POISONING OF HYDROGEN DISSOCIATION AT PD(100) BY ADSORBED SULFUR STUDIED BY AB-INITIO QUANTUM DYNAMICS AND AB-INITIO MOLECULAR-DYNAMICS

We report calculations of the dissociative adsorption of H-2 at Pd (10 0) covered with 1/4 monolayer of sulfur using quantum dynamics as well as molecular dynamics and taking all six degrees of freedom of the tw o H atoms fully into account. The ab-initio potential-energy surface ( PES) is found to be very strongly corrugated. In particular, we discus s the influence of tunneling, zero-point vibrations due to the localiz ation of the wave function of the nuclei when narrow valleys of the PE S are passed, steering of the approaching H-2 molecules towards low-en ergy barrier configurations, and the important role of subsurface abso rbates for the hydrogen dissociation. It is shown that ''established'' concepts derived from low-dimensional dynamical studies are not neces sarily valid in a high-dimensional treatment. (C) 1998 Elsevier Scienc e B.V. All rights reserved.