Single-Metal Atom Anchored on Boron Monolayer - ACS Publications

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Single-Metal Atom Anchored on Boron Monolayer (#12) as an Electrocatalyst for Nitrogen Reduction into Ammonia at Ambient Conditions: A First-Principles Study haoran zhu, Yan-Ling Hu, Shi-Hao Wei, and Dayin Hua J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11696 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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The Journal of Physical Chemistry

Single-Metal Atom Anchored on Boron Monolayer (β12) as an Electrocatalyst for Nitrogen Reduction into Ammonia at Ambient Conditions: A First-Principles Study Hao-Ran Zhu,† Yan-Ling Hu,† Shi-Hao Wei,∗,† and Da-Yin Hua‡ Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo University, Ningbo, 315211, P.R. China, and Department of Physics, Faculty of Science, Ningbo University, Ningbo, 315211, P.R. China E-mail: [email protected]

Abstract Based on the first-principles calculation, single transition metal (TM) atoms of first and second transition series are anchored on the β12 phase of the boron monolayer (BM) electrocatalyst for the sustainable production of ammonia (NH3 ) by reducing nitrogen (N2 ). We have found a new type of electrocatalyst, i.e. V atom support on the β12 -BM (V/β12 -BM), which has a low onset potential (0.28 V), low cost, high stability, high selectivity and high efficiency under mild conditions. The difference charge density and local density of state (LDOS) are further demonstrated that there is a “acceptance-donation" interaction between TM atom and N2 and the ionization of 1π orbital of N2 can greatly elongate the N−N bond length, leading to ∗ To

whom correspondence should be addressed

† Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo University, Ningbo, 315211,

P.R. China ‡ Department of Physics, Faculty of Science, Ningbo University, Ningbo, 315211, P.R. China

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ACS Paragon Plus Environment

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the activity enhancement of N2 . We also point out that the bonding orbital (1π ) and antibonding orbital (1π ∗ ) of N2 play a crucial role in increasing the activity of N2 . Further, the CI-NEB calculation and MD simulation indicate that V/β12 -BM has highly dynamic and thermodynamic stability. Moreover, V/β12 -BM can also effectively suppress the hydrogen evolution reaction (HER) during the whole N2 reduction reaction (NRR) process. So we propose V/β12 -BM as an excellent and promising catalyst for N2 reduction to NH3 at ambient conditions.

Introduction Ammonia (NH3 ) is an essential chemicals that is widely used in agriculture and industry applications, particularly high energy density carrier and synthetic fertilizers. 1–4 Except for natural biological synthetic processes by nitrogenase enzymes, 5 the industrial NH3 production is predominantly synthesized via the Haber-Bosch process with Fe- or Ru-based catalysts, by which nitrogen (N2 ) and hydrogen (H2 ) molecules are processed at high pressures and temperatures, yielding roughly 500 million tons per year. 5 Due to the extreme inertness of N2 (N≡N bond energy of 940.95 kJ/mol) and carbon emission for producing the H2 precursor, the Haber-Bosch process accounts for ∼2% of global energy consumption as well as unavoidable CO2 emissions (approximate 2 tons of CO2 per ton of NH3 made). 5–8 Therefore, it is of great importance to develop an efficient and sustainable strategy for the activation and transformation of N2 under mild conditions. 8 The proton-assisted electrocatalytic N2 reduction reaction (NRR), ideally under ambient conditions, has been proposed as a sustainable alternative for nitrogen fixation and ammonia production which stem from N2 biological fixation with nitrogenase enzymes in bacteria that perform nitrogen fixation at room temperature and atmospheric pressure. 5,9–11 Single-atom catalysts (SACs) supported on nanosheet have become a promising way to improve their catalytic performance and show distinct selectivity of some special reactions, 6,12–16 however, reduced metal-particle size will make the SACs more prone to aggregation into metal clusters because of their high surface free energy. 12 A substrate that can deposit the individual metal atom firmly to prevent aggregation of metal atom is of great important for maintaining their 2


