Nanostructured Catalytic Materials: Design and Synthesis

Hua Chun Zeng

National University of Singapore, Singapore

INTRODUCTION

Natural and manmade catalytic materials are among the
oldest ‘‘nanostructured materials’’ known long before the
era of nanoscience and nanotechnology. The classic het-
erogeneous catalysts consist of active nanoscale metal
component(s) and solid carriers,

[1]

namely, ‘‘inert’’ oxide

supports such as alumina, silicates, or magnesium oxide,
to increase the reaction surface area and metal utilization.
Conventional processing techniques for fabrication of
supported catalysts contain one or more of the following
steps: impregnation, precipitation, coating, rewashing,
ion-exchange, pulverization, dying, and calcination, etc.

[2]

Nanoscale metal clusters or particles can be thus formed
on the oxide carriers after these processes. It is now well
known that the particle size, local composition, and
structure (shape) of nanoscale catalysts determine the
ultimate catalytic activity and selectivity. For example, it
has been demonstrated that the activity of TiO

2

-supported

gold particles is very sensitive to their size (2 to 3 nm) in
the CO oxidation reaction with oxygen at ambient con-
ditions.

[3]

It is also well known that the structure and

composition of nanocatalysts may change under the
reaction conditions, and thus their performance could be
time-dependent. Nonetheless, prevailing catalyst prepara-
tion still remains largely as a technological art rather than
a science, although surface science has significantly
deepened our general understanding of heterogeneous
catalysis using single-crystal model catalysts.

[4]

Over the past 15 to 20 years, we have witnessed

exciting advances in the design and synthesis of low-
dimensional nanostructured materials, ranging from full-
erenes, carbon nanotubes, supramolecular assemblies,
mesoporous structures, to various organic–inorganic hy-
brid materials. Taking advantage of the rapid development
in nanoscience and nanotechnology, a wide range of syn-
thetic techniques are now in place. For example, nano-
structured materials can be prepared with constrained or
unconstrained synthetic methods, in which inorganic or
organic templates (e.g., porous oxides, organic ligands,
well-oriented crystal planes, and supramolecule-directing
agents) are commonly employed. With these newer ap-
proaches and knowledge, a huge variety of nanostructured
catalytic materials have been designed and synthesized,
which will be the main review topic of the present article.

With the emphasis on catalytic prospect, the objective of
this article thus aims at introducing various design and
synthesis strategies for this new class of materials. Future
challenges and research directions in this area will also
be addressed.

DESIGN AND SYNTHESIS
OF NANOSTRUCTURED
CATALYTIC MATERIALS

Because of the advances in nanostructured catalytic ma-
terials and assembling methodologies, we may have to
reexamine the roles of each component in the traditional
heterogeneous catalysts and redevelop the technology. In
fact, a practical solid catalyst is normally not a simple
chemical compound, but a highly organized multicompo-
nent materials system (e.g., active components and car-
rier). In this regard, a modern view of solid catalysts is
different from the traditional one. An organized assembly
of catalytic materials can be considered as a ‘‘catalyst
device’’,

[5]

and the ways of chemical and structural

organizations in the device will give profound impacts on
its ultimate performance. In the foreseeable future, a
transformation from the traditional catalyst preparation to
a more sophisticated ‘‘assembly’’ technology is antici-
pated in view of the rapid progress of this field. As the
first steps toward this end, nonetheless, various nanocom-
ponents with desired chemical and structural properties
and organization programmability must be fabricated and
investigated for the constitution of a nanocatalyst ‘‘tool-
box.’’ In the following sections, we will look into the
current trends of research in this important area.

Architectures of Porous Materials

Many significant progresses in porous materials have been
made over the past two decades, which extends the com-
monly known zeolites (chemical formula M

x/n

n+

[(AlO

2

)

x

-

(SiO

2

)

y

]

x

zH

2

O, where M

n +

represents a metal cation;

pore diameter < 1 nm) from microporous regime (pore
diameter < 2 nm) to mesopores (2–10 nm),

[6]

large meso-

pores (> 10 nm), and macropores (> 50 nm). In particu-
lar, crystalline VPI-5 with uniform pores larger than
1.0 nm had been synthesized for the first time in 1988,

Dekker Encyclopedia of Nanoscience and Nanotechnology

2539

DOI: 10.1081/E-ENN 120009427
Copyright

D 2004 by Marcel Dekker, Inc. All rights reserved.

