Suppression of Hydrogen Evolution Reaction in Electrochemical N2

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Suppression of Hydrogen Evolution Reaction in Electrochemical N2 Reduction Using Single-Atom Catalysts: A Computational Guideline Changhyeok Choi, Seoin Back, Na-Young Kim, Juhyung Lim, Yong-Hyun Kim, and Yousung Jung ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00905 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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ACS Catalysis

Suppression of Hydrogen Evolution Reaction in Electrochemical N2 Reduction Using Single-Atom Catalysts: A Computational Guideline

Changhyeok Choi†, Seoin Back†, Na-Young Kim‡, Juhyung Lim†, Yong-Hyun Kim‡ and Yousung Jung*† †

‡

Graduate school of Energy Environment Water and Sustainability (EEWS) and

Nanoscience and Technology, Korea Advanced Institute of Science and Technology

(KAIST), 291 Daehakro, Daejeon 305-701, Korea

Abstract We studied electrochemical nitrogen reduction reactions (NRR) to ammonia on single atom catalysts (SACs) anchored on defective graphene derivatives by density functional calculations. We find significantly improved NRR selectivity on SACs than existing bulk metal surface due to the great suppression of hydrogen evolution reaction (HER) on SACs with the help of ensemble effect. In addition, several SACs including Ti@N4 (0.69 eV) and V@N4 (0.87 eV) are shown to exhibit lower free energy for NRR than that of the Ru (0001) stepped surface (0.98 eV) due to a strong back-bonding between hybridized d-orbital metal

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atom in SAC and π* orbital in *N2. Formation energies as a function of nitrogen chemical potential suggest that Ti@N4 and V@N4 are also synthesizable under experimental conditions.

KEYWORDS nitrogen reduction, electrocatalysts, ammonia synthesis, single-atom catalysts, hydrogen evolution reaction, density functional theory calculations

1. Introduction Ammonia (NH3) is one of the most produced chemicals in chemical industry.1-3 Ammonia has been produced primarily by Haber-Bosch process, whose main usage is the production of fertilizers.2 In this process, nitrogen (N2) and hydrogen (H2) react in high temperatures and pressures. Although this reaction (N2 (g) + 3H2 (g) → 2NH3 (g)) is exothermic (∆H0298 = 92.4 kJ/mol), it requires large energies in order to dissociate strong triple bond of nitrogen. The operating conditions for this process is typically around 400℃ and 150 bar on Ru- or Febased catalyst, for example. Due to this highly energy-intensive nature, ammonia production is responsible for 1-2% of worldwide energy consumption, and thus significant efforts have been made to alleviate these harsh reaction conditions.4 In nature, ammonia is synthesized by an enzymatic system called nitrogenase. Contrary to industrial condition for ammonia production described above, enzymes can produce ammonia under ambient conditions along with the following electrochemical reactions: N2 + 8 (H+ + e-) → 2NH3 + H2.4 This enzymatic and electrochemical reaction mechanisms are different from that of Harber-Bosh process. In Haber-Bosch process, N2 and H2 gas molecules are dissociated first (dissociative mechanism) before N and H atoms react to form bonds, 2


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requiring large energies in particular to dissociate the strong N-N triple bonds. By contrast, N2 molecules are hydrogenated (associative mechanism) sequentially in enzymatic reactions, weakening the N-N bonds (rather than dissociating them), requiring less energies than to break the N-N triple bonds.4 Thus, electrochemical synthesis of ammonia is considered a promising alternative to Harber-Bosch process. Many studies for electrochemical nitrogen reduction reaction (NRR) using noble metal catalysts have been reported.4-9 In experiments, NRR using Ru cathode and a solid polymer electrolyte cell at 90℃ was reported.8 The maximum NH3 yield rate and faradaic efficiency (FE) were 1.30 µg·h-1·cm-2 at -1.02 V vs. (Ag/AgCl) and 0.92 % at -0.96 V vs. (Ag/AgCl), respectively. More recently, NRR using Pt cathode/anode and a mixed NH4+/H+ conducting Nafion 211 membrane at ambient conditions with various reactants, N2 (or air) and H2 (or H2O), was reported.5 At 0.2 V, a maximum ammonia yield rate of 1.14x10-5 mol·m-2·s-1 and maximum FE of 2% were obtained by using N2 and H2 as reactants. A (110)-oriented Mo nanofilm was used as a catalyst for NRR.10 A maximum rate of NH3 formation of 3.09x1011

