Tuning the performance of Pt–Ni alloy/reduced graphene oxide catalysts for 4-nitrophenol reduction

Abstract

An environmentally benign and economic reaction system with an effective catalyst for 4-nitrophenol reduction is highly desirable. Here, we synthesized reduced graphene oxide (RGO) supported Pt–Ni alloy catalysts with different atomic ratios of Pt and Ni, investigated their morphology, size, dispersity, structure and elemental valence, and studied their catalytic activity in order to tune their performance for 4-nitrophenol reduction. It is worth pointing out that the RGO support can efficiently avoid the aggregation of Pt–Ni alloy nanoparticles, and the most dispersed and smallest Pt–Ni particles on RGO can be obtained when the atomic ratio of Pt to Ni is 1 : 9. The Pt–Ni/RGO (1 : 9) nanocatalyst also shows a higher catalytic activity toward the conversion of 4-NP to 4-AP with a catalytic rate constant of 0.3700 min⁻¹ than Pt–Ni/RGO (1 : 3) and Pt–Ni/RGO (1 : 25), and much higher than that of Pt/RGO, Ni/RGO and bare Pt–Ni owing to the well-defined composition, small particle size (10 nm), good dispersion, synergistic effect between Pt and Ni, and electron transfer between RGO and Pt–Ni alloy nanoparticles. In addition, the catalyst possesses good stability and recyclability for the catalytic reduction reaction. The Pt–Ni/RGO nanocatalyst, with well-defined composition, small particle size, uniform dispersity, high catalytic rate, and recyclability, should be an ideal catalyst for specific applications in liquid phase reactions.

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1. Introduction

As is well known, 4-aminophenol (4-AP) is an important intermediate for the fabrication of diverse analgesic and antipyretic drugs,^1,2 anticorrosion lubricants, photographic developers and hair drying agents.^3,4 Conventionally, it is usually produced by multi-step reduction of 4-nitrophenol (4-NP) in the presence of iron–acid, which creates serious environmental problems by generating a large amount of Fe–FeO.⁵ Alternatively, an environmentally benign and economic reaction system that produces 4-AP by direct catalytic hydrogenation of 4-NP in the presence of sodium borohydride (NaBH₄) has been developed in recent years, which avoids using corrosive acid by introducing nano-scaled metallic or bimetallic nanoparticles catalysts.⁶ Bimetallic nanoparticles, especially expensive noble metals alloying with cheap transition metals, such as Pd–Ni,⁷ Pd–Co,⁸ Au–Ni,⁹ have attracted growing attention owing to the enhanced catalytic performance resulting from a synergistic effect compared with those of the respective individual metal nanoparticles.^10,11 Among the various synthesized bimetallic nanomaterials, Pt–Ni alloy nanoparticles have been widely exploited attributed to their potential application in catalysis as well as other useful properties.¹² However, the practical applications of these catalysts are hampered due to their high costs with the problems of aggregation and coalescence which reduces the catalytic activity and surface area. An attractive strategy to deal with these problems could be immobilization of these nanoparticles on an appropriate solid support (such as carbon materials, polymers, metal oxides and so on).^13,14 Interactions between supports and active species benefit the stability and catalytic performance of a catalyst by modifying the physical and chemical status of the active sites, which largely prevents the agglomeration of active species and greatly enhances the catalytic ability.¹⁵

