This journal is©The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 13319--13322 | 13319

Cite this:Chem. Commun., 2014,

50, 13319

Nitrogen-fixation catalyst based on graphene:every part counts†

Yuan-Qi Le, Jia Gu and Wei Quan Tian*

The catalytic profile and function of each component of a

molybdenum–graphene based catalyst (Mo/N-doped graphene)

for nitrogen fixation, which combines the merits of these two

components, is evaluated computationally. The Mo/N part acts as

an active centre for N2 bond breaking and the graphene part works

as an electron transmitter and electron reservoir.

The reduction of dinitrogen to ammonia is crucial to all life.1 Onlya few prokaryotic organisms can translate nitrogen gas into fixednitrogen (ammonia or nitride) with nitrogenase, i.e. nitrogenfixation.2 The most common industrial transformation method,the Haber process, converts N2 into NH3 under high pressure andtemperature, requiring tremendous amounts of energy.3 Thus,simple and efficient catalysts for nitrogen fixation under ambientconditions are still highly desired.

The mechanism of the reduction of N2 to NH3 at a singlemolybdenum(0) or tungsten(0) centre has been reviewed,4 andmany works5 about nitrogen fixation based on different metalcatalysts have been reported. However, the mechanism is stillelusive.6 A mixture of hydrazine and ammonia was generated7

through metal catalysis. The two catalysts Mo[HIPTN3N]8 and[Mo(L)(N2)2]2(m-N2)9 are able to fix nitrogen under ambientpressure and temperature, but the catalytic efficiency still needsimprovement. All of the protonations of the Mo[HIPTN3N]system are exothermic, while most of the reduction processesare endothermic,10 so the efficiency of nitrogen fixation dependson reduction processes. On the other hand, the loss of ligands

leads to the decreased reactivity of the catalysts (Mo[HIPTN3N]and [Mo(L)(N2)2]2(m-N2)).5a Thus the mechanism of nitrogenfixation awaits elucidation and the function of each componentof the catalysts needs identifying.

Graphene, a single atomic honeycomb layer of sp2-hybridizedcarbon with high stability and carrier mobility, has been extensivelystudied for its potential application in microelectronics andnanoscience.11 The required protonation in nitrogen fixation andthe subsequent reduction involves charge transfer, and the highcarrier mobility of graphene might facilitate this charge transfer, asthe conjugated p electrons serve as a bridge for charge transferduring the reduction of graphene oxide.12 The multi-radical char-acter of graphene nanoribbons causes the frontier molecular orbitalsto locate at the edge of the ribbons,13 which could donate and acceptelectrons. Recent studies found that metal oxide doped graphene isa good catalyst for oxygen reduction.14 Further mechanistic investi-gation of the catalytic application of graphene will certainly help toopen up a new field for the application of this new nano-material.

A nitrogen-fixation catalyst based on graphene, which is hoped tocombine the merits of molybdenum (efficient catalytic activity)and graphene (electron bridge and reservoir), is computationallyevaluated in the present work. With the aid of molecular orbitaltheory, bonding and population analysis, the possible mechanism ofnitrogen fixation catalyzed by the Mo/N-doped graphene system isexplored. The energetics (with zero-point vibrational energy correc-tion) and properties of the possible intermediates are studied, andthe role of each component of the catalyst is identified to providemechanistic information for experimental endeavour. The densityfunctional theory based method B3LYP is employed for geometryoptimization and property characterization. The pseudopotentialbased basis set SDD is used for Mo, and the Gaussian basis set6-31G(d,p) is used for N, C and H.

This catalyst (C33H15MoN3) contains a molybdenum atomand three ligand nitrogen atoms as shown in Fig. 1. Two biggermodels (C69H21MoN3 and C117H27MoN3 as shown in Fig. S1, ESI†)with bigger graphene sheets have similar electronic structures.For computational efficiency, C33H15MoN3 is investigated in detailfor nitrogen fixation in the present work.

