Catalyst design method for co2 reduction to co by mo si2n4 doped with transition metal atoms

Abstract

The application belongs to the technical field of electrochemical energy materials, and particularly relates to a design method of a MoSi2N4 doped with transition metal atoms to promote CO2 reduction into CO, and aims to screen out a catalyst with excellent CO catalytic activity and product selectivity, and analyze the source of the catalytic activity. The main scheme comprises the following steps: constructing a calculation model containing different single-atom catalyst active centers; calculating the binding energy and formation energy of the catalysts by using the density functional theory, so as to judge the thermal stability of the materials; exploring the CO2 adsorption energy, intermediate binding energy and limiting potential of the reaction process of different single-atom active sites; and analyzing the activity and selectivity of the catalyst in the CO production process. The application provides a theoretical basis for designing a novel and efficient catalyst.

Description

Technical Field

[0001] This invention relates to the field of computational materials chemistry, and in particular to a method for designing a catalyst that promotes the reduction of CO2 to CO by doping MoSi2N4 with transition metal atoms. Background Technology

[0002] Electrochemical carbon dioxide reduction reaction (CO2RR) is one solution to the problem of carbon dioxide (CO2) emission and utilization. It can convert CO2 into high-value chemical fuels through multiple protonation steps. However, the C=O bond in the CO2 molecule is very stable and requires high energy to break, which makes CO2RR suffer from slow reaction rate and low Faraday efficiency. At the same time, CO2RR with multiple reaction products and the inevitable competitive hydrogen evolution reaction (HER) in the process also lead to low product selectivity. Among the multiple product conversions, CO has very high industrial value as a raw material for various chemical products and industrial reactions. Noble metal catalysts such as Au and Ag are excellent catalysts for selectively reducing CO2 to CO and effectively suppressing HER, but their large-scale application is limited due to their high cost. Therefore, the development of low-cost, efficient, stable, and noble metal-free CO2RR electrocatalysts remains a current research focus. Single-atom catalysts (SACs) have a series of excellent properties, such as extremely high atom utilization efficiency, high product selectivity, and uniform distribution of catalytic active sites. Among them, the activity of single-atom catalysts is closely related to the coordination environment of the central atom. Since the discovery of graphene, two-dimensional (2D) materials, due to their unique structural features and distinctive physical, chemical, and electronic properties, have been considered highly promising substrates for supporting highly active transition metal atoms in the development of novel CO catalysts that can replace noble metals. MoSi2N4 is a novel two-dimensional semiconductor material experimentally synthesized via chemical vapor deposition (CVD). Composed of seven atomic layers (N-Si-N-Mo-N-Si-N), it exhibits very high environmental stability. However, the CO2RR activity of pristine MoSi2N4 is not good; therefore, various methods are needed to modulate its surface coordination environment and activate its CO2RR activity.

[0003] Against this backdrop, the present invention designs a series of potential CO2RR catalysts by doping highly reactive transition metal atoms on the surface of MoSi2N4, and screens out a single-atom catalyst with high stability, high CO catalytic activity and selectivity according to the corresponding steps, thus effectively solving this problem. Summary of the Invention

[0004] Based on experimentally synthesized MoSi₂N₄, this invention investigates a series of transition metal atom-doped two-dimensional MoSi₂N₄ single-atom catalysts TM@MoSi₂N₄-Nv (TM=Sc-Zn) for CO₂RR using density functional theory (DFT). First, their stability was analyzed. Then, the reaction pathway for CO production was calculated, the product selectivity relative to HCOOH, CH₄, and H₂ was studied, and the source of the optimal catalyst's catalytic performance for CO production was analyzed.

[0005] To achieve the above objectives, this invention provides a method for designing catalysts that dope MoSi2N4 with transition metal atoms to promote the reduction of CO2 to CO, comprising the following steps: (1) Constructing a computational model for catalysts containing active single-atom centers The process includes several steps, such as model selection, establishing a single-layer supercell, setting a vacuum layer, and model optimization.

[0006] The model is a two-dimensional layered structure composed of seven atomic layers (N-Si-N-Mo-N-Si-N), and the single-layer supercell is a 3×3×1 MoSi2N4 supercell doped with transition metals, such as... Figure 1 As shown in (a) and (b); the vacuum layer is 20 Å; the model optimization refers to the optimization of lattice parameters and the optimization of the forces of atoms in the x, y, and z directions after fixing the z-axis and optimizing the x and y axes, to obtain the TM@MoSi2N4-Nv monolayer cell structure.

[0007] In one embodiment of the present invention, after model optimization, the model of Mn@MoSi2N4-Nv is shown in Figure 2 After structural optimization, the Mn atoms did not move away from the MoSi2N4 monolayer, indicating that the Mn atoms are stably bonded to the MoSi2N4 surface, proving that the modeling and optimization methods of this invention are reasonable.

[0008] (2) Evaluate the thermal stability of different catalysts Thermal stability is one of the key properties of a catalyst. The binding energy E of the catalyst is calculated using formulas (I) and (II). bind and formation energy E form To evaluate the thermal stability of the catalyst.

[0009] E bind =E(TM@MoSi2N4-Nv)–E(MoSi2N4-Nv)–E(TM) (I) E form =E(TM@MoSi2N4-Nv)–E(MoSi2N4-Nv)–μ(TM) (II) Where E(TM@MoSi2N4-Nv) and E(MoSi2N4-Nv) represent the energies of the TM@MoSi2N4-Nv monolayer and the MoSi2N4 monolayer with N vacancies, respectively. E(TM) and μ(TM) represent the energies of a single TM atom in vacuum and a single TM atom in a stable bulk phase, respectively.

