Thermally Stable Hierarchical Nanostructures of Ultrathin MoS2Nanosheet-Coated CeO2 Hollow Spheres as Catalyst for AmmoniaDecompositionXueyun Gong,†,‡,§ Ying-Qiu Gu,∥ Na Li,† Hongyang Zhao,† Chun-Jiang Jia,*,∥ and Yaping Du*,†,§

†Frontier Institute of Science and Technology jointly with College of Science, State Key Laboratory for Mechanical Behavior ofMaterials, Xi’an Jiaotong University, Xi’an 710054, China‡College of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454000, China§Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry &Materials Science, Northwest University, Xi’an 710069, China∥Key Laboratory for Colloid and Interface Chemistry, Key Laboratory of Special Aggregated Materials, School of Chemistry andChemical Engineering, Shandong University, Jinan 250100, China

*S Supporting Information

ABSTRACT: MoS2 ultrathin nanosheet-coated CeO2 hollow sphere (CeO2@MoS2) hybrid nanostructures with a 3D hierarchical configuration weresuccessfully constructed from a facile two-step wet chemistry strategy: first,CeO2 formed on a silica core which served as a template and was subsequentlyremoved by NaOH solution to attain hollow spheres, and then few-layeredultrathin MoS2 nanosheets were deposited on the CeO2 hollow spheresthrough a hydrothermal process. As a proof of concept application, the as-prepared CeO2@MoS2 hybrid nanostructures were used as catalytic material,which exhibited enhanced catalytic activity in ammonia decomposition for H2production at high temperature. It was demonstrated that, even with astructural transformation from MoS2 to MoNx under harsh conditions ofammonia decomposition at high temperature (700 °C), the 3D hierarchicalnanostructures of the CeO2@MoNx were well kept, indicating the importantrole of the CeO2 support.

■ INTRODUCTION

In recent years, two-dimensional (2D) layer structurednanomaterials have attracted increasing interest due to theirunique properties and promising applications.1−8 As animportant two-dimensional (2D) layer structured nanomaterial,MoS2 has been widely used for diverse applications, such ascatalysis,9−14 solid-state lubrication,15 energy storage,16 tran-sistors,17,18 sensing,19−21 and lithium batteries.22−24 As far asthe catalysis applications for MoS2 are concerned, boththeoretical calculations25 and experimental results26 haveindicated that the catalytic activity mainly originated fromactive sites located along the edges of MoS2 layers. However,the freshly prepared MoS2 layers would easily tend torestacking and aggregation, especially during the high-temper-ature reaction like ammonia decomposition, because of theirhigh surface energy and interlayer van der Waals attractions,which severely reduced the active sites and damped theperformances concerning the practical applications. Therefore,how to avoid the stacking and aggregation, thus increasing theexposed active sites, is of special importance. Practically,rational design and synthesis of three-dimensional (3D)hierarchical architectures based on 2D nanosheets is aneffective approach for preventing stacking and aggregation,

thus opening the door of realizing practical applications of thiskind of nanostructure.27−29 Ammonia (NH3), which can beeasily liquefied under moderate conditions, has been consideredas an excellent storage medium for hydrogen compared withtraditional carbonaceous materials.30 Catalytic ammoniadecomposition is an efficient method to generate COx-freehydrogen. Up to now, Ru has been found to be the bestammonia decomposition catalyst.31 Transition metals based onFe and Ni have also been investigated as excellent catalysts forNH3 decomposition.

