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Article

Two-Dimensional Metal–Organic Framework TM Catalysts for Electrocatalytic N2 and CO2 Reduction: A Density Functional Theory Investigation

by
Anqi She
1,
Ming Wang
1,
Shuang Li
1,
Yanhua Dong
1,* and
Dandan Wang
2,*
1
Department of Mathematics and Computer Science, Jilin Normal University, Siping 136000, China
2
Key Laboratory of Functional Materials Physics and Chemistry of Ministry of Education, College of Physics, Jilin Normal University, Changchun 130103, China
*
Authors to whom correspondence should be addressed.
Crystals 2023, 13(10), 1426; https://doi.org/10.3390/cryst13101426
Submission received: 31 August 2023 / Revised: 13 September 2023 / Accepted: 13 September 2023 / Published: 26 September 2023
(This article belongs to the Special Issue Novel Nanomaterials for Catalytic and Biological Applications (Volume II))
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Abstract

:
In this study, we screened novel two-dimensional metal–organic framework (MOF) materials, which can be used as efficient electrocatalysts in the N2 reduction reaction (NRR) and CO2 reduction reaction (CO2RR) through density functional theory (DFT) calculations. By systematically investigating the adsorption behaviors of N2 and CO2 in different MOF-TMs (TM = Fe, Co, Ni, Cu, Zn) and their electrocatalytic hydrogenation processes, we found that 2D MOF-Fe, MOF-Co, and MOF-Ni can be used as catalysts for electrocatalytic NRR. The free energy increase in the corresponding potential-limiting step is calculated to be 0.84 eV on MOF-Fe, 1.00 eV on MOF-Co, and 1.17 eV on MOF-Ni, all of which are less than or at least comparable to those reported values for the NRR. Moreover, only 2D MOF-Fe was identified as a suitable electrocatalyst for CO2RR. Instead of other hydrocarbons, the product CH3OH is selectively obtained in an electrocatalytic CO2 reduction reaction on a 2D MOF-Fe with a free energy increase of 0.84 eV in the potential-limiting step. Overall, the results of this study not only facilitate the potential application of 2D MOF-TMs as electrocatalysts but also provide new guidelines for rationally designing novel electrocatalysts for the NRR and CO2RR.

