**1. Introduction**

Over the past centuries, CO2 has become the main carbon resource due to the decreases of limited resources such as coal, oil, and natural gas [1]. However, the CO2 concentration in the atmosphere has consequently increased from ~280 ppm (preindustrial) to ~390 ppm in 2010

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

at a rate of ca. 1% per year [2], which arguably contributes to the "greenhouse effect," and increases the global temperatures and climate change. CO2 emissions are still existing threat to humans; it is high time that effective measures should be taken to decrease the emission of CO2.

Hence, the carbon capture and sequestration (CCS) system is considered to be an efficient method for CO2 utilization [3, 4]. Nevertheless, the hydrogenation reaction is the most important chemical conversions of CO2; moreover, which offers a good opportunity for sustainable development in the energy and environmental sectors. Indeed, the reaction process not only reduces the CO2 amount in the atmosphere but also produces fuels and valuable chemicals [5].

As a promising fuel energy, methane, a simple hydrocarbon, has a wide range of applications in the industry and civil use, which also used to produce some downstream products, such as ethyne, hydrogen, and ammonia [6, 7]; therefore, the strategy of CO2 methanation is signifi‐ cantly meaningful. Undeniably, the resources of fossil fuels are diminishing and fuel prices have undergone strong fluctuation in recent years. Therefore, developing alternative fuels from nonfossil fuel sources and processes are highly desirable. The products from CO2 hydrogenation, such as methane, hydrocarbons, methanol, and dimethyl ether, are excellent fuels in internal combustion engines, and are easily stored and transported, but the literature studies showed that the CO2 conversion to methanol and dimethyl ether is still very low (~20%) and it is difficult to achieve higher conversion of CO2 [8, 9]. CO2 methanation is a simple reaction, generating methane under atmospheric pressure with several advantages over other chemicals. Although the conversion was still very low, the CH4 formation from CO2 at low temperature has become an important breakthrough in the utilization of CO2 [10].

CO2 methanation is a significant catalytic hydrogenation process, as is shown in Eq. (1).

$$\text{CH}\_2 + 4\text{H}\_2 \rightarrow \text{CH}\_4 + 2\text{H}\_2\text{O}, \qquad \Delta\text{H}\_{296\text{K}} = -2\text{S} \text{2.9K} \cdot \text{mol}^{-1} \tag{1}$$

The methanation of CO2 has a wide range of applications including the production of syngas and the formation of compressed natural gas [1]. A prototype CO2 recycling plant to supply clean energy preventing global warming has been built in 1996 using these key materials and has been operating successfully [11]. Without doubt, CO2 methanation is the key pathway for CO2 recycling, which requires a catalyst to achieve acceptable rates and selectivities. And extensive studies have been conducted on metal‐based catalytic systems in the hydrogenation of CO2 to methane.

Noble metals (e.g., Ru, Rh, Pd) supported on oxide supports (e.g., TiO2, Al2O3, CeO2) were the most effective catalysts for CO2 methanation under relatively mild operating conditions [12– 14]; however, the high cost of the catalysts limited their practical applications [15]. Therefore, to obtain a feasible and cost‐effective catalytic process, nonnoble metal catalysts (e.g., Ni, Co) were focused by many scholars [16, 17]. This review attempts to present the catalytic reactivity and reaction mechanism over the catalysts, particularly over the heterogeneous catalysts with an emphasis on the effects of supports and the second metal additives, as well as an overview regarding the challenges and opportunities for future research in the field.
