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

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 (DS) 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 (DS-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.


Introduction
2][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 (NH 3 ) and carbon dioxide (CO 2 ) as reactants.Another key issue of this route lies in its dependence on NH 3 as the nitrogen source, which is industrially produced via the energy-and capital-intensive Haber-Bosch process under harsh conditions. 4It consumes ∼2% of the global fossil energy and emits ∼300 million tons of CO 2 annually during the inert N 2 xation to NH 3 due to the usage of gray hydrogen, and ultimately impacts the sustainability of the whole urea production industry. 4Therefore, developing a low-carbon and alternative sustainable urea synthesis protocol is signicant for achieving and maintaining the sustainable development of human society.
][7][8][9][10][11][12][13][14][15][16][17][18][19][20] For example, Chen et al. experimentally prepared an electrocatalyst by anchoring PdCu nanoparticles on TiO 2 nanosheets for aqueous N 2 and CO 2 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 CO 2 and N 2 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-BiVO 4 heterostructures and ower-like nickel borate [Ni 3 (BO 3 ) 2 ], respectively. 6,7Zhu et al. theoretically predicted Mo 2 B 2 , Ti 2 B 2 , and Cr 2 B 2 as potential efficient electrocatalysts for urea production via N 2 and CO 2 coupling under ambient conditions based on the data from mechanistic calculations with the computational hydrogen electrode (CHE) model. 8][15][16][17][18][19][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][6][7][21][22][23] while the direct coupling of N 2 with CO represents an ideal approach to generating this crucial intermediate. Unfortnately, this route faces several formidable challenges due to the intrinsic difference in electronic structures of N 2 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 N 2 and CO.Mechanistically, the binding of the polarized CO molecule with the metallic active site is usually more favorable than the nonpolarized N 2 molecule, forming a s bond via the C atom due to the signicant 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 N 2 molecule, resulting in a more stable metal-CO s bond.As a result, N 2 is usually a poorer p-acceptor ligand than CO, 25 while achieving horizontal N 2 adsorption is even more challenging.In principle, the horizontal N 2 adsorption should be energetically ensured (Fig. 1A) to achieve the efficient formation of the *NCON intermediate via N 2 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 N 2 at the catalytic site thereby guaranteeing the dominant N 2 horizontal adsorption represents one of the key challenges for efficient urea electrosynthesis.Besides, as shown in Fig. 1A, even if the dominance of horizontal N 2 adsorption is guaranteed, the competition between C-N coupling and N 2 direct electroreduction to *N 2 H 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.
In this work, we proposed a strategy to properly address the aforementioned challenges by steering the competitive N 2 and CO adsorption with a conned dual active site, which provided an elegant proposal to achieve efficient urea production from N 2 and CO.As presented in Fig. 1C, the conned dual active site could stabilize non-polar N 2 adsorption by forming one additional stable TM-N bond, whereas the polar CO molecule binding was hardly affected or even destabilized, thereby making horizontal N 2 adsorption very competitive and eventually promoting the formation of the *NCON intermediate.A group of metals, mainly rst-row transition metals, was chosen as active centers and anchored on nitrogen-doped graphene to construct our conned dual-site catalysts shown in Fig. 1C.Our simulations successfully predict the dual-site VN 4 (D S -VN 4 ) 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 -VN 4 catalyst shows tremendous suppression of undesirable ammonia formation and hydrogen evolution, enabling the high reaction selectivity toward urea products.This work not only identied the spatially isolated dual site with the connement effect for steering the competitive N 2 and CO adsorption but also rationalized this theoretical strategy to ne-tune the strengths of chemical bonds, thus paving a promising route to obtain green urea electrosynthesis via rational catalyst design.

