Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2018, 9, 570−576
Non-Transition-Metal Catalytic System for N2 Reduction to NH3: A Density Functional Theory Study of Al-Doped Graphene Yong-Hui Tian,*,† Shuangli Hu,† Xiaolan Sheng,† Yixiang Duan,† Jacek Jakowski,‡ Bobby G. Sumpter,‡ and Jingsong Huang‡ †
Research Center of Analytical Instrumentation, College of Life Sciences, Sichuan University, Chengdu, Sichuan 610064, PR China Center for Nanophase Materials Sciences and Computational Sciences & Engineering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
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‡
S Supporting Information *
ABSTRACT: The prevalent catalysts for natural and artificial N2 fixation are known to hinge upon transition-metal (TM) elements. Herein, we demonstrate by density functional theory that Al-doped graphene is a potential non-TM catalyst to convert N2 to NH3 in the presence of relatively mild proton/ electron sources. In the integrated structure of the catalyst, the Al atom serves as a binding site and catalytic center while the graphene framework serves as an electron buffer during the successive proton/ electron additions to N2 and its various downstream NxHy intermediates. The initial hydrogenation of N2 can readily take place via an internal H-transfer process with the assistance of a Li+ ion as an additive. In view of the recurrence of H transfer in the first step of N2 reduction observed in biological nitrogenases and other synthetic catalysts, this finding highlights the significance of heteroatom-assisted H transfer in the design of synthetic catalysts for N2 fixation.
T
the TM centers. Therefore, the catalytic conversion of N2 to NH3 also benefits from the versatile redox capability of TMs. In contrast, scant attention has been paid to the capacity of nonTMs to functionalize N2. Except for a few examples of photocatalytic systems,17,18 non-TM systems mimicking nitrogenases to catalyze nitrogen fixation under ambient conditions have not been reported. Non-TM alkyl aluminum (AlR3) compounds display strong Lewis acidity, making it possible to bind a N2 ligand in an endon manner by a dative bond. However, simple calculations of the reaction thermodynamics for a trimethylaluminum model indicate that, although it binds with N2, the Al center alone is incompetent to catalyze the transformation of N2 to NH3 because of its comparatively poor redox properties to mediate the formation of the NxHy intermediates as observed in the Mo−N2 or Fe−N2 catalyzed systems (see Figure S1 in the Supporting Information). On the other hand, graphene is expected to exhibit versatile redox capability.19 Therefore, the non-TM Al center could become catalytically active if it is embedded in a graphene framework. In fact, substitutional doping has been demonstrated to be an effective measure to modulate the catalytic properties of graphene-based materials. For instance, foreign atom doped graphene has been reported to display remarkable catalytic ability for oxygen reduction,20,21 N2O reduction,22 or oxidative coupling of amines.23 Moreover, the hypothetical Mo- or Fe-doped graphene materials have been shown to be active for catalyzing N2 reduction to NH3.24,25
he conversion of dinitrogen (N2) to ammonia (NH3) is one of the most important chemical processes in both the biosphere and the fertilizer industry. Tremendous research efforts have been made to overcome the inertness of the N2 molecule, leading to the identification of a number of catalytic systems for the reduction of N2 to NH3.1−4 The industrial Haber−Bosch process employs mainly the heterogeneous Fe catalysts for ammonia production from N2 and H2.5 In the nitrogenase enzyme family, Fe has also been elucidated as the essential element for N2 binding and fixation.6 Further, some molecular catalysts mimicking the biological nitrogenases have been synthesized, in which the conversion of N2 to NH3 occurs on Mo or Fe centers under ambient conditions in homogeneous systems.7−12 Unlike the Haber−Bosch process where the N≡N bond is broken on Fe catalysts before any hydrogen additions occur, the nitrogen fixations on nitrogenases and molecular catalysts involve six sequential proton and electron transfers without first dissociating N2 to adatoms.13,14 It is noteworthy that all of these catalytic systems are based on transition-metal (TM) centers. In the synthesized homogeneous catalytic systems featuring TM-containing Fe−N2,10−12 Mo−N2,7,9 and bimetallic Mo− N2−Mo complexes,8 the TM catalytic centers are capable of back-donating d-electrons to the antibonding π*-orbitals of the N≡N bond, thereby promoting the binding and activation of the N2 ligands.15 The bound N2 can be then functionalized to catalytically produce ammonia in the presence of external proton and electron sources. Mechanistic studies have revealed that the TMs exist in diverse oxidation states during a catalytic turnover,16 which is facilitated by the multiple d-electrons on the active TM centers and meanwhile is associated with the NxHy intermediates of various degrees of reduction ligated to © 2018 American Chemical Society
