Synthesis of Pt–Pd Core–Shell Nanostructures by Atomic Layer

Article pubs.acs.org/cm

Synthesis of Pt−Pd Core−Shell Nanostructures by Atomic Layer Deposition: Application in Propane Oxidative Dehydrogenation to Propylene Yu 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 employed to synthesize supported Pt−Pd bimetallic particles in the 1 to 2 nm range. The metal loading and composition of the supported Pt−Pd nanoparticles were controlled by varying the deposition temperature and by applying ALD metal oxide coatings to modify the support surface chemistry. Highresolution scanning transmission electron microscopy images showed monodispersed Pt−Pd nanoparticles on ALD Al2O3and TiO2-modified SiO2 gel. X-ray absorption spectroscopy revealed that the bimetallic nanoparticles have a stable Pt-core, Pd-shell nanostructure. Density functional theory calculations revealed that the most stable surface configuration for the Pt− Pd alloys in an H2 environment has a Pt-core, Pd-shell nanostructure. In comparison to their monometallic counterparts, the small Pt−Pd bimetallic core−shell nanoparticles exhibited higher activity in propane oxidative dehydrogenation as compared to their physical mixture. KEYWORDS: atomic layer deposition, platinum, palladium, bimetallic nanoparticles, catalyst

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INTRODUCTION Bimetallic catalysts offer the possibility to combine the unique advantages of each component, allowing the catalyst activity, selectivity, and stability to be tuned by precisely controlling the bimetallic composition and structure. Moreover, owing to the changes in electronic and geometric structure, supported bimetallic catalysts often exhibit enhanced catalytic properties compared to simple mixtures of their monometallic counterparts.1−4 Supported Pt−Pd nanoparticles are among the most widely studied and implemented bimetallic heterogeneous catalysts in important technological areas,5 including aromatics hydrogenation,6,7 petroleum hydrocracking,8 emission control,9,10 hydrogen storage,11,12 and electrocatalysis in fuel cells.13,14 Pt−Pd bimetallic nanocatalysts not only show enhanced selectivity and activity, but also better tolerance to poisons such as sulfur.6,15 The high activity of under-coordinated surface atoms has motivated efforts to synthesize supported precious metal nanoparticles in the size range of a few nanometers. Moreover, the high price of precious metals dictates that the catalyst should be finely dispersed to have a very high ratio of surface atoms to bulk atoms. More fundamentally, catalysts with welldefined size, composition, and structure are necessary to build precise structure−reactivity relationships and provide a more © 2012 American Chemical Society

complete understanding of bimetallic nanocatalysts. Unfortunately, the synthesis of uniform bimetallic nanoparticles with diameters below 2 nm has proved challenging for traditional catalyst synthesis methods such as wet impregnation,16,17 and colloidal chemistry.18−22 Atomic layer deposition (ALD) is a promising technique for producing uniform precious metal nanoparticles on high surface area supports because of its unique feature of sequential, selflimiting surface reactions.23,24 ALD allows the nanoparticle size and composition to be controlled precisely by adjusting the number and sequence of ALD cycles of each component. In addition, the deposition of precious metal nanoparticles is affected by the deposition temperature and the surface chemistry of the underlying support. The chemical properties of the support materials can be modified by coating a few ALD cycles of an oxide without significantly changing the porosity of the template material. By combining ALD processes for metal oxides and noble metals, it is possible to engineer nanocatalysts with unique structure and properties by depositing a series of discrete layers, which each performs a specific function such as Received: January 9, 2012 Revised: April 30, 2012 Published: August 20, 2012 3525

