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STM ELUCIDATES MECHANISM
Molecule-at-a-time study reveals fundamentals of classic reaction
Want to validate a reaction mechanism? Why not prepare the reaction intermediate, trap it, take a picture of it, and then watch it react--one molecule at time? Researchers in California have taken this approach to study the oxidation of carbon monoxide to carbon dioxide.
One of the simplest and most well-studied reactions, oxidation of CO to CO2 stands as an archetypal reaction in heterogeneous catalysis. But more than a model system, the elementary reaction is central to automobile emissions control, air purification, and chemical sensing.
For decades, surface scientists have used nearly every tool in their bags to get a handle on the reaction mechanism. Yet a clear picture of the fundamental steps remained elusive.
Now, using a scanning tunneling microscope (STM) with unique capabilities, University of California, Irvine, physics and chemistry professor Wilson Ho and postdoctoral associate Jae Ryang Hahn have synthesized and studied an O-CO-O complex. And they've identified the previously proposed but unobserved surface species as an intermediate in the CO oxidation reaction [Phys. Rev. Lett., 87, 166102 (2001)].
To date, only a few groups--among them Ho's and that of Karl-Heinz Rieder, a professor of physics at Free University in Berlin--have reported examples of step-by-step synthesis using an STM. According to Rieder, the present study is "important work in the field of single-molecule engineering due to the model character of the reaction."
Taking advantage of the single-molecule dexterity for which scanning probe instruments have become famous, Ho and Hahn use an STM tip to gently coax a CO molecule toward a pair of closely spaced oxygen atoms on a frigid silver surface. Very low temperatures minimize interfering vibrations and help confine reactants and intermediates to the surface. As the molecule is moved to within 2 Å of the oxygen atoms, the atoms and molecule form an O-CO-O complex. By applying a brief electron pulse to the complex, the team energizes the species and causes it to react. From its excited state, O-CO-O goes on to form CO2, which desorbs from the surface, leaving behind a lone oxygen atom.
Identifying a second reaction pathway, the Irvine group shows that CO2 can be formed by depositing a CO molecule from an STM tip onto an isolated oxygen atom.
Ordinarily, scanning probe techniques cannot provide chemical information. But in recent years, Ho and his associates worked to overcome that limitation by pioneering procedures by which an STM can be used to measure highly sensitive vibrational data. Vibrational analysis of O-CO-O plays a central role in the present study.
"Not only does the STM allow us to make the complex, but it allows us to image it and characterize it spectroscopically via single-molecule vibrational spectroscopy and microscopy," Ho says.
NUDGE, NUDGE! A pair of oxygen atoms (oblong) and a CO molecule (circular) appear as distinct features in an STM image (top left). As the species are brought closer together (top right), the image changes until, at less than 2 Å separation, an OCOO complex forms (bottom left). Exciting the complex causes CO2 to form and desorb, leaving behind a lone O atom (bottom right).
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Copyright © 2001 American Chemical Society