Metal-Free Single Atom Catalyst for N2

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Metal-Free Single Atom Catalyst for N2 Fixation Driven by Visible Light Chongyi Ling, Xianghong Niu, Qiang Li, Aijun Du, and Jinlan Wang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07472 • Publication Date (Web): 04 Oct 2018 Downloaded from http://pubs.acs.org on October 4, 2018

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Journal of the American Chemical Society

Metal-Free Single Atom Catalyst for N2 Fixation Driven by Visible Light Chongyi Ling,a,b Xianghong Niu,c Qiang Li,a Aijun Du,*,b Jinlan Wang*,a aSchool

of Physics, Southeast University, Nanjing 211189, People’s Republic of China. of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Gardens Point Campus, Brisbane, QLD 4001, Australia. cSchool of Science, Nanjing University of Posts and Telecommunications, Nanjing 210046, People’s Republic of China. bSchool

ABSTRACT: Solar nitrogen (N2) fixation is the most attractive way for the sustainable production of ammonia (NH3), but the development of a highly active, long-term stable and low-cost catalyst remains a great challenge. Current research efforts for N2 reduction mainly focus on the metal-based catalysts using electrochemical approach, while metal-free or solar-driven catalysts have been rarely explored. Herein, on the basis of a concept of electron “acceptance-donation”, a metal-free photocatalyst namely, boron (B) atom decorated on the optically active graphitic-carbon nitride (B/g-C3N4), for the reduction of N2 is proposed by using extensive first-principles calculations. Our results reveal that gas phase N2 can be efficiently reduced into NH3 on B/g-C3N4 through the enzymatic mechanism with a record low onset potential (0.20 V). Moreover, the B-decorated g-C3N4 can significantly enhance the visible light absorption, rendering them ideal for solar-driven reduction of N2. Importantly, the as-designed catalyst is further demonstrated to hold great promise for synthesis due to its extremely high stability. Our work is the first report of metal-free single atom photocatalyst for N2 reduction, offering cost-effective opportunities for advancing sustainable NH3 production.

Introduction The element nitrogen is essential for plants, animals and other life forms on Earth.1-4 Although the atmosphere consists of more than 78% of N2, the inherent inert character of N2 makes its utilization very difficult. As the reduction product of N2, NH3 is not only an important chemical in various fields, but also a promising energy storage intermediate.5-6 Currently, industrial N2 fixation to produce NH3 mainly relies on the Haber–Bosch process, which, however, not only requires heavy energy consumption, but also generates a large number of greenhouse gases.7-10 A promising way for the sustainable production of NH3 under ambient condition is the photocatalytic or electrocatalytic reduction of N2.11-15 During these processes, efficient catalysts to enhance the rate, increase the selectivity and decrease energy consumption of the reactions play a key role.16 Therefore, the exploration of catalysts for N2 reduction reaction (NRR) has attracted tremendous attention during the past decade.4, 10-12, 17-22 To date, numerous catalysts have been fabricated, such as FeMoS chalcogels,23 BiOBr nanosheets,24 Gold 25 nanoparticles/black Si/Cr, Au nanorods,26 Fe2O3/carbon nanotubes,27 Au@TiO217 and so on. However, these efforts mainly focus on metal based catalysts and metal-free catalyst has been rarely explored. Considering the fact that metal-free catalysts can possess excellent activity, long durability and free from the poisoning and cross-over effects in addition to their intrinsic advantages of low cost and environmental friendliness28-40, the development of metal-free catalysts for N2 fixation is therefore of great economic interest and scientific importance. Especially for the photocatalysts, they can directly drive the reaction by harvesting sunlight to produce NH3 from N2 and H2O.41

