A General Synthetic Strategy for Oxide-Supported Metal Nanoparticle
Nanfeng Zheng and Galen D. Stucky*
Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106
Received August 17, 2006; E-mail: firstname.lastname@example.org
Gold nanoparticles, particularly with dimension less than 10 nm,
exhibit unexpectedly high catalytic activities toward different types
of reactions, a property not revealed in bulk gold.1-4In order to
obtain high catalytic activity, gold nanoparticles are generally
dispersed on support materials, among which oxides are commonly
used.5The overall performance of a supported gold nanoparticle
catalyst highly depends on the size and shape of the gold
nanoparticles, the structure and properties of oxide supports, and
the gold-oxide interface interactions.5,6Although several techniques
have been developed to prepare oxide-supported gold nanoparticles,
they do not allow precise control over these parameters. Conse-
quently, some reported results on gold catalysis are controversial
and even partly contradictory.7,8A new synthetic strategy is needed
that makes it possible to tune each of the above factors for supported
We have developed a general strategy to prepare oxide-supported
metal nanoparticle catalysts, which is applicable to acidic and basic
oxides. In addition to its versatility, the approach permits facile
control over different parameters of a supported metal nanoparticle
catalyst (e.g., particle size, size-distribution, loading). The effect
of oxide supports in determining the size and size dispersion of
gold nanoparticles is minimized because the gold nanoparticles are
synthesized before they are immobilized on the oxide surface. The
overall strategy allows us to isolate the catalytic effect of each
individual parameter involved in a supported gold nanoparticle
catalyst. We have successfully applied this methodology to develop
green, efficient gold catalysts, such as the selective oxidation of
ethanol by oxygen.
Supported gold nanoparticles are traditionally synthesized from
single-atom gold precursors using aqueous chemistry. In the two
most popular preparation methods (i.e., coprecipitation and deposi-
tion-precipitation), single-atom gold precursors are either copre-
cipitated with oxide precursors or directly deposited on an oxide
surface. The size and size-distribution of gold nanoparticles formed
during the calcination process, and the degree of dispersion on the
support, are highly dependent on the following conditions: (1) pH
and concentration of the precursor solution; (2) isoelectric point
and type of the oxide support; (3) calcination temperature and
procedure.5For example, the deposition-precipitation method (DP)
requires the adjustment of pH value within the range of 6∼10 and
is not applicable to acidic and hydrophobic supports such as SiO2,
WO3, SiO2-Al2O3, and activated carbon. A major challenge of
conventional preparations is the difficulty of controlling the average
size of gold nanoparticles and their size distribution on different
oxide supports. The gold nanoparticles in the oxide-supported gold
catalysts prepared by Haruta and co-workers typically have a
particle-size standard deviation above 30%. Furthermore, although
different loadings of gold nanoparticles can be achieved by changing
the molar ratio of gold precursor to oxide support, they invariably
do not have the same-sized gold nanoparticles.9
In contrast to aqueous-solution synthesis, Goodman and co-
workers have successfully created model supported gold nanopar-
ticle catalysts in high vacuum chambers for better fundamental
understanding of the origin of their catalytic activity.10In both types
of catalysts, however, the formation of gold nanoparticles is heavily
influenced by the oxide supports because of the in situ formation
of nanoparticles.5,10In comparison, the more controllable formation
of isolated gold nanoparticles has been extensively studied through
different wet-chemistry approaches during the past two decades.11-13
The large-scale synthesis of nearly monodisperse gold nanoparticles
with size standard deviation less than 10% has been recently
achieved.14-19However, the lack of a general strategy to homo-
geneously disperse these gold nanoparticles on supports has limited
their applications in catalysis.7,20-25
Prior to this work, presynthesized gold colloidal particles have
been occasionally applied as precursors for the preparation of oxide-
supported gold catalysts.7,21-24The presynthesized nanoparticles,
which have a broad particle-size distribution, are generally deposited
on oxide supports through mechanical mixing followed by evapora-
tion.21,22As a result, the gold nanoparticles are not homogeneously
dispersed on the oxide surface. These gold nanoparticles are
frequently observed to sinter after thermal treatment to remove the
organic ligands. The prepared catalysts do not show advantages in
terms of controlling size or dispersion of nanoparticles.
In order to overcome the size-control problem, nearly monodis-
perse gold nanoparticles were synthesized using the single-phase,
one-step synthesis that we recently developed.16The narrow size-
distribution of gold nanoparticles is readily generated by using weak
reducing agents (amine-borane complexes) rather than strong
reductants (e.g., NaBH4, LiBH4) typically used. By selecting
different reaction solvents and controlling the reaction temperatures,
three different-sized monodisperse gold nanoparticles (i.e., 3.5 (
0.5, 6.3 ( 0.5, and 8.2 ( 0.9 nm) were synthesized (Figure S1).
