Finding one bowl of porridge too hot, a second too cold, and a third "just right," Goldilocks articulated a general preference that turns out to govern catalytic reactions. Ideal catalysts need to exhibit surface reactivity that is neither too high nor too low. Like the children's story character with her porridge, researchers have found a bimetallic system whose chemical properties are just right--and can be manipulated through subtle modification of the material's thickness.

	JUST RIGHT Chen (center) and graduate students Brian Polizzotti (left) and Hwu discuss X-ray absorption experiments at Brookhaven National Laboratory's National Synchrotron Light Source. PHOTO BY MITCH JACOBY

"In order to do catalysis, you need a surface that's active enough to hold onto reactants, but not so tightly that they can't move around and react," says Jingguang G. Chen, an associate professor of materials science and engineering and of chemical engineering at the University of Delaware, Newark, describing a nickel-platinum catalyst that his research group is investigating.

The team reports that coating a certain crystal face of platinum, Pt(111), with a monolayer film of nickel produces a catalytically active surface whose properties differ from pure platinum crystals and from thick nickel films. Specifically, the group finds that the single-monolayer Ni/Pt specimen promotes hydrogenation of cyclohexene to cyclohexane. In contrast, on pure platinum or on platinum crystals that have been coated with more than two atomic layers of nickel, the hydrogenation reaction does not occur [J. Am. Chem. Soc., 124, 702 (2002)].

Clues to the origin of the unique chemistry come from several sets of experiments. Hydrogen's behavior was probed via desorption measurements in which cold specimens were dosed with hydrogen and then slowly warmed to desorb H₂ from the surface. Desorption temperatures are used as indications of the relative bond strength of a surface species.

Chen, graduate student Henry H. Hwu, and postdoctoral researcher Joseph Eng Jr. find that H₂ desorbs at a temperature of 287 K from Pt specimens, whereas thick Ni films need to be warmed above 320 K for H₂ to desorb. Surprisingly, adding just a single monolayer of nickel to platinum lowers the desorption temperature to 226 K. "This is a clear indication that hydrogen is more weakly bonded to the monolayer Ni/Pt surface," Chen says.

The Delaware group turned to surface vibrational spectroscopy and X-ray absorption methods to examine cyclohexene's surface behavior. The team finds that on the single-monolayer Ni/Pt surface, cyclohexene is weakly p-bonded. In contrast, pure platinum and nickel films break the alkene's double bond and hold the organic species tightly via a di-s bonding arrangement.

"Overall, the unique chemistry of the monolayer Ni/Pt surface can be explained by the weak interaction between adsorbates and the monolayer-thick nickel film," Chen remarks. He adds that the study suggests that chemical properties of metal catalysts can be manipulated by controlling film thickness.

Applying the results to industrial catalytic problems, Chen and coworkers find that the monolayer Ni/Pt system functions more efficiently in hydrodesulfurization of thiophene than Pt crystals or thick Ni films [J. Catalysis, published electronically Jan. 3, http:// www.idealibrary.com/links/doi/10.1006/jcat.2001.3453]. The group is now investigating the fundamental basis of the Ni/Pt system's weakened interactions with adsorbates.

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