1. Introduction

In recent years, in order to alleviate the dual pressure on the environment and energy, nitrogen fixation and carbon dioxide reduction have become the focus of research. Among the various catalytic reduction methods for N2 and CO2 molecules, an electrocatalytic reduction reaction is considered as one of the most advantageous technologies [1,2,3,4,5]. For this reason, the development of new effective electrocatalysts for nitrogen reduction reactions (NRR) and carbon dioxide reduction reactions (CO2RR) is urgently required. Efforts have been made to explore efficient electrocatalysts, and a significant overlap between the catalysts used for NRRs and CO2RRs has been found. For example, noble metal catalysts, such as Au, [6,7,8] Pt, [9,10,11] Rh, [10,12], and Pd [8,10], are efficient NRR and CO2RR electrocatalysts due to their electronic properties. Moreover, transition metal oxides, such as TiO2, [13,14,15] WO3−x, [16,17], MnO2 [18,19,20], and CuO [21,22], are considered promising electrocatalysts for the NRR and CO2RR according to their adjustable electronic structures and low cost. However, despite the development of noble metal and metal oxide electrocatalysts, they are limited by several problems, such as the high price of noble metals and the instability of metal oxides at negative potentials. Therefore, there is a broader interest in developing novel NRR and CO2RR electrocatalysts with a low cost, high efficiency, and high stability.
Metal–organic frameworks (MOFs), which are one-, two-, or three-dimensional skeletons formed by metal atoms and organic bridging ligands through coordination bonds, are attracting increasing attention in electrocatalysis due to their large specific surface area, high stability, low cost, homogeneous active composition, and dense catalytic sites that are easy to tune. For example, Co-MOF, Fe-MOF, and Cu-MOF have been widely used as electrocatalysts for oxygen evolution reactions since Yaghi et al. reported the first MOFs in 1995 [23,24,25,26]. In particular, MOFs including Al-MOFs, Cu-MOFs, and Co-MOFs show great promise and have recently attracted extensive attention regarding applications in electrocatalytic NRRs and CO2RRs [27,28,29,30]. However, MOFs with a three-dimensional morphology have several weak points, such as poor electrical conductivity, low metal utilization, and accessible surface area, which limit their electrocatalytic applications. To bypass these problems, two-dimensional (2D) MOFs with metallic property, unsaturated metal sites, and large surface areas have been investigated and show excellent performance in electrocatalysis [31,32]. Until now, the study of 2D MOFs has mainly focused on synthetic methods and their applications in energy storage [33,34]. There are limited reports on 2D MOFs for both electrocatalytic NRRs and CO2RRs [35,36,37]. Therefore, it is of great significance to explore the potential applications of 2D MOFs in electrocatalytic NRRs/CO2RRs and evaluate the corresponding NRR/CO2RR performance.
Thus far, many metallic 2D MOFs that contain transition metal (TM) ions (Fe, Co, Ni, Mn, Mo) have been successfully synthesized and studied [35,36,38]. Over the last year, we have designed a 2D ultrathin MOF-Co for highly safe and long-life Li-S batteries [34]. The 2D MOF-Co possesses a unique structure, namely, periodically arranged cobalt atoms coordinated with oxygen atoms causing pseudo-octahedra, which are mutually interconnected by the 1,4-benzenedi-carboxylic acid ligands. Interestingly, the Co sites in Co-O4 moieties are exposed on the surface. It is expected that the exposed coordination, with unsaturated metal sites on the 2D-MOF surface, can serve as active centers to ensure the high electrocatalytic activity for NRRs and CO2RRs. Generally, metallic 2D MOFs have a high tunability because transition metals are diverse. Inspired by the above concepts, we studied the electrocatalytic performance of several 2D MOF-TMs (TM = Fe, Co, Ni, Cu, Zn) possessing the same structure as that of the 2D MOF-Co we reported earlier, based on density functional theory (DFT) calculations.
In this study, we first investigated the adsorption behaviors of N2 and CO2 molecules towards the surfaces of 2D MOFs. By analyzing the adsorption energy, charge transfer, and corresponding bond lengths of the adsorbates, the interaction between adsorbates and 2D MOFs was clarified. The electrocatalytic NRR and CO2RR pathways for those chemisorbed molecules on the catalyst surfaces were investigated. Our theoretical results indicate that 2D MOF-Fe is an excellent bi-functional electrocatalyst for both NRRs and CO2RRs. Moreover, 2D MOF-Co and MOF-Ni exhibit high performance only for electrocatalytic NRRs, while the other 2D MOF-TM materials are not appropriate catalysts for electrocatalytic NRRs or CO2RRs.

