Photocatalytic Ammonia Synthesis using Fe-Based MOFs: The Role of Ligand Functionalization

Area of Science:

• Materials Science
• Catalysis
• Renewable Energy

Background:

• The Haber-Bosch process for ammonia synthesis is energy-intensive and a major source of CO2 emissions.
• Photocatalytic ammonia synthesis presents a promising carbon-neutral alternative, but efficiency remains a challenge.
• Understanding active sites in photocatalysts is key to improving solar-to-ammonia conversion.

Purpose of the Study:

• To investigate the role of the Fe cluster in MIL-101(Fe) for photocatalytic ammonia synthesis.
• To explore how ligand functionalization of MIL-101(Fe) impacts catalytic activity.
• To establish structure-activity relationships for designing efficient solar-driven ammonia synthesis systems.

Main Methods:

• Experimental and computational investigations of MIL-101(Fe) and its functionalized derivatives.
• Systematic ligand functionalization with groups like -NH2, -Br, -NO2, -F, and -CF3.
• Spectroscopic, surface characterization, and time-resolved infrared spectroscopy to analyze electronic properties, N2 adsorption, proton availability, and excited-state lifetimes.

Main Results:

• The μ3-oxo-centered trinuclear Fe cluster in MIL-101(Fe) acts as the active site for N2-to-NH3 conversion.
• Ligand functionalization modulates Fe electron density, N2 adsorption, and proton availability, influencing catalytic performance.
• Fluorine functionalization of MIL-101(Fe) optimally balanced key factors, leading to a ~60% increase in ammonia production.

Conclusions:

• MIL-101(Fe) metal-organic frameworks with functionalized ligands can significantly enhance photocatalytic ammonia synthesis.
• Structure-activity relationships were uncovered, highlighting the importance of ligand design for optimizing catalyst performance.
• This work provides design principles for developing advanced materials for sustainable solar-driven ammonia production.