Effects of Surface Defects
on Photocatalytic H<sub>2</sub>O<sub>2</sub> Production by Mesoporous
Graphitic Carbon Nitride
under Visible Light Irradiation
- Publication date
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Abstract
Photocatalytic production of hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) from ethanol (EtOH) and molecular oxygen (O<sub>2</sub>)
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<sub>2</sub> (H<sub>2</sub>O<sub>2</sub> formation). The GCN catalysts with 56 and 160
m<sup>2</sup> g<sup>–1</sup> surface areas exhibit higher activity
for H<sub>2</sub>O<sub>2</sub> production than the catalyst prepared
without silica template (surface area: 10 m<sup>2</sup> g<sup>–1</sup>), but a further increase in the surface area (228 m<sup>2</sup> g<sup>–1</sup>) decreases the activity. In addition, the selectivity
for H<sub>2</sub>O<sub>2</sub> 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<sub>2</sub>, thus decreasing the
H<sub>2</sub>O<sub>2</sub> selectivity. Furthermore, these defects
also behave as the active sites for photocatalytic decomposition of
the formed H<sub>2</sub>O<sub>2</sub>. Consequently, the GCN catalysts
with relatively large surface area but with a small number of surface
defects promote relatively efficient H<sub>2</sub>O<sub>2</sub> formation
