Enhanced catalytic reduction of p-nitrophenol on ultrathin MoS2

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Enhanced catalytic reduction of p-nitrophenol on ultrathin MoS2 nanosheets decorated noble-metal nanoparticles Xiu-Qing Qiao, Zhen-Wei Zhang, Feng-Yu Tian, Dong-Fang Hou, Zheng-Fang Tian, Dongsheng Li, and Qichun Zhang Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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Crystal Growth & Design

Enhanced catalytic reduction of p-nitrophenol on ultrathin MoS2 nanosheets decorated noble-metal nanoparticles Xiu-Qing Qiao†, Zhen-Wei Zhang†, Feng-Yu Tian†, Dong-Fang Hou†, Zheng-Fang Tian$, Dong-Sheng Li†,‡,* and Qichun Zhang§,*A †

College of Materials and Chemical Engineering, Hubei Provincial Collaborative Innovation Center for New

Energy Microgrid, Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang, 443002, China ‡

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,

Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. $

Hubei Key Laboratory of Processing and Application of Catalytic Materials, Huanggang Normal

University,Huanggang 438000, China §

School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798,

Singapore KEYWORDS: MoS2; catalytic reduction; p-nitrophenol; noble metal-free; ABSTRACT: In this work, ultrathin MoS2 nanosheets (MoS2 NSs) with defect-rich structure were prepared and used as catalysts for the reduction of p-nitropheonol. Also, thanks to the defect sites, noble metal (Au, Ag, Pd, Pt) nanoparticles (NPs) can successfully deposited on the MoS2 NSs via a facile and efficient one-step photochemical reduction process without the assistance of stabilizer. The morphology and crystal structure of as-synthesized materials were characterized by X–ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopic (XPS) and N2 physisorption mearurements. Catalytic results demonstrate that as-prepared MoS2 NSs and noble metal modified MoS2 NSs (NM/MoS2 NSs) effectively catalyze the reduction of p-nitrophenol (p-NP) to p-

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aminophenol (p-AP) with NaBH4. The Pd/MoS2 NSs shows the best catalytic performance, exhibiting an apparent rate constant (κapp) of 0.386 min-1 even with moderate chemical stability. Interestingly, the activity of MoS2 NSs is comparable and even higher than previous studied noble metal-based catalysts. Moreover, the good reusability without significant activity loss suggests that MoS2 NSs can be a strong candidate for sustainable chemical catalysis. The origins of the high catalytic activity are also discussed and a probable reaction mechanism is suggested. The present work opens up new opportunities for the future design and fabrications of versatile MoS2-based composite materials.

INTRODUCTION p-Nitropheonol (p-NP) is one of the most common and stubborn organic pollutant in industrial wastewaters, which is poisonous for human beings and animals. On the contrary, its reductive derivative, p-aminophenol (p-AP), is a valuable intermediate product used in the manufacture of paracetamol and clofibrate drugs, developing agent, antioxygens and petroleum additives.1-3 To this end, the catalytic transformation of p-NP into p-AP has been extensively investigated during the past years,4-6 and most of these studies were focused on the activity of noble-metal nanoparticles (NPs)such as Ag, Au, Pd and Pt. Although the high catalytic potential of noble metal NPs, however, their used may lead to depletion of rare resources, environmental contamination and health issues. In addition, the agglomeration of colloidal noble metal NPs during the catalytic reaction is often inevitable. In order to overcome these problems, the immobilization of NPs onto a high-surface-area inorganic support, such as carbon nanotube,7 MOFs,8 reduced graphene oxide,3 TiO29 and graphene4, has been proved to be an effective method. Among all these supports, two-dimensional (2D) substrates are most forceful contestants owing to the large surface, on which metal NPs can be anchored and endowed the material with catalytic properties.10 Atomically 2D thin-layered structures, such as graphene, carbon nitride (C3N4), hexagonal boron nitride (h-BN), and transition-metal dichalcogenides (TMDCs) are emerging as fascinating materials for a wide range of applications due to their unique physical properties. In particular, TMDCs such as MoS2, CoS211 and Ni2S312,

