Electron-Withdrawing Ligand Modulation of Pt Sites in a Covalent Organic Framework for Photocatalytic CO2-to-CH4 Conversion.

Abstract

Photocatalytic CO2-to-CH4 conversion in water remains challenging because selective multielectron proton-coupled reduction requires efficient charge separation and controlled intermediate hydrogenation. Herein, we report an electron-withdrawing-ligand-regulated Pt-site covalent organic framework, Pt-LEW/TBCOF, for selective CO2-to-CH4 photoconversion in CO2-saturated pure water without sacrificial reagents. The donor-acceptor TBCOF scaffold, constructed from tris(4-aminophenyl)benzene-derived donor units and bipyridine acceptor units, provides visible-light absorption and well-defined N-coordination sites for Pt immobilization. A preformed Pt complex bearing an electron-withdrawing fluorinated ligand was incorporated into the bipyridine-containing framework to generate highly dispersed Pt coordination sites with a regulated local electronic structure. TBCOF mainly produces CO with negligible CH4 formation, while ligand-free Pt/TBCOF exhibits only limited CH4 generation and low selectivity. In contrast, Pt-LEW/TBCOF shows a markedly enhanced CH4/CO ratio at the same Pt loading. Increasing the Pt loading to 8.1 wt % affords a CH4 production rate of 53.9 μmol g-1 h-1 with 89% CH4 selectivity among gaseous carbon products under visible-light irradiation. Spectroscopic, photoelectrochemical, and theoretical analyses suggest that the donor-acceptor COF backbone facilitates charge separation, whereas the electron-withdrawing ligand environment modulates the electronic structure and intermediate conversion behavior of Pt sites, favoring further hydrogenation toward CH4. This work suggests that electron-withdrawing ligand modulation of Pt sites may provide a useful approach for regulating product selectivity in the present COF-based photocatalytic system.