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superior performance. Currently, metal oxides, metal dichalcogenides, carbon-based nanomaterials, etc., have been investigated as the support for SACs. 12–17 Compared to other nanomaterials, 2D nanomaterials have a wealth of superiorities such as high specific surface areas, robust mechanical properties and easily fabricated, and so on. 11,18,19 Recently, 2D nanomaterials have been employed for a great deal of electrocatalytic processes (water-splitting reaction, CO2 reduction reaction, N2 reduction reaction, etc.). 19 And the boron monolayer (BM) with two types of borophene has been successfully grown on the Ag(111) surface under ultrahigh vacuum, one is a striped phase, the other is a homogeneous phase (β12 and

χ3 ). 20,21 On account of inherently metallic conductivity of BM, 22–24 it can be predicted to apply to the field of electrocatalytic H2 production 25 and superior electrode material for Li-ion, 26,27 and has the prospect of constructing nano-superconducting devices. 24,28 Considering the native porous structure for β12 and χ3 BM, this special characteristic makes them great potential to bind single metal atom tightly. 22–24 In this work, in view of the fact that β12 phase is thermodynamic more stable than other two phases (striped and χ3 phases), single transition metal atom (the atom of the first and second transition series) support on β12 phase is applied to NRR by employing first-principles calculation. Density functional theory (DFT) calculations demonstrate that SACs can effectively catalyze N2 reduction to NH3 , and V/β12 -BM is suggested as most efficient catalyst with low overpotential of 0.28 V, high thermal stability up to 800K, large diffusion barrier of 2.50 eV for V atom diffusion.

Computational Details The spin-polarized first-principles calculations are implemented in the Vienna Ab initio Simulation Package (VASP) 29,30 with projector-augmented-wave (PAW) pseudopotentials. 31 The generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) form 32,33 is used for simulating the electronic exchange-correction effect and a cutoff energy of 600 eV for the plane-wave expansion is adopted. The convergence criteria is 10−5 eV and 0.03 eV Å−1 for energy and force,

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respectively. To avoid interactions between two periodic units, a vacuum space with a thickness at least 20Å is set. Supercell consisting of 4×3×1 unit cells are used for geometrical optimization and static electronic structure calculation, and the Brillouin zones are sampled by Monkhorst-Pack k-point mesh with a 4×3×1 k-point grid. Also, explicit dispersion correction terms to the energy are employed through the use of the DFT-D2 method with the standard parameters programmed by Grimme and co-workers. 34 The climbing nudged elastic band method (CI-NEB) is applied to calculate the energy barrier. 36 In order to simulate the stability, we carry out Isobaric-isothernal (NPT) ensemble molecular dynamics (MD) simulation at 300 K and 800 K with time step of 2 fs by using Forcite code. 35 The binding energy (Eb ) of single metal atom adsorbed on β12 -BM is calculated by

Eb = ET M/β12 −BM − Eβ12 −BM − ET M

(1)

where ET M/β12 −BM , Eβ12 −BM and ET M are the energies of TM/β12 -BM, pure β12 -BM and single transition metal atom. The cohesive energy (Ecoh ) of metal crystal can be obtained by

Ecoh = (EM(bulk) − nET M )/n

(2)

where the EM(bulk) is the energy of metal crystal and n is the number of metal atoms in the crystal. The energy difference (∆Eb ) between Eb and Ecoh is defining as

∆Eb = Eb − Ecoh

(3)

SM ) of small molecule (SM) (including N and H) adsorbed on The adsorption energy (Eads 2

4


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TM/β12 -BM can be calculated by

SM Eads = ESM+T M/β12 −BM − ET M/β12 −BM − ESM

(4)

where ESM+T M/β12 −BM and ESM are the energies of SM adsorbed on TM/β12 -BM and isolated SM, respectively. For the NRR process, six net coupled proton and electron transfer (CPET) steps are needed to participate in (N2 + 6H+ + 6e− = 2NH3 ). According to previous theoretical studies, 37 for simplicity, gaseous H2 is employed as the source of protons due to its convenience to simulate the anode reaction (H2 = 2H+ + 2e− ), although different proton sources may affect the rate and yield of NH3 production. 38 Each CPET step involves the transfer of a proton coupled with an electron from solution to an adsorbed species on the surface of catalyst. The Gibbs free energy change of every elemental steps are calculated by using the standard hydrogen electrode (SHE) model proposed by Norskøv et al, 39,40 which the chemical potential of (H+ + e− ) pair is equal to half of the free energy of H2 . In addition, the NRR performance is evaluated by the reaction free energy change (∆G) through the spin-polarized calculation for each step via the following equation