mol· s-1·cm-2 at -0.49 V (vs. RHE) and maximum FE of 0.72 % at -0.29 V (vs. RHE) were

reported. Recently, ultrathin Rh nanosheet nanoasembilies (RhNNs) was reported as an effective electrocatalyst for NRR, showing 23.88 µg·h-1·mgcat-1 of maximum ammonia yield rate at -0.2 V (vs. RHE) and ~0.7 % of maximum FE at 0.0 V (vs. RHE), respectively.11 These results indicate NRR is possible at mild conditions, however, the faradaic efficiency (FE) of NRR at low temperatures was significantly low and most of the measured current densities were instead originated from hydrogen evolution (HER) rather than NRR. An improved FE was reported (35%) using N2 and steam in a molten hydroxide suspension of nano-Fe2O3 electrolyte and Ni electrodes, but the operating temperature was high (200℃).6 3


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In theoretical investigations, density functional theory (DFT) studies have offered useful insights for NRR mechanism at atomic levels and proposed several potential candidates as efficient catalysts.4, 7, 9, 12-16 For example, several pure metals (e.g. Ru, Mo) were shown to yield low overpotential for NRR, however all pure metals showed more negative limiting potential for NRR than HER.7, 9 Abghoui et al. suggested transition metal nitrides including VN, ZrN, NbN, CrN, RuN and WN as efficient NRR catalysts via Mars-van Krevelen mechanism with low overpotential and high selectivity for NRR.15-17 Howalt et al. suggested several transition metal nano-clusters including Mo12 and molybdenum nitride nano-cluster as efficient catalysts for NRR.18-19 Recently, a theoretical study has shown that non-aqueous proton donor such as 2,6-lutidinium (LuH+) can suppress the HER during NRR.12 Despite of these previous efforts to synthesize ammonia under ambient conditions, finding NRR catalysts having high FE with suppressed HER at low temperatures is still extremely challenging. Recently, single atom catalyst (SAC) has been prominent as a promising candidate for various catalytic reactions.20-29 It surpasses the conventional catalysts in terms of significantly improved specific activity and reduced amount of loaded noble metals. Dramatic modification of electronic structures of SACs from those of bulk metal surfaces can alter catalytic activity and selectivity.20-24, 29-30 For NRR using SACs, a theoretical study on several transition metal atoms at boron nitride (BN) sheet has been reported13 without experimental results. The authors focused on overpotential for NRR and suggested Mo single atom at BN sheet as promising NRR catalysts with low overpotential,13 however, HER which is the most problematic side reaction in real experiments was not considered.

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In the present study, we investigated catalytic activity of NRR on single transition metal atom anchored at the defected graphene sites containing carbon vacancies and nitrogen. DFT calculations and computational hydrogen electrode (CHE) model were used to study the stability of SACs at each defect sites and thermodynamics of the NRR on SACs. We find that many SACs can effectively suppress HER with the help of electronic effects and the lack of ensemble effects. Among SACs considered in this study, several SACs including Ti@N4 and V@N4 are suggested as efficient catalysts with low overpotential and high selectivity for NRR. Also, these SACs are suggested to be synthesizable in experimental conditions on the basis of formation energy calculations.

2. Computational details Structure relaxation and total electronic energy calculations were performed using spinpolarized density-functional theory (DFT) methods implemented in the Vienna Ab initio Simulation Package (VASP) with projector-augmented wave pseudopotential (PAW).31-33 We used the RPBE exchange functional with the van der Waals (D3) correction.34-36 A cut-off energy was set to 500 eV and k-points were sampled using the 3x3x1 Monkhorst-Pack mesh.37 We modelled four defected graphene as a support for single metal atom; single-vacancy surrounded by three carbon (M@C3 in Fig. 1a), double-vacancy surrounded by four carbon (M@C4 in Fig. 1b), single-vacancy surrounded by three N (M@N3 in Fig. 1c), and doublevacancy surrounded by four N like porphyrin-center (M@N4 in Fig. 1d). We first used periodic supercell of rectangular graphene containing 60 carbons to model SACs. The c axis of graphene supercell was set to 20 Å in order to eliminate artifactual interactions along c5


