Nanoscale

PAPER

Cite this: DOI: 10.1039/c8nr00532j

Received 19th January 2018,Accepted 2nd March 2018

DOI: 10.1039/c8nr00532j

rsc.li/nanoscale

Synthesis of ultrathin Ni nanosheets forsemihydrogenation of phenylacetylene to styreneunder mild conditions†

Jing-Wen Yu,‡a Xin-Yu Wang,‡a Chen-Yue Yuan,a Wei-Zhen Li,b Yu-Hao Wanga andYa-Wen Zhang *a

The synthesis of ultrathin metal nanosheets (NSs) attracts broad scientific and technological interest, and

it still remains a challenge for non-noble metals like nickel due to their intrinsic cubic symmetry and high

surface energy. Herein, we report a NiO intermediated solvothermal method towards the synthesis of

ultrathin Ni NSs (thickness < 3 nm) using N,N-dimethylformamide as the solvent and n-butylamine as the

shape controlling reagent. The growth of the ultrathin Ni NSs follows an intermediate mechanism which

was proved by the results obtained by means of transmission electron microscopy (TEM), X-ray diffraction

(XRD), X-ray absorption spectra (XAS) and X-ray photoelectron spectroscopy (XPS). Under solvothermal

conditions, the nickel acetylacetonate precursor was first reduced to a NiO NS intermediate, then

reduction occurred and NiO NSs were reduced to Ni NSs. The synthesized ultrathin Ni NSs predominately

in a metallic state showed high selectivity (88.0–92.0%) towards styrene (ST) in the phenylacetylene (PA)

semihydrogenation reaction under mild conditions (323 K, 1 atm of hydrogen) in a broad PA conversion

range (2.0–98.0%). The low coverage of oxygen atoms on the Ni NS surface is proposed to account for

the high ST selectivity, as indicated by density functional theory (DFT) calculations.

Introduction

Transition metals such as Mn, Fe, Co, and Ni in the fourthperiod of the periodic table are indispensable materials formodern technologies. Related materials are applied to variousareas such as microelectronic devices, magnetic recordingmedia, lithium batteries, catalysis, and superconductors.1–6

Among them, Ni is considered as a potential substitute fornoble metal catalysts like Pt and Pd for its fine catalytic activityin hydrogenation reactions.7–10 Therefore, diverse methodshave been developed for the synthesis of Ni based nano-materials, such as electrodeposition, borohydride reduction ofmetal salts, thermal decomposition of metal complexes,etc.3,11,12 Multiple Ni nanomaterials with various morphologies

were synthesized, including nanotubes, nanorods, nanowires,monodisperse nanoparticles, etc.13–16

The fabrication of ultrathin (several or even one atom layerthick) two-dimensional (2D) nanomaterials has been attractinggreat attention in the past few years since 2D nanomaterialswere proved to have distinct physical and chemical properties.However, an ultrathin structure would severely increase thesurface free energy of nanomaterials,17 and only limitedachievements have been reported so far. The 2D metalnanosheets (NSs) acquired at present are mainly noble metalNSs, including monometallic Au,18 Pd,19,20 Rh,21–23 Ru,24 andIr23 and bimetallic Pt–Cu,25 Pd–Ag,26 Pd–Au,27 Au–Ag,28 andIr–Rh NSs.23 The synthesis of transition metal oxides and saltNSs such as TiO2-NS, ZnO-NS and transition metal (Fe, Cu,and Ni) vanadate NSs, and supported layered Ni nanosheetarrays has also been reported recently.29–33 However, the con-trolled synthesis of free-standing ultrathin non-noble metalNSs still remains a great challenge.

Phenylacetylene (PA) removal through the semihydrogena-tion reaction is an important process in polystyrene pro-duction because PA is a poisonous impurity in styrene (ST)feed stocks. The existence of small quantities of PA will resultin the deactivation of polymerization catalysts and affects thefinal product distribution of polystyrene. A number of noblemetal catalysts have already been developed and applied to the

†Electronic supplementary information (ESI) available: More XRD patterns andTEM images, XANES spectra, EXAFS results, and other data. See DOI: 10.1039/c8nr00532j‡These authors contributed equally.

aBeijing National Laboratory for Molecular Sciences (BNLMS), College of Chemistry

and Molecular Engineering, Peking University, Beijing 100871, China.

