Mo2TiC2 MXene: A Promising Catalyst for Electrocatalytic Ammonia Synthesis

Accepted Manuscript Title: Mo2 TiC2 MXene: A Promising Catalyst for Electrocatalytic Ammonia Synthesis Authors: Yijing Gao, Yongyong Cao, Han Zhuo, Xiang Sun, Yongbing Gu, Guilin Zhuang, Shengwei Deng, Xing Zhong, Zhongzhe Wei, Xiaonian Li, Jian-guo Wang PII: DOI: Reference:

S0920-5861(18)31013-7 https://doi.org/10.1016/j.cattod.2018.12.029 CATTOD 11846

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

17 July 2018 15 October 2018 12 December 2018

Please cite this article as: Gao Y, Cao Y, Zhuo H, Sun X, Gu Y, Zhuang G, Deng S, Zhong X, Wei Z, Li X, Wang J-guo, Mo2 TiC2 MXene: A Promising Catalyst for Electrocatalytic Ammonia Synthesis, Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.12.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mo2TiC2 MXene: A Promising Catalyst for Electrocatalytic Ammonia Synthesis Yijing Gao, Yongyong Cao, Han Zhuo, Xiang Sun, Yongbing Gu, Guilin Zhuang, Shengwei Deng, Xing Zhong, Zhongzhe Wei, Xiaonian Li, Jian-guo Wang* Institute of Industrial Catalysis, College of Chemical Engineering, State Key Laboratory Breeding Base

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of Green-Chemical Synthesis Technology, Zhejiang University of Technology, Hangzhou 310032,China.

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E-mail: [email protected]

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Graphical abstarct

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Electrocatalytic Ammonia Synthesis Process on the Mo2TiC2 MXene.

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Highlights

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Nineteen different possible pathways (five association pathway and fourteen dissociation pathway) analyzed by DFT calculation and Gibbs free energy

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calculation.

Valid N2-philicity, N≡N triple bond of the N2 molecule (the optimal distance) is

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sufficiently activated, from 1.11 Å to 1.268Å

Mo2TiC2 MXene can reduce the overpotential by changing the reaction pathway.

Mo2TiC2 as an ordered, double transition metals carbides is an eligible

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electrocatalyst for the NRR

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Abstract

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Electrocatalytic ammonia synthesis provides an energy-efficient alternative to the Haber−Bosch process. The aim is to find promising electrocatalysts which are able to change the reaction pathway and reduce the overpotential. Here, based on density functional theory, a comprehensive mechanism study of the N2 activation and NH3 synthesis on the Mo2TiC2 MXenes is presented. For catalytic reaction mechanism, nineteen different possible pathways are screened for the lowest overpotential, where the corresponding potential-determining step are compared by Gibbs free energy calculation. The result reveals Mo2TiC2 MXenes exhibit both valid N2-philicity and high catalytic activity for electrocatalytic ammonia synthesis through a dissociation mechanism with a low overpotential of 0.26 V. Further, the competing reaction of H2 evolution is simultaneously suppressed which shows a relatively high potentials of 0.74 V. This study shows a brand new material for catalyzing NH3 synthesis under ambient conditions and provides the theory background to reduce the overpotential by changing the reaction pathway.

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Keywords: Mo2TiC2 MXene NH3 Synthesis Electrochemistry Gibbs Free Energy Density Functional Theory Overpotential Introduction

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Haber‐ Bosch process is one of the most significant scientific realizations of the 20th century, contributing to the annual production of ammonia [1-3]. To achieve a sufficient rate, the Haber-Bosch process operates at high temperatures and pressures, which is driven by heat energy from fossil fuel[4]. It always results in heavy energy consumption. In contrast, the introduction of renewable electric energy to assist the activation of nitrogen molecule or change the reaction pathways are in the practical and theoretical significance [5-7]. Thus, electroreduction as an energy-efficient alternative for synthesizing ammonia has drawn more and more attention. In the electrocatalytic process, it is a hot topic to reduce the overpotential via designing and choosing

