Amorphous NiB/carbon nanohybrids: synthesis and catalytic enhancement induced by electron transfer

Abstract

Catalytic conversion of hazardous 4-nitrophenol (4-NP) to 4-aminophenol over cost-effective catalysts is environmentally and commercially desired. Herein, we have synthesized amorphous NiB/C nanohybrids through a solution-phase reduction using pre-oxidized carbon nanoparticles as support. The amorphous catalyst shows promising activity towards the reduction of 4-NP with a high k_n (3.640 s⁻¹ mg⁻¹) and low activation energy (35.4 kJ⁻¹ mol⁻¹). The catalytic activity highly depends on the carbon support and the crystalline degree of NiB alloy. An electron-transfer-induced catalytic mechanism is proposed to explain the catalytic performance of NiB/C. It is found that the electron transfer in metal/support interfaces makes NiB alloy electrophilic, favoring the adsorption of reactants, generation of active hydrogen and hydrogenation of 4-NP. The amorphous NiB/C catalyst can be recycled five times without obvious loss of activity. Our work demonstrates an example to explore the catalytic behavior of amorphous NiB/C catalyst, which may shed light on developing other advanced amorphous catalysts for 4-NP reduction.

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Introduction

Converting hazardous pollutants to useful materials is desired for environmental protection and sustainable development.¹ 4-Nitrophenol (4-NP) is a toxic pollutant and poses public health hazards with carcinogenic character, while 4-aminophenol (4-AP) is a typical intermediate for agrochemicals, pharmaceuticals and dyestuffs.² 4-NP can be reduced to 4-AP by sodium borohydride over proper metal catalysts.^3–8 Noble metals are usually selected to catalyse the reaction due to the high efficiency and selectivity,^9–14 but the scarcity and high cost limit their practical usage, which pushes forward the intensive study on cost-effective alternative catalysts.

Ni is a promising candidate due to its cost-effectiveness and earth-abundance, but shows poor activity towards the reduction of 4-NP.^15,16 The catalytic activity for a given catalyst is governed by geometric structures and electronic states.¹⁷ Prior works placed much attention on geometric parameters of Ni catalysts, and various Ni nanostructures were designed to explore the structure-dependent catalytic performance.^18–20 Up to date, however, little work has been done to boost the 4-NP reduction by altering the intrinsic electronic states of Ni-based catalysts, although electronic states are believed to exert much more importance on catalysis than geometric structures.¹⁷

Forming alloy and coupling with carbon support are two effective strategies to modify electronic states of Ni catalysts.^21–24 Boron (B) is a typical alloying element, because B bears less valence electrons that would affect the electronic states of catalytic metals to generate a significant enhancement on catalysis.²⁵ For example, a Pd–B catalyst was found to boost the decomposition of formic acid, which was ascribed to the alternation of chemical states of Pd 3d electrons induced by the B alloying and carbon support.²⁶ Moreover, Ni alloying with B usually generates amorphous NiB alloy,^27,28 having a unique short-range ordering but long-range disordering structure,²⁹ which may bring many possibilities to boost the 4-NP reduction.

Inspired by these, we have synthesized an amorphous NiB alloy supported by pro-oxidized carbon nanoparticles (NPs) via a solution-phase reduction method. The amorphous NiB/C catalyst shows a promising activity towards the 4-NP reduction as compared to its crystalline counterparts and unsupported NiB alloy. An electron-transfer-induced catalytic mechanism is rationally proposed to elucidate the promising catalysis. The amorphous NiB/C catalyst possesses the merits of high efficiency, cost-effectiveness and good recyclability, which provides a new insight into the design and synthesis of advanced non-noble-metal nanocatalysts.

Experimental section

Synthesis of amorphous NiB/C catalyst

The carbon NPs (Vulcan XC-72R, ∼50 nm in sizes) were obtained from Cabot Corporation (USA). All the chemicals were purchased from Shanghai chemical reagent company and used as received. To generate surface functional groups, the carbon NPs were oxidized in the concentrated nitric acid at a ratio of 50 mL of acid per gram of carbon. The carbon/acid dispersion was heated at 80 °C for 1 hour under stirring. The oxidized carbon NPs were centrifuged and washed with deionized water for several times and then vacuum-dried at room temperature.

