Iowa State University
From the SelectedWorks of Wenyu Huang
2011
Rh1−xPdxnanoparticle composition dependence
in CO oxidation by oxygen: catalytic activity
enhancement in bimetallic systems
James Russell Renzas, University of California - Berkeley
Wenyu Huang, University of California - Berkeley
Yawen Zhang
Michael E. Grass
Dat Tien Hoang, University of California - Berkeley, et al.
Available at: http://works.bepress.com/wenyu_huang/9/
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Rh1 xPdx nanoparticle composition dependence in CO oxidation
by oxygen: catalytic activity enhancement in bimetallic systems
James Russell Renzas,ab Wenyu Huang,ab Yawen Zhang,bc Michael E. Grass,de
Dat Tien Hoang,a Selim Alayoglu,a Derek R. Butcher,af Franklin (Feng) Tao,waf
Zhi Liud and Gabor A. Somorjai*af
Received 18th September 2010, Accepted 2nd December 2010
DOI: 10.1039/c0cp01858a
Bimetallic 15 nm Rh1 xPdx nanoparticle catalysts of five different compositions and supported
on Si wafers have been synthesized, characterized using TEM, SEM, and XPS, and studied in CO
oxidation by O2 in two pressure regimes: atmospheric pressure and 100–200 mTorr. The RhPd
bimetallic nanocrystals exhibited similar synergetic effect of increased reaction activity at both
atmospheric (760 Torr) and moderate (100–200 mTorr) pressures compared with pure Pd or Rh.
The magnitude of the effect depends on the relative pressures of the CO and O2 reactant gases
and the reaction temperature. The catalytic activity of the nanocrystals measured at moderate
pressure is directly correlated to the APXPS studies, which were carried out in the same pressure.
The APXPS studies suggest that the Pd–Rh interfaces are important for the enhanced activity
of the bimetallic nanoparticles.
Introduction
Bimetallic catalysts are frequently utilized for industrial
applications and they are important model systems for
furthering the understanding of catalytic mechanisms and
processes. These catalysts often exhibit increased activity,
favorable selectivity, and reduced rates of deactivation.1–9
For example, one recent study by Stamenkovic et al. on a
Pt3Ni(111) single crystal in the oxygen reduction reaction
demonstrated a 10-fold enhancement in catalytic activity over
the corresponding Pt(111) surface.1 This effect was determined
to be a result of compositional oscillation in the top three
layers of the catalysts. Another study by Gao et al. found that
CO oxidation kinetics on AuPd(100) depend on whether
Pd sites are isolated or contiguous, because of significant
differences in CO binding on the two types of sites.2
a
Department of Chemistry, University of California, Berkeley,
CA 94720, USA
b
Chemical Sciences Division, Lawrence Berkeley National
Laboratory, Berkeley, CA 94720, USA
c
College of Chemistry and Molecular Engineering,
and the State Key Lab of Rare Earth Materials Chemistry and
Applications & PKU-HKU Joint Lab in Rare Earth Materials and
Bioinorganic Chemistry, Peking University, Beijing 100871, China
d
Advanced Light Source, Lawrence Berkeley National Laboratory,
Berkeley, CA 94720, USA
e
Department of Applied Physics, Hanyang University, Ansan,
Korea 426-791
f
Materials Science Division, Lawrence Berkeley National Laboratory,
Berkeley, CA 94720, USA. E-mail: [email protected]
w Current address: Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA.
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The studies by Stamenkovic and Gao both used bimetallic
single crystals. Single crystals, however, are known to behave
quite differently from nanoparticles.10 Nanoparticle reactivity
is often dependent on size and oxidation state,11 in part
because small particles can oxidize much more easily than
large particles and, especially, single crystals. In order to
expand control and understanding of bimetallic nanoparticle
catalysts there is a need for detailed studies on the roles of
nanoparticle size, surface composition, and in situ oxidation
state on catalytic behavior.
