Artificial Photosynthesis of C1–C3 Hydrocarbons from Water and CO_2 on Titanate Nanotubes Decorated with Nanoparticle Elemental Copper and CdS Quantum Dots

Abstract

The conversion of CO_2 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–x)Ti_3O_7) decorated with elemental copper deposits followed by an overcoat of CdS quantum dot deposits. This ternary photocatalyst is capable of catalyzing the conversion of CO_2 and water into C1–C3 hydrocarbons (e.g., CH_4, C_2H_6, C_3H_8, C_2H_4, C_3H_6) upon irradiation with visible light above 420 nm. With this composite photocatalyst, sacrificial electron donors are not required for the photoreduction of CO_2. We have shown that water is the principal photoexcited-state electron donor, while CO_2 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)CO_2, then the product hydrocarbons are built upon a ^(13)C backbone. However, free molecular H_2 is not observed over 5 h of visible light irradiation even though proton reduction in aqueous solution is thermodynamically favored over CO_2 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 CO_2 to surface exchange sites on the ternary catalyst leads to the formation of surface-bound CO_2 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 CO_2. The methyl radical (CH_3•), 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.

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