ChemComm View Article Online
Published on 07 May 2014. Downloaded by University of Windsor on 26/10/2014 22:41:52.
COMMUNICATION
Cite this: Chem. Commun., 2014, 50, 13319
View Journal | View Issue
Nitrogen-fixation catalyst based on graphene: every part counts† Yuan-Qi Le, Jia Gu and Wei Quan Tian*
Received 16th March 2014, Accepted 7th May 2014 DOI: 10.1039/c4cc01950d www.rsc.org/chemcomm
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 Only a few prokaryotic organisms can translate nitrogen gas into fixed nitrogen (ammonia or nitride) with nitrogenase, i.e. nitrogen fixation.2 The most common industrial transformation method, the Haber process, converts N2 into NH3 under high pressure and temperature, requiring tremendous amounts of energy.3 Thus, simple and efficient catalysts for nitrogen fixation under ambient conditions are still highly desired. The mechanism of the reduction of N2 to NH3 at a single molybdenum(0) or tungsten(0) centre has been reviewed,4 and many works5 about nitrogen fixation based on different metal catalysts have been reported. However, the mechanism is still elusive.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 ambient pressure and temperature, but the catalytic efficiency still needs improvement. All of the protonations of the Mo[HIPTN3N] system are exothermic, while most of the reduction processes are endothermic,10 so the efficiency of nitrogen fixation depends on reduction processes. On the other hand, the loss of ligands
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: [email protected]; 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 charge of the MoN3 part in C33H15MoN3, C69H21MoN3 and C117H27MoN3. Major structures of the intermediates in nitrogen-fixation. Charge and bond order variation of function groups during addition of hydrogen in C33H15MoN3/N2. Frontier molecular orbitals of C69H21MoN3 and C117H27MoN3. See DOI: 10.1039/c4cc01950d
This journal is © The Royal Society of Chemistry 2014
leads to the decreased reactivity of the catalysts (Mo[HIPTN3N] and [Mo(L)(N2)2]2(m-N2)).5a Thus the mechanism of nitrogen fixation awaits elucidation and the function of each component of the catalysts needs identifying. Graphene, a single atomic honeycomb layer of sp2-hybridized carbon with high stability and carrier mobility, has been extensively studied for its potential application in microelectronics and nanoscience.11 The required protonation in nitrogen fixation and the subsequent reduction involves charge transfer, and the high carrier mobility of graphene might facilitate this charge transfer, as the conjugated p electrons serve as a bridge for charge transfer during the reduction of graphene oxide.12 The multi-radical character of graphene nanoribbons causes the frontier molecular orbitals to locate at the edge of the ribbons,13 which could donate and accept electrons. Recent studies found that metal oxide doped graphene is a good catalyst for oxygen reduction.14 Further mechanistic investigation of the catalytic application of graphene will certainly help to open up a new field for the application of this new nano-material. A nitrogen-fixation catalyst based on graphene, which is hoped to combine the merits of molybdenum (efficient catalytic activity) and graphene (electron bridge and reservoir), is computationally evaluated in the present work. With the aid of molecular orbital theory, bonding and population analysis, the possible mechanism of nitrogen fixation catalyzed by the Mo/N-doped graphene system is explored. The energetics (with zero-point vibrational energy correction) and properties of the possible intermediates are studied, and the role of each component of the catalyst is identified to provide mechanistic information for experimental endeavour. The density functional theory based method B3LYP is employed for geometry optimization and property characterization. The pseudopotential based basis set SDD is used for Mo, and the Gaussian basis set 6-31G(d,p) is used for N, C and H. This catalyst (C33H15MoN3) contains a molybdenum atom and three ligand nitrogen atoms as shown in Fig. 1. Two bigger models (C69H21MoN3 and C117H27MoN3 as shown in Fig. S1, ESI†) with bigger graphene sheets have similar electronic structures. For computational efficiency, C33H15MoN3 is investigated in detail for nitrogen fixation in the present work.
Chem. Commun., 2014, 50, 13319--13322 | 13319
View Article Online
Communication
Published on 07 May 2014. Downloaded by University of Windsor on 26/10/2014 22:41:52.
