Single Mo Atom Supported on Defective Boron Nitride Monolayer as

Aug 11, 2017 - The molecular electrocatalysts that contain Fe–, Co–, and Mo–Nx complexes as mentioned above are promising catalysts for N2 fixat...
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Single Mo Atom Supported on Defective Boron Nitride Monolayer as an Efficient Electrocatalyst for Nitrogen Fixation: A Computational Study Jingxiang Zhao*,† and Zhongfang Chen*,‡ †

Key Laboratory of Photonic and Electronic Bandgap Materials, Ministry of Education, and College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin, 150025, China ‡ Department of Chemistry, University of Puerto Rico, Rio Piedras Campus, San Juan, PR 00931, United States S Supporting Information *

ABSTRACT: The production of ammonia (NH3) from molecular dinitrogen (N2) under mild conditions is one of the most attractive and challenging processes in chemistry. Here by means of density functional theory (DFT) computations, we systematically investigated the potential of single transition metal atoms (Sc to Zn, Mo, Ru, Rh, Pd, and Ag) supported on the experimentally available defective boron nitride (TM−BN) monolayer with a boron monovacancy as a N2 fixation electrocatalyst. Our computations revealed that the single Mo atom supported by a defective BN nanosheet exhibits the highest catalytic activity for N2 fixation at room temperature through an enzymatic mechanism with a quite low overpotential of 0.19 V. The high spinpolarization, selective stabilization of N2H* species, or destabilizing NH2* species are responsible for the high activity of the Moembedded BN nanosheet for N2 fixation. This finding opens a new avenue of NH3 production by single-atom electrocatalysts under ambient conditions.

1. INTRODUCTION Nitrogen fixation, which can convert the abundant dinitrogen (N2) in the Earth’s atmosphere to ammonia (NH3), is crucial to sustain all forms of life because nitrogen is required to biosynthesize basic building blocks of plants, animals, and other life forms.1 Also, NH3 synthesis from atmospheric nitrogen in the industrial Haber−Bosch process for N2 fixation is primarily used in production of fertilizers2,3 that directly enables the population growth, and thus is considered as the most important invention of the twentieth century.4 Currently, roughly 500 million tons of NH3 per year is produced by the Haber−Bosch process,5 which requires extreme reaction conditions, such as high temperatures and high pressures of its reactant N2 and H2 gases.6 The electrochemical N2 reduction reaction (NRR, N2 + 6H+ + 6e− → 2NH3), which originates from N2 biological fixation with nitrogenase enzymes in bacteria, is rather promising for N2 fixation because it can occur at ambient temperature and pressure.7,8 Especially, the NH3 product can be easily separated from hydrogen feed gas, and the NRR process can be effectively tuned by changing the operating potential, the electrolyte, the pH, etc., thus greatly improving the production yield of ammonia.9 The Fe-, Co-, and Mo-based materials are the widely used catalysts to activate the inert N2.7,10−32 For example, Nishibayashi’s group10,12,19,22,24 proposed that the transition metal−dinitrogen (TM−N2, TM = Fe, Co, and Mo) complex bearing an anionic PNP-pincer (PNP = 2,5© 2017 American Chemical Society

bis(ditertbutylphosphinomethyl)pyrrolide)) ligand can serve as an effective catalyst toward the catalytic nitrogen fixation under mild reaction conditions. The molecular electrocatalysts that contain Fe−, Co−, and Mo−Nx complexes as mentioned above are promising catalysts for N2 fixation and conversion, in which suitable organic ligands are essential to confine the TMs to essure their good stability and high activity.7,26,29 Compared to the molecular electrocatalysts, heterogeneous electrocatalysts possess better durability and can be more easily incorporated into functional energy conversion devices such as fuel cells and electrolyzers.33,34 Therefore, incorporating Fe−, Co−, and Mo−Nx moieties into the surface to construct well-defined and tunable active sites that inherit the advantages of both molecular and heterogeneous catalysts is highly desirable for N2 fixation under ambient conditions. Notably, the impressive advances in large-scale DFT computations not only facilitate the search of efficient catalysts for N2 reduction but also gain deeper insights into the NRR mechanism.26,35 For example, the widely used Fe-based catalysts for NH3 synthesis in the Haber−Bosch process were obtained by testing more than 2500 compounds and 6500 experiments.36,37 For comparison, by examining the adsorption energies of NRR intermediates on transition metal surfaces, Received: May 19, 2017 Published: August 11, 2017 12480

