Nickel nanoparticles decorated on electrospun polycaprolactone/chitosan nanofibers as flexible, highly active and reusable nanocatalyst in the reduction of nitrophenols under mild conditions

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ContentslistsavailableatScienceDirect

Applied

Catalysis

B:

Environmental

jo u r n al ho me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p c a t b

Nickel

nanoparticles

decorated

on

electrospun

polycaprolactone/chitosan

nanoﬁbers

as

ﬂexible,

highly

active

and

reusable

nanocatalyst

in

the

reduction

of

nitrophenols

under

mild

conditions

Kadir

Karakas

a

_,

_Asli

_Celebioglu

b

_,

_Metin

_Celebi

a

_,

_Tamer

_Uyar

b,c,∗∗,2

_,

Mehmet

Zahmakiran

a,∗,1

a_{Nanomaterials}_and_Catalysis_(NanoMatCat)_Research_Laboratory,_Department_of_Chemistry,_Yüzüncü_Yıl_University,_Van,_65080,_Turkey b_{UNAM-National}_{Nanotechnology}_Research_Center,_Bilkent_University,_Ankara,_06800,_Turkey

c_Institute_of_Materials_Science_&_{Nanotechnology,}_Bilkent_University,_Ankara,_06800,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received3July2016

Receivedinrevisedform2October2016 Accepted8October2016

Availableonline13October2016 Keywords: Electrospinning Nanoﬁbers Nickel Nitrophenols Reduction Nanocatalyst

a

b

s

t

r

a

c

t

Today,thereductionofnitroaromaticsstandsamajorchallengebecauseofthepollutantanddetrimental natureofthesecompounds.Inthepresentstudy,weshowthatnickel(0)nanoparticles(Ni-NP)decorated onelectrospunpolymeric(polycaprolactone(PCL)/chitosan)nanoﬁbers(Ni-NP/ENF)effectivelycatalyze thereductionofvariousnitrophenols(2-nitrophenol,2,4-dinitrophenol,2,4,6-trinitrophenol)undermild conditions.Ni-NP/ENFnanocatalystwasreproduciblypreparedbydeposition-reductiontechnique.The detailedcharacterizationoftheseNi-NP/ENFbasednanocatalysthavebeenperformedbyusingvarious spectroscopictoolsincludingICP-OES,P-XRD,XPS,SEM,BFTEM,HRTEMandBFTEM-EDXtechniques. Theresultsrevealedtheformationofwell-dispersednickel(0)NP(dmean=2.71–2.93nm)onthesurface

ofelectrospunpolymericnanoﬁbers.ThecatalyticactivityoftheresultingNi-NP/ENFwasevaluated inthecatalyticreductionofnitrophenolsinaqueoussolutioninthepresenceofsodiumborohydride (NaBH4)asreducingagent,inwhichNi-NP/ENFnanocatalysthasshownhighactivity(TOF=46.2mol

2-nitrophenol/molNimin; 48.2mol2,4-dinitrophenol/molNimin;65.6mol2,4,6-trinitrophenol/mol Nimin).Moreimportantly,duetothenanoﬁbrouspolymericsupport,Ni-NP/ENFhasshownaﬂexible characteristicsalongwithreusabilityproperty.TestingthecatalyticstabilityofNi-NP/ENFrevealedthat thisnewcatalyticmaterialprovideshighreusabilityperformance(at3rdreuse86%for2-nitrophenol, 83%2,4-dinitrophenoland82%2,4,6-trinitrophenol)forthereductionofnitrophenolsevenatroom tem-peratureandunderair.Thepresentstudyreportedherealsoincludesthecompilationofwealthykinetic dataforNi-NP/ENFcatalyzedthereductionofnitrophenolsinaqueoussodiumborohydridesolution dependingontemperatureandtypeofsupportmaterial(Al2O3,C,SiO2)tounderstandtheeffectofthe

supportmaterialanddeterminetheactivationparameters.

1. Introduction

Nitrophenolsareakindofthemostwidelyusedindustrialnitro aromaticcompounds(NAC’s)andfrequentlyemployedas

interme-∗ Correspondingauthor.

∗∗ Correspondingauthorat:UNAM-NationalNanotechnologyResearchCenter, BilkentUniversity,Ankara,06800,Turkey.

E-mailaddresses:tamer@unam.bilkent.edu.tr(T.Uyar),zmehmet@yyu.edu.tr

(M.Zahmakiran).

1 _Website:_{www.nanomatcat.com}_. 2 _Website:_{http://unam.bilkent.edu.tr/∼uyar}_.

diatesintheproductionofexplosives,pharmaceuticals,pesticides, pigments, dye,wood preservativesand rubberchemicals [1,2]. Although,theyareusefulintermediatesinthefabricationofvarious aforementionedmaterials,theyalsoactascommon environmen-talpollutantsbecauseoftheirtoxicityandresistancetomicrobial degradation[3,4].Forthesereasons,nitrophenolsareconsideredas aprioritypollutantbytheEnvironmentalProtectionAgency(EPA) ofUSA,anditsconcentrationinnaturalwatersisrestrictedtoless than10mg/L[5,6].Uptodate,varioustechniquessuchas mem-braneﬁltration[7,8],microbialdegradation[9,10],photocatalytic degradation [11,12], electro-Fenton method [13,14], electroco-agulation[15,16],adsorption[17,18],electrochemicaltreatment http://dx.doi.org/10.1016/j.apcatb.2016.10.020

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[19,20]and chemical reduction [21,22]have been reported for removingnitrophenols from contaminated water. Amongthese techniques,thechemical reductionofnitrophenols seemstobe mostefficientwaytoremovenitrophenolsfromwaste contami-natedwater.Additionally,aminophenolsformedviathereduction ofnitrophenolsusuallyfindapplicationsasaphotographic devel-operoffilms,corrosioninhibitor,dryingagent,precursorforthe manufactureofanalgesicandantipyreticdrugs[23–25].

