Single Molybdenum Atom Anchored on N-Doped Carbon as a

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Single Molybdenum Atom Anchored on N-Doped Carbon as a Promising Electrocatalyst for Nitrogen Reduction into Ammonia at Ambient Conditions Chongyi Ling, Xiaowan Bai, Yixin Ouyang, Aijun Du, and Jinlan Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05257 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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The Journal of Physical Chemistry

Single Molybdenum Atom Anchored on N-Doped Carbon as a Promising Electrocatalyst for Nitrogen Reduction into Ammonia at Ambient Conditions Chongyi Ling,1,2 Xiaowan Bai,1 Yixin Ouyang,1 Aijun Du,*,2 Jinlan Wang*,1 1

School of Physics, Southeast University, Nanjing 211189, People’s Republic of China

2

School of Chemistry, Physics and Mechanical Engineering, Science and Engineering

Faculty, Queensland University of Technology, Gardens Point Campus, Brisbane, QLD 4001, Australia

Abstract: Ammonia (NH3) is one of the most important industrial chemicals owning to its wide applications in various fields. However, the synthesis of NH3 at ambient conditions remains a coveted goal for chemists. In this work, we study the potential of the newly synthesized single atom catalysts (SAC), i.e. single metal atoms (Cu, Pd, Pt, and Mo) supported on N doped carbon for N2 reduction reaction (NRR) by employing first-principles calculations. It is found that Mo1-N1C2 can catalyze NRR through an enzymatic mechanism with an ultra-low overpotential of 0.24 V. Most importantly, the removal of the produced NH3 is rapid with a free energy uphill of only 0.47 eV for the Mo1-N1C2 catalyst, which is much lower than those ever-reported catalysts with low overpotentials and endows Mo1-N1C2 excellent durability. The coordination effect on activity is further evaluated, showing that the experimentally realized active site, single Mo atom coordinated by one N atom and two C

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atoms (Mo-N1C2), possesses the highest catalytic performance. Our study offers new opportunities for advancing electrochemical conversion of N2 into NH3 at ambient conditions. Introduction NH3 is one of the most important industrial chemicals, which has wide applications in various fields, including plastic, textile, fertilizer, precursor for nitrogen-containing compounds as well as hydrogen carrier.1-2 The industrialized production of NH3 is mainly based on the Haber-Bosch process using atmospheric N2 and H2 as raw materials.3-4 Although the N2 from the atmosphere is almost unlimited, the extremely high stability of N2 makes the Haber-Bosch process require a heavy energy-consumption and extreme reaction conditions (150−350 atm, 350−550 °C).5-6 A more promising and attractive strategy for NH3 production from N2 is the electrochemical N2 reduction reaction (NRR), which is originated from the nitrogenase enzymes in bacteria that can run nitrogen fixation (N2 + 6H+ + 6e- = 2NH3) at atmospheric pressure and room temperature.7-9 Therefore, NH3 production through such a process can not only reduce the high reaction pressure and temperature, but also lower the energy consumption. More importantly, the production yield of NH3 can be effectively improved by controlling the operating potential and reaction environment.10-11 Thus, searching for a catalyst that can perform nitrogen fixation like nitrogenase enzymes is of paramount importance, which remains an aspirational goal for chemists until now.5, 12 Metal based catalysts play critical roles in catalysis.13-15 For NRR, various transition metal based molecular catalysts, such as Fe- and Mo-nitride complexes, have been synthesized and shown good performance for NRR, where the single metal atoms coordinated by N atoms (M-Nx) are the active centers.16-22 However, molecular catalysts generally exhibit poorer


