Encapsulation of Metal Nanoparticles (MNPs) as Catalyst

Abstract

Metal nanoparticles (MNPs) are the main agents in heterogeneous catalysis. Hence, utilizing the effective physico-chemical methods to engage them to achieve the highest catalysts performance with well-controlled size, shape, and surface properties seems to be essential. The encapsulation of metal nanoparticles is a promising approach that enhances the catalytic activity of the materials. Not only the encapsulating structures can adjust the catalytic properties of metal nanoparticles, particularly selectivity, but also prevents them from agglomeration and sintering. In this chapter, the various encapsulating structures consist of yolk/core-shell and mesoporous structures, and encapsulating materials that are divided into three parts, including inorganic materials, metal–organic frameworks, and organic materials are presented.

1. Introduction

Catalysts have a significant impact on contemporary chemical processes. The role of nano-catalysts in the form of metal nanoparticles (MNPs), such as nanoparticles of transition metals and particularly the group VIII metal elements in the fourth period with d-band center electronic structure, is rising significantly in the heterogeneous catalysis, due to their high active surface areas and special electronic properties [1, 2]. For instance, Fe-, Co-, Ni-, Pt-, Rh-, and Pd-based catalysts have been extensively utilized in various processes to produce varieties of fuels and chemicals, consisting of Fischer Tropsch Synthesis, steam and dry reforming, methanation, and CO oxidation [1]. MNPs expose influential activities in various catalytic reactions. One of the main difficulties in using the MNPs is their weak stability in the practical catalytic processes. In recent years, an exquisite and effective strategy has been applied to optimize the performance of MNPs by encapsulating them that provides unique advantages toward catalysis, particularly under harsh conditions by restricting the aggregation and sintering of MNPs. The concept of the encapsulated metals nanoparticles that improve a wide range of catalytic reactions, especially under rough environments has been presented as “chainmail nano-catalyst” in some of the literatures [3].

In addition, Encapsulation adjusted the energy distinctions between the highest occupied molecular orbital to the lowest unoccupied molecular orbital [4]. In fact, inside the encapsulated structure of MNPs, the electron of MNPs can penetrate through the shell to enhance the catalytic performance on the external surface. Therefore, the shell is able to restrict the medium of reactants and products from direct contact by the MNPs and protect the MNPs from damage in harsh conditions. Overall, these encapsulated MNPs represent impressive catalysts that can be applied in numerous catalytic reactions under harsh conditions and also propose a tunable structure alongside unique electronic properties [3].

In this chapter, a comprehensive and compressed presentation of the encapsulated MNPs advantages, a categorization of the encapsulating layers, and some methods of encapsulated nano-catalysts preparation are reported, respectively.

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.

1. Provide effective metal-support interactions (MSI)

2. Prevent from the coalescence nanoparticles

3. Protect from catalyst deactivation by stabilizing the active metal species in catalysts against sintering

4. Tandem reactions can be enabled and promoted by encapsulated configuration

5. Improvement of catalytic selectivity

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, 7, 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, 7, 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, 9, 10, 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, 13, 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, 13, 14, 15, 16, 17, 18].

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.

Figure 1.

Encapsulation of MNPs as catalyst.

3. Encapsulating structures

Encapsulation provides a unique catalyst immobilization technique that by encapsulating of MNPs with porous layers creates an impressive porosity for the reactants to be able to reach the metal surface. Various strategies and materials have been utilized to make these layers. An encapsulated structure can conduct by designing and fabricating the unique chainmail catalyst via a wide range of shells [3]. As a result, a vast majority of structural designs and strategies in such encapsulated catalysts have been thrived. Although a large range of various materials, such as metallic state of metals, alloys of metals, metal carbides, metal oxides, metal phosphides, and metal nitrides, can be encapsulated, a variety range of coating layers can be utilized for the shells that contain various forms, such as shells, tubes, sheaths, matrices, and films. In this chapter, the encapsulated catalysts structures on the basis of the morphology are categorized into two main groups: (1) Yolk/Core-Shells, and (2) Mesoporous Structures [1, 2, 3], as shown inFigure 1.

3.1 Yolk/core-shell structures

Encapsulation compels the complex of support and MNPs to implement a nonplanar geometry, which leads to a higher reactivity. The core-shell morphology is allocated to the encapsulated structure that encases nanoparticles in a confined case by an outer shell [1]. In contrast, another approach to encapsulate MNPs inside an individual architecture shell which is a state of eggs with a void space between the MNP “yolk” and the porous coating “shell,” this structure has added profits due to their specific role as nanoreactors [19]. Although Yolk-shell is a “core-void-shell” structure, which is similar to the core-shell structure, a void space between the core and the shell depicts the distinction between these two structures [1]. Recently, yolk-shell materials have attracted interest due to their particular combination of high thermal stability with monodisperse and narrow particle-size distributions that convert them to an effective catalyst for heterogeneous catalysis. Not only do these specifications have a significant role in catalyst performance, but also they can facilitate kinetic and mechanistic researches [20].

In these yolk/core-shell catalysts, the vast majority of the catalytic attributes of the throughout catalyst are the same and may include a tiny alteration through the change from active particle to active particle. Thus, in these structures the resulting catalytic activity of the system can be the same all over the surface of the catalysts, in comparison, in traditional solid catalysts there are no homogenous sites and to evaluate the activity of the catalysts, an average of all possible morphologies should be considered. Furthermore, the yolk/core-shell structures provide catalytic particles with a uniform size that leads to an effective SMSI for all the particles. They contribute to the new catalytic sites at the interface of themselves and MNPs and modify their actual catalytic characteristics of them. Therefore, these features cause these kinds of encapsulation structures to be placed at the noticeable position as support of MNPs catalysts and remarkable efforts have been done in synthesizing base-metal yolk/core-shell catalysts in recent years [1, 21]. In addition, there are some significant ways toward the improvement of the design and fabrication, in particular, yolk-shells to produce effective catalysts:

3.1.1 Enhance the porosity by etching

Although the encapsulation of nanoparticles by applying an additional coating layer may decline the adsorption of the reactants on the active phase, the porosity of the shells exhibits a crucial role in the improvement of mass transfer across the shells. Therefore, porosity is an essential specification that should be considered when synthesizing yolk/core-shell catalysts. Various methods to make porose yolk/core-shell, such as layer-by-layer deposition techniques and sacrificial templating procedures, in particular for yolk-shells, have been reported that provide adequate stability even under harsh reaction conditions. Moreover, the “surface-protected etching” is a contemporary strategy that outputs catalysts with porous shells and enhanced stability simultaneously. This process protects the oxide shells, in particular the inner part of the shells, by utilizing a suitable layer of the polymeric ligand as an etching agent. Thus, the oxide shells would have their original size and the selective etching of the interior creates the porous structure [22]. On the other hand, this procedure may have a noticeable restriction in which the metal loading of the catalyst in most cases is placed at a low level. To dispel this drawback, first of all, an inert chemical linker should be utilized to fix the MNPs onto the surface of the initial support with another layer of these inert chemical linkers. Next, the surface-protected etching procedure would be utilized to modify the outer shell into a mesoporous shell that exposes the MNPs to the reactant active species. Silica is one of the most popular inert chemical linkers [22, 23, 24, 25].