[0010] The binding energy E of the catalyst bind E < 0 indicates that the catalyst is stably bound; conversely, E bind If the density of states (PDOS) is greater than 0, the catalyst bonding is unstable. Furthermore, the thermal stability of the catalyst was verified by calculating the projected density of states (PDOS) of the catalyst to analyze the orbital hybridization between the transition metal atoms and the MoSi₂N₄ surface. The full hybridization of the p orbitals of Si atoms and the d orbitals of the transition metal atoms near the Fermi level indicates that the bonding between the transition metal atoms and the MoSi₂N₄ surface is stable, further demonstrating the thermal stability of the catalyst.

[0011] The formation energy of a catalyst can indicate the difficulty of its experimental synthesis. The formation energy of a catalyst is E. form A formation energy <0 indicates that the catalyst can be easily synthesized under mild experimental conditions. However, a positive formation energy does not necessarily mean that the system cannot be synthesized experimentally. For example, the calculated formation energy of Co-MoS2 is 1.80 eV, but it can be synthesized under relatively mild hydrothermal conditions. If the formation energies of catalysts are all less than this value, it indicates that they can be synthesized without demanding experimental conditions.

[0012] (3) Evaluate the CO2 activation ability of the catalyst. The first and most crucial step in CO2RR is the initial activation of carbon dioxide molecules, i.e., CO2 adsorption. The adsorption configuration of carbon dioxide significantly influences the subsequent reduction process. Two initial CO2 adsorption configurations are considered: vertical and horizontal. The degree of deformation of the CO2 structure after optimization is used to determine whether CO2 is activated. If the CO2 changes from a linear to a V-shaped configuration (∠OCO < 180°), it can be preliminarily determined that the CO2 molecule has been activated. Simultaneously, the CO2 adsorption energy of the two initial adsorption configurations is calculated using formula (III) to further determine which configuration to use in subsequent studies. The adsorption configuration with a greater degree of CO2 deformation and a more negative CO2 adsorption energy was used in subsequent research.

[0013] E ads =E(CO2_TM@MoSi2N4-Nv)–E(TM@MoSi2N4-Nv)–E(CO2) (III) Wherein, E(CO2_TM@MoSi2N4-Nv) refers to the total energy of the system in which the catalyst adsorbs CO2, E(TM@MoSi2N4-Nv) refers to the energy of the TM@MoSi2N4-Nv monolayer, and E(CO2) refers to the energy of a single CO2 molecule.

[0014] The adsorption capacity of transition metal atoms can be described by the d-band center. According to the classical d-band center theory, the closer the d-band center value is to the Fermi level, the stronger the adsorption capacity. In one embodiment of the present invention, the d-band centers of TM atoms in the TM@MoSi2N4-Nv (TM=Sc-Zn) monolayer were calculated. It was found that the d-band centers of Sc, Ti, Mn, and Fe@MoSi2N4-Nv monolayers are relatively close to the Fermi level. Therefore, the present invention predicts that their adsorption capacity will be stronger. The optimization results of CO2 adsorption configuration and the calculation results of CO2 adsorption energy also verify the prediction of the present invention.

[0015] (4) Preliminary screening of CO2RR catalysts CO has high industrial value as a raw material for various chemical products and industrial reactions. However, previous studies have shown that when carbon dioxide molecules are adsorbed onto the catalyst surface, the resulting CO2RR can produce a variety of products, including two-electron products CO and HCOOH, six-electron product CH3OH, eight-electron product CH4, and C generated through C-C coupling. 2+ Products. This paper does not consider C. 2+ The products, because CC coupling requires two or more active sites, are difficult to achieve in single-atom catalysts. As shown in the figure, in the CO2RR process, the two-electron products CO and HCOOH are generated via the pathways CO2→*COOH→*CO→CO and CO2→*OCHO→*HCOOH→HCOOH, respectively. Besides desorption, *CO and *HCOOH can also generate the *CHO intermediate, which is a key intermediate for the generation of multi-electron products such as CH4. Furthermore, HER consumes active sites on the catalyst surface, thus reducing the reaction rate, and is the main competing reaction for CO2RR. Therefore, an ideal CO catalyst should not only have high CO2RR activity but also be able to suppress other side reactions, such as the generation of HCOOH, further hydrogenation of *CO, and HER. Here, this invention first evaluates the activity of CO generation on a TM@MoSi2N4-Nv monolayer.

[0016] As can be seen from the above analysis, CO can be generated according to the following steps, namely *+CO2+H + +e - →*COOH (1) *COOH+H + +e -→*CO+H2O (2) *CO→*+CO (3) To evaluate the catalytic activity of catalysts in producing CO and to screen for superior CO catalysts, this invention first calculated the free energy diagrams for CO production. The free energy change at each step was calculated according to formula (IV), establishing a free energy diagram for the CO2 reduction to CO reaction pathway at an applied potential of 0V.

[0017] The free energy of each elementary reaction was calculated based on the computational hydrogen electrode (CHE) model proposed by Nørskov et al. The free energy change ΔG between any two steps in the CO2 reduction process was calculated using the free energy formula (IV), i.e. ΔG = ΔE + ΔZPE – TΔS (IV) Here, ΔE is the electronic energy difference for each reaction step calculated using DFT. T is the temperature (298.15 K), and ΔZPE and ΔS are the zero-point energy change and entropy change, respectively, which can be obtained through frequency calculation.

[0018] Specifically, this includes a free energy diagram of the CO2 to CO reaction pathway, with *+CO2, *COOH, *CO, and *+CO as the x-axis and the free energy changes of different steps in the CO2 to CO reaction pathway as the y-axis, constructed under an applied voltage of 0V. (Theoretical U) L It has been proven to be an effective indicator for evaluating catalyst activity. Therefore, the value with the maximum free energy change (ΔG) is found from the free energy diagram. max The steps are as follows: then calculate U according to formula (V). L Comparing the U of CO production in TM@MoSi2N4-Nv monolayers L Filter out U L The catalyst closest to 0 has the best CO catalytic activity.