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ORDER REPRINTS

as illustrated in Fig. 1.

[7]

Subsequently, periodic meso-

porous solids were discovered by Mobil researchers in
1992.

[8]

These ordered porous materials with large pore

openings had significantly enhanced our ability in uti-
lizing intrapore chemical reactivity and space confine-
ment for catalytic reactions, sorption processes, ion
exchange, as well as materials synthesis, including archi-
tectures for growth and organization of functional nano-
materials. For example, hexagonal MCM-41 and cubic
MCM-48 (Mobil codes) can be prepared with cationic
surfactants under basic conditions. These mesoporous
silicates together with various modifications (i.e., with
metal cations via ion exchange, complexation, and direct
‘‘planting’’)

[9]

allow us to tailor chemical and thermal

properties to meet different working environments. For
instance, isomorphic substitution and postsynthesis incor-
poration of active metal species are the two major
methods for metal introduction to mesoporous silica.

[10]

Furthermore, surface functionalization with organic
groups will change the chemical properties of the surfaces,
forming hybrid inorganic–organic mesoporous silicates.

In the latter cases, the mesopores of the prepared solids
can be viewed as nanoscopic reactors, separators, or host
templates for chemical processes and nanostructured
materials fabrication.

Various methods have been developed to introduce

organic surface groups onto the mesoporous hosts, on the
basis of chemical reactions between the hydroxyl-covered
surfaces and reactive silane coupling agents. For example,
covalent grafting, coating, and co-condensation reactions
are common methods to tailor surface properties these
days.

[11]

In grafting processes, surface modification with

organic ligands and functional groups is normally carried
out by silylation that takes place on free and geminal
silanol groups [i.e.,

Si–OH and

Si(OH)

2

] under dry

conditions. On the other hand, coating provides a means to
introduce organics onto hydrated pore surfaces where a
monolayer of water is present to form organosilanes.
Compared to the above two methods, the co-condensation
process is a more direct method. The surface organic
functional groups can be anchored together with the for-
mation of mesoporous silicates under so-called ‘‘one pot’’

Fig. 1

(a) Pore characteristics in some representative aluminophospates AlPO

4

-11, AlPO

4

-5, and VPI-5. (b) Measured pore sizes by

argon adsorption techniques, noting that the VPI-5 shows a pore diameter greater than 1.0 nm. (From Ref. [7].) (View this art in color at
www.dekker.com.)

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Nanostructured Catalytic Materials: Design and Synthesis

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synthetic conditions.

[12]

These organic–inorganic hybrids

possess a number of advantages for catalytic applications
that organic polymers and amorphous/nonporous silica
counterparts do not have.

[11]

In many cases, space con-

finement and stabilization of the catalyst in the mesopo-
rous solid can be attributed to the observed enhancement
of catalytic activity. These materials have demonstrated
utility as catalysts in acid/base catalysis, oxidations, re-
ductions, enantioselective catalysis, stereospecific po-
lymerizations, and fine chemicals synthesis.

[11]

Rich

inclusion chemistry of this class of materials with guest
species in the internal space had been reviewed recently.

[9]

Over the past decade, increasing research activities

have been shown in the fabrication of hierarchical porous
structures that possess ordered pores ranging from micro-
to macroscale. There have been a number of techniques
developed.

[7]

For example, microporous colloidal parti-

cles have been used for shape-, film-casting to prepare a
range of hierarchical porous structures. On the other hand,
bulk dissolution and restructuring of preexisting oxides
have been used to fabricate shaped hierarchical structures
of zeolite. Furthermore, structures with mesoporosity can
be prepared by using surfactants as structuring directing
agents.

[7]

In addition to these approaches, new film

formation methods allow us to fabricate various zeolite
and molecular sieve layers and membranes. In recent
years, pore sizes of ordered mesoporous oxides have been
extended up to 10 nm with the use of block copolymers,
and from 100 nm to 1 mm with Latex spheres as tem-
plating structures.

