Single Mo1(Cr1) Atom on Nitrogen-Doped Graphene Enables Highly

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Research Article Cite This: ACS Catal. 2019, 9, 3419−3425

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Single Mo1(Cr1) Atom on Nitrogen-Doped Graphene Enables Highly Selective Electroreduction of Nitrogen into Ammonia Wanghui Zhao,†,§ Lifu Zhang,‡,§ Qiquan Luo,† Zhenpeng Hu,‡ Wenhua Zhang,*,†,Π Sean Smith,Π and Jinlong Yang*,†

ACS Catal. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 03/19/19. For personal use only.

†

Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Materials for Energy Conversion and Synergetic Innovation Centre of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China ‡ School of Physics, Nankai University, Tianjin 300071, China Π Department of Applied Mathematics, School of Physics and Engineering, Australian National University, Canberra, Australian Capital Territory 2600, Australia S Supporting Information *

ABSTRACT: Searching for new types of electrocatalysts with high stability, activity, and selectivity is essential for the production of ammonia via electroreduction of nitrogen. Using density functional theory (DFT) calculations, we explore the stability of single metal atoms (M1) supported on nitrogen-doped graphene (N3-G); the competitive adsorption of dinitrogen and hydrogen; and the potential competition of first dinitrogen protonation and hydrogen adsorption on metal sites. Consequently, we identify Mo1/N3-G and Cr1/N3-G as candidate electrocatalysts for nitrogen reduction reaction (NRR). The theoretically predicted selectivities (overpotentials) are 40% (0.34 V) and 100% (0.59 V) on Mo1/N3-G and Cr1/N3G, respectively. The electroreduction of nitrogen proceeds via distal-toalternating hybrid mechanism with two spectator dinitrogen molecules. The high stability, high selectivity to ammonia, and relatively low overpotentials for NRR suggest Mo1(Cr1)/N3-G as the most promising electrocatalyst among those studied for electroreduction of nitrogen. KEYWORDS: electroreduction of nitrogen, ammonia selectivity, single atom catalysts, density functional theory, graphene selectivity toward ammonia production (less than 10%).11−14 Fortunately, a bridge between molecular catalysts and traditional heterogeneous catalysts was built through stable singleatom catalysts (SACs) well dispersed on different substrates in recent years.15−23 SACs not only maximize the utilization of metal but may perform high activity and achieve much higher selectivity for target product for several reactions compared with their nanocrystal and nanocluster counterparts.17−19 It is expected that SACs can exhibit excellent performance with lower overpotential and higher selectivity for a nitrogen reduction reaction. As for the substrate for a single atom, a substrate with high conductivity is preferable to obtain high current density, and a graphene-based material could be a good candidate. Recently, theoretical works have been performed to evaluate the performance of stable SACs as electrocatalysts for NRR.24−27 However, the overpotential is typically the only point of focus for the theoretical evaluation of SACs, and the selectivity has not been considered until now. On metal

1. INTRODUCTION The catalytic synthesis of ammonia as fertilizer is of enormous significance for global agriculture.1 The widely used welloptimized Haber−Bosch process in industry for ammonia synthesis proceeds at high temperature and high pressure (500 °C and 20 MPa) and contributes to around 3% of global CO2 emissions.1−4 Therefore, the search for better ammonia synthesis under ambient condition becomes a topic of great interest, albeit challenging.5 In nature, nitrogenase enzymes with special active sites can fix nitrogen from the atmosphere under ambient conditions via an associative mechanism rather than a dissociative mechanism.6 As a biomimetic process, electroreduction of nitrogen with a six-proton−electron process working at ambient condition becomes quite attractive for ammonia synthesis.7−10 Additionally, the development of renewable energy such as solar energy or wind energy makes electroreduction become a competitive way to produce ammonia. Stable electrocatalysts with high ammonia selectivity (i.e., faradic efficiency) and low NRR overpotential are necessary for large-scale synthesis of ammonia.11,12 Metal-based nanocrystal and nanoparticle electrocatalysts used for nitrogen electroreduction are more stable than molecular catalysts or enzymes, but they suffer from low © XXXX American Chemical Society