[0019] U L =–ΔG max / e (V) Wherein, ΔG max It is the largest free energy change among all elementary reactions.

[0020] In one embodiment of the present invention, such as Figure 6 As shown in (b), the calculation results indicate that the UL values ​​for CO production by the three TM@MoSi2N4-Nv (TM=Sc,Ti,Mn) catalysts are relatively small, at -0.28 V, -0.34 V, and -0.16 V, respectively. This suggests that they require relatively low applied voltages to reduce CO2 to CO and have better reactivity.

[0021] (5) Product selectivity analysis: Compare the selectivity of CO production with that of HCOOH, CH4 and H2 production. The first step in CO2RR, the hydrogenation of the CO2 molecule, occurs via two pathways: CO2 + H+ + +e - →*COOH and CO2+H + +e - →*HCOO, leading to CO and HCOOH products respectively. Therefore, this invention compares the free energy changes of the catalysts producing *COOH and *HCOO, and further calculates the free energy change of *HCOO producing *HCOOH, to comprehensively evaluate the catalyst's product selectivity for CO and HCOOH.

[0022] Meanwhile, after confirming the selectivity of the catalyst for these two products, it is still necessary to consider whether the products can be successfully desorbed and whether they tend to be further hydrogenated and reduced. That is to say, after *CO is formed, in addition to the desorption of *CO to produce CO products, it is also possible to carry out the reaction of further hydrogenation of *CO to produce C1 products. *CHO is a key intermediate for the production of C1 products such as CH4. Therefore, this invention mainly considers the reaction tendency of *CO desorption and *CO hydrogenation to produce *CHO intermediates, and calculates the CO binding energy and the free energy diagram of CH4 production of these catalysts with better CO catalytic activity screened in step (4).

[0023] Calculate the CO binding energy E using formula (VI) b (*CO): E b (*CO)=E(*CO)-E(*)-E(CO2)+E(H2O)-E(H2) (VI) Where E(*CO) is the total DFT energy of TM@MoSi2N4-Nv with *CO intermediate adsorbed, and E(*) is the energy of the clean TM@MoSi2N4-Nv surface. E(CO2), E(H2O), and E(H2) are the total DFT energies of gaseous CO2, H2O, and H2, respectively.

[0024] Finally, as is well known, the most likely competing reaction in the CO2RR process is hydrogenation, i.e., * + H. + +e - →*H. However, in CO2RR applications, H atom absorption is unavoidable, but the initial hydrogenation step must be more favorable for the formation of *COOH or *OCHO. To evaluate the reaction selectivity of the catalyst for CO2RR versus HER, this invention compares the free energy changes of the first-step hydrogenation step in CO2RR and the first-step reaction step in HER, and incorporates the limiting potential U for the production of CO and H2 by the catalyst. L The difference, i.e., UL (CO2RR)-U L (H2) was used to further evaluate the product selectivity of the catalyst relative to CO and H2.

[0025] After the above screening process, the present invention is able to screen out catalysts with excellent CO catalytic activity and product selectivity.

[0026] (6) Analysis of the source of catalytic activity This invention further explores the source of activity of the catalyst with optimal CO catalytic activity. The activation ability of this catalyst for CO2 was analyzed by comparing CO2 adsorption energy and charge transfer, and the PDOS of the catalyst after CO2 adsorption was analyzed to determine the interaction between CO2 and the catalyst. Because after CO2 adsorption, the bonding orbitals of CO2 split and overlap with the 3d orbitals of the transition metal atom, electrons from the πg orbitals of CO2 are transferred to the empty d orbitals of the transition metal atom. This synergistic effect of electron acceptance, feedback, and d orbital occupancy ensures stable adsorption and effective activation of carbon dioxide, but also contributes to the difference in catalytic activity.

[0027] In one embodiment of the present invention, the Mn@MoSi2N4-Nv monolayer exhibits optimal CO catalytic activity. For example... Figure 11 As shown, the PDOS of adsorbed CO2 reveals that after CO2 adsorption, the bonding orbitals of CO2 split and overlap with the 3d orbitals of Mn atoms, causing electrons from the πg orbitals of CO2 to transfer to empty d orbitals of Mn atoms. Simultaneously, the presence of empty antibonding orbitals near the Fermi level makes it easier for Mn@MoSi2N4-Nv to accept electrons and continue hydrogenation, thus reducing the difficulty of further hydrogenation.

[0028] This invention discloses a catalyst design method for promoting the reduction of CO2 to CO by doping MoSi2N4 surface with transition metal atoms. Through theoretical modeling and structural optimization of the doped structure, a series of potentially high-CO-catalyzing single-atom catalysts were designed. Based on traditional density reaction theory calculations, advanced computational methods, including DFT+U and van der Waals force corrections, were applied to reasonably evaluate the thermal stability, CO-catalyzing activity, and product selectivity of these catalysts. By combining the above steps in a progressive screening process, the range of research models was narrowed, and the desired model was quickly and accurately identified, resulting in CO catalysts with excellent theoretical catalytic performance. This provides design guidance for the experimental preparation of catalysts with high stability, high CO-catalyzing activity, and high selectivity. Attached Figure Description

