Low reduction barrier and exothermic ammonia desorption of violet phosphorene nanosheets for nitrogen photoreduction with high reduction rate and long durability

Photocatalytic nitrogen reduction is an environmentally friendly strategy but faces limitations in reaction rate and durability. Herein, violet phosphorene nanosheets (VPNS) were demonstrated to be efficient photocatalysts with high reduction rate of 371 µmol g -1 h -1 (1.0 wt.% rhodium co-catalyst, no sacrificial agent) and high reaction durability (16 h continuous reaction for four cycles). Water and nitrogen were demonstrated to be easily absorbed on the photocatalysts and further reduced into ⁎NH-NH, ⁎NH-NH 2 , and ⁎NH 2 -NH 2 to yield ammonia from in-situ FTIR and density functional theory calculation to follow an associative alternating pathway. The active sites either with defects or not in VPNS were found to energetically favored for nitrogen reduction to obtain ammonia. Defects were found to enhance rhodium deposition and nitrogen adsorption, but resulting in hard desorption of ⁎NH 3 . The first (⁎N-N →⁎N-NH) and second hydrogenation step (⁎N-NH →⁎NH-NH) were found to be endothermic with a moderate energy barrier around 0.60 eV and a small energy barrier (0.18-0.3 eV), respectively. All following steps were found to be exothermic, especially the exothermic desorption of ammonia is well consistent with the long continuous photocatalytic durability. No structure variation was observed for the VPNS after photocatalytic reactions (16 h⁎4). Violet phosphorus is first utilized for photocatalytic nitrogen fixation. With rhodium cocatalyst, the system achieves 371 μmol h⁻¹ g⁻¹ ammonia yield sustained over 16 hours, accompanied by a minimal energy barrier of 0.59 eV. In situ FTIR and DFT calculations reveal the underlying reaction mechanism. • Defects of VPNS were considered to affect the nitrogen reduction performance. The defects of VPNS were found to significantly enhance the deposition of cocatalyst rhodium and further absorption of nitrogen. Only one endothermic hydrogenation step (⁎N-NH →⁎NH-NH) is 0.12 eV, which is easily proceeded. However, the defects were found to make the desorption of ⁎NH3 extremely difficult (2.21 eV). • The absorption and reduction of nitrogen on cocatalyst rhodium close to undefected VPNS were also demonstrated to be energetically favoured. The most endothermic step of first hydrogenation step (⁎N-N →⁎N-NH) was found to have a moderate energy barrier around 0.60 eV and the second hydrogenation step (⁎N-NH →⁎NH-NH) with a small energy barrier (0.18-0.3 eV) and with all following steps exothermic. Especially the exothermic detachment of ammonia is consistent well with the long continuous photocatalytic durability. • The photoreduction of nitrogen by both defected or undefected VPNS was found to follow an associative alternating pathway (end on). Most active sites should be from undefected ones since the cocatalyst rhodium atoms are far away from phosphorus vacancy. • A high nitrogen reduction rate of 371 µmol g -1 h -1 has been obtained by VPNS with 1.0 wt.% rhodium as co-catalyst without any sacrificial agent. No degradation was detected even after 16 h continuous reaction for four cycles, much better than reported photocatalysts which are usually limited by 4 h. • The reaction pathways were confirmed by in situ FTIR spectroscopy and density functional theory (DFT) calculations. • No structural degradation of VPNS was detected even after 64 hours of continuous operation. The robust structure and easily desorption of ⁎NH3 from VPNS ensure the long reaction durability for nitrogen reduction.