Rh-Pt bimetallic catalysts: Synthesis, characterization, and catalysis of core-shell, alloy, and monometallic nanoparticles

Article abstract of DOI:10.1021/ja8061425

Rh?Pt core-shell, RhPt (1:1) alloy, and Rh + Pt monometallic nanoparticles (NPs) were prepared using standard polyol reduction chemistry in ethylene glycol (EG) with standard inorganic salts and polyvinylpyrrolidine (PVP _550oo) stabilizers. PVP-free colloids were also prepared but less stable than the PVP-protected NPs. Rh?Pt core-shell particles were prepared from 2.7, 3.3, and 3.9 nm Rh cores with varying shell thicknesses (～1 and ～2 ML). The particles were characterized by a combination of TEM, single-particle EDS, EDS line scans, XRD analysis, Debye Function simulations, FT-IR, and micro-Raman CO-probe experiments. The three different architectures were evaluated for preferential oxidation of CO in hydrogen (PROX) using 1.0 wt % Pt loadings in Al₂0₃ supports. For hydrogen feeds with 0.2% CO and 0.5% 0₂ the Rh?Pt NP catalyst has the best activity with complete CO oxidation at 70 °C and very high PROX selectivity at 40 °C with 50% CO conversion.

Full text of DOI:10.1021/ja8061425

Published on Web 12/02/2008
Rh-Pt Bimetallic Catalysts: Synthesis, Characterization, and
Catalysis of Core-Shell, Alloy, and Monometallic
Nanoparticles
Selim Alayoglu and Bryan Eichhorn*
Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park,
Maryland 20742
Received August 5, 2008; E-mail: eichhorn@umd.edu
Abstract: Rh@Pt core-shell, RhPt (1:1) alloy, and Rh + Pt monometallic nanoparticles (NPs) were prepared
using standard polyol reduction chemistry in ethylene glycol (EG) with standard inorganic salts and
polyvinylpyrrolidine (PVP55000) stabilizers. PVP-free colloids were also prepared but less stable than the
PVP-protected NPs. Rh@Pt core-shell particles were prepared from 2.7, 3.3, and 3.9 nm Rh cores with
varying shell thicknesses (∼1 and ∼2 ML). The particles were characterized by a combination of TEM,
single-particle EDS, EDS line scans, XRD analysis, Debye Function simulations, FT-IR, and micro-Raman
CO-probe experiments. The three different architectures were evaluated for preferential oxidation of CO in
hydrogen (PROX) using 1.0 wt % Pt loadings in Al
.5% O
PROX selectivity at 40 °C with 50% CO conversion.
2
O
3
supports. For hydrogen feeds with 0.2% CO and
0
2
the Rh@Pt NP catalyst has the best activity with complete CO oxidation at 70 °C and very high
Introduction
dramatic effects on the catalytic activities. For example,
8
,9
1
-3
Mavrikakis et al. have shown that hydrogen-bond enthalpies
on Pt monolayers were reduced by as much as 0.4 eV depending
on the substrate, which makes them far more active for
hydrogenation than bulk Pt. In a study of Pt overlayers on
Ru(0001) hosts Jakob and co-workers elegantly showed how
the structural and electronic influences of the host could be
Rodriguez, Goodman, and others
performed detailed
studies of metal thin film overlayers deposited on bulk metallic
surfaces. The studies were designed to probe the electronic
structures and nature of metal-metal bonding between the metal
overlayers and the metallic substrates. The results showed that
work functions of metallic films were altered from their bulk
values due to the structural and electronic effects of the
underlying metal host. Through a combination of thermal
desorption studies and photoelectron experiments the studies
documented significant intermetallic electron transfers between
overlayer and substrate metals having different d-band electronic
populations. In general, it was shown that electron density was
transferred from those metals with filled d states to those with
less filled d states. These findings differ from the electronic
interactions in bulk intermetallics and alloys where electrone-
gativity considerations dictate the magnitude and direction of
3
experimentally differentiated. They showed that the electronic
influence of the host Ru metal on the Pt overlayer was
maximized with single monolayer films but quickly disappeared
after three or more monolayers of coverage. In contrast, the
structural influences of the substrate persisted to greater over-
3
layer thickness due to long-range pseudomorphic growth.
Translation of these results from bulk monolayer surface
structures to nanoparticles (NPs) with monolayer shells on
metallic cores (i.e., M@M′ core-shell NPs, see drawing) is
not completely obvious. For example, how will deviations from
bulk band structures and finite electronic states affect charge
transfer between the core and shell metals? How will the surface
strain of the shell of a core-shell NP compare to that of a bulk
thin film overlayer grown pseudomorphically on a faceted
substrate? Mavrikakis and co-workers developed remarkably
accurate “near surface alloy” theoretical models for describing
trends in activity, adsorbate binding, and mechanistic pathways
3
-8
electron transfer. More recent studies
have shown that
lowering the energy of the metal overlayer’s d-band center has
a significant impact on the interactions of the metal overlayer
with adsorbate molecules on the surface. The resulting changes
in activation energies and bond enthalpies of adsorbates have
9
(
(
(
1) Rodriguez, J. A.; Goodman, D. W. Science 1992, 257, 897–903.
2) Campbell, C. T. Annu. ReV. Phys. Chem. 1990, 41, 775–837.
3) Schlapka, A.; Lischka, M.; Gross, A.; Kasberger, U.; Jakob, P. Phys.
ReV. Lett. 2003, 91, 016101/1-016101/4.
on various core-shell structures. However, several approxima-
tions must be made in order to perform such detailed analysis
on these large, complicated systems. The approximations, such
as perfectly faceted substrates and uniform interfaces between
core and shell metal layers, may cause complications in
modeling nonuniform, multifaceted core-shell NPs made from
solution. It is clear that additional comparative experimental
investigations of core-shell and alloy NP systems are needed
(
(
4) Jakob, P.; Schlapka, A. Surf. Sci. 2007, 601, 3556–3568.
5) Rodriguez, J. A.; Campbell, R. A.; Goodman, D. W. J. Phys. Chem.
1
991, 95, 5716–5719.
(
(
(
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1
990, 94, 6936–6939.
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Technol. A 1991, 9, 1698–1702.