Fig. 1
ChemComm
The Mo/N-doped graphene catalyst (C33H15MoN3).
Fig. 2 Hydrogenation sequences in the Schrock10 and the enzymatic17 reactions.
Two possible pathways for nitrogen fixation have been studied: the Schrock cycle5e,15,16 and the nitrogenase reaction.17 The Schrock cycle is based on a MoIII complex with a specifically designed ligand [HIPT] that provides a sterically shielded site for N2 bonding and reduction. The reaction mechanism of nitrogenase has been simulated for a simplified nitrogenase (FeMoco model system).17 These two reaction pathways, both consisting of six consecutive protonation and reduction steps with different acids as proton sources, are described in Fig. 2 with different hydrogenation (protonation and reduction) sequences, and were investigated for the 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 present work. The proton energy ( 984.97 kJ mol 1) was calculated in eqn (1), and the electron energy ( 433.73 kJ mol 1) was taken as the ionization energy of [CoCp*2] in eqn (2).5e The reaction energy of 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 other steps 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 the N–N bond distance along the reaction pathways up to the fourth H addition are plotted in Fig. 3 (relevant data are listed in Table S5 (ESI†) and are plotted in Fig. S5, ESI†). According to the variation of N–N bond length and bond order after the adsorption of N2
13320 | Chem. Commun., 2014, 50, 13319--13322
Fig. 3 The energetics and N–N bond distance during the N2 fixation in the Schrock and enzymatic reaction paths. 1-catalyst, 2-N2 adsorption, 3/4-1st hydrogenation (protonation and reduction), 5/6-2nd hydrogenation, 7/8-3rd hydrogenation, 9/10-4th hydrogenation.
on Mo, the end-on adsorption of N2 on Mo in the Schrock reaction essentially retains the N–N triple bond [the bond length of N2 is 1.11 Å from B3LYP/6-31G(d) prediction]. The addition of the first two protons elongates the N–N bond to 1.33 Å. The addition of the third 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. The addition of the fourth H to the N (Na) atom bonded to Mo breaks the NH3 apart, with a N–N bond distance of 3.01 Å. On the other hand, the side-on adsorption in the enzymatic reaction path with Mo bonding to the two N atoms of N2 elongates the N–N bond to nearly a double bond (the N–N bond length of N2H2 is 1.24 Å from the B3LYP/6-31G(d,p) prediction). The alternate hydrogenation on the two N atoms leads to the formation of a hydrazine structure with a N–N bond length of 1.46 Å [the N–N bond in hydrazine is 1.49 Å from the B3LYP/6-31G(d,p) prediction]. The majority of the steps in these two reaction paths are exothermic, except for the third protonation in the Schrock reaction (absorbing 103.4 kJ mol 1) and the reduction of the fourth H in the enzymatic reaction (absorbing 19.8 kJ mol 1). The unfavorable energetic requirement in the third protonation of the NH2 (step 7 in Fig. 3) in the Schrock reaction path hinders this process. On the other hand, the energy released from the fourth protonation (31.9 kJ mol 1) (step 9 in Fig. 3) is enough for the reduction of this intermediate (step 10 in Fig. 3). Thus, in terms of reaction energies, the catalyst takes the enzymatic reaction path for nitrogen fixation. Hydrazine (N2H4) could easily form after the fourth hydrogenation, 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
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 07 May 2014. Downloaded by University of Windsor on 26/10/2014 22:41:52.
ChemComm
Communication
Fig. 4 Possible reaction paths for H4R [MoNH2NH2] in the enzymatic reaction.
