Fabrication of two- and three-dimensional model catalyst systems with monodispersed platinum nanoparticles as active metal building blocks

Article abstract of DOI:10.1016/j.ica.2005.10.057

Well-defined Pt monodispersed nanoparticles within the catalytically relevant 1-10 nm size regime were synthesized in solution phase by several synthetic methods which differed in the choice of reducing agent, surface stabilizer, reaction temperature and

Full text of DOI:10.1016/j.ica.2005.10.057

Inorganica Chimica Acta 359 (2006) 2683–2689
www.elsevier.com/locate/ica
Fabrication of two- and three-dimensional model catalyst systems with
monodispersed platinum nanoparticles as active metal building blocks
Krisztian Niesz, Matthias M. Koebel, Gabor A. Somorjai *
Department of Chemistry, University of California, Berkeley, D56 Hildebrand Hall, Berkeley, CA 94720-1460, United States
Chemical and Materials Science Divisions, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, United States
Received 19 September 2005; accepted 17 October 2005
Available online 19 December 2005
Dedicated to Professor Brian James.
Abstract
Well-defined Pt monodispersed nanoparticles within the catalytically relevant 1–10 nm size regime were synthesized in solution phase
by several synthetic methods which differed in the choice of reducing agent, surface stabilizer, reaction temperature and solvent. Three-
dimensional model catalysts were fabricated by incorporating the metal nanoparticles into ordered channels of high surface area mes-
oporous oxides such as SiO , Al O and Ta O through either sonication or direct synthesis of the oxide support around the particles.
2 2 3 2 5
Deposition of the same nanocrystals onto silica supports by means of the Langmuir–Schaeffer technique produced two-dimensional
model catalysts.
ꢀ
2005 Elsevier B.V. All rights reserved.
Keywords: Platinum nanoparticles; Oxide supports; Langmuir–Schaeffer technique
1
. Introduction
10 nm) under well-defined conditions (i.e., solution phase)
provides superior control of metal particle size and surface
morphology. Over the past decade, many such synthetic
methods have become available through advances of nano-
technology [1–12] delivering nanomaterials of tunable size,
shape and very sharp particle size distributions (r ± 5%).
Here, we report on solution phase synthesis of monodi-
spersed Pt-nanoparticles within the catalytically important
size range (1–10 nm) and their use in making novel model
catalysts. The different techniques employed in this work
can be differentiated in terms of the methods of growth
(direct reduction versus seeded growth), the reducing
agents (linear alcohols, ethylene glycol, L(+)-ascorbic acid,
In modern industrial heterogeneous catalysis, one can
observe a shift of focus from high turnover rates towards
high selectivity for a desired target molecule, resulting in
fewer pollutants and side products, thus minimizing cost
and maximizing process efficiency. The synthesis of sup-
ported metal catalysts with tunable properties, having great
potential to produce more selective catalysts, has attracted
attention. For the fabrication of such model catalyst sys-
tems, a combination of knowledge from two distinct fields
in materials chemistry, metal nanoparticle and mesoporous
oxide support material synthesis, is required. Catalysts syn-
thesized by standard techniques such as impregnation or
incipient wetness typically contain a wide distribution of
particle sizes. The synthesis of monodispersed noble metal
nanoparticles in the catalytically relevant size range (1–
trisodium citrate, H2 or NaBH ) or the capping agent
4
(poly-N-vinylpyrrolidone (PVP) versus Pluronic L64
(EO PO EO ) triblock copolymer). Purified nanoparti-
1
3
30
13
cles obtained in this way were used to fabricate novel three-
(3D) and two-dimensional (2D) model catalyst systems.
*
The synthesis of mesoporous oxide materials and their
Corresponding author. Tel.: +1 510 6424053; fax: +1 510 6439668.
E-mail address: somorjai@socrates.berkeley.edu (G.A. Somorjai).
potential applications also represent major focus of numer-
0
020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.10.057

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K. Niesz et al. / Inorganica Chimica Acta 359 (2006) 2683–2689
ous research groups. One application of mesoporous mate-
rials is as a catalyst support [13]. The surface chemistry of
SBA-15 (mesoporous SiO ) is analogous to amorphous
2
SiO which is used for supporting small metal nanocrystals
2
but the former has properties (i.e., pore size) which are easy
to control by varying the synthesis conditions. Ultimately,
we are interested in controlling the pore size of SBA-15-like
oxide support materials based on SiO , Al O and Ta O
5
2
2
3
2
while maintaining long range order of the mesopores. Here,
we report that a variety of mesoporous oxides were synthe-
sized in our laboratory and used as supports for platinum
nanoparticles for our 3D model catalyst systems. The tech-
niques that have been used to characterize the platinum
nanoparticle/mesoporous oxide systems include X-ray dif-
fractometry (XRD), small angle X-ray scattering (SAXS),
UV-Raman and infrared spectroscopies, transmission elec-
tron microscopy (TEM), physisorption and chemisorption
measurements.
