Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

Program Scope
THEORY OF THE REACTION DYNAMICS OF SMALL MOLECULES
ON METAL SURFACES
Bret E. Jackson
Department of Chemistry
104 LGRT
710 North Pleasant Street
University of Massachusetts
Amherst, MA 01003
jackson@chem.umass.edu
Our objective is to develop realistic theoretical models for molecule-metal interactions
important in catalysis and other surface processes. The dissociative adsorption of molecules on
metals, Eley-Rideal and Langmuir-Hinshelwood reactions, recombinative desorption and sticking
are all of interest. To help elucidate the experiments that study these processes, we examine how
they depend upon the nature of the molecule-metal interaction, and experimental variables such
as substrate temperature, beam energy, angle of impact, and the internal states of the molecules.
Electronic structure methods based on Density Functional Theory (DFT) are used to compute the
molecule-metal potential energy surfaces. Both time-dependent quantum scattering techniques
and quasi-classical methods are used to examine the reaction dynamics. Effort is directed
towards developing improved quantum methods that can accurately describe reactions, as well as
include the effects of temperature (lattice vibration) and electronic excitations.
Recent Progress
The dissociative chemisorption of methane on a Ni catalyst is the rate-limiting step in the
chief industrial process for H 2 production. However, even on this catalyst the reaction probability
is very small under typical conditions, and the dynamics are not fully understood. Most of our
recent efforts have focused on the dissociative chemisorption of methane on metals, in an attempt
to understand how methane reactivity varies with the temperature of the metal, the translational
and vibrational energy in the molecule, and the properties of the metal surface. Initial studies of
methane dissociation on Ni(111) used DFT to compute the barrier height and explore the
potential energy surface (PES) for this reaction, with an emphasis on how it changes due to lattice
motion. At the transition state for dissociation, we found that if the metal lattice is allowed to
relax, the Ni atom over which the molecule dissociates puckers out of the surface by a few tenths
of an Å. Put another way, thermal motion of this Ni atom causes the barrier to dissociation to
change, which should lead to a strong variation in the reactivity with temperature. High
dimensional quantum scattering calculations which allowed for the inclusion of several key
methane degrees of freedom (DOF), as well as the motion of the metal atom over which the
reaction occurs, found that the reaction probability was significantly larger than for a static lattice,
and strongly increased with temperature.
In an attempt to better understand the role of lattice motion on methane dissociation, we
implemented a variety of mixed quantum-classical models [1,3]. The best approach was to treat
both the lattice motion and the molecular center of mass motion classically, and the remaining
molecular DOF quantum mechanically. While this lost some tunneling contributions from
motion normal to the surface, it allowed us to directly observe the motion of the lattice at a given
collision energy. The majority of the reaction probability came from collisions between a
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