A robust platinum carbonyl cluster anion [Pt3(CO)6]52- encapsulated in an ordered mesoporous channel of FSM-16: FTIR/EXAFS/TEM characterization and catalytic performance in the hydrogenation of ethene and 1,3-butadiene

Article abstract of DOI:10.1021/jp9718919

A robust trigonal prismatic cluster anion [Pt₃(CO)₆]₅^2- (v_CO = 2078, 1878 cm^-1; ??_max = 405 and 702 nm; R₁(Pt - Pt) = 2.68 Aì? (CN = 2.0); R₂(Pt - Pt) = 3.08 Aì? (CN = 1.5)) is selectively synthesized in the ordered hexagonal channel of FSM-16 by the reductive carbonylation of H₂PtCl₆TNR₄⁺/FSM-16 under a CO + H₂O atmosphere at 323 K. The Pt cluster anion extracted in THF solution by cation metathesis was identified as [Pt₃(CO)₆]₅^2- by FTIR and UV - vis spectroscopic data. TEM observation showed that [Pt₃(CO)₆]₅^2- was uniformly dispersed and aligned in the mesoporous channels of FSM-16. The Pt₁₅ cluster anions formed in FSM-16 were relatively stabilized by the coimpregnated quaternary alkylammonium cations as NR₄⁺ in the following order for the alkyl groups: butyl > ethyl > methyl, methyl viologen (MV) > hexyl a?? no countercations. The EXAFS and FTIR studies demonstrated that [Pt₃(CO)₆]₅^2- in FSM-16 was transformed by the controlled removal of CO at 300-343 K into the partially decarbonylated Pt₁₅ cluster (R₁(Pt - Pt) = 2.69 Aì?, CN = 2.2; R₂(Pt - Pt) = 3.10 Aì?, CN = 1.4). This sample showed IR spectra indicating a linear CO (v_CO = 2062 cm^-1) without a bridged one. The Pt carbonyl cluster was eventually converted by thermal evacuation exceeding 463 K to naked Pt particles (15 Aì? diameter; R(Pt - Pt) = 2.76 Aì?, CN = 7.8). The controlled removal of CO from [Pt₃(CO)_6-x]₅^2- in FSM-16 by thermal evacuation at 300-423 K yields samples with marked catalaytic activities for hydrogenation of ethene and 1,3-butadiene at 300 K. 1,3-Butadiene is selectively hydrogenated to 1-butene on the partially decarbonylated Pt carbonyl clusters in FSM-16, whereas it is preferentially converted to n-butane on the naked Pt particles in FSM-16.

Full text of DOI:10.1021/jp9718919

3866
J. Phys. Chem. B 1998, 102, 3866-3875
2-
A Robust Platinum Carbonyl Cluster Anion [Pt3(CO)6]5 Encapsulated in an Ordered
Mesoporous Channel of FSM-16: FTIR/EXAFS/TEM Characterization and Catalytic
Performance in the Hydrogenation of Ethene and 1,3-Butadiene
Takashi Yamamoto,† Takafumi Shido,† Shinji Inagaki,‡ Yoshiaki Fukushima,‡ and
Masaru Ichikawa*,†
Catalysis Research Center, Hokkaido UniVersity, Sapporo 060, Japan, and Toyota Central R&D Laboratories,
Inc., 41-1 Nagakute, Aichi 480-11, Japan
ReceiVed: June 10, 1997; In Final Form: March 6, 1998
2-
A robust trigonal prismatic cluster anion [Pt3(CO)6]5 (νCO ) 2078, 1878 cm-1; λmax ) 405 and 702 nm;
R1(Pt-Pt) ) 2.68 Å (CN ) 2.0); R2(Pt-Pt) ) 3.08 Å (CN ) 1.5)) is selectively synthesized in the ordered
+
hexagonal channel of FSM-16 by the reductive carbonylation of H2PtCl6/NR4 /FSM-16 under a CO + H2O
atmosphere at 323 K. The Pt cluster anion extracted in THF solution by cation metathesis was identified as
2-
2-
[Pt3(CO)6]5 by FTIR and UV-vis spectroscopic data. TEM observation showed that [Pt3(CO)6]5 was
uniformly dispersed and aligned in the mesoporous channels of FSM-16. The Pt15 cluster anions formed in
FSM-16 were relatively stabilized by the coimpregnated quaternary alkylammonium cations as NR4 in the
+
following order for the alkyl groups: butyl > ethyl > methyl, methyl viologen (MV) > hexyl . no
countercations. The EXAFS and FTIR studies demonstrated that [Pt3(CO)6]52- in FSM-16 was transformed
by the controlled removal of CO at 300-343 K into the partially decarbonylated Pt15 cluster (R1(Pt-Pt) )
2.69 Å, CN ) 2.2; R2(Pt-Pt) ) 3.10 Å, CN ) 1.4). This sample showed IR spectra indicating a linear CO
(νCO ) 2062 cm-1) without a bridged one. The Pt carbonyl cluster was eventually converted by thermal
evacuation exceeding 463 K to naked Pt particles (15 Å diameter; R(Pt-Pt) ) 2.76 Å, CN ) 7.8). The
controlled removal of CO from [Pt3(CO)6-x]5
2- in FSM-16 by thermal evacuation at 300-423 K yields samples
with marked catalaytic activities for hydrogenation of ethene and 1,3-butadiene at 300 K. 1,3-Butadiene is
selectively hydrogenated to 1-butene on the partially decarbonylated Pt carbonyl clusters in FSM-16, whereas
it is preferentially converted to n-butane on the naked Pt particles in FSM-16.
1. Introduction
and CO hydrogenation to C1-C5 alcohols in comparison with
conventional metal catalysts.13,18
Zeolites are aluminosilicate crystallites consisting of micro-
porous cavities and channels of molecular dimensions (6-13
Å) with smaller windows. Such micropores can supply “a
templating circumstance” for the selective synthesis of some
bulky metal carbonyl clusters which fit into the interior cages
as ultimate “nanometer-size reaction vessels”. According to the
analogy of a ship model in a bottle, an intrazeolitic preparation
of metal complexes and clusters is referred to as a “ship-in-
bottle” synthesis.1,14 The metal ions and subcarbonyls in the
microcavities of NaY and NaX zeolites were subsequently
converted by the reductive carbonylation reactions with CO +
H2O or CO + H2 to form metal carbonyl clusters such as
Recently, mesoporous molecular sieves such as MCM-4119,20
and FSM-1621,22 have been synthesized using different micelle
surfactant templates. These materials consist of the ordered
mesoporous channels of 20-100 Å diameter, which are much
larger than those of the conventional zeolites such as ZSM-5,
AlPO-5, and NaY. They are potential hosts for synthesizing
bulky organometallic complexes23 and metal particles which are
accessible to larger substrates in the catalytic reactions. Ad-
ditionally, they have been used for the templating fabrication
of the quantum dots and wires of metals24 and chalcogenides.25
Chini type Pt carbonyl clusters such as [Pt3(CO)6]n2-(n )
2-10) are synthsized by the reductive carbonylation of Pt salts
such as H2PtCl6 and Pt(CO)Cl3 in solution.34 Gates et al.39 have
previously reported that a mixture of the Chini complexes was
formed in the reaction of H2PtCl6 impregnated on MgO with
CO and water. Furthermore, we have recently demonstrated
Rh6(CO)16,2 [Pt3(CO)6]n2- (n ) 3, 4),3-6 Pd13(CO)x,7 Ir6(CO)16,8
2- 11
Ir6(CO)152-,9 Rh6-xIrx(CO)16 (x ) 0-6),10 Ru6(CO)18
, and
HRuCo3(CO)12;12 these carbonyl cluster complexes are encap-
sulated in supercages of 13 Å diameter. The “ship-in-bottle”
synthesis of homometallic and bimetallic clusters may open new
opportunities for the rational design of tailoring metal catalysts
having a uniform distribution of particle sizes and metal
compositions.1,6 The resulting metal clusters in the micropores
of NaY and NaX zeolites exhibit higher activities and catalytic
stabilities for the water-gas-shift reaction,4 NO + CO reaction,5
alkane hydrogenolysis,14 olefin hydroformylation reaction,15,16
that three-stacked ([Pt3(CO)6]32-) and four-stacked ([Pt3(CO)6]42-
clusters are selectively synthesized in the micropores of NaY
by the reductive carbonylation of [Pt(NH3)4]2+/NaY4 and Pt2+
)
/
NaY,5 respectively. However, owing to the size limitation of
the zeolite micropores, so far greater than five-stacked clusters
([Pt3(CO)6]n2-) have not been synthesized by the ship-in-bottle
technique. In this study, we extended the works for the
intrazeolitic synthesis of larger Pt carbonyl clusters such as
[Pt3(CO)6]n2- (n > 5)) using the mesoporous channels of FSM-
* To whom correspondence should be addressed.
