Photosensitized Reduction of CO₂to CH₄and H₂Evolution in the Presence of Ruthenium and Osmium Colloids: Strategies To Design Selectivity of Products Distribution

Photoreduction of CO₂to methane and higher hydrocarbons is accomplished in aqueous solutions by using visible light and Ru or Os colloids as catalysts. One system is composed of Ru(II) tris(bipyridine), Ru(bpy)₃²⁺, as photosensitizer, triethanolamine, TEOA, as electron donor, and one of the following bipyridinium charge relays: N,N’-dimethyl-2,2^/-bipyridinium, MQ²⁺(1), N,N-trimethylene-l^’-bipyridinium, TQ²⁺(2), N,N'-tetramethylene-2,2’-bipyridinium, DQ²⁺(3), or N,N’(S-sulfonatopropy1)-3,3'-dimethyl-4,4-'bipyridinium, MPVS⁰(4). Illumination of these systems under CO₂in the presence of Ru or Os colloids results in the formation of methane and ethylene and in H₂evolution. In the second system, illumination of an aqueous solution under CO₂that includes Ru(II) tris(bipyrazine) as sensitizer, TEOA as electron donor, and the Ru colloids leads to the formation of methane, ethylene, and ethane, and no H₂-evolution occurs. The reduction process of CO₂proceeds via electron transfer of metal-activated CO₂rather than through a hydrogenation route. Detailed studies show that the H₂-evolution process can be inhibited by the addition of bipyrazine, while CO₂reduction is inhibited in the presence of added thiols. Methanation of CO₂by hydrogen proceeds in the dark in the presence of Pt and Ru or Os colloids and in the presence of MQ²⁺(1). The need for the electron relay implies that the methanation process occurs through an electron-transfer mechanism.