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Article

A “Goldilocks Zone” in Bilayer Cobalt Phthalocyanine: Optimizing Confinement for Efficient CO2RR

by
Longlei Yin
1,
Dongdong Qi
1,*,
Tianyu Wang
1,* and
Jianzhuang Jiang
1,2,*
1
Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, Department of Chemistry and Chemical Engineering, School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, China
2
Guizhou Key Laboratory of Macrocyclic and Supramolecular Chemistry, School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China
*
Authors to whom correspondence should be addressed.
Catalysts 2026, 16(4), 328; https://doi.org/10.3390/catal16040328
Submission received: 3 March 2026 / Revised: 26 March 2026 / Accepted: 1 April 2026 / Published: 2 April 2026
(This article belongs to the Section Computational Catalysis)
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Abstract

In this study, the electrochemical reduction of CO2 to CO within a bilayer cobalt phthalocyanine (CoPc)2 confinement system was systematically investigated using density functional theory (DFT). The results reveal that the (CoPc)2 architecture creates a well-defined catalytic microenvironment, in which the synergy between vertical spacing (regulated by moderate interlayer interactions) and lateral displacement gives rise to an optimal “Goldilocks zone”. This zone is characterized by a vertical distance (D) of 4.25–4.5 Å and a parallel displacement (L) of approximately 1 Å. Within this confined environment, the adsorption and desorption of key intermediates are optimally balanced, leading to enhanced catalytic activity. Electronic structure analysis further demonstrates that such spatial confinement induces asymmetric charge redistribution in the CO2 molecule, resulting in distinct regioselectivity. This work provides a general design strategy for developing high-performance and site-selective catalysts through precise engineering of interlayer geometric environments.

Graphical Abstract

1. Introduction

Against the backdrop that environmental issues have evolved into a critical challenge threatening human survival [1,2,3], the concept of carbon neutrality has become a universal consensus [2,3,4,5,6]. In recent years, scientists have developed various catalysts to drive the carbon dioxide reduction reactions (CO2RRs) in order to reduce carbon emissions [7,8,9,10,11], among which cobalt-containing complexes have been proven to be an effective kind of catalytic material [12,13,14,15,16,17,18].
Our theoretical investigation on cobalt-containing catalysts starts from 2018; in that year we applied an overall investigation to a mono-cobalt catalytic site [19]. In fact, in the past several years, we demonstrated various CO2RR catalyzing mechanisms with transition metal complexes using density functional theory (DFT) calculations [20,21,22,23,24,25,26,27]. Along this research line, our group has further extended such atomic-level understandings into crystalline covalent organic frameworks (COFs), particularly those based on metallophthalocyanine (MPc) building blocks. We have systematically revealed how substituent engineering, interlayer hydrogen-bond networks, hierarchical nanostructures, and local microenvironment modulation collectively govern the activity, selectivity, and stability toward both CO and CH3OH production in electrocatalytic and photocatalytic CO2 reduction. These accumulated findings strongly motivate the present theoretical study. Among these works, a special work based on dual-cobalt catalyzing site showed a completely different mechanism and outstanding catalytic performance (~2.4 μmol g−1 s−1 CO and ~90% CO/H2 selectivity) [13]. This dual-cobalt confined space provides a synergistic catalyzing environment to lower the Gibbs free energy barrier of CO2RR. Other research groups also found that this type of synergistic catalytic environment is indeed significantly influential to the catalytic performance [28,29,30,31,32,33,34,35]. For instance, Dong et al. [33] constructed a Z-scheme heterojunction system by combining cobalt phthalocyanine (CoPc) with Zn-Salen-COF through π-π stacking, where synergistic dual metal sites (Co2+ and Zn2+) reduced the Gibbs free energy barrier for *COOH formation from 0.30 eV to 0.11 eV. Lu et al. [34] further confirmed through density functional theory (DFT) calculations that the interlayer spacing plays a crucial regulatory role in the selectivity and thermodynamic properties of (FeN4)2 in the catalytic reduction of carbon dioxide. Huang et al. [35] employed a thermally induced strategy to obtain three distinct interlayer configurations—IHEP-21 (5.97 Å), IHEP-22 (5.31 Å), and IHEP-23 (5.13 Å)—with photocatalytic activities following the order IHEP-22 > IHEP-23 > IHEP-21. Specifically, IHEP-22(Co) exhibits the highest CO production rate (350.9 μmol·h−1·g−1), 3.60 and 1.46 times higher than IHEP-21(Co) and IHEP-23(Co), respectively. This experimental trend confirms that moderate interlayer spacing optimally balances electronic interactions and steric accessibility for dual-site synergistic catalysis. Collectively, these studies establish that the distance and spatial configuration between adjacent catalytic motifs constitute decisive factors governing reaction thermodynamics and product selectivity in CO2 reduction, which could be viewed as our initial idea of the present work.
Although our previous basic research provided important insights into the carbon dioxide reduction reaction (CO2RR), and several of the above studies have also demonstrated the significance of the synergistic environment in bimetallic catalysts, they were largely confined to specific bimetallic structure models and have not explored the influence of the complex spatial interactions within the structure on the catalytic performance. This study aims to fill this gap through two major innovative breakthroughs. Firstly, we introduced two variables, vertical distance (D) and parallel distance (L), to define the bimetallic geometric structure. This two-dimensional variable reveals how the coordinated evolution of the spatial structure determines the configuration of the catalyst and its subsequent impact on the thermodynamics of the reaction. Secondly, this study aims to go beyond the description limitations of specific materials (such as a certain type of specific COF) and construct a representative bimetallic macrocycle (MN4)2 confined catalytic model. This model provides a universal structural design guideline for dual cobalt catalysts or dual metal macrocycle catalysts (such as dual cobalt phthalocyanines, etc.) with similar geometric structures. Through the continuous simulation of the evolution of spatial geometric parameters, we revealed the intrinsic relationship between the confined microenvironment and the catalytic reduction of carbon dioxide to carbon monoxide performance. This work fills the theoretical gap in our team and other research teams’ previous studies regarding “the regulatory laws of spatial geometric topology on catalytic activity”.

