Ball-milling jackpot holds hydrogen, Bucky diamonds

Nanocrystalline diamonds appear at the end of a process to produce and store hydrogen using anthracite coal. "The idea we explored was based on ball milling graphite processes found in the hydrogen storage literature," said Angela D. Lueking, assistant professor of energy and geoenvironmental engineering at Penn State. "We substituted anthracite coal for graphite because it is abundant and inexpensive. Now. with 20/20 hindsight, we are struck by the fact that coal gasification is currently the most economical way to produce hydrogen."

Interest in hydrogen as a vehicular fuel has researchers investigating ways to create hydrogen inexpensively; other researchers are looking at ways to transport and store hydrogen in a safe manner.

Lueking's group was exploring a way to store hydrogen in carbon-based materials and inadvertently stumbled upon a method that combines production and storage and produces nanocrystalline diamonds as a by-product.

Lueking and her team ball milled powdered anthracite coal with cyclohexene.

Ball milling involves mixing a slurry of anthracite powder and cyclohexene with small steel balls, and mixing so the steel balls pound the coal particles and the cyclohexene, causing physical and chemical changes.

"Ball milling imparts a lot of energy to the slurry," Lueking said. "There is high pressure and temperature in every impact of the balls on the slurry, but we do not really understand the structural changes in the carbon that occur in the process."

Unlike the graphite experiments, Lueking's anthracite experiment has hydrogen gas evolving from the mixture at room temperature. Either the hydrogen is within the material in a tight pore structure, or they are forming a new carbon structure. The hydrogen outgassing continued for about a year and increased with addition of moderate heat.

"At first, we thought the mass spectrograph was broken because hydrogen was just coming off," Lueking said. "We tried another mass spec, and the same thing happened."

Wanting to know the structure of the ball milled product, and looking for carbon nanotubes, the researchers used transmission electron microscopy to investigate the small particles.

What the researchers had were Bucky diamonds, a nanocrystalline diamond surrounded by onion-like layers of graphite. Diamonds are a natural form of pure carbon, but with a differing molecular structure than graphite or the graphite-like coal.

The ball milling process seems a simpler and gentler way of creating nanodiamonds and especially Bucky diamonds. Lueking's team said it hopes once they understand how they are forming, they can increase the yield of diamonds in the process.

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