Nanotechnology in Catalysis: Applications in the Chemical Industry, Energy Development, and Environment Protection

By Marcel Van de Voorde

Reflecting the R&D efforts in the field that have resulted in a plethora of novel applications over the past decade, this handbook gives a comprehensive overview of the tangible benefits of nanotechnology in catalysis. By bridging fundamental research and industrial development, it provides a unique perspective on this scientifically and economically important field.
While the first three parts are devoted to preparation and characterization of nanocatalysts, the final three provide in-depth insights into their applications in the fine chemicals industry, the energy industry, and for environmental protection, with expert authors reporting on real-life applications that are on the brink of commercialization.
Timely reading for catalytic chemists, materials scientists, chemists in industry, and process engineers.

• Language: English
• Publisher: Wiley
• Release date: Jun 20, 2017
• ISBN: 9783527699834

Book preview

Series Editor Preface

Since years, nanoscience and nanotechnology have become particularly an important technology areas worldwide. As a result, there are many universities that offer courses as well as degrees in nanotechnology. Many governments including European institutions and research agencies have vast nanotechnology programmes and many companies file nanotechnology-related patents to protect their innovations. In short, nanoscience is a hot topic!

Nanoscience started in the physics field with electronics as a forerunner, quickly followed by the chemical and pharmacy industries. Today, nanotechnology finds interests in all branches of research and industry worldwide. In addition, governments and consumers are also keen to follow the developments, particularly from a safety and security point of view.

This books series fills the gap between books that are available on various specific topics and the encyclopedias on nanoscience. This well-selected series of books consists of volumes that are all edited by experts in the field from all over the world and assemble top-class contributions. The topical scope of the book is broad, ranging from nanoelectronics and nanocatalysis to nanometrology. Common to all the books in the series is that they represent top-notch research and are highly application-oriented, innovative, and relevant for industry. Finally they collect a valuable source of information on safety aspects for governments, consumer agencies and the society.

The titles of the volumes in the series are as follows:

Human-related nanoscience and nanotechnology

Nanoscience and Nanotechnology for Human Health

Pharmaceutical Nanotechnology

Nanotechnology in Agriculture and Food Science

Nanoscience and nanotechnology in information and communication

Nanoelectronics

Micro- and Nanophotonic Technologies

Nanomagnetism: Perspectives and Applications

Nanoscience and nanotechnology in industry

Nanotechnology for Energy Sustainability

Metrology and Standardization of Nanomaterials

Nanotechnology in Catalysis: Applications in the Chemical Industry, Energy Development, and Environmental Protection

The book series appeals to a wide range of readers with backgrounds in physics, chemistry, biology, and medicine, from students at universities to scientists at institutes, in industrial companies and government agencies and ministries.

Ever since nanoscience was introduced many years ago, it has greatly changed our lives – and will continue to do so!

March 2016

Marcel Van de Voorde

About the Series Editor

Marcel Van de Voorde, Prof. Dr. ir. Ing. Dr. h.c., has 40 years' experience in European Research Organisations, including CERN-Geneva and the European Commission, with 10 years at the Max Planck Institute for Metals Research, Stuttgart. For many years, he was involved in research and research strategies, policy, and management, especially in European research institutions.

He has been a member of many Research Councils and Governing Boards of research institutions across Europe, the United States, and Japan. In addition to his Professorship at the University of Technology in Delft, the Netherlands, he holds multiple visiting professorships in Europe and worldwide. He holds a doctor honoris causa and various honorary professorships.

He is a senator of the European Academy for Sciences and Arts, Salzburg, and Fellow of the World Academy for Sciences. He is a member of the Science Council of the French Senate/National Assembly in Paris. He has also provided executive advisory services to presidents, ministers of science policy, rectors of Universities, and CEOs of technology institutions, for example, to the president and CEO of IMEC, Technology Centre in Leuven, Belgium. He is also a Fellow of various scientific societies. He has been honored by the Belgian King and European authorities, for example, he received an award for European merits in Luxemburg given by the former President of the European Commission. He is author of multiple scientific and technical publications and has coedited multiple books, especially in the field of nanoscience and nanotechnology.

Foreword

At present the world is facing enormous problems concerning climate, energy, environment, and food and is waiting for solutions provided by technology, in particular chemistry. Catalysis is the motor for chemical transformations; for technical applications, it is mostly heterogeneous catalysis where the processes occur at the surfaces of solids. Since for a given quantity of material the accessible surface area increases with decreasing particle size, catalyst particles exhibit typically dimensions in the nanometer range, and catalysis has been a nanotechnology already long before this term was introduced. The controlled preparation, stabilization, and characterization of well-defined nanoparticles is hence one of the important tasks of research. The electronic and thereby chemical properties of very small particles may differ considerably from those of the bulk material, leading to altered catalytic properties as shown, for example, by gold and impressively demonstrated in Chapter 2 by Masatake Haruta. An alternative possibility consists in the use of porous bodies, as, for example, offered by the large family of zeolites and related compounds that are widely used in industrial applications.

This book presents a series of contributions from experts of the field reflecting the current state of the art and demonstrating the potential of nanocatalysis for a number of practical applications. It will therefore be of considerable value for those interested in the development of this area.

Gerhard Ertl

Fritz-Haber-Institut der Max-Planck-Gesellschaft Berlin

Foreword

A View from the Ridge: Catalysis in a Modern Society

Today's inspection of bookshelves with catalysis books in a traditional science library or of databases with catalysis review collections shows a pleiad of keywords covering almost every aspect of modern science. A personal selection of keywords in the area of (heterogeneous) catalysis looks as follows:

principles and practice, fundamentals and applications, experiment and theory;

basis, concepts and applications;

(challenges in) design, synthesis, characterization, modeling, application;

surface chemistry, solid-state chemistry, in situ spectroscopy and characterization;

combinatorial chemistry; photochemistry, electrochemistry, kinetics, modeling, stereochemistry, dynamics; environmental chemistry, synthetic chemistry, materials science;

fine chemicals synthesis, fuels, industrial feedstock, biomass, energy applications, sustainable and clean technology.

