NETL researcher Chris Matranga.

Nano Solutions

August / September 2010 By: Tim Palucka Volume 8 Number 4

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Most carbon capture and sequestration technologies are focused on removing the CO₂ from fossil-fuel-based power plant flue gases and storing them underground in stable geologic formations. But Christopher Matranga and his research team at the National Energy Technology Laboratory are investigating nanotechnology solutions in the form of hybrid photocatalysts to reuse the captured CO₂ and transform it into useful, valuable chemical products such as methane or methanol. If it works, and preliminary simulations indicate it will, sales of the chemical products could help to pay for the costs of capturing CO₂, storing it and monitoring the storage sites. “Instead of CO₂ being a waste product, it becomes a chemical feedstock that can be converted into something that consumers can buy or use,” Matranga says. “Methane and methanol can both be used as a fuel or sold back into the industrial chemical stream for other applications.”

The process involves a photocatalyst—a material that uses light to help speed up a reaction—along with hydrogen or water to convert the CO₂ to CH4 (methane), CH₃OH (methanol), and other chemicals through a process called “photoreduction.” A well-known, inexpensive photocatalyst for this purpose is titanium dioxide, which is available as nanoparticles in commercial quantities; titanium oxide is commonly used as a pigment in white paint, among other things. But it suffers from the disadvantage of only being photocatalytically active in the UV range of the solar spectrum. “This means that you have to hit it with a high amount of energy to excite it and to initiate the photocatalytic process,” Matranga says. Also, because UV photons comprise only about 1 to 5 percent of the light that reaches the earth’s surface from the sun, only a small fraction of incoming energy is available for this CO₂ photoreduction process.

But Matranga and his colleagues realized that they could use more of the solar spectrum by combining titanium oxide with other nanoparticles to form a hybrid or “heterostructured” material. They chose nanoparticles of the semiconducting material cadmium selenide (CdSe) as the second component of the heterostructure because they are optically active in the lower-energy visible portion of the spectrum. The CdSe material acts as a photosensitizer for the TiO₂, giving visible light photocatalytic activity to the heterostructured system.

What’s more, the researchers can “tune” the CdSe component to absorb different colors of visible light simply by varying the size of the nanoparticles. “If we want the photocatalyst to be active in the green region of the solar spectrum we use a small CdSe nanoparticle to form the heterostructure; if we want it to be active toward the red region of the spectrum we use a larger CdSe nanocrystal,” Matranga says. “It’s easy to control nanoparticle sizes using synthetic chemistry.”

Besides expanding the range of light wavelengths that can be used in the photoreduction process, the addition of CdSe to titanium oxide helps with the crucial requirement of “charge separation” in photochemistry. Basically, when a photocatalyst absorbs UV light energy, a negatively charged electron is excited and kicked out of its place on the crystal lattice, leaving behind a positively charged “hole”—essentially the absence of an electron in a crystal structure. If the electron immediately resumes its place on the lattice—a process called “recombining” of the electron and the hole—then the electron is not available for the photoreduction reaction of CO₂. So it’s necessary to spatially separate these positive and negative charges for the process to work efficiently. By adding a CdSe nanocrystal to the titanium oxide to form a heterostructure, a photon of visible light can kick an electron from the CdSe lattice into the TiO₂ material. The hole left behind in the CdSe and the electron in the TiO₂ are thereby separated by the boundaries between these two materials. Matranga and his coworkers have already demonstrated the visible light photoreduction of CO₂ into methane and methanol using this technology, and have applied for a provisional patent. Now they’re investigating the use of other semiconducting materials in place of CdSe to broaden the range of light and materials that can be used for CO₂ photoreduction. “Using this type of nanotechnology,” Matranga says, “we have been able to push the photoactivity of a CO₂ reuse photocatalyst from the UV edge all the way out to the red edge of the solar spectrum.”

The first applications for this technology would be to retrofit coal-fired power plants with this nanosized photocatalyst somewhere in the flue gas stream to capture and remove CO₂. Preliminary process simulations of this scenario show that the technology is viable, and the implementation and operating costs would be competitive with or better than other state-of-the-art CO₂ capture technologies. This fits in very well with DOE’s requirements that any new technology retrofitted into existing power plants must have a minimal impact on the cost of electricity.

Further down the road, when geological CO₂ sequestration becomes more viable, other possibilities open up for this technology. With a readily available reservoir of CO₂ and a photocatalyst that can turn it into useful chemicals like methane, methanol and other products, the sequestration site could also become a chemical plant. “As long as the sequestered CO₂ is there as molecular CO₂, and it hasn’t reacted with the rock to form a carbonate or some other mineral species,” Matranga says, “then you have time to think about converting it back into something that can be sold to offset the costs associated with carbon capture.”

Matranga acknowledges that the work so far is lab scale in demonstration and that there’s no guarantee that the technology will be efficient on a large industrial scale. On the other hand, all signs suggest its future is promising.

Tim Palucka is a science writer for NISC, an IBM company, which is a site support contractor for NETL.