MIT Material Could Aid High-Temperature Processes

CAMBRIDGE, Mass. -- With a proud smile, MIT's Professor Jackie Ying held up a small glass jar containing a white fluffy powder resembling fresh snow on a very cold day. Yet this powder, far from melting at the touch of a warm hand, can withstand temperatures up to 1300 degrees Celsius.

The object of her attention is a new material that could make it easier to burn methane (the principal component of natural gas) while drastically cutting emissions of pollutants from natural gas power plants. Further, the procedure for creating the material paves the way for materials that could improve other high-temperature processes such as the production of some chemicals.

The work relies on microscopic "reactors" so small that over a hundred could fit on the period at the end of this sentence. It will be reported in the January 6 Nature by Dr. Ying, an MIT Associate Professor of Chemical Engineering, and Andrey J. Zarur. Dr. Zarur (MIT PhD 2000) is Research Manager and Chief Scientist at Starlab in Belgium.

INDUSTRIAL CATALYSTS

Many industrial processes rely on catalysts, materials that facilitate key chemical reactions by, for example, lowering the temperature at which a given reaction can proceed. It is difficult, however, to use catalysts at extremely high temperatures (over about 1,000 degrees C). Most are destroyed.

Enter the case of natural gas as a fuel for power plants. Natural gas is very abundant in the United States and environmentally friendly. It produces less of the greenhouse gas carbon dioxide than any other fossil fuel, Professor Ying said.

However, it is difficult to burn, requiring very high temperatures (around 1400 degrees C) to produce a stable flame. Temperatures can go even higher as combustion continues. Among other disadvantages, at such temperatures "nitrogen and oxygen in the air combine to form nitrogen oxide, a pollutant that contributes to acid rain and smog," Professor Ying said. Burning at lower temperatures would create little NOx and cut emissions of greenhouse gases like unburned methane.

Two groups of catalysts have been studied to that end. Those based on noble metals, however, are unstable at the temperatures needed for efficient ongoing combustion (about 1,300 degrees C). Catalysts based on other metals allow the combustion process to start (known as light-off) at 700 degrees C, but the Holy Grail for light-off is actually around 400 degrees C.

THE CHALLENGE

That was the challenge the MIT researchers decided to tackle. Could they create a catalyst that would allow light--off at a low temperature but would also be stable at operating temperatures up to about 1300 degrees C?

The researchers' solution drew from years of work on nanoporous and nanocrystalline materials--those containing pores or crystals only billionths of a meter in diameter. "I'm interested in using these materials to realize goals that aren't possible with conventional technologies," Professor Ying said.

They created the new barium hexaaluminate (BHA) catalyst via a reverse microemulsion, in which water droplets only nanometers in diameter are suspended in oil. Each droplet is, in effect, a miniscule reactor. That's because when added to the water--oil mixture, the principal "ingredients" for the catalyst preferentially move from the oil into the water droplets, where they react. A final heat treatment and powder recovery complete the process.

Professor Ying explained that conventional approaches for producing BHA result in a material that isn't well mixed before the heat treatment. As a result, the crystallization must be conducted at temperatures so high that particles undergo severe growth. That decreases their surface area, which in turn decreases their reactivity and limits light--off to about 700 degrees C.

In the new process, however, the diffusion of ingredients into the water droplets creates a much more homogenous mixture, which means that the final crystallization heat treatment can be conducted at a lower temperature. That lower temperature, Professor Ying hoped, would suppress particle growth, maintain the high BHA surface area and reactivity, and ultimately allow a lower light--off temperature.

She was right. The new BHA crystals were only 30 nanometers in diameter, even at 1,300 degrees C, giving them a surface area ten times higher than the surface area for BHA produced by conventional processing. Light--off went down to 600 degrees C.

MICROSCOPIC TINKERING

"But that wasn't good enough," Professor Ying said. Enter a little more microscopic tinkering.

The researchers knew that another material--ceria--is an active catalyst at low temperatures and might allow the desired light--off. Above about 600 degrees C, however, ceria crystals agglomerate, destroying the material.

The solution: add ceria to the reverse microemulsion used to produce the BHA particles. The ceria, too, diffuses from the oil into the water droplets, resulting in BHA particles covered with discrete deposits of ceria that in micrographs remind one of fingerprints over a circle. Because the ceria crystals are anchored to the BHA and are separated from one another, even at very high temperatures they cannot fuse together.

The end result: Professor Ying and Dr. Zarur's ceria--coated BHA has a light--off of just about 400 degrees C and can withstand temperatures higher than 1,100 degrees C. The two also found that their catalyst is stable in the presence of water vapor and other potential poisons.

"This combination of low-temperature catalytic activity, high-temperature thermal stability, and poisoning resistance renders our catalysts interesting for potential practical applications in ultralean catalytic combustion of methane," the authors conclude in Nature. Further, Professor Ying said, the overall process "could be extended to other systems and reactions of interest."

The process reported in Nature, which is patent-pending, was funded by the David and Lucile Packard Foundation through a $550,000 Packard Fellowship Professor Ying received in 1995. She emphasized the importance of these funds, which are discretionary, in allowing her to pursue potentially risky areas of research like that described in Nature. The authors acknowledge the NSF-funded MIT Center for Materials Science and Engineering and Michael Frongillo for use of and assistance with the electron microscopy facilities.

Professor Ying is the first Packard Fellow at MIT in engineering. Another honor: in 1999 she was named one of the world's 100 best young innovators by Technology Review magazine.

Her current work in this area is funded by ABB Alstom Power.