Graphene, consisting of a single atomic layer of sp² carbon atoms,¹⁶ has many unique properties such as high specific surface area,¹⁷ charge carrier mobility,¹⁸ good electrical conductivity,¹⁹ high mechanical strength and special chemical properties,²⁰ making it an ideal support for growth of functional nanoparticles, which shows great potential for diverse applications in catalysis,^21,22 energy storage devices^23,24 and sensing.²⁵ As graphene could not only stabilize metals but also facilitate electron transfer between graphene and metals in catalytic reaction process, the catalytic performance of the nanocatalysts could be highly improved.²⁶ However, graphene sheets are unstable with respect to scrolling or restacking and tend to form agglomerations through van der Waals interactions,²⁷ and their poor dispersity in water solution originated from hydrophobic surface leads to weak interaction between metals and graphene.²⁸ As is well known, graphene sheets can be produced by solution-based chemical reduction of graphene oxide (GO), known as reduced graphene oxide (RGO). Unless graphene, the hydrophilicity of GO allows it to be readily uniformly dispersed in water and GO possesses abundant oxygen-containing functional groups and a large surface area,²⁹ which provides chemically active sites available for nanoparticles anchoring and dispersion.^27,30 Nevertheless, GO exhibits a significant loss of conductivity, which limits electron transfer between metal nanoparticle and support and thus leads to low catalytic activity. It needs to be reduced to restore the sp² hybrid network (RGO) to reintroduce the conductive property.³¹ Although intensive effort has focused on graphene- or graphene oxide-based materials, the goal of direct growth and anchoring bimetals on their surfaces with high dispersity, uniform distribution, improved catalytic activity and good recyclability is still full of challenges.^32,33

Herein, we report a facile and simple in situ co-reduction approach for one-pot synthesis of Pt–Ni bimetals supported on RGO with different atomic ratios using GO as intermediate support, in which metal ions and GO were reduced simultaneously by NaBH₄, for catalytic hydrogenation of 4-NP. The oxygen functional groups on GO are in favor of metal ions anchoring through electrostatic adsorption, and after co-reduction, RGO avoids the aggregation of Pt–Ni nanoparticles and facilitates the reactant adsorption and electron transfer to the well dispersed Pt–Ni nanoparticles catalyst.³² The as-synthesized Pt–Ni/RGO nanocatalyst with atomic ratio of Pt and Ni 1 : 9 displays better catalyst activity and good stability for the reduction of 4-nitrophenol than other Pt–Ni/RGO nanocatalysts, monometallic Pt/RGO, Ni/RGO and bare Pt–Ni, owing to the well-defined composition, small particle size, good dispersion, synergistic effect between Pt and Ni, and electron transfer between RGO and Pt–Ni alloy nanoparticles.

2. Experimentation and characterization

2.1 Materials

Graphite powder and potassium tetrachloroplatinate(II) (K₂PtCl₄) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Nickel chloride hexahydrate (NiCl₂·6H₂O) and sodium borohydride (NaBH₄) were obtained from Kermel Chemical Reagent Co. Ltd. (Tianjin, China). 4-Nitrophenol was purchased from Macklin Biochemical Co. Ltd. All the other chemicals were analytical reagent grade and used without further purification.

2.2 Characterization

The morphology and crystal structure of the samples were observed by transmission electron microscopy (TEM) and high resolution TEM (HRTEM) on a JEM-2100 TEM operating at an acceleration voltage of 200 kV. The crystal structures of the products were characterized by X-ray diffraction (XRD) on a D8ADVANCE in the range of 5–85°. The chemical compositions of catalysts were analyzed by using a ELAN 9000 inductively coupled plasma atomic mass spectrometer (ICP-MS). Fourier transform infrared spectras (FTIR) were recorded on a PWP 110-40 FTIR spectrophotometer. Raman scattering was performed on a LabRAM HR Raman spectrometer using a 532 nm laser source. X-ray photoelectron spectroscopy (XPS) measurement was performed with a Thermo Scientific-ESCALAB 250XI multifunctional imaging electron spectrometer to study the surface properties. The catalytic activity of as-prepared catalysts was measured by UV absorption spectroscopy on a UV-2450 spectrophotometer.

2.3 Synthesis of GO

GO was prepared from purified natural graphite using a modified Hummers' method.³⁴ In this method, 2 g of graphite flakes was added into a 9 : 1 mixture of concentrated H₂SO₄/H₃PO₄ (90 : 10 mL) under stirring. Then, 14 g of KMnO₄ was added gradually into the above solution and the mixture was kept at 50 °C and continued stirring for 12 h. After that, the suspension was cooled to room temperature and poured into 100 mL ice-water mixture with stirring to obtain a homogeneous suspension. Subsequently, 10 mL 30% H₂O₂ was added drop-wise into the mixture to reduce the residual KMnO₄ until the color became yellow. After stirring for another 2 h, the suspension was centrifuged and washed with 10% HCl and deionized water, and finally freeze-dried and stored in a vacuum oven at room temperature.