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0030] Figure 1The diagram shows the model and stability analysis of the present invention, where (a) and (b) are the top view and front view of TM@MoSi2N4-Nv, respectively; (c) and (d) are the binding energy and formation energy of TM@MoSi2N4-Nv (TM=Sc-Zn), respectively; and (e) is the d-band center value of TM@MoSi2N4-Nv (TM=Sc-Zn). Figure 2 A schematic diagram of the optimized structure of TM@MoSi2N4-Nv (TM=Sc-Zn); Figure 3 A schematic diagram of PDOS for TM@MoSi2N4-Nv (TM=Sc-Zn), including TM-3d and Si-2p; Figure 4 Schematic diagram of the optimized structure for vertical CO2 adsorption on TM@MoSi2N4-Nv (TM=Sc-Cu) and the original MoSi2N4; Figure 5 Schematic diagram of the optimized structure for CO2-level adsorption on TM@MoSi2N4-Nv (TM=Sc-Cu) and the original MoSi2N4; Figure 6 This is a schematic diagram of CO activity analysis, where (a) represents the CO2RR and HER reaction pathways, (b) is the Gibbs free energy diagram of CO2 reduction to CO on a Sc,Ti,Mn@MoSi2N4-Nv monolayer, and (c) is the U-energy diagram of CO2 reduction to CO on a TM@MoSi2N4-Nv (TM=Sc-Cu) monolayer. L ; Figure 7 This is a schematic diagram of the product selectivity analysis of CO relative to HCOOH and H2, where (a) is the ΔG formed by *COOH and *OCHO, and (b) is the *OCHO+H2O on the TM@MoSi2N4-Nv (TM=Sc-Cu) monolayer. + +e - →HCOOH's ΔG, (c) is the ΔG of the first protonation step in CO2RR (ΔG(*COOH)) and HER (ΔG(*H)), and (d) is the limiting potential difference (U) between CO2RR and HER. L (CO2RR)-U L (HER)); Figure 8 A schematic diagram of the reaction selectivity for CO desorption and reduction to CH4 on Sc@MoSi2N4-Nv, with insets showing the optimized configuration of the intermediate; Figure 9 A schematic diagram of the reaction selectivity for CO desorption and reduction to CH4 on Ti@MoSi2N4-Nv, with the inset showing the optimized configuration of the intermediate; Figure 10 A schematic diagram of the reaction selectivity for CO desorption and reduction to CH4 on Mn@MoSi2N4-Nv, with the inset showing the optimized configuration of the intermediate; Figure 11 This is a schematic diagram of the activity difference analysis of Cr,Mn,Co@MoSi2N4-Nv, where (a) is the Gibbs free energy diagram of CO2 conversion to CO on the Cr,Mn,Co@MoSi2N4-Nv monolayer, (b) is a comparison of CO2 adsorption energy and CO2 transfer charge on the Cr,Mn,Co@MoSi2N4-Nv monolayer, (c) is the molecular orbital of CO2 molecule, and (d)-(f) are the PDOS of CO2 adsorbed on the Cr,Mn,Co@MoSi2N4-Nv monolayer, respectively. Detailed Implementation

[0031] The embodiments of the present invention will be described in detail below. Although the present invention will be described and illustrated in conjunction with some specific embodiments, it should be noted that the present invention is not limited to these embodiments. On the contrary, any modifications or equivalent substitutions made to the present invention should be covered within the scope of the claims of the present invention.

[0032] Furthermore, to better illustrate the present invention, numerous specific details are set forth in the following detailed embodiments. Those skilled in the art will understand that the present invention can be practiced without these specific details.

[0033] This invention discloses the design, screening, and analysis process of a catalyst for promoting the reduction of CO2 to CO by doping the surface of MoSi2N4 with fourth-period transition metal atoms (Sc-Zn), which consists of the following steps: (1) Constructing a computational model for catalysts containing active single-atom centers The process includes several steps, such as model selection, establishing a single-layer supercell, setting a vacuum layer, and model optimization.

[0034] A 3×3×1 TM@MoSi2N4-Nv (TM=Sc-Zn) supercell structure model was constructed by replacing the N atoms on the surface of the two-dimensional MoSi2N4 composed of seven atomic layers (N-Si-N-Mo-N-Si-N) with fourth-period transition metal atoms (Sc-Zn). Figure 1 As shown in (a) and (b), and set to a vacuum layer of 20 Å.

[0035] The model optimization refers to the optimization of lattice parameters and the optimization of the forces between atoms in the x, y, and z directions within the unit cell after fixing the z-axis and optimizing the x and y axes, to obtain a stable TM@MoSi2N4-Nv (TM=Sc-Zn) supercell structure. Figure 2As shown, after model optimization, most transition metal atoms did not stably move away from the MoSi₂N₄ monolayer, indicating that the transition metal atoms are stably bonded to the MoSi₂N₄ surface, proving the rationality of the modeling and optimization method of this invention. It is necessary to explain the situation of Zn atoms here. The optimization results initially indicate that it cannot stably bond to the MoSi₂N₄ surface; its thermal stability will be further analyzed through binding energy and PDOS calculations.

[0036] (2) Evaluate the thermal stability of TM@MoSi2N4-Nv (TM=Sc-Zn) Thermal stability is one of the key properties of a catalyst. The binding energy E of the catalyst is calculated using formulas (I) and (II). bind and formation energy E form To evaluate the thermal stability of the catalyst.

[0037] E bind =E(TM@MoSi2N4-Nv)–E(MoSi2N4-Nv)–E(TM) (I) E form =E(TM@MoSi2N4-Nv)–E(MoSi2N4-Nv)–μ(TM) (II) Where E(TM@MoSi2N4-Nv) and E(MoSi2N4-Nv) represent the energies of the TM@MoSi2N4-Nv monolayer and the MoSi2N4 monolayer with N vacancies, respectively. E(TM) and μ(TM) represent the energies of a single TM atom in vacuum and a single TM atom in a stable bulk phase, respectively.

