A visible-light-induced electron transfer system was constructed in aqueous solution to reduce carbon dioxide. The electron transfer system consisted of tris(2,2`-bipyridine)ruthenium(II) chloride, methyl viologen, ethylenediaminetetraacetic acid (EDTA, a sacrificial electron donor), and a colloidal metal nanoparticle as a catalyst. When the colloidal metal nanoparticle catalysts prepared by alcohol reduction of metal salts were used for this system, methane was generated by direct decomposition of the contained ethanol. Hence, the colloidal platinum and ruthenium nanoparticle catalysts protected by micelle and liposome were prepared by photoreduction of corresponding metal salts in aqueous media without ethanol and used for the electron transfer system to generate methane. The methane generation from carbon dioxide was confirmed by isotopic experiments using NaH13CO3 as a carbon dioxide source. In addition to methane-13C, methane-12C was also detected in this experiment. The methane-12C is considered to be generated from carbon dioxide produced by the decomposition of EDTA. Micelle-protected ruthenium and liposome-protected platinum nanoparticle catalysts showed relatively high efficiency for methane generation from carbon dioxide in this study.
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