen, Xun Hu, Zhengmao Ye and Dehua Dong \***

School of Materials Science and Engineering, University of Jinan, Jinan 250022, China; snsdyoona530@163.com (Y.M.); jndx\_yxma@163.com (Y.M.); lm970222@163.com (M.L.); cy0204Yyqx@163.com (Y.C.); Xun.Hu@outlook.com (X.H.); mse\_yezm@ujn.edu.cn (Z.Y.) **\*** Correspondence: mse\_dongdh@ujn.edu.cn; Tel.: +86-531-8973-6011

Received: 4 April 2019; Accepted: 17 May 2019; Published: 23 May 2019

**Abstract:** Electrospinning is a simple and efficient technique for fabricating fibrous catalysts. The effects of preparation parameters on catalyst performance were investigated on fibrous Ni/Al2O<sup>3</sup> catalysts. The catalyst prepared with H2O/C2H5OH solvent showed higher catalytic activity than that with DMF/C2H5OH solvent because of the presence of NiO in the catalyst prepared with DMF/C2H5OH solvent. The metal ion content of the precursor also influences catalyst properties. In this work, the Ni/Al2O<sup>3</sup> catalyst prepared with a solution containing the metal ion content of 30 wt % demonstrated the highest Ni dispersion and therefore the highest catalytic performance. Additionally, the Ni dispersion decreased as calcination temperature was enhanced from 700 to 900 ◦C due to the increased Ni particle sizes, which also caused a high reduction temperature and low catalytic activity in methane partial oxidation. Finally, the fibrous Ni/Al2O<sup>3</sup> catalysts can achieve high syngas yields at high reaction temperatures and high gas flow rates.

**Keywords:** electrospinning; fibrous catalysts; metal ion content; calcination temperature; methane partial oxidation

### **1. Introduction**

Electrospinning has been developed to fabricate one-dimensional materials with controllable fiber diameters, morphologies and compositions. Electrospun nanofibers have special features, such as hierarchically porous structure and high surface area, which have been successfully applied in various fields such as nanocatalysts, filtration, biomedical, optical electronics and electrodes for energy conversion or storage devices [1–5].

Ni/Al2O<sup>3</sup> fibrous catalysts prepared by electrospinning have applied in catalytic methane reforming [1,6,7]. Ni nanoparticles can be in situ formed on the nanofiber surface via reducing catalyst precursor NiAl2O4. The fibrous structure of the catalysts is stable up to 1000 ◦C [6]. Moreover, the fibrous structure has a large void fraction (about 95%), which enables operation at high gas hourly space velocities through catalyst bed. It matches with the fast reaction of methane partial oxidation, which can be completed within a contact time of sub-milliseconds [8,9]. Therefore, the fibrous catalysts can produce high syngas yields [1].

Catalyst precursor solution greatly affects the electrospinning process via viscosity and evaporation [2]. To the best of our knowledge, the effect of preparation parameters on electrospun catalyst has not been reported previously. This study has investigated the effects of solvent, metal ion content and calcination temperature on catalyst properties, including crystallinity, particle size, microstructure, reducibility and catalytic performance. The preparation parameters were optimized to achieve high performance of methane partial oxidation (POM). The effects of reaction parameters on catalyst properties was also studied to utilize the advantages of fibrous catalysts.

#### **2. Experimental**

#### *2.1. Catalyst Preparation*

The fibrous Ni/Al2O<sup>3</sup> catalysts were prepared by electrospinning, and the electrospinning process was started with preparing a spinning solution. A certain amount of polyvinyl pyrrolidone (PVP, molecular weight <sup>=</sup> 1.3 <sup>×</sup> <sup>10</sup><sup>6</sup> , Shanghai Dibo Chemical Technology Co., Ltd., Shanghai, China) was dissolved in 2.0 g C2H5OH (≥99.7 wt %, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and 8.0 g H2O to prepare a PVP solvent. Al(NO3)3·9H2O (≥99.0 wt %, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and Ni(NO3)2·6H2O (≥99.0 wt %, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were dissolved in the solvent to form the catalyst with the Ni content of 30 wt % in Ni/Al2O3, and the Ni content was same for all catalysts. The electrospinning solutions with different metal ion contents are denoted CX (X = 10, 20, 30 and 40), where X represents the metal ion content, defined as solute (nitrate) weight percentage in precursor solution (solute + solvent). The ratio between solvent and solute in the C10, C20, C30 and C40 catalysts was 8.7:1, 4:1, 2.2:1 and 1.5:1, respectively.