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durability and are less readily incorporated into electrolyzers in comparison with heterogeneous catalysts.23-24 Thus, incorporating the active centers of the molecular catalysts (M-Nx) into conductive solids to construct well-defined and tunable heterogeneous catalysts is attractive and desirable for NRR. Among numerous heterogeneous catalysts, the newly emerged single atom catalysts (SACs) also hold the single atoms as the active centers.25-31 Very recently, Li’s group reported the synthesis of various single metal atoms (Cu, Pd, Pt, and Mo) supported on N doped carbon with the help of chitosan and metal precursor (CuCl2•2H2O, PdCl2, K2PtCl6 and Na2MoO4•2H2O).32 The single metal atoms in the as-prepared catalysts were coordinated by one nitrogen atom and two carbon atoms (M1-N1C2). Moreover, Mo1-N1C2 is found to be highly active for hydrogen evolution reaction under alkaline condition, with a low overpotential of 132 mV at a current density of -10 mA/cm2. Actually, such a M1-N1C2 structure is analogous to that of active center for NRR in metal-nitride complexes,16 and might exhibit similar outstanding catalytic performance for NRR. Investigating the NRR activity of M1-N1C2 is therefore of great economic interest and scientific importance. In this work, we systematically investigate the catalytic performance of the experimentally realized single atom catalyst, M1-N1C2 (M = Cu, Pd, Pt, and Mo)32, for NRR on the basis of first-principles computations. Mo1-N1C2 stands out due to its strong binding strength with N2 (-1.19 eV). Further calculations show that Mo1-N1C2 possesses ultra-high catalytic activity for NRR with a rather low overpotential of 0.24 V, which ensures the efficient reduction of N2 on Mo1-N1C2 at ambient conditions. Surprisingly, the free energy change (∆G) for NH3 desorption is only 0.47 eV, leading to the rapid removal of produced NH3 and better



durability of Mo1-N1C2. The coordination effect on the activity is further studied and the experimentally synthesized single Mo atom coordinated by one N atom and two C atoms (Mo1-N1C2) is supposed to own the highest activity for NRR. Computational Details All the first-principles calculations were performed by using Vienna ab initio simulation package (VASP).33-34 The ion-electron interactions were described by the projector augmented wave (PAW) method.35 The generalized gradient approximation in the Perdew-Burke-Ernzerhof (PBE) form36-37 and a cut-off energy of 500 eV for plane-wave basis set were adopted. Spin-polarized calculations were employed for all the systems and the convergence criterion for the residual force was set to 0.01 eV/Å. To simulate the M1-N1C2, a single metal atom anchored on 5 × 5 × 1 supercell graphene with an N atom doped near the metal atom is used, which is similar to the theoretical model presented in the original work.32 The vacuum space was larger than 15 Å, which is enough to avoid the interaction between two periodic units. The climbing nudged elastic band method was used to locate saddle points and minimum energy paths.38 The Brillouin zone was sampled with Monkhorst-Pack mesh with a 4 × 4 × 1 k-point grid. The calculations of Gibbs free energy change (∆G) for each elemental step was based on the computational hydrogen electrode model proposed by Nørskov et al.,39 which can be computed by: ∆G = ∆E + ∆EZPE - T∆S + eU + ∆GpH where ∆E is the electronic energy difference before and after the adsorption of reaction intermediates; ∆EZPE and ∆S are the changes in zero point energies and entropy respectively,


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which are obtained from the vibrational frequency calculations and presented in Table S1; T is the temperature, which is set to be 298.15 K in this work; e and U are the number of electrons transferred and the electrode potential applied; ∆GpH is the free energy correction of pH, which can be calculated by: ∆GpH = kBT × pH × ln10, and the pH value is set to be zero. The overpotential (η) of the whole reduction process is determined by the potential-limiting step which has the most positive ∆G (∆Gmax) as computed by: η = Uequilibrium - ULimiting, where Uequilibrium is the equilibrium potential of NRR (about -0.16 V) and ULimiting is the limiting potential obtained by: ULimiting = -∆Gmax/e. Results and Discussion

Figure 1. Top and side views of the structures of (a) M1-N1C2. Mo1-N1C2 with N2 adsorption through (b) side-on and (c) end-on patterns. N-N bond lengths and charge transfer from Mo1-N1C2 to N2 are also presented. Gray, cyan, yellow and blue balls represent the C, Mo, doped N and adsorbed N atoms, respectively.

