DFT Studies on C–C Bond Formation Mechanism in Electrochemical CO2 Reduction on Single Cu‐Modified Covalent Triazine Frameworks
Authors: Ohashi Keitaro , Nagita Kaito , Harada Takashi , Nakanishi Shuji , Kamiya Kazuhide 
Published: December 2024

The electrochemical reduction of CO2 into C2+ products represents a promising solution to completing the carbon cycle, thereby fostering a sustainable energy supply. Since Hori and coworkers demonstrated that Cu-based catalysts can efficiently catalyze the conversion of CO2 to C2+,1 numerous studies on developing Cu-based electrocatalysts for C2+ production and identifying their reaction mechanism have been reported. With respect to catalysts composed of bulk Cu or one of its alloys, the mechanism for C–C bond formation, a key step in the production of C2+ compounds, has been generally agreed upon as a Langmuir-Hinshelwood-type CO dimerization process. In contrast to these traditional bulk metal electrocatalysts for CO2RR, single-atom electrocatalysts (SAECs), which comprise single metal atoms dispersed on heterogeneous substrates, has garnered significant attention as next-generation electrocatalysts since SAECs exhibit unique catalytic features stemming from fine tuning of the adsorption strength of the reaction intermediates through manipulation of the coordination environment. Our group has focused on one class of SAECs – metal-modified pyridine-containing covalent triazine frameworks (M–CTFs) – which show good mechanical robustness and high chemical durability stemming from their conjugated nitrogen bonding.2, 3 Because SAECs lack adjacent metal sites, the C–C bond formation mechanism commonly believed to occur on bulk Cu does not unequivocally proceed on Cu-based SAECs. However, given that C–C bond formation represents the pivotal elementary step for C2+ production, a comprehensive study is warranted. Herein, we carried out a first-principles study on the C–C bond formation mechanism on single-Cu-atom-modified CTFs (Cu-CTFs), which are a promising platform for SAECs.4

We proposed several possible mechanisms based on a previous paper in Fig. 1 and the progression of these reactions was first evaluated with static DFT. In Mechanism (a), the co-adsorption of two CO molecules onto the single Cu atom (step a1) is not feasible. Furthermore, *OCCO in Mechanism (b) was cleaved into two COs during the relaxation. On the basis of the aforementioned considerations, we ruled out the CO dimerization process on Cu-CTFs. We next considered the formation of C–C bonds via the one-electron CO reduction reaction (Mechanisms (c) and (d)) and calculated the free energy diagram for *CHO and *COH formations (Fig. 2). At the electron transfer to *CO, although there is a large endergonic barrier (1.93 eV at U=–0.69 V vs. computational hydrogen electrode(CHE)) for *COH formation, *CHO formation is exergonic. The energy diagram for *CHO⇀*COCHO shows an endergonic reaction of ~0.40 eV. *COCHO was adsorbed onto the Cu site and the C–C bond was not cleaved after the structural relaxation, which is in contrast to the result for the *OCCO intermediate formed by CO dimerization.
Static DFT, without water and performed under vacuum conditions, is a suitable method for identifying a metastable reaction intermediate with low calculation cost. In contrast, considering that *CHO and *COCHO are hydrophilic groups, incorporating the accurate solvation effects using explicit water molecules would allow more accurate assessment of reaction energies and lead to more efficient SAECs design. Ab intio molecular dynamics (AIMD) is a powerful tool to simulate elementary steps with explicit water molecules. Thus, we thought that the C–C bond formation of CO2RR on Cu-CTFs can be further explored by hybridizing AIMD with static DFT. We attempt to elucidate the reaction mechanism for *CHO+CO⇀*COCHO through a blue moon ensemble using constrained AIMD as shown in Fig. 3. When we imposed a constraint on the C–C bond distance between *CHO and CO molecule (d
C–C) and when the potential energy at a d
C–C of 2.6 Å was set to zero, the highest energy recorded was 0.09 eV at a d
C–C of 1.8 Å, while the minimum energy was –0.14 eV at a d
C–C of 1.5 Å as the final state. To elucidate the origin of the d
C–C-related changes in the potential energy, we divided the energy profile into four regions, which can be interpreted as the *CHO+CO⇀*COCHO reaction proceeding as follows: a transition of co-adsorption (local minimum) between CO and *CHO undergoes, CO starts to interact with the adsorbed *CHO on the Cu center, with a concomitant increase in potential energy and is followed by an insertion reaction to form *COCHO.

References

(1) Hori, Y.; Murata, A.; Takahashi, R.; Suzuki, S. Chem. Lett.
1987, 16, 1665–1668.
(2) Kamiya, K. Chem. Sci.
2020, 11, 8339–8349.
(3) Kato, S.; Hashimoto, T.; Iwase, K.; Harada, T.; Nakanishi, S.; Kamiya, K. Chem. Sci.
2023, 14, 613–620.
(4) Ohashi, K.; Nagita, K.; Yamamoto, H.; Nakanishi, S.; Kamiya, K. ChemElectroChem
2024, 11.

Figure 1