Going big to study small

It takes a big computer to model very small things. And, like its namesake state, New York Blue is big. Made up of 36,864 processors, the massively parallel IBM Blue Gene/L is housed at DOE’s Brookhaven National Laboratory (BNL) on New York’s Long Island, where, among other things, it’s used to model quantum dots, or nanoparticles, just a few atoms in size.

Overseeing New York Blue’s care and feeding is Brookhaven’s Computational Science Center, headed by James W. Davenport. Davenport is spearheading an effort to model nano-sized slivers of metallic material, such as gold and palladium, on the supercomputer.

Modeling nano-sized particles is necessary because conventional diagnostic tools have not yet been able to accurately determine the makeup of such tiny bits, which differ from larger chunks of the same materials. When large pieces of gold and palladium are X-rayed their crystal structure becomes apparent. “The atoms are arranged in a certain periodic array that is well defined. But small particles are not the same. It is not known exactly what the atomic structures of most of these particles are,” Davenport says.

Learning the physical properties of these nanocrystals is the first step in determining how they may be used as catalysts — materials which manipulate chemicals — and in turn create new applications in DOE-related fields such as advanced energy technology.

Hydrogen storage turns golden

Gold, for example, is a noble inert metal that normally does not react with other chemicals. Yet when gold particles made up of fewer than about 1,000 atoms are placed in the presence of certain chemicals, catalytic reactions take place.

A particular catalytic interaction involving gold nanoparticles may prove useful for producing hydrogen, most of which is currently made from natural gas. Hydrogen separated from natural gas often also contains carbon monoxide, which poisons the fuel cells used to produce electricity. Gold nanocrystals may be used as a catalyst to scrub carbon monoxide from the hydrogen stream. This could greatly expand clean electricity production, since water is the only byproduct of fuel cell operation.

There also is evidence that nanoparticles might be a good hydrogen storage medium. Hydrogen storage currently requires considerable space, presenting a major obstacle to its use as a significant alternative energy source. Storing hydrogen in a large tank, for example, makes it less practical as an automotive fuel. Cutting the space requirement by getting hydrogen to adhere to tiny particles of metal would be a major step toward creation of a hydrogen-based economy.

All of this makes it important “to understand how the chemical bond of hydrogen to these nanoparticles differs from the chemical bond of hydrogen to a large chunk of metal,” Davenport says.

To accomplish these goals first requires understanding how the properties of the very small differ from the big. And that is where the modeling on New York Blue comes in.

Searching for low energy

“We are studying the shapes of the different particles as a function of their size by calculating the energies of these different structures and seeing which one is the lowest,” Davenport says.

The energy in this case is potential energy — the force drawing the atoms together — as opposed to kinetic energy, which involves actual movement of the atoms. “If I allow two atoms to come close together the bond energy increases and that’s what makes chemical bonds — that’s what holds our world together,” Davenport says. “We are trying to calculate that chemical bond energy.”