1260

J. Phys. Chem. C 2010, 114, 1260–1267

Synthesis and Site Structure of a Replica Platinum-Carbon Composite Formed Utilizing
Ordered Mesopores of Aluminum-MCM-41 for Catalysis in Fuel Cells
Kazuki Oka,† Yoshiyuki Shibata,‡ Takaomi Itoi,§ and Yasuo Izumi*,†Department of Chemistry, Graduate School of Science, Department of Nanomaterial Science, Graduate Schoolof AdVanced Integration Science, and Department of Mechanical Engineering, Graduate School of Engineering,
Chiba UniVersity, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
ReceiVed: October 16, 2009

Platinum nanoparticles have been reported with mean sizes between 1.5 and 7 nm supported on carbon. The
contact between Pt nanoparticles and C has never been controlled and monitored nanoscopically. In thispaper, stable Pt nanoparticles with a mean size of 1.2 nm were synthesized embedded on/in a C matrix
catalytically produced from acetylene over the Pt nanoparticles. The replica Pt-C composite was synthesized
inside of the ordered mesopores (2.7 nm) of Al-MCM-41 followed by removal of the template. The contact
between the Pt nanoparticle and C was experimentally observed by high-energy resolution Pt L2-edge XANES
spectra tuned to 11065.7 eV, at a lower energy by 5 eV than the Pt Lβ1 peak top for the replica Pt-C pressed
to electrolyte polymer (Nafion). The spectra were nicely reproduced in a theoretical spectrum using ab initio
multiple scattering calculations for the interface Pt site between cuboctahedral Pt38 and graphite layers. Other
Pt sites detected in state-selective Pt L2-edge XANES were exclusively metallic for replica Pt-C/Nafion
either in air or in H2. The thus-characterized replica Pt-C composite was tentatively tested as a cathode ofa H2-air polymer electrolyte fuel cell in comparison to commercial 20 wt % Pt/Vulcan XC-72 as the cathode.
The improvement of Pt dispersion stabilized on/in a C matrix, effective contact of Pt with C, and diffusionof O2 in a few nanometers of replica Pt-C powder was suggested.
Introduction
The control of platinum nanoparticle size between 1.5 and 7
nm supported on carbon is essential in the application to fuelcells1 and detoxification of exhaust gas from automobiles.2 Pt
nanoparticles on C nanofibers (mean Pt size of 1.7-4 nm),3,4 C
nanotubes (mean Pt size of 2-6.7 nm),4–10 ordered mesoporous
C (mean Pt size of 1.5-3 nm),11–13 C powder (mean Pt size of
1.6-4.8 nm),14–17 and spherical C (mean Pt size of 2.9 nm)18
and Pt monolayer on nanoparticles of other metals19 have beensynthesized in narrow particle size distribution.
The polymer electrolyte fuel cell (PEFC) is expected to be aportable and compact power source to start at atmospherictemperature and be operated at lower than 373 K for automobileand home use.20 In the design of PEFCs, the cathode catalyst
for the oxygen reduction reaction (O2 + 4H+ + 4e- f 2H2O)
is most critical because the physical contact of Pt with C (electriccontact), the electrolyte (wet polymer to transport protons), andoxygen gas needs to be enabled at the same time.1 The
morphologic effects of Pt nanoparticles, films, and alloys on
the oxygen reduction reaction were reported.1,21 In contrast that
the Pt nanoparticles have been formed on various types ofcontrolled C materials,3–17 the electric contact of Pt with C has
never been controlled and monitored nanoscopically.
In this work, the contact of Pt with C was controlled basedon the idea of catalytic acetylene decomposition over Pt
nanoparticles in a narrow size distribution centered at 1.2 nm
formed inside of Al-exchanged ordered mesoporous silica Al* To whom correspondence should be addressed. E-mail: yizumi@
faculty.chiba-u.jp. Phone and Fax: +81-43-290-3696.
†
Graduate School of Science.
‡
Graduate School of Advanced Integration Science.
§
Graduate School of Engineering.

MCM-41 to produce Pt-C composites in mesopores. The
replica Pt-C composite was separated via the removal of AlMCM-41 using hydrofluoric acid. The synthesis of the replica
Pt-C composite was monitored using high-resolution transmission electron microscope (TEM), X-ray diffraction (XRD), andBrunauer, Emmett, and Teller (BET) adsorption of N2. The local
structure and heterogeneity of Pt sites were analyzed based onconventional Pt L3- and L2-edge X-ray absorption fine structure(XAFS) and state-selective Pt Lβ1-selecting Pt L2-edge XAFS,
respectively. State-selective XAFS was applied to Pt catalysts
for the first time.
The Pt nanoparticles in a narrow size distribution centeredat 1.2 nm are advantageous to use most of the Pt sites (80%)
for oxygen reduction;1 however, mass-specific activity foroxygen reduction is still under debate and/or critically depends
on reaction conditions. The rate constant values were nearly
constant for mean Pt particles between 1.6 and 4.8 nm on C.17
The mass-specific activity gradually decreased when the mean
Pt particle size decreased from 30 to 1 nm22 or reached amaximum at 3.5 nm.23 The instability of Pt particles as small
as 1 nm was suggested to be transformed into amorphous andless active.9 In this paper, stabilization of Pt nanoparticles (mean
1.2 nm) was tried on/in a C matrix catalytically formed fromacetylene in mesoporous space. The electronic structure of theinterface Pt site between Pt nanoparticles and C was investigated. The replica Pt-C was pressed as a tentative cathodecatalyst of membrane electrolyte assembly (MEA).
Experimental Section
Synthesis. Ten milliliters of an 8.4 mM aqueous solution oftetraammineplatinum(II) hydroxide hydrate PtII(NH3)4(OH)2 · 2H2O(Strem Chemicals) and 1.0 g of Al-MCM-41 (molar ratio SiO2/

