Introduction

p-Nitrophenol is a type of highly toxic and potentially carcinogenic compound produced from versatile industries of explosive, pharmaceutical, pesticide, dye, pigment, rubber chemical and wood preservative process, and has been listed as priority pollutants due to its high toxicity and non-biodegradation (Bae et al. 2016; Chen et al. 2016; Sun and Lemley 2011). p-Aminophenol is an important intermediate for the fabrication of pharmaceuticals, photographic developers, anticorrosion lubricants and other industrially important products. Over the past few years, the heterogeneous catalytic reduction of p-nitrophenol to p-aminophenol by NaBH4 in the aqueous solution was regarded as a green and sustainable manufacture process to dispose p-nitrophenol and produce p-aminophenol (Saha et al. 2010; Wu et al. 2011). Therefore, it is important to develop novel effective catalysts for the reaction.

In recent years, bimetallic nanocatalysts, especially Pt-based alloys, have been getting increased attentions due to their enhanced activity, stability and selectivity compared with pure metals in many heterogeneous catalytic processes. Pt-based alloys are kind of important catalyst systems and usually composed of Pt and transition metals, such as PtNi, PtFe, PtCu, PtCo, PtRu, and so on (Zou et al. 2015; Shukla et al. 2004; He and Li 1988; Ohkubo et al. 2013; Kageyama et al. 2011; Yang et al. 2016). The size and dispersion of Pt-based alloy nanoparticles make significant difference on their specific catalytic properties, and highly dispersed nanoparticles with small size and narrow size distribution usually show excellent catalytic performance owing to their large surface area and specific electronic structure. However, the Pt-based alloy nanoparticles are not stable and easy to aggregate because of their high surface energy, which subsequently leads to decline of their catalytic activity (Sahoo et al. 2014).

In order to solve the above problems, Pt-based alloy nanoparticles can be immobilized on the surface of supports with large surface area (Sahoo et al. 2014; Zhao et al. 2016). Nanostructured supports with various morphologies such as sphere, sheet, rod, tube and fiber have been used as novel supports to load metal catalysts (Hu and Chua 2016; Yoo et al. 2009; Li et al. 2012; Zhuang et al. 2016; Ogunlaja et al. 2016). Electrospinning is a promising, simple and low-cost strategy to produce fibers with diameters ranged from nanoscale to micrometer scale. The nanofibers, the products of pyrolysis from electrospun nanofibers, have been considered as an alternative for the support materials owing to their large surface area, controllable size, high mechanical strength and good stability (Guo et al. 2014). Previous research revealed that highly porous structure of nanofibers provided numerous attachment sites for nanocatalysts and also ensured the permeability of the support matrix and the accessibility of the reactant molecules (Fu et al. 2016; Lee et al. 2011; Zhao et al. 2008).

In the present work, we prepared mesoporous SiO2 nanofibers by pyrolyzing precursor of SiO2 electrospun nanofibers and then employed it as support to immobilize PtNi nanocatalyst. Characterization results showed that PtNi alloy nanoparticles could be homogeneously supported on the surface of SiO2 nanofibers. PtNi/SiO2 nanofibers showed superior catalytic activity and good reusability towards the reduction reaction of p-nitrophenol to p-aminophenol by NaBH4 in the aqueous solution.

Materials and methods

Reagents and instruments

Polyvinylpyrrolidone (PVP, M w = 1,300,000), tetraethyl orthosilicate (TEOS), anhydrous ethanol, Ni(NO3)2·6H2O, H2PtCl6·6H2O, NaOH, HCl, hydrazine hydrate, p-nitrophenol and NaBH4 were all purchased from commercial suppliers and used without further purification. Double-distilled water was used throughout the experiment.

Atomic force microscopy (AFM) study was carried out using Bruck Dimension FastScan scanning probe microscope. N2 adsorption/desorption isotherms and the corresponding pore size distribution were recorded on Quantachrome NOVA4200 specific surface area and pore size distribution analyzer. Field emission scanning electron microscopy (FE-SEM) images were observed by JEOL JSM-6701F FE-SEM. Transmission electron microscopy (TEM) images were taken using a JEOL JEM-2100 TEM. X-ray Powder Diffraction (XRD) data was obtained with a Rigaku Ultima III X-ray diffractometer. High-resolution X-ray photoelectron spectroscopy (XPS) was carried out on a VG ESCALAB250 X-ray photoelectron spectrometer. The composition of the catalyst was determined using Shimadzu ICPS-7500 inductively coupled plasma-atomic emission spectrometry (ICP-AES) technique. The magnetic measurement was performed using a Lake Shore 7404 vibrating sample magnetometer. The UV-Vis adsorption spectra were determined on UV-2450 UV-Vis spectrophotometer.

