Synthesis of Pt−Pd Core−Shell Nanostructures by Atomic LayerDeposition: Application in Propane Oxidative Dehydrogenation toPropyleneYu Lei,† Bin Liu,‡ Junling Lu,† Rodrigo J. Lobo-Lapidus,§ Tianpin Wu,§ Hao Feng,† Xiaoxing Xia,∥

Anil U. Mane,† Joseph A. Libera,† Jeffrey P. Greeley,‡ Jeffrey T. Miller,§ and Jeffrey W. Elam*,†

†Energy Systems Division, Argonne National Laboratory, Lemont, Illinois 60439, United States‡Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States§Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States∥Department of Physics, The University of Chicago, Chicago, Illinois 60637, United States

ABSTRACT: Atomic layer deposition (ALD) was employedto synthesize supported Pt−Pd bimetallic particles in the 1 to2 nm range. The metal loading and composition of thesupported Pt−Pd nanoparticles were controlled by varying thedeposition temperature and by applying ALD metal oxidecoatings to modify the support surface chemistry. High-resolution scanning transmission electron microscopy imagesshowed monodispersed Pt−Pd nanoparticles on ALD Al2O3-and TiO2-modified SiO2 gel. X-ray absorption spectroscopyrevealed that the bimetallic nanoparticles have a stable Pt-core,Pd-shell nanostructure. Density functional theory calculationsrevealed that the most stable surface configuration for the Pt−Pd alloys in an H2 environment has a Pt-core, Pd-shellnanostructure. In comparison to their monometallic counterparts, the small Pt−Pd bimetallic core−shell nanoparticles exhibitedhigher activity in propane oxidative dehydrogenation as compared to their physical mixture.

KEYWORDS: atomic layer deposition, platinum, palladium, bimetallic nanoparticles, catalyst

■ INTRODUCTIONBimetallic catalysts offer the possibility to combine the uniqueadvantages of each component, allowing the catalyst activity,selectivity, and stability to be tuned by precisely controlling thebimetallic composition and structure. Moreover, owing to thechanges in electronic and geometric structure, supportedbimetallic catalysts often exhibit enhanced catalytic propertiescompared to simple mixtures of their monometallic counter-parts.1−4 Supported Pt−Pd nanoparticles are among the mostwidely studied and implemented bimetallic heterogeneouscatalysts in important technological areas,5 including aromaticshydrogenation,6,7 petroleum hydrocracking,8 emission con-trol,9,10 hydrogen storage,11,12 and electrocatalysis in fuelcells.13,14 Pt−Pd bimetallic nanocatalysts not only showenhanced selectivity and activity, but also better tolerance topoisons such as sulfur.6,15

The high activity of under-coordinated surface atoms hasmotivated efforts to synthesize supported precious metalnanoparticles in the size range of a few nanometers. Moreover,the high price of precious metals dictates that the catalystshould be finely dispersed to have a very high ratio of surfaceatoms to bulk atoms. More fundamentally, catalysts with well-defined size, composition, and structure are necessary to buildprecise structure−reactivity relationships and provide a more

complete understanding of bimetallic nanocatalysts. Unfortu-nately, the synthesis of uniform bimetallic nanoparticles withdiameters below 2 nm has proved challenging for traditionalcatalyst synthesis methods such as wet impregnation,16,17 andcolloidal chemistry.18−22

Atomic layer deposition (ALD) is a promising technique forproducing uniform precious metal nanoparticles on high surfacearea supports because of its unique feature of sequential, self-limiting surface reactions.23,24 ALD allows the nanoparticle sizeand composition to be controlled precisely by adjusting thenumber and sequence of ALD cycles of each component. Inaddition, the deposition of precious metal nanoparticles isaffected by the deposition temperature and the surfacechemistry of the underlying support. The chemical propertiesof the support materials can be modified by coating a few ALDcycles of an oxide without significantly changing the porosity ofthe template material. By combining ALD processes for metaloxides and noble metals, it is possible to engineer nanocatalystswith unique structure and properties by depositing a series ofdiscrete layers, which each performs a specific function such as

Received: January 9, 2012Revised: April 30, 2012Published: August 20, 2012

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© 2012 American Chemical Society 3525 dx.doi.org/10.1021/cm300080w | Chem. Mater. 2012, 24, 3525−3533

serving as the catalyst support, providing or promoting catalyticactivity, and imparting thermal stability.25,26

