This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys. Cite this: DOI: 10.1039/c3cp50361e Selective catalytic oxidation using supported gold–platinum and palladium–platinum nanoalloys prepared by sol-immobilisation Virginie Peneau, a Qian He, b Gregory Shaw, a Simon A. Kondrat, a Thomas E. Davies, a Peter Miedziak, a Michael Forde, a Nikolaos Dimitratos, c Christopher J. Kiely b and Graham J. Hutchings* a Supported nano-alloys have been prepared using the sol-immobilisation method for two bimetallic combinations, namely gold–platinum and palladium–platinum, using activated carbon and titania as supports. Some of the materials were prepared using a method where both metals are simultaneously reduced, thereby leading to homogeneous alloys being formed. In addition, sequential reduction of the metal combinations has also been investigated to facilitate the formation of core–shell structures. The materials have been characterized using X-ray photoelectron spectroscopy and aberration-corrected scanning transmission electron microscopy. The supported nanoparticles have been tested for a two selective oxidation reactions, namely the oxidation of toluene and benzyl alcohol using tertiary butyl hydroperoxide at 80 1C, in order to elucidate any potential structure–activity relationships. Introduction The use of noble metal nanoparticles containing Au, Pd or Pt in the field of catalysis is now well demonstrated for a number of demanding chemical transformations such as the direct synthesis of hydrogen peroxide, 1–4 selective alkane activation, 5,6 alkene epoxidation, 7–9 and selective alcohol oxidation 10–15 amongst others. There is significant interest in the develop- ment of synthetic techniques that can be used to reproducibly prepare supported metal nanoparticles of small size with well- defined structure and composition. In catalysis applications where noble metals are to be employed, a low metal loading can mitigate the cost involved with the use of precious metals provided that the deposited particles have a high catalytic activity for the target reaction. 16 Achieving a high dispersion of 2–5 nm highly faceted metal nanoparticles increases the number of low coordination metal sites which can lead to increased catalytic activity, provided of course that such low coordination metal sites are the active sites employed in the catalytic cycle. Where the interface periphery regions between the supported nanoparticles and support material are impor- tant in the catalytic cycle, then a higher metal dispersion also has a positive effect on the observed activity by increasing the number of such sites provided that the support surface and metal nanoparticle have strong interactions. However, typically in the process of supporting metal nanoparticles there may be a modification of the nanoparticle morphology and agglomera- tion is often difficult to avoid when using ‘‘wet’’ chemical techniques. Furthermore, the modification (promotion) of the activity of one catalytic component of the system by the inclu- sion of a second metal or other inorganic additive, a central feature of noble metal catalysis, 17–19 can be difficult to control reliably. Thus the preparation of bimetallic nanoparticles is of major importance, but their use can greatly increase the com- plexity of the catalyst, especially if there is a high level of inhomogeneity in the material. We have previously shown that Au–Pd nanoparticles pre- pared by colloidal methods can be immobilized onto supports with minimal change in the particle size, although the nature of the support is thought to direct the shape of the supported particles. 5 For example, we have shown that when homo- geneous icosahedral Au–Pd colloid nanoparticles having a mean size of 2.9 nm are supported on TiO 2 , there is a slight increase in the mean particle size to 3.9 nm; but, more importantly, the particles have a tendency to ‘wet’ the TiO 2 a Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK. E-mail: hutch@cardiff.ac.uk; Fax: +44 (0)29 2087 4030; Tel: +44 (0)29 2087 4805 b Department of Material Science and Engineering, Lehigh University, 5 Easter Packer Avenue, Bethlehem PA 18015-3195, USA c Department of Chemistry, University College London, 20 Gordon Street, WC1H 0AJ London, UK Received 25th January 2013, Accepted 28th March 2013 DOI: 10.1039/c3cp50361e www.rsc.org/pccp PCCP PAPER Downloaded by Cardiff University Libraries on 15/04/2013 14:27:37. Published on 02 April 2013 on http://pubs.rsc.org | doi:10.1039/C3CP50361E View Article Online View Journal
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This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys.