2.4 Synthesis of Pt–Ni/RGO nanocatalysts

Pt–Ni/RGO nanocatalysts were synthesized by a one-step solution-based chemical reduction method in the absence of capping agents. Briefly, 50 mg of the as-synthesized GO was added to 50 mL of distilled water and sonicated for 2 h to obtain an exfoliated and well dispersed GO suspension. Then, 11.5 mg of K₂PtCl₄ and 59.2 mg of NiCl₂·6H₂O were poured into the GO suspension, and the mixture was kept stirring for 1 h. Subsequently, 25 mL 6 mg mL⁻¹ NaBH₄ solution was added dropwise into the suspension with vigorous magnetic stirring for about 20 min. The synthesized Pt–Ni/RGO catalyst with a mass loading of 40% Pt and Ni was obtained by collecting the resultant black suspension by centrifugation, thoroughly washing with distilled water and ethanol for several times, and freeze-drying. This sample was denoted as Pt–Ni/RGO (1 : 9). To determine the optimized atomic ratio of Pt : Ni, Pt–Ni/RGO catalysts with atomic ratios of Pt to Ni 1 : 3 and 1 : 25 were synthesized by adjusting the amount of the respective metal precursors without changing the total metal mass loading. The corresponding samples were denoted as Pt–Ni/RGO (1 : 3) and Pt–Ni/RGO (1 : 25), respectively. For comparison, monometallic Pt/RGO and Ni/RGO with equal total metal loading amount (40%) and bare Pt–Ni (1 : 9) were also synthesized using the same method.

2.5 Catalytic reduction of 4-NP

The reduction reaction of 4-nitrophenol (4-NP) was tested to quantitatively evaluate the catalytic activity of the as-synthesized nanocomposites. In a typical procedure, 2 mL of 4-NP (4 mM) and 2 mL of freshly prepared ice-cold NaBH₄ solution (1.5 M) were mixed with 100 mL of distilled water by magnetic stirring at 25 °C. Subsequently, 3 mg of the as-prepared catalyst was added into the mixture solution to trigger the reaction. During the reaction process, 2 mL of the reaction solution was withdrawn at a regular time intervals and immediately diluted with 2 mL of distilled water. Then the solution was immediately measured by the UV-vis spectrophotometer in a scanning range of 250–500 nm, and the intensity of the absorption peak at 400 nm was used to examine the concentration change of 4-nitrophenol.

3. Results and discussion

3.1 Synthesis and characterization of Pt–Ni/RGO nanocatalysts

In this study, Pt–Ni/RGO nanocatalysts were synthesized through a simple one-pot co-reduction method, in which Pt²⁺, Ni²⁺ and GO were reduced by NaHB₄ simultaneously (Fig. 1). In a typical process, GO was first sonicated to obtain an exfoliated and well dispersed suspension consisting mostly of single-layered GO sheets, which have abundant negatively charged oxygen functional groups such as hydroxyl, epoxide, carbonyl and carboxyl groups. The electrostatic interactions between the positively charged Pt²⁺ and Ni²⁺ and negatively charged GO sheets offer a strong driving force for adsorption of Pt²⁺ and Ni²⁺ ions onto GO surface, and GO sheets provide reactive sites for the nucleation of metal nanoparticles. After addition of NaHB₄, Pt²⁺ and Ni²⁺ ions were in situ reduced to Pt–Ni alloy on GO surface and GO was reduced to RGO simultaneously.³⁵ As a result, RGO-supported alloy Pt–Ni alloy nanoparticles were successfully synthesized owing to the high surface area and abundant oxygen functional groups of GO nanosheets.