Received: November 22, 2017 Accepted: January 16, 2018 Published: January 16, 2018 570
DOI: 10.1021/acs.jpclett.7b03094 J. Phys. Chem. Lett. 2018, 9, 570−576
Letter
The Journal of Physical Chemistry Letters
relatively mild proton and electron sources, in agreement with previously studies.29 Before detailed descriptions of the catalytic processes are provided, an overview of the calculated reaction pathway based on the fragment G-Al model system C120H27Al is illustrated in Scheme 1, which shows the most favorable elementary steps computationally identified in terms of thermodynamics. Basically, N2 was first bound to G-Al 1, and then six-protons and six-electrons were alternately added to the N2 ligand in a so-called symmetric manner, giving various intermediates NxHy 2−16 of different degrees of reduction, and ultimately leading to the formation of two equivalents of NH3. According to the calculated energetics, protonations were found to occur prior to electron additions. In Scheme 1, both the intermediate structures and the respective reaction energies are presented; the charge of +1 is shown for the intermediate species at each step of protonation; the oxidation states are displayed as well for the graphene segment denoted by “G” for ease of tracking charge transfer. It is worth noting that among all of the intermediates, 4 or 4-Li+ was identified as the effective catalysts, the reason for which is clarified below. Accordingly, the catalytic reactions form a loop at the key intermediate 4 instead of going back to the starting G-Al. The reaction steps involved in the catalytic cycle are generally thermodynamically feasible, resembling the catalytic N2 reduction systems based on the synthetic TM-based complexes.14 For the detailed reaction mechanisms, we start by discussing the binding of N2 to the Al center, an essential elementary step for the occurrence of N2 reduction. Three Al−N2 complexes involved in the reaction pathway were inspected regarding their binding properties as summarized in Table 1. The (G0)Al(N2) adduct 2 displays an end-on (η1-N2) coordination mode. The adsorption of N2 onto the Al center was identified to be a barrierless process, in contrast to the existence of an activation barrier for the N2 absorption onto Al nanoclusters.30 The binding energies for both the fragment and periodic models are consistently predicted to be −0.28 eV,31 in line with the previous DFT calculations.32 With a thermal correction, this binding process became nearly thermally neutral (ΔG° = +0.07 eV, Scheme 1) as a result of the entropy contribution. Such small magnitude of Gibbs energy changes is quite common for N2 binding on other TM-based catalytic systems,12 which indicates the Al−N2 adduct is thermally labile. However, the bond distance between G-Al and N2 is relatively short at 2.169 Å, which is indicative of a binding interaction, although the corresponding Gibbs energy change is slightly positive. In comparison, another intermediate structure (G0H)Al(N2) 4 (Figure 2a), i.e., the Al−N2 complex supported by hydrogenated graphene (G0H), shows a stronger binding energy of −0.37 eV, and accordingly a shorter distance of 2.142 Å between G-Al and N2. More importantly, when a Li+ cation is incorporated as an additive into the model as denoted by 4-Li+ (Figure 2b), the binding strength of the Al−N2 complex is significantly enhanced, as demonstrated by the much stronger binding energy of −1.02 eV and the much shorter distance of 1.979 Å between G-Al and N2. According to the qualitative charge analysis on the N2 unit, the presence of Li+ can induce an electron transfer from graphene to N2, leading to a partially reduced N2 moiety that can be formally written as (N=N)−, which has a stronger coordinating capability to Al than the neutral N2 ligand (see Table S1 for detailed charge analysis). Concurrently, N2’s triple bond is appreciably activated as indicated by the elongated N≡N distance of 1.185 Å and the
