(646a) The Influence of Nickel Catalyst Geometry on the Dissociation Barriers of H2 and CH4: Ni13 Vs. Ni(111)

     The role of catalyst size and geometry in hydrocarbon decomposition is elucidated using density functional theory (DFT). For the sake of clarity, the work focuses on the dissociative adsorption of H₂ and CH₄ on an icosahedral Ni₁₃ cluster and a Ni(111) surface.  Both H₂ and CH₄ adsorb molecularly at t₁ sites of the cluster. Subsequent dissociation leaves H atoms most strongly bound to c₃ sites, while CH₃ favors t₁ and b₂ sites.  A hybrid linear synchronous transit/quadratic synchronous transit (LST/QST) transition state search algorithm allows transition state structures to be located and dissociation barriers to be obtained.  The lowest H₂ and CH₄ dissociation barriers are estimated to be 3.45 kcal/mol and 9.9 kcal/mol, respectively. The corresponding values for Ni(111) are 0.57 kcal/mol and 21.43 kcal/mol.  Molecular orbital (MO) theory is employed to explain how local distortion in the electronic structure of the nickel atoms lowers these barriers. Meanwhile, a modified (111) surface is investigated to explore its effects on H₂, and CH₄ dissociation.