∆G = ∆E + ∆EZPE − T ∆S + ∆GU + ∆G pH

(5)

Where ∆E is the different of adsorption energy of a given group, ∆EZPE and ∆S are the differences in the zero-point energy and the change of entropy, between the adsorbed state and the corresponding free-standing state, respectively, T is the temperature (T = 298.15 K). ∆GU is the contributions of electrode potential to shift the free energy ∆G at the applied electrode potential (U). ∆G pH is the correction of the H+ free energy by the influence of the H+ concentration, ∆G pH = 2.303 × kB T × pH ≈ 0.059 × pH, where kB is the Boltzmann constant and the value of pH is assumed to zero in this work. The zero-point energies and entropies of the NRR species are computed from the vibrational frequencies, in which only the adsorbate vibrational modes are calculated explicitly, 5



and catalyst sheet is fixed. The entropies and vibrational frequencies of molecules in the gas phase are taken from the NIST database.[http://cccbdb.nist.gov/]

Results and discussion Structure and Stabilities of TM/β12 -BM 2.02

Y

Zr

Nb Mo Tc

Ru Rh

Pd Ag Cd

1.6 1.2 0.8 ∆Eb (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4 0.00 -0.4 -0.8 first transition series

-1.2

second transition series

-1.6 -2 -2.0

Sc

Ti

V

Cr

Mn Fe

Co

Ni

Cu

Zn

Figure 1: The screening of a single transition metal atom support on β12 -BM with high stability, red square and blue triangle represent the elements of the first and second transition series, respectively. The negative ∆Eb means that the single transition metal atom can be adsorbed on the substrate steadily, and vice versa.

The optimized geometric and energy-band structures of β12 -BM are presented in Figure S1 in the Supporting Information. The optimized lattice parameters for β12 -BM are: a = 2.925 Å; b = 5.062 Å, and the optimized morphology is shown in Figure S1a, which are in good consistent with previous reported results. 21,22 Figure S1b shows the energy band of β12 -BM. Obviously, two bands cross the Fermi level, exhibiting a metallic band character, coinciding with previous results. 22,23 Due to its superior conductivity, it is anticipated that β12 -BM can be applied to electro-catalytic 41 2 reaction of NRR. On the basis of weak interaction between β12 -BM and N2 (EN ads =-0.09 eV),

the hexagonal holes of BM anchoring SACs are adopted to overcome this weak interaction. Taking into account the isolated transition metal atoms tend to aggregate into clusters, 12 the stability of

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SACs need to be access. The computational screening of a single transition metal atom (including the elements of the first and second transition series) supported on β12 -BM for NRR reaction are shown in Figure 1. From Figure 1, it is obvious that these structures of TM/β12 -BM are stable when TM = Sc, Ti, V, Mn, Co, Y, Zr. Therefore, SACs in this work are carried out around these seven atoms supported on β12 -BM.

Figure 2: (a) The N−N bond length and (b) the adsorption free energy (∆GN2 ) for N2 adsorbed on TM/β12 -BMs.

N2 Adsorption on TM/β12 -BM It is well established that N2 molecule adsorbed on the catalyst is the initial step for N2 activation in NRR reaction. The reaction free energy for N2 adsorbed on TM/β12 -BMs are shown in Figure 2a. For the same transition metal atom, the end-on patterns are more energy favorable than the side-on patterns. As for the end-on patterns, only the adsorption free energy of N2 (∆GN2 ) adsorbed on Sc/β12 -BM is greater than zero (∆GN2 =0.08 eV), which means that Sc/β12 -BM can’t 7



Figure 3: Difference charge density of N2 adsorbed on (a) V/β12 -BM and (b) Mn/β12 -BM, where the isosurface value is set to be 0.01 e/Å3 . The positive and negative charges are shown in yellow and cyan, respectively. Salmon, green and violet, blue balls represent the B, V, Mn and N atoms, respectively.