Cite this: DOI: 10.1039/c3cp50361e
Selective catalytic oxidation using supportedgold–platinum and palladium–platinum nanoalloysprepared by sol-immobilisation
Virginie Peneau,a Qian He,b Gregory Shaw,a Simon A. Kondrat,a Thomas E. Davies,a
Peter Miedziak,a Michael Forde,a Nikolaos Dimitratos,c Christopher J. Kielyb andGraham J. Hutchings*a
Supported nano-alloys have been prepared using the sol-immobilisation method for two bimetallic
combinations, namely gold–platinum and palladium–platinum, using activated carbon and titania as
supports. Some of the materials were prepared using a method where both metals are simultaneously
reduced, thereby leading to homogeneous alloys being formed. In addition, sequential reduction of the
metal combinations has also been investigated to facilitate the formation of core–shell structures. The
materials have been characterized using X-ray photoelectron spectroscopy and aberration-corrected
scanning transmission electron microscopy. The supported nanoparticles have been tested for a two
selective oxidation reactions, namely the oxidation of toluene and benzyl alcohol using tertiary butyl
hydroperoxide at 80 1C, in order to elucidate any potential structure–activity relationships.
Introduction
The use of noble metal nanoparticles containing Au, Pd or Pt inthe field of catalysis is now well demonstrated for a numberof demanding chemical transformations such as the directsynthesis of hydrogen peroxide,1–4 selective alkane activation,5,6
alkene epoxidation,7–9 and selective alcohol oxidation10–15
amongst others. There is significant interest in the develop-ment of synthetic techniques that can be used to reproduciblyprepare supported metal nanoparticles of small size with well-defined structure and composition. In catalysis applicationswhere noble metals are to be employed, a low metal loading canmitigate the cost involved with the use of precious metalsprovided that the deposited particles have a high catalyticactivity for the target reaction.16 Achieving a high dispersionof 2–5 nm highly faceted metal nanoparticles increases thenumber of low coordination metal sites which can lead toincreased catalytic activity, provided of course that such lowcoordination metal sites are the active sites employed in the
catalytic cycle. Where the interface periphery regions betweenthe supported nanoparticles and support material are impor-tant in the catalytic cycle, then a higher metal dispersion alsohas a positive effect on the observed activity by increasing thenumber of such sites provided that the support surface andmetal nanoparticle have strong interactions. However, typicallyin the process of supporting metal nanoparticles there may be amodification of the nanoparticle morphology and agglomera-tion is often difficult to avoid when using ‘‘wet’’ chemicaltechniques. Furthermore, the modification (promotion) of theactivity of one catalytic component of the system by the inclu-sion of a second metal or other inorganic additive, a centralfeature of noble metal catalysis,17–19 can be difficult to controlreliably. Thus the preparation of bimetallic nanoparticles is ofmajor importance, but their use can greatly increase the com-plexity of the catalyst, especially if there is a high level ofinhomogeneity in the material.
We have previously shown that Au–Pd nanoparticles pre-pared by colloidal methods can be immobilized onto supportswith minimal change in the particle size, although the nature ofthe support is thought to direct the shape of the supportedparticles.5 For example, we have shown that when homo-geneous icosahedral Au–Pd colloid nanoparticles having amean size of 2.9 nm are supported on TiO2, there is a slightincrease in the mean particle size to 3.9 nm; but, moreimportantly, the particles have a tendency to ‘wet’ the TiO2
a Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building,
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support and develop a cub-octahedral morphology. Whenactivated carbon is used as the support, the mean Au–Pdparticle size is also modestly increased to 3.7 nm, but nowthe particles retain the morphology of the starting colloid as thedegree of substrate wetting in this case is minimal. Interest-ingly, the carbon supported Au–Pd nanoparticles were found tobe more active for the solvent-free oxidation of toluene usingmolecular oxygen under conditions which are much milderthan those used previously, although this achievement requiredthe use of elevated temperatures and high pressure O2.6 Wehave also reported the straightforward preparation of core–shellas well as homogenous random alloy Au–Pd structures using asol-immobilisation method.14,20 Homogeneous alloys are formedwhen the two metals are reduced simultaneously during thepreparation, whereas core–shell structures are formed when thetwo metals are reduced consecutively. These nanoparticles whensupported on TiO2 or carbon catalyse the oxidation of benzylalcohol using molecular oxygen as the oxidant.6,14
We have now used the sol-immobilisation method to pre-pare Au–Pt and Pd–Pt based catalysts and tested these materialsfor the oxidation of benzyl alcohol and toluene using tert-butylhydroperoxide (TBHP). These two alloy compositions and theirvarious structural variants show differences in both catalyticactivity and selectivity which we can relate to the structure ofthe catalyst.