3.1.2 Surface-protected calcination

In most cases, the various yolk/core-shells that are prepared through the sol–gel deposition method are amorphous. Due to the remarkable effects of the crystalline phases on the performance of catalysts, it is essential to enhance shell crystallinity. The effective pathway to modify the amorphous shells to their crystallized counterparts is calcination at high temperatures. Although calcination would improve the crystalline phases of the shell, it might reduce the porosity of the original amorphous shell considerably. Therefore, to prevent this difficulty, first of all, it is important to conduct another inert chemical layer (such as silica layer) on the top of the shell through a sol–gel process, then the calcination treatment should be applied. Eventually, the final porous morphology that will be a yolk-shell structure can be achieved by scarifying the inert chemical layers through chemical etching [22, 23, 24, 25]. Figure 2 depicts the procedure of fabrication of a porous yolk-shell through employing sacrificed layer, calcination, and etching treatment. Two TEM images of a core-shell (a1) and yolk-shell (a2) are also illustrated in Figure 3 [26].

Figure 2.

Synthetic procedure of MNP@Void@M’O₂ (M’O₂: TiO₂, CeO₂, ZrO₂, …) Yolk-Shell in four steps; 1: Encapsulation with sacrificed shell (m-SiO₂) nanoparticles, 2: Encapsulation with exterior shell (M’O₂), 3: Calcination, and 4: Etching.

Figure 3.

TEM images of (a1) MNP@SiO₂ core-shell and (a2) MNP@Void@M’O₂ Yolk-Shell. Reprinted with permission from Ref. [26]. Copyright 2021 American Chemical Society.

3.2 Mesoporous structures

Mesoporous materials are special types of nanomaterials with ordered arrays of uniform nanochannels that are fabricated by participating self-assembly of surfactants and framework precursors. Mesoporous materials include pores with diameters in the range of 2–50 nm that by dispersion of MNPs into this porous matrix a range of heterogeneous catalysts can be formed [2]. These structures by supplying a great surface area provide a dramatic spatial dispersion of the MNPs that leads to stability enhancement in contrast to nanoparticle aggregation and coalescence in a catalytic process. In addition, the prominent porosity of these structures in catalysts paves the way to prepare an effective mass transfer that improves the catalytic performance by facilitating the contact of reactants with the MNPs as active sites of catalysts. Furthermore, the appropriate pore size distribution puts the MNPs adjacent to the mesoporous material, which leads to the enhancement of catalytic activity and stability through boosting the strong MSIs. The mesoporous materials have pores with an adequate size that cause the adsorption of pre-synthesized MNPs with small sizes directly [1, 2]. Meanwhile, the size and volume of the mesopores or in overall the size of the whole mesoporous matrix, and metal charge have a significant impact on the immobilization of metals [27]. Eventually, the specific morphology of the pores in these structures exposes the sites of active metals, which leads to facilitating the catalytic process [1, 2]. Thus, these features of mesoporous materials convert them to outstanding supports that can encapsulate the MNPs in catalysts.

Although the incipient wetness impregnation is a popular strategy for the encapsulation of MNPs in the mesoporous materials as heterogeneous catalysts, the self-assembly-based approach is the other useful strategy [2].

4. Encapsulating materials

4.1 Encapsulation of MNPs in inorganic materials

In recent decades, noticeable progress in synthesizing various inorganic nanomaterials to encapsulate the nanoparticles with unique catalytic performance, owing to their principal advantages, such as simplicity of fabrication, tunability of formation, and cost-effectiveness, converts them to popular supports. Although inorganic oxides (SiO₂, TiO₂, CeO₂, ZrO₂, etc.) are mostly applied as shell structures in the yolk/core-shell morphology, some of them, in particular silica, exist in mesoporous in some cases. In contrast, zeolite mesoporous structures are the common morphology beside the yolk/core-shell structure of them.

4.1.1 Silica

4.1.1.1 Silica nano-shells

Silica has attracted an increasing number of researcher attention owing to its inimitable physicochemical properties, such as tunable morphology, extensive surface area (≈1500 m² g⁻¹) and pore volume, adjustable sizes (50–150 nm), shapes (hexagonal, wormhole-like, cubic, and lamellar) and morphologies (spheres, helical fibers, tubules, gyroids, crystals), ease of surface functionalization (both interior as well as exterior), unique topology, colloidal and thermal stabilities, and high dispersity [27]. Silica with each of these desirable features can be achieved through tuning the synthesis conditions, such as the temperature, pH, stirring speed, and type of silica source, and particularly the surfactant [27]. Therefore, these unique attributes of silica besides the simplicity in controlling the SiO₂ precursors convert it to a principal and useful inorganic shell for the encapsulation of MNPs. Moreover, owing to SiO₂ inert chemical features, its structure would be resistant under harsh conditions that lead to the protection of the size of MNPs under this condition. Furthermore, not only would not the chemical inertness of silica that encapsulates MNPs is influenced by the metal-oxide interactions but also intensify their catalytic properties [1, 3]. Although silica supports due to their chemical inertness have a tiny interaction with reactants that may reduce the adsorption of the reactive molecules, their porosity can restrict the effects of their weakness by intensifying their diffusion across the capsule [1]. Moreover, stability of the metal-silica nanoparticles is an outstanding challenge, particularly in catalytic applications that deal with diverse physicochemical properties especially the features of the final catalyst compounds, such as the type of MNPs, particle size, degree of silica condensation, and chemical functionalization [27].

The principal method to form the silica shells is the sol–gel. The synthesis of the silica shell is conducted through a modified Stober procedure, in which the hydrolysis and condensation of tetra ethyl orthosilicate (TEOS) take place in aqueous ethanol in the presence of a base as a catalyst, such as ammonia, to control the growth of silicate on the surface of the MNPs [1, 2]. To coat some unstable metals, such as Ag, with silica, it is essential to utilize the amines, such as dimethylamine or di-ethylamine, instead of ammonia as the base catalyst [2]. Overall, as opposed to mesoporous silica materials to encapsulate the MNP in the shell spheres of mesopores silica, firstly the metal precursors should be dispersed and the surfactant alongside the silica precursor (TEOS) should be added. This core-shell strategy contributes the best results in encapsulating some metal oxides, such as Mn, Co, and Ni [27].

Although the general configuration of silica shells achieved through the sol–gel process is microporous, the mesopore textures could also be formed that are more appropriate for catalysis applications because this shape of texture rectifies the mass transfer limitations [1, 4]. The encapsulated metal-silica nanostructure gaining through the sol–gel pathway makes a thin shell of silica which owing to the high interfacial energy a fragile interaction exists between silica shell and MNPs. Nucleation of silica prevents from occurring a suitable interaction between silica and MNPs. However, to pave the way and achieve an improved interfacial interaction, utilizing some bridging agents as surface primers is essential [4]. Generally, the surface primers are applied for encapsulation of silica over large MNPs (>10 nm) that can refer to some of them, such as amino propyltrimethoxy silane (APS), methoxy poly (ethylene glycol) thiol (MPEG-SH), and polyvinylpyrrolidone (PVP). PVP is applied for the smooth coating of silica on the MNPs [1, 2]. Despite the fact that surface primers have a critical role to create a stable and homogeneous silica coating, in some cases, they can provide a selective silica coating on the surface [4, 28]. Furthermore, the type of the silica source and the added surfactants have a crucial impact on the size of MNPs and the thickness of the SiO₂ shell in yolk/core-shell structures. Although some surfactants, such as PVP, cetyltrimethylammonium bromide, and chloride (CTAB and CTAC), could not be influential on the dispersion of the MNPs, they could have an enhancement effect on the porosity of the SiO₂ shells [1, 4].