The excellent performance of transition metal based catalysts can be ascribed to the co-existence of empty and occupied d orbitals, which on one hand can accept the lonepair electrons of N2 and on the other hand can donate electron into anti-bonding orbitals of N2 to weaken the N ≡ N triple bond.1 For non-metal element, B atom in molecular catalyst with sp2 hybridization has exhibited great potential for N2 fixation, where N2 can interact with two B site through the end-on pattern and be reduced into B2N2 or [B2N2]2-.1 Actually, B atom with sp3 hybridization also contains occupied and empty orbitals simultaneously, indicating its potential for N2 fixation. More importantly, sp3 hybridized B atom can bind N2 through side-on pattern due to the compatibility of the symmetry of the orbitals, which may result in a different reduction product. Therefore, decorating single B atom with sp3 hybridization onto an optically active and metal-free substrate may achieve the goal of metal-free N2 fixation driven by solar. Since graphitic-carbon nitride (C3N4) is a well-known metal-free photocatalyst for various reactions,31-33 the incorporation of single B atom onto C3N4 (B/g-C3N4) holds great potential for converting N2 into NH3 driven by solar. So in this work, we investigate the catalytic activity of the first metal-free single atom photocatalyst, B/g-C3N4, for the reduction of N2. As expected, B/g-C3N4 can bind the N2 strongly, -1.04 and -1.28 eV through side-on and end-on patterns, respectively. Surprisingly, the adsorbed N2 can be further reduced into NH3 with a rather low onset potential of 0.20 V through the enzymatic mechanism. In addition, the decoration of B atom on g-C3N4 can significantly enhance the visible light absorption, making the reduction process possibly occur under visible light. The stability of B/g-C3N4 is systematically evaluated, and shows that the designed catalyst is very promising to be synthesized.

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Figure 1. (a) Simplified schematic of N2 bonding to transition metals. (b) Electronic configuration of pure B atom and B atom with sp3 hybridization. sp3 hybridization of B will result in four sp3 hybrid orbitals, where three of them are half filled and the rest one is empty. (c) N2 binding motifs to the B atom which is stabilized on the substrate. (d) Design concept of as a photocatalyst for N2 fixation. When incorporating into g-C3N4, B atom will interact with two N atoms to form two B-N bonds, leaving an empty sp3 orbital and a half occupied sp3 orbital. Therefore, the incorporated B atom will have strong binding strength with N2 molecule. The arrows along different directions represent single electrons of opposite spin and spindle in different colours describe the orbital lobes of opposite phase. Gray, blue and pink balls represent the C, N and B atoms, respectively.

Computational Details All the first-principles spin-polarized calculations were performed by using Vienna ab initio simulation package.42-43 The ion-electron interactions were described by the projector augmented wave method.44 The generalized gradient approximation in the Perdew-Burke-Ernzerhof form45-46 and a cut-off energy of 500 eV for plane-wave basis set were adopted. The convergence criterion for the residual force and energy was set to 0.01 eV/Å and 10-5 eV, respectively, during the structure relaxation. Supercells consisting of 2 × 2 × 1 gC3N4 unit cells were used and the Brillouin zones were sampled by a Monkhorst-Pack k-point mesh with a 3 × 3 × 1 kpoint grid. A vacuum space exceeds 15 Å was employed to avoid the interaction between two periodic units. The band structures and optical adsorption spectra are calculated by using the hybrid functionals based on the Heyd-ScuseriaErnzerhof method.47 Ab initio molecular dynamics simulations (AIMD) were employed to evaluate the thermodynamic

stability of the materials. Details on the calculations of the free energy diagrams and the onset potentials can be found in the supporting information (SI). Results and Discussion Generally, the chemisorption of gas phase N2 onto the surface of the catalysts is the prerequisite for an efficient NRR process. For transition metal (TM) based catalysts, their strong binding strength with N2 can be ascribed to their advantageous combination of empty and occupied d orbitals (Figure 1a). On one hand, due to the existence of lone-pair electrons of N2, TM centers need to have empty d orbitals to accept the lonepair electrons. On the other hand, to enhance the N-TM bonds, the TM atoms should have separate d electrons that can be donated into the anti-binding orbital and weaken the N ≡ N triple bond.1 Therefore, “acceptance-donation” of electrons is the nature of the interaction between the TM and N2, where


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Figure 2. Top and side view of the structures of (a) B/g-C3N4 and the adsorption of N2 on B/g-C3N4 through the (b) side-on and (c) end-on patterns. Difference charge density of B/g-C3N4 with the adsorption of N2 via (d) side-on and (e) end-on patterns, where the isosurface value is set to be 0.005 e/Å3 and the positive and negative charges are shown in yellow and cyan, respectively. Gray, blue and pink balls represent the C, N and B atoms, respectively.

