Regulating the Unsaturated Co Sites on MOF-74(Co) for Enhancing Photocatalytic N<sub>2</sub> to NH<sub>3</sub> Conversion
Bing-Hua Ye, Cheng Liu, Yueling Chen, Qi Chen, Kaiqiang Jing, Jimmy C. Yu, Jionghua Wu, Ling Wu
Abstract
Photocatalytic N 2 fixation is a green and sustainable pathway for the synthesis of ammonia. However, its efficiency is considerably restricted by the energy-intensive activation of N 2 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 N 2 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 H 2 O from MOF-74(Co). These Co sites can function as active sites to promote chemisorption and activation of N 2 molecules. The sample MOF-74(Co)-200 exhibits the highest NH 4 + 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 N 2 activation at the molecular level.