Typically, as-oxidized carbon NPs (20 mg) were ultrasonically dispersed in 5 mL of deionized water and subsequently mixed with an aqueous solution of NiCl₂ (0.10 M, 0.5 mL). The resulted suspension was further homogenized under sonication for half an hour. A freshly prepared NaBH₄ solution (0.50 M, 2.0 mL) was then added dropwise to the mixture with vigorous stirring. The mixture was stirred for another 1 hour at room temperature to fully deposit amorphous NiB alloy onto the carbon NPs. The desired catalyst was collected by centrifuging and washed with deionized water and ethanol for several times and then vacuum-dried at room temperature. For convenience, the amorphous NiB/C sample is labelled as NiB/C.

Crystallization of amorphous NiB/C catalyst

NiB/C was loaded into a quartz tube furnace and subjected to crystallization under a flow of 1% H₂/N₂ at 50 mL min⁻¹. The temperature was increased from 30 °C to the desired values with a ramp rate of 5 °C min⁻¹ and then held for 2 hours to crystallize the amorphous NiB/C. As-calcinated samples were then cooled down to room temperatures in the same flow. The corresponding NiB/C samples calcinated at 300 °C, 400 °C, 500 °C and 600 °C are labelled as NiB₃₀₀/C, NiB₄₀₀/C, NiB₅₀₀/C and NiB₆₀₀/C, respectively.

Characterization

The morphology was investigated by a JEOL JEM-2100F high-resolution transmission electron microscopy (HRTEM). EDX spectrum was recorded with an energy dispersive X-ray spectroscopy (EDX) attached to TEM. The powder X-ray diffraction (XRD) patterns were recorded by using a Bruker D8 (German) diffractometer with a Cu Kα radiation source. The contact angles (CAs) were measured on a Krüss contact angle instrument (Easydrop DSA 20) using the Laplace–Young fitting model. Samples are pressed at 10 kPa to obtain a flat surface before CA measurements. The zeta potential was measured on a Marvel Zetasizer Nano ZS90 equipment. The Fourier transform infrared spectra (FTIR) were recorded on a Nicolet Avatar 370 spectrometer. The X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Thermo ESCALAB 250Xi instrument with an Al K_α X-ray source. The C 1s signal (284.6 eV) was used to calibrate the binding energies (BE). The UV-vis absorption spectra were recorded using a double-beam UV-vis spectrophotometer (Analytik Jena, Specord 210 Plus). The bulk composition was determined using a Perking-Elmer Optima 4300DV inductively coupled plasma optical emission spectroscopy (ICP-OES). The accurate concentration of 4-AP product was determined by an Ultimate 3000 HPLC system (Thermo Fisher Scientific, USA) with a C18 packed column and UV detector.

Catalytic reduction of 4-nitrophenol

The catalytic reduction of 4-NP was performed in aqueous solution in a standard quartz cell with 1 cm path length. Typically, an aqueous mixed solution of 4-NP and NaBH₄ was introduced to the quartz cell. The total volume of the reaction solution is 2.5 mL, and the initial concentrations of 4-NP and NaBH₄ are 0.05 mM and 5 mM, respectively. Then, a given amount of NiB/C dispersion was injected and the concentration of Ni is kept at 2 μg mL⁻¹ in the system. The time-dependent UV-vis absorption spectra for the 4-NP reduction were recorded using an UV-vis spectrophotometer in a scanning range of 250 to 500 nm. The variation of 4-NP concentration with reaction time was monitored at a wavelength of 400 nm. To study the recyclability, the catalyst was recovered by high-speed centrifugation, washed with water and then redispersed in water for the next catalytic cycle. This procedure was repeated five times.

Results and discussion

Phase and structures

Fig. 1a shows the XRD patterns of the amorphous NiB/C and carbon support. Two samples show similar XRD patterns. The weak and broad peaks located at 2θ = 26° are ascribed to the typical (002) facets of graphite carbon (JCPDS file no. 41-1487). No diffraction peaks can be indexed to crystalline NiB alloy, indicating the presence of amorphous NiB alloy in the catalyst. The EDX spectrum (Fig. 1b) proves the presence of Ni and C with a mass ratio (m_Ni/m_C) of 0.313, close to the value of 0.327 measured by ICP-OES. B is not detected due to its low content and less sensitivity of B in the EDX analysis. Note that the m_Ni/m_C ratio measured by XPS is as high as 0.455 (shown below), larger than the bulk composition (EDX or ICP-OES results), which implies that the amorphous NiB alloy is coating on the carbon.

Fig. 1 (a) XRD patterns of the amorphous NiB/C and carbon support. (b) EDX spectrum of the amorphous NiB/C.