† Hokkaido University.
‡ Toyota Central R&D Laboratories, Inc.
S1089-5647(97)01891-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/28/1998

2-
[Pt3(CO)6]5 Encapsulated in a Mesoporous Channel
J. Phys. Chem. B, Vol. 102, No. 20, 1998 3867
TABLE 1: Crystallographic Data Characterizing the
Reference Compounds and Fourier Transform Range Used
in the EXAFS Analysisa
16 as a host material. The resulting Pt carbonyl cluster anions
formed in FSM-16 were characterized by FTIR, UV-vis, and
EXAFS spectroscopy. The structural transformation of Pt
carbonyl clusters toward Pt particles has been also studied by
EXAFS, TPD, and FTIR measurements and TEM observation
during the controlled thermal evacuation of the Pt carbonyl
clusters in FSM-16. The catalytic behaviors of the partially
decarbonylated Pt cluster anions and Pt particles in FSM-16
are discussed for the hydrogenation of ethene and 1,3-butadiene
in conjunction with their hydrogenation kinetics, activities, and
selectivies, which are different from those of conventional metal
catalysts.
crystallographic data
Fourier transform
samples
Pt foilb
W(CO)6
shell
CN
R/Å
∆k/Å-1
∆R/Å
n
Pt-Ptd
W-Ce
W-Of
12
6
6
2.7742
2.063
3.206
3.5-18.0
3.5-18.0
3.5-18.0
1.9-3.0
1.2-2.1
1.2-2.1
3
3
3
c
a Notation: CN, coordination number for absorber-backscatterer
pair; R, absorber-backscatterer distance; ∆k, limits used for forward
Fourier transformation; ∆R, limits used for shell isolation (R is distance);
n, power of k used for Fourier transformation. b From ref 28. c From
ref 29. d Reference for Pt-Pt. e Reference for Pt-C. f Reference for
Pt-O.
2. Experimental and Procedures
The diffuse reflectance UV-vis spectra for the powdered
sample of Pt carbonyl complexes formed in FSM-16 charged
in a quartz-plated cell (1 mm thickness) and the absorption
spectra of the extracted species in solution were recorded using
a Shimadzu UV-vis spectrometer under nitrogen and CO
atmosphere at 300 K.
2.1. Sample Preparation. The host FSM-16 was synthe-
sized using a commercial-grade Kanemite as a layered alumi-
nosilicate (SiO2/Al2O3 ) 320; Fe, below 0.02%, supplied by
Hokkaido Soda Co. Ltd.) and C16H33NMe3Cl as a surfactant
template according to the published procedures.21,22 Kanemite
was mixed with a water solution of C16H33Me3N+Cl (0.143 mol
L-1, 280 mL; EP-Grade Tokyo Kasei Co. Ltd.) and was
subsequently heated at 343 K for 3 h with vigorously stirring.
The precipitated material was filtered out and dispersed into
500 mL of water; this was heated at 343 K for 3 h, keeping a
pH value of 8.5. After washing with water to remove residual
Na+, the resulting materal of FSM-16 thus prepared was calcined
to remove the organic template under an O2 flow (25 mL/min)
at 823 K for 14 h. The N2-adsorption isotherm showed that
the calcined FSM-16 powder has a surface area of 950 m2/g
and a pore size of 27 Å. It consists of the well-defined
hexagonal mesoporous channels (27.5 Å diameter) which are
characterized by X-ray powder patterns in the low-angle region
(2θ ) 2.26, 3.44, 4.50 and 5.90) and by TEM observation.
The powdered FSM-16 was impregnated with a quaternary
alkylammonium salt NR4X (R ) Me, Et, Bu, Hex; X ) Br, Cl,
OH; 99% purity, purchased from Nakarai Co. Ltd.) from an
aqueous solution, and the impregnated material was dried by
vacuum evaporation; this is refererd to as NR4X/FSM-16. The
loading amount of NR4X in FSM-16 was 1.37 × 10-4 mol/gcat.
The 5.0 mass wt % Pt samples of H2PtCl6/NR4X/FSM-16 were
prepared by the conventional impregnation of NR4X/FSM-16
with an aqueous solution of H2PtCl6 (99%, Wako Co. Ltd.).
The sample of H2PtCl6/FSM-16 without alkylammonium cation
was similarly prepared by the impregnation of FSM-16 with
H2PtCl6. Each powdered sample was exposed to 26.7 kPa of
CO (99.9999%, Takachiho Co. Ltd.) and/or a mixture of CO
(26.7 kPa) and H2O (2.7 kPa) in a closed circulating reactor
for 5-15 h by ramping the temperature from 300 to 323 K.
The Chini-type Pt carbonyl cluster anions were independently
synthesized from H2PtCl6 in an alkaline solution of methanol
and isolated as [NEt4]2[Pt3(CO)6]n2- (n ) 3, 4, and 5) according
to the literature method.32 The reference samples of [Pt3(CO)6]32-
Transmission electron micrographs were taken using a JEOL
JEM-2000EX at an accelerating voltage of 200 kV. The power
samples were dispersed on a carbon foil with a microgrid from
the suspended solution of ethanol and/or acetone. The TEM
images were observed with minimum damage to the structures
and morphology of the samples by the electron beams.
2.3. EXAFS Measurement and Analysis. The powdered
and disk samples were charged under N2 in an in-situ EXAFS
cell with a Kapton film window (500 µm) to prevent exposure
of the sample to air. Pt-LIII edge (11 562 eV) EXAFS (extended
X-ray absorption fine structure) spectra were measured on BL-
10B at the Photon Factory of the National Laboratory for High
Energy Physics (KEK-PF; Tsukuba, Japan). The energy and
the current of the electron (or positron) were 2.5 GeV and 250
mA, respectively. The Si(311) channel cut monochromator was
used. The transmission X-ray absorption spectrum was detected
using the ion chambers filled with a mixture gas of Ar and N2.
