The dissociation of ${\mathrm{H}}_2$ at metal surfaces is discussed with particular reference to the role of $d$ electrons. It is shown that the $d$ holes of a transition metal serve initially as sinks for $s$ electrons. This reduces the Pauli repulsion and so the entrance-channel activation barrier. $s-d$ transfer continues into the chemisorption region and allows the formation of metal-H bonds with only a moderate weakening of the metal-metal bonds.

• Received 23 April 1985

DOI:https://doi.org/10.1103/PhysRevLett.55.1583