**1. Introduction**

The main aim of current research on catalysis is to find out long-lasting catalyst that can consistently work without compromising its catalytic activity [1–3]. The catalytic activity mostly depends on a number of active sites and their accessibilities. Catalytic activity can be tuned by manipulating the nanoparticles sizes or by producing shaped nanoparticles with exposed facets [2–5]. Keys factors for catalytic properties are not very well known and atoms with the different environments are mostly showed different catalytic activity. For example, an atom on the edges and on corners mostly showed different properties, more catalytic activity compared with the atoms present in the middle of exposed facet.

For catalysis purpose, mostly surface atoms are accessible and catalytically active, remaining bulk atoms are providing support. The amount of metal required to produce catalytic activity can be reduced by bringing down the particle size at the atomic level, it may be enhanced by using single-site metal catalyst. Singlemetal atom catalysts (SMACs) have shown better activity compared with subnanometer nanoparticles materials. As limited by the change in morphologies of nanoparticles, a concept of SMACs has emerged because it only way remaining to maximize efficiency of catalyst [4, 5]. Many single Co atoms homogeneous catalysts have been reported (**Figure 1a, b**) [7], in which single metal atom is supported by the bulky organic functional groups; their activity and accessibility depends on the organic environment around the single metal atom. These organic functional groups do not allow to agglomerate, provide thermodynamic stability and chemical stability (**Figure 1**). However, it is very tedious and laborious to recover back homogeneous catalyst from solution mixture and hard to reuse it again. Thus we need a catalytic system that is easy to recover and reusable. So, heterogeneous catalysts can be a good choice. However, to achieve the catalytic efficiency equal to homogeneous catalyst, a single-atom catalyst preparation is still challenging.

The preparation of stable SMACs is still challenging because high energy of a single atom leads to agglomeration and makes them unstable under reaction conditions at the industrial level. An effective way for a synthesis of SMACs via increasing the interaction between a metal atom and support [4]. In general process, by reducing the size of nanocatalyst and bring them at atomic scale, we are trying to heterogenize homogeneous catalysts on heterogeneous supports. This can be done by overcoming surface energy of a single atom by anchoring on a support substrate, which might sufficiently active like a homogenous catalyst. Graphene is a good support to stabilize coordinately unsaturated single metal sites (CUMS). Graphene is a

#### **Figure 1.**

*(a and b) Co-PNP complex catalyzed the hydrogenation of CO2 to formate and formamides (Reproduced with the permission, Copyright 2018, American Chemical Society [7]).*

**73**

**Figure 2.**

*Cobalt Single Atom Heterogeneous Catalyst: Method of Preparation, Characterization, Catalysis…*

perfect two-dimensional structure with high specific surface area, high mechanical strength, and thermal stability. Its unique structure and electronic properties are promising for the synthesis of stable CUMS metal atom. Several recent works have

Overall support substrate and metal interaction could help to design longlasting, stable SMACs which could work without loss of catalytic activity under the operational condition for specific reaction or applications. Using support-metal interaction concept, Sahoo *et al*. investigated a theoretical model of graphene supported transition metal (TM) atom including Iron (Fe) single atom [8]. DFT result showed that graphene supported single-atom catalyst display relative lower activation energy barrier for methane activation. However, of its excellent catalytic performance, it has been proved that a single metal atom can migrate on a surface of graphene defect (**Figure 2**), which allows them to agglomerate into bigger nanoparticles, resultant in loss of catalytic activity. It has been proved that a single metal atom can migrate from one position to another position on graphene layer. Zhao *et al*. observed single Fe atom diffusion on graphene edge [12]. *In situ* electron microscopy was used to investigate diffusion of Fe along edges of graphene via either adding carbon atoms or by removing carbon atom (**Figure 2A**–**G**). Besley *et al.* also studied the dynamic behavior of single Fe atoms embedded in graphene sheet [13]. Migration of Fe atom was also observed by the aberrationcorrected high-resolution transmission electron microscopy (AC-HRTEM) experiments. Anchoring mechanism using heteroatoms has found most effective tool

*One cycle growth of graphene edge: (A–D) a series of high-resolution TEM images for 4 s, Fe atom are highlighted with the red dots, nearby carbon atoms are highlighted by black dots. (E) Corresponding atomic structure of A–D. (F) The combination of A–D, which showed the trace of the Fe atom during one unit cell translocations, (G) the atomic structure of the whole growth process (Reproduced with the permission, 2014,* 

*Proceedings of the National Academy of Science of the United States of America [12]).*

been demonstrated the preparation of SMACs using graphene [6–11].

