Bimetallic Nanoparticles as Efficient Catalysts

25 Pages·2016·7.36 MB·English

materials
Article
Bimetallic Nanoparticles as Efficient Catalysts:
Facile and Green Microwave Synthesis
MagdaBlosi1,SimonaOrtelli1,AnnaLuisaCosta1,MicheleDondi1,AliceLolli2,
SaraAndreoli2,PatriciaBenito2andStefaniaAlbonetti2,*
1 ISTEC-CNR,InstituteofScienceandTechnologyforCeramics,NationalResearchCouncil,ViaGranarolo64,
Faenza48018,Italy;[email protected](M.B.);[email protected](S.O.);
[email protected](A.L.C.);[email protected](M.D.)
2 DepartmentIndustrialChemistry“TosoMontanari”,BolognaUniversity,VialeRisorgimento4,
Bologna40136,Italy;[email protected](A.L.);[email protected](S.A.);[email protected](P.B.)
* Correspondence:[email protected];Tel.:[phone]
AcademicEditor:NikolaosDimitratos
Received:13June2016;Accepted:4July2016;Published:8July2016
Abstract: Thisworkdealswiththedevelopmentofagreenandversatilesynthesisofstablemono-
andbi-metalliccolloidsbymeansofmicrowaveheatingandexploitingecofriendlyreagents: water
as the solvent, glucose as a mild and non-toxic reducer and polyvinylpirrolidone (PVP) as the
chelatingagent. Particlesize-control, totalreactionyieldandlong-termstabilityofcolloidswere
achievedwiththismethodofpreparation. Allofthematerialsweretestedaseffectivecatalystsinthe
reductionofp-nitrophenolinthepresenceofNaBH astheprobereaction. Asynergisticpositive
4
effect of the bimetallic phase was assessed for Au/Cu and Pd/Au alloy nanoparticles, the latter
showingthehighestcatalyticperformance. Moreover,monoandbi-metalliccolloidswereusedto
prepareTiO -andCeO -supportedcatalystsfortheliquidphaseoxidationof5-hydroxymethylfufural
2 2
(HMF) to 2,5-furandicarboxylic acid (FDCA). The use of Au/Cu and Au/Pd bimetallic catalysts
ledtoanincreaseinFDCAselectivity. Finally,preformedPd/Cunanoparticleswereincorporated
into the structure of MCM-41-silica. The resulting Pd/Cu MCM-41 catalysts were tested in the
hydrodechlorinationofCF OCFClCF CltoCF OCF=CF . TheeffectofCuonthehydrogenating
3 2 3 2
propertiesofPdwasdemonstrated.
Keywords: Au/Cu;Pd/Au;Pd/Cu;nanosols;microwave-assistedsynthesis;supportedcatalysts;
HMF;hydrodechlorination
1. Introduction
Inrecentyears,noblemetalnanoparticleshaveattractedmuchattentionowingtotheirpotential
applicationinmanydifferentfieldsduetotheiroptical,electrical,mechanicalandcatalyticproperties.
Bimetallicnanoparticles,composedoftwodifferentmetalelements,areofagreatinterestfromboththe
scientificandtechnologicalpointsofview. Thestructureofbimetallicnanoparticlesisdefinedbythe
distributionmodesofthetwoelementsandcanbeorientedinrandomalloy,alloywithanintermetallic
compound,cluster-in-clusterandcore-shellstructures,leadingtoenhancedapplications,comparedto
monometallicnanoparticles[1–4].
In this context, bimetallic nanostructures show remarkably improved optical, catalytic and
electronicpropertieswithrespecttomonometalliccounterparts[5].Infact,itisexpectedthatbimetallic
nanoparticlescouldshownotonlythecombinationofthepropertiesrelatedtothepresenceoftwo
individualmetals,butalsonewpropertiesduetoasynergybetweenthetwometals. Theshapeand
sizeofmono-andbi-metallicnanoparticlesarestrictlydependentonthepreparationmethodsand
conditionsandaffectthephysicochemicalpropertiesofthefinalnanomaterial. Thecombinationoftwo
Materials2016,9,550;doi:10.3390/ma9070550 www.mdpi.com/journal/materials
Materials2016,9,550 2of25
metalsresultsinmuchmorepossibilitiesinshapeandstructureduetothemiscellaneousdistribution
ofeachmetalwithinaparticleandtheirvariousorganization[6].
Pre-formed metal nanoparticles can be used in catalysis in a wide range of reactions.
Thedepositionoftheseparticlesontodifferentsupportsisfundamentaltoguaranteeparticles’stability,
mechanical resistance and easier work-up procedure at the end of the process. Thus, a synthetic
approach,whichallowsahomogeneousnanoparticlesuspension,isofcrucialimportanceforcatalytic
applications,inordertoexploittherealeffectoftheactivephaseontheactivity. Thiseffectiseven
moresignificantwhenitcomesfromthecomparisonofbimetallicandmonometallicnanoparticles.
In particular, the development of a colloidal synthesis that can guarantee precise composition,
morphologyandparticlesizeisofcrucialrelevanceforbimetallicnanoparticles[7].
Inthisstudy,wereportthedevelopmentofbimetallicnanostructures(Au/Cu,Pd/Au,Pd/Cu)by
meansofapatentedenvironment-friendlyapproach[8],easilytransferabletolarge-scaleproduction
and allowing the achievement of stable nanosols, even at high solid loading. Water is the benign
solvent;glucoseservesasamild,renewableandnon-toxicreducingagent;andpolyvinylpyrrolidone
(PVP)asawater-soluble,inexpensiveandnon-toxicchelatingadditive. Theusesofinexpensiveand
nontoxicchemicals,environmentally-benignsolventsandrenewablematerialsaresomeoftheessential
issuesinthenanomaterialssciencefieldinlightofthe“green”syntheticstrategyforindustrial-scale
manufacturing[9]. Mostofthesyntheticroutesreportedintheliterature, evenifscalabletolarge
production, are based on organic solvents, thus implying a complex environmental path to the
industrialproduction[10–14]. Sofar,someexpensiveand/ortoxicchelatingagents(thiols,oleicacid,
hexadecylamine, trioctylphosphineoxide)[15–17]areemployedtopreparemetalnanoparticlesin
organicsolvents,makingthesesynthesisprocesseslesspromisingforasubsequentindustrialscale
up. Moreover,concerningthereducingagents,alargebodyofliterature[18–20]proposedstrongand
hazardousreducingagents,suchashydrazine,sodiumborohydride(NaBH )anddimethylformamide
4
(DMF),whicharehighlyreactiveandpresentpotentialenvironmentalandbiologicalrisks.