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axis. Then, we modeled carbon defect sites (@C3, @C4) and nitrogen defect sites (@N3, @N4) at aforementioned graphene supercell. Gas molecules were calculated in 15x15x15 Å cell with gamma point sampling. Calculation details for H adsorption and NRR on bulk metal surfaces are in Supporting Information. The computational hydrogen electrode (CHE) model was used to establish a free energy profile for the electrochemical reduction reactions, as pioneered by Nørskov and coworkers.38 In this model, free energy of proton and electron pair G(H+ + e-) is equivalent to that of a half of gaseous hydrogen (0.5 G(H2)) under standard reaction conditions (pH = 0, 298.15 K, 1 atm) with no external potential. The free energy of reaction intermediates of electrochemical reduction is shifted by –neU when the external potential U is applied and n electrons are involved in the reaction. Thermal correction terms to convert electronic energies into free energies were calculated only for adsorbents and listed in Table S1. A previous study showed that water can stabilize *NNH, *NH and *NH2 by 0.1 eV, 0.1 eV and 0.2 eV, respectively (affecting the overpotential for NRR by ~0.1 eV).9 Due to this small effect of water in NRR and large calculation cost for considering kinetics in calculations, we did not include solvation effects in this study and focused on thermodynamic overpotentials.

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Figure 1. Four calculation models for SACs, (a) M@C3, (b) M@C4, (c) M@N3, (d) M@N4. Rectangular cell denotes repeating unit. Red, brown and blue balls represent transition metal, carbon and nitrogen atom, respectively.

3. Results and discussions 3.1. Stability of SAC Metal-support interactions are important in heterogeneous catalysis, and especially so for single-atom catalysts due to a direct alteration of the electronic structure of the metal atoms. Diffusion and aggregation of metal can be problematic in catalyst-support systems since these

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can reduce catalytic activity and stability of catalytic cycles. We thus first calculated binding energies (Eb) of each metal atom at the defected graphene supports to assess the stability. Although binding of metal atom at defect sites is thermodynamically favorable, metal atoms can aggregate on support. Thus, we compared Eb and the cohesive energies of metal (Ec) of each metal atom in order to determine SACs that prevent both diffusion and aggregation. The Eb and Ec are calculated by thefollowing equations: (1) Eb = - (E(M@Cx or M@Nx) – E(Msingle) – E(@Cx or @Nx)) (2) Ec = - (E(Mbulk) / N - E(Msingle)), where E(Mbulk), E(Msingle) and N indicate the energy of the bulk crystal unit cell of the corresponding transition metal, the energy of single transition metal atom in vacuum, and the number of atoms in the unit cell, respectively. If Eb > Ec, the binding of metal atom to a support is thermodynamically more favorable than aggregation of metal atoms. We considered thirty d-block transition metals, thus total 120 of Eb (4 different defect sites for each metal atom) and 30 of Ec were calculated. Among 120 SACs, only SACs meeting the following two conditions were chosen for further considerations: Eb is exothermic and Eb – Ec > 0. Calculated Ec, Eb – Ec and experimental Ec values are in supporting information (Table S2). Binding energies at all defect sites are exothermic, but a limited number of metals showed negative Eb – Ec at various defect sites. In summary, 23 metals for @C3, 26 metals for @C4, 6 metals for @N3, and 20 metals for @N4 were identified to be stable without aggregation at each defect site, respectively. We considered the NRR activities and selectivity for these smaller sets of species for the rest of this paper.

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3.2. Reaction mechanism and free energy for NRR on SACs To investigate catalytic activity for NRR on SACs, we first evaluate reaction free energies for NRR on SACs. The net reaction of NRR is N2 + 6 (H+ + e-) → 2NH3, a six-electron reduction reaction. For the thermodynamically stable SACs, we considered all possible reaction intermediates including isomers (e.g. *NNH2 vs. *NHNH) in Fig. S1 to calculate free energy change at potential determining step (PDS) for the lowest energy required reaction pathway. Because both end-on and side-on adsorbed geometries are possible for *N2 and *NNH,13, 39 we added the suffix (s) for *N2 and *NNH (*N2(s) and *NNH(s)) when the side-on adsorbent is more stable than the end-on adsorbent. The reaction free energy change at PDS (∆GPDS) on Ru (0001) stepped surface which showed the lowest NRR overpotential among bulk metal surfaces is shown for comparison.12 It has been shown that the first reduction step (*NNH formation) is the PDS, representing an elementary electrochemical reaction step with the largest positive free energy change, of NRR for most metal surfaces,4, 7, 9

and, a large reaction free energy for first reduction step can hinder initiation of NRR.