ALD has been successfully developed to synthesizemonometallic nanocatalysts, such as Pt,27−31 Pd,32−37 andIr,38 as well as bimetallic Pt-Ru,39,40 supported by TiO2, Al2O3,SrTiO3, ZnO, SiO2, carbon, etc., for various catalyticapplications. However, in comparison to the typical layer-by-layer behavior of metal oxide ALD, noble metal ALD can becomplicated by the different reactivity and nucleation behaviorfor the noble metal growth on metal oxide support surfaces andthe high mobility of metal atoms and clusters.25,41 In this work,we present a novel method using ALD to synthesize supportedPt−Pd nanocatalysts in the size range of 1−2 nm with a narrowsize distribution and core−shell structure.Propylene is one of the most important chemical

intermediates in the petrochemical industry. Propane oxidativedehydrogenation (ODH) to propylene has been extensivelystudied, and platinum and palladium monometallic catalystshave both been investigated as propane ODH catalysts.42−44 Inthis study, we used propane ODH as a probe reaction toevaluate our bimetallic catalysts and we found that thebimetallic Pt−Pd nanoparticles are more efficient in propaneODH than an equivalent physical mixture of the monometallicPt and Pd.

■ EXPERIMENTAL SECTIONAtomic Layer Deposition. The ALD was performed in a viscous

flow reactor that has been described in detail elsewhere.45 Briefly, ALDsamples were prepared in a hot-walled vacuum chamber equipped withan in situ quartz-crystal microbalance (QCM) and quadrupole massspectrometer (QMS). Ultrahigh purity N2 carrier gas (Air-gas,99.999%) was further purified using an Aeronex Gatekeeper InertGas Filter to trap oxygen-containing impurities before entering thereactor.The high surface area support used in this work was Silicycle

S10040 M silica gel with ∼100 m2/g surface area, a particle size of 75−200 μm, and a pore diameter of 30 nm. Before each experiment, theSiO2 was baked in an oven at 200 °C overnight to desorb water andachieve a consistent density of surface hydroxyl groups.46 PrebakedSiO2 gel (0.5 g) was uniformly spread onto a stainless steel sampleplate with a mesh top to contain the powder while still allowing accessto the precursor vapors. The powder samples were loaded into thecenter of the reactor and kept for at least 30 min at 200 °C in a 350sccm flow of UHP N2 at 1 Torr pressure to allow temperaturestabilization and to further outgas the SiO2 gel. Next, the sample wascleaned by exposure to 30 sccm of flowing ozone at 1 Torr pressure at200 °C for 15 min. After cleaning, the SiO2 gel surface was modifiedusing either 5 ALD cycles of Al2O3 or 5 ALD cycles of TiO2. TheAl2O3 ALD used alternating exposures to trimethyl aluminum (TMA,Sigma-Aldrich, 97%) and deionized water at 200 °C. The TiO2 ALDused alternating exposures to TiCl4 (Sigma-Aldrich, 99.9%) anddeionized water at 150 °C. Five cycles of Al2O3 and TiO2 yield filmthicknesses of 6 and 3 Å, respectively. The surface area of the substratewas assumed the same before and after the ALD coating.47,48 Thethicknesses of the TiO2 and Al2O3 films were obtained in two ways.The first method was to measure the coating thickness on witnessSi(100) wafers coated simultaneously with the powder usingspectroscopic ellipsometry. The second method was to measure theweight gain of the SiO2 powder after the Al2O3 or TiO2 ALD. Fromthese weight changes and the density and known surface area of theSiO2, the ALD film thicknesses could be calculated. The thicknessesobtained using these two methods were typically within 10%.The Pt ALD used alternating exposures to trimethyl-

(methylcyclopentadienyl) platinum (Pt(MeCp)Me3, Sigma-Aldrich,98%) and O2 (Air-gas, 99.9%). The Pd ALD used alternatingexposures to palladium hexafluoroacetylacetonate (Pd(hfac)2, Sigma-Aldrich, 99.9%) and formalin (Sigma-Aldrich, HCHO, 37 wt.% in H2O

with methanol added for stability). The deposition temperature for themetal ALD was varied from 100 to 300 °C. For the mixed-metal ALD,the adsorbed Pt precursor was reacted with O2 at 250−300 °C, and theadsorbed Pd precursor was reacted with HCHO at 200 °C prior todepositing the second metal. The Pt and Pd metal loadings weredetermined by X-ray fluorescence spectroscopy (XRF, OxfordED2000) and inductively coupled plasma (ICP, Varian Vista-MPXinstrument). In this work, the catalysts prepared using ALD aredesignated as, (e.g.) Pt1Pd1/5c Al2O3/SiO2, to represent bimetallicPt−Pd nanoparticles deposited on 5-cycle ALD Al2O3-coated SiO2 gelwith a Pt/Pd = 1:1 molar ratio.

