来源：ACS Publications
We report the activity of transition metals (Fe, Co, and Ru) supported on CeO2 (CeO) and Gd-doped CeO2 (CeGdO) for ammonia synthesis. Compared with BM/CeO (BM = Fe + Co), the Ru/BM/CeGdO catalyst achieved 3.1-fold higher rates at 450 °C and 0.8 MPa. Such high activity was attributed to the basicity of the catalysts and their reversible hydrogen storage ability. Specifically, built-in oxygen vacancies and partially reduced ceria support readily transfer electrons to the metals. Further, the electron-rich metal surface easily dissociates N2 and forms NH3 at rapid rates. Kinetic analysis indicates the positive hydrogen order for Ru/BM/CeGdO, which indicates the mitigation of the hydrogen poisoning effect and shift of the rate-determining step from nitrogen dissociation to NHx formation. These catalysts demonstrate stable performance for long durations. This work highlights the potential of mixed transition metal-based catalysts supported on oxide materials for enabling efficient ammonia synthesis.
In the present work, we investigated Co/Fe bimetallic catalysts supported on two different materials: CeO2 (CeO) and Gd-doped CeO2 (CeGdO). Additionally, we incorporated <5 wt % Ru into the most active support to further enhance performance. Between the support materials, metals loaded on the CeGdO support exhibited superior activity. At 450 °C and 0.8 MPa, the ammonia synthesis activity of BM/CeGdO and Ru/BM/CeGdO was 1.51 and 3.13 times greater, respectively, than that of BM/CeO2. Notably, Ru/BM/CeGdO achieved the highest ammonia synthesis rate of 9.50 mmol g–1 h–1 at 500 °C and 1.5 MPa. Moreover, kinetic analysis, supported by both theoretical and experimental data, revealed that the developed catalysts exhibited efficient activation of the N≡N bond. As a result, the rate-determining step (RDS) shifted from nitrogen dissociation to NHx formation, marking an important advancement in the quest for efficient ammonia synthesis under mild conditions. Further, the reaction was confirmed to be free from diffusion resistances, as validated by the Mears and Weisz–Prater criteria. Experimental findings disclosed the vital role of Ov and Ce3+ species for the transfer of electrons from the support to metal particles, which facilitates ammonia synthesis under milder conditions.