**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.
