Reaction Control

THE MUCH-SOUGHT-AFTER ability to pick and choose the bonds broken in a chemical reaction in the gas phase has been getting closer to becoming part of the chemists' repertoire over the past 20 years. In a forward jolt, researchers now have selectively broken C-H bonds in the catalytic dissociation of methane on a nickel surface.

The control of such a complex reaction involving so many atoms had been thought to be intractable. But a team led by chemistry professor Arthur L. Utz of Tufts University used a laser to vibrationally excite C–H bonds in a beam of triply deuterated methane molecules, causing the C–H bonds to break as the molecules glance off the metal surface (Science 2008, 319, 790).

The research not only demonstrates the ability to control a reaction on a surface, but also reveals how energy flows during heterogeneous catalysis. This insight is important, because catalytic reactions of methane with metals are popular commercial and laboratory methods for producing H₂.

Chemists have been seeking to selectively control bond breaking for decades. The first successful experiments involved simple reactions of deuterated water (HOD) with hydrogen atoms in the gas phase. Depending on the choice of energy, a bombarding laser could excite stretching in either the O–D or O–H bonds. An O–H stretch produced OD and H₂, while an O–D stretch produced OH and HD.

A major impediment to progress in this area has been the tendency of the excitation energy to redistribute among a molecule's other bonds. Surface reactions have been thought to be even more problematic, as scores of surface atoms soak up the energy and thwart the bond-breaking process.

The Utz group overcame the problem by shooting triply deuterated methane molecules from a fast-moving beam at a nickel surface while exciting C–H bond stretches with a laser. The combination of translational and vibrational energy allowed the surface-catalyzed reaction to happen quickly, before the energy had a chance to redistribute.

The work is "a lovely additional example of bond-selective chemistry," says Richard N. Zare, chemistry professor at Stanford University.

The work's "punch line is that one can perform bond-selective chemistry even in the face of extensive interactions with the surface," says F. Fleming Crim, chemistry professor at the University of Wisconsin, Madison, and a bond-selective chemistry pioneer.

Even a few years ago, Utz says, chemists believed the great number of surface atoms in such a complex reaction would prevent control of bond breaking. Utz, who was a graduate student of Crim's during the early years of selective chemistry, notes, "We surprised ourselves with the extent to which we could control reactivity on a metal surface."