Fig. 1 Schematic illustration of the synthesis of Pt–Ni/RGO nanocatalysts.

To investigate the morphology and microstructure of the as-synthesized nanocatalysts, Pt–Ni/RGO nanocatalysts and bare Pt–Ni were characterized by TEM and HRTEM (Fig. 2). As seen from Fig. 2a–c, the RGO nanosheets are clearly distinguished from the background and are transparent monolayer with a few wrinkles, which is a characteristic feature of RGO sheets. Meanwhile, the Pt–Ni nanoparticles are homogeneously and uniformly anchored on the surface of RGO nanosheets and well separated from each other although some of them tend to stack together. Notably, the sizes of the Pt–Ni nanoparticles on RGO nanosheets strongly depend on atomic ratios of Pt and Ni (inset in Fig. 2a–c). Pt–Ni/RGO (1 : 9) has the smallest average particle size of about 10 nm and narrower size distribution than Pt–Ni/RGO (1 : 3) of ∼21 nm and Pt–Ni/RGO (1 : 25) of ∼16 nm, which means that Pt–Ni particle size decreases at first but increases later with increasing of Ni content. Similar result was also reported by Zhang et al.³⁶ Particle size plays an important role on the catalytic activity of the catalysts. A decrease of metal particle sizes leads to an increase in their surface area, edges on their surface and corner atoms, which can improve the catalytic properties. For comparison, TEM image of the bare Pt–Ni (1 : 9) prepared in the absence of GO is shown in Fig. 2d. The bare Pt–Ni (1 : 9) nanoparticles without graphene are aggregated severely and assembled into chain-like particles with an average particle size of about 120 nm, which is much bigger than the Pt–Ni nanoparticles in Pt–Ni/RGO (1 : 9) nanocatalysts (Fig. 2b). This result indicates that GO nanosheets have significant influence on the size of the Pt–Ni nanoparticles. GO can serve as an effective dispersing agent which can anchor Pt–Ni nanoparticles and thus control their sizes and distribution on graphene during the synthetic process. In addition, the lattice spacing of the particle of the Pt–Ni/RGO (1 : 9) nanocatalyst was measured to be 0.215 nm from a high resolution of TEM image (the inset in the top of Fig. 2b). The value is smaller than that of the (111) plane of Pt (0.227 nm, JCPDS-04-0802) but larger than that of the (111) plane of Ni (0.203 nm, JCPDS-46-0850), indicating that the Pt–Ni nanoparticles are in crystalline alloy state.³⁰ The molar ratio and actual total loading content of Pt and Ni in the Pt–Ni/RGO nanocatalysts were further determined by ICP-MS, as shown in Table 1. The results are in good agreement with the theoretical molar ratio and total amount of Pt and Ni (40%), meaning that most of the Pt and Ni were loaded on the surface of RGO.

Fig. 2 TEM images of (a) Pt–Ni/RGO (1 : 3), (b) Pt–Ni/RGO (1 : 9), (c) Pt–Ni/RGO (1 : 25), (d) bare Pt–Ni (1 : 9). The insets in the bottom of (a–c) show the size distribution of the corresponding nanocatalysts. The inset in the top of (b) shows the HRTEM image of the Pt–Ni/RGO (1 : 9).

Table 1 Catalysts composition determined by ICP-MS

Catalysts	Pt loading (wt%)	Ni loading (wt%)	Pt : Ni (molar ratio)	PtNi loading (wt%)
Pt–Ni/RGO (1 : 3)	20.23	17.36	1 : 2.85	37.59
Pt–Ni/RGO (1 : 9)	10.45	27.48	1 : 8.74	37.93
Pt–Ni/RGO (1 : 25)	4.55	33.54	1 : 24.47	38.09