10.1021/jp909934r  2010 American Chemical Society
Published on Web 12/17/2009

Analysis and Catalysis of a Replica 1.2 nm Pt-C Composite

J. Phys. Chem. C, Vol. 114, No. 2, 2010 1261

Al2O3 ) 0.9875/0.0125, 1.11 wt % Al; Aldrich) were mixed andstirred at 353 K for 2 days. The suspension was filtered, washed,and heated in vacuum at 573 K for 2 h. The obtained powder(Pt-Al-MCM-41, 1.3 wt % Pt) was in a 30 mL min-1 H2(>99.99%; 101 kPa) flow at 573 K for 2 h and then in a 200 mL
min-1 C2H2 (>98%) + N2 (>99.999%) flow (molar ratio 1:9, total
101 kPa) at 973 K for 1 h. The obtained black powder (0.72 wt %Pt) was treated with 30 mL of hydrofluoric acid (15%; special
grade, Wako Pure Chemical) to give the replica Pt-C composite(0.84 wt % Pt). Fortunately, in view of electric conductivity,
amorphous C dissolved in hydrofluoric acid, and Pt-graphite waspreferably obtained.6,24 Somanathan et al. prepared Pt nanoparticles
starting from hydrogen hexachloroplatinate(IV) in Al-MCM (molar
ratio SiO2/Al2O3 ) 0.995/0.005) followed by calcination at 823
K. In 200 mL min-1 of C2H2 + N2 flow (molar ratio 3:7), the startof C decomposition at 923 K, carbon nanotube formation between
973 and 1073 K, the maximum C decomposed at 1073 K, andpreferable formation of graphitic sheets and amorphous C at 1173
K were reported.6 The starting temperature of C decomposition
and the maximum decomposed C yield were reproduced in thiswork, but we chose a relatively low temperature of 973 K for C2H2
decomposition to prevent the thermal growth of Pt nanoparticles.
Replica Pt-C powder (76 mg) was dispersed in 1.0 mL of
15% Nafion dispersion solution (DE2021-CS, Wako Pure
Chemical) using ultrasonic treatment (85 W, 28 kHz) and was
mounted on 190 µm thick C paper coated with polytetrafluoroethylene (TGP-H-060H, Chemix). Similarly, 25 mg of commercial 20 wt % Pt supported on C (Vulcan XC-72, Cabot)
was dispersed on TGP-H-060H using a 5-10% Nafion dispersion solution. As a reference, mechanically dispersed Pt/Vulcan
XC-72 on TGP-H-060H was also prepared. A 50 µm thickelectrolyte polymer Nafion film (NR-212, Dupont; >95%; acidcapacity > 9.2 × 10-4 equiv g-1) was pressed with replica Pt-C/
TGP-H-060H as the cathode and 20 wt % Pt/Vulcan XC-72/
TGP-H-060H as the anode for MEA. Another MEA was madeusing the same Nafion film pressed with 20 wt % Pt/Vulcan
XC-72/TGP-H-060H on both sides. The area of the electrodecatalysts was 5 cm2.
Characterization. Nitrogen adsorption measurements were
performed at 77 K with the pressures between 1.0 and 90 kPa
in a vacuum system connected to diffusion and rotary pumps(10-6 Pa) and equipped with a capacitance manometer (models
CCMT-1000A and GM-2001, ULVAC). The samples were
evacuated at 393 K for 2 h before measurements. Highresolution TEM images were taken using LaB6 source TEM
equipment (JEOL, Model JEM-4000FX) with an accelerating
voltage of 400 kV. Samples were dispersed in ethanol (>99.5%,
Wako Pure Chemical) and mounted on amorphous C-coatedcopper mesh (CU150 Mesh, JEOL).
XRD data were obtained using a Rigaku MiniFlex diffractometer at a Bragg angle of 2θB ) 2-10° for ordered mesopores
and 10-75° for crystallites of C and Pt. The conditions involved
were 30 kV and 15 mA, Cu KR emission, and a nickel filter.
The replica Pt-C powder and each intermediate composite
during the synthesis were set in a dip of 18 × 2 × 1 mm3 on
glass plate to measure the electric conductivity with the twopoint method.25
The powder samples for synchrotron X-ray measurements
were prepared in the vacuum system (10-6 Pa) and transferredin situ to a Pyrex glass cell equipped with 25-50 µm thick
Kapton (Dupont) windows on both sides. The samples in H2,air, or argon were sealed with fire and transported to thebeamline. The replica Pt-C powder was mounted on TGP-H060H, as described above, but pressed to only one side of Nafion(NR-212) for synchrotron X-ray study. The Nafion and protection polyester films (50 µm) were used as a window of Pyrex
glass cells.
Conventional Pt L3- and L2-edge XAFS spectra were measured at 290 K in transmission mode in the Photon Factory at
the High-Energy Accelerator Research Organization (Tsukuba,
Japan) on beamlines 9C and 12C. The storage-ring energy was
2.5 GeV, and the ring current was between 450 and 250 mA.
A Si(111) double-crystal monochromator and cylindrical double
mirror were inserted into the X-ray beam path. The parallelnessof the double crystals was set to 65% intensity and to the
maximum flux using a piezo translator. The slit opening size
was 1 mm × 1 mm in front of the I0 ionization chamber. The
I0 and It ionization chambers were purged with the mixture ofAr(15%) + N2(85%) and Ar, respectively. The scan steps were
∼7.8, ∼0.46, and ∼2.5 eV in the pre-edge, edge, and postedge
regions, respectively. The accumulation time was 1-10 s for adata point. The Pt L3- and L2 absorption edge energy values
were calibrated to 11562 and 13272.3 eV, respectively, for the
spectra of Pt metal.26,27
State-selective Pt L2-edge XAFS measurements were performed at 290 K at the beamline 37XU of SPring-8 (Sayo,Japan). The storage ring energy was 8 GeV at the top-up ring
current of 100 mA. A Si(111) monochromator and rhodiumcoated mirror were used. The undulator gap was optimized tomaximize the X-ray beam flux at each data point. To stabilize
the X-ray beam position on the surface of the sample, themonochromator stabilization mechanism was used.28
The Pt Lβ1 emission spectra were measured using a homemade Rowland-type fluorescence spectrometer set at beamline
37XU.28–30 A Johann-type spherically bent Ge(844) crystal(Saint-Gobain; curvature radius of 450 mm, d ) 0.57737 Å)
and NaI(Tl) scintillation counter (SC; Model SP-10, Oken) were
mounted. The energy resolution of the fluorescence spectrometer
was estimated to be 3.4 eV at Pt Lβ1, including the contributionof the beamline. The apparent core-hole lifetime width for Pt
Lβ1-selecting XAFS was estimated to be 2.0 eV based on theformula31

