- Email: [email protected]
MoS2 nanosheets supported on carbon hybridized montmorillonite as an efficient heterogeneous catalyst in aqueous phase
Applied Clay Science 183 (2019) 105346
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research Paper
MoS2 nanosheets supported on carbon hybridized montmorillonite as an efficient heterogeneous catalyst in aqueous phase
T
⁎
Kang Penga, Jianwei Wanga, Hongjie Wanga, , Xiaoyu Lib, Pengfei Wana, Hanyi Zhanga, Longqi Baia a b
State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China School of Materials Science and Engineering, Chang'an University, Xi'an 710064, China
A R T I C LE I N FO
A B S T R A C T
Keywords: MoS2 nanosheets Carbon material Montmorillonite Heterogeneous catalyst Reduction reaction
Heterogeneous catalysis in aqueous phase plays a pivotal role in energy conversion, environmental remediation and pharmaceutical production. MoS2 has been considered to be a promising catalyst, however its catalytic performance in aqueous phase is greatly restricted by the hydrophobicity and low electrical conductivity. The electrical conductivity and aqueous dispersity of MoS2 nanosheets could be improved by supporting it on the carbon hybridized montmorillonite, which was synthesized through the hydrothermal carbonization and calcination pyrolysis of sucrose with the montmorillonite. Herein, the MoS2 nanosheet supported on carbon hybridized montmorillonite was successfully prepared via in-situ hydrothermal method, which exhibits 2D composite structure with good interface integration. The catalytic performance of nanocomposites was synergistically improved by montmorillonite and carbon material, and the apparent reaction rate constant for the reduction of p-nitrophenol reaches 0.874 cm−1 with nanocomposites as the catalyst. This material design strategy in this study could provide novel idea to develop efficient heterogeneous catalyst in aqueous phase.
1. Introduction Catalysis plays a pivotal role in chemical industry and technology for generation and conversion of energy resource, protection and harnessing of ecological environment and pharmaceutical production. Heterogeneous catalysts have a wide range of applications in the diverse fields, such as solar energy conversion (Yu et al., 2018), hydrogen evolution reaction (Zhao et al., 2018), carbon dioxide reduction (Wang et al., 2019), degradation of organic pollutants (Rokicińska et al., 2016) and synthesis of medicines (Peng et al., 2017b). Nanocomposite materials as efficient heterogeneous catalysts have attracted great attention via the synergistic effects of different components, which exhibit promising prospect in the modern science and technology. Transition metal dichalcogenides (TMDs) exhibit a lot of attractive physicochemical properties and application prospects in the field of catalysis, electronics, photonics, biomedicine and sensing (Guardia et al., 2014). As one of the typical TMDs, molybdenum disulfide (MoS2) is a promising catalyst due to its low cost, earth abundance and high catalytic activity, which is considered as a potential substitution of precious metal catalysts for hydrogen evolution reaction. It has a sandwich interlayer structure composed with covalently bonded S-Mo-S
⁎
nanosheets, which are held together with van der Walls interactions (Li and Peng, 2018a). The single or few layered MoS2 nanosheets possesses unexpectedly excellent catalytic activity different from the bulk materials, which is attributed to the sufficient exposure of catalytic active edge sites. However, the catalytic performance of MoS2 in aqueous phase is greatly restricted by its low electrical conductivity and poor dispersity in aqueous phase for its hydrophobicity. Loading the MoS2 nanosheets on the surface of support materials is one of effective solution. The common support materials consist of carbon nanotubes (Ekspong et al., 2016), graphene (Han et al., 2018) and other carbon materials (Yang et al., 2016; Xu et al., 2019), which could enhance electronic transmission but not improve the dispersity of MoS2 nanosheets in aqueous phase. Designing composite support materials to synchronously improve the electrical conductivity and dispersity of MoS2 nanosheets might be an efficient approach for promoting its catalytic performance in aqueous phase. Natural clays are series of layered aluminosilicate minerals with unique structure, stable physicochemical property, low cost and abundant resources (Li et al., 2018a; Li et al., 2018b). As one of the most common clay minerals, montmorillonite (Mt) consists of a central octahedral sheets (AlO6) sandwiched between two tetrahedral sheets
Corresponding author. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.clay.2019.105346 Received 28 August 2019; Received in revised form 12 October 2019; Accepted 18 October 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
of thioacetamide and 1.210 g of sodium molybdate under stirring conditions, and the suspension was hydrothermally treated at 220 °C for 24 h. The products were collected by centrifugation and dried at 60 °C for 24 h. For comparison, mono-component bare MoS2 was synthesized via hydrothermal method at 220 °C for 24 h. The carbon material (C) was prepared via the hydrothermal carbonization and calcination pyrolysis of sucrose, and the MoS2 nanosheets supported on the carbon material (MoS2@C) or Mt (MoS2@Mt) were prepared by the similar process.
(SiO4) with specific nanosheet-like structures (Yan et al., 2017). It possesses excellent hydrophilicity (Peng et al., 2019b), swelling ability, adsorbability, large specific surface area, intercalation and cation exchange properties (Tahir, 2017), which are valuable in the applications of catalyst supports (Hu et al., 2012; Liu et al., 2014; Rokicińska et al., 2016; Li and Peng, 2018b; Zhang et al., 2018; Peng et al., 2019a), functional assembly (Peng et al., 2017a; Li et al., 2018c), adsorption of pollutants and drug delivery systems. Kang Peng et al. (Peng et al., 2016) in situ hydrothermally synthesized MoS2 nanosheets on the surface of Mt, in which the dispersity of MoS2 nanosheets in aqueous phase was improved by the hydrophilicity of Mt. However, the electrical conductivity of Mt is also very poor, which is harmful to the electronic transmission in the catalytic reaction. Hybridization of hydrophilic Mt and conductive carbon material might obtain the composite support materials with excellent hydrophilicity and electrical conductivity (Ruiz-Hitzky et al., 2011; Peng and Yang, 2017). Supporting MoS2 nanosheets with this carbon hybridized Mt could synchronously improve the electrical conductivity and dispersity, thereby synergistically enhancing the catalytic performance of MoS2 nanosheets. Heterogeneous catalysis in aqueous phase plays an important role in the fields of degradation of organic pollutants (Li and Peng, 2018c), pharmaceutical production and the acquisition of hydrogen energy. As one of the nitroaromatics, p-nitrophenol (4-NP) is a kind of dangerous water pollutants due to its solubility, chemical stability and degradation-resistant, which leads to significant environment and health risk. One of effective treatment is catalytic reduction of 4-NP to 4-aminophenol (4-AP), which is an important intermediate in the production process of analgesic and antipyretic drugs. And the catalytic reduction of 4-NP to 4-AP is commonly used as a probe reaction for catalysis in aqueous phase due to its complete conversion and easy measurement. In this study, MoS2 nanosheets were in-situ hydrothermally synthesized on the surface of carbon hybridized Mt, and the catalytic performance of prepared composites was evaluated in the aqueous phase via the reduction reaction of 4-NP with NaBH4 as the reductant. The electrical conductivity and aqueous dispersity of composites could be synergistically improved by the hydrophilic Mt and conductive carbon material. The microstructures, morphologies and the interfacial characteristics of composites were characterized. The roles of Mt and carbon material for enhancing the catalystic performance were investigated, and the catalystic mechanism in the aqueous phase was explored and illustrated.