Fromtheprocessintensificationviewpoint,microwaveirradiationwasexploitedastheheating
source [21–23], so enabling rapid achievement of the desired temperatures and the homogenous
heatingofthetreatedvolumes.
Onallofthepreparedbimetallicnanosols,thecatalyticperformanceswereassessedandcompared
exploitingtheprobereactionof4-nitrophenole(4-NP)hydrogenation[24]. Then,onthesupported
catalysts,adeepercatalyticcharacterizationwascarriedout.
Gold-supportednanoparticleshavebeenwidelyusedascatalystsinthefieldofalcoholoxidation
reactions[25,26]. Morerecently,thesecatalystshavebeentestedin5-hydroxymethylfurfural(HMF)
oxidationtoproduce2,5-furandicarboxyicacid(FDCA)[27,28], amonomerusedforthesynthesis
of a new class of polymers made from renewables [29,30]. In fact, HMF is derived from sugars’
dehydration, anditcanbetransformedintoaseriesofinterestingbuildingblocks, bothexploited
inthepharmaceuticalandinthechemicalindustries[31]. Inordertoimprovecatalystperformance
in terms of activity and stability, researchers have focused their attention on the development of
bimetallicnanoparticleswithcontrolledsize,compositionandmorphologyandtheuseofdifferent
supports [7]. For example, Au/Pd-supported catalysts are considered to be efficient for alcohols
oxidationreactions[32].
The goal of this paper was thus to describe the synthetic approach developed in our group.
Thismicrowave(MW)-assistedmethodallowstheformationofcolloidalmonometallicandbimetallic
nanoparticles with controlled size, composition and different morphologies (alloy and core-shell)
to be used as is and for the preparation of supported catalysts. In particular, supported Au/Cu
and Au/Pd nanoparticles were used in HMF oxidation, while Pd/Cu-MCM-41 catalysts were
tested in the hydrodechlorination of CF OCFClCF Cl to produce trifluoromethyl trifluorovinyl
3 2
ether(CF OCF=CF ).
3 2
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2. ResultsandDiscussion
2.1. PreparationandCharacterizationofMetallicSols
All of the prepared bimetallic sols exploited the green glucose as the reducing agent, which
maximizes its reducing performance in alkaline conditions [33,34]. A simple, microwave-assisted
strategy was developed for the preparation of different metal nanoparticles stabilized by
polyvinylpyrrolidone (PVP), with various morphologies (alloy and core-shell). For lack of space,
inthiswork,wewilljustdiscusstheresultsobtainedusingmetallicalloys;nevertheless,recentworks
reportourresultsobtainedutilizingcore-shellsystems[35,36]. Asageneralobservation,microwave
heatinghasbeenshowntoprovidehomogeneousparticlenucleationandshortersynthesistimethan
traditionalheating, andthedevelopedsynthesisrouteshowedseveraladvantageswithrespectto
othermethods(FigureS1). Indeed,itissimple,eco-friendly,carriedoutatlowtemperatureandeasily
transferable to large-scale production [8]. Particle size-control, total reaction yield and long-term
stabilityofcolloidswereachievedthankstoanaccuratereactionoptimizationandthecombination
withmicrowaveheating,promotingtheintensificationoftheprocessevenforlarge-scaleproduction.
ThecharacteristicsofthesynthesizedbimetallicsamplesarelistedinTable1.
Table1.Particlesizedistributionofthesynthesizednanoparticlesbymeansofdifferenttechniques:
dynamiclightscattering(DLS),transmissionelectronmicroscopy(TEM),X-raydiffraction(XRD).
Sample1 d-DLS(nm) PDI3 d-TEM(nm) d-XRD(nm)
Au 17 0.2 4˘2[37] 4[38]
Cu 350,802 0.4 3˘1 70
Pd 23 0.3 2˘1[35] 6
Au Cu 17[38] 0.2 - 4.0[38]
3 1
Au Cu 23 0.3 3˘2 3.5[38]
1 1
Au Cu 61 0.3 - 3.0[38]
1 3
Au Cu 80 0.5 - n.d.[38]
1 6
Pd Au - - 3˘1[35] 3
3 1
Pd Au 36 - - 5
1 0.5
Pd Au 25 0.4 3˘1[35] 3
1 1
Pd Au 71 - - 3
1 2
Pd Au 35 0.3 4˘2 4
1 3
Pd Au 34 0.2 3˘2[35] 5
1 6
Pd Au - - 4[35] 5
1 9
Pd Cu 73 0.3 - 5
1 0.5
Pd Cu 79 0.4 - 3
1 1
Pd Cu 88 0.4 - 4
1 2
Pd Cu 99 0.2 - 5
1 3
Pd1Cu6 133,402 0.5 - 6
1Thenominalmolardistributionofthebimetallicstructureisreported;2theDLSdiameterreferstothebimodal
distribution;3polydispersionindex.
2.1.1. Au/CuSystem
AuandCunanosolsshowedverydifferentcolloidalstability. Infact, theAusampleshowed
anoptimaltimestabilityofalmost12months,whileCunanosolwasaffectedbystrongaggregation
phenomenathatinvolvedprecipitationwithin24h.
Indetail,AunanoparticleshadameanhydrodynamicdiameterbyDynamicLightScattering
(DLS) of 17 nm, with a low polydispersity index (PDI) value typical of a narrow size distribution.
Onthecontrary,Cunanoparticlesshowedabimodaldistributionwiththemajorpeaksetat350nm.
This marked difference was not reflected by TEM analysis, which is reliable for the detection
of Cu primary nanoparticles with an average dimension of 3 nm, so neglecting the coarser and
sedimentedparticles.
Materials2016,9,550 4of25
Theco-synthesesperformedforachievingthealloycompounds,Au/Cu,showedaverygood
colloidalstability(severalmonths),despitetheaggregationtendencyobservedforCunanoparticles
alone. Probably,thepresenceofAuseedactedwithasortofstabilizingeffect,preventingtherapid
aggregationofthenewly-formedbimetallicparticles.Anyway,theinfluenceofthecopperpresencewas
observedbyDLS.Forgoldandgold/coppersuspensions,uptoaAu:Cuatomicratio1:1,arelatively
monodispersedsignal,withanaveragediameterof17–20nm,wasdetected. Increasingthecopper
content led to polydispersed particles with a larger mean DLS diameter. Moreover, as indicated
bythePDI,thesizedistributionwascloselyrelatedtotheaverageparticlediameter: thelargerthe
particlediameter,thebroadertheparticlesizedistribution. Actually,theseresultsseemstoindicate
thataqueoussols,uptoaAu:Cuatomicratio1:1,arepredominantlyconstitutedbyhomogeneous
bimetallicnanoparticleswithaunimodalparticlesizedistribution,whileincreasingthecoppercontent
inthesolcausestheformationofnanoclusterswithinhomogeneoussizeand/orcomposition,asshown
byXRDanalysisdiscussedinthefollowing. Furthermore,adeepenedstudyontheinfluenceofPVP
concentrationoverthehydrodynamicdiameterwasperformedandreportedintheSupplementary
Materials(TableS1andFigureS2).