1
ΓApp

2

)

1
ΓPt 3d3/2

2+

1
ΓPt 2p1/22

The gas for the ion chamber was 30% N2 and 70% He in
front of the sample. The slit opening in the front of the ionizationchamber was 0.3 mm (horizontal) × 1.0 mm (vertical). The
sample was placed in a plane near horizontal, tilted by 6° toward
the incident X-ray and by 7° toward the Ge crystal. The sample
surface, Ge crystal, and slit in front of the SC were controlled
on a Rowland circle (radius 225 mm) in the vertical plane. The
openings of the receiving slit and the slit in front of the SC
were 8.0 mm (h) × 0.1 mm (v). The sample and SC werecovered with a lead plate housing, except for the X-ray paths.
With the excitation energy set to 13298.8 eV, Pt Lβ1 emission(M4 f L2; θB ) 75.895°) spectra were measured. The scanstep was ∼0.58 eV, and the accumulation time was 60 s for adata point. The fluorescence spectrometer was tuned to eachenergy around the Pt Lβ1 emission peak, and Pt L2-edge XANES(X-ray absorption near edge structure) spectra were measured.
The scan step was ∼0.46 eV with an accumulation time of
40-60 s for a data point. The Pt Lβ1 fluorescence energy value
was calibrated to 11070.84 eV for the spectrum of Pt metal.26,27
The energy positions of the monochromator and the fluorescence1262

J. Phys. Chem. C, Vol. 114, No. 2, 2010

Oka et al.

spectrometer were reproduced within (0.1 and (0.2 eV,
respectively.
Analyses. The XAFS data were analyzed with XDAP (XAFS
Services International).32 The pre-edge background was approximated by a modified Victoreen function C2/E2 + C1/E +C0. The background of the postedge oscillation was approximated by a smoothing spline function, calculated by anequation for the number of data points where k was thewavenumber of photoelectrons.
data_points(µxi - BGi)2i)1exp(-0.075ki2)∑

e smoothing factor

Multiple shell curve fit analyses were performed for the
Fourier-filtered k3-weighted EXAFS data in k and R space using
empirical amplitude and phase shift parameters extracted fromEXAFS of PtO2 and Pt metal for Pt-O [bond distance R(Pt-O)) 0.1993 nm with the coordination number (N) of 6]33 and
Pt-Pt bonds [R(Pt-Pt) ) 0.2775 nm with the N of 12]4 basedon reported crystal structures. The many-body reduction factor
S02 was assumed to be equal for the sample and reference. The
goodness of fit was given as requested by the Committee onStandards and Criteria in X-ray Absorption Spectroscopy.
The Pt L3- and L2-edge XANES spectra were theoreticallygenerated using ab initio calculation code FEFF 8.4,34 operated
in a self-consistent field and full multiple scattering modes.
Theoretical calculations of Pt XANES were reported to clarify
the Pt cluster size effect and spectral change upon H/CO
adsorption.35–37 The exchange-correlation potential of HedinLundqvist was chosen and corrected by adding a constant shiftof +1.0 eV to the Fermi level. The calculated potential was
also corrected by adding a constant shift of -1.6 eV to the “pureimaginary optical potential” to compare to Pt Lβ1-selecting Pt
L2-edge spectra, but no shift was given for comparison toconventional XANES. The energy of the theoretically generated
spectrum was shifted by +4.7 eV to compare to conventional
Pt L3/2-edge spectra and Pt L2-edge spectra tuned to the Pt Lβ1
peak top (11070.7-11070.8 eV). Corresponding to the Pt L2edge spectra tuned to 11065.7 and 11075.4 eV, the energies of
the theoretically generated spectra were shifted by 0 and +8.2
eV, respectively.
The Pt metal site model consisted of 38 atoms of a facecentered cubic (fcc) Pt metal cuboctahedron. Among the Pt
atoms, N(Pt-Pt) values of 6, 8, and 24 atoms were 12, 9, and6, respectively.38 The central Pt atom in the (111) surface (N )9) was considered as a representative Pt site for calculations(Figure 1) compared to an average N of 7.6 for all of the Pt
sites. A central Pt atom in the (111) surface was in vacuum orat the interface with 177 atoms of three layers of graphite[R(C-C) ) 0.1418 nm]. The Pt-C bond length was varied
between 0.16 and 0.22 nm based on the crystal structures oforganometallic compounds.39 Relatively long R(Pt-C) valuesof 0.320-0.359 nm were used in the calculation model of Pt37
on graphite.40 The Pt L2-edge spectrum tuned to 11065.7 eV
for replica Pt-C/Nafion was best reproduced theoretically by
setting the R(Pt-C) to 0.18 nm. Thus, the nearest Pt-C bonddistance was fixed to 0.18 nm. Average R(Pt-C) and N values
were 2.0 nm and 1.9, respectively, for seven Pt atoms at theinterface (Figure 1).
Results
N2 Adsorption, XRD, TEM, and Conductivity. The specificsurface area (SBET) of 970 m2 g-1 for Al-MCM-41 decreased

