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Theoretical study on double-atom catalysts supported with graphene for electroreduction of nitrogen into ammonia
Journal Pre-proof Theoretical study on double-atom catalysts supported with graphene for electroreduction of nitrogen into ammonia Weijie Yang, Hanyu Huang, Xunlei Ding, Zhao Ding, Chongchong Wu, Ian D. Gates, Zhengyang Gao PII:
S0013-4686(20)30058-X
DOI:
https://doi.org/10.1016/j.electacta.2020.135667
Reference:
EA 135667
To appear in:
Electrochimica Acta
Received Date: 14 November 2019 Revised Date:
24 December 2019
Accepted Date: 7 January 2020
Please cite this article as: W. Yang, H. Huang, X. Ding, Z. Ding, C. Wu, I.D. Gates, Z. Gao, Theoretical study on double-atom catalysts supported with graphene for electroreduction of nitrogen into ammonia, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135667. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Theoretical study on double-atom catalysts supported with graphene for electroreduction of nitrogen into ammonia Weijie Yanga, Hanyu Huanga, Xunlei Dingb, Zhao Dingc*, Chongchong Wud, Ian D.Gatesd, Zhengyang Gaoa* a
School of Energy and Power Engineering, North China Electric Power University, Baoding 071003, China
b
School of Mathematics and Physics, North China Electric Power University, Beijing 102206, China
c
Department of Mechanical, Materials and Aerospace Engineering, Illinois Institute of Technology, Chicago, IL,
60616 d
Department of Chemical and Petroleum Engineering, University of Calgary, T2N 1N4, Calgary, Alberta, Canada
*Corresponding author: Zhengyang Gao ([email protected]), Zhao Ding ([email protected])
Abstract Developing electrocatalyst with high catalytic activity for the nitrogen reduction reaction (NRR) is the key step to accelerate the application of electrocatalytic nitrogen fixation. Eight kinds of double-atom Fe catalysts supported with graphene-based substrate (Fe-TMDA/GS, TM=Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) were constructed for electroreduction of nitrogen into ammonia. The bonding mechanism of Fe- transition metal dimer on substrates was studied from the perspectives of orbital hybridization and electron transfer. Based on the analysis of reaction paths and Gibbs free energy variation, the potential determining step of the NRR is the transfer process of the first H+/e- pair. Fe-TiDA/GS has the highest NRR catalytic activity among the eight kinds of Fe-TMDA/GS, with a Gibbs energy variation of 0.88 eV. Based on the analysis of electron density difference, band structure and work function, electron transfer is the main factor affecting nitrogen adsorption, and the second metal atom improves the catalytic activity of NRR through promoting electron transfer between N2 and the graphene surface. This theoretical research provides new insights into developing highly efficient electrocatalyst for NRR.
Key words: double-atom catalysts; electrocatalysis; nitrogen reduction reaction; catalytic activity; density functional theory
1. Introduction Ammonia (NH3) synthesis, based on the Haber–Bosch process (N2 + 3H2 → 2NH3), is one of the most important catalytic reactions used industrially today consuming about 1% of global energy consumption [1, 2]. To save energy and reduce emissions, electroreduction of N2 into NH3 (N2 + 6H+ + 6e- → 2NH3) has recently attracted interests due to its mild reaction conditions [3, 4]. In theory, the electrochemical N2 reduction reaction (NRR) can take place at room temperature and atmospheric pressure if sufficient voltage is applied [5]. However, there is a lack of efficient catalysts which can produce NH3 with significant yield and high Faradaic efficiency. Therefore, rational design and synthesis of electrocatalysts for NRR is a crucial step to promote the development of ammonia synthesis. Recently, single-atom catalysts (SACs), such as Fe[6, 7], Co[8], Mn[9], Ru[10], Au[11], Mo[12, 13] and B[14] have aroused extensive attention for NRR owing to its high catalytic activity and selectivity. Similar to SACs, as atomic-scale catalysts, double-atom catalysts (DACs) have shown excellent performance in hydrolytic dehydrogenation
of
ammonia
borane
(Pt2/Graphene[15]),
CO
oxidation
(Ni-V/Graphene[16]), formic acid dehydrogenation (Pd2/Graphene[17]), oxygen reduction
(Fe-Co/Graphene[18]),
and
CO2
reduction
(Mn-Cu/Graphene[19])
reactions. According to Ma et al. [8], DACs supported on graphdiyne have obvious advantages with respect to the free energy variation (△G) of the potential determining step (PDS) for NRR than SACs supported with graphdiyne. Similarly, Chen et al.[9] investigated DACs consisting of transition metals (TM, TM= Cr, Mn, Fe, Co, and Ni) supported with C2N for NRR and concluded that the △G of the PDS on TM2/C2N is only half that on TM1/C2N. Inspired by these results, DACs seem to be much stronger for the NRR than is the case for SACs. In addition, the graphene-based substrate has excellent conductivity which can further promote the NRR. Therefore, it is reasonable to assume that DACs supported with graphene substrate (DACs/GS) are a promising candidate for the NRR. In addition, doping metal atoms on both sides of a single layer
graphene can be realized in the roll-to-roll production of graphene according to Zhi et al. [20]. Warner et al.[21] observed the atomic structures of Fe-FeDA/GS through aberration-corrected transmission electron microscopy. According to Nørskov et al. [22], metals such as Rh, Ru, Ir, and Fe were calculated to lie near the top of the volcano plot, suggesting that these metals are promising candidate catalysts for the NRR. Considering its low cost and broad storage capacity, iron has more obvious advantages in practical application than those noble metals. In addition, the high catalytic activity of single-atom iron catalysts has been proven in experimental [6, 23] and theoretical studies [24, 25]. Therefore, to further improve the catalytic activity of single-atom iron catalysts for the NRR, eight kinds of Fe-TMDA/GS (TM= Ti, V, Cr, Mn, Fe, Co, Ni, Cu) were constructed to investigate their catalytic performance. In this work, geometric structures and electronic characteristics were first studied to identify the bonding mechanism of the Fe-TM dimer on the substrate. Secondly, the reaction paths of the NRR on Fe-TMDA/GS were established based on the enzymatic reaction mechanism. Subsequently, the Gibbs free energy variation, potential determining step, electron transfer and bond length in the process of the NRR were discussed to analyze the catalytic activity of the NRR. Fourth, based on the volcano map, the relationship between the catalytic activity of the NRR and the adsorption energy of N2 was analyzed. Finally, the bonding mechanism of N2 on Fe-TMDA/GS was investigated by analyzing orbital hybridization and electron transfer. 2. Method All density functional theory (DFT) calculations were performed in the Vienna ab initio Simulation Package (VASP) with the generalized gradient-corrected PBE exchange-correction functional [26] and projector augmented wave (PAW) basis set [27, 28]. Spin polarization and Van Der Waals interactions were considered to study the interaction between N2 and surface. Consistent with our previous work [29-34], a 4×4 supercell of graphene with a vacuum layer of 15 Å was built and a kinetic energy cutoff of 500 eV with the width of Gaussian smearing of 0.05 eV was adopted to
describe the occupation of electronic levels. For geometry optimization, the Brillouin zone was sampled with a 7×7×1 Γ-centered k-point grid and the convergence standard of force was 0.02 eV/Å. For the calculation of energy and density of states, the Brillouin zone was sampled with a 15×15×1 Γ-centered k-point grid and the energy convergence standard of self-consistent field computations were less than 1×10-5 eV. The adsorption energy (Eads), defined by Eads = Etot – Esur – EN2, was used to measure the adsorption strength between N2 and catalyst surface where Etot, Esur, and EN2 represent the ground-state energy of the adsorbed system, catalyst surface, and nitrogen, respectively. The Bader charge was calculated to study the electron transfer between N2 and surface [35]. According to the standard hydrogen electrode (SHE) model proposed by Nørskov et al. [36], the Gibbs free energy variation, △G, was calculated to describe the energy change of the NRR. In detail, one-half of the chemical potential of hydrogen is equivalent to the chemical potential of the proton-electron pair. The calculation of △G is given by △G = △E + △ZPE - T△S + △GU + △GpH where △E is the electronic energy difference directly acquired from the self-consistent field calculation. △ZPE is the change in zero-point energy, T is the absolute temperature (298.15 K). △S is the entropy change, both △ZPE and T△S can be computed by vibrational frequencies. △GU is the contribution of the external potential which can be calculated by -n e U, where n is the transfer number of electron and U is the applied voltage. △GpH is the correction of the H+ free energy in the aqueous solution, which can be calculated by △GpH = 2.303 kBT pH, where kB is the Boltzmann constant and the value of pH is assumed to be zero.