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with methanol added for stability). The deposition temperature for the metal 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 the adsorbed Pd precursor was reacted with HCHO at 200 °C prior to depositing the second metal. The Pt and Pd metal loadings were determined by X-ray fluorescence spectroscopy (XRF, Oxford ED2000) and inductively coupled plasma (ICP, Varian Vista-MPX instrument). In this work, the catalysts prepared using ALD are designated as, (e.g.) Pt1Pd1/5c Al2O3/SiO2, to represent bimetallic Pt−Pd nanoparticles deposited on 5-cycle ALD Al2O3-coated SiO2 gel with a Pt/Pd = 1:1 molar ratio. Scanning Transmission Electron Microscopy. Scanning Transmission Electron Microscopy (STEM) measurements were made on both the as-prepared and reduced samples. A few milligrams of catalyst were sonicated in 10 mL of isopropanol for 10 min to obtain a welldispersed slurry. A drop of this mixture was deposited onto a lacey carbon copper sample grid (SPi Supplies, 400 mesh) and thoroughly dried with an ultrainfrared lamp. STEM images were obtained using a JEOL JEM-2100F FEG FasTEM (EPIC at Northwestern University). The histograms of particle sizes were generated from the STEM images 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 Research Collaborative Access Team (MRCAT) at Sector 10 of the Advanced Photon Source, Argonne National Laboratory. The XAS measurements were made in transmission mode with the ionization chamber optimized for the maximum current with linear response. Spectra at both the Pt L3 edge (11.564 keV) and the Pd K edge (24.35 keV) were acquired for the bimetallic samples. Pt and Pd foils were used to calibrate the monochromator. The amount of sample used was optimized to achieve an edge step of at least 0.2. The samples were fully reduced using 50 sccm 3.5% H2 in He as balance gas at 250 °C for one hour. Next, the reactor was purged using 150 sccm ultrahigh purity He for 10 min at 250 °C. The samples were cooled to room temperature in He and measured as the “reduced” sample to obtain precise information on the metal-metal bond distances and coordination numbers and to facilitate determination of the particle structure and size. Standard procedures based on WINXAS 3.1 software were used to fit the data in the EXAFS regime.50 The Pt−Pt and Pd−Pd scattering phase shift and amplitude were obtained from reference Pd foil for Pd−Pd (NPd−Pd= 12 at 2.75 Å) and Pt foil for Pt−Pt (NPt−Pt = 12 at 2.77 Å). Commercial software for EXAFS data analysis (FEFF) was used to build Pt−Pd and Pt−Pd scattering phase shift and amplitudes. A homogeneous Pt−Pd alloy model for FEFF fitting was built by carefully substituting Pt with Pd in an fcc bulk structure. 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. The composition weighted average first shell coordination number (CN) for the 1:1 bimetallic nanoparticles was calculated using: CN = (CNPt−Pt + CNPt−Pd)/ 2 + (CNPd−Pd + CNPt−Pd)/2. Density Functional Theory Calculations. Density functional theory (DFT) calculations were performed using the Vienna Ab-initio Simulation 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-correlation interactions.57,58 The Kohn−Sham valence states were expanded in a plane wave basis set up to 25 Ry (or 340 eV). The surface Brillouin zone was sampled with 4 × 4 × 1 k points based on the Monkhorst-Pack sampling scheme; we consider these results to be fully converged with respect to k points.59 Benchmark tests on k points showed that the statistical error was within 10 meV. The self-consistent iteration was converged to within a criterion of 1 × 10−7, and the ionic steps were converged to 0.02 eV/Å. The Methfessel−Paxton smearing scheme was used, 60 and with a Fermi population of the Kohn−Sham states of kBT = 0.2 eV, with the total energies extrapolated to 0 eV.

serving as the catalyst support, providing or promoting catalytic activity, and imparting thermal stability.25,26 ALD has been successfully developed to synthesize monometallic nanocatalysts, such as Pt,27−31 Pd,32−37 and Ir,38 as well as bimetallic Pt-Ru,39,40 supported by TiO2, Al2O3, SrTiO3, ZnO, SiO2, carbon, etc., for various catalytic applications. However, in comparison to the typical layer-bylayer behavior of metal oxide ALD, noble metal ALD can be complicated by the different reactivity and nucleation behavior for the noble metal growth on metal oxide support surfaces and the high mobility of metal atoms and clusters.25,41 In this work, we present a novel method using ALD to synthesize supported Pt−Pd nanocatalysts in the size range of 1−2 nm with a narrow size distribution and core−shell structure. Propylene is one of the most important chemical intermediates in the petrochemical industry. Propane oxidative dehydrogenation (ODH) to propylene has been extensively studied, and platinum and palladium monometallic catalysts have both been investigated as propane ODH catalysts.42−44 In this study, we used propane ODH as a probe reaction to evaluate our bimetallic catalysts and we found that the bimetallic Pt−Pd nanoparticles are more efficient in propane ODH than an equivalent physical mixture of the monometallic Pt and Pd.

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EXPERIMENTAL SECTION

Atomic Layer Deposition. The ALD was performed in a viscous flow reactor that has been described in detail elsewhere.45 Briefly, ALD samples were prepared in a hot-walled vacuum chamber equipped with an in situ quartz-crystal microbalance (QCM) and quadrupole mass spectrometer (QMS). Ultrahigh purity N2 carrier gas (Air-gas, 99.999%) was further purified using an Aeronex Gatekeeper Inert Gas Filter to trap oxygen-containing impurities before entering the reactor. 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, the SiO2 was baked in an oven at 200 °C overnight to desorb water and achieve a consistent density of surface hydroxyl groups.46 Prebaked SiO2 gel (0.5 g) was uniformly spread onto a stainless steel sample plate with a mesh top to contain the powder while still allowing access to the precursor vapors. The powder samples were loaded into the center of the reactor and kept for at least 30 min at 200 °C in a 350 sccm flow of UHP N2 at 1 Torr pressure to allow temperature stabilization and to further outgas the SiO2 gel. Next, the sample was cleaned by exposure to 30 sccm of flowing ozone at 1 Torr pressure at 200 °C for 15 min. After cleaning, the SiO2 gel surface was modified using either 5 ALD cycles of Al2O3 or 5 ALD cycles of TiO2. The Al2O3 ALD used alternating exposures to trimethyl aluminum (TMA, Sigma-Aldrich, 97%) and deionized water at 200 °C. The TiO2 ALD used alternating exposures to TiCl4 (Sigma-Aldrich, 99.9%) and deionized water at 150 °C. Five cycles of Al2O3 and TiO2 yield film thicknesses of 6 and 3 Å, respectively. The surface area of the substrate was assumed the same before and after the ALD coating.47,48 The thicknesses of the TiO2 and Al2O3 films were obtained in two ways. The first method was to measure the coating thickness on witness Si(100) wafers coated simultaneously with the powder using spectroscopic ellipsometry. The second method was to measure the weight gain of the SiO2 powder after the Al2O3 or TiO2 ALD. From these weight changes and the density and known surface area of the SiO2, the ALD film thicknesses could be calculated. The thicknesses obtained 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 alternating exposures to palladium hexafluoroacetylacetonate (Pd(hfac)2, SigmaAldrich, 99.9%) and formalin (Sigma-Aldrich, HCHO, 37 wt.% in H2O 3526