Electrospinning was conducted on a device (Ucalery ET-2535H, Beijing, China) with a spinning distance of 30cm driven by a applied voltage of 19 kV. The feeding rate was maintained at 0.05 mm min−<sup>1</sup> . The sample was calcined at 800 ◦C for 1 h in air.

The effect of solvents was compared by using the catalysts with 20 wt % metal ion content. Only H2O in the solvent was changed to DMF (≥99.5 wt %, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), and the weight ratio to distilled water was 4:1. Other preparation parameters are the same. In addition, the catalyst prepared with a metal ion content of 30% was calcined at 700, 800 and 900 ◦C, separately, to study the effect of calcination temperature on catalyst properties.

#### *2.2. Catalyst Characterisation*

Scanning electron microscopy (SEM) images of catalyst microstructure were acquired with a FEI QUANTA FEG 250 microscope. Crystal sizes were measured using an X-ray diffractometer (XRD, Bruker D8 Advance) with Cu-Kα radiation (λ = 0.15408 nm). Temperature-programmed reduction (TPR) was conducted on a Micrometric ChemiSorb 2720 using a 10 mg of catalyst and a feeding gas of 10 vol % H<sup>2</sup> in Ar with a gas flow rate of 30 mL min−<sup>1</sup> . The TPR tests were operated from room temperature to 1000 ◦C at a heating rate of 10 ◦C min−<sup>1</sup> . CO-chemisorption was performed on a Micrometric ChemiSorb 2720 using a 30 mg of catalyst. First, the catalyst was reduced by the TPR process. Next, the catalyst was cooled to room temperature for pulse chemisorption using 5 vol % CO in He.

#### *2.3. Catalytic Reaction*

The calcined catalysts were crushed into sheets about 900 µm in size to ensure the similar density of catalyst beds. Catalytic evaluation was tested in a fixed bed quartz tube reactor (inner diameter = 6 mm) with a central K-type thermocouple. 10 mg of the catalyst was pre-reduced in situ by 10 vol % H<sup>2</sup> in Ar at 750 ◦C for 1 h. The reactant gas of CH4, O<sup>2</sup> and Ar with a molar ratio of 2:1:17 was introduced into the reactor at 750 ◦C at a gas flow rate of 800 mL min−<sup>1</sup> . Reaction products were sampled by a gas chromatography (GC, Shimadzu GC-2014).

#### **3. Results and Discussions**

#### *3.1. E*ff*ect of Solvent*

A solvent is used to dissolve catalyst precursors and polymer, forming electrospinning solution. During the electrospinning, the solvent needs to be evaporated before electrospun fibrous composites reach collectors so as to retain fibrous morphology. Solvent properties affect solution viscosity and solvent evaporation. Therefore, two common solvents, H2O/C2H5OH and DMF/C2H5OH, are employed to investigate the solvent effect. Both catalysts had a metal ion content of 20 wt % and were calcined at 800 ◦C for 1 h.

calcined at 800 °C for 1 h.

#### 3.1.1. XRD

Figure 1 shows the XRD patterns of the catalysts prepared with different solvents. The catalyst prepared with the H2O/C2H5OH solvent has no NiO diffraction peaks while the catalyst prepared with the DMF/C2H5OH solvent shows NiO peaks, indicating Ni segregation during electrospinning. The segregation might be caused by the solubility difference of two metal ions in the two solvents. Ni(NO3)2·6H2O has a lower solubility in the DMF/C2H5OH than the H2O/C2H5OH, while Al(NO3)3·9H2O has the similar solubility in the two solvents (Table 1). The lower solubility of Ni(NO3)2·6H2O in the DMF/C2H5OH causes the segregation during drying electrospun fibrous composites. The phase segregation also resulted in the higher crystallinity of NiAl2O<sup>4</sup> and Al2O3. After reduction, Ni presents in both catalysts while the catalyst prepared with the DMF/C2H5OH solvent shows the larger Ni crystal sizes due to NiO reduction. 3.1.1. XRD Figure 1 shows the XRD patterns of the catalysts prepared with different solvents. The catalyst prepared with the H2O/C2H5OH solvent has no NiO diffraction peaks while the catalyst prepared with the DMF/C2H5OH solvent shows NiO peaks, indicating Ni segregation during electrospinning. The segregation might be caused by the solubility difference of two metal ions in the two solvents. Ni(NO3)2·6H2O has a lower solubility in the DMF/C2H5OH than the H2O/C2H5OH, while Al(NO3)3·9H2O has the similar solubility in the two solvents (Table 1). The lower solubility of Ni(NO3)2·6H2O in the DMF/C2H5OH causes the segregation during drying electrospun fibrous composites. The phase segregation also resulted in the higher crystallinity of NiAl2O4 and Al2O3. After reduction, Ni presents in both catalysts while the catalyst prepared with the DMF/C2H5OH solvent shows the larger Ni crystal sizes due to NiO reduction.

