Tim White is currently Associate Professor at the School of Materials Science and Engineering and Director of the Facility for Advanced Characterisation, Testing ...
3YNTHESISOF(IGH 0ERFORMANCE (YDROXYAPATITE 'OLD #ATALYSTSFOR #//XIDATION .OPPHAWAN0HONTHAMMACHAI A :HONG:IYI B'UO*UN A(AN9I&AN B AND4*7HITEA A
3CHOOLOF-ATERIALS3CIENCEAND%NGINEERING .ANYANG4ECHNOLOGICAL5NIVERSITY 3INGAPORE B )NSTITUTEOF#HEMICALAND%NGINEERING3CIENCE !GENCYFOR3CIENCEAND2ESEARCH 3INGAPORE
# ORRESPONDING!UTHOR.OPPHAWAN0HONTHAMMACHAI 4EL &AX % MAILADDRESSNOPPHAWAN NTUEDUSG
!BSTRACT #ATALYSTSFORLOWTEMPERATURE#/OXIDATIONWERE PREPAREDBYDECORATINGHYDROXYAPATITE(!P CERAMIC FOAMSCAFFOLDSWITHHIGHLYDISPERSEDGOLD NANOCRYSTALSUSINGADEPOSITION PRECIPITATION$0 PROCESS#ATALYTICACTIVITY MICROSTRUCTUREAND CRYSTALLINITYWERESTUDIEDASAFUNCTIONOFREAGENT P( ANDAGINGTIME MIN FORPOWDERS ANDPOROUSSUPPORTS3UPERIORPRODUCTSWITHSMALL NM GOLDCRYSTALSDISTRIBUTEDHOMOGENEOUSLYOVER (!PFOAMWEREOBTAINEDATP( ,ARGERCRYSTAL SIZESANDCOLLOIDALGOLDAGGLOMERATIONAPPEAREDAT LONGERAGINGTIMES4HEOPTIMIZEDCATALYSTPREPARED BYREACTIONATP(FORMINSHOWED#/ CONVERSIONTO#/AT#4HE!U (!PCOMPOSITE DEMONSTRATEDEXCELLENTDURABILITYBYRETAINING STRUCTURALANDCRYSTALLOGRAPHICINTEGRITYWITHNOLOSS OFACTIVITYWHENTESTEDAT#OUTTOH
+EYWORDS #/OXIDATION DEPOSITION PRECIPITATIONPROCESS GOLD NANOCRYSTAL HYDROXYAPATITE
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)NTRODUCTION 4HE CATALYTIC ACTIVITY OF OXIDE SUPPORTED GOLD NANOPARTICLES HAS BEEN EXPLOITED IN MANY APPLICATIONS INCLUDING CHEMICAL SENSING HYDROGENPRODUCTION FUELCELLS ANDVARIOUSCHEMICAL PROCESSES;=OFWHICHCARBONMONOXIDEOXIDATIONISTHEBEST KNOWN;=3EVERALMECHANISMSHAVEBEENPROPOSEDTOEXPLAIN THE ENHANCED ACTIVITY AT THE METAL OXIDE INTERFACE INCLUDING THEPRESENCEOFHIGHLYREACTIVESURFACEGOLDATOMSATGROWTH STEPS;= THEEASEOFCHARGETRANSFERATTHEBOUNDARY;= AND IN THE CASE OF REDUCIBLE SUPPORTS THE PROVISION OF ACTIVATED OXYGENTHATCANBETRAPPEDATREACTIONSITES;=7HATEVERTHE MEANS ITISEVIDENTTHATINTERACTIONSBETWEENTHESUPPORTAND THE GOLD MUST BE TAILORED FOR PERFORMANCE ENHANCEMENT 7HILEPOROUSBINARYOXIDESSUCHASALUMINA;=ORZIRCONIA;= ARE FREQUENTLY USED AS CATALYTIC SCAFFOLDS THE SEARCH FOR SUPPORTS WHOSE PROPERTIES WILL AMPLIFY THE REACTIVITY OF NANOSIZEDGOLDCONTINUES/NESUCHMATERIALISHYDROXYAPATITE (!P WHOSE POTENTIAL AS A COMPONENT OF ENVIRONMENTALLY