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appropriate catalyst [8-11]. As a 2D materials, MXenes[12-15] exhibit similar electrical conductivity to multilayer graphene[16, 17], easily tunable structure, and hydrophilic nature. Apart from the wide application in batteries [18-21] or electrochemical supercapacitors [2224], MXenes have also been considered as the promising candidates for electrochemical catalyst [25-28]. In the previous experiment report, composite [29-31] and functional groups [32, 33] are benefit to improve catalytic activity. In fact, bare MXenes monolayers have also been proved to exhibit a valid N2-philicity, CO2-philicity, and H2O-philicity [34, 35] by DFT calculation. Recently, the new ordered layered MAX phase (Mo2TiAlC2)[36] and Mo2TiC2 MXene[37] are emerged, in which surface Mo layer acts as the active site. Some studies [38-40] have proved that ordered Mo2TiC2 MXene are structurally and energetically stable. However, there is seldom study about application. As Mo is an important element in catalyzing NH3 synthesis in biological system [41, 42], more and more work has been focus on the possible application in electrocatalytic ammonia synthesis. The experiment study of Mo Nanofilms[43] and Mo based sulfides[44], nitrides[45, 46], oxides[47] represent a remarkable N2 reduction reaction (NRR) activity. It gives us great courage to study the activity of NRR on the Mo2TiC2 surface. Based on the above consideration, the performance of electrocatalytic ammonia synthesis on Mo2TiC2 MXene have been investigated by the DFT calculation. A comprehensive mechanism study has been performed on both association and dissociation mechanism, which nineteen different possible pathways have been analyzed by the Gibbs free energy. The free energies of corresponding potentialdetermining step are compared to screen the best reaction pathway with the lowest overpotential. As the results demonstrated, N2 molecule is sufficiently activated on Mo2TiC2 monolayer, and the length of N≡N bond from 1.11 Å to 1.268Å . Mo2TiC2 MXenes exhibits high catalytic activity for electrocatalytic ammonia synthesis through a dissociation mechanism with a low overpotential of 0.26 V. Further, the overpotential of H2 evolution reaction (HER) is considered. A relatively high potentials (0.74 V) means the competing reaction is effectively suppressed by the optimal overpotential (0.26 V). Thus, this study reveals the mechanism of electrocatalytic ammonia synthesis on Mo2TiC2 monolayer, and provides a new guideline for reducing the overpotential by reaction pathway change.

2. Computational Methods

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Based on density functional theory (DFT), all of the calculations were performed by using the Vienna ab-initio Simulation Package (VASP)[48].The exchangecorrelation energy was described with the Perdew, Burke, Ernzerhof (PBE) version of the generalized gradient approximation (GGA)[49]. Core electrons effect on the valence electron density were represented by using The Projector Augmented Wave (PAW)[50] method. The cutoff energy for the plane wave basis sets was 450 eV. The position of atoms and the cell parameters were analyzed by using the conjugated gradient method. The convergence threshold was set to be 10−5 eV in energy and 10−2 eV/Å in force. In the optimizations of monolayers, the Brillouin zone[51] was sampled by Monkhorst–Pack k-points mesh of 6x6x1. The electronic structures were obtained by a set of 12x12x1 and using the tetrahedron method. DFT-D3 method was employed to calculate the van der Waals (vdW) interaction[52]. The vacuum space was larger than 20 Å so as to avoiding interactions between simulated 2D Mo2TiC2 monolayer sheet and the periodic images. The charge density difference is analyzed by the VESTA code[53]. In this work, the calculations of Gibbs free energies followed the approach of Nørskov[54-56] et al. The reference potential is standard hydrogen electrode (SHE), which was theoretically defined in solution (pH=0, p (H2) =1bar). The chemical potential of a proton and electron pair (μ (H++e-)) equivalent to that of a half of gaseous hydrogen (μ (H2)) under standard reaction condition. The Gibbs free energy (ΔG) for each elemental step is calculated at 298.15 K from the following equation: ΔG = ΔEDFT + ΔZPE+ΔH−TΔS + ΔGpH + ΔGU, Where E is the total energy calculated by VASP software, ZPE is the zero point energy, the change in the heat capacity and entropy are present as ΔH, ΔS, T is the temperature (298.15 K). ΔGpH is the Gibbs free energy change caused by pH value. ΔGpH = −kBTln[H+] = pH × kBT ln10, Where kB is the Boltzmann constant. All calculation presented in this work, pH value is equal to zero. ΔGU is caused by electrode potential U. ΔGU = −neU, Where n is the transferred electron number. The adsorption energies (Ead) of the different adsorbates, the following equation was considered: Ead=Etotal-(Esub+ Emol) Where Etotal shows the total energy of substrate after adsorption, Esub is the energies of substrate, and the Emol denotes the total energies of an adsorbate molecule in the gas phase including the N2, NxHy.

3. Results and Discussion

3.1 Structures and Properties of Mo2TiC2

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Currently, MXenes monolayer are usually obtained by exfoliating MAX phase [5759]. Due to the similarity of the atomic structure, the Mo2TiC2 sheet is built up via substituting two surface Ti-atomic layers on the Ti3C2 by Mo atoms. As Figure1 shows, the atomic model of geometry is built with quintuple layers in a sequence of Mo (s) − C−Ti (c)−C−Mo (s), where Mo (s) represents the surface Mo atoms and Ti (c) represents the center Ti atoms,. The optimized lattice parameter of relaxed structure is a = b = 2.961 Å. The Mo2TiC2 MXene is highlighted in a 4 × 4 × 1 supercell. Four different highly symmetric adsorption sites have been studied, including the atop site above the Mo atom (T), the bridge site of the Mo–Mo bond (B) and two different hollow sites with the centre of C atom (H1) and Ti atom (H2).