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Fig. 3 shows the FTIR spectra of the as-synthesized GO and Pt–Ni/RGO (1 : 9) nanocatalyst. In the spectrum of GO, the broad and intense band around 3410 cm⁻¹ and the band at 1623 cm⁻¹ can be attributed to the –OH stretching vibration due to the surface adsorbed water molecules, while the peak at 1733 cm⁻¹ is assigned to the C=O stretching vibration of the carboxylic groups. The peaks at 1224 cm⁻¹ and 1053 cm⁻¹ correspond to the stretching vibration peaks of C–O (epoxy) and C–O (alkoxy), respectively. The results clearly confirm that oxygen-containing groups were successfully bound to the edges of the graphene nanoplates through overoxidation. After the reduction, the intensity of the band at 3410 cm⁻¹ for Pt–Ni/RGO (1 : 9) composite decreased dramatically, suggesting the subsequent removal of surface adsorbed water molecules during the reduction process.³⁷ Additionally, all the other peaks at 1733, 1623, 1224 and 1053 cm⁻¹ corresponding to the oxygen functionalities disappeared and new peaks at 1636, 1560, 1390, 1128 cm⁻¹ occurred, confirming the formation of the Pt–Ni nanoparticles and RGO from GO. The results also demonstrate that the nanoparticles attach to the graphene support via the oxygen functional groups, and the interactions were strong enough to ensure the nanoparticles remained attached even after chemical cleaning and ultrasonication.³⁸

Fig. 3 FTIR spectra of GO and Pt–Ni/RGO (1 : 9) nanocatalyst.

The reduction of GO to RGO in the Pt–Ni/RGO nanocatalyst is further verified by Raman spectroscopy. As shown in Fig. 4, both the GO and Pt–Ni/RGO (1 : 9) display two prominent peaks of the D and G bands, which are associated with disorder features induced by lattice defect and vibrations of the graphite sp² carbon domains respectively. The intensity ratio (I_D/I_G) of the D to the G band correlates with the average size of sp² domains.³⁰ The D and G bands of GO are located at 1348 and 1603 cm⁻¹ with I_D/I_G ratio of 1.02. While in the case of Pt–Ni/RGO (1 : 9), the two bands move to lower wavenumber of 1345 and 1589 cm⁻¹. Besides, the I_D/I_G ratio increases from 1.02 for GO to 1.38 for Pt–Ni/RGO (1 : 9) because the average size of in-plane sp² domains become smaller when GO is effectively reduced to RGO.³⁹ The results indicate that the GO has been well deoxygenated and reduced in Pt–Ni/RGO (1 : 9) nanocatalyst.

Fig. 4 Raman spectra of GO and Pt–Ni/RGO (1 : 9) nanocatalyst.

In order to further ascertain the formation of Pt–Ni/RGO nanocatalysts, the XRD of the as-synthesized GO, Pt/RGO and Pt–Ni/RGO was performed as shown in Fig. 5. The characteristic diffraction peak of GO at around 2θ = 10.2° corresponding to (001) reflection disappeared and no new peak occurred at 2θ = 22.5° corresponds with the (002) reflection of graphene in the Pt/RGO and Pt–Ni/RGO nanocatalysts, indicating that the GO nanosheets were reduced and the restacking and aggregation of RGO sheets has been prevented by the supported metal alloy nanoparticles.³⁵ The Pt/RGO has four characteristic diffraction peaks at 40.09°, 46.59°, 67.87° and 81.51° corresponding to the (111), (200), (220) and (311) planes of face-centered-cubic crystalline Pt nanoparticles (JCPDS 04-0802). In the case of Pt–Ni/RGO nanocatalysts, the diffraction peaks continuously shift to higher angles with elevated loading of Ni content in contrast with Pt/RGO, but no characteristic peaks of Ni or its oxides were detected. This shift reveals that Ni atoms have entered into Pt lattice and the formation of Pt–Ni alloy, which is consistent with the observation result of TEM.⁴⁰ Similar shift can also be observed by Xu et al. on SiO₂ supported Pt–Ni alloy and Pt–Ni/C nanocrystallites.^41,42 The diffraction peak intensity decreases with the increasing of Ni content and even disappears when the atomic ratio of Pt to Ni is 1 : 25, indicating that excess Ni in the Pt–Ni alloys will lead to weaker crystallinity.³⁵

Fig. 5 XRD patterns of GO, Pt/RGO and Pt–Ni/RGO nanocatalysts with Pt–Ni atomic ratios of 1 : 3, 1 : 9 and 1 : 25.