As-treated carbon NPs show a quasi-spherical shape with diameters of ∼50 nm and smooth surfaces (Fig. 2a). The HRTEM image (Fig. S1, ESI†) shows the carbon NPs have slightly-distorted lattice fringes with an interplanar distance of 0.356 nm corresponding to the (002) facets of graphite carbon. After the reaction, amorphous NiB alloy deposits on the carbon and forms amorphous NiB/C nanohybrids (Fig. 2b). To prove the presence of amorphous NiB alloy, we carefully observed the lattice fringes of NiB/C using HRTEM. Fig. 2c shows the graphite (002) lattice fringes with a distance of 0.356 nm, indicating the presence of graphite carbon in the area labelled by yellow rectangle in panel (b). In contrast, the area labelled by white rectangle that contacts intimately with carbon does not show any lattice fringes belonging to graphite or crystalline NiB alloy (Fig. 2d). The results prove the presence of amorphous NiB alloy coating on the carbon NPs.

Fig. 2 TEM images of (a) the carbon NPs and (b) NiB/C nanohybrids. (c) HRTEM image of the area indicated by the yellow rectangle in panel (b), showing clear (002) crystalline planes belonging to the graphite carbon (XC-72R). (d) HRTEM image of the area indicated by the white rectangle in panel (b). (e) HAADF-STEM image of an individual NiB/C nanohybrid. (f) SAED pattern of the amorphous NiB/C catalyst.

The high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image (Fig. 2e) of an individual NiB/C nanohybrid shows that the carbon NP is evenly covered by numerous bright spots. Generally, a HAADF-STEM image represents the Z-contrast of materials. Metals can easily be distinguished from the lighter carbon support using HAADF-STEM imaging, due to the strong Z-dependence of HAADF signals.³⁰ The bright spots are reasonably originated from NiB alloy, proving the uniform distribution of amorphous NiB alloy on the carbon. Fig. 2f shows the SAED pattern of NiB/C, which only contains a halo diffraction belonging to graphite (002) facets, instead of clear diffraction rings or distinct spots originated from crystalline NiB alloy.³¹ The SAED result further proves the amorphous nature of NiB alloy on the carbon, in good agreement with the HRTEM and XRD analyses.

Chemical states of amorphous NiB/C

The survey XPS spectrum (Fig. 3a) shows that NiB/C is composed of Ni, B, C and O elements. The O element is originated from oxygen-containing groups on the carbon. The m_Ni/m_C ratio (0.455) is higher than that measured by ICP-OES (0.327), indicating the occurrence of NiB alloy coating on the carbon. The C 1s XPS spectrum (Fig. 3b) shows the presence of several functional groups (C=O, C–O, COOH) on NiB/C. Fig. 3c shows the Ni 2p XPS spectra of NiB/C and unsupported NiB alloy. The peaks with binding energies (BEs) at 853.6 eV and 852.7 eV in NiB/C correspond to zerovalent Ni 2p_3/2 and Ni 2p_1/2, respectively. In contrast, Ni in unsupported NiB alloy shows two chemical states, zerovalent state and Ni²⁺ oxidation state.

Fig. 3 (a) The survey spectra of NiB/C and unsupported NiB alloy, and the high-resolution XPS spectra of (b) C 1s, (c) Ni 2p and (d) B 1s.

The B 1s XPS spectra (Fig. 3d) show that both NiB/C and unsupported NiB alloy have zerovalent and oxidation states of B. Generally, NiB alloy should have a zerovalent Ni and B. The presence of oxidation states is caused by the partial oxidization by air due to the high reactivity of zerovalent Ni and B. Note that the BE of Ni 2p_3/2 for NiB/C exhibits an obvious positive shift by ∼0.7 eV as compared to that of unsupported NiB alloy. The positive shift in BE suggests that some electrons have been transferred from Ni to the carbon support due to the SMSI effect.^32,33

We have also characterize the oxygen-containing groups by FTIR to probe the SMSI effect in NiB/C, because these groups could induce a charge transfer between metal and support.³⁴ Fig. 4 shows the FTIR spectra of as-oxidized carbon support and NiB/C. Both samples show characteristic absorption bands associated with oxygen-containing groups.³⁵ The bands at 3438, 1380 and 1036 cm⁻¹ can be assigned to carboxylic O–H stretching vibration, O–H bending vibration of phenolic and carboxylic OH, and phenolic C–O stretching vibration.³⁶ It should be noted that the carboxylic C=O stretching vibration (1626 cm⁻¹) for NiB/C positively shifts by ∼26 cm⁻¹ as compared with that (1600 cm⁻¹) for carbon support. This implies the presence of SMSI effect in NiB/C and the electron transfer from NiB alloy to carbon support, which is consistent with the XPS analyses. The detailed discussion about the band shift is provided in ESI.†

Fig. 4 FTIR spectra of as-oxidized carbon and the NiB/C catalyst.

