A team of German scientists has found that commercial carbon nanotubes containing residual amounts of cobalt and iron nanoparticles are highly effective in ammonia (NH3) decomposition. The discovery could prove useful in removing NH3 from coal gasification streams, and in the production of hydrogen gas for use as a green fuel.
Liquid NH3 is seen as a promising carrier for molecular hydrogen gas. Traditional commercial catalysts for NH3 decomposition are based on iron and nickel, and ruthenium is another highly effective catalyst. But high metal loading and complicated synthesis procedures for these catalysts are a problem, as is the steep price and limited availability of ruthenium.
What Jian Zhang and his colleagues at the Fritz Haber Institute in Berlin have done is exploit the impurities found in carbon nanotubes. The nanotubes require no additional chemical treatment, and the researchers claim that the catalytic performance of the material is comparable with, or even better than, that of currently available catalysts.
Also, the microstructure and chemical composition of the nanotubes is unchanged following long exposure to NH3 at high temperatures. This raises the possibility that the nanotubes could be recycled and used further.
“From a economic perspective, the cost is zero as we did not waste anything, and just used the metallic residues in a commercial product,” says Zhang. “Actually, the capital cost of carbon nanotubes could be decreased if we fully considered the price saving during the NH3 decomposition process.”
While production volumes of commercial carbon nanotubes remain relatively low, and costs high, the situation is steadily improving. For example, Bayer Materials can produce nanotubes for between €50 and €100 per kilogramme, and the company’s goal is €5/kg, with a production rate of several thousand tonnes per year.
Zhang says that large-scale production of carbon nanotubes will be achieved in the near future, and that the efficient use of nanotubes in applications such as hydrogen gas production will promote the development of nanomaterials-based technologies.
Zhang et al. – Commercial Fe- or Co-containing carbon nanotubes as catalysts for NH3 decomposition, ChemComm 2007 (subscription required for full article)
Article first published in Nanomaterials News.