Thus, it seems that at the end of the 2nd millennium, catalysis research was making effective use of many major advances offered by progress in various areas of science. Catalysis, and in particular heterogeneous catalysis, appeared as a truly multidisciplinary science that was profoundly affecting in a positive way environment and daily activities. In combination with modern materials science, very advanced materials were designed, applicable as catalysts in many petrochemical, chemical, and fine chemical sustainable industrial processes. As a result, catalysis was susceptible for being defined as a mature science.

An overview of the science and technology of heterogeneous catalysis at the end of the 1990s culminated in the publication of an impressive standard work denoted as Handbook of Heterogeneous Catalysis, published by the present science editor (scientific eds. G. Ertl, H. Knözinger, F. Schüth, and J. Weitkamp, 2nd edition). It covered all aspects of heterogeneous catalysis ranging from physical and physicochemical concepts to large-scale process technology.

Although the catalytic community of research scientists usually picks up quickly breakthroughs from other scientific domains, and the scientific literature at the end of previous century already showed clear awareness of the potential use of nanosciences for advancement in (heterogeneous) catalysis, today's revolution caused by nanotechnology in catalysis among many other domains was not fully and explicitly realized.

Nanoheterogeneous Catalysis in a Sustainable Society

In the traditional heterogeneous catalysis of the twentieth century, microsized and microstructured objects of catalyst entities were determining activity and selectivity. Enhanced strength of Brønsted acid sites is encountered in solids with regular microporosity. The deposition of metal clusters on supports gave rise to the concept of structure-sensitive and structure-insensitive reactions, depending on the size, morphology, and surface properties of (noble) metal particles.

The many reviews in the area invariably show that the aim of nanomaterials science in the past few decades has been related to the control of size, shape, structure, morphology, and composition of matter, leading to nanoproducts with a high degree of perfection. Therefore, selective implementation of nanoconcepts into catalyst design and preparation strategies should show significant advances in the design, synthesis, and characterization of catalytic nanomaterials. Obviously, this transfer of concepts will help to settle major standing issues in catalysis for a sustainable society, such as preparation of novel catalyst compositions with enhanced/superior potential for alternative chemicals production and alternative feedstock conversion, for achieving a sustainable energy future, and for environment protection.

Inspection of search results in a science database for the nanocatalysis concept (SciFinder, January 24, 2017) yielded from 2000 onward 472 hits in English written books and reviews. A similar search for specific research papers and patents up to 2000 amounted to only 12 hits, while from 2000 onward a total number of 7001 documents appeared. When the concepts of nanotechnology and catalysis are closely coupled in a similar database search, in total some 5400 reviews show up, of which 536 were published recently, that is, later than 2015.

Many typical topics concerning properties of nanomaterials for biomass conversion, energy, and environmental processing appear, resulting in newly structured materials with unprecedented properties allowing us to achieve hyperselectivity in many catalytic reactions. A personal grasp in the many available topics points to the high potential of nanocatalysts with mono- and multimetal sites, for selective conversion of hydrocarbons and biomass components, newly designed photocatalytic materials in energy (carrier) conversions:

crystal structure and morphology control of nanoparticles for designing catalysts; control of size, shape, composition, and surface properties of nanometal catalysts

support-induced enhancement, and size and shape control of catalytic properties of (precious) monometallic nanoparticle catalysts; lifetime and reactivity increase using atomic layer overcoating of metal nanoparticles; integration of nanocatalysts with multicomponent and hierarchically complex structures

copper-based nanoparticles for multicomponent reactions; structure sensitivity of Au in environmental catalysis

photocatalysis and photoelectrocatalysis for water splitting; photo-oxidation with nanosized semiconductors; functional nanostructures for catalysis; nanocatalysts for solar hydrogen

concepts of photodeposition for preparation of catalytic nanoparticles on semiconductors

noble metal nanocatalysts on metal oxide-based supports for (low temperature) fuel cell applications

atomically precise nanoclusters

It is understandable that the present editors, given their activity as catalyst researcher and science policy maker, respectively, felt the necessity of bringing this abundant information together in an easily digestible format. I am pleased to be able to express my personal appreciation for this initiative and I am looking forward to get fully acquainted with the detailed content of this new three-volume handbook in catalysis.

Pierre Jacobs

Honorary Professor

Faculty of Bio-Engineering

KU Leuven

Belgium

January 25, 2017

Part One

Preparation of Nanocatalysts and Their Potential in Catalysis

1

Liquid-Phase Synthesis of Nanocatalysts

Jean-Francois Hochepied

ENSTA ParisTech UCP, MINES ParisTech MAT/SCPI, 828 Bd des Maréchaux, 91762 Palaiseau Cedex, France

1.1 Introduction

The control of catalysts structure at the nanoscale is the key to increase performances and improve the fundamental knowledge about reaction mechanisms. Thanks to powerful nanocharacterization tools, especially high-resolution transmission electron microscopy [1], chemists can check their ability to control critical parameters as particle size, composition, shape, exposed crystalline faces, and particle–support interfaces. This has boosted studies linking processes, nanostructures, catalytic properties measurements, and modelization, paving the way for the rational design of nanocatalysts.

Liquid-phase processes offer a compromise between industrial constraints and fine control of nanostructures. It is impossible to encompass in a few pages all processes and materials relevant to catalysts and many are considered with more details elsewhere in this book, so the point of view is to focus on two families of materials – metallic oxides and metals – and show by selected examples how they can be shaped and interfaced at the nanoscale using simple and industrially relevant processes to create nanocatalysts. In the case of oxides – either catalysts or catalysts supports – we will consider both particles and porous structures, whereas we will rather focus on particles in the case of metals, considering they can be either colloidal catalysts or supported by oxides.

Liquid-phase syntheses are bottom-up approaches consisting in condensing soluble species. The formation of solid can be described in terms of nucleation and growth (eventually followed by agglomeration) and basic theories and concepts help to understand the strategies relevant to catalysts design.

