Temperature effect on the photoinduced reduction of methyl viologen with several sensitizers and the evolution of hydrogen from water

Irradiation by visible light of an aqueous solution containing a photosensitizer, methyl viologen (MV²⁺) and ethylenediaminetetraacetic acid leads to the formation of the reduced form of methyl viologen (MV⁺). The quantum yield for the formation of MV⁺ depends strongly on the time during which the formation is observed owing to the reaction of MV⁺ with oxidative products and its reduction to MV⁰. Proflavin, acridine yellow and ruthenium(II)tris(2,2-bipyridyl) were used as photosensitizers and showed the same ability to promote hydrogen evolution. When CdS was used as a sensitizer a factor of 10 less hydrogen was obtained than when the dyes were used. The redox catalysts platinum, PtTiO₂RuO₂ and PtCdS in colloidal systems showed approximately the same activity towards the reduction of water. The reduction of MV²⁺ and the evolution of hydrogen were enhanced at higher temperature (70 °C). The optimum conditions for water reduction on redox catalysts in colloidal systems under continuous illumination are analysed.