EAU Sustainable Supported Metal Nanoparticles & Their Applications in Catalysis Summary

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DOI: 10.1002/cssc.200800227
Sustainable Preparation of Supported Metal Nanoparticles
and Their Applications in Catalysis
Juan M. Campelo, Diego Luna, Rafael Luque,* Jos M. Marinas, and Antonio A. Romero[a]
18
 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2009, 2, 18 – 45
Metal nanoparticles have attracted much attention over the last
decade owing to their unique properties as compared to their
bulk metal equivalents, including a large surface-to-volume ratio
and tunable shapes. To control the properties of nanoparticles
with particular respect to shape, size and dispersity is imperative,
as these will determine the activity in the desired application.
Supported metal nanoparticles are widely employed in catalysis.
Recent advances in controlling the shape and size of nanoparti-
1. Introduction
The scientific community has witnessed an explosion of interest and investment in the field of nanoscience and nanotechnology over the last few years. The nanoscience revolution (in
terms of sheer interest and investment) is one of the biggest
things to happen since the beginning of modern science,[1]
and it is nowadays at the core of future technological progress
owing to the increasing ability to manipulate matter on the
nanometer scale. The ability to directly work and control systems at the same scale as nature (e.g. mitochondria, DNA,
cells) can potentially provide a very efficient approach to the
production of chemicals, energy and materials. Over a billion
years, natural systems have evolved nanoscale biological entities for the efficient production of materials (i.e. enzymes) and
energy (i.e. chlorophyll). By mimicking these systems, scientists
may be able to reach the aims of a future sustainable society.
Nanomaterials have therefore been regarded as a major step
forward to miniaturisation and nanoscaling with various subfields that have been developed to study such materials. Every
different subdiscipline plays its hand in modern nanoscience
and technology (Figure 1). The nanotechnology field is highly
multidisciplinary; inputs from physicists, biologists, chemists
and engineers are required for the advancement of the understanding in the preparation, application and impact of new
nanotechnologies.
Figure 1. Nanomaterials encompass all fields, from materials science and engineering to (bio)medical applications.
A nanomaterial can be defined as a material that has a structure in which at least one of its phases has one or more dimensions in the nanometer size range (1–100 nm). Such materials include polycrystalline materials with nanometer-sized
crystallites, materials with surface protrusions spatially separated by distances on the order of nanometers, porous materials
ChemSusChem 2009, 2, 18 – 45
cles have opened the possibility to optimise the particle geometry
for enhanced catalytic activity, providing the optimum size and
surface properties for specific applications. This Review describes
the state of the art with respect to the preparation and use of
supported metal nanoparticles in catalysis. The main groups of
such nanoparticles (noble and transition metal nanoparticles) are
highlighted and future prospects are discussed.
with particle sizes in the nanometer range or nanometer-sized
metallic clusters dispersed within a porous matrix (supported
metal nanoparticles).
Among them, metal nanoparticles have attracted much attention over the last decade owing to their relatively high
chemical activity and specificity of interaction. Furthermore,
the properties of metal nanoparticles are very different to
those of their bulk equivalents, such as a large surface-tovolume ratio.[2] Bearing in mind the briefly mentioned advantages and many outstanding features of metal nanoparticles, it
is not surprising that the number of publications dealing with
metal nanoparticles has increased almost exponentially over
the last few years (Figure 2), with over 5000 publications so far
Figure 2. The growth in the number of publications dealing with metal
nanoparticles (1990–2007). Source: SciFinder Scholar.
in 2008 alone. Research efforts are expected to continue increasing as the benefits of the chemical properties achieved at
the nano level become increasingly apparent for applications.
One of the key driving forces for the rapidly developing field
of nanoparticle synthesis is the already mentioned distinctly
differing physicochemical properties presented by metal nanoparticles as compared to their bulk counterparts. Nanoparticles
typically provide highly active centers, but they are very small
and not in a thermodynamically stable state. Structures at this
size regime are indeed unstable as a result of their high surface
[a] Prof. J. M. Campelo, Prof. D. Luna, Dr. R. Luque, Prof. J. M. Marinas,
Dr. A. A. Romero
Departamento de Qumica Orgnica, Universidad de Crdoba,
Campus de Rabanales, Edificio Marie Curie (C-3),
Ctra Nnal IV, Km 396, E14071, Crdoba (Spain)
Fax: (+ 34) 957-212066
E-mail: q62alsor@uco.es
 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
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R. Luque et al.
energies and large surfaces.[1, 2] To produce stable particles, it is
necessary to terminate the particle growth reaction and there
are a number of methods by which this has been achieved.
The addition of organic ligands or inorganic capping materials,
or other metal salts, creating core–shell-type particle morphology as well as colloids and soluble polymers has been utilized
to achieve this aim.[3–5] These materials can be grouped in the
so-called “unsupported” metal nanoparticles. However, a problem arises with regard to the activity and reuse of these materials: the nanoparticles may undergo aggregation and suffer
from poisoning under the reactions conditions resulting in deactivation and loss of catalytic activity.
A significant amount of research with the expressed aim of
inhibiting aggregation and producing highly active nanoparticles with homogeneous size dispersity has been published.[3, 5, 6]
The control of size, shape and dispersity of nanoparticles is key
to selective and enhanced activity. A mechanism to achieve
this control is to utilise another nanotechnology, that of (nanoProf. Juan Manuel Campelo completed
his PhD at the Universidad Complutense
de Madrid (Spain) and then moved to
Crdoba, where he is presently Head of
the Departamento de Qumica Orgnica. For three decades, he has been involved in acid–base heterogeneous catalysis research applied to organic
chemistry and the use of supported
metal nanoparticles in heterogeneous
catalysis.
Dr. Antonio Angel Romero obtained his
PhD in chemistry from the Universidad
de Crdoba. Following a year working
with Jacek Klinowski at Cambridge University, UK, he was appointed a lecturer
at the Departamento de Qumica
Orgnica at the Universidad de Cordoba
in 1999. His main research interests are
related to heterogeneous catalysis applied to organic chemistry and mesoand nanoporous materials including
supported nanoparticles.
Prof. Diego Luna obtained his PhD in
chemistry from the Universidad de Crdoba (Spain) and later began his teaching career there. His research activities
have focused on heterogeneous catalysis and heterogenized homogeneous
catalysis. He has (co-)authored over
140 publications and seven patents.
From 2006, he has been involved in the
technology-based SME, Seneca Green
Catalyst, SL.
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porous) supports. These materials can be grouped as the socalled supported metal nanoparticles. The unique properties of
supported metal nanoparticles are directly related to the specific particle morphology (size and shape), metal dispersion,
concentration and the electronic properties of the metal
within their host environment (Figure 3).[1, 7]
Immobilisation and stabilisation of the nanoparticle form
allows exploitation of the special properties that occur at this
size regime. The fusion between porous materials and nanoparticle technology is potentially one of the most interesting
and fruitful areas of this interdisciplinary research. The potential for increased efficiency from nanoparticle catalysts, in combination with the advantages of such heterogeneous supports,
increases the “green” credentials of the process, with higher
selectivity, conversion, yield and catalyst recovery being proposed advantages and targets. This provides the opportunity
to develop specific devices for specific applications in various
fields including medicine,[8] sensors[9] and catalysis.[5, 10] Support-
Dr. Rafael Luque completed his PhD at
the Universidad de Crdoba in 2005. He
then joined James H. Clark at the Green
Chemistry Centre of Excellence at the
University of York, UK, as a research
fellow. He recently returned to Crdoba,
where he was appointed a senior researcher. His research interests include
nanomaterials, biofuels, heterogeneous
catalysis and microwave chemistry.
Prof. Jos Mara Marinas has been a
professor at the Departamento de Qumica Orgnica at the Universidad de
Crdoba since 1976. He has published
over 450 manuscripts related to his
main research interests: applied heterogeneous catalysis and photocatalysis for
the preparation of fine chemicals, sustainable and green chemistry and environmental remediation.
 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2009, 2, 18 – 45
Supported Metal Nanoparticles for Catalysis
Figure 3. Nanoparticles: size, shape and composition (the quoted nanoparticle sizes are all approximate).[1] Reproduced with permission.
ularly attractive candidates for catalytic applications. Their
main features also include a unique transition between molecular and metallic states (providing a local density of states
(LDOS)), a short-range ordering and increasing number of
kinks, corners and edges.[1, 2, 7] Futhermore, recent advances in
controlling the particle size and shape have opened the possibility to optimise the geometry of the particles for enhanced
activity, providing the optimum size and surface properties for
specific reactions. Decreasing metal cluster or nanoparticle size
also results in an increase in the available surface of the
system. The relationship between this surface, intraparticle
metal–metal bonding, the particle shape and atom-packing geometry ultimately determines the efficacy of these nanoparticles in their catalytic applications.
2.2. Density of States and Surface Plasmon Resonance
ed metal nanoparticles play an important role in catalysis,
which is the most widely studied application of these materials, as they have been extensively employed in many industrial
processes.[11]
Far from providing a comprehensive revision of all the reported protocols and catalytic applications of supported metal
nanoparticles, this Review describes the state of the art with
respect to the preparation of supported metal nanoparticles
and provides an overview of the recently reported key preparation protocols and applications of such materials in catalysis.
Owing to the rapidly expanding nature of this field, we hope
that this Review provides a helpful overview and introduction
to readers in this exciting research area.
2. Supported Metal Nanoparticles
2.1. What Is a Metal Nanoparticle?
Quantisation of the electronic states of nanoparticles, and manipulation of these states through size and shape control, is
one of the main factors driving research in this field. Changes
in properties at the nano level occur by different mechanisms
for different materials. As a general rule, as we progress from
the bulk metal to ever decreasingly smaller metal nanoparticles
the energy continuum of the bulk metal changes to produce
increasingly more discrete energy levels; that is, the density of
the electronic states decreases (Figure 4). This phenonemon is
important as, for example, the lowering of the density of states
within Ag nanoparticles in medical applications facilitates the
existence of migratory Ag+ ions, which have a high affinity for
sulfur and phosphorus.
One of the primary reasons that nanoparticles have received
such attention in the field of analytical science and sensors is
that the adsorption of an analyte onto the nanoparticle surface
will disrupt this resonance oscillation and generate a change in
The term metal nanoparticle is
normally used to described
nanosized metals with dimensions (length, width or thickness)
within the nanometer size range
(1–100 nm). Bulk metals are typically ductile and possess high
thermal and electrical conductivity; properties resulting from
electron delocalisation within
the bulk matrix. In contrast, such
physical properties are not typical amongst metal nanoparticles,
as the delocalisation observed in
the bulk is typically absent, thus
giving rise to properties that are
completely different to those of
the bulk equivalent.
Metal nanoparticles have a
large
surface-area-to-volume
ratio as compared to the bulk
equivalents, making them partic-
Figure 4. Relationship between nanoparticle size, energy and the principle of energy of states. Stabilisation and
control of nanoparticle size can be achieved by selecting a nanoporous support potentially leading to enhanced
catalytic activity and selectivity and ultimately designer catalysts.[7] Reproduced with permission from the Royal
Society of Chemistry.