Preparation of porous SiO2 nanofibers

For the preparation of TEOS spinning solution, PVP (3.0 g) and TEOS (3.0 g) were dissolved into anhydrous ethanol (30.0 mL) followed by being stirred for 2 h at 25 °C. Then, HCl (0.26 mL) was added dropwise into the above solution to control the hydrolysis/gelation of TEOS; this was followed by continuous stirring the system for 12 h. The electrospinning was performed under a positive high voltage of 20 kV with a solution feeding rate of 30 μL min−1, and the distance between the collector and the tip of the needle was set as 15 cm. Subsequently, the as-electrospun nanofibrous mat was calcined at 550 °C in air for 2 h to obtain porous SiO2 nanofibers.

Preparation of PtNi/SiO2 nanofibers

SiO2 nanofibers (0.28 g) were added into ethylene glycol (160 mL) and ultrasonicated for 1 h to form a stable suspension of SiO2 nanofibers. H2PtCl6·6H2O aqueous solution (0.03352 mol L−1, 4.3 mL) and Ni(NO3)2·6H2O (416.3 mg) were dissolved in ethylene glycol (40 mL). This metal salt solution was added into the above SiO2 nanofiber suspension followed by addition of hydrazine hydrate (85 wt%, 6.4 mL) and NaOH ethylene glycol solution (0.375 mol L−1, 7.2 mL). This mixture was kept in an ultrasonic bath for 10 min and then stirred at 110 °C for 3 h under N2 atmosphere. The above reaction suspension was cooled and subsequently separated by centrifugation. The prepared solid product was thoroughly washed with water and anhydrous ethanol and then dried in a vacuum oven at 50 °C for 24 h to obtain PtNi/SiO2 nanofibers. For comparison, Pt/SiO2 nanofibers and Ni/SiO2 nanofibers were also prepared using the same method.

Catalytic reduction of p-nitrophenol

The reduction of p-nitrophenol into p-aminophenol by NaBH4 is widely used as a model reaction for monitoring the catalytic activity of metal nanoparticles (Deka et al. 2017). So this reaction was adopted to quantitatively evaluate the catalytic activity of the as-prepared catalysts at 25 °C. In a typical procedure, freshly prepared p-nitrophenol aqueous solution (2 × 10−3 mol L−1, 20 mL) and NaBH4 aqueous solution (0.25 mol L−1, 20 mL) were mixed to form a uniform solution by stirring for 2 min, and then SiO2 nanofiber-supported nanocatalyst (5.0 mg) was added into the above solution. During the reaction process, the reaction solution (0.5 mL) was taken from the reaction system at a regular interval of 1 min, and subsequently diluted with double-distilled water (9.5 mL). The reaction process was monitored by measuring the change of UV-Vis absorbance intensity of p-nitrophenol at 400 nm. The sampling procedure was continued until the reaction solution became colorless. The reusability experiment was conducted by the same procedure.

Results and discussion

Characterization of the as-synthesized target products

Figure 1 shows the typical AFM images of SiO2 nanofibers precursor and SiO2 nanofibers. It could be observed that the precursor of SiO2 nanofibers showed a smooth surface (Fig. 1a, c), while SiO2 nanofibers revealed a rough and irregular surface (Fig. 1b, d). Surface roughness parameters of mean roughness (Ra) and root mean square roughness (Rq) were presented in Fig. 1. The Rq and Ra values for SiO2 nanofibers (Rq = 21.02 nm, Ra = 17.10 nm) were three times higher than those of SiO2 nanofibers precursor (Rq = 6.08 nm, Ra = 5.12 nm). The roughness variation of SiO2 nanofibers could be attributed to the formation of porous structure on the surface due to pyrolyzing of precursor. The rough surface and porous structure could help catalyst nanoparticles to support uniformly on the surface of SiO2 nanofibers.