The surface composition and the electronic properties of Pt–Ni on Pt–Ni/RGO nanocatalyst were further analyzed by X-ray photoelectron spectroscopy (XPS). Fig. 6a and b shows the Pt 4f and Ni 2p XPS spectra of the Pt–Ni/RGO (1 : 9) nanocatalyst, respectively. In Fig. 6a, the diffraction peak at the binding energy of 68.4 eV can be assigned to the XPS peak position of Ni 3p according to the report by Ma et al.⁴³ The two intense peaks located at 71.37 eV (Pt 4f_7/2) and 74.79 eV (Pt 4f_5/2) could be assigned to metallic Pt⁰ species, and two more weak peaks detected at 72.91 eV and 76.83 eV are attributed to Pt²⁺ species in PtO and Pt(OH)₂. A comparison of the relative intensities of the peaks due to Pt⁰ and Pt²⁺ indicated that Pt in Pt–Ni/RGO was predominately metallic although there exists Pt²⁺ species in Pt–Ni/RGO nanocatalyst.⁴⁴ In Fig. 6b, the peaks located at 855.80 eV and 873.50 eV are assigned to elemental Ni²⁺ 2p_3/2 and 2p_1/2 of Ni(OH)₂, respectively. The other two peaks at 856.90 eV and 874.80 eV could be assigned to NiOOH. This indicates that Ni⁰ on the surface of the nanocatalyst is low irrespective, which because bulk Ni is easily oxidized in the atmosphere. Similar results can also be observed in previous report.³⁰ Additionally, there exists a characteristic intense shake up satellite signal adjacent to the main peaks at 861.42 eV and 879.99 eV, which is due to the multielectron excitation.⁴⁴ Comparison of Pt–Ni/RGO and Pt/RGO nanocatalysts as shown in Fig. 6c, it can be observed that a lower binding energy shift in Pt⁰ 4f_7/2 peak of the Pt–Ni/RGO nanocatalysts in comparison to that of Pt/RGO, which signifies the electronic modification of Pt in the Pt–Ni alloy. The electrons transfer from Ni to Pt due to the electronegative difference between Ni (1.91) and Pt (2.28) has lowered the density of state on the Fermi level and thus modify the electronic properties of Pt, which is expected to enhance the catalytic activity.³² For instance, it has been reported by Park et al. that a negative binding energy shift is responsible for an electron donation of Ni to Pt, resulting in a change in the electronic properties of the Pt 4f peaks and improving the electrocatalytic activity of Pt–Ni alloys.⁴⁵

Fig. 6 XPS spectra of (a) Pt 4f, (b) Ni 2p of the Pt–Ni/RGO (1 : 9) nanocatalyst and (c) Pt 4f_7/2 peak shift in Pt/RGO, Pt–Ni/RGO (1 : 3), Pt–Ni/RGO (1 : 9) and Pt–Ni/RGO (1 : 25).