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°C to investigate the low temperature limit for ALD nanoparticle synthesis and the effect on loading. Lower deposition temperatures reduce the mobility of surface species and the degree of thermal decomposition of the ALD precursors, and should yield a more uniform coverage of smaller nanoparticles. The amount of Pt and Pd deposited during the initial 1−5 ALD cycles was examined using in situ QCM at 100 °C. Note that at these low deposition temperatures, the precursor ligands remain on the surface so that the QCM measures the weight gains of the adsorbed precursor molecules. Consequently, these weight gain values should be interpreted as relative measurements of the metal loadings. Figure 1a shows that during the

The DFT calculations were performed with periodic boundary conditions. The Pt−Pd alloy surfaces were represented by 5-layer slabs with p(2 × 2) unit cells and close-packed (111) facets. The top three layers were allowed to relax. A vacuum distance equivalent to approximately nine metal layers was used between successive metal slabs. The lattice constant of the 50:50 Pt/Pd bulk alloy (L10 ordering) was optimized to be 3.96 Å. To model the well-mixed core of the Pt−Pd nanoparticles in our experiment, the distribution of atoms in the bottom two layers was kept fixed in its bulk arrangement. The configurations in the top three layers were sampled via permutations of the arrangements of the Pt and Pd atoms. Single point energy calculations were first performed to screen out the thermodynamically unfavorable configurations. All structures with total energies per unit cell within ∼0.05 eV of the energy of the most stable configurations at each Pd concentration were then fully optimized to determine their energies; the relative energies of the configurations were not found to change significantly due to the optimization. The most 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 the optimizations 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 on a 3-fold fcc site (4 fcc sites in total on a 2 × 2 unit cell) for each permuted configuration. The most stable configurations in the presence of hydrogen were then determined. H diffusion into the alloy sublayer (2nd layer) region was also considered for the most stable configurations at each surface coverage of Pd by placing one H from 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 inside diameter of ∼4 mm at atmospheric pressure. Ten milligrams of the bimetallic catalyst Pd1Pt1/5c TiO2/SiO2 was homogeneously diluted in 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 particle size. XRF was employed to ensure the same amounts of Pd and Pt in the mixture as compared to the bimetallic samples. Twenty milligrams of 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 1 sccm 10% O2 were used as reactants. Online gas chromatographic analysis was performed on a Hewlett-Packard 5890 GC equipped with a TCD and a FID detector. The conversion of the reaction was defined as the percentage of propane consumed to propane fed. The yield of propylene was obtained as Y = X × S, where X is the propane conversion and S is the selectivity to propylene.

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RESULTS AND DISCUSSION ALD Al2O3- and TiO2-Coated SiO2. The purpose of coating the SiO2 surface with ALD Al2O3 and TiO2 is to promote the ALD Pt and Pd nucleation. Under identical preparation conditions, one ALD Pt or Pd cycle on the bare Silicycle S10040 M silica gel yielded only ∼0.1 wt% Pt or Pd loading. This loading was too low for most heterogeneous catalytic studies and characterization. It has been shown that the nucleation of Pt and Pd ALD is relatively prompt on Al2O333,39 and TiO2.41,61 Consequently, modifying the SiO2 surface with a few layers of ALD TiO2 and Al2O3 can increase the efficiency of the ALD Pt and Pd nucleation without decreasing the surface area.47,48 Bimetallic Pt−Pd Nanoparticle Synthesis. We first examined the effects of adjusting the number of ALD cycles, the deposition temperature, and the support surface on the metal loading and composition of the supported Pt−Pd nanoparticles. Pt62,63 and Pd64−66 ALD are typically conducted at 300 and 200 °C, respectively. In this work, the Pt and Pd ALD were performed at deposition temperatures as low as 100

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

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, in agreement with previous measurements.61 However, the Pd weight gain was greatly attenuated for the subsequent cycles on both supports. This finding is consistent with previous studies showing that 100 °C is not sufficient for the HCHO to remove the hfac ligands from the Pd and the support, thereby blocking additional 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 both surfaces the mass gains were negligible during the subsequent cycles. It is noteworthy that of all the systems studied, Pt ALD 3527

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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.