On the other hand, ultrasmall MNPs (<10 nm) are unstable and have an aggregate tendency in alcoholic solutions. As a result, the formation of a coating of silica cannot be implemented by an appropriate outcome. To get rid of this imperfection, the silica coating should be conducted in a reverse micelles (or microemulsion) system by using polyoxyethylene nonylphenyl ether (Igepal CO-520) as a surfactant [2]. Mostly, owing to there not adequate interaction between silica and the metal surface this technique of coating may have some weaknesses that may lead to an undesirable and imperfection coating. Liu et al. [29] presented an effective procedure, ship-in-a-bottle, to fabricate a robust thin silica coating on the sub-3 nm MNPs in reverse micelles. In this method, the synthesis of MNPs and silica coating in the presence of water/cyclohexane/reverse micelle system will participate simultaneously. The combination of microemulsion system and ship-in-a bottle technique pave the way to achieve a range of various metal-silica core-shell composites [2].

4.1.1.2 Mesoporous framework of silica

Mesoporous silica nanoparticles are presented as effective support for the encapsulation of MNPs due to their outstanding specifications, such as well-ordered framework, tunable pores, high surface area, stability, and thermally toughness. In addition, they contribute a well-made 3D matrix that provides a monotonous distribution of MNPs and confinement them from aggregating there that lead to noticeably protect against the sintering of them. Moreover, in contrast to the other inorganic supports through the encapsulation of MNPs, the mesoporous silica nanoparticles can represent a wide range of applicable characteristic surfaces in both the exterior (on the surface) and interior (in the mesopore) space, where MNPs can be organized by a chemical linkage or physically immobilized by electrostatic interactions [27].

The mesoporous silica nanoparticles can be synthesized through cooperative self-assembly of surfactant and silica species. The morphology and dimensions of mesoporous silica nanoparticles are significantly influenced by the factors of reaction kinetics of sol–gel chemistry, such as assembly kinetics, silica condensation, nucleation, growth rates, and surfactant-silica interactions, in addition to pH value of the reaction medium, water content, and temperature. Mobil composition of matter (MCM)-41, and the Santa Barbara amorphous type material (SBA)-15 are the famous mesoporous silica nanoparticles that are synthesized by applying quaternary ammonium salts and Pluronic copolymer-based surfactants, respectively [27].

There are two common techniques to encapsulate the MNPs in mesoporous silica materials. First of all, in the way of an incipient wetness impregnation method dealing with the encapsulation of MNPs, a solution of the metal salt is exposed to a powder of mesoporous silica including the same pore volume as the volume of the metal salt solution. Then, the MNPs are transferred from the metal salt solution into the mesopores by utilizing the capillary force. Finally, the implementation of the calcination and reduction in H₂ subsequently will pave the way to achieve a mesoporous silica matrix that encapsulates MNPs. In some cases of the incipient wetness impregnation procedures, electrostatic interactions are the main force to conduct the diffusion of MNPs through the meso-channels that can be implemented by modifying the mesopore surface by positively charged quaternary ammonium groups [2].

The other procedure is the participation of self-assembly of the MNPs and silica precursors with a surfactant-mediated condensation technique. Firstly, a dilution of the silica precursor is mixed with the aqueous ammonia including the CTAB surfactant molecules for initial nucleation and on the contrary of the core-shell silica architecture then the desired metal precursor is subsequently added to the mixture. Eventually, to achieve the desired mesoporous silica material, the surfactant can be eliminated by either calcination procedure at high temperatures (550°C) or a variety of chemical solutions, such as acidic ethanol or ammonia in ethanol/isopropanol. Although isopropanol, as a solvent for ammonium nitrate, can be utilized to extract the surfactant, it is able to save the well-order of the mesostructures and create a dramatic impact on the surface area and pore volume of mesoporous silica nanoparticles. Hence, not only there is not any essential tuning in the pore size or volume of the mesoporous silica nanoparticles, but also there is no noticeable change in the final particle size or pore sizes in the presence of the encapsulated MNPs in the siliceous frameworks that leads to uniform encapsulating of metals in mesoporous silica nanoparticles at the basic PH [2, 27].

4.1.2 Titania nano-shells

Among the non-silica coating materials, titania is the most popular metal oxide. Despite the fact that TiO₂ is utilized as a coating layer of MNPs in a wide range of catalysts, this combination has illustrated noticeable synergy in various catalytic reactions. The synthetic routes and precursors are the main effective agents to produce a specific architecture of yolk/core-shell. The encapsulation of MNPs in titania nano-shells is the outcome of the direct coating of TiO₂ on them. Similar to the method of silica coating, sol–gel procedure is the major process of titania coating. Thus, through this method by applying titanium alkoxides like tetraisopropoxide (TTIP) in a nonaqueous solution while the existence of water, the TiO₂ coating will be implemented. Due to titanium alkoxide hydrolyze in water immediately, applying chelating agents like acetylacetone, which is a chelating agent of titanium butoxide (TBOT), controlled hydrolysis can be provided that leads to fabricate a core-shell nanostructure with a stable coating of titania [2].

To develop stable yolk/core-shells of titania, utilizing appropriate precursors have a crucial role. As evidence, a chelated complex of titanium glycolate, which is formed by reacting titanium alkoxide with ethylene glycol and is much more stable than titanium alkoxide, can provide an outstanding precursor for the titania coating of MNPs that can undertake a very controllable sol–gel process to fabricate identical metal@TiO₂ core-shell nanoparticles that are catalyzed by acetone. Moreover, this technique is applicable for a controlled coating of titania on small MNPs with a diameter from tiny sizes to 50 nm [2, 4]. Furthermore, to achieve a uniform and thin shell of titania that can be adjusted with the thickness ranging from 3 to 12 nm in the sol–gel process, an acid catalyst like citric acid to hydrolyze the TBOT at the presence of alcohol can be applied [2].

Another suitable precursor that can slow down the hydrolyze in water, titanium (IV) bis (ammonium lactate) dihydroxide (TALH), can provide the titania coating with shell thickness ranging from sub-50 nm on MNPs. Although the TALH in aqueous solutions at room temperature is stable, it can be hydrolyzed at high temperatures (approximately 65°C) that lead to control of the sol–gel process [2].

Although Zeng et al. [30] fabricated a yolk-shell nanostructure of the Au@TiO₂ through the hydrolysis of TiF₄ at high temperatures (180°C), the most popular pathway to obtain a metal/TiO₂ yolk-shell nanostructure forms a coating layer of titania on metal/SiO₂ composites, which the silica will be sacrificed [2]. The principal benefit of this procedure of MNPs encapsulation with titania at the presence of sacrificial silica is the easy formation on the contrary of the direct formation of titania nano-shell. In addition, owing to the chemical sympathy between the two oxides to achieve a suitable coating layer of TiO₂, it is not essential to implement any rectification on the silica surface to set up the interactions between titanate species and silica. Despite this coating pathway being a successful process, to improve the coating of titania in this way, utilizing a surfactant such as hydroxypropyl cellulose (HPC) is presented that enhances the colloidal dispersion of silica nanoparticles [2].