adsorb N2 for the end-on pattern spontaneously. As shown in Figure 2b, the N−N bond length of side-on patterns are longer than that of the end-on patterns. It means that the elongated bond length of N−N will weaken the N≡N triple bond, and then N atom is more easily to react with other molecules, indicating the signal for the activation of N2 : the longer the bond length, the greater the activity. In general, the end-on patterns are more favorable in energy, while the side-on patterns have more advantages in the activation of N2 . For the side-on patterns, only V/β12 -BM and Mn/β12 -BM can spontaneously adsorb N2 under mild conditions (owing to ∆GN2 < 0 eV), so the difference charge density of N2 adsorbed on these two systems are selected to illuminate the interaction between N2 and TM/β12 -BM (Figure 3). Because N2 molecule has a lone-pair elec8


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trons and transition metal atom has partially-occupied d-orbitals, it can be predicted that there is a “acceptance-donation" interaction between TM atom and N2 when N2 molecule is adsorbed on the TM/β12 -BM. As shown in Figure 3, the charge accumulation and depletion can be obviously observed for both N2 molecule and TM atom, which means that V (or Mn) atom will accept lonepair electrons of N2 and simultaneously donate electrons into the antibonding orbital of N2 . The net effect is that a charge density is accumulated around the N2 , making N−N bonds easily broken, which means the activity of N2 is improved. The Bader charge analysis confirm this conclusion (see Table S1), there is significant net charge transfer from TM atom to N2 molecule, about 0.20 and 0.33 electrons through end-on and side-on patterns for N2 adsorbed on V/β12 -BM, respectively. There are about 0.23 and 0.26 electrons transfer from Mn to N2 through end-on and side-on patterns for N2 adsorbed on Mn/β12 -BM, respectively.

Electrocatalytic NRR The electrocatalytic reaction of N2 reduction to NH3 on TM/β12 -BMs will be followed five possible reaction pathways as shown in Figure 4. Table 1 presents the minimum energy reaction pathway (MERP), the rate limiting step (RLS) and the corresponding onset potential (U) for the electrocatalytic reaction of N2 reduction to NH3 on TM/β12 -BMs, and the corresponding adsorption energy 2 (EN ads ) of N2 are also listed. For the sake of simplicity in this work, we only use the side-on ad-

sorbed N2 over V/β12 -BM as an example to illustrate the whole NRR reaction process. Gibbs free energy change (∆G) for N2 reduction to NH3 on V/β12 -BM is shown in Figure 5. First, one N2 2 molecule binds to V atom with EN ads of -0.63 eV (Table 1). When ∆EZPE and entropy are taken

into account, the ∆GN2 is equal to -0.24 eV. In this step, N2 molecule will be activated. Second, the activated N2 will be hydrogenated to ∗ N-∗ NH by adsorbing a proton coupled with an electron transfer. The Gibbs free energy uphill in this step is 0.28 eV. Subsequently, the H+ /e− pair will unceasingly attack the N atom of ∗ N-∗ NH species, resulting in the formation of either ∗ N-∗ NH2 or ∗ NH-∗ NH species with a free energy uphill of 0.23 and 0.14 eV for ∗ N-∗ NH2 and ∗ NH-∗ NH, respectively. It is indicated that ∗ NH-∗ NH formation is more thermodynamically feasible than 9



∗ N-∗ NH 2

formation.

Figure 4: Proposed schematic elementary reactions for N2 reduction to NH3 . There are five possible reaction pathways.

1.01

*N

0 0.0

(0.79)

(0.27) *N-*NH2

0.5

Free energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.00

* *N2 (-0.24)

-0.5

(-0.15) *NH2-*NH2

*NH-*NH *N-*NH (0.18) (0.04)

(-0.99) *+NH3

*NH-*NH2 (-0.67)

-1 -1.0

*NH (-1.11)

-1.5

-2 -2.0

(-2.09)

(-2.13)

*NH2

*NH3

-2.5 0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

Reaction Pathway Figure 5: Gibbs free energy change (∆G) for N2 reduction to NH3 on V/β12 -BM. The reaction pathway IV (the red one) is the minimum free energy pathway. The energy of the entrance, including the isolated V/β12 -BM and free N2 molecule, is set to zero as the reference.

