the selectivity of a reaction that potentially avoids the unwanted side reactions. Drivers for development of advanced catalysts include (i) production of high value products with inexpensive raw materials, (ii) energy efficient and environmentally benign chemical conversion processes, (iii) increasingly stringent environmental regulations, and (iv) low-cost catalysts such as with reduction or replacement of precious metals [1].
Catalysis occupies an important place in chemistry, where it develops in three directions, i.e., heterogeneous, homogeneous and enzymatic. Both the homogeneous and heterogeneous catalysis has its own merits and demerits due to which there is urgent need of a new catalytic system, which should be active like homogeneous catalysis, and should also be easily recoverable like heterogeneous catalyst. 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 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.
Introducing a catalyst increases the speed of a reaction in one of three ways;
It can lower the activation energy for the reaction,
Act as a facilitator and bring the reactive species together more effectively, or
Create a higher yield of one species when two or more products are formed.
3.2 Nanocatalysts
Nanomaterials are more effective than conventional catalysts for two reasons. First, their extremely small size (typically 10–80 nm) yields a tremendous surface area-to-volume ratio. Also, when materials are fabricated on the nanoscale, they achieve properties not found within their macroscopic counterparts. Both of these reasons account for the versatility and effectiveness of nanocatalyst.
Since the end of the 1990s, with the development of Nanoscience, nanocatalysis has emerged as a domain at the interface between homogeneous and heterogeneous catalysis, which offers unique solutions to answer the demanding conditions for catalyst improvement [2, 3]. 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 homogeneous and heterogeneous catalysts, namely high efficiency and selectivity, stability and easy recovery or recycling. Specific reactivity can be anticipated due to the nano dimension that can afford specific properties which cannot be achieved with regular, non-nano materials. Nano-catalytic system allows rapid, selective chemical transformations with excellent product yield coupled with the ease of catalyst separation and recovery. Because of its nano size (high surface area) the contact between reactants and catalyst increases dramatically. Insolubility in the reaction solvent makes the catalyst heterogeneous and hence can be separated out easily from the reaction mixture.
Nanoparticles are recognized as the most important industrial catalyst and have wider application ranging from chemical manufacturing to energy conversion and storage. Its variable and particle-specific catalytic activity is due to their heterogeneity and their individual differences in size and shape. The fine tuning of nanocatalyst, in terms of composition (bimetallic, core-shell type or use of supports), shape and size has accomplished greater selectivity. Thus, the question here is how the physical properties of nanoparticles affect their catalytic properties, and how fabrication parameters can in turn affect those physical properties. By a better understanding of these, a scientist can design nanocatalysts, which are highly active, highly selective, and highly resilient. All these advantages will enable industrial chemical reactions to become more resource efficient, consume less energy, and produce less waste, which also helps to counter the environmental impact caused by our reliance on chemical processes.
3.2.1 Concept of Nanocatalysis
The concept behind nanocatalysis may be understood by considering the impact of the intrinsic properties of nanomaterials (Figure 3.1) that have a vital impact on their catalytic activity and may be categorized as:
1 (i) Quantities that are directly related to bond length, such as the mean lattice constant, atomic density, and binding energy. Lattice contraction in a nano-solid induces densification and surface relaxation.
2 (ii) Quantities that depend on the cohesive energy per discrete atom, such as self-organization growth; thermal stability; coulomb blockade; critical temperature for phase transitions, evaporation in a nano-solid; and the activation energy for atomic dislocation, diffusion, and chemical reactions.
3 (iii) 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, photo-absorption, and photoemission.
4 (iv) 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 nano-solid, as well as the magnetic performance of a ferromagnetic nano-solid.
Figure 3.1 Diagram of effect of intrinsic property on nanocatalyst. (Source: https://www.slideshare.net/foolishcrack/nanocatalyst-108342252) [55].
By precisely controlling the size, shape, spatial distribution, surface composition and electronic structure, and thermal and chemical stability of the individual nano components, they can be widely used in catalysis with newer properties and activity.
3.2.2 Metallic Nanoparticles (NP) as Catalyst
About two-thirds of chemical elements are metals. Using the molecular orbital description, 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. The most important properties of a metal are:
Its ability to transport electrons, i.e., the property of conductivity: The conductivity is based on 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.
Magnetism: 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 (for example, the well-known ferromagnetism of iron, cobalt and nickel); while non-oriented free spins produce paramagnetic materials (for example, copper and gold).
Metallic NPs, also called nanoclusters, are pieces of metal at the nanometer scale. They can be nanocrystalline, 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. (As a result,
