*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 metalsupport 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-yolkshell 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 nanoshells in the form of yolk-shell [25, 35–37]. Similar to the other yolk-shell nanocatalysts, 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 onepot 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

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

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 [39–41].

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 grapheneencapsulated 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 structuredirecting 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 aluminumcontaining 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 [50–53].

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

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 [50–53]. 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].
