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Surface Cluster Catalyst for N2-to-NH3 Thermal Conversion Xue-Lu Ma, Jin-Cheng Liu, Hai Xiao, and Jun Li J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10354 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017
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Journal of the American Chemical Society
Surface Cluster Catalyst for N2-to-NH3 Thermal Conversion Xue-Lu Ma, Jin-Cheng Liu, Hai Xiao* and Jun Li* Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China
Supporting Information ABSTRACT: The ammonia synthesis from N2 is of vital importance, with imitating biological nitrogen fixation attracted much interest. Herein, we investigate the catalytic mechanisms of N2-to-NH3 thermal conversion on the singly dispersed bimetallic catalyst Rh1Co3/CoO(011), and find that the preferred pathway is an associative mechanism analogous to the biological process, in which alternating hydrogenations of the N2 occur, with H2 activation on both metal sites. We propose that the singly dispersed bimetallic M1An catalyst, in which the doped metal atom M substitutes an oxygen atom on the oxide surface of metal A, serves as a new surface single-cluster catalyst (SCC) design platform for the biomimetic N2-to-NH3 thermal conversion. The catalytic ability of M1An catalyst is attributed to both the charge buffer capacity of doped metal M and the complementary role of synergic metal A in catalysis. Our work provides insights and guidelines for further optimizing the M1An catalyst.
Ammonia is one of the most important and widely produced chemicals worldwide, and a primary ingredient in production of fertilizer.1 The large scale synthesis of ammonia starts with N2 activation, which requires overcoming its inertness due to the strong triple-bonds, apolarity, a negative electron affinity, a large HOMO-LUMO gap (10.82 eV), and a high ionization energy (15.58 eV).2 Thus the catalyst plays a key role in activating N2 to produce ammonia.3 In contrast to the N2 dissociative mechanism involving metal surface in the Haber-Bosch process,4 most of the studies of the enzyme-catalyzed ammonia synthesis propose an associative mechanism via N2/H+/e- reaction system, wherein the N≡N bond breaks after partial hydrogenation of the N2 molecule.5 Inspired by this moderate biological nitrogen fixation process, extensive efforts have been made to study the activation and functionalization of bound N2,6 while a high-efficiency catalytic nitrogen cycle is still a long-standing and challenging goal, and the mechanism of catalytic N2-to-NH3 conversion remains elusive. The electron-donating ability of materials has been proved to play an important role in the redox mechanism of N2 activation.7 Active metal sites with low chemical valence can contribute to enhancing the electron-donating ability to the antibonding π* orbitals of N2, which further weakens the N≡N bond and drives the eventual cleavage of N2.8 However, the scaling relation of hydrogen-containing intermediates on transition-metal surfaces precludes the independent optimization of adsorption and transition-state energies that would deliver the ideal catalyst.9 Wang et al. very recently demonstrated that this scaling relation can be broken by a second catalytic site that intervenes in the TM-
mediated catalysis.10 Particularly, the bimetallic sites with metals in low oxidation state can deliver the cooperative or synergistic effects that arise from the sequential or simultaneous actions on different metal centers.11 Each metal center is specifically responsible for different elementary steps that compose the overall reaction.12 Furthermore, a bimetallic metal-metal bond possesses an intrinsic polarity due to metal-doping at O-vacancy that induces unique reaction pathways.13
Figure 1. Isolated Rh1Co3 bimetallic sites. (a) The top view. (b) The side view. (c, d) Single dispersion of Rh1Co3 bimetallic sites anchored on CoO(011) surface to form SCC. Recently, the singly dispersed bimetallic site (M1An) was identified to facilitate dissociation of a reactant molecule and then accelerate a subsequent coupling with another reactant molecule to form desired product molecule.14 Isolated Rh1Co3 bimetallic sites exhibit a distinctly different catalytic performance in reduction of nitric oxide with carbon monoxide. 14a The Rh atom in the dispersed bimetallic sites, which substitutes an oxygen atom on the CoO surface as shown in Figure 1, can be regarded as both an electron-donating site and a catalytic site. This catalyst was stable at reducing condition with H2 and at higher temperature.14 Here, different catalytic mechanisms of N2-to-NH3 thermal conversion are explored on the isolated Rh1Co3 site using firstprinciples methods of density functional theory (DFT) (see computational details in the Supporting Information, SI). We find that the preferred reaction pathway is the associative mechanism analogous to the biological process, which is driven by both the charge buffering ability of doped metal M in M1An and the complementary role of A in catalysis. Thus, we propose that the singly dispersed bimetallic M1An catalyst can serve as a new surface single-cluster catalyst design platform (Figure S1) for the
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biomimetic N2-to-NH3 thermal conversion. Guidelines for optimizing its performance is also provided. An isolated bimetallic site supported on a non-metallic surface (e.g., metal oxides) can exhibit a distinctly different catalytic performance, owing to the ionic state of the singly dispersed bimetallic sites and the minimized choices of binding configurations of reactants compared with continuously packed bimetallic sites.14a Therefore, the fundamental features concerning the N2 and H2 chemisorption are first investigated.