3. Results and Discussion 3.1 Catalyst models The optimized structures of Fe-TMDA/GS are plotted in Fig. 1, and the corresponding key geometric and electronic parameters are listed in Table 1. From Fig. 1 (a) to (h), the included angles between Fe-TM line and carbon-based substrate (θ) of
Fe-TMDA/GS gradually increase, which is mainly due to the radius difference of metal atoms, suggesting that doping different transition metal atoms can significantly regulate the geometric structures of Fe-TMDA/GS. Moreover, the magnetic moments of Fe-TMDA/GS increase first and then decrease, with a variation range of 5 µB, suggesting that doping different transition metal atoms enables different electronic structures of Fe-TMDA/GS. Therefore, doping different transition metal atoms provides a way to further promote the activity of the NRR. In detail, the binding energies between the Fe-TM dimer and carbon-based substrate are all larger than that of the single-atom Fe (-7.14 eV) [35]. Compared with previous studies, according to the research of Yuan et al.[16], the bond length of Ni-C in the system of Ni-CuDA/GS is about 1.81 Å, which is similar to the calculated bond length of Fe-C in the system of Fe-CuDA/GS (1.80 Å), suggesting that our calculated results are reasonable. The theoretical simulation of ab initio molecular dynamics (AIMD) was employed at 298.15 K under 10 ps to check the structural stability of eight Fe-TMDA/GS catalysts [37], as shown in Fig. 2. From Fig. 2, the average bond length of Fe-C and TM-C and system energy were plotted, respectively. The system energy and the corresponding average bond length oscillated in a small area, suggesting that Fe-TMDA/GS should have high stability. In detail, the bond length was basic consistent with the value in Table 1.
Fig. 1 Structures of Fe-TMDA/GS
Table 1 Bond lengths, charge variation (△q), magnetic moments (M), and included angles between Fe-TM line and carbon-based substrates (θ) for Fe-TM DACs/GS. Properties
FeTi
FeV
FeCr
FeMn
FeFe
FeCo
FeNi
FeCu
Fe-C(1) (Å) Fe-C(2) (Å) Fe-C(3) (Å) Fe-M (Å) M-C(1) (Å) M-C(2) (Å) M-C(3) (Å) △q-Fe (e) △q-TM (e)
2.00 1.86 1.86 2.39 2.09 2.05 2.05 -0.70 -1.32 2.00 65.35 -10.58
1.97 1.84 1.84 2.36 2.07 2.03 2.03 -0.69 -1.15 3.06 65.54 -9.84
2.00 1.89 1.89 2.54 1.97 2.08 2.08 -0.71 -0.94 5.98 74.31 -8.95
1.93 1.83 1.83 2.41 1.98 2.11 2.11 -0.67 -0.89 5.00 73.34 -8.79
1.94 1.89 1.89 2.31 1.92 1.91 1.91 -0.71 -0.71 3.98 75.82 -9.29
1.96 1.96 1.96 2.38 1.84 1.85 1.85 -0.70 -0.51 2.88 88.46 -10.10
1.88 1.89 1.89 2.39 1.92 1.92 1.93 -0.69 -0.47 2.00 87.11 -10.21
1.80 1.80 1.80 2.44 2.10 2.10 2.10 -0.67 -0.53 0.98 89.71 -8.64
M (µB) θ (°) Eb (eV)
Fig. 2 Energy and bond length of the eight Fe-TM /GS catalysts in the simulation of AIMD
To further study the binding mechanism of Fe-TM on single-vacancy graphene-based substrate, the projected density of states (PDOS) of Fe-TMDA/GS were calculated, as shown in Fig. 3. The p orbital of the C, d orbital of the Fe, and d
orbital of the TM are plotted with the brown, green and blue lines, respectively. With the exception of Fe-CuDA/GS, there were obvious orbital hybridizations among Fe(d), TM(d) and C(p), suggesting that the substrate can hold Fe-TM firmly through covalent bonding. In addition, the smallest binding energy of Fe-Cu on the substrate can be understood by non-orbital hybridization.