Chemisorption of N2 on the surface of catalyst to ensure the sufficient activation of the inert N≡N triple bond is the prerequisite for an efficient NRR catalyst.40 Therefore, the adsorption of N2 on M1-N1C2 (M = Cu, Pd, Pt and Mo) is first investigated. Two stable adsorption configurations are taken into consideration and respectively labeled as side-on and end-on configurations as shown in Figure 1b and 1c. For the side-on configuration, both two N atoms interact with metal atom, forming two M-N bonds (Figure 1b). For the end-on



structure, only one N atom binds with metal atom (Figure 1c). The calculated binding energies of N2 through side-on and end-on adsorption are presented in Table S2. For Cu1-N1C2, Pd1-N1C2 and Pt1-N1C2, adsorption of N2 can only adopt the end-on pattern with a rather weak binding strength, the corresponding adsorption energies are just -0.58, -0.40 and -0.58 eV, respectively. For Mo1-N1C2, N2 can be tightly fixed through both patterns, with adsorption energies of -1.19 and -1.18 eV for side-on and end-on adsorption, respectively. The corresponding N≡N bond lengths are elongated to 1.18 and 1.14 Å compared to that of isolate N2 molecule (1.12 Å). Moreover, Bader charge analysis41 show that the adsorbed N2 gains 0.49 and 0.32 e from Mo1-N1C2 for side-on and end-on configurations, respectively. These evidences imply the potential activation of the inert N≡N triple bond. Therefore, only Mo1-N1C2 is selected as the potential electrocatalyst, which is in accordance with experimental results that Mo-nitride complex catalysts play crucial role in NRR.16-21 Then we move on to the assessment of the catalytic performance of Mo1-N1C2 for the reduction of N2 into NH3. The reduction processes through both side-on and end-on N2 absorption configurations are investigated due to their very similar binding strengths. For each configuration, two possible reduction mechanisms are taken into consideration as shown in Figure S1, labeled as enzymatic, consecutive mechanisms for side-on configuration and alternating, distal mechanisms for end-on configuration, respectively. For the enzymatic and alternating mechanisms, the proton-electron pairs (H+ + e-) attack at two N atoms alternatively, while for consecutive and distal mechanisms, the proton-electron pairs first attack one N atom consecutively to form a NH3 and then attack at the remaining N atom to form another NH3.


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Figure 2. Free energy diagrams for N2 reduction through (a) alternating and (b) distal mechanisms at different applied potential as well as the corresponding (c) structures of the reaction intermediates. Gray, cyan, yellow and blue balls represent the C, Mo, doped N and adsorbed N atoms, respectively.

Free energy diagrams for N2 reduction through alternating and distal mechanisms as well as the structures of reaction intermediates are presented in Figure 2. The first two steps along the alternating and distal routes are basically the same, i.e. N2 adsorption and the reduction into *N2H, respectively. It can be clearly seen that the free energy for N2 adsorption is rather negative (-0.75 eV), indicating that Mo1-N1C2 can capture gas phase N2 molecule efficiently. However, the hydrogenation of *N2 into *N2H need a high energy consumption with ∆G increase by 0.73 eV, which is the potential-limiting step for the distal mechanism (Figure 2b). For the alternating mechanism, the potential-limiting step is the reduction of *NH-NH2 into *NH2-NH2 with a ∆G of 0.78 eV (Figure 2a). Therefore, the reduction of end-on adsorbed N2 on Mo1-N1C2 is overall inefficient, with a η of 0.62 and 0.57 V for alternating and distal mechanisms, respectively.



Figure 3. Free energy diagrams for N2 reduction through (a) enzymatic and (b) consecutive mechanisms at different applied potential as well as (c) the corresponding structures of the reaction intermediates. Gray, cyan, yellow and blue balls represent the C, Mo, doped N and adsorbed N atoms, respectively.

