Enhanced N2-Fixation by Engineering the Edges of Two-Dimensional

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Enhanced N-Fixation by Engineering the Edges of Two-Dimensional Transition Metal Disulfides Feifei Li, Li Chen, Hongmei Liu, Dongchao Wang, Changmin Shi, and Hui Pan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04730 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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The Journal of Physical Chemistry

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Enhanced N2-fixation by Engineering the Edges of Two-dimensional Transition

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Metal Disulfides

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Feifei Li,a,b Li Chen,c Hongmei Liu,c Dongchao wang,c Changmin Shi, c* and Hui Pan a,d*

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aJoint

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Engineering, University of Macau, Macao SAR

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bState-owned

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cInstitute

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University, Linyi, Shandong, P. R. China

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dDepartment

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Macao SAR

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*Hui Pan: E-mail: [email protected]; Fax: +86 853-88222425; Tel: +86 853-88224427

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*Changmin Shi: E-mail: [email protected]

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Abstract

Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials

Assets and Laboratory Management, Linyi University, Linyi, Shandong, P. R. China

of Condensed Matter Physics, School of Physics and Electric Engineering, Linyi

of Physics and Chemistry, Faculty of Science and Technology, University of Macau,

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Design of novel catalysts for the reduction of N2 to ammonia has been urgently pursued because

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of various issues related to the industrial reduction technology. In the work, we perform

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first-principles calculations on the basis of density-functional theory (DFT) to control the edges of

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two-dimensional (2D) transition-metal disulfide (TMDs), including MoS2, WS2, VS2, NbS2, TiS2,

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and TaS2, for the achievement of optimal efficiency in nitrogen fixation. Our calculations show

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that nitrogen molecules prefer to stay at the bridge-on sites of the metal edges of TMD

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nanoribbons because of exothermic reactions. The calculated energy barrier at each step illustrates

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that VS2 has the lowest potential-determine-step (PDS) of 0.16 eV in distal pathway, leading to its

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best catalytic activity in N2 reduction reaction (NRR). Additionally, we find that the trend of 1

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catalytic activity of 2D TMD nanoribbons is following: VS2 > NbS2 > TiS2 > MoS2 > WS2 >

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TaS2. We show that the charge transfer is critical to the reduction reaction. We further

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demonstrate that the edges of TMDs, especially VS2, show a higher selectivity for NRR over

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hydrogen evolution reaction (HER) by investigating the competition between HER and NRR. Our

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findings not only reveal the effect of the edges of TMDs on NRR, but also provide theoretic

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support to the reported experimental results in literatures. It is expectable that the 2D TMD

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nanoribbons, especially VS2, may find application for efficient N2 fixation. At the same time, our

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work may guide the design of new catalysts for NRR.

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1. Introduction

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Ammonia (NH3) is an essential inorganic compounds in the world, which is widely used in making

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agricultural fertilizer, fibers, plastics, and so on.1-3 Industrial NH3 is synthesized from N2 and H2 in the

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presence of high pressure, high temperate and catalysts due to the inert triple N≡N bond,4 referred to the

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Harber-Bosch process.5, 6 To date, a lot of methods on nitrogen fixation have been developed to improve

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the efficiency and reduce the cost, such as, electrocatalytic, photocatalytic and biological methods.7-9

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Among them, the electrocatalytic nitrogen reduction reaction (NRR, N2 + 6H+ + 6e- →NH3) has

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attracted extensive attention,10, 11 because this process can occur at ambient condition and be controlled

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by electrocatalysts, applied potential, pH values, and electrolyte, etc..12

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Many transition-metal species (TM), such as Fe, Ru, Co, Mo-based complexes, have been used for N2

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reduction reaction,13-17 because their partially occupied d-orbital of TM can donate electrons to the empty

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π*-orbital of N2 and accept electron from its σ-orbital, leading to enhanced adsorption.18 However, the

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strong binding between N2 and TM was harmful to the protonation of nitrogen, leading to low efficiency

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N2-fixation.19 Two-dimensional (2D) transition-metal disulfides (TMDs), such as MoS2, MoSe2, and 2


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WS2, have triggered wide interests and application in hydrogen evolution reaction, carbon dioxide

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reduction reaction and oxygen reduction reaction.20-22 Recently, Sun and co-workers reported that

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defect and edge of 2D MoS2 nanostructure played important role on the N2-fixation and enhanced

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Faradic efficiency and NH3 yield could be achieved by creating defects and edges.23, 24However,

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the mechanism of the effect of the edges of 2D TMDs on the performance of N2-fixation is still

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unclear. For example, which kind edge plays positive role in N2-fixation? Can we optimize the

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catalytic performance by edge-engineering?

