In situ reduction of well-dispersed nickel nanoparticles on hierarchical nickel silicate hollow nanofibers as a highly efficient transition metal catalyst

Abstract

Despite being a promising substitute for noble metals used in nanocatalysts, the inexpensive and earth-abundant transition-metal catalysts are still impractical, mainly due to their low catalytic activity and durability. Therefore, acquiring a highly active and stable transition metal catalyst is urgently desirable. In this paper, we describe a mild method for the synthesis of small-sized nickel nanoparticles (NiNPs) immobilized on hierarchical double-shell nickel silicate hollow nanofibers (NSHNFs) in a large scale. The NiNPs/NSHNFs catalysts show high catalytic activities and excellent stabilities towards the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The reduction has a pseudo-first-order rate constant of 13.21 × 10⁻³ s⁻¹ and an activity parameter of 13.21 × 10⁻³ s⁻¹ mg⁻¹, which are higher than those of the previously reported Ni-based catalysts. In particular, the NiNPs/NSHNFs catalysts can be easily separated from the solution by gravitational sedimentation owing to their unique structure. Therefore, our NiNPs/NSHNFs nanocomposites hold promise for further industrial applications as cheap and effective catalysts.

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Introduction

In recent years, nanocatalysts,^1,2 especially noble metal-based (e.g. Au, Ag, Pt, Pd, etc.) nanocatalysts,^3–7 have attracted much attention due to their excellent physicochemical characteristics, along with the rapid development of nanotechnology. Notwithstanding the fact that these noble metal nanoparticles possess high catalytic activities, the significant cost, relatively low abundance and time consuming recovery of these nanoparticles hinder their applications in industry.^8,9 The substitute of precious noble-metal catalysts with cost effective transition-metal catalysts maintaining excellent catalytic activities is appealing, however, the development of such strategy is challenging.^10–14 The nano-sized nickel nanoparticles (NiNPs) may provide a valuable way to address this challenge due to their excellent catalytic and magnetic properties.^15–17 However, the magnetic NiNPs are easy to be aggregated, resulting in a remarkable reduction in their activities during the catalytic process,^18–20 and very difficult to be removed and recycled from the reaction media. An efficient way to overcome the above issues is to disperse well-defined NiNPs onto a suitable support.^21,22 As an ideal support, the following characteristics are essential: a high specific surface area; a strong affinity with the catalyst particles to ensure efficient immobilization of the nanoparticles in a well-dispersed way; an excellent stability and flexibility for resisting external stimuli, easy separation for recovery, low cost and abundant supply.^23–27

Such a perfect support material has been less reported due to the design and synthetic bottlenecks to satisfy all these demands mentioned above. The recent success of tuning the physicochemical properties of silicate nanomaterials with new strategies developed by our group has shined a light on this barren research area.^28–30 We have developed a universal route to fabricate the hierarchical metal silicates (Mg, Ni, Cu) hollow nanofibers with double shells assembled by building units (tiny nanosheets^31,32 or nanotubes³³), which exhibited a high surface area, excellent stability and could be easily recovered by gravitational sedimentation. Specifically, the successfully synthesis of hierarchical nickel silicate hollow nanofibers (NSHNFs) assembled by flexible nanosheets inspire us to achieve fabricating NiNPs/NSHNFs, in which nickel silicate nanofibers acting as both a catalyst support and the precursor of NiNPs. Since the NiNPs come from the in situ reduction of nickel ions of the nickel silicate support, it is reasonable to predict that uniform small size NiNPs with good dispersion could be easily formed on the surface of NSHNFs. Very recently, Yang's group prepared NiNPs/silica nanotubes by a novel in situ thermal decomposition and reduction method from nickel silicate nanotubes precursor.^34,35 However, after the high temperature calcination process in H₂ atmosphere, the above nickel silicate nanotubes assembled by ultrathin nanoflakes converted into the silica nanotubes consisting of numerous nanoparticles, losing their hierarchical structure which are beneficial for loading small particles. The survival of hierarchical structure composed of flexible nanoflakes is desirable for supports being able to prevent the aggregation of particles, provide more active sites for NiNPs loading and enhance the stability of the catalysts.^36,37 Thus, it is necessary to develop a mild method to fabricate the nickel nanoparticles supported catalysts while keeping the microstructures of the nickel silicate support unchanged during the in situ reduction process.