BENIGN CATALYSTS HAS NOT BEEN WIDELY CONSIDERED ALTHOUGH PHOTOCATALYTIC MINERALISATION OF POLLUTANTS ;= AND OXIDATIVE DECOMPOSITION OF METHANOL ;= HYDROCARBON ;= TRICHLOROBENZENE AND CHLOROETHYLENE ; = HAVE BEEN DEMONSTRATED0ROGRESSINTHIS½ELDHASBEENHAMPEREDPARTLY BECAUSE (!P IS CRYSTALLOCHEMICALLY COMPLEX ;= AND ALSO BECAUSE IT HAS BEEN DIF½CULT TO PREPARE WITH POROUS ARCHITECTURES ALTHOUGHMETHODSTOCONSTRUCTINVERSEOPALINE STRUCTURES; =ANDFOAMS; =ARENOWAVAILABLE)NDEED APATITICCHEMISTRYISDIVERSEWITHSOMEANALOGUESINTRINSICALLY CATALYTIC; = ANDTHESEFORMSMAYINTERACTFAVORABLYWITH METALLICCO CATALYSTSSUCHASGOLDTOINVIGORATEREACTIVITY $EPOSITION PRECIPITATION $0 IS A RELIABLE METHOD TO PREPARE OXIDE SUPPORTED GOLD ; = IN WHICH THE KEY PARAMETERISTHEISOELECTRICPOINT)%0 OFTHESCAFFOLDMATERIAL -OREAU ET AL ;= FOUND THAT THE DEPOSITION RATE OF AGGLOMERATEDGOLDCRYSTALSONTITANIAACCELERATEDWITHP( )%0ASTHEPOSITIVESURFACECHARGEATLOWERP(ATTRACTEDGOLD BEARING ANIONS !U#L (OWEVER WITH P( )%0 THE TITANIA SURFACE CARRIED A NET NEGATIVE CHARGE AND ELECTROSTATIC REPULSIONOFANIONSRESULTEDINHIGHLYDISPERSEDGOLDCRYSTALS WITHTHECOVERAGEIN¾UENCEDBYMANYFACTORSINCLUDINGTHE CONCENTRATIONOF(!U#L THEVOLUMETOMASSRATIOOF(!U#L TOSUPPORT THEBASETYPE.A/( .(/( UREA AGINGTIME AND TEMPERATURE PROCESSING STEPS ½LTRATION WASHING AND DRYINGSTEPS ANDCALCINATIONTEMPERATURE;=3ELECTIONOF THE ALKALINE SOLUTION PROVES ESPECIALLY CRITICAL IN CONTROLLING THEYIELD DISPERSIONANDSIZEOFGOLDCRYSTALS&OREXAMPLE :ANELLAETAL;=FOUNDTHATDURING$0IN.A/(THEGOLDWAS PARTIALLY REDUCED TO METAL AFTER DRYING AT # BUT A COMPARABLE EXPERIMENT IN UREA DELAYED METALLIZATION TO CALCINATIONTEMPERATURES # 7HILE $0 SYNTHESES OF GOLD NANOPARTICLES ON REDUCIBLE 4I/ &E/ #O/ ; =ANDNON REDUCIBLEOXIDES3I/ !L/ ; =HAVEBEENWIDELYREPORTEDANDTHECATALYTIC MECHANISMSINVESTIGATED CORRESPONDINGEXAMINATIONOF(!P SUPPORTS ARE SPARSE 4HE NOTABLE EXCEPTION IS THE STUDY OF
6EMUGOPALETAL;=WHERE!UAND2U DEPOSITEDFROM (!U#LX(/ AND 2U#L(/ AT P( FOR H PRODUCED A CATALYSTUSEDTOPROMOTEALOWTEMPERATUREWATER GASSHIFT REACTION)TWASFOUNDTHAT!U(!PSHOWEDTHEHIGHERACTIVITY WITHCONVERSIONTO#/AND(ACHIEVEDAT# BUT CORRELATIONOFPREPARATIVECONDITIONSWITHRESPECTTOTHESIZE DISPERSIONANDCATALYTICACTIVITYOFTHEGOLDCRYSTALSREQUIRES CLARI½CATION 4HIS REPORT DESCRIBES A $0 METHOD THAT HOMOGENEOUSLYDISTRIBUTESGOLDNANOCRYSTALSNMAVERAGE DIAMETER OVER POROUS (!P TO CREATE A HIGHLY EF½CIENT AND DURABLECATALYSTFOR#/OXIDATION4ECHNIQUESTOCONTROLTHE SIZEANDDISPERSIONOFMETALLICCRYSTALSON(!PAREDETAILED PARTICULARLYWITHRESPECTTOTHEMANNEROFSUPPORTSYNTHESIS P(CONTROLANDAGINGTIME