2. Results and Discussion

2.1. Structural Stability and Interlayer Interactions of (CoPc)2

The present model, face-to-face dimer, is a normal structure existing in the catalyzing field. Scientists found that the layer-layer distance of two layers is located in the range between about 3.0 and 5.0 Å range [36,37,38,39]. It is worth noting that the D and L distances are not only determined by the refined catalyzing center itself. This structure is always embedded into a peripheral supporting framework. So, the shape of this refined catalyzing center is not only determined by itself but also significantly affected by the peripheral supporting structures. But we must pay attention to the fact that the structural stability is also exerting positive or negative effects to the whole stability. Here, we start our discussion from the refined space stability together with its positive/negative effects on the whole stability. Details regarding the selection of spin multiplicities for (CoPc)2 are provided in Table S1.
We systematically investigated the evolution of binding energy (ΔE) under the coherent of vertical distance (D = 3–8 Å) and parallel distance (L = 0, 1 Å). Our results indicate that the system stability is predominantly governed by the vertical distance D, Figure 1. In contrast, the parallel distance L exerts a marginal influence. As L increases from 0 to 1 Å, the energy profiles remain essentially parallel, exhibiting only minor numerical deviations. Based on the response of ΔE to D, the structural evolution can be categorized into three distinct physical regimes:
(1)
Attracting force range (D = 3.0–4.5 Å): Within this range, the negative ΔE confirms an attracting force between the two CoPc layers. The system reaches its thermodynamic minimum of −2.64 eV at D = 3.5 Å (L = 0 Å). But here we must remind all the readers that, although D = 3.5 Å performs best in stability, the space between Co-Co catalyzing sites is too narrow to accommodate CO2 molecules, which brings great hindrance to the interlayer catalyzing processes. This difficulty is directly emphasized in the subsequent discussions of the present article.
(2)
Repulsive force range (D = ~5 Å): Along with the continuous elongation of D, ΔE undergoes a process of first increasing and then decreasing, resulting in a repulsive range of D = ~5 Å.
(3)
The range of weakly coupled double catalytic layers (D = 5.5–8.0 Å): As D increases further, the binding energy rapidly diminishes and plateaus toward zero. This indicates a dissipation of interlayer electronic coupling, where the system virtually transitions into two quasi-independent molecules.
Based on this stability analysis, the D = 3.5–5.0 Å range was selected for evaluating CO2RR performance. Configurations corresponding to D < 3.5 Å were not considered, as such small interlayer distances give rise to intense steric repulsion and provide no kinetic space for reactants to enter. We also excluded structures with D > 5.0 Å, since these fail to retain the required geometric constraints and interlayer synergy.
Finally, clarifying the physical differences between these ranges is necessary. As shown in IGMH (independent gradient model based on Hirshfeld partitioning) [40] surface distributions (Figure 2), interlayer isosurfaces evolve significantly as D increases in 0.5 Å steps. For both L = 0 Å and L = 1 Å models, continuous, dense planar overlap at D = 3.5 Å transitions gradually into fragmented local distribution at D = 4.5 Å. Eventually, these interactions become sparse at D = 5.0 Å. Meanwhile, models at the same vertical distance show slightly weaker interlayer interaction at L = 1 Å compared to L = 0 Å.