As the further protonation, there are three competitive reaction pathways proceeding through ∗ N-∗ NH : 2

(I) ∗ N-∗ NH2 −→ ∗ N + NH3 (g) −→ ∗ NH −→ ∗ NH2 −→ ∗ NH3 10


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(II) ∗ N-∗ NH2 −→ ∗ NH-∗ NH2 −→ ∗ NH + NH3 (g) −→ ∗ NH2 −→ ∗ NH3 (III) ∗ N-∗ NH2 −→ ∗ NH-∗ NH2 −→ ∗ NH2 -∗ NH2 −→ ∗ NH2 + NH3 (g) −→ ∗ NH3 and two competitive reaction pathways proceeding through ∗ NH-∗ NH: (IV) ∗ NH-∗ NH −→ ∗ NH-∗ NH2 −→ ∗ NH + NH3 (g) −→ ∗ NH2 −→ ∗ NH3 (V) ∗ NH-∗ NH −→ ∗ NH-∗ NH2 −→ ∗ NH2 -∗ NH2 −→ ∗ NH2 + NH3 (g) −→ ∗ NH3 The reaction pathway IV for the protonation of ∗ N-∗ NH is the minimum free energy pathway with the maximal free energy uphill of 0.14 eV for ∗ NH-∗ NH, and the residual reaction steps are spontaneous processes, as shown in Figure 5. The equilibrium structures of V/β12 -BM in the minimum free energy pathway, i.e., the reaction pathway IV, are shown in Figure S2. The values of ∆G∗ NH−∗ NH , ∆G∗ NH−∗ NH2 , ∆G∗ NH , ∆G∗ NH2 , ∆G∗ NH3 are 0.14 eV, -0.85eV, -0.44eV, -0.98 eV, -0.04 eV, respectively. Those results not only mean that the high efficiency of the minimum free energy pathway but also demonstrate that the side-on adsorbed N2 may be reduced under a very low onset potential. Moreover, the reaction pathway I, III and V have the same highest free energy uphill of 0.52 eV for forming ∗ N, ∗ NH2 -∗ NH2 and ∗ NH2 -∗ NH2 , respectively, and the reaction pathway II has a free energy uphill of 0.23 eV for ∗ N-∗ NH2 . Therefore, regarding of the alternating mechanism and an onset potential (0.28 V), the reaction pathway I, III and V are not efficient enough, and the reaction pathway IV is the minimum free energy pathway. 2 Table 1: The N2 adsorption energy (EN ads , in eV), the minimum energy reaction pathway (MERP), the rate limiting step (RLS) and corresponding onset potential (U, in V) for the electrocatalytic reaction of N2 reduction to NH3 on TM/β12 -BMs.

end-on Ti/β12 -BM V/β12 -BM Mn/β12 -BM Co/β12 -BM Y/β12 -BM Zr/β12 -BM

2 ENads -0.66 -1.02 -1.24 -0.79 -0.54 -0.53

MERP ∗ N → ∗ N-NH → ∗ N-NH → ∗ N → ∗ NH → ∗ NH → ∗ NH 2 2 2 3 ∗ N → ∗ N-NH → ∗ N-NH → ∗ NH-NH → ∗ NH → ∗ NH → ∗ NH 2 2 2 2 3 ∗ N → ∗ N-NH → ∗ N-NH → ∗ N → ∗ NH → ∗ NH → ∗ NH 2 2 2 3 ∗ N → ∗ N-NH → ∗ N-NH → ∗ NH-NH → ∗ NH −NH → ∗ NH → ∗ NH 2 2 2 2 2 2 3 ∗ N → ∗ N-NH → ∗ N-NH → ∗ NH-NH → ∗ NH −NH → ∗ NH → ∗ NH 2 2 2 2 2 2 3 ∗ N → ∗ N-NH → ∗ NH-NH → ∗ NH-NH → ∗ NH → ∗ NH → ∗ NH 2 2 2 3

2 side-on ENads MERP V/β12 -BM -0.63 ∗ N2 → ∗ N-∗ NH → ∗ NH-∗ NH → ∗ NH-∗ NH2 → ∗ NH → ∗ NH2 → ∗ NH3 Mn/β12 -BM -0.52 ∗ N2 → ∗ N-∗ NH → ∗ N-NH2 → ∗ N → ∗ NH → ∗ NH2 → ∗ NH3