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employed (Table S1). The van der Waals interactions were described using the empirical correction in the Grimme scheme,69 and the density functional semicore pseudopotential (DSPP) was adopted for the relativistic effects of TM atoms, in which the core electrons are replaced by a single effective potential and some degree of relativistic corrections are introduced into the core.70 The double numerical plus polarization (DNP) was chosen as the basis set for other elements, whose accuracy can be comparable to that of Pople’s 6-31G** basis set.71 Self-consistent field (SCF) calculations were performed with a convergence criterion of 10−6 au on the total energy and electronic computations. To ensure high quality results, the real-space global orbital cutoff radius was chosen as high as 5.2 Å. Because of the high computational cost, the bulk water layer was not considered because it only slightly stabilizes the NRR intermediates via hydrogen bonding (∼0.10 eV).72 Instead, a conductor-like screening model (COSMO) was used to simulate a H2O solvent environment,73 whose dielectric constant was set as 78.54. Other solvents, such as THF and Et2O for NRR, are also used for NRR, which will affect the amount of produced ammonia.74 To model the defective BN monolayer with boron monovacancy, we first built a periodic supercell containing 16 boron and 16 nitrogen atoms with a vacuum of 15 Å in the z-direction, and then, one boron atom was removed to create a B monovacancy to provide an anchoring site for a single TM atom, in which all of the atoms were fully optimized without any constraints. In order to examine the validity of computations in a 4 × 4 supercell, we recalculated the adsorption energies of some adsorbates on the BN nanosheet in a 5 × 5 supercell. The rather small differences in the computed adsorption energies by using 4 × 4 and 5 × 5 supercells (Table S2) ensure the reliability of the conclusions in this work. The k-point sampling of the Brillioun zone was done using a 5 × 5 × 1 grid centered at the gamma (Γ) point for geometry optimizations, while a 12 × 12 × 1 k-points grid was used for electronic structure computations. The Hirshfeld charge analysis was performed to compute the charge transfer and magnetic moment.75 The N2 dissociation minimum energy path (MEP) was obtained by LST/QST tools in DMol3 code.76 Six net coupled proton and electron transfer (CPET) steps are involved in the NRR process (N2 + 6H+ + 6e− → 2NH3). According to previous theoretical studies,72 for simplicity, gaseous H2 was employed as the source of protons due to its convenience to simulate the anode reaction (H2 ↔ 2(H+ + e−)), although different proton sources may affect the rate and yield of NH3 production.77 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 (ΔG) of every elemental step was calculated by using the standard hydrogen electrode (SHE) model proposed by Nørskov et al.,78−80 which uses one-half of the chemical potential of hydrogen as the chemical potential of the proton−electron pair. According to this method, the ΔG value can be determined as follows: ΔG = ΔE + ΔZPE − TΔS + ΔGU + ΔGpH, where ΔE is the electronic energy difference directly obtained from DFT calculations, ΔZPE is the change in zero-point energies, T is the temperature (T = 298.15 K), and ΔS is the entropy change. ΔGU is the free energy contribution related to electrode potential U. ΔGpH is the correction of the H+ free energy by the concentration, which can be determined as ΔGpH = 2.303 × kBT × pH (or 0.059 × pH), where kB is the Boltzmann constant and the value of pH was assumed to be zero. According to this equation, the free energies of each elementary step are increased with increasing pH values, while the overpotential is unchanged (for a detailed explanation, see Note 1 in the Supporting Information). The zero-point energies and entropies of the NRR species were computed from the vibrational frequencies, in which only the adsorbate vibrational modes were calculated explicitly, while the catalyst sheet was fixed. The entropies and vibrational frequencies of molecules in the gas phase were taken from the NIST database.81

