Uncovering the Mechanism for Urea Electrochemical Synthesis by Coupling N2 and CO2 on Mo2C-MXene
Jiahe Peng , Xiao Wang , Zheng Wang , Bin Liu , Peng Zhang , Xin Li , Neng Li
Urea, the first organic compound produced from inorganic raw material, [1] is widely utilized in fertilizers as the source of nitrogen[2-5] due to its high nitrogen content (46%) and easy spontaneous conversion to NH3 in soil, while its industrial synthesis requires very large energy consumption under high temperature and pressure and emits a massive amount of green-house gas CO2.[6, 7] In addition, the conversion rate and urea production efficiency are quite low through this method.[8, 9] Therefore, the exploration of high-performance catalysts for urea production is necessary to alleviate the energy and environmental crisis.[10, 11] Aiming at this challenging field, tremendous efforts have been devoted to realizing urea synthesis through electrocatalytic co-activation of N2 and CO2 under normal temperature and pressure conditions, [12-14] which can not only alleviate the pressure of urea industrial production to a certain extent, but also have important significance for solving environmental problems.[15-17]
Compared with other reactions, urea electrosynthesis reactions have higher requirements for catalysts, so finding and developing catalysts with good selectivity and excellent catalytic activity are the primary task of studying urea synthesis catalytic reactions. At present, there has been some progress in the international electrocatalytic synthesis of urea.[18-20] Chen et al. have fixed Pd-Cu alloy nanoparticles on TiO2 nanosheets as catalysts, which reached the high urea production rate of 3.36 mmol g-1 h-1 and excellent Faradaic efficiency of 8.92% at -0.4 V versus reversible hydrogen electrode.[21, 22] This research opened up a precedent for the production of urea under mild conditions. Motivated by the different charging properties of N2 and CO2, Yuan et al. have achieved a higher urea formation rate and faradaic efficiency at the same applied potential via Mott-Schottky Bi-BiVO4 heterostructures (5.91 mmol g-1 h-1 and 12.55%) and BiFeO3/BiVO4 heterojunctions (4.94 mmol g-1 h-1 and 17.18%), respectively.[23, 24] The novel free noble-metal-based MXenes are potential candidates for electrocatalysts.[25-27] Recently, Zhu et al. have proposed MBene (Mo2B2, Ti2B2 and Cr2B2), a new branch of two-dimensional (2D) materials MXenes, as excellent electrocatalysts for urea formation with limiting potentials ranging from -0.49 to -0.65 eV by means of DFT methods.[28] They have clearly revealed the reaction mechanism of urea direct synthesis using N2 and CO2 and provided theoretical guidance for the design and development of carbon-nitrogen reduction reaction (CNRR) catalysts.
In the work, the mechanism of urea synthesis reaction was systemically analyzed by DFT approach, and the MXenes-based CNRR catalyst was firstly reported. The influence of catalyst's geometry, element composition and surface state on the catalyst's reactivity and reaction process was analyzed by means of energy and transition state analysis, etc. By adjusting the elemental composition and morphological structure of the catalyst, we tried to realize the targeted adsorption, effective activation, and highly selective C-N coupling steps in the whole urea electrosynthesis process of MXene-based catalyst and look for a low-cost MXenes-based urea synthesis catalyst with excellent CNRR performance. This work is dedicated to the improvement and development of the theoretical research of MXenes for urea electrosynthesis. We hope this research will have important guiding significance for the design and synthesis of high-performance CNRR catalysts.