ExperimentalCatalyst preparation using sol-immobilisation
For the preparation of sols, aqueous solutions of HAuCl4
(Johnson Matthey), PdCl2 (Johnson Matthey) and PtCl2 (JohnsonMatthey) were prepared at the desired concentrations. Polyvinylalcohol (PVA, 1 wt% aqueous solution, Aldrich, MW = 10 kDa)was freshly prepared and used as the stabilizer. NaBH4 (SigmaAldrich, 0.1 M aqueous solution) was also freshly prepared andused as the reducing agent. Different morphological variants ofthe colloid were prepared by either simultaneous or sequentialaddition of metal precursor and reducing agent. H2SO4 (FischerScientific) was used for acidification of the sol prior to additionof the support materials. Carbon (Darco GC60, Sigma Aldrich)and TiO2 (P25, Degussa) were used in the ‘‘as received’’ state asthe support materials.
Homogeneous Au–Pt alloy structures. To an aqueous mixtureof HAuCl4 and PtCl2 of the desired concentration (1 : 1 metalweight ratio, 1 wt% total metal in final catalyst) the PVA solution(1 wt%) was added (PVA/(Au + Pt) (wt/wt) = 1.2) with vigorousstirring for 2 min. NaBH4 was then added rapidly such that theNaBH4 : total metal ratio (mol/mol) was 5. The solution wasstirred for 30 min and then H2SO4 was added, followed by theintroduction of the desired support materials. After 2 h ofstirring the mixture was filtered, washed with distilled waterand dried at 120 1C for 16 h.
Aushell–Ptcore structures. To an aqueous solution of PtCl2 ofthe desired concentration (1 : 1 metal weight ratio, 1 wt% totalmetal in final catalyst) the PVA solution (1 wt%) was added(PVA/(Au + Pt) (wt/wt) = 1.2) with vigorous stirring for 2 min.
NaBH4 was then added rapidly such that the NaBH4 : Pt ratio(mol/mol) was 5. The solution was stirred for 30 min and thenan aqueous solution of HAuCl4 was added, followed by NaBH4
such that the NaBH4 : Au ratio (mol/mol) was 5. The solutionwas stirred for an additional 30 min before H2SO4 was added.Finally the desired support materials were introduced. After 2 hof stirring the mixture was filtered, washed with distilled waterand dried at 120 1C for 16 h.
Ptshell–Aucore structures. To an aqueous solution of HAuCl4
of the desired concentration (1 : 1 metal weight ratio, 1 wt%total metal in final catalyst) the PVA solution (1 wt%) was added(PVA/(Au + Pt) (wt/wt) = 1.2) with vigorous stirring for 2 min.NaBH4 was then added rapidly such that the NaBH4 : Au ratio(mol/mol) was 5. The solution was stirred for 30 min beforeaddition of an aqueous solution of PtCl2 followed by NaBH4
such that the NaBH4 : Pt ratio (mol/mol) was 5. The solution wasstirred for 30 minutes and then H2SO4 was added, followed bythe desired support materials. After 2 h of stirring the mixturewas filtered, washed with distilled water and dried at 120 1Cfor 16 h.
Pd–Pt nanoalloys with random alloy, Pdshell–Ptcore andPdcore–Ptshell morphologies were prepared in a similar mannerto the three procedures described above using an aqueoussolution of PdCl2 in place of the HAuCl4.