32−35 Recently, molybdenum nitrides werefound exhibiting high performance as catalyst for catalyticdecomposition of ammonia to produce COx-free hydrogen.36

Molybdenum nitrides can be obtained by ammonolysis ofMoS2,

37 which gave the possibility for MoS2 used for catalyticammonia decomposition.Ceria (CeO2) has been studied intensively for its unique

properties and found to act as a highly effective support orpromoter for catalytic ammonia decomposition to hydro-gen.38,39 Up to now, diverse morphologies of CeO2nanostructures have been fabricated, such as nanocubes,40

Received: February 1, 2016

Article

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© XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.6b00265Inorg. Chem. XXXX, XXX, XXX−XXX

nanorods,41,42 nanotubes,43−45 etc. Among the variousmorphologies, the porous hollow spheres46,47 with remarkableinterior space and pore structure are particularly attractivebecause of their low density, high specific surface area, andsuperior permeation.Herein, for the first time, we reported the growth of 2D

ultrathin MoS2 nanosheets on uniform CeO2 porous hollowspheres to form ultrathin MoS2 nanosheet−CeO2 hollowsphere hybrid nanostructures with a 3D hierarchical config-uration, which is denoted as CeO2@MoS2. The CeO2@MoS2hybrid nanostructures and MoS2 nanosheets were used ascatalysts for the production of hydrogen via catalyticdecomposition of ammonia. It is found that, even through astructural transformation from MoS2 to MoNx under a harshcondition of ammonia decomposition at high temperature (700°C), the 3D hierarchical nanostructures of the CeO2@MoNxwere kept well (Scheme 1), indicating the important role of the

CeO2 support, which induced the enhanced catalytic activity.However, the pure MoS2 nanosheets exhibited poor catalyticactivity due to the serious aggregation during the catalyticprocess. Although the synergistic effect between Mo- and Ce-based materials is still not clear, the design and synthesis ofhybrid hierarchical materials spot a light on the preparation ofhighly efficient and stable catalysts in heterogeneous catalysis.

■ EXPERIMENTAL SECTIONMaterials and Reagents. Cerium(III) nitrate hexahydrate

(Ce(NO3)3·6H2O, 99.99%, Sigma-Aldrich), tetraethylorthosilicate(Si(OC2H5)4, 98%, Acros Organics), sodium molybdate (Na2MoO4·2H2O, 99%, Fluka), thioacetamide (CH3CSNH2, 98%, Sigma-Aldrich),ammonium hydroxide (NH3·H2O, 25%), absolute ethanol (C2H6O,99.7%), sodium hydroxide (NaOH, 96%), and ethylene glycol(C2H6O2) were purchased from Tianjin Zhiyuan Chemical Company.All chemicals were used without any further purification. Deionizedwater was used throughout this study.Synthesis of CeO2 Hollow Spheres. The CeO2 hollow spheres

were synthesized via a slightly modified method developed by Stuckyet al.47 In a typical procedure, a mixture of deionized water (56 mL)and ammonium hydroxide (8.4 mL) was added into a flask containingthe mixture of ethanol (280 mL) and tetraethylorthosilcate (8 mL)under vigorous stirring at room temperature for 24 h; the productswere then collected by centrifugation (8500 rpm), followed by dryingat 60 °C for 6 h. A 100 mg portion of dried SiO2 was dispersed in 13mL of ethylene glycol with ultrasonication in a beaker. Then, 0.5 g ofcerium nitrate hexahydrate and 0.75 mL deionized water were added,and the mixture was stirred for 30 min to form a homogeneoussolution. The mixture was then transferred to a 25 mL Teflon-lined

stainless steel autoclave and heated in an electric oven at 130 °C for 24h. The autoclave was cooled down to room temperature. The SiO2@CeO2 particles were collected by centrifugation (10 000 rpm) andwashed with absolute ethanol. The products were dispersed in 5 mol/L NaOH solution for 2 days to attain CeO2 hollow spheres for furtheruse.

Synthesis of MoS2 Nanosheets. In a typical procedure, 50 mg ofsodium molybdate, 100 mg of thioacetamide, and 14 mL of deionizedwater were added into a 25 mL Teflon-lined stainless steel autoclave.After stirred for 30 min, the suspension solution in Teflon-linedstainless steel autoclave was heated in an electric oven at 200 °C for 24h. The autoclave was then cooled down to room temperature. Theblack precipitate was collected by centrifugation, washed thoroughlywith absolute ethanol, and dried at 65 °C overnight in vacuum oven.The as-prepared MoS2 nanosheets were further treated at 600 °C in Aratmosphere for 4 h to obtain high crystallinity.

