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; Na_xH_2-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% ¹³CO₂, then the product hydrocarbons are built upon a ¹³C 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.