ChemSusChem 2009, 2, 18 – 45
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R. Luque et al.
the electric field which can then be detected. This resonance
behaviour is a phenomenon described as the surface plasmon
adsorption (also termed the localised surface plasmon resonance (LSPR); Figure 5), and typically occurs as the nanoparticles decrease in size below the 100 nm threshold and commonly generates a strong colour in the visible region, the
result of specific scattering interactions.
Figure 5. Oscillation of a metal nanoparticle’s electron cloud (background)
relative to the metal core (foreground), in response to the electromagnetic
field—the basis for the surface plasmon resonance effect observed in nanoparticles.[7] Reproduced with permission from the Royal Society of Chemistry.
The electric-field component of the incident light induces
oscillation of the nanoparticle’s electron cloud.[13] Therefore,
the size and shape of the metal nanoparticles will have a
direct impact on this oscillation as this will affect the electron
cloud density. Contributions from effective metal nuclei charge
and polarisation within the electron cloud (affected by the dielectric constant of the metal) are also expected, as is influence
from the support material surrounding the nanoparticle.
2.3. Porosity of the Support
2.3.1. Porous Materials
A porous material is normally a solid that comprises an interconnected network of pores (voids). Many natural substances
such as rocks, clays, biological tissues (e.g. bones) and synthetic materials including ceramics, metal oxides, carbonaceous
materials and membranes can be considered as porous materials. A porous material is characterised by its porosity (e.g.
macro, meso-, microporosity or combinations thereof) as well
its textural and physical properties which are dependent on its
constituents.
The use of porous materials with defined pore sizes and
characteristics as supports for nanoparticles allows the generation of specific adsorption sites, creating a partition between
the exterior and the interior pore structure.[5, 14] It also has the
added advantage of inhibiting particle growth to a particular
size regime as well as reducing particle aggregation.[5–7, 10, 14]
Furthermore, by selecting and manipulating the textural properties of the porous support (sometimes in unison with a reduction step), it should be possible to control the size and
shape of the resulting nanoparticles. For example, Au has been
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shown to demonstrate strong size selectivity, with the metal–
oxygen interaction, which is important in oxidation reactions,
altering as a result of the particle size. This behaviour leads to
the possibility of size-selective and reusable heterogeneous
catalysts based on the size of the nanoparticle rather than the
pore size.
A variety of porous materials have been utilised as the support media for the controlled preparation of metal nanoparticles. Each support offers its own advantage, with specific thermally stable materials such as carbons receiving greater interest in high-temperature and high-pressure catalytic transformations (i.e. hydrogenations).[5, 10, 15] Some of the most commonly
used supports and features associated with these materials regarding the preparation of supported metal nanoparticles will
now be discussed. Among the wide range of solid supports
employed for the deposition of metal nanoparticles, carbonaceous materials, metal oxides and polymers are the three main
families of widely reported solid supports.
2.3.1.1. Carbonaceous Materials
Carbon-related materials offer great advantages as supports.
First, recent advances in the field have allowed the preparation
of carbon nanostructures with well-defined porosities and high
surface areas. Second, the carbonaceous surface can be conveniently modified through different approaches (e.g. ozonolysis, plasma, doping with heteroatoms, acid or basic treatment)
to stabilise catalyst–support interactions.[16] Despite the conventional use of microporous carbons as supports for metal
nanoparticles,[17] there have been recent advances in the preparation of a range of carbonaceous materials as flexible supports. Budarin et al. have recently reported the preparation of
a wide range of supported metal nanoparticles[18] on a novel
family of mesoporous carbonaceous materials called Starbon[19]
prepared from controlled carbonisation (under nitrogen atmosphere) of mesoporous starch.[20]
Endo et al. have also described the preparation of Pt nanoparticles (< 3 nm) using carbon-fiber-type materials and, interestingly, a carbon cup-stack motif to effectively trap the growing nanoparticles between the cups.[21] Pt nanoparticles were observed inside and outside of the carbon-fiber structure. Carbon nanotubes have also been investigated as supports for metal nanoparticles.[10, 22–24] Their intrinsic properties include high surface areas, unique physical properties and morphologies, high electrical conductivity and inherent size and hollow geometry that makes them extremely attractive as supports for heterogeneous catalysts (Figure 6).[24] 2.3.1.2. ACHTUNGRE(Bio)Polymers Polymers are another group of extensively employed supports for metal nanoparticles (Figure 7).[3–5, 25] They have been widely employed as a result of their availability, enhanced stabilisation properties of metal nanoparticles and resistance to particle sintering/agglomeration. Recently, the use of novel engineered polymers such as polyorganophosphazenes with an inorganic  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2009, 2, 18 – 45 Supported Metal Nanoparticles for Catalysis Figure 6. Transmission electron microscopy (TEM) images at different magnifications of Ru nanoparticles homogeneously dispersed inside the channels of carbon nanotubes.[24] Reproduced with permission from the Royal Society of Chemistry. Figure 8. TEM images of 5 wt % Pd supported on mesoporous starch prepared in A) ethanol and B) acetone.[30] Reproduced with permission from the Royal Society of Chemistry. Figure 7. Four examples of polymer-stabilised gold nanoparticles and clusters: a) gold clusters stabilised by water-soluble polymers; b) gold nanoparticles immobilised in the pores of a functionalised resin; c) gold clusters supported inside a polymer particle; and d) gold nanoparticles deposited on polymer surfaces.[41] Reproduced with permission. backbone,[26] polyvinylpyridine,[5, 27] fibers[5] and dendrimers[5, 28] as supports has become increasingly popular. Alternative supports including biopolymers and biomass-related polymers have been recently employed for the preparation of supported metal nanoparticles. Biopolymers are indeed attractive candidates for use as supports for catalytic applications.[29] They offer several advantages compared to traditional supports including low toxicity and cost as well as high biocompatibility, availability and abundance. Much work has been devoted to the preparation of different metal nanoparticles on various biopolymers including our own work on mesoporous starch[30] that afforded highly dispersed Pd metal nanoparticles with a narrow particle size distribution (Figure 8), and natural porous materials (e.g. diatomite).[31] 2.3.1.3. Metal Oxides In general, metal oxides offer high thermal and chemical stabilities combined with a well-developed porous structure and high surface areas (> 100 m2 g 1), meeting the requirements
for most applications. They can also be easily prepared and
ChemSusChem 2009, 2, 18 – 45
further functionalised, adding value to their use as support or
catalyst. Depending on the chemical reactivity of the support,
metal oxides can be classified as inert (e.g. SiO2) and reactive
(e.g. CeO2) metal oxides. Among the metal oxides, silica,[32–35]
alumina,[32, 34, 36–38] , titania,[34, 35, 39–42] ceria[32, 34, 36, 43] and zirconia[36, 37, 44] are the most commonly employed supports.
Superparamagnetic oxides (e.g. Fe3O4) have recently
emerged as new materials for the immobilisation of metal
nanoparticles with improved separation capabilities.[45] Welldistributed and stabilised supported Pd[45a] and Rh[45b] MNPs in
the magnetisable support surface (particle size in the 2–3 nm
range) were found to be very active in the hydrogenation of
cyclohexene[45] and benzene.[45a] The most attractive feature of
the protocol is that the materials can be easily recovered using
a permanent magnet in the reactor and can be reused in up
to 20 runs without a significant loss in catalytic activity.
Mesoporous aluminosilicates from the SBA and M41S families have been also reported to be good supports for metal
nanoparticles. We recently reported the preparation of Pt
nanoparticles on the mesoporous aluminosilicate MCM-48.[46]
The Pt metal nanoparticles were highly dispersed on the support with a very narrow (4–6 nm) particle size. Materials with
low Pt loading (typically 0.5 %; Figure 9 A) were found to be
highly active in the hydroisomerisation of n-octane. More recently, metal phosphates have also been reported as a novel
family of efficient supports for Au metal nanoparticles.[47]
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and less energy-intensive protocols have been the subject of
many recent reports. In this section, we briefly discuss the
main sustainable approaches for the preparation of supported
metal nanoparticles. These can be subdivided into physical
(e.g. sonication, microwaves, UV), chemical (e.g. electrochemical, impregnation) and physicochemical (i.e. sonoelectrochemical) routes.
3.1. Physical Routes
Figure 9. TEM images of Pt/Al-MCM-48 materials with different Pt loadings
prepared by the traditional impregnation/reduction method: A) 0.5 wt % Pt;
B) 1 wt % Pt; C) 3 wt % Pt; D) 5 wt % Pt.[46] Reproduced with permission.
The development of a range of green technologies including
sonication, microwaves, UV, laser, plasma and supercritical
fluids in the preparation of supported metal nanoparticles is
evident in the literature. Some examples of reported protocols
are examined herein.
3.1.1. Sonochemistry
2.3.2. Nonporous Materials
Nonporous materials can be defined as materials that do not
have any voids or pores in their structure. These include
metals, foils, glass, hard plastic and some polymers including
polyethylene, polypropylene and other engineered polymers
not included in the porous polymer section (e.g. poly(N,N-diACHTUNGREalkylcarbodiimide)).[48]
Despite the use of these common supports for the preparation of supported metal nanoparticles, other biomaterials and
biomass have recently been reported as supports for metal
nanoparticles. Following the initial reports by Raveendran
et al.,[49] and He and Zhao,[50] much work has been devoted to
the preparation of different metal nanoparticles on various biopolymers including cellulose,[51, 52] chitosan[53–55] and poly(allylamine) gels.[56]
3. Preparation Routes
The preparation of supported metal nanoparticles for a wide
range of catalytic applications has been well developed over
the last few years. However, with sustainability emerging as a
design criterion in nanoparticle synthesis and applications
since the mid-1990s, there is still room for improvement in the
methodologies employed for this purpose.[3, 57]
In the context of nanomaterials, the sustainability concept is
reflected in many of the green chemistry principles.[58] Ideally,
the metal nanoparticle should be prepared with less toxic precursors, in water or more environmentally benign solvents (e.
g. ethanol) using the least number of reagents and a reaction
temperature close to room temperature and as few synthetic
steps as possible (one-pot reaction) as well as minimising the
quantities of generated by-products and waste.[3, 58] Also, the
nanoparticles should be well dispersed on the support surface
and highly active in their catalytic applications. The use of less
toxic precursors in benign solvents (e.g. metal acetate or nitrate solutions in water and/or ethanol) using more environmentally friendly supports (e.g. biopolymers and biomaterials)
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Sonochemistry deals with the understanding of the effect of
sonic waves and wave properties on chemical systems. Ultrasounds remarkably enhance mass transport, reducing the
thickness of the diffusion layer, and also affect the surface morphology of the treated materials, normally enhancing the surface contact area.[59] Deposition and reduction of the particles
(favoured by ultrasonic radiation) takes place almost consecutively so that the heating step normally employed in other protocols can be avoided,[39] making the preparation of supported
metal nanoparticles more energy-efficient and environmentally
friendly. Another interesting feature of the methodology is the
controllable nanoparticle size distribution that can be achieved. These advantages are related to the acoustic cavitation
phenomena, that is, the formation, growth and collapse of the
generated bubbles in a liquid medium. The extremely high
temperatures (> 5000 8C), pressure (> 20 MPa) and cooling
rates (> 1010 8C s 1) lead to many unique properties in the irradiated solution.[60] The preparation of metal nanoparticles is
usually performed using a conventional ultrasonic cleaning
bath or a high-power probe (Figure 10).[61] However, these ultrasonic-assisted protocols sometimes require the additional
Figure 10. Traditional ultrasonic bath (left) and ultrasonic probe (right) employed for the preparation of supported metal nanoparticles.