[13,14]

For example, nonionic polyethyl-

ene oxide surfactants were used to prepare wormhole-like,
MSU-type SBA-n and CMI-1 mesoporous materials. With
decaoxyethylene-cetyl-ether or polyoxyethylene(6)tride-
cyl-ether surfactants, hierarchical macroporous metal
oxides had been synthesized without using polymeric
sphere template.

[13]

It is believed that the macro–meso-

structured metal oxides were formed through an interme-
diate phase of supermicelles (the length in the micrometer
range and the diameter in the submicrometer range) and
mesostructured nanoparticles of metal oxides. The re-
moval of the organic components resulted in the formation
of hierarchical mesoporous–macroporous structures.

[13]

The approach had been successfully applied to ZrO

2

,

Nb

2

O

5

, Ta

2

O

5

, Al

2

O

3

, and CeO

2

oxide systems, and the

networks of the pores can be preserved at elevated
temperatures to meet catalytic applications. Very recently,
hierarchical and self-similar growth of self-assembled
mesophase crystals in micrometer size had been investi-
gated with the assistance of glass substrate. Ordered
octahedral crystal building units can be assembled into
various high-ordered stack structures via edge-sharing.
It is found that the large structures are formed through
stepwise nucleation from the edges of the previous
crystals.

[15]

In addition to the general interest in the synthesis and

design of micro-, meso-, and macroporous materials, there
are interests in the preparation of single-site molecular
receptors in silica matrix for specific adsorption and
catalysis applications. Using molecular imprinting tech-
niques, both microporosity and chemical functionality
have been achieved for this new class of organic–
inorganic hybrid materials. Typically, bulk-imprinted
silica with hydrophilic framework can be prepared via
copolymerization of the imprint organosilane(s) with a
silica source (normally, silicon alkoxides such as tetraethyl
orthosilicate, TEOS), followed by a mild thermal treat-
ment.

[16]

Fig. 2 shows a flowchart of this type of synthesis,

where organic functional groups (such as amines) can be
immobilized into the silica matrix possessing high specific
surface area. The imprinted silicas can act as shape-
selective base catalysts,

[17]

and the hydrophilic framework

prepared in this way offers an interesting potential to
stabilize polar reactive intermediates and transition states
at the active sites.

[16]

Finally, it should be mentioned that the mesoporous

materials can be used as traditional high-surface-area
catalytic supports for active metal loading. On the other
hand, nanostructured metallic catalysts encapsulated can
nucleate the expansion of the mesopore channels. For
example, prefabricated gold and platinum nanoparticles
had been investigated to tune the pore size of the meso-
porous silica (SBA-15) that are grown around them (e.g.,
in the range of 9.2 to 11.6 nm).

[18]

Other important applications of the porous materials

include template synthesis of nanostructured catalytic
materials used as either individual (freestanding) catalysts
or basic building blocks for self-assembly, as will be
discussed in subsequent sections.

Designs of Layered
Organic–Inorganic Nanohybrids

In recent years, layered organic–inorganic materials have
attracted increasing research attention not only because of
the fundamental interest in general supramolecular chem-
istry, but also because of their potential usage as
precursors for catalytic nanomaterials processing. There
are two basic types of clay materials: cationic and anionic
clays. The cationic clays, which have been investigated
extensively over the past decades, were conventional
catalysts used in oil cracking before the replacement of
zeolites in 1964.

[19]

They have been used in many in-

dustrial chemical processes such as isomerization, liquid
refining, and Friedel–Crafts alkylation, including some
emerging environmental technologies (such as cation
exchange and waste carriers).

[19]

In this class of materials,

cations are located in the lamellar space (interlayer space)
formed by negatively charged alumino-silicate layers. As

Nanostructured Catalytic Materials: Design and Synthesis

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Fig. 2

Process of molecular imprinting in bulk silica. (a) Sol–gel hydrolysis and condensation catalyzed by HCl. (b) Removal of the

aromatic core and creation of a cavity with spatially organized aminopropyl groups covalently anchored to the pore surfaces. (From
Ref. [17].)