Scheme 1. Schematic representation of the three-dimensional catalyst
preparations.
2
D model catalysts are promising tools to study cata-
inic acid (H PtCl Æ 6H O) was used as inorganic precursor
2 6 2
lytic reactions on surfaces: the planar geometry of the sys-
tem allows direct in situ probing of surface species and
intermediates which occur during chemical reactions by
means of optical (sum frequency generation spectroscopy)
and scanning probe (STM and AFM) techniques. Such
studies have great potential to provide insight into catalytic
reactions and further our understanding of underlying
reaction mechanisms. The Langmuir–Schaeffer technique
and reduced by various types of alcohols (methanol, etha-
nol and ethylene glycol) at reflux temperatures. PVP
(poly(vinylpyrrolidone)) of different molecular weights
was used to stabilize the particles by capping them in aque-
ous media. After the synthesis, the nanoparticles were puri-
fied from the excess amount of PVP by alternating
precipitation with hexane/2-propanol mixture and redi-
spersion in water.
[
14] is an excellent tool to produce high density metal nano-
particle arrays [15].
2.1.2. Synthesis of platinum nanoparticles with PVP capping
agent by ascorbic acid reduction
2
. Experimental
PVP stabilized Pt nanoparticles with sizes between 2.0
and 5.3 nm were synthesized using direct reduction of
H PtCl Æ 6H O with ascorbic acid in a water/methanol sol-
2.1. Fabrication of three-dimensional Pt/mesoporous oxide
catalyst systems
2
6
2
vent mixture. The H PtCl precursor was added to a reflux-
2 6
ing solution of PVP and ascorbic acid under vigorous
stirring. After a few minutes, a color change from yellow
to dark brown was observed indicating the formation of
nanoparticles. After a 15 min reaction time, the solution
was cooled to room temperature and nanoparticles were
isolated by precipitation with acetone. Residual PVP was
removed by repeated suspension of the crude product in
ethanol and precipitation with hexanes. The particle size
was controlled by the methanol content in the solvent mix-
ture (boiling temperature), the concentration of the Pt-pre-
cursor and the reducing agent.
First monodisperse platinum nanoparticles in the size
range of 1–10 nm were synthesized using a variety of wet
chemical synthetic recipes. For each particle size, the reac-
tion conditions (direct reduction or seeded growth method,
capping agents, reducing agents, solvents, and reaction
temperatures) were optimized to produce highly monodis-
perse samples. The Pt-nanoparticles produced this way
were then incorporated into mesoporous silica (SBA-15)
using low power sonication. Alternatively, the silica sup-
port was synthesized around the Pt nanoparticles (Scheme
1
) in a process which is referred to as the nanoparticle
encapsulation method. The synthesis of mesoporous
Al O and Ta O with ordered, SBA-15-like structure
was developed and provides additional materials which
can be used as catalyst supports.
2.1.3. Synthesis of platinum nanoparticles with PVP capping
agent by citrate reduction
2
3
2
5
PVP stabilized Pt-nanoparticles of 1.5 nm were
obtained by direct reduction of H PtCl Æ 6H O by triso-
2
6
2
dium citrate in boiling water in the presence of PVP.
After a reaction time of 45 min, the mixture was cooled
to room temperature and the nanoparticles precipitated
with acetone. Purification of nanoparticles was affected
by threefold suspension in ethanol and precipitation with
hexanes.
2.1.1. Synthesis of platinum nanoparticles with PVP capping
agent by alcohol reduction
Pt nanoparticles in the size range of 1.7–7.1 nm were
synthesized by an alcohol reduction method as described
earlier [16–18]. Briefly, aqueous solution of hexachloroplat-

K. Niesz et al. / Inorganica Chimica Acta 359 (2006) 2683–2689
2685
2.1.4. Synthesis of platinum nanoparticles with Pluronic L64
capping agents
Petri dish and aged for 5 days at 313 K in an incubator
and calcined at 673 K overnight.