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ChemSusChem 2009, 2, 18 – 45
Supported Metal Nanoparticles for Catalysis
use of a reductant including sodium borohydride,[62] hydrogen[39] and hydrogen/polyalcohols,[60–63] to further ensure the
reduction of the nanoparticle on the support.
3.1.2. Microwave Irradiation
It has been recently demonstrated that microwaves are a very
effective technology in applied chemistry. Several examples of
microwave-assisted deposition of metal nanoparticles on supports have been reported, mainly employing solutions of
metal salts as precursors. Microwave irradiation has several advantages over conventional methods, including short reaction
times, small particle sizes, narrow size distributions and high
purity.[18, 64] El-Shall and co-workers have extensively investigated the use of microwaves for the preparation of a range of
supported metal nanoparticles including Au and Pd.[32, 65] They
have also prepared capped Au and Pd nanoparticles on metal
oxides using polyethylene glycol and poly(N-vinyl-2-pyrrolidone) as protective polymers prior to microwave-heating to
further stabilise the nanoparticles from agglomeration. In this
way, the obtained metal nanoparticles were better dispersed
and had a narrower particle size distribution, which in turn increased their activity for the investigated application (e.g. oxidation of CO). They claimed that fast and uniform heating (due
to high dielectric constants of PEG and PVP) achieved under
microwave irradiation allows a quicker reduction of the metal
precursor on the support.[32, 65]
We recently reported the preparation of a range of metallic
nanoparticles on an ordered mesoporous silica SBA-12 structure.[66] The metallic Au, Ag and Pd nanoparticles were prepared in a very short time (< 2 min) under microwave irradiation of a solution of the metal salt precursor in ethanol/water or ethanol/acetone mixtures without the need of additional reductant.[66] Both the ethanol and the hydroxy-rich silica surface facilitate the reduction of the metal nanoparticle on the support as previously reported.[30, 67] The microwave protocol afforded dispersed and relatively small metal nanoparticles (2, 3.8 and 11.3 nm average particle size for Au, Ag and Pd, respectively; Figure 11), which were highly active catalysts for oxidation reactions. The time of microwave irradiation is a critical parameter in the preparation of these materials as longer reaction times lead to substantial particle agglomeration (Figure 12). Our recent studies were in good agreement with findings by ElShall and co-workers.[65] This methodology has, in general, difficulties with respect to controlling the particle size and distribution of the metal nanoparticle on the support. However, the polyalcohols added as stabilisers/capping agents have been shown to help to achieve a more homogeneous and narrow particle size distribution.[68] 3.1.3. Pulsed Laser Ablation The laser approach involves the vaporisation of metals employing a pulsed laser (e.g. Nd-YAG) and subsequent controlled deposition on the surface of the support under well-defined conditions of temperature and pressure.[32, 69] Savastenko et al. ChemSusChem 2009, 2, 18 – 45 Figure 11. Top: TEM images of “bare” SBA-12 ( 160 000, left;  300 000, right). Bottom: TEM images of Ag metal nanoparticles (left) and Au metal nanoparticles (right) supported on SBA-12 ( 300 000, 50 nm) prepared in a domestic microwave oven.[66] Reproduced with permission from the Royal Society of Chemistry. Figure 12. TEM micrographs of Pd/SBA-12 prepared in a domestic microwave oven at different times: A) 2 min; B) 10 min; and C) 20 min.[66] Reproduced with permission from the Royal Society of Chemistry. recently reported the preparation of monometallic Pt and Rh nanoparticles (1 wt % loading) by the direct deposition on different SiO2 supports by means of pulsed ultraviolet (248 nm) excimer laser ablation of Pt and Rh bulk metals.[70] The supported metal nanoparticles had a narrow particle size distribution (centered at ca. 2.5 nm) and exhibited high activities in the reduction of NOx compounds. The method has several advantages for the synthesis of supported metal nanoparticles. First, it does not usually involve the use of chemical precursors or solvents and therefore it provides a simple, environmentally friendly and effective synthetic route for supported contamination-free crystalline metal nanoparticles.[32] Second, almost any metal or mixtures in any composition and form (e.g. sheets,  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemsuschem.org 25 R. Luque et al. films and powders) can be turned into metal nanoparticles. Third, metal nanoparticles can directly be supported on catalysts as they are created with a significant number of dangling bonds and they are strongly adsorbed and anchored onto supports. Last, and most importantly, no side products are created and the technique can be scaled up for industrial applications.[36] The sizes and compositions of the generated metal nanoparticles can be adjusted to generate materials for specific catalytic applications.[36, 70] Figure 13. A) TEM and B) HRTEM images of multiwalled carbon nanotubes decorated with Rh metal nanoparticles, prepared using supercritical CO2.[76] Reproduced with permission from the Royal Society of Chemistry. 3.1.4. Supercritical Fluids 3.1.5. Plasma Another efficient and green alternative to prepare a wide range of supported metal nanoparticles has been the use of supercritical fluids. A very good revision on the subject was recently reported by Zhang and Erkey.[71] The conventional procedure involves the dissolution of a metal precursor in a supercritical (sc) fluid (e.g. scCO2) and its subsequent incorporation on a substrate/support under various conditions.[71, 72] The impregnated metal precursor can be reduced to its elemental form by three different approaches: 1) chemical reduction in the supercritical fluid (using a reducing agent such as H2 or ethanol);[71, 73] 2) thermal reduction in the supercritical fluid;[71, 74] and 3) thermal decomposition (in an inert gas) or chemical reduction with hydrogen or air after depressurisation.[71, 72, 75] Supercritical fluids offer several advantages as compared to traditional methods. First, they can provide enhanced masstransfer properties owing to their higher diffusivities compared to liquids and lower viscosities. Second, the lower surface tension allows better penetration and wetting, avoiding problems related to partial structure shrinkage or pore collapse on certain materials (e.g. silica aerogels) that are present in conventional chemical methodologies. Third, it is possible to control the particle dispersion and morphology on various supports by employing different metal precursors and by varying the metal content and reduction temperatures and chemistry.[71] Supercritical CO2 has been widely employed for the preparation of supported metal nanoparticles as it is abundant, inexpensive, non-flammable and non-toxic.[71–76] Wai and co-workers have shown that it is possible to decorate multiwalled carbon nanotubes with ruthenium and rhodium nanoparticles through hydrogen reduction of the respective metal precursors and by using scCO2 as the delivery medium for the deposition of the metal nanoparticles.[76] Supported particles with good dispersity along the external surface of the tube structure were produced in the 5–10 nm range (Figure 13). The resultant materials were shown to be active in the hydrogenation of a range of arenes.[76] However, more studies are needed in terms of the solubility of the organic precursors in the supercritical fluid and the reduction step in order to further develop this technique. Another major issue in the widespread use of supercritical fluid is the cost of the equipment. 26 www.chemsuschem.org A novel plasma reduction method at room temperature has been used to prepare supported metal nanoparticles. Legrand et al. employed a dihydrogen microwave plasma to reduce various metal solutions (Au, Pt and Pt-Au) on zeolites.[77] The afterglow of such microwave plasma (2.45 GHz) was found to contain hydrogen atoms at a sufficiently low temperature to effectively reduce the metal ions in solution to small metal nanoparticles (< 5 nm) on NaY and HY zeolites. The supported metal nanoparticles were found to be very stable to thermal treatment. An Ar-glow discharged plasma has also been employed to support Pt, Pd, Ag and Au nanoparticles on a range of supports including nonporous TiO2, g-alumina and H-ZSM-5.[78] The metal nanoparticles were found to be homogeneously distributed on the surface of the support. Oxygen-glow discharge plasma allowed the preparation of supported metal nanoparticles, but small quantities of metal oxides were also found in their preparation. This technique is a very promising and straightforward way to prepare metal nanoparticles as it is an environmentally friendly, fast and simple methodology and also a promising alternative to hydrogen reduction at high temperatures. However, the specialised equipment needed makes difficult its widespread use. 3.1.6. Other Physical Routes A range of other physical routes to prepare supported metal nanoparticles have also been reported, including the use of gamma radiation[79] and Au–Ag exchange.[80] 3.2. Chemical Routes The classical chemical synthesis methods are co-precipitation, impregnation and deposition-precipitation. Some other novel greener routes include precipitation from reverse-micelle (water-in-oil) emulsions, photochemistry, chemical vapour deposition and electrochemical reduction.  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2009, 2, 18 – 45 Supported Metal Nanoparticles for Catalysis 3.2.1. Traditional Methods 3.2.1.1. Impregnation This methodology entails the “wetting” of the solid support with a solution containing the metal precursor. The common method of chemical impregnation is the so-called wetness impregnation. In this method, the metal nanoparticle precursor, which is normally a salt (e.g. metal nitrate, chloride), is dissolved in the minimum quantity of solvent. The resulting metal salt solution is then added to the porous support, filling its pores so that a thick paste is formed. The solvent is then removed in a rotary evaporator and the final solid is oven-dried and subsequently calcined and reduced (if needed) before being tested as a catalyst (Figure 9 and Figure 14).[33, 46, 81] The metal nanoparticles obtained by this methodology are dispersed depending on the metal, support and loading of the final solid.[33, 46, 81, 82] the sol-gel technology cannot be easily applied to polymeric substrates.[71] Bao and co-workers recently reported a modified one-pot co-precipitation approach to the preparation of supported Ag metal nanoparticles on silica.[86] The template-directed route comprises the capping of Ag + cations in solution with dodecylamine, subsequent reduction to Ag metal using formaldehyde (CH2O) and eventual self-assembly of the material after addition of the silica source (e.g. tetraethyl orthosilicate (TEOS); Figure 15). Compared to conventionally impregnated materials that were found to have large particles (15–18 nm) unevenly dispersed on the support (Figure 16), the uniformly supported metallic silver nanoparticles prepared using this novel methodology had an average size of 3.5 nm and were found to be well dispersed and embedded in the silica matrix (Figure 16). Figure 15. Schematic depiction of the preparation of Ag metal nanoparticles on silica using a modified co-condensation protocol.