Nucleation and growth mechanism of Pd/Pt bimetallic clusters in sodium Bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles as studied by in situ x-ray absorption spectroscopy

Article abstract of DOI:10.1021/jp054062t

We report in situ X-ray absorption spectroscopy (XAS) investigations on the formation of palladium-platinum (Pd/Pt) bimetallic clusters at the early stage within the water-in-oil microemulsion system of water/AOT/nheptane. The reduction of palladium and platinum ions and the formation of corresponding clusters are monitored as a function of dosage of reducing agent, hydrazine (N₂H₅OH). Upon successive addition of the reducing agent, hydrazine (N₂H₅OH), five distinguishable steps are observed in the formation process of Pd/Pt clusters at the early stage. Both in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis for both the Pd K-edge and Pt Lm-edge revealed the formation of Pd/Pt bimetallic clusters. A corresponding structural model is proposed for each step to provide a detailed insight into the nucleation and growth mechanism of Pd/Pt bimetallic clusters. We also discussed the atomic distribution of Pd and Pt atoms in Pd/Pt bimetallic clusters based on the calculated XAS structural parameters.

Full text of DOI:10.1021/jp054062t

21566
J. Phys. Chem. B 2005, 109, 21566-21575
Nucleation and Growth Mechanism of Pd/Pt Bimetallic Clusters in Sodium
Bis(2-ethylhexyl)sulfosuccinate (AOT) Reverse Micelles as Studied by in Situ X-ray
Absorption Spectroscopy
†
,†,‡
†
†
Ching-Hsiang Chen, Bing-Joe Hwang,* Guo-Rung Wang, Loka Subramanyam Sarma,
‡
‡
‡
Mau-Tsu Tang, Din-Goa Liu, and Jyh-Fu Lee
Nanoelectrochemistry Laboratory, Department of Chemical Engineering,
National Taiwan UniVersity of Science and Technology, Taipei 106, Taiwan, R.O.C.
ReceiVed: July 22, 2005; In Final Form: September 14, 2005
We report in situ X-ray absorption spectroscopy (XAS) investigations on the formation of palladium-platinum
Pd/Pt) bimetallic clusters at the early stage within the water-in-oil microemulsion system of water/AOT/n-
heptane. The reduction of palladium and platinum ions and the formation of corresponding clusters are
monitored as a function of dosage of reducing agent, hydrazine (N OH). Upon successive addition of the
reducing agent, hydrazine (N OH), five distinguishable steps are observed in the formation process of
(
2
H
5
2 5
H
Pd/Pt clusters at the early stage. Both in situ X-ray absorption near-edge structure (XANES) and extended
X-ray absorption fine structure (EXAFS) analysis for both the Pd K-edge and Pt LIII-edge revealed the formation
of Pd/Pt bimetallic clusters. A corresponding structural model is proposed for each step to provide a detailed
insight into the nucleation and growth mechanism of Pd/Pt bimetallic clusters. We also discussed the atomic
distribution of Pd and Pt atoms in Pd/Pt bimetallic clusters based on the calculated XAS structural parameters.
1. Introduction
mechanism. A thorough understanding of the formation mech-
anism is necessary to control the factors such as size, shape,
1
Bimetallic clusters are of great interest from both scientific
and surface morphology which govern the catalytic activity of
the nanoparticles. Success in either particle design or scaling-
up also requires a detailed knowledge of the particle formation
mechanism. Although a large number of studies were focused
on the synthesis and characterization of bimetallic clusters, e.g.,
and technological perspectives because of the modification of
physical and chemical properties not only due to quantum-size
effects but also as a result of the combination of different
It was observed that the activity and selectivity of
the metal catalysts can be drastically influenced by the presence
of a second metal component. In our previous study we
demonstrated that the structure of bimetallic catalysts can be
significantly influenced by the degree of alloying between the
2
-4
metals.
1
5
16
17,18
19
20
5
Ag/Cu and Au/Cu, Pd/Pt, Pt/Ru,
Rh/Pt, and Pd/Au,
we know of less reports focusing on the formation mechanism
2
1,22
of bimetallic clusters at early stages.
6
One of the methods suitable to study the structure of metal
clusters is the in situ X-ray absorption spectroscopy (XAS)
comprising both X-ray absorption near-edge (XANES) and
extended X-ray absorption fine structure (EXAFS) regions.
XANES can reveal the oxidation state and d-band occupancy
of a specific atom as well as the size of the metallic cluster,23
while the local atomic structure can be obtained from the
analysis of EXAFS. The XAS technique has been proved as a
powerful technique for the characterization of bimetallic
two constituent elements. Controlling the size and morphology
of the metal nanoparticle is a key theme of the nanoparticle
research, as it has a strong influence on optical, magnetic,
electronic, and catalytic properties of the nanoparticles. Metha-
nol, ethanol, and formic acid are some of the more commonly
used alternative fuels, and recent work has shown that formic
7
acid is a particularly attractive candidate for fuel cells. Such
oxidation reaction of the fuel requires a catalyst. Although Pt
is the most well-known fuel cell catalyst, in its pure form it
suffers from serious surface poisoning due to the adsorption of
reaction intermediates. The catalytic activity of Pt surfaces
24-26
catalysts,
since it is difficult to obtain structural information
on such systems by means of conventional material analysis
methods at the early stages. Moreover, due to the change in
cluster structure during the course of formation, ex situ
techniques such as transmission electron microscopy (TEM) and
X-ray diffraction (XRD) have difficulty giving real-time struc-
tural information in a reaction atmosphere. Meanwhile, the
catalytic activity of bimetallic clusters is strongly influenced
8
,9
toward formic acid oxidation has been studied in detail, and
many infrared spectroscopic studies1
0-12
have established that
CO adsorption is primarily responsible for surface poisoning.