the combination of empty and occupied d orbitals plays a key role.1 Among the non-metal atoms, B atom also possesses the combination of empty and occupied orbitals. As presented in Figure 1b, the electronic configuration of B atom is 2s22p1 and the sp3 hybridization of these orbitals will result in three half occupied and one empty orbitals. When decorating the B atom into a suitable substrate by forming two B-substrate chemical bonds, the B atom would have one occupied and one empty sp3 left, which can drive the “acceptance-donation” process as shown in Figure 1c. Therefore, the incorporated B atom can sever as active center for the reduction of N2. In fact, N2 fixation on B site has been realized experimentally in a molecular catalysts.1 To gain a metal-free catalyst for NRR, what we need to do is to find a suitable substrate that consists of non-metal elements. As mentioned above, g-C3N4 is a promising active and stable metal-free photocatalyst and has been applied to various reactions, such as water splitting, Friedel–Crafts reaction of benzene, oxygen reduction reaction, NO decomposition and so on.31-33, 38, 48-49 It is composed of 3-fold-coordinated and 2-foldcoordinated N atoms and 3-fold-coordinated C atoms which

are bonded to 3 N atoms (see Figure 1d). The 2-foldcoordinated N atoms endow g-C3N4 the capacity for the incorporation of B atom. During this process, two N-B bonds are generated to stabilize the B atom onto g-C3N4 and meanwhile, leaving one occupied and one empty sp3 orbitals. Therefore, the anchored the B atoms will have strong interaction with the gas phase N2 molecules. Moreover, large periodic vacancies of g-C3N4 offer enough space for the adsorption and subsequent reduction of N2 (Figure 1d). In addition, g-C3N4 possesses the ability of excellent solar light absorption, which can generate photo-excited electrons for the reduction of N2. Thus, g-C3N4 is an ideal substrate for the incorporation of B to achieve the N2 fixation and the produced B/g-C3N4 may be an excellent metal-free photocatalyst for the reduction of N2.



Figure 3. (a) Schematic depiction of distal, alternating and enzymatic mechanisms for N2 reduction to NH3. Free energy diagrams for N2 reduction on B/g-C3N4 through (b) distal, (c) alternating and (d) enzymatic mechanisms at different applied potentials.

The optimized structures of B/g-C3N4 are presented in Figure 2a. Clearly, the framework structure of g-C3N4 can be maintained well with the decoration of B atom. Figure 2b and 2c illustrate structures of N2 adsorbed B/g-C3N4 via side-on and end-on patterns, respectively. As expected, the binding strength between N2 and B/g-C3N4 is indeed very strong with the adsorption energy of -1.04 and -1.28 eV for side-on and end-on adsorption, respectively. Charge density difference of B/g-C3N4 with the adsorption of N2 is further calculated as shown in Figure 2d and 2e. Significant charge transfer between the anchored B atom and N2 can be observed for both adsorption patterns. Generally, the charge transfer is one-way that from one species to another depending on their relative electron accepting and donating abilities.50-54 Interestingly, charge transfer here is found to be two-way that charge accumulation and depletion can be observed for both N2 molecule and B atom. This phenomenon is actually in perfect accordance with the “acceptance-donation” process as described above: B atom will accept lone-pair electrons and simultaneously donate electrons into the anti-bonding orbital of N2. Therefore, these calculation results, including high binding strength and two-way charge transfer between N2 and B/g-C3N4, strongly support our design concept. Next we move on to the assessment of the catalytic performance of B/g-C3N4 for the reduction of N2 into NH3. Three typical mechanisms, named as enzymatic, alternating and distal (Figure 3a), which include all possible reaction intermediates and have been widely investigated, are taken into consideration. The end-on adsorbed N2 can be reduced through two different ways, alternating and distal. The electron pairs attack at one N atom preferentially to produce a NH3 and then attack at another N atom for distal mechanism. On the contrary, the electron pairs attack at two N atoms alternatively for alternating pathway. For enzymatic mechanism, the N2 adsorbed on B/g-C3N4 through the side-on pattern and then two N atoms are hydrogenated by the proton-