Fig. 3 PDOS of Fe-TMDA/GS
3.2 Analysis of reaction paths There are three recognized pathways of the NRR in the system of non-SACs, including distal, alternating, and enzymatic reaction mechanisms [3, 22]. However, previous studies [38-40] proved that the enzymatic reaction mechanism is dominant in the system of SACs. In detail, according to the theoretical research of Wang et al., the PDS of the NRR in the system of N1/C2N-h2D with the enzymatic reaction mechanism is the lowest [38]. Similarly, in the experimental and theoretical research of Liu et al., the lowest PDS of the NRR in the system of Fe1/N4-C is 1.69 eV, corresponding to the enzymatic reaction mechanism [39]. Furthermore, according to the theoretical research of Zhou et al., in the double atom system of Mo-TM/C (TM= B, N, P, S, Se, and Cl), the preferred reaction path is the enzymatic mechanism [40]. Therefore, here, the reaction paths of the NRR using the system of Fe-TMDA/GS were calculated according to the enzymatic mechanism. Before calculating the reaction path of nitrogen reduction reaction, the selectivity of catalyst between hydrogen evolution reaction (HER) and NRR was analyzed through comparing the adsorption Gibbs energy of H and N2 [41], as shown in Fig. S1. Obviously, the N2 will preferentially adsorb on the surface of Fe-TMDA/GS compared with H, and Fe-TMDA/GS have high selectivity for NRR through inhibiting the adsorption of H. According to the similarity of reaction paths in the system of Fe-TMDA/GS, we take Fe-TiDA/GS as an example (as shown in Fig. 4) to study the reaction path of the NRR, and the reaction paths of the NRR in other surfaces were plotted in Figs. S2 to S8. In Fig. 4, the sum of Gibbs energies of Fe-TiDA/GS, N2, and 6 H was selected as the zero point of energy. Consistent with the enzymatic reaction mechanism [39], the H+/e- pair was alternately added to the adsorbed nitrogen atoms, and the reaction cycle of the NRR was completed with the desorption of NH3.
Fig. 4 Configurations and Gibbs energy variation along the reaction path of NRR on Fe-TiDA/GS
To study the Gibbs energy variation of the NRR on different surfaces of Fe-TMDA/GS, Gibbs free energy diagrams of the NRR on Fe-TMDA/GS at zero and applied potential were calculated, as shown in Fig. 5. In addition, the detail information of free energy and activation barriers were summarized in Table S2 to S10. According to the Gibbs free energy variation of the NRR at zero potential, we can conclude that PDS of the NRR on Fe-TMDA/GS is the transfer process of the first H+/e- pair, labeled in red arrows. To facilitate the analysis of catalytic activity differences, the max Gibbs free energy variations (△Gmax) associated with the PDS were summarized in Fig. 6.
Fig. 5 Gibbs free energy diagrams of NRR on Fe-TMDA/GS at zero and applied potential
1.6
1.54
1.4
△Gmax (eV)
1.2
1.14 1.06
1.0
1.06
1.10 1.03
0.95 0.88
0.8
0.6
0.4
Fe-Ti Fe-V Fe-Cr Fe-Mn Fe-Fe Fe-Co Fe-Ni Fe-Cu
Fig. 6 Max Gibbs energy variation of NRR over Fe-TMDA/GS.
The larger the △Gmax of the NRR, the lower is the catalytic activity. Consequently, the catalytic activities of Fe-TiDA/GS and Fe-CuDA/GS are the highest and lowest cases, respectively, among the eight cases of Fe-TMDA/GS. The △Gmax ranges from 0.88 to 1.54 eV, indicating that we can regulate the doped metal atoms to tune the catalytic activity of the NRR. In the next discussion, we selected Fe-TiDA/GS as an example to study reaction details, such as bond lengths and charge variations. In Fig. 7(a), the zero point of charge was selected as the charge of Fe-TiDA/GS before N2 adsorption. The electron number variations of Fe-Ti dimer are all negative in the NRR, indicating that the Fe-Ti dimer acts as the electron donor in the NRR, which is consistent with the theoretical research of Chen et al. [12]. In detail, the number of electrons donated by the Fe-Ti dimer gradually increased in the transfer process of the first three H+/e- pairs and decreased in the transfer process of the last three H+/e- pairs. In most cases, the electron number variations of the substrate are also negative, suggesting that the substrate also acts as the electron donor. In detail, the increased electrons of intermediate (NxHy) are equal to the reduced electrons of Fe-Ti dimer and substrate. In Fig. 7(b), the bond lengths of N-N in the process of the NRR were obviously stretched compared with an isolated nitrogen molecule. In detail, there was a sharp increase of the N-N bond length in the transfer process of the third of H+ and
e- pairs, suggesting the breakage of the N-N band in this process, as shown in Fig. 4. Moreover, the electron number variation and bond length of Fe-Ti dimer are both constantly changing in the process of NRR, indicating that the active site is variable and the catalytic process is dynamic, which is similar to the dynamic catalytic process of gold nanoparticles [42, 43].
Fig.7 (a) Charge variation of Fe-Ti dimer and substrate (b) Bond lengths of N-N and Fe-Ti dimer.
3.3 Analysis of catalytic activity To evaluate the regulation effect of TM, the △Gmax of other single-atom Fe catalysts were listed in Table 2. Compared with other single-atom Fe catalysts, the △Gmax of Fe-TiDA/GS (0.88 eV) is obviously lower than that of other single-atom Fe catalysts (larger than 1 eV), indicating the catalytic activity of single-atom Fe catalysts in the NRR can be further improved via reasonable construction of the Fe-TM double-atom catalyst. To further evaluate the performance of catalyst, the △Gmax of other metal catalysts were listed in Table S1. The catalytic activity of Fe-TiDA/GS located at the medium level, indicating that catalytic activity should be further improved through other control mode of electronic structure, such as defect and doping other atoms.
Table 2 The △Gmax of PDS for NRR over different catalysts
Catalysts FeSA@SVG[44] FeSA/MoS2[24]
△Gmax (eV) 1.37 1.02
Reference Electrochim. Acta. 2018, 284, 392e399 Chem. Eur. J. 2017, 23, 8275 – 8279
FeSA/N4-C[39] FeSAN3[25] Fe3/θ-Al2O3(010)[23] FeSA/MoS2[45]
1.69 1.45 1.14 1.01
FeSA/Graphdiyne[8] FeSA/C2N Fe(110)[46] Fe-TiDA/GS
1.16 1.06 1.39 0.88
Nano Energy. 2019, 61, 420–427 J. Am. Chem. Soc. 2016, 138, 8706−8709 Nat. Comm. 2018, 9:1610 Phys. Chem. Chem. Phys. 2018, 20, 9248-9255 J. Phys. Chem. C 2019, 123, 19066−19076 Small Methods 2018, 1800291 Catal. Sci. Technol. 2019, 9, 174-181 This work
According to the Sabatier principle [47], a medium adsorption strength between reactants and catalyst surface is most suitable for the catalytic reaction. Therefore, the △Gmax versus adsorption energy of N2 for Fe-TMDA/GS were plotted, as shown in Fig. 8. An inverted volcano map of catalytic activity based on △Gmax and adsorption energy of N2 can be found, which can explain the catalytic activity order of Fe-TMDA/GS in the NRR. In addition, the data dot for single-atom Fe catalyst was added in Fig. 8, and it was represented by the purple circle. Obviously, Fe-TMDA/GS, expect for Fe-CuDA/GS, has higher catalytic activity than single-atom Fe catalyst, indicating that the regulation effect of TM improves the catalytic activity for NRR. 1.6 Cu
1.5
△Gmax (eV)
1.4
Fe@SVG
1.3 1.2
PDS=-2.58*Eads(N2)-1.35 R2=0.99
Cr
Co
1.1 1.0
Mn Ni
Fe V
0.9 0.8
Too Strong
Ti
PDS=3.99*Eads(N2)+4.29 R2=0.95 Too Weak
-0.95 -0.90 -0.85 -0.80 -0.75 -0.70 -0.65 -0.60 Eads(N2) (eV)
Fig. 8 △Gmax as a function of adsorption energy of N2.
