Mechanism of CO₂ Electroreduction to Multicarbon Products over Iron Phthalocyanine Single-Atom Catalysts

Carbon dioxide reduction reaction (CO₂RR) is a promising method for converting CO₂ into value-added products. CO₂RR over single-atom catalysts (SACs) is widely known to result in chemical compounds such as carbon monoxide and formic acid that contain only one carbon atom (C1). Indeed, at least two active sites are commonly believed to be required for C-C coupling to synthesize compounds, such as ethanol and propylene (C₂₊), from CO₂. However, experimental evidence suggests that iron phthalocyanine (PcFe), which possesses only a single metal center, can produce a trace amount of C₂₊ products. To the best of our knowledge, the mechanism by which C₂₊ products are formed over a SAC such as PcFe is still unknown. Using density functional theory (DFT), we analyzed the mechanism of the CO₂RR to C1 and C₂₊ products over PcFe. Due to the high concentration of bicarbonate at pH 7, CO₂RR competes with HCO₃^- reduction. Our computations indicate that bicarbonate reduction is significantly more favorable. However, the rate of this reaction is influenced by the H₃O⁺ concentration. For the formation of C₂₊ products, our computations reveal that C-C coupling proceeds through the reaction between in situ-formed CO and PcFe(“0”)-CH₂ or PcFe(“-I”)-CH₂ intermediates. This reaction step is highly exergonic and requires only low activation energies of 0.44 and 0.24 eV for PcFe(“0”)-CH₂ and PcFe(“-I”)-CH₂. The DFT results, in line with experimental evidence, suggest that C₂₊ compounds are produced over PcFe at low potentials whereas CH₄ is still the main post-CO product.