Atomically dispersed s-block metal calcium site modified mesoporous g-C3N4 for boosting photocatalytic N2 reduction

Abstract

Alkaline-earth metal elements in the s-block of the periodic table have rarely been studied as active sites for nitrogen (N₂) photofixation. Herein, we report a single-atom calcium (Ca)-modified mesoporous g-C₃N₄ (Ca/m-g-C₃N₄) for promoting the photocatalytic N₂ reduction reaction (pNRR) under ambient conditions. Moreover, the atomically dispersed Ca as active sites of Ca/m-g-C₃N₄ could achieve high adsorption of N₂ molecules, which could be confirmed by nitrogen temperature-programmed desorption test (N₂-TPD). In this regard, Ca single atoms can not only serve as the active centers of N₂ but also optimize the energy band structure of m-g-C₃N₄, facilitating the photocatalytic synthesis of ammonia (NH₃). Therefore, the optimal 0.5 Ca/m-g-C₃N₄ demonstrates a remarkable NH₃ generation amount of 42.23 μg g_cat.⁻¹ h⁻¹, which is 2.1 times that of pure g-C₃N₄ (20.41 μg g_cat.⁻¹ h⁻¹). Furthermore, the present 0.5 Ca/m-g-C₃N₄ demonstrates good stability, and the NH₃ production is still remarkably unvaried after four cycling tests. More interestingly, it was also found that other alkaline earth metal elements, including magnesium (Mg), strontium (Sr), and barium (Ba), can also activate N₂ molecules. Accordingly, the photocatalytic NH₃ synthesis yield of Ba/m-g-C₃N₄ is 29.52 μg g_cat.⁻¹ h⁻¹, slightly more than that of Mg/m-g-C₃N₄ (20.52 μg g_cat.⁻¹ h⁻¹) and Sr/m-g-C₃N₄ (20.88 μg g_cat.⁻¹ h⁻¹). We hope that this work could provide a novel insight for developing other high-performance N₂-photofixation systems based on low-cost s-block alkaline-earth metal materials in the future.