Overthelastdecade,nanotechnology,whichdealprimarilywith thesynthesis,characterization,andexplorationofnanomaterials, hasbecameapopularresearcharea,wheresignificanteffortshave beenmadetousenanoscale systems[26].Theimpact ofthese researchestobothfundamentalscienceandpotentialapplications hasbeentremendousandisstillgrowing[27].Nanomaterialshave alreadyfoundmanyfascinatingapplicationsinawidevarietyfield ofchemistry,physics,electronics,biology,medicineandcatalysis. Thelatterapplicationfieldofnanomaterialsisofparticularinterest asitisthekeyforthedevelopmentofstartingchemicals,fine chem-icalsandpharmaceuticaldrugsfromrawmaterials[28].Compared totheirbulk-counterparts,metalnanoparticleshavemuchhigher surface-to-volumeratio,thus,largerfractionofcatalyticallyactive atomsonthesurface,andthesesurfaceatomsofnanoparticlesdo notorderthemselvesinthesamewayasthoseinbulkmetal[29]. Furthermore,theelectronsinnanoparticlesareconfinedtospaces thatcanbeassmallasafewatomswidthsacrossgivingriseto quan-tumsizeeffects[30]andhigherFermipotentialwhichleadstothe loweringofreductionpotentialvalue,andhencemetal nanoparti-clescanfunctionasacatalystformanyelectron-transferreactions suchasreductionofnitrophenols[31].

Hitherto,varioustypesofligandsorsurfactantsstabilizedAu [32],Ag[33],Pd[34],Pt[34,35]andNi[36]nanoparticleshave alreadybeentestedasactivenanocatalystinthereductionof nitro-phenolsin thepresence of sodiumborohydrideasa reductant. However,the recoveryof noble metal nanoparticlesfromsuch stabilizers-containingsystemsisnoteasyandalsomakesUV/vis spectroscopymonitoringofthereactionextremelydifficultdueto thepresenceofsuspendednanoparticlesinthereactionsolution. Inaddition,thereisanothercriticalmattertoobtainpureactive metalsurfacesbystayingawayfromsurfacecontamination result-ingfromsurfaceprotectinggroups,whichoftenleadtoadecrease inthecatalyticperformanceresultingfromtheblockingofactive sites.Moreover,thelowstabilityofthesecolloidalnanoparticles againstagglomerationdiminishestheirreusabilityperformances, whichisoneofthemostimportantcriteriasintheheterogeneous catalysis.Inthiscontext,thegenerationofthemetal nanoparti-clesonsolidsupportmaterialshasalreadybeenacknowledgedfor preventingaggregationofactivemetalnanoparticlesand provid-inghighreusabilityperformancesinthenitrophenols reduction [37–40].Additionally,theuseofsupportedmetalnanoparticlesas nanocatalystinthereductionofnitrophenolsmayprovideakinetic controlofthecatalyticreaction,whereasitisalmostimpossiblein thecatalyticemploymentofcolloidalmetalnanoparticles.Despite alltheadvancementinsupportedmetalnanocatalystpreparation [41],there isstill great interestin developingcatalyst supports withhighstabilityandlargesurfacearea.Atthisconcern, electro-spinninghasbeenconsideredasoneofthepowerfulmethodsto generatenanofibroussupportmaterialswithahugespecificsurface area[42–45].Recentstudieshavealreadyshownthatelectrospun polymericnanofibersactasefficientsupportmaterialinthe stabi-lizationofFe(0)[46],Au(0)[47],Pd(0)[48],Pt(0)[49]andAg(0)[50] nanoparticlesforenvironmentalremediationapplications[46–51]. Thecatalyticstabilityresultsobtainedinthesestudiesprompted ustouseelectrospunpolymericnanofibersascatalystsupportfor guestcatalyticallyactivemetalnanoparticlesinthecatalytic reduc-tionofnitrophenols.

In this study, we report a facile synthesis of nickel nanoparticles(Ni-NP)decoratedonelectrospunpolymeric (poly-caprolactone(PCL)/chitosan) nanoﬁbers, hereafterreferred toas Ni-NP/ENF, and their excellent catalysis for the reduction of variousnitrophenols including2-nitrophenol,2,4-dinitrophenol, 2,4,6-trinitrophenolinaqueoussodiumborohydride(NaBH4;as

reducing agent)solution.Chitosansubunithasbeenselectedin thepreparation ofelectrospunpolymericnanoﬁbersbecauseof its NH2 groups, which mayact as ligandstabilizer tosurface

boundNi-NP.Someofushavealreadyreportedthattheexistence ofsurfacegrafted NH2groupsonsolidsupportmaterialscanact

asstabilizingagentstosupportedPdAuCr[52],PdAg-MnOx[53],

PdAu-MnOx[54]andPd-MnOx[55]nanoparticles.Ni-NP/ENF

cata-lystwassimplyandreproduciblypreparedthroughsurfactant-free deposition-in-situ reduction technique [56] at room tempera-ture, and characterized by inductively coupled plasma-optical emission spectroscopy (ICP-OES), powder X-ray diffraction (P-XRD),X-rayphotoelectronspectroscopy(XPS),scanningelectron microscopy(SEM),brightﬁeldtransmissionelectronmicroscopy (BFTEM)and high resolution transmission electron microscopy (HR-TEM).Thesumoftheirresultsrevealed thattheformation of nickel(0)nanoparticles (dmean=2.71–2.93nm)on thesurface

ofelectrospunPCL/chitosannanoﬁbers.TheseNi-NP/ENFcatalyst provideexceptional activities(TOF=46.2mol2-nitrophenol/mol Nimin; 48.2mol 2,4-dinitrophenol/mol Nimin; 65.6mol 2,4,6-trinitrophenol/molNimin)inthereductionofnitrophenolseven atroomtemperatureandunderair.Moreover,theexcellent dura-bilityofNi-NP/ENFandtheirﬂexibilenatureenablethemreusable nanocatalystforthenitrophenolsreduction.

