Graphene Oxide-Supported Transition Metal Catalysts for Di-Nitrogen

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Graphene Oxide-Supported Transition Metal Catalysts for Di-Nitrogen Reduction Tongtong Yang, Shaobin Tang, Xiyu Li, Edward Sharman, Jun Jiang, and Yi Luo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08149 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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The Journal of Physical Chemistry

Graphene Oxide-Supported Transition Metal Catalysts for Di-Nitrogen Reduction Tongtong Yang†§, Shaobin Tang‡§, Xiyu Li†, Edward Sharman‖, Jun Jiang*†, Yi Luo† †Hefei

National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation

Center of Chemistry for Energy Materials), CAS Center for Excellence in Nanoscience, Department of Chemistry and Materials Science, University of Science and Technology of China (USTC), Hefei, Anhui 230026, China ‡Key

Laboratory of Organo-Pharmaceutical Chemistry of Jiangxi Province, Gannan Normal University,

Ganzhou 341000, China ‖Department

of Neurology, University of California, Irvine, California 92697, USA

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ABSTRACT: Developing single metal atom catalysts with high stability and efficiency for the electroreduction of dinitrogen (N2) to ammonia (NH3) has attracted extensive attention but remains a challenge. In this work, we employed density functional theory calculations to design the first graphene oxide- (GO-) supported transition metal catalysts (TM@GO) for N2 fixation. Both single TM atoms and trimers (TM = Pt, Cu, Ni and Co) are considered. The calculated results show that owing to the active sites provided by the epoxy functional group, GO can serve as an ideal substrate to stabilize TM atoms, as it affords larger binding energies and higher diffusion barriers, compared to pristine graphene. The strong interaction of TMs with GO is ascribed to the large polarization of the positive charges on deposited TM atoms. Deposited TM3 trimers possess higher stability than single TM atoms. Interestingly, Ni3@GO exhibits the highest electrocatalytic activity for converting N2 to NH3 among the TM atoms considered. The predicted reaction pathways show that the reduction of N2 to NH3 at deposited TMs follows a Heyrovský associative rather than a dissociative mechanism.


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1. Introduction Ammonia (NH3) is an important raw material for synthetic fertilizers, and for the past century it has been mainly produced from dinitrogen (N2) and hydrogen (H2) by the Haber–Bosch1 process. However, this process requires extreme reaction conditions (350° to 550°C and 150 to 350 atm) to activate N2 when iron-based heterogeneous catalysts are used, making it energy intensive. Indeed, Haber-Bosch ammonia synthesis consumes a significant amount of the global annual energy supply (~1.4%). In contrast to the energy-consuming Haber–Bosch process, biological N2 fixation is known to occur at ambient pressure and mild temperatures. Certain microorganisms can reduce N2 to NH3 by using nitrogenase enzymes. In this transformation, six electrons and six protons are required to produce two equivalent NH3 molecules per N2.N2 + 6H+ + 6e - → 2NH3. Inspired by the natural biological process, various transition metal–dinitrogen complexes have been designed for the transformation of N2 into NH3 under ambient conditions. Banerjee et al.2 , for example, prepared FeMoS-chalcogels that were capable of reducing N2 to NH3 under mild conditions. Then in 2016 ， Li et al.3 proposed using FeN3-embedded graphene as a catalyst to convert N2 into NH3. Recently, Zhao et al. 4 employed single Mo atom-embeded BN nanosheets as a catalyst for fixing N2. Due to their high activity, however, transition metal (TM) atoms generally prefer to agglomerate on surfaces, thus resulting in catalyst deactivation. Therefore, it is necessary to employ suitable substrates (such as metal hydride clusters or organic ligands) to stabilize the TM atoms. Graphene (Gr) has attracted considerable attention owing to its outstanding structural and electronic properties and potential applications in nanoscale electronics.5-9 A notable drawback of Gr is the lack of active defect sites on its surface, leading to weak interaction with atoms or molecules. Such weak interactions allow TM atoms to aggregate easily. To avoid this, a large number of strategies have been proposed for stabilizing unaggregated TM atoms on Gr. Tawfik and his coworkers10 reported that the introduction of carbon vacancies into Gr sheets can enhance the binding of TM. In order to increase the ACS Paragon Plus Environment