13

have received significant attention because are semiconductors with


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sizable bandgaps and are naturally abundant. For example, molybdenum disulfide (MoS2), which possess a distinct structure of molybdenum atoms sandwiched between two layers of hexagonally packed sulphur atoms, has drawn a considerable attention because of its intriguing electronic properties and potential applications in electrocatalysis,14-16 photocatalysis,17-19

supercapacitors,20

lithium-ion batteries16, 21 and chemical sensors.22, 23 Recently, elaboration works have been devoted to fabricating MoS2 nanostructures with different electronic characteristics, such as defect-rich MoS2 NSs,24 oxygen-incorporated MoS2 NSs,14 metallic 1T-MoS2 NSs,25 and hybrid structures with other nanomaterials.26,

27

However, the application of MoS2 in chemical catalysis is still limited and the

development is underway. For example, Xu et al. recently reported the antibacterial activity of MoS2 NSs by the colony counting method.28 Chen et al. found that MoS2 NSs possess an intrinsic peroxidase-like activity and can catalytically oxidize 3,3´,5,5´-tetramethylbenzidine (TMB) with H2O2 to produce a color reaction.29 Our group successfully synthesized ultra-thin 2D MoS2 NSs with excellent adsorbent properties for dyes.30, 31 Li Zhou used MoS2 NSs as a versatile support to deposit various inorganic NPs

32

. All these studies unraveled the potential of ultra-thin MoS2 layers as large

surface area in organic substrates for deposition of metal NPs. One the other hand, several only reports indicated that MoS2-based hybrid materials can be used as catalysts for the degradation of tetracycline 33

and, recently, p-NP

34

. In general, decorating 2D materials with nanocrystals to form hybrid

nanostructures can improve the inherent properties as well as bring some elegant properties and functions. So, it is expected that a high-efficient catalyst may be obtained through the synergistic effect of binary composition of noble-metal NPs and MoS2 NSs. Up to now, only few papers have reported the deposition of noble metals onto MoS2 NSs. For example, Wang et al. developed a general and facile method for noble metal nanocrystal modified MoS2 NSs with the assistance of stabilizers and showed that the Pd/MoS2 NSs is an excellent electrocatalyst for methanol oxidation.

35

Zhang et al. reported the solution-phase epitaxial growth of

Pd, Pt, and Ag NPs on the surface of single-layer MoS2 NSs and showed that Pt/MoS2 NSs catalyst exhibits high catalytic activity for the hydrogen evolution reaction.

36

Nevertheless, several challenges

still be there, such as the complicated procedures and special stabilizers are necessary, which impede


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the popularization of MoS2-based hybrid materials. Thus, a simple and effective strategy for the largescale production of noble metal modified MoS2 NSs is highly desired. In this contribution, we present the synthesis of several noble metal (Ag, Au, Pd, Pt) modified MoS2 NSs (NM/MoS2) by a facile photochemical reduction method and investigate their catalytic performance for the reduction of p-NP to p-AP using sodium borohydride (NaBH4) as a reducing agent. It is found that the as-prepared NM/MoS2 NSs exhibit excellent catalytic performance for the reduction of p-NP, with an apparent rate constant of 0.386 min-1 (Pd/MoS2). More importantly, the ultra-thin MoS2 NSs we reported show very good activity and chemical stability for long-term catalytic reactions, indicating the MoS2 NSs as a potential metal-free catalyst. Furthermore, a possible reaction mechanism that contributes to the improvement of catalytic performance for MoS2 and NM/MoS2 NSs is discussed. The study presented here provides an impetus for the application of MoS2-based composite material in redox catalysis.

2. EXPERIMENTAL SECTION

2.1 Chemicals Ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O), thiourea (CH4N2S), sodium borohydride (NaBH4), p-Nitropheonol(C6H5NO3), absolute ethyl alcohol (CH3CH2OH) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Silver nitrate (AgNO3), gold (III) chloride trihydrate

(HAuCl4·3H2O),

chloroplatinic

acid

hexahydrate

(K2PtCl6·6H2O),

potassium

hexachloropalladate (K2PdCl6) were purchased from Sigma-Aldrich. All the reagents are analytical reagent grade and used without further purification.