Synthesis of CeO2@MoS2 Hybrid Nanostructures. In a typicalprocedure, a 20 mg portion of the obtained CeO2 hollow spheres wasdispersed in 14 mL of deionized water with ultrasonication for 30 min.Then, 50 mg of sodium molybdate and 100 mg of thioacetamide wereadded. After being stirred for 30 min, the suspension solution wastransferred into a 25 mL Teflon-lined stainless steel autoclave and thenheated in an electric oven at 200 °C for 24 h. The autoclave was thencooled down to room temperature. The black precipitates werecollected by centrifugation, washed thoroughly with absolute ethanol,and dried at 65 °C overnight in a vacuum oven. The as-preparedCeO2@MoS2 hybrid nanostructures were further treated at 600 °C inAr atmosphere for 4 h to obtain high crystallinity.

Characterization. The morphologies of the as-obtained productswere examined by the transmission electron microscopy (TEM,Hitachi HT-7700) and scanning electron microscopy (SEM, FEIQuanta F250). The detailed microscopic structure and the chemicalcomposition of the products were analyzed using high-resolutiontransmission electron microscopy (HRTEM, Tecnai G2 F20 S-TWIN)operated at 200 kV. The crystallinity and crystal phase of the driedpowders of samples were examined by X-ray diffraction (XRD, RigakuD/MAX-RB) with a scanning rate of 5°/min from 10° to 80°, usingCu Kα radiation (λ = 1.5418 Å). Nitrogen adsorption−desorptionisotherms were measured on a Micromeritics TriStar 3000porosimeter (mesoporous characterization) and Micromeritics ASAP2020 (microporous characterization) at 77 K. The samples wereoutgassed at 60 °C for 12 h under vacuum prior to measurements. Thespecific surface areas were calculated on the basis of the Brunauer−Emmett−Teller (BET) method, and the pore size distributions weremeasured from the desorption branch of isotherms using the Barrett−Joyner−Halenda method (BJH). Energy dispersive X-ray spectrometry(EDX) characterizations were performed on a transmission electronmicroscope (Tecnai G2 F20 S-TWIN). X-ray photoelectron spectros-copy (XPS) data were obtained using a Escalab 250 xi photoelectronspectrometer using Al K radiation (15 kV, 225 W, base pressure ∼5 ×1010 Torr).

In Situ XRD Experiments. In situ XRD experiments underhydrogen atmosphere were carried out on a PANalytical X’Pert3

Powder diffractometer operating in reflection mode with Cu Kαradiation (λ = 1.54178 Å, 40 kv, 40 mA). The powder sample wasloaded into a ceramic tube which was attached to an in situ flow cell(Anton Paar HRK-900 reaction cell). A small resistance heating wirewas installed right below the tube, and the temperature was monitoredwith a chromel−alumel thermocouple that was placed inside the tubenear the sample. The in situ XRD tests (5% H2/Ar, 30 mL/min) werecarried out following a temperature-programmed mode: 25 °C → 100°C → 200 °C → 300 °C → 400 °C → 500 °C → 600 °C → 700 °C→ 800 °C → cool down (ramping rate: 30 °C/min) with stabilizationat each temperature for 60 min. Data were collected with a step widthof 0.013°, and a counting time of 50 s per step (20 min/run). Dataobtained from the last run at each temperature was used for plotting.

Catalytic Testing. For a typical ammonia (NH3) decompositionexperiment, 100 mg (20−40 mesh) of the catalyst was loaded in aquartz tube (I.D. = 6 mm) fixed bed reactor, and pure gaseous NH3was passed through the catalyst bed. The reactor temperature was

Scheme 1. Schematic Illustration for the Synthesis of CeO2@MoS2 Hybrid Nanostructures and Its Use in CatalyticDecomposition of Ammonia for H2 Production

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increased from 400 to 700 °C in 50 °C steps. At each step, the reactionwas allowed to equilibrate for 60 min to reach the steady-stateconditions, and data obtained from the last gas chromatography run ateach temperature were used to calculate the conversion value.Reaction temperature was maintained at 700 °C for 58 h to evaluatethe stability of the catalyst, and the NH3 conversion was recordedcontinuously. After the stability test, the temperature was decreased toambient temperature under NH3 flow and then increased from 400 to700 °C in 50 °C steps. The NH3 conversion data were collected againduring the heating procedure. The concentrations of outlet gases wereanalyzed by an online gas chromatograph (Ouhua GC 9160), whichwas equipped with a thermal conductivity detector (TCD) andPorapark Q column (1.5 m of length).

