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.