2.2 Synthesis of ultra-thin MoS2 NSs The ultrathin MoS2 NSs were synthesized following the same method used in our previous work. Typically, 1 mmol of (NH4)6Mo7O24·4H2O and 30 mmol of CH4N2S were added in 35 mL of deionized water to form a clear light-blue solution. After stirred for 15mins, the solution was transferred into autoclave (50 mL) and heated at 180℃ for 24 hours. Then the reactor was cooled


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down to room temperature naturally and black powders were collected by centrifugation, washed and then dried.

2.3 Synthesis of NM/MoS2 NSs (NM = Ag, Au, Pd and Pt) For Ag/MoS2 NS synthesis, 90mg of the as-prepared ultrathin MoS2 NSs were first dispersed into 20 mL of the mixed solution (the volume ratio of ethyl alcohol and water is 2:18) by ultrasonication for 30 mins to give a black suspension. Subsequently, certain amounts of AgNO3 aqueous solution (5mg/L) was added into the above MoS2 suspension at a rate of 30 drops per min under magnetic stirring for 30 mins. Ag content in the Ag/MoS2 NSs hybrid was calculated to be 10 wt%. The mixture was then irradiated directly with a 500-W mercury lamp for 30mins. Then the suspension was rinsed thoroughly with ultrapure water and dried to obtain Ag decorated MoS2 NSs. Other NM/MoS2 NSs were also be synthesized according to the similar procedures, changing AgNO3 to HAuCl4·3H2O, K2PdCl6 and H2PtCl6·6H2O and noble metal loadings are always 10 wt% if not specified.

2.4 Sample Characterizations The crystal structure and phase of the products were characterized by Rigaku Ultima IV powder X-ray diffraction (XRD, Cu Kα, λ=0.15418nm) at 40 kV and 40 mA. The morphologies were monitored by JEOL-JSM-7500F field emission scanning electron microscopy (FESEM, 10 kV) and Tecnai G2 F20 S-TWIN transmission electron microscopic (TEM, 200 kV). ESCALAB 250X-ray photoelectron spectroscopic (XPS, Al Kα) was used to analyze the surface chemical valence state. The Gold APP Vsorb 2800 system was employed to measure the Brunauer–Emmett–Teller (BET) specific surface area by N2 adsorption at 77 K. Barrerr-Joyner-Halenda (BJH) method was adopted to calculate the pore size distribution from the desorption branch of the isotherms. Shimadzu UV-vis 2550 spectrophotometer (UV-vis, Japan) was used for the determination of p-NP concentration. Zeta Sizer Nano ZS-90 (Malvern Instruments Inc., UK) with a HeNe light source (632nm) was used to determine the zeta-potential of the MoS2 NSs. The data was obtained by averaging the values of three measurements.


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2.5 Catalytic reduction of p-NP The reduction of p-NP to p-AP by excess NaBH4 was used as a model reaction to evaluate the catalytic activity of ultra-thin MoS2 NSs and NM/MoS2 NSs. Typically, 1.6mg of NaBH4 was added into 3ml of p-NP aqueous solution (0.1mM) in the quartz cell (1.0 cm path length and 4 mL volume). Immediately, the color of the solution changed from light yellow to yellow-green due to the formation of p-nitrophenolate ions under alkaline condition. Next, 1.5mg of MoS2 NSs or NM/MoS2 NSs were introduced into the above solution, and UV-vis absorption spectra at different time intervals were recorded on a Shimadzu UV2550 spectrophotometer (scanning range: 200-500nm, 20°C). The reduction of p-NP was monitored by the decrease of the absorption at 400nm. To perform the reusability study, after each catalytic reaction, the catalyst was isolated by centrifugation, washed with water and directly used for the next catalytic run.

3. RESULTS AND DISCUSSION

3.1 Synthesis of ultra-thin MoS2 NSs and NM/MoS2 NSs In this work, noble metal (Ag, Au, Pd and Pt) NPs were loaded on defect-rich MoS2 NSs via a photodeposition method to form NM/MoS2 heterostructures. Importantly, the deposition process was performed under UV light irradiation (λ< 380 nm) without the addition of any stabilizer. The synthesis procedure of NM/MoS2 NSs is illustrated in Figure 1. First, ultra-thin MoS2 NSs were synthesized via a one-step hydrothermal process, similar to our recently reported method.

