ELECTRON-SPIN-RESONANCE STUDIES OF PHOTOCATALYTIC INTERFACE REACTIONS OF SUSPENDED M/TIO(2) (M=PT, PD, IR, RH, OS, OR RU) WITH ALCOHOL AND ACETIC-ACID IN AQUEOUS-MEDIA

We present here the results of photocatalytic interface reactions between the catalysts shown in the title and acetic acid or alcohol (methanol, ethanol, and 2-propanol) in aqueous solution. Irrespective of loaded metal species in the platinum group, the same radical intermediates, a methyl radical (.CH3) and a carboxymethyl radical (.CH2COOH), were recognized by observation of their ESR absorption from aqueous solution of the acetic acid at room temperature. Moreover transient intermediates of alpha-hydroxymethyl radical (.CH2OH), alpha-hydroxyethyl radical (CH3.CHOH), and alpha-hydroxypropyl radical (CH3.COHCH3) were observed in reactions with organic ingredients of methanol, ethanol and 2-propanol, respectively. Our results suggest that a photocatalytic active site in the generation of the radical intermediates under UV illumination is not on the metal surface but the TiO2 surface of the photocatalysts. As a ''preintermediate'', existence of a hydroxyl radical (.OH) was presumed on the surface of the photocatalyst; the radical is created by electron transfer from a hydroxy anion (OH-) to an electron hole (p+) of TiO2. Thus, the most probable photocatalytic reaction mechanism has been proposed which indicates the resultant electron transfer from OH- to H+ over the photoexcited M/TiO2.