Platinum nanoparticles within a size range of 3.5–6.6 nm
were synthesized according to a recently reported scheme
2.1.7. Capillary inclusion (CI) method
[
19] which uses the seeded growth method. First, homoge-
Previously prepared platinum nanoparticles in the size
range of 1–10 nm were embedded into highly-ordered mes-
oporous silica (SBA-15) and other oxides by using low
power sonication for 3 h [18]. The as-synthesized materials
were calcined at 723 K under air to remove the template
polymer (PVP) from the nanoparticle surface and reduced
neous Pt seeds (3.5 nm) were produced by reducing
H PtCl Æ 6H O with NaBH in the presence of Pluronic
L64 (EO PO EO , EO = ethylene oxide, PO = propyl-
ene oxide) triblock copolymer dissolved in H O (Pluronic
L64/Pt = 10). A subsequent slow addition of the precur-
sor hexachloroplatinic acid water solution to the Pt seed sol
2
6
2
4
1
3
30
13
2
+
4
by H . The reduced catalysts were characterized by XRD,
2
while H was flowing through the reaction vessel led to lar-
ger nanoparticles (5–6.6 nm). After the second addition of
TEM, nitrogen physisorption and selective gas
chemisorption.
2
the platinum precursor was completed, H was bubbled for
2
an additional 5 min and the reaction mixture was stirred
overnight. The particles were isolated by precipitation with
a 2-propanol/hexane mixture and redispersed in water.
2.1.8. Nanoparticle encapsulation (NE) method
An alternative technique to produce high surface area
Pt/SBA-15 catalysts is the nanoparticle encapsulation
method [24,25]. Monodisperse PVP-capped platinum
nanoparticles (size range of 1.7–7.1 nm) obtained by alco-
hol reduction were used as templates and the silica support
was synthesized around them. Briefly, 2.5 g of Pluronic
P123 was completely dissolved in 50.5 mL of deionized
2
.1.5. Synthesis of SBA-15 mesoporous silica support
Mesoporous silica SBA-15 was prepared according to
the method reported in the literature [20]. Pluronic P123
EO PO EO ) triblock copolymer (4.0 g) was dissolved
(
2
0
70
20
water. The Pt colloidal aqueous solution (27.0 mL,
in 30 g of water followed by the addition of 120 ml of a
À3
3
· 10 M) was mixed together with the polymer solution
2
3
M HCl (aq.) solution while stirring at 313 K for
0 min. Subsequently, 9.0 g of tetraethoxysilane (TEOS)
at 313 K, and stirred for 1 h to ensure complete dispersion
of the particles. Then 0.375 mL of 0.5 M aqueous NaF
solution and 3.91 mL of tetramethylorthosilicate (TMOS)
were quickly added to the colloidal solution. The mixture
was stirred for 20 h at 313 K. The resulting slurry was aged
for an additional day at 373 K. The brown precipitates
were separated by centrifugation, thoroughly washed with
water and acetone, and dried in an oven at 373 K. Finally,
the catalysts were cured by a recently developed, low
temperature (373 K) oxidation–reduction treatment which
effectively removes the PVP from the surface of the
particles and residual polymer from the pores of the silica
support [26].
was added to the solution with stirring at 313 K for 20 h.
The mixture was aged at 373 K for 24 h. The white powder
was recovered through filtration, washed with water and
ethanol, and dried in air. The product was calcined at
8
23 K overnight to produce SBA-15.
2
.1.6. Synthesis of mesoporous alumina and tantalum oxide
supports
Highly ordered mesoporous alumina with SBA-15-like
structure was prepared by a sol–gel based self-assembly
method which was reported earlier [21]. To summarize,
1
g of Pluronic P123 was dissolved in 12 ml of absolute
ethyl alcohol and stirred for 15 min at 313 K. At the same
time a second solution was prepared containing various
amounts of concentrated hydrochloric acid and 6 ml of
absolute ethyl alcohol. In this method, no water was used
in addition to the water present in the concentrated HCl
solution. Aluminum tri-tert-butoxide (2.46 g) was added
slowly to this solution under vigorous stirring. The two
solutions were mixed and stirred further at 313 K. In the
2.2. Fabrication of two-dimensional Pt/SiO model catalysts
2
by means of the Langmuir–Schaeffer technique
2D model catalysts were fabricated by depositing PVP
stabilized 7.1 nm Pt nanoparticles, synthesized by the ethyl-
ene glycol reduction, onto a silicon substrate exhibiting a
native oxide layer by using the Langmuir–Schaeffer tech-
nique [14,27]. A schematic of the process is shown in
Scheme 2. A Langmuir-trough was thoroughly cleaned
and filled with ultrapure water. The silicon substrate was
then placed onto an L-shaped aluminum holder which
was attached to a motorized stage (dipper) and submerged
in the water phase. A dilute suspension of freshly synthe-
sized and purified 7.1 nm Pt-nanoparticles (ethylene glycol
reduction) in chloroform was added dropwise to the water
phase of the trough until the surface force reached a value
of ꢀ3–5 mN/m. Following a 1 h equilibration period, the
film was compressed at low compression rates (0.3 mN/
m/min) until the desired surface force value was obtained.
