Development of the ReaxFF Reactive Force Field for Describing Transition Metal Catalyzed Reactions, with Application to the Initial Stages of the Catalytic Formation of Carbon Nanotubes

With the aim of developing a computationally inexpensive method for modeling the high-temperature reaction
dynamics of transition metal catalyzed reactions we have developed a ReaxFF reactive force field in which
the parameters are fitted to a substantial quantum mechanics (QM) training set, containing full reaction pathways
for relevant reactions. In this paper we apply this approach to reactions involving carbon materials plus Co,
Ni, and Cu atoms. We find that ReaxFF reproduces the QM reaction data with good accuracy while also
reproducing the binding characteristics of Co, Ni, and Cu atoms to hydrocarbon fragments. To demonstrate
the applicability of ReaxFF we performed high-temperature (1500 K) molecular dynamics simulations on a
nonbranched all-carbon feedstock in the presence and absence of Co, Ni, and Cu atoms. We find that the
presence of Co and Ni leads to substantial amounts of branched carbon atoms, leading eventually to the
formation of carbon-nanotube-like species. In contrast, we find that under the same simulation conditions Cu
leads to very little branching and leads to products with no nanotube character. In the absence of metals no
branching is observed at all. These results suggest that Ni and Co catalyze the production of nanotube-like
species whereas Cu does not. This is in excellent agreement with experimental observations, demonstrating
that ReaxFF can provide a useful and computational tractable tool for studying the dynamics of transition
metal catalytic chemistry.