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Three-dimensional NiS-MoS2/Graphene heterostructured nanohybrids as high-performance hydrodesulfurization catalysts Sunil P. Lonkar, Vishnu Pillai, and Saeed M. Alhassan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00287 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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ACS Applied Nano Materials

Three

Dimensional

NiS-MoS2/Graphene

Heterostructured Nanohybrids as High-performance Hydrodesulfurization Catalysts Sunil P. Lonkar*, Vishnu V. Pillai, and Saeed M. Alhassan*

*Corresponding author

Department of Chemical Engineering, Khalifa University (The Petroleum Institute), P.O box 2533, Abu Dhabi, UAE

E-mail: [email protected], [email protected] Tel:+971-26075944, Fax: +971-26075200

A simplistic and scalable method for constructing three-dimensional highly porous aerogels composed of in-situ formed NiS nanoparticles anchored onto 2D MoS2/graphene assembly was reported. This process involves simple hydrothermal treatment followed by moderate thermal annealing. The as-synthesized nanohybrids were deeply investigated using several characterization techniques. The favorable interfacial interactions promoted the graphene surface to control the aggregation of layered MoS2 and uniform decoration of in-situ formed NiS nanoparticles was achieved in the resulting nanohybrids. The resulting nanostructured 3D NiS-MoS2/graphene assembly produced synergistic effects and displayed remarkably enhanced hydrodesulfurization (HDS) performance towards DBT conversion. The content of

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NiS co-catalyst controls the hydrogen spillover in the resulting nano-heterojunction catalysts and exhibits an enhanced HDS performance. The presented synthetic approach can be applied into the large-scale production of 3D ternary heterostructured nanohybrids composed of metal sulfide/oxide, 2D MoS2 and graphene aerogels and opens new avenues for plethora of applications including electrocatalysts and high-performance electrode materials for energy storage, in conjunction it can also offer innovative pathways to fabricate hierarchical and newer functional hybrids composed of 2D materials.

Keywords: 3D Heterostructure, Nanohybrids, MoS2, Graphene, Hydrodesulfurization

1. Introduction In recent years, owing to its fascinating physicochemical and electronic properties, , the two dimensional (2D) layered inorganic compounds composed of transition metal dichalcogenides (TMDs) at nanoscale have engrossed substantial in the field of energy conversion, storage and in electrocatalysis applications1-4. Amongst TMDs, a layered molybdenum disulfide (MoS2), due to its unique layered structure, wherein every molybdenum atom is present at the center of a trigonal prismatic coordination and covalently bonded to the sulfide ions, this specific electronic structure make MoS2 a competent material as hydrotreating catalysts

5-6

and electrocatalyst

7-9

. With ever-increasing demand for cleaner

fuels and stringent specifications applied to acceptable sulfur limits in petroleum fuels are becoming a challenging task for the refineries 10. Therefore, the development of active, stable and low-cost catalyst systems for efficient hydrodesulfurization (HDS) is highly requisite 11. So far, most of the industrial HDS catalyst systems have been based on MoS2 either supported or promoted with other active metals such as nickel (Ni) or cobalt (Co), widely noted as NiMoS and CoMoS phases

12-14

. The HDS catalytic

mechanism using these hybrid catalysts was widely investigated by using classical phase models i.e active phase model

15

and remote-control model (RC)

16-17

. According to phase decoration model, the


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higher activity for the Ni or Co promoted MoS2 catalyst is ascribed due to the realization of active CoMo-S and Ni-Mo-S phases

18

. Nevertheless, the existence of such active phases were not directly

identified and considered to be vulnerable at higher temperature which is detrimental to the final catalytic activity

19

. Another model is Remote Control (RC) model, which highlights the migration of

hydrogen (hydrogen spillover, Hso), over a donating phase (NiSx or CoSx) to the MoS2 accepting phase and displayed higher catalytic activity of acceptor compared to the Co/Ni-Mo-S phase 20-21. Hence, in RC model, the high HDS catalytic activity of such heterostructured mixed sulfides is ascribed due to the catalytic synergy that is brought up by the individual sulfides species (i.e donor and acceptor) in the resulting nanohybrid catalysts. Recently, several reports have confirmed the advantages of using such bi-component sulfide catalytic system in hydrotreating reactions

22-23

. These reports underline the role

of high surface contact between two sulfides for improved catalytic activity. Hence, development of donor phase hybridized with acceptor phase at nanoscale could improve their surface area contact which is key to achieve high HDS catalytic activity. Recently, Wang et al.

