In chemical reactions, water adds speed without heat

Posted on 29. Aug, 2012 by in Academic Departments, Annual Report, Chemical and Biological Engineering, Issues, Research

molecules

Through an interaction with hydrogen atoms (green), a water molecule (magenta and blue) moves rapidly across a metal oxide surface. This atomic-scale speed leads to more efficient chemical reactions. Image: Guowen Peng.

In industries such as the petrochemical, pharmaceutical, food and agricultural industries, hydrogen-based chemical reactions have huge applications. “For example, upgrading of oil to gasoline, and in making various biomass-derived products, you need to hydrogenate molecules—to add hydrogen—and all this happens through catalytic transformations,” says Paul A. Elfers Professor of Chemical and Biological Engineering Manos Mavrikakis.

A chemical reaction transforms a set of molecules (the reactants) into another set of molecules (the products), and a catalyst is a substance that accelerates that chemical reaction, while not itself being consumed in the process. How quickly those reactions occur affects how quickly products are produced.

And while scientists long have known that adding trace amounts of water can speed up chemical reactions in which hydrogen is one of the reactants, Mavrikakis and his collaborators recently discovered why.

He and Flemming Besenbacher, a professor of physics and astronomy at the University of Aarhus, Denmark, drew on their respective theoretical and experimental expertise to study metal oxides, a class of materials often used as catalysts or catalyst supports. They found that when water is present, hydrogen diffuses rapidly via proton transfer, or proton “hopping,” in which hydrogen atoms from the oxide surface “jump” onto nearby water molecules and make hydronium ions, which then deliver their extra proton back to the oxide surface (at a different position than the hydrogen started on that surface) and liberate a water molecule. In fact, at room temperature, water makes hydrogen diffuse 10,000 trillion times faster on metal oxides than it would have diffused without water.

In the absence of water, heat is needed to speed up that motion.

The process works in the hydrogenated surface area: The team saw that, on a nanoscale “path” on iron oxide templated with hydrogen atoms, water found the path, stayed on it, and kept moving within its boundaries.

The UW-Madison researchers—including chemical and biological engineering research scientist Guowen Peng, PhD student Carrie Farberow, and PhD alumnus Lars Grabow (now an assistant professor at the University of Houston)—received funding from the U.S. Department of Energy Office of Basic Energy Sciences.

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