EXAFS in situ: The effect of bromide on Pd during the catalytic direct synthesis of hydrogen peroxide

(1)

Pleasecitethisarticleinpressas:P.Centomo,etal.,EXAFSinsitu:TheeffectofbromideonPdduringthecatalyticdirectsynthesisof hydrogenperoxide,Catal.Today(2014),http://dx.doi.org/10.1016/j.cattod.2014.10.001

ARTICLE IN PRESS

G Model

CATTOD-9281; No.ofPages4

CatalysisTodayxxx(2014)xxx–xxx

ContentslistsavailableatScienceDirect

Catalysis

Today

jo u rn al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / c a t t o d

EXAFS

in

situ:

The

effect

of

bromide

on

Pd

during

the

catalytic

direct

synthesis

of

hydrogen

peroxide

P. Centomo

a,∗

_,

_C.

_Meneghini

b,c

_,

_S.

_Sterchele

a,d

_,

_A.

_Trapananti

e

_,

_G.

_Aquilanti

f

_,

_M.

_Zecca

a

a_Dipartimento_di_Scienze_Chimiche,_Università_degli_Studi_di_Padova,₃₅₁₃₁_Padova,_Italy b_Dipartimento_di_Scienze,_Università_degli_Studi_Roma_TRE,_Roma,_Italy

c_CNR-TASC,_c/o_CRG-GILDA_(ESRF),_Grenoble,_France

d_Industrial_Chemistry_and_Reaction_Engineering,_Process_Chemistry_Center_(PCC),_Department_of_Chemical_Engineering,_Åbo_Akademi_University, FI-20500Turku/Åbo,Finland

e_CNR-IOM-OGG,_c/o_European_Synchrotron_Radiation_Facility,_71,_avenue_des_Martyrs,₃₈₀₀₀_Grenoble,_France f_Sincrotrone_Trieste,_ELETTRA,₃₄₀₁₂_Basovizza,_TS,_Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received16May2014

Receivedinrevisedform6October2014 Accepted7October2014

Availableonlinexxx

InmemoryofthelateProfessorBenedetto Corain(July8th1941-September24th 2014),passionatechemistandteacher Keywords: Palladium Hydrogenperoxide Directsynthesis EXAFS Insitu

a

b

s

t

r

a

c

t

Thedirectsynthesisofhydrogenperoxideiscatalyzedbypalladiumcatalystssupportedoverdifferent solids.Withtheaidofasuitableplug-ﬂowreactioncell,wecarriedoutapreliminaryX-rayabsorption ﬁnestructurespectroscopy(EXAFS)characterizationofapalladiumcatalystsupportedonthecommercial resinK2621underreactionconditions(insitu).Whereasthecatalyst,whichinthedrycatalystpresents metalPdandPdOwhenfresh,ispracticallyunaffectedbythereactionmedium(methanol)orbythe reactionmixture(CO2/H2/O2,86/10/4,v/v)itundergoesanapparentreductionofpartofPdOtometal

Pdandsomemetalleachingduringthereactioninthepresenceofbromideions.Theseﬁndingssuggest theroleofbromideionsasenhancersoftheselectivityofpalladiumcatalystsinthedirectsynthesisof hydrogenperoxidecouldnotbelimitedtotheselectiveblockingofthesitesresponsiblefortheundesired formationofwater,butcouldalsoentailphasemodiﬁcationsoftheactivemetal.

1. Introduction

Heterogeneous catalysts supported on commercial ion-exchangeresinsforthedirectsynthesis(DS)ofhydrogenperoxide fromtheelements,H2+O2→H2O2,hasbeenunderinvestigation forseveralyears[1–5].

Thisreaction is of interest to both industry [6–11] and the academia [12–24] for the small scale, on-site and on-demand production of this commodity, for which the anthraquinone autooxidation(AO)processisnotconvenient[12].Thekeyissuein thisreactionistheselectivitytowardsH2O2,becausetheactivePd orbimetallicPd–AuorPd–Ptcatalystsalsopromotetheformation ofwater,thatisthethermodynamicallyfavouredproduct.

