Effects of Surface Defects on Photocatalytic H2O2 Production by Mesoporous Graphitic Carbon Nitride under Visible Light Irradiation

Photocatalytic production of hydrogen peroxide (H2O2) from ethanol (EtOH) and molecular oxygen (O2) was carried out by visible light irradiation (λ > 420 nm) of mesoporous graphitic carbon nitride (GCN) catalysts with different surface areas prepared by silica-templated thermal polymerization of cyanamide. On these catalysts, the photoformed positive hole oxidize EtOH and the conduction band electrons localized at the 1,4-positions of the melem unit promote two-electron reduction of O2 (H2O2 formation). The GCN catalysts with 56 and 160 m2 g–1 surface areas exhibit higher activity for H2O2 production than the catalyst prepared without silica template (surface area: 10 m2 g–1), but a further increase in the surface area (228 m2 g–1) decreases the activity. In addition, the selectivity for H2O2 formation significantly decreases with an increase in the surface area. The mesoporous GCN with larger surface areas inherently contain a larger number of primary amine moieties at the surface of mesopores. These defects behave as the active sites for four-electron reduction of O2, thus decreasing the H2O2 selectivity. Furthermore, these defects also behave as the active sites for photocatalytic decomposition of the formed H2O2. Consequently, the GCN catalysts with relatively large surface area but with a small number of surface defects promote relatively efficient H2O2 formation.