In this article, we demonstrate a mild route to construct one dimensional (1D) nanofibrous catalysts with small-sized NiNPs supported on the hierarchical NSHNFs with double shells. As illustrated in Scheme 1, uniform silica hollow nanofibers (SHNFs) are firstly prepared as the precursor by a facile single capillary electrospinning method. Then, the hierarchical NSHNFs with double shells assembled by ultrathin nanosheets are fabricated by a simple hydrothermal treatment by using SHNFs as the sacrificial template. Finally, uniform-sized NiNPs with well-dispersed distributions are formed on the surface of ultrathin nanosheets of NSHNFs through an in situ reduction under hydrothermal conditions. The structures of the NSHNFs support can be retained quite well after the reductive reaction. The as-prepared hierarchical NiNPs/NSHNFs exhibit a high catalytic activity and excellent stability in catalyzing the reduction of 4-NP. Additionally, the synthesized catalyst can be easily separated from the solution by gravitational sedimentation due to their high length-to-diameter ratio.

Scheme 1 Schematic diagram of the formation of NiNPs/NSHNFs.

Experimental

Synthesis of SiO₂ hollow nanofibers (SHNFs)

The SHNFs were prepared through a single capillary electrospinning method. 0.95 g of polyvinylpyrrolidone (PVP, M_n = 1 300 000) power was dissolved in 10 mL of ethanol. Then, 1.6 mL of tetraethyl orthosilicate (TEOS) was slowly dropped into the above PVP solution to obtain the precursor. After that, the precursor was transferred into a plastic syringe for electrospinning under the voltage of 9.5 kV. The products were collected at a distance about 20 cm to the syringe tip. Finally, the above composites of PVP/TEOS were calcined at a rate of 25 °C h⁻¹ and remained for 2 h at 550 °C. Thus, the SHNFs were obtained.

Synthesis of hierarchical nickel silicate hollow nanofibers (NSHNFs)

The NSHNFs were prepared through a simple hydrothermal process. In a typical synthesis, nickel acetate tetrahydrate (0.2 mmol), ammonia chloride (2 mmol), and NH₃·H₂O (0.2 mL, 28%) were added under stirring to 10 mL distilled water, the resulting solution and the as-prepared SHNFs (0.02 g) were transferred into a 15 mL Teflon-lined autoclave. The autoclave was sealed and maintained at 100 °C for 10 h. After the autoclave was cooled down to room temperature, the resulting green precipitates were collected and washed several times with distilled water and absolute ethanol. The final products were dried at 60 °C for 12 h.

Synthesis of NiNPs/NSHNFs

The NiNPs/NSHNFs were prepared through a simple hydrothermal process. Firstly, 0.02 g of dried as-obtained NSHNFs was dispersed in 10 mL of deionized water. Then, 0.2 g of NaBH₄ was added and the solution was thoroughly mixed. The resulting suspension was transferred into a 15 mL Teflon-lined autoclave. The autoclave was sealed and maintained at 180 °C for 5 h. After the autoclave was cooled down to room temperature, the products were collected and washed several times with deionized water and ethanol and dried at 60 °C for 12 h.

Catalytic properties of the NiNPs/NSHNFs

A given amount (1 mg) of the NiNPs/NSHNFs was added to a 0.1 mM aqueous solution of 4-nitrophenol (4-NP) (5 mL). Then, a fresh 40 mM solution of NaBH₄ (5 mL) was rapidly poured into the above mixture under stirring. Subsequently, 3 mL mixture solution was added to a standard quartz curette for analysis by UV-Vis absorption spectra. The above experiments were carried out at room temperature, approximately 20 °C. The colour of the mixture gradually vanished, indicating the reduction of 4-NP. Different amount of NiNPs/NSHNFs were also used as catalysts adding to the reaction solution for the reduction of 4-NP.