[86] Reproduced with permission from the Royal Society of Chemistry. Figure 14. TEM images of gold metal nanoparticles on different oxide supports prepared by the impregnation method using citrate as reducing agent. a) Au/ZrO2 ; b) Au/CeO2 ; c) Au/Fe2O3 ; d) Au/SiO2. Arrows indicate gold nanoparticles.[81] Reproduced with permission. 3.2.1.2. Co-precipitation The co-precipitation method involves the simultaneous precipitation of the metal and the support.[83] In this way, the metal nanoparticles can be incorporated into the structure of various mesoporous materials.[33, 84] Barau et al.[33] recently prepared supported Pd metal nanoparticles on hexagonal mesoporous silicas (HMS) using a sol-gel approach previously reported.[85] Such metallic Pd nanoparticles, with particle sizes around 4– 5 nm, were found to be relatively well dispersed on the silica surface. However, the presence of the metal precursors in solution interfere with the polymerisation chemistry of the material, often resulting in samples with undesirable properties. Also, ChemSusChem 2009, 2, 18 – 45 Figure 16. TEM and HRTEM images of a, c) Ag metal nanoparticles supported on SiO2 prepared by a modified co-condensation protocol; b, d) Ag metal nanoparticles supported on SiO2 prepared by conventional impregnation. The insets show the particle size distributions of silver nanoparticles.[86] Reproduced with permission from the Royal Society of Chemistry. 3.2.1.3. Precipitation-Deposition This method was initially reported by Haruta et al.[87] and involves the dissolution of the metal precursor followed by ad-  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemsuschem.org 27 R. Luque et al. justment of the pH (i.e. 5–10) to achieve a complete precipitation of the metal hydroxide (e.g. Au(OH)3), which is deposited on the surface of the support. The hydroxide formed is subsequently calcined and reduced to the elemental metal (Figure 17).[84, 88, 89] are subsequently taken up upon interaction with the support.[38, 90, 92–94] The microemulsion–support interaction can be enhanced by increasing the hydrophobicity of the support (e.g. silylation of hydroxy-rich surfaces), making it more chemically compatible with the microemulsion during the deposition step.[95] Wang et al. have also recently reported another interesting approach of this methodology employing a water– liquid CO2 (as oil phase) microemulsion stabilised by sodium bis(2-ethylhexyl)sulfosuccinate as surfactant and hexane.[96] In this way, Pd, Rh and Pd-Rh nanoparticles with sizes ranging from 2 to 10 nm could be homogeneously deposited on the surface of multiwalled carbon nanotubes. 3.2.3. Photochemistry Figure 17. TEM micrographs of a) Au/TiO2 (violet powder) and b) Au/Al2O3 (grey-blue powder) prepared by the deposition/precipitation method. Scale bars: 50 nm.[89] Reproduced with permission. In general, these methodologies often provide a broad nanoparticle size distribution and it is difficult to tune the particle size for a particular application owing to a poor control of the nanoparticle size (Figures 9, 14 and 17), that also affects the dispersion and size of the metal nanoparticle with increasing metal loadings (Figure 9). Particle agglomeration is quite a common phenomena, and the use of liquid solutions has been reported to create a collapse of fragile supports (e.g. organic or silica aerogels) as a result of the high surface tension of the liquid solution.[90] Furthermore, many of the reported protocols require the use an excess of external reductant (e.g. NaBH4, H2, hydrazine) to ensure the complete formation of the supported metal nanoparticle, that has to be removed after the reaction. 3.2.2. Microemulsions Microemulsions can be described as homogeneous-like combinations of water, oils and/or surfactants (often in the presence of alcohol- or amine-based compounds).[91] The formation of reverse micelles was confirmed to be an interesting and environmentally friendly alternative to the preparation of metal nanoparticles. Thus, a solid support is impregnated with a microemulsion containing a dissolved metal salt precursor,[90, 92, 93] in a similar way to that of the previously described traditional chemical impregnation. Metal nanoparticles obtained using this methodology have a more controllable, narrow crystallite distribution as compared to those obtained through the traditional impregnation, co-precipitation and precipitation-deposition methods.[92–94] This has been attributed to the confined location of a limited amount of metal salt in the micelles that 28 www.chemsuschem.org Only a few reports can be found on photochemical protocols to prepare supported metal nanoparticles. Kohsuke et al. reported a photoassisted deposition method that allows the formation of Pt and Pd nanoparticles on the photoexcited tetrahedrally coordinated titanium species within the framework.[97] Similarly, He et al. demonstrated that metallic Au nanoparticles could be supported on a TiO2 support by decomposition and photochemical deposition of a gold precursor (HAuCl4) employing a 125 W high-pressure mercury lamp.[98] Yu et al. recently prepared bimodal metallic Pt nanoparticles (1–3 nm) supported on titania nanotubes through photochemical deposition of hexachloroplatinic acid (H2PtCl6·6 H2O) as the metal precursor on the titania nanotube.[99] These materials were found to be highly active in the hydrogenation of CO2 to methane under mild reaction conditions (100 8C). In general, the photochemical deposition of metal nanoparticles minimises the use of chemicals and solvents and is therefore more environmentally friendly than many of the reported chemical routes. However, it is still not clear how well the size and distribution of metal nanoparticles on a solid support can be controlled. 3.2.4. Chemical Vapour Deposition Chemical vapour deposition is another promising route for the preparation of supported metal nanoparticles. It has been regarded as a powerful method to generate highly dispersed metal catalysts in a controlled and reproducible manner.[100] This procedure involves the vaporisation (sublimation) of metals and growth of the metal nanoparticles under high vacuum in the presence of an excess of stabilising organic solvents (e.g. aromatic hydrocarbons, alkenes and tetrahydrofuran) and/or reducing agents (e.g. H2).[26, 101, 102] The metal nanoparticles have a relatively narrow particle size distribution (2– 8 nm, Figure 18). Chemical vapour deposition is claimed to allow the preparation of metal nanoparticles on a wide range of organic and inorganic supports under very mild conditions (< 50 8C) to afford highly active heterogeneous catalysts,[102, 103] thereby avoiding the formation of large agglomerated nanoparticles from other protocols. Nevertheless, the method is often limited by the vapour pressure of the precursor and mass-transfer-limited kinetics.[73]  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2009, 2, 18 – 45 Supported Metal Nanoparticles for Catalysis methodologies offer a better control of the growth and distribution of nanoparticles. However, some of them involve the use of an excess of additional reductant (e.g. hydrazine, NaBH4) to ensure the complete reduction of the metal nanoparticles and must be removed prior to the use of the supported nanoparticles in the catalytic reaction. 3.3. Physicochemical Routes Figure 18. TEM images of untreated (left; scale bar: 50 nm) and functionalised carbon nanofibers (right, 2.04 wt % Pd; scale bar: 20 nm) with Pd metal nanoparticles through chemical vapour deposition.[103] Reproduced with permission from the Royal Society of Chemistry. A few examples of greener combined physicochemical routes have been reported, the most common ones being sonoelectrochemistry and flame spray pyrolysis. 3.3.1. Sonoelectrochemistry 3.2.5. Electrochemical Reduction The electrochemical deposition of metal nanoparticles, although not widely employed, has also been mainly reported for carbon-based supports.[104] The electrodeposition of Pt nanoparticles under potentiostatic conditions takes places from acidic aqueous solutions of H2PtCl6. The use of stabilisers (e.g. tetraalkylammonium salts) is needed to prevent the deposition of particles at the surface of the cathode.[90] 3.2.6. Other Chemical Methods Alternative chemical protocols have been reported to prepare supported metal nanoparticles. Sunagawa et al. prepared a range of supported metal nanoparticles (Au, Ru, Rh, Pd, Ir and Pt) using an ion-exchange/reduction approach that involves the adsorption of metal ions or complexes on the surface of the support which are subsequently reduced to the metallic state by heating the solution.[105] In this way, metal loadings up to 20 wt % were achieved, allowing a substantial control of the particle size (below 2 nm). Metallic Pd nanoparticles have also been prepared by confination/reduction of palladium acetate onto a urea cross-linked imidazolium-functionalised silica material.[106] These supported Pd metal nanoparticles were highly active, stable and reusable in Suzuki–Miyaura couplings (Figure 19) as compared to conventionally prepared silica-supported Pd metal nanoparticles, which suffered Pd leaching after one reaction cycle.[106] These This approach involves a combination of ultrasound waves and electrochemistry. Comprehensive reviews about the use of sonoelectrochemistry have been recently reported.[107, 108] Ultrasounds have beneficial effects on electrochemistry. They enhance mass transport, therefore altering the rate and sometimes the mechanism of the electrochemical reaction.[59] They also affect surface morphology through cavitation jets at the electrode–electrolyte interface, increasing the surface area. Furthermore, ultrasounds reduce the diffusion layer thickness and therefore ion depletion. Silver-coated TiO2 nanoparticles have been prepared in this way.[109] 3.3.2. Flame Spray Pyrolysis This alternative method of preparation was initially reported by Madler et al.,[110] and further reports on the technique appeared recently.[111] The liquid precursor mixture was fed in the centre of a methane/oxygen flame by a syringe pump and dispersed by oxygen, forming a fine spray (Figure 20). The spray flame was surrounded and ignited by a small flame ring issuing from an annular gap (0.15 mm spacing at a radius of 6 mm). Product particles were then collected on a glass fibre filter (Whatmann GF/D, diameter 25.7 cm) with the aid of a vacuum pump. The technique afforded highly stable supported metallic Pd nanoparticles with small particle size (< 5 nm) that were suitable for various applications.[110, 111] 3.3.3. Other Physicochemical Routes Figure 19. TEM images of Pd metal nanoparticles supported on functionalised silica: a) before (scale bar: 20 nm); b) after first reuse (scale bar: 100 nm); and c) after fifth reuse (scale bar: 20 nm) in the Suzuki reaction.[106] Reproduced with permission from the Royal Society of Chemistry. ChemSusChem 2009, 2, 18 – 45 Ishida et al. recently reported the deposition of metallic Au nanoparticles on porous coordination polymers using the socalled solid grinding approach.