E-mail: ywzhang@pku.edu.cnbNational Institute of Clean-And-Low-Carbon Energy, Future Science & Technology

City, Changping District, Beijing 102211, China

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PA semihydrogenation reaction over the past few decades,including Pd,34–38 Pt,39–42 Ru,41,42 and Rh.43 However, becauseof their high cost and poisoning sensitivity, they are expectedto be replaced by low-cost non-noble metal catalysts. Novelcatalytic materials for PA semihydrogenation with high STselectivity under mild conditions are highly desired. Therefore,a series of studies have been carried out on developing Nibased PA semihydrogenation catalysts, including Au@Ni core–shell nanoparticles,44 nickel phosphides,45 nickelsilicides,46–48 and supported and unsupported Ninanoparticles.7,49–51

Herein, we demonstrate a novel solvothermal methodtowards the synthesis of ultrathin monometallic Ni NSs (thick-ness < 3 nm) using N,N-dimethylformamide as the solvent andn-butylamine as the shape controlling reagent. It was demon-strated by the results from transmission electron microscopy(TEM), X-ray diffraction (XRD), X-ray absorption spectra (XAS)and X-ray photo electron spectroscopy (XPS) that the growth ofthe Ni NSs follows a NiO intermediate mechanism. The nickelacetylacetonate precursor was first reduced to the NiO NSintermediate, and then transformed to Ni NSs. The syn-thesized Ni NSs showed 88–92% ST selectivity for the PA semi-hydrogenation reaction under mild conditions with 1 atm H2.We also performed density functional theory (DFT) calcu-lations based on reported theoretical studies;52–55 the resultsindicated that low coverage of oxygen atoms on the Ni NSsurface should be responsible for the high ST selectivity.

Results and discussionSolvothermal synthesis of ultrathin Ni NSs

As is known, metallic Ni has no intrinsic driving force to growinto ultrathin 2D nanostructures attributed to its fcc symmetryand high surface energy. The solvothermal method is a work-able scheme to synthesize such unstable structures.Meanwhile, shape controlling reagents should also be used toconfine the growth direction of the nanocrystals which hasbeen reported in the synthesis of layered Co nanocrystals.56

Therefore, Ni(acac)2 was chosen as the precursor, and n-butyla-mine, which has a moderate coordination effect to the Niatom in Ni(acac)2 compared to other strong surfactants, waschosen as the shape controlling reagent. Meanwhile, N,N-di-methylformamide was chosen as the solvent due to its goodsolubility and reducing capacity to Ni(acac)2.

The ultrathin Ni NSs were obtained through 4 hours ofsolvothermal reaction at 473 K. As illustrated in TEM images(Fig. 1a and b), the as-obtained Ni NSs are well dispersed andshowed a sheet like morphology. The Ni NSs are about 2.4 nmthick measured by TEM (Fig. 1a) and 2.1 nm thick measuredby AFM (Fig. S1†). The Ni NSs acquired are in the size rangefrom several hundred nanometers to the micron scale(Fig. 1b). The Ni (111) lattice was observed on a high resolu-tion transmission electron microscopy (HRTEM) image(Fig. 1c), and Ni (220) diffraction was observed on a selectedarea electron diffraction (SAED) pattern (Fig. 1d).

Ni NSs with various kinds of amine additions and noamine addition were also synthesized under the same con-ditions to reveal the role of n-butylamine in the solvothermalreaction. As the TEM images illustrate (Fig. S2†), the Ni NSssynthesized with no amine addition are 10 times thicker thann-butylamine added samples. It proved the confinement effectof n-butylamine in the growth of Ni NSs. Other primaryamines including n-octylamine and benzylamine were alsoproved effective for generating Ni NSs, but the NSs formed arealso thicker than NSs synthesized with n-butylamine (Fig. S3aand b†). Secondary amine, tertiary amine, diamine and hydra-mine including 2-butylamine, triethylamine, ethanediamine,and cholamine were also used for synthesizing Ni NSs, butproved inefficient for generating Ni NSs (Fig. S3c–f†) thoughamines like cholamine and hydrazine were reported to beeffective for the synthesis of C-dots57 and barium titanate,58

which may be attributed to their geometrical configurationsthat make coordination with Ni(acac)2 difficult.

Structure and growth mechanism of Ni NSs

To further investigate the structure and growth mechanism ofNi NSs, Ni NS samples formed at different solvothermal reac-tion times were synthesized and characterized by TEM, XAS,XRD and XPS. As illustrated in TEM images (Fig. 2), thesamples formed at 0.5–6 hours of solvothermal reaction allexhibited sheet-like morphologies. This indicates that thereduction of the Ni(acac)2 precursor was completed within0.5 hours from the beginning of the solvothermal reaction.The concentration of Ni in the centrifugation supernatant ofthe 0.5 h sample was tested by ICP, and proved the completereduction of the Ni(acac)2 precursor. Samples formed in the8.5 h and 18 h solvothermal reaction were also synthesized,but as TEM images show (Fig. S4†), the samples have agglom-erated to large particles (average diameter > 500 nm).

