Nanomaterials in Catalysis E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 1025

Veröffentlichungsjahr: 2012

Beschreibung

Nanocatalysis has emerged as a field at the interface between homogeneous and heterogeneous catalysis and offers unique solutions to the demanding requirements for catalyst improvement. Heterogeneous catalysis represents one of the oldest commercial applications of nanoscience and nanoparticles of metals, semiconductors, oxides, and other compounds have been widely used for important chemical reactions. The main focus of this fi eld is the development of well-defined catalysts, which may include both metal nanoparticles and a nanomaterial as the support. These nanocatalysts should display the benefits of both homogenous and heterogeneous catalysts, such as high efficiency and selectivity, stability and easy recovery/recycling. The concept of nanocatalysis is outlined in this book and, in particular, it provides a comprehensive overview of the science of colloidal nanoparticles. A broad range of topics, from the fundamentals to applications in catalysis, are covered, without excluding micelles, nanoparticles in ionic liquids, dendrimers, nanotubes, and nanooxides, as well as modeling, and the characterization of nanocatalysts, making it an indispensable reference for both researchers at universities and professionals in industry.

Leseprobe

Contents

Cover

Related Titles

Title Page

Copyright

Foreword

Preface

References

List of Contributors

Chapter 1: Concepts in Nanocatalysis

1.1 Introduction

1.2 The Impact of the Intrinsic Properties of Nanomaterials on Catalysis

1.3 How can Nanocatalyst Properties be Tailored?

1.4 Nanocatalysis: Applications in Chemical Industry

1.5 Conclusions and Perspectives

References

Chapter 2: Metallic Nanoparticles in Neat Water for Catalytic Applications

2.1 Introduction

2.2 Synthesis of Nanoparticles in Water: The State of The Art

2.3 Water-Soluble Protective Agents and their use in Nanocatalysis

2.4 Conclusion and Perspectives

References

Chapter 3: Catalysis by Dendrimer-Stabilized and Dendrimer-Encapsulated Late-Transition-Metal Nanoparticles

3.1 Introduction

3.2 Synthesis

3.3 Homogeneous Catalysis with DENs Generated from PAMAM and PPI Dendrimers

3.4 Highly Efficient ‘click'-Dendrimer-Encapsulated and Stabilized Pd Nanoparticle Pre-Catalysts

3.5 Heterogeneous Catalysis

3.6 Electrocatalysis

3.7 Conclusion and Outlook

Acknowledgments

References

Chapter 4: Nanostructured Metal Particles for Catalysts and Energy-Related Materials

4.1 General Survey

4.2 Nanostructured Clusters and Colloids as Catalyst Precursors

4.3 Nanostructured Materials in Energy-Related Processes

4.4 Characterization of Nanostructured Metallic Catalyst Precursors and their Interaction with Coatings and Supports Using X-ray Absorption Spectroscopy

Acknowledgments

References

Chapter 5: Metallic Nanoparticles in Ionic Liquids – Applications in Catalysis

5.1 Introduction

5.2 Interactions between Ionic Liquids and Metallic Nanoparticles

5.3 Catalytic Applications

5.4 Conclusions

Acknowledgments

References

Chapter 6: Supported Ionic Liquid Thin Film Technology

6.1 Introduction

6.2 Nanoparticle Catalysis with Supported Ionic Liquids

6.3 Benefits for Synthesis and Processes

6.4 Conclusion

References

Chapter 7: Nanostructured Materials Synthesis in Supercritical Fluids for Catalysis Applications

7.1 Introduction: Properties of Supercritical Fluids

7.2 Synthesis of Nanopowders as Nanocatalysts in SCFs

7.3 Synthesis of Supported Nanoparticles as Nanocatalysts in SCFs

7.4 Supercritical Microfluidic Synthesis of Nanocrystals

7.5 Conclusion

References

Chapter 8: Recovery of Metallic Nanoparticles

8.1 Introduction

8.2 Immobilization on a Solid Support

8.3 Multiple Phases

8.4 Precipitation and Redispersion

8.5 Magnetic Separation

8.6 Filtration

8.7 Conclusions

References

Chapter 9: Carbon Nanotubes and Related Carbonaceous Structures

9.1 Introduction

9.2 Carbon Nanotubes as Nanosupport

9.3 Purification and Functionalization

9.4 Preparation of CNT-Supported Catalysts

9.5 Applications of CNT-Supported Catalysts

9.6 Other Related Carbonaceous Materials

9.7 Summary

References

Chapter 10: Nano-oxides

10.1 Introduction

10.2 Synthesis and Characterization of Nano-oxides

10.3 Catalytic Applications of Nano-oxides

10.4 Conclusions and Perspectives

References

Chapter 11: Confinement Effects in Nanosupports

11.1 Introduction

11.2 Confinement Effects in Carbon Nanotubes

11.3 Metal Catalyst-Free Chemical Reactions inside Carbon Nanotubes

11.4 Catalytic Reactions over Metal Particles Confined Inside Carbon Nanotubes

11.5 Summary

Acknowledgment

References

Chapter 12: In Silico Nanocatalysis with Transition Metal Particles: Where Are We Now?

12.1 Introduction

12.2 Surface Chemistry and Chemistry on Facets of Nanoparticles: Is it the Same ?

12.3 Electronic and Geometric Factors that Determine the Reactivity of Metal Surfaces

12.4 Theoretical Studies of Multistep Pathways

12.5 Conclusion

Acknowledgments

References

Index

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Foreword

Catalysis has been the leading science and technology in the 60s, associated with the raise of the use of oil. Refineries and petrochemistry have then known their golden age. The rarefaction of energy sources and the need for a more rational use of the available energy provides a new opportunity for catalysis to play a leading role in society. If catalysis is traditionally divided into three main fields: heterogeneous, homogeneous and enzymatic, the present tendency is the convergence of these fields towards a molecular approach.

Thus, heterogeneous catalysis that usually is carried out at the solid-gas interface and is perhaps the technologically most utilized form as it provides high turnovers to yield products and low deactivation, which provides their long life. Homogeneous catalysis is usually carried out on transition metal ions surrounded by ligands and dissolved in organic solvents. These catalysts show very high selectivity and they play a major role in the pharmaceutical industry. Enzyme catalysts, of which there are about 3000 in the human body, usually operating in water solution under physiological conditions at room temperature and neutral pH. Catalysis is life! It accounts for most of the processes on this planet, whether operating under planetary conditions of near room temperature or making products with high selectivity by the chemical technologies. Revolution in the synthesis of nanomaterials that provides the opportunity to produce catalysts with controlled size and shape has led to the discovery that the catalytic selectivity and turnover correlates with the size and shape of nanoparticles. This observation, along with characterization techniques, allow us to carry out molecular or atomic scale studies of catalyst particles under reaction conditions and provides revolutionary developments in the field of catalysis science, which is named “nanocatalysis”. The result is a rapidly increasing interest for the molecular aspect of catalysis aiming at a new understanding of how catalysts work on molecular and atomic scales. The challenge for the future will be to develop technologies that are very (totally?) selective while little energy demanding and respecting the environment. This process in our contemporary world is commonly called “green chemistry” and can rely on nanocatalysis.