XRDanalyseswithdifferentAu:Cumolarratiosshowedthepeakbroadeningtypicalofnanosized
crystallites. Thepatternsareconsistentwiththesingle-phasefacecenteredcubicstructure(fcc)ofgold
andcopper(spacegroupFm-3m),withnoobservableimpuritiesofCuO ofthephase-segregated
x
metalsuptotheAu Cu sample. Increasingthecopperamount,ashiftofthemaingoldpeakwas
1 3
progressivelyobserved;thisbehaviorisconsistentwiththeformationofthealloycompounds[39].
For higher Cu content (Au Cu , Au Cu ), the presence of a reflection at 43.1˝ 2θ suggests the
1 3 1 6
segregationofcopperwiththeformationofcopperoxide(Cu O).
2
TheinterplanardistancesassessedforthedifferentsamplesarereportedinTable2. Probablyin
the presence of an excess of copper, only part of the Cu forms the alloy, while the remaining part
couldbeoxidizedbygold,formingaredoxcoupleCu/Au. Thedecreasingoftheinterplanardistance
forahighcopperamountwouldbecausedbyCu-Auisomorphoussubstitutionwithcopperatoms
characterizedbyaloweratomicradius.
Table2.Interplanardistances(100)detectedforAu,CuandAu/Cucompounds.
Sample d(Å)
Au 2.352
Au Cu 2.314
1 1
Au Cu 2.291
1 3
Au Cu 2.278
1 6
Cu 2.080
Moreover,thebimetallicmaterialsgivebroaderXRDpeaks,suggestingthattheAu/Cusystem
hasasmallercrystallitesizethanthemonometallicone. Ineffect,crystalsizevalues,estimatedusing
theScherrerequation,indicated~4nmforgoldand~3–3.5nmforAu/Cusols,confirmingthecoarser
sizeofthemonometallicsystem. TheseresultswerealsoconfirmedbyTEManalysisthatevidencedfor
Au Cu aparticlesizelowerthanthemonometalAu,supportingtheformationofthealloyAu Cu
1 1 1 1
(Figure 1). As well as the gold nanoparticles [40], also the bimetallic structure (Au Cu ) showed
1 1
asphericalmorphology. Theirregularshapeobservedforthelargerparticlesprobablyresultsfromthe
aggregationduringthepermanencefor40minat90˝C.
Theselectedareaelectrondiffractionanalysis(SAED)supportedtheformationoftheAu/Cu
alloy,correspondingtocrystalliteswithafacecenteredcubicstructure. Thecollectedsignalsevidenced
interplanardistancesof0.22,0.20and0.14nm,correspondingrespectivelytotheplans(111),(200)and
(220)andconsistentbothwiththeXRDresultsandwiththevaluesreportedintheliteratureforthe
Au Cu alloy(JCPDSCardNo. 65-0937)(Table3).
1 1
Materials 2016, 9, 550 5 of 25
Materials2016,9,550 5of25
Materials 2016, 9, 550 5 of 25

Figure 1. HRTEM images of sample Au1Cu1 at different magnifications (a,b). The inset shows the FFT
of the observed nanoparticle (distances at 0.22 and 0.20 nm are circled); (c) selected area electron
diffraction analysis (SAED) analysis performed on several nanoparticles.
The selected area electron diffraction analysis (SAED) supported the formation of the Au/Cu
alloy, corresponding to crystallites with a face centered cubic structure. The collected signal s
evidFeFiniggcuuerrdee 11in.. HtHeRrRpTTElEaMMn aimrim adagigesstea sonfo csfeasmsa mopflpe 0 lAe.2uA21Cu, u0C.12 au0t daaintffdder ife0fn.e1tr 4emn natgmmn,ia fgicconaritfiiroecnasstpi (ooann,bsd)(.i anT,gbhe) .r ienTsshpeete scinhtisovewteslsy hth otewo F stFhTthe e plans
(111)o, f( 2th0e0 )o basnedrv (e2d2 0n)a naonpda rctoicnles i(sdtiesntatn bceost ha1 tw 0.i12th2 athnde X0.R20D n rme saurlet sc iarcnlded w); i(tch) stehlee cvteadl uaerse ar eepleoctrrtoend in the
FFToftheobservednanoparticle(distancesat0.22and0.20nmarecircled);(c)selectedareaelectron
literadtiuffrrea cftoiorn t hane aAlyusi1sC (uSA1 aElDlo) ya n(aJClyPsiDs pSe Crfaorrdm eNdo o.n 6 s5e-v0e9r3a7l )n (aTnaobplaer t3ic)l.e s.
diffractionanalysis(SAED)analysisperformedonseveralnanoparticles.
TTTahabebl elsee3 3l.e. IcIntnettederr ppalrlaaennaaa rer lddeicisstttraaonnncce edss iofofffr tathhcetei AoAnuu a1CCnuaul1y sssaaimsm p(pSlelAe..E TTDhhe)e svvuaalpluupeeossr cctoeoldllle ectchtteeedd f bobyrym TTaEEtMMio naa nnoddf XtXhRReDD A aaurre/eC u
1 1
alloy,c ocmorpraersepdo tnod thineg t abtou lactreyds tdaaltlait.e s with a face centered cubic structure. The collected signals
comparedtothetabulateddata.
