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November 2008

The Repulsive Behavior of Hydrogen Atoms

Quick, complex diffusion process sheds light on reactions on popular catalyst

Results: When water breaks apart on a well-known catalyst, the once-close hydrogen atoms quickly part company in a complicated process, according to scientists at Pacific Northwest National Laboratory and the University of Texas at Austin. Their detailed study shows the hydrogen atoms are slightly repelled by each other on the surface of the rutile titanium dioxide catalyst. The atoms move apart by first sliding an electron over to their new home. Then, the rest of the atom follows.

Why it Matters: Understanding what happens on the surface of rutile titanium dioxide could help researchers tailor this material to use sunlight to speed the reaction that splits water into hydrogen and oxygen. The resulting hydrogen can be used to power fuel cells that could replace gasoline-powered engines in cars and trucks.

Hydrogen Movement.
Hydrogen movement shown by the scanning tunneling microscope. By taking snapshots before (left) and after (middle) of the motion of the hydrogen atoms on the catalyst, the scientists can determine where and how fast the atoms moved (final).

Methods: The research team took a two-prong approach to studying hydrogen movement or diffusion. They began with experimental studies and followed up with theoretical calculations.

For the experiments, they used temperature-dependent measurements to track hydrogen movement on the catalyst's surface. They performed these measurements using a scanning tunneling microscope that can resolve single atoms on a surface in the Department of Energy's EMSL, a national scientific user facility at PNNL. For the theoretical studies, they used density functional theory calculations and other models.

The results from the experimental and theoretical studies did not agree. While both show hydrogen diffusing across the surface at the same rates, the underlying parameters controlling the rates differed.

The researchers believed the differing results were because of how the hydrogen moves. They speculated that the hydrogen diffused via a two-step process. A hydrogen atom has two parts: a positive nucleus and a negative electron. In diffusion, the lone electron nimbly hops over to the new location first. Then, the larger nucleus hefts itself to the new location.

The first step, the electron moving, is not accounted for in the theoretical calculations. "This [inequality in the results] clearly provides a motivation for future theoretical studies." said Bruce Kay, a PNNL chemical physicist on the project.

What's Next: The team continues to study water on rutile titanium dioxide, focusing on its reactions with oxygen to get one step closer to understanding titanium dioxide as a water splitting catalyst.

Acknowledgments: The U.S. Department of Energy's Office of Basic Energy Sciences, Chemical Sciences Division funded the experimental work done by Zhenrong Zhang, Bruce D. Kay, Yingge Du, Igor Lyubinetsky, and Zdenek Dohnálek. of PNNL. This work was funded under two BES projects: Chemical Transformations at Complex Interfaces and Fundamental Investigations of Water Splitting on Model TiO2 Photocatalysts Doped for Visible Light Absorption. The experimental team used the isothermal scanning tunneling microscope and other instruments at DOE's EMSL, a national scientific user facility at PNNL.

The Robert A. Welch Foundation; and National Science Foundation funded the theoretical work by Shao-Chun Li, Daniel Sheppard, and Graeme Henkelman of University of Texas at Austin. Mike White, former director of PNNL's Institute for Integrated Catalysis, who passed away suddenly, was a key player in this study.

Reference: Li SC, Z Zhang, D Sheppard, BD Kay, JM White, Y Du, I Lyubinetsky, G Henkelman, and Z Dohnálek. 2008. "Intrinsic Diffusion of Hydrogen on Rutile TiO2(110)." Journal of the American Chemical Society 130(28):9080-9088.

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