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Nanostructured Catalytic Materials: Design and Synthesis

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they are generally prepared starting from the natural
minerals, these clays and modified clay catalysts promise
widespread industrial applications in the future owing to
the low cost. Similar to cationic clays, anionic clays also
have alternate lamellar structures, but with an opposite
charge arrangement. Anionic clays are mainly synthetic.
The preparation is relatively simple and inexpensive; they
can be synthesized primarily with coprecipitation, anion
exchange, and structure reconstruction methods. Hydro-
talcite-like compounds (HTlcs), for example, have been
investigated extensively over the past decade, and they
have been widely used as catalysts, flame retardants,
molecular sieves, anion adsorbents, ion-exchangers, and
medicine stabilizers.

[20]

The sheet-like structure contain-

ing cations can be derived from layered Mg(OH)

2

(brucite) structure. When divalent cations are partially
substituted by trivalent ones, positive charges will be
built-up within the octahedron sheets. For solid charge
neutrality, anions have to be intercalated into the
interlayer space, as illustrated in Fig. 3. The HTlcs have
a chemical formula of [M

1

x

II

M

x

III

(OH)

2

]

x+

(A

x/n

n

)

mH

2

O

(where M = metal, A = interlayer anions),

[20]

whose prop-

erties can be tuned with variations of cationic brucite-like
sheets and anionic interlayer species. For example, or-
ganics- and polymer-containing HTlcs should be viewed
as supramolecular assemblies or nanocomposites com-
posed of organic parts (anionic) and inorganic molecular
sheets (cationic).

[21]

It is important to realize that the

intercalative ability in these alternately arranged layers
is based on electrostatic interaction, and the inorganic
parts are dissolvable in acidic environment while the or-
ganic parts are removable by ionic exchange or oxida-
tion at elevated temperatures. Furthermore, layer charge
density can be tuned with the content of M

III

cations in

the synthesis. The basal spacing (the distance between
two brucite-like sheets) depends on the size of the
intercalants. For instance, large-sized biomolecular anions
(up to several nanometers), such as nucleoside monophos-

phates and deoxyribonucleic acid (DNA), have been
intercalated into the interlayer space of Mg

0.68

Al

0.32

(OH)

2

NO

3

)

0.32

1.2H

2

O (a pristine HTlc).

[22]

Because of

their synthetic versatility in compositional tailoring, this
class of organic–inorganic hybrids has been used as
precursor compounds for the synthesis of functionalized
metal-oxide nanomaterials and nanocomposites after the
thermal removal or conversion of polymeric interca-
lants.

[23]

The chemical nature of the intercalated anions,

such as oxidative or nonoxidative, had been investigated
with respect to thermal reactions, in which nanostructured
Co

3

O

4

spinel oxides were formed from Co

II

Co

III

–HTlc

precursors.

[24]

Nanocrystalline Co

3

x

Al

x

O

4

can also be

incorporated into the g-Al

2

O

3

matrix via the simultaneous

formation of catalyst and support from the Co

II

Co

III

–HTlc

and alumina xerogel.

[25]

In addition to the above bulk materials preparations,

organic–inorganic hybrid materials can also be prepared
into thin films to meet new applications. The major
synthetic methods employed in this area are sol–gel-based
techniques, intercalation reactions, layer-by-layer assem-
bly (e.g., Langmuir–Blodgett technique and electrostatic
self-assembly), and evaporation techniques, as exempli-
fied in Fig. 4.

[26]

Although the primary objective of these

film-fabrication developments is for organic–inorganic
electronics, it is believed the hybrid structures prepared
can be extended to catalytic applications, including
chemical sensors, in the near future.

Very recently, single-crystal bulk organic–inorganic

hybrids had been investigated for the fabrication of cat-
alytic nanocomposites and large-scale organization of

Fig. 3

The hydrotalcite-like anionic-clays have a layered

structure similar to that of brucite [Mg(OH)

2

] but with some

positive charges. To maintain the electrical neutrality for the
solid, intercalation of anions (e.g., nitrate and carbonate or
even DNA) into the interbrucite layers (i.e., interlayer space)
occurs, which leads to a hydrotalcite-like structure. (View this art
in color at www.dekker.com.)

Fig. 4

(a) Electrostatic assembly of anionic inorganic layers

and cationic organic layers. (b) A similar assembly of (a) but
with negatively charged layers formed from nanoparticles (e.g.,
CdSe or CdS). (From Ref. [26].)