2. Experimental

2.1. Materials

Nickel(II) chloride hexahydrate (NiCl2·6H2O), methanol

(CH3OH), sodium borohydride (NaBH4), 2-nitrophenol

(O2NC6H4OH), 2,4-dinitrophenol ((O2N)2C6H3OH),

2,4,6-trinitrophenol (picric acid; (O2N)3C6H2OH), acetic acid (AA;

CH3CO2H), formic acid (FA; HCOOH), polycaprolactone

((C6H10O2)n PCL, Mw: 80,000g/mol) and chitosan

(Poly(d-glucosamine), low molecular weight) were purchased from Sigma-Aldrich® _and _used _without _any _{puriﬁcation.} _Deionized

waterwasdistilledbywaterpurificationsystem(Milli-Q Water Purification System). All glassware and Teflon-coated magnetic stir bars werewashed withacetone and copiouslyrinsed with distilledwaterbeforedryinginanovenat323K.

2.2. Characterization

NickelcontentsoftheNi-NP/ENF,Ni/C,Ni/SiO2 andNi/Al2O3

samples weredetermined by ICP-OES (Leeman, Direct Reading Echelle)after each sample was completelydissolved in a mix-tureofHNO3/HCl(1/3ratio). Thescanningelectronmicroscope

(SEM) (Quanta 200 FEG, FEI) was used for the morphological characterizationsof nanofibers.Prior toimaging, sampleswere sputteredwith5nmAu/Pd(PECS-682)andtheaveragefiber diam-eter(AFD)wascalculatedfromtheSEMimagesbyanalyzingat least100fibers.DuringtheSEMcharacterization,energydispersive X-ray(EDX)measurementswerealsoperformedforthe elemen-talanalysisofthenanofibers.Brightfield transmissionelectron microscopy (BFTEM) and high resolution transmission electron microscopy(HR-TEM)wereperformedonFEI-TecnaiG2F30, oper-atingat300kV.ForimagingofTEM,electrospinningofnanofibers wasperformeddirectlyontotheTEMgrids.Then,thenanofibers collectedontoTEMgridswereexposedtotreatmentforthe

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grow-ingofNi-NPandimagingwasperformedthereafter.Thechemical compositionofnanofiberswasinvestigatedbyX-ray photoelec-tronspectrometer(XPS)(ThermoFisherScientific).XPSwasused bymeansofafloodgunchargeneutralizersystemequippedwith amonochromatedAlK␣X-raysource(h=1486.6eV).Thepass energy,stepsize,andspotsizewere30eV,0.1eV,and400mm, respectively.Inordertoobtaineddetailedinformation,thehigh resolutionspectrawererecordedforthespectralregionsrelating toNi,O,CandNatpassenergyof50eV.

2.3. PreparationofelectrospunPCL/chitosannanoﬁbers

Thebead-freeanduniformPCL/chitosannanofiberswere pro-ducedbyusingbinarysolventsystem;AA/FA.Thehomogenous electrospinningsolutionswerepreparedbydissolvingPCLand chi-tosaninAA/FA(1/1(v/v))solventmixtureat6.5%(w/v)and2% (w/v)polymerconcentration,respectively.Afterwards,theseclear solutionswereloadedin3mLsyringefittedwithametallic nee-dleof0.4mminnerdiameterandtheywerelocatedhorizontally ona syringepump(modelKDS-101,KDScientific,USA).Oneof theelectrodesofhigh-voltagepowersupply(Spellman,SL30,USA) wasclampedtothemetallicneedleandtheplatealuminum col-lectorwasgrounded.Electrospinningparameterswerearrangedas follows:feedrateofsolutions=0.5mL/h,appliedvoltage=15kV, tip-to-collector distance=10cm. Thegrounded stationarymetal collector covered with an aluminum foil was used to deposit the electrospun nanofibers.The electrospinning apparatus was enclosedinaPlexiglasboxandelectrospinningwascarriedoutat about23◦Cat20%relativehumidity.Thecollectednanofiberswere driedovernightatroomtemperatureinafumehood.

2.4. In-situformationofnickelnanoparticlesdecoratedon electrospunpolycaprolactone/chitosannanoﬁbers(Ni-NP/ENF) andtheircatalysisinthereductionofnitrophenols

Ni-NP/ENF nanocatalyst was obtained by the conventional impregnation and subsequent reduction steps [56]. Typically, 2.0mL methanol solution containing NiCl2·6H2O (4.75mg,

19.9␮molNi)andENF(3.0mg)ismixedfor3h.Then,methanol wasremovedfromthemixturebyapplyinglowvacuum(10−1Torr) atroomtemperature; theresulting solidwasremoved,washed with water (3×20mL) and dried in vacuum-oven at 303K (10−1Torr).Anaqueoussolutionofnitrophenol(2.0mM;20␮mol nitrophenolcorrespondsto2.78mgforO2NC6H4OH,3.68mgfor

(O2N)2C6H3OH and 4.58mgfor (O2N)3C6H2OHin 10.0mLH2O,

whichweredoneinseparateexperiments),and2.0mgNi(II)/ENF were taken in a jacketed one necked reaction ﬂask (20.0mL) containing a Teﬂon-coated stir bar was placed on a magnetic stirrer(HeidolphMR-3004)andthermostatedto298Kbyusinga constanttemperaturebath(LabCompanionRW-0525).Then,the mixturewasstirred for 15min toachieve thermal equilibrium. Next,1.0mLaqueoussolutionofsodiumborohydride(0.2mmol NaBH4correspondsto77.2mgNaBH4)wasaddedintothereaction

flaskandthecatalyticreactionwasstarted(t=0min)bystirring the mixtureat >600rpm. At each predetermined time interval 100␮L of the reaction solution waswithdrawn and dilutedto 2.0mLfortheanalysisoftransformationefficiencyofnitrophenol using a Shimadzu UV-3600 UV–vis spectrometer. The optical absorption spectra were measured at fixed wavelengths 416, 359and 393nm,which are thecharacteristic absorptionpeaks for 2-nitrophenol, 2.4-dinitrophenol and 2,4,6-trinitrophenol, respectively. The concentration of nitrophenol was calculated usingacalibrationcurveconstructedwithabsorbanceofstandard

solutions. The extent of catalytic reduction was expressed as conversion,whichwascalculatedasEq.(1)shows;

conversion= A/A0 (1)

whereA0istheinitialconcentrationandAistheconcentrationof

nitrophenolatcertaintimepoint.