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J. AM. CHEM. SOC. 2008, 130, 17479–17486 9 17479
10.1021/ja8061425 CCC: $40.75  2008 American Chemical Society

A R T I C L E S
Alayoglu and Eichhorn
1
2
to unravel the underlying structural and electronic principles
governing the activities of bimetallic nanoparticles and help
refine modeling studies.
software package. The ATOMS program was used to generate
the spherical core/shell nanoclusters of desired Rh core sizes and
1
Pt shell thicknesses. Fractional atom coordinates in a P cubic lattice
of 100 Å cell size were simulated using the Debye function with
no symmetry constraints. Details of the simulations are given in
the Supporting Information.
For the IR-CO probe experiments the colloidal solutions of the
desired nanoparticle architectures were saturated with CO using a
stainless steel needle submerged in the solution and bubbling CO
at a flow rate between 20 and 40 sccm for ca. 20 min. A 100 µL
aliquot of CO-saturated colloidal solution was transferred into a
liquid IR cell and monitored in a Nexus 870 FT-IR spectrometer.
The liquid IR cell consisted of a 0.5 mm Teflon spacer sandwiched
While some bimetallic NP catalysts show expected and
somewhat systematic changes, other systems are less predictable
and have surprising activities. For example, CO oxidation
2
between two rectangular-shaped CaF windows. The colloidal
solution prior to CO saturation was used for the background spectra.
A Renishaw Raman Microscope was employed to measure the
micro-Raman spectra of CO-saturated nanoparticles. The nanopar-
ticle powders were dispersed on glass slides and then sealed in a
gastight Linkam temperature control stage. A 632.8 nm He:Ne or
x
catalyzed by Pt1-xRh alloy NPs show activities intermediate to
1
0
those of the Pt and Rh NP end members. In contrast,
preferential oxidation of CO in hydrogen (PROX) activities of
x
the Pt1-xRu NPs are much better than those of the Pt and Ru
end members due to synergistic effects of the alloy surfaces.
4
88 nm Ar ion laser was focused on the particles through 0.5 mm
Moreover, the Ru@Pt core-shell NPs are far superior to the
quartz window using Leica N plan L50×/0.50 objective lens. The
1
1
Pt1-xRu
x
alloy NPs and monometallic mixtures. To experi-
backscattered light was monitored with a resolution of 20 expo-
-1
mentally probe the importance of architecture in bimetallic NP
systems we embarked on a systematic evaluation of bimetallic
particles in different architectural configurations, namely,
core-shell, alloy, and mixed monometallic NPs (see drawing).
We report here the full synthetic, spectroscopic, and structural
characterization of Rh@Pt core-shell NPs with different core
sizes and shell thicknesses with and without surfactant stabiliz-
ers. In addition, our preliminary catalysis studies show that the
Rh@Pt core-shell NPs are superior to the alloy and monome-
tallic mixtures for PROX applications. Full catalytic evaluations
and theoretical studies will be reported in a subsequent
publication.
sures/s between 300 and 2400 cm . The particles were dosed with
CO or CO/air mixtures for ca. 20 min. The spectra were recorded
under flowing gas streams. Both 488 and 633 nm lasers yielded
spectra with identical peak positions but different peak intensities.
Only the 633 nm data are presented here.
TEM samples were prepared directly from the reaction solutions
by diluting 20 µL of colloids to ∼2 mL with deionized water. A 3
µL amount of such mixtures were drop cast and dried on continuous
carbon film-deposited copper grids. A JEM 2100 LaB6 TEM
operating at 200 kV was used for both low- and high-resolution
imaging. A Jeol 2100F Field Emission Transmission Electron
Microscope (FE-TEM) equipped with an Inca Energy Dispersive
Spectrometer (EDS) was used for line scans and point spectra of
nanoparticles. The FE-TEM was operated at 200 kV and in the
scanning mode.
Experimental Section
2 3
The catalysts were prepared by adding γ-Al O to colloidal
All reactions were carried out under N
2
atmosphere using
suspensions of nanoparticles and drying the slurry under vacuum.
In a typical preparation, 10 mL of Rh@Pt nanoparticle colloidal
suspension and 973 mg of γ-Al O were mixed overnight and
standard Schlenk line techniques on a Fisher Scientific Isotherm
hot plate stirrer with a temperature control unit using a Teflon-
2
3
coated K-type thermocouple. Chemicals, PtCl
3.09%), Pt(acac) (Strem, 98%, acac ) acetylacetonate),
Rh(NO ·2H O (Alfa Aesar, 99.9% pure, Rh 31.1%), RhCl
Engelhard, Rh 39.46%), Rh (CO) Cl (Fluka, >97%), polyvi-
nylpyrrolidine (Aldrich, typical M ) 55 000), ethylene glycol
VWR, H O > 0.02%), acetone (Pharmco Aaper, HPLC-UV grade),
ethanol (Pharmco Aaper, 200 proof), and γ-Al (Alfa Aesar,
9.97% metal basis) were purchase and used as received.
The colloidal suspensions described below were diluted with
2
(Engelhard, Pt
vaccuum dried at ca. 100 °C while vigorously stirring. Such
composition yielded a 1.0% by weight Pt alumina-supported
catalyst. The catalyst was washed with acetone several times with
equivolume mixtures of acetone and ethanol and then baked at 60
°C overnight. Catalysis runs were carried out using 105 mg of
catalyst charges in all cases. A standard fixed bed flow-through
7
2
3
)
3
2
3
(
2
4
2
w
(
2
11,13,14
2
O
3
reactor was employed and has been described elsewhere.
An
9
inlet velocity for gases of 0.21 m/s and a total flow rate of 400
NmL/min was employed. The gas hourly space velocity (GHSV)
5
-1 -1
acetone and centrifuged at 6000 rpm using a Hermle Z 300. The
supernatants were clear. The precipitates were washed with acetone
and acetone-ethanol mixture upon sonication using a Fisher
Scientific FS30H sonicator bath. The cycles of dilution by
acetone-ethanol mixtures, sonication, and centrifugation were
repeated 4-5 times. The precipitates were dried in open air prior
to characterization.