3.2 Catalytic properties of Pt–Ni/RGO nanocatalysts

In the present study, the reduction of 4-NP to 4-AP by NaBH₄ was carried out to characterize the catalytic performance of Pt–Ni/RGO nanocatalysts with different Pt–Ni atomic ratios. For comparison, the catalytic performances of Pt/RGO, Ni/RGO and bare Pt–Ni (1 : 9) were also investigated under the identical conditions. It has been reported that the reduction reaction did not proceed without any catalysts or in the presence of pure GO or RGO.³² The reaction progress was monitored by UV-vis spectroscopy at different time intervals since 4-NP exhibits a strong absorption peak at 400 nm in alkaline solution (Fig. 7). As the reduction reaction proceeds, the intensity of the absorption peak at 400 nm decreases significantly owing to the reduction of 4-NP, whereas a new peak appears at 300 nm with continuously increased intensity because of the formation 4-AP. Furthermore, it is noticed that the absorption intensities at 400 nm in the case of the Pt–Ni/RGO nanocatalysts decrease much faster in comparison to Ni/RGO, Pt/RGO and Pt–Ni (1 : 9), which means that Pt–Ni/RGO nanocatalysts have better catalytic activity. The reduction rates of Pt–Ni/RGO nanocatalysts follow an order of Pt–Ni/RGO (1 : 9) > Pt–Ni/RGO (1 : 3) > Pt–Ni/RGO (1 : 25), indicating the performance of the catalysts depends on the atomic ratio of Pt and Ni. 4-NP can be transformed into 4-AP with a conversion of at least 94% within 18 min for Pt–Ni/RGO (1 : 3), 8 min for Pt–Ni/RGO (1 : 9), 30 min for Pt–Ni/RGO (1 : 25), 40 min for Ni/RGO and 50 min for Pt/RGO, respectively, whereas the conversion of 4-NP for bare Pt–Ni (1 : 9) in 50 min is only 23%.

Fig. 7 Time-dependent UV-vis absorption spectra of the reduction of 4-nitrophenol by NaBH₄ catalyzed by Pt–Ni/RGO (1 : 3), Pt–Ni/RGO (1 : 9), Pt–Ni/RGO (1 : 25), Pt/RGO, Ni/RGO and bare Pt–Ni (1 : 9).

To further compare the catalytic properties of different nanocatalysts and investigate the mechanism, a pseudo-first-order kinetics equation is applied to calculate the rate constant (k) since excess NaBH₄ was used in this study, which is given as follows:³²

kt = ln(C_t/C₀) = ln(A_t/A₀)

where C_t and C₀ represent the concentration of 4-NP at time t and t = 0, which will be calculated from the absorbance of 4-NP at 400 nm at time t (A_t) and t = 0 (A₀) respectively.

Fig. 8 shows the plots of ln(A_t/A₀) vs. reaction time of the as-synthesized nanocatalysts. It can be clearly seen that all the plots fit well with the pseudo-first-order kinetics model with R² values larger than 0.99. The turnover frequency (TOF) of the catalyst, which is the number of reactant molecules that 1 g of catalyst can convert into products with one second, is used to compare catalyst efficiencies. The rate constant and TOF of different catalysts are listed in Table 2. It is noted that the rate constants and TOF values for Pt–Ni/RGO nanocatalysts are higher than those of monometallic Pt/RGO and Ni/RGO, indicative of higher catalytic activity of Pt–Ni/RGO nanocatalysts. This may be due to the synergetic chemical coupling effects of Pt–Ni alloy. According to the traditional theory about the catalytic reduction of 4-NP to 4-AP, BH₄⁻ would adsorb onto the catalyst surface and donate electrons to noble metal nanoparticles, and then transfer to the 4-NP adhered to catalysts surfaces via chemical adsorption. As a result, when Ni atoms enter into Pt lattice in Pt–Ni alloy, it can enhance and transfer electrons to Pt, facilitating the transfer process of electrons to the substrate and thus enhancing the catalytic effect.³² In view of this, the rate constant can be improved by increasing the content of Ni (Pt–Ni/RGO (1 : 9) > Pt–Ni/RGO (1 : 3)). However, it doesn't continually increase (Pt–Ni/RGO (1 : 25) < Pt–Ni/RGO (1 : 9)), because fewer metallic Pt atoms are available on the catalysts surface for the reduction of 4-NP.¹² The apparent activation energy and pre-exponential factor of the Pt–Ni/RGO nanocatalysts were also estimated based on the linear correlation between ln k and the 1/T (as shown in Fig. S1†) which follows the Arrhenius equation: ln k = ln A − E_a/RT.⁴⁶ And their values are listed in Table S1.† The apparent activation energy of the Pt–Ni/RGO (1 : 9) was measured to be 29.1 kJ mol⁻¹, which is lower compared to Pt–Ni/RGO (1 : 3) (34.1 kJ mol⁻¹) and Pt–Ni/RGO (1 : 25) (44.3 kJ mol⁻¹). The results are in well agreement with the order of the rate constants of the Pt–Ni/RGO nanocatalysts, which further indicates the optimum molar ratio of Pt–Ni is 1 : 9. In addition, researchers also found that nanoparticles with the smaller size and better dispersion may have greatly enhanced catalytic properties owing to their higher surface area, more edges and corner atoms, since heterogeneous catalysis takes place only on the surface of the nanoparticles.⁴⁷ Based on the above, the Pt–Ni/RGO (1 : 9) nanocatalyst which has the optimum Pt–Ni atomic ratio and the smallest particle size as illustrated in TEM image (Fig. 2) shows the best catalytic performance. Moreover, its rate constant of 0.3700 min⁻¹ is about 70 times higher than that of bare Pt–Ni (1 : 9) of 0.0054 min⁻¹, which indicates that the enlarged surface area and high conductivity of RGO can not only facilitate the reactant adsorption but also make the Pt–Ni nanoparticles well dispersed and transfer electron to them, thereby dramatically improving the catalytic effect.³⁰