The spectra were analyzed by a computer program supplied by
Technos Co. Ltd.27 The background absorption was subtracted
using a McMaster function, and the baseline was estimated by
the cubic sprain method. The k3-weighted EXAFS function was
Fourier transformed into R-space using the k-range from 3.5 to
18 Å-1. The Hanning function used was δ ) 0.5 Å-1. The
phase shift was not corrected for the preliminary Fourier
transformation. The inverse Fourier transform was calculated
to obtain a filtered EXAFS function. The R range of an inverse
Fourier transformation was taken from 1.09 to 3.31 Å. The
Hanning function of ∆ ) 0.05 Å was used as a window function.
The filtered EXAFS function of the sample is analyzed by the
curve-fitting method using the following equation:28
ø (k) )
(N [f (k)][S (k)])/(kr 2) sin(2 kr + φ (k))
∑
T
i
i
i
i
i
i
2-
i
and [Pt3(CO)6]4 encapsulated in NaY zeolite were prepared
by the similar reductive carbonylation of [Pt(NH3)4]2+/NaY and
Pt2+/NaY according to the “ship-in-bottle technique”.4,5
2.2. IR and UV-Vis Spectroscopy and TEM Observation.
The impregnated samples such as H2PtCl6/FSM-16 or H2PtCl6/
NR4X/FSM-16 were pressed into a self-supporting wafer (1.5
mm i.d., 12-20 mg/cm2) with pressure of 300 kg f/cm2. The
wafer was mounted in an in situ IR cell equipped with CaF2
windows. IR spectra were recorded using a Fourier transform
infrared spectrometer (Shimadzu FTIR 8100 M) with the
resolution of 2 cm-1 and co-accumulation of 100 interferograms
to improve the signal/noise ratios of the IR spectra.
exp(-2σ2 k2) (1)
where øT(k), Ni, r, σ, fi(k), Si(k), and φi(k) are the calculated
EXAFS function, the coordination number, the interatomic
distance, the Debye-Waller factor, the backscattering amplitude,
the reduction factor, and the phase shift function, respectively.
The fitting EXAFS parameters of the sample were determined
for Ni and ri to minimize σi and for the correction of threshold
energy (∆E0). The backscattering amplitudes and phase shift
functions of the Pt-Pt, Pt-C, and Pt-O (of Pt-CO) bonds
were calculated using those of Pt foil (Pt-Pt)28 and W(CO)6

3868 J. Phys. Chem. B, Vol. 102, No. 20, 1998
Yamamoto et al.
(Pt-C and Pt-O).29 Table 1 presents crystallographic data and
the Fourier transform range of the some reference compounds
used in the EXAFS analysis. Although the atomic weight of
W atoms is four times smaller than that of platinum, W(CO)6
is taken as a good reference compound because of the narrow
distribution of W-O(CO) distances. The effect of multiple
scattering can be included by using the backscattering amplitude
and phase shift function of W(CO)6. To evaluate the justifica-
tion of a curve-fitting, the residual factor R is defined and used
in the following equation:
je
2
|k nøobs(kj) - kjnøF(kj)|
∑
j
j)js
R )
(2)
je
2
|k nøobs(kj)|
∑
j
x
j)js
where øobs is a Fourier filtered EXAFS function, øF is a
calculated EXAFS function using equation 1, j is an index of
data points, kj is the k value of the jth data, and js and je are
initial and final data points of the curve-fitting. The EXAFS
parameters for discussion were fixed around the optimum value,
and the residual factor R was calculated to offer the optimum
values for the other parameters. The real value was estimated
using a residual factor R which is smaller than twice the
optimum value.
2.4. Catalytic Reactions and TPD Measurements. The
catalytic reactions were carried out using a closed-circulating
system equipped with a Pyrex-glass tubing reactor, which
mounted each sample of 50 mg. The hydrogenation of ethene
and 1,3-butadiene proceeded at the temperatures of 278-303
K. The pressures of reactant gases were p(H2) ) p(olefin) )
13.3 kPa, where olefins are ethene and 1,3-butadiene. The dead
volume of the reactor was 250 cm3. Products were analyzed
by a Shimadzu 8A gas chromatograph (gc) with a TCD (thermal
conductive detector) using the columns VZ-10 (4 m, 313 K)
and VZ-7 (6m, 300 K) for ethene, ethane, 1- and cis/trans-2-
butenes, and 1,3-butadiene, respectively. Helium was used as
a carrier gas (30 mL/min) for the gc analysis.
Figure 1. IR spectra of carbonyl species formed by the reductive
carbonylation of H2PtCl6/NEt4Cl/FSM-16. The H2PtCl6/NEt4Cl/FSM-
16 sample was exposed to CO (p(CO) ) 26.6 kPa) at 323 K for (a) 1
h and (b) 3 h. Then the sample was exposed to CO + H2O (p(CO) )
26.6 kPa, p(H2O) ) 2.0 kPa) at 323 K for (c) 1 h, (d) 3 h, (e) 12 h, and
(f) 24 h. The background IR absorption spectrum of H2PtCl6/NEt4Cl/
FSM-16 was subtracted from each spectrum recorded after the reductive
carbonylation of the sample.
TABLE 2: Wavenumber of ν(CO) of Pt Carbonyl Clusters
Pt carbonyl
clusters
NR4X/FSM-16
or NaY
ν(CO)terminal
/
ν(CO)bridge/
cm-1
cm-1
2- a
[Pt3(CO)6]5
FSM-16
2086
2075
2075
2076
2070
2076
2079
2074
2080
2056
2055
2045
1882
1877
1875
1879
1878
1881
1884
1882
1824
1798
1870
1860
Me4NCl/FSM-16
Et4NCl/FSM-16
Et4NOH/FSM-16
Bu4NCl/FSM-16
Bu4NOH/FSM-16
He4NBr/FSM-16
MVCl2/FSM-16
NaY
2- b
2- b
2- c
2- c
TPD (temperature programmed decomposition) measurements
using a mass spectrometer (ANELVA-100) were conducted to
study the reactivities of the Pt carbonyl clusters in FSM-16.
[Pt3(CO)6]4
[Pt3(CO)6]3
[Pt3(CO)6]5
[Pt3(CO)6]4
NaY
in solution (THF)
in solution (THF)
a This work. b From refs 4 and 5. c From ref 32.
3. Results and Discussion
the sample consists of two intense bands of the linear CO at
2079 and bridging CO at 1880 cm-1 as shown in Figure 1c-f.
As it was previously reported,26 an olive-green sample was
similarly formed by the reductive carbonylation of H2PtCl6//
FSM-16 with CO + H2O at 300-323K. The resulting sample
of Pt carbonyl cluster anions showed two intense IR carbonyl
bands at 2086s and 1882 m cm-1. The CO bands observed in
the sample of H2PtCl6/NEt4Cl/FSM-16 after the prolonged
reaction closely resemble those of the Chini-type Pt carbonyl
clusters such as [Et4N]2[Pt3(CO)6]n, where n ) 3-5 in crystal
and solution,32 and [Pt3(CO)6]n2- (n ) 3, 4) in NaY,4,5 although
the band positions of linear and bridging CO are relatively
shifted to lower frequencies, as presented in Table 2.