Pd surfaces have been found to possess better catalytic activity
toward formic acid oxidation, but a very slow deactivation
results in reduced oxidation current.13 The Pd/Pt bimetallic
system also exhibits a high catalytic ability for hydrogen
oxidation.14
6
by their atomic distribution and structure. In our previous
works, we have successfully evaluated the formation mechanism
of monometallic Pt and bimetallic Pt/Cu clusters in AOT reverse
Owing to its wide application as a heterogeneous catalyst,
we chose the Pd/Pt bimetallic system to study the formation
2
1,27
microemulsions by in situ XAS.
In line with our ongoing effort to evaluate the formation
mechanism of metal nanoparticles, we have aimed here to
investigate the formation mechanism of Pd/Pt bimetallic clusters
by in situ XAS at the early stages and to provide a detailed
*
Corresponding author. Fax: +886-2-27376644. E-mail: bjh@
mail.ntust.edu.tw.
†
‡
National Taiwan University of Science and Technology.
National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan.
1
0.1021/jp054062t CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/29/2005

Pd/Pt Bimetallic Clusters in AOT Reverse Micelles
J. Phys. Chem. B, Vol. 109, No. 46, 2005 21567
insight into the nucleation and growth mechanism. In addition,
we also attempted to provide detailed information on an atomic
distribution and structural model of Pd/Pt bimetallic clusters.
thickness of the liquid layer) so that the proper edge jump step
could be achieved during the measurements. All of the spectra
were recorded at room temperature in a transmission mode.
Higher harmonics were eliminated by detuning the double
crystal Si(111) monochromator. The incident and transmission
X-ray intensities were, respectively, detected by ion chambers
that were installed in front of and behind the sample cell. Energy
was scanned from 200 eV below the edge to 1000 eV above
the edge. Standard compounds, Pd foil and Pt foil for the
respective edges, were measured simultaneously by using the
third ionization chamber so that energy calibration could be
performed scan by scan.
2. Experimental Section
2
.1. Preparation of Monometallic Pd Clusters in AOT
Reverse Micelles. The monometallic Pd clusters were prepared
in a microemulsion containing n-heptane as the continuous oil
component, water, and surfactant. The surfactant used was 1
M sodium bis(2-ethylhexyl)sulfosuccinate (AOT) (99%). The
n-heptane and the surfactant were thoroughly mixed, and an
aqueous solution of 1 mL of 0.6 M K2PdCl4 was subsequently
added to form a well-defined microemulsion phase with the Pd
complex in the water pool. The volume ratio of the aqueous
phase to the organic phase was kept at 1:10. An important
parameter characterizing the microemulsion, i.e., the water-to-
surfactant molar ratio, W ) {(water)/(surfactant)}, was equal
to 5.5 in the present experiment. This microemulsion solution
was placed in the liquid cell, which was carefully designed to
govern the edge jump, for the in situ XAS measurement. A
microemulsion of the same composition of oil, water, and the
surfactant that contained the reducing agent was also prepared.
The reducing agent solution was composed of 1 M hydrazine
B. XAS Data Analysis. Standard procedures were followed
to analyze the EXAFS data. First, the raw absorption spectrum
in the pre-edge region was fit to a straight line and the
background above the edge was fit using a 11-knot cubic spline
function calculated from AUTOBK code. The EXAFS function,
ø, was obtained by subtracting the postedge background from
the overall absorption and then normalizing with respect to the
edge jump step. The normalized ø(E) was transformed from
2
energy space to k-space. The ø(k) data were weighted by k in
order to compensate for dampening of the XAFS amplitude with
2
increasing k. Subsequently, k -weighted ø(k) data in the k-space
(N2H5OH) and 1 M sodium hydroxide (NaOH), and their
-1
ranging from 3.23 to 10.86 Å for the Pd K-edge and from
volume ratio was kept at 1:1. An appropriate amount of reducing
agent containing microemulsion was then added to the micro-
emulsion containing Pd complex via microsyringe every hour
starting at 1 h and ending at 5 h, whereby the Pd complex was
-1
3
.37 to 11.44 Å for the Pt LIII-edge were Fourier transformed
(FT) to r-space to separate the EXAFS contributions from
different coordination shells. A nonlinear least-squares algorithm
was applied to curve fitting of EXAFS in r-space between 1.1
and 3.0 Å for the Pd K-edge and between 1.1 and 3.3 Å (without
phase correction) for the Pt LIII-edge. All the computer programs
0
reduced to Pd clusters. As the significant changes in the
reduction process occurred during the progressive addition of
the reducing agent at 3 h and 5 h, we will show and discuss the
XAS data at these two time periods. The final product obtained
28
were implemented in the UWXAFS 3.0 package with the
backscattering amplitude and the phase shift for the specific
(at 5 h) in section 2.1 was marked as “S1”.