Scanning Transmission Electron Microscopy. Scanning Trans-mission Electron Microscopy (STEM) measurements were made onboth the as-prepared and reduced samples. A few milligrams of catalystwere sonicated in 10 mL of isopropanol for 10 min to obtain a well-dispersed slurry. A drop of this mixture was deposited onto a laceycarbon copper sample grid (SPi Supplies, 400 mesh) and thoroughlydried with an ultrainfrared lamp. STEM images were obtained using aJEOL JEM-2100F FEG FasTEM (EPIC at Northwestern University).The histograms of particle sizes were generated from the STEMimages using ImageJ software.49

X-ray Absorption Spectroscopy. X-ray absorption spectroscopy,including extended X-ray absorption fine structure spectroscopy(EXAFS) and X-ray absorption near edge structure spectroscopy(XANES), was conducted at the beamline of the Materials ResearchCollaborative Access Team (MRCAT) at Sector 10 of the AdvancedPhoton Source, Argonne National Laboratory. The XAS measure-ments were made in transmission mode with the ionization chamberoptimized for the maximum current with linear response. Spectra atboth the Pt L3 edge (11.564 keV) and the Pd K edge (24.35 keV) wereacquired for the bimetallic samples. Pt and Pd foils were used tocalibrate the monochromator. The amount of sample used wasoptimized to achieve an edge step of at least 0.2. The samples werefully reduced using 50 sccm 3.5% H2 in He as balance gas at 250 °Cfor one hour. Next, the reactor was purged using 150 sccm ultrahighpurity He for 10 min at 250 °C. The samples were cooled to roomtemperature in He and measured as the “reduced” sample to obtainprecise information on the metal-metal bond distances andcoordination numbers and to facilitate determination of the particlestructure and size.

Standard procedures based on WINXAS 3.1 software were used tofit the data in the EXAFS regime.50 The Pt−Pt and Pd−Pd scatteringphase shift and amplitude were obtained from reference Pd foil forPd−Pd (NPd−Pd= 12 at 2.75 Å) and Pt foil for Pt−Pt (NPt−Pt = 12 at2.77 Å). Commercial software for EXAFS data analysis (FEFF) wasused to build Pt−Pd and Pt−Pd scattering phase shift and amplitudes.A homogeneous Pt−Pd alloy model for FEFF fitting was built bycarefully substituting Pt with Pd in an fcc bulk structure. A two-shellmodel fit of the k2-weighted EXAFS data was obtained between k = 2.8− 12 Å−1 and r = 1.3 − 3.0 Å, respectively. The composition weightedaverage first shell coordination number (CN) for the 1:1 bimetallicnanoparticles was calculated using: CN = (CNPt−Pt + CNPt−Pd)/2 + (CNPd−Pd + CNPt−Pd)/2.

Density Functional Theory Calculations. Density functionaltheory (DFT) calculations were performed using the Vienna Ab-initioSimulation Package (VASP), a periodic plane wave-based code.51−54

The ionic cores were treated with the projector augmented wave(PAW) formalism.55,56 The PW91 generalized gradient functional(GGA-PW91) was used to describe the electron exchange-correlationinteractions.57,58

The Kohn−Sham valence states were expanded in a plane wavebasis set up to 25 Ry (or 340 eV). The surface Brillouin zone wassampled with 4 × 4 × 1 k points based on the Monkhorst-Packsampling scheme; we consider these results to be fully converged withrespect to k points.59 Benchmark tests on k points showed that thestatistical error was within 10 meV. The self-consistent iteration wasconverged to within a criterion of 1 × 10−7, and the ionic steps wereconverged to 0.02 eV/Å. The Methfessel−Paxton smearing schemewas used, 60 and with a Fermi population of the Kohn−Sham states ofkBT = 0.2 eV, with the total energies extrapolated to 0 eV.

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The DFT calculations were performed with periodic boundaryconditions. The Pt−Pd alloy surfaces were represented by 5-layer slabswith p(2 × 2) unit cells and close-packed (111) facets. The top threelayers were allowed to relax. A vacuum distance equivalent toapproximately nine metal layers was used between successive metalslabs. The lattice constant of the 50:50 Pt/Pd bulk alloy (L10ordering) was optimized to be 3.96 Å. To model the well-mixedcore of the Pt−Pd nanoparticles in our experiment, the distribution ofatoms in the bottom two layers was kept fixed in its bulk arrangement.The configurations in the top three layers were sampled viapermutations of the arrangements of the Pt and Pd atoms. Singlepoint energy calculations were first performed to screen out thethermodynamically unfavorable configurations. All structures with totalenergies per unit cell within ∼0.05 eV of the energy of the most stableconfigurations at each Pd concentration were then fully optimized todetermine their energies; the relative energies of the configurationswere not found to change significantly due to the optimization. Themost favorable Pd coverage in the top layers, for example, Pd/(Pd +Pt) = 0.0, 0.25, 0.5, 0.75, and 1.0 (c.f. Table 2), was identified from theoptimizations of these configurations.To model the hydrogen reduction conditions, the alloy surface was

