Steering competitive N₂ and CO adsorption toward efficient urea production with a confined dual site

Abstract

Electrocatalytic urea synthesis under mild conditions via the nitrogen (N₂) and carbon monoxide (CO) coupling represents an ideal and green alternative to the energy-intensive traditional synthetic protocol. However, this process is challenging due to the more favorable CO adsorption than N₂ at the catalytic site, making the formation of the key urea precursor (*NCON) extremely difficult. Herein, we theoretically construct a spatially isolated dual-site (D_S) catalyst with the confinement effect to manipulate the competitive CO and N₂ adsorption, which successfully guarantees the dominant horizontal N₂ adsorption and subsequent efficient *NCON formation via C–N coupling and achieves efficient urea synthesis. Among all the computationally evaluated candidates, the catalyst with dual V sites anchored on 4N-doped graphene (D_S-VN₄) stands out and shows a moderate energy barrier for C–N coupling and a low theoretical limiting potential of −0.50 V for urea production, which simultaneously suppresses the ammonia production and hydrogen evolution. The confined dual-site introduced in this computational work has the potential to not only properly address part of the challenges toward efficient urea electrosynthesis from CO and N₂ but also provide an elegant theoretical strategy for fine-tuning the strength of chemical bonds to achieve a rational catalyst design.

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1. Introduction

As a crucial artificial fertilizer, urea (NH₂CONH₂) has laid a solid foundation for the development of agriculture to produce food and avoid the mass starvation of human beings.^1–3 Meanwhile, it is an essential feedstock for manufacturing high-value-added chemicals such as plastics, adhesives, potassium cyanate, and urea nitrate. In industry, large-scale urea synthesis was achieved at a high temperature of 150–200 °C and high pressure of 150–250 bar using ammonia (NH₃) and carbon dioxide (CO₂) as reactants. Another key issue of this route lies in its dependence on NH₃ as the nitrogen source, which is industrially produced via the energy- and capital-intensive Haber–Bosch process under harsh conditions.⁴ It consumes ∼2% of the global fossil energy and emits ∼300 million tons of CO₂ annually during the inert N₂ fixation to NH₃ due to the usage of gray hydrogen, and ultimately impacts the sustainability of the whole urea production industry.⁴ Therefore, developing a low-carbon and alternative sustainable urea synthesis protocol is significant for achieving and maintaining the sustainable development of human society.

Recently, the direct coupling of carbon-based feedstocks (CO₂ and CO₂-derived CO) with NH₃-free nitrogen sources (N₂, NO, nitrite, and nitrate) has been proposed as a promising and efficient one-step method to produce urea via electrochemical reduction, which has attracted increasing attention from academia due to the sustainability and affordability.^5–20 For example, Chen et al. experimentally prepared an electrocatalyst by anchoring PdCu nanoparticles on TiO₂ nanosheets for aqueous N₂ and CO₂ coupling to synthesize urea, which achieved a formation rate of 3.36 mmol g⁻¹ h⁻¹ and a faradaic efficiency of 8.92% at −0.4 V versus RHE.⁵ From electrochemical CO₂ and N₂ coupling, Zhang and co-workers reported the urea yield rates/faradaic efficiency of 5.91 mmol g⁻¹ h⁻¹/12.55% and 9.70 mmol g⁻¹ h⁻¹/20.36% on the Mott–Schottky Bi–BiVO₄ heterostructures and flower-like nickel borate [Ni₃(BO₃)₂], respectively.^6,7 Zhu et al. theoretically predicted Mo₂B₂, Ti₂B₂, and Cr₂B₂ as potential efficient electrocatalysts for urea production via N₂ and CO₂ coupling under ambient conditions based on the data from mechanistic calculations with the computational hydrogen electrode (CHE) model.⁸ Moreover, compared with the chemically inert N₂ molecule, higher urea yields can be generally obtained by using more activated nitrite and nitrate, but have trouble in large-scale applications due to the limited availabilities of those nitrogen sources.^13–20

