Green nanotechnology - turning diesel soot into carbon nanotubes

(Nanowerk Spotlight) Diesel-burning engines are a major contributor to environmental pollution. They emit a mixture of gases and fine particles that contain some 40, mostly toxic chemicals, including benzene, butadiene, dioxin and mercury compounds. Diesel exhaust is listed as a known or probable human carcinogen by several state and federal agencies in the United States. Wouldn't it be nice if we could render diesel soot harmless before it gets released into the environment? Wouldn't it even be nicer if we could use this soot to manufacture something useful? Japanese scientists have come up not only with a unique technique for effectively collecting diesel soot but also a method for using this soot as a precursor for the production of single-walled carbon nanotubes. How is that as a practical example for green nanotechnology?
Green Nanotechnology is usually defined as a set of technologies that offer the possibility of changing the manufacturing process in two ways: "Incorporating nanotechnology for efficient, controlled manufacturing would drastically reduce waste products; and the use of nanomaterials as catalysts for greater efficiency in current manufacturing processes by minimizing or eliminating the use of toxic materials and the generation of undesirable by-products and effluents." (from the EPA website).
While the EPA and others look into the future and see optimized manufacturing processes that reduce pollution at the source, it seems a third way, much more relevant to today's environmental problems, is to deal with the remediation of existing pollutants. It will take decades to develop and roll out across the existing industrial infrastructure the nanotechnology-enabled, 'green chemistry'-type manufacturing technologies that will have a real impact on the environment.

Most of the nanotechnology-related presentations at last week's Cleantech 2007 conference dealt with future applications for solar cells, batteries, fuel cells etc. There was only one that was remotely concerned with addressing existing pollution remediation, dealing with filtration media.

Standing knee-high in diesel soot is not as sexy as installing highly efficient solar cells in Southern California, that's for sure. I would argue, though, that we need more emphasis on immediate, stop-gap type of nanotechnology applications for today's environmental problems. Nanotechnologies certainly hold the keys to a green future but how many years can we wait until these technologies have a meaningful impact on our environment; and what happens to pollution levels and their effects on health and environment in the meantime?

One immediate area of application of course is filtration. Nanofiltration devices are in – very limited – use already. You might have a nanotech water filter installed on your kitchen tap, there might be a nanotech air filter in your car, or some coal- and gas-fired plants might use nanoparticulate sorbents in their smokestack filters. While filters keep the bad stuff out of the air or water, the questions remains what happens to the filter, and the pollutants in it, after use. If they are just dumped in a landfill than this only shifts the problem from one medium, e.g. air, to another, e.g. soil or ground water.
Filtration – nano-enabled or not – and recycling have to go hand in hand. Otherwise the solution is only half-baked. A group of researchers in Japan have come up with an interesting nanotechnology solution for dealing with diesel soot that could serve as an example for novel, short-term solutions to existing environmental problems.
There is an excellent diesel pollution primer over at the Union of Concerned Scientists's website, so suffice it to say here that diesel soot is nasty and dangerous stuff.
In 2005, Tetsuro Nishimoto of Juon Co. Ltd, a Japanese company specialized in the development and production of environmental equipment, filed a patent application for a unique technique for effectively collecting diesel soot from internal combustion engines. Subsequently, researchers at Yokohama City University and Nissan Arc, Ltd. have shown that diesel soot can be recycled as a carbon source for the synthesis of single-walled carbon nanotubes (SWCNTs).
The researchers first collected the exhaust soot from the diesel engines of an electric generator and a vessel by means of the technique developed by Nishimoto. Particulate matter (PM) from diesel engines is effectively trapped on ceramic filters in the exhaust pipe. The PM, mainly composed of soot, soluble organic fractions (SOF), and sulfates, is separated from the filters by ultrasonic washing with water or ethanol. The diesel soot was predominantly collected by Soxhlet extraction of the PM with ethanol. The collected diesel soot recycled by this method was then subjected to laser vaporization to synthesize SWCNTs. The extracted SOF can be recycled as diesel fuel.
"We synthesized SWCNTs by laser vaporization of diesel soot that includes the fragments of various fullerenes formed during the combustion of light or heavy oil" Dr. Masaru Tachibana explained to Nanowerk. "We demonstrated that the fragments in the diesel soot are suitable precursors for the synthesis of SWCNTs. The success in the synthesis of SWCNTs from diesel soot shows that diesel soot can be recycled as a carbon source for the synthesis of SWCNTs."
Tachibana is a professor of physics in the nanoscale science and technology group at Yokohama City University in Japan. Together with his collaborators he published a paper on these findings, titled ("Synthesis of Single-Wall Carbon Nanotubes from Diesel Soot"), in the Japanese Journal of Applied Physics.
The Japanese researchers' synthesis method also provides an important insight for the growth mechanism of SWCNTs. Tachibana explains: "In electric arc-discharge and laser vaporization methods, graphite can be generally used as a carbon source. However, a few studies have shown the synthesis of SWCNTs from other carbon sources, among them synthesis from fullerenes by a laser vaporization method. It was considered that the laser decompositions of fullerenes, or their fragments, are suitable precursors for the synthesis of SWCNTs. When we found fullerenes in diesel soot this motivated us to try synthesizing SWCNTs and thus find a recycling solution for the soot."
The scientists found that diesel soot contains C60, C70 and other fullerenes. These fullerenes are thought to be formed during the combustion of light or heavy oil. They also found that diameters of SWCNTs synthesized from diesel soot are smaller than those of SWCNTs synthesized from the graphite.
Although the researchers didn't examine the detailed behavior of the toxic chemicals contained in diesel soot in the synthesis process for SWCNTs, they believe that almost all toxic chemicals get destroyed during the laser vaporization process. They concede the possibility that some might remain in the SOF, though.
Tachibana hopes that the recycling applications of diesel soot and its modification prove useful not only to the synthesis of SWCNTs but also to electrodes for fuel cells and gas storage materials.
"Although our paper's contribution to the scientific field might be small, we hope it will prove to be very important for its contribution to a cleaner environment" says Tachibana.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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