3.1. Synthesis and Characterization of Pt Carbonyl
Complexes in FSM-16 by the Reductive Carbonylation of
H2PtCl6/NEt4Cl/FSM-16. A sample of H2PtCl6/NEt4Cl/FSM-
16 contaning 5.0 mass % Pt was prepared by a coimpregnation
of NEt4Cl/FSM-16 and H2PtCl6. This sample was exposed to
CO and/or a saturated water vapor in a closed circulating system
by ramping the temperature from 300 to 323 K. After the
exposure of H2PtCl6 in NEt4Cl/FSM-16 to CO the reaction was
measured by in-situ IR spectroscopy. The CO bands at 2188,
2149, and 2119 cm-1 were gradually developed as shown in
Figure 1a,b. From the analogy of the reference Pt carbonyl
species,31 the IR bands at 2188, 2149, and 2119 cm-1 can be
assignable to cis-Pt(CO)2Cl2 (2188 and 2148 cm-1) and Pt(CO)-
Cl3 (2121 cm-1), respectively. The mono-Pt carbonyls were
subsequently converted by the admission of a mixture of CO
(26.7 kPa) and H2O (2.7 kPa), leading to the homogeneous
formation of an olive-green product. The final IR spectrum of
The attempts to extract the Pt carbonyl species from the
resulting sample of H2PtCl6/NEt4Cl/FSM-16 after reductive
carbonylation was conducted using THF (tetrahydrofuran) and
MeOH as a polar solvent were unsuccessful. By contrast, the
greenish-blue complex was extracted from the sample under

2-
[Pt3(CO)6]5 Encapsulated in a Mesoporous Channel
J. Phys. Chem. B, Vol. 102, No. 20, 1998 3869
N2 atmosphere by cation metathesis with a quaternary alkyl-
ammonium salt in THF and MeOH. Figure 2 shows the IR
spectrum of Pt carbonyl cluster complexes (Figure 2a) which
were extracted using [(Ph3P)2N]Cl in THF solution from the
sample of H2PtCl6/NEt4Cl/FSM-16 after reductive carbonylation,
comparing with that of [NEt4]2[Pt3(CO)6]5 in MeOH (Figure
2b) (2056 vs, 1896 w, 1873 s, and 1848 w cm-1). It was found
that the IR spectrum of the extracted sample (2054 vs for linear
CO and 1896 w, 1872 s, and 1845 w cm-1 for bridging CO) is
in good agreement with that of a five-stacked [Pt3(CO)6]5 anion
only excepting for the minor shoulder bands at 2046 and 1860
cm-1. The weak shoulder bands at 2046 and 1860 cm-1 are
reasonably assigned to a smaller Chini Pt cluster anion such as
[Pt3(CO)6]42-, which was a minor byproduct in the reductive
carbonylation of H2PtCl6 in FSM-16. The UV-vis absorption
spectrum of the extracted sample in THF (Figure 3a) was
observed at 405 and 702 nm, which resembles that of
[Pt3(CO)6]52- in MeOH (Figure 3b; λmax ) 408 and 697 nm).34
The IR carbonyl bands and UV-vis spectra of the family of
2-
Chini-type complexes [Pt3(CO)6]n (n ) 2-10) in solution
depend basically on the size of Pt cluster dianions. In
Figure 2. IR spectra of (a) Pt carbonyl species extracted with TPP-
(Ph3P)2NCl in THF from the sample C of H2PtCl6/NEt4Cl/FSM-16 after
comparison with the characteristic UV-vis bands at 367, 422,
2-
506, and 562 nm for [Pt3(CO)6]3 and those at 394, 513, and
the reductive carbonylation at 323
[NEt4]2[Pt3(CO)6]5 in MeOH.
K and (b) the reference
620 nm for [Pt3(CO)6]42-, respectively, the extracted species is
2-
2-
assigned to [Pt3(CO)6]5 in THF solution. Accordingly, the
IR and UV-vis data as shown in Figures 1-3 and Table 2
2-
reasonably suggest that [Pt3(CO)6]5 was homogeneously
formed in the mesoporous channel of FSM-16 by the reductive
carbonylation of H2PtCl6/NEt4Cl/FSM-16, similarly in solution,
as follows:
H2PtCl6 + 3CO + H2O f cis-Pt(CO)2Cl2 + CO2 + 4HCl
(3n)cis-Pt(CO)2Cl2 + (3n + 1)H2O + (6n)HCl +
(3n + 1)CO2 f H2[Pt3(CO)6]5 + (6n)HCl + (3n + 1)CO2
In the first stage of the cluster formation, the preadsorbed
water in H2PtCl6/FSM-16 was consumed for the reduction of
H2PtCl6 with CO to form cis-Pt(CO)2Cl2 with a small amount
of Pt(CO)Cl3. As shown in Figure 1, it is suggested that an
excess amount of water with CO is enough to promote the water-
gas shift reaction and the successive conversion of cis-
Figure 3. UV-vis spectra of (a) Pt carbonyl species extracted with
(Ph3P)2NCl in THF from the sample C of H2PtCl6/NEt4Cl/FSM-16 after
2-
Pt(CO)2Cl2 to [Pt3(CO)6]5 in the channels of FSM-16. The
the reductive carbonylation at 323
[NEt4]2[Pt3(CO)6]5 in MeOH solution.
K
and (b) the reference
differences in the positions of the CO bands (shifting to higher
2-
2-
frequency ∆ν ) 18-8 cm-1) for the sample of [Pt3(CO)6]5
2-
encapsulated in FSM-16, compared with those in THF solution,
may be explained due to the interaction of Pt cluster anions4,5
with the acidic sites such as protons and Al3+ on the internal
study, it is suggested that a robust [Pt3(CO)6]5 is formed by
the reductive carbonylation of H2PtCl6 in the mesoporous
channels of FSM-16. The Pt15 cluster anion is accommodated
with the alkylammonium cations in FSM-16.
2-
wall of FSM-16 (Si/Al ) 320), similarly for [Pt3(CO)6]n (n
) 3 and 4) in NaY.4,5 There is a similar band shift and
difference for the relative peak intensity in the reflectance UV-
vis spectrum for the dark-brownish powdered sample of
3.2. EXAFS Characterization. EXAFS measurements
were conducted to obtain direct information about the structures
of the Pt carbonyl clusters synthesized in FSM-16. Figure 4
shows the Fourier transforms of k3-weighted EXAFS of [NEt4]2-
[Pt3(CO)6]5/BN (sample A), [Pt3(CO)6]42-/NaY (sample B), and
[Pt3(CO)6]52-/NEt4Cl/FSM-16 (sample C). Figure 5 shows the
EXAFS spectra of each sample as presented in Figure 4. The
backscattering amplitude and phase shift of Pt-Pt and Pt-CO
were extracted from the EXAFS spectra of Pt foil and W(CO)6,
respectively. It was interesting to find that the observed Pt
L-edge FT patterns of samples B and C were similar to that of
the reference sample of [NEt4]2[Pt3(CO)6]5 diluted in boron
nitride (sample A). The R-range of 1.09-3.31 Å in the spectra
of Figure 5 was inverse Fourier transformed, which was
reasonably fitted by five shells, consisting of two Pt-Pt
2-
[Pt3(CO)6]5 encapsulated in FSM-16, compared with the
absorption spectrum of the green solution of the extracted
species in THF (Figure 3a). Nevertheless, it is interesting to
find that the dark-brownish sample (λmax ) 261 (w), 444 (s)
and 760 (w) nm) changed reversibly to a dark-green one (λmax
) 405 (s) and 705 (m) nm) by the exposure of THF, which
resembles that of [Pt3(CO)6]52- in MeOH solution (Figure 3b).
The difference in the reflectance UV-vis spectrum of the dried
sample may be caused by an assembling of Pt cluster anions in
the ordered mesoporous channels of FSM-16 like the dark-
brownish solid/crystal of the corresponding Pt15 cluster anion
salt. According to the FTIR and UV-vis data obtained in this

3870 J. Phys. Chem. B, Vol. 102, No. 20, 1998
Yamamoto et al.
