Photoactivity and photo selectivity of a dielectric metal-oxide photocatalyst (ZrO2)probed by the photoinduced reduction of oxygen and oxidation of hydrogen

The photoinduced reduction of oxygen and photooxidation of hydrogen occurring on oxidized and reduced surfaces of the metal-oxide dielectric material ZrO2 (in monoclinic form) have been examined to probe the spectral variations of the photoactivity and photoselectivity of this metal oxide by determining (true) photochemically defined quantum yields for the two redox reactions at various wavelengths of photoexcitation (200 nm < lambda < 400 nm). Irradiation of zirconia in the fundamental absorption (intrinsic; lambda < 260 nm) region leads predominantly to the photoreduction of oxygen, whereas the photooxidation of hydrogen predominates on irradiation in the extrinsic spectral region (260 mm < lambda < 360 nm). A pathway for the formation of electron-trapped states (Zr3+) and hole-trapped states (O center dot-) in the extrinsic spectral region is described since both oxidation and reduction take place in this extrinsic region with oxidation predominating. The reported experimental results provide the long sought-after spectral variations into the photoactivity and photoselectivity of metal-oxide photocatalysts in gas/solid heterogeneous systems. (c) 2005 Elsevier B.V. All rights reserved.