This book entitled Nanomaterials in Catalysis by K. Phillippot and P. Serp is a very timely exposure of the new science and technologies of nanocatalysis. Following an excellent introduction of the concepts in nanocatalysis, the fabrication of nanocatalysts in various media is discussed in several important papers ranging from nanoparticles in aqueous phase, nanoclusters and colloids as catalysts precursors, nanoparticles in ionic liquid and supercritical fluids, dendrimers that serve as excellent polymeric supports for nanoparticles and finally nanocatalysts recovery, which addresses the problem of deactivation and regeneration. A section is dedicated to nanoparticle supports like carbon nanotubes and nano oxides which are discussed. The last chapter reviews modeling of nanocatalysts to show the foundation of theoretical treatment of nanocatalysis and nanomaterials that are used as catalysts.

This is a timely book and it will be a very useful addition to those interested in the field of catalysis and its most important extension by the use of nanomaterials to carry out heterogeneous, homogeneous and enzyme catalysis. Studies of nanocatalysts and characterization of these nanomaterial systems lead to an atomic and molecular level understanding of how catalyst materials work. The book describes the frontiers of catalysis on a broad front, and we believe it adds to our knowledge and perhaps rapid evolution of the field of catalysis for the near future.

Berkeley and ToulouseOctober 2012

G. A. SomorjaiB. Chaudret

Preface

Process catalysts, for petroleum products, chemicals, pharmaceuticals, synthetic rubber and plastics, among others, represent a $13 billion-per-year business worldwide, and as such play a vital role in the economy. Thus, the importance of catalysis in the chemical industry is reflected by the following points:

– >95% of all products (volume) and >70% of all products (processes) are synthesized by means of catalysis,

– >80% of the added value in chemical industry is based on catalysis,

– ~20% of the world economy depends directly or indirectly on catalysis.

Catalysts can be classified into two main groups: heterogeneous catalysts (solid-state catalysts) and homogeneous catalysts (transition metal complexes). Of increasing importance are the biocatalysts, enzymes that are protein molecules of colloidal size, which can be classified somewhere between molecular homogeneous catalysts and macroscopic heterogeneous catalysts. There are also intermediate forms, such as homogeneous catalysts attached to solids, also known as immobilized catalysts.

The suitability of these catalysts for an industrial process is governed mainly by the following properties:

– recovery (heterogeneous catalysts homogeneous catalysts),

– activity (heterogeneous catalysts < homogeneous catalysts),

– selectivity (heterogeneous catalysts homogeneous catalysts),

– stability (heterogeneous catalysts > homogeneous catalysts).

By far, and mainly because of the recovery aspect, the most applied catalysts at an industrial level are the heterogeneous ones. Indeed, approximately 80% of all catalytic processes involve heterogeneous catalysts against 15% homogeneous catalysts and 5% biocatalysts. The homogeneous systems are currently limited to reactions for which i) heterogeneous catalysts are not stable with a dissolution of the active phase (methanol carbonylation, hydroformylation), ii) the catalyst can be lost (polymerization), or iii) heterogeneous catalysts are inefficient (asymmetric catalysis).

In the past century, catalysis became the basis of large-scale processes in bulk chemistry and petrochemistry. In the XXI century, shifting demands, energy and new environmental challenges require new catalytic solutions. The two major issues are related to the depletion of raw materials and to the environment/health.

On one hand the depletion of raw materials drives researches:

– on catalysis for energy aiming at: i) improvements in the catalytic processes across the chemical and petroleum industries that will increase resource and energy utilization efficiencies; ii) cost-effective, environmentally sound utilization of energy resources such as coal and biomass; iii) efficient photocatalysts for water splitting and electrocatalysts for fuel cells, and iv) hydrogen storage and carbon dioxide capture and sequestration could benefit from new abilities to catalyze solid-state reactions.

– on new catalytic systems which: i) show high selectivity (atom economy), ii) are able to transform biomass in various chemicals, and iii) are based on cheap and abundant metals.

In parallel, the growing concern over environmental issues and the successful implementation of legislation drives researches in catalysis on two ways to improve the quality of our environment by:

– cleaning up emissions from current processes (catalytic converters, VOC, ozone, pesticides, dioxin..., indoor air quality, water pollution, organic particulate), and

– developing alternative cleaner catalytic processes (safer reagents, limited by-products, reduced mass and energy inputs...).

Although incremental improvements to catalytic processes will result in a better control of desired chemical transformations, in order to fully realize the needed advancements real breakthroughs must also be achieved:

in photocatalysis to successfully and economically use solar energy to convert water into oxygen and hydrogen;in interfacial electrocatalysis to realize, at a realistically low cost, the efficiency potentially attainable by fuel cells;in the gasification and conversion of coal/biomass into transportation fuels;in the catalytic conversion of biomass to more useful products;in the development of catalysts that operate in aqueous media to fully enable the promise of “green chemistry”;in the construction of multi-component (nanoscale) catalysts.

In such a challenging context, it is obvious that any new branches of catalysis science should be considered as a potential spur to reach the objectives. A promising approach consists in bridging the gap between homogeneous and heterogeneous catalysis, in order to combine the advantages of each ones. The first efforts date from the 70's with the first International Symposium on Relations between Homogeneous and Heterogeneous Catalysis in 1973. In the middle of the 90's, catalysis joined the nanosciences and nanotechnologies wave, which significantly contributed to reinforce the connections between molecular and solid state catalysis communities.

At that time, we both started our scientific carrier, during a period in which the prefix nano was poorly used (Figure 1). One of us (Karine) as a PhD student and after as a postdoc at Rhodia worked on the homogeneous rhodium catalyzed hydroamination reaction and the synthesis of carbonates with tin-supported catalysts, respectively, before integrating the Centre National de la Recherche Scientifique where she develops ligand-functionalized colloidal nanoparticles for application in catalysis. The other (Philippe) as a PhD and then a post-doc student, investigated the potential of chemical vapor deposition methods to prepare supported catalysts or carbon nanomaterials, and is now the team leader of a group that develops new catalytic systems for both homogeneous and supported catalysis. We are both very excited by the catalysis with nanomaterials, due to our double scientific background that allows to conciliate the molecular and solid state point of view, and to work on the bridge in between homogeneous and heterogeneous catalysis.

Figure 1 Number of articles including the prefix nano- in their title (from Web of Knowledge).

Thus, the terms “nanocatalyst” and “nanocatalysis” have appeared some years ago as a continuation of the development of nanotechnologies. Today, few books are dedicated to the subject, [1–4] and a recent article [5] raises this interesting question: “Nanocatalysis: Mature science revisited or something really new?” In the present book, the concepts of nanocatalysis are defined, to give to the reader a comprehensive overview of what is a nanocatalyst, and to rationalize the advantages of nanocatalysts related to their activity, selectivity and stability. Thus, each chapter will provide a critical overview of a specific domain of nanocatalysis through the most relevant examples of the literature.

In that sense, this book is the first one that introduces concepts and main achievements, and covers the main aspects of nanocatalysis in general, considering both the active phase, and the support as well as their modeling and characterization.