29
atom pairs being theoretically calculated by using FEFF7 code.
2
.2. Preparation of Bimetallic Pd/Pt Clusters in AOT
Palladium and platinum foils were employed as references for
the Pd-Pd and Pt-Pt bonds. Pd-Pt and Pt-Pd bonds are
modeled based on a face-centered cubic (fcc) model containing
six Pd and six Pt atoms in symmetric positions. The amplitude
Reverse Micelles. Half of the amount of product S1 was placed
in the liquid cell designed for in situ XAS study, and to this an
equal amount of the Pt precursor, H2PtCl6 emulsion, was
introduced (at 6.5 h). Appropriate amounts of reducing agent
were then added each hour to form bimetallic Pd/Pt clusters.
The H2PtCl6 microemulsion solution was prepared by using a
procedure similar to that for the preparation of the K2PdCl4
microemulsion. The X-ray absorption spectra were recorded at
three time intervals, 8.3, 10.3, and 15.3 h, during the progressive
addition of the reducing agent. The final product obtained (at
2
reduction factor, S0 , values for Pt and Pd were obtained by
analyzing the Pt and Pd foils, respectively, and by fixing
2
coordination numbers in the FEFFIT input file. The S0 values
were found to be 0.88 and 0.95 for Pd and Pt, respectively.
3
. Results and Discussion
15.3 h) in section 2.2 was marked as “S2”. All reactions were
3.1. In Situ XANES and EXAFS at the Pd K-Edge of Pd
carried out at room temperature. Nitrogen gas was purged into
the microemulsion system throughout the experiment to prevent
the clusters from oxidation by air.
Clusters. The Pd K-edge XANES spectra of the Pd micro-
emulsion system as a function of dosage of reducing agent,
hydrazine, are shown in Figure 1a. In the XANES spectra the
absorption transition in the pre-edge region is a 1s to 4d dipole
forbidden transition according to the selection rule, ∆l ) 1 and
∆j ) 1, where l is the orbital angular momentum and j is the
total angular moment of the local density of states.30 The
absorption threshold resonance, called the white line, appearing
between 24360 and 24380 eV corresponds to the electronic
transitions that arise from the 1s state to the unoccupied 4p states
above the Fermi level. The second (24385 eV) and third (24435
eV) peaks correspond to 1s f dp and 1s f dsp transitions,
respectively. This white line intensity is sensitive to changes in
electron occupancy in the valance orbital and ligand field
A. In Situ XAS Measurements. The X-ray absorption spectra
at the Pd K-edge and the Pt LIII-edge were recorded at beamline
BL12B2 at the Spring-8, Japan. The electron storage ring was
operated at an energy of 8 GeV and a beam current of 100 mA.
A double crystal Si(111) monochromator was employed for
-
4
energy selection with a resolution ∆E/E better than 1 × 10
at both the Pd K-edge (24352 eV) and the Pt LIII-edge (11564
eV). Formation of Pd clusters and the corresponding Pd/Pt
bimetallic clusters in reverse micelles proceeded in a homemade
cell made with PTFE for in situ XAS study. Two holes were
made, one on top of the cell and the other on one side. After
placing the liquid sample, the top hole was closed with a Teflon
rod to avoid the exposure of the sample to the outer atmosphere.
A hollow Teflon rod with a Kapton film cap at one end was
inserted into the other hole in the in situ cell. The position of
the Teflon rod was adjusted to reach the optimum absorption
thickness (∆µx ≈ 1.0, ∆µ is the absorption edge, x is the
3
1,32
environments of the absorber.
Hence, it can be used to
calculate the density of the unoccupied states and the changes
in the oxidation state of the Pd absorber. As can be seen from
Figure 1a, the white line intensity of the initial Pd2 solution
gradually decreases as the reducing agent amount increases from
+
-
5
0 to 61.6 × 10 mol, indicating the progressive reduction of

21568 J. Phys. Chem. B, Vol. 109, No. 46, 2005
Chen et al.
Figure 2. Normalized XANES spectra near the Pd K-edge of the Pd/
Pt bimetallic microemulsion system.
increases. This implies that all of the palladium ions were
0
completely reduced to Pd and were transformed to Pd clusters.
The Pd clusters thus obtained are marked as “S1”.
3
.2. In Situ XANES at the Pd K-Edge and the Pt LIII-
Edge of Bimetallic Pd/Pt Clusters. Half of the amount of Pd
clusters labeled as “S1” is placed in the liquid cell, and an equal
amount of the H2PtCl6 emulsion is introduced into the system.
The Pd K-edge XANES spectra of the bimetallic Pd/Pt
microemulsion system as a function of dosage of the reducing
agent are shown in Figure 2. After the addition of the Pt4
microemulsion, the white line intensity increases slightly due
to the oxidation of the Pd clusters. The white line energy position
is found to be at 24375 eV, and it matches closely with the
white line energy position of the PdCl species of the [PdCl4]2
microemulsion, indicating the formation of Pd-Cl species in
this state. There was also a change in the white line energy
+
-
-
5
position with addition of 132.2 × 10 mol of the reducing
agent at 8.3 h, indicating the generation of new species around
the Pd core atoms. We also checked the white line energy
position which is found to be at 24367 eV, and it is close to the
white line energy position of the Pd-OH species of the Pd-
2
Figure 1. (a) XANES spectra and (b) FT of k -weighted EXAFS data
at the Pd K-edge of the Pd microemulsion system as a function of
hydrazine dosage.
