Adsorption dynamics of H2 on Pd(100) from first principles

Abstract

We study H₂ dissociative adsorption on Pd(100) through classical molecular dynamics (MD) calculations, using density functional theory (DFT) to describe the molecule–surface interaction potential. We employ two methods to evaluate the forces acting on the atoms along the trajectories: (i) by doing a DFT calculation (and using the Hellman–Feynman theorem) every time step, and (ii) by computing the gradient of a six-dimensional potential energy surface (PES) obtained first, by interpolation of DFT total energy results using the corrugation reducing procedure (CRP). The corresponding MD calculations, hereafter referred to as ab initio MD (AIMD) and CRP-PES-MD, respectively, provide very similar dissociative adsoption probabilities (P_diss) as a function of the impact energy (E_i) for initial rotational states characterized by 0 ≤J≤ 4 indicating that the interpolated CRP-PES gives a faithful representation of the underlying ab initioPES. Thus, we make use of the computationally cheaperCRP-PES-MD for a detailed analysis of rotational effects on dissociative adsorption for 0 ≤J≤ 12. In agreement with available experimental data for H₂ interacting with Pd surfaces, we have found that P_diss barely depends on J for E_i≥ 200 meV, and that it follows a non-monotonic J-dependence at low energies. Our simulations show that two competing dynamical effects which were previously suggested based on lower-dimensional model calculations are indeed also operative at low energies in a realistic high-dimensional treatment. For low values of J, a shadowing effect prevails that entails a decrease of P_diss when J increases, whereas for J > 6, rotational effects are dominated by the adiabatic energy transfer from rotation to perpendicular motion that provokes the increase of P_diss with increasing J.