NH3 adsorption on PtM (Fe, Co, Ni) surfaces: cooperating effects of charge transfer, magnetic ordering and lattice strain

0.15 MB·

by  Satadeep Bhattacharjee

NH adsorption on PtM (Fe, Co, Ni) surfaces: cooperating effects of charge
3
transfer, magnetic ordering and lattice strain
SatadeepBhattacharjee1,S.J.Yoo2,UmeshV.Waghmare3andS.C.Lee1,4
1Indo-KoreaScienceandTechnologyCenter(IKST),Bangalore,India
2FuelCellResearchCenter,KoreaInstituteofScience,Korea
3JawaharlalNehruCentreforAdvancedScientificResearch(JNCASR),Bangalore,India
4ElectronicMaterialsResearchCenter,KoreaInstituteofScience&Tech,Korea
6
1
0
2

n
a Abstract
J
Adsorptionofamoleculeorgroupwithanatomwhichislesselectronegativethanoxygen(O)anddirectlyinteracting
4
withthesurfaceisveryrelevanttodevelopmentofPtM(M=3d-transitionmetal)catalystswithhighactivity.Here,we
1
presenttheoreticalanalysisoftheadsorptionofNH3molecule(NbeinglesselectronegativethanO)on(111)surfaces

] ofPtM(Fe,Co,Ni)alloysusingthefirstprinciplesdensityfunctionalapproach.Wefindthat,whileNH -Ptinteraction
i 3
c isstrongerthanthatofNH withtheelementalM-surfaces,itisweakerthanthestrengthofinteractionofNH with
3 3
s
M-siteonthesurfaceofPtMalloy.
-
l
r Keywords: Bi-metallicalloys,Fuelcell
t
m PACS:82.65.My,82.20.Pm,82.30.Lp,82.65.Jv
.
t
a
m Bimetallic surfaces of Pt alloyed with first row transition metals (M) such as Fe, Co, Ni etc. are interesting and
promisingfor theirpotentialapplicationsascatalytic cathodesin ProtonExchangeMembraneFuel Cell (PEMFC).
-
d These alloys not only ensure the cost effectiveness but also reduce the over-potential and increase the oxidation
n reductionreaction (ORR) activity[1, 2, 3]. In a cathode which is made up of pure Pt, the accumulationof oxygen
o
species, suchasO,OH etc, decreasetheavailabilityofactivesites, aswellasincreasetheactivationbarrierforthe
c
ORR.InrecentstudyitwasshownthatcarbonsupportedPtMalloysexhibitimprovedcatalyticactivitybypreventing
[
the oxidation and the dissolution of the Pt in the aqueous medium[2]. However, yet the catalytic activity is much
1
weakerthanwhatisexpectedtheoretically. Thisisbecause,theoxophilicnatureofMatomsresultsintheformation
v
of surface M-oxides leading to degradation of catalytic activity of these bi-metals. To preventsuch effects, it was
0
9 proposedthatselectivecoverageofM-siteswithligandscontaininglesserelectro-negativeelementssuchasnitrogen
4 (N)wouldreducetheoxidationofMspeciesandtherebyhelpmorePtsitestoremainactive[4]. Theenhancement
3 of catalytic activity along this route involves two key steps: (1) proper selection of M which can provide Pt-sites
0
suitableenvironmenttoretaintheirORRactivityand(2)tuningtheelectronicstructureofM-sitesinordertoprevent
.
1 formationofM-oxides.
0 The first step is emphasized as follows: the electro negativity difference between M and Pt causes the charge
6
transferfromMtoPtsite, resultinga down-shiftofthePt-dstatesviafillingofthePtd-band,therebyreducingthe
1
accumulation of oxygen species over Pt sites. To achieve the second step, one requires preferential adsorption of
:
v
N-containingligandonM-sites,i,eMshouldbemoreN-philicthanPt,inaPtMalloycondition.
i
X The electronic, chemical and structural properties of the transition metal (M) are therefore very important to
achieving this two mechanisms. The predictability of the modification of electronic and chemical properties of M
r
a in an alloy environment with respect to its elemental state is a challenging task. It is therefore necessary to find
thedescriptor(s)whichproperlyindicatethechangesinelectronic,magneticaswellaschemicalbehaviourofthese
transitionmetalsin thealloyedform. Thesamedescriptor(s)canalso beused inpredictionofthe catalyticactivity
ofthePtMcatalyst. Changeincatalyticactivityoftransitionmetalsurfacesormorepreciselytheadsorptionenergy
ofsmallmoleculesaredescribedintermsofsocalledd-bandcentermodelofHammerandNørskov[5,6,7],which
predicts a linear correlation between the shift of the d-band center of one metal with respect to the other with the
energyofadsorption. Predictionsofchemisorptionpropertiesintermsofthecenterofthed-bandwhichisanenergy
PreprintsubmittedtoElsevier January15,2016
pointaveragedoverspin-upandspin-downd-bandcenters.thereforetheeffectsofthespinpolarizationareneglected.