2
+
33
(
H2O)4 complex. These observations indicate that one of
Pd2+ to Pd . The Fourier transformed (FT) k -weighted EXAFS
0
2
the possibilities is the formation of Pd-OH species. After further
addition of the reducing agent, the white line intensity decreases,
indicating that the oxidation state of the Pd core ions is reduced
-1
spectra (∆k ) 3.23-10.86 Å ) of Pd microemulsions, as shown
in Figure 1b, reveal a peak between 1.5 and 2.0 Å corresponding
0
-
5
to Pd .
to the Pd-Cl bond. After the addition of 26.4 × 10 mol of
The Pt LIII-edge XANES spectra of the bimetallic Pd/Pt
microemulsion system as a function of the reducing agent dosage
are shown in Figure 3. The white line at the Pt LIII-edge is an
absorption threshold resonance, attributed to the electronic
transition from the 2p3/2 state to unoccupied states above the
Fermi level and is sensitive to changes in electron occupancy
N2H5OH at 3 h, the magnitude of the Pd-Cl bond is decreased
and a new peak appears between 2.0 and 3.0 Å related to the
Pd-Pd bond, indicating that [PdCl4]2- ions were reduced and
Pd clusters were formed directly without any intermediate. The
corresponding chemical reaction can be written as follows:
3
1
2-
-
in the valence orbitals of the absorber. Hence, changes in the
white line intensity have been directly related to the density of
unoccupied 5d states and indicate the changes in the oxidation
state of the Pt absorber. As can be seen from the Figure 3 inset,
the decrease of the white line intensity with an increase in the
reducing agent dosage suggested the decrease in the electron
vacancy. Generally speaking, the lower the white line intensity,
N H OH + 2[PdCl ] + 4OH f
2
5
4
0
-
N + 2Pd + 8Cl + 5H O (1)
2
2
In the final step, after the addition of 61.6 × 10- mol of
N2H5OH at 5 h, the FT peak corresponding to the Pd-Cl bond
disappears completely and the magnitude of the Pd-Pd bond
5

Pd/Pt Bimetallic Clusters in AOT Reverse Micelles
J. Phys. Chem. B, Vol. 109, No. 46, 2005 21569
Figure 4. FT-EXAFS spectra at the Pd K-edge of the bimetallic Pd/
Pt system as a function of reducing agent dosage.
into the system, and it is ascribed to the Pt-Cl bond, consistent
Figure 3. Normalized XANES spectra near the Pt LIII-edge of the
Pd/Pt bimetallic microemulsion system.
27
-5
with our previous work. After the addition of 132.2 × 10
mol of reducing agent at 9 h, the magnitude of the Pt-Cl bond
decreased gradually and a new peak appeared at around 1.4-
1.8 Å, which is attributed to the Pt-OH bond and increases in
the higher the electron density and the lower the oxidation state.
Hence, changes in the white line intensity may be regarded as
an indication of the reduction of the Pt core ions in the bimetallic
Pd/Pt microemulsion system (labeled as “S2”).
-
5
magnitude. Further addition of 229 × 10 mol of reducing
agent at 11.5 h decreases the magnitude of the Pt-OH bond
and increases the magnitudes of both the Pt-Pt and the Pt-Pd
3
.3. In Situ EXAFS at the Pd K-Edge and the Pt LIII-
-
5
Edge of Bimetallic Pd/Pt Clusters. At the beginning of the
bonds. After the addition of reducing agent to 325.8 × 10
experiment, half of the amount of Pd clusters “S1” is placed in
mol at 15.8 h, the magnitudes of both the Pt-Pt and Pt-Pd
bonds are increased and the magnitude of the Pt-OH bond is
decreased. As can be seen from both Figures 4 and 6, there are
two peaks between 2.0 and 3.5 Å in both the Pd K-edge and
the Pt LIII-edge FT-EXAFS spectra, and this shows that the
bonding distances of Pd-Pd and Pt-Pt from the spectra of S2
are larger than those of the Pd foil and the Pt foil, respectively.
This indicates that not only Pd atoms but also Pt atoms contain
heteroatomic bonds at the state of S2. We have shown the
imaginary and absolute magnitudes of Fourier transforms of
sample S2 and the Pt foil in the top and bottom panels,
2
the liquid cell. The Fourier transformed k -weighted EXAFS
-
1
spectra (∆k ) 3.23-10.86 Å ) at the Pd K-edge of the
bimetallic Pd/Pt microemulsion system, as shown in Figure 4,
demonstrate a peak between 1.5 and 2.0 Å corresponding to
the Pd-Pd bond. After the addition of the Pt4 microemulsion
into the system, the magnitude of the Pd-Pd bond is decreased
and a new peak corresponding to the Pd-Cl bond, which has
+
2
a magnitude comparable with the Pd K-edge k -weighted
EXAFS spectrum of the [PdCl4]2 microemulsion, appears
between 2.0 and 3.0 Å. The magnitude of Pd-Cl bond is
decreased and a new peak related to the Pd-OH bond is formed
-
2
respectively, of Figure 7. The k -weighted EXAFS spectra
-
5
after the addition of 132.2 × 10 mol of reducing agent at 8.3
h. We have fitted the Pd-OH bond with the FEFF7 software
package, and the data are shown in Table 2. Upon further
recorded at the Pd K-edge and the Pt LIII-edge for the state of
S2 are shown in Figures 8 and 9, respectively. The EXAFS
oscillation frequencies at both the Pd K-edge and the Pt LIII-
edge of S2 are different from those of Pd and Pt foils, indicating
the existence of correlations between Pd and Pt. All these
observations indicate that the metallic ions have been reduced
and form Pd/Pt bimetallic clusters.
