Catalytic Ammonia Synthesis from N₂ and H₂ over Fullerene-Supported Rh_nCo_4-n (n = 0-4) Clusters: A Theoretical Study

The development of new, efficient, and environmentally friendly ammonia synthesis catalysts is crucial, as the widely utilized Haber-Bosch process requires high temperature and high pressure for N₂ dissociation, leading to significant energy consumption and carbon emissions. In this work, we systematically studied the catalytic ammonia synthesis from N₂ and H₂ over C₆₀-supported Rh_nCo_4-n (n = 0-4) clusters through density functional theory calculations. Our findings reveal that the reactions follow a distal association mechanism, with NH₂^* hydrogenation being identified as the highest energy barrier step and almost barrierless N₂ dissociation. All four metal atoms are the active sites in NH₃ synthesis. The C₆₀-support effectively promotes nitrogen reduction and lowers the N-H formation barriers. The synergistic effect of Rh_n, Co_4-n, and the C₆₀-support significantly improves the efficiency of the ammonia synthesis reaction. Higher Rh/Co ratios reduce the NH₂^* hydrogenation barrier. The C₆₀Rh₄ exhibits superior catalytic performance with the lowest NH₂^* hydrogenation barrier and highest turnover frequency (TOF) values; however, C₆₀Co₄ emerges as a more promising catalyst candidate for large-scale applications due to its low cost, unique ability to achieve thermal N₂-to-NH₃ conversion (unattainable by Rh-doped clusters), and TOF values exceeding all Rh-doped clusters except for Rh₄. Our findings provide fundamental insights into catalytically active sites, optimal Rh/Co ratios, and C₆₀'s synergistic role, guiding improved catalyst design for efficient NH₃ synthesis.