covered with 1 monolayer (ML) of atomic H. Each H atom adsorbs ona 3-fold fcc site (4 fcc sites in total on a 2 × 2 unit cell) for eachpermuted configuration. The most stable configurations in thepresence of hydrogen were then determined. H diffusion into thealloy sublayer (2nd layer) region was also considered for the moststable configurations at each surface coverage of Pd by placing one Hfrom the surface ML into the octahedral site of the sublayer region.Catalytic Activity Testing. Propane oxidative dehydrogenation

(ODH) was carried out in a microflow fixed-bed reactor with insidediameter of ∼4 mm at atmospheric pressure. Ten milligrams of thebimetallic catalyst Pd1Pt1/5c TiO2/SiO2 was homogeneously dilutedin 90 mg silicon carbide with a particle size of 44 μm. For comparison,a catalyst mixture was prepared using ALD Pt/5c TiO2/SiO2 (∼2 wt %Pt) and ALD Pd/5c TiO2/SiO2 (∼1 wt % Pd) with similar particlesize. XRF was employed to ensure the same amounts of Pd and Pt inthe mixture as compared to the bimetallic samples. Twenty milligramsof this mixture was diluted in 80 mg silicon carbide for catalytic testing.The catalysts were calcined in 10% O2 and further in 10% H2 at 250°C for one hour, respectively. Typically, 2 sccm 10% propane and 1sccm 10% O2 were used as reactants. Online gas chromatographicanalysis was performed on a Hewlett-Packard 5890 GC equipped witha TCD and a FID detector. The conversion of the reaction was definedas the percentage of propane consumed to propane fed. The yield ofpropylene was obtained as Y = X × S, where X is the propaneconversion and S is the selectivity to propylene.

■ RESULTS AND DISCUSSION

ALD Al2O3- and TiO2-Coated SiO2. The purpose ofcoating the SiO2 surface with ALD Al2O3 and TiO2 is topromote the ALD Pt and Pd nucleation. Under identicalpreparation conditions, one ALD Pt or Pd cycle on the bareSilicycle S10040 M silica gel yielded only ∼0.1 wt% Pt or Pdloading. This loading was too low for most heterogeneouscatalytic studies and characterization. It has been shown thatthe nucleation of Pt and Pd ALD is relatively prompt onAl2O3

33,39 and TiO2.41,61 Consequently, modifying the SiO2

surface with a few layers of ALD TiO2 and Al2O3 can increasethe efficiency of the ALD Pt and Pd nucleation withoutdecreasing the surface area.47,48

Bimetallic Pt−Pd Nanoparticle Synthesis. We firstexamined the effects of adjusting the number of ALD cycles,the deposition temperature, and the support surface on themetal loading and composition of the supported Pt−Pdnanoparticles. Pt62,63 and Pd64−66 ALD are typically conductedat 300 and 200 °C, respectively. In this work, the Pt and PdALD were performed at deposition temperatures as low as 100

°C to investigate the low temperature limit for ALDnanoparticle synthesis and the effect on loading. Lowerdeposition temperatures reduce the mobility of surface speciesand the degree of thermal decomposition of the ALDprecursors, and should yield a more uniform coverage ofsmaller nanoparticles.The amount of Pt and Pd deposited during the initial 1−5

ALD cycles was examined using in situ QCM at 100 °C. Notethat at these low deposition temperatures, the precursor ligandsremain on the surface so that the QCM measures the weightgains of the adsorbed precursor molecules. Consequently, theseweight gain values should be interpreted as relative measure-ments of the metal loadings. Figure 1a shows that during the

first cycle of Pd ALD, the Pd weight gains were 70 and 50 ng/cm2 on the 3 nm TiO2 and 3 nm Al2O3 surfaces, respectively, inagreement with previous measurements.61 However, the Pdweight gain was greatly attenuated for the subsequent cycles onboth supports. This finding is consistent with previous studiesshowing that 100 °C is not sufficient for the HCHO to removethe hfac ligands from the Pd and the support, thereby blockingadditional Pd(hfac)2 adsorption in the subsequent cycles.64−66

The mass gains for Pt on Al2O3 and TiO2 were 0 and 45 ng/cm2, respectively, during the first ALD cycle, and on bothsurfaces the mass gains were negligible during the subsequentcycles. It is noteworthy that of all the systems studied, Pt ALD

Figure 1. Metal uptake results from (a) in situ QCM analysis of firstfive cycles of Pt and Pd ALD performed on planar TiO2 and Al2O3surfaces at 100 °C, (b) XRF/ICP results measured on bimetallic Pd−Pt nanoparticles synthesized by one cycle of Pt and Pd ALDperformed on TiO2- and Al2O3-coated SiO2 gel surface. The datapoints are results averaged from multiple samples.