2.5. ReusabilityperformanceofNi-NP/ENFinthecatalytic reductionofnitrophenols

Afteronecompletereactioncycle,Ni-NP/ENFwasisolatedby suctionﬁltrationusingWhatmannﬁlterpaperandwashedwith excessethanol-watermixtureanddriedinvacuum-ovenat303K (10−1Torr).Then,thedriedcatalystweighedandusedforthenext cycleofcatalyticreactionwithfreshsubstrates.

2.6. DeterminationofactivationparametersforNi-NP/ENF catalyzedreductionofnitrophenols

Inordertodeterminetheactivationparameters(Ea,_H*and S*)fortheNi-NP/ENF(2mg;0.9%wtNicorrespondsto0.31␮mol Ni)catalyzedreductionofnitrophenols(2.0mM;20␮mol nitro-phenol corresponds to 2.78mg for O2NC6H4OH, 3.68mg for

(O2N)2C6H3OHand 4.58mg for(O2N)3C6H2OH in10.0mLH2O,

which were donein separateexperiments), the catalytic reac-tion was performed at different temperatures in the range of 298K–318Kand theinitialrateconstantsweredeterminedand usedtoconstructArrheniusandEyring-Polonyiplotstoﬁnd acti-vationparameters.

2.7. Preparationofnickelnanoparticlessupportedoncarbon (Ni/C),alumina(Ni/Al2O3)andsilica(Ni/SiO2)catalysts

Inathreeseparateexperiments5.0mLaqueoussolution con-taining NiCl2.6H2O (6.2mg, 25.9␮mol Ni) and solid support

(100mgC,Al2O3,SiO2)ismixedfor3h.Afterthat,thefresh

aque-ousNaBH4solution(1.0mL,14mg,0.38mmol)wasaddedtothese

mixturesandtheresultingsolutionswerestirredforhalfanhour underambientconditions.Aftercentrifugation(6000rpm,5min), copiouswashingwithwater(3_×20mL),ﬁltration,anddryingin ovenat373K,Ni/C,Ni/Al2O3andNi/SiO2catalystswereobtained

aspowders.

2.8. TheuniquenessofNi-NP/ENFnanocatalystinthecatalytic reductionofnitrophenols

To investigatethe uniqueness of Ni-NP/ENF withrespect to Ni/C,Ni/Al2O3 andNi/SiO2 catalysts;thecatalytic reductionsof

2-nitrophenol,2.4-dinitrophenoland2,4,6-trinitrophenol, respec-tivelyperformedinasetofexperimentsunderidenticalconditions (0.31␮molNiand20␮molnitrophenol)byusingNi/C,Ni/Al2O3

andNi/SiO2catalystsinthepresenceof0.2mmolNaBH4at298K.

3. Resultsanddiscussion

3.1. PreparationandcharacterizationofNi-NP/ENF

Thebead-freeanduniformPCL/chitosanpolymericnanofibers werefabricatedbyusingelectrospinningtechnique(Scheme1). The morphology of the as-prepared electrospun PCL/chitosan nanofiberswasinvestigatedbySEMandthecollectedSEMimages indifferentmagnificationsweregiveninFig.1(a–c).Theseimages indicativeoftheformationofsmoothanduniformfibrous struc-turewithameanfiberdiameterof95±45nm(Fig.1(d)).Nickel nanoparticlessupportedonPCL/chitosannanofibers(Ni-NP/ENF)

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Scheme1. SchematicillustrationforthefabricationofPCL/chitosannanoﬁbersby electrospinning.

wereobtainedbytheconventionalimpregnationandsubsequent reduction technique [56]. For this purpose, ﬁrstlywe prepared Ni(II)/ENF precatalyst by the wet-impregnation of NiCl2·6H2O

precursorontothesurfaceofPCL/chitosannanoﬁbersatroom tem-perature.

ThewhitecolorofPCL/chitosannanoﬁbrouswebwasturninto greenattheendofthesurfacedepositionofNi(II)andtheamount of Ni(II)loadingwas foundto be0.9% wt by ICP-OESanalysis. TheformationofNi-NPonthesurfaceofPCL/chitosannanoﬁbers and the concomitant reduction of nitrophenols (2-nitrophenol, 2,4-dinitrophenoland2,4,6-trinitrophenol)weredoneunder in-situconditions byusing sodiumborohydride asreducing agent (Scheme2).

Fig.2. P-XRDpatterns of (a)PCL/chitosan nanoﬁberand Ni-NP/ENF samples obtainedunderin-situconditionsfromthereductionsof(b)2-nitrophenol,(c) 2,4-dinitrophenoland(d)2,4,6-trinitrophenolinthe2␪rangeof5–90◦_.

Fig. 2 depicts P-XRD patterns of PCL/chitosan nanofibers in additiontothoseofNi-NP/ENFsamplesobtainedunderin-situ con-ditionsfrom thereductions of 2-nitrophenol, 2,4-dinitrophenol and 2,4,6-trinitrophenol. The PCL/chitosannanofibers exhibited twostrongdiffractionpeaksatBraggangles2␪=21.3◦and23.9◦, which represent the (110) and (200) reflections respectively of a polyethylene-like crystal structure of PCL polymer with

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Scheme2.SchematicillustrationforthepreparationofNi-NPonthesurfaceofelectropunPCL/chitosannanoﬁbers.