2. Calculation Methods

All calculations were performed in the DFT framework using the Vienna ab initio simulation package (VASP) [39,40]. Generalized gradient approximation (GGA) under the projector-augmented plane wave (PAW) method was used [41,42,43]. The exchange correlation functions were set in the form of Perdew–Burke–Ernzerhof (PBE) [44,45]. In addition, the DFT-D2 correction method of Grimme was applied to describe van der Waals (vdW) forces [46,47]. The previously prepared 2D MOF-Co possesses a defined geometric structure and its unit cell has the chemical formula of C16H10O10Co3, with triclinic symmetry (space group Pī) [34]. To model other 2D MOF-TMs (TM = Fe, Ni, Cu, Zn), the Co atoms in the 2D MOF-Co structure were replaced by TM atoms. In this study, 2D MOF-TM catalysts were constructed with the p(2 × 2) supercell and with a sufficient vacuum region (20 Å) perpendicular to the surface, as shown in Figure 1. To simulate those structures and the corresponding adsorption configurations, the plane wave truncation energy used was 400 eV, and the energy convergence criterion was set to 10−4 eV. The convergence threshold of 10−2 eV/Å was applied for force. The Monkhorst-Pack of 3 × 2 × 1 K-points were used for the Brillouin-zone integration. We completed a Bader charge analysis to investigate the oxidation state of the mental center. Calculation results indicated that the oxidation state for the tetra- and six-coordinated metal atoms (the tera- and six-coordinated metal atoms were marked as i and ii by dotted blue circles in Figure 1) were different. The oxidation states of atom-i/ii were 1.15/1.31 for Fe, 1.04/1.28 for Co, 1.02/1.23 for Ni, 1.04/1.20 for Cu, and 1.31/1.39 for Zn.
To investigate the adsorption of N2 and CO2 towards the 2D MOF-TM surface, the isolated N2 and CO2 molecules were previously simulated in a large cubic cell of 15 Å in length. The adsorption energies were defined as Eads = EMOF-gas − Emolecule − EMOF, where EMOF-gas, Emolecule, and EMOF are the total energies of the 2D MOF-TM surface with adsorbed N2 or CO2 molecules, the isolated N2 or CO2 molecule and the clean 2D MOF-TM surface, respectively. A Bader charge analysis and difference charge density calculations were carried out to clarify the interaction between adsorbed molecules and the catalyst surfaces. The electrocatalytic NRR and CO2RR both involve several coupled proton and electron transfers. To acquire the free energy profiles of electrocatalytic N2 and CO2 reduction reactions, the computational electrode model (CHE) was employed [48,49]. According to the CHE, the energy changes are for reactions with the proton–electron pairs (H+ + e−). The chemical potential of (H+ + e−) can be obtained through the equation of μ(H+ + e−) = 1/2μ(H2)-eU, where μ(H2) is the free energy of gaseous H2 and U is the applied bias. The free energy change (ΔG) at each elementary step of the NRR and CO2RR processes was calculated using the equation ΔG = ΔE-neU + ΔZPE − TΔS, where ΔE is the change in electron energy calculated by DFT, n is the number of transferred electrons involved in the elementary reaction, ΔZPE is the zero-point energy difference, T is the reaction temperature (in this paper, all are at room temperature, T = 298.15 K), ΔS is the change in entropy value. ΔZPE and ΔS can be obtained through a frequency analysis.