2.3. Characterization The X-ray diffraction (XRD) patterns of the samples were obtained on a RIGAKU D/max-2550 PC X-ray diffractometer, using Cu Kα radiation (λ = 0.15418 nm) at 40 kV and 40 mA with a scanning rate of 1.2°/min. Fourier transform infrared (FTIR) data were recorded on a Nicolet Nexus 670 FTIR spectrophotometer using KBr pellets in the range of 400–4000 cm−1 with a resolution 2 cm−1. The Raman spectra were obtained using a Renishaw Micro-Raman System 2000 spectrometer with a 532 nm laser in the range of 300–2000 cm−1 with a resolution 2 cm−1. The morphologies of the samples were observed by scanning electron microscope (SEM, JEOL JSM-6360LV) equipped with energy dispersive spectrometry (EDS) under the accelerating voltage of 5 kV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were operated with a JEOL JEM-2100F transmission electron microscope at 200 kV. X-ray photoelectron spectroscopy (XPS) was used to measure the chemical states of the samples on an ESCALAB 250 spectrometer. The contact angles of the samples were measured using the sessile-drop technique using a goniometer (GBX, France). 2.4. Catalytic-activity evaluation In order to evaluate the catalytic activity of the samples, the reduction of 4-NP was used as a model reaction. 4-NP (0.12 mmol/L) and NaBH4 (72 mmol/L) were dissolved in 50 mL of distilled water under stirring, and 1 mL of catalyst with different concentration was dropped in the solution. The solution was withdrawn regularly from the reactor, and the concentration of 4-NP was determined by the absorbance measured at 400 nm. The decoloration rate (%) was calculated from the formula: decoloration rate (%) = (C0eC)/C0, where C0 is the initial concentration of 4-NP, and C was the concentration of 4-NP at homologous times. The catalyst was centrifuged for the next cycle.
2. Materials and methods 3. Results and discussion 2.1. Materials The synthetic process of the MoS2 nanosheet supported on carbon hybridized montmorillonite (MoS2@C/Mt) is illustrated schematically (Fig. 1). Firstly, the carbon hybridized montmorillonite (C/Mt) was prepared through the hydrothermal carbonization and calcination pyrolysis of sucrose on the Mt. Then, C/Mt was added in the aqueous solution of sodium molybdate and thioacetamide, and mixed with stirring. The molybdate ion and sodium molybdate could be adsorbed on the surface of C/Mt. Under the hydrothermal conditions, the MoS2 nanosheets were nucleated and grown on the C/Mt. Finally, MoS2@C/ Mt was successfully prepared with MoS2 nanosheets supported on the C/Mt. The general morphologies of samples were observed by scanning electron microscope (SEM). Mt possesses smooth surface and 2D layered morphology with 5–20 μm (Fig. 2a). The SEM image of C/Mt (Fig. 2b) indicates that Mt and carbon material stack with each other, which presents hybrid layered morphology. The uniform hybridization of Mt and carbon in the C/Mt would be beneficial to the improvement of electrical conductivity. The morphology of hydrothermally prepared MoS2 (Fig. 2c) exhibits aggregate sphere constituted by the assembly of nanosheets, in which the edge active sites of MoS2 nanosheets could not
The clay used in this work was natural Na-montmorillonite obtained from the Zhejiang, China. Sucrose (C12H22O11), thioacetamide (CH3CSNH2), sodium molybdate (Na2MoO4·2H2O), sodium borohydride (NaBH4) and 4-NP (NO2C6H4OH) were provided from Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytical grade and used as received. 2.2. Preparation MoS2 nanosheets were in situ hydrothermally synthesized on the surface of the carbon hybridized montmorillonite (C/Mt), which was prepared via hydrothermal carbonization and calcination pyrolysis of sucrose with Mt. In a typical preparation experiment, 1.000 g of Mt and 1.000 g of sucrose as carbon source were dispersed in 60 mL of distilled water under magnetic stirring for 30 min. Then, the suspension was transferred into a Teflon-lined stainless steel autoclave (100 mL in capacity) and heated at 200 °C for 24 h. The precipitate was collected and calcined at 800 °C for 3 h in a nitrogen atmosphere to obtain C/Mt. The obtained C/Mt was added in the 60 mL of aqueous solution with 1.503 g 2
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
Fig. 1. Schematic representation for the synthesis of MoS2@C/Mt.
the HRTEM image of MoS2, a lattice fringe with spacing of 0.62 nm is found from the stacked layered structure of MoS2, which is corresponding to the (002) plane of hexagonal MoS2. From the TEM image of MoS2@C/Mt (Fig. 3d), the MoS2 nanosheets are well supported on the C/Mt with 2D composite structure. The HRTEM image reveals the good interface integration between the MoS2 nanosheets and carbon materials. The crystal structures of Mt, C/Mt, MoS2 and MoS2@C/Mt were measured by X-ray diffraction (XRD). The obvious reflection at 7.26° in the XRD pattern of Mt (Fig. 4a) is corresponding to the d001-value of Na-montmorillonite, which is calculated to be 1.22 nm by Bragg's equation. In the XRD pattern of C/Mt, the reflection corresponding to
be exposed fully. In the SEM image of MoS2@C/Mt (Fig. 2d), the MoS2 nanosheets were grown uniformly on the surface of C/Mt. The energy dispersive spectrometer (EDS) result indicates MoS2@C/Mt contains the elements of Mo, S, C, Si, Al, O and Mg. The microstructures of C/Mt and MoS2@C/Mt were further investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The TEM image of Mt (Fig. 3a) indicates the dispersed Mt possesses 2D nanosheet structure. In the TEM image of C/Mt (Fig. 3b), it could be observed that the surface of Mt is surrounded with carbon materials. Hydrothermally prepared MoS2 exhibits the nanosheet with 100–200 nm in width (Fig. 3c), which is the building unit of MoS2 sphere in the SEM image. In
Fig. 2. SEM images of (a) Mt, (b) C/Mt, (c) MoS2 and (d) MoS2@C/Mt, the inset shows the corresponding EDS spectrum. 3
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
Fig. 3. TEM images of (a) Mt, (b) C/Mt, (c) MoS2 and (d) MoS2@C/Mt, the inset shows the corresponding HRTEM images.
C/Mt, CeC vibration band is found at 1554 cm−1 besides the vibration bands of Mt. For hydrothermally prepared MoS2, the band at 445 cm−1 could be assigned to stretching vibration of MoeS, and the vibration band of hydroxyl group is hardly found in the FTIR spectrum. The FTIR spectrum of MoS2@C/Mt exhibits more vibration bands of hydroxyl group differed from that of pure MoS2, which is conducive to the aqueous dispersibility of composites. The further phase structures of samples were studied by Raman spectroscopy. The Raman spectrum of hydrothermally prepared MoS2 (Fig. 4c) exhibits two obvious peaks at 380 and 406 cm−1, which are corresponding to the E12g in-plane vibration modes and A1g out-of-plane vibration modes of MoeS bonds, respectively. The E12g and A1g vibration modes of MoS2@C/Mt are red-shifted to 377 and 402 cm−1, and the E12g-A1g frequency difference becomes slightly smaller, which could be attribute to the fewer stacked layer of MoS2 nanosheets in MoS2@C/Mt. The Raman spectra of C/Mt and MoS2@C/Mt (Fig. 4d) show two characteristic scattering bands of G-band and D-band, which are assigned to the all sp2 carbon forms and the disorder in sp2-hybridized carbon systems, respectively. The higher interlayers of G-band
the basal reflection of Na-montmorillonite becomes weaker, and the basal spacing decreases, which could be attributed to the partial delamination of Mt under the hydrothermal and calcining conditions. The XRD pattern of hydrothermally prepared MoS2 indicates the hexagonal MoS2 phase with high crystallinity, and the reflection at 14.38° is assigned to the (002) basal spacing of 0.62 nm, in accord with the HRTEM result. In the XRD pattern of MoS2@C/Mt, the reflection of MoS2 is stronger than that of Mt, which might because hydrothermally prepared MoS2 possesses higher crystallinity and wraps on the surface of C/Mt. And, another reason might be that the intensity of reflection corresponding to Mt decreased significantly after hydrothermal and calcining treatment (Peng and Yang, 2017) The vibrational bands and functional groups of samples were analyzed by Fourier transform infrared (FTIR) (Liu et al., 2018). In the FTIR spectrum of Mt (Fig. 4b), the bands at 472 and 1021 cm−1 are ascribed to bending-vibration of SieO and the stretching-vibration of O-Si-O due to the silicon‑oxygen tetrahedron of Mt. The bands of –OH stretchingvibration and bending-vibration are observed at 3620 and 1637 cm−1, which contributes to the hydrophilicity of Mt. In the FTIR spectrum of 4
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
Fig. 4. (a) XRD patterns and (b) FTIR spectra of Mt, C/Mt, MoS2 and MoS2@C/Mt, (c) Raman spectra of MoS2 and MoS2@C/Mt, and (d) Raman spectra of C/Mt and MoS2@C/Mt.