Received: December 19, 2018 Revised: February 14, 2019

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DOI: 10.1021/acscatal.8b05061 ACS Catal. 2019, 9, 3419−3425

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ACS Catalysis

ZPE is defined as ZPE = ∑i 12 hvi , where i is the frequency number, vi is the frequency with unit cm−1. The entropies of gas phase H2, N2, NH3, and NH2NH2 are obtained from the NIST database39 with standard condition, and the adsorbed species were only taken vibrational entropy (Sv) into account, as shown in the following formula: l ÅÄÅ ÑÉÑ−1 o o ÑÑ o hvi ÅÅÅ ijj hvi yzz Ñ Sv = ∑ R o m ÅÅÅexpjjj k T zzz − 1ÑÑÑ o k T ÑÑÖ o B ÅÅÇ k B { i n É| ÅÄÅ ij hvi yzÑÑÑÑo o ÅÅ o j z Å − lnÅÅ1 − expjj− zÑ} o j kBT zzÑÑÑÑo ÅÅ k {ÑÖo ÅÇ ~

nanocrystals, the low selectivity is revealed to be associated with two issues. One is that the stronger binding of hydrogen than dinitrogen greatly reduces the surface concentration of activated dinitrogen (*N2). The second is that the higher overpotential of NRR in the presence of surface hydrogen relative to the hydrogen evolution reaction (HER) causes the HER to be an unwanted competitive channel.11,12 On SACs, atomic hydrogen and dinitrogen molecules compete with each other for the active metal sites, which determines the surface concentration of *N2. Furthermore, for the low-coordination of metal atoms in SACs,28−32 more than one nitrogen containing fragment (*NxHy, x = 1−2, y = 0−6) and hydrogen atoms may be simultaneously captured. The potential favorable adsorption of hydrogen on metal site rather than the protonation of *NxHy will open a path for HER and reduce the selectivity toward ammonia. These competitive processes should be considered when evaluating SACs for NRR with high selectivity. In this work, three criteria are adopted to theoretically screen for the best electrocatalyst among supported single atom metal catalysts (M1) on N3-G: (1) the high stability of M1/N3-G; (2) the stronger adsorption of dinitrogen than hydrogen; (3) a less negative potential for the first nitrogen protonation in comparison with hydrogen adsorption. It is suggested that Mo1/N3-G and Cr1/N3-G may be potential electrocatalysts for nitrogen electroreduction with high stabilitiy, high selectivity and relative low overpotential. In addition, a new reaction mechanism of NRR proceeding via hybrid mechanism with two spectator dinitrogen molecules on Mo1(Cr1)/N3-G is proposed.

. Among which R = 8.314 J·mol−1·K−1, T = 298.15 K, h = 6.63 × 10−34 J·s, kB = 1.38 × 10−23 J·K−1, i is the frequency number, vi is the vibrational frequency (unit is cm−1). The energy of electron and proton pair is calculated as 1/2 H2 according to calculated hydrogen electrode (CHE) by Nørskov, which corresponds to reverse hydrogen electrode (RHE).40−42 Therefore, in the RHE, the chemical potential of a proton− electron pair was defined as ΔG(H+ + e−) =

1 G(H 2) − eU 2

, where U is the electrochemical potential relative to RHE. The adsorption Gibbs free energies of electron−proton pair is defined as 1 i y ΔGads,e − p = E H‐M1/N3‐G − jjjE M1/N3‐G + E H2 zzz + ΔZPE 2 k {

