Reaction Mechanism of Cu(II)-Grafted Visible-Light Responsive TiO2 and WO3 Photocatalysts Studied by Means of ESR Spectroscopy and Chemiluminescence Photometry

The reaction mechanism of visible-light responsive Cu(II)-grafted TiO2 and WO3 photocatalysts was analyzed by detecting trapped holes and electrons by means of electron spin resonance (ESR) spectroscopy and O-2(center dot-) and H2O2 by chemiluminescence photometry. At first, the photocatalytic activities were confirmed for acetaldehyde decomposition in gas phase under 470 nm light emitting diode (LED) irradiation. The reduction of grafted Cu2+ ions and the alternative reduction of O-2 by the resultant Cu+ ions were shown by ESR measurements at 77 K for both photocatalysts under irradiation of visible light (500 W high pressure Hg-lamp with a 500 nm sharp-cut-filter). One-electron reduction product for O-2, that is, O-2(center dot-), was detected in aqueous suspension by 442 nm irradiation for Cu(II)/TiO2. On the other hand, only two-electron reduction was observed for Cu(II)/WO3 where Cu2+ species mainly act as a catalyst since the optical absorption mainly causes band gap excitation. Based on these observations, the difference in the reaction mechanism of Cu(II)/TiO2 photocatalyst and Cu(II)/WO3 photocatalysts was discussed.