Since this is a multi-authored book, significant differences in style from chapter to chapter are inevitable, but we have tried to avoid overlaps as much as possible. We thank all the authors for their efforts to meet deadlines, and to follow the format defined for the book. We would also like to acknowledge the assistance of Anne Brennfueher and Lesley Belfit at Wiley-VCH, whose advice has been most helpful at the various stages of preparation of the manuscript. Finally, we hope that the book will be useful to fellow scientists and practitioners and will stimulate further research and discussion on the development of nanomaterials for catalysis.

ToulouseOctober 2012

Philippe SerpKarine Philippot

References

1. Scott, S.L., Crudden, C.M., Jones, C.W. (Eds.) (2003) Kluwer Academics/Plenum Publishers, Nanostructured catalysts, New-York.

2. Zhou, B., Hermans, S., Somorjai, G.A. (Eds.) (2004) Kluwer Academics/Plenum Publishers, Nanotechnology in catalysis, New-York.

3. Heiz, U., Landman, U. (Eds.) (2008) Springer-Verlag, Berlin, Nanocatalysis, Heidelberg, 2007.

4. Astruc, D. (Ed.) (2008) Wiley-VCH Verlag GmbH & Co KGaA, Nanoparticles and catalysis, Weinheim.

5. Schlögl, R., Abd Hamid S.B. (2004) Nanocatalysis: Mature Science Revisited or Something Really New? Angew. Chem. Int. Ed.43 (13), 1628–1637.

List of Contributors

Didier Astruc

Université Bordeaux 1

ISM, UMR CNRS 5255

351 Cours de la Libération

33405 Talence Cedex

France

Cyril Aymonier

ICMCB-CNRS-Université

de Bordeaux

87 avenue du docteur Albert

Schweitzer

33608 Pessac Cedex

France

Xinhe Bao

State Key Laboratory of Catalysis

Dalian Institute of Chemical Physics

Zhongshan Road 457

Dalian 116023

China

Helmut Bönnemann

Max-Planck-Institut für

Kohlenforschung

Kaiser-Wilhelm-Platz 1

45470 Mülheim an der Ruhr

Germany

Dirk E. De Vos

K. U. Leuven

Dept. M2S - Faculteit

Bio-ingenieurswetenschappen

Postbus 2461, Kasteelpark

Arenberg 23

3001 Heverlee

Belgium

Audrey Denicourt-Nowicki

Ecole Nationale Supérieure

de Chimie de Rennes

CNRS, UMR 6226

Avenue du Général Leclerc,

CS 50837

35708 Rennes Cedex 7

France

Abdou Diallo

Université Bordeaux 1

ISM, UMR CNRS 5255

351 Cours de la Libération

33405 Talence Cedex

France

Emil Dumitriu

Technical University of Iasi

Faculty of Chemical Engineering and Environment Protection

71 D. Mangeron Ave

700050 Iasi

Romania

Isabelle Favier

Université Paul Sabatier, Laboratoire

Hétérochimie Fondamentale et

Appliquée, UMR CNRS 5069

118 route de Narbonne

31062 Toulouse Cedex 9

France

and

CNRS, LHFA UMR 5069

31062 Toulouse Cedex 9

France

Iann C. Gerber

Université de Toulouse

Laboratoire de Physique et Chimie

des Nano-Objets (LPCNO-IRSAMC, INSA, UPS, CNRS-UMR 5215)

Équipe Modélisation Physique et Chimique

135 avenue de Rangueil

31077 Toulouse Cedex

France

Inge Geukens

K. U. Leuven

Dept. M2S - Faculteit

Bio-ingenieurswetenschappen

Postbus 2461, Kasteelpark

Arenberg 23

3001 Heverlee

Belgium

Montserrat Gómez

Université Paul Sabatier, Laboratoire

Hétérochimie Fondamentale et

Appliquée, UMR CNRS 5069

118 route de Narbonne

31062 Toulouse Cedex 9

France

and

CNRS, LHFA UMR 5069

31062 Toulouse Cedex 9

France

Marco Haumann

FAU Erlangen-Nuremberg

Chemical Reaction Engineering

Egerlandstr. 3

91058 Erlangen

Germany

Josef Hormes

University of Saskatchewan

Canadian Light Source, Inc.

101 Perimeter Road

Saskatoon, SK S7N 0X4

Canada

Timna-Joshua Kühn

University of Saskatchewan

Canadian Light Source, Inc.

101 Perimeter Road

Saskatoon, SK S7N 0X4

Canada

Vasile Hulea

Ecole Nationale Supérieure

de Chimie de Montpellier

Institut Charles Gerhard

Equipe MACS, UMR 5253

8, rue de l'Ecole Normale

34296 Montpellier Cedex 5

France

Guram Khelashvili

Strem Chemicals, Inc.

7 Mulliken Way

Newburyport, MA 01950-4098

USA

David Madec

Université Paul Sabatier, Laboratoire

Hétérochimie Fondamentale et

Appliquée, UMR CNRS 5069

118 route de Narbonne

31062 Toulouse Cedex 9

France

and

CNRS, LHFA UMR 5069

31062 Toulouse Cedex 9

France

Samuel Marre

ICMCB-CNRS-Université

de Bordeaux

87 avenue du docteur Albert

Schweitzer

33608 Pessac Cedex

France

Catia Ornelas

Université Bordeaux 1

ISM, UMR CNRS 5255

351 Cours de la Libération

33405 Talence Cedex

France

Xiulian Pan

State Key Laboratory of Catalysis

Dalian Institute of Chemical Physics

Zhongshan Road 457

Dalian 116023

China

Karine Philippot

Laboratoire de Chimie de

Coordination du CNRS

205 route de Narbonne

BP44099

31077 Toulouse Cedex 4

France

Romuald Poteau

Université de Toulouse

Laboratoire de Physique et Chimie

des Nano-Objets (LPCNO-UMR5215, IRSAMC)