The stable monodisperse gold nanoparticles were capped by long-
chain alkyl thiols (e.g., dodecanethiol) and soluble in organic
solvents (e.g., chloroform, dichloromethane, toluene, hexanes). The
synthesized gold nanoparticles are hydrophobic owing to capping
with long-chain alkyl thiols. The homogeneous dispersion of these
hydrophobic nanoparticles on hydrophilic oxides can therefore be
Fortunately, we have found a route to circumvent this dispersion
problem. The approach is based on the general concept of utilizing
relatively weak interactions between metal nanoparticles and the
substrates in an aprotic solvent to create a homogeneous loading
of the nanoparticles. The dispersion is then locked in place by
calcination. For the usual aqueous preparation procedures, columbic
charge determined by the isoelectric points of the supporting oxides
dominates the interaction with the metal nanoparticle precursors.
In an aprotic solvent environment, oxide surfaces have a common
feature, namely abundant permanent dipoles on their surface, which
means that they preferably adsorb charged, polar, and highly
Published on Web 10/13/2006
10.1021/ja0659929 CCC: $33.50 © xxxx American Chemical Society
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polarizable species, such as metal nanoparticles, through dipole-
charge, dipole-dipole, and dipole-induced dipole interactions,
respectively. When oxide powders (e.g., TiO2, SiO2, ZnO, Al2O3)
are added to a solution of dodecanethiol-capped gold nanoparticles
in an aprotic solvent (e.g., chloroform, methylenechloride), the gold
solution is decolored while the color of the oxide powders darkens
with stirring time (Figure S2). This adsorption behavior confirms
the interaction between the hydrophobic gold nanoparticles and the
hydrophilic oxides. We found that this interaction is rather weak
and not competitive with conventional hydrogen bonding. The
addition of a protic solvent (e.g., ethanol) can easily release the
adsorbed gold nanoparticles from the oxide surface into the solution
as evidenced by increased darkening of the liquid phase. This weak
interaction between the gold nanoparticle and the metal oxide was
observed for both charged and neutral organic-capped gold nano-
particles whose charge was characterized by electrophoretic mobility
measurements (Figure S3). Therefore, the weak interaction between
oxide particles and hydrophobic metal nanoparticles most likely is
due to dipole-induced dipole or dipole-charge interactions.
By utilizing the weak interaction between metal-oxide particles
and metal nanoparticles, as illustrated in Figure 1, we have
successfully deposited gold nanoparticles on different supports
ranging from very acidic oxides (e.g., zeolite in its acid form) to
very basic oxides (e.g., ZnO) and from insulators (e.g., SiO2) to
semiconductors (e.g., TiO2). The adsorption and desorption of metal
nanoparticles kinetically occurs on the oxide surface when a solvent
is present, which allows metal particles to migrate on the oxide
surface during stirring. Therefore, it is not surprising that this kinetic
process leads to the homogeneous dispersion of metal nanoparticles
on oxide particles in all the metal-oxide composites we prepared.
Another benefit of our general strategy is that same-sized metal
nanoparticles can be easily loaded on supports in varying amounts
by simply changing the amount of the oxide materials added into
the metal nanoparticle solutions (Figure S4), which is technically
difficult by conventional preparation methods. In addition to the
deposition of gold nanoparticles, this assembly approach is also
valid for the deposition of other metal nanoparticles (e.g., silver,
gold-silver, platinum, palladium) on different oxides.
As noted above, the metal nanoparticles in the as-prepared metal-
oxide composites are capped by organic thiols and do not possess
catalytic properties. The capping ligands are removed by heating
the prepared metal nanoparticle-oxide composites, typically in air
at 300 °C for 1 h. After calcination, the ligands are decomposed
and no sulfur is detected by XPS analysis. As illustrated in Figure
1, for both basic and acidic metal oxide supports, no obvious
aggregation of gold nanoparticles is observed by TEM even though
the gold nanoparticles in the calcined samples tend to wet the oxide
surface during calcination. After the removal of the organic capping
ligands, the gold nanoparticles are strongly bound to oxide supports
and cannot be removed by protic organic solvents (e.g., methanol,
ethanol). The supported gold nanoparticles even survive under acidic
conditions (e.g., in 1 M HCl) when they are supported on nonbasic
oxides (e.g., SiO2, TiO2).
