Plants everywhere use solar energy to remove carbon dioxide from the atmosphere, turning it into water and building materials like wood and bamboo. For decades, chemists have dreamed of doing the same, or even converting the carbon dioxide into fuels. Now.a University of Southern California team has received funding to further develop a promising research approach into this possibility.
In June, 2011 Stephen Cronin and USC colleagues published a paper in ACS Catalysis on “Photocatalytic Conversion of CO2 to Hydrocarbon Fuels via Plasmon-Enhanced Absorption and Metallic Interband Transitions.”
Stephen Cronin. Click photo for link to lab website.
Cronin, an associate professor in the Viterbi School's Ming Hsieh Department of Electrical Engineering/Electrophysics, is a specialist on plasmons, which are naturally bundled electric charges that can be generated on the surface of a solid, usually a metal, when the surface is energized by oscillating electric fields.
The plasmons are the surface electrical charge energy equivalent of photons, the units of light energy. In a series of papers including that of June 11, Cronin’s group found that if they created plasmons with the same frequency as photons hitting a metal, the catalytic characteristics of the material were often greatly enhanced for a variety of reactions.
Catalysis is a process by which the energy requirements for a chemical reaction can be drastically changed by the addition of a substance, a catalyst. The catalyst isn’t consumed in the reaction; instead, it plays an intermediary role, creating an alternate lower energy route.
For example, carbon dioxide can be turned into methane in a laboratory by simple procedures, adding heat or other energy that breaks the tight bonds between the carbon and oxygen molecules. But much energy is required this way – as much energy as was produced when (for example) the carbon, in the form of oil or coal was burnt.
Plants have an efficient chemical process that uses the photocatalyst chlorophyll to break these bonds using sunlight.
Plasmons in action, top to bottom: unenergized TiO2 surface on gold; same surface energized, closeup of the energy interaction at the border of the two substances.
Chemists have been using titanium oxide with added gold particles to try to duplicate chlorophyll’s prowess, but the results have been uneven and unpredictable.
The June 2011 paper found that if plasmons were energized on the gold particles to fit the wavelength of the light coming in to power the reaction the effects were dramatic – as much as 66-fold increase in conversion. Even more interesting, if the energy pulled from the light coming in was high enough, the reaction products changed, with fuel substances like the gas ethane produced.
The problems: much of the visible light hitting the mixture was wasted, because the titanium dioxide did not absorb it. And also, the gold particles offered very little surface area for the plasmon enhancement to occur.
The new study will attempt to optimize the reaction by creating more plasmonically active surface area and “doping” the titanium in order to increase sensitivity to visible light.
The Office of Naval Research is funding the project, which in addition to the creation of fuel from carbon dioxide may also yield a way to recycle carbon dioxide back into oxygen on submarines. Cronin also has a CAREER grant from the National Science Foundation to pursue plasmon catalysis research that runs through 2014.
Other plasmon catalysis routes he is investigating include improvement of dye-based solar cells and using the sun to split water into hydrogen and oxygen, which can stored and later be recombined to yield energy.
In addition to Cronin, the team includes Wenbo Hou of the USC Dornsife College Departement of Chemistry, and Wei Hsuan Hung, Prathamesh Pavaskar, Alain Goeppert, and Mehmet Aykol, all of the Viterbi School.
