Structural insights into mononuclear Cu₁ motifs for efficient CO electroreduction

Abstract

This study identified a fundamental structure-stability–activity triad correlation for CO electrocatalysis at site-specific mononuclear Cu₁–C₂N motifs. The privileged σ-π bonding synergy coupled with spatial confinement simultaneously optimizes reactivity-stability trade-offs from both electronic and geometric aspects, contrasting the conventional metal–Nx structures.

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Wei Liu

Wei Liu received his BE degree from Northwestern Polytechnical University (NPU) in 2012, and PhD from the University of Science and Technology of China (USTC) in 2017. After postdoctoral research at the Hefei National Research Center for Physical Sciences at the Microscale, USTC, he was promoted to an associate professor at the School of Chemistry, Dalian University of Technology (DUT) in 2020. His research interests focus on synchrotron radiation and its applications in photo-electrocatalysis for clean energy conversion.

The electrocatalytic upcycling of carbon monoxide (CO) into high-value chemicals has garnered considerable interest as a promising approach for energy storage and conversion.^1,2 Among the reported electrocatalysts, Cu-based systems are mostly employed due to their exceptional activity toward hydrocarbon production.^3–7 Nevertheless, achieving high product selectivity remains a fundamental challenge, which is primarily hindered by the complex, dynamic chemical states of Cu surfaces under specific reaction conditions.⁸ Crucially, a comprehensive understanding of the underlying reaction mechanisms remains elusive due to ambiguous conformational relationships.

Atomic metal–nitrogen–carbon (M–N–C) catalysts, featuring atomically dispersed metal centres embedded onto a carbon matrix, have emerged as promising platforms for selectivity control and mechanistic insights within multistep reactions. Recent elegant study on site-specific configurations such as Cu₁–N₄,⁹ Cu₁–N₂O₂ (ref. 10) and Cu₁–N₂B₂ (ref. 11) demonstrated notable efficacy for selective CO₂-to-CH₄ conversion. Despite these advances, isolated Cu₁ centres typically exhibit relatively weak binding of the key *CO intermediates, which impedes efficient consecutive hydrogeneration, particularly at moderate cathodic potentials. Strengthened *CO binding is therefore essential for the energetically favourable CO reduction reaction (CORR). However, enhanced *CO interactions also introduce significant stability concerns, as strong-field ligands like CO can irreversibly alter the electronic structure of metal sites, aggravating the catalyst deactivation at CORR-relevant potentials. These persisting activity–stability trade-offs have plagued the development of high-performance CORR catalysts.

In this study, we prototypically exploited the site-specific adsorption characteristics of C1 intermediates and thermodynamic trend across various mononuclear Cu₁ motifs. Importantly, we showcased the suppressed Jahn–Teller distortion in robust Cu₁–C₂N₁ moieties and their pivotal role in achieving selective CO-to-CH₄ conversion, in contrast to their conventional Cu₁–N₂ counterpart. The unique Cu–C interaction confers backdonation feedback, which mediates weakened adsorption strength and thereby lowers the reaction energy barrier in the initial proton-coupled electron transfer (PCET) step. Furthermore, we validate the broader applicability of this model across diverse carbon nitride (CN) architectures, highlighting the synergic bonding interaction and topological confinement as an efficient means to address stability–activity trade-offs within the realm of single atom catalysis.

In the covalent framework featuring a triazine backbone, the Lewis-basic nitrogen atoms act as robust σ-donor ligands that effectively stabilize isolated Cu atoms within the holey structure (Fig. 1a, b and S1a). This Cu₁–N₂ motif, displaying off-center distortion, represents a common structural paradigm in CN hosted transition metal-based single-atom catalysts (SACs).¹² As revealed by prior theoretical study, the Cu²⁺ ions are prone to leaching under aqueous or redox-active conditions, often transforming into hydrated Cu₁–N₂·2H₂O complexes.¹³ Such instability has been widely linked to the Jahn–Teller distortion associated with Cu²⁺ species (3d⁹ configuration), where the degenerate e_g orbitals (d_z² and d_x²−y²) render the system particularly sensitive to local ligand field perturbations.^14,15 Upon adsorption of small molecules like CO, a spontaneous symmetry breaking may occur to minimize the overall electronic energy, resulting in lower-symmetry yet electronically more stabilized complexes. This ligand field distortion not only perturbs the geometric structure of the active site but also modulates key aspects of the catalytic mechanism, including intermediate binding/desorption and overall reactivity.^16,17

Fig. 1 DFT optimized structures of (a) Cu₁–N₂/CN, (b)Cu₁–N₂/PHI, CO chemisorbed (c) Cu₁–N₂/CN and (d) Cu₁–N₂/PHI. (e) Time-dependent evolution of Cu–N bond lengths (left) and total energies (right) in the Cu₁–N₂/CN system. (f) Calculated binding energies of CO onto Cu₁–N₂/CN, Cu₁–N₂/PHI and Cu₁–N₂/melon (left) and the corresponding Cu–C bond lengths (right).