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on Al2O3 showed almost no mass gain during the first cycle,suggesting a very low reactivity of Pt(MeCp)Me3 on Al2O3 at100 °C.Next, Pt−Pd nanoparticles were synthesized at different

temperatures on the Al2O3- and TiO2-coated SiO2 with anominal surface area of 100 m2/g. Figure 1b shows the metalloadings of Pt and Pd prepared using one ALD cycle of eachmetal at different temperatures and using the different supportmaterials. The data points represent average values recordedfrom multiple samples. The Pt metal loading on the 5c Al2O3-coated SiO2 increased exponentially with increasing depositiontemperature. There was barely any loading of Pt metal on 5cAl2O3/SiO2 after 5 min Pt(MeCp)Me3 exposures at 100 °C,

which was consistent with the in situ QCM results. The weightloading of Pt reached ∼1 wt % at 250 °C and further increasedto 2.5 wt % at 300 °C. On the basis of the steady-state ALD Ptgrowth rate of 0.5 Å/cycle at 300 °C, the specific surface area ofthe SiO2 gel, and the density of Pt, we expect a maximum Ptloading from 1 ALD Pt cycle of 10 wt %. The lower metalloading of 2.5 wt % may result from subsaturating Pt(MeCp)-Me3 exposures or from a lower density of reactive sites on theALD Al2O3 compared to the ALD Pt surface. The exponentialincrease in Pt loading with deposition temperature suggests anexponential increase in chemisorption rate for the Pt(MeCp)-Me3 precursor, and supports the idea that all of the Pt metalloadings in Figure 1b result from subsaturating exposures.

Figure 2. STEM image of the as-prepared (a) and reduced (b) Pd1Pt0.5/5c Al2O3/SiO2, and as-prepared (d) and reduced (e) Pd1Pt1/5c TiO2/SiO2.The normalized Pd size distribution (c) and (f) is deduced from more than 500 particles.

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The relationship between Pt loading and depositiontemperature provides a convenient method for preparing Pt−Pd bimetallic catalysts with different Pt loading using only asingle ALD Pt cycle on the Al2O3-coated SiO2 support.Surprisingly, temperature had very little effect on the Pt loadingon the TiO2-coated SiO2. The Pt metal loading reached ∼1.9wt% for depositions at 100 °C and increased only slightly to∼2.1 wt% for deposition at 200 °C. However, the Pd loadingwas ∼0.9 wt% after one ALD Pd cycle independent ofdeposition temperature or substrate. From these metalloadings, the surface density of Pt and Pd on high surfacearea supports can be calculated to be ∼20 ng/cm2 for Pt onTiO2, ∼10 ng/cm2 for Pd on TiO2, and ∼10 ng/cm2 for Pd onAl2O3. These values are significantly lower than thecorresponding values of 45 ng/cm2 for Pt on TiO2, 50 ng/cm2 for Pd on TiO2, and 70 ng/cm2 for Pd on Al2O3

determined from the QCM studies for the first cycle of Ptand Pd ALD. One explanation for this discrepancy is that theexposures used for the SiO2 gel samples were subsaturating.However, if we assume that the weight gains measured in theQCM studies represent dissociatively chemisorbed precursors(i.e., all of the ligands remain on the surface), then the metalloadings from the QCM measurements become 28 ng/cm2 forPt on TiO2, 14 ng/cm2 for Pd on TiO2, and 10 ng/cm2 for Pdon Al2O3, which are very similar to the values determined fromthe SiO2 gel. This good agreement lends confidence to thevalidity of using the simple and convenient QCM measure-

ments to understand the ALD metal growth on high surfacearea supports.