electron pairs (H+ + e-) alternately. The second NH3 molecule will be released just following the generation of the first one. Free energy diagrams through these three pathways are illustrated in Figure 3b, 3c and 3d, respectively, while the corresponding structures of the reaction intermediates are presented in Figure S1-S3 in the Supporting Information. Owing to the high binding strength between N2 and B/g-C3N4, the hydrogenation of the adsorbed N2 is very easy with a free energy uphill of 0.22 and 0.09 eV for end-on and side-on adsorption way, respectively. For the subsequent elementary reaction steps through the distal pathway, the generation of the first (*N-NH2 + H+ + e- = *N + NH3) and second (*NH2 + H+ + e- = *NH3) NH3 are endothermic and the rest elementary steps are exothermic. Moreover, the free energy uphill for the production of the second NH3 (0.87 eV) is much higher than that of the first one (0.29 eV). Therefore, the generation of the second NH3 is the potential-determining step and the corresponding onset potential is as high as 0.87 V (Figure 3b). Regarding of the alternating mechanism, the hydrogenation of *NH-NH2 into *NH2-NH2 presents an even higher free energy than the protonation of *NH2, leading to a much higher onset potential (1.21 V) than the distal pathway (Figure 3c). Therefore, the NRR processes on B/g-C3N4 via these two mechanisms are not efficient enough. For the case of enzymatic mechanism, the subsequent steps are all exothermic except the final step (*NH2-*NH3 + H+ + e- = *NH3 + NH3) as shown in Figure 3d. The occurrence of this elementary reaction only demands the energy injection of 0.20 eV, leading to a rather low onset potential (0.20 V). To the best of our knowledge, this value is much lower than all the other catalysts that have been reported, demonstrating that B/g-C3N4 is a very promising metal-free catalyst for NRR with excellent performance.


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Figure 4. Optical adsorption spectra of pure g-C3N4 and B/gC3N4, which are illustrated by black and red line, respectively. It should be noted that the actual reaction pathway is very complicated and may include all these three reaction pathways. Therefore, we further calculate the minimum free energy diagram of N2 reduction starting with an end-on adsorbed N2. As shown in Figure S4a, the first three electrochemical steps are via the alternating mechanism, while the last three electrochemical steps are through the enzymatic mechanism due to the extremely low free energy of *NH2-*NH2 (2.80 eV lower than *NH2-NH2 intermediates). The potentialdetermining step is the hydrogenation of N2 with an onset potential of 0.22 V. This result not only indicates the high efficiency of the minimum free energy pathway, but also demonstrates that the end-on adsorbed N2 may also be reduced under a very low onset potential. Furthermore, to evaluate the accuracy of PBE results, revised-PBE (RPBE) functional which describes the binding energies with sufficient accuracy is also employed.55-56 As shown in Figure S4b, the RPBE gives similar results to that of PBE (the calculated onset potentials are 0.21 and 0.20 V, respectively), demonstrating the reliability of PBE for qualitatively describe the NRR activity of B/g-C3N4. This may be ascribed to the fact that the onset

potential comes from the difference between the energies of two intermediates. As for photocatalysts, the photoconversion efficiency is very important. However, it is well known that g-C3N4 shows only marginal absorption in the visible light range owing to its relatively large band gap.32, 57-58 The improvement of the optical adsorption activity is therefore highly desirable to achieve better photocatalytic performance of g-C3N4. Interestingly, the decoration of B on g-C3N4 can greatly enhance the visible (VIS) light and infrared (IR) light harvesting. As shown in Figure 4, the main light adsorption peak of pure g-C3N4 is located at ~300 nm wavelength, indicative of its strong absorbance of ultraviolet (UV) light and very limited absorbance of visible light. These results are in good agreement with previous studies.32, 57 Although the absorbance of ultraviolet light will be slightly weakened, both the visible light and IR light absorbance can be greatly improved for g-C3N4 with the decoration of B. The enhancement of the light absorbance of B/g-C3N4 can be ascribed to the decrease of band gap. As shown in Figure S5, band gap of pure g-C3N4 is 2.98 eV, which is in good agreement with previous studies.32, 58 Thus, it can only adsorb the light with short wavelength (ultraviolet light). With the decoration of B atom, the band gap decreases to 1.12 eV, which endows B/g-C3N4 the ability of capturing the light with longer wavelength (visible and infrared light). Therefore, B/gC3N4 would possess a higher photoconversion efficiency than pure g-C3N4, which can ensure more efficient generation of photo-excited electrons and better photocatalytic performance. Finally, the possibility of experimental realization and the stability of B/g-C3N4 are systematically evaluated. The decorations of B atom on different sites of g-C3N4 are first investigated and two distinct sites are taken into consideration. One is the decoration on the hexagonal hole region that interact with three N and three C atoms (Hex site, Figure 5a, upper panel); the other is in the vacancy of g-C3N4, bonding to two 2-fold-coordinated N atoms and forming a N2C1B1 fourmember ring (VN2C1, Figure 5a, lower panel). For the former case, significant reconstruction of the structure can be observed after full relaxation, where the B atom will occupy the position of a neighbouring C atom (Figure 5a, right panel).