Among the large number of possible nanoparticle compositions, Rh, Pt, and Pd-based mono-, bi-, and trimetallic nanocrystals with controlled composition, size, and shape are of
special interest because of their role in three-way automotive
exhaust catalysis.12 Although CO oxidation has been studied
extensively, many open questions remain regarding catalyst
behavior in these systems, especially with regard to composition
and oxidation state13,14 and how these factors affect catalyst
deactivation,15 synergism,16 and surface segregation.17
Previous work in our lab on 15 nm Rh0.5Pd0.5 bimetallic
Pd-core Rh-shell nanoparticles supported on Si wafers used
ambient pressure X-ray photoelectron spectroscopy (APXPS)
to examine the dependence of surface composition and oxidation state on the local chemical environment.17 This study
found significant reversible changes in the catalyst composition when the nanoparticles were exposed to NO, CO, O2, H2,
and mixtures of NO + CO at 300 1C. However, the catalytic
reaction kinetics and surface composition of these nanoparticles
has not previously been studied under the CO oxidation
reaction conditions.
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In this work, the synthesis of near-monodisperse Rh1 xPdx
(x = 0–1) nanocrystals at a constant size of 15 nm by a
one-step polyol method is reported. The as-synthesized
bimetallic nanocrystals have Rh-rich surfaces. The RhPd
bimetallic nanoparticle catalysts showed activity enhancement
(synergy) in CO oxidation by O2 compared with pure Pd or
Rh. APXPS studies showed that CO interacts more strongly
with Pd than with Rh, while O2 has the opposite behavior.
Based on the observation from APXPS studies, the reaction
synergy could be explained by spillover of O atoms from Rh
onto CO- and/or C-covered Pd reaction sites under reaction
conditions.
Experimental details
(1)
Nanoparticle synthesis
Rhodium(III) acetylacetonate (Rh(acac)3, 97%), palladium(II)
acetylacetonate (Pd(acac)2, 99%), 1,4-butanediol (99%), and
poly(vinylpyrrolidone) (PVP, Mw = 55 000) were purchased
from Sigma-Aldrich. All the solvents, including acetone, ethanol,
hexane, and chloroform, were of analytical grade and were
used without further purification.
Given amounts of Rh(acac)3 and Pd(acac)2 (0.1 mmol total)
and 1 mmol PVP (monomer concentration) were added to
20 ml of 1,4-butanediol in a 50 ml three-necked flask at room
temperature (Table 1). The stock solution was heated to 50 1C
using a Glas-Col electromantle (60 W, 50 ml) with a
Cole-Parmer temperature controller (Diqi-senses), and was
evacuated at this temperature for 20 min to remove water and
oxygen under magnetic stirring, resulting in an optically
transparent orange-yellow solution. The flask was then heated
to 220 1C at a rate of 10 1C min 1 and maintained at this
temperature (2 1C) for 1.5 h under Ar. During the reaction
the color of the solution gradually turned from orange-yellow
to black. When the reaction was complete an excess of acetone
was poured into the solution at room temperature to form a
cloudy black suspension. This suspension was separated by
centrifugation at 4200 rpm for 6 min and the black product
was collected by discarding the colorless supernatant. The
precipitated Rh1 xPdx nanocrystals were washed with acetone
once then re-dispersed in ethanol.
The synthetic procedure used to produce Pd nanocrystals
was the same as that used to synthesize Rh1 xPdx (x = 0.2–0.8)
nanocrystals, except that 0.1 mmol Pd(acac)2 was added to
Table 1 Synthetic conditions, lattice constants, and crystallite sizes of
PVP-capped Rh1 xPdx (x = 0–1) nanocrystals
Sample
Rh(acac)3/ Pd(acac)2/
mmol
mmol
T/1C t/h
a/nm
Size/nm
Rh
Rh0.8Pd0.2
Rh0.6Pd0.4
Rh0.5Pd0.5
Rh0.4Pd0.6
Rh0.2Pd0.8
Pd
0.2
0.08
0.06
0.05
0.04
0.02
0
0.385
0.385
0.387
0.387
0.390
0.397
0.395
14.4
15.1
15.4
16.0
15.1
14.9
15.6
a
0
0.02
0.04
0.05
0.06
0.08
0.1
205
220
220
220
220
220
220
2
1.5
1.5
1.5
1.5
1.5
0.5
The standard deviation statistic from 100 nanocrystals.