State Key Laboratory of Urban Water Resource and Environment, Institute of

Theoretical and Simulational Chemistry, Academy of Fundamental and

Interdisciplinary Sciences, Harbin Institute of Technology, Harbin, 150080,

P. R. China. E-mail: tianwq@hit.edu.cn; Fax: +86-451-86403305;

Tel: +86-451-86403445

† Electronic supplementary information (ESI) available: Citations of Gaussian 03,B3LYP and basis sets 6-31G(d,p) and SDD. The structural parameters, Mulliken chargeof the MoN3 part in C33H15MoN3, C69H21MoN3 and C117H27MoN3. Major structures ofthe intermediates in nitrogen-fixation. Charge and bond order variation of functiongroups during addition of hydrogen in C33H15MoN3/N2. Frontier molecular orbitals ofC69H21MoN3 and C117H27MoN3. See DOI: 10.1039/c4cc01950d

Received 16th March 2014,Accepted 7th May 2014

DOI: 10.1039/c4cc01950d

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13320 | Chem. Commun., 2014, 50, 13319--13322 This journal is©The Royal Society of Chemistry 2014

Two possible pathways for nitrogen fixation have been studied:the Schrock cycle5e,15,16 and the nitrogenase reaction.17 The Schrockcycle is based on a MoIII complex with a specifically designed ligand[HIPT] that provides a sterically shielded site for N2 bonding andreduction. The reaction mechanism of nitrogenase has beensimulated for a simplified nitrogenase (FeMoco model system).17

These two reaction pathways, both consisting of six consecutiveprotonation and reduction steps with different acids as protonsources, are described in Fig. 2 with different hydrogenation(protonation and reduction) sequences, and were investigated forthe Mo/N-doped graphene system (Fig. S2 and S3, ESI†).

Lutidinium ([2,6-LutH]+)5e was used as proton source and[CoCp*2]16b was used for electron energy calculation in the presentwork. The proton energy (�984.97 kJ mol�1) was calculated ineqn (1), and the electron energy (�433.73 kJ mol�1) was taken asthe ionization energy of [CoCp*2] in eqn (2).5e The reaction energyof the first protonation, accordingly, is E(H1) � E[N2(ad)] �E(proton), while that of the first reduction is E(H1R) � E(H1) �E(electron). The protonation and reduction energies of othersteps were calculated in a similar way.

[2,6-LutH]+ - [2,6-Lut] + H+; DEH+ = �984.97 kJ mol�1 (1)

[CoCp*2] - [CoCp*2]+ + e; DEe = �433.73 kJ mol�1 (2)

The reaction energies of these two reaction pathways and theN–N bond distance along the reaction pathways up to the fourthH addition are plotted in Fig. 3 (relevant data are listed in Table S5(ESI†) and are plotted in Fig. S5, ESI†). According to the variationof N–N bond length and bond order after the adsorption of N2

on Mo, the end-on adsorption of N2 on Mo in the Schrock reactionessentially retains the N–N triple bond [the bond length of N2 is1.11 Å from B3LYP/6-31G(d) prediction]. The addition of the firsttwo protons elongates the N–N bond to 1.33 Å. The addition of thethird H stretches the N–N bond to a single bond (1.43 Å).However, this H addition requires 103.4 kJ mol�1 of energy. Theaddition of the fourth H to the N (Na) atom bonded to Mo breaksthe NH3 apart, with a N–N bond distance of 3.01 Å.

On the other hand, the side-on adsorption in the enzymaticreaction path with Mo bonding to the two N atoms of N2

elongates the N–N bond to nearly a double bond (the N–Nbond length of N2H2 is 1.24 Å from the B3LYP/6-31G(d,p)prediction). The alternate hydrogenation on the two N atomsleads to the formation of a hydrazine structure with a N–N bondlength of 1.46 Å [the N–N bond in hydrazine is 1.49 Å from theB3LYP/6-31G(d,p) prediction].

The majority of the steps in these two reaction paths areexothermic, except for the third protonation in the Schrockreaction (absorbing 103.4 kJ mol�1) and the reduction of thefourth H in the enzymatic reaction (absorbing 19.8 kJ mol�1).The unfavorable energetic requirement in the third protonationof the NH2 (step 7 in Fig. 3) in the Schrock reaction pathhinders this process. On the other hand, the energy releasedfrom the fourth protonation (31.9 kJ mol�1) (step 9 in Fig. 3) isenough for the reduction of this intermediate (step 10 in Fig. 3).Thus, in terms of reaction energies, the catalyst takes theenzymatic reaction path for nitrogen fixation.

Hydrazine (N2H4) could easily form after the fourth hydro-genation, as the energy difference between H4R(E)[MoNH2NH2]and its transition state (TS1 in Fig. 4) is only 21.9 kJ mol�1.The Mo–N(a) and Mo–N(b) distances of the product P(N2H4) are

Fig. 1 The Mo/N-doped graphene catalyst (C33H15MoN3).

Fig. 2 Hydrogenation sequences in the Schrock10 and the enzymatic17

reactions.

Fig. 3 The energetics and N–N bond distance during the N2 fixation inthe Schrock and enzymatic reaction paths. 1-catalyst, 2-N2 adsorption,3/4-1st hydrogenation (protonation and reduction), 5/6-2nd hydrogena-tion, 7/8-3rd hydrogenation, 9/10-4th hydrogenation.