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stability of TM atoms, heteroatoms, such as B11 , N12,13 and P14, have been introduced into vacancy defect sites on the Gr surface. However, the introduction of vacancy defects may have negative effects on the electronic properties of Gr, thus lowering the activity of TM atoms for the reduction of N2 to NH3. Thus, developing simple and more moderately active substrates to support TM atoms that thereby retain high catalytic activity, still remains a large challenge. Graphene oxide (GO)15,16, modified as it is by oxygen-containing groups that provide active defect sites, can act as an excellent substrate for transition metal–dinitrogen complexes. Indeed, Palladium (Pd) clusters are calculated by Tang and Zhu17 to exhibit strong binding to GO, and GO so decorated strongly adsorbs nitrogen oxides. Moreover, Chen et al.18 showed that Magnesium (Mg) atom-doped GO has improved hydrogen storage capacity. In addition, Shown et al.19 reported that GO decorated with copper (Cu) nanoparticles has been used to enhance the photocatalytic reduction of carbon dioxide (CO2) by visible light. GO-based catalysts were widely applied in various fields, such as catalytic decomposition, photosensitive ignition, catalytic combustion, solar cells, and biotherapeutics.20-24 He et. al.20 have proposed the GO supported transition metal complexes of triaminoguanidine which showed high catalytic performance on RDX decomposition. Kymakis et. al.23 fabricated flexible organic photovoltaic devices with in situ nonthermal photoreduction of spin-coated GO electrodes. Jung et al.24 reported the photothermal ablation therapy of skin cancer by using GO-hyaluronic acid conjugate. However, reports of using GO-supported TM clusters as catalysts for N2 fixation are scarce. In this work, we present the design of an excellent dinitrogen reduction catalyst consisting of a three TM atoms cluster supported on GO (TM3@GO) using density functional theory (DFT) calculations. Based on the large adsorption energy and high transfer barrier calculated for this system, our results show that GO can effectively anchor small clusters of TM atoms. Furthermore, following the associative Heyrovský pathway, 25 TM3@GO can efficiently convert N2 into NH3. 2. Computational methods ACS Paragon Plus Environment

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All calculations have been performed using the plane-wave technique as implemented in the Vienna ab-initio simulation package (VASP).26 The generalized gradient approximation (GGA) with PerdewBurke-Ernzerhof (PBE) functional was employed to describe the exchange-correlation potential in these calculations.27 Spin-polarized calculations were carried out. The projector augmented wave (PAW)28 method with an energy cutoff of 500 eV was used for the plane-wave expansion. Considering the strong coulomb and exchange corrections required for TMs, the GGA+U method was employed to describe partially filled d-orbitals.29 The long-range van der Waals (vdW) correction was made using Grimme's parameters30 (referred to as DFT+D2). The climbing image nudged elastic band (CI-NEB) method was applied to search for transition states.31 The force and energy convergence criteria were set to 0.01 eV/Å and 10-5 eV, respectively. The Gr model (see Figure S1 in Supporting Information) was constructed using unit cell parameters a=b=2.46Å, c=6.80Å; α=β=90 °, γ=120°. A 4 × 4 supercell with 32 carbon atoms was used. The Brillouin zone was sampled with 5 × 5 × 1 Monkhorst–Pack32 k-meshes. The effect of k-mesh on energy has been presented in Figure S1. Continue to increase k-mesh from 5 × 5 × 1, the energy is almost unchanged. Therefore, we believed that k-mesh of 5 × 5 × 1 is reasonable for the calculation accuracy. Since GO is always obtained by strong oxidation of graphite by the modified Hummer method, there are various oxygenated functional groups on its surface, the two dominant ones generally considered to be hydroxyl (OH) and epoxy (–O–) groups.33-39 In this work, GO was constructed by adsorption of an oxygen atom at the carbon-carbon bridge site. Compared to Gr, the surface of GO is slightly distorted (Figure S2). To evaluate the binding strength between TM and the substrate, the binding energy (Eb) was calculated as: Eb = ETM + EG –ETM@G Where ETM, EG, and ETM@G represent the energies of TMn (n = 1 or 3), of Gr or GO, and of TMn@Gr or TMn@GO , respectively. ACS Paragon Plus Environment