Figure 6. Fourier filtered k3-weighted EXAFS spectrum (solid line)
and their curve fittings (dotted line) for the Pt-Pt, Pt-C, and Pt-O
shells of [Pt3(CO)6]52-/NEt4Cl/FSM-16 (sample C) in the Fourier
transformed k range of 4-14 Å-1
.
TABLE 3: Structural Parameters of Platinum Clusters
Derived by Pt LIII-Edge EXAFS
shell
CN
R/Å
∆E0/eV
∆σ2/10-3 Å2
residual (%)
[NEt4]2[Pt3(CO)6]5
Pt-Pt
2.1
1.5
1.5
1.0
1.0
2.68
3.07
2.10
1.89
2.99
6.5
-8.5
0.4
-1.1
7.5
Pt-Pt
5.8
2.0
1.3
4.5
Pt-C(b)
Pt-C(t)
Pt-O
-5.9
4.6
Figure 4. Fourier transforms of k3-weighted EXAFS of (a) [NEt4]2-
[Pt3(CO)6]5/BN (sample A), (b) [Pt3(CO)6]42-/NaY (sample B), and (c)
[Pt3(CO)6]52-/NEt4Cl/FSM-16 (sample C).
[Pt3(CO)6]42-/NaY
Pt-Pt
2.2
1.4
1.6
0.9
1.0
2.67
2.99
2.14
1.99
2.99
6.5
-8.5
0.4
-5.8
-4.6
-1.6
12.2
9.6
Pt-Pt
7.4
Pt-C(b)
Pt-C(t)
Pt-O
2.0
0.9
1.3
[Pt3(CO)6]52-/NEt4Cl/FSM-16
Pt-Pt
Pt-Pt
Pt-C(b)
Pt-C(t)
Pt-O
2.3
1.7
2.2
1.3
1.0
2.68
3.08
2.08
1.90
3.01
3.7
-6.4
-7.6
-10.0
-2.3
-0.7
7.0
5.8
2.6
3.0
The number of independent parameters is determined by the
following equation (Nyquist law):30
Nmax ) 2(∆k)(∆R)/(π + 1)
(3)
In this study, with ∆k and ∆R being 14.5 Å-1 and 2.22 Å,
respectively, Nmax is calculated to be 12. Table 2 shows the
coordination number (CN), interatomic distance (R), correction
of threshold energy (∆E0), and difference of Debye-Waller
factor (∆σ2) from the synthesized samples and reference
compounds which are determined by the curve-fitting analysis
of EXAFS spectra. The experimental errors in the Pt-Pt
distance and CN were estimated as 0.02 Å and 0.08 for Pt-Pt,
and those for Pt-C and Pt-O were estimated as 0.05 Å and
0.2, respectively. The Chini-type complexes Pt3(CO)6]n2-are
composed of Pt3(CO)6 triangular units which are stacked
together to form trigonal prismatic Pt cluster frameworks, as
depicted in Table 3. According to the X-ray crystal analysis,
the cross section of Pt3(CO)6 units is about 8 Å in diameter
and the interfacial distances of “Pt3(CO)6” units are varied from
3.08 to 3.10 Å for [Pt3(CO)6]n2- (n ) 3-5).34 As summerized
in Table 3, the observed intra-Pt-Pt atomic distrance (R1 )
Figure 5. k3-weighted EXAFS spectra of (a) [NEt4]2[Pt3(CO)6]5/BN
2-
(sample A), (b) [Pt3(CO)6]42-/NaY(sample B), and (c) [Pt3(CO)6]5
/
NEt4/FSM-16 (sample C) in the Fourier transformed k range of 3.5-
18 Å-1
.
contributions (Pt-Pt contribution in the Pt3 triangle and between
the triangles), two Pt-C contributions (terminal and bridge CO),
and one Pt-O contribution. Figure 6 shows the results of the
curve-fitting analysis. The Fourier transform of the filtered k3-
weighted EXAFS for the sample of [Pt3(CO)6]52-/NEt4Cl/FSM-
16 is in a good agreement with that of the curve-fitting.
2.68 Å) and interfacial distances between adjacent triplatinum
2-
plane (R2 ) 3.08 Å) for the synthesized sample of [Pt3(CO)6]5
/
NEt4Cl/FSM-16 (sample C) are in a good agreement with those
of the reference [NEt4]2[Pt3(CO)6]5 in BN (sample A) (R1 )
2.68 Å; R2 ) 3.07 Å in the X-ray analysis) within experimental

2-
[Pt3(CO)6]5 Encapsulated in a Mesoporous Channel
J. Phys. Chem. B, Vol. 102, No. 20, 1998 3871
studies, it was found that [Pt3(CO)6]52- cluster anions are formed
uniformly by the reductive carbonylation of H2PtCl6 in FSM-
16, which were encapsulated in the ordered channel of FSM-
16.
3.3. Thermal Stability of [Pt3(CO)6]52- with Quaternary
Alkylammonium in FSM-16. As it was previously reported,26
an olive-green Pt15 carbonyl cluster anion was formed similarly
by the reductive carbonylation of H2PtCl6/FSM-16 without any
quaternary alkylammonium cation under CO + H2O atmo-
sphere, whereas the Pt cluster anion is readily decomposed by
evacuation even at 300-323 K. This is caused by the
irreversible transformation due to the successive removal of CO,
resulting in a brownish product (2063 s and 1820 w cm-1). Its
IR spectrum resembled those of higher nuclear Pt carbonyl
clusters such as [Pt55(CO)x]n- and [Pt38(CO)44]2- (2060-2043
s and 1832-1820 w cm-1) in THF solution.35 By contrast, it
was demonstrated that a thermostable [Pt15(CO)30]2- was
successfully prepared using the modified FSM-16 as a host
which was coimpregnated with the quaternary ethylammonium
salts used in the present work. As shown in Figure 1, each
coimpregnated sample of H2PtCl6/NR4Cl/FSM-16 (R ) CH3,
C2H5, C4H9, methyl viologen (MV), and C6H13) was converted
by reductive carbonylation to form an olive-green product,
showing the intense IR carbonyl bands characteristic of linear
and bridging carbonyls which appear around 2075-2079 (s)
and 1870-1884 (m) cm-1, respectively. Table 2 summerizes
the IR data for CO bands for Pt15 carbonyl clusters in FSM-16,
without and in the presence of various countercations NR4Cl.
The linear and bridging CO bands of [Pt15(CO)30]2- in FSM-
16 relatively shifted to higher frequencies by varying the larger
quaternary alkylammonium cations. The red-shift of CO bands
is possibly due to the electronic interaction of Pt carbonyl anions
with the countercations. In particular, both CO bands of the
Pt15 carbonyl cluster anions formed in FSM-16 without a
quaternary alkylammonium cation shift to much higher
frequencies(∆ν(CO) ) 28-10 cm-1). These frequency shifts
are observed for the formation of contact ion pairs between the
Pt carbonyl anions and acidic protons located on the silicate
wall of FSM-16. The thermostability of [Pt3(CO)6]n2- formed
in FSM-16 was studied with the change of IR spectra before
and after the thermal evacuation for 1 h by ramping the
temperature 323-363 K. It was found that the Pt carbonyl
clusters in FSM-16 without the countercations are readily
transformed to higher nuclear carbonyl clusters by removal of
Figure 7. Transmission electron micrograph of [Pt3(CO)6]52-/NEt4-
Cl/FSM-16 (sample C).
error (0.02 Å). In fact, the EXAFS parameters of the Pt-Pt
interatomic distances and CN were determined in good accuracy
based on the Debye factor (∆σ2) and R factor of less than 10%.