evidenced interplanar distances of 0.22, 0.20 and 0.14 nm, corresponding respectively to the plans
Interplanar Distance d(Å) TEM d(Å) XRD d(Å) Literature
(111), (200) and (220) and consistent both with the XRD results and with the values reported in the
InterplanarDistance d(Å)TEM d(Å)XRD d(Å)Literature
literature for the Au1Cu1 adll o(1y11 )( JCPDS Card N2o.2. 76 5-0937) (T2a.b3l1e 3). 2.23
dd(1 (1210)0) 21.2.977 22..3013 21..2938
d 1.97 2.03 1.98
Table 3. Interplanar ddids(t2 a(02n02)0c)e s of the Au1C1u1.41. 41s1a mple. The1 1.v.44a22lu es collected 1b1.y.44 00T EM and XRD are
compared to the tabuladtde(2 d(23 01d)1a) ta. 11.2.200 11..2222 11..1199
(311)
Interplanar Distance d(Å) TEM d(Å) XRD d(Å) Literature
The evolution of the surface plasmon resonance bands (SPR) for the Au/Cu samples is shown in
FiguTreh e2.e Bvyo laudtidoinngo fCtuhe, dtsh u(e1r1 1fS)a PcRe pmlaasxmimounmre2 sp.o2en7a akn acbesboarnpdt2is.o3n(1S sPhRi)ftfeodr ftrhoem2A. 2tuh3/ eC Auus acmhaprlaecsteisrisshtiocw vnaliune
F(5ig2u5r nem2.)B tyowadadrdins gthCeu C,tudh e(t2y0S0p)P iRcaml vaaxliumeu, mbe1cpo.e9ma7k inagb seovrepnt 2iio.n0n 3s shhaipftee dmfororem 1s.ti9hm8e ilAaru tcoh tahrea cCteur irsetiscovnaalnucee
(b5a25ndn mch)atroawctaerrdizsetdhe bCy ua tdny op(2n2i0c-)as hlvaraplu per,obfeicleo m[14.i04n]1g. evenins1h.4a2p emoresimi1la.4r0t otheCuresonanceband
characterizedbyanon-shda (r3p11)p rofile[40]. 1.20 1.22 1.19
The evolution of the surface plasmon resonance bands (SPR) for the Au/Cu samples is shown in
Figure 2. By adding Cu, the SPR maximum peak absorption shifted from the Au characteristic value
(525 nm) towards the Cu typical value, becoming even in shape more similar to the Cu resonance
band characterized by a non-sharp profile [40].

FFiigguurree2 2..U UVV-V-VISISs sppeecctrtraac coollleleccteteddf oforrA Auu//CCuu aallllooyy ssaammpplleess..
Thereactionyieldforthesesampleswascalculatedbyestimationoftheunreactedionsassessed
byICPanalysisontheultrafilteredsamples. Theresultsshowedatotalreactionyieldforallofthe
consideredsamples.
Figure 2. UV-VIS spectra collected for Au/Cu alloy samples.
Materials 2016, 9, 550 6 of 25
The reaction yield for these samples was calculated by estimation of the unreacted ions assessed
by ICP analysis on the ultrafiltered samples. The results showed a total reaction yield for all of the
considered samples.
Materials2016,9,550 6of25
2.1.2. Pd/Au and Pd/Cu Systems
2.1.2. Pd/AuandPd/CuSystems
Exploiting the same green reaction, both Pd/Au and Pd/Cu compounds were synthesized in the
formE oxfp laoliltoiynsg. tAhes soabmseergvreede nforre aActuio/Cnu,b soothls,P dd/esApuitea ntdheP dag/gCruegcaotmiopno tuennddsenwceyr eosf ytnhteh eosnizlye dCiun
nthaenofopramrtioclfeasl, lothyes .coAuspolibnsge rwveitdh fPodr Apruo/dCuucesdo lhsi,gdhelsyp sittaebtlhee saoglsg, reevgiadteionncitnegn da epnocsyitoivfet hseynoenrlgyisCtiuc
enfafnecotp oanr ttihclee sc,otllhoeidcaolu sptalibniglitwy.i thPdproducedhighlystablesols, evidencingapositivesynergistic
effectFoignutrhee 3c oslhlooiwdsa lssotmabei liotfy .the prepared sols for both Pd/Au and Pd/Cu series. The addition of
pallaFdiiguumre, d3useh toow itss stoympiecaol fdtahrek pcroelpora,r weditsho als vfeorryb bortohadP da/bsAourbaanndceP idn/ tCheu vsiesriibelse. rTanhgeea, dednihtiaonnceodf
pthaell addaiurkmn,edsus eotfo itthsety psuicsaplednasrikoncos,l omr,wakitihnga vveerryyb rdoiaffdicaublsto rtbhaen cceolilnecthtieonvi soibf leUrVan-VgeI,Se nshpaenctcread tfhoer
tdhaersken nesasnoofsothlse. suspensions,makingverydifficultthecollectionofUV-VISspectraforthesenanosols.

Figure 3. Images of (a) the Pd/Au series; and (b) the Pd/Cu series.
Figure3.Imagesof(a)thePd/Auseries;and(b)thePd/Cuseries.
The hydrodynamic diameters evaluated by DLS are listed in Table 1. For the Pd/Au sols, the
ThehydrodynamicdiametersevaluatedbyDLSarelistedinTable1. ForthePd/Ausols,the
resulting curves were monomodal with mean diameters comparable each other (from 24 to 35 nm)
resultingcurvesweremonomodalwithmeandiameterscomparableeachother(from24to35nm)and
and slightly larger than the single metal ones (17 nm for Au and 23 nm for Pd). Only Pd1Au2
slightlylargerthanthesinglemetalones(17nmforAuand23nmforPd). OnlyPd Au evidenced
evidenced a larger dimeter, 71 nm, probably due to some aggregation phenomena. 1 2
alargerdimeter,71nm,probablyduetosomeaggregationphenomena.
A different behavior was observed for the Pd/Cu series, as already shown in Au/Cu samples;
AdifferentbehaviorwasobservedforthePd/Cuseries,asalreadyshowninAu/Cusamples;
the higher is the Cu amount, the larger is the detected hydrodynamic diameter (from 73 to 133 nm)
thehigheristheCuamount,thelargeristhedetectedhydrodynamicdiameter(from73to133nm)
with an increased polydispersion well evidenced by the PDI values. The size distribution curves
with an increased polydispersion well evidenced by the PDI values. The size distribution curves
(Figure 4a,b) showed the marked influence produced by the copper addition, which for the
(Figure4a,b)showedthemarkedinfluenceproducedbythecopperaddition,whichforthecomposition
composition Pd1Cu6 evidenced the bimodal size distribution typical of copper sols.
Pd Cu evidencedthebimodalsizedistributiontypicalofcoppersols.
1 W6e hypothesized that the smaller peak could corresponded to a different phase, for example
We hypothesized that the smaller peak could corresponded to a different phase, for example
Cu2O; probably, with a large excess of copper the total reduction of the precursor is too difficult to be
Cu O;probably,withalargeexcessofcopperthetotalreductionoftheprecursoristoodifficulttobe
com2pleted, and an oxidized part remained in the sample. This hypothesis was strengthened by the
completed,andanoxidizedpartremainedinthesample. Thishypothesiswasstrengthenedbythe
XRD results that evidenced the formation of Cu2O in the sample containing the highest amount of
XRDresultsthatevidencedtheformationofCu OinthesamplecontainingthehighestamountofCu
Cu (Figure 5). 2
(Figure5).
InFigure5,boththediffractionpatterncollectedforthePd/Cusamples(a)andthetablelisting
theshiftsdetectedfortheprincipalpeakthatsupportstheformationofthealloys(b)arereported.