Similar to metal surfaces, large free energies are required for the first protonation step (*N2 + (H+ + e-) → *NNH or *NNH(s)) on SACs, in which only 28 SACs require less than 1 eV of free energy for the first protonation step (Fig. S2). To find SACs showing an improved catalytic performance compared to the bulk metal catalysts, we focused on SACs showing

∆GPDS ≤ 1.0 eV, corresponding to better or similar catalytic activity than that of Ru(0001) (0.98 eV). The ∆GPDS of all SACs are shown in Fig. 2.

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As a result, we found 23 SACs that satisfied ∆GPDS ≤ 1.0 eV and the corresponding results (∆G(*H), ∆G(*N2), ∆GPDS and PDS) are shown in Fig. 3 and Table S3. Among 23 SACs, the PDS of 16 SACs (Mo@C3, Nb@C3, V@C3, Ir@C3, Co@C4, Ir@C4, Mo@C4, Nb@C4, Os@C4, V@C4, La@N3, Mn@N3, Cr@N4, Sc@N4, V@N4 and Y@N4) is the first protonation step (*NNH or *NNH(s) formation) and the PDS of 6 SACs (Re@C4, W@C4, Sc@N3, Ti@N3, Y@N3 and Ti@N4) is the final protonation step (*NH3 formation). Only Ta@C4 shows a different PDS (*N + NH3), indicating that PDS of most SACs is the first or last protonation steps. For *N2, only La@N3 prefers side-on adsorbed *N2(s) to end-on adsorbed *N2, and all other SACs favor end-on *N2. Although *N2(s) is not preferred to *N2 on most SACs, *NNH(s) is preferred to *NNH on several SACs. At second reduction step, *NHNH and *NNH2 are possible. For the *NHNH-preferred SACs, both *NH + NH3 and *NH2NH2 are possible at fourth reduction step via *NHNH2. All the *NHNH-preferred SACs favor a *NH + NH3 pathway. The *NNH2-preferred SACs follow a distal pathway for remaining reactions via *N + NH3. At final reaction step, *NH3 can be either desorbed (NH3) or further protonated to NH4+. Due to the extensive computational costs for explicit solvation model, we could not consider reaction free energy for further protonation of *NH3 to be released to solution in the form of NH4+. However, several previous studies on nitric oxide (NO) electrochemical reduction on Pt surfaces have shown that adsorbed NH3 on Pt surfaces can be easily further protonated to NH4+ and released into solution,40-42 suggesting that *NH3 on SACs may also be desorbed easily by forming NH4+ in solution. Of course, experiments in acidic media would be helpful to further protonate *NH3 to NH4+.16 Thus, NH3 desorption may not be a problematic obstacle in NRR, hence not considered in detail here.

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Among the finally chosen 23 SACs that met ∆GPDS ≤ 1.0 eV, we found several SACs (Ir@C3, Mo@C4, Nb@C4, Os@C4, Re@C4, V@C4, La@N3 and Ti@N4) which can show significantly reduced ∆GPDS than that on Ru(0001) stepped surface by more than 0.20 eV.

Figure 2. The ∆GPDS (eV) on 120 SACs considered in this work. SACs filled with patterned represent thermodynamically unstable SACs (metal-support anchoring energy less favorable than metal-metal cohesive energy).

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Figure 3 Reaction free energy change at PDS (∆GPDS) on SAC candidates. Color codes represent the product at the PDS of each SAC. Green, blue, red and black represent that *NNH (s), *NNH, *NH3 and *N+NH3 formation being the PDS, respectively. For comparison, ∆GPDS on Ru (0001) stepped surface is shown in horizontal dashed line.