Jacobsen et al. discovered cobalt−molybdenum nitride (Co3Mo3N), one of the most active catalysts for ammonia synthesis.38 Clearly, both theoretical and experimental investigations are highly important to develop effective electrocatalysts for NRR and other related electrocatalytic reactions.39 Recently, single-atom catalysts (SACs), in which the catalytically active individual and isolated metal atoms are anchored to supports, have emerged as a new frontier in heterogeneous catalysis, and demonstrated distinguishing performances for various reactions due to their high catalytic activity with a significantly reduced amount of metals used.40−44 Zhang and co-workers first demonstrated that the isolated single Pt atom anchored on the surfaces of iron oxide nanocrystallites exhibits excellent stability and high activity for CO oxidation.44 Afterward, the catalytic performances of various single TM atoms deposited on different substrates have been extensively explored both experimentally and theoretically.45−54 However, to our best knowledge, there are only very few explorations on the catalytic performance of SAC for N2 fixation and conversion, in which the single Mo and Fe atoms supported by N-doped defective graphene are revealed to exhibit high catalytic activity for NRR due to the formation of MoN3 and FeN3 moieties.55,56 As an analogue of graphene, the boron nitride (BN) nanosheet possesses many inherent advantages such as high chemical stability and superior oxidation resistance.57−60 Especially, BN nanosheet exhibits much higher thermal stability (up to 1000 K) than graphene (below 600 K).61 Interestingly, some point defects, such as B-vacancy and N-vacancy, can be identified during the growth of BN nanosheet, or be deliberately produced by electron beam irradiation62 or solvent exfoliation,63 in which B-vacancy is preferably formed.57,62 In particular, the introduction of these point defects endows BN nanosheet higher chemical activity, thus making it an excellent substrate to support metal particles for various catalytic reactions.63−65 For example, Sun et al. have reported the synthesis of Pd nanoparticles supported on BN nanosheet, which exhibit excellent catalytic activity for the hydrogenation of nitro aromatics;63 Fu et al. used BN nanosheet as a catalyst substrate to anchor Pd−Fe nanoparticles for Suzuki−Miyaura coupling catalysts.64 Since TMNx moiety exhibits high catalytic activity for N2 reduction, can the unsaturated N atoms around the B monovancy in a defective BN nanosheet be utilized to immobilize the single metal atom to activate the inert N2 molecule? To answer this question, by means of density functional theory (DFT) computations, we investigated the potential of single TM atoms (Sc ∼ Zn, Mo, Rh, Ru, Pd, and Ag) anchored on the boron monovacancy in a BN monolayer as a N2 reduction catalyst. Our DFT results demonstrated that, due to its good performance in activating N2 molecules, stabilizing N2H while destabilizing NH2 species, the anchored Mo atom exhibits the highest catalytic activity for N2 fixation through an enzymatic mechanism with a low overpotential of 0.19 V. Therefore, the single Mo atom supported by the defective BN monolayer with B-vacancy is a promising singleatom catalyst for N2 fixation.

2. MODELS AND METHODS All of the computations were performed by means of spin-polarized density functional theory (DFT) methods using the DMol3 code.66,67 The Perdew, Burke, and Ernzerhof (PBE)68 exchange-correlation functional within a generalized gradient approximation (GGA) was

3. RESULTS AND DISCUSSION 3.1. Screening TM/Defective BN Nanosheets as NRR Electrocatalyst. According to previous investigations,72,82 the 12481

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Figure 1. Calculated (a) Gibbs free energies of N2 molecule and (b) adsorption energies of N2H and NH2 species on various single TM atoms supported by defective BN nanosheets.