The VASP software package of DFT theory calculation tool is used in this chapter, with software version 5.4.4.[29, 30] The projection plus plane wave PAW method is applied to describe the potential, and the exchange and association between electrons are also processed by GGA-PBE functional.[31, 32] The determination of the input parameters in this chapter has also been tested for convergence, and when setting the cutoff energy of the plane wave base group to be 450 eV, the faster calculation result has little effect. The structure-optimized energy convergence standard is 10-5 eV, while the residual stress convergence standard is 0.01 eV/Å. Considering the magnetic nature of transition metal atoms, in all calculations ISPIN=2 is used for spin polarization calculations.[33] The DFT-D3 method was also adopted to correct the van der Waals interaction in the structure.[34, 35] The Monkhorst-Pack method to sample the K-spot in Brillouin district was used here, considering the accuracy of the calculation results and using the K-point of 5×5×1 for structural optimization.[36] The reaction transition state was calculated based on the CINEB method.[37, 38]
Structural Model of MXene. In this work, Mo2C-based MXenes were modeled, including Mo2CS2 and Mo2CO2-MXene with unburdened single atoms on the surface (Figure 1a and 1b), Fe@Mo2CS2-MXenes formed by S-vacancy defect-loaded Fe atoms (Figure 1c) and O-vacancy defect-loaded Fe atom formation Fe@Mo2CO2-MXene (Figure 1d). The Ti@Mo2C and Fe@Mo2C-MXene are formed by Mo vacancy defect and the support Fe and Ti single atoms corresponding from Figure 1g-1i, respectively. Of course, surface-pure Mo2C and Ti2C-MXenes, as shown in Figure 1e and 1f, are also within the scope of this research. To ensure the accuracy and repeatability of the calculations, the structural model of the catalyst is composed of 3×3 supercells. Since MXene belongs to the two-dimensional layered structure, the interlayer distance of the catalyst should be large enough to avoid interaction between adjacent sheet layers, and the vacuum layer thickness along the z-axis direction is set to 20 Å when modeling.
For MXenes synthesis in experiment, the surface -O functional groups of materials can be effectively replaced or removed by molten inorganic salt etching.[39] The S-cover MXene has been successfully synthesized in experiment and applied in electrocatalysis.[40, 41] Moreover, both theoretical and experimental researches have proved that Mo2CTx is a reliable single atom support, [42-45] indicating the realistic significance of Mo2CTx-based single atom catalysts. As for bare Mo2C and Ti2C model construction, we have referred to former theoretical researches, [46-49] and recent researches have successfully used bare Mo2C as the superior catalyst.[50] In addition, the single atom-doped MXene models referred to these former researches.[14, 51-54] The Mo atom can be easily replaced by other transition metals in experiment, which proved the experimental synthesis possibility of our model.
Mechanism of Electrochemical Urea Synthesis. According to former researches, [22, 28] the entire reaction of urea electrosynthesis can be artificially divided into four stages: (1) the catalyst activity center adsorbs and activates N2 and CO2 molecules, (2) the activity center successfully reduces C*OO to C*O intermediate, (3) the coupling of N*N and C*O forms N*CON, (4) N*CON completes the four-step protonation hydrogenation to form urea. The catalytic activity of the urea synthesis together with the urea synthesis reaction mechanism on the MXenes-based catalyst is shown in Scheme 1.
The complex reaction steps put forward higher requirements for catalyst performance. Firstly, N2 and CO2 activation on catalyst activity center is the primary condition for electrochemical urea synthesis. Secondly, the NRR protonation energy barrier on catalyst activity center must be higher than the maximum reaction energy barrier of urea synthesis to forbidden the NH3 formation, and CO2 should be reduced to C*O intermediates to ensure that the catalyst activity center preferentially undergoes urea synthesis reactions instead of generating by-products. Subsequently, the C-N coupling reaction of *CO and *N2 to form a *NCON intermediate is a kinetically determining factor in synthesizing urea, and the catalyst must promote this process. The *NCON intermediate is continuously hydrogenated to *NCONH, *NHCONH/*NCONH2, *NHCONH2, and *NH2-CONH2 through four proton-coupled electron (PCET) steps, and finally urea is released. In order to investigate the activity of Mo2C-based catalysts for urea synthesis, the catalyst performance from three aspects was estimated as: the adsorption and activation capacity of CO2 and N2 molecules, the energy barrier at the C-N coupling reaction and the catalytic selectivity toward urea formation.