%XPERIMENTAL (YDROXYAPATITE(!P SUPPORTFABRICATION (!P WAS SYNTHESIZED BY MIXING G CALCIUM NITRATE #A./ -ERCK AND G .( (0/ !LDRICH IN ML .(/( -ERCK AND ML DOUBLY DEIONIZEDWATER4HESUSPENSIONWASHEATEDTO#WITH MAGNETIC STIRRING FOR MIN AFTER WHICH THE TEMPERATURE WASRAISEDTO#ANDSTIRRINGCONTINUEDFORMIN4HE PRECIPITATED(!PWASSEPARATEDUSINGVACUUM½LTRATIONAND WASHED SEVERAL TIMES PRIOR TO OVEN DRYING OVERNIGHT AT # #RYSTALLIZATION WAS PROMOTED BY DEWATERING AT # WITH A HEATING RATE OF # MIN FOR MIN FOLLOWEDBYSOAKINGAT#FORHRWITHAHEATINGRATE OF#MIN 4HE (!P FOAM SUPPORT WAS PREPARED IN A TWO STAGE PROCESS;=&IRST METHYLCELLULOSEC0SVISCOSITY !LFA !ESAR WASDISSOLVEDINMLOFDEIONIZEDWATERTHENAERATED WITHASTAINLESSSTEELBLADEAGITATORAT RPMFORMIN 3ECOND A SUSPENSION OF ML (/ AND G (!P WAS ADDEDTOTHEPOLYMERSOLUTIONWITHAGGRESSIVEMIXINGFOR MIN4HEPOLYMERFOAMIMPREGNATEDWITH(!PWASCALCINED AT##MIN FORMINTOREMOVEORGANICSPRIORTO ½NALSINTERINGFORHRS##MIN 'OLDDEPOSITION PRECIPITATION$0 'OLDSTOCKSOLUTIONSWEREPREPAREDBYADDING-.A/( 3IGMA !LDRICH DROPWISE TO ML OF M- (!U#L (!U#L(/ !LFA !ESAR UNTIL THE P( STABILIZED AT PREDETERMINEDVALUESRANGINGFROMTO(!PG WAS DISPERSED IN THE SOLUTION WHICH WAS AGED AT # WITH VIGOROUS STIRRING FOR MIN AFTER WHICH THE GOLD LOADED CATALYSTSWERECOLLECTEDBY½LTRATIONANDWASHEDREPEATEDLY WITH DEIONIZED WATER TO REMOVE CHLORIDE 4HE POWDER WAS OVEN DRIED OVERNIGHT AT # PRIOR TO AIR CALCINATION AT ##MIN FORH -ATERIALSCHARACTERIZATION 4HE CRYSTALLOGRAPHIC PARAMETERS AND PHASE CONTENT OF THE (!P SUPPORT AND GOLD LOADED CATALYSTS WERE DETERMINED
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FROMPOWDER8 RAYDIFFRACTION82$ PATTERNS3HIMADZU,!" 82$ COLLECTEDATK7USING#U+αRADIATION3PECIMENS WEREPREPAREDFORDIFFRACTIONBYPACKINGPOWDERINTOAGLASS HOLDER × CM WITH INTENSITY COLLECTED BY STEP SCANNING OVERTHEθRANGETOININCREMENTSOFANDA DWELL TIME OF S 2IETVELD ANALYSIS AS IMPLEMENTED IN 4/0!3; =WASUSEDFORQUANTITATIVEPHASEDETERMINATION WITH HYDROXY¾UORAPATITE ;= AND GOLD ;= EMPLOYED AS CRYSTALLOGRAPHIC MODELS IN A SIMULATION THAT INCLUDED A BACKGROUND POLYNOMIAL SCALE FACTOR CELL PARAMETERS