Fig. 1
figure 1

Two-dimensional and three-dimensional AFM images of SiO2 nanofibers precursor (a, c) and SiO2 nanofibers (b, d)

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Figure 2 shows the XRD patterns of Pt/SiO2 nanofibers and PtNi/SiO2 nanofibers. A broad diffraction peak at two theta in the range of 15–30° was corresponded to the amorphous SiO2 (JCPDS 29-0085). For Pt/SiO2 nanofibers, the diffraction peaks detected at around 40.0, 46.5, 68.0, and 81.5° were attributed to the (111), (200), (220) and (311) crystal planes of Pt face-centered cubic crystal structure (JCPDS 04-0802), respectively. Diffraction pattern of PtNi/SiO2 nanofibers showed similar peaks as those of Pt/SiO2 nanofibers. It is worth noting that the four diffraction peaks of PtNi/SiO2 nanofibers were slightly shifted to higher 2 theta values with respect to those of Pt/SiO2 nanofibers, which suggested the formation of PtNi alloy (Kim et al. 2009; Zhang et al. 2014a).

Fig. 2
figure 2

XRD patterns of Pt/SiO2 nanofibers and PtNi/SiO2 nanofibers

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The porous structure of the obtained SiO2 nanofibers and PtNi/SiO2 nanofibers were characterized by N2 adsorption/desorption analysis. SiO2 nanofibers and PtNi/SiO2 nanofibers exhibited type-IV adsorption isotherm pattern with hysteresis loop in the range of P/P0 0.4–1.0 (Fig. 3a). According to the classification by the International Union of Pure and Applied Chemistry (IUPAC) (Leofanti et al. 1998), the shape of the hysteresis loop was identified as type-H1, corresponding to the narrow mesoporous pores within SiO2 nanofibers and PtNi/SiO2 nanofibers. The SiO2 nanofibers had a specific Brunauer-Emmett-Teller (BET) surface area of 644.93 m2 g−1 and average pore diameter of 3.8 nm. The large surface area of SiO2 nanofibers provided an ample space for supporting of metal nanoparticles on their surface. However, the BET surface area and average pore diameter of PtNi/SiO2 nanofibers were 351.37 m2 g−1 and 3.4 nm, respectively. The declination of surface area and average pore diameter of PtNi/SiO2 nanofibers could be attributed to the loading of metal nanoparticles and the blockage of mesopores by metal nanoparticles (Wang and Dai 2009).

Fig. 3
figure 3

N2 adsorption/desorption isotherms (a) and the corresponding pore size distribution (b) of SiO2 nanofibers and PtNi/SiO2 nanofibers

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To reveal the morphology and microstructure of SiO2 nanofibers and PtNi/SiO2 nanofibers, SEM, TEM and HRTEM observations were carried out in Fig. 4. It could be seen that one-dimensional SiO2 nanofibers with average diameter of 400 nm were obtained in Fig. 4a. Compared with the SEM image of SiO2 nanofibers precursor (Fig. S1), the enlarged SEM image of SiO2 nanofibers (the inset of Fig. 4a) further revealed that the last pyrolysis process created a rough surface, which was in accordance with the results of AFM analysis. TEM image of PtNi/SiO2 nanofibers showed that metal nanoparticles were successfully and uniformly loaded on the surface of SiO2 nanofibers (Fig. 4b). The statistical result showed that distribution range of PtNi particles size was narrow and the average diameter of them was 18.7 nm (Fig. 4c). To further reveal the microstructure of PtNi nanoparticles, HRTEM observation was carried out. As shown in Fig. 4d, the lattice spacing of 0.214 nm was larger than that of the pure Ni (0.203 nm) and was smaller than that of the (111) plane of Pt (0.23 nm), which provided additional evidence for the formation of PtNi alloy (Bai et al. 2012; Zhang et al. 2010, and the lattice fringe was attributed to the (111) plane of face-centered cubic PtNi alloy (Zhu et al. 2016).

Fig. 4
figure 4

SEM and enlarged SEM images of SiO2 nanofibers (a), TEM image of PtNi/SiO2 nanofibers (b), the size distribution of PtNi nanoparticles (c), and HRTEM lattice image of PtNi catalyst (d)