Characterization

X-ray power diffraction (XRD) analysis was measured on a Siemens D5005 Diffractometer with Cu Kα radiation (λ = 1.5418 Å). Field-emission scanning electron microscope (FE-SEM) images were obtained with a HITACHI SU8010 microscope. Transmission electron microscopic (TEM) images, high-resolution transmission electron microscopy (HRTEM) images were obtained on a JEM-2100F microscope with an accelerating voltage of 200 kV. Brunauer–Emmett–Teller (BET) surface area was measured on a Micromeritics Tristar 3000 analyzer at 77.4 K. A Thermo ESCALAB 250 X-ray photoelectron spectroscope (XPS) equipped with a standard and monochromatic source (Al Kα hν = 1486.6 eV) was employed for surface analysis. UV-Vis absorption spectra were measured at room temperature with a UV-Vis-NIR (Purkinje General, TU-1900) spectrophotometer.

Results and discussion

Synthesis and characterization of hierarchical NiNPs/NSHNFs nanocomposites

As shown in Fig. 1a, the as-prepared SiO₂ nanofibers have nearly uniform diameters (500–700 nm), the mean length of the fibers is dozens of micrometers. The high-magnification FESEM (Fig. 1b) demonstrates that the surface of the SiO₂ nanofibers is relatively smooth. And the corresponding TEM image in Fig. 1c indicates that the SiO₂ nanofibers have a typical hollow structure with a wall thickness of approximately 100 nm. Moreover, the XRD pattern (Fig. S1†) of the SiO₂ hollow fibers exhibits a broad band located at 2θ = 22°, which can be assigned to the characteristic diffraction peak of amorphous SiO₂.

Fig. 1 Typical FESEM (a and b) and TEM (c) images of SiO₂ hollow nanofibers.

After hydrothermal treatment of the SiO₂ nanofibers in an alkaline solution containing nickel ions at 100 °C for 10 h, the NSHNFs were obtained. The XRD pattern of the obtained fibers shows characteristic broad diffraction peaks which can be indexed to nickel silicate (Ni₃Si₄O₁₀(OH)₂·5H₂O, JCPDS card no. 43-0664) (Fig. 2, black line). The FESEM image shows that (Fig. 3a) the fibrous nanostructure of the sample is perfectly retained after the hydrothermal treatment. From the high-magnification FESEM image (Fig. 3b), we can find that the surface of the fibers becomes rougher compared with the original SiO₂ nanofibers. Fig. 3c clearly shows that the fibers possess double-walled structure and the walls are composed of numerous randomly oriented ultrathin nanosheets forming a hierarchical architecture. In agreement with the above FESEM observations, the low-magnification TEM image (Fig. 3d) exhibits that the outer diameter of the fiber further increase to 600–800 nm, and the gap between outer and inner walls can be clearly observed, approximately 100 nm, which agrees well with the wall thickness of hollow SiO₂ nanofibers. The high-magnification TEM image (Fig. 3e) demonstrates that the outer and inner walls of the fiber are consisted of multitudinous ultrathin nanosheets. Fig. 4 displays the N₂ adsorption–desorption isotherm of the obtained NSHNFs. In virtue of the double-wall structure and the ultrathin sheet-like subunits, the hierarchical nanofibers possess a relatively large Brunauer–Emmett–Teller (BET) surface area of 350 m² g⁻¹. As mentioned in the preceding, the NSHNFs possess the following features: one-dimensional nanofibrous morphology, high length-to-diameter ratio, unique hierarchical hollow structure, ultrathin nanosheets units and the large specific surface area. These features probably make the sample be a high-performance catalyst support.

Fig. 2 XRD patterns of NSHNFs (black line) and NiNPs/NSHNFs (blue line).

Fig. 3 Typical FESEM (a–c) and TEM (d and e) images of NiSiO hollow nanofibers.

Fig. 4 Nitrogen adsorption–desorption isotherm of NiSiO hollow nanofibers.