3
+
final reaction mixture, the ratio of Al /Pluronic P123/
EtOH/H O/HCl was 1/0.017/30/6/1. The resulting homo-
2
geneous sol was poured into a teflon container and aged
for three days at 313 K under N flow. Eventually the white
2
sticky material that was produced was calcined at 673 K in
a flow of oxygen, overnight to remove the polymers from
the pores. Mesoporous Ta O material with well-ordered
2
5
structure was prepared according to the method in the lit-
erature [22,23]. One gram of Pluronic P123 was dissolved in
2
0 g of ethanol before the addition of 1.97 g of TaCl and
5
0
.32 g H O. After stirring, the solution was poured into a
2

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K. Niesz et al. / Inorganica Chimica Acta 359 (2006) 2683–2689
molecules was carried out by NaBH at room temperature,
4
the formation of small nanoparticles of 3.5 nm was
observed. These seeds were used as nuclei for producing
larger particles (5.4, 6.0 and 6.6 nm); this is achieved by
adding additional H PtCl precursor to the reaction mix-
2
6
ture under H flow. Pluronic triblock copolymer was used
2
as surface protective agent instead of the commonly used
poly-N-vinylpyrrolidone (PVP). Because the former poly-
mer does not have a pure carbon backbone it may decom-
pose under milder oxidative conditions. This is a crucial
step because it helps to eliminate the buildup of amorphous
carbon/graphite on the nanoparticle surface during the sta-
bilizer removal process.
An alternative synthetic route to fabricate Pt-nanoparti-
cles in the 1–10 nm size range was explored. Sodium citrate
and ascorbic acid reducing agents are often used for syn-
thesizing Au and Ag nanoparticles [28–31]. They are both
mild reducing agents and are typically used in aqueous
solution, a very convenient fact, since most commonly used
polymeric capping agents such as PVP, Pluronic L64, poly-
acrylic acid, poly-acrylate and poly-acrylamide materials
are water soluble. Both sodium citrate and ascorbic acid
were used to produce small (1.5–2.0 nm) and medium-sized
(3.5–5.3 nm) Pt-nanoparticles. Fig. 1 shows TEM images
of selected Pt nanoparticle samples synthesized by different
methods.
Scheme 2. Schematic representation of the two-dimensional catalyst
fabrication by the Langmuir–Schaeffer technique.
The substrate was then lifted off by means of the motorized
dipper stage at a rate of 0.1 mm/min. 2D-nanoparticle films
obtained in this way were left to dry in air and character-
ized by SEM.
3
. Results and discussion
3
.1. Synthesis of Pt nanoparticles
The size of the platinum nanocrystals was tuned uni-
3.2. Synthesis of mesoporous oxide supports
formly in the range of 1–10 nm using different reaction con-
ditions such as reaction temperature, solvent, reducing
agent and surface stabilizing agent. The characteristics of
the samples are shown in Table 1. Using alcohol reduction
technique and PVP as the protective agent, Pt nanoparti-
cles of five different sizes (1.7, 2.6, 2.9, 3.6 and 7.1 nm) were
produced with narrow size distribution (r ꢁ 0.3 nm).
When Pluronic L64 triblock copolymer was used as cap-
ping agent and the reduction of the platinum precursor
Well-ordered mesoporous oxide materials were prepared
by sol–gel techniques for further use as catalyst supports.
SBA-15 with a pore diameter of 9 nm, a surface area of
800 m g and a pore volume of 1.2 cm g was used as
a model because of its well defined and easily tuneable
properties.