on Al2O3 showed almost no mass gain during the first cycle, suggesting a very low reactivity of Pt(MeCp)Me3 on Al2O3 at 100 °C. Next, Pt−Pd nanoparticles were synthesized at different temperatures on the Al2O3- and TiO2-coated SiO2 with a nominal surface area of 100 m2/g. Figure 1b shows the metal loadings of Pt and Pd prepared using one ALD cycle of each metal at different temperatures and using the different support materials. The data points represent average values recorded from multiple samples. The Pt metal loading on the 5c Al2O3coated SiO2 increased exponentially with increasing deposition temperature. There was barely any loading of Pt metal on 5c Al2O3/SiO2 after 5 min Pt(MeCp)Me3 exposures at 100 °C,

which was consistent with the in situ QCM results. The weight loading of Pt reached ∼1 wt % at 250 °C and further increased to 2.5 wt % at 300 °C. On the basis of the steady-state ALD Pt growth rate of 0.5 Å/cycle at 300 °C, the specific surface area of the SiO2 gel, and the density of Pt, we expect a maximum Pt loading from 1 ALD Pt cycle of 10 wt %. The lower metal loading of 2.5 wt % may result from subsaturating Pt(MeCp)Me3 exposures or from a lower density of reactive sites on the ALD Al2O3 compared to the ALD Pt surface. The exponential increase in Pt loading with deposition temperature suggests an exponential increase in chemisorption rate for the Pt(MeCp)Me3 precursor, and supports the idea that all of the Pt metal loadings in Figure 1b result from subsaturating exposures. 3528

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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 in comparison to the monometallic nanoparticles.

ments to understand the ALD metal growth on high surface area supports. Bimetallic Pt−Pd Nanoparticle Characterization. The bimetallic nanoparticles were synthesized using one ALD Pt cycle at 250 °C followed by one ALD Pd cycle at 100 °C over 5-cycle Al2O3-coated SiO2 gel, or one ALD Pt cycle at 100 °C followed by one ALD Pd cycle at 100 °C over 5-cycle TiO2coated 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, as determined using XRF and ICP. Thus, these bimetallic nanoparticle samples are designated according to their molar ratios 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 measuring more than 500 particles from multiple images recorded for each sample. 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 hydrogen reduction at 250 °C for 1 h, the mean size of the bimetallic particles remained almost the same at 1.3 ± 0.3 nm (Figures 2b and 2c). The as-prepared Pd1Pt1/5c TiO2/SiO2 nanoparticles had an average size ∼1.2 ± 0.4 nm, with over 55% around 1 nm (Figure 2d). After reduction, the particles aggregated slightly so that the particle size increased to 1.7 ± 0.5 nm (Figures 2e and 2f). These particle sizes will be further discussed below in combination with the X-ray absorption spectroscopy results.

The relationship between Pt loading and deposition temperature provides a convenient method for preparing Pt− Pd bimetallic catalysts with different Pt loading using only a single ALD Pt cycle on the Al2O3-coated SiO2 support. Surprisingly, temperature had very little effect on the Pt loading on the TiO2-coated SiO2. The Pt metal loading reached ∼1.9 wt% for depositions at 100 °C and increased only slightly to ∼2.1 wt% for deposition at 200 °C. However, the Pd loading was ∼0.9 wt% after one ALD Pd cycle independent of deposition temperature or substrate. From these metal loadings, the surface density of Pt and Pd on high surface area supports can be calculated to be ∼20 ng/cm2 for Pt on TiO2, ∼10 ng/cm2 for Pd on TiO2, and ∼10 ng/cm2 for Pd on Al 2 O 3 . These values are significantly lower than the corresponding 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 Pt and Pd ALD. One explanation for this discrepancy is that the exposures used for the SiO2 gel samples were subsaturating. However, if we assume that the weight gains measured in the QCM studies represent dissociatively chemisorbed precursors (i.e., all of the ligands remain on the surface), then the metal loadings from the QCM measurements become 28 ng/cm2 for Pt on TiO2, 14 ng/cm2 for Pd on TiO2, and 10 ng/cm2 for Pd on Al2O3, which are very similar to the values determined from the SiO2 gel. This good agreement lends confidence to the validity of using the simple and convenient QCM measure3529

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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 Pd−Pt Pt−Pt Pt−Pd Pd−Pd Pd−Pt Pt−Pt Pt−Pd Pd−Pd Pd−Pt Pt−Pt Pt−Pd Pd−Pd Pd−Pt Pt−Pt Pt−Pd

2.2 3.7 4.9 2.6 3.0 3.5 5.3 2.8 2.2 5.1 7.3 2.4 1.5 4.9 6.7 2.1

2.72 2.69 2.70 2.69 2.73 2.69 2.71 2.69 2.70 2.69 2.72 2.69 2.71 2.69 2.73 2.69

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

0.4 −7.8 −3.5 7.1 1.2 −7.7 −2.8 6.9 0.3 −6.7 −1.4 6.1 1.4 −5.8 −1.7 7.6

1 wt% Pt Pt0.5Pd1/5c Al2O3/SiO2

1 wt% Pd 1 wt% Pt

Pd1Pt1/5c TiO2/SiO2

1.7 ± 0.5

1 wt% Pd 2 wt% Pt

Pd1Pt1.5/5c Al2O3/SiO2

1 wt% Pd 2.5 wt% Pt

Pd K edge and Pt L3 edge were measured. Particle size was determined by STEM on the samples after hydrogen reduction at 250 °C. CN is coordination numbers. R is bond distance. Debye−Waller factor was obtained from measurement of Pt and Pd foil and fixed at 0.002. E0 is energy shift. 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%. c Error bar ± 0.02 Å. a