3.3. H adsorption on SACs In previous section, we focused on reaction free energy change for NRR on SACs and found several promising SACs which can show better catalytic activity than Ru(0001) stepped surface in terms of ∆GPDS. Now, we turn to NRR selectivity on SACs by considering H adsorption free energy related to HER. Previous studies have shown that FE of NRR on 12


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most metal surface is extremely low, and most current densities are originated from HER, not NRR.5-6, 8, 14, 43 The required free energy for *H is more negative than that for NRR on most metal surfaces.7, 9 Metal surfaces are easily covered by *H and *H can block active sites for NRR. Thus, suppressing H adsorption can be a reasonable way to increase NRR selectivity in addition to lowering ∆GPDS for NRR.12, 44 As an estimation to determine if SACs can suppress H adsorption better than metal surfaces, we compared free energy change for H adsorption (∆G(*H)) on SACs (∆GSAC(*H)) to that on metal surfaces (∆Gsurface(*H)) comprised of same metal atom in SAC. The difference between ∆G(*H) on SACs and that on metal surfaces (∆GSAC(*H) – ∆Gsurface(*H)) were plotted in Fig. 4. It is shown that ∆GSAC(*H) – ∆Gsurface(*H) are positive for most metals and highly delocalized in positive region, indicating that H adsorption can be highly suppressed on SACs. Thus, enhanced NRR selectivity is expected on SACs by sufficiently suppressing H adsorption. In addition to suppressed H adsorption on SACs, we investigated NRR selectivity on SACs in more detail by considering *N2/*H selectivity. If SACs cannot sufficiently bind N2, it seems difficult to achieve high NRR selectivity although H adsorption is suppressed. As such, the *N2/*H selectivity, we suggest, is an important metric to consider in NRR, although in literature the overpotentials for NRR are often mainly discussed. We compared ∆G(*N2) with ∆G(*H) for promising SACs (∆GPDS ≤ 1 eV) (Fig. 5). As can be seen in Fig. 5, several SACs are in *N2 dominant region (∆G(*N2) < ∆G(*H) at 0 V (vs. RHE)), showing improved selectivity. For these SACs, N2 adsorption would be less hindered by H adsorption at low overpotential region. However, Ir@C3, Re@C4 and Os@C4 known to show low ∆GPDS (Fig. 3) are in *H dominant region (∆G(*N2) > ∆G(*H)) due to their highly positive ∆G(*N2), thus, 13


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*N2 activation is expected to be difficult. Consequently, we suggest Mo@C3, Nb@C3, V@C3, Mn@N3, La@N3, Sc@N4 and V@N4 as satisfying both the superior catalytic activity and *N2/*H selectivity. Also, V@C4, Ti@N3 and Ti@N4 seem to be promising which are in near the *N2/*H selectivity border line (∆G(*N2) = ∆G(*H)) in Fig. 5 with significantly low ∆GPDS (Fig. 3 and Table S3). The free energy diagram for these SACs both at zero and applied potentials are shown in Fig. S3. We note that, under operating experimental negative potentials to initiate NRR, however, ∆G(*H) would become more negative than ∆G(*N2), and HER will be dominant over NRR (also shown in Fig. S3) as in almost all bulk metal catalysts. However, due to a significant suppression of H adsorption found on SACs compared to bulk metal cases, even at experimental negative potentials, the ∆G(*H) of promising SACs would still be relatively less negative than those of bulk metal surfaces. Thus, one would still expect advantages of SACs for improved NRR selectivity at these operating potentials compared to bulk metals albeit with lesser degree as the potentials become more negative. For example, SACs comprised of Sc, Ti, V, Mo and Nb, promising SACs near the *N2/*H selectivity borderline in Fig. 5, show less negative ∆G(*H) than that of bulk metal surfaces at applied potentials (Fig. S4). The applied potential (U) was set to limiting potential (UL) for NRR on SAC or metal surface. We found that all of the SACs near the borderline (Mo@C3, Nb@C3, V@C3, V@C4, Ti@N3, Sc@N4, Ti@N4 and V@N4) show ∆GSAC(*H) - ∆Gsurface(*H) > 0 at UL. Thus, we expect less H adsorption on these SACs than that on metal surfaces both at zero and applied potentials.