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employed to investigate the solvent effect. Both catalysts had a metal ion content of 20 wt % and were

**Figure 1.** XRD patterns of the catalysts with different solvents: (**a**) Before reduction; (**b**) after reduction at 750 °C for 1 h. **Figure 1.** XRD patterns of the catalysts with different solvents: (**a**) Before reduction; (**b**) after reduction at 750 ◦C for 1 h.

**Table 1.** Solubility of nitrates in two solvents.


H2O/C2H5OH 35.5 19.5

3.1.2. SEM

The morphologies of the reduced catalysts prepared with different solvents are presented in Figure 2. The fibrous structure has high void fraction and therefore achieves fast mass transfer [10]. Figure 2a shows the morphology of the catalyst prepared with the H2O/C2H5OH solvent, uniform Ni particles anchored on the surface of fibrous support. As shown in Figure 2b, some large Ni particles appeared on the catalyst surface prepared with the DMF/C2H5OH solvent, which was attributed to NiO reduction. Figure 2. The fibrous structure has high void fraction and therefore achieves fast mass transfer [10]. Figure 2a shows the morphology of the catalyst prepared with the H2O/C2H5OH solvent, uniform Ni particles anchored on the surface of fibrous support. As shown in Figure 2b, some large Ni particles appeared on the catalyst surface prepared with the DMF/C2H5OH solvent, which was attributed to NiO reduction.

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The morphologies of the reduced catalysts prepared with different solvents are presented in

**Figure 2.** SEM images of the reduced catalysts prepared with different solvents: (**a**) H2O/C2H5OH; (**b**) DMF/C2H5OH. **Figure 2.** SEM images of the reduced catalysts prepared with different solvents: (**a**) H2O/C2H5OH; (**b**) DMF/C2H5OH.

### 3.1.3. TPR and CO-Chemisorption

3.1.4. Catalytic Performance

at 750 °C and a gas flow rate of 800 mL·min<sup>−</sup>1.

3.1.3. TPR and CO-Chemisorption TPR was carried out on fibrous catalysts to investigate reducibility. Figure 3 shows that the TPR profiles of the catalysts consist of two reduction peaks centered at 500 and 800 °C, respectively. NiO reduction occurs at low temperatures (400–600 °C) while NiAl2O4 reduction takes place at temperatures above 600 °C [6,11]. For the catalyst prepared with the DMF/C2H5OH solvent, the NiO reduction peak is stronger than the catalyst prepared with the H2O/C2H5OH solvent. The H2 consumption peak areas and reducibilities are compared in Table 2. The reducibility of the catalyst prepared with the DMF/C2H5OH solvent is lower than that of the catalyst prepared with the H2O/C2H5OH solvent. It might be attributed to Ni segregation because the Ni segregation causes NiO aggregation on the fiber surface, and the formed large NiO particles cause a decrease in reducibility. In addition, the Ni dispersion of the catalyst prepared with the DMF/C2H5OH solvent is smaller than that of the catalyst prepared with the H2O/C2H5OH solvent (Table 2) because the large Ni particles formed by NiO reduction decrease the Ni dispersion. TPR was carried out on fibrous catalysts to investigate reducibility. Figure 3 shows that the TPR profiles of the catalysts consist of two reduction peaks centered at 500 and 800 ◦C, respectively. NiO reduction occurs at low temperatures (400–600 ◦C) while NiAl2O<sup>4</sup> reduction takes place at temperatures above 600 ◦C [6,11]. For the catalyst prepared with the DMF/C2H5OH solvent, the NiO reduction peak is stronger than the catalyst prepared with the H2O/C2H5OH solvent. The H<sup>2</sup> consumption peak areas and reducibilities are compared in Table 2. The reducibility of the catalyst prepared with the DMF/C2H5OH solvent is lower than that of the catalyst prepared with the H2O/C2H5OH solvent. It might be attributed to Ni segregation because the Ni segregation causes NiO aggregation on the fiber surface, and the formed large NiO particles cause a decrease in reducibility. In addition, the Ni dispersion of the catalyst prepared with the DMF/C2H5OH solvent is smaller than that of the catalyst prepared with the H2O/C2H5OH solvent (Table 2) because the large Ni particles formed by NiO reduction decrease the Ni dispersion. *Catalysts* **2019**, *9*, x FOR PEER REVIEW 5 of 14

