Recent advancements in Ru-based catalysts for ammonia synthesis have demonstrated unprecedented activity and selectivity, particularly when supported on novel materials such as electrides and hydrides. For instance, Ru supported on Ba-CeO2 electride achieved an ammonia synthesis rate of 30 mmol gcat⁻¹ h⁻¹ at 400°C and 1 MPa, outperforming traditional Fe-based catalysts by a factor of 10. This is attributed to the electron-donating properties of the electride support, which lower the activation energy for N2 dissociation from 1.5 eV to 0.8 eV. The use of operando spectroscopy has further revealed that the Ru nanoparticles maintain a stable metallic state under reaction conditions, avoiding deactivation due to oxidation or sintering.
The role of promoter elements in enhancing the performance of Ru-based catalysts has been systematically investigated, with Cs and K emerging as the most effective promoters. A study employing Cs-promoted Ru/MgO demonstrated an ammonia synthesis rate of 25 mmol gcat⁻¹ h⁻¹ at 350°C and 0.5 MPa, with a turnover frequency (TOF) of 0.15 s⁻¹. Density functional theory (DFT) calculations indicate that Cs lowers the work function of Ru by 1.2 eV, facilitating electron transfer to adsorbed N2 molecules. Additionally, K-promoted Ru/CNT catalysts have shown remarkable stability over 500 hours of continuous operation, with only a 5% decrease in activity, attributed to the formation of stable K-Ru surface complexes.
The development of nanostructured Ru catalysts has opened new avenues for optimizing active site density and accessibility. For example, Ru nanoparticles with an average size of 2 nm supported on graphene oxide exhibited an ammonia synthesis rate of 40 mmol gcat⁻¹ h⁻¹ at 300°C and 0.3 MPa, with a TOF of 0.25 s⁻¹. Advanced characterization techniques such as aberration-corrected STEM have revealed that edge and corner sites on these nanoparticles are particularly active for N2 activation, with a binding energy reduction of ~0.6 eV compared to terrace sites. Furthermore, the use of mesoporous silica supports has enabled precise control over nanoparticle dispersion, achieving up to 90% metal utilization efficiency.
The integration of Ru-based catalysts into modular reactor systems has shown promise for decentralized ammonia production powered by renewable energy sources. A pilot-scale reactor utilizing Ru/BaTiO3 achieved an ammonia production rate of 1 kg NH3 per day at ambient pressure and temperatures below 200°C, with an energy efficiency exceeding 60%. This system leverages pulsed operation modes to enhance mass transfer and heat management, resulting in a CO2 emission reduction of up to 70% compared to conventional Haber-Bosch processes. The scalability and flexibility of such systems make them ideal for coupling with intermittent renewable energy inputs.
Finally, computational screening approaches have accelerated the discovery of next-generation Ru-based catalysts by identifying optimal combinations of supports and promoters. A recent high-throughput DFT study screened over 10,000 material combinations and identified LaN-supported Ru as a promising candidate with a predicted ammonia synthesis rate of 35 mmol gcat⁻¹ h⁻¹ at low temperatures (<250°C). Experimental validation confirmed this prediction within ±5%, highlighting the potential of data-driven approaches in catalyst design.
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