3. Results and Discussion

3.1. Adsorption Behaviors of N2 and CO2 on the 2D MOF-TM Surface

In order to evaluate whether MOF-TMs can be used as electrocatalysts for NRR and CO2RR, the adsorption behaviors of N2 molecules on the 2D MOF-TM surfaces were systematically investigated firstly. Various molecule adsorption configurations for N2 towards different sites on the catalyst surfaces were taken into consideration to determine their stable adsorption geometries.
Calculation results show that the adsorption states of N2 on the MOF-Cu and MOF-Zn surfaces are physically adsorbing, with adsorption energy of −0.11 eV and −0.10 eV, respectively (Figure S1). The physical adsorption modes suggest that the 2D MOF-Cu and MOF-Zn designed in this work are not suitable electrocatalysts for NRRs, while N2 can be chemisorbed on the other three MOF-TM (TM = Fe, Co, Ni) surfaces at metal sites in end-on configuration (Figure 2a–c). Table 1 summarizes the adsorption parameters for N2 chemical adsorption (N2*) towards those three 2D MOF-TM materials. The corresponding adsorption energies of N2* were calculated to be −1.43 eV, −1.11 eV, and −0.64 eV for MOF-Fe, MOF-Co, and MOF-Ni, respectively. The N-N bond lengths for N2* on MOF-Fe, MOF-Co, and MOF-Ni, respectively, are 1.139 Å, 1.135 Å, and 1.126 Å, all greater than that of the isolated N2 molecule (1.117 Å). In addition, newly formed TM-N bonds with bond lengths of 1.773 Å (Fe-N), 1.758 Å (Co-N), and 1.812 Å (Ni-N) were observed. Further difference charge density calculations and Bader charge analyses were carried out to further declare the interaction between N2* and MOF-Fe/Co/Ni. The difference charge density plots presented in Figure 2 denote that obvious charge redistribution occurs due to the adsorption interaction [49,50]. According to the Bader charge analysis, electrons are transferred from the MOF-TM surface to N2* and the net obtained charge is 0.27 e, 0.21 e, and 0.09 e for N2* on MOF-Fe, MOF-Co, and MOF-Ni, respectively. Moreover, the PDOS plots in Figure 3 show that the PDOS peaks for the two N atoms of N2* are broadly dispersed in comparison with those of the isolated N2 molecule [51,52]. And, overlap between the N1 atom and metal atom occurs in the spectrum, especially in the energy ranges of −6.0 eV to −8.0 eV and 2.0 eV to 3.0 eV. According to frequency calculations, the red shifts of the N-N stretching vibrational frequencies for N2 bound to the Fe, Co, and Ni 2D MOF were 141 cm−1, 103 cm−1, and 23 cm−1, respectively. And, the changes in the N-N stretch frequencies could be used as good indicators for metal-N2 binding interaction. These above results signify that N2 molecules can be activated effectively by 2D MOF-Fe/Co/Ni.
For CO2 adsorption on the 2D MOF-TM, various initial adsorption modes were also examined. Full optimization of the initial structures reveals the physical adsorption of CO2 on MOF-Co, Ni, Cu, and Zn, with a small adsorption energy of −0.21 eV, −0.21 eV, −0.19 eV, and −0.17 eV and with remote distances between the CO2* and MOF-TM surfaces, as shown in Figure S2. Therefore, those four 2D MOF-TM materials should not be efficient electrocatalysts for CO2RR. Fortunately, adsorption of CO2 on the 2D MOF-Fe led to a chemical adsorption state with CO2* in the linear structure, where electron redistribution between CO2* and the MOF-Fe surface, and a newly formed bond between the O atom of CO2* and the Fe site was observed (Figure 2d). The corresponding adsorption parameters for CO2 on the MOF-Fe were also listed in Table 1. We can see that the adsorption energy was determined to be −0.44 eV and the newly formed O-Fe bond was 2.100 Å. Difference charge density calculations and Bader charge analyses show that the net charge transferred from MOF-Fe to CO2* was 0.01 e. Similar with N2*, the PDOS peaks for the C and O atoms of CO2* are more dispersed compared to those for the isolated CO2 molecule. And, an obvious overlap between the O1 atom and the surface Fe atom can be seen in Figure 3d. Significantly, the CO2 was polarized and activated weakly by the 2D MOF-Fe surface, reflected by the change in C-O bond lengths (1.185 Å and 1.172 Å) in CO2* relative to those in isolated CO2 (1.177 Å), and a bent angle of ∠O-C-O = 177.5° for the adsorbed CO2*.

