| AVS 63rd International Symposium & Exhibition | |
| Fundamental Discoveries in Heterogeneous Catalysis Focus Topic | Thursday Sessions |
| Session HC+SS-ThM |
| Session: | Dynamics of Gas-surface Interactions in Heterogeneous Catalysis |
| Presenter: | Arthur Utz, Tufts University |
| Authors: | A.L. Utz, Tufts University N. Chen, Tufts University E.H. High, Tufts University |
| Correspondent: | Click to Email |
Vibrational state resolved reactivity measurements have established that mode-selective chemistry, in which the reaction probability, S0, depends on the identity of the reactant’s vibrational state, and bond selective chemistry, in which the product identity depends on the reagent’s vibrational state, is widespread in the dissociation of methane and its isotopologues on Ni and Pt surfaces. Two factors lead to the observed mode- and bond-selectivity. First, methane’s distorted transition state geometry introduces a bias that favors those vibrational motions that best access the transition state geometry. The sudden vector projection (SVP) model of Guo and coworkers predicts selectivity based on this factor. As the methane molecule approaches the surface, the molecule-surface interaction potential can also perturb the molecule’s vibrations and lead to vibrational energy redistribution in the entrance channel for the reaction. Reaction path Hamiltonian calculations by Jackson and coworkers, quantum dynamics calculations by Kroes et al., and the vibrational adiabatic predictions of Halonen et al. focus on how this second factor impacts reactivity. In all of these calculations, the incident molecule’s vibrational state symmetry can influence the vibrational coupling channels and energy flow pathways for the molecule as it approaches the surface.
This talk will focus on state-resolved experimental measurements of CH2D2 dissociation on a 90K Ni(111) surface. Unlike CH4, the C2v symmetry of the CH2D2 molecule results in both the ν1 symmetric- and ν6 antisymmetric C-H stretching vibrations being infrared active. Therefore, we can use state-resolved infrared laser excitation of CH2D2 in a supersonic molecular beam to measure the reaction probability for these two C-H stretching states as a function of incident translational energy (Etrans). By performing the measurements at 90K, we observe a sharp energy threshold for reaction that permits an unusually precise measure of the efficacy of each vibration in promoting reaction. Our choice of excitation transitions further reduces experimental error in comparing the two states' reactivity. Contrary to the predictions of a vibrationally adiabatic model, the two states have nearly identical reaction probability. We will compare these results with recent reaction path Hamiltonian calculations from the Jackson group to explore how the symmetry of these two vibrational states impacts their reactivity.