PREFERREDORIENTATIONANDZEROPOINTCORRECTION)NTHISWAY POWDER82$WASUSEDTODETERMINEPHASECONTENT CONDUCT MASSBALANCEANDESTIMATECRYSTALSIZE#ATALYSTMORPHOLOGY WAS EXAMINED USING A ½ELD EMISSION SCANNING ELECTRON MICROSCOPE3%- *%- OPERATEDINSECONDARYELECTRON IMAGING3%) MODEATK64OAVOIDCHARGING THEPOWDERS WERE PRESSED INTO A CARBON TAPE ATTACHED TO AN ALUMINUM STUB FOLLOWED BY GOLD COATING USING A LOW ENERGY PLASMA -ICROGRAPHS OF THE PYROLYZED SAMPLES WERE OBTAINED AT MAGNI½CATIONSOF× AND× "RIGHT½ELDTRANSMISSION ELECTRONMICROSCOPY4%- *%-& WASCONDUCTEDAT K6.ITROGENADSORPTION DESORPTIONISOTHERMSWEREUSEDTO DETERMINE SURFACE AREA AND POROSITY -ICROMERITICS !3!0 VIATHE"RUNAUER %MMETT 4ELLER"%4 METHOD!MIXED GAS OF NITROGEN AND HELIUM WAS ALLOWED TO ¾OW THROUGH THE ANALYZER AT A CONSTANT RATE OF CCMIN .ITROGEN WAS USED TO CALIBRATE THE ANALYZER AND ALSO SERVED AS THE ADSORBATEATLIQUIDNITROGENTEMPERATURE4HESAMPLESWERE THOROUGHLYOUTGASSEDFORHAT#BEFOREEXPOSURETOTHE ADSORBENTGAS #ATALYTICTESTING #ATALYTIC OXIDATION OF #/ WAS CONDUCTED IN A ½XED BED MICROREACTORUSINGACATALYSTCHARGEOFMG0RIORTOTESTING THE MATERIAL WAS HEATED IN AIR AT # FOR H TO REMOVE IMPURITIES!FTERCOOLINGTOROOMTEMPERATURE AREACTANTGAS CONTAININGVOL#/INAIRWASPASSEDTHROUGHTHECATALYST BEDWITHAGASHOURLYSPACEVELOCITY'(36 OFH 4HE OUTLETGASESWEREANALYZEDBYANON LINEGASCHROMATOGRAPH '# 3HIMADZU " EQUIPPED WITH THERMAL CONDUCTIVITY 4#$ AND¾AMEIONIZATION&)$ DETECTORS#ATALYSTDURABILITY WASESTABLISHEDBY½XINGTHETEMPERATUREOFTHEREACTORAT # FOR H UNDER THE SAME FEED CONDITIONS USING THE !U (!PMATERIALPREPAREDATP(
2ESULTSAND$ISCUSSION 3TRUCTUREOF(!PSUPPORT "OTHTHEDRIEDANDCALCINEDPOWDERSCONTAINEDSINGLEPHASE (!P AS DETERMINED BY POWDER 8 RAY DIFFRACTION &IGURE WITHTHEAVERAGECRYSTALLITESIZEINCREASINGFROMTONM AFTER HEAT TREATMENT 4ABLE -ATCHING OF SIMULATED AND EXPERIMENTALDIFFRACTEDPATTERNSINVARIABLYREQUIREDCORRECTION OFL RE¾ECTIONINTENSITIESTHATWEREENHANCEDASARESULT OFACICULARFACETING3UBSEQUENTINCORPORATIONOF(!PINTHE
4ABLE 0HYSICALPROPERTIESOFDRIED CALCINEDANDFOAM(!PSUPPORTS
3AMPLE
"%4 0ORESIZE (!P SURFACEAREA NM CRYSTALSIZE
NM MG
$RIED(!P
#ALCINED(!P
#ALCINEDFOAM(!P
DETERMINEDBY82$ 800
