Microenvironment Engineering Vinylene-Linked Covalent Organic Frameworks for Highly Efficient CO₂ Photoreduction.

Area of Science:

• Materials Science
• Catalysis
• Photochemistry

Background:

• Efficient photocatalytic reduction of carbon dioxide (CO2) to carbon monoxide (CO) in gas-solid systems without solvents, cocatalysts, or sacrificial agents remains a significant challenge.
• Covalent organic frameworks (COFs) offer tunable structures for photocatalytic applications, but optimizing their performance under demanding conditions requires precise control over the microenvironment.

Purpose of the Study:

• To design and construct a robust pyridazine-based COF platform for tunable CO2 photoreduction.
• To establish a structure-reactivity relationship by modulating the nitrogen content in aldehyde linkers.
• To investigate the performance of a hybrid COF material with coordinated rhenium complexes for enhanced CO2-to-CO conversion.

Main Methods:

• Synthesis of vinylene-linked pyridazine COFs with varying nitrogen content in aldehyde linkers.
• Photocatalytic CO2 reduction experiments under gas-solid conditions without solvents, cocatalysts, or sacrificial agents.
• Characterization using in situ infrared spectroscopy and isotopic labeling.
• Computational analysis using density functional theory (DFT).
• Coordination of Re(CO)5Cl to specific nitrogen sites within the COF structure.

Main Results:

• A pyridine-containing COF demonstrated superior charge separation and the highest CO production rate among metal-free photocatalysts under gas-solid conditions.
• The optimized COF microenvironment, when functionalized with Re(CO)5Cl, created molecular catalytic centers enhancing the acceptor-donor-acceptor (A-D-A) charge-transfer pathway.
• The resulting hybrid material achieved a record gas-solid CO2-to-CO activity, outperforming existing heterogeneous CO2 reduction systems.
• DFT calculations and spectroscopic studies elucidated the mechanism, highlighting the role of N-site configuration in intermediate binding and transition state energetics.

Conclusions:

• N-microenvironment engineering in COFs is a versatile strategy for developing high-performance photocatalysts for CO2 conversion.
• The designed pyridazine COF platform enables precise tuning of electronic properties and catalytic activity.
• This work presents a significant advancement in heterogeneous photocatalysis for CO2 reduction, offering a solvent-free and additive-free approach.