Rh atom is not saturated in its coordination shell, co-adsorption is in fact more stable than mono-adsorption, leading to a thermodynamic advantage for the coadsorption of N2 and H2 on the same Rh center of Rh1Co3 bimetallic site as shown in Figure 2(c), which then drives unique associative reaction pathways. In an associative mechanism, the two nitrogen centers in N2 remain bound to each other as the molecule is hydrogenated stepby-step, with NH3 being released only when the final N-N bond is broken. Hydrogenation in an associative mechanism can be envisaged to occur through two possible pathways. In the first type (the alternating pathway), each of the two nitrogen centers undergoes single hydrogenation event in turn, until one of them is converted into NH3 and the N-N bond is broken. The second type is in analogy with an evolutionary distal associative pathway. Hydrogenation occurs preferentially on one of the two N atoms, leading to the release of one NH3 and leaving behind a metal nitrido (M-N) unit, which is subsequently hydrogenated to deliver the other NH3. Feasible associative pathways of N2-to-NH3 conversion on the Rh1Co3 bimetallic site were all investigated as shown in the reaction network in Figure S4. In either associative mechanism, the reaction pathway involves common elementary steps as follows: H2/N2 chemisorption steps, N2 hydrogenation (N-H bond formation) steps, N-N cleavage step and NH3 desorption steps. The sequence of the elementary steps determines the mechanism. The energy diagrams for the alternating and evolutionary distal hydrogenation mechanisms are shown in Figures S5 and S6, respectively, and the optimized intermediates and transition states along the reaction pathway are also presented.
Figure 2. (a) The optimized N2 adsorption structures and corresponding adsorption energies. (b) The relationship between N2 unit charge and N–N bond length. (c) The proposed favorable coadsorption of N2 and H2 on the Rh center of Rh1Co3 bimetallic site. Based on the our ab initio molecular dynamics (AIMD) simulations and static DFT calculations (Movie S1 and Figure 2(a)), N2 activation is spontaneously achieved when it is chemically adsorbed on the surface isolated Rh1Co3 sites. Chemisorbed N2 on isolated Rh1Co3 sites experiences significant weakening of the N ≡ N triple bond, promoting its catalytic conversion into NH3. Specifically, a high degree of N2 activation is embodied in a bridging manner, such as configurations (1), (2) and (5) as shown in Figure 2(a) and Table S1. The increased negative charge and N-N distance of N2 indicate reductive activation of the coordinated N2 through an enhanced backdonation from the bridging synergic metal centers, as depicted in the charge density difference on adsorbates and surface shown in Figure S2. Thus, there is elongation of N–N bond lengths in each N2 adsorption configurations caused by transfer of electron from metal d states to the antibonding orbitals of N2, and the relationship between N2 unit charge and N–N bond length of six chemisorbed N2 states is shown in Figure 2(b). The activation of N2 at the isolated Rh1Co3 sites is consistent with the principles of dinitrogen hydrogenation.15 H2 absorbs dissociatively at Rh1Co3 bimetallic sites (Rh site and Co1/Co4 sites) and absorbs molecularly at sites far away from Rh1Co3 bimetallic sites (Co2/Co3 sites) as shown in Figure S3 and Table S2. The results indicate that the isolated Rh active center of Rh1Co3 bimetallic site is efficient for the activation of H2 to H active species. In the H2 adsorption configurations, the adsorbatesurface bonding is interpreted as a combination of the filled 1σ MO of H2 interacting with the empty π-states of the metal and πback-donation into the antibonding orbital of H2.16 Because the
Starting from the stable coadsorption of N2 and H2 on the Rh1Co3 bimetallic site (i) in Figure 2(c), the preferred reaction pathway was verified to be the alternating hydrogenation mechanism as shown in Figure 3. The key intermediates are illustrated in Figure 4. The active H* at Rh center is further transferred to the nitrogen atom from 2 to 3 with a barrier of 1.85 eV. The second H2 either adsorbs at Co1 site on 3, leading to the alternating pathway, or adsorbs at Rh center of 3, leading to the evolutionary distal pathway. It is the extra accommodation offered by the Co sites that draws the reaction to the alternating pathway.
