*3.3. E*ff*ect of Calcination Temperature*

Ni catalysts are formed via reducing NiAl2O4, and catalytic activity is affected by catalyst crystallinity, which is determined by calcination temperature. To study the effect of calcination temperature on catalytic activity, the fibrous catalysts were calcined at temperatures ranging from 700 to 900 ◦C. All catalysts were prepared with a metal ion content of 30 wt % and the H2O/C2H5OH solvent. As shown in Figure 11a, NiAl2O<sup>4</sup> has a low crystallinity when calcined at 700 ◦C. With the increase of calcination temperature, the crystallinity is enhanced, resulting in the increase of crystal sizes. Accordingly, the reduced catalysts show the increased Ni crystal sizes with calcination temperature according to the diffraction intensity in Figure 11b. Calculated by the Scherrer equation, the Ni crystal sizes are 7.4, 8.1 and 9.2 nm at the calcination temperatures of 700, 800 and 900 ◦C, respectively.

TPR profiles are shown in Figure 12. According to the XRD results, NiAl2O<sup>4</sup> crystallinity enhances with the increase of calcination temperature, resulting in the increase of reduction temperature. As shown in Figure 11a, Ni reducibility increases slightly due to the presence of NiO with the increase of calcination temperature. Additionally, the Ni dispersion of catalysts reduces with the increase of calcination temperature, which is attributed to the increase of Ni crystal size [6].

The POM was conducted at 750 ◦C and a gas flow rate of 800 mL min−<sup>1</sup> , and the methane conversions are shown in Figure 13. According to the TPR results in Figure 12, the catalysts calcined at 700, 800 and 900 ◦C were reduced for 1 h at a reduction peak temperature of 615, 750 and 800 ◦C, respectively, which ensures that the catalysts were pre-reduced to the same extent. The catalyst calcined at the higher temperature showed the lower catalytic activity due to the decrease of Ni dispersion (Table 4).

dispersion (Table 4).

*3.3. Effect of Calcination Temperature* 

Ni catalysts are formed *via* reducing NiAl2O4, and catalytic activity is affected by catalyst crystallinity, which is determined by calcination temperature. To study the effect of calcination temperature on catalytic activity, the fibrous catalysts were calcined at temperatures ranging from 700 to 900 °C. All catalysts were prepared with a metal ion content of 30 wt % and the H2O/C2H5OH solvent. As shown in Figure 11a, NiAl2O4 has a low crystallinity when calcined at 700 °C. With the increase of calcination temperature, the crystallinity is enhanced, resulting in the increase of crystal sizes. Accordingly, the reduced catalysts show the increased Ni crystal sizes with calcination temperature according to the diffraction intensity in Figure 11b. Calculated by the Scherrer equation,

**Figure 11.** XRD patterns of the catalysts with different calcination temperatures: (**a**) Before reduction; (**b**) after reduction at 750 °C for 1 h. **Figure 11.** XRD patterns of the catalysts with different calcination temperatures: (**a**) Before reduction; (**b**) after reduction at 750 ◦C for 1 h. *Catalysts* **2019**, *9*, x FOR PEER REVIEW 12 of 14

**Figure 12.** TPR profiles of the catalysts prepared at different calcination temperatures. **Figure 12.** TPR profiles of the catalysts prepared at different calcination temperatures.

**Table 4.** Reducibility and Ni dispersion of Ni/Al2O3 catalysts. **Temperature (°C) Peak Area Reducibility (%) Ni dispersion (%)**  700 50.8 81.7 0.63 800 52.7 85.7 0.60 900 55.5 91.5 0.38

conversions are shown in Figure 13. According to the TPR results in Figure 12, the catalysts calcined at 700, 800 and 900 °C were reduced for 1 h at a reduction peak temperature of 615, 750 and 800 °C, respectively, which ensures that the catalysts were pre-reduced to the same extent. The catalyst calcined at the higher temperature showed the lower catalytic activity due to the decrease of Ni

**Figure 13.** Methane conversion of the catalysts prepared at different calcination temperatures during

The fibrous structure has high thermal stability and large void fraction, which makes it possible to operate at high temperatures and gas flow rates. The C30 catalyst was chosen to investigate the

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

*3.4. Effect of Reaction Temperature and Gas Flow Rate* 

**4. Conclusions** 

decision to publish the results.

dispersion (Table 4).

**Figure 12.** TPR profiles of the catalysts prepared at different calcination temperatures.

**Table 4.** Reducibility and Ni dispersion of Ni/Al2O3 catalysts. **Temperature (°C) Peak Area Reducibility (%) Ni dispersion (%)**  700 50.8 81.7 0.63 800 52.7 85.7 0.60 900 55.5 91.5 0.38

The POM was conducted at 750 °C and a gas flow rate of 800 mL min−1, and the methane conversions are shown in Figure 13. According to the TPR results in Figure 12, the catalysts calcined at 700, 800 and 900 °C were reduced for 1 h at a reduction peak temperature of 615, 750 and 800 °C,

**Figure 13.** Methane conversion of the catalysts prepared at different calcination temperatures during the POM for 10 h at 750 °C and a gas flow rate of 800 mL min<sup>−</sup>1. **Figure 13.** Methane conversion of the catalysts prepared at different calcination temperatures during the POM for 10 h at 750 ◦C and a gas flow rate of 800 mL min−<sup>1</sup> .