∗ Correspondingauthor.Tel.:+390498275206. E-mailaddresses:paolo.centomo@unipd.it(P.Centomo),

carlo.meneghini@ﬁs.uniromatre.it(C.Meneghini),ssterche@abo.ﬁ(S.Sterchele),

angela.trapananti@esrf.fr(A.Trapananti),giuliana.aquilanti@elettra.eu

(G.Aquilanti).

Theadditionofgoldtothepalladiumcatalysts[25]orthe addi-tionofhalideions(Cl−,Br−)tothereactionmixture,especiallyin combinationwithmineralacids[13],arethemosteffectivewaysto enhancetheselectivityreportedsofar.Whileenhancingthe selec-tivity,thehalideionsdecreasethereactionrate[13].Purportedly theyavoidthedissociativechemisorptionofdioxygenbyblocking preferentiallythesiteswhereittakesplace[13,25].Thishypothesis issupportedbytheoreticalmodelsobtainedfromDFTcalculations

[26],butthemechanismisstillunclearandsounddirectevidence onthenatureoftheiractionnotachievedyet.

Tothebestofourknowledge,noinformationontheroleplayed by halide ions as DS promoters hasbeen gathered in situ and reportedsofar.Thiskindofinformationishighlydesirable[27]and

http://dx.doi.org/10.1016/j.cattod.2014.10.001

(2)

ARTICLE IN PRESS

G Model

2 P.Centomoetal./CatalysisTodayxxx(2014)xxx–xxx inthiscontextwehaverecentlyreportedontheimplementationof

acellfortheXAFSanalysisofsolidcatalystsundergas–liquid–solid conditions[28].

TheXAFS(X-ray absorptionfinestructurespectroscopy)is a wellascribed[29,30]chemicallyselectivetechniquetoprobethe chemicalnatureoftheabsorber(valencestate) itscoordination chemistry,andthelocalatomicstructurearoundit.TheXAFS sig-nalisconstitutedbythefineoscillationsthatcanbeobservedon theatomicabsorptioncoefficientofnotisolatedatoms.The scat-teringtheorydescribesthesefeaturesascausedbytheinterference betweenthephotoelectronwavesoutgoingfromtheabsorberand scatteredbythesurroundingatomicpotentials[31].TheXAFS sig-nalis traditionallydistinguishedin two regions:the nearedge (XANES:X-rayabsorptionnearedgestructure)andtheextended (EXAFS:extendedX-rayabsorptionfinestructure)regions.Inthe EXAFS region a relatively simple and well ascribed formalism allowstounderstandthelocalatomicstructurearoundthe aver-ageabsorberinthesampleasacombinationofcoordinationshells whoseparameters(coordinationnumbers,coordinationdistances anddisorderfactorscanbeaccuratelydeterminedbyleastsquare refinementprocedures[31–33].Thetheoryrequiredforab-initio modellingtheXANESfeaturesisgenerallycomplexbecausethey areaffectedbythefinestdetailsofinteratomicpotentialsandfull multiplescatteringeffects takeplace [31].However theXANES regionisdenseofstructuralandelectronicinformationthatcanbe oftenunderstoodcomparingthedataofreferencecompound[30]. WereporthereinontheinsituXAFS investigationwiththis cellof a nanostructured Pd catalyst for theDS workingin the presenceofsodiumbromide.Inviewofourlong-standing inter-estintopolymer-supportedmetalcatalysts[34–38]andintotheir applicationtoDS[5],thispreliminaryinvestigationwasfocused onamonometallicPdcatalystsupportedonthecommercial ion-exchangeresinLewatitK2621.Theexperimentsperformedwith theinsitu XAFS cell,have beencarried outunderatmospheric pressure.Althoughthisconditioncouldnotbeoptimalfor prac-ticalapplications,valuableinformationcanbeanywaycollectedas shownbytheachievementsofLunsford’sandStrukul’sgroupsin

theﬁeld[19,20,23].