Competitive N 2 and CO adsorption on single and dual sites
To achieve an efficient urea electrochemical synthesis via N 2 and CO coupling, the rst priority is to ensure preferential N 2 adsorption with horizontal conguration.However, as mentioned in Fig. 1, the polar CO molecule is more likely to cover the catalytic site than the non-polar N 2 molecule based on the analysis from molecular orbital theory.To this end, we rst 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., TMN 4 , TMN 3 C 1 , TMN 2 C 2 , and TMN 1 C 3 ) shown in Fig. S1.† Then, we calculated and compared their binding strengths with N 2 and CO.As presented in Fig. 2 and the detailed energetic data in Table S1, † the N 2 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 N 2 molecule.The two exceptions, NiN 4 and NiN 3 C 1 , with similar CO and N 2 binding strengths are due to their physical adsorptions toward adsorbates.In general, our calculated binding strengths of CO and N 2 on the single active site follow the vertical CO > vertical N 2 > horizontal N 2 , 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 N 2 , thereby eliminating the N 2 attacking and blocking the C-N coupling for urea synthesis.
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 N 2 and CO adsorption.Noteworthily, our recent work has successfully validated this strategy to enhance N 2 adsorption and activation with the dual-site catalyst, making near-ambient conditions of ammonia synthesis possible. 26In 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][28][29][30] for O 2 electroreduction 27 and CO 2 electroreduction. 28Thus, the adsorption of N 2 and CO molecules was systematically examined on the designed dual-site catalysts.As presented in Fig. 2 and Table S2,  catalyst in regulating the competitive N 2 and CO adsorption, which lays a good basis for efficient C-N coupling.

Functionality of the conned dual site
Before moving forward to urea synthesis mechanism calculations on different conned dual-site catalysts, we further explored the origins behind these ne-tuned N 2 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 -VN 4 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 signicant change in spin-charge represents a stronger bonding interaction.For the case of N 2 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 aer 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 -VN 3  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 N 2 adsorption more competitive than CO.
Meanwhile, the conned dual-site catalysts will transfer more electrons to the adsorbed N 2 molecule via the additionally constructed stable TM-N bond shown in Fig. 3C.By contrast, no signicant difference was found in the amount of charge transfer for the adsorbed N 2 and CO molecules on the singlesite catalysts shown in Fig. S6.† Clearly, the enhanced N 2 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 N 2 molecule as the energy reference.As listed in Table S6, † the core active sites of the ve candidates with dominant N 2 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 ne-tuning of N 2 and CO adsorption using our proposed conned dual-site catalysts.

Feasibility of conned dual-sites for urea electrosynthesis
Since our proposed dual-site strategy was already able to properly address the challenge from the N 2 and CO competitive adsorption, we further evaluated the capability of this strategy in tackling the competition between CO-N 2 coupling and  However, the formation of the *N 2 H intermediate via N 2 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-N 2 coupling.In other words, the free energy change of N 2 protonation will become more negative at the high U, making N 2 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 *N 2 H and *NCON intermediates on the D S -VN 4 catalyst, which indicates that at least a U of −1.16 V is needed to make the formation of *N 2 H more competitive than *NCON.In principle, the D S -VN 4 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 -VN 3 C 1 , D S -VN 2 C 2 , D S -CrN 2 C 2 , and D S -CrN 1 C 3 is calculated to be −0.64,−0.71, −0.38, and −0.39 eV, respectively.Therefore, the CO-N 2 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 N 2 dissociation into two isolated N atoms was also considered over these ve candidates shown in Fig. S8.† Due to the signicant energy requirements, N 2 dissociation is thermodynamically very unfavorable to occur under electrochemical conditions on these model catalysts.
The above results clearly show that the ve designed dualsite 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 -VN 4 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 VN 4 site was recently reported experimentally, 32 laying a good basis for the synthesis of the D S -VN 4 catalyst. 27,28Therefore, we focus on the D S -VN 4 catalyst to specically 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 -VN 4 catalyst, while the atomic structures of important intermediates were also provided.As mentioned above, the N 2 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 Co 2 @N 6 G and 1.35 eV for FeNi@N 6 -G), 21  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-VN 4 catalyst (Fig. S10 †) are only +0.22 eV, and much weaker than the chemisorption of the N 2 molecule (−2.05 eV), making the binding of H atoms much less competitive than N 2 molecules.Despite the electrochemical binding of H with the catalyst being enhanced with increasing applied potential, N 2 adsorption will remain dominant on the D S -VN 4 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 -VN 4 catalyst could effectively and selectively produce urea in the limiting potential range of −0.50-−1.135V, which are able to suppress both the N 2 reduction and the HER.We anticipate that our proposed strategy based on conned dual-site catalysts will shed light on the experimental catalyst design with high activity and selectivity toward urea production via CO and N 2 coupling.