-
5
addition of 229 × 10 mol of the reducing agent at 10.3 h, the
magnitude of the Pd-OH bond decreases and the magnitudes
of not only the Pd-Pd bond but also the Pd-Pt bond increase.
-
5
However, after the addition of 325.8 × 10 mol of reducing
agent at 15.3 h, the magnitudes of both the Pd-Pd and Pd-Pt
bonds increase and the Pd-OH bond completely disappears.
The imaginary part and absolute magnitude of the Fourier
transform of the Pd K-edge for the final sample S2 and for the
Pd foil were compared in the top and bottom panels, respec-
tively, of Figure 5.
3.4. Formation Mechanism of Monometallic Pd Nano-
clusters in Reverse Microemulsions. The structural parameters
(coordination number N, bond distance R, Debye-Waller factor
2
σ , inner potential shift ∆E0) derived from the Pd K-edge
EXAFS data analysis are shown in Table 1. The coordination
number NPd-Cl is found to be 3.86 in the absence of reducing
2
-5
The Fourier transformed k -weighted EXAFS spectra (∆k )
agent. However, after the addition of 26.4 × 10 mol of N2H5-
-
1
3
.37-11.44 Å ) at the Pt LIII-edge of the bimetallic Pd/Pt
OH at 3 h, NPd-Cl is decreased to 1.89 and contributions from
Pd coordination begin to appear (NPd-Pd, 4.14), indicating that
Pd clusters are formed. Further increasing hydrazine dosage
microemulsion system are shown in Figure 6. A peak appears
4+
in the range of 1.8-2.1 Å when the Pt microemulsion is added

21570 J. Phys. Chem. B, Vol. 109, No. 46, 2005
Chen et al.
TABLE 1: Structural Parameters Derived from the Pd K-Edge EXAFS Data Analysis of the Pd Microemulsion System with
Various Contents of Reducing Agenta
σ2
(×10 Å2)
-3
R factor
amount of N
2
H
5
OH (mol)
time (h)
shell
N
R
j
(Å)
j
0
∆E (eV)
0
0
3
Pd-Cl
Pd-Cl
Pd-Pd
Pd-Pd
Pd-Pd
3.86 (0.13)
1.89 (0.02)
4.14 (0.04)
8.20 (0.32)
12.00 (0.50)
2.136 (0.003)
2.304 (0.001)
2.739 (0.001)
2.733 (0.003)
2.748 (0.004)
3.1 (0.3)
3.5 (0.1)
4.7 (0.1)
5.4 (0.5)
6.4 (0.6)
2.13 (0.40)
0.13 (0.11)
-2.11 (0.08)
-5.01 (0.28)
2.51 (0.29)
0.00041
0.00009
-
-
5
5
2
6
6.4 × 10
1.6 × 10 (S1)
5
0.00293
0.00433
Pd foil
a
2
2
N ) coordination number, R (Å) ) bonding distance, σ (Å ) ) Debye-Waller factor, and ∆E (eV) ) inner potential shift.
0
TABLE 2: Structural Parameters Derived from the Pd K-Edge EXAFS Data Analysis of the Pd/Pt Bimetallic Clusters System
with Various Contents of Reducing Agent
σ2
(×10 Å2)
-3
∆E
(eV)
R factor
amount of N
S1
add equal mole of Pt emulsion
2
H
5
OH (mol)
time (h)
shell
N
R
j
(Å)
j
0
5
6.5
Pd-Pd
Pd-Cl
Pd-Pd
Pd-OH
Pd-Pd
Pd-OH
Pd-Pd
Pd-Pt
Pd-Pd
Pd-Pt
Pd-Pd
8.20 (0.32)
3.89 (0.08)
0.90 (0.51)
3.51 (0.32)
0.90 (0.61)
2.31 (0.08)
2.88 (0.11)
1.01 (0.12)
5.24 (0.28)
4.42 (0.61)
12.00 (0.50)
2.733 (0.003)
2.303 (0.002)
2.825 (0.030)
2.058 (0.008)
2.821 (0.037)
2.068 (0.003)
2.744 (0.003)
2.811 (0.010)
2.747 (0.003)
2.750 (0.010)
2.738 (0.004)
5.4 (0.5)
2.3 (0.4)
5.6 (0.6)
4.9 (1.3)
6.5 (0.4)
9.5 (0.7)
3.3 (0.4)
0.0 (1.2)
4.8 (0.4)
7.8 (1.2)
6.4 (0.6)
-5.01 (0.28)
2.26 (0.26)
16.87 (3.11)
2.09 (1.13)
18.01 (5.09)
1.06 (0.31)
-5.69 (0.24)
6.34 (1.03)
-3.56 (0.52)
-4.00 (1.21)
2.51 (0.29)
0.00293
0.00171
-
5
1
2
32.2 × 10
8.3
0.03147
0.00118
-
5
29 × 10
10.3
-
5
3
25.8 × 10 (S2)
15.3
0.00526
0.00433
Pd foil
-
a
2
2
N ) coordination number, R (Å) ) bonding distance, σ (Å ) ) Debye-Waller factor, and ∆E
0
(eV) ) inner potential shift.