Roughly, two key parameters, supersaturation S and solid surface tension γ (or free energy), are sufficient to provide expressions for homogeneous nucleation and growth rates in solution. Surface tension is related to the energy needed to create interfaces. In the case of a liquid (drop model), the surface chemical potential of a droplet with radius r is given by the following equation:

equation

where Vm is the molar volume of the condensing molecule. Similar expressions can be obtained with solids, keeping in mind that surface tension is no more isotropic if they are crystallized but depends on the exposed faces. In any case, the important point is the γ/r dependence law. Supersaturation may be defined as the ratio between the actual quotient of the reaction in solution and the equilibrium constant, and gives the driving force of precipitation. In addition, in nucleation an additional free energy term comes from the creation of a solid surface, so the expression of the free enthalpy ΔGi for the condensation of i soluble units A into a cluster Ai can be written as follows:

equation

where s is the surface covered by one unit. Here a simplified approach considers a critical germ i* defined by the maximum of ΔGi and derive a kinetics expression for the nucleation rate J. In fact, in a more rigorous approach, the thermodynamical system with all population from i = 1 to i = Nmax should be considered [2], but the derived expression for nucleation kinetics is the same as in the simplified approach. The critical germ can be considered as the smallest possible solid particle; in theory, it practically does not exist in solution, but its formation is the kinetics bottleneck. So in this approach, the nucleation rate J, that is, the number of germs produced per second and per volume unit, is as follows:

equation

The expression is simple, but unfortunately supersaturation and surface tension are in general not easy to determine. If mixing of solutions is used, even with rapid mixers characteristic times for mixing are in general longer than characteristic times for nucleation. Surface tension depends on the germ facies, which is by no means similar to the equilibrium (Wulff) facies. Nevertheless, the expression gives some clues about sensitiveness of nucleation kinetics to supersaturation and surface tension. In the case of heterogeneous nucleation, a prefactor (f<1) is used in the expression of ΔGi to quantify the fact that it is generally easier than homogeneous nucleation (the interfacial energy between substrate and germ is lower than between solvent and germ). It is important to keep in mind that if heterogeneous nucleation is wanted, the supersaturation must be moderate to avoid homogeneous nucleation, which explains the strategies used for controlling subtrate–nanoparticles interfaces.

If we consider the evolution of a crystalline germ, crystal growth depends on the nature of the faces. If we consider perpendicular growth (addition of new layers on faces), high-energy faces grow faster than low-energy faces, because they can grow by continuous incorporation of matter from the solution, whereas low-energy faces grow by 2D nucleation. So the surface proportion of high-energy faces tends to lower during crystal growth. Trying to modify the relative growth rates of different faces is the basis of kinetic facies tuning. In order to tune the facies of nanoparticles, several strategies are possible. Some recipes produce germs with well-defined facets, additives that will selectively poison some surfaces and prevent their growth are also known more or less empirically. If we consider general expressions, nucleation, especially when initial supersaturation is very high, can consume the main part of reactants and only a small fraction can remain available for growth. This seems favorable if one just wants to synthesize nanoparticles, but a problem is the first precipitate may frequently be metastable (according to the Ostwald rule of stages [3], less stable products are kinetically favored), transforming into the stable product via redissolution under conditions of much lower supersaturation, hence more favorable to growth. It is also important to keep in mind that even when there is no more average supersaturation after precipitation, nanoparticles may still evolve due to size polydispersity: Small particles are more soluble than bigger ones due to the 1/r dependence law of their surface chemical potential; hence, in a medium supposed to be at the solubility equilibrium, the average solubility level is in fact undersaturated for small particles that tend to dissolve, whereas supersaturated for big particles that tend to grow: This phenomenon is known as Ostwald ripening [4]. The same applies for facies: The chemical potential of high-energy faces is higher than that of low-energy faces, and the facies tends to change in favor of low-energy faces. Uncontrolled ripening is therefore in general detrimental to catalysts activity by lowering the surface area and exposing less active faces (Figure 1.1).

Figure depicting ripening of nanoparticles in solution without apparent supersaturation that is an important cause for catalysts performance loss.

Figure 1.1 Ripening of nanoparticles in solution without apparent supersaturation. An important cause for catalysts performance loss.

1.2 Metallic Oxides

1.2.1 (Co)Precipitation by Mixing (Aqueous Solutions) and Ripening

The mixing of a concentrated aqueous solution of metallic salts and a basic solution is the most direct way to (co)precipitate metallic (hydr)oxides. The strategy is simple: generate a very high supersaturation favoring nucleation over growth, which is relatively easy when resulting (hydr)oxides are poorly soluble. In general, as precipitated products (often metastable, sometimes amorphous) need post-treatment (ripening and calcination) to be chemically and crystallographically stable, rebuild their surfaces and meet the specifications needed for the application in catalysis (Figure 1.2).

Figure 1.2 From soluble species to solid (hydr)oxides: the frequent occurrence of intermediate metastable solid (in green) must be considered in the size and shape control of final particles (in red).

The way mixing is done is critical and can change drastically the products starting from the same solutions and considering the same final bulk conditions, as evidenced, for instance, with boehmite (mesoporous or fibrillar) precipitated by mixing aluminum nitrate with soda [5]. The most direct way of mixing two solutions consists in the injection of the basic solution into a batch containing the metallic acidic solution, but even with performant mixers the physicochemical conditions (pH, concentrations, and consequently supersaturation) of the bulk and locally at the injection point vary from the beginning to the end of the injection (even if rapid), with the risk that corresponding precipitated particles may be different. To circumvent this, in order to stabilize the physicochemical conditions in the mixing zones, a separated double-jet system is appropriate, as evidenced, for instance, for nickel hydroxide precipitation where the bulk pH was shown to control particle size and crystallinity [6]. The extreme case of double-jet consists in rapid (static) (micro)mixers, where both fluids are injected and mixed in a confined volume. The design of microreactors at various scales (from laboratory-scale microfluidics to industrial mixers) has been recently boosted by the increasing computing power for hydrodynamic modelization and improvements of fabrication techniques allowing precise control of device dimensions (channels, cavities) and interface quality (Figure 1.3) [7]. These continuous systems are the cornerstones of process intensification in precipitation [8]. For instance, Fang et al. [9] coprecipitated Fe(II) and Fe(III) to produce magnetite nanoparticles and evidenced that their rapid mixer (impinging jets) led to good crystallinity and homogeneity, Palanisamy and Paul [10] precipitated ceria continuously in a T-mixer and linked particle characteristics (homogeneity, size distribution, agglomeration) to the quality of mixing (engulfment flow conditions).

Figure depicting the example (a-d) of geometries of Hartridge–Roughton (vortex) micromixers.

