Photochemical reduction of low concentration CO2 to solar fuel in a metal free 2D-covalent organic framework

Abstract

Converting diluted CO₂ into valuable fuels using solar energy is an energy-efficient approach to mitigate global warming. Covalent organic frameworks (COFs) hold great potential as photocatalysts for CO₂ reduction due to their pre-designable, porous, and robust structures. This study presents the synthesis and characterization of a 2D BDT-TTA COF, made from 1,3,5-tris-(4-aminophenyl)triazine (TTA) and benzo[1,2-b:4,5-b′]-dithiophene-2,6-dicarboxaldehyde (BDT), with a high surface area (1886 m² g⁻¹) and notable CO₂ uptake. The COF has suitable band positions for visible-light absorption and CO₂ reduction to CO. Its periodic nanochannels, decorated with heteroatoms, influence CO₂ binding and diffusion with a heat of adsorption of 29 kJ mol⁻¹. The as-synthesized COF exhibits photoreduction of CO₂ to CO at a rate of 533 μmol g⁻¹ h⁻¹ with a total yield of 12.80 mmol g⁻¹ and a selectivity of ∼99% in the presence of sacrificial electron donors. Under a diluted CO₂ atmosphere (5% CO₂/95% Ar), it produced 3.44 mmol g⁻¹ CO over 36 h, a feat not previously achieved by any metal-free photocatalytic system. The study also provides mechanistic insights through DFT calculations and in situ DRIFT spectroscopy, highlighting the synergistic interaction between CO₂ and catalytically active sites within the COF pore. This metal-free COF system demonstrates a breakthrough in photocatalytic CO₂ reduction under diluted conditions.