Figure 1 The Schematic of different adsorption sites of metal atoms on Mo 2TiC2 monolayer on the top view (a) and side view (b). The supercell indicates the high symmetry Top (T), Bridge (B), Hollow (H1, H2) adatom site. The

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blue, grown and purple balls represent titanium, carbon and molybdenum atom.

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To analyze the effect of Mo substitution, the density of states (DOS) of Mo2TiC2 and Ti3C2 are presented and compared in Figure 2. Both pristine Ti3C2 and Mo2TiC2 are metallic system, as we can see a high electron density near the Fermi level. The electron density of Mo2TiC2 layer is smaller than Ti3C2, implying Mo atoms benefit to the stability. To further understand these changes, the partial electron density of states (PDOS) is also examined. In the Ti3C2 monolayer, the electron density of states around the Fermi level are mainly made of the Ti-3d orbitals. The C-2p orbitals mainly hybridize with Ti-3d orbitals under Fermi level (around -3.0 eV). Whereas in the Mo2TiC2 monolayer, both Ti-3d orbitals and Mo-4d orbitals contribute to the Fermi level, and Ti-3d orbitals become smaller compared to Ti3C2. It can be considered that the electronic properties of Mo2TiC2 are strongly associated with Mo atom. It is noteworthy that the hybridization between C-2p orbitals and Ti-3d orbitals is lower in energy (around -4.0 eV), which means a strong C-Ti interaction n in Mo2TiC2. But C2p orbitals and Mo-4d orbitals has an obvious overlap around the Fermi level. It can be concluded that Mo atom can enhances the C-Ti bond and stabilizes the structures.

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Figure 2. Partial density of states (PDOS) of (a) Mo2TiC2 and (b) Ti3C2.

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3.2 N2 Adsorption

Figure 3. (a), (b), (c) Optimized structures of different N2 adsorption configuration. (d), (e), (f) The distribution of

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transferred charges between N2 molecular and Mo2TiC2 monolayer. The isovalue is all the same, equal to 0.005 eÅ-3. Green stands for getting charge, yellow stands for losing charge. (g), (h), (i) Electron localization function (ELF)

molybdenum and nitrogen atom.

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plots for the corresponding configuration. The blue, grown, purple and silver balls represent titanium, carbon,

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N2 activation is the fundamental requisite to start the NRR process. The initial adsorption manner has a significant impact on the subsequent reaction pathway. Thus, different initial configurations of N2 adsorption on the Mo2TiC2 and functionalization monolayer with O/OH/F groups are examined, including end-on and side-on initial configurations. MXene coated by functional groups can hardly adsorb nitrogen, due to the active sites of Mo surface occupied (Figure S1). As depicted in Figure 3, eight different initial configurations trend to form three kind of optimized structure H2-side on (Figure 3(a)), H1-side on (Figure 3(b)), T-end on (Figure 3(c)). All of the examined structures, adsorption energy are negative, from -1.268 to -0.903eV, which means the strong N2-philicity of the materials. The DFT+D3 results suggest that N2 goes from the gas phase to a chemisorbed state preferring to be the horizontal configuration anchoring at the H2 site, as Figure 3(a) shows . The N≡N bond (the optimal distance) is sufficiently activated from 1.11 Å to 1.268Å . Even on the Mo/BN single atom catalyst, N≡N triple bond is slightly activated with near 1.12 Å [60]. In order to understand the bonding strengths, electron localization function (ELF) analysis are carried out to describe the localization of electrons pairs. The value of ELF is set in the range of 0 to 1, where 1, 0.5, and 0 represent covalent, metallic and nobonding character, respectively. The weak localization of ELF∼0.3 is observed between Mo and N atoms, which means the metallicionic character. In addition, there is a strong

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localization of electrons near the hollow site (ELF~0.7) on the surface of Mo2TiC2 MXene. In the Figure 3(g), localization electron between surface and nitrogen match well, compared with others. Thus, H2 (side on) is considered to be the most favorable structure for N2 adsorption. To further explain the strong adsorption of N2 molecular, the charge density differences and bader charge analysis code[61] are carried out. Green stands for getting charge, yellow stands for losing charge. It can be seen that N2 molecular acts as the electron acceptor, and the electron are transferred from the surface Mo layer of Mo2TiC2 in all of structure. As for the most energy favorable structure (Figure 3(d)), around the N2 molecular, the electrons are mainly located around N atoms, imply noticeable activation of N2 molecular. Since the amount of charge transfer up to 1.08 e, the adsorption of N2 molecular on the Mo2TiC2 MXene is considered to be chemical.