2.2. Catalytic Performance Evaluation Based on Gibbs Free Energy Profiles

Using the above (CoPc)2 refined catalyzing space, we tried to introduce CO2, H+, and e− to complete Equation (6). DFT calculations were applied in three steps, leading to a gradually deepening understanding.
First, we calculated the Gibbs free energy curves based on the eight structures selected from the stability analysis. As can be found (Figure 3a), the vertical distance (D) significantly affects the Gibbs free energy curve, leading to different rate-determining steps (RDSs) and adsorption stabilities. Particularly, when the changing range of L is within 1 Å, it is obvious that the main affecting factor is D rather than L. When D is as small as 3.5 Å, an important intermediate *COOH cannot exist between such narrow CoPc planes. When D is increased to 4.0 Å, the RDS is identified as the step of CO2 absorption because of the great steric hindrance. We continued to investigate the situation with D = 4.5 Å and found that its RDS is located at the step of CO desorption with ΔG between 1 and 1.5 eV. When D is enlarged to 5 Å, the RDS is still CO desorbing step, but ΔG is increased to the range of >2 eV. Here we must emphasize three rules in catalysis science [9,41]. If the adsorption energy of catalyst for reactants is too low, the reactants are difficult to capture by the active sites. If the adsorption energies of intermediates are too low, the intermediates are desorbed in advance, which destroys the integrity of catalytic reaction. If the adsorption energy of the product is too high, the product will retain and block the active site and hinder the catalysis of new reactants, which is the nature of so-called catalyst poisoning. Since CO2RRs are often carried out at room temperature, we can judge that D = 4.5 Å is much better than other catalyst models. The detailed Gibbs free energy data for all evaluated structures are summarized in Table S2.
Then, we proceeded to the second step: constructing a more finely tuned catalyst model by narrowing the sampling intervals (Figure 3b). As can be found, we added D = 4.25 Å and D = 4.75 Å beyond D = 4.5 Å; the results show that D = 4.25 Å (L = 0/1 Å) exhibits a lower rate-determining step energy barrier compared to CoPc. Finally, in the third step, to further explore the influence of parallel distance, L = 0.5/1.5 Å were increased in the two models (D = 4.5/4.25 Å) with better catalytic performance (Figure 3c,d). We can observe that the Gibbs free energy curves of the D4.25L1.5 and D4.25L1.0 are highly similar to each other. This suggests that parallel position adjustments within ~1 Å have a much weaker impact on the thermodynamic trends compared to vertical distance adjustments. Nevertheless, a proper parallel displacement also plays a beneficial role in facilitating the CO2RR. (From here, we start to use the symbol DXLY. For example, D4.25L1.5 denotes a configuration with D = 4.25 Å and L = 1.5 Å).
To test the limitation of a larger parallel distance, we extended our calculations to a more significantly displaced structure (D4.25L2) (Figure 3c,d). We found a notable discrepancy in the comparison between D4.25L2 and D4.25L1/1.5. Despite a lower barrier for CO2 activation, the reaction is hindered by a prohibitively high CO desorption energy (ΔG = 2.032 eV), leading to catalyst poisoning. This phenomenon was also observed for D = 4.5 Å, where the onset of poisoning occurs at a smaller offset (L = 1.5 Å), as the CO desorption barrier already reaches 2.50 eV.
This study evaluated the actual selectivity of the initial adsorption step in a mixed gas environment by calculating the adsorption energies of N2 and H2O on a representative D4.5L1 bilayer cobalt phthalocyanine (CoPc)2 model. The calculation results showed that compared to CO2 (ΔEads = −0.80 eV), the adsorption of N2 is extremely weak (ΔEads = 0.03 eV), indicating that it hardly produces competitive adsorption. Although the adsorption of H2O was stronger than that of N2, it was still 0.14 eV weaker than that of CO2. Given that the main competing reaction in the CO2RR process is the hydrogen evolution reaction (HER) rather than the simple binding of water molecules, the HER activities of the monolayer cobalt phthalocyanine (CoPc) and the bilayer model were evaluated at different vertical distances and parallel distances (0 Å and 1 Å). As shown in Figure 4a, for the (CoPc)2, when the vertical distance is 4.0–4.5 Å, the Gibbs free energy change in HER is at a moderate level. It is neither as easy to occur as in the single-layer case, nor as excessive adsorption as at 4.75 Å and 5 Å, which might lead to the occupation of reactive sites. Further, the selectivity was evaluated by combining the thermodynamic limiting potential difference U L C O 2 − U L H 2 [42,43]. As shown in Figure 4b, among the studied models, D4.25L0/1 and D4.5L0/1 exhibit more positive limiting potential values, indicating higher selectivity for the carbon monoxide (CO) product. Although the models at 4.75 Å and 5.0 Å also have relatively positive limiting potentials, their overall catalytic performance is not ideal due to the site occupation issue caused by overly negative ΔG(*H) as analyzed previously.
In summary, only when the geometric configuration is optimized within an appropriate range of D and L can we achieve a favorable balance among reactant adsorption, intermediate binding, product desorption and the inhibition of the hydrogen evolution reaction (HER). The maximum Gibbs free energy change (ΔG) for each model, representing the rate-determining step (RDS) [44] is summarized in Figure 3e. Notably, at D = 4.25 Å with L in the range of 0–1.5 Å, the RDS barriers for the CO2RR are consistently below 1.0 eV, leading to superior catalytic efficiency. In particular, the D4.25L1 configuration exhibits the most significant interlayer promotion effect, with an RDS barrier of only 0.98 eV, which is 0.29 eV lower than that of the monolayer model Although the RDS barriers for the D4.5 (L = 0/0.5/1 Å) systems are slightly higher than that of the CoPc monolayer, these configurations offer more stable CO2 adsorption without the risk of catalyst poisoning, making them our sub-optimal models.