The catalytic performances of MoS2 and MoS2@C/Mt in the aqueous phase were estimated via the reduction of 4-NP to 4-AP with NaBH4 as the reductant. The reduction reaction of 4-NP is an ideal model reaction for the assessment of catalytic activity in aqueous phase due to its feasible thermodynamics but hindered kinetics and easy detectability. Mt and C/Mt have little decolorizing ability for 4-NP (Fig. 6a), and the physical adsorption capacity of C/Mt is slightly higher than Mt, which could be attributed to the 2D porous structures and synergistic adsorption of Mt and carbon material. 4-NP is catalyzed reduction by MoS2 and MoS2@C/Mt, and catalytic performance of MoS2@C/Mt is remarkably higher than that of MoS2. It might because that supporting on C/Mt could improve the electronic conductivity and aqueous dispersibility of MoS2 nanosheets and expose more edge catalytic active sites. The reduction of 4-NP with excess reductant was reported to conform with pseudo-first-order kinetics (Peng et al., 2017b), and the apparent reaction rate constant could be calculated to evaluated the catalytic performance. The apparent reaction rate constant of MoS2@C/ Mt at concentration of 1.0 g/L is 0.874 cm−1 (Fig. 6b), which is much higher than that of MoS2 (0.235 cm−1). The catalytic performance exhibits a trend of improvement with the increase of MoS2@C/Mt concentration (Fig. 6c). The apparent reaction rate constants at different MoS2@C/Mt concentrations (Fig. 6d) indicate that the catalytic performance improves sharply below the concentration of 1.0 g/L and tend to be stable at concentration higher than 1.0 g/L. MoS2@C and MoS2@Mt were prepared by the similar process to investigate roles of Mt and carbon material in the improvement of the catalystic performance. The SEM and TEM images of MoS2@C (Fig. S2) indicates that MoS2 nanosheets were assembled on the surface of
compared to D-band indicates the existence of carbon form with graphene-like structures, which benifits the improvement of the electronic conductivity. The surface element chemical state and interfacial characteristic of MoS2@C/Mt were investigated by X-ray photoelectron spectroscopy (XPS). XPS survey spectra (Fig. 5a) in the range of 0–700 eV reveal the chemical element types in the samples, and the elements of Si, Al, O and C in C/Mt and the elements of S and Mo in MoS2 are found. The peaks corresponding to Si, Al, O, C, S and Mo elements are all observed in the XPS survey spectrum of MoS2@C/Mt. As shown in the high-resolution scans for Mo 3d electrons (Fig. 5b), the Mo 3d3/2 and Mo 3d5/2 peaks of hydrothermally prepared MoS2 and MoS2@C/Mt are at binding energies of 233.1, 229.9, 233.0 and 229.4 eV, which indicates that the chemical valance state of molybdenum atom is Mo(IV). The high-resolution scan for S 2p electrons of MoS2 (Fig. 5c) exhibits two peaks at 163.9 and 162.8 eV, which are attributed to the S 2p1/2 and S 2p3/2 of S2−. The S 2p1/2 and S 2p3/2 peaks of MoS2@C/Mt shift to 163.7 and 162.4 eV with large peak width, which suggest electronic interactions in the composites. In the high-resolution scans for C1s electrons (Fig. 5d), the peaks corresponding to CeC, C-OH and C]O bonds could be curve-fitted from the XPS spectrum of C/Mt. For MoS2@C/Mt, the peaks of C-OH and C]O bonds become weaker obviously, which might prove the formation of chemical bond between the carbon and MoS2. In the high-resolution XPS scans for Si 2p and Al 2p electrons of Mt, C/Mt and MoS2@C/Mt (Fig. S1), the Si 2p and Al 2p peaks of C/Mt and MoS2@C/Mt are shifted to higher binding energies than that of Mt, which could result from the interlamellar spacing change of Mt. 5
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
Fig. 5. XPS survey spectra of (a) Mt, C/Mt, MoS2 and MoS2@C/Mt, high-resolution scans for (b) Mo 3d and (c) S 2p electrons of MoS2 and MoS2@C/Mt, (d) C1s electrons of C/Mt and MoS2@C/Mt.
synergistically improves the catalytic performance. The contact angle of MoS2@C/Mt (38.2o) is lower than that of MoS2 (53.9o), indicating the better hydrophilicity of MoS2@C/Mt. The hydrophilic Mt could enhance the dispersibility of composites in the aqueous phase to facilitate the catalytic reaction.
carbon material with serious aggregation, and the interfaces of MoS2 nanosheets and carbon material were observed from the HRTEM image. Carbon material has only physical adsorption for 4-NP (Fig. S3a), and MoS2@C and MoS2@Mt exhibits excellent catalytic performance for reduction of 4-NP. The apparent reaction rate constants of MoS2@C and MoS2@Mt for reduction of 4-NP (Fig. S3b) are higher than that of hydrothermally prepared MoS2, but lower than that of MoS2@C/Mt, indicating that Mt and carbon material could synergistically improve the catalytic performance of MoS2 nanosheets. As shown in Fig. S4, the catalytic ability of MoS2@C/Mt does not show obvious change after three cycles, indicating the good reusability of MoS2@C/Mt for decoloration of 4-NP. The possible mechanism for the catalytic reduction reaction of 4-NP with MoS2@C/Mt as the catalyst is schematically illustrated (Dong et al., 2019). Firstly, NaBH4 as the reductant hydrolyzes spontaneously to produce hydrogen and BH3OH− anion. BH3OH− as the actual reductant injects excess electrons to MoS2 nanosheets for catalytic reduction, and the conductive carbon material could promote the electron transfer in this process. Finally, 4-NP is catalyzed reduction to be 4-AP by electrons transferred from the catalyst. The reaction might proceed according to the following equation:
BH 4− + H2 O → BH3 OH− + H2
4. Conclusions In summary, the MoS2 nanosheets supported on carbon hybridized Mt were successfully prepared via the in-situ hydrothermal growth of MoS2 on the surface of C/Mt. MoS2@C/Mt exhibits 2D composite structure with good interface integration between the MoS2 nanosheets and carbon materials. Supporting on C/Mt could improve the electronic conductivity and aqueous dispersibility of MoS2 nanosheets and expose more edge catalytic active sites. The composites possess excellent catalytic performance for the reduction reaction of 4-NP, and the apparent reaction rate constant reaches 0.874 cm−1 at concentration of 1.0 g/L. The strategy in this study could be extended to the modification of other clay mineral material and the design of efficient catalyst in aqueous phase. Acknowledgements
(1)
This work was supported by National Natural Science Foundation of China (51804242, 51772237, 51704030), the China Postdoctoral Science Foundation (2018T111054, 2017M623182 and 2017M610617), Natural Science Basic Research Plan in Shaanxi province of China (2018JQ5155) the Shaanxi Postdoctoral Science
BH3 OH− + NO2 C6 H 4 OH (4 − NP) → NH2 C6 H 4 OH (4 − AP) −
+ BO2 + H2 O
(2)
The excellent adsorptive property of C/Mt could increase the apparent concentration of 4-NP around the composites, which further 6
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
Fig. 6. (a) Decoloration of 4-NP and (b) the apparent reaction rate constants of Mt, C/Mt, MoS2 and MoS2@C/Mt at concentration of 1.0 g/L, (c) decoloration of 4-NP and (d) the apparent reaction rate constants at different MoS2@C/Mt concentrations.