30

The zeta potential of as-

synthesized MoS2 NSs (Figure S1) was always negative for pH values ranging from 1 to 14, indicating that the surface of MoS2 NSs is negatively charged; this surface charge can exert repulsion forces between the MoS2 NSs providing a highly dispersed solution. It was reported that metal NPs can be selectively deposited on the edge sites or defective sites of MoS2 layers.

37

Therefore, the defect-rich

structure of MoS2 NSs gives us the opportunity to modify the surface of MoS2 with various noble metals. When noble metal salts were added in the reaction mixture, the metal ions can be anchored on the surface of MoS2 NSs by electrostatic interactions. Then, noble metal NPs were grown on the MoS2


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NSs surface by a photochemical reduction method. Thanks to the defective structure of ultra-thin MoS2 NSs, no surface stabilizer was necessary for the reduction of noble metals. A similar procedure has also been reported for the growth of various noble metal NPs on graphene layers.4, 38, 39

Figure 1. Schematic illustration of the synthesis of NM/MoS2 NSs: (I) synthesis of MoS2 NSs, (II) adsorption of metal ions onto the MoS2 NSs surface and (III) photochemical reduction for the synthesis of noble metal NPs on the MoS2 NSs.

3.2 Characterizations Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were used to characterize the morphology and crystal structure of the as-synthesized ultra-thin MoS2 NSs and NM/MoS2 NSs. As can be seen from Figure 2a, the MoS2 NSs consisted of crumpled nanosheets with plenty of folded edges, indicating that MoS2 NSs are ultra-thin structures.

30

The discontinued fringes

of the curled edges in HRTEM image (Figure 2b) indicate the existence of plenty of defects.

31

The

lattice fringes of MoS2 with a lattice spacing of 6.3Å correspond to the (002) plane. Besides, the selected-area electron diffraction (SAED) pattern, where six independent and distinguishable diffraction arcs are clearly observed (Figure 2c), implies the formation of a quasiperiodic structure which originates from high density of defect states.

40

Scanning electron microscopy (SEM) image of

the MoS2 NSs shows the lateral size of sample is uniform and falls in the range 100–200nm (Figure


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S2). A typical photograph of MoS2 suspension standing for 24 hours is shown in Figure 2d, indicating that the dispersion is stable during a long time owing to the electrostatic repulsion between negatively charged MoS2 NSs.

Figure 2. Typical (a)TEM and(b) HRTEM images and (c) SAED pattern of the ultra-thin MoS2 NSs.(d) Photograph of MoS2 NSs dispersion in water after standing for 24 hours. Typical (e) TEM and (f) HRTEM images of the Ag/MoS2 NSs.

For the Ag/MoS2 hybrid NSs, after photochemical reduction of AgNO3, uniform Ag NPs with an average size of about 5nm were formed and homogeneous distributed on the surface of the MoS2. TEM analysis on the Ag/MoS2 NSs clearly demonstrates the formation of the Ag-MoS2 composite structure, where in Figure 2e the Ag NPs appear as dark spots and MoS2 as bright areas. In agreement to this, in Figure 2f, the lattice spacing of an individual nanoparticle is 0.236 nm, which can be assigned to the (111) crystal plane of Ag (JCPDS no. 65-2871). The uniform dispersion of Ag NPs without stabilizers may originate from the fact that the defect and edge sites of the MoS2 NSs surface tend to bind metal ions with high affinity.41 To further demonstrate the general applicability of this method, we have grown also other noble metals, such as Au, Pd and Pt, on the surface of MoS2 NSs using a similar procedure. Figure 3 shows the TEM and HRTEM images of Au/MoS2 NSs, Pd/MoS2 NSs and Pt/MoS2 NSs composites. As shown in Figures 3a-3c, all the metal NPs are homogeneous


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distributed on the MoS2 NSs surface. The lattice spacings of 0.234 nm (Figure 3d), 0.225 nm (Figure 3e) and 0.229 nm (Figure 3f) can be assigned, respectively, to the (111) planes of Au (JCPDS no. 652870), Pd (JCPDS no. 65-2867) and Pt (JCPDS no. 65-2868) metals.

Figure 3. Typical (a-c) TEM and (d-f) HRTEM images of the Au/MoS2 (a and d), Pd/MoS2 (b and e) and Pt/MoS2 (c and f) NSs.