XRD patterns showed the phase change of the samples(Fig. 3a). The 1 h sample does not show any obvious diffractionpeaks which indicate its amorphous nature. The diffractionpeaks of fcc Ni started to appear on the pattern of the 2 hsample, and the intensities of the patterns increase with thesolvothermal reaction time for 3–6 h samples, which indicate

Fig. 1 (a and b) TEM images, (c) HRTEM image and (d) SAED patterns ofNi NSs synthesized through 4 hours of solvothermal reaction at 473 K.

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the formation and growth of crystallized fcc Ni. The decreaseof the full width at half maximum (FWHM) of the 3–6 hsamples also indicates their gradually enlarged crystal size.

XAS spectra (Fig. 3b and c) were recorded for samplesformed in 1 to 5.5 h solvothermal reactions to better under-stand the component and the phase transformation of Ni NSsduring the solvothermal reaction, especially for those amor-phous samples with no obvious X-ray diffraction peaks. TheXANES spectra of the 1 h and 1.5 h samples are completelyidentical to the NiO standard sample, which proved that theNi(acac)2 precursor was reduced to NiO within 1 h (Fig. 3b).The spectra of the 2 h and 3 h samples are also consistentwith the NiO sample but the intensity of the white line peakdecreases with the reaction time, which indicates that the gen-erated NiO are being partially reduced. Unlike 1–3 h samples,

the 4 h and 5.5 h samples are more consistent with the Ni foilsample, which demonstrates that most of the NiO in the bulkphase is reduced to metallic Ni. Linear combination fitting(LCF) analysis was done for all collected XANES spectra usingmetallic Ni foil and NiO as standards to provide a quantitativedescription of the solvothermal process. As Fig. 4a illustrates,during 1–1.5 h, the content of Ni and NiO almost remainsunchanged, the content of NiO exceeds 90%, which corres-ponds to the quick reduction of the Ni(acac)2 precursor andthe formation of NiO. After 1.5 h, as the NiO are reduced tometallic Ni, the Ni content increases rapidly and the NiOcontent decreases relatively. After 4 h, over 90% NiO is reducedto Ni, and the gradient of Ni and NiO content curves starts toslow down.

Fourier transformed EXAFS spectra gave out the same infor-mation as the XANES spectra and LCF analysis. Only Ni–O andNi–Ni (NiO) coordinations can be observed in the 1–2 hspectra and the intensity of the coordinations is weakenedwith the reaction process (Fig. 3c). No apparent coordinationcan be observed in the range of 4–6 Å which indicates theamorphous structure of the samples. In the spectra of 3–5.5 hsamples, Ni–O and Ni–Ni (NiO) coordinations disappear whileNi–Ni (Ni) coordination appears. All EXAFS spectra were fittedto calculate the related coordination numbers (Table 1 andFig. 4b). From 1 h to 3 h, the coordination number of Ni–O(NiO) decreased from 4.6 to 1.3. Meanwhile, the Ni–Ni (Ni)coordination number increased from 1.8 to 9.9 during 2–5.5 h.

Fig. 3 (a) XRD patterns, (b) XANES spectra, (c) Fourier transformedEXAFS spectra and (d) XPS spectra of Ni NSs formed at different reactiontimes. Black vertical lines in (a) represent the standard data for fcc Ni(JCPDS No.04-0850).

Fig. 2 TEM images of Ni NSs formed at different reaction times, (a–f )corresponding to 0.5 h, 1 h, 2 h, 3 h, 4 h, and 6 h, respectively.

Fig. 4 (a) LCF analysis results and (b) the EXAFS fitted coordinationnumber of Ni NSs formed at different reaction times.

Table 1 EXAFS fitting results of Ni NSs formed at different reactiontimesa

Sample Coordination R (Å) C.N.