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The XPS measurement was used to evaluate the surface structure and chemical states of as-prepared catalysts. Figure 5a shows the XPS spectra of Pt4f in Pt/SiO2 nanofibers and PtNi/SiO2 nanofibers. It could be observed that Pt existed predominantly in Pt0 form, demonstrating that Pt4+ precursor was successfully reduced to form metallic Pt on the surface of SiO2 nanofibers. And the new strong peak at the binding energy value of 67.3 eV for PtNi/SiO2 nanofibers could be assigned to the XPS peak of Ni3p (Ma et al. 2014). It is notable that the Pt4f peaks of PtNi/SiO2 nanofibers showed negative shifts compared to those of Pt/SiO2 nanofibers, which may be caused by the transfer of electrons from Ni to Pt in PtNi alloy (Sahoo et al. 2014). The electronic transfer can result in the downshift of the Pt d-band center position which plays a vital role in determining the catalytic performance. The downshift is directly related to the adsorption energies of the reactants on the catalyst as well as their activation barriers, which may be good for the improvement of catalytic activity (Ahmadi et al. 2015; Zhang et al. 2014b). In the fine spectra of Ni2p in PtNi/SiO2 nanofibers (Fig. 5b), the peaks at the binding energies of 855.04 and 872.72 eV were ascribed to Ni2p3/2 and Ni2p1/2, respectively. Besides, there were two shake-up satellite signals caused by multi-electron excitation at 860.86 and 878.68 eV (Song et al. 2015a). These results were found to be in agreement with other literature values of PtNi alloys (Sahoo et al. 2014; Ahmadi et al. 2015; Shukla et al. 2001).

Fig. 5
figure 5

The XPS spectra of Pt4f in Pt/SiO2 nanofibers and PtNi/SiO2 nanofibers (a) and the XPS spectra of Ni2p in PtNi/SiO2 nanofibers (b)

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Catalytic activity of PtNi/SiO2 nanofibers

The time-dependent UV-Vis absorption spectra changes of p-nitrophenol catalyzed by different catalysts are shown in Fig. 6. The UV-Vis absorption peak at 400 nm is the characteristic of p-nitrophenolate ions in alkaline solution (Ye et al. 2016). It was observed that the intensity of absorption peak at 400 nm almost had no change in the presence of pure SiO2 nanofibers after 24 h of experimentation (Fig. 6a), indicating that SiO2 nanofibers itself could not be used as catalyst and this reduction reaction could not proceed without catalyst. When Ni/SiO2 nanofibers and Pt/SiO2 nanofibers were added into the reaction system, the absorption peak intensity decreased gradually with time increase, and the conversion rates (%) of p-nitrophenol to p-aminophenol after 6 min for catalysts of Ni/SiO2 nanofibers and Pt/SiO2 nanofibers were 17.9 and 43.7%, respectively (Fig. 6b, c). Notably, PtNi/SiO2 nanofibers showed the highest catalytic activity compared with Pt/SiO2 nanofibers and Ni/SiO2 nanofibers and can transform p-nitrophenol to p-aminophenol completely within 6 min (Fig. 6d). Moreover, the catalytic properties for the physical mixture of Pt/SiO2 nanofibers and Ni/SiO2 nanofibers (Pt/SiO2 + Ni/SiO2, with mass ratio of Pt/SiO2:Ni/SiO2 = 1:3) were measured. The result showed that the catalytic activity of Pt/SiO2 + Ni/SiO2 was far less than PtNi/SiO2 nanofibers, and the conversion rate of p-nitrophenol to p-aminophenol only reached 52.4% at 6 min (Fig. S2). The simple physical mixture of Pt/SiO2 + Ni/SiO2 could not improve catalytic activity effectively and the excellent catalytic activity of PtNi/SiO2 nanofibers could be attributed to the synthetic effect of Pt and Ni within PtNi alloy (Bae et al. 2012; Kang et al. 2016).

Fig. 6
figure 6

Time-dependent UV-Vis adsorption spectra changes of p-nitrophenol for different catalysts

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The kinetics of this reduction reaction was studied for evaluating the catalytic rate of different catalysts. It is usually followed the pseudo-first-order kinetics with respect to the concentration of p-nitrophenol (Song et al. 2015b), which is given as follows:

$$ \ln \left({C}_t/{C}_0\right)= \ln \left({A}_t/{A}_0\right)=-{k}_a t $$
(1)

where C t and C 0 represent the concentration of p-nitrophenol at times t and t = 0, respectively. A t and A 0 are the absorbance of p-nitrophenol (at peak of 400 nm) at times t and t = 0, respectively. k a is the apparent rate constant. C t /C 0 is calculated from A t /A 0.

A linear relationship of ln(C t /C 0) versus reaction time (t) was obtained in Fig. 7. The ln(C t /C 0) showed a good linear correlation with the reaction time for all catalysts, which indicated that the reduction reaction of p-nitrophenol followed the pseudo-first-order kinetics. The k a of different catalysts were calculated from the slopes of the linearly fitted plots of ln(C t /C 0) versus time. The k a values of Ni/SiO2 nanofibers, Pt/SiO2 nanofibers and PtNi/SiO2 nanofibers for the reduction of p-nitrophenol were 0.55 × 10−3, 1.61 × 10−3 and 12.84 × 10−3 s−1, respectively.