Fig.3. SEMimagestakenatdifferentmagniﬁcationsforNi-NP/ENFcatalystsformedunderin-situconditionsfromthecatalyticreductionsof2-nitrophenol(a–c), 2,4-dinitrophenol(d–f)and2,4,6-trinitrophenol(g–i)inaqueousNaBH4solutionatroomtemperatureunderair.

orthorhombicunitcellparameters[57,58].P-XRDpatternsof Ni-NP/ENF samplesdid not show Ni(0) phases due tothe low Ni loadingonPCL/chitosannanoﬁbers(<5.0% wt),theirP-XRD pat-ternsaresimilartothatofthePCL/chitosannanoﬁbroussupport, thusindicatingthatnonewphasesappearaftertheformationof

Ni-NP/ENFandthat,afterreaction,noobservablealterationsforthe PCL/chitosannanoﬁbroussupportisdetected[59,60].

ThepreservationofthenanoﬁbrousstructureofPCL/chitosan webattheendof theNi-NP/ENFformationwasalsoconﬁrmed by SEM analyses.Fig.3 shows SEM imagesof Ni-NP/ENF

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sam-Fig.4. BFTEMimages(a–c)takenatdifferentmagniﬁcationsandHRTEMimage(d)forNi-NP/ENFcatalystformedunderin-situconditionsfromthecatalyticreductionof 2,4-dinitrophenolinaqueousNaBH4solutionatroomtemperatureunderair.

ples formed under in-situ conditions from the reductions of 2-nitrophenol,2,4-dinitrophenol and2,4,6-trinitrophenolat dif-ferentmagnifications.Theinspectionoftheseimagesrevealsthat thepreservationof fibrousstructure and formationof clumped Ni-NPonthesurfaceofPCL/chitosannanofibers.Although,SEM imagesindicated thattheformationof clumpedNiparticleson thesurface of PCL/chitosannanofibers, BFTEM analysesreveals thatthepresenceof surfacesupportedNi-NP.Fig.4(a–c)shows therepresentativeBFTEMimagesofNi/ENFsampleformedunder in-situ conditions from the reduction of 2,4-dinitrophenol (see Fig.10(videinfra)Fig.S1andforBFTEMimagesofsamplesformed underin-situconditionsfromthereductionsof2-nitrophenoland 2,4-dinitrophenol,respectively),whoseSEMimagesdisplayedthe existenceofsomeclumpedparticles.TheseBFTEMimagestaken atdifferent magnificationsclearly show theformation of well-dispersed,smallsized(dmean=2.79nm)NiNPsonthesurfaceof

PCL/chitosannanofibers.EDXanalysisperformedduringtheBFTEM observationofNi-NP/ENF frommany differentareas confirmed thepresence ofnickel metalintheanalyzedregions(Fig. S2in theSupportinginformation).Thecrystallinityofthesein-situ gen-eratedNi-NPonthesurfaceofPCL/chitosannanofiberswasalso analyzedbyHRTEMandthecollectedHRTEMimageofNi-NP/ENF sample formed under in-situ conditions from the reduction of 2,4-dinitrophenolwasgiveninFig.4(d).ThisHRTEMimageis dis-playingthehighlycrystallinenatureoftheNi-NPandthecrystalline fringedistanceof0.18nmwasmeasuredfortheindividualNi-NP, whichcanbeassignedto[200]dspacingoffccNi-NP[61].Inorder todeterminethechemicalenvironmentthechemicalenvironment andtheoxidationstateofnickelintheNi-NP/ENFsamples,we

per-formedsurveyandNi2pcorelevelXPSmeasurements(Fig.5). Fig.5(a)showsthesurveyXPSspectrumofthatrevealsthe exis-tenceofNiinadditiontothePCL/chitosannanoﬁberselements(C, NandO).TheinspectionofNi2pcorelevelXPSspectrumgivenin Fig.6(b)givesthreepeakscenteredaround856,862and878eV, whichcanreadilybeassignedtoNi(0)3p3/2,Ni(II)2p3/2andNi(0)

2p1/2,respectively[62].ThepeaksobservedforNi(II)2p3/2show

thepresenceofNiO,thatmayoriginatefromthesurface oxida-tion of Ni(0) nanoparticlesduring theXPS sampling procedure [63].Additionally,theslightshiftstowardshigherenergyregion observedinthesequenceofthesamplesrecoveredfromthe reduc-tion of 2-nitrophenol→2,4-dinitrophenol→2,4,6-trinitrophenol canbeexplainedbytheinteractionofnickel(0)nanoparticleswith formed NH2groups,whichisexpectedtoinduceapositivecharge

onthesurfacemetalandthatwouldincreasethebindingenergies ofNi(0)3p3/2andNi(0)2p1/2.

3.2. ThecatalyticactivityofNi-NP/ENFinthereductionof 2-nitrophenol,2,4-dinitrophenoland2,4,6-trinitrophenolinthe presenceofNaBH4

Thecatalyticreductionoftoxicnitrophenolstothe correspond-ingaminophenolderivativesbysodiumborohydride(NaBH4)inthe

presenceofNi-NP/ENFcatalystwasselectedasamodeltest reac-tiontoexaminethecatalyticperformanceofNi-NP/ENFcatalyst. In alkaline solution nitrophenols show their strongest absorp-tionbandsat415nm(2-nitrophenol),359nm(2,4-dinitrophenol), 393nm(2,4,6-trinitrophenol).Thus,bytracingandmonitoringthe changeoftheabsorptionpeak,therelativekineticparameterscan