Figure 1.3 Example of geometries of Hartridge–Roughton (vortex) micromixers. (Reproduced with permission from Ref. [7]. Copyright 2016, Elsevier.)

Considering aqueous solutions, in general mixing is performed under atmospheric pressure and consequently at moderate temperatures. Some batch systems (autoclaves) allow mixing under pressure in hydrothermal conditions, but most of the time mixing and hydrothermal ripening are separated steps. This may appear as a constraint, but also offers a lever to tune the product characteristics by choosing independently the physicochemical conditions for each step. The way precipitates are heated up to hydrothermal ripening is critical and some products may be sensitive to the ramp used because they can begin to transform before reaching the desired temperature, so there is a specific interest in controlling the temperature ramp accurately and if possible in reaching high heating rates. For batch systems, heating by the walls is less appropriate if fast heating is wanted, microwave heating is much more relevant but limited to relatively small volumes and as a consequence not well spread at industrial scale. Continuous systems allow fast heating even using classical transfer by the walls if exchange surfaces are high enough. In such systems, a precipitate of amorphous titanium hydroxide can be transformed into well-crystallized and well-dispersed anatase nanoparticles (10 nm) at relatively low temperature (120 °C) in a few minutes [11]. An important variation is the continuous supercritical hydrothermal synthesis (in general 400–500 °C, 30 MPa). Aimable et al. [12] precipitated in a few seconds nanocrystallized particles (or agglomerates) of ZrO2, TiO2, ferrites, and perovskites in their system combining a T-mixer and a continuous flow reactor (cf. Figure 1.4). Similarly, Kawasaki et al. [13] produced with very short residence time (order of magnitude 1 s) well-crystallized TiO2 anatase nanoparticles (in the range of 13–30 nm). It is noteworthy that these authors also used micromixers to mix the Ti(IV) solution with supercritical KOH solution to improve the control of nucleation.

Schematic diagram depicting a typical equipment coupling micromixer and continuous hydrothermal ripening.

Figure 1.4 A typical equipment coupling micromixer and continuous hydrothermal ripening. (Reproduced with permission from Ref. [12]. Copyright 2009, Elsevier.)

Catalysts or catalyst supports are often solid solutions or doped oxides. Mixed oxides where cations have close hydrolysis reactivity can be obtained by aqueous coprecipitation. Cationic doping is sometimes more difficult when the dopant has a very different solubility compared to the cation(s) of the host matrix. If some cations are highly soluble in water (mainly from columns 1 and 2 of the classification), different strategies should be used. A cheap and convenient way consists in coprecipitating poorly soluble species as citrates, acetates, and so on if column 2 elements are involved, but particle size control is generally poor and calcination is necessary to get the oxide phase.

The advantages of aqueous (co)precipitation are obvious: Huge quantities of particles with high yields may be produced (generally >10% in mass), which is industrially relevant, in general a good average composition control can be achieved, and satisfactory crystallinity and specific surface area values may be obtained for most oxide materials (especially for tri- or tetravalent metallic cations) after appropriate posttreatments. The control of reactor geometry, propellers, and so on ensures the robustness of the process. Nevertheless, this strategy implies limitations: In general, no fine control of crystallite facies or particle size and shape is obtained just after precipitation and ripening conditions cannot always offer levers to change the facies in a favorable way (this depends on the mechanisms involved in the solid transformations), precipitates and powders may be strongly agglomerated, which may hinder their use.

Facies or shape control may be obtained during ripening, along with crystal growth. Of course, equilibrium facies will be naturally favored, but some additives may adsorb specifically on certain faces and poison their growth. Facies is also pH dependent, since the charge of oxide faces is pH dependent, which in turn changes the surface tension. Just to give one example, Wang et al. [14] obtained a nice kinetic-controlled facies for SnO2 particles just by tuning pH and studied the impact of high-energy faces exposition to promote catalytic properties (Figure 1.5).

Figure depicting example of pH-controlled facies of submicrometer particles.

Figure 1.5 Example of pH-controlled facies of submicrometer particles. (Reproduced with permission from Ref. [14]. Copyright 2012, John Wiley & Sons, Inc.)

It is also important to keep in mind that at the nanoscale, some nano-objects do not result from classical crystal growth but from other mechanisms (self-rolling of nanosheets in the case of titanate nanotubes [15], etc.).

1.2.2 Hydrolytic Sol–Gel

In classical sol–gel, a metallic alkoxide is mixed with water in alcohol. The hydrolysis of the precursor (the replacement of at least one ligand by one hydroxyl) is the first step and allow further condensation to get an oxide at the end of polymerization:

equationequation

The control of the balance between the kinetics of precursor hydrolysis and condensation is the key to control the particle size and their agglomeration (formation of a gel by collisions and attachment of reactive particles). This can be achieved by the choice of the ligand, the base/acid concentration (catalyst), the choice of the alcohol, and the water/alcohol ratio. The most common precursors are Si or Ti alkoxides, and the synthesis of TiO2 nanoparticles, nanocoatings, or nanostructures by sol–gel has been thoroughly studied. Of course, previous considerations about mixing are still valid, and the control of TiO2 gel formation by rapid Vortex mixers was carefully studied by Marchisio et al. [16]. In such systems, the alkoxides of cations difficult to coprecipitate in water have very often close reactivity and copolymerize. Classical sol–gel methods lead in general to amorphous nano-oxide particles with a few nanometers in diameter, and connected by necks. Depending on the way solvent is removed, one can get nanoparticles, xerogels, or aerogels. Aerogels are obtained by CO2 supercritical drying, keeping the integrity of the nanostructure built in solution, whereas xerogels are much denser and result from classical drying where capillary forces make the nanoarchitecture collapse. Highly porous metallic oxide aerogels offer interesting nanostructures for catalysis [17], the drawbacks being the cost of the process and their brittleness, which probably explain other families of porous materials that are nowadays hotter topics as shown elsewhere in this book. Sol–gel systems are also compatible with soft or hard templates to make ordered or hierarchical porous materials.