■ RESULTS AND DISCUSSION

The crystal phases of as-harvested CeO2@MoS2 hybridnanostructures, CeO2 hollow spheres, as well as ultrathin

MoS2 nanosheets were examined with powder X-ray diffractionanalysis (XRD, Figure 1a). The diffraction peaks of CeO2

hollow spheres can be readily indexed to the cubic phase withlattice constant of a = b = c = 5.41 Å (space group Fm3 m,JCPDS 34-0394). As for the pure ultrathin MoS2 nanosheets,the crystal phase mainly consists of hexagonal MoS2, but thediscernible peaks cannot match the standard MoS2 card(JCPDS 37-1492) exactly. The diffraction peaks correspondingto MoS2 were not observed in the XRD pattern of CeO2@MoS2 hybrid nanostructures, indicating that MoS2 nanosheetscoating on the CeO2 hollow spheres may consist of only fewlayers, which are too thin to be detected.9,22,48−51

Due to the reducibility of produced H2 in catalytic NH3

decomposition conditions, it is necessary to monitor the crystalphase changes of catalyst under H2 atmosphere at the reactiontemperature. The corresponding in situ XRD results were

Figure 1. (a) XRD patterns of CeO2@MoS2 hybrid nanostructures, CeO2 hollow spheres, and pure ultrathin MoS2 nanosheets. In situ XRD patternscollected under hydrogen atmosphere 5% H2 in Ar of (b) pure MoS2 nanosheets and (c) CeO2@MoS2 hybrid nanostructures.

Figure 2. SEM images of (a) CeO2 hollow spheres and (b) CeO2@MoS2 hybrid nanostructures. TEM images of (c) CeO2 hollow spheres and (d)CeO2@MoS2 hybrid nanostructures and (e, f) HRTEM images. (g) HAADF-STEM image of CeO2@MoS2 hybrid nanostructures.

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shown in Figure 1b,c. The diffraction peaks of pure MoS2nanosheets (Figure 1b) cannot match the standard MoS2exactly at room temperature; with the temperature increase,the peak shift occurred. When the temperature was higher than300 °C, the detected peaks can be assigned to the (002), (100),(103), and (110) planes in the hexagonal MoS2 phase. As forCeO2@MoS2 hybrid nanostructures (Figure 1c), diffractionpeaks assigned to hexagonal MoS2 phase also appeared whenthe temperature was higher than 200 °C. The above resultsdemonstrated that the deviation of ultrathin MoS2 nanosheetsdiffraction peaks from the standard card may be caused by thelow crystallinity.52 The crystallinity was improved as thetemperature increased and the low-crystalline MoS2 nanosheetstransformed to pure hexagonal MoS2 phase finally.The morphology of as-obtained CeO2 hollow spheres and

