Core–Shell Covalently Linked Graphitic Carbon Nitride–Melamine–Resorcinol–Formaldehyde Microsphere Polymers for Efficient Photocatalytic CO₂ Reduction to Methanol

Photocatalytic reduction of CO2 with light and H2O to form CH3OH is a promising route to mitigate carbon emissions and climate changes. Although semiconducting metal oxides are potential photocatalysts for this reaction, low photon efficiency and leaching of environmentally unfriendly toxic metals limit their applicability. Here, we report metal-free, core-shell photocatalysts consisting of graphitic carbon nitride (g-C3N4, CN) covalently linked to melamine-resorcinol-formaldehyde (MRF) microsphere polymers for this reaction. Covalent linkage enabled efficient separation of photo-generated carriers and photocatalysis. Using 100 mg of a photocatalyst containing 15 wt % CN, a CH3OH yield of 0.99 μmol·h-1 was achieved at a reaction temperature of 80 °C and 0.5 MPa with external quantum efficiencies ranging from 5.5% at 380 nm to 1.7% at 550 nm. The yield was about 20 and 10 times higher than that of its components CN and MRF, respectively. Characterization with X-ray photoelectron spectroscopy, transmission electron microscopy, and bulk and surface elemental analyses supported the formation of a core-shell structure and the charge transfer in the C-N bond at the CN-MRF interface between the methoxy group in the 2,4-dihydroxylmethyl-1,3-diphenol part of MRF and the terminal amino groups in CN. This enhanced ligand-to-ligand charge transfer resulted in 67% of the photo-excited internal charge transferred from CN to the hydroxymethylamino group in MRF, whose amino group was the catalytic site for the CO2 photocatalytic reduction to CH3OH. This study provides a series of new metal-free photocatalyst designs and insights into the molecular-level structure-mediated photocatalytic response.