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 N 2 molecule with the catalytic dual-site can be enhanced, which guarantees the effective coupling of the preadsorbed N 2 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 -VN 4 , D S -VN 3 C 1 , D S -VN 2 C 2 , D S -CrN 2 C 2 , and D S -CrN 1 C 3 were computationally found to exhibit dominant N 2 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 *N 2 H formation from electrochemical N 2 hydrogenation.Among all the theoretically predicted promising candidates, D S -VN 4 stands out as an efficient electrocatalyst for urea synthesis with high activity and selectivity.This study demonstrates the feasibility and functionality of the conned 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.

Computational details
All the density functional theory (DFT) calculations with spinpolarization were performed using the Vienna Ab Initio Simulation Package (VASP) code. 33The revised Perdew-Burke-Ernzerhof (RPBE) functional was employed to describe the exchange-correlation interactions within the generalized gradient approximation. 34,35The electron-ion interactions were represented by the projector augmented wave (PAW) method. 36he 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. 37The transition state with only one imaginary frequency was identied using the climbing image nudged elastic band (CI-NEB) method. 38Bader charge calculation was performed to analyze the charge population and charge transfer. 39Other computational details can be found in the ESI.†

Fig. 1 (
Fig. 1 (A) Typical challenges to urea electrosynthesis by N 2 and CO coupling from a view of the reaction mechanism.(B) HOMO electron density of N 2 and CO. (C) Schematic illustration of N 2 /CO adsorption steering with the confined dual-site strategy.
Fig. 2 The calculated adsorption energy of N 2 and CO molecules on the (A) TMN 4 moiety, (B) TMN 3 C 1 moiety, (C) TMN 2 C 2 moiety, and (D) TMN 1 C 3 moiety with single site and spatially isolated dual sites.The black dashed line represents the equivalent adsorption, and above the line is the N 2 -dominated adsorption region.
Fig. 3 (A) The spin-charge density of pure Ds-VN 4 , N 2 adsorption, and CO adsorption, where the isosurface value was set to be 0.009 e Å −3 .(B) The calculated COHP between D S -VN 4 and the N 2 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 N 2 and CO molecules.
N 2 electrochemical hydrogenation, where the ve candidates with dominant N 2 adsorption were chosen as model systems.At rst, we examined the free energy changes of the coupling process via the Eley-Rideal mechanism (*N 2 + CO(g) / *NCON) and the N 2 electroreduction process (*N 2 + H + + e − / *N 2 H) for convenient comparison.As shown in Fig. 4A and S7, † negative reaction free energies can be observed for the coupling process (−0.49eV for D S -VN 4 , −0.34 eV for D S -VN 3 C 1 , −0.43 eV for D S -VN 2 C 2 , −0.01 eV for D S -CrN 2 C 2 , and −0.15 eV for D S -CrN 1 C 3 ) on the ve catalysts, indicating the thermodynamic feasibility.In contrast, the protonation of the adsorbed N 2 molecule to the *N 2 H intermediate needs to overcome the positive free energy.Thus, at 0 V potential, the C-N coupling is thermodynamically more favorable than N 2 electroreduction on the ve dual-site catalysts.

Fig. 4 (
Fig. 4 (A) The calculated free energy change of C-N coupling and N 2 electroreduction at 0 V vs. RHE.(B) Reaction competition between C-N coupling and N 2 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 -VN 4 catalyst, as well as the reaction competition between urea production and H 2 evolution under different applied potentials.
indicating the kinetic feasibility of generating the *NCON intermediate.Aerward, the *NCON intermediate undergoes consecutive electrochemical protonation to produce the *NHCON, *NHCONH, *NHCONH 2 , *NH 2 CONH 2 intermediates.The corresponding free energy changes are +0.05,−0.04, +0.47, and +0.50 eV, respectively.In this case, the nal protonation process (*NHCONH 2 + H + + e − / *NH 2 CONH 2 ) shows the largest uphill free energy of +0.50 eV, and is therefore the theoretical potential-determining step.Finally, the desorption of the *NH 2 CONH 2 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 -VN 4 catalyst.