Formation of amorphous NiB/C

We found that the pre-oxidation of carbon NPs exerts much importance on the formation of amorphous NiB/C. As-received and as-oxidized carbon NPs exhibit different surface properties (Fig. 5a). As-received carbon NPs are hydrophobic and the surface obtained by pressing as-received carbon NPs shows a contact angle (CA) of 95°. The zeta potential is measured to be ∼5 mV. In contrast, the carbon NPs oxidized by HNO₃ are superhydrophilic. A 5 μL water droplet rapidly spreads out on the corresponding surface. Such hydrophilic carbon NPs can be easily dispersed in aqueous solution and have a zeta potential of −50 mV.

Fig. 5 High-resolution XPS spectra of (a) C 1s and (b) O 1s for as-oxidized carbon NPs. (c) Zeta potentials of as-received and as-oxidized carbon NPs. Insets are the photographs of a 5 μL water droplet sitting on the pressed carbon surfaces. (d) Schematic illustration of the adsorption, in situ reduction of Ni²⁺ and coating of NiB onto the negatively-charged carbon NPs.

The XPS survey spectra (Fig. S2†) show that both samples contain C and O elements with a C/O atomic ratio of 0.9/0.1 for as-received carbon and 0.7/0.3 for as-oxidized carbon, respectively. The high-resolution C 1s XPS spectrum of as-oxidized carbon NPs (Fig. 5b) can be deconvoluted to several peaks related to carboxylic (–COOH, 288.7 eV), carbonyl (–C=O, 286.9 eV), hydroxyl (–OH, 285.9 eV) groups together with a majority of graphitic carbon (284.6 eV).³² The O 1s XPS spectrum (Fig. 5c) can be deconvoluted to two peaks corresponding to carbonyl (O=C, 534.1 eV) and O–C (533.1 eV).

Fig. 5d schematically illustrates the formation of NiB/C. As-oxidized carbon NPs contain many oxygen-containing functional groups, which behave as negatively-charged sites. The Ni²⁺ ions are easily anchored on carbon through an electrostatic attraction and in situ reduced by NaBH₄. When the supersaturation degree of NiB reaches a critical point, amorphous NiB alloy will heterogeneously nucleate on the carbon NPs.³⁷ As the reduction goes on, NiB species adds onto previously-formed nuclei and finally forms amorphous NiB alloy coating on the carbon (NiB/C).

Catalytic performance of amorphous NiB/C

NiB/C shows a promising catalytic activity towards the reduction of 4-NP by NaBH₄. The yellow aqueous solution of 4-NP has a strong adsorption peak at 400 nm due to the formation of 4-nitrophenolate anions in the presence of BH₄⁻ ions. This reaction is kinetically restricted without catalysts in spite of its thermodynamical feasibility.³⁸ When NiB/C is added (2 μg mL⁻¹ of Ni in solution), the solution fades gradually and becomes colourless within 180 s. Fig. 6a shows the time-dependent UV-vis absorption spectra for the reduction of 4-NP over NiB/C. The absorption of 4-NP anions at 400 nm decreases along with a concomitant increase of 4-AP product at 303 nm. Note that these spectra show two isosbestic points at 280 and 314 nm, suggesting that the catalytic reduction of 4-NP over NiB/C has no byproduct.³⁹ We also used HPLC to determine the accurate concentration of 4-AP so as to calculate the yield of 4-AP and the conversion rate. The equilibrium yield of 4-AP is estimated to be 98.5% and the conversion rate of 4-NP is as high as 99.7% for the reduction reaction. This is reasonable because the reduction of 4-NP by NaBH₄ over metal catalysts is a model reaction and can exclusively generate 4-AP product.^6,40

Fig. 6 (a) Time-dependent UV-vis absorption spectra for the reduction of 4-NP over NiB/C at 25 °C. (b) Plots of ln(A_t/A₀) versus reaction time for the reduction of 4-NP over NiB/C, NiB and carbon support.