Adsorption of N2 and CO2 on the Surface of Fe-Mo2CS2. Former researches have reported that N2 or CO2 molecule can be captured and activated by the active center when adsorbed separately on the surface of Fe-Mo2CS2.[55, 56] But the co-adsorption may lead to different adsorption behaviors. According to our calculation, CO2 and N2 molecules are located on both sides of the Fe atom when co-adsorbed (Figure 2). The adsorption energy of N2 on the surface of Fe-Mo2CS2 is -0.72 eV, and the bond length between two N atoms is extended from 1.115 to 1.132 Å, indicating it can be stably adsorbed and activated on this surface; while for CO2 adsorption, the O-C bond is from 1.176 Å to 1.182 and 1.172 Å, and the angle change is very small, which can be ignored. The adsorption energy of CO2 molecules on the surface of Fe-Mo2CS2 is only -0.06 eV. Although CO2 molecules have low adsorption energy under the action of N2, they can be still spontaneously adsorbed. The above data suggest that N2 and CO2 can be spontaneously adsorbed on the surface of Fe-Mo2CS2 at the same time.
CO/N2 Coupling Reaction Based on Mxenes Materials. After revealing the co-adsorption behavior of N2 and CO2 on the surface of Fe-Mo2CS2, we also need to evaluate the activation possibility of C*OO conversion to C*O on the same surface with the presence of N2 molecule. According to the previous research and a large number of literature reports on the CO2 reduction reaction (CO2RR), [57] C*OO reaction to C*O requires a two-step protonation process: C*OO + H+ + e- → C*OOH and C *OOH + H+ + e- → C*O + H2O. Based on to our calculations, C*OO + H+ + e- → C*OOH process belongs to the endothermic reaction when adsorbed with N2 molecules, and the reaction free energy of this protonation step changes to 0.17 eV. The reaction energy barrier of this endothermic reaction is relatively low compared to the C*OO first step protonation barrier (0.21-0.24 eV) on the MBenes material reported by Zhu et al.[28] The process of converting C*OOH into C*O and releasing H2O molecules is exothermic in the second step, and the reaction energy changes to -0.77 eV. Therefore, the C*O intermediates can be efficiently generated on the Fe-Mo2CS2 surface. In the presence of N2, the adsorption energy of C*O on this surface is -1.34 eV, leading to the stable adsorption of C*O intermediates on such a surface without desorption, which can meet the basic conditions for the coupling reaction of N*N and C*O.
Subsequently, the ground state energy and the free energy change of C-N coupling reaction on the surface of the catalyst have been calculated. The atomic structure diagram of each intermediate and the free energy change value of the corresponding reaction intermediate ΔG are shown in Figure 2. According to former researches, [22, 28] the most important and difficult reaction process in the synthesis of urea is the coupling reaction of N*N and C*O. However, we can clearly see from Figure 2 that the N*N and C*O coupling reactions on the Fe-Mo2CS2 surface are endothermic with an insurmountable reaction barrier (2.72 eV), indicating that the C-N coupling reaction is difficult to carry out on this surface, and therefore such a single-atom catalyst is not suitable for urea synthesis.
Besides Fe-Mo2CS2, considering the thermodynamic process of Mo2CS2 for C-N coupling reaction covered by surface -S functional groups without Fe atoms, Mo2CO2 covered by surface-O functional groups was compared. It can be found that pure -O (Figure 3b) and -S (Figure 3b) functional groups covered Mo2CTx cannot stably co-adsorb N2 and CO molecules. N2 and CO move away from the Mo2CS2 surface by more than 3.2 Å after structural optimization. As for Mo2CO2, C*O is bonded with the -O surface groups and tends to form CO2 molecules, while the N2 molecules keep still far away from the surface of Mo2CO2 by more than 3.2 Å. Moreover, the energy barriers of C-N coupling reaction are 3.15 and 2.98 eV for Mo2CS2 and Mo2CO2, respectively. Therefore, the C-N coupling reaction is difficult to perform on the Mo2CTx passivated by the functional group, and the subsequent study attempted to complete the optimization of catalyst in the form of a defect-loaded transition metal (Figure 3c, d).
