Migration mechanism of aromatic-edge growth

A new reaction pathway is suggested for the five-member ring migration along a graphene edge. The migration sequence is initiated by H atom addition to the adsorbed cyclopenta group. The elementary steps of the migration pathway were analyzed using quantum-chemical calculations. Based on the obtained energetics, the dynamics of the system were investigated by solving the master equations. The reaction rates of the new pathway show values sufficiently high to compete with, and even dominate, other surface reactions. The kinetics results indicate that the rate-limiting step of the migration sequence is the P-scission of the five-member ring after the addition of an H atom, whereas the other steps reach partial equilibrium at higher temperatures. Employing the new migration kinetics along with adsorption, desorption, and growth steps, sterically resolved kinetic Monte Carlo simulations were carried out at conditions typical of soot growth in hydrocarbon flames. The simulation results further support the critical role of five-member ring migration in the growth of graphene layers. The evolving surface morphology and ensuing growth rate are determined by competition between the migration of five-member rings and nucleation of six-member rings at surface corners. An important implication of this migration phenomenon is that while five-member rings are constantly being formed on the growing edge, they do not accumulate; rather they are converted to six-member rings. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.