Selective oxidation catalysis
All reactions were performed in a stirred glass round bottomflask (50 ml) fitted with a reflux condenser and heated in an oilbath. Typically, the substrate, either toluene (47 mmol, SigmaAldrich, 99.9%) or benzyl alcohol (47 mmol, Sigma Aldrich,99.8%), and the required amount of catalyst were suspended inthe solution. TBHP (47 mmol, 70% in water, Sigma Aldrich) wasadded when the reaction mixture reached the reaction tem-perature of 80 1C. The reaction was carried out in air at atmo-spheric pressure. The identification and analysis of theproducts was carried out using GC-MS and GC (a Varian star3400 cx with a 30 m CP-Wax 52 CB column). The products wereidentified by comparison with known standards. For the quan-tification of the amounts of reactants consumed and productsgenerated, the external calibration method using a standard(1,3,5-trimethyl benzene, Sigma Aldrich) were used. 1,3,5-Trimethyl benzene was added after the reaction and beforethe samples were analysed. All reaction mixtures were dissolvedin methanol (1 : 1 weight ratio) before analysis due to the solidform of the product obtained after longer reaction times. TheTBHP content at the end of reaction was quantified by iodo-metric titration and in all cases less than 5% of the initialamount of TBHP added to the reaction mixture was detected.
Catalyst characterization
Samples for examination by scanning transmission electronmicroscopy (STEM) were prepared by dispersing the dry catalystpowder onto a holey carbon film supported by a 300 meshcopper TEM grid. STEM high angle annular dark field (HAADF)images of the metallic particles were obtained using an aberra-tion corrected JEOL 2200FS (S)TEM operating at 200 kV.
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X-ray energy dispersive (XEDS) spectra were acquired fromindividual nanoparticles larger than 2 nm in size while rasteringthe beam over the entire particle, using a Thermo Noran Si(Li)XEDS detector.
X-Ray Photoelectron Spectroscopy (XPS) measurements weremade on a Kratos Axis Ultra DLD spectrometer. Samples weremounted using double-sided adhesive tape, and bindingenergies were referenced to the C(1s) binding energy of adven-titious carbon contamination that was taken to be 284.7 eV.Monochromatic AlKa radiation was used for all analyses. Theintensities of the Au(4f), Pt(4f) and Pd(3d) features were used toderive the Pd/Pt, and Au/Pt surface atom ratios.
Results and discussionOxidation of benzyl alcohol
The oxidation of benzyl alcohol was performed with TBHP asthe oxidant at 80 1C in air. We compared the activity of Au–Ptnanoparticles supported on carbon and TiO2 (see Fig. 1(a)and (b)). Amongst the Au–Pt catalyst variants, the supportednanoalloys prepared using simultaneous reduction gave the bestcatalytic performance. However, the Ptshell–Aucore/C catalyst,prepared by the sequential addition of Pt to Au ‘seeds’, displaysan enhanced catalytic activity when compared with the TiO2
supported analogue; whereas, for the Aushell–Ptcore variant(prepared by the sequential addition of Au to the Pt ‘seeds’)and the Au–Pt variant (prepared by simultaneous metalreduction) the catalytic activity is higher for TiO2-supportednanoparticles. The product distribution for supported Au–Pt
catalysts is very different from that obtained with the supportedAu–Pd nanoalloys we have reported previously.21 When Au–Pdcatalysts are used, a significant amount of toluene is formed,whereas when the Pd component of the alloy is replacedwith Pt, toluene is not observed and the main products arebenzaldehyde and reaction products that derive from thealdehyde (Scheme 1). In addition, the catalytic performanceof the Au–Pt and Aushell–Ptcore materials are markedly differentwhen supported on carbon- and TiO2, as benzoic acid ispredominantly formed by the TiO2 supported Au–Pt nano-particles. This is not the case for the Ptshell–Aucore nanoparticlessupported on both TiO2 and carbon, where benzaldehyde is themajor oxidation product in both cases. This clearly shows theimportance of the alloy composition and morphology, as wellas the support identity.