Figure 3. Energy diagram for the alternating hydrogenation of N2 on a singly dispersed Rh1Co3 bimetallic site anchored on CoO(011). (see Figure S7 for free energies at different temperatures) After the alternating hydrogenation of both Rh-connected nitrogen and Co1-connected nitrogen to form the NH2NH2 hydrazine species (5), N-N bond cleavage occurs from 7 to 8 with the accessible barrier of 0.78 eV. The resulted two neighboring NH2 adsorbates locate between Rh and Co4, as well as Co1 and
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Journal of the American Chemical Society Co6, respectively. The calculation suggests that the third H2 is again activated by the Rh center, and delivers the active H* species to generate the first NH3 at Co4 site (from 10 to 11), with the barrier of 1.05 eV. The second NH3 is then produced at Co1 site (from 12 to 14) with the barrier of 2.03 eV. It is worth noting here that the final production step of both NH3 takes place on Co sites, as the NH3 desorption on Co site is easier than on Rh center. Thus, the Co sites serve as an important complement to the catalytic kernel. In the evolutionary distal associative pathway, N-N cleavage after successive hydrogenation of Rh-connected nitrogen from 17 to 25 has a high barrier of 3.35 eV, as shown in Figure S3. With the increase of hydrogenation degree on the Rh-connected nitrogen, the activation energy of N-N bond splitting decreases (with barrier of 2.94 eV from 18 to 26, and 2.19 eV from 20 to 27), but it is still not as favorable as the N-N bond cleavage from 7 to 8 in the alternating pathway. Hence, the alternating pathway as presented in Figure 4 dominates the mechanism for N2-to-NH3 conversion on the singly dispersed bimetallic site. As shown in Figure S8 the Rh1Co3/CoO(011) catalyst is stable and recovers after the catalytic reaction, despite surface Rh-Co distance alteration along the reaction coordinate due to surface reconstruction.
The activation energies for the series of reactions NHxNHx + H → NHxNHx+1 and NHxNHx+1 + H → NHx+1NHx+1 show approximately linear dependence on the Bader charge difference on the doped metal (Figure 5a). When the Rh center is substituted by Co forming a Co4 monometallic site (energy diagram for the alternating hydrogenation is shown in Figure S11), such linear dependence still exists, as shown in Figure 5b. This indicates that the charge buffer capacity of the doped metal M in M1An can be used to tune the barriers in the alternating mechanism, and the doped metal with large charge buffer capacity and low N2 reduction activation energy is the ideal candidate for ammonia synthesis. These requirements constitute the base to select Rh1Co3/CoO(011) for N2-to-NH3 conversion. Further search for the optimal design of surface single-cluster catalysts with singly dispersed bimetallic M1An sites is under progress.
Figure 5. The relationship of N2 hydrogenation energy barriers and the corresponding doped metal charge difference. (a) Rh1Co3 bimetallic site. (b) Co4 monometallic site. H1-H6 stand for six elementary steps of N2 hydrogenation (N-H bond formation).
Figure 4. The preferred N2-to-NH3 conversion mechanism on the singly dispersed bimetallic site of SCC. To gain further insights, charge population analysis was performed and shown in Figure S9 and Figure S10. The results suggest that the preferred alternating pathway is dictated by the electronic effect of doped metal M in M1An. Due to the substitution of an oxygen atom (i.e., filling the oxygen vacancy) on the CoO surface, the Rh atom in isolated bimetallic site lies in a low oxidation state and is negatively charged, and serves as an electron reservoir capable of electron donation or releasing, which buffers the charge variation during N2-to-NH3 conversion. The counteraction of the charges on NxHy moiety by the Rh center manifests at the steps of hydrogenation (N-H bond formation) and N-N bond cleavage. According to Bader charge analysis, the charge on Rh center increases from -0.13 to 0.13 |e|, when the charge on NxHy moiety decreases from 0.09 to -0.93 |e| during N-N bond cleavage. The charge on Rh center decreases from 0.39 to 0.28 |e|, when the charge on NxHy moiety increases from -1.06 to -1.03 |e| during the first hydrogenation, whereas the charges on the Co sites show little change throughout the N2-toNH3 conversion. Thus, the ability of doped metal M in M1An as a charge buffer underlies the catalytic ability of M1An for the this conversion.
