Photocatalytic performance of alkali metal doped graphitic carbon nitrides and Pd-alkali metal doped graphitic carbon nitride composites

Although there are several reports about the beneficial performance of metal doped graphitic carbon nitrides (g-C3N4) in various photocatalytic reactions, the effect of different alkali metal dopants has not been studied systematically. Series of undoped, Li-, Na- and K-doped samples was synthetized and used for the preparation of novel type of photocatalysts, composed of palladium nanoparticles supported on alkali metals-doped graphitic carbon nitride. Palladium loading was achieved via citrate reduction of palladium precursor in the presence of the carbon nitride material. Several physicochemical characterization methods such as attenuated total reflection infrared-(ATR-IR), diffuse reflectance UV–visible spectroscopy, high-resolution transmission microscopy (HRTEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), and thermogravimetric analysis (TGA) were used to get information about the morphology and composition of the novel composites. According to the optical characterization results, the band gap of g-C3N4 decreased upon Na- and K doping but increased after Li-doping. The palladium nanoparticles were distributed over the support and appeared in dispersed (undoped g-C3N4, Li-g-C3N4) or agglomerated (Na-g-C3N4, K-g-C3N4) form with individual particle size below 5 nm. The catalytic performance of these composite materials was tested in two processes: (i) photodegradation of methyl orange and (ii) hydrogen production from methanol. The Na- doping of graphitic carbon nitride was found to be a key issue to enhance hydrogen production. The carbon nitride matrix was found to be stable during the photocatalytic experiments, while the Pd component underwent certain changes during the photocatalytic reforming reaction of methanol. Our results indicated that the really acting metallic Pd cocatalyst was formed mainly in situ.