2. MODELS AND METHODS 2.1. Computational Details. All spin-polarized calculations were performed by using the density functional theory (DFT) method implemented in DMol3 package.33 DFT Semicore Pseudopotential method (DSPP),34 Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional and double numerical basis sets with polarization functions (DNP) were adopted.35,36 A DFT-D semiempirical correction with Grimme method is applied to account for the dispersion interaction.37 However, for the lack of database, the calculations including Re, Pt, and Au are performed without vdW corrections. Solvent effect is included by conductor-like screening model (COSMO) with a dielectric constant of 78.54.38 A hexagonal supercell containing (5 × 5) unit cells of graphene monolayer with about 15 Å vacuum layer was used as a support. Three carbon atoms near one single carbon vacancy are substituted by three nitrogen atoms (N3-G). Further, 3 × 3 × 1 k-points grids were used to represent the Brillouin zone for geometric optimization, and 7 × 7 × 1 k-points grids were used to perform energy calculations and vibrational frequency calculations. The real-space global cutoff radius is set to be 5.2 Å. Fermi occupation is applied to achieve electronic convergence. The convergence tolerances of energy, force, and displacement for the geometry optimization were 1 × 10−5 Ha, 0.002 Ha/Å, and 0.005 Å, respectively. All the atoms are relaxed for geometric optimization. And for adsorbed system, the adsorbed atoms combined with the metal atom and the three first neighboring nitrogen atoms are relaxed to calculate the vibrational frequencies. 2.2. Gibbs Free Energy Calculations. The adsorption or reaction Gibbs free energy is defined as ΔG = ΔE + ΔZPE − TΔS, where ΔE is the adsorption or reaction energy based on DFT calculations, ΔZPE is the zero point energy (ZPE) correction, T is the temperature, and ΔS is the entropy change.

− T ΔS

, where EH‑M1/N3‑G, EM1/N3‑G and EH2 represent the energies of HM1/N3-G, M1/N3-G and gas-phase hydrogen, respectively. 2.3. Overpotential Calculations. The potential needed for each protonation step was defined as U = −ΔG/e. The limiting potential (UL) was calculated as UL = −ΔGL/e,24,43 where ΔGL is the reaction Gibbs free energy of potential-limiting step. For nitrogen reduction reaction N2 + 6H+ + 6e− → 2NH3, UL is negative. The overpotential (η) of the whole electroreduction process can be calculated as the: η =Uequilibrium − UL, where Uequilibrium is the equilibrium potential of NRR (−0.16 V for NRR). The limiting potential of HER (UL,HER) can be calculated as UL,HER = −|ΔGH|/e, where ΔGH is the adsorption free energy of electron−proton pair. And the overpotential of HER can be calculated as ηHER = Uequilibrium − UL = |ΔGH|/e.40−42 2.4. Method To Evaluate the Selectivity. The selectivity of NRR is simply estimated according to Boltzmann distribution that is f NRR = 1/(1 + exp{ −δG/kBT). Where, δG is the Gibbs free energy difference between two competitive reactions at the same time, kB is Boltzmann constant, and T is temperature. Taking the competitive formation of M1-H and *NNH as an example, if the possibility of the formation of *NNH is set as 1, the possibility of the formation of M1-H would be exp{−δG/ kBT}, where δG = ΔGM1‑H − ΔG*NNH. If δG is positive, the possibility of the formation of M1-H is less than 1, and the selectivity toward NRR( f NNR) is larger than 50%.

3. RESULTS AND DISCUSSION 3.1. Stability of N-Doped Graphene Supported Single Metal Atom. Eighteen kinds of metal atoms (M = Sc−Zn, Mo, 3420

DOI: 10.1021/acscatal.8b05061 ACS Catal. 2019, 9, 3419−3425

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Table 1. Binding Energy Difference (ΔEb) of Various Metal Atoms Anchored at N3-G Site and Neighboring Graphene Site metal atom

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

ΔEb (eV) metal atom

−3.75 Zn

−3.47 Mo

−3.71 Ru

−2.79 Rh

−4.66 Pd

−2.62 Ag

−2.97 Re

−2.75 Pt

−1.92 Au

ΔEb (eV)