2.3. Evolution of Reaction Intermediates and Microscopic Mechanisms

After the detailed catalyzing performance sort based on Gibbs free energy curves, now the key task naturally changed into the catalyzing mechanism analysis. As can be found, the CO2 molecular structure is always obviously changed in the double layer (CoPc)2 refined catalyzing space, in which the bond angle is significantly decreased from 180° (linear formation for free CO2 with the C=O bond length of 1.16 Å) to 130–140° (bent configuration for the adsorbed CO2). It is worth noting that the bond lengths of adsorbed CO2 are sensitive to the shape of refined space, which can be divided into two relationships (Table 1 and Figure 5a,b): when D is 3.5 or 4 Å, the C=Oa (Oa, the one farther from the non-coordinating CoPc layer) bond is elongated to 1.26–1.27 Å, while the C=Ob (Ob, the one closer to the non-coordinating layer) bond remains 1.16–1.19 Å. In contrast, when D is 4.25 or 4.5 Å, this adsorbed CO2 molecule is able to keep the quasi-symmetric structure with both C=O bond lengths ranging from 1.20 to 1.24 Å. But please pay attention that this quasi-symmetry is only valid in the geometric viewpoint. If we change the viewpoint into Mayer bond order [45,46], it is easily found out that the bond order of left and right C=O bond is ~1.40 and ~1.70, respectively, indicating the state of “geometric equilibrium but electronic polarization”. The above bond length and bond angle parameters are in excellent agreement with the structural reference of the activated state of carbon dioxide on the metal macrocycle catalyst [47]. This bent adsorption configuration is a typical feature of the transition from physical adsorption to chemical adsorption in cobalt-based systems [48,49], further validating the rationality of our structural model.
The next step, Proton Electron Cooperative Transfer (PECT), process is deeply modulated by the above disequilibrium and polarization. There are two proton attacking sites in the *(Co)-Oa-C(Co)-Ob complex (Figure 5c,d). Although the C=Ob bond length has a little greater elongation than C=Oa, For the D range of 4.0–4.5 Å, the key intermediate *(Co)-Ob-C(Co)-OaH always shows greater thermodynamic stability in comparison with *(Co)-ObH-C(Co)-Oa with the binding energy reduction of 0.01–0.15 eV (Figure 5h). This preference exists because the Ob site has two disadvantages: (1) the crowded space around Ob, which brings obvious steric hindrance on the H+ attacking route; (2) the special electronic structure of Ob, which is discussed in detail in the section of “electronic structural viewpoints to (CoPc)2 catalyzing mechanism”. When the distance increases to 4.75–5.0 Å, the system loses its preference for a specific intermediate. The energy difference drops to nearly zero.
The CO release phase at the end of the reaction is as important as the adsorption process of CO2 at the beginning of the reaction. Structural analysis reveals that the C=O bond length of the *CO intermediate (1.15–1.18 Å) closely approaches that of a gas-phase molecule. The adsorption configuration exhibits a significant spatial response: when L = 0, the adsorbed species tends to be horizontally centered; when L ≠ 0, while the C atom remains firmly bonded to the Co center, the molecule adopts an outward “tilted” (tilting-up) deviation state. This tilted geometry effectively reduces the energy barrier for subsequent CO release, which accelerates active site recovery (Figure 5e,f). To quantify how the confinement effect regulates catalytic patterns, we developed a volcano plot (Figure 5g). Here, CO adsorption energy (ΔE*CO) serves as the descriptor to correlate with the Gibbs free energy barrier of CO desorption (ΔGdes(*co)). The volcano plot clearly demonstrates the Sabatier principle [50,51]: in our best-performing system (D4.25L1), the subtle geometric tuning introduced by L = 1.0 Å nicely balances interlayer repulsion and adsorption strength, placing the catalyst close to the volcano summit. By comparison, strong spatial confinement at D = 4 Å brings about excessive steric repulsion and correspondingly weak adsorption, pushing these systems toward the right slope. On the contrary, structures with larger interlayer distances lie on the left “poisoning” side. Without sufficient interlayer modulation, they bind *CO far too strongly. Even though monolayer CoPc shows reasonably balanced adsorption and desorption, it still cannot outperform our optimized structure.
In short, a suitable degree of interlayer confinement helps strike an optimal balance between *CO adsorption and desorption. In this way, the intrinsic activity bottlenecks can be overcome, leading to significantly improved catalytic performance.