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Figure 4. Difference between H adsorption free energy on the SAC (∆GSAC(*H)) and adsorption free energy on the surface (∆Gsurface(*H)) of the same metal atoms (eV). Horizontal dashed line denote ∆GSAC(*H) = ∆Gsurface(*H).

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Figure 5. Calculated ∆G(*H) and ∆G(*N2) on SACs that satisfying ∆GPDS ≤ 1.0 eV. Dashed line indicates ∆G(*H) = ∆G(*N2). SACs in ∆G(*H) > ∆G(*N2) region (*N2 dominant region), under the dashed line, correspond to N2 adsorption is favorable than *H formation at 0 V (vs. RHE).

3.4. Origin of improved activity on the SACs 3.4.1. Ensemble effect One of the biggest differences between SAC and metal surface is atomic ensemble at active site. On SAC, literally single metal atom exists at active site, and thus only top site 16


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adsorption is possible (unless there is an explicit adsorbate-support interaction), while there are many metal atoms in bulk metal surface and several adsorption sites (e.g. top, bridge and hollow) are available. In previous section, it is shown that H adsorption can be highly suppressed on SACs (Fig. 4). To investigate the origin of suppressed H adsorption on SACs, we examined the role of ensemble effect on *H by comparing ∆G(*H) on different adsorption sites at several metal surfaces. Relative energies of *H on the top site compared to those on the most stable adsorption site at metal surfaces are shown in Fig. S5. All metal surfaces except for Pt(111) and Ir(111) show positive relative energies on top site for *H, indicating that *H prefer bridge or hollow sites to top site, and *H can be destabilized on top site (Fig. S5). The suppressed H adsorption is thus originated from availability of only the top adsorption sites on SACs, meaning that atomic ensemble effect can play an important role in suppressing HER. As can be seen in Fig. 5, most SACs in *N2 (near-)dominant region (V@C3, Mo@C3, Nb@C3, V@C4, Ti@N3, Ti@N4, V@N4 and Y@N4) consist of transition metals in the left– leg of volcano plot (Sc, Ti, V, Y, Nb and Mo), showing moderate to strong *N binding strength.7 It has been shown that metal surfaces showing stronger *N binding than *H binding (e.g. Sc, Ti, Y and Zr) should be able to electrochemically reduce N2 (without making too much H2 at the same time) at -1 V to -1.5 V (vs. NHE), although Sc, Ti, and Zr still showed relatively strong *H binding.7 For the latter metal species, we find significantly suppressed H adsorption on the corresponding SACs compared to metal surfaces as can be seen in Table S4. Interestingly, it seems that the ensemble effect on *H is more significant for aforementioned SACs. The difference in *H adsorption between the top site and the most favorable site shows that *H can be highly destabilized on top site at strongly *N binding 17


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metal surfaces (Fig. S5), indicating that ensemble effect on *H is large for strongly *N binding metals. In addition to *H, ensemble effect can help to reduce ∆GPDS for strongly *N binding metals. Most of promising SACs in Fig. 3 consist of strongly *N binding metals. Previous studies have suggested that selectively stabilizing *NNH and (or) destabilizing *NH2 are important to design NRR catalysts with low overpotential.9, 12-13 In especially, for strongly *N binding metals, destabilization of *NH2 is required rather than stabilization of *NNH to reduce overpotential for NRR.9 Similar to *H on metal surfaces, *NH2 also prefers bridge or hollow site to top site. Thus, unfavorable adsorption site on SACs can destabilize *NH2 and reduce ∆GPDS for strong *N binding metals. Consequently, ensemble effect on SACs is especially favorable on strongly *N binding metals, improving both catalytic activity and selectivity.

3.4.2. Electronic effect Previous studies of N2 binding at single metal sites have shown that interactions between the metal atom and ligands can play an important role in improving the N2 binding ability of metal atoms by modified electronic structures.39 Electronic structures of SAC are quite different from those of bulk metal or nano-catalysts due to strong metal-support interaction.27 It can lead to charge transfer between metal and support, thus anchored metal atom usually carries some positive charge.20, 27 Thus, for several SACs, it seems that electronic structure of SAC can affect N2 adsorption ability of metal. Among several promising SACs, we investigated electronic structure of Ti@N4, which shows very low ∆GPDS and ∆G(*N2) as representative. As can be seen in Fig. 6a and Fig. S6a, 18