1.2.3 Homogeneous (Co)Precipitation by Thermohydrolysis (Aqueous Solutions)

Nanostructured microparticles may spontaneously result from homogeneous (co)precipitation in aqueous solutions, when supersaturation is generated relatively slowly (compared to mixing) in the whole volume of a solution. Generally, the precipitation is triggered by heating (using conventional or microwave sources), with different possible effects such as lowering the solubility of cations in acidic conditions (forced hydrolysis), generating a base by in situ decomposition (urea as main example), and removing ligands that stabilized the cations (ammonia for instance). Forced acidic hydrolysis is relevant to poorly soluble cations (Ti⁴+, Fe³+, etc.), ammonia removal to divalent transition elements (Ni²+, etc.) [18], and urea decomposition to cations that can be solubilized in the initial weakly acidic conditions (Mg²+, Y³+,Ni²+, etc.) [19]. The products may be hydroxides or hydroxycarbonates and their nanostructure may be kept when calcined to oxides. If some nanostructured multiscale particles sometimes spontaneously appear without template agents, homogeneous precipitation can be performed with hard or soft templates to generate nanoarchitectures, with possible hierarchical porosity considered as beneficial to control the diffusion of species toward catalytic surfaces. The most usual hard templates are colloidal crystals of silica or polymer spheres, and soft templates are mesophases of surfactants such as CTAB, SDS, and so on, especially used for their hexagonal phases. After precipitation and calcination, resulting oxide architectures may exhibit both meso- and macroporosity and be optimal catalyst supports as shown by Zhou et al. [20] with Pd/Al2O3 for the selective hydrogenation of pyrolysis gasoline. Template removal (by calcination or dissolution) is often delicate and the desired porous structures may collapse during this operation.

1.2.4 Homogeneous (Co)Precipitation and/or Ripening in Nonaqueous Solvents

Aqueous-based syntheses often face two main problems: (i) Some mixed oxide materials are difficult to obtain by coprecipitation due to the difference in solubility and reactivity of metallic precursors (cations). (ii) Hydrothermal conditions, often necessary to obtain well-crystallized catalysts, require equipment working under pressure and the consequent technological, safety, and economical impacts may be heavy. Nonaqueous solvents may circumvent both issues. Solvothermal crystallization is often possible under atmospheric pressure (up to reflux) or at least under lower autogenous pressures compared to water, but of course chemistry must be adapted by choosing or synthesizing appropriate precursors in terms of solubility and reactivity. In general, the precursors decompose thermally, while the solvent and/or additives limit and/or orient particle growth after nucleation. These processes are often better than aqueous syntheses for controlling the size, composition, crystallinity, crystal habit, and architectures of particles, especially at the nanoscale, but their economical and environmental costs are higher. Production of oxides in polyols generally uses water as reagent, but in other nonaqueous systems metallic oxides can be produced using an organic molecule instead of water as the source molecule for oxygen atoms: for instance, alkoxides or ethers with metallic chloride precursors dissolved in CH2Cl2 [21], benzylalcohol [22], or even a polyol [23] also playing the role of solvent. This nonhydrolytic sol–gel is interesting to produce well-crystallized nanoparticles and mixed oxides. If research works seem more focused on nanoparticles, some studies show that these routes are also interesting for textured or porous materials without need for templates nor supercritical drying [21].

1.2.5 Anodization

Anodization of some metallic foils may spontaneously lead to nanostructured oxide surfaces. As main examples relevant to catalysis, we may cite alumina porous membranes and titania nanotube arrays, which have drawn a lot of attention in the field of photocatalysis [24]. The setup is simple, and the concentration of the electrolytes, pH, and anodization potential control the formation of arrays [24]. For instance, TiO2 nanotubes arrays in Figure 1.6 were produced by anodic oxidation at a potential of 25 V of a Ti foil in a KF electrolyte at pH 4, during 17 h.

Figure depicting anodization setup (a) and TiO2 nanotubes arrays produced by this technique (b).

Figure 1.6 Anodization setup (a) and TiO2 nanotubes arrays produced by this technique (b). (Reproduced with permission from Ref. [24]. Copyright 2006, Elsevier.)

1.3 Metallic Nanoparticles

The size reduction of metallic particles to the nanolevel induces geometrical and electronic effects, both playing a role in their catalytic properties (activity and selectivity). Well-faceted polyhedral metallic nanoparticles have a significant fraction of atoms not only on crystal faces but also on edges or even corners, sites that are likely favorable to high catalytic activity. In addition, some high-index faces may exhibit steps and kinks as active sites for catalytic reactions. Syntheses allowing size, shape, and facies control of very small NP (<10 nm) are consequently highly valuable for fundamental understanding and applications. Nevertheless, a disturbing feature is that very small NPs exposing high-energy faces are thermodynamically unstable, which means their facies may undergo some evolution detrimental to performances. In some conditions used for catalytic reactions, Ostwald ripening can lead to both size increase and shape transformation into more stable (and much less catalytic) particles so quickly that studies about optimizing the facies could be considered as vain. For instance, the transformation of tetrahedral Pd particle into spherical ones was already observed after the first cycle of the Suzuki reactions [25]. The stabilization of metallic NPs (against ripening or irreversible aggregation in liquid media) with organic molecules is possible, but their surface modification changes their catalytic behavior. Fortunately enough, if in general they lower the catalytic activity on the one hand, capping agents may promote selectivity as underlined in Ref. [26] on the other hand. As far as solvents are concerned, most of metallic nanoparticles relevant to catalysis can be obtained in aqueous systems. An industrially acceptable alternative is offered by polyol-mediated syntheses. Other nonaqueous systems may give better control on particle dimensions at the nanolevel, but are not industrially well spread and since they are less useful in the field of catalysis compared to other applications, they will not be considered here.

1.3.1 Aqueous Syntheses

Some metallic nanoparticles can be produced by adding hydrides to metallic cations, and (as for oxide precipitation) the control of mixing is critical. Thiele et al. [27] have shown that well-controlled silver seeds could be obtained in their microfluidic homemade device using micromixers to mix AgNO3 and NaBH4 solutions (Figure 1.7). The size control is found better than obtained with a batch reactor. These seeds can be used for growing nanotriangles in a second and separated step, with precise dimensions control.

Figure depicting continuous flow synthesis of silver nanoparticles using micromixers.

Figure 1.7 Continuous flow synthesis of silver nanoparticles using micromixers. (Reproduced with permission from Ref. [27]. Copyright 2015, John Wiley & Sons, Inc.)

