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CBE team designs improved catalysts for hydrogen chemistry

Manos Mavrikakis (Larger image.)

Metal alloys are denoted as solute/host pairs. The x-axis indicates the energy (Eseg) for a single solute atom to move from the bulk to the surface layer of the host metal. The y-axis denotes the difference between the magnitudes of the hydrogen binding energies (.H = 1/4 monolayer) on the pure solute (|BEHsol|) and on the pure host (|BEHhost|) close-packed metal surfaces. Regions in which hydrogen-induced segregation is expected are hatched. The * symbol denotes overlayers; otherwise, subsurface alloys are present. (Graphic reprinted from Nature Materials.) (Larger image.)

Writing in the Oct. 17 Nature Materials, graduate student Jeff Greeley and Chemical and Biological Engineering Assistant Professor Manos Mavrikakis identify a new class of near surface alloys (NSAs) that bind hydrogen atoms loosely. The weak bonds are useful for catalysis because they make subsequent reactions easier. A standard rule for weak bonds is that a higher energy cost is paid in breaking up the H2 molecule; by using density functional theory calculations, the team has determined a new class of NSAs that can yield superior catalytic behavior for hydrogen-related reactions.

"We found that selected NSAs offer an exciting exception to this rule by simultaneously allowing weak hydrogen binding and low H2 dissociation barriers. Weak binding of hydrogen, in turn, can make subsequent reaction steps easier, thereby allowing lower temperatures to be used for reactions on these NSAs," Mavrikakis says.

Near surface alloys that react at lower temperatures with higher selectivity would be a boon to many chemical and pharmaceutical processes. The team believes its discovery could be used to design an NSA that binds carbon monoxide weakly. When combined with the properties of improved hydrogen dissociation, the alloy could be useful as a robust fuel-cell anode.

The methodology was developed in the College of Engineering in order to identify nanostructures for easy hydrogen chemistry, but has a broad base of application and could be applied to identify promising catalysts for a variety of other chemical reactions. NSAs can be prepared with state-of-the-art nanosynthesis techniques, allowing layer-by-layer control of the desired nanostructures.

Content by perspective@engr.wisc.edu

Date last modified: Tuesday, 26-Apr-2005 17:06:42 CDT
Date created: 26-Apr-2005

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