Regulating Electron Transfer in a POM-Based MOF Photocatalyst with Dual Active Sites for Enhanced N₂ Oxidation

Area of Science:

• Materials Science
• Catalysis
• Green Chemistry

Background:

• Nitric acid (HNO₃) is a crucial industrial chemical, but its synthesis often involves energy-intensive processes.
• Photocatalysis offers an environmentally friendly route for nitrogen (N₂) fixation, yet faces challenges like high activation barriers and catalyst recombination.
• Polyoxometalates (POMs) and metal-organic frameworks (MOFs) are promising materials for catalytic applications.

Purpose of the Study:

• To develop an efficient photocatalytic system for direct nitric acid (HNO₃) synthesis from nitrogen (N₂).
• To investigate the dual-site mechanism in POMs-based MOFs for enhanced N₂ activation and oxidation.
• To explore the synergistic effects of Mo₇₂Cr₃₀ and UiO-66 in a composite material for sustainable artificial N₂ fixation.

Main Methods:

• Fabrication of a composite photocatalyst combining polyoxometalates (Mo₇₂Cr₃₀) and a metal-organic framework (UiO-66).
• Characterization of the material's structure and electronic properties, focusing on oxygen vacancies and metal oxidation states.
• Evaluation of the photocatalytic activity for nitric acid (HNO₃) synthesis under simulated solar irradiation.

Main Results:

• The Mo₇₂Cr₃₀/UiO-66 composite exhibited a high nitric acid (HNO₃) synthesis rate of 646.3 μg g⁻¹ h⁻¹.
• A dual-site mechanism was identified: oxygen vacancies on UiO-66 capture electrons, facilitating N₂ activation, while holes on Mo₇₂Cr₃₀ oxidize water to generate hydroxyl radicals (•OH).
• The composite significantly outperformed individual components, showing ~18-fold and ~6-fold improvements over Mo₇₂Cr₃₀ and UiO-66, respectively.

Conclusions:

• The synergistic interaction between Mo₇₂Cr₃₀ and UiO-66 in the composite material is highly effective for artificial nitrogen (N₂) fixation.
• This POMs-based MOF design offers a novel strategy for overcoming the challenges in N₂ activation and electron-hole recombination.
• The developed photocatalyst presents a promising and sustainable approach for industrial nitric acid (HNO₃) production.