Figure 4. Partial density of states (PDOS) of differnet N2 adsorption structure. (a) H2-side on (b) H1-side on (c)

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Further, electron density of states (DOS) calculation are carried out to examine the electronic effect between N2 molecular and Mo2TiC2 monolayer, as shown in Figure 4. Compared pristine Mo2TiC2 in Figure 2(a), Ti-3d orbitals, Mo-4d orbitals present more overlaps with C-2p orbitals, which means the synergistic effect of Mo-Ti atom. N-2p orbitals present an effective overlap with Mo-4d orbitals validated the strong interaction, in the vicinity of -6 eV below the Fermi level. The electrons of Mo2TiC2 were transferred into anti-bond level of N2, resulting in the decrease of bond order in N2.This is in line with the results of previous structural calculations. Thus, we can conjecture that the Mo2TiC2 is an eligible electrocatalyst for the Nitrogen Reduction Reaction (NRR) due to strong N2-philicity than most of existing catalyst [60, 62-64].

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3.3 N2 Reduction Mechanism

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Figure 5 The schematic of ninteen possible pathway and possible reaction intermediates for NRR. The annotation in parentheses represents the arrowhead color of the previous step.

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To further illustrate the NRR performance the Mo2TiC2, a mechanism study has been performed. Reaction intermediates for N2 Reduction Reaction on the Mo2TiC2 monolayer have been researched and the most stable structures are obtained as Figure 5 shows. Nineteen different arrows represent nineteen possible pathways. And the short blue line means the interaction between NxHy species and substrate[65, 66]. To further evaluate the potential as the electrocatalyst for electrocatalytic ammonia synthesis, the subsequent NRR elementary steps through two reaction mechanisms are canvassed, including associate mechanism and dissociative mechanism. We define the pathway which N2 do not break until the ammonia generate as associate one, otherwise define as dissociation.Associative mechanism has five possible pathways (Association A~Association F) and dissociative mechanism pathways including fourteen possible pathways (Dissociation A~Dissociation N). The DFT calculations are also carried to obtain a reasonable approximation of the Gibbs free energy in the Figure 6. Correction terms (zero-point energy, enthalpic temperature, entropy corrections) of adsorbates and gas-phase molecules are listed in the Table S1and Table S2. Providing that the NRR follows the associative mechanism, once N2 molecule

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presents a side-on configuration on Mo2TiC2 monolayer, the first H+ + e- pair from the electrolyte is added to one of the N atom to generate NNH species. The length of N−H bond is 1.027 Å. Along with the H+ attacks, the N−N bond is further elongated to 1.354 Å. The Gibbs free energy of this step increase by 0.22 eV. For the second H+ + e- pair attacking *NNH species, there are two possible production: one is added into the bare N atom to generate *NHNH and the Gibbs free energy slightly decrease by -0.01 eV, another is added into the -NH terminal to form NNH2 species and the Gibbs free energy increase by 0.06 eV. On the Association A pathway,*NNH2 species break into *N and NH3 by the H+ + e- pair attacking. The free energy is downhill by -1.78 eV. The subsequent elementary steps is hydrogenating the remaining *N species into the second NH3 by three steps. The ΔG values of three steps are -0.49 eV, 0.56 eV and 0.26 eV. On the Association B pathway, NNH2 species hydrogenated into *NHNH2 species with the ΔG values 0.20 eV. While on the Association C pathway, the H+ + e- pair attacks *NHNH species , the *NHNH2 species generate with the ΔG values 0.17 eV. The subsequent elementary steps is same. The H+ + e- pair adds to the –NH terminal of *NHNH2 species, *NH2NH2 species yield. This elementary step has a big barrier in the free energy calculation (0.95 eV). The following step is ammonia generate one by one. In fact, H+ + e- pair can also add to the –NH2 terminal of *NHNH2 species which leads to the N-N bond break (Association D Association F). This elementary step is downhill by -2.24 eV. It is obvious that *NHNH2 species trend to break into *NH and NH3 in thermodynamics. When the NRR follows the dissociative mechanism, the situation become more complex. Along with the first H+ + e- pair attacks, N2 separate into N atom and NH species with the Δ G values -2.03 eV. On the Dissociation A pathway, the hydrogenation do not happen on the bare N terminal until first NH3 yield. On the Dissociation B pathway, the H+ + e- pair adds to the bare N terminal first to form *NH+*NH species. It can also be generated by *NHNH break up (Dissociation H). The following step generate *NH2 + *NH species with the ΔG values 0.95 eV. It is worth mentioning that *NH2 + *N species can hydrogenate into *NH2 + *NH species (Dissociation M, Dissociation N). The Δ G values of this step is -0.43 eV. The *NH2NH species can also break up into *NH2 + *NH species (Dissociation D, Dissociation E) The Gibbs free energy of this elementary step decreases by -2.3 eV. The next hydrogenation step has two product: one is *NH2 + *NH2 species (Dissociation B, Dissociation F, and Dissociation G), anther is *NH species and NH3 (Dissociation I, Dissociation J, Dissociation K, and Dissociation L). On the Dissociation C pathway, *N+*NH2 species is formed by NNH2 breaking.