NiNPs/NSHNFs were fabricated via the in situ reduction process. During the reduction process, Ni ions were reduced to neutral Ni partially, thus the NiNPs/NSHNFs composites were formed. The XRD pattern (Fig. 2, blue line) of the obtained composites differs from that of the NSHNFs, it shows one more peak located at 2θ = 44.52° than that of NSHNFs, which can be assigned to the characteristic diffraction peak of nickel (JCPDS card no. 65-2865). The result indicates the formation of NiNPs/NSHNFs nanocomposites after the reduction process. The FESEM image (Fig. 5a) demonstrates the structure of NSHNFs is maintained quite well during the reductive treatment. Moreover, it is clear that randomly scattered spherical particles with uniform size-distribution of the NiNPs are nicely dispersed on the surface of the NSHNFs (Fig. 5b). The TEM image (Fig. 5c) clearly shows that the NiNPs are well-dispersed on the surface of NSHNFs, and the corresponding size distribution histogram (Fig. 5d) exhibits that the size of Ni particle is 37.9 ± 5.9 nm. The N₂ adsorption–desorption isotherm of the NiNPs/NSHNFs composites is shown in Fig. S2,† the BET surface area of the NiNPs/NSHNFs is 278 m² g⁻¹, which is smaller compared with that of the NSHNFs (350 m² g⁻¹), since some pores of the NSHNFs are occupied by the NiNPs. To obtain more structural information about the NiNPs, the HRTEM was also performed. As shown in Fig. 5e, the interplanar distance of the nanoparticle is 0.203 nm, which can be corresponded to the (111) plane of nickel, further confirming the formation of Ni nanoparticles. The NiNPs/NSHNFs composites were also examined by EDS (Fig. 5f), which exhibits the existence of Ni, Si, O elements (C element is ascribed to carbon conductive tape). Furthermore, the chemical states of nickel species in NSHNFs and NiNPs/NSHNFs were examined by XPS analysis (Fig. 6). The Ni 2p XPS spectrum of NSHNFs (Fig. 6a) has four peaks which are corresponded to the spin orbit components Ni 2p_3/2 and Ni 2p_1/2 with binding energies at 880.8 eV, 863.0 eV and 874.5 eV, 857.4 eV, respectively, indicating the existence of Ni(II) species. Compared with the Ni 2p XPS spectrum of NSHNFs, there are two more peaks for that of NiNPs/NSHNFs (Fig. 6b), which are respectively related to the spin orbit components of Ni 2p_3/2 and Ni 2p_1/2 with the binding energies at 856.5 eV and 876.4 eV, demonstrating the formation of Ni(0) species on the NSHNF surface. Moreover, these two peaks exhibit a positive shift relative to those of the pure NiNPs (854.5 eV for 2p_3/2 and 872.1 eV for 2p_3/2),³⁸ suggesting the strong affinity between NiNPs and NSHNF.

Fig. 5 Typical FESEM (a and b) and TEM (c) images of NiNPs/NSHNFs; (d) the size distribution histogram of NiNPs calculated from the corresponding TEM image; (e) HRTEM image of the NiNP; (f) EDS patterns of NiNPs/NSHNFs.

Fig. 6 XPS spectra in the Ni 2p level of (a) NSHNFs and (b) NiNPs/NSHNFs.

Application of NiNPs/NSHNFs nanocomposites for catalytic reduction of 4-NP

It is well-known that 4-aminophenol (4-AP) is a vital intermediate for the manufacture of analgesic^39,40 and antipyretic drugs,^41,42 while 4-NP is a common organic pollutant.^43,44 Direct reduction of 4-NP to 4-AP in the presence of NaBH₄ is an efficient and environment friendly conversion route. Here, we utilized the reduction of 4-NP to 4-AP by NaBH₄ in an aqueous solution as a model system to quantitatively evaluate the catalytic activity and stability of the as-prepared NiNPs/NSHNFs nanocomposites. As shown in Fig. 7a, the solution of 4-NP exhibited an absorption at 317 nm under neutral conditions. Upon the addition of NaBH₄, the absorption peak shifts to 400 nm owing to the formation of 4-nitrophenolate ions. The corresponding colour of the solution changed from light yellow to bright yellow. When the NiNPs/NSHNFs nanocomposites were added, the absorption of 4-NP at 400 nm decreased gradually with a concomitant increased in the peak of 4-AP at 295 nm as shown in Fig. 7b. When the reduction reaction finished, the peak of 4-NP at 400 nm vanished, and the colour of the solution turned to colourless, which indicated that the catalytic reduction of 4-NP had proceeded successfully. For comparing, the pure NSHNFs have been used as the catalysts at the same conditions and the corresponding UV/Vis absorption spectra during the catalytic reduction of 4-NP are shown in Fig. S3.† No absorption peak at 295 nm (4-AP) is observed even after adding pure NSHNFs as the catalysts for two hours, which indicates that the Ni nanoparticles are the catalytic active sites. The slight decrease of absorption peak at 400 nm may be caused by the physical adsorption between NSHNFs and 4-NP.