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before N2 adsorption on Ti@N4, partial density of states (PDOS) of Ti (3d) is broad rather than sharp, it suggests that 3d orbitals of Ti are hybridized due to an interaction with the support. Through this interaction, charge transfer between metal and support is expected. In our Bader charge analysis and charge density difference map (Fig. S6b), 1.8 e- are transferred from Ti to support. This interaction makes Ti carry some positive charge and easy for N2 adsorption. Likewise, other SACs carry some positive charge on metal atom in our Bader charge analysis (e.g. +1.8 and +1.7 for V@N4 and Mo@C3) and differential charge density map (Fig. S7). Also, positive charge on metal can hinder H+ approaching metal and *H formation due to electrostatic repulsion between positively charged metal and H+.43 In addition to ensemble effect, positive charge on metal also can thus play an important role in less H adsorption. After N2 adsorption on Ti@N4, two overlapped PDOS between Ti (3d) and *N2 (2p) are observed in just below the fermi energy (Fig. 6a). These overlapped PDOS mainly consist of Ti (3dxz)-*N2 (2px) coupling and Ti (3dyz)-*N2 (2py) coupling (Fig. S6c), all components corresponding to back-bonding between Ti and *N2. As can be seen in Fig. 6b, electron density isosurface at the aforementioned overlapped PDOS region in Fig. 6a indeed shows back-bonding between Ti (3dxz)-*N2 (2px) and Ti (3dyz)-*N2 (2py). In Bader charge analysis and charge difference map (Fig. 6c), 0.4 e- is accumulated on *N2, suggesting charge transfer from Ti to *N2 through π back-donation. Consequently, we found that support-modified electronic structure of SAC can bind N2 strongly for several SACs, and charges on metal due to metal-support interactions can play a great role in the binding of N2.

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Figure 6. (a) PDOS of Ti (3d) and *N2 (2p) before and after N2 adsorption on Ti@N4, (b) the electron density isosurface of bonding region noted in (a), and (c) Charge density difference between before and after N2 adsorption. Isosurface level in (b) and (c) are 0.07 and 0.02 e Å-3 and charge density difference is computed as ρ (Ti-N2@N4) - ρ (Ti@N4) - ρ (N2). Cyan and yellow represents positive and negative region in (c), respectively.

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3.4. 3. Formation energies of SAC candidates In order to establish a connection with experimental synthesis of the SACs predicted here, we calculated the formation energies (Eform) of the several candidates using the following equation,

Eform = (EM@Cx or EM@Nx) – Egraphene + lµC – mµN – µM, where l and m represent the number of carbon defects and nitrogen in SAC and E is the total energy of each system, and µC, µN and µM are the chemical potential of carbon, nitrogen, and metal defined as the total energy per atom in pristine graphene, nitrogen gas, and bulk metals (M= Ti, V, Fe, Co), respectively. For comparison, we note that Fe@N4 and Co@N4 have been synthesized/observed experimentally in previous reports,29, 45-46 and thus compared the formation energies of these species to those of Ti@N4 and V@N4 as representative, which exhibit good catalytic activity for nitrogen reduction in our calculations (Table S3). Both Fe@N4 and Co@N4 have similar formation energy values which are about 0.28 eV higher than Ti@N4 and about 0.31 eV lower than V@N4 (Table 1). Thus, we expect that Ti@N4 and V@N4 can also be synthesized in experimental conditions due to relatively low formation energies. We plotted the formation energy graph as a function of nitrogen chemical potential to investigate in more detail (Fig. S8). Basically, M@C3 has a lower formation energy than M@C4, while M@N3 exhibits a higher formation energy than M@N4. In particular, Co@C3 and Ti@C3 can be easily formed in nitrogen-deficient environments. As the nitrogen chemical potential increases, M@N4 can appear more easily than M@C3, and all candidates (Ti, Fe, Co, and V) are synthesizable in nitrogen-rich conditions at the @N4 sites.