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Figure 6. Gibbs free energy diagram at U=0V. The orange line indicates the favorable pathway.

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This Gibbs free energy diagram consists of two parts: reaction and diffusion. The diffusion part shows that ammonia desorption is not spontaneous and the free energy increases at least 0.56 eV. In the reaction part, the value of free energy change is much more complexed. The Gibbs free energy of different hydrogenated product is displayed clearly in the Figure 6. When the first H+ + e- pair attacks activated N2 molecule, breaking into N atom and NH species is thermodynamically spontaneous (-2.034 eV), generate NNH species is non-spontaneous (0.217 eV). On the NNH species hydrogenation process, generating NNH2 species is non-spontaneous (0.059 eV)， generating NHNH species，breaking into N atom and NH2 species is spontaneous in thermodynamics (-0.095 eV). On the NHNH species hydrogenation process, breaking into NH and NH2 species (-2.29 eV) is spontaneous in thermodynamics, too. To a certain extent, every hydrogenation results more in cracking. However, the possibility of reaction pathways cannot be judged simply. Thus, evaluating the potential-determining step (PDS) and the corresponding minimum overpotential are necessary. In the associative mechanism, the potentiallimiting step is *NH species hydrogenation to form *NH2 species with the free energy of 0.56 eV. The potential-limiting step in the Association B,C pathway (*NHNH2 species hydrogenation to form *NH2NH2 species) has a larger ΔG values(0.95 eV). In the dissociative mechanism, there are three different potential-limiting step: *NH+NH2 species hydrogenate into form 2*NH2 species withΔG values 0.6 eV (Dissociation B,D,E,N,M ), *NH species hydrogenate into *NH2 species with ΔG values 0.56 eV (Dissociation A,C,H), *NH2 species hydrogenate into *NH3 species with ΔG values 0.26 eV (Dissociation F,G). Obviously, the NRR of the Mo2TiC2 MXene prefers to be

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Dissociation F or G due to the smallest overpotential (0.26 V). Further, the *NNH species more trend to be *NHNH, due to the smaller ΔG. Thus, the optimal NRR mechanism is considered as Dissociation F pathway as the orange line in the Figure 6. Different with the traditional Haber‐ Bosch process ， along the favorable Dissociation F or G pathway ， N≡N triple bond do not break down until the ∗ 𝑁𝐻𝑁𝐻2 + 𝐻 + + 𝑒 − →∗ 2𝑁𝐻2 step. Hydrogenation elementary step occur effectively causes N≡N triple bond more prone to breakage. Compared with other pathway, the most difficult step (*NH species hydrogenation) have been avoided, which benefits to the reduction of overpotential.

Figure 7. Gibbs free energy diagram at U=0.26 V. The orange line indicates the favorable pathway..

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Furthermore, the influence of applied electric potential (0.26 V) are discussed. As Figure 7 shows, most pathway still have the non-spontaneous elementary step, except the orange line pathway (only discuss the reaction part). In other words, the orange line pathway become thermodynamically feasible at the potential of 0.26 V. Compared with other Mo-base catalyst [46, 47, 67-71], the overpotential is effectively reduced. Thus, from the Gibbs free energy analyses, it can concluded that Mo2TiC2 would be a new, effective materials of ammonia synthesis through dissociation F pathway with 0.26 V overpotential. Suitable catalyst is effective to reduce the overpotential by changing the reaction pathway. 3.4 H2 Evolution Reaction In the above section, the reaction free energy change for NRR have been studied

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to illustrate the excellent catalytic activity on Mo2TiC2 catalyst. Now, the competing hydrogen evolution reaction (HER) should be considered to study the NRR selectivity. The Gibbs free energies of Mo2TiC2 MXenes have been calculated to investigate the HER activity (Figure 8). The optimized H* structure with a large negative ΔG of 0.74 eV has been represent in the Figure 8. The required free energy for *N2 (-0.80 eV) is slightly negative than that for *H. In other word, the Mo active site is easier to be occupied by N2, which benefits to the NRR performance. Further, the interaction between H* and the Mo active site is so strong that prevents the further hydrogen generation. Thus, the HER with a large barrier of 0.74 V can be highly suppressed when the overpotential of NRR performance is 0.26 V.