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 °C followed by one ALD Pd cycle at 100 °C over 5-cycle Al2O3coated SiO2 gel to yield metal loadings of 2.5% Pt and 0.9% Pd (Pd1Pt1.5/5c Al2O3/SiO2). The detailed EXAFS model fittings for the Pt−Pd bimetallic particles with three different molar ratios (Pd/Pt = 1:0.5, 1:1, and 1:1.5) are listed in Table 1. The EXAFS 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−Pd is probably due to different degrees of Pd aggregation and particle size. For Pt0.5Pd1, Pd was first deposited and later treated in 250 °C in oxygen in the Pt ALD step, and this probably 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 bulk standards. The Pt−Pt bond length for the 1.3 nm Pd1Pt0.5 bimetallic particles decreases as much as 0.07 Å, which occurs only 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. For Pd1Pt0.5, Pt0.5Pd1, Pd1Pt1, and Pd1Pt1.5, the composition weighted 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, 2.2 nm, and 1.9 nm, 71 which is within the error of the STEM results. The fact that CNPt−Pt > CN Pt−Pd and CNPd−Pt > CNPd−Pd suggests that the bimetallic nanoparticles preferentially form a Pt core - Pd shell structure after reduction in H2. DFT. The bimetallic samples were prepared using both one cycle Pt ALD followed by one cycle Pd ALD, and one cycle Pd ALD followed by one cycle Pt ALD. In both cases, the XAS measurements of the reduced bimetallic nanoparticles indicated Pt core-Pd shell structures regardless of the deposition order. DFT calculations revealed that Pd surface segregation is modestly more favorable in the presence of hydrogen from the reduction performed prior to the XAS measurements. The most stable configurations shown in Table 2 indicate that Pt− Pd (1:1) alloys with a Pt-rich shell are very slightly more stable in the absence of hydrogen (the surface composition is 25% Pd

It is necessary to obtain detailed structural information on the supported Pt−Pd bimetallic nanoparticles to build precise structure−reactivity relationships for these catalysts. A recent study determined the structure of 2.5−5 nm unsupported Pt− Pd nanoparticles synthesized by colloidal chemistry using HAADF-TEM.67 The supported Pt−Pd bimetallic particles in our study are in the size range of 1−2 nm. It is extremely challenging to identify the structure of these smaller particles using high resolution TEM because of the greatly reduced number of metal atoms (∼900 Pt atoms for a 3 nm Pt cluster versus ∼150 atoms for a 1.5 nm Pt cluster), as well as attenuation and scattering by the underlying support. Consequently, we turned to synchrotron X-ray absorption spectroscopy to elucidate the structure of the ALD Pt−Pd nanoparticles. Because of the small size, the ALD Pt−Pd nanoparticles became partially or fully oxidized upon air exposure. The presence of Pt−O and Pd−O bonds in the nanoparticles would make it almost impossible to interpret the structure from the EXAFS results, especially for these very small particles. Thus, to obtain unambiguous results regarding the Pt−Pd metal bond coordination numbers and bonding, the as-prepared Pt−Pd nanoparticles were fully reduced prior to XAS measurement using 3.5% H2 at 250 °C in a quartz reaction tube. However, before cooling down, the catalysts were purged at 250 °C in He to ensure total desorption of the hydrogen. Moreover, the XAS measurements were performed using an ultrahigh purity He flow 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 small shift in edge position and change in shape of the bimetallic sample on both the Pt and Pd edges imply the formation of bimetallic particles. Moreover, the significant change in the magnitude and imaginary parts of the EXAFS signals for the bimetallic sample compared to the monometallic samples indicate a second scatterer, that is, a bimetallic nanoparticle.68,69 To gain a better understanding of the Pt−Pd nanoparticle structure, two additional samples were prepared: (1) one ALD 3530

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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)

relative energy [in eV]b

Pd ratio (first− second−third layer)

