Atomically dispersed s-block metal calcium site modified mesoporous g-C3N4 for boosting photocatalytic N2 reduction†
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 gcat.−1 h−1, which is 2.1 times that of pure g-C3N4 (20.41 μg gcat.−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 gcat.−1 h−1, slightly more than that of Mg/m-g-C3N4 (20.52 μg gcat.−1 h−1) and Sr/m-g-C3N4 (20.88 μg gcat.−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.