Nanostructured Catalytic Materials: Design and Synthesis

2543

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catalysts under high-temperature environments.

[27,28]

In

particular, individual MoS

2

(used in hydrodesulfurization)

layers can be converted from direct sulfidation of single-
molecular sheets of MoO

3

supported with organic ‘‘pil-

lars’’ in the interlamellar layers. As there has been a large
variety of metal-oxide-organic hybrids available nowa-
days, this method provides a new means for the indirect
synthesis of lamellar organic–inorganic catalytic hybrid
materials consisting of building units of transition metal
dichalcogenides.

[27]

To make the catalytic materials us-

able in high-temperature environments, large-scale orga-
nization of metal-oxide nanostructures in a controllable
manner at elevated temperatures is highly desirable but
particularly challenging, because of the difficulties in
controlling the interconnectivity among individual crys-
tallites due to random nucleation and grain growth upon
heating. An organized condensation of single-molecular
MoO

3

sheets can be achieved with a controlled removal of

the organic intercalants.

[28]

This controlled condensation

takes place within the space provided by the pristine
organic–inorganic hybrid single-crystals. In this sense, the
role of precursor crystal can be viewed as a microscopic
‘‘green compact’’ analogous to the macroscopic one in
ceramic processing.

[28]

These new types of catalyst

organization may be useful for microreactor systems
where fluid stream stability is crucial, because the stack-
ing of nanostructured catalysts can be controlled to reduce
flow resistance.

In addition to the above chemical conversion and or-

ganization of nanostructured catalytic materials, organic–
inorganic hybrids have also been used in the preparation
of freestanding nanostructured materials. For example,
catalytically important materials such as tungsten oxide
nanowires, tungsten disulfide nanotubes, and VO

x

-nano-

tubes had also been prepared from organic–inorganic hy-
brid precursors that were preorganized in the forms of
lamellar mesostructures.

[29–31]

The final nanostructures

are formed either with high-temperature processing or
with hydrothermal treatment. These synthesized individ-
ual inorganic nanostructures will be the subject of the
next section.

Nanobuilding Blocks and Mesoscale
Self-Organizations

The research in low-dimensional catalytic materials has
blossomed in many new directions since Iijima’s discovery
of carbon nanotubes in 1991.

[32]

For example, syntheses of

low-dimensional catalytic nanostructures of MoS

2

, WS

2

,

MoO

3

, TiO

2

, V

2

O

5

, ZrO

2

, Co

3

O

4

, ZnO, etc. have been

carried out,

[33–41]

together with fabrications of nanostruc-

tured catalyst carriers such as MgO, Al

2

O

3

, and SiO

2

.

[42–44]

While many significant breakthroughs have been made for

the metal oxides and chalcogenides, the search for new
types of nanostructures and self-assemblies continues,
aiming at complementary functionalities and perform-
ances. Technologically, many known catalytic materials
are expected to gain better utilizations, because of their
new properties and possible high catalytic activities in the
nanometer regime (quantum confinement effect).

[39]

Dis-

crete, freestanding nanostructures can be viewed as basic
structural units or construction building blocks; they can be
used individually or collectively (after a proper organiza-
tion/assembly, will be addressed soon) in heterogeneous
catalysis and chemical-sensing applications.

Many synthetic strategies have been developed for the

fabrication of nanostructures. For example, metal oxide
nanoparticles (zero-dimension, 0-D) can be prepared
rather routinely via sol–gel methods

[45]

and direct precip-

itation.

[46]

A great variety of core-shell nanostructures

(0-D) have also been prepared via sol–gel and other
coating methods (e.g., layer-by-layer deposition) with the
assistance of removable inner-core supports.

[47]

On the

other hand, one-dimensional (1-D) nanomaterials can be
fabricated with the following techniques. 1) Utilizing
intrinsic structural anisotropy: Inorganic materials with
low structural symmetries can be prepared into 1-D
morphology along certain crystallographic axes.

[37,40]

The unidirectional growths can be further manipulated

Fig. 5

Synthesis of barcoded 1-D materials with alumina

membrane template where 1-D channels are parallel to each
other. (From Ref. [50].) (View this art in color at www.dekker.
com.)