leads to a decrease in Pd-Cl coordination and an increase in
Pd-Pd coordination, indicating growth of Pd clusters. Metallic
nanoparticles synthesized from the reverse micellar solutions
SO3- groups of the AOT surfactant. After the addition of 26.4
-
5
2+
× 10 mol of N2H5OH (reduction reaction, I), Pd ions are
0
reduced to Pd nuclei through intermicellar exchange processes
2
1,27
38,39
are expected to possess a uniform morphology.
The
(see stage B in the model).
After the addition of 61.6 ×
-
5
2+
coordination number derived from XAS is a nonlinear function
10 mol of N2H5OH (reduction reaction, II), Pd ions are
completely reduced to Pd clusters (see stage C in the model).
0
of particle diameter, and it has been widely used in EXAFS
analysis to determine the size of the nanoparticle.3
4-37
In the
3.5. Reaction between Monometallic Pd Nanoclusters and
Pt4 Ions in Reverse Micelles. The XAS structural parameters
derived from the Pd K-edge and Pt LIII-edge EXAFS data
analysis and are shown in Tables 2 and 3, respectively. The
coordination number of Cl around Pd and Pt is found to be
3.89 and 4.10, respectively, after the addition of the Pt4 micro-
emulsion into the system. And we also found the coordination
number of Pd around Pd as 0.9 at this stage, indicating that the
Pd clusters are not completely dissolved and the Pd clusters
+
present study, the Pd-Pd coordination number is found to be
-
5
8
.2 with the reducing agent dosage of 61.6 × 10 mol at 5 h,
indicating that the particle diameter of Pd clusters is between
1
.5 and 2.0 nm.
+
The formation mechanism of Pd nanoclusters is proposed to
be step by step in this work based on the XAS results, and a
schematic representation of the formation process is depicted
in Scheme 1. Two reaction steps of three stages were proposed
in this model. In the initial stage (see stage A, Scheme 1), the
2+
-
microemulsion system contains Pd ions, Cl ions, and the
Figure 5. Imaginary part and real magnitude of Fourier transforms of
the Pd K-edge for the sample S2 (top panel) and for the Pd foil (bottom
panel).
Figure 6. FT-EXAFS spectra at the Pt LIII-edge of the bimetallic Pd/
Pt microemulsion system as a function of reducing agent dosage.

Pd/Pt Bimetallic Clusters in AOT Reverse Micelles
J. Phys. Chem. B, Vol. 109, No. 46, 2005 21571
EXAFS data of the Pd K-edge. Since XAS is an average
technique, the coordination numbers of both the Pd-Cl bond
and the Pd-Pd bond would be averaged out. Therefore, the
real coordination number of the Pd-Pd bond would be under-
estimated. This implies that the coordination number of the Pd-
Pd bond should be larger than 0.9 at this stage. The reason the
Pd cluster would not be dissolved is that the equilibrium
2
-
-
potential (E0) of the electrochemical reaction [PtCl6] + 2e
2
-
-
f [PtCl4] + 2Cl , 0.680 V/NHE, is slightly higher than the
equilibrium potential of the electrochemical reaction [PdCl4]
2e f Pd + 4Cl , 0.591 V/NHE. The higher equilibrium
2-
-
0
-
40
+
potential for the platinum complex indicates that the palladium
clusters would be oxidized and formed a local corrosion cell in
the system. The real coordination number of Cl around Pt in
the system is lower than that in H2PtCl6 solution, whose coordin-
27
ation number is 6, indicating that the electrochemical reaction,
2
-
-
0
-
[
PdCl4] + 2e f Pd + 4Cl , has proceeded with the addition
4
+
of the Pt microemulsion into the system in order to keep the
-
5
charge balance. After the addition of 132.2 × 10 mol of
reducing agent at 8.3 h, the coordination of Cl around Pd
disappears and the coordination of OH around Pd appears and
its value is found to be 3.51 without changing the valance state,
as shown in Figure 2. This is consistent with the formation of
a [Pd(OH)4] complex. We also found a decrease in coordina-
tion of Cl around Pt to 2.17, and coordination of OH around Pt
is observed and its value is 1.17 at 9 h. These observations are
Figure 7. Imaginary part and real magnitude of Fourier transforms of
the Pt LIII-edge for the sample S2 (top panel) and for the Pt foil (bottom
panel).