Fe single atom loading on the surface of Mo2CS2 can reduce CO2 to CO products at a lower maximum reaction energy barrier, so we attempt to fix Fe atoms on the surfaces of Mo2CS2 and Mo2CO2 in the form of S and O atom defects, which may improve the performance of urea synthesis catalyst. The thermodynamic processes of these two Fe atom-loaded catalysts for urea synthesis coupling reactions are shown in Figure 3c-3d. Analysis of the geometry shows that after introducing Fe atoms, Fe@Mo2CS2 and Fe@Mo2CO2 obtain better adsorption properties of CO and N2, and the tower-like N*CON can also be stably present on the surface of the two catalysts. However, compared with the energy change of the system before and after coupling reaction, the energy difference between "N*CON*" and "N*N-C*O" on the two catalysts needs to absorb more than 1.8 eV, which means they are not suitable for urea synthesis. Comprehensive analysis of the above data indicates that Mo2C-MXenes with functional group covered on the surface do not have the ability to adsorb and activate N2 and CO2 or complete C/N coupling at lower energy barriers. The passivation of functional groups is an important factor affecting the urea electrosynthesis performance of MXene-based catalysts.
Then, the urea synthesis properties of Mo2C and Ti2C-MXenes without surface functional groups (Ti was chosen because its single atoms can obtain C*O intermediates in our former research[56]) were investigated, and the atomic structure diagrams of C-N coupling reaction are shown in Figure 3e and 3f. Both N*N and C*O can be stably adsorbed on the surfaces of Mo2C and Ti2C, and the structures of N*CON intermediates are also stable. The energy differences of coupling reaction on the Mo2C and Ti2C surfaces are -0.168 and 0.594 eV, respectively. The relatively lower reaction energy barrier is beneficial to completing the C/N coupling reaction on these two pure MXenes materials, meaning they have urea electrosynthesis potential.
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| N*N-C*O(eV) | N*CON*(eV) | ΔE(eV) | N≡N(Å) | |
| Fe-Mo2CO2 | -426.847 | -424.166 | 2.681 | 1.132 |
| Mo2CO2 | -457.712 | -454.731 | 2.981 | 1.114 |
| Mo2CS2 | -418.706 | -415.563 | 3.143 | 1.115 |
| Fe@Mo2CO2 | -458.031 | -456.210 | 1.821 | 1.207 |
| Fe@Mo2CS2 | -420.696 | -418.493 | 2.203 | 1.245 |
| Mo2C | -296.674 | -296.842 | -0.168 | 1.256 |
| Ti2C | -267.059 | -266.465 | 0.594 | 1.346 |
| Mo vacancy | -286.487 | -284.988 | 1.499 | 2.382 |
| Fe@Mo2C | -294.204 | -293.902 | 0.302 | 1.239 |
| Ti@Mo2C | -295.606 | -296.129 | -0.523 | 1.186 |
Reduction Reaction of CO2 and N2 on the Surfaces of Mo2C and Ti2C. Firstly, the co-adsorption of N2 and CO2 on these two MXenes has been studied. The N≡N bond is activated to 1.26 Å when adsorbed on the surface of Mo2C, and the N atom forms a chemical bond with the surrounding Mo atom. When N2 is adsorbed on the surface of Ti2C, the N≡N bond is expanded to 1.35 Å and is also bonded with the surrounding Ti atoms. The result shows a strong interaction between the N2 molecule and the two MXenes (Figures 4 and 5). As for CO2 molecules, the bond length is elongated when adsorbed on the surfaces of Mo2C and Ti2C, and the bond angle ∠OCO changes from 180° to 111° (Mo2C surface) and 114.9° (Ti2C surface), indicating strong interaction between the CO2 molecule and these MXenes. Therefore, both CO2 and N2 can be stably co-adsorbed and efficiently activated on the surfaces of the Mo2C and Ti2C-MXenes.
Then, we explored the performance of reducing C*OO to C*O on the surfaces of Mo2C and Ti2C under the action of surface N2 molecules. The first step of C*OO protonation on the surfaces of both Mo2C and Ti2C MXenes is a spontaneous process. The free energy change ΔG is -1.07 and -0.32 eV for Mo2C and Ti2C, respectively. As for the further protonation of C*OOH to C*O, it is endothermic on both Mo2C and Ti2C MXenesthe surfaces. The ΔG is 0.69 eV for Mo2C, and the reaction energy barrier is relatively low. But the reaction requires large energy of 1.58 eV for Ti2C, which is obviously not a high-performance CNRR catalyst.