3.4 Binding mechanism of N2
According to the above discussion of catalytic activity, the adsorption energy of N2 is crucial for the catalytic activity of the NRR in the system of Fe-TMDA/GS. Therefore, it is necessary to further study the bonding mechanism of N2 on the surface of Fe-TMDA/GS. According to our previous studies [29, 31], there are two main contributions for adsorption energy of N2, namely, orbital hybridization and electron transfer. To study the effect of orbital hybridization on the bonding mechanism of N2 on the surface, the projected density of states (PDOS) for the adsorbed system were plotted, as shown in Fig. 9. In Fig. 9, the density of states of the total, d orbital of Fe and p orbital of N are plotted in black, green and red lines, respectively. In the region near the Fermi level, there was no obvious hybrid peak between Fe(d) and N(p), indicating that the contribution of orbital hybridization to the bonding process of N2 on the surface can be neglected.
Fig. 9 PDOS of N2 and Fe in the adsorption system of N2
To study electron transfer between N2 and the substrate in the bonding process, the electron density difference of N2 adsorption on Fe-TMDA/GS was plotted, as shown in Fig. 10. In Fig. 10, the yellow and cyan colors denote the electron density accumulation and depletion regions, respectively. There is obvious electron transfer between N2 and Fe-TMDA/GS, suggesting that electron transfer acts as the dominant role in the process of N2 bonding. In detail, electron density depletion mainly appears in the region of the Fe atom and N-N bond, indicating that the N-N bond strength weakened and the Fe atom acts as an electron donor in the adsorption process. Electron density accumulation mainly appears in the region of the Fe-N bond, indicating that the Fe-N bond was strengthened with the electron transfer. Based on the analysis of PDOS and electron density difference, we conclude that electron transfer is the main factor in the adsorption process of N2 on Fe-TMDA/GS. Therefore, to explore the difference of catalytic activity of the NRR between Fe-TMDA/GS and single-atom Fe catalyst, the electron transfer of N2 was calculated, as shown in Fig. 11. Compared with the single-atom Fe catalyst, Fe-TMDA/GS has significant advantages in the electron transfer of N2, which accounts for the higher catalytic activity of Fe-TMDA/GS than that of single-atom Fe catalyst. To further investigate how the TM atom accelerate the electron transfer between N2 and graphene, the band structures of the eight Fe-TMDA/GS and single-atom Fe catalyst were calculated, as shown in Fig. 12. Compared with single-atom Fe catalyst, the band number near Fermi level in the system of Fe-TMDA/GS catalysts significantly increased and the band gap obviously decreased, suggesting that electron transfer is more likely to occur in the system of Fe-TMDA/GS. Furthermore, the work function (Φ) was calculated to evaluate the ability of electrons to escape from a solid surface, was shown in Fig. S9. The work function is defined as Φ = Ev - Ef, where Ev and Ef are the vacuum level Fermi energy, respectively, and the smaller the value of Φ, the more likely to lose electrons. From Fig. S9, the value of Φ for eight Fe-TMDA/GS catalysts obviously lower than that of single-atom Fe catalyst, indicating that Fe-TMDA/GS is more likely to provide electrons to N2. However, based on Fig. 12 and Fig. S9, Fe-CuDA/GS should have higher catalytic activity of NRR than single-atom
Fe catalyst, which is in contradiction to the calculated value of △Gmax, implying that catalytic activity of NRR should depend on other factors, such as interaction between catalyst and N2. Therefore, we calculated the d-band center of Fe-CuDA/GS and single-atom Fe catalyst to study the interaction between catalyst and N2. The d-band center of Fe-CuDA/GS (-1.98 eV) is farther from Fermi level than that of single-atom Fe (-0.97 eV), which can explain the above contradiction in the system of Fe-CuDA/GS and single-atom Fe catalyst. Therefore, we infer that the second metal atom improves the catalytic activity of NRR mainly through promoting electron transfer between N2 and the surface.
Fig. 10 Electron density difference of N2 adsorption on Fe-TMDA/GS catalysts (contour lines in plots are drawn at 0.003e/Å3 intervals). 0.45 0.41
Electron transfer (e)
0.40
0.39
0.39
0.38
0.38 0.35
0.35
0.35 0.33
0.30 0.25 0.20 0.15 0.12
0.10 0.05 0.00 FeTi
FeV
FeCr FeMn FeFe FeCo FeNi FeCu
Fig. 11 Electron transfer of N2 in the adsorption process.
Fe[43]
Fig. 12 The band structures of the eight Fe-TMDA/GS catalysts and single-atom Fe catalyst
4. Conclusions Eight kinds of double-atom Fe catalysts were constructed through doping transition metal atom on graphene substrates (Fe-TMDA/GS) and the geometry structures and electronic characteristics of these eight catalysts were studied. Reaction path, potential determining step, Gibbs free energy variation, catalytic activity and bonding mechanism of the nitrogen reduction reaction (NRR) in the system of Fe-TMDA/GS were systematically investigated through density functional theory calculations. The results indicate that the potential determining steps of the NRR on Fe-TMDA/GS are associated with the transfer process of the first H+/e- pair. Compared with the single-atom Fe catalyst, Fe-TMDA/GS has more obvious advantages for its lower Gibbs free energy variation for the potential determining step of the NRR. Fe-TiDA/GS has the highest catalytic activity for the NRR among the eight kinds of Fe-TMDA/GS considered here, with a Gibbs energy variation of 0.88 eV. Based on the
volcano map of catalytic activity, the catalytic activity order of Fe-TMDA/GS can be understood from the adsorption energy of N2. Electron transfer is the main factor affecting nitrogen adsorption, and the second metal atom in Fe-TMDA/GS improves the catalytic activity of the NRR by promoting electron transfer between N2 and the surface. Acknowledgements This work was supported by Beijing Natural Science Foundation (2182066), Natural Science Foundation of Hebei Province of China (B2018502067). IDG and CCW acknowledge support from the University of Calgary’s Canada First Research Excellence Fund (CFREF) program entitled the Global Research Initiative for Sustainable Low Carbon Unconventional Resources.