CERAMIC FOAM AND ½RING AT # DID NOT PERCEPTIBLY CHANGE THE CRYSTALLINITY OR TEXTURE 4ABLE 4HE CALCINED POWDERCONTAINEDLARGEAGGREGATES M OF(!PCRYSTALS WHILETHECERAMICFOAMCONSISTEDOFSPONGYMACROPOREWALLS OFHIGHPOROSITY&IGURE 4HEHIGHESTSURFACEAREAWITHTHELARGESTPORESIZE NM ASDETERMINEDBYTHE"%4PROCEDUREWASOBTAINEDFROM DRIED (!P BUT DECREASED DRAMATICALLY AFTER CALCINATION AT #DUETOCRYSTALGROWTH4ABLE 0RIORTOCALCINATION THEDRIED(!PPOWDERSHOWEDATYPE)6NITROGENADSORPTION DESORPTIONISOTHERM&IGUREA WITHAHYSTERESISLOOPOFTYPE ( CONSISTENT WITH SLIT PORE MESOPOROSITY ; = !FTER
(a)
Hydroxyapatite 100.00%
600
Counts
400
200
0
-200
10
20
30
40
50
60
70
80
2Th Degrees 2,500 (b)
Hydroxyapatite 100.00%
2,000
Counts
1,500 1,000 500 0 -500
10
20
30
40
50
60
70
80
2Th Degrees &IGURE 2IETVELDPLOTSOF82$PATTERNSOFA POWDER(!PANDB FOAM(!PCALCINEDAT#4HEEXPERIMENTALINTENSITYISSHOWNASCIRCLESANDTHESIMULATED PATTERNASASOLIDLINE4HEINTENSITYDIFFERENCEBETWEENTHESEPATTERNSISSHOWNBELOW4HE"RAGGDIFFRACTIONMARKERSFOR(!PARESHOWNATTHEBOTTOM OFEACHPLOT
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4ABLE #ORRELATION OF (!P SUPPORT AND DEPOSITION PRECIPITATION CONDITIONS ON GOLDCRYSTALSIZEANDLOADING
(!P
P( !GINGTIME !ULOADING !UCRYSTALSIZE MIN NM
0OWDER
&OAM
&OAM
&OAM
&OAM
&OAM
&OAM
&OAM
DETERMINEDBY82$ CALCINATION THE POWDER AND FOAM GAVE TYPE )) ISOTHERMS &IGUREB ASTHEMESOPORESCOLLAPSEDLEADINGTOUNRESTRICTED MONOLAYER MULTILAYER ADSORPTION ;= 4HIS CON½RMS THAT WHILE THE CERAMIC FOAM IS MACROPOROUS ITS PORE WALLS ARE COMPOSED OF DENSE (!P PARTICLES RATHER THAN MESOPOROUSMATERIAL
%FFECTOF(!PSUPPORTON#/OXIDATION 'OLD NANOCRYSTALS DEPOSITED ON CERAMIC FOAM (!P WERE GENERALLYEVENLYDISTRIBUTED ANDOFSMALLERANDMOREREGULAR SIZE THANTHOSE½XEDTOTHEPOWDER4ABLE &IGURE 4HE !U LOADEDFOAMWASALWAYSMOREACTIVEWITHTHEOPTIMIZED MATERIAL ACHIEVING #/ CONVERSION AT # A TEMPERATURE # LOWER THAN THE ANALOGOUS CALCINED POWDER&IGURE !STHEAPATITEFOAMWITHOUTGOLDWASNOT CATALYTIC IT IS CLEAR THAT THE SIZE AND DISPERSION OF GOLD PARTICLESCONTROL#/OXIDATIONACTIVITY 4HEPRECISEFUNCTIONOFTHE(!PSUPPORTDURINGCATALYSIS WILLREQUIRECLARI½CATION BUTRECENTSTUDIESOFGOLDNANOCRYSTALS LOADEDONPHOSPHATE DOPEDTITANIA ORATTACHEDTOMONOCLINIC ANDHEXAGONAL,A0/ SUGGESTTHEPHOSPHATEGROUPSSTABILIZE !U CRYSTALS PREVENTING THEIR AGGLOMERATION AND ENHANCING #/CONVERSIONATHIGHTEMPERATURES; =4HEFUNCTIONOF THECRYSTALSTRUCTURESUPPORTISSUBTLE ASINTHECASEOF,A0/ GREATER#/CONVERSIONWASACHIEVEDONLYWITHCATALYSTSUSING THEHEXAGONALDIMORPH4HESERESULTSAREINACCORDWITHTHE PRESENT INVESTIGATION WHERE THE SIZE OF THE GOLD CRYSTALS DECORATING (!P REMAINED CONSTANT AFTER CALCINATION TO #(OWEVER THEMECHANISMOF#/CONVERSIONMUSTBE DISTINCT DUETOTHEDIFFERENTCHEMISTRY CRYSTALSTRUCTUREAND OXYGEN TRANSFERENCE EF½CIENCY OF APATITE !S (!P IS BOTH