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In general, 2D TMD nanoribbons have two kinds of edges, armchair edge and zigzag edge,25

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where zigzag edge is favorable in energy and easy to be formed.26, 27 In this work, we focus on the

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effect of zigzag edge on the efficiency of N2-fixation. Six 2D TMDs, including MoS2, WS2, VS2,

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NbS2, TiS2, and TaS2, are systematically investigated by first-principles calculations. Our

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calculation shows that N2 can easily adsorb on the edge of XS2 (X=Mo, W, V, Nb, Ti, and Ta)

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with 2H phase and the bridge-on adsorption is more energy favorable. VS2 show the best NRR

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catalytic activity with the PDS of 0.16eV. Furthermore, the Bader charge analyses show the

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relationship between charge transfer and free energy.

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2. Computational method and models

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2.1 Calculation methods

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All calculations were performed using Vienna ab initio simulation package (VASP) on the basis

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of density-functional theory (DFT).28, 29 The generalized gradient approximation pseudopotential

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with Perdew-Burke-Ernzerhof (GGA-PBE) formation was used to describe the electron-electron

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exchange and correlation energy.30 The valence electrons are 4s24p64d55s1, 4s24p64d45s1,

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5p65d36s2, 3s23p63d44s1, 3s23p63d24s2 and 5s25p65d44s2 for Mo, Nb, Ta, V, Ti and W, 3



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respectively. The cut-off energy of 500 eV for the plain-wave basis sets was adopted and the

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convergence threshold was 10-4 eV per atom. A 7×1×1 Monkhorst-pack k-point mesh was used

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for structural optimizations. DFT-D2 method was used to evaluate van der Waals (vdW)

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interaction in the systems.31 To model the zigzag edges of XS2 (X=Mo, W, V, Nb, Ti, and Ta),

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their nanoribbon slabs with 16 X and 32 S atoms are used in our calculations (Fig. 1). The vacuum

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was set to 12Å and 15Å along b- and c-directions, respectively, to avoid the interlayer interaction

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between two periodic units. Then, the structures are fully relaxed without other restrictions.

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Fig. 1 The geometry of the zigzag edge of XS2.

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The adsorption energy is defined as: Eads = E(XS2+N2)-E(XS2)-E(N2)

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where E(XS2+N2), E(XS2) and E(N2) are the total energies of XS2 nanoribbon with N2 adsorbed,

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pure XS2 nanoribbon and N2, respectively.

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The reaction energy (ΔE) is defined as: ΔE=E(XS2+N2Hn)- E(XS2+N2Hn-1)- ½E(H2)

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where E(XS2+N2Hn), E(XS2+N2Hn-1) and E(H2) are the total energies of XS2 nanoribbon with N2

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and n adsorbed H atoms, XS2 nanoribbon with N2 and n-1 adsorbed H atoms and H2 gas,

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respectively.

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The Gibbs free energy (ΔG) was calculated using the following equation: 32 ΔG =ΔE+ΔZPE 4


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– TΔS, where ΔE is the reaction energy, ΔZPE is the difference in zero-point energies (ZPE), T is

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the temperature (T=298K), and ΔS is the difference in entropy and calculated from vibrational

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frequency calculation. For simplicity, ZPE and entropy (TS) were only calculated at Γ-point with

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the catalyst fixed. The values of ZPE and entropy (TS) for H2 and N2 were obtained from standard

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molecular table. 33, 34 The Gibbs free energy change (ΔG) for each elemental step as a function of

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the electrode potential (U) can be calculated by ΔG(U) = ΔE + ΔZPE – TΔS + eU = ΔG(U=0) +

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eU, where U is the electrode potential respect to the standard hydrogen electrode. The limiting

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potential (UL) is calculated as UL = -ΔGmax/e. A smaller UL value indicates a better NRR.