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1.4a
2.9
2.4
1.9
2.0
2.0
2.4
20 ml of 1,4-butanediol in a 50 ml three-necked flask at room
temperature for reaction at 220 1C for 0.5 h (Table 1).
The synthetic procedure used for Rh nanocrystals was the
same as that used to synthesize Pd nanocrystals, except that
0.2 mmol Rh(acac)3 was added to 20 ml of 1,4-butanediol in a
50 ml three-necked flask at room temperature and evacuated
at 140 1C for 20 min, followed by reaction at 205 1C for
2 h (Table 1).
(2)
Preparation of the Langmuir–Blodgett film
The Rh1 xPdx nanocrystals were washed several times by
repeated cycles of precipitation and dispersion. Precipitation
was performed by adding 1 ml of the nanocrystal solution to
4 ml of hexane. The supernatant was decanted and the
remaining precipitated nanoparticles were dispersed by sonication in 1 ml of ethanol (in early wash cycles) or chloroform
(in later wash cycles) to remove the impurities and excess PVP.
Monolayer of Rh1 xPdx nanocrystals was formed by placing
drops of Rh1 xPdx nanocrystal chloroform solution onto the
water surface of a LB trough (Nima Technology, M611) at
room temperature.18 The surface pressure was monitored with
a Wilhelmy plate, which was adjusted to zero prior to spreading
the nanocrystals. The resulting surface layer was compressed
by moving the mobile barrier at a rate of 15 cm2 min 1. A
typical surface pressure was 11 mN m 1, which resulted in
nanoparticle surface coverage of 15–30%. The Rh1 xPdx
nanocrystals were deposited onto Si wafers (1 cm 1 cm)
by lift-up of the substrates at a rate of 1 mm min 1 using the
Langmuir–Schäffer horizontal liftoff method. This 2D sample
geometry was used for reaction studies and the APXPS
studies.
(3) CO oxidation reactions at atmospheric (760 Torr) and
moderate (100–200 mTorr) pressure
CO oxidation reactions were performed using two setups: one
atmospheric-pressure setup, and one moderate-pressure setup.
The first setup was used to obtain turnover frequency (TOF)
and activation energy using gas chromatography (GC) at a
total pressure of one atmosphere. The second setup was used
to obtain light-on temperatures and qualitative comparison of
reaction rates at total pressures of 100–200 mTorr. This setup
was built specifically to mimic the conditions used in APXPS
studies of surface segregation and oxidation behavior in
bimetallic nanoparticle catalyst systems, such as Rh1 xPdx
catalysts,17 in order to bridge the pressure gap between
conventional kinetic studies and photoelectron-based studies.
The atmospheric-pressure setup consisted of a laboratory
scale batch reactor with continuous gas recirculation between
160 and 200 1C. Samples were loaded into quartz reactors with
a type-K thermocouple touching the reactor near the sample.
Prior to the reaction, the manifold was filled with 40 Torr CO
(Praxair, UHP), 100 Torr O2 (Praxair, UHP), and a balance
of He (Praxair, UHP) initially at atmospheric pressure, all
regulated by mass flow controllers. Gas composition was
analyzed with a HP 5890 Series II gas chromatograph
equipped with a thermal conductivity (TCD) detector. Turnover frequency (reported as product molecules site 1 s 1) was
calculated by extrapolating the conversion data to the initial
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time and by correcting the Rh1 xPdx surface area using the
method described above.
The moderate-pressure setup, in contrast, uses a twochamber system in order to feed very small pressures of the
reaction gas mixture to a low-pressure chamber for detection.
Reactants are fed to a high-pressure reaction chamber (0.3 L)
using two leak valves—one for O2 and one for CO. The
contents of the high-pressure chamber are sampled into a
low-pressure chamber through a leak valve. The low-pressure
chamber is kept at a pressure of 5 10 8 Torr using a
turbomolecular pump. Reactant and product concentration
is measured using a mass spectrometer (Stanford Research
Systems) in the low-pressure chamber. Sample temperature is
controlled using a ceramic button heater (Momentiv), and a
type-K thermocouple. Gas mixing was found to be very
efficient and changes in the reaction chamber are registered
by the mass spectrometer almost immediately. The system
functions similarly to a batch reactor due to the low flow rate
of gas in and out of the reaction chamber.