Équipe Modélisation Physique et Chimique

135 avenue de Rangueil

31077 Toulouse Cedex

France

Wolf-Jürgen Richter

Max-Planck-Institut für

Kohlenforschung

Kaiser-Wilhelm-Platz 1

45470 Mülheim an der Ruhr

Germany

Alain Roucoux

Ecole Nationale Supérieure

de Chimie de Rennes

CNRS, UMR 6226

Avenue du Général Leclerc,

CS 50837

35708 Rennes Cedex 7

France

Judith Scholz

FAU Erlangen-Nuremberg

Chemical Reaction Engineering

Egerlandstr. 3

91058 Erlangen

Germany

Philippe Serp

Laboratoire de Chimie de

Coordination - UPR8241 CNRS

composante ENSIACET

4 allée Emile Monso

Toulouse University

31030 Toulouse Cedex 4

France

Dang Sheng Su

Chinese Academy of Science

Institute of Metal Research

Shenyang National Laboratory for Materials Science

72 Wenhua Road

Shenyang 110016

China

and

Fritz Haber Institute of the Max Planck Society

Department of Inorganic Chemistry

Faradayweg 4–6

14195 Berlin

Germany

Chapter 1

Concepts in Nanocatalysis

Karine Philippot and Philippe Serp

1.1 Introduction

Catalysis occupies an important place in chemistry, where it develops in three directions, which still present very few overlaps: heterogeneous, homogeneous and enzymatic. Thus, homogeneous and heterogeneous catalysis are well-known as being two different domains defended by two scientific communities (molecular chemistry and solid state), although both are looking for the same objective, the discovery of better catalytic performance. This difference between homogeneous and heterogeneous catalysis is mainly due to the materials used as catalysts (molecular complexes in solution versus solid particles, often grafted onto a support), as well as to the catalytic reaction conditions applied (for example liquid-phase reactions versus gas-phase ones). Considering the advantages of these two catalytic approaches, on the one hand heterogeneous catalysts are easy to recover but present some drawbacks, such as the drastic conditions they require to be efficient and the mass transport problems; on the other hand, homogeneous catalysts are known for their higher activity and selectivity, but the separation of expensive transition metal catalysts from substrates and products remains a key issue for industrial applications [1]. The first attempts to bridge the gap between these two communities date from the 1970s to the early 1980s. From one side chemists working in the molecular field, such as J.M. Basset, M. Che, B.C. Gates, Y. Iwasawa and R. Ugo, among others, initiated pioneering works on surface molecular chemistry to develop single-site catalysts, and/or reach a better understanding of conventional supported catalyst preparation through a molecular approach; from the other side, chemists of the solid state, such as G. Ertl and G. Somorjai, were interested in the molecular understanding of surface chemical catalytic processes. For the latter, the revolutionary development of surface science at the molecular level was possible thanks to the development of techniques of preparation of clean single crystal surfaces and characterization of structure and chemical composition under ultrahigh vacuum [(X-ray photoelectron spectroscopy (XPS), atomic emission spectroscopy (AES), low energy electron diffraction (LEED) etc]. Once again, although these scientists aimed at a common objective, little interaction or cross-fertilization action has appeared during the last 20 years. One should however cite the first International Symposium on Relations between Homogeneous and Heterogeneous Catalysis, organized on Prof. Delmon's initiative in Brussels (Belgium) in 1973. Interestingly, this event appeared 17 years after the first International Congress on Catalysis (Philadelphia, 1956) and 5 years before the first International Symposium on Homogeneous Catalysis (Corpus Christi, 1978). In parallel, although colloidal metals of Group 8 were among the first catalysts employed in the hydrogenation of organic compounds, the advent of high pressure hydrogenation and the development of supported and skeletal catalysts meant that colloidal catalysis has hardly been explored for many years [2–4].

Since the end of the 1990s, and with the development of nanosciences, nanocatalysis has clearly emerged as a domain at the interface between homogeneous and heterogeneous catalysis, which offer unique solutions to answer the demanding conditions for catalyst improvement [5, 6]. The main focus is to develop well-defined catalysts, which may include both metal nanoparticles and a nanomaterial as support. These nanocatalysts should be able to display the ensuing benefits of both homogenous and heterogeneous catalysts, namely high efficiency and selectivity, stability and easy recovery/recycling. Specific reactivity can be anticipated due to the nanodimension that can afford specific properties which cannot be achieved with regular, non-nano materials (Figure 1.1).

Figure 1.1 Nanoarchitecture: an avenue to superior precision. Axes are: A: composition of functional sites; B: ordering level of sites; C: functional properties of material.

In this approach, the environmental problems are also considered. Definitions can be given: the term ‘colloids’ is generally used for nanoparticles (NPs) in liquid-phase catalysis, giving rise to ‘colloidal catalysis,’ while ‘nanoparticle’ is more often attributed to NPs in the solid state, thus related to the heterogeneous catalysis domain. The terms ‘nanostructured’ or ‘nanoscale’ materials (and by extension ‘nanomaterials’) are any solid that has a nanometer dimension. Despite these differences in nomenclature, NPs are always implicated and ‘nanocatalysts’ or ‘nanocatalysis’ summarize well all the different cases.

In the nanoscale regime, neither quantum chemistry nor the classical laws of physics hold. In materials where strong chemical bonding is present, delocalization of electrons can be extensive, and the extent of delocalization can vary with the size of the system. This effect, coupled with structural changes, can lead to different chemical and physical properties, depending on size. As for other properties, surface reactivity of nanoscale particles is thus highly size-dependent. Of particular importance for chemistry, surface energies and surface morphologies are also size-dependent, and this can translate to enhanced intrinsic surface reactivity. Added to this are large surface areas for nanocrystalline powders and this can also affect their chemistry in substantial ways [7]. Size reduction to the nanometer scale thus leads to particular intrinsic properties (quantum size effect) for the materials that render them very promising candidates for various applications, including catalysis. Such interest is well established in heterogeneous catalysis, but colloids are currently experiencing renewed interest to get well-defined nanocatalysts to increase selectivity.

Much work in the field has focused on the elucidation of the effects of nanoparticle size on catalytic behavior. As early as 1966, Boudart asked fundamental questions about the underlying relationship between particle size and catalysis, such as how catalyst activity is affected by size in the regime between atoms and bulk, whether some minimum bulk-like lattice is required for normal catalytic behavior, and whether an intermediate ideal size exists for which catalytic activity is maximized [8]. Somorjai's group has studied this issue extensively. Although there is tremendous variation in the relationships between size and activity depending on the choice of catalyst and choice of reaction, these relationships are often broken into three primary groups: positive size-sensitivity reactions, negative size-sensitivity reactions, and size-insensitive reactions. There is also a fourth category composed of reactions for which a local minima or maxima in activity exists at a particular NP size (see Figure 1.2) [9, 10]. Positive size-sensitivity reactions are those for which turnover frequency increases with decreasing particle size. The prototypical reaction demonstrating positive size-sensitivity is methane activation. Dissociative bond cleavage via σ-bond activation as the rate-limiting step is a common feature in reactions with positive size-sensitivity. Negative size-sensitivity reactions are those for which turnover frequency decreases with decreasing particle size. In this case, formation or dissociation of a π-bond is often the rate-limiting step. The prototypical reactions for this group are dissociation of CO and N2 molecules, which each require step-edge sites and contact with multiple atoms. These sites do not always exist on very small NPs, in which step-edges approximate adatom sites. These reactions also sometimes fall into the fourth category of those with a local maximum in turnover frequency versus particle size because certain particle sizes geometrically favor the formation of these sorts of sites. The third type of reaction is the size-insensitive reaction, for which there is no significant dependence of turnover frequency on nanoparticle diameter. The prototypical size-insensitive reaction is hydrocarbon hydrogenation on transition metal catalysts, for which the rate-limiting step is complementary associative σ-bond formation. Although these effects are often referred to as structure-sensitivity effects, they are referred to as size-sensitivity effects here in order to further distinguish them from another type of structure-sensitivity, which is derived from differences in crystal face and which is discussed below.

Figure 1.2 Major classes of size-sensitivity, which describe the relationships between NP size and turnover frequency for a given combination of reaction and NP catalyst. (------) negative size-sensitivity; (- - - -) positive size-sensitivity; (·······) Mix of negative and positive sensitivity.