300 Quantity adsorbed (cm3 g-1)
(a)
(a)
250 200 150 100 50 0 0.0
(b)
Quantity adsorbed (cm3 g-1)
6
0.2
0.4 0.6 0.8 Relative pressure (P/P0)
1.0
0.2
0.4 0.6 0.8 Relative pressure (P/P0)
1.0
(b)
5 4 3 2 1 0 0.0
&IGURE &IGURE 3%)MICROGRAPHSOFCALCINEDA POWDER(!PANDB FOAM(!P
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.ITROGENADSORPTION DESORPTIONISOTHERMSOFA DRIEDANDB CALCINED FOAM(!P
(a)
(a)
100 nm
(b)
20 nm
(b)
100 nm
(c)
20 nm
&IGURE "RIGHT½ELD4%-IMAGESOFGOLDDECORATINGA CALCINEDPOWDERANDB FOAM(!PSYNTHESIZEDATP(ANDANAGINGTIMEOFMIN
140
Au-powder HAp, pH8, 60 min, 0.1% Au (14 nm) Au-foam HAp, pH8, 60 min, 4.8% Au (8 nm) Calcined foam HAp
120 %CO Conversion
100 nm
(d)
100 80 60 40 20 0 0
50
100 150 200 250 Temperature (°C)
300
100 nm
350
&IGURE
&IGURE
#OMPARISONOFTHE#/CONVERSIONEF½CIENCYOFCALCINEDFOAM(!P GOLD LOADEDCALCINEDPOWDERANDFOAM(!P
(!PLOADEDGOLDNANOCRYSTALGROWTHANDDISPERSIONATP(A B C ANDD AGINGTIMEMIN
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Separated colloidal gold cluster
20 nm
100 nm
&IGURE
&IGURE
4%-IMAGEOFANHEDRALGOLDNANOCRYSTALSP(
,ARGECOLLOIDALGOLDCLUSTERFORMATIONP( AGINGTIMEMIN
POLYMORPHICANDZEOLITIC;=WITHAWIDERANGEOFCHEMICAL ANALOGUES A SYSTEMATIC STUDY OF CRYSTALLOCHEMICAL VARIANTS EG IRON ZINC OR MAGNESIUM REPLACEMENT OF CALCIUM AND PHOSPHOROUS MAYPRODUCESUPERIORCATALYTICSUPPORTS
;!U#L/( = ISPREVALENT DEPOSITINGONMANYNUCLEATIONSITES THATLEADSTOGOLDCRYSTALSOFSMALLERSIZEANDHIGHERDISPERSION 7HEN THE P( INCREASES TO WHICH EXCEEDS THE )%0 GOLD DEPOSITIONISIMPEDEDBYELECTROSTATICREPULSIONBETWEENTHE (!PSURFACEANDGOLDHYDROXIDECOMPLEXES4HEGEOMETRICAL CON½GURATIONOFTHEGOLDNANOCRYSTALSCONTROLSTHENUMBER OFCO ORDINATIVELYUNSATURATEDSITES3PHERICALTOPOLOGYWAS DOMINANT &IGURE AND PROVIDES HIGHER SELECTIVITY AND TURNOVER FREQUENCY 4/& ; = COMPARED TO FACETED OR MULTIPLY TWINNEDPARTICLES-40S 1UANTITATIVE 8 RAY DIFFRACTION CLEARLY SHOWS THE P( DEPENDENCEOFGOLDLOADINGANDCRYSTALLINITY4ABLE &IGURE &ROMP( THEGOLDLOADINGINCREASESBYMORETHANAN ORDER OF MAGNITUDE BUT DECLINES AT HIGHER P( 4HE GOLD CRYSTAL SIZE DETERMINED FROM THE 82$ PEAK WIDTH IS CHARACTERISTICALLYSLIGHTLYSMALLERTHANDIRECTOBSERVATIONSBY 4%- ASEACHPARTICLEISCOMPOSEDOFCRYSTALDOMAINMOSAICS