2-
2
-
27
consistent with the formation of [Pt(OH)4] species. This
indicates that the ligand exchange rate in palladium is higher
-
5
than that in platinum. Further addition of 229 × 10 mol of
reducing agent decreased the coordination of OH around both
Pd and Pt and generated the Pt-Pd bond and the Pd-Pd bond,
indicating the existence of correlations between Pd and Pt. The
corresponding reactions could be written as follows:
2
-
2-
N H OH + [Pd(OH) ] + [Pt(OH) ]
+
2
5
4
4
+
0
0
4
H f Pd /Pt + N + 6H + 3H O (2)
2 2 2
Finally, NPd-Pd and NPd-Pt are found to be 5.24 and 4.42,
respectively, from the Pd K-edge and NPt-Pd and NPt-Pt are found
to be 2.91 and 6.78, respectively, from the Pt LIII-edge with the
2
Figure 8. k -weighted EXAFS spectra recorded for Pd/Pt bimetallic
clusters (S2) at the Pd K-edge.
-
5
addition of 325.8 × 10 mol of reducing agent.
We checked the reliability of the Pd-Pt and Pt-Pd bonds
of the bimetallic Pd/Pt clusters at the state of S2 by comparing
the FEFF theoretical fit with the back-transformed experimental
EXAFS data of the Pd K-edge and the Pt LIII-edge, as shown
in Figures 10 and 11, respectively. The two-shell theoretical fit
(hollow circle line) matches closely with the back-transformed
experimental data (solid square line); this good fit quality
indicates that not only the Pd-Pd and Pt-Pd bonds at Pd
K-edge but also the Pd-Pt and Pt-Pt bonds at the Pt LIII-edge
are reliable.
3
.6. Formation Mechanism of Pd/Pt Nanoclusters in
Reverse Microemulsions. From the XAS results discussed
above a model is proposed for the formation of bimetallic Pd/
Pt clusters in microemulsions, as shown in Scheme 2. Four
reaction steps of five stages are proposed in this model. Initially,
the Pd cluster “S1” is placed in the liquid cell. The microemul-
0
-
sion system contains Pd clusters and the SO3 group of the
AOT surfactant (see stage A, Scheme 2). After the addition of
2
Figure 9. k -weighted EXAFS spectra recorded for Pd/Pt bimetallic
4
+
clusters (S2) at the Pt LIII-edge.
the Pt microemulsion in the first reaction step (reaction, I),
some of the Pd is dissolved and forms Pd-Cl bonds, and
[PtCl4] complexes are formed in order to keep the charge
balance (see stage B, Scheme 2). The coordination numbers
NPd-Cl and NPt-Cl are found to be 3.81 and 4.10. We also found
0
2
-
would be the nuclei in the system. Because there are two kinds
of chemical species, PdCl42- and Pd clusters, dispersed in the
2
solution, a two-shell model was used to fit the k -weighted

21572 J. Phys. Chem. B, Vol. 109, No. 46, 2005
Chen et al.
SCHEME 1: Model Proposed for the Formation Mechanism of Monometallic Pd Clusters at Early Stages
TABLE 3: Structural Parameters Derived from the Pt LIII-edge EXAFS Data Analysis of the Pd/Pt Bimetallic Clusters System
with Various Contents of Reducing Agent
σ2
(×10 Å2)
-3
∆E
(eV)
R factor
amount of N
2
H
5
OH (mol)
time (h)
shell
N
R
j
(Å)
j
0
add equal mole of Pt emulsion
7
9
Pt-Cl
Pt-OH
Pt-Cl
Pt-OH
Pt-Pd
Pt-Pt
Pt-Pd
Pt-Pt
Pt-Pt
4.10 (0.17)
1.17 (0.11)
2.17 (0.09)
0.88 (0.14)
1.99 (0.29)
6.64 (0.38)
2.91 (0.32)
6.78 (0.58)
12.00 (0.29)
2.317 (0.007)
1.914 (0.007)
2.317 (0.003)
1.914 (0.011)
2.671 (0.011)
2.672 (0.004)
2.748 (0.007)
2.758 (0.006)
2.778 (0.007)
2.2 (0.4)
1.6 (0.9)
4.2 (0.4)
4.3 (1.3)
8.8 (1.5)
7.3 (0.5)
6.3 (0.8)
7.3 (0.8)
4.8 (0.2)
9.15 (0.50)
-5.73 (1.35)
9.03 (0.57)
0.00641
0.00499
-
5
1
32.2 × 10
-
5
2
29 × 10
11.5
-11.69 (1.71)
0.01384
0.24 (1.36)
-2.29 (0.63)
2.96 (1.21)
3.74 (0.89)
-
5
3
25.8 × 10 (S2)
15.8
0.02235
0.00394
Pt foil
6.48 (0.27)
a
2
2
N ) coordination number, R (Å) ) bonding distance, σ (Å ) ) Debye-Waller factor, and ∆E
0
(eV) ) inner potential shift.
the coordination number NPd-Pd is 0.9. After the addition of
between the micelles. These collisions result in fusion of the
reverse micelles, an exchange of the contents within the cores,
-
5
1
32.2 × 10 mol of N2H5OH in the second reaction step
4
1-44
(reaction, II), both the Pd-Cl and Pt-Cl bonds are transferred
and a redispersion of the micelles.