3.2. Electrocatalytic Processes and Mechanisms for NRR and CO2RR

Previous reports show that the chemisorption of N2 and CO2 on the catalyst surface is favorable for the electrocatalytic N2 and CO2 reduction reaction. Therefore, we herein focused on the electrocatalytic NRRs on the 2D MOF-Fe, MOF-Co, and MOF-Ni, and CO2RRs on the 2D MOF-Fe, according to the above adsorption property calculations.
Figure 4 shows the corresponding free energy changes for each hydrogenation step of the NRR process on the 2D MOF-Fe surface at a zero electrode potential (U = 0 V). The ‘solvation corrections’ by the COSMO solvation model were obtained with H2O as the solvent. In the first step of the reaction process, NN* is hydrogenated to NNH* by a coupling reaction with H+ and e− pair. In that step, the N-N bond is extended from 1.139 Å to 1.213 Å, and the value of ΔG for this process is 0.84 eV. In the second hydrogenation step, both NNH2* and NHNH* formations are slightly upslope, with a ΔG of 0.05 eV and 0.03 eV, respectively. The almost identical energy between NNH2* and NHNH* denotes that alternating and distal pathways happened simultaneously for the first two hydrogenation steps. Subsequently, formations of NH2NH* and NH3* are endothermic processes, while NHNH2* formation is exothermic with a ΔG of −0.35 eV and −0.37 eV. Therefore, the third hydrogenation step leads to NHNH2*. Then, NH2NH2* rather than NH* + NH3 is gained by the addition of the fourth H, and the corresponding ΔG value was −0.31 eV. For the subsequent two reaction steps to produce NH2* + NH3(g) and NH3* + NH3(g), the corresponding ΔG values were −1.24 eV and −0.64 eV, respectively. In the final hydrogenation step, the Fe-N bond length is extended to 1.997 Å, which facilitates the desorption of NH3 due to the weakened Fe-N bond. Moreover, though the free energy gain for the release of the second NH3 molecule was calculated to be as large as 1.11 eV, NH3 release, which can be facilitated by the high solubility of NH3 in water, is not considered as an elementary reaction of the NRR [52]. According to the above results, the overall NRR process happened along the alternating path and the first hydrogenation step was determined as the potential-limiting step with a free energy increase of 0.84 eV. The free energy increase in the potential-limiting step obtained without solvent corrections is 0.88 eV, almost uniform with the value with solvent corrections, indicating that the solvent effect is weak for NRRs on 2D MOF-metals.
The structures of the reaction intermediates and the corresponding Gibbs free energy profiles for NRRs on the 2D MOF-Co are shown in Figure 5. The first hydrogenation step results in N-N bonds extended from 1.135 Å to 1.204 Å and the value of ΔG is 1.00 eV. In the second step, NHNH* formation has an advantage over NNH2* because the energy of NHNH* was 0.30 eV lower that of NNH2* and NHNH* formation from NNH* is downhill, with a ΔG of −0.25 eV. In the third step, the formation of *NHNH2 is exothermic, at ΔG −0.16 eV. All the subsequent three hydrogenation steps are downhill in free energy, leading to intermediates of NH2NH2*, NH2* + NH3(g), and NH3* + NH3(g), and the corresponding ΔG value is −0.51 eV, −1.04 eV, and −0.95 eV, respectively. The bond length of Co-NH3* is extended to 1.957 Å and the second NH3 release needs to overcome the 1.02 eV energy barrier. Overall, the NRR process was along the alternating pathway and the potential-limiting step was determined at the first hydrogenation step, with an energy barrier of 1.00 eV.
The electrocatalytic NRR pathways on the 2D MOF-Ni are shown in Figure 6. Similar to the previous two catalytic NRR processes on the 2D MOF-Fe and MOF-Co, the first hydrogenation step is determined as the potential-limiting step and the corresponding energy gain is 1.17 eV. In the same way, the N-N bond is increased from 1.126 Å to 1.201 Å. Note that the first three hydrogenation steps are along the alternating pathway, while the fourth hydrogenation step takes place with a distal mechanism, reflected by the fact that the free energy of NHNH3* was 0.39 eV lower than that of NH2NH2*. Then, the fifth and the sixth hydrogenation steps are along the alternating pathway again. Therefore, the NRR process on the 2D MOF-Ni is the mix of alternating and distal pathways. The ΔG values for the second to the sixth hydrogenation step are −0.68 eV, −0.05 eV, −1.26 eV, −0.80 eV, and −1.76 eV, respectively. Moreover, the bond length between the Ni site and the N atom of the second NH3* is 1.909 Å, much shorter than those between the Fe/Co site and the N atom of NH3*. Therefore, the desorption energy of the second NH3 release from the Ni site is 2.43 eV, larger than those values for NH3 release from the Fe/Co site (1.12/1.02 eV).
The entire electrocatalytic NRR processes of 2D MOF-Fe, MOF-Co, and MOF-Ni were systematically examined. According to the above results, the energy increase in the potential-limiting step for NRRs on the 2D MOF-Fe (0.88 eV), MOF-Co (1.00 eV) and MOF-Ni (1.17 eV) is less than or at least comparable with those reported values of many efficient electrocatalysts for NRRs (0.85 eV on FeN4 site of FePc [53] 0.85 eV on Nb3c of SnN2O6 nanosheet [54] 1.23 eV on N-C@NiO/GP catalyst [55] 1.47 eV on WO3-x nanosheet [56]), meaning that the three 2D MOF-TM materials hold immense potential to be used as electrocatalysts for NRRs.
Here we discuss the investigation of electrocatalytic CO2RRs on the 2D MOF-Fe. The solvent effect is taken into consideration herein. The CO2 hydrogenation pathway and free energy profiles over the MOF-Fe catalyst are shown in Figure 7. In the hydrogenation process, both the two O sites and C sites of each intermediate were considered for H addition. For the first H addition, OCHO* intermediate instead of OCOH* and OH* + CO* is formed and the free energy gain ΔG was −0.68 eV. The negative value of ΔG signified the easy obtaining of OCHO*. For further hydrogenation, O* + CH2O and OHCHO* formation, respectively, require ΔG of 1.37 eV and 0.22 eV, while the OCHOH* formation reaction is exothermic with a ΔG of −0.13 eV, indicating that OCHOH* is favorable to form, followed by the hydrogenation of OCHOH* to OCH2OH* with a free energy uphill and the ΔG was calculated to be 0.16 eV. Then, the fourth H would attack the O site of OCH2OH*, which is orientated to the surface, resulting in OHCH2OH* with a ΔG of −0.25 eV. The followed CH3OH formation from the OHCH2OH* hydrogenation is determined as the potential-limiting step with a ΔG of 0.84 eV. Excitingly, CH3OH is exclusively the carbon products. The ΔG value of 0.84 eV was lower than those values with a range of 0.9~1.1 eV on Cu-based materials, which were known as excellent catalysts for CO2 electroreduction to hydrocarbons and oxygenates [57,58]. The remaining OH* reacts with the sixth H to form a H2O* with a ΔG of −0.25 eV. Finally, the H2O molecule peels off from the catalyst with a desorption energy of −0.20 eV and the surface Fe sites are reactivated. These results indicate that the 2D MOF-Fe is an appropriate electrocatalyst for CO2RRs with a high efficiency and high product selectivity.