2.23 Å and 3.03 Å, thus the N2H4 releases feasibly from the catalyst. The desorption energy of N2H4 is 148.2 kJ mol 1, close to that of ammonia (139.5 kJ mol 1). The energy barrier of the fifth 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 environments, while NH3 is more likely to form in strongly acidic environments because of the abundance of H+. The breaking of the N–N bond takes place in the fourth hydrogenation with maximum bond distance and minimum bond order in the Schrock reaction path. Specifically, the N–N bond length is 1.43 Å and bond order is 0.99 in the Schrock reaction path after the third hydrogenation. The catalytic mechanism of the Mo/N-doped graphene catalyst in the present work is different from that of the Schrock reaction.10 The most evident differences are the different hydrogenation site and order of hydrogenation. The release of the first molecular ammonia takes place after the third hydrogenation experimentally,10 while it occurs after the fourth hydrogenation using the Mo/N-doped graphene catalyst. Most of the steps in the Mo/N-doped graphene system are exothermic while the reduction processes 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 be divided into three groups. The charge of each group increases and decreases regularly along the reaction path. Groups 1 (grp1, Mo/N in Fig. 5) and 2 (grp2, surrounding carbons) have small charge fluctuation during the N2 fixation. Group 3 has regular alternating and significant variation of atomic charge during protonation and reduction, i.e. group 3 loses about 0.75 electrons during protonation and gains about 0.81 electrons during reduction (as listed in Tables S6 and S7, ESI†). During protonation, a proton attacks the N2 moiety, forming a N–H bond with the gain of electrons from group 3, and leaves group 3 positively charged. The reduction of the system leads to the injected electron remaining at group 3. Group 3 acts as an electron donor during protonation and an electron acceptor during reduction, i.e. an electron reservoir. Ascribed to the high charge carrier mobility of graphene, group 2 serves as an electron transmitter. Thus far, according to the structure of reactive intermediates and charge variation in these intermediates during N2 fixation, the roles of the three groups can be identified as: group 1 – active centre, group 2 – electron transmitter, group 3 – electron reservoir.
This journal is © The Royal Society of Chemistry 2014
Fig. 5 The variation of atomic charge (the Mulliken charge difference of the present step from that of the previous step) of different groups (grp) in Mo/N-doped graphene during the enzymatic catalytic N2 fixation.
The HOMO (highest occupied molecular orbital) of Mo/N-doped graphene (Fig. 6) matches the charge variation of the system during hydrogenation. The periphery of the graphene part
Fig. 6 The LUMOs (left) and HOMOs (right) of the intermediates in the enzymatic reaction. Hn means the nth protonation. HnR is the reduction step following the nth protonation. The top two are the LUMOs (left) and HOMOs (right) of Mo/N-doped graphene.
Chem. Commun., 2014, 50, 13319--13322 | 13321
View Article Online
Published on 07 May 2014. Downloaded by University of Windsor on 26/10/2014 22:41:52.
Communication
(group 3) dominates in the HOMO. Group 1 (Mo/N) also contributes to the HOMO, while group 2 has little population in the HOMO. The Mo/N is the active centre for protonation according 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 has the dominant contribution. Electrons would travel to the Mo/N moiety during reduction. Such an orbital distribution in the HOMO and LUMO of those intermediates highlights the Mo/N as the active centre and the periphery region as an electron reservoir. In summary, the possible reaction mechanism of N2 fixation by a graphene catalyst (Mo/N-doped graphene) has been studied with density functional theory. The enzymatic route is energetically more feasible for N2 fixation on Mo/N-doped graphene than the Schrock path, as it is more energetically favorable for the simultaneous bonding of two N atoms to Mo. Six protons are introduced alternately to the two nitrogen atoms forming three intermediates: [MoNH2NH2], [MoNH2NH3] and [MoNH3]. [MoNH2NH2] tends to generate NH2NH2 in weakly acidic environments because of its weak Mo–Na and Mo–Nb interaction, while [MoNH2NH3] and [MoNH3] could form in strongly acidic environments with the abundance of H+. The variation of atomic charge along the reaction processes reveals that the catalyst can be divided into three functional parts with different catalytic roles. The Mo/N is the active centre, and the graphene body serves as an electronic bridge for electron transmission, while the graphene periphery region stores and provides electrons for protonation and reduction. With high charge carrier mobility, the use of graphene functioning as an electron transmitter and an electron reservoir could broaden its applications in catalysis. This work is supported by the State Key Lab of Urban Water Resource and Environment (HIT) (2014TS01) and the Open Project of State Key laboratory of Supramolecular Structure
13322 | Chem. Commun., 2014, 50, 13319--13322
ChemComm
and Materials (JLU) (SKLSSM201404). Dr Lei Liu is gratefully acknowledged for his constructive suggestion and for reading this work.