[0038] The binding energy E of the catalyst bind E < 0 indicates that the catalyst is stably bound; conversely, E bind If the value is greater than 0, the catalyst binding is unstable. For example... Figure 1 As shown in (c), the binding energies of TM@MoSi2N4-Nv (TM=Sc-Zn) range from -4.42 eV to -0.44 eV, with the Ni@MoSi2N4-Nv system exhibiting the highest binding energy at -4.42 eV, while the Zn@MoSi2N4-Nv system exhibits the lowest binding energy at -0.44 eV. Furthermore, the orbital hybridization of transition metal atoms with the MoSi2N4 surface is analyzed by calculating PDOS, verifying the thermal stability of the catalyst. Figure 3 As shown, the full hybridization of the p orbitals of Si atoms and the d orbitals of transition metal atoms near the Fermi level indicates that the transition metal atoms are stably bonded to the MoSi2N4 surface, and also proves the thermal stability of the catalyst.

[0039] After screening for thermal stability, Zn@MoSi2N4-Nv was excluded because its binding energy is low, and PDOS also showed that the bonding between Zn atoms and the MoSi2N4 surface is unstable. Therefore, it was not used in the subsequent CO2RR process study.

[0040] The formation energy of a catalyst is used to determine the difficulty of its experimental synthesis. Formation energy E form A value <0 indicates that the catalyst can be easily synthesized under mild experimental conditions. For example... Figure 1 As shown in (d), only the Mn@MoSi2N4-Nv system has a negative formation energy of -0.38 eV, indicating that this system is easy to produce. However, the formation energy E form A value greater than 0 does not mean that the system cannot be synthesized experimentally. For example, the calculated formation energy of Co-MoS2 is 1.80 eV, but it can be synthesized under relatively mild hydrothermal conditions. The formation energies of TM@MoSi2N4-Nv (TM=Sc-Cu) monolayers are all lower than this value, indicating that they can be synthesized without demanding experimental conditions.

[0041] (3) Evaluate the CO2 activation ability of the catalyst. The first and most crucial step in CO2RR is the initial activation of carbon dioxide molecules, i.e., CO2 adsorption. The adsorption configuration of carbon dioxide significantly influences the subsequent reduction process. Consider two initial CO2 adsorption configurations: vertical initial adsorption (using *CO... 2_1 (represented by) and initial adsorption of CO2 level (using *CO) 2_2 (This is indicated by...) The degree of deformation of the CO2 molecule after structural optimization is used to determine whether CO2 has been activated. If CO2 changes from a linear to a V-shaped form (∠OCO < 180°), it can be preliminarily determined that the CO2 molecule has been activated. For example... Figure 4 As shown, except for Mn@MoSi2N4-Nv, in the optimized *CO 2_1 In this configuration, the CO2 molecule retains its linear shape. In contrast, as... Figure 5 As shown, in the TM@MoSi2N4-Nv (TM=Sc-Cu) monolayer *CO 2_2 In the configuration, the shape of the CO2 molecule undergoes a significant change; the originally linear CO2 molecule becomes V-shaped (∠OCO < 180°), forming a bidentate C-TM-O structure. Therefore, it can be preliminarily determined that in *CO 2_2 The CO2 molecules in the configuration have been activated. Simultaneously, the CO2 adsorption energies of the two initial CO2 adsorption configurations were calculated using Equation (III) to further determine which configuration to use in subsequent studies. The adsorption configuration with a greater degree of CO2 deformation and a more negative CO2 adsorption energy was used in the subsequent research.

[0042] Eads =E(CO2_TM@MoSi2N4-Nv)–E(TM@MoSi2N4-Nv)–E(CO2) (III) Wherein, E(CO2_TM@MoSi2N4-Nv) refers to the total energy of the system in which the catalyst adsorbs CO2, E(TM@MoSi2N4-Nv) refers to the energy of the TM@MoSi2N4-Nv monolayer, and E(CO2) refers to the energy of a single CO2 molecule.

[0043] As shown in Table 1, the CO2 adsorption energies for the two initial CO2 adsorption configurations were calculated, and it was found that *CO 2_2 Configuration ratio * CO 2_1 The configuration exhibits a larger adsorption energy. Therefore, this invention uses the initial adsorption configuration at the CO2 level for subsequent studies.

[0044] In addition, the optimized CO2 configuration and CO2 adsorption energy of the original MoSi2N4 monolayer were calculated, such as... Figure 5 As shown in Table 1, in the optimized CO2 adsorption configuration of the original MoSi2N4 monolayer, the CO2 molecules always maintain a linear structure, indicating that CO2 molecules are difficult to activate on its surface. However, after surface doping with transition metal atoms, the CO2 molecules are deformed and the CO2 adsorption energies are all more negative than those of the original MoSi2N4 monolayer, indicating that this can enhance its CO2 adsorption capacity.

[0045] Table 1 Metal MoSi2N4 Sc Ti V Cr Mn Fe Co Ni Cu <![CDATA[*CO2_1]]> 0.13 -0.18 -0.21 -0.47 -0.22 -0.47 -0.27 -0.23 -0.18 -0.16 <![CDATA[*CO2_2]]> 0.20 -1.32 -0.78 -0.76 -0.28 -0.46 -0.71 -0.31 -0.45 -0.45 The adsorption capacity of transition metal atoms can be described by the d-band center. According to the classical d-band center theory, the closer the d-band center value is to the Fermi level, the stronger the adsorption capacity. For example... Figure 1 As shown in (e), this invention calculated the d-band centers of TM atoms in the TM@MoSi2N4-Nv (TM=Sc-Cu) monolayer and found that the d-band centers of Sc, Ti, Mn, Fe@MoSi2N4-Nv monolayers are relatively close to the Fermi level. Therefore, this invention predicts that their adsorption capacity will be stronger. The corresponding calculation results also verify the prediction of this invention. After adsorbing CO2, the bond angle of CO2 in Sc, Ti, Mn, Fe@MoSi2N4-Nv changes from 180° to 132.09~143.50°, and the CO2 adsorption energy is -0.46~-1.32 eV.