24

prepared NiS2/MoS2

nanohybrids by hydrothermal method and used an efficient catalyst for hydrodeoxygenation p-cresol. In this study, the importance of highly active sulfides catalysts with enhanced surface area and the role of NiS phases were highlighted. Furthermore, to achieve enhanced performance and stability promoted MoS2 is often depended on various catalyst supports such as alumina and carbonaceous materials 25. Similarly, support also can play a vital role as Hso carrier from donor to acceptor as demonstrated by RC model

26-28

. Amongst, graphene support, owing to its high surface area, rich porosity, and exceptional

Sp2 network structures that can significantly enhance the catalytic properties were observed

29

.

Recently, we have successfully demonstrated the use of highly porous and stable three dimensional and interconnected MoS2/graphene aerogels as excellent HDS catalysts 30. But, the use of the promoter such as NiSx decorated 3D porous HDS catalyst system has not been explored yet. Hence, it is anticipated that using graphene-based aerogels in which donor (nickel sulfide) and acceptor (MoS2) was uniformly


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distributed could lead to synergism and improve the activity of the embedded heterostructures as HDS catalysts. In present work, we report a simplistic one-step approach that involves a hydrothermal process to produce of 3D NiS-MoS2/graphene heterostructured nanohybrids. The effect of NiS as a promoter in enhancing the catalytic efficiency of few-layered MoS2 supported on the 3D graphene on the hydrodesulfurization of dibenzothiophene was investigated. The critical effect of graphene layers in controlling the aggregation of in-situ formed MoS2 layers and NiS nanoparticles uniformly distributed on 3D MoS2/graphene hybrids was also observed. Furthermore, the introduction of NiS nanoparticles enhances the overall HDS activity of the resulting catalysts.

2. Experimental

2.1 Preparation of 3D NiS-MoS2/graphene heterostructured nanohybrids First, the graphite oxide was obtained by graphite oxidation as per the procedure mentioned in our earlier work 31. In the second step, the 3D porous hybrid aerogels of graphene were fabricated using a scalable hydrothermal process followed by freeze-drying. In typical reaction, a highly dispersed aqueous GO suspension was obtained by sonicating the GO stock (5 mg mL-1). Similarly, in a separate vial, 0.4 gm ammonium tetrathiomolybdate (ATTM) was dissolved in 5 mL of deionized water. The resulting two solutions were homogenized under stirring and ultrasonication. To this mixture, nickel salt and aqueous NaOH solutions with varying the stoichiometric ration were added. The final GO/ATTM/Ni salt mixed solution was hydrothermally treated in a sealed Teflon autoclave at 200 ◦C for 4h. After cooling, a black solid macroporous hydrogel was recovered and subsequently immersed in water followed by a washing treatment under running de-ionized water to remove any by-products and metal ions. The obtained hydrogels were then freeze-dried for 24 h and were further subjected to thermal annealing driven


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crystallization at 600 ◦C for 2h to ensure complete thermal reduction of RGO into graphene and formation of highly crystalline MoS2 and NiS phases. In the preparations with a variable Ni/Mo ratio, the amount of molybdenum was fixed, but that of Ni varied. The nanohybrids having 1, 5 and 20 wt-% of NiS was prepared and labeled as NMG-1, NMG-2, and NMG-3, respectively. For comparison, pristine 3D graphene and MoS2 nanosheets were prepared under similar protocol and named as 3D-graphene and nano-MoS2, respectively.