CeO2@MoS2 hybrid nanostructures is shown in Figure 2a,b,respectively. As seen from the scanning electron microscopy(SEM) image of Figure 2a, the CeO2 hollow spheres werecomposed of small CeO2 particles. Figure 2b showed the SEMimage of CeO2@MoS2 hybrid nanostructures. As seen fromFigure 2b, the MoS2 nanosheets were uniformly coated on thesurfaces of CeO2 hollow spheres to form a 3D hierarchicalconfiguration. The inset in Figure 2b illustrated that theCeO2@MoS2 hybrid nanostructures possessed an inner hollowstructure. The structures of as-synthesized CeO2 hollowspheres and CeO2@MoS2 hybrid nanostructures were furtherstudied by transmission electron microscopy (TEM) (Figure2c,d). From Figure 2c it can be seen that the average thicknessof CeO2 was approximately ∼40 nm and the diameter of thehollow spheres was ∼260 nm. Figure 2d exhibited the typicalTEM image of CeO2@MoS2 hybrid nanostructures; it wasclearly demonstrated that the ultrathin MoS2 nanosheets havecovered the surface of CeO2 hollow spheres. As seen fromFigure S1, the pure MoS2 obtained by the same hydrothermalmethod without the presence of CeO2 hollow spherespresented the nanosheet morphology. From the high-resolutiontransmission electron microscopy (HRTEM) images of CeO2@MoS2 hybrid nanostructures in Figure 2e,f, the lattice fringeswere clearly visible with a spacing of ∼0.27 nm, correspondingto the spacing of the (200) planes of CeO2, and the latticefringes with a spacing of ∼0.27 nm and ∼0.64 nmcorresponded to the (100) and (002) plane of hexagonalMoS2 (Figure 2f). High-angle annular dark-field scanning TEM(HAADF-STEM) image in Figure 2g indicated that the CeO2@MoS2 hybrid nanostructures assumed the well-defined 3Dhierarchical morphology.Meanwhile, energy dispersive X-ray spectrometry (EDX)

mapping analysis of the CeO2@MoS2 hybrid nanostructuresalso verified the core (CeO2 hollow spheres) and shell (MoS2nanosheets) hierarchical structure (Figure 3). From theelemental mapping images, it can be seen that the innerhollow core consists of Ce and O elements, and Mo and Selements are homogeneously dispersed in the shell, furtherconfirming that the ultrathin MoS2 nanosheets were uniformlycoated on the surfaces of CeO2 hollow spheres.X-ray photoelectron spectroscopy (XPS) was applied to

investigate the chemical states of Ce, O, Mo, and S in theCeO2@MoS2 hybrid nanostructures (Figure 4). As shown inFigure 4a, the Ce 3d electron core level was characterized bytwo series of peaks: 3d5/2 and 3d3/2. For 3d5/2 of Ce(IV), thepeaks of the Ce 3d94f2Ln−2 (882.8 eV), Ce 3d94f1Ln−1 (887.3eV), and Ce 3d94f0Ln (899.0 eV) states were labeled v, v2, andv3, respectively. For 3d5/2 of Ce(III), the Ce 3d

94f2Ln−1 (882.8

eV) and Ce 3d94f1Ln (885.5 eV) states corresponded to vo andv1. The u (u = u, u0, u1, u2, u3) structures corresponding to theCe 3d3/2 levels could be characterized by the same assign-ment.53,54 Therefore, the XPS results indicated that Ce in theas-synthesized CeO2@MoS2 hybrid nanostructures was mainlycerium(IV) and cerium(III). The fitting of the O 1s region withtwo-peak contribution elucidated that at least two kinds ofoxygen species were present in the near surface domain ofCeO2@MoS2 hybrid nanostructures (Figure 4b). The peak atabout 530.0 eV was due to crystal lattice oxygen of CeO2, whilethe peak at about 532.5 eV was due to chemisorbed oxygen onthe hybrid nanostructure surfaces. The binding energies of Mo3d3/2 and Mo 3d5/2 peaks were located at 231.9 and 228.7 eV,respectively, suggesting that the oxidation state of the Mo ion ispositive quadrivalence in the synthesized CeO2@MoS2 hybridnanostructures (Figure 4c).28 Meanwhile, the peaks at 161.6and 162.6 eV in Figure 4d could be assigned to the bindingenergies of S 2p3/2 and S 2p1/2, respectively; thus, the S wasnegative divalent in the CeO2@MoS2 hybrid nanostructures.Full nitrogen sorption isotherms of as-synthesized CeO2@