Despite the impressive recent progress in electrochemical urea synthesis, the sluggish reaction kinetics and limited yield still seriously impede its industrial integration. In principle, the formation of key *NCON species is a prerequisite for urea synthesis,^{5–7,21–23} while the direct coupling of N₂ with CO represents an ideal approach to generating this crucial intermediate. Unfortunately, this route faces several formidable challenges due to the intrinsic difference in electronic structures of N₂ and CO molecules, as well as their similar bonding modes with active sites. As shown in Fig. 1A, the foremost challenge is the competitive adsorption of N₂ and CO. Mechanistically, the binding of the polarized CO molecule with the metallic active site is usually more favorable than the non-polarized N₂ molecule, forming a σ bond via the C atom due to the significant character of the highest occupied molecular orbital (HOMO) of C 2s (Fig. 1B).²⁴ Meanwhile, the HOMO of the CO molecule is energetically closer to the metal d-orbital than that of the N₂ molecule, resulting in a more stable metal-CO σ bond. As a result, N₂ is usually a poorer π-acceptor ligand than CO,²⁵ while achieving horizontal N₂ adsorption is even more challenging. In principle, the horizontal N₂ adsorption should be energetically ensured (Fig. 1A) to achieve the efficient formation of the *NCON intermediate via N₂ coupling with CO, but unfortunately is much less competitive than CO adsorption. Clearly, this dilemma greatly restricts the feasibility of *NCON formation, leading to either CO poisoning or CO reduction to hydrocarbons during the electrochemical process. Therefore, manipulating the strong competitive CO adsorption with N₂ at the catalytic site thereby guaranteeing the dominant N₂ horizontal adsorption represents one of the key challenges for efficient urea electrosynthesis. Besides, as shown in Fig. 1A, even if the dominance of horizontal N₂ adsorption is guaranteed, the competition between C–N coupling and N₂ direct electroreduction to *N₂H is the second grand challenge to obtain a preferential formation of the *NCON intermediate. Then, the production of urea via the protonation of the *NCON intermediate should be achieved at an affordable energy cost, thereby requiring high reactivity of the catalyst, which clearly denotes the third challenge. Noteworthily, the hydrogen evolution reaction (HER) is inevitable in almost all electrocatalytic processes, true also for urea electrosynthesis, which represents the fourth formidable challenge.

Fig. 1 (A) Typical challenges to urea electrosynthesis by N₂ and CO coupling from a view of the reaction mechanism. (B) HOMO electron density of N₂ and CO. (C) Schematic illustration of N₂/CO adsorption steering with the confined dual-site strategy.

In this work, we proposed a strategy to properly address the aforementioned challenges by steering the competitive N₂ and CO adsorption with a confined dual active site, which provided an elegant proposal to achieve efficient urea production from N₂ and CO. As presented in Fig. 1C, the confined dual active site could stabilize non-polar N₂ adsorption by forming one additional stable TM–N bond, whereas the polar CO molecule binding was hardly affected or even destabilized, thereby making horizontal N₂ adsorption very competitive and eventually promoting the formation of the *NCON intermediate. A group of metals, mainly first-row transition metals, was chosen as active centers and anchored on nitrogen-doped graphene to construct our confined dual-site catalysts shown in Fig. 1C. Our simulations successfully predict the dual-site VN₄ (D_S-VN₄) catalyst to be a very promising candidate to drive the electrochemical production of urea via the *NCON protonation with a low limiting potential of −0.50 V. Meanwhile, this D_S-VN₄ catalyst shows tremendous suppression of undesirable ammonia formation and hydrogen evolution, enabling the high reaction selectivity toward urea products. This work not only identified the spatially isolated dual site with the confinement effect for steering the competitive N₂ and CO adsorption but also rationalized this theoretical strategy to fine-tune the strengths of chemical bonds, thus paving a promising route to obtain green urea electrosynthesis via rational catalyst design.