Figure 1. The Pt38 cluster model on three layers of graphite for XAFS
analyses.

by 21% upon impregnation of Pt (Table 1). In contrast to thispartial block of mesopores, the SBET for Pt-Al-MCM-41
decreased by 95% after catalytic C2H2 decomposition, suggesting that a (nearly) complete block of mesopores by C formed.
The loss of SBET was recovered to 540 m2 g-1 with the HF
treatment (Table 1).
The regularity of the MCM-41 framework was monitored by
XRD during the synthesis step of replica Pt-C. The distance
between the pore and neighboring pore (a0 of the unit cell) was
4.7 nm (d100 ) 4.1 nm) for Al-MCM-41 (Table 1).6,41 The value
decreased to 4.5 nm (d100 ) 3.9 nm) upon impregnation of Pt,
in a similar trend to ref 6. In contrast, no peak was detected in
XRD for Pt-C-Al-MCM-41 probably because the electron
density of C formed in Al-MCM-41 mesopores was similar tothat of the Al-SiO2 template. The lower-angle peaks recoveredat d ) 3.7 and 3.2 nm for replica Pt-C composites, but theintensity was weaker compared to the peak for Pt-Al-MCM41. No distinct peaks derived from the Pt or C crystallite wereobserved throughout the synthesis step of replica Pt-C. Aftera fuel cell test for 3 h, the replica Pt-C composite used as thecathode was re-examined by XRD. No peak was observed in
the range of 2θB ) 10-75°, suggesting the Pt particle size
remained small.
The TEM image for replica Pt-C is depicted in Figure 2A.
Darker dots of Pt particles are clearly observed between 0.38
and 3.33 nm (Figure 2C). The mean particle size is 1.2 nm,with a standard deviation of 0.5 nm. The mean size corresponds
to a total of 66 atoms in one particle.38 Most of the Pt particles
are within the pore size of Al-MCM-41 (2.7 nm) (Figure 2C).
The replica C is not clear in TEM images probably because theaxis of replica Pt-C may randomly distribute, and the diameterof the rod/tube-like structure should be less than 3 nm dispersed
over amorphous C coated on Cu mesh. However, the straightarrangement of darker Pt spots is observed, reminiscent of theordered mesoporous structure of the Al-MCM-41 framework(Figure 2B).
The electric conductivity for Pt-Al-MCM-41 of less than 1µS cm-1 dramatically increased to 263 mS cm-1 upon catalytic
C2H2 decomposition (Table 1). The value was by 17% superior

Analysis and Catalysis of a Replica 1.2 nm Pt-C Composite

J. Phys. Chem. C, Vol. 114, No. 2, 2010 1263

TABLE 1: Physicochemical Characterization of Intermediate and Final Materials in the replica Pt-C Composite Synthesis

samplespecific
SBET(m2 g-1)

Al-MCM-41
Pt-Al-MCM-41
Pt-C-Al-MCM-41
replica Pt-C
Pt/Vulcan XC-72
hexagonal Pt-block copolymer composite

970
767
35
540
18

TEMd
RPore-Pore(nm)a

Pt loading(wt%)b

4.7
4.5
n.p.f
30

1.3
0.72
0.84
20
74

NPt-Ptc

NPt-O or
NPt-Cc

5.1
6.1
12

2.1
2.2
-

XRDe

dPt-Pt (nm)
1.2
-

n.p.f
n.p.,f n.p. f,g
4.8

electric
conductivity(mS cm-1)<0.001<0.001
263
259
224
2.5

ref
this work
this work
this work
this work
this work
25

aDistance from the center of the pore to the center of the neighboring pore based on XRD. b Based on the Pt L3-edge jump value.
Coordination number based on the Pt L3-edge EXAFS curve fit analysis. d Average particle size of Pt based on TEM image. e Average particle
size of Pt based on XRD. f No peak detected. g Sample after PEFC cell test for 3 h.

cFigure 3. Normalized Pt L3-edge XANES spectra for Pt-Al-MCM41 (a), Pt-C-Al-MCM-41 (b), replica Pt-C in air (c), replica Pt-C/
Nafion in air (d), Pt metal of 5 µm thickness (e), PtO2 (f), and 20 wt% Pt/Vulcan XC-72/Nafion (g).

Figure 2. High-resolution TEM image for replica Pt-C (A). Theinternal size of mesopores of Al-MCM-41 was added in (B). The size
distribution of the darker dots of Pt particles (C).

to that for 20 wt % Pt/Vulcan XC-72. The value for replica
Pt-C (259 mS cm-1) was comparable to that for Pt-C-AlMCM-41 (Table 1).
Conventional XANES. Conventional Pt L3-edge XANES
spectra were measured during synthesis steps of the replica
Pt-C composite (Figure 3). The peak positions of XANES for
Pt-Al-MCM-41 (11565.7, 11580.5, and 11594.2 eV; spectrum(a)) were similar to those for Pt metal (spectrum (e)). The
intensity of three postedge peaks was weaker for Pt-Al-MCM41 compared to that for corresponding peaks for Pt metal,
demonstrating that the Pt particles in Pt-Al-MCM-41 were on
the nanometer scale.