We have previously shown that Au–Pd/TiO2 materials pro-duce surface bound superoxide radicals in the oxidation oftoluene with TBHP and proposed that the TiO2 support was akey component of the active catalyst due to its stabilizing effecton these radical species.6 The morphology of the depositedAu–Pd alloy nanoparticles (which changes when supportedon TiO2)20 and their interface with TiO2 could also be crucialfactors in allowing the spill-over of radical species generated onthe metal surface onto defect sites in the support material. Forthe supported Au–Pd nanoparticles, benzoic acid was the majorproduct when TBHP was used as oxidant.6 Hence, it is inter-esting to note that the Au–Pt and Aushell–Ptcore particles sup-ported on TiO2 generally show a higher activity in the oxidationreaction (as compared to the carbon supported materials) andalso give higher yields of the sequential oxidation product(i.e. benzoic acid) in a similar manner to Au–Pd/TiO2. Inaddition, the Ptshell–Aucore materials exhibit similar behaviourwith both supports, indicating that the identity of the support
Fig. 1 Oxidation of benzyl alcohol using the various Au–Pt catalyst variantstructures supported on (a) activated carbon and (b) TiO2 with TBHP as oxidant.Reaction conditions: reaction time: 24 h; TBHP : substrate = 1 : 1; substrate : metal =6500; reaction temperature: 80 1C.
Scheme 1 Reaction network for the oxidation of benzyl alcohol.
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in benzyl alcohol oxidation is less pronounced for this binarymetal combination. Since all these materials exhibit very highTBHP decomposition (o5% left at the end of the reaction,although it is important to note that in no case is total TBHPdecomposition observed) and we previously observed thatcarbon-supported materials are usually the more active foralcohol and hydrocarbon oxidation,5,6,10,14 it is possible thatit is the lifetime and nature of the radical species formed(tBuO�, tBu�, tBuOO�, �OH, �OOH), rather than the ability todecompose TBHP using metallic nanoparticles, that is impor-tant for determining the oxidative ability of the catalysts.
Furthermore, we expected that the Pd–Pt catalysts wouldshow higher activity for the oxidation of benzyl alcohol ascompared to the Au–Pt system since metallic Pd nanoparticlesare more effective than Au nanoparticles for this transforma-tion using molecular oxygen (see Table 1). The data presentedin Fig. 1 and 2 show that carbon-supported Pd–Pt materials
have similar catalytic activity and selectivity to their Au–Ptcounterparts, except for the Pdshell–Ptcore variant which showssignificantly a higher conversion accompanied by high selec-tivity to benzoic acid. Since the Pd–Pt catalysts were preparedwith a 1 : 1 weight ratio of metal it follows that the molarquantity of Pd is higher than Pt by a factor of 1.8, and whenpreparing supported Pdshell–Ptcore particles versus Ptshell–Pdcore
particles, this should result in differences in the shell thicknessand possibly the surface composition. One could postulate thatthe Pdshell–Ptcore catalysts should behave more like the Pd-onlynanoparticles, which in the reduced state (as in Pd catalystsprepared by sol-immobilisation) are highly active for the for-mation of benzaldehyde from benzyl alcohol, since the highermolar amount of Pd may translate into a more completecoverage of the Pt core in the nanoparticles. With thesecatalysts we observe that only the Pdshell–Ptcore/C materialshows the expected behavior which is possibly linked to theretention of the nanoparticle morphology prepared by sol-immobilisation on activated carbon and the presence of anouter shell consisting Pd0. This proposition will be examinedfurther using XPS studies in a subsequent section.