Foundation (192522) the China Scholarship Council (201906285057) and Shaanxi Innovation Capacity Support Program (2018TD-031). We thank Dr. Jiamei Liu, Zijun Ren and Chao Li at Instrument Analysis Center of Xi'an Jiaotong University for XPS, SEM and TEM measurements.
evolution reaction on hybrid catalysts of vertical MoS2 nanosheets and hydrogenated graphene. ACS Catal. 8, 1828–1836. Hu, K.-h., Zhao, D.-f., Liu, J.-s., 2012. Synthesis of nano-MoS2/bentonite composite and its application for removal of organic dye. Trans. Nonferrous Metals Soc. China 22, 2484–2490. Li, X., Peng, K., 2018a. Hydrothermal synthesis of MoS2 nanosheet/palygorskite nanofiber hybrid nanostructures for enhanced catalytic activity. Appl. Clay Sci. 162, 175–181. Li, X., Peng, K., 2018b. MoSe2/montmorillonite composite nanosheets: hydrothermal synthesis, structural characteristics, and enhanced photocatalytic activity. Minerals 8, 268. Li, X., Peng, K., 2018c. Synthesis of 3D mesoporous alumina from natural clays for confining CdS nanoparticles and enhanced photocatalytic performances. Appl. Clay Sci. 165, 188–196. Li, C., Sun, Z., Zhang, W., Yu, C., Zheng, S., 2018a. Highly efficient g-C3N4/TiO2/kaolinite composite with novel three-dimensional structure and enhanced visible light responding ability towards ciprofloxacin and S. aureus. Appl. Catal. B-Environ. 220, 272–282. Li, X., Peng, K., Chen, H., Wang, Z., 2018b. TiO2 nanoparticles assembled on kaolinites with different morphologies for efficient photocatalytic performance. Sci. Rep. 8, 11663. Li, X., Peng, K., Dou, Y., Chen, J., Zhang, Y., An, G., 2018c. Facile synthesis of wormholelike mesoporous tin oxide via evaporation-induced self-assembly and the enhanced gas-sensing properties. Nanoscale Res. Lett. 13, 14. Liu, Y., Murata, K., Inaba, M., 2014. Synthesis of mixed alcohols from synthesis gas over alkali and Fischer–Tropsch metals modified MoS2/Al2O3-montmorillonite catalysts. React. Kinet. Mech. Catal. 113, 187–200. Liu, W., Xu, J., Han, J., Jiao, F., Qin, W., Li, Z., 2018. Kinetic and mechanism studies on pyrolysis of printed circuit boards in the absence and presence of copper. ACS Sustain. Chem. Eng. 7, 1879–1889. Peng, K., Yang, H., 2017. Carbon hybridized montmorillonite nanosheets: preparation, structural evolution and enhanced adsorption performance. Chem. Commun. 53, 6085–6088. Peng, K., Fu, L., Ouyang, J., Yang, H., 2016. Emerging parallel dual 2D composites: Natural clay mineral hybridizing MoS2 and interfacial structure. Adv. Funct. Mater. 26, 2666–2675.
Declaration of Competing Interest The authors declare no competing interests. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2019.105346. References Dong, X., Ren, B., Sun, Z., Li, C., Zhang, X., Kong, M., Zheng, S., Dionysiou, D.D., 2019. Monodispersed CuFe2O4 nanoparticles anchored on natural kaolinite as highly efficient peroxymonosulfate catalyst for bisphenol A degradation. Appl. Catal. BEnviron. 253, 206–217. Ekspong, J., Sharifi, T., Shchukarev, A., Klechikov, A., Wågberg, T., Gracia-Espino, E., 2016. Stabilizing active edge Sites in semicrystalline molybdenum sulfide by anchorage on nitrogen-doped carbon nanotubes for hydrogen evolution reaction. Adv. Funct. Mater. 26, 6766–6776. Guardia, L., Paredes, J.I., Munuera, J.M., Villar-Rodil, S., Ayán-Varela, M., MartínezAlonso, A., Tascón, J.M.D., 2014. Chemically exfoliated MoS2 nanosheets as an efficient catalyst for reduction reactions in the aqueous phase. ACS Appl. Mater. Interfaces 6, 21702–21710. Han, X., Tong, X., Liu, X., Chen, A., Wen, X., Yang, N., Guo, X.-Y., 2018. Hydrogen
7
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
Wang, J., Gan, L., Zhang, Q., Reddu, V., Peng, Y., Liu, Z., Xia, X., Wang, C., Wang, X., 2019. A water-soluble Cu complex as molecular catalyst for electrocatalytic CO2 reduction on graphene-based electrodes. Adv. Energy Mater. 9, 1803151. Xu, Q., Liu, Y., Jiang, H., Hu, Y., Liu, H., Li, C., 2019. Unsaturated sulfur edge engineering of strongly coupled MoS2 nanosheet-carbon macroporous hybrid catalyst for enhanced hydrogen generation. Adv. Energy Mater. 9, 1802553. Yan, Z., Yang, H., Ouyang, J., Tang, A., 2017. In situ loading of highly-dispersed CuO nanoparticles on hydroxyl-group-rich SiO2-AlOOH composite nanosheets for CO catalytic oxidation. Chem. Eng. J. 316, 1035–1046. Yang, L., Zhou, W., Lu, J., Hou, D., Ke, Y., Li, G., Tang, Z., Kang, X., Chen, S., 2016. Hierarchical spheres constructed by defect-rich MoS2/carbon nanosheets for efficient electrocatalytic hydrogen evolution. Nano Energy 22, 490–498. Yu, S., Fan, X.-B., Wang, X., Li, J., Zhang, Q., Xia, A., Wei, S., Wu, L.-Z., Zhou, Y., Patzke, G.R., 2018. Efficient photocatalytic hydrogen evolution with ligand engineered allinorganic InP and InP/ZnS colloidal quantum dots. Nat. Commun. 9, 4009. Zhang, C., Yu, Y., Wei, H., Li, K., 2018. In situ growth of cube-like AgCl on montmorillonite as an efficient photocatalyst for dye (Acid Red 18) degradation. Appl. Surf. Sci. 456, 577–585. Zhao, G., Rui, K., Dou, S.X., Sun, W., 2018. Heterostructures for electrochemical hydrogen evolution reaction: a review. Adv. Funct. Mater. 28, 1803291.
Peng, K., Fu, L., Li, X., Ouyang, J., Yang, H., 2017a. Stearic acid modified montmorillonite as emerging microcapsules for thermal energy storage. Appl. Clay Sci. 138, 100–106. Peng, K., Fu, L., Yang, H., Ouyang, J., Tang, A., 2017b. Hierarchical MoS2 intercalated clay hybrid nanosheets with enhanced catalytic activity. Nano Res. 10, 570–583. Peng, K., Wang, H., Li, X., Wang, J., Cai, Z., Su, L., Fan, X., 2019. Emerging WS2/montmorillonite composite nanosheets as an efficient hydrophilic photocatalyst for aqueous phase reactions. Sci. Rep. https://doi.org/10.1038/s41598-019-52191-9. Peng, K., Wang, H., Li, X., Wang, J., Xu, L., Gao, H., Niu, M., Ma, M., Yang, J., 2019b. One-step hydrothermal growth of MoS2 nanosheets/CdS nanoparticles heterostructures on montmorillonite for enhanced visible light photocatalytic activity. Appl. Clay Sci. 175, 86–93. Rokicińska, A., Natkański, P., Dudek, B., Drozdek, M., Lityńska-Dobrzyńska, L., Kuśtrowski, P., 2016. Co3O4-pillared montmorillonite catalysts synthesized by hydrogel-assisted route for total oxidation of toluene. Appl. Catal. B-Environ. 195, 59–68. Ruiz-Hitzky, E., Darder, M., Fernandes, F.M., Zatile, E., Palomares, F.J., Aranda, P., 2011. Supported graphene from natural resources: easy preparation and applications. Adv. Mater. 23, 5250–5255. Tahir, M., 2017. Synergistic effect in MMT-dispersed Au/TiO2 monolithic nanocatalyst for plasmon-absorption and metallic interband transitions dynamic CO2 photo-reduction to CO. Appl. Catal. B-Environ. 219, 329–343.
