Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

In recent years we have studied H-graphite reactions, important in the formation of
molecular Hydrogen on graphitic dust grains in space, the etching of graphite walls in fusion
reactors, and the modification of the electronic properties of graphene. Two recent studies
examined the physisorption of H. We have developed a powerful approach to these types of
problems based on the reduced density matrix, which allows us to evolve a quantum system
weakly coupled to a bath over relatively long times. Thus, we can not only compute, quantum
mechanically, the scattering into free and bound states, we can observe the relaxation and/or
desorption from these states over long times. In a study of H physisorption on graphite, we
showed that sticking is enhanced at low energies by diffraction mediated trapping states, and
examined the adsorption and subsequent desorption of H on graphite in great detail. We
compared our reduced density approach with more traditional wavefunction-based approaches,
and demonstrated the utility and accuracy of these various methods [5]. More recently, these
wavefunction-based close coupling approaches were used to examine H sticking on single-layer
graphene at very low energies. We showed that support of graphene on a substrate, or suspension
of graphene over a hole in a substrate modifies the dispersion properties of the lattice vibrations,
stabilizing the two-dimensional structure. This in turn modifies the H-graphene phonon coupling,
leading to anomalously large sticking probabilities at very low temperatures [7].
A collaboration with the Kroes group (Leiden) used AIMD, ab initio molecular
dynamics, to study H 2 dissociation on Cu(111). In AIMD, one computes the instantaneous forces
on the particles “on the fly”, using DFT in our case. This eliminates the need to construct
approximate many-dimensional PESs, which is extremely difficult when lattice motion is
involved. Our studies showed that lattice motion and temperature could effect some aspects of
H 2-Cu(111) scattering, particularly the quadrupole alignment parameter [8]. A book chapter
discussing the effects of lattice motion on gas-surface reactions is in press [9]. The primary focus
is on our methane studies, where the molecule-phonon coupling is unusually strong.
Two major projects are near completion, both applying our full-dimensional RPH
approach to methane reactions on metals. Studies on Ni(111) were motivated by experiments
from the Utz group (Tufts), where they were able to measure dissociative sticking probabilities
over a wide range of surface temperatures. This was the first real test of the model we developed
for including lattice motion effects in these reactions [1,3]. Not only did our ab initio methods
accurately reproduce the observed variation with temperature, they were able to explain why the
variation in reactivity with temperature was strong for certain energies and weak for others. In
addition, we were able to model and explain the variation in reactivity with nozzle temperature in
terms of contributions to sticking from vibrationally excited molecules. Another set of studies
focused on Pt(110)-(1x2), where the missing-row reconstruction leads to a very large surface
corrugation, making this an excellent model for real (rough) catalysts. Our focus was on
comparing the reactivity and the molecule-phonon coupling at the step edges with that on the
terrace sites of smooth Pt(111) and Pt(100) surfaces, as well as comparing with experimental
work from the Beck group (EPFL). We found that the edge sites were more reactive, as expected,
but that the effects of lattice motion were not any larger than on smoother surfaces, even though
the phonon coupling was far more complicated, involving the motion of several lattice atoms.
This coupling increases the dissociative sticking probability by about an order of magnitude when
the surface temperature increases from 400 K to 600 K, in agreement with the experiments.
Future Plans
Two on-going projects involve methane chemisorption on Pt(111), which we compared
with Ni(111) in an early low-dimensional study. While we found the Pt surface to be more
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