[102] This methodology entails the grinding of volatile organogold complexes (e.g. [Me2AuACHTUNGRE(acac)]) with porous coordination polymers in an agate mortar in air for 20 min at room temperature and subsequent reduction of the resulting material in a stream of 10 vol % H2 in N2 at 120 8C for 2 h. The so-prepared materials exhibited small and well-distributed metallic Au nanoparticles with an average size of 2 nm compared to larger and less evenly distributed supported Au metal nanoparticles prepared by chemical vapour deposition (Figure 21). The reported materials were also highly active in the aerobic oxidation of alcohols.[102]  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemsuschem.org 29 R. Luque et al. cles and catalysis was recently edited by Astruc.[112] Here, we initially divide them into two main groups: supported noblemetal nanoparticles (i.e. Au, Ag, Pt and Pd) and transitionmetal nanoparticles (i.e. Fe, Ni and Cu). This section, far from providing a comprehensive revision of all the reported catalytic applications of supported metal nanoparticles, gives an overview of the key applications of such materials in catalysis. Thus, each metal will be briefly introduced and some of the different reported methodologies/applications will be described. 4.1. Noble-Metal Nanoparticles 4.1.1. Gold Figure 20. Scanning transmission electron microscopy images of Pd metal nanoparticles on a La2O3/Al2O3 support: A) as-made material; B) annealed at 800 8C; C) after reaction (five cycles from 200 to 1000 8C); d) annealed at 1000 8C.[111] Reproduced with permission from the Royal Society of Chemistry. Gold nanoparticles have become increasingly popular in catalysis. Au was originally considered to be chemically inert and regarded as a poor catalyst. However, when gold is prepared as very small (less than 10 nm) and dispersed particles on a suitable support, it can be a highly active catalyst in a wide range of reactions including oxidations, hydrogenations and related reactions.[112] The most interesting methodologies and applications of Au nanoparticles has been recently reviewed by Daniel and Astruc.[113] Metallic Au nanoparticles have been reported to be well dispersed on different supports with nanoparticle sizes ranging from 1 to 10 nm.[40, 41, 113, 114] They have been widely employed in the oxidation of CO.[40, 41, 113] The optimum size of metallic Au nanoparticles is highly dependent on their potential applications. Haruta and co-workers prepared supported metallic Au nanoparticles by a precipitation-deposition method using a basic solution of the gold precursor (HAuCl4) which is gelated after adjusting the pH (6–10) to the corresponding deposition of the Au(OH)3 hydroxide on the support (Figure 17). The catalyst is then washed and dried to give the supported metallic Au nanoparticles after calcinations at 300– 400 8C.[89] Stable spherical metal nanoparticles are formed and their size can be controlled by the calcination temperature. Haruta and co-workers showed that the optimum particle size for the oxidation of CO is 3 nm. This size can be obtained by calcining the material at 297 8C.[5, 114] TiO2 is the dominant support for Figure 21. TEM images and the respective size distributions of Au metal nanoparticles supported on a porous coordination polymer ([Cu2ACHTUNGRE(pzdc)2ACHTUNGRE(bpy)]n ; pzdc = pyrazine-2,3-dicarboxylate; bpy = 4,4’-bipyridine) prepared by a, b) solid grinding (1 wt % Au) and c, d) chemical vapour deposition (0.5 wt % Au).[102] Reproduced with permission. 4. Catalytic Applications Herein, we aim to highlight some of the most commonly reported supported metal nanoparticles stemming from the number of publications and applications in catalysis from the last few decades. An elaborated monography about nanoparti- 30 www.chemsuschem.org Figure 22. Turnover frequencies per surface gold atom at 273 K for CO oxidation using a) Au/TiO2 (*); b) Au/Al2O3 (~), and c) Au/SiO2 (&) as a function of moisture concentration. The superior performance of Au/TiO2 in the CO oxidation is shown (arrow).[89] Reproduced with permission.  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2009, 2, 18 – 45 Supported Metal Nanoparticles for Catalysis metallic Au nanoparticles employed for the catalytic oxidation of CO,[5, 89, 114] regardless of the reaction conditions (Figure 22). Numerous supported metallic Au nanoparticles have been prepared and utilised in a wide range of catalytic applications, indicating the potential in utilising such supported metal nanoparticles in catalysis. Most of the work carried out with Au in the last few years has been carried out by the groups of Corma, Haruta and Hutchings. Some of these are briefly summarised in Table 1. 4.1.2. Palladium Palladium is probably the most versatile metal in promoting or catalysing reactions, particularly those involving C C bond formation (e.g. Suzuki, Heck and Sonogashira), many of which are not easily achieved with other transition-metal catalysts.[126] Thus, the preparation of supported metallic Pd nanoparticles on different supports has been extensively investigated. Several supported metallic Pd nanoparticles have been employed in Table 1. Applications of various supported Au metal nanoparticles in catalysis Application Support, method, nanoparticle size Ref. Hydrogenations of alkenes, alcohols and aldehydes Various supports Various methods NP size: variable [115] Oxidations alcohols Metal oxides (e.g. Fe2O3, Al2O3, TiO2), carbon Co-precipitation, impregnation/reduction NP size: 2–10 nm [116, 117] aldehydes Al2O3, SiO2, activated carbon Impregnation (from metal sols)/reduction NP size: 2–13 nm [116d] aromatic amines Polymers Impregnation NP size: 5–10 nm [118] alkenes Metal oxides (e.g. Al2O3, SiO2), activated carbon, polymers Various methods NP size: variable [117, 119, 120] nitroarenes TiO2, Fe2O3, activated carbon Deposition/precipitation, impregnation NP size: 3–5 nm [121] Oxidative decomposition of alkylamines and dioxins Fe2O3/La2O3 Deposition/precipitation NP size:1–10 nm [114] Direct epoxidation of propylene TiO2 (MCM-48) Deposition/precipitation NP size: 1–10 nm [114] C C coupling reactions Oxides (Ce, Ti, Zr, SiO2) Deposition/precipitation NP size: 10 nm [122] Hydroamination of terminal alkynes Chitosan, chitosan/SiO2 Impregnation NP size: 2–6 nm [53] Benzannulation of 2-(phenylethynyl)benzaldehyde and phenylacetylene CeO2, TiO2 and C Impregnation/reduction, precipitation NP size: 3–17 nm [123] Synthesis of methanol/dimethyl ether from syngas ZnO, ZnO/Al2O3 H-Y zeolite Impregnation [124] Hydrochlorination of ethyne Activated carbon Impregnation [117] Water gas shift reaction Mesoporous TiO2, CeO2 Deposition/precipitation, coprecipitation and gelation NP size: 2–5 nm [125] ChemSusChem 2009, 2, 18 – 45 Substrate  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemsuschem.org 31 R. Luque et al. C C bond reactions including the Suzuki, Heck, Sonogashira and related C C couplings (Scheme 1).[127] Figure 23. Top: TEM images of Pd metal nanoparticles supported on polyaniline (PANI) fibers: A) before reaction; C) halfway through reaction; D) after two reuses.[133] Reproduced with permission. Scheme 1. Selected C C coupling reactions catalysed by supported Pd metal nanoparticles. The Suzuki–Miyaura reaction involves the coupling of aryl halides (reactivity: iodides > bromides > chlorides) with aryl
boronic acids (Scheme 1). Metallic Pd nanoparticles supported
on alumina- and silica-based oxides,[35a, 37, 128] commercial magnetic nanoparticles,[129, 130] carbonaceous materials,[18, 131] siliceous mesocellular foams (MCF),[132] mesoporous biopolymers,[30] natural porous materials,[31] and other polymers including polyaniline nanofibers,[133] polysilane,[134] and related polymers[25e, 48] have been reported as highly active and reusable
catalysts in the coupling of various aryl bromides and chlorides
with aryl boronic acids. Budarin et al. prepared highly dispersed, active and reusable metallic Pd nanoparticles on biopolymers (Figure 8) that afforded quantitative conversion of
bromobenzene (starting material) into the cross-coupled product (biphenyl) within a few minutes of reaction
(Scheme 2).[18, 30]
triflates to alkenes (Scheme 1). Several examples of catalytically
active metallic Pd nanoparticles for Heck reactions supported
on similar supports to those reported for the Suzuki reaction
can also be found.[18, 30, 31, 33, 51, 101, 129, 130a, 131, 132, 135]
Budarin et al. recently reported the preparation of catalytically active Pd nanoparticles on silica and starch (Figure 8) for
the Heck reaction under microwave irradiation.[18, 30, 33] This protocol afforded very good conversions and selectivities to the
C C coupled products using iodobenzene and methyl acrylate
(Scheme 3 and Table 2) and styrene in a few minutes of reaction, in a similar way to those of related protocols of Cejka and
co-workers using Pd/MCM-41 materials (Scheme 4).[136]
Scheme 3. Heck coupling of iodobenzene and methyl acrylate using Pd
metal nanoparticles on expanded starch[18, 30] and silica.[33]
Scheme 4. Heck coupling of iodobenzene and styrene using Pd metal nanoparticles on expanded starch.[30]
Scheme 2. Suzuki coupling of bromobenzene and phenylboronic acid using
Pd metal nanoparticles on expanded starch.[18, 30]
Gallon et al. prepared supported metallic Pd nanoparticles
on polyaniline nanofibers as semi-heterogeneous catalysts for
C C couplings in water.[133] The highly dispersed low-loaded
metal nanoparticles were very effective in the Suzuki coupling
of aryl chlorides and phenylboronic acids (Figure 23) as well as
in the formation of phenols from aryl halides.[133] The catalysts
were also highly stable and reusable up to 10 times in the
Suzuki reaction.
The Heck reaction is another key C C bond-forming reaction. It predates the Suzuki methodology and is one of the
most useful derivations of palladium chemistry, giving access
to new extended alkenes through the addition of halides and
32
www.chemsuschem.org
Table 2. Heck reaction of iodobenzene and methyl acrylate using Pd/
starch supported catalysts (adapted from ref. [30]; reproduced with permission from the Royal Society of Chemistry).[a]
Entry
Conv. [mol %]
Select. [mol %][b]
0.5 % Pd/starch-1
2.5 % Pd/starch-2
5 % Pd/starch-3
50
> 90
> 99
> 99
> 90
85
0.5 % Pd/starch-4
2.5 % Pd/starch-5
5 % Pd/starch-6
70
> 95
> 95
> 99
> 95
85
[a] Reaction conditions 8 mmol iodobenzene, 8 mmol methyl acrylate,
5 mmol triethylamine, 0.1 g catalyst, microwave irradiation, 300 W, 90 8C,
5 min. [b] Selectivity for methyl acrylate.
 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2009, 2, 18 – 45
Supported Metal Nanoparticles for Catalysis
Another key C C bond-forming reaction is the Sonogashira
coupling of terminal alkynes (Scheme 1). It involves the alkynylation of aryl or alkenyl halides with alkynes to afford crosscoupled alkynes together with homocoupled dialkynes as byproducts. Chinchilla and Najera recently reported the use of
supported Pd nanoparticles in the Sonogashira reaction.[137]
Many other protocols for the synthesis of various organic com-
pounds of pharmaceutical interest can be found elsewhere.[18, 30, 51, 130, 134]
Supported Pd nanoparticles have also been used in a wide
range of other catalytic applications including hydrogenations,
oxidations, hydrodechlorinations and C H activation. The list
of applications is enormous. Some of them are summarised in
Table 3.
Table 3. Selected applications of supported Pd metal nanoparticles.
Application
Substrate
Support, method, nanoparticle size
Ref.