2. Results and discussion

2.1. Competitive N₂ and CO adsorption on single and dual sites

To achieve an efficient urea electrochemical synthesis via N₂ and CO coupling, the first priority is to ensure preferential N₂ adsorption with horizontal configuration. However, as mentioned in Fig. 1, the polar CO molecule is more likely to cover the catalytic site than the non-polar N₂ molecule based on the analysis from molecular orbital theory. To this end, we first chose ten transition metals (TM) as active sites (Ti, V, Cr, Mn, Fe, Co, Ni, Mo, Ru, and Rh) and embedded them in graphene with different coordination environments (i.e., TMN₄, TMN₃C₁, TMN₂C₂, and TMN₁C₃) shown in Fig. S1.† Then, we calculated and compared their binding strengths with N₂ and CO. As presented in Fig. 2 and the detailed energetic data in Table S1,† the N₂ molecule energetically favors the one-sided vertical binding mode to the single active site, and the binding strength of CO is much stronger than that of the N₂ molecule. The two exceptions, NiN₄ and NiN₃C₁, with similar CO and N₂ binding strengths are due to their physical adsorptions toward adsorbates. In general, our calculated binding strengths of CO and N₂ on the single active site follow the vertical CO > vertical N₂ > horizontal N₂, which is consistent with the theoretical expectation shown in Fig. 1A. Therefore, the CO adsorption and subsequent reduction will be the dominant reaction route on the single-site catalyst if co-feeding CO with N₂, thereby eliminating the N₂ attacking and blocking the C–N coupling for urea synthesis.

Fig. 2 The calculated adsorption energy of N₂ and CO molecules on the (A) TMN₄ moiety, (B) TMN₃C₁ moiety, (C) TMN₂C₂ moiety, and (D) TMN₁C₃ moiety with single site and spatially isolated dual sites. The black dashed line represents the equivalent adsorption, and above the line is the N₂-dominated adsorption region.

Based on the proposed strategy in Fig. 1C, we spatially introduced a second active site with weak O binding strength but strong binding with N, namely the dual active site catalyst, to steer the competitive N₂ and CO adsorption. Noteworthily, our recent work has successfully validated this strategy to enhance N₂ adsorption and activation with the dual-site catalyst, making near-ambient conditions of ammonia synthesis possible.²⁶ In our model catalyst shown in Fig. S2,† we sterically linked two localized single TM site catalysts by using two benzene rings. Noteworthily, this designed catalyst is very similar to the experimentally reported Pacman dinuclear porphyrins^27–30 for O₂ electroreduction²⁷ and CO₂ electroreduction.²⁸ Thus, the adsorption of N₂ and CO molecules was systematically examined on the designed dual-site catalysts. As presented in Fig. 2 and Table S2,† different from the cases of the single-site catalyst dominated by CO adsorption, a group of dual-site catalysts form the N₂-dominated adsorption such as D_S-VN₄, D_S-VN₃C₁, D_S-VN₂C₂, D_S-CrN₂C₂, and D_S-CrN₁C₃. The para-configuration of the TMN₂C₂ moiety was also considered (Fig. S3†), which showed similar functionality and activity to the ortho-configuration in Fig. 2. Furthermore, we applied a Boltzmann function to evaluate the distribution of N₂ and CO on the active sites. Our results in Fig. S4, Tables S3, and S4† clearly indicate the dominant population of N₂ on the D_S-VN₄, D_S-VN₃C₁, D_S-VN₂C₂, D_S-CrN₂C₂, and D_S-CrN₁C₃ catalysts, which effectively suppresses the CO adsorption. The aforementioned results clearly reveal the capability of the designed dual-site catalyst in regulating the competitive N₂ and CO adsorption, which lays a good basis for efficient C–N coupling.

2.2. Functionality of the confined dual site

Before moving forward to urea synthesis mechanism calculations on different confined dual-site catalysts, we further explored the origins behind these fine-tuned N₂ and CO adsorptions. To this end, the spin-charge density maps were plotted to reveal the interactions of the adsorbates with the active sites, where the D_S-VN₄ catalyst was chosen as an example. As shown in Fig. 3A, the dual V site has an obviously large spin charge but gradually decreases as the adsorbates approach, indicating the involvement of spin electrons in V sites during the adsorption process. In principle, a more significant change in spin-charge represents a stronger bonding interaction. For the case of N₂ adsorption, the V sites on both sides retain small and equal spin-charge density. For CO adsorption, the spin charge of the V site on the C atom side is negligible, while that on the O atom side is still obvious. Thus, based on the changes in the spin-charge density of the V site before and after adsorption, the bonding strength will follow the order of V–C > V–N > V–O. A similar phenomenon could be observed on the D_S-VN₃C₁, D_S-VN₂C₂, and D_S-CrN₁C₃ catalysts as shown in Fig. S5.†

