Application of the ReaxFF reactive force field to reactive dynamics of hydrocarbon chemisorption and decomposition

We report here reactive dynamics (RD) simulations of the adsorption and decomposition of a gas of 20-120 methane, ethyne, ethene, benzene, cyclohexane, or propene molecules interacting with a 21 Å diameter nickel nanoparticle (468 atoms). These RD simulations use the recently developed ReaxFF reactive force field to describe decomposition, reactivity, and desorption of hydrocarbons as they interact with nickel surfaces. We carried out 100 ps of RD as the temperature is ramped at a constant rate from 500 to 2500 K (temperature programmed reactions). We find that all four unsaturated hydrocarbon species chemisorb to the catalyst particle with essentially no activation energy (attaching to the surface through π electrons) and then proceed to decompose by breaking C-H bonds to form partially dehydrogenated species prior to decomposition to lower order hydrocarbons. The eventual breaking of C-C bonds usually involves a surface Ni atom inserting into the C-C bond to produce an atomic C that simultaneously with C-C cleavage moves into the subsurface layer of the particle. The greater stability of this subsurface atomic C (forming up to four Ni-C bonds) over adatom C on the particle surface (forming at most three Ni-C bonds) is critical for favorable cleaving of C-C bonds. For the two saturated hydrocarbon species (methane and cyclohexane), we observe an activation energy associated with dissociative chemisorption. These results are consistent with available experimental reactivity data and quantum mechanics (QM) energy surfaces, validating the accuracy of ReaxFF for studying hydrocarbon decomposition on nickel clusters.