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

Abstract

Photocatalytic production of hydrogen peroxide (H 2 O 2 ) from ethanol (EtOH) and molecular oxygen (O 2 ) 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 O 2 (H 2 O 2 formation). The GCN catalysts with 56 and 160 m 2 g –1 surface areas exhibit higher activity for H 2 O 2 production than the catalyst prepared without silica template (surface area: 10 m 2 g –1 ), but a further increase in the surface area (228 m 2 g –1 ) decreases the activity. In addition, the selectivity for H 2 O 2 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 O 2, thus decreasing the H 2 O 2 selectivity. Furthermore, these defects also behave as the active sites for photocatalytic decomposition of the formed H 2 O 2 . Consequently, the GCN catalysts with relatively large surface area but with a small number of surface defects promote relatively efficient H 2 O 2 formation.