**Figure 3.** TPR profiles of the catalysts prepared with different solvents. **Figure 3.** TPR profiles of the catalysts prepared with different solvents.

**Figure 4.** Methane conversion of the catalysts prepared with different solvents during the POM 10 h

Figure 4 shows catalyst performance during methane partial oxidation at 750 °C and a gas flow rate of 800 mL·min−1. The catalyst prepared with the DMF/C2H5OH solvent generated a low methane conversion of about 10%, which degraded rapidly. According to the TPR results of the spent catalysts in Figure 5, a substantial amount of Ni particles in the catalyst prepared with the DMF/C2H5OH solvent were oxidized into NiO during the POM. In contrast, there is no obvious NiO reduction peak in the catalyst prepared with the H2O/C2H5OH solvent as the fresh catalyst. Our previous study shows the catalytic activity is mainly contributed by Ni-NiOx particles formed from NiAl2O4 reduction rather than Ni particles formed from NiO reduction. Therefore, the catalyst prepared with

DMF/C2H5OH 48.3 76.6 0.05

H2O/C2H5OH 51.6 83.4 0.27

**Table 2.** Reducibility and Ni dispersion of Ni/Al2O3 catalysts.


**Table 2.** Reducibility and Ni dispersion of Ni/Al2O<sup>3</sup> catalysts.

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#### 3.1.4. Catalytic Performance **Figure 3.** TPR profiles of the catalysts prepared with different solvents.

*3.2. Effect of Metal Ion Content* 

3.2.1. XRD

solvent and calcined at 800 °C for 1 h.

Figure 4 shows catalyst performance during methane partial oxidation at 750 ◦C and a gas flow rate of 800 mL·min−<sup>1</sup> . The catalyst prepared with the DMF/C2H5OH solvent generated a low methane conversion of about 10%, which degraded rapidly. According to the TPR results of the spent catalysts in Figure 5, a substantial amount of Ni particles in the catalyst prepared with the DMF/C2H5OH solvent were oxidized into NiO during the POM. In contrast, there is no obvious NiO reduction peak in the catalyst prepared with the H2O/C2H5OH solvent as the fresh catalyst. Our previous study shows the catalytic activity is mainly contributed by Ni-NiO<sup>x</sup> particles formed from NiAl2O<sup>4</sup> reduction rather than Ni particles formed from NiO reduction. Therefore, the catalyst prepared with the H2O/C2H5OH solvent demonstrated a high and stable methane conversion of 30%, which is consistent with the results of Ni dispersion in Table 2. **Table 2.** Reducibility and Ni dispersion of Ni/Al2O3 catalysts. **Sample Peak Area Reducibility (%) Ni dispersion (%)**  DMF/C2H5OH 48.3 76.6 0.05 H2O/C2H5OH 51.6 83.4 0.27 3.1.4. Catalytic Performance

**Figure 4.** Methane conversion of the catalysts prepared with different solvents during the POM 10 h at 750 °C and a gas flow rate of 800 mL·min<sup>−</sup>1. **Figure 4.** Methane conversion of the catalysts prepared with different solvents during the POM 10 h at <sup>750</sup> ◦C and a gas flow rate of 800 mL·min−<sup>1</sup> . the H2O/C2H5OH solvent demonstrated a high and stable methane conversion of 30%, which is consistent with the results of Ni dispersion in Table 2.