3.2. Structures, Stabilities, and Properties of MoEmbedded BN Nanosheet. Figure 2 presents the fully

following criteria can be proposed for an eligible electrocatalyst for NRR: (1) the catalyst can facilitate the chemisorption of N2 molecule to guarantee the sufficient activation of its inert N−N triple bond and (2) the catalyst can selectively stabilize N2H* and (3) destabilize NH2* species to guarantee the reduction of the overpotential. According to the above criteria, we screened a series of TM atoms (TM = Sc to Zn, and Mo, Ru, Rh, Pd, and Ag) supported by a defective BN nanosheet. Following criterion 1, we computed the ΔG values of N2 adsorption on TM atoms supported on a defective BN nanosheet (Figure 1a) by considering both end-on and side-on initial adsorption configurations, in which only the low coverage of N2 molecule is considered, since the single TM could be distributed on the surface of the BN nanosheet separately and regularly. Our results showed that, except for that on an anchored Mo atom, the side-on N2 adsorption is more favorable energetically than the end-on adsorption for all of the other metal atoms. As shown by Figure 1a, the ΔG values of N2 adsorption on anchored Sc, Cr, Co, Ni, Cu, Zn, Pd, and Ag are positive, suggesting that these anchored TM atoms are not appropriate as the NRR electrocatalysts due to their poor performance for N2 activation. Following criterion 2, we can also rule out Rh atom, since it exhibits relatively weak stabilization for the N2H* species (Eads = −1.12 eV, Figure 1b). Following criterion 3, we can find that Ti, V, Mn, and Ru atoms on a defective BN nanosheet are also not eligible because they possess relatively strong interaction with NH2* species with adsorption energies of −4.18, −3.78, −3.43, and −3.29 eV, respectively (Figure 1b). Therefore, a Mo-embedded BN nanosheet is the only eligible electrocatalyst for the NRR satisfying all three screening criteria, which will be focused on in the following discussions. In addition, we examined the possibility of a larger TM cluster supported on a defective BN nanosheet as the NRR electrocatalyst by taking the Mo13 cluster as an example. Our computations showed that the adsorption energies of N2, N2H, NH2, and NH3 species on the anchored Mo13 cluster are −1.37, −2.93, −4.07, and −1.68 eV, respectively (Figure S1). Though the Mo13 cluster sufficiently activates the inert N2 molecule, it is unsuitable for N2 reduction due to its too strong interaction with NH2 species (not satisfying criterion 3). Though these computations cannot rule out the possibility of using Mo clusters anchored on BN nanosheets as NRR catalysts, it is better to avoid clustering problems of Mo atoms to fully release the potential of our designed catalysts for NRR.

Figure 2. Optimized structure of Mo-embedded BN monolayer. The unit of bond length is Å.

optimized structure of the Mo-embedded BN nanosheet. Our results demonstrated that the single Mo atom can be stably adsorbed by the three unsaturated N atoms around the boron vacancy. In the newly formed Mo−N3 moiety, the Mo−N bond length is 1.99 Å. Since the radius of the Mo atom is much larger than that of the missing B atom, the anchored Mo atom is outward from the BN monolayer surface by 1.14 Å. An ultimate prerequisite for SAC to be an effective catalyst is its good stability for long-term uses. In this sense, strong binding between the TM atom and the anchoring material is essential to prevent its aggregation. Thus, we examined the binding energy of a single Mo atom on a defective BN monolayer to evaluate its structural stabilities, where Eb = EMo@BN − EMo − EBN, and EMo@BN, EMo, and EBN denote the energies of the adsorbed structures and isolated parts. Our results showed that the binding energy of the single Mo atom on the defective BN monolayer with a boron vacancy is −8.95 eV. Furthermore, we examined the thermal stability of the Moembedded BN nanosheet by performing the first-principles molecular dynamics simulations at 500 K using the Nosé− Hoover heat bath scheme, in which the time step was set at 1.0 fs for a total period of 20 ps. No notable geometry buckling was observed after MD simulations (Figure S2). The large binding energy and excellent thermal stability indicate that the Bvacancy site in the BN monolayer can serve as a good anchoring site for a single Mo atom, ensuring its high stability. Kinetically, the diffusion barrier of the adsorbed Mo atom from the B-vacancy site to a neighboring hollow site was also calculated. Due to the high barrier (7.31 eV) and large endothermicity (7.24 eV), this process is unlikely to occur (Figure S3). Recently, Fe and Pd nanoparticles have been immobilized and uniformly distributed on the surface of BN nanosheets;63,64 12482

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and activation on this catalyst surface can easily take place at room temperature, which can be written as N2(g) → N2*, where the asterisk * denotes an adsorption site. Interestingly, the spin-polarized ground state of the Moembedded BN monolayer is retained even after N2 adsorption. The spin densities of the N2-adsorbed system are mainly localized at the central Mo atom (about 74%), while the adsorbed N2 molecule also contributes a certain amount of spin moment (about 13%, Figure S7). The spin density transfer from the Mo-embedded BN nanosheet to the adsorbed N2 molecule leads to the activation of N2 molecule. Though the inert N−N triple bond of the N2 molecule is sufficiently activated, its dissociation into two separated N atoms on the Mo-embedded BN monolayer is still considerably difficult. For example, the dissociation barrier for the side-on configuration is as high as 4.25 eV, accompanied with a large endothermicity of 2.68 eV (Figure S8). 3.4. Electrocatalytic Nitrogen Reduction. To evaluate the potential of the Mo-embedded BN monolayer as the electrocatalyst for the conversion of the activated N2 to NH3, we canvassed the subsequent NRR steps through three possible pathways, including the distal, alternating, and enzymatic mechanisms, in which six consecutive protonation and reduction processes are involved (Scheme 1). Figure 4 presents