2.4. Electronic Effect Analysis and Site-Preference Mechanisms

Further analysis of NPA charges and charge density difference (CDD) [52,53] reveals how the interlayer confinement modulates the intensity of CO2 activation and adsorption stability. As shown in Figure 6a, at a short vertical distance (D = 4.0 Å), the charge transfer is minimal, leading to the lowest adsorption stability. This arises from the strong electron cloud repulsion induced by overly narrow confinement, which restricts efficient charge injection into the CO2 molecule. In contrast, when the vertical distance lies within D = 4.25–5.0 Å, the charge transfer to CO2 rises evidently, implying that this vertical spacing effectively optimizes the electronic coupling between (CoPc)2 and the CO2 molecule. Notably, the D4.5L1 catalytic configuration shows a clear increase in charge transfer compared with the monolayer CoPc. This confirms, from an electronic perspective, the unique advantage of D4.5L1 in stabilizing adsorption. Specifically, it strengthens the binding while evading the risk of over-adsorption. As shown in Figure 6b, within the D = 4.25 and 4.5 Å models (optimal and sub-optimal vertical models for catalysis in this work), the L = 1.0 Å parallel distance demonstrates more abundant charge transfer than other parallel distances in the same vertical distance. This confirms that for these two vertical distances, the “moderate parallel distance offset” of L = 1 Å provides a more accessible electron transport channel, inducing more efficient substrate activation and enhanced adsorption stability.
As illustrated in Figure 6c, comparing the CDD plots of D4.5L0/1, D4.25L0/1, and monolayer CoPc reveals clear evidence of interlayer synergistic polarization in the bilayer configurations. Electrons are transferred from the d z 2 orbitals of the directly involved Co atom into the energy-matched π ∗ antibonding orbitals of CO2, forming a stable Co-C bond. Simultaneously, the CoPc layer not in direct contact with CO2 contributes to even more significant charge transfer. Notably, the dual-cobalt synergistic mechanism within these bilayer models relies on a precisely confined environment to take effect effectively. Optimized geometries allow the bilayer architecture to disrupt the isolated electronic states typical of monolayer systems. Through interlayer charge compensation, substantial increases in electron flow toward CO2 significantly boost its activation. This interlayer electronic synergy induced by spatial confinement represents the main cause for the higher catalytic performance of the bilayer architecture relative to its monolayer architecture.
The bilayer architecture further induces significant differences in the reactivity of individual atomic sites within the CO2 molecule. To understand the origin of the site selectivity during *COOH generation under moderate interlayer confinement (D = 4.0–4.5 Å), we carried out a thorough electronic analysis. Specifically, the nucleophilic sites of the optimal catalytic model (D4.25L1) were quantitatively characterized using the Fukui function (f−) [54,55,56]. Figure 6d demonstrates that the distal oxygen atom (Oa, farther from the non-coordinating CoPc layer) displays a slightly higher f− value than the proximal oxygen (Ob, closer to the non-coordinating layer). This suggests that Oa has a greater susceptibility to electrophilic attack (H+) during nucleophilic responses, a finding strongly corroborated by the electrostatic potential (ESP) [57] distribution. The 2D contour maps (Figure 6e) clearly reveal that Oa is situated within a more negative and spatially extended ESP environment, providing a stronger electrostatic attraction for the approaching H+. Altogether, the alignment between high local reactivity (as indicated by f−) and the pronounced negative ESP identifies Oa as the preferential site for protonation. It further underscores that the regioselectivity in this bilayer system is not an intrinsic property of the CoPc substrate, but is rather a unique phenomenon emerged from synergistic confinement effects at this particular spatial scale (D = 4.0–4.5 Å).

2.5. Self-Assessment and Outlook to This (CoPc)2 Confinement System

At the methodological level, this study adopts an implicit solvent model for simplification, aiming to eliminate the interference of complex environmental factors and focus on the essential influence of interlayer geometric parameters (D and L) on the fundamental mechanism of (CoPc)2 cooperative catalysis. This approach enables us to separate the core variable of spatial topological structure from the complex reaction system and independently analyze its control laws on catalytic performance. Although the above simplification was carried out, the “Goldilocks Zone” determined in this study and the trend of structure-activity evolution still have high reliability in the real electrochemical environment. This conclusion is strongly supported by experimental research: in structure-related porphyrin-based COF materials, researchers have observed excellent long-cycle stability [27,58,59,60]; due to the high dependence of catalytic performance on the geometric distribution of active sites, its long-term stable performance means that the core structural parameters remain intact throughout the reaction process and are not significantly disturbed by external potential or solvent fluctuations. It is undeniable that the real electrochemical interface is a highly complex and dynamic system.
Future studies using explicit solvation models or ab initio molecular dynamics (AIMD) simulations can conduct more detailed analyses of the transient behavior of the interface. However, the quantitative topological rules established in this study provide reliable and universal design principles for bimolecular electrocatalysts, laying a solid theoretical foundation for the development of high-performance electrocatalysts with geometric constraints.

