Steric effect in dissociative chemisorption of hydrogen on Cu

Previous quantum dynamics studies showed that the sticking coefficient of hydrogen on metals is strongly influenced by the initial rotational state of the hydrogen molecule. In order to further elucidate this steric effect, we report an analysis of the sticking coefficient of the ground vibration of hydrogen on Cu in its relation to the spatial distribution of the initial wavefunction using a 3D quantum mechanical model. Snap-shots of the wavefunction are plotted at various times of the propagation and they show that the initial orientation of the angular momentum eigenfunction is maintained during a molecule-surface collision. Thus the initial orientation of the molecule plays an important role in adsorption. Significant enhancement of the dissociation probability at threshold energies is predicted for highly oriented hydrogen molecules on metals whose molecular axis is parallel to the metal surface. This orientational enhancement of the dissociation probability for molecules on a surface may have a significant impact in heterogeneous catalysis, in particular, when activated chemisorption of the reagent molecule on a metal is a rate limiting step.