Artificial photosynthesis of C1-C3 hydrocarbons from water and CO2 on titanate nanotubes decorated with nanoparticle elemental copper and CdS quantum dots.

The conversion of CO2 and water into value-added fuels with visible light is difficult to achieve in inorganic photocatalytic systems. However, we synthesized a ternary catalyst, CdS/(Cu-TNTs), which is assembled on a core of sodium trititanate nanotubes (TNTs; NaxH2-xTi3O7) decorated with elemental copper deposits followed by an overcoat of CdS quantum dot deposits. This ternary photocatalyst is capable of catalyzing the conversion of CO2 and water into C1-C3 hydrocarbons (e.g., CH4, C2H6, C3H8, C2H4, C3H6) upon irradiation with visible light above 420 nm. With this composite photocatalyst, sacrificial electron donors are not required for the photoreduction of CO2. We have shown that water is the principal photoexcited-state electron donor, while CO2 bound to the composite surface serves as the corresponding electron acceptor. If the photochemical reaction is carried out under an atmosphere of 99.9% (13)CO2, then the product hydrocarbons are built upon a (13)C backbone. However, free molecular H2 is not observed over 5 h of visible light irradiation even though proton reduction in aqueous solution is thermodynamically favored over CO2 reduction. In terms of photocatalytic efficiency, the stoichiometric fraction of Na(+) in TNTs appears to be an important factor that influences the formation of the observed hydrocarbons. The coordination of CO2 to surface exchange sites on the ternary catalyst leads to the formation of surface-bound CO2 and related carbonate species. It appears that the bidentate binding of O═C═O to certain reactive surface sites reduces the energy barrier for conduction band electron transfer to CO2. The methyl radical (CH3(•)), an observed intermediate in the reaction, was positively identified using an ESR spin trapping probe molecule. The copper deposits on the surface of TNTs appear to play a major role in the transient trapping of methyl radical, which in turn self-reacts to produce ethane.