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Energies 2020, 13, 420 44 of 96 CO2 + 4H2 → CH4 + 2H2O −164 kJ/mol CO2 methanation reaction CO2 + H2 → CO + H2O 41 kJ/mol Reverse water–gas shift reaction (67) (68) Although the Gibbs free energy value of methanation reactions is highly negative in a wide temperature range, the reaction rate is aided by catalysts addition [555]. The selection of the catalyst material regulates the reaction activity and selectivity to methane products, avoiding the generation of heavier hydrocarbon compounds. The nickel-based catalyst was first used by Sabatier and Senderens for hydrogenation of carbon oxides and numerous organic compounds (i.e., ketones, aldehydes, alkenes, aromatics) [556]. Elements mainly located in groups 8–10 of the periodic system of elements are suitable materials for methanation catalysis. Mills and Steffgen investigate the catalytic activity and methane selectivity of the most suitable metals for methanation reactions. The activity order is: Ru > Fe > Ni > Co > Mo, while the selectivity ranking is Ni > Co > Fe > Ru [557]. Ruthenium and rhodium are among the most active catalysts for carbon oxides methanation. Yet, the high cost of noble metals obstructs their use in commercial-scale plants [558]. The nickel-based catalyst is highly selective towards methane production, is relatively cost-effective, and the catalytic activity is suitable for enhancing the reaction at low temperature [559]. The high activity of iron-based metals promotes the carbon oxides conversion, but nickel-based catalysts are preferred for the higher selectivity [560]. Finally, the catalytic activity of molybdenum compounds (MoS2, MoO2) is lower than that of the other metal-based materials considered suitable for methanation reaction, and the selectivity is toward higher hydrocarbons production [561]. However, molybdenum experiences a high resistance to acids compared to nickel and are commonly used in sulfidic environments [562]. Supports and promotors and preparation techniques are a fundamental feature for an efficient methanation process and a high methane yield [563]. Typically, the support is made of porous material to increase the number of the active site and the surface area of the catalyst nanoparticles exposed to reactants [564]. Several metal oxides such as TiO2, SiO2, Al2O3, CeO2 and ZrO2 are adopted [565]. Zeolites are commonly used as supports to the metal catalyst as they improve the methane selectivity [566]. Although the methanation catalysts are mostly supported, Raney nickel and MoS2 compounds are prepared without support material. Raney nickel is a Ni-Al alloy prepared with the alkaline dissolution of Al that has high activity in hydrogenation and methanation reactions. Indeed, the enhanced ability to dissociate CO increases the activity and selectivity [567]. Molybdenum sulfides are active catalytic materials attractive for the high resistance to sulfuric acid poisoning [568]. Dopants act as electronic or structural promoters. Structural promoters alter the formation and stabilization of catalytic materials to promote a finer nanoparticles dispersion and thus, a higher conversion yield. The electronic promoter, instead, alters the electron density of the catalyst promoting the conversion rate and modifying the product selectivity [569]. Promotors are also able to prevent catalyst deactivation, including sintering, fouling and poisoning mechanisms. Dopants are typically metal oxides such as Zr, Ce, La, V and Mg. The addition of zirconia to catalyst materials reduces the carbon formation and the activity deactivation due to sintering [570]. Ceria remarkably prevents the formation of coke deposits and improves the catalytic activity and stability [571]. Magnesium incorporation increases the dispersion of the catalytic particles, enhancing activity and stability [572]. Similarly, vanadium oxides addition results in a remarkable increase in thermal stability, resistance to coke formation and activity. Indeed, the catalyst and dopant calcination led to the formation in the substrate of particles small and highly dispersed [573]. Finally, adding lanthanum, the activity is improved since it allows higher CO2 adsorption on the surface, and the selectivity increases up to 100% at low temperature [574].PDF Image | Green Synthetic Fuels
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