2
À1
3
À1
Al O and Ta O with hexagonally ordered channel sys-
2
3
2
5
tem, similar to that of SBA-15, were successfully synthe-
Table 1
The synthesis conditions and the characteristic sizes of the different Pt nanoparticle samples
a
b
c
Surface stabilizing agent
Reducing agent
Temperature (K)
TEM size (nm)
XRD size (nm)
PVP
PVP
PVP
PVP
Ethylene glycol
Ethanol
Methanol
469
351
338
338
469
338
353
353
373
293
293
293
293
1.7 ± 0.26
2.6 ± 0.22
2.9 ± 0.21
3.6 ± 0.26
7.1 ± 0.37
2.0 ± 0.35
3.4 ± 0.71
5.3 ± 1.20
1.5 ± 0.25
3.5 ± 0.38
5.4 ± 0.77
6.0 ± 0.98
6.6 ± 0.67
1.9
2.5
3.0
3.8
7.8
2.1
3.6
5.5
Methanol
PVP
Ethylene glycol
Ascorbic acid
Ascorbic acid
Ascorbic acid
Sodium citrate
Ascorbic acid (no PVP)
PVP
PVP
PVP
Pluronic L64
Pluronic L64
Pluronic L64
Pluronic L64
a
NaBH
NaBH
NaBH
NaBH
4
4
4
4
3.5
5.0
5.5
6.2
+ H
+ H
+ H
2
2
2
With Pluronic L64 stabilizing agent first Pt seeds were synthesized by reducing the precursor with NaBH
as reducing agent.
4
. During the nanoparticle growth H
2
was used
b
The size of the particles determined by TEM measurements by counting 300 particles.
The size of the nanoparticles determined by XRD using the Debye–Scherrer equation: d = Ak/Dhcos h, where d is the particle diameter, A is a constant
c
˚
(generally = 1), k is the X-ray wavelength (Co Ka = 1.79 A), Dh is the full width at half-max of the (111) peak in radians and h is half the Bragg angle of
the peak.

K. Niesz et al. / Inorganica Chimica Acta 359 (2006) 2683–2689
2687
Fig. 1. TEM images of Pt nanoparticles synthesized by different techniques: (A) PVP coated Pt (7.1 nm) nanoparticles made by ethylene glycol reduction;
B) Pluronic L64 coated Pt (6.0 nm); (C) PVP coated Pt (3.5 nm) made by ascorbic acid reduction; (D) PVP coated Pt (1.5 nm) made by citrate reduction.
(
sized in powder form. The high long-range order of the
organized mesoporous structures after the removal of the
polymer (calcinations at 673–823 K under air) were
revealed by transmission electron microscopy (TEM) and
small-angle X-ray scattering (SAXS) measurements (not
shown here). Physisorption measurements on the mesopor-
ous alumina and tantalum oxide samples revealed a BET
surface area of 400 and 120 m g with a pore size of 7
and 4.0 nm, respectively. Although mesoporous alumina
and tantalum oxide show lower thermal stability at higher
temperatures than SBA-15, these oxides were stable during
the template removal at 673 K and kept their hexagonally
ordered structure intact.
located on the outer surface of the oxide supports and only
a smaller portion of them are incorporated into the channel
system (Fig. 2).
Nanoparticle encapsulation is a ‘‘one-pot’’ synthetic
method (Scheme 1). Template assisted nanoparticles are
reduced in aqueous solution, upon which a triblock copoly-
mer Pluronic P123, a silica source (TMOS) and NaF are
added. The entire synthesis is conducted at pH 7 with the
fluoride anion acting as a catalyst for the hydrolysis of sil-
icate species. In the final material, all of the Pt particles are
encapsulated within the SBA-15 framework. TEM investi-
gations (Fig. 3A) and nitrogen physisorption measure-
ments (not shown here) indicate that the nanoparticles
are homogeneously dispersed inside the channels of the
SBA-15, and neither aggregation nor blocking of the pore
openings are key points in the design of high surface area
model catalysts. From SAXS measurements, it has been
determined that the lattice spacing (d100) of the SBA-15
increases from 10.1–13.4 nm as Pt particles of 7.1 nm diam-
eter are incorporated (Fig. 3B). The presence of the Pt-
nanoparticles during the self-assembly of the Pluronic
2
À1
3
.3. Fabrication of three-dimensional catalysts
When using the capillary inclusion method (Scheme 1)
to prepare catalysts, it was found that the size of the nano-
particles and the structure of the support (SiO , Al O and
2
2
3
Ta O ) are unaffected by the sonication. However, TEM
2
5
measurements also show that the nanoparticles mostly
2 3 2 5
Fig. 2. Pt (2.9 nm)/SBA-15 (A), Pt (2.9 nm)/Al O (B), Pt (2.9 nm)/Ta O (C) catalysts prepared by capillary inclusion (CI) method.

