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Analytical Chemistry

Making Water Step By Step

Atomic resolution study reveals sequence of events

by Mitch Jacoby
February 16, 2009 | A version of this story appeared in Volume 87, Issue 7

Credit: Mitch Jacoby/C&EN
University of Aarhus' Matthiesen prepares vacuum equipment used to probe surface reaction mechanisms.
Credit: Mitch Jacoby/C&EN
University of Aarhus' Matthiesen prepares vacuum equipment used to probe surface reaction mechanisms.

PROVIDING A VIEW of the inner workings of a molecular transformation with unprecedented detail, researchers in Denmark have recorded all of the intermediate steps of a surface chemical reaction with atomic-scale resolution (ACS Nano, DOI: 10.1021/nn8008245).

Studies that reveal the subtle molecular events that comprise reaction mechanisms provide insights that can aid scientists in improving a reaction's yield or selectivity or help block undesirable reactions. Such studies generally elucidate a single bonding event or a key reaction intermediate.

Using scanning tunneling microscopy and quantum calculations to aid image interpretation, physicists Jesper Matthiesen, Stefan Wendt, Flemming Besenbacher, and coworkers at Aarhus University's Interdisciplinary Nanoscience Center "watched" step by step as hydrogen reacted with oxygen to form water on a titania crystal surface. That deceptively simple-sounding reaction—oxidation on a hydrated form of TiO2, which is a high-volume commercial catalyst—lies at the heart of numerous catalytic processes in wide-ranging applications including self-cleaning surfaces and TiO2-based dye-sensitized solar cells.

With a time-lapse series of STM images, the team strung together videos that zoom in on a complex dance of surface species. The videos depict adsorption, dissociation, diffusion, and reaction of oxygen with hydrogen on TiO2. They also capture formation of HO2, H2O2, and H3O2 intermediates and the final reaction product—water molecule dimers—and subsequent desorption of water from the catalyst surface. The study reveals that trace quantities of coadsorbed water help facilitate the reaction by opening low-energy pathways for diffusion of hydrogen atoms.

"Mapping out the entire sequence of elementary steps—not just some of them—in a surface-catalyzed reaction is a remarkable accomplishment," says Manos Mavrikakis, a professor of chemical engineering at the University of Wisconsin, Madison. He adds that TiO2's widespread use in catalysis and the presence of oxygen and moisture in so many applications makes the chemistry studied here highly relevant.



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