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 IrO2. The polymer-colloidal IrO2 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 Na2SiF6 and NaHCo3. 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 Na2SiF6-NaHCO3 buffer during the reaction, and this aggregation gradually decreased the photocatalytic activity of the system. Heterogeneous photosystems composed of this polymer and colloidal IrO2 were also prepared using 70 nm diameter SiO2 particles as supports. Photocatalysts made by the sequential loading of colloidal IrO2 and the photosensitizer polymer onto SiO2 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 IrO2 particles under these conditions. The most efficient heterogeneous photocatalyst was obtained by the adsorption of a mixture of the polymer and colloidal IrO2 onto SiO2 in Na2SiF6-NaHCO3 solution. This composite had a high activity, comparable to that of the polymer-colloidal IrO2 system. Transmission electron microscopy showed that the colloidal IrO2 particles were covered with the polymer, which had aggregated in the solution. This result indicates that the polymer-IrO2 aggregates retain their activity when immobilized on a support that might be used to organize overall water splitting systems.