Photocatalytic Oxidation of Water by Silica-Supported Tris(4,4‘-dialkyl-2,2‘-bipyridyl)ruthenium Polymeric Sensitizers and Colloidal Iridium Oxide

A cationic polymer containing tris(4,4‘-dialkyl-2,2‘-bipyridyl)ruthenium groups linked by aliphatic spacers was studied as a photosensitizer for the catalytic oxidation of water in the presence of colloidal IrO 2 . The polymer−colloidal IrO 2 system photocatalytically reduced persulfate, a sacrificial electron acceptor, and oxidized water to O 2 and H + in solutions that were buffered at pH 6 by Na 2 SiF 6 and NaHCO 3 . The quantum efficiency for O 2 evolution and turnover number with respect to the Ru complex in the polymer reached 25% and 100, respectively. The polymer gradually aggregated in the Na 2 SiF 6 −NaHCO 3 buffer during the reaction, and this aggregation gradually decreased the photocatalytic activity of the system. Heterogeneous photosystems composed of this polymer and colloidal IrO 2 were also prepared using 70 nm diameter SiO 2 particles as supports. Photocatalysts made by the sequential loading of colloidal IrO 2 and the photosensitizer polymer onto SiO 2 particles at ca. pH 6 had much lower photocatalytic activity than did the unsupported system, presumably because there was little physical contact between the polymer and colloidal IrO 2 particles under these conditions. The most efficient heterogeneous photocatalyst was obtained by the adsorption of a mixture of the polymer and colloidal IrO 2 onto SiO 2 in Na 2 SiF 6 −NaHCO 3 solution. This composite had a high activity, comparable to that of the polymer−colloidal IrO 2 system. Transmission electron microscopy showed that the colloidal IrO 2 particles were covered with the polymer, which had aggregated in the solution. This result indicates that the polymer−IrO 2 aggregates retain their activity when immobilized on a support that might be used to organize overall water splitting systems.