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3. Results and discussion First, we placed one TM atom (Pt, Cu, Ni, or Co) on Gr (geometrical structures are shown in Figure S3). The results of binding energies are listed in Table 1. The values of Pt@Gr, Cu@Gr, Ni@Gr and Co@Gr are 3.04, 0.59, 2.35 and 2.78 eV, respectively. When the substrate was converted to GO (the geometrical structures are showed in Figure S4), binding energies for these metals are increased to 3.89, 1.98 3.57 and 5.55 eV, respectively. The results clearly show that the oxygen functional group on GO can stabilize atoms of these metals. Deposition of a single, isolated metal atom may cause loss of the active site. Thus, we also investigated the adsorption of a group of three TM atoms on Gr and GO. Starting from the initial configuration of TM3@GO in which TM and O atoms are put on the bridge site above C-C bond (Figure S5), we carried out geometric optimization which results in relatively stable structures of Pt3@GO, Cu3@GO, Ni3@GO and Co3@GO in Figure 1 (and TM3@Gr in Figure S6). In Table 1, compared to pristine Gr, it is found that the TM3@GO systems exhibit larger binding energies. The largest Eb of TM3 on GO is 6.92 eV, indicating strong coupling and good stability. The differences in binding energies of TM atoms adsorbed onto GO or Gr substrates is shown in Figure 2. Obviously, positive difference values show that GO improves the stability of a metal compared to its deposition on Gr. More importantly, GO stabilizes metal trimers to a greater degree than single metal atoms. In addition, from Bader charge analysis (Table 1), it is found that charges of 0.01, 0.18, 0.47 and 0.57 e- were transferred from TM atoms to the Gr substrate for adsorbed Pt, Cu, Ni, and Co, respectively. For the GO support, these charges increase to 0.31, 0.58, 0.60 and 0.80 e-, respectively, indicating an enhanced interaction with the TM atom. Compared to single TM atoms, TM3 trimers transfer more charge from metal to GO, ranging from 0.81 to 1.64 e-. The distribution of exchange/polarized charges is displayed in Figure 3. These charges were obtained by subtracting the calculated electronic charges of the individual TM3 trimers and GO from those of the adsorbed TM3@GO. Clearly, the redistributed ACS Paragon Plus Environment

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negative charge densities primarily accumulate on the oxygen atom of GO, while the positive densities accumulate on the metal atoms. Meanwhile, the structures of TM@Gr, TM@GO, TM3@Gr, and TM3@GO have also been investigated after increasing their supercell size to 6 × 6 (Figure S7-10). The binding energies of TMn (n = 1 or 3) and polarized charges of TM taken from the Gr or GO support were listed in Table S1. Compared with the results of 4 × 4 supercell, one can conclude that their catalysis performances are independent of the size of supercell. Table 1. The computed binding energies (Eb) of TMn (n = 1 or 3) and polarization positive charges of TM taken from the Gr or GO support in TM@Gr, TM@GO, TM3@Gr, and TM3@GO, respectively. TM Pt Cu Ni Co

Eb (eV) TM charge (e+) TM@Gr TM@GO TM3@Gr TM3@GO TM@Gr TM@GO TM3@Gr TM3@GO 3.04 3.89 5.16 6.38 0.01 0.31 0.03 0.81 0.59 1.98 1.52 4.34 0.18 0.58 0.50 1.57 2.35 3.57 2.63 6.06 0.47 0.60 0.57 1.53 2.78 5.55 1.98 6.92 0.53 0.80 0.74 1.64


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Figure 1. Top and side views of the optimized structures of TM3@GO (TM= Pt, Cu, Ni or Co).


