Photosensitised oxidation of water by CdS-based suspensions

Abstract

Dispersions of CdS powder with or without metal or metal oxide deposits, such as Pt, Au, Ag, Rh, Rh₂O₃ and RuO₂, have been used to sensitise the oxidation of water by PtCl^2–₆. The most active of the CdS-based photosensitisers was found to be one in which Pt was deposited by precipitation of a Pt colloid onto the surface of a CdS sample which had been annealed in air for 3 h prior to platinisation. A study was made to determine the optimum conditions for O₂ evolution. The initial rate of O₂ evolution was found to depend upon a number of factors including: pH, [CdS/Pt], [PtCl^2–₆], [O₂], Pt loading and CdS annealing temperature and environment. Although a number of different electron acceptors were tried, including PtCl^2–₆, PtCl^2–₄, Pt(OH)^2–₆, H₂AuO^–₃, S₂O^2–₈, Co(NH₃)₅Cl²⁺ and Fe(CN)^3–₆, O₂ photogeneration was observed with only PtCl^2–₆ and Fe(CN)^3–₆. A number of other materials were tested as photocatalysts for the oxidation of water by PtCl^2–₆, including TiO₂, TiO₂/Pt, TiO₂/Rh₂O₃, CdO, CdO/Pt, HgS/Pt, SiO₂/Rh₂O₃ and Al₂O₃/Pt; however, only the TiO₂-based materials showed any activity. The results of this work are discussed with respect to the current controversy over the mechanism for O₂ evolution.