relative energy [in eV]b

0.0−1.0−0.5 0.25−1.0−0.25 0.5−0.5−0.5 0.75−0.5−0.25 1.0−0.25−0.25

0.01 0 0.02 0.08 0.09

0.0−1.0−0.5 0.25−1.0−0.25 0.5−1.0−0.0 0.75−0.5−0.25 1.0−0.0−0.5

0.44 0.20 0.07 0 0.07

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

and 75% Pt). When the alloy surface is covered by H, however, the configurations with a Pd-rich shell become more thermodynamically favored compared to the Pt-rich shell systems; 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 became oxidized upon air exposure, and this necessitated hydrogen reduction to obtain meaningful XAS results. The DFT findings suggest that the Pd-rich shell might form during the hydrogen reduction step regardless of the as-deposited bimetallic nanoparticle structure. Moreover, Somorjai and co-workers reported that Pd segregation in 15 nm bimetallic Pt0.5Pd0.5 nanoparticles was not reversible5 so that once the Pd-rich shell formed, the structure would not change with the surrounding chemical environment. Consequently, the as-deposited structure of our ALD Pt−Pd nanoparticles is not known, and it might be possible to control the structure by adjusting the deposition conditions (e.g., Pt0.5Pd1 versus Pd1Pt0.5). In-situ XAS studies of the Pt−Pd nanoparticle ALD are underway to explore this possibility. Oxidative Dehydrogenation of Propane. The catalytic activity of the Pd1Pt1/5c TiO2/SiO2 bimetallic nanoparticles were evaluated in oxidative dehydrogenation of propane to propylene. For comparison, we also measured the catalytic activity of a physical mixture of Pt and Pd monometallic catalysts of identical Pt and Pd loading and similar particle size on the same 5c TiO2/SiO2 support. Carbon dioxide was detected as the major byproduct, but propylene was also observed at up to 22% concentration. Figure 4a shows that the bimetallic Pd1Pt1 catalyst exhibited a higher selectivity to propylene in the temperature range of 300−400 °C compared to the physical mixture. The largest difference occurred at 300 °C where the selectivity of the Pd1Pt1 catalyst was ∼70% higher relative to the mixture, and the highest overall propylene selectivity of 22% was observed for the Pd1Pt1 catalyst at 400 °C. Figure 4(b) shows that the propylene yield from the Pd1Pt1 bimetallic catalyst was higher than from the physical mixture over the temperature range 350−500 °C. The maximum propylene yield observed was 5.7% at 450 °C for the Pd1Pt1 catalyst, ∼20% higher relative to the physical mixture of monometallic catalysts. These results are encouraging not simply because higher propylene yields are technologically relevant, but because they demonstrate that Pt and Pd behave differently in bimetallic form. A fundamental understanding of these differences would require further studies employing a

Figure 4. (a) Selectivity and (b) yield of propylene of Pd1Pt1/5c TiO2/SiO2 bimetallic catalyst and the physical mixture of Pd and Pt monometallic catalysts.

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

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CONCLUSIONS ALD 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−Pd nanoparticles could be controlled by varying the deposition temperature and support surface chemistry. X-ray absorption spectroscopy revealed a Pt core−Pd shell nanostructure in reduced form, independent of the deposition sequence and composition. Density functional theory calculations suggest that the Pd surface segregation may result from adsorbed hydrogen following the H2 reduction. The Pt core−Pd shell nanoparticles show higher selectivity and yield to propylene in propane oxidative dehydrogenation as compared to the physical mixture of monometallic ALD Pt and Pd catalysts. ALD is a promising method to prepare size- and composition-controlled supported bimetallic metal nanoparticles with diameter less than 2 nm.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 1-630-252-3520. E-mail: [email protected]. 3531

