Density functional theory of N2 fixation on B doped g-C₉N₁₀― 23 ―[ 共同研究成果 ]Density functional theory of N2 fixation on B doped g-C9N10
Yuelin Wang* and Yoshitada Morikawa
Department of Precision Engineering, Graduate School of Engineering, Osaka University, 2-1
Yamada-oka Suita, Osaka 565-0871
Email: wangyl262@cp.prec.eng.osaka-u.ac.jp
N2 fixation to produce NH3 is vital for fertilizers and energy storage. Converting N2 toNH3 under ambient conditions is challenging due to the stable N≡N bond. The industrial HaberBosch method, though effective, requires extreme conditions and generates CO2 [1-2].
Photocatalysis and electrocatalysis of N2 provide eco-friendly alternatives, improving reaction
rates, selectivity, and energy efficiency while avoiding CO2 emissions. Discovering a suitable
N2RR catalyst is crucial for sustainable development [3]. Metal-free carbon materials havegained significant attention over metal-based catalysts due to their excellent stability, low cost,high surface area, and environmental friendliness [4]. B atom is electron-deficient atom, and it
has Lewis-acid characteristics, which can drive the “σ donation-π backdonation” process whenthe B atom is attacked by Lewis base (such as N2) [5]. Thus, in this project, we havesystematically investigated N2RR on B doped g-C9N10 with three doping configurations, i. e. B
substituted N (BN1), B substituted C (BC1), and B anchored (BA) on g-C9N10 using density
functional theory (DFT) calculations [6].
First, we construct the seven B atom doping structures, namely B substituted C or N andanchored on g-C9N10 (Figure 1(a-h)). The stability of different doping sites is evaluated byformation energy, and BN1 and BA (BC1) are stable at N-poor (rich) conditions. (Figure 1(i)). We
found that for BC1, the N2 molecule is physisorbed at B site due to the large boron-nitrogen
distance (3.298 Å), while N2 adsorbed on BN1 only has an end-on configuration with anadsorption energy of −1.53 eV and N2 adsorbed on BA has two different end-on and side-on
configurations with Eads‘s of −1.48 eV and −0.91 eV, respectively. we found that N2 is preferably

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adsorbed in the end-on configuration than the side-on one by a large energy difference of 0.57
eV and the transition from end-on to side-on N2 requires a barrier of 1.00 eV by CI-NEB [7-8]..

Figure 1. The possible site of B doped g-C9N10 (a). The optimal structure of BC1 doped g-C9N10 (b), BC2 doped gC9N10 (c), BC3 doped g-C9N10 (d), BN1 doped g-C9N10 (e), BN2 doped g-C9N10 (f), BN3 doped g-C9N10 (g) and BA
doped g-C9N10 (h). (i) The formation energy of seven B doped g-C9N10 structures as a function of N chemical
potential. ΔμN = 1/2(μN2 − EN2) where EN2 is the total energy of a gas-phase N2 molecule at 0 K. The bottom axes
show the corresponding N2 chemical potentials at the absolute temperature T and partial pressure P (with P° = 1atm), µN2 = H°(T) − H°(0) − TS°(T) + kBTln(P/P°), where the enthalpy H° and the entropy S° are obtained fromref. 9.

Then, we investigated the five possible N2RR pathways [10-12] starting by end-on N2
adsorption, including alternating, distal, mixed Ⅰ, mixed Ⅱ and mixed Ⅲ pathways. The free energy
calculations show that BN1 and BA doped g-C9N10 proceeds via mix I mechanism (Figure 2) startingfrom the stable end-on N2 with low limiting potentials of −0.62 V and −0.44 V, respectively. Importantly,
H blocks active site in the case of BA doped g-C9N10 due to stronger Eads of H* (-1.95 eV), resulting inDensity functional theory of N2 fixation on B doped g-C₉N₁₀― 25 ―lower N2RR selectivity, while BN1 doped g-C9N10 can effectively prevent the H poisoning due to theweaker H adsorption relative to N2 adsorption (-0.25 eV vs. -0.9 eV), thus improving the N2RR activity
and selectivity.
Finally, we also perform the band structure and adsorption spectrum to investigate thephotocatalytic activity. The band structures and absorption spectra indicate that introducing B
atom can decrease the band gap and enhance the light absorption ability in the visible range.
In summary, in this project, partly DFT works were performed by Simulation Tool for
the Atom Technology (STATE) program using AOBA-B system of the supercomputer intohoku university to study atomistic insights into the N2RR on B-doped g-C9N10. The
supercomputer offers very effective and time-saving to run the work. We think that our work
would motivate experimental work to prove and explore the more carbon nitride materials for
N2RR. Detailed results are published in reference [6].

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SENAC Vol. 56, No. （3 2023. 7）

Figure 2. (a) Schematic depiction of distal, alternating, enzymatic and Mixed mechanisms for N2RR. (b) Free
energy diagrams for optimal N2 reduction pathway on BN1 doped g-C9N10 through mixed Ⅰ mechanisms with theoptimized structure of each intermediate. (c) Free energy diagrams for optimal N2 reduction pathway on BA doped
g-C9N10 through enzymatic mechanism with the optimized structure of each intermediate.

Density functional theory of N2 fixation on B doped g-C₉N₁₀― 27 ―

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