Materials2016,9,550 7of25
Materials 2016, 9, 550 7 of 25
Materials 2016, 9, 550 7 of 25
(a) (b)
(a) (b)

FiFgFiuiggrueurer4e .4 P.4 P.a Partraitcriclteliecs lseizi zseeiz ddeiis sdttrriisibbtruuittbiioounnti oaasnsss eaesssssseeedsd sfefoodrr PfPoddr/C /PuCd ub/Cibmuiem btaiemltlaicel ltniacallnnicoa snnooalssno: o(lass:)o (lssaa:)m (sapa)lm essap mflreopsmlfer sPo dmfr1oCPmud0 .5PC du1Cu0.5
to Pd1Cu3 compared with Pd and Cu; (b) Pd1Cu6 sample compared to Pd and Cu. 1 0.5
totPod PCd1uCuc3o cmompapraerdedw withitPh dPdan adndC uC;u(b; ()bP)d PdC1uCus6 asmampplelec ocmompapraerdedto toP dPda nadndC Cu.u.
1 3 1 6

(a) (b)
(a) (b)

Figure 5. (a) Focusing of the XRD spectra on the principal peak for the Pd/Cu series; (b) List of the
Figure5. (a)FocusingoftheXRDspectraontheprincipalpeakforthePd/Cuseries;(b)Listofthe
progressive shift of the principal detected peaks. Progressive shift of the principal detected peak.
proFgigreusrsei v5e. s(ah)i fFtoocfuthsienpg roinf ctihpea lXdReDte cstpeedctpreaa okns. tPhreo pgrreinssciivpeals hpiefatko ffothr ethper iPndci/pCaul dseerteiecst;e d(bp) eLaikst. of the
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theA shniyfths odwet,etchteedp foera kthseh pirfitnwcipasalw peeallk atphaptr seucpiaptoerdtst itlhlet fhoermPadtioCnu of stahme apllleo,ysb u(bt) faorre arecpooprtpeder. molar
1 2
concenAItnrna yFthiiogonuwrh,e it gh5he, beproettahhk at hsnhe6i fd7t %iwff,raatshc wetioedlnal tpaapasptutreegrcgniae ctseotdell detcitltlhe tadht efto hPred t1ahCdeu dP2e dsda/Cmcoupp lsepa, embrupdtl oefsoe rs( ana) o catonepdnp tteherr em tthaoeblallrea tltiiscteing
strctuhocnetc usehrneitf,rtbastu idotenct oehncitgterhidbe ruf otther satnhto e6 t7ph%rei,n stcehigepr adelga patate iasoukng tpghheasett nesodum pthpeanotro ttnhs eot hafedC dfuoerdOm c.aoOtpinponesr a odmfo tpehlsee n Paoldtl oeCynsut e(rb, t)th haeer elaa nrtetaipclyeo sritseodf.
2 1 6
thsetrduacttaAucrneoy,n hbfiourwtm c,eo tdnhtterhi bpeueptaerkse stseohn tichfete owsfeagtswr ewogaedtliilfo fnaep rpephnretenpcoihamatesedenso ;tniil nlo fft ahCceut ,2POthd.e 1OCmnua 2s iansmapmpelpaekl ePa,d tb1C3u6ut. 65,f ˝othr2 eθa a ccnooarplryepsseipsr o mndoslar
of the data confirmed the presence of two different phases; in fact, the main peak at 36.5° 2θ
toctohnecCeuntrOatoioxnid ehipghhaesre t(hJCanP D67S%C, atrhdeN doa.ta6 5s-u3g2g88e)s,tewdh tilheaat tt4h1e. 5a2d4d˝e2dθ ,ctohpepdeert edcoteesd npoeta kenctoerrr etshpeo lnadttsice
2
tocstothrreurecPstdpuoCrneu,d bas lutloot yctho(enJC tCrPiubD2uOSt eCosxa itrdode t Nhpeho a.sse4eg8 -(rJ1eC5gP5a1Dti)oS. nC parhde nNoom. 6e5n-o3n28 o8)f, Cwuh2iOle. aOt n4 1s.a52m4p° l2eθ P, dth1eC ude6,t ethctee da nalysis
peak corresponds to the PdCu alloy (JCPDS Card No. 48-1551).
of Bthyem deaatnas ocofnthfiermScehde rrtehre epqureasteionnc,et hoef mtweaon dcrifyfsetraelnlitt epshizaesewsa; sinca lfcauclta, ttehdea nmdarinep poretaekd iant T3a6b.5le° 12.θ
By means of the Scherrer equation, the mean crystallite size was calculated and reported in
ExccoerprtefsoproPnddsC tou th,et hCeur2eOsu oltxsidaere pchoahseer e(nJCtPwDitSh Cthaerdtr eNnod. o6b5s-e3r2v8e8d), bwyhDilLe Sadt a4t1a.;5i2n4d° e2eθd,, tthhee hdiegthecetred
Table 1. Exce1pt f0o.5r Pd1Cu0.5, the results are coherent with the trend observed by DLS data; indeed,
watphseeta hkhe icgcohoreprrep swepraosan mtdhose u tcono ttp,hptehe ePr dlaaCmrguoe uarnlwlto, aytsh (etJhC leaPrmDgeSer a Cnwacarrsdy tNshteao l.lm 4it8eea-1sni5 z5cer1.y)Ts. thaelliptee askizseh. iTfthew paseadke tsehcitfet dwaalss ofor
thdeePtedc/tBeAyd u maslseeoar nifeoss r,o atfhn etdh Pefd oS/rActhuhe irssreserairem se,qp aulnead,tn iofoonrs, etthghirese gsmaameteapdnle pc, hrnyaoss tesaselglwirteeeg rsaeitzedede twpehcatases decsa; ltwchueelrapet redodeg treaecnstdesdi vr; eetphseoh ritfetdis in
copTnrasoibsgltreee ns1st.i wvEexi tcshhepitfhtt eifsof coro rPnmdsia1sCttieounn0t.5 wo, ftihtPhed trAheeus ufoallrtlsmo ayart(ieoF ncigo ouhfr ePerde6An).tu w aliltohy t(hFeig turreen 6d) .o bserved by DLS data; indeed,
the higher was the copper amount, the larger was the mean crystallite size. The peak shift was
detected also for the Pd/Au series, and for this sample, no segregated phases were detected; the
progressive shift is consistent with the formation of PdAu alloy (Figure 6).