(4)
APXPS studies
APXPS was carried out on the Scienta 4000-HiPP endstation at
beamline 9.3.2 of the Advanced Light Source.19 A Rh0.5Pd0.5,
pure Rh, and pure Pd sample were mounted together on a 1 cm
button heater. Pd 3d, Rh 3d, C 1s, and Si 2p spectra were
monitored at 490 eV photon energy for all three samples as the
O2 pressure, CO pressure, and temperature were varied. At each
pressure/temperature condition, the Pd 3d/Rh 3d ratio was
monitored until it reached steady state, typically after 3 h at
each condition.
Characterization of the RhPd bimetallic
nanoparticles
The size, shape, and lattice structure of the Rh1 xPdx nanocrystals were analyzed using a Philips FEI Tecnai 12 transmission
electron microscope (TEM) and Philips CM200/FEG highresolution TEM (HRTEM), operated at 100 kV and 200 kV,
respectively. The samples were prepared by placing a drop of
the nanocrystal solution in ethanol onto a continuous carboncoated copper TEM grid. Powder X-ray diffraction (XRD)
patterns were recorded on a Bruker D8 GADDS diffractometer using Co-Ka radiation (l = 1.79 Å). Lattice parameters
of the nanocrystals were calculated with the least-squares
method. X-Ray Photoelectron Spectroscopy (XPS) experiments were performed on a Perkin-Elmer PHI 5300
XPS spectrometer with a position-sensitive detector and a
hemispherical energy analyzer in an ion-pumped chamber
(evacuated to 2 10 9 Torr). The Al-Ka (1486.6 eV) X-ray
source of the XPS spectrometer was operated at 350 W with a
15 kV acceleration voltage. XPS was also carried out in UHV
at the Advanced Light Source Beamline 9.3.2 for energydependent (and therefore depth-dependent) composition
analysis. This system has been described previously.17 The
binding energy (BE) for the samples was calibrated by setting
the measured BE of the broad C 1s, which is dominated by
carbon from the PVP capping agent, to 285 eV. The elemental
ratio was calculated from the integrated peak areas of the Rh
3d, Pd 3d, and O 1s core levels. The morphological changes of
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Phys. Chem. Chem. Phys., 2011, 13, 2556–2562
the Rh1 xPdx nanocrystals, Rh thin films, and Pd thin films
before and after reaction were observed with a Zeiss Gemini
Ultra-55 analytical scanning electron microscope (SEM). The
total number of surface sites was calculated from coverage
determined by a minimum of ten SEM images taken from
randomly selected locations on each sample and taking into
account the lattice parameters determined from XRD. This
calculation is purely geometric and does not account for site
blocking by strongly-bound capping agent molecules or other
species.
Panels a–f of Fig. 1 display TEM images of the Rh1 xPdx
(x = 0–0.8) nanocrystals. The bimetallic nanocrystals are
faceted, but show a mix of various shapes in 2D projection,
such as hexagons, pentagons, triangles, and cubes. All the
bimetallic nanocrystals have an average crystallite size near 15 nm,
with standard size distributions of around 15% (Table 1).
XRD measurements determined that all the bimetallic nanocrystals have a face-centered cubic (fcc) structure with lattice
parameters lying between 0.385 and 0.397 nm (Table 1).
