Electrons grab unexpected energy share

Iron rusts. Silver tarnishes. Such low-energy transformations of metals are among the most obvious and ubiquitous examples of surface chemistry. Yet currently, “no one can predict what [chemical] reactions will occur on a pure metal surface,” notes Eric W. McFarland of the University of California, Santa Barbara. Consequently, all solid metal-based catalysts have been found empirically rather than designed from scratch.

New experiments by McFarland and his colleagues in California and Germany are clarifying what happens when an atom or molecule settles onto a metallic surface, even briefly, and then reacts with the metal. Such adsorption reactions (SN: 12/10/94, p. 390) play a vital role in industrial processes that use metal catalysts to create many important chemicals.

By shooting beams of various atoms and molecules onto ultrathin films of silver on silicon, the researchers have demonstrated that low-energy adsorption reactions inject much more of their energy into electrons in a metal surface than anyone had previously realized. The scientists report their findings in the Dec. 21, 2001 Science. Researchers had long thought that the lion’s share of the released energy simply heated the metal.

As it turns out, however, up to 100 percent of the released adsorption energy can pass to electrons or other electric-charge carriers in the metal, the new study shows. These activated charge carriers may be previously unrecognized instigators of chemical reactions on metal surfaces–the subject of current studies by McFarland and his colleagues.

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