Bimetallic Pt−Pd Nanoparticle Characterization. Thebimetallic nanoparticles were synthesized using one ALD Ptcycle at 250 °C followed by one ALD Pd cycle at 100 °C over5-cycle Al2O3-coated SiO2 gel, or one ALD Pt cycle at 100 °Cfollowed by one ALD Pd cycle at 100 °C over 5-cycle TiO2-coated SiO2 gel. The metal loadings were 1 wt% Pd and 1 wt%Pt on Al2O3, and 1 wt% Pd and 2 wt% Pt on TiO2, asdetermined using XRF and ICP. Thus, these bimetallicnanoparticle samples are designated according to their molarratios as Pd1Pt0.5/5c Al2O3/SiO2 and Pd1Pt1/5c TiO2/SiO2,respectively. These samples were characterized using STEM,and histograms of particle sizes were prepared by measuringmore than 500 particles from multiple images recorded for eachsample. The mean size of the as-prepared Pd1Pt0.5/5c Al2O3/SiO2 nanoparticles was ∼1.1 ± 0.2 nm, as shown in Figure 2a.The as-prepared sample has a very narrow size distribution,with ∼78% of the particles ∼1 nm (Figure 2c). After hydrogenreduction at 250 °C for 1 h, the mean size of the bimetallicparticles remained almost the same at 1.3 ± 0.3 nm (Figures 2band 2c). The as-prepared Pd1Pt1/5c TiO2/SiO2 nanoparticleshad an average size ∼1.2 ± 0.4 nm, with over 55% around 1 nm(Figure 2d). After reduction, the particles aggregated slightly sothat the particle size increased to 1.7 ± 0.5 nm (Figures 2e and2f). These particle sizes will be further discussed below incombination with the X-ray absorption spectroscopy results.

Figure 3. Pd1Pt1/5c TiO2/SiO2 XANES of (a) Pt edge and (b) Pd edge and Fourier transform of EXAFS of (c) Pt edge and (d) Pd edge incomparison to the monometallic nanoparticles.

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It is necessary to obtain detailed structural information onthe supported Pt−Pd bimetallic nanoparticles to build precisestructure−reactivity relationships for these catalysts. A recentstudy determined the structure of 2.5−5 nm unsupported Pt−Pd nanoparticles synthesized by colloidal chemistry usingHAADF-TEM.67 The supported Pt−Pd bimetallic particles inour study are in the size range of 1−2 nm. It is extremelychallenging to identify the structure of these smaller particlesusing high resolution TEM because of the greatly reducednumber of metal atoms (∼900 Pt atoms for a 3 nm Pt clusterversus ∼150 atoms for a 1.5 nm Pt cluster), as well asattenuation and scattering by the underlying support.Consequently, we turned to synchrotron X-ray absorptionspectroscopy to elucidate the structure of the ALD Pt−Pdnanoparticles.Because of the small size, the ALD Pt−Pd nanoparticles

became partially or fully oxidized upon air exposure. Thepresence of Pt−O and Pd−O bonds in the nanoparticles wouldmake it almost impossible to interpret the structure from theEXAFS results, especially for these very small particles. Thus, toobtain unambiguous results regarding the Pt−Pd metal bondcoordination numbers and bonding, the as-prepared Pt−Pdnanoparticles were fully reduced prior to XAS measurementusing 3.5% H2 at 250 °C in a quartz reaction tube. However,before cooling down, the catalysts were purged at 250 °C in Heto ensure total desorption of the hydrogen. Moreover, the XASmeasurements were performed using an ultrahigh purity Heflow so that the bimetallic Pt−Pd nanoparticles were “clean”,and not hydrogen terminated.Figure 3 shows the XANES and EXAFS spectra for the Pt,

Pd, and Pd1Pt1/5c TiO2/SiO2 nanoparticle samples. The smallshift in edge position and change in shape of the bimetallicsample on both the Pt and Pd edges imply the formation ofbimetallic particles. Moreover, the significant change in themagnitude and imaginary parts of the EXAFS signals for thebimetallic sample compared to the monometallic samplesindicate a second scatterer, that is, a bimetallic nanoparticle.68,69

To gain a better understanding of the Pt−Pd nanoparticlestructure, two additional samples were prepared: (1) one ALD

Pd cycle at 100 °C, followed by one ALD Pt cycle at 250 °C(Pt0.5Pd1/5c Al2O3/SiO2), and (2) one ALD Pt cycle at 300 °Cfollowed by one ALD Pd cycle at 100 °C over 5-cycle Al2O3-coated SiO2 gel to yield metal loadings of 2.5% Pt and 0.9% Pd(Pd1Pt1.5/5c Al2O3/SiO2). The detailed EXAFS model fittingsfor the Pt−Pd bimetallic particles with three different molarratios (Pd/Pt = 1:0.5, 1:1, and 1:1.5) are listed in Table 1. TheEXAFS fitting results of Pt0.5Pd1/5c Al2O3/SiO2 and Pd1Pt0.5/5c Al2O3/SiO2 are fairly close and represent the same structure.The slight difference between coordination numbers of Pd−Pdis probably due to different degrees of Pd aggregation andparticle size. For Pt0.5Pd1, Pd was first deposited and latertreated in 250 °C in oxygen in the Pt ALD step, and thisprobably led to slightly larger particles compared to Pd1Pt0.5.The bimetallic particles show 1−2% contraction in both the