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Weijie Yang: Conceptualization, Writing - Original Draft, Investigation Hanyu Huang: Formal analysis, Visualization Zhao Ding: Validation, Data Curation Xunlei Ding：Supervision, Resources, Software Chongchong Wu: Writing - Review & Editing Ian D.Gates: Supervision, Writing - Review & Editing Zhengyang Gao: Supervision, Resources, Software, Funding acquisition
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0013-4686(20)30058-X
DOI:
https://doi.org/10.1016/j.electacta.2020.135667
Reference:
EA 135667
To appear in:
Electrochimica Acta
Received Date: 14 November 2019 Revised Date:
24 December 2019
Accepted Date: 7 January 2020
Please cite this article as: W. Yang, H. Huang, X. Ding, Z. Ding, C. Wu, I.D. Gates, Z. Gao, Theoretical study on double-atom catalysts supported with graphene for electroreduction of nitrogen into ammonia, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135667. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Theoretical study on double-atom catalysts supported with graphene for electroreduction of nitrogen into ammonia Weijie Yanga, Hanyu Huanga, Xunlei Dingb, Zhao Dingc*, Chongchong Wud, Ian D.Gatesd, Zhengyang Gaoa* a
School of Energy and Power Engineering, North China Electric Power University, Baoding 071003, China
b
School of Mathematics and Physics, North China Electric Power University, Beijing 102206, China
c
Department of Mechanical, Materials and Aerospace Engineering, Illinois Institute of Technology, Chicago, IL,
60616 d
Department of Chemical and Petroleum Engineering, University of Calgary, T2N 1N4, Calgary, Alberta, Canada
*Corresponding author: Zhengyang Gao ([email protected]), Zhao Ding ([email protected])
Abstract Developing electrocatalyst with high catalytic activity for the nitrogen reduction reaction (NRR) is the key step to accelerate the application of electrocatalytic nitrogen fixation. Eight kinds of double-atom Fe catalysts supported with graphene-based substrate (Fe-TMDA/GS, TM=Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) were constructed for electroreduction of nitrogen into ammonia. The bonding mechanism of Fe- transition metal dimer on substrates was studied from the perspectives of orbital hybridization and electron transfer. Based on the analysis of reaction paths and Gibbs free energy variation, the potential determining step of the NRR is the transfer process of the first H+/e- pair. Fe-TiDA/GS has the highest NRR catalytic activity among the eight kinds of Fe-TMDA/GS, with a Gibbs energy variation of 0.88 eV. Based on the analysis of electron density difference, band structure and work function, electron transfer is the main factor affecting nitrogen adsorption, and the second metal atom improves the catalytic activity of NRR through promoting electron transfer between N2 and the graphene surface. This theoretical research provides new insights into developing highly efficient electrocatalyst for NRR.
Key words: double-atom catalysts; electrocatalysis; nitrogen reduction reaction; catalytic activity; density functional theory
1. Introduction Ammonia (NH3) synthesis, based on the Haber–Bosch process (N2 + 3H2 → 2NH3), is one of the most important catalytic reactions used industrially today consuming about 1% of global energy consumption [1, 2]. To save energy and reduce emissions, electroreduction of N2 into NH3 (N2 + 6H+ + 6e- → 2NH3) has recently attracted interests due to its mild reaction conditions [3, 4]. In theory, the electrochemical N2 reduction reaction (NRR) can take place at room temperature and atmospheric pressure if sufficient voltage is applied [5]. However, there is a lack of efficient catalysts which can produce NH3 with significant yield and high Faradaic efficiency. Therefore, rational design and synthesis of electrocatalysts for NRR is a crucial step to promote the development of ammonia synthesis. Recently, single-atom catalysts (SACs), such as Fe[6, 7], Co[8], Mn[9], Ru[10], Au[11], Mo[12, 13] and B[14] have aroused extensive attention for NRR owing to its high catalytic activity and selectivity. Similar to SACs, as atomic-scale catalysts, double-atom catalysts (DACs) have shown excellent performance in hydrolytic dehydrogenation
of
ammonia
borane
(Pt2/Graphene[15]),
CO
oxidation
(Ni-V/Graphene[16]), formic acid dehydrogenation (Pd2/Graphene[17]), oxygen reduction
(Fe-Co/Graphene[18]),
and
CO2
reduction
(Mn-Cu/Graphene[19])
reactions. According to Ma et al. [8], DACs supported on graphdiyne have obvious advantages with respect to the free energy variation (△G) of the potential determining step (PDS) for NRR than SACs supported with graphdiyne. Similarly, Chen et al.[9] investigated DACs consisting of transition metals (TM, TM= Cr, Mn, Fe, Co, and Ni) supported with C2N for NRR and concluded that the △G of the PDS on TM2/C2N is only half that on TM1/C2N. Inspired by these results, DACs seem to be much stronger for the NRR than is the case for SACs. In addition, the graphene-based substrate has excellent conductivity which can further promote the NRR. Therefore, it is reasonable to assume that DACs supported with graphene substrate (DACs/GS) are a promising candidate for the NRR. In addition, doping metal atoms on both sides of a single layer
graphene can be realized in the roll-to-roll production of graphene according to Zhi et al. [20]. Warner et al.[21] observed the atomic structures of Fe-FeDA/GS through aberration-corrected transmission electron microscopy. According to Nørskov et al. [22], metals such as Rh, Ru, Ir, and Fe were calculated to lie near the top of the volcano plot, suggesting that these metals are promising candidate catalysts for the NRR. Considering its low cost and broad storage capacity, iron has more obvious advantages in practical application than those noble metals. In addition, the high catalytic activity of single-atom iron catalysts has been proven in experimental [6, 23] and theoretical studies [24, 25]. Therefore, to further improve the catalytic activity of single-atom iron catalysts for the NRR, eight kinds of Fe-TMDA/GS (TM= Ti, V, Cr, Mn, Fe, Co, Ni, Cu) were constructed to investigate their catalytic performance. In this work, geometric structures and electronic characteristics were first studied to identify the bonding mechanism of the Fe-TM dimer on the substrate. Secondly, the reaction paths of the NRR on Fe-TMDA/GS were established based on the enzymatic reaction mechanism. Subsequently, the Gibbs free energy variation, potential determining step, electron transfer and bond length in the process of the NRR were discussed to analyze the catalytic activity of the NRR. Fourth, based on the volcano map, the relationship between the catalytic activity of the NRR and the adsorption energy of N2 was analyzed. Finally, the bonding mechanism of N2 on Fe-TMDA/GS was investigated by analyzing orbital hybridization and electron transfer. 2. Method All density functional theory (DFT) calculations were performed in the Vienna ab initio Simulation Package (VASP) with the generalized gradient-corrected PBE exchange-correction functional [26] and projector augmented wave (PAW) basis set [27, 28]. Spin polarization and Van Der Waals interactions were considered to study the interaction between N2 and surface. Consistent with our previous work [29-34], a 4×4 supercell of graphene with a vacuum layer of 15 Å was built and a kinetic energy cutoff of 500 eV with the width of Gaussian smearing of 0.05 eV was adopted to
describe the occupation of electronic levels. For geometry optimization, the Brillouin zone was sampled with a 7×7×1 Γ-centered k-point grid and the convergence standard of force was 0.02 eV/Å. For the calculation of energy and density of states, the Brillouin zone was sampled with a 15×15×1 Γ-centered k-point grid and the energy convergence standard of self-consistent field computations were less than 1×10-5 eV. The adsorption energy (Eads), defined by Eads = Etot – Esur – EN2, was used to measure the adsorption strength between N2 and catalyst surface where Etot, Esur, and EN2 represent the ground-state energy of the adsorbed system, catalyst surface, and nitrogen, respectively. The Bader charge was calculated to study the electron transfer between N2 and surface [35]. According to the standard hydrogen electrode (SHE) model proposed by Nørskov et al. [36], the Gibbs free energy variation, △G, was calculated to describe the energy change of the NRR. In detail, one-half of the chemical potential of hydrogen is equivalent to the chemical potential of the proton-electron pair. The calculation of △G is given by △G = △E + △ZPE - T△S + △GU + △GpH where △E is the electronic energy difference directly acquired from the self-consistent field calculation. △ZPE is the change in zero-point energy, T is the absolute temperature (298.15 K). △S is the entropy change, both △ZPE and T△S can be computed by vibrational frequencies. △GU is the contribution of the external potential which can be calculated by -n e U, where n is the transfer number of electron and U is the applied voltage. △GpH is the correction of the H+ free energy in the aqueous solution, which can be calculated by △GpH = 2.303 kBT pH, where kB is the Boltzmann constant and the value of pH is assumed to be zero.
