Adsorption of molecular nitrogen on clean and modified Ru(001) surfaces: The role ofσbonding

Abstract

A detailed study of the adsorption of ${\mathrm{N}}_2$ on clean and chemically modified Ru(001) surfaces suggests that the mechanism of ${\mathrm{N}}_2$ adsorption is qualitatively different from that of the isoelectronic CO molecule. A multitude of experimental techniques performed on ruthenium preadsorbed with well-characterized coverages of electron donors (potassium) and acceptors (oxygen) have produced the following principal findings: (1) On all surfaces studied, ${\mathrm{N}}_2$ adsorption produces a negative work-function change which results principally from a transfer of charge from the ${\mathrm{N}}_2$ molecule to the surface. (2) The charge transferred per ${\mathrm{N}}_2$ admolecule is reduced in the presence of potassium, increased in the presence of oxygen. (3) ${\mathrm{N}}_2$ interacts repulsively with potassium adatoms, attractively with oxygen adatoms. (4) The observation that potassium precoverages as low as ${\mathrm{CTHETA}}_{\mathrm{K}>0.08\mathrm{}}$ completely suppress the adsorption of ${\mathrm{N}}_2$ at 85 K enables us to determine a minimum K-${\mathrm{N}}_2$ interaction distance of 4.25 Å. (5) The ${\mathrm{N}}_2$ adsorption bond is weakened in the presence of potassium, strengthened in the presence of oxygen. (6) N—N bond strengthening (weakening) as observed in the vibrational spectrum is always accompanied by ${\mathrm{N}}_2$ adsorption bond strengthening (weakening). These experimental results indicate that the adsorption bond of ${\mathrm{N}}_2$ is formed principally through σ donation in contrast to that of CO which is widely believed to be mediated via synergistic charge donation from the CO 5σ orbital to the metal and ‘‘back-donation’’ from the metal d band to the CO 2π orbital. This mechanism contradicts most theoretical models of ${\mathrm{N}}_2$ adsorption, which predict similar bonding mechanisms for these two diatomic molecules.