The formation of a noble metal-MoS2 composite crystal structure was also confirmed by X-ray diffraction (XRD) (Figure 4). The MoS2 NSs showed three weak but resolved diffraction peaks at 14.09° (001), 33.27° (100) and 59.01° (110), which could be indexed to the standard diffraction pattern of MoS2 (JCPDS no.73-1508). The broad and low intensity of the (001) reflection are related to the thin layered structure of MoS2.

30

For the NM/MoS2 NSs materials, new obvious diffraction peaks

at 38.17°, 38.24°, 40.21° and 39.60° were observed, which could be assigned to the (111) diffraction of cubic Ag (JCPDS no. 87- 0720), Au (JCPDS no. 89-3697), Pd (JCPDS no. 87-0639), and Pt (JCPDS no. 87-0642), respectively. The XRD results further illustrate that noble metals have been successfully decorated on the surface of MoS2 NSs, consistent with TEM observations. Meanwhile, for the NM/MoS2 NSs, the XRD peak intensities of MoS2 are similar to those of the pristine MoS2 NSs sample, demonstrating that the crystal structure of MoS2 does not changed after surface modification.


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Figure 4. XRD patterns of MoS2 and NM/MoS2 NSs.

More detailed information regarding the chemical environment of the MoS2 NSs and NM/MoS2 NSs was ascertained using X-ray photoelectron spectroscopy (XPS). The high-resolution XPS spectra of noble metals NPs in NM/MoS2 NSs samples are shown in Figure 5. Careful fitting of the Ag 3d core level spectrum of Ag/MoS2 NSs gives two peaks at 374.1 eV and 368.0 eV (Figure 5a), with a splitting of 6.1 eV, which can be attributed to the Ag 3d3/2 and Ag 3d5/2 signals of zero-valence metallic Ag.42 For Au/MoS2 NSs (Figure 5b), the peaks located at 87.7 eV and 84.1 eV are assigned to the binding energy of Au 4f5/2 and Au 4f7/2, respectively, of the zero valence Au.43 With respect to Pd/MoS2 NSs, peaks near 340.1 eV and 334.7 eV shifted to the lower binding energy compared with the characteristic peaks for metallic Pd0 (343.1 eV and 335 eV) can be assigned to the Pd 3d3/2 and Pd 3d5/2.44 This shift (by 0.3 eV) may arise from the electron transfer from MoS2 NSs to the Pd NPs. Since the work function of MoS2 (4.1~4.7 eV)40 is lower than that of Pd (5.2 eV)45, an electron transfer from the MoS2 NSs to Pd NPs would occur for Pd-decorated MoS2 NSs. Whereas, for Pt/MoS2 NSs, peaks at 75.9 eV and 72.6 eV with a 3.3eV peak separation could be attributed to the Pt 4f5/2 and Pt 4f7/2 core-level signals which are largely shifted to higher energies compared to Pt0 (74.6eV and 71.3 eV).


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These unexpected higher binding energies may be attributed to both larger final-state effects and nanoparticle charging due to a loss of conductivity as the Kubo gap increases with diminishing particlesize.46 The XPS results clearly demonstrate that noble metal NPs, which had been deposited on the MoS2 NSs surface, can preserve their metallic nature without undergo further oxidation. For MoS2 NSs (Figure S3), the binding energies located at 229.2 eV (3d5/2) and 232.6 eV (3d3/2) can be assigned to Mo4+ in 2H-MoS2, while the peaks at 231.6 eV and 228.6 eV can be ascribed to Mo4+ in 1T-MoS2.35 The relatively weak peak located at 235.9 eV is assigned to Mo3d signal of Mo6+ ions, indicating the partial oxidation of Mo atoms, possibly at the edge or defect sites of the MoS2 lattice.14 Meanwhile, the

Figure 5. High-resolution XPS spectra of (a) Ag/MoS2, (b) Au/MoS2, (c) Pd/MoS2 and (d) Pt/MoS2 NSs.

S2p3/2 and S2p1/2 signals centered at 161.5 eV and 162.8eV, respectively, demonstrate the existence of the MoS2 phase. No obvious shift in the binding energies is observed in the Mo 3d and S 2p region,


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indicating that deposition of noble metal NPs does not significantly alter the chemical state and bonding environment of MoS2.