A Bruker C2 Discover (Parallel Beam) General Area Diffraction
Detection (GADDS) system was used for measurement of powder
diffraction profiles. A monochromatic Cu KR radiation source (40
mV and 40 mA) was employed along with a Bruker ACS Hi-Star
detector. The diffraction patterns were acquired between 33° and
was calculated to be 2.3 × 10 mL g
h
with a corresponding
residence time of ca. 35 ms. The gas mixture for the PROX reaction
was composed of 0.2% CO (99.5% pure, Al tank), 0.5% O2
(99.999% pure), 50% H (99.999% pure), and the balance Ar
2
(99.999% pure). Catalytic performance was evaluated by way of
temperature-programmed reactions (TPR) between room temper-
ature and 200 °C with a heating ramp rate of 1.8 °C/min. The gases
were introduced to the reactor using carefully calibrated mass flow
controllers. The gas products were monitored online using a Prima
δB mass spectrometer. Supported catalysts were used “as prepared”
and cycled through several TPR PROX runs (1-5 times) to attain
optimal activity.
9
0° by integrating four frames with 14° 2θ per frame. The samples
Synthesis. 2.7 nm Rh NPs. (1) Method 1. In a typical synthesis,
were oscillated in the xy axis to homogenize the diffracting grains.
33.0 mg of Rh(NO ) ·2H O was dissolved in 20 mL of ethylene
3
3
2
Powder diffraction patterns were simulated using the DISCUS
(
12) Proffen, T.; Neder, R. B. J. Appl. Crystallogr. 1997, 30, 171–175.
(
(
10) Park, J. Y.; Zhang, Y.; Grass, M.; Zhang, T.; Somorjai, G. A. Nano
Lett. 2008, 8, 673–677.
(13) Zhou, S. G.; McIlwrath, K.; Jackson, G.; Eichhorn, B. J. Am. Chem.
Soc. 2006, 128, 1780–1781.
(14) Zhou, S. H.; Jackson, G. S.; Eichhorn, B. AdV. Funct. Mater. 2007,
17, 3099–3104.
11) Alayoglu, S.; Nilekar, A. U.; Mavrikakis, M.; Eichhorn, B. Nat. Mater.
2
008, 7, 333–338.
1
7480 J. AM. CHEM. SOC. 9 VOL. 130, NO. 51, 2008

Rh-Pt Bimetallic Catalysts
A R T I C L E S
glycol (EG) in a 50 mL 3-neck round-bottom flask along with 28.0
mg of PVP55000. The Rh3 precursor salt was loaded in a dry box;
ramped to 130 °C with a heating rate of 1-2 °C/min and aged for
4 h. The colloidal suspension was quenched in an ice bath.
4.3 nm Rh@Pt NPs with 2 ML Thick Pt Shells. A 27.8 mg
+
all other transfers were done on a Schlenk line under N
2
atmospheres. The mixture was heated to ∼80 °C and kept
isothermal 10-15 min to dissolve the contents of the reaction
mixture. The light brown solution was brought to boil in EG and
2
amount of PtCl was dissolved in 10 mL of EG, and 10 mL of 3.3
nm Rh NPs suspension was added as described above. Pt was
deposited at 130 °C. The colloidal suspension was quenched in an
ice bath after 4 h of aging.
2
refluxed for ca. 90 min. in flowing N with vigorous stirring. The
solution turned black and colloidal at about 110 °C. The reaction
was quenched over ice. The colloids are stable for months without
any precipitation.
2.7 nm PVP-Free Rh@Pt NPs with ca. 1 ML Thick Pt
Shells. The same procedures were used as described above, except
the 2.2 nm PVP-free Rh colloids were used, the deposition
temperature was 130 °C, and the deposition time was 2 h. A slow
temperature ramp of ∼1 °C/min was employed for Pt deposition.
The PVP-free colloids were stable for only short periods of time
and precipitate over 2 h unless stirred.
(
2) Method 2. In a typical reaction, 53.3 mg of RhCl
mg of PVP55000 were dissolved in 10 mL of EG in a 50 mL 2-neck
round-bottom flask at 80 °C in flowing N . In a separate 100 mL
-neck flask 30 mL of EG was heated to 100 °C in flowing N
3
and 56.0
2
3
2
.
Approximately 40 mg of granular NaBH
4
was then added into hot
PtRh (1:1) Alloy NPs. A 40.0 mg amount of Pt(acac)
2
and 19.6
EG, and the mixture was heated to ∼150 °C. The 80 °C RhCl
solution was then syringed into the 150 °C NaBH solution. The
resulting mixture instantly turned black colloidal and was refluxed
for about 90 min in flowing N . The reaction was quenched on ice.
Rh NPs colloids are stable for months without any precipitation.
.7 nm Pt NPs. In a typical synthesis, 54.0 mg of PtCl and
5.0 mg of PVP55000 in 40 mL of EG were refluxed for 1 h. To
3
mg of Rh (CO) Cl salts were dissolved in 20 mL of EG along
2
4
2
with 55.0 mg of PVP55000. The temperature was ramped to a boil
in less than 10 min. The solution turned black colloidal at about
4
1
70 °C. The colloidal suspension was refluxed for about 2 h and
2
quenched over ice.
5
2
5
Results
prepare the physical mixture, the monometallic colloidal suspensions
of Pt and Rh NPs were mixed and stirred overnight.
In this study, monometallic NPs of Rh and Pt were prepared
15
by modifications of known polyol methods. The Rh NPs were
used directly in catalytic evaluations and also as the core “seeds”
in the preparation of Rh@Pt core-shell particles. Uniform 2.7
nm Rh NPs were prepared by reducing Rh(NO ) ·2H O in
3
3
.3 and 3.9 nm Rh NPs. Typically, 20.3 mg of RhCl was
dissolved in 8 mL of Rh NP colloidal suspension (see Method 1
above) and diluted to 16 mL with EG. The mixture was heated to
8
2 3
0 °C in flowing N to fully dissolve the RhCl . The mixture was
3
3
2
then heated to 130 °C and aged for 4.5 h at 130 ( 2 °C. The reaction
was quenched by immersing the flask in an ice bath, yielding 3.3
nm Rh NPs. Larger 3.9 nm Rh NPs were synthesized as described
above, except 41.0 mg of RhCl was dissolved in 16 mL of EG
3
and mixed with 8 mL of Rh colloidal suspension (Method 1) and
ethylene glycol in the presence of PVP stabilizers. Changing
the temperature and/or PVP:Rh ratio did not significantly affect
the resulting NP sizes or size distributions. A second method
for making 2.7 nm Rh NPs from RhCl was also employed to
3
make catalysts and seeds for subsequent growth studies. While
both methods gave NPs that were structurally and spectroscopi-
the mixture was aged at 130 ( 2 °C for 4 h.