Notes and references 1 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. ¨gl, Angew. Chem., Int. Ed., 2003, 42, 2004–2008. 3 R. Schlo 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. ¨ller, Chem. 5 (a) T. J. Hebden, R. R. Schrock, M. K. Takase and P. Mu Commun., 2012, 48, 1851–1853; (b) T. Ogawa, Y. Kajita, Y. WasadaTsutsui, H. Wasada and H. Masuda, Inorg. Chem., 2013, 52, 182–195; (c) A. Itadani, M. Tanaka, T. Mori, H. Torigoe, H. Kobayashi and Y. 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. Kirchner and 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. Pasˇalic´, M. H. Gerzabek, F. Libisch, R. Reiter, ¨rfer, T. Mu ¨ller, R. Shepard and H. Lischka, Angew. Chem., J. Burgdo Int. Ed., 2013, 52, 2581–2584. 14 Y. Y. Liang, H. L. Wang, J. G. Zhou, Y. G. Li, J. Wang, T. Regier and H. J. Dai, J. Am. Chem. Soc., 2012, 134, 3517–3523. 15 (a) A. Magistrato, A. Robertazzi and P. Carloni, J. Chem. Theory Comput., 2007, 3, 1708–1720; (b) B. L. Guennic, B. Kirchner and M. Reiher, Chem. – Eur. J., 2005, 11, 7448–7460; (c) F. Studt and F. Tuczek, Angew. Chem., Int. Ed., 2005, 44, 5639–5642. 16 (a) D. V. Yandulov, R. R. Schrock, A. L. Rheingold, C. Ceccarelli and W. M. Davis, Inorg. Chem., 2003, 42, 796–813; (b) D. V. Yandulov and R. R. Schrock, Inorg. Chem., 2005, 44, 1103–1117. 17 B. Hinnemann and J. K. Nørskov, Top. Catal., 2006, 37, 55–70.
This journal is © The Royal Society of Chemistry 2014
Published on 07 May 2014. Downloaded by University of Windsor on 26/10/2014 22:41:52.
COMMUNICATION
Cite this: Chem. Commun., 2014, 50, 13319
View Journal | View Issue
Nitrogen-fixation catalyst based on graphene: every part counts† Yuan-Qi Le, Jia Gu and Wei Quan Tian*
Received 16th March 2014, Accepted 7th May 2014 DOI: 10.1039/c4cc01950d www.rsc.org/chemcomm
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 Only a few prokaryotic organisms can translate nitrogen gas into fixed nitrogen (ammonia or nitride) with nitrogenase, i.e. nitrogen fixation.2 The most common industrial transformation method, the Haber process, converts N2 into NH3 under high pressure and temperature, requiring tremendous amounts of energy.3 Thus, simple and efficient catalysts for nitrogen fixation under ambient conditions are still highly desired. The mechanism of the reduction of N2 to NH3 at a single molybdenum(0) or tungsten(0) centre has been reviewed,4 and many works5 about nitrogen fixation based on different metal catalysts have been reported. However, the mechanism is still elusive.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 ambient pressure and temperature, but the catalytic efficiency still needs improvement. All of the protonations of the Mo[HIPTN3N] system are exothermic, while most of the reduction processes are endothermic,10 so the efficiency of nitrogen fixation depends on reduction processes. On the other hand, the loss of ligands
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: [email protected]; 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 charge of the MoN3 part in C33H15MoN3, C69H21MoN3 and C117H27MoN3. Major structures of the intermediates in nitrogen-fixation. Charge and bond order variation of function groups during addition of hydrogen in C33H15MoN3/N2. Frontier molecular orbitals of C69H21MoN3 and C117H27MoN3. See DOI: 10.1039/c4cc01950d
This journal is © The Royal Society of Chemistry 2014