Another important effect observed when using Pt containingalloys was that the formation of toluene was completely avoidedin the presence of Pt. We have shown previously that Au–Pdcatalysts prepared by different techniques and supported onvarious materials tend to produce significant amounts oftoluene from benzyl alcohol via hydrogenolysis or dispropor-tionation pathways (the latter being particularly promotedby Pd).10,13,14 To the best of our knowledge, this is the firstreport of switching-off toluene production in this reaction,which is important for applications where benzaldehyde orbenzoic acid are required with high selectivity. It should also benoted that the choice of oxidant (i.e. TBHP) and not necessarilythe presence of Pt increases the production of benzoic acid withall catalysts tested, as the three monometallic standard cata-lysts all show high activity towards the production of benzalde-hyde from benzyl alcohol when molecular oxygen is used as theoxidant (Table 1).
Oxidation of toluene
We have also compared the activity of the various Au–Pt andPd–Pt materials supported on carbon and TiO2 for tolueneoxidation (Table 2 and Fig. 3). In all cases the carbon-supportedmaterials were more active than the TiO2 supported catalysts,which is in keeping with our previous reports on the subject.6
We propose that the promotional effect of carbon, which itselfshows low activity when used with molecular oxygen at 160 1C,and is only mildly active (ca. 7% conversion) when using TBHPas the oxidant at 80 1C,5,6 is independent of the metal alloycomposition since both Au–Pt and Pd–Pt have similar conversionand selectivity profiles with benzoic acid being generated as themajor product. For the carbon-supported materials, which displayhigher catalytic activity, we observed benzaldehyde-dimethylacetaland methyl benzoate as reaction products (Scheme 2), which werenot present in the products for the TiO2-supported catalysts. Thismay suggest differences in the mechanism, or availability of
Table 1 Oxidation of benzyl alcohol using monometallic Au, Pd and Pt catalystssupported on activated carbon with molecular oxygen as the oxidant
Test conditions: catalyst: 0.05 g; reaction temperature: 120 1C; p(O2): 150 psi;stirring rate: 1500 rpm; reaction time: 2 h.
Fig. 2 Oxidation of benzyl alcohol using the various Pd–Pt catalyst variantstructures supported on (a) activated carbon and (b) TiO2 with TBHP as oxidant.Test conditions: reaction time: 24 h; TBHP : substrate = 1 : 1; metal : substrate =6500; reaction temperature: 80 1C.
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radical species affecting the oxidation, since in the reactionswith benzyl alcohol (which would be the primary product oftoluene oxidation) the acetal species was observed in signifi-cant amounts for all catalysts tested (Fig. 3). Benzaldehydedimethylacetal can be produced by the interaction of methyland methoxy radicals (�CH3 and �OCH3) derived from TBHP,22
with benzaldehyde or benzoic acid. It is also possible that the–CH3 moiety is cleaved from toluene in the oxidation processresulting in �CH3 and benzyl radicals. However, considering theabsence of biphenyl (coupled product of two benzyl radicals)and the well-known propensity for TBHP to produce �CH3 and�OCH3 methoxy radical species,20 we do not consider that thispathway is operating to produce the acetal species we observe.The production of methyl benzoate can also be explained by theaction of the radical products derived from TBHP decomposi-tion or an esterification reaction involving methanol (derivedfrom TBHP) and benzoic acid. It is interesting to note that thesenew products are only observed with the carbon supportedmaterials, even though both carbon and TiO2 supported cata-lysts display similar levels of TBHP decomposition for thesereactions.
Furthermore, from the data presented on benzyl alcoholoxidation one might be tempted predict that only those materialswhich had high activity towards benzoic acid production frombenzyl alcohol would show a high activity in the tolueneoxidation reaction. However, we observed that benzoic acid isthe major product in all cases, even though there are majordifferences in the product distributions for benzyl alcoholoxidation as discussed previously. This is also different fromour previous findings using Au–Pd/TiO2 catalysts where theproduct distribution for toluene oxidation with TBHP wasrather diverse at short reaction times, but tended towards benzoicacid production for longer reaction times,5 showing the gradual
sequential oxidation of benzaldehyde through to benzoic acid.For our TiO2 supported materials, and more so for the Au–Ptthan the Pd–Pt alloy, there is a clear tendency to producebenzoic acid (Table 2, entries 1 and 2). In the case of carbon
Table 2 Comparison of the activity of Au–Pt and Pd–Pt random alloy catalystssupported on TiO2 and C for toluene oxidation at 80 1C using TBHP
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supported materials, all catalysts produced benzoic acid asthe major product, even at short reaction times (Table 2,entries 3 and 4).
