Microenvironment Modulation of Cu₃-based Metal-Covalent Organic Frameworks via Methyl Substitution for Enhanced...

Photocatalytic CO₂ reduction offers a compelling strategy for carbon neutrality, yet its efficiency is critically limited by both inefficient charge utilization and insufficient active-site modulation. In this study, a methyl-functionalized trinuclear copper building unit was introduced to successfully construct Me₂-Cu₃-PY-COF, while the unmodified Cu₃-PY-COF was prepared as the control sample. The introduction of methyl groups enhances electron density, narrows the band gap, and promotes charge separation and transfer, thereby improving photocatalytic CO₂ reduction performance. As a result, Me₂-Cu₃-PY-COF exhibited a CH₄ production rate of 135.15 µmol·g^-1·h^-1 with a selectivity of 91.6%, which was higher than that of the control sample (86.09 µmol·g^-1·h^-1, 81.9%). Mechanistic studies reveal that CO₂ is converted to CH₄ via a stepwise hydrogenation pathway, with Cu₃ clusters serving as the active sites undergoing partial reduction from Cu²⁺ to Cu⁺, while imine sites facilitate CO₂ adsorption. Density functional theory (DFT) calculations demonstrate that methyl-induced electronic modulation improves CO₂ and intermediate adsorption, which in turn promotes multi-electron reduction. This work promotes the research on the application of trinuclear copper cluster-based metal-covalent organic frameworks as photocatalysts.