MMMaaatteteerrriiaiaallslss2 22000111666,,,9 99,,,5 55555000 888 o oofff 2 22555

(a)
(a) (b)
(b)

FFiigguurree 66.. ((aa)) FFooccuussiinngg oofft thheeX XRRDDs sppeeccttrraao onnt htheep prrininccipipaallp peeaakkf oforrt htheeP Pdd//AAuu sseerriieess;; ((bb)) LLiisstt ooff tthhee
Figure 6. (a) Focusing of the XRD spectra on the principal peak for the Pd/Au series; (b) List of the
pprrooggrreessssivivees shhififtto offt thheep prrininccipipaalld deeteteccteteddp peeaakkss..
progressive shift of the principal detected peaks.
22..22.. CCaattaallyyttiicc AAccttiivviittyy
2.2. Catalytic Activity
22..22..11.. CCaattaallyyttiicc TTeesstto offP PrriissttiinneeN Naannooppaarrtticiclelessi nint thheeH Hyyddrrooggeennaatitoionno off4 4-N-Nitirtoropphheennooll
2.2.1. Catalytic Test of Pristine Nanoparticles in the Hydrogenation of 4-Nitrophenol
TThhee ccaattaallyyttiicc ppeerrffoorrmmaannccee ooff tthhee ssyynntthheessiizzeedd nnaannooppaarrttiicclleess wwaass tteesstteedd iinn tthhee rreedduuccttiioonn ooff
The catalytic performance of the synthesized nanoparticles was tested in the reduction of
444---nnniiitttrrroooppphhheeennnooolll (((444---NNNPPP))) tttooo 444---aaammmiiinnnoooppphhheeennnooolll (((444---AAAPPP))) wwwiiittthhh aaannn eeexxxccceeessssss ooofff NNNaaaBBBHHH444,,, aaasss ttthhheee mmmooodddeeelll rrreeeaaaccctttiiiooonnn
((SScchheemmee1 )1.)A. cAcocrcdoirndgilnyg,ltyh,e rtehdeu crteidonucrtaitoens craantebse rceagna rdbeed raesgbaeridnegdi nadse pbeenidnegn tinodftehpeecnodnecnetn toraf titohne
(Scheme 1). Accordingly, the reduction rates can be regarded as being independent of the
occfootnnhcceeenrnettrdraauttciiooinnng ooaff g tthehnee t rr,eeNddauuBcciHinn4gg [aa4gg1ee].nntt,, NNaaBBHH44 [[4411]]..
NO NH
NO2 NH2
2 2
+ NaBH + H O
+ NaBH4 + H2O
4 2
OH OH
OH OH
4-Nitrophenol 4-Aminophenol
4-Nitrophenol 4-Aminophenol

SSSccchhheeemmmeee1 11... 4 44---NNNiitittrrroooppphhheeennnooolllr rreeeddduuucccttitioioonnnw wwitiiththhN NNaaaBBBHHH444...
This reaction is particularly easy to follow because only the product 4-aminophenol (4-AP) is
TThhiiss rreeaaccttiioonn iiss ppaarrttiiccuullaarrllyy eeaassyy ttoo ffoollllooww bbeeccaauussee oonnlyly ththe epprordoducutc t4-4a-maminionpohpehneonlo (l4(-4A-PA)P i)s
formed, and the extent of the reaction can be determined by measuring the change in UV-VIS
ifsofromrmede,d a,nadn dthteh eexetxetnetn tofo fththee rreeaacctitoionn ccaann bbee ddeetteerrmmiinneedd bbyy mmeeaassuurriinngg tthhee cchhaannggee iinn UUVV--VVIISS
absorbance at the 400-nm wavelength typical of 4-NP in alkaline condition.
aabbssoorrbbaanncceea attt thhee4 40000--nnmmw waavveelelennggththt ytyppicicaallo off4 4-N-NPPi nina alklkaalilnineec coonndditiitoionn..
Preliminary tests without the catalyst indicated that no reaction occurs in the studied
PPrreelilmimininaarryyt etsetsstws itwhiotuhtouthte tchaeta lcyasttailnydsti cainteddictahtaetdn othraeat ctnioon roecaccutirosnin othcceusrtsu diine dthcoen dstiutidoinesd,
conditions, confirming the absence, during our tests, of non-catalytic phenomena. On the contrary,
ccoonnfidrimtioinngs, tchoenafibrmseinncge ,thdeu raibnsgenocuer, dteusrtsin,go fonuor nte-csatst,a loyft incopnh-ceantaolmyteinc ap.hOennomtheencao. nOtrna rthy,ea clolnotfrathrye,
all of the nanoparticles acted as effective catalyst for the hydrogenation of 4-nitrophenol in the
naalln oopf atrhteic lneasnaocpteadrtiacsleesf faecctteivde acsa teaflfyescttivfoer ctahtealhyysdt rfoogr etnhaet ihonydorfog4-enniatrtioopnh eonf o4l-ninitrthopehperneosel nicne tohfe
NpprareeBssHeenn4cc.eeI noodff eNNedaaB,BtHHh4e4.. aIIndndddeeiteeidodn,, ttohhfee m aadedtddaiilttliiioocnns ooolffs mmtoeettthaalellliircce assocotllisso tntoo m tthhixee t urreeraaecccttiaioounns emmsiiaxxttguurrraeed cucaaauulssdeeessc aoa lgogrrraaadtdiuuoanall
decoloration of the solution and ultimately a bleaching of the yellow color, due to the formation of
odfetchoelosroalutitoinon oaf nthdeu slotilmutaitoenly aandb luealtcihminatgeloyf ath beleyaecllhoiwngc oolfo trh,ed uyeeltloowth ceofloorrm, dautieo ntoo tfh4e- AfoPr,mshaotiwonin ogf
4-AP, showing a weak absorption of violet wavelength stemming from its peak at 300 nm.
a4w-AePa,k sahbosworinpgti oan woefavki oalbestowrpatvioelne nogf tvhioslteetm wmaivneglefnrogmth isttsepmemakinagt 3fr0o0mn mits. peak at 300 nm.
In Figure 7, typical degradation spectra of 4-nitrophenolate are reported. On the basis of the
IInn FFigiguurree 77,, tytyppicicaall ddeeggrraaddaattiioonn ssppeeccttrraa ooff 44-n-nitirtroopphheennoollaattee aarree rreeppoorrtteedd.. OOnn tthhee bbaassiiss ooff tthhee
collected UV-VIS spectra, the plot of ln (A/Ao) versus time was obtained and the kinetic constants
ccoolllleecctteedd UUVV--VVIISSs sppeeccttrraa,,t htheep plolotto offl nln( A(A//AAoo)) vveerrssuuss ttiimmee wwaass oobbttaaiinneedd aanndd tthheek kinineettiiccc coonnssttaannttss
calculated. The resulting kinetic constants for all of the bimetallic nanosols are listed in Table 4
ccaallccuullaatteedd.. TThhee rreessuullttiinnggk kiinneettiiccc coonnssttaannttssf foorra allllo offt thheeb bimimeetatalllilcicn naannoossoolslsa arreel ilsisteteddi ninT Taabblele4 4.