leads to the decreased reactivity of the catalysts (Mo[HIPTN3N] and [Mo(L)(N2)2]2(m-N2)).5a Thus the mechanism of nitrogen fixation awaits elucidation and the function of each component of the catalysts needs identifying. Graphene, a single atomic honeycomb layer of sp2-hybridized carbon with high stability and carrier mobility, has been extensively studied for its potential application in microelectronics and nanoscience.11 The required protonation in nitrogen fixation and the subsequent reduction involves charge transfer, and the high carrier mobility of graphene might facilitate this charge transfer, as the conjugated p electrons serve as a bridge for charge transfer during the reduction of graphene oxide.12 The multi-radical character of graphene nanoribbons causes the frontier molecular orbitals to locate at the edge of the ribbons,13 which could donate and accept electrons. Recent studies found that metal oxide doped graphene is a good catalyst for oxygen reduction.14 Further mechanistic investigation of the catalytic application of graphene will certainly help to open up a new field for the application of this new nano-material. A nitrogen-fixation catalyst based on graphene, which is hoped to combine the merits of molybdenum (efficient catalytic activity) and graphene (electron bridge and reservoir), is computationally evaluated in the present work. With the aid of molecular orbital theory, bonding and population analysis, the possible mechanism of nitrogen fixation catalyzed by the Mo/N-doped graphene system is explored. The energetics (with zero-point vibrational energy correction) and properties of the possible intermediates are studied, and the role of each component of the catalyst is identified to provide mechanistic information for experimental endeavour. The density functional theory based method B3LYP is employed for geometry optimization and property characterization. The pseudopotential based basis set SDD is used for Mo, and the Gaussian basis set 6-31G(d,p) is used for N, C and H. This catalyst (C33H15MoN3) contains a molybdenum atom and three ligand nitrogen atoms as shown in Fig. 1. Two bigger models (C69H21MoN3 and C117H27MoN3 as shown in Fig. S1, ESI†) with bigger graphene sheets have similar electronic structures. For computational efficiency, C33H15MoN3 is investigated in detail for nitrogen fixation in the present work.
Chem. Commun., 2014, 50, 13319--13322 | 13319
View Article Online
Communication
Published on 07 May 2014. Downloaded by University of Windsor on 26/10/2014 22:41:52.
Fig. 1
ChemComm
The Mo/N-doped graphene catalyst (C33H15MoN3).
Fig. 2 Hydrogenation sequences in the Schrock10 and the enzymatic17 reactions.
Two possible pathways for nitrogen fixation have been studied: the Schrock cycle5e,15,16 and the nitrogenase reaction.17 The Schrock cycle is based on a MoIII complex with a specifically designed ligand [HIPT] that provides a sterically shielded site for N2 bonding and reduction. The reaction mechanism of nitrogenase has been simulated for a simplified nitrogenase (FeMoco model system).17 These two reaction pathways, both consisting of six consecutive protonation and reduction steps with different acids as proton sources, are described in Fig. 2 with different hydrogenation (protonation and reduction) sequences, and were investigated for the 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 present work. The proton energy ( 984.97 kJ mol 1) was calculated in eqn (1), and the electron energy ( 433.73 kJ mol 1) was taken as the ionization energy of [CoCp*2] in eqn (2).5e The reaction energy of 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 other steps 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 the N–N bond distance along the reaction pathways up to the fourth H addition are plotted in Fig. 3 (relevant data are listed in Table S5 (ESI†) and are plotted in Fig. S5, ESI†). According to the variation of N–N bond length and bond order after the adsorption of N2
13320 | Chem. Commun., 2014, 50, 13319--13322
Fig. 3 The energetics and N–N bond distance during the N2 fixation in the Schrock and enzymatic reaction paths. 1-catalyst, 2-N2 adsorption, 3/4-1st hydrogenation (protonation and reduction), 5/6-2nd hydrogenation, 7/8-3rd hydrogenation, 9/10-4th hydrogenation.
