Dynamics of the Dissociation of Hydrogen on Stepped Platinum Surfaces Using the ReaxFF Reactive Force Field

The dissociation of hydrogen on eight platinum surfaces, Pt(111), Pt(100), Pt(110), Pt(211), Pt(311), Pt(331),
Pt(332), and Pt(533), has been studied using molecular dynamics and the reactive force field, ReaxFF. The
force field, which includes the degrees of freedom of the atoms in the platinum substrate, was used unmodified
with potential parameters determined from previous calculations performed on a training set exclusive of the
surfaces considered in this work. The energetics of the eight surfaces in the absence of hydrogen at 0 K were
first compared to previous density functional theory (DFT) calculations and found to underestimate excess
surface energy. However, taking Pt(111) as a reference state, we found that the trend between surfaces was
adequately predicted to justify a relative comparison between the various stepped surfaces. To assess the
strengths and weaknesses of the force field, we performed detailed simulations on two stepped surfaces,
Pt(533) and Pt(211), and compared our findings to published experimental and theoretical results. In general,
the absolute magnitude of reaction rate predictions was low, a result of the force field's tendency to underpredict
surface energy. However, when normalized, the simulations show the correct linear scaling with incident
energy and angular dependence at collision energies where a direct dissociation mechanism is observed.
Because ReaxFF includes all degrees of freedom in the substrate, we carried out simulations aimed at
understanding surface-temperature effects on Pt(533). On the basis of the results on Pt(533)/Pt(211), we studied
the reaction of hydrogen at normal incidence on all eight surfaces in a range of energies where we anticipated
the force field to give reasonable qualitative trends. These results were subsequently fit to a simple linear
model that predicts the enhanced reactivity of surfaces containing 111-type atomic steps versus 100-type
atomic steps. This model provides a simple framework for predicting high-energy/high-temperature kinetics
of complex surfaces not vicinal to Pt(111).