**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 [1–3], as shown in **Figure 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/coreshell, 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 [22–25].

#### *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

#### **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.*

*Encapsulation of Metal Nanoparticles (MNPs) as Catalyst DOI: http://dx.doi.org/10.5772/intechopen.103184*

**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.*

achieved by scarifying the inert chemical layers through chemical etching [22–25]. **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 coreshell (a1) and yolk-shell (a2) are also illustrated in **Figure 3** [26].

#### **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 presynthesized 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 selfassembly-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].

#### *Encapsulation of Metal Nanoparticles (MNPs) as Catalyst DOI: http://dx.doi.org/10.5772/intechopen.103184*

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 nanoshells 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].

#### *Encapsulation of Metal Nanoparticles (MNPs) as Catalyst DOI: http://dx.doi.org/10.5772/intechopen.103184*

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 nanoshell. 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].