MoS2 hybrid nanostructures were recorded to attaininformation on the pore size distribution and specific surfacearea. A type IV nitrogen isotherm shown in Figure 5a suggesteda characteristic of mesoporous materials.55 Accordingly, theBrunauer−Emmett−Teller (BET) specific surface area ofCeO2@MoS2 hybrid nanostructures was about 54.9 m2 g−1.The pore size distribution derived from adsorption data andcalculated by the Barrett−Joyner−Halenda (BJH) method wasshown in Figure S2. The plot indicated that most pores were inthe mesoporous structure with the pore diameter ofapproximately ∼4 nm. Such a high specific area of hollowstructures with mesoporous structure could be desirable forcatalytic applications, which may facilitate the transportation ofmass and reactants.The catalytic activity of each sample for the hydrogen

production via decomposition of ammonia was tested in twoidentical runs, in which the first heating run was regarded as the

Figure 3. EDX elemental mapping images of Ce, O, Mo, and S in theselected area showing in Figure 2g.

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self-activation procedure. The catalytic tests were performedover 100 mg of CeO2@MoS2 samples with ammonia (NH3)space velocity of 12 000 cm3 gcat

−1 h−1. After activation, thecatalytic performance of CeO2@MoS2 hybrid nanostructuresfor NH3 decomposition reaction, tested as a function oftemperature, was presented in Figure 5b. As can be seen, theactivity in terms of NH3 conversion increased with elevating thereaction temperature. The content of MoS2 in CeO2@MoS2hybrid nanostructures was ca. wt 62%; however, the NH3

conversion of CeO2@MoS2 hybrid nanostructures wasobviously higher than that of pure MoS2 nanosheets whenthe same amount of catalysts was adopted (100 mg), indicatingCeO2 serves as an excellent promoter which could effectivelyimprove the catalytic activity of MoS2. To further demonstratethe high-temperature stability of catalysts, long-durationstability tests were performed for pure MoS2 nanosheets andCeO2@MoS2 hybrid nanostructures at 700 °C with a gashourly space velocity (GHSV) of 12 000 cm3 gcat

−1 h−1 over 58h. As depicted in Figure 5c, the catalytic activity of CeO2@MoS2 hybrid nanostructures was much higher than that of pureMoS2 nanosheets during the whole process. The conversion oftwo samples remained increased over the initial 34 h andsubsequently stabilized at nearly 100% conversion, indicatingthat nitridation of MoS2 to molybdenum nitride was a very slowprocess. Figure 5d showed the comparison of catalytic activityfor CeO2@MoS2 hybrid nanostructures between activation by afirst heating procedure (Figure 5b) and by the stability test at700 °C for 58 h. The NH3 conversion of the latter was

obviously higher than that of the former, demonstrating thatthe transformation of MoS2 to molybdenum nitride in Figure5b was incomplete and a simple activated process would notfully exhibit the actual catalytic ability of catalysts. It may be thereason that the catalytic activity of CeO2@MoS2 hybridexhibited lower activity compared with the previously reportedMoOx catalyst.

56

To investigate the phase changes, structure, and texture ofthe CeO2@MoS2 hybrid nanostructures, characterizations onthe reacted catalysts after the NH3 decomposition have beencarried out. As seen from Figure 6a, the crystal phases of theused CeO2@MoS2 hybrid nanostructures and pure ultrathinMoS2 nanosheets after a stability test were examined with XRD.Ammonolysis of MoS2 at 700 °C for 58 h led to a mixture ofthe molybdenum nitride phase, and the XRD diffractionpatterns were assigned to hexagonal MoN (JCPDS 25-1367)and hexagonal Mo5N6 (JCPDS 51-1326), respectively. Therestill exists unreacted molybdenum sulfide phase whichcorresponded to monoclinic Mo2S3 (JCPDS 40-972). ForCeO2@MoS2 hybrid nanostructures, molybdenum sulfidetransformed to molybdenum nitride completely after reactionat 700 °C for 58 h, and CeO2 phase was transformed tohexagonal Ce2O2S (JCPDS 04-626). For Mo-based catalysts,Tagliazucca et al. have performed in situ XRD to study thephase transformations of MoO3 catalyst during the NH3

decomposition reaction.56 The phase transformations formolybdenum oxide during the NH3 decomposition processwas very complex because nitridation of molybdenum and

Figure 4. XPS spectra of CeO2@MoS2 hybrid nanostructures: (a) Ce 3d, (b) O 1s, (c) Mo 3d, and (d) S 2p.