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RLS ∗ N → ∗ N-NH 2 ∗ N → ∗ N-NH 2 ∗ N → ∗ N-NH 2 ∗ N → ∗ N-NH 2 ∗ N → ∗ N-NH 2 ∗ N → ∗ N-NH 2

U 1.17 1.01 0.87 1.09 1.16 1.11

RLS U ∗ ∗ 2 → N- NH 0.28 ∗ N → ∗ N-∗ NH 0.83 2 ∗N


As shown in Table 1, for the end-on patterns, the MERP of Ti/β12 -BM and Mn/β12 -BM adopt the same reaction pathway: ∗ N2 → ∗ N-NH → ∗ N-NH2 → ∗ N → ∗ NH → ∗ NH2 → ∗ NH3 ; the MERP of V/β12 -BM and Zr/β12 -BM are ∗ N2 → ∗ N-NH → ∗ N-NH2 → ∗ NH-NH2 → ∗ NH → ∗ NH2 → ∗ NH3 and ∗ N2 → ∗ N-NH → ∗ NH-NH → ∗ NH-NH2 → ∗ NH → ∗ NH2 → ∗ NH3 , respectively; and the MERP of Co/β12 -BM and Y/β12 -BM proceed along ∗ N2 → ∗ N-NH → ∗ NH-NH → ∗ NHNH2 → ∗ NH2 -NH2 → ∗ NH2 → ∗ NH3 . The RLS of them all are initial protonation, indicating the activation of N2 is committed step. As to the side-on patterns, due to that only the adsorption free energy for N2 adsorbed on V/β12 -BM and Mn/β12 -BM are less than zero (Figure 2), Table 1 only shows the results of these two systems. For Mn/β12 -BM, as shown in Figure S2a, up to the initial protonation of N2 , two nitrogen atoms in N2 are directly adsorbed on Mn atom. However, in the subsequent elementary reaction steps, only one nitrogen atom in N2 is directly bonded on Mn atom, while the other nitrogen atom is as far away from the Mn atom as possible. This means that, after the initial protonation of N2 , the subsequent reaction steps is following the end-on pattern. The RLS is the initial protonation, and the free energy uphill is 0.83 eV. For V/β12 -BM, as shown in Figure S2b, during the whole reaction process, two nitrogen atoms in N2 are directly bonded to V atom, and the RLS is still the initial protonation with a free energy uphill of 0.28 eV. Moreover, the free energy uphill for the production of ∗ NH-∗ NH is equal to 0.14 eV. We notice that both free energy uphills (0.28 and 0.14 eV for the productions of ∗ N-∗ NH and ∗ NH-∗ NH, respectively) in the reaction pathway IV are far less than that of other systems (Table 1), and also better than a wealth of catalysts, (Figure S8), such as graphene, MoS2 , V3 C2 , MoC6 and so on,. 5,10,11,42–55 The Gibbs free energy change (∆GNNH ) of ∗ N-NH (for end-on pattern) or ∗ N-∗ NH (for side-on pattern) species with relation to ∆GN2 is shown in Figure 6. It can be found that ∆GNNH decreases almost linearly with the increasing of ∆GN2 . At the same time, ∆GNNH is also the free energy uphill of RLS for N2 reduction to NH3 on TM/β12 -BMs. It is interesting to find that the slope of the line for the side-on pattern is larger than that of end-on pattern, which means that the catalytic performance of the side-on pattern is better than that of end-on pattern when the adsorption free energies are similar. This is due to the fact that the 1π 1π ∗ orbital of N2 molecule has better contact

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1.5

Y

Zr

Co V

∆GNNH (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ti

1.01 Mn

0.5

Mn

Side-on adsorption End-on adsorption V

-0.75

-0.5 -0.50

∆GN (eV)

-0.25

0.00 0

2

Figure 6: Gibbs free energy change (∆GNNH ) of ∗ N-NH or ∗ N-∗ NH species as a linear function of ∆GN2 . The red and blue lines represent the end-on and side-on patterns, respectively.