2544

Nanostructured Catalytic Materials: Design and Synthesis

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with the assistance of inorganic salts or ionic/nonionic
organic surfactants present in the solution synthesis.

[48]

2)

Template-directed growth: In addition to the soft templates
(such as self-assembled molecular structures, e.g., micelles
discussed in the previous sections), this method utilizes
solid templates for growth depositions or chemical inser-
tions. For example, nonporous single-crystalline NaCl
substrate and porous supports such as zeolites, MCM-41,
and anodic alumina membranes (with parallel 1-D chan-
nels) have been used in the synthesis of 1-D nanostruc-
tures,

[49]

including semiconductor metallic barcodes

(Fig. 5).

[50]

The resultant nanostructures can be harvested

by template-removal. In certain cases, however, the
template can become a part of the resulting 1-D nano-
structures. These latter cases had recently been demon-
strated in the synthesis of 1-D bimorph composites of TiO

2

–

SnO

2

and Co

0.05

Ti

0.95

O

2

–SnO

2

,

[51]

and in the preparation

of Ag/MoO

3

catalytic nanostructures.

[52]

3) Surfactant-

assisted synthesis: Preferred electrostatic and chemical
interactions between organic surfactants and certain crys-
tallographic surfaces will restrict the growth along certain
directions, resulting in the kinetic control of the growth
anisotropy. This method had been investigated extensively
for metals (e.g., Au) and semiconductor nanomaterials such
as CdSe, ZnSe, CdS, and ZnS in recent years.

[53,54]

4)

Oriented attachment: This growth mechanism had recently
been investigated for the synthesis of 1-D materials such as
ZnO nanorods and b-Co(OH)

2

nanoplatelets, respectively,

from smaller building blocks.

[55,56]

Individual nanostructure units can be used as model

catalysts or chemical-sensing devices. For example, a gas
sensor for NO

2

detection had been developed with a single

nanoribbon of SnO

2

. In the illuminated state (UV light),

photo-generated holes recombine with trapped electrons at
the SnO

2

surface, desorbing NO

2

and increasing the

sensing current.

[57]

The general advantages of nanode-

vices are small size, fast response, and high sensitivity. If
required, 2-D- or even 3-D nanostructures can be further
constructed from starting 0-D- or 1-D subunits via meso-
scale self-assembly processes. There have been numerous
investigations in this area using organic surfactants.

[40,41]

As illustrated in Fig. 6, direct crystallite coupling via
oriented attachment process provides another possible
means to serve this purpose.

[58]

More examples in this

area can be found in the literatures.

[55,59]

Unlike the

layered nanostructures (2-D), self-assembled 2-D nano-
structures must possess different properties, because of the
presence of discrete low-dimensional subunits and organic
capping molecules within the assemblies. The retention or
removal of these organizing agents is an important issue in
catalytic applications, as the presence of organic compo-
nents will change the inorganic nature of catalysts
(hybridization). Future detailed comparisons for different
design strategies are urgently needed.

In addition to the ordered organic–inorganic assem-

blies, nanoarchitectures of multifunctional catalysts can
be further achieved by introducing dispersible organic-
capped inorganic nanostructures into 3-D gel matrixes,
which will also create the necessary chemical function-
ality and porosity in addition to the better material
utilization. In particular, structurally shaped (or faceted)
nanocatalysts may provide predesigned active catalytic
sites for the desired chemical reactions, although they may
appear to be random in the composite matrixes.

Interfacial Engineering and Self-Assembly

Our understanding of the catalyst carrier has now gone
beyond its supporting role; various levels of participation
of carriers in catalytic reactions have been known in
atomic and molecular scales.

[60]

As mentioned earlier, the

surface functionalities of catalytic materials (in either
planar or spherical form) can be further modified with the
guide of nanochemistry principles

[61]

and a wide range of

surface-engineering processes available.

[47]

In addition to

metal single-crystal surfaces that have been investigated
extensively over the past three decades, the synergetic
effects of catalyst–carrier systems have been investigated
with model catalysts, which may be closer to the reality of
a heterogeneous catalytic reaction environment. Taking

Fig. 6

(A) Flowchart of synthesis of forklike a-MoO

3

nanostructures via manipulating growth directions with TiO

2

capping. (B) Oriented attachment between two forklike
nanostructures along the [010] direction (perpendicular to
the paper). (From Ref. [58].) (View this art in color at www.
dekker.com.)