Aside from considerations of NP size, a second major area of inquiry is that of the effect of nanoparticle shape on reaction rate, selectivity, and deactivation. This work is derived from the abundance of research done on single crystal surfaces, which has demonstrated what is known as structure sensitivity in catalysis. Experiments on a wide variety of catalysts have determined that the atomic arrangement of atoms on a surface has a significant effect on catalyst behavior. As demonstrated in Figures 1.3 and 1.4, the type of crystal face dramatically affects the coordination, number of nearest neighbors, and both two- and three-dimensional geometry of the catalytically active surface atoms. The availability of particular types of adsorption sites can have a large effect on catalysis, as it is common for adsorbates to differ in their affinity for each type of adsorption site. Consequently, the presence or absence of a particular type of site can affect not only reaction rates, but also selectivity. However, not all reactions are structure sensitive and some reactions are known to be structure sensitive only within a range of specific conditions. In the case of nanoparticle catalysts, structure-sensitivity is manifested in terms of NP shape. When little attention is given to shape, most NPs adopt roughly spherical shapes, often referred to as polyhedra or octahedra, in order to minimize surface energy.

Figure 1.3 Two of the most common fcc crystal faces, (111) (left) and (100) (right).

Figure 1.4 Four of the most common adsorption sites found on single crystal terraces.

These NPs predominately feature (111)-oriented surface atoms, which is the lowest energy crystal face. Under certain conditions, however, nanoparticle catalysts can be synthesized such that the shape, and consequently the surface atom orientation, is kinetically trapped into a nonequilibrium shape, such as a cube, triangle, platelet, or rod [11]. Nanoparticles of different shape have been shown to have different activity and selectivity [12, 13], as well as stability [14] in catalytic reactions. Shape-controlled NPs play an important role as model catalysts in furthering the large-scale effort to bridge the ‘materials gap’ between the real-world systems and scientific understanding in catalysis.

Since nanocatalysts are made of nanoparticles or/and nanomaterials, as a metal or metal oxide active phase or as a support or a combination of both, nanoparticles and nanomaterials have been the object of an ever increasing interest during recent decades. The common goal is the development of well-defined nanoparticles/nanomaterials displaying well-controlled properties to get efficient and selective nanocatalysts for numerous relevant catalytic reactions (as examples arene hydrogenation, carbon–carbon coupling, CO oxidation...).

1.2 The Impact of the Intrinsic Properties of Nanomaterials on Catalysis

Involvement of interatomic interaction causes the performance of a solid, or a cluster of atoms, to vary from that of an isolated atom. Adjustment of the relative number of the under-coordinated surface atoms provides an additional freedom that allows one to tune the properties of a nanosolid with respect to that of its bulk counterpart. Hence, contribution from the under-coordinated atoms and the involvement of interatomic interaction can be the starting point of consideration to bridge the gap between an isolated atom and a bulk solid in chemical and physical performances. The impact of atomic coordination reduction (deviation of bond order, length, and angle) is tremendous. It unifies the performance of a surface, a nanosolid, and a solid in amorphous state consistently in terms of bond relaxation and its consequences on bond energy [15, 16]. The unusual behavior of a surface and a nanosolid has been consistently understood and systematically formulated as functions of atomic coordination reduction and its derivatives (size dependence) on the atomic trapping potential, crystal binding intensity, and electron–phonon coupling. If one could establish the functional dependence of a detectable quantity, Q. on atomic separation or its derivatives, the size dependency of the quantity Q is then certain. One can hence design a nanomaterial with desired functions based on such prediction. The physical quantities of a solid can be normally categorized as follows:

Quantities that are directly related to bond length, such as the mean lattice constant, atomic density, and binding energy. Lattice contraction in a nanosolid induces densification and surface relaxation.Quantities that depend on the cohesive energy per discrete atom, such as self-organization growth; thermal stability; Coulomb blockade; critical temperature for phase transitions, and evaporation in a nanosolid; and the activation energy for atomic dislocation, diffusion, and chemical reactions.Properties that vary with the binding energy density in the relaxed continuum region such as the Hamiltonian that determine the entire band structure and related properties such as band gap, core level energy, photoabsorption, and photoemission.Properties from the joint effect of the binding energy density and atomic cohesive energy such as the mechanical strength Young's modulus, surface energy, surface stress, extensibility and compressibility of a nanosolid, as well as the magnetic performance of a ferromagnetic nanosolid.

Structural miniaturization has indeed given a new freedom that allows us to tune the physical properties that are initially nonvariable for the bulk chunks by simply changing the shape and size to make use of the effect of atomic coordination reduction.

The intrinsic properties of nanomaterials and their size dependency will induce, directly or not, several effects on catalysis (Figure 1.5), that will be discussed in detail throughout this book. Some relevant examples are given below, which will be developed in more details in the following chapters.

Figure 1.5 The impact of the intrinsic properties of nanomaterials on catalysis.

1.2.1 Metallic Nanoparticles

About two-thirds of chemical elements are metals. Using the molecular orbital description, as is usual for covalently bonded atoms in molecules, the generation of a metallic material can simply be understood as the formation of an infinitely extended molecular orbital, leading to energy bands. The development of a metallic band structure requires a minimum number of electronic levels, which have to be very similar in energy so that electrons can move by only thermal activation. All the properties that we know for a bulk metal derive from the existence of such a band. The most important property of a metal is its ability to transport electrons, namely the property of conductivity. To understand what conductivity is based on, it is necessary to consider the relation between occupied and unoccupied electronic bands, as electrons can become mobile only if the energy band of which they are part is not fully occupied. Most of the d-type transition metals are characterized by only partially filled d-orbitals so that incompletely filled bands result in any case. d10 elements such as palladium, platinum or gold have nearby s-bands that can be used for electron transport. Another important property of metals, at least for some of them, is magnetism, for example the well-known ferromagnetism of iron, cobalt and nickel. The existence of unpaired electrons is a condition for magnetism; however, only the uniform orientation of free spins over a large area results in ferromagnetism while non-oriented free spins produce paramagnetic materials. Copper and gold are the only colored metals, the others looking ‘silvery’ when they have smooth surfaces. Finely dispersed metals are all dark brown or black. The silvery luster and the dark appearance are caused by the total reflection of light in the first case, and by the total absorption of light in the latter. Color is caused by the partial absorption of light by electrons in matter, resulting in the visibility of the complementary part of the light. On smooth metal surfaces, light is totally reflected by the high density of electrons and no color results; instead a mirror-like effect is observed [17].

The description of bulk materials is made by means of the laws of classical physics. A metal particle will present properties different from those of metal bulks, because of the reduction in the size (quantum size effect). Indeed, if a size range is attained where the band structure begins to disappear and discrete energy levels become dominant, quantum mechanical rules, which are well-established for describing electronic situations in molecules and atoms have to replace those of classical physics suitable for bulk materials (Figure 1.6). Nevertheless, small particles are parts of a material and not atoms nor molecules, and are thus considered as intermediate species.

Figure 1.6 Electron levels of atom, cluster, nanoparticle and bulk metal.

Metallic NPs, also called nanoclusters, are pieces of metal at the nanometer scale, of one to a few nanometers in size. They can be noncrystalline, aggregates of crystallites or single crystallites (nanocrystals). Due to the number of bound metal atoms they contain, metallic nanoparticles display intermediate electronic energy levels in comparison with molecules and metal bulks [18]. As a result, particular physical and chemical properties are expected for metallic nanoparticles that can lead to applications in various areas such as in catalysis [19]. In this latter domain, metallic nanoparticles are generally considered as intermediate species between metal complexes and metal surfaces, and the term ‘nanocatalysts’ is now commonly used to describe them.