−1.36

−3.05

−3.06

−1.96

−0.97

−1.22

−3.19

−0.78

−0.41

N3-G, hydrogen binds stronger with Sc than dinitrogen molecule. Therefore, Sc1/N3-G is excluded as an effective NRR catalyst candidate. For the peculiarity of SACs, the adsorption Gibbs free energies of the second and third dinitrogen combined with the adsorption Gibbs free energies of hydrogen with the presence of one or two dinitrogen molecules are all calculated (Supporting Information, Table S2). The negative adsorption Gibbs free energy of each dinitrogen and the more negative adsorption Gibbs free energy than hydrogen are considered to determine the saturated numbers of adsorbed dinitrogen molecules on each M1/N3-G catalyst. On V1(Ni1, Cu1)/N3-G, only one dinitrogen can be captured, and on Co1(Ru1, Rh1)/N3-G, up to two dinitrogen molecules can be strongly bonded. Most interestingly, on Ti1(Cr1, Mn1, Fe1, Mo1 and Re1)/N3-G, three dinitrogen molecules can be effectively adsorbed on the active center of metal atoms. Then the NRR processes on M1/N3-G with saturated number (Table 2) of dinitrogen molecules are investigated. 3.3. Selectivity of the Formation of *NNH and M1-H. After the saturated adsorption of dinitrogen molecules, the first protonation of *N2 to form *NNH, one of the potential-limiting steps of NRR,44,45 is considered. The competitive process here is the attachment of hydrogen on dinitrogen saturated M1/N3-G active sites, which opens a pathway for further HER. The reaction Gibbs free energy of the first protonation of one of the binding dinitrogen molecules (ΔG*NNH) and the adsorption Gibbs free energy of hydrogen on metal atom (ΔGM‑H) with the presence of multiadsorbed dinitrogen molecules are listed in Table 2. The potential vs RHE needed for each protonation step is defined as U = −ΔG/e. The potential difference of two steps are denoted as ΔU = U*NNH − UM1‑H, and a positive value corresponds to higher selectivity. As revealed in Table 2, the formation of *NNH is potential preferable to the adsorption of hydrogen on metal atom of Cr1/N3-G and Re1/N3-G, which indicates that Cr1(Re1)/N3-G are expected as potential electrocatalysts with high NRR selectivity. On Cr1/N3-G, the potential needed for *N2 protonation is 0.63 eV less negative than that on Re1/N3-G and the selectivity toward *NNH is estimated as close to 100% at room temperature. It is also noticed the potential required for protonation of *N2 on Mo1/ N3-G is the least among all the M1/N3-G SACs, and the selectivity toward *NNH is estimated as 40% at room temperature. Although the selectivity on Mo1/N3-G is not as high as that Cr1(Re1)/N3-G, it is higher than that obtained by

Ru, Rh, Pd, Ag, Re, Pt, and Au) are accommodated on the center of the three nitrogen atoms doped in graphene (Supporting Information, Figure S1). The binding energy differences (ΔEb) defined as the energy difference of M1 atom anchored at N3-G site and nearby graphene site (Table 1 and Supporting Information, Figure S2). The lower ΔEb indicates the lower probability of the diffusion of M1 on N3-G and the higher stability of M1/N3-G. For Zn1, Pd1, Ag1, Pt1, and Au1 anchored on N3-G, the binding energy differences are higher than −1.5 eV, which indicates that these atoms are not well anchored at N3-G site and thus ready to diffuse and aggregate. Therefore, in the following steps, Zn1, Pd1, Ag1, Pt1, and Au1 supported on N3-G are not considered. 3.2. Competitive Adsorption between Hydrogen and Nitrogen. The adsorption Gibbs free energies of hydrogen (Supporting Information, Figure S3) and dinitrogen with endon and side-on modes (Supporting Information, Figure S4) on M1/N3-G are calculated to evaluate if dinitrogen can be effectively captured. The calculated Gibbs free energies of hydrogen and dinitrogen are shown in Figure 1 (also listed in

Figure 1. Adsorption Gibbs free energy of H and N2 adsorbed on 13 kinds of M1/N3-G. Black, red, and blue lines represent the adsorption of atomic hydrogen, end-on mode nitrogen, and the side-on mode nitrogen, respectively.