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3. Results and discussion

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3.1 Nitrogen adsorption

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Fig. 2 The optimized configurations of N2 bonded on the edge of XS2.

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The adsorption of N2 on the surface of catalyst is the first step for the N2-fixation.35,36

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Therefore, the initial adsorption configuration is important for the following reaction pathways

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and investigated accordingly first.37 N2 is difficult to adsorb on the S edge, but can be chemisorbed

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on the metal (X) edge with three possible configurations, including end-on, side-on and bridge-on

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(Figure 2). The adsorption energies for the chemisorption (Table1) are all negative, indicating that

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these reactions are exothermic. Especially, the bridge-on adsorption is more energy favorable

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than other adsorption configurations. The calculated N-N bond lengths and charge transfers 5



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between N2 and XS2 (Figure 3) show that the N-N bond is elongated by 7.2 to 17.1 % than free N2

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(1.11 Å) for all the configurations. In general, the adsorption on the bridge-on pattern shows the

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largest extension of the N-N bond length for all XS2(X=Mo, W, V, Nb, Ti, and Ta). The extension

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of the triple N-N bond indicates the initial activation of N2 in N2-fixation 38, 39. The analysis of

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Bader charge shows that electrons transfer to N2 molecule for all adsorption configurations.

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Consistent with the extension of bond length, N2 gets more electrons at the bridge-on site than

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other adsorption configurations, which can also be confirmed by the charge density differences

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(Fig.4 (a)).

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Fig. 3 The charge transfers (a) and N-N bond length (b) of N2 bonded on the XS2 edge with

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different configurations.

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Table 1 Adsorption energies of N2 adsorbed on XS2 with different configurations (eV). end

side

bridge

MoS2

-1.24

-0.92

-1.72

WS2

-1.30

-1.20

-2.05

VS2

-0.68

-0.19

-0.71

NbS2

-0.30

-0.28

-0.96

TiS2

-0.38

-0.07

-0.86

TaS2

-0.76

-0.51

-1.50

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To get a deep insight into the bonding nature of different adsorption pattern, the projected

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electronic densities of states (PDOSs) of the 2D TMD nanoribbon with N2 adsorbed were

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calculated. Here, VS2 (Fig.4 (b)) is discussed as an example. The electronic structure of N atom is

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2s22p3. The number of the electrons in s and p orbitals are obtained by integrating the PDOS

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below the Fermi energy. The transferred electrons in every orbital are listed in Table S1. We see

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that the electron transfer mainly occurs in p orbitals. Further analysis shows that both of the pz and

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py orbitals gain electrons and px loses electrons, indicating that the charge transfer mainly takes

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place in the 2σ and 2π* orbitals near the Fermi level. After N2 adsorbed on the V edge of VS2

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nanoribbon, the overlaps of PDOSs are mainly from V-3d and N-2p coupling. Moreover, electrons

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transfer from the V-3d orbital to the empty N-2π*, leading to partially occupied 2π*. We see that

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the overlaps around the Fermi level are different for different adsorption configurations. For the

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end-on pattern, some electrons transfer from V-to-N2 (3d→2π*, mainly to N_2py and N_2pz) and

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some electrons from N2-to-V (2σ→3d, mainly from N_2px), which are named π-backdonation to

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N2 and σ-donation from N2, resulting in weakened triple N-N bond. Comparing the PDOSs of the

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side-on and bridge-on configurations, we find that the overlaps below the Fermi energy are one N

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orbital for side-on (N-pz), but two (N-py and N-pz) for bridge-on. As shown in Table S1, the

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charges donate to N-pz for side-on, but N-py and N-pz for bridge-on because only one metal atom

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is connected to N2 and overlaps well with one of orbitals for the side-on adsorption. When N2

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interacts with two metal atoms, it is possible to have two orbitals overlapped, resulting in

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increased charge transfer and extended N-N bond length. Based on the above analysis, the

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bridge-on site is the preferred adsorption site for activating N2. Clearly, we see that the more

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charge transfer, the larger extension of the N-N bond length. Therefore, the bridge-on

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configuration with the largest N-N bond length, more charge transfer, and favorable energy, is

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selected as the initial configuration for N2 adsorption in the following NRR. 7



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Fig.4 (a) Charge density difference of N2 adsorbed on the edge of VS2 with different pattern. The

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value of isosurface is 0.004e/Å. (b) PDOS of free N2, 3d orbital of V and their interaction with

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different patterns.