2. Experimental

LewatitK2621(sulfonatedpolystyrene-divinylbenzene macro-reticular ion-exchange resin; exchange capacity=1.92mmol/g) waskindlyprovidedbyLanxess.Allthereagentsandthematerials wereusedasreceivedfromSigma–Aldrich.

ThepreparationofthecatalystPd/K2621fisdescribedinref.

[4].ThedetaileddescriptionofthecatalyticcellforinsituXAFS measurementsisreportedinref.[28].Inatypicalexperiment,an aqueoussolutionsof[Pd(NH3)4]SO4(0.0256g,0.188mmol, corre-spondingto1%Pd (w/w),in thefinal catalysts)wasaddedtoa suspensionof1.0104gofK2621in10mlofdistilledwater,after swellingfor2h.Thesuspensionwaslettoreactovernightona swirlingplate.Thesolidwasrecoveredbyvacuumfiltration, care-fullywashedwithdistilledwater(3×10ml)onthegoochfilter and dried overnight at 110◦C. The filtrate wascombined with thewashingliquorandanalyzedwithICP-MSforthequantitative determinationoftheunreactedmetal.Aftertheion-exchange,the residualamountofPdinthemotherliquorwaslessthan0.1%ofits startingamounts.Consequently,theuptakeofpalladiumwas com-pleteandtheexperimentalmetalloadingpracticallycorresponds tothenominaloneforeachsample.

The beige (very light brown-yellow) solid was suspended in 50ml of 37%aqueous formaldehyde solutionand heated up toreﬂux conditionwith anoil bathfor 3h. The black product wasrecovered byvacuum ﬁltrationand carefully washedwith

distilledwater(3×10ml).Thematerialwasdriedinovenat110◦C overnight.

XAFSmeasurementsat PdK-edge (24,350eV) intransmission geometryhave beencollectedatELETTRA-XAFS [39] and ESRF-BM08[40]beamlines.ThestructuralEXAFS(extended-XAFS)signal hasbeenextractedandquantitativelyanalyzedusingESTRAand Fitexaprograms,respectively[41].

ThecatalystwascharacterizedbyEXAFSatroomtemperature in thedry state(polyvinylpirrolydonepellet), in the methanol-swollenstate,withthealcoholflowingthroughthecatalystplaced inside themeasurement cellafter swellingwithmethanol, and underreactionconditions,respectivelywithorwithout10ppmof NaBrdissolvedinmethanol.Forthelatterexperimentamixtureof methanol(ora10ppmNaBrsolutioninmethanol),pre-saturated withCO2,H2 andO2,ofH2 andof O2 werecontinuouslyletto flow through the catalyst bed inside the measurement cell.As described inRef. [28]each gaswasseparatelyfed witha dedi-catedsyringeatatmosphericpressureatthedesiredflowrates. Flowrates (CO2:1.72ml/min, O2:0.20ml/min,H2:0.08ml/min; CO2/H2/O2=86/10/4,v/v)weresetastokeepthegasmixtureout oftheexplosiverange[42].Theliquidwasfedusingaperistaltic pump.

Thesameprocedurehasbeenemployedalsofortheoff-line catalyticrunsforthequantitativeanalysisofH2O2andthe con-centrationofH2O2hasbeenmeasuredbyiodometrictitration.

3. Resultsanddiscussion

InthisworkweinvestigatedwithEXAFSinsituapalladium cat-alystobtainedduringa previousinvestigationofours[4]under halogen-free conditions to prevent the accidental retention of halideions.Forthispurpose[Pd(NH3)4]2+wasusedasthemetal precursorandion-exchangedforH+ _in_the_resin_Lewatit_K2621. Theimmobilizedmetalcomplexwasthereducedwith37%aqueous formaldehydeatreﬂux.