we expect that Mo atoms can also be anchored on the defective boron nanosheets using a similar procedure (see Figure S4 in the Supporting Information). The electron properties of catalysts, such as charge and spin density distribution, and band gap, were shown to be vital for catalysts to promote N2 adsorption and activate its inert N−N triple bond.55,56 According to the Hirshfeld charge analysis, about 0.70 |e| is transferred from the Mo atom to the defective BN nanosheet, and each of the three nearest neighboring N atoms has a charge of −0.26 |e| (Figure S5). The Mo-embedded BN monolayer has a magnetic ground state with a total spin moment of 3.00 μB, which is 0.71 eV lower in energy than the nonmagnetic state. Remarkably, the spin density mainly locates on the Mo atom (2.88 μB, about 96%), whereas the N atoms (−0.03 μB) are polarized antiferromagnetically (Figure S5). The band structures of pristine BN and Mo-embedded BN monolayers are presented in Figure S6. The calculated band gap of the pristine BN monolayer is 4.71 eV, which is well consistent with the previous theoretical reports (4.78 eV).83,84 For the Mo-embedded BN monolayer, the band gap is greatly reduced to 1.52 eV due to the introduction of the occupied d bands of the Mo atom to the band gap. The large and localized spin moment as well as the decreased band gap could play a vital role in activating the N2 molecule on the Mo/BN monolayer and lowering the overpotential of the whole NRR, which will be discussed in detail later. 3.3. N2 Adsorption. After knowing the geometric and electronic structures as well as the stabilities of the Mo embedded BN monolayer, we then moved to investigate its catalytic activity for NRR. It is well-established that the N2 adsorption on the catalyst surface is the first step to initialize the NRR72,82 and its initial adsorption manner plays an important role in the subsequent reaction pathway. Thus, we examined N2 adsorption on the Mo atom supported by the defective BN monolayer. After full structural relaxation, we found that end-on and side-on configurations are energetically favorable. For the endon configuration, the N2 molecule and the central Mo atom anchored on the defective BN nanosheet form a Mo−N bond with a length of 2.12 Å (Figure 3), whereas, for the side-on

Scheme 1. Schematic Depiction of Three Mechanisms for N2 Electroreduction to NH3 on the Single Mo Atom Anchored on Defective BN Monolayer

the atomic configurations at various states of each elementary step on the surface of the Mo-embedded BN monolayer through distal, alternating, and enzymatic mechanisms, while Figure 5 summarizes the corresponding free energy profiles. Providing that the NRR follows the distal pathway, the adsorbed N2 will be hydrogenated by adsorbing a proton coupled with an electron transfer, and form a N2H* species adsorbed on the Mo site (Figure 4a). The hydrogen is attached to the distal N site with a N−H bond length of 1.04 Å, and the N−N bond is further elongated to 1.24 Å. This elementary step is slightly uphill in the free energy profile by 0.75 eV. In the following step, the (H+ + e−) consecutively attacks the distal N atom of the N2H* species. As a result, the N2H2* species is yielded and the Gibbs free energy decreases by 0.76 eV. Subsequently, the first NH3 molecule can be released after the interaction of the third (H+ + e−) with the prehydrogenated N site in the N2H2* group, and one N atom remains on the Mo atom with a Mo−N length of 1.72 Å. The free energy of N2H2* reduction to NH3 + N* is downhill by 0.12 eV. In the subsequent steps, the remaining N* species will be hydrogenated to the second NH3 by reacting with another three protons coupled with electrons. The ΔG values for the three

Figure 3. Optimized structures of N2 adsorption on Mo-embedded BN monolayer for (a) end-on and (b) side-on configurations. The key bond lengths (Å) are also given.

