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.

Ultrathin 2D Ti3C2 MXene Co-catalyst anchored on porous g-C3N4 for enhanced photocatalytic CO2 reduction under visible-light irradiation.

Constructing an efficient photocatalyst is critical for photocatalytic carbon dioxide (CO2) into valuable fuel. Herein, a high-efficiency catalyst was synthesized by a simple one-step electrostatic self-assembly method, in which Ti3C2 (TC) was anchored on porous g-C3N4 (PCN) with rich -NHx via NHx-Ti bond. Such a chemical interaction made the optimized TC/PCN-2 with 2 wt% loading of Ti3C2 possess highest CH4 production (0.99 µmol·h-1·g-1catalyst) under visible light (>420 nm), which was 14 times higher than that of pure PCN (0.07 µmol·h-1·gcatalyst-1) at the same condition. More importantly, the TC/PCN-2 photocatalyst still maintained satisfied activity after four cycles. Besides the formation of NHx-Ti chemical bonding and superior conductivity of Ti3C2 as a co-catalyst, which facilitated interfacial charges separation and migration, the exceptional performance could also attribute to the enhanced CO2 adsorption/activation and improved light-harvesting capability. This work provided a potential application in energy conversion with MXene as an efficient co-catalyst.

In situ fabrication of amorphous TiO2/NH2-MIL-125(Ti) for enhanced photocatalytic CO2 into CH4 with H2O under visible-light irradiation.

A series of amorphous TiO2/NH2-MIL-125(Ti) (Am-TiO2/NM) composites were in-situ fabricated via one-pot facile water (H2O) bath method. The photocatalytic performance of as-synthesized samples was evaluated by CO2 reduction with H2O under visible-light irradiation. The Am-TiO2/NM with 22.6 wt% Am-TiO2 loading had the highest photo activity for CO2 reduction into CH4 (1.18 µmol·h-1·g-1catalyst), which was 1.7 times higher than that of pure NM. The enhanced photocatalytic activity was attributed to the formation of the strong interactions between NM and Am-TiO2, which not only improved the ability of electron transfer, but also inhibited the recombination of electron-hole pairs. Furthermore, the optimum sample showed satisfied stability within five times recycling tests during the photo reaction under visible-light irradiation. These kinds of MOF-based composites provided potential applications in the field of energy conversion.