Since thegoalof this investigationwastosimplycheckthe reliabilityoftheapparatusforthecharacterizationinsituofthe catalystsfortheDSreaction,wedidnotattemptanyquantitative estimateoftheproductionofH2O2andoftheselectivitytowards itduring theXAFS measurements.Theproduction ofH2O2 was onlyqualitativelyshownupontheadditionofanacidicsolution of TiOSO4 [43] to samples of theliquid phase collectedat the celloutlet.However,representativeoff-lineexperimentswiththe sameexperimentalset-up,carriedoutinourlaboratory,showed thatundertheseconditionstheconcentrationofH2O2 isaround 0.06mMand0.02mMintheabsenceandinpresenceofbromide ions,respectively.Thesevaluesarequitelow,butitshouldbeborne inmindthatthereactorwasnotoptimized forcatalytic perfor-mance.Nevertheless,theseoutcomesareinagreementwiththe literature,showingthatbromideionsastheselectivityenhancers produceasigniﬁcantdecreaseofthecatalyticactivity[12,13].

TheresultsoftheanalysisoftheEXAFSmeasurementscollected underdifferentexperimentalconditionsatroomtemperatureare summarizedinTable1.InFig.1,EXAFSsignalsfromtheasprepared catalysts(a)andafterthetreatmentwith1.0ml/minmethanolfor 2h(b)arereportedwiththeirbestﬁtcurves,asanexample.The contributionstothetotalEXAFSfromthemetalandoxidized(PdO) Pdphasesarealsoshown(lowerpartofeachpanel).

Intheas-preparedcatalystwefoundbothmetalPdandPdO. Aftersomepreliminary tests,themetal phase wasassumed to havethefccstructure.TheinteratomicdistancesofmetalPdare constrained tothefcc structure,leaving onlythefccedge (aPd) asfreeparameterinthereﬁnement[44].Thea_Pd=3.88(1) ˚Aisin goodagreementwiththeliterature value(aPd=3.89 ˚A)for bulk Pd.The oxidephase(PdO)wasincludedwithsignals fromtwo

(3)

ARTICLE IN PRESS

G Model

P.Centomoetal./CatalysisTodayxxx(2014)xxx–xxx 3

Table1

PdK-edgeEXAFSdataanalysisresultsforthecatalystPd/K2621funderdifferentexperimentalconditions(insituresultsinshadowedlines).

t(h) Pd PdO Leastsquare

Atom% a(Å)e _R Pd O(Å)f RPd Pd(Å)f R2wg Dry,asprepareda _– ₆₂₍₃₎ _3.88(1) _2.01(1) _3.06(1) _5.93_×₁₀−2 Flowingmethanolb ₂₊_0.75 ₆₄₍₃₎ _3.86(1) _1.99(1) _3.04(1) _0.137 UnpromotedReactionc _2.25–3 ₆₅₍₃₎ _3.86(1) _2.05(1) _2.98(1) _6.12_×₁₀−2 PromotedReactiond _2.25–3 ₇₆₍₄₎ _3.86(1) _2.06(1) _2.97(1) _7.87_×₁₀−2 a_{Polyvinylpirrolydone}_pellet. b_Flow_rate_1.0_ml/min.

c _Flow_rates_(ml/min):₁_(methanol),_1.72_(CO

2),O20.20,H20.08. d _The_same_as_c₊₁₀_ppm_NaBr_dissolved_in_methanol.

e_Bulk_Pd_lattice_parameter.

f _Pd _O_and_Pd _Pd_are_the_first_and_the_next_neighbour_shells_of_the_PdO_phase. g_Quantifies_the_best_fit_quality_[28]_.

0.0 0.2 3 5 7 9 11 kχ (k) [Å −1 ] k [Å−1]

Dry Pd/K2621f MeOH − Pd/K2621f met

a b Pdfcc PdO Residual EXAFS 0.0 0.2 0 2 4 6 FT (k χ (k)) [Å −2 ] R [Å] a b Imm−FT(Pd_fcc) Imm−FT(PdO) Imm−FT(b) FT

