CHAPTER 1: Introduction: Synthesis and Catalysis on Metal Nanoparticles

Heterogeneous catalysis is critical for chemical and energy transformations. It has played a cornerstone role in the chemical industry for more than one century. Many industrial catalysts were developed on the basis of trial-and-error. An industrial catalyst is typically a combination of a few or more components. From a materials science point of view, an industrial catalyst is very heterogeneous in terms of composition, structure, size, shape, and dispersion of catalyst particles on their support. In addition, a chemical reaction with a heterogeneous catalyst is performed on the surface of a catalyst particle at a high temperature while the catalyst particle is in a gaseous environment or in a liquid. An oxidizing and/or reducing reactant very likely restructures its surface and/or subsurface and/or bulk before a stable catalytic performance is obtained. In many cases, the chemistry and structure of a catalyst particle during a catalytic reaction could be different from those before catalysis. They could be very different from those of a catalyst under ex situ conditions after catalysis. Here, the ex situ condition is defined as a status at which the catalyst is at room temperature and all reactant gases are purged. Due to these potential differences between in situ and ex situ conditions and the heterogeneity of an industrial catalyst in chemistry and structure, understanding heterogeneous catalysis at a molecular level has been quite challenging. Despite the complications with catalyst materials and their reaction pathways, heterogeneous catalysts have played a cornerstone role in chemical and energy transformations, and heterogeneous catalysis has been one of the most important fields since the beginning of the last century.

The measured catalytic performance (activity, selectivity, and stability) is the outcome of many structural and chemical factors that interact with each other. However, a catalytic event is performed on a catalytic site which has a specific geometrical packing of catalyst atoms to give a suitable electronic structure for an appropriate molecular or dissociative adsorption with a subsequent coupling to form a product. Essentially, a catalytic event is determined by a molecular reaction with a catalytic site at a microscopic level. The correlation between the macroscopic catalytic performance and microcosmic “picture” of catalytic sites is lacking due to the heterogeneity of an industrial catalyst. Experimental and theoretical simulations of industrial catalysis with single crystal model catalysts have been an important approach to understanding how a specific site on the surface of a catalyst particle participates in a catalytic reaction at a solid (a catalyst)–gas or liquid (reactants) interface. This approach has provided a tremendous amount of information on catalytic reactions from a surface science point of view. It has been the cornerstone for understanding heterogeneous catalysis at a molecular level. However, due to the limited variability of structure, composition, and size, there exists a gap in materials between a well-defined single crystal model catalyst and an industrial catalyst with heterogeneous structure and chemistry.

In the last two decades, the significant advance in nanoscience and nanotechnology was partially driven by both the quantum effect of semiconductor nanoparticles with different sizes and plasmonic effect of some noble metal nanoparticles with different sizes. Accompanying this, spectacular achievements have been made in syntheses toward controlling the size, shape, or composition of nanomaterials. These achievements in the materials synthesis of nanomaterials, particularly the control of size, shape, structure, and composition of metal or bimetallic nanoparticles, have offered the possibility to bridge the gap between materials in the studies of heterogeneous catalysis. Colloidal chemistry allows tuning of the size, shape, structure, and composition readily. As metal atoms on different crystallographic faces of a metal nanoparticle pack differently, tuning the shape of metal nanoparticles to expose different crystallographic faces can offer different types of catalytic sites. For example, Pt atoms on a Pt nanocube pack into a (100) surface of a fcc lattice. An octahedral nanoparticle only exposes its (111) face. Both (100) and (111) faces can be found on a cubo-octahedral nanoparticle. A concave nanocube, in fact, offers a stepped surface with a high density of under-coordinated catalyst atoms. The capability of tuning size with synthesis provides a method to distinguish the sites of under-coordinated catalyst atoms at corners and edges from the site on the surface since there is a size-dependent density of these under-coordinated atoms at corners and/or edges.

Chapter 2 describes structural factors of catalyst nanoparticles, which could influence catalytic performances including catalytic activity and selectivity. In addition, it reviews correlations between these structural factors and catalytic performance, and discusses the potential restructuring of the surfaces of catalyst particles. A few examples are discussed from a structural point of view.

Chapter 3 reviews a few synthetic routes of metal nanoparticle catalysts. It presents three methods for the preparation of metal nanoparticles supported on a substrate including photo-assisted deposition methods using single-site photocatalysts, a microwave-assisted deposition method, and deposition of size-controlled metal nanoparticles as colloidal precursors. In addition, the syntheses of multi-functional catalysts are discussed.

In Chapter 4, syntheses of noble metal nanoparticles are reviewed from an organometallic point of view. It emphasizes the nature of the ligand stabilizing the nanocatalysts in solution and the role of the support for a supported catalyst. Syntheses of Pt, Rh, Ru, Ir and other metals are exemplified. Factors influencing the synthesis of metal nanoparticles and their performance in catalysis are discussed in detail.

After the review of syntheses in Chapters 2–4, Chapter 5 discusses the catalytic transfer hydrogenation of organic compounds. It focuses on the utilization of nanoparticle catalysts of the earth-abundant metal, Ni, a replacement for noble metal catalysts in hydrogen-transfer reductions of functional groups.