To gain insight into these effects in the context of the CORR, we performed density functional theory (DFT) calculations on three representative substrates—CN, poly(heptazine imide) (PHI), and melon—each supporting Cu₁–N₂ active centers (Fig. 1c, d and S1b). In all cases, CO binding induces a pronounced structural deformation. As shown in Fig. 1b, the Cu center, initially confined within a planar triazine-based cavity, is displaced out of plane following CO adsorption. Typically, in the CO chemisorbed Cu₁–N₂/CN system, the axial Cu–C(O) bond measures only ca. 1.77 Å, while the equatorial Cu–N bonds elongate on average from 2.36 Å to 2.44 Å, resulting in local symmetry breaking and lifting of orbital degeneracy. These observed geometric distortions are consistent with the expected compressed Jahn-Teller distortion behavior of the Cu²⁺ center under strong ligand interactions.

To evaluate the dynamic stability of these configurations under realistic conditions, we conducted ab initio molecular dynamics (AIMD) simulations at room temperature. Across all three frameworks, the Cu₁ center exhibited marked instability, characterized by significant energy fluctuations and abrupt structural transitions throughout the simulation trajectory (Fig. 1e and S2–S4). Mean square displacement (MSD) analysis further revealed extensive migration of the Cu atom away from its original coordination site, accompanied by persistent oscillations in the Cu–N bond length (Fig. S5–S7). These findings confirm that CO adsorption not only induces static geometric distortion but also fosters dynamic destabilization of the active center. Consistently, all systems display strong Cu–CO binding, with adsorption energies exceeding −1.0 eV—values typically associated with hindered intermediate desorption and restricted catalytic turnover (Fig. 1f).

To explore the strong binding effects on reaction energetics, we computed the free energy profiles for CO reduction across the three substrates. All systems exhibited the highest reaction barrier at the first proton-coupled electron transfer (PCET) step, with activation energies of 1.04 eV (Cu₁–N₂/CN), 0.83 eV (Cu₁–N₂/PHI), and 0.97 eV (Cu₁–N₂/melon) (Fig. 2a). This shared bottleneck suggests that the strong stabilization of the *CO intermediate increases the thermodynamic cost of initiating electron–proton transfer, thereby impeding catalytic progression.

Fig. 2 (a) Free energy diagram of the CORR over Cu₁–N₂/CN and Cu₁–N₂/PHI. (b)–(c) Projected density of states (PDOS) for CO (upper panel) and CHO (lower panel) adsorption over (b) Cu₁–N₂/CN and (c) Cu₁–N₂/PHI. (d) d-band center for a clean surface and adsorbed CO.

To probe the electronic basis of this phenomenon, we examined the projected density of states (PDOS) of the Cu₁ center before and after CO and CHO adsorption (Fig. 2b, c and S8–S10). While the three frameworks exhibit minor variations in their unbound electronic structures, the core Cu₁–N₂ motif remains electronically consistent across the systems. Upon CO adsorption, a notable downward shift in the d-band center (ε_d) is observed—for instance, from −1.46 eV to −3.32 eV in Cu₁–N₂/CN—indicative of strong electron withdrawal from the metal center (Fig. 2d). This shift reflects the dual nature of the Cu–CO interaction, comprising both σ-donation from the CO lone pair and π-back donation from the Cu d-π orbitals into the 2π* antibonding orbital of CO. This synergistic bonding results in orbital splitting, partial depletion of the d_z² orbital, and a transition in hybridization from square-planar dsp² toward pseudo-octahedral d²sp³.¹⁸ The resulting Jahn–Teller distortion drives the Cu atom out of the planar cavity, thereby compromising the geometric stability of the active site and ultimately influencing catalytic performance.

To address the challenges posed by the excessive binding of CO at Cu₁–N₂ centers, we explored alternative coordination environments capable of attenuating adsorbate interactions while maintaining structural integrity. Prior studies have shown that introducing axial ligation can disrupt the chemisorption of reactants/intermediates by modifying ligand field symmetry to reduce electronic overlap.^19–21 However, achieving precise and scalable axial coordination architectures has turned to be synthetically demanding, and they are reasonably labile under CORR-relevant bias. An alternative approach involves enhancing the intrinsic metal–support interaction. In this context, we developed a defect-engineered Cu₁–C₂N moiety by introducing nitrogen vacancies into the triazine-based framework. Defect-trapped Cu₁ sites proved to be thermodynamically more stable, aligning with earlier studies.²² The migration of the Cu atom from its original Cu₁–N₂ site towards the vacancy site is predicted to be an energetically favorable process, which indicates great potential for controllable synthesis (Fig. S12).