Fig. 8 Plot of ln(A_t/A₀) versus time for the reduction of 4-NP catalyzed by (a) Pt–Ni/RGO (1 : 9), (b) Pt–Ni/RGO (1 : 3), (c) Pt–Ni/RGO (1 : 25), (d) Ni/RGO, (e) Pt/RGO, (f) bare Pt–Ni (1 : 9).

Table 2 Rate constants (k) of the reaction catalyzed by different nanocatalysts, TOF and the correlation coefficient (R²) for the ln(C_t/C₀) vs. t plots

Catalysts	Pt–Ni/RGO (1 : 3)	Pt–Ni/RGO (1 : 9)	Pt–Ni/RGO (1 : 25)	Pt/RGO	Ni/RGO	Bare Pt–Ni (1 : 9)
k (min^−1)	0.1890	0.3700	0.0850	0.0575	0.0729	0.0054
R^2	0.9973	0.9907	0.9926	0.9903	0.9905	0.9908
TOF(×10^17) s^−1	49.8	110.9	28.8	17.5	22.2	4.4

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For the consideration of practical application, the recyclability of Pt–Ni/RGO (1 : 9) nanocatalyst was examined for four times. As shown in Fig. 9, the conversion of 4-NP to 4-AP maintains higher than 86.27%, showing that there is no obvious loss of catalytic activity after each recycling. This result also indicates that the as-prepared Pt–Ni/RGO (1 : 9) nanocatalyst has a high stability in the reduction of 4-NP. Therefore, the recyclability of Pt–Ni/RGO (1 : 9) makes it a promising candidate for the catalytic reduction of 4-AP to 4-NP.

Fig. 9 The reusability of the Pt–Ni/RGO (1 : 9) nanocatalyst for the catalytic reduction of 4-NP.

4. Conclusions

In summary, well-dispersed Pt–Ni alloyed nanoparticles with tunable Ni loading and particle size uniformly supported on RGO (Pt–Ni/graphene) were synthesized via a simple in situ co-reduction approach without using any agent. GO can provide chemically active sites available for metal nanoparticles anchoring and dispersion in the synthesis process. The as-synthesized Pt–Ni/RGO (1 : 9) nanocatalyst exhibited remarkable catalytic activity for the reduction of 4-nitrophenol attributed to the well-defined composition, small particle size, good dispersion, synergistic effect between Pt and Ni, and electron transfer between RGO and Pt–Ni nanoparticles. In addition, it can be readily recycled and reused, implying its good stability and reusability. These findings highlight the importance of support, elemental composition, particle size and dispersity on the catalytic enhancement for reduction reaction, and may be regarded as a first step toward the synthesis of more efficient catalyst for specific applications in liquid phase reaction.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21271158 and 21576247) and Postdoctoral Research Sponsorship of Henan Province (Grant No. 2015015).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16045j

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