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Figure 8. Free energy diagram for the HER on Mo2TiC2.

4. Conclusions

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In summary, structure and electronic calculation have been employed to study the feasibility of Mo2TiC2 electrocatalyst of ammonia synthesis. N2 molecule present a side-on configuration on Mo2TiC2 monolayer, and N≡N is sufficiently activated, from 1.11 to 1.268Å . Further, the catalytic reaction mechanism are analyzed by Gibbs free energy calculation. The result shows rate-determining steps of nineteen possible pathways (five association and fourteen dissociation included) are not identical. The lowest energy barrier step is the NH2 hydrogenated into ammonia with 0.26 eV. Thus, Mo2TiC2 as an ordered, double transition metals carbides exhibits high NRR activity through a dissociation mechanism with the overpotential of 0.26 V, which satisfies the balance of N2 activation and the reduction of the overpotential. Further, the reaction free energy change for the competing HER reveals that the Mo active site is easier to be occupied by N2. The relatively high potentials (0.74 V) also means the competing reaction is highly suppressed by the optimal overpotential (0.26 V). Thus, the Mo2TiC2 is an eligible and promising electrocatalyst for ammonia synthesis. The mechanism study provides a new guideline for reducing the overpotential by reaction pathway change.

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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No 21625604, 21776251, 21671172, and 21706229).

Reference

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[1] T. Travis, Chem. Ind-london., (1993) 581-585. [2] A. Vojvodic, A.J. Medford, F. Studt, F. Abild-Pedersen, T.S. Khan, T. Bligaard, J.K. Norskov, Chem. Phys. Lett., 598 (2014) 108-112. [3] W. Patkowski, W. Rarog-Pilecka, Przem. Chem., 96 (2017) 1042-1046. [4] A. Azimi, A. Arabkhalaj, R.S. Markadeh, H. Ghassemi, Fuel, 230 (2018) 52-63. [5] S. Giddey, S.P.S. Badwal, A. Kulkarni, Int J Hydrogen Energy, 38 (2013) 1457614594. [6] M.A. Shipman, M.D. Symes, Catal. Today, 286 (2017) 57-68. [7] H.Z. Liu, Chinese J Catal, 35 (2014) 1619-1640. [8] V. Kyriakou, I. Garagounis, E. Vasileiou, A. Vourros, M. Stoukides, Catal. Today, 286 (2017) 2-13. [9] Y.Y. Cao, Y.J. Gao, H. Zhou, X.L. Chen, H. Hu, S.W. Deng, X. Zhong, G.L. Zhong, J.G. Wang., Adv. Theory Simul, 1 (2018) 1870012. [10] X. Zhong, H.Y. Yu, X.D. Wang, L. Liu, Y. Jiang, L. Wang, G.L. Zhuang, Y.Q. Chu, X.N. Li, J.G. Wang, Acs Appl. Mater. Inter., 6 (2014) 13448-13454. [11] L.N. Zhai, C.N. Cui, Y.T. Zhao, X.L. Zhu, J.Y. Han, H. Wang, Q.F. Ge, J Phys. Chem. C, 121 (2017) 16275-16282. [12] N.K. Chaudhari, H. Jin, B. Kim, D. San Baek, S.H. Joo, K. Lee, J. Mater. Chem. A, 5 (2017) 24564-24579. [13] M. Khazaei, A. Ranjbar, M. Arai, T. Sasaki, S. Yunoki, J Mater. Chem. C, 5 (2017) 2488-2503. [14] M. Naguib, Y. Gogotsi, Accounts. Chem. Res., 48 (2015) 128-135. [15] M. Naguib, O. Mashtalir, J. Carle, V. Presser, J. Lu, L. Hultman, Y. Gogotsi, M.W. Barsoum, Acs Nano, 6 (2012) 1322-1331. [16] A.H. Khan, S. Ghosh, B. Pradhan, A. Dalui, L.K. Shrestha, S. Acharya, K. Ariga, B. Chem. Soc. Jpn., 90 (2017) 627-648. [17] X. Zhong, Y.Y. Qin, X.L. Chen, W.L. Xu, G.L. Zhuang, X.N. Li, J.G. Wang, Carbon, 114 (2017) 740-748. [18] Q.Q. Meng, J.L. Ma, Y.H. Zhang, Z. Li, C.Y. Zhi, A. Hu, J. Fan, Nanoscale, 10 (2018) 3385-3392. [19] B. Ahmed, D.H. Anjum, Y. Gogotsi, H.N. Alshareef, Nano Energy, 34 (2017) 249256. [20] C. Eames, M.S. Islam, JACS, 136 (2014) 16270-16276. [21] M. Naguib, J. Come, B. Dyatkin, V. Presser, P.L. Taberna, P. Simon, M.W. Barsoum, Y. Gogotsi, Electrochem. Commun., 16 (2012) 61-64. [22] C. Zhan, M. Naguib, M. Lukatskaya, P.R.C. Kent, Y. Gogotsi, D.E. Jiang, J. Phys. Chem. Lett, 9 (2018) 1223-1228. [23] J.M. Luo, X.Y. Tao, J. Zhang, Y. Xia, H. Huang, L.Y. Zhang, Y.P. Gan, C. Liang, W.K. Zhang, Acs Nano, 10 (2016) 2491-2499. [24] R.T. Wang, S.J. Wang, Y.B. Zhang, D.D. Jin, X.Y. Tao, L. Zhang, J. Mater. Chem. A, 6 (2018) 1017-1027. [25] L. Lorencova, T. Bertok, E. Dosekova, A. Holazova, D. Paprckova, A. Vikartovska,