Supporting Information, Table S1). Dinitrogen molecule prefers to adsorb with end-on mode rather than side-on mode on all the considered M1/N3-G. The adsorption of dinitrogen on M1/N3G is more than 0.3 eV negative than that on M1 anchored on the boron vacancy of h-BN,24 which indicates that dinitrogen can be trapped more effectively on M1/N3-G. Furthermore, on most M1/N3-G, the binding of dinitrogen with end-on mode is stronger than that of hydrogen, which corresponds to a high surface concentration of *N2 for further NRR. While, on Sc1/

Table 2. Numbers of nitrogen molecules effectively binding on M1/N3-G, the reaction Gibbs free energy (eV) of the formation of *NNH (ΔG*NNH), the adsorption free energy of H on M1/N3-G (ΔGM1‑H), and the potential difference (V) between the formation of *NNH and M1-H (ΔU = U*NNH − UM1‑H = −ΔG*NNH/e − [−ΔGM1‑H/e]) metals

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Mo

Ru

Rh

number

3

1

3

3

3

2

1

1

3

2

2

3

ΔG*NNH ΔGM‑H ΔU

1.15 0.98 −0.17

0.89 0.26 −0.63

0.75 1.10 0.35

1.75 1.42 −0.33

1.46 0.15 −1.31

1.28 0.12 −1.16

0.79 0.02 −0.77

1.82 1.40 −0.42

0.50 0.49 −0.01

1.13 −0.58 −1.71

1.52 0.04 −1.48

1.38 1.77 0.39

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ACS Catalysis recent experiments.46−49 Therefore, the detailed electroreduction processes with three dinitrogen (3N2-M1/N3-G) are investigated on both Mo1/N3-G and Cr1/N3-G SACs. 3.4. Electroreduction Mechanism of NRR. For the electroreduction of nitrogen molecule, four possible (distal, alternating, hybrid, and enzymatic)24,26,50 mechanisms via various intermediates have been proposed (Supporting Information, Figure S5). Along each electroreduction pathway, atomic hydrogen without adsorbing on catalyst directly attacks *NxHy (x = 1, 2; y = 0−5) including the formation of *NNH. While for the end-on adsorption mode of dinitrogen on Mo1(Cr1)/N3-G, only distal, alternating, and hybrid mechanisms are considered. On both Mo1/N3-G and Cr1/N3-G, the minimum Gibbs free energy potential (MGFEP) of NRR proceeds via a hybrid distal-to-alternating pathway along *N2 → *NNH → *NNH2 → *NHNH2 → *NH2NH2 → *NH2···NH3 → *NH3, which agrees well with the mechanism of NRR on Fe/ yatris (phosphino) silyl ligand.51 The structures of possible intermediates with N−N bond lengths along hybrid reaction path are shown in Figure 2. The N−N distance increases with the increasing of hydrogen numbers on 3N2-Mo1(Cr1)/N3-G and N−N bond breaks when the *NH2···NH3 forms.

Figure 3. Minimum Gibbs free energy surfaces of NRR proceeds via a hybrid distal-to-alternating pathway at zero and rate-limiting potential on (a) 3N2-Mo1/N3-G and (b) 3N2-Cr1/N3-G. Shaded part indicates the recovery of catalyst (the replacement of *NH3 by *N2), which is not an electrochemical step and does not change with the applied potential.