Finally, similar to encapsulation of ultrasmall MNPs with silica nano-shells, titania coating will be conducted in the same way. First of all, in a reverse micelles system nanoparticles are formed and then a coating of silica on the nanoparticles at the presence of TEOS will be done and the last layer, which is titania, will be formed through applying TBOT. Eventually, the yolk-shell nano-capsule can be achieved by thermal reduction and etching of the silica templates [2].

4.1.3 CeO₂ nano-shells

Another metal oxide that attracted significant interest is cerium oxide. The presence of oxygen vacancies at the terminating of its planes is affected considerably on the adsorption of reactant molecules in the catalytic reactions by controlling the energetics of the surface interactions [31]. Despite the fact that the encapsulation of MNPs in CeO₂ nano-shells tackle the sintering of MNPs, it provides strong metal-support interactions (SMSI) that lead to enhancing the stability and the performance of the catalysts at the series of catalytic oxidation reactions, particularly at high temperatures [2].

The core-shell nanoparticles of metal@CeO₂ could be synthesized based on the self-assembly procedure by applying a supramolecular [2]. Gorte et al. [32] implemented this strategy by applying a capping ligand of a thiolate (11-mercaptoundecanoic acid, MUA) to fabricate Pd nanoparticles that are mixed in tetrahydrofuran (THF) [2]. The carboxylic groups of MUA conducted the self-assembly of the cerium (IV) alkoxides around the Pd nanoparticles directly by exchanging the alkoxy group on the Ce (IV) salt with the carboxylic group on the surface of the Pd nanoparticles as a result of the presence of the carboxylic group, which is a stronger ligand for Ce (IV) than the alkoxy group. Furthermore, various ranges of metal@CeO₂ core-shell nanoparticles can be fabricated by controlling and designing precisely the effective parameters of the self-assembly strategy and the sol–gel process, particularly by utilizing the appropriate chelating agent. So, some of these chelating agents and their role in the encapsulation of MNPs by CeO₂ yolk/core shells are presented as follows [2].

Ethylenediaminetetraacetic acid (EDTA) is applied as a chelating agent that chelates Ce (III) salt. Although EDTA slows down the hydrolysis of the Ce³⁺ ions, due to the negative charges of the EDTA-Ce (III) complex, the electrostatic interactions between the Ce complex precursor and MNPs have a significant role in the self-assembly strategy [2]. Moreover, triethanolamine as a chelating agent to achieve a cerium-atrane precursor has exhibited its effect on fabricating the desired metal@CeO₂ core-shell nanoparticles by adjusting the sol–gel kinetics. Another chelating agent for the CeO₂ coating on MNPs is citric acid. It puts its effects by controlling the self-assembly way through conducting the adsorption of Ce³⁺ ions on the MNPs that were gradually oxidized to CeO₂ nanoparticles [2].

Another principal approach to synthesize metal@CeO₂ core-shell nanoparticles is the auto-redox strategy, which is based on the reduction of the high valence metal species and the oxidation of low-valence Ce (III) species. Some of metal@CeO₂ core-shell nanoparticles that are synthesized by utilizing this mechanism depict a “rice-ball” shape-like Ag@CeO₂. In addition, to encapsulate the ultrasmall MNPs and construct the uniform metal@CeO₂ core-shell nanoparticles, the auto-redox strategy could be undertaken in a reverse micelle system. Furthermore, due to any surfactants not engaged in the auto-redox methods, multicore-shell nanospheres with a diameter larger than 85 nm could be formed. Although CeO₂-encapsulated bimetallic MNPs can be formed by applying the auto-redox strategy, the self-assembly approach and the salting-out effect can be influenced the formation of these core-shell nanoparticles [2].

Another ideal encapsulation nanostructure with a hollow space between the metal core and the outer porous CeO₂ shell is the yolk-shell architecture that has an effective impact on tackling the aggregation and sintering of tiny noble MNPs in catalytic reactions [2]. To implement this kind of encapsulation, a templating method should be conducted by coating a layer of silica firstly, and then the layer of CeO₂ could participate on the MNPs through a sol–gel process. Eventually, the metal@SiO₂@CeO₂ nanospheres can be modified into multi-yolk-shell metal@CeO₂ nanospheres by eliminating the silica template. In addition, through this process, multi-yolk-shell structured nano-catalysts can be formed, for instance, Pd@hm-CeO₂, which is presented by Zheng et al. [2, 33]. Despite silica being the most popular sacrificial template, polystyrene (PS) fibers and resorcinol-formaldehyde (RF) can be utilized as a removable template [2].

4.1.4 ZrO₂ nano-shells

Zirconium dioxide (ZrO₂) is a metal-based inorganic material that is presented as an insulator in some applications. Due to the chemical inert feature of ZrO₂, it has outstanding resistance to acids and alkalis environments that convert it to crucial catalyst support at harsh reaction conditions. In addition, it has significant heat stability that is suitable for a high thermal catalytic reaction to encapsulate and support the MNPs. Although it has low thermal conductivity and is utilized as thermal barrier coatings, it has a high refractive index, which is well-suited for various optical applications [34].

Recently, zirconia as catalyst support, particularly in the encapsulation shape, attracts numerous interests. In most cases, MNPs are encapsulated in zirconia nano-shells in the form of yolk-shell [25, 35, 36, 37]. Similar to the other yolk-shell nano-catalysts, metal@ZrO₂ yolk-shell can be synthesized which this catalyst with robust zirconia shells illustrates noticeable catalytic activity and outstanding anti-aggregation features during the time of the catalytic process and upon thermal treatment or reduction.

4.1.5 Carbon nano-shells

Apart from inorganic oxide shells, encapsulating MNPs in carbon nano-shells participated in numerous investigations. Although the core-shell nanostructures manufactured from inorganic oxide have noticeable advantages, in some conditions they illustrate some weaknesses, for instance, the dissolution of silica coatings at strong basic conditions. In contrast, carbon yolk/core-shells can tackle these difficulties and additionally demonstrate some outstanding specifications, such as high physical and chemical stability under harsh conditions, high surface area, tunable electronic structures, high electrical conductivity, good biocompatibility, and relatively low manufacturing costs. Therefore, they can be one of the best materials to encapsulate the MNPs [1, 2, 38]. With regard to the type of crystallinity of carbon shells, this section is allocated into metal @amorphous carbon and metal @graphitic carbon.

4.1.5.1 Metal @amorphous carbon

Metal @amorphous carbon can be fabricated by encapsulated MNPs through a polymer coating layer. Due to their low cost, rich chelating groups, and high compatibility with MNPs, these polymers are presented as leading carbon precursors that include resorcinol-formaldehyde (RF) resin, tannic acid, and polydopamine (PDA) [2]. To fabricate a noticeable-performance M@carbon catalysts, carbon precursors should be utilized through a suitable controlled sol–gel process to provide the target coating layer and then carbonization should be implemented which the output would be a carbon shell with appropriate thickness and tunable pore structure. For instance, the carbon nano-shell with 63 wt% would be formed during the carbonization of the RF coating shell under an inert atmosphere [1, 2]. In addition, to adopt the congruity between the inorganic cores and the RF shells, it is essential to modify the surface with CTAB or 3-aminopropyltriethxoysilane (APS) in the coating process [2].

