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Sound waves for activating nickel.

Sound waves for activating nickel

An ultrasonic cleaning bath is a convenientway to scrub items such as delicate jewelry. Now a group of chemists has shown that a similar technique can be used to strip an oxide coating from particles of nickel powder. That cleansing action turns nickel into a better catalyst for certain chemical reactions.

"We went out looking for the world'sworst hydrogenation catalyst,' says chemist Kenneth S. Suslick of the University of Illinois at Urbana-Champaign. The researchers came up with nickel, which normally forms a protective oxide coating when exposed to air. This tough, invisible film makes nickel ideal for coins and other applications where resistance to chemical attack is prized. However, the metal is known to be quite reactive if clean, single crystals can be obtained in a high vacuum.

Working with Dominick J. Casadonte,Suslick discovered that irradiation with sound waves at a frequency of 20 kilohertz increases nickel's activity as a catalyst by more than 100,000 times. Their experiments involved the addition of hydrogen to organic compounds called alkenes, a reaction widely used in the chemical industry. The reactions took place in solvents such as octane.

The researchers also examinednickel particles before and after irradiation to see what changes had occurred. They noticed that the initially rough, crystalline particles were quickly smoothed out until they were practically spherical. Particles also tended to clump together to form larger units. At the same time, each particle's nickel oxide coating, originally 200 to 250 angstroms thick, was made much thinner and more fragmented.

The results, reported in the May 27JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, support the view that a process called cavitation plays a major role. When sound waves pass through a liquid, they cause tiny bubbles to form. These bubbles suddenly collapse to generate highly energetic shock waves.

"This throws the particles into oneanother at high velocities,' says Suslick. Particles brushing against one another do the smoothing, while direct collisions lead to aggregation. The process also helps scrape away the metal's oxide layer.

"This is the first time,' says Suslick,"that anyone has really answered the question of where the increase in reactivity [due to ultrasound irradiation] is coming from.'
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Title Annotation:ultrasonic cleaning process for nickel powder
Author:Peterson, Ivars
Publication:Science News
Date:Jun 20, 1987
Words:365
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