Finally, our new catalysts have been benchmarked to thosedescribed previously in the literature (Table 3). Benzoic acidwas the major product obtained for all the sol-immobilisation-type materials, as opposed to benzaldehyde for the Cr- andCo- based catalysts. We also note that the catalytic activities ofthe Au–Pd, Pt–Pd and Au–Pt materials are significantly lowerthan when these materials are used at high temperature withmolecular oxygen as the oxidant. The conversion levels achieved(under comparable conditions) are lower for our new TiO2
based catalysts as compared to results obtained using thesupported Au–Pd system. This is linked to facile decompositionof TBHP observed with the Pd–Pt and Au–Pt catalysts, and may
be specifically due to the Pt component in the bimetallicformulation. Additionally, all the carbon supported metal nano-particles display higher catalytic activity levels than their TiO2
analogues, which is in keeping with our previous studies.5,6,21
Characterisation
Scanning transmission electron microscopy. A systematicsub-set of random alloy Au–Pt/C and Pt–Pd/C bimetallic samplesprepared by the sol-immobilisation method were examined bySTEM analysis. Fig. 4 shows representative microstructural datafor the Au–Pt/C catalyst system. Here the metal particle sizeranged between 1 and 6 nm with a mean value of 2.3 nm. XEDSanalysis from numerous supported particles showed them all tobe Au–Pt random alloys. We have previously shown that no signi-ficant systematic composition variation develops with particle
Table 3 Comparison of the activity of our PdPt and AuPt catalysts to other catalysts used previously in the literature with TBHP as oxidant
Fig. 4 Electron microscopy analysis of the Au–Pt/C catalyst made by sol-immobilisation. (a) A low magnification STEM-HAADF image; (b, c) higher magnificationSTEM-HAADF images of representative Au + Pt particles; (d) particle size distribution derived from HAADF images; (e) a typical XEDS spectrum acquired from anindividual Au–Pt alloy particle.
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size in the Au–Pt system prepared by sol-immobilisation23
and this was confirmed to be the case here also. The corre-sponding microscopy data for the Pt–Pd/C catalyst system ispresented in Fig. 5. Here the metal particle size was found tovary between 1 and 12 nm with a mean value of 3 nm. Onceagain multiple XEDS analyses confirmed each supportedparticle to be a Pt–Pd alloy, and as has noted previously25 asystematic composition/size variation was detected with thelarger particles consistently being Pd-rich and the smaller onesbeing Pd-deficient. No evidence of elemental segregationwithin individual Pt–Pd particles was detected by STEM-HAADFimaging.
X-ray photoelectron spectroscopy. All the catalysts werecharacterised using XPS and the data concerning bindingenergies and surface atomic metal ratios are presented inTable 4. In all cases the measured binding energies of Au, Pdand Pt indicate that the metal is in the zero-valent state, i.e. allthe nanoparticles are metallic in nature, which might beexpected considering the vigorous chemical reduction stepsused in the preparation of the sol-immobilised catalysts. ForPd–Pt materials supported on carbon, the XPS data show thatthe Pdshell–Ptcore/C catalyst has a similar Pd/Pt surface atomicratio to the Pd–Pt random alloy, but the Ptshell–Pdcore particleshows a lower Pd/Pt ratio than the other two variants (i.e. 1.79and 1.74 versus 1.34 respectively). The surface Pd/Pt ratios ofthe Pd–Pt random alloy and Pdshell–Ptcore variants are very closeto the theoretical value for a homogeneous alloy with a 1 : 1weight metal ratio, but the surface enrichment in Pt for the
Ptshell–Pdcore/C possibly indicates that the desired morphologyhad been achieved. However, for TiO2 supported Pd–Ptmaterials, the Pd/Pt surface atom ratio varied widely indicatingvery different nanoparticle compositions linked to both thesequence of metal addition and support choice in the catalystpreparation. The Pd–Pt/TiO2 random alloy catalyst exhibits aPd/Pt surface ratio that is very close to the theoretical value expectedfor a homogeneous alloy nanoparticle. The Ptshell–Pdcore/TiO2 cata-lyst clearly showed Pt surface enrichment as the Pd/Pt ratio waslowered to 1.50 (compared to the homogeneous alloy variant)
Fig. 5 Electron microscopy analysis of the Pt–Pd/C catalyst made by sol-immobilisation. (a) A low magnification STEM-HAADF image; (b, c) higher magnificationSTEM-HAADF images of representative Pt + Pd particles; (d) particle size distribution derived from HAADF images; (e) a typical XEDS spectrum acquired from anindividual Pt–Pd alloy particle.