Materials2016,9,550 9of25
Materials 2016, 9, 550 9 of 25

Figure 7. Typical evolution of UV-VIS spectra as a function of time during the hydrogenation of
Figure 7. Typical evolution of UV-VIS spectra as a function of time during the hydrogenation of
4-nitrophenole(4-NP)inthepresenceofcatalyst.
4-nitrophenole (4-NP) in the presence of catalyst.
As rAesp orerpteodrteedls eewlsehwerheer[e4 2[4],2]t,h tehere raecatciotinond dooeessn noott ssttaarrtt iimmmmeeddiiaatteellyy aaffteter rththe eadaddditiiotino nofo tfhteh e
cataclyatsat,lybsut,t bountl oynalyft earftaern ao ntiocteiacebalebltei mtimeela lga,gi,. ei..e,.,t htheei nindduuccttiioonn ttiimmee.. TThhiiss mmaayy bbee aascsrcirbiebde dtot osesveevrearl al
reasroenass:otnhse: tphree pserensceenocef oofx oyxgyegnedn idssisoslovlevdedin inw waateterro orr aa tthhiinn nnaannooxxiiddee ccooaatitningg oonn thteh epapratirctliec lseusrufarcfea.c e.
InorInd eorrdtoerr etod ruecdeutchee thined inudctuicotniotnim time,es,e sveevrearlasl tsrtarateteggieiessh haavvee bbeeeenn iinnvveessttiiggaatteedd [4[400,4,43,34,44]4, ]o,bosbesrevrivngin g
that it was significantly reduced for all of the prepared samples only by bubbling nitrogen for 1 min
thatitwassignificantlyreducedforallofthepreparedsamplesonlybybubblingnitrogenfor1min
befobreefothree tahde daidtidointioonf oNf aNBaHBHa4 nadnda daddidninggt htheer ereaaggeennttss iinn tthhiiss oorrddeerr:: ccaatatalylysts, t,4-4N-NP Panadn, df,infianllayl, ly,
4
NaBNHaB.HP4.r oPbroabbalyb,lyth, itships rporcoecdeudruerea lalollowwsst htheer eremmoovvaall ooff ooxxyyggeenn ddiissssoollvveedd inin ththee sosolulutitoino,n w, whihchic h
4
would involve NaBH4 in a competitive reaction with 4-NP. The results, referring to data collected
wouldinvolveNaBH inacompetitivereactionwith4-NP.Theresults, referringtodatacollected
4
after the pre-treatment with bubbling nitrogen, are summarized in Table 4 in terms of kinetic
afterthepre-treatmentwithbubblingnitrogen,aresummarizedinTable4intermsofkineticconstant,
constant, conversion and turn over frequency (TOF, defined as the number of moles of reduced 4-NP
conversionandturnoverfrequency(TOF,definedasthenumberofmolesofreduced4-NPpermole
per mole of catalyst per second).
ofcatalystpersecond).
Table 4. Catalytic performances of the prepared bimetallic nanosols. TOF, turn over frequency.
Table4.Catalyticperformancesofthepreparedbimetallicnanosols.TOF,turnoverfrequency.
Sample Catalyst Concentration× 10−5(M) K × 10−2(s−1) TOF × 10−2 (s−1)
Sample ACua talystConcentratio1n.1ˆ 10´5(M) Kˆ101´.720 (s´1) TO5.F3 ˆ10´2(s´1)
Cu 1.1 0.20 0.4
Au Pd 1.1 0.11 11..7905 34.1 5.3
Cu Au1Cu1 1.1 1.1 02..2700 5.8 0.4
Pd 0.11 1.95 34.1
Au1Cu3 1.1 1.70 4.9
Au1Cu1 Au1Cu6 1.1 1.1 20..7700 1.2 5.8
Au1Cu3 Pd1Cu0.5 1.1 0.11 11..7104 32.2 4.9
Au1Cu6 Pd1Cu1 1.1 0.11 00..7800 15.7 1.2
Pd Cu Pd1Cu2 0.11 0.11 10..1246 7.8 32.2
1 0.5
Pd Cu Pd1Cu3 0.11 0.11 00..8109 5.5 15.7
1 1
Pd Cu Pd1Cu6 0.11 0.11 00..2067 3.1 7.8
1 2
Pd Cu Pd1Au0.5 0.11 0.11 02..1190 40.7 5.5
1 3
Pd Cu Pd1Au1 0.11 0.11 02..0279 49.5 3.1
1 6
Pd1Au2 0.11 1.35 37.5
Pd Au 0.11 2.10 40.7
1 0.5 Pd1Au3 0.11 0.59 30.3
Pd Au 0.11 2.29 49.5
1 1 Pd1Au6 0.11 0.35 10.3
Pd Au 0.11 1.35 37.5
1 2
DPatda rAefuer to the following conc0en.1t1rations: 4-NP 9 × 10−5 M and N0.a5B9H4 0.715 M. For all 3o0f .t3he samples,
1 3
TOPdF vAaluues are calculated consi0d.e1r1ing the first 60 s of the reacti0o.n3.5 10.3
1 6
Datarefertothefollowingconcentrations: 4-NP9ˆ10´5 MandNaBH4 0.715M.Forallofthesamples,
TOFvaluesarecalculatedconsideringthefirst60softhereaction.
Materials 2016, 9, 550 10 of 25
Materials2016,9,550 10of25
All of the prepared nanoparticles act as catalysts in the hydrogenation of 4-NP; among the
singlAe lml oeftatlhs eApur, eCpua raenddn Padn,o tphaer btiecslet spearcftoarms caantcaelsy swtserien otbhseerhvyeddr ofogre nPadt,i ofonlloowf 4e-dN bPy; Aamu oanngd tChue.
sAinsg elveimdeentacelsdA inu ,thCeu liatnerdatPudr,et [h4e5,b4e6s]t, bpiemrfeotramlliacn ncaensowpearretioclbesse crovuedldf sohroPwd ,nfootl loonwlyed thbey cAomubainndatCioun.
