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December 2008

Alcohol Exploits Defects to Its Advantage

Scientists unlock mystery of how alcohol splits, travels on surface of catalyst

Results: On the surface of a common catalyst, alcohol molecules do the unexpected: they hop with the help of defects, according to scientists at Pacific Northwest National Laboratory, University of Texas at Austin, and Southern Illinois University. The catalyst's surface has holes or vacancies where an oxygen atom should be, but isn't. Instead of ignoring these vacancies, the alcohol discards the hydrogen from its only oxygen and jumps into the vacancy. Then, the molecule hops from one vacancy to the next as they come nearby. 

Why it matters: This study is the first to show how the vacancies on the surface of a titanium dioxide catalyst can aid in moving molecules. Learning how atoms behave on the surface of the catalyst could help tailor molecular delivery systems. For example, scientists could design a vacancy-rich system to lead the desired portions of an alcohol to sites where they are converted into hydrogen fuel and carbon dioxide. In addition, this atom-by-atom understanding can help in designing or refining technologies that use titanium dioxide, such as water purifiers, self-cleaning glass, and air purifiers.

Methods: The researchers combined experiment and theory to study the behavior of alcohol on rutile titanium dioxide. For the experiments, they used imperfect titanium dioxide with holes or vacancies on the surface where oxygen atoms should be. Then, they added a 4-carbon alcohol, called 2-butanol.

Using a state-of-the-art scanning tunneling microscope at the Department of Energy's EMSL, they found that the alcohol did not behave as expected. The alcohol shed its hydroxyl hydrogen (see sidebar), leaving the oxygen exposed. The oxygen, still attached to the alcohol's carbons and hydrogens, jumped into the nearest hole. But, the oxygen did not stay put for long. It hopped from vacancy to vacancy as the vacancies came nearby.

Scanning electron micrograph and model.
Images from a scanning electron microscope (top) and a simple model show how the oxygen from the alcohol (green with R on top) diffuses on the surface of titanium dioxide (the oxygen is represented by the blue spheres and the titanium by the purple).

Next, the team analyzed these results with detailed theoretical calculations. They found that this travelling behavior at low temperatures required less energy than the other route: breaking off the whole hydroxyl group and moving the hydrocarbon along the surface. To confirm these results, the team studied a range of alcohols, one to eight carbons in length. They found the same pattern of movement.

What's next? The team is moving on to determine how other molecules would break and diffuse on more active oxides, such as tungsten trioxide.

Reference: Zhang Z, R Rousseau, J Gong, SC Li, BD Kay, Q Ge, and Z Dohnálek. 2008. "Vacancy-assisted Diffusion of Alkoxy Species on Rutile TiO2(110)." Physical Review Letters 101:156103.

Acknowledgments: The DOE's Office of Basic Energy Sciences, Chemical and Material Sciences Division funded the experimental and theoretical work by Zhenrong Zhang, Roger Rousseau, Bruce Kay, and Zdenek Dohnálek at PNNL. The Robert A. Welch Foundation and National Science Foundation funded the work by Jinlong Gong and Shao-Chun Li at the University of Texas at Austin and Qingfeng Ge at Southern Illinois University. In addition, Jinlong Gong received funding from PNNL through the Summer Research Institute.

The experimental work, including use of the scanning tunneling microscopy, and the theoretical calculations were done in DOE's EMSL, a national scientific user facility at PNNL.

This work supports PNNL's mission to strengthen U.S. scientific foundations for innovation by developing tools and understanding required to control chemical and physical processes in complex multiphase environments.

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What is an alcohol?

While you may only think of alcohol as the kick in your Long Island Ice Tea, it is actually an entire class of molecules. All alcohols have an oxygen atom bound to a hydrogen atom. These two atoms, bound together, are known as a hydroxyl group. The rest of the molecule is typically carbon and hydrogen atoms.