Pyrolytic formation of TiO2/carbon nanocomposite from kraft lignin: Characterization and photoactivities

This article reports on the formation of pyrolytic carbon/TiO2 nanocomposite (p-C/TiO2) by pyrolysis of a mixture of the P25 TiO2 and kraft lignin at 600 °C. The result was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, UV-visible spectroscopy, electron paramagnetic resonance spectrometry (EPR), thermogravimetry (TGA) and SEM microscopy. Its photocatalytic activity was ascertained using three classes of chemical probes, namely (i) degradation of methylene blue (MB) and rhodamine-B (RhB) dyes in UV light-irradiated aqueous suspensions, (ii) depletion of phenol and (iii) degradation of antibiotics. The p-C/TiO2 nanocomposite is a strong phisisorbent of both MB and RhB nearly twofold with respect to neat TiO2. Although it is nearly twofold more photoactive toward the degradation of MB (0.091 min−1 versus 0.047 min−1), it is not with regard to RhB degradation (0.064 min−1 versus 0.060 min−1). For the degradation of phenol in aqueous media (pH 3), pristine TiO2 was far more effective than p-C/TiO2 for oxygenated suspensions (17.6 × 10−3 mM min−1 versus 4.3 × 10−3 mM min−1). Under an argon atmosphere, the kinetics were otherwise identical. The activity of the material was tested also for a real application in the degradation of a fluoroquinolone antibiotic such as enrofloxacin (ENR) in tap water. It is evident that the photoactivity of a semiconductor photocatalyst is not a constant, but it does depend on the nature of the substrate used and on the experimental conditions. It is also argued that the use of dyes to assess photocatalytic activities when suspensions are subjected to visible light irradiation is to be discouraged as the dyes act as electron transfer photosensitizers and or can undergo photodegradation from their excited states.