**Table 4.** Reducibility and Ni dispersion of Ni/Al2O<sup>3</sup> catalysts.


#### *3.4. E*ff*ect of Reaction Temperature and Gas Flow Rate*

The fibrous structure has high thermal stability and large void fraction, which makes it possible to operate at high temperatures and gas flow rates. The C30 catalyst was chosen to investigate the effect of reaction conditions on catalytic activity. Figure 14 shows that CH<sup>4</sup> conversion increases with reaction temperature at a gas flow rate of 1000 mL min−<sup>1</sup> , and the catalytic reaction rate increases with gas flow rate at 750 ◦C. Under the reaction conditions of 800 ◦C and a flow rate of 1000 mL min−<sup>1</sup> , the selectivity of H<sup>2</sup> and CO was 97% and 87%, respectively, and the yield was 9.8 <sup>×</sup> <sup>10</sup><sup>5</sup> L Kg−<sup>1</sup> <sup>h</sup> <sup>−</sup><sup>1</sup> and 4.4 <sup>×</sup> <sup>10</sup><sup>5</sup> L Kg−<sup>1</sup> <sup>h</sup> −1 , respectively. The H<sup>2</sup> and CO yields were calculated according to H<sup>2</sup> and CO amounts in the product gas. Therefore, the fibrous Ni/Al2O<sup>3</sup> catalyst can generate high syngas yields owing to the fibrous structure. *Catalysts* **2019**, *9*, x FOR PEER REVIEW 13 of 14 effect of reaction conditions on catalytic activity. Figure 14 shows that CH4 conversion increases with reaction temperature at a gas flow rate of 1000 mL min−1, and the catalytic reaction rate increases with gas flow rate at 750 °C. Under the reaction conditions of 800 °C and a flow rate of 1000 mL min−1, the selectivity of H2 and CO was 97% and 87%, respectively, and the yield was 9.8 × 105 L Kg−1 h−1 and 4.4 × 105 L Kg−1 h−1, respectively. The H2 and CO yields were calculated according to H2 and CO amounts in the product gas. Therefore, the fibrous Ni/Al2O3 catalyst can generate high syngas yields owing to

**Figure 14.** Catalytic performance of the C30 catalyst during the POM changing with operation temperature and gas flow rate. **Figure 14.** Catalytic performance of the C30 catalyst during the POM changing with operation temperature and gas flow rate.

electrospun fibrous Ni/Al2O3 catalysts. The catalyst prepared with the H2O/C2H5OH solvent mainly consisted of NiAl2O4, while the catalyst prepared with the DMF/C2H5OH solvent formed NiO due to Ni segregation. The catalytic performance is mainly contributed by the Ni from NiAl2O4 reduction, and therefore the catalytic activity of the catalyst prepared with the H2O/C2H5OH solvent was higher than that of the catalyst prepared with the DMF/C2H5OH solvent. The metal ion content affects catalyst composition, microstructure, reducibility and dispersion and therefore catalytic performance during the POM. The C30 catalyst had the highest catalytic performance. In addition, the higher calcination temperature produced the larger Ni particles due to the larger crystal size of NiAl2O4, which required a high reduction temperature. Therefore, the catalytic activity during the POM decreased with the increase of calcination temperature. Finally, it has been confirmed that the fibrous Ni/Al2O3 catalysts can achieve high syngas yields through the POM owing to the fibrous structure. **Author Contributions: C**onceptualization, Yuyao Ma and D.D.; methodology, Yuyao Ma, Yuxia Ma, M.L. and Y.C..; software, Yuyao Ma; validation, X.H., Z.Y. and D.D.; formal analysis, Yuyao Ma, Yuxia Ma and D.D.; investigation, Yuyao Ma, Yuxia Ma, M.L. and Y.C.; resources, D.D.; data curation, Yuyao Ma, M.L. and Y.C.; writing—original draft preparation, Yuyao Ma; writing—review and editing, D.D.; visualization, X.H., Z.Y. and

D.D.; supervision, X.H., Z.Y. and D.D.; project administration, D.D.; funding acquisition, D.D.

and Shandong Province Key Research and Development Program (2018GGX102037).

Shandong Province Key Research and Development Program (2018GGX102037).

**Funding:** This research was funded by Natural Science Foundation of Shandong Province (ZR2017MEM022)

**Acknowledgments:** D. H. Dong acknowledges the startup funding provided by the University of Jinan. The study is a part of the projects of Natural Science Foundation of Shandong Province (ZR2017MEM022) and

**Conflicts of Interest:** The authors declare no conflict of interest. The funder played a decisive role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript and in the