2
3 3
.2 nm PVP-Free Rh NPs. A 49.0 mg amount of Rh(NO ) ·
3
cally identical, the RhCl method produced superior catalysts.
2
2
H
2
O and 15.4 mg of Rh
2 4 2
(CO) Cl were transferred into a 50 mL
Chloride or other impurities were not detected by EDS in any
of the Rh NPs. To prepare larger Rh NPs, a sequential growth
method was employed in which additional Rh layers were
deposited over preformed Rh seeds at temperatures that did not
favor self-nucleation. The procedure is similar to that described
-neck round-bottom flask in a dry box. The precursor salts were
dissolved in 22.5 mL of EG on a Schlenk line under positive N
pressure. The solution was slowly brought to a boil and aged over
0 min with vigorous stirring. The reaction was quenched over ice.
2
9
Some degree of aggregation/precipitation occurred after the reaction
was quenched, but the colloidal suspension was restored with
stirring.
by Somorjai and Tilley but with different precursors and less
16,17
PVP.
3
The less oxidizing RhCl precursor was used for the
2
2 6
.2 nm PVP-Free Pt NPs. A 126.1 mg amount of H PtCl was
sequential growth step. By controlling the stoichiometry,
uniform Rh NPs of 3.3 or 3.9 nm were prepared. Representative
TEM images and particle size histograms are found in Figures
a and S1, Supporting Information.
Pt shells were deposited on the Rh seeds to generate Rh@Pt
dissolved in 22 mL of EG. The yellow solution was slowly ramped
to 130 °C and aged for 90 min to give a give a black colloidal
suspension. The colloidal suspension of Pt NPs was aged for an
additional 30 min at 180 °C and then quenched in an ice bath. The
colloids were not stable in suspension and precipitated after 2 h at
room temperature.
1
2
core-shell NPs using a PtCl precursor and similar seeded
1
8
3
.2 nm Rh@Pt NPs with 1 ML Thick Pt Shells. In a typical
synthesis, 14.0 mg of PtCl was dissolved in 10 mL of 2.7 nm Rh
NP colloidal suspension and charged with an additional 10 mL of
EG. The mixture was stirred at ∼60 °C to dissolve the PtCl . The
solution was brought to 130 °C with a temperature ramp of ∼2
C/min and aged for 4 h. The reaction was quenched over ice. The
growth techniques. To achieve a desired shell thickness the
2
PtCl concentrations were adjusted to accommodate the size of
2
the Rh core particles. The stoichiometric ratios were calculated
by employing a modified form of Schmid’s Magic Number
2
algorithm for cubeoctahedral clusters and the density of the FCC
°
19-22
metal (see Table S1, Supporting Information).
The Pt shells
Rh@Pt nanoparticle colloids were not stable in solution and
precipitated after 24 h.
5
.1 nm Rh@Pt NPs with 2 ML Thick Pt Shells. In a typical
reaction, 47.1 mg of PtCl was dissolved in 10 mL of EG. A 10
mL amount of 3.9 nm Rh NP colloidal suspension was then added
under flowing N atmosphere at room temperature. The mixture
was heated to 80 °C to ensure dissolution of the PtCl . The
temperature was then ramped to 130 °C at a ramping rate of ∼1
C/min. The mixture was aged at 130 ( 3 °C for 4.5 h and then
quenched to room temperature in an ice bath.
.4 nm Rh@Pt NPs with 1 ML Thick Pt Shells. A 17.7 mg
amount of PtCl was dissolved in 10 mL of EG. A 10 mL amount
of 3.9 nm Rh NPs suspension was added. Finally, the mixture was
(15) He, B. L.; Chen, Y. X.; Liu, H. F.; Liu, Y. J. Nanosci. Nanotechnol.
2
005, 5, 266–270.
2
(
(
16) Humphrey, S. M.; Grass, M. E.; Habas, S. E.; Niesz, K.; Somorjai,
G. A.; Tilley, T. D. Nano Lett. 2007, 7, 785–790.
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Lett. 2005, 5, 435–438.
2
2
(18) Lima, F. H. B.; Gonzalez, E. R. Appl. Catal. B: EnViron. 2008, 79,
3
41–346.
°
(
(
(
19) Schmid, G. Nanostruct. Mater. 1995, 6, 15–24.
20) Schmid, G. Chem. ReV. 1992, 92, 1709–1727.
4
21) Martin, T. P.; Bergmann, T.; Gohlich, H.; Lange, T. J. Phys. Chem.
2
1
991, 95, 6421–6429.
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J. AM. CHEM. SOC. 9 VOL. 130, NO. 51, 2008 17481

A R T I C L E S
Alayoglu and Eichhorn
Figure 1. Representative TEM images of (a) 3.3 nm Rh NPs prepared by sequential deposition, (b) PVP-stabilized 5.1 nm Rh@Pt core/shell NPs comprising
.9 nm Rh cores and 2 ML Pt shells, and (c) PVP-free 3.3 nm Rh@Pt core/shell NPs comprising 2.2 nm Rh cores and 2 ML Pt shells. High-resolution
3
images of the particles in a-c are shown directly below each figure.
Figure 2. Representative STEM-EDS line spectra of (a) a 5.1 nm Rh@Pt NP with 3.9 nm Rh core and 2 ML Pt shell and (b) a 5 nm PtRh alloy nanoparticle.
Relative atomic % composition (vertical axis) of Pt (red) and Rh (blue) are plotted against the line scan probe position (horizontal axis) and given next to
STEM images. A 1.5 nm probe was used to trace 10-15 nm scans across each particle. The particle center is at ∼6.5 nm in a and ∼ 5.5 nm in b.
were also deposited at under-nucleation temperatures to avoid
self-nucleation of monometallic Pt NPs. Aging the colloidal
solutions up to 3-4 h was required to guarantee clear/colorless
alloy structures and avoided monometallic particle contaminates
and the graded alloy formation produced from other methods.
TEM analysis shows 4.9 nm particles but some shape anisotropy
(see Figure S3, Supporting Information).