3.3 Catalytic properties of MoS2 and NM/MoS2 NSs To evaluate the catalytic activity of MoS2 and NM/MoS2 NSs, the reduction of p-NP to p-AP in the presence of excess NaBH4 was chosen as the model reaction. The light-yellow p-NP aqueous solution (0.1mM) shows an intense absorption peak at 317nm, which upon addition of NaBH4, the maximum of this peak shifted to 400nm due to the formation of p-nitorphenolate ions (Figure 6a). During this period the color of the solution changes from light yellow to bright yellow. After MoS2 and NM/MoS2 NSs catalysts were introduced into the p-NP/NaBH4 solution, the intensity of the absorption peak at 400nm gradually decreases as the reduction reaction proceeds (Figure 6b and c). Meanwhile, a new peak appears at 300nm with the intensity increasing with time, indicating the gradual transformation of p-NP to p-AP. This catalytic process can be easily characterized, and its reaction kinetics was monitored by measuring the absorbance at 400nm at different time intervals.3, 47 The overall reduction reaction could be expressed as follows:

Because the initial concentration of NaBH4 greatly exceeded that of p-NP, it was assumed that the concentration of BH4− remained almost constant during the course of the reaction. In other words, the catalytic reduction of p-NP into p-AP can be considered as pseudo-first-order reaction, which can be expressed as: ln(Ct/C0)= -κapp·t Where Ct and C0 are the p-NP concentrations at time t and 0, respectively and κapp is the apparent rate constant (min-1).


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Figure 6. (a) UV-vis spectra of p-NP before and after adding NaBH4 solution. (b, c) Successive UV-vis absorption spectra of the reduction of p-NP by NaBH4 in the presence of ultra-thin MoS2 NSs (b) and Pd/MoS2 NSs (c). (d) Plot of ln(Ct/C0) versus time for MoS2 and Pd/MoS2 NSs systems.

As shown in Figures 6b, 6c and Figure S8, over 80% of p-NP can be reduced within 12 mins, while complete conversion was achieved in 10 mins for Pd/MoS2 NSs. Also, the catalytic reduction performance of other noble metal (Ag, Au and Pt) NP-decorated MoS2 NSs were also evaluated (Figure S9-11) and the corresponding plots of ln(Ct/C0) versus time are shown in Figure 6d. It can be observed that ln(Ct/C0) values show good linear correlation with the reaction time for all catalysts, indicating that the reduction follows a first-order reaction law. The rate constant (κapp) values calculated from the slope of the linear region were found to be 0.386 min-1, 0.290min-1, 0.188 min-1, and 0.196 min-1 for Pd/MoS2, Ag/MoS2, Au/MoS2 and Pt/MoS2 NSs, respectively, while the κapp value for MoS2 NSs is estimated to be 0.155min-1. It is clear that Pd/MoS2 NSs catalyst shows the best catalytic activity among the examined materials, which is 2.5 times higher than that of MoS2 NSs.


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Interestingly, the catalytic activity of MoS2 NSs is comparable and even higher than those of some reported metal-based catalysts and some new metal-free catalysts such as BN10 and graphene48, 49.

Table 1. Comparison of the activity of our catalysts with other reported catalysts in terms of the rate constant κapp value and activity factor.

Sample

κapp ×102 (min-1)

Quality of

Activity factor

catalyst

(κapp×102/quality)

used(mg)

(min–1·mg–1)

References

RGO-Ni25Co75

9.30

6.00

1.55

[50]

h-BN NSs

negligible

10.0

negligible

[10, 51]

Ni NPs

0.310~0.850

6.50

0.0500~0.130

[52]

Ag NPs

0.300

2.00

0.150

[2]

Reduced GO–Ni hybrid

0.620~0.0148

6.50

0.100~2.20

[52]

Meso-CeO2–Au(0)

21.0

15.0

1.40

[5]

Meso-HAP–Au(0)

26.4

15.0

1.76

[5]

RGO/Ag

2.17~3.88

4.94~13.84

0.210~0.440

[53]

Graphene carbocatalyst

5.70

2.50

2.28

[48]

γ-Al2O3–Pt hybrid

3.20

1.09

2.94

[54]

NAP-MgO–Au(0)

