Mar
13
More Sunlight to Fuel Technology Progress
March 13, 2009 | 4 Comments
One goal of science is to develop an artificial version of photosynthesis to produce chemicals that would be fuels, hopefully for many a CO2 and water to a hydrocarbon or alcohol.
The Sandia Lab, MIT, Penn State and now Feng Jiao and Heinz Frei, chemists with Lawrence Berkeley National Laboratory’s Physical Biosciences Division who are announcing they have now taken a critical step towards this goal with the discovery that nano-sized crystals of cobalt oxide can effectively carry out the critical photosynthetic reaction of splitting water molecules.
Frei and Jiao have reported the results of their study in the journal Angewandte Chemie, in a paper entitled: “Nanostructured Cobalt Oxide Clusters in Mesoporous Silica as Efficient Oxygen-Evolving Catalysts.” (Yes, I believe its in German) The Helios Solar Energy Research Center, a scientific program at the Berkeley Lab supports the work, which is aimed at developing fuels from sunlight.
As regular readers know, artificial photosynthesis for the production of liquid fuels offers a route to a renewable and carbon-neutral source of transportation energy, thus it would not contribute added CO2, rather it would recycle CO2 into, out of and back into the atmospheric carbon cycle.
Dr. Frei explains, “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. Effective photooxidation requires a catalyst that is both efficient in its use of solar photons and fast enough to keep up with solar flux in order to avoid wasting those photons. Clusters of cobalt oxide nanocrystals are sufficiently efficient and fast, and are also robust (last a long time) and abundant. They perfectly fit the bill.”
Dr. Frei discusses the project’s path, “To take advantage of the flexibility and precision by which light absorption, charge transport and catalytic properties can be controlled by discrete inorganic molecular structures, we have been working with polynuclear metal oxide nanoclusters in silica. In earlier work, we found that iridium oxide was efficient and fast enough to do the job, but iridium is the least abundant metal on earth and not suitable for use on a very large scale. We needed a metal that was equally effective but far more abundant.” That explains the excitement and news value in the catalysts made from nanostrutured cobalt oxides in mesoporous silica, both far more economical than iridium.
The Berkeley Lab’s press release briefly explains the natural process by saying:
Green plants perform the photooxidation of water molecules within a complex of proteins called Photosystem II, in which manganese-containing enzymes serve as the catalyst. Manganese-based organometallic complexes modeled off Photosystem II have shown some promise as photocatalysts for water oxidation but some suffer from being water insoluble and none are very robust.
From that lead in:
In looking for purely inorganic catalysts that would dissolve in water and would be far more robust than biomimetic materials, Frei and Jiao turned to cobalt oxide, a highly abundant material that is an important industrial catalyst. When Frei and Jiao tested micron-sized particles of cobalt oxide, they found the particles were inefficient and not nearly fast enough to serve as photocatalysts. However, when they nano-sized the particles it was another story. Dr. Frei points out, “The yield for clusters of cobalt oxide (Co3O4) nano-sized crystals was about 1,600 times higher than for micron-sized particles, and the turnover frequency (speed) was about 1,140 oxygen molecules per second per cluster, which is commensurate with solar flux at ground level (approximately 1,000 Watts per square meter).”
Cobalt Oxide Nanotube Structures For Fuel. Click image for more information.
To hold the cobalt oxide nanoparticles together Frei and Jiao used mesoporous silica as their scaffold, growing their cobalt nanocrystals within the naturally parallel nanoscale channels of the silica via a technique known as “wet impregnation.”
The results are the best performance yet and comes from rod-shaped crystals measuring 8 nanometers in diameter and 50 nanometers in length, which are interconnected by short bridges to form bundled clusters shaped like a sphere with a diameter of 35 nanometers. Dr. Frei said while the catalytic efficiency of the cobalt metal itself was important, the major factor behind the enhanced efficiency and speed of the bundles was their size.
The surface area matter comes up again in this form of solar collection, too. Dr. Frei explains, “We suspect that the comparatively very large internal area of these 35 nanometer bundles (where catalysis takes place) was the main factor behind their increased efficiency, because when we produced larger bundles (65 nanometer diameters), the internal area was reduced and the bundles lost much of that efficiency gain.”
In continuing the effort Frei and Jiao will be doing further studies to gain a better understanding of why their cobalt oxide nanocrystal clusters are such efficient and high-speed photocatalysts while also looking into other metal oxide catalysts. But, the next big step is to integrate the water oxidation half of the reaction with the carbon dioxide reduction step into an artificial leaf type system.
Dr. Frei and Dr. Jiao have brought another method to science for artificial photosynthesis with efficiency, speed and size that compares well with natural plant processes. Using cobalt for the raw material at low cost, a stable constructed structure in a process that has no extreme demands in operating environments bolds well for this artificial photosynthesis innovation.
While none of the researchers are offering production rates or other investment and market measures, that is a relief. Absent hype the shear innovation can be appreciated. But all the scientists know that kind of information will be needed in the coming years. Maybe its best not to get into that, now. But a rooftop system that might displace some or all of the home heating and transport costs to a family would have significant value. Fossil fuels won’t be cheap forever and with each passing year we’ll get a better idea of the running annual costs. Just having the option to miss the high peaks will be significant. That makes this field of research important, especially if the investment to income ratios have an advantage over solar photovoltaic cells.
Solar is getting to be a bigger field than anticipated just a few short years ago.
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Fotosíntesis artificial con un nanocatalizador: combustible líquido a partir de luz solar un poco más cerca (ING)…
Heinz Freies y Feng Jiao trabajan para lograr la “fotosíntesis artificial". Han creado un catalizador basado en nanocristales de óxido de cobalto con una matriz de sílice para separar moléculas de agua eficientemente. Los resultados son los …
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