3. Results and Discussion 3.1 Catalyst models The optimized structures of Fe-TMDA/GS are plotted in Fig. 1, and the corresponding key geometric and electronic parameters are listed in Table 1. From Fig. 1 (a) to (h), the included angles between Fe-TM line and carbon-based substrate (θ) of
Fe-TMDA/GS gradually increase, which is mainly due to the radius difference of metal atoms, suggesting that doping different transition metal atoms can significantly regulate the geometric structures of Fe-TMDA/GS. Moreover, the magnetic moments of Fe-TMDA/GS increase first and then decrease, with a variation range of 5 µB, suggesting that doping different transition metal atoms enables different electronic structures of Fe-TMDA/GS. Therefore, doping different transition metal atoms provides a way to further promote the activity of the NRR. In detail, the binding energies between the Fe-TM dimer and carbon-based substrate are all larger than that of the single-atom Fe (-7.14 eV) [35]. Compared with previous studies, according to the research of Yuan et al.[16], the bond length of Ni-C in the system of Ni-CuDA/GS is about 1.81 Å, which is similar to the calculated bond length of Fe-C in the system of Fe-CuDA/GS (1.80 Å), suggesting that our calculated results are reasonable. The theoretical simulation of ab initio molecular dynamics (AIMD) was employed at 298.15 K under 10 ps to check the structural stability of eight Fe-TMDA/GS catalysts [37], as shown in Fig. 2. From Fig. 2, the average bond length of Fe-C and TM-C and system energy were plotted, respectively. The system energy and the corresponding average bond length oscillated in a small area, suggesting that Fe-TMDA/GS should have high stability. In detail, the bond length was basic consistent with the value in Table 1.
Fig. 1 Structures of Fe-TMDA/GS
Table 1 Bond lengths, charge variation (△q), magnetic moments (M), and included angles between Fe-TM line and carbon-based substrates (θ) for Fe-TM DACs/GS. Properties
FeTi
FeV
FeCr
FeMn
FeFe
FeCo
FeNi
FeCu
Fe-C(1) (Å) Fe-C(2) (Å) Fe-C(3) (Å) Fe-M (Å) M-C(1) (Å) M-C(2) (Å) M-C(3) (Å) △q-Fe (e) △q-TM (e)
2.00 1.86 1.86 2.39 2.09 2.05 2.05 -0.70 -1.32 2.00 65.35 -10.58
1.97 1.84 1.84 2.36 2.07 2.03 2.03 -0.69 -1.15 3.06 65.54 -9.84
2.00 1.89 1.89 2.54 1.97 2.08 2.08 -0.71 -0.94 5.98 74.31 -8.95
1.93 1.83 1.83 2.41 1.98 2.11 2.11 -0.67 -0.89 5.00 73.34 -8.79
1.94 1.89 1.89 2.31 1.92 1.91 1.91 -0.71 -0.71 3.98 75.82 -9.29
1.96 1.96 1.96 2.38 1.84 1.85 1.85 -0.70 -0.51 2.88 88.46 -10.10
1.88 1.89 1.89 2.39 1.92 1.92 1.93 -0.69 -0.47 2.00 87.11 -10.21
1.80 1.80 1.80 2.44 2.10 2.10 2.10 -0.67 -0.53 0.98 89.71 -8.64
M (µB) θ (°) Eb (eV)
Fig. 2 Energy and bond length of the eight Fe-TM /GS catalysts in the simulation of AIMD
To further study the binding mechanism of Fe-TM on single-vacancy graphene-based substrate, the projected density of states (PDOS) of Fe-TMDA/GS were calculated, as shown in Fig. 3. The p orbital of the C, d orbital of the Fe, and d
orbital of the TM are plotted with the brown, green and blue lines, respectively. With the exception of Fe-CuDA/GS, there were obvious orbital hybridizations among Fe(d), TM(d) and C(p), suggesting that the substrate can hold Fe-TM firmly through covalent bonding. In addition, the smallest binding energy of Fe-Cu on the substrate can be understood by non-orbital hybridization.
Fig. 3 PDOS of Fe-TMDA/GS
3.2 Analysis of reaction paths There are three recognized pathways of the NRR in the system of non-SACs, including distal, alternating, and enzymatic reaction mechanisms [3, 22]. However, previous studies [38-40] proved that the enzymatic reaction mechanism is dominant in the system of SACs. In detail, according to the theoretical research of Wang et al., the PDS of the NRR in the system of N1/C2N-h2D with the enzymatic reaction mechanism is the lowest [38]. Similarly, in the experimental and theoretical research of Liu et al., the lowest PDS of the NRR in the system of Fe1/N4-C is 1.69 eV, corresponding to the enzymatic reaction mechanism [39]. Furthermore, according to the theoretical research of Zhou et al., in the double atom system of Mo-TM/C (TM= B, N, P, S, Se, and Cl), the preferred reaction path is the enzymatic mechanism [40]. Therefore, here, the reaction paths of the NRR using the system of Fe-TMDA/GS were calculated according to the enzymatic mechanism. Before calculating the reaction path of nitrogen reduction reaction, the selectivity of catalyst between hydrogen evolution reaction (HER) and NRR was analyzed through comparing the adsorption Gibbs energy of H and N2 [41], as shown in Fig. S1. Obviously, the N2 will preferentially adsorb on the surface of Fe-TMDA/GS compared with H, and Fe-TMDA/GS have high selectivity for NRR through inhibiting the adsorption of H. According to the similarity of reaction paths in the system of Fe-TMDA/GS, we take Fe-TiDA/GS as an example (as shown in Fig. 4) to study the reaction path of the NRR, and the reaction paths of the NRR in other surfaces were plotted in Figs. S2 to S8. In Fig. 4, the sum of Gibbs energies of Fe-TiDA/GS, N2, and 6 H was selected as the zero point of energy. Consistent with the enzymatic reaction mechanism [39], the H+/e- pair was alternately added to the adsorbed nitrogen atoms, and the reaction cycle of the NRR was completed with the desorption of NH3.