45.6

15.0

3.04

[5]

AuNPs/TWEEN

9.05

not available

not available

[38]

AuNPs/TWEEN/GO

25.4

not available

not available

[38]

Ag/iron oxide NPs

13.3～18.5

2.00

6.65~9.25

[55]

MoS2 NSs

15.5

1.50

10.33

This work

Ag/graphene oxide

49.3

2.00

24.6

[2]

Pd/MoS2 NSs

38.6

1.50

25.7

This work

Au nanodots

1.60

0.0600

26.3

[56]

Pt black

4.20

0.0500

84.0

[3]

Ag/MoS2 NSs

29.0

1.50

19.3

This work

Au/MoS2 NSs

18.8

1.50

12.5

This work

Pt/MoS2 NSs

19.6

1.50

13.1

This work


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In order to compare our results with some other reports, we also listed the ratio of rate constants (κapp) over the quality of

catalyst, named activity factor, as summarized in Table 1.From this

comparison data it can be observed that the catalytic activity of MoS2 NSs reported here is about 79.5 times higher than that of Ni NPs

52

and 68.9 times higher than Ag NPs.2 This supports the notion that

unmodified MoS2 NSs could be considered as an alternative promising catalyst to replace metal-based catalysts in the reduction of p-NP. After noble metal modification, however, the catalytic activity was remarkably improved, illustrating the photochemical reduction method reported here can effectively improve the catalytic performance of MoS2.

Figure 7. Apparent rate constants (κapp) of four different NM/MoS2 NSs catalysts with varying noble-metal contents.

We also studied the catalytic properties of NM/MoS2 NSs with varying noble-metal contents and the corresponding κapp values of the catalytic reactions are shown in Figure 7. For NM/MoS2 NSs, the κapp increased with increasing the noble-metal loading and highest values are obtained when the weight percentage is 10wt %. While, the further increase of the metal content leads to the reduction of the


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catalytic performance. This may be attributed to the agglomeration of noble metal NPs (at high concentrations) and the decreased number of exposed active sites of the MoS2 substrate.

3.4 Catalyst stability and mechanism study The stability and reusability of the catalyst are of great importance for practical applications. In order to perform the stability tests, the pristine MoS2 and Pd-decorated MoS2 NSs were selected as catalysts for the reduction of p-NP. Impressively, the metal-free MoS2 NSs catalyst exhibited excellent stability and retained more than 95% of its initial activity after five successive catalytic runs (Figure 8a), revealing high durability and reusability. In contrast, when Pd/MoS2 NSs were recovered and reused for the reduction of p-NP, it took longer time to complete the reaction than the previous run (Figure 8b). Moreover, PXRD pattern of the MoS2 NSs and Pd/MoS2 NSs after five successive catalytic runs illustrates the high stability of MoS2, while for Pd/MoS2 NSs the main diffraction peaks of Pd were remarkably decreased (Figure S12). As a result, the decrease in catalytic efficiency of Pd/MoS2 NSs may be results of the peeling off and coagulation of Pd NPs form the surface of the MoS2 NSs during the centrifugation. Thus, it is suggested that the as-prepared MoS2 NSs can be used as an alternative active and stable metal-free catalyst for the catalytic reduction of p-NP.

Figure 8. The apparent rate constants (κapp) within five cycles of (a) MoS2 NSs and (b) Pd/MoS2 NSs.

According to the traditional theory about the catalytic reduction of p-NP by noble metals, BH4− acts as an electron donor and provides hydrogen atoms for the catalytic reaction. The surface hydrogen and electron then reacted with the adsorbed p-NP to yield p-AP. On the basis of the above experimental


16

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results and the theory analysis, the origin of the high catalytic activity of MoS2 NSs was analysed, and a possible mechanism is proposed as shown in Scheme 1. First, BH4− anions adsorbed on the edge sites of MoS2 NSs may provide active hydrogen species and electrons for the reduction of p-NP.