on Mo, the end-on adsorption of N2 on Mo in the Schrock reaction essentially retains the N–N triple bond [the bond length of N2 is 1.11 Å from B3LYP/6-31G(d) prediction]. The addition of the first two protons elongates the N–N bond to 1.33 Å. The addition of the third 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. The addition of the fourth H to the N (Na) atom bonded to Mo breaks the NH3 apart, with a N–N bond distance of 3.01 Å. On the other hand, the side-on adsorption in the enzymatic reaction path with Mo bonding to the two N atoms of N2 elongates the N–N bond to nearly a double bond (the N–N bond length of N2H2 is 1.24 Å from the B3LYP/6-31G(d,p) prediction). The alternate hydrogenation on the two N atoms leads to the formation of a hydrazine structure with a N–N bond length of 1.46 Å [the N–N bond in hydrazine is 1.49 Å from the B3LYP/6-31G(d,p) prediction]. The majority of the steps in these two reaction paths are exothermic, except for the third protonation in the Schrock reaction (absorbing 103.4 kJ mol 1) and the reduction of the fourth H in the enzymatic reaction (absorbing 19.8 kJ mol 1). The unfavorable energetic requirement in the third protonation of the NH2 (step 7 in Fig. 3) in the Schrock reaction path hinders this process. On the other hand, the energy released from the fourth protonation (31.9 kJ mol 1) (step 9 in Fig. 3) is enough for the reduction of this intermediate (step 10 in Fig. 3). Thus, in terms of reaction energies, the catalyst takes the enzymatic reaction path for nitrogen fixation. Hydrazine (N2H4) could easily form after the fourth hydrogenation, 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
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 07 May 2014. Downloaded by University of Windsor on 26/10/2014 22:41:52.
ChemComm
Communication
Fig. 4 Possible reaction paths for H4R [MoNH2NH2] in the enzymatic reaction.
2.23 Å and 3.03 Å, thus the N2H4 releases feasibly from the catalyst. The desorption energy of N2H4 is 148.2 kJ mol 1, close to that of ammonia (139.5 kJ mol 1). The energy barrier of the fifth 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 environments, while NH3 is more likely to form in strongly acidic environments because of the abundance of H+. The breaking of the N–N bond takes place in the fourth hydrogenation with maximum bond distance and minimum bond order in the Schrock reaction path. Specifically, the N–N bond length is 1.43 Å and bond order is 0.99 in the Schrock reaction path after the third hydrogenation. The catalytic mechanism of the Mo/N-doped graphene catalyst in the present work is different from that of the Schrock reaction.10 The most evident differences are the different hydrogenation site and order of hydrogenation. The release of the first molecular ammonia takes place after the third hydrogenation experimentally,10 while it occurs after the fourth hydrogenation using the Mo/N-doped graphene catalyst. Most of the steps in the Mo/N-doped graphene system are exothermic while the reduction processes 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 be divided into three groups. The charge of each group increases and decreases regularly along the reaction path. Groups 1 (grp1, Mo/N in Fig. 5) and 2 (grp2, surrounding carbons) have small charge fluctuation during the N2 fixation. Group 3 has regular alternating and significant variation of atomic charge during protonation and reduction, i.e. group 3 loses about 0.75 electrons during protonation and gains about 0.81 electrons during reduction (as listed in Tables S6 and S7, ESI†). During protonation, a proton attacks the N2 moiety, forming a N–H bond with the gain of electrons from group 3, and leaves group 3 positively charged. The reduction of the system leads to the injected electron remaining at group 3. Group 3 acts as an electron donor during protonation and an electron acceptor during reduction, i.e. an electron reservoir. Ascribed to the high charge carrier mobility of graphene, group 2 serves as an electron transmitter. Thus far, according to the structure of reactive intermediates and charge variation in these intermediates during N2 fixation, the roles of the three groups can be identified as: group 1 – active centre, group 2 – electron transmitter, group 3 – electron reservoir.
This journal is © The Royal Society of Chemistry 2014
Fig. 5 The variation of atomic charge (the Mulliken charge difference of the present step from that of the previous step) of different groups (grp) in Mo/N-doped graphene during the enzymatic catalytic N2 fixation.
The HOMO (highest occupied molecular orbital) of Mo/N-doped graphene (Fig. 6) matches the charge variation of the system during hydrogenation. The periphery of the graphene part
Fig. 6 The LUMOs (left) and HOMOs (right) of the intermediates in the enzymatic reaction. Hn means the nth protonation. HnR is the reduction step following the nth protonation. The top two are the LUMOs (left) and HOMOs (right) of Mo/N-doped graphene.