2
supernatants with no PtCl deposits. Rh@Pt NPs were selec-
tively prepared from 2.7, 3.3, and 3.9 nm Rh cores with ∼1
and ∼ 2 ML Pt shells. Representative TEM images and particle
size histograms are found in Figures 1b and S1, Supporting
Information. The size evolutions of the core-shell particles are
in agreement with the calculated shell thicknesses and thus agree
well with expectations based on the Schmid model. While CO
probe experiments show that the core Rh particles are com-
pletely encapsulated by the Pt shells (see below), the monolayer
coverages described here will certainly have variations from
particle to particle due to core size dispersity and are only
intended to represent mean values. After Pt deposition the
colloids persisted for ca. 24 h and then began to agglomerate
and deposit on the bottom of the flask.
PVP-free samples of Rh and Rh@Pt NPs were also prepared
using different precursors and slightly different protocols. The
resulting particles had mean sizes and size distributions similar
to the PVP analogs but the particles agglomerated (see Figures
1
c and S2, Supporting Information) and the colloids persisted
for short time periods (ca. 1-2 h) before deposition as expected.
The NPs described above have been fully characterized by a
combination of TEM, single-particle EDS, EDS line scans, XRD
analysis, FT-IR, and micro-Raman CO probe experiments and
catalytic evaluation. These experiments are described below.
The composition and architecture (core-shell vs alloy) of
individual NPs were evaluated using a 1.5 nm EDS probe in a
JEOL 2100 FE TEM operating in the STEM mode. Multiple
single-particle EDS analysis of core-shell and alloy NPs from
various syntheses confirmed that all particles are bimetallic in
nature, and stray monometallic particles were not observed in
any of the samples. STEM-EDS line scans were recorded from
x 2
Pt1-xRh alloy NPs were prepared by coreduction of Pt(acac)
1
0,23,24
and Rh
2 4 2
(CO) Cl in EG solutions with PVP stabilizers.
We found that these precursors gave the most homogeneous
(
(
23) Bergounhou, C.; Blandy, C.; Choukroun, R.; Lecante, P.; Lorber, C.;
Pellegatta, J. L. New J. Chem. 2007, 31, 218–223.
5
.1 nm Rh@Pt NPs containing 2 ML Pt shells (Figure 2a). The
24) Harada, M.; Einaga, H. J. Colloid Interface Sci. 2007, 308, 568–572.
data clearly show the Pt M line with a bimodal Pt distribution
1
7482 J. AM. CHEM. SOC. 9 VOL. 130, NO. 51, 2008

Rh-Pt Bimetallic Catalysts
A R T I C L E S
Figure 3. (a) Ex-situ FT-IR spectra of CO-saturated colloidal suspensions sampled as a function of time during synthesis of the Rh@Pt NPs. The stacked
plots show (i) parent Rh core NPs at 0 min and (ii) Rh@Pt NPs in 10th min, (iii) 40th min, (iv) 160th min, (v) and 240th min. (b) FT-IR spectra of
CO-saturated colloidal suspensions comparing core-shell (red), 50:50 alloy (blue), and a mixture of monometallic Rh and Pt nanoparticles (green).
that reaches a maximum at the edge of the particle (i.e., the
shell), whereas the Rh L line shows maximum Rh concentration
at the center of the particle. STEM-EDS point spectra acquired
from different regions of a distinct nanoparticle show the highest
atomic % Pt at the edge with more atomic % Rh at the center
(
Figure S4, Supporting Information), which is consistent with
the EDS line scan. In contrast, the PtRh alloy NPs show single
Gaussian distributions of X-rays across the particle for both
elements as expected from the random arrangement of atoms
Figure 4. Micro-Raman (633 nm) spectra of CO-saturated PVP-free NPs
showing (a) the υ(M-C) region (M ) Pt, Rh) and (b) the υ(CO) region.
The data were recorded for 2.2 nm Pt NPs (black), 2.2 nm Rh NPs (green),
and 2.7 nm Rh@Pt NPs (red) in 2.5:1 air-to-CO gas mixtures.
(
Figure 2b). Definitive EDS line scan spectra could not be
obtained for the 3-4.5 nm Rh@Pt NPs due to limitations of
the 1.5 nm probe size. However, correlations of XRD and CO
probe experiments (see below) show that the smaller core-shell
particles have the same structure.
shell exists as small islands on the Rh core at short reaction
times, the FT-IR data suggest that the sorbed CO on the two
surfaces are not strongly coupled and give rise to separate υ(CO)
modes. This observation is in contrast to spectra for the PtRh
alloy NP colloids that show a single, atop υ(CO) peak that is
intermediate to those on monometallic Pt and Rh (Figure 3b).
Importantly, the core-shell architecture is conveniently distin-
guished from both the alloy and the monometallic rhodium
phases by FT-IR CO probe experiments. As the Rh cores
increase in size and/or thicker Pt shells are deposited, the Rh@Pt
To evaluate the surface structures of the various NPs we
2
5-27
employed CO-probe experiments
as a qualitative tool to
differentiate Pt from Rh on the surface of the particles. CO is
a well-known and long-studied molecular probe and particularly
convenient to monitor by FT-IR and micro-Raman spectroscopy.
The core/shell architecture with only one type of surface atom
can qualitatively be distinguished from other bimetallic archi-
tectures, namely, alloys and physical mixtures, which have both
elements on the NP surface. CO-saturated colloidal Rh suspen-
sions give rise to two features in their FT-IR spectra. The strong
broad peak at 2034 cm- is assigned to CO in the atop position
-1
atop mode is blue shifted from 2061 to 2073 cm but is always
distinct from the alloy atop υ(CO) mode, which appears at
-1
2055-2058 cm . As such, the IR experiments provide a
connection between small particles for which TEM line scans
are not possible and the larger particles where line scans clearly
distinguish the core-shell architecture from the alloy. The blue
shift of the atop mode may be due to differential faceting of
the Pt shell as the particles become larger or lateral effects
associated with altered equilibrium CO coverages. Interestingly,
the FT-IR spectra for the PVP-free Rh@Pt core-shell NPs are
virtually identical to those of PVP-protected particles (Figure
S6b, Supporting Information), suggesting that PVP does not
significantly affect the equilibrium CO coverage or the electronic
structure of the surface.