34

Meanwhile, unsaturated sulfur atoms on the surface act as active centers that capture p-NP molecules through ion-dipole interactions. Then the dissociated hydrogen species could efficiently diffuse from MoS2 to nitro groups, thereby inducing the formation of p-AP. In this step, the p-AP product desorbs away from the MoS2 surface quickly. In our work, the synthesized defect-rich MoS2 NSs may provide a large number of synergistic sites such as catalytic sites, adsorption sites, and centers with electron storage and shuttling capability,48 which would be the origin of the superior catalytic performance. When modified with noble metals, the surface of MoS2 NS shaving a large number of defects and active sites is advantageous for the uniform growth and distribution of metal NPs. The decoration of noble metals results larger surface areas of NM/MoS2 NSs (Figure S13, Table S1) and a number of interfaces. It is known that a Fermi level alignment occurs whenever a metal and semiconductor are placed in contact. The Fermi level of Ag, Au, Pd and Pt is 4.7 eV,57 5.1 eV,58 5.2 eV45 and 5.65 eV,59 respectively, while the value for MoS2 is 4.0-4.7eV.40 Therefore, in noble metal–MoS2 junctions, electrons will transfer from MoS2 to noble metal NPs, resulting in the formation of an electronenriched region surrounding the noble metal NPs and a depletion layer near MoS2 surface. This means that noble metal NPs accept electrons and show rich electrons effect, which is beneficial to the electron transfer between the metal NPs and p-NP molecules. While the electron-depleted layer at the MoS2 NSs surface can facilitate the electron transfer processes from BH4− to MoS2 and then to the noble metal NPs. Meanwhile, a portion of the catalytic reduction may also proceed on the unoccupied active sites of MoS2. As a result, the synergistic effect between noble metal NPs and ultra-thin MoS2 NSs is believed to be in charge of the high catalytic activity. However, further studies including theoretical calculations are needed to better understand the mechanism of these complex interactions.


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Scheme 1. (a) A probable reaction mechanism for the reduction of p-NP over MoS2 and NM/MoS2 NSs catalysts. (b, c) Energy band positions of MoS2 and Pd.

4. CONCLUSIONS In conclusion, we reported an effective and stable noble metal-free MoS2 catalyst for the reduction of p-NP with NaBH4 under mild reaction conditions. Further the activity of MoS2NSs can be greatly enhanced after decorating the MoS2 surface with noble metal (Au, Ag, Pd, Pt) NPs. The method used for the decoration of noble metals is simple and versatile while no stabilizer was necessary for the growth of the metal NPs, which is of considerable technological importance. Our results suggested that although NM/MoS2 NSs show impressive catalytic performance, they display moderate chemical stability. Instead, the MoS2 NSs could be used as efficient and durable metal-free catalyst for the transformation of p-NP to p-AP. It is expected that the MoS2 NSs may also have potential applications in other fields including sensors, fuel cells and electronics due to their unique electronic and optical properties. In addition, the present approach for the development of NM/MoS2 NSs can provide a generic synthetic path way for the facile production of other functional hybrid nanomaterials.

ASSOCIATED CONTENT


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Supporting Information Electronic supplementary information (ESI) available: Zeta potential plot and SEM image of MoS2 NSs, XPS spectra and N2 adsorption-desorption isotherms of MoS2 NSs and NM/MoS2 NSs samples.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (D.-S. Li) and [email protected] (Q.-C. Zhang)

Notes The authors declare no competing financial interest

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was financially supported by the NSF of China (Nos. 51502155, 51572152, 21373122, 21673127 and 21671119) , the Research Project of HPDE (No. D20151203) and the State Key Laboratory of Structural Chemistry, FJIRSM (20170020).

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Table of Contents In this work, noble metal-free catalyst MoS2nanosheets (MoS2 NSs) were synthesized and evaluated by the catalytic conversion of p-nitrophenol (p-NP) to p-aminophenol (p-AP). Interestingly, the MoS2 NSs exhibits comparable and even better catalytic activity than those of previously reported catalysts. Furthermore, the catalytic performance can be further enhanced by noble metal nanocrystals (Au, Ag, Pd, Pt) decoration via a facile and efficient one-step photochemical reduction approach without the assistance of stabilizer.Thereinto,Pd/MoS2 NSs has the highest catalytic activity with an apparent rate constant (κapp) of 0.35 min-1. The present work opens up enormous opportunities for the large-scale production of diverse MoS2based nanohybrids for many technological applications.


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Enhanced catalytic reduction of p-nitrophenol on ultrathin MoS2

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