On the other hand, although conducting the coating process after synthesizing MNPs will be yielded a controllable shell, implementing this process through a one-pot in which the formation of MNPs and the polymer coating will be done in a single step leads to achieving a more convenient pathway to synthesis M@RF core-shell nanospheres in the absence of surfactants. Through this procedure first of all, MNPs are formed from metal salts by adding formaldehyde. Next in the presence of the other precursor, ammonia, the polymerization of the RF precursors on the surface of MNPs will be undertaken. The concentration of resorcinol and formaldehyde can control the size and thickness of the RF shells. Moreover, resorcinol can reduce the surface activity of MNPs and prevent them from aggregating. Eventually, M@carbon core-shell nanospheres with a wide range of metals can be obtained after carbonization [2].

Furthermore, impregnation is another principal approach to synthesizing encapsulated MNPs in amorphous carbon. Overall, in this method firstly metal ions could be adsorbed at the sites of amino groups in pre-synthesized mesoporous aminophenol formaldehyde (APF) nanospheres. To create hollow carbon shells which, encapsulate MNPs before conducting carbonization a mesoporous silica layer should be done. Moreover, yolk-shell structures with MNPs can be achieved by coating another layer of APF on the APF@SiO₂ nanospheres. This mechanism is flexible and can be used to fabricate monometallic Au, Pt, Rh, and Ru and bimetallic Au-Pt, Au-Rh, and Pt-Rh nanoparticles [2]. What is more, some other sources of carbon such as D-glucose, saccharides including fructose and sucrose, and dopamine can be involved. Dopamine owing to its catechol and amine groups and the ability to self-polymerize on various substrates is considered as a remarkable carbon source, particularly in synthesizing yolk-shell structures containing MNPs in the yolk that encapsulated with carbon nano-shells [1, 2].

4.1.5.2 Metal @graphitic carbon

Although amorphous carbon represents significant specification in catalysis applications, graphitic carbon has further conductivity and stability, particularly in electrocatalysis. The high-temperature pyrolysis promotes the crystallization of carbon through the process of fabrication of MNPs encapsulated in graphitic carbon nano-shells (M@GC). One of the most common procedures to fabricate the M@GC is the pyrolysis of metal–organic frameworks (MOFs) in an inert or reductive atmosphere directly. During the pyrolysis of MOFs which are the assembly of metal ions as nodes that are linked together through organic ligands as linkers, MNPs are achieved by the reduction of metal nodes, and then these MNPs can catalyze the generation and configuration of graphitic carbon from organic linkers on their surface. In addition, bimetallic alloy nanoparticles could be encapsulated in graphitic carbon through pre-encapsulating noble MNPs or metal salts in MOFs and subsequent pyrolysis [2].

Graphene is a solo layer of graphitic carbon atoms that bonded together in a honeycomb crystal lattice, which this unique structure intensifies its specifications much higher than the other carbon material and converts it to an outstanding material for encapsulating MNPs to improve their catalytic activities. As a result of diffusion restriction and chemical inertness of graphene to a various oxidizing gas, it conducts as a passivation layer to intercept some metal (Cu, Ni, etc.) from oxidation. Although the potential energy surface of graphene can transfer from 0.15 to 1 eV on various metal substrates (Ni, Co, etc.), the principal metal has an influential impact on the electronic structure of the graphene coating layer. The electronic specification of graphitic carbon can exhibit its critical role on the catalytic activity when the shell includes no more than three to four carbon layers. Hence, it is crucial to fabricate carbon encapsulated catalysts with a controllable number of graphene layers which one of the usable procedures is the chemical vapor deposition (CVD) technique [39].

Although through CVD techniques a thin film can be formed on the substrate surface which has a dramatic influence on the fabrication of carbon nano-shells, these methods have multistep synthesis processes, which lead to being complex, expensive, and difficult to implement large-scale commercialization. In addition, to achieve a graphene shell on MNPs through the CVD procedure, it is essential to provide a high temperature above 800°C, this condition may lead to melting the MNPs and gathering them together [39, 40]. Thus, it is essential to utilize a functional synthesis pathway that is presented by an arc-discharge method. Through this technique, MNPs encapsulated in graphene shells have large sizes and extensive distribution of particle sizes, in some cases empty carbon cages and CNTs were achieved simultaneously. To tackle this difficulty, applying a long pulse laser in methane or a mixture of methane and helium at room temperature are presented, the efficiency of this approach is the formation of an ordinary size of 5 nm and appropriate size distribution of 3–10 nm of core-shell structure M@GC [39, 40, 41].

On the other hand, the metal alloy can be encapsulated in highly nitrogen-doped graphene layers by one-step annealing under a nitrogen flow without adding any other carbon sources by utilizing bimetallic complexes with CN- group linkers in the form of metal–organic framework (MOF) precursors [39]. In addition, electrostatic interactions between negatively charged graphene oxide and positively charged metal oxide nanoparticles can be applied for encapsulating metal oxide in graphene shells, which this method can be followed by chemical reduction. First of all, amino propyl-trimethoxy silane (APS) paves the way for the metal oxide nanoparticles to exhibit an oxide surface positively charged. Then, electrostatic interactions put the modified metal oxide nanoparticles with negatively charged graphene oxide together. Eventually, the accumulation of them will chemically be reduced with hydrazine to gain the metal oxide nanoparticles encapsulated in graphene. Because this mechanism can provide all the applicable features, such as simplicity of operation, low cost, and optimal efficiency, it can be a functional strategy to produce a variety of graphene-encapsulated catalysts on a large scale [39, 40].

4.1.6 Zeolites

Zeolites are presented as highlight catalyst supports due to their highly crystalline, well-distributed pore structure and adjustable acidity. In general cases, the zeolite structure embodies TO₄ tetrahedra (T defines Si, Al, and P, etc.). Due to adjusting the T-O-T linkage a wide range of zeolite structures can be formed via tuning the synthesis conditions such as the composition of the gel, the nature of the structure-directing agent (SDA), or the temperature [42, 43].

Although the catalytic activity of the catalysts often depends on the host nanoparticles, it can be modified by the zeolite framework features. In particular, the modification of local geometry around active sites which is derived from steric constraints affected by the size of zeolite cavities can effectively influence the reactivity of catalysts [43]. Zeolites based on the size of their pores can be classified into small, medium, large, and extra-large pores. The catalytic activities of zeolites deeply depend on the structural and compositional features, consisting of pore sizes, channel types, and framework compositions. In comparison with the other catalyst supports, zeolites are presented as a shape selective that can selectively interact with reactants, products, and transition-states that this attribute has a significant impact on the catalytic performance of zeolites [42]. Due to zeolites being composed of tetrahedrally [SiO₄]⁴⁻ and [AlO₄]⁵⁻ primary units, to balance the overall electric charge of the zeolitic skeleton, some free cations are accommodated into the channels of the 3D framework, which can be substituted by other cations. Al content in the zeolite framework has the main effect on the ion exchange capacity of zeolites when the cations of zeolites are exchanged by protons, zeolites conducted as solid Brønsted acid catalysts [44]. In addition, the existence of charges in the zeolite framework, as well as extra-framework cations, can have a significant impact on the electronic and redox properties of the encapsulated complex [43]. Despite the fact that zeolites can apply as a shell through the coating of MNPs with layers of zeolite, in some cases, the encapsulation of MNPs can operate in the regular cavities and nanochannels of zeolites [42]. Hence, the encapsulation of MNPs in zeolites is considered in two parts—yolk/core-shells and mesoporous structures.