−0.16 V, respectively. The reaction Gibbs free energies of each elementary step via distal mechanism and alternating mechanisms are listed in Supporting Information, Tables S3, S4, and the Gibbs free energy surfaces are shown in Supporting Information, Figures S6, S7. When intermediates *NNH, *NNH2, *NHNH2, *NH2NH2, *NH2NH3, and *NH3 present on 2N2-Mo1(Cr1)/N3-G, the protonation of *NxHy rather than Mo1(Cr1) metal atom (Supporting Information, Figure S8 and Table S5) makes the dominant selectivity toward ammonia at these steps. Furthermore, the adsorption free energies of *NH2NH2 on 2N2-Mo1/ N3-G and 2N2-Cr1/N3-G are calculated as −0.91 and −0.85 eV, respectively. The strong adsorption can effectively prevent the production of byproduct NH2NH2 and guarantee the high ammonia selectivity. Thus, the theoretically predicted selectivity is kept as 40% on 2N2-Mo1/N3-G and 100% on 2N2-Cr1/N3-G at room temperature. According to our proposed reaction mechanisms, the other two dinitrogen molecules act as spectators of NRR. With the presence of the other two dinitrogen molecules, the adsorption free energies of H atoms are greatly weakened from −0.10 (0.34) eV to 0.49 (1.10) eV on N2-Mo1(Cr1)/N3-G and on 3N2-Mo1(Cr1)/N3-G as listed in Table S2, respectively. While, the free energy of the formation of *NNH almost does not change. Thus, the adsorption of H atom on M1 site is greatly inhibited, and the selectivity toward NRR is enhanced. To some extent, this is an example of self-enhanced selectivity reaction. The charge states of *NxHy(moiety 1), 2N2-M1/N3 (moiety 2) and graphene (moiety 3) are analyzed by Hirshfeld charge (as listed in Supporting Information, Tables S6, S7). Charge variations are observed for each moiety as shown in Figure 4. *N2 on 2N2-Mo1/N3-G accumulates 0.03 e− more electrons than that on 2N2-Cr1/N3-G, and the corresponding required limiting potential is lowered by 0.25 V, which indicates the more negative electrons accumulated on *N2, the easier for the formation of *NNH. 2N2-M1/N3 moiety always gains electrons from other parts, while *NxHy moiety and graphene moiety can

Figure 2. Optimized structures of possible intermediates *N2, *H-*N2, *NNH, *NNH2, *NHNH2, *NH2NH2, *NH2···NH3, and *NH3 with two preadsorbed spectator dinitrogen molecules on Mo1/N3-G (a) and Cr1/N3-G (b).

The potential-limiting step of NRR on both 3N2-Mo1/N3-G and 3N2-Cr1/N3-G (Figure 3) is the first protonation of nitrogen to form *NNH with reaction Gibbs free energies of 0.50 and 0.75 eV, respectively. The limiting potentials (UL) vs RHE on 3N2-Mo1/N3-G and 3N2-Cr1/N3-G are calculated as −0.50 and −0.75 V according to UL = −ΔGL/e, respectively. When UL is applied, all the elementary steps are downhill as shown in Figure 3. The UL obtained on Mo1(Cr1)/N3-G is less negative than that of gold based heterogeneous electroreduction catalyst (about −1.7 V on Au(310) surface).14 The overpotential (η) of NRR on 3N2−Mo1/N3-G and 3N2−Cr1/N3-G are calculated as 0.34 and 0.59 V according to η = Uequilibrium − UL, where Uequilibrium = 3422

DOI: 10.1021/acscatal.8b05061 ACS Catal. 2019, 9, 3419−3425

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ACS Catalysis

4. CONCLUSIONS In summary, based on DFT calculations, we systematically investigated the nitrogen electroreduction activity on M1 (M = Sc−Zn, Mo, Ru, Rh, Pd, Ag, Re, Pt, and Au) supported by Ndoped graphene (M1/N3-G). Among 18 types of metal atoms by screening with three criteria, Mo1/N3-G and Cr1/N3-G are theoretically predicted as advantageous stable electrode materials for NRR with low overpotential and high selectivity. The electroreduction of nitrogen proceeds via distal-toalternating hybrid pathway with two spectator dinitrogen molecules. On Mo1/N3-G, the overpotential is only 0.34 V with a selectivity about 40%. As for Cr1/N3-G, the overpotential is 0.59 V, and the selectivity of NRR is greatly increased to about 100%. The theoretically predicted selectivity of NRR on Mo1 (Cr1)/N3-G is much higher than the selectivity obtained on current heterogeneous catalysts. The experimental selectivity may depend on the uniformity of the synthesized structures. Herein, our calculations suggest that stable Mo1 (Cr1)/N3-G may be strongly competitive materials for the electroreduction of nitrogen with high selectivity, low overpotential, and high conductivity. It is noticed that different with the widely investigated Mo-based catalysts for N2 reduction, the investigation of Cr-based catalysts just started,52 and this is the first report of Cr-based solid catalyst for N2 reduction. It is also worthy of note that, if only from the selectivity point of view, Re1/N3-G may also be a potential candidate.

