Fig. 4. DFT calculations on CO₂ protonation to CH₄.

a Free energy diagram for CH₄ production on Cu active site in the complex structures of [C₁₀H₁₄CuN₂O₈] and [C₁₀H₁₅CuN₂O₈]⁺. The inserted figures represent the protonation/deprotonation between [C₁₀H₁₄CuN₂O₈] and [C₁₀H₁₅CuN₂O₈]⁺. b Corresponding atomic configurations for each elementary step, including [C₁₀H₁₄CuN₂O₈], *CO₂, *COOH, *CO, *CHO, *OCH₂, *OCH₃, *O, *OH, and [C₁₀H₁₅CuN₂O₈]⁺. Orange, red, gray, white, blue sphere represent Cu, O, C, H, N atoms, respectively.