%FFECTOFP(ON#/OXIDATIONACTIVITIES 4HEDISTRIBUTIONANDSIZEOFGOLDNANOCRYSTALSWASCONTROLLED BY ADJUSTING THE P( FROM TO TO INCREASE HYDROLYSIS OF
;!U#L= TO;!U/( = ANDREDUCETHEADSORPTIONOFCHLORIDE &IGURE 4ABLE 7HILE NEUTRAL !U#L(/ IS THE PRINCIPLE SPECIESATP( ^ ; =HYDROXYCHLORIDEANIONS
!U#LX/( X OCCUR FROM MID P( UNTIL AT P( !U/( IS DOMINANT4HEREFORE INACIDICCONDITIONSADSORPTIONOFGOLD COMPLEXESON(!PSURFACESTAKESPLACESLOWLYANDISOFLIMITED
EXTENT !T P( AND THE CONCENTRATION OF ;!U#LX/( X= INCREASESASDOESTHENUCLEATIONANDGROWTHOFGOLD!TP( WHICH SPANS THE )%0 OF (!P AROUND ;= ANIONIC
5000
120 2θ = 38.2°, Au (111)
3000
f e d c b a
2000 1000 0 20
40 60 2θ (Degree)
80
%CO Conversion
4000 Count
pH6, 60 min, 1.5% Au (8 nm) pH8, 60 min, 4.8% Au (8 nm) pH9, 60 min, 2.4% Au (5 nm) pH12, 60 min, 0.8% Au (6 nm)
140
100 80 60 40 20 0 0
50
100 150 200 250 Temperature (°C)
300
&IGURE
&IGURE
82$PATTERNSOFA THEPUREFOAM(!PANDAFTERGOLDDEPOSITIONATP(S B C D E ANDF AGINGTIMEMIN
#ORRELATIONOFP(WITH#/CONVERSIONOFGOLDLOADED CALCINED FOAM(!P
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350
pH8, 60 min, 4.8% Au (8 nm) pH8, 10 min, 3.1% Au (8 nm)
100 %CO Conversion
Reaction rate (mmol CO/gAu/s) x 10-3
120
80 60 40 20 0 0
3.0 pH9, 60 min, 2.4%Au (5 nm)
2.5 2.0 1.5 1.0 0.5 0.0
20 40 60 80 100 120 140 160 180 200 Temperature (°C)
0
20
40
60 80 100 120 140 160 180 Reaction time (h)
&IGURE
&IGURE
#ORRELATIONOF!UCRYSTALGROWTHTIMEWITH#/CONVERSIONEF½CIENCYOF (!PLOADEDGOLD
#/CONVERSIONOF!U LOADED(!PATP(ASAFUNCTIONOFREACTIONTIME AT#
)N ADDITION THE LARGE GOLD CLUSTERS THAT APPEAR AT P( INDEPENDENT OF THE (!P SUPPORT ARE ONLY RECOGNIZED BY MICROSCOPY&IGURE #OMPLETE#/CONVERSIONWASOBTAINEDAT^#FORTHE CATALYSTSPREPAREDATP(AND&IGURE WITHTHELATTER PROVINGMARGINALLYSUPERIOR ASTHEGOLDCRYSTALSWERESLIGHTLY SMALLER NM VERSUS NM 4HE LEAST #/ CONVERSION WAS OBSERVED FOR P( MATERIAL WHICH HAD THE LARGEST GOLD SIZENM WITHLEASTDISPERSIONASEVIDENTFROM4%-IMAGES &IGURE
REACTIONRATEWITH#/CONVERSIONATATEMPERATUREOF #&IGURE
%FFECTOFGOLDPARTICLESIZEON#/OXIDATIONACTIVITY &ORTHEOPTIMALP(CATALYSTS THE$0AGINGTIMEWASVARIED FROM ANDMINTOSYSTEMATICALLYADJUSTCRYSTALSIZE 4HE SMALLEST UNIFORM !U SIZE NM WAS OBTAINED AT THE SHORTESTAGINGTIMEMIN 4HISMATERIALGAVETHEFASTEST