Fig. 3 (A) The spin-charge density of pure Ds-VN₄, N₂ adsorption, and CO adsorption, where the isosurface value was set to be 0.009 e Å⁻³. (B) The calculated COHP between D_S-VN₄ and the N₂ molecule as well as the CO molecule. The Fermi level (E_F) was set to zero, and the bonding and antibonding orbitals were presented on the right and left, respectively. (C) The calculated charge transfer to adsorbed N₂ and CO molecules.

To further quantitatively evaluate the strength of chemical bonds, we performed the crystal orbital Hamilton population (COHP) analysis³¹ on the D_S-VN₄ catalyst. As shown in Fig. 3B, fewer bonding orbitals of V–O can be observed compared with V–N and V–C, showing a weak V–O interaction. Furthermore, the integrated COHP (ICOHP) values of V–N, V–C, and V–O bonds were calculated to be −3.04, −3.07, and −1.65, respectively. Typically, a more positive ICOHP value represents a weaker chemical bond. Thus, the strength of the chemical bonds at the D_S-VN₄ sites follows the order of V–C > V–N > V–O. The corresponding ICOHP values for the other four catalysts with N₂-dominant adsorption are summarized in Table S5.† A general trend of bonding strength of TM–N > TM–O can be obtained on the designed dual-site candidates. Indeed, this trend is in line with our proposal in Fig. 1C that the sterically introduced second site can better enhance the binding strength of the N atom compared to the O atom, and eventually makes N₂ adsorption more competitive than CO.

Meanwhile, the confined dual-site catalysts will transfer more electrons to the adsorbed N₂ molecule via the additionally constructed stable TM–N bond shown in Fig. 3C. By contrast, no significant difference was found in the amount of charge transfer for the adsorbed N₂ and CO molecules on the single-site catalysts shown in Fig. S6.† Clearly, the enhanced N₂ adsorption and its dominance on the active site by the dual-site strategy are again supported by the electron transfer analysis. Note that the implementation of this strategy is based on the active site with strong binding to the N atom, i.e., guaranteeing a strong TM–N bond. We further examined it at the single TM site by calculating the binding energy of a single N atom using the N₂ molecule as the energy reference. As listed in Table S6,† the core active sites of the five candidates with dominant N₂ distribution all exhibit strong N binding, implying the effectiveness of the dual-site strategy for modulating competitive adsorption. As a summary, the aforementioned spin-charge density, COHP, and charge transfer analysis provide the rationale for the fine-tuning of N₂ and CO adsorption using our proposed confined dual-site catalysts.

2.3. Feasibility of confined dual-sites for urea electrosynthesis

Since our proposed dual-site strategy was already able to properly address the challenge from the N₂ and CO competitive adsorption, we further evaluated the capability of this strategy in tackling the competition between CO–N₂ coupling and N₂ electrochemical hydrogenation, where the five candidates with dominant N₂ adsorption were chosen as model systems. At first, we examined the free energy changes of the coupling process via the Eley–Rideal mechanism (*N₂ + CO(g) → *NCON) and the N₂ electroreduction process (*N₂ + H⁺ + e⁻ → *N₂H) for convenient comparison. As shown in Fig. 4A and S7,† negative reaction free energies can be observed for the coupling process (−0.49 eV for D_S-VN₄, −0.34 eV for D_S-VN₃C₁, −0.43 eV for D_S-VN₂C₂, −0.01 eV for D_S-CrN₂C₂, and −0.15 eV for D_S-CrN₁C₃) on the five catalysts, indicating the thermodynamic feasibility. In contrast, the protonation of the adsorbed N₂ molecule to the *N₂H intermediate needs to overcome the positive free energy. Thus, at 0 V potential, the C–N coupling is thermodynamically more favorable than N₂ electroreduction on the five dual-site catalysts.