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3.2 Nitrogen Reduction

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Fig. 5 The two possible ways for N2 fixation to NH3 on XS2.

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Fig.6 Free energy diagrams for NRR at electrode potenatial U = 0 V on: (a) VS2, (b) NbS2, (c) MoS2, (d) TiS2, (e) TaS2, and (f) WS2.

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Generally, there are three possible reaction pathways for N2 reduction to NH3, including distal,

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alternating and enzymatic ways.40 As the bridge-on adsorption shows the largest N-N bond length

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and adsorption energy, we only consider the distal and enzymatic pathways in following

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discussion (Fig.5). In the distal pathway, H adsorbs on one N atom until the N-N bond breaks in

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4th step with the formation of *N and NH3; then, H atoms continue to adsorb on another N atom to 9



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form the second NH3. In the enzymatic pathway, H atoms adsorb on the two N atoms

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alternatively, and the N-N bond breaks in 5 or 6th step to form 2*NH2 or *NH2+*NH3. The Gibbs

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free energies for each reaction step in N2 fixation on XS2 edge were calculated to evaluate their

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catalytic performances (Fig. 6). For the N2-fixation on the V edge of VS2 nanoribbon (Fig. 6a), the

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adsorption energy at the first step is very strong (-0.71 eV). After considering the ZPE and TS, the

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free Gibbs energy is calculated to be -0.17eV because of the binding of hydrogen from the gas

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phase.33 At the same time, the triple bond is extended from 1.11Å to 1.20Å, indicating the

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activation of the N2 molecule. At the 2nd step, *NN + H+ + e- → *NNH, an uphill barrier with a

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small energy of 0.13 eV is required. At the 3rd step, there are two paths, that the distal and

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enzymatic pathways. The *NNH can be further protonated into *NNH2 or *NHNH with Gibbs

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energy changes (ΔG) of 0.15 or -0.23 eV for the distal and enzymatic pathways, respectively,

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indicating the enzymatic path is easy to happen. For the enzymatic pathway, the values of ΔG are

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0.01, 0.58, -2.33, and 0.16 eV for the 4-7th steps, respectively. We see that the formation of

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*NH2NH2 at the 5th step needs the largest free energy of 0.58 eV, which is the potential-limiting

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step (PDS). For the distal pathway, the reactions at the 4th, 5th and 6th steps are downhill with a ΔG

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of -0.64, -1.06, and -0.42 eV, respectively. Especially, the N-N bond breaks with the energy of

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-0.64 eV at the 4th step. The reaction at 7th step, *NH2+*NH3→2*NH3, needs the largest ΔG (0.16

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eV). By comparing the energy at each step for both pathways, we see that the distal pathway is

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more energy favorable than enzymatic pathway.

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From the calculated energy barriers for all considered nanoribbons (Fig. 6), the PDS of the

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distal pathway for VS2, NbS2 MoS2, TiS2, WS2, and TaS2 are 0.16, 0.41, 0.53, 0.47, 0.94, and 1.0

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eV, respectively. The PDS of enzymatic pathway for VS2, NbS2 MoS2, TiS2, WS2, and TaS2 are

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0.59, 0.41, 0.48, 0.58, 0.94 and 1.0 eV respectively. Clearly, the PDS increases in the trend of: 10


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VS2 (0.16 eV) ＜ NbS2 (0.41 eV) ＜ TiS2 (0.47 eV) ＜ MoS2 (0.48 eV) ＜ WS2 (0.94eV) ＜

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TaS2 (1.0 eV). We see that VS2 shows the best catalytic activity due to the lowest PDS among the

3

considered systems. In addition, the NH3 desorption is critical for the active site recycling in the

4

next catalytic cycle. The NH3 desorption (*NH3 ↔* + NH3 (g)) process is a thermodynamic