[0046] (4) Preliminary screening of CO2RR catalysts CO has high industrial value as a raw material for various chemical products and industrial reactions. However, previous studies have shown that when carbon dioxide molecules are adsorbed onto the catalyst surface, the resulting CO2RR can produce a variety of products, including two-electron products CO and HCOOH, six-electron products CH3OH, eight-electron products CH4, and C2+ products generated through CC coupling. This paper does not consider C2+ products because CC coupling requires two or more active sites, which is difficult to achieve in single-atom catalysts. Figure 6 As shown in (a), in the CO2RR process, the two-electron products CO and HCOOH are generated via the pathways CO2→*COOH→*CO→CO and CO2→*OCHO→*HCOOH→HCOOH, respectively. Besides desorption, *CO and *HCOOH can also generate the *CHO intermediate, which is a key intermediate for the generation of multi-electron products such as CH4. Furthermore, HER consumes active sites on the catalyst surface, thus reducing the reaction rate, and is the main competing reaction for CO2RR. Therefore, an ideal CO catalyst should not only possess high CO2RR activity but also be able to suppress other side reactions, such as the generation of HCOOH, further hydrogenation of *CO, and HER. Here, this invention first evaluates the activity of CO generation on a TM@MoSi2N4-Nv (TM=Sc-Cu) monolayer.

[0047] As can be seen from the above analysis, CO can be generated according to the following steps, namely... *+CO2+H + +e - →*COOH (1) *COOH+H + +e - →*CO+H2O (2) *CO→*+CO (3) To evaluate the catalytic activity of catalysts in producing CO and to screen for superior CO catalysts, this invention first calculated the free energy diagrams for CO production. The free energy change at each step was calculated according to formula (IV), establishing a free energy diagram for the CO2 reduction to CO reaction pathway at an applied potential of 0V.

[0048] The free energy of each elementary reaction was calculated based on the computational hydrogen electrode (CHE) model proposed by Nørskov et al. The free energy changes between any two steps in the CO2 reduction process were calculated using the free energy formula (IV). ΔG = ΔE + ΔZPE – TΔS (IV) Here, ΔE is the electronic energy difference for each reaction step calculated using DFT. T is the temperature (298.15 K), and ΔZPE and ΔS are the zero-point energy change and entropy change, respectively, which can be obtained through frequency calculation.

[0049] Specifically, this includes constructing a free energy diagram of the CO2 to CO reaction pathway with *+CO2, *COOH, *CO, and *+CO as the x-axis and the free energy changes of different steps in the CO2 to CO reaction pathway as the y-axis, under an applied voltage of 0V. The diagram is used to identify the step with the maximum free energy change (ΔG). max The steps are as follows: then calculate the limiting potential U according to formula (V). L .

[0050] U L =–ΔG max / e (V) Wherein, ΔG max It is the largest free energy change among all elementary reactions.

[0051] Comparison of CO production U in TM@MoSi2N4-Nv monolayer L Filter out U L The catalyst whose activity is closest to zero exhibits the best CO catalytic activity. For example... Figure 6 As shown in (b) and (c), TM@MoSi2N4-Nv (TM=Sc,Ti,Mn) produces a smaller ΔGmax for CO, and ΔG max The corresponding steps are * + CO2 + H. + +e - →*COOH and*COOH+H + +e - →*CO+H2O. U L The values ​​are -0.28V, -0.34V, and -0.16V, respectively. This indicates that they require relatively low applied voltage to reduce CO2 to CO, and therefore exhibit better reactivity. The U values ​​compared to the Ni−N4B2−Cx catalyst are... L Compared to (-0.2V), Mn@MoSi2N4-Nv still exhibits better reactivity.

[0052] (5) Product selectivity analysis The first step in CO2RR, the hydrogenation of the CO2 molecule, occurs via two pathways: * + CO2 + H+ + e - →*COOH and*+CO2+H + +e - →*HCOO, leading to CO and HCOOH products respectively. Therefore, this invention compares the free energy changes of the catalysts producing *COOH and *HCOO, and further calculates the free energy change of *HCOO producing *HCOOH to comprehensively evaluate the catalyst's product selectivity for CO and HCOOH. Figure 7As shown in (a), all systems tend to produce the *OCHO intermediate rather than the *COOH intermediate, i.e., ΔG(*OCHO) < ΔG(*COOH). However, as Figure 7 As shown in (b), the *OCHO intermediate in almost all systems requires overcoming a very high free energy barrier to generate the *HCOOH intermediate, especially in Sc,Ti,Mn@MoSi2N4-Nv monolayers. Clearly, although the *OCHO intermediate is easier to generate than the *COOH intermediate, the high free energy barrier required to generate the HCOOH product means that, according to the BEP relation, the CO product will still be the preferred product in these systems.

[0053] Meanwhile, after confirming the selectivity of the Sc,Ti,Mn@MoSi2N4-Nv monolayer for these two products, it is still necessary to consider whether the products can be successfully desorbed and whether they tend to be further hydrogenated and reduced. That is to say, after *CO is formed, in addition to *CO desorption to produce CO products, it is also possible to carry out the reaction of *CO further hydrogenation to produce C1 products. *CHO is a key intermediate for the production of C1 products such as CH4. Therefore, this invention mainly considers the reaction tendency of *CO desorption and *CO hydrogenation to produce *CHO intermediates, and calculates the CO binding energy and the free energy diagram of CH4 production of these catalysts with better CO catalytic activity screened in step (4).

[0054] Calculate the CO binding energy E using formula (VI) b (*CO): E b (*CO)=E(*CO)-E(*)-E(CO2)+E(H2O)-E(H2) (VI) Where E(*CO) is the total DFT energy of TM@MoSi2N4-Nv with *CO intermediate adsorbed, and E(*) is the energy of the clean TM@MoSi2N4-Nv surface. E(CO2), E(H2O), and E(H2) are the total DFT energies of gaseous CO2, H2O, and H2, respectively.