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Notes

(22) Yin, A. X.; Min, X. Q.; Zhang, Y. W.; Yan, C. H. J. Am. Chem. Soc. 2011, 133 (11), 3816−3819. (23) Puurunen, R. L. J. Appl. Phys. 2005, 97 (12), 121301−121352. (24) Leskela, M.; Ritala, M. Thin Solid Films 2002, 409 (1), 138−146. (25) Lu, J. L.; Lei, Y.; Elam, J. W. In Noble Metals; Su, Y.-H., Ed.; INTECH: Janeza Trdine, Slovenia, 2012; pp 159−178. (26) Lei, Y.; Mehmood, F.; Lee, S.; Greeley, J.; Lee, B.; Seifert, S.; Winans, R. E.; Elam, J. W.; Meyer, R. J.; Redfern, P. C.; Teschner, D.; Schlogl, R.; Pellin, M. J.; Curtiss, L. A.; Vajda, S. Science 2010, 328 (5975), 224−228. (27) Enterkin, J. A.; Setthapun, W.; Elam, J. W.; Christensen, S. T.; Rabuffetti, F. A.; Marks, L. D.; Stair, P. C.; Poeppelmeier, K. R.; Marshall, C. L. ACS Catal. 2011, 1 (6), 629−635. (28) Li, J. H.; Liang, X. H.; King, D. M.; Jiang, Y. B.; Weimer, A. W. Appl. Catal., B 2010, 97 (1−2), 220−226. (29) Setthapun, W.; Williams, W. D.; Kim, S. M.; Feng, H.; Elam, J. W.; Rabuffetti, F. A.; Poeppelmeier, K. R.; Stair, P. C.; Stach, E. A.; Ribeiro, F. H.; Miller, J. T.; Marshall, C. L. J. Phys. Chem. C 2010, 114 (21), 9758−9771. (30) Hsu, I. J.; Hansgen, D. A.; McCandless, B. E.; Willis, B. G.; Chen, J. G. J. Phys. Chem. C 2011, 115 (9), 3709−3715. (31) King, J. S.; Wittstock, A.; Biener, J.; Kucheyev, S. O.; Wang, Y. M.; Baumann, T. F.; Giri, S. K.; Hamza, A. V.; Baeumer, M.; Bent, S. F. Nano Lett. 2008, 8 (8), 2405−2409. (32) Feng, H.; Libera, J. A.; Stair, P. C.; Miller, J. T.; Elam, J. W. ACS Catal. 2011, 1 (6), 665−673. (33) Feng, H.; Elam, J. W.; Libera, J. A.; Setthapun, W.; Stair, P. C. Chem. Mater. 2010, 22 (10), 3133−3142. (34) Rikkinen, E.; Santasalo-Aarnio, A.; Airaksinen, S.; Borghei, M.; Viitanen, V.; Sainio, J.; Kauppinen, E. I.; Kallio, T.; Krause, A. O. I. J. Phys. Chem. C 2011, 115 (46), 23067−23073. (35) Buchbinder, A. M.; Ray, N. A.; Lu, J. L.; Van Duyne, R. P.; Stair, P. C.; Weitz, E.; Geiger, F. M. J. Am. Chem. Soc. 2011, 133 (44), 17816−17823. (36) Lu, J. L.; Stair, P. C. Angew. Chem., Int. Ed. 2010, 49 (14), 2547−2551. (37) Lu, J. L.; Fu, B. S.; Kung, M. C.; Xiao, G. M.; Elam, J. W.; Kung, H. H.; Stair, P. C. Science 2012, 335 (6073), 1205−1208. (38) Silvennoinen, R. J.; Jylha, O. J. T.; Lindblad, M.; Osterholm, H.; Krause, A. O. I. Catal. Lett. 2007, 114 (3−4), 135−144. (39) Christensen, S. T.; Feng, H.; Libera, J. L.; Guo, N.; Miller, J. T.; Stair, P. C.; Elam, J. W. Nano Lett. 2010, 10 (8), 3047−3051. (40) Jiang, X. R.; Gur, T. N.; Prinz, F. B.; Bent, S. F. Chem. Mater. 2010, 22 (10), 3024−3032. (41) Lee, H. B. R.; Bent, S. F. Chem. Mater. 2012, 24 (2), 279−286. (42) Vajda, S.; Pellin, M. J.; Greeley, J. P.; Marshall, C. L.; Curtiss, L. A.; Ballentine, G. A.; Elam, J. W.; Catillon-Mucherie, S.; Redfern, P. C.; Mehmood, F.; Zapol, P. Nat. Mater. 2009, 8 (3), 213−216. (43) Sugiyama, S.; Hirata, Y.; Nakagawa, K.; Sotowa, K. I.; Maehara, K.; Himeno, Y.; Ninomiya, W. J. Catal. 2008, 260 (1), 157−163. (44) Sadykov, V. A.; Pavlova, S. N.; Saputina, N. F.; Zolotarskii, I. A.; Pakhomov, N. A.; Moroz, E. M.; Kuzmin, V. A.; Kalinkin, A. V. Catal. Today 2000, 61 (1−4), 93−99. (45) Elam, J. W.; Groner, M. D.; George, S. M. Rev. Sci. Instrum. 2002, 73 (8), 2981−2987. (46) Gajan, D.; Coperet, C. New J. Chem. 2011, 35 (11), 2385−2722. (47) Elam, J. W.; Libera, J. A.; Huynh, T. H.; Feng, H.; Pellin, M. J. J. Phys. Chem. C 2010, 114 (41), 17286−17292. (48) Lu, J. L.; Kosuda, K. M.; Van Duyne, R. P.; Stair, P. C. J. Phys. Chem. C 2009, 113 (28), 12412−12418. (49) Figueroa, S. J. A.; Stewart, S. J.; Rueda, T.; Hernando, A.; de la Presa, P. J. Phys. Chem. C 2011, 115 (13), 5500−5508. (50) Friebel, D.; Miller, D. J.; O’Grady, C. P.; Anniyev, T.; Bargar, J.; Bergmann, U.; Ogasawara, H.; Wikfeldt, K. T.; Pettersson, L. G. M.; Nilsson, A. Phys. Chem. Chem. Phys. 2011, 13 (1), 262−266. (51) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47 (1), 558−561. (52) Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49 (20), 14251−14269. (53) Kresse, G.; Furthmuller, J. Phys. Rev. B 1996, 54 (16), 11169− 11186.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported as part of the Institute 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 Energy Sciences. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. The computational portion of this research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory, the National Energy Research Scientific Computing Center supported by the Office of Science of the U.S. Department of Energy, Fusion operated by the Laboratory Computing Resource Center at Argonne National Laboratory; and the Center for Nanoscale Materials supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