2.2 Characterizations

As prepared samples of NMG nanohybrids and MoS2 nanoparticles were extensively characterized using different characterization techniques. The X-ray powder diffraction (PXRD) analyses were conducted on a Philips X’Pert Pro X-Ray diffractometer equipped with a scintillation counter and Cu-Kα radiation reflection mode. The microscopic morphology and structures of the nanohybrids were characterized using a FEI Tecnai (G20) transmission electron microscope (TEM/HRTEM) and Zeiss (1540 XB) scanning electron microscope (SEM) coupled with energy dispersive X-ray analysis (EDX). The X-ray photoelectron spectra (XPS) were recorded using a SSX-100 system (Surface Science Laboratories, Inc.) equipped with a monochromated Al Kα X-ray source, a hemispherical sector analyzer (HSA) and a resistive anode detector. LabRAM HR (Horiba Scientific) was used to obtain Raman spectra. Typically, a 50x objective was used with 633 nm excitation line. The specific surface area and porosity of the resulting 3D NMG nanohybrids were obtained using ASPS 2010 (Micromeritics, USA) Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption at liquid N2 temperature. A pre-treatment under high vacuum at 100 oC for 24 h before N2 adsorption was carried using a Quantachrome Autosorb gas-sorption system. Temperature programmed desorption of ammonia (NH3-TPD) experiments were performed by a Micromeritics Autocue 2910 system. First, 100 mg of the catalyst sample treated at 500 °C for 1 h in a 50 mL min−1 helium flow in order ensure moisture free sample and subsequently cooled to 100 °C. In


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second step, sample was conditioned in presence of a probe gas (5% NH3 in helium) for 1 h in a 50 mL min−1 flow and then the reactor temperature was increased to 650 °C at a rate of 10 °C min−1 in a 30 mL min−1 helium flow. The HDS catalytic efficiency of resulting NMG nanohybrid catalysts were tested in a 150-cm3 batch reactor. In typical experiment, 0.040 g catalysts immersed in 40 mL hexadecane solution containing 1.0 wt.% dibenzothiophene 32. The detailed HDS procedure is provided as supporting information (S1).

3. Results and Discussion

3.1 Fabrication of 3D NiS-MoS2/graphene (NMG) nanohybrids The formation mechanism of the as-prepared NMG heterostructured nanohybrids prepared via simple hydrothermal method is shown in Scheme. 1. In this study, ammonium tetrathiomolybdate, nickel salt, and graphite oxide were used as the source of Mo and Ni and graphite oxide as graphene precursor. Under a hydrothermal process, ATTM is expected to decompose into MoS3 and sulfur species

33

and

similarly, graphite oxide undergo partial hydrothermal reduction to form reduced graphite oxide (RGO) and under basic condition, nickel salt hydrolyze to form a nickel hydroxide. Subsequently, under thermal treatment, a complete reduction of RGO to graphene was achieved accompanied with the reduction of MoS3 to MoS2. Also, Ni(OH)2 was converted into nickel oxide which in-situ reacts with the excess sulfur species liberated during the reduction MoS3 to form nanosized NiS particles. The surface of the thermally reduced GO (graphene) sheets was expected to provide support for both MoS2 and NiS nucleation sites, resulting in a uniform dispersion of nanosized heterostructured particles onto graphene.


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Scheme1. Schematic representation for preparation of 3D NiS-MoS2/graphene heterostructured nanohybrid

3.2 Characterization of 3D NiS-MoS2/graphene nanohybrids To understand the crystal structure of 3D NiS-MoS2/graphene samples, XRD patterns were studied in detail and showed in Fig.1. The XRD patterns present several broadened diffraction peaks imply a relatively poor crystallinity and nanoscale dimensions of NiS-MoS2 on the graphene support. For 3D MoS2/Graphene (Fig.1a), the broader peaks at 14.34, 33.36, 39.86, 49.62 and 58.69 degrees correspond (002), (100), (103), (105), and (110) positions confirm the existence of nano MoS2 (Fig.S1 and JCPDS 371492). Additionally, a broad peak around 26 ° (002) planes demonstrates thermal reduction of graphite oxide to graphene. The peak broadening indicates that the presence of MoS2 controlled the graphene layers from possible stacking. At lower Ni salt loading, XRD patterns do not show any remarkable changes and no typical peaks of Ni and its sulfides was observed, probably due to the low amount of NiS loaded. However, at higher Ni salt loading (Fig.1c-d), a new set of peaks was observed at 30.71, 35.2,