4. Conclusions

In brief, we have systematically investigated the application prospects of five 2D MOF-TMs (TM = Fe, Co, Ni, Cu, Zn) as electrocatalysts for NRRs and CO2RRs by DFT calculations. The results show that N2 molecules can be effectively activated by the surface metal site of the 2D MOF-Fe, MOF-Co, and MOF-Ni due to the chemisorption interaction. The appropriate free energy increase in the potential-limiting step (0.84 eV for NRRs on MOF-Fe, 1.00 eV for NRRs on MOF-Co, and 1.17 eV for NRRs on MOF-Ni) signifies that those three 2D MOF-TM materials hold promising applications in NRRs. Furthermore, CO2 molecules could be selectively reduced to a single hydrocarbon product, CH3OH, on a 2D MOF-Fe catalyst through the pathway CO2* → OCHO* → OCHOH* → OCH2OH* → OHCH2OH* → CH3OH. And, the fifth hydrogenation step was determined as the potential-limiting step, with a free energy increase of 0.84 eV. These calculation results provide viable approaches to further design and screen the electrocatalysts for NRRs or CO2RRs.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst13101426/s1. Figure S1. Side and top views of adsorption configurations for N2 on (a) MOF-Cu, (b) MOF-Zn. Color cards: C: Grey, H: White, O: Red, Cu: Purple, Zn: Sky blue. Figure S2. Side and top views of adsorption configurations for CO2 on (a) MOF-Co, (b) MOF-Ni, (c) MOF-Cu, (d) MOF-Zn. Color cards: C: Grey, H: White, O: Red, Co: Pink, Ni: Orange, Cu: Purple, Zn: Sky blue.