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Fig.5. (a)Survey(b)Ni2pcorelevelXPSspectraofNi-NP/ENFcatalystsformedunderin-situconditionsfromthecatalyticreductionsof(i)2-nitrophenol,(ii)2,4-dinitrophenol and(iii)2,4,6-trinitrophenolinaqueousNaBH4solutionatroomtemperature.

beobtained.BeforetestingthecatalyticactivityofNi-NP/ENFone hastocheckwhether(i)self-hydrolysis ofsodiumborohydride (NaBH4)reducesthenitrophenols and (ii) thesupportmaterial

PCL/chitosan catalyzes the reduction of nitrophenols. For that reason,weperformedsixcontrolexperimentsinwhichthe reduc-tionsof2-nitrophenol,2,4-dinitrophenoland2,4,6-trinitrophenol

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Table1

Theturnoverfrequency(TOF)valuesofNi-NP/ENFcatalystinthereductionofof2-nitrophenol,2,4-dinitrophenoland2,4,6-trinitrophenolinaqueousNaBH4solutionat

differenttemperatures.

Substrate 298KTOF(min−1) 303KTOF(min−1) 308KTOF(min−1) 313KTOF(min−1)

2-Nitrophenol 46.2 61.2 71.6 117.2

2,4-Dinitrophenol 48.2 54.9 65.5 181.2

2,4,6-Trinitrophenol 65.6 75.7 93.6 160.9

Scheme3.ThechangeinthecolorofthesolutionthroughtheNi-NP/ENFcatalyzed reductionof2-nitrophenol,2,4-dinitrophenoland2,4,6-trinitrophenol.

were investigated in the presence of NaBH4 and PCL/chitosan nanofibers+NaBH4.Wefoundthatthecompletelyidenticalresults forNaBH4assistedreductionofnitrophenolsintheabsenceand presenceofPCL/chitosannanofibers(Fig.S3intheSupporting infor-mation)andtheseresultsindicatedthatPCL/chitosannanofibers arecatalyticallyinactiveinthereductionofnitrophenols.Inthe absenceofNi-NF/ENFcatalysttheconsumptionrateof nitrophe-nolsinthepresenceofequivalentamountsofNaBH4wasfoundto be2,4,6-trinitrophenol (55min)>2,4-dinitrophenol (65min)> 2-nitrophenol(240min),theseresultsrevealedthattheconsumption ofnitrophenolsoccursveryslowlybytheself-hydrolysisofsodium borohydride(Eq.(1)).

NaBH4+2H2O→ NaBO2+4H2 (1)

However,inthepresenceofverylowamountofNi(II) impreg-natedonthesurfaceofPCL/chitosannanoﬁbers(Ni=0.31␮mol) thein-situformationofNi-NP/ENFandconcomitantreductionof nitrophenolsoccurrapidlyat298K(Fig.6).Thecolorofthe reac-tionsolutionschangedfromyellowtocolorless,grayorlightbrown dependingonthesubstratethroughouttheNi-NP/ENFcatalyzed reductionofnitrophenols(Scheme3).

The reduction of nitrophenols in the presence of catalysts withanexcessNaBH4 proceedsviatheformation of phenolate

ionsby the addition of NaBH4 and the transformation of

phe-nolate ionsto aminophenols [46–51]. The absorption bands of 2-nitrophenol,2,4-dinitrophenoland2,4,6-trinitrophenolat415, 359and393nm,respectivelyshiftedto283,448and306nmupon

Table2

ActivationparametersfortheNi-NP/ENFcatalyzedreductionof2-nitrophenol,2, 4-dinitrophenoland2,4,6-trinitrophenolinaqueousNaBH4solution.

Substrate Ea(kJmol−1) H#(kJmol−1) S#(Jmol−1K−1)

2-Nitrophenol 45.6 43.1 −103.4 2,4-Dinitrophenol 63.9 61.3 −43.1 2,4,6-Trinitrophenol 44.8 42.3 −103.6

theadditionoffreshNaBH4solution.Thisindicatestheformationof 2-nitrophenolate,2,4-dinitrophenolateand2,4,6-trinitophenolate inthereactionsolutionandtheintensitiesoftheirabsorptionbands graduallydecreasedasthereductionproceedsinthepresenceof Ni-NP/ENFcatalyst(Fig.6(a–c)).Theconsiderationoftheconversion graphsgiveninFig.6(d)revealsthat(i)thecomplete consump-tionofnitrophenolsoccurredwithin3min,and(ii)theinitialrate ofNi-NP/ENFcatalyzedreductionofnitrophenolsatthesame tem-perature(298K)andnickelconcentration(0.31␮mol)followedthe order of 2,4,6-trinitrophenol>2,4-dinitrophenol>2-nitrophenol. The initial turnover frequency (TOF) of Ni-NP/ENF catalyst in the catalytic reduction of 2-nitrophenol, 2,4-dinitrophenol and 2,4,6-trinitrophenolwerefoundtobe46.2mol2-nitrophenol/mol Nimin, 48.2mol 2,4-dinitrophenol/mol Nimin, 65.6mol 2,4,6-trinitrophenol/mol Nimin. It should be noted that, these TOF valuesrecordedatroomtemperaturearehigherthanthosefound withr-GOsupportedCu2Onanoparticles[64],polyanilinecoated

Aunanorods[65],hydrogelsupportedNinanoparticles[66]and hydrogelsupportedConanoparticles[67]catalystforthecatalytic reductionofthesamesubstrates.