Pt−Pt and Pd−Pd bond distances as compared to their bulkstandards. The Pt−Pt bond length for the 1.3 nm Pd1Pt0.5bimetallic particles decreases as much as 0.07 Å, which occursonly for very small nanoparticles, typically less than 3 nm.70

The total coordination numbers for Pt (CNPt−Pt + CNPt−Pd)and Pd (CNPd−Pd + CNPt−Pd) are less than bulk value of 12. ForPd1Pt0.5, Pt0.5Pd1, Pd1Pt1, and Pd1Pt1.5, the compositionweighted average first shell CN’s are 6.4, 7.0, 8.5, and 7.8,respectively, corresponding to particles size 1.2 nm, 1.4 nm, 2.2nm, and 1.9 nm, 71 which is within the error of the STEMresults. The fact that CNPt−Pt > CN Pt−Pd and CNPd−Pt >CNPd−Pd suggests that the bimetallic nanoparticles preferentiallyform a Pt core - Pd shell structure after reduction in H2.

DFT. The bimetallic samples were prepared using both onecycle Pt ALD followed by one cycle Pd ALD, and one cycle PdALD followed by one cycle Pt ALD. In both cases, the XASmeasurements of the reduced bimetallic nanoparticles indicatedPt core-Pd shell structures regardless of the deposition order.DFT calculations revealed that Pd surface segregation ismodestly more favorable in the presence of hydrogen fromthe reduction performed prior to the XAS measurements. Themost stable configurations shown in Table 2 indicate that Pt−Pd (1:1) alloys with a Pt-rich shell are very slightly more stablein the absence of hydrogen (the surface composition is 25% Pd

Table 1. EXAFS Data Fittings of Four Supported Pd−Pt Bimetallic Samplesa

sample particle size (nm) metal loading scatter CNb Rc (Å) DWF (×103) E0 (eV)

Pd1Pt0.5/5c Al2O3/SiO2 1.3 ± 0.3 1 wt% Pd Pd−Pd 2.2 2.72 2 0.4Pd−Pt 3.7 2.69 2 −7.8

1 wt% Pt Pt−Pt 4.9 2.70 2 −3.5Pt−Pd 2.6 2.69 2 7.1

Pt0.5Pd1/5c Al2O3/SiO2 1 wt% Pd Pd−Pd 3.0 2.73 2 1.2Pd−Pt 3.5 2.69 2 −7.7

1 wt% Pt Pt−Pt 5.3 2.71 2 −2.8Pt−Pd 2.8 2.69 2 6.9

Pd1Pt1/5c TiO2/SiO2 1.7 ± 0.5 1 wt% Pd Pd−Pd 2.2 2.70 2 0.3Pd−Pt 5.1 2.69 2 −6.7

2 wt% Pt Pt−Pt 7.3 2.72 2 −1.4Pt−Pd 2.4 2.69 2 6.1

Pd1Pt1.5/5c Al2O3/SiO2 1 wt% Pd Pd−Pd 1.5 2.71 2 1.4Pd−Pt 4.9 2.69 2 −5.8

2.5 wt% Pt Pt−Pt 6.7 2.73 2 −1.7Pt−Pd 2.1 2.69 2 7.6

aPd K edge and Pt L3 edge were measured. Particle size was determined by STEM on the samples after hydrogen reduction at 250 °C. CN iscoordination numbers. R is bond distance. Debye−Waller factor was obtained from measurement of Pt and Pd foil and fixed at 0.002. E0 is energyshift. A two-shell model fit of the k2-weighted EXAFS data was obtained between k = 2.8−12 Å−1 and r = 1.3−3.0 Å, respectively. bError bar ± 10%.cError bar ± 0.02 Å.

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and 75% Pt). When the alloy surface is covered by H, however,the configurations with a Pd-rich shell become morethermodynamically favored compared to the Pt-rich shellsystems; the optimized surface composition is 75% Pd and 25%Pt, which is consistent with the binding energy of BEPd−H >BEPt−H.

72

In our experiments, the bimetallic nanoparticles becameoxidized upon air exposure, and this necessitated hydrogenreduction to obtain meaningful XAS results. The DFT findingssuggest that the Pd-rich shell might form during the hydrogenreduction step regardless of the as-deposited bimetallicnanoparticle structure. Moreover, Somorjai and co-workersreported that Pd segregation in 15 nm bimetallic Pt0.5Pd0.5nanoparticles was not reversible5 so that once the Pd-rich shellformed, the structure would not change with the surroundingchemical environment. Consequently, the as-deposited struc-ture of our ALD Pt−Pd nanoparticles is not known, and itmight be possible to control the structure by adjusting thedeposition conditions (e.g., Pt0.5Pd1 versus Pd1Pt0.5). In-situXAS studies of the Pt−Pd nanoparticle ALD are underway toexplore this possibility.Oxidative Dehydrogenation of Propane. The catalytic