Aofs tehvei dpernocpeedrtiinest hreelalitteedr attou trhee[ 4p5r,e4s6e]n,cbeim ofe ttawlloic inndanivoipdauratli cmleestaclosu, lbdust haolswo nneowto pnrloyptehreticeosm dbuien taot itohne
osfo-tchaellpedro psyenrteiregsisretilca teefdfetcot tbheetwpereesne nthcee otwf tow moeintadlsiv. iAdu malamrkeetda lss,ybnuertgailsstoic neefwfecpt roonp etrhtiee scadtualeyttioc
tpheerfsoor-mcaallnecdess yfnoerr gthisitsi cperfofebcet breeatwcteioenn twheast weovimdeentacelsd. Afomr aAruke/Cdus yannedrg Pisdti/cAeuff encatnoonsttrhuecctuartaelsy. tiIcn
pAeurf/oCrum, adnecsepsitfeo rththei spoporor bceatraelaycttiico pnewrfaosrmevaidnecen caesdsefsosreAd ufo/rC tuhea nCduP mdo/nAoumneatnaol,s tthrue catdurdeisti.oInn oAfu C/uC uto,
dAeusp pirteodthuecepdo aonr ecnahtaalnyctiecmpeenrtf oorfm thaen accetiavsistyes fsoerd thfoe rcothmepCousitmioonn oofm Aeut1aCl,ut1h. eAa fdudrtihtieorn inocfreCauset oofA Cuu
pcroondteuncte dpraonduencehda,n icnesmteeandt, oaf dtehteriamcteinvtiatyl effofrectth. eTchoem laptotesri twioans ojufsAtiufieCdu by. Athefu mrtahrekreidn cinrecaresemoefnCt ouf
1 1
ctohnet ehnytdprrooddyuncaemd,iicn sdtieaamd,eatedr eatrnimd etnhtea lfeofrfmecat.tiTohne olaf ttCeur2wOa ssejugsrteifigaedtedby pthheasmesa.r kSeimdiilnacrrleym, ienn tPodf/tAheu
hsyerdireosd, ythnea msyicnedrigamic eetfefrecatn dwtahse afsosremssaetdio ntilol ftCheu cOomsepgoresigtaiotend Ppdh1aAsues1,. Swimhiiclhar lrye,aicnhPedd /tAheu msearixeism,tuhme
2
spyenreforgrmicaenffceec t(Fwigausraes s8eas)s.e Adgtialilnt,h ae cfuomrthpeors iitniocrnePasde Aofu a,dwdehdic hAure amcehteadl dtheecrmeaasxeism purmogpreesrfsoivrmelya ntchee
1 1
(cFaitgaulyreti8c a)a.cAtigvaitiyn,, adfeusrpthiteer inthcere assiemoilfaard dneadnoApuarmtiectlael ddeicmreeansseisonp,r ohgyredsrsoivdeylnyatmheicc atdailaymticetaecrt ivaintyd,
dheosmpiotegethneeistiym oilfa trhnea bniompeatratlilcilce pdhimaseen (sTioanb,leh y1d).r o dynamicdiameterandhomogeneityofthebimetallic
phaseO(nTa tbhlee o1t)h.er hand, the Pd/Cu compounds did not show any kind of catalytic improvement with
respeOcnt tthoe tohteh esrinhagnled ,Ptdh emPdet/aCl.u Icnodmeepdo,u na dcsodnisdtannott sdheocwreaasninygk intrdenodf cwataasly toicbsimerpvreodv eamddenintgw iCthu
r(eFsipgeucrtet o8bt)h ceosminbgilneePdd wmitehta slt.rIuncdteuerda,l avacroinastitoannt, sduecchre aass itnhge threynddrowdaysnoabmsiecr vdeiadmadetdeirn ignCcruea(Fseig aunrde 8thbe)
coocmcubrirneendcwe iothf sthtreu csteugrraelgvaatrioianti opnh,esnuocmhaesnath e(Thaybdlero 1d)y, nwamhiicchd iparmobeatebrlyin cprreeavsaeialendd othne aonccyu rproetnecnetioafl
tshyenseerggriestgiac toiountcpohmeen.o mena(Table1),whichprobablyprevailedonanypotentialsynergisticoutcome.
(a) (b)
FFiigguurree 88.. EEvvoolluuttiioonn ooff tthhee kkiinneettiicc ccoonnssttaanntt ffoorr tthhee 44--NNPP hhyyddrrooggeennaattiioonn rreeaaccttiioonn ffoorr PPdd bbiimmeettaalllliicc
ccoommppoouunnddss wwiitthh ddiiffffeerreenntt aammoouunnttss ooffA Auua annddC Cuua dadddeded::( a(a))t htheeP Pdd//AAuus syysstteemma anndd( b(b))t htheeP Pdd//CCuu
ssyysstteemm.. DDataat arefreerf etro tthoe tfhoellofwolilnogw cionngcecnotnrcaetinotnr:a 4ti-oNnP: 94 ×- N10P−5 9Mˆ, ca1ta0l´y5st M1.1, ×c a1t0a−l6y Mst, N1.1aBˆH41 00.7´165 MM,.
NaBH 0.715M.
4
It is well know that the synergistic effect on the catalytic activity depends on the surface
electronic structure that can be modified by the interactions between the two kinds of atoms in the
Itiswellknowthatthesynergisticeffectonthecatalyticactivitydependsonthesurfaceelectronic
bimetallic alloy owing to the strain and ligand effects. The “strain effect” involves the average bond
structurethatcanbemodifiedbytheinteractionsbetweenthetwokindsofatomsinthebimetallic
length of metal atoms, which is typically different from those in the monometals, while the so-called
alloyowingtothestrainandligandeffects. The“straineffect”involvestheaveragebondlengthof
“ligand effect” involves the hetero-metallic bonding interactions of atoms in the surface of
metalatoms,whichistypicallydifferentfromthoseinthemonometals,whiletheso-called“ligand
nanoparticles, which modifies their surface electronic structure [6]. Even if the mechanism is still
effect”involvesthehetero-metallicbondinginteractionsofatomsinthesurfaceofnanoparticles,which
under debate, both effects result in changes in the width and average energy of the d-band that may
modifiestheirsurfaceelectronicstructure[6]. Evenifthemechanismisstillunderdebate,botheffects
account for the enhancement in catalytic activity.
resultinchangesinthewidthandaverageenergyofthed-bandthatmayaccountfortheenhancement
The hydrogenation reaction involving 4-NP can be divided into two main parts: firstly, the
incatalyticactivity.
decomposition of NaBH4 on the surface of nanoparticles, producing H-atoms, which turn to be
The hydrogenation reaction involving 4-NP can be divided into two main parts: firstly, the
available to 4-NP molecules, and, finally, the addition of protons to the 4-NP and the simultaneous
decomposition of NaBH on the surface of nanoparticles, producing H-atoms, which turn to be
4
removal of its oxygen, following one of the typical mechanisms of heterogeneous reactions, like the
availableto4-NPmolecules,and,finally,theadditionofprotonstothe4-NPandthesimultaneous
Eley–Rideal or the Langmuir Hinshelwood [42]. An overall comparison among the catalysts can be