Oxide photocatalysts with wide band gaps, such as alkaline earth oxides, convert the energy of ultraviolet (UV) photons into electron-hole pairs. For nanoparticles of MgO, the energy needed to generate the hole state (which converts an O2- anion to O-) depends on the coordination of the O2- anion: A three-coordinate corner site (O3C2-) requires less energy to form the radical (4.6 eV) than does a four-coordinate edge site (5.5 eV).
Sterrer et al. have used three techniques--electron paramagnetic resonance, infrared spectroscopy (IR), and diffuse UV reflectance spectroscopy--to study the optical excitations on clean MgO nanoparticles (with diameters of 5 to 8 nm). Photoexcitation of either site led to EPR signals corresponding only to O3C- sites. Exposure to hydrogen during 5.5-eV irradiation fully bleached the EPR signal, depleted the UV absorption associated with the O3C2- sites (but not the O4C2- sites), and led to the appearance of a single O-H stretching band in the IR spectrum. The authors conclude that the initially formed edge site exciton (or the oxygen radicals that form from it) migrates to the corner sites rather than reacts with the hydrogen donor species. Such studies should prove useful in understanding more complex reactions of these catalysts, such as the activation of C-H bonds. -- PDS
J. Am. Chem. Soc. 10.1021/ja028059o (2002).