1
-1
and the lower intensity band at 1940 cm to bridging CO on
Rh (see Figure 3). PVP-free Rh NPs have identical spectra. Gem
dicarbonyl modes were not observed in any of our colloidal
2
8-30
Rh NPs.
The CO-probe technique has been used to spectroscopically
monitor formation of the Pt shell in the synthesis of the Rh@Pt
NPs. The colloidal solutions were sampled at various times in
the reaction, saturated with CO, and evaluated by FT-IR (Figure
3
). The υ(CO) bands associated with the Rh NPs decrease in
intensity as the terminal and bridging CO bands associated with
CO sorbed to Pt surfaces emerge. After 4 h of reaction CO bands
associated with exposed Rh surfaces have disappeared and only
Pt-CO modes are observed. Assuming that the incomplete Pt
Micro-Raman spectra of Rh@Pt, Rh, and Pt NPs saturated
with CO are shown in Figure 4. Raman analyses of PVP-coated
NPs were not extensively studied due to formation of PVP-
37
derived graphite-like deposits that diminish CO signal intensi-
ties and obscure the M-C regions of the spectrum. The Raman
data described below were collected from PVP-free samples
under flowing CO atmospheres. Maximum peak intensities were
observed with 2.5:1 air:CO gas mixtures, but the peak positions
were the same in pure CO. Peaks in both regions of the spectrum
disappeared when the CO atmospheres were removed. Mono-
(
(
25) Wang, Y.; Toshima, N. J. Phys. Chem. B 1997, 101, 5301–5306.
26) Dassenoy, F.; Casanove, M. J.; Lecante, P.; Pan, C.; Philippot, K.; Amiens,
C.; Chaudret, B. Phys. ReV. B 2001, 63, 235407/1–235407/7.
(
27) Pan, C.; Dassenoy, F.; Casanove, M. J.; Philippot, K.; Amiens, C.;
Lecante, P.; Mosset, A.; Chaudret, B. J. Phys. Chem. B 1999, 103,
1
0098–10101.
(
(
28) Pasztor, M.; Solymosi, F. Magy. Kem. Foly. 1986, 92, 321–328.
29) Vantblik, H. F. J.; Vanzon, J. B. A. D.; Huizinga, T.; Vis, J. C.;
Koningsberger, D. C.; Prins, R. J. Am. Chem. Soc. 1985, 107, 3139–
metallic Pt NPs give rise to two peaks, namely, a sharp peak at
-1
2
4
095 cm assigned to υ(CO) on atop Pt sites and a broader
1
3
147.
-
80 cm peak due to the υ(Pt-C) mode (Figure 4). Similarly,
Rh NPs give rise to two υ(CO) features at 2060 and ∼1920
(
30) Siepen, K.; Bonnemann, H.; Brijoux, W.; Rothe, J.; Hormes, J. Appl.
Organomet. Chem. 2000, 14, 549–556.
J. AM. CHEM. SOC. 9 VOL. 130, NO. 51, 2008 17483

A R T I C L E S
Alayoglu and Eichhorn
Figure 5. XRD profiles comparing (a) Rh NPs via polyol reduction of Rh(NO
3
)
3
2
·2H O to give (bottom) 2.7 nm as-synthesized Rh NPs, (middle) 3.3 nm
Rh NPs, and (top) 3.9 nm Rh NPs grown from sequential deposition. (b) XRD profiles of Rh@Pt core-shell (blue pattern) and (1:1) PtRh alloy (purple
pattern) NPs of similar particle diameters. The inset shows an enlargement of the 111 and 200 reflections. JCPDS lines for Pt (black), Rh (green), and 1:1
PtRh alloy (purple) are also shown.
cm- for linear atop CO and bridging CO, respectively. In
1
(see light blue profile in Figure 6a). However, this shift does
not result from PtRh alloy formation but instead is a conse-
quence of mass-averaged diffraction effects associated with the
core-shell structure. The IR CO-probe data for these 4.4 nm
Rh@Pt NPs clearly show a pure Pt surface and are inconsistent
with alloy formation (see Figure S5, Supporting Information).
To further illustrate this point we performed Debye function
(DF) simulations of model core-shell systems generated from
idealized spherical particles of FCC metals with the desired core
-1
addition, a very broad 440 cm peak for the υ(Rh-C) mode
is present. The Rh@Pt core-shell NPs yield features that are
characteristics of Pt surfaces, namely, broadened peaks at 2090
and 480 cm , which are assigned to linear υ(CO) and υ(M-C)
bands, respectively, and may be indicative of electronically
altered Pt surfaces.
Powder X-ray diffraction analysis (XRD) coupled with Debye
function simulations (DF) provides an effective means of
differentiating alloys from core-shell NPs and monitoring shell
growth in multilayer particles. Powder diffraction profiles of
as-made 2.7 nm Rh NPs exhibit strong broad 111 reflections
centered ca. 41° (Figure 5a). The 200 reflection and other high-
angle peaks are weak, but metallic Rh is evident. Figure 5a
shows that crystallinity increases with increasing size of the
Rh NPs; however, the peaks remain broad and denote disorder.
The core-shell and alloy NPs of identical composition and
similar size are easily differentiated by their XRD patterns
-1
3
1-33
and shell configurations.
Details of the models are given
in the Supporting Information. The simulated powder diffraction
patterns were calculated from nondistorted model particles using
1
2
the DF sub-routine in the DISCUS software package. The
simulations are shown in Figure 6b and qualitatively show
systematic peak shifts toward higher angles as the core size
increases and/or the shell thickness decreases. For example, two
Rh@Pt NPs with 3.9 nm Rh cores are shown in Figure 6. Those
particles with 2 ML Pt shells (dark blue) show diffraction peaks
that are almost perfectly aligned with bulk Pt, whereas those
with 1 ML Pt shells (light blue) are noticeably shifted toward
Rh peak positions in both the simulated and the observed XRD
patterns. The quantitatiVe differences in peak positions most
likely result from lattice distortions (primarily in the shell)
associated with the pseudoepitaxial growth of thin Pt shells on
the particle cores.
(
Figure 5b).