InPtMalloys,thedbandfillingofthemetals(M)aredifferentfromtheirparentstate. Thelatticemismatchbetween
theM-Ptresultsastraininthesystem,whichcanalsoinfluencethecatalyticactivity.
TounderstandthepossibilityofselectiveM-coveragewithN-containingligand,wehavestudiedtheadsorptionof
NH onPtM(Fe,Co,Ni)surfaces. OurcalculationsrevealsareversibilityofadsorptionbehaviourofNH molecule
3 3
on M sites of PtM in comparisonto the pure M(Fe,Co,Ni) surfaces. NH has larger adsorptionenergieson M-site
3
of PtM surface than of an M-site of elemental surfaces. We relate the such changes in adsorption energies to the
physical mechanisms such as charge transfer, magnetic moment and strain effects. Although the effects of charge
transfer and strain can be related to the shift of the d-band center, the effect spin polarization is not included in
the d-band model. Most prominently, we have quantified the effects of these mechanisms from our first-principles
calculations. For designinghighly efficientalloy catalysts such as PtM, we suggestthat one shouldgather insights
fromamulticomponentdescriptorratherthanascalerdescriptorsuchasd-bandcenter.
Our first-principles calculations were within the framework of density functional theory (DFT) with Perdew-
BurkeErnzerhoffunctionalofexchangecorrelationenergyderivedwithinageneralizedgradientapproximation[15]
andprojectoraugmentedwave (PAW) methodas implementedin Viennaab initio simulationpackage(VASP)[16].
The PAW potentials include the following valence electrons: 3d84s1 for Co, 5d95s1 for Pt, 2s22p3 for N, and 1s1
for H. Surfacesof Pt, M and PtM were simulated using periodic supercellscontainingslabs of their 4 4in-plane
×
unitcellsandthicknessoffouratomicplanes,thusconsistingof64atoms. Thetoptwoatomicplaneswererelaxed
whilethebottomtwoatomicplaneswerekeptfixedintheirbulkstructure. Wavefunctionsofvalenceelectronswere
expandedin a plane wave basis set truncatedwith an energycut-offof 450 eV. The integrationsover the Brillouin
zoneweresampledonuniformgridof3 3 1k-pointsusingMethfessel-Paxtonsmearingwithawidthof0.1eV.
× ×
Ionicrelaxationareperformedsuchthattheforceoneachionissmallerthan0.02eV/A˚.Thedipolecorrectionwas
appliedalongthedirectionperpendiculartothemetalsurfacestocorrecttheelectricfieldsarisingfromthestructural
asymmetryandperiodicity.Theadsorptionenergiesweredeterminedusingtherelation,
E =E (E +E ), (1)
ad S+A S A
−
where E is the energy of the surface plus adsorbate while E and E are energies of the surface and adsorbate
S+A S A
respectively.MagneticinteractionsamongM-MandM-Ptareconsideredtobeferromagnetic.
ForeachPtMalloyconsideredhere,wefirstobtaineditsoptimizedstructurefor(1:1)composition.Weoptimized
itsstructurewithtetragonalsymmetrywithP4/mmmspacegroup.Intheoptimizedstructure,(seeTable. I), c √2,
a ≃
andM-atomoccupiesthe1a(0,0,0)siteandPtoccupies1d(0.5,0.5,0.5)site. ItisknownthattheL1 phasebecome
0
stable for FePt below 1300 C, for CoPt below 825 C and for NiPt below 1300 C [8]. The (111) surface was
◦ ◦ ◦
constructedfromthisrelaxedbulkstructure. Inthe TableII,wereporttheadsorptionenergiesofNH moleculeon
3
elementalmetalsurfacesaswellasonMPtsurfacesfortheatoppositions,whichisthemostpreferredadsorptionsite
forNH on the transitionmetal surfaces[9, 10, 11]. For the elementalmetalsurfaces, NH has highestadsorption
3 3
energyonPt(111)surfacefollowedbyNi(111),Fe(110)andCo(0001). However,onMPtsurfacesNH isfound
3
to havestrongerbindingatM sites thanPt. The strengthof NH -Pt bindingon MPtsurfacesdecreaseas we move
3
fromNitoFe. Suchreversalofadsorptionbehaviourisinterestingandalso giveusscopefortuningtheactivityof
PtMalloythroughmanipulatingtheelectronicstructureofM-sitesusingsuitableligands.Thequestionis:howdowe
understandsuchreversalofchemisorption?
