Microenvironment Modulation of Cu3-based Metal-Covalent Organic Frameworks via Methyl Substitution for Enhanced Photocatalytic CO2 Reduction to CH4.

Abstract

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.