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MARGINALIA

A Really Moving Story

Roald Hoffmann

Chemistry with a Hammer

Methane, CH4, "wants" to fall apart or dissociate on a nickel surface to produce CH3 and H strongly bound (chemisorbed) to that surface—the process is thermodynamically downhill. But bleed a little CH4 into the vacuum above a clean nickel surface and nothing happens. Oh, the methane settles down on the surface (physisorption) but refuses to decompose. We say there is a barrier to dissociative chemisorption. Breaking up is hard to do; that we know. But breaking up methane is crucial; if one wants to make complex molecules from abundant natural gas, one better begin by disrupting at least one C–H bond in ever-so-stable methane.

Figure 1. Argon crashesClick to Enlarge Image

The nature and size of that barrier was studied independently by three groups (Gert Ehrlich at the University of Illinois, Robert J. Madix at Stanford and Sylvia T. Ceyer at MIT). They seeded CH4 in a helium beam in a way such that the speed of the CH4 could be controlled. When that beam of CH4 molecules impinged on the surface, nothing happened until the kinetic energy of the methane exceeded a threshold value (about 60 kilojoules per mole), at which point the CH4 fell apart, generating the products CH3 and H, both of which were chemisorbed on the surface.

Sylvia Ceyer and her coworkers then went on to do a beautifully simple experiment. They laid down a layer of gently physisorbed (and thus essentially unperturbed) methane on that surface. Then they trained a beam of argon atoms (the hammer!) of controlled energy on the unsuspecting quiescent methane. When the argon atoms provided somewhat more collisional oomph than the moving methane molecules had in the absence of the noble gas, chemistry took place.





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