High-Intensity Ultrasound Creates Better Catalyst For Cleaning Fuels

Molybdenum disulfide normally consists of long, flat layers of molybdenum metal atoms sandwiched above and below by single atomic layers of sulfur. Because the interaction between the sulfur planes is weak, they can easily slide on one another, providing excellent high-temperature lubrication.

But molybdenum disulfide's other important commercial application is as a catalyst used by the petroleum industry to remove sulfur-containing compounds in gasoline. Upon combustion, these unwanted sulfur compounds would contribute to the formation of ecologically damaging acid rain.

"The flat planes of molybdenum disulfide that make it such a good lubricant also interfere with its ability to react with fuels to remove sulfur," said Kenneth Suslick, a U. of I. professor of chemical sciences. "This is because all the reactions necessary for sulfur removal occur along the edges of the long planes, and the bigger the planes, the less relative edge there is."

Suslick and students Millan Mdleleni and Taeghwan Hyeon discovered a way to make molybdenum disulfide with many more edge atoms using a technique called sonochemistry -- the chemical application of high-intensity ultrasound. The technique produces very small particles of molybdenum disulfide, 1,000 times smaller than the thickness of a human hair, that subsequently do not form into planes.

The sonochemical synthesis arises from acoustic cavitation -- the formation, growth and implosive collapse of small gas bubbles in a liquid blasted with sound. The collapse of these cavitating bubbles generates intense local heating, forming a hot spot in the cold liquid with a transient temperature of about 9,000 degrees Fahrenheit, the pressure of about 1,000 atmospheres and the duration of about a billionth of a second.

"When the bubbles collapse, the vapor of volatile molybdenum-metal-containing compounds inside the bubbles is decomposed into hot metal atoms," Suslick said. "These atoms then react with sulfur dissolved in the liquid to form clusters of molybdenum disulfide that contain a few thousand atoms and are about a millionth of an inch in diameter."

As the researchers reported in the June 24th issue of the Journal of the American Chemical Society, these clusters are too small to have extended planes of atoms and consequently possess many more edge atoms that can participate in the sulfur-removal process.

"Our sonochemically prepared molybdenum disulfide is 10 times more active than the standard industrial catalyst," Suslick said. "The sonochemical synthesis is simple, quick and easy to scale up."