Homogeneous precipitation of metallic particles uses less reactive reductors than hydrides, and is generally activated by heating after mixing in nonreactive conditions (low temperature). For instance, a simple recipe to produce gold nanoparticles consists in dissolving HAuCl4 in water, adding sodium citrate, adjusting pH to a desired value, and heating at moderate temperatures; of course, from the stem work by Turkevich [28], many studies were however necessary to describe thoroughly this system, explain the mechanisms at the basis of size and shape tuning [29].

Size, shape, and facies control is now mastered for many metallic NPs, often via seed-mediated growth. Well-faceted germs can be obtained by controlling nucleation conditions, and then additives as anions, metallic cations, polymers, surfactants, or other organic molecules, chosen or discovered more or less empirically, selectively adsorb on well-defined crystallographic face and impede or accelerate their growth. Of course, the downside of this method is the risk of persistence of directing agents after synthesis and the consequent alteration of the catalytic activity of the relative faces. Some articles only deal with the synthesis of particle with well-defined facets, but some study continuous shape tuning (from spherical to cubes, from cubes to octahedra, etc.), generally going along with limited growth, showing the intermediate states, which is of course of the highest interest for catalytic properties studies. For instance, CTAB preferential adsorption to (100) rather than (111) faces of gold allows kinetic shape tuning [30]. In addition to poisoning, oxidative etching, attacking preferential faces, can be used to dissolve selectively particles with specific facies [31]. As an example of shape control directly applied to catalysis, Berhault et al. [32] studied the effect of Pd NP shape and facies on the selective hydrogenation of buta-1,3-diene by synthesizing nanocubes, nanotetrahedra, and nanorods, thanks to a seed-mediated approach and the surfactant CTAB (easily removed after synthesis) as inhibitor of (100) facets growth. They proved that the preferential exposure of (100) planes in rods allowed a high selectivity for the conversion into butenes. At the nanoscale, one must also keep in mind that some unusual facies and shapes may be promoted compared to their bulk counterpart. For instance, seed-mediated growth approach was found very efficient to produce gold nanorods. Gold has a cubic structure, and the existence of nanorods results in fact from growth of fivefold twinned gold seed or other seeds (Pt, Ag, etc.).

1.3.2 Polyol-Mediated Syntheses

Among these systems, we may cite polyol-mediated syntheses, polyols being both reducing agents and solvents. For instance, Bonet et al. [33] demonstrated the relevance of polyols to produce gold, platinum, palladium, ruthenium, and iridium nanoparticles. Surfactants, capping ligands, or even well-chosen anions can be added to tune particle size and shape in the same logic as in aqueous solutions. Wiley et al. [34] successfully tuned the shape of Ag nanocrystals by chloride addition. Interestingly, some additives may accelerate specific face growth (instead of poisoning). Song et al. [35] evidenced that Ag+ ions could promote (100) face growth in Pt nanocrystals, allowing shape tuning from cubes, cuboctahedra, and octahedra just by changing the quantity of Ag+. Ag is first specifically reduced on (100) Pt faces, and then Ag(0) accelerates the reduction of Pt(IV) and is reoxidized and dissolved simultaneously. This process is interesting because the exposed faces do not result from poisoning, avoiding issues about possible persistent adsorption of additives. The combination of poisoning and oxidative etching gave spectacular results in polyol with fine shape tuning for, for instance, Pd [36]. Another interesting point about polyol is their high compatibility with microwave heating.

1.3.3 Hollow Particles by Galvanic Replacement

Galvanic replacement has recently been used to produce and study hollow metallic particles and metallic nanoframes [37]. A metallic nanoparticle (often Ag, easy to oxidize) playing the role of a sacrificial template is oxidized (generally in aqueous solutions) by a metallic cation that is reduced on the surface: Au, Pt, or Pd nanocages can be obtained this way, as well as alloys and shell–shell nanocages. These hollow particles and nanoframes are supposed to enhance their catalytic activity by confinement of the reactants in their cavity. Nevertheless, for the time being, such methods cannot produce significant amounts of materials, consume relatively expensive templates, and as a consequence are still industrially irrelevant.

1.4 Oxide–Metal Interfaces

Nowadays, the variety of synthetic methods, the availability of characterization techniques at the nanoscale [38], and the increasing modelization of heterogeneous catalysis open the way to the rational design of metal–oxide catalysts with optimization of the oxide architecture (or particle size and shape) and exposed faces, optimization of the metallic particles size and shape, and last but not least the interface between metallic NP and the oxide support. See, for example, the case of CeO2/Au nanocatalysts in Ref. [39].

An important point is that the oxide is often more than a support and can be a catalyst itself as with TiO2 photocatalysts. So the oxide material can be a porous structure, a microparticle, or a nanoparticle, and the catalyst is in fact a hybrid (or heterostructure) between the oxide and the metal, where the nature of the junction (coherent interface or not) may play a key role in electron transfers and consequently catalytic activity. In the case of oxide semiconductor photocatalysts, the photoelectron migration to the metallic particle, preventing the recombination with the hole located in the valence band of the oxide, is assumed to be the main cause for the much higher photocatalytic activity of TiO2 (or ZnO) when covered with metallic NP (Ag, for instance). Generally, in order to create metal–oxide hybrids, it is easier to precipitate metallic particle on oxide substrate than the opposite, with some exceptions including surface oxidation of metallic substrates.

