Electron Spin Resonance Studies of Photocatalytic Interface Reactions of Suspended M/TiO₂ (M=Pt, Pd, Ir, Rh, Os, or Ru) with Alcohol and Acetic Acid in Aqueous Media

M. Kaise, H. Nagai, K. Tokuhashi, S. Kondo, S. Nimura, and O. Kikuchi
[Langmuir, Vol.10, p.1345, 1994]

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 a-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.