with the d orbital of TM atom when N2 is adsorbed on TM/β12 -BMs by the side-on pattern, which leads to the result that more electron in the d orbital of TM can transfer to the 1π ∗ antibonding orbital of N2 , and the more efficient activation of N2 . To get further insight into the catalytic behavior of TM/β12 -BMs, the spin-polarized local density of state (LDOS) of free N2 molecule and TM/β12 -BM (TM = V, Mn) with adsorbed N2 molecule are calculated and presented in Figure 7. The molecule orbitals of free N2 molecule mainly including 2σ , 2σ ∗ , 1π , 3σ , 1π ∗ , 3σ ∗ around the Fermi level are shown in Figure 7a. Due to that transition metals have a coexistence of occupied and empty d orbitals, when N2 molecule is adsorbed on V/β12 -BM and Mn/β12 -BM, the d orbitals of TM will hybridize with s,p orbitals of N, which results in consequence that on one hand the empty d orbitals can accept the lone-pair electrons of N2 and on the other hand the occupied d orbitals can donated electrons into antibonding orbitals of N2 , weakening the N≡N triple bond. The “acceptance-donation" interaction mechanism and strong orbitals hybridization interaction between the d orbitals of TM and the s,p orbitals of N can easily be seen in Figure 7. When N2 molecule is adsorbed on V/β12 -BM with the end-on pattern, the strength and shape of the occupied 2σ ∗ 3σ and unoccupied 1π ∗ 3σ ∗ orbitals of N2 have changed dramatically, while the shape of 1π orbital of N2 almost remains unchanged (Figure 7b). At the

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Figure 7: Spin-polarized local density of state (LDOS) of (a) free N2 molecule, the adsorption of N2 on (b) V/β12 -BM and (c) Mn/β12 -BM through end-on pattern, and on (d) V/β12 -BM and (e) Mn/β12 -BM through side-on pattern, respectively. The brown regions represent the total LDOS of N2 , green and violet lines represent 3d orbital of V and Mn atom, respectively. The blue, salmon and grey balls in upper-right corner represent nitrogen, boron and TM (V, Mn) atom, respectively. The Fermi level is set to 0 eV.

same time, in the range from ∼-0.9 eV to 0.0 eV under the Fermi level, the empty orbitals of free N2 molecule are partially occupied. This means that the empty d orbitals can accept the lone-pair electrons of N2 while the occupied d orbitals can donate electrons into antibonding orbitals of N2 . For N2 molecule adsorbed on Mn/β12 -BM with the end-on pattern, except that the empty orbitals of free N2 molecule are partially occupied by more electrons, the rest of LDOS are similar to that of V/β12 -BM with the end-on pattern (Figure 7c). The Bader charge analysis confirm this conclusion: for the end-on pattern, the charge transfer from Mn/β12 -BM to the adsorbed N2 (0.23 |e|) is more than that of V/β12 -BM (0.20 |e|), as shown in Table S1. The charge transfer from TM/β12 -BM to

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the adsorbed N2 results in repulsive interaction between the two N atoms, so the N≡N triple bond is weaken. Therefore, for NRR reaction process, the onset potential of Mn/β12 -BM (0.87 V) is less than that of V/β12 -BM (1.01 V), as shown in Table 1, which means that the catalytic performances of Mn/β12 -BM is better than that of V/β12 -BM. When N2 molecule is adsorbed on V/β12 -BM with the side-on pattern, the 1π orbital of N2 splits greatly, and it’s strength also decreases (Figure 7d). At the same time, compared to LDOS of N2 adsorbed on V/β12 -BM with the end-on pattern, the empty orbitals of free N2 molecule are partially occupied by more electrons, as shown in 7d, in the range from ∼-0.9 eV to 0.0 eV under the Fermi level. This leads to a significant charge transfer from V/β12 -BM to the adsorbed N2 molecule. Bader charge analysis shows that the charge transfer increases from 0.20 to 0.33 |e| (Table S1). It is well known that the results of ultraviolet photoelectron spectroscopy analysis show that the ionization of 1π orbital in free N2 molecule will greatly affect the N−N bond length. As shown in Figure 3a, some electrons of 1π orbital in N2 are transferred to V/β12 -BM, which leads to the N−N bond length is greatly elongated. The decrease in LDOS strength for 1π orbital of N2 also confirms this (Figure 7d). Therefore, the activity of N2 is greatly improved, and the onset potential is drastically reduced from 1.01 V (for end-on pattern) to 0.28 V (for side-on pattern). This means that the catalytic performances of V/β12 -BM with the side-on pattern is greatly improved. We notice that the 1π orbital of N2 also splits greatly when N2 molecule is adsorbed on Mn/β12 -BM with the side-on pattern (Figure 7e). However, the charge transfer for N2 molecule adsorbed on Mn/β12 -BM with the side-on pattern slightly increases from 0.23 to 0.26 |e| (Table S1), while the partial occupation orbitals of N2 are very similar to that of the end-on pattern. So the catalytic performances of Mn/β12 -BM with the side-on pattern is slightly improved. This result can be confirmed by the change of the onset potential: the onset potential is decreased from 0.87 V (for end-on pattern) to 0.83 V (for side-on pattern), as shown in Table 1.