5

equilibrium process because it is not driven by voltage and also not involved in the proton and

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electrons transfer, where it is determined by the thermal energy. The energies of NH3 desorption

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are 1.13, 0.79, 1.96, 1.05, 1.12, and 1.54 eV for VS2, NbS2 MoS2, TiS2, WS2, and TaS2,

8

respectively. According to previous reports, the NH3 desorption is usually endothermic, but can be

9

easily overcome by the released energy at the protonation step.41 Also, under strong acidic

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conditions (pH=0), *NH3 can be easily soluble in aqueous electrolyte.42 Therefore, the catalyst can

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be recycled successfully.

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Fig.7 Charge transfer at each step for VS2 and TaS2 via (a) Distal and (b) Enzymatic pathways.

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Free energy difference at each step for VS2 and TaS2 via (c) Distal and (d) Enzymatic pathways.

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The charge transfer plays an important role in catalytic performance.43-45 To reveal the

16

mechanism, we calculate the charge transfers at each reaction step by Bader charge analysis (Figs.

17

7 and S1). The ΔQ is the electrons difference of the N atom between the present and previous 11



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steps, and a positive value indicates that the N atom gains electrons. As discussed above, VS2 and

2

TaS2 show the best and poorest catalytic performance, respectively, which are used as examples in

3

following discussion on the charge transfer (Fig. 7). At the 1st step, ΔG is -0.17 and -0.96 eV for

4

VS2 and TaS2, respectively, which corresponds to a ΔQ value of 0.66e and 1.01e, respectively. In

5

the following reduction steps, for example, the Gibbs free energy (ΔG) at the 7th step

6

(*NH2+*NH3 → 2*NH3) for VS2 (0.17 eV) is lower than that for TaS2 (1.0 eV). At the same time,

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we see that the N atom gains electrons on VS2 (0.07 e), while it losses electrons on TaS2 (-0.1 e).

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As shown in Fig.S1, the Gibbs free energy (ΔG) for the steps in enzymatic pathway,

9

2→3(*NNH→*NHNH),

4→5(*NHNH2→2*NH2),

5→6(2*NH2→*NH2+NH3),

and

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6→7(*NH2+*NH3→2*NH3) show approximately linear relationship with the charge transfer

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(ΔQ). Clearly, the charge transfer plays an important role in tuning catalytic performance of

12

catalysts in NRR. If the N atoms gain more electrons, the reaction is easier to happen at this step.

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3.3 Competition with the hydrogen evolution reaction

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The hydrogen evolution reaction (HER) is a major competing reaction with NRR on the surface

15

of catalyst. In order to understand the competition behavior between HER and NRR, we calculated

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the free energies of adsorbed N2 molecule (*N2) and H adatom (*H). As shown in Fig.8 (a), the

17

values of ΔG (*N2) are more negative than ΔG (*H) in the *N2 favorable region. For example, the

18

value of ΔG (*N2) is -0.17 eV for VS2, which is more negative than ΔG (*H) (0.26 eV), indicating

19

that the edge of VS2 prefers to be covered by N2 rather than H. Therefore, HER will be

20

suppressed, indicating the superior selectivity of VS2 for NRR. To further evaluate the selectivity

21

of NRR, the value differences between UL(NRR) and UL(HER) are calculated (Table 2). The more

22

positive of UL(NRR)- UL(HER) indicates a higher selectivity of NRR over HER. As shown in

12


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Table 2, MoS2 and VS2 show positive UL(NRR)- UL(HER) and, therefore, have the most

2

selectivity for NRR over HER. However, the edge of MoS2 will be more likely to be covered by

3

H, which would hinder its NRR performance. Therefore, VS2 shows a much higher selectivity on

4

NRR.

5 6

Fig.8 (a) Comparison of the Gibbs free energies of adsorbed N2 molecule (*N2) and H adatom

7

(*H). The *H favorable region indicates ΔG(*H) ＜ΔG(*N2). The *N2 favorable region indicates

8

ΔG(*N2)＜ΔG(*H). (b) calculated adsorption free energy diagram of hydrogen evolution for XS2

9

at U=0.

