467of O2 over Ti02 it would seem reasonable to speculate that O2 may act as a recombination center at the surface under these conditions.In conclusion, we believe that this extraordinary photochromic effect in Ti02, reported here in brief, is of fundamental importance in photocatalysis and warrants thorough investigation. Possibilities that immediately spring to mind are studies of sensitization, Le., charge injection into Ti02 by visible-light-absorbing surface compounds or dopant ions; the effect of surface-deposited metals;~~ (1 7) Pichat, P. In Heterogeneous and Homogeneous Photocatalysis; Pelizzetti, E., Serpone, N., Eds.; Reidel: Dordrecht, in press. and more fundamental characterization of the trapping states and the role of the chemical state, and physical texture, of the TiOz surface in controlling electron-hole recombination.Acknowledgment. We thank Dr. C. Dimitropoulos and Professor w. Benoit in the Dgpartement de Physique for the use and adaption of the single-beam PAS spectrometer, and Dr P. P. Infelta for his generous assistance in the development of the software programming for data acquisition and manipulation.The possibility of using the photosensitive properties of a highly divided MOO, (30 m2 8-I) to prepare hydrogen-molybdenum bronzes has been explored. Ultraviolet illumination of MOO, suspensions in an alcohol (methanol, 2-propanol) as a source of hydrogen atoms produces Ho,~&~OO~. Addition of Ti02 allows the formation of H0,93M003, which indicates interparticle hydrogen atom transfer. The presence of Pt metal, either directly deposited on MOO, or by adding Pt/TiO,, suppresses any hydrogen-molybdenum bronze formation. The origin of the detrimental role of Pt, which contrasts with the necessary presence of a group VI11 (groups 8-10) metal for the preparation of bronzes by hydrogen spillover, is discussed.