Table 4 XPS surface compositional data from the various Au–Pt and Pd–Ptcatalysts supported on TiO2 and C
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but the Pdshell–Ptcore/TiO2 variant had an increased surface Pdcontent (Pd/Pt = 2.42). The latter result suggests either that thedesired alloy structure has been achieved or that there may bePd-only particles present on the TiO2 surface in addition toalloyed particles. We note that for the carbon-supported mate-rials, the changes in Pd/Pt surface atomic ratio do not as clearlyindicate the formation of core–shell structures as compared tocorresponding data for the TiO2-supported materials.
In the case of Au–Pt materials, the nominal surface Au/Ptatomic ratio should be close to 1 for a 0.5Au–0.5Pt wt%material. XPS shows that for Au–Pt supported on carbon orTiO2, the Au/Pt surface atomic ratio is 0.96 indicating a highprobability that a homogenous Au–Pt alloy had been formed.The Aushell–Ptcore variant supported on both TiO2 and carbonshow a lower Au/Pt surface ratio as compared to the homo-genous alloy configuration, suggesting a change in morphologywhich we interpret is the result of the formation of a core–shellconfiguration. Notably, the Au/Pt ratio is higher for the TiO2
versus C supported material (0.84 versus 0.75) which we con-sider is linked to the ability of the nanoparticles to wet the TiO2
surface upon deposition thereby changing the particle shape.For the reverse Ptshell–Aucore configuration, the Au/Pt surfaceratio was higher than the homogeneous alloy material indi-cating that a reverse core–shell structure was successfullyfabricated. The XPS data supports this hypothesis for thePtshell–Aucore configuration, but we point out that when carbonis used as a support, the particles have a higher Au/Pt ratio thanthe TiO2-supported counterparts which may also possibly bedue to some Au diffusion into the Pt-shell in the nanoparticles.There is also the possibility that the actual particle size in thePtshell–Aucore configuration sample has been modified uponimmobilisation and this affects the surface atomic metal ratiosthat we measure.
It should also be noted that our XPS analyses on all of thesematerials detected small amounts of residual Cl� originatingfrom the chloride salts used in the sol-immobilisation prepara-tion. In addition, S was also detected which arises from thesulphuric acid added to the sol in order to increase theinteraction of the particles with the support surface.
Conclusions
We have used the sol-immobilisation method to prepare a setof Au–Pt and Pd–Pt nanoparticles which that have been sup-ported on carbon and TiO2. When the two metals are addedsimultaneously homogeneous alloy nanoparticles are formedas confirmed by XPS and STEM characterization. Sequentialaddition of the metals can lead to core–shell structures asevidenced by XPS analysis. In terms of catalytic application,it is clear that the order of metal addition and the nature of thesupport play important roles, although for toluene oxidationboth these sets of materials are less active that the corre-sponding Au–Pd materials which we have previously reported.6
However, for the oxidation of benzyl alcohol we discovered anunexpected effect in which the presence of Pt switches-off theformation of toluene in this reaction. This finding is important
in facilitating more efficient production of the desired pro-ducts, which is essential in the potential application of thesecatalytic systems.
Acknowledgements
We thank the EPSRC and Cardiff University for financialsupport.
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