1 h Ni–O 2.055 ± 0.011 4.6 ± 0.6Ni–Ni (NiO) 3.126 ± 0.008 5.9 ± 0.9

1.5 h Ni–O 2.049 ± 0.012 4.8 ± 0.5Ni–Ni (NiO) 3.119 ± 0.009 4.8 ± 0.6

2 h Ni–Ni (Ni) 2.516 ± 0.025 1.8 ± 1.7Ni–O 2.066 ± 0.025 3.2 ± 0.5Ni–Ni (NiO) 3.130 ± 0.010 2.0 ± 0.6

3 h Ni–Ni (Ni) 2.481 ± 0.007 5.4 ± 0.74 h Ni–Ni (Ni) 2.485 ± 0.003 7.8 ± 0.65.5 h Ni–Ni (Ni) 2.486 ± 0.003 9.9 ± 0.8

a For details, see Table S1.

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The variation of the fitted coordination number is identical tothe XANES, LCF analysis and EXAFS results.

XPS measurements of 3 h and 4 h samples were taken tocharacterize their surface compositions. As can be observed inFig. 3d, the peak around 852 eV is assigned to Ni0 and peaksbetween 855 and 861 eV are assigned to NiO.59–62 The 4 hsample showed both metallic Ni and NiO peaks, while the 3 hsample showed only a NiO peak. The XPS results indicate thatthe metallic Ni phase in the 3 h sample mainly distributed inthe bulk. The sample is wrapped up with a thin NiO layer onthe surface. The surface of the 4 h sample consists of both Ni0

and NiO.Combined with all TEM, XRD, XAS and XPS results, the

growth mechanism of Ni NSs can be concluded as shown inScheme 1. Under solvothermal conditions, the Ni(acac)2 pre-cursor was reduced to the NiO intermediate within 0.5 hours,then the generated NiO intermediate was further reduced to NiNSs at 1.5–4 h. At 3 h, most of the NiO phases were reduced tometallic Ni, but a thin NiO layer still remains on the surface ofthe NSs. At 4 h, the surface NiO layer was partially reduced tometallic Ni. After 4 h, the Ni NSs started to agglomerate toform larger Ni particles.

Catalytic performance of ultrathin Ni NSs in PAsemihydrogenation

To estimate the intrinsic catalytic performance of the syn-thesized Ni NSs, PA semihydrogenation reactions were con-ducted based on normally used reaction conditions in theliterature.7,44–51 Detailed reaction conditions of each reactionare listed in Table 2. Ethanol was chosen as the solventbecause of its excellent solubility of both reactants andproducts, and its dispersibility of the Ni nanosheets andenvironmental friendliness. Other green solvents including

ionic liquids are also potential choices.63 Table 2 and Fig. 5aand b show the PA semihydrogenation performance of the NiNSs formed at different solvothermal reaction times and Ninanoparticles reduced with sodium borohydride (Ni-B NPs) asthe comparison sample. Ni NSs formed at 3 h (Ni-3h), 4 h (Ni-4h), and 6 h (Ni-6h) were tested. Among all tested catalysts, Ni-4h showed the highest activity (89%), and the PA conversionwas 98.0% after 20 hours of reaction (Table 2, entry 2). The as-obtained highest selectivity value is comparable to those ofvarious nickel nanoparticles reported in literaturestudies.7,44,49–51 The slightly agglomerated Ni-6h sample with arelatively large thickness compared to the Ni-4h sample exhibi-ted only 43.0% PA conversion through even longer reactiontime (Table 2, entry 4). The Ni-3h sample showed 5.0% PA con-version through 12.5 hours of reaction, which is a relativelylow activity (Table 2, entry 3). Our investigations showed thatmetallic Ni is the active phase for the Ni based hydrogenationcatalysts. Therefore, the wrapped NiO layer on the surface ofNi-3h is responsible for its low activity. The NiO layer preventsthe contact of the PA molecule on the active metallic Ni phasesublayers. The PA conversion change versus the reaction time(Fig. 5a) also demonstrates the different activities of Ni andNiO wrapped Ni NSs. The PA conversion by Ni-4h and Ni-6hincreases with the reaction time, but the PA conversion byNi-3h almost stayed constant over time which proved the in-activeness of NiO to PA semihydrogenation. The PA conversionis also related to the particle size of the catalyst in terms of thesurface atom ratio. XRD and XAS results showed that Ni-6hconsisted of a pure metallic Ni phase, but the agglomerationof NSs enlarged its average particle size and thus reduced itsactivity. Similarly, the activity of Ni-B NPs may also be affectedby both the factors. The Ni-B NPs showed 55.0% PA conversion(Table 2, entry 5) which is lower than Ni-4h but higher thanother catalysts. The TEM image (Fig. S5a†) showed the agglom-

Table 2 Activities and selectivities of Ni NSs and Ni-B NPs towards PAsemihydrogenation

Entry Catalyst Time (h) Conv. (%) ST sel. (%) EB sel. (%)

1 None 24 0 — —2 Ni-4h 20 98.0 89.0 11.03 Ni-3h 12.5 5.0 93.0 7.04 Ni-6h 22 43.0 93.0 7.05 Ni-B NPs 20 55.0 82.0 18.0

Reaction conditions: 1.0 mmol PA as the reactant, 20 mg of Ni NSs orNi-B NPs as the catalyst, 4.5 mL of ethanol as the solvent, 1 atm of H2as the reductant, the reaction temperature was kept at 323 K. The con-version and selectivity data were calculated based on GC results.