Figure 3 presents the free energy diagrams for the reduction of N2 via the side-on adsorption as well as the corresponding structures of the reaction intermediates. Owing to the strong binding strength, the ∆G for N2 adsorption via side-on configuration is also very negative (-0.73 eV), potentially ensuring efficient N2 capture and activation. For *N2 reduction into *N2H, the free energy just slightly uphills by 0.40 eV, which is much smaller than that of end-on adsorbed N2 (0.73 eV). Subsequently, both reduction mechanisms present similar tendency: the protonation of *N2H and *NH2 are endothermic and the others are exothermic. During the whole reduction process by enzymatic mechanism, the potential-limiting step is the hydrogenation of *N2 into *N2H with an ultra-low overpotential of 0.24 V (Figure 3a), which is much lower than that for the best metal catalyst (greater than 0.50 V).12 Such a low overpotential is an indication of the high catalytic performance of Mo1-N1C2 for NRR. While for the consecutive route, the potential-limiting step is the hydrogenation of *NH2 into *NH3 with a ∆G of 0.53 eV, which is slightly higher than that by enzymatic mechanism (0.40 eV) as shown in Figure 3b. Nevertheless, the η through the consecutive mechanism (0.37 V) is


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still attractive. Besides, band structure and density of states (DOS) of Mo1-N1C2 (Figure 4a) are further calculated and show that Mo1-N1C2 owns a quality of strong metallicity, which ensures efficient electron transfer42-44 during the reduction process. Collectively, low overpotential and high conductivity endow Mo1-N1C2 a very promising catalyst for N2 reduction into NH3 at ambient conditions. Except for N2 electrochemical reduction process, the desorption of the produced NH3 is another determining factor for an efficient NRR catalyst.12 Generally, to efficiently catalyze NRR, the catalyst should have relatively high surface activity to bind N2 tightly enough in order to activate the inert N≡N bond sufficiently.40 However, this high surface activity will also result in high binding strength between NH3 and the catalyst.12 For example, V3C2, Nb3C2 and single Mo atom supported on defected BN were predicted to be highly active catalysts for NRR. However, the free energies for NH3 desorption are relatively high, 0.92, 1.16 and 0.70 eV, respectively.11,

45

As a result, their catalytic performance will be

significantly limited by the removal of the produced NH3 and the durability of catalysts can be also lowered. Surprisingly, the ∆G for NH3 desorption on Mo1-N1C2 is only 0.47 eV despite of the high binding strength between N2 and Mo1-N1C2. Moreover, ∆G for NH3 adsorption on Mo1-N1C2 is more positive than that for N2 adsorption (-0.47 vs -0.73 eV), indicating the produced NH3 can be removed rapidly. In addition, experimental results have shown that Mo1-N1C2 possesses superior stability against long-period electrocatalytic process at both alkaline and acidic conditions.32 Therefore, Mo1-N1C2 may possess excellent durability for NRR as well.



Figure 4. (a) Band structure and density of states of Mo1-N1C2, where the red and blue lines in band structure respectively represent the majority spin and minority spin components. The Fermi level is set to zero. (b) Charge variation of the three moieties and (c) N-N bond length along the alternative mechanism via end-on adsorption, where the bond length increases linear before broken.

Charge analysis is further explored to gain insight into the excellent performance of Mo1-N1C2. According to previous studies,11, 46 each intermediate can be divided into three moieties: graphene substrate without Mo-N1C2 (moiety 1), Mo-N1C2 (moiety 2) and the adsorbed NxHy species (moiety 3) as shown in Figure S2. The charge variation here is defined as the charge difference of each moiety between the present step and the previous step, which is calculated by using Bader charge analysis41. As shown in Figure 4b, N2 gains 0.49 e through the adsorption on Mo1-N1C2, where the charge is donated by both moiety 1 and moiety 2. For the hydrogenation of *N2 into *N2H, Mo-N1C2 moiety contributes electrons to both graphene and *N2H. On the contrary, Mo-N1C2 moiety will only act as the transmitter to transfer electrons between graphene and NxHy species in the subsequent 4 hydrogenation process, as the charge variation of moiety 2 are close to 0. The graphene serves as an electron reservoir that donates or accepts electron in these four steps ascribing to its high charge density.46-47 Besides, N−N bond lengths in each intermediate along the pathway (Figure 4c) increase monotonously, indicative of the gradual activation process of N2. Moreover, from gas phase N2 to *NH-*NH2 (where the N-N has not been broken), N-N bond length presents a nearly linear increase, implying that the stretching effect of adsorption is comparable to that