Fig. 4 Configurations and Gibbs energy variation along the reaction path of NRR on Fe-TiDA/GS
To study the Gibbs energy variation of the NRR on different surfaces of Fe-TMDA/GS, Gibbs free energy diagrams of the NRR on Fe-TMDA/GS at zero and applied potential were calculated, as shown in Fig. 5. In addition, the detail information of free energy and activation barriers were summarized in Table S2 to S10. According to the Gibbs free energy variation of the NRR at zero potential, we can conclude that PDS of the NRR on Fe-TMDA/GS is the transfer process of the first H+/e- pair, labeled in red arrows. To facilitate the analysis of catalytic activity differences, the max Gibbs free energy variations (△Gmax) associated with the PDS were summarized in Fig. 6.
Fig. 5 Gibbs free energy diagrams of NRR on Fe-TMDA/GS at zero and applied potential
1.6
1.54
1.4
△Gmax (eV)
1.2
1.14 1.06
1.0
1.06
1.10 1.03
0.95 0.88
0.8
0.6
0.4
Fe-Ti Fe-V Fe-Cr Fe-Mn Fe-Fe Fe-Co Fe-Ni Fe-Cu
Fig. 6 Max Gibbs energy variation of NRR over Fe-TMDA/GS.
The larger the △Gmax of the NRR, the lower is the catalytic activity. Consequently, the catalytic activities of Fe-TiDA/GS and Fe-CuDA/GS are the highest and lowest cases, respectively, among the eight cases of Fe-TMDA/GS. The △Gmax ranges from 0.88 to 1.54 eV, indicating that we can regulate the doped metal atoms to tune the catalytic activity of the NRR. In the next discussion, we selected Fe-TiDA/GS as an example to study reaction details, such as bond lengths and charge variations. In Fig. 7(a), the zero point of charge was selected as the charge of Fe-TiDA/GS before N2 adsorption. The electron number variations of Fe-Ti dimer are all negative in the NRR, indicating that the Fe-Ti dimer acts as the electron donor in the NRR, which is consistent with the theoretical research of Chen et al. [12]. In detail, the number of electrons donated by the Fe-Ti dimer gradually increased in the transfer process of the first three H+/e- pairs and decreased in the transfer process of the last three H+/e- pairs. In most cases, the electron number variations of the substrate are also negative, suggesting that the substrate also acts as the electron donor. In detail, the increased electrons of intermediate (NxHy) are equal to the reduced electrons of Fe-Ti dimer and substrate. In Fig. 7(b), the bond lengths of N-N in the process of the NRR were obviously stretched compared with an isolated nitrogen molecule. In detail, there was a sharp increase of the N-N bond length in the transfer process of the third of H+ and
e- pairs, suggesting the breakage of the N-N band in this process, as shown in Fig. 4. Moreover, the electron number variation and bond length of Fe-Ti dimer are both constantly changing in the process of NRR, indicating that the active site is variable and the catalytic process is dynamic, which is similar to the dynamic catalytic process of gold nanoparticles [42, 43].
Fig.7 (a) Charge variation of Fe-Ti dimer and substrate (b) Bond lengths of N-N and Fe-Ti dimer.
3.3 Analysis of catalytic activity To evaluate the regulation effect of TM, the △Gmax of other single-atom Fe catalysts were listed in Table 2. Compared with other single-atom Fe catalysts, the △Gmax of Fe-TiDA/GS (0.88 eV) is obviously lower than that of other single-atom Fe catalysts (larger than 1 eV), indicating the catalytic activity of single-atom Fe catalysts in the NRR can be further improved via reasonable construction of the Fe-TM double-atom catalyst. To further evaluate the performance of catalyst, the △Gmax of other metal catalysts were listed in Table S1. The catalytic activity of Fe-TiDA/GS located at the medium level, indicating that catalytic activity should be further improved through other control mode of electronic structure, such as defect and doping other atoms.
Table 2 The △Gmax of PDS for NRR over different catalysts
Catalysts FeSA@SVG[44] FeSA/MoS2[24]
△Gmax (eV) 1.37 1.02
Reference Electrochim. Acta. 2018, 284, 392e399 Chem. Eur. J. 2017, 23, 8275 – 8279
FeSA/N4-C[39] FeSAN3[25] Fe3/θ-Al2O3(010)[23] FeSA/MoS2[45]
1.69 1.45 1.14 1.01
FeSA/Graphdiyne[8] FeSA/C2N Fe(110)[46] Fe-TiDA/GS
1.16 1.06 1.39 0.88
Nano Energy. 2019, 61, 420–427 J. Am. Chem. Soc. 2016, 138, 8706−8709 Nat. Comm. 2018, 9:1610 Phys. Chem. Chem. Phys. 2018, 20, 9248-9255 J. Phys. Chem. C 2019, 123, 19066−19076 Small Methods 2018, 1800291 Catal. Sci. Technol. 2019, 9, 174-181 This work
According to the Sabatier principle [47], a medium adsorption strength between reactants and catalyst surface is most suitable for the catalytic reaction. Therefore, the △Gmax versus adsorption energy of N2 for Fe-TMDA/GS were plotted, as shown in Fig. 8. An inverted volcano map of catalytic activity based on △Gmax and adsorption energy of N2 can be found, which can explain the catalytic activity order of Fe-TMDA/GS in the NRR. In addition, the data dot for single-atom Fe catalyst was added in Fig. 8, and it was represented by the purple circle. Obviously, Fe-TMDA/GS, expect for Fe-CuDA/GS, has higher catalytic activity than single-atom Fe catalyst, indicating that the regulation effect of TM improves the catalytic activity for NRR. 1.6 Cu
1.5
△Gmax (eV)
1.4
Fe@SVG
1.3 1.2
PDS=-2.58*Eads(N2)-1.35 R2=0.99
Cr
Co
1.1 1.0
Mn Ni
Fe V
0.9 0.8
Too Strong
Ti
PDS=3.99*Eads(N2)+4.29 R2=0.95 Too Weak
-0.95 -0.90 -0.85 -0.80 -0.75 -0.70 -0.65 -0.60 Eads(N2) (eV)
Fig. 8 △Gmax as a function of adsorption energy of N2.