3. Model and Computational Details

3.1. Ideal Model and Reaction Mechanism

In order to describe the ideal CO2RR catalysts based on dual cobalt complexes, a theoretical model was built up using two parallel phthalocyanine cobalt moieties (Figure 7e). In this model, two CoPc moieties are located in the face-to-face manner, between which the CO2RR reaction occurs. The shape and size of this refined reaction space are defined by the vertical distance (D) and parallel distance (L). It is worth noting that the translation operation for L change is always along with the Co-N bond.
It is worth noting that the computational model used in this study is derived from the typical phthalocyanine-based covalent organic frameworks (COFs) and face-to-face stacked metal porphyrin/phthalocyanine dimer structures that are widely present in experiments [38,61,62,63,64,65]. In these systems, the relative positions of adjacent building units are mainly determined by the interlayer vertical distance (D) and parallel distance (L). Due to the significant geometric constraints imposed by the topological connection of the covalent framework on interlayer rotation, the model ignores the influence of the relative rotation angle between layers. The construction process of the computational model used in this study is illustrated in Figure 7, in which we selected cobalt phthalocyanine (CoPc) [66,67,68,69,70], which has excellent catalytic activity and is representative, as the benchmark computational model.
Catalysts 16 00328 g007
Figure 7. Structures of the phthalocyanine-based materials observed in experiments and their corresponding theoretical simulation models. (a–d) Typical bilayer phthalocyanine metal materials as shown in the experimental synthesis (the images are successively taken from References [38,63,64,65], with copyright permission granted). (e) Theoretical structure model of (CoPc)2 with geometric parameters.
Figure 7. Structures of the phthalocyanine-based materials observed in experiments and their corresponding theoretical simulation models. (a–d) Typical bilayer phthalocyanine metal materials as shown in the experimental synthesis (the images are successively taken from References [38,63,64,65], with copyright permission granted). (e) Theoretical structure model of (CoPc)2 with geometric parameters.
Catalysts 16 00328 g007
The stability of the theoretical models was determined by the binding energy ΔE, as defined in Equation (1). The more negative the ΔE value, the stronger the intermolecular binding and the more stable the structure [29,71].
∆ E = E D o u b l e − C o P c − 2 E S i n g l e − C o P c
The catalytic pathways (Figure 8) are represented by the following steps [67,72,73]:
PcCo-(empty)-CoPc + CO2 → PcCo-CO2-CoPc
PcCo-CO2-CoPc + H+ + e− → PcCo-CO2H-CoPc
PcCo-CO2H-CoPc + H+ + e− → PcCo-CO-CoPc + H2O
PcCo-CO-CoPc → PcCo-(empty)-CoPc + CO
Total reaction: CO2 + 2H+ + 2e− → CO + H2O
For Equations (2)–(6), the Gibbs free energy change ΔG could be calculated in the following manner:
Δ G = ∑ i   ∈   p r o d u c t s G i − ∑ j   ∈   r e a c t a n t s G j

3.2. Computational Details

All the calculations were performed using the Gaussian 16 Revision A.03 software package [74] based on density functional theory (DFT), and electronic structure analyses were conducted via Multiwfn 3.8 [75]. To simulate the aqueous environment of the carbon dioxide reduction reaction (CO2RR), the SMD implicit solvation model [76] was employed throughout all the structural optimizations. To balance computational accuracy and efficiency, a mixed basis set was adopted at the ωB97XD [77] level, including LanL2TZ(f) [78] for Co and 6-311G(2d) [79,80] for C, H, N, and O atoms. To precisely control the key parameters of vertical distance (D) and parallel distance (L), a constrained optimization strategy was adopted using the optimizing strategy of “fixed Pc framework”, “relaxed CoN4 active sites but constrained the distances between the Co/N atoms and their counterparts in the opposite layer”, and “relaxed CO2 and intermediates”. Our calculations followed the logic of “initial focus on core intervals followed by in-depth fine-tuning”. A screening of the dual distance parameters for the bilayer structures was first performed. Subsequently, theoretical simulations and analyses of CO2 reduction to CO were conducted based on the preliminary screened structural models.

