Nanoparticles in Catalysis

Supported gold nanoparticles (Au NPs) exhibit unique catalytic properties for the oxidation of organic compounds. The catalytic activities and the selectivities of the supported Au catalysts largely depend on the kind of support and the particle size of Au. For oxidation of alcohols to aldehydes and ketones, the reducibility of metal oxide (MO_x) supports plays a prominent role, while the basicity of the supports or the size of Au particles are more important factors for non-reducible MO_x, non-oxides, and other supports. The size effect is more pronounced for dehydrogenation than aerobic oxidation because dehydrogenation takes place mainly on the low-coordinated edge and corner Au atoms. Oxidation of alkenes to epoxides using O₂ as a sole oxidant has been achieved by supported Au clusters having a diameter of 2 nm or less. For oxidation of cyclohexane using O₂ as a sole oxidant, the presence of Brønsted acid sites contributes to the production of K/A oil. The size of Au particles also largely affects the reaction rate and product selectivity; sub-nanometer Au clusters exhibited significantly higher catalytic activity and K/A oil selectivity than Au NPs.

Reduction and oxidation reactions (redox reactions) are fundamental and important transformation of chemicals in both laboratory and industrial chemistries. With regard to atom economy and the environmental demands, an ultimate goal of these reactions is to employ molecular hydrogen (H₂) or molecular oxygen (O₂). High-performance heterogeneous catalysts with high activity, selectivity, recoverability, and reusability are ideal for the development of green sustainable processes using H₂ or O₂. Moreover, the heterogeneous catalyst systems are the promising approach to solve the disadvantage of homogeneous ones, such as short lifetimes (low stability), risk of contaminating products with metals (low recoverability), tedious workups for reuse (low reusability), and so on. For the design of high-performance heterogeneous catalysts under liquid-phase redox reactions, metal nanoparticles (NPs) is the most promising strategy because of their unusual properties compared to bulk metal. This review provides an overview of metal NP heterogeneous catalysts developed for redox reactions using H₂ or O₂. The state-of-the-art metal NP catalysts show higher activity and selectivity for the chemoselective hydrogenations of carbonyl, nitro, and alkynyl compounds while retaining C=C bonds, and the aerobic oxidation of alcohols and the Wacker type oxidation of alkenes, which overcome the limitations of the conventional catalyst systems. This improved catalytic performance is due to significant advances in the precise fabrication of nanoscale metals, which has made it possible to explore novel catalysis and design metal active centers.

This chapter is an overview focusing on the preparation and use of transition metal-containing nanoparticles (NPs) described in the literature over the past decade or so. It is organized according to the metal, including NPs that feature catalysis based on Pd, Ni, Pt, Cu, Au, Rh, Ru, Co, and Fe. Nanoparticles that involve metals on various supports are discussed, as are those derived solely from precursor metals salts. Experimental procedures from these reports detailing both the preparation and use of several of these NPs are also contained herein. Exciting developments associated with mixed metal NPs and their applications that highlight synergistic effects of synthetic value offer a glimpse of what is likely to be an increasingly important direction for catalysis in the near future.

This review provides a summary of the main concepts employed to prepare nanoparticles (NPs) and clusters encapsulated by dendrimers, their catalytic applications, and interesting research and trends in this field. Dendrimers not only serve as synthesis templates for the preparation of NPs but also stabilize the NPs against leaching and aggregation, which makes it possible to tune the functionality of the peripheral groups and bridge the gap between homogeneous and heterogeneous catalyses. In particular, π-conjugated dendrimers, referred to as dendritic poly(phenylazomethine)s (DPA), are regarded as reactors for synthesizing sub-nanoscale clusters with atomic-level precision, even using different elements. This new synthesis technology has significant potentials for enabling enhanced activity, selectivity, and stability of catalysts.

This chapter reviews the state of the art in the use of nanoparticles in cross-coupling catalysis, with a focus on C—H activation chemistry. Current understanding of the mechanistic role of heterogeneous catalyst sources as applied to classic cross-coupling process is discussed. Understanding of the factors that influence speciation of catalysts under conditions commonly employed in C–H activation/functionalisation chemistry is highlighted. An overview of the tests that an experimentalist can perform to gain insights on whether a reaction is heterogeneously or homogeneously catalysed is given, along with their respective caveats. Six detailed case studies are presented, aimed at introducing the reader to the breadth and variety of recent developments of both synthetic utility and mechanistic understanding of nanoparticles applied to the field of C—H activation/functionalisation chemistry.

By taking advantage of the high catalytic activity and high turnover frequency (TOF) of heterogeneous metal nanoparticle catalysts, continuous-flow systems, in which introduced reactants are converted into the desired product in high yield, can be realized. These continuous-flow reactors possess high compatibility with sequential continuous-flow systems, which enable multistep flow synthesis of biologically active compounds such as active pharmaceutical ingredients (APIs) and natural products.

This chapter will review the currently available strategies for interfacing transition metal nanoparticles with enzymes and other more complex biological systems, as well as the applications of such biometal hybrids in the areas of catalysis, energy production, environmental remediation, and medicine. In the first part of this chapter, the focus will be on the many nanometal-enzyme hybrids that have been developed for applications in organic synthesis. Within the field of organic chemistry, nanometal-enzyme hybrids are often used as bifunctional catalysts to mediate different multistep transformations, as for example the dynamic kinetic resolution of alcohols and amines. The second part of this chapter will offer an overview of nanometal-enzyme hybrids that are used as bioelectrodes in biofuel cells. This area of research has grown significantly during the past decades, much because of the many potential future applications of such devices for medical purposes. Here, nanometal-enzyme hybrid based biofuel cells hold particular promise for biosensing applications, as well as for replacing battery-based solutions in actuator devices such as mechanical valves and pacemakers. In the final part of this chapter, the different strategies to use bacteria to synthesize metal nanoparticles will be reviewed. As will be shown by the many examples in this part, biologically synthesized and supported transition metal nanoparticles constitute interesting catalytic systems that could for example be used for energy production, pollutant degradation, and small molecule synthesis.

Metal nanoparticles modified by chiral ligands or chiral polymers, called as “chiral metal nanoparticles,” are promising catalysts for asymmetric organic transformations. In this chapter, the class of chiral modifiers is focused to overview advance in the field of metal nanoparticle-catalyzed asymmetric reactions.