Direct deposition of metallic NPs on oxide substrates is possible, but the nature of the interface and the dispersion is far from optimal so far as the catalytic activity and reusability are concerned. Another common approach, wet impregnation, consists in mixing noble metal soluble precursors with the substrates, evaporating the solvent, and finally reducing in dry form by gas-phase reactions. In order to improve the control of the oxide–metal interface, a more relevant strategy is the chemisorption of metallic precursors on the surface followed by in situ reduction. The knowledge of the oxide surface physicochemistry (pH-dependent surface charge (zeta potential)), measured or evaluated (for instance, by multisite complexation models), guides the choice for the precursor and the best conditions to adsorb it. As an example of electrostatically favorable adsorption, ammonia can solubilize by complexation of many metallic cations (transition metals, Pt, Pd, Ru,) in basic conditions, a positively charged complex being the main species (Pt(NH3)4²+, etc.), whereas oxide surfaces are negatively charged. The opposite approach, adsorbing anionic species on positively charged oxide (in acidic conditions), is of course possible and was found very relevant in the case of gold. Adsorbed species can therefore act as favored nucleation sites for further reduction and growth of metallic particles, or even be directly calcined. It is noteworthy that the reducing agent can also be the molecule complexing the metal cation. Recently, many studies compared such adsorption methods with more brutal methods as wet impregnation, and the better dispersion of NPs onto the surface and interface coherence systematically leads to better performances, as shown by Li et al. for Pd/TiO2 for the Heck coupling reaction [40]. As an interesting variation, Zhong et al. [41] first adsorbed Sn(II) onto TiO2 surface, which reduced Pd in situ to get well-dispersed Pd nanoparticles supported by TiO2, with high loading and activity for Suzuki cross-coupling reaction. Zhang et al. [42] used the photocatalytic properties of TiO2 to photodeposit Pd and evidenced the role of pH in the control of Pd particle size and dispersion. In aqueous systems, both the speciation of the metallic precursors and the surface charge of the oxide substrate are pH dependent and it is not too surprising that pH is the key physicochemical parameter when dealing with in situ reduction. A good knowledge of the system physicochemistry helps to control heterogeneous nucleation and growth by smooth tuning of supersaturation, avoiding shifting to homogeneous nucleation. Depending on the crystalline nature of the oxide and metal, the resulting interface may be coherent (with lattice matching), which in general changes drastically the catalytic or photocatalytic properties (electron transfer through the interface, nature of the metal–oxide medium triple line, etc.). As an example, Haruta et al. [43] deposited gold nanoparticles on TiO2 (anatase) substrates by direct impregnation, resulting in ill-defined interface and precursor adsorption followed by reduction resulting in coherent interface (Figure 1.8). Propylene epoxidation was possible only with the catalyst with coherent interface, whereas the other one catalyzed only the full oxidation of the organic molecule.

Figure depicting the example of process–catalytic activity relationship and interpretation by the quality of the oxide–metal interface.

Figure 1.8 Example of process–catalytic activity relationship and interpretation by the quality of the oxide–metal interface. (From Ref. [43].)

Interestingly, besides NP deposition or in situ synthesis on preexisting surface, it is possible to synthesize the oxide matrix around metallic particles, Kónya et al. [44] prepared this way nanocatalysts with well-defined Pt shape in SBA-15 matrix.

One-pot synthesis of oxide matrix and metal nanoparticles with controlled oxide matrix architecture, NP particle size and shape, and metal–oxide interface is possible in some cases. As an example, Chen et al. [45] produced Pt-SBA-15 by calcining a hybrid prepared by solution chemistry: A hexagonal direct mesophase was produced with Pt source embedded, thanks to a thiol with the templating block copolymers in the hydrophobic channels, whereas tetraethylorthosilicate TEOS polymerized in the continuous aqueous phase. Then calcination removed all organics and reduced Pt simultaneously, via decomposition of intermediate platinum sulfide. These catalysts were found better for methylcyclohexane dehydrogenation compared to conventional Pt-SiO2.

1.5 Conclusion

The understanding of catalysts activity and selectivity is making continuous progress, thanks to nanocharacterization and modelization, and structure–activity (or selectivity) relationships are more and more evidenced. The rational design of new catalyst needs a trustworthy toolbox of processes allowing nanolevel control of key characteristics as composition, crystallographic structure, dimensions, architectures, and interfaces. In this toolbox, liquid-phase (mainly aqueous) syntheses benefit from a rather long history in precipitation, with many thermodynamic and kinetic data available for chemical engineers. Many recipes have been optimized, and they offer other practical advantages: equipment are in general simple and affordable, process parameters are easily tunable, and continuous processes allow industrial scale-up…From the academic point of view, research proved that intriguing interfacial reactivity at the nanoscale could produce original nanostructures of high interest in catalysis. Of course, a compromise must be found between industrial and environmental costs on the one hand and the performances on the other, but in any case liquid-phase syntheses are versatile enough to support both academic discoveries and industrial developments of nanocatalysts.

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2

Supported Gold Nanoparticles Leading to Green Chemistry

Tamao Ishida¹ and Masatake Haruta¹,²

¹Tokyo Metropolitan University, Graduate School of Urban Environmental Sciences, Research Center for Gold Chemistry, 1-1 Minami-osawa, Hachioji, Tokyo 192–0397, Japan

²Dalian Institute of Chemical Physics, Gold Catalysis Research Center, 457 Zhongshan Road, Dalian 116023, P.R. China

2.1 Introduction

Since the discovery of catalysis by zero-valent gold nanoparticles (Au NPs) in 1987 [1] and by supported Au(III) in 1988 [2], Au has attracted many researchers in the field of catalysis. Research on catalysis by Au started with gas-phase reactions: Supported Au NPs on reducible metal oxides exhibited extremely high catalytic activity for CO oxidation even at −70 °C [1,3], and supported Au(III) on carbon-catalyzed hydrochlorination of acetylene to vinyl chloride [2,4].

Application of heterogeneous Au catalysts into fine chemical syntheses has also been rapidly developed involving selective oxidations and hydrogenations. Similar to gas-phase reactions, Au-catalyzed reactions in liquid phase are influenced by the size of Au NPs, the nature of supports, and the oxidation states in some cases. Recent developments enable the preparation and deposition of Au clusters with diameters of smaller than 2 nm onto supports and discussion of size dependence of Au clusters. It has been found that there are different suitable size for certain reactions and that suitable size may also change by the substrate molecules. The kind of supports also significantly affects the reactions in terms of oxygen vacancy, acid/base properties, adsorption capability of substrates, and electronic interactions with Au.

Understanding of size and support effects will contribute to rational design of highly active and selective Au catalysts. This chapter focuses on the size dependence of Au particles and role of supports for oxidation and hydrogenation reactions with abundant reagents such as O2 and H2 in liquid phase.