Further the addition of
-
5
to Pd-OH and Pt-OH, respectively (stage C, Scheme 2). This
indicates that the ligand exchange happens in both metallic
cores. Reverse micelles in solution will exchange the contents
229 × 10 mol of N2H5OH (reaction III) resulted in the
formation of Pd-Pt and Pt-Pd bonds. We found NPd-Pt and
NPt-Pd to be 1.01 and 1.99, respectively, at this stage; however,
Pt-OH is still found in reaction III (see stage D, Scheme 2).
At the final step (reaction IV, formation of pure Pd/Pt bimetallic
clusters), NPd-Pd and NPd-Pt are found to be 5.24 and 4.42,
respectively. From the Pt LIII-edge, NPt-Pd and NPt-Pt are found
to be 2.91 and 6.78, respectively. In the resulting Pd/Pt clusters,
the total coordination number of Pd and Pt around absorbing
“Pd” atoms (NPd-Pd + NPd-Pt, 9.66) is almost similar to the total
4
1-44
of their cores via both fusion and redispersion processes.
As a result, the reduction of metal ions within the cores of
reverse micelles can result in the growth of nanosized metal
particles. This particle growth within the cores of the reverse
micelles depends strongly on the exchange of the reactants
between micelles. The exchange process occurs when micelles
collide because of the Brownian motion and the attractive forces
Figure 10. Two-shell fits with back-Fourier transformed experimental
Figure 11. Two-shell fits with back-Fourier transformed experimental
EXAFS data for Pd/Pt bimetallic clusters (S2) at the Pd K-edge.
EXAFS data for Pd/Pt bimetallic clusters (S2) at the Pt LIII-edge.

Pd/Pt Bimetallic Clusters in AOT Reverse Micelles
J. Phys. Chem. B, Vol. 109, No. 46, 2005 21573
SCHEME 2: Model Proposed for the Formation Mechanism of Bimetallic Pd-Pt Clusters at Early Stages
coordination number of Pd and Pt around “Pt” atoms (NPt-Pd +
NPt-Pt, 9.69), indicating that the size of the Pd/Pt cluster is
between 2.5 and 3.0 nm.34
tion numbers of the Pd and Pt atoms around the Pt atom are
determined as 2.91 and 6.78, respectively, and the total
coordination number of B ) ∑NPt-i is 9.69. The total coordina-
tion numbers of A and B are almost the same, indicating that
the particle size in both “Pd” and “Pt” is the same. From these
values the structural parameters Pobs (NPd-Pt/∑NPd-i) and Robs
(NPt-Pd/∑NPt-i), which can be used to determine the atomic
distribution, were calculated as 0.46 and 0.30, respectively. The
higher value of Pobs, 0.46, indicates the higher extent of atomic
dispersion of “Pd” atoms when compared to Pt. In other words,
the segregation of Pt atoms is more serious, as the lower value
of Robs, 0.30, suggests. It is also interesting to discuss the extent
of metallic interaction based on the alloy extent of “Pd” and
3.7. Composition, Atomic Distribution, and Nanostructure
of Pd/Pt Bimetallic Clusters. The composition of the Pd/Pt
cluster is calculated from XAS by measuring the edge jump
(∆µx) at the Pd K-edge and the Pt LIII-edge, and it is found
that the atomic ratio of Pd:Pt is 1.2:1 (Pd1.2Pt). Recently, we
have explored an XAS based methodology to determine the
alloying extent or atomic distribution of bimetallic nanopar-
6
ticles. We have applied this methodology to the Pd/Pt system
investigated here. Basically, in the case of a homogeneous
system for which the core of the cluster is composed of N atoms
of A (NA) and the surface is made of N atoms of B (NB), the
total coordination (NAA + NAB) is equal to 12 for the A atom
and less than 12 for the B atom.45 More generally, we have the
following relationships:
“Pt”. In this case, the homoatomic coordination number of “Pd”
atoms around the Pd core is slightly higher than the heteroatomic
coordination number of “Pt” atoms around the Pd core,
indicating that there are probably two regions on the local Pd
cluster structure. One of the regions looks like Pt-Pd alloy
particles stacked together, and the other is a Pd-Pd stack. As
we can see in Tables 2 and 3, the time taken for Cl ligand
transfer to OH ligand in the case of the Pd is shorter than the
time taken for the same in the case of the Pt. The higher ligand
exchange rate implies that the Pd-OH can be first reduced to
NAA + NAB ) 12 > NBA + N , (NA)N ) (NB)NBA
,
BB
AB
σAB ) σBA, RAB ) RBA (3)
In the case of the Pd/Pt bimetallic clusters of the present study,
the coordination numbers of the Pd and Pt atoms around the
Pd atom are found to be 5.24 and 4.42, respectively, and the
total coordination number of A ) ∑NPd-i is 9.66. The coordina-
0
Pd and that it would be in the core of the bimetallic cluster.
From the Pt LIII-edge XAS, we found that the homoatomic

21574 J. Phys. Chem. B, Vol. 109, No. 46, 2005
Chen et al.
out in this direction and provides a detailed insight into the
mechanism and growth of Pd/Pt bimetallic clusters.
Acknowledgment. The financial support from the National
Science Council (under Contract Numbers NSC93-2811-E-011-
0
08, NSC94-2214-E-011-010, and NSC94-2120-M-011-002),
the National Taiwan University of Science and Technology, and
the National Synchrotron Radiation Research Center (NSRRC)
is gratefully acknowledged.
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(
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(
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