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46.2, 53.97 degrees corresponds to (100), (101), (102), (110) which is attributed to the nanosized NiS particles (JCPDS 37- 1492). The formation intermediate hybrid Ni(OH)2/MoS3 was also confirmed by XRD analysis (Fig.S2). Furthermore, the crystallite size of NiS in resulting NMG nanohybrids was determined from X-ray diffraction peak broadening using Debye–Scherrer equation 34 and found to be 14 nm (NMG2) and 21 nm (NMG-3). This result indicates that the NiS particle size significantly decreased with increasing the graphene content which underlines the size controlling effect of graphene nanosheets for these in-situ formed nanoparticles. Because at lower Ni precursor content, better nucleation and interaction with graphene functionalities were expected which in-turn resulted in smaller NiS size. In contrast, at higher Ni precursor loadings, these interactions may not be that influential which may lead to a poor control over the size of the resulting NiS nanoparticles. The morphology and elemental composition of resulting NMG nanohybrids were studied by SEM-EDS as shown in Fig. 2. A highly corrugated and porous structure of the resulting nanohybrids is evident, which is composed of uniformly distributed in-situ formed MoS2 layers within ultrathin graphene layers was observed (Fig.2a). Also, in compared to pristine graphene aerogel, the surface robustness in NMG nanohybrids was clearly noticeable from lower magnification SEM images as showed in Fig.S3. The EDS mapping displayed in Fig.2b further reveals the existence of molybdenum, nickel and sulfur elemental species with uniform distribution.


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Figure 1. XRD patterns of 3D MoS2/G a) and NiS-MoS2/G nanohybrids, NMG-1 b), NMG-2 c), and NMG-3 d). The elemental composition obtained from EDS analysis is tabulated in table-1. Moreover, the Ni-Mo mixed element mapping (2c) clearly evidence that the NiS nanoparticles were in-situ formed in close proximity with Mo sites having uniform distribution with MoS2 nanolayers in the resulting 3D MoS2/ graphene heterostructured nanohybrids. However, highly agglomerated Ni domains were observed in case of NMG-3 (Fig.S3) in compared to NMG-2. This observation clearly indicated that the lower NiS loading can lead to its uniform distribution in resulting nanohybrid which could facilitate the higher HDS catalysis through improved promotion effect. The EDX analysis (Fig. 2d) showed C, O, Mo, S, Ni peaks in the same domain, which indicates that the graphene surface is uniformly decorated with NiS-MoS2 heterostructure assembly without any external impurity. To further explore the microstructure of the resulting nanohybrids, the TEM and HRTEM imaging was used (Fig. 3).


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Figure 2. SEM images a), EDS mapping of MoL, SK, NiL b), mixed Mo-Ni EDS mapping c), and EDAX


10

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spectra d) of NMG-2 nanohybrids

For, 3D MoS2/G, it was clearly evident that the MoS2 sheets were uniformly inserted within the ultrathin graphene surface (Fig.3a). The curved MoS2 layers embedded into dense graphene layers were clearly visible. For NMG-2 nanohybrid (Fig.3b), we notice that there exists a uniform distribution of the particles having sub-50 nm size which is attributed to the NiS particles. In addition, the few-layered MoS2 sheets supported on graphene was also observed (Fig.3c), confirming the successful in-situ formation of NiSMoS2 heterostructured uniformly supported unto 3D graphene support.