The peak at 11566.5 eV for Pt-C-Al-MCM-41 (spectrum(b)) became weaker and broader compared to that at 11565.7
eV in spectrum (a). An attempt was made to fit spectra (a) and(b) with theoretical spectra for a metallic Pt site or interface Ptsite between the metal and graphite. It was difficult to fit because
spectrum (a) seemed to be a mixture of metallic and oxidic Pt
sites. Spectrum (b) seemed to be a mixture of metallic Pt and
Pt at the interface with C. Instead, Pt Lβ1-selecting XANES
spectra were compared to theoretical data because the Pt state
was selected and the spectral pattern was sharpened.28,29,42–47
The XANES spectrum for the replica Pt-C composite in air
is shown in Figure 3c. The energy of first peak above theabsorption edge (11566.7 eV) is intermediate between the peakat 11565.6 eV for the Pt metal (spectrum (e)) and the one at
11567.3 eV for PtO2 (spectrum (f)). The first peak for spectrum(c) is more intense (1.47) than those for spectra (a) and (b)(1.33-1.19), and the postedge pattern of spectrum (c) becamefeatureless. Thus, oxidation of part of metallic Pt in Pt-C-AlMCM-41 was suggested after the removal of the Al-MCM-41
framework. In the XANES spectrum for Pt sites in replica Pt-C/
Nafion in air (Figure 3d), the first peak above the absorption
edge shifted down to 11566.5 eV, and the intensity decreased(1.34). The peak intensity was essentially identical to that forspectrum (a) (1.34 versus 1.33). No peak feature at 11590.5 eV
for PtO2 (spectrum (f)) was detected in spectrum (d). The
postedge pattern of spectrum (d) was most like that of spectrum(b).
In comparison, a XANES pattern of 20 wt % Pt/Vulcan XC72/Nafion (Figure 3g) corresponded exactly to that for Pt metal(spectrum (e)). The mean Pt particle size in the sample was 4.8
nm based on the peak width of XRD, much greater than that in
the replica Pt-C composite (1.2 nm) and in less contact with
Nafion.

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Oka et al.

Figure 4. Pt Lβ1 emission spectra for Pt/Vulcan XC-72/Nafion in H2 (a), replica Pt-C in air (b), replica Pt-C/Nafion in H2 (c), and replica
Pt-C/Nafion in air (d). The arrows indicate tune energy values for Pt Lβ1-selecting XAFS measurements (Figure 5).

X-ray Emission and State-Selective XANES. The Pt Lβ1emission peaks for replica Pt-C/Nafion in H2 or in air appeared
at 11070.7-11070.8 eV (Figure 4c, d), essentially identical tothat for Pt metal (11070.84 eV). The emission peak for replica
Pt-C powder in air was 11071.0 eV (spectrum (b)), shifted by+0.2 eV from that for Pt metal. The fwhm (full width at halfmaximum) values were relatively greater (7.5-7.6 eV) for
replica Pt-C/Nafion and replica Pt-C powder compared to
7.1-7.2 eV for standard Pt compounds and 20 wt % Pt/Vulcan
XC-72/Nafion (spectrum (a)).
Then, Pt L2-edge XANES spectra were measured (Figure 5B)
tuned to the Pt Lβ1 peak top (11070.7-11071.0 eV; the arrows
in Figure 4). The absorption edge position progressively shiftedfrom 13272.3 eV for Pt metal (spectrum (a)) to 13272.8 eV for
replica Pt-C/Nafion in H2 (spectrum (f)), 13272.9 eV for replica
Pt-C/Nafion in air (spectrum (e)), or 13273.4 eV for replica
Pt-C in air (spectrum (d)) and then to 13274.2 eV (PtO2,
spectrum (b)). Despite the gradual edge shift, postedge peaks
appeared at almost the same position (13287-13289, 13302,
13312, and 13327 eV) for replica Pt-C/Nafion in H2/in air andreplica Pt-C in air (spectra (d)-(f)) as those for Pt metal(spectrum (a)). Thus, metallic Pt states were selected in replica
Pt-C samples (as powder or on Nafion) in XANES spectra
tuned to 11070.7-11071.0 eV.
Pt L2-edge XANES spectra tuned to the lower-energy sideof Pt Lβ1 peaks (11065.7-11067.0 eV; the arrows in Figure 4)
are shown in Figure 5A. The first intense peak at 13272.3-13273.3
eV above the absorption edge (spectra A(e), (f)) became weakercompared to the peaks in spectra tuned to the Pt Lβ1 peak top(panel B(e), (f)). The absorption edge position for replica Pt-C/
Nafion shifted from 13267.9 eV in H2 to 13268.1 eV in air.
The postedge pattern for 20 wt % Pt/Vulcan XC-72/Nafion(Figure 5A(c)) resembled that of metallic Pt tuned to the Pt
Lβ1 peak top (Figure 5B(a)) if the spectrum B(a) shifted by 2.3
eV toward the lower-energy side. In contrast, the postedge
pattern of spectra A(e) and (f) for replica Pt-C/Nafion becamefeatureless in panel A and did not resemble that of metallic Pt