In heterogeneous catalysis, the use of metallic nanoparticles is well established, mainly based on their high reactivity. One of their properties is their high number of surface atoms that increases with decreasing particle size (Table 1.1), these surface atoms being the active sites for catalysis. In addition, surface atoms which are at the edges or in the corners are more active than those in planes, and their number also increases with decreasing particle size. Since the number of surface atoms present in NPs will govern their catalytic reactivity, control of the size of NPs is thus of high importance. For industrial processes, the particles are generated on supports such as alumina, silica or charcoal by impregnation from solutions of the corresponding metal salts followed by a reduction procedure giving rise to zero-valent nanoparticles. Since the larger particles are less active than the smaller ones, only a portion of the metal particles contribute to the catalytic process [17]. To overcome this problem, other ways of synthesis have been explored like solution procedures, but in that case, the addition of a stabilizing agent (polymer, surfactant, ligand...) is necessary to keep the NPs stable. Besides their protecting role, and although they occupy some active sites at the surface of nanoparticles, stabilizers can tune their reactivity by influencing their morphology or/and their surface chemistry. When the nanoparticles are deposited onto a support, the organic ligands can be eliminated from the nanomaterial by washings with appropriate solvents or by calcination under air at high temperature or under plasma conditions, to obtain naked NPs.

Table 1.1 Number of surface atoms in relation with the total number of atoms in full shell clusters.

A modern approach of colloid chemistry is presently being developed to increase the reactivity of NPs in a limited size up to 10 nm, using several types of stabilizer as well as several types of support for their heterogenization. It appears that a fundamental understanding of the surface properties of such metallic nanoparticles is needed to get more efficient and selective nanocatalysts in the future.

1.2.2 Metal Oxide Nanoparticles

The metal elements can form a large diversity of oxide compounds, which can adopt structural geometry with an electronic structure that can exhibit metallic, semiconductor, or insulator character [20]. Most of the catalysts used in industrial applications involve an oxide as active phase, promoter, or support. At the nanoscale, these compounds can exhibit unique physical and chemical properties due to their limited size and a high density of defect sites such as edges, corners and point defects [21]. As for other materials, the process of size reduction is expected to dictate structural, transport and chemical properties, which themselves will influence the final catalytic performance. In this section we will concentrate on pure oxide nanoparticles, but one should keep in mind the important place of mixed nano-oxides, such as nanozeolites [22] or others [23–26] in catalysis.

Bulk oxides are usually robust and stable systems with well-defined crystallographic structures. However, the growing importance of surface free energy and stress with decreasing particle size must be considered, since changes in thermodynamic stability associated with size can induce modification of cell parameters and/or structural transformations [27], and in extreme cases, the NP can disappear because of interactions with its surrounding environment and of its high surface free energy [28]. To display mechanical or structural stability, a NP must have a low surface free energy. As a consequence of this requirement, phases that have a low stability in bulk materials can become very stable in nanostructures. This structural phenomenon has been detected in TiO2, VOx, A12O3, or MoOx [29–33]. Thus, in the case of alumina the stable structure for micro-sized samples is the α phase while γ appears more stable for nanostructured materials [14]. Size-induced structural distortions have been observed in NPs of A12O3 [28], Fe2O3 [34], ZrO2 [35] or CeO2 [36].

The NP size is also related to the transport properties of the oxide, since, as already stated, the nanostructure produces the so-called quantum size or confinement effects, which essentially arise from the presence of discrete, molecular-like electronic states. Additional general electronic effects of quantum confinement experimentally probed on oxides are related to the energy shift of exciton levels and optical bandgap [37]. Oxide materials can present ionic or mixed ionic/electronic conductivity and it is experimentally well established that both can be influenced by the nanostructure of the solid. The number of electronic charge carriers in a metal oxide is a function of the band gap energy according to the Boltzmann statistics. The electronic conduction is referred to as n- or p-hopping-type depending on whether the principal charge carrier are, respectively, electrons or holes. In an analogous manner to hopping-type conduction, ionic conduction takes place when ions can hop from site to site within a crystal lattice as a result of thermal activation. As a result of the nanoscale derived effects, it is well known that CeO2 exhibits an improved n-type conductivity, which may be four orders of magnitude greater than that corresponding to bulk/micro-crystalline ceria, and is ascribed to a significant enhancement of the electronic contribution [38]. The strong size-dependence observed for the electrical conductance in the context of gas-sensing devices has been recently reviewed [39–41]. Some of the most dramatic effects of the nanostructure on ionic transport in oxides are observed in the field of Li-ion batteries. An outstanding enhancement of Li-ion vacancy conductivity has been achieved using Li-infiltrated nanoporous Al2O3 [42].

Structural and electronic properties obviously drive the physical and chemical properties of the solid, and this last group of properties is influenced by size in a simple classification. In their bulk state, many oxides have wide band gaps and a low reactivity [43]. A decrease in the average size of an oxide particle does in fact change the magnitude of the band gap (Figure 1.7), with strong influence on the conductivity and (photo)chemical reactivity [41, 44–46]. Surface properties of oxides nanomaterials are of central importance in catalysis. Solid–gas or solid–liquid chemical reactions can be mostly confined to the surface and/or subsurface regions of the solid. As mentioned, the two-dimensional (2D) nature of surfaces has notable structural consequences, typically a rearrangement or reconstruction of bulk geometries, and electronic consequences, such as the presence of mid-gap states, which may act as trapping centers in photocatalysis, whose behavior depend on the relative position of their energy with respect to the valence and conduction band edge position [46]. In the case of nanostructured oxides, surface properties are strongly modified with respect to 2D-infinite surfaces, producing solids with unprecedented sorption [47, 48] and acid/base characteristics [49], or metal– support interaction/epitaxy [50, 51]. Finally, the presence of under-coordinated atoms or O vacancies in an oxide NP should produce specific geometrical arrangements as well as occupied electronic states located above the valence band of the corresponding bulk material, enhancing in this way the chemical reactivity of the system [35, 45, 52–54]. This latter remark also concerns the cytotoxicity of these materials [55].

Figure 1.7 Optical band gap energy as a function of the inverse squared primary particle size for common metal oxides. (Adapted from [56]).

We feel that it is important to stress the need for a fundamental understanding of the properties of nanostructured oxides, particularly for sizes in which the atoms directly affected in their properties are a significant percentage of the total number of atoms present in the solid particle; this usually implies a dimension limited to about or below 10 nm. When this fact occurs exclusively in one dimension, we are dealing with a surface or film, whereas in two dimensions, nanotubes, nanowires, and other interesting morphologies are obtained. Finally, when the three dimensions are limited to the nanoscale, nanoparticles are formed.

1.2.3 Carbon Nanoparticles

Carbon is unique in the number and the variety of its polymorphs. Figure 1.8 illustrates how the inorganic (nano)carbons can result from the extension of organic materials through large molecules [57]. These inorganic (nano)materials are very different in structure and properties, and their structural as well as surface chemistry is extremely complex.

Figure 1.8 C–C bonds to form a large number of hydrocarbons and their extension to carbon families. (Adapted from [58]).