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of the hydrogenation. Therefore, Mo1-N1C2 possesses the considerate ability to activate the adsorbed N2 sufficiently. Finally, the coordination effect on the performance is evaluated as single Mo atoms in different molecular catalysts for NRR normally have distinct coordination.16-17,

20

Two

different kinds of active centers, Mo-N2C1 and Mo-N3, are constructed as shown in Figure S3. As displayed in Figure S4 and S5, the binding strength of N2 on Mo-N2C1 and Mo-N3 are very strong for both side-on and end-on patterns and are constantly enhanced with the increase of number of coordinated N atom (from Mo-N1C2 to Mo-N2C1 to Mo-N3). The ∆G of the protonation of *N2 into *N2H on Mo-N2C1 and Mo-N3 are further calculated as an initial evaluation of their catalytic activity for NRR since this process always has a relatively high ∆G in the whole reduction reaction. As shown in Figure S6, ∆G for the hydrogenation of side-on adsorbed N2 into *N2H on both Mo-N2C1 and Mo-N3 are rather positive (0.72 and 0.73 eV, respectively). On the contrary, the protonation of end-on adsorbed N2 is much easy with ∆G increase by 0.48 and 0.43 eV for Mo-N2C1 and Mo-N3, respectively. Nevertheless, the side-on adsorption of N2 is much more stable than the end-on adsorption for both cases of Mo-N2C1 and Mo-N3, making the proportion of end-on adsorbed N2 rather rare. Therefore, Mo-N2C1 and Mo-N3 are not efficient sites for catalyzing NRR and the Mo-N1C2 site which has been realized experimentally owns the highest activity. Conclusions In summary, we have systematically investigate the potential of a series of experimentally realized single atom catalysts (M1-N1C2, M = Cu, Pd, Pt, and Mo) for electrochemical N2 reduction into NH3. Mo1-N1C2 exhibits the outstanding performance with a calculated



overpotential of only 0.24 V, indicating the NRR can efficiently take place on Mo1-N1C2 at ambient conditions. More strikingly, the produced NH3 can be removed rapidly with a ∆G as low as 0.47 eV, much lower than ever reported catalysts with low overpotentials. Meanwhile, the metallic nature of Mo1-N1C2 will ensure the efficiency in electron transfer as well. These excellent features (strong binding strength with N2, low overpotential for NRR, rapid removal of NH3 and excellent electrical conductivity) endow Mo1-N1C2 as a compelling high-efficient and durable catalyst for electrochemical N2 reduction reaction. ASSOCIATED CONTENT Supporting Information. Schematic depiction of different reduction mechanisms for N2 reduction to NH3; definition of three moieties of NxHy adsorbed Mo1-N1C2; structures of Mo-N2C1 and Mo-N3 active centers; structures of Mo-N2C1 and Mo-N3 with N2 and N2H adsorption; free energy diagrams for N2 adsorption and protonation on Mo-N2C1 and Mo-N3. This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author * [email protected] (J.W.); *[email protected] (A.D). Notes The authors declare no competing financial interest. Acknowledgements This work is supported by the National Key R&D Program of China (Grant No. 2017YFA0204800), Natural Science Foundation of China (Grant Nos. 21525311, 21373045, 21773027), Jiangsu 333 project (BRA2016353), China Scholarship Council (CSC,


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201706090115), the Fundamental Research Funds for the Central Universities and Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_0044) in China. A.D also acknowledges the support by Australian Research Council under Discovery Project (DP170103598). The authors acknowledge the computational resources provided by NCI National Facility and The Pawsey Supercomputing Centre through the National Computational Merit Allocation Scheme supported by the Australian Government and the Government of Western Australia, and National Supercomputing Center in Tianjin.

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Single Molybdenum Atom Anchored on N-Doped Carbon as a

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