**2. Advantages of encapsulated MNPs**

In view of an increasing range of recent research, implementing an encapsulated structure for nano-catalysts is the efficient way to protect these catalysts' properties from any deactivation agents and prevent them from the decrease of catalytic activity. The advantages of encapsulated structure can be expressed as follows and in going into detail their effective specifications and involved parameters in this structure will be explained.


One of the most crucial characteristics of the encapsulated nano-catalysts is the strong interaction between the MNPs and the encapsulating materials. Interactions between MNPs and support materials, metal-support interaction (MSI), can profoundly enhance the catalytic activity and tunability to selective reactions and products [5]. MSI can modify electronic properties, geometric morphologies, or chemical compositions of MNPs to make active sites have specific properties and catalytic activities [6, 7]. The geometric result of MSI is the decoration of the metal surface, partial coverage or total encapsulation of the metal can be conducted by the support. Since morphological modifications can take place on the catalyst as a result of the MSIs and encapsulated structure paves the way to enhance them by contributing a maximal interfacial area through participating the MNPs in close contact with the encapsulating materials [6–8]. Furthermore, strong metal-support interaction (SMSI) has been mostly applied to develop bifunctionality at the interface of metal–metal oxides systems. The bifunctional outcome leads to a synergistic effect through an improvement of catalyst activity and selectivity by creating new reaction sites at the interface between the metal and the support, and the spillover phenomenon occurs at the interface by transferring reactants from metal or support to the interface, which all of these phenomena are soared at encapsulated structure [6–8]. Therefore, the optimized catalytic performance can be gained by the unique MSI among the MNPs and the encapsulating materials [5].

Metal nano-catalysts have a high surface-to-volume ratio, which leads to coalescence during catalysis, especially at high reaction temperatures where this is prone to modification, resulting in a noticeable decrease in active surface areas and, eventually, a reduction in catalytic activity and selectivity [2]. In contrast, encapsulation by encasing of MNPs in nano-shells or nanopores prohibits them from coalescence, therefore, the efficiency of the active nano-catalysts surface area is preserved remarkably.

Catalyst deactivation is a principal difficulty in the heterogenous catalytic processes since the catalyst activity and selectivity will be reduced with time on stream and the catalyst regeneration or replacement will be included at a noticeable rate of time and resources [8, 9]. Therefore, a great number of researches have been conducted to gain long-term stability in heterogeneous catalysis, particularly by applying an optimized structure that includes the maximized performance. In the midst of the principal reasons of catalyst deactivation, sintering of MNPs, which occurs by an irreversible mechanism that significantly reduces catalyst recyclability, has attracted more attention toward preventing it from happening. Catalyst sintering reduces the metal's active surface area and can occur via particle coalescence or Ostwald ripening [10, 11]. Not only may the MNPs engage in migration and coalescence particularly when they are exposed to severe conditions, but also the deactivation of catalysts via sintering can be intensified. Nevertheless, by encapsulating MNPs in a highly nanopores support, this spatial confinement can dramatically suppress their migration and coalescence and can prevent sintering without limiting the catalysis and thus stabilize MNPs under harsh reaction conditions [2, 8–11].

Chemical processes are conducted by multiple steps. To minimize energy consumption and economic cost, integrating multiple steps of reactions can be a crucial solution. A tandem catalysis reaction engages sequential reactions within one condition through the coupling of appropriate catalysts in which sequential transformation of the substrate is conducted via two or more mechanistically distinct reaction steps [12–14]. The encapsulated structure of MNPs provides an integration of various interfaces in one nanostructure in a controllable manner that converts them as effective and principal catalysts in the tandem catalysis reactions. Moreover, encapsulating MNPs in porous shells could promote tandem reactions via modifying the reaction sequences or the molecular-sieving effect [2, 15].

Due to preventing the formation of unwanted products, increasing the waste of chemicals, reducing the essential refinement stages, and optimizing the selective products, it is critical to control the chemical reactions. The heart of a chemical reaction is its catalysts so it is crucial to design an appropriate catalyst to enhance the selectivity. The activity and particularly selectivity of heterogeneous catalysts depend on their surfaces structure and their active sites. First of all, the encapsulated structures in various catalysts support by providing their unique morphology and can act as sieves for molecules that allow the transfer of molecules with selective sizes, resulting in high shape selectivity. In addition, catalytic selectivity of the encapsulated nano-catalysts can be improved by utilizing suitable functional groups in their materials that have a significant influence on the reactant adsorption and reduce the mass transfer limitations by increasing their coefficient diffusions. In addition to the controlled porosity of the encapsulated structures, owing to high active surface areas of the encapsulating materials, there are more sites to involve the suitable functional groups to enhance the selectivity of the catalysts [12–18].

**Figure 1.**

*Encapsulation of MNPs as catalyst.*

Although each one of mentioned nano-catalysts encapsulated structure benefits are effective separately, there is a remarkable relationship among their impacts. Indeed, each of them causes the other one or implements it in parallel and intensifies each other.

In contrast, although the encapsulation structures can enhance the catalytic performances, mass transfer may be restricted by this structure to some extent, which is disadvantageous to the catalytic process. This issue should be modified by the precise sketch through the choice of materials and methods synthesis of the catalysts with encapsulation structures [1]. In this chapter, the classification of the encapsulation of catalysts is undertaken with the type of encapsulating materials because some of them can form into two distinct groups of morphologies or exhibit with an individual structure, such as organic materials. Thus, in follow firstly describe each type of morphology and then the encapsulation of catalysts is presented in three comprehensive parts from the view of encapsulating materials: (1) Inorganic Materials, (2) Metal–Organic Frameworks (MOFs), and (3) Organic Materials as it is depicted in **Figure 1**.