After the formation of N*CON intermediates on the surface of the catalyst, the reaction process becomes relatively simple, and two N atoms undergo protonation process. We investigated the reaction free energy changes of different hydrogenation reactions, and the hydrogenation of N atoms basically conforms to the alternating mechanism. In addition, we also consider the hydrogenation of oxygen atoms. This reaction is endothermic, and the reaction energy barrier is much higher than that of the protonation of N atom. By comparing the PCET energy changes at each step of the entire reaction path, the C*OOH protonation is the potential determining step with the largest energy barrier. The maximum reaction energy barrier for the electrochemical reduction reaction of CO2 and N2 on the Mo2C and Ti2C surfaces is 0.69 and 1.58 eV respectively, meaning the catalytic activity of Mo2C is higher than that of Ti2C.
Transition State of CO2/N2 Coupling Reaction. The results show that the CO2/N2 coupling reaction can be carried out spontaneously on the surface of Mo2C, but the energy analysis of key intermediates only studies the relatively stable structure in the reaction process, and the calculated energy barrier is the thermodynamic reaction barrier, which can be used to indicate the thermodynamic CNRR activity of MXenes. When studying the reaction process of C/N coupling reaction which has a large change in atomic configuration, it is necessary to analyze the transition state to estimate the kinetic energy barrier. In this section, the kinetic process of the coupling reaction of N*N and C*O on the surface of Mo2C is studied via CI-NEB transition state search. Eight intermediate points were set between the initial and final states, and the calculated energy change of the reaction transition state is depicted in Figure 4. The transition state energy barrier for N*N and C*O to couple reaction is 1.5 eV on the Mo2C surface, much higher than the values for other CNRR catalysts reported so far. Clearly, bare Mo2C-MXene is not an excellent catalyst for urea synthesis.
Effect of Single Atoms on Mo2C Urea Synthesis Performance. Through previous research, although Mo2C as a catalyst for urea electrosynthesis has a low thermodynamic barrier of only 0.69 eV, the transition state barrier of the most complex C/N coupling reaction on Mo2C is 1.5 eV, which is the biggest obstacle restricting Mo2C to be an excellent catalyst for urea synthesis. Therefore, in this section, an attempt will be made to improve the CNRR performance of Mo2C-MXene through single atom doping, including Ti and Fe atoms. By calculating the Gibbs free energy of the reaction, the reaction process and maximum reaction energy barrier of Mo2C-MXene loaded with single atoms for CNRR were studied, and the reaction transition state of the C/N coupling reaction of the key reaction of urea synthesis was analyzed by the CINEB method.
As shown in Figures 6 and 7, the minimum energy pathway of the urea synthesis reaction and the atomic configuration of the reaction intermediates on the surfaces of Fe@Mo2C and Ti@Mo2C were demonstrated. And Figure 8 shows the transition state energy changes of the C/N coupling reaction on the surfaces of these two catalysts. The calculation results show that the potential determining step changes from C*OOH + H+ + e- → C*O + H2O to N*N-C*O coupling reaction on Fe@Mo2C, and the maximum reaction energy barrier for urea synthesis also drops from 0.69 to 0.366 eV, indicating that adding Fe atoms is conducive to the formation of urea. The transition state energy barrier for the C/N coupling reaction reduced from 1.5 to 0.78 eV. As for Ti@Mo2C, the addition of Ti single atom also changes the potential determining step of the catalytic reaction. The C*OO first protonation step is the potential determining step with 0.362 eV energy barrier, and the transition state energy barrier for the C/N coupling reaction reduced from 1.5 to 0.74 eV. In general, the results show that the addition of single atoms can affect the activation and reduction process of C*OO, which is conducive to forming the C*O intermediate and reducing the maximum reaction energy barrier of CNRR.