Fig. 6b shows the kinetic plots of the reaction over NiB/C, NiB and carbon support. The reactions follow a pseudo-first-order kinetics with respect to 4-NP due to the excessive NaBH₄. The rate constant (k) is determined by the slope of a linear ln(A_t/A₀) verses reaction time (t) plot, where A₀ is the initial absorbance of 4-NP and A_t is the absorbance at a given t. The carbon support shows no catalytic activity. NiB/C shows a k of 0.0182 s⁻¹, which is ∼4 times higher than that of the unsupported NiB alloy, implying the occurrence of the so-called strong metal/support interaction (SMSI) effect in NiB/C.⁴¹

Crystallization-dependent catalysis

The crystalline degree of NiB/C exerts great influence on the catalytic activity. Fig. 7a shows the XRD patterns of NiB/C samples calcinated at different temperatures. The intensity of graphite (002) peak maintains unchanged during the calcining process. Thus, the crystalline degree of NiB alloy is defined as the intensity ratio of NiB alloy (111) and graphite (002) diffraction peaks. NiB/C calcinated at high temperatures shows intensive diffraction peaks belonging to crystalline NiB alloy, and the crystalline degree of NiB/C increases with the calcination temperature. Fig. 7b shows the enlarged (111) peaks of as-obtained NiB alloys, which shift to a higher 2θ angle relative to the standard pattern of pure Ni (JCPDS file no. 04-0850). This implies that B atoms are incorporated into Ni lattices and reduces the (111) interplanar spacing as B has a smaller atomic radius than Ni.

Fig. 7 (a) XRD patterns of the crystalline NiB/C catalysts obtained by calcining the amorphous NiB/C at different temperatures. (b) The enlarged (111) peaks of the NiB/C catalysts, showing a positive 2θ shift relative to the standard pattern of Ni.

Fig. S3† shows plots of ln(A_t/A₀) versus reaction time for the 4-NP reduction over the crystalline NiB/C catalysts. The kinetic constants together with crystalline degrees are summarized in Table 1. Clearly, the catalytic activity of NiB/C catalysts highly depends on the crystalline degree. The amorphous NiB/C has a better catalytic performance than the crystalline NiB/C counterparts, which highlights the important role of the amorphous nature of NiB/C.

Table 1 Crystalline degrees, catalytic activities and Ni 2p binding energies of NiB/C and NiB alloy

Samples	Crystalline degree^a	k (s^−1)	kn^b (s^−1 mg^−1)	BE of Ni 2p3/2 (eV)
a Crystalline degree is defined as the intensity ratio of NiB (111) facet and graphite (002) facet.b kn is the normalized reaction rate constant, kn = k/mmetal.
NiB/C	0	0.0182	3.640	853.6
NiB300/C	0.62	0.00822	1.644	853.4
NiB400/C	0.86	0.00752	1.504	853.3
NiB500/C	2.65	0.00424	0.848	853.1
NiB600/C	4.61	0.00122	0.244	853.0
NiB	—	0.00431	0.862	852.7

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Such a crystallization-dependent activity can be ascribed to the modification of electronic states of NiB/C catalysts. Fig. 8 shows the Ni 2p XPS spectra for NiB/C catalysts with different crystalline degrees. Each spectrum consists of two asymmetric peaks corresponding to Ni 2p_3/2 and Ni 2p_1/2 of zerovalent Ni. Note that the BE of Ni 2p_3/2 for the crystalline NiB/C exhibits a gradual shift towards a high BE as the calcination temperature decreases from 600 °C to 300 °C. This means the crystalline degree of NiB/C alters the electronic states of Ni. As discussed above, the positive shift of BE suggests the electron transfer from Ni species to the carbon support, which makes Ni more electrophilic. Such electron transfer intrinsically modifies the catalytic activity of NiB/C. The detailed mechanism is discussed below.

Fig. 8 High-resolution Ni 2p XPS spectra for the crystalline NiB/C catalysts obtained by calcining amorphous NiB/C at various temperatures.

Electron-transfer-induced catalytic mechanism

Fig. 9 schematically illustrates the electron-transfer-induced catalytic enhancement for the 4-NP reduction over NiB/C. It is believed that the 4-NP reduction by BH₄⁻ is governed by the efficient adsorption of reactants and rapid hydrogen transfer from BH₄⁻ to 4-NP over metal catalysts.⁴² As discussed above, some electrons of amorphous NiB alloy migrate to carbon support due to the SMSI effect. The electron transfer makes the amorphous NiB electrophilic. 4-NP molecules exist in the form of 4-nitrophenolate ions because of the weak basicity of NaBH₄ solution. The reactants, 4-nitrophenolate and BH₄⁻ ions, are anions and can be adsorbed by the electrophilic NiB. Such a strong adsorption facilitates the subsequent hydrogenation of 4-NP over NiB/C, because the kinetics of this reaction is believed to follow a Langmuir–Hinshelwood mechanism, where both reactants need to be adsorbed on the catalyst prior to reaction.^43,44

Fig. 9 Schematic illustration of the catalytic mechanism for the 4-NP reduction over NiB/C.