FromtheaveragemagneticmomentsattheM-sitesinPtMsurfacesandthoseinthepureM-surfaces(shownin
bracketin theTableI),we see thatthemagneticmomentsonM-sitesarelargerin PtM thaninpureM.Thisisdue
to charge transfer from the minority d-band of M to d-band of Pt as is evident in spin-resolved Bader charges[12]
reported in Table III. It is seen that about 0.82 electrons transfer from Fe-d bands to Pt d-bands, of which is 0.45
electronstransfer to the majority spin channeland 0.37 electronsare transfer to the down spin channelof the Pt-d
bands. Similarly,0.48electronstransferfromCosite tothePtsite, ofwhich0.41electronstransfertothe majority
spin channel and only 0.07 electrons of Pt-d band. In the case of NiPt, 0.34 electrons transfer from Ni to Pt, out
of which 0.31 electrons are transferred to to the majority Pt-d band and only 0.03 electronstransfer to Pt minority
d-band. Thischargetransferalso inducesa smallmagneticmomentonthe Ptatom, themagnitudeofthe magnetic
momentbeing0.5,0.45and0.34m respectivelyforFePt,CoPtandNiPt.
B
Fromtheoptimizedlatticeconstants(Table.I)weexpectstraineffects,asreflectedinthemagnitudeofthesurface
vectorsforthepuremetal(M)aswellasPtM surfaces(Table. IV),itisclearthattheM-Mdistancesarelongerfor
2
PtMthanthatinpureMsurface(byabout8%forFe,6%forCoand7%forNi). Accordingtothed-bandmodelof
heterogeneouscatalysts,forthelatetransitionmetalssuchasFe,CoandNitheincreaseininteratomicdistanceforM
latticeleadstonarrowingofMd-bandwidthwhichwillcausetheincreaseinthestrengthofchemisorptionduetoan
upwardshiftofthed-bandcentertopreservethechargeconservation[13].
Fromtheprojecteddensityofstates(DOS),itisclearthatthemajorityspind-bandsarecompletelyfullinFePtand
CoPtanddeterminetheM-magneticmomentwhile magneticmomentis determinedbybothmajorityandminority
spinelectronsinNiPt.
Inpuremetallicsurfacesthereisnointer-sitechargetransfersincethereisnodifferenceintheelectronegativity
amongthesesites. HoweverforthePtMsystemsthereischargetransfereffectarisingfromthedifferenceintheelectro
negativityofMandPtatoms.OnthePaulingscale,theelectronegativitydifferencebetweenPtandM(Fe,Co,Pt)are
0.45,0.40,0.37respectively.ThemagneticmomentsonM-sitesinPtMsurfacesareingenerallargerthanwhatisthere
inM-sitesinapureM-surface. AlsotherearestraineffectsinduetothelatticemismatchbetweenPtandMinPtM.
ThedifferenceintheadsorptionenergyofNH moleculefromMtothePtMsurfacesariseduetoacombinedeffects
3
oftheabovementionedphenomena.Touncovertheindividualcontributionsofchargetransfer,magnetismandstrain
weapplythefollowingmethodology.
TheadsorptionenergyoftheNH moleculeontheMsurfaceisgivenby,
3
EM =E E E (2)
ad M+A− M− A
whilethatonaPtMsurfaceiswrittenas,
EPtM=E E E (3)
ad PtM+A− PtM− A
ThechangeoftheadsorptionenergyofNH moleculefromtheMtoPtMsurfacecanbewrittenas[9]
3
D E =EPtM EM =(E E ) (E E )
ad ad − ad PtM+A− M+A − PtM− M
=D Eq+D Em+D Ee, (4)
whereD Eq, D Em andD Ee arerespectivelythecontributionstothechangesin adsorptionenergyD Ead comingfrom
thechargetransfer,magneticmomentsandstrain. TocalculateD Eq,D Em andD Ee wehaveusedfollowingmethod:
We assume that one can simulate the effects of Pt-coordination on M, by considering the pure M surface with an
enhancedsurfacearea(tocapturetheeffectofstrain),alargermagneticmomentandalessnumberofelectronsper
Matom. Weexpresssuchassumptionsinamathematicalformgivenby,
D Ee =EaMd(SPtM)−EaMd(SM)
D E =EM(m ) EM(m ) (5)
m ad PtM − ad M
D E =EM(q ) EM(q )
q ad PtM − ad M
D Ee involves calculation of adsorption energy of NH3 on the M-surface at its equilibrium surface area SM and on
the M-surface with enhanced surface area S (which is the surface area of PtM). To obtain D E , we perform
PtM m
calculation of adsorption energies for the M-surface surface with an enhanced magnetic moment, by con-straining
the overallmagneticmomentof the system such that the magnetic momentper M ion is aboutm (which is the
PtM
magneticmomentperMioninaPtMsurface). SincethereisnochangeintheelectronicchargeinsidetheMsphere
(only redistribution of majority and minority spin electrons), no charge transfer effect is involved. Also since the
structureisunchanged,thestraineffectsisabsenttoo. ThelasttermoftheEq.5(D E )isobtainedbysubtractingthe
q
magneticandlatticecontributionsfromthetotalD E (fromtheEq.(4)).