*DOI: http://dx.doi.org/10.5772/intechopen.85773*

*Cobalt Single Atom Heterogeneous Catalyst: Method of Preparation, Characterization, Catalysis… DOI: http://dx.doi.org/10.5772/intechopen.85773*

perfect two-dimensional structure with high specific surface area, high mechanical strength, and thermal stability. Its unique structure and electronic properties are promising for the synthesis of stable CUMS metal atom. Several recent works have been demonstrated the preparation of SMACs using graphene [6–11].

Overall support substrate and metal interaction could help to design longlasting, stable SMACs which could work without loss of catalytic activity under the operational condition for specific reaction or applications. Using support-metal interaction concept, Sahoo *et al*. investigated a theoretical model of graphene supported transition metal (TM) atom including Iron (Fe) single atom [8]. DFT result showed that graphene supported single-atom catalyst display relative lower activation energy barrier for methane activation. However, of its excellent catalytic performance, it has been proved that a single metal atom can migrate on a surface of graphene defect (**Figure 2**), which allows them to agglomerate into bigger nanoparticles, resultant in loss of catalytic activity. It has been proved that a single metal atom can migrate from one position to another position on graphene layer. Zhao *et al*. observed single Fe atom diffusion on graphene edge [12]. *In situ* electron microscopy was used to investigate diffusion of Fe along edges of graphene via either adding carbon atoms or by removing carbon atom (**Figure 2A**–**G**). Besley *et al.* also studied the dynamic behavior of single Fe atoms embedded in graphene sheet [13]. Migration of Fe atom was also observed by the aberrationcorrected high-resolution transmission electron microscopy (AC-HRTEM) experiments. Anchoring mechanism using heteroatoms has found most effective tool

#### **Figure 2.**

*Cobalt Compounds and Applications*

still challenging.

For catalysis purpose, mostly surface atoms are accessible and catalytically active, remaining bulk atoms are providing support. The amount of metal required to produce catalytic activity can be reduced by bringing down the particle size at the atomic level, it may be enhanced by using single-site metal catalyst. Singlemetal atom catalysts (SMACs) have shown better activity compared with subnanometer nanoparticles materials. As limited by the change in morphologies of nanoparticles, a concept of SMACs has emerged because it only way remaining to maximize efficiency of catalyst [4, 5]. Many single Co atoms homogeneous catalysts have been reported (**Figure 1a, b**) [7], in which single metal atom is supported by the bulky organic functional groups; their activity and accessibility depends on the organic environment around the single metal atom. These organic functional groups do not allow to agglomerate, provide thermodynamic stability and chemical stability (**Figure 1**). However, it is very tedious and laborious to recover back homogeneous catalyst from solution mixture and hard to reuse it again. Thus we need a catalytic system that is easy to recover and reusable. So, heterogeneous catalysts can be a good choice. However, to achieve the catalytic efficiency equal to homogeneous catalyst, a single-atom catalyst preparation is

The preparation of stable SMACs is still challenging because high energy of a single atom leads to agglomeration and makes them unstable under reaction conditions at the industrial level. An effective way for a synthesis of SMACs via increasing the interaction between a metal atom and support [4]. In general process, by reducing the size of nanocatalyst and bring them at atomic scale, we are trying to heterogenize homogeneous catalysts on heterogeneous supports. This can be done by overcoming surface energy of a single atom by anchoring on a support substrate, which might sufficiently active like a homogenous catalyst. Graphene is a good support to stabilize coordinately unsaturated single metal sites (CUMS). Graphene is a

*(a and b) Co-PNP complex catalyzed the hydrogenation of CO2 to formate and formamides (Reproduced with* 

*the permission, Copyright 2018, American Chemical Society [7]).*

**72**

**Figure 1.**

*One cycle growth of graphene edge: (A–D) a series of high-resolution TEM images for 4 s, Fe atom are highlighted with the red dots, nearby carbon atoms are highlighted by black dots. (E) Corresponding atomic structure of A–D. (F) The combination of A–D, which showed the trace of the Fe atom during one unit cell translocations, (G) the atomic structure of the whole growth process (Reproduced with the permission, 2014, Proceedings of the National Academy of Science of the United States of America [12]).*

for synthesis of stable SMACs, which could avoid their agglomeration and helps in remain catalytically active. However, anchoring mechanisms is not known completely and this mechanism can vary with substrates. In case of pristine graphene-SMACs void or defect in the single-layer graphene served as a trap site for the single metal atom and help them to stabilize.
