Influence of Nickel Catalyst Geometry on the Dissociation Barriers of H₂ and CH₄: Ni₁₃ versus 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 H 2 and CH 4 on an icosahedral Ni 13 cluster and a Ni(111) surface. Both H 2 and CH 4 adsorb molecularly at t 1 sites of the cluster. Subsequent dissociation leaves H atoms most strongly bound to c 3 sites, while CH 3 favors t 1 and b 2 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 H 2 and CH 4 dissociation barriers are estimated to be 3.5 and 9.9 kcal/mol, respectively. The corresponding values for Ni(111) are 0.6 and 21.4 kcal/mol. Molecular orbital (MO) theory is employed to explain how local distortion in the electronic structure of the nickel atoms lowers these barriers.