Chapter 6 reports the recent progress achieved in nanocatalysis, particularly use of quaternary ammonium salts as water-soluble capping agents of rhodium nanoparticles. Hydrogenation under biphasic liquid–liquid conditions and asymmetric catalysis including ethylpyruvate hydrogenation and prochiral arene hydrogenation are described.

As well as hydrogenations on Ni and Rh nanoparticles in Chapters 5 and 6, the catalysis of C–C coupling on Pd nanoparticles is reviewed in Chapter 7. The importance of Pd-catalyzed C–C coupling pioneered by Heck, Suzuki and Negishi was recognized by the 2010 Nobel prize in chemistry. Thanks to the advance in the synthesis of metallic Pd nanocatalysts with specific size and shape, efficient catalysis on Pd nanoparticles in contrast to traditional small molecules was revealed. Due to the limited space, only C–C coupling on selected examples are discussed in Chapter 7, though a large number of applications of Pd nanoparticles in C–C coupling have been reported in the literature.

Gold nanoparticles supported on oxide substrates exhibit exciting catalytic activities for many inorganic and organic reactions in contrast to the inert nature of macroscopic gold particles. Tremendous effort has been put into exploration of the origin of catalytic activity of Au nanoparticles following the discovery of activity for many reactions on them. Numerous papers on this topic have been published in the literature. In these reports, most of the Au catalyst particles were supported on an oxide. Other than metal oxides, salts have been used as supports for Au nanoparticles. Some salts are solid acids, which are used in acid catalysis. In fact, integration of the acidic support of a salt with gold nanoparticles forms a bi-functional catalyst that is highly active in organic reactions. Chapter 8 reviews the preparation and catalysis of salt-based Au catalysts. These salts in the Au/salt catalysts are carbonate, phosphate, hydroxyapatite, hydroxylated fluoride, metal sulfate, and heteropolyacid. A comparison between the roles of salts and oxides in gold catalysis is made.

All the metal nanoparticle catalysts discussed so far are metal nanoparticles with a solid core. From a structural point of view, a nanoparticle with a hollow or porous core could provide different catalytic activity. A porous metal nanoparticle consist of an external surface and an internal surface. Thus, the total surface area is significantly increased due to the existence of the internal surface in the hollow or porous core of the particle. More importantly, the density of catalyst atoms with a low coordination number in such a metal particle is in fact much higher than that of a metal nanoparticle with a solid core. In addition, its inner surface could be covered with much less capping molecules due to the limited space in a pore. Thus, the density of active sites could be significantly larger. In addition, the limited space at a scale of nanometres or a sub-nanometre, in fact increases the rate of collision of reactant molecules with catalyst atoms. These structural features could offer porous metal nanoparticles different catalytic activity and selectivity in contrast to a metal nanoparticle with a solid core. Chapter 9 presents the synthesis of porous metal nanocatalysts and discusses the catalysis of the nanoparticles.

As discussed in Chapter 9, the local structure of a catalyst nanoparticle is critical for its catalytic activity. Other than the localized structure of a metal nanoparticle, such as the surface of a catalyst particle with a solid core and the surface and inner surface of a porous nanoparticle, the external environment of a particle could influence catalytic shape to some extent. A metal particle together with its surrounding layers or shell can be considered as a single reactor at the nanoscale. The surroundings could be a scaffold of a dendrimer around the loaded metal nanoparticles, a cross-linked linear polymer of microgels, a polymer with a hydrophobic or hydrophilic substrate around a metal particle, or a shell of a metal particle. Chapter 10 describes several types of nanoreactors and discusses the effect of the surroundings on the catalysis of metal nanoparticles.

Other than these catalytic reactions, metal nanoparticles can also catalyze clock reactions. Chapter 11 reviews the clock reactions on metal and oxide particles, and discusses the mechanisms.

Computational chemistry is significant for a mechanistic understanding of chemical reactions. The application of a computational approach to catalysis studies is certainly the most successful synergy in chemical sciences. A tremendous effort has been made in this field in recent decades and spectacular achievements have been obtained. Theoretical studies are able to not only rationalize experimental findings and provide insights into reaction pathways and catalytic performance, they are also able to offer guidance for the design of new catalysts. Computation-aided screening of catalysts can largely narrow the coverage of potential composition and structure and accelerate the design and optimization of catalysts. Chapter 12 briefly introduces computational methods and presents examples to demonstrate how computational approaches were used to determine reaction pathways that could not be tracked with current experimental techniques. This chapter also reviews applications of the computational approach in studying catalytic reactions on metal nanoparticles supported on reducible oxides and demonstrates the essential role of the metal/oxide interface in promoting the catalytic performance of metal nanocatalysts.

Catalysis on manganese oxide octahedral molecular sieves exhibits high catalytic activity in many catalytic reactions for chemical and energy transformations. Chapter 13 reviews the synthesis of these molecular sieves and discusses catalytic reactions including selective oxidation and fine chemical synthesis, C–H activation and CO₂ activation, environmental remediation, and green chemistry.