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.