Nanostructured Catalytic Materials: Design and Synthesis

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advantage of the electron beam lithography technique, for
example, metal nanoparticles have been arrayed on silicon
surface at predesignated locations.

[62]

Pt and Ag nano-

particle arrays were prepared at different interparticle
distances. By varying the particle size and distances
between particles, the structural factors responsible for
the selectivity and activity in diverse catalytic reactions
had been investigated.

[62]

Other versions of model ca-

talysts on oxidized silicon wafers have also been fabri-
cated by spin-coating technique in recent years.

[63]

These

wafer-supported catalytic metal and metal-oxide thin
films have been tested under catalytic reaction conditions
and investigated with state-of-the-art surface analytical
techniques. Various catalyst systems have been studied
over the past few years.

[63]

In view of its versatility, this

evaluation approach is expected to be suitable for the
characterization of various nanostructured catalytic ma-
terials that can be prepared into colloidal suspensions
for spin-coating.

Apart from electron/ion beam lithography techniques,

nanoelectrochemical patterning process and photolitho-
graphy methods have also been widely used in the
fabrication of substrate patterns for the deposition of nano-
structured materials. These processes have been conducted
in both vapor-phase (e.g., chemical vapor deposition) and

liquid phase (e.g., solution growth), producing various
patterned 2-D nanostructures on the supports. For ex-
ample, the hierarchical self-assembly of gold nanopar-
ticles onto an organic bilayer template pattern on silicon
had been demonstrated.

[64]

Furthermore, ordered porous

structures had been fabricated by self-assembly of zeo-
lite nanocrystals on micropatterned silicate film sur-
faces.

[65]

Without demanding pretreatments, single-

crystal surfaces have recently been proven to be suitable
for nanoparticle self-aligned growth. In particular, a
hexagonal superlattice of anatase TiO

2

nanospheres has

been arranged on a-MoO

3

(010) surface without any

surfactants and surface patterns,

[66]

noting that both TiO

2

and a-MoO

3

are important catalysts in this material

combination. With well-developed surface science tech-
niques, in-depth investigations of supported nanostruc-
tured catalysts can be further pursued. Elegant examples
in this area have been reported in the literature, as
shown in Figs. 7 and 8 for the investigations on CO

Fig. 8

A scanning tunneling microscopic image of a MoS

2

nanocluster exposed to atomic hydrogen at 600 K, which
resulted in the formation of S vacancies (circled). Models below:
side and top views of S vacancies. (From Ref. [67].) (View this
art in color at www.dekker.com.)

Fig. 7

Scanning tunneling microscopic images of TiO

2

(110)-

(1

1): (A) clean surface before oxygen exposure, and (B) after

oxygen exposure at 650 K. Scanning tunneling microscopic im-
ages of Au/TiO

2

(110)-(1

1): (C) before CO:O

2

exposure, and

(D) after CO:O

2

exposure at 300 K. (From Ref. [3].)

2546

Nanostructured Catalytic Materials: Design and Synthesis

ORDER REPRINTS

oxidation and hydrodesulfurization.

[3,67]

On the (110)

surface of TiO

2

, Au particles show high activity at 2–3

nm for CO oxidation. However, Au particles in the Au–
TiO

2

composite aerogel retain high activity for the same

reaction at a size of 6 nm, because of an increase in the
interfacial contact area of the gold with multiple
domains of TiO

2

in the aerogel matrix.

[68]

The catalytic

bifunctionality of an active composite can thus be
explored when the support and catalyst have comparable
sizes (within a factor of 2).

[68]

Similar to vertical arrays of carbon nanotubes, catalytic

materials have also been prepared into 2-D arrays, but
with an extruding direction perpendicular to the surface.
Recently, ZnO nanorods and GaN nanotubes had been
prepared with gold nanoparticle catalysts in the vapor
phase and with a subsequent thermal treatment to remove
inner templates (e.g., in the synthesis of GaN nanotubes,
ZnO nanorods were used as templates);

[69]

the diameter of

the prepared 1-D nanostructures is proportional to the size
of metal catalysts in the synthesis. This method is now
widely used in the synthesis of 1-D-nanostructures,
including the control of periodic chemical component
variation along a nanostructure.

