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
■
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
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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.
■
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
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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.
■
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
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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.
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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
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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.
■
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.
■
AUTHOR INFORMATION
Corresponding Author
*Phone: 1-630-252-3520. E-mail: [email protected].
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The authors declare no competing financial interest.
■
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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