Researchers Develop Method for Higher-Rate Solar Photocatalytic Conversion of CO2 and Water Vapor to Hydrocarbon Fuels
|Product generation rates from a nitrogen-doped nanotube array film surface-loaded with both Pt and Cu catalysts. Credit: ACS. Click to enlarge.
Researchers at Penn State have developed a method for the more efficient solar conversion of carbon dioxide and water vapor to methane and other hydrocarbons using nitrogen-doped titania nanotube arrays. The arrays feature a wall thickness low enough to facilitate effective carrier transfer to the adsorbing species, and are surface-loaded with nanodimensional islands of co-catalysts platinum (Pt) and/or copper (Cu).
A paper on their work was published online 27 January in the ACS journal Nano Letters.
|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)
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