3.4 Binding mechanism of N2
According to the above discussion of catalytic activity, the adsorption energy of N2 is crucial for the catalytic activity of the NRR in the system of Fe-TMDA/GS. Therefore, it is necessary to further study the bonding mechanism of N2 on the surface of Fe-TMDA/GS. According to our previous studies [29, 31], there are two main contributions for adsorption energy of N2, namely, orbital hybridization and electron transfer. To study the effect of orbital hybridization on the bonding mechanism of N2 on the surface, the projected density of states (PDOS) for the adsorbed system were plotted, as shown in Fig. 9. In Fig. 9, the density of states of the total, d orbital of Fe and p orbital of N are plotted in black, green and red lines, respectively. In the region near the Fermi level, there was no obvious hybrid peak between Fe(d) and N(p), indicating that the contribution of orbital hybridization to the bonding process of N2 on the surface can be neglected.
Fig. 9 PDOS of N2 and Fe in the adsorption system of N2
To study electron transfer between N2 and the substrate in the bonding process, the electron density difference of N2 adsorption on Fe-TMDA/GS was plotted, as shown in Fig. 10. In Fig. 10, the yellow and cyan colors denote the electron density accumulation and depletion regions, respectively. There is obvious electron transfer between N2 and Fe-TMDA/GS, suggesting that electron transfer acts as the dominant role in the process of N2 bonding. In detail, electron density depletion mainly appears in the region of the Fe atom and N-N bond, indicating that the N-N bond strength weakened and the Fe atom acts as an electron donor in the adsorption process. Electron density accumulation mainly appears in the region of the Fe-N bond, indicating that the Fe-N bond was strengthened with the electron transfer. Based on the analysis of PDOS and electron density difference, we conclude that electron transfer is the main factor in the adsorption process of N2 on Fe-TMDA/GS. Therefore, to explore the difference of catalytic activity of the NRR between Fe-TMDA/GS and single-atom Fe catalyst, the electron transfer of N2 was calculated, as shown in Fig. 11. Compared with the single-atom Fe catalyst, Fe-TMDA/GS has significant advantages in the electron transfer of N2, which accounts for the higher catalytic activity of Fe-TMDA/GS than that of single-atom Fe catalyst. To further investigate how the TM atom accelerate the electron transfer between N2 and graphene, the band structures of the eight Fe-TMDA/GS and single-atom Fe catalyst were calculated, as shown in Fig. 12. Compared with single-atom Fe catalyst, the band number near Fermi level in the system of Fe-TMDA/GS catalysts significantly increased and the band gap obviously decreased, suggesting that electron transfer is more likely to occur in the system of Fe-TMDA/GS. Furthermore, the work function (Φ) was calculated to evaluate the ability of electrons to escape from a solid surface, was shown in Fig. S9. The work function is defined as Φ = Ev - Ef, where Ev and Ef are the vacuum level Fermi energy, respectively, and the smaller the value of Φ, the more likely to lose electrons. From Fig. S9, the value of Φ for eight Fe-TMDA/GS catalysts obviously lower than that of single-atom Fe catalyst, indicating that Fe-TMDA/GS is more likely to provide electrons to N2. However, based on Fig. 12 and Fig. S9, Fe-CuDA/GS should have higher catalytic activity of NRR than single-atom
Fe catalyst, which is in contradiction to the calculated value of △Gmax, implying that catalytic activity of NRR should depend on other factors, such as interaction between catalyst and N2. Therefore, we calculated the d-band center of Fe-CuDA/GS and single-atom Fe catalyst to study the interaction between catalyst and N2. The d-band center of Fe-CuDA/GS (-1.98 eV) is farther from Fermi level than that of single-atom Fe (-0.97 eV), which can explain the above contradiction in the system of Fe-CuDA/GS and single-atom Fe catalyst. Therefore, we infer that the second metal atom improves the catalytic activity of NRR mainly through promoting electron transfer between N2 and the surface.
Fig. 10 Electron density difference of N2 adsorption on Fe-TMDA/GS catalysts (contour lines in plots are drawn at 0.003e/Å3 intervals). 0.45 0.41
Electron transfer (e)
0.40
0.39
0.39
0.38
0.38 0.35
0.35
0.35 0.33
0.30 0.25 0.20 0.15 0.12
0.10 0.05 0.00 FeTi
FeV
FeCr FeMn FeFe FeCo FeNi FeCu
Fig. 11 Electron transfer of N2 in the adsorption process.
Fe[43]
Fig. 12 The band structures of the eight Fe-TMDA/GS catalysts and single-atom Fe catalyst
4. Conclusions Eight kinds of double-atom Fe catalysts were constructed through doping transition metal atom on graphene substrates (Fe-TMDA/GS) and the geometry structures and electronic characteristics of these eight catalysts were studied. Reaction path, potential determining step, Gibbs free energy variation, catalytic activity and bonding mechanism of the nitrogen reduction reaction (NRR) in the system of Fe-TMDA/GS were systematically investigated through density functional theory calculations. The results indicate that the potential determining steps of the NRR on Fe-TMDA/GS are associated with the transfer process of the first H+/e- pair. Compared with the single-atom Fe catalyst, Fe-TMDA/GS has more obvious advantages for its lower Gibbs free energy variation for the potential determining step of the NRR. Fe-TiDA/GS has the highest catalytic activity for the NRR among the eight kinds of Fe-TMDA/GS considered here, with a Gibbs energy variation of 0.88 eV. Based on the
volcano map of catalytic activity, the catalytic activity order of Fe-TMDA/GS can be understood from the adsorption energy of N2. Electron transfer is the main factor affecting nitrogen adsorption, and the second metal atom in Fe-TMDA/GS improves the catalytic activity of the NRR by promoting electron transfer between N2 and the surface. Acknowledgements This work was supported by Beijing Natural Science Foundation (2182066), Natural Science Foundation of Hebei Province of China (B2018502067). IDG and CCW acknowledge support from the University of Calgary’s Canada First Research Excellence Fund (CFREF) program entitled the Global Research Initiative for Sustainable Low Carbon Unconventional Resources.
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Weijie Yang: Conceptualization, Writing - Original Draft, Investigation Hanyu Huang: Formal analysis, Visualization Zhao Ding: Validation, Data Curation Xunlei Ding：Supervision, Resources, Software Chongchong Wu: Writing - Review & Editing Ian D.Gates: Supervision, Writing - Review & Editing Zhengyang Gao: Supervision, Resources, Software, Funding acquisition
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: