Regulating the Unsaturated Co Sites on MOF-74(Co) for Enhancing Photocatalytic N2 to NH3 Conversion

Photocatalytic N₂ fixation is a green and sustainable pathway for the synthesis of ammonia. However, its efficiency is considerably restricted by the energy-intensive activation of N₂ molecules. Herein, a series of MOF-74(Co) with varying contents of coordinatively unsaturated Co sites were synthesized through the thermal treatment at different temperatures (T = 100, 150, 200, 250 °C). These photocatalysts were used for the photocatalytic fixation of N₂ in the absence of a sacrificial agent. The experimental results and theoretical computations indicate that the amount of coordinatively unsaturated Co sites increases as the temperature elevates by the removal of coordinated H₂O from MOF-74(Co). These Co sites can function as active sites to promote chemisorption and activation of N₂ molecules. The sample MOF-74(Co)-200 exhibits the highest NH₄⁺ production rate of 84 μmol·g^–1·h^–1, which is six times higher than that of the pristine MOF-74(Co) (13.9 μmol·g^–1·h^–1). The enhanced performance can be ascribed to the abundance of active sites and the optimal mobility of the photogenerated charges. Finally, a mechanism is proposed for the photocatalytic N₂ activation at the molecular level.