10

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Table 2 Calculated limiting-potential for NRR and HER. UL(NRR)(V)

UL(HER)(V)

UL(NRR)- UL(HER)(V)

VS2

-0.16

-0.26

0.10

NbS2

-0.41

-0.15

-0.26

TiS2

-0.47

-0.08

-0.39

MoS2

-0.48

-1.30

0.82

TaS2

-1.0

-0.77

-0.23

WS2

-0.94

-0.72

-0.22

4. Conclusion

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In summary, we systematically investigate the N2 reduction reaction on the edge of XS2(X=Mo,

13

W, V, Nb, Ti, and Ta) based on DFT calculations. We show that the bridge-on sites at the metal

14

edges of TMD nanoribbons is much more favorable to adsorb N2 molecules. By calculating the 13



Page 14 of 29

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energy barrier at each step, we find that VS2 has the best catalytic activity in NRR because of the

2

lowest limiting potential (-0.16V). We also show that the catalytic activity of 2D TMD

3

nanoribbons has the following trend: VS2 > NbS2 > TiS2 > MoS2 > WS2 > TaS2 because of the

4

increase PDS with the trend. By comparing the free energies of adsorbed N2 molecule and H

5

adatom and investigating the competition between NRR and HER, we predict that VS2 show a

6

higher selectivity on NRR. Our findings demonstrate that the edges of TMDs are critical to NRR

7

and may be responsible for the reported phenomena in literatures. We expect that 2D TMD

8

nanoribbons, especially VS2, may find application for efficient N2 fixation and our work may

9

provide guidance on the design of new catalysts for NRR.

10

Supporting Information

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Charge transfer between free N2 and adsorbed N2 on VS2, Relationship of charge transfer and free

12

energy difference in the steps for enzymatic pathway are available in Supporting Information.

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Conflicts of interest

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The authors declare no competing financial interest.

15

Acknowledgements

16

Hui Pan is thankful for the support of the Science and Technology Development Fund from Macau

17

SAR (FDCT-0102/2019/A2) and Multi-Year Research Grants (MYRG2017-00027-FST and

18

MYRG2018-00003-IAPME) from Research & Development Office at University of Macau.

19

This study was also supported by Natural Science Foundation of Shandong Province, China

20

(ZR2019BA027, ZR2017QA003), National Natural Science Foundation of China (No. 51431004,

21

11634007, 11804006).

22

References 14


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[1] Yan, D.; Li, H.; Chen, C.; Zou, Y.; Wang, S. Defect engineering strategies for nitrogen

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TOC Graphics

3

21



Fig.1 The geometry of the zigzag edge of XS2. 49x69mm (300 x 300 DPI)


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Fig.2 The optimized configurations of N2 bonded on the edge of XS2. 75x38mm (300 x 300 DPI)



Fig. 3 The charge transfers (a) and N-N bond length (b) of N2 bonded on the XS2 edge with different configurations. 149x52mm (300 x 300 DPI)


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Fig.4 (a) Charge density difference of N2 adsorbed on the edge of VS2 with different pattern. The value of isosurface is 0.004e/Å. (b) PDOS of free N2, 3d orbital of V and their interaction with different patterns. 149x105mm (300 x 300 DPI)



Fig.5 The two possible ways for N2 fixation to NH3 on XS2. 85x81mm (300 x 300 DPI)


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Fig.6 Free energy diagrams for NRR at electrode potenatial U = 0 V on: (a) VS2, (b) NbS2, (c) MoS2, (d) TiS2, (e) TaS2, and (f) WS2. 149x176mm (300 x 300 DPI)



Fig.7 Charge transfer at each step for VS2 and TaS2 via (a) Distal and (b) Enzymatic pathways. Free energy difference at each step for VS2 and TaS2 via (c) Distal and (d) Enzymatic pathways. 246x161mm (300 x 300 DPI)


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TOC Graphic: The calculated energy barrier at each step illustrates that VS2 has the lowest potentialdetermine-step (PDS) of 0.16eV in distal pathway, leading to its best catalytic activity in NNR. 83x39mm (300 x 300 DPI)


Enhanced N2-Fixation by Engineering the Edges of Two-Dimensional

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