ad
Usingthemethodologiesabove,wehavesplittheD E intoitslattice,magneticandchargetransfercontributions
ad
(seeresultstabulatedinTable.IV).ItisseenthatforFePtandCoPttheincreaseofNH adsorptionenergyw.r.ttheFe
3
andCosurfacecanberelatedmostlytothestrainandthechargetransfereffects,magnetismplaysweakerrolehere.
ForNiPt,duetorelativesmallerdifferenceofelectronegativitybetweenNiandPtthechargetransfereffectissmall.
Thechangeinadsorptionenergyismainlydominatedbystraineffect. Also,amongallthree,FePtisexpectedtobe
bestintermsofactivitysincetheselectiveM-coveragewithNH iseasiestinthiscase.
3
3
IntheFig.2,weshowtheDOSofNH inthegasphase(2a),theCo-dprojectedDOSandNH projectedDOSfor
3 3
NH adsorbedCo(0001)surface(2b)andd-projectedDOSfortheCo(0001)surface(2c). From2(a)and2(c)itis
3
evidentthatthelowestunoccupiedlevels(4a1,2elevels)ofNH arewellabovethed-statesandareweaklyinvolved
3
inthechemisorption. Itisthenon-bondinglonepair(3a1level)whichdescribestheM-NH bonding. Onthemetal
3
surfacethelonepairishighlystabilized(byabout4eV),whilethethreeNH bonds(doublydegenerated1eleveland
3
2a1level)arealsostabilizedslightly.
SinceNH moleculeinteractswiththesurfacethroughthelone-pairelectrons,itiseasytounderstandwhyNH
3 3
prefersto M-sites in PtM surface. The lone pair electrons prefer to interact more strongly to the electron deficient
M-atomsthanPt atomswhichare morenegativelychargeddueto electrontransferfromthe M-atoms. Thisis also
demonstrated in the Fig.3, where we show that DOS of the lone pair on CoPt surface. The lone pair is mainly
composed of N-P orbital with little contribution from N-s orbital[14]. From the figure it can be understood that,
z
when the NH molecule is adsorbedon one of the Co, it donateslone pair electronsto thatCo and thereforelone-
3
pair-DOS moves upwards in energy w.r.t the case when it is adsorbed on Pt-site. NH molecule is therefore less
3
nucleophiliconnegativelychargedPtsites.
In conclusion, we have studied the cooperative roles of charge transfer, magnetism and strain in adsorption of
NH onFePt, CoPtandNiPtsurfaces. SinceNH hasalargeradsorptionenergyonM-sitescomparedtoPtsites, it
3 3
is possibleto achievethe selective NH coverageof M sites on PtM surfaces. Such selective coverageis easiest in
3
thecase of FePtdueto the largestM spinmoments, strongestchargetransferandassociatedstrain effects. Among
thethreemechanisms,theeffectsofchargetransferandstrainarethelargest,whilethecontributionofmagnetismis
relativelymodest. WethereforepredictFePttobethemosteffectiveamongthethree. Inprinciple,onecanpropose
athreecomponentdescriptorD=D(D q,D m,D e ),wherethreecomponentsofthedescriptormeasurethechangein
charge,magneticmomentandstraineffectsinPtMsurfacescomparedtothatofelementalM-surface.
ThisworkwassupportedbyNRFgrantfundedbyMSIP,Korea(No.2009-0082471andNo.2014R1A2A2A04003865)
andtheConvergenceAgendaProgram(CAP)oftheKoreaResearchCouncilofFundamentalScienceandTechnology
(KRCF).UVWacknowledgesfundingfromtheIndo-KoreaInstituteofScienceandTechnology(IKST).