The surface compositions and structure of the bimetallic
nanocrystals were measured experimentally by XPS in ultrahigh vacuum. Two systems were used—one system with an
Al K-a source with X-ray Photon Energy of 1486.6 eV
(photoelectron mean free path B1.6 nm) and another system
with a tunable synchrotron source set to 645 eV (photoelectron
mean free path B0.7 nm). As shown in Fig. 2, the atomic
fraction of Pd is significantly lower at the surface of the
bimetallic nanoparticles than in the bulk. This demonstrates
that the as-synthesized nanoparticles have clear core–shell
geometry, with a Pd-rich core and Rh-rich shell. The transition
between these regions is gradual and comprised of mixed
Rh–Pd phases. Changes in the local chemical environment
dramatically affect the surface composition and oxidation
state of the nanocrystals by inducing selective phase-segregation
of the nanoparticle components.17
Reaction and APXPS results and discussion
CO oxidation tests in 40 Torr CO, 100 Torr O2 and 620 Torr
He using the atmospheric-pressure reactor revealed that all the
bimetallic nanocrystals are active for CO oxidation between
170–210 1C. SEM measurements before and after reaction
showed that no severe aggregation occurred, as shown in
Fig. 3. The turnover frequency was strongly dependent on
nanoparticle composition, as shown at 180 1C and 190 1C in
Fig. 4a. The maximum turnover frequency was 1.34 molecules
site 1 s 1 at 190 1C on the Rh0.6Pd0.4 nanocrystals. Turnover
frequencies were also compared to the theoretical activity of a
linear combination of pure Rh and pure Pd nanocrystals,
x)) +
calculated as TOFLinear-combination = (TOFRh (1
(TOFPd x). The maximum measured enhancement was
5.0 at 180 1C for the Rh0.2Pd0.8 nanocrystals versus the
theoretical activity of a linear combination of pure Rh and
pure Pd nanoparticles. The enhancement factor decreased with
increasing temperature for all samples.
The CO oxidation reaction was also carried out at in both a
mixture of 100 mTorr O2 and 40 mTorr CO, and a mixture of
100 mTorr O2 and 100 mTorr CO. Rh and Rh0.5Pd0.5 nanoparticles were both very active in these conditions, while the
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Fig. 1 TEM images and size distribution histograms (insets) of Rh1 xPdx nanocrystals: (a) x = 0; (b) x = 0.2; (c) x = 0.4; (d) x = 0.5;
(e) x = 0.6; (f) x = 0.8.
Pd nanoparticles had a very low turnover frequency. The turnover frequency measured from each nanoparticle catalyst at
175 1C and 100 mTorr O2 in 40 mTorr CO and 100 mTorr CO is
compared in Fig. 4b. At 175 1C in 100 mTorr O2 and 40 mTorr
CO, the Rh nanoparticles had a turnover frequency of
0.078 molecules site 1 s 1, as compared to a turnover frequency
of 0.211 molecules site 1 s 1 for the Rh0.5Pd0.5 nanoparticles
and a turnover frequency of 0.0018 molecules site 1 s 1 for the
Pd nanoparticles. These turnover frequencies are lower than
those measured at high pressure, as expected from entropic
considerations. Notably, the turnover frequency measured on
the bimetallic nanoparticles was significantly larger than on
either the Rh or Pd monometallic nanoparticles. The maximum
measured enhancement was 5.3 for the Rh0.5Pd0.5 nanoparticles versus the theoretical activity of a linear combination
of pure Rh and pure Pd nanocrystals in these conditions.
Apparent activation energy was measured both at atmosphere and low pressure, which showed strong dependence
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on nanoparticle composition. At atmosphere pressure, it
decreased with increased Pd molar fraction, from a maximum
of 108 kcal mol 1 for pure Rh to a minimum of 51 kJ mol 1
for pure Pd nanoparticles. This trend is consistent with
studies on pure Rh and Pd nanoparticles.20,21 At low pressure
(100 mTorr O2 and 100 mTorr CO), the apparent activation
energy measured for the bimetallic nanoparticles followed the
same trend as those obtained at atmosphere pressure.
The turnover frequencies measured using bimetallic nanoparticles were greater than those measured on the monometallic nanoparticles. This indicates that the enhancement
effect is too large to be explained simply by complete coverage
of the nanoparticle surface with Rh. Consequently, the synergetic
effect must be due to interaction between Rh and Pd or their
adsorbates. These results are consistent with previous studies,
which found synergistic behavior in Rh–Pd catalysts prepared
by sequential impregnation and co-impregnation7,16,22,23 and
lower ignition temperature for Rh nanoparticle catalysts as
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Fig. 2 Pd atomic fraction of the Rh1 xPdx nanocrystals in UHV
determined at X-ray energies of 1486.6 eV and 645 eV by XPS versus
as-synthesized total Pd composition. The X-ray with higher energy can
probe deeper surface than that with lower energy. For RhPd bimetallic
nanoparticle, the 645 eV X-ray can only probe less than 0.7 nm
surface, and thus are very surface sensitive. These results clearly
demonstrated that the bimetallic nanoparticles posses a Rh rich
surface.
Fig. 3 SEM images of Rh0.6Pd0.4 nanocrystals (a) before and (b) after
CO oxidation. No obvious particle aggregation is observed after CO
oxidation.
compared to Pd nanoparticle catalysts studied in similar
conditions.23–25 Previous studies also determined that synergism
does not occur in physical mixtures in this system.23,26
To better understand the mechanism of the observed synergetic
effect, we used APXPS to study the surface composition and
the oxidation state of Pd and Rh under reaction conditions.
The as-synthesized Rh0.5Pd0.5 nanoparticles have a Rh rich
surface (Fig. 2). When we first introduce 100 mTorr O2 and
40 mTorr CO at 175 1C, the surface composition of Pd
increased slightly from 10% to 11%. When the CO pressure
was increased to 100 mTorr, the surface Pd increased to 23%,
which indicates that CO has a stronger interaction with Pd
than with Rh. Further evidence for the stronger interaction of
O2 with Rh and CO with Pd was observed by exposing the
NPs to 100 mTorr O2 or 100 mTorr CO separately or to a
mixture of 100 mTorr of both gases. The Pd 3d and Rh 3d
spectra are shown in Fig. 5. Rh 3d is 70% oxidized in pure O2
while Pd 3d is only 29% oxidized. In pure CO, however, the Pd
3d peak shows increased intensity at higher BE, indicating a
strong interaction with CO or dissociated C atoms. It is
possible, then, that the preferential adsorption of CO on
isolated Pd atoms could lead directly to the synergetic increase
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Phys. Chem. Chem. Phys., 2011, 13, 2556–2562
Fig. 4 (a) Turnover frequency at 180 1C and 190 1C as a function of
Pd molar fraction for Rh1 xPdx nanocrystals in 100 Torr O2, 40 Torr
CO, 620 Torr He. Dashed lines represent the theoretical activity of a
linear combination of pure Rh and pure Pd nanocrystals. A significant
synergetic effect was observed for the bimetallic catalysts in these
reaction conditions. (b) Turnover frequency in 100 mTorr O2 and
175 1C versus CO pressure and Pd molar fraction for Rh1 xPdx
bimetallic nanoparticle catalysts. Dashed lines represent the theoretical
activity of a linear combination of pure Rh and pure Pd nanocrystals.
The degree of the synergetic effect observed for the bimetallic catalyst
increased with O2 to CO ratio.
Fig. 5 (a) Pd 3d and (b) Rh 3d spectra from Rh0.5Pd0.5 NPs at 220 1C
in 100 mTorr O2 (red), 100 mTorr CO (blue) and a reaction mixture of
nominally 100 mTorr O2 + 100 mTorr CO (black). In pure CO, the Pd
3d peak shows increased intensity at higher BE, indicating a strong
interaction with CO or dissociated C atoms. In pure O2, however, the
Rh 3d indicates a large degree of oxidation (approximately 70%) while
the Pd 3d indicates only a relatively modest degree of oxidation (29%).
in the reaction rate observed because isolated Pd on the
Rh-dominated surface ensures an adequate supply of adsorbed
CO for the reaction while oxygen dissociatively adsorbs on
Rh. In this model, the reaction rate is increased by reaction
at Pd sites between oxygen spillover from Rh and the
CO-covered Pd sites. In this case, the synergetic effect is
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our high pressure work, which was carried out at an intermediate PO2/PCO ratio, found an intermediate synergetic
enhancement, and our low pressure work found a significantly higher enhancement when PO2/PCO = 2.5 than when
PO2/PCO = 1 under otherwise-identical conditions. It is
important to note that this correlation is independent of
reaction order because these enhancements are relative to
linear combinations of pure Pd and pure Rh catalyst activity.
Fig. 6 (a) Pd 3d and (b) Rh 3d spectra from Rh0.5Pd0.5 NPs at 175 1C
in reaction mixtures of nominally 100 mTorr O2 + 100 mTorr CO
(red) and 100 mTorr O2 + 40 mTorr CO (black). At 175 1C in the
mixtures of 100 mTorr O2 and 40 mTorr CO, the surface was 26% Pd.