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Figure 3. TEM images of 3D MoS2/G a) and NMG-2 (b and c) nanohybrids. Moreover, the resultant selected area electron diffraction (SAED) pattern of NMG-2 nanohybrid clearly reveals the NiS-MoS2 heterostructure as shown in Fig. S4. The observed rings clearly demonstrate a monocrystalline structure and distinct phases of MoS2 and NiS, respectively. For higher NiS loading, the agglomerations of size enhanced NiS nanoparticles were clearly observed (Fig.S5). Hence, the TEM and SAED studies clearly indicated the formation of the distinct phases of individual NiS and MoS2 layers, ruling out the possibility of formation of Ni-Mo-S phase during the synthesis of 3D NiS-MoS2/graphene nanohybrids. The chemical compositions of resulting nanohybrids were further elucidated by XPS analysis. The high resolution XPS survey spectrum (Fig.4 and Fig.S6) of the resulting NiS-MoS2/graphene nanohybrid clearly showed the presence of characteristic C, Mo, S, Ni and O elements. Further, the absence of any other foreign element indicated of the purity of the as-synthesized NiS-MoS2 supported on the graphene surface in the resulting 3D assembly. Moreover, the significant increase in C1s peak intensity with simultaneous decrease of O1s peak, in compared to the corresponding pristine GO peaks, highlight the thermal reduction of GO into graphene.

35

. Also, the deconvoluted XPS spectra of Mo 3d peak indicates

two characteristic peaks at binding energies of 232.3 and 229.2 eV, attributing to the Mo 3d3/2 and Mo 3d5/2 orbitals of Mo in the Mo

4+

oxidation state 36. Furthermore, the deconvoluted S 2p spectra has

resulted in to two peaks at binding energies of 163.2 and 162.2 eV, conforming to the S 2p1/2 and S 2p3/2 orbitals of the divalent sulfide ions (S2–) oxidation state 37, which reveals the co-existence of NiSMoS2 in the resulting 3D graphene frameworks. The characteristic of Ni 2p3/2 and 2p1/2 for NiS containing samples were located around 855 and 876 eV, and the peak around 561 eV corresponds to its “shake-up” which indicated that synthesis of NiS nanoparticles was achieved 38. Similarly, in compared to the pristine GO (285.2 eV), a slight shift in binding energy (0.7 eV) of characteristic C-C and C=C peak


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was observed in 3D NiS-MoS2/graphene hybrid (284.5 eV). Such binding energy shift indicated the presence of highly conductive Sp2 networks that is formed after the reduction of GO. This conductivity enhancement can improve the graphene charge transport ability to MoS2 and thereby increase the overall current density, expecting the higher catalytic activities of the nanohybrid. The surface property features and porosity structures of the 3D NMG heterostructured nanohybrids were also studied and results are shown in Figure 5. The specific surface area obtained from BrunauerEmmett-Teller (BET) isotherm for the resulting 3D NiS-MoS2/graphene hybrids and nano-MoS2 was measured to be 125.2, 117.9, 85.3 and 7.6 m2/g for NMG-1, NMG-2, NMG-3, and Nano-MoS2, respectively (Table 1). It can be noted that the NMG nanohybrids possess significantly higher surface area in compared to the Nano-MoS2 and other reported porous 3D MoS2/graphene structures 39.


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Figure 4. XPS survey spectrum of GO a), 3D MoS2/G b) and NMG-2 c), and high resolution spectra of Mo 3d, Ni 2p and S 2p, respectively for NMG-2 nanohybrid. It can be seen that the presence of graphene has a significant influence on the increased surface area for NMG nanohybrids. For nanohybrids, the content of NiS nanoparticles is controlling the surface area; higher loading tends to lower the surface area. The 3D NiS-MoS2/graphene hybrid (NMG-2) display the features of type IV IUPAC isotherms having a very small H3 hysteresis loop, indicating that the pores are mainly mesopores and originating from the increased volume mesoporous channels in the resulting 3D architecture (Fig.5a). Further, he pore-size distribution of NMG-2 nanohybrid obtained from BJH method showed one sharp distribution around 17 nm and one broad distribution ranging from 25 to 45 nm,


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indicating the presence of mesopores, respectively (Inset of Fig. 5A). Hence, the superior BET specific surface area for NiS-MoS2/graphene signifies the development of highly porous networks during the thermal treatment that resulted into a unique 3D structure for these hybrid materials.