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REFERENCES

(1) Chen, M. S.; Kumar, D.; Yi, C. W.; Goodman, D. W. Science 2005, 310 (5746), 291−293. (2) Kitchin, J. R.; Norskov, J. K.; Barteau, M. A.; Chen, J. G. Phys. Rev. Lett. 2004, 93, 15. (3) Campbell, C. T. Annu. Rev. Phys. Chem. 1990, 41, 775−837. (4) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 22 (11), 1179− 1201. (5) Tao, F.; Grass, M. E.; Zhang, Y. W.; Butcher, D. R.; Renzas, J. R.; Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.; Somorjai, G. A. Science 2008, 322 (5903), 932−934. (6) Navarro, R. M.; Pawelec, B.; Trejo, J. M.; Mariscal, R.; Fierro, J. L. G. J. Catal. 2000, 189 (1), 184−194. (7) Barrio, V. L.; Arias, P. L.; Cambra, J. F.; Guemez, M. B.; Pawelec, B.; Fierro, J. L. G. Fuel 2003, 82 (5), 501−509. (8) Fiermans, L.; De Gryse, R.; De Doncker, G.; Jacobs, P. A.; Martens, J. A. J. Catal. 2000, 193 (1), 108−114. (9) Roy, S.; Baiker, A. Chem. Rev. 2009, 109 (9), 4054−4091. (10) Jelic, J.; Meyer, R. J. Catal. Today 2008, 136 (1−2), 76−83. (11) Kobayashi, H.; Yamauchi, M.; Kitagawa, H.; Kubota, Y.; Kato, K.; Takata, M. J. Am. Chem. Soc. 2010, 132 (16), 5576−5577. (12) Kobayashi, H.; Yamauchi, M.; Kitagawa, H.; Kubota, Y.; Kato, K.; Takata, M. J. Am. Chem. Soc. 2008, 130 (6), 1818−1819. (13) Peng, Z. M.; Yang, H. J. Am. Chem. Soc. 2009, 131 (22), 7542− 7543. (14) Lim, B.; Jiang, M. J.; Camargo, P. H. C.; Cho, E. C.; Tao, J.; Lu, X. M.; Zhu, Y. M.; Xia, Y. N. Science 2009, 324 (5932), 1302−1305. (15) Yasuda, H.; Yoshimura, Y. Catal. Lett. 1997, 46 (1−2), 43−48. (16) Castillo, N.; Perez, R.; Martinez-Ortiz, M. J.; Diaz-Barriga, L.; Garcia, L.; Conde-Gallardo, A. J. Alloys Compd. 2010, 495 (2), 453− 457. (17) Adams, B. D.; Ostrom, C. K.; Chen, A. C. J. Electrochem. Soc. 2011, 158 (4), B434−B439. (18) Liu, Y.; Chi, M. F.; Mazumder, V.; More, K. L.; Soled, S.; Henao, J. D.; Sun, S. H. Chem. Mater. 2011, 23 (18), 4199−4203. (19) Lim, B.; Wang, J. G.; Camargo, P. H. C.; Cobley, C. M.; Kim, M. J.; Xia, Y. N. Angew. Chem., Int. Ed. 2009, 48 (34), 6304−6308. (20) Zhang, H.; Jin, M. S.; Wang, J. G.; Kim, M. J.; Yang, D. R.; Xia, Y. N. J. Am. Chem. Soc. 2011, 133 (27), 10422−10425. (21) Wang, L.; Nemoto, Y.; Yamauchi, Y. J. Am. Chem. Soc. 2011, 133 (25), 9674−9677. 3532

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

Chemistry of Materials

Article

(54) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6 (1), 15− 50. (55) Blochl, P. E. Phys. Rev. B 1994, 50 (24), 17953−17979. (56) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59 (3), 1758−1775. (57) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46 (11), 6671−6687. (58) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1993, 48 (7), 4978−4978. (59) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13 (12), 5188− 5192. (60) Methfessel, M.; Paxton, A. T. Phys. Rev. B 1989, 40 (6), 3616− 3621. (61) Lu, J. L.; Stair, P. C. Langmuir 2010, 26 (21), 16486−16495. (62) Aaltonen, T.; Ritala, M.; Sajavaara, T.; Keinonen, J.; Leskela, M. Chem. Mater. 2003, 15 (9), 1924−1928. (63) Aaltonen, T.; Rahtu, A.; Ritala, M.; Leskela, M. Electrochem. Solid-State Lett. 2003, 6 (9), C130−C133. (64) Elam, J. W.; Zinovev, A.; Han, C. Y.; Wang, H. H.; Welp, U.; Hryn, J. N.; Pellin, M. J. Thin Solid Films 2006, 515 (4), 1664−1673. (65) Goldstein, D. N.; George, S. M. Thin Solid Films 2011, 519 (16), 5339−5347. (66) Goldstein, D. N.; George, S. M. Appl. Phys. Lett. 2009, 95, 14. (67) Sanchez, S. I.; Small, M. W.; Zuo, J. M.; Nuzzo, R. G. J. Am. Chem. Soc. 2009, 131 (24), 8683−8689. (68) Bernardi, F.; Traverse, A.; Olivi, L.; Alves, M. C. M.; Morais, J. J. Phys. Chem. C 2011, 115 (25), 12243−12249. (69) Bando, K. K.; Kawai, T.; Asakura, K.; Matsui, T.; Le Bihan, L.; Yasuda, H.; Yoshimura, Y.; Oyama, S. T. Catal. Today 2006, 111 (3− 4), 199−204. (70) Lei, Y.; Jelic, J.; Nitsche, L. C.; Meyer, R.; Miller, J. T. Top. Catal. 2011, 54, 334−348. (71) Frenkel, A. I.; Hills, C. W.; Nuzzo, R. G. J. Phys. Chem. B 2001, 105 (51), 12689−12703. (72) Greeley, J.; Mavrikakis, M. Nat. Mater. 2004, 3 (11), 810−815.

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