March 14th, 2009

"Let's Clear the Air"


Artificial Photosynthesis may Assist in Methanol Production

by Eric Underwood, NAFTC Curriculum Developer

Artificial leaves and artificial trees may hold the key for future energy generation. Green plants have been undergoing a process called photosynthesis for millions of years. Photosynthesis is the method by which plants produce food by reacting water with carbon-dioxide. Solar energy serves as the prime mover for the process. Plants absorb light primarily using the pigment chlorophyll, which is the reason that most plants have a green color. The main products of photosynthesis include sugars (food) and oxygen. In other words, photosynthesis splits water to liberate oxygen and fixes carbon dioxide into sugar. Scientists want to imitate the process of photosynthesis to produce clean and green liquid methanol.

Researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory are now in the process of a major breakthrough towards artificial photosynthesis. They think the secret lies in nano-sized crystals of cobalt oxide and believe that the material can effectively carry out the crucial photosynthetic reaction of splitting water molecules to produce methanol. The cobalt oxide will serve as a catalyst to capture solar photons (light particles) to oxidize water. This breakthrough process will not contribute to green house gas emissions or global warming. The idea is to create an artificial leaf that can duplicate the process of photosynthesis. That leaf will be able to capture the solar photons and have a catalytic system in place that can oxidize water.

By means of an artificial photosynthesis scenario, nanotubes embedded within a membrane would act like green leaves, using incident solar radiation (Hv) to split water molecules (H2O), freeing up electrons and oxygen (O2) that then react with carbon dioxide (CO2) to produce methanol (CH3OH). The result is a renewable green energy source that can also scrub the atmosphere of excessive carbon dioxide from the burning of fossil fuels.

nanotubes-for-fuel

Diagram of Green Leaves System. Photo Courtesy of U.S. DOE.

Heinz Frei and Feng Jiao carried out the research with the help of the Helios Solar Energy Research Center (Helios SERC), a scientific program under the direction of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory. Frei, deputy director and chemist of Helios SERC, explains the process as “Photooxidation of water molecules into oxygen, electrons and protons (hydrogen ions) is one of the two essential half reactions of an artificial photosynthesis system – it provides the electrons needed to reduce carbon dioxide to a fuel.” He notes that using cobalt oxide as a catalyst for the reaction is efficient in capturing solar photons and is fast enough to keep up with solar flux in order to prevent wasting protons. The clusters of cobalt oxide nanocrystals are robust and abundant.

However, the success story did not happen overnight. The micron-sized particles of cobalt oxide that were first proposed were ineffective and slow to act as catalysts. To find a solution, Frei and Jiao turned to nano-sized cobalt oxide. The yield for clusters of cobalt oxide nano-sized crystals is about 1,600 times higher than for micron-sized particles. In addition, the turnover frequency (speed) is about 1,140 oxygen molecules per second. The next big step will be to integrate the water oxidation half reaction with the carbon dioxide reduction step in an artificial leaf-type system.




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