3.3. TheactivationparametersforNi-NP/ENFcatalyzedreduction of2-nitrophenol,2,4-dinitrophenoland2,4,6-trinitrophenol

TheeffectoftemperatureontheNi-NP/ENFcatalyzed reduc-tionof2-nitrophenol,2,4-dinitrophenoland2,4,6-trinitrophenol wasalsoinvestigatedbyperformingaseriesofexperimentsfor Ni-NP/ENFcatalyzedreductionof2-nitrophenol,2,4-dinitrophenol and2,4,6-trinitrophenolat298,303,308and313K(Fig.7). Expect-edly(Fig.7(a–c)),thecatalyticactivityofNi-NP/ENFenhancedby theincreaseofthetemperatureinthereductionofallnitrophenol substratesand thedeterminedTOFvaluesatdifferent tempera-turesweregiveninTable1.Theobservedrateconstants(kobs)at

eachtemperaturewerecalculatedfromthelinearportionofeach plotandtheserateconstantswereusedtoconstructArrheniusand Eyring-PolonyiplotsgiveninFig.8toﬁndtheactivationenergy (Ea),activationenthalpy(H#)andactivationentropy(S#).The

determinedEa,H# andS# valuesweretabulatedinTable2.

Assumingthattheapparentactivationparameterscalculatedfrom themacroscopickineticdatagivenabovearerelevanttothemost criticalactivationstepinnitrophenolsreductionmechanism,one can argue that the positive magnitude of theapparent activa-tionenthalpyandlargenegativevalueoftheapparentactivation entropyimplythepresenceofanassociativereactionsteprevealing atransitionstate[68,69].

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Fig.6.UV–visspectrafortheNi-NP/ENF(0.31␮molNi)catalyzedreductionsof(a)2-nitrophenol(20␮mol),(b)2,4-dinitrophenol(20␮mol),(c)2,4,6-trinitrophenol (20␮mol)(d)theremainingfractionofnitrophenolsversustimegraphforNi/ENFcatalyzedreductionofnitrophenolsintheaqueoussodiumborohydride(0.2mmol) solutionatroomtemperatureunderair.

Table3

ThecomparisonofthecatalyticactivitiesofNi(0)nanoparticles-basedsupportedcatalysts(Ni-NP/ENF,Ni/Al2O3,Ni/Al2O3,Ni/SiO2)inthecatalyticreductionof2-nitrophenol,

2,4-dinitrophenoland2,4,6-trinitrophenolinaqueousNaBH4solutionatroomtemperatureunderair.

Substrate Ni-NP/ENFTOF

(min−1₎

Ni/Al2O3TOF

(min−1₎

Ni/CTOF(min−1₎ _Ni/SiO₂_TOF

(min−1₎

2-Nitrophenol 46.2 2.4 7.6 7.0

2,4-Dinitrophenol 48.2 6.5 8.7 8.3

2,4,6-Trinitrophenol 65.6 3.1 3.2 6.3

3.4. ThecatalyticdurabilityanduniquenessofNi-NP/ENFinthe reductionof2-nitrophenol,2,4-dinitrophenoland

2,4,6-trinitrophenolinthepresenceofNaBH4

The isolability and reusability as crucial measures of cat-alytic durability werealso tested for Ni-NP/ENF catalystin the catalyticreductionof2-nitrophenol,2,4-dinitrophenoland 2,4,6-trinitrophenol at 298K. For this purpose, after the complete reduction of nitrophenols, Ni-NP/ENF catalyst was isolated as black ﬁlms,washed withwater, driedand bottled underargon atmosphere.TheisolatedNi-NP/ENFcatalystwasweighedand re-dispersedinaqueousfreshnitrophenolssolutionsandyetanactive inthecatalyticreductions(Fig.9).TheinitialTOFvaluesprovided byNi-NP/ENFcatalystatthe3rdreusewerecalculatedas39.7mol

2-nitrophenol/molNimin,40.0mol2,4-dinitrophenol/molNimin, 54.0mol2,4,6-trinitrophenol/mol Nimin at298K.These results arerevealing thatNi-NP/ENFcatalyst retains86, 83and 82%of itsinherentactivityevenat3rdcatalyticreuseinthereductionof 2-nitrophenol,2,4-dinitrophenoland2,4,6-trinitrophenol, respec-tively. The reusabilityperformance of Ni/ENFcatalyst is higher thanthatofobtainedbyAu@SiO2[70],polygonalshapedAu[71]

andcitratestabilizedAunanoparticles[72]usedascatalystinthe reductionofnitrophenols.

TheremarkablereusabilityperformanceofNi-NP/ENFcatalyst maybeattributedtothehighstabilityofthein-situgeneratedNi NPsagainsttobulkNiformation.Indeed,arepresentativeBFTEM images of Ni/ENF sample harvested after the third reuse from thereductionof2-nitrophenolstillshowstheexistenceof

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non-Fig.7. Theremaining fraction ofnitrophenols versustime graphfor Ni/ENF (0.31␮molNi)catalyzedreductionsof2-nitrophenol(20␮mol),2,4-dinitrophenol (20␮mol)and2,4,6-trinitrophenol(20␮mol)intheaqueoussodiumborohydride (0.2mmol)solutionatdifferenttemperaturesintherangeof298–313K.

Fig.8.(a)Arrheniusand(b)Eyring-PolonyiplotsforNi/ENF(0.31␮molNi) cat-alyzedreductionsof2-nitrophenol(20␮mol), 2,4-dinitrophenol(20␮mol)and 2,4,6-trinitrophenolintheaqueoussodiumborohydride(0.2mmol)solution.

agglomeratedNi(0)NPsinNi-NP/ENFcatalyst(Fig.10).Theslight decrease(∼15%)inthethirdcatalyticreusemaybeattributedtothe decreaseinthenumberofactivesurfaceatomsduetothe clump-ingofsurfacesupportedNi(0)nanoparticles,whichwerelabelled inFig.10(c)and(d).Moreimportantly,Niwasnotdetectedinthe ﬁltratecollectedfromeachcyclebytheICP-MStechnique(witha detectionlimitof28ppbforNi)conﬁrmingthatthereisno indica-tionofleachingofNi(0)nanoparticlesintoreactionsolutionwithin thedetectionlimitofICP-MS.

