The role played by surface structure in breaking molecules

Herokazu Ueta
Many industrial chemicals are made by breaking other molecules up and sticking them back together in different ways. The problem is that the precursor chemicals—the ones that are used to make the end product—are quite stable so a lot of energy is needed to break them up. To overcome this problem catalysts are used. Often, these are metallic surfaces, which act to make the desired reaction require less energy. However, catalysts, and their interactions with the chemicals they are reacted with, are often poorly understood. Thus, much theoretical and experimental research effort is devoted to understanding catalyst systems.

Some recent work has focused on the interaction between methane and nickel, which is important for steam reforming of natural gas (methane is reacted with water to generate hydrogen and carbon monoxide). Recent experimental work has shown that the efficiency of this reaction depends not only on how high the kinetic energy of methane is but also on how the methane molecule is vibrating as it hits the nickel surface. Unfortunately, this is a difficult problem to model because the quantum mechanical description becomes quite complex once the surface, the molecule, and its vibrations are included.

However, in a recent issue of Physical Review Letters, researchers are reporting the effect of nickel lattice motion and surface reconstruction on methane dissociation. These theoretical calculations used a combination of approaches, where the nickel surface atoms are described exactly by their quantum mechanical state and the methane is approximated as a quasi-diatomic—CH₃-H, where CH₃ is treated as an atomic entity. The simulations show that the atoms in the nickel surface rearrange themselves during methane bond breaking. They also show that the reconstruction changes the local environment of the methane and reduces the barrier allowing the reaction to proceed. Their results indicate that, not only the excitation state of methane, but also the configuration of the surface atoms plays a role in methane dissociation.

These results need to be followed up with experiments utilizing molecules with a well-defined vibrational state. Such experiments can be used to observe the behavior of the surface during the interaction with the molecule.