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 O₂ and H⁺ in solutions that were buffered at pH 6 by Na₂SiF₆ and NaHCO₃. The quantum efficiency for O₂ 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.