or PtO2. The spectra (e) and (f) resembled theoretical spectra
for the interface Pt atom between Pt metal and graphite (Figures
1 and 5A(h)) when the energy was shifted by -4.7 eV relative
to XANES tuned to the Pt Lβ1 peak top.
Next, Pt L2-edge XANES spectra tuned to the higher-energy
side of Pt Lβ1 peaks (11074.1-11075.4 eV; the arrows in Figure4) were measured for replica Pt-C/Nafion compared to 20 wt% Pt/Vulcan XC-72/Nafion (Figure 5C). The postedge pattern
for spectra C(e) and (f) in Figure 5 resembled that of corresponding spectra tuned to the Pt Lβ1 peak top (Figure 5B(e),(f)) if the spectra B(e) and (f) shifted by 3.5 eV toward thehigher-energy side. The spectra (e) and (f) resembled theoretical
spectra for a Pt atom in a Pt nanoparticle (Figure 5C(g)) whenthe energy was shifted by +3.5 eV relative to XANES tuned
to the Pt Lβ1 peak top (Figure 5B(g)). The absorption edge
position for replica Pt-C/Nafion in air (13276.4 eV) was by
1.0 eV greater than that in H2. Postedge peak shifts were within
1 eV toward the higher-energy side by switching the ambientgas from H2 (spectrum (f)) to air (spectrum (e)).
The postedge pattern for 20 wt % Pt/Vulcan XC-72/Nafion(Figure 5C(c)) resembled that of metallic Pt tuned to the Pt
Lβ1 peak top (Figure 5B(a)) if the spectrum B(a) shifted by 2.7
eV toward the higher-energy side. The Pt Lβ1 tune energy forspectrum C(c) was greater by 3.4 eV compared to the peak top.
The Pt Lβ1 tune energy for spectrum A(c) was smaller by 3.7
eV compared to the peak top, and the XANES peaks shift was
2.3 eV. The reduction of the tune energy difference versus the
XAFS energy difference was 62-80% mainly due to the finalstate relaxation effect.48
Discussion
Replica Pt-C Morphology. The Pt loading in Pt-Al-MCM41 was 1.3 wt % after cation exchange and reduction in H2 at
573 K (Table 1). The amount corresponds to 67.5 µmol of Pt
substituting on the proton sites for 1 g of Al-MCM-41 (412µmol of Al g-1). Therefore, 32.8% (or 16.4%) of the proton

Analysis and Catalysis of a Replica 1.2 nm Pt-C Composite

Figure 5. Normalized Pt L2-edge XANES spectra for Pt metal (a),PtO2 (b), 20 wt % Pt/Vulcan XC-72/Nafion in H2 (c), replica Pt-C in
air (d), replica Pt-C/Nafion in air (e), and replica Pt-C/Nafion in H2(f). The tune energy was 11067.0 eV (c) and 11065.7 eV (e, f) in panelA, 11070.8 eV (a), 11070.6 eV (b), 11071.0 eV (d), 11070.7 eV (e),and 11070.8 eV (f) in panel B, and 11074.1 eV (c) and 11075.4 eV (e,f) in panel C (see the arrows in Figure 4). Theoretically generated Pt
L2-edge XANES spectrum for a central Pt atom in the (111) face of a
cuboctahedral Pt38 cluster (g) and the atom in contact with graphite
layers [R(Pt-C) ) 0.18 nm, R(C-C) ) 0.1418 nm] (h). The energy
shifts were 0, 4.7, and 8.2 eV for spectrum A(h), B(g), and C(g),respectively.

sites of Al-MCM-41 were exchanged to [Pt(NH3)4]2+ species(or [Pt(NH3)4(OH)]+ species). This exchange ratio was greaterthan values for Pt zeolites,49 but the Pt amounts introduced into
the samples were similar between 0.6 and 2.0 wt % due to thedifference of Al concentrations, 10 - 12 wt % for zeolites49
versus 1.11 wt % for Al-MCM-41 of this work.
The Pt loading for Pt-C-Al-MCM-41 was 0.72 wt % after
catalytic C2H2 decomposition at 973 K over Pt particles in AlMCM-41 (Table 1). Therefore, the C content in the Pt-C-AlMCM-41 was 44.6%. If the pore volume for Al-MCM-41 was
0.6 cm3 g-1,41 the density of formed C was 2.25 g cm-3
(graphite), and catalytically formed C remained exclusively
inside of the mesopores of Al-MCM-41 (HR-TEM), it isestimated that 60.5% of the mesopores was stuffed. No C rod/
tube pushed out of mesopores was detected in TEM images for
Pt-C-Al-MCM-41 (not shown). The SBET value for Pt-AlMCM-41 (767 m2 g-1) decreased by 95% after catalytic C2H2
decomposition (Table 1), supporting the effective block ofmesopores. Only the external surface of Al-MCM-41 adsorbed
N2.
The Pt loading for replica Pt-C was 0.84 wt % after theremoval of the Al-MCM-41 framework (Table 1). Therefore,
47.4% of Pt in Pt-C-Al-MCM-41 was lost during the washingwith HF solution. The specific SBET value for replica Pt-C