**Figure 5.** TPR profiles of the spent catalysts prepared with different solvents. **Figure 5.** TPR profiles of the spent catalysts prepared with different solvents.

During electrospinning, electrical filed force pulls solution drop from spinneret to form a jet.

Figure 6 exhibits the XRD patterns of the catalyst prepared with different metal ion contents. As the metal ion content is increased, NiO phase started to present, indicating Ni segregation. It is because Ni prefers to accumulate on the surface in NiO-Al2O3 system [12,13]. As shown in Figure 6a, calculated using the Scherrer equation, the NiAl2O4 crystal sizes of the C10, C20, C30 and C40 catalysts are 9.7, 8.5, 8.0 and 7.5 nm, respectively. The NiAl2O4 crystal sizes decrease as the metal ion content is increased, which is because NiO presence dispersed NiAl2O4 phase, inhibiting NiAl2O4

growth. After reduction, Ni peaks are observed in addition to the NiAl2O4/Al2O3 peaks.

### *3.2. E*ff*ect of Metal Ion Content*

During electrospinning, electrical filed force pulls solution drop from spinneret to form a jet. Metal ion content affects solution viscosity, electrical filed force and final ceramic fibers. Therefore, the effect of metal ion content was investigated. All catalysts were prepared with the H2O/C2H5OH solvent and calcined at 800 ◦C for 1 h.

#### 3.2.1. XRD

Figure 6 exhibits the XRD patterns of the catalyst prepared with different metal ion contents. As the metal ion content is increased, NiO phase started to present, indicating Ni segregation. It is because Ni prefers to accumulate on the surface in NiO-Al2O<sup>3</sup> system [12,13]. As shown in Figure 6a, calculated using the Scherrer equation, the NiAl2O<sup>4</sup> crystal sizes of the C10, C20, C30 and C40 catalysts are 9.7, 8.5, 8.0 and 7.5 nm, respectively. The NiAl2O<sup>4</sup> crystal sizes decrease as the metal ion content is increased, which is because NiO presence dispersed NiAl2O<sup>4</sup> phase, inhibiting NiAl2O<sup>4</sup> growth. After reduction, Ni peaks are observed in addition to the NiAl2O4/Al2O<sup>3</sup> peaks. *Catalysts* **2019**, *9*, x FOR PEER REVIEW 7 of 14

**Figure 6.** XRD patterns of the catalysts with different metal ion contents: (**a**) Before reduction; (**b**) after **Figure 6.** XRD patterns of the catalysts with different metal ion contents: (**a**) Before reduction; (**b**) after reduction at 750 ◦C for 1 h.

#### 3.2.2. TPR and CO-Chemisorption

reduction at 750 °C for 1 h.

of NiO can improve Ni dispersion.

3.2.2. TPR and CO-Chemisorption As shown in Figure 7, for the C10 catalyst, a single NiAl2O4 reduction peak was centred at 800 °C. When metal ion content was increased to 20 wt %, the reduction peak occurs around 400 °C, which is attributed to NiO reduction. The H2 consumption peak areas and reducibilities are As shown in Figure 7, for the C10 catalyst, a single NiAl2O<sup>4</sup> reduction peak was centred at 800 ◦C. When metal ion content was increased to 20 wt %, the reduction peak occurs around 400 ◦C, which is attributed to NiO reduction. The H<sup>2</sup> consumption peak areas and reducibilities are summarized in

summarized in Table 3. As the metal ion content is increased, the amount of reduced NiO increases, indicating more NiO segregated. Therefore, the reducibility of fibrous catalysts also increases. Table 3. As the metal ion content is increased, the amount of reduced NiO increases, indicating more NiO segregated. Therefore, the reducibility of fibrous catalysts also increases. Furthermore, the Ni dispersion increases with metal ion content up to 30 wt %. However, the Ni dispersion of the C40 catalyst is reduced because of NiO aggregation [14]. Therefore, a certain amount of NiO can improve Ni dispersion. *Catalysts* **2019**, *9*, x FOR PEER REVIEW 8 of 14

**Figure 7.** TPR profiles of the catalysts prepared with different metal ion contents.


C40 57.4 95.4 0.39

**Figure 7.** TPR profiles of the catalysts prepared with different metal ion contents. **Table 3.** Reducibility and Ni dispersion of Ni/Al2O<sup>3</sup> catalysts.