Upon increasing the CO pressure back to 100 mTorr, the surface Pd
increased to 31%. This indicates that at higher oxygen pressures, Pd
sites are increasingly isolated on an ever more Rh-dominated surface
for the Rh0.5Pd0.5 nanoparticles.
heavily dependent on the distribution of Rh and Pd in the
top layers. This observation also supports the mechanism
originally proposed for Rh–Pt by Oh and Carpenter26 and
later extended to Rh–Pd by Araya and Diaz,7 that surface
oxygen from RhOx species may react with CO adsorbed on
reduced Pt atoms.
In order to gain a better understanding of the equilibrium
Rh/Pd ratio under these reaction conditions, the NPs were
heated to 220 1C in 100 mTorr O2 + 100 mTorr CO for
3 hours, followed by checking again the original conditions of
100 mTorr O2 + 40 mTorr CO at 175 1C and then increasing
the CO pressure to 100 mTorr. After 3 hours at 220 1C in
100 mTorr O2 + 100 mTorr CO, the surface Pd increased to
30%. After decreasing the CO pressure to 40 mTorr and the
temperature to 175 1C, the surface was only 26% Pd. Upon
increasing the CO pressure back to 100 mTorr, the surface Pd
again increased to 31% (Fig. 6). This indicates that at higher
relative oxygen pressures, Pd sites are increasingly isolated on
an ever more Rh-dominated surface for the Rh0.5Pd0.5 nanoparticles. It is also evident from Fig. 6 that at lower CO
pressure, the Rh 3d peak shifts to higher BE (approximately
8% oxidized), while the Pd 3d peak does not shift
(no oxidation). This observation further supports our claim
that more RhOx species form at higher PO2/PCO ratio, leading
to an enhanced synergistic effect. Both the formation of RhOx
species and the decreased surface Pd content may contribute to
the enhanced synergistic effect with increasing PO2/PCO ratios.
The measured enhancements in both this study and similar
studies by Yoon et al.23 and Araya and Weissman16 are
consistent with the APXPS observations. In each study, the
PO2/PCO is different and the measured enhancement changes
as we would predict. Our reaction, as previously stated, was
carried out in PO2/PCO = 2.5 at 180 1C and 190 1C at high
pressure, and PO2/PCO = 2.5 and 1 at 175 1C, while Weissman
et al.’s study was carried out at PO2/PCO = 5 at 135 1C and
Yoon et al.’s study was in PO2/PCO = 1 at 180 1C. Weissman
et al.’s study, which had the highest PO2/PCO ratio, also found
the highest synergetic enhancement, while Yoon et al.’s study,
with the lowest PO2/PCO ratio, found the lowest synergetic
enhancement. The present study confirms this correlation, as
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Conclusions
Rh1 xPdx nanocrystals or 15 nm in diameter were synthesized,
characterized, and studied in catalytic CO oxidation in two
pressure regimes. A model based on APXPS studies was
proposed to help explain the observed catalytic behavior.
The nanocrystals were found to exhibit significant compositiondependent synergetic effects in CO oxidation. This observed
synergy could be related to preferential adsorption of CO on
isolated Pd atoms or islands on the bimetallic catalyst surface
and spillover of oxygen adsorbates from Rh to CO-covered
Pd reaction sites. Our APXPS and reaction results resolve
questions arising from discrepancies in the degree of synergetic
enhancement measured on bimetallic nanoparticles in the
CO oxidation reaction by demonstrating a clear correlation
between the O2 : CO ratio, surface composition and bimetallic
nanoparticle synergy. This unique and desirable synergetic
behavior in bimetallic catalysts will hopefully lead to greater
understanding of the many complex factors governing
reactivity in these systems.
Acknowledgements
This work was supported by the Director, Office of Science,
Office of Basic Energy Sciences, of the U.S. Department of
Energy under Contract No. DE-AC02-05CH11231. Y.W.Z.
gratefully acknowledges the financial aid of Huaxin Distinguished
Scholar Award from Peking University Education Foundation
of China.
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