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SC R

IP T

V. Sasinkova, J. Filip, P. Kasak, M. Jerigova, D. Velic, K.A. Mahmoud, J. Tkac, Electrochim. Acta, 235 (2017) 471-479. [26] Z.W. Zhang, H.N. Li, G.D. Zou, C. Fernandez, B.Z. Liu, Q.R. Zhang, J. Hu, Q.M. Peng, Acs Sustain. Chem. Eng, 4 (2016) 6763-6771. [27] G.P. Gao, A.P. O'Mullane, A.J. Du, Acs Catal., 7 (2017) 494-500. [28] Z.W. Seh, K.D. Fredrickson, B. Anasori, J. Kibsgaard, A.L. Strickler, M.R. Lukatskaya, Y. Gogotsi, T.F. Jaramillo, A. Vojvodic, Acs Energy Lett., 1 (2016) 589594. [29] V.M.H. Ng, H. Huang, K. Zhou, P.S. Lee, W.X. Que, J.Z. Xu, L.B. Kong, J. Mater. Chem. A, 5 (2017) 3039-3068. [30] Q. Xue, Z. Pei, Y. Huang, M. Zhu, Z. Tang, H. Li, Y. Huang, N. Li, H. Zhang, C. Zhi, J. Mater. Chem. A, 5 (2017) 20818-20823. [31] M. Yu, S. Zhou, Z. Wang, J. Zhao, J. Qiu, Nano Energy, 44 (2018) 181-190. [32] M.K. Han, X.W. Yin, H. Wu, Z.X. Hou, C.Q. Song, X.L. Li, L.T. Zhang, L.F. Cheng, Acs Appl. Mater. Inter., 8 (2016) 21011-21019. [33] S.Z. Zhang, J. Liu, Y. Xie, Y.J. Lu, L. Li, L. Lu, J.H. Yang, S.H. Wei, Acta Physchim. Sin., 33 (2017) 2022-2028. [34] N. Li, X.Z. Chen, W.J. Ong, D.R. MacFarlane, X.J. Zhao, A.K. Cheetham, C.H. Sun, Acs Nano, 11 (2017) 10825-10833. [35] L.M. Azofra, N. Li, D.R. MacFarlane, C. Sun, Energy Environ. Sci., 9 (2016) 2545-2549. [36] B. Anasori, J. Halim, J. Lu, C.A. Voigt, L. Hultman, M.W. Barsoum, Scripta Mater, 101 (2015) 5-7. [37] H. Kim, B. Anasori, Y. Gogotsi, H.N. Alshareef, Chem. Mater., 29 (2017) 64726479. [38] M. Khazaei, A. Ranjbar, M. Arai, S. Yunoki, Phys. Rev. B, 94 (2016) 125152. [39] Y.F. Li, Y.C. Ding, B. Xiao, Y.H. Cheng, Phys Lett A, 380 (2016) 3748-3755. [40] T.L. Tan, H.M. Jin, M.B. Sullivan, B. Anasori, Y. Gogotsi, Acs Nano, 11 (2017) 4407-4418. [41] B.M. Hoffman, D.R. Dean, L.C. Seefeldt, Accounts. Chem. Res., 40 (2009) 609619. [42] R.R. Schrock, Science, 301 (2003) 76-78. [43] D. Yang, T. Chen, Z. Wang, J. Mater. Chem. A, 5 (2017) 18967-18971. [44] L. Zhang, X. Ji, X. Ren, Y. Ma, X. Shi, Z. Tian, A.M. Asiri, L. Chen, B. Tang, X. Sun, Adv. Mater., 30 (2018) e1800191. [45] Q. Li, L. He, C. Sun, X. Zhang, J Phys. Chem. C, 121 (2017) 27563-27568. [46] I. Matanovic, F.H. Garzon, N.J. Henson, Phys. Chem. Chem. Phys., 16 (2014) 3014-3026. [47] J. Han, X. Ji, X. Ren, G. Cui, L. Li, F. Xie, H. Wang, B. Li, X. Sun, J. Mater. Chem. A, 6 (2018) 12974-12977. [48] G. Kresse, J. Furthmuller, Comp. Mater. Sci., 6 (1996) 15-50. [49] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett, 77 (1996) 3865-3868. [50] G. Kresse, J. Furthmuller, Phys. Rev. B, 54 (1996) 11169-11186. [51] H.J. Monkhorst, J.D. Pack, Phys. Rev. B, 13 (1976) 5188-5192.

