Mechanistic aspects of ammonia synthesis on Ta3N5 surfaces in the presence of intrinsic nitrogen vacancies

Graphdiyne Enabled Nitrogen Vacancy Formation in Copper Nitride for Efficient Ammonia Synthesis

The electrocatalytic reduction of nitrate is promising for sustainable ammonia synthesis but suffers from slow reduction kinetics and multiple competing reactions. Here, we report a catalyst featuring copper nitride (Cu3N) anchored on a novel graphdiyne support (termed Cu3N/GDY), which is used for electrocatalytic reduction of nitrate to produce ammonia. The GDY absorbed hydrogen and enabled nitrogen (N) vacancy formation in Cu3N for the fast nitrate reduction reaction (NO3RR). Further, the distinct absorption sites formed by GDY and N vacancy enabled the excellent selectivity and stability of NO3RR. Notably, the Cu3N/GDY catalyst achieved a high ammonia yield (YNH3) up to 35280 μg h-1 mgcat.-1 and a high Faradaic efficiency (FE) of 98.1% using 0.1 M NO3- at -0.9 V versus a reversible hydrogen electrode (RHE). Using electron paramagnetic resonance (EPR) technology and in situ X-ray absorption fine structure (XAFS) spectroscopy measurement, we visualized the N vacancy formation in Cu3N and electrocatalytic NO3RR enabled by GDY. These findings show the promise of GDY in sustainable ammonia synthesis and highlight the efficacy of Cu3N/GDY as a catalyst.

Nitrogen-vacancy-modulated efficient ammonia desorption over 3d TM-anchored BC3N2 monolayer

Nitrogen fixation using electrochemical methods on the surface of single-atom catalysts (SACs) provides a highly feasible strategy for green and low-energy-consumption ammonia (NH3) production. Herein, using density functional theory (DFT) calculations, we explored in detail the potential of monolayer BC3N2 SACs supported with 3d transition metal (TM) atoms (TM@BC3N2) to facilitate nitrogen reduction. The results revealed that the TM@BC3N2 systems exhibited remarkable catalytic activity in the nitrogen-reduction reaction (NRR). The fine NRR activity was related to the just-right bonding/antibonding orbital interactions between the 2π* of N2 and the d orbitals of the TM ions. The nitrogen-adsorption configurations were found to have different activation mechanisms. In addition, the effects of convectively formed convex nitrogen defects (VN) on the interaction between N2 and VN-TM@BC3N2 and the NRR process of VN-TM@BC3N2 were studied, and it was found that VN could fine-tune the reaction efficiency of the eNRR because after N atom dissociation to form VN, the interaction of TM-C3 was enhanced, and the activation of nitrogen and adsorption of NH3 by the TM-active centers were weakened. The present study can be used as a motivation for further experimental and theoretical research of 2D monolayers as NRR electrocatalysts.

Theoretical investigation of borane compounds mimicking transition metals for N2 fixation and activation

N₂ fixation is very difficult because of the nonpolarity and high stability of N₂. Traditionally, it is achieved by transition metal (TM) systems utilizing the back donation from the d orbitals of the TM to the antibonding π* orbitals of N₂ to activate N₂. This back donation is rare for main group compounds due to the lack of high-lying valence d orbitals. In the present study, we show that borane compounds with weak B-X (X = H, Si, Ge, and Sb) bonds can mimic TM systems and be used to fix and activate N₂. This is achieved by the back donation from the σ bonding orbitals of the B-X bonds to the antibonding π* and σ* orbitals of N₂. There is even a linear relationship between the number of B-X bonds and the binding potential energy of N₂ with BR¹R²R³ (R¹, R², R³ = H, CH₃, SiH₃, GeH₃, and SbH₂). Based on these findings, we designed several stable silylborane compounds that are feasible for N₂ fixation and activation under mild reaction conditions, i.e., room temperature and 1 atm. In some sandwich-like complexes formed between N₂ and silylborane compounds, N₂ is even activated from the triple bond to double bond.