By contrast, as shown in Table 3, it is worthy to note that, for
[Pt3(CO)6]42-/NaY (sample B), the Pt-Pt distance of an inter-
Pt3 triangle was substantially shorter (R(Pt-Pt) ) 2.99 Å) than
that of the reference compound [NEt4]2[Pt3(CO)6]5 in BN (R2
) 3.07 Å) and [Pt3(CO)6]52- synthesized in the mesoprous FSM-
16 (R1 ) 2.68 Å; R2 ) 3.08 Å). This may be caused by the
confining effect due to the intrazeolitic constraint in the
microcavity of NaY (13 Å in diameter). On the contrary, the
mesoporous channels of FSM-16 are large enough (28 Å in
diameter) to accommodate Pt15 carbonyl clusters in the channel
with negligible structural distortion. Heaton et al.36 studied by
2-
the 195Pt NMR technique the structure of [Pt3(CO)6]5 in
solution and demonstrated that the Pt3 triangle was fluxionally
rotated in the Pt3 plane of the trigonal prismatic framework,
which causes the wide distribution of the Pt-Pt interatomic
distances of the cluster frameworks. Accordingly, it is suggested
that the slight elongation of the Pt-Pt interatomic distance in
the [Pt3(CO)6]52-/NEt4Cl/FSM-16 may be associated with the
2-
2-
fluxional rotation of the [Pt3(CO)6]5 anions (8 × 12 Å van
CO at 300 K. On the other hand, the samples of [Pt3(CO)6]n
der Waals diameter) in the mesoporous channels of FSM-16
(27.5 Å).
with the bulky countercations such as NBu4 and NEt4 are
thermostable up to the higher temperatures above 343 K. The
order of thermal stability for [Pt3(CO)6]52-/NR4/FSM-16 was
measured by TPD (temperature-programmed decomposition)
studies, in terms of the temperatures to start the cluster
decomposition owing to the removal of CO, as follows:
To elucidate the location of the [Pt3(CO)6]52- cluster anions
in the mesoporous channel of FSM-16, a transmission electron
2-
micrograph has been determined for the sample of [Pt3(CO)6]5
/
NEt4Cl/FSM-16. Figure 7 shows a TEM picture of [Pt3(CO)6]52-
clusters in NEt4Cl/FSM-16. It was found by close observation
of the TEM picture that the Pt clusters as small speckles of
10-15 Å size were uniformly scattered and aligned in the
ordered channel of FSM-16. The relatively large particles of
20 Å may be induced by the exposure of the electron beams
under the TEM observation. Nevertheless, no Pt particles were
observed at the external surface of the FSM-16 channel.
Moreover, it was demonstrated by TEM observation that the
Pt carbonyl cluster anions such as [NEt4]2[Pt3(CO)6]5 were
difficultly introduced inside the mesoporous channels of FSM-
16 from the THF solution. Most of the Pt carbonyl cluster
anions preferentially agglomerated as large particles and rods
of more than 100 Å diameter, which were deposited on the
external surface of FSM-16. According to the EXAFS and TEM
butyl > ethyl > methyl, methyl viologen (MV) .
hexyl > no countercation
This order of the cluster stability depends on the size of NR4
cations used, which is consistent with that of the carbonyl
2-
frequency shift of [Pt3(CO)6]5 as shown in Table 2. This
implies that a Pt carbonyl cluster anion is protected with NR4
cations from the strong acidic protons on the hydrophobic
channels of FSM-16. The tetrahexylammonium cation as an
exception is possibly not accessible to the Pt carbonyl cluster
anions formed in the mesoporous channels of FSM-16 (28 Å)
owing to its size limitation.
3.4. Cluster Transformation of [Pt3(CO)6]52-/NEt4Cl/

3872 J. Phys. Chem. B, Vol. 102, No. 20, 1998
Yamamoto et al.
Figure 9. Fourier transforms of k3-weighted EXAFS of [Pt3(CO)6]5
/
2-
NEt4Cl/FSM-16 (sample C) evacuated at (a) 300 K, (b) 343 K, (c) 363
K, (d) 393 K, (e) 473 K, and (f) Pt foil in the k range of 3.5-18 Å-1
.
Figure 8. IR spectra of [Pt3(CO)6]52-/NEt4Cl/FSM-16 (sample C) after
the thermal evacuation for 1 h at various temperatures (a) 300 K, (b)
323 K, (c) 343 K, (d) 373 K, (e) 423 K, (f) 473 K, and (g) 523 K,
respectively.
TABLE 4: Structural Parameters of [Pt3(CO)6]52-/NEt4Cl/
FSM-16 Evacuated at Various Temperatures Derived by Pt
LIII-Edge EXAFS
shell
CN
R/Å
∆E0/eV
∆σ2/10-3 Å2
residual (%)
FSM-16. It was demonstrated by the in-situ FTIR and EXAFS
observation that the thermostable [Pt15(CO)30]2-/Et4NCl/FSM-
16 (2075 s and 1875 m cm-1) was gradually transformed by
evacuation at 10-4 Torr by ramping the temperature from 300
to 343 K, resulting in a partially decarbonylated species which
showed the characteristic CO IR bands (2060 s and 1870 vw
cm-1). Figure 8 represents the change of IR spectra of
[Pt3(CO)6]52-/NEt4Cl/FSM-16 by the thermal evacuation at
various temperatures. When the sample was evacuated at 323
K, a band intensity of bridging CO preferrentially decreased
and eventually disappeared, compared with those of terminal
CO. The linear CO band was relatively enhanced and broad-
ened. In addition, the peak position of linear CO shifted to
lower frequencies around 2062 cm-1 by the prolonged thermal
evacuation. By the ramping of temperatures above 523 K, the
terminal CO gradually decreased and eventually disappeared.