configuration, two Mo−N bonds with a length of 2.11 Å are formed. Since there is about 0.10 (end-on configuration) and 0.29 |e| (side-on configuration) transfer from Mo-embedded BN monolayer to N2 molecule, the N−N bond length is elongated from 1.12 Å in an individual N2 molecule to 1.14 and 1.20 Å in the adsorbed ones. The N2 adsorption energies in the two configurations are about −0.86 and −0.87 eV, respectively. When taking account of ΔEzpe and entropy, the ΔG values for the N2 adsorption on the Mo-embedded BN monolayer are −0.21 eV for the end-on configuration and −0.22 eV for the side-on configuration, respectively. Therefore, N2 adsorption 12483

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(0.75 eV) and alternating (0.88 eV) pathways. Furthermore, we calculated the overpotential (η) for the NRR on the Moembedded BN monolayer via these three mechanisms. The overpotential (η) is a good indicator of catalytic reactivity, and here a smaller η value indicates a faster N2 reduction reaction. According to the computational hydrogen electrode (CHE) model, the η value can be determined by the equation η = Uequilibrium − Ulimiting, where Uequilibrium is the equilibrium potential of NRR (about −0.16 V in this work for the reaction N2 + 6H+ + 6e− → 2NH3) and Ulimiting is the applied potential required to eliminate the energy barrier of the rate-limiting step, which can be determined as Ulimiting = −ΔG/e, where ΔG is the free energy of the potential-limiting step. As discussed above, the limiting potentials of NRR on the Mo-embedded BN monolayer via distal, alternating, and enzymatic mechanisms are −0.75, −0.88, and −0.35 V; thus, the η values are (−0.16 V) − (−0.75 V) = 0.59 V, (−0.16 V) − (−0.88 V) = 0.72 V, and (−0.16 V) − (−0.35 V) = 0.19 V, respectively. Obviously, the NRR occurring on the Mo-embedded BN monolayer prefers to proceed through the enzymatic mechanism due to the smallest overpotential (0.19 V), which is even smaller than that on the well-established Re(111) surface (0.50 V).82 Therefore, the anchored Mo atom on the defective BN monolayer is expected to be a very promising single-atom catalyst for the NRR. Recently, a new pathway has been proposed for N2 reduction,15,85−88 namely, the mixed pathway, in which the N2H2* species in the distal pathway would shuttle to an alternating pathway (dotted arrows in Scheme 1). Thus, we also examined this hybrid mechanism for the Mo-embedded BN monolayer. According to our computations, whatever the subsequent pathway is, the free energy for the protonation of the adsorbed N2 to N2H* species is unchanged (+0.75 eV) in this hybrid mechanism, which is always much larger than that of the potential-determining step in the enzymatic mechanism (0.35 eV). Thus, the hybrid mechanism is less favorable energetically than the enzymatic mechanism. 3.5. Mechanisms of High NRR Activity on the Mo−BN Monolayer. In order to further understand the superior catalytic performance of the Mo-embedded BN monolayer for the NRR, we inspected the variations of the atomic charges in each elementary step along the favorable enzymatic pathway (Figure 6). According to previous studies,55,56 each intermediate was divided into three moieties, including moiety 1 (BN monolayer), moiety 2 (MoN3, i.e., Mo and its surrounding three N atoms), and moiety 3 (the adsorbed NxHy species). Our computations demonstrated that, for N2 adsorption on the Mo-embedded BN monolayer via the side-on configuration, the N2 molecule and MoN3 moiety can gain 0.29 and 0.07 electrons from the BN monolayer. In the following hydrogenation and reduction steps, obvious charge fluctuation occurs for the three moieties (Figure 6). For example, when a proton coupled with an electron interacts with the adsorbed N2 molecule, the formed N2H* species can gain about 0.16 electrons from the BN monolayer, while about 0.14 electrons will be transferred from the achieved H2N*−*NH to BN monolayer in the fourth step. Thus, the BN monolayer servers as an electron donor or acceptor during the NRR, i.e., an electron reservoir, while the MoN3 moiety acts as a transmitter to charge transfer between the adsorbed NxHy species and BN monolayer. In addition, we also examined the N−N bond lengths in each step along the enzymatic pathway. For a free N2 molecule, its N−N bond length is about 1.12 Å. Upon