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reduction of molybdenum oxide occurred simultaneously. Theyconcluded that many factors, including the active phase,domain size, and surface species/area, could influence theactivity of the catalyst. It would be too simple to correlate thecatalytic activity only to the phase composition. For the CeO2@MoS2 hybrid, the phase transformation was more complicatedthan that for MoO3, so it is impossible to distinguish thecontribution of each component. The morphology of CeO2(Ce2O2S) hollow spheres had no substantial changes due to itsexcellent thermal stability (Figure 6b). However, pure MoS2(MoNx) nanosheets seemed to severe restacking andaggregation after the harsh reaction conditions (temperatureup to 700 °C, large concentration of ammonia and/orhydrogen) as can be observed from the TEM and HAADF-STEM (inset) image in Figure 6c. For CeO2@MoS2 (Ce2O2S@MoNx) hybrid nanostructures (Figure 6d), the CeO2 (Ce2O2S)hollow structure was well-maintained, and no major agglom-eration occurred in the MoS2 (MoNx) nanosheets coated onthe surfaces of CeO2 (Ce2O2S) spheres. Noticeably, even afterreaction at 700 °C for 58 h, the morphology of CeO2@MoS2(Ce2O2S@MoNx) hybrids were still well-maintained (STEMimage in inset of Figure 6d). Meanwhile, the specific surfacearea of the used CeO2@MoS2 (Ce2O2S@MoNx) hybridnanostructures calculated from nitrogen sorption isothermswas 37.5 m2g−1. It has been reported that CeO2 acts as adispersant in the Ni−Al catalyst system and supplies “spacers”to inhibit the mobility of Ni atoms, preventing the particles

from aggregating.38 In the CeO2@MoS2 hybrid nanostructures,we believe that the CeO2 spheres inside can also enhance thedispersion of the active molybdenum sites and suppress MoS2nanosheets from restacking and aggregation under harshreaction conditions. Also, the possible electron transfer onthe Ce−Mo interface could be accelerated, through which theperformance of the CeO2@MoS2 hybrid nanostructures forcatalytic decomposition of ammonia was promoted.In addition, XPS was applied to investigate the surface state

of the corresponding catalyst after the stability test of Ce, O, S,Mo, and N in CeO2@MoS2 hybrid nanostructures (FigureS3a−e) and pure MoS2 nanosheets (Figure S4a−c). As shownin Figure S3a, only v (v = vo, v1) and u (u= uo, u1) forcharacteristics of Ce(III), which could be observed in the Ce 3dXPS spectrum, indicate that the oxidation state of Ce(IV) wasreduced to Ce(III).54 Meanwhile, XRD (Figure 6a) indicatedthat CeO2 (Ce(IV)) was reduced to Ce2O2S (Ce(III)) byammonolysis. The fitting of the O 1s region with the peak atabout 530.2 eV is due to the crystal lattice oxygen of Ce2O2S(Figure S3b). Meanwhile, the discernible peak at 161.5 eV inFigure S3c could be assigned to the binding energies of S 2p3/2of Ce2O2S. Figure S3d showed the peak at 229.8 eV (Mo4+

3d5/2), 232.28 eV (Mo6+ 3d5/2), and 235.38 eV (Mo6+ 3d3/2),suggesting the mixed valence state of the Mo species.57 Thepeaks at 394.4 and 397.9 eV can be assigned to Mo 3p3/2 and N1s, respectively, which were characteristic of molybdenumnitride (Figure S3e),58 indicating that the molybdenum sulfide

Figure 5. (a) N2 adsorption/desorption isotherms of CeO2@MoS2 hybrid nanostructures. (b) Temperature dependent NH3 conversion at a GHSVof 12 000 cm3 gcat

−1 h−1 after activation by a first heating process. (c) Long-term catalytic stability of the catalysts measured at 700 °C at a GHSV of12 000 cm3 gcat

−1 h−1 (100 mg, 20 mL/min). (d) Comparison of catalytic activity for CeO2@MoS2 hybrid nanostructures between activation by afirst heating procedure and by a stability test at 700 °C for 58 h.

