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-NH₂, and *NH₂-NH₂ 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 *NH₃. 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).