Novel polyoxometalate@g-C₃N₄ hybrid photocatalysts for degradation of dyes and phenolics

Graphitic carbon nitride (g-C3N4) is an emerging metal-free catalyst, and has attracted considerate research interests in photocatalysis. For improving the low photocatalytic activity due to the polymeric nature, a variety of methods have been developed. In this study, polyoxometalate (POMs) functionalized g-C3N4 were synthesized using a facile hydrothermal method as novel photocatalysts. The photocatalysts were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), N2 sorption isotherms, thermogravimetric analysis (TGA), and UV-vis diffusion reflectance spectroscopy (UV-vis DRS). The photocatalytic properties were evaluated in photodecomposition of aqueous methylene blue (MB) and phenol under UV-visible light irradiations. Compared to pristine g-C3N4, POMs modified samples demonstrated enhanced efficiencies in photodegradation of MB and phenol. It was suggested that increased specific surface area, porous volume and efficient charge transfer would be responsible for the improved photocatalysis. This study proves the promising role of POMs in modification of novel photocatalysts.

Nitrogen-doped graphene for generation and evolution of reactive radicals by metal-free catalysis

N-Doped graphene (NG) nanomaterials were synthesized by directly annealing graphene oxide (GO) with a novel nitrogen precursor of melamine. A high N-doping level, 8-11 at. %, was achieved at a moderate temperature. The sample of NG-700, obtained at a calcination temperature of 700 °C, showed the highest efficiency in degradation of phenol solutions by metal-free catalytic activation of peroxymonosulfate (PMS). The catalytic activity of the N-doped rGO (NG-700) was about 80 times higher than that of undoped rGO in phenol degradation. Moreover, the activity of NG-700 was 18.5 times higher than that of the most popular metal-based catalyst of nanocrystalline Co3O4 in PMS activation. Theoretical calculations using spin-unrestricted density functional theory (DFT) were carried out to probe the active sites for PMS activation on N-doped graphene. In addition, experimental detection of generated radicals using electron paramagnetic resonance (EPR) and competitive radical reactions was performed to reveal the PMS activation processes and pathways of phenol degradation on nanocarbons. It was observed that both (•)OH and SO4(•-) existed in the oxidation processes and played critical roles for phenol oxidation.