Fig. 7
figure 7

Plots of ln(C t /C 0) versus time in the presence of different catalysts

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In order to compare the catalytic activity with different catalysts of the previous reports, k a should be further normalized to another comparable kinetic parameter, k n , which can be defined as (Liu et al. 2013):

$$ {k}_n=\left({10}^{-3}{C}_0 V/ m\right){k}_a $$
(2)

where C 0 (mM) is the initial concentration of p-nitrophenol, V (mL) is the volume of reactant solution, and m (g) is the metal mass of catalyst. Based on the above formula and ICP-AES results (Table S1), the calculated k n value of PtNi/SiO2 nanofibers was 434 × 10−3 mmol s−1 g−1, which was larger than those of Ni/SiO2 nanofibers (18 × 10−3 mmol s−1 g−1) and Pt/SiO2 nanofibers (55 × 10−3 mmol s−1 g−1). We compared the catalytic activity of PtNi/SiO2 nanofibers with those of PtNi and supported PtNi nanocatalysts for p-nitrophenol reduction (Table 1) (Sahoo et al. 2014; Song et al. 2015b; Ghosh et al. 2004). The PtNi/SiO2 nanofibers showed superior catalytic activity, and its k n value was about 6.5 times higher than that of PtNi nanosnowflakes/RGO. The improved catalytic activity of PtNi/SiO2 nanofibers was attributed to the following reasons: (1) the high surface area and porous structure of SiO2 nanofibers benefited the adsorption of p-nitrophenol molecules, which helped reactant molecules to access the surface of catalyst; (2) the small size and uniform dispersion on the support surface of PtNi alloy nanocatalyst also contributed to the increasing of catalytic activity.

Table 1 Comparison of rate constant of PtNi/SiO2 nanofibers with those of PtNi and supported PtNi nanocatalysts for p-nitrophenol reduction
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Recyclability and stability of PtNi/SiO2 nanofibers

Magnetic property of PtNi/SiO2 nanofibers was investigated at 25 °C using a vibrating sample magnetometer with an applied field −18,000 Oe < H < 18,000 Oe. The magnetic hysteresis loop of PtNi/SiO2 nanofibers is shown in Fig. 8a. The PtNi/SiO2 nanofibers exhibited typical ferromagnetic curve and its saturation magnetization (Ms) value was calculated to be 11.48 emu g−1. Figure 8b shows the magnetic separation image of PtNi/SiO2 nanofibers. When an external magnetic field was applied, the catalyst nanoparticles were attracted to the wall of vial and the suspension became clear and transparent, which indicated that PtNi/SiO2 nanofibers could be easily recollected from reaction systems by an external magnet.

Fig. 8
figure 8

Magnetic hysteresis loop of PtNi/SiO2 nanofibers measured at 25 °C (a) and magnetic separation image of PtNi/SiO2 nanofibers (b)

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The stability of a catalyst is an important characteristic for practical applications. So the stability of PtNi/SiO2 nanofibers was also evaluated for the reduction of p-nitrophenol. Figure 9 shows the conversion rates of p-nitrophenol with PtNi/SiO2 nanofibers as catalyst in eight successive cycles at 6 min. It could be seen that the conversion rate value slightly decreased with the increase of the cycle number, and the conversion rate of p-nitrophenol still could maintain 94.2% at the eighth cycle, which indicated that PtNi/SiO2 nanofibers possessed excellent stability. This may be attributed to the strong nanocatalyst-support interaction strengthened by the rough surface of SiO2 nanofibers, which contributed significantly to anchoring nanoparticles and protected them from leaching in recycling process.

Fig. 9
figure 9

Conversion rates of p-nitrophenol with PtNi/SiO2 nanofibers as catalyst in eight successive cycles

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Conclusion

In summary, PtNi alloy nanoparticles were homogeneously supported on the surface of porous SiO2 nanofibers to form PtNi/SiO2 nanofibers. The as-prepared PtNi/SiO2 nanofibers exhibited highly catalytic activity towards the reduction of p-nitrophenol when only trace amount of PtNi catalyst was used. In addition, the catalyst could be easily recycled from heterogeneous reaction system based on its good magnetic properties; it also had excellent stability and still could maintain high catalytic activity for eight cycles. Considering its high catalytic activity, good recycling and excellent stability, the PtNi/SiO2 nanofibers has promising applications in the area of advanced heterogeneous catalysis.