8
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research Paper
MoS2 nanosheets supported on carbon hybridized montmorillonite as an efficient heterogeneous catalyst in aqueous phase
T
⁎
Kang Penga, Jianwei Wanga, Hongjie Wanga, , Xiaoyu Lib, Pengfei Wana, Hanyi Zhanga, Longqi Baia a b
State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China School of Materials Science and Engineering, Chang'an University, Xi'an 710064, China
A R T I C LE I N FO
A B S T R A C T
Keywords: MoS2 nanosheets Carbon material Montmorillonite Heterogeneous catalyst Reduction reaction
Heterogeneous catalysis in aqueous phase plays a pivotal role in energy conversion, environmental remediation and pharmaceutical production. MoS2 has been considered to be a promising catalyst, however its catalytic performance in aqueous phase is greatly restricted by the hydrophobicity and low electrical conductivity. The electrical conductivity and aqueous dispersity of MoS2 nanosheets could be improved by supporting it on the carbon hybridized montmorillonite, which was synthesized through the hydrothermal carbonization and calcination pyrolysis of sucrose with the montmorillonite. Herein, the MoS2 nanosheet supported on carbon hybridized montmorillonite was successfully prepared via in-situ hydrothermal method, which exhibits 2D composite structure with good interface integration. The catalytic performance of nanocomposites was synergistically improved by montmorillonite and carbon material, and the apparent reaction rate constant for the reduction of p-nitrophenol reaches 0.874 cm−1 with nanocomposites as the catalyst. This material design strategy in this study could provide novel idea to develop efficient heterogeneous catalyst in aqueous phase.
1. Introduction Catalysis plays a pivotal role in chemical industry and technology for generation and conversion of energy resource, protection and harnessing of ecological environment and pharmaceutical production. Heterogeneous catalysts have a wide range of applications in the diverse fields, such as solar energy conversion (Yu et al., 2018), hydrogen evolution reaction (Zhao et al., 2018), carbon dioxide reduction (Wang et al., 2019), degradation of organic pollutants (Rokicińska et al., 2016) and synthesis of medicines (Peng et al., 2017b). Nanocomposite materials as efficient heterogeneous catalysts have attracted great attention via the synergistic effects of different components, which exhibit promising prospect in the modern science and technology. Transition metal dichalcogenides (TMDs) exhibit a lot of attractive physicochemical properties and application prospects in the field of catalysis, electronics, photonics, biomedicine and sensing (Guardia et al., 2014). As one of the typical TMDs, molybdenum disulfide (MoS2) is a promising catalyst due to its low cost, earth abundance and high catalytic activity, which is considered as a potential substitution of precious metal catalysts for hydrogen evolution reaction. It has a sandwich interlayer structure composed with covalently bonded S-Mo-S
⁎
nanosheets, which are held together with van der Walls interactions (Li and Peng, 2018a). The single or few layered MoS2 nanosheets possesses unexpectedly excellent catalytic activity different from the bulk materials, which is attributed to the sufficient exposure of catalytic active edge sites. However, the catalytic performance of MoS2 in aqueous phase is greatly restricted by its low electrical conductivity and poor dispersity in aqueous phase for its hydrophobicity. Loading the MoS2 nanosheets on the surface of support materials is one of effective solution. The common support materials consist of carbon nanotubes (Ekspong et al., 2016), graphene (Han et al., 2018) and other carbon materials (Yang et al., 2016; Xu et al., 2019), which could enhance electronic transmission but not improve the dispersity of MoS2 nanosheets in aqueous phase. Designing composite support materials to synchronously improve the electrical conductivity and dispersity of MoS2 nanosheets might be an efficient approach for promoting its catalytic performance in aqueous phase. Natural clays are series of layered aluminosilicate minerals with unique structure, stable physicochemical property, low cost and abundant resources (Li et al., 2018a; Li et al., 2018b). As one of the most common clay minerals, montmorillonite (Mt) consists of a central octahedral sheets (AlO6) sandwiched between two tetrahedral sheets
Corresponding author. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.clay.2019.105346 Received 28 August 2019; Received in revised form 12 October 2019; Accepted 18 October 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
of thioacetamide and 1.210 g of sodium molybdate under stirring conditions, and the suspension was hydrothermally treated at 220 °C for 24 h. The products were collected by centrifugation and dried at 60 °C for 24 h. For comparison, mono-component bare MoS2 was synthesized via hydrothermal method at 220 °C for 24 h. The carbon material (C) was prepared via the hydrothermal carbonization and calcination pyrolysis of sucrose, and the MoS2 nanosheets supported on the carbon material (MoS2@C) or Mt (MoS2@Mt) were prepared by the similar process.
(SiO4) with specific nanosheet-like structures (Yan et al., 2017). It possesses excellent hydrophilicity (Peng et al., 2019b), swelling ability, adsorbability, large specific surface area, intercalation and cation exchange properties (Tahir, 2017), which are valuable in the applications of catalyst supports (Hu et al., 2012; Liu et al., 2014; Rokicińska et al., 2016; Li and Peng, 2018b; Zhang et al., 2018; Peng et al., 2019a), functional assembly (Peng et al., 2017a; Li et al., 2018c), adsorption of pollutants and drug delivery systems. Kang Peng et al. (Peng et al., 2016) in situ hydrothermally synthesized MoS2 nanosheets on the surface of Mt, in which the dispersity of MoS2 nanosheets in aqueous phase was improved by the hydrophilicity of Mt. However, the electrical conductivity of Mt is also very poor, which is harmful to the electronic transmission in the catalytic reaction. Hybridization of hydrophilic Mt and conductive carbon material might obtain the composite support materials with excellent hydrophilicity and electrical conductivity (Ruiz-Hitzky et al., 2011; Peng and Yang, 2017). Supporting MoS2 nanosheets with this carbon hybridized Mt could synchronously improve the electrical conductivity and dispersity, thereby synergistically enhancing the catalytic performance of MoS2 nanosheets. Heterogeneous catalysis in aqueous phase plays an important role in the fields of degradation of organic pollutants (Li and Peng, 2018c), pharmaceutical production and the acquisition of hydrogen energy. As one of the nitroaromatics, p-nitrophenol (4-NP) is a kind of dangerous water pollutants due to its solubility, chemical stability and degradation-resistant, which leads to significant environment and health risk. One of effective treatment is catalytic reduction of 4-NP to 4-aminophenol (4-AP), which is an important intermediate in the production process of analgesic and antipyretic drugs. And the catalytic reduction of 4-NP to 4-AP is commonly used as a probe reaction for catalysis in aqueous phase due to its complete conversion and easy measurement. In this study, MoS2 nanosheets were in-situ hydrothermally synthesized on the surface of carbon hybridized Mt, and the catalytic performance of prepared composites was evaluated in the aqueous phase via the reduction reaction of 4-NP with NaBH4 as the reductant. The electrical conductivity and aqueous dispersity of composites could be synergistically improved by the hydrophilic Mt and conductive carbon material. The microstructures, morphologies and the interfacial characteristics of composites were characterized. The roles of Mt and carbon material for enhancing the catalystic performance were investigated, and the catalystic mechanism in the aqueous phase was explored and illustrated.