4.1.6.1 Yolk/core-shell structures of zeolites

In the approach of nanotechnology, zeolites can be utilized to make novel nanostructure synthetic materials, zeolite core-shell structured materials being the outstanding structure among them [45]. The capability to synthesize core-shell zeolite composites has depicted the principal importance of chemical adaptability and structural likeness between core and shell crystals, as well as their close crystallization conditions [46]. On one side, a core-shell structure of zeolite can be formed via crystal overgrowth in which an aluminum-free zeolite (core) was coated with aluminum-containing zeolite (shell). Fluoride ions as mineralizers can conduct the accomplished passivation of acid sites on the external surface to minimize the imperfections of the core-shell zeolite structure, so applying them is essential [47]. To increase the selectivity and catalytic activity of the core-shell zeolite with TON structure, a novel high silica zeolite, for skeletal isomerization of n-tetradecane, it is essential to break needle-like particles for the formation of new acid sites on the pore mouths of smaller broken particles since, the acid sites on the side surface of the needle-like particles, which principally catalyzed the cracking of alkanes, were passivated [47]. In addition, through utilizing the techniques of layer by layer self-assembly of polyelectrolyte the core-shell zeolite-zeolite composites consisting of single-crystal core and polycrystalline shells of various zeolite structure types can be fabricated. This approach occurs based on the coulombic forces that lead to enhancing the surface charge of the core particles by coating the layers of zeolite [45]. Moreover, core-shell structures of zeolites can perform as a multi-purpose catalyst that have several impacts in various functions simultaneously. For instance, the shape-selective attribute of zeolite shell provides this ability that a catalytically active nano/micro-sized core encapsulated with a thin selective zeolite shell can be potentially utilized as a tiny membrane reactor. To achieve this purpose, first of all, the catalytic active materials such as metal oxides will be replaced in the core. Then secondary growth will be conducted through a precursor solution to coat a layer of zeolite, subsequently, the growth of this layer can continue until a dense and well-intergrown zeolite shell is formed that can also play its role as a highly efficient zeolite membrane. Not only does this zeolite shell provide a prominent selective mass transfer between the encapsulated core and the extension, but also the participating catalytic reaction on the core can promote each of such zeolite core-shell structured particles into a tiny membrane reactor which can present appropriate potential in a wide range of reaction systems [45].

On the other hand, the other nano-shell catalyst structure of zeolite which can be pointed to it is the yolk-shell zeolite-based catalysts. This structure can be achieved by silica-zeolite core-shell materials. First of all, silica particles are partially dissolved under high pH conditions, then through controlling the recrystallization of the surface of silica spheres by the zeolite, new agglomerated zeolite nanocrystals can be formed which depict hollow capsules. MNPs prior to recrystallization can be located at the core [43]. Overall, to synthesize spherical hollow zeolitic structures should apply the sacrificial templates such as organic polymers or silica whose particles within the core can be removed in the final step respectively by calcination or etching [45].

4.1.6.2 Mesoporous framework of zeolites

Zeolites with a mesoporous matrix are a family of porous materials with an effective crystalline framework containing a finite number of well-defined and small cavities with sub-nanometer to 2 nm size. The most interesting attribute of their structures is the possibility to tune and choose the similar size of their micropores with the size of MNPs that would be encapsulated in them. In addition, it can intensify the catalytic selectivity due to enhance the efficiency of reactants and products diffusions [2]. Meanwhile, by encapsulating the MNPs inside the micropores of the zeolite framework they would be effectively enclosed through their interconnected cavities [42, 48]. Not only can mesoporous zeolites perform as an immobilize or stabilize framework to encapsulate the nanoparticle catalysts, but also they represent as a molecular sieve via molecular selecting with the proper size and shape or as a hybrid catalyst for transforming products formed firstly [43].

There are various procedures to implement the encapsulation of MNPs through the zeolite pores which are selected on the basis of nanoparticle size and their nature [43]. Although the formation of MNPs and the growth of zeolites are two parallel pathways, in most synthesis strategies both of them are simultaneously conducted in one process synthesis, due to the pore sizes of most zeolites being too small, less than 2 nm, which is not appropriate for directly encapsulating MNPs [1, 2]. Here some popular approaches to undertake the encapsulation of MNPs in zeolites are presented.

4.1.6.2.1 Hydrothermal strategy

Hydrothermal synthesis strategy is a common procedure that is done under a thoroughly alkali condition. In this technique the metal precursor is directly added to the synthetic solution, to prevent premature reduction or precipitation of the metal salt through the crystallization of the zeolite, a mercaptosilane ligand like 3-mercaptopropyl tri-methoxy silane (MPTMS) should be applied into the hydrothermal synthetic system. Despite the mercapto groups supporting the metal precursors from reducing untimely through the alkaline synthetic solution, the other group of MPTMS that is alkoxysilane prepares an appropriate condition to fabricate crystalline frameworks with silicate and aluminate. Thus, the MPTMS paves the way to conduct the encapsulation of the MNPs in the mesoporous framework of zeolite monotonously. Eventually, the crystalline framework of metal precursor-zeolite which is gained by this procedure should be calcined in air to eliminate the organic agents and reduced in H₂ to create novel MNPs through the micropores of zeolite. In addition, LTA-type zeolites including micropores with the approximate size of 0.41 nm are the appropriate option to encapsulate the MNPs through this hydrothermal synthesis strategy. To enhance this hydrothermal procedure with the highest efficiency (>90%) of zeolite-encapsulated MNPs, initially, alcohol is added to eques mixture of the mercaptosilane and metal salt, then a pre-hydrolyze at low temperature will be performed to achieve a uniform gel [2, 48].

4.1.6.2.2 Solvent-free crystallization

Solvent-free crystallization is another popular strategy to encapsulate MNPs in mesoporous zeolites. By exposing a metal/silica/alumina hybrid in the vapor of water at a high temperature the solid-phase transformation of the amorphous silica and/or alumina into a crystalline zeolite that encapsulates MNPs is initiated. Furthermore, this synthesis method can provide a pathway to fully encapsulate the pre-synthesized MNPs inside the zeolite single crystals more reliably [2]. FAU- and MFI-type zeolites are the main mesoporous zeolite types that are applied for this procedure [2, 43]. In particular, Chen et al. [49] utilized nanocrystals of MFI-type zeolite (silicate-1 or S-1) in a strong alkaline environment. The outstanding feature that promoted the crystallization involved the Kirkendall effect which led to growing the pore size of mesoporous inside the S-1 crystals to around 3 nm and may impact on the enhancement of the mass transfer in catalytic applications [2].