We implemented this strategy on both CN and PHI frameworks. Structural optimization shows that the Cu–C bond in Cu₁–C₂N/CN (1.91 Å) is significantly shorter than the Cu–N bond in the original Cu₁–N₂/CN (2.11 Å), indicating a stronger interaction between Cu and the defected carbon site. To probe the thermodynamic and kinetic stability of these new motifs, we carried out AIMD simulations (Fig. 3e and S11a). The time course of total energy evolution in both Cu₁–C₂N/CN and Cu₁–C₂N/PHI systems displays smooth convergence without abrupt fluctuations, suggesting high structural stability. Further analysis of Cu–C bond length distributions reveals minimal fluctuation around the initial mean value, with no signs of structural degradation or significant distortion throughout the simulation. As shown in the COHP analyses (Fig. S13), the Cu–support interaction is strengthened when the coordination changes from N₂ to C₂N, indicated by more negative ICOHP values. This enhancement arises from reduced occupation of antibonding states in the C₂N structure. To evaluate the electrochemical stability of Cu single-atom sites, we calculated the reduction potentials for five representative configurations (Table S1). The results show that C₂N-coordinated Cu sites exhibit more negative reduction potentials than N₂-coordinated ones, indicating stronger resistance to over-reduction. This suggests that C₂N environments better stabilize Cu centres at negative potentials, which is crucial for maintaining atomic dispersion and preventing aggregation.

Fig. 3 DFT optimized structures of (a) Cu₁–C₂N/CN, (b) Cu₁–C₂N/PHI, CO chemisorbed (c) Cu₁–C₂N/CN and (d) Cu₁–C₂N/PHI. Time-dependent evolution of Cu–N bond lengths (left) and total energies (right) in (e) pristine Cu₁–N₂/CN and (f) the CO adsorbed system.

We proceeded to examine the adaptive behavior of metal sites upon CO adsorption. In contrast to the Cu₁–N₂ systems, where CO binding induces significant Jahn–Teller distortion and off-plane displacement of the Cu atom, the Cu₁–C₂N involved frameworks exhibit only marginal structural perturbation. In both Cu₁–C₂N/CN and Cu₁–C₂N/PHI cases, CO adsorption does not significantly displace the Cu atom from its original site (Fig. 3c and d). An AIMD trajectory spanning 6 ps post–CO binding confirms this observation: the Cu atom remains within 0.3 Å of its initial location, and the local coordination is predominantly preserved (Fig. 3f and S11b). Importantly, the system exhibits rapid structural recovery following minor fluctuations, with the Cu atom returning to its original coordination geometry within 0.5 ps. This behavior hints at a level of self-healing or reconstructive stability intrinsic to the Cu₁–C₂N moiety (Fig. S16). From a thermodynamic perspective, the adsorption of CO on Cu₁–C₂N/CN and Cu₁–C₂N/PHI yields binding energies of −0.73 eV and −0.98 eV, respectively. These values fall within an optimal window for catalytic reactions, being sufficiently strong to facilitate effective charge transfer yet not overly strong as to impede intermediate desorption or inhibit turnover. In addition, we examined alternative reduction pathways, which were found to be thermodynamically disfavoured (Fig. S18 and Table S2). The corresponding thermodynamic diagrams under different pH conditions are also provided (Fig. S19).

To further evaluate the catalytic implications of the modified Cu₁ sites, we investigated the full CORR pathway, using Gibbs free energy calculations to assess the CO-to-CH₄conversion (Fig. 4a). Notably, the potential-determining step (PDS)—the initial PCET step—exhibits a significantly reduced barrier: from 1.04 eV to 0.35 eV on the CN substrate, with a similar reduction observed on PHI. Decomposition of the binding energy reveals that this barrier reduction is primarily attributed to a decrease in deformation energy. In the Cu₁–N₂ system, deformation accounts for nearly 50% of the total binding energy, whereas in Cu₁–C₂N, the stronger native coordination leads to minimal structural rearrangement upon CO binding (Fig. S20). This lowered deformation cost results in moderate binding energies that are more conducive to catalytic turnover. Collectively, these findings underscore that engineering well-defined Cu₁–C₂N sites through nitrogen vacancy-induced restructuring offers a promising avenue for achieving improved stability and activity in CO reduction catalysis.