Fig. 5 (a) Activities and (b) selectivities of Ni NSs and Ni-B NPs towardsPA semihydrogenation. (c) ST selectivity versus the PA conversion plotfor Ni-4h and Ni-B NPs. (d) Activities and selectivities of Ni-4h for 5reaction cycles. For each cycle, the reaction was conducted at 1 atm H2,323 K for 16 h. In (a–c), red ( ), blue ( ), green ( ), and brown ( ) curvesrepresent Ni-4h, Ni-B NPs, Ni-6h, and Ni-3h, respectively.

Scheme 1 Schematic illustration of the growth mechanism of Ni NSs.

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eration and larger average particle size of Ni-B NPs. The XRDpatterns (Fig. S5b†) and the XANES spectrum (Fig. S6†) of Ni-BNPs showed the characteristic signals of Ni but they are notidentical to the standard Ni sample. Amorphous nickel borideor the NiO phase may also exist in Ni-B NPs. The larger particlesize and the co-existing NiO with metallic Ni should be respon-sible for the lower activity of Ni-B NPs.

All tested catalysts showed ST selectivity higher than 80%.However, in consideration of the consecutive hydrogenationmechanism of PA semihydrogenation (Scheme 2), low PA con-version means low ST concentration in the system which willcertainly result in a low ST hydrogenation rate and low ethyl-benzene (EB) selectivity. ST selectivity under high PA conver-sion can better evaluate the authentic selectivity of the catalyst.Therefore, the ST selectivities under different PA conversionswere tested for Ni-4h and Ni-B NPs (Fig. 5b). Through 20 and28 hours of reaction, the PA conversion by Ni-4h and Ni-B NPsreached 98.0% and 99.0%, respectively. From the ST selectivityversus the PA conversion plot (Fig. 5c), it can be observed thatthe ST selectivity of Ni-4h remains above 88.0% along with theincreased PA conversion, but the ST selectivity of Ni-B NPsdecreases with the increasing PA conversion. When the PAconversion reached 99.0%, the ST selectivity of Ni-B NPs isonly 49.0%. The results indicated that the as-synthesized Ni-4hhad high selectivity to ST. To further investigate the selectivityof Ni-4h, ST hydrogenation tests were carried out for Ni-4h andNi-B NPs. The reaction conditions were consistent with the PAhydrogenation tests, except that the reactant was changed tothe same amount of ST; the results are listed in Table 3. ForNi-4h, the ST conversion was 5.9% for 1 hour of reaction and25.2% for 14.5 hours of reaction, but for Ni-B NPs, the ST con-version reached 99.0% within 1 hour. These results indicatethat Ni-4h has a strong adsorption effect on PA molecules, andthus reduced the ST hydrogenation rate; the Ni-4h sample is apromising catalyst for the semihydrogenation of PA.

The stability of Ni-4h was tested by recycling 5 times underthe same reaction conditions. The PA conversion and ST

selectivity are shown in Fig. 5d. Before each cycle, a certainamount of PA was added in the reaction tube as the reactant.After each reaction, the Ni nanosheets were separated comple-tely by magnetic field, and then the remaining solution wasanalysed by GC to acquire the amount of PA left. The conver-sions were calculated based on the PA content change beforeand after each reaction. The ST selectivity remains above89.0% after 5 cycles, but the PA conversion decreases from95.0% to 32.0%. The decrease of the PA conversion is probablydue to the inevitable agglomeration caused by the high surfaceenergy of the nanosheet structure. It can be observed from theTEM image and XRD patterns (Fig. S7†) that particle agglom-eration had occurred on the recycled sample.

