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Theoretical study of gas-surface reaction dynamics: H reaction with Cl adsorbed on Au(111) dissociative chemisorption of methane on Ni(111)
Part I. The Eley-Rideal reaction of H atoms with Cl adsorbed on Au(111) surface is examined. Electronic structure calculations based on density functional theory are used to construct a model potential energy surface. Both a flat and corrugated surface are considered. Single adsorbate quantum mechanical and quasi-classical methods are used to calculate the reaction cross section and product state distributions. A reaction cross section of 2–3 Å2 is found. Steering of the incident H atom towards the adsorbed Cl plays an important role in this large cross section. The product state distribution of the HCl has over an eV in vibrational energy and less than one eV in each of the translational and rotational distributions. Reactivity at impact parameters less than 1.0 Å is found to be dependent on the incident energy and vibrational state of the adsorbate. Single adsorbate corrugated surface studies show an increase in reactivity at small impact parameters. Using multiadsorbate quasi-classical methods and a corrugated potential energy surface, we have found that H atoms react with Cl atoms adsorbed onto a Au(111) surface to produce HCl via Eley-Rideal (ER), Hot Atom (HA), and Langmuir-Hinschelwood (LH) pathways. We observe two ER mechanisms. At small normal incidence energies, reaction results from a more-or-less direct collision with the Cl, leading to a large amount of product vibration, and relatively cold rotation and translation. In the second mechanism, more dominant at near-normal incidence and/or large incident energies, the H atom passes near the Cl, recoils from the metal, and is pulled into orbit about the Cl. This leads to broader product state distributions, and a more even distribution of the 3.0 eV of available energy among the product degrees of freedom, similar to products formed via the HA pathway. Overall, ER processes tend to contribute less than 10% to the reactivity, and most of the HCl is formed via HA processes. There is an increase in HCl formation with surface temperature for both the ER and HA mechanisms, but this increase is relatively weak. We observe typically about 12% H atom sticking, which would lead to HCl formation via a LH process in the experiments, above 140 K. We observe a weak forward scattering due to the direct ER component, as in the experiments. However, unlike the experiments, we observe a dip in our product angular distributions about &thetas;f = 0°, which we ascribe to our quasi-classical approximation. While we tend to see more energy in the hot products than in the experiments, our product translational, rotational, and vibrational distributions are in relatively reasonable agreement with those measured. One major disagreement with experiment is that there is apparently a significant sticking of the H atom at low temperatures, leading to a large LH component. In addition, the ER and HA components increase much more strongly with temperature than in the calculations. It is possible that electon-hole pair excitations in the metal strongly relax both the H atom and the excited HCl molecules formed.^ Part II. A quantum mechanical method is presented for studying the dynamics of the dissociative chemisorption of methane on metal surfaces. An oscillating transition state barrier that incorporates electronic effects is introduced. The minimum energy path for the dissociative chemisorption of CH4 on a Ni(111) surface is presented. Two scenarios are considered. The first includes the surface in its minimum energy geometry. The second looks at the effect on the transition state barrier when a Ni atom is moved 0.2 Å above the surface plane. The transition state barrier's height and width are found to decrease when the Ni atom at which the reaction occurs is perturbed from its equilibrium geometry.^
Joseph G Quattrucci,
"Theoretical study of gas-surface reaction dynamics: H reaction with Cl adsorbed on Au(111) dissociative chemisorption of methane on Ni(111)"
(January 1, 2008).
Electronic Doctoral Dissertations for UMass Amherst.