Study on Nanofibrous Catalysts Prepared by Electrospinning for Methane Partial Oxidation

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/Al₂O₃ 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/Al₂O₃ 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/Al₂O₃ catalysts can achieve high syngas yields at high reaction temperatures and high gas flow rates.

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 [1,2,3,4,5].

Ni/Al₂O₃ 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/Al₂O₃ 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 = 1.3 × 10⁶, 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⁻¹. 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 % H₂ in Ar with a gas flow rate of 30 mL min⁻¹. The TPR tests were operated from room temperature to 1000 °C at a heating rate of 10 °C min⁻¹. 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 % H₂ in Ar at 750 °C for 1 h. The reactant gas of CH₄, O₂ 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⁻¹. Reaction products were sampled by a gas chromatography (GC, Shimadzu GC-2014).

3. Results and Discussions

3.1. Effect 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.

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. Solubility of nitrates in two solvents.

Solvent (8 g/2 g)	Ni(NO3)2·6H2O (g)	Al(NO3)3·9H2O (g)
DMF/C2H5OH	24.5	18.5
H2O/C2H5OH	35.5	19.5

). 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.

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.

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. 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

. 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.

3.1.4. Catalytic Performance

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-NiO_x particles formed from NiAl₂O₄ 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.

3.2. Effect 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-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.

3.2.2. 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 summarized in

Table 3. Reducibility and Ni dispersion of Ni/Al₂O₃ catalysts.

Sample	Peak Area	Reducibility (%)	Ni Dispersion (%)
C10	50.8	81.7	0.17
C20	51.6	83.4	0.27
C30	52.7	85.7	0.60
C40	57.4	95.4	0.39

. 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.

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 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⁻¹ 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 CH₄ 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].

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.

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, 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, NiAl₂O₄ 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⁻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, 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. 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

).

3.4. Effect 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 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 the fibrous structure.

4. Conclusions

In this study, the effects of preparation parameters on catalyst properties were investigated on 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.