$URABILITY 4HE DURABILITY AND STABILITY OF !U LOADED (!P AT P( WAS TESTEDBYMAINTAININGTHETEMPERATUREOFTHEREACTORAT# FOR H &IGURE 4HE #/ OXIDATION ACTIVITY REMAINS CONSTANT AS A FUNCTION OF REACTION TIME UNLIKE METAL OXIDE SUPPORTED !U CATALYSTS SUCH AS #U-N/ ;= AND 4I/ ; =WHEREDEACTIVATIONOCCURREDRAPIDLYDUETOGROWTHOFTHE !UCRYSTALS; =4HISISASIGNI½CANTPROBLEMFORNANOGOLD CATALYSTSTHATMAKESTHEMUNSUITABLEFORLONGTERMUSEHAS BEENOVERCOMEUSINGTHE(!PSCAFFOLD&ROMTHE4%-IMAGE OFTHECATALYSTAFTERTHEDURABILITYTEST&IGURE ITISEVIDENT THATTHEAVERAGESIZEANDDISPERSIONOF!UPARTICLESHASNOT CHANGED4HEREASONFORTHEENHANCEDSTABILITYOFNANOGOLD ON(!PREMAINSTOBEINVESTIGATED
#ONCLUSIONS
100 nm
&IGURE "RIGHT½ELD4%-IMAGESHOWINGTHATTHESMALLSIZEANDHIGHDISPERSION OF!UNANOCRYSTALSISMAINTAINEDAFTERTHEDURABILITYTESTWASCONDUCTED AT#FORH
'OLD"ULLETIN
#ATALYSTS COMPOSED OF HIGHLY DISPERSED GOLD NANOCRYSTALS ON A MACROPOROUS (!P FOAM WERE PREPARED USING A $0 METHODFROMA.A/(BUFFEREDSOLUTIONOFGOLDCHLORIDE4HE PHASE PURE (!P WAS SYNTHESIZED AS CALCINED POWDER AND CERAMICFOAMWITHCRYSTALSIZESOFANDNMRESPECTIVELY !LTHOUGH THE SURFACE AREAS WERE EQUIVALENT THE FOAM FAVOUREDTHEDEPOSITIONOFANHEDRALGOLDCRYSTALSOFSMALLER SIZE AND HIGHER DISPERSION TO CREATE EFFECTIVE CATALYSTS 4HE P( OF THE (!U#L SOLUTION CONTROLS THE SPECIATION OF !U#LX/( XANIONSWHICHATTENUATESTHERATESOFDEPOSITION SUCH THAT SMALLER CRYSTALS NM AND HOMOGENEOUS DISTRIBUTIONS WERE OBTAINED FROM P( NEAR THE )%0 OF (!P,ONGERREACTIONTIMESRESULTEDINLARGERCRYSTALSIZESAND COLLOIDALGOLDCLUSTERSSEPARATEDFROMTHE(!P&ORSAMPLES PREPARED AT P( WITH AN AGING TIME OF MIN CONVERSION OF #/ TO #/ AT THE LOW REACTION TEMPERATURE # WAS ACHIEVED /VER A TEST PERIOD OF H AND AREACTIONTEMPERATUREOF#THECATALYSTSHOWEDSTRUCTURAL
AND CRYSTALLOGRAPHIC STABILITY WITHOUT MEASURABLE LOSS OFACTIVITY
2EFERENCES #7#ORTI 2*(OLLIDAY $44HOMPSON !PPL#ATAL! 4 6#HOUDHARY #3IVADINARAYANA ###HUSUEI !+$ATYE *0&ACKLER *R
!CKNOWLEDGEMENTS
$7'OODMAN *#ATAL ).2EMEDIAKIS .,OPEZ *+.RSKOV !PPL#ATAL!
4HISRESEARCHWORKWASSUPPORTEDUNDERGRANTNUMBER FROMTHE!GENCYFOR3CIENCE 4ECHNOLOGYAND2ESEARCH ! 34!2 3INGAPORE THROUGH THE 'ERMAN 3INGAPORE $&' *OINT2ESEARCH0ROGRAMME±/PTIMISATIONOF1UANTUM%FFECTS IN!PATITE BASED.ANOCOMPOSITE-ETALLICAND3EMICONDUCTOR #ATALYSTSFOR%NVIRONMENTAL0ROTECTION²
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