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2.23 Å and 3.03 Å, thus the N2H4 releases feasibly from thecatalyst. The desorption energy of N2H4 is 148.2 kJ mol�1, closeto that of ammonia (139.5 kJ mol�1). The energy barrier of thefifth protonation (TS2 in Fig. 4) is as high as 205.5 kJ mol�1,and the reaction energy is exothermic by 292.8 kJ mol�1.Therefore, N2H4 tends to be generated in weakly acidic environ-ments, while NH3 is more likely to form in strongly acidicenvironments because of the abundance of H+.

The breaking of the N–N bond takes place in the fourthhydrogenation with maximum bond distance and minimumbond order in the Schrock reaction path. Specifically, the N–Nbond length is 1.43 Å and bond order is 0.99 in the Schrockreaction path after the third hydrogenation. The catalyticmechanism of the Mo/N-doped graphene catalyst in the presentwork is different from that of the Schrock reaction.10 The mostevident differences are the different hydrogenation site andorder of hydrogenation. The release of the first molecularammonia takes place after the third hydrogenation experi-mentally,10 while it occurs after the fourth hydrogenation usingthe Mo/N-doped graphene catalyst. Most of the steps in theMo/N-doped graphene system are exothermic while the reductionprocesses in the Schrock reaction10 need energy.

According to the change of atomic charge during the N2

fixation (Fig. 5), the Mo/N-doped graphene system can bedivided into three groups. The charge of each group increasesand decreases regularly along the reaction path. Groups 1(grp1, Mo/N in Fig. 5) and 2 (grp2, surrounding carbons) havesmall charge fluctuation during the N2 fixation. Group 3 hasregular alternating and significant variation of atomic chargeduring protonation and reduction, i.e. group 3 loses about 0.75electrons during protonation and gains about 0.81 electronsduring reduction (as listed in Tables S6 and S7, ESI†).

During protonation, a proton attacks the N2 moiety, forminga N–H bond with the gain of electrons from group 3, and leavesgroup 3 positively charged. The reduction of the system leads tothe injected electron remaining at group 3. Group 3 acts as anelectron donor during protonation and an electron acceptorduring reduction, i.e. an electron reservoir. Ascribed to the highcharge carrier mobility of graphene, group 2 serves as anelectron transmitter. Thus far, according to the structure of reactiveintermediates and charge variation in these intermediatesduring N2 fixation, the roles of the three groups can be identifiedas: group 1 – active centre, group 2 – electron transmitter,group 3 – electron reservoir.

The HOMO (highest occupied molecular orbital) of Mo/N-dopedgraphene (Fig. 6) matches the charge variation of the systemduring hydrogenation. The periphery of the graphene part

Fig. 4 Possible reaction paths for H4R [MoNH2NH2] in the enzymatic reaction.

Fig. 5 The variation of atomic charge (the Mulliken charge difference ofthe present step from that of the previous step) of different groups (grp) inMo/N-doped graphene during the enzymatic catalytic N2 fixation.

Fig. 6 The LUMOs (left) and HOMOs (right) of the intermediates in theenzymatic reaction. Hn means the nth protonation. HnR is the reductionstep following the nth protonation. The top two are the LUMOs (left) andHOMOs (right) of Mo/N-doped graphene.

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(group 3) dominates in the HOMO. Group 1 (Mo/N) alsocontributes to the HOMO, while group 2 has little populationin the HOMO. The Mo/N is the active centre for protonationaccording to the HOMO and the bonding character of Mo and N2,and the periphery part (grp3) provides electrons for protonation.In the LUMO (lowest unoccupied molecular orbital), the Mo/N hasthe dominant contribution. Electrons would travel to the Mo/Nmoiety during reduction. Such an orbital distribution in theHOMO and LUMO of those intermediates highlights the Mo/Nas the active centre and the periphery region as an electronreservoir.

In summary, the possible reaction mechanism of N2 fixation bya graphene catalyst (Mo/N-doped graphene) has been studied withdensity functional theory. The enzymatic route is energetically morefeasible for N2 fixation on Mo/N-doped graphene than the Schrockpath, as it is more energetically favorable for the simultaneousbonding of two N atoms to Mo. Six protons are introducedalternately to the two nitrogen atoms forming three intermediates:[MoNH2NH2], [MoNH2NH3] and [MoNH3]. [MoNH2NH2] tendsto generate NH2NH2 in weakly acidic environments because ofits weak Mo–Na and Mo–Nb interaction, while [MoNH2NH3]and [MoNH3] could form in strongly acidic environments withthe abundance of H+.