DFT calculations of the PA semihydrogenation reaction on theNi surface

DFT calculations were performed to further explain the cata-lytic performance of Ni-4h towards PA semihydrogenation. Thecalculations were done on the most thermodynamic stable Ni(111) surface.64 First we compared the most stable adsorptionsof PA, ST and EB on the Ni (111) surface. As shown in Fig. 6,the aromatic rings of these adsorbates are strongly bonded tothe Ni surface, with the H atoms away from the surface. Thiscorresponds to the changes of the hybridization of C–H bondsfrom the sp2 to sp3 state of the benzene ring, indicating chemi-sorption of these aromatics on the Ni surface. The alkynylgroup of PA and alkenyl groups of ST exhibits differentchanges before and after being adsorbed on the Ni (111)surface. The alkynyl group of PA is bonded strongly with theNi surface, with a distance to the surface of 1.4 Å (Fig. 6a andc). This is closer than the distance of the aromatic ring to thesurface which is 2.0 Å. The hydrogen atom of the terminalalkynyl group was 60° away from the surface, which indicatesthe C–H hybridization from sp to sp2. When the PA moleculeis semihydrogenated to ST, the stable adsorption geometry isobserved as shown in Fig. 6b and d. We can see a weakerbonding effect of the alkenyl group than the alkynyl group onthe surface. The distance between the alkenyl group of the STand Ni surface is 2.2 Å, which is more away from the surfacethan the alkynyl group of PA. From this point of view, STshows a stronger hydrogenation ability than PA.

To further evaluate the activity and the selectivity of thehydrogenation of phenylacetylene, we computed the reaction

Fig. 6 Top and side views of the stable adsorption configurations of (aand c) PA and (b and d) ST on the Ni (111) surface. Ni, C and H atoms aremodelled by blue, black and white balls, respectively.

Scheme 2 The consecutive hydrogenation mechanism of PA semihy-drogenation. ST will be further hydrogenated to form by-productethylbenzene.7,39–41,43,46–48

Table 3 ST hydrogenation activities of Ni-4h and Ni-B NPs

Entry Catalyst Time (h) ST conv. (%)

1 Ni-4h 1 5.92 Ni-4h 14.5 25.23 Ni-B NPs 1 99.6

Reaction conditions: 1.0 mmol of ST as the reactant, 20 mg Ni-4h orNi-B NPs as the catalyst, 4.5 mL of ethanol as the solvent, 1 atm of H2as the reductant, the reaction temperature was set to 323 K. The STconversions were calculated based on GC results.

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pathways using the DFT method. As the schematic potentialsurface shows (Fig. 7), PA is stabilized on the Ni (111) surfacewith an adsorption energy of −2.66 eV, and is then convertedto ST. The hydrogenation steps of ST are an endothermicprocess, with the H first adding to the terminal carbon of theC–C double bond. The energy of the direct desorption of ST is1.69 eV, which is comparable to the energy of converting ST toa desorbed EB molecule of 1.65 eV. These two pathways arecompeting reactions. Noting that the reaction rate of ST hydro-genation is also related to the surface H concentration, theunsaturation adsorption and dissociation of H2 at 1 bar slowdown the ST hydrogenation steps. Here the Ni (111) surfaceoffers a high selectivity of PA to ST.

The low coverage of oxygen on the Ni surface in Ni-4hsamples played a key role in the reactivity and selectivity of thesemihydrogenation. We further considered the effect of thelow monolayer (1/16 ML) coverage of oxygen on the Ni (111)surface. The adsorbates showed a weakened adsorption on theNi–O surface. Oxygen on the Ni surface may hinder the directC–Ni interaction of the aromatic ring with the Ni atoms(Fig. S8†). This gives a large increase of the adsorption ener-gies of ST and EB, from −1.69 eV and −0.97 eV to −0.56 eVand −0.08 eV, respectively. The surface oxygen on Ni (111) alsochanges the adsorption of PA, from −2.66 eV to −2.07 eV. PA isstill the most stable adsorbate among the hydrogenation path-ways, which can be well stabilized on the Ni–O surface. STdesorbs on the Ni–O surface easily, which could be more facilethan overcoming the barrier of further hydrogenation to EB.The low coverage of oxygen on the Ni surface may give rise tothe selectivity of semihydrogenation. The intermediates withweakened adsorption are also removed in time, which givesmore sites on Ni (111) for further reaction. The sample of Ni-4hwith a low content of oxygen on the surface may promote boththe activity and selectivity compared with Ni-B as a reference.