With this motivation, we propose in this Letter Al-doped graphene (G-Al) as the first potential non-TM catalytic system for N2 reduction under the operative conditions similar to those of the homogeneous Mo−N2 or Fe−N2 systems. This idea may pan out for the following reasons. The binding energies of an Al atom with a single vacancy in graphene are calculated to be −5.97 and −2.58 eV in terms of atomic Al or bulk Al, respectively (see details in the Supporting Information). Additionally, the thermal stability of G-Al at room temperature has been demonstrated in previous studies by using ab initio molecular dynamics simulations.26,27 Further, the binding of a N2 ligand on Al is facilitated by the Lewis acidity and the positive charge of the Al center,28 while the versatile redox capability that is characteristic of TM-based catalysts is enabled by the graphene framework. Using density functional theory (DFT), we demonstrate the catalytic capacity of G-Al for NH3 synthesis from N2 in the presence of relatively mild proton and electron sources by examining the reaction energetics and mechanisms. According to our DFT calculations along with the electronic structure analysis, the G-Al mediated reduction of N2 to NH3 displays some unique catalytic features, especially in the initial hydrogenation step for the first bond breaking of N2. In addition, an alkali metal Li+ ion as an additive in the proposed G-Al catalytic system was found to greatly facilitate the activation and fixation of N2. To explore the catalytic N2 fixation based on the proposed non-TM catalyst, we carried out theoretical calculations using both fragment and periodic G-Al models substitutionally doped with an Al atom (Figure 1). The fragment model denoted as
Figure 1. Structure of the Al-doped graphene (G-Al) models with a C atom in the graphene framework substituted by an Al atom: (a) fragment model C120H27Al; (b) periodic model C127Al. A N2 molecule is ligated to the Al center in an end-on fashion. C, gray; N, blue; Al, pink; H, white.
C120H27Al allowed us to obtain the thermal corrections giving Gibbs free-energy changes. The energetics without vibrational corrections calculated from the complementary periodic system denoted as C127Al were used to confirm the trend in energy changes. For the fragment system, the structure optimizations and energetics were both calculated at the DFT level using the B3LYP hybrid functional, while for the periodic one, the energetics were calculated by the B3LYP functional based on the structures optimized by the PBE functional. The details of calculations are provided in the Supporting Information. 2,6Dimethylpyridinium (abbreviated as [2,6-Lutidinium]+ or simply [LutH]+) and decamethylcobaltocene (i.e., CoCp*2) were used as the proton and electron sources, respectively, to probe the reactivity of the model systems; the corresponding proton affinity of the conjugate base of [LutH]+ and the ionization energy of the reductant CoCp*2 were calculated to be 10.44 and 4.28 eV, respectively, showing that they are 571
DOI: 10.1021/acs.jpclett.7b03094 J. Phys. Chem. Lett. 2018, 9, 570−576
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The Journal of Physical Chemistry Letters
Scheme 1. Reaction Pathway of Alternating Protonations (H+) and Electron Additions (e) Calculated Based on the Fragment G-Al Model C120H27Al 1 for the Reduction of N2 to Various NxHy Intermediates 2−16 and Finally to NH3, in the Presence of Mild Proton and Electron Sources ([LutH]+ and CoCp*2, Respectively)a
a The reaction energy for each step is reported as ΔG° in electronvolts. A positive charge symbol “+” is shown for the intermediate species enclosed in square brackets after each step of protonation. The graphene fragment denoted as G is accompanied by a superscript of 0, I, II, or −I to represent the oxidation state of 0, + 1, + 2, or −1, respectively. The schematic bond orders shown in NN, NN, and NN are indicative of the structural characteristics of the respective NxHy intermediates. For all of the intermediates, the NxHy components without parentheses carry a formal charge of −1, while those enclosed in the parentheses are all formally neutral except in 9′ where the NHNH2 component has a charge of +1 due to the presence of a proton H+ bound to the formally neutral NHNH. Note that the (G0H)Al(NN) intermediate 4 with a hydrogenated graphene fragment was identified as an effective catalyst because of the strong Al−N2 binding, especially in the presence of Li+, as described in the text. The reaction step from 4 to 5 involves an internal H transfer, while the step from 16 to 4 involves a NH3−N2 exchange. For both of these steps, the values of ΔG° shown in the parentheses represent the Gibbs reaction energies in the presence of the Li+ additive.