The variation of atomic charge along the reaction processesreveals that the catalyst can be divided into three functionalparts with different catalytic roles. The Mo/N is the activecentre, and the graphene body serves as an electronic bridgefor electron transmission, while the graphene periphery regionstores and provides electrons for protonation and reduction. Withhigh charge carrier mobility, the use of graphene functioning asan electron transmitter and an electron reservoir could broadenits applications in catalysis.

This work is supported by the State Key Lab of Urban WaterResource and Environment (HIT) (2014TS01) and the OpenProject of State Key laboratory of Supramolecular Structure

and Materials (JLU) (SKLSSM201404). Dr Lei Liu is gratefullyacknowledged for his constructive suggestion and for readingthis work.

Notes and references1 R. R. Schrock, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 17087.2 (a) B. K. Burgess and D. J. Lowe, Chem. Rev., 1996, 96, 2983–3011;

(b) R. R. Eady, Coord. Chem. Rev., 2003, 237, 23–30.3 R. Schlogl, Angew. Chem., Int. Ed., 2003, 42, 2004–2008.4 (a) J. Chatt, J. R. Dilworth and R. L. Richards, Chem. Rev., 1978, 78,

589–625; (b) M. Hidai, Coord. Chem. Rev., 1999, 185–186, 99–108;(c) F. Neese, Angew. Chem., Int. Ed., 2006, 45, 196–199.

5 (a) T. J. Hebden, R. R. Schrock, M. K. Takase and P. Muller, Chem.Commun., 2012, 48, 1851–1853; (b) T. Ogawa, Y. Kajita, Y. Wasada-Tsutsui, H. Wasada and H. Masuda, Inorg. Chem., 2013, 52, 182–195;(c) A. Itadani, M. Tanaka, T. Mori, H. Torigoe, H. Kobayashi andY. Kuroda, J. Phys. Chem. Lett., 2010, 1, 2385–2390; (d) P. Avenier,M. Taoufik, A. Lesage, X. Solans-Monfort, A. Baudouin, A. de Mallmann,L. Veyre, J.-M. Basset, O. Eisenstein, L. Emsley and E. A. Quadrelli,Science, 2007, 317, 1056–1060; (e) S. Schenk, B. L. Guennic, B. Kirchnerand M. Reiher, Inorg. Chem., 2008, 47, 3634–3650.

6 J. B. Howard and D. C. Rees, Proc. Natl. Acad. Sci. U. S. A., 2006, 103,17088–18093.

7 T. A. Bazhenova and A. E. Shilov, Coord. Chem. Rev., 1995, 144, 69–145.8 D. V. Yandulov and R. R. Schrock, Science, 2003, 301, 76–78.9 K. Arashiba, Y. Miyake and Y. Nishibayashi, Nat. Chem., 2011, 3, 120–125.

10 R. R. Schrock, Angew. Chem., Int. Ed., 2008, 47, 5512–5522.11 A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183–191.12 S.-Y. Xie, X.-B. Li, Y. Y. Sun, Y.-L. Zhang, D. Han, W. Q. Tian,

W.-Q. Wang, Y.-S. Zheng, S. B. Zhang and H.-B. Sun, Carbon, 2013,52, 122–127.

13 F. Plasser, H. Pasalic, M. H. Gerzabek, F. Libisch, R. Reiter,J. Burgdorfer, T. Muller, R. Shepard and H. Lischka, Angew. Chem.,Int. Ed., 2013, 52, 2581–2584.

14 Y. Y. Liang, H. L. Wang, J. G. Zhou, Y. G. Li, J. Wang, T. Regier andH. J. Dai, J. Am. Chem. Soc., 2012, 134, 3517–3523.

15 (a) A. Magistrato, A. Robertazzi and P. Carloni, J. Chem. TheoryComput., 2007, 3, 1708–1720; (b) B. L. Guennic, B. Kirchner andM. Reiher, Chem. – Eur. J., 2005, 11, 7448–7460; (c) F. Studt andF. Tuczek, Angew. Chem., Int. Ed., 2005, 44, 5639–5642.

16 (a) D. V. Yandulov, R. R. Schrock, A. L. Rheingold, C. Ceccarelli andW. M. Davis, Inorg. Chem., 2003, 42, 796–813; (b) D. V. Yandulov andR. R. Schrock, Inorg. Chem., 2005, 44, 1103–1117.

17 B. Hinnemann and J. K. Nørskov, Top. Catal., 2006, 37, 55–70.

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