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Stability of V/β12 -BM Except for the catalytic performance, the stability of catalyst also plays a key role for the application of catalyst. The CI-NEB and MD methods are used to check the dynamical and thermal stabilities of V/β12 -BM. As shown in Figure S3, there is a large energy barrier of 2.50 eV when the V atom diffuses to the nearest hole, which indicates that the single V atom can bind to the

β12 -BM robustly, showing outstanding dynamical stability. The MD simulation for 100 ps with a time step 2fs is carried out at 300 K and 800 K, as shown in Figure S4 and S5, respectively. The geometric structure of V/β12 -BM has a slight amount of distortion, but it’s surface morphology is almost not changed, indicative of the favourable thermodynamic stability. As we all known, the hydrogen evolution reaction (HER) is the side reaction for NRR reaction process, 5 so we also calculate the adsorption free energy (∆GH ) for H adsorbed on the V atom in V/β12 -BM. The ∆GH is equal to 0.09 eV, which means that H atom cannot be adsorbed onto the V/β12 -BM stably. It is worth noting that, for the initial adsorption, the free energy change for N2 adsorption on V/β12 -BM (∆GN2 = -0.24 eV) is obviously lower than the free energy change for HER on V/β12 -BM (∆GH = 0.09 eV). It means that N2 adsorption is energy favorable than H adsorption. Therefore, when the minimum potential U (-0.28 V) has been applied toward NRR, N2 is more advantageous to occupy the active position, and NRR is easier to carry out than HER, indicating the superior selectivity of V/β12 -BM.

Conclusions In summary, based on the first-principles calculation, by anchoring single transition metal atom of first and second transition series on the β12 phase of the boron monolayer, We have designed a new type of electrocatalyst with low cost, high stability, high selectivity and high efficiency under mild conditions. By computing the free energy change of each elementary step during N2 reaction reduction, the V atom support on the β12 -BM (V/β12 -BM), which has the best ability for the activation of N2 among all TM/β12 -BMs, can provide amazingly low onset potential (0.28 V). The 16


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difference charge density and the spin-polarized local density of state for N2 adsorption on V/β12 BM and Mn/β12 -BM are further demonstrated that there is a “acceptance-donation" interaction between TM atom and N2 and the ionization of 1π orbital of N2 can greatly elongate the N−N bond length, leading to the activity enhancement of N2 . This is why V/β12 -BM for NRR reaction process has very low overpotential of 0.28 V. Besides, the CI-NEB calculation and MD simulation indicate that V/β12 -BM has highly dynamic and thermodynamic stability. Further, V/β12 -BM can also effectively suppress the hydrogen evolution reaction during the whole N2 reduction reaction (NRR) process. So we propose V/β12 -BM as an excellent and promising catalyst for N2 reduction to NH3 .

Acknowledgement This research is supported by the Natural Science Foundation of China (Grants NSFC 11375091), the K.C. Wong Magna Fund in Ningbo University. The computation is performed in the Supercomputer Center of NBU.

Supporting Information Available Details about the free energy and Bader charge of N2 adsorbed on the TM/β12 -BMs; geometric and band structure of β12 -BM; reaction process for the N2 reduction by TM/β12 -BM (TM = V, Mn) on the side-on pattern; diffusion barrier for single V atom on β12 -BM; AIMD simulation of β12 -BM with the adsorption of V atom; recent reports of overpotential for N2 reduction.

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