Fig. 4 (A) The calculated free energy change of C–N coupling and N₂ electroreduction at 0 V vs. RHE. (B) Reaction competition between C–N coupling and N₂ electroreduction under the applied potential. (C) The calculated free energy diagram of urea synthesis by C–N coupling and reduction at 0 V vs. RHE on the designed D_S-VN₄ catalyst, as well as the reaction competition between urea production and H₂ evolution under different applied potentials.

However, the formation of the *N₂H intermediate via N₂ reduction would be electrochemically promoted by increasing the applied potential (U), while that of *NCON was less affected due to the noninvolvement of the proton-electron coupling step during the CO–N₂ coupling. In other words, the free energy change of N₂ protonation will become more negative at the high U, making N₂ reduction more favorable than the C–N coupling at a certain U. Thus, the effect of U was further considered to explore this competitive process. As shown in Fig. 4A, there is a free energy gap of 1.16 eV between *N₂H and *NCON intermediates on the D_S-VN₄ catalyst, which indicates that at least a U of −1.16 V is needed to make the formation of *N₂H more competitive than *NCON. In principle, the D_S-VN₄ catalyst will preferentially form the *NCON intermediate at U < −1.16 V (Fig. 4B), which is named the potential tolerance limitation (PTL) in this work. Similarly, the PTL of D_S-VN₃C₁, D_S-VN₂C₂, D_S-CrN₂C₂, and D_S-CrN₁C₃ is calculated to be −0.64, −0.71, −0.38, and −0.39 eV, respectively. Therefore, the CO–N₂ coupling process through the Eley–Rideal mechanism will be preferentially triggered unless the applied limiting potential exceeds the PTL (Fig. 4B). Besides, the possibility of adsorbed N₂ dissociation into two isolated N atoms was also considered over these five candidates shown in Fig. S8.† Due to the significant energy requirements, N₂ dissociation is thermodynamically very unfavorable to occur under electrochemical conditions on these model catalysts.

The above results clearly show that the five designed dual-site catalysts have the potential to achieve a high reaction selectivity for C–N coupling to form the *NCON intermediate, which could be converted into valuable urea by the subsequent electrochemical reduction process. Clearly, the D_S-VN₄ catalyst with the largest potential tolerance range in Fig. 4B is the most interesting candidate in our framework. Notably, the successful synthesis of a single VN₄ site was recently reported experimentally,³² laying a good basis for the synthesis of the D_S-VN₄ catalyst.^27,28 Therefore, we focus on the D_S-VN₄ catalyst to specifically explore the subsequent reaction pathway and catalytic activity for urea synthesis in the following section.

As presented in Fig. 4C, we plotted the free energy diagram of the most favorable reaction pathway for urea electrochemical production over the D_S-VN₄ catalyst, while the atomic structures of important intermediates were also provided. As mentioned above, the N₂ molecule forms a horizontal adsorption mode on the dual site with two TM–N bonds, which is exothermic by 2.05 eV. Then, the formation of the *NCON intermediate from the C–N coupling occurs via the Eley–Rideal mechanism with a free energy change of −0.49 eV. Notably, a moderate reaction energy barrier of 1.21 eV is observed for this coupling process (Fig. S9†), which is lower than that of reported in-plane dual site catalysts (e.g., 1.62 eV for Co₂@N₆G and 1.35 eV for FeNi@N₆G),²¹ indicating the kinetic feasibility of generating the *NCON intermediate. Afterward, the *NCON intermediate undergoes consecutive electrochemical protonation to produce the *NHCON, *NHCONH, *NHCONH₂, *NH₂CONH₂ intermediates. The corresponding free energy changes are +0.05, −0.04, +0.47, and +0.50 eV, respectively. In this case, the final protonation process (*NHCONH₂ + H⁺ + e⁻ → *NH₂CONH₂) shows the largest uphill free energy of +0.50 eV, and is therefore the theoretical potential-determining step. Finally, the desorption of the *NH₂CONH₂ intermediate is endothermic by 1.25 eV. Theoretically, only a low limiting potential of −0.50 V is required to drive this reduction process toward urea synthesis, indicating the high activity of the designed D_S-VN₄ catalyst.