Fig.1.AnalysisofthePdK-edgeEXAFSspectra(left)moduliofthek(k)FourierTransforms(right)ofexperimental(dottedlines)andbestfitcurves(solidlines)forthedry Pd/K2621fcatalystasprepared(a)andafterthetreatmentwith1.0ml/minmethanolfor2h(b).Curvesaandbareverticallyshiftedforclarityinbothpanels.Thelowerparts ofthepanelsshowthePdmetal(Pdfcc)andPdOcontributionsusedtofitthebdatasetandtheresidualk(exp−fit)(left)andtheFTimaginarypartsoftheFTofexperimental data(Imm-FT(b))ofthebestfitcurveforthebsample,ofthePdmetal(Imm-FT(Pdfcc))andPdO(Imm-FT(PdO))contributions.(Forinterpretationofthereferencestocolour inthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

coordinationshells,with4nearest neighboursoxygenatomsat about2andwith4nearestneighbourspalladiumatomsatabout 3 ˚A,respectively.ThereﬁnedRPd OandRPd Pddistances(2.01(1), 3.06(1) ˚A)arealsoingoodagreementwiththeliteraturevalues (dPd O=2.023 ˚AanddPd Pd=3.043 ˚A[45])forbulkPdO.Thefraction ofPdinthemetalphasehasbeenestimatedbyconstrainingthe coordination numbers of metal Pd as xNbulki, being Nbulki the ith coordination number of bulk fcc structure. The estimated proportionofPdintheaspreparedcatalystisaround62%.

Swellinginmethanoldoesnotappreciablyaffectthecatalyst. TheEXAFSdataobtainedfromthemeasurementonthe alcohol-swollenmaterialloadedinthecellfedwith1.0ml/minofmethanol (Fig.1b)arepracticallythesameasthoseofthedrycatalyst(Fig.1a). Inthis casethedatacollection(45min) wasstarted after2hof conditioningofthecatalystwiththeﬂowingsolvent.Accordingly thecatalystappearstobestableinthereactionsolvent.

Noappreciablechangeswereobservedwhenthereagents(H2 andO2,dilutedwithCO2;CO2/O2/H2=1.72/0.20/0.08ml/min)were fedwithmethanol(1.0ml/min).Inthiscasethecollectionofthe EXAFSdata(lasting∼45min)wasstartedafter2.25hofreaction. Undertheseconditionstheliquidphasecollectedatthecelloutlet turnedyellowuponadditionofanacidicsolutionofTiOSO4. indi-catingtheformationofH2O2,i.e.thatthecatalystisactive.These ﬁndingsindicatethatwiththisexperimentalset-upthecatalyst

doesnotundergoanychangeor,mostlikely,thatthechangesare toosmalltobedetected.

Bycontrast,when10ppmofNaBrweredissolvedinmethanol ﬂowweobservedanappreciableincreaseoftheatomicpercentage ofmetalpalladiumandloweringoftheedgejumpto0.66 from theinitialvalueof0.73.Theseoutcomeclearlydemonstratethat bromideionshaveatleasttwoeffects:ontheonehandtheyreduce theoxidationstateofpalladiumand,ontheotherone,theypromote palladiumleaching.Thisisinagreementwiththeresultsobtained withthesamecatalystinabatch-wisereactorunderconditions morerelevanttopracticalexploitationofthereaction[46].Itis possiblethattheseeffectsareconnectedwiththeroleofbromide ionsaspromotersoftheDSreaction.

4. Conclusion

Theresultsobtainedinthisworkshowthatourinsituapproach totheEXAFScharacterizationofheterogeneouscatalystsforthe DS reactioncan provideusefulinformation ontheevolution of thecatalyst underduty. Althoughtheapparatusemployed was notoptimizedforcatalytic performance,thepreliminaryresults reported herein support the hypothesis that bromide ions in low concentration(10ppm),atwhich theyareknowntoactas effectivepromoters,favourthereductionofthecatalyst(increase

(4)

ARTICLE IN PRESS

G Model

4 P.Centomoetal./CatalysisTodayxxx(2014)xxx–xxx oftheatomicfractionofmetalPd)andthemetalleaching.Thisis

inlinewithotherﬁndingswhichwereobtainedinacompletely independentinvestigationofoursontheDSreaction,suggesting thattheseeffectsareconnectedwiththeabilityofbromides(and chlorides) to enhance the selectivity towards H2O2 of the DS catalysts.

