A triazole-based covalent organic framework as a photocatalyst toward visible-light-driven CO₂ reduction to CH₄

Solar-light driven reduction of CO₂ to CH₄ is a complex process involving multiple electron and proton transfer processes with various intermediates. Therefore, achieving high CH₄ activity and selectivity remains a significant challenge. Covalent organic frameworks (COFs) represent an emerging class of photoactive semiconductors with molecular level structural tunability, modular band gaps, and high charge carrier generation and transport within the network. Here, we developed a new heterocyclic triazole ring containing COF, TFPB-TRZ, through the condensation reaction between 1,3,5-tris(4-formylphenyl)benzene (TFPB) and 3,5-diamino-1,2,4-triazole (TRZ). The TFPB-TRZ COF with multiple heteroatoms shows suitable visible light absorption, high CO₂ uptake capability and an appropriate band diagram for CO₂ photoreduction. Photocatalysis results reveal a maximum CO₂ to CH₄ conversion of 2.34 mmol g^-1 with a rate of 128 μmol g^-1 h^-1 and high selectivity (∼99%) using 1-benzyl-1,4-dihydronicotinamide (BNAH) and triethylamine (TEA) as sacrificial agents. Under similar reaction conditions in the presence of direct sunlight, the TFPB-TRZ COF displays a maximum CH₄ yield of 493 μmol g^-1 with a rate of 61.62 μmol g^-1 h^-1, suggesting the robustness and light-harvesting ability of the COF photocatalyst. A femtosecond transient absorption (TA) spectroscopy study shows fast decay of excited state absorption (ESA) in the COF compared to the TFPB building unit due to efficient electron transfer to the catalytic site in the framework. The mechanism of CO₂ reduction to CH₄ is studied by DFT-based theoretical calculation, which is further supported by an in situ diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) study. The DFT results reveal that the lone pair of electrons on nitrogen heteroatoms present in the triazole ring of the TRZ moiety help in the stabilization of the CO intermediate during CO₂ to CH₄ conversion. Overall, this work demonstrates the use of a metal-free, recyclable COF-based photocatalytic system for solar energy storage by CO₂ reduction.