Figure 4. Optimized geometric structures of various intermediates (such as N2H*, N2H2*, N*···NH3, NH*, NH2*, etc.) along the reaction path of NRR proceeded on Mo−BN monolayer through (a) distal, (b) alternating, and (c) enzymatic mechanisms.

steps are −0.90, −0.67, and +0.63 eV, respectively. Finally, the second NH3 molecule can be desorbed from the Mo-embedded BN nanosheet after overcoming a positive ΔG value of 0.31 eV. Remarkably, in this distal pathway, the protonation of N2 to form N2H* species is the potential-limiting step due to the maximum ΔG values (+0.75 eV) among all elementary steps. When the NRR follows the alternating and enzymatic pathways, the protonation alternately occurs between the two N atoms, resulting in the release of the first NH3 at the sixth step (Scheme 1). The potential-limiting step (ΔG) of N2 fixation on the Mo-embedded BN monolayer in the alternating mechanism is the formation of the N2H4* group, and that in the enzymatic mechanism is the reduction of the NH2* group to NH3 (Figure 5). At zero electrode potential (U = 0 V), along the enzymatic pathway, there is a key energy barrier of 0.35 eV required for the protonation step of the NH2* species (NH2* + H+ + e− → NH3*), which is much lower than those of distal 12484

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Figure 5. Free-energy diagrams for the NRR on Mo−BN monolayer at zero and applied potential (limiting potential) through (a) distal, (b) alternating, and (c) enzymatic mechanisms.

revealed that the single Mo-atom-embeded BN nanosheet possesses outstanding NRR catalytic activity, and the whole reactions prefer to take place through the enzymatic mechanism with a considerably small overpotential of 0.19 V. Therefore, our computations suggest a quite promising singleatom catalyst with high efficiency and good stability catalyst for N2 fixation. We hope that our studies could motivate more experimental and theoretical research to further explore the potential of two-dimensional monolayers as NRR electrocatalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b05213. Test computations for choosing density functional and supercell size; detailed explanation about the effect of pH values to the overpotential; variation of total energy and geometry snapshot of the Mo-embedded BN monolayer with boron monolayer at 20 ps during MD simulation at 500 K; the minimum energy path for the diffusion of the adsorbed Mo atom from the defect binding site to a neighboring hollow site; NRR species adsorption on Mo13/BN nanosheet; charge distribution and spinpolarized density for Mo-embedded BN nanosheet; calculated band structures of Mo-embedded BN nanosheets; isosurface of the spin-resolved density of N2 adsorption on Mo-embedded BN nanosheet; the N2 dissociation minimum energy path on Mo−BN monolayer; the feasibility to synthesize a Mo-embedded boron nitride monolayer by the proposed procedure; the variation of the N−N bond length along the enzymatic

Figure 6. Charge variation of the three moieties along the enzymatic pathway. Moieties 1, 2, and 3 represent the BN nanosheet, the MoN3 unit, and the adsorbed NxHy species, respectively. 0, 1, 2, 3, 4, and 5 represent the N2*, N2H*, NH−NH*, NH2−NH*, NH2−NH2*, and (NH2* + NH3) intermediates along the reaction pathway, as depicted in Figure 4c.

electroreduction, the N−N bond is consecutively stretched until it is ruptured at the sixth step (Figure S9), implying the feasibility of N2 activation on the surface of the Mo-embedded BN monolayer.

4. CONCLUSIONS In summary, by means of spin-polarized DFT computations combined with a CHE model, we systematically investigated the potential of single transition metal atoms (Sc to Zn, Mo, Ru, Rh, Pd, and Ag) supported on the experimentally available defective boron nitride (TM−BN) monolayer with a boron monovacancy as a N2 fixation electrocatalyst. Our computations 12485

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pathway; the absolute energies (in Hartrees) and the coordinates of these mentioned geometries (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Jingxiang Zhao: 0000-0001-6023-8887 Zhongfang Chen: 0000-0002-1445-9184 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in China by the Excellent Young Foundation of Harbin Normal University (Grant No. XKYQ201304) and in USA by Department of Defense (Grant W911NF-12-1-0083). This paper is dedicated to the memory of Professor Panwen Shen, who recently passed away.

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