Solar light-driven photocatalytic degradation of phenol on S-doped nanoporous carbons: The role of functional groups in governing activity and selectivity

Metal-free nanoporous carbons, either as received or sulfur doped, have been studied as photocatalysts for the degradation of phenol under solar light irradiation. The introduction of sulfur significantly enhanced both phenol adsorption and photodegradation. The texture and surface chemistry of the catalysts tested were evaluated using adsorption of nitrogen, thermal analysis, potentiometric titration and XPS. Various parameters related to the kinetics of phenol adsorption and photodegradation were investigated. They include the kinetics order for the adsorption and the intraparticle diffusion coefficient. The effect of the surface density of the molecules adsorbed on the efficiency of photodegradation was also evaluated. The photocatalytic activity for the degradation of phenol in terms of the distribution of main intermediate products was linked to the type and content of S-containing functional groups. It was found that thiophenic groups in small pores enhance phenol adsorption via providing basicity and hydrophobicity and contribute to photoactivity by inducing defects altering the energy band gap. Sulfoxides and sulfone, on the other hand, increase the selectivity of phenol oxidation to catechol and participate to oxidation via increasing the population of holes, which might promote the formation of reactive sulfur and oxygen species.