Although the detailed mechanism for the reduction of 4-NP by BH₄⁻ ions is an open question, the active H arising from the BH₄⁻ ions is believed as the exact species for the hydrogenation of 4-NP.^45,46 Such active H atoms are generated through the cleavage of B–H bond in the BH₄⁻.⁴⁷ The BH₄⁻ ions can donate electrons to metallic Ni due to their strong electron-injection capability⁴⁸ and generate active H species on the NiB/C. Nevertheless, the step of electron donation is kinetically restricted and needs to overcome the energy barrier for the cleavage of B–H bond.⁴⁹ In this case, Ni is electron-deficient and electrophilic in NiB/C. The electrophilic Ni can effectively induce the electron donation from BH₄⁻ ions to NiB alloy in NiB/C, which could substantially reduce the kinetic barrier of the H-generation over NiB/C.

Once the active H atoms are generated, they chemisorb on NiB/C through a strong Ni–H chemical interaction.^50,51 The H atoms are thermodynamically unstable and readily react with 4-NP to yield 4-AP product through a conventional hydrogenation route. The released H₂ bubbles might evidence the presence of the active H species, since unreacted H atoms could combine with each other and form H₂ during the reaction. In a word, the rapidly-generated active H atoms and abundant adsorption of reactants induced by the electron transfer from amorphous NiB to carbon support account for the promising activity of NiB/C.

Fig. 10a shows the kinetic plots of the 4-NP reduction over NiB/C at various temperatures. The linear relationship between ln k and 1/T exhibits a conventional Arrhenius-type dependence on reaction temperatures. The activation energy (E_a) for the reaction is estimated to be 35.4 kJ mol⁻¹. Such a low E_a means the resulting NiB/C can reduce the energy barrier for the 4-NP reduction. To compare the catalytic performance of the current catalyst and those reported previously, it is not appropriate to compare apparent rate constant k, because the apparent k is proportional to the mass of active metals in catalysts (m_metal). Hence, a normalized activity parameter k_n, defined by k_n = k/m_metal, is used to compare the catalytic performance. Table S1† summarizes the catalytic activities of typical metals (or -based) catalysts previously reported for the 4-NP reduction. It is clear that NiB/C (3.640 s⁻¹ mg⁻¹) outperforms most of the catalysts and even shows a competitive performance as compared to some noble metal catalysts.

Fig. 10 (a) Time plots for the reduction of 4-NP over NiB/C at different temperatures. The insert shows the linear fitting of ln k versus 1/T. (b) Catalytic activities of NiB/C with five times of cycling uses.

The recyclability of NiB/C for the 4-NP reduction was also investigated with five consecutive cycles. Fig. 10b shows NiB/C can be recycled five times without significant loss of the catalytic performance. The rate constant of the catalyst in the fifth run is 0.0166 s⁻¹, which shows a little decrease as compared to that in the first run (0.0182 s⁻¹). We collected the NiB/C catalyst after five catalytic cycles and analysed its phase and composition. Fig. S4† shows the XRD pattern of the NiB/C collected by centrifugation, which shows no obvious change in phase. ICP-OES result shows that the recovered NiB/C is composed of 22.4 wt% of Ni, a slight decrease as compared to the Ni composition (23.5 wt%) in the fresh catalyst. Such a slight decrease in kinetic rate with recycling is probably caused by the slight leaching of Ni²⁺ or detaching of active NiB alloy from the carbon support after every usage. The finding shows that the amorphous NiB/C offers both good catalytic activity and acceptable recyclability for reduction of 4-NP to 4-AP under the current catalytic conditions.

Conclusions

In summary, the amorphous NiB/C nanocatalyst has been synthesized through a conventional chemical reduction process using pre-oxidized carbon NPs as support. As-oxidized carbon NPs are negatively charged, which facilitates the adsorption of Ni²⁺ and in situ deposition of amorphous NiB alloy. The resulting amorphous NiB/C shows a promising activity for catalytic reduction of 4-NP to 4-AP. The catalytic performance highly depends on the carbon support and the crystalline degree of NiB alloy. Electron transfer occurs in the interface between amorphous NiB and carbon. Such an interfacial electron transfer makes NiB electrophilic, favouring the adsorption of reactants, the generation of active H species by cleaving B–H bond and hydrogenation of 4-NP. The resultant NiB/C shows good recyclability and can be recycled five times without obvious loss of the activity. The cost-effective and high-efficiency NiB/C nanocatalyst has the potential application for the industrial reduction of 4-NP.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51201116 and No. 51310105015).

