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Author

Han GuoFollow

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Chemistry

Year Degree Awarded

2018

Month Degree Awarded

September

First Advisor

Bret Jackson

Subject Categories

Materials Chemistry | Physical Chemistry

Abstract

The dissociative chemisorption of small molecules on metal surfaces is an important step in many heterogeneous catalytic processes, and has received considerable scientific attention. In this thesis, a quantum approach based on the reaction path Hamiltonian is used to explore the dissociative chemisorption of methane and its deuterated isotopologues on several metal surfaces.

The theoretical approach is described in Chapter 2. This approach treats all 15 degrees of freedom of the methane molecule, and includes the effects of lattice motion, allowing us to examine the translational and vibrational enhancements, mode- and bond-selectivity and the surface temperature dependence observed in experiments.

In Chapter 3, this approach is used to explore the dissociation of CH4, CHD3and CH2D2on Ni(111). The symmetric stretch mode is found to be more effective at promoting dissociation of CH4and CHD3than the antisymmetric stretch mode, while for CH2D2the two modes have similar efficacies. This mode specificity has also been observed for C-H stretch overtone and combination states of different symmetry. In addition, the dissociation of methane isotopologues shows bond selective behavior. With our model, mode specificity and bond selectivity is explained in terms of mode softening, the nonadiabatic couplings and symmetry in vibrationally adiabatic normal modes.

The dissociative chemisorption of methane on Pt(111) is examined in Chapter 4, where the computed sticking probabilities and vibrational efficacies are compared between reactions on Pt(111) and Ni(111). The variation in reactivity with surface temperature is investigated with our improved treatment of lattice motion. In addition, to achieve quantitative agreement with experiment, a semi-empirical specific reaction parameter density functional with van der Waals corrections is used and the results are compared with those using the PBE functional.

Finally, the effect of surface defects, i.e. step sites, is investigated in Chapter 5. Different reaction paths have been located for CH4dissociation on the stepped Pt(211) and Ni(211) surfaces. For both surfaces, dissociation along the step edge dominates the total sticking at all but the highest incident energies, due to the lower activation energy.

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