[0055] Figures 8-10The diagram shows the free energy diagram for CH4 formation from Sc,Ti,Mn@MoSi2N4-Nv. The reaction pathway is *CO→*CHO→*OCH2→*OCH3→*O+CH4→*OH→*+H2O. For Sc,Ti@MoSi2N4-Nv, although *CO→*CHO is exothermic relative to *CO desorption, high free energy barriers of 1.52 eV and 1.64 eV need to be overcome in the subsequent hydrogenation process to complete the CH4 formation. For Mn@MoSi2N4-Nv, the protonation of *CO to form *CHO requires a lower energy input (0.07 eV), but this is still larger than the thermodynamic barrier of CO desorption (0.04 eV). Furthermore, similar to Sc,Ti@MoSi2N4-Nv, the free energy barrier for CH4 formation from Mn@MoSi2N4-Nv is 1.12 eV, which is unfavorable for CH4 formation. Furthermore, according to the Sabatier principle, adsorbed CO exhibits poor stability, favoring its desorption rather than further reduction. The CO binding energies of Sc,Ti,Mn@MoSi2N4-Nv are -0.21, -0.24, and -0.19 eV, respectively, which are significantly lower than the CO binding energy calculated on marginalized Fe-N4 single-atom catalysts with a Faraday efficiency of 94% (-0.75 eV). This indicates that CO products can be readily desorbed after formation on their surface. Therefore, CO on Sc,Ti,Mn@MoSi2N4-Nv tends to desorb to produce CO products and can suppress the formation of other products.

[0056] Finally, as is well known, the most likely competing reaction in the CO2RR process is hydrogenation, i.e., * + H. + +e - →*H. However, in CO2RR applications, H atom absorption is unavoidable, but the initial hydrogenation step must be more favorable for the formation of *COOH or *OCHO. To evaluate the reaction selectivity of the catalyst for CO2RR versus HER, this invention compares the free energy changes of the first-step hydrogenation step in CO2RR and the first-step reaction step in HER, and incorporates the limiting potential U for the production of CO and H2 by the catalyst. L The difference, i.e., U L (CO2RR)-U L (H2) was used to further evaluate the catalyst's product selectivity relative to CO and H2. For example... Figure 7 As shown in (c), except for the Sc@MoSi2N4-Nv system, all other systems are located above the diagonal, tending towards the first protonation step of HER. Figure 7As shown in (d), this invention further calculated the limiting potential difference between CO2RR and HER for all TM@MoSi2N4-Nv systems to evaluate the selectivity of CO2RR relative to HER. A more positive result indicates higher CO2RR selectivity of the catalyst. Compared to the Sc,Ti@MoSi2N4-Nv system, the Mn@MoSi2N4-Nv system performed better in CO2RR production than in HER production, indicating that the Mn@MoSi2N4-Nv system has excellent CO2RR selectivity. Therefore, considering all competing reactions, CO will be the major product on Mn@MoSi2N4-Nv.

[0057] After the above screening process, the present invention was able to screen out Mn@MoSi2N4-Nv as a catalyst with excellent CO catalytic activity and product selectivity.

[0058] (6) Analysis of the source of catalytic activity This invention further explores the source of the catalyst with optimal CO catalytic activity. The activation ability of this catalyst for CO2 is analyzed by comparing CO2 adsorption energy and charge transfer, and the PDOS of the catalyst after CO2 adsorption is analyzed to determine the interaction between CO2 and the catalyst. Specifically, as... Figure 11 As shown in (a), after CO2 adsorption by the Mn@MoSi2N4-Nv monolayer, a ΔG of 0.16 eV is required to generate *COOH, where *COOH represents the carboxyl intermediate adsorbed on the catalyst surface. Then, the formation of *CO requires 0.01 eV, followed by desorption to generate CO products by overcoming a low ΔG of 0.04 eV. However, the ΔGs for *COOH generated by Cr and Co@MoSi2N4-Nv monolayers, which have the same PDS as the Mn@MoSi2N4-Nv monolayer, are 0.64 eV and 0.71 eV, respectively, which are very large. To investigate the superior performance of the Mn@MoSi2N4-Nv monolayer in CO2 RR, it is necessary to analyze the reasons for this situation. First, as... Figure 11 As shown in (b), the Mn@MoSi2N4-Nv monolayer exhibits a higher CO2 adsorption energy and more charge transfer than the other two monolayers, meaning it can adsorb CO2 molecules more stably. Secondly, Figure 11 (d)-(f) show the PDOS after CO2 adsorption in a Cr, Mn, Co@MoSi2N4-Nv monolayer. Figure 11(c) The PDOS of the CO2 molecule was also plotted for comparison. After CO2 adsorption, the bond orbitals of CO2 split and overlap with the 3d orbitals of the TM atom, resulting in electrons transferring from the πg orbitals of CO2 to the empty d orbitals of the TM atom. Compared with the Cr, Co@MoSi2N4-Nv monolayer, the 3d orbitals of Mn@MoSi2N4-Nv completely overlap with the 2p orbitals of CO2, resulting in hybridized bonding orbitals. This synergistic effect of electron acceptance, feedback, and d orbital occupancy ensures stable adsorption and effective activation of CO2, leading to the difference in catalytic activity. Furthermore, the presence of empty antibonding orbitals near the Fermi level makes it easier for the Mn@MoSi2N4-Nv monolayer to accept electrons and continue hydrogenation, thereby reducing the difficulty of generating *COOH.

