  We present Monte Carlo simulations on Eley-Rideal abstraction reactions of
atomic hydrogen chemisorbed on graphite. The results are obtained via a hybrid
approach where energy barriers derived from density functional theory
calculations are used as input to Monte Carlo simulations. By comparing with
experimental data, we discriminate between contributions from different
Eley-Rideal mechanisms. A combination of two different mechanisms yields good
quantitative and qualitative agreement between the experimentally derived and
the simulated Eley-Rideal abstraction cross sections and surface
configurations. These two mechanisms include a direct Eley-Rideal reaction with
fast diffusing H atoms and a dimer mediated Eley-Rideal mechanism with
increased cross section at low coverage. Such a dimer mediated Eley-Rideal
mechanism has not previously been proposed and serves as an alternative
explanation to the steering behavior often given as the cause of the coverage
dependence observed in Eley-Rideal reaction cross sections.