Hydrogenations
alkynes, cinnamaldehyde
Polyaniline
Deposition/reduction
NP size: 200–500 nm (13 nm agglomerates)
[138]
hydroxyaromatic derivatives
Hydrophilic carbon
Co-precipitation/reduction
NP size: 20–80 nm
[139]
nitroarenes
Carbon nanofibers
Impregnation/reduction
NP size: 3–7 nm
[140]
C H activation and C C coupling
Alumina nanoparticles
Impregnation, precipitation
[141]
Decomposition of H2O2
Polymer resin beads
Impregnation/reduction
NP size: 30–146 nm
[142]
Synthesis of H2O2
Metal oxides (e.g. Al2O3)
Impregnation, sol-gel method
[143, 144]
alcohols
Metal oxides, mesoporous materials (e.g. SBA-15)
Impregnation, microwaves
NP size: 1–20 nm
[66, 144–146]
glycerol
Metal oxides, mesoporous materials, polymers
All methods
NP size: 1–200 nm
[147]
CO/NO oxidation
–
[148]
benzene oxidation
SBA-15
Impregnation, grafting
NP size: 5–20 nm
[149]
alkenes
SBA-12
Microwave irradiation
NP size: 2 nm
[66]
Conversion of CHClF2
(gas-phase dismutation, hydrodehalogenation and pyrolysis)
Metal fluorides (e.g. CaF2)
Sol-gel synthesis/reduction
NP size: 3–8 nm
[150]
Hydrodechlorination
Polymers, carbons, Al2O3, TiO2, ZrO2
Various procedures
NP size: 2–50 nm
[44a, 151]
Hydrogen sorption
carbon nanotubes, activated carbon, yttrium oxides
Various procedures
NP size: variable
[152]
Synthesis of vitamin intermediates
Polymers
Impregnation/reduction
NP size: 1–2 nm
[153]
Oxidations
ChemSusChem 2009, 2, 18 – 45
 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
33
R. Luque et al.
4.1.3. Platinum
The majority of supported Pt nanoparticles have been employed in hydrogenations[70, 140, 154, 155] or in electrocatalytic oxidations for fuel cell applications.[156, 157] Recently, Campelo et al.
reported the preparation of metallic platinum nanoparticles
supported on acidic Al-MCM-48 for the hydroisomerisation of
n-alkanes.[46] Metallic Pt nanoparticles were found to be highly
dispersed on the support (Figure 9), and their activities were
comparable to those of silicoaluminophosphates and zeolites.[158, 159]
Metallic Pt nanoparticles have also been supported on magnetic nanoparticles through modification of Fe3O4 with ionic
liquid groups and subsequent ion-exchange of platinum salts
(e.g. K2PtCl4) with the linked groups and reduction of the Pt
ions with hydrazine (Figure 24) to give highly stable and active
Figure 24. Top: Preparation of Pt metal nanoparticles supported on magnetic nanoparticles (top scheme). Bottom: TEM images of Pt metal nanoparticles supported on ionic liquid modified magnetic nanoparticles (MNP-IL-C8Pt); scale bars: 50 nm (left) and 10 nm (right), respectively.[154d] Reproduced
with permission.
materials for the hydrogenation of alkynes and a,b-unsaturated aldehydes (Table 4).[154d]
Supported metallic Pt nanoparticles were also very active
and selective in the oxidation of CO,[160] alcohols,[161, 162] alkenes[163] and other compounds[164] including the oxidation of
vitamin precursors such as l-sorbose to 2-keto-l-gulonic
acid,[152] and the dehydrogenation of methylcyclohexane.[165]
Layered double hydroxide supported nanoplatinum catalysts
were also reported to be good catalysts for the allylation of aldehydes, giving moderate to good yields of homoallylic alcohols.[166]
4.1.4. Silver
Silver and silver-based compounds are highly antimicrobial by
virtue of their antiseptic properties to several bacterial strains
including Escherichia coli and Staphylococcus aureus. As a
result, most of the recent applications of metallic Ag nanoparticles have been related to biological/medical areas.[167] Nevertheless, supported metallic Ag nanoparticles have been reported for a variety of catalytic applications.
Supported metallic Ag nanoparticles have become increasingly important in the selective oxidation of alkanes and alkenes for the synthesis of industrially interesting products including epoxides and aldehydes.[66, 168] Metallic Ag nanoparticles supported on alumina[168c] and calcium carbonate[168b] are
very active and selective in the heterogeneous epoxidation of
ethylene. More recently, Chimentao et al. prepared highly
active and dispersed supported Ag metal nanoparticles on alumina and MgO for the selective gas-phase oxidation of styrene.[169] Loadings ranging from 11 to 40 % Ag were obtained
as well as different particle sizes and morphologies (from nanowires to nanopolyhedra). The activity and selectivity of the
supported metallic Ag nanoparticles was found to be very
strongly dependent on the morphology of the nanoparticles.
Mitsodume et al. recently reported the activity and reusability of low-loaded supported metal nanoparticles on hydrotalcites (prepared by conventional impregnation-reduction with
hydrogen) in the oxidant-free dehydrogenation of alcohols.[170]
Materials with less than 0.01 wt % metal and an average silver
particle diameter of 3.3 nm provided extremely good conversions and selectivities to the ketones with exceedingly higher
turnover numbers (TON = 600–22 000) than previously reported
values for the dehydrogenation of alcohols, and also were reusable four times with no decrease in reaction rates. Supported metallic Ag nanoparticles exhibited a superior performance
as well as improved selectivities to dehydrogenation in the reaction compared to its Ru and Pd analogues (Figure 25).[170]
Supported Ag metal nanoparticles have also been employed
in hydrogenations of dyes including methylene blue, eosin,
Table 4. Hydrogenation of various alkynes and a,b-unsaturated aldehydes[a] using platinum metal nanoparticles supported on functionalised magnetite
nanoparticles.[154d] Reproduced with permission.
Entry
Substrate
t [h]
Products (yield [%])
1
2
3
4
5
6
7
diphenylacetylene
1-ethynyl-4-methylbenzene
2-ethynyl-6-methoxynaphtalene
3-phenylprop-2-yn-1-ol
methyl 3-phenylpropiolate
cinnamaldehyde
2-methyl-3-phenylacrylaldehyde
16
4.5
4.5
4.5
4.5
12
12
cis-stilbene (95), trans-stilbene (5)
1-methyl-4-vinylbenzene (88), 1-ethyl-4-methylbenzene (12)
2-methoxy-6-vinylnaphtalene (67), 2-ethyl-6-methoxynaphtalene (33)
(Z)-methylprop-2-en-1-ol (78), (E)-methylprop-2-en-1-ol (11), 3-phenylpropanol (11)
(Z)-methyl 3-phenylacrylate (67), (E)-methyl 3-phenylacrylate (15), methyl 3-phenylpropanoate (18)
3-phenylprop-2-en-1-ol (99)
2-methyl-3-phenylprop-2-en-1-ol (90)
[a] Reaction conditions: 0.5 mmol substrate, 5 mL methanol, 90 8C, 200 psi H2, 22 mg catalyst.
34
www.chemsuschem.org
 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2009, 2, 18 – 45
Supported Metal Nanoparticles for Catalysis
Figure 25. Oxidation of cinnamyl alcohol using supported metal nanoparticles on hydrotalcites (HT).[170] Reproduced with permission.
rose Bengal[171] and Rhodamine 6G[172] as well as in the hydrogenation of acrolein.[173]
4.1.5. Rhodium
Catalytically active supported metallic Rh nanoparticles have
mostly been employed in hydrogenations including the reduction of 1-alkenes,[174] arenes,[45b, 175] and ketones[175] (Figure 26),
CO hydrogenation,[176] and in the reduction of various a,b-unsaturated compounds.[177, 178]
Savastenko et al. reported the lean NOx reduction using supported metallic Rh nanoparticles on SiO2 in the 100–400 8C
temperature range and employing a representative test gas
Figure 26. TEM images and particle size distribution of Rh metal nanoparticles on aluminum oxyhydroxide nanofibers, used in the hydrogenation of
arenes at room temperature with a hydrogen balloon.[175] Reproduced with
permission.
ChemSusChem 2009, 2, 18 – 45
mixture of oxygen-rich diesel engine exhaust gas.[70] The results
demonstrated that under cyclic lean/rich operating conditions,
a laser synthesized PtRh/SiO2 bimetallic catalyst showed the
highest activity for NOx reduction in the low temperature
range 100–300 8C. At 150 8C, the activity of this catalyst was
found to be around three times higher than that of a monometallic Rh/SiO2 reference catalyst prepared by wet impregnation. The low-temperature NOx reduction activity and the selectivity to N2 of the conventional Rh catalyst could also be enhanced by additional laser deposition of Pt nanoparticles.[70]
Bao and co-workers prepared catalytically active Rh metal
nanoparticles confined inside carbon nanotubes for the conversion of CO and H2 into ethanol.[179] The overall formation
rate of ethanol (30 mol molRh 1 h 1) was found, for the first
time, to be an order of magnitude superior for metallic Rh
nanoparticles supported inside the carbon nanotubes than for
metallic Rh nanoparticles supported on the surface of the
nanotubes, despite the latter supported metal nanoparticles
being more accessible.
Alumina-supported metallic Rh nanoparticles (particle size
4.4 nm) have been investigated in the ring opening of cyclohexane giving n-hexane, n-pentane and benzene as major
products with minor quantities of methylcyclopentane and
light alkenes (C1–C4).[180] Rh/TiO2 materials with extremely low
Rh loadings (0.004 % Rh) were also found to be active in the
partial oxidation of propylene, giving yields of about 13 % at
275 8C.[36]
4.1.6. Ruthenium
Supported metallic Ru nanoparticles are highly active and selective in various catalytic processes. Alumina- or silica-supported ruthenium selectively reduces nitrogen oxide to nitrogen.[181] The hydrogenation of aromatic compounds including
tetralin,[182] methyl benzoate,[182] 2-methoxycarbonylphenyl-1,3dioxane[183] and CO[184] using Ru/HY and Ru/Al2O3 catalysts has
also been reported. Miyazaki et al. prepared uniformly supported metallic Ru nanoparticles on g-Al2O3 with an average diameter of 5 nm and a maximum loading of 6.3 wt % for ammonia
synthesis.[185] The materials were prepared through reduction
of a ruthenium colloid using ethylene glycol and subsequent
deposition of the Ru metal nanoparticles on alumina. The rate
of ammonia formation with these supported Ru nanoparticles
was found to be 10 times higher than the rate obtained with
non-promoted Ru/Al2O3 catalysts prepared by conventional impregnation methods.
Li et al. successfully prepared metallic Ru nanoparticles on
SBA-15 using a simple ultrasound-assisted polyol method,
where the ultrasounds were claimed to provide both the
energy for the reduction of the RuIII ions (by ethylene glycol)
and the driving force for the loading of the Ru0 nanoparticles
into the pores of the SBA-15.[60] A loading of 14 % Ru ensured
highly active and selective materials (> 60 % conversion,
> 80 % selectivity to CO) in the partial oxidation of methane by
oxygen to give a mixture of CO and H2 (syngas).