Chem. Commun., 2014, 50, 13319--13322 | 13321
View Article Online
Published on 07 May 2014. Downloaded by University of Windsor on 26/10/2014 22:41:52.
Communication
(group 3) dominates in the HOMO. Group 1 (Mo/N) also contributes to the HOMO, while group 2 has little population in the HOMO. The Mo/N is the active centre for protonation according 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 has the dominant contribution. Electrons would travel to the Mo/N moiety during reduction. Such an orbital distribution in the HOMO and LUMO of those intermediates highlights the Mo/N as the active centre and the periphery region as an electron reservoir. In summary, the possible reaction mechanism of N2 fixation by a graphene catalyst (Mo/N-doped graphene) has been studied with density functional theory. The enzymatic route is energetically more feasible for N2 fixation on Mo/N-doped graphene than the Schrock path, as it is more energetically favorable for the simultaneous bonding of two N atoms to Mo. Six protons are introduced alternately to the two nitrogen atoms forming three intermediates: [MoNH2NH2], [MoNH2NH3] and [MoNH3]. [MoNH2NH2] tends to generate NH2NH2 in weakly acidic environments because of its weak Mo–Na and Mo–Nb interaction, while [MoNH2NH3] and [MoNH3] could form in strongly acidic environments with the abundance of H+. The variation of atomic charge along the reaction processes reveals that the catalyst can be divided into three functional parts with different catalytic roles. The Mo/N is the active centre, and the graphene body serves as an electronic bridge for electron transmission, while the graphene periphery region stores and provides electrons for protonation and reduction. With high charge carrier mobility, the use of graphene functioning as an electron transmitter and an electron reservoir could broaden its applications in catalysis. This work is supported by the State Key Lab of Urban Water Resource and Environment (HIT) (2014TS01) and the Open Project of State Key laboratory of Supramolecular Structure
13322 | Chem. Commun., 2014, 50, 13319--13322
ChemComm
and Materials (JLU) (SKLSSM201404). Dr Lei Liu is gratefully acknowledged for his constructive suggestion and for reading this work.
Notes and references 1 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. ¨gl, Angew. Chem., Int. Ed., 2003, 42, 2004–2008. 3 R. Schlo 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. ¨ller, Chem. 5 (a) T. J. Hebden, R. R. Schrock, M. K. Takase and P. Mu Commun., 2012, 48, 1851–1853; (b) T. Ogawa, Y. Kajita, Y. WasadaTsutsui, H. Wasada and H. Masuda, Inorg. Chem., 2013, 52, 182–195; (c) A. Itadani, M. Tanaka, T. Mori, H. Torigoe, H. Kobayashi and Y. 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. Kirchner and 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. Pasˇalic´, M. H. Gerzabek, F. Libisch, R. Reiter, ¨rfer, T. Mu ¨ller, R. Shepard and H. Lischka, Angew. Chem., J. Burgdo Int. Ed., 2013, 52, 2581–2584. 14 Y. Y. Liang, H. L. Wang, J. G. Zhou, Y. G. Li, J. Wang, T. Regier and H. J. Dai, J. Am. Chem. Soc., 2012, 134, 3517–3523. 15 (a) A. Magistrato, A. Robertazzi and P. Carloni, J. Chem. Theory Comput., 2007, 3, 1708–1720; (b) B. L. Guennic, B. Kirchner and M. Reiher, Chem. – Eur. J., 2005, 11, 7448–7460; (c) F. Studt and F. Tuczek, Angew. Chem., Int. Ed., 2005, 44, 5639–5642. 16 (a) D. V. Yandulov, R. R. Schrock, A. L. Rheingold, C. Ceccarelli and W. M. Davis, Inorg. Chem., 2003, 42, 796–813; (b) D. V. Yandulov and R. R. Schrock, Inorg. Chem., 2005, 44, 1103–1117. 17 B. Hinnemann and J. K. Nørskov, Top. Catal., 2006, 37, 55–70.
This journal is © The Royal Society of Chemistry 2014