Fig. 4 (a) Free energy diagram of the CORR over Cu₁–C₂N/CN and Cu₁–C₂N/PHI. (b)–(c) Projected density of states (PDOS) for CO (upper panel) and CHO (lower panel) adsorption over (b) Cu₁–C₂N/CN and (c) Cu₁–C₂N/PHI. (d) d band center for a clean surface and adsorbed CO. Differential charge densities of (e) CO adsorbed Cu₁–C₂N/CN and (f) CO adsorbed Cu₁–C₂N/PHI. The charge depletion and accumulation are depicted in cyan and yellow, respectively. The isosurface value is 0.004 e Å⁻³. (g)–(h) Interactions between CO molecular frontier orbitals and Cu 3d orbitals of (g) Cu₁–N₂/CN and (h) Cu₁–C₂N/CN.

In the case of Cu₁–C₂N/CN, the defect-confinement effectively stabilizes pseudo-square-planar (D_4h) Cu₁ sites, maintaining structural integrity and inhibiting out-of-plane displacement upon CO adsorption. The Cu–C establishes a synergic σ-π bonding interaction, particularly beneficial in strong ligand environments. The energy separation between the Cu d_x²−y² and d_z² becomes sufficiently large so that the hole forms a singlet in the d_x²−y² orbital. As compared to Cu₁–N₂, the d_z² orbital of Cu₁–C₂N is fully occupied, and CO adsorption begins with electron donation from the 5σ orbital of CO to the d_x²−y² orbital of Cu. This modified interaction leads to weakened CO adsorption, resulting in a Cu–C(O) bond length measuring 1.97 Å. Differential charge density analysis confirms the charge transfer contribution of the d_x²−y²–5σ component and reduced π back-donation from Cu to *CO (Fig. 4b). The COHP calculated after CO adsorption also supports this view (Fig. S23).

The electronic structures of the Cu₁–C₂N moiety on various supports were investigated based on PDOS analysis. As shown in Fig. 4c, the calculated ε_d of Cu₁–C₂N/CN undergoes significant downshift to −2.99 eV compared to that of Cu₁–N₂/CN (−1.46 eV), with occupied states notably distant from the Fermi energy level, indicating a destabilized Cu–CO interaction. Following CO adsorption, a minor downshifted ε_d aligns with the observed suppressed electron feedback behavior. Additionally, the hybridized peaks below the Fermi level demonstrate strong electronic resonance between C-2p (*CHO, *CO) and Cu-3d_x²−y², suggesting enhanced reactivity on the Cu₁–C₂N sites (Fig. 4d).

To evaluate whether the structural response is specific to CO, we extended our calculations to NO and NO₃⁻ (Fig. S24–S26). Similar to CO, both species caused notable distortion in the Cu₁–N₂ structure, while the Cu₁–C₂N structure remained largely intact, indicating enhanced rigidity and resistance to adsorbate-induced perturbation conferred by the C₂N coordination.

Collectively, these findings demonstrate that the unique Cu₁–C₂N moiety effectively mitigates CO overbinding, while preserving charge transfer efficiency and stabilizing the Cu active site, offering a rational strategy to address stability–activity trade-offs. The structure insights here inform further improvements to this catalyst system. Moreover, the energetically favored metal restructuring establishes a fascinating paradigm for controllable synthesis of SACs and is currently undergoing investigation.

Conclusions

In summary, DFT calculations unveil that excessive CO binding induces compressed Jahn–Teller distortions and impedes turnover of the CORR on conventional atomic Cu₁–N₂ sites. This unfavorable interaction arises from strong electronic coupling within the consonant 3d_z²–σ and 3d_xz/d_yz–2π* contributions, which suppresses the PCET process and destabilizes the coordination structure. To address these challenges, we exploited enhanced metal–support interactions and proposed a unique Cu₁–C₂N moiety by incorporating nitrogen vacancies. The privileged σ-π bonding synergy coupled with spatial confinement proves to be advantageous for suppressed Jahn–Teller distortion and prevented demetallation. The pseudo-square-planar Cu₁ sites interact with C1 intermediates predominantly through the 3d_x²−y²–5σ bond and significantly reduce the rate-determining barrier for CO reduction. The synergic bonding interaction and topological confinement present a promising avenue for tackling stability–activity tradeoffs in the realm of single atom catalysis and indicate great potential for exploring multi-electron and proton transformations.

Author contributions

Wei Liu and Yantao Shi: conceptualization, supervision, funding acquisition, writing, review & editing. Wenzhe Shang: methodology, data curation, formal analysis, writing – original draft. Xiangyang Zhang and Tianna Liu: data curation. All the authors contributed to the overall scientific interpretation and edited the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The data supporting this article have been included as part of the SI.

Additional calculation methods, and supplementary figures, tables, and supporting discussion referenced in the main text. See DOI: https://doi.org/10.1039/d5ta04412j.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22472014 and 51773025), the Natural Science Foundation of Liaoning Province (2024-MSBA-13), Fundamental Research Funds for the Central Universities (DUT22LAB602), and the China Postdoctoral Science Foundation (Grant No. 2023M740496).

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