A

CC E

PT

ED

M

A

N

U

SC R

IP T

[52] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, J Chem. Phys., 132 (2010) 154104. [53] K. Momma, F. Izumi, J Appl. Crystallogr, 44 (2011) 1272-1276. [54] J. Rossmeisl, A. Logadottir, J.K. Nørskov, Chem. Phys., 319 (2005) 178-184. [55] J.K. Norskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J.R. Kitchin, T. Bligaard, H. Jonsson, J Phys. Chem. B, 108 (2004) 17886-17892. [56] A.A. Peterson, F. Abild-Pedersen, F. Studt, J. Rossmeisl, J.K. Norskov, Energ. Environ. Sci., 3 (2010) 1311-1315. [57] B. Akuzum, K. Maleski, B. Anasori, P. Lelyukh, N.J. Alvarez, E.C. Kumbur, Y. Gogotsi, Acs Nano, 12 (2018) 2685-2694. [58] M. Alhabeb, K. Maleski, B. Anasori, P. Lelyukh, L. Clark, S. Sin, Y. Gogotsi, Chem. Mater., 29 (2017) 7633-7644. [59] M. Alhabeb, K. Maleski, T.S. Mathis, A. Sarycheva, C.B. Hatter, S. Uzun, A. Levitt, Y. Gogotsi, Angew. Chem. Int. Edit., 57 (2018) 5444-5448. [60] Z.X. Wang, Z.G. Yu, J.X. Zhao, Phys. Chem. Chem. Phys., 20 (2018) 12835-12844. [61] W. Tang, E. Sanville, G. Henkelman, J. Phys-condens. Mat., 21 (2009) 084204. [62] J.R. Han, Z.C. Liu, Y.J. Ma, G.W. Cui, F.Y. Xie, F.X. Wang, Y.P. Wu, S.Y. Gao, Y.H. Xu, X.P. Sun, Nano Energy, 52 (2018) 264-270. [63] Y. Zhang, W.B. Qiu, Y.J. Ma, Y.L. Luo, Z.Q. Tian, G.W. Cui, F.Y. Xie, L. Chen, T.S. Li, X.P. Sun, Acs Catal., 8 (2018) 8540-8544. [64] H. Cheng, L.X. Ding, G.F. Chen, L. Zhang, J. Xue, H. Wang, Adv. Mater., (2018) 1803694. [65] D. Bao, Q. Zhang, F.L. Meng, H.X. Zhong, M.M. Shi, Y. Zhang, J.M. Yan, Q. Jiang, X.B. Zhang, Adv. Mater., 29 (2017) 1604799. [66] J. Zhao, Z. Chen, JACS, 139 (2017) 12480-12487. [67] I. Matanović, F.H. Garzon, N.J. Henson, Phys. Chem. Chem. Phys., 16 (2014) 3014-3026. [68] X. Ren, G. Cui, L. Chen, F. Xie, Q. Wei, Z. Tian, X. Sun, Chem. Commun., 54 (2018) 8474-8477. [69] J. Zhao, J. Zhao, Q. Cai, Phys. Chem. Chem. Phys., 20 (2018) 9248-9255. [70] Z.W. Chen, X. Lang, Q. Jiang, J. Mater. Chem. A, (2018) 9623-9628. [71] X. Zhang, Z. Yu, S.S. Wang, S. Guan, H.Y. Yang, Y. Yao, S.A. Yang, J. Mater. Chem. A, 4 (2016) 15224-15231 .