Notes and references

1. L. M. Gilbertson, J. B. Zimmerman, D. L. Plata, J. E. Hutchison and P. T. Anastas, Chem. Soc. Rev., 2015, 44, 5758–5777.
2. T. Aditya, A. Pal and T. Pal, Chem. Commun., 2015, 51, 9410–9431.
3. Y. W. Yang, Y. Y. Mao, B. Wang, X. W. Meng, J. Han, C. Wang and H. W. Yang, RSC Adv., 2016, 6, 32430–32433.
4. A. B. Vysakh, C. L. Babu and C. P. Vinod, J. Phys. Chem. C, 2015, 119, 8138–8146.
5. D. Saikia, Y. Y. Huang, C. E. Wu and H. M. Kao, RSC Adv., 2016, 6, 35167–35176.
6. A. M. Kalekar, K. K. K. Sharma, M. N. Luwang and G. K. Sharma, RSC Adv., 2016, 6, 11911–11920.
7. Z. Ji, X. Shen, G. Zhu, H. Zhou and A. Yuan, J. Mater. Chem., 2012, 22, 3471–3477.
8. A. Roy, B. Debnath, R. Sahoo, K. R. S. Chandrakumar, C. Ray, J. Jana and T. Pal, J. Phys. Chem. C, 2016, 120, 5457–5467.
9. M. M. Zhang, X. Lu, H. Y. Wang, X. L. Liu, Y. J. Qin, P. Zhang and Z. X. Guo, RSC Adv., 2016, 6, 35945–35951.
10. W. Dong, L. Zhang, B. Li, L. Wang, G. Wang, X. Li, B. Chen and C. Li, Appl. Catal., B, 2016, 180, 13–19.
11. J.-J. Lv, A.-J. Wang, X. Ma, R.-Y. Xiang, J.-R. Chen and J.-J. Feng, J. Mater. Chem. A, 2015, 3, 290–296.
12. F. Yang, C. Wang, L. Wang, C. Liu, A. Feng, X. Liu, C. Chi, X. Jia, L. Zhang and Y. Li, RSC Adv., 2015, 5, 37710–37715.
13. W. C. Ye, J. Yu, Y. X. Zhou, D. Q. Gao, D. A. Wang, C. M. Wang and D. S. Xue, Appl. Catal., B, 2016, 181, 371–378.
14. Z. Ji, X. Shen, J. Yang, G. Zhu and K. Chen, Appl. Catal., B, 2014, 144, 454–461.
15. Y.-g. Wu, M. Wen, Q.-S. Wu and H. Fang, J. Phys. Chem. C, 2014, 118, 6307–6313.
16. S. Vivek, P. Arunkumar and K. S. Babu, RSC Adv., 2016, 6, 45947–45956.
17. J. Greeley, J. K. Norskov and M. Mavrikakis, Annu. Rev. Phys. Chem., 2002, 53, 319–348.
18. J. W. Xia, G. Y. He, L. L. Zhang, X. Q. Sun and X. Wang, Appl. Catal., B, 2016, 180, 408–415.
19. Y. Li, Q. Y. Liu and W. J. Shen, Dalton Trans., 2011, 40, 5811–5826.
20. A. Wang, H. Yin, H. Lu, J. Xue, M. Ren and T. Jiang, Langmuir, 2009, 25, 12736–12741.
21. X. Fan, G. Zhang and F. Zhang, Chem. Soc. Rev., 2015, 44, 3023–3035.
22. J. Carrasco, D. Lopez-Duran, Z. Liu, T. Duchon, J. Evans, S. D. Senanayake, E. J. Crumlin, V. Matolin, J. A. Rodriguez and M. Veronica Ganduglia-Pirovano, Angew. Chem., Int. Ed., 2015, 54, 3917–3921.
23. A. Mondal and N. R. Jana, ACS Catal., 2014, 4, 593–599.
24. E. Gracia-Espino, X. Jia and T. Wagberg, J. Phys. Chem. C, 2014, 118, 2804–2811.
25. X. Z. Li, K. L. Wu, Y. Ye and X. W. Wei, Nanoscale, 2013, 5, 3648–3653.
26. K. Jiang, K. Xu, S. Z. Zou and W. B. Cai, J. Am. Chem. Soc., 2014, 136, 4861–4864.
27. Y. Zhu, F. P. Liu, W. P. Ding, X. F. Guo and Y. Chen, Angew. Chem., Int. Ed., 2006, 45, 7211–7214.
28. M. Lewandowski, Appl. Catal., B, 2015, 168–169, 322–332.