Acknowledgments

We acknowledge N. Novello, L. Olivi (XAFS beamline at Elettra-SincrotroneTriesteS.C.p.A.,Italy,Elettraproposalnumber: 20110243)and A. Rizzo (BM08beamline at ESRF, France,ESRF proposalnumber:CH3388)forthesupportduringtheXAFS mea-surements.

References

[1]G.Blanco-Brieva,E.Cano-Serrano,J.M.Campos-Martin,J.L.G.Fierro,Chem. Commun.(2004)1184–1185.

[2]C.Burato,P.Centomo,M.Rizzoli,A.Bifﬁs,S.Campestrini,B.Corain,Adv.Synth. Catal.348(2006)255–259.

[3]C.Burato,S.Campestrini,Y.-F.Han,P.Canton,P.Centomo,P.Canu,B.Corain, Appl.Catal.Gen.358(2009)224–231.

[4]S.Sterchele,P.Biasi,P.Centomo,P.Canton,S.Campestrini,T.Salmi,M.Zecca, Appl.Catal.Gen.468(2013)160–174.

[5]S.Sterchele,P.Biasi,P.Centomo,S.Campestrini,A.Shchukarev,A.-R.Rautio, J.-P. Mikkola,T.Salmi,M.Zecca,Catal.Today(2014),http://dx.doi.org/10. 1016/j.cattod.2014.02.021.

[6]B.Zhou,M.Rueter,L.-K.Lee,B.Pelrine,Utilizinganactivesupportednoblemetal phase-controlledcatalystinaliquidmediumcontaininganorganicsolvent andwaterforprovidinghighactivityandproductselectivitytotheprocess US20020106320A1,2002.

[7]D.A.Rino,D.A.Giordano,P.Giuseppe,Directsynthesisofhydrogenperoxidein amulticomponentsolventsystemWO2002092501A1,2002.

[8]G.Paparatto,R.D’Aloisio,G.D.Alberti,R.Buzzoni,Processforthedirect synthe-sisofhydrogenperoxideUS6630118B2,2003.

[9]R.D’Aloisio,G.D.Alberti,G.Paparatto,Fromhydrogenandoxygenin halo-genatedoracidpromotersandheterogeneitycatalystUS20040151659A1, 2004.

[10]H.Thomas,J.Robert,S.Guido,Processforthedirectsynthesisofhydrogen peroxideWO2005108285A1,2005.

[11]T.Haas,G.Stochniol,R.Jahn,Processforthedirectsynthesisofhydrogen per-oxideUS7241908B2,2007.

[12]J.M.Campos-Martin,G.Blanco-Brieva,J.L.G.Fierro,Angew.Chem.Int.Ed.45 (2006)6962–6984.

[13]C.Samanta,Appl.Catal.Gen.350(2008)133–149.

[14]J.K.Edwards,G.J.Hutchings,Angew.Chem.Int.Ed.47(2008)9192–9198.

[15]J.K.Edwards,B.E.N.N.Solsona,A.F.Carley,A.A.Herzing,C.J.Kiely,G.J.Hutchings, Science323(2009)1037–1041.

[16]P.Landon,P.J.Collier,A.F.Carley,D.Chadwick,A.J.Papworth,A.Burrows,C.J. Kiely,G.J.Hutchings,Phys.Chem.Chem.Phys.5(2003)1917–1923.

[17]P.J.Miedziak,S.A.Kondrat,N.Sajjad,G.M.King,M.Douthwaite,G.Shaw,G.L. Brett,J.K.Edwards,J.D.Morgan,G.Hussain,G.J.Hutchings,Catal.Sci.Technol. (2013).

[18]Y.-F.Han,J.H.Lunsford,J.Catal.230(2005)313–316.

[19]Q.Liu,J.C.Bauer,R.E.Schaak,J.H.Lunsford,Angew.Chem.Int.Ed.47(2008) 6221–6224.

[20]S.Abate,M.Freni,R.Arrigo,M.E.Schuster,S.Perathoner,G.Centi, Chem-CatChem5(2013)1899–1905.