2.3. Characterization The X-ray diffraction (XRD) patterns of the samples were obtained on a RIGAKU D/max-2550 PC X-ray diffractometer, using Cu Kα radiation (λ = 0.15418 nm) at 40 kV and 40 mA with a scanning rate of 1.2°/min. Fourier transform infrared (FTIR) data were recorded on a Nicolet Nexus 670 FTIR spectrophotometer using KBr pellets in the range of 400–4000 cm−1 with a resolution 2 cm−1. The Raman spectra were obtained using a Renishaw Micro-Raman System 2000 spectrometer with a 532 nm laser in the range of 300–2000 cm−1 with a resolution 2 cm−1. The morphologies of the samples were observed by scanning electron microscope (SEM, JEOL JSM-6360LV) equipped with energy dispersive spectrometry (EDS) under the accelerating voltage of 5 kV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were operated with a JEOL JEM-2100F transmission electron microscope at 200 kV. X-ray photoelectron spectroscopy (XPS) was used to measure the chemical states of the samples on an ESCALAB 250 spectrometer. The contact angles of the samples were measured using the sessile-drop technique using a goniometer (GBX, France). 2.4. Catalytic-activity evaluation In order to evaluate the catalytic activity of the samples, the reduction of 4-NP was used as a model reaction. 4-NP (0.12 mmol/L) and NaBH4 (72 mmol/L) were dissolved in 50 mL of distilled water under stirring, and 1 mL of catalyst with different concentration was dropped in the solution. The solution was withdrawn regularly from the reactor, and the concentration of 4-NP was determined by the absorbance measured at 400 nm. The decoloration rate (%) was calculated from the formula: decoloration rate (%) = (C0eC)/C0, where C0 is the initial concentration of 4-NP, and C was the concentration of 4-NP at homologous times. The catalyst was centrifuged for the next cycle.
2. Materials and methods 3. Results and discussion 2.1. Materials The synthetic process of the MoS2 nanosheet supported on carbon hybridized montmorillonite (MoS2@C/Mt) is illustrated schematically (Fig. 1). Firstly, the carbon hybridized montmorillonite (C/Mt) was prepared through the hydrothermal carbonization and calcination pyrolysis of sucrose on the Mt. Then, C/Mt was added in the aqueous solution of sodium molybdate and thioacetamide, and mixed with stirring. The molybdate ion and sodium molybdate could be adsorbed on the surface of C/Mt. Under the hydrothermal conditions, the MoS2 nanosheets were nucleated and grown on the C/Mt. Finally, MoS2@C/ Mt was successfully prepared with MoS2 nanosheets supported on the C/Mt. The general morphologies of samples were observed by scanning electron microscope (SEM). Mt possesses smooth surface and 2D layered morphology with 5–20 μm (Fig. 2a). The SEM image of C/Mt (Fig. 2b) indicates that Mt and carbon material stack with each other, which presents hybrid layered morphology. The uniform hybridization of Mt and carbon in the C/Mt would be beneficial to the improvement of electrical conductivity. The morphology of hydrothermally prepared MoS2 (Fig. 2c) exhibits aggregate sphere constituted by the assembly of nanosheets, in which the edge active sites of MoS2 nanosheets could not
The clay used in this work was natural Na-montmorillonite obtained from the Zhejiang, China. Sucrose (C12H22O11), thioacetamide (CH3CSNH2), sodium molybdate (Na2MoO4·2H2O), sodium borohydride (NaBH4) and 4-NP (NO2C6H4OH) were provided from Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytical grade and used as received. 2.2. Preparation MoS2 nanosheets were in situ hydrothermally synthesized on the surface of the carbon hybridized montmorillonite (C/Mt), which was prepared via hydrothermal carbonization and calcination pyrolysis of sucrose with Mt. In a typical preparation experiment, 1.000 g of Mt and 1.000 g of sucrose as carbon source were dispersed in 60 mL of distilled water under magnetic stirring for 30 min. Then, the suspension was transferred into a Teflon-lined stainless steel autoclave (100 mL in capacity) and heated at 200 °C for 24 h. The precipitate was collected and calcined at 800 °C for 3 h in a nitrogen atmosphere to obtain C/Mt. The obtained C/Mt was added in the 60 mL of aqueous solution with 1.503 g 2
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
Fig. 1. Schematic representation for the synthesis of MoS2@C/Mt.
the HRTEM image of MoS2, a lattice fringe with spacing of 0.62 nm is found from the stacked layered structure of MoS2, which is corresponding to the (002) plane of hexagonal MoS2. From the TEM image of MoS2@C/Mt (Fig. 3d), the MoS2 nanosheets are well supported on the C/Mt with 2D composite structure. The HRTEM image reveals the good interface integration between the MoS2 nanosheets and carbon materials. The crystal structures of Mt, C/Mt, MoS2 and MoS2@C/Mt were measured by X-ray diffraction (XRD). The obvious reflection at 7.26° in the XRD pattern of Mt (Fig. 4a) is corresponding to the d001-value of Na-montmorillonite, which is calculated to be 1.22 nm by Bragg's equation. In the XRD pattern of C/Mt, the reflection corresponding to
be exposed fully. In the SEM image of MoS2@C/Mt (Fig. 2d), the MoS2 nanosheets were grown uniformly on the surface of C/Mt. The energy dispersive spectrometer (EDS) result indicates MoS2@C/Mt contains the elements of Mo, S, C, Si, Al, O and Mg. The microstructures of C/Mt and MoS2@C/Mt were further investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The TEM image of Mt (Fig. 3a) indicates the dispersed Mt possesses 2D nanosheet structure. In the TEM image of C/Mt (Fig. 3b), it could be observed that the surface of Mt is surrounded with carbon materials. Hydrothermally prepared MoS2 exhibits the nanosheet with 100–200 nm in width (Fig. 3c), which is the building unit of MoS2 sphere in the SEM image. In
Fig. 2. SEM images of (a) Mt, (b) C/Mt, (c) MoS2 and (d) MoS2@C/Mt, the inset shows the corresponding EDS spectrum. 3
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
Fig. 3. TEM images of (a) Mt, (b) C/Mt, (c) MoS2 and (d) MoS2@C/Mt, the inset shows the corresponding HRTEM images.
C/Mt, CeC vibration band is found at 1554 cm−1 besides the vibration bands of Mt. For hydrothermally prepared MoS2, the band at 445 cm−1 could be assigned to stretching vibration of MoeS, and the vibration band of hydroxyl group is hardly found in the FTIR spectrum. The FTIR spectrum of MoS2@C/Mt exhibits more vibration bands of hydroxyl group differed from that of pure MoS2, which is conducive to the aqueous dispersibility of composites. The further phase structures of samples were studied by Raman spectroscopy. The Raman spectrum of hydrothermally prepared MoS2 (Fig. 4c) exhibits two obvious peaks at 380 and 406 cm−1, which are corresponding to the E12g in-plane vibration modes and A1g out-of-plane vibration modes of MoeS bonds, respectively. The E12g and A1g vibration modes of MoS2@C/Mt are red-shifted to 377 and 402 cm−1, and the E12g-A1g frequency difference becomes slightly smaller, which could be attribute to the fewer stacked layer of MoS2 nanosheets in MoS2@C/Mt. The Raman spectra of C/Mt and MoS2@C/Mt (Fig. 4d) show two characteristic scattering bands of G-band and D-band, which are assigned to the all sp2 carbon forms and the disorder in sp2-hybridized carbon systems, respectively. The higher interlayers of G-band
the basal reflection of Na-montmorillonite becomes weaker, and the basal spacing decreases, which could be attributed to the partial delamination of Mt under the hydrothermal and calcining conditions. The XRD pattern of hydrothermally prepared MoS2 indicates the hexagonal MoS2 phase with high crystallinity, and the reflection at 14.38° is assigned to the (002) basal spacing of 0.62 nm, in accord with the HRTEM result. In the XRD pattern of MoS2@C/Mt, the reflection of MoS2 is stronger than that of Mt, which might because hydrothermally prepared MoS2 possesses higher crystallinity and wraps on the surface of C/Mt. And, another reason might be that the intensity of reflection corresponding to Mt decreased significantly after hydrothermal and calcining treatment (Peng and Yang, 2017) The vibrational bands and functional groups of samples were analyzed by Fourier transform infrared (FTIR) (Liu et al., 2018). In the FTIR spectrum of Mt (Fig. 4b), the bands at 472 and 1021 cm−1 are ascribed to bending-vibration of SieO and the stretching-vibration of O-Si-O due to the silicon‑oxygen tetrahedron of Mt. The bands of –OH stretchingvibration and bending-vibration are observed at 3620 and 1637 cm−1, which contributes to the hydrophilicity of Mt. In the FTIR spectrum of 4
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
Fig. 4. (a) XRD patterns and (b) FTIR spectra of Mt, C/Mt, MoS2 and MoS2@C/Mt, (c) Raman spectra of MoS2 and MoS2@C/Mt, and (d) Raman spectra of C/Mt and MoS2@C/Mt.
