PHOTOSENSITIZED REDUCTION OF CARBON-DIOXIDE IN SOLUTION USING NOBLE-METAL CLUSTERS FOR ELECTRON-TRANSFER

Carbon dioxide was reduced to methane by visible-light irradiation of a solution composed of tris(bipyridine)ruthenium(III) as photosensitiz er, ethylenediaminetetraacetic acid disodium salt as sacrificial reage nt, methyl viologen as electron relay, and a colloidal dispersion of p olymer-protected noble-metal clusters, prepared by alcohol-reduction, as catalyst. Among the noble-metal clusters examined, Pt clusters show ed the highest activity for the formation of methane as well as hydrog en. In order to improve the activity, oxidized clusters and bimetallic clusters were also applied. For example, the CH4 yield in 3-h irradia tion increased from 51 x 10(-3) mu mol with unoxidized Pt clusters to 72 x 10(-3) mu mol with partially oxidized ones. In the case of Pt/Ru bimetalic systems, the improvement of the catalytic activity by air tr eatment was much greater than in case of monometallic clusters.