4. Conclusions

This study systematically explores the influence of the bilayer cobalt phthalocyanine (CoPc)2 on the electrocatalytic carbon dioxide reduction reaction (CO2RR) through density functional theory (DFT) calculations. Our results show that non-covalent interactions between layers stabilize the (CoPc)2 structure and form a “refined catalyzing space”, which plays a key role in regulating the reaction thermodynamics. We observed that the best catalytic behavior appears in a “superimposed” region, where the vertical distance D falls into a moderate interlayer interaction range and is coupled with an appropriate parallel distance L. In this confined environment, the cooperation between geometric restriction and electronic activation reaches a favorable balance, providing a unique “Goldilocks Zone” for the catalytic reaction, which is summarized as the vertical distance (D) at the range of 4.25–4.5 Å and parallel distance (L) near 1 Å. Among all the structural models, D4.5L1 (D = 4.5 Å, L = 1 Å) shows improved CO2 activation and effectively stabilizes the initial adsorption of CO2 molecules. D4.25L1 (D = 4.25 Å, L = 1 Å) delivers the highest catalytic activity: it markedly reduces the energy barrier of the rate-limiting step and achieves a good balance between intermediate adsorption and product desorption.
Further electronic analysis clarifies how optimized bilayer confinement regulates charge transfer and CO2 activation. This confined microenvironment breaks the intrinsic electronic symmetry of the CO2 molecule, naturally determining the selectivity of reaction sites. Ultimately, this study shows that regioselectivity is not an intrinsic property of the material but an induced property caused by geometric constraints. These findings open a new avenue for the design of highly active and efficient CO2 reduction catalysts and provide a novel design principle for the precise control of site-selectivity through fine-tuning the interlayer spacing environment.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal16040328/s1, Table S1: Energy differences (ΔE/eV) between the triplet (M = 3) and singlet (M = 1 ) states for various (CoPc)2 configurations (ΔE = E(M=3) − E(M=1)). Table S2: Calculated Gibbs free energy values (ΔG/eV) of all reaction intermediates for CO2 reduction reaction (CO2RR) and hydrogen evolution reaction (HER) for different double-layer cobalt phthalocyanine (CoPc)2 configurations and single-layer cobalt phthalocyanine CoPc. Note: All Gibbs free energies (ΔG) are referenced to the zero point of the total initial energy (G = 0 eV). For the carbon dioxide reduction reaction (CO2RR), the initial state is defined as [* + CO2 + 2(H+ + e−)]; for the hydrogen evolution reaction (HER), the initial state is defined as [* + H+ + e−]. All values are rounded to two decimal places.

Author Contributions

Conceptualization, L.Y. and D.Q.; theoretical simulation, L.Y.; data curation, L.Y. and D.Q.; writing—original draft preparation, L.Y. and D.Q.; writing—review and editing, D.Q., T.W. and J.J.; project administration, T.W. and J.J.; funding acquisition, D.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Fundamental Research Funds for the Central Universities (FRF-BD-25-014).