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Figure 2. The computed differences in binding energies of TM and TM3 between Gr and GO supports.


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Figure 3. The distribution of exchange/polarized charges between TM3 and GO with the isosurfaces of 0.009 e Å-3. Cyan and yellow bubbles represent positive and negative charges, respectively.


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Figure 4. The diffusion barriers of deposited TM atoms on Gr and GO. To gain further insight into the stability of TM anchored graphene systems, we also investigated the diffusion barriers of metal atoms as shown in Figure 4. From the potential energy profile of TM3 diffusion on GO (see details in Figure S11), the calculated diffusion barriers show that deposition of trimers or single metal atoms onto the GO substrate is much more stable dynamically than onto pristine Gr. Compared with a given single metal atom, diffusion of its TM3 cluster has a much higher barrier. For example, barriers for Pt3 and Co3 can be up to about 3.4 eV, suggesting that the GO substrate effectively hinders further TM aggregation. Based on binding energies and barriers, one can assume that TM atoms preferably are deposited on the GO surface in the form of a trimer cluster. For TM3@GO, if the functional group of GO is converted from epoxy (–O–) to hydroxyl (OH), one can get the optimized structure of TM3@GO-OH (Figure S12). The binding energies of Pt@GO-OH, Cu@GO-OH, Ni@GO-OH and Co@GO-OH are 5.28, 4.03, 5.17 and 4.91 eV,


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respectively (Table S2). For TM3@GO, the corresponding values are increased to 6.38, 4.34, 6.06, 6.92 eV, respectively. Therefore, TM3@GO is more stable than TM3@GO-OH. Bader charge analysis indicated that the coupling between TM atoms and substrate in TM3@GO was stronger than TM3@GO-OH. TM3@GO thus represents the most stable catalyst configuration among all designs. We have investigated the catalytic activity of highly stable TM3@GOs as catalysts for N2 fixation. At the cathode, a N2 molecule is reduced to NH3 by transfer of six proton-electron pairs according to the following overall reaction: N2 + 6H+ + 6e- → 2NH3

(1)

In this study, following a Heyrovský-type mechanism, we investigated two pathways for reduction of N2 to NH3: associative and dissociative. Accordingly, the adsorbed NHx or N2Hx species (x = 0-2) are directly hydrogenated by attachment of protons from the electrolyte and electrons from the electrode. The detailed Heyrovský mechanisms and the process for free energy calculation can be found in Supporting Information. The associative mechanism for N2 fixation is first discussed. Free energy profiles are shown in Figure 5, and corresponding optimized structures of various intermediates can be found in Figure S13. The free energies for N2 adsorption are -1.87, -0.56, -1.30 and -1.43 eV for Pt, Cu, Ni and Co trimers, respectively, indicating strong interaction with the TM3@GO catalyst. Beginning with the proton-electron pair transfer, the free energy changes for the first three steps of the associative process are uphill for all considered TM3@GO. For Pt, Cu, and Co trimers supported on GO, the addition of the first hydrogen atom to form *N2H is the rate-determining step among the first three steps, with increased free energies of 0.68, 1.52, and 1.24 eV, respectively. In the


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case of Ni3@GO, the *N-NH2 bond cleavage to form *N and NH3 by hydrogenation has the highest free energy change of 1.23 eV, and thus becomes the rate-determining step. In the next three steps, the free energy pathways for all TM3@GO catalysts are downhill. However, the last step, desorption of *NH3, is endothermic. By analyzing the entire reduction process for nitrogen fixation, clearly, one can see that the rate-determining step depends on the particular metal involved. For Pt3 and Co3 systems, the adsorbed NH3 desorption must overcome the highest free energies of 1.80 and 1.57 eV among all reaction steps, respectively. In the case of Cu3@GO, the formation of *NNH has a more positive free energy change (1.52 eV) than the *NH3 desorption step, determining limiting-potential for N2 reduction. For the Ni3 system, the transfer of the third proton-coupled electron to *NNH2 to form *N and NH3 becomes the rate-determining step, with a free energy barrier of 1.23 eV. Furthermore, we plotted the free energy profiles for N2 reduction to NH3 on Ni3@GO at applied potentials U = 0 and -1.23 V vs the standard hydrogen electrode (SHE) (see Figure S14). Clearly, the key step in the reduction of N2 to NH3 is cleavage of the N-N bond in *N-NH2 at -1.23 V. Therefore, Ni deposited on a GO surface possesses the highest catalytic performance for N2 fixation among all considered metal systems.