Fig. 7 (a) UV/Vis spectra of 4-NP before and after the addition of an aqueous solution of NaBH₄. (b) UV/Vis absorption spectra during the catalytic reduction of 4-NP over NiNPs/NSHNFs nanocomposites. (c) The circles and squares indicate ln(C/C₀) and C/C₀ versus reaction time for the reduction of 4-NP over NiNPs/NSHNFs nanocomposites, respectively. C₀ and C are the absorption peak at 400 nm initially and at time t, respectively. (d) The reusability of NiNPs/NSHNFs catalyst.

Initially, excess NaBH₄ was added to the reaction system, so the concentration of BH₄⁻ can be considered as a constant throughout the whole reduction process. Thus, the reduction rate can be assumed to be independent of NaBH₄. Therefore, pseudo-first-order kinetics could be applied for the evaluation of rate constant of our catalytic reaction.⁴⁵ Fig. 7c displays the linear relationship between ln(C/C₀) and reaction time in the reaction, the corresponding kinetic equation is ln(C/C₀) = −0.7928t + 0.1293, the rate constant k is calculated to be 13.21 × 10⁻³ s⁻¹. The activity parameter κ = k/m, which is defined as the ratio constant k to the mass of the catalyst,^34,35,46,47 is 13.21 × 10⁻³ s⁻¹ mg⁻¹. The reported activity parameters were 1.5 × 10⁻³ and 12 × 10⁻³ s⁻¹ mg⁻¹ for Ni/SiO₂ magnetic hollow microspheres and Ni/SiO₂ nanotubes, respectively, which are lower than that of NiNPs/NSHNFs fabricated in this work.^34,35 Moreover, we also investigated the reusability of NiNPs/NSHNFs. As displayed in Fig. 7d, the conversion remains higher than 80% after five cycles, indicating the high stability of the catalysts. TEM image and XRD pattern (Fig. S4†) of the NiNPs/NSHNFs composites after five catalytic cycles show that NiNPs are remained dispersed quite well on the surface of NSHNFs and the structure of NSHNFs remain unchanged. The good stability is probably due to the following two reasons. Firstly, the unique in situ reduction method leads to the tight assembly of NiNPs on the surface of NSHNFs support. Secondly, the hierarchical structure could inhibit the aggregations of NiNPs efficiently.

Conclusions

In conclusion, we have demonstrated a simple and efficient method for preparing nickel silicate double-walled hollow nanofibers consisted of numerous ultrathin nanosheets. Then, the well-defined NiNPs/NSHNFs composites were obtained by a mild in situ reduction method. The as-prepared nanocomposites exhibited an excellent catalytic activity and stability due to the unique structure of the NSHNFs support and high dispersion of the small NiNPs. The hierarchical nanofibers possess a relatively large specific surface area and their ultrathin sheet-like subunits could facilitate the dispersion of NiNPs efficiently. Due to the facile synthesis and excellent catalytic performance of NiNPs supported hierarchical double-walled hollow nanofibrous silicate materials, these NiNPs based transition-metal catalysts are expected to replace noble-metal catalysts for further industrial applications.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant No. 21473027); Jilin Provincial Key Laboratory of Advanced Energy Materials (Northeast Normal University); The project development plan of science and technology of Jilin Province (No. 20140101110JC); Research Fund for the Doctoral Program of Higher Education of China (No. 20120043110005); Opening Fund of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01529h

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