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transformed to molybdenum nitride after reaction at 700 °C for58 h. Figure S4a−c showed the XPS signals taken from the Mo3d, S 2p, Mo 3p, and N 1s regions of the corresponding catalystafter the stability test of pure MoS2 nanosheets. As seen fromFigure S4a, XPS signals at 235.6 and 232.5 eV corresponded toMo6+ 3d3/2 and Mo6+ 3d5/2, respectively. The peaks at 228.8 eVin Figure S4a could be assigned to the binding energies of Mo4+

3d5/2 of MoS2. Double peaks at 162.8 and 161.7 eV (FigureS4b), attributable to the core levels of S 3p1/2 and S 3p3/2,respectively, were characteristic of S2− in MoS2. While the peakslocated at 394.3 and 397.8 eV were ascribed to the levels of Mo3p3/2 and N 1S of molybdenum nitride (Figure S4c), it isnoticed that the XRD results showed the samples after thecatalytic tests were mainly composed of Mo2S3 and MoNx,indicating that Mo should be reduced from +4 to even loweroxidation state in the reductive reaction condition. However,Mo6+ was observed from the XPS spectra, which is perhapsbecause the surface of the sample was oxidized when it wasexposed to air since XPS is a surface-sensitive technique.

■ CONCLUSION

In summary, for the first time, we reported the synthesis of 2Dultrathin MoS2 nanosheets grown on uniform CeO2 hollowspheres to form CeO2 hollow spheres@MoS2 nanosheetshybrid nanostructures with a 3D hierarchical configuration. Theas-prepared CeO2@MoS2 hybrid nanostructures possessed high

surface area and exhibited enhanced activity in catalyticammonia decomposition for H2 production. The preparationof CeO2@MoS2 hybrid nanostructures and use of it as catalystfor ammonia decomposition may pave the way toward facileand efficient production of ultrathin transition metaldichalcogenides and rare earth materials-based catalysts withexcellent performance.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.inorg-chem.6b00265.

Figures S1−S4 (TEM, pore size distribution, and XPSdata) (PDF)

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: jiacj@sdu.edu.cn.*E-mail: ypdu2013@mail.xjtu.edu.cn.

Author ContributionsX.G., Y.-Q.-G., and N.L. contributed equally to this work.

NotesThe authors declare no competing financial interest.

Figure 6. TEM images of the reacted (a) XRD patterns of the reacted CeO2@MoS2 (Ce2O2S@MoNx) hybrid nanostructures and pure MoS2nanosheets after stability test. (b) CeO2 (Ce2O2S) hollow spheres, (c) MoS2 (MoNx) nanosheets, and (d) CeO2@MoS2 (Ce2O2S@MoNx) hybridnanostructures after two heating runs (insets in c and d represent HAADF-STEM images of corresponding catalyst after stability test).

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■ ACKNOWLEDGMENTS

We gratefully acknowledge the financial aid from the start-upfunding from Xi’an Jiaotong University, the FundamentalResearch Funds for the Central Universities (2015qngz12),Fundamental Research Funding of Shandong University (Grant2014JC005), the Taishan Scholar project of Shandong Province(China), the NSFC (Grants 21371140, 21301107), theNational Science Funds of China for Excellent Young Scientists(Grant 21522106), and the Open Foundation of KeyLaboratory of Synthetic and Natural Functional MoleculeChemistry of Ministry of Education (Grant 338080051). Wealso appreciated Dr. Xinghua Li at Northwest University for hiskind help to obtain HRTEM images.

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Inorganic Chemistry Article

DOI: 10.1021/acs.inorgchem.6b00265Inorg. Chem. XXXX, XXX, XXX−XXX

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