To see whether the improved selectivity is due to theexposed facet of nanosheets, further DFT computations of PAhydrogenation on Ni (100) and Ni–O (100) surfaces were con-ducted (Fig. S9–S11†). PA, ST and EB are adsorbed on the Ni(100) surface with similar configurations to those on the Ni

(111) surface (Fig. S9†). However, the binding strengths of theintermediates on Ni (100) are stronger than those on Ni (111).This makes ST harder to desorb from the Ni (100) without thehelp of O atoms. As shown in Fig. S10,† the low coverage ofoxygen atoms on the Ni (100) surface increases the distancebetween the benzene ring and the surface. So the adsorbedoxygen atoms weaken the adsorption of the intermediates. Theaddition of oxygen on Ni (100) also improves the selectivity toST, due to a more facile desorption of ST from the Ni–O (100)surface (Fig. S11†). However, the selectivity on Ni (100) maynot be as good as that on Ni (111) owing to the fact that thebarrier of ST hydrogenation is similar to the ST desorptionenergy. From this point of view, the close-packed Ni (111)surface with the low coverage of oxygen can give a larger extentof selectivity enhancement.

Conclusions

In this work, we demonstrated the synthesis of ultrathin NiNSs via a NiO intermediated solvothermal method. The for-mation of the Ni NSs follows a NiO intermediated mechanism.The nickel acetylacetonate precursor was first reduced to theNiO NS intermediate and then reduced to Ni NSs. The syn-thesized Ni-4h NSs exhibit high activity and ST selectivitytoward the PA semihydrogenation reaction under mild con-ditions, owing to their nanosheet structure and the lowcontent of oxygen on their surface. Our work provides a robustmethod to acquire ultrathin nickel nanosheet materials andan efficient catalyst for the semihydrogenation of PA.

ExperimentalMaterials

Nickel acetylacetonate (Ni(acac)2, 96%, J&K Scientific), nickelchloride (NiCl2·6H2O, A.R., Beijing Chemical Works Co. Ltd,China), sodium borohydride (NaBH4, A.R., SinopharmChemical Reagent Co. Ltd, China), N,N-dimethylformamide(DMF, A.R., Xilong Chemicals Co. Ltd, China), n-butylamine(A.R., Sinopharm Chemical Reagent Co. Ltd, China), n-octyl-amine (A.R., Sinopharm Chemical Reagent Co. Ltd, China),benzylamine (A.R., J&K Scientific), triethylamine (A.R., XilongChemicals Co. Ltd, China), 2-butylamine (A.R., SinopharmChemical Reagent Co. Ltd, China), cholamine (A.R., XilongChemicals Co. Ltd, China), ethanediamine (A.R., XilongChemicals Co. Ltd, China), phenylacetylene (PA, 98%+, AlfaAesar), ethylbenzene (EB, A.R., Sinopharm Chemical ReagentCo. Ltd, China), styrene (ST, A.R., Xilong Chemicals Co. Ltd,China), and polyethylene glycol 200 (PEG200, M̄w = 200, C.P.,Sinopharm Chemical Reagent Co. Ltd, China) were all used asreceived.

Synthesis of ultrathin Ni nanosheets (NSs)

The ultrathin Ni NSs were synthesized using a solvothermalmethod. In a typical case, 0.05 g of nickel acetylacetonate was

Fig. 7 Schematic potential surface of PA semihydrogenation on Ni (111)and Ni (111) with 1/16 ML oxygen coverage. (*) represents the adsorbedintermediates, and (g) represents the gas phase intermediates and pro-ducts. All zero-point correlated energies are with respect to the gasphase PA and two gas phase hydrogen molecules (0 eV).

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dispersed in a mixture of 10 ml N,N-dimethylformamide and2.0 ml deionized water, then 0.50 ml n-butylamine was added.The solution was transferred into a 25 ml Teflon-lined con-tainer sealed in a stainless steel autoclave. The autoclave waskept at 473 K for a fixed time. The as-obtained grey precipitatewas then centrifuged and washed twice with ethanol. Finally,the as-obtained products were freeze-dried at 273 K. About10 mg Ni nanosheets can be acquired via a batch of solvo-thermal reactions.

Synthesis of sodium borohydride reduced Ni nanoparticles(Ni-B NPs)

For comparison, Ni NPs were also synthesized using a sodiumborohydride reduction method reported previously.65

Typically, 5.0 mL NaBH4 solution (13 mmol NaBH4 dissolvedin the mixture of 1.0 mL water and 4.0 mL PEG200) was addedinto NiCl2 solution (1.7 mmol NiCl2·6H2O dissolved in 10 mLPEG200) drop by drop under magnetic stirring. The yieldedblack precipitates were then separated by using a magnet andwere washed 3 times with water. The final product was alsofreeze-dried at 273 K.