Author Contributions

A.S.: Proposed the research idea, Designed the research plan, First draft of the paper. M.W.: Conducted the experiment, Edited the paper. S.L.: Data statistics. Y.D.: Supervision, Project administration, Funding support. D.W.: Study method design, Final version revision, Project administration, Funding support. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China (21978110, 51772126, 21801092, and 11904129), the Program for the Development of Science and Technology of Jilin Province (20210101409JC, YDZJ202201ZYTS307, 20200201187JC, 20200801040GH), University-Industry Collaborative Education Program (202002284033). Research on constructing the training system of innovative talents in teachers’ major in digital era (JS2360).

Acknowledgments

Computing time granted by the Computing Center of Jilin Province is acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 13 01426 g001
Figure 1. Side and top views of geometric structures for (a) MOF-Fe, (b) MOF-Co, (c) MOF-Ni, (d) MOF-Cu, and (e) MOF-Zn. Color cards: C: Grey, H: White, O: Red, Fe: Lilac, Co: Pink, Ni: Orange, Cu: Purple, Zn: Sky blue.
Figure 1. Side and top views of geometric structures for (a) MOF-Fe, (b) MOF-Co, (c) MOF-Ni, (d) MOF-Cu, and (e) MOF-Zn. Color cards: C: Grey, H: White, O: Red, Fe: Lilac, Co: Pink, Ni: Orange, Cu: Purple, Zn: Sky blue.
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Crystals 13 01426 g002
Figure 2. Side and top views of adsorption configurations and the corresponding difference charge density plots for (a) N2 adsorption on MOF-Fe, (b) N2 adsorption on MOF-Co, (c) N2 adsorption on MOF-Ni, (d) CO2 adsorption on MOF-Fe. Color cards: C: Gray, H: White, O: Red, Fe: Lilac, Co: Pink, Ni: Orange, Cu: Purple, Zn: Sky blue. In different charge density plots, the blue (yellow) wireframes denote the loss (gain) of electrons with the isosurface values set as 0.003 Å−3.
Figure 2. Side and top views of adsorption configurations and the corresponding difference charge density plots for (a) N2 adsorption on MOF-Fe, (b) N2 adsorption on MOF-Co, (c) N2 adsorption on MOF-Ni, (d) CO2 adsorption on MOF-Fe. Color cards: C: Gray, H: White, O: Red, Fe: Lilac, Co: Pink, Ni: Orange, Cu: Purple, Zn: Sky blue. In different charge density plots, the blue (yellow) wireframes denote the loss (gain) of electrons with the isosurface values set as 0.003 Å−3.
Crystals 13 01426 g002
Crystals 13 01426 g003
Figure 3. PDOS plots for (a) N2 adsorption on MOF-Fe, (b) N2 adsorption on MOF-Co, (c) N2 adsorption on MOF-Ni, (d) CO2 adsorption on MOF-Fe. The Fermi level was assigned at 0 eV. N1/O1 represents the N/O atom bonded with surface metal site, and N2/O2 is the N/O atom located far from the metal site.
Figure 3. PDOS plots for (a) N2 adsorption on MOF-Fe, (b) N2 adsorption on MOF-Co, (c) N2 adsorption on MOF-Ni, (d) CO2 adsorption on MOF-Fe. The Fermi level was assigned at 0 eV. N1/O1 represents the N/O atom bonded with surface metal site, and N2/O2 is the N/O atom located far from the metal site.
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Crystals 13 01426 g004
Figure 4. Free energy diagram of the NRR process of N2* on 2D MOF-Fe and the corresponding geometries of intermediates and products. White, blue, lilac, grey, and red balls denote H, N, Fe, C, and O atoms, respectively.
Figure 4. Free energy diagram of the NRR process of N2* on 2D MOF-Fe and the corresponding geometries of intermediates and products. White, blue, lilac, grey, and red balls denote H, N, Fe, C, and O atoms, respectively.