[21]S.Abate,R.Arrigo,M.E.Schuster,S.Perathoner,G.Centi,A.Villa,D.Su,R.Schlögl, Catal.Today157(2010)280–285.

[22]S. Abate,P. Lanzafame,S.Perathoner, G. Centi, Catal. Today169 (2011) 167–174.

[23]S.Melada,R.Rioda,F.Menegazzo,F.Pinna,G.Strukul,J.Catal.239(2006) 422–430.

[24]P. Biasi, J. García-Serna, A. Bittante, T. Salmi, Green Chem. 15 (2013) 2502–2513.

[25]E.N.Ntainjua,M.Piccinini,J.C.Pritchard,Q.He,J.K.Edwards,A.F.Carley,J.A. Moulijn,C.J.Kiely,G.J.Hutchings,ChemCatChem1(2009)479–484.

[26]T.Deguchi,M.Iwamoto,J.Phys.Chem.C117(2013)18540–18548.

[27]G.A.Somorjai,C.Aliaga,Langmuir26(2010)16190–16203.

[28]P.Centomo,C.Meneghini,M.Zecca,Rev.Sci.Instrum.84(2013)054102.

[29]D.C.Koningsberger,R.Prins,X-rayAbsorption:Principles,Applications, Tech-niquesofEXAFS,SEXAFSandXANES,Wiley,NewYork,1988.

[30]G.Bunker,IntroductiontoXAFS,CambridgeUniversityPress,Berlin,2010.

[31]J.J.Rehr,R.C.Albers,Rev.Mod.Phys.72(2000)621–654.

[32]B.-K.Teo,EXAFS:BasicPrinciplesandDataAnalysis,Springer,Berlin,1986.

[33]A.Filipponi,A.DiCicco,C.R.Natoli,Phys.Rev.B52(1995)15122–15148.

[34]M.Králik,B.Corain,M.Zecca,Chem.Pap.(2000)254–264.

[35]D.Gaˇsparoviˇcová,M.Kralik,M.Hronec,A.Bifﬁs,M.Zecca,B.Corain,J.Mol. Catal.A:Chem.244(1–2)(2006)258–266.

[36]P.Centomo,M.Zecca,B.Corain,J.Clust.Sci.18(2007)947–962.

[37]B.Corain,M.Zecca,P.Canton,P.Centomo,Philos.Trans.R.Soc.Math.Phys.Eng. Sci.368(2010)1495–1507.

[38]M.Bortolus,P.Centomo,M.Zecca,A.Sassi,K.Jeˇrábek,A.L.Maniero,B.Corain, Chem.Eur.J.18(2012)4706–4713.

[39]A.D.Cicco,G.Aquilanti,M.Minicucci,E.Principi,N.Novello,A.Cognigni,L.Olivi, J.Phys.Conf.Ser.190(2009)012043.

[40]S.Pascarelli,F.Boscherini,F.d’Acapito,J.Hrdy,C.Meneghini,S.Mobilio,J. SynchrotronRadiat.3(1996)147–155.

[41]C.Meneghini,F.Bardelli,S.Mobilio,Nucl.Instrum.MethodsPhys.Res.Sect.B: BeamInteract.Mater.At.285(2012)153–157.

[42]V.Schröder,B.Emonts,H.Janßen,H.-P.Schulze,Chem.Eng.Technol.27(2004) 847–851.

[43]I.R. Cohen, T.C. Purcell, A.P. Altshuller, Environ. Sci. Technol. 1 (1967) 247–252.

[44]C.Battocchio,C.Meneghini,I.Fratoddi,I.Venditti,M.V.Russo,G.Aquilanti,C. Maurizio,F.Biondino,R.Matassa,M.Rossi,S.Mobilio,G.Polzonetti,J.Phys. Chem.C116(2012)19571–19578.

[45]J.Waser,H.A.Levy,S.W.Peterson,ActaCrystallogr.6(1953)661–663.

[46]S.Sterchele,P.Biasi,N.Gemo,A.Shchukarev,A.-R.Rautio,J.-P.Mikkola,P.Canu, P.Centomo,K.Kordas,T.Salmi,unpublishedresults.