Application of the ReaxFF Reactive Force Field to Reactive Dynamics of Hydrocarbon Chemisorption and Decomposition † Jonathan E. Mueller, ‡ Adri C. T. van Duin, § and William A. Goddard III* ,‡ Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125, and Department of Mechanical and Nuclear Engineering, Penn State UniVersity, PennsylVania 16801 ReceiVed: September 14, 2009; ReVised Manuscript ReceiVed: December 16, 2009 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. 1.0. Introduction Nickel is the primary catalyst in the steam reforming process 1 for converting methane and water into synthesis gas (carbon monoxide and hydrogen) which is then used in such important industrial processes as the Haber Bosch synthesis of ammonia and the Fischer-Tropsch formation of higher hydrocarbons. 2 In addition, nickel catalysts are used in high temperature solid oxide fuel cells using hydrocarbon fuels, and more recently nickel has been used to catalyze the formation and growth of carbon nanotubes (CNTs) from hydrocarbons. 3 These applica- tions have stimulated numerous studies of hydrocarbon rear- rangements on nickel, resulting in a good understanding of the fundamental processes of simple hydrocarbon molecules reacting on low index surfaces of nickel. 4–7 Nevertheless, there remain many questions about the chemistry on the defect rich surfaces of nanoparticles, used, for example, as catalysts for growing CNTs. During CNT growth, the nickel particle catalyst is responsible for catalyzing at least three processes: decomposition of the hydrocarbon feedstock, transport of the activated hydrocarbon species to the edge of the growing nanotube, and addition of the activated carbon species to the growing end of the nanotube. Each of these steps could play a rate limiting role depending on the growth conditions; however, experimental evidence suggests that feedstock decomposition is the limiting step for low temperature (350 °C) CNT growth. 8 While the adsorption and decomposition of hydrocarbons on low index surfaces has been examined in many experiments 2,4,7,9–12 there has been little in the way of application of these results to larger catalytic problems, such as the role of feedstock decomposition in CNT growth. Thus, surface science studies of hydrocarbon chemisorption and decomposition on low index nickel surfaces try to limit the number of defects, whereas a nickel catalyst particle used in CNT growth may have many surface defects not present on the perfect (111) surface. These defects likely play important roles in catalyzing reactions on the particle surface, but experimental studies of CNT growth typically cannot isolate just one part of the process (feedstock decomposition) from the subsequent rearrangements, making it difficult to obtain a detailed chemical mechanism including the key steps involved in feedstock decomposition. We show here that reactive dynamics (RD) simulations provide mecha- nistic information about these heterogeneous catalytic processes, which we expect to be useful for understanding more complex reactions, such as CNT growth. Here, we present RD simulations of six representative hydrocarbon species (methane, ethyne, ethene, benzene, cyclo- hexane, and propene) as they chemisorb and decompose on a 468 atom nickel nanoparticle (21 Å diameter). These six examples were chosen to cover a variety of hydrocarbon types. Ethyne and ethene allow us to compare the reactivity for species with one or two π bonds. Propene allows us to consider the effect of the weak allylic C-H bond. Benzene brings in effects of aromaticity and ring structures. For the saturated hydrocar- bons methane and cyclohexane, we can examine the initial CH bond cleavage for systems that do not chemisorb strongly. With the exception of propene, the chemisorption and decomposition † Part of the “Barbara J. Garrison Festschrift”. * Corresponding author. E-mail: wag@wag.caltech.edu. ‡ California Institute of Technology. § Penn State University. J. Phys. Chem. C 2010, 114, 5675–5685 5675 10.1021/jp9089003  2010 American Chemical Society Published on Web 01/27/2010