Kinetics of hydrogen production upon reduction of aqueous TiO₂ nanoparticles catalyzed by Pd⁰, Pt⁰, or Au⁰ coatings and an unusual hydrogen abstraction; steady state and pulse radiolysis study

Reduction of H⁺ by TiO₂ electrons (e _TiO2^-) in aqueous colloidal solution takes place in the presence of surface metal catalysts. The catalytic reduction gives rise to adsorbed hydrogen atoms. In the presence of Pd⁰ or Pt⁰, material balance shows that most of the adsorbed H atoms combine to molecular hydrogen. When the TiO₂ nanoparticles are partially coated with Au⁰ instead of Pd⁰ or Pt⁰, a higher than expected molecular hydrogen level is observed, attributed to a short chain reaction involving hydrogen abstraction from 2-propanol. This unusual hydrogen abstraction reaction has not been reported before. The mechanism and energy balance are discussed. The surface modification of TiO₂ nanoparticles was carried out by reduction of K₂PdCl₄, HzPtCl ₆, or HAuCl₄ with e_TiO2. The latter had been generated through electron injection from hydrated electrons, hydrogen atoms, or 2-propanol radicals, produced by γ or pulse radiolysis prior to the addition of the metal compounds. Upon addition of the metal compounds, immediate reactions take place producing metals clusters (M⁰) by multistep reductions reactions on the TiO₂ surface. The chemical kinetics involving the different metals and the reaction rate constant of e _aq^- and e_TiO2^- with AuCl ₄^- is also reported.