4.1.6.2.3 Secondary growth of zeolite on metal/zeolite seeds

The encapsulation of MNPs in zeolites can be involved in the seeded growth of zeolites on zeolite seeds that already include MNPs. This synthetic strategy is implemented in two steps—first of all, the impregnation of the zeolite seeds with a metal salt is implemented, then desiccated the mixture to achieve a dry powder, and next conducted the reduction process at a high temperature in H₂ to convert the metal salts to the MNPs, eventually the achievement products are the zeolite seeds included the MNPs. In the second step, a hydrothermal system in the presence of aluminosilicate or silicate gels and the zeolite seeds including the MNPs are involved. Consequently, the encapsulation of MNPs such as Pt, Pd, Rh, and Ag in zeolites like MFI, MOR, and BEA, particularly at the interface between the zeolite seed and sheath could be effectively conducted. Moreover, a great core-sheath interface can enhance the loading amount of MNPs which can be obtained by employing a zeolite type with a high surface area when providing the metal-containing seeds through the synthesis process [2, 48].

4.2 Metal: Organic frameworks (MOFs)

Metal–organic frameworks (MOFs) are outstanding microporous materials including two major components: bridging organic linkers and inorganic secondary building units (SBUs) of metal ions or oxo-clusters (3d transition metals, 3p metals, or lanthanides). MOFs provide an exceptional combination of inorganic and organic components with synergistic interactions among them which create great usability for a myriad of purposes. These microporous materials are fabricated by gathering metal ions with organic ligands together in appropriate solvents often during a self-assembly strategy. In addition, the organic linkers are di-topic or polytopic organic ligands like carboxylate, nitrogen-donor groups, sulfonate, or phosphonate that are able to bind with metal-containing SBUs to form crystalline framework structures with open pores. Although MOFs present crystalline structures with dramatic large and uniform internal surface areas, their porosity and chemical features can adjust respectively by tuning the pore size and modifying the organic linkers according to our requirements on various catalytic applications. Moreover, the functional groups of organic ligands such as -NH₂, -NO₂, -SO₃H, -Cl, and -OCH₃ groups can be linked on the pore walls through the one-step assembly or post-synthetic modification which can impact on the catalytic performance. As a result, more than 20,000 MOFs with various compositions and topologies have been reported in recent decades. Meanwhile, metal nodes can present Lewis acidity features which are emerged by utilizing transition metals, additionally, they can engage in redox catalysis or support the progression of coupling reactions. Furthermore, metal nodes make various coordination positions by the participation of solvent molecules which can be eliminated through a thermal approach while saving their frameworks [50, 51, 52, 53].

Encapsulating guest particles into MOFs provides a wide range of potential for various applications, particularly in catalysis. A great number of particles can be encapsulated such as inorganic MNPs, coordination complexes, quantum dots, polyoxometalates, enzymes, and polymers through a pre- and post-synthetic strategy. In comparison with the other encapsulating materials, MOFs exhibit confinement effects and shape selectivity in a more effective route, and their synthesis conditions are more moderate. In addition, owing to the existence of a wide range of MOF structures, it is simple to adopt a suitable MOF as the encapsulating material. Hence, the encapsulation of MNPs in MOFs converts them to prominent catalysts that attract considerable attention, due to they present the unique attributes of MOFs alongside the chemical and physical properties of MNPs simultaneously. In addition, this combination of active nanoparticles and functional organic linkers of MOFs can facilitate the charge transfer interactions with active components by coordination or π···π forces, which lead to present a significant enhancement in their catalytic performances [50, 51, 52, 53]. The composites of nanoparticles encapsulated in MOFs can be fabricated through two main strategies—(1) stabilizing pre-synthesized nanoparticles in organic or inorganic agents as core and then enclosed in the shell of MOFs which generate a core-shell structure; (2) utilizing MOFs as mesoporous templates to encapsulate nanoparticles within their cavities [50].

4.2.1 Yolk/core-shell structures of MOFs

Despite the outstanding attributes of the composites of nanoparticles encapsulated in MOFs being able to conduct the effective catalytic activity, some restricting issues still exist that should be noticed. First of all, the pores can be blocked by the encaged nanoparticles during their growth, thus restricting the diffusion of the reaction medium in the catalytic process. Secondly, the loading of guest particles is limited to them which are smaller than the pore dimensions. Furthermore, the other specifications of guest particles like shape and morphology may not have adequate adoption with the host cavities. Moreover, it is not possible to control the deposition of guests mostly and eventually the guest nanoparticles may leach in liquid phase. To tackle these difficulties, the core-shell encapsulation strategy is offered [53].

The core of a core-shell MOFs-based composite can consist of inorganic nanoparticles like metal oxides, carbon materials, and polymers or other MOFs which are encapsulated in a MOF shell. Although the shape, size, morphology, and composition of the core have a significant effect on the catalytic performance of the catalyst, this structure provides effective encapsulation due to the integration of chemical/physical properties of two distinct materials that lead to the synergetic effects. Recently, the yolk-shell or hollow structures attracted further attention because the properties of core-shell MOF-based architecture are optimized in this type of structure that presented added conductivity, hierarchical porosity, and effective diffusion. These structures can be achieved through a controllable etching of the core materials based on core-shell structures. Although carbonizing techniques at high temperature can destroy the MOFs, by employing this approach yolk-shell or hollow structures can be generated in which their porosity and active sites have been maintained [53].

4.2.1.1 Growth of MOFs on pre-synthesized MNPs

Growth of MOFs on pre-synthesized MNPs is the main approach to encapsulate MNPs in shell of MOFs which are implemented in three main strategies that are presented in follow. Although the prominent benefit of this approach is the participation of MNPs with various sizes and shapes, the control of the assembly of metal-support interface is a noticeable attribute. In addition, the principal role in fabricating an appropriate core-shell MOFs-based composite in this strategy belongs to the consistency and conformity between the MNP core and the MOF shell [2].

4.2.1.1.1 Capping agent-assisted synthesis (CAAS)

First of all, the pre-synthesized core materials and capping agents/surfactants like PVP which is an amphiphilic nonionic polymer, are simultaneously provided earlier the growth of MOF shell around them due to prevent the aggregation of active NPs and/or the self-nucleation of MOF particles [53]. In addition, capping agents intensify the compatibility between the MNPs and the MOF shell. While PVP is the most popular capping agent for encapsulation of inorganic materials in MOFs, depending on the design process of core materials can be utilized the other type of surfactants like cetyltrimethylammonium bromide (CTAB), tetradecyltrimethylammonium bromide (TTAB), cetylpyridinium bromide (CPB), and sodium dodecyl sulfate (SDS). The CTAB is the optimal surfactant in terms of shape/overgrowth control, as well as PVP, owing to the sizes and shapes of the final composite can be adjusted by changing the growth time and the quantity of CTAB in solution. Overall, despite CAAS being a popular method due to the ability to control the shape, size, and chemical nature of the encapsulated core, the multiple steps of this method restricted its utilization [53].

4.2.1.1.2 Inorganic template-assisted synthesis (ITAS)

In this method, firstly, the core material is coated by employing an inorganic material like SiO₂ as binding sites for shell MOF, or metal oxide as the metal source for the shell MOF growth, and then synthesis of MOF will be implemented. Although in contrast CAAS procedure applying metal oxide prepare some advantages due to their act as metal source or protection of reactive nanoparticles in the core, there are some restrictions to fabricate wrapped nanoparticles owing to the shell MOF chemical stability will be endangered through the etching of the sacrificing metal oxide. Despite the fact that SiO₂ can play as a useful protective layer, it can represent as a sacrificial layer to fabricate a yolk-shell structure by conducting selective etching of SiO₂. In addition, in some cases, metal oxide/metal(0) nanoparticles have also been utilized as sacrificing agents to fabricate core-shell architectures [53].

