**Acknowledgements**

*Cobalt Compounds and Applications*

mental applications [31].

**3. Conclusions and future directions**

tation using experimental data and theoretical models.

Single-atom catalyst has established a connection between homogeneous and heterogeneous processes. A single atom catalyst can be better than a homogeneous catalyst because it is easy to recover at the end of the process and equal catalytic activity. Single-atom catalyst can have many fascinating characteristics such as maximize the atomic efficiency, a high activity like a homogeneous system, and selectivity. A single-atom catalyst has been proved to be a very efficient and powerful catalyst for various applications including the ORR, OER, HER, CO2 conversion, and organic transformation. Significant progress is made to understand the catalyst preparation, characterization using advanced techniques and mechanism interpre-

The microenvironment of a single atomic site needs to investigate more in detail, *in situ* spectroscopy technique can be utilized to study the active site in operation condition, and a specific reaction mechanism can be proposed [29]. It can be understood from literature, the interaction between the metal single atom and support is necessary to stabilize a single metal atom. If we can manipulate these interactions, it can tune the catalytic activity, stability, and selectivity for multiple catalytic applications. MOFs are the fascinating precursor for single metal atom catalyst design. Multi-

metal MOFs can also use for the synthesis of multi-metallic single atom heterogeneous catalyst to utilized the one catalyst for numerous applications [30]. To further change the catalytic phenomena of a single metal catalyst, two-dimensional materials such as MXenes, MoS2, HBN, black phosphorus, and many more support materials can be successfully explored for the synthesis, industrial, and environ-

Porous metal oxide support for the preparation of a single-metal atom catalyst can be also utilized and use for organic transformation, gas phase, and electrochemical applications. Although porosity of metal oxide support is an issue, however two-dimensional metal oxides can be utilized for single-atom catalyst preparation [32]. Silica (mesoporous spherical or nanosheets) can be a good support for the single-atom catalytic application, however, it needs to be explored further [33, 34]. Non-conducting and passive nature of silica might a problem for single-atom catalysts. Porous carbon nanocage using KCC-1 as a hard template was used to prepare a three-dimensional accessible surface carbon sphere [35]. Same carbon morphology can be utilized for the preparation of mesoporous carbon-based SAMCs for various applications. **Figure 8** is demonstrating the overall picture of different ways of single-atom catalyst preparation and their utilization in various applications.

Recently, metal-air batteries have been received to consider a potential candidate for future energy storage materials due to their theoretical high energy density, excellent safety, and abundant TM resources. Typically, metal-air batteries' performance

*General discerption of SMACs, method of preparation, support used and applications.*

**84**

**Figure 8.**

We thank SERB - DST, New Delhi for their support under EMEQ Scheme.