J. Phys. Chem. C, Vol. 114, No. 2, 2010 1265
recovered as much as 540 m2 g-1 (Table 1). When the catalytic
C2H2 decomposition temperature was 1073 K, thermally decomposed C was formed as a byproduct but could be separated
from Pt-C-Al-MCM-41. The C content in the Pt-C-AlMCM-41 was 66.0%. It is estimated that 143.9% of themesopores for Al-MCM-41 were stuffed, suggesting excessive
C was pushed out of mesopores. At 873 K, acetylene did notreact either catalytically or thermally, consistent with ref 6.
When catalytic C2H2 decomposition was performed at 973
K in a closed circulation system or in a batch for Pt-Al-MCM41, the decomposition was by 75% slower than that in the flow
setup in this study. Formed H2 due to C2H2 decomposition
remained in the system and inhibited further C2H2 decomposition
due to equilibrium. When MCM-41 or FSM-16 was used insteadof Al-MCM-41, ion exchange was not applicable to introduce
Pt species. Catalytic C2H2 decomposition was performed on the
Pt nanoparticles, followed by HF treatment. The Pt content in
obtained replica Pt-C was only 0.09 wt %.
The mean Pt particle size was 1.2 nm for replica Pt-C (Figure
2A, C). The average N value for Pt-Pt bonds by EXAFS (6.1;Table 1) suggested even smaller Pt nanoparticles.38 This
discrepancy is due to the contribution of NPt-O or NPt-C.
Pt-C-Al-MCM-41 and replica Pt-C samples both measured
in air provided NPt-O values of 2.1-2.2, including the undividable contribution of NPt-C (Table 1) because the exterior of these
Pt nanoparticles should be oxidized and/or carburized.
Pt Sites in Replica Pt-C. Partial oxidation of Pt sites wasdemonstrated in conventional Pt XANES for a replica Pt-C
composite (Figure 3c). In contrast, a high energy resolution Pt
L2-edge XANES spectrum (Figure 5B(d)) tuned to the Pt Lβ1
peak top (11071.0 eV; Figure 4b) preferably selected metallic
Pt. The emission of Pt(IV) sites appeared at 11070.6 eV (notshown), and the spectral pattern tuned to 11070.6 eV for PtO2
(Figure 5B(b)) was totally different from that in spectrum (d).
Conventional XANES for replica Pt-C/Nafion (Figure 3d)
was similar to that for Pt metal (spectrum (e)) in the absorption
edge region or that for Pt-C-Al-MCM-41 (spectrum (b)) in
the postedge region. The XANES spectra tuned to the Pt Lβ1
peak top for replica Pt-C/Nafion both in air and in H2
exclusively detected metallic Pt sites (Figure 5B(e), (f)). The
spectra resembled well theoretical spectrum B(g) generated forthe surface Pt site on the (111) face of a cuboctahedral Pt38
cluster.
When the tune energy was on the lower energy side of thePt Lβ1 peak for replica Pt-C/Nafion (Figure 4c, d), the firstintense peak at 13272.3-13273.3 eV became weaker (Figure
5A(e), (f)), suggesting greater occupation of the Pt 5d band.
The spectra (e) and (f) resembled well theoretical spectrum
5A(h) calculated for the interface site between the Pt nanoparticle and graphite layers (Figure 1). Thus, interface Pt sites werepredominantly selected in Pt L2-edge XANES tuned to 11065.7
eV for replica Pt-C/Nafion, and negative charge transfer fromC to Pt nanoparticles was suggested. However, the selective
detection of interface Pt sites was unsuccessful for Pt(mean size
4.8 nm, Table 1)/Vulcan XC-72/Nafion. Only dominant metallic
Pt sites appeared in XANES tuned to 11067.0 eV.
EXAFS, in general, provides statistical bonding information
for Pt in a sample irradiated with X-ray. Therefore, Pt-C bonds
are statistically preferably detectable for the Pt sites in Pt
particles in a narrow size distribution centered at 1.2 nm
compared to Pt sites in Pt particles of mean size 4.8 nm. Evenso, the contribution of Pt-C bonds was statistically minor for
replica Pt-C compared to that of Pt-Pt bonds (Figure 3a, b).
High energy resolution XAFS in this work still provides

1266

J. Phys. Chem. C, Vol. 114, No. 2, 2010

statistical bonding information; however, the information is
specific for Pt sites corresponding to a lower Pt Lβ1 emission
energy of 11065.7 eV for replica Pt-C/Nafion (Figure 5A(e),(f)) and Pt/Vulcan XC-72/Nafion (Figure 5A(c)). Thus, thediscussion is reasonable that Pt sites in contact with C werediscriminated among smaller Pt particles in replica Pt-C/Nafion,
and the XANES spectrum was compared to theoretical XANES
data (Figure 5A(h)) for the interface Pt sites depicted in Figure
1. Even after being tuned to a lower Pt Lβ1 energy of 11067.0
eV, the Pt-C interface site was not discriminated for Pt/Vulcan
XC-72/Nafion because the site population was too small among
Pt sites corresponding to lower Pt Lβ1 emission energy. It isapparent that the reason was the greater mean Pt particle sizeand predominant population of Pt-Pt bonds in Pt/Vulcan XC72/Nafion.
The XANES spectra tuned on the higher-energy side of Pt
Lβ1 (Figure 4c, d) for replica Pt-C/Nafion resembled thetheoretical spectrum in Figure 5C(g) generated for surface thePt site on the (111) face of Pt38, shifted by +3.5 eV fromspectrum B(g). This XANES shift was rationalized based on
the difference of the fluorescence tune energy and the finalstate relaxation effect. Metallic Pt state was selected in the
XANES tuned to 11075.4 eV.
Due to the selection rule of the electronic transition (azimuthal
quantum number ∆l ) (1; total angular momentum ∆j ) 0,(1), the near-edge transition from L2 (2P1/2) is limited to O4(2D3/2) at a deeper energy than O5 (2D5/2), whereas those fromL3 (2P3/2) are both limited to O4 and O5.49 This is the reason
why the Pt L2-edge white line is weaker than that in Pt L3-edge
spectra.35 By changing the fluorescence tune energy between
11065.7 and 11075.4 eV (Figure 4c, d), electron-sufficient Ptsites in contact with C (Figure 5A(e), (f)) and relatively electron
deficient Pt sites on/in Pt nanoparticles (Figure 5B, C(e), (f))
were discriminated, respectively. In contrast, a negligible
difference was detected by switching the ambient gas from H2
to air for replica Pt-C/Nafion, except for an absorption edge
shift less of than 1 eV (Figure 5e, f). The shape resonance peaknear the L3-edge was reported, for example, by H adsorption
on Pt.35,36,49 One of the possibilities is that the antibonding level
between Pt 5d3/2 and the frontier level of the adsorbent was
occupied for replica Pt-C/Nafion samples. It is also possiblethat a similar shape resonance peak appeared due to adsorbed
H in H2 and adsorbed O2 in air. A technical reason may be that
the XAFS measurements for Pt catalysts on Nafion were done
as in a half-cell and no water was supplied/catalytically producedin samples during the measurements. The contact of Pt withC(solid) and Nafion(liquid) was found to be secure; however,
Nafion in pure H2 or air was not entirely mobile and thus may
block the Pt/gas interface sites.
Implications to the PEFC Cathode Catalyst of Replica
Pt-C. The electric conductivity for Pt-C-Al-MCM-41 wasby 17% superior to that for conventional 20 wt % Pt/Vulcan
XC-72 (Table 1) probably because catalytically formed C inmesopores was regularly oriented in the [001] direction of AlMCM-41, despite the low Pt content in Pt-C-Al-MCM-41(0.72 wt %). The comparable conductivity values for Pt-C-AlMCM-41 and replica Pt-C (Table 1) were also understandable
if we think that the nonconducting Al-MCM-41 part wasremoved, but regular orientation of the C rod/tube would be
lost for the latter.
To enable 0.13 mg Pt cm-2 as the cathode catalyst, 76 mg of
replica Pt-C (0.84 wt % Pt) was mounted on C paper versus
25 mg of 20 wt % Pt/Vulcan XC-72 on C paper (1 mg Pt cm-2)(see Supporting Information). The greater amount of replica