Due to the inevitable occurrence of the hydrogen evolution reaction (HER) in any electrochemical reactions, we further took it into consideration. Our calculated binding free energies of two H atoms on the Ds-VN₄ catalyst (Fig. S10†) are only +0.22 eV, and much weaker than the chemisorption of the N₂ molecule (−2.05 eV), making the binding of H atoms much less competitive than N₂ molecules. Despite the electrochemical binding of H with the catalyst being enhanced with increasing applied potential, N₂ adsorption will remain dominant on the D_S-VN₄ catalyst at U_L < −1.135 V and properly suppress the HER due to the lack of active sites to bind H. Based on the above energetic analysis of the whole reaction pathways for urea synthesis, the D_S-VN₄ catalyst could effectively and selectively produce urea in the limiting potential range of −0.50–−1.135 V, which are able to suppress both the N₂ reduction and the HER. We anticipate that our proposed strategy based on confined dual-site catalysts will shed light on the experimental catalyst design with high activity and selectivity toward urea production via CO and N₂ coupling.

3. Conclusion

In summary, we proposed a theoretically feasible strategy to steer the competitive adsorption of reactants by constructing a catalyst with a spatially isolated dual-site. With the help of the spatial formation of a more stable TM–N bond than TM–O, the interaction of the N₂ molecule with the catalytic dual-site can be enhanced, which guarantees the effective coupling of the pre-adsorbed N₂ with CO and facilitates the formation of the *NCON intermediate as the key precursor for urea electrosynthesis. Based on our systematic calculations on a group of transition metal-based dual-site catalysts, D_S-VN₄, D_S-VN₃C₁, D_S-VN₂C₂, D_S-CrN₂C₂, and D_S-CrN₁C₃ were computationally found to exhibit dominant N₂ distribution in a horizontal mode, effectively blocking CO adsorption. Furthermore, the formation of the *NCON intermediate on these screened catalysts is thermodynamically more favorable than the *N₂H formation from electrochemical N₂ hydrogenation. Among all the theoretically predicted promising candidates, D_S-VN₄ stands out as an efficient electrocatalyst for urea synthesis with high activity and selectivity. This study demonstrates the feasibility and functionality of the confined dual-site strategy in regulating the competitive adsorption of reactants to achieve efficient C–N coupling and urea production, which can bring important theoretical guidance for the design of experimental catalysts for practical and sustainable urea synthesis.

4. Computational details

All the density functional theory (DFT) calculations with spin-polarization were performed using the Vienna Ab Initio Simulation Package (VASP) code.³³ The revised Perdew–Burke–Ernzerhof (RPBE) functional was employed to describe the exchange–correlation interactions within the generalized gradient approximation.^34,35 The electron–ion interactions were represented by the projector augmented wave (PAW) method.³⁶ The kinetic energy cutoff of the plane wave was set to be 500 eV and the convergence criterion for the residual forces and total energies were set to be 0.03 eV Å⁻¹ and 10⁻⁵ eV, respectively. The empirical correction in Grimme's method (DFT + D3) was adopted to describe the van der Waals interaction.³⁷ The transition state with only one imaginary frequency was identified using the climbing image nudged elastic band (CI-NEB) method.³⁸ Bader charge calculation was performed to analyze the charge population and charge transfer.³⁹ Other computational details can be found in the ESI.†

Data availability

Computational data supporting the findings can be found in the article and ESI,† and are available from the authors upon reasonable request.

Author contributions

T. W. led and supervised this project; Z. C. performed all the DFT computations and data analysis; Y. H. L. contributed to the plot of Fig. 1; all authors discussed the results, and contributed to the writing of the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2022YFA0911900) and the National Natural Science Foundation of China (22273076); T. W. is thankful for the start-up packages from Westlake University and the Research Center for Industries of the Future (RCIF) at Westlake University for supporting this work. We thank Westlake University HPC Center for computation support. This work is dedicated to Prof. Haijun Jiao on the occasion of his 60th birthday.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sc04688e

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