The Role of Thermally Excited Vibrations in Gas-Surface Reactions: Methane on Ni(111) and Ir(111).
Del Sesto, Deno.
Abstract: Eigenstate-resolved molecular beam experiments have been very
effective at probing the dynamics of gas-surface reactions. This is especially true for
methane reactions on nickel surfaces due to its importance in the field of heterogeneous
catalysis, and its ability to be modeled theoretically. These experiments are able to shed
light on the process of breaking the C-H bond, and revea... read morel how the partitioning of internal
energy within the reactant gas can affect the overall probability of reaction. These
results can then be used to predict reactivity of an ensemble of states, which is more
applicable to industrial conditions. In this paper, we utilize this information to create a
simple model to predict the relative contributions of each individual vibrational state
present in a thermal reactant gas. In a steam-reforming reactor, the methane is heated to
high temperatures above a nickel catalyst. This heating creates reactants that are
vibrationally hot, yet have little translational energy along the reaction coordinate. The
simple model predicts that for a hot methane gas with little translational energy, the
vibrationally excited molecules dominate the reactivity, while ground state molecules
contribute little. As translational energy is increased, vibrations become less important
and the ground state dominates simply because of its higher population. The second part of
this paper addresses the effects of vibrations on the second possible reaction pathway -
the precursor-mediated pathway. This pathway is studied on an Ir(111) surface. The low
activation barrier to reactivity, along with a high methane trapping probability at low
translational energy allows for a measurable number of molecules to react via this pathway.
Preliminary results seem to indicate that thermally excited vibrations within the methane
gas have a larger effect on the reactivity of the trapping-mediated pathway than they do on
the direct pathway.
Thesis (Ph.D.)--Tufts University, 2011.
Submitted to the Dept. of Chemistry.
Advisor: Arthur Utz.
Committee: Jonathan Kenny, Elena Rybak-Akimova, and Ricardo Metz.
Keyword: Chemistry.read less