Artificial Photosynthesis of C1–C3 Hydrocarbons from Water and CO_2 on Titanate Nanotubes Decorated with Nanoparticle Elemental Copper and CdS Quantum Dots
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.
© 2015 American Chemical Society. Received: November 12, 2014; Revised: January 21, 2015; Published: January 22, 2015. Special Issue: Mario Molina Festschrift. Research support was funded by a National Science Foundation grant awarded to M.R.H. (Grant No. CHE-0924597). We acknowledge Dr. Nathan Dalleska for helping gas composition analysis at the Environmental Analysis Center, Environmental Science and Technology of California Institute of Technology (Caltech). We also acknowledge the technical staffs for the assistance of EPR and XRD works in the Division of Chemistry and Chemical engineering of Caltech. XPS and DRIFT experiments were carried out at the Molecular Materials Research Center of the Beckman Institute of Caltech. H.P. is grateful to the Korea National Research Foundation (2013K2A1A2052901, 2014S1A2A2027802, and 2014M1A3A3A02034875), the Korea Center for Artificial Photosynthesis (KCAP) (No. 2009-0093880), and the Korea CCS R&D Center (KCRC) (No. 2014M1A8A1049354) for financial support, and the Seoul Broadcasting System (SBS) Foundation in support of his sabbatical leave at Caltech.
Supplemental Material - jp511329d_si_001.pdf