In summary, in the synthetic strategy described here, both weak
and strong interactions between metal-nanoparticles and oxides are
involved in the different steps of the preparation, which, we believe,
is crucial in order to obtain supported metal catalysts with well-
defined physical and chemical properties. The weak interaction
between hydrophobic metal nanoparticles and oxide nanoparticles,
which we exploit for their cooperative assembly, allows controllable
manipulations over both metal nanoparticles and oxide components
of the catalysts since they are presynthesized individually. We
believe that this assembly approach can help to bring together the
well-developed nanomaterials synthesis with catalysis and sensing
applications. Furthermore, the preparation of catalysts through
cooperative assembly is also desirable to create multifunctional
catalysts. For example, more than two types of metal nanoparticles
can be simultaneously deposited on the same oxide. While the
assembly step allows more control over individual components of
the catalysts, the calcination step activates and stabilizes the metal
nanoparticles. The collective benefit of this approach is desirable
for the design of supported metal nanoparticle catalysts.
Figure 1. TEM images of 6.3 nm (A-G) and 3.5 nm (H) gold nanoparticles (5% in weight) supported on different oxides: (A) zeolite (CBV600); (B)
R-Fe2O3; (C) TiO2 (P25); (D) hydroxyapatite; (E) Al2O3; (F) ZnO; (G) fumed SiO2; (H) SiO2. The images were taken from the samples after thermal
treatment at 300 °C in air for 1 h. All scale bars are 20 nm.
C O M M U N I C A T I O N S
B J. AM. CHEM. SOC.
To examine the catalytic properties of the supported gold
nanoparticles prepared by the general strategy described above, we
have chosen the selective oxidation of ethanol by oxygen. In order
to study the size-dependent catalysis, three SiO2-supported catalysts
with 3.5, 6.3, and 8.2 nm gold nanoparticles were prepared by
depositing the nanoparticles on chromatography silica gel in 0.5%
(in weight) from their corresponding chloroform solution followed
by calcination in air at 300 °C for 1 h. The catalytic properties of
these three catalysts at 200 °C are shown in Figure 2. The smallest
gold nanoparticles (3.5 nm) do not exhibit the high catalytic activity
observed from 6.3 nm gold nanoparticles (conversion of 45% of
ethanol). Both 3.5 and 8.2 nm nanoparticles have lower ethanol
conversion (24% and 22%, respectively) but much higher selectivity
to acetaldehyde (90%) than do the 6.3 nm particles (75%). While
6.3 nm Au nanoparticles give TOF (turn-over frequency to
acetaldehyde) of 113 h-1, TOFs are 80 and 73 h-1for 3.5 and 8.2
nm particles, respectively.
After noting that 6.3 nm gold nanoparticles have a higher activity,
we used them for ethanol oxidation under even milder conditions
and found that SiO2-supported 6.3 nm Au nanoparticles (2.5% of
Au in weight) exhibit prominent catalysis with 21% ethanol
conversion at 100 °C. The selectivity to ethyl acetate and acetal-
dehyde is 86% and 14%, respectively. When the gold loading is
increased to 5%, the ethanol conversion reaches 39% with 90%
selectivity to ethyl acetate. This catalytic production of ethyl acetate
directly from ethanol by SiO2-supported gold nanoparticles is
unusual in its mild conditions used and the high selectivity that is
realized. The only byproduct is acetaldehyde, which can also be
used as a feedstock for ethyl acetate. The selective oxidation of
alcohols catalyzed by SiO2-supported gold nanoparticles prepared
by other methods requires much harsher conditions, which might
be due to their larger size.26It should also be noted that both copper-
and palladium-based catalysts for the transformation of ethanol to
ethyl acetate are also operated at temperatures higher than 200 °C
and generate diverse byproducts.27,28
Acknowledgment. This work was supported by the NASA
University Research Engineering and Technology Institute on Bio
Inspired Materials (BiMat) under Award No. NCC-1-02037 and
made use of the MRL central facilities supported by the MRSEC
Program of the National Science Foundation under Award No.
DMR05-20415. We thank Prof. Horia Metiu and Prof. Jacob
Israelachvili for helpful discussions and Dr. Peter Stoimenov for
XPS measurement. We also thank Degussa for providing oxide
Supporting Information Available: Materials used for this study,
synthesis and catalysis of oxide-supported metal nanoparticles, the
decoloring of gold nanoparticles upon the addition of oxide powders,
electrophoretic mobility measurements, gold nanoparticle size-distribu-
tion, and XPS spectra before and after calcination. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Figure 2. The size-dependent catalysis of ethanol oxidation by O2at 200
°C. Three catalysts with 3.5, 6.3, and 8.2 nm gold nanoparticles supported
on SiO2(0.5% in weight) were used. Catalysis conditions: 1.000 g catalyst
(0.5% Au on SiO2), ethanol @ 0.6 mL/hour, and O2@10 mL/min.
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