In heterogeneous catalysis, carbon materials are unique catalyst supports, allowing the anchoring of the active phase, and can also be catalysts or catalyst poisons (carbon deposits) by themselves [58]. Although activated carbon and carbon blacks (CBs) are the most commonly used carbon supports, there is an increasing interest in the application of new carbon nanoparticles [fullerenes [59], carbon nanotubes (CNTs), carbon nanofibers (CNFs) [60], and graphene [61]] as supports for catalysis or catalysts since the nanostructure of these materials can offer a unique combination of properties. The catalytic behavior of solid carbons depends of course on their surface properties, but these surface properties are to a large extent a direct consequence of their bulk properties.

The controlled curvature or the orientation of the graphene layers in carbon nanoparticles dictated important properties. The curvature in fullerenes, CBs or CNTs is introduced by including pentagons and heptagons, together with hexagons, as starting fragments. This curvature of the graphene sheets induces strong modifications of the electronic properties; and comparison with graphite shows modification of the π-electron cloud [62]. The rolling-up of the graphene sheet to form a CNT causes a rehybridization of carbon orbital's (nonplanar sp2) configuration, thus leading to modification of the π density in the graphene sheet, which will depends on CNT diameter. It is worth noting that the theoretically predicted electronic properties are often modified by the presence of defects such as pentagons, heptagons, vacancies or impurities [63]. Similarly, in order to account for the bonding of the carbon atoms of a fullerene molecule, the hybridization must be a modification of the sp3 hybridization of diamond and sp2 hybridization of graphite. It is such that the σ orbital no longer contain all of the s-orbital character and the π orbital is no longer of the purely p-orbital character, as they are in graphite. Unlike the sp3 or sp2 hybridizations, the fullerene hybridization is not fixed but has variable characteristics depending on the number of carbon atoms in the molecule and consequently of its diameter. The number of carbon atoms, the pyramidization angle (θ − 90°), and the nature of the hybridization are related and this relationship (in this case the s character in the π-orbital) is given in Figure 1.9 [64].

Figure 1.9 Hybridization of fullerene molecules as a function of pyramidization angle (θσπ − 90°). θσπ is the common angle of the three σ bonds. (Adapted from [64]).

The rehybridization plays an important role in determining the electronic structure of the fullerene's family and it is the combination of topology and rehybridization that together account for the possible specific reactivity of all the curved sp2 nanostructures. The influence of carbon curvature on molecular adsorption of hydrogen has been reported [65, 66]. The hydrogen adsorption energy barrier is found to strongly depend on the local curvature of the carbon network whereby the barrier is lowered with increasing curvature. Whereas in the case of C60 and CNTs, hydrogen chemisorption can be achieved by exposure to atomic hydrogen, the chemisorption on graphite (0001) requires hydrogen ions of low kinetic energy (~1 eV).

For CNTs, the presence of relatively well-defined and nanometric inner hollow cavities can also induce differences of reactivity between the convex (external) and concave (internal) surfaces. Thus, it has been experimentally proven that hematite NPs located inside the CNT inner cavity are more easily reduced (873 K) by the support than those on the outer surface (1073 K) [67]. Beside CNTs, other carbon nanostructures with the negative curvature analog of fullerenes have been proposed as materials with interesting structural and functional properties [68]. Another property of CNTs is the possibility to perform reactivity in a well-defined confined space (see Chapter 11), and to take advantage of plausible confinement effects [69, 70]. The confinement effects that influence chemical reactions can be classify into three groups: (i) shape-catalytic effects, that is, the effect of the shape of the confining material and/or the reduced dimensionality of the porous space; (ii) physical (or ‘soft’) effects including the influence of dispersion and electrostatic interactions with the confining material; and (iii) chemical (or ‘hard’) effects that involve significant electron rearrangement, including the formation and breaking of chemical bonds with the confining material [71]. The latter is usually considered to be the actual catalytic effect, and it is the one that has the most obvious influence on the reaction rates, as it alters the reaction mechanism. However, the first and second types of effect can also have a strong influence on both the rates and equilibrium yields, as has been shown in several recent theoretical calculations [72] and experimental studies [73].

By careful manipulation of various synthesis parameters, it is possible to generate filamentous carbon nanostructures in assorted conformations and also to control their crystalline order (Figure 1.10). The tunable orientation of the graphene layers can directly affect catalytic activity and selectivity, for example by specific metal catalyst crystallographic face exposure according to the support [60].

Figure 1.10 Different carbon nanostructures produced by catalytic chemical vapor deposition: (a) multiwalled carbon nanotubes; (b) fishbone-carbon nanofibers (f-CNFs); and (c) platelet-carbon nanofibers (p-CNFs).

Finally, the thermal control on nanocatalysts becomes increasingly important as the size of the system diminishes. Therefore, for exothermic reactions the thermal conductivity of CNTs or graphene should play a critical role in controlling the performance of the catalyst.

Besides CNTs, graphene and fullerenes, catalytic applications of other carbon nanomaterials such as carbon nano-onions [74], or recently nanodiamonds [75, 76], and carbon nanohorns [77] have been much less studied.

1.3 How can Nanocatalyst Properties be Tailored?

The use of metallic/oxide NPs in catalysis is crucial as they mimic metal surface activation and catalysis at the nanoscale and thereby bring selectivity and efficiency to heterogeneous catalysis [78]. But, to be of interest, NPs should at least: (i) have a specific size (1–10 nm); (ii) have a well-defined surface composition; (iii) have reproducible syntheses and properties; and (iv) be able to be isolated and redissolved [79]. If the nanoparticle is supported, the question of precise control of its location, and thus of its spatial and chemical environment should also be addressed. Tailoring nanocatalysts properties thus necessitates being able to control these NP characteristics as well as their morphology, crystalline structure and composition (intrinsic composition and surface state).

1.3.1 Size, Shape and Surface Chemistry of Nanoparticles

Nowadays, to develop efficient catalytic systems, two important concepts are considered in nanocatalysis, namely the bottom-up strategy for the synthesis of well-controlled in size/shape NPs and the molecular approach to obtain more selective nanocatalysts. The bottom-up strategy allows the building of metallic nanoparticles from monometallic species. The control of NPs size is made possible by addition of a stabilizing agent, also called a capping agent (ligand, surfactant, polymer, dendrimer...). The introduction of ligands as nanoparticles stabilizers is of special interest because it focuses on the precise molecular definition of the catalytic materials. This strategy potentially allows optimization of the parameters that govern the efficiency in catalytic reactions, including enantioselectivity [80, 81]. As for homogeneous catalysts, an appropriate choice of the protecting agent means that the surface properties of the NPs can be tuned, as it can modify the nature of active sites (morphology) and the surface chemical environment (steric and/or electronic effect). As a result, NPs are very soluble in water or classic solvents, depending on the way of preparation and the stabilizer. The surface of nanoparticles can also be modified to render them more soluble in specific media (for example ionic liquids (IL)s or scCO2). They can also be handled and even characterized as molecular compounds by spectroscopic techniques [nuclear magnetic resonance (NMR), infrared (IR), ultraviolet-visible (UV-vis) spectroscopy, electrochemistry] in addition to solid-state techniques [transmission electron microscopy (TEM), wide-angle X-ray scattering (WAXS), powder-X-ray diffraction (XRD), X-ray excited photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS)...].