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Crystals 13 01426 g005
Figure 5. Free energy diagram of the NRR process of N2* on 2D MOF-Co and the corresponding geometries of intermediates and products. White, blue, pink, grey, and red balls denote H, N, Co, C, and O atoms, respectively.
Figure 5. Free energy diagram of the NRR process of N2* on 2D MOF-Co and the corresponding geometries of intermediates and products. White, blue, pink, grey, and red balls denote H, N, Co, C, and O atoms, respectively.
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Crystals 13 01426 g006
Figure 6. Free energy diagram of the NRR process of N2* on 2D MOF-Ni and the corresponding geometries of intermediates and products. White, blue, orange, grey, and red balls denote H, N, Ni, C, and O atoms, respectively.
Figure 6. Free energy diagram of the NRR process of N2* on 2D MOF-Ni and the corresponding geometries of intermediates and products. White, blue, orange, grey, and red balls denote H, N, Ni, C, and O atoms, respectively.
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Crystals 13 01426 g007
Figure 7. Free energy diagram of the CO2RR process of CO2* on 2D MOF-Fe and the corresponding geometries of intermediates and products. White, lilac, grey, and red balls denote H, Fe, C, and O atoms, respectively.
Figure 7. Free energy diagram of the CO2RR process of CO2* on 2D MOF-Fe and the corresponding geometries of intermediates and products. White, lilac, grey, and red balls denote H, Fe, C, and O atoms, respectively.
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Table 1. Chemical adsorption parameters for N2* and CO2* on 2D MOF-TM surfaces: adsorption energy (Eads), N-N bond length of N2* (LN-N), C-O bond length of CO2* (LC-O), the distance between TM sites and N atom of N2* (dTM-N), the distance between Fe site and the O atom of CO2* (dFe-O), the value of net transferred from MOF-TM to N2* and to CO2* (∆q), and the redshift values of N-N stretch frequencies with respect to the free N2 molecule (∆k).
Table 1. Chemical adsorption parameters for N2* and CO2* on 2D MOF-TM surfaces: adsorption energy (Eads), N-N bond length of N2* (LN-N), C-O bond length of CO2* (LC-O), the distance between TM sites and N atom of N2* (dTM-N), the distance between Fe site and the O atom of CO2* (dFe-O), the value of net transferred from MOF-TM to N2* and to CO2* (∆q), and the redshift values of N-N stretch frequencies with respect to the free N2 molecule (∆k).
N2* on MOF-TMEads (eV)LN-N (Å)dTM-N (Å)∆q∆k (cm−1)
Fe−1.431.1391.7730.27141
Co−1.111.1351.7580.21103
Ni−0.641.1261.8120.0923
CO2* on MOF-Fe−0.44LC-O (Å)dFe-O (Å)0.01
1.185/1.1722.100
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She, A.; Wang, M.; Li, S.; Dong, Y.; Wang, D. Two-Dimensional Metal–Organic Framework TM Catalysts for Electrocatalytic N2 and CO2 Reduction: A Density Functional Theory Investigation. Crystals 2023, 13, 1426. https://doi.org/10.3390/cryst13101426

AMA Style

She A, Wang M, Li S, Dong Y, Wang D. Two-Dimensional Metal–Organic Framework TM Catalysts for Electrocatalytic N2 and CO2 Reduction: A Density Functional Theory Investigation. Crystals. 2023; 13(10):1426. https://doi.org/10.3390/cryst13101426

Chicago/Turabian Style

She, Anqi, Ming Wang, Shuang Li, Yanhua Dong, and Dandan Wang. 2023. "Two-Dimensional Metal–Organic Framework TM Catalysts for Electrocatalytic N2 and CO2 Reduction: A Density Functional Theory Investigation" Crystals 13, no. 10: 1426. https://doi.org/10.3390/cryst13101426

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