Synergistic effect of coordinating interface and promoter for enhancing ammonia synthesis activity of Ru@N–C catalyst

Abstract

Triruthenium dodecacarbonyl (Ru₃(CO)₁₂) was applied to prepare the Ru-based ammonia synthesis catalysts. The catalyst obtained from this precursor exhibited higher activity than the other Ru salts owing to its unique atomic reorganization under mild temperatures. Herein, Ru₃(CO)₁₂ as a guest metal source incorporated into the pore of ZIF-8 formed the Ru@N–C catalysts. The results indicated that the Ru nanoparticle (1.7 nm) was dispersed in the confined N coordination environment, which can increase the electron density of the Ru nanoparticles to promote NN bond cleavage. The promoters donate the basic sites for transferring the electrons to Ru nanoparticles, further enhancing ammonia synthesis activity. Ammonia synthesis investigations revealed that the obtained Ru@N–C catalysts exhibited obvious catalytic activity compared with the Ru/AC catalyst. After introducing the Ba promoter, the 2Ba–Ru@N–C(450) catalyst exhibited the highest ammonia synthesis activity among the catalysts. At 360 °C and 1 MPa, the activity of the 2Ba–Ru@N–C(450) is 16 817.3 µmol h⁻¹ g_Ru⁻¹, which is 1.1, 1.6, and 2 times higher than those of 2Cs–Ru@N–C(450) (14 925.4 µmol h⁻¹ g_Ru⁻¹), 2K–Ru@N–C(450) (10 736.7 µmol h⁻¹ g_Ru⁻¹), and Ru@N–C(450) (8604.2 µmol h⁻¹ g_Ru⁻¹), respectively. A series of characterizations were carried out to explore the 2Ba–Ru@N–C(450) catalysts, such as H₂-TPR, XPS, and NH₃-TPD. These results suggest that the Ba promoter played the role of an electronic and structural promoter; moreover, it can promote the NH₃ desorption from the Ru nanoparticles.