Figure 5. Nitrogen adsorption–desorption isotherms and pore size distribution of (a) NMG-2 and (b) nano MoS2.

Table 1. The BET surface area, BJH pore diameter, pore volume properties and elemental analysis determined by EDS of the Nano-MoS2 and NMG nanohybrids

Sample

BET surface area (m2/g)

Pore diameter (nm)

Pore volume (cm3/g)

Atomic ratio (Ni:Mo)*

Mo (wt%)

S (wt%)

Ni (wt%)

MoS2 (wt%)

Nano MoS2

7.6

1.86

0.094

-

57.10

42.90

-

100

NMG-1

125.2

14.81

0.226

0.029

35.96

24.50

0.64

60.1

NMG-2

117.9

17.12

0.202

0.148

35.99

25.73

1.34

59.8

NMG-3

85.3

20.09

0.154

0.586

34.04

35.03

12.23

59.4

*Atomic ratio, determined by XPS analysis


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The Raman spectroscopic studies were also carried out to confirm the successful fabrication of the heterostructured 3D NiS-MoS2/graphene (Fig.6). The nanohybrid exhibit two characteristic Raman peaks corresponding to 371 and 395 cm−1 , ascribed primarily due to the in-plane E12g and out-ofplane A1g vibrational modes arises from the MoS2 crystal, respectively

40

. Moreover, for NMG-2

nanohybrid, a decreasing difference from 27 to 19 cm−1 within peaks of E12g and A1g was noted in comparted to the pure MoS2 associated peaks at 376 and 403 cm−1 as showed in Fig.S6. This difference indicated the increased interfacial affinity between Mo precursors and GO which could effectively minimize the aggregation in the layered MoS2

41

. Moreover, these results further confirm that the

edge-exposed MoS2 slabs were uniformly distributed on graphene support via a layer confinement method. The Raman peak centered at 220 cm−1 is ascribed to the NiS and other characteristic peaks at 335 and 376 cm−1 are not apparent, which could be due to the possible overlapping with the peaks of MoS2.

Figure 6. Raman spectra of NMG-2 nanohybrids a) and NH3-TPD profile of NMG nanohybrid catalysts b). In addition, graphene also shows characteristic Raman bands arising at 1351 cm-1(D-band) and about 1578 cm−1 (G-band). The G-band is known for its first-order scattering of the E2g mode arising from sp2 carbon network, whereas the D band is arising from vibrations of sp3 carbon atoms located at the


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defective edge sites from graphene

42

. It is well know that the graphitization degree in carbon based

materials is associated with the Raman D-band and G-band peak ratio i.e ID/IG ratio. Hence, in present study, the increased in D/G ratio from 0.96 (GO) [Fig. S5) to 1.52 (NiS-MoS2/Graphene) attributed to the significant reduction of GO i.e increase in graphitization under hydrothermal process, which is further reduced under thermal annealing. The reduction of GO to graphene will improve the electron mobility, which can promote the effective enhancement of HDS catalytic process. This also signifies the increased interphasic attraction within the graphitic plane of graphene and in-situ formed NiS- MoS2 nanostructures. Moreover, a downshift of Raman G band by around 6 cm−1 was observed for NiSMoS2/Graphene, compared graphene (Fig.S7), indicates the possible charge transfer from graphene to MoS2 43. It was well demonstrated that the higher catalyst acidity can lead to enhanced HDS performance44. Hence, the temperature-programmed desorption of ammonia (NH3-TPD) analysis was performed in order to measure the strength of various acidic sites in NMG nanohybrids. The resulting NH3-TPD data for the NMG nanohybrids are plotted in Fig. 6b. It can be seen that the noticeable changes were observed in terms of NH3 desorption among these hybrids and the acidities can be categorized as the weak (

Graphene heterostructured

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