Table 1. Binding Energies, Bond Distances, and Stretching Frequencies of Three Al−N2 Complexes in the Catalytic Cycle c, d
2 4c,d 4-Li+c
Ebind (eV)a
dNN (Å)b
dAl−N (Å)
υNN (cm−1)b
−0.28 −0.37 −1.02
1.104 1.105 1.185
2.169 2.142 1.979
2444 2431 1937
a
The reported energy values were calculated by the fragment model without thermal corrections: for 2 and 4, Ebind = E(2 or 4) − E(N2) − E(G-Al); for 4-Li+, Ebind = E(4-Li+) − E(N2−Li+) − E(G-Al), where E(N2−Li+) refers to the total energy of the N2−Li+ complex containing a solvent benzene molecule. bThe bond distance and stretching frequency of N2 in the gas phase were calculated to be 1.106 Å and 2458 cm−1, respectively. Note that the NN bond length in free N2 is slightly longer than those in 2 and 4. The subtle difference is possibly due to the basis set incompleteness for computation or physically due to the σ-donation from N2 to Al. c2 and 4 represent the Al−N2 complexes supported by G and GH, respectively, while 4-Li+ refers to structure 4 but with a Li+ additive incorporated (see Figure 2 for the structures). dFor the neutral species 2 and 4, periodic models were also used for calculations, giving similar results as the fragment models (see Table S2).
Figure 2. Optimized structures of the Al−N2 complexes: (a) (G0H)Al−N2 4 without Li+ and (b) (G0H)Al−N2/Li+ 4-Li+ with Li+, both supported by hydrogenated graphene (G0H). Note that a benzene molecule was included in the 4-Li+ model to simulate the explicit solvation effect of cation−π interaction. The fragment C120H27Al model was used for computations, and only the core parts of the structures are displayed for clarity.
short at 1.06(1) Å, which is close to that of N2 in gas phase, suggesting a weak interaction between Li+ and N2.33 The difference between 4-Li + and [Li−N2 −Li]2+ implies a cooperative effect between Al-doped graphene and Li+ to strengthen the binding and activate the N2 ligand. The stronger Al−N2 binding in the (G0H)Al(N2) intermediates 4 and especially 4-Li+ than the (G0)Al(N2) adduct 2 make them an effective catalyst in the catalytic cycle (Scheme 1). As will be seen below, the presence of Li+ can also significantly facilitate
reduced wavenumber of 1937 cm−1 for the stretching mode of N2, compared to those of 2, 4, and N2 in the gas phase (Table 1). In contrast, according to the crystal structure of a previously reported [Li−N2−Li]2+ compound, the N≡N distance is rather 572
DOI: 10.1021/acs.jpclett.7b03094 J. Phys. Chem. Lett. 2018, 9, 570−576
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The Journal of Physical Chemistry Letters
Figure 3. (a) Gibbs energy profile corresponding to the key Al−NxHy intermediates formed by a sequence of proton/electron additions as displayed in Scheme 1. A similar energy profile without thermal corrections was predicted by using the periodic model as shown in Figure S2. Red triangular symbols indicate the results with Li+ additive. (b) Free-energy changes from intermediates 4 to 5 without and with Li+ additive. The fragment C120H27Al model was used for computation, and only the core part of the structures are displayed for clarity. Note that the N=NH group in 5 and 5Li+ are tilted toward G because of the H−π interaction with the π-electron density on the sp2-hybrized carbon atoms.