activity of the Pd1Pt1/5c TiO2/SiO2 bimetallic nanoparticleswere evaluated in oxidative dehydrogenation of propane topropylene. For comparison, we also measured the catalyticactivity of a physical mixture of Pt and Pd monometalliccatalysts of identical Pt and Pd loading and similar particle sizeon the same 5c TiO2/SiO2 support. Carbon dioxide wasdetected as the major byproduct, but propylene was alsoobserved at up to 22% concentration. Figure 4a shows that thebimetallic Pd1Pt1 catalyst exhibited a higher selectivity topropylene in the temperature range of 300−400 °C comparedto the physical mixture. The largest difference occurred at 300°C where the selectivity of the Pd1Pt1 catalyst was ∼70% higherrelative to the mixture, and the highest overall propyleneselectivity of 22% was observed for the Pd1Pt1 catalyst at 400°C. Figure 4(b) shows that the propylene yield from the Pd1Pt1bimetallic catalyst was higher than from the physical mixtureover the temperature range 350−500 °C. The maximumpropylene yield observed was 5.7% at 450 °C for the Pd1Pt1catalyst, ∼20% higher relative to the physical mixture ofmonometallic catalysts. These results are encouraging notsimply because higher propylene yields are technologicallyrelevant, but because they demonstrate that Pt and Pd behavedifferently in bimetallic form. A fundamental understanding ofthese differences would require further studies employing a

broader range of ALD samples, surface science probes, andadditional DFT calculations.

■ CONCLUSIONSALD was employed to synthesized ultrasmall (1−2 nm),supported Pt−Pd nanoparticles with a narrow size distribution.The metal loading and composition of the supported Pt−Pdnanoparticles could be controlled by varying the depositiontemperature and support surface chemistry. X-ray absorptionspectroscopy revealed a Pt core−Pd shell nanostructure inreduced form, independent of the deposition sequence andcomposition. Density functional theory calculations suggestthat the Pd surface segregation may result from adsorbedhydrogen following the H2 reduction. The Pt core−Pd shellnanoparticles show higher selectivity and yield to propylene inpropane oxidative dehydrogenation as compared to the physicalmixture of monometallic ALD Pt and Pd catalysts. ALD is apromising method to prepare size- and composition-controlledsupported bimetallic metal nanoparticles with diameter lessthan 2 nm.

■ AUTHOR INFORMATIONCorresponding Author*Phone: 1-630-252-3520. E-mail: jelam@anl.gov.

Table 2. Most Stable Surface Configurations for a Pt−Pd(1:1) Alloya

clean surface 1 ML H on surface

Pd ratio (first−second−third layer)

relativeenergy [in

eV]bPd ratio (first−

second−third layer)

relativeenergy [in

eV]b

0.0−1.0−0.5 0.01 0.0−1.0−0.5 0.440.25−1.0−0.25 0 0.25−1.0−0.25 0.200.5−0.5−0.5 0.02 0.5−1.0−0.0 0.070.75−0.5−0.25 0.08 0.75−0.5−0.25 01.0−0.25−0.25 0.09 1.0−0.0−0.5 0.07

aThe Pd ratio for the 4th and 5th layers are both fixed at the bulkcomposition. Bold values represent the most stable configuration. Pdratio is defined by NPd/(NPd + NPt) in each layer. bRelative energiesare per unit cell.

Figure 4. (a) Selectivity and (b) yield of propylene of Pd1Pt1/5cTiO2/SiO2 bimetallic catalyst and the physical mixture of Pd and Ptmonometallic catalysts.

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NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis material is based upon work supported as part of theInstitute for Atom-efficient Chemical Transformations (IACT),an Energy Frontier Research Center funded by the U.S.Department of Energy, Office of Science, Office of Basic EnergySciences. Use of the Advanced Photon Source was supportedby the U.S. Department of Energy, Office of Science, Office ofBasic Energy Sciences, under Contract No. DE-AC02-06CH11357. MRCAT operations are supported by theDepartment of Energy and the MRCAT member institutions.The computational portion of this research was performedusing EMSL, a national scientific user facility sponsored by theDepartment of Energy’s Office of Biological and EnvironmentalResearch located at Pacific Northwest National Laboratory, theNational Energy Research Scientific Computing Centersupported by the Office of Science of the U.S. Department ofEnergy, Fusion operated by the Laboratory ComputingResource Center at Argonne National Laboratory; and theCenter for Nanoscale Materials supported by the U.S.Department of Energy, Office of Science, Office of BasicEnergy Sciences.

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