Characterization

Transmission electron microscopy (TEM) and high-resolutiontransmission electron microscopy (HRTEM) were carried outon JEM-2100 and JEM-2100F transmission electron micro-scopes, respectively, (JEOL, Japan) operating at 200 kV. Thesamples were prepared by drying a drop of the Ni NSs/Ni-B NPdispersion in ethanol onto a copper grid coated with a carbonfilm. An atomic force microscopy (AFM) image was collectedon a dimension ICON-PT atomic force microscope (Bruker,USA). X-ray diffraction (XRD) were collected on an X’Pert3

diffractometer (PANalytical, Netherlands) with Cu Kα radiation(λ = 1.54056 Å) operating at 40 kV, 100 mA. The scan range wasset to 2θ = 10–90°. X-ray photoelectron spectroscopy (XPS)measurements were carried out on an Axis Ultra imagingphotoelectron spectrometer (Kratos Analytical Ltd, UK) with amonochromatic Al Kα (1486.7 eV) X-ray source operating at 225W with 15 kV acceleration voltage. The C 1s line at 284.8 eVwas used to calibrate the binding energies (B.E.). X-ray absorp-tion spectra (XAS) at the Ni K edge (8333 eV) were collectedusing transmission mode at the 1W1B beamline of the BeijingSynchrotron Radiation Facility (BSRF), Beijing, China and theBL14W beamline of the Shanghai Synchrotron RadiationFacility (SSRF), Shanghai, China. A double Si (111) crystalmonochromator was used for energy selection. All collectedspectra were analysed using the Athena program within theIfeffit package.66 Inductively coupled plasma-atomic emissionspectroscopy (ICP-AES) analysis was conducted on a ProfileSpec ICP-AES spectrometer (Leeman, USA).

PA semihydrogenation tests

The PA semihydrogenation reaction was carried out in a glassreaction tube (2.5 cm in diameter). Typically, in order to testthe intrinsic catalytic properties of the Ni NSs without loadingto a support, about 20 mg of freshly prepared Ni NSs or Ni-B

NPs were dispersed into 4.5 ml of ethanol, then 1.0 mmol PAwas added as the reactant. The reaction mixture was stirredand heated with an oil bath. The reaction occurred when H2

was purged into the reaction tube with a H2 balloon. The as-obtained products were analysed by GC (GC 7820A, AgilentTechnologies, equipped with a HP5 column and a FIDdetector).

Density functional theory (DFT) computational method

The density functional calculations were performed using theVienna Ab initio Simulation Package (VASP).67,68 The exchange–correlation energy functional of Perdew–Burke–Ernzerhof gen-eralized gradient approximation (GGA-PBE) was employed forthe electronic exchange.69 The core electron interactions weredescribed using projector augmented-wave (PAW) pseudopo-tentials with a kinetics cut-off energy of 400 eV.70 The close-packed Ni (111) and Ni (100) surfaces were exposed to a 4 × 4unit cell with four layers in the slab model. The top two layerswere relaxed during the optimization process, and the bottomtwo layers were fixed. The Brillouin zone was automaticallygenerated on a 3 × 3 × 1 Monkhorst–Pack k-point sampling.The vacuum region was around 15 Å between the slabs toavoid artificial interactions. Each structure was relaxed untilthe residual force was less than 0.05 eV Å−1.

All the transition states were characterized via theClimbing-Image Nudged Elastic Band (CI-NEB) method.71 Thevibration calculations were performed to validate the zero-point energy (ZPE) correlation. The adsorption energy (Eads) isdefined as Eads = ENi-slab/adsorbate − ENi-slab − Eadsorbate, whereENi-slab/adsorbate, ENi-slab and Eadsorbate are the total energy of theadsorbate on the Ni surface, the energy of the Ni slab, and theenergy of the adsorbate calculated in the gas phase, respect-ively. The reaction energy (ΔE) is defined as the energy differ-ence of the initial states and all final products adsorbed separ-ately on the metal surfaces. The reaction barrier (Ea) is theenergy difference of the transition state and initial states.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundation ofChina (No. 21573005, 21771009, and 21621061), the NationalKey Research and Development Program of the MOST of China(No. 2016YFB0701100), and the Beijing Natural ScienceFoundation (No. 2162019). The XAS experiments were con-ducted in the Beijing Synchrotron Radiation Facility and theShanghai Synchrotron Radiation Facility. The computationalwork is supported by High-performance Computing Platformof Peking University.

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