the first hydrogenation of N2 ligand, which should be ascribed to the stronger Al−N2 binding and the activated N2 ligand. Next we examine more closely the catalytic performance of G-Al for NH3 synthesis from N2. Figure 3a shows the Gibbs reaction energy profile accounting for the key Al−NxHy intermediates formed by a sequence of protonation and electron-transfer steps. Note that the energies for respective protonation and electron addition shown in Scheme 1 are summed up in Figure 3a. Consistent energetics without thermal corrections were obtained based on the fragment C120H27Al model and the periodic C127Al model (Figure S2). Similar to other well-studied catalytic systems,14 the reaction steps involved are thermodynamically favorable except for the first step of hydrogenation of N2 and the last step of NH3−N2 exchange. Once N2 is hydrogenated in the first step from 4 to 5, the downstream protonation and electron addition steps are exergonic. Although the last step of NH3−N2 exchange is endergonic, the energy cost can be locally compensated by the energy releases from previous steps. In general, the Gibbs energy profile suggests the feasibility of the proposed reaction pathway. Let us focus on the first hydrogenation of N2. This step is generally considered the most difficult but also the most important step in the entire reaction processes for both the synthetic and the enzymatic N2 fixation, because it generates the first activated N2H species facilitating the downstream reactions. Shilov has estimated that direct proton and electron addition to N2 (N2 + H+ + e = N2H) is kinetically unattainable.34 Presumably, the first pronation of (G0)Al(N2) 2 would take place on N2, directly giving rise to an intermediate [(GII)Al−N=NH]+ or [(G0)Al(N≡NH)]+. However, the protonated N2 species was not observed, as geometry optimization attempting to obtain these putative structures led to the protonated graphene, yielding [(GIH)Al(N2)]+ 3 (Scheme 1), which upon subsequent electron addition led to the neutral (G0H)Al(N2) 4 (see Figure 2a for the structure). Noticing the proximity of the H atom attached to the graphene framework G to N2 in 4, a H-atom transfer pathway may exist for producing the first hydrogenated N2 specie (GI)Al−N=NH 5. This hypothesis was computationally confirmed as shown in Figure 3a by identifying the corresponding transition state that connects the intermediates 4 and 5. The corresponding barrier
height (TS4−5) of 1.08 eV suggests that the H-transfer process is viable at relatively low temperature for a typical homogeneous catalyst to remain stable.35 The relatively low barrier also indicates that the hydrogenation of N2 was facilitated by the Al-doped graphene. The observation that graphene hydride 4 serves as a key intermediate for subsequent hydrogenation of the N 2 coordinated to Al center is not alone. Similar scenarios have also been observed for the first pronation step of the N2 ligand both in Schrock’s synthetic Mo catalysts and the biological nitrogenase systems.36 In the former case, the protonation of an amido ligand of the catalyst was found to be thermodynamically more favorable than the direct protonation of the N2 ligand; upon subsequent electron addition of the complex, the hydrogen atom is transferred from the hydrogenated heteroatom to N2, rendering the formation of N=NH− species.37,38 Similarly, in the E4 state of the nitrogenase catalyzed reaction, the protonated sulfide (SH−) bridge has been proposed to be the source of protons and electrons for the initial hydrogenation of N2 ligand to yield a diazene adduct.39 Most recently, Chalkley et al. proposed that metallocenemediated proton-coupled electron transfer (PCET) might play a crucial role for the initial generation of N=NH−, where the C atoms on CoCp*2 serve as a mediator to realize the PCET event.40 In short, the heteroatoms such as C, N, or S in the vicinity of the N2 ligand, including the graphene framework of our proposed non-TM G-Al catalyst, may play a critical role in the first hydrogenation of N2. Such heteroatom-assisted H transfer should be fully exploited in the design of synthetic catalysts for N2 fixation. To