The catalytic performances of MoS2 and MoS2@C/Mt in the aqueous phase were estimated via the reduction of 4-NP to 4-AP with NaBH4 as the reductant. The reduction reaction of 4-NP is an ideal model reaction for the assessment of catalytic activity in aqueous phase due to its feasible thermodynamics but hindered kinetics and easy detectability. Mt and C/Mt have little decolorizing ability for 4-NP (Fig. 6a), and the physical adsorption capacity of C/Mt is slightly higher than Mt, which could be attributed to the 2D porous structures and synergistic adsorption of Mt and carbon material. 4-NP is catalyzed reduction by MoS2 and MoS2@C/Mt, and catalytic performance of MoS2@C/Mt is remarkably higher than that of MoS2. It might because that supporting on C/Mt could improve the electronic conductivity and aqueous dispersibility of MoS2 nanosheets and expose more edge catalytic active sites. The reduction of 4-NP with excess reductant was reported to conform with pseudo-first-order kinetics (Peng et al., 2017b), and the apparent reaction rate constant could be calculated to evaluated the catalytic performance. The apparent reaction rate constant of MoS2@C/ Mt at concentration of 1.0 g/L is 0.874 cm−1 (Fig. 6b), which is much higher than that of MoS2 (0.235 cm−1). The catalytic performance exhibits a trend of improvement with the increase of MoS2@C/Mt concentration (Fig. 6c). The apparent reaction rate constants at different MoS2@C/Mt concentrations (Fig. 6d) indicate that the catalytic performance improves sharply below the concentration of 1.0 g/L and tend to be stable at concentration higher than 1.0 g/L. MoS2@C and MoS2@Mt were prepared by the similar process to investigate roles of Mt and carbon material in the improvement of the catalystic performance. The SEM and TEM images of MoS2@C (Fig. S2) indicates that MoS2 nanosheets were assembled on the surface of
compared to D-band indicates the existence of carbon form with graphene-like structures, which benifits the improvement of the electronic conductivity. The surface element chemical state and interfacial characteristic of MoS2@C/Mt were investigated by X-ray photoelectron spectroscopy (XPS). XPS survey spectra (Fig. 5a) in the range of 0–700 eV reveal the chemical element types in the samples, and the elements of Si, Al, O and C in C/Mt and the elements of S and Mo in MoS2 are found. The peaks corresponding to Si, Al, O, C, S and Mo elements are all observed in the XPS survey spectrum of MoS2@C/Mt. As shown in the high-resolution scans for Mo 3d electrons (Fig. 5b), the Mo 3d3/2 and Mo 3d5/2 peaks of hydrothermally prepared MoS2 and MoS2@C/Mt are at binding energies of 233.1, 229.9, 233.0 and 229.4 eV, which indicates that the chemical valance state of molybdenum atom is Mo(IV). The high-resolution scan for S 2p electrons of MoS2 (Fig. 5c) exhibits two peaks at 163.9 and 162.8 eV, which are attributed to the S 2p1/2 and S 2p3/2 of S2−. The S 2p1/2 and S 2p3/2 peaks of MoS2@C/Mt shift to 163.7 and 162.4 eV with large peak width, which suggest electronic interactions in the composites. In the high-resolution scans for C1s electrons (Fig. 5d), the peaks corresponding to CeC, C-OH and C]O bonds could be curve-fitted from the XPS spectrum of C/Mt. For MoS2@C/Mt, the peaks of C-OH and C]O bonds become weaker obviously, which might prove the formation of chemical bond between the carbon and MoS2. In the high-resolution XPS scans for Si 2p and Al 2p electrons of Mt, C/Mt and MoS2@C/Mt (Fig. S1), the Si 2p and Al 2p peaks of C/Mt and MoS2@C/Mt are shifted to higher binding energies than that of Mt, which could result from the interlamellar spacing change of Mt. 5
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
Fig. 5. XPS survey spectra of (a) Mt, C/Mt, MoS2 and MoS2@C/Mt, high-resolution scans for (b) Mo 3d and (c) S 2p electrons of MoS2 and MoS2@C/Mt, (d) C1s electrons of C/Mt and MoS2@C/Mt.
synergistically improves the catalytic performance. The contact angle of MoS2@C/Mt (38.2o) is lower than that of MoS2 (53.9o), indicating the better hydrophilicity of MoS2@C/Mt. The hydrophilic Mt could enhance the dispersibility of composites in the aqueous phase to facilitate the catalytic reaction.
carbon material with serious aggregation, and the interfaces of MoS2 nanosheets and carbon material were observed from the HRTEM image. Carbon material has only physical adsorption for 4-NP (Fig. S3a), and MoS2@C and MoS2@Mt exhibits excellent catalytic performance for reduction of 4-NP. The apparent reaction rate constants of MoS2@C and MoS2@Mt for reduction of 4-NP (Fig. S3b) are higher than that of hydrothermally prepared MoS2, but lower than that of MoS2@C/Mt, indicating that Mt and carbon material could synergistically improve the catalytic performance of MoS2 nanosheets. As shown in Fig. S4, the catalytic ability of MoS2@C/Mt does not show obvious change after three cycles, indicating the good reusability of MoS2@C/Mt for decoloration of 4-NP. The possible mechanism for the catalytic reduction reaction of 4-NP with MoS2@C/Mt as the catalyst is schematically illustrated (Dong et al., 2019). Firstly, NaBH4 as the reductant hydrolyzes spontaneously to produce hydrogen and BH3OH− anion. BH3OH− as the actual reductant injects excess electrons to MoS2 nanosheets for catalytic reduction, and the conductive carbon material could promote the electron transfer in this process. Finally, 4-NP is catalyzed reduction to be 4-AP by electrons transferred from the catalyst. The reaction might proceed according to the following equation:
BH 4− + H2 O → BH3 OH− + H2
4. Conclusions In summary, the MoS2 nanosheets supported on carbon hybridized Mt were successfully prepared via the in-situ hydrothermal growth of MoS2 on the surface of C/Mt. MoS2@C/Mt exhibits 2D composite structure with good interface integration between the MoS2 nanosheets and carbon materials. Supporting on C/Mt could improve the electronic conductivity and aqueous dispersibility of MoS2 nanosheets and expose more edge catalytic active sites. The composites possess excellent catalytic performance for the reduction reaction of 4-NP, and the apparent reaction rate constant reaches 0.874 cm−1 at concentration of 1.0 g/L. The strategy in this study could be extended to the modification of other clay mineral material and the design of efficient catalyst in aqueous phase. Acknowledgements
(1)
This work was supported by National Natural Science Foundation of China (51804242, 51772237, 51704030), the China Postdoctoral Science Foundation (2018T111054, 2017M623182 and 2017M610617), Natural Science Basic Research Plan in Shaanxi province of China (2018JQ5155) the Shaanxi Postdoctoral Science
BH3 OH− + NO2 C6 H 4 OH (4 − NP) → NH2 C6 H 4 OH (4 − AP) −
+ BO2 + H2 O
(2)
The excellent adsorptive property of C/Mt could increase the apparent concentration of 4-NP around the composites, which further 6
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
Fig. 6. (a) Decoloration of 4-NP and (b) the apparent reaction rate constants of Mt, C/Mt, MoS2 and MoS2@C/Mt at concentration of 1.0 g/L, (c) decoloration of 4-NP and (d) the apparent reaction rate constants at different MoS2@C/Mt concentrations.