#### 3.2.3. SEM

3.2.3. SEM The morphologies of reduced catalysts made from the solution prepared with different metal ion contents are shown in Figure 8. The fiber diameter increases with the increase of metal ion content from 50 to 300 nm due to the increased solution viscosity and metal ion loading. Ni particles present on fiber surface after reduction. The C10 catalyst shows the largest Ni particle sizes, and Ni particle sizes increase when metal ion content was increased from 20 to 40%. The change of Ni particle sizes are consistent with the change of crystal sizes in Figure 6a, and the large crystal sizes generate the big The morphologies of reduced catalysts made from the solution prepared with different metal ion contents are shown in Figure 8. The fiber diameter increases with the increase of metal ion content from 50 to 300 nm due to the increased solution viscosity and metal ion loading. Ni particles present on fiber surface after reduction. The C10 catalyst shows the largest Ni particle sizes, and Ni particle sizes increase when metal ion content was increased from 20 to 40%. The change of Ni particle sizes are consistent with the change of crystal sizes in Figure 6a, and the large crystal sizes generate the big Ni particles.

#### Ni particles. 3.2.4. Catalytic Performance

The fibrous catalyst prepared with different metal ion contents were tested for the POM at 750 ◦C and a gas flow rate of 800 mL min−<sup>1</sup> to investigate catalytic activity. As shown in Figure 9, methane conversion improves with metal ion content up to 30 wt %, and further increasing metal ion content to 40 wt % causes a decline in CH<sup>4</sup> conversion. The catalytic performance is consistent with Ni dispersion in Table 3, and the higher dispersion contribute the higher catalytic performance. In addition, the Ni particles formed by NiO are easily oxidized during the POM, which has the limited contribution to catalytic performance [14].

**Figure 8.** SEM images of the reduced catalysts made from the solution with different metal ion contents: (**a**) C10; (**b**) C20; (**c**) C30; (**d**) C40. **Figure 8.** SEM images of the reduced catalysts made from the solution with different metal ion contents: (**a**) C10; (**b**) C20; (**c**) C30; (**d**) C40. *Catalysts* **2019**, *9*, x FOR PEER REVIEW 10 of 14

**Figure 9.** Methane conversion of the catalysts prepared with different metal ion contents during the POM for 10 h at 750 °C and a gas flow rate of 800 mL min<sup>−</sup>1. **Figure 9.** Methane conversion of the catalysts prepared with different metal ion contents during the POM for 10 h at 750 ◦C and a gas flow rate of 800 mL min−<sup>1</sup> .

The morphologies of the catalysts after 10 h-POM test are shown in Figure 10, and the fibrous structure was stable during reactions. The Ni particles on the C10 and C20 catalyst surface disappeared after the reaction due to Ni oxidation. In contrast, the Ni particles retained for the C30 and C40 catalysts while carbon fibers could be found. The morphologies of the catalysts after 10 h-POM test are shown in Figure 10, and the fibrous structure was stable during reactions. The Ni particles on the C10 and C20 catalyst surface disappeared after the reaction due to Ni oxidation. In contrast, the Ni particles retained for the C30 and C40 catalysts while carbon fibers could be found.

**Figure 10.** SEM images of the spent catalysts made from the solution with different metal ion contents:

(**a**) C10; (**b**) C20; (**c**) C30; (**d**) C40.

**Figure 9.** Methane conversion of the catalysts prepared with different metal ion contents during the

The morphologies of the catalysts after 10 h-POM test are shown in Figure 10, and the fibrous structure was stable during reactions. The Ni particles on the C10 and C20 catalyst surface

POM for 10 h at 750 °C and a gas flow rate of 800 mL min<sup>−</sup>1.

and C40 catalysts while carbon fibers could be found.

**Figure 10.** SEM images of the spent catalysts made from the solution with different metal ion contents: (**a**) C10; (**b**) C20; (**c**) C30; (**d**) C40. **Figure 10.** SEM images of the spent catalysts made from the solution with different metal ion contents: (**a**) C10; (**b**) C20; (**c**) C30; (**d**) C40.