further reduce the activation barrier for TS4‑5 as mentioned above, we examined the cooperative effect of alkali metal ions by studying Li+ as an additive in the proposed catalytic processes. In some homogeneous N2 reduction systems involving alkali metals as reductants, alkali metal ions have been found to cooperatively boost N2 activation together with transition-metal centers.41 As explained above based on Table 1, the incorporation of Li+ remarkably strengthens the binding and induces the activation of N2 in (G0H)Al(N2) 4-Li+. Moreover, it is noteworthy that the energy of activation (TS4‑5) corresponding to the H atom transfer from 4-Li+ to 5-Li+ is reduced to 0.84 eV, lower than that of 1.08 eV in the absence of 573
DOI: 10.1021/acs.jpclett.7b03094 J. Phys. Chem. Lett. 2018, 9, 570−576
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The Journal of Physical Chemistry Letters
intermediates and the final NH3 product. To this end, a qualitative charge analysis was performed as presented in Figure 4, in which the G-Al model was divided into three components
Li+ as shown in Figure 3a. Meanwhile, with the assistance of Li+, the H atom transfer reaction became exergonic with a ΔG° = −0.36 eV, which is in stark contrast to the endergonic character when Li+ is absent with a ΔG° = +0.94 eV (Figure 3b). Thus, the first step of N2 reduction to N=NH− is predicted to be facilely realized by Al-doped graphene in the presence of Li+, which is in line with the important role known for alkali cations to activate N2 in a few catalytic systems based on transition-metal complexes.42,43 It is worth noting that in the present study, Li+ was used only as an additive in the proposed model system, instead of being introduced as a reductant, which is the case in the existing homogeneous catalytic systems involving alkali metals. On the basis of the present work, it would be interesting to compare Li+ with other alkali metal ions in future studies. It is beneficial to compare our non-TM catalyst with TMbased catalysts regarding the first and the most difficult step of hydrogenation. We notice that in the hypothetical Fe-doped graphene catalyst studied previously,25 the first hydrogenation of N2 forming N2H is slightly exothermic (ΔE = −0.22 eV), in contrast to the much more exothermic character in the downstream reaction steps. The difference in the magnitude suggests the difficulty of the initial hydrogenation of N2. In comparison, the first hydrogenation of N2 is endergonic (ΔG° = +0.94 eV) in our non-TM system when Li+ is absent. As (G0H)Al(N2) 4 is transformed to (GI)Al-NNH 5, one electron is transferred along with a proton from the graphene G fragment to N2 to form the N=NH− moiety, but the electron prefers to be partially shifted back to reduce the zwitterionic character. However, when Li+ is present, the monopole of Li+ may stabilize the zwitterion and thus may favor the charge transfer from graphene to N2H. This speculation is supported by a qualitative charge analysis, which shows the charges on the N2H moiety as −0.076 and −0.857 |e| in the absence and presence of Li+, respectively. Also because of the presence of Li+, the first hydrogenation of N2 becomes exergonic (ΔG° = −0.36 eV), similar to the Fe-doped graphene catalyst. Once N2 is activated into the reactive N=NH− state, the downstream reactions are thermodynamically favorable, as shown in Scheme 1 and Figure 3a, which bears similarity to the Chatt or the Schrock cycle.35,44 However, the reactions proceed via an alternating pathway in the present G-Al system, which is distinguished from the distal mechanism in the Schrock cycle. This can be readily understood in that some intermediates such as nitride species, available for the Mo center in Schrock complexes, are difficult to form between the N3− ligand and the Al center. On the other hand, the exchange of NH3 with N2 in the final stage of a reaction cycle is strongly disfavored because of a positive ΔG° of 1.20 eV, when Li+ is absent from the catalytic system, due to the strong Lewis acid-base adduct AlNH3. However, the presence of Li+ made it much less endergonic for the conversion from (G0H)Al(NH3) 16-Li+ to (G0H)Al(N2) 4-Li+ as the ΔG° is reduced to 0.80 eV. Therefore, the Li+ as an additive not only assists the initial hydrogenation of N2 but also is beneficial for the exchange of NH3 and N2. Although it is positive, this energy may be locally compensated by the energy releases from previous steps. Note that the exchange of NH3 and N2 may become thermodynamically favored, if an acid stronger than [LutH]+ is used. Finally, it is critical to pay a closer inspection to the charge variations along with the proton- and electron-transfer events in order to disclose the role of graphene in the proposed non-TM catalyst, as the N2 ligand is gradually reduced to various NxHy
Figure 4. (a) Structure partition and (b) Mülliken charge (Q) of the reaction intermediates involved in the reaction pathway as displayed in Scheme 1. The variation of NN bond lengths is also shown, verifying the single, double, and triple bond orders shown in Scheme 1. The fragment C120H27Al model was used for the computation.
including the graphene block, the Al atom, and the NxHy ligands. It can be seen that the charges on Al are within a narrow range around 0.3|e| during the entire course of reactions, which is close to the positive charges on the Al center previously calculated for Al-doped graphene.28 The NxHy moieties in the (G0/G0H/GI/GIH)Al(NxHy) intermediates 2, 3, 4, 6, 7, 10, 11, 14, and 15 are also within a narrow range around 0.2|e|. These moieties correspond to the formally neutral N≡N, NH=NH, NH2−NH2, and NH3 moieties that are coordinated to the Al center, leading to partially positive charge on them as a result of the dative bond between N and Al. The slightly negatively charged NxHy moieties can be found in (GI)Al−N=NH 5, [(GII)Al−NH−NH2]+ 8, (GI)Al−NH−NH2 9, [(GII)Al−NH2]+ 12, and (GI)Al−NH2 13. The NxHy moieties in these intermediates must carry a negative formal charge of −1 in order to satisfy the octet rule on each N atom. The extra electrons needed for the valence saturation of each intermediate should come from the charge transfer from graphene to the NxHy components. The graphene segments indeed display a drastic fluctuation of charges as expected from the charge transfer, which varied in a zigzag manner upon alternate proton and electron additions. This observation is similar to the variation of charges found in previous studies on TM-doped graphene systems.24,25 For the protonated species, the [(GI/GIH)Al(NxHy)]+ intermediates 3, 6, 10, and 14 supported on GI or GIH, and the [(GII)Al− NxHy]+ intermediates 8 and 12 supported on GII, the electrons consumed in the course of reductive protonation process were supplied by graphene, thereby leading to graphene segments bearing more positive charges than before protonation. Interestingly, 8 and 12 display the most positive charges, in accordance with their GII support that has the highest oxidation state. On the other hand, in the neutral (G0/G0H)Al(NxHy) intermediates 2, 4, 7, 11, and 15 supported on G0 or G0H, the graphene segments bear some negative charges, which stores the electrons to be used for the subsequent protonation processes to yield NxHy+1 in a more reduced form. All of the calculated charges for these intermediates are consistent with the expected oxidation states on the graphene segment. For the rest of the (GI)Al−NxHy intermediates 5, 9, and 13 supported 574
DOI: 10.1021/acs.jpclett.7b03094 J. Phys. Chem. Lett. 2018, 9, 570−576
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The Journal of Physical Chemistry Letters on GI, their graphene moieties are expected to be positively charged. However, the calculated charges are slightly negative or nearly zero. For these zwitterions, the electrons on the NxHy moieties are partially shifted back onto the graphene segment to reduce the zwitterionic characters, as can be seen from the small magnitude of negative charges on their respective NxHy moieties. In short, for all of these intermediates, graphene displays a wide range of oxidization states in the optimized reaction pathway, implicating the versatile redox property of graphene. A further piece of evidence indicating the versatile redox capability of the graphene