XRD analyses of core-shell particles are hindered by
similarities of the FCC unit cells of the two metals (Pt, a )
3
.916 Å; Rh, a ) 3.814 Å). However, discernible shifts in peak
positions in both experimental and simulated diffraction patterns
provide nonintuitive insight into the diffraction behavior of
core-shell bimetallic NP structures. In particular, the data show
that (1) the diffraction patterns are dominated by the thin (1-3
ML) Pt shells, (2) shifts in the XRD profiles can be accurately
described through DF simulations, and (3) DFA/XRD analysis
can be used to evaluate changes in shell thicknesses in particles
having the same core size. These findings are described below.
The Rh@Pt NPs all show a single 111 FCC peak centered at
ca. 40° for which distinct Pt and Rh phases cannot be discerned
due to the inherent broadness of the diffraction patterns (Figure
As in other core-shell and alloy NP systems multiple
analytical methods are required to fully characterize and
differentiate the different architectural configurations. While
STEM-EDS line scans shown in Figure 2 clearly differentiate
the 5.1 nm Rh@Pt core-shell NPs from the 4.9 nm (1:1) PtRh
alloy NPs, the smaller catalytically relevant NPs cannot be
evaluated through line scans due to limitations of the probe size.
However, the micro-Raman data and comparisons of IR-CO
probe experiments and XRD data show that the small particles
have the same core-shell architectures as the larger particles
evaluated in Figure 2. Moreover, the activities of the core-shell
and alloy particles are distinguished in the catalytic evaluations
described below.
6
a). While the peak positions of the Rh@Pt core-shell particles
are dominated by Pt diffraction, the peaks shift toward Rh peak
positions as the mass % Rh increases. For example, as the Rh
cores become larger with Pt shells of the same size, the
diffraction peaks shift toward those of bulk Rh. Likewise, as
the Pt shell thickness is increased on Rh cores of the same size
the diffraction peaks shift further toward those of bulk Pt. In
certain cases, such as the 4.4 nm Rh@Pt NPs comprising 3.9
nm Rh cores and 1 ML thick Pt shells, the XRD patterns
approach the peak positions of the bulk (1:1) PtRh alloy phase
(
(
31) Bonnemann, H.; Britz, P.; Vogel, W. Langmuir 1998, 14, 6654–6657.
32) Reetz, M. T.; Winter, M.; Breinbauer, R.; Thurn-Albrecht, T.; Vogel,
W. Chem. Eur. J. 2001, 7, 1084–1094.
(33) Vogel, W. Cryst. Res. Technol. 1998, 33, 1141–1154.
1
7484 J. AM. CHEM. SOC. 9 VOL. 130, NO. 51, 2008

Rh-Pt Bimetallic Catalysts
A R T I C L E S
Figure 6. (a) XRD profiles and (b) DFA simulations of Rh@Pt NPs of various core diameters and shell thicknesses: (red) 2.7 nm Rh core, 1 ML Pt shell;
reddish brown) 2.7 nm Rh core, 2 ML thick Pt shell; (light blue) 3.9 nm Rh core, 1 ML Pt shell; (orange) 3.3 nm Rh core, 2 ML Pt shell; (dark blue) 3.9
nm Rh core, 2 ML Pt shell. JCPDS peak positions for Pt (black), 1:1 RhPt alloy (purple), and Rh (green) are also presented.
(
Figure 7. TPR plots showing (a) H
particles, 4.9 nm Pt50Rh50 alloy NPs, and 3.2 nm Rh@Pt NPs for H
calculated from the limiting reactant, O , and % of maximum CO formation is relative to the CO inlet concentration. See text for details.
2
O formation and (b) CO
2
formation for monometallic Pt NPs, monometallic mixtures of Pt NPs, and 2.7 nm Rh
2
streams contaminated with 2000 ppm CO. Percent of maximum H O formation was
2
2
2
To compare and contrast the activity of the core-shell NPs
Rh@Pt catalysts the temperature for complete CO conversion
varied from 60 to 90 °C with H light-off varying from 35 and
60 °C (see Supporting Information, Figure S7). In contrast, CO
and H oxidation occur almost simultaneously for the mono-
metallic mixture, which gives very poor PROX selectivities
along with low activities. For reference, our pure Pt NP catalysts
with that of the alloys and monometallic NPs we evaluated the
2
PROX reaction (preferential oxidation of CO in H
feeds contaminated by 0.2% CO by volume along with 0.5%
. The Pt-Rh catalysts were supported on γ-Al , and each
2 2
) using H
2
O
2
O
2 3
were loaded with 1.0 wt % Pt and equimolar Pt:Rh ratios. The
reactions were conducted in a fixed-bed, flow-through reactor,
and catalytic activity was monitored by way of temperature-
under these conditions show CO and H
2
oxidation onsets (light
off) at 155-160 and 175-180 °C, respectively, which is
3
4
programmed reactions (TPR) measuring the evolution of H
and CO . The results are shown in Figure 7. The maximum
CO formation (100% in Figure 7) is based on the 0.2% inlet
CO concentration. The maximum H O formation was calculated
from the limiting reactant O , so that 100% H O formation
would result in conversion of all O to water. Using the gas
mixtures described above, complete conversion of CO to CO
followed by hydrogen oxidation with the remaining O would
result in an 80% H O formation.