3.5. ThecatalyticuniquenessofNi-NP/ENFinthereductionof 2-nitrophenol,2,4-dinitrophenoland2,4,6-trinitrophenol

TheuniquenessofNi-NP/ENFwascomparedwiththemost com-monlyusedsolidsupportmaterials(Al2O3,SiO2andcarbon).The

catalyticactivitiesofNi/Al2O3(1.1%wtNiloading),Ni/SiO2(1.0%wt

Niloading)andNi/C(0.9%wtNiloading)weretestedinthe reduc-tionof2-nitrophenol,2,4-dinitrophenoland2,4,6-trinitrophenolin thepresenceofequalamountsofNiandNaBH4atroom

tempera-ture.TheresultsoftheseexperimentsweregiveninFig.11,which indicatesthat thereduction of2-nitrophenol, 2,4-dinitrophenol

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Fig.9.TheremainingfractionofnitrophenolsversustimegraphforthereusabilityperformanceofNi/ENFcatalystinthecatalyticreductionsof2-nitrophenol, 2,4-dinitrophenoland2,4,6-trinitrophenol.

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Fig.10.BFTEMimages(a–d)ofNi/ENFsampleharvestedafterthethirdreusefromthereductionof2-nitrophenol.

and2,4,6-trinitrophenolproceedsmorerapidlybyNi-NP/ENF cata-lyst.TheinitialTOFvaluesofNi/Al2O3,Ni/SiO2andNi/Cdetermined

in the reduction of 2-nitrophenol, 2,4-dinitrophenol and 2,4,6-trinitrophenolwerecomparedwithNi-NP/ENFcatalystinTable3. ThecomparisonofthesevaluesshowsthatNi-NP/ENFcatalystis atleast6andathighest20timesmoreactivethannickelbased Ni/Al2O3,Ni/SiO2andNi/Ccatalysts.BFTEManalysesofNi/Al2O3,

Ni/SiO2 andNi/Ccatalystsshowedthatformationoflargersized

Ni(0)nanoparticles(Fig. S4intheSupportinginformation)with respecttoNi-NP/ENF catalyst,which explainshigheractivityof Ni-NP/ENF catalysts than Ni/Al2O3, Ni/SiO2 and Ni/C catalysts.

Although,these catalyticmaterials wereprepared by thesame methodologyunderidenticalconditions,theformation ofsmall sizedNi-NPonPCL/chitosannanoﬁberscanbeexplainedbythe existenceofchitosan NH2 grouponthesurface ofnanoﬁbers,

whichactsasligandstabilizertosurfaceboundnickel(0) nanopar-ticles[73,74]andpreventtheirsurfaceagglomerationinNi-NP/ENF catalyst.

4. Conclusions

In the current study, PCL/chitosan nanoﬁbers supported nickel(0)nanoparticles(Ni-NP/ENF)wereprepared,characterized andusedasnanocatalystinthecatalyticreductionofvarious nitro-phenolsinthepresenceofsodiumborohydrideasareducingagent. Someofthemajorﬁndingsofthisstudycanbesummarizedas follows:

(a)Ni-NP/ENF catalyst can reproducibly be prepared by the conventional wet-impregnation of Ni(II) onto PCL/chitosan nanoﬁbersandtheirborohydridereductionunderin-situ condi-tionsduringthecatalyticreductionofnitrophenolsinaqueous solutionallatroomtemperature,

(b)Ni-NP/ENFcatalystwascharacterizedbyusingICP-OES,P-XRD, XPS,BFTEM,BFTEM-EDXandHRTEManalyses.Theresultsof these multi-pronged analysesreveal theformation of well-dispersedandhighlycrystallinenickel(0)nanoparticlesonthe surfaceofPCL/chitosannanoﬁbers,

(c)The catalytic performance of Ni-NP/ENF in terms of activ-ity and stability was tested in the catalytic reduction of 2-nitrophenol, 2,4-dinitrophenol and 2,4,6-trinitrophenol in aqueous sodium borohydride solution under mild condi-tions (atroom temperature and under air). Ni-NP/ENF was found to be highly active nanocatalyst in these catalytic transformations.Theyprovideexceptionalturnover frequen-cies in the catalytic reduction of 2-nitrophenol (46.2mol 2-nitrophenol/molNimin), 2,4-dinitrophenol (48.2mol 2,4-dinitrophenol/molNimin) and 2,4,6-trinitrophenol (65.6mol 2,4,6-trinitrophenol/mol Nimin), which are thehighest TOF values amongtheheterogeneouscatalyststestedinthe cat-alyticreductionofthesesubstrates,

(d)Moreover,thesenewnickel(0)nanoparticlesshowexceptional stabilitythroughoutthecatalyticrunsagainstleachingand sin-teringsothattheyretain>82%oftheiractivityevenatthe3rd catalyticreuse.

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Fig.11.Theremainingfractionofnitrophenolsversustime graphforNi/ENF, Ni/Al2O3, Ni/SiO2 and Ni/C (in all 0.31␮mol Ni) catalyzed reductions of

2-nitrophenol(20␮mol),2,4-dinitrophenol(20␮mol),2,4,6-trinitrophenol(20␮mol) intheaqueoussodiumborohydride(0.2mmol)solutionatroomtemperatureunder air.

Overall,Ni-NP/ENFcatalystisavailablebyasimpleprocedure and is foundtobesuperior heterogeneouscatalyst interms of activityandstabilityinthecatalyticreductionof2-nitrophenol, 2,4-dinitrophenol and 2,4,6-trinitrophenol in aqueous sodium borohydridesolutionundermildconditions.

Acknowledgements

MZthankstothepartialsupportsbyFevziAkkayaScientiﬁc Activities Support Fund (FABED), Science Academy and Turk-ishAcademyofSciences (TUBA).TUalsothankstoTheTurkish Academyof Sciences—OutstandingYoung ScientistsAward Pro-gram(TUBA-GEBIP)forpartialfunding.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound, in theonlineversion,at http://dx.doi.org/10.1016/j.apcatb.2016. 10.020.

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