Colloidal semiconductors in systems for the sacrificial photolysis of water. 2. Hydrogen production with platinum/titanium dioxide catalysts

Pt/TiO/sub 2/ aggregates prepared by heterocoagulation catalyzed the oxidation of the donors EDTA and oxalate when irradiated with light of energy greater than the band-gap of TiO/sub 2/. The rate of hydrogen production decreased with prolonged irradiation not only because of donor consumption but also because of loss of catalyst activity. This loss of activity seems to be induced by hydrogen produced during irradiation. The greatest yield of hydrogen from a Pt/TiO/sub 2//EDTA system corresponds to the donation of ca. 23 electrons from each molecule of EDTA. The yield of carbon dioxide confirmed that the oxidation of EDTA extended beyond decarboxylation. The maximum rate of hydrogen production (R/sub max/) was directly related to the adsorption density of EDTA on TiO/sub 2/ surfaces. This reflects the ability of adsorbed EDTA to reduce photogenerated holes before their combination with photogenerated electrons. R/sub max/ increased when the adsorption of EDTA on Pt surfaces was suppressed by an increase in the ionic strength. Stabilization of the Pt/TiO/sub 2/ catalysts against coagulation, using PVA, lowered the photolysis efficiency apparently because PVA retarded the exchange between EDTA in solution and oxidation products at TiO/sub 2/ surfaces. When the concentration of Pt was varied, an optimum R/submore » max/ was observed. R/sub max/ first increased with Pt concentration due to the increasing number of TiO/sub 2/ particles carrying Pt. At the higher concentrations of Pt, R/sub max/ decreased partly because of the decrease in surface coverage of EDTA on TiO/sub 2/ which accompanied the increase in Pt concentration. An optimum R/sub max/ was also found when the concentration of TiO/sub 2/ was varied. R/sub max/ at first increased due to the increasing number of absorbing TiO/sub 2/ particles and then decreased, at higher TiO/sub 2/ concentrations, due to the reducing surface coverage of EDTA. 18 references, 9 figures, 4 tables.« less