More recently, Pan et al. reported the use of Ru multiwalled
carbon nanotubes for the hydrogenation of glucose to sorbi-
 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
35
R. Luque et al.
tol.[186] The nanotubes were impregnated with a solution of
RuCl3 in ethanol for 24 h and then treated at 300 8C for 2 h
under He flow. Prior to the hydrogenation reaction, the catalyst
was reduced under H2 flow at 400 8C for 2 h. The supported Ru
catalyst showed higher activity in the hydrogenation of glucose than Raney Ni and metallic ruthenium nanoparticles supported on Al2O3 and SiO2.
The activity of Ru/g-Al2O3 materials was also investigated in
the Fischer–Tropsch synthesis at 170 8C and 4 bar (space velocity 7.2 mL min 1 g, H2/CO 1:2 molar ratio).[38] The obtained CO
conversion under the reaction conditions was less than 1 %,
and the turnover frequency (TOF) of the catalyst was found to
be strongly dependent on the nanoparticle size. The optimum
activity was found for the catalysts with an average particle
size of 10 nm.[38] Che and co-workes reported Ru nanoparticles
supported on hydroxyapatite as an efficient and reusable catalyst for cis-dihydroxylation and oxidative cleavage of alkenes.[187] The catalyst showed excellent activities and selectivities in these oxidations.
4.2. Transition-Metal Nanoparticles
A wide range of supported transition-metal nanoparticles have
been reported.[112] The most common metal nanoparticles reported to date include Fe, Ni and Cu. These supported metal
nanoparticles have been extensively employed in industrial
processes including hydrogenations, reforming and Fischer–
Tropsch synthesis.[112, 188]
4.2.1. Iron
Iron is a well-known example of supported transition-metal
nanoparticles. However, most of the reported protocols deal
with the preparation of iron oxide nanoparticles as the complete reduction of Fe2 + /Fe3 + to metallic iron is highly challenging as compared to related transition/noble metals owing to
its high electropositive standard reduction potential (Fe2 + /Fe =
0.44 V; Fe3+/Fe = 0.037 V). Thus, only a few reports can be
found on the preparation, characterisation and activity of supported metallic Fe nanoparticles. Fe on activated carbon and
on Al2O3 materials are highly dispersed, with different particle
size distributions (from 3 to 16 nm) depending on the method
of preparation employed.[38] The materials were found to be
differently active in the Fischer–Tropsch process, with the
smaller Fe particles (< 9 nm) being remarkably less active than larger crystallites. Of note was also the influence of the support in the stability and activity of the supported Fe nanoparticles, with activated carbon being a better stabiliser and promoter of the Fe nanoparticle activity compared to alumina.[38] Gonzalez-Arellano et al. recently reported a facile and environmentally friendly methodology to prepare supported metallic Fe nanoparticles on a range of supports including MCM-41, silica, starch and cellulose (Figure 27).[189] Fe/MCM-41 materials were found to be extremely active and selective in the oxidation of a variety of alcohols under microwave irradiation using hydrogen peroxide as green oxidant, with turnover numbers between 50 and 300 (Table 5). The oxidation was carried out 36 www.chemsuschem.org Figure 27. TEM images of Fe metal nanoparticles supported on A) MCM-41; B) DARCO; C) starch and D) cellulose. The bottom plot shows the TON value for the supported Fe nanoparticle after the oxidation of benzyl alcohol.[189] Reproduced with permission. under mild reaction conditions (200 W, 70–90 8C, 1 h), and the Fe/MCM-41 catalyst was recyclable, preserving its activity and structure after three reuses (Figure 27, bottom). These significant results were a clear improvement on reported protocols for the oxidation of alcohols,[14, 190] and provided exceedingly higher TON values as well as an important reduction in reaction times (from 8–24 h to a maximum of 1 h). There are other interesting applications of supported Fe metal nanoparticles, including environmental remediation[191] . Metallic Fe nanoparticles on anionic hydrophilic carbon (Fe/C) and polyacrylic acid (Fe/PAA), with a particle size ranging between 20–100 nm, were employed in the remediation of soil and groundwater through the dehalogenation of chlorinated hydrocarbons,[192] and the remediation of metal ions such as CrIV and AsV.[191, 193] Both trichloroethylene and metal ions (CrVI and AsV) were effectively and rapidly reduced under the reaction conditions. 4.2.2. Nickel Nickel metal nanoparticles have unusual properties and exhibit excellent catalytic activities.[194] Carbon- and silica-supported Ni metal nanoparticles have been widely investigated in the gasphase hydrogenation of aromatic compounds.[195] Although  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2009, 2, 18 – 45 Supported Metal Nanoparticles for Catalysis Table 5. Efficient and selective oxidation of alcohols using Fe/MCM-41[a] as catalyst.[189] TON[b] Conv. ACHTUNGRE[mol %] 1 170 31 > 90
2
294
60
> 95
3
162
38
> 99
4
290
54
> 99
5
46
< 10 > 99
Entry
Substrate
Product
Select.
ACHTUNGRE[mol %][c]
fectively dissociate hydrogen
molecules in the gas phase to
provide atomic hydrogen that
can be bonded to the carbonaceous surface.[201] Hydrogen desorption spectra showed that
around 3 wt % hydrogen was released in the range of 67–247 8C.
The hydrogen chemisorption
process facilitated by Ni metal
nanoparticles was suggested as
an effective reversible hydrogenstorage method.[201]
4.2.3. Copper
[a] 0.2 g substrate, 4 mmol H2O2 (0.4 mL, 30 wt % in water), 2 mL acetonitrile, 0.050 g catalyst, microwave irradiation, 200 W, 70–90 8C, 1 h. [b] Number of moles of product produced per mole of catalyst. [c] Selectivity based
on alcohol conversion.
the porous silica investigated had a low surface area
(15 m2 g 1), the supported Ni metal nanoparticles were found
to be relatively small (< 10 nm) and well dispersed, providing interesting activities that were dependent on the thermal pretreatment step in such a way that the presence of NiO particles has some influence in the hydrogenation process. Ni and Ni-Cu metal nanoparticles supported on inorganic materials (e.g. sepiolite, AlPO4) were also investigated in the liquid-phase hydrogenation of fatty acid ethyl esters[196] and propargyl alcohols.[197] Ni/ZrO2 and Ni/Al2O3 materials have been investigated in the steam reforming of methane and combined steam and CO2 reforming of methane.[198] The two supported Ni catalysts (Ni/ ZrO2-CP and Ni/Al2O3-C) exhibited fairly stable catalysis under low gas hourly space velocities (GHSVs) of CH4, but they are easily deactivated under high CH4 GHSVs. Results of the combined steam and CO2 reforming reaction of methane pointed out that the Ni/ZrO2-AN catalyst may be a promising catalyst for the production of syngas with flexible H2/CO ratios (H2/ CO = 1.0–3.0) to meet the requirements of various downstream chemical syntheses. Supported nanosized nickel on carbon nanotubes can also catalyse the thermal decomposition of ammonium perchlorate, which is the most common oxidizer in composite solid propellants.[199] The addition of the supported Ni nanoparticles decreased the high decomposition temperature of ammonium perchlorate from 447 8C to 346 8C, increasing the total heat release (differential thermal analysis) by 1.21 kJ g 1. Further burning-rate experiments revealed that the addition of Ni nanoparticle/carbon nanotube as catalyst increased the burning rate as well as lowered the pressure exponent of ammonium perchlorate based solid-state propellants.[200] Hydrogen storage is another interesting application of Ni metal nanoparticles supported on carbonaceous materials including (multiwalled) carbon nanotubes and mesoporous carbons. Kim et al. recently prepared 6 wt % Ni on multiwalled carbon nanotubes that can efChemSusChem 2009, 2, 18 – 45 Supported Cu metal nanoparticles have been prepared on a range of supports and shown activity in various catalytic processes. Although some preliminary investigations on the preparation of supported Cu metal nanoparticles appeared in the late 1970s and 1980s,[202] most of the reported protocols have been published only recently. Driessen and Grassien prepared Cu metal nanoparticles on silica and found that their activities in reactions of methyl fragments from methyl iodide dissociation on the catalyst surface were highly dependent on the oxidation state of the copper, the hydroxy group coverage on the silica support and the surface roughness of the Cu particles.[203] They obtained methane, ethane (and ethylene only on samples with a low content of hydroxy groups) as the main reaction products on a reduced Cu/SiO2 surface containing only Cu metal nanoparticles. Cu nanoclusters (typically 2–4 nm size, 0.8 nm height) supported on Al2O3 films were employed in the reduction of NO.[204] Also, Cu metal nanoparticles on metal oxides have been reported as highly active and selective catalysts in the selective dehydrogenation of methanol. The materials, prepared by reduction under hydrothermal synthesis conditions (promoted by the surfactant cetyltrimethylammonium bromide) without the use of additional reductant, were found to selectively yield formaldehyde and H2 with almost 100 % selectivity.[205] Aziridination and cyclopropanation reactions are other interesting catalytic applications of Cu metal nanoparticles.[206] Cu-Al2O3 materials were found to be active and selective in the aziridination and cyclopropanation of a range of alkenes including styrene, cyclohexene and related aromatics, and 1hexene in typically 3–4 h, with the cyclopropanation providing a 30:70 cis/trans ratio. Similarly, the same metal nanoparticles were employed in the synthesis of 1,2,3-triazoles, affording the products in moderate to high yields after 3–8 h reaction.[207] The catalysts were easily recovered by centrifugation and reused several times without a significant loss in activity. More recently, supported Cu metal nanoparticles were reported to be active in the water gas shift reaction (Scheme 5). The reaction involves water splitting after reacting with carbon monoxide to give carbon dioxide and hydrogen. It is part of  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemsuschem.org 37 R. Luque et al. Scheme 5. Water gas shift reaction catalysed by supported Cu metal nanoparticles.[208] steam reforming of hydrocarbons and has been considered an interesting research avenue to produce hydrogen, although it is limited by the high temperatures used in the process. Cu/ molybdena materials are five to eight times more active than CuACHTUNGRE(100), with activities that are comparable to those of Cu/ CeO2 (111) and superior to those of Cu/ZnO (0001) surfaces.[208] 5. Future Prospects and Outlook 5.1. Sustainable Preparation The preparation of supported metal nanoparticles for a wide range of catalytic applications has been well developed over the last few years. Thus section aims to provide an overview of the future prospects of supported metal nanoparticles in view of their methods of preparation and applications in catalysis. As it has been highlighted through this Review, the sustainable preparation of supported metal nanoparticles (e.g. the use of less toxic precursors in benign solvents,[30, 66, 189] environmentally friendly supports,[30, 31, 51, 53, 54] less energy-intensive protocols[30, 66]) has been the subject of many recent reports and reviews.[3, 7, 57, 112] Bioreducing agents including sugars, glutathione, starch and a range of plant and algal extracts have also been employed in the preparation of Au, Ag and Ni metal nanoparticles with reasonably good control over particle size and, in some cases, shape.[57] Recently, there has been a significant growing interest in the development of bio-inspired approaches to achieve designer hybrid nanomaterials.[3, 57, 209] For example, Bale et al. reported the protein-directed preparation of supported Ag metal nanoparticles on carbon nanotubes.