4.2.1.1.3 Epitaxial growth synthesis (EGS)

EGS is a synthetic strategy to encapsulate MOF particles as a core with the second layer of MOF. In addition, in most cases, the shell MOF topology is similar to the core MOF and active nanoparticles would be placed at the interface of the two layers prior to the epitaxial growth. Overall, this approach leads to effective integration of MOFs because synergies of various MOFs properties are simultaneously presented by an exceptional composite of MOFs @MOFs. Despite this method presenting a wide range of similarities to the CASS strategy, it can enhance the diffusion of the reaction medium toward the active sites due to the ability to fabricate a super-thin shell with a thickness size less than 10 nm [53, 54].

4.2.2 Encapsulation of the MNPs in the micropores of MOFs

Not only would MOFs provide a fully available pore space to optimize the diffusion of the reaction medium, but they also support the MNPs by enclosing them in their mesoporous matrix that increases the accessibility of active sites and enhances the catalytic activity [53]. Due to MOFs can play the role of host materials and be able to provide confined spaces for nucleation of MNPs, the encapsulation of MNPs into the cavities or channels of MOF matrix can be conducted by impregnating metal precursors in the pre-synthesized MOFs and subsequent reduction of the metal precursors in the micropores of MOFs [2]. Although the precise control of this encapsulation type of MNPs in MOFs seems to be difficult, the most effective pathway can be achieved by the double-solvent procedure prior to the reduction step. As a result of this double-solvent approach, the capillary force intensifies the mass transfer of metal precursors into the micropores of MOFs, which prevents from aggregation of MNPs and minimizes the dispersion of them on the outer surface of MOF [52].

The effective route to utilize noble metals (Pd, Ru, or Pt) in the MOFs-based catalysts is in the form of alloys with low-cost transition metals (Cu, Co, or Ni) owing to decreasing the essential amount of expensive noble metals during the catalyst’s fabrication. Furthermore, the atomic and electronic specifications of their structures can be adjusted which leads to improved catalyst activity. In this approach, firstly low-cost transition MNPs are impregnated in the pre-synthesized MOFs and then are reduced by NaBH₄. In the next step, the salts of noble metals were exposed into a solution of MOF in which the galvanic replacement reaction with the transition MNPs with the help of an excitement process like sonication can be started. Finally, it leads to the fabrication of alloy nanoparticles like Co-Ru which is encapsulated in MOF [2, 55].

4.3 Encapsulation of MNPs in organic materials

One of the most effective approaches of ultrasmall MNPs encapsulation with meticulously composed sizes in catalysts is the encapsulation of them in organic materials, which is presented in two parts as follows [2]:

4.3.1 MNPs @organic capsules

Organic capsules, such as dendrimers, which are a family of hyperbranched polymers, have a spherical structure, which is compressed on the exterior and creates hollow space in the interior. By applying organic groups, such as tertiary amines, the interior cavity can be put into particular operation, such as interception of metal ions from the solution through encapsulating them. In addition, the dendrimers can provide a monodic encapsulation of MNPs due to form mono dispersing of them with appropriate adjustment of ultrasmall sizes. Not only can dendrimers operate as effective encapsulating materials owing to the consuming amount of them being approximately the same as the metal ions amount, but also they are able to present a stable form that is unchanged for some months. Thus, these specifications convert them as useful stabilizers in the catalytic process to prepare a monotonous dispersion of nano-catalysts particles with suitable stability [2].

Polyamidoamine (PAMAM) is the most used dendrimer for encapsulating the ultrasmall MNPs. Not only can dendrimers, particularly PAMAM, provide homogeneous catalysis, but an effectual mass transfer will also happen at their exterior surface that leads to a significant improvement in the catalytic activity of the ultrasmall MNPs. To present applicable heterogeneous catalysts of encapsulated ultrasmall MNPs in dendrimers, it seems essential to utilize mesoporous supports, such as silica. Not only do they supply high surface area, but also they prepare constructive enclosures to intensify the stabilization of ultrasmall MNPs in harsh catalytic processes. When applying the silica mesoporous matrix as support, the electrostatic interaction and hydrogen bonding between silica molecules and dendrimers enforce MNPs@ dendrimers into the mesopores of the silica matrix. Not only do these supported catalysts without removing the dendrimer present highlighted stability, selectivity, and activity in a range of catalysis processes, they illustrate adjustable catalytic specifications that are possible to alter in order to enhance the catalyst activity by modifying the active groups on dendrimers, for instance, the tertiary amines can improve the activity of the catalyst due to their electron enrichment attributes. In addition, the unique spatial morphology of dendrimers in the shape of tree may have a significant impact on the reaction substrate stability, including reactant molecules or intermediate species that can lead to decrement of the level of activation energy and improve the turnover frequency [2, 19, 56]. Furthermore, click dendrimers consist of a great ratio of triazole rings, are the other type of dendrimers that can be fabricated from the azide-alkyne cyclo addition. Their rings provide an appropriate situation to accomplish the encapsulation by adsorbing metal ions and grafting the MNPs. In addition, by adding triethylene glycol (TEG) termini to the click dendrimer convert them to an effective soluble one in aqueous solutions which are suitable to fabricate the encapsulation of ultrasmall monometallic or bimetallic like Pt-Co in water solution. Thus, the yield of catalyst performance in an aqueous solution will dramatically enhance [2].

4.3.2 Porous organic cages (POCs)

POCs with intrinsic porosity have prominent attributes, such as high surface areas, shape durability, structural adjustability, and in particular including active practical groups in their cavities that lead them to encapsulate ultrasmall MNPs in an effective way. Moreover, POCs can exhibit either in a crystalline or an amorphous structure. Furthermore, the heterogeneous catalysts that are formed by encapsulating ultrasmall nanoparticles in these POCs would be homogenized in the solutions and have a significant influence on the enhancement of the performance of catalysts. Their specifications can be modified or developed through post-synthetic modification strategies, such as polymorph selection, modular co-crystallization, and the fabrication of composite materials. Not only does the cage wrapping significantly impact porosity, during this route by employing various crystalline polymorphs distinctive physical properties can be exhibited. In addition, to fabricate a cage in the correct form, at least it is essential to provide precursors with proper and precise geometry, for instance, a mild alter in bond angles of the precursors can lead to a distinct cage that might be in size and stoichiometry [2, 57, 58].

To fabricate POGs with polyhedral organic molecules, triamine is utilized to form the top and bottom prism that include a dialdehyde with a long alkyl chain and a thioether group at the three peripheral sides. Although the long alkyl chain provides a cage profoundly soluble in an oil phase due to thriving their hydrophobic properties, the thioether group leads the cage to adsorb the metal ions and prepare suitable sites to graft the MNPs. Therefore, these specifications develop POGs capability to utilize as promising catalysts, particularly in catalyzing organic reactions [2].

Acknowledgments

A very special thanks to Mohammadreza Joharkesh, who helped to depict schemes 1, and 2.