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References
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[3] D.He,S.MuandS.Pan,Carbon,49,82-88(2011)
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[8] TBMassalski,H.Okamoto,Binaryalloyphasediagrams[CDROM].MaterialsPark,OH:ASMInternational1996
[9] S.Bhattacharjee,K.Gupta,N.Jung,S.J.Yoo,U.V.WaghmareandS.C.Lee,PhysicalChemistryChemicalPhysics,17,9335-9340(2015)
[10] G.Novell-Leruth,A.Valcarcel,J.Perez-Ramrez,andJ.M.Ricart,J.Phys.Chem.C,111,860-8682007
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[12] W.Tang,E.Sanville,G.Henkelman,J.Phys.:Condens.Matter,21,084204-0842042009,
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PtM Latticeconstants Magneticmoments
a(A˚) c(A˚) onM-site(m )
B
FePt 2.70 3.82 3.01(2.58)
CoPt 2.65 3.74 1.9(1.7)
NiPt 2.67 3.78 0.8(0.7)
Table1:ThelatticeconstantsandmagneticmomentsontheM-siteofM(Fe,Co,Ni)PtintheirL10. Inthebracket,wealsoshowthevaluesofthe
magneticmomentsforpureM-surfaces.
Surface AdsorptionSite Adsorptionenergies(eV)
Fe(110) Fe -0.72
Co(0001) Co -0.69
Ni(111) Ni -0.80
Pt(111) Pt -0.95
FePt Fe -0.85
Pt -0.53
CoPt Co -0.80
Pt -0.66
NiPt Ni -0.87
Pt -0.74
Table2:Calculatedadsorptionenergieson’ontop’sitesfordifferenttransitionmetalsurfaces.
Surface Site n n (n +n )
↑ ↓ ↑ ↓
Pt Pt 5.00 5.00 10.00
Fe(011) Fe 5.30 2.71 8.01
Co(0001) Co 5.35 3.65 9.00
Ni(111) Ni 5.32 4.68 10.00
FePt Pt 5.45 5.37 10.82
Fe 5.32 1.86 7.18
CoPt Pt 5.41 5.07 10.48
Co 5.22 3.30 8.52
NiPt Pt 5.31 5.03 10.34
Ni 5.22 4.43 9.65
Table3:SpinresolvedBaderchargeperatomfordifferentatomsondifferentsurfaces.
Metal MagnitudeofthesurfacevectorinA˚
M PtM
Fe (2.87,2.02) (2.70,2.70)
Co (2.50,2.50) (2.65,2.65)
Ni (2.48,2.48) (2.67,2.67)
PtM D Ead D Eq D Em D Ee
(meV) (meV) (meV) (meV)
FePt 130 50 30 50
CoPt 110 40 20 50
NiPt 70 20 10 40
Table4:Theroleofchargetransfer,magnetismandstraineffectsinthereversalbehaviourofadsorption.
6
60
FePt
40
20
0
-20
Spin-up
-40
Spin-down
) -60
V
60
e CoPt
/ 40
s
e 20
t
a 0
t
-20
s
( -40

S -60
O
60
D
40 NiPt
20
0
-20
-40
-60
-8 -6 -4 -2 0 2 4 6
Energy (eV)
Figure1:(Coloronline)ThedensityofstatesprojectedonM(Fe,Co,Ni)d-statesofFePt,CoPtandNiPt
7
NH molecule NH /Co(0001) Co(0001) surface
3 3
8 8 8
2e
6 6 6
c
O
4 M 4a1 a 4 b 4
U
2 L 2 2
3a1
0 0 Co-d-DOS 0
-2
-2 -2
)
V
-4 1e -4 -4
e
(
-6
F -6 -6
E
-8
- O -8 -8
E
M
-10
O -10 -10
H NH -DOS
3
-12
-12 -12 Co-d-DOS
-14 2a1 -14 Spin Spin -14
-16 DOS DOS
-16 -16
-18
-18 -18
-20
-20 -20
DOS
DOS DOS
Figure2: (Coloronline)(a)ThedensityofstatesofNH3 moleculeingasphase(b)theCo-dprojectedDOSandNH3projectedDOSforNH3
adsorbedCo(0001)surfaceand(c)d-projectedDOSfortheCo(0001)
8
1
NH on Co
3
NH on Pt
3
0.8
)
V
e 0.6
/
s
e
t
a
t
s
(

S 0.4
O
D
0.2
0
-10 -8 -6 -4 -2 0 2 4 6 8
Energy (eV)
Figure3:(Coloronline)ThedensityofstatesprojectedtothelonepairofNH3moleculeadsorbedonCoPtsurface.
9
Figure4:(Coloronline)NH3adsorbedonFePtsurface(viewedalongc-axis)asarepresentativeoftheadsorptiongeometry.
10