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Figure 5. Free energy profiles for nitrogen fixation on TM3@GO via the Heyrovský associative mechanism. The free energy profiles for N2 fixation via Heyrovský's dissociative mechanism for the TM3@GOs are shown in Figure 6. There are obvious differences in the stabilities of intermediates between the associative and dissociative mechanisms. For deposited Cu3, Ni3 and Co3 clusters, the free energy pathways for N-N cleavage to form two adsorbed *N are uphill with barriers of 5.05, 3.64, and 1.74 eV, respectively, while such N-N cleavage for Pt3@GO is downhill in the free energy profile by 0.39 eV. In the following six proton-coupled electron transfer steps, the formation of intermediates is thermodynamically favorable. The final two non-


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electrochemical steps releasing the adsorbed NH3 are endergonic for all metals. The desorption of the first adsorbed NH3 must overcome the highest barrier of 1.80 eV among all metals. Therefore, comparing the associative with the dissociative mechanism, electroreduction of N2 to NH3 on TMs supported on GO is achieved more readily by the associative mechanism, which offers a lower energy barrier.

Figure 6. Free energy profiles for nitrogen fixation on TM3@GO via the Heyrovský dissociative mechanism.


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Moreover, simulations of using TM3@GOs as catalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) were performed, which however exhibited relatively low catalytic activities for both reaction ( Figure S15 and Figure S16). 4. Conclusion In summary, first-principles calculations have been employed to investigate the stability of TM (TM = Pt, Cu, Ni and Co) atoms anchored on Gr or GO and their electrocatalytic activity for N2 fixation. The calculated results show that owing to the active defect sites provided by the epoxy functional group, TM atoms can strongly bind to the GO surface based on their large binding energies and high diffusion barriers. Compared to single TM atoms, TM3 trimers possess higher stability. Calculations of free energy pathways show that Ni3@GO is stable and exhibits high electrocatalytic activity for N2 fixation, as it has a relatively low limiting-potential of -1.23 V following the Heyrovský associative mechanism, compared to the Co, Cu, and Pt trimers. Our results open up a new strategy for the design of several metal atoms catalysts for N2 fixation. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Calculation of free energy, optimized structures of graphene and graphene oxide, configurations of TM@G, diffusion barrier of TM3 on GO, various relevant species for nitrogen reduction reaction, free-energy diagrams of oxygen reduction reaction and hydrogen evolution reaction for TM3@GO, including Table S1−S2 and Figures S1−S16.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Shaobin Tang: 0000-0002-2969-8698 Jun Jiang: 0000-0002-6116-5605 §Author Contributions Tongtong Yang and Shaobin Tang contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the MOST 973 Program (No. 2014CB848900), the National Natural Science Foundation of China (NSFC) (No. 21633006, 21473166 and 21463004), Hefei Science Center CAS (2016HSC-IU012), CAS Key Research Program of Frontier Sciences (QYZDB-SSW-SLH018), the Steady High Magnetic Field Facilities, High Magnetic Field Laboratory, CAS, and the Fundamental Research Funds for the Central Universities. The numerical calculations in this paper have been done on the supercomputing system in the Supercomputing Center of University of Science and Technology of China. REFERENCES (1) Aika, K.-i.; Tamara, K. Ammonia Synthesis over Non-iron Catalysts and Related


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Graphene Oxide-Supported Transition Metal Catalysts for Di-Nitrogen

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