Figure 4. Charge states analyzed by Hirshfeld charge of different moieties at each NRR elementary step. Charge variation of adsorbed NxHy species (moiety 1), the 2N2-M1/N3 unit (moiety 2) and the graphene substrate (moiety 3) on (a) Mo1/N3-G and (b) Cr1/N3-G at different species states along the minimum Gibbs free energy surface. Negative and positive values indicate electron gain and lose.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b05061.

gain or lose electrons with different surface species. *N2, *NNH, *NH2NH3 gain electrons on 2N2-Mo1/N3-G and 2N2-Cr1/N3G, while *NNH2, *NHNH2, *NH2NH2 and *NH3 lose electrons. Graphene always lose electrons except for the presence of *NNH2 on 2N2-Mo1/N3-G or the presence of *NNH2 and *NH3 on 2N2-Cr1/N3-G. Graphene plays as an electron reservoir, and 2N2-Mo1/N3, 2N2-Cr1/N3 transfer electrons between graphene substrate and *NxHy. The high conductivity of graphene is advantage for the charge transfer process during the electroreduction of nitrogen. To recover the catalyst, the produced NH3 should be ready to departure from catalysts. The replacement of produced NH3 with N2 is endothermic by 0.20 eV on 2N2-Mo1/N3 and 0.31 eV on 2N2-Cr1/N3 (Figure 3), which can easily be conquered at room temperature. For the adsorption of NH3 and N2, NH3 molecule donates its lone pair electrons to the empty bands of metal atom while N2 gains electrons from metal atom. Accordingly, the reduction condition with negatively charged electrocatalyst enhances the adsorption of N2 while weaken the adsorption of NH3 (Supporting Information, Tables S8, S9), which is an advantage for the production of NH3, and the recovery of 3N2-Mo1/N3-G and 3N2-Cr1/N3-G. Furthermore, the solvation of ammonia also releases some energy to stabilize the desorbed ammonia, which also promotes the leaving of produced NH3. At last, ab inito molecular dynamics simulations at 300 and 500 K are performed to verify the stability of Mo1/N3-G and Cr1/N3-G. Each simulation lasts 10 ps, and Mo1 and Cr1 are well anchored at N3-G site during simulation. The snapshots at 2.0, 4.0, 6.0, 8.0, and 10.0 ps for each simulation are shown in Supporting Information Figure S9.

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Figures S1−S9 and Tables S1−S16; all the corrections for ZPE and TS for our systems are listed in Table S10−S16 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhenpeng Hu: 0000-0002-8469-1683 Wenhua Zhang: 0000-0002-0075-385X Sean Smith: 0000-0002-5679-8205 Jinlong Yang: 0000-0002-5651-5340 Author Contributions §

(W.Z., L.Z.) These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program (Grant Nos. 2018YFA0208600, 2016YFA0200600), the National Natural Science Foundation of China (21473167, 21688102, 21703222), the Fundamental Research Funds for the Central Universities (WK3430000005) and partially by the support of China Scholarship Council (CSC) (File No. 201706345015). The calculations were performed on the supercomputing system in USTC-SCC and Guangzhou-SCC. 3423

DOI: 10.1021/acscatal.8b05061 ACS Catal. 2019, 9, 3419−3425

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