2.2 Oxidation

Oxidation reaction is a key process in chemical industry and occupies about 30% of the total value production in industrial processes. Although some oxidation reactions are operated by using O2, stoichiometric oxidizing agents have still been used predominantly, in particular for fine chemical syntheses. Replacing these oxidizing agents with O2 or air is highly desired to minimize wastes. At the same time, high selectivity should be achieved when the substrates have more than one functional groups that can be oxidized. Since gold catalysts generally have advantages over Pd and Pt catalysts in terms of higher product selectivity, control of the particle size and understanding the effect of support or stabilizers are great concerns for designing high-performance Au catalysts.

2.2.1 Oxidation of Alcohols

2.2.1.1 Size Dependence

Rossi and coworkers first reported the size dependence on the catalytic activity of naked colloidal Au without protecting agents for aerobic oxidation of glucose (Scheme 2.1) in the range of 3–10 nm in order to exclude the effect of supports and stabilizers [5]. Au NPs showed catalytic activity when its size became smaller than 5 nm, and the reaction rate increased with a decrease in the size of Au. However, the reaction rate per surface Au atom, turnover frequency (TOF), did not change with its size. They concluded that the apparent increased reaction rates were due to an increase in the number of surface Au atoms.

A chemical reaction depicting aerobic oxidation of glucose.

Scheme 2.1 Aerobic oxidation of glucose.

On the other hand, Tsukuda and coworkers studied the size dependence of poly(N-vinyl-2-pyrrolidone)-stabilized Au NPs and clusters (Au:PVP) on the aerobic oxidation of 4-hydroxybenzyl alcohol. They found that the TOF increased with a decrease in their size in the range of 1–6 nm (Figure 2.1) [6]. This result suggested that the catalytic activity of the surface Au atoms changed with the size. Experimental results, such as X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), and FT-IR spectroscopies using CO as a probe molecule, revealed that Au:PVP are negatively charged due to the electron donation from PVP [6c]. Theoretical studies also demonstrated that Au13 clusters are negatively charged by electron donation from the carbonyl oxygen of PVP units [7]. Anionic Au core is considered to be beneficial for the activation of O2 [8].

Size dependence on Au:PVP for the aerobic oxidation of 4-hydroxybenzyl alcohol.

Figure 2.1 Size dependence on Au:PVP for the aerobic oxidation of 4-hydroxybenzyl alcohol. (Adapted with permission from Ref. [6c]. Copyright 2009, American Chemical Society.)

Gold on carbonaceous materials is also studied to discuss size effects [9]. For glucose oxidation, the TOF increased with a decrease in the size of Au NPs in the range of 1.9–15 nm [9b]. Since glucose oxidation is performed under alkaline conditions (pH > 9), the contribution of basic properties of the support is not remarkable; thus, the TOF increased with a decrease in the size of Au regardless of the kind of metal oxide supports [10]. Smaller Au particles tend to show higher catalytic activity than larger ones in many reports [6,9,10], although different size dependence have also been reported; Au NPs with 7 nm in diameter showed higher activity than smaller or larger Au NPs on CeO2 and TiO2 [11].

2.2.1.2 Electronic State of Au and Ligand Effect

Gold nanoparticles have size distributions due to the aggregation of Au during deposition or calcination. Therefore, precisely size-controlled Au clusters have recently been focused to discuss the size dependence of Au clusters. Various kinds of atomically precise Au clusters protected by organic ligands, such as Au9(PPh3)8(NO3)3, Au11(PPh3)8Cl3, and Au25(SPh)18, have been used and deposited onto supports. It should be noticed that the above Au clusters have cationic characters due to the presence of counteranions or S—Au(I)—S bond; thus, electronic states of Au should be considered for the evaluation of catalytic properties of these Au clusters. In order to discuss the size dependence of bare Au particles, removal of the ligands is required. Phosphine and thiolate ligands can be removed by calcination in air or H2 typically in the range of 200–400 °C, although thermal treatment often causes aggregation of Au clusters.

Golovko and coworkers reported that Au9(PPh3)8(NO3)3 and Au101(PPh3)21Cl5 deposited on TiO2 showed low catalytic activity for aerobic oxidation of benzyl alcohol, but calcination in O2 or followed by H2 reduction significantly improved their activity, whereas the Au particle size became larger (2.5–5.7 nm) [12]. They concluded that calcination effect was not due to exposure of bare Au surface but due to changing chemical state of Au from cationic to metallic by removal of phosphine ligands. In addition, Au9(PPh3)8(NO3)3/TiO2 showed much lower catalytic activity than Au101(PPh3)21Cl5/TiO2 even after calcination, implying that nitrate anions poisoned the catalytic activity of Au.

Protected Au clusters have been sometimes confined in the nano- and mesoporous materials to be inhibited from aggregating. However, the aggregation could not be completely inhibited. Tuel and coworkers prepared Au25(p-aminobenzenethiolate)17 deposited on zeolite (SBA-15) and tested them for benzyl alcohol oxidation [13]. They reported that the catalytic activity significantly increased with calcination temperature due to the removal of thiolate ligands from Au, which coincided with the reduction of S–Au(I)–S to metallic Au and an increase in the size of Au NPs. Xie and coworkers reported the oxidative removal of alkyl thiolate ligands from Au clusters as sulfonic acid by treating with t-butyl hydroperoxide (TBHP) at 50 °C [14]. The thiolate ligands could be almost and completely removed after 36 and 48 h, respectively, preserving the Au cluster size (1.2–1.5 nm) on hydroxyapatite (HAP). The catalytic activity of thiolate-protected Au clusters was negligible. Even though most of thiolates were removed by TBHP treatment for 36 h, the catalyst showed inferior activity to Au/HAP treated with TBHP for 48 h, suggesting that very small amount of thiolate remained significantly affect the catalytic activity of Au clusters.

2.2.1.3 Catalytic Activity and Selectivity Tunable by Support Materials

Oxidation of alcohols is initiated by the deprotonation of hydroxy group of alcohols to form alkoxide. Since carbon and polymer supports generally lack surface functionalities in terms of acid/base properties, Au/C and Au/polymers have required base for the alcohol oxidation. However, Au on metal–organic frameworks (MOFs) [15] and modified carbonaceous materials [16] have recently proved to catalyze alcohol oxidation even in the absence of base.

Tang and coworkers proposed that the aryl rings in the MOF affected Au clusters electronically to form anionic Au, which accounts for the activation of O2 and high catalytic activity [15c]. van der