Concerning the control of nanoparticle size on which the number of surface active sites will be dependent, chemists have developed several methods, and very small NPs are already produced by different procedures [82–84]. The most well-known method is the reduction of a metal salt which gives rise to nanoparticles in an aqueous phase followed by the decomposition of metal-organic precursors also called the organometallic approach, which is more appropriate for obtaining NPs in organic media [85]. Size control is attained by the use of a large variety of capping agents, which limit the growth of nanoparticles. One can cite as examples of stabilizing agents, ions, polyoxoanions, surfactants (ammonium salts), polymers (polyvinylalcohol, polyvinylpyrrolidone, block-copolymers...), dendrimers such as polyamidoamine (PAMAM) and ligands (thiols, phosphines, amines...).

Considering reactivity and selectivity, the control of the surface state of the NPs is of critical importance as it can influence the course of a reaction. Since nanocrystals in their native form are dominated by the surface species [86], the protective agents used during the synthesis of metallic nanoparticles in solution play an important role. Two points are concerned, the control of the morphology and crystal structure [87] and the control of the surface composition [88].

The catalytic activity of metal nanocrystals is highly dependent on the nature of their surface structure [89, 90], exposure of different crystallographic facets, together with the increased number of edges, corners and faces, being key parameters. Therefore, NPs of different shapes are highly desirable as catalysts. While the effect of metallic nanoparticles size on the catalytic activity is well documented, knowledge about the influence of metallic nanoparticle shapes has started to develop only recently [91, 92]. Thus, the efficient control of the morphology is an on-going project all over the world and very interesting works have appeared in recent years [93, 94]. Solution-phase based methods have been shown to have great capability and flexibility to produce metal nanocrystals with well-defined morphologies with crystallographic control [87]. Capping agents are chosen for their influence on the shape of the particles, and consequently on the nature of surface active sites (edges, corners, faces, kinks, terraces, defects...) [95–98]. Controlling precisely the kinetics of the reaction appears as a key point to control NP shape [99, 100]. This is illustrated on Figure 1.11 that depicts different Co nano-objects synthesized from the same precursor and using the same stabilizing agents by careful adjustment of kinetic parameters.

Figure 1.11 Various cobalt nano-objects synthesized from [Co2{N(SiMe3)22(THF)]/lauric acid/hexadecyl amine mixtures by careful control of the reaction conditions, (a) spherical Co NPs (scale bar = 200 nm); (b) Co nanorods (scale bar = 200 nm); (c) Co nano-urchins (scale bar = 1 mm). With courtesy of K. Soulantica (LPCNO, France).

Second, the intrinsic composition of metallic nanoparticles has also to be controlled to tune their reactivity and selectivity. For example, the synthesis of alloyed versus core-shell bimetallic systems or the synthesis of NPs with a well-controlled surface state (meaning that the influence of stabilizing agents and/or the eventual poisoning of active sites are perfectly known and directed) are key points of current interest. Bimetallic nanocrystals with core-shell, heterostructure, or intermetallic and alloyed structures are emerging as a new class of nanocatalysts. They are expected to display not only a combination of the properties associated with two distinct metals, but also new properties and capabilities due to a synergy between the two metals [101]. More importantly, bimetallic nanocrystals usually show composition-dependent surface structure and atomic segregation behavior, and therefore more interesting potential applications. Compared with monometallic nanocrystals, preparation of bimetallic ones is much more complicated and difficult to achieve. In recent years, many research groups have made great efforts in this area, and bimetallics with controllable structures could be obtained, following different ways of synthesis [102]. For example, the preparation of uniform bimetallic Rh/Fe NPs in a phenylazomethine dendrimer, which provides improved catalytic reactivity for the hydrogenation of olefins and nitroarenes compared with monometallic RhNPs in a dendrimer cage has been reported recently [101]. Nevertheless, one of the most challenging problems is the comprehension of nucleation and growth mechanism of nanocrystals in solution, which would make the synthesis more efficient and better control the catalytic properties.

The properties of nanocrystals are also dependent on the surface chemistry. Chemical modifications of nanoparticles, such as by the use of ligands or adatoms to decorate their surface can thus provide new catalytic properties [87]. Thus, one can expect to be able to modify the chemical properties of nanoparticles by an appropriate choice of the capping agents, due to their own electronic or/and steric properties. At least, the coordination of ligands on surface atoms can block some metallic active sites and further orient catalytic reactions. Such studies are presently emerging, comparing for example the influence of a polymer and simple ligands in the dynamics and reactivity of carbon monoxide at the surface of small ruthenium NPs [103] or the influence of more sophisticated ligands as carbenes [104] or alkyl/arylphosphines [105] in the hydrogenation of aromatic derivatives. In asymmetric catalysis, the chiral capping agent used for the stabilization of metallic nanoparticles is of fundamental importance as it is expected to induce enantioselectivity. This area of nanocatalysis should be developed as only a very few examples of enantioselective nanocatalysts are known, mainly for the hydrogenation of ethyl pyruvate with cinchonidine-stabilized NPs [106]. This aspect will be treated in a specific section below.

The use of colloidally synthesized nanoparticles for the preparation of supported catalysts offers several advantages (e.g., precise control of particle size and morphology) when compared with traditional preparation techniques. Although such NPs have already been successfully used for catalytic applications in the liquid phase, applications in heterogeneous gas-phase catalysis are still scarce [107]. This is mainly due to the fact that in heterogeneous gas-phase catalysis organic stabilizers are often considered to have a detrimental effect on catalytic activity since, due to their presence, the active centers on the NPs are partly blocked. But recent studies have been published, in which the influence of ligands on heterogeneously catalyzed reactions in the gas phase was investigated with positive impact on catalytic reactivity and selectivity.

The development of new reaction media and recovery of the nanocatalysts are also crucial aspects that are widely studied to solve environmental problems. There are presently many investigations to develop metallic nanoparticles for catalysis in green solvents [108, 109]. For example, ILs [110], in particular imidazolium-based ILs, have proven to be suitable media for the generation and stabilization of soluble metallic nanoparticles. Such metallic nanoparticles immobilized in ILs appeared as efficient green catalysts for several reactions in multiphase conditions [111]. This aspect of nanocatalysis will be presented in Chapter 5 by M. Gomez et al. as well as in Chapter 6 by M. Haumann et al. dealing with supported IL-phase catalysis. Fluorous solvents, perfluorinated alkanes being the most representative, are also green solvents which have been used as reaction media for nanocatalysts [112]. The first work was reported by R. Crooks and co-workers who described dendrimer-encapsulated PdNPs for alkene hydrogenation in a mixture THF/perfluoro butyltetrahydrofuran with successful recycling tests up to 12 runs, with no loss of activity nor detectable leaking of the catalyst into the organic phase [113]. Other groups stabilized metallic nanoparticles with heavily fluorinated ligands or polymers to increase their solubility in fluorous solvents for catalysis [114–117]. Nevertheless, applications of fluorous solvents in nanocatalysis are practiced to a much smaller extent compared with other green solvents. In another way, some groups are developing the synthesis of metallic nanoparticles and nanomaterials soluble in supercritical fluids [118, 119]. Chapter 7 by C. Aymonier et al. is dedicated to this subject.