framework of G-Al can be seen from the two isomers found in the third H+/e addition step corresponding to the formation of (GI)Al−NH−NH2 9 and (G−I)Al(NH=NH2) 9′, which are nearly thermodynamically equivalent (Scheme 1) but correspond to N2H3 moieties in different reduced states (detailed description in Figure S3). Put together, all of these observed variations of charges demonstrate that the proposed Al-doped graphene system possesses the potential to mediate the catalytic N2 reduction, i.e., the catalytic Al center binds with various NxHy ligands, while the graphene framework serves as an electron buffer to supply or store electrons for generating NxHy ligands at various reduced states, when external protons or electrons are successively transferred to the system. In conclusion, we have demonstrated the feasibility of utilizing Al-doped graphene as a non-TM catalyst for N2 reduction to NH3 in a manner similar to that for the homogeneous catalysis based on TM complexes operated under mild conditions. The first hydrogenation of N2 was readily realized via an internal H atom transfer from the graphene framework to the N2 ligand bound to the Al binding site. A similar scenario has also been observed in Schrock’s catalysis system and in the nitrogenase enzyme, where hydrogenated heteroatoms were suggested to be vital for the first hydrogenation step of N2. Therefore, heteroatom-assisted hydrogenation of N2 appears to be a ubiquitous process in homogeneous N2 reduction systems, although more cases would be needed to solidify this concept. A Li+ ion was used as an example to show that alkali metal ions as an additive can substantially facilitate the binding and activation of N2 at the initial reaction stage and the exchange of NH3 with N2 at the final stage of the proposed catalytic cycle. Electronic structure analysis disclosed that the integrated structure of Al embedded in a graphene framework is indispensable to realize the catalytic N2 reduction to NH3. During the course of reactions, the graphene moiety showed multiple oxidation states, whereas that of Al approximately remains constant. In the proposed model catalyst, the Al dopant provides a binding center to ligate the various NxHy ligands, while the graphene framework with a versatile redox property serves as an electron buffer to mediate the formation of nitrogenous intermediates at different reduced states. This work highlights the significance of heteroatomassisted H transfer in the design of synthetic catalysts for N2 fixation and invites experimental work to confirm the first nonTM catalytic system for the reduction of N2 to NH3.
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Computational details; optimized structure of the Me3Al(N2)-[LutH]+ complex (Figure S1); thermodynamic value of direct reduction of Me3Al(N2) by CoCp*2; charge analysis for 4 and 4-Li+ (Table S1); comparison of binding energies for 2 and 4 calculated using the fragment and periodic models (Table S2); comparison of energy profiles calculated without thermal corrections using the fragment and periodic models (Figure S2); structures for the two isomers 9 and 9′ (Figure S3) (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yong-Hui Tian: 0000-0002-1261-3192 Yixiang Duan: 0000-0001-8346-0421 Jacek Jakowski: 0000-0003-4906-3574 Bobby G. Sumpter: 0000-0001-6341-0355 Jingsong Huang: 0000-0001-8993-2506 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Faculty Startup Grant of Sichuan University and by the National Science Foundation of China (Grant No. 21443012). Part of the work was performed at the Center for Nanophase Materials Sciences (CNMS), which is a U.S. Department of Energy Office of Science User Facility. Y.-H.T. thanks the National Supercomputing Center in Shenzhen for providing the computational resources and Gaussian09 program (version of ES64LG09RevD. 01). J.J., B.G.S., and J.H. acknowledge resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DEAC02-05CH11231.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b03094. 575
DOI: 10.1021/acs.jpclett.7b03094 J. Phys. Chem. Lett. 2018, 9, 570−576
Letter
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