[210] A range of proteins were investigated, including poly-l-lysine, bovine serum albumin, soybean peroxidase and a1-acid glycoprotein. An aqueous dispersion of multiwalled carbon nanotubes and protein/polypeptide were stirred with a solution of silver nitrate for 24 h, and the conjugates were reduced using sodium borohydride Figure 28. TEM images of Ag metal nanoparticle formation on a) multiwalled carbon nanotubes-poly-l-lysine; b) multiwalled carbon nanotubes-bovine serum albumin; c) multiwalled carbon nanotubes-a1-acid glycoprotein, and d) pristine multiwalled carbon nanotubes.[210] Reproduced with permission. (Scheme 6). The dispersion of the Ag metal nanoparticles on the support was dependent on the protein employed for the synthesis (Figure 28). Table 6. Top twelve sugar-derived platform molecules.[213] Platform molecule Structure 1,4-diacids (succinic, fumaric and malic acids) 2,5-furandicarboxylic acid 3-hydroxypropionic acid aspartic acid glucaric acid glutamic acid itaconic acid levulinic acid 3-hydroxybutyrolactone glycerol Scheme 6. Schematic depiction of the protein-mediated formation of Ag/ multiwalled carbon nanotube materials: a) generation of multiwalled carbon nanotube-protein conjugates by incubating the nanotubes with an aqueous solution of protein; b) formation of Ag metal nanoparticles by exposing the nanotube-protein conjugates to a solution of silver nitrate, washing and subsequent reducing with sodium borohydride.[210] Reproduced with permission. 38 www.chemsuschem.org sorbitol xylitol/arabinitol  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2009, 2, 18 – 45 Supported Metal Nanoparticles for Catalysis 5.2. Sustainable and Future Catalytic Applications Some of the most interesting catalytic trends and future prospects for supported metal nanoparticles include applications in fuel cells, the transformation of platform molecules, environmental remediation and more recently NMR imaging. Pd/Pt nanoparticles supported on carbonaceous materials have been reported as promising materials for hydrogen sorption, to be employed as fuel cells in cars.[151] However, there is still quite a way to go until we see the full implementation of such systems. Recent studies from Shao-Horn et al. demonstrated that the stabilities of supported Pt metal nanoparticles in lowtemperature fuel cells were compromised by the degradation of Scheme 7. Plan for the development of glycerol-based products from specialty to commodity chemicals. the electrochemically active surface area in the material through loss of Pt from fuel cell electrodes and coarsening of Pt partiSupported metal nanoparticles have also been recently emcles.[156] Manipulation of the surface structure[211] and chemistry ployed in a truly sustainable application: environmental remediation. Fe nanoparticles supported on chitosan and silica,[54] (through deposition of Au clusters on Pt nanoparticles)[212] of the supported metal nanoparticles can improve the durability. and Fe and Ni-cellulose acetate materials[218] were employed in Transformations of platform molecules (e.g. 1,4-diacids, glycthe removal of chlorinated compounds in water. Chen et al. erol) is another interesting research avenue for supported devised their oxide-supported Au metal nanoparticles metal nanoparticles. Platform molecules are generally com(Figure 14) to work, for the first time, in the light-driven roompounds with various functionalities that can be turned into a temperature oxidation of volatile organic compounds such as plethora of chemicals and value-added products through difformaldehyde.[81] Upon irradiation with six light tubes of blue ferent catalytic transformations including oxidations, hydrogelight (wavelength 400–500 nm, 0.17 W cm 2 irradiation energy), [213] nations, amidations and esterifications. the initial CH2O concentration decreased by 64 % in 2 h and a The US Department of Energy has generated a list of the so-called 12 top sugar-deparallel increase in CO2 concentration was observed, confirming the oxidation of CH2O (Figure 29). The activity of the materived main platform molecules (or building blocks) that should rials was found to be highly dependent on the support, with receive attention in the next few years (Table 6). Glycerol and ZrO2 being the most effective support regardless of the type succinic acid are two examples of such platform molecules. of light employed in the reaction (Figure 30). This effect is asGlycerol is a well-known compound that has recently attractsociated with the oxygen adsorbed on the more active suped a great deal of attention as it is produced in large quantiports (Zr and Ce oxides) that can migrate to the gold nanoparties as by-product in the preparation of biodiesel. The glycerol ticles, thus accelerating the oxidation.[81] surplus has challenged both industrial and organic chemists to come up with potential routes to obtain high-value-added Supported Pt and Pd nanoparticles on Al2O3 were recently products from glycerol (Scheme 7).[147] The oxidation and hyreported to provide para-hydrogen induced polarisation (PHIP) signals in heterogeneous catalysed gas-phase hydrogenations drogenolysis of glycerol have been extensively investigated (Figure 31).[219] PHIP can improve the NMR signals of reaction with a wide range of Pt, Pd and Au metal nanoparticles sup[18b, 116a, 147, 214] ported on carbonaceous materials. intermediates and products by several orders of magnitude, providing a high sensitivity which is ideal to follow reaction Succinic acid (1,4-dibutanoic acid) is another important platmechanisms.[220] Furthermore, the combination of para-hydroform molecule. Clark and co-workers recently reported the preparation of high-value-added products from succinic acid gen with supported metal nanoparticles in hydrogenations will through different transformations,[18a, 215, 216] and further investiaid the development of new research tools for fundamental and practical applications including kinetic and mechanistic gations are ongoing using supported noble-metal nanopartistudies and the production of polarized fluids for advanced cles (Pt, Pd, Rh, Ru) in the hydrogenation of succinic acid.[217] ChemSusChem 2009, 2, 18 – 45  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemsuschem.org 39 R. Luque et al. Figure 29. Oxidation of 100 ppm CH2O on Au metal nanoparticles supported on ZrO2 under blue light or sunlight at 25 8C. a) Changes in reactant concentration (CH2O, four upper lines) and product (CO2, four lower lines) under different light intensities as a function of irradiation time. b) Relationship between CH2O conversion and light intensity.[81] Reproduced with permission. Figure 30. Influence of the support on the activity of the HCHO oxidation reaction. Black bars: CH2O conversion (%) under illumination of blue light. Striped bars: conversion under red light.[81] Reproduced with permission. magnetic resonance imaging (MRI).[221] MRI techniques suffer from a low intrinsic sensitivity, and many efforts have been devoted to the use of homogeneous hydrogenations with parahydrogen to produce hyperpolarised contrast agents for biomedical and related applications.[221–223] In this context, the use of active and reusable heterogeneous catalysts will produce catalyst-free polarised fluids (after separation of the catalyst from the reaction products) that can be used to prepare polarised gases. MRI has also been employed to study the morphology of catalysts and synthesis techniques.[224, 225] 5.3. Implications of Supported Metal Nanoparticles Despite the development of more sustainable routes to their preparation as well as their high activities and selectivities in catalysis, the toxicity issue associated with engineered metal 40 www.chemsuschem.org Figure 31. Left: 1H NMR spectra recorded in PASADENA[220] experiments during the in situ hydrogenation of propylene using Pt/Al2O3 catalysts with a) 1.1 nm Pt metal nanoparticles and b) 0.6 nm Pt metal nanoparticles. The two hydrogen atoms in the product from the para-hydrogen molecule are labeled A and B, and the residual NMR signals of the reactant (propylene) are labeled 1–3. Right: ALTADENA[220] experiments during propylene hydrogenation in the Earth’s magnetic field using Pt/Al2O3 catalysts with a) 8.5, b) 3.5, c) 1.1 and d) 0.6 nm Pt particle size. The two hydrogen atoms in the product from the para-hydrogen molecule are labeled A and B. A broad band in the low-field part of the spectrum corresponds to H2.[219] Reproduced with permission. nanoparticles also needs to be addressed. It is not clear whether the highly active and dispersed supported metal nanoparticles desired for catalytic applications are hazardous in contact with human tissues (specially as a result of prolonged exposure) as a consequence of their unique physicochemical properties or the chemical nature of the metal nanoparticles and/or support surfaces.[226] Results from unsupported metal nanoparticles point to the absence of cytotoxicity and cellular oxidative stress, although there is in vitro uptake as well as retention in cells and tissues of metal nanoparticles (Table 7). Thus, the approach of covering potentially toxic surface groups with more innocuous ones to improve the biocompatibility of nanomaterials has recently been explored.[3, 57, 227, 228] 6. Conclusions The 21st century has brought a great interest and expansion of the nanomaterials field. The timeliness of being “green and nano” for nanomaterials synthesis has become evident from the latest developments and publications in the field. Among them, supported metal nanoparticles are important owing to their unique properties and various methods of preparation. Recent advances in the design and preparation of supported metal nanoparticles confirmed that a numerous variety of metal nanoparticles can nowadays be synthesized through different preparation routes and supports to give tailored sizes, shapes and distributions, overcoming the main drawbacks of  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2009, 2, 18 – 45 Supported Metal Nanoparticles for Catalysis Table 7. Summary of acellular, in vitro, and in vivo findings with Pt nanoparticles.[226] Reproduced with permission. Property Technique Result NP singlet size BET surface area Agglomerate size in cell culture medium Cell-free oxidant capacity Cytotoxicity Cellular oxidative stress or inflammatory mediator release in vitro uptake in vivo acute inflammation Retention of Pt nanoparticles in lung tissue Retention of Pt nanoparticles in lavage cells TEM N2 isotherm adsorption Dynamic light scattering 2’,7’-dichlorofluorescin oxidation Release of lactate dehydrogenase Luciferase reporter activity; release of IL-6 TEM Intratracheal instillation Atomic emission spectroscopy Atomic emission spectroscopy 11–35 nm 1–27 m2 g 1 92 (flowers) and 67 nm (multipods) low no no flowers > multipods
low; flowers and multipods
flowers > multipods
flowers > multipods
traditional synthetic methodologies. Such designer materials
will have a significant impact in many areas including increasing applications in industrial catalytic processes.
However, we must not forget that the preparation of supported metal nanoparticles should be promoted in a more sustainable way, reducing waste generation and the use of toxic
compounds with room-temperature aqueous solution protocols, improving manufacturing safety as well as decreasing the
production costs. In addition to that, the implementation of
such catalysts may not come without a price and the environmental impact associated with the preparation of supported
metal nanoparticles must be assessed from many different
points of view (transport, waste, biomagnification in the food
chain) as well as their potential toxicity when released in ecosystems and humans.
Acknowledgements
This research was subsidised by grants from the Direccin General de Investigacin (project CTQ2007-65754/PPQ), the Ministerio
de Ciencia y Tecnologa, FEDER funds, and from the Consejera de
Innovacin, Ciencia y Empresa (Junta de Andaluca, projects
FQM-191 and P07-FQM-2695).
Keywords: heterogeneous catalysis · nanostructures
supported catalysts · sustainable chemistry
·
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