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

Abstract

Carbon dioxide reduction reaction (CO 2 RR) is a promising method for converting CO 2 into value-added products. CO 2 RR 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 (C 2+ ), from CO 2 . However, experimental evidence suggests that iron phthalocyanine (PcFe), which possesses only a single metal center, can produce a trace amount of C 2+ products. To the best of our knowledge, the mechanism by which C 2+ products are formed over a SAC such as PcFe is still unknown. Using density functional theory (DFT), we analyzed the mechanism of the CO 2 RR to C1 and C 2+ products over PcFe. Due to the high concentration of bicarbonate at pH 7, CO 2 RR competes with HCO 3 − reduction. Our computations indicate that bicarbonate reduction is significantly more favorable. However, the rate of this reaction is influenced by the H 3 O + concentration. For the formation of C 2+ products, our computations reveal that C−C coupling proceeds through the reaction between in situ-formed CO and PcFe("0")−CH 2 or PcFe("-I")−CH 2 intermediates. This reaction step is highly exergonic and requires only low activation energies of 0.44 and 0.24 eV for PcFe("0")−CH 2 and PcFe("-I")−CH 2 . The DFT results, in line with experimental evidence, suggest that C 2+ compounds are produced over PcFe at low potentials whereas CH 4 is still the main post-CO product.

Exploring the Mechanism of the Electrochemical Polymerization of CO2 to Hard Carbon over CeO2(110)

“…Accordingly, also later oligo- and polymerization steps can proceed without the need for interactions with the catalyst provided an electron transfer from the cathode to the terminating carboxylate (R–COO) group is feasible and CO 2 is still available as feedstock. This sets CeO 2 apart from classical CO 2 reduction catalysts which contribute more directly to the reaction path by stabilizing the carbene intermediates through a metal–carbon bond , and adsorption sites for the coupling partner . Once the chain has grown through subsequent reactions with CO 2 , the reduction and conversion of the keto group into their enol form becomes possible.…”