Evacuation at 300 K
Pt-Pt
Pt-Pt
Pt-C(b)
Pt-C(t)
Pt-O
2.3
1.7
2.1
1.3
1.0
2.68
3.10
2.10
1.90
3.03
3.0
-2.1
3.8
-9.9
-0.8
-0.8
5.6
6.2
2.6
2.8
11.8
Evacuation at 343 K
2.69
3.10
2.13
1.91
3.02
Pt-Pt
Pt-Pt
Pt-C(b)
Pt-C(t)
Pt-O
2.2
1.4
1.1
1.5
1.0
-2.5
-7.5
-1.6
-8.9
-1.3
0.4
6.0
6.4
5.8
4.5
9.5
Evacuation at 363 K
2.73
1.87
3.07
Pt-Pt
Pt-C
Pt-O
7.3
0.4
0.3
-2.9
-7.8
4.5
3.5
-2.8
2.2
8.8
0.2
9.0
Evacuation at 393 K
2.73
1.88
3.07
2-
The results suggest that the bridging CO of [Pt3(CO)6]5 in
Pt-Pt
Pt-C
Pt-O
8.3
0.3
0.3
-2.9
-7.8
4.5
3.5
-2.8
2.2
FSM-16 was partially switched to the linear one by controlled
thermal evacuation, which was followed with a subsequent
removal of the linear CO. Both CO bands characteristic of
[Pt3(CO)6]52- were not regenerated by the admission of CO upon
the decarbonylated sample, implying that an irreversible trans-
formation of the Pt carbonyl cluster proceeds by thermal
evacuation. Figure 9 shows the Fourier transforms of k3-
weighted EXAFS of [Pt3(CO)6]52-/NEt4Cl/FSM-16 before and
after the sample was evacuated at 300, 343, 363, 393, and 473
K, respectively. The peaks at 2.4, 2.7, and 3.0 Å in the Pt
L-edge Fourier transforms are mainly attributed to Pt-Pt of
the intra-Pt3 triangle, Pt-C (for the bridging CO), and Pt-Pt
of inter-Pt3 units, respectively. Table 4 shows the structural
parameters obtained by the EXAFS analysis of the samples
before and after the thermal treatments in a vacuum. When
the sample of [Pt3(CO)6]52-/NEt4Cl/FSM-16 was evacuated at
343 K, the CN of Pt-C(bridging CO) decreased from 2.1 to
1.1 owing to the linear-bridging transformation and removal of
CO, while the interatomic distances and coordination numbers
Evacuation at 473 K
2.72 -3.2 2.7
Pt-Pt
7.8
of two Pt-Pt contributions (R1 and R2), Pt-C(t) (terminal CO),
and Pt-O(CO) were basically unchanged. This suggests that
the trigonal prismatic Pt cluster framework may be retained after
the controlled removal of CO at 343 K. This may be explained
because the lens effect of carbon of terminal CO is much larger
than that of bridged CO, and that of the oxygen of bridge CO
was relatively small in the EXAFS spectra. Hence Pt-O
contribution mainly comes from terminal CO. The EXAFS
evaluation showed that the decrease of peak height of the Pt-O
contribution in Figure 9b was not caused by the decrease of
CN of Pt-O but by the increase of the Debye-Waller factor
of the Pt-O bond. This may be because of enhancing the
mobility of CO by the partial removal of CO of [Pt3(CO)6]5
These EXAFS data are consistent with those of IR observation
about the switching of bridging CO to linear CO by the
2-
.

2-
[Pt3(CO)6]5 Encapsulated in a Mesoporous Channel
J. Phys. Chem. B, Vol. 102, No. 20, 1998 3873
Figure 10. Temperature-programmed desorption (TPD) profiles of CO
2-
and CO2 formation monitored by a mass spectrometer on [Pt3(CO)6]5
/
NEt4Cl/FSM-16 (sample C) by heating from 300 to 523 K (ramping
rates of 5 K/min).
Figure 11. Transmission electron micrograph of [Pt3(CO)6]52-/NEt4-
Cl/FSM-16 (sample C) after the thermal evacuation at 473 K for 1 h.
controlled evacuation of [Pt3(CO)6]52-/NEt4Cl/FSM-16. Fur-
thermore, when the sample of [Pt3(CO)6]52-/NEt4Cl/FSM-16
was evacuated at 363 K, the Fourier transform pattern of the
Chini cluster complexes drastically changed to that of a single
Pt-Pt peak (R ) 2.73 Å, CN ) 7.3) which is characteristic of
metal Pt particles. The averaged value of CN of the Pt-Pt bond
was evaluated as 7.3. This suggests that a Pt particle formed
in FSM-16 by the thermal evacuation at 363 K consist of 50-
60 Pt atoms having a diameter of 15 Å. Besides, the Pt-C
and Pt-O contribution was observed at 1.87 and 3.07 Å in the
Fourier transformed k3 function in Figure 9c-e. The CN of
Pt-C and Pt-O being 0.3-0.4 implies that all the surface Pt
atoms of the particle were coordinated with CO (CO/Pt ) 0.4).
The estimated dispersion (CO/Pt ) 0.4-0.45) of Pt particles
in FSM-16 was roughly consistent with the average size of Pt
particle of 15 Å because of the Pt-Pt coordination number of
7.3. The adsorbed CO on the Pt particles is decreased by the
further evacuation at temperatures exceeding 473 K, resulting
in the naked Pt particles (Pt-Pt CN ) 7.8; Pt-C(O), CN ) 0)
owing to the complete removal of CO (Figures 8g-h and 9f).
The EXAFS, TEM, and IR data in Figures 9 and 8 suggest that
the Pt15 carbonyl clusters in the channels of FSM-16 were
subsequently transformed and converted to a robust carbonyl
cluster of Pt55 at 363 K (Figure 9d) and eventually to a naked
Pt particle of 15 Å diameter (CN ) 7.8) at 473 K (Figure 9e).
3.5. TPD and TEM Studies on [Pt3(CO)6]52-/NEt4Cl/
FSM-16 during the Thermal Transformation. To study the
reactivities of Pt carbonyl clusters in FSM-16, a temperature-
programmed desorption (TPD) measurement was conducted on
[Pt3(CO)6]52-/NEt4Cl/FSM-16, as shown in Figure 10. Three
desorption peaks of CO (m/e ) 28) were observed at 359, 397,
and 454 K, while the peak of CO2 (m/e ) 44) was negligible.
Peaks at 397 and 454 K, correspond to the desorption peak of
CO chemisorbed on Pt particle because the position of the CO
peaks in TPD measurement was similar to that of the conven-
tional Pt/SiO2.37 The peak at 359 K can be assigned to the
partial desorption of CO due to the cluster transformation from
[Pt3(CO)6]52- to a carbonylated cluster of Pt55 in FSM-16. The
carbonylated Pt cluster was transformed by the complete
removal of CO to a naked Pt particle at 454 K. Figure 11 shows
a TEM photograph of [Pt3(CO)6]52-/NEt4Cl/FSM-16 after
thermal evacuation at 473 K for 2 h. Small Pt particles of 15-
20 Å diameter were observed uniformly scattered and aligned
SCHEME 1: Proposed Cluster Transformation of
2
[Pt3(CO)6]5 - Encapsulated in FSM-16 by the Controlled
Removal of CO with Thermal Evacuation at Various
Temperatures, Deduced by EXAFS, FTIR, and TEM
Characterization Data
in the mesoporous channels of FSM-16. This TEM observation
was consistent with the EXAFS results in terms of the CN )
7.8. According to the data of the EXAFS, FTIR, TPD, and
2-
TEM studies, [Pt3(CO)6]5 encapsulated in the channels of
FSM-16 is proposed to be transformed as depicted in Scheme
1, owing to the controlled removal of CO by the thermal
evacuation at 300-473 K. [Pt3(CO)6]52- in FSM-16 is partially
decarbonylated at 343 K in keeping the Pt cluster framework
of a trigonal prism. When the sample was evacuated above
2-
363 K, [Pt3(CO)6]5 was subsequently converted to a larger
cluster such as Pt50-60, which is bound with linear CO. The
resulting Pt carbonyl cluster was completely desorbed by the
evacuation at 473 K, resulting in the naked Pt particles having
diameters of 15-20 Å. Domen et al.38 previously reported by
2-
IR study that [Pt3(CO)6]5 impregnated on SiO2 was trans-

3874 J. Phys. Chem. B, Vol. 102, No. 20, 1998
Yamamoto et al.
Figure 12. Turnover frequency (TOF) of ethene and 1,3-butadiene
hydrogenation at 300 K on [Pt3(CO)6]52-/NEt4Cl/FSM-16 after thermal
evacuation at various temperatures for 1 h. p(C2H4) ) p(H2) ) 13.3
kPa or p(C4H6) ) p(H2) ) 13.3 kPa.
