Fig. 2.

Graphical representation of solar methanation reaction schemes and energetics. a In a first reaction step, water is split into reducing equivalents [H] and O₂. [H] can be molecular H₂ or a H⁺/e⁻ pair. If [H] is molecular H₂, then CO₂ is reduced to CH₄ via the “molecular pathway”. If [H] is a H⁺/e⁻ pair, then CO₂ is reduced to CH₄ via the “proton/electron pathway”. b Graphical representation of the thermodynamic states of the solar methanation reaction. The energy level of the system is elevated from S0 to S1 during the water splitting step. Energy is released during CO₂ reduction, reaching the energy level of the products, S2. c Reactions occurring during the solar methanation pathways shown in a. The “molecular” pathway represents the state-of-the-art of industrial methanation, where H₂ is produced during water electrolysis, as discussed in the “State-of-the-art industrial CO₂ methanation” section