Foundation (192522) the China Scholarship Council (201906285057) and Shaanxi Innovation Capacity Support Program (2018TD-031). We thank Dr. Jiamei Liu, Zijun Ren and Chao Li at Instrument Analysis Center of Xi'an Jiaotong University for XPS, SEM and TEM measurements.
evolution reaction on hybrid catalysts of vertical MoS2 nanosheets and hydrogenated graphene. ACS Catal. 8, 1828–1836. Hu, K.-h., Zhao, D.-f., Liu, J.-s., 2012. Synthesis of nano-MoS2/bentonite composite and its application for removal of organic dye. Trans. Nonferrous Metals Soc. China 22, 2484–2490. Li, X., Peng, K., 2018a. Hydrothermal synthesis of MoS2 nanosheet/palygorskite nanofiber hybrid nanostructures for enhanced catalytic activity. Appl. Clay Sci. 162, 175–181. Li, X., Peng, K., 2018b. MoSe2/montmorillonite composite nanosheets: hydrothermal synthesis, structural characteristics, and enhanced photocatalytic activity. Minerals 8, 268. Li, X., Peng, K., 2018c. Synthesis of 3D mesoporous alumina from natural clays for confining CdS nanoparticles and enhanced photocatalytic performances. Appl. Clay Sci. 165, 188–196. Li, C., Sun, Z., Zhang, W., Yu, C., Zheng, S., 2018a. Highly efficient g-C3N4/TiO2/kaolinite composite with novel three-dimensional structure and enhanced visible light responding ability towards ciprofloxacin and S. aureus. Appl. Catal. B-Environ. 220, 272–282. Li, X., Peng, K., Chen, H., Wang, Z., 2018b. TiO2 nanoparticles assembled on kaolinites with different morphologies for efficient photocatalytic performance. Sci. Rep. 8, 11663. Li, X., Peng, K., Dou, Y., Chen, J., Zhang, Y., An, G., 2018c. Facile synthesis of wormholelike mesoporous tin oxide via evaporation-induced self-assembly and the enhanced gas-sensing properties. Nanoscale Res. Lett. 13, 14. Liu, Y., Murata, K., Inaba, M., 2014. Synthesis of mixed alcohols from synthesis gas over alkali and Fischer–Tropsch metals modified MoS2/Al2O3-montmorillonite catalysts. React. Kinet. Mech. Catal. 113, 187–200. Liu, W., Xu, J., Han, J., Jiao, F., Qin, W., Li, Z., 2018. Kinetic and mechanism studies on pyrolysis of printed circuit boards in the absence and presence of copper. ACS Sustain. Chem. Eng. 7, 1879–1889. Peng, K., Yang, H., 2017. Carbon hybridized montmorillonite nanosheets: preparation, structural evolution and enhanced adsorption performance. Chem. Commun. 53, 6085–6088. Peng, K., Fu, L., Ouyang, J., Yang, H., 2016. Emerging parallel dual 2D composites: Natural clay mineral hybridizing MoS2 and interfacial structure. Adv. Funct. Mater. 26, 2666–2675.
Declaration of Competing Interest The authors declare no competing interests. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2019.105346. References Dong, X., Ren, B., Sun, Z., Li, C., Zhang, X., Kong, M., Zheng, S., Dionysiou, D.D., 2019. Monodispersed CuFe2O4 nanoparticles anchored on natural kaolinite as highly efficient peroxymonosulfate catalyst for bisphenol A degradation. Appl. Catal. BEnviron. 253, 206–217. Ekspong, J., Sharifi, T., Shchukarev, A., Klechikov, A., Wågberg, T., Gracia-Espino, E., 2016. Stabilizing active edge Sites in semicrystalline molybdenum sulfide by anchorage on nitrogen-doped carbon nanotubes for hydrogen evolution reaction. Adv. Funct. Mater. 26, 6766–6776. Guardia, L., Paredes, J.I., Munuera, J.M., Villar-Rodil, S., Ayán-Varela, M., MartínezAlonso, A., Tascón, J.M.D., 2014. Chemically exfoliated MoS2 nanosheets as an efficient catalyst for reduction reactions in the aqueous phase. ACS Appl. Mater. Interfaces 6, 21702–21710. Han, X., Tong, X., Liu, X., Chen, A., Wen, X., Yang, N., Guo, X.-Y., 2018. Hydrogen
7
Applied Clay Science 183 (2019) 105346
K. Peng, et al.
Wang, J., Gan, L., Zhang, Q., Reddu, V., Peng, Y., Liu, Z., Xia, X., Wang, C., Wang, X., 2019. A water-soluble Cu complex as molecular catalyst for electrocatalytic CO2 reduction on graphene-based electrodes. Adv. Energy Mater. 9, 1803151. Xu, Q., Liu, Y., Jiang, H., Hu, Y., Liu, H., Li, C., 2019. Unsaturated sulfur edge engineering of strongly coupled MoS2 nanosheet-carbon macroporous hybrid catalyst for enhanced hydrogen generation. Adv. Energy Mater. 9, 1802553. Yan, Z., Yang, H., Ouyang, J., Tang, A., 2017. In situ loading of highly-dispersed CuO nanoparticles on hydroxyl-group-rich SiO2-AlOOH composite nanosheets for CO catalytic oxidation. Chem. Eng. J. 316, 1035–1046. Yang, L., Zhou, W., Lu, J., Hou, D., Ke, Y., Li, G., Tang, Z., Kang, X., Chen, S., 2016. Hierarchical spheres constructed by defect-rich MoS2/carbon nanosheets for efficient electrocatalytic hydrogen evolution. Nano Energy 22, 490–498. Yu, S., Fan, X.-B., Wang, X., Li, J., Zhang, Q., Xia, A., Wei, S., Wu, L.-Z., Zhou, Y., Patzke, G.R., 2018. Efficient photocatalytic hydrogen evolution with ligand engineered allinorganic InP and InP/ZnS colloidal quantum dots. Nat. Commun. 9, 4009. Zhang, C., Yu, Y., Wei, H., Li, K., 2018. In situ growth of cube-like AgCl on montmorillonite as an efficient photocatalyst for dye (Acid Red 18) degradation. Appl. Surf. Sci. 456, 577–585. Zhao, G., Rui, K., Dou, S.X., Sun, W., 2018. Heterostructures for electrochemical hydrogen evolution reaction: a review. Adv. Funct. Mater. 28, 1803291.
Peng, K., Fu, L., Li, X., Ouyang, J., Yang, H., 2017a. Stearic acid modified montmorillonite as emerging microcapsules for thermal energy storage. Appl. Clay Sci. 138, 100–106. Peng, K., Fu, L., Yang, H., Ouyang, J., Tang, A., 2017b. Hierarchical MoS2 intercalated clay hybrid nanosheets with enhanced catalytic activity. Nano Res. 10, 570–583. Peng, K., Wang, H., Li, X., Wang, J., Cai, Z., Su, L., Fan, X., 2019. Emerging WS2/montmorillonite composite nanosheets as an efficient hydrophilic photocatalyst for aqueous phase reactions. Sci. Rep. https://doi.org/10.1038/s41598-019-52191-9. Peng, K., Wang, H., Li, X., Wang, J., Xu, L., Gao, H., Niu, M., Ma, M., Yang, J., 2019b. One-step hydrothermal growth of MoS2 nanosheets/CdS nanoparticles heterostructures on montmorillonite for enhanced visible light photocatalytic activity. Appl. Clay Sci. 175, 86–93. Rokicińska, A., Natkański, P., Dudek, B., Drozdek, M., Lityńska-Dobrzyńska, L., Kuśtrowski, P., 2016. Co3O4-pillared montmorillonite catalysts synthesized by hydrogel-assisted route for total oxidation of toluene. Appl. Catal. B-Environ. 195, 59–68. Ruiz-Hitzky, E., Darder, M., Fernandes, F.M., Zatile, E., Palomares, F.J., Aranda, P., 2011. Supported graphene from natural resources: easy preparation and applications. Adv. Mater. 23, 5250–5255. Tahir, M., 2017. Synergistic effect in MMT-dispersed Au/TiO2 monolithic nanocatalyst for plasmon-absorption and metallic interband transitions dynamic CO2 photo-reduction to CO. Appl. Catal. B-Environ. 219, 329–343.
8