Figure 13. Selectivity of C4 hydrocarbon products in the hydrogenation
of 1,3-butadiene at 300 K on [Pt3(CO)6]52-/NEt4Cl/FSM-16 after
thermal evacuation at various temperatures for 1 h. p(C4H6) ) p(H2)
) 13.3 kPa.
formed by thermal evacuation at 573 K to Pt particles of 20 Å
in a high dispersion via the mechanism of the removal of CO.
3.6. Catalytic Performances for Olefin Hydrogenation on
[Pt3(CO)6]52-/FSM-16 after the Controlled Removal of CO.
2-
To investigate the catalytic performances of [Pt3(CO)6]5 in
FSM-16 concerning their structural transformations caused by
the controlled removal of CO, we have carried out the
2-
hydrogenation of ethene and 1,3-butadiene on [Pt3(CO)6]5
/
NEt4Cl/FSM-16 after thermal evacuation for 1 h at various
2-
temperatures. It was found preliminarily that [Pt3(CO)6]5
/
NEt4Cl/FSM-16 is catalytically inactive for the hydrogenation
of ethene and 1,3-butadiene at 300-323 K.
Figure 12 shows the structural dependence of the rates of
formation in terms of turnover frequencies (TOF) for the
hydrogenation of ethene and 1,3-butadiene as a function of
evacuation temperatures of [Pt3(CO)6]52-/NEt4Cl/FSM-16. The
TOFs were calculated by the rate of olefin consumption divided
by the total number of Pt atoms used. The reaction rates of
ethene hydrogenation increased by the evacuation of
[Pt3(CO)6]52-/NEt4Cl/FSM-16 after thermal evacuation above
320 K, whereas the hydrogenation of 1,3-butadiene proceeded
appreciably only after the controlled removal of CO by
evacuation above 363 K. For both reactants, the hydrogenation
rates substantially increased during the cluster transformation
of [Pt3(CO)6]52-/NEt4Cl/FSM-16 caused by thermal evacuation.
Actually, the rates of ethene hydrogenation at 300 K were
enhanced by a 30 times magnitude and that of 1,3-butadiene
hydrogenation by 15 times by the thermal evacuation of
[Pt3(CO)6]52-/NEt4Cl/FSM-16 at 393 K, compared with those
evacuated at 343 K. The hydrogenation of both olefins was
almost completely suppressed by the exposure of CO (10 Torr)
at 300 K to the resulting partially decarbonylated samples.
Figure 13 shows the product selectivity in the hydrogenation
of 1,3-butadiene. The molar selectivity toward butenes as the
half-hydrogenated products was above 97% for the sample
[Pt3(CO)6]52-/NEt4Cl/FSM-16 after evacuation at 323, 343, and
363 K. The butene products consist of 1-butene (76%), cis-2-
butene (10%), and trans-2-butene (14%). n-Butane was formed
in less than 5% selectivity, regardless the reaction temperatures
and time on stream. Nevertheless, as shown in Figure 13, the
selectivities of 1,3-butadiene hydrogenation markedly changed
after the resulting sample was further decarbonylated above 393
K; the selectivity for 1-butene decreased from 76% to 22%,
Figure 14. Arrhenius plots for the hydrogenation of 1,3-butadiene on
[Pt3(CO)6]52-/NEt4Cl/FSM-16 (sample C) after thermal evacuation at
(a) 343 K (sample D) and (b) 363 K (sample E) for 1 h.
but those of n-butane and trans-2- and cis-2-butenes increased
to 38%, 21%, and 19%, respectively. The results suggest that
the isomerization of 1-butene proceeded simultaneously to trans-
and cis-2-butenes on the partially decarbonylated sample. On
the naked Pt particles of 15-20 Å size in FSM-16 which was
prepared by thermal evacuation above 523 K, the hydrogenation
of 1,3-butadiene proceeded nonselectively at 300 K toward
n-butane in 70-90% selectivity. Hence, it was sugggested that
2-
[Pt3(CO)6]5 in FSM-16 after the controlled removal of CO
exhibited catalytic activities for the selective hydrogenation of
1,3-butadiene toward butenes mainly consisting of 1-butene,
while the naked Pt particles (15-20 Å size) in FSM-16 lost
selectivity for 1-butene, resulting in the complete hydrogenation
of butadiene to n-butane even at 300 K.
2-
As depicted in Scheme 1, it is suggested that [Pt3(CO)6]5
in FSM-16 is transformed to form a partially decarbonylated
Pt clusters by the thermal evacuation at 343 K (catalyst D) and
to Pt50-60 particles bound with CO (catalyst E) above 363 K.
They are active for the selective hydrogenation of 1,3-butadiene
toward 1-butene at 300 K. On the other hand, the reaction
proceeded nonselectively toward butane on the naked Pt
particles, similar to the conventional metal catalysts.
Figure 14 shows Arrhenius plots for 1,3-butadiene hydrogen-
ation on samples D and E. The activation energy of the

2-
[Pt3(CO)6]5 Encapsulated in a Mesoporous Channel
J. Phys. Chem. B, Vol. 102, No. 20, 1998 3875
hydrogenation on sample D was 95.2 kJ mol-1, and that of
sample E was 59.3 kJ mol-1. For both catalysts, the selectivities
of butenes (above 97%) such as 1-butene were independent of
the reaction temperatures. The difference of the activation
energy for the hydrogenation of 1,3-butadiene may be owing
to the stronger adsorption of butadiene on the partially car-
bonylated Pt carbonyl clusters in FSM-16. It is of interest to
find that ethene as a smaller molecule than 1,3-butadiene is
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2-
readily hydrogenated on the sterically less crowded [Pt3(CO)6]5
in FSM-16 (samples D and E) by controlling the removal of
CO as a function of evacuation temperature, as shown in Figure
12. The active site area of sample E accessible for 1,3-butadiene
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4. Conclusion
2-
(1) Bulky Pt carbonyl clusters such as [Pt3(CO)6]5 are
synthesized in the mesoporous channels of FSM-16 by the
reductive carbonylation of H2PtCl6 with CO + H2O at 323 K.
Guest [Pt3(CO)6]5 was extracted from the sample with TPP
2-
salt into the THF solution by cation metathesis. This was
characterized by FTIR, EXAFS, and UV-vis spectroscopy.
(2) [Pt3(CO)6]52- in FSM-16 is stabilized by coimpregnated
countercations NR4+. The thermal stability of the Pt cluster
anion in FSM-16 was evaluated in the following order for the
alkyl groups: R ) butyl, ethyl > methyl, methyl viologen (MV)
. hexyl > no countercations.
(3) EXAFS, TEM, and FTIR studies suggest that [Pt3(CO)6]52-
anions in FSM-16 are transformed by the controlled removal
of CO to the partially decarbonylated Pt clusters and eventually
converted into naked Pt particles of 15 Å size by evacuation
exceeding 473 K, as indicated in Scheme 1.
2-
(4) [Pt3(CO)6]5 in FSM-16 exhibits catalytic activities for
the hydrogenation of ethene and 1,3-butadiene selectively toward
butenes (above 97% selectivety) after the controlled removal
of CO by thermal evacuation that are different from those on
conventional metal catalysts.
References and Notes
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Phys. Chem. 1990, 94, 5317.
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(4) Li, G.-J.; Fujimoto, T.; Fukuoka, A.; Ichikawa, M. J. Chem. Soc.,
Chem. Commun. 1991, 1337.
(39) Chang, D. C.; Koningberger, D. C.; Gates, B. C. J. Am. Chem.
Soc. 1992, 114, 6460.