AVS 60th International Symposium and Exhibition
    Surface Science Thursday Sessions
       Session SS2-ThA

Paper SS2-ThA11
Vibrational Efficacy in Methane Dissociative Chemisorption Dynamics

Thursday, October 31, 2013, 5:20 pm, Room 202 A

Session: Surface Dynamics and Non-adiabatic Processes
Presenter: I.A. Harrison, University of Virginia
Authors: S.B. Donald, University of Virginia
I.A. Harrison, University of Virginia
Correspondent: Click to Email
Dynamics play an important role in the activated dissociative chemisorption of methane and can modulate the thermal reactivity by about an order of magnitude as compared to statistical expectations based on energetics alone. A dynamically-biased precursor mediated microcanonical trapping (d-PMMT) model of activated dissociative chemisorption has sufficed to quantitatively replicate the behavior of methane interacting with a number of metal surfaces (M) for a variety of non-equilibrium and thermal equilibrium experiments. It was found that molecular translation parallel to the surface and rotations are spectator degrees of freedom and vibrational energy is typically less efficacious than translational energy directed along the surface normal in promoting reactivity [e.g., the vibrational efficacy relative to normal translational energy is ηv = 0.4 for CH4/Pt(111)]. This latter kind of dynamical effect is well-documented for gas-phase atom + diatomic molecule reactions where it is codified by the “Polanyi rules” which state that “early” transition state barriers are preferentially surmounted by translational energy and “late” barriers by vibrational energy. The threshold energy for chemisorption, E0, and the thermally state averaged vibrational efficacy, ηv, derived from d-PMMT analysis of different CH4/M experiments are shown to have interesting and systematic correlations with the results of GGA-DFT electronic structure calculations of CH4/M transition state properties as M is varied. The Polanyi rules apparently extend well to the CH4/M chemistry and may thereby serve to guide ab initio predictions of experimental dissociative sticking coefficients.