Structures, Energetics and Reactions of Hydrocarbons on Nickel

Citation

Mueller, Jonathan Edward (2010) Structures, Energetics and Reactions of Hydrocarbons on Nickel. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/RVXX-Z341. https://resolver.caltech.edu/CaltechTHESIS:05262010-132424180

Abstract

To better understand and improve reactive processes on nickel surfaces such as the catalytic steam reforming of hydrocarbons, the decomposition of hydrocarbons at fuel cell anodes, and the growth of carbon nanotubes, we have performed atomistic studies of hydrocarbon adsorption and decomposition on low index nickel surfaces and nickel catalyst nanoparticles. Quantum mechanics (QM) calculations utilizing the PBE flavor of density functional theory (DFT) were performed on all CH_x and C₂H_y species to determine their structures and energies on Ni(111). In good agreement with experiments, we find that CH is the most stable form of CH_x on Ni(111). It is a stable intermediate in both methane dehydrogenation and CO methanation, while CH(2,ad) is only stable during methanation. We also find that nickel surface atoms play an important catalytic role in C-H bond formation and cleavage. For the C₂H_y species we find a low surface coverage decomposition pathway proceeding through CHCH_ad, the most stable intermediate, and a high surface coverage pathway which proceeds through CCH_3,ad, the next most stable intermediate. Our enthalpies along these pathways are consistent with experimental observations.

To extend our study to larger systems and longer time scales, we have developed the ReaxFF reactive force field to describe hydrocarbon decomposition and reformation on nickel catalyst surfaces. The ReaxFF parameters were fit to geometries and energy surfaces from DFT calculations involving a large number of reaction pathways and equations of state for nickel, nickel carbides, and various hydrocarbon species chemisorbed on Ni(111), Ni(110) and Ni(100). The resulting ReaxFF description was validated against additional DFT data to demonstrate its accuracy, and used to perform reaction dynamics (RD) simulations on methyl decomposition for comparison with experiment. Finally ReaxFF RD simulations were applied to the chemisorption and decomposition of six different hydrocarbons (methane, acetylene, ethylene, benzene, cyclohexane and propylene) on a 468 atom nickel nanoparticle. These simulations realistically model hydrocarbon feedstock decomposition and provide reaction pathways relevant to this part of the carbon nanotube growth process. They show that C-C π bonds provide a low barrier pathway for chemisorption, and that the low energy of subsurface C is an important driving force in breaking C-C bonds.

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