Catalytic Selectivity of Fe@Mo2C and Ti@Mo2C toward Urea Production. Besides the outstanding activity, the catalytic selectivity toward urea formation is another intrinsic characteristic that influences the faradaic efficiency. The catalytic selectivity toward urea production was compared to the competitive NRR (N2 reduc-tion reaction). As shown in Figure 9a, two possible reduction products (NH2NH2 and NH3) are considered in the NRR side reaction. For the 4e- reduction product NH2NH2, the reaction *NH2NH + H+ + e- → *NH2NH2 is its potential determining step (PDS) with ΔG values of 0.66 and 0.67 eV for Fe@Mo2C and Ti@Mo2C, respectively, which are larger than their maximum ΔG values (0.366 eV for the former and 0.362 eV for the latter) for urea production. For the 6e- reduction product NH3, *NH + H+ + e- → *NH2 is the potential determining step with ΔG values of 0.36 and 0.60 eV for Fe@Mo2C and Ti@Mo2C, respectively. The higher ΔG value of the potential determining step on Ti@Mo2C indicates the NH3 production is difficult. Unfortunately, the potential determining step and limiting potential for the byproduct NH3 and the targeted urea are both identical on the Fe@Mo2C monolayer.
The catalytic selectivity toward urea production compared to the competitive CO2RR to the HCOOH product and CORR (CO reduction reaction) to the CH4 product is also assessed. For reduction product HCOOH, the first hydrogenation process (*OCOH + H+ + e- → *HCOOH) is its potential determining step with the ΔG values of 0.76 and 0.95 eV for Fe@Mo2C and Ti@Mo2C respectively (Figure 9b), which is larger than that of the second CO2 PCET step in urea production. The higher energy barriers can effectively block the generation of HCOOH, which is beneficial for CO formation. As shown in Figure 9c, reactions *CH2O + H+ + e- → *CH2OH (0.58 eV) and *CH3 + H+ + e- → *CH4 (0.57 eV) are the potential determining steps of Fe@Mo2C and Ti@Mo2C for CORR, respectively. Both values are larger than the maximum ΔG of Fe@Mo2C (0.366 eV) and Ti@Mo2C (0.362 eV) for urea production.
Moreover, the energy barriers of CO further reduction to *CHO intermediate and N2 hydrogenation to *NNH on Ti@Mo2C are -0.16 and -0.09 eV, respectively. Both of them are larger than the ΔG value of -0.47 eV for key C/N coupling reaction in urea production, which indicates that the adsorbed N2 and CO are more inclined to form N*CO*N than the hydrogenation step on Ti@Mo2C. As for Fe@Mo2C, the energy barriers of CO further reduction to *CHO intermediate and N2 hydrogenation to *NNH on Ti@Mo2C are respectively -0.22 and -0.01 eV. But the C/N coupling reaction on Fe@Mo2C is endothermic, meaning the hydrogenation step is easier to occur on Fe@Mo2C. Overall, the formation of byproducts can be greatly suppressed on the Ti@Mo2C monolayer, thus indicating its excellent catalytic selectivity.
In this work, the structural models of Mo2C-based MXenes with different surface states were constructed, and the C/N coupling reaction energy barriers and adsorption states of key intermediates of these catalysts were studied using the first principles method. The results show that the presence of surface functional groups was not conducive to the coupling reaction of C*O and N*N intermediates on the surface of the catalyst, regardless of whether Fe atoms were supported or not. The C/N coupling reaction energy barrier on the surface of Mo2CTx is greater than 1.8 eV, so the thermodynamic process of coupling reaction is difficult to achieve. Subsequently, the urea electrosynthesis performance of bare Mo2C and Ti2C-MXenes was studied, finding they both could effectively activate CO2 and N2. Our calculations show that Mo2C has a good activity to convert CO2 and N2 into urea with a reaction energy barrier of 0.69 eV, while Ti2C does not have the ability to reduce N2 and CO2 to urea due to the high maximum reaction thermodynamic energy barrier (> 1.5 eV).
Due to the complexity of the coupling reaction process, we can not only study its thermodynamic process, and the transition state search method was also used to estimate its kinetic process. The result shows that N*N and C*O coupling transition state energy barrier on the surface Mo2C is difficult to overcome. Therefore, the bare Mo2C doped with single transition metal atom is constructed to study the effect of single atoms on the CNRR activity, showing the addition of Fe and Ti atoms can markedly reduce the maximum reaction energy barrier and transition state energy barrier of urea synthesis. The maximum reaction energy barrier of CNRR of Fe@Mo2C and Ti@Mo2C is about 0.36 eV. Between these single atoms, Ti@Mo2C exhibits excellent catalytic selectivity. Therefore, two-dimensional Ti@Mo2C monolayer can be applied as a promising CNRR catalyst.