In summary, a new heterogeneous catalyst design, surface single-cluster catalyst with singly dispersed bimetallic M1An sites, was proposed for the N2-to-NH3 thermal conversion to imitate the associative reaction mechanism of nitrogenase. We have demonstrated with isolated Rh1Co3 cluster on CoO(011) surface that the preferred pathway indeed follows the analogous enzymecatalyzed associative mechanism, in which hydrogenations of the two N atoms of N2 occur in an alternating manner, with H2 activation on both metal sites to deliver the active H species. The catalytic ability of M1An catalyst arise from both the electronic effect of doped low-valent metal M that serves as a charge buffer, and the complementary role of synergic metal A in catalysis. We suggest that the ideal, doped metal candidate should be with large charge buffer capacity and low N2 reduction activation energy.
ASSOCIATED CONTENT
Supporting Information The Supporting Information (SI) is available free of charge on the ACS Publications website. Additional comments and discussion of the findings in the manuscript, and supplemental data as noted in the text (PDF). AUTHOR INFORMATION
Corresponding Author *E-mail for J.L.:
[email protected]; H.X.:
[email protected] Notes The authors declare no competing financial interest.
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ACKNOWLEDGMENT This work is financially supported by NSFC (21590792, 91645203, and 21521091) and China Postdoctoral Science Foundation (No. 2017M610863). The calculations were performed using supercomputers at Tsinghua National Laboratory for Information Science and Technology and the Supercomputer Center of the Computer Network Information Center at the Chinese Academy of Sciences. REFERENCES (1) (a) Ertl, G. Catal. Rev. 2006, 21, 201. (b) Tanaka, H.; Nishibayashi, Y.; Yoshizawa, K. Acc. Chem. Res. 2016, 49, 987. (2) Jia, H. P.; Quadrelli, E. A. Chem. Soc. Rev. 2014, 43, 547. (3) van der Ham, C. J.; Koper, M. T.; Hetterscheid, D. G. Chem. Soc. Rev. 2014, 43, 5183. (4) (a) Schlogl, R. Angew. Chem. Int. Ed. Engl. 2015, 54, 3465. (b) Ertl, G. Angew. Chem. Int. Ed. Engl. 2008, 47, 3524. (c) Logadóttir, Á.; Nørskov, J. K. J. Catal. 2003, 220, 273. (d) Honkala, K.; Hellman, A.; Remediakis, I. N.; Logadottir, A.; Carlsson, A.; Dahl, S.; Christensen, C. H.; Nørskov, J. K. Science 2005, 307, 555. (5) (a) Kuriyama, S.; Arashiba, K.; Nakajima, K.; Matsuo, Y.; Tanaka, H.; Ishii, K.; Yoshizawa, K.; Nishibayashi, Y. Nat. Commun. 2016, 7, 12181. (b) Coric, I.; Mercado, B. Q.; Bill, E.; Vinyard, D. J.; Holland, P. L. Nature 2015, 526, 96. (c) Hoffman, B. M.; Lukoyanov, D.; Yang, Z. Y.; Dean, D. R.; Seefeldt, L. C. Chem. Rev. 2014, 114, 4041. (d) Schrock, R. R. Angew. Chem. Int. Ed. Engl. 2008, 47, 5512. (6) (a) Knobloch, D. J.; Lobkovsky, E.; Chirik, P. J. Nat. Chem. 2010, 2, 30. (b) Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84. (c) Rodriguez, M. M.; Bill, E.; Brennessel, W. W.; Holland, P. L. Science 2011, 334, 780. (d) Burford, R. J.; Yeo, A.; Fryzuk, M. D. Coord. Chem. Rev. 2017, 334, 84. (e) Falcone, M.; Chatelain, L.; Scopelliti, R.; Živković, I.; Mazzanti, M. Nature 2017, 547, 332.
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