Metal-free carbon nitride photocatalysis with in situ hydrogen peroxide generation for the degradation of aromatic compounds

The photocatalytic performance of a thermally exfoliated carbon nitride material was investigated in aqueous solution and employing visible-light emitting diodes (LEDs) as radiation source (λmax = 417 nm). The operating conditions were studied using phenol as model compound. The increase on the surface area of the catalyst with the exfoliation treatment promotes faster degradation and mineralization rates and an easier reduction of O2 into H2O2. The H2O2 production takes place only in the presence of both phenol and dissolved oxygen. The study was followed by assessing the photocatalytic degradation of ten organic compounds (individually or in a mixture) commonly found in agro-industrial wastewaters (phenol, catechol, resorcinol, hydroquinone, benzoic acid, 4-hydroxybenzoic acid, protocatechuic acid, gallic acid, 4-methoxyphenol and tyrosol). These compounds were selected to study the position, order and nature of substitution on the aromatic ring as well as the possible influence of the pKa. Generally, more reactive compounds yield higher amounts of H2O2 formed, whereas the pKa does not affect photocatalysis owing to the amphoteric properties of the catalyst. Thus, the successful oxidation of the organic compounds achieved in situ generation of H2O2 with relatively high productivities using a metal-free carbon nitride material stable in consecutive runs.