Data Availability Statement

The original contributions presented in this study are included in this article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The binding energies of (CoPc)2 dimer with different D and L (D = vertical distance, L = parallel distance).
Figure 1. The binding energies of (CoPc)2 dimer with different D and L (D = vertical distance, L = parallel distance).
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Figure 2. IGMH isosurfaces (0.0012 a.u.) of interlayer interactions in (CoPc)2. Atomic color code: C (cyan), H (white), N (pink), Co (purple). The green isosurfaces represent interlayer weak interactions.
Figure 2. IGMH isosurfaces (0.0012 a.u.) of interlayer interactions in (CoPc)2. Atomic color code: C (cyan), H (white), N (pink), Co (purple). The green isosurfaces represent interlayer weak interactions.
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Figure 3. Gibbs free energy profiles and evaluation of catalytic performance. (a) Gibbs free energy pathways of the eight models selected from stability analysis compared with the CoPc; (b) Comprehensive comparison of all catalyst models, incorporating additional structures at D = 4.25 and 4.75 Å (for both L = 0, 1 Å) to pinpoint the optimal catalyst model; (c,d) Effect of parallel distance (L) on (c) the sub-optimal D = 4.5 Å and (d) the optimal D = 4.25 Å models; (e) Rate-determining step (RDS) barriers for all investigated configurations versus the CoPc monolayer. The asterisk (*) denotes the catalyst model without adsorbates.
Figure 3. Gibbs free energy profiles and evaluation of catalytic performance. (a) Gibbs free energy pathways of the eight models selected from stability analysis compared with the CoPc; (b) Comprehensive comparison of all catalyst models, incorporating additional structures at D = 4.25 and 4.75 Å (for both L = 0, 1 Å) to pinpoint the optimal catalyst model; (c,d) Effect of parallel distance (L) on (c) the sub-optimal D = 4.5 Å and (d) the optimal D = 4.25 Å models; (e) Rate-determining step (RDS) barriers for all investigated configurations versus the CoPc monolayer. The asterisk (*) denotes the catalyst model without adsorbates.
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Figure 4. (a) Free energy curves corresponding to the hydrogen evolution reactions for different catalyst models. (b) The limiting electrode potentials of the carbon dioxide reduction reaction and the hydrogen evolution reaction under the condition of voltage U = 0 V. The asterisk (*) denotes the catalyst model without adsorbates.
Figure 4. (a) Free energy curves corresponding to the hydrogen evolution reactions for different catalyst models. (b) The limiting electrode potentials of the carbon dioxide reduction reaction and the hydrogen evolution reaction under the condition of voltage U = 0 V. The asterisk (*) denotes the catalyst model without adsorbates.
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Figure 5. (a,b) Geometric comparison of *CO2 adsorption, as exemplified by D4L0 and D4.5L0 models; (c,d) Two distinct types of *COOH configurations; (e,f) Contrasting *CO adsorption geometries under L = 0 and L ≠ 0 conditions (demonstrated via D4.5L0 and D4.5L1; red arrows highlight the CO liberation pathway from the catalyst surface); (g) Volcano-shaped activity trends for the 2e− CO2RR process; (h) Comparison of energy differences (ΔE) between the two *COOH adsorption modes across the investigated catalyst models.
Figure 5. (a,b) Geometric comparison of *CO2 adsorption, as exemplified by D4L0 and D4.5L0 models; (c,d) Two distinct types of *COOH configurations; (e,f) Contrasting *CO adsorption geometries under L = 0 and L ≠ 0 conditions (demonstrated via D4.5L0 and D4.5L1; red arrows highlight the CO liberation pathway from the catalyst surface); (g) Volcano-shaped activity trends for the 2e− CO2RR process; (h) Comparison of energy differences (ΔE) between the two *COOH adsorption modes across the investigated catalyst models.
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Figure 6. Charge distribution and electronic property analysis. (a) Total charge transfer from the catalyst to the CO2 molecule for all investigated models; (b) Charge transfer amounts (catalyst to CO2) for the optimal and sub-optimal models at different parallel displacements (L); (c) Charge density difference (CDD) maps for the CoPc monolayer and the optimal/sub-optimal configurations (D = 4.25/4.5 Å, L = 0/1 Å). Purple and green regions represent charge density depletion and accumulation; (d) Fukui function (f−) distribution for the D4.5L1model, characterizing the reactivity toward electrophilic attack (e.g., by H+) at the Oa and Ob atoms; (e) Electrostatic potential (ESP) maps illustrating the electrostatic attraction of the Oa and Ob environments toward protons (H+).
Figure 6. Charge distribution and electronic property analysis. (a) Total charge transfer from the catalyst to the CO2 molecule for all investigated models; (b) Charge transfer amounts (catalyst to CO2) for the optimal and sub-optimal models at different parallel displacements (L); (c) Charge density difference (CDD) maps for the CoPc monolayer and the optimal/sub-optimal configurations (D = 4.25/4.5 Å, L = 0/1 Å). Purple and green regions represent charge density depletion and accumulation; (d) Fukui function (f−) distribution for the D4.5L1model, characterizing the reactivity toward electrophilic attack (e.g., by H+) at the Oa and Ob atoms; (e) Electrostatic potential (ESP) maps illustrating the electrostatic attraction of the Oa and Ob environments toward protons (H+).
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Figure 8. The catalytic mechanism of CO2 → CO at the interlayer of (CoPc)2.
Figure 8. The catalytic mechanism of CO2 → CO at the interlayer of (CoPc)2.
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Table 1. Calculated geometric parameters and Mayer bond orders (MBO) of *CO2 adsorption configurations on different (CoPc)2 models.
Table 1. Calculated geometric parameters and Mayer bond orders (MBO) of *CO2 adsorption configurations on different (CoPc)2 models.
Geometric ParametersMBO
Oa-C-Ob (°)C-Oa (Å)C-Ob (Å)C-OaC-Ob
D4L0132.541.191.261.771.15
D4L1133.181.191.271.761.15
D4.25L0134.801.211.231.691.40
D4.25L0.5137.111.201.231.711.42
D4.25L1134.941.211.241.681.40
D4.25L1.5136.351.211.231.691.46
D4.25L2135.901.211.231.691.48
D4.5L0132.581.211.241.681.42
D4.5L0.5133.001.211.241.681.43
D4.5L1133.351.211.241.681.43
D4.5L1.5133.781.221.241.671.46
D4.75L0134.601.211.231.701.50
D4.75L1135.541.211.231.711.51
D5L0134.761.211.231.701.54
D5L1137.731.211.221.701.54
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Yin, L.; Qi, D.; Wang, T.; Jiang, J. A “Goldilocks Zone” in Bilayer Cobalt Phthalocyanine: Optimizing Confinement for Efficient CO2RR. Catalysts 2026, 16, 328. https://doi.org/10.3390/catal16040328

AMA Style

Yin L, Qi D, Wang T, Jiang J. A “Goldilocks Zone” in Bilayer Cobalt Phthalocyanine: Optimizing Confinement for Efficient CO2RR. Catalysts. 2026; 16(4):328. https://doi.org/10.3390/catal16040328

Chicago/Turabian Style

Yin, Longlei, Dongdong Qi, Tianyu Wang, and Jianzhuang Jiang. 2026. "A “Goldilocks Zone” in Bilayer Cobalt Phthalocyanine: Optimizing Confinement for Efficient CO2RR" Catalysts 16, no. 4: 328. https://doi.org/10.3390/catal16040328

APA Style

Yin, L., Qi, D., Wang, T., & Jiang, J. (2026). A “Goldilocks Zone” in Bilayer Cobalt Phthalocyanine: Optimizing Confinement for Efficient CO2RR. Catalysts, 16(4), 328. https://doi.org/10.3390/catal16040328

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