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Table 1. The formation energy values of M@N4 (M= Ti, V, Fe, Co) from the equation of

Eform = EM@N4 – Egraphene + 6µC – 4µN – µM. M@N4

Eform (eV)

Ti@N4

1.98

V@N4

2.58

Fe@N4

2.26

Co@N4

2.27

3.5. Application of SACs for other electro-catalytic reactions In the previous section (3.4.1), we found that ensemble effect of SACs can suppress H adsorption, suggesting that the present SACs can also be used for other electro-catalytic reactions that require the careful control of ∆G(*H) (e.g. catalysts for HER itself). In case of HER, the optimal catalytic activity appears for ∆G(*H) = 0 eV. For example, the well-known superior HER catalysts like Pt or Pt-based catalysts show |∆G(*H)| < 0.1 eV.47-49 Thus, to further increase the HER activity of reactive metals with ∆G(*H) < 0 in the left-leg of the HER volcano, shifting them towards more positive ∆G(*H) is required by destabilizing *H adsorption47-49, an aspect shown in Fig. 4 that ensemble effects of SACs can help highly destabilize *H close to 0 eV for reactive metals.

4. Conclusion

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Key finding of the present density functional investigations on the catalytic activity of SACs for NRR can be summarized as follows. (i) SACs exhibit more positive ∆G(*H) than most metal surfaces, suggesting a potential suppression of HER and dramatic improvements in the NRR/HER selectivity. We find that several SACs (Mo@C3, Nb@C3, V@C3, V@C4, La@N3, Mn@N3, Ti@N3, Sc@N4, Ti@N4 and V@N4) are promising NRR catalysts with similar or lower ∆GPDS than that of the Ru (0001) stepped surface, also with suppressed HER. In particular, we suggest Ti@N4 and V@N4 as promising NRR catalysts. Moreover, these two SACs are suggested to be synthesizable in experimental conditions on the basis of formation energy results that are comparable to those of Fe@N4 and Co@N4 previously made experimentally. (ii) A main origin of the improved catalytic activity and selectivity on SACs lies in the significant suppression of HER due to a highly unstable *H and *NH2 on SACs (i.e., atop site) due to the lack of atomic ensemble. With the help of electronic effect (positive charges on metal) and the lack of ensemble effect on SACs, H adsorption is significantly suppressed and N2 adsorption will be less hindered by *H on SACs compared to those of bulk metal surfaces comprised of the same metal atoms. Although HER will be dominant on SACs at sufficiently negative potentials, H adsorption will still be suppressed on SACs than that of bulk metal surfaces even at applied potentials and improved NRR selectivity is expected. (iii) Several SACs show highly negative ∆G(*N2) with low ∆GPDS due to electronic structure effects. In PDOS and Bader charge analysis for Ti@N4, metal-support interaction leads to positive charges on metal, making the corresponding metal d-orbitals easier to polarize and bind N2 more favorably. It allows a large overlap between Ti and *N2 via a favorable back-bonding, which can be clearly seen in PDOS and electron density isosurfaces. 23


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The limiting potential for NRR on Ti@N4 (-0.69 V) found in this study is comparable to that of other catalysts suggested by previous theoretical studies, such as molybdenum nitride nano-cluster (-0.8 ~ -0.5 V), ZrN (-0.76 V), VN (-0.51 V) and RuN (-0.23 V)15-18, but with improved NRR selectivity over HER.

Author Information Corresponding Authors *Y. Jung, E-mail: [email protected], Tel: +82-42-350-1712

Acknowledgment We acknowledge generous support through the National Research Foundation of Korea from the Korean Government (NRF-2016M3D1A1021147, NRF-2017R1A2B3010176).

Supporting Information Computational details for H adsorption and NRR on metal surfaces. Calculated binding energies and cohesive energies of metals. Free energy changes at the first protonation step. Illustration of reaction intermediates for NRR. Summary of reaction free energies and the free energy diagram for NRR on SACs. Relative energies of *H on top site compared to *H on the most stable adsorption sites. The ∆G(*H) on metal surface (Sc, Ti, V, Y, Mo) and those on SACs. The individual PDOS of Ti 3d orbital and N 2p (*N2) orbital in Ti@N4 before and after N2 adsorption. The differential charge density map upon adsorption of transition metal

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atom on defective graphene (Ti@N4, Mo@C3 and V@N4). The formation energy diagram of M@Cx and M@Nx (M = Ti, V, Fe, Co) as a function of nitrogen chemical potential.

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