Depiction of sunlight-driven photocatalytic carbon dioxide conversion to hydrocarbon fuels using nitrogen-doped titania nanotube arrays surface-loaded with Cu and/or Pt cocatalyst nanoparticles. Credit: ACS. Click to enlarge.

Using outdoor global AM (air mass) 1.5 sunlight (100 mW cm^-2), they obtained hydrocarbon production rate of 111 ppm cm^-2 h^-1 when the nanotube array samples are loaded with both Cu and Pt nanoparticles.

This rate of CO₂ to hydrocarbon production obtained under outdoor sunlight is at least 20 times higher than previous published reports, which were conducted under laboratory conditions using UV illumination, according to the team, led by Professor Craig Grimes.

The photocatalytic methane-forming reaction requires eight photons, with additional photons required for other hydrocarbons; achieving significant hydrocarbon yields requires an efficient photocatalyst that utilizes a maximum amount of solar energy.

...we sought to enhance photocatalytic carbon dioxide conversion rates by using the following strategies: (i) employ high surface area titania nanotube arrays, with a wall thickness low enough to facilitate efficient transfer of photogenerated charge carriers to the surface species; (ii) modify the titania band gap to absorb and utilize the visible portion of the solar spectrum where the bulk of the solar energy lies; (iii) distribute cocatalyst nanoparticles on the nanotube array surface to adsorb the reactants and help the redox process.

—Varghese et al. (2009)

Gas sample analysis of the reaction products predominately showed methane, while ethane, propane, butane, pentane, and hexane as well as olefins and branched paraffins were also found in low concentrations.

The team found that while cocatalysts are essential for high rate CO₂ conversion, uniform nitrogen doping of the titania did not significantly contribute to the process.

The team performed a series of experiments to obtain a more fundamental understanding of the CO₂ conversion process.

...a likely process in the photocatalytic reduction of CO₂ by Cu or Pt loaded samples is the reduction of CO₂ via the reaction CO₂ + 2e^- → CO + ½O₂, which involves a free energy change of about 257 kJ/mol (1.33 eV per electron). The CO, thus formed, would react with atomic hydrogen to form hydrocarbons...Further rigorous studies are required to verify the validity of this hypothesis and understand the role of OH radicals and O₂ in the possible back reactions.

We hope this work opens new avenues for carbon recycling using renewable sources.

—Varghese et al. (2009)

Resources

• Oomman K. Varghese, Maggie Paulose, Thomas J. LaTempa, and Craig A. Grimes (2009) High-Rate Solar Photocatalytic Conversion of CO₂ and Water Vapor to Hydrocarbon Fuels. Nano Lett., Article ASAP • DOI: 10.1021/nl803258p