Superpolyhedral Clusters and Their
Organizational Forms

Compared with the nanobuilding blocks discussed above,
inorganic super fullerene and polyhedral clusters, which
have been synthesized in recent years, are generally much
smaller but with more distinct geometrical features. This
class of materials is formed from even smaller metal–
ligand polyhedra, such as tetrahedrons, octahedrons
through edge-, corner-sharing, etc. On the other hand,
these super inorganic ‘‘molecules’’ can be further packed
into repeated arrangements, giving away larger crystal-
lites. Fig. 9 shows a representative packing of this type
of superclusters.

[70]

Instead of oxygen in zeolites, basic

Fig. 10

Models of the [Cd

16

In

64

S

134

]

44

anionic clusters: a) a ball-and-stick model, b) the same view as a) shown as metal-centered

tetrahedra, and c) crystal packing of the clusters, viewed along the c axis (the unit cell is framed). The large sphere indicates the central
cavity. (From Ref. [71].) (View this art in color at www.dekker.com.)

Fig. 9

Packing of ball-like anions [{Mo

2

V

O

4

(CH

3

COO)}

30

-

{(Mo)Mo

5

O

21

(H

2

O)

6

}

12

]

42

in the crystal lattice (space filling

model viewed along [111] (top) and perpendicular to [111]
(bottom)). (From Ref. [70].)

Nanostructured Catalytic Materials: Design and Synthesis

2547

N

ORDER REPRINTS

units of tetrahedrons can now be extended to sulfur, noting
that the ionic radius of S

2

is much larger than that

of O

2

. For example, metal sulfides can now be pre-

pared into supertetrahedral clusters with a large cavity,
as shown in Fig. 10 for [Cd

16

In

64

S

134

]

44

(3.1 nm in edge

length).

[71]

A great variety of this type of superpolyhedral

clusters and molecules has been prepared nowadays and
has been reviewed recently.

[72]

One common feature in

these superpolyhedral clusters is their inner cavity. If
treated as nanobuilding blocks in their further architec-
tures, the superclusters with inner space will create even
more complex structures in addition to their chemical
complexity in nanoconfinement regime. Many of these
solids show a semiconducting character.

[72]

Apart from the

assembly shown in Fig. 9, there have existed a great
number of organization schemes of inorganic clusters with
organic counterparts.

[45]

These tailor-made organic–inor-

ganic nanocomposites could be potential candidates for
future heterogeneous catalysis applications.

CONCLUSION

We have examined various design and synthesis strategies
for the fabrication of nanostructured catalytic materials.
Generally speaking, nanostructured ‘‘kits-and-parts’’ as
well as their organization methodologies have been
available to constitute a nanocatalyst toolbox, i.e., they
are ready for direct usage or further assembly. To
transform ‘‘catalytic materials’’ to real custom ‘‘cata-
lysts’’, however, a large research endeavor is needed, and
this includes systematic investigations on multicomponent
assemblies and resultant chemico-physical properties,
including hierarchical pore structures. It is noted that the
catalytic performance of these materials remains largely
untested, despite the significant progress in materials
research. While traditional methods will still dominate the
industrial-scale catalyst processing at the present time, we
are beginning to see nanostructured catalytic materials in
practical applications of ‘‘chemical plant/laboratory-on-a-
chip’’ and chemical-sensing technology where the amount
of catalysts required is small, and supported type catalysts
may not be needed. For larger-scale industrial chemical
processes, for instance, it is believed that the traditional
‘‘metal ions impregnation’’ can be replaced with ‘‘nano-
structures impregnation,’’ as the nanobuilding blocks can
be prepared into colloidal suspensions for metal loading
on general catalyst supports. Furthermore, nanoarchitec-
tures of these building units can be achieved with the well-
developed sol–gel technology. Perhaps, simultaneous
formation of nanobuilding blocks and supporting oxide
matrix should be considered as a next level of architec-
tures, where the catalyst and support could be better
integrated in the new-generation catalysts.

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