Figure 5. (a) Formation energy of the decoration of B atom on different sites of g-C3N4 and the optimized structures are illustrated on the right panel. The energies presented in the figure are the corresponding formation energies as respect to that of the structure in Figure 2a. (b) Variations of temperature and energy against the time for AIMD simulations of B/g-C3N4, insert are top and side views of the snapshot of atomic configuration. The simulation is run under 1000 K for 10 ps with a time step of 2 fs.



On the contrary, the initial geometric structure can be maintained well for the latter case (Figure 5a, right panel). The calculated binding energies of B at the three different positions are presented in Table S3. It can be observed that the binding energy of B atom on VN3C2 site (Figure 2a) of g-C3N4 is as high as -4.33 eV, indicative of the strong binding strength. Although the binding energies of B atoms on Hex and VN2C1 sites are also considerable (-3.57 and -0.91 eV, respectively), their binding energies are much less negative than that on VN3C2 site (0.75 and 3.42 eV, respectively). Therefore, high binding energy and large energy difference between the formation energies provides significant advantages in terms of experimental synthesis of the designed B/g-C3N4 catalyst. The stability of B/g-C3N4 is further evaluated by using AIMD simulations. As shown from Figure S6 to S8, the structure remains very well at 400, 600 and 800 K. Even when the temperature increases to 1000 K, there is still no significant distortion of the geometric structure (Figure 5b), indicative of the high thermodynamic stability of B/g-C3N4. We further performed a MD simulation for 100 ps, as shown in Figure S9, the energy and the temperature are oscillating near the equilibrium state and the structure of B/g-C3N4 remains very well. This temperature is a slight higher than the experimental measured critical temperature (~900 K)59, which may be ascribed to the fact that the synthesized g-C3N4 may contain many defects, while the model we used here is the perfect one. It should be also noted that incorporation of various non-metal atoms into g-C3N4, including B, has been already realized experimentally.60-62 Therefore, we can conclude that the asdesigned B/g-C3N4 holds great promise for synthesis. Conclusions In summary, we have designed the first metal-free single atom photocatalyst, B/g-C3N4, with excellent activity for N2 fixation. The two-way charge transfer confirms the “acceptance-donation” process between N2 and B/g-C3N4, which plays a key role in the capture and activation of N2. For the subsequent N2 reduction process, the computed onset potential is only 0.20 V through the enzymatic mechanism. Moreover, the decoration of B on g-C3N4 can greatly enhance the visible light and infrared light harvesting, which endows them ideal candidates for visible light reduction of N2. In addition, the as-designed B/g-C3N4 presents much low formation energy than other B decorated g-C3N4 structures, offering significant advantages in terms of experimental synthesis. Besides, this catalyst exhibits a high thermal stability over 1000 K. Low onset potential, strong visible light absorbance, great promise for synthesis and high stability render B/g-C3N4 a very compelling photocatalyst for N2 fixation, which may pave a new way for advancing sustainable NH3 production.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Details on the calculations of the free energy diagrams and the onset potentials; calculated zero point energies, entropy and the structures of different reaction intermediates; variations of temperature and energy of against the time for the ab initio molecular dynamics simulations of B/g-C3N4 under 400, 600 and 800 K.

AUTHOR INFORMATION

*[email protected] (J.W.); *[email protected] (A.D).*

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by National Natural Science Foundation for Distinguished Young Scholar (Grant No. 21525311), the National Key R&D Program of China (Grant No. 2017YFA0204800), Natural Science Foundation of China (Grant No. 21773027), Jiangsu 333 project (BRA2016353), China Scholarship Council (CSC, 201706090115) and the Fundamental Research Funds for the Central Universities and Postgraduate Research ＆ Practice Innovation Program of Jiangsu Province (KYCX17_0044) in China. A.D also acknowledges the support by Australian Research Council under Discovery Project (DP170103598). The authors acknowledge the computational resources provided by NCI National Facility, the Pawsey Supercomputing Centre through the National Computational Merit Allocation Scheme supported by the Australian Government and the Government of Western Australia, National Supercomputing Center in Tianjin.

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Metal-Free Single Atom Catalyst for N2

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