2
O
consistent with literature reports. While the monometallic
mixture of Rh + Pt shows somewhat better PROX activity (160
°C complete CO oxidation) than that of pure Pt (190 °C
complete CO oxidation), it is far less active than the PtRh alloy
(105 °C complete CO oxidation) or Rh@Pt core-shell (70 °C
complete CO oxidation) catalysts under identical conditions and
loadings (Figure 7). In particular, the best Rh@Pt core-shell
NP catalyst shows complete CO oxidation by 40 °C and a
2
2
2
2
2
2
2
2
2
2
subsequent H oxidation that is complete by 80 °C with
The TPR data for the core-shell Rh@Pt NPs with ca. 1 ML
thick Pt shells, (1:1) PtRh alloy NPs, and monometallic mixtures
of Pt and Rh NPs are shown in Figure 7. In general, CO
exceedingly high PROX selectivity at room temperature (see
Figure S7, Supporting Information). However, a representative
catalyst with activity that is intermediate in the four independent
oxidation significantly precedes H oxidation for the core-shell
2
and alloy catalysts, which gives rise to high PROX selectivities
at relatively low temperatures. For the four different batches of
(
34) Minemura, Y.; Ito, S.; Miyao, T.; Naito, S.; Tomishige, K.; Kunimori,
K. Chem. Commun. 2005, 1429–1431.
J. AM. CHEM. SOC. 9 VOL. 130, NO. 51, 2008 17485

A R T I C L E S
Table 1. Comparative PROX Selectivities for the Different Rh-Pt and Ru@Pt Core/Shell Catalysts
Alayoglu and Eichhorn
a
b
c
d
e
d
f
d
T
50(5(O
2
), °C
selectivity at T50(5
T
50(5(CO
2
), °C
selectivity at T50(5
T
comp(0.1(O
2
), °C
selectivity at Tcomp(1
Ru@Pt (1:1)
Rh@Pt (1:1)
Rh@Pt (1:2)
Pt + Rh
66 ( 4
79 + 4
44 ( 6
163
74 ( 3
90 ( 3
99.8 ( 0.1
76
45 ( 4
65 ( 3
83 ( 1
95.6 ( 0.4
99.6 ( 0.3
79 ( 1
178 ( 3
136 ( 1
122 ( 1
192 ( 7
215 ( 3
40.4 ( 0.4
49.7 ( 0.1
48.3 ( 0.1
47.0 ( 0.5
46.2 ( 0.3
33 ( 4
162 ( 3
176 ( 3
Pt50Rh50
174 ( 6
55.2 ( 0.4
55.2 ( 0.1
a
b
c
d
PROX feed is composed of 1% CO, 1% O
2
, 50% H
2
, and an Ar balance. See ref 11. Temperature of 50 ( 5% O
2
consumption. Selectivity is
defined as [ꢀCO2/(ꢀCO2 + ꢀH2O)] × 100 where ꢀCO2 is the fraction of O used to convert CO to CO
O. Temperature of 50 ( 5% CO conversion. Temperature of complete (99.9 ( 0.1%) O
estimated due to abrupt light off (see Figures S8 and S9 in the Supporting Information for TPR data).
2
2
and ꢀH2O is the fraction of O
2
used to convert H
consumption. Temperatures and selectivities were
2
to
e
f
g
H
2
2
batches tested is shown in Figure 7 for comparison. A full
analysis of the PROX activity under different conditions,
comparisons with other core-shell particles, and a theoretical
analysis of these systems will be described in a subsequent
publication.
mechanisms of reaction in which different atoms catalyze
different steps (e.g., the bifunctional mechanism).
A comparison of the PROX activities and selectivities for a
3
5,36
1% CO, 1% O , and 50% H feed of the different Rh-Pt
2
2
1
1
catalysts and the Ru@Pt catalyst is given in Table 1. While
the activities of the Rh@Pt NPs toward PROX are better than
the RhPt alloy, the differences are not as dramatic as the Ru/Pt
Discussion
11
Through the use of various precursors and protocols PtRh
bimetallic nanoparticles with different architectures (i.e.,
core-shell, alloy, and mixtures of monometallic NP) can be
prepared selectively. Combinations of analytical techniques are
required to identify and characterize the different configurations,
especially for small bimetallics where EDS line scans are not
feasible. The complete architectural series of Rh and Pt
bimetallic NPs represents the third such complete series along
system. Although it is clear that the Pt-Rh bimetallic particles
have a clear synergistic effect for PROX activity, the enhance-
ment of the core-shell structure is far less pronounced in the
present case. However, the presence of room-temperature CO
oxidation in all of the Rh@Pt systems tested and lack of such
activity for the alloys clearly demarcates the differences in the
architectures.
Finally, we note that PVP-free NPs of various architectures
are easily accessible by the methods described herein; however,
the stability of the colloids is limited, and the resulting particles
show significant agglomeration (see Figure 1c). While the FT-
IR CO probe data of the PVP-protected and PVP-free NPs are
virtually identical, the Raman signals of the CO-saturated PVP-
free NPs are significantly more intense. The lack of graphitic
1
3,14
11
with the PtAu
and PtRu systems. While the sizes,
compositions, and projected surface areas of the NPs within
these three series are quite similar, the catalytic activities are
markedly different. For both the PtRu and PtRh bimetallic
systems the core-shell particles were most active for PROX
followed by the alloys, the mixture of monometallic NPs, and
finally the pure Pt NPs. The enhancements in activities are
directly related to the changes in architectural configurations,
which alter the electronic structures of the surface metals and/
or facilitate alternate reaction mechanisms relative to pure Pt.
However, the changes in activities for the different Rh-Pt
37
deposits associated with PVP degradation may explain the gain
in intensity of the latter. Catalytically, the PVP-free particles
did not perform as well as the PVP-protected materials due to
agglomeration.
1
1
Acknowledgment. This material is based upon work supported
by the National Science Foundation under grant no. CHE0401850
and the DOE HFI program under grant no. DE-FG02-05ER15731.
We thank Dr. Wen-An Chou for assistance with TEM data
collection and gratefully acknowledge the NSF MRI for funding
through grant 0619191.
architectures are less dramatic than the Ru-Pt systems, but
the PROX selectivities are better (see Table 1). Mavrikakis has
8
,9
shown that core-shell particles containing Pt shells on Rh
and Ru cores have lower equilibrium CO coverages and lower
Pt-H bond enthalpies relative to pure Pt, which explains the
2
enhancements in CO and H oxidation kinetics. These differ-
ences in substrate binding are correlated with shifts in the Pt
d-band center due to charge transfer between the Pt shell and
the core metals. The situation is directly analogous to the thin
Supporting Information Available: Additional TEM data and
particle size histograms, core-shell synthetic charts, IR data,
and details of the diffraction models. This material is available
free of charge via the Internet at http://pubs.acs.org.
5
-7
film studies of Goodman and Campbell
in which electron
transfer between metal overlayers and their metallic hosts altered
the metal-metal interactions and surface electronic structures.
The authors noted that the electronic charge transfer in the
overlayer structures differed from that in the bulk alloys,
suggesting that the Rh-Pt electronic interactions in Rh@Pt NPs
and RhPt alloy NPs may also be different. In addition to possible
differences in electronic structure, the alloy NPs also have two
types of surface atoms and can therefore promote alternate
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7486 J. AM. CHEM. SOC. 9 VOL. 130, NO. 51, 2008