Oka et al.
Pt-C mounted caused difficulty in making physical contact of
Pt with the electrolyte (wet polymer to transport protons) andoxygen gas at the same time.1 Hence, to enable practical electric
power generation using replica Pt-C catalyst, the Pt content in
replica Pt-C powder should be increased to 20 wt % Pt typicalfor other commercial Pt/C catalysts while keeping the Pt particle
size of 1.2 nm. One possibility is to increase the Al content
from 1.11 wt % to the zeolite level of 10-12 wt % to augment
the cationic Pt complexes exchanged. Another possibility is toutilize a three-dimensional mesoporous template, for example,
MCM-48, to stabilize Pt nanoparticles on/in formed C in thethree-dimensional mesopores. The advantage to synthesize metal
particles in a narrow size distribution centered at ∼1 nm and tosecure contact of the metal and C demonstrated in this paper
can be applied to nonprecious metal-C catalysts, for example,
cobalt or iron porphyrin decomposition50 in mesoporous SiO2based templates. Carbonization of block copolymer selfassembled with Pt, Pb, and Nb species is another possibility to
produce a Pt-C composite, as recently reported for an anodematerial resistive to CO poisoning.51
Conclusions
Pt nanoparticles introduced by cation group exchange andformed by heating in vacuum at 573 K inside of Al-MCM-41
catalyzed C2H2 decomposition to form a C rod/tube. The AlMCM-41 template was removed with a 15% HF solution. Pt
nanoparticles with a mean size of 1.2 nm were obtained
embedded on/in the C matrix. The Pt loading was 0.84 wt %.
The replica Pt-C synthesis procedure was supported by thechanges of the specific SBET value from 767 m2 g-1 for Pt-AlMCM-41 to 35 m2 g-1 for Pt-C-Al-MCM-41 and then to 540
m2 g-1 for replica Pt-C. The Pt particles could not be monitored
by XRD both before and after cathode catalytic tests in PEFC,
suggesting the stability of 1.2 nm Pt particles embedded on/in
the C matrix. The Pt sites in replica Pt-C pressed to Nafion
were analyzed by Pt Lβ1-selecting high energy resolution Pt
L2-edge XANES spectra. The first peak above the absorption
edge for the spectra tuned to 11065.7 eV was weak, suggestingthat interface Pt site received electron transfer from the C incontact. The high energy resolution XANES spectra for bothinterfacial Pt with C and metallic Pt sites were nicely reproducedin theoretical spectra generated using FEFF 8.4. The shaperesonance peak in H2 or in air was not certain probably because
the antibonding levels were below the Fermi level or the shaperesonance peaks for H and O2 adsorbed were similar. Superior
turnover numbers were tentatively evaluated for the Pt-C
replica in the cathode compared to Pt/Valcun XC-72 due to thedifferences of effective contact of Pt with C versus Pt impregnation on C and the diffusion efficiency of O2 in replica Pt-C
powder (see Supporting Information).
Acknowledgment. The authors are thankful for financial
support from the Sumitomo Foundation for Basic Scientific
Research (No. 070110). The conventional X-ray absorption
experiments were performed under the approval of the Photon
Factory Proposal Review Committee (No. 2008G167). The Pt
Lβ1 emission and high energy resolution Pt L2-edge X-ray
absorption experiments were performed under the approval of
the SPring-8 Program Review Committee (No. 2008B1111).
The authors thank Dr. Uruga, Dr. Tanida, and Dr. Terada forthe adjustment of beamline optics and technical help at SPring-8
and Prof. Kaneko and Prof. Kanoh for the XRD apparatus. This
paper is part 31 of a state-selective X-ray absorption finestructure series.

Analysis and Catalysis of a Replica 1.2 nm Pt-C Composite

J. Phys. Chem. C, Vol. 114, No. 2, 2010 1267

Supporting Information Available: Supporting text andFigure S1 for PEFC tests. This material is available free ofcharge via the Internet at http://pubs.acs.org.

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