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Figure 1 Structural models of MXenes: (a) Mo2CS2 of the S crown energy group, (b) Mo2CO2 of the O crown energy group, (c) Mo2CS2 loaded Fe atom Fe@Mo2CS2 of the S defect, (d) Mo2CO2 loaded Fe atom by Fe@Mo2CO2 with O atom defect, (e) Mo2C, (f) Ti2C, (g) Mo2C of Mo atom vacancy, (h) Mo2C loaded Fe atom Fe@Mo2C of Mo defect, (i) Mo2C-loaded Ti atoms for Mo defects Ti@Mo2C.
Scheme 1 Four important reaction processes for CO2 and N2 electrocatalytic urea synthesis through four proton-coupled electron (PCET) steps.
Figure 2 Atomic structure diagram of urea synthesis reaction energy change, N2 and CO2 common adsorption and coupling reaction on the surface of Fe-Mo2CS2.
Figure 3 Mo2C-MXenes coupling reaction atomic structure diagram for different surface states. (a) -O functional group Mo2CO2, (b) -S functional group Mo2CS2, (c) S defect Mo2CS2 support Fe atom Fe@Mo2CS2, (d) O defect Mo2CO2 load Fe atom Fe@Mo2CO2, (e) Mo2C, (f) Ti2C.
Figure 4 Free energy change diagram of the minimum energy path of CO2/N2 urea synthesis reaction, the atomic configurations of the reaction intermediates and the transition state energy change of C*O and N*N coupling reactions on the surface of Mo2C (The selected bond lengths are in Å. Black represents the N-N bond and red represents C-O).
Figure 5 Free energy change diagram of the minimum energy path and the atomic configurations of the reaction intermediates for CO2/N2 urea synthesis reaction on the surface of Ti2C. The selected bond lengths are in Å. Black represents the N-N bond and red represents C-O.
Figure 6 The optimal energy path free energy change pattern and the atomic configurations of the reaction intermediates for the urea synthesis reaction occurring on the surface of Fe@Mo2C. The selected bond lengths are Å. Black and red represent the N-N and C-O bonds, respectively.
Figure 7 The optimal energy path free energy change pattern and atomic configurations of the reaction intermediates for the urea synthesis reaction occurring on the surface of Ti@Mo2C. The selected bond lengths are in Å. Black represents the N-N bond and red represents C-O.
Figure 8 (a) The transition state energy change in the C/N coupling reaction on the surface of Fe@Mo2C, (b) the transition state energy change in the C/N coupling reaction on the surface of Ti@Mo2C.
Figure 9 Gibbs free energy diagrams for the N2 reduction reaction (NRR) to (a) NH2NH2, (b) NH3 products, (c) the CO2 reduction reaction to HCOOH, (d) the CO reduction reaction to CH4 on Fe@Mo2C and Ti@Mo2C monolayer.
Table 1. The Energy of the Reaction Intermediate before and after the C*O and N*N Coupling Reactions on Mxenes, the Energy Difference between "N*CON*" and "N*N-C*O" (ΔE = E(N*CON*) - E(N*N-C*O)), and the Bond Length of N≡N When N2 Molecules are Adsorbed
| N*N-C*O(eV) | N*CON*(eV) | ΔE(eV) | N≡N(Å) | |
| Fe-Mo2CO2 | -426.847 | -424.166 | 2.681 | 1.132 |
| Mo2CO2 | -457.712 | -454.731 | 2.981 | 1.114 |
| Mo2CS2 | -418.706 | -415.563 | 3.143 | 1.115 |
| Fe@Mo2CO2 | -458.031 | -456.210 | 1.821 | 1.207 |
| Fe@Mo2CS2 | -420.696 | -418.493 | 2.203 | 1.245 |
| Mo2C | -296.674 | -296.842 | -0.168 | 1.256 |
| Ti2C | -267.059 | -266.465 | 0.594 | 1.346 |
| Mo vacancy | -286.487 | -284.988 | 1.499 | 2.382 |
| Fe@Mo2C | -294.204 | -293.902 | 0.302 | 1.239 |
| Ti@Mo2C | -295.606 | -296.129 | -0.523 | 1.186 |
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