2688
K. Niesz et al. / Inorganica Chimica Acta 359 (2006) 2683–2689
Fig. 3. TEM image (A) and SAXS spectra (B) of Pt(7.1 nm)/SBA-15 synthesized by nanoparticle encapsulation (NE).
P123 into its hexagonal micellar arrangement causes a
swelling of individual micelles. This is a strong evidence
that Pt is incorporated into the channel of the SBA-15 dur-
ing growth. The nanoparticle encapsulation represents a
more convenient procedure for the synthesis of metal nano-
particles/SBA-15 materials. The maximum Pt particle size
that may be embedded in the SBA-15 channels by the
inclusion method is ꢁ7 nm, while the upper limit on nano-
particle inclusion for the encapsulated synthetic method is
determined by the maximum expansion that the micelles
can undergo without destruction (ꢁ15–20 nm). All Pt-
nanoparticles are synthesized using a polymeric capping
agent which controls the growth and stabilizes them in
solution. Upon sonication in the mesoporous oxide chan-
nels or after the direct condensation of silica around the
Pt nanoparticles, the nanoparticles are coated with ꢁ1
monolayer of this capping agent. The stabilizing polymers
are tightly bound to the Pt surface for entropic reasons.
Removing these species from the Pt surface is difficult
and constitutes a major effort of still ongoing research.
Typically, heterogeneous catalysts are calcined in flowing
oxygen or air prior to reduction in hydrogen which
removes adsorbed oxygen in the form of water. The calci-
nation step is used to remove carbonaceous deposits and
other impurities from the Pt surface but the particle shape
can be altered if the calcination temperature exceeds 600 K.
We have recently developed a procedure (a series of oxida-
tion–reduction cycles at relatively low temperatures,
Fig. 4. SEM image of 2D Pt nanoparticle array deposited on silica wafer.
compression, the surface coverage of the substrate with Pt
nanoparticles can be adjusted. Using this methodology, a
surface coverage up to 75% of a monolayer with Pt-nano-
particles was achieved. Removal of the PVP capping agent
on 2D Pt-nanoparticle arrays however is more difficult than
in the case of the 3D-analogues. When freshly prepared
densely packed nanoparticle-films are exposed to mild
(
O -plasma or Ozone treatment at room temperature) or
2
harsher oxidative conditions (O -annealing at 723 K), the
2
particles aggregate. This behavior is highly undesirable for
catalytic model studies since under normal reaction condi-
tions such clusters would most probably fuse to form larger
metal conglomerates. In order to keep individual particles
finely dispersed on the substrate, they must be immobi-
lized/chemically bound to the substrate surface prior to
the capping agent removal. This can be accomplished by
burying them with a thin film oxide (e.g., silica or alumina)
deposited by either chemical vapor deposition (CVD), elec-
tron beam evaporation or atomic layer deposition (ALD).
3
73 K) which enables us to remove the PVP from the par-
ticle surface and the Pluronic P123 from the mesopores of
the SBA-15 support at the same time. The observed cata-
lytic activity of an untreated catalyst during ethylene
hydrogenation increased by an order of magnitude
À1 À1
increases (17–202 lmol g
s ) after undergoing five oxi-
dation–reduction cycles at 373 K.
.4. Fabrication of two-dimensional catalysts
.1 nm cubic PVP stabilized Pt-nanoparticles synthesized
3
4. Conclusion
7
Several different fabrication methods used to produce
model catalysts are reported. Such catalysts will be used
in a systematic study of the variables that influence reaction
selectivity: (1) the synthesis of Pt nanoparticles with tun-
by the ethylene glycol reduction method were deposited
onto native oxide covered silicon wafers by means of the
Langmuir–Schaeffer technique (Fig. 4). By varying the film

K. Niesz et al. / Inorganica Chimica Acta 359 (2006) 2683–2689
2689
able size and narrow size distribution was developed using
a variety of synthetic conditions; (2) the synthesis of a high
surface area mesoporous support materials with well-
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2
1
Acknowledgments
1
This work was supported by the National Science Foun-
dation under Contract No. DMR-0244146, the Director,
Office of Energy Research, Office of Basic Energy Sciences,
Chemical Sciences Division, of the US Department of En-
ergy under Contract No. DE-AC02-05CH11231 and the
Swiss National Science Foundation (SNF). The authors
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