[0059] It should be noted and understood that various modifications and improvements can be made to the invention described in the detailed description above without departing from the spirit and scope of the invention as claimed in the appended claims. Therefore, the scope of the claimed technical solutions is not limited to any specific exemplary teachings given.

[0060] The applicant declares that the detailed method of the present invention is illustrated by the above embodiments, but the present invention is not limited to the above detailed method, that is, it does not mean that the present invention must rely on the above detailed method to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for designing a catalyst in MoSi2N4 doped with transition metal atoms to promote the reduction of CO2 to CO, comprising the following steps: Step (1) Construct a computational model of the catalyst containing active single-atom centers. First, a model is selected, then a single-layer supercell is built, a vacuum layer is set, and the model is optimized to obtain a single-layer supercell model after transition metal atom doping, denoted as TM@MoSi2N4-Nv; Step (2) Evaluate the thermal stability of the catalyst The thermal stability of the catalyst was evaluated by using the calculated binding energy and formation energy of the catalyst. The thermal stability of the catalyst was verified by calculating the projected density of states (PDOS) of the catalyst and analyzing the orbital hybridization between the transition metal atoms and the MoSi2N4 surface. Step (3) Evaluate the CO2 activation capacity of different catalysts Two initial CO2 adsorption configurations were used. The degree of CO2 deformation after structural optimization was used to determine whether CO2 was activated. The CO2 adsorption energy of the two initial CO2 adsorption configurations was calculated to further determine which configuration should be used in subsequent studies. Step (4) Preliminary screening of CO2RR catalysts The catalytic activity for CO production was analyzed, the free energy diagram for CO production was calculated, the step with the largest change in free energy was identified based on the free energy diagram, and then the limiting potential U was calculated. L Comparing the U of CO production in TM@MoSi2N4-Nv monolayer L The catalyst with the best CO catalytic activity was screened out. Step (5) Product selectivity analysis Compare the selectivity of CO production with that of HCOOH, CH4, and H2 production to evaluate the product selectivity of the catalyst; Step (6) Analysis of the source of catalytic activity By comparing CO2 adsorption energy and charge transfer, the activation ability of the catalyst for CO2 is analyzed, and the PDOS of the catalyst after CO2 adsorption is analyzed to determine the interaction between CO2 and the catalyst. The model described in step (1) is a two-dimensional layered structure composed of seven atomic N-Si-N-Mo-N-Si-N, with a single-layer supercell being a 3×3×1 MoSi2N4 supercell doped with transition metal atoms, and a vacuum layer of 20 Å. The model optimization refers to the optimization of lattice parameters by fixing the z-axis and optimizing the x-axis and y-axis, and the optimization of the forces of atoms in the x, y, and z directions within the cell, to obtain the TM@MoSi2N4-Nv single-layer cell structure.

2. The design method according to claim 1, characterized in that, The binding energy E of the TM@MoSi2N4-Nv monolayer in step (2) bind and formation energy E form Calculated according to formulas (I) and (II): E bind =E(TM@MoSi2N4-Nv)–E(MoSi2N4-Nv)–E(TM) (I) IT form =E(TM@MoSi2N4-Nv)–E(MoSi2N4-Nv)–μ(TM) (II) Where E(TM@MoSi2N4-Nv) and E(MoSi2N4-Nv) represent the energy of the TM@MoSi2N4-Nv monolayer and the energy of the MoSi2N4 monolayer with N vacancies, respectively, and E(TM) and μ(TM) represent the energy of a single TM atom in vacuum and the energy of a single TM atom in a stable bulk phase, respectively.

3. The design method according to claim 1, characterized in that, Step (2) Using the calculated binding energy E bind and formation energy E form To determine the bonding between transition metal atoms and the substrate, as well as the ease of catalyst synthesis in experiments, and to analyze the stability of the bonding between transition metal atoms and the substrate by calculating the PDOS of the TM@MoSi2N4-Nv monolayer.

4. The design method according to claim 1, characterized in that, The CO2 adsorption energy in step (3) is calculated according to formula (III): E ads =E(CO2_TM@MoSi2N4-Nv)–E(TM@MoSi2N4-Nv)–E(CO2) (III) Wherein, E(CO2_TM@MoSi2N4-Nv) refers to the total energy of the system in which the catalyst adsorbs CO2, E(TM@MoSi2N4-Nv) refers to the energy of the TM@MoSi2N4-Nv monolayer, and E(CO2) refers to the energy of a single CO2 molecule.

5. The design method according to claim 1, characterized in that, The free energy change ΔG of each elementary reaction in step (4) is calculated based on the hydrogen electrode model. ΔG for any two steps in the CO2 reduction process is calculated using the free energy formula (IV), i.e. ΔG = ΔE + ΔZPE – TΔS (IV) Where ΔE is the electronic energy difference for each reaction step calculated by DFT, T is the temperature, and ΔZPE and ΔS are the zero-point energy change and entropy change, respectively, obtained by frequency calculation. The limiting potential is calculated using the formula (V): U L =–ΔG max / e (V) Wherein, ΔG max It is the maximum value of the free energy change in all elementary reactions.

6. The design method according to claim 1, characterized in that, Step (5) compares the free energy changes of the first protonation step of CO2RR to produce *COOH and *OCHO, and the free energy change of *OCHO→*HCOOH to evaluate the product selectivity of CO and HCOOH. The product selectivity of CO and CH4 is evaluated by comparing the free energy changes of CO desorption and further hydrogenation to *CHO. To evaluate the reaction selectivity of the catalyst for CO2RR and HER, the free energy changes of the first hydrogenation step of CO2RR and the first step of the HER reaction are compared, and the limiting potential U for the catalyst to produce CO and H2 is also considered. L The difference, i.e., U L (CO2RR)-U L (H2) was used to further evaluate the product selectivity of the catalyst relative to CO and H2.

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