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Langmuir 1991, 7, 2528-2533
Dissociation of H2 on Metal Surfaces J. Harris Institut fur Festkorperforschung, Forschungszentrum Jiilich, 0-51 70 Julich, BRD Received April 10, 1991 Cluster calculationsimply that the only stable state of H2 on simple and noble metals is fully dissociated. Calculations using a jellium substrate predict a stable state of molecular chemisorptton. A plausible reason for the difference is proposed. By use of an energy diagram constructedon the baslspf small cluster calculations, it is shown how molecules with very low kinetic energy can utilize vibrational energy to overcome a high entrance channel energy barrier to dissociation. Potential Energy Surface for Hz on Metal Surfaces To date, calculations of the interaction of H2 with metal surfaces have included either the atomic nature of the substrate (cluster models)’-S or its extended nature Gellium models)4 but not both together. Cluster calculations for a Cu “substrate” show similarities and differences with the predictions of the jellium model. As the H2 molecule approaches the surface, the energy increases, goes through a maximum (the top of the “activation barrier”), and then decreases into the chemisorption well. In cluster models it is found, regardless of the number of atoms that are used in the surface cluster, that the optimal interproton distance, D’, increases suddenly as the activation barrier is overcome so that the chemisorption region corresponds to more or less complete dissociation. With a jellium description of the substrate, D increases slightly toward the top of the barrier, but then decreases again sharply into the chemisorption region,which corresponds therefore to molecular chemisorption. These features can be understood as follows. The increase in the energy a t large molecule-surface separations is due to Pauli repulsion between the most extended cluster orbitals (or jellium orbitals with energy near the Fermi energy) and the fully occupied, low-lying la, orbital of H2. The metal orbitals, which are much higher-lying than la,, are pushed upward in energy (primarily by the requirement of orthogonality to the la,) and this is the source of the Pauli repulsion. Superposed on the repulsion is a weak, long-range van der Waals attraction and these two components form an at least quasistable state of physisorption. The existence of this state has been established experimentally and unequivocally for H2 on CU(~OO)~ and there can be little doubt that it exists also for other simple and noble metals. The physisorption well, of depth typically 30 meV, forms a minimum quite far from the surface (-6 au above the uppermost layer of ions) and so is practically onedimensional with the underlying lattice structure of the substrate causing only a weak ripple in directions parallel to the surface. The “back wall” of the physisorption well serves also as the “front wall” of the activation barrier which separates physisorption and chemisorption regions. The chemisorption region proper corresponds to protonsurface distances
