Title: Structures, energetics and reactions of hydrocarbons on nickel

Speaker: Dr. Jonathan E. Mueller, California Institute of Technology

Date/Time: Thursday, September 23, 2010, 9:30 am Mountain Time        

Location: CSRI/90 SNL/NM - Videoconferenced to 942/1341 SNL/CA

Brief 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 CHx and C2Hy species to determine their structures and energies on Ni(111). We find that CH is the most stable form of CHx on Ni(111). This is in good agreement with experiments, where CH is a stable intermediate in both methane dehydrogenation and CO methanation, while CH2 is only stable during methanation. Furthermore, nickel surface atoms catalyze the formation and cleavage of C-H bonds by moderating the energy difference between reacts and products, and also by stabilizing the transition state. For the C2Hy species we find a low surface coverage decomposition pathway proceeding through CHCH, the most stable intermediate, and a high surface coverage pathway which proceeds through CCH3, the next most stable intermediate. Both pathways are observed experimentally.

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 the early stages 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.

CSRI POC: John Aidun, 505-844-1209, and Rick Muller, 505-284-3669