29. R. Caputo, F. Guzzetta and A. Angerhofer, Inorg. Chem., 2010, 49, 8756–8762.
30. C. Kubel, A. Voigt, R. Schoenmakers, M. Otten, D. Su, T. C. Lee, A. Carlsson and J. Bradley, Microsc. Microanal., 2005, 11, 378–400.
31. L. Wang, W. Li, M. Zhang and K. Tao, Appl. Catal., A, 2004, 259, 185–190.
32. S.-j. Li, Y. Ping, J.-M. Yan, H.-L. Wang, M. Wu and Q. Jiang, J. Mater. Chem. A, 2015, 3, 14535–14538.
33. K. V. Kovtunov, D. A. Barskiy, O. G. Salnikov, D. B. Burueva, A. K. Khudorozhkov, A. V. Bukhtiyarov, I. P. Prosvirin, E. Y. Gerasimov, V. I. Bukhtiyarov and I. V. Koptyug, ChemCatChem, 2015, 7, 2581–2584.
34. P. Serp, M. Corrias and P. Kalck, Appl. Catal., A, 2003, 253, 337–358.
35. N. Lakshmi, N. Rajalakshmi and K. S. Dhathathreyan, J. Phys. D: Appl. Phys., 2006, 39, 2785–2790.
36. R. L. Jia, C. Y. Wang and S. M. Wang, React. Kinet. Catal. Lett., 2005, 86, 135–139.
37. L. J. Liu, J. G. Guan, W. D. Shi, Z. G. Sun and J. S. Zhao, J. Phys. Chem. C, 2010, 114, 13565–13570.
38. S. Gu, S. Wunder, Y. Lu, M. Ballauff, R. Fenger, K. Rademann, B. Jaquet and A. Zaccone, J. Phys. Chem. C, 2014, 118, 18618–18625.
39. P. Herves, M. Perez-Lorenzo, L. M. Liz-Marzan, J. Dzubiella, Y. Lu and M. Ballauff, Chem. Soc. Rev., 2012, 41, 5577–5587.
40. S. Bae, S. Gim, H. Kim and K. Hanna, Appl. Catal., B, 2016, 182, 541–549.
41. H. Yen, Y. Seo, S. Kaliaguine and F. Kleitz, ACS Catal., 2015, 5, 5505–5511.
42. S. K. Ghosh, M. Mandal, S. Kundu, S. Nath and T. Pal, Appl. Catal., A, 2004, 268, 61–66.
43. F.-h. Lin and R.-a. Doong, Appl. Catal., A, 2014, 486, 32–41.
44. N. Bingwa and R. Meijboom, J. Phys. Chem. C, 2014, 118, 19849–19858.
45. P. Zhao, X. Feng, D. Huang, G. Yang and D. Astruc, Coord. Chem. Rev., 2015, 287, 114–136.
46. Q. Zhang, X. F. Fan, H. Wang, S. O. Chen and X. Quan, RSC Adv., 2016, 6, 41114–41121.
47. S. Fountoulaki, V. Daikopoulou, P. L. Gkizis, I. Tamiolakis, G. S. Armatas and I. N. Lykakis, ACS Catal., 2014, 4, 3504–3511.
48. C. Deraedt, L. Salmon, S. Gatard, R. Ciganda, R. Hernandez, J. Ruiz and D. Astruc, Chem. Commun., 2014, 50, 14194–14196.
49. Z. Zhang, F. Xiao, J. Xi, T. Sun, S. Xiao, H. Wang, S. Wang and Y. Liu, Sci. Rep., 2014, 4, 4053.
50. P. Ferrin, S. Kandoi, A. U. Nilekar and M. Mavrikakis, Surf. Sci., 2012, 606, 679–689.
51. K. Christmann, O. Schober, G. Ertl and M. Neumann, J. Chem. Phys., 1974, 60, 4528–4540.

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Footnote

† Electronic supplementary information (ESI) available: HRTEM images and XPS spectra of carbon NPs. Detailed discussion of FTIR results. Catalytic performance of crystalline NiB/C. XRD pattern of recycled NiB/C. Table of normalized rate constants for NiB/C and previously-reported catalysts. See DOI: 10.1039/c6ra19262a

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